Geology of the country around Coventry and Nuneaton. Memoir for 1:50 000 geological sheet 169 (England and Wales)

By D McC Bridge J N Carney R S Lawley and A W A Rushton

Bibliographical reference: Bridge, D McC, Carney, J N, Lawley, R S, and Rushton, A W A. 1998. Geology of the country around Coventry and Nuneaton. Memoir of the British Geological Survey, sheet 169 (England and Wales).

London: The Stationery Office 1998. NERC copyright 1998. First published 1998. ISBN 0 11 884520 9. Printed in the UK by The Stationery Office J 53234 C6 6/98

The grid used on the figures is the National Grid taken from the Ordnance Survey map. (Figure 1) is based on material from Ordnance Survey 1: 50 000 scale map number 140 © Crown copyright reserved Ordnance Survey licence no. GD272191/1998.

(Front cover) Cover photograph: Panoramic view looking east across Judkins' Quarry, Nuneaton, towards Hinckley. The face to the left exposes the Precambrian Caldecote Volcanic Formation which is unconformably capped (near top of conveyor belt) by Triassic strata. The conical spoil heap of 'Mount Jud' overlooks a modern terraced waste disposal operation from which landfill gas is being generated for public supply. (MN27927) (Photographer: T P Cullen)

(Rear cover)

Other publications of the Survey dealing with this and adjoining districts

Books

Maps

Acknowledgements

In the memoir, the chapters on Precambrian rocks, Ordovician intrusions, and Devonian and Jurassic sedimentary rocks were largely written by Dr J N Carney, whilst the section on Cambro-Ordovician sedimentary rocks was written jointly by that author and Dr A W A Rushton. The Carboniferous and Economic Geology chapters are by Mr D McC Bridge, and the account of the Triassic rocks is by Mr R S Lawley. Dr Carney and Mr Bridge combined to write the remaining chapters. The many other contributors are listed on the title page, and acknowledged in the appropriate sections of the text. The field survey was carried out under the direction of Drs R A Old and J W Baldock. Data from BGS open-file reports written by Mr M G Sumbler and Drs J W Baldock, R A Old and J G Rees (listed in Appendix 2) have been freely used in this account. The memoir was edited by Messrs T J Charsley and J I Chisholm.

We gratefully acknowledge information and assistance generously provided by the geological staff of British Coal, and by the major quarrying companies for allowing access to their sites. Thanks are also due to Mr A F Cook for providing palaeontological specimens and for sharing with us his wide local knowledge. The local authorities and site investigation companies have provided a wealth of information, without which the resurvey would have been seriously disadvantaged. We also acknowledge the access and help given by numerous farmers and landowners throughout the district during the course of the geological survey.

Notes

Preface

This memoir, written to accompany the Coventry 1:50 000 geological map (sheet 169), provides a comprehensive account of the geology of a distinctive part of the English Midlands, where the exposed geological record spans more than 600 million years. Deep quarries in the Nuneaton Inlier afford exposure of the only surface representatives of Precambrian basement rocks between Charnwood Forest and the Malverns. The inlier is flanked to the west by the Warwickshire Coalfield, and to the east by the Hinckley Basin, a Triassic-age basin fill, largely concealed by a thick mantle of Pleistocene glacial deposits.

Mineral extraction has long been important to the economy of the district, but many of the traditional industries are in decline, and the redevelopment of former colliery sites, brickpits and quarries close to or within urban centres, has itself created geological problems that can be assessed only by reference to modern geological maps.

The information presented in this memoir incorporates the results of detailed mapping by the British Geological Survey, as well as subsurface information from numerous site investigations and from exploration by the coal and water industries. It is is intended to be of practical value to a wide range of users, from specialists in the earth sciences or related disciplines, to others who may use the findings as an aid to planning and development or as a basis for mineral exploration. It should also be of interest to local naturalists and amateur geologists.

The memoir contains much hitherto unpublished material, and the many specialist contributions highlight the increasing emphasis placed by the Survey on a multidisciplinary approach to geological mapping.

The memoir represents a major advance in the understanding of the geological history of the district, and I am confident it will play its part for many years to come in serving the needs of the scientific, planning and commercial communities.

David A Falvey, PhD Director, British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham. NG12 5GG.

Geology of the country around Coventry and Nuneaton—summary

In a region rich in natural resources, and with a long history of mineral extraction, there is an evident need for up-to-date geological information, as a foundation for the planning of land use and development. This memoir is intended to meet this basic need, while also providing an overview of the geology of part of the English Midlands, and should be of interest to amateur and professional geologists alike.

The district described in this memoir extends northwards from the city of Coventry to encompass the adjoining towns of Bedworth, Nuneaton and Hinckley; this urbanised tract contrasts sharply with the surrounding rural areas which include parts of north Warwickshire, Leicestershire and the West Midlands.

In terms of geological structure, the western part of the district lies within the Warwickshire Coalfield, and is separated from the Hinckley Basin farther east by Precambrian and Lower Palaeozoic basement rocks exposed in the Nuneaton Inlier. This last structure preserves remnants of a Charnian volcanic arc and includes one of the most complete Cambrian successions in Britain, together with Upper Devonian continental and marine deposits.

The Warwickshire Coalfield, on the western flanks of the Nuneaton Inlier, remains one of Britain's few active coalfields, with an estimated 400 million tonnes of coal still recoverable. The coal-bearing strata are overlain by Upper Carboniferous redbeds which are important hydro-geologically, providing Coventry with large volumes of water for public and industrial use.

The Hinckley Basin contains a sedimentary fill of Triassic age, comprising redbeds of fluviatile, lacustrine and wind-blown origin. It is bounded to the west by the Polesworth Fault, a structure that has a long history of movement above a major line of weakness in the basement.

An extensive marine transgression towards the end of Triassic times is represented by the mudstones and limestones of the Penarth Goup; similar rocks persist through-out the Jurassic succession.

Parts of the west of the district are drift free but in the east there are extensive spreads of Pleistocene glacial deposits. Postglacial deposits are restricted to terrace deposits and alluvial tracts bordering the main rivers.

This memoir also describes the tectonic history and deep structure of the district.

(Front cover) Panoramic view looking east across Judkins' Quarry, Nuneaton, towards Hinckley. The face to the left exposes the Precambrian Caldecote Volcanic Formation which is unconformably capped (near top of conveyor belt) by Triassic strata. The conical spoil heap of 'Mount Jud' overlooks a modern terraced waste disposal operation from which landfill gas is being generated for public supply. (MN 27927) (Photographer: T P Cullen)

(Frontispiece) Panoramic view of Sudeley Opencast Site, prior to its restoration in 1991. Strata dip at about 12° into the western face (at left), which is about 80m high. The visible worked coal seams in ascending order are the Low Main, Nine Feet, Ell (two leaves), Two Yard (being worked at foot of main face) and Four Feet (A14575).

(Geological succession) Summary of geological sequence.

History of survey of the Coventry sheet

The district covered by the Coventry (169) sheet of the 1:50 000 geological map of England and Wales was origi­nally surveyed by A C Ramsay, W T Aveline and H H Howell between 1855 and 1859 and the results published on the one-inch Old Series sheets 53NW, 53NE, 54NE, 62SE, 63SW and 63SE.

The primary six-inch survey of the district was carried out between 1912 and 1915 by T C Cantrill, C H Cun­nington, T Eastwood, T H Whitehead and W Gibson. New Series one-inch Sheet 169 appeared in separate Drift and Solid editions in 1922 and 1926, respectively. The coalfield area was revised by G H Mitchell in 1940.

The present resurvey at six-inch or 1:10 000 scale was carried out between 1963 and 1992. The surveyors were J N Carney, D McC Bridge, R S Lawley, R A Old, M G Sumbler, J G Rees, J W Baldock, K Taylor, E G Poole, D J Lowe, B C Worssam, B A Hains, and A Horton.

The sheet is published at 1:50 000 scale in two editions: Solid and Drift (on which Drift and Solid deposits are both coloured) and Solid with Drift (on which only the Solid rocks are coloured).

Geological 1:10 000 scale National Grid maps included wholly or in part in 1:50 000 sheet 169 (Coventry) are listed below, together with the initials of the geological surveyors and dates of survey. In the case of marginal sheets, only those surveyors who mapped within the area covered by sheet 169 are listed. Copies of the maps have been deposited in the BGS libraries at Keyworth and Edinburgh for public reference and may also be inspected in the BGS London Information Office in the Natural History Museum Earth Galleries, South Kensington, London. Copies may be purchased directly from BGS as black and white dyeline sheets.

SP 27 NW RAO 1978, 1980
SP 27 NE RAO 1978, 1987
SP 28 NW RAO 1988
SP 28 NE JGR 1988
SP 28 SW MGS 1980
SP 28 SE JGR 1987
SP 29 NW KT 1965
SP 29 NE KT 1963
SP 29 SW DMcCB 1991
SP 29 SE DMcCB 1991
SP 37 NW RAO 1978, 1986–87
SP 37 NE MGS 1978
SP 38 NW DMcCB 1988
SP 38 NE DMcCB 1990
SP 38 SW RAO 1986–88
SP 38 SE DMcCB 1987
SP 39 NW KT, JWB 1964–65, 1990
SP 39 NE JWB, RSL 1990–91
SP 39 SW JWB 1989–90
SP 39 SE DJL, RSL 1990
SP 47 NW MGS 1979
SP 47 NE MGS 1976, 1979
SP 48 NW JNC 1991
SP 48 NE JNC 1991–92
SP 48 SW JNC 1990
SP 48 SE JNC 1991
SP 49 NW AH, RSL 1972–73, 1991
SP 49 NE BCW 1965
SP 49 SW RAO 1989
SP 49 SE RSL 1991
SP 57 NW Aims 1976, 1979
SP 58 NW BAH, EGP, RSL 1961, 1963, 1992
SP 58 SW JNC 1991
SP 59 NW BAH, EGP 1961, 1964
SP 59 SW BAH, RSL 1961, 1991

Chapter 1 Introduction

This memoir describes the geology of the district covered by the 1:50 000 Coventry Geological Sheet (169) published in 1993. The district lies mostly in the county of Warwickshire, but includes parts of Leicestershire and the West Midlands. The northern suburbs of the densely populated and industrialised city of Coventry dominate the south-centre of the district and merge northwards with the towns of Bedworth, Nuneaton and Atherstone, which have grown up on the exposed margin of the Warwickshire Coalfield. Hinckley, lying farther to the east, is the only other large centre of population.

The district can be divided into three areas of contrasting scenery, illustrating well the close relationship between physiography and geology (Figure 1) and (Figure 2).

The central part of the Warwickshire Coalfield occupies the western half of the district. It consists of a dissected plateau region of Upper Carboniferous sandstones and mudstones, bordered to the north-east by a narrow peripheral outcrop of Coal Measures and Millstone Grit. The plateau, which rises to a maximum height of 181 m above OD near High Ash Farm, forms part of the so-called Meriden Gap, an important greenbelt area separating the city of Coventry from the West Midlands conurbation. Sandstones within the succession define the structural and topographical grain of the coalfield, producing north-east- and north-facing scarps, and long dip slopes; the valleys are floored by mudstones. Much of this land is in agricultural use.

The Nuneaton Inlier flanks the coalfield to the northeast; its more resistant Late Proterozoic and Lower Palaeozoic basement rocks give rise to a tract of elevated ground, which has long been the focus of a hardrock quarrying industry. Lamprophyre sills that have invaded this group of rocks form upstanding ridges trending parallel to the regional strike.

To the east is an area of more subdued relief underlain by relatively soft sandstones and mudstones of Triassic and Jurassic age. The character of this area reflects not so much the bedrock geology but rather the distribution of the Pleistocene glacial drift, which forms a thick and varied mantle throughout this largely rural area.

The main watershed of England crosses the district from Corley Moor in the west to Ashby Parva in the east.

To the north of this divide the rivers Anker and Soar flow northwards into the Trent; to the south, the rivers Sowe, Swift, Sherbourne and Smite Brook form part of the River Avon catchment.

Mineral extractive industries are important to the local economy. Although coal and clay have been dug in the district probably since Roman times, it was the Industrial Revolution that produced the most dramatic change in the district. By the eighteenth century, the exposed coalfield was the industrial heart of the region, with upwards of fifty pits sited on the outcrop producing both coal and ironstone. Output reached its highest level in 1939 when 5.8 million tonnes of coal were produced from 20 mines (National Coal Board, 1985). Today, though large reserves of coal remain, successive mergers and closures have occurred, so that only Daw Mill in the west of the coalfield now remains in production. Apart from coal, the district is an important source of construction materials with six hardrock quarries in operation at the time of the survey. Sand and gravel aggregate is produced from glaciofluvial deposits at Gibbet Hill Quarry in the south-east of the district, and clay for brickmaking is dug in the centre of Coventry.

Geological sequence

(Geological succession) Summary of geological sequence.

The geological formations described in this memoir are listed on the inside of the front cover. The solid rocks range from Late Proterozoic to Jurassic in age; their distribution is shown in (Figure 2).

Outline of geological history

The oldest rocks exposed in the Coventry district form part of an igneous basement complex of latest Precambrian (Vendian) age. They are the remnants of the Charnian volcanic arc, formed by the eruption of calcalkaline magmas generated along a subduction zone marking the site of convergence between two tectonic plates. When particularly silica-rich, dacitic magmas reached the surface the eruptions were explosive, giving rise to voluminous subaqueous pyroclastic flows composed of coarse-grained, crystal-enriched material, but there were also periods of less vigorous activity when andesitic magmas were erupted, releasing clouds of volcanic ash or dust. The volcaniclastic materials accumulated in fairly deep waters in basins flanking the volcanic arc, building up the bedded sequence of the Caldecote Volcanic Formation. As volcanism began to wane, the basinal sequence was gently folded and faulted, and the magmas became more basic, forming a complex of basaltic-andesite and granophyric diorite intrusions of which the youngest — signalling the end of magmatic activity along the arc — is dated to 603 Ma. Palaeogeographical reconstructions suggest that the Charnian arc probably lay off the western margin of the Gondwana continent, forming part of a larger system of volcanic chains separated by microplates. Such tectonic configurations are usually short-lived, geologically speaking, and eventually the plate boundary came under compression, signalling the onset of the Avalonian–Cadomian orogeny of late Precambrian times. These movements brought about convergence and collision between the Charnian arc and the other volcanic systems, so forming a larger crustal block — the Midlands Microcraton — that now constitutes the basement to central England. Structures formed within the microcraton during the orogeny are of a deep-seated nature, and were to influence the tectonic development of this area later in Palaeozoic and early Mesozoic times.

The Avalonian upheavals elevated the Charnian igneous rocks within the now-enlarged Gondwana landmass, exposing them to a long episode of erosion and chemical degradation. By latest Proterozoic or earliest Cambrian times, however, the crust of the Midlands Microcraton began to extend, causing this landmass to subside beneath the waters of the expanding Iapetus Ocean. Deposition of the Hartshill Sandstone Formation followed. At first the landmass may have subsided differentially along faults, and into the graben so formed were poured highly immature sediments derived by the down-slope flowage of weathered material. A more general phase of subsidence then followed, laying down a typical marine transgressive sequence consisting firstly of shore-face sandstones and then, as the waters deepened, of more glauconitic and muddy sandstones deposited on the inner shelf. Towards the end of this cycle, pronounced changes in relative sea-level temporarily interrupted the supply of sandy material to the basin, producing a condensed sequence of richly fossiliferous, glauconitic and phosphatic limestone hardgrounds. There was then a brief return to sandstone deposition before the waters of the basin deepened significantly, and muddy environments of the outer shelf became prevalent.

In a basinal area that was now remote from the shoreline, muddy sediments of the Stockingford Shale Group accumulated over a period extending from the Lower Cambrian to at least the Lower Ordovician (Tremadoc), forming a sequence broken only by a minor disconformity associated with an influx of more sandy material. Conditions of deposition fluctuated between those of anoxia (oxygen starvation), and those in which the sediments were oxygenated, and it is possible that these environmental changes contributed to the richness and diversity of the trilobite fauna.

Between the Tremadoc and Upper Devonian there is an hiatus in the geological record corresponding to a period of about 115 million years. The sequence of events during this period can be reconstructed only with difficulty: subsidence may have continued, or been accelerated, forming across the western part of the area a basin in which considerable thicknesses of sediment or volcanic material accumulated, so raising the geothermal gradient and causing some recrystallisation in the underlying rocks. The nature or age of this basin fill cannot be determined, but the record within the surviving Cambro-Ordovician rocks shows that it was affected by folding and faulting in the early phases of the Caledonian orogeny. In late Ordovician times, about 440 to 450 million years ago, the Precambrian and Cambro-Ordovician rocks were invaded by magmas that solidified at depth to form dioritic and lamprophyric intrusive rocks, now represented, respectively, by the South Leicestershire Diorites and the Midlands Minor Intrusive Suite. Palaeogeographical considerations indicate that this magmatism must have resulted from the direction of heat towards the crust of the Midlands Microcraton, which lay between active subduction zones situated in the west, beneath the Welsh Basin, and to the north-east, along the edge of Tornquist’s Sea.

At the end of the Caledonian Orogeny, the Acadian tectonism of mid-Devonian age signified the distant events that closed the Iapetus Ocean farther west, and was transmitted to the Midlands Microcraton as a relatively non-penetrative type of deformation. The movements probably caused inversion of the earlier basin, with large-scale low-amplitude flexuring of the Cambro-Ordovician rocks, and was associated with regional uplift that removed all the Lower Palaeozoic strata younger than Tremadoc.

The Caledonian upheavals were followed by tectonic quiescence, and sedimentation was re-established across the North Wales–North Pennines landmass in late Devonian times; in the present district meandering river systems deposited the fine- and coarse-grained continental sediments of the Oldbury Farm Sandstone Formation. A marine incursion from the south is also recorded in these deposits. A change in the fluviatile regime towards the end of the Devonian resulted in the deposition of carbonate hardpans, or calcretes, in the soil profiles of the time.

The Lower Carboniferous period in the British Isles was a time of major crustal extension, which saw the emergence of the Wales–Brabant High as a stable land barrier, bounded to the north by a series of small basins with intervening highs. By late Dinantian times, as active extension was replaced by a pattern of general thermal sagging, an area of more continuous sedimentation the Pennine Basin — began to evolve to the north of the land barrier.

Throughout the early development of the Pennine Basin, the Coventry district occupied a position on the northern fringes of the Wales–Brabant High and was unaffected by early Carboniferous transgressions. Sedimentation only began in late Namurian times, when an attenuated sequence of the deltaic Millstone Grit was laid down in embayments on the edge of the land barrier. Marine incursions occasionally flooded the basin margin depositing Lingula-bearing mudstones. These indicate that only the higher stages of the Namurian, R2 and G1, are represented.

By Westphalian times the hingeline of the Pennine Basin had moved farther south, and coal-swamp conditions existed throughout the region. For a period of about 3 million years conditions of slow subsidence in an alluvial or delta-plain setting favoured accumulation of peat and associated lacustrine sediments of the Coal Measures. In areas of reduced subsidence close to the Wales–Brabant High, peat accumulation was sustained over long periods, a factor that was to prove significant to the eventual generation of the thick and economically important coal seams that make up the Warwickshire Thick Coal. Eustatic marine transgressions took place across the delta plain on several occasions, successive incursions encroaching progressively farther south on to the Wales–Brabant High.

During the Bolsovian Stage (Westphalian C) the style of sedimentation changed as a conseqence of Variscan tectonic uplift in areas to the south and west of the coal basin. This led to the regression of the coal-swamps, and to the deposition of redbeds in freer-draining parts of the floodplain and in alluvial fans at the basin margin. Sedimentation was accompanied locally by episodic outpourings of volcaniclastic material.

A period of uplift and erosion followed, before conditions reverted to those more typical of the early Westphalian. The fluvial sandstones and back-swamp mudstones that characterise the Halesowen Formation point to a period of fairly slow subsidence, when the alluvial plain was crossed by large river systems, probably fed from the south.

Towards the end of the Carboniferous Period, redbed deposition was re-established as the northward advance of the Variscan deformation front caused renewed uplift and basin formation within the Variscan foreland area. Silts, sands and gravels eroded from the adjacent upland areas to the south were delivered to the region by ephemeral rivers and floods to form the thick detrital complexes of the Meriden Formation and overlying red-bed sequences.

The culmination of the Variscan orogeny, in late Carboniferous to early Permian times, produced a complex pattern of uplifted and flexured, fault-bounded crustal blocks, the most important structures being the synclines which preserve the coal seams of the present Warwickshire Coalfield. Deformation extended to deep levels within the crust, rejuvenating many of the Precambrian basement structures and tilting the mantling Palaeozoic strata to high angles along linear zones of monoclinal flexuring.

Relative quiescence prevailed over the next 50 million years, spanning most of the Permian Period. During this time, processes of erosion acted on the rugged Variscan landmass, stripping away the Carboniferous rocks from the most elevated parts, exposing large expanses of Cambro-Ordovician strata and even forming a small structural 'window' into the Precambrian rocks.

In early Triassic times the crust began once more to extend, causing reactivation of some of those Variscan faults with favoured orientations. In the east of the district, the former area of Variscan uplift subsided along the line of the Polesworth Fault, creating the Hinckley Basin as a negative inversion structure. The Polesworth Fault remained active during the subsequent filling of the basin, so influencing the character and distribution of the deposits formed.

The Triassic infill to the Hinckley Basin accumulated largely under continental conditions, when the region lay some 15 to 20° north of the equator. The deeper parts of the basin have not been investigated but, by analogy with adjacent areas, are thought to consist of conglomeratic sandstones laid down by northerly flowing braided streams. The sediment was ultimately derived from the Armorican mountains far to the south, in what is now the English Channel and Brittany. With time, the river system evolved into a more mature meandering system, and by middle Triassic times, when the Mercia Mudstone was deposited, the subdued landscape was one of broad floodplains and ephemeral lakes, linked on occasion to the sea.

Late in the Triassic Period, a marine transgression spread across the region, depositing siltstones, mudstones and shelly limestones within a progressively subsiding, tropical shelf sea. This persisted into the Jurassic Period, with deposition of the Lias Group, of which only the lower part is proved in the present district.

The Tertiary (Neogene and Palaeogene) Period was predominantly one of uplift and erosion, during which the Jurassic formations were gently tilted and removed from all but the south-eastern corner of the district.

During Middle Pleistocene times the region was glaciated, and a widespread blanket of glacial drift was deposited across much of the terrain. The glacial deposits are the product of a single glaciation, here ascribed to the Anglian Stage. Tills contain erratics that suggest ice advanced from both northerly and north-easterly directions at different times, with intervening periods of glacial lake deposition. Sands and gravels were laid down as morainic accumulations, as deltas in glacial lakes, and as proglacial sandurs. Late Pleistocene erosion has destroyed most of the glacial landforms, and glacial debris has contributed to postglacial river terrace and alluvial deposits.

Chapter 2 Precambrian (Neoproterozoic III)

The Nuneaton Inlier includes the only surface representatives of Precambrian basement rocks between Charnwood Forest and the Malvern Hills. The Precambrian outcrop covers a limited area of 2.8 km2, between Nuneaton and Hartshill, but the extent of the exposure afforded by two deep quarries is proportionately very large, so enabling a comprehensive range of lithologies and structures to be demonstrated. The occurrence is of particular geological significance in that it forms part of a sequence which preserves intact the transition between the Proterozoic and Lower Palaeozoic erathems.

The Precambrian rocks consist of volcaniclastic strata with minor intrusions, and are correlated with the Charnian Supergroup whose type area is in Charnwood Forest, some 23 km to the north-east (Moseley and Ford, 1985). They are the local representative of the 'Charnwood Terrane' (Pharaoh et al., 1987a), which itself is part of a larger crustal massif, the 'Midlands Microcraton', that resisted the Caledonian deformation (Pharaoh et al., 1987b). Within this microcraton, the Charnwood Terrane is in tectonic contact with belts of similar volcanic rocks that were joined together as a result of plate convergences during the Avalonian/ Cadomian orogeny, in latest Proterozoic time. Multidisciplinary studies, summarised by Pharaoh et al. (1987a and b), support an earlier view of Turner (1949) that the crust of the Midlands Microcraton occupies a triangular-shaped area, tapering northwards, extending from eastern Leicestershire to the Longmynd.

In the western part of their outcrop the Precambrian rocks pass unconformably beneath south-westerly dipping Lower Cambrian strata of the Hartshill Sandstone Formation. Farther east, a greater unconformity is developed with Triassic strata, which overlap westwards across the upthrown side of the Polesworth Fault. The Precambrian rocks themselves dip south-south-west or south-east, in angular discordance with the Lower Cambrian strata, and although they were affected in Precambrian time by folding and faulting, with a spaced fracture cleavage locally developed, they do not possess a penetrative slaty cleavage of the type described by Worssam and Old (1988) from the Charnwood Forest outcrop. Nevertheless, a minor metamorphic discordance, between Precambrian volcaniclastic rocks at lower anchizonal metamorphic grades and Lower Cambrian mudstones in the late diagenetic metamorphic grade, has been detected in the Nuneaton area (Merriman et al., 1993): the metamorphism of these rocks is discussed at greater length in Chapter 11.

It was Lapworth (1882) who first demonstrated the considerable age of these rocks by his discovery that they were unconformably overlain by strata yielding Cambrian fossils. However, their Precambrian age was not finally confirmed until Lapworth (1898) had established the magnitude of the unconformity from the fact that the highest beds in the overlying Hartshill Sandstone Formation contain Lower Cambrian fossils. Some indication of the diversity of these Precambrian volcanic rocks was given in the early descriptions by Lapworth (1886, 1898), who also utilised a remarkably modern and precise terminology. Later petrographic and chemical studies largely concentrated on an intrusion of granophyric diorite, classified as markfieldite (Jones, 1935), and on the nature of the unconformity between the Precambrian and Lower Cambrian rocks (Wills and Shotton, 1934). The studies by Allen (1957, 1968a) are among the most detailed published accounts of the volcaniclastic sequences and their interpretation.

There are no longer any natural exposures of the Precambrian rocks, but sections up to 100 m deep were available at the time of survey in Judkins' and Boon's roadstone quarries, and the following account is largely based on detailed studies of these (Carney and Pharaoh, 1993). The Precambrian is divided into the Caldecote Volcanic Formation, of bedded fine- and coarse-grained volcaniclastic rocks, and a later complex of small basic and intermediate intrusive stocks and sheets which include the body of markfieldite referred to above.

Caldecote Volcanic Formation

These rocks were called the 'Caldecote Volcanic Series' (e.g. Lapworth, 1898; Allen, 1968a) before being given their present name by Brasier et al. (1978). The formation belongs to the Charnian Supergroup (see below).

This memoir designates as the type area for the Caldecote Formation the whole of the Precambrian outcrop, from the south-eastern end of Judkins' Quarry [SP 3501 9266] north-westwards to just south of Atherstone Road [SP 3283 9509]. The partial type section ((Figure 3)a) comprises the rocks exposed at the Site of Special Scientific Interest (SSSI) on the upper northern levels of Boon's Quarry [SP 3301 9467], 50 m due south of Grange Farm ((Figure 4), locality 1). Other sections illustrated are based on the succession measured in the northeastern part of Judkins' Quarry [SP 3469 9308] (Figure 3)b; ((Figure 5), locality 1) and on a composite section measured in the north-western part of the same quarry [SP 343 933] ((Figure 3)c, (Figure 5), but these are periodically inaccessible and in any case are not expected to survive further quarrying and landfilling operations. The Caldecote Formation is at least 130 m thick beneath the unconformable Lower Cambrian or Triassic cover in Judkins' Quarry and its base is neither seen nor has been encountered in boreholes.

The age of the Caldecote Formation is no younger than 603 ± 2 Ma, which is the value obtained radiometrically by the U-Pb zircon method on a granophyric diorite intruding the succession in Judkins' Quarry (Tucker and Pharaoh, 1991). This limiting age supports some broad lithological and geo chemical correlations between the Caldecote Formation and the Maplewell Group of the Charnian Supergroup (Moseley and Ford, 1985), the latter yielding fossils indicative of the latest Precambrian, Vendian Stage (Ford, 1958, and subsequent reviews in Worssam and Old, 1988, and Brasier, 1989).

The Uriconian volcanic group of Shropshire has also been correlated with the Charnian (e.g. Greig et al., 1968) but is now known to be a younger Precambrian extrusive sequence, with U-Pb zircon ages grouped around 566 ± 2 Ma (Tucker and Pharaoh, 1991). The Uriconian rocks also differ geochemically from the Charnian (Thorpe, 1972) and are placed within a separate Vrekin Terrane', which forms the crust in the western part of the Midlands Microcraton (Pharaoh et al., 1987a). Further Precambrian volcanic rocks dated at 612 and 616 Ma occur within the concealed Caledonide orogenic terrane on the north-eastern flank of the Midlands Microcraton, as revealed in boreholes between Leicestershire and Norfolk; these may not be precise equivalents of the Charnian but may constitute a separate and third Precambrian volcanic arc terrane (Noble et al., 1993).

Subdivision and lithology

The Caldecote Formation is a bedded volcaniclastic succession in which massive or stratified crystal-lapilli tuff and its coarse-grained variants predominate over well-bedded to finely laminated tuffaceous sandstone, siltstone and mudstone. The latter were originally thought to constitute a lower bedded succession (Allen 1968a), but the present particularly deep quarry sections demonstrate that in fact they are interspersed at various stratigraphical levels within the coarser-grained deposits, as shown in (Figure 3). The sequence in the quarries is devoid of lava flows, although Allen (1968a) described brash apparently composed of flow-banded rhyolite lava in fields to the north of Grange Farm [SP 3303 9473] .

The lithological nomenclature followed here is based on that of Fisher (1961), and utilises a pyroclastic terminology for those volcaniclastic rocks consisting predominantly of fragments that owe their disaggregation to a volcanic process, irrespective of what processes led to their final accumulation. For the finer-grained beds, which are admixtures of pyroclastic material and modified or reworked (epiclastic) constituents, the prefix 'tuffaceous' is used, as recommended by Fisher (1966).

The range of volcaniclastic rocks within the Caldecote Formation is indicated in (Figure 3), and examples of their disposition within small areas is shown by the maps of Boon's Quarry (Figure 4) and Judkins' Quarry (Figure 5). These maps also indicate the various localities mentioned in the text. The formation cannot be subdivided into formal members but is seen here to be composed of two principal lithological associations, a crystal-lapilli tuff facies grouping and a tuffaceous siltstone facies grouping.

Crystal-lapilli tuff facies grouping

These rocks correspond to the feldspar-quartz-crystal-tuff (Welded Tuff) category of Allen (1957) and to the quartz-felsite of Lapworth (1882). In their coarse grain size and crystal-rich composition they are similar to certain massive beds within the Beacon Hill Formation, the lower division of the Maplewell Group in Charnwood Forest (Moseley and Ford, 1985), but are more thickly developed.

This facies grouping is the most important in volumetric terms, constituting about 85 per cent of the exposed thickness of the formation. It characteristically forms massive beds, which are up to 50 m thick in Judkins' Quarry ((Figure 3)b and c), but there are also stratified facies, described later. Exposures north-east of the Lower Cambrian unconformity at the SSSI in Boon's Quarry ((Figure 4), locality 2) show a grey, compact, massive rock crammed with crystals that commonly are of lapilli size (about 2 to 6 mm). The most abundant of these are plagioclase, forming between 50 and 60 per cent of the rock and seen as white to pale pink crystals with subhedral or rounded outlines; many are fractured into mosaics of subgrains which when disaggregated appear as clusters of small angular crystal fragments ((Plate l)a, see page 147). They are accompanied by between 15 and 20 per cent of grey and glassy quartz crystals with rounded margins and spherical, ovoid or sometimes euhedral, bi-pyramidal shapes; many exhibit a radially disposed internal micro-fracture pattern. A thin section of this rock (E62363) shows plagioclase to be clouded and in part albitised, with strained extinction caused by the extensive development of fractures that separate many crystals into a number of subgrain domains. The larger and more intact plagioclase and quartz crystals are commonly part-surrounded by narrow rims of fine-grained and turbid chloritic material representing adherences of the former host magma, and it is noticeable that many of the crystal margins so enclosed preserve original magmatic embayment textures. The original matrix constituents of this sample are largely recrystallised to felts of secondary white mica, chlorite and epidote, but in the matrix of an identical crystal lapilli tuff from the Judkins' Quarry (E62283) the outlines of abundant glass shards are preserved from such obscuration. They occur in random orientation and although possibly slightly flattened they are otherwise undeformed by secondary compression; they are replaced internally by grainy microcrystalline aggregates of quartz, feldspar, white mica and chlorite ((Plate 2)a, see page 148).

Dark inclusions are ubiquitous although minor (less than 10 per cent) constituents of the crystal-lapilli tuffs, and are of two types. Porphyritic inclusions, described by Waller (1886) and Allen (1957), have ovoid, discoid or equidimensional shapes, measure from a few millimetres to several decimetres in size and in detail have ragged margins with the enclosing crystal-lapilli tuff (Plate l)a. They have a fine-grained, black to dark green matrix enclosing plagioclase and quartz crystals similar in size and degree of brecciation to those in the host rock, although not as abundant. A thin section from Judkins' Quarry (E62181) shows magmatically embayed quartz and subrounded plagioclase crystals in a highly chloritic matrix which preserves 'fluidal' textures in some parts (Plate 2)b, see p.148 and vitric shards in others. This matrix resembles in appearance the chloritic material surrounding some of the crystals in the host tuff, as described above, suggesting that the dark porphyritic inclusions are fragments of scoria that have remained intact. Lithic blocks, forming the second type of inclusion, are commonly angular with sharp margins against the host tuff; these blocks include fine-grained aphyric to sparsely porphyritic andesite or dacite, devitrified glass with relict perlitic texture, and welded tuff. The last-mentioned shows a fluidal texture truncated at the margin of the clast (E62339), suggesting that welding and compression of vitric shards had occurred before incorporation into the host rock.

A rather less homogeneous variety of crystal-lapilli tuff forms the highest bed beneath the Triassic unconformity in the central north-eastern face of Judkins' Quarry ((Figure 3)b). It has a diffuse stratification which is caused by layers with crystal-dominant and lithic-dominant compositions. Many of the lithic fragments are microcrystalline aggregates which nevertheless have relict perlitic textures, indicating that they are composed of devitrified and recrystallised volcanic glass. Others are composed of altered fine-grained hornblende-andesite or dacite with quartz and plagioclase microphenocrysts. Particularly distinctive to this tuff are dark maroon inclusions showing a strong preferred orientation parallel to adjacent bedding planes; fluidal textures (E62334) suggest that these clasts are fiamme of the type found in welded tuffs. Lower in the same section, a 1 m-thick bed of stratified and graded crystal-vitric tuff contains abundant dark maroon, vesicular vitroclasts pseudomorphed by secondary chlorite and epidote aggregates. That this bed was produced by volcanism involving more basic magma compositions is suggested in a thin section (E62787) by the paucity of quartz crystals and presence of lithic clasts composed of hornblende-andesite and dark brown oxidised tachylyte glass.

The best examples of stratification within the crystallapilli tuff facies are displayed at outcrop by two beds which together cap the massive crystal-lapilli tuff succession in the north-western part of Judkins' Quarry ((Figure 3)c). In the lower bed of tuff-breccia, about 12 m thick ((Figure 5), locality 2), the stratification consists of a diffuse, 20 to 50 mm-scale layering defined by pale and dark grey colour variations. Particularly distinctive to this rock are the abundant (about 25 per cent) dark porphyritic inclusions; these are disc-shaped, up to 0.3 m long, and in places exhibit contorted or hooked shapes indicative of plastic deformation whilst still hot. Such inclusions become numerous in some layers where they amalgamate to form discontinuous beds. The upper stratified bed, 10 m thick, lies immediately beneath the Cambrian unconformity (locality 3): it is composed of alternations of pale grey coarse-grained crystal tuff and darker grey or greyish green crystal-vitric lapilli tuff in layers up to several centimetres thick. In a thin section of a crystal-vitric tuff layer (E62180) about 20 to 35 per cent consists of dark grey-green vitroclasts between 3 and 7 mm in size; they have filamentous matrix textures, preserved in secondary aggregates of chlorite, quartz and feldspar, and also enclose large quartz and plagioclase crystals with magmatically embayed boundaries. The other constituents of this rock are individual crystals of plagioclase (40 per cent) and quartz (20 per cent), both forming small fracture rhombs showing in-situ breakdown into mosaics of small anhedral grains.

Large-scale disruption of bedding within the Caldecote Formation is shown by the sediment-raft breccias which crop out in the north-western part of Judkins' Quarry. These beds occur within an otherwise homogeneous sequence of massive crystal-lapilli tuff ((Figure 3)c), and although only a few metres thick they form distinctive stratigraphical markers that can be traced over a strike length of about 200 m in the northern part of the quarry, as shown by the continuation of the bed at locality 4 (Figure 5). These rocks are similar to the 'slump breccias' found in the Blackbrook Group and the Bradgate Formation of the Maplewell Group, in Charnwood Forest (Moseley and Ford, 1985). The Judkins' Quarry breccias contain rafts up to a few metres long of laminated tuffaceous siltstone and mudstone, hosted within a heterogeneous lithology composed of crystal-lapilli tuff admixed with finer-grained tuff. The larger sediment rafts have ragged or feathered lateral terminations, suggesting that they have been pulled apart within the enclosing tuff, while smaller sediment clasts commonly show folding and contortions indicative of deformation in a semiconsolidated state. Some larger rafts dip at up to 75° southwest, as opposed to about 45° south-west for the succession as a whole, suggesting that the rafts are imbricated. At one contact between crystal-lapilli tuff and a sediment raft, the tuff shows a strongly aligned fluidal fabric caused by the mixing together and parallel entrainment of crystal-rich and finer-grained material; the beds at the base of the included raft are highly disturbed ((Plate 1)b, see p.147).

Tuffaceous siltstone facies grouping

Where displayed at the partial type section in Boon's Quarry ((Figure 4), locality 1), these rocks are characterised by a parallel stratification picked out by pale grey, dark grey and olive-green weathering tints. They form a 5.5 m-thick sequence composed of tuffaceous mudstone, siltstone and sandstone in regular alternations that define a series of upward-coarsening cycles (Figure 3)a. The highest of these cycles is surmounted by a bed of massive crystal-lapilli tuff about 4 m thick, which becomes finer grained towards its basal contact. The latter is in part defined by a 4 mm-thick layer of mudstone whose lower boundary truncates microfaulted laminae in the underlying tuffaceous siltstone.

At the Boon's Quarry section, tuffaceous mudstone is a compact, dark olive-green rock with conchoidal fracture. Although when exposed it appears massive or only faintly laminated, in polished slabs (e.g. (E62346) it usually contains a lamination, between mud and fine silt-grade components, which is intensely convoluted. In a thin section of this sample, about 15 per cent of the rock is composed of fine silt-size plagioclase and quartz crystals occurring in a matrix recrystallised to microcrystalline aggregates of quartz, chlorite and white mica studded with abundant granular clusters of highly birefringent, iron-rich epidote. In more silty areas (E62355), abundant sliver-shaped microcrystalline aggregates may be altered devitrified glass shards. X-ray diffraction (XRD) analysis of the finest grained (< 21.1 m) matrix material of the tuffaceous mudstone shows it to be composed of K-mica (2M1 polytype), chlorite and quartz, with minor pyrophyllite (information supplied by R J Merriman, 1993).

Tuffaceous siltstone beds at the partial type section possess a millimetre-scale lamination and are variegated in pale green, pale grey and greenish grey colours. The lamination is commonly parallel and extremely regular, although rarely is convoluted. Other sedimentary structures observed in the lower part of the section (Figure 3)a are domical lamination, produced by water-escape processes, and low-angle planar cross-lamination. A single measurement of cross-lamination dip gave a palaeocurrent flow almost due south. In a thin section of diffusely laminated siltstone (E62358), sorting occurs between those laminae with 15 to 20 per cent plagioclase and quartz crystals, and those with only 5 to 10 per cent crystals; other laminae are composed of vitric tuff crammed with y-shaped devitrified glass shards up to 1 mm in size.

Tuffaceous sandstone forms a number of discrete parallel-sided and sharp-margined beds, 5 to 20 mm thick, in tuffaceous siltstone at the Boon's Quarry section. Most sandstone beds are poorly sorted and matrix-supported, and some are upwardly injected by, and mixed with, mudstone from the underlying beds. Normal grading occurs within beds that define rapidly alternating sequences of tuffaceous mudstone, siltstone and sandstone (Plate 1)c; these structures suggest the presence of 'Bouma' B to E divisions of distal turbidites (e.g. Walker, 1967). The sandstone component of one particular graded bed is largely epiclastic, with lithic clasts predominant over plagioclase and quartz crystals; in a thin section from this bed (E62356) the lithic clasts, of lapilli- to coarse sand-size, are subrounded to angular fragments of plagioclase-hornblende-phyric andesite or dacite showing groundmass textures varying between microcrystalline and intergranular types.

In Judkins' Quarry, representatives of the tuffaceous siltstone grouping occur at a number of stratigraphical levels but are most prominent within a 7 m-thick series of beds (Figure 3)b which can be followed for about 300 m along the north-eastern quarry face (e.g. at locality 1, (Figure 5)). Descriptions of 'Bedded Tuffs' by Allen (1957), and of similar sequences by Lapworth (1886) and Wills and Shotton (1934) were based on earlier sections exposed near this last locality. Farther to the north-west (locality 5) the sequence contains fine-grained vitric tuff, in beds up to 2 m thick, showing distinctive green weathering colours; fresh surfaces show a pale brown rock with a sharply-defined, millimetre-scale parallel- and cross-laminated structure (Plate 1)d. In a thin section (E62335), silt-size quartz and plagioclase crystals constitute less than 10 per cent of the rock, which is crammed with devitrified glass shards showing sliver, crescentic, bubble-wall and y-shapes (Plate 2)c, see p.148); the abundant microcrystalline material between these shards is unresolvable but interpreted to be finely comminuted shard debris.

Accretionary lapilli up to 4 mm in size form a single lapilli-thick layer in tuffaceous siltstone from a sequence, now no longer existing, in the uppermost northern levels of Judkins' Quarry (Carney and Pharaoh, 1993).

Precambrian intrusive rocks

Intrusive igneous rocks occupy an estimated 25 per cent by volume of the total Precambrian outcrop in Judkins' Quarry (Figure 5). Two separate intrusive phases are demonstrated by the relationships seen here. They comprise an early complex of porphyritic to sparsely phyric basaltic-andesite and microdiorite intrusions, and a younger intrusive stock of granophyric diorite (or markfieldite). Of the five types of intrusion described from earlier quarry sections by Allen (1957), the 'feldspar-porphyry' and 'quartz-gabbro' categories were not recognised in the present survey. These rocks were collectively termed the 'Blue Hole Intrusive Series' by Allen (1957), but such a name is inappropriate, and they are referred to here on the basis of lithological type.

From the granophyric diorite body in Judkins' Quarry an age of 603 ± 2 Ma was obtained by the U-Pb zircon dating method, suggesting that the final intrusive events occurred in latest Precambrian times (Tucker and Pharaoh, 1991).

Basaltic-andesite and microdiorite intrusions

These intrusions are similar in field appearance and geochemistry (below) to the 'North Charnwood Diorites', which mostly are confined to the outcrops of the Black-brook and Beacon Hill Tuff divisions of the Charnian Supergroup (Worssam and Old, 1988). Their Precambrian age is demonstrated, uniquely in the Midlands region, at the SSSI in Boon's Quarry ((Figure 4), locality 3) which shows that a dyke of fine-grained dark grey to green-grey basaltic-andesite intruding crystal-lapilli tuff becomes reddened in its upper part and is unconformably overlain by basal beds of the Lower Cambrian sequence.

In Judkins' Quarry the intrusions form a complex of interlinked sheets (Figure 5). They are between 0.5 and 50 m thick, a variation which commonly occurs along the length of a single body. The larger intrusions form subvertical dykes or steeply inclined sheets and have northerly trends parallel to, and in some cases coincident with, Precambrian faults (e.g. locality 6, (Figure 5)). Subhorizontal sheets, which tend to be thinner, exploit lithological contacts or zones of low-angle fracturing in the Caldecote Formation. Most intrusions are affected by fracturing, many containing brecciated zones infilled by quartz-carbonate veins; more extensive are the shear zones localised along contacts with the host rocks. In the northwestern face of Judkins' Quarry, some of these shear systems are cross-cut by Ordovician lamprophyre sheets.

The more extensive exposures of these intrusions in Judkins' Quarry commonly show variation within a single body, from types which are aphyric to those containing up to 30 per cent phenocrysts of small, pale grey plagioclase and less-common greenish altered mafic crystals. Chilled margins between 10 and 20 mm thick border many intrusions, but are commonly obscured by contact-related shearing. A thin section of a porphyritic basaltic-andesite intrusion (E62167) contains about 30 per cent of small plagioclase phenocrysts totally pseudomorphed by aggregates of albite, white mica, epidote and pumpellyite. Less common chlorite-rich areas probably represent pseudo-morphs of mafic phenocrysts. The intergranular-textured groundmass shows relatively less-altered andesine laths, between which untwinned feldspar occurs interstitially. Interspersed throughout this groundmass are rounded to acicular iron-titanium oxides, lath-shaped chlorite aggregates, which may be pseudomorphic after primary pyroxene, and quartz pools; the interstitial secondary minerals mainly comprise chlorite and epidote. In a further basaltic-andesite (E62185), small rectangular clinopyroxene phenocrysts show partial alteration to chlorite-rich aggregates. The coarser development of albite plates in the groundmass of some intrusions produces textures verging towards that of microdiorite, as in a thin section of sample (E62284.

Granophyric diorite (markfieldite)

The intrusion of granophyric diorite in Judkins' Quarry was first discovered in 1932 by F Jones (1935). The field relations, which show the intrusion to be of Precambrian age, are of particular significance to the regional geology of the English Midlands for they enabled Wills and Shotton (1934) to assign a Precambrian age to the comparable diorite intrusions found in the Charnian of the southern part of the Charnwood Forest, between Newton Linford and Stanton under Bardon.

The Charnwood Forest diorites were termed 'markfieldite' by Hatch (1909) but Wills and Shotton (1934) preferred 'granophyric diorite' as the more accurate name. They are placed within a 'South Charnwood Diorite' association of intrusions which are more leucocratic, contain visible quartz, and are less sheared than the North Charnwood Diorites (Worssam and Old, 1988). The South Charnwood Diorites are described in some detail by Worssam and Old (1988); they comprise granodiorite, quartz-diorite and monzodiorite variants with quartz ranging from 15 to 27 per cent and alkali feldspar between 4 and 17 per cent. Most samples that have been chemically analysed are quartz-monzodiorites, as shown below.

The granophyric diorite in Judkins' Quarry maps out as being younger than the basalt and basaltic-andesite sheets which are truncated along its south-eastern contact (Figure 5); although jointed, it is also rather less brecciated internally than the earlier intrusions. The two outcrops of the intrusion are separated by a north-east-trending Precambrian fault. On the south-east side of this, the intrusion is a sinuous stock-like body, elongated north-eastwards, which extends from the base of the quarry upwards to the Triassic unconformity. To the north-west, the fault throws up a subhorizontal intrusive contact ((Figure 5), locality 7) along which the diorite overlies massive crystal-lapilli tuff of the Caldecote Formation; its upper surface hereabouts is sculpted into gullies infilled by pebbly and conglomeratic sandstone of the Lower Cambrian Hartshill Sandstone.

Granophyric diorite from the central part of the intrusion forms dark grey quarry faces with a well-spaced, irregular fracture system. When fresh it is medium- to coarse-textured, containing visible pale green feldspar and dark grey mafic minerals enclosed within a pink finely crystalline (granophyric) mesostasis, giving the rock its characteristic inequigranular, colour-mottled appearance. In a thin section (E62159), plagioclase (55 per cent) forms large (2 to 5 mm) euhedral laths and plates extensively converted to aggregates of albite, epidote, white mica and carbonate. The plagioclase is zoned outwards to clear albite rims. Mafic silicate minerals comprise chlorite pseudomorphs (20 per cent), forming euhedral laths suggestive of original hornblende, and sporadic small carbonated pseudomorphs representing original pyroxene; opaque minerals, including sulphides, form several per cent of the rock. All of these minerals are in sharp contact with an interstitial residuum, comprising about 25 per cent of the rock, composed of granophyric intergrowths between quartz and a turbid brown K-feldspar, with some apatite needles ((Plate 3), see p.149). Electron microprobe analysis (EMPA) of the medium-textured rock (E60128) identified primary amphibole (ferroedenite), augite and titanium magnetite, and secondary prehnite, pumpellyite, iron-epidote and pycnochlorite (information supplied by R J Merriman, 1993).

The diorite becomes finer grained towards its contact with the Caldecote Formation, developing a porphyritic facies with only minor granophyric areas. In a thin section (E62343) from this marginal facies, plagioclase euhedra (about 55 per cent) are accompanied by colourless clinopyroxene (20 per cent) forming small euhedra or larger plates which sub-poikilitically enclose the plagioclase. Chloritic aggregates (about 10 per cent) preserve euhedral outlines suggesting they are pseudomorphic after original hornblende crystals. Secondary actinolite with pale green to straw yellow pleochroism rims the hornblende pseudomorphs and forms laths intergrown within epidote/carbonate aggregates in the interstitial areas.

Xenoliths form dark, fine-grained, angular inclusions in granophyric diorite from the north-western part of the intrusion (near locality 7). Although in hand-specimen they resemble certain of the basalt or basaltic-andesite intrusive rocks, their mineralogy is more appropriate to hornblende-microdiorite. In a thin section (E62183), about 50 per cent of this rock consists of microgranular aggregates of plagioclase and quartz poikilitically enclosed by large chlorite plates (40 per cent) which it is thought are pseudomorphic after hornblende. Scattered iron-titanium oxides form a further few per cent of the rock.

Thermal metamorphism of the Caldecote Formation by the granophyric diorite is suggested by darkened rocks which form an aureole extending for several metres from the intrusive contacts. In a thin section (E62311) of altered crystal-lapilli tuff, plagioclase crystals are converted to white mica aggregates, and quartz shows marginal blebby outgrowths; matrix constituents have recrystallised to fine-grained, saccharoidal quartzofeldspathic aggregates. The conditions of secondary alteration within the intrusion itself are discussed in Chapter 11.

Geochemistry

In this section Drs T C Pharaoh and T S Brewer report on the compositions of 37 whole-rock samples that were analysed to determine the petrogenetic history, geochemical affinities and tectonic setting of the Nuneaton Precambrian rocks. The detailed results of this study are given elsewhere (Carney and Pharaoh, 1993), and only selected representative compositions are included here (Table 1) and (Table 2). Those samples prefixed 'IUD' were collected by TCP in 1985–86 and analysed in 1987 by TSB, and those prefixed by 'E' were collected by JNC in 1989–90 and analysed at the BGS in 1990–91. The geochemical data include a full range of major and trace elements, with rare earth elements (REE) additionally determined for six of the rocks. The results are presented in variation diagrams (Figure 6), (Figure 7) which facilitate comparison with the Charnian rocks from the Charnwood Forest district.

Caldecote Volcanic Formation

The dataset for the Caldecote Volcanic Formation is based on samples from the crystal-lapilli tuff and tuffaceous siltstone facies groupings, with one sample of a dark porphyritic inclusion. The geochemical classification of the samples is determined using ((Figure 6)a and b, which are based on major element composition and CIPW normative mineralogy respectively. Both parameters may be affected by the mobility of major elements under low-grade metamorphism, and the derived classifications should therefore be regarded as approximate only. (Figure 7) displays the data normalised to mid-ocean ridge basalt (MORB) values (Pearce, 1982), with large ion lithophile (LIL) elements (e.g. K, Rb and Ba) grouped on the left of the diagram and high field strength (HFS) elements (e.g. Nb, Zr, Ti and Y) on the right, facilitating lithological comparison across a range of elements.

The crystal-lapilli tuff facies grouping (selected analyses in (Table 1)) is predominantly of dacitic composition ((Figure 6)a and b), most samples containing 63 to 70 per cent by weight SiO2 and 1 to 1.6 per cent by weight MgO. Very little chemical variation is observed through the sampled succession, even in those crystal tuffs in which bedding is apparent, or which show variation in grain size. Three samples of darker tuff (II, VI and VII) have slightly lower content of Si02 (61–64 wt per cent), and markedly higher MgO (2.3–3.9 wt per cent), Fe2O3Tot (4.8–6.3 wt per cent), CaO (2.9–4.2 wt per cent) and loss on ignition (LOI), which may reflect the presence of less felsic inclusions, or hydrothermal alteration, or both. It is notable, however, that 3 samples with 'dark inclusions' (III, IV and V) are geochemically indistinguishable from the majority of crystal lapilli tuff samples. (Figure 7)a clearly demonstrates the limited geochemical variation exhibited by the crystal tuff.

The geochemical patterns ((Figure 7)a) show strong enrichment of LIL elements (K, Rb, Ba) and Th, and slight enrichment of Ce with respect to Nb, a pattern commonly attributed to volcanic arc suites (Pearce, 1982). Content of the HFS elements is comparable to, or less than, that of MORB, despite the felsic composition. Depletion of P and Ti may be due to incorporation of these elements in phases fractionated from felsic magmas, such as apatite and titano-magnetite. The chondritenormalised plot of REE content in one sample (I) shows a small negative Eu anomaly, which indicates a small degree of plagioclase removal, either in the fractionating source magma or as a result of sedimentary reworking (Carney and Pharaoh, 1993).

Analyses of the tuffaceous siltstone facies grouping (Table 1) indicate that these rocks are generally less felsic than the crystal lapilli tuffs and of broadly andesitic-dacitic composition ((Figure 6)a and b). They typically contain lower amounts of Si02 than the crystallapilli tuffs (60–64 wt per cent), but show slightly higher MgO (1.88–2.88 wt per cent), CaO (3.22–6.79 wt per cent) and LOI (6.52–7.35 wt per cent). The geochemical patterns of these samples ((Figure 7)b) are very similar to those of the crystal tuffs, however, indicating a common source magma.

Data are available for one dark, porphyritic inclusion of andesitic composition within the crystal lapilli tuff ((Table 1), VIII). The geochemical pattern of this sample resembles that of other rock types in the Caldecote Volcanic Formation ((Figure 7)c), except for the content of Yb, which is anomalously high. The REE pattern of this sample is also anomalous, being enriched in heavy REE.

Precambrian intrusive rocks

Samples from two suites of Precambrian intrusive rocks exposed in Judkins' and Boon's quarries were analysed: microdiorite (basaltic-andesite) sheets (8 samples, 5 represented in (Table 2)) and the granophyric diorite (markfieldite) stock (4 samples, all represented in (Table 2)). To facilitate comparison with the volcaniclastic rocks, geochemical data for the intrusive rocks are plotted on the same variation diagrams (Figure 6) and (Figure 7).

The microdiorite (basaltic-andesite) intrusive sheets ((Figure 6)a and b) contain SiO2 in the range 49 to 55 wt per cent, MgO between 3.2 and 5.6 wt per cent, and Fe2O3Tot from 10.6 to 11.4 wt per cent. (Figure 6)a suggests that they belong to a low/medium-K calcalkaline series. The most primitive compositions in (Table 2) (e.g. II, III and V) have MgO + CaO > 12 wt per cent, falling within the chemical definition of basalt (Pearce and Cann, 1973), and low content ( below MORB) of HFS elements ((Figure 7)d). However, even these samples have low contents of Cr (< 50 ppm) and Ni (< 10 ppm), and are fractionated with respect to the parental liquid. Significant chemical differences are observed between the interior and marginal chilled zones of some sheets; compare, for example, analysis I (chill) with II (centre of same sheet) and IV (chill) with III (centre of same sheet) in (Table 2). The chilled margins have lower CaO and Fe2O3Tot, and higher SiO2, Na2O and LOI than their respective interiors. MgO is only slightly lower in the chills, while K2O, Rb and Ba exhibit more variable behaviour, being considerably enriched in JUD13 but not in JUD34. By contrast, the content of HFS elements such as Ti, Nb, Zr and Y is identical (within the analytical precision) in chill and interior, confirming the relative immobility of these elements under the prevailing metamorphic conditions. The mobility of the LIL elements is also indicated by the dispersion in their geochemical patterns ((Figure 7)d) and is compatible with the subgreenschist metamorphic alteration observed petrographically, particularly at the margins of the minor intrusions. The geochemical signatures of the microdiorites ((Figure 7)d), show strong enrichment in LIL elements, Th and Ce with respect to Nb and other HFS elements, as observed in the felsic rocks. The HFS elements exhibit a very flat profile, with content below MORB, and a relatively primitive mantle source region is again inferred.

The granophyric diorite ((Table 2), VI–IX) has higher contents of SiO2 (55–57.6 wt per cent), K2O (> 2.3 wt per cent), Rb (> 65 ppm), Ba (> 593 ppm) and Th (3–7.3 ppm) than the microdiorites. It forms part of a high-K calc-alkaline series ((Figure 6)a), and is classified as quartz monzodiorite in (Figure 6)b. The enhanced enrichment in LIL elements and Th is clearly visible in (Figure 7)e, and P is also strongly enriched in comparison to the microdiorites.

These features are interpreted to reflect the increasing maturity of the volcanic arc with time (Pharaoh et al., 1987b). The changes would be the result partly of progressive enrichment of the mantle source region with LIL, Th and Ce carried by subduction-derived fluids (Pearce, 1982), and partly the consequence of crustal contamination resulting in the enrichment in Si02 and LIL elements. The limited variation displayed by the LIL elements suggests that the granophyric diorite is less affected by alteration than the finer-grained microdiorite sheets. Two samples of the granophyric diorite exhibit small negative Eu anomalies (Carney and Pharaoh, 1993), indicative of plagioclase removal from the parental liquid.

Geochemical correlation with Charnwood Forest

Correlation of the Nuneaton Precambrian rocks with comparable rock types in Charnwood Forest has been proposed by Carney et al. (1992) and Pharaoh and Gibbons (1994), and is further apparent when the geochemical data reviewed above are plotted together with the data fields occupied by Charnian magmatic suites (Figure 8). It should be noted, however, that these diagrams are strictly only valid for magmatic rocks, whereas many of the Nuneaton samples are volcaniclastic. The Charnwood data are those published by Pharaoh et al. (1987b), Webb and Brown (1989), and include some unpublished data (Pharaoh and Brewer, in preparation).

Data for the Caldecote Volcanic Formation fall within the field of Charnian felsic magmatic rocks in the AFM diagram ((Figure 8)a), exhibiting a calc-alkaline trend of evolution, and on the Nb-Y diagram ((Figure 8)b), where they occupy the field of volcanic arc and syn-collision felsic volcanic rocks. The geochemical patterns shown in (Figure 7)a–c also demonstrate that the Caldecote tuffaceous rocks are comparable in composition to Charnian felsic lavas ((Figure 7)f). This fact indicates that the Caldecote Volcanic Formation and Charnian Supergroup had a similar (probably the same) magmatic source, whose distinctive geochemical signature was little modified by post-eruptive sedimentary and alteration processes.

Data for the microdiorite sheets and the granophyric diorite from Nuneaton respectively fall within the fields of the North Charnwood Diorites and South Charnwood Diorites (Worssam and Old, 1988), if due allowance is made for overlap between these fields in (Figure 8)a and (Figure 8)b. The geochemical patterns shown in (Figure 7)d and e are also comparable with those of the North Charnwood and South Charnwood Diorites (Pharaoh et al., 1987b, fig. 2b). Both volcaniclastic and intrusive rocks exhibit the geochemical signature of Precambrian rocks of the Charnwood Terrane (Pharaoh et al., 1987b), distinct from other Precambrian magmatic suites in southern Britain.

Tectonic setting

The variation diagrams presented above indicate that the tectonic setting of the Precambrian rocks in the Nuneaton area is comparable to that of their Charnian correlatives, as reviewed by Pharaoh et al. (1987b). That the source magmas of the Caldecote volcaniclastic rocks were erupted within a relatively primitive volcanic arc is indicated by the fact that the geochemical patterns ((Figure 7)a-c) are characteristic of a fairly depleted mantle source region which yielded magmas with HFS content lower than MORB, but with contents of LIL elements, Th and Ce enriched by subduction magmatism.

The microdiorite sheets were intruded into this volcanic arc soon after eruption of the volcaniclastic succession. The granophyric diorite, however, exhibits further progressive enrichment in LIL elements, Th, Ce and P, and belongs to a high-K calc-alkaline series emplaced into a volcanic arc that had achieved greater maturity in terms of its magmatic development. Increasing maturity of the Charnian volcanic arc with time has also been described by Pharaoh et al. (1987b).

Mode of formation of the Precambrian rocks

The principal conclusions of the geochemical study are that the Caldecote Volcanic Formation was probably derived from the same magmatic source that supplied the Charnian Supergroup, by subduction magmatism, and that these rocks are the remnants of a fairly primitive volcanic arc. Palaeogeographic lines of evidence further suggest that this arc may originally have lain off the western margin of the developing Gondwana continent, although the precise geological configuration of this complex system is at present conjectural (e.g. Abalos, 1992).

The Nuneaton exposures show two phases of Charnian volcanic arc evolution, the first extrusive and highly acidic in character and the second being one of faulting and intrusion of more basic magmas. The Caldecote Volcanic Formation, representing the initial extrusive phase, is here viewed as a markedly bimodal lithological association in which the beds of massive crystal-lapilli tuff represent the proximal deposits of pyroclastic eruptions that periodically were directed into a basin where the finer-grained and more distal tuffaceous siltstones and mudstones were accumulating.

That the whole sequence was deposited in water is suggested by the presence in the tuffaceous siltstones and mudstones of graded bedding, water-escape structures, convolute lamination and cross-lamination. A basinal setting, in moderate to deep waters well below wave base, is suggested by the paucity of structures normally attributed to wave or tidal current activity, although the influence of such processes on sediment transportation cannot be entirely ruled out. The repetitive normal grading is interpreted to indicate deposition from distal, high-density turbidity currents (Lowe, 1982). The thicker upward-coarsening sedimentary cycles may have formed when rapidly accumulated material sloughed off the flanks of the volcanic axis, at times of progressively increasing seismic or eruptive activity. In other sandstone beds, sediment-mixing phenomena are probably related to the development of convolute lamination in the finer-grained sediments (Lowe, 1975), and are taken to indicate widespread mobilisation of poorly consolidated material.

Contemporaneous pyroclastic activity within the volcanic arc contributed the glass shards locally concentrated in siltstone laminae, and formed thicker beds of fine-grained vitric tuff. The latter are the deposits of pyroclastic ash falls into water, followed by reworking by bottom currents to form the characteristic laminated and cross-laminated structures. The geochemical data indicate such eruptions to have been relatively less acidic than those that produced the crystal-lapilli tuffs. The appearance of accretionary lapilli is also indicative of subaerial pyroclastic surge activity in the volcanic source region; according to Heinrichs (1984), such lapilli can assume a protective outer coating and then be deposited subaqueously without disintegrating.

Rocks of the crystal-lapilli tuff facies are both conformable and intimately layered with the tuffaceous siltstone beds and therefore were similarly deposited under water. Their relatively constant crystal proportions — about 55 per cent plagioclase and 20 per cent quartz — indicates a strong source control over their composition and suggests that most beds are compositionally immature and, despite submarine reworking, are essentially the primary eruptive products of dacitic magmas. This conclusion is supported by the abundance of fragmentary juvenile volcanic material (shattered euhedral crystals and glass shards) and by the close geochemical similarities of these rocks to Charnian felsic lavas. Previously these rocks were considered to be welded tuffs (Allen, 1957; 1968a), but, although welding and plastic deformation are seen in some of the dark porphyritic inclusions, true matrix shards are generally little deformed and mainly show random orientation; the textural evidence suggests that there was no significant welding after deposition. These rocks are therefore probably not welded tuffs as defined by Wright et al. (1980), but more likely represent subaqueous pyroclastic flows of the type described from basins marginal to volcanic arcs (Fiske and Matsuda, 1964). These may have originated either from submarine eruptions or from subaerial pyroclastic flows which subsequently entered water (see discussions in Fisher, 1984, and Stix, 1991). In the Caldecote Formation the occurrence of stratified and graded tuff at the top of the succession in Judkins' Quarry ((Figure 3)c) is particularly reminiscent of subaqueous pyroclastic flows, as are the distinctive cracked and brecciated crystals (Fisher, 1984). A high crystal content, as found in the coarse-grained rocks of the Caldecote Formation, is a feature not uncommon in volcaniclastic successions, and has been attributed by Cas and Wright (1987) to processes causing the further concentration of crystals during mass-flow transportation. The rapid emplacement of the Caldecote flows caused liquefaction and flowage of the underlying and semiconsolidated fine-grained sediments, resulting in contorted bedding and in places giving rise to sediment-raft breccias, as seen in Judkins' Quarry. The imbrication seen in some of these breccias is consistent with transport towards the north-east, whereas the single palaeocurrent direction measured from a tuffaceous siltstone indicates a flow to the south. It is noteworthy that north-eastwards and southerly current transport directions were determined from volcaniclastic sedimentary rocks in the Precambrian outcrops of Charnwood Forest (Moseley and Ford, 1989, p.272).

Later in the arc's evolution the magmas became more basic and were introduced as Precambrian intrusive rocks belonging to two chemically distinct magma types. The basaltic-andesite (microdiorite) sheets, geochemically correlated with the North Charnwood Diorites, were emplaced within a relatively juvenile arc but the granophyric diorite, geochemically correlated with the South Charnwood Diorites, represents a high-K calcalkaline magma emplaced in an arc of increased maturity subsequent to further subduction-enrichment of the mantle source region. These intrusions were preferentially located along fault zones, demonstrating a type of structural control on igneous activity that is also observed in Phanerozoic intra-oceanic island arc systems in the south-western Pacific. In these more recent examples, volcanic activity was governed by adjustments to the stress regime prevailing prior to cessation of magmatism (e.g. Titley and Heidrick, 1978). In the case of the Nuneaton volcanic arc, the cessation of magmatic activity can be dated at, or soon after, 603 ± 2 Ma, the age of the granophyric diorite stock constituting the youngest intrusion in Judkins' Quarry.

Chapter 3 Cambrian and Lower Ordovician (Tremadoc)

Sedimentary rocks of this age crop out within the Nuneaton Inlier and include one of the most extensive and complete Cambrian sequences in Britain. The succession has a south-westerly regional dip and forms an elongated outcrop which extends north-westwards for 16 km from Bedworth to Atherstone, on the northern edge of the map sheet, and thence for a further 2.3 km into the adjoining Coalville district. The basal beds are located along the eastern margin of the outcrop, where they rest unconformably on eroded Precambrian basement; to the south-west, the highest strata are overstepped by Upper Palaeozoic rocks of the Warwickshire Coalfield. Although there are few natural exposures, the Cambro-Ordovician strata have been studied in detail in various quarries, temporary sections and boreholes.

The succession commences with the Hartshill Sandstone Formation which is assigned to the Comley Series (Lower Cambrian). It is of particular significance in providing the clearest evidence in Britain of the cycles of arenaceous sedimentation that followed the Cambrian marine transgression across Precambrian rocks of the Midland Platform (Brasier, 1985). The succeeding Stockingford Shale Group represents relatively more continuous deposition, predominantly of mudstone, from late Comley to early Tremadoc times. The simple structure and comparatively complete stratigraphical and faunal succession make the Stockingford Shale Group an important reference section for the stratigraphy of the Cambrian in England and Wales.

The Cambro-Ordovician rocks of the Nuneaton Inlier were at first referred to the Carboniferous System (Conybeare and Phillips, 1822); however, Yates (1829), by making lithological comparisons between the Lickey Quartzite and the Hartshill Quartzite, suggested they should be assigned to the 'Transition Rocks' — equivalent to the Lower Palaeozoic of present usage. The first Geological Survey maps retained the Hartshill Formation and Stockingford Group as Carboniferous, but with some reserve (Howell, 1859). It was Lapworth (1882) who proved the true age of these rocks when he found Cambrian fossils in the Stockingford Shale Group. It was not the only time Lapworth's work had called the Geological Survey maps into question (Oldroyd, 1990), and a revision was soon undertaken by Strahan (1886), who was able to demonstrate a previously unrecognised unconformity between the Cambrian and Carboniferous rocks.

Classification

Lithostratigraphy

In keeping with modern practice, most of the rock units shown on the new geological maps of this district have

been assigned formal lithostratigraphical names in accordance with the hierarchical scheme recommended by Holland et al. (1978). The two principal Cambro-Ordovician units that have been formally named are the Hartshill Sandstone Formation and the Stockingford Shale Group, and these are in turn subdivided into members and formations respectively. This nomenclature has evolved over a period of about a century, and is the outcome of work by Lapworth (1898), Illing (1913, 1916), Taylor and Rushton (1971), and Brasier et al. (1978), as shown in (Table 3). Further details for the Nuneaton–Hartshill area are given by Carney (1992a) and Baldock (1991a).

Chronostratigraphy and biostratigraphy

The rocks of Cambrian and earliest Ordovician age are divided into four series in accordance with the scheme of Cowie et al. (1972), as shown in (Table 3). The boundaries between these are recognised biostratigraphically, and in the case of the Stockingford Shale Group depend largely on the stratigraphical distribution of trilobites (Thomas et al., 1984). These may be referred to the standard scheme of zones (Cowie et al., 1972) adapted for the most part from successions in Scandinavia (Westergard, 1944). Illing (1916), Rushton (1966, 1967, 1978, 1979, 1983) and Taylor and Rushton (1971) have described most of the trilobites and discussed their distribution and significance. The oldest macrofossils in the succession are those of the Home Farm Member, near the top of the Hartshill Sandstone. Those beds can be assigned to certain of the faunal stages recognised on the Siberian Platform (e.g. Brasier et al., 1992), but beneath this datum the beds are unfossiliferous, save for trace fossils and a few non-diagnostic acritarchs of uncertain chronostratigraphical position.

Hartshill Sandstone Formation

The Hartshill Sandstone is an arenaceous succession 260 m thick which crops out for 5.2 km between Nuneaton and Hartshill. Formerly this unit was known as the 'Hartshill Quartzite' before its name was changed to 'Hartshill Formation' by Brasier et al. (1978). Those authors proposed a lithostratigraphical division of the unit into five members (Table 3) and this scheme is followed here, but with two minor changes; a lithological qualifier has been added to the formation name, and a sixth and basal unit—the Boon's Member—has been recognised. To assist description, a further informal division of the strata has been made into subunits A to L, each representing beds that show similar types of sedimentary structures indicative of a common origin (Carney, 1992a, 1995). A listing of the various members and subunits, with brief description of the bedforms and interpretation of environments for some of them, is given in the summary stratigraphical column of (Figure 9).

The formation is best observed in quarry sections, there being very few natural exposures. Two of the disused quarries contain geological Sites of Special Scientific Interest (SSSIs) and those exposures have been chosen as partial type sections. For the lower beds of the Hartshill Sandstone, which include the Boon's and Park Hill members and the Precambrian–Cambrian unconformity, the type section is the SSSI in Boon's (formerly ManAbell's) Quarry [SP 3279 9471] to [SP 3299 9467]. The other type section is in Woodlands Quarry [SP 3248 9480]: it includes the Home Farm and Woodlands members and the upper contact with the Stockingford Shale Group (Brasier et al., 1978). Descriptions of the remaining and largest part of the formation, encompassing the upper Park Hill Member and the whole of the Tuttle Hill and Jee's members, are based on the extensive, but temporary, quarry sections exposed between the south-western face of Boon's Quarry [SP 331 944] and the north- and south-eastern faces of the adjacent Hartshill Quarry (formerly Jee's Quarry) [SP 336 937].

The age of the Hartshill Sandstone Formation is constrained by the faunas of the Home Farm Member, which contains in the Hyolithes Limestone ('Hyolite' Limestone of Lapworth, 1898) the oldest assemblage of shelly fossils to be found at outcrop in England. On a chronostratigraphical timescale the faunas indicate that the Hartshill Sandstone lies at the base of the early Cambrian Comley Series, being largely within the Non-trilobite Zone but including, in its uppermost beds, the basal part of the Olenellid Zone (Cowie et al., 1972). The Home Farm Member itself spans the time boundary between upper Tommotian and lowermost Atdabanian Stages as defined on the Siberian Platform (Brasier et al., 1992). The underlying strata are barren of diagnostic body fossils, but the trace fossil assemblages suggest a position within the upper part of the Nemakit-Daldynian Stage (Brasier et al., 1992). For these beds a maximum depositional age of 560 Ma is indicated by a U-Pb zircon radiometric determination obtained on the Ercall Granophyre in Shropshire (Tucker and Pharaoh, 1991) which is unconformably overlain by the Wrekin Quartzite correlative of the Hartshill Sandstone Formation (Brasier, 1989).

Correlations between the Hartshill Sandstone and other Lower Cambrian strata in England and Wales are summarised by Cowie et al. (1972) and in a number of later publications (e.g. Brasier, 1989). The faunas in the Home Farm Member are equivalent to those at the base of the Lower Comley Sandstone Formation of Shropshire, suggesting a correlation between the underlying Hartshill beds and the Wrekin Quartzite (Lapworth, 1898; Brasier, 1989). However, the Lickey Quartzite, formerly correlated with the Hartshill Formation (Lap-worth, 1898), is now assigned a post-Cambrian (probably Ordovician) age by Molyneux (in Old et al., 1991). The Withycombe Formation, intersected in a borehole in Banbury, 55 km south-south-east of Hartshill, contains faunas indicative of an earliest Cambrian age (Brasier et al., 1992), older than the Home Farm Member (Rushton and Molyneux, 1990), but is lithologically dissimilar to the Hartshill Sandstone. Worldwide correlations are summarised by Brasier (1992) amongst others.

The Precambrian-Cambrian unconformity

The hardrock quarries of the Nuneaton Inlier provide some of the most instructive exposures of this unconformity to be found in Britain. Moreover, in the Boon's and Judkins' quarries there are developed two contrasting styles of unconformable relationship, each of which reflects a certain stage in the tectonic subsidence of the Cambrian depositional basin (Carney, 1992a).

At the SSSI in Boon's Quarry, the unconformity is exposed on both sides of the corner between two quarry faces [SP 3299 9467] , about 80 m south-south-west of Grange Farm. The surface of unconformity is in places disturbed by bedding plane shears and faults, but is preserved intact on the quarry corner itself ((Figure 4), locality 4). There, convex protrusions, which correspond to the tops of weathering spheroids or corestones (Brasier and Hewitt, 1979), are developed in the upper 2 m of the Precambrian volcaniclastic succession ((Plate 4)a, see p.150). Between individual spheroids are seen reddened, clay-rich and oxide-impregnated weathering rinds showing a tangential 'onion skin' foliation: the only original minerals to survive in the rinds are rounded quartz granules which do not change in shape, distribution or abundance when traced into fresh tuff forming the spheroid interiors. This type of clayey weathering is analogous to the horizon seen below the saprolitic zone, at the base of modern lateritic profiles in Uganda (e.g. McFarlane, 1983), suggesting that the unconformity is preserving the remnants of a land surface that had formed in a tropical or subtropical climate. The coarse-grained and compositionally immature nature of the mantling beds of the Boon's Member, which principally contain angular clasts, are further palaeoenvironmental indicators suggesting deposition near to steep, possibly fault-controlled slopes.

The unconformity in Judkins' Quarry [SP 343 932] is exposed for about 200 m near the base of the southwestern quarry face, between localities 8 and 9 (Figure 5), which may correspond to a more deeply excavated equivalent of 'section 1' and 'section 2' described in detail by Wills and Shotton (1934), and to the section described by Allen (1968b). The spheroidal weathering described in the Precambrian rocks here by Wills and Shotton (1934) was not observed during this survey; the rocks beneath the unconformity are fresh. The erosion surface is mantled by 1.5 m of boulder conglomerate and pebble conglomerate in upward-fining sequence, in turn succeeded by the normal grey, medium-grained lithic sandstones of the Park Hill Member. In the north-west of this locality the unconformity truncates a faulted contact between Precambrian crystal-lapilli tuff and granophyric diorite (markfieldite) and becomes highly irregular where developed on the latter (locality 9). Joints and fractures in the diorite were widened by erosion, producing on the fresh rock surface a number of pinnacles (Allen, 1968b) separated by narrow (about 0.5 m wide) gullies up to 3 m deep. Filling the gullies are grey, coarse-grained pebbly sandstones and cobble conglomerates basal to the Park Hill Member. Such relationships suggest a rather later stage in the marine transgression than that seen at Boon's Quarry, corresponding to a more widespread subsidence which allowed the Park Hill Member to overstep the Boon's Member on to the Precambrian land surface. The basal conglomerates and pebbly sandstones, being composed of well-rounded clasts, are interpreted as beach or foreshore deposits that accumulated within hollows on a wave-cut coastal platform (Wills and Shotton, 1934; Allen, 1968b).

Boon's Member

This member was named (Carney, 1992a) for the coarse-grained red beds that formerly were included within the Park Hill Member (Brasier and Hewitt, 1979). It is uniquely exposed in Boon's Quarry, where it is divided into three informal units (A to C, (Figure 9)), each with a partial type section on the north-eastern quarry face. The member's aggregate measured thickness is at least 19 m, and structural cross-sections suggest that almost 40 m may be present in Boon's Quarry. The member is only locally developed, however, being absent from the Judkins' Quarry sequence.

The Boon's Member is devoid of trace fossils but contains sporadic sphaeromorph acritarchs in thin mudstone layers sampled from the very base (Unit A) and middle to upper parts (Units B and C). All may be assigned to the genus Leiosphaeridia, with the exception of a single possible acanthomorph acritarch. The fauna has no particular biostratigraphical value but has considerable environmental significance in demonstrating the existence of a marine influence during deposition of the Boon's Member (Molyneux, 1992).

The basal beds, comprising Unit A, are at least 3 m thick in the partial type section at the SSSI ((Figure 4), locality 4), where they unconformably overlie the Precambrian rocks. The most conspicuous beds (e.g. at locality 5) are the discontinuous lenses of massive, bouldery breccio-conglomerate. These contain subspherical cobbles and boulders of Precambrian crystal-lapilli tuff up to 2 m across, supported in a poorly sorted matrix of generally highly angular, sand- to gravel-size rock fragments mainly composed of Precambrian volcanic material ((Plate 4)b, see p.150). The bases of the lenses are mostly planar, but some show asymmetrical scour marks indicative of south-westwards flowing currents. Not so distinctive, but more common in Unit A, are granule-stone beds, between 0.06 to 0.6 m thick, which are also very poorly sorted but are made up of angular rock fragments with smaller maximum diameters than clasts in the breccio-conglomerates. The plane-bedded structure of the granulestones is defined by the parallel orientation of 'floating' platy clasts, by concentrations of angular clasts within breccia layers, and by layers impoverished in the larger clasts and/or enriched in the coarse-grained sand component. In a thin section of granulestone (E62354), about 50 per cent consists of granule-size Precambrian volcanic quartz crystals and angular clasts of fine-grained volcanic rock, many with oxide rims; these same lithologies also form grains within the sand-size feldspathic litharenite matrix, along with plagioclase and orthoclase feldspar, and several per cent of amorphous oxide grains. The interstitial minerals comprise white mica and subordinate quartz overgrowths.

The red sandstones and breccias of Unit B form a sequence at least 9 m thick at the partial type section in the south-eastern part of Boon's Quarry ((Figure 4), locality 6). The beds are between 0.17 and 1.15 m thick and have parallel sides and flat tops. Many beds, particularly in the lower part of the unit, comprise repetitive depositional couplets of massive to plane-bedded or rarely trough cross-bedded sandstone alternating with layers of massive to graded matrix-rich breccia 2 to 20 cm thick ((Plate 4)c, see p.150). Some breccias occur as lenticular aggregations with basal scours whose asymmetry of profile indicates south-westward current flow. Between these beds, partings are commonly defined by thin breccias with mudstone clasts; many of these are impersistent, resulting in the amalgamation of some beds (Figure 9), but others may contain thin mudstone drapes and from one of these, in the road cutting leading to the shore of the Boon's Quarry lake [SP 3313 9445], acritarchs were recovered. Petrographically the sandstones are litharenites or feldspathic litharenites (McBride, 1963); in a thin section (E62711) they contain subrounded quartz grains (50 per cent) and Precambrian volcanic fragments (35 per cent) together with several per cent of amorphous oxide or partly-oxidised lithic grains. Many grains are oxide-rimmed, with quartz overgrowths and white-mica aggregates appearing as common interstitial constituents. The breccia content decreases upwards through the unit, commensurate with the incoming of grey sandstone beds marking the gradational passage to Unit C.

The grey or pink sandstones of Unit C are 7 m thick in their type section by the lake shore on the lower southeastern face of Boon's Quarry [SP 3310 9442]; they can also be observed north-west of the Precambrian–Cambrian unconformity at the SSSI ((Figure 4), locality 7). The sandstone beds are tabular and, like those of Unit B, mainly massive, with thin breccia layers or 'floating' Precambrian volcanic clasts; low-angle planar cross-bedding is present in some, however. Normal grading to siltstone occurs in some beds, while others coarsen upwards, with top surfaces showing undulations indicative of wave or current action. Mudstone drapes and thin beds are sporadically present in this sequence. The sandstones are litharenites, with about 25 per cent lithic grains but only a few per cent of opaque constituents.

Park Hill Member

These sandstones form the eastern margin to the Hartshill Sandstone crop but are seldom exposed outside the quarries (Baldock, 1991a). Between Boon's and Hartshill quarries the member is about 56 m thick, but to the southeast it oversteps the Boon's Member and is only 30 m thick where it overlies Precambrian rocks in Judkins' Quarry.

The partial type section, which includes the lower to middle part of the member, is taken at the SSSI on the north-western upper levels of Boon's Quarry [SP 3279 9463] to [SP 3286 9475], 150 m west-north-west of Grange Farm. The basal contact with the Boon's Member is only exposed near the lake shore in the south-eastern part of Boon's Quarry [SP 3312 9442]. It is drawn at the lower boundary of the first bed with significant cross-bedding and a rippled top surface; this change also coincides with the incoming of detrital glauconite. In Judkins' Quarry the basal beds directly overlie the Precambrian–Cambrian unconformity and, as previously noted, comprise an upward-fining cobble conglomerate and pebbly sandstone sequence 1.5 m thick. The latter differs from the Boon's Member in being thinly developed and containing a preponderance of rounded, as opposed to angular, clasts; it is therefore regarded as a basal transgressive lag deposit of the Park Hill Member. The upper contact with the Tuttle Hill Member is taken at the base of a prominent sheared mudstone bed on the southeastern face of Hartshill Quarry [SP 3366 9374], which may also be the 'summit of the double shale band' described by Lapworth (1898).

The Park Hill sandstones are typically pale grey, with weathered surfaces speckled by white or pink feldspar and grey to black lithic or oxide grains. Although they are amongst the most quartz-rich of the Hartshill sandstones, true quartzites are absent and most are sublitharenites or lithic subarkoses in the petrographic classification of McBride (1963). Relative to the overlying Tuttle Hill Member, this succession contains very little mudstone and only sporadic glauconite.

Trace fossils on bedding planes in the north-eastern part of Hartshill Quarry comprise Psammichnites, Neonereites, Arenicolites and Planolites (Brasier and Hewitt, 1979), and Diplocraterion (Brasier, written communication, 1990); body fossils have not been found.

Unit D, at the base of the Park Hill Member, comprises a succession of sandstones and conglomeratic sandstones which is 17 m thick in Boon's Quarry and 8 m thick in Judkins' Quarry. Sandstones exposed by the lake shore at the foot of the south-western face in Boon's Quarry [SP 3303 9441] occur in beds between 0.2 and 0.6 m thick bounded by megarippled top surfaces and scoured bases; mudstone partings are only rarely developed. Such beds typically show low-angle tabular-planar cross-bedding or trough cross-bedding. The intercalated beds of conglomeratic sandstone contain angular to sub-rounded small pebble-size clasts of Precambrian volcanic rock concentrated into planar layers of matrix-supported conglomerate. These beds are either massive or show normal grading from grey, coarse-grained pebbly sandstone to cappings of micaceous siltstone.

Unit E, about 37 m thick, forms the main part of the Park Hill Member. The partial type section in Boon's Quarry shows sandstone beds between 0.2 and 1.8 m thick bounded by scoured bases and rippled top surfaces. Most are cosets of tabular-planar cross-bedding with between 2 and 4 individual sets present in each bed. The foreset beds commonly show regular reversals in direction of inclination (Figure 9), producing the herringbone pattern of cross-bedding which is so typical of this unit; north-east and south-west to south-south-west inclinations are predominant in beds at the type section (e.g. locality 8, (Figure 4)). Interspersed between these beds are cosets of trough cross-bedding with foresets usually dipping east-south-eastwards. Bedding plane structures in the middle to upper parts of Unit E are presently most extensively exposed along the northeastern face of the Hartshill Quarry [SP 3354 9390] to [SP 3366 9374]. They include symmetrical wave ripples and asymmetrical to linguoid current ripples, the latter types showing north-easterly and south-westerly current directions (Brasier and Hewitt, 1979). Other structures include nests of lunate, scoop-shaped scour pits which are exposed at various stratigraphical levels; they indicate currents flowing from north-east to south-west (Carney, 1995).

The top of the Park Hill Member is exposed on the south-eastern face of Hartshill Quarry [SP 3366 9374] and comprises the upward-coarsening sequence of beds in Unit F. The basal 2 m is a heterolithic association between beds of mudstone (0.03 to 0.06 m thick) interleaved with thin (about 0.10 m) flat-topped sandstones showing normal grading, plane-bedding and load-casting at their bases. They are overlain by wavy-topped and mudstone-draped compound cross-bedded sandstones which in turn pass up to thick (0.5 to 1.0 m) beds with tabular-planar cross-bedding.

Tuttle Hill Member

This member occupies the largest part of the Hartshill Sandstone Formation outcrop and is about 150 m thick. There are small natural exposures at the top of Caldecote Hill on the west side of the B4111 road [SP 3391 9346] , and east of the Atherstone Road north of Hartshill [SP 3275 9492] (Baldock, 1991a), but the beds are best viewed within the Hartshill and Midland [SP 349 925] quarries. The principal section, and the most accessible, is that exposed on the south-east face of Hartshill Quarry [SP 3349 9360] to [SP 3366 9375], although this cannot be designated a type section because of its temporary nature, within a working quarry. In this section the member is divided into six packages of beds, designated Units G to L in (Figure 9). The base is a sheared mudstone bed which rests sharply on the Park Hill Member, and the upper boundary is a gradational passage into the Jee's Member.

This succession shows a diversity of sedimentary rock types that is not seen in the underlying Park Hill Member. Compared to the latter it shows an increased mudstone content, with mudstone draping most bedding planes, and the sandstones characteristically weather to darker colours. Medium-grained grey and pink sandstones are typical in the lower units (G–I) whereas the younger beds become dark maroon, finer grained and more micaceous higher up (Units J–L), a trend commensurate with an increased abundance of glauconite and of oxide rims to grain boundaries. The sandstones are more feldspathic than in the Park Hill Member, being subarkoses and lithic subarkoses on the petrographic scheme of McBride (1963).

Brasier and Hewitt (1979) found a similar trace fossil assemblage to that in the Park Hill Member, with the addition of Gordia in the upper part of the Tuttle Hill Member. During the present survey ten mudstone drapes were sampled for acritarchs but none was found.

The oldest beds, comprising the 5.3 m-thick succession of Unit G, represent a passage from a lower heterolithic sequence of mudstone containing thin, flat-topped, plane-bedded and graded sandstone layers, into thicker, plane-bedded or compound cross-bedded sandstones with subordinate mudstone interbeds higher up. Overlying these, Unit H is a 7.5 m-thick succession of pebbly coarse-grained sandstones in highly lenticular, erosively-based beds between 0.2 and 1.0 m thick. Between these sandstones are lenses, up to 0.7 m thick, of finely laminated mudstone and siltstone. Mudstone also forms clasts in the pebbly sandstones, either randomly distributed or defining ripple-marked abandonment surfaces. The sandstones may be structureless but some show herringbone cross-bedding; this consists of very poorly defined north-east dipping avalanche foresets capped by a thin development of south-west dipping foresets representing the rippled top of the bed.

The succeeding Unit I comprises 39 m (excluding igneous intrusions) of pink to grey glauconitic sandstones occurring mainly in thick (0.8 to 2.0 m) beds showing large-scale (0.8 to 2.0 m) tabular-planar or trough cross-bedding (Figure 9). No significant channelisation occurs in this unit, although the tops of many beds show evidence of scouring before deposition of the overlying sandstone ((Plate 5)a, see p.151), the adjacent beds being invariably separated by a mudstone drape 5 to 20 mm thick. Throughout this unit the foreset beds are generally inclined north-eastwards, indicating a unidirectional current distribution pattern; the cross-bedding is simple in type, although reactivation surfaces occur sporadically. In the lower part of Unit I, the tabular-planar cross-bedded sandstones occur in packages several metres thick alternating with similar thicknesses of compound lenticular cross-bedded sandstones. Separating these alternating packages are thin (about 0.7 m) heterolithic intervals of mudstone with sandstone ripple trains.

Unit J is a 10.5 m-thick heterolithic association of mudstone beds, 0.01 to 0.08 m thick, that separate parallel-sided flat-topped sandstone beds 0.05 to 0.8 m thick. The sandstones are maroon and glauconitic, with significant amounts of heavy minerals (E62477). They are plane-bedded, or have gently inclined cross-bedding, and some show normal or inverse grading from coarse-grained pebbly sandstone to medium-grained sandstone. Lenses of very coarse-grained pebbly sandstone also occur within the mudstone layers.

Unit K comprises a 39 m-thick sequence of maroon, feldspathic, glauconitic and micaceous sandstones; beds are between 0.07 and 0.2 m thick in the lower part of the unit, thickening to 0.8 m in the higher part. It is sedimentologically uniform, consisting of beds showing compound lenticular cross-bedding (Figure 9), many with wavy, bioturbated tops. These beds characteristically amalgamate or split laterally, and are bounded by mudstone-draped partings. Subordinate partings which appear as trails of mudstone clasts are akin to the lower order discontinuities of Class V sandwaves described by Allen (1980). Foreset beds show dominant north-easterly inclinations.

Unit L sharply succeeds Unit K, the change occurring across a single bedding plane ((Plate 5)b). It is a 56 m-thick succession (excluding intrusions) of maroon, glauconitic, micaceous subarkosic sandstones occurring in beds 0.1 to 0.8 m thick, which are remarkably flat-topped and parallel-sided throughout. A plane-bedded or laminated structure is characteristic of these beds, although some show very low-angle cross-bedding inclined north-eastwards or south-westwards. Mudstone drapes occur sporadically along partings, and the top several centimetres of many beds are bioturbated, as shown in a thin section (E65250) which indicates homogenisation of the mud and fine sand components of the sediment. Near the top of the unit, planar and trough cross-bedding become more accentuated in beds gradational to the overlying Jee's Member.

Jee's Member

This member is exposed along the upper south-western faces of the Hartshill and Midland quarries, and at the SSSI in Woodlands Quarry [SP 3249 9475]. Its type section, following that for the Tuttle Hill Member, is on the south-eastern face of Hartshill Quarry [SP 3349 9360]. The member, between 5 and 6 m thick, passes gradationally downwards into the Tuttle Hill Member; its top surface is disconformably overlain by the basal conglomerate of the Home Farm Member.

The extensively bioturbated beds of the member contain trace fossils corresponding to the 'Cruziana facies' of Seilacher (1967), with Isopodichnus, Arenicolites, Planolites and Didymaulichnus (Brasier and Hewitt, 1979).

The member mainly comprises maroon, fine-grained, micaceous and glauconitic subarkosic sandstones which develop a calcareous poikilotopic cement about 1 m below the Home Farm Member. The beds are between 0.2 and 0.3 m thick and have undulating upper surfaces on which may occur pebbly winnowed lags. Some sandstones comprise complex cosets in which plane-bedding and trough or tabular-planar cross-bedding may all be represented (Figure 9). Siltstone and mudstone sequences commonly form the bed tops and have a millimetre-scale internal lamination picked out by grey, pink and maroon colours.

Home Farm Member

The Home Farm Member is a carbonate sequence 2 to 3 m thick. It is continuously exposed in the Midland, Hartshill and Woodlands quarries, and is therefore inferred to be present throughout the outcrop of the Hartshill Sandstone Formation. Its type section was designated by Brasier et al. (1978) as 'exposure 1' of the SSSI located on the north-eastern face of Woodlands Quarry [SP 3249 9475]. The member rests erosively and with a basal conglomerate on a surface of ravinement developed on the Jee's Member, and its top is a hardground surface sharply overlain by sandstones of the Woodlands Member.

Although it is thin, this member is significant in three major respects. First, its fossil assemblage has enabled the Hartshill Sandstone to be correlated with early Cambrian successions elsewhere in the world, as originally recognised by Lapworth (1898) and further refined in studies by Brasier (1984, and review in Brasier, 1989). Second, the faunas show the Home Farm Member to be a highly condensed sequence, equivalent to successions 40 m thick in south-east Newfoundland (Brasier et al., 1992). Third, the faunas show that the Home Farm Member belongs to an important sedimentary hiatus and phosphogenic episode within the European and eastern Canadian Cambrian basins, and is of a similar age to the major phosphatic accumulations of south-east Asia and Australia (Brasier, 1980).

The palaeontology of the Home Farm Member is described by Matthews and Missarzhevsky (1975), and by Brasier (1984). Brasier (1986, 1989) shows a chart which summarises the vertical succession of the beds and the range of the most significant fossils that they have yielded; this stratigraphy is followed here, with the names used by that author shown in italics.

Above an erosional disconformity at the base of the member are the Quartzose Conglomerate and the Sandstone Bed which together are about 1 m thick. On the central south-western face of Hartshill Quarry [SP 3302 9398] the interval occupied by these beds consists of two upward-fining depositional couplets. The lowermost conglomerate bed erosively overlies calcareous Jee's Member sandstones and is the coarsest deposit of the Hartshill Sandstone to be seen above the Boon's Member. It is a hard grey rock consisting of well-rounded Precambrian volcanic pebbles supported in a coarse-grained quartz-sand matrix. Planar to tangential cross-bedding is prominent, with foresets inclined towards the north-west. The conglomerate grades upwards into a calcareous, highly glauconitic and bioturbated sandstone, which in turn is overlain by a further upward-fining conglomerate-sandstone couplet.

From these beds, Brasier (1989) reports the trace fossil Planolites in mudstone partings. There also occur small fragments of tommotiids, brachiopods, hyolithids and an internal cast of Coleoloides typicalis ((Plate 7)i, see p.153).

The Hyolithes Limestone, about 1 m thick, is divided into 12 beds by Brasier (1986). It is the most fossiliferous unit in the Hartshill Sandstone Formation, containing a rich fauna of tommotiids, tubes (Hyolithellus, Coleoloides, Torellella), protoconodont, hyolith and brachiopod fossils. For an extensive review, the reader is referred to Brasier (1989). The base of the Hyolithes Limestone is a gullied hardground surface on which rests the Phosphatised Limestone Conglomerate. The latter marks the beginning of calcareous deposition in the Home Farm Member; it comprises red sandy limestone (Bed li) and limestone with black phosphatised limestone intraclasts (Beds lii–iii), with a phosphatised limestone hardground at the top ((Plate 6), see p.152). The succeeding beds (2–10ii) constitute the Coleoloides Limestones, Siltstones and Shales, in which maroon or dark grey sandy limestones and nodular limestones pass up to more massive limestones with several hardground discontinuities. The hardground at the base of the Hyolith Shell Bed (Bed 10iii) is an iron-impregnated, stromatolitic-encrusted discontinuity marking an important faunal change, with the incoming of the index protoconodont Rhombocorniculum insolutum (Brasier, 1989).

The Sparry and Algal Limestones of Beds 11 and 12 are together about 0.15 m thick. They show patchy recrystallisation to coarsely crystalline carbonate segregations. From these beds, Brasier (1989) describes 'meadows' of Coleoloides together with a sparser fauna of hyolithids, tubes, tommotiids, inarticulate brachiopods and sponge spicules. Many of the hardground discontinuities in these beds are encrusted with stromatolites, one exposure in the Hartshill Quarry [SP 3302 9398] showing stromatolitic structures of the laterally linked hemispheroid type (Carney, 1992a). The top of the Home Farm Member is a ferromanganese crust draped with detrital mica from the overlying Woodlands Member (Brasier, 1989).

Woodlands Member

This youngest component of the Hartshill Sandstone Formation is exposed in its entirety in the disused Woodlands Quarry [SP 3248 9480], which is the type section designated by Brasier et al. (1978). Exposures in strike sections along the upper south-western face of Hartshill Quarry, although equally complete, are accessible only in a few places. The member, between 9 and 14 m thick, rests sharply and disconformably on the upper hard-ground surface of the Home Farm Member and is in turn sharply overlain by the Purley Shale Formation at the base of the Stockingford Shale Group.

In Woodlands Quarry the member forms a succession of dark grey, highly glauconitic and micaceous subarkosic sandstones in beds between 0.7 and 2.8 m thick. Cross-bedding and planar bedding are defined by fine-scale laminae, also seen in a thin section (E62475) to have a depositional microfabric formed by the alignment of platy grains, glauconite and red siltstone clasts. Bed tops have thin mudstone drapes and are ripple-marked; there are also large-scale linguoid and lunate ripple structures on certain planes, indicating alternating current flows towards the north-east and south-west. On the south-western face of Hartshill Quarry [SP 3303 9396], beds in the upper 1.5 m of the Woodlands Member are much reduced in thickness, to between 0.07 and 0.15 m. They also show extensive bioturbation and contain intervals of red sandy limestone or sparry carbonate. Some exposures show the upper calcareous sandstone to be sharply overlain by red mudstone of the Purley Shale Formation, whereas in others, sediment mixing is suggested by flame-like structures comprising lenticles of glauconitic sandstone in the mudstone and tongues of the latter projecting downwards into the sandstone. Brasier (1989) reports a sparse shelly fauna containing Coleoloides and Torellella from these beds.

Depositional environment

Palaeogeographical reconstructions and faunal correlations together suggest that the Hartshill Sandstone Formation represents the initial deposits of a marine transgression across the Precambrian landmass of 'Avalonia', which bordered the western margin of the Gondwana continent (McKerrow et al., 1992). It is most likely that this transgression commenced in latest Precambrian time, following a phase of rifting that fragmented the western margin of the Gondwana continent (Brasier, 1980; McKerrow et al., 1992) and led to expansion of the Iapetus Ocean (Wilson, 1966).

The coarse-grained red beds in the lower part of the Boon's Member are believed to contain the detritus shed from fault scarps formed during an early rifting event as the Gondwana continent began to break up. In keeping with such an environment, the angularity of the smaller clasts and abundance of lithic and unstable oxide grains indicate short distances of sediment transport and minimal reworking at the depositional site. The particularly distinctive bouldery breccio-conglomerates of Unit A are interpreted as the highly immature chaotic deposits of debris flows, whilst the included rounded boulders are corestones that became detached during the collapse of a crumbly, tropically weathered regolith developed on the Precambrian rocks. This style of sedimentation requires steep slopes; the available palaeocurrent directions indicate that these faced towards the south-west. The granulestone beds with plane stratification are akin to the deposits of high-concentration sediment gravity flows (Walker, 1978; Lowe, 1982) which possibly were triggered by the collapse of talus-covered slopes that mantled the fault escarpments. The depositional regime envisaged for Unit A is that of an alluvial fan which prograded into the sea, the marine influence being indicated by the occurrence of acritarchs; such an environment would be comparable to the 'scree-apron delta' system of Nemec (1990, fig. 1) or to the debris cones formed by deep-water Gilbert-type fan deltas (e.g. Postma, 1990, fig. 2). The overlying Unit B beds also comprise sediment gravity flows and resemble the deposits of proximal turbidite sequences (Hiscott and Middleton, 1979; Ghibaudo, 1992). Their accumulation in a marine environment is indicated by an intercalated acritarch-bearing mudstone, and is in keeping with deposition on the more medial parts of a submarine fan, or fan delta (Carney, 1992a). The better compositional maturity of Unit C sandstones suggests a hinterland made more subdued by continued erosion, allowing sediment to be reworked in alluvial plains or along the shoreline prior to eventual deposition. Fan delta influences persisted to form the more massive beds, but some show reworking by wave and/or current processes suggesting that the basin had shoaled to near wave base depths.

The transition from the Boon's to the Park Hill member reflects widespread regional subsidence and submergence of the former rifted topography. This caused a transgressive sequence to be laid down in a wide epeiric sea within a depositional province known locally as the 'Midland Platform' (Haim and Horton, 1969), and regionally as the 'Avalon platform' (Brasier, 1980).

The beds of the Park Hill Member are the initial post-rifting deposits of the main marine transgression. At the base of Unit D the persistence of a fan delta influence is suggested by the conglomeratic sandstone beds, but these are interspersed with cross-bedded sandstones indicative of reworking in marine tidal environments. A shoreface (nearshore) setting is suggested for the succeeding sandstone beds of Unit E: they typically show herringbone cross-bedding, resembling that described from sections through modern sandwaves in the Bay of Fundy (Dalrymple, 1984), and are suggested to have been constructed from sandy material transported by alternating north-eastward and south-westward tidal currents. The lunate scour pits seen on some bedding surfaces, if formed by ebb—flow currents (Carney, 1992a), would indicate that the north-easterly direction was that of the flood tide. It follows that the trough cross-bedded cosets, showing east-south-east current directions, are sections through sinuous-crested megaripples formed by a longshore current regime. A relative rise of sea-level caused the shoreface to retreat landwards, heralding a change to proximal inner shelf environments in which were deposited the mudstone and sandstone beds of Unit F. The lower, normally graded beds suggest deposition from sediment-laden currents induced by storms, whereas the younger and thicker beds up-section are the deposits of a prograding sediment wedge that had been worked into dune-like sandwaves analogous to the Class I or II bedforms of Allen (1980).

The Tuttle Hill Member was deposited on the inner part of a shelf made wider by continued transgression. The heterolithic (sandstone-mudstone) successions of Units G, J and L contain graded beds and may represent minor intervals of shelf flooding during which arenaceous material was transported basinwards by storm-induced currents. At the base of the member, the transition from Unit G to H is analogous to the upward-coarsening trend produced during the growth of an offshore shelf sand ridge in waters that had deepened significantly following transgression (Hein et al., 1991). The thick packages of tabular-planar cross-bedded sandstones in Unit I represent a post-transgressional highstand of sea-level during which sediments that had prograded into the deepening basin were worked into dune-like sandwaves (Allen, 1980): these migrated under the influence of a unidirectional, northeastwards flowing current which had penetrated the wider shelf. At times of shoaling on this shelf, the compound lenticular cross-bedded sandstones of Units I and K were formed in more fluctuating currents, perhaps as stacked sandwaves with scoured and megarippled topsurfaces of the type described by Dalrymple (1984). The plane-bedded sandstones of Unit L represent further storm-induced sedimentation; the thickness and homogeneity of this unit suggests an extended period of equilibrium between subsidence and the rate of sediment supply, with long intervals of low sedimentation during which the muddy tops of sand sheets were deeply bioturbated.

The Jee's Member represents a return to wave- or current-agitated environments, signifying lower sea-levels relative to those prevailing in the upper part of the Tuttle Hill Member. One consequence of this may have been erosion of the upper Jee's Member beds, since these are sharply overlain by basal conglomerates of the Home Farm Member. The deposition of the latter was a response to a rise in the relative sea level, which deepened the waters on the shelf and caused the shoreline to retreat landwards, thereby cutting off the arenaceous supply to the basin and allowing a condensed carbonate-rich sequence to build up. Overlying a surface of possible submarine erosion occupied by the Quartzose Conglomerate and Sandstone beds, the Coleoloides Limestones, Siltstones and Shales represent slow, intermittent sedimentation punctuated by episodes of submarine scouring of the carbonate banks. Their record of stable isotopes matches the pattern seen in the age-equivalent beds of Newfoundland, and suggests that the shelf was shoaling from subtidal to peritidal environments (Brasier et al., 1992).

The final stages of the marine transgression are seen in the beds of the Woodlands Member. They represent the deposits formed when currents carrying arenaceous material on to the shelf became re-established. The top 1.5 m of the member records a period of low sediment accumulation, reflected by the bioturbated and calcareous nature of the sandstones. There followed a major episode of flooding, during which the arenaceous source once more retreated landwards. Outer shelf environments eventually prevailed, with the accumulation of the mudstone-dominated Stockingford Shale Group.

Stockingford Shale Group

At outcrop the Stockingford Shale Group is about 700 m thick and succeeds the Hartshill Sandstone Formation with a sedimentary break but no obvious unconformity. It ranges in age from early Cambrian (mid-Comley Series) to early Ordovician (low Tremadoc Series) and represents relatively continuous argillaceous deposition, mainly in low-energy environments, over the Midland Platform. The Stockingford Shale Group is subdivided on the basis of colour, fissility and sandstone content into seven formations (Baldock 1991a), as follows:

Merevale Shale Formation over 90 m at outcrop
Monks Park Shale Formation 80 m
Moor Wood Sandstone Formation 15 m
Outwoods Shale Formation 250 m
Mancetter Shale Formation 30–75 m
Abbey Shale Formation 10–40 m
Purley Shale Formation 200 m

The four lowest formations strike north-westwards from Nuneaton, where they emerge from beneath the overstepping Triassic rocks, to just west of Atherstone (Sheet 155), where they are truncated by the Polesworth Fault. Dips are moderate to steep, towards the south-west. The younger formations emerge from beneath the Upper Palaeozoic north-west of Oldbury [SP 308 948] and extend beyond Atherstone, where the Merevale Shale Formation is overstepped by Triassic strata. The Monks Park and Merevale formations reappear to the west of the district in a small inlier at Dosthill, on the western margin of the Warwickshire Coalfield (Sheet 154). South of Nuneaton the only division exposed is the Outwoods Shale Formation, which there forms a southward-trending anticline; it is overstepped by Triassic strata at Bedworth.

Natural exposures are few and poor. Road and railway cuttings that formerly exposed parts of the group are now degraded and overgrown. The best exposures at present are in the roadstone quarries at Purley [SP 3055 9634], Mancetter or Oldbury [SP 3100 9540] and Griff No. 4 [SP 3610 8670]. Most of the detailed knowledge of the Stockingford Group has been obtained from such quarries and other temporary exposures, and above all from study of the three Merevale Boreholes (Taylor and Rushton, 1971).

Numerous exploration boreholes drilled by British Coal show that the Stockingford Shale Group lies unconformably below Upper Palaeozoic rocks throughout the Warwickshire Coalfield. The distribution of the main provings is shown in (Figure 10) and the diagnostic faunas are given in (Table 4). Biostratigraphical studies combined with dipmeter measurements indicate that the Cambrian strata beneath the Coal Measures of the Fillongley area occur within a faulted anticline or dome. The central part of this structure is occupied by the Outwoods Shale Formation whereas around its flanks younger units of the Stockingford Group subcrop beneath the Coal Measures. An extensive development of the Monks Park Shale Formation beneath Corley youngs southwards into the expanse of Merevale Shale Formation that underlies the Carboniferous strata of the Coventry and Warwick districts (Old et al., 1987). The anticlinal structure does not coincide with the other known areas of Variscan uplift, and its significance is discussed in Chapter 12. To the west, the Monks Park Shale is proved in the Fillongley Anticline, and a complementary syncline farther west accounts for the Merevale Shale subcrops.

There are few records of the Stockingford Group or its equivalents east of the Polesworth Fault. The Aston Flamville Borehole proved Tremadoc strata; but Brown's (1889, p.30) circumstantial record that Stockingford Shales were formerly exposed beneath drift in a railway cutting [SP 484 959] between Elmesthorpe and Stoney Stanton has not been confirmed, and a trial boring made there by M J Le Bas (University of Leicester) in 1974, though 4 m deep, did not reach bedrock.

The regional correlation of the Stockingford Shale Group offered by Cowie et al. (1972), and Rushton (1974) remains essentially unchanged by more recent work. In general, correlation in the Comley Series is rather tentative, but is more secure in the St David's Series and good in the Merioneth, though the Merioneth–Tremadoc boundary is not well constrained. Towards the top of the Monks Park Shale Formation, where fossils are rare and trilobites almost absent, a tenuous local correlation has been drawn using the distribution of various brachiopods, protoconodonts and problematical fossils as seen in the Merevale Boreholes Nos. 1, 1A and 2; these fossils, together with assemblages of acritarchs, have been of value in the correlation of short Lower Palaeozoic cores from several British Coal boreholes.

In the following description some typical examples of trilobites, and new discoveries, are shown on (Plate 7) to (Plate 8), (Plate 9), together with some other types of fossils.

Purley Shale Formation

This formation, named Purley Shales by Lapworth (1898, p.345), was recognised mainly on the basis of its red or purple colouration; it consists of blocky and shaly mudstones about 180 to 210 m thick which outcrop between Atherstone and Nuneaton. Reddish brown mudstones proved beneath Triassic rocks in boreholes at Bar Pool Valley in Nuneaton have been assigned to the Purley Formation, but it is not proved farther south.

The Purley Shale Formation overlies the Woodlands Member of the Hartshill Sandstone Formation along a sharply defined contact which is visible intermittently at the top of the south-west faces of the Hartshill and Midland quarries and in Woodlands Quarry. The top of the Woodlands Member is calcified and bioturbated in the two former sections, suggesting a break in sedimentation prior to deposition of the Purley Shale. The top of the formation grades up into the Abbey Shale Formation (Illing, 1916, p.398). The SSSI at Woodlands Quarry is taken as the basal stratotype of the Purley Shale Formation [SP 3244 9482].

The mudstones are commonly blocky, structureless and poorly laminated. Burrows are present but bioturbation is not generally conspicuous. Dark grey (anoxic) mudstones have not been observed. There are sporadic thin beds of siltstone and fine-grained sandstone, of which one, formerly exposed in Purley Park [SP 3120 9612], showed current ripples. Three informal divisions within the Purley Shale, of subequal thickness, were described by Baldock (1991a), and comprise blocky to shaly red or maroon mudstones with sporadic red-brown siltstones, overlain by pinkish grey to grey-green fissile mudstones, in turn followed by interbedded purple, red and green shaly mudstones.

The red colour of many of the strata is considered to be primary, possibly resulting from the weathering process in the source area (Allen, 1968b), but part of the red colour may be the result of staining by Triassic rocks; such staining caused Illing (1916, pl. 38) to place the top of the Purley Shale in Stockingford railway cutting, about 400 m west of its position on the present map.

Biostratigraphy

Fossils are generally very scarce in the Purley Shale Formation, though faunas indicative of the higher Comley and lower St David's series have been found at four principal horizons, as listed in detail by Rushton (1966, pp. 2–6).

  1. Calcareous nodules close to the base of the formation have yielded Coleoloides and trilobite fragments attributed with doubt to Callavia.
  2. About 70 m above the base greenish grey beds of mudstone have yielded a fauna of brachiopods, hyolithids and trilobites at Camp Hill (Rushton's Locality 1B) [SP 3389 9299] and these indicate a level near the base of the Protolenid-strenuellid Biozone of Cowie et al. (1972). A similar fauna collected by R J Kennedy from a temporary exposure [SP 3500 9218] close to Judkins' Quarry includes Strenuella sabulosa and Serrodiscus bellimarginatus, allowing correlation with Locality 1B, together with the brachiopod Alisina atlantica and the trilobite Hebediscus attleborensis ((Plate 7)e–g, j-1, see p.153).
  3. About 140 m above the base of the formation another bed of greenish mudstone [SP 3468 9234] yielded a prolific fauna, mainly of eodiscid trilobites (including Chelediscus acifer, (Plate 7)c, d and h, see p.153), which suggest correlation with high Lower Cambrian strata in New York and south-east Newfoundland. From the same locality, Potter (1974) obtained an acritarch microflora with Eliasum llaniscum and Skiagia ciliosa, suggesting correlation with the late Lower Cambrian Rausve horizon of the East European Platform (Volkova et al., 1979).
  4. The highest fauna, of sponge spicules, brachiopods and trilobites, occurs 55 m above the C. aciferhorizon and within about 10 m of the top of the formation. It is of St David's age and indicates correlation with a high level in the Paradoxides oelandicus Zone. The Comley–St David's Series boundary appears to lie at an undetermined level within a 40 m thickness of featureless red mudstones (Rushton, 1966).

Abbey Shale Formation

The Abbey Shale Formation is relatively thin and comprises greenish and bluish grey and black shaly mudstones. It passes down into the Purley Shale Formation from which it is distinguished principally by colour, better lamination, greater fissility and the presence of numerous layers of dark pyritous mudstone. The top of the formation is marked by a minor unconformity.

Although the outcrop extends for over 6 km, the formation is very poorly exposed. Detailed knowledge of the succession has been obtained from Merevale No. 3 Borehole and from temporary exposures, notably the trenches made by Illing (1916) at Hartshill Hayes, traces of which can still be seen [about [SP 3240 9423], and which is taken as the stratotype. The section in Purley Park where Illing (1916, p.393) described the base of the formation [SP 3101 9610] is now much overgrown.

The thickness varies markedly; it is less than 20 m thick in the Camp Hill area [SP 346 923], was measured at about 30 m at Illing's trenches, thins northwards to about 10 m west of White Hall Farm [SP 3191 9502] (Baldock, 1991b), and thickens again to about 40 m near Atherstone.

Lithologies were described in detail by Illing (1916), and Taylor and Rushton (1971). They include dark grey pyritous mudstone interbedded with greenish or bluish grey mudstones. There are thin calcareous and glauconitic sandstone beds, and impure limestones 1 to 5 cm thick at intervals of a few metres. Pyritic and phosphatic nodules occur throughout. Two thin (less than 5 mm) white clay beds recorded in Merevale No. 3 Borehole are probably of bentonitic origin.

Biostratigraphy

The Abbey Shale Formation is notable for its rich fauna of sponge spicules, horny brachiopods, hyolithids, Stenotheca and bradoriid ostracods, and especially trilobites, of which Illing (1916) recorded more than 50 species collected from 22 horizons. He grouped the horizons as shown below (inferred correlation after Rushton, 1979).

Horizons Faunas Biozones
G1-G3 Upper

Paradoxides davidis fauna

 Ptychagnostus punctuosus
Fl–F3 Lower Hypagnostus parvifrons
E1–E3 Hartshillia (passage) fauna
D1–D3 Upper

Paradoxides hicksii fauna

Tomagnostus fissus
B1–C3 Lower
A4 Paradoxides aurora fauna Ptychagnostus gibbus

Detailed distribution of the species was listed by Illing (1916). The agnostid trilobites were reviewed by Rushton (1979) and most of the other forms by Lake (1906–1946).

The lowest fauna contains Paradoxidid fragments but is not very diverse, and its correlation is not well established. The P. hicksii faunas are diverse and were detected by A F Cook and P H Whitworth at temporary exposures in an industrial estate at Camp Hill [SP 3447 9236] and at a bridge crossing on the Coventry Canal (Vernons Lane) [SP 3494 9203] ((Plate 8) g–k, see p.154). The Hartshillia Fauna is a low-diversity fauna with abundant Hartshillia inflata ((Plate 8)f); it is developed at Hartshill Hayes but is probably only of local occurrence because it was not observed either in Merevale No. 3 Borehole or at the Coventry Canal locality (Cook, 1977). The lower P. davidis Fauna was observed at temporary exposures near Vernons Lane, where Mr Cook collected Paradoxides (s.1.) abenacus ((Plate 8)a, see p.154) and good material of centropleurid trilobites, including Clarella impar (Cook, 1977), Luhops expectans and Luhops? pugnax ((Plate 8)b–e). The Upper P. davidis fauna, which has been detected only at Hartshill Hayes, is very similar to the P. davidis faunas of the St David's area, South Wales, and in south-east Newfoundland.

Mancetter Shale Formation

The Mancetter Shale Formation consists mainly of smooth grey mudstones that weather to a khaki or greenish colour, but is characterised particularly by the presence of thin beds of hard sandstone. It is up to 30 m thick in the Camp Hill area north-west of Nuneaton, thickens to about 45 m at Hartshill Hayes, and attains about 75 m to the west of White Hall Farm [SP 316 955]. The base of the formation rests on the Abbey Shale with a slight unconformity at which one or two trilobite zones are missing. The top of the formation passes up into the Outwoods Shale. The Mancetter Formation is poorly exposed but its outcrop can be traced by the distribution of blocks of sandstone and microconglomerate in field brash, especially in the area west of White Hall Farm where three beds of gritty arkosic sandstone were mapped locally (Baldock, 1991b). The basal unconformity is poorly visible at the site of Illing's trenches in Hartshill Hayes [SP 3237 9423]. Temporary exposures were seen in an industrial estate at Camp Hill [SP 3457 9227] and in an excavation at Vernons Lane canal crossing [SP 3492 9204] (information from A F Cook). The whole thickness of the formation was studied in Merevale No. 3 Borehole (Taylor and Rushton, 1971, p.7), which accordingly supplies the best reference section of the formation.

The mudstones of the Mancetter Formation are mostly grey, shaly and burrowed, and they alternate commonly with thin beds and wisps of fine-grained sandstone. Dark pyritous mudstone layers are thin and rare. Hard sandstone beds 5 to 20 cm thick occur at intervals of one to a few metres; they are tabular, with lobate, load-cast bases and sharp tops. They were described by Hawkes (in Taylor and Rushton, 1971, p.8) as locally silicified glauconitic subgreywackes. They were probably deposited rapidly as intermittent high-density gravity-flows bringing coarse detritus on to the Midland Platform shelf area. The basal conglomerate of the Mancetter Shale is 2 to 8 cm thick and was described in detail by Illing (1916, p.395). It contains igneous material as well as fragments of the underlying shale.

Biostratigraphy

The fauna of the Mancetter Shale Formation is sparse. It includes horny brachiopods, the large bradoriid Anabarochilina primordialis near the base and the agnostid trilobites Ptychagnostus fumicola and Tomagnostella sulcifera. The whole formation is assigned to the highest St David's Series Biozone of Lejopyge laevigata. The macrofauna was described and the stratigraphical distribution listed by Rushton (1978). An acritarch microflora recorded by Potter (1974) from the Mancetter Formation in Merevale Borehole No. 3 has species in common with assemblages of equivalent age from eastern Newfoundland (lower A2 microflora of Martin and Dean, 1981, 1988), including Timofeevia lancarae?, T. phosphoritica and Cristallinium cambriense.

Outwoods Shale Formation

This formation is a thick succession of interbedded dark grey and pale grey mudstones, with rare sandstone beds that are much finer grained and thinner than those of the Mancetter Shale. It is the thickest and most widely exposed formation of the Stockingford Group, and the only one exposed south of Nuneaton. The thickness has been estimated at 300 m, but if intrusive sills are discounted 250 m may be a better estimate.

The principal exposures are in quarries where igneous sills are being worked, for example in the areas of Griff [SP 360 894] and [SP 364 886] and Oldbury [SP 310 950] and [SP 305 960], but there are numerous smaller exposures, often of thermally altered shale associated with intrusions, in road, railway and canal cuttings and in several small pits and quarries; localities are listed by Taylor and Rushton (1971, p.12) and by Baldock (1991a, 1991b), Bridge (1991) and Lawley (1992a). The formation was formerly well seen in Stockingford railway cutting (now much overgrown) and temporarily in a trench along the brook that flows through Camp Hill sampled from [SP 3436 9232] to [SP 3424 9225]. Dips vary from 20° to 85° (commonly 35°) and are overturned at one locality [SP 3401 9248]. The greater part of the succession was seen in the Merevale Boreholes, the upper part in No. 1 and the lower in No. 3. The Outwoods Shale Formation was encountered at depth in boreholes at Park House, Fillongley Hall, and possibly at Birchley Hall and Well Green Farm.

The base and top of the formation are both gradational. The base is taken at a level where dark mudstones become conspicuous and form more than a small part (about 2 per cent) of the sequence. In Merevale No. 1 Borehole the top lies below the level at which the sandstones of the Moor Wood Sandstone Formation appear (Taylor and Rushton, 1971) but in the Fillongley Hall Borehole the topmost beds pass up into the Monks Park Shale without the intervention of the Moor Wood Sandstone.

The mudstones are described by Taylor and Rushton (1971, p.10). They consist of millimetre- to centimetre-scale alternations of pale burrowed mudstone and dark pyritous mudstone. Each type locally predominates through thicknesses of several metres of beds. The burrows in the pale mudstones are mainly parallel to bedding and often pyritised (Taylor and Rushton, 1971, pl. 5, fig. 2; pl. 6, figs. 3, 6). The dark grey mudstones contain a little more carbon and sulphur than the pale mudstones and generally lack burrows; they include many bedding planes covered with organic debris, including algal filaments (Taylor and Rushton, 1971, pl. 5, fig. 6; pl. 6, figs. 1, 2); trilobites and bradoriid ostracods tend to be commoner in these rather than in the pale mudstones. There are nodules of pyrite and carbonate in the paler mudstones, and of phosphate in the darker mudstones. The contacts between the paler and darker mudstones are commonly sharp, with indications of pauses in deposition, especially in the lower part of the formation (Taylor and Rushton, 1971, p.12). Raiswell and Berner (1986) showed that the sulphur-to-carbon ratio in the Outwoods Shale Formation is significantly higher than in later (post-Lower Palaeozoic to Holocene) marine shales.

Thin micaceous layers and sandstone beds occur locally. Near the top of the formation, quarries at Griff expose a local development of fine-grained micaceous sandstone beds up to 0.6 m thick. These show ripple-drift bedding and some convolute bedding, and their bases show flute casts (Bridge, 1991). Although these sandstone beds resemble those of the overlying Moor Wood Sandstone, the interbedded mudstones make up a far higher proportion of the succession, and this, together with the lack of Orusia in the overlying mudstones, indicates that they lie at a lower stratigraphical level.

Biostratigraphy

Apart from the lowest 9 m of the formation, which have not yielded diagnostic fossils, the faunas of the Outwoods Formation indicate low horizons in the Merioneth Series (Upper Cambrian). The trilobite fauna includes some 36 species that allow recognition of all the lower biozones of the Merioneth Series, those of Agnostus pisiformis, Olenus (including 4 subzones) and, at the top of the formation, the basal part of the Parabolina spinulosa Biozone, which is characterised also by the appearance of the brachiopod Orusia lenticularis. Rushton (1978) described the fauna of the A. pisiformis Biozone and gave the stratigraphical distribution of the fossils. The trilobites of the Olenus Biozone ((Plate 9)i, see p.155) were described by Rushton (1983), who gave their stratigraphical distribution and highlighted the cosmopolitan character of the fauna. Acritarch assemblages recorded from the Olenus Biozone by Potter (1974) show the successive appearance of species such as Timofeevia pentagonalis and Leiofusa stoumonensis, at about the same levels as they appear in eastern Newfoundland (upper A2 microflora of Martin and Dean, 1981, 1988).

Moor Wood Sandstone Formation

The Moor Wood Sandstone Formation is the only arenaceous division in the Stockingford Shale Group and consists of 15 m of sandstone and siltstone with interbeds of shaly micaceous mudstone. It overlies the Outwoods Shale gradationally and passes upwards into the Monks Park Shale, the limits being taken arbitrarily at the lowest and highest sandstone beds that are greater than 0.15 m in thickness. Although the formation is not exposed, the outcrop, which extends from Oldbury [SP 3087 9469] to Merevale Hall [SP 2954 9733], can be traced by means of landform features and the distribution of sandstone brash. A borehole at Griff Colliery No. 4 Shaft reached sandy shales with Orusia lenticularis, suggestive of the Moor Wood Formation, at a depth of 100.3 m below OD.

The entire thickness of the formation was studied in Merevale No. 1 Borehole (Taylor and Rushton, 1971, p.22). The sandstone beds, up to about 0.3 m thick, are fine grained and well sorted, and are composed mainly of quartz and mica with some pyrite and feldspar. The bedding is typically convoluted (Taylor and Rushton, 1971, pl. 7), the convolutions mostly showing overturning towards the north-east (when the core is orientated with respect to the regional dip), suggestive of slumping in the same direction. The micaceous mudstone inter-beds are commonly wavy-bedded, showing sharp contacts with the sandstones, and some are seen to truncate the tops of the convolute-bedded sandstone units. The Moor Wood Sandstone was not intersected by the Fillongley Hall Borehole, which passed down from the base of the Monks Park Shale directly into strata assigned to the Outwoods Shale. It is inferred that the Moor Wood Sandstone is only of local development.

Biostratigraphy

The Moor Wood Sandstone Formation is assigned to the Parabolina brevispina Subzone of the Parabolina spinulosa Biozone, because fossils indicative of this subzone occur both below and above the Moor Wood Sandstone. The only fossils found within the formation are the brachiopod Orusia lenticularis, which ranges both below and above the limits of the formation, and fragments of Parabolina sp.

Monks Park Shale Formation

The Monks Park Shale Formation is composed largely of black mudstones and is about 80 m thick. Like correlative deposits in other part of England and Wales, it represents a relatively condensed succession that formed under poorly oxygenated conditions.

The formation is very poorly exposed, but its outcrop has been traced from near Oldbury Farm to Merevale Hall, mainly by the presence of distinctive black mudstone fragments in the soil. The Merevale No. 1 Borehole provides the best reference section. There are poor exposures in Monks Park Wood [SP 2990 9625], in a road cutting to the north [SP 2979 9652] and in an adjoining wood [SP 2960 9674], but the outcrop was mainly delineated by means of augering and by use of a scintillometer to detect the relatively high gamma radiation emitted by beds in the upper part of the formation (Taylor and Rushton, 1971, pp. 24, 64). The formation is proved at depth beneath Coal Measures in several boreholes near Corley and Fillongley, north-west of Coventry ((Figure 10), (Table 4)).

The lowest 15 m of the formation, where it passes up from the Moor Wood Sandstone, consists of grey micaceous shaly mudstones. These pass up into the black pyritous mudstones that particularly characterise the formation. The top of the formation is taken arbitrarily at the top of a transition zone in which black mudstones alternate with greenish grey mudstones typical of the overlying Merevale Shale.

The greater part of the formation is composed of black mudstones which show a black streak when scratched. They are commonly blocky and structureless, but finely laminated beds also occur. These mudstones, which were described by Taylor and Rushton (1971, p.28), are thought to represent deposition in a strongly dysaerobic (oxygen-poor) environment. They contain more than 2 per cent carbon and much sulphur (equivalent to more than 20 per cent pyrite). Nodules of dolomite (associated with baryte) and phosphate are common. The black mudstones of the Monks Park Shale differ from those of correlative strata in Wales (Cope and Rushton, 1992) in the rarity of fine sedimentary lamination and the lack of large calcareous concretions (stinkstones).

At intervals there are thin beds (1 to 8 cm thick) of pale grey mudstone. These are burrowed and commonly contain horny brachiopods, and indicate relatively brief periods of oxygenation. Thin interbeds of fine-grained sandstone are present at the base of the formation where it passes down into the Moor Wood Sandstone. Sandstone beds are very rare in the high (black) part of the Monks Park Shale, in contrast to the equivalent beds in the Croft Borehole, Shropshire (Rushton et al., 1988).

Biostratigraphy

The lower grey mudstones of the Monks Park Shale Formation contain abundant Orusia lenticularis ((Plate 9)e) at numerous horizons (about 90 levels in Merevale No. 1 Borehole), and trilobites of the Parabolina spinulosa Biozone. The biostratigraphy of the black mudstones depends principally on the distribution of olenid trilobites, of which there are more than 20 species referable to the genera Parabolina, Leptoplastus, Eurycare, Ctenopyge and Sphaerophthalmus. They represent several subzones of the Biozones of Parabolina spinulosa, Leptoplastus, Protopeltura praecursor, Peltura minor and P. scarabaeoides, as detailed by Taylor and Rushton (1971, pp. 25–33). The fauna of the black mudstones includes horny brachiopods, notably the large form Broeggeria ((Plate 9)a), whereas small acrotretids and lingulellids are commoner in the pale burrowed beds. Bradoriids occur, and, in the upper beds, the protoconodont Phakelodus tenuis and a small coarse-ribbed orthoid brachiopod. Acritarchs recorded by Potter (1974) from the brevispina Subzone of the P. spinulosa Biozone include Dasydiacrodium caudatum and Veryhachium cf. dumontii, which appear at about the same level, near the base of the P. spinulosa Biozone, in eastern Newfoundland (Martin and Dean, 1981; 1988). Trunculumarium revinium was also recorded from the P. brevispina Subzone, slightly lower than its appearance in eastern Newfoundland near the top of the P. spinulosa Biozone. No determinable microflora was recovered from the higher zones of the formation.

The highest part of the formation lacks diagnostic fossils and its correlation is uncertain, though it is homotaxial with the highest part of the Dolgellau Formation of North Wales (Allen et al., 1981) and the Acerocare Biozone of Scandinavia. The Monks Park Shale nevertheless affords the most complete faunal succession in Britain of the Upper Merioneth Series (Thomas et al., 1984, fig. 6, p.15).

Merevale Shale Formation

This formation is composed mainly of grey and greenish grey mudstones. It includes the local representatives of the Tremadoc Series and is partly equivalent to the Shineton Shales of Shropshire, though it includes older strata than are known in Shropshire. The outcrop extends north-west from the Arley Fault System to about 1 km north-west of Merevale Church (beyond the district boundary), where it is overstepped by Millstone Grit; it exposes only the lower part of the formation, which is estimated to be about 100 m thick, but younger strata occur at depth, and the aggregate thickness is probably much greater.

The mudstones are grey and shaly when fresh but weather to greenish or buff tints. There are rare dark grey interbeds and also thin beds of siltstone and sandstone. The lithologies are described by Taylor and Rushton (1971, p.34) and Old et al. (1987, p.3). Trace fossils and bioturbation are widespread. The base of the formation, seen in Merevale No. 2 Borehole, is taken at the top of a zone of transition 5 m thick, in which black to dark grey mudstones typical of the Monks Park Formation alternate with greenish grey mudstone (Taylor and Rushton 1971, p.24). These alternations indicate fluctuations in oxicity at the sea floor. The formation is poorly exposed and the base is not seen at outcrop, so the core of the Merevale No. 2 Borehole is taken as the basal stratotype of the formation.

Many boreholes reached the subcrop of the Merevale Shales, particularly in the area between Coventry and the Fillongley Anticline (Figure 10). The Chapel Green, Moat House Farm and Hawkes End boreholes appear to have reached the transition beds between the Monks Park and Merevale formations (Table 4); higher strata with Rhabdinopora ((Plate 9)d) were encountered to the south and west, for example at the Outwoods, Meriden and Gables Farm boreholes. To the south of Coventry, borehole records suggest that a south-east-younging subcrop of lower Tremadoc rocks over 10 km wide forms one limb of a broad syncline–anticline pair located under the Warwick district (Old et al., 1987, fig. 2). In the east of the present district the only borehole record of the Merevale Formation is in Aston Flamville Borehole, 3 km east-south-east of Hinckley. There the mudstones, unlike those of the outcrop and Warwickshire Coalfield subcrop, show a weak but persistent vertical cleavage.

Biostratigraphy

The Merevale Shale Formation has yielded a few fossils typical of the lower part of the Tremadoc Series at outcrop, notably Rhabdinopora flabelliformis cf. socialis (Bulman 1927, p.28; Bulman and Rushton, 1973, p.10), and a wider range of species from boreholes (Table 4), but the age of the lowest part of the formation remains uncertain. The boundary between the Merioneth and Tremadoc series (approximately equivalent to the Cambrian–Ordovician boundary) is not closely constrained in the district, and does not necessarily coincide with the Monks Park to Merevale transition zone. In Merevale No. 2 Borehole there are about 30 m of black Monks Park Formation mudstones above the highest Merioneth zonal trilobites, and at outcrop the lowest horizon yielding Rhabdinopora, of definite Tremadoc age, was a quarry [SP 2889 9765], now filled in] which Taylor and Rushton (1971, p.35) estimated to be 90 m or more above the base of the Merevale Formation. The intervening strata, about 120 m thick, are homotaxial with the highest Merioneth Acerocare Biozone and the basal Tremadoc; a number of macrofossils have been found, but none of diagnostic value (Table 4). Acritarch floras recovered from these strata are as follows. From the Park Hill Lane Borehole, R E Turner recorded Acanthodiacrodium angustum, Cymatiogalea bellicosa, C. cristata, Stelliferidium cortinulum and Vulcanisphaera capillata. A similar assemblage, with A. angustum, C. bellicosa, C. cristata, S. cortinulum and Dasydiacrodium cf. caudatum, was recorded from the Blabers Hall Borehole. In each case, an earliest Tremadoc age is suggested by comparison with acritarch assemblages from the Shineton Shales of Shropshire and the Upper Cambrian and lower Tremadoc of eastern Newfoundland, possibly predating all or part of the Rhabdinopora flabelliformis Biozone as recognised in the Shineton Shales succession of Shropshire. In the Berryfields Farm Borehole, Dr Turner recorded cf. Timofeevia pentagonalis, which ranges into the latest Cambrian Acerocare Biozone in eastern Newfoundland, accompanied by species such as A. cf. angustum and Micrhystridium robustum which are not known below the Tremadoc. This suggests the presence of strata close to Merioneth-Tremadoc series boundary. The Hawkes End Borehole yielded an acritarch assemblage with Acanthodiacrodium spp., Cymatiogalea bellicosa?, C. cuvillieri, C. multarea?, Stelliferidium cf. cortinulum and an undescribed species of Dasydiacrodium. The latter resembles an unpublished species from the earliest Tremadoc of eastern Newfoundland. The other species, particularly C. bellicosa? and S. cf. cortinulum, also indicate an early Tremadoc age, though both of these range into the tenellus Biozone.

The Aston Flamville Borehole yielded one trilobite, Platypeltoides, and a Tremadoc acritarch assemblage which included Acanthodiacrodium angustum, Arbusculidium destombesii, Cymatiogalea cristata, C. cuvillieri, C. velifera, Micrhystridium shinetonense and Vulcanisphaera cirrita. Arbusculidium destombesii s.s. is known only from the Rhabdinopora flabelliformis Biozone in the Shineton Shales, although a similar form, A. frondiferum, ranges as high as the Brachiopod Beds (mid-Tremadoc) there.

Depositional environment

There has been no detailed sedimentological study of the conditions under which the Stockingford Group was deposited and they are known only in outline. As a whole the Group accumulated at fairly shallow depths on a tectonically stable marine shelf that received little arenaceous detritus. Variations in lithological character of the mudstone formations reflect changes in the conditions of deposition: three phases during which relative anoxia prevailed (parts of the Abbey, Outwoods and Monks Park formations) alternated with the accumulation of oxic, more sand-rich sediments. The oldest division, the Purley Shale Formation, is bioturbated (though this not always very evident) and contains a sparse fauna of benthic habit (brachiopods, hyolithids, thick-shelled trilobites); these are indicative of oxygenated conditions. The red-beds evidently include detritus, derived from an area of subaerial erosion, that was not subsequently reduced. The overlying Abbey Shale Formation includes evidence of poorly oxygenated conditions in some black pyritous beds, whereas others, including thin beds of glauconitic sandstone, indicate the influence of tidal currents (Allen, 1968b, p.35). The trilobite fauna probably owes its richness and diversity to the fluctuating environmental conditions.

The top of the Abbey Shale Formation is a surface of erosion and possible emergence, the mildly transgressive base of the overlying Mancetter Formation being marked by a thin basal conglomerate reminiscent of a shore face deposit (Allen, 1968b). The succeeding shales are bioturbated and were deposited in oxygenated conditions whilst the sandstone interbeds represent occasional influxes of coarse material by gravity flow.

The overlying Outwoods Shale Formation contains many anoxic layers, which, at some levels in the middle part of the formation, continue through several metres of beds. These represent the 'Olenid biofacies' (Fortey, 1975) which is characterised by olenid trilobites that were adapted to living near or on the sea floor in areas of exceptionally low oxygenation (Henningsmoen, 1957).

The Moor Wood Sandstone Formation represents a short-lived and apparently local episode of sandstone deposition, at a time of instability and slumping, on a north-easterly facing slope (Taylor and Rushton, 1971, p.22). The Monks Park Shale Formation marks a return to low-energy conditions that became relatively anoxic during the deposition of the upper part of the formation. At this time the olenid biofacies was re-established; bioturbation is conspicuous only in thin layers that represent brief periods of relatively higher oxygenation.

The change from black Monks Park to grey or greenish grey Merevale Formation mudstones corresponds to renewed ventilation of the sea floor. Although the Merevale Formation (Tremadoc Series) is not proved to be of great thickness in the present area, correlative strata have a remarkably wide subsurface distribution, and are known from boreholes and seismic reflection evidence to be very thick (up to about 2 km), for example in a graben or half-graben beneath the Worcester Basin (Smith and Rushton, 1993, fig. lb). The same authors suggest that renewed oxygenation seen in the basal Tremadoc strata in the Nuneaton area (and elsewhere), and the rapid rate of sedimentation, may be related to an early Tremadoc phase of rifting.

Chapter 4 Ordovician intrusive igneous rocks

All of the known Palaeozoic intrusive rocks in the Coventry district are of Ordovician age; the intrusion found between Cambrian and Carboniferous rocks in the Dale Wood Borehole was previously thought to be younger than Westphalian A (Old et al., 1990), but is now interpreted to be weathered in its topmost part, and unconformably overlain by the basal Carboniferous beds.

This area and the Coalville district to the north contain two associations of Ordovician intrusions, different from each other both in petrography and chemical composition. One set of intrusions, cropping out around Sapcote, Stoney Stanton and Croft, are oversaturated with respect to silica and belong to the association of rocks known locally as the South Leicestershire Diorites (Le Bas, 1968; Worssam and Old, 1988). The other set, which form numerous concordant intrusive sheets cropping out in the Nuneaton Inlier or concealed beneath Carboniferous strata of the Warwickshire Coalfield farther west, together comprise the quartz-deficient Midlands Minor Intrusive Suite. The latter do not occur within Upper Devonian strata, and were therefore attributed a post-Tremadoc, pre-Devonian age by Worssam and Old (1988). New radiometric data discussed below further constrains their age, and that of the South Leicestershire Diorites, to the late Ordovician.

Previous workers have suggested the possibility that all of these intrusions, together with the Mountsorrel igneous complex of Leicestershire, were related to the same magma source (e.g. Le Bas, 1968; Hawkes, in Taylor and Rushton, 1971). However, Allport (1879) had earlier shown that the intrusions of the Nuneaton Inlier (Midlands Minor Intrusive Suite) differed from the 'neighbouring syenites of Leicestershire' in being olivine bearing, and therefore quartz free, and in containing only very minor amounts of alkali feldspar. The two plutonic associations may have had different magmatic parents, therefore.

Probably the first to be intruded were the South Leicestershire Diorites, for which a U-Pb zircon radiometric determination on samples from Enderby Quarry, in the adjacent Coalville district (sheet 155), gives an age of 449 ± 18 Ma (Pidgeon and Aftalion, 1978, re-evaluated by Noble et al., 1993), or late Caradoc on the timescale of Cowie and Bassett (1989). The very similar age of the Midlands Minor Intrusive Suite in the Nuneaton Inlier is indicated by a U-Pb zircon and baddeleyite determination of 442 ± 3 Ma, or mid-Ashgill, from a pegmatitic segregation in the sill at Griff No. 4 Quarry [SP 361 894] (Noble et al., 1993). These results demonstrate that in a European-wide context both intrusive phases are early Caledonian, as had been concluded by Le Bas (1972) for the South Leicestershire Diorites. The U-Pb zircon method is considered to be a high-precision technique for dating the emplacement of acid or intermediate magmas; the above results therefore supersede an earlier age estimate, of 546 ± 22 Ma based on the Rb-Sr method, for the South Leicestershire Diorite pluton near Enderby in the Coalville district (Cribb, 1975). On the other hand, the rather low value of around 400 Ma, obtained by the K-Ar method on separated hornblendes from the Griff No. 4 Quarry, suggests a disturbance of this particular isotopic system at about the time of the Acadian (late Caledonian) orogenic episode (C Rundle, written communication, 1990).

Geochemical studies utilising major and trace element analyses have indicated that magma compositions parental to the South Leicestershire Diorites show the involvement of a subduction zone in their generation (Webb and Brown, 1989). It will be shown here that the Midlands Minor Intrusive Suite also has a subduction zone geochemical signature, but that this is accompanied by a strong within-plate component of element enrichment. These considerations serve to underline the importance of the petrographic differences originally noted by Allport (1879) and assist in constraining the regional plate tectonic setting of this area in late Ordovician times, as explained below.

South Leicestershire Diorites

These intrusions form two belts of outcrop, currently being exposed in the line of quarries between Calver Hill [SP 496 931] and Stoney Stanton [SP 490 950] (Figure 11), and at Croft Quarry [SP 510 965] farther to the north-east. Before quarrying, these rocks gave rise to small natural outcrops and to the hill 123 m high near Croft (Bosworth, 1912; Eastwood et al., 1923). This topography reflects partial exhumation of an old land surface, upon which the intrusions formed ridges or inselbergs that were later buried beneath Triassic strata (Watts, 1947; LeBas, 1993). The unconformity with the Triassic rocks is well exposed in some of the quarries, as originally described by Bosworth (1912), but there are no exposures of the country rock that hosted the intrusions at the time of their emplacement. In the Mountsorrel area, farther north, it has been demonstrated that intrusions of the South Leicestershire Diorites were emplaced into hornfelsed argillaceous sedimentary rocks comparable to the Stockingford Shale Group (Le Bas, 1972). There is indeed a possibility that the Stockingford Shales hosted the intrusions described here, for they have been reported (Brown, 1889) beneath drift deposits in a railway cutting [SP 484 959] between Elmesthorpe and Stoney Stanton, within a kilometre of the buried intrusive ridge. However, this occurrence has not been confirmed in recent surveys (p.30).

In the first detailed petrographic study by Hill and Bonney (1878), these intrusions were classified as syenites, although with some of the attributes of quartz diorite. The view of Whitehead (in Eastwood et al., 1923) was that, as no alkali feldspar could be identified, these rocks compared better with quartz diorite or tonalite; this opinion was supported by the geochemistry of one sample. Considerations of mineralogy and texture led the same author to conclude that these intrusions did not compare with those of South Charnwood, as had been suggested by Hill and Bonney (1878), and which, in any case, are now known to be Precambrian in age, but bore instead a 'very distinct resemblance' to the intrusive rocks in the Mountsorrel area. The same cornparison was acknowledged in the suggestion of Le Bas (1972) that the South Leicestershire and Mountsorrel rocks, although representing different plutons, were part of the same 'Caledonian' magmatic episode.

Although the South Leicestershire Diorites occupy separate outcrops in this district, geophysical studies summarised by Allsop and Arthur (1983) confirm the view of Le Bas (1968, 1972) that they are linked at depth to form a single pluton, which extends south-eastwards into the Market Harborough district and includes the microtonalitic rocks proved in the Countesthorpe (Cottage Homes) Borehole (Poole et al., 1968). Le Bas (1972) envisaged this pluton to be a complex of overlapping, roughly circular intrusive bodies consisting of quartz diorite, tonalite and microtonalite, with intrusion having possibly taken place in that order. The intrusions in the Coventry district are mainly quartz diorites and represent the western-marginal, and possibly the earliest, part of this complex.

Intrusive rocks in Croft Quarry

The igneous rocks in Croft Quarry are currently excavated to a depth of about 100 m below the original ground surface. They are grey-weathering rocks, more massive than the intrusions of Sapcote and Stoney Stanton, but nevertheless transected by east–west and north-east to south-west joint systems (Eastwood et al., 1923). Layering, which is sporadically present, is generally subhorizontal, consisting on fresh surfaces of decimetres-thick alternations between pink and blue-grey quartz diorite. Modal analyses show that six samples from Croft contain between 15 and 22 per cent free quartz and 0 to 8 per cent alkali feldspar (Worssam and Old, 1988, table 6), suggesting a broad classification of the intrusion as quartz diorite (nomenclature after Streckeisen, 1976) although with affinities also to tonalite and granodiorite.

The Croft intrusions show only small variations in texture and mineral proportions, and no further sampling was undertaken during the present survey. A thin section (E4978) of a particularly fresh quartz diorite shows a hypidiomorphic-inequigranular texture. About 45 per cent of the rock consists of relatively large (3 to 4 mm) plates of labradorite, showing concentric zoning to more sodic margins, with sporadic acicular crystals of similar size representing chlorite pseudo-morphs after original amphibole. The larger crystals are contained in a medium-grained hypidiomorphic-granular base of small plagioclase plates, abundant quartz aggregates and acicular crystals of a pale yellow to brownish green pleochroic amphibole. The quartz aggregates develop blebby, but not graphic, intergrowths with an adjacent turbid alkali feldspar, the latter amounting to only a few per cent of the rock ((Plate 10)a, see p.156). Secondary minerals are variably developed within the Croft intrusion; in the above sample they comprise white mica and chlorite-epidote overprints to the plagioclase and mafic constituents respectively, whereas in a more strongly altered sample (E19167) there is an additional patchy replacement of plagioclase by albite, with pumpellyite occurring interstitially. Radiating prehnite infilling cavities, and zeolite veining, were noted in the Croft rocks by Webb and Brown (1989).

In a mafic xenolith from the Croft Quarry described by Le Bas (1972), over one half of the assemblage consists of centimetre-size brown pleochroic hornblende which is accompanied by plagioclase and minor augite, quartz, calcite and epidote. This mineralogy was compared by Le Bas (1972) to certain mafic facies of the Midlands Minor Intrusive Suite, implying that the latter may be older than the South Leicestershire Diorites. An alternative possibility, however, is that the xenolith represents a fragment of mafic cumulate that crystallised early in the history of the plutonic complex, and was subsequently stoped out by the more voluminous quartz diorite magmas.

Intrusions between Calver Hill and Stoney Stanton

In the present district these rocks are exposed over a 2 km length of strike within a series of small quarries, of which only those at Sapcote, Stoney Cove and Granitethorpe still have accessible walls (Figure 11). The occurrences are probably all interconnected, forming a subdued, sinuous ridge within Triassic rocks and drift deposits. It extends northwards for at least 1.8 km beneath the cover to emerge in the Yennards and Barrow Hill quarries in the Coalville district (Worssam and Old, 1988).

The rocks form grey, massive quarry faces on which two types of structure were described by Eastwood et al. (1923). The first consists of three sets of joints: one set trends east–west and dipping between 50 and 60° to the south; a second set trending north-east to south-west; and a third set, orientated north–south, is observed only in the quarries at Cary Hill and Clint Hill (now merged together as the Stoney Stanton Quarry). These joint systems were also recognised during the present survey (Lawley, 1992b) and their distribution is shown in (Figure 11). In an earlier study, Bosworth (1912) observed joint systems orientated north-north-west and north-west in the Calver Hill and Sapcote quarries, and suggested that the jointing may have determined the overall trend of the sub-Triassic ridge between Calver Hill and Clint Hill.

The second type of structure noted by Eastwood et al. (1923) is planar, and geometrically related to the east–west set of joints; it comprises decimetre-scale alternations between pink and blue-grey varieties of intrusive rock. In a thin section containing both varieties (E11929), Eastwood et al. (1923) noted that in the pink type, feldspars are more turbid and altered, and epidote is more abundant, than in the blue-grey type. This was largely confirmed by Lawley (1992b), who also suggested that the turbidity of the feldspars was caused by the presence in abundance of orientated haematite inclusions.

Petrographic details

A review of 21 thin sections, mainly collected by Eastwood et al. (1923), shows that most of these intrusions are quartz diorites with between 10 and 15 per cent modal quartz. Compared with the rocks at Croft, the specimens examined have lower contents of quartz and increased proportions of mafic minerals. They are also texturally and compositionally more diverse, containing very fine-grained as well as coarse-grained pegmatitic varieties.

Medium-grained hypidiomorphic and inequigranular textures are typically found in quartz diorites from Granitethorpe Quarry [SP 495 937]. In a thin section (E11930) large (2 to 3 mm) plates of labradorite, extensively converted to white mica aggregates, are zoned outwards to sodic rims. They occur in a fine- to medium-grained hypidiomorphic-granular groundmass consisting of small plagioclase euhedra, chlorite pseudomorphs after original acicular amphibole (about 15 per cent of the rock) and scattered opaque grains. Within the groundmass are abundant poikilitic quartz pools together with interstitial chlorite and aggregates of strongly birefringent epidote.

The Cary Hill (Stoney Stanton) Quarry [SP 490 946] contains a variety of intrusive lithologies. A medium- to coarse-grained hypidiomorphic and inequigranular-textured quartz diorite (E11939) contains both large (1 to 2 mm) and smaller, interstitial plagioclase crystals extensively converted to white mica and showing prominent rims of turbid, untwinned (?alkali) feldspar. The other minerals comprise poikilitic quartz pools (between 15 and 20 per cent of the rock), acicular amphibole crystals, now chlorite pseudomorphs (10 to 15 per cent), and disseminated iron-titanium oxides. In contact with this rock are fine-grained lithologies, one of which is described as a 'rammel rock' by Eastwood et al. (1923). In a thin section (E11938) it has a decussate groundmass of heavily altered plagioclase laths interspersed with iron-titanium oxides and with interstitial areas of poikilitic quartz (5 to 10 per cent) and chlorite; there are sporadic phenocrysts of plagioclase and of acicular amphibole now totally replaced by aggregates of chlorite and opaque minerals. Adjacent to this is a further fine-grained, sparsely porphyritic rock which in a thin section (E11937) displays strong fluxional orientation of the small plagioclase laths ((Plate 10)b). The other ground-mass constituents are interstitial quartz pools, acicular to equant iron-titanium oxides and chlorite, some of the latter being pseudomorphic after small amphibole laths; also present are sporadic chloritised mafic phenocrysts and carbonate-quartz amygdales. These finer-grained rocks are suggested by Eastwood et al. (1923) to be of hypabyssal aspect though it is not clear whether they represent a separate intrusive sheet or are part of a chilled margin within a larger multiphase intrusive complex.

The intrusions sampled by Eastwood et al. (1923) from the western part of the outcrop, formerly known as the 'Top Quarry' but now merged as the westernmost salient of the Stoney Cove Quarry [SP 4920 9410], constitute a particularly diverse assemblage. A thin section (E11933) of a mafic quartz diorite shows a medium-grained hypidiomorphic-granular texture with plagioclase crystals totally converted to white mica, and with about 20 per cent of small, acicular to equant, chloritised grains representing pseudomorphed amphibole; approximately 10 to 15 per cent quartz is present as poikilitic pools. From the same locality, a thin section (E11934) of a coarse-grained 'vein' intruding the quartz diorite shows a pegmatitic lithology composed of large (15 to 20 mm) interlocking plates of orthoclase microperthite together with subordinate sodic plagioclase and quartz, the latter occurring either as interstitial pools or as graphic to blebby intergrowths in the orthoclase. Secondary epidote and chlorite are coarsely developed in this rock. In a further pegmatitic sample (E9702); precise location uncertain), large plates of sodic plagioclase are rimmed by a fine-grained aplitic mesostasis composed of crystals and aggregates of turbid alkali feldspar, plagioclase and quartz which locally show vermicular intergrowth textures ((Plate 10)c).

Midlands Minor Intrusive Suite

The Midlands Minor Intrusive Suite may have been emplaced soon after the South Leicestershire Diorites, although the error ranges of the dating methods discussed above suggest that no great period of time had elapsed between the two magmatic events. Its outcrop in this district is confined to the Nuneaton Inlier, where it comprises numerous fine- and coarse-grained intrusive sheets which invade the Precambrian and CambroOrdovician successions. The intrusions do not extend into the Devonian and Carboniferous rocks that lie unconformably above. Their age relations are well demonstrated at the north-western end of Griff No. 4 Quarry [SP 3619 8885] by exposures showing the upper part of an intrusion, weathered to a chlorite-clay mineral-carbonate assemblage, overlain by pale grey calcareous sandstone (E62669) basal to the Lower Coal Measures. A similar weathered profile occurs at the junction between a sill and overlying Carboniferous rocks at about 38 m in the Daw Mill Borehole, and in the nearby Dale Wood Borehole, both near Fillongley.

These rocks constitute the 'diorites' of Eastwood et al. (1923, p.36) and lamprophyres' of Worssam and Old (1988, p.84). The name used here was proposed by Carney et al. (1992), and emphasises the widespread distribution of the intrusions across the central and western Midlands region of England. They are recognised in the Wrekin area, 70 km to the west-north-west of this district (Watts, in Lapworth, 1898), and are also proved beneath Carboniferous strata of the Warwickshire Coalfield in several deep boreholes. The eastward extent of the Suite is not known; in the present district representatives have been proved in the Combe Abbey No. 2 Borehole (Carney, 1991), and in the Twycross Borehole farther north (Worssam and Old, 1988). Within the Midlands Suite there are only minor petrographic and geochemical differences between the individual intrusions or groups of intrusions, further suggesting that they are part of one broadly consanguineous association.

Allport (1879) was the first to carry out a detailed petrographic study of these rocks. The coarse-grained variants were classified as diorites, but he noted that their unusual mineralogy, of hornblende coexisting with pseudomorphs after augite and olivine, was comparable with the 'hornblende and augite andesites' or 'basalts' of Bohemia. The rocks were subsequently placed within the camptonitic variety of lamprophyre by Watts (in Lapworth, 1898), and compared with lamprophyres found as far afield as Scotland and Ireland. The classification adopted here follows that by Hawkes (in Taylor and Rushton, 1971) in assigning the lineage of the

Midlands Minor Intrusive Suite as a whole to the spessartite variety of calc-alkaline lamprophyre, and its coarse-grained dioritic equivalents. However, one criterion justifying Hawkes' reclassification — the supposed absence of olivine among the chloritised mafic mineral constituents — was incorrect; in the present survey, chlorite pseudomorphs with outlines characteristic of olivine have been recognised. The presence of olivine is in any case compatible with the classification of spessartite (calc-alkaline) lamprophyre advocated by Rock (1987).

In the Nuneaton Inlier these intrusions attain their greatest development within the Stockingford Shale Group, where they may locally constitute up to 20 per cent of the outcrop. They comprise two main intrusive types, forming either fine-grained sheets of spessartite lamprophyre, or thicker composite sheets of hornblende diorite. The thicker, coarse-grained intrusions are chilled at their margins to a fine-grained rock identical in texture and mineralogy to spessartite lamprophyre, which also forms minor intrusions within the coarse-grained facies. Thus the lamprophyres are thought to represent the more rapidly cooled products of the magmas that supplied the thicker sheets.

Many intrusions form bold landform features and can be traced for some considerable distance. They map out as sheets concordant with, or gently transgressive to, the surrounding bedding (Baldock, 1991a, 1991b). Descriptions of small exposures throughout the Nuneaton Inlier are given by Howell (1859), Allport (1879) and Eastwood et al. (1923). Nowadays they seldom form natural exposures, and the following descriptions are based on studies in quarries or on rocks sampled from boreholes.

Spessartite lamprophyre intrusions

In quarry exposures spessartite lamprophyre is a grey or greenish grey, fine- to medium-textured, aphyric or sparsely microporphyritic rock. Small plagioclase laths are visible on fresh surfaces, together with much dull green chloritic alteration and sporadic carbonate and/or chlorite-filled amygdales. The intrusive sheets, between 1 and 5 m thick, have dark grey, very fine-grained chilled margins usually about 10 mm thick. The wider bodies may be massive but some show internal chilled contacts indicative of a multiple emplacement history, as in the 5 m-thick body which invades the Precambrian–Cambrian unconformity in the north-western face of Judkins' Quarry ((Figure 5), locality 10). In the latter intrusion, there is also developed parallel to the intrusive contacts a faint foliation which in places is emphasised by narrow (10 to 20 mm thick) segregations of pink medium-grained feldspathic material. A larger-scale development of this texture is suggested by the descriptions of a lamprophyre dyke exposed in a former working, known as the 'Anchor Quarry', situated in the vicinity of the present entrance to Boon's Quarry [SP 334 946]: according to Watts (in Lapworth, 1898; p.395) it is a variety of diorite, termed 'anchorite', consisting of pebble-sized spheroids of hornblende diorite embedded within a white, feldspar-rich matrix.

Concordant or gently transgressive lamprophyre sills are concentrated at certain stratigraphical levels, either as single thick bodies or as multiple intrusions of up to four thinner sheets. They commonly exploit the Precambrian–Cambrian unconformity in Judkins' Quarry, the Boon's Member at the SSSI in Boon's Quarry ((Figure 4), locality 7), and the Tuttle Hill and Woodlands Members (Hartshill Sandstone Formation) in Hartshill Quarry. They are most concentrated in the sheeted complexes adjacent to the margins of the thicker dioritic intrusions in the Stockingford Shale Group, and are described together with these below.

Discordant lamprophyre sheets mainly follow northwest trends and are between 2 and 4 m thick. The exposed examples comprise, in the south-east of Harts-hill Quarry [SP 3362 9368], a dyke cutting across bedding in the Hartshill Sandstone and inclined at 70° to the northeast; and a further dyke in massive Precambrian rocks in the north-western part of Judkins' Quarry ((Figure 5), locality 11), inclined at 50° to the south-west. Other minor sheets follow joint planes with shallower dips, to the north-east or south-west, in these quarries. Thin lamprophyre dykes are observed to cut the axes of chevron folds in the north-western part of Hartshill Quarry [SP 3305 9417], providing evidence for an Ordovician compressional deformation (Hartshill Event), which is discussed in Chapter 12.

Petrographic details

In a thin section (E62517) of a 1 m-thick lamprophyre sill from Boon's Quarry [SP 3312 9442], the occurrence of olivine is suggested by the habit of some scattered microphenocrysts ((Plate 11)a, see p.157), though these are now totally replaced by aggregates of chlorite, carbonate and opaque minerals. The groundmass consists mainly of plagioclase laths, now converted to carbonate and white mica, together with abundant lath-shaped chlorite pseudo-morphs representing amphibole, and opaque aggregates; carbonate and chlorite are also abundant interstitial minerals. Another thin section (E62502) of fine-grained lamprophyre, from a sill at the junction between the Hartshill Sandstone Formation and Stockingford Shale Group in Hartshill Quarry [SP 3304 9386], shows a sparse microphenocryst assemblage comprising pseudomorphs that may have been pyroxene or olivine, accompanied by sporadic chloritised amphibole laths. In the groundmass, pseudomorphs of plagioclase and amphibole laths occur in fluxional orientation, with small elliptical carbonate amygdales elongated parallel with this fabric.

A thin section (E62378) of a more medium-grained lamprophyre, from a sill in the south-east part of Hartshill Quarry [SP 3360 9365], contains about 55 per cent of randomly orientated plagioclase laths altered to white mica and unidentifiable brown, grainy secondary minerals. Chlorite and iron-titanium oxide aggregates form pseudomorphs after mafic silicate microphenocrysts (about 10 to 15 per cent) which have outlines resembling olivine or pyroxene. Scattered through the slide are several per cent of euhedral to anhedral iron-titanium oxide grains, and accessory apatite needles; the interstices between plagioclase euhedra are filled by aggregates of carbonate, epidote and chlorite.

Diorite intrusions

These intrusions are found only within Cambrian to Lower Ordovician (Tremadoc) strata of the Stockingford Shale Group, where they attain thicknesses of up to 70 m. Some indication of their lateral extent is given by one sill which can be followed along strike for 3.7 km, from an old quarry north-west of Atherstone [SP 301 9746], in the adjacent Coalville district, south-eastwards through the Purley West, Mancetter and Oldbury quarries before splitting into thinner bodies near Hartshill Hayes [SP 348 943].

Measured sections (Figure 12) show that many of the thickest intrusions are composite layered bodies, usually consisting of a leucocratic upper facies resting on a maficenriched, hornblendic lower layer. This diagram further illustrates a type of cryptic compositional variation, detected with a portable magnetometer in the Purley East and Griff No. 4 sills; the magnetic susceptibility increases downwards within the mafic-enriched basal layer of each body. A complication to this overall pattern is seen in the south-eastern part of Griff Quarry [SP 365 884], where the hornblendic facies is interleaved within more leucocratic hornblende diorite and locally develops a subvertical contact with the latter (Figure 13).

The upper, hornblende diorite facies of many sills exposed in the quarries, although appearing to be massive, is in detail commonly seen to be composite, as in Mancetter Quarry where a number of internal chill zones divide the body into separate intrusive phases (Figure 12). The north-western continuation of this sill in Purley West Quarry [SP 3052 9610] forms a 45 m-thick body of hornblende diorite that is separated from the underlying maficrich Purley East sill (Figure 12) by a 15 to 20 m-thick screen of country rock. This relationship suggests that the Mancetter/Purley West and Purley East sills are part of a layered complex similar to the Griff No. 4 sill, but that on emplacement the mafic fraction became separated and was intruded along a lower bedding plane in the host rocks.

Contact relationships further demonstrate the importance of multiple intrusion during emplacement of the larger sills. For example, in the upper few metres of the mafic sill in Purley East Quarry [SP 3061 9621], the greyish green hornblende diorite becomes fine to medium grained and develops a contact-parallel igneous foliation defined by small but perceptible grainsize variations. This type of structure suggests multiple intrusion with relatively insignificant chilling between different magma batches. It is related to the occurrence, adjacent to the upper and lower contacts of the sill, of sheeted complexes usually composed of two to five lamprophyre sills, each about a metre thick and separated by equally thin screens of the country rock. In the similar sheeted complex forming the lower contact of the sill at the southern end of Griff No. 4 Quarry (Figure 13), there are numerous thin (0.1 to 0.3 m) chilled lamprophyre bodies, and between these the country rock mudstones show evidence of thermal softening and induced plasticity, sometimes forming narrow septa within the sills. However, no evidence has been found that these sediments were other than highly consolidated at the time of intrusion. Exposures in Mancetter Quarry [SP 3095 9505] demonstrate that listric normal faulting accompanied the successive emplacement of thick hornblende diorite intrusions ((Plate 17b), see p.162). In the same locality, the country rocks are in places tilted to high angles and affected by normal or reverse faults near to transgressive sill contacts.

The Gruff No. 4 sill [SP 361 867] is the type example of a layered Midlands Suite intrusion. In detail it consists of two nested wedge-shaped bodies, each with separate maficenriched layers (Figure 13). The north-westernmost sill comprises an upper facies, about 24 m thick, of pale grey, pyritous, coarse-grained hornblende diorite. White plagioclase and dark amphibole laths are prominent in this rock, which in the lower part of the quarry face shows a patchy development of coarser-grained facies containing larger (15 mm) amphibole crystals. Beneath the hornblende diorite, the poikilitic hornblende meladiorite, 22 m thick, shows a downwards increase in mafic minerals and in its lower part is a black, pyritous coarse-grained rock, with amphibole crystals up to 25 mm long. In places it develops a heterogeneous texture, with coarse-grained segregations enriched in pink sodic feldspar accompanied by large pyrite crystals and aggregates. The contact between the hornblende diorite and poikilitic hornblende meladiorite was obscured at the time of survey in Griff No. 4 Quarry but visible in the sill's continuation farther north, at Griff No. 2 Quarry, Arpac landfill site [SP 361 894]. There, the hornblende diorite becomes heterogeneous at the base, with pegmatitic mafic and felsic segregations appearing as stringers or ovoid patches: lower down this rock passes into the more typical poikilitic hornblende meladiorite lithology.

Immediately above the lower contact of the northwestern intrusion in Griff No. 4 Quarry, the poikilitic facies rests on a layer of dark grey non-poikilitic hornblende meladiorite 4 to 5 m thick; this layer is highly magnetic (Figure 12), and is characterised by a decimetre-scale igneous foliation, parallel to the contact, consisting of dark grey and paler grey (more feldspathic) rock.

Petrographic details

A section through the north-western part of the Griff No. 4 sill [SP 3618 8670] (Figure 12) typifies the petrographic range that characterises the Midlands Minor Intrusive Suite within the Nuneaton Inlier. Above the basal lamprophyric chill zone, 0.2 m thick, a thin section (E62685) of hornblende meladiorite shows a fine- to medium-grained hypidiomorpic-granular textured rock ((Plate 11)b, see p.157): randomly-orientated laths and plates of strongly zoned labradorite (50 per cent of the rock), heavily altered to white mica and carbonate, are accompanied by (partially chloritised) laths of hornblende (25 per cent), showing pale yellow to brown pleochroism, and small euhedral white mica/chlorite pseudomorphs representing former olivine and/or pyroxene (together 15 per cent). These latter pseudomorphs are rimmed by aggregates of chlorite intergrown with red-brown biotite. Iron-titanium oxides form a further few per cent, as euhedra or anhedral aggregates. Small carbonate rhombs are a late interstitial phase, and sulphides are sporadically distributed in hand specimen. Hornblende meladiorite from 4 m above the basal contact (E62681) shows better preservation of primary minerals and a more open framework, with interstitial carbonate and chlorite aggregates. In addition to amphibole and olivine and/or pyroxene pseudomorphs (together about 40 per cent), about 5 per cent of iron-titanium oxides form clusters of small euhedra; their presence may in part account for the high magnetic susceptibility and the large Fe203 content (15 per cent after recalculation for loss on ignition LOI, see (Table 5)) of this sample.

Poikilitic hornblende meladiorite, sampled from 11 m above the base of the sill, is a texturally heterogeneous rock; a thin section (E62678) shows that medium-grained and plagioclase-rich, hypidiomorphic-granular areas, similar to hornblende meladiorite described above, occur together with areas in which large (10 to 15 mm) poikilitic hornblende plates have developed. The hornblende, 50 to 60 per cent in total, has pale brown to dark reddish brown pleochroism (see below) and is commonly fringed by a colourless to pale yellow, or sometimes greenish blue, secondary amphibole. It encloses plagioclase laths (Ab92--93 determined by electron microprobe analysis; information from RJ Merriman, 1993) and small mafic euhedra (20 per cent), formerly olivine and pyroxene but now chlorite and white mica pseudomorphs. Accessory minerals include iron-titanium oxides and acicular apatite, both having crystallized before hornblende and plagioclase. Interstitial secondary minerals comprise aggregates of fibrous chlorite (diabantite-pycnochlorite; information from R J Merriman), carbonate, euhedral pale green to colourless actinolite and clusters of pumpellyite; coarse-grained disseminated sulphides are also of late-stage interstitial origin. This rock grades upwards into a more feldspathic hornblende meladiorite in which the original pyroxenes have largely survived alteration. In a thin section (E62680) from 17 m above the base of the sill, aggregates of poikilitic brown hornblende (45 per cent of the rock) enclose small euhedra of only partially altered colourless clinopyroxene (10 per cent), together with small chloritised euhedra inferred to represent former olivine (5 per cent). Plagioclase (30 per cent) comprises aggregates or single crystals, the latter enclosed by hornblende ((Plate 11)c, see p.157).

Pegmatitic segregations within this more feldspathic variant of the poikilitic hornblende meladiorite consist of large hornblende crystals intergrown with coarsely crystalline carbonate, sulphides and pink feldspar. In a thin section (E62293) the host rock coarsens towards the segregation, which has a margin 10 mm wide composed of a monomineralic plagioclase-rock. The central part, 10 to 20 mm in width, comprises coarse-grained aggregates of clear untwinned feldspar (about 75 per cent), coarse-grained euhedral sulphide and aggregates of white mica that may be pseudomorphic after an earlier. silicate mineral. In previous accounts (e.g. Thomas, in Eastwood et al., 1923, p.40), perthite or orthoclase were stated to occur in these segregations, but during the present study electron microprobe analyses have indicated only sodium-rich feldspar (Ab97–98) to be present.

A thin section (E62671) of hornblende diorite from the upper layer of the Griff sill shows a coarse-grained, hypidiomorphic-granular texture ((Plate 11)d, see p.157), with strongly zoned laths and randomly orientated plates of oligoclase/andesine (65 per cent), part-enclosed by 3 to 4 mm-size plates of highly altered reddish brown pleochroic horn- blende (20 per cent). The other mafic minerals are represented by small chlorite pseudomorphs (5 per cent), possibly after olivine. Secondary interstitial minerals comprise chlorite, carbonate, epidote, biotite, minor quartz and sulphides. Similar types of hornblende diorite are prevalent in the Mancetter and Purley West sills.

Electron microprobe studies and chemical analyses of separated crystals have shown a range of hornblende species to be present in the composite sills. From the poikilitic hornblende meladiorite facies in Griff No. 4 Quarry, described above, the hornblende compositions range from edensitic to pargasitic (E59856), (E59857); information from RJ Merriman), whereas pargasite was identified in a hornblende diorite from Mancetter Quarry and magnesio-hastingsite from Purley Quarry (Thorpe et al., 1993).

Intrusive rocks in boreholes near Fillongley

Further provings of the Midlands Minor Intrusive Suite have been made in deep boreholes sunk in the Fillongley area, 8 km west of the Nuneaton Inlier. The cores show variations between lamprophyre, hornblende diorite and poikilitic hornblende meladiorite, as in the intrusions described above, but there appear to be greater proportions of iron-titanium oxides and mafic silicate microphenocrysts in the Fillongley samples.

The intrusion interleaved between Cambro-Ordovician and Carboniferous rocks in the Dale Wood Borehole has a top surface altered to aggregates of chlorite-saponite, leucoxene, clay minerals and carbonates (Old et al., 1990). Such assemblages are considered to indicate pre-Carboniferous weathering of the intrusion, being also identical to those developed in the north-western part of Griff No. 4 Quarry, where the top of a sill is unconformably overlain by Carboniferous sandstone (Figure 13). Lower down in this borehole, a sill about 23 m thick develops a mafic-enriched facies towards the base; a thin section (E67243) 4 m above the base shows abundant large plates of brown pleochroic hornblende tinged with late-stage pale green hornblende. The hornblende subpoikilitically encloses plagioclase laths, small and partly altered euhedra of colourless clinopyroxene (about 10 per cent of the rock), and slightly less abundant chloritised olivine euhedra. These minerals occur within an interstitial fine-grained turbid base of brownish alterational mineral, accompanied by epidote and carbonate.

In Daw Mill Underground Borehole, a bioturbated weathering profile developed at the top of the intrusion is erosively overlain by a clay-pebble conglomerate forming the base of the Coal Measures ((Plate 12)a, see p.158). A thin section (E65546) shows that the underlying intrusion is altered to carbonate-impregnated aggregates of chlorite, clay mineral and oxides. A further thin section (E65549) from near the base of this intrusion shows a fine- to medium-grained intergranular-textured and highly altered rock with about 5 to 10 per cent of small chloritised euhedra that may represent pseudomorphs after olivine or pyroxene. Lower down in the same borehole a fine- to medium-textured sill 0.3 m thick is highly altered, but nevertheless shows in a thin section (E65550) three types of pseudomorphs after mafic microphenocrysts: large carbonated euhedra, smaller chloritised euhedra or aggregates, and laths altered to white mica ((Plate 12)b, see p.158) possibly represent original olivine, pyroxene and amphibole respectively.

Geochemistry

In this section Dr P J Henney compares the geochemistry of the two suites of late Ordovician intrusions and outlines the magmatic processes that might account for the geochemical differences between them. Data from representative samples of the Midlands Minor Intrusive Suite, collected during the present survey from quarries in the Nuneaton Inlier and from the Fillongley Borehole, are listed in (Table 5). Other data, not tabulated, are incorporated into the accompanying set of comparative geochemical variation diagrams (Figure 14), (Figure 15), (Figure 16), (Figure 17); they include analyses of Midlands Suite intrusions from the Purley and Mancetter quarries, taken from Thorpe et al. (1993) and from unpublished BGS studies (information from T C Pharaoh), and analyses of samples collected from the South Leicestershire Diorites in the Croft, Enderby and Mountsorrel quarries. These latter figures, obtained during the course of a BGS–Open University collaborative study funded by the European Community and the Department of Trade and Industry, were kindly made available by Dr P C Webb of the OU, who is acknowledged as the second contributor to this section. Representative analyses of South Leicestershire Diorites are also given in Worsam and Old (1988, (Table 7)).

Chemical classification of the Ordovician intrusions

A plot of total alkalis versus silica (Figure 14) shows that rocks of the Midlands Minor Intrusive Suite have higher total alkali abundances, for any given silica content, than the South Leicestershire Diorites (with the single exception shown on (Figure 14)). Both associations show increasing alkali contents (2 to 10 weight per cent Na2O + K2O) with increasing silica, however. The rocks of the Midlands Minor Intrusive Suite show a greater range of chemical composition and include, at the silica-poor end of the spectrum, the most primitive compositions, those that correspond to mafic-enriched poikilitic hornblende meladiorite facies in the Griff and Mancetter quarries. (Figure 14) also indicates that the Midlands Suite lies on a 'trachyte' trend and the South Leicestershire Diorites on a 'rhyolite' trend which also encompasses calc-alkaline rocks such as basaltic-andesite and andesite (note that the plutonic equivalents of these names are not given in (Figure 14)).

The geochemical characteristics of the Midlands Minor Intrusive Suite, as exemplified by the Griff Quarry samples, are most completely summarised in a spider-diagram normalised to mid-ocean ridge basalt (MORB) (Figure 15). This illustrates the relatively high content of large ion lithophile elements (LILE) (e.g. Sr, K, Ba) and light rare earth elements (LREE) (e.g. Ce) of these rocks, coupled with their high content of compatible elements (e.g. Cr, Ni); these are features typical of calc-alkaline lamprophyres (Rock, 1991; Thorpe et al., 1993). Contents of K, Rb and Ba are highly variable, indicating either a variation in primary magmatic compositions or scatter produced by varying degrees of late or post magmatic hydrothermal alteration. The same diagram compares the Griff Quarry rocks with representative data from the South Leicestershire Diorites, and shows that the latter have higher abundances of K, Rb, Y and Th, and significantly lower Sc, Cr, Ni and Ti. Although the Sr, Ce and Ba contents of the two groups overlap, the Midlands Suite generally shows a greater range of values for Ba and Sr.

Some of the differences in high field strength element (HFSE) (e.g. Ti, Y) abundances between the two suites may reflect differences in the magma sources, as discussed below. The South Leicestershire Diorites exhibit a linear correlation between TiO2 and MgO (on a diagram not included here), suggesting that fractionation processes operating within the plutons may also have modified HFSE abundances, in particular Ti. The role of crystal fractionation in generating the diverse compositions that characterise the Midlands Minor Intrusive Suite is more ambiguous, however. A further test for fractionation is to consider the distribution of rare earth elements (REE) on a chondrite-normalised plot (Nakamura, 1974): for the samples from Griff Quarry (Table 5), this shows steep profiles and enrichment in LREE, with CeN/YbN ratios of 6 to 8. None of the samples shows a negative Eu anomaly, supporting the suggestion that in these intrusions there has been little or no plagioclase fractionation. One sample (E62674) did in fact show a slight enrichment in Eu, but whether this is due to feldspar accumulation or is a source characteristic is unclear (Rogers et al., 1985). Further clues to the extent to which crystal fractionation may have modified the compositions of the Midlands Suite intrusions can be obtained from their compatible element (MgO, Ni and Cr) contents, as these are very sensitive to removal of olivine, pyroxene and amphibole. These elements show a range in values (Table 5) which could indicate that crystal fractionation, particularly of mafic silicate phases, may well have occurred in some samples (e.g. (E62676), (E62674). However, in the majority of samples the values for MgO, Ni and Cr suggest that there has been only minor fractionation, while in some (e.g. (E62682), (E62683) the abundances of these elements are comparable with those of essentially unfractionated, primitive mantle-derived melts (Rhodes, 1981).

Geochemical comparison with calc-alkaline lamprophyres

There has been some argument about whether the rocks of the Midlands Minor Intrusive Suite show affinities to the camptonite or spessartite varieties of lamprophyre, as discussed above. In order to constrain further the geochemical attributes of this suite, the Griff Quarry samples are plotted on a spider diagram (Figure 16), this time normalised to an average calc-alkaline lamprophyre (CAL) of hornblende-plagioclase (spessartite) type derived from a compilation of over 5000 analyses (Rock, 1991). On this plot the average lamprophyre composition is represented as a line with a value of 1, parallel to the x-axis, and relative enrichments and depletions of the compared data, representing departures from this average composition, appear as values above or below this line. The plot shows that the Griff Quarry rocks have LILE and LREE abundances lower than those found in most CALs (except for Sr and, in some instances Ba), but that HFSE and compatible element abundances are generally close to or even exceed (particularly Ti, Cr and Ni) those in the average CAL. The relative LILE depletions of the Griff hornblendic rocks are less pronounced in a comparison with average spessartite, as this category does not include LILE-enriched mica-bearing lamprophyre types. However, the Rb, K, Th, Ce, Y, Sm and Hf contents of the Griff rocks are still lower than might be expected for calc-alkaline hornblende lamprophyres and Ti values are noticeably higher.

These discrepancies in LILE content may be explained if the Griff rocks have experienced significant late to post-magmatic alteration involving selective removal of low field strength divalent ions such as Rb and K. Although such processes are indicated by the petrography of these rocks they cannot account for the variations in HFSE and LREE, which are generally considered as immobile during low-temperature alteration (Pearce, 1983). A more likely explanation is that the Griff rocks are calc-alkaline lamprophyres whose HFSE and LREE contents may, at least in part, reflect variations in the composition of the deep-seated source region from which these magmas were drawn. For example, the low Y and Yb contents of the Griff samples, relative to the average spessartite, may indicate the presence of residual garnet, left behind during the partial melting process that extracted these magmas from the source area (Thorpe et al., 1993). The high TiO2 of the Griff samples may also reflect a contribution from a mantle source comparable to the Ocean Island Basalt (OIB) type mantle of Leat and Thorpe (1989), as well as variations in the degree of partial melting. Similarly, LREE concentrations in the source areas of melts may be significantly modified if elements are added by metasomatic processes during subduction, and then further altered by variable degrees of partial melting (Pearce, 1983).

Tectonic setting

One of the principal indicators of the tectonic setting of these rocks is the MORB normalised spidergram (Figure 15). For the Midlands Minor Intrusive Suite it shows a pattern characteristic of subduction-related magmatism, particularly in the pronounced depletions of Nb and Ta relative to LILE and LREE, resulting in the high La/Ta and Ba/Ta ratios typical of volcanic arc magmas. Similar patterns are shown by the South Leicestershire Diorites, whose volcanic arc characteristics are also emphasised in the study by Webb and Brown (1989, fig. 7.16). The differences between the two intrusive suites are highlighted by the Ti-Zr-Y triangular plot (Figure 17). This shows that the Midlands Minor Intrusive Suite falls within the field for within-plate basalts, whereas the South Leicestershire Diorites fall just outside the field for calc-alkaline basalts. However, it should be noted that the tectonic setting of the South Leicestershire Diorites cannot be properly inferred from this diagram, which is intended mainly for use with rocks of basaltic composition.

The relatively high TiO2 content of the Midlands Minor Intrusive Suite, indicated by (Figure 17), is one of its most distinctive features; it suggests that these magmas originated from a mantle source that contained a within-plate chemical component, as well as a significant arc or subduction-related component (Thorpe et al., 1993), the latter being more prominent in the source of the magmas that produced the South Leicestershire Diorites.

Although the foregoing discussion suggests that a subduction zone was involved in generating the magmas of both intrusive associations, the direction of dip, or polarity, of this subduction is difficult to determine. One possibility is that it descended beneath the Midlands area from the west and north, and was associated with the volcanic arc systems bordering the Iapetus Ocean in the Welsh Basin (e.g. Woodcock, 1990) and the Lake District (Branney and Soper, 1988), but it could alternatively have descended from the north-east, where further volcanic arcs existed along the former margin of Tornquist's Sea (Evans, 1979; Pharaoh et al., 1991). Apparently all of these volcanic arcs had reached their peak of activity during the Caradoc, and had ceased by earliest Ashgill times. Therefore, from the radiometric age data discussed earlier, only the South Leicestershire Diorites may be truly contemporary with subduction. The slightly younger Midlands Minor Intrusive Suite, with its pronounced within-plate geochemistry, may represent magmatism generated in the closing stages of subduction, when plate convergence had almost ceased and an intraplate tectonic regime was being established.

Chapter 5 Devonian

In the Coventry district there are no strata between those of early Ordovician (Tremadoc) and late Devonian age. The beds described here form the only outcrop of Late Devonian rocks east of Titterstone Glee in Shropshire, although similar beds are known at depth in boreholes to the east and south of the Coventry district (Mortimer and Chaloner, 1972; Butler, 1981), and to the north-west (Mitchell, 1954) and are interpreted from seismic records to be present in the Widmerpool Gulf area to the north (e.g. Ebdon et al., 1990).

Oldbury Farm Sandstone Formation

The discovery of Late Devonian strata in the Merevale area was made during a revision survey conducted in 1964. The mapped outcrop extends for 2.5 km along the western margin of the Nuneaton Inlier and attains a maximum width of 450 m to the north of Oldbury Farm [SP 303 953] before terminating to the north-west against the Arley Fault System. The beds dip at 20° to the south-west and form an unconformity-bounded sequence; to the north-east they overstep southwards across Upper Cambrian strata of the Monks Park, Moor Wood and Outwoods Formations, and they are in turn overstepped in the south-west by coarse-grained sandstones of the Millstone Grit.

Lithostratigraphy

These beds were originally referred to the Upper Old Red Sandstone (Taylor and Rushton, 1971) and are here formally named the Oldbury Farm Sandstone Formation. The type section is designated as the interval between 53.9 and 213.4 m in the BGS Merevale No. 2 Borehole (stratigraphical thickness 152 m), and consists of greyish green conglomerates, sandstones and siltstones, and reddish brown to green silty and sandy mudstones (Figure 18). Both continental and marine facies are represented, and the sequence becomes more calcareous towards the top (Taylor and Rushton, 1971). The lower contact, at the base of a conglomerate bed, is an erosion surface developed on Ordovician (Tremadoc) rocks of the Stockingford Shale Group; greyish green siltstones at the top of the formation are sharply overlain by a pale grey sandstone with mudstone flakes, at the base of the Millstone Grit. The sequence is here divided into four informal members (Figure 18).

The lithology of the formation in the Merevale area is described in detail by Taylor and Rushton (1971). The principal exposures occur in a small cutting [SP 3013 9572] 500 m north-north-west of Oldbury Farm, which serves as a reference section for the marine part of the sequence, and in a stream section farther west [SP 2976 9570] to [SP 2991 9587]. Both localities are described by Taylor and Rushton (1971) and are now much overgrown. Partial sections in the lower beds of the formation were proved in Merevale Nos. 1 and 1A boreholes (Taylor and Rushton, 1971).

Biostratigraphy

The Oldbury Farm Sandstone Formation has yielded a diverse fauna of invertebrate and vertebrate fossils, described by Taylor and Rushton (1971) and Butler (1981). The former authors also record an unsuccessful search for microfloral remains in 14 mudstone and siltstone samples from Merevale No. 2 Borehole.

The marine sandstones exposed in the stream [SP 2989 9585] north-west of Oldbury Farm, and correlated with Member 2 in (Figure 18), contain poorly preserved lingulids and bivalves, including Leptodesma cf. propinquum and L. lichas, both known from the Late Devonian of North America (Butler, in Taylor and Rushton, 1971). Fossiliferous material from a drainage trench nearby [SP 2999 9584], now overgrown, includes fragments of fish and ?phyllocarids, rhynchonellids, ?Cyrtospirifer, Leptodesma cf. propinquum, a ribbed pectinoid valve and cf. Mytilarca chemungensis. This fauna, like that obtained from a nearby cutting [SP 3013 9572], is again suggestive of a Late Devonian age (Butler, in Taylor and Rushton, 1971).

Marine invertebrates in Member 2 occur between 146.6 and 178.7 m in Merevale No. 2 Borehole. A particularly diverse fauna occupies the interval between 152.6 and 158.2 m, and includes Lingula punctata, L. sp. nov. 1, Cyrtospirifer cf. verneuili, Nuculoidea corbulifornzis, Leptodesma spinigerum, Palaeoneilo constricta, Prothyris stubblefieldi, and Sanguinolites cf. tiogensis, all of which occur in Late Devonian strata of mid- and/or late Frasnian age in the Willesden No. 1 Borehole, London (Butler, 1981). The phyllocarid crustacean remains were largely unidentifiable but some were provisionally placed in the typical Late Devonian genus Elymocaris by Rolfe (in Taylor and Rushton, 1971). The faunal assemblage suggested to Butler (1981) that these marine beds spanned the midFrasnian to early Famennian period.

Fish remains in Merevale No. 2 Borehole were reported on by Miles and Toombs (in Taylor and Rushton, 1971). They noted that the occurrence together of Bothriolepis sp., Holoptychius sp., H. cf. nobilissimus and Pseudosauripterus cf. anglicus was typical of Upper Old Red Sandstone fish assemblages; a late Frasnian age was suggested by the absence of Phyllolepis, although this could have been caused by environmental factors.

Lithology

The present account is largely based on the detailed log of Merevale No. 2 Borehole, and supplementary descriptions given by Taylor and Rushton (1971), augmented by the present writer's own observations on a suite of partial core samples curated at the BGS. In the borehole core, 23 sedimentary cycles were recognised by Taylor and Rushton (1971), each commencing with a basal conglomerate or coarse-grained sandstone, and passing upwards through fine-or medium-grained sandstone into mudstone or siltstone at the top. A further and broader division of the sequence is suggested here, into the four informal members shown in the summary stratigraphical column (Figure 18).

Member 1

Member 1 comprises beds of continental facies at the base of the formation, occurring between 213.4 and 178.9 m in Merevale No. 2 Borehole. In the borehole log this member is characterised by a progressive upward increase in gamma ray intensity, possibly indicating a more abundant mud component in the higher beds (Taylor and Rushton, 1971). The beds form repetitive cycles, each commencing with a conglomerate and then fining up into sandstone and finally mudstone. The lowermost conglomerate is a poorly sorted, sand-matrix supported deposit with intercalations of pebbly, coarse-grained, cross-bedded sandstone. The clasts, up to 0.1 m in diameter, mainly comprise extrabasinal types, with vein-quartz and igneous rocks prominent, but there is also a subordinate population of intrabasinal siltstone and mudstone clasts. In higher cycles, the conglomerates are thinner (about 0.2 m or less) and are associated with very coarse-grained sandstones containing mudstone and siltstone pebbles or flakes. Grey to pale green, fine-grained, calcareous and argillaceous sandstones form the larger part of each cycle. They contain wispy or pelleted mudstone layers and siltstone intervals showing climbing ripple lamination; most are burrowed and show contorted bedding, and some contain fish fragments. Subordinate beds of poorly sorted, coarse-grained, conglomeratic sandstones, too thin to be shown in (Figure 18), occur within the fine-grained sandstones. Laminated mudstones capping the two upper cycles in (Figure 18) are greyish green to reddish brown, with sand-filled burrows; desiccation cracks are preserved in those mudstone layers that also contain thin sandstone lamellae.

Member 2

Member 2 occurs between 178.9 and 144 m in Merevale No. 2 Borehole. It is distinguished by the presence of lingulid and phyllocarid fossils (Figure 18), and of crinoid columnals on one bedding plane, proving its deposition in a marginal marine environment. Conglomerates between 0.05 and 0.2 m thick form the base to many upward-fining sedimentary cycles; they have a silty or coarse-grained sand matrix containing shell and fish debris, and resemble lag deposits. Two clast suites are present: those of extrabasinal derivation comprise quartz, quartzite, igneous and metamorphic rock, whereas intrabasinal clasts are composed of mudstone and siltstone pellets or flakes. Many conglomerate beds have an irregular base and some contain rolled sandstone balls, indicative of loadcasting and sediment mixing during deposition. Pebbly sandstones that gradationally overlie the conglomerates contain siltstone, mudstone, quartz and igneous rock fragments, and concentrations of fish and shell debris. Taylor and Rushton (1971) described numerous brownish black collophane fragments in the basal sandstone of Member 2. Other sandstones are green to grey and argillaceous or calcareous, with drifted plant stems and intensely burrowed mudstone or siltstone laminae; most are massive, but sporadic cross-bedding and normal grading occur in sandstone beds near the top of the member. In a thin section (E34067) of a well-sorted fine-grained lithic subarkose from 174 m the quartz, orthoclase and lithic grains are separated by a granular carbonate matrix; accessory minerals comprise garnet, iron-titanium oxides, apatite and a little glauconite. Mudstones are greenish to reddish brown with thin (5 mm) beds of comminuted shell debris or with Lingula in growth position; wavy bedding and sand-filled burrows are commonplace.

In the reference section north-north-west of Oldbury Farm, Taylor and Rushton (1971) logged a marine sequence, 7.05 m thick, consisting mainly of red and pale green, massive to cross-bedded, fine- to medium-grained sandstone with subordinate beds of purplish red silty mudstone with thin red sandstone layers. The sandstones are fossiliferous, with fish fragments, lingulids, brachiopods and bivalves. The section is now partly obscured but in a recent visit linguoid ripples indicating a south-westward current flow were observed on a mudstone-draped sandstone bedding plane. Foreset bedding inclinations in the sandstones suggest a polymodal current regime, the main directions being to the north-east, south-west and south-east.

Member 3

Member 3, occurring between 144 and 115 m in Mere-vale No. 2 Borehole, is more thickly bedded than Member 2 and is devoid of marine fossils, suggesting a return to continental conditions of deposition. The gamma ray log (Taylor and Rushton, 1971) suggests an overall slight decrease in the mud component upwards through the succession. The member is dominated by cyclic repetitions, up to several metres thick, commencing with erosively based conglomerate and fining upwards to thin cappings of highly burrowed mudstone or siltstone. The conglomerates are sandstone matrix-supported and show an upward increase in carbonate cementation through the member; their clasts mainly comprise red or green siltstone, sandstone and mudstone of intrabasinal origin, but at the base of the two upper cycles there is a diverse assemblage of extrabasinal conglomerate clasts which include vein-quartz, quartzite, igneous and metamorphic rock fragments. The green to grey sandstone intervals are massive to cross-bedded and are poorly sorted overall, with sporadic 'floating' subangular pebble-size clasts. Well-bedded intervals comprise alternations between very coarse-grained and fine-grained layers, and many beds show normal grading to very coarse-grained or conglomeratic sandstone at the base ((Plate 13)a, see p.159). Such beds exhibit an irregular erosional basal contact, or may be loadcasted into the underlying beds. In a thin section of medium-grained sublitharenite (E34059) the well-rounded grains, of moderate to low sphericity, are separated by a granular carbonate cement; they comprise, in order of abundance: quartz and polycrystalline quartz, fine-grained felsitic volcanic material and feldspar (orthoclase, microperthite and granophyric quartz-feldspar intergrowths). Grey, green, red or purple mudstone and siltstone cappings to the sandstone beds contain cross-laminated wisps and slumped balls of sandstone; they are typically burrowed and otherwise disturbed. Fish fragments occur throughout this member, principally in conglomerate and sandstone beds.

Member 4

Member 4 occupies the interval between 115 and 53 m in Merevale No. 2 Borehole, at the top of the Oldbury Farm Sandstone Formation. The member is distinguished by containing beds of nodular carbonate and limestone-clast conglomerate, and its base is defined at the first occurrence of this nodular carbonate facies. The sequence is dominated in its lower part by thick upwards-fining sedimentary cycles but, higher up, the coarse-grained beds become less prominent and mudstone or siltstone beds are correspondingly thicker. Fish fragments are commonly found in the sandstone and conglomerate beds.

Carbonate concretions, and zones of carbonate veinlet formation, are scattered throughout Member 4 but at certain levels they become concentrated as nodular limestone in layers several centimetres thick, called 'cornstone' by Taylor and Rushton (1971). Such amalgamated nodular limestones are developed near the tops of mudstone, siltstone or (rarely) sandstone beds. They contain scattered quartz grains which are relicts of the host sediment, have convex margins against the host sediment, and show evidence for coalescence and entrapment of the host as arcuate slivers in the main body of the limestone; the limestone is milky and micritic, sometimes with concentric lamination and sparry, recrystallised areas ((Plate 13)b, see p.159). In a siltstone containing scattered nodules elongated parallel to the lamination, small fold structures are developed ((Plate 13)c, see p.159). That the carbonate-rich layers were of syndepositional or immediately postdepositional origin, and exposed to contemporaneous processes of erosion, is shown by the occurrence of thin (0.2 m or less) limestone-clast conglomerates. The conglomerates are either framework- or matrix-supported, the latter with an interstitial fill of medium- to coarse-grained calcareous sandstone; there are a few extrabasinal clasts but most pebbles are of a micritic limestone that is texturally identical to the limestone developed in the nodular carbonate beds ((Plate 13)d, see p.159).

Sandstones within Member 4 are greyish green to pale grey and micaceous, most possessing a hard calcareous cement. They are mainly fine-grained but commonly become medium- to coarse-grained and ill-sorted towards the base; massive and cross-bedded sandstones are present, some showing convoluted bedding. Many beds contain sporadic limestone pebbles and mudstone flakes, which may become concentrated into thin, matrix-supported conglomeratic layers. Capping the sandstones are beds of greyish green micaceous siltstone or greyish green to reddish brown mudstone, which may be cross-bedded but in which, more commonly, the laminae are much disturbed by burrows or by loadcasting. The 7 m-thick mudstone succession at the top of the formation is extensively burrowed, with wisps, pipes and amorphous patches of sandy material, and with calcareous concretions. Large rootlet casts are seen in the uppermost siltstone, which is sharply overlain by basal beds of the Millstone Grit.

Petrography of the extrabasinal clasts

Clasts from conglomerates at various levels in the Mere-vale No. 2 Borehole section were described by Dearnley (in Taylor and Rushton, 1971). In the limestone conglomerates (E34049), (E24052), (E34066), these clasts comprise devitrified acid to intermediate volcanic rock, tuff with small quartz xenocrysts, fragments of granophyric intergrowths, glass-shard tuffs which may exhibit compression of the shards, and schistose grits. From lithicrich calcareous sandstones were additionally described small clasts and grains of quartz-bearing spherulitic rhyolite lava and devitrified rhyolite, fine-grained tuff, granophyre, quartz-schist, meta-quartzite, and quartz with vermicular chloritic intergrowths (E34044), (E34046), (E34050), (E34051, (E34054), (E34055), (E34057), (E34058), (E34059), (E34060), (E34067), (E34070). Dark, fine-grained clasts in the basal conglomerate of the formation, identified as 'diorite' in the BGS sample collection, are in two thin sections (E67001), (E67002) respectively composed of fine-grained glauconitic sandstone and sericitised volcanic rock.

Depositional environment

Following the period of erosion and/or nondeposition that accompanied the Early to Middle Devonian (Acadian) deformation phase, continental sediments were accumulated within localised basins along the southern edge of a landmass extending from southern and central England and Wales to Scotland (Bluck et al., 1992). The Oldbury Farm Sandstone Formation was deposited in one of these. The cyclical nature of deposition in this formation was emphasised by Taylor and Rushton (1971), who proposed that these beds mainly comprise fluviatile deposits. In Member 1, the upward-fining sequences resemble those within the Devonian successions of Wales, which are interpreted to be sections through the point-bar systems of river channels (Allen, 1965). In the alluvial environment the burrowed and sun-cracked mudstone cappings to the sequences would correspond to overbank floodplain deposits.

The beds of Member 2 are distinctive in containing marine fossils, reflecting the influence of a Late Devonian marine transgression which is documented over a large area of central and southern England and Wales (Allen, 1964). Evidence from several boreholes suggests that the transgression advanced northwards, not reaching the Merevale area until the late Frasnian (Butler, 1981). It would be expected that such a change in the depositional environment would be mirrored in the sedimentary record, but this is only true in part; Taylor and Rushton (1971), in describing Merevale No. 2 Borehole, note that the lithology and cyclicity of beds in the marine interval 'is similar to that in the remainder of the succession except that the sandstones are better sorted and contain shell beds'. However, they go on to say that 'the marine argillaceous rocks are even more intensely burrowed than their nonmarine equivalents; they contain lingulids in the position of growth indicating deposition under quiet conditions — possibly in intertidal mud-flats'. In such environments, preservation of cyclic sedimentary repetitions could occur if arenaceous and gravelly material from rivers in flood was spread across the coastline, building local strand plains across the intertidal areas. This would be in keeping with palaeogeographic reconstructions by Bluck et al. (1992), which place the southern edge of the landmass immediately to the north of Merevale at this time.

The sea withdrew southwards in early Famennian time (Butler, 1981), when continental deposition was resumed in Members 3 and 4. The style of deposition reverted to that seen in Member 1, with thick upward-fining sequences representing channelised flow in meandering river systems. The difference, however, was that carbonate became increasingly sequestered in the sediments to form the beds of nodular and conglomeratic limestone that characterise Member 4. The nodular carbonates are similar to the calcretes that form in tropical and subtropical soil profiles at the present day; fossil equivalents are described from the continental Devonian successions of the Anglo-Welsh outcrop (Allen, 1974) and the Scottish Borders basin (Leeder, 1976). The intercalated beds crammed with limestone clasts indicate erosion of the contemporary calcretes, further attesting to the syndepositional origin of this carbonate and the continental environment of deposition. The calcrete development was ascribed by Allen (1974) to a changing fluviatile regime, with parts of the alluvial plain starved of clastic material for long periods. The drainage reorganisation implied for the Oldbury Sandstone Formation may reflect the onset of the late Devonian earth movements that formed rift systems farther north, such as the Widmerpool Gulf (Ebdon et al., 1990).

The provenance of detritus in the continental beds of the Oldbury Farm Sandstone Formation cannot be established through palaeocurrent measurements owing to the lack of suitable exposures; nor have any geochemical or isotopic studies been undertaken to compare them with possible source rocks in the region. A study of the larger clasts shows that, although many indicate derivation from an acid volcanic terrain, their petrography does not resemble that of any of the lithologies in the nearby Precambrian Caldecote Volcanic Formation. They have been compared by Dearnley (in Taylor and Rushton, 1971) with clasts in the Old Red Sandstone of the Church Stretton district, whose ultimate source was believed to have been the Precambrian Rushton Schist, Longmyndian and Uriconian terrains (Greig et al., 1968), located 85 km to the west of Merevale. An alternative possibility is that the provenance of the clasts lay in a different direction; they may, for example, have been derived from a postulated late Ordovician volcanic system located on the eastern margin of the Midlands Microcraton (Pharaoh et al., 1993).

Chapter 6 Carboniferous: Namurian

No strata of early Carboniferous (Dinantian) age are known within the district, and late Carboniferous strata rest unconformably on pre-Carboniferous rocks. An attenuated Millstone Grit succession of late Namurian age, and probably no more than 25 m thick, lies conformably beneath the Westphalian Coal Measures in a narrow belt along the south-west flank of the Nuneaton Inlier (Figure 2). As the Millstone Grit outcrop is traced southwards across the Arley Fault it passes from Cambrian on to Devonian basement. Towards Oldbury, the sequence oversteps the Devonian back on to Cambrian, and nearer to Nuneaton the outcrop narrows as the Millstone Grit is overlapped by Coal Measures. Exposure is generally poor, particularly in the drift-covered area to the south of the Arley Fault, and Merevale No. 2 Borehole, described by Taylor and Rushton (1971), provides the only detailed section through the succession.

Palaeogeography and depositional setting

During Carboniferous times, the Coventry district was situated on an area of stable basement, termed the Wales–Brabant High, which extended from the Irish Sea eastwards into Belgium (Cope et al., 1992, p.81). To the north, separated by an intervening shelf, lay a developing basin, the Widmerpool Gulf, one of a series of fault-bounded basins that developed in central and northern Britain in response to late Devonian and Dinantian crustal extension (Leeder, 1982). Marine Dinantian sedimentation did not extend southwards beyond the Widmerpool Gulf until late Asbian or Brigantian times, when limestones and mudstones were deposited as a southerly thinning succession along the margins of the Wales–Brabant High (Worssam and Old, 1988).

During the Namurian, the Widmerpool Gulf and adjoining shelf continued to receive sediment, first from the Wales–Brabant High (Trewin and Holdsworth, 1973), and later from turbidite-fronted deltas that prograded from the east, depositing quartzofeldspathic detritus of northerly provenance (Jones, 1980). Once the main basin infilling was completed, in late Marsdenian times, the delta systems transgressed southwards on to the old landmass. In the present district, a condensed sequence of shallow-water deltaic sediments was deposited in a south-east-trending embayment in the Wales–Brabant landmass but outside the embayment no Namurian strata were deposited (Fulton and Williams, 1988, p.183, fig. 14.6).

Stratigraphy

The stratigraphy of the Millstone Grit is illustrated by reference to Merevale No. 2 Borehole (Figure 19). The succession is 21 m thick, and only the latest two stages of the Namurian are represented, the Marsdenian (R2) and the Yeadonian (G1). In the lower part of the sequence, coarse-grained pebbly sandstones are stacked into upward-fining compound bodies with pebble lags of quartz, quartzite and quartz-feldspar-porphyry; the sandstones are mainly feldspathic, poorly sorted and argillaceous but include some white or greyish white, quartzose varieties. The upper part of the sequence is predominantly argillaceous and consists of grey mudstone, grey to greyish brown seatearth and a thin coal. Dark grey and black fossiliferous mudstones with bases at 52.96 m and 39.37 m were identified tentatively by Ramsbottom (in Taylor and Rushton, 1971, p.55) as the Bilinguites superbilinguis (R2c1) and Cancelloceras cumbriense (G1b1) marine bands, respectively. The former has yielded Lingula mytilloides together with turreted gastropod spat. The latter contains a diverse benthonic fauna including Lingula mytilloides, Orbiculoidea sp. and Productus carbonarius, together with turreted gastropods and Sphenothallus. Also associated is the nautiloid Peripetoceras along with the ammonoids Anthracoceratites and Cancelloceras. Unfortunately the ammonoids are too poorly preserved to allow specific identification. The Gastrioceras subcrenaturn Marine Band, defining the base of the overlying Coal Measures, has not been positively identified but is assumed to be represented by a thin mudstone with Lingula and fish debris at 30.63 m.

The outcrop of the Millstone Grit is marked by a yellowish brown pebbly sandstone, which can be traced southwards as a ridge landform through Monks Park Wood to the Arley Fault. The presence of a massive basal quartz pebble conglomerate on the north side of the fault and only a thin basal grit layer on the south side was cited by Taylor and Rushton (1972, p.62) as evidence that the fault was active in the Namurian. The southeastward continuation of the crop from Ridge Lane [SP 300 950] to Oldbury and beyond is obscured by till. The only exposures are just south-west of a path [SP 3190 9368] by the disused Moor Wood Quarry and in a disused railway cutting [SP 3206 9360] at Moorwood Lane, Chapel End. At the former locality, the base of the succession is exposed in a 3.2 m section (Baldock, 1991a), which shows a basal conglomerate with rounded vein-quartz and angular Cambrian mudstone and lamprophyre clasts, overlain by medium-grained yellow feldspathic sandstone. This is succeeded by further conglomerate, lateritised in the upper part, overlain in turn by medium-grained brown sandstone and mudstone.

At the second locality noted above, about 2 m of very coarse-grained pebbly sandstone are exposed. Clast imbrication (80° to the east) and a single dipping reactivation surface (25° to north-east) give a tentative indication of palaeocurrent flow; although the exposure is very poor, the implication is that at least some of the detritus may have been derived from the Wales–Brabant landmass.

The westward extent of the Millstone Grit subcrop beneath the Coal Measures is poorly constrained. Pebbly sandstones, noted in the shaft records for Griff No. 4 and Charity Colliery pits, are tentatively referred to the Namurian by Fulton (1987a). To the east of the outcrop, the only borehole that may have encountered Namurian strata beneath the Triassic cover is one at Well Green Farm, where the basal Carboniferous yields a miospore assemblage of late Namurian to Langsettian age (McNestry, 1992). To the south, the Millstone Grit is absent and the overlying Coal Measures rest directly on pre-Carboniferous rocks. The curving feather edge of Namurian deposition is shown by Fulton and Williams (1988, fig. 14.6) to run round the east and south sides of Nuneaton and then north-west through New Arley to the district boundary.

Chapter 7 Carboniferous: Westphalian-Stephanian

Introduction

A sequence of Westphalian strata, forming the central part of the Warwickshire Coalfield, is preserved in a shallow southward-plunging syncline in the west of the district. Some 250 m of Coal Measures rest conformably on thin Namurian strata in the north-east, but the latter die out southwards and the Coal Measures then rest unconformably on pre-Carboniferous (Cambrian and Tremadoc) rocks. The coalfield is bounded to the west by the Western Boundary Fault System, to the north-east by outcrop and to the south-east by incrop below the Trias. The Coal Measures are overlain by over 1000 m of Barren Measures, consisting mainly of redbeds.

(Figure 20) gives a generalised vertical section of the succession. As is the case elsewhere in Britain, the basal limits of the Lower and Middle Coal Measures are taken at the bases of the Subcrenatum and Vanderbeckei marine bands, respectively (Stubblefield and Trotter, 1957). The additional presence of the Aegiranum Marine Band allows precise chronostratigraphical subdivision of the measures into three stages (Langsettian, Duckmantian and Bolsovian). The use of the stratotype stage names here, rather than the more familiar synonyms ( Westphalian A, B and C), follows the recommendations of Owens et al. (1985). The Middle Coal Measures contain most of the workable coal seams, of which the Warwickshire Thick Coal has been the most important. The top of the Coal Measures is a lithostratigraphical boundary, drawn at the base of the first primary redbed formation (in this case, the Etruria Formation).

Throughout the overlying Barren Measures faunas are scarce, and precise chronostratigraphical correlation is impractical at present. It is convenient to take the base of Westphalian D at the base of the Halesowen Formation, which is marked locally by an unconformity. The overlying redbeds may be Westphalian D or Stephanian in age but have yielded no stratigraphically significant fossil evidence.

Previous research

The first geological survey of the coalfield was carried out at the one-inch scale in 1855 to 1859, and an account of the geology was subsequently published by Howell (1859). The primary six-inch survey was completed between 1912 and 1915, and the results for the Coventry district were described by Eastwood et al. (1923). Early papers on the coalfield included contributions on palaeontology (Vernon, 1912), possible coalfield extensions beneath the Triassic cover (Boulton, 1926), and the use of miospores for correlation (Paget, 1936, 1937). Coal seam and interseam thicknesses, based on shaft records and underground workings, were given by

Mitchell (1942), and an atlas detailing coal thickness and composition on a seam-by-seam basis was published by the National Coal Board (1957). The structure of the Thick Coal group of seams was further amplified in a paper by Cope and Jones (1970). More recent sedimentological syntheses have been given by Fulton (1987a and b) and Fulton and Williams (1988) on the Coal Measures, and by Besly (1983, 1988) on the redbeds. A geological appraisal of the southern part of the coalfield, intended as a foundation for land-use planning, has also been published (Old et al., 1990).

Technical reports prepared by Hallsworth (1992), Haslam and Lumb (1992), Jones (1992), Riley (1992) and McNestry (1992) have been used extensively in the compilation of this chapter.

Depositional environment and sedimentary facies

Coal Measures

The palaeogeography established during the Namurian (Chapter 6) continued to influence sedimentation throughout the early Westphalian. The Wales–Brabant High persisted as a topographical barrier but was of low relief and was dissected by at least one southerly directed embayment extending into South Oxfordshire and Berkshire (Guion, 1992, figs. C8 and C9).

Low subsidence rates on the northern flanks of this landmass are reflected in the southward attenuation of the Coal Measures from 250 m in the north to around 125 m in the south-east, and also in reduced water depths, as indicated by the nearshore or brackish-water faunas of the main marine bands.

Sedimentation throughout the Langsettian and Duckmantian occurred on a low-lying, slowly subsiding delta plain or lower alluvial plain (Fulton and Guion, 1990; Jones, 1992). Lakes were the largest component of this environment, forming expanses of water tens of kilometres in diameter, but with depths of usually only a few metres. The fine-grained nature of many of the lacustrine deposits, coupled with the presence of well-preserved articulated bivalves and the rarity and small scale of the associated sedimentary structures, suggest that the lakes were filled dominantly by sediment transported in suspension. Sediment was dispersed between the lakes by a network of highly sinuous and mobile distributary channels, and bedload material of silt and sand accumulated where deltas fed by minor channels prograded into the lakes.

Periods of lake filling and channel abandonment produced emergent or shallow-water areas (mires) in which extensive plant colonisation took place. Within these mires, chemical and organic processes acted on the root-rich sediments, converting them to siliciclastic palaeosols (seatearths). Peat development followed but was interrupted at times by flood events, allowing input of siliciclastic sediment to the mire. Further development and raising of the mire above the influence of groundwater led to a change in the vegetation type and the eventual accumulation of thick, laterally extensive, raised peat bogs. In the case of the Warwickshire Thick Coal, peat growth is thought to have kept pace with subsidence over long periods, creating the deposits that, after burial and compaction, ultimately formed the main leaves of this important group of seams.

Marine incursions took place infrequently, resulting in deposition of dark, Lingula-bearing mudstones. The basinwide occurrence of many of these marine bands suggests that eustatically induced highstands of sea level were responsible for their deposition.

Eight main sedimentary facies can be recognised from the Warwickshire Coalfield (Jones, 1992). (Table 6) summarises their diagnostic features and notes some important examples identified in the sequence. Whilst the facies types are similar to those described from the Pennine Basin to the north (Fielding, 1984, 1986; Guion, 1984, 1987a and b; Haszeldine, 1984), they differ significantly in the following ways.

  1. Major channel facies, forming thick and laterally extensive sandbodies, are absent; any thick sandbodies present represent multistorey minor channel systems.
  2. Siliciclastic palaeosols (seatearths) are generally thicker and more evolved, forming sequences that may occupy entire interseam intervals. Some possess characteristics showing that they formed under better-drained conditions than their counterparts farther north in the Pennine Basin. Such features include red or brown colouration and an abundance of sphaerosiderite.
  3. Lacustrine sequences have characteristics (such as rooting) showing that shallow water conditions were more prevalent than within the main part of the Pennine Basin. The thickest lacustrine sequences occur in the Duckmantian, indicating greater subsidence rates during this period.

Although these features are primarily a response to reduced subsidence rates at the margins of the Pennine Basin, other factors, discussed by Fulton and Williams (1988), may have influenced sedimentation; they include tectonics, eustacy, compaction and local sedimentary controls.

Barren Measures

In late Duckmantian times, differential uplift along the southern margin of the Pennine Basin led to the development of a block-and-basin style of palaeogeography, and a change of depositional environment from poorly drained fluviodeltaic plains (Coal Measures) to better drained alluvial redbed environments (Etruria Formation). A fault-bounded horst to the west of the present coalfield is thought to have shed detritus eastwards to form a series of alluvial fans that are now preserved in the footwall block of the Western Boundary Fault (Besly, 1988, p.207, fig. 15.9c); it is highly probable that other upfaulted emergent areas lay to the east of the coalfield and were also subject to erosion at this time. Beyond the influence of these structures, sediments accumulated in an alluvial floodplain setting (Besly, 1988). The occurrence of coals, and of grey and buff poorly drained palaeosols, in the lower parts of the Etruria sequence suggest an environment of Coal Measures type, initially prone to flooding; however, the upward increase in proportion of redbeds points to an overall improvement in drainage through Bolsovian time. At the eastern margin of the basin, sedimentation was interrupted by episodic volcanic activity (Bridge, 1991).

There was then some uplift and erosion of this sequence, followed in early Westphalian D times by widespread deposition of the Halesowen Formation. An alluvial association of southerly provenance, dominated by stacked channel sandstones, forms the lower part of this unit. Overbank deposits include thin coals and gley palaeosols. In the upper parts of the unit, argillaceous beds are widespread and resemble the interdistributary lake-fills of the underlying Coal Measures. The remainder of the Barren Measures, up to and including the Tile Hill Mudstone Formation, consist entirely of redbeds deposited on well-drained alluvial plains and fans. They accumulated in a rapidly subsiding foreland basin that was developing in advance of the Variscan Front. The succession is divided into a number of cycles, each corresponding to a major phase of fan progradation. Individual cycles consist of a distal mudstone facies below, with ephemeral channelised sandstones, overlain by more proximal debris-flow and sheetflood sandstones; conglomerates containing extrabasinal clasts occur in the upper parts of each cycle. The occurrence of calcretes and desiccation structures, particularly in the lower part of the succession, has been interpreted as indicating a climate of increased aridity (Besly, 1988).

Faunas

Biostratigraphical data in this chapter are taken from a report by Riley (1992).

Marine faunas

Because of its marginal position in the Pennine Basin, only the strongest marine transgressions reached the district and most of the Westphalian marine bands are missing. Of the six that have been identified (Table 7), Vanderbeckei and Aegiranum are the only ones to develop a fully marine assemblage, characterised by calcareous brachiopods. The remainder contain a brackish-water Lingula fauna, sometimes accompanied by agglutinating foraminifera.

All the marine bands die out southwards into palaeosol facies as the Wales–Brabant High is approached (Fulton and Williams, 1988). However, the distribution of the faunal belts in successive marine bands shows that the contemporary shoreline was not static but shifted southwards during the early Westphalian as the influence of the Wales–Brabant High diminished (Figure 21). The westward failure of the Aegiranum Marine Band towards the Western Boundary Fault is noteworthy, indicating relative uplift here at the time of this incursion.

Nonmarine faunas

Nonmarine faunas, characterised by bivalves ('mussels') and ostracods, are useful in correlating lacustrine sequences throughout north-west Europe. However, as with the marine bands, the lacustrine faunas are not well developed in the Coventry district (Table 7). The principal bivalve occurrences have been documented by Mitchell (1942, table 1), and the zones represented are indicated on the generalised vertical sections (Figure 25), (Figure 28). Fish remains are commonly associated with the bivalves.

Miospores

Miospores are present in all the fine-grained clastic lithologies and coals. Although the palynological zonation of the Silesian has less resolution than that achieved by faunal biostratigraphy, miospores are important, because of their abundance, in sequences where fauna is absent or nondiagnostic of age. Some of the coals have particular miospore facies suites which allow local correlation, even though the component taxa are stratigraphically long-ranging.

Database

The original colliery shaft sinkings, summarised by Mitchell (1942), give the most complete records for the older parts of the coalfield, though the correlation of the coals is in some cases doubtful. The six-inch or 1:10 000 geological maps of the area, listed at the front of the memoir, contain summaries of the principal shafts and boreholes, and additional details are given in the accompanying open-file reports. Up-to-date information on the extent of former mining, and details of current workings, are held by British Coal.

Stratigraphy

Coal Measures

The Coal Measures crop out in a narrow belt around the north-east of the coalfield syncline. Exposures are rare but their crop is well defined by shaft and borehole records, as well as by more recent opencast data. North of the old Ansley Hall Colliery, the measures are predominantly overlain by thin till, but were worked by opencast methods in a series of contiguous workings stretching for 4 km from Oldbury Spinney [SP 310 940] to Holly Park [SP 289 969] (Figure 22). South-eastwards, they can be traced from Ansley Common (small opencast), through Stockingford Colliery at Chapel End, and the Haunchwood and Nuneaton collieries, to Stockingford railway cutting [SP 341 920], and then, partly beneath till, through the suburbs of Heath End to the Sudeley opencast site. Thick drift conceals the outcrop south of this point, and from Hawkesbury the measures incrop beneath Triassic strata.

The Coal Measures overlie presumed Millstone Grit north-west of Chapel End [SP 323 933], but south-east of there they rest unconformably on Cambrian Outwoods Shales. The concealed coalfield has been proved westwards as far as the Western Boundary Fault, and extends southwards into the adjoining Warwick district. Deep mining is now centred on a single colliery, at Daw Mill [SP 260 899], from where workings are extending southwards.

A section through the Warwickshire Coalfield (Figure 23) shows the distribution of major coal seams and marine bands, and illustrates the effect of thinning and onlap against the basement rocks; the Half Yard coal is used as a datum level in this diagram on account of its widespread distribution. Isopach maps for the Coal Measures of the district (Figure 24) show a north-eastward thickening towards the Pennine Basin; this effect is illustrated in a wider context by Fulton and Williams (1988, figs. 14.10 and 14.11)

Lower Coal Measures

The Lower Coal Measures are of Langsettian age and range in thickness from over 150 m in the north-east of the district to less than 30 m in the south, with about 50 m present over much of the central part of the coalfield area ((Figure 24)a). The attenuation of the sequence to less than 20 m in the area of the Arley Fault may be due to contemporary uplift, though onlap against a preexisting basement high cannot be ruled out.

The sequence is predominantly argillaceous but includes some thicker sandstones towards the base. Coal seams, present mainly in the upper part of the sequence, were formerly extensively worked in opencast operations but only two seams (the Bench and Seven Feet) have been exploited by deep mining. (Figure 25) gives a generalised vertical section.

The Subcrenatum Marine Band, definitive of the base of the Westphalian, has not been conclusively recognised but is thought to be represented by 0.15 m of mudstone with Lingula and fish debris encountered at 30.63 m in Merevale No. 2 Borehole; the bed has also yielded a diverse conodont assemblage including cavusgnathids.

The beds between the Subcrenatum Marine Band and the Stanhope Coal attain a maximum thickness of 40 m at crop but attenuate rapidly down-dip as strata in the lower part of the sequence are progressively overlapped. A prewar shaft sunk at Griff Colliery provides one of the few records of the sequence; it found sandstones near the base, with pale to dark grey mudstones in the middle and near the top. Faunal evidence from these lower beds is meagre.

The Stanhope Coal is the lowest of a group of named seams that have been worked from outcrop; in the succeeding 60 m of strata are found the Stumpy, Bench, Double, Deep Rider, Yard, Trencher, Seven Feet and Thin coals. Opencast abandonment plans provide details of the thicknesses of these coals, which are summarised in (Table 8). Many of the coals are thin and impersistent, so exact correlation of individual seams between sites is difficult. The Bench Coal usually occurs in two leaves, separated by 0.4 to 1.9 m of grey mudstone or seatearth. The upper (Top Bench) leaf is overlain by 7 to 8 m of grey mudstone with sandstone beds, and is succeeded by the Double Coal, also in two leaves, with 0.5 to 2.5 m of mudstone or seatearth intervening. The interval between the Double and the Deep Rider varies from 2 to 10 m and is composed predominantly of mudstone and seatearth. The Deep Rider is a double or triple seam but locally becomes insignificant or absent. A persistent sandstone over 10 m thick forms the roof to the Deep Rider in the Hollypark and Monks Park workings. Farther south, about 5 to 6 m of grey mudstone and seatearth with sandstone, ironstone and impersistent thin coals (Yard and Trencher) intervene before the Seven Feet Coal. The latter seam, once the mainstay of the coalfield, splits and becomes thinner in the Stockingford–Gruff area where it is overlain by seatearth and the Thin Coal.

A series of underground boreholes was sunk from Arley, Newdigate and Coventry collieries in the 1960s and 70s as part of a programme to prove the Seven Feet and Bench seams. These found that the intervals and the lithologies between seams vary widely, and that the only reliable marker bed is a bivalve-bearing black shaly mudstone in the roof of the Stumpy Coal. The survey found that the Bench Coal varies from 0.7 to 1.5 m thick, and the Seven Feet from a few millimetres up to 1.2 m. None of the intervening seams was deemed workable.

Representative sections, based mainly on modern exploration boreholes, are illustrated for two transects in (Figure 26) and (Figure 27). The bulk of the strata are grey planty mudstones of lacustrine facies, and brown seatclays; sandstones, presumed to be mainly channel-fills, form upward-fining sequences generally no more than 5 m thick. Sideritic ironstone is common in the finer-grained lithologies, as scattered or dense aggregates (sphaero-siderite) and as large irregular nodules up to about 0.5 m across. It occurs in sufficient concentration at several levels to have been formerly worked as a source of iron ore (see Chapter 13).

In the subcrop, the only recorded occurrence of Lingula, other than that thought to represent the Subcrenatum Marine Band in Merevale No. 2 Borehole, comes from a depth of 568.45 m in the Fillongley Borehole, and may represent the Amaliae Marine Band. The identification is based on the occurrence in the overlying beds of a poorly preserved nonmarine fauna containing cf. Anthracosia regularis, Carbonicola sp., Naiadites sp. and Carbonita spp.. Carbonicola is unknown from post-Langsettian strata and homeomorphs of the late Langsettian bivalve A. regularis have been reported elsewhere from a mussel band which overlies the Amaliae Marine Band (Eagar, 1962, fig. 8a). Other bivalve records are restricted to Naiadites cf. quadrata and N. aff. producta from beneath the Vanderbeckei Marine Band at Tunnel Pit, Haunchwood. However, biostratigraphical correlation of some of the Langsettian coals can be extended into the district from thicker bivalve-bearing sequences in adjacent areas. This allows the base of the Carbonicola pseudarobusta Subzone to be taken above the Stanhope Coal and that of the C. crista-galli Subzone above the Deep Rider. Two thin fish beds, yielding scales, skull bones and teeth, have been described by Cook (1976) from the lower part of the succession north of Bermuda [SP 356 905]; one lies 1.2 m beneath the Lower Bench Coal, the other some 75 m lower, below the Stanhope Coal.

Middle Coal Measures

The Middle Coal Measures, of Duckmantian and early Bolsovian age, extend from the base of the Vanderbeckei Marine Band, or, its presumed horizon above the Seven Feet Coal, up to the base of the first primary redbeds or coarse-grained sandstone ('espley') of the Etruria Formation. Thicknesses for the major (Duckmantian) part of the sequence range from 90 m at outcrop in the northeast to 60 m at depth in the south of the district ((Figure 24)b). The Thick Coal occurs in the lower part of the sequence and is the main source of deep-mined coal in the district. (Figure 28) gives generalised vertical sections.

Strata below the Thick Coal

The Vanderbeckei Marine Band can be traced throughout the northern and central parts of the coalfield but fails southwards; the Pickford Green Borehole marks its approximate southern limit. The marine band is typically represented by a grey or dark grey mudstone lying close above, or in contact with, the Seven Feet Coal. The definitive ammonoid Anthracoceratites vanderbeckei has not been recovered in this district; faunas are impoverished,

being generally restricted to a brackish-water assemblage of Lingula, agglutinating foraminifera and fish debris. Exceptions include the additional presence of the bivalve Myalina compresses at Griff Colliery No. 4 Shaft, Paraconularia at Ansley Hall Colliery, sponge spicules in the Meriden Borehole and the ostracod Holinella sp. at the former Haunchwood Brick and Tile Company No. 3 Pit. This last locality also contained an intimate association of nonmarine assemblages including Anthraconaia sp., Anthracosia sp., Naiadites sp. and Geisina arcuata.

The interval between the Vanderbeckei Marine Band and the Thick Coal ranges from around 30 m at outcrop, to between 5 and 8 m in the area of the Prime Thick Coal (see below). It consists mainly of seatclays and mudstones but includes some thin sandstones and impersistent coals. In the Ansley Hall Colliery, a seam known as the Smithy (or Low Main), occurs some 19 m above the base of the succession. Though not found in the Stockingford area, the seam was worked at Sudeley opencast site and is mentioned in shaft records for Griff Colliery.

Farther south, at Exhall Colliery, two thin coals are recorded in this same interval; the Fungus Coal, 0.7 m thick, lies between 3.5 and 6.6 m above the Seven Feet and may be the equivalent of the Smithy (Mitchell, 1942); at a slightly higher level is the 0.6 m Lady Coal. Sideritic ironstone in nodules and thin layers is concentrated in beds overlying the Fungus Coal, and was worked as the White Ironstone during the nineteenth century.

Thick Coal

Over the central and south-western part of the Warwickshire Coalfield, a number of coal seams combine to form the Thick Coal, the constituent leaves of which are at some point either in contact, or separated by only thin dirt partings. Fulton (1987b) restricted use of the term Thick Coal to those areas where no more than 0.3 m of siliciclastic sediment intervenes between leaves. As thus defined, the Thick Coal (or Prime Thick Coal) covers an area of approximately 100 km2, is up to 8.5 m in thickness, and is composed of 3 to 7 leaves. Geographically, it extends from Daw Mill Colliery in the west of the coalfield, south-east towards Coventry Colliery and south to the vicinity of Kenilworth. Outside the Prime Thick Coal area, the leaves separate and in the Sudeley and Binley areas, where they have been worked, they are spread out over a vertical interval of between 25 and 35 m. The aggregate thickness of coal at Binley is greater than in the Prime Thick Coal region, reaching over 10 m in Binley Colliery shafts. The constituent leaves of the Thick Coal were separately named following their mining in the north of the coalfield and were considered by Mitchell (1942, p.7) to comprise the Nine Feet (Slate), Ell, Ryder, Bare, Two Yard and Thin Rider coals. Cope and Jones (1970, p.585) also included the Smithy and High Main, although Fulton (1987b) excluded the former on compositional grounds. The area of the Thick Coal and lines of splitting of component leaves are illustrated in (Figure 22).

The leaves of the Thick Coal have been interpreted as long-residence histosols (Fulton, 1987b), which formed as a result of growth of successive raised bogs. Within individual leaves Fulton established a link between coal miospore assemblage and lithotype, and has shown that, in general, each leaf consists of coal and clastic sediment at the base, passing upwards into a clarain-dominated lithofacies with common fusain, then clarodurain, and then a duraindominated lithofacies towards the top. The sequence may be reversed at the top. The upward variation in lithofacies is a reflection of the evolution of the vegetation types within the mire and records the rise of the bog surface above groundwater (Fulton, 1987a,b). Peat growth is usually terminated by an increase in the rate of subsidence, which may cause a reversal of the lithofacies trend at the top of the peat. Interbedded siliciclastic sediment represents a return to conditions in which the surface of the mire is covered with water, allowing sediment input and development of a siliciclastic palaeosol.

Within the Thick Coal group, poorly preserved bivalves recovered from the roof of the High Main seam at Sudeley [SP 3565 8893] have been identified as Anthracosia spp. and A. retrotracta, and are characteristic of the A. phrygiana Subzone. Nonmarine faunas suggesting an horizon near the base of the Lower Anthracosia similis-Anthraconaia pulcra Zone are present above the Nine Feet Coal. They include Anthracosia cf. aquilina., A. cf. fulva and A. cf. phrygiana from Haunchwood Colliery. These faunas may represent the A. caledonica Subzone, but the eponymous taxon has not been found.

Faunas of the Anthracosia atra Subzone are widespread and well preserved. The lowest fauna is represented by Anthracosia sp. and Naiadites sp. below the Bare Coal in an opencast trench near Bentley. Many localities have yielded diverse faunas in the roof of the Two Yard Coal, including Anthracosia atra, A. cf. aquilina, A. concinna, A. fulva, A. phrygiana, Anthracosphaerium affine, Anthracosphaerium gibbosa, Anthracosphaerium turgida and Naiadites sp..

Thick Coal to Four Feet Coal

The beds in this interval thicken from 15 m on the flanks of the coalfield to a maximum of 30 m locally in the centre. The zone of maximum thickness coincides with a northwest to south-east pattern of washouts within the Two Yard Coal. These define a distributary channel belt up to 1.2 km wide, characterised by erosively-based upward-fining sandstones (Figure 29). Within this channel belt, small 'islands' of non-channelised deposits occur. Guion and Fulton (1986) have described a minor channel from within this system from the main Dexter Manrider Road linking Dexter and Daw Mill collieries. The channel washes out the Two Yard seam, is strongly erosive, and its base is marked by a locally developed lag conglomerate of mudstone and ironstone clasts. The lower 5 m of the channel fill is dominated by sandstone and characterised by a series of low-angle lateral accretion surfaces. Palaeocurrent measurements on ripple cross-lamination indicate a flow direction towards the south-east. Using palaeohydraulic parameters Guion and Fulton estimated that the channel was 5 m deep and between 28 and 81 m wide.

The distributary channel system passes laterally and vertically into beds that are predominantly argillaceous, and represent various lacustrine associations. A section recorded from above the Thick Coal at Coventry Colliery

North Rock Head [SP 322 847] by Fulton and Guion (1990) is summarised as follows:

Thickness (m)
Four Feet Coal —
Palaeosol (gleysol) with roots 3.0
Mudstone and siltstone alternations with plant fragments and fronds 14.0
Sandstone, trough cross-laminated 5.5
Mudstone with increasing sandstone laminae, coarsening upwards 4.0
Mudstone, black, siderite-rich, becoming paler grey towards top 1.3
Thick Coal —

The sequence is interpreted as that of a prograding delta, with distal and proximal mouth bar elements, overlain by lacustrine suspension deposits. Other similar examples have been identified from cored boreholes, such as Birch Tree Farm (Jones, 1992, p.13 )

Four Feet Coal to Aegiranum Marine Band

The Four Feet Coal is a laterally impersistent high ash coal that ranges in thickness from 0.6 to 1.8 m, but is split in the Newdigate, Coventry and Binley workings. The mudstones forming its roof usually contain nonmarine bivalves and fish scales; Anthracosia atra was collected from this level at Sudeley opencast site. Several British Coal boreholes (see (Figure 26) and (Figure 27)) have also yielded Lingula sp. from about this horizon. This may be a local development of the Maltby Marine Band, though Riley (1992) believes that it is more likely to be the Haughton Marine Band, since it overlies the main interval of Anthracosia atra-bearing strata. This is in continuity with the discovery of a marine band above the P33 coal in the Coalville district to the north, also correlated with the Haughton Marine Band (Worssam and Old, 1988, p.41).

The Four Feet Coal is separated from the Half Yard Coal by some 15 to 20 m of strata which include brown palaeosols and, locally, a considerable thickness of sandstone. At several localities in the north-east of the coalfield, the beds above the Four Feet Coal are locally reddened. For example, Salutation No. 13 Borehole, to the south of Chapel End, encountered redbeds overlain by grey measures. Farther south, at Black Bank [SP 362 863], site investigation boreholes drilled on a former brickworks found reddening extending from rockhead to within a few metres of the Four Feet Coal. It is uncertain whether the coloured strata represent primary redbeds of Etruria facies, or merely original grey measures secondarily reddened beneath Triassic cover (now eroded). The absence of redbeds from this interval in the deeper part of the coalfield suggests that they are more likely to be secondary.

The Half Yard Coal is the highest formally named seam in the coalfield and is generally split into 2 leaves. At Sudeley opencast site, the lower and upper leaves (0.7 and 0.8 m respectively) are separated by variegated palaeosols of Etruria aspect, and the top of the Coal Measures, hereabouts, is taken beneath a coarse-grained channel sandstone, which locally cuts out the upper leaf of the Half Yard Coal.

Aegiranum Marine Band to base of Etruria Formation

The Aegiranum Marine Band lies between and 10 and 15 m above the Half Yard Coal and is the highest and most widespread marine horizon in the Westphalian of the Warwickshire Coalfield. It dies out towards the Western Boundary Fault as the diachronous base of the Etruria Formation redbed facies descends through the sequence to lie at levels stratigraphically below the marine horizon. Marine faunas are relatively diverse and dominated by shelly bottom-dwelling forms. Typical components include foraminifera (Rectocornuspira sp.), horny brachiopods (Lingula mytilloides, Orbiculoidea nitida), the conulariid Paraconularia sp., calcareous brachiopods (cf. Dictyoclostus craigmarkensis, Levipustula rimberti, Productus carbonarius, Plicochonetes sp., Rugosochonetes skipseyi, Tornquistia diminuta), bivalves (Anthraconeilo sp., nuculoid debris), the gastropod Platyconcha hindi and the ammonoid Donetzoceras aegiranum, together with indeterminate ostracods, crinoid ossicles, echinoid fragments, conodont and fish debris. Ichnofauna is also present in the form of the burrow Tomaculum.

Beds above the Aegiranum Marine Band, where still in grey measures, consist of brownish grey palaeosol mudstones, with a few thin coals and stacked channel sandstones of variable thickness.

Barren Measures

Strata overlying the productive Coal Measures in the Warwickshire Coalfield range in age from Bolsovian to early Permian, though the youngest two divisions, the Kenilworth Sandstone and Ashow formations of the Warwick district (Old et al., 1987) do not enter the Coventry district. The names and boundaries of the two lowest divisions (Etruria and Halesowen formations) are essentially unchanged from those used at Warwick, but the nomenclature of the post-Halesowen redbeds, the former 'Keele' and 'Enville' divisions, has been revised (Figure 20).

Early workers on the coalfields of North and South Staffordshire and the Forest of Dean (King, 1898; Gibson, 1899; Newell Arber, 1917) recognised two broad subdivisions above the Halesowen Formation, one predominantly argillaceous (Keele 'Series'), the other arenaceous (Enville 'Series'). Over the years these names came to be widely accepted, and were applied to sequences throughout the Midlands. In the Warwickshire Coalfield, local stratigraphical names were introduced for parts of the sequence by Eastwood et al. (1923), and the stratigraphy was further refined by subsequent workers (Shotton, 1929; Old et al., 1987; 1990), according to whether sandstone or mudstone formed the predominant lithology. A summary of the later schemes is included in (Figure 20). Common to all the classifications, however, was the basic tenet of separate 'Keele' and 'Enville' subdivisions. The acquisition for the current study of geophysical logs from boreholes in the central part of the Warwickshire Coalfield has allowed more detailed correlation of the post-Halesowen redbeds than would be possible from surface mapping alone. These show that the succession is divisible into a number of cycles in which the proportion of sandstone to mudstone increases upwards. The upper part of each cycle is commonly conglomeratic and the boundary with the overlying cycle is relatively sharp, whereas the upwards transition from mudstone to sandstone within any given cycle is, by contrast, gradational and highly diachronous. In the light of these observations it was considered necessary to revise the nomenclature of the lower part of the post-Halesowen redbed sequence. In the scheme that has been adopted for this account (Figure 20), the Keele Formation and Coventry Sandstone of Old et al. (1987; 1990) are replaced by a single formation (Meriden Formation), which is divided into three members (Whitacre, Keresley and Allesley), each corresponding to a major upward-sanding cycle. The status of the overlying Tile Hill Mudstone Formation is unchanged.

Etruria Formation

The Etruria Formation consists of a rather ill-defined group of beds that mark the transition from the poorly drained alluvial backswamp setting of the Coal Measures to the freer-draining regime of an elevated floodplain. Their appearance in the sequence provides the first widespread evidence of Variscan uplift in source areas marginal to the coal basin. The genesis of the redbeds has been discussed by Besly and Turner (1983), and further overviews are given by Besly and Fielding (1988), and Besly (1988).

In the Warwickshire Coalfield, the Etruria Formation is represented by a group of mudstones, sandstones and breccio-conglomerates, mainly grey in colour but also variegated in shades of brown, red, purple and yellow. The boundary with the Coal Measures is diachronous and commonly gradational, but is conventionally drawn at the base of the more consistently reddened strata, or beneath the associated coarse-grained sandstones known in the Midlands as 'espleys'. The top, defined locally by an unconformity, is taken at the incoming of grey sandstones of the Halesowen Formation. Most of the formation is probably of Bolsovian age, but the basal beds may locally belong to the late Duckmantian Stage (Figure 20).

The combination of sedimentary thinning, diachronous base, and pre-Halesowen denudation makes it difficult to produce meaningful isopach maps for the Etruria Formation; in (Figure 30) separate diagrams have been drawn to illustrate the variations in thickness of the Etruria Formation ((Figure 30)b) as compared with the overall thickness of Bolsovian strata ((Figure 30)a). The onset of redbed deposition occurred earliest on the flanks of the coalfield, where the base of the formation lies within a few metres of the Half Yard Coal. Over the remainder of the coalfield, the first indications of reddening occur some 25 m above the Aegiranum Marine Band (Figure 26), (Figure 27) and (Figure 30). The effects of subHalesowen erosion are clearly evident in the vicinity of the Arley Fault (Figure 30), but are difficult to quantify elsewhere. The local thickening of the sequence to over 170 m adjacent to the Western Boundary Fault coincides with the development of a sequence of mudstone-flake breccias interpreted as debris fans (see below), and may, in part, be facies related.

The crop of the Etruria Formation north-west of Nuneaton was formerly well defined by a series of old brick pits, now all long since backfilled and largely built over. The main diggings extended in a line northwards from Bermuda [SP 351 894] to just south of Chapel End [SP 328 926]. Farther to the north-west the outcrop diminishes rapidly in thickness as it is cut out by the subHalesowen unconformity. South of Bermuda the outcrop is concealed beneath a thick cover of glacial deposits. Exposures are rare, and archival data from the old brick pits (Eastwood et al., 1923, pp.70–72) and shaft sinkings are the only sources of information. Exploration boreholes drilled by British Coal in the south-west of the coalfield provide additional data, though in many cases the Etruria Formation was largely uncored, and interpretation of the sequence is dependent on geophysical log signatures. A BGS borehole at Weston Hill Farm encountered Etruria Formation beneath Triassic strata, and is the only proving of the Etruria Formation to the east of the outcrop.

From the available evidence, it is possible to subdivide the Etruria Formation into three facies associations (Figure 30); two correspond with the 'alluvial plain' and 'alluvial fan' associations described by Besley (1988, pp.204–209). The third is a volcaniclastic association, proved only in the Weston Hill Farm Borehole. The latter provides the first evidence of Westphalian volcanicity in this part of the Midlands and is, therefore, of considerable interest. Typical sections of the three associations are included in (Figure 26), (Figure 27) and (Figure 34).

Alluvial fan deposits

These are restricted to the western margin of the coalfield and all lie within 3 km of the Western Boundary Fault. Boreholes in this zone (e.g. Priory Wood and Outwoods) record a westward-thickening wedge of red-brown and grey mudstones and siltstones, intercalated with mudstone-flake breccias. Evidence of pedogenic alteration is ubiquitous, and most commonly takes the form of colour mottling; plant traces are preserved as carbonaceous films, and sphaerosiderite is a widespread accessory mineral. The associated breccias, usually less than 0.5 m thick, are probably mainly intraformational but include discoidal clasts of undoubted Cambrian mudstone as well as other minor exotic constituents of 'Lower Palaeozoic' aspect. The gamma log for the succession is gently indented and shows a general upwards-coarsening signature but otherwise shows little discrimination between beds. The facies is notable for its lack of channel sandstones.

The distribution of the deposits is consistent with the palaeogeographic reconstruction proposed by Besly (1988), whereby the area to the west of the Warwickshire Coalfield was uplifted during the late Duckmantian, and then proceeded to shed material eastwards into the coal basin during the remainder of the Duckmantian and Bolsovian. The effect of the development of the fault scarp and uplift of the horst to the west was to provide a source for flash floods, during which predominantly fine-grained material (weathered Coal Measures and Cambrian mudstones) was redistributed as broad debris fans along a narrow belt parallel to the Western Boundary Fault.

Alluvial plain deposits

Over the central part of the coalfield, the Etruria Formation is more variable in character and includes thin coals and grey palaeosols more typical of the Coal Measures. Red mottling is both laterally and vertically restricted and in some boreholes is completely absent. Where the sequence is thickest, in a north-easterly trending belt from Allesley through Brownshill Green to Sudeley, up to half the formation consists of erosive-based multi-stacked sandstones. In borehole records these are usually described as greyish green, fine to medium grained, and poorly sorted with a coarse basal lag.

A more distinctive suite of sandstones, grouped under the general term of 'espley rocks', is recorded by Eastwood (1923, pp.70–72) from shafts and brick pits in the Stockingford area. These locally carry 50 per cent or more of exotic clasts (vein quartz, quartzite, chert, green shale). Records indicate that these sandstone bodies are well developed towards the base of the sequence but are laterally impersistent. A thick (13 to 14 m) basal or near-basal 'espley' is noted in Nuneaton Colliery Nos. 3 and 4 shafts and forms the prominent landform feature of Plough Hill [SP 320 930]. Farther north-west this 'espley' rapidly diminishes in thickness and cannot be traced much beyond Ansley Common [SP 315 932].

The lithologies are interpreted as recording deposition on an alluvial plain subject to periodic flooding and receiving sediment from a network of channels, at least some of which were draining a nearby hinterland. Palaeocurrent data are extremely sparse but measurements taken in former quarries at Heath End [SP 348 903] and Bermuda [SP 351 894] (Besly, 1983) suggest sediment transport dominantly from the south, and imply a source area outside the district. This does not preclude the possibility that locally derived material in some of the basal 'espley' sandstones may have been derived from emergent Lower Palaeozoic sources in the position of the present Hinckley Basin. The abundance of sandstone in the sequence implies that there was only limited contribution of material from the uplifted horst to the west of the coal basin.

Nonmarine faunas are recorded from near the top of the Etruria Formation in Haunchwood Brickpits; they include Anthraconauta sp., Anthraconaia sp., Carbonita fabulina, C. rankiana, C. secans and Spirorbis pusillus. It is not clear whether these faunas lie within the Anthraconaia adamsi-hindi or Anthraconauta phillipsi zones.

Volcaniclastic association

The Weston Hill Farm Borehole drilled by BGS in 1991 provides the first evidence for contemporaneous volcanism within the Etruria Formation of the Warwickshire Coalfield. The sequence, illustrated graphically in (Figure 34), comprises alternations of red and grey palaeosol mudstones and greyish green, thin-bedded tuffaceous sandstones, siltstones and mudstones. The volcaniclastic detritus is concentrated in discrete thin, sharp-based beds, which constitute about 25 per cent of the total sequence and increase in frequency upwards. The individual particles range up to coarse-sand grade, are subangular to subrounded, and have a distinctive pearly lustre. Some of the beds show normal grading, and in others small-scale asymmetrical folds are evident. Although there is some intermixing of volcaniclastic and siliciclastic detritus at bed boundaries, most contacts are sharp and show no cross-contamination. Ochreous root traces and cylindrical plant stems up to 1 cm diameter are locally preserved, some in their position of growth.

In thin section, the coarser-grained lithologies are seen to contain abundant highly altered (kaolinised) clasts of vitric and hyalocrystalline lava, with subordinate euhedral plagioclase crystals. Very delicate bubblewall shards are present in the matrix, indicating that much of the detritus is of primary origin and has undergone little reworking ((Plate 14)a, see p.160).

The deposits are interpreted as airfall tuffs generated, perhaps, during a series of phreatomagmatic eruptions. The abundance of juvenile glassy fragments implies rapid quenching of the eruptive products, which may have occurred through the interaction of groundwater and magma. The unusually good preservation of plant remains, including large stems, is consistent with rapid burial. Some reworking of the tuffs by surface water may have occurred but is not thought to have been a major factor. Analogous deposits, containing well-preserved conifer-like stems, and also interpreted as airfall tuffs, have been described from the Etruria Formation in the West Midlands (Glover et al., 1993).

Halesowen Formation

The Halesowen Formation is dominated by thick beds of grey coarse-grained sandstone, interbedded with grey mudstones, siltstones, seatearths and rare thin coals. Many of the boreholes penetrating the full thickness of the formation have cored only its lower part, and the top of the formation is imprecisely known. Cores of the entire formation, recovered in the Birch Tree Farm Borehole, are held by BGS and provide one of the few reliable records of the succession (Figure 31). The sedimentology of the formation has not been studied in any great detail but deposition in lacustrine and coal-bearing alluvial environments has been suggested by Besly (1988, p.210).

The base of the formation is taken at the base of a thick massive sandstone, known colloquially as the '100-foot' (Eastwood et al., 1923). This persists throughout most of the area, and is readily identified both on the ground and in geophysical logs. The top of the formation can be placed at one of two levels depending on the criteria applied. As a mapping expedient, the boundary is drawn at a colour change, from grey in the Halesowen Formation to predominantly chocolate-brown in the Meriden Formation above. This junction is gradational, over about 25 m, and can only be located approximately on the ground. The boundary is more accurately defined on geophysical logs, but is placed some 30 m lower, at a regional gamma-ray marker characterised by values up to twice the local background (Figure 31). The geochemical and petrological evidence, reviewed later in this chapter, suggests that the gamma peak is the more significant boundary in terms of provenance, as it marks a significant change in sandstone composition. However, to maintain consistency with the accompanying 1:50 000 map, the grey measures above the gamma peak are here included within the Halesowen Formation.

The thickness of the formation, from the basal sandstone to the gamma peak, ranges between 70 and 127 m, but averages about 100 m. Thinning occurs in the vicinity of the Arley Fault, and there are parallel zones, trending north-west to south-east, of thickening and complementary thinning centred respectively on the Staircase Lane and Flints Green boreholes. Isopachytes (Figure 31) strike obliquely to the Western Boundary Fault, suggesting that the uplifted area that lay to the west of the coalfield in Etruria times was denuded before Halesowen deposition commenced.

On a regional scale the formation gradually oversteps the older Westphalian formations southwards to rest eventually on Lower Palaeozoic rocks (Old et al., 1987). Sharp unconformable contacts at the base of the Halesowen Formation are visible on British Coal seismic lines around Fillongley, but details of these are confidential.

The succession proved in the Birch Tree Farm Borehole can be correlated across the basin (Figure 31) and is therefore described in some detail. It commences with a stacked sequence of channel sandstones, predominantly grey, fine- to coarse-grained, poorly sorted and micaceous. Individual sand bodies are erosively based, and commonly exhibit a variety of cross-bedding structures, or they may be massive. Coaly plant debris and thin intraformational breccias or conglomerates, mainly consisting of calcareous nodules of reworked calcrete, are widespread. The basal sandstone unit is overlain by 22 m of mainly pale to dark grey silty mudstones, with in-situ calcrete nodules and associated thin coals. Some of the mudstones are dark grey to black, and carbonaceous; others show diffuse colour lamination suggesting a lacustrine origin. In the borehole, two sandstones are present in the overlying sequence; at least one of these represents a major channelling episode that can be traced basinwide. Above, mudstones become the dominant lithology, suggesting more extended periods of lacustrine deposition; bivalves have been reported from one horizon (566.5 m). Above the gamma peak at about 560 m, the beds are predominantly argillaceous, and show local evidence of colour mottling as well as calcrete development. A 0.9 m bed of 'Spirorbis limestone', the Index Limestone, occurs 29 m above the gamma peak, and has yielded fish debris and ostracods.

Although coals within the Halesowen Formation have not generally been named, a seam found towards the base of the sequence and proved in Keresley 1A Borehole and Coventry Colliery shafts may be the Milton Coal, a seam which is widely developed in the area to the south (Old et al., 1987).

The main outcrop of the Halesowen Formation emerges from beneath drift near Sudeley, from where the component sandstones can be traced as a series of landform features to the northern margin of the district. The basal sandstone is some 18 to 22 m thick at Nuneaton Colliery, but increases to more than 40 m in the Haunchwood Tunnel Pit. North-westwards from Stockingford, it splits into two thinner beds which can be mapped on the south side of Plough Hill, with exposures in the old mineral railway cutting. However, north-west of the Bret's Hall cross-fault [SP 315 932] the higher bed dies out and the basal sandstone becomes thicker, again forming a prominent topographic feature south of the old Ansley Hall Colliery, where it progressively cuts down through the Etruria Formation to rest directly on Coal Measures.

Higher sandstones in the sequence vary in their lateral persistence but none extends along strike for more than about 5 km. The Index Limestone occurs in a mainly argillaceous part of the succession and can be traced along the length of the crop, either from surface brash or in boreholes. Limestone debris is particularly abundant in fields east of Manor House, and there may be two distinct beds in that area [SP 308 928] to [SP 316 925]. The limestone was formely mined at a locality [SP 345 896] just to the south of Lawn Cottage. The area over which the Index Limestone has been recognised covers at least 100 km2, which gives some indication of the size of the lakes that formed during this period. South of Sudeley, the Halesowen Formation is known only from old shaft records.

A small inlier of the Halesowen Formation comes to crop in the axis of the Arley Dome. Exposures of sandstone in Bourne Brook near Arley [SP 2784 8974], [SP 2786 8966] and adjacent to the roadside [SP 2794 9060] in Old Arley are poorly sorted, grey, and micaceous. Here again, the Index Limestone has been worked extensively, notably in Arley Wood [SP 2794 9074].

Faunas from the Halesowen Formation are poorly known and the Westphalian D nonmarine bivalves Anthraconauta tenuis and Anthraconaia prolifera are not recorded from this district, although the former is known in the south of the coalfield (Old et al., 1987).

The Halesowen Formation in the Meriden Borehole has yielded a diverse flora including Alethopteris lonchitica, Annularia stellata, Neuropteris scheuchzeri, N. flexuosa, N. heterophyllia, N. ovata and N. tenuifolia.

The petrography and bulk mineralogy of the Halesowen Formation are discussed in a later section, and provide the main evidence for provenance.

Meriden Formation

The outcrop of the Meriden Formation extends across the main part of the coalfield and continues under Mesozoic cover beneath the south-central part of the district. The formation comprises red and red-brown mudstones, siltstones and sandstones, with subordinate breccias, conglomerates and thin limestones. The combined thickness of the three component members is about 675 m. Gamma logs for selected boreholes (Figure 32) show the overall cyclic nature of the formation and the distribution of the main sandstone bodies.

As noted above, the base of the Meriden Formation is defined precisely on geophysical logs by a narrow zone of high gamma radiation; alternatively, as a mapping expedient, the boundary can be placed some 25 to 30 m higher in the sequence at the level where grey measures are replaced upwards by either chocolate brown mudstones or reddish brown sandstones. The top of the formation is taken at the highest mappable persistent sandstone beneath the predominantly argillaceous sequence of the overlying Tile Hill Mudstone Formation.

To assist description, informal names have been adopted for the sandstone-dominated cappings to the lower two cycles, following the scheme of Eastwood et al. (1923), but with minor modification. Thus the Whitacre Member is defined as an upward-sanding unit capped by the Arley and Exhall sandstones; whilst the second major cycle, the Keresley Member, has as its capping a packet of sandstones and conglomerates referred to as the Corley sandstone. The third cycle (the Allesley Member) is a more arenaceous unit overall and includes the 'Conglomerate of Allesley' (Eastwood et al., 1923). The Arley and Exhall and Corley sandstones are shown on the generalised vertical section of the 1:50 000 map, and on cross-sections; however they are not separately distinguished on the map face. The units correspond respectively to the 'Sandstones and conglomerates of Exhall and Arley' and the 'Sandstones and conglomerates of Corley' of Eastwood et al. (1923). The Corley Member, adopted by Old et al. (1990), is the stratigraphical equivalent of the Corley sandstone but is no longer retained as a formal lithostratigraphical subdivision.

In the absence of good exposure, the Birch Tree Farm Borehole, cored continuously from a depth of 300 m below surface, provides the best reference section for the lower part of the Meriden Formation. Other illustrative sections for the higher parts of the formation are listed later in this account. The geophysical signature of the formation is well illustrated by the Rough Close Borehole (Figure 32), which penetrated the complete formation and provides a useful characterisation of the sequence for correlation purposes.

The age of the lower part of the formation is likely to be Westphalian D, based on the occurrence of nonmarine bivalves found in the In Meadow Gate Borehole (Old et al., 1987, p.13), but it is suspected that the higher parts of the formation may include strata of Stephanian age. A jaw bone of the pelycosaur Ophiacodon discovered '¾ mile north-west of Coventry' (Murchison and Strickland, 1840, p.347) was included by Paton (1974) in the Autunian fauna obtained from the Kenilworth Sandstone, but if the locality quoted above is correct, the jaw bone belongs rather in the upper part of the Meriden Formation. On its own, the fossil indicates an age in the range late Stephanian to early Autunian (Paton, 1974, p.550), and therefore favours a Stephanian age for at least the higher part of the Meriden Formation.

Whitacre Member

The Whitacre Member consists of mudstones and subordinate sandstones in its lower part, but becomes increasingly arenaceous towards the top, where it includes beds assigned to the Arley and Exhall sandstones. Thicknesses obtained from geophysical logs are typically in the range 300 to 325 m. Good exposures in the argillaceous lower beds are rare, and the best record comes from the cores of the Birch Tree Farm Borehole. This reveals a sequence dominated in its lower part by mudstones showing desiccation cracks and green reduction spots, and containing abundant calcareous nodules, probably of pedogenic origin (calcrete). Interbedded sandstones form upward-fining units up to 5 m thick; they are typically cross-bedded, and some include intraclast mudstone and calcrete breccias. Many of the sandstones have a calcareous cement. In the northern part of the coalfield, the calcrete-rich sandstones form good, but impersistent, topographic features around the nose of the coalfield syncline.

In the primary survey of the district, particular attention was given to the mapping and correlation of the thin limestones that occur at intervals through the sequence. The present survey suggests that these beds are not as continuous as previously suspected. However, three of the more persistent beds occur at the following heights above the base of the member: Baxterley Limestone, 40 m; Whitacre Hall Limestone, 90 m; Maxstoke Limestone, 250 m. A thin bed of nodular limestone exposed in a cutting along the former Newdigate Colliery mineral railway [SP 3411 8676] has yielded fish debris, ostracods and the pulmonate gastropod Anthracopupa.

Throughout the upper 60 to 130 m of the member, sandstones grouped under the informal name of Arley and Exhall sandstones dominate the sequence. Their crop on the eastern flank of the coalfield syncline forms a strong scarp between Exhall and Goodyers End. From there, this feature can be traced north-eastwards through Newdigate to Astley but is partly drift-covered. Around the northern and north-western flank of the syncline, the sandstones form resistant landform features to the north of Daw Mill Colliery, though here the outcrop pattern is complicated by gentle folding. The geometry of the sand-bodies varies; some form persistent sheets that are traceable for 2 to 3 km, while others vary in thickness over short distances and include extraformational conglomeratic material. An analysis of the conglomerates, carried out by Shotton (1927), showed the main clast types to be Dinantian limestone, chert and Silurian greywackes, with subordinate vein quartz and quartzite. The Lickey Hills was suggested as a possible source of the greywacke clasts.

Exposures of the Whitacre Member are small and isolated in various minor streams, and few show more than a metre or so of strata.

A selection of the better sections is listed below.

  1. Bourne Brook [SP 2855 9007] to [SP 2880 9032]: intraclastbearing sandstones in lower part of member.
  2. Daw Mill Colliery [SP 2610 8990] : 5.8 m of interbedded sandstone and mudstone in upper part of member.
  3. Arley industrial estate [SP 2889 8981]: 4.2 m of interbedded sandstone and mudstone in lower part of member.
  4. Tipper's Hill quarries [SP 281 889] : 5.5 m of sandstones with mudflake breccias.
  5. Disused mineral railway, Bedworth Heath [SP 3429 8663] to [SP 3411 8676] : mudflake breccias, sandstones and thin beds of nodular limestone.
Keresley Member

This forms the second major upward-sanding cycle within the Meriden Formation and has a thickness in the south-centre of the coalfield of between 260 and 280 m, reducing to less than 200 m in the south-west. The lower argillaceous division stretches in an arc from north-east Coventry through Corley to Fillongley, where it is faulted out. The crop resumes in the core of the Fillongley Anticline to the west of the Arley Fault, though much reduced in width due to the decreased thickness there.

In the Coventry urban area, the only section of note is seen at Websters, Hemming and Sons Ltd Brickworks [SP 342 806] , which exposes beds about 100 m above the base of the member. A composite section described by Old (1989) and also figured by Besly (1988) includes between 15 and 20 m of strata showing multiple rapid variations between mudstone and sandstone. The thicker sandstones are cross-bedded and channelised; the thinner sheet sandstones show parting lineation, desiccation features, and evidence of bioturbation. The beds are consistently reddish brown in colour and show no evidence of contemporary pedogenesis. Besly (1988, p.213) interpreted the beds as 'an alluvial sequence in which most of the discharge was concentrated in large flood events'. The quarry is also of palaeobotanical importance; Vernon (1912) and Eastwood et al. (1923) both recorded the conifer Walchia here. This identification was later corrected by Florin (quoted in Wagner, 1983) to Lebachia piniformis; additional conifers were identified as L. frondosa var. zeilleri and Ernestiodendron filiciforme.

Farther to the north-west, towards Corley, channel and sheet sandstones within the argillaceous sequence form low ridges on the drift-free ground to the south of the M6 motorway. There are few exposures except in ditches and in the banks of the main streams. However, the main lithologies have been described in cored boreholes (Coy 1405 to 1408) drilled along the line of the motorway. In these, the sandstones are reported as being reddish brown or light greyish green, predominantly fine-grained, with parallel lamination or small-scale cross-bedding. The sand bodies are separated by red mudstones and more rarely by thin beds of pellety mudstone conglomerate. The Astley Court Limestone, described from this vicinity by Eastwood et al. (1923, p.90) was not found during the current survey, but limestone brash was noted at three localities farther west [SP 2618 8653], [SP 2573 8567], [SP 2829 8598].

The upper 85 m of the Keresley Member consist of thickly bedded sandstones with minor mudstone and conglomerate. These beds were termed 'Corley conglomerates and sandstones' by Eastwood et al. (1923), but unfortunately they also used the term 'Corley Conglomerate', which has been perpetuated by later writers (e.g. Shotton, 1929; Old et al., 1987). Because the proportion of conglomerate in these beds is always small, the latter description is inappropriate, and they are therefore referred to informally as the Corley sandstone in this account. The harder beds form elevated ridges from Holbrooks in the northern outskirts of Coventry through to Corley [SP 302 852]; farther west the unit is less readily defined at crop but can be traced in boreholes. In the agricultural areas, soils derived from these beds are very pebbly.

The escarpment at Corley Rocks [SP 3040 8522] furnishes good exposures through the basal beds of the Corley sandstone. In its lower part, the section comprises massive or trough cross-bedded sandstones with erosional bases, separated by thin mudstones or mudstone-flake conglomerates. Small pebbles, sideritic carbonate nodules and mudstone pellets occur as basal lags and as stringers on cross-sets. Lenticular pebble conglomerates occur in the upper part of the section. These are clast supported and comprise moderately well-rounded, very large pebbles (16 to 32 mm) set in a friable sandy matrix. The pebbles are dominantly of purple or grey, fine-grained sandstone, reportedly of Silurian age (Shotton, 1927). The composition of the pebbles is given by Shotton as follows:

A B C D E F
68% 11% 2% 16% — 3%

A= Silurian sandstone B= Quartzite C= Vein-quartz D= Chert E= limestone E= Other
  1. Selected sections in the Keresley Member are listed below:
  2. Corley Rocks [SP 3040 8520] to [SP 3037 8520]; 8 m of sandstone and conglomerate (Corley sandstone).
  3. Warehouse site, Coventry [SP 3313 7954]; 6 m of sandstone and conglomerate (Corley sandstone).
  4. Railway cutting, east of Coventry Station [SP 3369 7806] to [SP 3416 7795]: up to 6 m of cross-bedded sandstone (Corley sandstone).
  5. Websters, Hemming and Sons Ltd Brickworks [SP 342 806] : about 13 m of beds beneath the Corley sandstone.
  6. North side of railway cutting, south-west of Sandy Lane [SP 3311 8020]: 5 m of cross-bedded sandstone with channel lag conglomerate.
Allesley Member

The highest subdivision of the Meriden Formation is about 150 m thick and commences with a persistent mudstone interval of about 75 m, which can be traced from Coventry Station [SP 328 783] to Alton Hall Farm [SP 287 823]. The mudstones were formerly worked for brick clay in several shallow pits [SP 323 798] north of Spon End.

The overlying sandstones and interbedded mudstones form well-developed dip and scarp features to the east of Allesley. A section through these beds is provided by the Standard Motor Company well.

In Allesley village small exposures at Butcher's Lane [SP 3002 8082] and Birmingham Road [SP 3013 8061] are of pebbly sandstone and channel lag conglomerate. These are the ?Wesley Conglomerates' of Eastwood et al. (1923) and later writers, but they appear to be a local development with little value for stratigraphical correlation. Shotton (1927, p.613) gives the following pebble counts for these localities:

A B C D E F
Butcher's Lane [SP 3002 8082] 62% 0% 1% 12% 50 10%
Birmingham Road [SP 3013 8061] 20% 19% 0% 10% 39% 12%

A= Silurian sandstone B= Quartzite C= Vein-quartz D= Chert E= limestone E= Other

The Birmingham Road locality is notable for the occurrence of large prostrate trunks of silicified wood (Buckland, 1836; Eastwood et al., 1923). These are referred to as Dadoxylon by Vernon (1912) and Cordaites brandlingi by Eastwood et al. (1923). Mapping suggests that the fossil wood originates from a single horizon; it is recorded 96.7 m above the base of the member in a well at Mount Nod and is found in brash on the valley sides [SP 246 814]; [SP 248 811] north-west of Greenways Farm.

Tile Hill Mudstone Formation

The stratigraphical limits of this formation have been given by Shotton (1929; pp.171–172). The formation comprises the predominantly argillaceous sequence between the highest mappable sandstones of the Meriden Formation and the predominantly arenaceous Kenilworth Sandstone Formation (exposed in the Warwick district to the south). The thickness of the formation is about 280 m, of which the lowest 70 m come to crop in the south-west corner of the district. The age may be late Stephanian or early Autunian (see discussion on p.76).

There are few good exposures, and most of these are in sandstone, so that a detailed succession has not been established, particularly in the most heavily drift-covered areas. Mapping of individual sandstones shows that they tend to vary rapidly in thickness and die out laterally. The sandstones are predominantly red-brown, fine-grained, hard and flaggy, but include coarse-grained, poorly bedded varieties with channel-lag conglomerates. Both lithological types may occur within one unit and probably represent components of upward-fining fluvial cycles.

Small exposures are seen at two localities:

1. Stream at Whoberly [SP 3058 7940]; 1 m of blocky mudstone.

2. Stream near Lower Eastern Green [SP 2794 7974]; isolated exposures of flaggy red-brown sandstone.

Provenance and depositional setting of the Posthalesowen Redbeds

The post-Halesowen redbeds are interpreted as a sequence of stacked progradational alluvial fan systems, incorporating distal (argillaceous) and proximal (arenaceous and conglomeratic) elements. Besly (1988) considered the earliest redbeds, which he referred to the Keele Formation, to be of northerly provenance, derived perhaps from a region of low-grade meta-sediments such as the Lower Palaeozoic terrain of southern Scotland. The succeeding conglomeratic beds were studied by Shotton (1927), who concluded that the clasts in the Arley and Exhall sandstones were derived from a western source, whereas those in the Corley sandstone came from the east. Transport paths were deduced by matching clast type and abundance to lithologies in inferred source areas. Uplifted fault blocks occupying the sites of what are now Triassic basins at Knowle and Hinckley were identified as the principal sources of coarse detritus (Wills, 1956; Besly, 1988).

Palaeocurrent and heavy mineral data collected in the course of the present survey, support some of these findings but are not wholly consistent with the proposed models. The few sites that have yielded reliable palaeocurrent directions (Figure 33), point to sediment transport along the axis of the basin in a generally northward direction, but with a strong component of input from both margins. None of the sites examined showed any evidence for flow from the north. It is also significant that detrital input from both flanks appears to have been established at an early stage in the depositional history of the basin, allowing the possibility for mixing of different source-rock assemblages. Further evidence for simultaneous input of material from mineralogically distinct sourcelands is provided by the heavy-mineral studies, summarised in a later section.

Concealed extensions to the Warwickshire Coalfield

The Bulkington Prospect

The possibility of concealed Coal Measures existing to the east of the outcrop, beneath Triassic cover, was first debated around the turn of the century in reports to the Coal Commission. Later, Boulton (1926) argued on structural grounds for a continuation of the coalfield on the eastern limb of the Marston Jabbett Anticline; in particular, he drew attention to an area around the village of Bulkington, where in 1922 a water well, the Bulkington Borehole, had encountered grey measures beneath thin Triassic cover. Interest in the area revived in 1990 with the production by the BGS Regional Geophysics Group of the first detailed Bouguer Gravity Anomaly Map for the Coventry district. This showed a prominent gravity anomaly low, covering an area of some 5 km2 centred on the village of Bulkington (Figure 34). In the light of Boulton's earlier observations, it was decided to investigate the anomaly further and three boreholes were sunk. Two reached their target depth, but the third, located on the northern outskirts of Bulkington, had to be abandoned at 146 m because of poor drilling conditions. The investigation proved a westward-dipping Coal Measures sequence, comparable with that of the main coalfield, and concealed by only a thin cover of Triassic rocks.

The BGS Well Green Farm Borehole, drilled on the south side of the gravity anomaly, proved 107 m of Coal Measures resting unconformably on Cambrian rocks. Langsettian strata are 85 m thick and have yielded Lingula from three horizons (127.32, 149.21 and 149.34 m). It is not possible to determine which named marine bands these represent, though since generally it is the Subcrenatum, Listeri and Amaliae bands that penetrate farthest on to the Wales–Brabant High, these are regarded as the most likely candidates. Miospores recovered from between 47.78 and 117.02 m confirm the Langsettian age of the strata (McNestry, 1992); they include abundant Densosporites and Cristatisporites with common Laevigatosporites and Radiizonates. A single sample, from just above the basal unconformity at 149.4 m, contained a miospore assemblage tentatively assigned a late Namurian to Langsettian age. The possibility that the very basal beds (those below the lowest Lingula Band) are of Namurian age cannot, therefore, be dismissed

The sequence is shown graphically in (Figure 34). A minor channel system towards the base is composed of 9 m of thickly bedded grey and white quartzarenite. Some of the thicker bedforms appear structureless apart from a local coarsening towards the base, while others show parallel lamination, cross-bedding and climbing ripples. Successive sandstone tops are heavily bioturbated. Passing upwards, the sequence becomes predominantly argillaceous, with root-bearing grey-brown mudstones and siltstones of lacustrine facies, and interbedded thin coals in nine separate seams. Siderite and sphaerosiderite are ubiquitous. A gamma peak at about 67 m is associated with a dark grey mudstone and is taken to be the horizon of the Vanderbeckei Marine Band.

Only the lower part of the Duckmantian sequence is preserved at this locality, the higher beds being cut out by the sub-Triassic unconformity. Upward-fining channel sandstones, commonly red-tinged, are the dominant lithology. A coal, only partly cored but with an estimated aggregate thickness of 2.5 m, caps the sandstone sequence and may be correlatable with the Nine Feet seam. The succession proved in the Weston Hill Farm Borehole correlates well with that found in the exposed coalfield at Charity Colliery.

The structural setting of this prospect is conjectural but, on the evidence of the Bouguer anomaly data, it seems likely that the the strata are preserved in a northward-plunging syncline bounded to the west by a pre-Triassic fault.

The Hinckley Basin Prospect

Exploration of the Hinckley Basin so far has proved disappointing. In the mid-1970s, British Coal carried out a seismic survey of the region to the east of the Polesworth Fault, and followed this up with boreholes at Dadlington and Aston Flamville. Both failed to find Coal Measures, passing directly from Trias into Cambrian basement (Jones, 1981). The deeper parts of the basin have never been drilled; however, inspection of the gravity horizontal derivative map (Figure 51) suggests that the basin may be partitioned by major north-west-trending faults. In these circumstances, there is the possibility, as yet unproved, that Coal Measures are locally preserved in pre-Triassic grabens similar to that at Bulkington.

Petrography and heavy mineralogy

This section describes the petrography and heavy- mineral assemblages of the Carboniferous clastic rocks in the coalfield. The mineralogical signature of each of the main lithostratigraphical units is described, and assessed in relation to possible source areas for the sediment. Additional details are given in a technical report by Hallsworth (1992).

Eleven samples from the Birch Tree Farm Borehole, one from Merevale No. 2 Borehole and five from outcrop, were selected to provide coverage of all the main sandstone-bearing formations in the coalfield. Although neither the Kenilworth Sandstone nor the Ashow Formation crop out within the district, they form an integral part of the Barren Measures in the adjoining Warwick district, and so have been included in the study.

Thin sections were stained for K-feldspars and carbonates, and point counts of 200 grains were made to determine the bulk composition of the rocks. Analyses of heavy minerals were carried out by conventional optical methods: this involved a count of 200 non-opaque detrital grains per sample (where grain recovery permitted) to determine the overall composition of the suite, and a separate determination of specific provenance-sensitive mineral ratios. The petrographic results are summarised in (Figure 35) and the heavy-mineral data in (Table 9). Six mineralogically distinct units can be identified, corresponding broadly to the mapped formations. A summary of the distinguishing features of each unit follows.

Millstone Grit

The Millstone Grit sandstones are subarkoses, consisting of angular to subrounded monocrystalline and polycrystalline quartz grains, with subordinate feldspar (up to 20 per cent) showing varying degrees of dissolution, and partial or complete replacement by kaolinite; lithic fragments are rare.

Coal Measures

The Coal Measures sandstones are mineralogically relatively mature (sublitharenites), with comparatively low amounts of rock fragments, dominantly of chert; mica schist fragments are rare. The heavy-mineral suite is also of low diversity, with only the very stable minerals rutile, tourmaline and zircon present in any abundance. The scarcity of garnet cannot be attributed to depth-related dissolution, as the mineral is abundant 200 m higher in the sequence. It is either a provenance-related feature or may be a result of acidic weathering processes, which would also explain the observed lack of apatite. The presence of chrome spinel is unusual, as it is a generally rare accessory mineral; the rela lively high average CZi (chrome spinel/zircon index) value (3.5) is, therefore, a distinctive provenance signature.

The Coal Measures and Millstone Grit are both believed to have a northerly provenance (Besly, 1988) but the heavy-mineral suites of the two differ markedly, with the Millstone Grit lacking chrome spinel and containing high amounts of monazite. This implies a significant change in the source area between the Namurian and the Westphalian, either by unroofing or by a change in geographical location. Cliff et al. (1991) suggest that the Namurian was largely sourced from an area to the north of Scotland such as Greenland, whereas the Westphalian could have been derived from the Scottish basement. The Wales–Brabant High is known to have supplied small amounts of sediment to the southern Pennine basin during the early Namurian (Trewin and Holdsworth, 1973), but is unlikely to have been an important source during the late Namurian, when deposits in adjacent areas are known to have come from the north (Fulton and Williams, 1988).

Etruria Formation

The Etruria Formation sandstones are, in marked contrast to the Coal Measures, poorly sorted and relatively immature. Rock fragments are abundant and comprise chert and micromicaceous material ((Plate 14)b, see p.160). This petrographic change is consistent with the change to a more local provenance such as the Wales–Brabant High (Besly, 1988). However, the heavy-mineral assemblage apparently differs little from that of the Coal Measures. This may indicate that both formations were derived from a similar source but that tectonic uplift in Etruria times introduced less-weathered material to the area. Alternatively, separate sources may have supplied similar heavy-mineral assemblages.

The Halesowen Formation sandstones record the first input to the basin of metamorphic detritus of undoubted southerly derivation. The sandstones are poorly sorted, with an extremely high proportion of rock fragments (average 38.2 per cent). The detritus is dominated by quartz-schist and mica-schist fragments, with subordinate chert and pelites ((Plate 14)c, see p.160). The heavy-mineral suite is equally diverse, with up to nine mineral species recorded. Garnet is present in abundance (average 52 per cent) as well as rare source-specific chloritoid.

Halesowen Formation

The Halesowen Formation is thought to be related to the Pennant sandstones of South Wales, which were deposited by high-energy braided rivers flowing northwards from mountain ranges developing at the Variscan Front. Kelling (1974) proposed a source area in Devon or Cornwall, but the mineralogical evidence from rocks presently exposed in that region suggests that they could not have contributed significantly to the assemblage seen in the Halesowen Formation. The nearest outcrops with a comparable heavy-mineral assemblage, which includes chloritoid, are the glaucophane schists of the Ile de Groix in Brittany (Makanjuola and Howie, 1972).

Meriden Formation

The basal sandstones of the Meriden Formation are distinguished from those of the Halesowen Formation by a significant reduction in the proportion of metamorphic rock fragments, particularly the quartz-schist component. The most characteristic feature is the appearance of calcrete clasts ((Plate 14)d, see p.160), which are very common at the base of the sequence but decrease in abundance upwards; they are probably reworked from local penecontemporaneous soil profiles. The heavy-mineral suite indicates a major change in provenance, shown by a significant reduction in the proportion of garnet (average 10.5 per cent in the Whitacre Member) and the appearance of minor amounts of chrome spinel. Chloritoid is present at the base of the sequence but disappears upwards. Overall, the assemblage is quite similar to that of the Coal Measures.

The Arley/Exhall and Corley sandstones preserve a heavy-mineral assemblage of mixed affinity showing some of the attributes of the lower parts of the formation, but also including features more diagnostic of the higher redbed formations. The proportion of garnet is low, as is usual in the Meriden Formation, but the sandstones also contain staurolite and monazite, which are diagnostic minerals of the overlying Tile Hill Mudstones. The implication is that the detritus supplied to the basin during Meriden Formation times was derived from more than one source, with a significant contribution coming from a high-grade metamorphic terrain, either first cycle or reworked. This is in keeping with the palaeocurrent data, which indicate that northward-, eastward- and westward-directed transport systems all operated during this period.

Tile Hill Mudstones

A change in provenance can be inferred at the level of 2 the Tile Hill Mudstones. This' and the overlying formations are distinguished from the Meriden Formation by the disappearance of mica schist fragments, and by the presence of a high-grade metamorphic suite characterised by garnet (average 31.8 per cent), staurolite and the rare mineral monazite. Wills (1956) has suggested that that this assemblage was sourced from local fault-bounded blocks. If so, the mineralogy indicates that the most likely candidate is the 'Old Red Sandstone,' which is known to contain abundant garnet (Fleet, 1925).

Whether the increase in metamorphic grade of the detritus relates to unroofing and progressive denudation of an uplifted block of Devonian strata, or was triggered, instead, by nappe emplacement at the Variscan Front, is a matter for further research.

Geochemistry

The geochemical signatures of the Coal Measures and 200 succeeding beds, up to and including the Whitacre Member, have been established using material from the Birch Tree Farm Borehole (Haslam and Lumb, 1992). 100 Forty-nine samples, comprising more or less equal numbers of sandstone and mudstone/siltstone lithologies, were analysed by X-ray fluorescence for major elements (fused beads) and trace elements (pressed powders). Selected results are shown in (Figure 36). The elements Ti, V and Cr are particularly useful discriminants, with samples from each of the stratigraphical sub-divisions falling into distinct, if overlapping, fields on the triangular plot ((Figure 36)a). There is a general increase in Cr/V ratio upwards in the succession.

Coal Measures and Etruria Formation

The mudstones of this group are characterised by low values of the mobile elements Na, Ca, Mn and Sr, and a corresponding enrichment in the immobile suite Al, Ti, V, Cr, Co and Ni. This distribution suggests deposition under warm conditions with sufficiently good groundwater drainage to remove selected mobile elements. The succession has no equivalent of the more highly evolved palaeosols described from the Etruria Formation in Staffordshire (Haslam and Sandon, 1991; Haslam, 1993), which are more strongly depleted in K, Rb and Ba. This is probably because the alluvial plain in Warwickshire was subject to periodic waterlogging, and the sediments were not so strongly leached. The geochemical evidence provides no information on where the boundary should be drawn between the Coal Measures and the Etruria Formation.

Halesowen Formation

The Halesowen Formation mudstones show a somewhat wider range of composition than those of the beds below. Values of the mobile elements Na, Mg, K, Ca, Mn and Rb continue to be low, but are generally somewhat higher than in the Etruria Formation. The multi-element patterns compare closely with those of the Newcastle Formation, sampled in the Sidway Mill Borehole in North Staffordshire, and believed to be a correlative of the Halesowen Formation.

Sandstones in the Halesowen Formation show a consistent and distinctive pattern, with raised levels of Na, Mg, Al, P, K, Cr, Ni, Rb, Sr, Nb and Ba, compared with sandstones lower in the succession ((Figure 36)b). This association may partly reflect a higher clay content but contrasting Ti-V-Cr and Co-Ni-Cu patterns are more significant, reflecting the change in provenance noted in the section on mineralogy, above.

Meriden Formation: Whitacre Member

The transition to the Whitacre Member is marked primarily by a change in the concentration of mobile elements. Values of Ca and Mn are significantly higher in both mudstones and sandstones, suggesting a change in groundwater conditions from the freer drainage in the earlier sediments to more concentrated brines. Most of the sandstones contain calcite (represented by Ca, Mn, Sr) and a few contain baryte (Ba, Sr), probably as cement; calcrete clasts also occur in the lower part of the sequence. Compared with the Halesowen Formation sandstones, they generally show low values of most other elements. On the scale of the sampling, the junction between the Halesowen Formation and the Whitacre Member appears to be sharp.

Chapter 8 Triassic

Triassic strata belonging to the Sherwood Sandstone, Mercia Mudstone and Penarth groups crop out in the eastern part of the district (Figure 37), beneath an extensive cover of drift. The strata are mainly confined to the Hinckley Basin, a fault-bounded trough orientated north-west to south-east, which developed in Late Permian to Early Triassic times. Although the basin is comparatively shallow, with a Triassic fill estimated at between 350 and 600 m thick, the succession in the deeper parts of the basin is imperfectly known. The available evidence comes mainly from a coal exploration programme conducted in 1977–78 (Jones, 1981), which involved a seismic reflection survey and the drilling of two deep boreholes. More recent geophysical studies (Whitcombe and Maguire, 1981; Allsop and Arthur, 1983) have provided additional information on the possible configuration of the basin. The Triassic strata conceal a basement of deformed Precambrian, Cambrian and Carboniferous rocks.

Classification

The nomenclature of the Triassic rocks and likely stage correlations (based on Warrington et al., 1980) are summarised in (Figure 38); the earlier classification of Hull (1869) is shown for comparison (Table 10). Most of the rocks are of Triassic age but the basal subdivision, the Hopwas Breccia, is almost certainly diachronous and the Permian/Triassic system boundary is arbitrarily placed near the base of this deposit. The junction of the Sherwood Sandstone and Mercia Mudstone groups is gradational through a varying thickness of beds formerly termed 'Waterstones'. Warrington et al. (1980, p.39) included most of these transitional strata in the Bromsgrove Sandstone, following Wills (1970). In this account, however, the practice used in the Nottingham and Coalville districts is adopted, whereby beds of 'Water-stones' facies are included as the basal unit of the Mercia Mudstone Group (Charsley et al., 1990; Worssam and Old, 1988). The Mercia Mudstone outcrop is mainly drift-covered, and subdivision into formations has not proved possible. The basal unit has been mapped locally at Nuneaton as a lithofacies, however, and the Arden Sandstone and Blue Anchor formations can be identified in some borehole records. The overlying Penarth Group is also concealed beneath drift and is known mainly from boreholes.

It has not been possible to demonstrate any stratigraphical breaks in the Triassic sequence of the Hinckley Basin, though in the neighbouring Needwood and Knowle basins a widespread hiatus is recognised within the Sherwood Sandstone Group, approximately at the base of the Bromsgrove Sandstone (Rees and Wilson, in press; Old et al., 1991). A disconformity within the Mercia Mudstone Group, at the base of the Blue Anchor Formation, is also recognised in the adjacent Warwick district (Old et al., 1987).

Depositional setting

The Triassic sequence was deposited in a low-latitude continental setting, under and to semi-arid conditions. Except for some of the sandstones, the rocks are generally red in colour, a result of the breakdown by chemical and mechanical weathering to iron oxide (haematite) of detrital ferromagnesian silicates and iron-bearing clay minerals (Walker, 1976).

Deposition of the Sherwood Sandstone Group was controlled by palaeogeographical changes probably initiated during the Late Permian, when the basins that now surround the Warwickshire Coalfield formed by continental rifting (Whittaker, 1985; Warrington and IvimeyCook, 1992). On the eastern flank of the coalfield, synsedimentary movement on the Polesworth Fault initially controlled both the thickness and distribution of sediments accumulating in the adjoining Hinckley Basin. Other bounding faults may also have been active at this time. The downwarping of the Hinckley Basin was accompanied by fluvial sedimentation, with a major river bringing detritus from as far south as Brittany. At the same time, erosion of the uplifted basin margin caused fan conglomerates and flash flood deposits to form locally in the basin floor. Towards the end of Sherwood Sandstone times, the basin had filled sufficiently to allow fluvial sediments to overstep westwards across the Poles-worth Fault on to the flank of the Warwickshire Coalfield. The development of a coastal-plain sabkha in Mercia Mudstone times represented a major change in environment, and wind-blown silts with minor evaporite deposits (gypsum and anhydrite) accumulated. Towards the end of the Triassic, a marine incursion deposited littoral mudstones, sandstones and limestones of the Penarth Group. Marine conditions then continued into Jurassic times.

Sherwood Sandstone Group

The Sherwood Sandstone Group is subdivided into three formations ((Figure 38); (Table 10)), of which only the uppermost, the Bromsgrove Sandstone, is present at outcrop. The sequence varies markedly in thickness, due in part to relief on the underlying surface of unconformity, but mainly to movement on syndepositional faults.

The latter effect is most evident on the eastern margin of the basin where the Triassic fill, estimated at 500 to 600 m thick and consisting mostly of Sherwood Sandstone strata, thins westwards across the Polesworth Fault (Figure 37), to between 25 and 30 m thick on its upthrown side.

Hopwas Breccia

The Hopwas Breccia, regarded here as the basal subdivision of the Sherwood Sandstone Group, is not exposed in the Coventry district, and is only tentatively identified in a single borehole record. However, isopachyte contouring of borehole data from the neighbouring Coalville district suggests that breccias up to 16 m thick may locally floor the Hinckley Basin (Worssam and Old, 1988, fig. 14, p.53). The breccias thicken towards the Polesworth Fault, which would seem to indicate that deposition was influenced by tectonic subsidence.

The type area for the deposit is to the north-west, near the village of Hopwas [SK180 050] in the adjoining Lichfield district (Barrow et al., 1919). Here, and at outcrop in the northern part of the Hinckley Basin, the breccias consist of locally derived, poorly sorted, angular clasts of Carboniferous limestone, Cambrian quartzite and mudstone, and weathered igneous rocks. The detritus probably originated as accumulations of scree and talus material eroded from the basin margins and reworked by flash floods.

In the present district, the only recorded occurrence of material resembling the Hopwas Breccia is a 'conglomerate' 5 m thick, encountered near the base of the succession in the Stretton Baskerville Borehole. A sample (E27560) showed lithoclasts of lamprophyre, quartzite and limestone in a matrix of subangular quartz grains cemented with micritic and sparry calcite. The bed has previously been included in the overlying Polesworth Formation (Old, 1990), but the poor sorting, and the low sphericity and angularity of the clasts, suggest that it is more like a fan conglomerate of the Hopwas Breccia than an alluvial channel deposit.

The Hopwas Breccia has not yielded any fossils but is considered to be a diachronous deposit, in part late Permian, but mainly Triassic (Scythian–Anisian) in age.

Polesworth Formation

This name was introduced (Warrington et al., 1980) for the sequence of fluviatile sandstones and conglomerates, formerly termed Pebble Beds (Hull, 1869), which crop out between Polesworth and Warton in the adjoining district to the north (Worssam and Old, 1988).

The formation is present only in the deeper parts of the Hinckley Basin (Hains and Horton, 1969, fig. 14, p.67; Audley-Charles, 1970) and does not come to crop in the district. The subcrop is defined by the Polesworth Fault in the west and by the sub-Triassic topography elsewhere (Figure 37). The thickness of the formation, based on three deep boreholes and supported by seismic evidence, varies from 34 m at Stretton Baskerville (Figure 39) to about 90 m in the north of the district. This latter figure contrasts sharply with the 200 to 500 m thickness suggested by Worssam and Old (1980, fig. 15, p.57), who based their estimates mainly on gravity data. Published geophysical profiles (Whitcombe and Maguire, 1981) suggest that thicknesses of this order are only likely to be found close to the Polesworth Fault where the basin is at its deepest (see Chapter 12).

In its type area in the northern part of the Hinckley Basin, the Polesworth Formation comprises pebble-conglomerates, poorly cemented sandstones and thin beds of red mudstone. The well-rounded pebbles are mainly of grey-brown and liver coloured quartzites of distant provenance (known commonly as 'Bunter' pebbles) but there are also locally derived clasts of Cambrian quartzite and Carboniferous sandstone and mudstone. Views differ on the source of the 'Bunter' pebbles, though regional studies point to their ultimate derivation from northern France and south-west England. Sedimentological features indicate that deposition occurred in a low-sinuosity braided channel system (Worrsam and Old, 1988; Warrington and Ivimey-Cook, 1992).

Palaeogeographical reconstructions by Wills (1948; 1956) and Audley-Charles (1970, p.52, plate 7) suggest that the Polesworth Formation was deposited as a local alluvial fan, fed from the south-east by the 'Polesworth River', but a more recent reconstruction by Warrington and Ivimey-Cook (1992) suggests that there may have been a direct depositional link between the Hinckley Basin and the main sedimentary basins of the West Midlands, where the Kidderminster Formation is the equivalent stratigraphical unit.

No fossils have been recovered from the Polesworth Formation, and its age is uncertain; Warrington et al. (1980) inferred an early Triassic (Scythian) age.

Stratigraphy

The Polesworth Formation may locally rest conformably on the Hopwas Breccia, but generally it lies unconformably upon an eroded surface of Precambrian and Cambrian rocks (Figure 48) with Ordovician intrusions. Coal Measures may also be present beneath the deeper parts of the basin. The formation has only been encountered in two boreholes, one at Stretton Baskerville, the other at Hinckley Wharf (Figure 39). In both cases only a driller's log is available and lithological details are meagre. In the lower part of the Stretton Baskerville Borehole, 34 m of red and grey sandstone interbedded with conglomerate and red mudstone are assigned to the Polesworth Formation and, as noted earlier, 5 m of underlying breccia may represent Hopwas Breccia. The Hinckley Wharf Borehole, drilled for water in 1877, penetrated only the top 19.7 m of an estimated 80 m of Polesworth Formation. Apart from references to relatively harder and coarser sandstone beds, and a thin conglomerate of 'pea-sized' pebbles at the top (Old, 1990), the log provides few lithological details. The question as to whether or not there is a stratigraphical break between the Polesworth Formation and the overlying Bromsgrove Sandstone remains unresolved; seismic data suggest that there is no angular disconformity at this level.

Bromsgrove Sandstone

The Bromsgrove Sandstone forms a continuous outcrop along the western margin of the Hinckley Basin (Figure 37), and is proved at depth in the centre of the basin by several boreholes, as well as being imaged on seismic lines. West of the Polesworth Fault, the formation is between 25 and 30 m thick, but east of the fault it thickens, with between 37 and 100 m proved in boreholes and greater thicknesses implied by geophysical data. The Bromsgrove Formation is generally underlain by the Polesworth Formation, but it rests unconformably on pre-Triassic rocks to the west of the Polesworth Fault, and in the northeast of the district it onlaps the buried Triassic topography formed by the quartz diorite intrusions around Stoney Stanton [SP 498 948]. Sandstones within the exposed sequence form low ridge features in the open ground to the south of Nuneaton, with intervening argillaceous beds of the formation weathering down to form hollows. Elsewhere the succession is largely concealed beneath drift.

In its type area, to the south-west of the present district, the Bromsgrove Sandstone has yielded rich and diverse floral and faunal assemblages. Old et al. (1991) gave a comprehensive review of the main finds, which included plant remains, annelids, molluscs, arthropods, fish and reptiles, as well as a number of trace fossils. No fossils were found during the present survey but palynomorphs from elsewhere indicate a Mid-Triassic (Anisian) age (Old et al., 1991; Benton et al., 1994; Barclay et al., 1997).

Sedimentology and mode of deposition

The Bromsgrove Sandstone comprises numerous upward-fining sedimentary cycles (Figure 39). In the lower part of the formation these consist mainly of coarse-grained, trough cross-bedded sandstones, with a structureless conglomerate or breccia at the erosive base of each set. The presence of detrital feldspar in the sandstones distinguishes them from those of the underlying Polesworth Formation (Wills, 1948, p.533; 1970, p.249). The conglomerates contain pebbles of extraformational quartzite, feldspar, igneous rock (lamprophyre or diorite) and chert, as well as intraformational sandstone and mudstone. The detritus is commonly set in a litho-logically immature sand matrix cemented by sparry calcite. The lower cycles are rarely complete, any mudstone component commonly being eroded and incorporated as intraformational debris in the succeeding cycle. Upwards through the sequence, the cyclicity persists but the overall grain size decreases, as conglomerates become fewer and mudstones form a proportionately greater part of each cycle. The sandstones become finer grained and micaceous, showing planar or ripple cross-lamination (Figure 39). Small carbonate concretions, ranging in size from 1 to 6 mm, are common in these higher beds and are probably of pedogenic origin.

The sedimentary structures indicate deposition under fluvial conditions, where water flow and depth varied continuously. The basal conglomerates and breccias represent channel lag deposits, the overlying trough cross-bedded sandstone sets were formed as migrating sand dunes within the channel, and the succeeding finer-grained sandstones and mudstones represent waning flow, abandoned channel and overbank deposits. The truncated or incomplete cycles in the lower part of the sequence are consistent with deposition in a low-sinuosity braided channel system in which the potential for mudstone preservation is low. In the higher parts of the sequence the nature of the deposits suggests a reduced flow regime with deposition in meandering rather than braided channels, where overbank deposits are more likely to be preserved and periodic subaerial exposure allows caliche deposits to form. The currently accepted view of the Bromsgrove Sandstone (Warrington and Ivimey-Cook, 1992) is of a diachronous, fluvial succession deposited by a major northward-flowing river, draining from north-west France.

Stratigraphy

There is a stratigraphical break beneath the Bromsgrove Sandstone in neighbouring basins (Rees and Wilson, in press; Old et al., 1991), which is correlated with the Hardegsen Disconformity of Early Triassic sequences in Germany (Trusheim, 1963; Warrington, 1970). In this district the junction between the Bromsgrove Sandstone and the underlying Polesworth Formation appears to be gradational and there is no evidence for a non-sequence.

The Bromsgrove Sandstone is best developed to the east of the Polesworth Fault, with boreholes at Stretton Baskerville, Hinckley Wharf (Figure 39) and Sapcote Freeholt proving thicknesses of 37, 72 and 100 m, respectively. The logs show sandstone to be the dominant lithology, with mudstones forming only 7 to 14 per cent of the succession.

To the west of the Polesworth Fault, the Bromsgrove Sandstone is represented by a marginal facies, much thinner than in the basin and containing a higher proportion of locally derived breccias and conglomerates. At the northern end of the crop, the formation onlaps against a steep erosional palaeoslope of Cambrian and Precambrian rocks, and is dominated by calcretised breccio-conglomerates. These were encountered in the Caldecote Hill Farm boreholes, to the west of Caldecote village, where serial drilling by BGS was carried out to investigate the nature of the Precambrian/Triassic boundary. Similar deposits are exposed at Midland Quarry [SP 351 925] where the Cambrian Hartshill Sandstone is overlain unconformably by up to 3 m of polymict breccia and a further 6.2 m of trough cross-bedded, pebbly sandstone; both deposits contain angular clasts of Cambrian quartzite and lamprophyre. The unconformity is also exposed at several localities in the suburbs of Nuneaton and to the north of Hartshill [SP 325 940] (Baldock, 1991a; Lawley, 1992a). Southwards towards Coventry, the relief on the sub-Triassic basement is more subdued and the Bromsgrove Sandstone is consistently around 25 to 30 m in thickness. Cored sections from boreholes at Clifford Bridge, Grange Farm and Furlongs Farm (Figure 39) show upward-fining sequences of sandstone, siltstone and mudstone, each 2 to 6 m thick, with argillaceous beds comprising up to 30 per cent of the thickness. Although there are no stratigraphical markers in the sequence, the argillaceous nature of the cycles, coupled with the observed thickness variations, suggest that this area was only inundated in the later part of Bromsgrove Sandstone times. Exposures noted by Eastwood et al. (1923, p.99) in the Attleborough suburb of Nuneaton are now mostly obscured. However, small quarries around Marston Jabbett e.g. [SP 3778 8795] and cuttings along the Ashby de la Zouch Canal e.g. [SP 3875 8896] expose sections in buff-weathering cross-bedded sandstone (Bridge, 1991). Farther south, in Coventry, similar lithologies are exposed in a road cutting at Bell Green Road [SP 3574 8217] and in the railway cutting [SP 3499 8060] south of Mercer Avenue (Bridge, 1988). The geophysical log signature of the Bromsgrove Sandstone as shown by the gamma ray response for the Caldecote Hill Farm No. 3 and Grange Farm boreholes is noticeably serrated, reflecting the incidence of upward-fining sedimentary cycles (Figure 39). At the boundary with the Mercia Mudstone there is, in places, a sharp rise in the gamma ray values, providing a useful marker for correlation purposes.

Mercia Mudstone Group

The Mercia Mudstone Group consists predominantly of red-brown, unfossiliferous mudstones, and is generally poorly exposed. The strata rest conformably on the Bromsgrove Sandstone throughout the Hinckley Basin but overlap on to the Precambrian rocks at Hartshill [SP 344 945] (Baldock, 1991), and on to Ordovician quartz diorite intrusions around Sapcote [SP 490 935] in the north-east (Lawley, 1992b). Comparative sections are shown in (Figure 40). Borehole and seismic reflection data show that the sequence is over 200 m thick north of Hinckley, but less than 50 m on the flanks of the Hinckley Basin where the highest beds have been removed by erosion. Greater thicknesses may be attained towards the centre of the Hinckley Basin.

Palynomorphs recovered from rocks in the adjacent Coalville district suggest a late Anisian to Ladinian age range for the group in the Hinckley Basin (Worssam and Old, 1988, table 4), and indicate that beds equivalent to the Radcliffe, Carlton, Edwalton and Harlequin formations of Nottinghamshire (Elliott, 1961) are present.

In a study of the mineralogy of the Mercia Mudstone of Britain, which included a sample taken from Croft in the north-east of the present district, Jeans (1978) concluded that dolomite and calcite of evaporitic origin formed up to 30 per cent of individual beds. Clay minerals identified included chlorite, corrensite, sepiolite and smectite.

Stratigraphy

At the base of the Mercia Mudstone Group, 8 to 12 m of red and green micaceous mudstones, siltstones and thin, fine-grained sandstones form an upward passage from the Bromsgrove Sandstone Formation. This transitional facies, equivalent in part to the 'Waterstones' of Hull (1869) and correlatable with the Sneinton Formation in the Nottingham area (Charsley et al., 1990), gives rise to very subdued topographical features and micaceous, sandy to clayey soils; it forms a mappable unit along the north-east flank of the Nuneaton Inlier but is not extrapolated far beyond the known outcrop, as so much of the Triassic sequence is covered by glacial deposits. The beds are proved in several boreholes drilled on the edge of the crop (Figure 40); there are also exposures in Judkin's Quarry [SP 3477 9303] and in a disused quarry [SP 3806 8998] on the southern outskirts of Nuneaton (Baldock, 1991a; Bridge, 1991; Lawley, 1992a). The beds display an upward-fining cyclicity with ripple cross-lamination evident in the sandstones and subhorizontal planar lamination in the finer-grained lithologies. Soft sediment deformation, load casts and desiccation features are also common at some levels (Figure 40).

Above the basal unit is a sequence of colour-laminated mudstones and siltstones similar to those of the Radcliffe Formation of Nottinghamshire (Elliott, 1961); these have been proved in Grange Farm and Caldecote Hill Farm No. 3 boreholes (Figure 40), where they are some 16 to 18 m thick, but they have not been recognised at outcrop. The beds range in colour from purple through red to brown and possess a lamination that is commonly disrupted by soft-sediment deformation and desiccation structures. Thin green sandstones, also planar or ripple cross-laminated, may be present, and green reduction 'spots' are common throughout the sequence.

The colour-laminated beds are overlain by massive red-brown mudstones and siltstones forming the bulk of the Mercia Mudstone Group. Numerous boreholes have penetrated these beds but few record the succession in any detail. The BGS boreholes at Grange Farm and Caldecote Hill Farm show that within this apparently monotonous sequence there are subtle lithological variations which give rise to changes in shade, hardness and lamination of the beds (Figure 40). Green reduction zones are commonplace, affecting whole beds, but also forming irregular lenses or small 'spots'. Secondary satin-spar gypsum veins are ubiquitous and anhydrite nodules are present at some levels. The former presence of halite is indicated by the occurrence of pseudomorphs, but no economically significant deposits are known. Thin sandstone and siltstone beds (commonly termed 'skerries') occur sporadically, notably towards the base of the sequence. They form the few topographical features seen in the north of the district and, where exposed, tend be green or red and micaceous, occasionally exhibiting planar or ripple cross-lamination.

A marginal facies of coarse breccias is developed around the quartz diorite intrusions exposed in the Stoney Cove, Granitethorpe and Sapcote quarries [SP 494 942], [SP 495 937], [SP 497 934]. The breccias, consisting of diorite, mudstone and pale green 'skerry' clasts, are considered to be scree or talus deposits (Eastwood et al., 1923) that developed around upstanding masses of bedrock during Mercia Mudstone times.

The Arden Sandstone forms a distinct arenaceous unit some 66 to 121 m above the base of the Mercia Mudstone (Old, 1990). The outcrop lies beneath thick drift and is only known from borehole provings; it is 8 to 10 m thick in the Holy Well, Hinckley Wharf and Bond Street boreholes (Figure 40), where it is described in the logs as 'grey sandstone' or 'Upper Keuper Sandstone'. At outcrop in neighbouring basins to the west and south, the Arden Sandstone comprises erosively-based upward-fining sandstones with thin mudstones. Sedimentary structures include planar and ripple cross-lamination, and load casts. The diverse flora and fauna recorded in these areas indicate a brackish to marine environment of deposition, and miospores indicate a late Triassic, late Carnian (Tuvalian) age (Old et al., 1987; Old et al., 1991; Barclay et al., 1997).

The sequence above the Arden Sandstone continues in blocky red-brown mudstones similar to those beneath it. No single borehole in this district has proved the thickness of the overlying beds and they are here estimated to be approximately 50 m thick, a figure that compares well with the 40 m found in the Home Farm Borehole in the Warwick district, to the south (Sumbler, 1980).

Blue Anchor Formation

Formerly known as the Tea Green Marl, this is the highest formation in the Mercia Mudstone Group and is of ?late Norian to Rhaetian age (Warrington et al., 1980). It crops out in the south-east of the district, but is largely obscured by drift deposits. The formation, as proved by two boreholes near Lutterworth ((Figure 40), (Figure 48) and (Figure 49), comprises 2 to 3 m of green mudstone and a unit recorded in the driller's log as 'stone' (presumed sandstone or siltstone). The records are not detailed enough to confirm the basal disconformity seen in the neighbouring Warwick district. The top of the formation, and that of the Mercia Mudstone Group, is clearly marked in the boreholes by the change to dark mudstones of the overlying Westbury Formation of the Penarth Group.

Sedimentology and depositional environment

The basal Waterstones' facies reflects a gradational change in depositional environment from the cyclic fluviatile regime of the Bromsgrove Sandstone to the arid coastal plain conditions of the Mercia Mudstone Group. The sequence is dominated by fluviatile siltstones and sandstones, but conspicuous gamma-ray peaks in borehole logs point to the possibility of marine incursions. During deposition of the succeeding laminated beds, clastic input to the area was greatly reduced and evaporite deposits formed locally. It is generally accepted that these beds and the majority of the overlying red-brown mudstones were deposited on an arid coastal plain (sabkha) subject to periodic marine incursions and continental run-off, which saw the establishment of ephemeral playa lakes. Periods of subaerial deposition of windblown dust alternated with subaqueous deposition of fall-out dust in suspension, as described by Arthurton (1980). Upwards through the succession, subaerial deposition became dominant. However, the brackish to marine environment indicated by the Arden Sandstone represents at least one substantial marine incursion and the thinner 'skerry' beds may represent similar events. The green beds of the Blue Anchor Formation reflect depositional changes that preceded the Rhaetian marine transgression.

The Ranns of Kutch, India (Glennie and Evans, 1976), and the sabkhas of the north Egyptian Coast (Ali and West, 1983), where arid supratidal flats are annually flooded by storm tides, are possible modern analogues of the depositional environment of the Mercia Mudstone Group.

Penarth Group

This name was introduced by Warrington et al. (1980) for beds formerly termed 'Rhaetic' or 'Avicula contorta' Shales. The Penarth Group comprises deposits representing a marine transgression of late Triassic, Rhaetian, age. The blue-grey and black mudstones and fossiliferous limestones of the group distinguish it from the earlier Triassic rocks.

The Penarth Group crops out in the south-east of the district, where it is mainly covered with drift deposits. It is proved in several boreholes. A comparison of borehole data with records from the neighbouring Warwick and Market Harborough districts (Sheets 184 and 170 respectively) suggests that the group is approximately 20 to 27 m thick. In the Market Harborough district the succession was identified using different criteria (Poole et al., 1968), giving a thickness of 12 m. Though not divided on the published map, the sequence can be classified further in boreholes (Figure 40), as described below.

Palynomorph assemblages recovered from the Warwick district have been assigned a Rhaetian age and confirm that the group was deposited in a marine environment (Old et al., 1987, fig. 15).

Westbury Formation

The Westbury Formation rests conformably on the Blue Anchor Formation and consists of dark blue-grey and black fissile mudstone with lenticles of siltstone and fine-grained sandstone. The mudstone contains disseminated pyrite which weathers to give the formation an ochreous discolouration. The junction with the overlying Lilstock Formation is gradational.

Lilstock Formation

Cotham Member

The Gotham Member consists mainly of blue or green-grey calcareous mudstones with lenticles of white siltstone. In the borehole records ((Figure 40), 8 and 9) it is only distinguishable from the overlying Blue Lias Formation by an intervening limestone bed, 2.1 m thick, tentatively correlated here with the Langport Member (Carney, 1992b). At outcrop the deposit weathers to a heavy, green-grey or khaki clay with ochreous mottling.

Langport Member

The Langport Member is a hard, micritic limestone with thin partings of grey mudstone. It is pale grey to yellow in colour and contains disseminated pyrite. The borehole records from near Lutterworth ((Figure 40), (Figure 48) and (Figure 49) describe 1.7 to 2.1 m of hard blue-grey limestone resting on 'blue-grey clay and stone' of the Gotham Member.

Chapter 9 Jurassic

Lias Group

The Lias Group occurs in the south-east of the district, forming small outcrops in the River Swift and the neighbouring southwards-draining tributary valleys of the River Avon. Except in the valleys, the bedrock is concealed beneath a 10 to 30 m thickness of glacial deposits which has buried a north-west-facing escarpment, 15 to 20 m high, developed on the Lias and Penarth groups. Where this escarpment emerges from beneath the drift farther south, in the Warwick district, it is seen to have a scalloped and embayed course, due to erosion, suggesting that the rather straight incrop of these beds shown on the present map is a considerable oversimplification.

These strata are a continuation of the south-easterly dipping Lower Lias succession in the adjoining Warwick district, to the south (Old et al., 1987), which consists of alternating mudstone and limestone beds laid down within a progressively subsiding tropical shelf sea. The transition to more offshore, mud-dominated environments higher in this succession is a response to deepening of the basin during a marine transgression which proceeded south-eastwards across a former landmass represented by the London Platform (Donovan et al., 1979).

The nomenclature used here for the Lias Group (Powell, 1984) is based on that employed for the Redditch district (Old et al., 1991). The strata in the Coventry district belong to the lower part of the Lias Group, which includes at its base the Blue Lias Formation (defined by Cope et al., 1980). The overlying beds have not yet been formally named in this part of England. The lowermost part of the Lias Group may include a small thickness of Triassic-age (Rhaetian) strata but this has yet to be confirmed biostratigraphically; the evidence from the adjoining Warwick district indicates that there the succession belongs wholly to the Jurassic (Old et al., 1987).

Blue Lias Formation

The Blue Lias Formation consists of alternating beds of limestone and mudstone and is divided into two members. The Saltford Shale Member was named by Donovan (1956) and has its type section in the Saltford rail cutting near Bristol; the overlying Rugby Limestone Member, named by Old et al. (1991), has its type section in Parkfield Road Quarry [SP 493 759], near Rugby, as described by Old et al. (1987).

Saltford Shale Member

This member corresponds to the strata termed 'mudstones below the Blue Lias' farther south in the Warwick district (Old et al., 1987), where the stratigraphically lowest beds contain abundant specimens of the ammonite Psiloceras planorbis; the first occurrence of this genus defines the base of the Jurassic System (Cope et al., 1980). These beds unconformably overlie limestone correlated with the Langport Member of the Penarth Group (Old et al., 1987).

In the present district the equivalent beds crop out in the river valleys north and north-west of Harborough Magna [SP 4770 7983], but are not exposed. A borehole at The Orchard, Lutterworth proved the member to be 10.3 m thick, compared to between 11 and 20 m for the equivalent beds in the Warwick district (Old et al., 1987). The log shows 6 m of 'stone and shale' at the base, overlain by 4.3 m of 'hard shale'. The lower unit may correlate with the 5 to 8 m-thick Wilmcote Limestone Member at the base of the Blue Lias Formation in the Redditch district (Old et al., 1991).

Rugby Limestone Member

The estimated thickness for this member is just over 20 m. In the Warwick district the equivalent beds comprise a 25 to 40 m-thick division called 'Blue Lias', whose age ranges from late in the Laqueoceras laqueus Subzone of the Alsatites liasicus Zone to late in the Coroniceras rotiforme Subzone of the Coroniceras bucklandi Zone; the base of the Blue Lias is diachronous and youngs to the south (Old et al., 1987).

These beds were formerly exposed on the western side of a disused canal [SP 4814 7764], where 2.4 m of grey clay with limestone beds were seen. A further exposure of limestone occurs in the bed of the River Swift [SP 5023 7979]. In the Warwick district, the extensive sections at the type locality in Parkfield Road Quarry are described by Old et al. (1987).

At least 17.4 m of strata correlated with the Rugby Limestone Member were proved beneath drift in the borehole at The Orchard, Lutterworth; the record mentions 'hard rock' and 'stone', interpreted to be limestone beds, which alternate with beds of 'hard blue shale'.

Beds above the Blue Lias Formation

This mudstone-dominated sequence has an estimated minimum thickness of about 60 m in the south-eastern corner of the area. It corresponds to the lower (Sinemurian) part of a succession which in the adjoining Warwick district is informally termed 'mudstones above the Blue Lias', and which comprises 150 to 170 m of pyritic mudstones with a few limestone beds, and limestone and ironstone nodules (Old et al., 1987).

Little is known about these strata in the present district. They crop out along the valley of the River Swift but only form limited exposures in the river bed: blue-grey mudstones with comminuted shell debris are exposed south-west of Harborough Fields Farm [SP 5047 8005] and 0.4 m of dark blue-grey shaly mudstone is exposed south of the M6 motorway [SP 5013 7924]. A borehole east of

Gibbet Hill crossroads (Shawell Sand and Gravel Ltd) proved about 100 m of what is described as 'blue clay and stone beds', although it is likely that the lower part of this section includes the Blue Lias Formation and Penarth Group. A further borehole (Motorway No. 1332) where the M6 crosses the River Swift, proved about 12 m of stiff grey shaly clay with siltstone beds.

Chapter 10 Quaternary

The widespread cover of superficial (drift) deposits is mainly of glacial origin, but includes smaller areas of late-glacial to recent material.

Glacial deposits

Deposits of glacigenic origin cover much of the district, attaining thicknesses of up to 80 m in the east, but forming a more patchy cover to the higher ground of the Warwickshire Coalfield. The deposits were formed in a variety of settings, and include a complex suite of interbedded tills and associated meltwater sediments. All are considered to be the product of a single Middle Pleistocene glaciation, though variations in the composition and distribution of the tills suggest a depositional history involving separate ice advances, staged over perhaps several thousand years.

In the east of the district, the glacial drift is divisible into several members, each mappable over many square kilometres, and forming part of a larger drift province which stretches throughout the central and eastern Midlands. The succession was first described by Shotton (1953), who coined the term 'Wolston Series' for deposits that were later to be designated as the stratotype for the Wolstonian Stage (Shotton and West, 1969; Mitchell et al., 1973). The stratigraphical relationships between these deposits in the Coventry district are illustrated schematically in (Figure 41). The nomenclature differs slightly from that of Shotton (1953) to take account of later work (Shotton, 1976; Rice, 1968, 1981; Douglas, 1974, 1980; Sumbler, 1983; Rice and Douglas, 1991). (Table 11) compares the nomenclature used in this account with schemes that have been employed elsewhere in the Midlands. For convenience in description, the glacigenic elements of this eastern province are grouped under the informal title of Wolston Glacial Succession — a name used in preference to 'Wolston Series'.

The earliest Quaternary deposit, the Baginton Sand and Gravel, antedates the main glaciation of the area but shows evidence of having accumulated during a period of climatic deterioration: it is a body of fluvial sediment that occupies a broad north-east-trending palaeovalley excavated in Mercia Mudstone. With the onset of glaciation, ice advancing from the north and north-west overrode the Baginton deposits and deposited a Triasrich till over much of the district; in the lowland areas this is the Thrussington Till. Subsequent retreat of the ice sheet northwards allowed laminated silts and clays of the Wolston Clay to accumulate, first in transient proglacial lakes and later in a deeper and more extensive body of water covering the catchment area of the modern River Soar. Local developments of sand and gravel interrupted lacustrine sedimentation in the south and a more widspread sandur formed on the surface of the lake clays after the lake eventually drained. These deposits, together with morainic sand and gravel that accumulated along the eastern shoreline of the lake, are included within the Wolston Sand and Gravel. Towards the end of the glacial cycle ice again overrode the region, on this occasion from a north-east or easterly quadrant, depositing the blue-grey Oadby Till containing an abundance of chalk, flint and Jurassic limestone clasts. In the north-east of the district, the advance caused widespread glaciotectonic deformation of both the underlying drift and the local Mercia Mudstone bedrock, in a belt extending from Hinckley to Ullesthorpe. The Shawell Gravel comprises outwash deposits wholly intercalated within the Oadby Till.

The Dunsmore Gravel, which caps the glacial sequence, was deposited during the final stages of decay of the Oadby Till ice sheet, but has a discordant relationship to the earlier deposits and is not regarded as a member of the Wolston Glacial Succession.

As the Wolston Glacial Succession is traced westwards on to the higher ground of the Warwickshire Coalfield, the younger members pinch out, leaving a glacigenic sequence dominated by till. A more generalised terminology is adopted for the drift deposits of this western province, which are here distinguished as Western Glacial Drift. The boundary between the two glacigenic provinces is notionally taken at the 115 m rockhead contour, this being the highest level at which stratified deposits are still recognised.

The status and correlation of the glacial deposits within the overall framework of the British Quaternary is the cause of much debate. Shotton ascribed the deposits to the penultimate British glaciation, corresponding to a cold (Wolstonian) episode between the Hoxnian and Ipswichian interglacials. However, this view was challenged by Perrin et al. (1979) and Sumbler (1983), who considered the deposits to be older, and contemporaneous with the Anglian drift of East Anglia, which dates from about 450 000 years BP. This alternative view currently finds increasing support (e.g. Bowen et al., 1986; Rose, 1987; 1989; Jarvie, 1991), and is strengthened by the results obtained from amino-acid geochronometry at sites south-east of Coventry. Amino-acid ratios measured on molluscs from Waverley Wood Quarry [SP 364 714] suggest that the lower part of the Baginton Sand and Gravel formed in Oxygen Isotope Stage 15 (i.e. some 600 000 years BP). More recently, organic beds found overlying the glacial deposits at Frog Hall Quarry [SP 416 736] in the Warwick district, have been dated at Oxygen Isotope Stage 9 (Bowen, 1992), as have the type Hoxnian deposits of East Anglia. By implication it follows that the underlying (and consequently older) Wolston Glacial Succession is of probable Anglian age, a correlation accepted by the present authors.

Rockhead and drift thickness

The likely configuration of the rockhead prior to glaciation is illustrated in (Figure 42), which is based on BGS data and other published sources (Shotton, 1953; Douglas, 1980; Rice, 1981). Rockhead provings are sparse, particularly in the east, and the diagram purports to show only the general pattern of rockhead gradients. The subdrift contours outline a palaeovalley system running partly beneath the present River Avon but sloping towards the north-east and passing into the modern River Soar basin. Shotton (1953) interpreted this depression as a preglacial valley of the 'proto-Soar' river, forming part of a more extensive system that follows a north-eastward course from near Stratford-upon-Avon to Leicester and beyond. The floor of the valley falls from around 75 m above OD east of Brinklow to around 55 m at Huncote [SP 518 985], north of the district. A subsidiary buried valley (the Hinckley valley), its floor sloping southwards, merges with the proto-Soar valley in the vicinity of Wolvey. Around Croft, the main trunk valley splits into northerly and north-easterly trending arms. Both are locally overdeepened and were identified respectively as the Thurlaston and Narborough 'furrows' by Rice (1981), who assigned them a subglacial origin. A comparison of the rockhead map with the present-day topography shows that the thickest accumulations of drift are preserved on the subdued relief of the Jurassic and Triassic outcrops. In such areas the cover is commonly between 40 and 50 m thick but reaches 80 m locally, as in the axis of the proto-Soar valley east of Wolvey. The drift thins against the more resistant igneous intrusions around Croft and over the concealed Jurassic escarpment in the south-east.

In the western half of the district, the drift cover is more patchy and the rockhead surface is unlikely to differ fundamentally from that of the present day.

Baginton Sand and Gravel

These deposits are part of a suite of fluvial sands and gravels that Rose (1987, 1989, 1991) maintains can be traced across midland England from the area of Stratford-upon-Avon in the west to Suffolk in the east. In the present district the deposits occupy the buried valley of the proto-Soar, and consist of a basal Trias-rich gravel overlain by a sand, respectively the Baginton-Lillington Gravel and Baginton Sand of Shotton (1953). Near the margins of the deposit, the basal gravel may be absent, having been overlapped by the sand. Thicknesses range from about 10 m in the south, where the gravel and sand facies are equally developed, to a recorded maximum of 16.5 m measured in a working pit [SP 512 982], just to the north of the district. The surface of the deposit declines north-eastwards from a mean height of 84 m above OD around Coventry to 77 m at Croft (Rice, 1981).

In the south of the district, the palaeovalley with its associated infill is about 5 km wide and there are exten sive outcrops on both sides of the River Sowe, especially around Binley, and upstream along Smite Brook. Farther north, the deposit is almost entirely concealed beneath younger drift but re-emerges [SP 530 930] in a tributary of the River Soar north of Broughton Astley. In the intervening ground, the position of the palaeovalley is poorly constrained but some measure of control is given by boreholes at Cloudesley Farm and Lodge Farm, and also by a series drilled near Broughton Astley, all of which proved sand and gravel at the base of the drift. Around Croft, rockhead contours define possible alternative routes for the main river system. The Thurlaston furrow was interpreted by Rice (1981) as the main outlet for the proto-Soar, but Rice also reported Baginton Sand and Gravel from within the Narborough Furrow. In both cases, subglacial erosion may have cut down below the level of the Baginton Sand and Gravel and removed it. Indeed, Rice (1981) cites an example from motorway cuttings on the M69 [SP 500 975] where the Baginton Sand and Gravel is cut out over a distance of 900 m by later glacial deposits.

Details

Exposures in the Baginton Sand and Gravel are rare but the overall nature of the deposit is revealed in pits that lie just beyond the northern and southern boundaries of the district, at Huncote [SP 512 982] and at Pools Farm, Brandon [SP 382 762]. These have been described by Bridge et al. (1992) and by Maddy (1989), respectively. The Huncote pit provides the best section through the Baginton Sand and Gravel in the region, and it is appropriate to include a brief review of the sediments here to supplement earlier accounts by Shotton (1953) and Rice (1981). The schematic log is given in (Figure 43). The deposit rests on an uneven surface of Mercia Mudstone bedrock and in its lower part comprises some 10 m of massive, clast-supported gravel with only rudimentary bedding and lenses of coarse sand. Lithological analysis of the 11.2–16.0 mm size fraction (Lewis, 1989) indicates that the unit is made up predominantly of quartz (20 per cent) and quartzite (48 per cent) with minor constituents of locally derived diorite, Jurassic limestone and Carboniferous and Triassic sandstone; the fossil Gryphaea arcuata is also present throughout the gravel. Towards the base, the gravels contain much weathered mudstone debris derived from the local bedrock. Coal, mudstone and siltstone 'rip-up' clasts are conspicuous in the sands, particularly in concentrations along cross-bedding foresets. Reactivation surfaces in the sands indicate rapid changes in the rates of discharge and deposition within the river system.

The gravel is overlain by up to 6.5 m of yellow sands, many with basal pebble lag deposits. Overall, the unit fines upwards from coarse pebbly sands at the base, through finer and cleaner sands which, in turn, are capped by silts. The beds display a complex pattern of bedforms associated with meandering channel and sheet deposits. Planar cross-bedding and laminar bedding are predominant, with trough cross-bedding and climbing-ripple lamination better developed in the channelised sand bodies. Numerous cut-off and erosion surfaces are seen in the main working face.

Inclinations of planar cross-bedding foresets, climbing ripple laminations and imbricated pebbles indicate a dominant flow to the north-east, slightly at variance with the measurements of Rice (1981) which suggest northerly as well as north-easterly flow.

Depositional environment

The bedding structures in the Baginton Sand and Gravel reflect deposition in a low-sinuosity braided channel system dominated by longitudinal bars. The transition from gravel to sand deposition remains unexplained but clearly represents a major change in basin dynamics. Both Shotton (1953) and Rice (1981) have reported intraformational ice-wedge casts from within the sand facies at Huncote suggesting that deposition occurred, at least partly, under cold conditions.

Other evidence of the contemporary climate comes from pits in the Warwick district where the gravels contain mammalian faunas and include organic beds. The fauna and flora of these deposits described by Kelly (1968), Osborne and Shotton (1968), Coope (1989), Gibbard and Peglar (1989), and Shotton (1989) confirm that a change from a boreal to a cold climate took place during their deposition.

Wolston Glacial Succession

Thrussington Till

The term Thrussington Till was introduced by Rice (1968) to describe red Trias-rich or grey Lias-rich diamictons deposited during the first advance of ice after accumulation of the Baginton Sand and Gravel. Rice (1981) later broadened the definition to include all tills, irrespective of lithology, confined stratigraphically between Baginton Sand and Gravel and Wolston Sand and Gravel. Farther south, in the Warwick district, a lithostratigraphical scheme was preferred (Sumbler, 1983), by which tills with a predominance of Triassic material were placed within Thrussington Till, and most grey chalky types within the Oadby Till. Over most parts of the present district these red and grey facies are mutually exclusive, being separated by the Wolston Sand and Gravel (Figure 41). In a minority of cases, where the two are interleaved in complex fashion, the classification of the mixed deposit is based on an assessment of its likely stratigraphical position relative to the Wolston Sand and Gravel.

The Thrussington Till extends across most of the eastern part of the district, seemingly as a continuous sheet, but has only been exhumed where the headwaters of the rivers Soar and Avon have removed the cover of younger Quaternary deposits. The till sheet varies rapidly in thickness, particularly where it fills irregularities in the rockhead surface. At the margins of the Mesozoic subcrop, it is typically 3 to 7 m thick but is possibly over 20 m thick in some areas as, for example, immediately east of Smite Brook.

The base of the till sheet where it is in contact with the Baginton Sand and Gravel appears flat or gently undulating and sharp, suggesting that the underlying sand was frozen when the till was deposited. The upper contact with the Wolston Clay is gradational and marked by a zone, sometimes metres thick, in which till and brown stoneless clays interdigitate. In the urban areas of northeast Coventry, particularly around Stoke, lack of exposure precludes detailed mapping of this junction and the glacial drift is mostly shown as undivided till.

Trias-rich facies

Throughout most of the district, the Thrussington Till comprises a tough red-brown clay, with pebbles and blocks mostly of grey-green Triassic sandstone and siltstone, with rarer red mudstone, 'Bunter' pebbles, Carboniferous sandstone and small fragments of coal ((Plate 15)a, see p.161). Other erratics include Leicestershire granodiorites, Coal Measures ironstones and Lower Carboniferous limestones. The red clay component of the till derives mainly from erosion of redbeds within the Mercia Mudstone. As the till sheet is traced westwards on to the higher ground of the coalfield, it becomes darker brown in colour and the erratics include more locally derived Carboniferous and Lower Palaeozoic detritus.

Broad outcrops are present on both sides of the River Sowe and on the low interfluves that separate this river from Smite Brook. The field relations in this southern part of the district are illustrated in a section compiled from borehole records for part of the M6 motorway (Figure 44). This shows the till sheet thickening westwards from less than 4 m over the Jurassic escarpment to more than 20 m south of Pailton. Its upper surface slopes westwards and is inferred to lie at less than 70 m above OD beneath the M6 crossing of Smite Brook. Near Stretton under Fosse [SP 447 811], the sheet splits into three separate beds 2 to 3 m thick interleaved with sands, in an area of stratigraphical complexity showing features characteristic of an ice-margin contact zone.

In the Soar catchment, the Thrussington Till is affected by glaciotectonic disturbance (see below), but the Trias-rich facies is well developed west of Sapcote and is proved in numerous boreholes along the M69; thicknesses range from about 14 m (where undisturbed) to over 20 m. East of the River Soar, this same facies underlies a broad swathe of ground from Frolesworth [SP 504 906] to beyond Cosby Spinneys [SP 532 954].

Chalky facies

Brown and grey tills containing chalk are found in association with the more typical Trias-rich facies of the Thrussington Till in the axis of the proto-Soar valley, and, more widely, in the north-east of the district. Around Walsgrave [SP 385 815] a basal brown till containing chalk and flints is proved in boreholes (e.g. Motorway Borehole No. 1219) and comes to crop in a narrow lobe which extends from the M6/M69 interchange southwards to Walsgrave on Sowe and eastwards [SP 404 815] to just north of Hillfields Farm. Farther north at Wolvey Villa Farm, a borehole proved 19 m of grey chalky till beneath Wolston Clay. Other occurrences are described by Rice (1981) from the Narborough and Thurlaston furrows, where red, brown and grey chalk-bearing tills interdigitate and are accompanied by waterlaid glaciofluvial deposits. In the north-east of the district, grey clay with chalk is the predominant till lithology, especially in the area to the south and east of Frolesworth.

Depositional environment

The erratic content and composition of the bulk of the Thrussington Till suggests that it was laid down as a lodgement till by an ice-stream advancing up the proto-Soar valley from a mainly northerly or north-westerly direction. This is consistent with striations observed on rock surfaces at Croft (Rice and Douglas, 1991) which yield a direction of 125°, and with till fabric results of 120° and 155° quoted by Rice (1981) and Shotton (1983) for pits at Huncote and Wolston, respectively. However, in order to explain the more localised occurrences of chalky material, it is has to be assumed that ice lobes originating from the east or north-east must have encroached into the district from an early stage. The occurrences of chalky diamictons within the proto-Soar system remain enigmatic. Some may have been emplaced during the advance of the main Oadby Till ice-sheet, when according to Rice (1981), the Narborough and Thurlaston furrows were excavated. In other instances, where chalky till directly overlies bedrock, it is difficult to explain such a distribution other than by invoking the presence of an earlier tongue of chalk-bearing ice, possibly contemporaneous with the ice-advance that produced the Thrussington Till.

Wolston Clay

The Wolston Clay represents a major episode of glaciolacustrine deposition in the proto-Soar valley. It forms a widely mappable unit for which no standardised system of nomenclature currently exists. The name used here is the same as that proposed by Shotton (1953) and Sumbler (1983) for the clays and silts in the Warwick district. Farther north, however, the equivalent deposits in the Coalville district of Leicestershire are called Bosworth Clays and Silts (Shotton, 1976; Douglas, 1980; Worssam and Old, 1988). The Wolston Clay of the Coventry district is now known, as a result of the recent survey, to be a complex unit made up of clays deposited at various times in different parts of the proto-Soar valley.

Regional thickness variations suggest that deposition occurred in two principal sub-basins within a larger lacustrine province ((Figure 45)a). The northern sub-basin extends from near Stretton under Fosse to beyond Hinckley; its deposits comprise a thick and homogeneous succession of grey clays and silts which are identical to those mapped farther north by Douglas (1980). The southern sub-basin contains similar grey clays in its upper part, but these overlie redder-coloured clays whose type area is in the Warwick district farther south. Separating these two sub-basins is a ridge-like feature formed by a thickened development of the Thrussington Till.

Area north of Stretton under Fosse — the 'Bosworth tongue' of the Wolston Clay

The deposits of the northern sub-basin defined above mainly constitute grey or blue-grey clays and silts: the succession is homogeneous apart from some sandy intercalations in its lower part. The deposits thin across a subdued ridge south of Hinckley, but are otherwise continuous with the Bosworth Clays and Silts farther north (Rice and Douglas, 1991); they are therefore referred to here as the Bosworth tongue of the Wolston Clay. The deposits attain a maximum elevation of 114 m above OD in an outlier near Arbury Park [SP 335 893], near the western margin of the basin; from there they can be traced eastwards to near Sutton in the Elms [SP 515 946]. The distance between these two places is about 22 km, which probably represents a minimum width for the original lake basin at this point. To the south-east the lake was narrower, being confined by an escarpment of bedrock mantled by the Thrussington Till south of Broughton Astley, and by morainic accumulations of this till and Wolston Sand and Gravel near Stretton under Fosse.

The Bosworth tongue is stratigraphically confined between Thrussington Till below and Wolston Sand and Gravel above. Its junction with the latter, although a regular and planar surface in detail, is on a regional scale highly undulatory, being flexured into a system of structural domes and basins which become increasingly accentuated towards the main area of glaciotectonic disturbance in the north-east (see below and (Figure 45)c). The thickest proving is 28.5 m measured in a borehole at Bramcote Barracks, to the west of Wolvey.

Details

Outcrops of the Bosworth tongue form the gently concave profiles that characterise many of the lower slopes of valleys cut into the glacial succession. It is no longer exposed in the district, but auger samples from north of Stretton under Fosse show a generally uniform, bluish grey, sometimes silty clay with abundant small carbonate nodules ('race') and sporadic 'Bunter' pebbles. One of the best of the former exposures was in a temporary borrow pit near Aston Flamville [SP 455 921], opened for the construction of the M69. This section (Rice, 1981) showed that above a sequence of red Trias-rich till and brown clay (Thrussington Till), the Wolston Clay comprised 4 to 5 m of crudely stratified grey silts enclosing sporadic clay and till layers, and including pebbles interpreted as dropstones from the distortion of bedding beneath them. The erratics were mainly of Liassic types, but also included flints, chalk and oolite. Further exposures, now concealed, occurred in the old brickworks at Ashby Road in Hinckley [SP 432 949]. The 8 m section described from here by Old (1990) showed dark grey, stoneless and silty clay which was either massive or had weakly developed bedding. At the same locality, Eastwood et al. (1923) noted a thin seam of red sand just beneath the upper contact with the Wolston Sand and Gravel. They observed that the drift units here are thrown into sharp anticlinal structures as a result of glaciotectonic activity, with dips as high as 80°. A further excavation [SP 4389 9087], on the AS south of Burbage, revealed intensely frost-shattered dark grey and dark brown Wolston Clay overlain erosionally by Oadby Till and Dunsmore Gravel; the clay contained abundant flattened 'race' concretions and sporadic pebbles, one of diorite measuring 0.5 m across (Old, 1990). Core from the Weston Hill Farm Borehole in the Bedworth area shows massive to very finely laminated pale brown clay containing carbonate 'race', small dropstones, and inclusions of red till ((Plate 15)b); the latter occur up to 7 m from the base of the unit in this area (Bridge, 1991).

Sand is intercalated within the lower part of the Bosworth tongue over a wide area. Thicknesses of up to 11 m are encountered in boreholes at Wolvey Villa Farm, the crossing between the M69 and B4112 (Motorway Borehole No. 164), Cloudesley Farm and Lodge Farm. Buff to red silt and very fine-grained sand was also augered in the valley of the Wem Brook just to the south-east of the Coventry Road [SP 386 857]. To the north, indications of sand and silt in the clays were also found by augering near Nuneaton Fields [SP 397 915].

Area south of Stretton under Fosse

This area includes the Wolston Clay of the southern sub-basin, whose deposits crop out south of Stretton under Fosse and Bulkington ((Figure 45)a). The Wolston Clay hereabouts contains lensing beds of Wolston Sand and Gravel at various stratigraphical levels (Sumbler, 1983, fig. 2) and, unlike the predominantly grey clays farther north, it is commonly red- or brown-weathering. According to Sumbler (1983; 1985) the intercalated sands divide the Wolston Clay into two parts: a lower succession of red-weathering clays, which contains drop-stones principally of Triassic derivation, and an upper succession of bluish grey clays with erratics derived from Chalk and Jurassic rocks (Old et al., 1987). Farther south in the Warwick district, the upper sequence grades both vertically and laterally eastwards into the Hillmorton Sand (not recognised here), and is also replaced laterally by the chalky Oadby Till (Sumbler, 1983).

The principal variation in this outcrop occurs along the northern margin of the sub-basin, east of Stretton under Fosse, where the Wolston Clay is interleaved extensively with Thrussington Till and replaced laterally by the Wolston Sand and Gravel ((Figure 45)b). South of this zone of interdigitation the Wolston Clay thickens to about 30 m around Brinklow ((Figure 45)a), forming a succession that includes two intercalated beds of Wolston Sand and Gravel on the hill east of Brinklow [SP 436 795].

Wolston Clay of the southern sub-basin is not exposed in the Coventry district, but where augered it is typically a red to chocolate-brown clay or silty clay which characteristically weathers to a bright yellow clay in the upper 0.5 m. 'Bunter' pebbles and Triassic siltstone clasts occur sporadically.

Depositional environment

The glaciolacustrine character of the Wolston Clay has been commented on by many workers, although the nature of the lamination, considered as varying by Shotton (1976, p.248), has been disputed by Douglas (1980, p.282) who provided evidence that it may also have been produced by the action of turbidity currents. The presence of till inclusions and dropstones indicates that the ice front lay at no great distance, and in all probability formed part of the barrier that confined the glacial lake.

One of the principal controversies surrounding the Wolston Clay (Harwood, 1988) concerns its origin; whether it represents the accumulations of a single large lake — named the glacial 'Lake Harrison' by Shotton (1953) in honour of its discoverer (Harrison, 1898) or is instead a composite unit, containing the deposits of smaller transient glacial lakes ponded in front of, upon, or even within advancing ice sheets (Sumbler, 1983; Old et al., 1987). The field relationships described for the Coventry district suggest a more complex depositional history than the one envisaged by Shotton, but offer a means by which the two competing hypotheses can be accomodated.

The lake basin within the Coventry district was demonstrably narrower than the Lake Harrison proposed by Shotton (1953, figs. 6 and 7). The present survey also shows that it was compartmented, into northern and southern sub-basins, by the ridge of Thrussington Till extending between Stretton under Fosse and Bulkington. It is possible that the latter feature represents a former front of the Thrussington ice sheet, and that the lower, reddened part of the Wolston Clay succession in the southern sub-basin was deposited within an early lake, or a series of transient lakes, ponded against this barrier. Upon final retreat of this ice northwards, down the proto-Soar valley (Douglas, 1980), the lake waters would have occcupied the area vacated by ice north of the barrier and formed an enlarged body of water in which was laid down the Bosworth tongue of the Wolston Clay ((Figure 45)a). The sands at the base of the latter represent a brief phase of glaciofluvial deposition in the period between ice retreat and lake expansion.

This second lake eventually submerged the early glacial topography along the length of the proto-Soar valley, and is perhaps the equivalent of the large glacial lake envisaged by the earlier workers. The Chalk and Jurassic-derived dropstones in this younger development of the Wolston Clay indicate that the lake was bordered by an 'Oadby'-type ice sheet, which had advanced from the east or north-east (p.97), but the Thrussington Till ice-sheet was also nearby, and contributed the red till dropstones that characterise the basal part of the succession. The inferred minimum altitude of the lake surface was about 114 m above OD, corresponding to the maximum height attained by clays of the Bosworth tongue near to their pinch-out in the west of the outcrop. This is lower than the 410 feet (125 m) envisaged by Shotton (1953), but is comparable to elevations of between 110 and 115 m estimated for the Leicestershire outcrop by Douglas (1980, p.283). Areas where the top surface is at less than 100 m above OD approximately coincide with the thickest clay developments within the axis of the proto-Soar valley and may represent the deepest parts of the lake, which was not completely filled. The reduction in height is also, in part, a result of differential compaction (Douglas, 1980), and additionally reflects the influence of glaciotectonic compression, which induced regional flexuring of the glacial succession ((Figure 45)c).

Wolston Sand and Gravel

These glaciofluvial deposits have a widespread distribution within the Wolston Glacial Succession, and like the Wolston Clay, more than one name has been proposed for them. The name preferred here was suggested by Rice (1981) following an earlier, less formalised usage by Shotton (1953). It is used here to encompass all of the glaciofluvial deposits occurring between the Oadby Till and Thrussington Till, and is a complex, diachronous association of sediments deposited at different times and in various types of environment. Two principal facies associations occur in the area ((Figure 45)b); sand with intercalated clay and till crops out around and to the south-east of Monks Kirby whereas a tabular unit of yellow sand, constituting the Cadeby tongue of the Wolston Sand and Gravel, has a more widespread distribution across the central and northern parts of the area. The stratigraphical relationship between these two associations is shown schematically in (Figure 41).

Southern outcrop: sand with intercalated clay and till

This association of the Wolston Sand and Gravel consists of yellow sands interleaved with a particularly distinctive variety of red clayey sand, together with subordinate but persistent beds of stoneless red clay and till. Its type area is along the valley sides of the Smite Brook, between Newnham Paddox [SP 4794 8364] and Newbold Revel College [SP 4555 8090]. The deposits hereabouts form a lens-shaped body, up to 40 m thick around Monks Kirby ((Figure 45)b), which tapers gradually over a distance of 3 km south-eastwards (Figure 44), and thins out just beyond the River Swift. Stratigraphical relationships, deduced from boreholes along the M6 where it crosses the Smite Brook (Figure 44), suggest that the upper beds pass laterally into the Cadeby tongue of the Wolston Sand and Gravel, but that those beneath occupy a lower stratigraphical position and interdigitate with the Thruss-

ington Till and Wolston Clay. The beds are replaced southwards by Wolston Clay (compare (Figure 45)a and b), but persist as thin sandy intercalations within the Wolston Clay east of Brinklow [SP 436 795], and throughout the adjoining Warwick district (Old et al., 1987). To the south-east these beds overlie Thrussington Till with no intervening Wolston Clay, but nevertheless contain intercalations of stoneless clay together with red 'Thrussington-type' till, as shown in (Figure 44).

Details

The top surface of the body occupied by this facies of the Wolston Sand and Gravel rises to just over 120 m above OD around Monks Kirby, where the beds also attain their thickest development. They crop out extensively on the sides of the Smite Brook between Stretton under Fosse [SP 448 815], and Pailton [SP 4713 8180], but are not exposed. Their composition is demonstrated by auger sampling of the valley slopes above, and to the east of, Newbold Revel College [SP 4555 8090]; this survey shows a succession of pink to crimson-red, fine- to medium-grained clayey sands which become predominantly yellow or orange higher up. Interleaved within the sands are lenses of red stoneless silty clay and red till with abundant green Triassic siltstone clasts; commonly within a single bed the stoneless clay passes laterally into till.

In the succession of predominantly yellow and orange sands west of the Smite Brook (Figure 44), a motorway borehole (No. 1279B) showed the following deposits:

Thickness m
Gravel and sand containing many flint clasts 1.9
Sand, orange to brown, fine-grained and silty 9.6
Sand, brown, fine-grained and clayey, poorly sorted and with sporadic lamination 14.2
(end of hole)

The red clayey sand facies on the eastern side of the Smite Brook was penetrated in Motorway Borehole No. 1287, for which the log reads as follows:

Thickness m
Clay, blue-brown, with chalk clasts (Oadby Till) 4.6
Clay, reddish brown, sporadic silty partings and some erratics which include chalk; sandy at base 3.6
Gravel, red, poorly sorted and clayey 2.1
Silt and sand, red, mainly fine-grained, with thin beds of finely laminated clay and silt 7.3
Sand, red, fine-grained and silty 7.6
Clay, red and sandy, with boulders 0.6
Silt, brown, clayey but locally a clean sand 0.9
Clay, chocolate-brown to red, laminated and silty 0.3
Clay, red, with boulders (?Thrussington Till) (end of hole) 1.8

Deposits of the Wolston Sand and Gravel that may be the distal continuation of these beds are intercalated within Wolston Clay to the south of the main outcrop, between Brinklow and Harborough Magna. An old pit [SP 4387 7887] south-east of Brinklow exposed 1 m of brown loamy sand with flint and 'Bunter' pebbles, and a nearby borehole suggests that the sand may be about 7 m thick hereabouts (Sumbler, 1985). In the BGS Back Lane Borehole north of Harborough Magna, the proving was in 14.7 m of very fine-grained brown sand, almost devoid of pebbles, with a few thin beds of clay and silt (Sumbler, 1981).

Central and northern outcrop — the Cadeby tongue

The Cadeby tongue constitutes the predominantly yellow and orange sands which have a widespread distribution north of Monks Kirby ((Figure 45)b). It is stratigraphically confined between Wolston Clay below and Oadby Till above, and forms continuous outcrops which merge northwards into the Cadeby Sand and Gravel of Douglas (1980) and Worssam and Old (1988). To the south, the Cadeby tongue can be traced by means of outcrops and boreholes into the upper part of the Wolston Sand and Gravel body, as described above and shown in (Figure 44).

For most of its outcrop the Cadeby tongue forms a gently undulating, parallel-sided sheet lying between 97 and 115 m above OD. Along valley sides the sands form a composite topographical feature comprising a small scarplet surmounted by a narrow ledge. Seepage lines, marked by marshy or peaty ground, are common at the junction with the underlying Wolston Clay, and are well seen east of the B4112 at Withybrook [SP 434 844]. As the deposit is traced towards the east and south, it changes in morphology to a lensoid sand body, estimated from field relationships to be over 30 m thick east of Copston Magna ((Figure 45)b); however, the maximum subcrop provings hereabouts are 20 m in the borehole at Cloudesley Farm and 24.4 m in the Willey Fields Farm Borehole. The surface of the deposit rises to 125 m above OD south-east of Copston Magna [SP 461 881], to form a positive element of the glacial topography across which the Oadby Till has thinned; the lower boundary is a planar surface which has been warped into the dome and basin structures illustrated in (Figure 45)c. The ridgelike appearance of the deposit in this area suggests proximity to an area of glaciofluvial discharge, as discussed below.

Details

There are no longer any permanent exposures of the Cadeby tongue in this district. A section formerly exposed in the brickworks near Ashby Road in Hinckley [SP 4316 9490] showed up to 4 m of well-bedded brown sand with cross-bedding foresets dipping to north and south. Many beds were rich in coal pebbles up to 80 mm diameter, accompanied by 'Bunter' and red sandstone pebbles. The sands passed up into laminated clay overlain, in turn, by chalky till (Old, 1990). In the same area, Eastwood et al. (1923, p.111) described a further section 'behind the old toll-house on the Burbage Road' [SP 4414 9343] as follows:

Thickness m
Sand, red, loamy (1 ft) 0.3
Sand, red, pebbly, and fine gravel, false- bedded with streaks of coal fragments and red clay partings. The pebbles are up to 2 in (50 mm) and are chiefly small 'Bunter' quartz and quartzite (5 ft) 1.5
Sand, brownish-red, loamy (2 ft) 0.6
Sand, reddish-yellow, false-bedded (8 ft) 2.4

In a temporary excavation for a fishpond west of Wolvey [SP 4176 8819], the following section was reported (Carney, 1991):

Thickness

m
Red pebbly sand underlain by clayey diamicton (Dunsmore Gravel); erosion surface at base 1.7
Sand, red to yellow, medium- to coarse-grained passing down to medium-grained ripple cross-laminated sand with coaly laminae 0.6
Sand, yellow, medium- to coarse-grained and cross-bedded, with granule-sand defining foreset bedding 1.0
Sand, yellow, massive and coarse-grained with scattered 'Bunter' pebbles 0.8
Gravel, yellow to brown, massive and clast supported; abundant 'Bunter' and flint pebbles 0.7
Sand, yellow, medium-grained and cross-bedded 0.6
Sand, yellow, coarse-grained and pebbly 0.1
Sand, yellow, medium-grained (no base seen, but Wolston Clay believed to be 1 to2 m below) 0.3

The predominant dip of foreset bedding in the above section was to the west or west-south-west (250 to 260°).

In a further small temporary exposure east of Claybrooke Hall [SP 4967 8802], 1 m of yellow, massive coarse-grained pebbly sand was underlain by 0.5 m of yellow cross-bedded sand which coarsened upwards from medium-grained at the base; foresets in this latter bed dipped to the south-west (230°).

Auger sampling suggests that the upper part of the thickened Cadeby tongue east of Copston Magna is composed of a fine-grained yellow-brown sand with a significant clay content: this is further indicated by Wenner Array resistivity measurements (Raines, 1992) which suggest that up to 15 m of silty or clayey sand caps the Wolston Sand and Gravel between Cloudesley Bush [SP 4634 8637] and Willey Fields Farm [SP 4883 8558].

Depositional environment

The oldest deposits of the Wolston Sand and Gravel ('sand with clay and till' in (Figure 45)b), around and to the south of Monks Kirby, comprise a mixture of sand, till and possible glaciolacustrine material (stoneless clay), reminiscent of facies associations described from pro-glacial environments within or adjacent to an ice contact zone (e.g. Edwards, 1978). They are contained within a lensoid body which, in profile (Figure 44), is analogous to sections through some well-preserved Devensian moraine belts (Thomas, 1989, fig. 6) where ice-contact sediments have been deposited within troughs that were parallel and adjacent to till-cored moraine ridges. The latter component is recognised, west of the Smite Brook, by the rising surface of the Thrussington Till that is interleaved with the sands. East of this, the beds may represent a section through an ice-contact outwash apron that prograded south-westwards into an adjacent lake basin, causing lateral gradation into the Wolston Clay. Interaction between the glaciofluvial and glaciolacustrine environments would also explain the clayey nature of this facies of the Wolston Sand and Gravel, and the fact that it contains intercalations of stoneless clay representing temporary lake advances. Upon final stagnation and retreat of the Thrussington ice, the whole complex of deposits collapsed to form a moraine ridge, which subsequently became part of the shoreline to the northern lake basin in which was deposited the Bosworth tongue of the Wolston Clay.

The Cadeby tongue, which overlies the Wolston Clay, is amongst the youngest of the Wolston Sand and Gravel deposits. It forms a sheet-like body of wide lateral extent interpreted by Rice (1981) to be part of a glacial outwash plain, or sandur, deposited from fluvial systems that developed along a wide front at the margin of a stationary ice sheet. The sands prograded across a level Wolston Clay surface exposed after the glacial lake (s) had drained away. Foreset bed inclinations measured by Douglas (1980) and Rice (1981) for the area to the north indicate west to south-westward current flow across the sandur, a direction also indicated by the few measurements available in the Coventry district ((Figure 45)b). In the context of this flow pattern, the thickness trend within the Cadeby tongue suggests a lobate sand body, interpreted to be the wasted remnant of an outwash fan deposited by rivers debouching from a point source along an ice front located east or north-east of Copston Magna ((Figure 45)b). The abundance of flints in the Cadeby tongue, and the corresponding absence of red Triassic detritus, indicates that the source ice sheet had probably arrived from the east, and was therefore associated with an advance or readvance of the Oadby Till ice.

Shawell Gravel

The Shawell Gravel was named by Sumbler (1983) and correlated with the upper of two beds of the Wolston Sand and Gravel developed particularly to the east of Harborough Magna [SP 477 792]. It strongly resembles the yellow sands of the Cadeby tongue of the Wolston Sand and Gravel in composition and mode of origin, but unlike the latter it is intercalated between two sheets of the Oadby Till (Figure 41). For the Shawell Gravel to correlate with the Wolston Sand and Gravel would require this lower bed of Oadby Till to thin out farther west; indeed in the Warwick district Old et al. (1987) have shown that, beneath the Wolston Sand and Gravel, the Oadby Till passes laterally into the Wolston Clay, and there is evidence for a similar substitution in the area west of Newton [SP 527 786]. In the adjoining map sheets the Shawell Gravel is recorded in the north-eastern part of the Warwick district (Old et al., 1987), and in the Market Harborough district, to the east, it is thought to be a time-equivalent of the 'pre-Chalky Boulder Clay gravel' that occurs in pockets on the bedrock (Poole et al., 1968).

Outcrops of the Shawell Gravel commonly form a scarp-like feature which can be followed around the valley sides for 8.5 km along the eastern margin of the district south of gridline 86. The deposit cannot be traced very far to the west, more or less ending at the A5, and is in the form of a tabular sheet about 4 or 5 m thick lying at between 110 and 117 m above OD. It thickens in two areas: around Lutterworth [SP 534 845], where 11.1 m are proved in The Orchard Borehole, and in the vicinity of Gibbet Hill [SP 528 810], where 11 m were measured at the Gibbet Lane Quarry [SP 5409 8061] on the eastern boundary of the district. Coinciding with these thicker developments, the top surface of the bed becomes convex in profile and rises to elevations of around 122 m above OD. The base of the Shawell Gravel is irregular in detail at the Gibbet Lane Quarry, where borehole records show it to be channelised into the underlying Oadby Till surface. In the west the bed may also feather out laterally within the till, forming isolated lens-shaped bodies indicated by the outcrop patterns around, and to the south of, Moorbarns [SP 521 829], [SP 525 8211].

Details

Exposures of the Shawell Gravel were revealed in the working pit at Gibbet Lane Quarry [SP 5409 8061], where the section shown in (Figure 46) was measured. The gravel contains sporadic ice-wedge casts, is weathered at the sharply defined upper contact with the Oadby Till, and also has been incorporated as rafts 2 to 3 m across within the till. The lower sheet of Oadby Till was exposed about 1.5 m below the base of the measured section, but the contact was not seen. The Shawell Gravel at this locality consists of an 11 m-thick sequence of yellow to orange, or grey, medium- to coarse-grained cross-bedded sand and subordinate gravel occurring in tabular beds mostly less than 0.5 m thick. The main clast lithologies are 'Bunter' pebbles, sandstone and flint. Granule to small pebble-size coaly material, which may include fragments of Jurassic jet (Sumbler, 1983), is additionally concentrated along foresets.

The sequence of beds in (Figure 46) suggests that the Shawell Gravel represents three sedimentary phases: the beds below 8 m were deposited by north-east-flowing currents and are disposed in a series of upward-coarsening cycles; those between 6 and 8 m show a similar transport direction but consist of laterally continuous upward-fining beds ((Plate 16)a, upper part); the upper part of the section, above 6 m, comprises a sequence of sands overlain by gravels deposited by currents flowing south-westwards.

Two principal sedimentary facies occur at Gibbet Lane (Bridge et al., 1992). The clast-supported gravel facies mainly comprises beds at the base and top of the section. Some of these are laterally extensive and internally are either massive or show clast imbrication, whereas other beds are more lenticular with bases channelised into the underlying deposits ((Plate 16)b). Many of the gravel beds are sharply capped by medium-grained plane- or cross-bedded sand which is largely devoid of pebbles. The cross-bedded sand facies is the most prevalent, although it also contains flint-rich gravelly material in thin beds or as basal lag deposits. Most sand beds have planar tops and bases, but there also occur trough cross-bedded sands with strongly channelised bases. In the middle to upper part of the section many beds fine upwards from pebbly bases to cross-bedded tops with only sporadic floating pebbles ((Plate 16)a). Planar cross-bedding is typically developed within the sand facies; in the cross-bedded sets, grain size sorting and size grading produce alternations between medium-grained, and coarse-grained pebbly, foreset beds; avalanche foresets may also be developed. Many sand beds show reactivation surfaces that define increments of alternately well-sorted, medium-grained sand, and poorly sorted, pebbly sand or gravel. Fine-grained sand is confined to subordinate layers which commonly show climbing-ripple cross-lamination.

Further exposures of the upper beds of the Shawell Gravel occurred by the edge of the copse [SP 5309 7988] north of Coton Farm, as follows:

Thickness m
Topsoil 0.5
Sandy clay, brown to grey, with fragments of chalk, flint and 'Bunter' pebbles 1.4
Gravel, brown, with clay matrix (base of Oadby Till) 0.3
Clayey sand, yellow, with lenticular bodies of cross-bedded gravelly sand; becoming finer grained upwards 1.0
Sand, brown, trough cross-bedded, poorly sorted 0.15
Gravel, yellow, with locally abundant coarse- grained sand matrix 0.55
Sand, brown, medium-grained with granule-conglomerate lenses 0.07
Gravel, yellow, with coarse-grained sand matrix, massive and poorly sorted. Pebbles of 'Bunter' quartz and quartzite, flint, Jurassic sandstone and limestone seen 0.54

Information from ditch sections [SP 5281 8203] previously exposed during operations within the now-backfilled Cotesbach Quarry has been made available by M G Sumbler, as follows. One exposure showed about 3 m of medium-grained sand overlain by 1.1 m of brown and grey mottled stoneless clay which in turn was succeeded by a brown-grey till with abundant flint, Jurassic limestone and chalk fragments (Oadby Till). A further section, recording the base of the Shawell Gravel, showed about 2 m of medium- to coarse-grained sand above 0.3 m of grey to brown laminated clay and silt. This was underlain by a 0.4 m minimum thickness of bluish grey, sheared clay with rare chalk clasts.

Depositional environment

The Shawell Gravel has been interpreted as the deposits of braided streams that formed part of a proximal outwash system established during a phase of melting of the Oadby Till ice (Sumbler, 1983). The variety and complexity of bedforms in the Shawell Gravel invite comparisons with modern braided river systems fed by glacial meltwaters (Smith, 1971). For example, the planar cross-bedded sands with avalanche foresets resemble sections through low-profile gravel or sand bars whereas the medium-grained sands with climbing-ripple cross-lamination are low-discharge, bar-top deposits. Other low-discharge deposits comprise the channelised, trough cross-bedded sands which represent periods of erosion of the bars by meandering stream channels. Reactivation surfaces, and rapid changes in sediment grade within the regime of a single depositional unit, are indicative of hydrodynamic fluctuations that perhaps were caused by alternate freezing and thawing in the source region. Foreset inclinations here and farther south (Old et al., 1987) indicate sediment transport to the north and north-east, but changing to south-west in the upper beds of the Gibbet Lane Quarry. The reversal may reflect movements within the Oadby Till ice front as it commenced its final advance across the region.

Oadby Till

The Oadby Till represents the youngest component of lodgement till in the Wolston Glacial Succession of this region. Although it is primarily composed of a grey-matrix diamicton of Chalk and Jurassic derivation, it also includes lenses of red till rich in Triassic material, particularly in the basal parts. As discussed earlier, it is inappropriate to separate these lithologies, and the name Oadby Till is used here in the sense of Rice (1963) for all tills, both grey and red, that overlie the Wolston Sand and Gravel or Wolston Clay.

The Oadby Till underlies much of the gently undulating plateau forming the watershed between the Anker, Soar and Avon (Smite Brook) drainage systems: maximum elevations of about 140 m above OD are attained north of Cloudesley Bush [SP 465 867] and near Tythe Farm [SP 486 810]. The base of the till is a gently undulating surface which over much of its western and northern outcrop maintains an elevation between 100 and 115 m above OD. The Oadby Till is generally between 10 and 20 m thick where not significantly eroded, with maximum provings of 17.7 m in a borehole at Wolvey Lodge Farm and 17.8 m in a motorway borehole for the M6 (No. 1295). It is thicker in the south-east, beneath the relatively undissected plateau between Churchover [SP 511 807] and Gibbet Hill [SP 528 807], where 30 to 35 m of the till are estimated to be present. Two separate sheets of chalky grey till can be recognised hereabouts, with the Shawell Gravel occurring in between (Carney, 1992b).

Although the base of the Oadby Till is usually a planar surface, in places it cuts down through the underlying Wolston Sand and Gravel to rest directly on Wolston Clay, as, for example, in the west of the outcrop at Ansty [SP 400 836] and just to the north of Higham on the Hill [SP 383 959]. Farther east such relationships are more accentuated, as between Cloudesley Bush and Willey [SP 497 847], where the Oadby Till fills a narrow trough that coincides with a major downflexure of the Wolston Sand and Gravel south of Wibtoft ((Figure 45)c). Resistivity surveys in this area suggest that the till is locally extremely thick, perhaps up to 40 m in the vicinity of Willey (Raines, 1992, fig. 13). A similar till-filled trough in the Wolston Sand and Gravel is indicated on the spur east-south-east of Newnham Paddox [SP 482 835]. In the structurally complex eastern outcrop a further anomalous relationship is shown by the slope on the base of the Oadby Till (Carney, 1992, fig. 8), which declines north-eastwards into the zone of glaciotectonic disturbance described below. Thus, from a level of 117 m OD near Wibtoft [SP 480 872], the base drops to 100 m above OD north of Claybrooke Parva [SP 491 895], eventually reaching only around 75 m above OD north of Sutton in the Elms [SP 520 945], where the outcrops are also discontinuous, reflecting glaciotectonic movements and/or anomalous drift deposition.

In the south-eastern part of the district, where the Wolston Sand and Gravel and Wolston Clay are absent, the chalk-rich Oadby Till rests directly upon typical red Thrussington Till. This relationship is observed in the Lutterworth Borehole, which proves that beneath the Shawell Gravel, the lower sheet of grey facies Oadby Till, 10.4 m thick, is underlain by 8 m of a red till, classified as Thrussington Till.

Grey chalky till

Good exposures of the grey, chalky facies of the Oadby Till occur in the Gibbet Lane Quarry [SP 5409 8061] on the eastern margin of the district. They show that the lower part of the till immediately above the Shawell Gravel has a massive matrix composed of a tough dark blue-grey clay. About 25 per cent of the till consists of erratics, of which only about 10 per cent are of cobble or boulder size and the rest are no larger than small pebbles or granules ((Plate 15), see p.161). The principal rock fragments are of chalk, flint, Jurassic limestone, Jurassic fossils such as Gryphaea arcuata and Dactylioceras, and 'Bunter' pebbles. Locally, rafts of the Shawell Gravel up to 2 m across are incorporated into the till, the matrix of which then becomes brown and sandy. In a temporary exposure by an ornamental pool at the Magna Park Industrial Estate [SP 5170 8460], chalky Oadby Till has a slightly weathered grey to brown matrix; it contains cobble- and granule-size clasts of a similar type to those listed above. Narrow bodies of red, medium-grained sand form contorted rafts and pipe-like structures within the till matrix. In the north of the area, an excavation in Burbage [SP 4444 9258] showed a grey clay with erratics of chalk, grey fissile mudstone, angular quartzite, weathered pale yellow quartzite, ironstone, shelly Jurassic limestone, Jurassic fossils and a little flint (Old, 1990).

Brash in fields underlain by the grey till facies consists of 'Bunter' pebbles and flints, but chalk is seldom seen as it usually dissolves in the near-surface weathered zone. Other rock fragments observed in fields between Wibtoft and Cloudesley Bush [SP 47 87] include diorite and lamprophyre, Lower Cambrian sandstone and Carboniferous limestone.

Red Trias-derived till

These tills are not differentiated on the 1:50 000 Sheet but details of their distribution can be found on the unpublished set of 1:10 000 maps. They principally occur in association with grey tills in the Oadby Till outcrop north-east of a line between Hinckley, Burbage and Lut-

terworth, and are also described from farther north in the Coalville district (Douglas, 1980; Worssam and Old, 1987). In a second occurrence farther south, red till bodies are commonly encountered within a belt 4 to 5 km wide extending southwards from Newnham Lodge Farm [SP 4800 8490] to Harborough Magna [SP 478 792]. The elongation of this outcrop is parallel to a further north–south belt of mixed red and grey tills, located east of the district, between Lutterworth and Swinford. It is noteworthy that in their description of that area, Poole et al. (1968) do not mention red tills occurring farther than about 4 to 5 km east of the Coventry district, the approximate limit being an arcuate, north–south-trending front between Swinford, Gilmorton and Kilby.

The red tills typically form discontinuous masses at the base of the enclosing grey facies of Oadby Till. Those occurring near to the M6 south of Pailton have planar bases, continuous with the base of the surrounding grey till, but are markedly convex upwards (Figure 44). Individual lenses may attain considerable thicknesses, the maximum proven being 15 m in the Motorway Borehole No. 1295, and on the plateau nearby the red tills commonly protrude through the surrounding grey till to form irregular outcrops. The opposite relationship holds in the area north of Pailton, where the junction between Oadby Till and Wolston Sand and Gravel is highly irregular. As shown due east of Newnham Paddox [SP 483 834], the bodies of red till hereabouts have flat tops and concave bases, filling depressions in the top of the Wolston Sand and Gravel.

In the northern outcrops of mixed Oadby Till which are affected by glaciotectonic disturbance ((Figure 45)a and b), the red tills again occur at the base of the grey facies, although it is usually difficult to be sure of the original order of superposition prior to disturbance of the succession.

The red facies of Oadby Till was not exposed during this survey. Auger samples typically show a matrix of tough, chocolate-brown, red-brown or maroon clay, which changes to a bright yellow laminated clay in the weathering zone. Commonly this matrix is silty, or may contain layers of pink to red sand. Inclusions within the clay consist of small lenses of friable, crimson red 'millet seed' sand of Triassic derivation, together with slivers of pale green or grey siltstone and marly siltstone derived from Triassic skerries. Other larger fragments are of 'Bunter' quartz and quartzite, with shelly Jurassic limestone and chalk locally present in some tills. Many of the red till bodies appear homogeneous, but inclusions of grey chalky till were found by augering east of Newnham Paddox [SP 4817 8350]. This recalls the situation encountered in the Market Harborough district, where the red till facies commonly overlies grey chalky till (e.g. Poole et al., 1968, figs. 7 and 8), but is in some places interleaved with it.

Depositional environment

The Oadby Till is interpreted to be the lodgement deposits of the final ice advance across the region (e.g. Rice, 1968). The involvement of at least two separate ice sheets is suggested by the till fabrics developed to the north of the Coventry district: they show that the grey till was deposited by ice advancing from the north or north-north-east (Rice, 1981), whereas the red or 'Pennine' tills, basal to the grey tills, indicate an advance of ice from the north-west (Douglas, 1980). The intricate and close association between red and grey tills observed in this district, and elsewhere (Poole et al., 1968), is therefore interpreted as due to the interaction between separate but closely contemporaneous ice-sheets carrying north-westerly derived (red) and north-easterly derived (grey) material. That there may have been an earlier advance of the latter into the area, before glaciolacustrine sedimentation had ceased, is suggested by Shotton (1953) and Sumbler (1983) who note that the Oadby Till replaces laterally the Wolston Clay near Rugby. The debris from this earlier advance is possibly represented by the lower of the two Oadby till sheets, which lies beneath the Shawell Gravel in the south-east of the Coventry district.

Deposition of the Oadby Till was accompanied by significant erosion and glaciotectonic disruption of the preexisting glacial succession. The dynamic setting of this deformation provides a further insight into the depositional environment of the Oadby Till, as explained in the following section.

Glaciotectonic disturbances

Structures attributed to glaciotectonic deformation were described by Rice (1981) from two former exposures in the local drift sequence. In the quarry at Dunton Bassett, just outside the eastern boundary of the Coventry district (about 1.1 km due east of Leire), beds of the Thrussington Till, Wolston Clay and Wolston Sand and Gravel are thrown into asymmetrical folds and the whole sequence is repeated along low-angle thrust planes. Keels of Oadby Till within the downfolds contain a crude bedding, parallel to that in the underlying tectonised beds, proving not only that the Oadby Till ice sheet was involved in the deformation, but also that the ice must have advanced across the area before most of the tectonism occurred (Rice, 1981, fig. 7). A second glaciotectonic occurrence was demonstrated at excavations for the M69 motorway where it crosses the A5070 road west of Sapcote [SP 464 938]; it showed deeper structural sections, with slabs of Mercia Mudstone bedrock interleaved between Thrussington Till and Wolston (Bosworth) Clay in the manner of a series of imbricate thrust slices (Rice, 1981, fig. 9). At both of these localities, Rice (1981) deduced from the vergence of the fold and thrust structures that the direction of tectonic transport was from the north or north-north-east. This also gives the main movement direction of the Oadby Till ice.

Although no zones of disturbance were exposed during the present survey, their existence can be inferred from the highly anomalous outcrop patterns of the various drift members, particularly in the north-east of this district. The field relationships suggest that glaciotectonic deformation has operated at both a regional and a local level within the Wolston Glacial Succession.

Regional-scale deformation of the drift sequence is indicated by fluctuations in the elevation of the top surface of the Bosworth tongue of the Wolston Clay ((Figure 45)c). Shotton (1953, fig. 8) was first to show that this was a highly undulatory surface, but suggested that it had become so before the Woiston Sand and Gravel was deposited. It now appears, however, that thickening trends in the Woiston Sand and Gravel are independent of the undulations (compare (Figure 45)b and c), leaving tectonic deformation of an original near-planar contact as the more likely cause. The principal structures comprise a series of domes and basins developed in a zone trending north-west through Wibtoft, Burbage and Hinckley.

The main area of glaciotectonic disturbance (Figure 45)c, situated immediately north-east of the zone of undulations, shows a different structural style involving a more localised and intense type of deformation. It includes the ground intervening between the known occurrences of glaciotectonic folding and thrusting at Sapcote and Dunton Bassett. There are no current exposures in this area, but the extent of the deformation is recognised by the highly anomalous outcrop patterns of the drift members. A particularly pronounced zone of linear outcrops ((Figure 45)c) coincides with the north-draining tributary valleys of the Soar between Wibtoft [SP 481 873] and Claybrooke Parva [SP 493 880] : it consists of northerly orientated slivers of Oadby Till and Wolston Clay separated by Wolston Sand and Gravel, and is interpreted as an imbricated fold and fault complex. This linear zone curves to the northwest, shown by the outcrops around Fields Farm [SP 470 935], and extends in that direction to include the locality west of Sapcote where glaciotectonic thrusting was demonstrated by Rice (1981). To the south and west of the linear zone the outcrops more resemble the regional pattern, as illustrated by the circular outcrop of Wolston Clay [SP 475 893] south-east of Wigston Parva. The anticlinal folding of the Wolston Clay and Wolston Sand and Gravel described by Old (1990) in former exposures at the old brickworks at Ashby Road near Hinckley [SP 432 949] may be an example of this type of deformation.

No detailed analysis of these glaciotectonic phenomena has been undertaken, but it is suggested that the gentle regional-scale folding, coinciding as it does with thicker developments of the Wolston Clay (compare (Figure 45)a and c), is due to differential movements within this highly compressible material as it became compacted beneath, or squeezed along in front of, the main burden of the Oadby Till ice. In the zone of intense disturbance to the north-east, all of the glacial deposits have apparently been attenuated and depressed beneath the Oadby Till, the base of which is some 30 to 40 m lower than it is farther west. In such a subglacial setting, the drift sequence, which evidently was not in a frozen condition beneath the ice, may have been squeezed and extruded upwards along meltwater-lubricated sand layers that became zones of thrusting and folding at the ice front, forming linear features reminiscent of push-ridges (e.g. Croot, 1987). It is noteworthy that this area lies at the south-western termination of the Narborough Furrow (Rice, 1981, fig. 10), a major valley feature developed in the rockhead that is believed to have resulted from processes of scouring and glacigenic deposition in the sub-glacial environment.

Western Glacial Drift

Glacial deposits, consisting mainly of till but also including waterlaid sands, silts and clays, cover much of the Upper Carboniferous outcrop in the west of the district (Figure 42). Rockhead contours published for the area to the south of gridline 90 (Old et al., 1991) show the larger spreads of drift to be about 5 m thick, though some reach 10 m locally. Thicknesses of over 15 m proved in boreholes around Moat House Farm (e.g. Motorway Borehole No. 1077) are exceptional.

Till

Till forms the bulk of the glacial deposits and encloses, or in places is underlain by, impersistent beds of sand and gravel or laminated clay. The deposits occur as extensive remnants of what was probably a continuous drift plateau. The predominant lithology is a brown or reddish brown sandy clay with a variety of exotic and locally derived erratics. 'Bunter'pebbles and clasts of Meriden Formation sandstones and siltstones are ubiquitous. In most areas the till also contains a high proportion of brown-weathering blocks of Cambrian quartzite (Hartshill Sandstone Formation). Other common clasts include Cambrian mudstones, also probably derived from the Nuneaton Inlier, and skerry sandstone and siltstone fragments from the Mercia Mudstone Group. The distribution of clasts suggests that the till was deposited at the same time as the Thrussington Till, by ice advancing from the north or north-west.

In the Fillongley area a lower clay-rich, dark brown till is overlain by much more sandy and stony till in which sand and gravel sheets are common. Good examples of the latter occur on the plateau near New Arley [SP 293 892] and to the east of Arley Hall Farm [SP 273 903]. Ditch sections in these areas show gravels cryoturbated into the upper surface of the till sheet.

Around the ornamental pools [SP 3289 8920] to the west of Arbury Hall, the till is characterised by an abundance of chalk and flints in a grey or brown clay matrix. This represents one of the most westerly occurrences of this facies to be found in the district.

Details

Sections in the till are rare and tend to be transitory. Excavations for the coal plant at Coventry Colliery [SP 3203 8443] revealed:

Thickness m
Till: red-brown sandy clay becoming grey-brown with depth; erratics are mainly 'Bunter' pebbles with some Upper Carboniferous red sandstone, chalk and coal 1.5
Silt and silty sand, red-brown, weakly bedded 0.07
Till: red-brown highly calcareous clay with chalk pebbles and calcareous concretions, together with Upper Carboniferous sandstone, angular quartzite and ‘Bunter’ pebbles 0.6
Sand, red-brown silty and pebbly 0.2
Till: red-brown sandy clay with ‘Bunter’ pebbles; slightly calcareous  seen 0.3

The lowest 2 m of the excavation was unsafe to enter, but appeared to be in till resting on sandstone as the base.

The complexity of parts of the main till sheet is further illustrated by the Windmill House Farm and Ley's Farm boreholes, to the north-west of Allesley (Rees, 1989). These proved a sequence of silty and clayey tills, interbedded with laminated, stone-free silts and sands. Probably most of the sequence was waterlaid.

Glaciolacustrine deposits

Silts and clays, which are generally stoneless, occur sporadically at the base and also at the top of the main till sheet. The deposits are locally associated with sands and gravels, and appear to have been laid down in transient lakes that formed near the ice margin. The beds range in colour between whitish yellow and dark chocolate-brown, and the finer-graded material tends to be laminated. The deposits are rarely separable from the main till sheet, but discrete outcrops have been mapped around Wood End [SP 291 879], on the interfluve [SP 287 830] west of Hawkes End, and around Flint's Green [SP 266 801]. The occurrences appear to be unrelated to the main development of the Wolston Clay to the east, the surface of which is some 35 m lower in elevation.

Glaciofluvial deposits

Sand and gravel deposits are recorded at many sites but are in most cases laterally restricted and, as such, are not readily mappable. Some larger spreads are found in the urban areas of north Coventry and Bedworth, and to the east of Fillongley. Most of these deposits are known only from site investigation boreholes, and their precise distribution and affinity are uncertain. Some clearly underlie the till sheet, as at Tile Hill [SP 280 780], whereas others close to the eastern margin of the plateau [as at [SP 356 870] form outliers, and may correlate with either the Wolston Sand and Gravel or the Dunsmore Gravel (see below).

Details

One of the largest spreads of sand and gravel lies between Foleshill [SP 351 821] and Foxford [SP 353 836]. Trial pits in the grounds of Foxford School [SP 3548 8402] proved 1.9 m of poorly sorted cobble gravel (not bottomed), beneath a thin till cover. The gravel comprised blocks, up to 0.5 m across, of quartzite, sandstone and shale. Similar poorly sorted material has been recorded from the cemetery off Windmill Road [SP 3502 8280]. Farther to the north, the deposit is concealed beneath thicker till but can be traced in boreholes; one at the Coventry Canal–M6 crossing (Motorway Borehole No. 188) proved 9.1 m of sand and gravel, and another at Hawkesbury Lane Station passed from till into 8.8 m of sand and gravel. Rockhead contours suggest that the deposits hereabouts are channel infills, deposited by meltwater streams draining from the north, off the Nuneaton Inlier.

Other occurrences of sand and gravel, in a similar stratigraphical position but at a slightly lower elevation, crop out along the valley sides south of Potter's Green [SP 374820], and are proved in site investigation holes for the Manor Farm estate [SP 369 811].

Farther south. at Tile Hill, sand and gravel, probably not exceeding 3 m, crops out beneath till over a distance of about 3 km. The deposit is generally rather clayey, and includes laminated sands and silts. A similar association of lithologies was found to the west of Hawkes End, though this complex also includes red sands, which may have been derived from breakdown of the local bedrock.

Several spreads of sand and gravel form sheets interstratified with, or as a superficial capping to, till. These occur mainly in the Fillongley area and are mostly composed of variably sorted pale yellow or yellow-orange sands, as near Fillongley Grange [SP 295 870].

In a separate outlier at Bedworth, boreholes sited opposite the council offices (e.g. Library Borehole No. 1) proved up to 8.2 m of brown silty sand with layers of rounded to subangular gravel and laminated clay. The deposit lies close to the western boundary of the Wolston Glacial Succession and could legitimately be referred to either the Wolston Sand and Gravel or the Dunsmore Gravel, on the basis of its elevation. However, in the absence of other more definitive information, the deposit has been included with the other unclassified glaciofluvial deposits.

Late-glacial to postglacial deposits

The period of postglacial time represented in this district is believed to comprise the Hoxnian, Wolstonian, Ipswichian, Devensian and Flandrian stages. There was at least one cold period (the late Devensian) when ice sheets advanced again into areas not far to the north and periglacial conditions prevailed in the district. The status of the other cold phase (Wolstonian) is still in doubt.

Apart from small areas where solifluction ( 'head') deposits have accumulated, the late- and postglacial evolution of the district has mainly involved development of the (upper) Avon and Soar drainage systems. The forerunners to these rivers were glaciofluvial streams that established the main drainage lines as the last of the ice was melting. In the period that has since elapsed, these rivers have continued to evolve by a combination of terrace deposition and downcutting so that now they flow at levels some 50 m below the plateau surface. The river courses are independent of the rockhead configuration, and it is noteworthy that the modern River Avon flows in a reverse direction to that taken by the protoSoar, which drained the equivalent area in preglacial times. In the valley bottoms alluvium and peat constitute the most recent deposits.

Dunsmore Gravel

The name Dunsmore Gravel was applied by Shotton (1953) in the Warwick district to the ochreous, clayey gravels overlying the 'Wolston Series' drift succession. The notion that these deposits did not occur north of the River Avon (Shotton, 1976) was subsequently disproved (Sumbler, 1983), and the present survey has shown very extensive spreads of this material unconformably capping the Wolston Glacial Succession in the Coventry district. The equivalent deposits in the Coalville district are called 'flinty gravel' (Worssam and Old, 1988).

The principal outcrop of the Dunsmore Gravel underlies a dissected plateau at about 125 m above OD, extending from near Bulkington eastwards to High Cross (Figure 47). Many smaller outliers are scattered throughout the district, indicating that originally these deposits were more widespread. The Dunsmore Gravel may be up to 15 m thick on the undissected plateau to the north of Wolvey [SP 439 892], but elsewhere it probably averages between 5 and 10 m. The maximum proving is 7.8 m in a borehole near the junction of Wolvey and Rugby Road in Bulkington (Bulkington Sewerage No. 5).

The base of the Dunsmore Gravel is irregular and erosive; it is seen to cut down through the glacial sequence and into the Thrussington Till around Bulking-ton [SP 386 866], and to truncate contacts within the glaciotectonically disturbed drift between Wigston Parva and High Cross [SP 466 892] . A structure contour diagram (Figure 47) shows that the basal surface is sculpted into broad channels and valleys whose alignment coincides with the present courses of the Soar and Anker drainage, suggesting, as in the Warwick district (Old et al., 1987), that the modern river systems date from the time that the Dunsmore Gravel was laid down. Particularly good examples of channels filled by Dunsmore Gravel have been mapped around Bedworth [SP 383 867], and higher up on the plateau near Willey [SP 499 843] to [SP 500 857]. Features in the Dunsmore Gravel resembling terrace remnants are also indicated by the structure contours: they suggest an old terrace remnant at 125 to 135 m above OD, represented by the outliers around and to the south of High Cross, and a younger and more extensive terrace comprising the main outcrop farther to the north-west; the base of the latter, between 110 and 115 m above OD, slopes towards the present Anker drainage system. A further perched terrace remnant, with a base between 115 and 120 m above OD, comprises the series of small gravel outliers along the south side of the Smite Brook east of Pailton [SP 500 838].

Details

The Dunsmore Gravel is very poorly exposed in this district. A section through its base, in a temporary exposure on the western side of the River Anker south of the M69 [SP 4176 8819], showed 0.7 m of a red coarse-grained structureless sand, very poorly sorted, with 'floating' pebbles and cobbles of 'Bunter' quartz and flint. This rested on a red structureless diamicton, between 0.5 and 1 m thick, consisting of red mudstone slivers, aggregates of yellow friable sand, and pebbles of 'Bunter' derivation, flint and chalk. The diamicton rested on a highly irregular erosion surface cut into the underlying Wolston Sand and Gravel, and is interpreted as a solifluction deposit that mantled the floor of a palaeovalley later filled by the Dunsmore Gravel; the erosional contact sloped eastwards, towards the present Anker River.

Excavations at New Buildings in Hinckley [SP 4300 9412] revealed 1.8 m of brown, ochreous, very clayey sand and gravel, with flint, 'Bunter' pebbles, and Triassic sandstone and chalk pebbles; boreholes on this site proved 7.5 m of this deposit overlying the Oadby Till. Nearby, a flint-rich clayey sand and gravel, called 'Hinckley gravel' by Eastwood et al. (1923, p.111) was formerly dug near Friary Close [SP 4321 9402]. Further temporary sections at the Island Hotel [SP 4389 9087] showed about 2 m of very variable, ferruginous pebbly sand with thin irregular interbeds of pale brown clay and lenses of sand. The clasts included 'Bunter' pebbles, flint, weathered diorite and pale grey sandstone (Old, 1990).

Other deposits correlated with the Dunsmore Gravel infill a broad valley feature developed on the surface of the upper Oadby Till sheet in the Gibbet Lane Quarry, near Shawell [SP 5409 8061]. The basal bed, filling the central part of the valley and pinching out to either side, is an orange-brown, very poorly sorted, matrix-supported cobble conglomerate, possibly a debris-flow deposit. It is succeeded by 0.4 m of brown to yellow, poorly sorted, medium-grained pebbly sand with coaly lenticles. The upper bed, at least 1 m thick, grades from poorly sorted matrix-supported cobble conglomerate at the base to a medium-grained silty sand at the top.

Depositional environment

The high contents of flint, of clay-rich gravel and of very coarse-grained poorly sorted sand are in keeping with the suggestion of Shotton (1953) that the Dunsmore Gravel represents outwash from the last remnants of the Oadby Till ice. These deposits do not appear to have formed extensive sandurs or braided outwash plains, however, but more closely resemble the remnants of terraces related to the initiation of the modern Soar and Anker drainage systems. A revised model is therefore suggested, whereby the Dunsmore Gravel formed within

an immediately postglacial, or 'paraglaciar, cycle of sedimentation (process discussed by Church and Ryder, 1972). This involved initial formation of the older and higher terrace gravels, perhaps deposited from extensive and shallow aggradational meltwater systems that exploited irregularities on the Oadby Till plateau. Then, after further ice wastage and regional drainage readjustments, base levels were lowered and the valleys incised more deeply, resulting in the erosion and cannibalisation of the earlier deposits and their incorporation into systems of younger and lower terraces.

In keeping with the above model is the observation that in the Warwick district the surface of the Dunsmore Gravel appears to grade along the Avon valley into that of the 'Fifth terrace' (Tomlinson, 1925, fig. 2), the highest terrace of that particular system. Recent studies suggest that the deposits of Terrace 5, which have a temperate fauna (Whitehead, 1989), may correlate with Oxygen Isotope Stage 9. The implications of this for dating the underlying Wolston Glacial Succession are discussed by Maddy et al. (1991).

Anker Sand and Gravel

This new name is applied to sand and gravel deposits along the valley of the River Anker north-west of Bedworth. The deposits form a series of disconnected outcrops, resembling eroded terraces, and generally occur at less than 95 m above OD. For the most part they rest on bedrock, but some of the higher outliers between Nuneaton and Hinckley overlie Thrussington Till and Wolston Clay, demonstrating unconformity with the Wolston Glacial Succession. The Anker Sand and Gravel is overlain locally by more recent terrace and alluvial deposits of the present river.

These deposits appear to be very heterogeneous, and are not unlike the Dunsmore Gravel. A temporary section close to Wem Brook in Nuneaton [SP 364 908] showed:

Thickness m
Sandy gravel, red-brown to yellow 1.1
Sand, red, well-sorted 0.3
Sandstone (Bromsgrove Sandstone)

Elsewhere the nature of the Anker Sand and Gravel is revealed only by augering. The deposits in the vicinity of Marston Junction [SP 367 891] consist of red, fine-grained sand with scattered flints and 'Bunter' pebbles; they are difficult to distinguish from weathered Bromsgrove Sandstone and possibly were in part derived from this formation (Bridge, 1991). In the north-west, the terrace-like outcrops on either side of the River Anker consist of grey silty loam, with pockets of red sandy loam, containing rounded pebbles and cobbles together with subangular cobbles of a more local derivation. They range from matrix-supported to clast-supported types, the latter with pebbles and cobbles embedded in a silt matrix; flint is rare (Baldock, 1991a, 1991b).

The Anker Sand and Gravel closely resembles the Dunsmore Gravel, both in composition and in its mode of occurrence as terrace-like deposits related to the modern Anker valley (Figure 47). This similarity suggests that it was deposited towards the end of the same post-glacial fluvial cycle, after a further lowering of base levels had caused stream incision and erosion of the Dunsmore Gravel higher up the valley. The high content of angular, locally derived rock fragments is a distinguishing feature of the Anker deposits, and perhaps suggests that the valley received a significant additional contribution of detritus from the mass-movement of till or head material down the nearby steep eastern slopes of the Nuneaton Inlier.

Head

Head consists of weathered near-surface bedrock or drift material that has become mobilised during repeated episodes of freezing and thawing, and caused to move downslope by solifluction processes. Head deposits formed in the period immediately following retreat of the ice, when cold periglacial conditions would still have obtained, and again during later cold stages. In more recent times, further downslope surface movement and concentration of such debris has taken place, mainly by gravitational creep and downwashing.

Head is probably under-represented on the 1:50 000 geological map, and there are likely to be thin deposits of sandy wash on most slopes underlain by Upper Car boniferous sandstones, with thicker accumulations at concave breaks of slope.

Red silty sands, classified as head, occur in several minor valleys in the north-west of the district, and a more sizeable deposit is recorded in the east of the district, on the lower western slopes of Smite Brook valley [SP 464 834]. It consists of pebbles, cobbles and boulders of local derivation embedded in a matrix of yellow to brown silty and clayey sand.

River terrace deposits

River terraces are most common along the valleys of the Sowe, Anker and Soar, where there has been the greatest amount of postglacial erosion. All are broadly similar in lithology, being composed of gravelly sand and silt with 'Bunter' pebbles and flints. The numbering of the terraces along the River Sowe is based on a scheme adopted for the Avon (Old et al., 1987); in the other river basins a wider classification has not been attempted, and the numbering of the terraces is only of local significance. Some of the higher terraces may represent aggradations dating from pre-Ipswichian times, but most are either late Devensian or Flandrian in age.

Fourth Terrace

Small gravelly flats, correlated with the Avon Fourth Terrace, occur at heights of about 15 m above the flood-plain on either side of the River Sowe. Spreads on the south-east flank of Walsgrave Hill are conspicuously flinty and form low features overlapping on to till or Baginton Sand and Gravel. An associated thin patch of clayey pebbly sand capping the col [SP 399 810] to the north of Hillfields Farm is also designated as Fourth Terrace, though it is somewhat higher than the surrounding terrace deposits.

Based on evidence from the lower Avon, to the southwest, the Fourth Terrace deposits are known to be the oldest, and are considered to represent a cold stage aggradation following an undefined interglacial between the Hoxnian and Ipswichian stages (Bridgland et al., 1989).

Third Terrace

A spread of flinty gravel, tentatively correlated with the Avon Third Terrace, has been mapped in the Wyken area [SP 374 806]. The terrace flat lies about 8 m above the River Sowe floodplain and was identified as an area of sand and gravel during the primary survey of the district. The back of the terrace is indistinct, and thin gravels . noted in the adjoining school playing fields [SP 372 804] extend beyond the mapped feature.

No other terraces have been mapped at this level in the district, but an excavation [SP 3235 7945] described by Shotton (1929, p.212, 1953, pp.231–232) from the valley of the River Sherbourne in Coventry revealed about 5 m of sand in a bedrock channel, and yielded a skull of Hippopotamus. This site is now totally obscured by factory development.

Faunal evidence obtained from deposits underlying the Third Terrace in the lower Avon (Tomlinson, 1925; Bridgland et al., 1989) suggests that a temperate (Ipswichian) episode is represented in the early part of this aggradation.

Second Terrace

Narrow tracts of Second Terrace gravels occur on both sides of the River Sowe downstream of Foleshill. The deposits lie between 3 and 6 m above the floodplain but generally lack a well-developed terrace form, commonly sloping gently towards the river. Possibly the second terrace bench may have been destroyed during later downcutting. Although no sections are now visible, a former gravel pit [SP 363 811] near Bell Green showed 1.5 m of sandy gravel with 'Bunter' pebbles and rare flints. In a separate deposit nearby [SP 360 821], site investigation boreholes for the shopping precinct at Bell Green proved up to 3.6 m of fine to coarse gravel, with some cobbles. Cold climate mammalian faunas occur in the Second Terrace deposits downstream from the district (Tomlinson, 1925), and subarctic insect faunas have been recovered from Second Terrace deposits near Brandon [SP 390 754] (Coope, 1968). Associated peats have yielded a mid-Devensian radiocarbon date of about 30 000 years BP (Shotton, 1968, p.388).

Deposits mapped as undivided First and Second terrace are present along both banks of the River Soar. They form a bench 1 to 2 m above the floodplain and, in auger samples, consist of brown clayey silt and sand with layers of dark red sand and gravel.

First Terrace

First Terrace deposits occur along the River Sowe downstream of Wood End [SP 362 825]; along the River Anker and its tributary, Wem Brook; and (combined with Second Terrace deposits) along the River Soar below Stoney Bridge [SP 503 930]. The terraces lie 1 to 2 m above the adjacent floodplain, and their deposits, generally less than 3 m thick, consist of gravel, sand, silt and clay in varying proportions.

In the Sowe valley the effects of urban development, coupled with landfill tipping, have largely obscured the true form of these deposits. A borehole (Sowe Valley Sewer B1) drilled in a small outcrop of First Terrace floodplain at Stoke, proved 10.7 m of sand and gravel (not bottomed). Adjacent boreholes confirm that gravel here infills a local depression or channel extending well below the base of the modern alluvium. The First Terrace deposits in the Anker valley lie some 1 to 1.5 m above the modern alluvium, and consist of fine- to coarse-grained, yellow to brownish red sand, with some small exotic and local pebbles, including numerous flints.

There is no direct evidence for the age of the First Terrace deposits in this area, but they are probably late Devensian or early Flandrian.

Alluvium

Alluvial deposits occur in all but the smallest valleys. Their composition reflects local sources, both solid and drift, and even in the largest valleys they rarely exceed 6 m in thickness. The deposits are commonly divided into an upper bed of clay and silt, with scattered pebbles, underlain by sand and gravel. In parts of the Anker valley as at [SP 344 954] soft grey clays occur marginal to the modern stream alluvium. These contain little sand or silt and were probably deposited in small temporarily dammed lakes. Along the Sowe valley large tracts of the floodplain have been artificially built up, and the river has been straightened, to combat the effects of mining subsidence and to prevent flooding.

Peat

Deposits of peat or organic-rich clay have formed along zones of groundwater seepage within the drift. These are preferentially located where glacial sands and gravels overlie glacial clay or abut against clayey alluvium. Several small outcrops occur in the valleys of streams draining into the rivers Soar and Anker. In a tributary draining north-eastwards from Copston Magna the deposits form convex, wedge-like bodies at the junction between the river alluvium and the Wolston Sand and Gravel, and wider spreads cover the floor of the valley [as at [SP 4594 8934]. Hummocky peat deposits, higher on the valley side, may have formed during an earlier cycle of stream incision. Other similar examples are seen in the Anker valley north and west of Wolvey.

Peat is discontinuously developed at the springline formed between the Wolston Sand and Gravel and the underlying Wolston Clay [as at [SP 4076 8404] but is seldom thick enough to warrant delineation on the map.

An isolated area of peat and peaty clay [SP 525 947] near the water treatment works at Sutton in the Elms rests on alluvium and may represent material deposited in an abandoned stream meander.

Chapter 11 Regional metamorphism of the pre-Devonian rocks

In this section, R J Merriman comments on the assemblages of secondary minerals developed in the Precambrian, Lower Cambrian and late Ordovician (intrusive) rocks, and their bearing on the pre-Upper Devonian thermal history of this district.

The secondary alteration of primary minerals in the Precambrian rocks is consistent with the prehnitepumpellyite facies of regional very low-grade metamorphism. An equivalent grade of metamorphism is indicated by the lower anchizonal white mica crystallinity index of 0.370°20 obtained from a tuffaceous mudstone in the Caldecote Volcanic Formation. Alteration temperatures of 240 ± 20°C are indicated from chlorite (pycnochlorite) compositions, using the geothermometer of Cathelineau and Nieva (1985), and are within the range expected for the prehnite-pumpellyite facies (Bevins et al., 1991). Regional metamorphism of the Late Proterozoic basement is inferred to have occurred prior to deposition of Lower Palaeozoic strata, and may have been related to a late Avalonian orogenic event (Merriman et al., 1993). Although the Precambrian rocks of the Nuneaton Inlier were uplifted and subsequently buried again during late Ordovician basin development (see Chapter 12), it is unlikely that they suffered further significant alteration at the low temperatures that prevailed.

Lower Cambrian mudstones from the Hartshill Sandstone Formation of the Nuneaton Inlier, sampled from between sandstone beds in the Hartshill Quarry, gave white mica crystallinity values indicative of the late diagenetic grade. The mudstones also show some minor recrystallisation within about one metre of a lamprophyre sill belonging to the Midlands Minor Intrusive Suite. In contrast, samples of Tremadoc mudstone from the Stockingford Shale Group, collected from boreholes penetrating beneath the Carboniferous strata of the Warwickshire Coalfield, show late diagenetic to lower anchizonal metamorphic grades, higher than those in the outcrop rocks of the Nuneaton Inlier.

The secondary alteration of primary igneous minerals in rocks of the late Ordovician Midlands Minor Intrusive Suite is the result of very low-grade regional metamorphism. Although prehnite was not identified in these rocks, the occurrence of pumpellyite with epidote in the diorite sills in Mancetter Quarry, and of pumpellyite with minor actinolite in the poikilitic hornblende meladiorites in Griff No. 4 Quarry, is consistent with prehnitepumpellyite facies alteration. Chlorite compositions (determined by electron microprobe analysis, EMPA), range from diabantite to pycnochlorite and show very low concentrations of Ca, Na or K, typical of evolved chlorites in the prehnite-pumpellyite facies (Bevins et al., 1991). Using the geothermometer of Cathelineau and Nieva (1985), the chlorite compositions indicate that alteration of the Midlands Minor Intrusive Suite occurred at temperatures of 130 to 260°C, which broadly equates with the transition from late diagenetic to lower anchizonal conditions in metapelitic rocks.

Mineralogical data derived largely from borehole samples have enabled Merriman et al. (1993) to construct a contoured metamorphic map for the Precambrian and Palaeozoic rocks of central and eastern England. In relation to the Cambro-Ordovician sequence of the Coventry district, this shows that the higher-grade (late diagenetic to lower anchizonal) rocks occur within a narrow belt, broadly corresponding to the area occupied by the Warwickshire Coalfield, but which also extends south-eastwards, to Aylesbury. The rocks of the Nuneaton Inlier, with lower metamorphic grades, form part of the north-eastern margin of this belt; the western margin, which is poorly defined, was taken at the Western Boundary Fault.

Although these studies are still at an early stage, requiring more data to refine the boundaries between the areas of differing metamorphic grade, they nevertheless have important implications for unravelling the tectonic and stratigraphical evolution of the Coventry district during the period between the Ordovician and late Devonian, for which no sedimentary record is preserved. The low temperatures at which the secondary minerals developed, and absence of any penetrative cleavage or schistosity, are features typical of terrains affected by burial metamorphism (e.g. Winkler, 1979, p.5), where the degree of secondary recrystallisation at a given level within the succession is determined primarily by the thickness of the overlying rocks. With such a model, it can be suggested that the Tremadoc rocks with raised metamorphic grades beneath the Warwickshire Coalfield may originally have been buried beneath an overburden thickness of at least 5.5 km, and possibly as much as about 7 km. These thicknesses, estimated on the basis of a metamorphic field gradient of 36°C/km-1, are consistent with the pattern of regional metamorphism deduced for concealed strata in other parts of the Midlands Microcraton (Merriman et al., 1993), and are interpreted to have developed within an elongated sedimentary basin whose north-western extremity lay in the area now occupied by the Warwickshire Coalfield. The basin was subsequently inverted, and the overburden largely eroded away, presumably during the Caledonian phases of uplift in pre-late Devonian times.

Chapter 12 Structure

Structure

The structure of the Coventry district has been worked out by combining the results of surface mapping with data from near-surface investigations and from geophysical surveys. Because most of the main rock units are represented at outcrop, a virtually complete sequential structural evolution, from Precambrian 'basement' through Palaeozoic and Mesozoic 'cover' rocks, can be demonstrated. The sequence of events is summarised in (Table 12).

The district can be divided into two structural provinces (Figure 48): the Coventry Horst in the west includes the outcropping Palaeozoic and Precambrian rocks of the Warwickshire Coalfield and Nuneaton Inlier; the Hinckley Basin, farther east, is a shallow Mesozoic basin, one of a series of pull-apart basins that developed throughout the central England region between Permian and Cretaceous times (Whittaker, 1985). The margins of the Coventry Horst are defined by the north-westerly trending Polesworth Fault, crossing the central part of the area, and the northerly aligned Western Boundary Fault which lies just outside the western edge of the Coventry district and separates the Coventry Horst from another Triassic pull-apart structure, the Knowle Basin.

The Coventry Horst is a prominent structure in the north-west, where it remains a major element of present-day topography. It is less well defined along its south-east margin, where the Polesworth Fault is overstepped by Triassic strata and the bedrock mantled by drift.

Regional structural setting

The pattern of geophysical lineaments that has been established for the central England region by Lee et al. (1990) provides a framework for considering the tectonic evolution of the Coventry area. It shows that this part of the Midlands is located at the junction between two converging basement domains, respectively showing north-westerly and northerly structural 'grains'. The north-westerly basement trend, revealed at the surface by the Polesworth Fault (gravity lineament 9G of Lee et al., 1990) reflects the orientation of fold axes within Charnian rocks along the edge of the Midlands Microcraton and is also parallel to the Thringstone Fault (lineament 21G) which lies close to the eastern margin of the microcraton; the latter fault, in turn, is parallel with the north-westerly trend of 'eastern Caledonide' structures that lie concealed beneath younger rocks to the north-east (Pharaoh et al., 1987a). The northerly basement trend, exemplified by the Western Boundary Fault (gravity lineament 8G), is more an expression of discontinuities within the south-central part of the microcraton, of which the most important are the Malvern line and the Moreton Fault. The former has been shown to be a boundary between two Precambrian volcanic arc terranes (Pharaoh et al., 1987b; Lee et al., 1990). The coincidence between geophysical anomalies and surface structure, discussed more fully below, suggests that the margins of the Coventry Horst and Warwickshire Coalfield are defined by structures that extend to deep levels within the crust of this part of the Midlands Microcraton (Lee et al., 1990, 1991). This interpretation lends support to the model proposed by Turner (1949), that many surface structures are 'posthumous' in the sense that they represent displacements transmitted upwards during renewed movements, or 'rejuvenations', of preexisting lines of weakness in the basement.

Structure of the Precambrian rocks

The Precambrian rocks within the Nuneaton Inlier contain structures referrable to three closely related deformational episodes, each of a rather low intensity. The first (D1) involved folding and was followed by two phases of faulting (D2, D3).

In Judkins' Quarry the presence of a D1 folding episode is indicated by bedding dips of the Caldecote Volcanic Formation which change from south-westerly in the north to south-easterly or south-south-easterly farther south (Figure 5); the latter is also the dip direction of the bedded volcaniclastic rocks in Boon's Quarry [SP 3301 9467]. A coarse fracture cleavage, interpreted as having been formed during this folding, is sporadically developed in the central part of Judkins' Quarry and in the small northern pit of Boon's Quarry south-east of the SSSI ((Figure 4), locality 9): it comprises sets of subvertical, north-westerly-trending joints spaced about 1 m apart. A Precambrian age for the D1 folding is indicated by the absence of a similar structure affecting the overlying Lower Cambrian strata, which dip uniformly to the south-west. This is the first report of folding having affected the Precambrian succession at Nuneaton; although only a small part of the structure is exposed, the associated north-westerly fracture cleavage suggests a similar stress regime to that which produced the southeasterly plunging fold in the Charnwood Forest area (Worssam and Old, 1988), which is located adjacent to the eastern margin of the Midlands Microcraton. A significant difference between the two Precambrian outcrops, however, is that whereas the Nuneaton rocks are at a relatively low metamorphic grade, corresponding to the anchizone (metapelite stage II) of mica crystallinity (Chapter 11), those in Charnwood Forest were metamorphosed under higher grade, or epizonal (stage III) conditions (Pharaoh et al., 1987a). Further evidence that the Nuneaton DI event was a relatively low-intensity deformation compared to the Charnian folding is the absence at Nuneaton of the slaty cleavage found at Charnwood. As noted by Allen (1957), however, this could also be explained by the relative paucity of fine-grained, easily deformable sedimentary sequences in the Caldecote Formation.

Structures relating to the Precambrian D2 and D3 events comprise several faults in Judkins' Quarry, which cut across and therefore post date the D1 fold. None of these affect the Lower Cambrian strata, exposed on the south-western quarry face (Figure 5). The D2 episode was the most intense, producing several faults and many fractures with northerly or north-north-easterly 'Malvernian' orientations (004° to 013°); they are recognised in the south-eastern part of Judkins' Quarry (Figure 5) and in the small northern pit of Boon's Quarry [SP 3305 9467], south-east of the SSSI. The fault zones are subvertical, from 1 to 2 m wide, and are filled with slivers of broken country rock; most are normal displacements throwing between 7 and 20 m to the west, but in Judkins' Quarry there is also one reverse fault (south-west of locality 6, (Figure 5)) and a normal fault with easterly throw (between localities 1 and 5).

The second faulting episode (D3) produced fewer, larger structures with north-east to north-north-east orientation (018° to 030°). The D3 fault in Judkins' Quarry throws to the south-east and, in part, defines the northwestern margin of the granophyric diorite intrusion; it is truncated at the unconformity with Lower Cambrian strata (Figure 5). A similar structure occurs in the wall of the pit below and to the south-east of the SSSI in Boon's Quarry ((Figure 4), locality 10), but the nature of its throw is not known. The fault zones are quite distinctive; they are between 2 and 3 m wide, and composed of highly angular slivers of quartz-veined country rock loosely cemented by a red- or green-stained clayey gouge.

A relationship with igneous intrusive activity can be demonstrated for both of the faulting episodes in Judkins' Quarry: many of the early, northerly trending D2 faults and associated fractures are either parallel to, or are occupied by, basalt and basaltic-andesite intrusions, and the younger D3 fault is located along one side of the granophyric diorite intrusion, which has the same north-easterly elongation as the fault (Figure 5). Andesite intrusions are also reported from fault zones affecting the equivalent Precambrian rocks of Charnwood Forest (King, 1968, p.113), but all of the intrusive rocks in that area are overprinted by a late cleavage-forming deformation (Boulter and Yates, 1987).

Cambrian to early Devonian events

Cambrian extension

Early in Cambrian times, or possibly in the very latest Precambrian, the crust of the Midlands Microcraton came under tension and subsided, allowing the waters of the Iapetus Ocean to flood across it and deposit the transgressive marine sequence of the Hartshill Sandstone Formation. Field relations in Boon's Quarry suggest that the initial phase of this subsidence may have involved localised faulting, leading to the development of steep slopes down which travelled the highly immature debris flows and sediment gravity flows of the Boon's Member. The existence of faults of this age cannot be conclusively demonstrated in this quarry, but two north-westerly orientated faults with downthrows to the south-west, affecting Precambrian rocks close to the unconformity with the Lower Cambrian Hartshill Sandstone Formation, are possible candidates (Figure 4). The subsequent overstep of beds across the Boon's Member indicates that the faulting was followed by widespread subsidence, when the main part of the Hartshill Sandstone Formation was deposited.

Ordovician (Caledonian) deformation

Two styles of Ordovician deformation are recognised within the Nuneaton Inlier. The associated structures are not extensively developed, but are detectable in quarry exposures where intrusions of the Midlands Minor Intrusive Suite have fortuitously acted as tectonic markers. The regional tectonic context of this deformation is uncertain, but information about the metamorphic grade of the Cambro-Ordovician rocks, discussed in Chapter 11, indicates that the Nuneaton Inlier may have broadly corresponded with the north-eastern margin of a pre-mid-Silurian sedimentary basin (Merriman et al., 1993). The Ordovician deformational phases were possibly produced by the tectonic stresses associated with the evolution, and eventual inversion, of that basin.

Deformation belonging to the Hartshill Event, was compressional in type. It is identified in the northwestern part of Hartshill Quarry [SP 3305 9417], where the axis of a chevron fold is transected by a 1 m-thick lamprophyre dyke belonging to the Midlands Minor Intrusive Suite ((Plate 17a), see p.162). This fold is part of an array of similar structures of limited lateral extent, whose upright axes plunge towards the east or south-east. The folds lie in a steeply dipping sandstone sequence on the north-eastern side of a major silicified fault zone of north-westerly trend. The latter has an apparent normal component of throw since it dips north-eastwards, throwing the Hartshill Sandstone several metres down in the same direction. That it also had a transcurrent component, however, is shown by exposures in the main quarry face to the south-east [SP 3309 9394], where a sub- horizontal slickenside lineation is developed. If movements along this fault were synchronous with the generation of the chevron folds, as is suggested by the field relationships, a regime of dextral transpression along a northerly or north-westerly axis can be suggested for the Hartshill Event (Carney, 1992a).

The Hartshill Event is unlikely to have occurred before the Lower Ordovician (Tremadoc), because this is the faunal age of the youngest part of the Stockingford Shale Group, which rests conformably on the Hartshill Sandstone Formation; nor is it any younger than midAshgill (latest Upper Ordovician), which is the age determined for the Midlands Minor Intrusive Suite (Noble et al., 1993). Its age is therefore somewhat loosely constrained to between about 490 and 442 Ma. Within this time span, important tectonic changes occurred elsewhere in the region. The early Ashgill (Pusgillian) episode of folding and strike-slip faulting that closely followed cessation of volcanism in the Welsh Basin (Woodcock, 1990), is tentatively correlated with the Hartshill Event (Table 12); and a late Ordovician unspecified compressional event is inferred to have accompanied closure of Tornquists's Sea, to the north-east of this district (Pharaoh et al., 1995).

Other compressional structures occur in the southeastern part of Hartshill Quarry but their age is uncertain, owing to the absence of lamprophyre intrusions. That most were formed in a regime of north-east-directed compression is nevertheless indicated by the orientations of reverse faults, strike-slip faults and bedding-plane shear zones [SP 3353 9382]. Elsewhere in the Nuneaton Inlier, many reverse faults whose geometries indicate north-easterly compression clearly post date the Midlands Suite intrusions, for example in Griff No. 4 Quarry [SP 3618 8870] and Mancetter Quarry [SP 309 951]. The opposite sense of throw, indicative of south-west directed compression, is also observed along a reverse fault affecting Stockingford Shales and lamprophyre intrusions at the north-western end of Purley West Quarry [SP 3039 9636]. The age of these later movements is not known, but they could be related to Acadian or Variscan compressional events.

The other style of Ordovician deformation was extensional and occurred during emplacement of the Midlands Minor Intrusive Suite. Exposures in the central part of Mancetter Quarry [SP 3095 9505] show small listric extensional faults postdating some of the earlier phases of intrusion but truncated by the later bodies ((Plate 17b), see p.162). Their throws are consistently towards the south-west, indicating extension in that direction. The regional extent of this deformation is not known, but it was evidently closely associated with the emplacement of an igneous intrusive suite that can be recognised throughout the central and western parts of the Midlands Microcraton.

Silurian to early Devonian (Acadian) compression

The district may have been affected by the Acadian deformation of early to mid-Devonian times, dated at about 400 Ma (Soper et al., 1992). These movements inverted the pre-mid-Silurian sedimentary basin, whose remnants now lie concealed beneath the Warwickshire Coalfield, and in the ensuing erosion between 5.5 and 7 km of Lower Palaeozoic strata were removed from that area (Chapter 11). Along the western edge of the Nuneaton Inlier an angular unconformity of about 10° is developed between the Cambro-Ordovician sequence and overlying Late Devonian or Carboniferous rocks, suggesting rather limited pre-late Devonian deformation in that area.

Beneath the Warwickshire Coalfield, the records of deep boreholes provide evidence that the CambroOrdovician strata are commonly more fractured, and have steeper more variable dips, than the Carboniferous rocks that unconformably overlie them. The pre-Carboniferous structure of the central coalfield is revealed by a map of the Cambro-Ordovician subcrop compiled from the results of biostratigraphical and dipmeter surveys for these boreholes ((Figure 10), (Table 4)). These data do not extend over the northern part of the coalfield, and so the subcrop pattern is necessarily conjectural. Nevertheless, the evidence can be interpreted to indicate a faulted dome with the Outwoods Shale Formation in the core and younger strata (Monks Park and Merevale formations) flanking it. Although this structure is only imprecisely known, the data are sufficient to indicate a centre of uplift of the Cambro-Ordovician strata between Birchley Hall [SP 275 844] and Park House [SP 276 870]. The uplift is attributed to pre-Carboniferous deformation because its location does not coincide with the Carboniferous-age Fillongley Anticline or the nearby Arley Dome. The position of the northeast-trending fault shown between the Fillongley Hall and Park House boreholes is well constrained by the data. It juxtaposes the Outwoods Shale against the Merevale Shale, indicating a south-eastwards downthrow of at least 90 m. This fault does not appear to affect the Carboniferous strata, although it is parallel to the Arley Fault, suggesting that the latter is a rejuvenation of one part of a pre-existing, much wider fault zone.

There is no direct evidence that the above deformation was indeed Acadian in age. However, a resetting of isotope systems within the older rocks of the Nuneaton Inlier is suggested by ages of 411 ± 11 Ma and 405 ± 10 Ma obtained by the K-Ar method on separated amphiboles from the Ordovician hornblende-rich sill in Griff No. 4 Quarry. These and similar ages from other samples of the Midlands Minor Intrusive Suite suggest argon loss due to an Acadian disturbance at around 400 Ma (information from C Rundle, 1990), although they give no indication of the nature of this event.

Late Devonian to early Permian events

Between Devonian and early Permian times the Midlands area formed part of a foreland basin which lay to the north of the main Variscan thrust front. It is envisaged that compressional events originating within the latter (e.g. Shackleton et al., 1982) were transmitted to the foreland, in many cases causing older structures to be reactivated at various times throughout the Carboniferous period. The Variscan evolution of the Warwickshire Coalfield has been reviewed by Fulton and Williams (1988) and Besly (1988), and some parts of the following account draw heavily on their work.

Late Devonian to Dinantian extension

The tectonic cycle within this part of the Variscan foreland commenced in latest Devonian or earliest Dinantian times with a phase of rifting and extension, leading to the formation of the Pennine Basin. Although this period is represented by a non-sequence in the Coventry district (Chapter 6), evidence that the Arley Fault System was active is indicated by field relations in the Merevale area [SP 297 961], where the outcrop of the Oldbury Farm Sandstone terminates to the north-west against the fault, whereas the Millstone Grit outcrop continues across it. It follows that the Arley Fault System, although now a complex graben hereabouts, must earlier have thrown down to the south-east, preserving the Oldbury Farm Sandstone in the hanging-wall block. The field evidence constrains the age of this particular fault movement, which was presumably initiated during one of the late Devonian or Dinantian phases of rifting responsible for the structural development of the Widmerpool half-graben, north of the Coventry district (Ebdon et al., 1990). Movements along the Arley Fault which occurred later in the Carboniferous are described below.

Namurian, Langsettian and Duckmantian extension

By Namurian times the regime of extension-induced faulting had been replaced by one of slow subsidence (or thermal sag), which continued through to mid-Duckmantian times, allowing Namurian and Westphalian sediments to onlap southwards on to the northern margin of the Wales–Brabant High. Reduced subsidence rates above this basement high resulted in a southward attenuation in the Langsettian sequence and in the joining together of the various leaves of the Thick Coal to form a composite seam. Contemporary tectonism, involving movement on both the Arley and Western Boundary Fault systems, also exercised a control on sedimentation throughout this period. Taylor and Rushton (1971) suggested that syndepositional movement on the Arley Fault System was responsible for local facies variations in the Millstone Grit. They envisaged a further period of reactivation in Coal Measures times, citing as evidence the increase of multiple seam washouts towards the fault within the Monks Park opencast pit [SP 296 960]. On the southern segment of the fault system, thinning of the Lower Coal Measures across the structure provides further evidence of contemporary uplift.

Duckmantian movement on the Western Boundary Fault system is indicated by the seam split in the Nine Feet seam of the Thick Coal (Figure 22) which runs parallel to the fault for a distance of over 20 km.

Late Duckmantian to Bolsovian compression

Facies variations in the Aegiranum Marine Band and in the overlying Etruria Formation (Figure 21), (Figure 30) indicate that by late-Duckmantian times, the prevailing tectonic regime had changed from one of passive onlap to a block-and-basin-style geometry, with uplifted horsts lying to the west of the present coalfield and possibly also to the south and east. An overall compressional regime is envisaged for this period, possibly involving reverse movement on the Western Boundary Fault. Timing of the start of compression is fairly well constrained; Fulton and Williams (1988) show that it commenced between deposition of the Maltby and Aegiranum marine bands. The occurrence of contemporary volcaniclastic rocks in the east of the basin nevertheless implies volcanic activity which perhaps was related to a phase of localised extension centred on pre-existing faults.

Uplift, deformation and erosion of the northern part of the coal basin preceded deposition of the Westphalian D Halesowen Formation. The effects of this intra-Westphalian event are most evident in the area of the Fillongley Anticline, where seismic lines show a marked sub-Halesowen unconformity; they are also seen in the north-east of the coalfield where the Halesowen Formation cuts down to rest on Coal Measures. The effects of this deformational phase are less easily resolved in other parts of the coalfield, as virtually all the structures recorded at the level of the Two Yard Coal (Figure 49) are also represented in the post-Halesowen rocks (Figure 48).

Westphalian D to early Permian extension

Following intra-Carboniferous deformation, a regime of fairly uniform subsidence was re-established, allowing the basal fluviatile deposits of the Halesowen Formation to overstep across areas earlier occupied by horsts. The Western Boundary Fault appears not to have had any topographic expression at this time, for the Halesowen Formation isopachytes are orthogonal to its trend ((Figure 31), inset).

Subsidence rates increased within the Variscan foreland basin during the later stages of the Carboniferous and into the Permian, allowing thick redbed sequences to accumulate. Besly (1988) has suggested that block uplift on the flanks of the coalfield provided a source for the sediment, but the palaeogeography during this period remains very uncertain.

Early Permian compression

The culminating phase of the Variscan orogeny involved deformation of the entire Carboniferous sequence, as compression, probably from the north-east or south-west quadrants, caused folding and localised linear block uplift, and produced the main synclinal structures that now form the Warwickshire Coal Basin and subsidiary Bulkington Basin (Figure 48) and (Figure 49). It is probable that this tectonism reactivated some of the earlier folds and faults described above, so precluding accurate estimates of the age of the structures to be reviewed below.

The Hoar Park, Allesley and Benton Green synclines are broad, open folds, which together define the overall structure of the western part of the coalfield. The axial trend of the folds is towards the south-east or south-south-east, and they plunge in a similar direction; dips on their outer limbs are generally less than 10° and commonly reduce to less than 5°. The position of their axial traces can therefore be located only approximately.

The continuity of the two more northerly structures is disrupted by the cross-cutting Arley Fault System.

A belt of more complex subsidiary folding and faulting affects the eastern part of the coalfield and produces high dips locally in a zone lying between the Allesley Syncline and the Camp Hill and Binley monoclines. Within this belt, the most important structures are the Keresley Fault and the Astley, Pheasant's Nest Farm and Corley synclines, all trending south-south-east (Figure 48) and (Figure 49). The Keresley Fault has been proved in Coventry Colliery workings and on seismic lines. North of Brownshill Green [SP 312 834] it is a steeply dipping reverse fault with a throw down to the east of 27 m; farther south it changes to a near-vertical normal fault. Although there is seismic evidence of this fault well above the base of the Halesowen Formation, no displacement was detected at surface. However, gentle folding occurs parallel to the northern part of the fault, and surface faults with a parallel trend occur to the south-east (Old, 1989). The structure contour map (Figure 49) suggests that the Keresley Fault and flanking folds may form an arcuate en échelon replacement of the zone of steepening dips that converges towards the Binley Monocline. A possible north-westerly continuation of the Keresley Fault zone is seen in the small east-throwing fault south of the Arley Dome (Figure 49), and in a number of associated faults orthogonal to the Arley Fault in the same area (Figure 48). That it may continue farther in that direction is suggested by geophysical information which indicates a structure of the same trend displacing laterally the Western Boundary Fault (Figure 51).

The Arley Fault System is a north-east-trending structure that intervenes between the Hoar Park and Allesley synclines, transecting the Warwickshire Coalfield and linking the Western Boundary and Polesworth faults (Figure 48). It consists at surface of two principal faults, with numerous minor faults, arranged en échelon along its length. The south-western branch reverses its throw, which is a complexity attributed to the development of dome and basin structures on alternate sides of the fracture system. These structures are particularly well demonstrated on the structure contour plan of the Two Yard Coal (Figure 49). The most prominent, here termed the Arley Dome, is centred on the upthrown south-eastern side of the main fault [SP 280 905], bringing the Halesowen Formation to crop in its core; farther south-west along the same fault the throw reverses and a more elongated structure, the Fillongley Anticline, is developed on the upthrown north-western side (Rees, 1989). The association of en échelon fractures and anticlinal domes seen in the central sectors of the Arley Fault System may be explained by a combination of normal and transcurrent fault displacements; examples of similar structures where this style of deformation has been invoked have been described from certain oilfields in North America (Moody and Hill, 1956). In the case of the Arley Fault, evidence for transcurrent displacement is given by the dextral shift of 270 m in the outcrops of the Coal Measures and Millstone Grit in the Merevale area [SP 295 958], and by the set of curved faults, oblique to the Arley Fault [SP 265 870] (Figure 49), imaged on the seismic data to the east of the Fillongley Anticline. The timing of this transcurrent movement is more difficult to assess; it could have been initiated during intra-Carboniferous or end-Variscan compressional phases, perhaps acting as a lateral ramp structure to the Polesworth or Keresley faults.

The Camp Hill Monocline on the north-eastern rim of the Warwickshire Coalfield syncline has a north-westerly trend, parallel with the Polesworth Fault, and extends from the Arley Fault southwards into Nuneaton (Figure 48). The structure is shown on British Coal plans of the former Haunchwood opencast coal workings [SP 339 920], in which the south-westerly dip steepens progressively north-eastwards, from 14° to about 65° over a distance of 350 m. A further 350 m to the north-east of the workings [SP 3405 9246] in the vicinity of Camp Hill, there is a zone of subvertical dips in which strata of the Stockingford Shale Group are locally overturned, dipping to the northeast. This zone is interpreted to be a strike fault forming the upper hinge of the Camp Hill Monocline, against which the strata farther west have been rotated and steepened. Its sense of throw is not known, but the structural evidence is consistent with it being a reverse fault dipping to the north-east and throwing strata down in the opposite direction, having arisen within a regime of compression directed towards the south-west. Within the Haunchwood workings a strike fault is noted on the British Coal plans but it throws down towards the northeast, the opposite of that proposed above. Locally, it dips steeply in that direction, although it is evidently not a normal fault since a note on the plan describes it as a 'thrust'. This contradiction could be resolved by suggesting that it is a reverse fault, produced by north-east-directed compression, which then was passively rotated as the strata were later steepened towards the structure described above. The overall north-westerly trend of the Camp Hill Monocline, parallel to the Polesworth Fault, indicates the likelihood that both structures arose within the same phase of deformation and were perhaps instrumental in uplifting the narrow sliver of pre-Carboniferous rocks that defines the present Nuneaton Inlier. Variscan compression along the Polesworth Fault is suggested farther north in the Coalville district (Worssam and Old, 1988), and the evidence for it is discussed below.

To the south-east, the Camp Hill Monocline is deflected southwards and passes into the western limb of an upfold, the Marston Jabbett Anticline, developed on the upthrown side of the Bedworth Fault. To the northwest of the Arley Fault System, the monocline passes into the south-western flank of a complex anticlinal fold system bounded to the north-east by the Polesworth Fault (Worssam and Old, 1988).

The Binley Monocline, affecting the south-eastern rim of the Warwickshire Coalfield, has a more northerly orientation than the Camp Hill Monocline (Figure 48) and therefore may relate more closely to the Malvernian basement structural trend exemplified by the Western Boundary Fault. It is concealed beneath Triassic rocks, but structural subsurface information is available from British Coal plans of the Binley Colliery, in the south-east corner of the area depicted in (Figure 49). These plans show that like the Camp Hill Monocline, the Binley structure comprises strata whose westerly dips steepen progressively towards the east. Minor folding intensifies in the same direction, producing a series of open asymmetrical anticlines and synclines with axes parallel with the monocline and steep limbs facing eastwards. Such a style of deformation indicates east-directed compression, but an interpretation of the plans also shows the presence of strike-parallel normal faults throwing down to the west. The latter are possibly step faults, synthetic with respect to the developing monocline but displacing the earlier folds. The overall structural style of the Binley Monocline is compatible with the deformation of strata over a linear basement structure that was undergoing vertical uplift.

The Bulkington Syncline and Bedworth Fault are concealed beneath Triassic strata, and their precise configuration is conjectural. The Coal Measures within the syncline define a prominent oval-shaped negative Bouguer gravity anomaly (see below), and drilling has proved them to dip westwards on the eastern side of the syncline. The structural solution favoured in (Figure 48) shows a north-plunging syncline paired with the Bedworth Fault. The latter may be an extension of the inferred basement-related structures associated with uplift along the Binley Monocline, although if so the sense of throw has been reversed across a tectonic hinge lying between the two areas. Other structural configurations, perhaps involving periclinal folding in conjunction with linear basement uplifts, are possible for this little-known area.

The Polesworth Fault is known to have acted as a listric normal fault during the Triassic development of the Hinckley Basin, indicating that its plane of movement must dip towards the north-east. No further direct structural observations can be made in the present area, although circumstantial evidence already summarised suggests that it was in some way associated with the linear basement uplift inferred to have formed the Camp Hill Monocline. North of the district an anticline is developed in Coal Measures immediately adjacent to the Polesworth Fault, on its western side (Worssam and Old, 1988), suggesting that the fault was indeed a compressional structure in Variscan times. A similar structural style characterises the Thringstone Reverse Fault farther east; it has the same trend and may be of the same age of generation as the Polesworth Fault (Worssam and Old, 1988).

The Higham Anticline is a domal structure centred beneath the deepest part of the Hinckley Basin, to the north-east of the Polesworth Fault (Figure 48). Its presence is inferred from an interpretation of a British Coal seismic reflection profile across the north-eastern part of the structure. This shows that beneath the Triassic strata there occurs a series of reflectors dipping north-eastwards, away from the inferred axial region. Those reflectors are thought to represent a Cambrian succession intruded by sills; on the seismic section they are underlain by seismically isotropic material, probably representing Precambrian basement, which is believed to incrop beneath the Trias within the core of the anticline (information supplied by T C Pharaoh). This interpretation, shown in (Figure 48), is supported by a proving of Precambrian rocks directly beneath the Trias in the Stretton Baskerville Borehole, near Hinckley (Eastwood et al., 1923, p.99).

The Higham Anticline is primarily a Variscan axis of uplift, although the possibility of an earlier Caledonian or Acadian involvement in its development cannot be discounted. Its north-eastern flank dips relatively gently compared with the steepness of the Camp Hill Monocline, which is regarded as part of the western limb. These two structures together define a large-scale asymmetrical fold or dome, with south-westwards vergence, whose development is linked with that of the reverse faulting postulated to have occurred along the Polesworth Fault. The Higham Anticline probably continues north-westwards, separating the strata of the South Derbyshire and Warwickshire coalfields (Worssam and Old, 1988, fig. 27).

Structure of the Triassic rocks

Widespread uplift associated with the Variscan earth movements was followed by a period of extension during which Variscan structures in the east of the district were inverted. Subsidence was controlled primarily by movement on the Polesworth Fault, which is here taken as the bounding structure to the Hinckley Basin. Although this fault is not mapped far to the south-east beneath the drift, its continuation beneath Liassic strata in that direction is indicated by a large-amplitude linear gravity anomaly (Figure 51), to be discussed below. Triassic strata west of the Polesworth Fault are regarded as overlying a subsided edge to the Coventry Horst, which has only partially been inverted, and which therefore has the potential for preserving Coal Measures in structures such as the Bulkington Basin. East of the Polesworth Fault, the Carboniferous was probably entirely eroded from the axial region of the Higham Anticline prior to downfaulting and burial of the structure beneath the Triassic basin fill. Carboniferous rocks only reappear in the north-east of the Hinckley Basin (Worssam and Old, 1988), in the Leicestershire Coalfield, where they form the faulted eastern limb of the Higham Anticline.

Triassic faulting is usually only detected by surface mapping where suitable lithological markers exist, and therefore is probably under-represented on the published geological map. Small displacements are mapped in the vicinity of Marston Jabbett [SP 380 885] as a system of north-west- to west-north-west-orientated faults. These structures have imparted a dextral offset to Cambrian strata but may have had normal displacements in Triassic times. The fault complex has a comparable trend to the Princethorpe Fault farther south, which may have been active during the Triassic and was rejuvenated in post-Jurassic times (Old et al., 1987).

Triassic movements on the Polesworth Fault

The Polesworth Fault was the principal structure controlling subsidence along the western margin of the Hinckley Basin. It was a compressional structure in early Permian times (p.126) but was rejuvenated with a northeastward normal throw during the period of early Triassic crustal extension. Along the fault, both the amount of throw and the structural style are seen to vary in line with the changing structural configuration of the adjacent Hinckley Basin (Figure 48). Opposite the northwestern part of the basin (between Atherstone and Nuneaton), the fault had long been thought to define the contact between the Triassic and Precambrian–Cambrian rocks cropping out along the edge of the Nuneaton Inlier. However, the present remapping, accompanied by geophysical investigations (below) and drilling of the White Hall Farm and Hill House boreholes, has proved that the Triassic boundary here is a downflexured and step-faulted unconformity surface, with respective north-easterly dips of 10° and 30° in the two sets of boreholes (Baldock, 1991a, fig. 7). The same structural zone continues to the north-west and is indicated by exposures in a stream section [SP 3172 9625] north of White Hall Farm, which show a 30° northeasterly dip in the Bromsgrove Sandstone. The boundary is interpreted to lie in a monoclinal flexure on the upthrown (south-western) side of the Polesworth Fault, whose surface trace is located in the drift-covered Triassic bedrock a short distance to the north-east.

The amount of downthrow along the Polesworth Fault decreases south-eastwards, from 260 m estimated near Hartshill, to 130 m measured from a seismic line which crosses the fault north-east of Bedworth. The reduced amount of throw and less severe down-flexuring in the Bedworth sector are attributed to the influence of the Hinckley Basin col (see below), which has acted as a tectonic hinge, pinning movement along the fault and thereby restricting the main effects of half-graben development to the north-western part of the basin.

Structure of the Hinckley Basin

The Hinckley Basin is divided by a north-east-trending structural col into a north-western sector thickly infilled by Triassic strata and a south-eastern sector whose structure and sedimentary infill are largely unknown (Figure 48). The configuration of the basin floor is difficult to determine, because few borings penetrate to the base of the Triassic succession, and also because the available seismic reflection sections are restricted to the north-east of the basin. For the north-western sector, an interpretation based on the seismic refraction profiling of Whitcombe and Maguire (1981) is followed in the northerly line of cross-section shown on the 1:50 000 geological map. Those authors suggested that the basin is a half-graben structure, with a floor inclined southwestwards towards the Polesworth Fault. South-east of the structural col, however, the floor to the basin appears to slope gradually south-eastwards. The central col itself is in line of strike from north-east-trending minor fault systems on the Coventry Horst (Figure 48) and therefore may also be fault-bounded.

The age of subsidence within the north-western compartment of the Hinckley Basin is constrained by thickness variations observed in strata affected by flexuring and growth movements along the Polesworth Fault. North of the Coventry district, the basal Permo-Triassic Hopwas Breccia thickens towards the Polesworth Fault (Worssam and Old, 1988) suggesting that the half-graben structure of the basin came into existence in latest Permian or earliest Triassic times. High rates of subsidence were maintained in Scythian times, when about 200 m of Polesworth Formation accumulated on the downthrown side of the fault (Worssam and Old, 1988). Further subsidence, later in the Triassic, is suggested by evidence in the present district. The Hill Farm boreholes (Figure 39) show that the Bromsgrove Sandstone thickens north-eastwards towards the Hinckley Basin and is much affected by faulting, indicative perhaps of tectonic activity along the basin-margin in Anisian times (Baldock, 1991a). A later renewal of basin margin flexuring is indicated by the fact that all of the strata in these boreholes dip north-eastwards. No thickness data are available for the Sherwood Sandstone and Mercia Mudstone groups in the south-eastern part of the Hinckley Basin, but subsidence evidently continued through late Triassic times and into the Jurassic, resulting in marine transgression and the deposition of the Penarth and Lias groups.

Post-Jurassic movements

In the largely drift-covered Liassic succession, the only apparent structure is a 1 to 2° south-easterly dip estimated from borehole correlations in the Lutterworth area. The same gentle tilt was described from the area to the south, where the Lias Group is also affected by northwesterly trending faults (Old et al., 1987). Those faults converge towards the Polesworth Fault, suggesting that they are splays from this major structure. Although in the Coventry district geophysical studies prove that the Polesworth Fault continues south-eastwards beneath the Lias, due to the thick drift cover it is not known whether these strata are themselves affected by the fault.

Geophysical investigations

For this study an area larger than the Coventry district has been analysed, to provide a regional perspective. The main sources of geophysical information are the regional gravity and aeromagnetic survey data archived in the national BGS databanks. These have been used to derive the Bouguer anomaly and aeromagnetic maps (Figure 50), (Figure 51) and (Figure 53)." data-name="images/P1001250.jpg">(Figure 52). Additional local investigations were undertaken in support of the mapping programme. Full details of all surveys and interpretations are given by Cornwell and Royles (1993).

Two gravity and aeromagnetic profiles across the district have been interpreted simultaneously, using the 2.5 dimensional (2.5D) program 'Gravmag' in which the component bodies in the modelling process have specified strike extents (Busby, 1987). These profiles, shown in (Figure 53), were selected to include the main anomalies in the district and partly coincide with the lines of section chosen on the 1:50 000 geological map to illustrate the underlying structure. Lineaments have also been recognized from the geophysical data; they are indicated on (Figure 51) and (Figure 53)." data-name="images/P1001250.jpg">(Figure 52) and are discussed later.

Other sources of information include a series of long seismic refraction profiles centred on the Charnwood Forest area, which were recorded using quarry blasts as the main energy source. One of these profiles extends from quarries at Mancetter [SP 311 948], near the northern edge of the Coventry district (Figure 48), northeastwards to Bardon [SK 455 132] and provides evidence of the structure beneath the Hinckley Basin (Whitcombe and Maguire, 1981). Seismic reflection profiles in the district have been recorded mainly for coal exploration; most of these have not been published but Jones (1981) has reported some of the main conclusions.

Physical properties of rocks

Information bearing on the density, magnetic susceptibility and sonic velocity of the main rock types has been gathered during the present mapping, and from several earlier sources: Worssam and Old (1988), Cook et al. (1952), and research theses from the University of Leicester (Maroof, 1973; El-Nikhely, 1980; Arter, 1982). The data are summarised in (Table 13). The physical properties of the Precambrian Charnian rocks are variable and seldom particularly distinctive. A survey of values reported from the Charnwood area by Maroof (1973) and Whitcombe and Maguire (1980) showed seismic velocities of between 5.40 and 5.65 km/s, increasing to 6.4 km/s at 2.6 km depth. Densities of volcaniclastic rocks from the Maplewell Group were between 2.64 and 2.78 Mg/m3, comparable with those quoted for the Caldecote Volcanic Formation in (Table 13). In the Nuneaton Inlier magnetic susceptibility measurements indicate that the Caldecote Volcanic rocks are unlikely to give rise to distinct magnetic anomalies.

The magnetic properties and the palaeomagnetic significance of the remanent magnetisation of rocks from the Ordovician South Leicestershire Diorite suite within the district, and closely adjacent to it, have been examined by Duff (1980), while the Caldecote Volcanic Formation was included in a palaeomagnetic study by Piper and Strange (1989).

Regional gravity

The gravity coverage is good for the whole district, with an average station distribution of one per 1 km2. It is supplemented by measurements from in-fill and detailed traverses acquired either for the present project or derived from earlier surveys in the Coalville district (Cornwell and Allsop, in Worssam and Old, 1988), and in exploration for concealed igneous intrusions (Allsop and Arthur, 1983). These data are summarised in (Figure 50), for which a Bouguer correction density of 2.4 Mg/m3 was used to reduce the gravity data to sea-level datum. A useful presentation of these data to emphasise near-surface density contrasts is as a shaded relief plot of the first horizontal derivative (which shows gravity gradients), illuminated vertically. This diagram (Figure 51) illustrates the coincidence between linear gravity anomalies and certain of the structures mapped at the surface, as well as indicating the presence of other gravity lineaments, annotated for later discussion, which are the expression of structures hidden from view beneath younger deposits.

One of the largest anomalies on the Bouguer gravity map is the low centred on the north-western part of the Hinckley Basin (Al on (Figure 50)). An interpretation of this gravity anomaly simply in terms of Triassic thicknesses gives a basin fill of over 900 m (Worssam and Old, 1988). This figure is almost certainly an overestimate since it does not take into account the probable existence of high-density bodies on either side of the basin, beneath the Warwickshire and Leicestershire coalfields. If such high density bodies are included in the interpretation, their effect is to reduce the thickness of Triassic rocks required to explain the observed anomaly to about 600 m ((Figure 53), profile 1). This value is in better agreement with the seismic refraction evidence of Whitcombe and Maguire (1981), who estimate a maximum thickness of about 750 m for the Triassic sequence adjacent to the Polesworth Fault. The Bouguer gravity map also demonstrates well the compartmented nature of the Hinckley Basin, shown by the gravity lows to northwest (A1) and south-east (A2) of a central col defined by relatively high values (A3). The Knowle Basin of Triassic deposition lying to the west of the Coventry Horst is also characterised by a gravity low (Cook et al., 1952), but is not considered here as it lies outside the district.

In an attempt to resolve the uncertainty about the depth of the north-western part of the Hinckley Basin, and the nature of the underlying basement, two transient electromagnetic soundings (TEM) were made north of Nuneaton (sites I and II, (Figure 50)) (Cornwell et al., 1993). The results demonstrated the existence of low-resistivity material (10 to 32 ohm metre), interpreted as Triassic, and evidence of a higher-resistivity 'basement'. The interpreted Triassic thickness overlying this basement was about 540 m (top of basement at 450 m below OD), similar to the thickness indicated from the gravity evidence.

A further striking feature of the gravity data is the linear gradient zone coincident with the Polesworth Fault on the western margin of the Hinckley Basin (lineament L2A, (Figure 51)). Field investigations along the most accentuated part of this feature, north-west of Nuneaton, have shown that the contact between basement and Triassic rocks is an unconformity that has been tilted within a zone of north-eastward downflexuring (see above). Detailed gravity observations across this structure in the vicinity of Hill House [SP 3439 9367], interpreted using the 2.5D modelling programme, showed that the unconformable base of the Trias must steepen eastwards to angles well in excess of the 30° indicated by the borehole evidence. The modelling suggests that the Polesworth Fault could conceivably lie up to 250 m farther into the basin than is shown on the 1:50 000 map.

The Warwickshire Coalfield is indicated geophysically most clearly by the Bouguer gravity data. Generally the coalfield is associated with a gravity high, relative to the lows over the adjacent Triassic rocks. The highest gravity values occur as an elongated ridge just to the east of the coalfield, centred over the high density Precambrian and Cambrian rocks of the Nuneaton Inlier (B, (Figure 50)). Farther south a less prominent linear gravity high (C, see also lineament L8, (Figure 51)) coincides with the Binley Monocline, representing a further zone of basement uplift along the coalfield's eastern margin; it can be traced southwards into the Warwick district, where it changes direction and apparently merges with the Princethorpe Fault (Old et al., 1987). The western edge of the coalfield is also indicated as a gravity ridge (anomaly D) south of gridline 90; like the bounding lineaments farther east it gradually loses its expression when traced southwards into areas of thickening Triassic cover.

Within the main part of the Warwickshire Coalfield (anomaly E), gravity values decrease gradually towards the south, commensurate with an increase in thickness of the Barren Measures. This apparent simple relationship between the two (Cook et al., 1952) may be misleading, however, as is shown by the interpretations of profiles 1 and 2 (Figure 53), which indicate the probable presence of high-density rocks within the basement to the coalfield. These may be the basic igneous bodies inferred from the aeromagnetic data discussed below. Similar predictions of underlying high density basement rocks were made by Carruthers (in Old et al., 1987, fig. 23) for the southern part of the Warwickshire Coalfield, and by Cornwell and Allsop (in Worssam and Old 1988, fig. 29) for the South Derbyshire and Leicestershire coalfields. The gravity response of the Arley Fault is small and not well defined, probably because the data coverage is not sufficiently close.

The Bulkington Coal Basin is associated with a small but pronounced Bouguer gravity anomaly low due to a density contrast of approximately 0.2 Mg/m3 relative to the Cambrian and Precambrian basement rocks. Although the Bouguer gravity anomaly map based solely on the regional data gave an indication of the existence this basin, the definition of the anomaly was considerably improved (F, (Figure 50)) by the inclusion of gravity observations made during the course of the mapping of the Coventry district. The geological and geophysical setting of this basin are discussed more fully in Chapter 7.

Late Ordovician intrusions of the South Leicestershire Diorites are associated with local Bouguer anomaly highs around Sapcote, Stoney Stanton and Croft ((Figure 50), G). However, the larger part of these observed anomalies may be an effect of the concealed topography, in which the intrusions occur as inselbergs draped by lower-density Triassic rocks (Allsop and Arthur, 1983).

The linear alignments of anomalies or their abrupt terminations, shown by the regional Bouguer gravity and aeromagnetic maps (Figure 51), (Figure 53)." data-name="images/P1001250.jpg">(Figure 52), indicate the presence of several geophysical lineaments in the Coventry district. Some of these are obviously related to the mapped faults and have been mentioned briefly above. Others, tentatively interpreted as evidence of concealed structures, are now discussed.

Lineament L1 (Figure 51) comprises two segments interrupted by lineament L5; it has a north-westerly trend sub-parallel to the Polesworth Fault. The north-westerly segment forms a well-defined north-eastern geophysical margin to the gravity low associated with the Hinckley Basin, and farther north it coincides with a southwestwards-facing step on the basement surface imaged on the seismic refraction profile of Whitcombe and Maguire (1981). The concealed ridge of pre-Mesozoic rocks described by Allsop and Arthur (1983) is associated with this lineament. It is interpreted here to be an extension of the Boothorpe Fault, mapped in the Coalville district as a Carboniferous structure throwing down to the south-west. Farther west, the pronounced lineament L2A coincides with the Polesworth Fault; its south-eastward extension (L2B) indicates that this structure continues beneath the Mesozoic rocks, at least as far as the edge of the Coventry district and probably for a further 50 km south-eastwards. In the area between the Polesworth Fault–L2 system and lineament Ll, and also to the north-east of the latter, there occur less well-defined sets of linear west-northwest to north-north-west-trending anomalies, such as lineaments L3 and L4. These could be the expressions of other basement structures oblique to, or merging with, Ll and L2. Lineaments 1 to 4 are parallel with, and occur immediately to the south-west of, a major gravity lineament (No. 7 of Lee et al., 1991) associated with structures that define the junction between the Midlands Microcraton and the concealed eastern Caledonides structural domain (Lee et al., 1990). A lineament with a west-north-west trend (lineament L5) is also imaged in the deeper magnetic basement by the aeromagnetic data (Figure 53)." data-name="images/P1001250.jpg">(Figure 52). It appears to intersect lineament L2 near Bulkington and its continuation coincides with some faults in the Coventry Horst (Figure 48).

Lineament L6 appears to be a lateral displacement which interrupts the trend of the anomaly defining the Western Boundary Fault (L7) at about grid northing 95, just to the south of the large Dosthill magnetic feature (Figure 53)." data-name="images/P1001250.jpg">(Figure 52). The surface geology indicates a slight bend in the fault trace in this area but there is a possibility that a more well-defined structure exists at depth. This lineament is perhaps a continuation of anomaly L5, or of the north-west-trending Keresley Fault.

Aeromagnetic survey

The aeromagnetic data were acquired in 1955 by surveys at a constant barometric height of 549 m for the area to the south of gridline 00. The east-west flight lines were spaced 1.6 km apart and the north-south tie lines 9.7 km apart. The data are presented as a reduced-to-pole plot (Figure 53)." data-name="images/P1001250.jpg">(Figure 52), which shows the anomalies in their true position with respect to the source bodies, and without the distortion due to the inclination of the geomagnetic field.

The aeromagnetic anomaly over the Warwickshire Coalfield (A in (Figure 53)." data-name="images/P1001250.jpg">(Figure 52)) is one of the most pronounced geophysical features in the Coventry district and can be followed round into an even larger-amplitude anomaly centred near Walsall, farther west (not shown in (Figure 53)." data-name="images/P1001250.jpg">(Figure 52)). It is elongated towards the south-east and follows a slightly arcuate course which broadly mirrors that of the eastern margin of the coalfield and the Nuneaton Inlier. To the north-west it is truncated at the Western Boundary Fault, showing that the magnetic body responsible is an integral part of those structures that have determined the form of the coal basin. In considering the source of the magnetic material, it may be significant that the anomaly maximum (Al, (Figure 53)." data-name="images/P1001250.jpg">(Figure 52)), centred over the Dosthill Inlier 6 km from the north-western corner of the Coventry district, coincides with outcropping Cambrian rocks which contain a complex of dioritic sheets. These intrusions, identified with the Ordovician-age Midlands Minor Intrusive Suite (Chapter 4), were at first reported to have a low magnetisation (Worssam and Old, 1988), but detailed measurements taken during the present survey indicate that parts of one of the thicker Dosthill sills have high susceptibilities of 30–50 X 10−3 SI units. Although such values can give rise to significant aeromagnetic anomalies, interpretations of the Warwickshire Coalfield anomaly shown on profiles 1 and 2 (Figure 53) indicate that an accumulation of sill-like bodies would need to be excessively thick to explain the observed amplitude. The anomaly source is therefore modelled as a large trapezoidal body, or series of bodies (MB2, MB4) which, if related to the Midlands Minor Intrusive Suite, could represent deep-level feeders to the sills. An alternative explanation is that a linear block of Precambrian magnetic basement is responsible for the anomaly. This would require material with greater magnetic susceptibilities than have so far been measured within the Precambrian rocks of this district or the Charnwood Forest area. Nevertheless, evidence that such material is present in the region comes from the Withycombe Farm Borehole at Banbury, which proved highly magnetic Precambrian volcanic rocks at the base, as discussed by Lee et al. (1991).

The aeromagnetic response of bodies of the Midlands Minor Intrusive Suite cropping out within the Nuneaton Inlier is less obvious, but small-amplitude anomalies can be distinguished over some of the sills (Worssam and Old, 1988). Moreover, the general increase in the magnetic gradient in the Mancetter area of the Nuneaton Inlier [SP 312 957] coincides with the considerable development of such intrusions. Magnetic susceptibility measurements reveal variable magnetisations for these sills (Table 13), with high values of 20–50 X 10−3 SI units obtained for mafic-rich components in the lower parts of the thicker, composite intrusions within the Stockingford Shale Group (Figure 12).

A change in the amplitude of the Warwickshire Coalfield magnetic anomaly occurs across the Arley Fault, indicating that the magnetic source rocks are displaced at depth by that structure. South of this fault the trend of the source rocks coincides with that of fold axes (Astley Syncline and Pheasant's Nest Farm Syncline) in the Coal Measures.

Intrusions of the South Leicestershire Diorite suite in the north-eastern corner of the district have a weak or nonexistent magnetic signature, with the exception of the quartz diorites and granodiorites at Croft, which are the source of a pronounced aeromagnetic anomaly (B, (Figure 53)." data-name="images/P1001250.jpg">(Figure 52)). The varying aeromagnetic response is consistent with the observed magnetic susceptibility data which indicate values of greater than 10 X 10−3 SI units for the Croft intrusion and about 1 X 10−3 for the Barrow Hill-Stoney Stanton intrusions farther to the south-west (Table 13). The form of the Croft anomaly suggests that the intrusion probably increases in diameter at depth, even though the near-surface subcrop does not extend much beyond the mapped boundaries (Cornwell and Allsop, 1981). The large anomaly (C) to the east suggests the existence of a further plug or boss of a similar composition, and which is perhaps indicated by the quartz diorite proved in the Countesthorpe Borehole outside the Coventry district [SP 3567 2955]. The suggestion of Le Bas (1972) that the Croft-Countesthorpe anomalies indicate the presence of a single large pluton at depth is therefore supported by these data.

Chapter 13 Economic and applied geology

Coal

Historical background

A history of the Warwickshire Coalfield up to 1913 is given by Grant (1982) and summarised by Baldock (1991a); further details are given by Mitchell (1942), National Coal Board (1957), and Old et al. (1987).

The Romans were probably the first to exploit the coal in workings close to their main settlement of Manduessedum (Mancetter) on Watling Street (Grant, 1982). Medieval documents specifically mention coal being dug in the Stockingford–Haunchwood area prior to 1550, and in 1469 coal carts were prohibited from entering Coventry because of congestion. During the early development of the coalfield, coal was extracted entirely from shallow surface diggings located along the outcrop; according to Grant (1982) the steep dip of the seams precluded the use of bell-pits. By the 1700s, however, shaft mining had largely replaced crop working, with upwards of 13 collieries operating between Nuneaton Common and Wyken. The method of working is well documented in the case of Griff Colliery (Whitehead, 1969–70); it involved sinking a shaft to the base of the Two Yard Coal, and then driving levels horizontally to link with adjoining coal pits. Further roads were driven down dip for about 50 m, from which crossheads were cut parallel to the main level. The resulting panel of coal, measuring about 30 m in width, was then extracted using the longwall principle by retreating back up-dip. It was easier to sink new shafts and prepare new sites than to extend workings laterally; the resulting high density of shafts in some of the present-day urban areas (Old et al., 1990, Map 10) remains a potential problem to building works. Most of the pits suffered difficulties caused by flooding and were not in continuous operation; Griff, for example, operated for only 39 years between 1610 and 1710. A Newcomen steam engine was installed at Griff in 1714, and others followed shortly afterwards at sites in the southern part of the coalfield. However, the operational expense of these machines ruined several collieries (Grant, 1982). Gruff itself closed in 1730 and was not reopened until 1770.

Improvements in steam engine design and the expansion of the canal network led to a resurgence in mining in the 1770s, but the distribution of collieries from 1750 to 1850 remained much as before, along the exposed coalfield.

Deep mining and exploitation of the concealed coalfield really only began in the second half of the 19th century, first in the north at Baddesley (1851). By this time the railway system had largely replaced the canals as the principal means of transport, and branch mineral lines were constructed to provide direct access to the pits (such as Ansley Hall to Stockingford). Further deep mines were eventually opened, including the Haunchwood Tunnel Pit (1891) and Griff Clara (1893).

The exposed coalfield provided a strong locational force until the turn of the century, when new deep mines were sunk at Arley, Binley, Newdigate and Coventry collieries. Output reached its highest level in 1939, with 5.8 million tonnes produced from 20 mines. Since 1947, when the coal mining industry was nationalised, closures and mergers have progressively reduced the number of pits to the point where Daw Mill colliery, opened in 1964, is now the only operational deep mine in the coalfield. Annual output currently stands at about 2 million tonnes.

Opencast mining of coal was undertaken at several sites, mainly in the immediate post-war period but also more recently at Sudeley. All have been backfilled to provide agricultural or amenity land.

Current workings and resources

Recoverable reserves of the Warwickshire Thick Coal, estimated at 400 million tonnes (National Coal Board, 1985), all lie within the South Warwickshire Prospect, which extends from Daw Mill south towards Kenilworth and south-east to beyond Southam. The limits of the prospect are defined to the south by the current maximum workable depth of 1200 m; to the south-east by incrop against the Halesowen Formation and by thinning of the seams to less than 1.2 m; and to the east by former workings.

The coals are generally high-volatile, low-rank bituminous coals (British Coal Rank 902), with a moisture content that is generally quite high (over 10 per cent). The leaves of the Thick Coal generally have average sulphur contents of less than 4 per cent by weight, and average ash contents of less than 9 per cent by weight (air-dried basis) (Fulton, 1987). Coal quality deteriorates slowly towards the south-east, with ash and chlorine increasing and calorific value and rank decreasing from north-west to south-east (National Coal Board, 1957; 1985). The methane content of the coal (coal-bed methane) has been measured by Creedy (1986) as 2.5 m3/tonne in the Thick Coal at Daw Mill. An average figure of 1.7 m3/ tonne is given by Creedy (1991).

Brick clay and fireclay

Historical background

The Etruria Formation mudstones provided the chief raw material for brick and tile works in the Nuneaton area for nearly a century. Messrs Stanley's pits, established in 1867, and those of the Haunchwood Brick and Tile Company [SP 336 920], dating from about the same period, dominated the crop between Heath End and Whittleford until reserves were exhausted in the mid-1960s. All are now backfilled and partly redeveloped. Some of the pits reached depths of 25 to 30 m. In at least one case (Haunchwood), the original backfill of colliery waste has been re-excavated (for removal of coal by coal washing) and the pit refilled and landscaped.

In Stanley's No.1 Claypit [SP 343 913], between Haunchwood and Arbury roads, at least five different types of clay (varieties of red and mottled mudstone) were dug from open pits and mined from shallow underground adits that usually commenced from benches near the bottom of the pits. Most of the adits were abandoned by 1900, but some continued in use until 1937. The original open clay pits were amalgamated, extended and deepened, and working continued to at least the early 1960s. Parts of the combined pit were flooded by 1968 and it was backfilled in the early 1970s.

Workings in the more northerly Haunchwood pits (north and south of the railway line) ceased at an earlier date and were already backfilled or flooded by 1959. The southernmost workings in the Etruria Formation around Griff [SP 350 895] closed in 1967–68.

In other parts of the Nuneaton area, there were workings in Cambrian mudstones (Outwoods Shale Formation) at Chapel End [SP 322 937], and in Coal Measures mudstones and seatearths from small pits in the suburbs of Haunchwood and Heath End [e.g. [SP 344 913]. Material from the Lower Coal Measures (beneath the Seven Feet Coal) was more extensively excavated from Stanley's No. 4 Pit [SP 352 911] and from a pit at Chilvers Coton [SP 355 902]. In the Bedworth area, pits in Wolston Clay and Thrussington Till supplied the raw material for the former Hawkesbury and Bedworth Brick and Tile Works [SP 360 875] and Bedworth Brick Works [SP 363 871]. These excavations are now wholly backfilled, though at the latter site remedial work was carried out in 1987 to isolate contaminated fill. The Wolston Clay was also worked extensively in the Hinckley area, where, in 1913, 'wire-cut red bricks, facing and ornamental bricks, agricultural drainpipes, flower pots and glazed earthenware....were being manufactured' from clay dug from a pit [SP 431 950] near Ashby Road (Eastwood et al., 1923). Other pits nearby were disused at the time of the original six-inch survey.

Farther afield, the red mudstones of the Meriden Formation and Triassic Bromsgrove Sandstone were also exploited. (Table 14) summarises the principal localities of former brickpits for each worked formation.

Current workings and resources

Brick clay is currently only worked at one site, operated by Websters Hemming and Sons Ltd, Midland Brickworks, in Coventry [SP 342 805]. The company produces facing and semi-engineering bricks from mudstones within the Meriden Formation (Whitacre Member).

The Etruria Formation clays which were once the mainstay of the industry have now largely been worked out, except in the south of the crop where they lie beneath thick drift, and are probably too arenaceous to be utilised.

All the other outcropping formations noted in (Table 14) are potential sources of brick clay, but future exploitation is likely to depend much more on economic and environmental factors than on geological considerations.

Crushed rock

Quarrying is still a major industry in the Nuneaton area, based on the Precambrian volcanic rocks, Lower Cambrian Hartshill sandstones and Ordovician lamprophyre sills. The area constitutes one of the nearest sources of hardrock to London, and was already important in the early part of the century. Some of the early, smaller quarries in the Hartshill Sandstone Formation have been abandoned and backfilled, and many of the older quarries have been amalgamated to form the immense Hartshill Quarry [SP 334 942].

The former small quarries in the Precambrian rocks are now mainly within Judkins' Quarry [SP 343 933] , operated both as an active hardrock source (north end) and a methane-producing landfill site (south end).

The Oldbury and Griff lamprophyre sills are exploited in quarries at Mancetter [SP 310 950] and Griff [SP 363 884], respectively. At Griff, lamprophyre and associated hornfelsed mudstones are both taken.

Quartzite and lamprophyre were formerly also quarried west of Judkins' in the Midland Quarry [SP 350 924], and also at the small disused 'Council' Quarry [SP 340 933] at the junction of the Mancetter and Camp Hill roads.

Only one quarry is active outside the Nuneaton Inlier and that is the one at Croft [SP 512 965], which exploits a quartz diorite intrusion within the South Leicestershire Diorites suite. Other quarries in the same suite at Stoney Stanton and Sapcote are no longer worked. Stoney Cove Quarry [SP 493 941] is the largest within this group and is currently used for leisure purposes.

The following list gives the rock type and operating company for each of the working quarries noted above:

Quarry Rock type Operating company
Judkins' Sandstone, volcaniclastic and intrusive rocks ARC Central
Grill Lamprophyre and hornfelsed mudstone Pioneer Aggregates (UK) Ltd
Mancetter Lamprophyre Tilcon Ltd
Hartshill Sandstone Tarmac Roadstone Ltd, Western
Croft Quartz diorite ECC Quarries Ltd

The physical properties of aggregates supplied by three of the quarries are given in (Table 15). The figures are intended as a guide to material properties rather than a precise specification. Total annual output is estimated at about 1.9 million tonnes (BGS data).

All hardrock resources in the Nuneaton Inlier are tightly constrained by their geological distribution and by existing surface developments. The Precambrian rocks (and farther north, the Cambrian Hartshill sandstones) were long thought to be downfaulted along the base of Caldecote Hill. The present survey has shown that the Polesworth Fault lies to the east of its previously mapped line, and would suggest that additional hardrock resources exist beneath initially thin Triassic cover along the base of that slope. The implications of this reinterpretation of the geological structure may be of considerable importance to future planning and environmental interests.

Sand and gravel

The Shawell Gravel has been extensively worked in a series of quarries located on outcrops in the south-east of the district. Most are now worked out and are either backfilled or used as processing sites, but the Gibbet Lane Quarry [SP 536 804] remains active. Cotesbach Quarry [SP 526 818], which formerly extended along the eastern side of the A5 road between Gibbet Hill and Bransford Bridge, has recently been backfilled and restored to agricultural use.

Currently there is no exploitation of the Baginton Sand and Gravel in the district, though it was formerly extracted in pits adjoining [SP 384 788] and to the south [SP 388 780] of Coombe Pool. A BGS Mineral Assessment Report (Crofts, 1982) contains grading analyses for this deposit for the area to the south of gridline 80.

The Wolston Sand and Gravel was formerly worked from two pits in Hinckley (Leicestershire County Council, 1984). One [SP 410 947] has been backfilled and incorporated into a landfill site which now stands up to 8 m above the original surface. The other [SP 417 941] has been restored and its extent is unknown. Workings up to 5 m deep at the junction of Burbage Road and Sapcote Road [SP 4412 9344] and [SP 4416 9335] have been backfilled or built over.

Large potential resources of sand and gravel occur within the Wolston Sand and Gravel and Dunsmore Gravel on the plateau centred around Copston Magna. At the time of writing, a proposal to work these deposits was at the planning stage.

Building stone

The Bromsgrove Sandstone has been used locally as a freestone and, in its rough-cut form, is seen in many of the older houses and churches in the district; it is also incorporated in several bridges spanning the Coventry and Ashby canals. The rock later found fame as a superfine abrasive for rubbing down gold leaf on inlaid tombstones (A F Cooke, written communication, 1992). The stone was formerly worked in several shallow quarries between Bulkington and Nuneaton e.g. [SP 387 889], [SP 378 879] and [SP 381 899].

The Meriden Formation sandstones have also been quarried on a small scale for local use, as has the Cambrian Hartshill Sandstone. Small quarries in the former are found at Tipper's Hill [SP 281 889], at Corley [SP 304 852] and in Allesley [SP 304 812].

Limestone

Thin seams of 'Spirorbis' limestone within the Halesowen and Meriden formations were worked at one time presumably to satisfy the local demand for agricultural lime. Most workings are in the Index Limestone of the Halesowen Formation, which was formerly exploited in shallow pits along the outcrop and from shafts sunk east of Dennis Farm [SP 345 896].

Ironstone

Siderite nodules and bands are present in sufficient concentrations at several levels in the Coal Measures (Figure 25), (Figure 28) to have been worked as a source of ironstone during the late 19th century. Output reached a maximum of 100 000 tonnes in 1874–75, and then declined rapidly (Strahan, 1920). The White Ironstone, occurring between 6 and 18 m above the Seven Feet Coal, is the best known ironstone of the coalfield and was extensively exploited at Hawkesbury and Wyken collieries.

Non-ferrous metals

Manganese, in the form of small pockets of mixed oxides, was mined on a small scale from the Woodlands Member of the Hartshill Sandstone Formation and lower Purley Shales in the first half of the 19th century. 'Manganese pits' are shown on the Old Series one-inch geological map 63SW of 1855, south of Hartshill Castle [SP 326 942] and near Tuttle Hill Farm. Old workings were once seen in Hartshill Hayes, and Parkes (1824) reported that detached pieces up to '60 lbs in weight' were distributed in the red clayey soil.

Lead and copper sulphide mineralisation is associated with a small fault cutting a lamprophyre intrusion in Griff No. 4 Quarry [SP 3640 8872]. The basal Carboniferous sandstones at the same site also host disseminated sulphides.

At Judkins' Quarry [SP 346 930] small-scale occurrences of lead and copper sulphides were found by Ince et al. (1990) within 10 to 20 m of the Precambrian–Cambrian unconformity, a feature which those authors suggest has exerted a strong control over the mineralisation. The mineral assemblages are associated with calcite and baryte, and comprise a dominant calcium-barium-copper suite with minor zinc-lead-vanadium components. The sulphide species present include the copper-bearing sulphides bornite, chalcocite, chalcopyrite and tetrahedrite-tennantite, and the lead sulphide galena. Non-sulphide minerals include azurite and malachite (copper), cerussite (lead) and haematite (iron).

Zinc sulphide, represented by sphalerite, forms part of the Judkins' Quarry mineral assemblage (Ince et al., 1990).

Molybdenum sulphide mineralisation, in the form of molybdenite, occurs very rarely at Croft Quarry [SP 513 964]. The mineralisation occurs within Ordovician intrusive rocks of the South Leicestershire Diorites. It is characterised by complex albitisation and analcitisation, which is superimposed over an earlier assemblage attributed to the deuteric stage of alteration of the host rocks (King and Ford, 1968). The analcitisation takes the form of large veins from which smaller ones bifurcate, and it is typically located within the zones of highly-altered 'rammel' seen in the quarry. The molybdenite occurs within vein-infillings composed of analcime, calcite, laumontite and prehnite.

Vanadium-bearing minerals, which are normally of rare occurrence in the British Isles, are represented at Judkins' Quarry by mottramite and vanadinite. They form late-stage encrustations in association with the minerals described earlier (Ince et al., 1990).

Hydrogeology

Information on the hydrogeology of the district was compiled by C S Cheney. The central, northern and western parts of the district are drained by tributaries of the River Trent, which include to the north-east the River Soar, and in the north-west the River Anker. The western edge lies in the River Blythe catchment and the southern part is drained by south-flowing tributaries of the River Avon (Figure 1). Annual rainfall averages about 700 mm; the average annual potential evaporation is about 490 mm, possibly slightly higher in the south and slightly lower in the north and west.

As can be judged from the distribution of abstraction licences (Table 16), by far the most important aquifer in the district is the Meriden Formation, from which large volumes of water are abstracted for both industrial use and public supply. Currently this formation provides 24 per cent of Coventry's water requirements, the balance deriving from Oldbury Reservoir and the River Severn (Lerner, 1992).

The Halesowen Formation is now less important as an aquifer than it was in the past. Large quantities of water were once abstracted for public supply around Nuneaton and Bedworth, but this ceased because of declining yields.

Surprisingly little groundwater is abstracted from the Sherwood Sandstone Group, and that which is taken is used for industrial rather than agricultural purposes. Attempts to secure groundwater from this source for public supply to Nuneaton and Hinkley have been unsuccessful due to modest yields or poor water quality.

Glacial sands and gravels and, to a lesser extent, the Mercia Mudstone Group provide small yields which are important to the farming community. Small amounts of water have also been obtained from Cambrian and Liassic strata but yields are very low and suitable only for domestic use; there are no current abstraction licences relating to these rocks.

Barren Measures

The Halesowen and Meriden formations together form a complex multilayered aquifer, through which groundwater flow takes place primarily via fractures in the more persistent sandstone bodies; mudstones in the sequence are aquicludes or aquitards.

Sandstones of particular hydrogeological significance occur at the base of the Halesowen Formation and at the base of the Whitacre Member; they are known colloquially, as the '100 Foot' and '40 Foot', respectively (Eastwood et al., 1923). Other important sandstones occur at the top of the Whitacre Member (the Arley and Exhall sandstones), at the top of the Keresley Member (the Corley sandstone) and in the Allesley Member. Most high-yielding boreholes and wells penetrate one or more of these major sandstone units.

The overlying Tile Hill Mudstone Formation, consisting predominantly of red mudstones with subordinate thin sandstone interbeds, supplies small quantities of groundwater sufficient to meet local domestic requirements.

The Etruria Formation and the Coal Measures are dominantly argillaceous with thin, laterally impersistent sandstones. Yields are generally low, and water may be very hard or saline, with iron concentrations which may exceed 4 mg/l.

Halesowen Formation

The basal sandstone of the Halesowen Formation has been penetrated by numerous colliery shafts, water boreholes and wells. Initial yields were invariably large and occasionally represented a major problem for continued shaft sinking. At Whittleford [SP 3281 9186] the inflow was so great as to prevent completion of a pair of shafts, and these were later acquired by Nuneaton Corporation for use in the public water supply. The yield from one of the shafts was initially 23 l/sec but it declined as the water level fell. A heading was driven between the two shafts and this increased the yield to 30 l/s for a drawdown of only 6.8 m, but the improvement was only temporary and it was later necessary to drill narrow diameter boreholes into the bottom of the shafts. Shaft No. 2 is reported to have since collapsed, due to mining subsidence.

This gradual decline of yield with time is common to virtually all shafts, wells and boreholes penetrating the '100 Foot Sandstone' in the district. For example, at Baxterly Well an initial yield of 37 l/s declined to 3.4 l/s over a two-year period; at Exhall Colliery shaft a reported yield of 114 l/s declined to only 13 l/s over a similar period; and at Haunchwood Colliery Tunnel Pit a yield of 15 l/s declined to 3.7 l/s. The last example led Lapworth to conclude that the yield from this sandstone could be expected to decrease by more than half for each mile distance from the outcrop (Richardson, 1928).

At Hawkesbury Pumping Station a large-diameter well was initially capable of yielding almost 80 l/s. The well penetrated the Bromsgrove Sandstone Formation above the Halesowen Formation but little water was encountered until a very hard white sandstone at the bottom of the well was blasted, causing a massive inrush of water.

This well was later derogated by falling water levels due to heavy pumping at Exhall Colliery.

Water quality from the Halesowen Formation is generally good, if somewhat hard, containing high concentrations of iron as illustrated by the analysis of a water sample from Clara Well, Griff Colliery (Table 17). At Whittleford Pumping Station it was necessary to aerate the water to remove excessive concentrations of iron and manganese. At Exhall Colliery shaft water salinity increased considerably during the course of aquifer testing, due perhaps to the ingress of more saline water from the deeper Coal Measures. Indeed, a 250 mm diameter borehole at Longford was plugged at the base of the Halesowen Formation, presumably to prevent the poor-quality groundwater present in the Coal Measures from mixing with the better-quality water from the Halesowen Formation. Downing et al. (1966) reported evidence that water quality declines with depth in the Halesowen Formation. During shaft sinking at Griff Colliery, water was struck in fissures at 64 and 75 m; total dissolved solids in water from the upper level were 400 mg/l, and 660 mg/l in the lower, whilst total hardness values were 257 mg/l and 342 mg/l respectively.

Meriden Formation

Higher-yielding boreholes and wells in the Meriden Formation commonly penetrate one or more of the significant sandstone units. Severn Trent Water (1986) reported that yields tend to reflect the amount of sandstone encountered in a borehole. However, it is difficult to ascribe hydrogeological information from available drilling records to a particular part of the sequence. Most boreholes were drilled for industrial use or public supply. Many of the sources formerly used by industry are now abandoned and only 14 current abstraction licences remain (Table 16). Groundwater abstraction in the Coventry area, like that in many other industrial cities, has declined since the early 1960s.

A few supply boreholes, such as Stoney Road [SP 3331 7915], have become disused in recent years due to declining yields or contamination, but most have been in continual use for long periods. The earliest borehole at the Spon End/Doe Bank site was completed in 1855 and the construction of additional boreholes continued until 1875. These sources, which were discussed in some detail by Lyon (1949), are artesian and have remained in use to the present day. Other public supply sources currently providing water to the Coventry area are located at Mount Nod, Brownshill Green, Meriden Water Works and Watery Lane.

Lerner et al. (1992) reported that hydraulic conductivities measured on sandstone core plugs obtained from a coal exploration borehole drilled in the Meriden Formation lay in the range of 10–4 to 0.58 m/d with 80 per cent of values in the range of 10–4 to 10–2 m/d. Porosities were in the range of 5 to 22 per cent with a median of 11 per cent. An estimate of fracture porosity of 0.3 per cent was also provided. Hydraulic conductivities obtained from packer testing ranged from 0.01 to 2.88 m/d; the greater magnitude of these values as compared to those obtained from core plugs indicates the importance of fissure flow in the sandstone units. Bishop et al. (1992) reported the results of two constant-rate pumping tests carried out on sandstone units of the Meriden Formation. One provided a transmissivity of 26 m2/d and corresponding hydraulic conductivity of 3.3 m/d for the sandstones, whilst the second gave a transmissivity value of 17 m2/d, a hydraulic conductivity of 1.7 m/d and storage coefficient of 4 X 10−3.

Lerner (1992) attempted to interpret a regional pattern of groundwater levels in the Meriden Formation but found that the presence of strong vertical head gradients in the multilayered aquifer, in addition to water level responses to the pumping of deep boreholes made this difficult. A general trend was discernable from information on old rest water levels collected when boreholes were drilled, although major inconsistencies were present. A groundwater gradient exists (or existed) from recharge areas to the north and west of Coventry and on interfluves, towards discharge areas in streams in the centre and east of the outcrop area. Groundwater levels vary from artesian overflowing, to about 55 m below surface. Lerner noted that observed groundwater levels partly reflect borehole depth and strata penetrated, but also that the patterns of recharge and discharge had been considerably modified by human influence. In urban areas additional sources of recharge exist, in the form of leaking water mains, which more than compensate for reductions of recharge due to the presence of impermeable surfaces, whilst some deep sewers provide extra recharge points.

Yields from sandstones at the base of the Whitacre Member (the '40 Foot Sandstone') generally range between 8 and 15 l/s but for highly variable amounts of drawdown, ranging from only a few metres to over 70 m. The highest recorded yield obtained solely from this sandstone was almost 38 l/s for a drawdown of 10.5 m from a 250 mm diameter borehole at Foleshill (Goutaulds No. 8). Acid treatment carried out on another site in the Foleshill area (Little Heath No. 4) provided an increase in yield from 12.7 l/s for a drawdown of 103.9 m, to a yield of 14.1 l/s for the reduced drawdown of 78.6 m; the specific capacity was improved from 0.1 to 0.2 l/s/m.

There are indications that at some sites, yields and rest water levels have declined with time. This was illustrated most dramatically at Birchley Heath Well where a yield of 15.8 l/s in 1882 declined to only 2.3 l/s by 1914. Elsewhere the decline, if detectable, was more modest and was in any case of a smaller order than that observed for the Halesowen Formation. Shallow wells and boreholes that do not penetrate any of the major sandstone horizons yield between 0.3 and 0.5 l/s; dry boreholes are rare.

The quality of groundwaters from the Meriden Formation is generally good, although there is some variation between the different sandstones. This is illustrated in (Table 17) by analyses of groundwaters obtained from the Allesley Member, the Keresley Member (Corley sandstone) and the top and bottom of the Whitacre Member (the Arley/Exhall and '40 Foot' sandstones, respectively). These analyses broadly reflect a more extensive data set presented by Lyon (1949), which showed that water from the Whitacre Member, although potable, is generally more mineralised than water obtained from sandstones located higher in the Meriden Formation. Chloride concentrations, for example, only rarely exceed 20 mg/l in waters from the upper sandstones but commonly exceed 30 mg/l (ranging to over 100 mg/l) in waters from the Whitacre Member (Arley and Exhall sandstones). Calcium, sulphate and bicarbonate concentrations are also generally higher in ground-waters from the lower sandstone units. In contrast to the situation in the Halesowen Formation, dissolved iron is not a problem in the Meriden Formation.

Major ions (particularly calcium, magnesium, sulphate, chloride, nitrate) and metals are present at higher concentrations beneath the Coventry urban area than elsewhere in the present district but are not excessive in terms of water potability (Lerner and Tellam, 1992). Sewer leakage is the main source of inorganic pollution although industry may also contribute. The concentration of boron in urban boreholes is seven times that found in rural boreholes; although boron is associated with the metal-working industry, the more probable source is detergent contained in sewage (Burston et al., 1992).

As in many other industrialised areas, groundwater contamination by organic chlorinated solvents is widespread in the Coventry area. Almost all urban boreholes contain over 10 pg/1 of such solvents (Lerner and Tellam, 1992), whereas this type of contamination is almost absent in nearby rural locations. The solvents are a particularly insidious groundwater contaminant in that they are only very sparingly soluble in water, do not readily degrade under natural conditions, and are very persistent in the subsurface. The immiscible phase is significantly more dense and less viscous than water. This combination of properties can result in rapid and deep penetration of the aquifer, although the presence of mudstone beds limits the effect (Burston et al., 1992). Once in the aquifer the immiscible phase will provide a long-term source of contamination. The solvents are widely used for cleaning and degreasing in the heavy- engineering, metal-working and new high-technology industries, as well as for dry-cleaning in laundries. Of the five solvents found during a recent study (Burston et al., 1992), the most common was trichloroethene (TCE) which was found at concentrations over 1 pg/1 in all but two of a total of 23 sources; 38 per cent of boreholes exceeded the drinking water limit of 30 pg/l. This limit is exceeded in one public water supply borehole, where the water is treated to reduce the concentration to acceptable levels (Lerner and Tellam, 1992).

Sherwood Sandstone Group

The Sherwood Sandstone Group provides the most important and prolific aquifers in the English Midlands. In this district, however, the group is represented at outcrop by the Bromsgrove Sandstone, a fine- to medium-grained, well-cemented micaceous sandstone, with subordinate mudstones, which possesses relatively little primary or secondary permeability (except where fissured). Yields are therefore disappointingly low and there is only a small amount of licensed abstraction from this aquifer in the district (Table 16). Much of the outcrop to the south of Nuneaton is, in any case, obscured by thick argillaceous drift which limits aquifer recharge.

In aquifer outcrop areas, yields generally vary from about 3 l/s from a 200 mm diameter borehole to 6 l/s from a 500 mm diameter borehole. At the former Sterling Metals site to the south of Nuneaton two 300 mm diameter boreholes were subjected to blasting in an effort to improve their yield. This was to some degree successful; in the case of one borehole (Sterling Metals No. 2) an initial yield of 4.7 l/s for a drawdown of 16.8 m was increased to 10 l/s for a drawdown of 19.8 m, an appreciable improvement in specific capacity from 0.3 to 0.5 l/s/m. Wherever the aquifer is confined by the Mercia Mudstone Group, borehole yields are greater and a maximum recorded yield of 21 l/s for an unknown amount of drawdown from a trial water supply borehole was recorded at Hinckley Wharf.

Water quality at outcrop is good, but hard, and is illustrated by an analysis from a borehole from Sterling Metals No. 1 Borehole (Table 17). In the Hinckley area, several kilometres from outcrop, and where the aquifer is deeply confined by the Mercia Mudstone Group, groundwater from the Sherwood Sandstone Group is brackish or saline. These waters have high concentrations of sodium, chloride and sulphate ions. An analysis from the Hinckley Wharf trial borehole is shown in (Table 17). There is little information regarding water quality in areas to the south of Hinckley but it is expected that water quality there would be very similar to that found at Hinkley.

Mercia Mudstone Group

The Mercia Mudstone Group consists predominantly of mudstones interbedded with thin impersistent siltstones and sandstones (skerries). The thicker and more persistent Arden Sandstone, in the upper part of the mudstone sequence, is present in the subcrop notably around Hinckley.

The mudstones are effectively impermeable but usable supplies of water may be present within fissures in the sandstone beds. Yields are generally low, ranging from less than 0.3 to 1 l/s from boreholes of 150 mm diameter; drawdown ranges from about 2 to 20 m. The highest yield recorded in the district is 1.8 l/s for a draw-down of 18.3 m in a borehole at Hinckley [SP 426 941]; it penetrated about 18 m of Arden Sandstone, but was later abandoned owing to the collapse of unlined mudstone sections. Monkhouse (1984) in a study of borehole yields throughout the English Midlands suggested that there was a 50 per cent probability of obtaining a yield in excess of 0.8 l/s for a drawdown of less than 10 m from a borehole of 150 mm diameter penetrating 30 m of any saturated aquifer in the Mercia Mudstone, and a 75 per cent probability of a yield in excess of 0.2 l/s.

Little information is available regarding water quality in the Mercia Mudstone but it would appear to be generally potable, although very hard in places.

Superficial deposits

Argillaceous till deposits, together with sands and gravels, occur over much of the eastern part of the district. The Anker Sand and Gravel, Dunsmore Gravel, Wolston Sand and Gravel, and Baginton Sand and Gravel may offer limited amounts of water. These deposits rarely attain a thickness of 20 m and are commonly thinner.

At Villa Farm, Wolvey [SP 4274 8695] the Wolston Sand and Gravel varies between 3 and 12 m in thickness and intervenes between two till sheets. The sand is unconsolidated, with a median grain diameter of between 125 μm and 350 μm and is fairly well sorted. Permeability of the sands at this location lies in the range 1 to 15 m/d (Williams et al., 1984).

In the case of the Wolston and Baginton sands and gravels, the deposits are mostly concealed by impermeable till, with exposures restricted to valley sides. This configuration severely limits the potential for recharge, but permits rapid drainage from the sands and gravels into the valleys, thereby constraining the potential saturated thickness. Yields rarely exceed 1 l/s, and often decline rapidly in response to pumping. The very fine 'running' nature of some of the glacial sands may cause severe silting problems, which cannot readily be rectified even by the use of a standard screen and gravel pack.

Numerous springs issue from sand and gravel beds where they are underlain by impermeable clays. Some springs near Hinckley were reputed to have special medicinal properties. Golden Well, a slightly iron-rich (or chalybeate) spring located at Sapcote [SP 4900 9369], was used for bathing at the turn of the century, and the water had a considerable reputation for benefitting a wide variety of medical conditions (Richardson, 1931).

Groundwater quality data for the sands and gravels are sparse. The water is generally potable but hard, and may be iron-rich in some locations. These aquifers are particularly vulnerable to contamination from dispersed pollutants.

Made ground, landfill and waste disposal

Areas identified as Made Ground on the 1:50 000 geological map fall into two main categories. The first includes the numerous quarries and pits left by the extraction of opencast clay, hardrock for aggregate, or sand and gravel, and which over the years have been partly or wholly backfilled; the majority of these lie in a strip along the exposed coalfield and the Nuneaton Inlier. The second category comprises material built up above the original ground surface; it includes colliery spoil tips, now mostly landscaped, waste from tunnelling operations, and land reclamation schemes in areas affected by mining subsidence, such as the Sowe valley.

Landfill sites for domestic waste are virtually all sited on strata of low permeability and pose little risk to groundwater resources. The two sites currently operational are at Griff No. 2 Quarry [SP 361 895], sited on Cambrian mudstone and lamprophyre, and at Judkins' Quarry [SP 345 930] on the Hartshill Sandstone Formation and Precambrian basement rocks.

There is some evidence that potentially hazardous wastes were disposed of in sand and gravel pits prior to the licensing of landfill sites under the Control of Pollution Act, 1974. Such tips present a risk to groundwaters in the superficial deposits. For example, local groundwater contamination in the Wolston Sand and Gravel at Villa Farm, Wolvey, was caused by the disposal of a wide range of liquid wastes containing heavy metals and organic solvents into a lagoon that penetrated below the water table. Extensive research in the vicinity of the site showed that the development of the pollution plume was controlled by aquifer morphology, permeability distribution within the aquifer, and hydraulic head distribution in the vicinity of the lagoons. It was found that heavy metals were attenuated, probably by precipitation as sulphides and carbonates, within a short distance from the lagoon. Organic contaminants, however, were found up to 300 m from the lagoons. Changes in bacterial populations and variations in organic compounds present at the base of the aquifer suggested that some biodegradation to methane and carbon dioxide was taking place (Williams et al., 1984; Hitchman and Williams, 1988).

Engineering geology

The geotechnical properties of the bedrock and superficial deposits, as they affect engineering practice, are discussed in detail by P R N Hobbs in a technical report commissioned by the Department of the Environment, which covers the southern part of the Coventry district (Old et al., 1990). A summary of the main findings, together with additional information collected for the northern part of the district, has been compiled by S M M Fenwick for this memoir.

Mudrocks and clays

The mudrocks of the Cambrian, Devonian and coal-bearing Carboniferous formations consist of mudstones, siltstones and heavily overconsolidated clays. Pyrite is an ubiquitous mineral in this group of rocks, and its presence can result in high groundwater sulphate contents, requiring the use of special concrete and protective coatings. Taylor and Rushton (1972) quote analyses for the Outwood Shales Formation, which show carbon and sulphur present at levels equivalent to 7.9 per cent pyrite.

Slickensided surfaces are especially common in the softer seatclays of the Coal Measures and Millstone Grit. Where these occur in combination with steeply dipping strata and high-angle jointing, there is increased risk of failure in steep cut slopes. Such conditions are most likely to be found along the outcrop of the Coal Measures and in the rocks forming the western flank of the Nuneaton Inlier.

Amongst the younger, predominantly redbed mudstones of Upper Carboniferous to Triassic age, the Mercia Mudstone is best researched and is generally classified as a heavily overconsolidated clay when fresh; however, weathering decreases its residual strength, as well as causing structural changes which influence triaxial and consolidation results. Gypsum, present as nodules or veinlets, can result in locally high sulphate contents.

Sandstones and conglomerates

Sandstones in the district vary widely in strength depending on their composition, grain size, nature of the intergranular cement, and structural homogeneity. Massive cemented types may be very strong, whereas other types may be weak, with the least cemented behaving like sandy soils.

The calcium carbonate cement that generally binds the sandstones of the Meriden Formation and Bromsgrove Sandstone is easily weathered, causing breakdown of the rock to sand in the weathered zone and allowing the possibility of void migration by enlargement of joints.

Strength is also very sensitive to moisture content, particularly in the weaker sandstones. For example, the strength of the Bromsgrove Sandstone at the near dry state can be reduced by 80 per cent on saturation. Jointing strongly influences mass strength, and can lead to severe and unpredictable subsidence effects when undermining takes place.

All known cut slopes in sandstone appear stable; an example is the 10 m vertical cutting in the Bromsgrove Sandstone at the entrance to Judkins' Quarry [SP 3477 9303].

Igneous rocks

All the igneous rocks are strongly jointed, and this reduces their stability on steep cut slopes. In the lamprophyre sills, however, calcite joint-infillings may reduce the likelihood of failure. The South Leicestershire Diorites locally have a deep sub-Triassic weathering profile, which, in places, penetrates 3 to 15 m below the unconformity (Worssam and Old, 1988). This weathering is most marked in areas affected by earlier hydrothermal alteration; it results in a rubbly 'diorite sand' of dramatically different geotechnical character to that of the parent rock (Sylvester-Bradley and Ford, 1968).

Quaternary superficial deposits

The superficial deposits of the district are lithologically varied and are subject to rapid lateral variation. It is important, therefore, that detailed site investigations are carried out to determine foundation conditions prior to development.

The deposits broadly fall into four engineering geological groups based on their lithology and geotechnical characteristics (Table 18); landslips (including colluvium and solifluction deposits) and glaciotectonically disturbed ground are discussed separately.

Heterogeneous overconsolidated/dense deposits

This group includes the Thrussington and Oadby tills and parts of the Wolston Clay. These deposits are typically structureless and matrix-dominated, but can vary greatly in composition and thickness. Clasts up to 200 mm in size are supported in a firm to hard overconsolidated clay matrix, with some associated silt and sand. In the weathered zone, which is typically only 1 to 2 m thick, the till is softened so that the liquidity index and moisture content are increased.

The engineering behaviour of such tills is governed by the matrix. Excavation is reasonably easy, but can be hampered by unexpected pockets of running sand or silt. Additionally, swelling and heave may occur at the base of an excavation, particularly when nonsaturated, overconsolidated clays are exposed to the elements. Cut slopes of up to a maximum of 45° have been recommended for till where groundwater can be controlled. Reddish brown tills incorporating large amounts of Mercia Mudstone detritus may contain localised high sulphate contents.

Normally to slightly overconsolidated cohesive deposits

This group encompasses glacial lake clays, represented mainly by the Wolston Clay. Moisture contents may reach over 30 per cent in the upper 2 m; plasticities can be high, with strengths generally lower than for till but still implying slight or pseudo-overconsolidation.

Though their overall lithology is relatively uniform, glacial lake clays have a strongly layered structure. This results in anisotropy in drained strength and permeability values, as well as in swelling and consolidation behaviour. The Wolston Clay locally includes layers of gravelly till and laminae or lenses of sand, any of which may contain perched groundwater. The Wolston Clay was used extensively in the construction of embankments for the Coventry Eastern Bypass.

Dense non-cohesive deposits

This group encompasses the Baginton Sand and Gravel, the Shawell Sand and Gravel, and the Wolston Sand and Gravel, together with smaller unnamed glaciofluvial deposits in the west of the district. These deposits are generally dense and well sorted. The Wolston Sand and Gravel is the most laterally continuous, though where it is only a few metres thick foundation loading will normally be transmitted into the clay below.

The Shawell Sand and Gravel, confined to the east of the district, contains discontinuous ferruginous hard pans which may give very high standard penetration test (SPT) and cone penetration test (CPT) values.

Many of these granular beds contain perched aquifers, which may be artesian, and give rise to running sands. Such conditions are particularly prevalent in the Wolston Sand and Gravel and can pose a hazard to construction; in one instance an M69 motorway bridge founded in this unit had to be rebuilt, and in another case involving installation of underground services around the village of Barnacle, the completion of the scheme was considerably delayed by these unforeseen ground conditions.

Normally consolidated cohesive and loose non-cohesive deposits

Because of their minor representation in the Coventry district, there is little information on the geotechnical properties of the local alluvium and river terrace deposits, which make up the bulk of this group. In general terms, however, alluvial soils tend to be of high compressibility and low strength, particularly those having a high content of fines. Peat or organic clay lenses, which occur locally within the river terraces and alluvial tracts, are usually acidic (pH 4 to 5), tend to be highly compressible and are subject to long-term creep. Such layers are generally thin and of minor lateral extent, so can be stripped off. Small deposits of mainly superficial peat and organic-rich deposits may be expected where seepage from glaciofluvial sands and gravels has occurred on to clay slopes below; such deposits are mainly found near the contact between the Wolston Sand and Gravel and the Wolston Clay. More continuous peaty deposits may occur along valleys cut into these sands and gravels.

Head, colluvium and landslip deposits

Although head and colluvium are not widely mapped in the district, their presence should be anticipated, and their removal should be considered, since downslope movement may be reactivated by engineering work. Head has a higher moisture content and lower strength than its parent material, and is potentially unstable on slopes angled as low as 5°, particularly if derived from lithologies of high plasticity, such as the Wolston Clay. On steeper slopes, colluvium may 'creep' downslope, whilst on shallower slopes flow-type landslips can occur. Large natural landslips have not been identified in the district, though cut slope instability has occurred along the M69 motorway and the Coventry Eastern Bypass. Small degraded natural slips, and low-angle slips with little surface expression, may easily go unnoticed, but can form engineering hazards.

Glaciotectonically disturbed ground

Glaciotectonic movement has severely disrupted the drift sequence in the north-east of the district, causing displacement of large slabs of Mercia Mudstone and deforming the Thrussington Till into steep-sided diapirs up to 7 m high, injected into sand. High- and low-angle thrust planes, and steep normal faults associated with this deformation, may be reactivated as gravitational slip planes by engineering works, particularly cuttings.

A common effect of this deformation has been to juxtapose drift and bedrock deposits of widely differing geotechnical properties. Very detailed site investigations involving trial pits and trenches are recommended in the areas where glaciotectonic disturbance has been noted, in order to define planar features and lateral lithological changes.

Potential geological risk factors

Mining

The shallow pre-20th century workings along the eastern margin of the coalfield are not well documented, and present a potential subsidence hazard. Some areas of flooded and low-lying boggy ground close to the crop can be attributed to subsidence above old shallow workings [e.g. [SP 343 869]. The positions of many of the associated shafts are poorly recorded, and some may remain open beneath a near-surface capping. Records show that the majority of shafts lie within a corridor 500 to 700 m wide, running parallel to the outcrop (or incrop) of the Seven Feet Coal on its western (down-dip) side.

Modern total-extraction mining methods minimise the affect of subsidence but differential movement at surface must be expected, particularly in faulted ground, in areas of active undermining.

Earthquakes in the Coventry district

The following details have been supplied by Dr R M W Musson of the Global Seismology Unit. The Coventry district is not very active seismically, and no significant earthquakes are known to have epicentres here. BGS instrumental earthquake monitoring networks have only ever detected one small event in the district at a location [SP 292 885] north-west of Coventry; the magnitude was 0.4 ML on the Richter Local Magnitude scale and the event was not felt (Turbitt et al., 1990). However, a number of the larger British earthquakes have been felt in the district, notably those originating in the Hereford area (1863, 1896, 1926) and the Leicester–Derby area. For example, the earthquake of 11 February 1957, magnitude 5.2 ML, with epicentre near Castle Donnington, was strongly felt in Nuneaton though not so noticeable in Coventry (Neilson et al., 1984; Musson, 1990).

Landfill gas

The generation in modern landfill sites of methane gas through the decomposition of organic material is now carefully controlled, following the well-documented explosion at Loscoe, Derbyshire in 1986. However, in older sites lacking adequate ventilation, there is a continuing potential for lateral migration of landfill gases and associated leachates. In the present district, sites where methane generation is occurring are carefully managed, and gas levels are monitored on a regular basis by the local authorities.

Methane from abandoned mine workings

Methane is contained within coal either as a free gas or as an adsorbed layer on the surface of fissures and pores. Mining of the coal releases the methane, which during the lifetime of the mine is collected by the mine ventilation circuit. When mining ceases, the continued production of methane may lead to a buildup of high concentrations in collapsed workings and roadways. In the case of shallow mines, the gas may be able to migrate to surface, where it can pose a hazard, particularly if accumulations are unable to vent to the air. Experience suggests that most surface emissions originate from workings no deeper than 100 m below ground. In the context of the Warwickshire Coalfield, this means that the areas at possible risk are confined to a narrow belt extending parallel to the outcrop of the main seams, with another small area overlying shallow workings in the vicinity of the former Arley Colliery. Development within either of these areas should therefore include an assessment of the potential risk from mine gas, as well as the more usual geotechnical investigations.

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references can be purchased subject to the current copyright legislation.

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Appendix 1 Selected boreholes

This list includes the permanent record number, location, total depth and stratigraphical range (oldest formation listed first) of the boreholes referred to in this memoir. Boreholes with a suffix,

C are commercial-in-confidence and abstract logs are published with the permission of the owner. Most of the boreholes listed are shown on the relevant 1:10 000 geological maps listed under History of Survey; additional details are to be found in the open-file reports (Appendix 2). Copies of non-confidential records may be obtained from the British Geological Survey, Keyworth, Nottingham NG12 5GG at an advertised tariff.

Ansley Hall Colliery (SP39SW/52) [SP 3062 9369] 128.7 m Coal Measures, Halesowen Formation
Aston Flamville (SP49SW/49C) [SP 458 931] No log received; terminated in Stocking-ford Shale Group
Back Lane (SP47NE/58) [SP 4767 7968] 22.98 m Wolston Clay, Wolston Sand and Gravel, Oadby Till
Bar Pool Valley No. 3 (SP39SE/236) [SP 3526 9196] 11.0 m Stockingford Shale Group, Bromsgrove Sandstone Formation
Bar Pool Valley No. 4 (SP39SE/237) [SP 3534 9189] 9.5 m Stockingford Shale Group, Bromsgrove Sandstone Formation
Beanit Spinney (SP27NE/8C) [SP 2655 7658] 1138.0 m Stockingford Shale Group, Coa Measures, Etruria Formation, Halesowen Formation, Meriden Formation, Tile Hill Mudstone Formation
Bentley (SP29NE/2) [SP 2832 9607] 82.0 m No log
Berryfields Farm (SP28SW/179C) [SP 2498 8148] 1012.8 m Merevale Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Birch Tree Farm (SP38SW/161C) [SP 3102 8288] 862.7 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation and drift
Birchley Hall (SP28SE/135C) [SP 2743 8446] Full log not received; terminated in Stockingford Shale Group
Birchley Heath (SP29SE/13) [SP 2798 9421] 22.0 m Meriden Formation
Blabers Hall (SP28NE/122C) [SP 2533 8607] 687.5 m Merevale Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Blind Lane (SP27NW/2C) [SP 2450 7962] 1040.5 m Merevale Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation, Tile Hill Mudstone Formation
Bockendon (SP27NE/50C) [SP 2801 7525] 1167.7 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation, Tile Hill Mudstone Formation
Bond Street (SP49SW/56) [SP 4264 9415] 93.9 m Mercia Mudstone Group and drift
Bramcote Barracks (SP48NW/94) [SP 408 887] 122 m Mercia Mudstone Group, Thrussington Till, Wolston Clay, Wolston Sand and Gravel/Dunsmore Gravel
Brownshill Green Farm (SP38SW/100C) [SP 3069 8263] 929.0 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation and drift
Brownshill Green Pumping Station (SP38SW/109) [SP 3063 8270] 65.5 m Meriden Formation
Bulkington Borehole (SP38NE/52) [SP 3900 8672] 77.7 m ?Halesowen Formation, Bromsgrove Sandstone Formation, ?Mercia Mudstone Group and drift
Bulkington Sewerage No. 5 (SP38NE/300) [SP 3968 8665] 9.5 m Wolston Clay, Dunsmore Gravel
Caldecote Hill Farm No. 1 (SP39SW/247) [SP 3436 9363] 11.7 m Caldecote Volcanic Formation, Bromsgrove Sandstone Formation, Mercia Mudstone Group
Caldecote Hill Farm No. 2 (SP39SW/248) [SP 3439 9367] 34.1 m Caldecote Volcanic Formation and associated intrusives, Bromsgrove Sandstone Formation, Mercia Mudstone Group
Caldecote Hill Farm No. 3 (SP39SW/249) [SP 3441 9371] 76.0 m Caldecote Volcanic Formation and associated intrusives, Bromsgrove Sandstone Formation, Mercia Mudstone Group
Chantry Wood (SP28SE/3C) [SP 2580 8370] 876.7 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Chapel Green (SP28NE/128C) [SP 2741 8612] Full log not received; terminated in Stockingford Shale Group
Charity Colliery (SP38NE/2) [SP 3544 8768] 314.8 m Stockingford Shale Group, Millstone Grit, Coal Measures, Etruria Formation, Halesowen Formation and Thrussington Till
Clara Well (SP38NW/126) [SP 3478 8889] 69.2 m Halesowen Formation and drift
Clifford Bridge No. B1 (SP38SE/606) [SP 3761 8079] 159.0 m Stockingford Shale Group, Bromsgrove Sandstone Formation, ?Mercia Mudstone Group and drift
Cloudesley Farm (SP48NE/5) [SP 4588 8580] 65 m Mercia Mudstone Group, Glaciofluvial deposits, Wolston Clay, Wolston Sand and Gravel, Oadby Till
Combe Abbey No. 1 (SP37NE/103) [SP 3977 7891] 99.7 m Stockingford Shale Group, Bromsgrove Sandstone Formation, Mercia Mudstone Group
Combe Abbey No. 2 (SP48SW/1) [SP 4101 8268] 90.2 m Diorite intrusion, Stockingford Shale Group, Bromsgrove Sandstone Formation, Mercia Mudstone Group and drift
Corley Moor (SP28SE/2C) [SP 2819 8402] 843.7 m Monks Park Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Courtaulds No. 3 (SP38SW/134) [SP 3365 8083] 321.6 m Meriden Formation
Courtaulds No. 8 (SP38SW/137) [SP 3408 8079] 243.5 m Meriden Formation
Coventry Colliery No. 2 shaft (SP38SW/713) [SP 3217 8440] 662.0 m Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation and drift
Coventry Colliery No. 3 shaft (SP38NW/69) (underground) [SP 3108 8563] 57.4 m Stockingford Shale Group, Coal Measures
Dadlington (SP39NE/4) [SP 398 991] 358.0 m Stockingford Shale Group, Moira Breccia, Polesworth Formation, Bromsgrove Sandstone Formation, Mercia Mudstone Group and drift
Dale Wood (SP28NW/258C) [SP 2632 8701] 699.7 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Daniels Wood (SP28NW/258C) [SP 2467 8583] Full log not received
Daw Mill No. 122 (SP28NE/125C) (underground) [SP 2633 8759] 72.48 m Monks Park Shale Formation, Coal Measures
Dove House Farm (SP28NW/1) [SP 2472 8912] 643.9 m Monks Park Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Exhall Colliery (SP38NE/10) [SP 3572 8547] 265.8 m Coal Measures, Etruria Formation, Halesowen Formation and drift
Fillongley (SP28NE/1) [SP 2614 8700] 621.5 m Monks Park Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Fillongley Hall (SP28NE/127C) [SP 2673 8517] Full log not received; terminated in Stockingford Shale Group
Flints Green (SP27NE/47C) [SP 2626 7982] 1050.0 m Stockingford Shale Group, Coal Measures, Halesowen Formation, Etruria Formation,
Frank Shaft Meriden Formation, Tile Hill Mudstone Formation and drift (SP38NW/2) [SP 3346 8694] 476.3 m Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation and drift
Furlongs Farm (SP38NE/538) [SP 3975 8542] 111.4 m Stockingford Shale Group, Coal Measures, Bromsgrove Sandstone Formation, Mercia Mudstone Group and drift
Furnace Fields No. 1 (SP38NE/243) [SP 3681 8759] 6.0 m Stockingford Shale Group and drift
Furnace Fields No. 2 (SP38NE/244) [SP 3677 8755] 6.0 m Stockingford Shale Group and drift
Furnace Fields No. 3 (SP38 NE/245) [SP 3670 8757] 6.0 m Stockingford Shale Group and drift
Gables Farm (SP28SE/108C) [SP 2921 8175] 930.0 m Full log not received; terminated in Stockingford Shale Group
Grange Farm SP48SW/140 [SP 4228 8046] 132.16 m Merevale Shale Formation, Bromsgrove Sandstone Formation, Mercia Mudstone Group and drift
Greenways Farm (SP28SE/24C) [SP 2547 8075] 1047.8 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation, Tile Hill Mudstone Formation
Griff Colliery No. 4 shaft and borehole (SP39SW/10) [SP 3491 9052] 212.4 m Stockingford Shale Group, ?Millstone Grit, Coal Measures, Etruria Formation and drift
Griff Clara (SP38NW/1) [SP 3481 8896] 262.2 m Coal Measures, Etruria Formation, Halesowen Formation
Hartshill No. 1 (SP39NW/65) [SP 3267 9526] 13.9 m Hartshill Sandstone Formation, Bromsgrove Sandstone Formation
Hartshill No. 2 (SP39NW/66) [SP 3272 9539] 34.2 m Hartshill Sandstone Formation, Bromsgrove Sandstone Formation, Mercia Mudstone Group
Haunchwood Colliery (SP39SW/17) [SP 3143 9172] 338.8 m Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Hawkes End (SP28SE/81C) [SP 2997 8256] 952.6 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Hawkesbury Colliery (New Winning Pit) (SP38NE/11) [SP 3620 8546] 257 m Stockingford Shale Group, Coal Measures, Etruria Formation and drift
Hawkesbury Lane Station (SP38SE/317) [SP 3556 8489] 73.9 m Halesowen Formation, Meriden Formation and drift
Hawkesbury Pumping Station (SP38SE/318) [SP 3623 8461] 36.6 m Halesowen Formation, Bromsgrove Sandstone Formation and drift
Hazel Grove (SP38SW/162C) [SP 3015 8375] 872.0 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation and drift
Hillfields (SP28SE/137C) [SP 2762 8285] Full log not received
Hinckley Wharf (SP49SW/53) [SP 4128 9314] 245.4 m Polesworth Formation, Bromsgrove Sandstone Formation, Mercia Mudstone Group and drift
Hollyberry Hall (SP28SE/134C) [SP 2772 8383] Full log not received; terminated in Stockingford Shale Group
Hollyfast (SP38SW/13) [SP 3004 8368] 875.4 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Holy Well (SP49SW/52) [SP 4327 9395] 167.6 m Bromsgrove Sandstone Formation, Mercia Mudstone Group and drift
Keresley 1A (SP38SW/2) [SP 3158 8475] 662.6 m Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Kimberley's Grove (SP28NW/7C) [SP 2462 8764] 671.7 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Ley's Farm Nos. 1–4 (SP28SE/103) , (SP28SE/104) , (SP28SE/105) , (SP28SE/106) [SP 282 831] 7.0 m maximum: Till, sand and silt
Library No. 1 (SP38NE/295) [SP 3598 8681] 11.0 m Thrussington Till/Wolston Clay, Glaciofluvial sand and gravel
Little Heath No. 4 (SP38SE/4) [SP 3525 8270] 198.1 m Halesowen Formation, Meriden Formation and drift
Lodge Farm (SP48NE/6) [SP 4994 8623] 31.5 m Mercia Mudstone Group, Baginton Sand and Gravel, Thrussington Till, Wolston Clay, Wolston Sand and Gravel
Long Lady Wood (SP28SE/27C) [SP 2977 8419] 862.1 m Monks Park Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Longford (SP38SE/326) [SP 3624 8425] 122.3 m Coal Measures, Etruria Formation, Halesowen Formation, Bromsgrove Sandstone Formation and drift
Lutterworth (SP58SW/18) [SP 5472 8490] 123.9 m Mercia Mudstone Group, Penarth Group, Lias Group and drift
Merevale Nos. 1 & 1A (SP29NE/16) [SP 2994 9608] 217.6 m Stockingford Shale Group, Oldbury Farm Sandstone Formation
Merevale No. 2 (SP39NW/6) [SP 3001 9509] 258 m Monks Park Shale Formation, Merevale Shale Formation, Oldbury Farm Sandstone Formation, Millstone Grit, Coal Measures and drift
Merevale No. 3 (SP39NW/7) [SP 3071 9574] 249.6 m Abbey Shale Formation, Mancetter Shale Formation, Outwoods Shale Formation
Meriden (SP28SE/1C) [SP 2681 8187] 1028.0 m Merevale Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation and drift
Meriden No. 1 Well (SP28SE/6) [SP 2623 8262] 203.0 m Meriden Formation
Meriden No. 2 Well (SP28SE/7) [SP 2623 8262] 204.1 m Meriden Formation
Moat House Farm (SP28NE/68C) [SP 2776 8567] 716.5 m Merevale Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation and drift
Moor Farm (SP28NE/123C) [SP 2872 8500] 832.0 m Monks Park Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Morris Motors (SP38SE/5) [SP 3528 8270] 149.4 m Meriden Formation and drift
Motorway boreholes: Coy 1405 (SP38NW/130) [SP 3232 8576] 9.1 m Meriden Formation
Coy 1406 (SP38NW/131) [SP 3249 8573] 9.5 m Meriden Formation
Cov 1407 (SP38NW/132) [SP 3318 8558] 10.7 m Meriden Formation and drift
Cov 1408 (SP38NW/133) [SP 3334 8555] 12.1 m Meriden Formation and drift
No. 164 (SP48NW/38) [SP 4085 8697] 16 m Wolston Clay, Wolston Sand and Gravel, Oadby Till, Dunsmore Gravel
No. 188 (SP38SE/299) [SP 3540 8442] 9.1 m Thrussington Till and Glaciofluvial sand and *ravel
No. 1077 (SP28NE/14) [SP 2782 8560] 15.2 m Silt, laminated silt, sand and till
No. 1219 (SP38SE/65) [SP 3894 8237] 12.2 m Thrussington Till
No. 1279B (SP48SE/25) [SP 4548 8167] 25.8 m Wolston Sand and Gravel
No. 1287 (SP48SE/44) [SP 4640 8121] 29.0 m Thrussington Till, Wolston Sand and Gravel, Oadby Till
No. 1295 (SP48SE/53) [SP 4707 8083] 21.3 m Wolston Sand and Gravel, Oadby Till
No. 1332 (SP57NW/21) [SP 5039 7939] 15.2 m Lias Group and drift
Mount Nod (SP27NE/46) [SP 2922 7927] 215.6 m Meriden Formation, Tile Hill Mudstone Formation
Muzzard's Wood (SP28SE/26C) [SP 2987 8384] 852.3 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Newdigate No. 3 (SP38NW/67) (underground) [SP 3349 8688] Coal Measures
Nuneaton Colliery No. 3 shaft (SP39SW/20) [SP 3304 9211] 229.6 m Coal Measures, Etruria Formation, Halesowen Formation and drift
Nuneaton Colliery No. 4 shaft (SP39SW/21) [SP 3305 9207] 200.7 m Coal Measures, Etruria Formation, Halesowen Formation and drift
Outwoods (SP28NW/52C) [SP 2463 8529] 721.7 m Merevale Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Park Hill Lane (SP28SE/4C) [SP 2923 8046] 992.5 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Park House (SP28NE/129C) [SP 2762 8700] Full log not received; terminated in Stockingford Shale Group
Pickford Green (SP28SE/25C) [SP 2734 8103] 1022.0 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Priory Wood (SP28NW/55C) [SP 2361 8578] 603.8 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Ram Hall (SP27NW/3C) [SP 2469 7809] 1042.4 m Merevale Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation, Tile Hill Mudstone Formation
Rookery Farm (SP38SW/583C) [SP 3106 8161] 946.2 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation and drift
Rough Close (SP27NE/9C) [SP 2648 7851] c.1122.5 m Merevale Shale Formation, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation, Tile Hill Mudstone Formation and drift
Salutation No. 13 (SP39SW/60) [SP 3263 9268] 69.8 m Coal Measures, Etruria Formation
Sapcote Freeholt (SP49SE/1) [SP 4623 9431] 504.4 m Stockingford Shale Group, Sherwood Sandstone Group, Mercia Mudstone Group and drift
Shawell Sand and Gravel Ltd (SP58SW/23) [SP 5319 8062] 117 m Lias Group and drift
Solomon's Temple (SP28NE/63C) [SP 2610 8689] 655.5 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Southfields Road (SP49SW/51) [SP 4291 9331] 218.5 m Bromsgove Sandstone Formation, Mercia Mudstone Group and drift
Sowe Valley Sewer B1 (SP37NE/62) [SP 3690 7866] 10.7 m River Terrace Deposits
Spon End No. 2 (SP37NW/1043) [SP 3233 7912] 91.5 m Meriden Formation and drift
Staircase Lane (SP38SW/97C) [SP 3036 8129] 967.0 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Standard Motor Co. (SP37NW/265) [SP 3060 7811] 243.2 m Meriden Formation, Tile Hill Mudstone Formation
Sterling Metals No. 1 (SP38NE/54) [SP 3740 8974] 24.4 m Bromsgrove Sandstone Formation
Sterling Metals No. 2 (SP38NE/55) [SP 3736 8972] 30.8 m Stockingford Shale Group, Bromsgrove Sandstone Formation
Stretton Baskerville (SP49SW/45) [SP 4004 9115] 150.6 m ?Precambrian rocks, Polesworth Formation, Bromsgrove Sandstone Formation, Mercia Mudstone Group and drift
The Orchard (SP58SW/16) [SP 5380 8486] 96.3 m Mercia Mudstone Group, Penarth Group, Lias Group and drift
The Roughs (SP28NW/257C) [SP 2453 8651] 713.0 m Full log not received; terminated in Stockingford Shale Group
Wall Hill (SP28SE/136C) [SP 296 839] Full log not received
Watery Lane (SP38SW/5) [SP 3246 8350] 235.9 m Meriden Formation
Well Green Farm (SP38NE/539) [SP 3962 8618] 164.2 m Stockingford Shale Group, Coal Measures, Bromsgrove Sandstone Formation, ?Mercia Mudstone Group and drift
Weston Hill Farm (SP38NE/537) [SP 3911 8752] 146.9 m Coal Measures, Etruria Formation, Bromsgrove Sandstone Formation and drift
Whitehouse Farm (SP29SW/8C) [SP 2385 9302] 499.6 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Whitmore Park (SP38SW/12) [SP 3285 8126] 790 m Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation
Willey Fields Farm (SP48NE/4) [SP 4814 8572] 45.8 m Wolston Clay, Wolston Sand and Gravel, Oadby Till
Windmill House Farm Nos. 1–3 (SP28SE/100) , (SP28SE/101) , (SP28SE/102) [SP 288 813] 12.5 m maximum: Till, sand and silt
Wolvey Villa Farm (SP48NW/65) [SP 4272 8704] 50.5 m Mercia Mudstone Group, Thrussington Till, Wolston Clay, Wolston Sand and Gravel
Wolvey Lodge Farm (SP48NE/2) [SP 4503 8739] 45.8 m Wolston Clay, Wolston Sand and Gravel, Oadby Till
Woodcock Wood (SP28NW/56C) [SP 2425 8681] 678.0 m Stockingford Shale Group, Coal Measures, Etruria Formation, Halesowen Formation, Meriden Formation

Appendix 3 Geological Survey photographs

Photographs illustrating the geology of the Coventry district are deposited for reference in the headquarters library of the British Geological Survey, Keyworth, Nottingham NG12 5GG; in the library at the BGS, Murchison House, West Mains Road, Edinburgh EH9 3LA; and in the BGS Information Office at the Natural History Museum Earth Galleries, Exhibition Road, London SW7 2DE. The photographs show details of the various rocks and sediments exposed and also include general views and scenery. A list of titles can be supplied on request. The photographs can be supplied as black and white prints, colour prints, or 2 X 2 colour transparencies, at an advertised tariff.

Appendix 2 Open-file reports

Open-file reports containing geological details additional to those shown on the 1:10 000 maps are listed below. Each report is in the series 'Geological notes and local details for 1:10 000 sheets'; the more recent have been given BGS Technical Report numbers in the WA Series and these are shown in brackets. They can be consulted at BGS libraries or purchased from the sameoutlets as the dyeline maps.

SP 27 NW Berkswell and Balsall Common RAO 1987
SP 27 NE Coventry West (WA/88/47) RAO 1988
SP 28 NW Maxstoke (WA/89/20) RAO 1989
SP 28 NE Fillongley (WA/89/21) JGR 1989
SP 28 SW Meriden (WA/89/22) MGS 1989
SP 28 SE Allesley (WA/89/23) JGR 1989
SP 29 SE Old Arley (WA/92/28) DMcCB 1992
SP 37 NW Coventry Central (WA/88/48) RAO 1988
SP 37 NE Coventry South-east (WA/89/77) MGS 1989
SP 38 NW Bedworth West (WA/89/24) DMcCB 1989
SP 38 NE Bedworth (WA/91/58) DMcCB 1991
SP 38 SW Coventry North (WA/89/25) RAO 1989
SP 38 SE Coventry North-east (WA/88/51) DMcCB 1989
SP 39 NW Atherstone and Mancetter (WA/91/66) JWB 1991
SP 39 NE and SP 49 NW Higham on the Hil land Barwell (WA/92/09) RSL 1992
SP 39 SW Hartshill and Stockingford (WA/91/60) JWB 1991
SP 39 SE Central Nuneaton (WA/92/10) RSL 1991
SP 47 NW and SP 47 NE Rugby West MGS 1985
SP 48 NW, SW Wolvey and Shilton (WA/91/88) JNC 1991
SP 48 NE, SE and SP 58 SW Wibtoft, Pailton and Lutterworth (WA/92/24) JNC 1992
SP 49 SW Hinckley (WA/90/02) RAO 1990
SP 49 SE, SP 59 SW and SP 58 NW Sapcote, Broughton Astley and Ullesthorpe (WA/92/34) RSL 1993
SP 57 NW North-east Rugby MGS 1983

Authors: J W Baldock, D McC Bridge, J N Carney, R S Lawley, R A Old, J G Rees, M G Sumbler.

Author citations for fossil species

To satisfy the rules and recommendations of the international codes of botanical and zoological nomenclature, authors of cited species are listed below.

Chapter 3 Cambrian and Lower Ordovician (Tremadoc)

Chapter 5 Devonian

Chapter 6 Carboniferous: Namurian

Chapter 7 Carboniferous: Westphalian-Stephanian

Chapter 9 Jurassic

Chapter 10 Quaternary

Chapter 13 Economic and applied geology

Figures, plates and tables

Figures

(Figure 1) Principal physical features and drainage of the district.

(Figure 2) Sketch map of the geology of the district.

(Figure 3) Sections in the Caldecote Volcanic Formation.

(Figure 4) Locality map of the Precambrian and Cambrian rocks in Boon's Quarry. The SSSI extends along the upper northern quarry level, between localities 1 and 8.

(Figure 5) Locality map of the Precambrian, Cambrian and Triassic rocks in Judkins' Quarry.

(Figure 6) Geochemical classification of the Precambrian rocks, based on the variation of K20 against Si02, and a triangular plot of normative quartz, orthoclase and plagioclase values. The field boundaries are, for: A. after Peccerillo and Taylor (1976) and Gill (1981), and for B. after Streckeisen (1976, 1979). The relevant fields shown by the letters in (Figure 6)b are (as plutonic rocks, with volcanic equivalents in brackets): C. Granite (rhyolite); D. Granodiorite (dacite); E. Tonalite (dacite); F. Quartz monzodiorite/gabbro (andesite/basalt); G. Quartz diorite/gabbro (andesite/basalt); H. Monzodiorite/monzogabbro (andesite/basalt); I. Diorite/gabbro (andesite/basalt). Note that two microdiorite samples with normative olivine do not appear on (Figure 6)b.

(Figure 7) Geochemical patterns for the Caldecote Volcanic Formation and Precambrian intrusive rocks of Nuneaton, with felsic lavas from Charnwood Forest for comparison. Normalised to Mid Ocean Ridge Basalt (MORB) values after Pearce (1982). Includes data from Pharaoh et al. (1987b) and Pharaoh and Brewer (unpublished). Symbols as in (Figure 6).

(Figure 8) Comparative geochemical plots of the Nuneaton and Charnwood Forest Precambrian rocks.a. AFM diagram, showing the boundary between tholeiitic and calc-alkaline magma fields after Irvine and Baragar (1971). A. Variation of Niobium against Yttrium. The field boundaries for the different tectonic settings recognised by Pearce et al. (1984), are valid for the discrimination of felsic rocks only. Other symbols as in (Figure 6). Note that three microdiorite samples with Nb.1 are not shown on (Figure 8)b.

(Figure 9) Composite stratigraphical column in the Hartshill Sandstone Formation, showing the principal lithofacies and their interpretation.

(Figure 10) Map derived mainly from borehole information, showing Cambro-Ordovician formations and their probable extension beneath Carboniferous or Triassic cover.

(Figure 11) Geological map of the South Leicestershire Diorites between Calver Hill and Stoney Stanton.

(Figure 12) Vertical sections in composite sills of the Midlands Minor Intrusive Suite.

(Figure 13) Geological map of the composite intrusions in Griff No. 4 Quarry. Principal minerals (abbreviated) are shown in order of decreasing abundance from left to right.

(Figure 14) Plot of total alkalis against silica for the Ordovician intrusive suites. Compositional fields are from Le Bas et al. (1986).

(Figure 15) MORB normalised diagram comparing representative samples from the South Leicestershire Diorites with the Griff Quarry intrusion (shaded field), Midlands Minor Intrusive Suite. MORB normalising values (shown above the symbol for each element) are from Pearce (1983).

(Figure 16) Geochemical data for the Griff Quarry intrusion, Midlands Minor Intrusive Suite, normalised against an averaged hornblende-plagioclase (spessartite) lamprophyre composition. The normalising values are from Rock (1991).

(Figure 17) Ti-Zr-Y tectonic discrimination diagram for the Ordovician intrusions. The relevant fields are from Pearce and Cann (1973), and are: WPB, within-plate basalt; CAB, calc-alkaline basalt; LKT, OFB, low-potassium tholeiite, ocean-floor basalt.

(Figure 18) Type section of the Oldbury Farm Sandstone Formation in Merevale No. 2 Borehole. The vertical scale shows depths from the surface (modified from Taylor and Rushton, 1971, pl. III).

(Figure 19) Namurian sequence in Merevale No. 2 Borehole.

(Figure 20) Generalised vertical section of the Westphalian rocks showing current nomenclature in relation to previous schemes.

(Figure 21) Landward limits of marine bands in the Warwickshire Coalfield, as shown by the disappearance of Lingula. Composite diagram based on Fulton (1987a, figs. 5.8–5.11).

(Figure 22) Distribution of former opencast coaling sites, and extent of underground workings in the Thick Coal; seam split lines based on Cope and Jones (1970) and British Coal data.

(Figure 23) South-west to north-east section through the Warwickshire Coalfield, to show correlation of coal seams and marine bands (from Jones, 1992).

(Figure 24) Isopach maps of a) Langsettian, and b) Duckmantian strata for the central part of the Warwickshire Coalfield.

(Figure 25) Generalised vertical section of the Lower Coal Measures.

(Figure 26) Selected sections of the Coal Measures and Etruria Formation (north–south transect). Inset shows line of section.

(Figure 27) Selected sections of the Coal Measures and Etruria Formation (northeast–south-west transect). Inset shows line of section. See (Figure 26) for key.

(Figure 28) Generalised vertical section of the Middle Coal Measures. Key to ornaments as in (Figure 25).

(Figure 29) Isopach map of strata between the Two Yard and Four Feet coals, with associated sedimentary facies (based on Jones, 1992, figs. 7 and 8).

(Figure 30) Isopach maps: a) strata between Aegiranum Marine Band and base of Halesowen Formation (Bolsovian strata); b) Etruria Formation (also showing lithofacies associations).

(Figure 31) Selected sections of the Halesowen Formation with gamma ray logs. Inset shows line of section and approximate isopachs (base to gamma peak).

(Figure 32) Meriden and Tile Hill Mudstone formation log signatures for selected boreholes.

(Figure 33) Cross-bedding foreset dip directions for the post-Halesowen Formation redbeds.

(Figure 34) The geological and geophysical setting of the Bulkington Prospect: a) Bouguer anomaly map; b) Bouguer anomaly profile and interpreted model; c) selected boreholes.

(Figure 35) Classification of Carboniferous sandstones by formation, using scheme of Pettijohn et al. (1987).

(Figure 36) a) Part of Ti/50-V-Cr triangular diagram for samples from the Birch Tree Farm Borehole; b) Downhole geochemical profiles for selected elements in sandstone samples from the Birch Tree Farm Borehole.

(Figure 37) Sketch map of the geological setting of the Hinckley Basin (partly after Worssam and Old, 1988, fig. 27; Old et al., 1987, fig. 20).

(Figure 38) Generalised vertical section of the Triassic rocks of the Coventry district.

(Figure 39) Comparative borehole sections in the Sherwood Sandstone Group.

(Figure 40) Comparative sections in the Mercia Mudstone and Penarth groups. All but Judkins' Quarry section are boreholes.

(Figure 41) Summary of stratigraphical relationships in the glacial succession.

(Figure 42) Subdrift bedrock topography of the district.

(Figure 43) Section in the Baginton Sand and Gravel at Huncote Pit.

(Figure 44) Generalised section of superficial deposits along part of the M6 motorway (modified from an unpublished section by A Horton).

(Figure 45) Isopach maps for a) Wolston Clay, and b) Wolston Sand and Gravel; c) Structure contour map for the junction between the Wolston Clay and Wolston Sand and Gravel, incorporating selected glaciotectonic boundaries.

(Figure 46) Vertical section in the Shawell Gravel, Oadby Till and Dunsmore Gravel at Gibbet Lane Quarry.

(Figure 47) Structure contours on the base of the Anker Sand and Gravel and Dunsmore Gravel. Patches of Dunsmore Gravel occur also to east and south of the area shown.

(Figure 48) Principal structural elements of the Warwickshire Coalfield, and pre-Triassic geology of the Hinckley Basin.

(Figure 49) Structure of the Warwickshire Coalfield at the level of the Two Yard Coal.

(Figure 50) Bouguer gravity anomaly map of the Coventry district and surrounding area. Contours at 1 mGal intervals. Density for data reduction 2.40 Mg/m3. Anomalies identified by letters A–G are discussed in the text. I and II are the sites of TEM surveys.

(Figure 51) First horizontal derivative of the Bouguer gravity data for the Coventry district and surrounding area; shaded relief plot illuminated vertically. Broken lines, L1–8 are lineaments, discussed in the text. Solid lines are principal structural elements: ALS, Allesley Syncline; AF, Arley Fault; AS, Astley Syncline; BF, Bedworth Fault; BM, Binley Monocline; BS, Bulkington Syncline; CHM, Camp Hill Monocline; HPS, Hoar Park Syncline; HB, Hinckley Basin; KF, Keresley Fault: WBF, Western Boundary Fault; PF, Polesworth Fault; PNFS, Pheasant's Nest Farm Syncline; SLD, South Leicestershire Diorites.

(Figure 53)." data-name="images/P1001250.jpg">(Figure 52) Aeromagnetic map (reduced to pole) with contours at 20nT intervals. Anomalies A, B and C are discussed in the text. The other geological elements indicated by lettering are explained in the caption to (Figure 51). Profiles 1 and 2 are the locations of Gravmag profiles shown in (Figure 53).

(Figure 53) Geophysical profiles across the district. Gravity and aeromagnetic profiles and an interpreted shallow crustal section, based on the 2.5D program 'Gravmag', are shown for the two lines of section (profiles 1 and 2) given on (Figure 53)." data-name="images/P1001250.jpg">(Figure 52). Key to geophysical units: PT, Permo-Trias (density 2.40 Mg/m3, magnetic susceptibility 0 X 10–3 SI units); W, Westphalian (2.50, 0); C, Cambrian (2.65, 0); II, Groby diorite (2.73, 0.01); 12, (2.73, 0.01); 13, Stoney Stanton diorite (2.73, 0); 14, Croft diorite (2.70, 0.005); 15, Enderby diorite (2.72, 0); MB1, magnetic basement rocks (2.70, 0.03); MB2, (2.75, 30); MB3, (2.73, 0.03); MB4, (2.73, 0.01); NMB1, nonmagnetic basement rocks (2.70, 0); NMB2, (2.75, 0).

Plates

(Plate 1) Polished specimens of the Caldecote Volcanic Formation. Massive crystal-lapilli tuff, showing closely packed and fractured crystals of plagioclase (white to pink) and quartz (grey), with dark porphyritic inclusion at lower left (E62363), Boon's Quarry). Base of a sediment-raft breccia, showing flow-foliated sole (arrowed) resting on disturbed beds of tuffaceous mudstone, siltstone and sandstone. An angular dark porphyritic inclusion occurs at upper left of specimen (E62186), Judkins' Quarry). Repetitive normal grading shown by alternations of tuffaceous sandstone, siltstone and mudstone (E62356), Boon's Quarry). Fine-grained laminated vitric tuff, showing low-angle truncations between some of the laminae (E62307), Judkins' Quarry).

(Plate 2) Thin sections of specimens from the Caldecote Volcanic Formation (plane polarised light). a. Massive crystal-lapilli tuff, showing closely packed crystals of subhedral plagioclase (brown) and quartz (white). Most crystals show incipient brecciation, particularly quartz which is also seen as isolated fracture rhombs in the matrix. The enlarged inset of the matrix at X, to right, shows little-deformed to slightly flattened reticulate and sliver-shaped glass shards, which are internally replaced by quartz, feldspar, mica and chlorite (E62283), Judkins' Quarry). Dark porphyritic inclusion showing crystals of quartz (white) with pristine embayed outline and plagioclase (brown, similar to matrix). Quartz is surrounded by dark brown, fine-grained altered material which may be flow-foliated (E62181), Judkins' Quarry. Detail of undeformed sliver-shaped and bubble-wall glass shards in a fine-grained vitric tuff (E62335), Judkins' Quarry).

(Plate 3) Thin section (crossed polarisers) of granophyric diorite (markfieldite). Shows first-stage altered and corroded plagioclase and chloritised mafic crystals (middle of lower margin) surrounded by later-stage residuum characterised by granophyric intergrowths between quartz (white) and turbid K-feldspar (E62159), Judkins' Quarry).

(Plate 4) Field relations and main lithologies of the Boon's Member, basal to the Hartshill Sandstone Formation, as viewed in Boon's Quarry. The Precambrian–Cambrian unconformity surface (arrowed) at the SSSI, viewed looking north, showing pale red, plane-bedded granulestone and breccioconglomerate beds of the Boon's Member overlying Precambrian crystallapilli tuff. The Precambrian rocks are weathered to spheroidal masses immediately beneath the unconformity at the left of the picture (A14974). Polished slab showing highly angular clasts and low degree of sorting in the matrix of a bouldery breccio-conglomerate. The clasts are aligned parallel to a crude planar depositional fabric (E65259). Polished slab showing plane-bedded depositional couplets comprising pink sandstone alternating with thin, matrix-supported breccia layers (E65249).

(Plate 5) Contrasting bedforms within the Tuttle Hill Member exposed on the south-eastern face of Hartshill Quarry. A. Tabular-planar cross-bedded sandstone beds of Unit I, with foresets inclined towards the north-east (from right to left). The bed tops are scoured and mudstone-draped. B. Junction (arrowed) between lenticular, compound cross-bedded sandstone of Unit K and, to right, plane-bedded and parallel-sided sandstone of Unit L (A14960).

(Plate 6) Exposure of the Home Farm Member (Hyolithes Limestone division) on the south-western face of Hartshill Quarry. Phosphatised Limestone Conglomerate (with dark clasts) at the base is capped by a hardground on which rests nodular sandy limestones of the Coleoloides Limestones, Siltstones and Shales. Small, pale coloured, tubular fossils can be seen within the concretions below hammer head (A14949).

(Plate 7) Examples of fossils from the Comley Series. All examples are from the Purley Shale Formation except for i. The originals of a, c, d and h are in the Sedgwick Museum Cambridge (SM); the remainder are in the collection of the British Geological Survey; those prefixed Zv were presented by Mr R J Kennedy. a. Botsfordia pulchra (Matthew). Pedicle valve, showing divaricate granular ornament, SM A57215a, X 8. Camp Hill Industrial Estate [SP 3468 9234]. b. "Obolella" granulata Sharman. Lectotype, pedicle? valve, JR3150, X 8. "Purley Park Lane" about [SP 312 961]. Though described in 1886, this species has not previously been figured. The generic reference is not certain (Cocks, 1978). c, d and h. Chelediscus acifer Rushton. Side view and pygidium of enrolled holotype specimen, SM A57104, and cephalon SM A57107, all X 12. Camp Hill Industrial Estate [SP 3468 9234]. i. Alisina atlantica (Walcott). Ventral view of conjoined valves, Zv9656, X 5. Camp Hill Industrial Estate [SP 3500 9218]. J. Hebediscus attleborensis (Shaler and Foerste). Cranidium, Zv9659, X 10. Camp Hill Industrial Estate [SP 3500 9218]. g, j and k. Serrodiscu,s bellimaiginatus (Shaler and Foerste). Cephalon, Zv9661, and side and top view of fragmentary pygidium, Zv9657, all X 4. Camp Hill Industrial Estate [SP 3500 9218]. K. i. Coleoloides typicalis Walcott. TE47, X 5. Home Farm Member of Hartshill Sandstone Formation, Woodlands Quarry, Hartshill [SP 3248 9476]. L.1. Strenuella sabulosa Rushton. Cranidium, latex cast of intaglio, representing an internal mould, Zv9662, X 3. Camp Hill Industrial Estate [SP 3500 9218].

(Plate 8)Fossils from the St David's Series. All are from the Abbey Shale Formation and are in the collection of the British Geological Survey. A. Paradoxides abenacus Matthew. Disarranged exoskeleton, latex cast of external mould, Zs9857, X 1. The only known British example of this species. Excavation, Vernon's Lane [SP 3494 9003]. A F Cook coll. B. Luhops expectans (Barrande). Latex cast of external mould of pygidium, Zs823, X 2. Excavation, Coventry Canal [SP 3491 9204]. A F Cook coll. C. Luhops pugnax (Ming). Latex cast of external mould of cranidium, Zv9858, X 1. Excavation, Vernon's Lane [SP 3494 9003]. A F Cook coll. D. Cotalagnostus lens (Gronwall). Zs4577, X 5. Bar Pool Trench about [SP 3447 9236]. A F Cook coll. E. Onymagnostus ciceroides (Matthew). Latex cast of Zs828, X 5. The cephalic border is narrower and the pygidial axis better f. segmented and shorter than in C. lens. Excavation, Coventry Canal [SP 3491 9204]. A F Cook coll. G. Hartshillia inflata (Hicks). Latex cast of external mould, the right-hand specimen showing an axial spine on the sixth thoracic segment. GSM 57892, X 5. Hartshill Hayes [SP 3240 9425]. V C Ming coll. g, h and j. Plutonides [ParadoxidesJ hicksii (Salter), all X 2. g, Cranidium, latex cast of Zv9857. Excavation, Vernon's Lane [SP 3494 9003]. A F Cook coll. h and j, Librigena and pygidium, latex casts of Zs4533, 4540. Bar Pool Trench about [SP 3447 9236]. P H Whitworth coll. I. i. Stenotheca cornucopiae Hicks. Pyritised specimen, BDA2593, X 20. Merevale No. 3 Borehole.k and 1. Linnarssonia sagittalis (Davidson). k, external of pedicle valve and internal of brachial valve, Zs4523, X 4. Bar Pool Trench about [SP 3447 9236]. P H Whitworth coll. 1, internal of pyritised pedicle valve, BDA2463, X 10. Merevale No. 3 Borehole. m. Tomagnostus fissus (Linnarsson). Latex cast of GSM 57850, X 10. Hartshill Hayes [SP 3240 9425]. V C Illing coll.

(Plate 9) Selected fossils from the Merioneth and Tremadoc series. All the specimens are in the collection of the British Geological Survey. a. Broeggeria salteri (Ho11). Brachial? valve, BKE9299, X 4. Monks Park Shale Formation, Chapel Green Borehole. b and f. Protopeltura sp., both from Outwoods Shale Formation (Olenus Biozone, cataractes Subzone), Park House Borehole. b, cranidium, composite photograph of internal mould and latex cast of external mould, BKE9465 and 9466, X 4. f, librigena with short genal spine, BKE9470, X 8. c. Platypeltoides croftii (Callaway). BKE9213, X 6. Merevale Shale Formation (Tremadoc, flabelliformis Biozone?), Grange Farm Borehole. d. Rhabdinopora flabelliformis belgica (Bulman). BDU7015, X 2. Merevale Shale Formation (Tremadoc, flabelliformis Biozone), Outwoods Borehole. e. Orusia lenticularis (Wahlenberg). External moulds, showing radial and concentric sculpture, RU1279, X 5. Monks Park Shale Formation (spinulosa Biozone), Merevale No. 1 Borehole. g and h. Olenus? aff. solitaries (Westergard). Cranidium and latex cast of librigena, BKE9363A and B (counterparts), both X 4; the cranidium shows similarities to Protopeltura but the genal spine is long, and more like that of Olenus. Near the passage between the Outwoods Shale and Monks Park Shale formations (basal spinulosa Biozone), Fillongley Hall Borehole. i. Olenus wahlenbergi Westergard with Homagnostus obesus (Belt). Latex cast of BDA527. Outwoods Shale Formation (Olenus Biozone, wahlenbergi Subzone), Merevale No. 3 Borehole. j. Cyclotron lapworthi (Groom). Latex cast of GSM 90381, x 10. Outwoods Shale Formation ( Olenus Biozone, wahlenbergi Subzone (?)), excavation at Old Wharf Inn, Chivers Coton about [SP 362 905].

(Plate 10) Thin sections of specimens from the South Leicestershire Diorites suite (crossed polarisers). a. Quartz diorite with inequigranular texture, showing euhedral, concentrically zoned plagioclase crystals enclosed by a fine- to medium-grained base consisting of blebby quartz (white areas), plagioclase and lath-shaped hornblende (pale yellow grains) (E4978, Croft Quarry). b. Fine-grained intrusive facies, showing fluxional orientation of plagioclase laths around a chloritised mafic microphenocryst, probably originally pyroxene; quartz forms sporadic small white areas in the groundmass (E11937), Cary Hill Quarry). c. Pegmatitic 'vein' of granitic composition, showing large plates of sodic plagioclase rimmed by intergrowths, locally vermicular, between quartz (white areas) and turbid alkali feldspar; at lower left these minerals project into a cavity occupied by chloritic aggregates (E9702), Stoney Cove Quarry).

(Plate 11) Thin sections of specimens from the Midlands Minor Intrusive Suite outcropping in the Nuneaton Inlier (crossed polarisers). a. Fine-grained spessartite lamprophyre, showing pseudomorphed microphenocrysts with euhedral outlines suggesting original olivine (E62517), Boon's Quarry). b. Hornblende meladiorite forming the basal facies of a composite sill. The main constituents are small laths of plagioclase (white and grey areas) and larger acicular, altered hornblende (dark brown laths); other mafic minerals are represented by pseudomorphic chlorite (bluish birefringence) and white mica (pale yellow areas) (E62685), Griff Quarry). c. Feldspathic facies of poikilitic hornblende meladiorite from the lower, mafic-enriched part of a composite sill. Large hornblende plates (yellow and brown areas) enclose plagioclase laths, small and partly altered clinopyroxene euhedra (scrappy, reddish brown birefringent grains) and pseudomorphed olivine euhedra (deep blue chloritic grains) (E62680), Griff Quarry). d. Hornblende diorite from upper leucocratic facies of a composite sill. Euhedral plates and laths of plagioclase are part-enclosed by hornblende (pale yellow areas). The blue chloritic grains are mainly pseudomorphic after olivine (E62671), Gruff Quarry).

(Plate 12) Intrusive rocks from the Daw Mill underground borehole near Fillongley. a. Junction between the clay-weathered and bioturbated top surface of an Ordovician sill and overlying clay-pebble conglomerate forming the base of the Coal Measures sequence; 37.05–37.15 m depth. b. Lamprophyre sill (plane polarised light), showing three types of pseudomorphed mafic microphenocryst, characterised by carbonate alteration (large crystal left of picture), white mica (lath-shaped crystal at bottom right) and chlorite-opaque mineral (small crystals at upper left) (E65550), from 67.5 m depth).

(Plate 13) Slabbed core samples of the Oldbury Farm Sandstone Formation from Merevale No. 2 Borehole. a. Upward-fining bed of conglomerate, cross-bedded coarse-grained sandstone and fine-grained sandstone in Member 3 (By3599A), depth 133 m. b. Amalgamated nodular limestone in Member 4, showing included slivers of the host sediment in lower centre and at bottom left of picture (By3460), depth 75 m. c. Nodular carbonate in laminated fine-grained sandstone of Member 4, showing distortion of lamination around nodules (By3534), depth 100 m. d. Intraformational conglomerate of Member 4, showing abundant white clasts of micritic limestone (By 3463), depth 77 m.

(Plate 14) Thin sections of Upper Carboniferous sandstones and volcaniclastic sandstone from the Warwickshire Coalfield. Sections a, b and c are resin-impregnated and stained for carbonates and K-feldspar. a. Tuffaceous sandstone–Etruria Formation, Weston Hill Farm Borehole, depth 53.6 m (plane polarised light). Highly altered rounded clasts of vitric and hyalocrystalline lava, and ?scoria fragments, are set in a matrix largely kaolinitised. Scale bar is 0.2 mm. b. Etruria Formation, Birch Tree Farm Borehole, depth 716.0 m (crossed polarisers). Monocrystalline quartz, rock fragments (partially altered and replaced by kaolinite) and polycrystalline quartz are abundant in this poorly sorted, matrix-rich sandstone. Kaolinite is the most abundant cement, occurring as skeletal and vermicular forms. Scale bar is 0.4 mm. c. alesowen Formation, Birch Tree Farm Borehole, depth 634.0 m (plane-polarised light). Rock fragments are abundant and are dominantly mica schist (ms) and quartz schist but include pelite, chert, ferruginous micromicaceous fragments and rare igneous fragments. Monocrystalline and polycrystalline quartz (q) are rounded to subangular; rare feldspars (Id) are at various stages of dissolution; cement consists of sporadic patches of ferroan dolomite and kaolinite. Scale bar is 0.4 mm. d. Meriden Formation (Whitacre Member), Birch Tree Farm Borehole, depth 555.3 m (plane-polarised light). Monocrystalline and polycrystalline quartz are abundant. Lithic clasts include a high proportion of mottled calcrete fragments (c) as well as mica and quartz schist, pelite, chert and fine-grained igneous rocks. Cement consists mainly of large sparry pore-filling ferroan dolomite (fd) and small rhombic siderite (sr). Scale bar is 0.2 mm.

(Plate 15) Lithologies of the Wolston Glacial Succession.a. Thrussington Till; core from 19.0 m in the Weston Hill Farm Borehole.a. Wolston Clay; core from 16.35 m in the Weston Hill Farm Borehole.b. Exposure in Oadby Till above Shawell Gravel at the Gibbet Lane Quarry [SP 5409 8061].

(Plate 16) Shawell Gravel in the Gibbet Lane Quarry [SP 5409 8064]. A.(left) In the middle part of the section, trough cross-bedded yellow sands (above hammer) are overlain by laterally extensive beds of pebbly sand forming multiple upwards-fining depositional units. Hammer is at about 9.5 m in measured section (Figure 46). B. (right)Near the base of the succession, lenses of clast-supported gravel channelised into plane-bedded medium-grained sand.

(Plate 17a) Hartshill Sandstone Formation folded into chevron structures with upright axes; north-western part of Hartshill Quarry, view looking northwards. At lower left, the axial zone of an anticline is transected by a late Ordovician spessartite lamprophyre dyke (dark subvertical sheet). The lower quarry face is about 10 m high [SP 3305 9417].

(Plate 17b)Late Ordovician diorite sills (pale grey) in Mancetter Quarry. The sills are intruded along the bedding in Cambrian mudstones (dark areas), and comprise a lower set displaced by listric extensional faults (dipping from right to left) and a later, upper set that were emplaced after this faulting. The view is to the north-west [SP 3093 9505].

(Front cover) Cover photograph: Panoramic view looking east across Judkins' Quarry, Nuneaton, towards Hinckley. The face to the left exposes the Precambrian Caldecote Volcanic Formation which is unconformably capped (near top of conveyor belt) by Triassic strata. The conical spoil heap of 'Mount Jud' overlooks a modern terraced waste disposal operation from which landfill gas is being generated for public supply. (MN27927) (Photographer: T P Cullen)

(Rear cover)

(Geological sequence) Summary of geological sequence.

(Frontispiece) Panoramic view of Sudeley Opencast Site, prior to its restoration in 1991. Strata dip at about 12° into the western face (at left) , which is about 80m high. The visible worked coal seams in ascending order are the Low Main, Nine Feet, Ell (two leaves), Two Yard (being worked at foot of main face) and Four Feet (A14575).

Tables

(Table 1) Chemical compositions of selected whole-rock samples from the Caldecote Volcanic Formation.

(Table 2) Chemical compositions of selected whole-rock samples from Precambrian intrusive rocks.

(Table 3) Summary of previous and current nomenclature for the Hartshill Sandstone Formation and Stockingford Shale Group.

(Table 4) Cambro-Ordovician fossils from selected boreholes (see (Figure 10) for locations).

(Table 5) Chemical compositions of selected whole-rock samples from the Midlands Minor Intrusive Suite; La-Cs analysed by neutron activation, other elements by X-ray fluorescence.

(Table 6) Sedimentary facies within the Langsettian and Duckmantian of the Warwickshire Coalfield (based on Jones, 1992)

(Table 7) Biostratigraphical classification of the Westphalian. Marine bands and nonmarine bivalve subzones recognised in the Coventry district are shaded. No thicknesses are implied. Dotted boundaries denote uncertain correlation. Key: A. Anthracosia, An. Anthraconaia, At. Anthraconauta, C. Carbonicola

(Table 8) Former opencast coaling sites with details of seams worked and their thickness. Paradise [SP 355 902] , Sunnyside [SP 352 911] and Bermuda [SP 357 898] are excluded because of incomplete data.

(Table 9) Heavy-mineral assemblages of Namurian to ?early Permian sandstones of the Warwickshire Coalfield; provenance-sensitive heavy mineral ratios are also tabulated.

(Table 10) Current and former Triassic nomenclature applied to the Coventry district.

(Table 11) Glacial deposits of the Coventry district and their correlatives elsewhere in the Midlands.

(Table 12) A time scale for the principal structural events in the district. The absolute age values are in part based on Cowie and Bassett (1989).

(Table 13) Summary of physical properties of the main rock types. Data sources are indicated in the text.

(Table 14) Location of former brickpits, listed by formation.

(Table 15) Physical properties of aggregates. Information supplied by the quarry operators.

(Table 16) Groundwater abstraction licence data for the Coventry district. Figures derived from base data provided by the National Rivers Authority, Severn-Trent Region.

(Table 17) Typical chemical analyses of groundwater in the Coventry district.

(Table 18) Summary of geotechnical data for superficial deposits.

Tables

(Table 1) Chemical compositions of selected whole-rock samples from the Caldecote Volcanic Formation

Crystal-lapilli tuff facies grouping

 Porphyritic inclusion

Tuffaceous siltstone facies grouping
I II III IV V vi VII VIII IX X XI
SiO2 66.74 61.45 68.58 71.59 67.04 62.18 63.70 58.70 63.49 62.93 60.48
TiO2 0.50 0.46 0.43 0.39 0.47 0.48 0.45 0.56 0.42 0.73 0.64
A12O3 15.83 15.90 15.14 13.50 15.97 15.24 16.45 17.27 11.34 13.32 14.81
Fe2O3 5.05 6.25 3.77 3.01 3.90 5.05 4.78 7.64 5.26 6.47 7.07
MnO 0.06 0.16 0.13 0.11 0.12 0.16 0.12 0.20 0.12 0.13 0.13
MgO 1.57 3.94 1.14 1.35 1.33 2.54 2.35 4.46 1.88 2.84 2.84
CaO 1.67 2.94 2.05 2.49 3.52 4.23 2.97 3.20 6.79 4.15 4.25
Na2O 4.20 3.82 5.19 4.39 P3.65 1.78 3.74 1.64 2.45 0.54 0.24
K2O 2.02 1.06 1.22 0.82 1.74 1.49 1.39 2.42 0.94 2.61 3.28
P2O5 0.12 0.04 0.03 0.06 0.08 0.06 0.08 0.08 0.06 0.13 0.12
LOI 2.39 3.95 2.36 2.02 1.84 6.32 3.46 3.62 7.35 6.29 6.52
Rest 0.13 0.08 0.08 0.06 0.10 0.10 0.08 0.12 0.11 0.12 0.13
Total 100.28 100.05 100.12 99.79 99.76 99.63 99.57 99.91 100.21 100.26 100.51

Trace elements in parts per million
As 6 19 2 6 7 19 2 3 7 — —
Ba 461 93 203 111 288 31 190 215 228 355 506
Ce 11 20 19 25 25 27 23 23 23 20 26
Co 25 12 9 8 8 13 10 17 14 13 14
Cr 34 9 3 7 6 5 3 3 172 10 54
Cu 9 20 2 4 4 9 6 11 19 8 7
Hf — — — — — — — 5.57 — — —
Mo — — — 2 <2 1 <1 1 — — —
Nb 4 5 4 3 4 5 4 8 5 5 4
Ni 5 <3 <2 <2 <3 <3 <4 <4 9 3 6
Pb 5 1 4 2 7 <1 4 8 2 6 7
Rb 55 19 19 15 36 23 18 32 29 77 78
Sc 17 — — — — — — 19.20 22 — —
Sr 157 109 127 109 210 76 143 113 58 34 24
Ta — — — — — — — 0.53 — — —
Th — 4 3 2 2 5 2 7.55 6 6 1
V 48 60 44 29 46 60 54 65 68 72 82
Y 27 20 18 15 21 31 15 33 25 39 31
Zn 79 99 50 61 76 91 78 142 90 108 119
Zr 118 87 84 46 79 132 67 207 89 169 112
La 6.92 8 6 6 10 13 9 10.00 — — —
Ce 16.30 — — — — — — 23.20 — — —
Nd 9.40 — — — — — — 12.80 — — —
Sm 2.49 — — — — — — 3.40 — — —
Eu 0.66 — — — — — — 1.27 — — —
Gd 2.86 — — — — — — — — — —
Yb 2.13 — — — — — — 5.16 — — —
Lu 0.35 — — — — — — 0.80 — — —
  • Sample I, La-Lu analysed by ICP-ES (inductively coupled plasma emission spectrometry); sample VIII, La-Lu, Hf, Sc, Ta and Th analysed by neutron activation; all remaining data by XRF (X-ray fluorescence). LOI=loss on ignition. Rest=traces expressed as major elements.
  • I Crystal-lapilli tuff (JUD 15), Judkins' Quarry [SP 3431 9316].
  • II Crystal-lapilli tuff (E62336), Judkins' Quarry [SP 3454 9319]; beneath tuffaceous siltstones at Locality 5 (Figure 5).
  • III Crystal-lapilli tuff with dark inclusions (E62337), Judkins' Quarry [SP 3422 9319]; near Locality 7 (Figure 5).
  • IV Crystal-lapilli tuff (E62338), Judkins' Quarry [SP 3439 9318]; near Locality 4 (Figure 5).
  • V Crystal-lapilli tuff with dark inclusions (E62339), Judkins' Quarry [SP 3440 9340]; 130 m north of Locality 8 (Figure 5).
  • VI Crystal-lapilli tuff with dark matrix (E62342), Judkins' Quarry [SP 3414 9335]; 100 m north of Locality 10 (Figure 5).
  • VII Crystal-lapilli tuff (E62353), Boon's Quarry [SP 3308 9465]; 50 m south-east of Locality 1 (Figure 4).
  • VIII Dark porphyritic inclusion (E62352) in VII crystal-lapilli tuff.
  • IX Laminated tuffaceous siltstone (JUD 19), Judkins' Quarry [SP 3431 9335].
  • X Laminated tuffaceous siltstone (JUD 21), Judkins' Quarry [SP 3435 9334].
  • XI Laminated tuffaceous siltstone (JUD 22), Judkins' Quarry [SP 3435 9334].

(Table 2) Chemical compositions of selected whole-rock samples from Precambrian intrusive rocks.

Basaltic-andesite (microdiorite)

Granophyric diorite (markfieldite)
I II III IV V VI VII VIII IX
SiO2 54.13 50.93 49.44 50.02 49.95 55.78 56.44 57.61 56.47
TiO2 0.78 0.72 0.74 0.78 0.67 0.71 0.71 0.67 0.73
Al2O3 18.49 18.74 19.30 19.59 17.43 17.49 15.85 15.94 16.29
Fe2O3 10.62 11.38 11.76 10.71 11.07 8.49 9.37 7.26 8.93
MnO 0.17 0.21 0.22 0.21 0.21 0.15 0.18 0.18 0.22
MgO 3.66 3.85 4.22 4.20 5.68 2.99 3.97 2.14 3.69
CaO 4.14 9.05 7.88 5.47 7.69 5.86 5.45 4.53 5.51
Na2O 2.23 1.62 1.85 4.38 2.55 3.22 2.71 2.59 2.63
K2O 2.89 0.82 1.43 1.28 0.29 2.68 2.57 2.31 2.35
P2O5 0.08 0.07 0.07 0.07 0.08 0.24 0.25 0.22 0.20
LOI 2.89 2.49 2.96 3.63 4.96 2.53 2.71 5.79 2.58
Rest 0.18 0.17 0.17 0.15 0.09 0.20 0.25 0.15 0.20
Total 100.26 100.05 100.04 100.49 100.67 100.34 100.46 99.39 99.80

Trace elements in parts per million
As 17 8 — 10 — — 4 1 5
Ba 437 294 392 180 47 830 901 593 750
Ce 24 24 5 29 5 5 25 28 20
Co 26 37 48 31 32 29 37 18 23
Cr 84 47 17 37 27 18 152 11 17
Cu 42 84 25 41 49 92 72 22 54
Hf — — —— — — — — 2.13 1.69
Mo — — — — — — — — <1
Nb 2 3 1 — 1 2 3 3 3
Ni 9 5 4 7 10 4 11 <2 <1
Pb 16 4 10 13 6 8 10 4 6
Rb 44 22 26 20 6 70 76 70 65
Sc 47 44 — 48 — — 29 24.20 32.00
Sr 145 327 330 179 113 327 273 85 282
Ta — — — — — — — 0.18 0.12
Th — — 2 — 2 5 3 7.34 5.89
V 271 260 249 269 214 122 150 119 162
Y 17 16 17 17 20 21 24 18 17
Zn 136 97 135 147 111 83 106 101 94
Zr 36 30 30 32 21 63 73 74 58
La 5.80 — — — — — 14.49 12.40 13.30
Ce 11.53 — — — — — 29.22 27.10 27.30
Nd 6.92 — — — — — 15.24 15.80 14.50
Sm 1.99 — — — — — 3.53 3.35 3.12
Eu 0.84 — — — — — 0.94 0.72 0.93
Gd 2.33 — — — — — 3.52 — —
Yb 1.36 — — — — — 2.09 2.38 2.03
Lu 0.22 — — — — — 0.35 0.37 0.29
  • Samples I and VII, La-Lu analysed by ICP-ES (inductively coupled plasma emission spectrometry); samples VIII and IX, La-Lu, and Hf, Sc, Ta and Th analysed by neutron activation; all remaining data by XRF (X-ray fluorescence). LOI = loss on ignition. Rest = traces expressed as major elements
  • I Microdiorite (JUD 13), Judkins' Quarry [SP 3432 9317].
  • II Microdiorite (JUD 14), Judkins' Quarry [SP 3432 9317].
  • III Microdiorite (JUD 33), Judkins' Quarry [SP 3434 9319].
  • IV Microdiorite (JUD 34), Judkins' Quarry [SP 3434 9319].
  • V Microdiorite (JUD 42), Judkins' Quarry [SP 3433 9335].
  • VI Granophyric diorite (JUD 16A), Judkins' Quarry [SP 3421 9319].
  • VII Granophyric diorite (JUD 18), Judkins' Quarry [SP 3437 9331].
  • VIII Granophyric diorite, marginal facies (E62178), Judkins' Quarry [SP 3406 9316] 25 m east-north-east of Locality 2 (Figure 5).
  • IX Granophyric diorite (E62343), Judkins' Quarry [SP 3441 9333], north-easternmost part of intrusion (Figure 5).

(Table 5) Chemical compositions of selected whole-rock samples from the Midlands Minor Intrusive Suite; La-Cs analysed by neutron activation, other elements by X-ray fluorescence

Spessartite lamprophyre

 Intrusion in Griff No. 4 Quarry

Lamprophyres, Fillongley Borehole
Base Top
(E62341) (E62502) (E62517)

(E62676)

 (E62685) (E62681) (E62683) (E62678) (E62680) (E62682) (E62674) (BH465) (BH469) (BH477) (BH474)
SiO2 47.55 46.00

46.37

47.73

 43.63 40.40 41.93 42.39 45.36 42.22 47.67 55.02 48.93 49.08 47.28
TiO2 0.98 1.38

1.97

1.99

 1.54 2.24 1.37 1.33 1.07 1.50 0.95 2.17 2.40 1.78 2.21
Al2O3 16.41 16.26

15.38

14.54

 13.83 14.80 12.18 11.44 14.80 12.96 17.00 17.33 17.78 17.20 16.93
Fe2O3 7.53 8.66

11.40

10.38

 11.05 13.67 10.97 13.01 9.92 12.39 7.69 6.27 10.79 8.83 9.92
MnO 0.19 0.22

0.26

0.16

 0.36 0.45 0.24 0.20 0.20 0.20 0.25 0.09 0.23 0.19 0.18
MgO 4.92 4.52

6.41

9.34

 7.75 9.72 13.89 16.01 10.88 14.43 6.27 2.70 5.71 4.66 5.92
CaO 7.61 7.49

4.60

4.27

 8.12 5.84 8.11 6.77 7.22 6.45 6.11 2.21 2.47 4.50 4.68
Na2O 5.31 4.11

3.54

3.76

 2.42 2.78 2.11 2.06 3.42 2.25 5.02 8.38 5.78 5.01 4.45
K2O 0.96 1.58

2.39

0.39

 0.55 0.98 0.81 0.20 0.50 0.52 0.41 0.12 0.13 1.44 1.90
P2O5 0.85 0.53

0.47

0.54

 0.47 0.29 0.38 0.33 0.32 0.35 0.30 0.39 0.51 0.71 0.40
LOI 6.76 8.66

6.48

5.98

 9.30 8.25 7.10 5.44 5.51 5.78 7.34 3.72 5.02 5.42 5.11
Total 99.07 99.43

99.29

99.09

 99.06 99.48 99.17 99.28 99.23 99.13 99.21 98.82 99.75 98.82 99.51

Trace elements in parts per million
As 52 21

22

6

 2 — 1 0 0 0 0 7 — — 0
Ba 51 188

2236

1055

 288 207 762 80 270 204 120 283 174 51 184
Ce 66 49

32

44

 39 43 31 32 41 24 19 22 27 49 52
Co 26 41

41

42

 35 43 60 77 47 73 33 26 — — 24
Cr 6 106

102

103

 241 406 482 667 239 619 138 39 — — 28
Cu 23 76

48

49

 92 102 65 54 37 62 78 72 43 66 62
Mo — —

—

1

 1 1 0 0 1 1 1 5 2 2 3
Nb 10 9

8

8

 8 8 7 5 4 6 4 6 5 6 6
Ni 8 67

34

46

 122 133 297 334 135 257 55 22 11 2 11
Pb 5 17

12

2

 9 4 25 4 5 7 1 17 1 1 9
Rb 8 26

41

3

 5 15 6 1 5 1 7 4.01 1 14 25
Sc — —

—

21.90

 22.70 — — 33.70 28.50 — 21.30 22 — — 30
Sr 455 222

375

647

 550 469 721 386 639 419 647 295 398 554 979
Th 5 3

2

1.79

 2.05 4 2 2.22 2.31 2 2.68 1 2 2 0
V 169 201

276

314

 196 331 178 222 188 243 198 236 264 233 265
Y 15 16

14

16

 16 14 11 9 10 10 6 24 18 20 28
Zn 116 179

222

97

 76 88 150 105 96 101 73 144 79 55 82
Zr 102 103

103

121

 178 186 143 88 60 98 71 213 234 167 207
La 31 27

19

21.20

 21.90 25 22 16.90 18.80 19 9.90 0 23 23 19
Ce 66 49

39

48.10

 47.80 43 31 36 38.40 24 19.30 22 27 49 52
Nd — —

21

25.80

 25.10 — — 18.90 20 — 9 12 — — 22
Sm —

3.80

4.77

 4.58 — — 3.06 3.60 1.32 8 — — 19
Eu — —

1.26

1.54

 1.53 — — 1.02 1.14 — 0.54 — — — —
Gd — —

—

—

 — — — — — — — 4 — — —
Tb — —

0.55

0.61

 — — — 0.39 0.43 — 0.20 — — — 0
Yb — —

1.48

1.42

 1.40 — — 1.01 0.98 — 0.81 — — — —
Lu — —

0.27

0.21

 0.22 — — 0.15 0.13 — 0.10 — — — —
Ta — —

0.36

0.47

 0.39 — — 0.24 0.24 — 0.25 — — — —
Hf — —

2.49

2.63

 3.43 — — 2.21 1.65 — 1.75 — — — —
U — —

0.93

0.48

 0.48 — — 0.60 0.49 — 0.45 — — — —
Cs — —

2.52

0.46

 1.03 — — — 1.40 — — — — — —
  • Notes for (Table 5).
  • (E62341)From central facies of a 4 m-thick dyke [SP 3414 9334]; northwestern face of Judkin's Quarry, 100 m north of Locality 10, (Figure 5)
  • (E62502) Marginal, slightly porphyritic facies of 2 m-thick dyke [SP 3304 9387]; north-western part of Hartshill Quarry
  • (E62517) Concordant sheet 0.70 m thick in Boon's Quarry [SP 3311 9442]
  • (E62676) Fine-grained sheet, 0.30 m thick, intruded into poikilitic hornblende meladiorite facies (E62678) of the intrusion in Griff No. 4 Quarry [SP 3625 8870]
  • (E62685) Horneblende meladiorite, from chilled base of the northwestern intrusion in Griff No. 4 Quarry [SP 3625 8870]
  • (E62681) Hornblende meladiorite, from highly magnetic facies 3 m above (E62685) [SP 3625 8870]
  • (E62683) Horneblende meladiorite, same location as (E62685)
  • (E62678) Poikilitic horneblende meladiorite, from 11 m above (E62685)
  • (E62680) Feldspathic facies of poikilitic hornblende meladiorite, from 17 m above (E62685)
  • (E62682) Hornblende diorite immediately above poikilitic hornblende meladiorite, from the south-eastern intrusion in Griff No. 4 Quarry [SP 3650 8840]
  • (E62674) Hornblende diorite, from 30 m above base of the northwestern intrusion in Griff No. 4 Quarry. [SP 3625 8870]
  • (BH465) Lamprophyre chill zone from Fillongley Borehole [SP 2614 8700], depth 604 m
  • (BH469) Lamprophyre chill zone, location as above, depth 608 m
  • (BH477) Central part of lamprophyre sill, location as above, depth 613.9 m
  • (BH474) Lamprophyre chill zone, locations as above, depth 611.7 m

(Table 6) Sedimentary facies within the Langsettian and Duckmantian of the Warwickshire Coalfield (based on Jones, 1992)

Sedimentary facies

 Lithology Sedimentary structures Fauna/flora Geometry/ lateral relationships Examples

Lacustrine

 Pale to dark grey and black claystone and siltstone, with cannel coal and minor amounts of sandstone; siderite common Finely parallel laminated to massive Nonmarine bivalves, fish fragments and ostracods. Stems of Lepidodendron and Calamites; leaves of Neuropteris Up to 8 m thick and many km across Black carbonaceous mudstone sequence above the Two yard Coal

Lacustrine delta

 Prodelta Grey and pale grey siltstone with minor interlaminated sandstone forming part of an upward- coarsening sequence Parallel laminated with rare ripple cross-lamination; convolute lamination and load casts common Plant debris common Frequently passes into mouth bar facies above and laterally

Two Yard to Four Feet interseam interval in roadways at Daw Mill and also Coventry Colliery; studied in Birch Tree Farm Borehole
Mouth bar Thin- and thick-bedded sandstones with minor interbedded siltstones and claystones Ripple cross- lamination dominant with some small-scale cross-bedding, convolute lamination and scours Roots of Stigmaria and plant material common in upper parts Up to 5 m thick; coarsens upwards from underlying prodeltaic subfacies and may be overlain by overbank or palaeosol facies

Distributary channel

 Erosively based sandstones with lag conglomerates; fining upwards to siltstones and thin interbedded sharp- based sandstones Low-angle accretion surfaces representing point bar deposits Plant debris common Individual channels are up to 5 m thick and vary in width from 30 to 80 m A channel belt up 1.2 km wide washes out the Two Yard in the vicinity of the Dexter Manrider Road linking Dexter and Daw Mill collieries. Similar minor channels were visible above the Upper Half Yard Coal in the Sudeley Opencast Site, now restored

Overbank

 Grey and pale grey
siltstone with minor
(< 10%) sandstone		 Poorly laminated to massive Abundant well- preserved plant material including stems of Calamites and Cordaites. Some stems and trunks preserved upright in situ. Leaves of Neuropteris Typically occurs in elongate belts bordering palaeochannels and frequently characterised by rapid variations in interseam interval Identified from the Sudeley Opencast Site in Ell to Two Yard and Nine Feet to Ell interval

Crevasse splay

 Sharp-based sheet sandstones Unidirectional small-scale cross- bedding and ripple cross-lamination Generally less than 2 m thick, laterally extensive Examples occur in the lower part of the Nine Feet to Ell interval at Sudeley Opencast Site

Palaeosol (seatearth)

 Poorly drained Grey to dark grey mudstone and sandstone; siderite nodules common

Abundant listric surfaces; complete destratification common, though crude horizonation may be visible

Abundant oblique and subhorizontal Stigmaria roots

Passes transitionally upwards into mire facies

Widely developed throughout the Coal Measures
Partially drained Colour-mottled mudstones and sandstones; associated sphaerosiderite

Mire (coal)

 Coal, 'inferior coal' Banded Sheet-like deposits with leaves of coal up to 3 m thick extending over many tens of km2 Exemplified by the Thick Coal

Marine

 Dark grey to black mudstone Finely laminated to massive Goniatites, brachiopods, bivalves and foraminifera Laterally extensive over many thousands of km2; usually less than 1 m thick Vanderbeckei and Aegiranum marine bands

(Table 8) Former opencast coaling sites with details of seams worked and their thickness. Paradise [SP 355 902] , Sunnyside [SP 352 911] and Bermuda [SP 357 898] are excluded because of incomplete data

Opencast sites (with location and dates of working)
Holly Park

[SP 287 970]

1955–59

 Monks Park

[SP 293 961]

1949–53

 Bratts Waste

[SP 298 949]

1946–49

 Slack's Wood West

[SP 299 949]

1950–52

 Sudeley [SP 355 893] restored 1991

Seams worked (average thickness in m)

 Half Yard 0.9 — — — 0.79 0.74 (2 leaves)
Four Feet 1.8 1.7 1.3 — 0.8
Two Yard/Bare 4.8 7.3 2.4 2.3 1.0 (2 leaves) 3.0 (old workings)
Ryder — — 1.2 — —
Ell 0.5 — — — 1.27 (2 leaves)
Nine Feet 2.1 — — — 2.4 (old workings)
High Main 1.1 — 0.6 — 0.63
Low Main 1.1 — — — —
Thin 0.8 — — — —
Seven Feet 2.5 — 1.0 — —
Deep Rider 1.2 1.2 1.6 (2 leaves) 1.5 0.8 1.3 (2 leaves) —
Double 1.3 2.4 2.7 1.8 —
Top Bench 1.8 1.0 3.4 1.0 —
Lower Bench 4.4 1.1 1.4 —
Stumpy 0.9 — — —

Tonnage extracted

 475 461 209 443 not known 110 185 c. 1.3 million

(Table 9) Heavy-mineral assemblages of Namurian to ?early Permian sandstones of the Warwickshire Coalfield; provenance-sensitive heavy mineral ratios are also tabulated

Formation Locality/ Depth AP CH CS GT MO RU ST TO ZR Total count ATi Count RZi Count MZi Count CZi Count
Ashow Formation 3033 6940 9.0 — — 33.0 1.0 8.5 1.5 10.5 36.5 200 45.0 100 18.0 100 2.0 100 — 100
Kenilworth Sandstone Formation 2808 7209 10.5 — — 22.0 0.5 19.0 Rare 17.0 31.0 200 32.0 100 38.0 100 5.0 100 1.0 100
Tile Hill Mudstone Formation 3083 7735 8.0 — 0.5 40.5 — 13.5 3.5 5.5 28.5 200 50.0 100 25.4 134 — 100 1.0 100
Meriden Formation: Keresley Member (Corley sandstone) 3040 8522 7.0 — — 2.0 1.5 13.0 0.5 8.0 68.0 300 38.0 100 16.0 243 2.4 209 — 204

Meriden Formation: Whitacre Member

 BTF/325 m 13.5 — — 12.0 — 14.5 1.5 29.0 29.5 200 33.0 100 29.6 100 — 100 — 100
BTF/419 m

_

 5.5 — 1.0 18.0  0.5 12.0 — 10.5 52.5 200 41.0 100  18.6 129 0.9 106 1.9 107
BTF/491 m 13.5 — — 9.0 — 23.0 — 27.5 27.0 210 33.0 100 45.7 105 — 100 2.0 100
BTF/553 m 18.5 0.5 0.5 4.5 — 21.0 — 17.0 38.0 130 52.2 46 35.5 76 — 49 2.0 50

Halesowen Formation

 BTF/587 m 15.0 Rare Rare 58.5 0.5

_

 5.0  0.5 9.0  11.5

_

 200 60.0 100  35.0 100 1.5  66 1.5 66
BTF/600 m 19.0 0.5 — 51.5 — 6.5 — 7.0 15.5 200 63.1 111 32.5 123 1.2 84 — 83
BTF/634 m 13.5 0.5 — 47.5 — 5.0 — 11.5 22.5 250 50.0 100 21.0 100 — 100 — 100

Etruria Formation

 BTF/669 m — — — — —

—

 — — 32.0 68.0 25 — 8 — 17 — 17 — 17
BTF/716 m — — 1.0 8.0 — 19.0 — 21.0 51.0 100 — 21 27.1 70 — 51 1.9 52
Middle Coal Measures BTF/749 m — — 2.0 — — 10.0 — 11.0 77.0 200 — 50 11.5 174 — 154 2.5 158
Lower Coal Measures BTF/812.5 m — — 2.5 1.0 — 9.0 — 37.5 50.0 200 — 75 15.3 118 — 100 5.0 100
Millstone Grit MB/52.3 m 7.5 — — 9.0 1.5 10.5 — 16.0 55.5 225 29.5 61 13.2 144 2.3 128 — 125

AP = apatite, CH = chloritoid, CS = chrome spinel, GT = garnet, MO = monazite, RU = rutile (plus anatase), ST = staurolite TO = tourmaline, ZR = zircon, BTF = Birch Tree Farm Borehole, MB = Merevale No. 2 Borehole ATi = apatite/tourmaline index, calculated as (100 X apatite/apatite + tourmaline) RZi = rutile/zircon index; MZi = monazite/zircon index; CZi = chrome spinel/zircon index

(Table 10) Current and former Triassic nomenclature applied to the Coventry district

Current nomenclature (Warrington et al., 1980)

Former nomenclature (Hull, 1869)

PENARTH GROUP

 Lilstock Formation'

Rhaetic or Penarth Beds
Westbury Formation'

MERCIA MUDSTONE GROUP

 Blue Anchor Formation

KEUPER

New Red Marl
Unamed mudstone
Arden Sandstone
Unamed mudstone
siltstone 'Waterstones'

LOWER KEUPER SANDSTONE

 Waterstones

SHERWOOD

SANDSTONE GROUP

 Bromsgrove Sandstone Formation Building Stones Basement Beds
possible gap in sequence BUNTER Upper Red and Mottled Sandstone
Polesworth Formation Pebble Beds

Lower Red and Mottled Sandstone
Hopwas Breccia Lower Permian

1 Only recognised in boreholes

(Table 13) Summary of physical properties of the main rock types. Data sources are indicated in the text

Age and lithology Density (Mg/m3) Susceptibility (SIX 10−3) Seismic velocity (km/s)
SEDIMENTARY ROCKS
Quaternary 2.00 1.80
Jurassic
Lias Group 2.49 2.40
Triassic
Mercia Mudstone Group 2.45 3.20–3.40
Sherwood Sandstone Group 2.40?
Carboniferous
Westphalian 2.48 2.90
Devonian
Oldbury Farm
Sandstone Formation 2.47
Cambrian
Stockingford Shale Group 2.50 (Dosthill)
2.75 (Nuneaton) 0.1–0.3 3.8–4.4
Hartshill Sandstone Formation 2.61
Precambrian
Charnian 2.64–2.78 5.4–5.7
IGNEOUS ROCKS
South Leicestershire Diorites
Granodiorite (Croft) 2.70 10.2
Granodiorite (Mountsorrel) 2.65 26.4
Diorite (Barrow Hill) 2.73
Diorite (Stoney Stanton) 0.3–1.0
Midlands Minor Intrusive Suite
Diorite 2.74 0.4–50.0
Precambrian
Caldecote Volcanic Formation 2.64 0.2–0.4
Basic intrusions 2.80 0.4–2.0

(Table 14) Location of former brickpits, listed by formation

Formation Brickpit Location
Wolston Clay Victoria [SP 3582 8413]
Wolston Clay/ Bedworth [SP 3595 8770]
Thrussington Till [SP 3605 8745]
[SP 3630 8715]
[SP 3590 8610]
Barnacle Hall [SP 3845 8378]
Hinckley [SP 430 948]
[SP 443 953]
[SP 410 947]
Bromsgrove Sandstone Barras Heath [SP 3505 8055]
Meriden Formation Longford [SP 3560 8415]
Whittleford area [SP 3273 9191]
Wilson's Lane [SP 3495 8465]
Barker's Butts Lane [SP 3212 8000]
Old House Lane [SP 295 856]
Etruria Formation Stanley's Pits [SP 349 899 to [SP 342 915]
Haunchwood Pits [SP 340 917 to [SP 328 926]
Griff/Bermuda [SP 352 892 to [SP 348 904]
Coal Measures Stanley's No. 4 Pit [SP 351 912]
Outwoods Shale Formation Chapel End [SP 322 937]

(Table 15) Physical properties of aggregates. Information supplied by the quarry operators

Mancetter (lamprophyre) Hartshill (sandstone) Croft (quartz diorite)
Relative density:
Oven dried 2.72 2.61 2.62
Saturated surface dried 2.75 2.64
Apparent 2.80 2.69
Water absorption 0.99 1.1 0.95
Aggregate impact value 7–9 15 19
Aggregate abrasion value 8.0 3.5 4.9
Aggregate crushing value 13 14 20
Polished stone value 61 — 60
10% fines value (kn) 300 300 (dry) 200
260 (soaked)

(Table 16) Groundwater abstraction licence data for the Coventry district. Figures derived from base data provided by the National Rivers Authority, Severn-Trent Region

AGRICULTURE

SPORTS GROUNDS

INDUSTRY

PUBLIC SUPPLY

TOTALS
WATER USE

EXCLUDING SPRAY IRRIGATION

INCLUDING SPRAY IRRIGATION

SPRAY IRRIGATION

COLLIERIES

MANUFACTURING AND COOLING
AQUIFER ms/year No. licences m3/year No. licences ms/year

No. licences

 ms/year No. licences m3/year No. licences ms/year No. licences ms/year No. licences
GLACIAL SAND AND 70 438 57 15 456 2 —

—

 — — 30 000 1 — — 115 894 60
GRAVEL
MERCIA MUDSTONE 8 066 7 9092 2 364

1

 — — — — — — 17 522 10
GROUP
SHERWOOD — — — — —

—

 — — 233 889 3 — — 233 889 3
SANDSTONE GROUP
TILE HILL MUDSTONE 830 1 — — —

—

 — — — — — — 830 1
FORMATION
MERIDEN FORMATION 21 055 9 64 544 2 709

1

 226 900 2 6 412 327 12 11 957 900 4 18 683 435 30
HALESOWEN FORMATION 77 1 — — —

—

 — — — — — — 77 1
MIDDLE COAL — — — — —

—

 — — 13 638 1 — — 13 638 1
MEASURES
TOTALS 100 466 75 89 092 6 1073

2

 226 900 2 6 689 854 17 11 957 900 4 19 065 285 106

(Table 17) Typical chemical analyses of groundwater in the Coventry district

LOCATION WOLVEY VILLA FARM NUNEATON

Sterling Metals No. 1

 HINCKLEY

Southfields Road

 COVENTRY

Standard Motor Co. No. 2

 MERIDEN SHAFTS

No.1

 COVENTRY

Morris Motors

 BIRCHLEY HEATH CLARA WELL GRIFF
NATIONAL GRID REFERENCE [SP 4292 8699] [SP 3740 8974] [SP 4291 9331] [SP 3060 7811] [SP 2623 8262] [SP 3528 8106] [SP 2798 9421] [SP 3478 8889]
SOURCE TYPE Borehole Borehole Borehole Borehole Borehole Borehole Borehole Well
AQUIFER WOLSTON SAND AND GRAVEL BROMSGROVE SANDSTONE BROMSGROVE SANDSTONE ALLESLEY MEMBER KERESLEY MEMBER (CORLEY SANDSTONE) WHITACRE MEMBER (ARLEY/EXHALL SANDSTONE) WHITACRE MEMBER ( '40 FOOT SANDSTONE') HALESOWEN FORMATION ('100 FOOT SANDSTONE')
Date of analysis 1982 1943 1938 1941 1938 1939 1948 1915
pH 7.5 NA NA NA NA NA 7.2 NA
Total dissolved solids mg/l 510 450.0 4013.8 390.0 320.0 542.3 357 710
Calcium (Ca2+) mg/l 130 78.6 508.6 103.0 70 117.1 107 120
Magnesium (Mg2+) mg/l 13 25.0 102.6 16.2 13 34.7 3.9 42
Sodium (Na+) mg/l 11 34.4 609.0 13.4 32 30.1 6.0 48.7
Potassium (K+) mg/l 5.4 NA NA NA NA NA NA NA
Carbonate (C032−) mg/l 139 133.5 46.8 143.4 148 180.0 111 115
Sulphate (SO42−) mg/l 150 112.8 2543.7 74.1 32 95.5 59 320
Chloride (C1−) mg/l 22 24.0 203.0 15.0 12 55.0 20 38
Nitrate (NO3) mg/l 40 — trace 15.9 4 29.2 24 1.8
Iron (Fe3+) mg/l 0.05 2.3 0.14 1.5 NA 0.25 0.81 8.6
NA = Not available

(Table 18) Summary of geotechnical data for superficial deposits

Engineering group Type of deposit Moisture content

%

 Liquid limit

%

 Plastic limit

%

 Bulk density Mg/ms Dry density Mg/ms % Clay % Silt % Sand Undrained cohesion kPa SPT
N		 Max. dry density Mg/ms Optimum water content %
Normally Alluvium 23.8 36.6 18.7 1.92 1.54 2 9 36 50 32.5
consolidated and (18) (34) (18) (2.00) (1.67) (0) (6) (36) (29) (26)
cohesive and River Terrace 14.3 13.2 5.6 0.24 0.33 4.8 9.0 25.2 60 24
loose non- Deposits 66 68 26 1.0 1.11 19 31 89 248 80
cohesive deposits 63 43 43 38 30 30 36 30 30 25
Dense non- Baginton Sand 11.6 1 5 69 1.99 9
cohesive deposits and Gravel (13) (0) (4) (71) (2.06) (9)
9 10 13 74 0.39 4
9 8 8 8 5 5
Glaciofluvial 19.7 42 19.4 1.94 2 7 58
sand and gravel (16) (34) (19) (1.98) (0) (4) (57)
(undifferentated) 3.3 8.4 27
45 55 22 0.37 13 33 94
18 11 11 8 50 50 52
Normally to Wolston Clay 22.3 54 20 2.03 1.71 121
slightly (22.2) (53) (21) (2.05) (1.70) (110)
overconsolidated 4.7 16 4 0.13 0.16 73
cohesive 33 65 18 0.86 0.85 338
deposits 125 86 86 108 39 100
Laminated clay 19.7 34.8 18 2.05 90
(undifferentiated) (20) (32) (18) (2.07) 83
14 31 7 0.19 41
15 15 15 6 5
Heterogeneous Till (data refer 16.9 35.5 16.7 2.12 1.82 161 161* 1.87 14.6
overconsolidated/ mainly to (16) (34) (16) (2.13) (1.84) (140) (42) (1.88) (14)
dense deposits Thrussington Till) 4.1 8.7 3.4 0.11 0.11 107 0.08 2.6
43.3 86 43 1.16 0.88 690 661 0.3 11
342 284 284 252 175 219 14 33 33
  • Notes:
  • Datasets containing less than 5 points are not included
  • Standard deviation not listed for data containing less than 20 points * 6 out of 14 SPT N values are in excess of 50
  • Key:
  • 23.8 Arthmetic mean
  • (18) Median
  • 14.3 Standard deviation
  • 66 Range
  • 63 Number of data points