Geology of the country around Sunderland Memoir for 1:50 000 geological sheet 21 (England and Wales)

By D B Smith

Bibliographical reference: Smith, D B. 1994. Geology of the country around Sunderland. Memoir of the British Geological Survey, sheet 21 (England and Wales).

British Geological Survey

Geology of the country around Sunderland Memoir for 1:50 000 geological sheet 21 (England and Wales)

Author

D B Smith, BSc, DSc, CGeol, FGS formerly British Geological Survey

LONDON: HMSO 1994.  First published 1994. ISBN 0 11 884498 9. NERC copyright 1994. Printed in the UK for HMSO. Dd 292041 C8 8/94 3396/2 20249

Other publications of the Survey dealing with this and adjoining districts

Books

• Memoirs
• Geology of the Tynemouth district, Sheet 15, 1974
• Geology of the country between Durham and West Hartlepool, Sheet 27, 1967
• British Regional Geology
• Northern England, 4th Edition, 1971

Maps

• 1:625 000
• Solid geology (north sheet), 1979 Quaternary geology (north sheet), 1977 Aeromagnetic (north sheet), 1972
• 1:250 000
• Solid geology, Tyne-Tees Sheet 54°N-02°W, 1981
• Sea bed sediments and Quaternary geology, Tyne-Tees Sheet 54°N-02°W, 1986
• Aeromagnetic anomaly map, Tyne-Tees Sheet 54^°N-02^°W, 1985
• Aeromagnetic map of part of Great Britain and Northern Ireland (Sheet 9)
• Bouguer gravity anomaly map, 1978
• 1:50 000 and 1:63 350
• Sheet 14 Morpeth (Solid), 1955
• Sheet 14 Morpeth (Drift), 1977
• Sheet 15 Tynemouth (Solid), 1975
• Sheet 15 Tynemouth (Drift), 1968
• Sheet 20 Newcastle upon Tyne (Solid), 1989
• Sheet 20 Newcastle upon Tyne (Drift), 1992
• Sheet 21 Sunderland (Solid and Drift), 1978
• Sheet 26 Wolsingham (Solid), 1977
• Sheet 26 Wolsingham (Drift), 1977
• Sheet 27 Durham (Solid), 1965
• Sheet 27 Durham (Drift), 1965

Preface

The district described in this memoir comprises Wearside and parts of South Tyneside, and is dominated by the built-up area which extends from Sunderland north-westwards to Wallsend. It was once the heartland of the Northumberland and Durham Coalfield, but working is now restricted to a few small opencast pits. The published geological map which this memoir describes provides a guide to potential areas for further exploitation.

The availability of mining record plans, covering most land areas and much of the undersea area, has enabled the structure of the Carboniferous rocks at depth to be determined in considerable detail. Analysis of the large number of borehole and shaft sections has led to a detailed understanding of the Coal Measures succession, in which well-authenticated marine bands facilitate regional correlation.

In the built-up areas, up-to-date geological data are essential for planners and civil engineers to help effect urban renewal, particularly in those places where old coal workings exist beneath the surface, creating problems of ground instability. Foundation designs for new developments must also take into account the nature and thickness of the superficial deposits. This account helps meet all these needs by indicating the extent of former coal workings and by describing the lithologies and complex distribution of the superficial deposits.

The district is the type area for the marine Permian strata of the United Kingdom and includes fine exposures of the Magnesian Limestone, seen to best advantage in spectacular cliffs on the coast. Evaporites occur in the undersea area but have been dissolved elsewhere, causing widespread foundering. The interval between publication of the 1:50 000 map in 1978 and the preparation of this memoir, which will become a standard work of reference for these important and diverse rocks, has permitted a thoroughly modern and up-to-date treatment, including correlation with the equivalent Zechstein sequence in northern Europe.

Peter J Cook, DSc Director British Geological Survey Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG 4 January 1994

Acknowledgements

Dr Smith retired from the British Geological Survey in 1984 before this memoir was written, but subsequently completed the work under contract. The delay enabled the result of much new research, particularly on the Permian, and new offshore borehole data to be incorporated.

Dr M A Calver identified the Carboniferous fossils and Mr J Pattison was responsible for the identification of the Permian faunas. The memoir was compiled by Dr D V Frost and edited by Mr D H Land, Dr D V Frost, Mr R J Wyatt and Mr J I Chisholm.

For their helpful comments on aspects of early drafts of Chapter 3, sincere thanks are due to Mr T H Pettigrew and Dr P Turner (Marl Slate), Dr N T J Hollingworth (Ford Formation) and Dr C J R Braithwaite and Professor D J Shearman (Concretionary Limestone). Dr G A L Johnson is similarly thanked for his comments on drafts of Chapter 4 (Structure).

It is a pleasure to record our indebtedness to British Coal Corporation for their courtesy and cordial co-operation in allowing the wealth of detail contained in their records, plans and boreholes to be used to synthesise the description of the Coal Measures, and for offshore borehole data to be used freely in the description of the Permian rocks.

Our thanks are also due to landowners, quarry owners, local authorities, civil engineering consultants and contractors without whose permission to consult records, and to enter land, this memoir would not have been possible.

Notes

Throughout the memoir the word 'district' refers to the area covered by the 1:50 000 geological sheet 21 (Sunderland).

National Grid references are given in square brackets; all lie within 100 km square NZ.

Numbers preceded by the letter E refer to the sliced rock collection of the BGS.

Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, Keyworth.

History of the survey of the Sunderland sheet

The district covered by the Sunderland (21) sheet of the 1:50 000 geological map of England and Wales was originally surveyed on the six-inch County Series sheets Durham 3, 4, 7, 8, 13, 14, 20 and 21 and Northumberland 98 by H H Howell, and published in 1868 as Old Series one-inch sheet 105 SE. Maps at a scale of six inches to one mile were published for most of the district between 1867 and 1871, but there was no accompanying memoir.

Resurvey of the district on the six-inch scale was started in 1952 by G Armstrong and D S Buist, who incorporated earlier observations and recordings made by W Anderson and L H Tonks. The major share of the work was undertaken by D B Smith during 1953–1976. A small area was surveyed by E A Francis during 1958–1960. The 1:50 000 sheet was published in 1978.

Geological 1:10 560 scale National Grid maps included wholly or in part of 1:50 000 Sheet 21 are listed below together with the initials of the geological surveyors and dates of the survey; all surveyors are listed for marginal sheets. The surveyors were W Anderson, G Armstrong, D S Buist, G A Burnett, F C Cox, E A Francis, D H Land, D A C Mills, R H Price, G Richardson, D B Smith and L H Tonks.

Copies of the published and fair-drawn maps have been deposited in the BGS libraries at Keyworth and Edinburgh for public reference and may also be inspected in the London Information Office, in the Geological Museum, South Kensington, London. Copies may be purchased directly from BGS in published form or as black and white dyeline sheets.

Geology of the country around Sunderland—summary

The district described in this memoir comprises Wearside and parts of south Tyneside, and is dominated by the built-up area which extends from Sunderland north-westwards to Wallsend. The predominantly urban landscape is offset by an attractive coastline along which fine exposures of the Permian Magnesian Limestone are seen in spectacular cliffs, stacks and natural arches.

The district is the type section of the marine Permian succession in the United Kingdom. In addition to the Magnesian Limestone, it contains evaporites which, on land, have been dissolved away, giving rise to widespread collapse structures.

Carboniferous Coal Measures underlie the whole district and formed the basis of the coal mining industry, of which only a few small opencast workings survive.

Almost all the district is covered by glacial and periglacial deposits of Late Devensian age, which are locally complexly interbedded.

This memoir will be useful for planners and civil engineers involved in urban renewal, who are faced by problems of ground instability caused by old coal workings and the complex distribution of drift deposits.

(Inside front cover) Geological succession in the Sunderland district

(Front cover) Cover photograph Stack and natural arch formed of Permian Concretionary Limestone, Whitburn Colliery.

Chapter 1 Introduction

This memoir describes the geology of the district covered by the Sunderland (21) Sheet of the one-inch and 1:50 000 Geological New Series maps of England and Wales. Sea occupies slightly more than half of the district, with the land area lying mainly in the north-eastern part of the historic county of Durham. In addition to Sunderland, the district includes most of the heavily populated area of lower Tyneside (including parts of Newcastle upon Tyne), Washington New Town and the smaller towns of Chester-le-Street (part), Houghton-le-Spring and Seaham (Figure 1). Seaham was itself a new town in 1828 to 1831, built specifically to house the workers from the then-new Seaham Colliery and Seaham Harbour.

The main physical features of the area are closely related to the solid geology and comprise the hilly areas of Gateshead Fell and the East Durham Plateau, plus the less varied Wear Lowlands and a narrow coastal platform. Gateshead Fell and the East Durham Plateau (including its northward extension into the Fulwell and Cleadon hills) owe their existence to the resistance to erosion of thick Carboniferous sandstones and Permian limestones respectively, the Wear Lowlands being cut into relatively softer Carboniferous coal measures in which argillaceous rocks are dominant. The district is drained mainly by the River Wear and its tributaries, with the River Don and Lower Tyne draining the northern part. The East Durham Plateau is drained by a number of minor eastward-flowing streams. The land is relatively fertile and supports a mixed farming economy which has declined in its relative importance in recent years as more and more farm land has been built on.

The outlines of the present pattern of settlement are believed to have been established with the initiation of the many 'green villages' in the twelfth to fourteenth centuries (some probably on sites occupied previously), but the main growth in population and settlement began with the rapid development of the mining and heavy engineering industries in the eighteenth, nineteenth and early twentieth centuries. Recent declines of these industries have led to population stabilisation or a slight decline, but the settlements themselves have spread rapidly in response to modern pressures for greater living space.

The economic growth and prosperity of the region depended largely on its natural resources, coal in particular, and on the presence of the rivers Tyne and Wear that facilitated their exploitation. Though coal is known to have been used by the Romans, its use lapsed for several hundred years. From the thirteenth century onwards charters were granted to work coal and other minerals at a number of places in the west of the district. The coal was used mainly for industrial purposes such as lime burning, brewing and dyeing, the need being met mainly from adits and shallow pits in the High Main, Five-Quarter and Main seams along the banks of the River Tyne below Newcastle and from Wearside workings at Lumley. Primitive bell pits and associated galleries provided some coal from areas of thin drift such as Felling, Rickleton and Lumley, and deeper shafts were sunk from the mid-fourteenth century. Drainage and ventilation difficulties, however, limited output until improved pumps and engines became available, and the development of the coalfield was also hampered by poor inland transport. Markets initially depended on sea trade with east coast ports, especially London, and with ports in France and other European countries. Trade was beginning to blossom in the middle of the fourteenth century, but there was little change in total output between then and the sixteenth century. This situation changed in the middle of the sixteenth century, however, when demand from London and elsewhere began to increase sharply as population grew and the burning of wood was discouraged by its rapidly increasing price and by limiting statutes.

Steady improvements in ventilation and transport gradually allowed shafts to be sunk and worked at increasing distances from the waterways, so that by the seventeenth century major collieries had been developed in much of the west of the district, including Barmston, Gateshead, Fatfield, Felling, Harraton, Jarrow, Lambton, Lumley and Penshaw. Those at Harraton and Lumley were described as the most important in the district in the 1620s and very advanced for their day. Coal output and colliery development continued steadily throughout the seventeenth and eighteenth centuries, mainly to feed increasing demands from east coast users, and the working of the lower coals in the west and all the coals in the deeper parts, such as the Boldon Syncline, was facilitated by further improvements in pumping and ventilation techniques. Working methods improved, with the 40 to 50 per cent extraction by the early pillar and stall workings being supplanted in the late eighteenth century by panel workings that allowed up to 80 per cent extraction, and in turn by longwall methods of mining which allowed total extraction of the coal from worked areas. It was, however, the development of steam-driven machinery and transport that provided the impetus for the explosive growth of coal output in the early and middle parts of the nineteenth century. A further stimulus was provided by the development of high-capacity pumps (and later of freezing techniques) that made it possible to sink shafts through the highly water-bearing

Permian strata; this led to the extension of coal workings and the creation of new pits in the east of the land area. The first pit through Permian strata was sunk just south of the district in 1820, and was followed in due course by pits at Dawdon, Ryhope, Seaham, Silksworth, Wearmouth and Whitburn; that at Wearmouth was for some time the deepest coal mine in the world. With the progressive depletion of coal reserves in the western part of the district, the main locus of production gradually shifted eastwards and coal extraction became concentrated in the undersea areas where some workings extended up to 7 km from the coast.

In addition to supplying coal to east coast users by sea, the ready availability of cheap local fuel helped to maintain and nourish other important local industries that gave the district a wider industrial base. The mouths of the Tyne and Wear, for example, were major centres in the fourteenth to eighteenth centuries for the production of salt (essential for preserving food) which was produced by the evaporation of sea water in cast iron pans, and of glass making, providing employment for 4000 workers along the Tyne in the seventeenth and eighteenth centuries. Brick making from Quaternary clays and Carboniferous mudstones became important during the Industrial Revolution, most local demand being met by local production. Lime burning was an important industry from Roman times until the 1960s. Other significant industries were pottery making and iron smelting, the latter at one time using iron ore from the Coal Measures at Offerton. Local resource-based products that were not dependent on cheap coal included building stone (both Carboniferous sandstone and Magnesian Limestone), grindstones that commanded a large export market during the nineteenth century, Quaternary sand and gravel that met much of the early need for aggregate but is now almost worked out, and dolomite that was used for many years in the chemical and pharmaceutical industries. Aggregate is now obtained from crushed Magnesian Limestone at two quarries in the district and much of the demand for building sand is met from Permian Yellow Sands extracted at Houghton-le-Spring and Eppleton. Brewing remains an important industry in the Sunderland area, exploiting the plentiful supplies of groundwater from the Magnesian Limestone and Yellow Sands.

The general geological sequence of the district is shown on the inside front cover and on (Figure 2), and the position of a selection of the main boreholes and shafts that form much of the basis of this compilation is shown in (Figure 3). Drift deposits cover most of the dis trict, the main natural exposures of solid rock being along the coast and in some of the valleys. The superb coastal exposures of the Permian sequence have been vital to its understanding and interpretation. Almost all the quarries in the Carboniferous rocks were in sandstone and most are now abandoned and filled; many quarries in the Magnesian Limestone and Yellow Sands, though abandoned, remain at least partly open and a few are operational today.

This account is a summary of the findings of geological research in the district for the past 180 years. The early research was dominated by such figures as N J Winch, J Buddle and A Sedgwick, succeeded in the mid-nineteenth centry by W King, R Howse and J W Kirkby; these workers were followed in the late nineteenth and early twentieth centuries by G A Lebour, D Woolacott, C T Trechmann and W Hopkins. Their many publications provide a firm foundation for subsequent work. Much new information gathered during the resurvey of the district was combined with that from mine plans and borehole records in the construction of the 1:10 560 geological maps, of which the 1:50 000 map is a summary. Limitations of space preclude reproduction of most of these basic data here, but the 1:10 560 maps and additional information may be consulted at offices of the Geological Survey, while mine plans may be seen at Mining Records offices.

Chapter 2 Carboniferous

Visean and Namurian rocks

Strata of Visean and Namurian age underlie the whole district but have been deeply proved only by the Harton Borehole (Ridd et al., 1970). This proved a total thickness of nearly 400 m of Namurian beds and penetrated about 800 m of Visean strata (not bottomed); two leaves of the Whin Sill distend the Visean sequence. Uppermost Namurian strata have also been penetrated in a few widely spaced coal exploration boreholes.

Strata proved in the Harton Borehole are shown in (Figure 4), where the nomenclature used is slightly modified from that of Ridd et al. (1970); the classification is that suggested by Holliday et al. (1975), George et al. (1976) and Johnson (1980). The lowest 300 m of the Visean strata proved were assigned to the Orton Group by George et al. (1976), but are atypical in their high content of sandstone; no fossils were found below the uppermost third of this sandy sequence and their Asbian age is speculative. Higher parts of the Visean strata (Alston Group) and most of the Namurian sequence (Stainmore Group, formerly 'Millstone Grit') comprise typically cyclic Yoredale-type rocks in which most of the marine limestones and certain other beds are continuous across much of northern England and can be identified with some confidence. Correlation of the uppermost mainly sandy beds of the Namurian is generally less confident than that of the underlying, more regular, Yoredale-type sequence.

Sandstone is dominant in the few coal exploration boreholes that penetrate uppermost Namurian strata within the district; no marine fossils have been recognised in any of the thin argillaceous beds present and the choice of a possible equivalent to the Subcrenatum (= Quarterburn) Marine Band is based on comparisons with sequences proved outside the Sunderland district. Correlation between the various short sequences proved is generally uncertain.

All the Carboniferous strata of the Sunderland district were formed near the northern margin of the relatively stable Alston Block. By Asbian time the sharply differential rates of subsidence between the block and the adjoining Northumberland Trough to the north were reduced and thickness changes in Brigantian and younger strata (including Namurian) are relatively more gradual (e.g. Johnson, 1967, 1970, 1980; George et al., 1976). The depositional environments of the various Visean and Namurian sedimentary rocks of north-east England were discussed by Johnson, who also outlined the evolution of the structural framework and the history of subsidence of the region. Tectonic and sedimentary evolution are also discussed by Leeder (1988) and Fraser and Gawthorpe (1990).

Westphalian rocks

Rocks of Westphalian age underlie the whole district and are all of coal measures facies; about 850 m of these strata are present in the heart of the Boldon Syncline, northwest of Sunderland. The general sequence and classification are shown in (Figure 5). Most of the stratigraphical information on the Coal Measures of this district comes from boreholes, shafts and underground workings. Valuable additional data were obtained from scattered exposures in the banks of the Tyne and the Wear and their tributaries, and from a few abandoned quarries and clay pits.

The stratigraphical classification adopted here (Figure 5) is that of Ramsbottom et al. (1978), which summarises current views following many years of gradual evolution. It is based mainly on the faunal evidence of nonrnarine bivalves and of plant fossils (both macroscopic and microscopic), but the major boundaries are taken at the horizons of marine strata which, though thin, are generally more extensive than most other Coal Measures beds and which, in some instances, are sufficiently extensive to permit international correlation. On these criteria, rocks of Westphalian A, B and part of C are present in the Sunderland district, and comprise Lower Coal Measures (220 m), Middle Coal Measures (500 m) and Upper Coal Measures (135+m); there are no discernible major thickness changes in these sequences within the district.

The Lower Coal Measures in this district are generally similar to those of the younger parts of the underlying Namurian strata; sandstones dominate most of the sequence and coals are generally thin except in the uppermost 40 m. Sandstone similarly forms more than half of the succeeding Middle Coal Measures, but these also contain most of the workable coals, most of the marine strata and most of the well-studied floras and faunas. The Upper Coal Measures, by contrast, mainly comprise siltstones and mudstones with subordinate sandstones and only a few mainly thin coals; an unknown thickness of the younger part of the Upper Coal Measures is absent.

The lithology of the Coal Measures of the Northumbrian coalfield has not been described comprehensively but has been summarised by Armstrong and Price (1954), Smith and Francis (1967), Land (1974) and Jones and Magraw (1980). More detailed investigation of the lithology and sedimentary structures of some rock types, especially the sandstones, has been conducted by Haszeldine (1981, 1983a, b, 1984a) and Fielding (1982, 1984a, b) and the heavy mineral assemblages were studied by Kellett (1927). Sedimentary structures in the sandstones suggest overall sediment transport towards the south and south-east, though with many local variations, and this is consistent with Kellett's view that many of the heavy mineral grains present may have been derived mainly from the Cheviot area. The distribution, mineralogy and genesis of the uncommon and thin fragmental clay rocks in this and other areas were investigated by Richardson and Francis (1971) and the formation of calcitic and ferruginous concretions was discussed by Hemingway (1968) who quotes many references dealing with the sedimentology of coal-bearing strata. Some of the engineering characteristics of several of the main rock types were reviewed by Richardson (1983).

Palaeogeography

Work by many authors, summarised by Wills (1951) and Calver (1969), shows that the Coal Measures of the British Isles accumulated in a number of interconnected provinces; those of the Sunderland district were formed towards the northern margin of a Pennine Province, well north of a positive Wales–Brabant barrier that separated the subsiding areas of northern and southern Britain. Rates of subsidence (and consequently sediment thickness) varied sharply both within and between provinces, and subsidence in the Pennine Province during Westphalian A and B times was most rapid in Lancashire and North Staffordshire. Here, judging from the total thicknesses of strata present and also from isopachytes of some of the better-documented marine units and intervening sequences (Calver, 1968a, 1969), subsidence was generally at least twice as fast as in the Sunderland district, and at times was almost three times faster. Despite these variations, it is clear that all the measures of the Pennine Province were once continuous and did not form in a number of separate basins; there is no trace of influence by the block and basin mechanism that dominated the sedimentation of Lower Carboniferous strata in northern Britain (Fraser and Gawthorpe, 1990).

The northern margin of the Pennine Province is poorly documented, but the province appears to have been at least partly separated from a Scottish Province either by emergent sediment-source areas lying across parts of the present Southern Uplands and Cheviot Hills, or by a belt of reduced subsidence across the same area. The general palaeogeographical picture is thus of a broadly subsiding basin with its depocentre in the Manchester area, in which sedimentation almost matched subsidence as alluvial plains, composed mainly of elastic sediments, extended generally southwards into a shallow sea. At times of rising sea level or reduced sediment input relative to subsidence, the sea extended northwards, but the marine incursions into the Sunderland district were generally fewer and probably briefer than in more southerly parts of the basin (Calver, 1968a, 1969).

Conditions of deposition

The cyclothemic nature of Coal Measures deposition in Britain and elsewhere has been described and discussed in many publications; in north-east England it has been documented by, amongst others, Armstrong and Price (1954), Smith and Francis (1967), Johnson (1970, 1980) and Land (1974), who quote earlier references. Whilst no two cyclothems are identical, and each varies from place to place, a common pattern is for a coal seam or rootlet bed to be overlain in succession by a thin fish-bearing black siltstone, grey mudstone with nonmarine bivalves, grey or white sandstone (half the total thickness or more), grey siltstone and silty mudstone with abundant plant remains and, at the top, a rootlet bed or seatearth underlying the coal of the next cyclothem; marine strata, where present, lie near the base of the sequence, between the black siltstone and grey mudstone. At least 40 such cycles are present in the Coal Measures of the district and most of these can be further subdivided into two or more incomplete subcyclothems. According to Beerbower (1964), such cyclicity is a natural reflection of the interplay of sedimentary processes on alluvial plains and is not necessarily related to pulsed subsidence. Recent literature (e.g. Leeder, 1988) suggests periodic changes in global sea level as an important additional factor.

All reconstructions to date are agreed in postulating that the alluvial plain on which the Coal Measures of the Durham coalfield accumulated was part of a large delta complex that covered much of northern Britain; Durham is seen to lie in proximal, medial and distal parts of that plain at different times according to the position of the contemporary shoreline and the height or depth of the area relative to sea level. A detailed sedimentological study of the Coal Measures was not part of this survey, but work by Scott (1978), Haszeldine and Anderton (1980), Haszeldine (1981, 1984a) and Fielding (1982, 1984a, b, c, 1986) has furnished much useful sedimentological information either on or directly relevant to the depositional environment of the Coal Measures of the district. Haszeldine and Anderton (1980) differ somewhat from other workers in suggesting that some of the main sandstones accumulated on sandy braidplains in proximal parts of the coastal plain following periodic differential uplift of the source areas. Contemporary earth movements were also invoked by Fielding (1982) and Fielding and Johnson (1986) to account for some sharp changes in the character of regional to medium-scale coal-measure sedimentation.

Fielding (1984a) identifies 12 main sedimentary environments in the Westphalian A and B deltaic sequences of Durham, including almost all those found in modern deltas of comparable size; the sedimentary and other characteristics diagnostic of these environments are summarised in (Table 1). Fielding interprets the complex mosaic of facies in the Durham Coal Measures as a response to factors operating on large, medium and small scales; the large-scale influence is the evolution of the delta plain and of its pattern of major distributaries, the medium-scale influence is the interaction of tectonic and compactionally induced subsidence and the small-scale influence is local sedimentary processes and subsidence.

The relative importance of the main influences envisaged by Fielding varied greatly from time to time and from place to place on the delta plain, but they generally led to the accumulation of the thickest and most uniform coals in those parts of the delta where major channel sandstones are uncommon and compactional subsidence of peat and mud were correspondingly even and protracted. The complex pattern of seam splits that characterise parts of the Durham Coal Measures is interpreted as a response to differential subsidence, generally by compaction but in places by tectonic effects; Z-splits, in which part of a split seam joins another seam above or below, are interpreted by Fielding (1984c) as the product of a relatively unusual pattern of differential subsidence accompanied by and partly related to a specific sequence of channel establishment, filling and abandonment.

Application of the overall models outlined by Fielding and by Haszeldine and Anderton suggests that strata up to about the Brockwell Coal and perhaps also those above the High Main Coal accumulated mainly on the lower (i.e. distal) part of the Pennine delta plain, and those between and including the Brockwell and High Main coals were formed on middle and upper parts of the delta plain on which braidplains became established at times when the supply of sandy sediment temporarily increased.

Floras

Plant remains are abundant in the Coal Measures of the district but have received no systematic modern study. Plant fossils of the coalfield as a whole were listed by Kidston (1922) and from the Washington ‘J’ Pit sinking and boring by Hopkins and Philipson (1947). The general sequence of floras (including miospores) relative to the invertebrate zones is reasonably well established in other coalfields, however, and is summarised in (Figure 5).

The ecology of Westphalian B floras in County Durham and other parts of northern Britain was studied by Scott (1978, 1979); in Durham, this work was based on examination of opencast coal workings west and south-west of the district but is probably generally relevant to the Westphalian rocks throughout the coalfield. Scott analysed the macrofloras within the framework of a range of sedimentary environments similar to those recognised by Fielding (1984a, b) and distinguished between indigenous and transported remains. He tentatively concluded that floras on the floodplains were dominated by pteridosperms with some ferns, sphenopsids and lycopods; that lycopods were dominant in peat-forming swamps; that Calamites grew around lakes and on point bars; and that a range of pteridosperms grew on the levees alongside meandering rivers. Plant macrofossils are generally only poorly preserved in coal seams and the evidence here is mainly derived from miospores; detailed reviews of the miospore assemblages and environmental processes of Westphalian coal-forming peats were given by Smith (1962, 1963).

Faunas

The Coal Measures of Durham contain an abundant and varied fauna that includes both nonmarine and marine species. Nonmarine invertebrates include worms, gastropods, bivalves, eurypterids, crustaceans, insects and fish, and the marine faunas include foraminifera, worms, brachiopods, goniatites (only one report) and conodonts. All the faunas are concentrated in the argillaceous basal few metres of the various cyclothems, but many cyclothems have either no faunas or contain only a small range of invertebrates. Beds with abundant bivalves ('mussel bands') are most common in the main coal-bearing sequence above the Brockwell Coal and only the Victoria Shell Bed is widespread in earlier strata. A small range of vertebrate fossils has also been recorded.

The nonmarine bivalves of the Durham coalfield are well known through the pioneer work of Hopkins (1928, 1929, 1930), as refined by Calver in many internal Geological Survey reports, in Smith and Francis (1967) and in Calver (1968b). The nonmarine bivalve zones distinguished by Davies and Trueman (1927) and Trueman and Weir (1946) document the episodic evolution of these organisms and have been applied to the Westphalian sequence of the district with fair confidence ((Figure 5), mainly after Calver).

The detailed succession of nonmarine bivalves throughout the Durham Coal Measures has been discussed by Hopkins (1927, 1928, 1929, 1930), Calver (in Armstrong and Price, 1954 and Smith and Francis, 1967) and Land (1974). It is assumed that the non-marine bivalves lived mainly in lakes and lagoons on the surface of the delta plain, and that the impersistence of 'mussel bands' results from the limited size and lifespan of these water bodies. Their general ecology has not been widely studied in the district, though light is thrown on it in a study by Pollard (1969) of three 'mussel bands' that are widely associated with abundant ostracods. Pollard concluded that the mussel bands overlying the Brockwell and Harvey coals accumulated in large lagoons marginal to a coal swamp, but that the Claxheugh Shell Bed, near the top of the Middle Coal Measures, was formed in a shorter-lived pool on a delta-flat complex; insects were found in the Claxheugh Shell Bed by Kirkby (1864).

Marine fossils have been found at only seven levels in the Coal Measures of the district and at two further levels in adjoining areas. All are found in thin dark grey to black mudstones overlying coals, the Down Hill Marine Band (up to 5.9 m) being the thickest. The fossils have been recorded by, amongst others, Kirkby (1860), Hopkins (1934), Tonks (1939) and Armstrong and Price (1954).

The overall distribution and fossil assemblages of marine strata in the Westphalian of northern England were analysed by Calver (1968b, 1969), who showed that some marine incursions into the southern part of the Pennine Province failed to reach the Durham coalfield, and that most of those that did probably were relatively brief and featured a less varied and abundant fauna than farther south. Thus, some marine strata in Durham contain only microfossils (mainly foraminifera), others contain only Lingula and microfossils, and only the Ryhope Marine Band has a relatively varied fauna, including chonetoid brachiopods, gastropods, marine bivalves and nautiloids. Conodonts have been found in the Harvey, Hylton and Ryhope marine bands.

Further details of the fossils found in the Coal Measures of the district will be given in the systematic description of strata.

Coal

The coals of the Northumberland and Durham coalfield were formerly the subject of detailed research by officers of the Coal Survey, who summarised the main structural, mechanical and chemical properties of several of the main Westphalian coals and also investigated the quality of Namurian and Visean coals. In addition to the normal general downwards increase in rank and carbon content, they demonstrated a clear overall south-westwards increase in these aspects both regionally and in the Sunderland district, and ascribed this increase to proportionately greater heating over the Alston Block. According to Edwards (in Smith and Francis, 1967), the geographical pattern of rank variation is generally similar for all the coals investigated and the lateral variation greatly exceeds the variation between adjacent seams.

Based on unpublished Coal Survey report, Newcastle upon Tyne, 1959, and published by kind permission of British Coal.

Superimposed on the regional pattern, coals of the Sunderland district display evidence of locally intense thermal metamorphism (Edwards and Tomlinson, 1958; Jones and Cooper, 1970). Thermal alteration is most marked in a belt flanking the Muck Dyke, an extraordinarily discontinuous dolerite body intruded along a minor but persistent SW–NE fault through Fatfield. A narrower belt of less-altered coal flanks the WSW–ENE-trending Ludworth Dyke in the offshore area in the south-east of the district and an even narrower thermally metamorphosed belt lies astride the WNW–ESE Hebburn (or Monkton) Dyke in the north-west.

Only the lowest Westphalian coals were appreciably affected by the intrusion of the three leaves of the Whin Sill into Visean strata proved in the Harton Bore, but Ridd et al. (1970) and Jones and Cooper (1970) demonstrate an impressive range of thermal metamorphic effects on Namurian and Visean coals.

The pattern of variation in coal rank and volatile content that results from the combination of regional and thermal metamorphic effects is broadly similar in all seams investigated, though the ranks themselves increase progressively downwards; the pattern for the volatile content of the Top Busty Coal is shown in (Figure 6).

Desert reddening of Carboniferous strata took place in late Carboniferous and early Permian time, and is discussed in Chapter 3.

Connate brines

Surface brine springs emanating from the Coal Measures of north-east England were noted by Anderson (1945), who also reviewed earlier literature. The best known spring, at Birtley on the western fringe of the district, flowed at 26 000 gallons per day in 1794 but has now dried up; it was reported to be 'salter than the sea'. Other brine springs, including one at Walker, have also ceased to flow, presumably because of pumping in the expanding coal mines, but chloride-rich brines were encountered at various levels in most of the collieries in the north and west of the district. Analyses quoted by Anderson revealed that sodium chloride was the main salt present, but that calcium chloride was present in about half the brines analysed; total salinites averaged two to three times that of modern sea water, but sulphate was generally only a minor constituent. Anderson recorded that the composition of the brines, like that of many other brine springs in Britain, varied from time to time; thus in 1842 the Wallsend Spring was an almost pure calcium chloride brine whereas in 1848 it was only 30 per cent calcium chloride and almost 70 per cent sodium chloride. Some barium chloride is present in about half the brines quoted by Anderson.

A more comprehensive study of brines in the Coal Measures of north-cast England (Edmunds, 1975) revealed that the most concentrated brines tend to be found in the lowest workings and that the concentration increases eastwards away from the diluting effects of meteoric waters in the outcrop area. He found that most of the brines are now of calcium chloride with a high bromide content, but that potassium, magnesium and sodium are depleted relative to sea water. Local variations in the composition of the brines indicates relatively little interconnection between the various levels sampled. Edmunds concluded that most of the brines encountered in coal workings on land in north-east England were originally trapped in the sediments during Westphalian marine incursions into the area and their composition has gradually evolved through base exchange and dolomitisation. A smaller group of brines found mainly beneath Permian strata in the south-east of the coalfield were considered to owe their somewhat different composition to derivation from, or interaction with, Late Permian Zechstein brines. The possibility that brines similar to those in the main group found in the on-land Coal Measures might have been involved in Pennine mineralisation is briefly considered by Edmunds.

With the abandonment of many coal workings in the north and west of the district, and the consequent cessation of pumping, some former surface brine springs may, in time, begin to flow again.

Chalybeate (iron-rich) springs are also known in the Coal Measures of the district, one giving rise to the rivulet known as Spa Well Gill at Offerton and to a short-lived spa resort at its confluence with the River Wear.

Data quality and conventions

In a coalfield that has been worked for several hundred years, it is inevitable that there are enormous variations in the quality of geological records. Many early shafts and workings are unrecorded and in others only the thickest coals are noted; elsewhere, strata encountered in early shafts and boreholes were painstakingly recorded, some in picturesque vernacular, but their location cannot now be traced. Underground data in the old, mainly western part of the coalfield are therefore generally inadequate for detailed geological research, but the quality and quantity of data increased eastwards as the workings extended towards and beneath Permian strata and the recording of workings became mandatory. The most complete data are mainly in coastal and undersea areas, the youngest parts of the coalfield, where much exploration and recording has been the responsibility of trained geologists. A less welcome recent trend towards wireline logging of long sequences of unproductive Coal Measures has resulted in a marked decrease in the rate of accession of detailed scientific information on these strata.

Limitations of space preclude the quotation here of details used in the preparation of this account, including records of more than 2000 shafts and boreholes and of many surface exposures, but the non-confidential records are available for consultation in the British Geological Survey archives; similarly, copies of all mine plans covering the area and used in this compilation may be consulted in the appropriate Mining Records Office. These data are summarised on the seam maps included in the text, lines of split being shown where the separation between the leaves exceeds 0.5 m. In the accompanying representative sections of strata proved in shafts and boreholes, all argillaceous rocks except seatearths, but including siltstones, are undifferentiated because the quality of many records does not allow separation. For the same reason, the absence of faunal or seatearth symbols on many of the argillaceous beds shown probably results in many instances from a failure of recording rather than the absence of a fauna. Plant remains are almost ubiquitous and their presence is not indicated on the sections unless they are especially abundant.

Lower Coal Measures

These strata, between the base of the Quarterburn (Subcrenatum) Marine Band and the base of the Harvey (Vanderbeckei) Marine Band, are of Westphalian A age. They are mainly arenaceous and, in the few boreholes in which the whole sequence has been proved, sandstones generally comprise 60 to 80 per cent of the section and exceed 90 per cent in some boreholes. The lowest widespread sandstone is the uppermost or Third Grit of the Durham Millstone Grit Series as formerly defined (see Hull, 1968 for discussion).

Siltstones, mudstones, shales and coals make up most of the remainder of the Lower Coal Measures. In the lower part of the sequence the coals are relatively uniform and generally less than 30 cm thick, but from the Brockwell Coal upwards they are more variable and include several seams that are of workable thickness over substantial areas. The Harvey Coal, near the top of the Lower Coal Measures, is generally less than 1 m thick but is nevertheless one of the most consistent seams in the district.

'Mussel-bands', beds with abundant nonmarine bivalve faunas, overlie several coals, including the Victoria, Brockwell, Three-Quarter and Harvey seams, and are of considerable value in correlation. Fossil collections from these beds are meagre in most land areas of the district, but substantial collections from some of the deeper NCB offshore boreholes have revealed diverse and locally abundant faunas comparable with those known in the Durham district to the south (Calver in Smith and Francis, 1967). Ostracods are abundant above the Brockwell (locally) and Harvey coals (see Pollard, 1969 for summary and references) and fish remains have been recorded at several levels. Marine fossils have been noted in the Lower Coal Measures of the district in only one borehole and the position of the several marine bands thought to be present is inferred from provings in adjoining areas.

The general sequence of strata below the Victoria Coal

The general sequence of strata below the Victoria Coal is shown in (Figure 5), and the strata proved in a selection of the few boreholes into or through this sequence are shown in (Figure 7). White to pale grey sandstones predominate, several of which are coarse grained and some sparingly pebbly. The position of the Quarterburn Marine Band at the base is tentatively inferred at a gamma-ray peak at 487.1 m in the Harton No. 1 Borehole (Ridd et al., 1970). A marine fauna reported from the Washington J Pit borehole (Stobbs in Ford, 1927) includes only Lingula mytilloides and fish remains, and probably indicates the position of the Gubeon Marine Band. Elsewhere, this thin marine unit contains Planolites ophthalmoides, and the presence of this trace fossil in mudstone at the bottom of the Westoe Crown Shaft suggests a possible correlation with the Gubeon Marine Band (Figure 7).

The variable quality of recording and the equally variable character of the sequence, coupled with remoteness from the well-documented type localities in west Durham and the paucity of reliable intervening data, all contribute to uncertainty in the naming and correlation of the thin coal seams and other strata below the Victoria Coal. Because of these uncertainties, no reliable conclusions on thickness trends and sedimentary evolution may be drawn. The inferred position of the horizon of the Stobswood Marine Band as shown on (Figure 7), and of the Marshall Green Coal, may he one cyclothem too high.

The sequence between the Victoria Coal and Bottom Busty Coal

The sequence between the Victoria Coal and Bottom Busty Coal ranges from about 38 to about 56 m thick, averaging about 50 m; it is generally thickest in the northern and central parts of the district and thinnest in the south-west. Sandstone is the main rock type but it forms a relatively lower proportion where the sequence is thin.

The Victoria Coal

The Victoria Coal has been reached or penetrated by about 35 boreholes within the district and coal was proved in most of these; a number of boreholes just outside the district also prove the Victoria Coal. Many of the records are old and may be unreliable, but they indicate a seam generally less than 0.2 m thick and only locally greater than 0.3 m; the thickest proving was of 0.59 m in Harton Colliery No. 5 Bore, South Shields. Several boreholes encountered two thin coals up to 3 m apart at the appropriate level; the lower of these is generally taken as the Victoria.

The sequence between the Victoria Coal and the Brockwell group of coals

The sequence between the Victoria Coal and the Brockwell group of coals has also been proved by about 35 boreholes spread unevenly across the district, and a number of additional boreholes have penetrated the top of the sequence. Together, they show a thickness averaging about 20 m (maximum 25 m), but generally declining to 16 m or less offshore and in the coastal areas beneath Sunderland and Seaham. The Victoria Coal itself is almost everywhere overlain by the regionally extensive Victoria Shell Bed, comprising up to 3 m of dark grey, shaly, fossiliferous mudstone, but a single major channel sandstone up to 21 m thick occupies most of the Victoria-Brockwell interval in a broad belt across much of the northern and western parts of the district and also around Houghton-le-Spring. Mixed successions, including one or more thin coals and seatearths, characterise the thinner sequences in the east, but these also commonly contain substantial sandstones. The coals and seatearths tend to lie near the base and at about the middle of the sequence, and each level is widely overlain by mudstone rich in nonmarine bivalves.

The meagre collections from the Victoria Shell Bed permit few generalisations, but the bivalves include several species of Carbonicola including C. declivis and C. pseudorobusta, Curvirimula sp. and Naiadites cf. flexuasus. Additional species are found at this level in the districts immediately to the north (Land, 1974) and south (Smith and Francis, 1967), and are probably present in this district. The shell bed also contains Planolites aff. ophthalmoides (large form), ostracods and, particularly in its lower part, fish remains. Collections from the mudstones overlying the thin coals higher in the Victoria–Brockwell interval are similarly sparse, but from NCB Offshore No. 3 Borehole they collectively yielded Carbonicola aff. communis, C. declivis, C. pseudorobusta, Cnrviiirnuia subovata, C. cf. trapeziforma and fish remains.

The Brockwell Coal

The Brockwell Coal of the Sunderland district has been proved by 3 shafts and about 80 boreholes, which show it to be an unusually variable seam; it is generally in at least two leaves (commonly 3 or 4) that together aggregate less than 1 m. The individual coals are mainly less than 0.4 m thick (maximum 1 m) and are separated by up to 8 m of strata. Limited working has taken place near Wallsend in the north-west of the district and at Felling, Heworth and Ouston immediately to the west; here, two or more leaves unite to form a banded coal up to 2 m thick, but generally 0.8 to 1.2 m. Practice in the coalfield is to recognise a Top and Bottom Brockwell, but this is difficult to apply in parts of the district where either or both Top and Bottom seams are widely split and where one or both are locally absent through erosion or non-deposition. The complex pattern of seam splitting cannot be resolved fully on the available evidence, but the records of closely spaced boreholes at Birtley in the west of the district appear to show a substantial uppermost leaf of the Brockwell rising relatively sharply to just below the seatearth of the Three-Quarter Coal (Figure 8). A similar configuration at this level has been described from near Crook, 22 km south-south-west of Birtley (Fielding, 1984c).

Strata between the Brockwell Coal and Three-Quarter Coal

Strata between the Brockwell and Three-Quarter coals are generally thinner than between most of the main worked coals in the region, ranging from less than 1 m in Ravensworth Colliery No. 8 Underground Borehole at Birtley to about 15 m in places in the west and south of the district. Uncertainty in seam identification causes widespread difficulty in determining the seam separation, and the data do not show clear trends of thickness change across most of the district; total thicknesses generally range from 3.5 to 9 m.

These strata are varied and include one or more seatearths and thin coals which occur at several levels but which are most common in the upper part. Channel sandstones are not a major feature but are up to 14 m thick in the north-west of the district in a relatively narrow belt extending north-north-eastwards from near Fatfield towards Boldon Colliery (see also Fielding, 1984a, fig. 3). Small collections of fossils from mudstones overlying the Brockwell and the less extensive, thin, higher seams show the presence of limited bivalve faunas (Garbonicola, Curvirimula, Naiadites) and, particularly just below the Three-Quarter Coal, fish remains. The ostracod-rich bed widely found in or a few metres above the roof of the Brockwell in the district to the south (Smith and Francis, 1967; Pollard, 1969), has not yet been noted in its typical form in the Sunderland district, nor has the fragmental clayrock reported at this level farther south by Richardson and Francis (1971).

The Three-Quarter Coal

The Three-Quarter Coal of this district is readily identified and has been penetrated by about 100 boreholes and shafts, most of them situated west of grid line 35; these show that the seam is present almost everywhere and is generally 0.2 to 0.8 m thick. It is thickest in the west and has been worked locally in the extreme northwest of the district, in a smaller area immediately west of Heworth Colliery and in a few square kilometres between Birtley and Washington Village. The seam widely comprises an upper leaf and a generally thinner lower leaf separated by a thin mudstone parting, but east of Houghton-le-Spring the two leaves locally diverge by up to 4.5 m. Where worked in the north-west of the district, the upper leaf is 0.53 to 0.74 m thick, the parting 0.03 to 0.23 m thick and the lower leaf 0.05 to 0.10 m thick; figures for the Birtley–Washington workings are, respectively, 0.48 to 0.53 m, 0.03 to 0.43 m (but increasing sharply southwards) and 0.17 to 0.23 m. The seam contains two thin partings in the area beneath Penshaw and Shiney Row, and locally elsewhere, and cannel coal up to 0.61 m thick forms the upper part of the seam between Great Lumley, Bournmoor and Houghton-le-Spring.

Strata between the Three-Quarter Coal and Busty Coal

Strata between the Three-Quarter and Busty coals form a lithologically distinctive unit which ranges in thickness from 11 m in Offshore Borehole No. 4 to 33 m in Offshore Borehole No. 18, but is 16 to 22 m thick over most of the district. There are no clear thickness trends nor a consistent relationship between thickness and lithology (including that in underlying strata). Some of the thickness changes are surprisingly sharp and difficult to account for on current sedimentological or tectonic models.

Sandstone dominates the Three-Quarter to Busty interval (Figure 8), as in much of the district to the north (Land, 1974) and south (Smith and Francis, 1967). This sandstone widely forms a single unit 15 to 20 m thick, but is generally less than 15 m thick in the south-west of the district. In places it is coarse grained and gritty, and pebbles have been reported in some cores, but its base appears to be generally concordant and washouts of the Three-Quarter Coal are rare. Other rock types in the sequence include a thin, widespread, grey to black, argillaceous bed at the base, passing up locally into a few metres of silty mudstone and siltstone, with a seatearth or rootlet bed at the top. One or two thin coals and seatearths have been recorded in a few boreholes, mainly in the upper half of the sequence, but they are not extensive. The thin fragmental clayrock found immediately above the Three-Quarter Coal in the district to the north (Land, 1974) and west (Richardson and Francis, 1971) probably extends into the northern part of the Sunderland district, but has not been proved.

Plant remains are abundant in mudstones and siltstones at many levels between the Three-Quarter and Busty coals, and a limited suite of nonmarine bivalves (Carbonicola, Curvirimula, Naiadites) has been recorded widely near the base of the sequence. Fish remains and ostracods are locally abundant in the roof of the Three-Quarter Coal, but 'Estheria' , common at this level to the south (Smith and Francis, 1967), has not been recorded.

The Busty coals

The Busty coals, as in much of coastal Northumbria, comprise two main leaves, separated by up to 13 m of strata (Figure 9). There is a single composite seam only in the extreme south-west of the district and in a small area between Hetton-le-Hole and Seaham in the south, but the two seams approach to within 2 m in several places. The main workings are in the areas of composite Busty, but the Top Busty has been worked alone in an area of about 10 square kilometres beneath Washington, and both seams have been worked singly in small areas scattered across the district (Figure 9); only part of the seam was worked in some areas of composite Busty, but exact details are not known. According to an unpublished report (1959) by the NCB Coal Survey, the coals have an ash content of less than 7.5 per cent except in limited, mainly coastal, areas, where ash approaches 10 per cent, and east of Whitburn, where it reaches 15 per cent. Sulphur content is almost everywhere less than 1.5 per cent.

The thickness of the Busty coals is extremely variable and both the main leaves generally include one or more thin argillaceous beds or layers of inferior coal or carbonaceous shale. The Top Busty is somewhat more consistent than the Bottom Busty and tends to include fewer and thinner partings. The coal is generally less than 1.1 m thick (including partings) in the areas where the Top and Bottom Busty are worked separately, and it exceeds 1.4 m in aggregate thickness (including 2 widespread partings) only in the Hetton to Seaham area of composite Busty. Washouts are uncommon, the only reasonably well-documented example being in the Top Busty west of the Washington workings.

Identification of the Busty coals is fairly confident in most western, central and southern parts of the district, where two distinct seams or thin groups of seams are generally present and intervening strata contain few other coals or seatearths. In the north of the district, however, a widespread thin coal (probably a split) lies a short distance above the Bottom Busty and one or more other thin coals occur near the middle of the sequence both here and in parts of the south. In much of the undersea area in the north-east of the district, the Busty is represented by four or more banded coals spread over several metres of mainly argillaceous strata; here, seam identification is difficult, although it seems likely that the uppermost coal is the Top Busty of most other areas.

Strata between the Top and Bottom Busty coals of most of the district vary sharply in thickness (Figure 9), the divergence around the areas of composite Busty being particularly marked; ranges up to 10 m are normal. The strata themselves are generally mixed, with a slight tendency for sandstone to predominate in the west, where up to 8 m of sandstone locally occupies most of the Bottom Busty–Top Busty interval and is commonest in the lower part. The north-north-east-trending sandstone shown in the west and north of the district by Fielding (1984a, fig. 4d) has not been identified. Elsewhere, rhythmic alternations are of sandstone, siltstone and mudstone, with seatearths and thin coals being most common where argillaceous rocks predominate. Plant remains abound in the mudstones and siltstones, but nonmarine bivalves have been reported only locally in the roof of the Bottom Busty Coal and above one or more of the thin intermediate coals; no identifications are available.

Strata between the Busty Coal and Harvey Coal

Strata between the Busty and Harvey coals (Figure 10) range in thickness between about 24 m and 40 m, but are generally between 28 and 33 m thick; they include the highly variable Tilley group of coals and, towards the top, one or more thin coals that may equate with the Hodge Coal of west Durham.

The interval between the Top Busty and the lowest of the Tilley coals is dominated almost everywhere in the district by sandstone (The 'Busty Post') which is generally 8 to 12 m thick and locally exceeds 18 m. This sandstone is commonly coarse to very coarse grained, particularly in its lowest part, and in places lies on an erosion surface that has been cut down to below the Top Busty Coal. Generally, however, a thin argillaceous sequence intervenes between the Top Busty and the Busty Post, and argillaceous strata with one or more thin coals fill much of the Busty–Tilley interval locally in the north of the district and in the offshore area in the south-east (Figure 10). The seam interval is greatest where the Busty Post is thickest, presumably because of the resistance of the sandstone to compaction; it is here, too, that the Tilley coals are most closely spaced and least variable.

The Tilley Coals

The Tilley Coals lie in the low-middle to middle part of the Busty–Harvey interval, being highest where the Busty Post is thick. They generally comprise two to four separate coals or seatearths spread over up to 20 m of strata, but most commonly over 8 to 15 m. The great variability of the Tilley Coals and their complex pattern of splits makes seam identification difficult, especially in the east where provings are sparse. There is, moreover, doubt that the Top and Bottom Tilley coals or groups of coals are splits of the type Tilley Coal of west Durham. All the Tilley coals are generally less than 0.7 m thick, but up to four coals locally unite to form strongly banded composite seams up to 2.2 m thick (mainly 1.2 to 1.5 m). Workings in the Tilley of this district are confined to three small areas of such composite coal, one around the shafts at Usworth Colliery, another a short distance north-west of Herrington Colliery and a third about 1 km north-north-west of Houghton Colliery. Strata interbedded with the Tilley coals are mainly argillaceous where the seams are closely spaced, but they include sandstones up to a few metres thick in places where the seams are farthest apart.

Strata between the Tilley Coal and Harvey Coal

Strata between the Tilley and Harvey coals (Figure 10) include sandstones totalling up to 20 m thick (but generally less than 12 m) in much of the district; sandstone occupies almost the whole interval in parts of the extreme west, north-centre and south-east. Elsewhere, however, there is a mixed sequence of mainly argillaceous strata containing one or more thin coals. The possible equivalent of the Hodge Coal near the middle of the sequence is the most widespread of these coals, but is generally less than 0.4 m thick. An additional thin coal is widespread just below the Harvey seam in the north of the district and may be a Harvey split.

Plant fossils are common in argillaceous parts of the Busty–Harvey sequence, but invertebrate faunas are generally sparse at all levels. They comprise nonmarine bivalves that occur locally in mudstone between the Tilley coals, and ostracods that have been recorded in profusion in mudstones low in the sequence at Washington J Pit (Ford, 1927) and in Offshore Borehole No. 11 (Figure 10). Bivalves identified by Dr M A Calver from mudstones between the Tilley coals in Offshore Bore No. 3 include Carbonicola aff. crista-galli. Naiadites cf. subtruncatus was identified from mudstones overlying a thin coal high in the Busty–Harvey sequence of Offshore Borehole No. 4 (Figure 10). Fish scales have been recorded at several levels.

The Harvey Coal

The Harvey Coal is one of the most uniform seams in the coalfield and within the district it is almost everywhere between 0.5 and 1.0 m thick (Figure 11). Offshore, however, it is generally thin and split, except in the south-east where Offshore Boreholes D4 and VT11 prove a single Harvey Coal, 0.92 m and 1.09 m thick respectively. The coal is generally of good quality, with low ash and sulphur contents, but cannel up to 0.08 m thick occurs in the middle of an unusually thick Harvey seam east of Ryhope, and 'grey coal' up to 0.15 m thick lies near the top of the seam beneath Fulwell and Roker. No washouts have been proved.

In spite of its high quality, workings in the Harvey Coal have been limited by its general thinness and are markedly patchy (Figure 11). Between the worked-out areas, the seam is probably mainly too thin for economic working in the future and no extension of existing abandoned workings seems likely. Boreholes and scattered sections of the full seam suggest that some of the workings were in only upper or lower leaves of the coal, whereas others extracted the whole seam. In the north of the district, a widespread band up to 1.5 m thick separates a Top Harvey (0.6 to 0.8 m thick) from a thinner Bottom Harvey, but in the south and west, the seam is single; some of the workings here and immediately west of the district were in coal less than 0.5 m thick.

Strata between the Harvey Coal and the Harvey Marine Band

Strata between the Harvey Coal and the Harvey Marine Band (Figure 12) range in thickness from about 5 m in parts of the south-west of the district to more than 20 m in the south-eastern offshore area, where a single thick sandstone occupies most of the interval; thicknesses generally range between 10 and 15 m. In many areas the sequence includes parts of two or three sub-cyclothems.

The Harvey Coal is widely overlain by a thin kaolin-rich fragmental clayrock (FCR), which is succeeded by a few metres of fossiliferous shale and mudstone known as the 'Hopkins Band' (Carruthers, 1930) or the 'Hopkins Shell Bed' (Eastwood, 1935), following its discovery by Hopkins (1927). The FCR has been traced over an area exceeding 1500 km² by Richardson and Francis (1971), who found that it is generally only a few centimetres thick but locally reaches 0.3 m. They interpreted it as a lacustrine deposit modified by intense early diagenesis, but R K Taylor (in discussion on their paper), suggested that liquifaction of an abnormally sensitive clay was a more likely explanation. The Harvey FCR has yielded an unusual fauna of ostracods, fish remains, Spirorbis and the small gastropod Anthracopupa cf. britannica (M A Calver in Richardson and Francis, 1971).

The Hopkins Band

The Hopkins Band (Hopkins, 1927, 1929, 1930, 1960; Armstrong and Price, 1954; Calver in Smith and Francis, 1967; Pollard, 1966, 1969), contains an abundant and varied fauna, and is an extensive and reliable marker bed. It has yielded a diverse biota of ostracods, non-marine bivalves, serpulid worms, gastropods and plants. Detailed research by Pollard (1969) revealed that the bed has a distinctive lithological and faunal structure and composition over a central area of at least 1130 km², where a basal black carbonaceous shale is succeeded by an ostracod-bivalve coquina, which in turn passes up into grey mudstone containing abundant nonmarine bivalves. Fish remains are concentrated in the basal layers of the deposit and are from several species. The ostracods in the coquina are dominated by Geisina arcuata in common association with the bivalve Naiadites. The composition of the main bivalve faunas varies from place to place and at different levels in the deposit; they were summarised by Pollard (1969, table 2) who listed several species each of Anthraconaia (5), Anthracosia (4), Carbonicola (6) and Naiadites (5).

Remaining strata between the Harvey Coal and the Harvey Marine Band are largely siltstones and mudstones; sandstones are mainly subordinate but locally exceed half the total and, in the south-east, make up most of it. Representative sections are shown in (Figure 12). Nonmarine bivalves have been recorded in mudstones overlying thin coals in a few boreholes.

Middle Coal Measures

These strata, between the base of the Harvey (Vanderbeckei) Marine Band and the top of the Down Hill (Cambriense) Marine Band, range in age through the whole of Westphalian B and into the early part of Westphalian C. They occupy much of the outcrop and contain most of the widely worked coals of the Northumberland and Durham coalfield. These comprise, in ascending order, the Hutton, Brass Thill, Durham Low Main, Maudlin, Main, Five-Quarter and High Main Coals, the last being the coal upon which the early mining industry was established and first flourished along the banks of the Tyne.

The main coals all lie in the lowest third of the Middle Coal Measures, in a strongly rhythmic sequence in which there are also several major sandstones. Despite the widespread continuity and uniformity of the worked coals, extreme lateral variation and a complex pattern of seam splits characterise these strata. Thus, for example, the Durham Low Main Coal (or parts of it) unites in places with both the overlying Maudlin and underlying Brass Thill coals, and all the coals between the Main and the High Main are involved in splits and unions in different parts of the district. As in parts of the Lower Coal Measures, seam divergence at lines of split is commonly very sharp; this is well exemplified by the Maudlin and Durham Low Main seams, which maintain a generally constant separation exceeding 15 m over large parts of the district but then converge and unite within a few hundred metres.

Strata between the High Main Coal and the Wear Mouth Marine Band resemble the lower part of the Middle Coal Measures in their cyclicity and in the proportion of sandstone present, but the coals are too thin for widespread profitable working and lateral variability is generally less. They differ too in containing five marine bands, including the thick Ryhope (= Aegiranum) Marine Band with its unique content of goniatites. The massive gritty sandstones of the Seventy Fathom Post and Grindstone Post together make up much of the high ground in the north-west of the district, where they were formerly exploited in many quarries for the production of building stone and grindstones.

Middle Coal Measures above the Wear Mouth Marine Band are predominantly argillaceous and major channel sandstones are apparently absent. These strata form much of the low-lying ground between Hebburn and Washington, and are seen only in the banks of the River Wear downstream from Low Barmston and in a few minor stream sections and pits elsewhere. Beds exposed in the north bank of the Wear at North Hylton, previously regarded as the highest Coal Measures exposed in County Durham (Trechmann and Woolacott, 1919), are now known to lie high in the Middle Coal Measures, at least 60 m below the highest exposed strata.

The fauna of the Middle Coal Measures is strongly facies related; between the base and the Wear Mouth Marine Band it mainly comprises nonmarine bivalves, which are concentrated in mudstones and shales near the bases of many cyclothems. The most abundant and persistent 'mussel bands' are above the Brass Thill and High Main coals, and above Kirkby's and the Ryhope marine bands, but bivalves are more patchily abundant at many other levels. Above the Wear Mouth Marine Band, bivalves are generally less common, but fish debris, ostracods and 'Estheria', which occur sparingly in argillaceous beds of most of the underlying Middle Coal Measures, are relatively more abundant. Insect remains and small crustaceans have also been recorded from these higher beds (Woodward, 1918; Trechmann and Woolacott, 1919).

Marine fossils, mainly Lingula, foraminifera and conodonts, have been recorded at nine levels in the Middle Coal Measures of the district, but the area is far from the depocentre and most of the marine bands are thin and discontinuous. The main exception is the Ryhope Marine Band, which is up to 3 m thick in a number of boreholes and yielded immature goniatites at its type locality of Ryhope Colliery Shaft (Tonks, 1939); elsewhere it has yielded marine thin-shelled bivalves and conodonts (Armstrong and Price, 1954; Land, 1974).

The strata below the Hutton Coal

The strata below the Hutton Coal have been proved by about 140 shafts and boreholes, which are unevenly distributed across the district but are particularly concentrated in the west and south. In the land area, they range from about 24 m thick in boreholes at Springwell and Portobello to 42 m in the coastal area between Sunderland and Whitburn, but they are generally 29 to 35 m thick. The onshore data reveal a slight overall north-eastwards thickening, a tendency which is continued into the offshore area north and east of Sunderland, where a number of boreholes prove more than 42 m of strata. The lithological sequence is diverse but relatively constant over much of the west, south and south-east of the district; it is dominated by thick channel sandstone in the northern offshore area and in a broad tract between South Shields, Felling, Penshaw and Sunderland.

In the western and southern parts of the district there are three or more partial or complete minor cyclothems (Figure 14) for line of section)." data-name="images/P935963.jpg">(Figure 13), each capped by a seatearth with or without an overlying coal or carbonaceous shale and each commonly with a basal fossiliferous shale or shaly mudstone. Most of the coals are less than 0.3 m thick, but a relatively persistent seam slightly above the middle of the sequence locally exceeds 0.5 m in some northern parts of the district and is probably the Plessey Coal of Northumberland (Land, 1974). The equivalents of the Cheeveley Coal of Northumberland and the Ruler Coal of north-west Durham are doubtless present among the underlying thin coals.

Harvey (Vanderbeckei) Marine Band

The stratigraphically important Harvey (Vanderbeckei) Marine Band lies at the base of the sequence. First noted by G A Burnett in 1930 in an underground shaft at Hanoi' Colliery, South Shields, and named by Armstrong and Price (1954), this bed has been found to be widespread but not ubiquitous throughout the coalfield. It is generally composed of dark grey to black, silty, micaceous shale or shaly mudstone and is up to 0.6 m thick, though commonly 0.05 m to 0.2 m. The Harvey Marine Band lies in the Lingula facies of Calver (1968a) and in the district its fauna comprises Lingula mytilloides in association with fish debris and conodonts. Sponge spicules, marine bivalves and brachiopods such as Orbiculoidea, which are locally found in the district to the south (Smith and Francis, 1967), have not been recorded in the Sunderland district and only fish debris has been noted at this level in places.

Harvey (or Beaumont) Shell Bed

The marine band grades up into the Harvey (or Beaumont) Shell Bed, one of the most consistently fossiliferous units of the Middle Coal Measures in north-east England; it is generally 2 to 4 m thick and locally exceeds 6 m. Bivalves normally regarded as nonmarine are closely associated with marine fossils in the basal transitional zone, but most of the bed contains an abundant and diverse bivalve fauna in addition to burrows, fish debris, ostracods and Spirorbis. Bivalves collected from the Harvey Shell Bed in boreholes at Boldon and Wearmouth Colliery were listed by Armstrong and Price  (1954) and include Anthracosia cf. nitida, A. cf. regularis, Anthracosphaerium cf. turgidum and Naiadites quadratus. Additionally, Naiadites triangularis was identified from this bed in Offshore Bore No. 3 by Dr M A Calver, who also identified Anthraconaia modiolaris from an equivalent level in Offshore Bore No.4.

The Harvey Shell Bed persists over almost all of the district and is absent in only a few places in the centre and north-east where washouts occur locally below thick sandstone. In the west and south, strata overlying the shell bed commonly include two to four relatively closely spaced fossiliferous mudstones, and there are others above the Plessey equivalent and just below the Hutton Coal. The fauna in four such beds is recorded by Hopkins and Philipson (1947) at separate levels ranging from just above the Hutton to a few metres above the Harvey Shell Bed. It is difficult to distinguish which collection equates with that of the Plessey Shell Bed of Northumberland (see Land, 1974 for faunal list).

Sheet sandstones form up to half of several of the cyclic sequences below the Hutton Coal in the west and south of the district; one such sandstone, just below the Hutton seatearth, is commonly 3 to 6 m thick along much of the western margin. In a few places in the west and south, sandstones merge and occupy much of the sequence, but the cyclothems here generally remain recognisable. By contrast, the sandstone of the South Shields–Felling–Penshaw–Sunderland area and also offshore in the north is mainly indivisible and occupies much of the interval between the Shell Bed and the Hutton seatearth. It is commonly coarse-grained and contains mudstone clasts; though generally 15 to 30 m thick, it exceeds 35 m in a number of boreholes in the offshore area 5 to 8 km east of Sunderland (Figure 14).

The Hutton Coal

The Hutton Coal is the most widely worked seam of the Durham coalfield and has been extracted from almost all land areas of the district and also from a limited offshore belt east of Ryhope and Seaham (Figure 15). It is present throughout the district except for a small area in the extreme south-west and a small ill-defined area under Permian strata about 5 km east-north-east of the harbour at Seaham. An unpublished Coal Survey Report (1961) shows that the Hutton is mainly a bright coal, with a sulphur content generally of less than 2.5 per cent; cannel forms the upper part of the seam in a substantial area beneath the north-western suburbs of Sunderland and durain is common near the base of the seam.

The structure of the Hutton Coal (Figure 15) is relatively simple under most land areas of the district, where it is a single or composite seam 1 to 2 m thick. However, it thins to less than 1 m in a relatively narrow curved belt extending southwards from Jarrow Slake via Follingsby and Offerton to Seaton. There is no obvious relationship between the position of this belt and the nature of strata either above or below, but it is possible, as suggested in the unpublished Coal Survey report, that the on-land workings over most of the district exploit only the main part of the Hutton Coal, a Bottom Hutton having diverged eastwards and extensively failed roughly along the belt of thinner coal. The workings extend eastwards to a line roughly parallel with and close to the present coastline, beyond which complex splitting and general deterioration precluded profitable exploitation. Sparse data east of this line suggest that the seam here is mainly strongly banded and in part inferior, and extensively divides into a recognisable Top and Bottom Hutton; in places one or other of these is absent and it is not everywhere clear which seam is missing or whether a thin composite seam is present. The split seam appears to reunite in a broad north–south belt in the offshore area lying north of Ryhope and 4 to 6 km east of the coast, where a banded seam up to 2 m thick occurs. Information from the National Coal Board shows that the coal here has an ash content of 7 to 11 per cent and a sulphur content of 2.2 to 3 per cent.

Strata between the Hutton Coal and Brass Thill Coal

Strata between the Hutton and Brass Thill coals (Figure 16) vary markedly in lithology and thickness, assessment of the latter being greatly complicated by problems of seam identification. In most land areas this problem lies in the presence of a thin but widespread coal that commonly occurs 2 to 6 m below the worked Brass Thill Coal and locally appears to merge with it; this coal is here regarded as a separate seam and not as a split of the Brass Thill Coal. A different problem affects assessment in the southern offshore area, where thick sandstones intervene and parts of both Hutton and Brass Thill coals are absent. Given these reservations, the Hutton–Brass Thill interval in most land areas and in the northern offshore is generally 12 to 16 m thick, though locally as little as 5 m or as much as 20 m; it may increase to more than 25 m in the southern offshore area, 8 km and more east of Ryhope and Seaham.

Despite the lithological variability of the strata between the Hutton and Brass Thill coals, the lowest 1 to 3 m are generally argillaceous and have yielded scattered to abundant nonmarine bivalves at some localities. Dr M A Calver identified Anthraconaia cf. salteri, Anthracosia cf. aquilina, A. beaniana, A. cf. caledonica, A. ploygiana, Naiadites quadratus and a number of intermediate forms from mudstone above the Hutton in the Hylton Borehole (Armstrong and Price, 1954) and Anthracosia ovum from this level in Offshore Bore No.4. Stobbs (in Ford, 1927) recorded bivalves above the Hutton in Washington J Shaft, and also ostrocods, fish debris and Spirobis; Hopkins (1928) found this 'mussel bed' at Harton, Hylton,New Herrington, Springwell, Wearmouth and Westoe collieries.

The basal Shelly mudstone is overlain almost everywhere by sandstone, which is generally 2 to 6 m thick but locally absent and, mainly in a belt across the north of the district and also in the south-east, exceeds 9 m. In most of the land area south of Boldon and Cleadon, this sandstone is succeeded by a few metres of siltstonc and mudstone, which contain a sparse, patchy fauna of bivalves; this unit, in turn, is widely overlain by another sandstone (commonly 2 to 4 m thick), which underlies the thin coal mentioned above. This coal is generally 0.2 to 0.3 m thick hut thickens to up to 0.8 m in a limited area beneath Silksworth, Houghton-le-Spring and Hetton-le-Hole, where it is thicker than the Brass Thill Coal and has locally been called both the Bottom Brass Thill and the Brass Thill. In most southern land areas, it is separated from the Brass Thill Coal by mainly argillaceous strata, but sandstone up to 8 m thick lies at the top of the sequence in parts of the Follingsby–Washington–Hylton area. Atypically, a single sandstone up to 16 m thick occupies much of the Hutton–Brass Thill interval between Wardley and Boldon Colliery, and single or composite channel sandstones, together up to 30 m thick, occupy most or all of the interval in the offshore area well east of Ryhope and Seaham. Some impression of the variability of these strata is given in (Figure 16).

The Brass Thill Coal

The Brass Thill Coal, worked widely to the north as the Northumberland Low Main (Land, 1974) and on Tyneside as the Five-Quarter, is of workable thickness and quality in the Sunderland district only in the west and locally in the north (Figure 17). Immediately west of Wrekenton and Birtley, it converges sharply with and joins the overlying Low Main Coal to form the Pontop Hutton, but the composite seam does not extend into the present district where the Brass Thill is generally a single or simply divided seam. In a few places, including the north-west and south-west extremities of the district, it appears to be joined by the thin coal that commonly lies 2 to 6 m below, but many of the records are old and their interpretation equivocal. The seam crops out beneath drift in the extreme south-west of the district and beneath Permian strata in an anticline north-east of Seaham.

Where worked, the Brass Thill commonly included a thin median parting of mudstone or carbonaceous shale (Figure 17), but the place of this is taken by cannel in much of the South Shields area and beneath Wallsend and Hebburn; elsewhere, cannel formed the lowest part of the seam in the limited workings beneath South Hylton (Sunderland). In the main workings, it was customary to take both leaves of the coal and the intervening parting, but in some places only the upper leaf was worked; this upper leaf, too, locally contains a thin argillaceous parting. Two partings are common in the eastern fringes of the workings, where the coal became uneconomic to extract.

Information on the Brass Thill Coal east of the workings comes entirely from a thin and uneven scatter of shafts and boreholes and is difficult to interpret. Beneath much of the south-eastern land area, from Whitburn southwards to Houghton and Seaham, it seems most likely that the Brass Thill is generally a single banded coal less than 0.5 m thick, but locally up to 1.1 m. However, this area includes that between Silksworth and Hetton-le-Hole (see p.23) in which confusion has arisen with an abnormally thick underlying seam, which in a few places closely approaches the Brass Thill. The Brass Thill in the southern offshore area, identified by its overlying shell bed, also appears to be generally a single thin banded or otherwise inferior coal, but in the north, off South Shields, the seam in places reaches 1.7 m (including 2 to 4 bands) and has been worked (Figure 17). In the north-eastern offshore area, however, the limited data suggest that the Brass Thill there is in two leaves two metres or more apart and it is possible that the seam is also split, with the lower leaf failing (Figure 16), (Figure 18), east of Sunderland.

Strata between the Top Brass Thill Coal and the Bottom Low Main Coal

Strata between the Top Brass Thill Coal and the Bottom Low Main Coal range from 4 to 13 m in thickness but are generally 6 to 9 m thick; no regional trend is discernible. They are relatively uniform and comprise an almost ubiquitous unit of dark grey shelly mudstone (the Brass Thill Shell Bed) at the base overlain by a varied sequence which contains a widespread sheet sandstone up to 9 m thick, but generally 2 to 7 m. Argillaceous seatearth caps the sequence in most places, but seatearths are otherwise rare (Figure 18).

The Brass Thill Shell Bed is one of the most persistent 'mussel bands' of the Northumbrian coalfield and ranges up to 4 m in thickness. It was well known to Hopkins (1928), who noted that, although its bivalve fauna is similar to that overlying the Hutton Coal, it could be distinguished from the latter by the rarity of Spirorbis. This fossil is, however, recorded by Armstrong and Price (1954) from the Brass Thill Shell Bed in boreholes at Wearmouth Colliery and Hylton, together with a suite of bivalves including Anthracosia beaniana, A. aff. caledonica, A. cf. lateralis, A cf. phrygiana, A. cf. regularis and Naiadites quadratus; additionally, Dr M A Calver has recorded Anthracosia aff. disjuncts and A. aff. subrecta from this bed in Offshore No. 3 Bore.

The Low Main Coal

The Low Main Coal underlies the whole of the district except for small areas in the south-west and undersea off Seaham. It has been worked patchily along the western margin and more extensively in the south (Figure 19); elsewhere in the district the coal is generally too thin for profitable working, though limited extraction of parts of the seam has taken place at Whitburn and Ryhope. Where the whole seam has been worked, it was mainly a single coal 1 to 1.5 m thick, in which one or two thin partings were widespread, but workings ceased where the thickness of the combined coal (excluding partings) decreased to less than about 0.6 m.

Where the Low Main has been worked in the south of the district, it generally lies about midway between the Brass Thill and Maudlin coals and nomenclature here poses no problems. The Low Main and Maudlin (or Bottom Maudlin) converge northwards in the Seaham area and coalesce into a thick composite seam in a large triangular area that extends northwards to Sunderland (Figure 19). Here, the two seams are generally separated by an argillaceous parting 0.1 to 0.3 m thick, but the main workings arc in the Maudlin part of the seam and the Low Main has been worked only patchily.

The simple southern coal sequence of Brass Thill–Low Main–Maudlin persists northwards along the western margin of the district, but in most northern, central and eastern areas the sequence is diversified by one or more additional coals (Figure 18). In most of these areas it is possible to recognise a Top and Bottom Low Main Coal or group of coals, but it is difficult to prove within the district that they are leaves of the single southern coal. Beneath Washington, and in the east of the district, both leaves locally split further in places, leading to a confusing sequence of thin coals and to consequent problems of nomenclature. In some offshore areas, it is likely that the 'Top' and 'Bottom' Low Main coals are more correctly interpreted as top and bottom leaves of a Bottom Low Main Coal.

Little has been published about the quality of the Low Main coals, but information from the National Coal Board shows ash contents of 5–6 per cent and sulphur contents of 1–2 per cent in the undersea area off Seaham.

Washouts are uncommon in the worked areas of the Low Main Coal but a substantial NNW–SSE washout separates the two main areas of workings in the south of the district, and a narrow washout extends towards Jarrow from the north (Land, 1974, fig. 44). The Brass Thill Coals are generally less than 0.5 m thick in a broad curved belt which connects these two washouts and which passes beneath Follingsby, Washington and West Herrington.

Strata between the various leaves of the split Low Main Coal are mainly argillaceous and up to 11 m thick; scattered bivalves have been reported from above most of the various leaves in one or more boreholes. Fish debris and ostracods are less common, and Spirorbis was identified in the roof of the Bottom Low Main in Washington J Shaft (Stobbs, in Ford, 1927). Offshore Borehole No. 4 yielded Naiadites quadratus from the roof of the lowest leaf of the Low Main and Anthracosia aff. phrygiana, A. sp. intermediate between aquilina and disjuncta, Anthracosphaerium cf. exiguum, Naiadites quadratus, Carbonita humilis and fish debris from a thin coal 1.5 m higher in the sequence (identifications by Dr M A Calver). The sequence also includes sandstones up to 6 m thick (but generally less than 3 m) and several seatearths.

Strata between the Low Main Coal and Maudlin Coal

Strata between the Low Main and Maudlin coals range in thickness from a few centimetres where the seams are united to more than 21 m in the north-west of the district. Thicknesses of 9 to 15 m are normal in most western, northern and south-eastern parts of the district but decline to 3 to 9 m in the area of complexly split Low Main in the north-east. A thick sandstone, the Low Main Post, occupies most of the Low Main–Maudlin interval in the extreme south-west and north-west, where it locally reaches 15 m, and sandstone up to 12 m thick occupies this interval in offshore boreholes WM1A, WM3 and WM8. Generally, however, sandstone is subordinate to argillaceous strata, which occupy the interval in more than half the boreholes and shafts through this part of the sequence. A thin coal is present near the middle of the interval in parts of the north-west of the district. Plants are abundant, but bivalves occur only locally, generally in mudstone overlying the single Low Main or Top Low Main (Figure 18) but also above the thin, local, overlying coal. Dr M A Calver identified Anthraconaia sp. nov., Anthracosia beaniana?, A. aff. phrygiana, Naiadites quadratus and fish debris from mudstone above the 'Top Low Main' in Offshore Bore No. 4, and Spirorbis, Anthracosia beaniana, A. aff. ovum, A. aff. phrygiana, Anthracosphaerium cf. turgidum, Carbonita humilis and fish scales from above a thin coal a few metres higher in the sequence in Offshore Bore No. 3.

The Maudlin Coal

The Maudlin Coal is the most consistently thick seam in the district and has been worked out from almost all land areas (Figure 20). In these areas, it is generally 1.5 to 1.7 m thick, but commonly just exceeds 2 m in the triangular area between Sunderland and Seaham, in which it is united with the Low Main Coal. It is absent only in the south-west and offshore from Seaham. Many of the old and possibly incomplete records show that the coal commonly contains one or two thin argillaceous partings, including one that is widespread about 0.2 m above the base of the coal in the Washington area and another that lies a little below the middle of the coal in most central and northern land areas. In the south-west and north-west of the district, the seam thickens sharply as one or other of the argillaceous partings increase to more than 0.5 m, and working either ceased at this point or only the upper leaf was extracted. In the south-west, the leaves split further to form 3 or 4 thin coals spread over 1 to 3 m of strata, but in the north-west only the upper leaf appears to extend far beyond the line of split.

Splitting is also a major feature of the Maudlin Coal in the east of the district, where it divides into Top and Bottom Maudlin coals roughly along the line of the present coast (Figure 20). Both leaves are of workable thickness for up to 4 km seawards of the split, but the Top Maudlin is the more persistent and workings in it are more extensive than in the Bottom Maudlin. Both leaves are mainly thin in the eastern undersea parts of the district, where the Top Maudlin divides into two or more leaves and the Maudlin coals are spread over up to 15 m of largely argillaceous strata. The Bottom Maudlin is absent or very thin in several of the northern offshore boreholes.

Few data have been published on the quality of the Maudlin Coal, but Edwards (1963) records that in the Westoe area it is of moderate quality with ash and sulphur contents of 7 to 10 per cent and 1.3 to 2 per cent respectively. National Coal Board data indicate an ash content of 3 to 5 per cent and a sulphur content of 1 to 1.7 per cent in the undersea area east of Sunderland and Ryhope.

Strata between the Maudlin Coal and Main Coal

Strata between the Maudlin and Main coals (Figure 21) range between 6 and 28 m in thickness, but are widely 19 to 25 m thick beneath western Sunderland and along the north-western margin of the district. Maxima of up to 28 m coincide roughly with the area of combined Low Main and Maudlin coals, and minima of 6 to 7 m occur in parts of the offshore area well east of Sunderland and Ryhope, where both Maudlin and Main are split.

In most parts of the district these strata comprise two subcycles divided at a seatearth or thin coal that generally lies 3 to 7 m below the Main Coal. Both subcycles commonly include a sandstone, that in the lower subcycle locally reaching 16 m (the 'Maudlin Post'), but sandstones generally occupy less than half of the Maudlin–Main interval and are thin or absent in a few places. In most areas, the Maudlin Coal is succeeded by a metre or more of dark grey shale and shaly mudstone containing locally abundant bivalves and plant remains (Hopkins, 1928); Dr M A Calver identified Anthraconaia sp. cf. cymbula, Anthracosia. cf. beaniana, A. cf. nitida, A. aff. phrygiana and Naiadites cf. productus from this mudstone in Offshore Borehole No. 4 and Land (1974) quotes a somewhat larger assemblage from Offshore Borehole No. 5 a short distance north of the district boundary. Where argillaceous strata occupy most of the lower subcycle, scattered bivalves have been reported up to 10 m above the Maudlin Coal but do not equate with those of a relatively prolific but thinner 'mussel band' that lies low in the upper subcycle and also widely contains fish debris and 'Estheria'. This bed, the 'Blackhall Estheria Band' (Magraw et al., 1963), yielded Spirobis sp., Naiadites cf. productus, 'Estheria' sp., Carbonita sp. and fish debris from carbonaceous shale in Offshore Borehole No. 3 and was also recorded at Washington 'J' shaft by Ford (1927).

The Main Coal

The Main Coal, the correlative of the consistent Yard Seam of Northumberland, is present almost throughout the district and has been extensively worked (Figure 22). Structurally, the seam is divided into three belts, northern and southern areas of workable coal being separated by a 2 to 6 km-wide belt in which the coal is unworkably thin and widely split into two or more leaves.

The most widespread and continuous workings are in the south and west, where the Main Coal is 1.8 to 2.1 m thick over large areas and is generally split into three subequal leaves by thin argillaceous partings. Extraction of this thick uniform coal is virtually complete in all land areas, but workings ceased northwards and eastwards where one or both of the argillaceous partings gradually thicken as the leaves of coal thin and diverge. In a number of places, workings continued in all or parts of the seam for up to a few hundred metres beyond the line of split before becoming uneconomic. The combined coal thins eastwards to 1 to 1.5 m in the Seaham area, where up to three thin partings are present locally, and is generally 0.8 to 1.4 m thick in the south-eastern offshore area.

Information is scanty in much of the belt between the northern and southern worked areas, but it appears that between Sunderland and Ryhope a single or banded top leaf 0.4 to 0.9 m thick lies generally 2 to 7 m above one or two thin 'Bottom Main' coals. It is not clear whether this upper seam is a true split from the single worked seam to the south and west; it could be (a) a separate seam in its own right or (b) equivalent to a thin coal that lies a few metres above the worked Main Coal at Springwell and Usworth collieries and which may be a correlative of the Bentinck Coal of south Northumberland.

Workings in the Main Coal in the north of the district are less continuous than those in the south, but the coal shares with its Northumberland counterpart (Land, 1974) a remarkable uniformity of thickness (0.8 to 1.0 m) and quality. Partings are recorded only locally, mainly along the southern fringes of the workings, where gentle splitting and thinning occurs. In the extreme north, however, between Wallsend and Westoe, the Main and overlying Bentinck coals converge to form a composite seam of three or four leaves spread unevenly over 1 to 2 m of strata. Of the four leaves proved in Westoe Colliery shaft, it is probable that the lowest two are leaves of the Main Coal and generally equivalent to the two closely spaced leaves of the Main Coal in Whitburn Colliery shafts, the lower being the coal widely worked beneath South Shields. It is practice, therefore (Magraw, 1963; Edwards, 1963), to regard this worked coal (and also that at Wearmouth) as 'Bottom Main'; the Top Main is poorly documented and difficult to identify in the records of some other shafts and boreholes where Bentinck equivalents may also be present. The status of the Main Coal worked in the extreme north-west of the district, whether Main or Bottom Main, is difficult to determine on the limited evidence now available, but its uniform thickness suggests affinities with the Northumberland Yard Coal and the South Shields Bottom Main Coal; it too splits southwards into three or four leaves.

Data are scarce in the undersea area well east of South Shields, but it seems that the Main Coal there is a composite seam up to 2.5 m thick that contains 2 to 4 partings or beds of inferior coal; a uniformly thick (1.0 to 1.2 m) lower leaf is probably the equivalent of the worked 'Bottom Main' of the South Shields area.

Information on the quality of the Bottom Main Coal of the South Shields area and just offshore comes mainly from Edwards (1963) who quotes an average ash content of 6.2 per cent and an average sulphur content of 1.1 per cent; the range of variation is said to be very small. Farther east, NCB analyses of coals from several offshore bores indicate ash and sulphur contents of 3 to 6 per cent and 0.8 to 1.2 per cent respectively, figures that also apply to the offshore area east of Whitburn and Sunderland. However, ash increases to more than 10 per cent and sulphur to 2 per cent and more in some Main Coal samples from 4 to 6 km east of Ryhope.

Strata between the various leaves of the Main Coal are mainly argillaceous and include seatearths, but assessment of trends is complicated by the difficulty of identifying the leaves in some shafts and bores. Sandstone occupies most of the interval in the shafts at Wearmouth and Hylton collieries, and is also present in some other places where the Top and Bottom Main are more than 5 m apart. Bivalves and fish scales were noted in mudstone in the roof of the ?Bottom Main in the Hylton Borehole (Armstrong and Price, 1954), in an underground borehole [NZ 4389 5804] 2.5 km east of Sunderland, and in Offshore Borehole No. W7; no identifications are available.

Strata between the Main Coal and Five-Quarter Coal

Strata between the Main and Five-Quarter coals (Figure 23) average 18 to 24 m in thickness over most of the district, increasing to 25 to 32 m beneath Ryhope and Seaham, and in adjoining southern undersea areas; minimum thicknesses of 12 to 13 m occur in a small number of isolated shafts and boreholes in the south-west and north-west of the district. The thickness is difficult to assess in some central, north-eastern and eastern parts of the district where both Main and Five-Quarter coals are split and their identification problematical.

The Main to Five-Quarter rocks of the district generally comprise a single cyclothem, and this consists largely of sandstone in the Ryhope to Seaham and adjoining offshore area where the unit is thickest. The sandstone here reaches up to 27 m in thickness and in several boreholes rests directly upon the Main Coal without appreciable channelling. Elsewhere in the district, sandstone commonly occupies one quarter to one half of the Main to Five-Quarter interval, mainly in beds 2 to 6 m thick in the middle of the sequence, but locally reaching 12 m. Remaining strata are mainly argillaceous, and silt-stones and mudstones with only thin sandstones occupy most or all of the interval in the Hylton to Silksworth area, beneath northern parts of South Shields and in the south-eastern extremity of the district. Where argillaceous rocks predominate, they locally include seatearths and, uncommonly, thin coals, indicating the presence of two or more subcyclothems. None of the coals appears to be widespread, with the possible exception of the Bentinck coals, which are recognisable along the northern edge of the district and may extend (continuously or discontinuously) for some distance southwards.

Where proved in shafts between Wallsend and Westoe, the Bentinck appears to be in two closely spaced leaves, each 0.2 to 0.4 m thick.

These strata are not noted for their fossil content, but plant remains abound in the mudstones and siltstones, and bivalves occur patchily in mudstones at the base of the sequence. Ford (1927) recorded 2 species of bivalve at this level in Washington ‘J’ Pit and Dr M A Calver (in Geological Survey records) identified Anthraconaia cf. pulchella and Naiadites aff. productus, in addition to 'Estheria' and fish debris from the supposed Top Main in Offshore Borehole No. 4. Scattered bivalves and some fish debris also occur in mudstones higher in the Main to Five-Quarter sequence, especially in the northern offshore area.

The Five-Quarter Coal

The Five-Quarter Coal is recognisable in most of the district, but it has been worked only in the south-west where it is a single seam up to 1.25 m thick, but generally 0.8 to 1.1 m (Figure 24). It has also been worked in opencast sites near Lumley (in the south-west) and Portobello (centre-west). Only the upper part of the seam was extracted in the western workings at Harraton Colliery.

In the main worked areas the seam is generally unbanded, but argillaceous partings are present near the fringes of the workings; their incoming heralded the approach of uneconomic extraction. In all other parts of the district the seam is thin and/or split, and in a few places is absent. The data do not allow firm correlation of the various leaves of the split Five-Quarter except between closely spaced shafts and boreholes, and there is a risk of confusion with the Bentinck and/or Top Main seams. However, the leaves do not appear to diverge by more than a few metres and generally number only two; strata between them are almost exclusively argillaceous and locally contain scattered bivalves. Unlike the districts to the north (Land, 1974), west (Fielding, 1986) and south (Smith and Francis, 1967), there is no union of the Five-Quarter Coal or its leaves with either the underlying Main Coal or the overlying Metal Coal.

Strata between the Five-Quarter Coal and Metal Coal

Strata between the Five-Quarter and Metal coals (Figure 23) are generally 7 to 10 m thick but range from 5 to 6 m in underground boreholes at Shiney Row and east of Hendon and Ryhope to 14 m at Hebburn 'C' Pit. No clear thickness trends are apparent, but the seam interval is generally least in the extreme east, where the Five-Quarter Coal is split. The sequence is predominantly argillaceous throughout the district, and generally comprises a single cyclothem. However, sandstone up to 7 m thick is present at about the middle of the sequence in the area in which the Five-Quarter Coal has been worked and beneath Seaham, and was also proved at this level in a number of shafts and boreholes in the northern offshore area.

Strata interpreted to lie between the Five-Quarter and Metal coals are indifferently exposed in the bed and banks of the River Wear [NZ 29 52] south and south-west of Lambton Castle, where R H Price (in Geological Survey files) recorded more than 10 m of measures below the supposed Metal Coal; these include a median 3.5 m sandstone and, near the base, a 0.35 m coal.

Mudstones and siltstones in this interval contain abundant plant debris and have also yielded bivalves and fish debris. Dr M A Calver (in Geological Survey records) identified Naiadites cf. obliquus from mudstone overlying the Five-Quarter Coal in Offshore Borehole No. 4, and Naiadites sp. and debris of several species of fish from above a supposed upper leaf of the Five-Quarter Coal in Offshore Borehole No. 3.

The Metal Coal

The Metal Coal, though generally thin in the district, is one of the most consistent and readily recognisable seams of the Middle Coal Measures. It has been mined only at Wallsend in the north-west and in a small area at Ravensworth at the western margin of the district (Figure 25), but has also been worked opencast at Portobello and near Great Lumley.

The coal is thickest in the north-west of the district, where an upper leaf, commonly 0.15 to 0.20 m thick, is separated from a 0.6 to 0.8 m lower leaf by a 0.05 to 0.3 m argillaceous parting. A belt of coal 0.6 to 0.8 m thick extends southwards from the mined area to Great Lumley and beyond, but the coal thins gradually eastwards and is generally less than 0.3 m thick in central and eastern areas. Exceptionally, it maintains a thickness of 0.3 to 0.6 m in much of the north and north-east of the district where parts of the coal are inferior and up to three argillaceous partings are locally present. The Metal Coal in almost all central and eastern areas is a single seam and is absent in only a small number of shafts and boreholes. Only in Penshaw D Pit and in the undersea area off Sunderland is a double seam present, but it is not clear from the records whether the thin upper leaf, 2 to 3 m above the lower, is a split of the Metal or a separate seam. Strata between the two coals are argillaceous and have yielded 'mussels' in some offshore boreholes.

The Metal Coal and associated strata are known almost entirely from shafts and boreholes, but are patchily exposed for some distance in the bed and banks of the River Wear [NZ 29 52] west and south-west of Lambton Castle. Here, notes by R H Price (in Geological Survey files) record a 0.61 m coal, though seam identification is tentative.

Strata between the Metal Coal and High Main Coal

Strata between the Metal and High Main coals (Figure 23) are consistently 8 to 12 m thick in most of the western area in which the Metal Coal is thickest. They maintain this range across much of the south of the district; minima of 3 m at Houghton and 6 m at Lumley Sixth pit shafts are in areas of split High Main Coal. Elsewhere in the district, the seam interval is sharply variable, averaging 12 to 15 m but reaching more than 20 m in some boreholes in the north-east and more than 30 m in the Boldon–Harton area, where up to 17 m of sandstone are present.

In most western, central and southern areas, the strata between the Metal and High Main coals comprise a single cyclothem and are predominantly argillaceous. Sandstone, where present in these areas, is generally less than 7 m thick and is most common in the lower half of the sequence. Except for the Boldon–Harton area, sandstone is similarly subordinate to argillaceous strata in most northern and eastern parts of the district, but here the Metal–High Main interval commonly includes two or more subcyclothems separated by seatearths or thin coals. A thin argillaceous limestone was recorded in an underground borehole [NZ 4328 5558] south-east of Sunderland.

Surface exposures of these beds are mainly confined to stretches of the bed and banks of the River Wear [NZ 29 52] near Lambton Castle and of the Lumley Park Burn [NZ 28 51] north of Lumley Castle. Argillaceous strata predominate in both places.

Fossils other than plant remains have not been reported from the Metal to High Main interval in western and central parts of the district, where most of the records are old, but mudstones both above the Metal Coal and also above higher seatearths or thin coals have yielded bivalves, fish and ostracods from a number of boreholes offshore. Fossils were recorded at four levels between the supposed Metal and High Main coals in Offshore Borehole No. 3, the lowest being in the roof of the ?Metal at 386.4 m. From above a thin coal at 381.5 m in this bore, Dr M A Calver identified Anthracosia caledonica, A. simulans?, Naiadites sp. nov. plus the debris of three species of fish; and from mudstone between 373.3 m and 374.1 m he recorded Anthracosia sp. cf. aquilina Trueman & Weir non Sowerby, A. cf. simulans, Anthracosphaerium cf. propinquum, Naiadites sp. and Carbonita sp. The highest fossiliferous mudstone, 2 to 3 m below the High Main Coal, yielded Spirorbis sp., Anthracosia sp. cf. fulva, Naiadites alatus, Carbonita humilis and fish scales.

The High Main Coal

The High Main Coal was the mainstay of the early mining industry on Tyneside, and was worked from many pits. Most of these were north of the Tyne, because the coal splits southwards, and became too thin to extract a short distance south of the river (Figure 26). The High Main in this north-western part of the district was up to 2.7 m thick and contained a widespread argillaceous parting generally below the middle; the thicker upper leaf was also split near the southern margin of the workings. Some marginal workings apparently extracted only part of the seam, but which part has not everywhere been recorded.

The High Main Coal has also been worked in the Seaham and adjoining offshore areas, where it is generally 1.5 to 2.0 m thick and contains two to five argillaceous partings; some of these thicken north-westwards as the seam splays and becomes unworkable. East of Seaham, the High Main is mainly a single seam with only thin partings, but in most other offshore areas it contains one or more thicker argillaceous partings and leaves of inferior coal. Figures from the National Coal Board indicate ash contents of 7 to 10 per cent (not including argillaceous partings) in the undersea area off Whitburn, Sunderland and Ryhope, and sulphur contents of 2.1 to 3.5 per cent. Edwards (1963) quotes figures of 7.2 per cent ash and 1.4 per cent sulphur in the upper part of the High Main at Westoe Colliery, South Shields.

The two areas of worked High Main Coal are separated by a broad belt in which the seam is mainly less than 1 m thick. At the edges of this belt the seam is generally in two thin leaves a few metres apart; both leaves were worked in opencast sites near Portobello, Lumley Castle and Great Lumley. In much of the central area, however, there is only one thin High Main Coal and it is not clear whether the two main leaves have reunited here or one leaf has died out. In areas where two leaves are identifiable, the intervening strata are mainly argillaceous and include thin coals, carbonaceous shales and seatearths. Exceptionally, the 4 m seam interval at the Lumley Castle opencast coal working also included 2.5 m of sandstone, and sandstone is also present in the poorly exposed measures between the Top and Bottom High Main coals in Lumley Dene [NZ 29 50], south-east of Lumley Castle, and in the Wear Valley [NZ 29 52] south and south-west of Lambton Castle.

Strata between the High Main Coal and High Main Shell Bed

Strata between the High Main Coal and High Main Shell Bed (Figure 27) vary greatly in both thickness and lithology in the well-documented eastern and southern part of the district, and may be equally varied in the less well-known north-west. In thickness they range from about 15 m off South Shields to 36 m in a borehole midway between Ryhope and Seaham, but thicknesses of 18 to 22 m are widespread except in a WSW–ENE belt from Houghton-le-Spring and Hetton-le-Hole via northern Seaham into the undersea area off Ryhope and Sunderland, where these strata thicken sharply and generally exceed 30 m.

Lithological variation in strata between the High Main Coal and High Main Shell Bed increases gradually eastwards across the district. In much of the north-west, mid-west and mid-south, the interval is sandstone-dominated and in a few places sandstone (the High Main Post) occupies the whole sequence and widely exceeds 12 m in thickness. Generally, however, the High Main Coal is overlain by 1 to 4 m of mudstone and siltstone, and the single cyclothem is capped by further argillaceous strata, including seatearth and a thin coal. The High Main Post is patchily exposed in the Wear Gorge near Lambton Castle and in the sides of the valley of the Lumley Park Burn east and south-east of Lumley Castle. The uppermost 6 m of the sandstone were seen in a temporary excavation on the north side of the Burn [NZ 2991 5090], where it is overlain by an 11 m sequence of interbedded sandstones, siltstones and mudstones and one thin coal; plant remains and fish debris were the only fossils found.

The diverse strata in this stratigraphical interval in central and eastern parts of the district are widely divisible into three or more subcyclothems, and sandstone is patchy and generally subordinate to argillaceous strata (Figure 27). Nevertheless, a basal High Main Post up to 15 m thick is present in a few isolated boreholes, including two located 3 km off Sunderland. The subcyclothems mainly culminate in seatearths or thin coals, but in the north-east and locally in the east, the penultimate subcyclothem culminates in the supposed equivalent of the Ashington Coal, which is generally 0.3 to 0.6 m thick, but locally exceeds 1 m. This coal is widespread in the northern part of the Tynemouth district (Land, 1974), but is absent between there and the northern part of the Sunderland district. Along the north-eastern fringe of the latter it lies only 3 to 8 m above the High Main Coal, though this interval increases to 25 m in the offshore area farther south, where the seam is commonly banded and may locally be in two or more leaves. A thin equivalent of the Ashington Coal may also be present in some central parts of the district, where the single cyclothem of the west begins to divide eastwards.

Plant remains are abundant throughout the High Main Coal to High Main Shell Bed sequence, but invertebrate fossils are uncommon in most western and northern parts of the district. With increasing eastwards lithological diversity, however, shelly shales and mudstones form a considerable proportion of the strata, generally in at least two, and commonly in three or four separate beds. 'Mussels' are almost ubiquitous in these, though not everywhere abundant, and ostracods, 'Estheria' and fish debris are common at some levels. Mudstones directly above the High Main Coal in Offshore Borehole No. 3 yielded Anthraconaia sp. nov., Anthracosia sp. nov. cf. aquilinoides, A. caledonica, A. sp. in-termed. disjuncta/lateralis, Anthracosphaerium aff. propinquum, Naiadites alatus, N aff. obliquus, Carbonita sp. and cf. Planolites. Mudstones some 14 m higher in the same borehole yielded Anthraconaia rubida, Naiadites obliquus, N. cf. productus and Carbonita sp. (identifications by Dr M A Calver in BGS internal reports). A eurypterid found by Mr M Foster in the roof of the High Main Coal in undersea workings [NZ 5314 4640] east of Sunderland was identified by Dr N J Riley as a species of Lepidoderma.

Strata between the base of the High Main Shell Bed and the Ryhope Little Coal

Strata between the base of the High Main Shell Bed and the Ryhope Little Coal (Figure 27) range in thickness from about 25 to 35 m, with an average of about 32 m; no regional thickness trends are apparent.

The sequence comprises 2 to 5 subcyclothems (Figure 27), each generally a few metres thick and most numerous in the east of the district. This change accompanies an overall eastwards decrease in the proportion of sandstones, which form more than half of the sequence in most land areas, but considerably less than half in most offshore areas, where many individual sandstones are also thinner than those in the west. Argillaceous strata thicken complementarily eastwards and include up to five units of fossiliferous mudstones or shales and several seatearths and rootlet beds, many without overlying coals. Most of the coals present are less than 0.3 m thick and are inferior or banded; in the north they include one or more thin representatives of the Moorland Coal of Northumberland, but identification of these becomes increasingly uncertain southwards. Surface exposures of these strata are few and mainly small, the best being at Fence Houses [NZ 306 506] and [NZ 306 503], where 7 m+ of mudstones in about the middle of the sequence have been worked for the manufacture of bricks and tiles.

Both plant and invertebrate fossils abound in argillaceous strata between the base of the High Main Shell Bed and the Ryhope Little Coal. The High Main Shell Bed is one of the two most consistent and extensive 'mussel bands' of the Northumbrian and Durham Coalfield (Hopkins, 1928, 1930); it is generally 2 to 4 m thick, is rarely absent and locally reaches 8 m. Because of its thickness and the abundance of its fossils, identification of the Shell Bed is generally simple, but doubts over its identification arise in boreholes in some eastern areas where two or three closely spaced and relatively prolific 'mussel bands' occur. This difficulty is eliminated in part of the south and south-east of the district, where black and dark grey shaly mudstones at the base of the High Main Shell Bed contain Lingula (locally abundant) and some foraminifera. This, the High Main Marine Band, has been proved only in a WSW–ENE belt extending from Houghton-le-Spring towards the eastern margin of the district off Sunderland. The belt coincides closely with that in which strata between the High Main Coal and the High Main Shell Bed are thickest, suggesting that the marine incursion here was limited to an area of differentially rapid compaction and subsidence. A similarly patchy distribution of marine faunas at this horizon has been noted both to the north of the district (Land, 1974) and to the south (Smith and Francis, 1967).

The faunal assemblage from the High Main Shell Bed at Washington ‘J’ Pit was listed by Stobbs (in Ford, 1927) and by Hopkins and Philipson (1946); Anthracosia cf. simulans and Naiadites sp. nov. were identified by Dr M A Calver at this level in the Hylton Borehole (Armstrong and Price, 1954). Farther east, in cores from Offshore Borehole No. 3, Dr Calver (in Geological Survey records) identified Spirorbis sp., ?Anthraconaia cymbula, Anthracosia cf. aquilina True-man & Weir non Sowerby, A. cf. atra, A. concinna, A. aff. planitumida, Anthracosphaerium radiatum, Naiadites alatus, N. cf. obliquus, 'Estheria' sp., Carbonita humilis and fish scales in mudstone overlying a sandy mudstone containing fragmentary Lingula sp. A similar assemblage, but without Lingula, was identified by Calver in samples from Offshore Borehole No. 4, and he also recorded substantial bivalve assemblages from mudstones near the base of two succeeding subcyclothems in both boreholes. Fish debris, ostracods and 'Estheria' have been recorded (commonly with bivalves) in shales and mudstones at several levels between the High Main Shell Bed and the Ryhope Little Coal, and 'Estheria' is common (together with bivalves and fish debris) in the highest mudstone of the sequence, a few metres below the top. This bed may be equivalent to the Bullock Steads 'Estheria' Band of Northumberland.

The Ryhope Little Coal

The Ryhope Little Coal is 0.94 m thick at its type locality of Ryhope Colliery Shaft and it is 1.02 m thick in Silksworth Colliery No. 2 Shaft, 2.4 km to the west. Such thicknesses are typical only of a restricted belt stretching southwards from northern Sunderland to about Seaton (north-west of Seaham); elsewhere the seam is generally less than 0.6 m thick and commonly less than 0.3 m. It has not been deep mined, except on a local trial basis, but a possible lower leaf of the seam was worked in two opencast sites near Portobello, where the coal was 0.7 to 0.9 m thick; it is possible, however, that these workings were in the Ryhope Five-Quarter Coal.

At its type locality the Ryhope Little is a single seam, which lies about 5 m above a 0.18 m coal that thickens to 0.36 m at Silksworth. This lower seam is probably the coal mentioned previously which carries a bivalve / ‘Estheria’ / fish fauna, and which locally is thicker than the Ryhope Little. In places this lower coal has been called the 'Bottom Ryhope Little' or even the 'Ryhope Little', but it is not clear if the Ryhope Little of the type locality divides anywhere in the district.

Strata between the Ryhope Little Coal and the Ryhope Five-Quarter Coal

Strata between the Ryhope Little Coal and the Ryhope Five-Quarter Coal are about 14 to 17 m thick between Sunderland and Seaton; thicknesses of 10 to 20 m are recorded from most other parts of the district. Minima of 4 to 6 m appear probable in boreholes situated 4 to 6 km east of Whitburn and South Shields.

In most western, central and southern parts of the district, these strata comprise a single cyclothem made up predominantly of sandstone, which provides an almost unmistakable marker bed up to 18 m thick (Figure 27). Only in the northern offshore area, where the seams are closest together, is sandstone subordinate to argillaceous sediments. Here, in Offshore Boreholes 4 and 10, reworked Lingula in a few centimetres of black shale and shaly silty mudstone in the roof of the Ryhope Little Coal mark the position of the Little Marine Band, the most discontinuous of the district's marine bands. Lingula in both boreholes is associated with ostracods and fish debris, and in No. 4 Borehole the overlying fauna included Anthraconaia?, Anthracosia cf. aquilina Trueman & Weir non Sowerby, A. cf. barkeri and Naiadites angustus. Bivalves, with or without fish debris and ostracods, have also been reported from thin mudstones in the roof of the Ryhope Little Coal in a small number of other northern offshore boreholes. In this same area, a coal up to 0.5 m thick locally divides the sequence subequally; this coal may be a separate seam or the lower leaf of a split Ryhope Five-Quarter.

The Ryhope Five-Quarter Coal

The Ryhope Five-Quarter Coal is the highest worked seam in the district, being thickest (0.8 to about 1.5 m, excluding dirt bands) in coastal and offshore areas (Figure 28). At its type locality in Ryhope Colliery shafts, the seam is 1.27 m thick with a 0.05 m coal 0.2 m above, but in an area bounded by Silksworth and Houghton-le-Spring on the west and extending 3 to 6 km offshore, and southwards beyond Murton, the coal is generally a single seam averaging 0.9 to 1.3 m thick, with one, or exceptionally two, thin argillaceous partings; within this area it has been worked patchily at several collieries. The single seam persists northwards to Whitburn and Cleadon, but divides and thins towards the north and west, where it appears to pass into two or more leaves, each less than 0.5 m thick and up to 5 m apart. Splitting also occurs east and north-east of the main onshore area of single coal, but the leaves here generally remain close together and, in most boreholes, the seam is composite and contains inferior coal and one or more argillaceous partings. Locally, especially off Whitburn, there are additional thin coals just above and/or below giving rise to a compound banded seam up to 3.2 m thick, including two 0.6 m bands. Analyses of the coal from boreholes offshore from Sunderland indicate ash contents (not including partings) of 5 to 11 per cent and sulphur contents of 0.6 to 1.7 per cent (information from NCB sources).

Strata between the Ryhope Five-Quarter Coal and Kirkby's Marine Band

Strata between the Ryhope Five-Quarter Coal and Kirkby's Marine Band (Figure 29) are generally 14 to 18 m thick. Thickness variation is not great and the interval is only locally less than 10 m thick or more than 20 m. At least two cyclothems are widely present (Figure 29), the lower being thicker and more complete than the upper except east of Sunderland, where they are subequal. The lower cyclothem includes almost the full complement of cyclic rock types, surmounted by a coal that ranges up to 1.3 m in thickness but which is generally less than 0.4 m; it begins with a fossiliferous mudstone up to a few metres thick and includes a sheet sandstone that commonly makes up half or more of the cyclothem and locally exceeds 10 m in the coastal belt between Sunderland and Seaham. The upper cyclothem is generally less than 6 m thick but thins out locally offshore from Sunderland; its coal, underlying Kirkby's Marine Band, is mainly less than 0.4 m thick but reached 0.8 m in a borehole off Ryhope and 0.6 m off Seaham.

The fossiliferous mudstone at the base of the sequence, overlying the Ryhope Five-Quarter Coal, is commonly rich in plant debris but also, in about half its provings, yielded bivalves and ostracods; fish remains, Planolites and Leaia have been recorded locally. Naiadites and Carbonita humilis were both noted from this level in Offshore Boreholes 3 and 4. Bivalves have been reported from mudstone at the base of the upper cyclothem in a few boreholes.

Surface exposures of strata between the Ryhope Five-Quarter Coal and Kirkby's Marine Band are confined to the steep banks of the Tyne at St Anthony's [NZ 28 63], where 11 m or more of sandstone occupy most of the upper part of the interval.

Kirkby's Marine Band

Kirkby's Marine Band is up to 5 m thick and is one of the most widespread marine bands in the Durham Coal Measures. Named after its discoverer, J W Kirkby, who found it in 1858 (Kirkby, 1860) whilst examining grey mudstone from a depth of 279 m at Ryhope Colliery shafts, it is widely characterised by the presence of Lingula and other marine fossils at two levels separated by up to 1.3 m of mudstone containing nonmarine bivalves (Armstrong and Price, 1954); Kirkby's was the first record of Lingula in the Northumberland and Durham Coalfield. The common tripartite character of the bed was also noted in the districts to the south (Smith and Francis, 1967) and north (Land, 1974); Land (pp.99–100) also recorded variations ranging from a single marine stratum to a complexly interleaved bed with two nonmarine and one semimarine layer.

The marine band has been cored in more than 30 boreholes in the district, and large collections of fossils have been made. Two black beds containing Lingula mytilloides and foraminifera were separated by grey mudstone containing Anthracosia cf. acutella and A. cf. atra in the Hylton Borehole, and similar assemblages were recorded, together with many Planolites ophthalmoides and 'Estheria' , in the upper leaf of the bed from Offshore Boreholes 3 and 4 (identification by Dr C J Stubblefield and Dr M A Calver). Calver noted that foraminifera and small Anthracosia occur side by side on single bedding planes in Offshore Borehole No. 3. Fucoids, Lingula mytilloides, Planolites ophthalmoides and Lioestheria vinti were collected from the only good surface exposure [NZ 2816 6329] to [NZ 2840 6312] of the bed, on the north bank of the Tyne at St Anthony's. Kirkby (1860) also recorded fish remains and entomostraca from Ryhope.

Strata between Kirkby's Marine Band and the Hylton Marine Band

Strata between Kirkby's Marine Band and the Hylton Marine Band are 10 to 13 m thick over most of the district, where they generally comprise a single cyclothem (Figure 29); however, they thicken to 15 to 20 m in a NNE–SSW belt through Ryhope, where two or more cyclothems are present. Fossiliferous argillaceous beds passing down into Kirkby's Marine Band make up much of the lower part of the sequence, and sandstone is widespread in the upper part. Seatearths and thin coals occur most commonly towards the top of the sequence but, apart from plant remains, fossils are few and present only locally in these upper beds.

The Hylton Marine Band

The Hylton Marine Band, first discovered by Armstrong and Price (1954) in the cores of the Hylton Borehole, is a thin (up to 1.2 m) but relatively persistent grey to black shale and mudstone characterised by foraminifera, Lingula and Planolites. A marine fauna has since been recognised at this horizon in most cored boreholes, but apparently barren mudstones have been reported locally (see also Land, 1974); it is also patchily absent through penecontemporaneous erosion. The Hylton cores yielded only Lingula, but the nearby South Moor Farm Borehole [NZ 3435 5795] also yielded Planolites and, from closely associated strata, fish debris, ostracods and nonmarine bivalves. From Offshore Borehole No. 4, Dr M A Calver identified foraminifera including Ammonema sp., Lingula mytilloides, Planolites ophthalmoides and fish remains in thin grey mudstone. Of the only two known surface exposures of this bed, that on the north bank of the Tyne at St Anthony's was apparently barren, but that at Cross Rigg Quarry [NZ 325 539] , New Penshaw, yielded foraminifera (Rectocornuspira?, Ammonema sp.), Lingula mytilloides, Planolites ophthalmoides, Hindeodella sp., platformed conodonts and fish debris.

Strata between the Hylton Marine Band and Ryhope Marine Band

Strata between the Hylton and Ryhope marine bands range in thickness from about 20 to 37 m but are generally 29 to 32 m thick. No clear thickness trends are evident, but 3 to 5 cyclothems and subcyclothems are widely present (Figure 29) and thick sandstone forms extensive beds towards the bottom and top of the sequence. This is especially true in the west, where sandstone widely occupies most of the interval and underlies much of south-east Newcastle. The sandstone near the bottom of the sequence rests on a channelled erosion surface which, in places, as at Cross Rigg Quarry, New Penshaw, cuts sharply down into and through the Hylton Marine Band and its overlying nonmarine mudstones. The sandstone towards the top of the sequence is the coarse-grained Upper Seventy Fathom Post, which was formerly worked in quarries in south-east Newcastle (Byker, St Anthony's, etc.), Felling, Eighton Banks, Springwell, Usworth and Penshaw. Seatearths or thin coals cap the various cyclothems, with a banded coal in about the middle of the sequence, which thickens up to more than one metre in parts of the northern offshore area. Plants are common in these strata and some of the mudstones have also yielded sparse assemblages of Anthracosia and Naiadites; other bivalves, fish debris, 'Estheria' and Planolites have been recorded.

The Ryhope Marine Band

The Ryhope Marine Band is up to 3 m thick in this district and is by far the most faunally varied purely marine Westphalian stratum in the coalfield. It was named by Armstrong and Price (1954) following the discovery by Tonks (1939) of goniatite spat in a nodule preserved from the sinking in 1856 of Ryhope Colliery shafts. A list of marine fossils from part of this mudstone and shale bed in the Hylton Borehole included marine bivalves, foraminifera and conodonts. A longer faunal list from Offshore Borehole No. 9 (just north of the district border) was given by Land (1974, p.104), and from Offshore Borehole No. 4 Dr M A Calver identified foraminifera (including Ammonema, Glomospira and Hyperammina), Planolites ophthalmoides, Lachrymula pringlei, Orbiculoidea?, Rugosochonetes skipseyi, Straparolus sp. nov., Aviculopecten cf. gentilis, Dunbarella macgregori, Parallelodon cf. semicostatus, Pernopecten carboniferus, Posidonia sulcata, a coiled nautiloid, Anthracoceras cf. hindi, Paraparchites?, ostracods, Hindeodella sp., platformed conodonts and fish remains. The marine band has not been found at outcrop in the Sunderland district.

Strata between the top of the Ryhope Marine Band and the Usworth Coal

Strata between the top of the Ryhope Marine Band and the Usworth Coal (Figure 30) are mainly between 27 and 36 m thick in the district, with isolated extremes of 25 m and 39 m, but the absence of, or difficulty in recognising, the Usworth Coal in many boreholes precludes full assessment. They generally comprise a single cyclothem, although a thin subcyclothem is recognisable in some boreholes towards the top of the interval. Lowest strata, overlying the marine band, are predominantly argillaceous throughout the district and comprise grey silty mudstones with plant remains and nonmarine bivalves. In many central and eastern parts of the district these argillaceous beds persist to the seatearth of the Usworth Coal, or contain only a few metres of sandstone towards the top, but a thick coarse-grained massive sandstone, the Grindstone Post, occupies much of the upper part of the sequence in north-western areas. This sandstone forms conspicuous dip slopes beneath Springwell and Windy Nook, and was formerly worked at Pelaw [NZ 288 621] and Heworth [NZ 285 615].

Fossils from the main cyclothem have not been identified but Dr M A Calver reported Naiadites sp. cf. hindi, N. cf. productus, N. cf. triangularis and an acanthocladian fish spine from just below the supposed Usworth Coal in Offshore Borehole No. 4. Stobbs (in Ford, 1927) recorded a small range of plant remains and nonmarine bivalves in these strata.

The Usworth Coal

The Usworth Coal, named after the seam lying at 20 m in Usworth Colliery Shafts, is a widespread coal that ranges up to 2.3 m thick (including dirt bands), but is generally less than 1.5 m. However, in about one quarter of the shafts and boreholes through this horizon, mainly in central and north-western parts of the district, the coal is either absent or cannot be identified with certainty, its place being occupied by sandstone or a number of thin coals. Where present in western and southern parts of the district, the seam is commonly 0.8 to 1.3 m thick and generally includes at least two argillaceous partings. It thickens eastwards to 1.5 m at Silksworth and up to 2.3 m (including 3 argillaceous beds) in a borehole [NZ 3943 5164] near Seaham, east of which it appears to split into two banded seams up to 10 m apart in much of the undersea area. Here, the lower seam is up to 1.1 m thick and the generally thicker upper seam (including bands) is up to 1.8 m thick.

The Usworth Coal has not been mined in the district, mainly because of its banded character; no analyses of its ash and sulphur contents have been published.

Strata between the Usworth Coal and Hebburn Fell Coal

Strata between the Usworth and Hebburn Fell coals are generally 15 to 25 m thick (Figure 30) and are predominantly argillaceous; they comprise one to four incomplete cyclothems, each capped by a seatearth or thin coal, the lowest cyclothem commonly being the thickest. Sandstone is dominant mainly in the central part of the land area, between Wardley and Silksworth, and was formerly worked for grindstones at Cox Green beside the River Wear. Channel sandstone more than 20 m thick occupied most of the interval in Offshore Borehole No. 12 in the south-east of the district. Mudstones at the base of the various cyclothems contain a sparse nonmarine biota, with scattered bivalves (mainly Naiadites) and some fish debris; plant remains are abundant.

The Hebburn Fell Coals are a group of two or three seams spread over up to 10 m of strata (Figure 30) but which cannot be shown in this district to be splits of a single coal. It seems likely, indeed, that the original Hebburn Fell Coal is probably the thickest of the group at about this horizon in the type area in the north-west of the district, and that elsewhere the name has been applied to others of the group whichever is locally thickest. Of the two coals commonly present, the upper is usually the thicker, up to 1.4 m, with the lower more than 1 m thick in a few places only, and generally less than 0.5 m. Sandstone up to several metres thick occurs between the Top and Bottom Hebburn Fell Coals in the Washington and Follingsby areas.

The Hebburn Fell Coals

The Hebburn Fell Coals have not been mined underground in the district, but opencast workings near Usworth are believed to have extracted both top and bottom leaves of the seam, here 9 m apart. A coal tentatively identified as the Top Hebburn Fell is exposed in the south bank of the Wear [NZ 3353 5593] near Barmston; this seam was worked from small surface drifts in the nearby valley of Spa Well Gill.

Strata between the Hebburn Fell Coals and the Wear Mouth Marine Band

Strata between the Hebburn Fell Coals and the Wear Mouth Marine Band (Figure 30) are generally between 35 and 40 m thick in the few boreholes in which both bottom and top of the interval can be positively identified. About nine thin and mainly incomplete cyclothems are commonly present in this sequence, in which sandstone is generally subordinate to argillaceous strata except around Wardley and Follingsby, and also at Wearmouth. Most of the cyclothems are capped by a seatearth or thin coal, only the Dean Coal, named after its type locality at Dean (near West Park), South Shields, being thick enough to warrant a name. This coal, which is almost 2 m thick in its type area, has probably been mistaken for the Top Hebburn Fell Coal in some places. Fossiliferous grey to dark grey mudstones and black silty shales are present at the base of several of the cyclothems. A thin fragmental clayrock was recorded in about the middle of the sequence in the South Moor Farm Borehole, Washington.

Surface exposures of rocks between the Hebburn Fell Coals and the Wear Mouth Marine Band are uncommon in this district. The largest is a much-faulted sequence of varied strata that extends for some scores of metres north-north-eastwards from the confluence of Spa Well Gill with the River Wear [NZ 3364 5610], which yielded ostracods, fish debris and a few nomnarine bivalves from argillaceous beds overlying carbonaceous mudstones and thin coals. This may have been the exposure from which Hopkins (1929) recorded Anthracomya cf. phillipsii; but older rocks are exposed upstream and the precise locality cannot now be identified. The rocks exposed are fully 130 m below the base of the phillipsii zone (= base of Upper Coal Measures). These strata are also exposed in Spa Well Gill. Sandstone at a similar stratigraphical level was formerly worked in a quarry [NZ 313 641] near Hebburn Hall; faces up to 9 m high were still visible at the time of the resurvey.

Fossils reported in the shales and mudstones in cored boreholes through strata between the Hebburn Fell Coals and the Wear Mouth Marine Band include plants (abundant), nonmarine bivalves, ostracods, 'Estheria' and fish debris. Identifications by Dr M A Calver of specimens from the Hylton Borehole and from Offshore Borehole No. 4 showed that the nonmarine bivalves are mainly species of Anthraconaia and Naiadites, and that the ostracods are mainly referable to Carbonita and Geisina. A short list of fossils from these strata in Ryhope Colliery Shaft was given by Tonks (1939).

The Wear Mouth Marine Band

The Wear Mouth Marine Band (sometimes erroneously written as the Wearmouth Marine Band) was discovered in 1959 by Dr M A Calver in specimens collected by Dr D Magraw from Offshore Borehole No. 4, 5 km east of the mouth of the River Wear. It is equivalent to the Edmondia Band of the East Pennine Coalfield. In the district it comprises grey shaly mudstone with scattered to abundant arenaceous foraminifera. The Band has not been found in any surface exposures but has since been recognised in several offshore boreholes, in some of which it also contains (or is closely associated with) fish debris, Planolites and ostracods. Lingula was reported from Offshore Borehole No. 7. The Wear Mouth Marine Band was found to overlie a 1 cm tonstein in two boreholes [NZ 3117 6212]; [NZ 3097 6229] at Wardley (see also Jones and Magraw, 1980).

Strata between the Wear Mouth Marine Band and the top of the Down Hill Marine Band

Strata between the Wear Mouth Marine Band and the top of the Down Hill Marine Band have been fully cored only in the Down Hill Borehole [NZ 4350 5509] (Figure 31) and Offshore Borehole No. 12 [NZ 4993 4896], in which they are 105 m and 114 m thick respectively (both including small faults). Several other boreholes cored lower parts of the sequence, and further but less specific information comes from shafts such as those at Boldon and Silksworth collieries in the deepest parts of the Boldon Syncline. The strata have a broad outcrop, mainly beneath drift, that extends across much of the low-lying ground ('The Boldon Flats') between Hebburn and Castletown.

The sequence, exemplified by that proved in the Down Hill Borehole (Figure 31) is predominantly argillaceous and comprises 15 or more generally incomplete cyclothems. Most of these are capped with seatearth, carbonaceous shale or a very thin coal, but coals up to 0.7 m thick were proved in Offshore Borehole No. 12, in which sandstones were also thicker and more numerous than in the Down Hill Borehole. The offshore borehole also differed from that at Down Hill in proving a marine band some 40 m below the Down Hill Marine Band. This bed, the Dawdon Marine Band, was first identified and named in 1964 by Dr D Magraw (see also Jones and Magraw, 1980, p.31) and contains fragmentary Lingula and fish debris; it is probably equivalent to the Shafton Marine Band of the East Pennine Coalfield. The sequences proved in the various boreholes and shafts cannot yet be correlated in detail.

The main surface exposures of these strata lie on both banks of the River Wear between Low Barmston [NZ 338 564] and the Hylton Fault [NZ 353 573], and on the north bank of the Wear at Hylton (the so-called 'Claxheugh Exposure'); they are also visible in mainly small exposures scattered across the Boldon Flats and in the Don Valley between Hedworth [NZ 335 631] and Boldon Mill [NZ 343 627]. Good sections were also seen during the resurvey in pits worked for shale in the Wardley area, but these are now obscured.

The discontinuous exposures along the banks of the Wear between Low Barmston and the Hylton Fault span much of the lower part of the interval between the two marine bands. The most continuous sections are in the bed and banks of the River Wear [NZ 337 564] to [NZ 343 564] around the north side of Offerton Haugh, where R H Price (in Geological Survey files) measured sequences of mainly argillaceous strata up to 20 m thick low in the interval; carbonaceous shales and thin coals were also recorded. The presence of thicker coals (up to 0.7 m) slightly higher in the sequence is indicated by lines of old workings on the steep flanks of the deep incised meander immediately east of Offerton Haugh. Sandstones here are thick enough to form cliffs and they have been worked for grindstones in old quarries at North Hylton and Stony Heugh. The south-east side of the meander, White Heugh, also featured old ironstone nodule workings, which are said to have extended southwards to Offerton village (Potts, 1892). Potts recorded that the position of the workings high in the banks of the gorge facilitated initial transport of the nodules, which were delivered to boats waiting on the River Wear by the simple expedient of rolling them down the steep slope.

Strata exposed in the river bed at Offerton Haugh yielded abundant plant remains but only a few bivalves, fish and 'Estheria' . In contrast, Geisina? subarcuata, Lioestheria vinti, Naiadites sp. and fish scales were abundant in thin partly carbonaceous shale at a small exposure [NZ 3490 5606] in Offerton Gill on the southern lip of the gorge; this exposure is in beds about midway between the two marine hands.

The 'Claxheugh Exposure' [NZ 3634 3783], on the north bank of the Wear almost opposite to Claxheugh Rock, comprises a few metres of varied strata in a fault-bounded block; on structural evidence it probably lies 20 to 25 m below the Down Hill Marine Band. This interpretation is in agreement with that of Calver (in Armstrong and Price, 1954), suggesting that the fauna is closely comparable with that at the top of the Upper similis-pulchra Zone of the Pennine coalfields. The abundant and varied fauna from this exposure comes mainly from 0.8 to 0.9 m of dark grey and black mudstone and shale in the lower part of the exposed sequence, and particularly from two thin nodular beds of clay ironstone, in which most of the fossils are uncrushed. The beds at the Claxheugh Exposure were long regarded as the highest exposed Coal Measures in County Durham, and as belonging to the zone of Anthraconauta phillipsii, though both views are now shown to be incorrect. They and their fauna have been studied by Kirkby (1864a), Stobbs (1905), Trechmann and Woolacott (1919), Hopkins (1930), Armstrong and Price (1954) and Pollard (1969). Faunal lists by Calver in Armstrong and Price (p.989) and Pollard (pp.249–250) include mussels, arthropods, fish and insects; Pollard inferred that the fauna of the Claxheugh Exposure probably inhabited a freshwater to brackish shallow lake or lagoon. The record of the small limuloid arthropod Belinurus trechmanni (Woodward, 1918, Trechmann and Woolacott, 1919) is unique in the Durham Coal Measures.

Apart from that at the Claxheugh Exposure, the fauna of the strata between the Wear Mouth and Down Hill marine bands has not been examined systematically and full determinations are available only from two fossiliferous beds near the base of the sequence in a borehole [NZ 3932 5808] at Wearmouth Colliery. From the lower of the two fossiliferous beds in the Wearmouth Borehole, Dr M A Calver identified Anthraconaia sp. nov., Naiadites cf. elongatus, Carbonita cf. fabulina, Geisina? subarcuata, Hilboltina sp., 'Estheria' and fish debris; from the higher of the two he recorded cf. Anthraconauta phillipsii, Naiadites cf. daviesi, 'Estheria' and fish debris. The lower bed lies fairly close to the probable horizon of the Wear Mouth Marine Band (which was not identified) and the higher is about 22 m above the lower. Calver noted that neither assemblage indicated proximity to a marine environment.

The sections in the Wardley area were all in the lower part of the sequence and mainly comprised mudstones and siltstones, which were worked for brick making. Two main quarries [305 624; 309 625] exposed up to 4 m and 9 m of strata respectively, the latter including the basal 2 m of an overlying sandstone and a thin coal. A third quarry [NZ 305 621] exposed about 9 m of strata, including 0.3 m of overlying sandstone and a thin coal near the base of the excavation; dark grey partly ferruginous mudstone above the coal yielded pyritised bivalves, 'Estheria' , ostracods and fish scales.

The Down Hill Marine Band

The Down Hill Marine Band at the top of the Middle Coal Measures was discovered in 1963 in mudstones exposed [NZ 364 577] on the south bank of the River Wear near Claxheugh Rock, Sunderland, by students accompanying Dr J M Jones. Its stratigraphical level was uncertain because of the proximity of major faults, but the bed was subsequently proved at 101.3 m in the Down Hill Borehole [NZ 3501 6989], near West Boldon, and at 324 m in Offshore Borehole No. 12. Details of the bed in the Down Hill Borehole are summarised in (Figure 31). In Offshore Borehole No. 12, it was 5.9 m thick and comprised grey mudstone with Lingula, Orbiculoidea, Planolites ophthalmoides, fucoids and foraminifera. Marine fossils were not recognised in mudstone between 317.8 m and 319.1 m, and between 321.3 m and 322.5 m in this borehole, suggesting the possibility of multiple marine incursions reminiscent of those of Kirkby's Marine Band. Only Lingula was reported from the riverside exposure of the Down Hill Marine Band at Claxheugh (Jones and Magraw, 1980).

Upper Coal Measures

Upper Coal Measures strata in the district belong to the later part of Westphalian C. They occur mainly in the much-faulted Boldon Syncline, but their presence in a small area beneath Hendon (south-east Sunderland) and beneath a larger area east and north of Seaham is inferred on structural grounds. No stratigraphical information is available for either of the coastal occurrences, both of which are overlain by Permian strata.

The presence of Upper Coal Measures in the district had been suspected for many years following the identification by Stobbs (1905) of Anthraconauta phillipsii in mudstones exposed ?[NZ 3634 5783] on the north bank of the River Wear near North Hylton, Sunderland (the 'Claxheugh Exposure'). However, this identification was questioned by Calver (in Armstrong and Price, 1954) on the basis of further collections, and proof of the presence of Upper Coal Measures awaited the discovery of undoubted A. phillipsii in temporary exposures at Town End Farm Estate, north-west Sunderland (Smith, 1960). Subsequently, the discovery near Claxheugh Rock of the Down Hill Marine Band, which lies immediately beneath the Upper Coal Measures, and its confirmation in 1964 by the Geological Survey Down Hill Borehole [NZ 3501 5989], made it possible to calculate the distribution of Upper Coal Measures at their main outcrop (Figure 32). Apart from a few mainly small exposures in valleys, these strata are overlain by drift in all western and northern parts of this outcrop and by Permian strata in the remainder.

The few surface exposures and inadequately documented boreholes gave only a sketchy impression of the stratigraphy of the Upper Coal Measures north-west of Sunderland, and the Down Hill Borehole was drilled specifically to determine as much of the sequence as was practicable. It was sited so as to avoid major faults, and proved about 91.5 m of Upper Coal Measures strata. A further 50 m of these beds lie in the undrilled deepest part of the Boldon Syncline, a short distance east of Down Hill, but there are no exposures and nothing is known of them. The sequence of Upper Coal Measures cored in the Down Hill Borehole is summarised in (Figure 33); the several faults penetrated are probably all minor and do not materially shorten the sequence.

The main features of this sequence are the predominance of argillaceous strata, the presence of only a few thin coals and the general sparseness of nonmarine bivalves. The exact position of the base of the Upper Coal Measures falls within an apparently uniform mudstone bed and is difficult to define without detailed micro-palaeontological analysis. The highest Lingula, an isolated specimen, was found at a depth of 99.6 m, but appreciable numbers of Lingula occurred only below 100.3 m. A feature of particular interest was the presence of patchily stained sandstone between depths of 63.2 m and 74.0 m; the staining is of the type normally associated with reddening beneath the Carboniferous–Permian land surface and presumably results from this cause, but it lies unusually far beneath the unconformity.

Surface exposures of Upper Coal Measures occur in the valleys of the rivers Don and Wear. In the Don valley they are seen in generally small exposures in the valley sides at two places. The first is west of West Boldon [NZ 3467 6099] to [NZ 3489 6142], where a few metres of grey silty shale and mudstone lie a short distance above the Hylton Castle Coal. The second is cast of the West Boldon Fault, north of Boldon Colliery, where a number of exposures for about 400 m [NZ 344 627] to [NZ 348 627] are mainly in sandstone (some red stained) with subordinate grey sandy siltstones; these beds lie near the base of the Upper Coal Measures and dip eastwards.

The exposures in the Wear valley lie on both sides of the river near North and South Hylton, west of Sunderland. At Baron's Quay, north of the river, they extend for about 600 m [NZ 3528 5750] to [NZ 3580 5765]. The main exposures are of a few metres of fine- and medium-grained sandstone lying above the Hylton Castle Coal and immediately beneath the Permian Yellow Sands. The sandstone is yellow in the uppermost 0.4 to 0.6 m, grading down to purple, and is probably the highest Westphalian stratum seen in natural surface exposures in north-east England. Near Claxheugh Rock, south of the river, exposures extend for about 300 m north-north-east from the Claxheugh Fault [NZ 3632 5759] to [NZ 3646 5778]; here, the Down Hill Marine Band is succeeded by east-south-eastwards-dipping blocky mudstone (1.4 m), ferruginous siltstone (0.45 m) and grey shaly siltstone (1.8 m+).

Several temporary exposures at Town End Farm were mainly less than 2 m deep and revealed Upper Coal Measures under 1 to 1.9 m of drift. Sandstone was greatly subordinate to grey and dark grey siltstones and mudstones, and several thin beds of dark grey to black micaceous silty shale and carbonaceous shale were also present; cone-in-cone structure was noted in thin ferruginous siltstones. Anthraconauta phillipsii and 'Estheria' were found sparingly in the mudstones, and fish debris was particularly abundant in several of the dark grey to black silty shales. These strata all lie above the Hylton Castle Coal and young north-north-eastwards; the highest bed seen [NZ 3535 5964] was purple sandstone near the top of the sequence proved in the nearby Down Hill Borehole, lying immediately beneath Upper Permian dolomite.

Chapter 3 Permian

Introduction and stratigraphical classification

The classic Permian sequence of the district has been the subject of detailed study for more than 170 years, and has been described and discussed by many authors (see Smith and Francis, 1967 and Smith, 1980a, b for full references). The classification adopted here (Table 2) is that of Smith et al. (1986), which has evolved gradually from the pioneer work of Winch (1817) and Sedgwick (1829). The relationship between the current classification and an earlier version used on the published geological map is also shown in (Table 2), together with an approximate correlation with Permian strata in the southern North Sea and adjoining parts of Holland and Germany. Two main epochs are recognised, a lengthy Early Permian occupied mainly by erosion and by spasmodic desert sedimentation, and a relatively brief Late Permian in which the formation of thick marine strata was followed by the return of widespread continental sedimentation.

Perhaps 40 million years elapsed in the district between the deposition of the youngest preserved Coal Measures (and possibly later Carboniferous strata) and the formation of the oldest preserved Permian rocks. During this time, the British Isles drifted gradually northwards from the Carboniferous equatorial belt with its high rainfall into the semidesert and desert belts of the northern tropics (e.g. Lovell, 1977; Glennie, 1982, 1984). Concomitantly Variscan earth movements caused faulting, gentle folding and uplift of Carboniferous and older rocks, expelling the remaining Carboniferous seas and creating an uninterrupted land area across northern Laurasia. Perhaps at this time, but probably somewhat later, regional tension leading to crustal attenuation probably accounted for the initiation of subsidence in the Southern North Sea area; this subsidence resulted in gradual eastwards tilting of north-east England, parts of which were probably lowered to well below world sea level in late Early Permian times (Smith, 1970a, 1979).

Flooding of the sub-sea-level peneplain which formed the western slope of the main Southern North Sea Rotliegend desert basin is believed to have been extremely rapid (Smith, 1970a, 1979); it occurred when the Boreal Ocean broke in from the north, perhaps following a relative sea-level rise. Calculations by Steele (1981) suggest that the flooding may have taken 'several years to a few decades', and calculations by Glennie and Buller (1983) suggest that it may have taken three to six years, depending on the assumptions made regarding the dimensions of the seaway across the northern threshold.

The Late Permian tropical Zechstein Sea that was created by the flooding of the former desert basin extended eastwards to Lithuania and eastern Poland and may have been 300 m or more deep in the depocentres of its several main sub-basins. A complex history of sedimentary filling has been outlined by Smith (1970a, 1980b), who showed that, as in Germany (Lotze, 1934; Richter Bernburg, 1955), four main depositional cycles and many subcycles may be recognised. Smith suggested that the major cyclicity might be related to oscillations of world sea level, perhaps in response to contemporary glacial events in the Southern Hemisphere. The deposits of the first two main cycles almost filled the original topographical depression; they were characterised by marginal belts of thick carbonate and sulphate rocks that were succeeded in the second cycle by exceptionally thick basin-centre evaporites. The deposits of Cycles EZ3 and EZ4 accumulated more uniformly in space made available mainly by continuing subsidence, and are chiefly evaporites, but with a broad thin marginal carbonate wedge in Cycle EZ3. Rocks of a minor fifth cycle are found, in England, in eastern North Yorkshire only.

The western shores of the Zechstein Sea appear mainly to have lain along the gentle eastern flanks of a persistent, low proto-Pennine barrier, generally 20 to 40 km west of the present outcrop (Smith, 1989). This barrier may have persisted throughout deposition of the rocks of Cycles EZ1 and 2, but was probably breached when a connection was briefly established with the Vale of Eden during deposition of the carbonate rocks of Cycle EZ3 (Burgess, 1965; Pattison, 1970; Smith, 1970a). Throughout the Late Permian, desert conditions continued to dominate land areas around the Zechstein Sea, ensuring an episodic supply of wind-blown sand and silt to the adjoining seas. The presence of a sparse specialised biota of land plants and reptiles on this desert is attested by scattered remains in many nearshore Zechstein carbonate rocks.

Zechstein rocks exposed at the surface in the Sunderland district belong almost exclusively to Cycles EZ1 to EZ3 and constitute the well-known Magnesian Limestone sequence; rocks of Cycle EZ4 and the underlying Rotten Marl are known only from collapse pipes onshore and boreholes offshore. In general, the oldest rocks, those of the first subcycle of Cycle EZ1, are exposed in a narrow belt along the western escarpment (Figure 34), and are succeeded eastwards by progressively younger strata. The coastal cliffs are formed mainly of strata of Cycle EZ2, but are overlain in a syncline at Seaham by carbonate rocks of Cycle EZ3. The age of oolitic dolomite overlying the Hesleden Dene Biostrome, and of the biostrome itself, is uncertain, although a Cycle EZ2 age is favoured for the oolite; the exact mutual relationships of the Cycle EZ1 anhydrite and the Cycle EZ2 carbonate rocks have yet to be established. These uncertainties partly arise from the widespread dissolution of former evaporites and the anomalous stratigraphical juxtapositions brought about by the resultant foundering of younger strata. Thick Cycle EZ1 anhydrite remains undissolved in much of the undersea area (e.g. Magraw, 1975, 1978) and has been cored in many deep offshore coal exploration boreholes; anhydrite and salt of Cycle EZ2 were cored in one offshore borehole and may have been penetrated (but not recorded) in others. The presence of anhydrite of Cycles EZ3 and 4 has been inferred from the evidence of borehole cuttings and wire-line logs.

An attempt to show the original stratigraphical relationships of Permian strata in the Sunderland district is given in (Figure 35). In general, each of the main carbonate units forms an eastward-thickening wedge extending basinwards from a feather-edge and culminating eastwards in a shelf break and a marginal slope, but the geometry of the wedges differed in each cycle and subcycle in response to local and regional influences. The evaporites exhibit rather different patterns, with thickness changes that are partly complementary to those of the carbonate rocks. The full sequence between the basal unconformity and the top of the Cycle EZ3 carbonate unit is preserved only in the eastern undersea area, where offshore boreholes reveal that it thickens south-eastwards and east-south-eastwards from less than 150 m ten kilometres east of South Shields to more than 350 m twelve kilometres east of Seaham.

The Magnesian limestone, though widely covered by drift, has exerted a profound effect on the human development and natural history of the eastern part of the district. By its presence it hindered early exploitation of the underlying coal seams, thereby leading in the last two centuries to a different pattern of population growth and movement from that of the Wear lowlands; its thin dry upland soils were colonised by a restricted and specialised flora. A comprehensive review of these and many other aspects of the Magnesian Limestone of County Durham and adjoining areas was given by Dunn (1980); it includes an excellent summary of the geology by T H Pettigrew.

In addition to 1:50000 Sheet 21, this account of the Permian strata also embraces adjoining parts of the Tynemouth (Sheet 15) and Durham (Sheet 27) districts and includes a wealth of new information from coal exploration boreholes offshore. It is based on information from all available exposures and boreholes, detailed records of which are lodged in the files of the British Geological Survey; these records include drawings of all major quarry and coastal sections, and are also summarised in notes on the component 1:10 560 scale published maps.

Lower Permian

The Carboniferous–Permian unconformity

Little remains to chronicle the course of events in this district during latest Carboniferous and earliest Permian time, but it seems likely that an initial mountainous phase dominated by immature tabular landforms and resulting from the Variscan earth movements, was gradually superseded by a prolonged phase of decreasing relief as desert erosion first led to rapid downwasting and later to progressive peneplanation. During these processes, up to 630 m of Upper Carboniferous rocks were eroded from the Sunderland district, the resulting detritus doubtless accumulating in the early stages as screes, alluvial fans and spreads of valley-fill. These deposits, together with rare fossils from the sparse lifeforms that presumably inhabited this unremittingly harsh and inhospitable environment, were subsequently removed as base levels lowered and peneplanation proceeded.

The product of this prolonged phase of erosion was a mature, gently rolling peneplain, probably with a gentle eastwards slope into the subsiding North Sea Basin. This peneplain became the Carboniferous–Permian unconformity, a surface of generally low relief but with scattered minor eminences such as that at Newbottle, where a hill of Westphalian sandstone now rises a few metres above the general level. Small-scale relief had been virtually eliminated, too, judging from the configuration of the plane of unconformity in borehole cores and in the few available surface exposures; however, sand-filled fissures (neptunean dykes) up to 50 cm deep have been recorded locally (see Magraw, 1975). Mature peneplains in modern deserts commonly feature a few large hollows, which generally mark the sites of ephemeral watercourses (wadis); the possible occurrence of such a hollow was reported by Magraw (1978) from borehole evidence 7 km east of Ryhope.

The general environment in this district during the later phases of peneplanation was that of a barren, almost flat, bare rock pediment, swept by strong winds that carried abrasive sand flurries and perhaps small migrating dunes. Small quartz pebbles and other resistant rock fragments were thinly scattered on parts of this surface, particularly in the north and south of the district.

During the later phases of peneplanation, the surface of the desert was almost ubiquitously reddened to depths of 6 to 15 m, but locally much more. The reddening process affected different strata to varying degrees, being most penetrative in micaceous sandstones and least marked in coals and mudstones. On exhumed surfaces such as that of the Westphalian A sandstone at St Mary's Island, 8 km north of the district, reddening has penetrated preferentially far down into rock adjacent to joints and other fissures; unusually deep penetration to a depth of 66 m below the unconformity was noted alongside a minor fault in the Upper Coal Measures of the Down Hill Borehole, near West Boldon. The reddened rocks were regarded by some early workers as a stratigraphical unit lying between late Carboniferous and late Permian strata, but were shown by Howse (1848) to contain Carboniferous plant fossils. Daglish and Forster (1864), in a little-quoted but important contribution, were the first to suggest that the reddened strata were merely the subaerially weathered edges of Coal Measures rocks. Anderson and Dunham (1953) suggested that reddening may be attributed to desert weathering of pyrite and siderite already in the rocks and to the introduction of haematite and other iron oxides by percolating groundwater. An almost equally ubiquitous layer of grey rock immediately below the surface of the unconformity (Anderson and Dunham, 1953) was interpreted by Smith and Francis (1967) as a zone in which reddened strata had been bleached by the reducing waters of the Late Permian Zechstein Sea.

The only exposure in the district of the Carboniferous–Permian unconformity is a low cliff [NZ 357 576] on the north bank of the River Wear at Castletown, Sunderland. Here the unconformity is exposed at intervals for about 50 m in a low cliff, and is a gently dipping surface with a relief of a few centimetres (Plate 1); the underlying Carboniferous rock is a purple sandstone of the Upper Coal Measures, with an uppermost zone, 0.40 to 0.60 m thick, of yellow (reduced) sandstone.

Yellow Sands Formation

Distribution and age

This formation, first named the Yellow Sands by Hutton (1831) following work by Sedgwick (1829), covers about two-thirds of the Carboniferous–Permian unconformity in north-eastern England and is up to 68.7 m thick. At outcrop it is clearly discontinuous and, where data points are sufficiently abundant and closely spaced, the sand has been shown to form ridges of various heights (Smith and Francis, 1967). The recognition by Daglish and Forster (1864) that the Yellow Sands form hills on the old land surface, and do not generally fill hollows as had previously been thought, has been confirmed by later information. The crests of such hills are (or have been) visible in a number of exposures in both the Sunderland and Durham districts. As noted by Smith and Francis (1967) and Steele (1981, 1983), however, the data points in most parts of the area are too widely spaced and the sands too variable in thickness (Figure 37) to allow regional isopachytes to be drawn with any confidence. Nevertheless, Steele has interpreted the sands to lie in eight WSW-ENE belts, each 1.5 to 3.5 km wide and separated by corridors averaging 1 km wide in which sand is thin or absent. Later information reveals that this interpretation is not entirely correct and that the ridges may be less continuous and more variable in trend than shown by Steele; some, like that at Sherburn Hill in the district to the south, may comprise two or more parallel ridges with thick sand between. There is strong evidence in the north of the district for considerable modification of the primary shape of at least three of the ridges by Late Permian submarine slumping and, more generally, of slight modification during the Zechstein transgression.

Except for a thin bioturbated unit at their top (Plate 2), which contains an infauna of Zechstein brachiopods and bivalves (Bell et al., 1979; Steele, 1981), the Yellow Sands have yielded no fossils and their age is uncertain. However, they postdate the period of reddening (Anderson and Dunham, 1953), which probably followed prolonged peneplanation, and are older than the Marl Slate, which may be early to late Kazanian or even Tatarian (i.e. Late Permian–see Smith et al., 1974) in age. A late Early Permian age seems probable (Smith, 1972) and general equivalence to the Weissliegend of Germany (Geinitz, 1861; Howse, 1890; Hodge, 1932) and of the southern North Sea (Pryor, 1971; Glennie, 1983; Glennie and Buller, 1983) is widely accepted.

Cementation

At outcrop, the Yellow Sands are generally incoherent or only weakly cemented, but up to 2 to 3 m of sand at the top and bottom of the formation are moderately to strongly cemented in many boreholes and in some surface exposures; in some of the places where it is only a few metres thick, the whole formation is cemented. Cementation is generally stronger and more complete in much of the eastern offshore area, where several almost complete borehole cores have been recovered. Calcite, partly in large poikilotopic crystals (Waugh, 1978), is the most abundant cement, forming up to 35 per cent of the rock in some samples (Browell and Kirkby, 1866; Pryor, 1971; Steele, 1981), although up to 35 per cent of dolomite is present locally. Clay minerals (chiefly kaolinite and illite) effect weak cementation in some places (Pryor, 1971) and gypsum, commonly in large poikilotopic crystals, is extensive but patchy offshore (e.g. Smith, 1984) and was probably formerly widespread elsewhere. Both calcite and gypsum locally impart a strong lustre-mottling. In many surface exposures the presence of scattered, small, calcite-cemented patches gives the sand a nodular appearance (Sedgwick, 1829). The whole formation (5 cm thick) in Offshore Borehole W12A [NZ 5400 6599] is cemented and partly replaced by pyrite.

Colour

The characteristic yellow colour (10YR in the Munsell system; Steele, 1981) of the formation is almost ubiquitous at outcrop, but is locally diversified by laminae and thin lenses of dark red or black coarse grains coated with ferric or manganese oxides (Woolacott, 1912). The sands grade to their prevailing pale grey to subordinate blue-grey colours in the subsurface, and parts of the formation are grey at one of the deepest excavations (Steele, 1981). Pale brown and grey-brown tints are not uncommon in the offshore area and occur at all levels in the formation. The yellow colour at outcrop is a relatively recent feature and results from a number of causes, including yellow staining of the surface of the grains and the widespread occurrence of extremely thin yellow grain coatings, patches and menisci of iron-stained kaolinite (Steele, 1981); goethite and amorphous iron oxides are also present, partly replacing pyrite, but no haematite. The iron oxides in the grey Yellow Sands are ferrous, and much pyrite remains (Woolacott, 1912; Smith and Francis, 1967; Steele, 1981). Smith (1970b, 1972) speculated that the Yellow Sands were at one time red, like most other Lower Permian desert deposits in Britain, and that the present grey colour resulted from reduction of the ferric oxides during or after inundation by the Zechstein Sea; Steele (1981) concurred with this speculation. A measure of support for this hypothesis arose subsequently, when partly red gypsum-cemented Yellow Sands in two offshore boreholes north-east of Sunderland were reported by Smith (1984), who noted that undoubted reduction spots and patches in the otherwise red sandstone were exactly the shade of grey found in most of the formation. Partly red Yellow Sands have since been encountered in several other offshore boreholes in the north-east of the district.

The reddening of the Yellow Sands presumably resulted from the dissolution of ferromagnesian and other minerals, and the formation of coatings of iron oxide-impregnated clay minerals on the remaining grains in the manner reported by Walker (1967, 1976) and Walker et al. (1978) in more recent desert sands. Glennie et al. (1978) and Glennie and Buller (1983) concluded that most of the Weissliegendes and the supposedly equivalent Yellow Sands were never reddened. If the Yellow Sands were once red, it follows that they may not be strict correlatives of the Weissliegendes, as suggested by Pryor (1971) and Glennie and Buller (1983), unless the Weissliegendes were also once red.

Petrography

Following early work by Lebour (1902), Burton (1911), Haselhurst (1911) and Woolacott (1912), the petrography of the Yellow Sands was studied in detail by Hodge (1932), Pryor (1971) and Steele (1981). Pryor and Steele both classified the Durham Yellow Sands as mainly subarkoses with subordinate quartz arenites and subgreywackes; monocrystalline and polycrystalline quartz is predominant (more than 85 per cent of primary grains now preserved) with smaller proportions of feldspar (average 9 per cent, mainly potassic), rock fragments and heavy minerals. Diagenetic and infiltrated dolomite and clays, and local barite are also present (Steele, 1981). The suite of heavy minerals is relatively uniformly distributed and comprises 0.1 to 0.4 per cent of the Yellow Sands at outcrop; garnet is the most abundant heavy mineral noted by Pryor (1971) in samples from West Boldon and Claxheugh Rock, with progressively smaller proportions of tourmaline, zircon and rutile. A consistent diagenetic sequence identified by Steele (1981) was 1) the formation of pigment and thin kaolinite pellicles; 2) the formation of authigenic dolomite; 3) the formation of authigenic kaolinite; 4) the reduction of iron oxides, precipitation of pyrite and development of quartz overgrowths; 5) the crystallisation of sparry calcite; and 6) the oxidation of pyrite and some dissolution of carbonate minerals. Pervasive gypsum or anhydrite were introduced early in some areas, perhaps between stages 1 and 3; they inhibited or prevented further diagenesis until their uplift-related dissolution (Smith, 1984).

Grain-size analyses of bulk samples of Yellow Sands from near Hylton Castle and Castletown were presented by Hodge (1932), from Claxheugh Rock and from two exposures south of the district by Pryor (1971), and of 20 samples covering 22 m of grey Yellow Sands from Offshore Borehole No. 18 [NZ 4807 5345] by Magraw (1975, deposited at the British Library); their results indicate that the sands at these localities are mainly medium to fine grained with 15 to 35 per cent of coarse and very coarse sand. Average sorting is described by Pryor as moderate, with a predominant broadly peaked unimodal size distribution, but the pattern of sorting is very varied and includes bimodal (Hodge, 1932) and polymodal distributions. These results are probably strongly influenced by the bulk sampling technique employed and close inspection of quarry faces generally reveals that many individual laminae are well sorted (see also Steele, 1981). Nevertheless, although many of the coarsest grains are concentrated in individual laminae and groups of laminae, coarse grains are also scattered unevenly throughout many of the fine- and medium-grained parts of the rock.

In a few places, notably in the Tynemouth and adjoining undersea areas (just north of the district) and in an area several kilometres east of Ryhope and Seaham, basal parts of the formation contain scattered to abundant rounded to subrounded coarse granules and small pebbles of vein quartz and other rock types, locally passing into pebbly or conglomeratic sandstone. Pebbles are most common where the formation is less than 1 m thick, but 1.5 m of pebbly sandstone underlie 30 m of typical Yellow Sands in D8B Borehole, 8 km east of Ryhope; this borehole is recorded by Magraw (1978) as marking the site of a sharp depression in the unconformity. Quartz and other pebbles are abundant near the base and top of the Yellow Sands at Tynemouth Cliff, and Steele (1981) records pebbles at least 1 m above the base of the formation at Cullercoats, north of Tynemouth.

Several authors have noted that almost all of the coarse grains in the Yellow Sands are well rounded and that most smaller grains are noticeably less well rounded. These unquantified observations were confirmed by Pryor (1971) who found that less than 20 per cent of the grains in samples of sand from Sherburn Hill Sand Pit (south of the district) and from Claxheugh Rock are well rounded, with much of the remainder being rounded (25 to 35 per cent) and subrounded (28 to 33 per cent); subangular (14 to 21 per cent) and angular grains arc generally amongst the finest present.

The surface morphology of grains in the coarse sand fraction was examined by scanning electron microscopy by Pryor (1971) and by Krinsley and Smith (1981), with partly contrasting results. Both works revealed that many of the well-rounded coarse grains bear authigenic quartz overgrowths and had been pitted by chemical etching during the formation of diagenetic and cement minerals. Pryor commented that the entire surface of 20 grains examined by him were covered with diagenetic features including overgrowths; it is these features, he wrote, that give the coase grains their much-quoted frosting. By contrast, Krinslcy and Smith, with a sample of more than 70 coarse grains, reported extensive preservation of primary grain surface features including generally closely spaced cleavage plates. Authigenic feldspar overgrowths were reported by Waugh (1978). According to Steele (1981), authigenic overgrowths generally constitute 1 per cent or less of the rock.

Sedimentary structures

Apart from the widespread presence of up to a few metres of planar-laminated sand at the bottom and top of the formation, the most striking feature of all large surface exposures of the Yellow Sands is their distinctive style of large-scale cross-lamination. This varies greatly with the aspect of the faces, but in three dimensions it is identified as tangential trough cross-stratification (Pryor, 1971); the troughs are up to 6 m deep and 50 m across. Studies by Pryor (1971), Steele (1981) and Yardley (1984) revealed that set thicknesses between major bedding planes generally range from 0.25 to 3.0 m but that many sets locally exceed 3 m; it is clear that sets now preserved represent only the truncated remnants of beds that were formerly thicker. Steele (1981) quotes a maximum preserved set thickness of 11 m. Foresets are generally concave upwards, though exceptions have been noted, and dips (corrected for tilt) range up to about 37^°, but average about 18° (Pryor, 1971); steeper average dips undoubtedly prevailed in higher parts of the sets before truncation. Analyses of the trend of foresets by Opdyke (1961), Pryor (1971), Steele (1981, 1983), Glennie (1982, 1983) and Yardley (1984) in Yellow Sands in the Sunderland and adjoining districts are in general agreement that overall sediment transport was strongly bimodal, with clearly defined peaks towards the south and west-north-west (see (Figure 36)); a vector mean of 235^° was calculated by Steele (1981), parallel with the trend of the ridges. Yardley (1984) noted that the main set surfaces are erosional along most of their length, with trough axes oriented approximately NE–SW. Clemmensen (personal communication, 1990) supported this observation and inferred that the trough cross-bedding resulted from the infilling of migrating front-of-dune scour pits by seasonal winds alternating between north and south-east.

The 10 to 20 m-thick cemented Yellow Sands in two boreholes (W14A and W15) off Whitley Bay (Tynemouth district) and two boreholes (W7 and W11) east of South Shields were unusual in being mostly roughly horizontally laminated and in containing an atypically high proportion of coarse and very coarse, subspherical, frosted grains. It is possible that similar sand-sheet sedimentary features may have been present in other boreholes where the Yellow Sands were mainly incoherent and hence not recovered as cores.

Variants

The Yellow Sands generally comprise only sandstone, but Magraw (1978) recorded two thin beds of brown mudstone in Offshore Borehole No. D5 (7 km north-east of Seaham), and several laminae and thin beds of dark grey mudstone were proved in the lowest 2 m of the formation in Offshore Borehole No. W7 (7 km east of South Shields) and E4 (11 km east of Seaham Harbour). Small-scale sedimentary structures in sandstones associated with the mudstones in these boreholes, and also in the basal Yellow Sands of Offshore Borehole W8 (12 km east of South Shields), are consistent with subaqueous deposition or early postdepositional modification by water, and include probable liquefaction features.

Conditions of deposition

Despite the interpretation of the Yellow Sands by Pryor (1971) as a shallow-submarine sand wave complex, there has been general agreement since the perceptive observations of Daglish and Forster (1864) that they are an aeohart desert sand deposit; this view is accepted here. The recognition that the formation is disposed in ridges aligned generally parallel with the inferred mean direction of sand transport led Smith and Francis (1967) to deduce that the ridges were the partly planed-off remnants of longitudinal or seif dunes; this interpretation is still favoured by Glennie (1970, 1982) and Glennie and Buller (1983) as being consistent with the cross-stratification data. Steele (1981, 1983) and Yardley (1984), however, present strong evidence that the dunes were probably of a transverse or barchanoid type; they interpret the ridges as complex longitudinal draa built up of the superimposed remains of migrant transverse dunes. Steele considered the discontinuous basal unit of planar-laminated sand to be the product of a sand sheet formed during the initiation of the draa, and the more extensive upper unit of planar-laminated sand to be the product of a sand sheet formed during a terminal phase of draa stabilisation. The bioturbated to structureless uppermost few centimetres of the formation are thought to have been modified and partially reworked during or soon after the Zechstein transgression; Steele (1981) and Glennie and Buller (1983) attributed local deformation of sand near the top of the formation to liquefaction at the time of the transgression.

The thin clay beds and water-laid sandstones reported by Magraw (1978), and also found in Offshore Boreholes W7 and W8, were presumably formed in limited bodies of standing water, perhaps in an interdune sabkha following ephemeral rain; the rare pebbly layers within the Yellow Sands must indicate at least some episodes of water transport and perhaps deflation. Pebbly sands on the unconformity probably represent accumulations of litter and entrapped sand on the old land surface and the pebbly sandstone recorded in D8 Borehole may be a relic of water-laid wadi-fill. Steeply inclined, curved, thin dyke-like sheets of brown clay, which extend downward from the Marl Slate in a number of exposures from Sunderland northwards, are almost certainly Late Permian in age and are probably related to an episode of massive dislocation late during deposition of the Raisby Formation.

The Yellow Sands provide a major aquifer in the district, and have been tapped by many wells (some with side galleries) and boreholes. A complex of shallow boreholes and exceptionally long galleries beneath Claxheugh Rock and Ford Quarry at one time tapped Yellow Sands water to supply a large paper mill at Ford, Sunderland. According to Browell and Kirkby (1866), one cubic foot of Yellow Sands can hold 6 to 12.5 pounds (2.7 to 5.7 litres) of water, and Potter (quoted by Daglish and Forster, 1864) recorded that 10 000 gallons (45 4060 litres) per minute needed to be pumped from Murton Colliery shafts in order to allow sinking to proceed. The formation is also a major source of building sand, although it is currently being extracted from only three quarries in the district.

Upper Permian

Marl Slate Formation (part of EZ1a)

The Marl Slate, well known since the works of Sedgwick (1829) and Hutton (1831) as the equivalent of the Kupferschiefer of Germany, is the first widespread marine deposit of the Zechstein Sea. Because of its distinctive and abundant biota and its unusual content of metallic trace elements, the formation has been studied in greater detail than any other part of the English Zechstein sequence and it has been discussed by, amongst others, King (1850), Howse and Kirkby (1863), Kirkby (1866), Lebour (1902), Woolacott (1912, 1919b), Trechmann (1914 et seq.), Westoll (1941a, b, 1943), Deans (1950), Stoneley (1958), Dunham (1960, 1961), Love (1962), Hirst and Dunham (1963), Smith and Francis (1967), Smith (1970b, 1980b), Turner et al. (1978), Bell et al. (1979), Vaughan and Turner (1980), Evans (1982), Pettigrew (1980, 1985), Magaritz and Turner (1982) and Sweeney et al. (1987).

Distribution and general characteristics

The Marl Slate of the district and adjoining parts of the Tynemouth district to the north (Land, 1974) and Durham district (Smith and Francis, 1967) to the south is exposed (or has been recorded) in a score of quarries and natural exposures along the outcrop, and in more than 60 boreholes and shafts farther east (Figure 37). Although these provings are unevenly distributed and have not everywhere been fully or consistently described, they nevertheless show that the formation most commonly comprises a lower unit of dark grey to black, bituminous (sapropelic), extremely finely laminated, silty, argillaceous dolomite or dolomitic shale overlain gradationally by a slightly thicker unit of paler grey, less finely laminated, slightly bituminous, silty dolomite. Both units locally contain thin beds of microcrystalline dolomite, especially in western areas, and one or (rarely) more thin beds of dark grey to black, laminated or unlaminated, argillaceous, bituminous dolomite are not uncommon in the upper unit in the east. Turner et al. (1978) have shown that it may be possible to correlate parts of the Marl Slate between closely spaced boreholes on the basis of such beds and to identify two or more cyclic sequences, but the higher bed of bituminous rock has not been recorded in most boreholes. In a few boreholes, the lowest part of the formation is of the paler grey variety, with dark grey sapropelic rock first appearing up to 0.6 m above the base.

At outcrop, the Marl Slate is buff or paler grey than in unweathered borehole cores, and thin beds of brown plastic clay (some rich in fish remains) are present locally; grains, laminae and thin beds of reworked Yellow Sands have been widely reported in the basal few centimetres of the formation. Evidence of bioturbation in the Marl Slate is almost unknown, thus explaining the preservation of the lamination, but unmistakable evidence of penecontemporaneous minor submarine slumping has been noted in a few places, and the whole formation has been removed in parts of the Boldon and Sunderland areas by massive postdepositional events interpreted as submarine slides (Smith, 1970c). Probable minor erosion surfaces were noted in the upper part of the formation (one immediately below a black layer) in boreholes 10 km east of Marsden Bay and 7 km east of Whitburn, and the formation is absent in a borehole 8 km east of Ryhope and very thin in several others. The Marl Slate produces a strong and readily identified signature on wireline gamma-ray logs, with a sharp, single, symmetrical peak corresponding approximately with the middle of the formation.

The base of the Marl Slate is generally sharp and planar where it rests on Yellow Sands, its dip being influenced more by the configuration of the top of the latter than by tectonic deformation; where it rests directly on Coal Measures or Basal Permian Breccia, the base is commonly minutely uneven. The top of the formation is almost everywhere knife-sharp and planar, but it is slightly gradational in a few provings and locally grades by alternation over up to 1.5 m of strata. The presence of such gradational sequences led Kirkby (1866) and Magraw (1975) to recognise a thin group of passage beds between the Marl Slate and the Raisby Formation, but this is not necessary if, as is now customary, the top of the uppermost laminated bed is taken as the contact.

The Marl Slate in the Sunderland district is up to 2.6 m thick but is generally 0.8 to 1.2m (Figure 37). As Wilson (1881) noted, its considerable thickness variation is partly related to the relief of the buried topography and it is noticeable that most of the thickest provings are in places where the underlying Yellow Sands are less than 5 m thick (see also Turner et al., 1978). This broad relationship is, moreover, substantiated at a number of surface exposures where the dark grey lower unit of the formation (but not the paler grey upper unit) is seen to thin and locally die out against the flanks or crests of buried sand ridges and eminences on the unconformity. Regional comparison of the thicknesses of the Marl Slate and Yellow Sands shows, however, that this inverse thickness relationship is by no means always valid (Figure 37) and that it cannot be used as a basis for drawing Marl Slate isopachytes. In some boreholes where the formation is thin, only the upper unit is present, perhaps indicating overlap as seen in some surface exposures, but thin Marl Slate in some other boreholes is mainly or entirely of the dark grey to black type typical of the lower unit.

Petrography

Except for local thin beds of microcrystalline dolomite and limestone, the Marl Slate is a finely but discontinuously laminated rock of very varied composition. Bulk analyses (Browell and Kirkby, 1866; Trechmann, 1914; Hirst and Dunham, 1963; Smith and Francis, 1967) of the dark grey to black variety indicate contents of dolomite and/or calcite of 30 to 50 per cent, sand and clay minerals 10 to 40 per cent (generally greatest in the lower part), organic carbon 5 to 34 per cent and pyrite 2 to 5 per cent. The grey variety of Marl Slate generally comprises 60 to 90 per cent of dolomite and/or calcite and proportionately smaller amounts of sand, clay, organic carbon and pyrite than in the darker-coloured variety. Hirst and Dunham (1963), in discussing Marl Slate from boreholes south of the district, noted that the mutual proportions of dolomite and calcite vary widely and that, although dolomite generally greatly exceeds calcite, the latter predominates in a few places and both minerals are closely juxtaposed in some samples; these findings apply equally to the Marl Slate of this district. The carbonate component of the Marl Slate from boreholes off Seaharn was investigated in detail by Turner et al. (1978), who show that most of the dolomite is ferroan and calcium-rich; the close association of dolomite and calcite in adjoining laminae has been quoted by Sweeney et al. (1987) as evidence that both minerals are primary. Clay minerals were calculated to form a moderate proportion of the two samples analysed by Hirst and Dunham (1963) and to comprise kaolinite and illite with a little chlorite; their common close association with small concentrations of quartz grains suggests that they are probably mainly detrital.

The organic carbon in the Marl Slate is concentrated in discontinuous wavy films commonly 5 to 80 microns thick (Hirst and Dunham, 1963; Turner et al., 1978), which are separated by carbonate-rich or clay-rich lenses and also commonly by thin layers of quartz silt or very fine-grained sand. The organic-rich layers are very closely spaced in the dark grey to black variety of the Marl Slate, but are farther apart in the paler grey variety, in which the rock is characteristically formed of alternate laminae of silt-grade and mud-grade dolomite. There is general agreement (following Potonie, 1938) that the organic-rich layers are sapropelic. Estimates of the duration of Marl Slate deposition, based on the number of the laminae and on the premise that they are annual, range from 17 000 years (Oelsner, 1959) to 30 000 years (Strakhov, 1962). Gibbons (1978, 1983) showed that the organic carbon (kerogen) in the Marl Slate is mainly amorphous and capable of yielding up to 100 kg of hydrocarbons per tonne of rock upon pyrolysis; in north-east England the kerogen is immature.

Biota

The earliest biota of the Zechstein Sea comprises a small invertebrate assemblage that accompanied the transgression and survived (perhaps briefly) until the basin filled, and the much more varied fauna and flora of the Marl Slate itself.

Only Lingula has been recorded from the transgressive assemblage in the district; it has been found in the uppermost few centimetres of the reworked Yellow Sands at Hetton Downs Quarry and littering the uppermost surface of the Yellow Sands at Field House Sand Hole [NZ 355 506] (Steele, 1981). Bakevellia and Permaphorus occupy a similar stratigraphical position in the district to the south (Pettigrew, 1985).

The megascopic plant and animal fossils of the Marl Slate itself have been described and illustrated in many works (e.g. Sedgwick, 1829; Howse, 1848, 1858; King, 1848, 1850; Kirkby, 1866; Westoll, 1941a, b; Stoneley, 1958; Bell et al., 1979; Evans, 1982; Schweitzer, 1986) and have been summarised by Pettigrew (1980, 1985). The fossil fish are particularly well known and well preserved; isolated scales are common at most exposures of the Marl Slate and whole fossil fish are not uncommon at a few localities such as Cullercoats (in the North Shields district), Downhill Quarry [NZ 348 601], Houghton Quarry [NZ 341 506] and Hetton Downs Quarry [NZ 358 484]. Pettigrew (1985) recognised terrestrial, nearshore and offshore fossil assemblages, the terrestrial assemblages having been washed into the Marl Slate sea from land thought to have lain perhaps 15 to 30 km west of the present outcrop. Pettigrew's work relates to a much wider area than that described here, and only a small number of the fossils mentioned below have been found in the district.

The terrestrial assemblages recognised by Pettigrew (1985) include both plant and animal remains and have been found chiefly to the south of the district. The plant fossils are mainly the conifers Pseudovoltzia and Ullmannia, and include leaves and cones in addition to branches and stems up to 0.25 m in diameter; leaves of Callipteris (a probable ptefidosperm), Neocalamites (a horsetail), Sphenobaiera (a ginkgophyte), Pseudoctenis and Taeniopteris (both probable cycads) have also been recorded. All the plants, which have been reviewed by Stoneley (1958) and Schweitzer (1986), are adapted for life in arid conditions, except Neocalamites. Animal fossils in the terrestrial assemblage comprise the incomplete remains of two small reptiles, Protarosaurus and Coelurosauravus; the latter, found in 1978 near the base of the Marl Slate at Hetton Downs Quarry [NZ 358 484], Hetton-le-Hole (Pettigrew, 1979), is thought to have been capable of gliding flight and has been described in detail by Evans (1982).

The nearshore assemblage of Pettigrew (1985) comprises a restricted community of benthic organisms, together with a number of species of fish. The benthic forms, collected from Middridge and Eldon in the Barnard Castle district, include foraminifers, bryozoans, brachiopods, bivalves and ostracods, which are thought either to have lived in situ during brief periods when conditions became suitable or, more likely, to have been transported from marginal environments farther west. The fish are both bottom-living and free-swimming forms, and range from 0.1 to 0.8 m in length; they include the palaeoniscids Dorypterus (0.1 m) and Platysomus (0.3 m), the crossopterygiari fish Coelacanthus (0.6 m) , the ray-like Janassa (0.6 m) and the shark Wodnika (0.8 m).

The offshore assemblage recognised by Pettigrew (1985) comprises free-swimming fish that inhabited the oxygenated upper waters of the Marl Slate sea and settled to the bottom on death; they include the small palaeoniscid fish Palaeoniscum (0.2 m), which was much the most common form, and the larger carnivorous palaeoniscids Acrolepis (0.5 m) and Pygopterus (0.6 m). Acentrophorus (0.1 m) is of particular interest because it is the earliest known holostean fish.

Mineralisation

The Marl Slate has long been known for its abnormally high content of metallic ore minerals, principally pyrite, galena and blende, though the proportions of these in north-east England nowhere approach ore-grade. The mineralisation has been studied by Westoll (1943), Deans (1950), Dunham (1960, 1961), Hirst and Dunham (1963), Turner et al. (1978) and Sweeney et al. (1987), who each draw on the accumulated literature on the Kupferschiefer and other bituminous black shales. The metallic ores are mainly sulphides and are disseminated, replacive and void-filling. Pyrite is the main disseminated mineral and is mainly in framboidal, spherical aggregates 5 to 15 (typically 8) microns across (Deans, 1950; Love, 1962; Hirst and Dunham, 1963); these authors and Sweeney et al. (1987) ascribe the pyrite to bacterial reduction of the sulphate in sea water in the presence of organic matter and iron, and note that this pyrite is strongly concentrated in the organic-rich laminae. Hirst and Dunham (1963) found no evidence that any of the pyrite is a primary precipitate, believing it to be a product of diagenesis soon after burial in anoxic bottom sediments, but Sweeney et al. (1987) quote isotopic evidence in support of their alternative view that some of the pyrite was formed by reaction within the water column. Small crystals of pyrite, some idiomorphic, are widely disseminated in the Marl Slate.

Minerals found in voids in the Marl Slate include pyrite, blende, galena, chalcopyrite, barite, calcite, dolomite and anhydrite. These occur in narrow veins and as films on bedding planes and more widely in thin small lenses elongated parallel with the lamination. Hirst and Dunham (1963) record that the main mode of occurrence of blende, galena and chalcopyrite is in lenses, which they regard as diagenetic replacements of earlier pyrite. Westoll (1943) noted that chalcopyrite had replaced pyrite very early during the preservation of parts of fish fossils in the Marl Slate of south Durham. Turner et al. (1978) and Vaughan and Turner (1980) interpret small lenses of authigenic chalcedony, pyrite and dolomite as diagenetic replacements of early (perhaps primary) calcium sulphate.

In addition to its megascopic content of lead, zinc and copper sulphides and other minerals, the Marl Slate contains much higher proportions of other metallic trace elements than most black shales (Deans, 1950), although total amounts are less than those in the more highly mineralised parts of the Kupferschiefer; cobalt, manganese, molybdenum, nickel and vanadium are among the main trace elements present. Hirst and Dunham (1963) showed that the vertical distribution of some of these trace elements in the Marl Slate in several boreholes in south Durham is so distinctive that the various parts of the formation may be correlated on this basis for distances of at least 12 km. These authors also made the important observation that basal layers of the succeeding Raisby Formation do not carry comparable concentrations of metallic trace elements where the Marl Slate is absent; this probably implies that the Marl Slate does not pass laterally into normal marine carbonate rocks but is overlapped against sea-floor eminences and basin margins.

Magnetisation

The Marl Slate of eastern Durham has a weak natural remanent magnetisation, thought to be carried by detrital iron oxides or to be early diagenetic in origin (Turner and Vaughan, 1977; Vaughan and Turner, 1980); a lower normally polarised but varied unit is sharply overlain by a thinner unit of reversed polarity.

Conditions of deposition

The unusual lithology and biota of the Marl Slate and Kupferschiefer have prompted much speculation on its mode of formation, leading to the evolution of a shallow water lagoonal theory and an alternative deeper water barred stratified basin theory. The two theories are not, of course, mutually exclusive, for, even with a rapid initial flooding of the Zechstein Basin (Smith, 1970a, 1979), a wide range of water depths and sea-marginal embayments and lagoons must have existed. The prevailing view now is that the normal Marl Slate/Kupferschiefer is mainly a deposit of a barred basin perhaps 200 to 300 m deep over wide areas, but that over eminences and around the shelving margins it passes into a much more varied deposit. Water depths were particularly varied in eastern County Durham where local sea-floor relief of up to at least 67 m resulted from the inundation of pre-existing sand ridges. There is no evidence in the district of an approach to the contemporary shoreline, even where the formation is absent over the crests of former eminences.

The possibility that the lower parts of the Zechstein Sea might have been stagnant (anoxic), like those of the Black Sea, during deposition of the Marl Slate/Kupferschiefer was suggested by Pompeckj (1914, 1920) and has been accepted by most later workers on the Marl Slate including Deans (1950), Dunham (1961, 1962) Hirst and Dunham (1963), Smith (1970b, 1980a, b) Bell et al. (1979), Pettigrew (1980, 1985), Sweeney et al. (1987) and Turner and Magaritz (1986). In these works, especially from Dunham (1961) onwards, it is envisaged that an upper layer of oxygenated water supported a varied and abundant nektonic and planktonic biota, and overlay a generally thicker quiet lower layer of dark, cold, stagnant and perhaps hypersaline water in which little or no life other than bacteria could survive and in which organisms falling or being transported from the oxygenated upper layer would escape scavenging and rapid putrefaction. The two layers are thought to have met at a generally plane interface or transitional zone variously called a chemocline, halocline, pycnocline, thermocline or oxic/anoxic boundary; the level of this was governed by the depth of the basin-mouth sill and modified by the effect of currents, waves and organic productivity.

In his study of modern anoxic basins, Byers (1977) found that an oxic (or aerobic) layer about 50 m thick commonly overlies a dysaerobic layer about 100 m thick which in turn lies on the anoxic (anaerobic) stagnant bottom waters. He suggested that the top of the stagnant layer would rise and fall from time to time (perhaps seasonally) with variations in climate and basin hydrology, a mechanism previously proposed for the Paradox Basin of North America by Hite (1966), causing the area affected by anoxic conditions to expand and contract. Such fluctuation was invoked by Smith (1980b) to explain the interdigitation of shelly beds and 'normal' euxinic Marl Slate noted mainly in south Durham by Sedgwick (1829), Smith and Francis (1967), Mills and Hull (1976) and Bell et al. (1979), and more recently by Sweeney et al. (1987) to account for many of the unusual petrographical and diagenetic features of the Marl Slate. It is, however, probably not the cause of the almost ubiquitous very fine lamination of most basinal Marl Slate/Kupferschiefer, which probably results from seasonal (?annual) cycles of phytoplankton productivity and death in the oxic upper waters.

Analysis of data on anoxic basins led Demaison and Moore (1980) to conclude that silled basins should be prone to anoxia at times of worldwide transgression; the early stagnation of the Zechstein Sea supports this conclusion. This stagnation has been explained as an example of autoeutrophication brought about by the combination of limited circulation, the high productivity of surface waters and the rapid consumption of dissolved oxygen in lower waters following phytoplankton blooms (Brongersma Sanders, 1966, 1969, 1972; Smith, 1970b; Pettigrew, 1980). Such blooms may have seasonally raised the level of the top of the anoxic layer, perhaps leading to mass mortality of fish on occasions when the oxic layer became thin or was eliminated (see also Dunham, 1961).

In addition to the inferences of basin stagnation advanced on petrographical and biotic grounds, strong supporting isotopic data have been published by Magaritz et al. (1981), Magaritz and Turner (1982) and Turner and Magaritz (1986). These authors also suggest that there may have been periodic influxes of fresh water into the Zechstein Sea and that these events may have been associated with enhanced introductions of some of the metallic ions.

The source of the metallic trace elements remains controversial, but there is general agreement that the iron sulphide was formed either contemporaneously or very soon after deposition and that some of the other metallic sulphides were formed before the deposit was fully lithified. The suggestion by Brongersma-Sanders (1966, 1968) that the metallic ions were introduced into the Zechstein Sea basin in normal sea water and were concentrated by circulation cells and by phytoplankton seems inadequate to explain the abnormal levels observed and has found little favour. In contrast, the general coincidence of the most strongly mineralised areas of the Kupferschiefer with areas of known basement mineralisation has led to wide acceptance of the suggestion (e.g. Dunham, 1961) that the ions were introduced into the Zechstein Sea by submarine hydrothermal springs welling up from long-established fractures through already mineralised strata.

Raisby Formation (EZ 1 aCa )

This formation (formerly the Lower Magnesian Limestone) is the first major carbonate rock unit of the English Zechstein sequence. It forms a striking west-facing escarpment from West Boldon southwards to beyond Houghton-le-Spring and is dominated in the north by the Earl of Durham's Monument at Penshaw. The formation was recognised as a separate stratigraphical unit by Sedgwick (1829) and its general character and fauna have been reviewed by many later authors including Hutton (1831), King (1848, 1850), Howse (1848, 1858), Kirkby (1864a, 1866), Lebour (1902), Woolacott (1912), Trechmann (1914, 1925, 1945, 1954) and Smith (1970b, 1980a). There is a comprehensive review of the sedimentology, petrography and geochemistry of the formation by Lee (1990). The type locality is Raisby Quarries near Cornforth, in the district to the south (Smith et al., 1986). Rocks of the Raisby Formation were formerly widely worked for building stone and for agricultural and industrial purposes, but only two working quarries in the district survive, mainly supplying aggregate.

The base of the formation is taken at the top of the uppermost finely laminated dolomite of the Marl Slate and its top is taken at the successive incoming, from west to east, of oolitic, reef and oncolitic/stromatolitic dolomite.

Distribution and general characteristics

The ramp-like Raisby Formation has only a narrow outcrop (Figure 34), (Figure 38), which is mainly drift-free or only thinly drift-covered, but it extends beneath younger strata to the eastern margin of the district. It is (or has been) seen in more than 30 quarries, cuttings and natural exposures spaced unevenly along the outcrop, and has been penetrated by many shafts and boreholes farther east. Although most of the latter have been indifferently recorded, the data show that the formation is thickest (about 50 m) in the land area in the south of the district (Figure 38). It appears to maintain this thickness as far north as the southern outskirts of Sunderland, but then thins sharply to the north where it is locally absent. Thicknesses of 16 to 24 m are common in the southern and central offshore areas but, as on land, the formation is more variable in thickness and generally thinner in the northern offshore area. Most of the thickness variation in the south of the district is probably primary and related to the relief of the substrate and to distance from the Late Permian shoreline. In the north, these influences also control thickness maxima, but there is evidence here that the upper part and, in places, all of the formation was removed by postdepositional events.

The Raisby Formation is almost everywhere composed of dense, fine-grained, slightly calcitic dolomite, but in a few places the lower part of the formation contains substantial thicknesses of limestone (e.g. Woolacott, 1919a). Chemical analyses commonly show CaO contents of 54–58 per cent and MgO contents of 39–45 per cent (mainly 1–2 per cent more CaO than required by the dolomite ratio), with up to 3 per cent of iron and aluminium oxides and varied but generally low (<2 per cent) silica contents. An organic carbon content of about 0.5 per cent has been noted by Jones and Hirst (1972) in the district to the south. Small amounts of manganese oxides occur as black speckles and dendrites.

At outcrop, the Raisby Formation is characterised by generally parallel beds commonly 0.05 to 0.25 m thick; in a few places such beds make up the whole of the formation. Generally, however, it is more varied and in many places along the escarpment it is possible to recognise a broad tripartite sequence comprising a basal few metres of relatively uniform, evenly bedded, buff, mainly silt-grade dolomite, a thicker and much more varied middle unit of buff to brown mud- and silt-grade dolomite, and an upper unit of mainly cream and buff silt-grade dolomite. The contacts between these units are gradational and probably diachronous. The lowest unit is widely recognisable in boreholes offshore, but the distinction between the middle and upper units is commonly blurred or unrecognisable. Much of the formation is grey in the undersea area, suggesting that the prevailing buff and brown colours at outcrop may result from weathering. Many beds are sparingly fossiliferous and cavities after gypsum and anhydrite are ubiquitous.

Most beds of the Raisby Formation superficially appear to lack notable sedimentary structures, but inspection of etched or polished surfaces commonly reveals thin colour layering and widespread small-scale ripple-like lenticles and other minor sedimentary structures (Plate 3) at all levels in the formation. This suggests winnowing of carbonate grains during sedimentation, but undoubted cross-lamination is uncommon and the flaser effect may be partly or wholly of diagenetic (pressure-dissolutional) origin. Minor sedimentary discontinuities and possible hardgrounds have been noted at various levels in the formation by Lee (1990), indicating phases of nondeposition and perhaps local erosion. Evidence of widespread sediment instability and downslope movement has been described from many levels but especially from near the base and top (Smith, 1970c; Lee, 1990). Decimetre-scale rhythmic bedding is present in a few exposures and boreholes, and evidence of slight to intense bioturbation is found in places but does not appear to be ubiquitous. Subconcordant stylolites abound at all levels and are especially common in the thick middle unit; in places the stylolites are interpenetrant, locally creating stylolite breccias and indicating considerable volume loss.

A feature of many exposures is the presence of upwards-concave joints, some of which pass downwards into bedding planes; at some exposures they intersect, and blocks of strata between them are tilted. Upwards the joints cease abruptly at the base of the youngest disturbed stratum of the formation (Plate 4).

Lowest unit Rocks of the lowest unit are commonly 6 to 8 m thick and are in beds 0.03 to 0.25 m thick separated by flat to slightly stylolitic, black-coated bedding planes; thin to flaggy bedding predominates at most exposures. The rock is generally fairly hard and compact and is typical where exposed in most of Penshaw Hill Quarry [NZ 336 544] and in Frenchman's Bay [NZ 389 662], South Shields. Weak lamination and small-scale flaser structure occur locally and vague graded bedding in units 2 to 10 cm thick is present in part of this unit at Frenchman's Bay and in some offshore boreholes. The rocks are generally composed of equant mosaics of interlocking to idiomorphic dolomite, commonly in the range 0.03 to 0.06 mm, with small amounts of interstitial and patch-forming calcite microspar and a scattering of small subangular detrital quartz grains. Exceptionally, however, substantial thicknesses of calcite mudstone/wackestone were proved in Offshore Boreholes E3A (11 km east of Seaham) and W12B (16 km east of Marsden Bay), and also in boreholes near Offerton. In W12B Borehole, the transition from dolomite rock to limestone was in the middle of a bed and was not apparent macroscopically. Thinly and unevenly bedded calcite mudstones and finely crystalline dolomite are present near the middle of the unit in Penshaw Hill Quarry (Lee, 1990) and in nearby Dawson's Plantation Quarry [NZ 338 548].

Laminae and thin beds of dark grey to black, very fine-grained, argillaceous dolomite and dolomitic siliciclastic mudstone are common in this lowest unit, but cavities after calcium sulphate minerals are generally small and less common than in the other two units. Evidence of bioturbation is present only locally. A well-developed pseudofenestral fabric was noted in this unit or low in the middle unit in a borehole [NZ 388 571] at Beach Street, Sunderland. Another variant was formerly seen at Newbottle [NZ 337 514], where lowest beds are absent over an upstanding mass of Carboniferous sandstone and the onlapping dolomite of the Raisby Formation contains an abundant littoral fauna.

Rocks of the lowest unit in the west of the district contain widespread evidence of submarine slumping and sliding (Smith, 1970c; Lee, 1990), which is most common a few metres above the base. The disturbed strata range from sand-grade aggregates of microfossils to coarse imbricated breccias ('debris-flows') containing slabs up to 2 m long of fine-grained dolomite (Plate 5); they generally are overlain by a graded or rippled bed of mud-grade to sand-grade dolomite. Evidence of downslope sliding of whole groups of beds, but with only slight attendant dislocation and brecciation, is present at High Moorsley Quarry [NZ 333 455], just south of the district. The thin, graded beds at about this level at Frenchman's Bay and elsewhere may be distal equivalents of the disturbed beds in the west, but contorted strata and slump breccias are uncommon in this unit in the east. Imbrication patterns and contortion in the disturbed beds indicate transport generally east-north-eastwards, towards the basin. Distorted but otherwise well-preserved shells and microfossils are far more abundant in the disturbed strata than in the undisturbed beds, presumably because of introduction from more hospitable upslope environments and rapid burial. In places, as at Field House Sand Hole [NZ 354 586], Houghton-le-Spring, even the Marl Slate and the top of the Yellow Sands were involved in mass downslope movement, which produced hybrid rocks of fragmented dolomite and quartz sand; this occurrence was probably part of a regional pattern of submarine sediment instability. In contrast, thin (0.03 to 0.30 m) beds of medium- to coarse-grained sandstone, reported from low in the Raisby Formation at Tynemouth Cliff (Sedgwick, 1829; Burton, 1911) and in a number of offshore boreholes east of Seaton Sluice and Whitley Bay (1:50 000 Sheet 15), may be of more local origin, perhaps related to instability induced by the slopes of underlying aeolian dunes (Magraw, 1978, pp.166 and pl. 4, figs. 7–8); the sandstones offshore rest on strongly scoured surfaces (Plate 6) and contain little carbonate debris.

Hollow-cored tepee-like structures up to 15 m long and 0.5 m high and with dips of up to 45° occur in flaggy beds high in the unit on shore platforms [NZ 3888 6631] about 140 m north of Frenchman's Bay; they may be recent, resulting from stress release following unroofing.

Middle unit Rocks of this unit, commonly about 25 to 30 m thick in the west and south, are exposed in many quarries along the escarpment and in Houghton Cut [NZ 344 505]; they are typical in the main part of Houghton Quarry [NZ 342 506]. The dolomite is mainly in beds ranging from 0.03 to 0.30 m thick, separated by nodular, slightly stylolitic bedding planes coated with black clayey residues. Although individual beds are of roughly constant thickness for considerable distances, and recognisable sequences of beds may be traced between adjoining quarries and other exposures, there is nevertheless a striking local variability in bed thickness caused by sharp lateral passages from very thin-bedded and flaggy sequences into rocks with beds 0.8 m or more thick and lacking any obvious thinner bedding traces. Laminae and thin beds of dark grey, fine-grained, argillaceous dolomite and dolomitic siliciclastic mudstone occur sparingly in the lower part of the unit, but they die out upwards and traces of parallel thin lamination and small-scale flaser structure are widespread. Alternations of unlaminated and finely laminated, fine-grained dolomite (Plate 7) are a feature of several metres of strata low in this unit in the Seaham Borehole [NZ 4258 5031]; they closely resemble mid-slope strata of the Concretionary Limestone Formation (see p.93). A diffuse, nodular, thin layering characterises much of the rock in boreholes, especially in the east. Many sections in this unit contain one or more beds with a confused internal structure, probably related to downslope movement or soft-sediment deformation, and a few possible dewatering structures have been noted. Burrows are locally profuse and tracks and trails are abundant on some bedding surfaces at a number of exposures, such as Trow Point (Plate 8).

With few exceptions, rocks of the middle unit are of brittle, slightly calcitic dolomite, and are composed of dense mosaics of mainly aphanitic to microcrystalline dolomite with a little interstitial calcite microspar. Detrital silt-grade quartz grains are scattered unevenly throughout but are nowhere abundant, and small proportions of clay minerals are widespread. The appearance of the rock in the field is more varied than in thin section, ranging from porcellanous to finely saccharoidal. Pronounced mottling in shades of grey, buff and brown is a feature of this unit in many exposures in the south and west, but it is less common in the north and east; it has a wide range of patterns and is generally similar to that noted in this formation farther south (Smith and Francis, 1967) and in equivalent slope strata in Yorkshire (Smith, 1974b; Fuzesy, 1980; Kaldi, 1980; Harwood, 1986). Fuzesy and Kaldi illustrated the main types of mottling and interpreted the most common type (concordant strings of compressed, unlayered, pale-coloured dolomite nodules in a darker, slightly argillaceous, layered matrix) as the product of early diagenetic segregation and cementation followed by differential compaction and localised pressure-dissolution. Kaldi (1980) recognised a second major type of mottling produced by the redeposition of the first main type by downslope mass movement (slumps and debris-flows), but this type has yet to be recognised in the district. None of the mottling has been linked to burrow patterns. The mottled rocks commonly break with a subconchoidal 'chunky' or hackly fracture, well seen in Houghton Quarry [NZ 342 506], and are widely associated with slight to advanced autobrecciation; a close network of narrow calcite veins is present in the most severely autobrecciated rocks, in which only vestigial traces of primary bedding are preserved. Amoeboid and elongate cavities after anhydrite are abundant at most surface exposures of these strata, and similarly shaped anhydrite and gypsum patches form up to 5 per cent of much of the formation in the deep subsurface.

Highest unit This unit, generally only a few metres thick in the southern half of the district and absent in most of the north, is the least well-known part of the Raisby Formation. It has been cored in fewer boreholes than the underlying units and exposures are mainly restricted to the area around Herrington and Houghton-le-Spring; other possible exposures are at Humbledon Hill Quarry [NZ 3814 5528] , Gilleylaw Plantation Quarry [NZ 3755 5370] and Newport Dene [NZ 3852 5410], but these rocks more probably belong to the succeeding Ford Formation.

Rocks of this unit are almost exclusively dolomite and slightly calcitic dolomite and are generally cream or pale buff at outcrop, grading to pale grey and grey at depth in the east; brown varieties occur locally. The dolomite has not been studied petrographically during the resurvey, but in hand specimen appears mainly to be porous and finely saccharoidal. Cavities after anhydrite are abundant in the west, and anhydrite and gypsum commonly form up to 10 per cent of the unit in the east. Beds are more regular than in the lower units, and are commonly 0.20 to 0.35 m thick. Mottling is uncommon, and stylolites are less common than in the middle unit; however, traces of weak, small-scale flaser structure and possible primary sedimentary lamination occur throughout the unit. Fine planar lamination characterises a unique 0.12 m bed of calcite mudstone about 4 m below the top of the formation in a borehole [NZ 3954 5797] at Wearmouth Colliery. The exposures possibly of this unit at Humbledon Hill, Gilleylaw and Newport Dene are of porous, saccharoidal to oolitic, shelly dolomite in uneven beds 0.20 to 0.30 m thick, and may be the uppermost part of the formation.

Also atypical is the presence of thin beds of small, partly compressed ooids and pisoids (?oncoids) in grey and dark grey, slightly argillaceous dolomite 2 to 3 m below the top of the formation in Offshore Boreholes WI2B (15 km east of Marsden Bay) (Plate 9) and WM13 (7 km east of Whitburn). These beds, respectively 0.36 m and 0.15 m thick, strongly resemble the Trow Point Bed (Smith, 1986) that lies immediately below the Hartlepool Anhydrite; in both boreholes, this lower pisoidal bed has a gradational base. Judging from the records, it may also have been present in Offshore Boreholes D2 and D4 (east of Seaham), where 'black spherical bodies' and 'tiny black spherulites' were recorded at the appropriate level by geologists of British Coal. A bed of pisoids has also been recorded at a similar stratigraphical position in Yorkshire (Smith, 1974a) and Poland (Peryt and Piatkowski, 1977a, b).

The top of the formation in the south-western part of the district is taken at the incoming of coarsely saccharoidal and oolitic dolomite, but the contact is neither exposed nor seen in cored boreholes and so cannot be defined precisely. Elsewhere, especially from Houghton-le-Spring northwards, uppermost strata are widely disturbed and the top of the formation is taken at the top of the disturbed sequence. The disturbance, which has been attributed to massive downslope slumping and sliding (Smith, 1970c, 1985a) , is known informally as The Downhill Slide, after the former Downhill Quarry [NZ 348 601] where it was particularly well exposed (Smith, 1994). It locally involved and removed the whole of the Raisby Formation, Marl Slate and part of the Yellow Sands, leading to the creation of spectacular WSW–ENE slide canyons and to puzzling stratigraphical anomalies. For example, at the north end of Downhill Quarry, the whole formation comprised a complex of large lenticular slide-slices of bedded dolomite occupying an almost vertical-sided valley at least 8 m deep cut into the Yellow Sands, and large allochthonous blocks (olistoliths) of bedded dolomite rest on a discordant slide plane at Trow Point and Frenchman's Bay (Smith, 1970c, p1.2, fig. 1). Discordant probable erosion planes and disturbed strata at or near the top of the formation have been noted in some cored offshore boreholes (e.g. Magraw, 1978, pl. 4, fig. 4) and in the BGS Seaham Borehole [NZ 4258 5031]; a distinctive thin bed of sandy dolomite was encountered at a comparable level in a number of boreholes off Sunderland and Whitburn and might be the product of a tsunami generated by the Downhill Slide. The stratigraphical anomalies and apparently allochthonous blocks were noted by most early authors (e.g. Woolacott, 1903, 1918) and were interpreted by Woolacott (1909, 1912) and Trechmann (1954, with reservations) as evidence of tectonic thrusting. The most southerly record of these disturbed strata is at the top of the sequence at the south-eastern corner of Houghton Quarry [NZ 3436 5065]; anomalous stratigraphical relationships at this level persist northwards at least as far as Blyth (1:50000 Sheet 15).

Biota

The Raisby Formation has a varied biota which includes more than 60 species of plants and animals; it has been studied by Sedgwick (1829), Howse (1848, 1858, 1890), King (1848, 1850), Kirkby (1864b), Trechmann (1921, 1925, 1944), Logan (1962) and Pattison (in Smith and Francis, 1967). Extensive lists by J Pattison of fossils collected from many boreholes and surface exposures in this formation are retained at BGS offices and have been summarised by Patti-son (in Smith and Francis, 1967; in Smith, 1970b; and in Pattison et al., 1973). The palaeoecology has been reviewed by Pettigrew (1985). The summaries all include species identified in the adjoining districts to the north and south, as well as those found in the Sunderland district, but most species are likely to be present in all three districts.

Despite the large number of invertebrate species recorded in the district, the formation is not very fossiliferous; study of thin sections, however, generally reveals more fossils than are apparent in hand specimens. The lower beds are more fossiliferous than strata higher in the formation, although fossils persist to the extreme top in a few places; they are relatively abundant (presumably because of rapid burial and consequent protection from scavenging) in slumped strata and debris flows. The total invertebrate assemblage is similar to that of the succeeding Ford Formation but is less diverse than that of the reef faunas of the latter.

Plants are mainly preserved as impressions or as carbonaceous filaments and films, which are present at all levels in the formation. Most are the remains of land plants that were swept into the Zechstein Sea after their death, but marine algae also occur. The latter are preserved mainly as carbonaceous filaments. Palynomorphs, notably miospores, are preserved in most strata of the English Zechstein sequence, including the Raisby Formation of the district, but they have not been studied in detail; summaries of their general stratigraphical distribution have been given by Clarke (1965) and Pattison et al. (1973).

Animal fossils in the Raisby Formation are mainly invertebrates and are dominated by species of benthic brachiopods, bivalves and foraminifera. The most common brachiopods include Dielasma elongatum, Horridonia horrida and Strophalosia morrisiana. The bivalves include abundant Bakevellia spp. and Permophorus costatus, but the bivalve-rich coquinas characteristic of EZ1 carbonate strata of littoral facies are significantly absent. The foraminifera include species of Agathammina, Ammodiscus and Geinitzina. Other groups represented in lesser abundance include bryozoans, gastropods, nautiloids, ostracods, conodonts and crinoids. Vertebrates comprise two species of fish, both found low in the formation. Fossils recorded by Trechmann (1945) from Ford Quarry, and attributed by him to this formation, comprise a somewhat atypical assemblage (Pattison in Smith, 1970b); they were probably from the superficially similar strata of the overlying Ford Formation (see p.71).

Mineralisation

The presence of small quantities of barite, calcite, galena and sphalerite in lower beds of the Raisby Formation was noted by Winch (1817) and Sedgwick (1829). The more detailed distribution of these minerals, together with that of anhydrite, celestite, dolomite, fluorite, gypsum, iron oxides, pyrite and a number of less common minerals, has been documented and discussed for these and equivalent strata by Woolacott (1919b), Fowler (1943, 1957), Hirst and Smith (1974), Harwood (1980, 1983), Harwood and Smith (1986) and Lee and Harwood (1989). The minerals other than anhydrite and gypsum are most abundant in the vicinity of the Butterknowle Fault in southern County Durham (see Smith and Francis, 1967 for summary), but are also present in mainly small amounts throughout the Raisby Formation. Gypsum and anhydrite are widespread and abundant in the eastern undersea area, but have been dissolved in most inshore and land areas. Trechmann (1954) recorded layers and nodules of chert low in the formation at Claxheugh Rock, Sunderland.

The mode of occurrence of the minerals ranges from scattered small crystals in the rock mass to cavity-lining and cavity-fill. Veinlets and more complex fracture-fill also occur, and the gypsum and anhydrite form disseminated platy and poikilotopic crystals, intersecting aggregates and lenticular and amoeboid patches up to 10 cm across. Most of the cavities that are now lined or filled with other minerals were formed by the dissolution of gypsum and anhydrite. On land, calcite is by far the most abundant secondary mineral, both in vein form and as scalenohedral cavity lining; some calcite and celestite that has replaced anhydrite has distinctively castellated inherited outlines and some calcite has replaced dolomite. Dolomite, perhaps the second most abundant secondary mineral in the Raisby Formation onshore, is generally in rhombs which line cavities. Cubic pyrite (or its oxide breakdown products) is also present mainly as a cavity-lining. Galena and sphalerite are most common as cavity-fill and as vein minerals in lower parts of the formation, sphalerite generally persisting to higher levels than galena. Barite and fluorite, though nowhere abundant, are most common in the middle and higher parts of the formation (see also Fowler, 1943). Replacive pink barite is particularly abundant at the local top of the formation in a small coastal exposure [NZ 3941 6601] near South Shields, and iron minerals pervade a slide breccia in the Raisby Formation at the northern end of Downhill Quarry [NZ 348 601].

Views on the age and origin of the secondary minerals in the Raisby Formation and other parts of the Magnesian Limestone vary, and it is clear that more than one and, in places, several episodes of mineralisation took place. Some of the mineralisation may have been relatively early, as suggested by Harwood and Smith (1986) in their study of equivalent strata farther south, but host rock facies are less varied in the district than in those at outcrop in Yorkshire, and most of the mineralisation is probably epigenetic. The earliest major secondary mineral other than dolomite was generally calcium sulphate (probably anhydrite), that both displaced and replaced carbonate sediment early during lithogenesis. A replacive origin for much of the barite and fluorite and some of the galena has been argued (Fowler, 1943, 1957). Harwood (1983) has shown that the fluorite mineralisation in Yorkshire was multiphase, and Phemister (in Fowler, 1943) showed that, although fluorite was generally older than barite, this relationship was not universal. Calcitisation of some anhydrite probably took place during deep burial, and much calcite in cavities was formed at much the same time as fluorite and barite (Fowler, 1943); some calcite, however, may have been precipitated in voids and fractures by meteoric fluids during the present cycle of uplift, and some dolomite was probably calcitised during this phase (Lee and Harwood, 1989).

Work by Hirst and Smith (1974) and Harwood and Coleman (1983) on the mineralisation of the Raisby Formation in south Durham and on its Yorkshire correlative suggests that the sulphate ions were probably derived directly from Zechstein Sea water, and that the sulphides on the sites of former sulphate patches were formed by the bacterial reduction of anhydrite. The metallic ions are thought by most authors to have been derived mainly by the concentration in residual connate brines of detrital matter already in the host rock, but others have favoured a hydrothermal input to explain the concentrations of, for example, barium. The data do not show any strong geographical concentrations of secondary minerals in the district and there is no obvious relationship to known geological structures.

Conditions of deposition

The palaeogeographical setting, biota and sedimentary features of the Raisby Formation all suggest that it accumulated on a gently eastwards-inclined marine slope or ramp. By comparison with similar strata in Yorkshire, the rocks of the formation probably passed upslope into high energy grainstones at the seaward margin of a broad, shallow-water to peritidal carbonate shelf complex (Smith, 1974a, b; Harwood, 1981), the contemporary shoreline lying perhaps 20 to 40 km to the west (Smith, 1989). The origin of the lime silts and muds that made up most of the formation is not known, but derivation by selective storm, current and wave winnowing from the inferred shelf complex seems likely to have been a major process, with some hemipelagic and benthic input. Dolomitisation was probably mainly postburial and accomplished by large-scale and protracted seepage reflux.

The depositional slope in central parts of the district was steep enough, perhaps 1° to 4° (see Lewis, 1971; Carter, 1975), to sustain sediment creep and the down-slope movement of laminar and turbid suspension currents, debris flows and olistoliths. It has not been established whether this sediment movement was initiated by slope failure caused by sedimentary overloading or was triggered by external stimuli such as earth tremors, but tremors have been invoked by Smith (1970c) to account for the massive slope failure at the end of Raisby Formation time and to explain the origin of the curved low-angle joints; the inferred sea-level decline at about this time may also have been a factor. The average slope probably decreased in eastern parts of the district, where the shelf-marginal aprons merged imperceptibly into the carbonate muds of the basin floor. Crude estimates, based on minimum slopes of 1^°, suggest Permian water depths of at least 110 to 120 m in present coastal districts, deepening eastwards to perhaps 200 to 300 m in the basin centre. Initial variations of water depth resulting from buried aeolian sand bodies and other eminences appear to have been largely eliminated during the deposition of the Raisby Formation, leaving a slightly humped but generally smooth surface which was much diversified in the north by major WSW–ENE slump canyons and olistostromes.

Despite suggestions based on isotopic evidence (Margaritz et al., 1981) that the anoxicity of the early Zechstein Sea persisted from Marl Slate time throughout deposition of the Raisby and equivalent formations, the wide distribution of a moderately diverse indigenous benthic fauna indicates fairly normal marine conditions in most of the district, including offshore. This implies a partial or complete breakdown of the stratified, partly anoxic cells of the Marl Slate sea or a decline in the surface of the top of the anoxic layer and its withdrawal eastwards, perhaps through deepening of the water over the basin-margin sill and a consequent improvement in mixing and circulation.

Ford Formation (EZ1bCa)

Distribution and general characteristics

The Ford Formation (formerly the Middle Magnesian Limestone) is a classic wedge-shaped carbonate shelf complex, which crops out in a north–south belt up to 5 km wide (Figure 34). In most of eastern Durham, the main part of this complex comprises three facies, all dolomitised, with a narrow linear shelf-edge reef separating a broad belt of backreef and lagoonal beds (with patch-reefs) to the west from a belt of forereef talus aprons and off-reef beds to the east. However, this tripartite subdivision is almost certainly an oversimplification; in places, for example, early beds of this formation may pass beneath the reef, and the geometry of the reef itself is probably more varied than existing interpretations suggest. The type locality of the formation is Ford Quarry, Sunderland (Smith et al., 1986).

The youngest parts of the backreef strata of the Ford Formation, and almost all of the Heselden Dene Stromatolite Biostrome (see p.80) that succeeds the reef in the district to the south, have been eroded from the Sunderland district. Because of this, the exact stratigraphical relationships between the formation and younger Magnesian Limestone strata cannot now be established.

The youngest undoubted part of the Ford Formation is the Trow Point Bed (Smith, 1970b, 1986), an unusual thin deposit of peloidal, oncoidal and stromatolitic dolomite that is widespread in eastern parts of the district and which has counterparts in Germany and Poland.

Cavities after former secondary sulphates occur throughout the formation, and speckles and dendrites of manganese oxides are widespread on bedding planes and on the walls of fissures.

The scarcity of good borehole data and exposures in much of the outcrop of the Ford Formation, and the difficulty in defining its base in some areas, present considerable problems in the drawing of facies and formation boundaries. Only the steep eastern margin of the reef facies has a topographical expression that allows it to be traced with reasonable confidence; most other boundaries shown on the 1:10 560 and 1:50 000 scale maps are interpolated and in places are little more than speculative.

Backreee and lagoonal facies, with patch-reefs

Dolomitised carbonate rocks of this facies occupy most of the outcrop of the Ford Formation; they are classified here mainly as backreef because it cannot be demonstrated that the reef was an effective barrier for much of its early life, nor that truly lagoonal conditions were thereby established. These rocks are generally overlain by drift deposits and are the least well-documented part of the Magnesian Limestone sequence; their thickness is difficult to assess but probably reaches a maximum of more than 100 m in the east of the outcrop from Silksworth southwards.

Information on this facies in the district comes almost entirely from a scatter of mainly small temporary and permanent exposures in the Herrington, Houghton-le-Spring and Silksworth areas, plus the large but lithologically different faces at Ford Quarry [NZ 362 572], Sunderland. These sources suggest that most of the backreef and lagoonal beds are of pale cream and buff, granular and oolitic dolomite with a limited molluscan fauna, but that much of the lower part of the formation has undergone severe diagenetic changes, and little of the primary fabric has been preserved. All parts of the facies contain scattered to abundant cavities after former sulphates; many cavities are lined or filled with calcite and some with dolomite, but metallic sulphides are rare.

The stratigraphical uncertainties and highly variable diagenetic overprints preclude the recognition of a standard sequence in the backreef and lagoonal strata. However, judging from exposures in the Herrington, Silksworth and Houghton-le-Spring areas, lower beds (?20 to 30 m) comprise mainly finely saccharoidal dolomite with only local traces of ooids, whereas upper beds are mainly of cross-bedded shelly, granular and oolitic dolomite grainstone; patch-reefs occur, apparently, only in the middle or upper beds and are most common in the east. Most backreef beds at Ford Quarry differ from those exposed elsewhere, being mainly of dense silt-grade and mud-grade crystalline dolomite; they are discussed later.

The lower rocks of the backreef and lagoonal facies are exposed only in the west of the outcrop of the formation, chiefly near Houghton-le-Spring and Middle Herrington. Where least affected by diagenetic changes, these rocks arc mainly of soft, pale cream, slightly calcitic, saccharoidal (silt-grade to fine sand-grade) dolomite, and lie in even beds, commonly 0.10 to 0.35 m thick. Such rocks, totalling 18 m, were formerly exposed above supposed Raisby Formation in a temporarily exposed road cutting [NZ 35 53] near Middle Herrington and are also to be seen in Hasting Hill Quarry [NZ 3525 5442]. Traces of small-scale cross-lamination occur in some exposures of these beds, perhaps indicating that they are altered ooid grainstones.

The limited molluscan fauna of the early backreef beds is mainly in the form of scattered, barely identifiable moulds. From Hasting Hill Quarry, Trechmann (1945) recorded Astartella vallisneriana, which is characteristic of the lower backreef and lagoonal rocks. Pattison (BGS records) later collected Orthothrix sp., cf. Strophalosia morrisiana, Bakevellia hinneyi and B. cf. ceratophaga from the same locality.

Diagenetic changes in these lower backreef strata are most extreme in the south-west of the outcrop, near Houghton-le-Spring and Warden Law, where a few small quarries and natural exposures reveal a varied suite of rocks ranging from soft, cream, saccharoidal dolomite, to hard, thin-bedded to massive, grey, crystalline dolomite and limestone. Traces of primary bedding and cross-lamination are preserved in places, but a 'felted' fabric is widespread and obliterates most earlier structure. This fabric characterises many western exposures of this facies in the district to the south (Smith and Francis, 1967) and results from the replacement by calcite of earlier secondary aggregates of platy and discoidal anhydrite up to 5 mm across. Coarsely saccharoidal dolomite is also widespread, and parts of the rock have been slightly to severely autobrecciated during diagenesis; calcite fills many of the resulting fissures and cavities.

Higher beds (?60–80 m thick) of the backreef and lagoonal facies occupy most of the outcrop, but are (or have been) exposed mainly around the Herringtons and Silksworth. Although almost all the temporary exposures were small, the lithological and faunal consistency was sufficient to indicate that they were generally representative. Most of them exposed soft, pale cream, granular, shelly dolomite in even beds commonly 0.20 to 0.40 m thick, in which low-angle cross-bedding was widespread. A finely oolitic texture is present in many exposures and originally was probably even more prevalent; the centres of many ooids are now empty or contain only spray calcite. Large grapestones were present in oolitic dolomite in a trench [NZ 3755 5244] near Silksworth, but pisoids such as occur in equivalent strata farther south have not been noted. Diagenetic changes include the obscuring of ooids by the formation of saccharoidal mosaics of equant dolomite and, in many places, by the formation of irregular patches and lenses of dense dolomitic or calcitic autobreccias, in which little of the original rock remains.

Invertebrate fossils are abundant in the higher parts of the grainstone facies of the backreef and lagoonal beds, but are commonly comminuted and comprise a restricted range of species. Collections from a number of mainly temporary exposures around East Herrington and Silksworth show a dominance of bivalves and small gastropods, with foraminifers and ostracods at most places and brachiopods at some. The gastropods include specimens doubtfully referred to Coelostylina, Donaldina, Naticopsis and Yunnania, and the bivalves include Astartella sp., Bakevellia spp., Permophorus costatus and Pseudomonotis speluncaria. The brachiopods include Dielasma elongatum and Neochonetes? davidsoni, and the ostracods include many bairdiids. It is likely, on faunal and structural grounds, that thick-bedded, soft, shelly, granular dolomite exposed in Newport Dene [NZ 3852 5410] and below-reef oolitic dolomite at Gilleylaw Plantation Quarry [NZ 3755 5370] (New Silksworth) and Humbledon Hill [NZ 3814 5528] belong to this facies of the Ford Formation and not to the Raisby Formation (Smith, 1994). Several species of brachiopods, bivalves and gastropods were reported from these beds at Humbledon Hill by Trechmann (1945).

The backreef rocks at Ford, Sunderland

The backreef strata exposed at Ford Quarry and in an adjacent abandoned railway cutting differ from those at almost all other exposures of this facies. They comprise more than 55 m of unevenly, mainly thin- to medium-bedded dolomite with a sparse invertebrate fauna dominated by bivalves, but also featuring foraminifers and a few species of brachiopods including Orthothrix spp. and Neochonetes spp. (Pattison in Harwood et al., 1982; see also lists in Kirkby, 1866 and Trechmann, 1945). These beds bear a strong lithological resemblance to middle parts of the Raisby Formation, and were thus classified by Kirkby and Trechmann, but the unmistakable eastwards passage of at least the uppermost 35 m of bedded dolomite into massive reef dolomite excludes this classification unless the reef, too, is here part of the Raisby Formation. The fossil evidence is inconclusive as the fauna in the bedded dolomite would he an unusual one in either formation, but the presence of Orthothrix suggests that the Ford Formation is more likely. The fauna from the reef in the quarry is typical of Ford Formation (EZ1b) reef-core, but an EZ1 shelf-edge reef fauna is indicative of facies rather than age. The possible Raisby Formation age of the lowest 20 m of bedded strata, formerly exposed low in the north-west part of the quarry and apparently passing beneath the reef, cannot be ruled out, but they appeared to be part of an unbroken succession; the palaeontological evidence bearing on this is equivocal. The passage from bedded into reef rock, seen when the quarry was 30 m deep, was sharp and almost vertical in the south-east face (Smith, 1970b) but rather more irregular in the north-west face.

Although parts of the backreef dolomite at Ford Quarry are coarsely saccharoidal and might be of altered oolite, most of the rock is medium-bedded, very fine-grained to finely saccharoidal dolomite and is probably an altered mudstone/wackestone. A coarse nodular structure similar to that in parts of the Raisby Formation is present in some beds, and autobrecciation has locally obscured most primary features. Comminuted shell debris is abundant in many beds. The cause of the marked lithological difference between backreef strata of the Ford Formation at Ford and those elsewhere is unknown, but the reef at Ford occupies the floor of a major slide canyon and water immediately landward of the reef was presumably atypically deep and of lower energy than normal.

Large discordant masses of thin-bedded dolomite are present in the backreef beds at Ford Quarry and in the adjoining railway cutting, where they are particularly spectacular (Plate 10). Trechmann (1945) recorded thrust-like discontinuities and allochthonous masses of bedded dolomite at both places; these have been interpreted as submarine slide blocks (olistoliths) overlying slide planes (Smith, 1985a). Eastward movement, towards the reef, is suggested by spatial relationships (Trechmann, 1954) and by the configuration of slump rolls and folds in the disturbed strata. The inferred direction of movement implies a contemporary sea bed slope towards the reef here.

The distribution of the fine-grained backreef strata uniquely exposed at Ford is unknown, though presumably they were formerly extensive west of Sunderland and may have formed part of a belt extending southwards just landward of the reef.

Patch-reefs

A number of poorly exposed isolated bodies of autochthonous reef dolomite (boundstone), which occur in the south-west Sunderland and Silksworth area, have been interpreted as patch-reefs (Smith, 1981a). Knowledge about their exact relationships to surrounding strata is scanty, however, and some of the bodies could be erosionally isolated relics of former outlying parts of the main shelf-edge reef. All the known patch-reefs lie in shelly, mainly oolitic dolomite grainstone at least 30 m above the base of the formation; they range from a few metres to a few hundred metres across, but appear to be only a few metres thick.

The best exposed patch-reefs are at Gilleylaw [NZ 375 537], New Silksworth and nearby High Newport [NZ 387 538]. The Gilleylaw reef is a tabular body comprising up to 5.5 m of crudely thick-bedded stromatolite/bryozoan boundstone overlain by up to 1.3 m of coarsely pisolitic (?oncolitic) dolomite (Smith, 1958, pl. V1A; Smith, 1994), with a wedge of highly fossiliferous rubbly talus at the northern margin. The High Newport reef is arched in section and comprises a rigid core of bryozoan/algal boundstone flanked by aprons of rubbly shelly talus and laminated (?stromatolitic) dolomite. Other shelly patch-reefs occur at Newport Hill [NZ 379 539] and slightly to the north, and mainly stromatolitic, supposed patch-reefs are exposed about 1 km north-east of Middle Herrington and also in Silksworth Plantation [NZ 369 519]; very small patch-reefs were noted in temporary exposures [NZ 3755 5244] in oolitic dolomite south of Old Silksworth village. A cross-section of the Newport Hill patch-reef was exposed temporarily in 1983 and was seen to comprise a mound of grey-cream bryozoan boundstone flanked on both sides by outward-dipping shelly detritus (Hollingworth, 1987). Hollingworth noted that the presence of burrowing bivalves in the flanking talus of the Newport Hill patch-reef indicated a lack of early cementation, but the relief of the reefs and the presence of clasts of reworked boundstone in the talus of the Gilleylaw and High Newport reefs imply contemporaneous lithification of the reef-cores.

The fauna of the patch-reefs is both abundant and varied, and comprises frame-building and water-baffling bryozoans and many species of brachiopods (but not Horridonia), bivalves, gastropods and ostracods. Hollingworth (1987) noted striking differences between the faunas of the patch-reefs and of the shelf-edge reef, mainly in the relative proportions of the different taxa. For example, Kingopora is locally abundant in the patch-reefs but rare in the shelf-edge reef, and infaunal bivalves are much more abundant in the patch reefs than in the shelf-edge reef. Several species are peculiar to the patch-reefs, but crinoids are absent.

Dense multilayered encrustations of finely crystalline, turbid dolomite thickly coat bryozoans in the core of some of the patch-reefs and comprise most of the rock in a 1.5 m bed within the reef at Gilleylaw Plantation Quarry (Plate 11).

Reef facies

The shelf-edge reef of the Ford Formation has attracted attention since the time of Sedgwick (1829), mainly because of its abundant and varied fossils. Its biota has been studied by a long succession of authors including Howse (1848, 1858, 1890), King (1848, 1850), Kirkby (1858, 1860), Woolacott (1912, 1919b), Trechmann (1913, 1925, 1931, 1944), Logan (1962), Pattison (in Smith and Francis, 1967 and many unpublished BGS reports) and Hollingworth (1987). More than 90 invertebrate species have been recorded, some of which are not known outside the reef. Trechmann (1913, 1925), from large collections made during the sinking of Blackhall Colliery shafts in the district to the south, noted a progressive upwards faunal impoverishment which he attributed to increasing salinity. Subsequent work (Smith, 1981a; Hollingworth, 1987) has shown that the faunal variation is more complicated than Trechmann supposed and was a response to a wide range of factors including the depth, temperature, clarity and agitation of the water, in addition to food supply and competitive ecological pressures. Hollingworth (1987) suggested that increased salinity may have been an important limiting factor only in the youngest parts of the reef.

Few of the early workers speculated on the origin of the shelly rocks of the Ford Formation (the 'Shell Limestone') and it was left to Woolacott (in about 1908, personal communication to Trechmann and quoted by Trechmann, 1931) and Trechmann (1914) to suggest that they formed part of a bryozoan reef. This view was reiterated by Woolacott (1919b) and accepted in reviews by Smith and Francis (1967) and Smith (1981a). Smith recognised a range of subenvironments within the reef and Hollingworth (1987) described the detailed ecology of the faunal communities in these subenvironments. Much detailed information on faunal communities and local exposure details in the Tunstall Hills area of Sunderland is given by Hollingworth and Pettigrew (1988). Aplin (1985) detailed the petrology and geochemistry of the reef, and some aspects of reef diagenesis have been studied by Tucker and Hollingworth (1986).

Outcrops of reef rocks form a discontinuous irregular belt extending south-south-eastwards from West Boldon to beyond the southern margin of the district (Figure 34). Though partly obscured by drift, they give rise to a range of prominent hills from which a reef width of up to 1.3 km may be deduced. The top of the reef has been eroded from all exposures in the district but probably survives (though not exposed) in an isolated spur south of Seaham; preserved thicknesses of up to 40 m are known at Ford, Sunderland, increasing southwards to perhaps 100 m at Dalton-le-Dale in the south of the district.

Tentative reconstructions from widely scattered fragmentary evidence suggest that the reef is strongly asymmetrical in profile (Figure 39), with a steep seaward (eastern) face and a top that evolved from rounded to roughly flat as the reef grew (Smith, 1980c). The landward margin is marked by a sharp, lateral passage into backreef and lagoonal dolomite, and it is clear from the mutual relationships of reef and backreef strata that the latter accumulated at much the same rate as the reef grew upwards. In contrast, sediment accumulation at the seaward side of the reef lagged behind upwards reef growth, gradually increasing the asymmetry.

The patchy distribution of outcrops of hill-forming reef rock led Trechmann (1913 et seq.) to interpret them as a chain of reef knolls like those in the Possneck area of Germany (see Kerkmann, 1969), but the markedly asymmetrical profile is consistent with a more continuous shelf-edge reef. The argument based on topographical expression was, moreover, weakened by the discovery of reef dolomite in the bottom of a valley separating 'knolls' at West Boldon and Hylton Castle, which showed that the valley is probably relatively recent and not a re-excavated inter-reef channel. Nevertheless, it is clear that the steep seaward margin of the reef was deeply embayed, with probable channels and perhaps more than one line of reefs in places. The reef at Humbledon Hill differs from most other exposures in a number of respects, although these differences may be mainly secondary.

In many northern parts of the district, the reef rests on a deeply dissected surface left by the inferred massive submarine sliding that followed deposition of the Raisby Formation, the reef trend being approximately normal to that of the inferred slide canyons. Reef-base slopes of up to 45° occur against the flanks of these canyons in parts of Downhill Quarry [NZ 348 601] (West Boldon) and at Claxheugh Rock [NZ 362 575], and reef-base reliefs of up to 20 m are locally evident. A lesser but still somewhat discordant reef-base relief is noticeable where reef dolomite overlies shelly dolomite of probable early Ford Formation age at Humbledon Hill and Tunstall Hills, Sunderland (Smith, 1994).

The main subfacies of the reef (Figure 39) are mutually gradational, but their relative proportions differ greatly from place to place in response to the vicissitudes of reef growth. Many of the variations in lower parts of the reef undoubtedly stem from the substantial variation in water depth resulting from the relief of the surface on which it grew; there is considerable evidence of overlap as hollows were progressively filled. Reef growth thus almost certainly began at different times in different places, with early reefs gradually expanding and merging to form the present complex.

Basal coquina

Where a full range of reef subenvironments is present, the reef rests on a basal coquina composed mainly of well-preserved invertebrate remains (Trechmann, 1913, 1925; Smith and Francis, 1967; Smith, 1980a, 1981a; Hollingworth, 1987). The coquina, which has been recorded only beneath reef rocks, is only a few metres thick but has an exceptionally diverse fauna. Where exposed in temporary excavations [NZ 359 588] near Hylton Castle, Sunderland, it yielded 34 invertebrate species including foraminifers, bryozoans, brachiopods, gastropods and bivalves, and a similarly large faunal range is recorded by Hollingworth (1987) at a small exposure [NZ 3910 5432] near the north end of Tunstall Hills. The faunal community of the coquina was graphically depicted by Hollingworth (1987, fig. 6.3).

The coquina has been interpreted as a shell bank that accumulated on a break of slope in the substrate (Smith, 1980c, 1981a; Hollingworth, 1987); it is generally friable and completely dolomitised, but has remained undolomitised at the small exposure on Tunstall Hills, where Tucker and Hollingworth (1986) reported a range of early marine cements indicative of shallow-water accumulation punctuated by phases of subaerial exposure. The basal coquina has no framework and is not a reef; it did, however, by its early cementation, form a relatively rigid surface that permitted colonisation by frame-builders and hence played an essential part in reef initiation.

Reef-core and backreef transition

Massive buff to brown, hard, autochthonous dolomite (boundstone, biolithite) forms the bulk of the shelf-edge reef. In many field exposures, the rock appears to be finely crystalline and not particularly fossiliferous, but selective weathering, thin-sections and acetate peels reveal the almost ubiquitous presence of a variable framework of ramose and pinnate bryozoans (Plate 12) separated and surrounded by mud-grade to silt-grade turbid dolomite which itself contains a wide range of invertebrates. Pockets of detritus are abundant in places and the presence of reworked fragments of reef-core dolomite in the reef talus (Smith and Pray, 1977; Smith, 1981a) implies early lithification. Evidence of early marine fringing and cavity-filling cements in dolomitised reef rock was described by Aplin (1985), and in rare undolomitised pockets of reef-core rock by Tucker and Hollingworth (1986).

Many of the frame builders and other organisms in the reef-core are coated with lamellar encrustations, although the proportion of these varies greatly from place to place; in general, they are not as abundant as in most later subenvironments in the reef (Smith, 1958, 1981a). The encrustations comprise concentric alternating laminae of clear and turbid fine-grained dolomite and, in places, the whole rock is a mass of confused nodular encrustations with only small proportions of primary frame organisms (Plate 13). Such encrustations (' Stromaria') were noted by Brauch (1923) and Magdefrau (1933) in Zechstein reefs in Germany, and somewhat similar structures have since been described under the name Archaeolithoporella' from a number of places , including the Permian Capitan Reef of New Mexico and West Texas (Babcock, 1977) and the Permian rocks of the Caucasus (Pisera and Zawidzka, 1981). If all these encrustations are the same, and are organic in origin, Stromaria has historical priority (Smith, 1981a; Peryt, 1986). Whether the encrustations originated as submarine inorganic cements or were organic has yet to be established, although most authors favour a direct or indirect algal origin.

The presence of encrustations in fragments of reef-core rock in the sparse reef talus attests their early formation and lithification. They clearly strengthened and added bulk to the reef, supplementing the baffling, sediment-trapping and frame-building role of the bryozoans and assisting in the formation of a rigid body capable of building up from the sea floor.

All exposures of the reef-core contain a mixture of patches of autochthonous, tightly bound reef rock and sheets and pockets of shelly rubble; the mutual relationships of these were particularly clearly shown in 1973 during road widening on the north side of Humbledon Hill [NZ 380 533], Sunderland (Smith, 1981a). Here, about 8 m of the reef-core were exposed for about 125 m; upper parts were seen to comprise several concentrically layered, subspherical masses of autochthonous reef dolomite distributed unevenly amongst mainly low-dipping detritus (Figure 40). Careful collecting and faunal analysis by Pattison (1978, pl. 57; and internal BGS reports) showed that the reef masses are composed mainly of the in-situ remains of ramose bryozoans and the small pedunculate brachiopod Dielasma, but that the detritus contains an abundant and varied biota of bryozoans, brachiopods, bivalves and some gastropods. Pattison found little systematic change in overall faunal composition along the cutting.

In his analysis of faunal communities in the reef of the Ford Formation, Hollingworth (1987) recognised a lower reef-core and a main reef-core. He noted that the lower reef-core contains a fauna almost as diverse as that of the basal coquina, but that the relative proportions of some taxa differ. The bushy bryozoan Acanthocladia and the funnel-shaped fenestrate bryozoan Synocladia were the main early frame-building and baffling organisms which, by their growth, created a wide range of ecological niches and subenvironments. These, and the rubbly areas between them, were colonised by a variety of sessile and other invertebrates, including Fenestella, Kingopora, Dielasma, Horridonia, Pterospirifer, Bakevellia, Permophorus and Streblochondria (Hollingworth, 1987, fig. 6.12; Hollingworth and Pettigrew, 1988, fig. 8). Kingapora, Horridonia and Pterospirifer do not persist into the overlying main reef-core, in which Hollingworth recorded Acanthocladia and Fenestella in about equal proportions, and a varied shelly assemblage dominated by Dielasma, Stenoscisma, Strophalosia, Bakevellia, Pseudomonotis and Yunnania (Plate 14).

At its only surface exposure, at Ford Quarry (Sunderland), the landward margin of the reef is almost vertical, with limited interdigitation (Smith, 1970b, fig. 17); the lateral transition to bedded backreef strata is accomplished in a few centimetres, and beyond this there is very little mixing of reef and backreef components or clasts. There is, moreover, no evidence that the landward edge of the reef projected much above the level of equivalent backreef beds. The general verticality of the contact and the lack of evidence of mixing may indicate protracted upwards growth of the reef-core under relatively uniform conditions, probably initially well below wave base.

Reef-flat

Upper parts of the reef are crudely horizontally bedded, and these parts are designated as the reef-flat subfacies (Smith, 1981a). In the district, they are (or have been) exposed only at Ford Quarry and in a former small exposure at Quarry Heads [NZ 399 522] near Ryhope (Smith, 1958). Bedding is thick and uneven, and the rock appears to be mainly algal. In the district to the south (Smith and Francis, 1967; Smith, 1981a; Aplin, 1985; Hollingworth, 1987), upper parts of the reef comprise an unevenly thick-bedded complex assemblage of intensely encrusted bryozoan boundstone and algal stromatolites, with abundant shell and algal debris in sheets and pockets and with scattered rolled and encrusted cobbles and boulders of contemporaneously lithified reef rock; oncoids occur locally. This part of the reef is at least 10 m thick at Hawthorn Quarry [NZ 437 463], where its top is a near-horizontal erosion surface (Kitson, 1982; Smith, 1994). The thickness of reef that has been removed is unknown, although the erosion surface is overlain in part of the quarry by up to 4 m of conglomerate composed of clasts of reef-flat boundstone.

Compared with the reef-core, dolomite of the reef-flat subfacies features a low-diversity fauna (Smith and Francis, 1967; Smith, 1981a; Hollingworth, 1987). Detailed analysis of the fauna of reef-flat rocks to the south of the district (Hollingworth, 1987) revealed that Acanthocladia is the dominant bryozoan in the autochthonous rocks, with local patches rich in Fenestella and Dyscritella. Rubbly and algal-encrusted dolomite between and surrounding the masses of bryozoan boundstone contains a specialised fauna rich in Dielasma, a limited range of epibyssate bivalves (chiefly Bakevellia) and many browsing gastropods.

The bedded character, faunal communities, abundance of stromatolites and evidence of contemporaneous erosion all point to mainly vertical accretion of the reef-flat under shallow agitated water of normal to slightly enhanced salinity; phases of subaerial exposure were probable.

Rocks of the reef-flat, just landward of the reef-crest, feature tension gashes up to 0.6 m wide and parallel with the reef trend (Smith, 1980c, 1981a); these are impressively displayed at Maiden Paps, Tunstall Hills, where they extend downwards into the reef-core, and probably result from unequal stresses around the sharp reef-crest. The gashes are lined with laminar calcite (dedolomite) and include fallen blocks up to 30 cm long (Plate 15); some were not completely filled penecontemporaneously and a later partial filling of grains of coarse quartz and feldspar sand was noted by Burton (1911), Woolacott (1912) and Trechmann (1944). It is not clear whether the fissure-lining is a submarine cement or subaerial travertine, but the presumed low light levels probably exclude an algal origin; Hollingworth (1987) found no fissure-dwelling fossils in the fissure fill. Aplin (1985) postulated phases of karstification of reef-flat rocks during subaerial exposure, and Trechmann (1944) postulated a sea-level fall to account for the quartz sand ('aeolian') in the gashes at Maiden Paps. These postulations are in accord with the evidence of erosion of the reef-flat at Hawthorn Quarry, just south of the district.

Reef-crest and uppermost slope

Reef rocks of these facies are exposed in the district only at Ford Quarry, but are seen at several exposures in the district to the south. At all these localities, the rock is composed mainly of a complex mass of sinuous and concentric laminar algal sheets and masses, thickly encrusting a sparse framework of mainly ramose bryozoans. Bedding is highly lenticular, and the dip changes from the near-horizontal of the reef-flat subfacies to 45° or more in a few metres. The rock contains abundant pockets of shell and other debris, and early lithification and high energy are indicated by the local occurrence of reworked blocks of reef-crest boundstone. Narrow columnar stromatolites (Smith, 1981a, fig. 16) occur sparingly in the dipping rocks of the uppermost part of the slope, where they project upwards and slightly basinwards. Evidence from several exposures suggests that a sharp reef-crest exists only in upper parts of the reef, corresponding approximately to the bedded reef-flat rocks; it was strongly progradational, advancing basinwards over the reef talus much faster than it grew upwards (Smith and Francis, 1967). The sharpness and progradation is probably a response to the proximity of sea level and to a low rate of subsidence.

The highly specialised biota of the reef-crest, adapted to survive in high-energy conditions, has a moderate diversity. At Ford Quarry it comprises a dense mass of algal encrustations with only a few bryozoans, but with pockets of bioclastic debris rich in Dielasma and gastropods (especially Yunnania and Mourlonia) and with scattered bivalves and nautiloids (Pattison in Geological Survey records; Hollingworth, 1987). Farther south, Hollingworth (1987) recorded abundant Acanthocladia and Dyscritella at the reef-crest at Horden (1:50 000 Sheet 27).

Mid reef-slope

Rocks of this subfacies comprise thick to massive sheets of hard, encrusted, bryozoan boundstone interspersed with much thinner sinuous but generally steeply dipping sheets of laminated dolomite; the high dips are probably primary, and indicate the contemporary slope (45° to 90°) of the original reef-front. Most of the information on this subfacies comes from exposures south of the district, but early mid-slope reef dolomite is exposed at a number of places near Ryhope. The boundstone comprises a dense mass of ramose and pinnate bryozoans, with much reef debris and cavity-fill, and a specialised slope biota; its high primary dip, far exceeding the angle of rest, must indicate contemporaneous lithification.

The specialised biota comprises the remains of sessile and other organisms adapted to life on a steeply sloping substrate and in the many niches and cavities therein, plus the trapped remains of organisms washed down from the upper reef-slope and crest. Cainoids and Fenestella are abundant.

Lower reef-slope

Lower reef-slope rocks comprise talus derived from higher parts of the reef-slope and the reef-crest, plus the remains of the indigenous biota (Smith and Francis, 1967; Smith, 1981a; Hollingworth, 1987) and submarine and postburial cements. The talus comprises debris of a wide range of reef rock types, ranging from fallen blocks several metres across to silt-grade fragments, but much of the rock is a weakly cemented rubbly accumulation of granule to cobble-sized fragments (Plate 16); it dips basinwards at up to about 35^° and thins sharply away from the reef-front. In the district, the talus apron of the lower reef-slope is exposed only at an abandoned railway cutting [NZ 396 538] at Ryhope, but it is also known in the district to the south.

The biota of the talus includes both allochthonous assemblages characteristic of reef-slope and reef-crest boundstone, and an autochthonous assemblage of the organisms that lived there. Hollingworth (1987, fig. 6.34) and Hollingworth and Pettigrew (1987, fig. 16) analysed the indigenous fauna in pockets of rubble between the boundstone blocks and showed it to comprise a complex community of some 12 species including Acanthocladia, Fenestella, Synocladia, Cleiothyridina (abundant), Horridonia, Stenoscisma and Spiriferellina.

Diagenesis of the reef rocks (including patch-reefs)

Although the dolomitic character of the reef rocks was recognised by all the early writers, and evidence of partial dedolomitisation was noted by Woolacott (1912), the petrology of the reef received little study until the work of Aplin (1985). Analyses quoted by Woolacott (1912) and Trechmann (1914) of reef rock from the district reveal dolomite contents mainly in the range 95 to 98 per cent with only a little free calcite. In contrast, dedolomite from Tunstall Hills was shown by Trechmann (1914) to be 89 to 96 per cent calcite (two analyses). Diagenetic changes in many parts of the reef have led to the virtual obliteration of fossils in those parts.

The early onset of cementation in the reef rocks is indicated by the presence of derived blocks of encrusted reef debris both within the reef and in the talus (Smith and Pray, 1977; Smith, 1981a, 1985b), and is a prerequisite in the construction of reef slopes exceeding the angle of rest. Supporting petrographical evidence of the early precipitation of fibrous, fringing and botryoidal aragonite cements and of micritic and fringing high-Mg calcite cements was given by Aplin (1985) and Tucker and Hollingworth (1986), who regarded it as an integral part of reef development. Most of the cements were later replaced by dolomite, some of which was subsequently dedolomitised.

Pervasive dolomitisation of the reef and associated strata has been attributed to large-scale seepage reflux of magnesium-rich brines soon after burial (Smith, 1981a; Aplin, 1981, 1985), perhaps mainly during deposition of the succeeding Hartlepool Anhydrite. A similar mechanism has been proposed by Harwood (1986) to explain the pervasive dolomitisation of many of the contemporaneous Zechstein carbonate rocks of Yorkshire. Aplin considered that petrographical and carbon isotopic evidence rule out large-scale mixing-zone dolomitisation, although such a process could have occurred on a minor scale and may have modified the earlier dolomitisation fabrics.

Only minor relics of anhydrite are now preserved in the reef (Aplin, 1985; Hollingworth and Tucker, 1986) but the abundance of cavities is an indication of its former extent. The anhydrite patchily replaced carbonate minerals (probably mainly early dolomite) during burial and inferred reflux, and was subsequently dissolved by meteoric or mixed fluids during and following uplift. Much of the porosity thus created was later reduced by the precipitation of calcite spar (Aplin, 1985).

The dedolomitisation of reef rocks mainly affects the upper and eastern parts of the reef, and is especially extensive at Tunstall Hills, where the reef is rich in manganese oxides (Woolacott, 1912; Trechmann, 1914). It ranges from the patchy replacement of the rock by coarsely spherulitic and radial brown calcite to wholesale replacement by mosaics of coarse equant calcite (Trechmann, 1914, 1944; Smith, 1981a; Aplin, 1985; Tucker and Hollingworth, 1986). Fossils and other structures are well preserved in parts of the dedolomitised reef but almost obliterated in others. Much of the dedolomitisation is likely to have been accomplished by fluids enriched in calcium sulphate during dissolution of the IIartlepool Anhydrite, and of secondary anhydrite in the reef itself, but Aplin (1985) also suggested that some dedolomitisation may have occurred during penecontemporaneous subaerial exposure following early dolomitisation and by recent subaerial weathering.

Strata east of the reef

Scattered exposures up to about 3 km east of the reef in the East Boldon and Ryhope areas reveal the presence of sparingly to abundantly shelly dolomite of the Ford Formation. The exact relationship to the reef of these strata is not known, but their faunal assemblages probably exclude a post-reef age; they are thus either pre-reef or reef-equivalent.

The narrowness of the reef talus aprons is shown at an abandoned railway cutting [NZ 399 537] at Ryhope, where bedded saccharoidal dolomite, with a sparse fauna of bryozoans and brachiopods, and with a surprisingly low content of reef debris, lies less than 200 m from the foot of the reef slope. Slightly farther south, small exposures of off-reef bedded dolomite in the floor of Ryhope Dene [NZ 413 517], at least 1 km east of the reef, yielded a varied and abundant invertebrate fauna dominated by foraminifera (especially nodasariids) and ostracods.

Bedded dolomite east of the reef near East Boldon is also probably reef-equivalent, but a pre-reef age is possible. It is mainly cream and finely saccharoidal and, from exposures [NZ 3566 6075] and [NZ 3619 6011] about 1 km east of the reef, yielded abundant foraminiers, brachiopods, gastropods and bivalves; only Neochonetes? davidsoni was found in an exposure [NZ 374 606] about 2 km east of the reef.

In present coastal and undersea areas, the stratigraphical equivalent of the reef of the Ford Formation cannot be identified, unless the Trow Point Bed (see below) is of this age. Available exposure and borehole data suggest that the period of reef growth may not be represented by basin-floor sediment, or may be only very thinly represented.

Trow Point Bed

This bed, first noted in Offshore Borehole No.1, 7 km east of Blackhall Colliery (Smith and Francis, 1967, plate 8A), has been described in detail by Smith (1986). It is a heterogeneous deposit up to 0.6 m thick, but generally 0.05 to 0.20 m, mainly composed of grey to black compressed peloids and oncoids, but with scattered patches of small columnar stromatolites.

The Trow Point Bed lies between the Raisby Formation and the Hartlepool Anhydrite, and is necessarily part of the Ford Formation. There is no evidence, however, that it is a basinal equivalent of the reef or of the Hesleden Dene Stromatolite Biostrome; it may be younger than both.

The bed is widespread in the east of the district and has been proved in 13 offshore boreholes (Figure 38); on land it is exposed for one kilometre between its type locality at Trow Point [NZ 383 667], South Shields, and Frenchman's Bay, but it has not been recorded farther west. The surface exposures reveal that the bed is draped over the hummocky top of the Raisby Formation, being thickest in the hollows and thinning out against the tallest eminences. The peloids and oncoids are most abundant in the hollows, and comprise an ill-sorted assemblage of mixed grains enclosed in a laminar carbonaceous matrix (Plate 17) that also includes unevenly distributed foraminifers, bivalves and ostracods; columnar stromatolites (Plate 18), where present, are concentrated on the lower eminences, where they form fan-like arrays (Figure 41).

The Trow Point Bed generally has a sharp base and top, but the top in some offshore boreholes has been disrupted by the formation of replacive and displacive anhydrite (mainly now hydrated to gypsum) and veins of fibrous gypsum. Barite and fluorite replace upper parts of the bed in a number of places, forming semicontinuous concordant layers; at Marden Quarry (Tynemouth Sheet), a layer of barite at this level was reported by Smythe (1922) to be 0.6 m thick, but most occurrences are 1 to 3 cm thick. The mineral concentrations here presumably result from the impervious character of the formerly overlying Hartlepool Anhydrite Formation.

The Hesleden Dene Stromatolite Biostrome

Algal rocks of this member are exposed at Hawthorn Quarry, just south of the district, and almost certainly extend northward into the district in a prominent north-south ridge [NZ 42 47] south of Seaham. There are no exposures on the northern part of this ridge, within the Sunderland district, but at Hawthorn Quarry [NZ 436 463] the biostrome comprises a basal conglomerate (0 to 4.0 m) lying on the eroded but roughly horizontal top of the shelf-edge reef, conformably succeeded by about 22 m of algal-laminated dolomite. The conglomerate is composed of encrusted rolled cobbles and boulders of dolomite derived from the underlying reef, and has only a sparse matrix. The overlying algal rocks feature exceptionally large stromatolite domes and spectacularly crenulated algal (cyanophytic) laminites (Smith and Francis, 1967; Smith, 1981a, fig. 27).

Evidence on the age of the biostrome is conflicting and finely balanced, but it has yielded an apparently indigenous molluscan fauna of Ford Formation affinities from exposures at Blackhalls Rocks, 9 km south of the district. On this basis, and despite the unmistakable evidence of an erosional hiatus between reef and biostrome, the latter is here tentatively classed as a member of the Ford Formation. The main alternatives would equate it with part of the Hartlepool Anhydrite or with the Roker Dolomite Formation. The age of apparently unfossiliferous oolitic dolomite that overlies the biostrome at Hawthorn is discussed on p.88. It is shown as part of Cycle EZ2 on (Figure 34) and on the 1:10 560 and 1:50 000 maps.

History and conditions of deposition of the Ford Formation

The pattern of deposition of the Ford Formation differed markedly from that of the preceding Raisby Formation. The reason for this difference is not known, but the established pattern was probably broken by a sea-level decline of at least several metres and a subsequent sharp recovery, inferred in Yorkshire at about this stratigraphical level (Smith, 1968). The change also followed the major episode of slope failure and downslope sediment movement at the end of deposition of the Raisby Formation, which may have been triggered by earth movements related to the inferred sea-level changes.

Initial deposition of the Ford Formation took place on a broad gently eastwards-sloping surface, which was much diversified in the north of the district by WSW–ENE slide canyons, probably up to 50 m deep. Despite this diversification, the major influence on regional sedimentation was probably bathymetric and related to the initiation and evolution of the shelf-edge reef. In the south of the district, this appears to have formed at a minor slope convexity on the Raisby Formation substrate (Smith, 1980c, 1981a), but the northward extension of the reef across the trend of the slide canyons is more difficult to explain.

Lack of information resulting from poor exposure precludes full interpretation of the history of deposition of this formation, but it is possible that initial sediments over much of the area were granular shelly carbonates, including oolites; these may have been up to 50 m thick in places before the initiation of reef growth. Alternatively, however, these granular carbonates may be interpreted as the uppermost part of the Raisby Formation, and the base of the reef taken as the base of the Ford Formation.

The basal coquina upon which the reef was established has been interpreted as a shell bank (Smith and Francis, 1967), and petrographical evidence cited by Tucker and Hollingworth (1986) indicates shallow-water to peritidal accumulation. If this interpretation is correct, it implies a considerable (50 m+) sea-level decline from that deduced for the preceding Raisby Formation, and probably requires either strong regional tilting or emergence of large parts of that formation. Subsequent deepening drowned the initial shell bank, and much of the early part of the reef is thought to have grown under appreciable water depths (Smith, 1980c, 1981a; Hollingworth, 1987). These authors agree that the reef eventually grew up to (or almost to) sea level, with possible brief phases of emergence, before eventually being killed by a radical change of environmental conditions such as emergence or a sharp rise in salinity.

The upward growth of the reef resulted in the separation of backreef and basinal environments and, because of strongly contrasting sedimentation rates, in the evolution of an increasingly asymmetrical shelf wedge. Whilst reef growth was mainly subaqueous, the reef formed a step on the shelf floor and backreef sediments probably dipped steadily towards the reef from an inferred shoreline perhaps 20 to 40 km to the west. Water depths similarly increased eastwards, perhaps resulting in the profound lithological differences noted between the abundantly shelly grainstones of the Herrington area and the less fossiliferous, finer-grained carbonates immediately behind the reef at Ford Quarry. Early backreef deposits are thus interpreted as having been formed and accumulated on a stepped marine ramp, but later backreef sediments, formed after the reef approached sea level and thus became a barrier reef, were lagoonal. The evolution of the reef was summarised by Smith (1980c, fig. 1) and Hollingworth (1987, figure 7.1).

The varied and abundant biota, and the sedimentary structures, of most of the backreef and lagoonal beds and patch-reefs, all point to deposition in near-normal shallow sea water and at least moderate energy levels; some of the oolites may have formed peritidal shoals. In contrast, beds east of the reef were probably formed on a basin plain under more than 100 m of normal to slightly deoxygenated water. The Trow Point Bed, however, is thought to be the product of rather unusual conditions, and a maximum depth of perhaps 100 m may be set by the light requirements of the photosynthesising cyanophytes of which it is largely composed (Smith, 1986). A minimum water depth exceeding 15 m is indicated by the geometry of the bed where exposed between Trow Point and Frenchman's Bay, South Shields.

The presence of a marine planation on the top of the reef at Hawthorn Quarry and elsewhere almost certainly indicates a relative sea-level fall of at least a few metres, because about 18 m of coarse reef-derived conglomerate accumulated on the reef flat at Blackhalls Rocks in the adjoining district to the south; this fall inevitably led to emergence of the uppermost part of the reef and its consequent death. Reinundation was followed by the formation of the supposedly subaqueous Hesleden Dene Stromatolite Biostrome (Smith, 1981a; 1994).

Hartlepool Anhydrite Formation (EZ1A) and its residue (EZ1A(R))

Distribution, age and general characteristics

This formation is part of an almost continuous belt of anhydrite that surrounds the Zechstein Sea Basin and lies mainly within the similarly encircling Cycle EZ1 carbonate shelf wedge (Smith, 1974b, 1980b; Taylor and Colter, 1975; Taylor, 1980, 1984). It was proved at Hartlepool in 1888 (Marley, 1892; Trechmann, 1913) and later shown to extend both southwards under Cleveland and North Yorkshire (Napier, 1948; Taylor and Fong, 1969; Smith, 1974b) and northwards in the offshore parts of the Durham, Sunderland and Tynemouth districts (Raymond, 1962; Magraw et al., 1963; Smith and Francis, 1967; Smith, 1970b; Land, 1974; Magraw, 1975, 1978). The anhydrite has been dissolved from all inshore and land parts of the district, leaving a thin residue (Smith, 1971), but the distribution of the residue and of foundered strata show that it formerly extended westwards as far as the shelf-edge reef of the Ford Formation (Figure 35), (Figure 42).

The Hartlepool Anhydrite lies between the sharp top of the Trow Point Bed of Cycle EZ1 and the less well-defined base of the Roker Dolomite and the Concretionary Limestone of Cycle EZ2. It has generally been regarded as the sulphate phase of Cycle EZ1 (Smith et al., 1974, 1986). Taylor and Colter (1975) and Taylor (1980), however, showed that stratigraphically equivalent anhydrite in the Southern North Sea basin may be divided roughly equally into two main subunits, the upper of which appears to pass laterally into carbonate rocks of Cycle EZ2, and this casts doubts on the stratigraphical affinities of the anhydrite. This uncertainty, moreover, is compounded in the Durham offshore area where the anhydrite in several places contains beds of unfossiliferous dolomite that are lithologically indistinguishable from many of those in the overlying Concretionary Limestone Formation, but have little in common with the rocks of the underlying Raisby and Ford formations. In view of these uncertainties, it seems prudent to regard at least part of the Hartlepool Anhydrite Formation as transitional between Cycles EZ1 and EZ2, and possibly as partly coeval with the stromatolite biostrome and Concretionary Limestone. In this connection, an observation by Clark (1980a) in stratigraphically equivalent strata in the Netherlands may be relevant; he showed that, in places, the contacts between anhydrite rock and dolomite rock units had been shifted by up to several tens of metres up or down by the widespread formation of replacivc nodular anhydrite during and after burial.

Separate units of mainly secondary anhydrite, encountered some 13 m and 10 m above the supposed Hartlepool Anhydrite in, respectively, Offshore Borehole W15 (north of the district) and Offshore Borehole WM7A, are tentatively assigned to the Concretionary Limestone Formation and will be described in the appropriate part of the account.

In the district, the Hartlepool Anhydrite has been partly or wholly cored in 14 offshore boreholes, and additional information comes from wireline logs of several uncored boreholes and from boreholes in the adjoining districts. Although the quality of the wireline log response is variable and interpretation problematical in some cases, these data show that the formation is thickest (up to 150 m) in the south-east, but thins sharply northwards (Figure 42). The great variation in the thickness of the formation is undoubtedly partly primary, but much variation also stems from dissolution, from the formation of replacive anhydrite and some from ductile flow. The relative importance of these four factors varies from place to place, but the westwards thinning mainly results from dissolution and the north–south variation may be mainly primary. Because of the sharpness of some of the thickness changes, and uncertainty in the interpretation of some wireline logs and drillers' records, the isopachytes shown in (Figure 42) are speculative.

The overall appearance of the Hartlepool Anhydrite is remarkably uniform, the main variation being in its content of disseminated and bedded dolomite and rare limestone. Anhydrite generally comprises more than 85 per cent of the formation, which is a translucent blue-grey and grey rock with a faint to well-marked coarsely marbled appearance ((Plate 19); Raymond, 1962, p1.3) that results from an anastomosing (penemosaic) net of finely crystalline pale brown dolomite. Substantial parts of the anhydrite are massive and contain few traces of primary structure, but former bedding is widely indicated by discontinuous subparallel net elements; dips in the bedded parts are generally low but contortion and other evidence of dislocation are widespread. Beds of dolomite up to a few metres thick, but generally less than one metre, occur sparingly throughout the formation, but are most common in the lower part where one or two beds, together up to 5 m thick, are widespread 2 to 10 m above the base; they may be correlated for distances of a few kilometres (Plate 26))." data-name="images/P936014.jpg">(Figure 43). Crudely interbedded units of anhydritic dolomite and dolomitic anhydrite have been proved in most cored boreholes, with clear evidence of distension and fracturing in many of the dolomite beds. The lowest and uppermost parts of the Hartlepool Anhydrite have been hydrated to alabastrine gypsum, and scattered tabular selenite crystals up to 0.02 m long are concentrated at the margins of dolomite beds and stringers (Plate 20). Veins of fibrous gypsum are abundant in the dolomite beds in some boreholes.

Lithology and composition

The anhydrite rock is composed mainly of dense aggregates of very finely crystalline laths that form ovoid to cumuloid nodular masses, individually ranging up to a few centimetres across, but mostly being less than 2 cm (Plate 19). The nodules, which are of almost pure anhydrite, form a generally uncompressed mosaic in the massive parts of the formation, but are commonly slightly compressed in the more clearly bedded parts. Within the nodules, the anhydrite laths form felted interlocking aggregates with a strong tendency to lie in swarms disposed roughly parallel with the nodule margins. No unambiguous evidence of a gypsum precursor (either discoidal or upright-fibro-radiate) has been reported, although some of the nodules superficially resemble those formed in some modern sabkha settings by the replacement of primary sypsum. A weak enterolithic structure was present near the middle of atypically thin Hartlepool Anhydrite in Offshore Borehole W15 [NZ 4809 7622] in the district to the north, but it has not been recorded elsewhere.

The dolomite net in the anhydrite rock ranges from extremely delicate and discontinuous to coarse and almost unbroken; the coarser nets generally also contain abundant irregular inter-nodule dolomite patches and some of the thicker net elements contain two or more dolomite laminae. Mineral interrelationships show that the dolomite (presumably after aragonite or high-magnesian calcite) was the first of the present minerals to be formed and was later displaced and extensively replaced by anhydrite. Evidence of replacement of dolomite by anhydrite in this formation was first recognised by Phemister (in Fowler, 1944), who suggested that much of the anhydrite might be of replacive origin and therefore of only limited stratigraphical continuity. Nodular dolomite rock in a borehole 10.5 km east of Seaham appears to have replaced anhydrite. Halite occurs in small quantities, mainly as a vein filler associated with fibrous gypsum, and the anhydrite also contains up to 1 per cent of non-carbonate insolubles including clays, quartz and a few detrital heavy minerals (Smith, 1971b).

The dolomite beds (and rare limestones) in the Hartlepool Anhydrite include laminated (predominant) and unlaminated varieties; most have intricately fretted margins against the anhydrite, in response to varied replacement patterns. In some boreholes, individual dolomite beds include both laminated and unlaminated dolomite, with a tendency for the laminated part to be uppermost. The laminae range from fine, smooth and planar, where they are relatively anhydrite-free, to coarse and pustulose where secondary anhydrite is abundant. Laminae in the planar-laminated rocks comprise alternating couplets of silt-grade relatively pure dolomite and thinner layers of darker brown finer-grained carbonaceous dolomite, and similar but thicker couplets characterise the more varied anhydritic pustulose laminites. Unlaminated dolomite in the Hartlepool Anhydrite generally occurs in beds less than 0.2 m thick, and mainly comprises dense mosaics of equant silt-grade dolomite with widely varied proportions of replacive and subordinate displacive anhydrite and gypsum. Hints of graded (turbiditic?) bedding were noted in unlaminated dolomite high in the formation in a borehole 10.5 km east of Seaham.

The pustulose nature of the layers in some of the dolomite laminites gives them a stromatolitic aspect, and they were interpreted as algal (cyanophytic) in most earlier works. The complete gradation between pustulose and planar-laminated rocks, however, suggests that the pustulose character probably resulted from the distortion of planar-laminated rock by the growth of small anhydrite nodules and is therefore secondary. The lamination in many of the dolomite beds of the Hartlepool Anhydrite may thus be sapropelic and not cyanophytic.

Widespread and locally severe contortion of the laminites (see Smith and Francis, 1967, pl. 13B) may have been caused by plastic flow in the anhydrite, and evidence of synsedimentary slumping has been reported only at 323.5 m in Offshore Borehole E3 [NZ 5402 4900] and at 322.25 m in Offshore Borehole WI 2B [NZ 5600 6599]. Evidence of massive contemporaneous slope failure and synsedimentary slumping such as characterises parts of the equivalent Werraanhydrit in northern Germany (Herrmann and Richter-Bernburg, 1955) has not been recorded in Durham, but it might be difficult to recognise in cores.

Taylor (1980) recognised four subcycles in the Werraanhydrit of the Southern North Sea and traced them for more than 200 km across the Zechstein basin floor. They cannot be identified with certainty in the Hartlepool Anhdrite, although the widespread concentration of two or three beds of dolomite in the lower part of the formation invites comparison.

Conditions of deposition

The distribution of the Hartlepool Anhydrite shows that it is younger than the 100 m-high shelf-edge reef of the Ford Formation, construction of which is believed to have been terminated by a sea-level decline of at least a few metres (see p.82); the anhydrite is also younger than the Trow Point Bed which is thought (Smith 1986) to have been formed under water perhaps 25 to 100 m deep. The general depositional basin-floor setting of the earliest anhydrite is thus reasonably well established; what remains uncertain is the extent to which the anhydrite is secondary and, if it were mainly primary, the depth of water under which it was formed. The suggestion by Fowler (1944) that much or all of the anhydrite might be of replacive origin is supported by widespread macroscopic evidence of replacement in almost all the recent offshore cores, and convincing petrographical evidence of extensive replacement of dolomite by anhydrite in this formation has been reported by Dunham (1948), Raymond (1962) and Dearnley (in Magraw et al., 1963; Smith and Francis, 1967). Elsewhere, similar evidence relating to the equivalent Werraanhydrit in Holland has been presented by Clark (1980a), who suggested that the anhydritisation of the dolomite was probably accomplished after moderately deep burial, perhaps in the late Permian. Raymond (1962) disputed Fowler's view that the Hartlepool Anhydrite might be largely or wholly of replacive origin, suggesting instead that it is mainly a primary sulphate body that was much modified and perhaps enlarged by diagenetic changes, including extensive replacement of dolomite by anhydrite. Raymond's view accords better with the stratigraphical continuity of the anhydrite and is accepted here.

Early judgements that the Hartlepool Anhydrite was formed in a peritidal environment (e.g. Smith, 1970b; Magraw, 1978) were influenced by contemporary views on the supposed environmental restriction of nodular sulphates to supratidal sabkhas and on the supposed applicability of uniformitarianism to water-depth constraints on modern cyanophytes. These suppositions have since been shown to be misleading and this, together with the complete absence of recognisable sabkha cycles or evidence of emergence, and the recognition that the dolomite laminites could be sapropelic, allows a subaqueous interpretation.

Hypotheses on the genesis of the Hartlepool Anhydrite and the equivalent Werraanhydrit range from direct subaqueous chemical precipitation to elaborate combinations of marked sea-level oscillations, chemical precipitation and displacive growth in sabkha environments; all the hypotheses have strengths and weaknesses, and none is universally accepted. Thus, for example, the direct precipitation hypothesis is consistent with basin-floor deposition of the laminites, with the marginal thickening of the anhydrite and with the strong evidence in Germany of submarine slumping at the edge of the marginal shelf; however, it fails to account for the displacive character of much of the anhydrite both on the basin floor and around the margins. Similarly, the more elaborate theories account for the observed repetitions of nodular and laminated anhydrite, but require repeated sea-level falls of 150 m or more, which arguably ought to have left evidence of sabkha cycles and emersion; careful examination of many complete cores has so far failed to reveal these features. On balance, a combination of subaqueous chemical precipitation of sulphate and carbonate followed by displacive and replacive growth of sulphate below the sediment/brine interface, or after burial, seems to have fewest drawbacks and to offer a plausible compromise. It allows both shallow-water and deep-water accumulation, according to basin-floor configuration, the differences between the basinal and marginal deposits perhaps resulting from depth variations and chemical gradients.

The compromise interpretation proposed here receives a measure of support from the observed sharp lateral passage from evenly thin-bedded, upright-fibro-radiate selentic gypsum into typical chickenwire (mosaic) gypsum in the Messinian of parts of Sicily and Spain (Schreiber et al., 1976), and a similar passage in anhydrite (pseudomorphing upright-fibro-radiate selenitic gypsum) in parts of the Werraanhydrit of West Germany (J Paul, oral communication, 1987). If correct, the interpretation implies that much or all of the Hartlepool Anhydrite might once have been a subaqueous accumulation of mainly subvertical gypsum crystals up to a few centimetres long, divided into thin beds by dissolution and erosion surfaces, layers of gypsum crystal debris and accumulations of carbonate and organic matter. The competitive sea-floor growth of such gypsum crystals is believed to be restricted to the photic zone (Schreiber et al., 1976), and is probably most rapid in water only a few metres deep.

The cross-sectional configuration of the Hartlepool Anhydrite is comparable with that found in its counterparts all around the Zechstein Sea, with a flat top perhaps locally as little as 1 km wide passing eastwards into a relatively steep basin-margin slope. Instability on this slope was the cause of the massive and persistent submarine slumping of sulphate rocks in many places in Germany, and it may account for some of the deformation in parts of the Hartlepool Anhydrite.

Hydration and dissolution of the Hartlepool Anhydrite

The presence of gypsum at the base, top and within the Hartlepool Anhydrite at depths to 300 m and more below OD shows that undersaturated fluids have had some access to the formation via the enclosing and interbedded dolomite, but the extent of subsequent dissolution is difficult to judge. Such dissolution may be responsible for much of the thickness variation noted, but the top of the formation has been cored in only a few boreholes and convincing collapse-breccias are uncommon except where all the anhydrite is absent. Such breccias are present in several of the more westerly offshore boreholes and in the BGS Seaham Borehole (Smith, 1971) and occur widely onshore. Dissolution of the anhydrite has caused widespread foundering of all overlying strata, leading to the formation of an elongate syncline roughly parallel with the present coast and bringing carbonate rocks of Cycles EZ1 and EZ2 into juxtaposition. The characteristics of the foundered strata will be considered later.

On dissolution, the anhydrite left a residue of a few centimetres of roughly laminated mushy dolomitic and clayey sandy limestone (Plate 21), which is well exposed at Trow Point [NZ 383 667] , South Shields (Smith, 1970b, c, 1972). The residue also locally contains gypsum and a sparse suite of detrital grains including apatite, rutile and zircon; analyses by G M Tester (written communication) show that illite is the predominant clay mineral. Where exposed in an abandoned railway cutting [NZ 399 537] near the EZ1 reef front at Ryhope, the residue is up to 6 m thick and is composed mainly of fragments of soft dolomite in a flow-banded dolomite matrix; in places the residue has been squeezed up into the overlying breccias, presumably whilst plastic, to form laccolith-like bodies up to 3 m high and 5 m across (Figure 44).

Upper Magnesian Limestone'–general stratigraphy

As depicted on the 1:50000 map, this historic stratigraphical grouping comprises the Concretionary Limestone, the Hartlepool and Roker Dolomite, the Seaham Residue and the Seaham Formation. However, the recognition (Taylor and Fong, 1969; Smith, 1971a) that the Hartlepool and Roker Dolomite (now the Roker Dolomite Formation), the Concretionary Limestone Formation and the Seaham Residue (plus the equivalent Fordon Evaporites) form part of English Zechstein Cycle 2, whereas the Seaham Formation is part of Cycle EZ3, has led to the recommendation that the traditional classification be abandoned (Smith et al., 1986). Under the proposed new classification, the Concretionary Limestone and Roker Dolomite formations are seen respectively as broadly coeval slope and shelf facies of the carbonate phase of Cycle EZ2, with a possible time overlap at the base with higher parts of the Hartlepool Anhydrite Formation of Cycle EZ1 (see (Figure 35)). The four formations of the historic Upper Magnesian Limestone are treated separately in this account, the Concretionary Limestone being considered first because progradational sedimentation generally resulted in an upwards progression from slope (Concretionary Limestone) to shelf (Roker Dolomite) deposition. Following Smith (1971), the base of the Concretionary Limestone is taken at the top of the Hartlepool Anhydrite and not at the base of the 'Flexible Limestone' as in earlier classifications; detailed examination during the resurvey has shown that strata above and below this supposed datum are lithologically and faunally inseparable.

Doubts remain about the age and environmental significance of more than 20 m of oolitic limestone and dolomite that overlie the Hesleden Dene Stromatolite Biostrome (p.80) in a reef spur south of Seaham, which are exposed at Hawthorn Quarry, just south of the district. Similar oolite separates the biostrome and the overlying Seaham Residue at Blackhalls Rocks, 9 km farther south (Smith and Francis, 1967), and could, therefore, be part of either Cycle EZ1 or Cycle EZ2. Farther south still, at Hart Quarry [NZ 476 345], the evidence of age of ?equivalent oolitic limestone is even more uncertain, for there the oolite appears to rest against and overlap the seaward face of the Cycle 1 shelf-edge reef; from an adjoining quarry, however, the oolite yielded a sparse bivalve fauna of Roker Dolomite affinities (Trechmann, 1913). The view taken here, and on the 1:50 000 map, is that the evidence marginally favours a Cycle 2 age and, accordingly, these oolites are treated here as part of the Roker Dolomite Formation. This interpretation implies that the Concretionary Limestone Formation thins sharply to the south of Seaham and is thin or absent in most coastal areas in the adjoining Durham district, where its place is taken by coeval Roker Dolomite.

Except in the eastern undersea area, all formations younger than the Hartlepool Anhydrite have foundered as a result of the dissolution of the evaporites; the profound and spectacular lithological and stratigraphical changes thus induced are discussed on pp.111–115.

Concretionary Limestone Formation (part of EZ2Ca)

Distribution and general characteristics

Strata assigned to the Concretionary Limestone Formation (as broadly defined by Smith, 1971) crop out beneath drift in most of the coastal belt of the district except near Whitburn, under the eastern part of Sunderland town and from Seaham Grange southwards (Figure 34). They are exposed in coastal cliffs almost uninterruptedly from Trow Point to Souter Point, and from Hendon to a point about 3 km north of Seaham. Inland they are seen in several disused quarries between Westoe and Cleadon (South Shields), and at a number of other places, including the famous Fulwell Hill quarries at Sunderland. The formation is perhaps 95 to 100 m thick between Marsden Bay and Whitburn, but thins to about 45 m at Seaham and dies out between Seaham and Chourdon Point (just south of the district). To the north of the district, at Marden, Cullercoats and Tynemouth, it is present as thin collapse breccias (Land, 1974), and it is widespread offshore where, locally, it makes up all of the Cycle EZ2 carbonate wedge and may exceed 100 m.

Although readily defined in general terms, exact definition of the base and top of the formation is complicated by their apparent diachroneity, interdigitation and diagenetic changes. The base, for example, is commonly sharp where it rests on normal Hartlepool Anhydrite but is uncertain where thick beds of laminated carbonate rock similar to that of the Concretionary Limestone lie near the top of the Hartlepool Anhydrite, or where thick beds of anhydrite lie near the base of the Concretionary Limestone (see (Plate 26))." data-name="images/P936014.jpg">(Figure 43)). The top has not been seen in cored boreholes, except where Roker Dolomite is absent, and it is not clear in the key surface exposures at Roker (Sunderland) and Whitburn. At Roker, the top of the well-known 5 m-thick Cannonball Limestone [NZ 4070 5960] has traditionally been taken as the top of the Concretionary Limestone, but here, spheroidal calcite concretions persist for at least 3 m up into the succeeding undoubted Roker Dolomite; similar cream dolomite with spheroidal calcite concretions is periodically exposed on the foreshore beneath the Cannonball Limestone. Somewhat similar relationships exist at Rackley Way Goit [NZ 4129 6219], Whitburn, where a 5 m dolomite full of large spheroidal calcite concretions separates a varied sequence of crystalline concretionary limestones and concretion-bearing dolomites, below, from a succession of cream dolomites in which spheroidal calcite concretions are present in some beds. From this evidence it seems that the presence of spheroidal calcite concretions may not be diagnostic of the Concretionary Limestone Formation here and that there may be an interdigitating transition between it and the Roker Dolomite Formation. The recognition that cliffs and shore platforms from Whitburn southwards may be of Roker Dolomite and not Concretionary Limestone contradicts the 1:50 000 map and results from a recent reassessment of the local stratigraphy (Smith, 1994).

The Concretionary Limestone is by far the most varied carbonate formation of the English Zechstein sequence and has been described and discussed by many writers including Winch (1817), Sedgwick (1829), Howse (1848), King (1850), Kirkby (1864b), Browell and Kirkby (1866), Lebour (1884), Garwood (1891), Card (1892), Abbott (1903, 1907, 1914), Woolacott (1912, 1919b), Trechmann (1914, 1925, 1954), Holmes (1931), Tarr (1933), Smith (1970b, 1971, 1985a), Shearman (1971), Al-Rekabi (1982) and Braithwaite (1988); most of the key facts and relationships were noted by the early workers but many problems remain to be solved.

The variability of the Concretionary Limestone arises from the complex interplay of primary and secondary influences, as a result of which most individual beds display marked lateral changes in lithology. Detailed correlation between exposures is difficult, the only exception being the widespread 'Flexible Limestone' which is a thin and relatively distinctive marker bed slightly below the middle of the formation. The primary features of the formation include fine and even lamination in many beds and impressive evidence of widespread and repeated mass downslope movement of unconsolidated sediment; their distribution allows tentative recognition of upper, middle and lower slope subenvironments (Smith, 1985b). The secondary features include extensive collapse-breccias and a range of calcite concretions which give rise to what Abbott (1914) described as 'the most remarkable of all known concretionary formations' and Tarr (1933) described as 'the most remarkable patterns in sedimentary rocks anywhere in the world'.

Despite being the best known feature, concretions are rare or absent in many parts of the formation, although even in these parts the dolomite locally passes abruptly into massive crystalline concretionary limestones. Taking the formation as a whole, perhaps half is now mainly of dolomite with few concretions, a quarter is of mixed concretionary limestone (i.e. limestone with discrete concretions), secondary limestone and dolomite, and the remainder is mainly of concretionary limestone with subordinate dolomite. Many of the rocks have a fetid smell (Sedgwick, 1829), but the limestones are generally more fetid than the dolomites.

Primary and penecontemporaneous features

Where least altered by secondary effects, most of the Concretionary Limestone Formation comprises alternations of finely laminated, slightly bituminous, mud-grade to silt-grade dolomite and unlaminated, silt-grade to sand-grade dolomite; the interbedding is on a wide range of scales and oolite grainstones and packstones are also locally present. The relative proportions of laminated and unlaminated rock vary greatly both geographically and stratigraphically, but the proportion of laminated rock generally increases downslope and is greatest on the basin floor. The presence of abundant fine parallel lamination uniquely distinguishes the Concretionary Limestone from all other Permian carbonate formations in north-east England.

Laminated dolomite

Lamination in these rocks is generally fine (10 to 30 couplets per centimetre), planar and even, but in many beds the lamination is somewhat thicker and uneven, and in others the appearance of lamination is imparted by strongly compressed subparallel carbonaceous flakes and discontinuous sheets that are reminiscent of those in the Marl Slate. The laminae comprise alternations of almost pure dolomite and slightly to strongly bituminous dolomite, together with scattered detrital (?wind-blown) grains of quartz and mica. The paler-coloured nonbituminous layers range from predominant to subordinate, perhaps partly in response to early differential dissolution, compaction and lithification; in some beds the laminae are so thin that it is difficult to distinguish and count them. Total organic contents of 1.6 to 3.37 per cent were reported by Al-Rekabi (1982) in weathered samples of Concretionary Limestone from Marsden Hall Quarry and Marsden Bay, but apparently much greater contents than this occur in unweathered lower parts of the formation in some offshore boreholes. Examination by J Robson and Dr A Douglas (written communication) of a bituminous laminite from Offshore Borehole B2 (1:50 000 Sheet 15) suggested that the kerogens present were largely derived from marine plants (probably algae) and that the hydrocarbons are 'near the maturity level associated with the onset of petroleum formation'. Robson and Douglas further comment that the kerogen isolated from the B2 sample was amorphous and of marine type which probably indicated accumulation in a reducing environment.

The exceptionally thin and parallel lamination of the 2 to 4 m 'Flexible Limestone' and its content of fish remains at Fulwell Hills, Marsden and Hendon readily distinguishes it from most (but not all) laminated rocks of the Concretionary Limestone. Similar evenly laminated and probably equivalent beds are (or have been) exposed at Cleadon Pumping Station, at Horsley Hill (South Shields), in temporary excavations 1.2 km north of East Boldon, in Fulwell railway cutting, for several hundred metres on both sides of the Wear Gorge at Sunderland, in many temporary exposures in Sunderland town centre, at Mowbray Park (Sunderland) and in the coastal cliffs near Ryhope. They also occur as fragments in collapse breccias as far west as the western outskirts of Sunderland, indicating the minimum former extent of laminated strata. The moderate flexibility of moist dolomitic Flexible Limestone, particularly at Hendon, was noted by Winch (1817) and subsequent authors. Card (1892) claimed that it probably resulted from the interlocking character of the component dolomite grains (not substantiated by Dr CJ R Braithwaite, written communication, 1987) coupled with the presence of considerable empty pore space (i.e. lack of cement). Analyses (not including organic carbon) by Browell and Kirkby (1866), Card (1892) and Trechmann (1914) showed that the flexible Limestone at Fulwell, Marsden and Hendon is a dolomite rock containing only a little calcite and 1.6 to 6.7 per cent of insolubles. As Woolacott (1912) noted, many other finely laminated dolomite rocks of the Concretionary Limestone are slightly flexible when moist and if thin enough, but their flexibility does not compare with that of the Flexible Limestone at Hendon.

Dolomite laminites occur widely in boreholes offshore, and differ in the east from those exposed onshore mainly in their high content of gypsum and anhydrite; all these offshore rocks are now brittle, but slight former flexibility is indicated by local strong contortion. Dolomite laminites in some beds separating leaves of the Hartlepool Anhydrite contain abundant displacive/replacive gypsum and anhydrite, and are indistinguishable from those in the main mass of the Concretionary Limestone Formation.

Both onshore and offshore, minor to major plication and contortion of dolomite laminites has been noted at a number of places (see (Plate 22)), and minor truncation surfaces are common. These features probably result from downslope sediment movement, ranging from slow progressive creep to actively downcutting turbid suspension currents and minor to massive submarine slides.

Unlaminated dolomite

Unlaminated beds of the Concretionary Limestone on land are mainly of almost pure dolomite and range from a few millimetres in thickness to rare masses up to 12 m thick; most are 0.02 to 0.30 m thick. Examination of thin sections shows that the dolomite is commonly composed mainly of an equant mosaic of silt-grade crystals with little or no signs of grading. Many thin unlaminated beds, however, display normal or inverse grading, with concentrations of organic matter near the top of some layers (Plate 23); these beds are probably the distal product of turbidity currents (Smith, 1980b) or storm winnowing. Comparable grading has been reported in equivalent Zechstein strata (the Stinkdolomit) in Denmark (Clark and Tallbacka, 1980) and Holland (Clark, 1980a) and was interpreted similarly. Clark also interpreted some laminated and graded beds in the Stinkdolomit as the settling product of high-slope and shelf sediment that had been stirred up and taken into suspension by storm-generated turbulence.

The thicker unlaminated beds generally feature uneven and lenticular bedding, which superficially resembles low-angle cross-stratification. These beds are interpreted as the proximal and medial products of substantial submarine slumps, slides and turbidity currents (Smith, 1970b, 1971a, 1985b). They commonly overlie a scoured surface with a relief of up to 0.5 m, and some have coarse-grained basal layers which include bivalves, gastropods and small fragments of laminite. Sole marks and coarse debris flows are uncommon but contortion and eastwards-directed overfolds ((Plate 24) and (Figure 45)) and thrust-like glide planes are relatively common. Such slumped and slid unlaminated strata are present in most exposures of the Concretionary Limestone, but are particularly abundant and well exposed in coastal cliffs between Potter's Hole [NZ 412 636] and Wheatall Way [NZ 4133 6295], Whitburn.

Here, several disturbed beds and their underlying truncation surfaces may be traced for scores to hundreds of metres along the depositional strike. Contorted strata have also been noted in cores from a number of boreholes offshore (Plate 25), but they appear to be less abundant than onshore.

Erosion and truncation beneath slumped strata in the Concretionary Limestone generally appear to have been slight, but evidence of the downslope removal of up to 5 m of strata was noted during the resurvey at a number of coastal exposures. Slump scars and canyons are relatively uncommon, the best exposed example in 1987 being in Marsden Quarry [NZ 405 642], where a steep-sided west–east channel some 7 m deep, and at least 50 m across, is cut into laminated carbonates and filled with silt to sand-grade altered oolitic dolomite packstone containing a coarse basal lag of fragments of laminated dolomite.

Study of the cliffs between Marsden Bay and Whitburn gives the impression that most or all of the unlaminated beds bear some evidence of mass downslope movement if traced far enough, perhaps implying a slump origin even where evidence of slumping is lacking. The presence of oolites is clearly anomalous in the slope environment, and they too may be allochthonous. They include the several beds and thick lenses of oolitic dolomite that form much of the cliffs [NZ 4010 6478] to [NZ 4078 6436] south of Marsden Bay; a source in the shelf facies (i.e. the Roker Dolomite) at the top of the slope has been postulated (Smith, 1970b, 1980b, 1985b).

There is strong evidence in the cores of several offshore boreholes, especially those of WM7A (6 km east of Whitburn), that some unlaminated, turbiditic ooid grainstones became fluidised after burial by laminites and escaped upwards so as to intrude the latter; or, perhaps, to flow out on to the sediment surface as oolite sand volcanoes. The fluidised sediment now forms complex injection veins, lenses and sheets, some of which are up to 30 cm thick and contain disoriented rafts of laminite.

Biota

The biota of the Concretionary Limestone Formation comprises a limited suite of poorly preserved plant remains which are common to all facies, and a strongly facies-linked fauna. The macroscopic plant remains are mainly present as carbonised filaments, some possibly algal but mostly unidentifiable, and isolated and very uncommon examples of Calcinema permiana. A few specimens of Calamites and Ullmannia (both land plants) were reported in and a few metres above the 'Flexible Limestone' at Fulwell Quarry by Kirkby (1864c), and poorly preserved plants are also present in thin-bedded limestones at Byer's Quarry [NZ 410 637], Whitburn. Much of the organic content of the laminites is probably planktonic in origin, but it has not been investigated in detail. The fauna comprises the remains of nektonic fish which are found only in laminated strata, and a shelly benthic fauna which is found only in unlaminated beds. Burrows have not been reported.

Fish in the Concretionary Limestone are restricted to a 16 m sequence including and overlying the 'Flexible Limestone' at Fulwell Hill (Kirkby, 1863, 1864b; Howse, 1880) and to the supposed Flexible Limestone in Marsden Bay and at The Crags, Hendon (Sunderland). They are relatively abundant at Fulwell only in 0.6 m of laminated limestone, the Tulwell Fish Bed of Kirkby, some 2.4 m above the base of the fish-bearing sequence, other levels and other localities having yielded only a few widely scattered individuals. Most of the fish are less than 0.1 m long, well preserved and whole, and are referred to Acentrophorus varians, according to Pettigrew (1985), these were probably free-swimming shoaling fish that formed at least part of the diet of the other fish species present, namely Acrolepis sedgwickii (0.30 m). The excellent preservation of most of the fish suggests that they were buried in quiet anoxic conditions lacking in both scavengers and a burrowing benthos.

The benthic fauna is known on land mainly from coastal cliffs and adjoining quarries between the northern end of Marsden Bay and the former site of Whitburn Colliery, but smaller collections have also been made from coastal cliffs between Salterfen Rocks and Ryhope. Offshore, good assemblages have been noted in the Concretionary Limestone in Offshore Boreholes 3, 4,10, B7 and WM7A but not in several other boreholes. Near Whitburn and Marsden, the fossils are unevenly but widely distributed amongst predominantly unlaminated beds, but in most other places the fossils are concentrated in the basal parts of unlaminated beds within otherwise laminated sequences and may have been transported there by turbidity currents or debris-flows. The benthic macrofauna includes five species of bivalve, of which the most common are Liebea squamosa, Pennophorus costalus and Schizodus obscurus, and at least four species of gastropods; the benthic microfauna, which is very abundant in places, but otherwise sparse, includes nodosariid foraminifera and at least eight species of ostracod (T H Pettigrew, written communication, 1987). The common occurrence of bivalves (either open or shut) as nucleii in calcite spherulites in the Whitburn and Marsden areas was recorded by Sedgwick (1829), and the near-perfect preservation of shell ornament on these bivalves was noted by Howse (1848) and Garwood (1891).

Conditions of deposition

The recognition of the Concretionary Limestone as a basin-margin slope deposit is based on the almost ubiquitous evidence of sediment instability and the pattern of distribution of the biota and of the main primary rock types. Similar features characterise equivalent strata in most marginal parts of the Southern North Sea basin, including Holland, Germany and Poland. In north-east England, the three main slope subfacies may be defined on the basis of primary and penecontemporaneous features:

Upper slope, comprising thin- to thick-bedded, mainly mud- and silt-grade rocks, some laminated, with a scattered to abundant well-preserved shelly fauna and a few beds of oolite; many (locally most) beds exhibit evidence of downslope movement of sediment (whilst unconsolidated or semiconsolidated) and related truncation surfaces are abundant; some slump scars and slide canyons.

Middle slope, comprising about equal proportions of alternating (a) finely laminated carbonate with rare fish remains and some evidence of downslope movement (plications, overfolds, etc.) and (b) unlaminated carbonate (including some oolites) with many disturbed and contorted beds, some with invertebrate remains near the base; contemporaneous truncation surfaces and glide planes are abundant and some slide canyons also occur; graded beds (?turbidites) are rare to common, according to locality.

Lower slope, comprising finely laminated, mainly mud-grade carbonate with rare fish remains and many (mainly thin) beds of unlaminated, mud-grade to silt-grade carbonate, some graded; a few thick lenses of slumped strata, including toe-of-slope oolite fans; some evidence of mass downslope movement in upper parts of this subfacies.

An attempt to reconstruct the various depositional environments of the Cycle EZ2 carbonate rocks of the district, including the Concretionary Limestone, is given in (Figure 46). In this, the Roker Dolomite Formation is interpreted as a shelf deposit, which, at its basinward margin, grades into the upper slope subfacies of the Concretionary Limestone Formation. The belt of rocks of slope facies may be as much as 15 to 20 km wide. The very finely laminated basin-plain facies known from the Southern North Sea (Taylor and Colter, 1975; Taylor, 1980, 1984) is probably not typically developed even in the extreme east of the district, but cores from a number of the more easterly offshore boreholes bear features indicative of an approach to basinal deposition.

In terms of the subenvironments defined here, the upper slope subfacies. is represented mainly by Concretionary Limestone strata exposed between Marsden Hall Quarry and Whitburn, and these strata probably progradationally overlie rocks of middle slope facies (perhaps with a little of the lower slope) that form most other exposures including those inland at Fulwell Hill, Carley Hill, Hendon, Ryhope and Southwick.

The evidence from limited borehole cores from offshore suggests that most of the undersea area lies in the lower slope belt, though the Concretionary Limestone cores in Offshore Borehole B7 (1:50000 Sheet 15) were probably of upper slope or high middle slope subfacies. As in the coastal exposures, sedimentary progradation ensured that successive beds at each borehole site were formed on progressively higher parts of the slope.

The geographical distribution of laminites, especially their concentration in the north of the district and as far west as Fulwell, suggests that the various facies belts of the Cycle EZ2 carbonate rocks may trend west–east or WNW–ESE rather than NNW–SSE, as might be expected from the trend of the Cycle EZ1 shelf-edge reef. If this were so, it implies that the belt of thickest Hartlepool Anhydrite may have lain west of the present coastline from Ryhope northwards, and may have been less than 2 km wide.

The depth of the Cycle EZ2 basin has been estimated (Smith, 1980b) at perhaps 300m; with marginal shelving, a depth of 200 m is reasonable at the foot of the slope and is consistent with (a) minimum slopes required to initiate and maintain downslope sediment movement and (b) the thickness of the Cycle EZ2 (Fordon) evaporites in North Yorkshire and Humberside. Upper parts of the slope sustained a fauna that was probably at least partly indigenous, indicating oxygenated conditions, but beds of much of the middle slope and all of the lower slope contain only the remains of nektonic fish and phytoplankton and were almost certainly formed under anoxic conditions. Supporting evidence of stagnation and tranquil deposition during formation of the laminites comes from the excellent preservation of the fish and of the planar lamination (indicative of the absence of scavengers and infauna), the preservation state of the kerogens (noted previously) and the absence of a benthic fauna. Similar evidence of stagnation has long been accepted in the equivalent Stinkkalk in Germany, and has been further supported in Germany by isotopic analyses (Magaritz and Schulze, 1980). A stratified water column is thus implied, the lamination probably indicating seasonal (?annual) changes, including planktonic blooms and auto-eutrophication. Deposition on the marginal slopes must have been slow and rhythmic for most of the time, but was periodically interrupted (and many of the newly formed deposits swept away) during brief episodes of sediment instability. Estimates of the number of laminae in slope rocks thus give a poor indication of the possible duration of Concretionary Limestone sedimentation, but estimates based on low-slope laminites in Offshore Borehole B8 (1:50 000 Sheet 15), where slumping was not excessive, and in Offshore Borehole WM7A, suggest a possible time span of at least 120 000 years.

The origin of the fine sedimentary particles of the Concretionary Limestone is uncertain. Most early authors regarded them as a chemical precipitate of the open sea, and this view was accepted by Al-Rekabi (1982), who also thought that much of the original carbonate mud was produced by the disaggregation of algal skeletons. A hint of an alternative view was given by Shearman (1971), who suggested that some of the calcite of the laminated beds might originally have been a near-surface gypsum precipitate that was converted to calcite whilst falling to, or lying on, the sea floor.

Whatever the origin of the carbonate mud grains, their concentration on the higher parts of the basin-margin slope must indicate disproportionately high marginal mud productivity. The view favoured here is that much of the carbonate mud was probably generated on the broad shelf of the equivalent Roker Dolomite Formation and periodically was swept over the shelf edge, together with shelly grainstones, into the slope domain of the Concretionary Limestone Formation. This view of a hemipelagic origin is consistent with the high incidence of slope failure and sediment instability, which is a natural consequence of slope oversteepening through differentially rapid slope-top accumulation, and also with the observed basinward thinning of the carbonate laminae. The fact that no comparable thinning has been observed in the organic-rich laminae supports the view that these were pelagic and that their deposition was related to more widespread events such as seasonal planktonic blooms.

Secondary features other than major collapse phenomena

Diagenetic modifications to the Concretionary Limestone include the addition of large quantities of calcium sulphate, most of which has since been dissolved from land and inshore areas, but which persists in the east of the district, and the formation of secondary limestones and calcite concretions for which the area is justly famous. Sulphate impregnation appears almost entirely to have preceded the growth of concretions and to have followed dolomitisation.

Gypsum and anhydrite

These minerals widely comprise 5 to 15 per cent of both laminated and unlaminated dolomite (and locally limestone) of the Concretionary Limestone in the east of the district, where they fill most interstices and form sparse to abundant replacive/displacive lenses and irregular patches. Mineral relationships indicate that the sulphate was originally anhydrite and that the gypsum is a hydration product related to uplift. Anhydrite is most common in the extreme east of the district, but even here part of it generally has been altered to selenite. At depth the anhydrite is mainly composed of tightly-packed tiny laths and encloses many small corroded rhombs and patches of dolomite; it hydrates to coarsely crystalline poikilotopic gypsum. The evidence of displacive growth lies in the common disruption of laminae (see Raymond, 1962, fig. 7c). Pyrite is common in the sulphate/carbonate rocks, particularly at or near mutual contacts.

In addition to forming replacive/displacive patches, sulphate in the Concretionary Limestone forms irregular beds created by the coalescence of nodular patches and, exceptionally, forms units up to 6.5 m thick. Such units in Offshore Boreholes WM7A and W15 lie about 10 to 15 m above the base of the formation and comprise a strongly layered sequence in which thin irregular sheets of coalesced nodules are crudely interbedded with subordinate irregular laminae and thin beds of dark grey bituminous dolomite, and with beds up to 0.06 m thick in which the anhydrite features tightly packed enterolithic folds (Plate 26). There is a complete gradation between the nodular sheets and the enterolithically folded beds, probably indicating that the latter, too, were at one time discontinuous layers of nodules. Similar enterolithic anhydrite was illustrated by Raymond (1962, pl. 4) from low in the equivalent Kirkham Abbey Formation of the Robin Hood's Bay Borehole (North Yorkshire) and by Clark (1980b) in the equivalent Z2 carbonate in Holland. Such structures are common in sabkha environments, but the basinal location of WM7A and W15 boreholes excludes such a sabkha origin, unless multiple major sea-level oscillations were involved. The writer believes that the location, style and fabric of the enterliths is consistent with displacive growth of sulphate just beneath the sediment surface, under anoxic conditions at water depths of perhaps 100–200 m; organic matter and sulphate-reducing bacteria may have been involved. The possibility of the deep subaqueous or post-burial diagenetic formation of nodular and thinly bedded anhydrite was discussed by Dean et al. (1975) in an attempt to reconcile apparent inconsistencies resulting from the overenthusiastic application of the sabkha concept.

Veins of fibrous gypsum occur in many of the cores of offshore boreholes, generally along planes of weakness such as joints, carbonaceous and micaceous laminae, and stylolites. They are most abundant in the lower part of the formation, where they commonly form a dense rectilinear network; dissolution of this network leaves a semi-breccia of blocky fragments such as is exposed at many places along the coast (see Smith, 1972, pls. 5, 6). The gypsum veins probably were emplaced following hydrofracture during unroofing, in the manner proposed by Shearman et al. (1972), but some may result from sulphate-rich brines created by the dehydration and later dissolution of the Hartlepool Anhydrite.

Concretions and secondary limestones

The calcite concretions of the Concretionary Limestone have been the subject of repeated description and comment, and were freely described and illustrated by Abbott (1903, 1907, 1914), Holtedahl (1921) and Braithwaite (1988). They occur at all levels in the formation but on land appear to be most abundant in units 3.6 m and 16.5 m thick, about 27 m and 55 m above the base of the formation respectively (Smith, 1971). Of these two units, the main or upper unit is that which overlies the Flexible Limestone of Carley Hill, Fulwell and Southwick quarries, Marsden Bay and Hendon; it is also exposed in Mowbray Park, Sunderland, and formerly in old quarries and railway cuttings at Grangetown, Sunderland.

The concretions have a bewildering range of mutually gradational forms, and attempts to describe them in words have generally fallen far short of conveying their variety and character. The simplest types are lithologically similar to early diagenetic calcite concretions found in many carbonate sequences, and are perhaps best exposed at Cannon-ball Rocks [NZ 4071 5960], at the top of the formation or just above it at Roker, Sunderland. Most of these concretions are concentrically layered, finely crystalline, spheroidal masses 0.05 to 0.20 m in diameter (but locally 0.3 m), many with a small irregular central cavity; the concretions are mainly in point contact or mutually interfere. By far the most common concretions elsewhere, however, are calcite spherulites ((Plate 27)B), which are mainly less than 0.05 m across but locally exceed 0.15 m; these are found in both unlaminated and laminated beds of the Concretionary Limestone, and are dominant in upper slope and high mid-slope rocks in the Cleadon, Marsden, Whitburn and northern offshore areas. Most spherulites lack concentric layering and, where especially large, pass peripherally into irregular masses of coarsely crystalline white calcite. Spherulites around Marsden are preferentially nucleated on bivalve shells (Woolacott, 1912), but such nucleation is uncommon elsewhere.

Most of the remaining types of concretion result mainly from the intersection of radial and concentric (rhythmic) elements and are most abundant in mid-slope rocks such as those at Carley Hill, Fulwell and Southwick quarries, Mowbray Park and Hendon. Here, unlaminated beds mainly contain relatively simple globular ('cannon-ball') and club-like to rod-like concretions ((Plate 27)A, F), which in places form almost all of the rock, and laminated beds most commonly contain highly complex 'reticulate' concretions ((Plate 27)C, E); other concretions include radiating tubes and bizarre combinations of two or more of the main types.

Workers from Sedgwick (1829) onwards have noted that primary lamination passes uninterruptedly through many concretions and that their distribution and pattern is strongly affected in many places by the incidence of joints, fractures ((Plate 27)G) and major bedding planes.

The irregular areas between concretions and within the framework of reticulate concretions are generally partly or wholly filled with uncemented dolomite of fine silt grade, which is readily removed by surface weathering. Braithwaite (1988) interprets this dolomite as infiltrated internal sediment, and notes that it predates stylolites in the host rock. The presence of idiomorphic twinned scalenohedra of calcite up to 50 mm long in interconcretion dolomite at Fulwell Quarry was noted by Trechmann (1914); such calcite has since been found at Marsden and several other localities. Offshore, interconcretionary areas range from thinly lined cavities to complete fill of a range of types; the cavity-lining here mainly comprises internal sediment and dog-tooth spar, and the fill is of finely granular calcite or dolomite, commonly with a gypsum or anhydrite cement.

The long faces formerly seen at Carley Hill, Fulwell and Southwick quarries showed that there is a strong tendency for individual beds to bear the same general type of concretion for considerable distances and that certain concretionary patterns are almost restricted to some beds. Thus, for example, concretions like that in (Plate 27)D are virtually confined to a bed near the top of the Flexible Limestone, and the most complex reticulate concretions are found mainly in a 4.5 to 5.0 m group of beds immediately overlying the Flexible Limestone. Local sequences may be constructed by using these features and, more readily, by tracing non-concretionary 'marl’ beds, but, as Kirkby (1864c) noted, such sequences are subject to sharp lateral variation. Evidence of downslope sediment movement is more difficult to recognise in the concretion-rich beds than in their dolomite equivalents, because of the destruction of most primary fabrics, but many of the larger-scale features such as slump rolls are preserved locally.

In addition to the limestones with abundant concretions, considerable parts of the Concretionary Limestone are of hard grey and brown limestone almost without concretions; some or all of these may be secondary limestones, having passed through a dolomite stage. They include flaggy and thin-bedded, mainly unlaminated, high-slope deposits, including those that make up much of the cliffs between Lizard Point and Potter's Hole [NZ 412 636], Whitburn; they comprise patches of mid-slope rocks in cliffs towards the southern end of Marsden Bay and elsewhere; and they also comprise considerable sequences in low-slope laminated rocks in some offshore boreholes such as B8 (off Blyth, 1:50000 Sheet 15). The Whitburn rocks, including those in the well-known Byer's Quarry (now filled), contain an abundant and surprisingly well-preserved bivalve-foraminifer-ostracod fauna and many obscure plant remains (see 'Biota', p.92). The rocks are generally very finely crystalline, whereas the laminated secondary limestones cored offshore are generally lustre mottled and coarsely crystalline. A rare type of coarsely crystalline limestone was seen during the resurvey at the northern extremity [NZ 403 644] of Marsden Quarry, where beds up to 0.3 m thick were formed entirely of apparently replacive upright-fibro-radiate calcite.

A feature of many laminated limestones of the Concretionary Limestone is the widespread occurrence (and local abundance, up to 15 per cent) of mainly displacive thin lenses, tongues and more complex 'Christmas-tree' patterns of white coarsely crystalline blocky calcite. These have sharply pointed feathery terminations and are mainly concordantly elongated; they are present at all levels in the formation and are particularly abundant in laminated beds in parts of Fulwell Quarry where many are transected by large, younger, radiate, prismatic calcite (Figure 47). Some calcite lenses have shapes similar to those of displacive calcium sulphate (Plate 28); it is probable that this calcite has replaced anhydrite. Most of the lenses, however, are more acutely terminated and thinner than the anhydrite lenses preserved in the undersea areas, and bear little evidence of a replacive origin; they may be void-fill.

Evidence of both increases and decreases in volume is present in many parts of the formation. Volume increases are most clearly indicated by the displacement of laminae around lenses of secondary calcite and anhydrite; it results in distortion of the bedding and the creation of minor tepee-like structures in some laminites (Plate 29). Volume decreases, apparently resulting from dissolution of both carbonate and sulphate rocks, are also on a wide range of scales. The main large-scale dissolution effects (other than those caused by dissolution of anhydrite beds) affect both dolomite and limestone, but are most common in dolomite; they are seen in many parts of the coastal cliffswhere in places, parts of beds have been dissolved so as to cause foundering and brecciation of overlying strata (Plate 30). This type of dissolution may be related to circulating meteoric groundwaters, but many of the small-scale dissolution effects probably result from leaching and stylolite formation at depth (Plate 31), (Plate 32) (see also Clark, 1980b and Braithwaite, 1988). Some strongly spherulitic limestones have been partly to wholly fragmented by volume loss; breccia fragments created by these small-scale processes are generally only a few centimetres across. Intervening areas are either empty or comprise buff, brown or grey finely granular dolomite cavity-fill, together with complex arrays of displacive calcite (Plate 32) in both tongues and downward-pointing, void-lining, dog-tooth prisms. Much of the cavity-fill is multiphase, indicating episodic accumulation, and some was contorted or fractured by continuing settlement and compaction.

Composition, petrology and mineralisation of the concretionary and other secondary limestones

Analyses of the Concretionary Limestone (Browell and Kirkby, 1866; Garwood, 1891; Trechmann, 1914; Al-Rekabi, 1982) show that the concretions are of almost pure calcite (mainly >95 per cent), with minor amounts of dolomite, metallic oxides and silica. The powdery matrix, by contrast, is mainly of dolomite with up to 25 per cent of calcite. AIRekabi (1982) also investigated the silica (1.2 to 2.4 per cent), iron (0.1 to 0.8 per cent) and strontium (256 to 1021 ppm) contents, and reported traces of contaminating Ti, Ba, Rb, Zn,. Cu, Pb, Ni, Mn, K and Na. Rough calculations by Garwood (1891) suggested that the bulk composition of concretion-bearing Concretionary Limestone, including interstitial dolomite, is about the same as that of the almost pure dolomite rock of the Roker Dolomite at its type locality. From this, Garwood inferred that the striking secondary changes in the mineralogy of much of the Concretionary Limestone could have taken place in a closed system in a dolomite host rock.

The calcite of the concretions varies widely in grain size and crystal geometry, and includes dense mosaics of interlocking silt-grade crystals, radial assemblages of fibrous and prismatic crystals up to 0.20 m long and a complete spectrum of intermediate types. Some globular concretions are composed almost entirely of small calcite subhedra, but most globular and club-like forms have cores (commonly concentrically layered) of small crystals, surrounded by outwards-thickening fibrous and prismatic crystals. Most of the megascopic crystals are turbid and brown, because of the presence of abundant small dolomite rhombs, microcavities (Al-Rekabi, 1982) and specks of limonite, but the outermost parts of the largest prisms commonly contain relatively fewer inclusions and are white or colourless; Al-Rekabi noted that many calcite crystals are strongly zoned in their distal parts. A large proportion of the crystals bifurcate outwards, particularly in outer parts of the larger concretions, and all have scalenohedral terminations where facing the powdery dolomite matrix. Many of the longer calcite crystals are curved ((Plate 27)D), presumably in response to competitive growth pressures, and crystals in some of the coarsest mosaics also have curved (strained) faces. Spherulites differ from the globular and club-like concretions mainly in being radially crystalline to the core (or nearly so) and in commonly being nucleated on bivalves; they too, though, have scalenohedral terminations, exhibit crystal bifurcation and, in the largest spherulites, display crystal curvature and outwards-increasing purity. Phemister (Geological Survey internal report, 1939) detected traces of aragonite in spherulites from Whitburn, and suggested that the calcite crystals had replaced aragonite; Al-Rekabi (1982), however, found no evidence of past or present aragonite in the Concretionary Limestone, other than in molluscan shells.

Patches of replacive anhydrite are present in some incipient concretionary limestones cored in boreholes offshore, including W15, and appear to have exerted an inhibiting effect on the growth of the concretions.

Small amounts of secondary minerals other than calcite and anhydrite are widely dispersed throughout the Concretionary Limestone. Pyrite is probably the most abundant, but other minerals include celestite, chalcopyrite, fluorite, galena, halite, sphalerite, chalcedony, chert and quartz. Most occur in cavities, but Al-Rekabi (1982) reported that some chalcedony is probably secondary after anhydrite and that some of the quartz probably replaced calcite and dolomite. The halite fills a few narrow veins offshore, but may formerly have been more widespread. A lens of chert, 9 m long and 0.15 m thick, was reported by Burton (1911) in coastal cliffs near Ryhope.

Despite the oily smell of much of the crystalline concretionary limestone, the advanced maturation of the kerogens, and the fact that equivalent strata in Germany and Poland are regarded as hydrocarbon source rocks, mineral oil has not been recorded in this district; some of the bitumen is concentrated in stylolites and may be a dissolution residue.

Age and origin of the concretions and other secondary limestones

Progress towards understanding the age and mode of origin of the calcite concretions and other secondary limestones of the Concretionary Limestone has been slow and not entirely successful. Their age cannot be closely estimated, but Sedgwick (1829) and Howse (1848) deduced that they postdated lithification of the rock, and this view is now generally accepted for most of the complex concretions. Petrographical evidence, additionally, shows that the calcite of the complex concretions is younger than most or all of the anhydrite in the rocks, which in turn is younger than the main phase of pervasive dolomitisation; the latter is regarded by Al-Rekabi (1982) as having resulted from seepage reflux. Much of the secondary calcite must have formed, therefore, after appreciable burial, and the local presence of sutured contacts between concretions shows that the concretions are older than at least some stylolites. From their field relationships, they clearly predate the formation of gypsum veins, anhydrite dissolution and faulting, all of which may be Cretaceous to Recent in age and probably accompanied uplift. The growth of the concretions presumably was slow and may have been episodic.

Few of the early workers speculated on the mode of formation of the calcite concretions, though it is clear that they understood them to have resulted from segregation of calcite in a rock that previously may have been a more even mixture of calcite and dolomite. Garwood (1891) understood the complexity of the problem and was the first to suggest that carbonic acid and other gases given off during the decomposition of organic matter in the deposit may have played a significant part in facilitiating the redistribution of calcium carbonate. Young (in discussion on Abbott, 1903, p.52) commented on the possible involvement of gypsum in the formation of the concretions, and the presence of both colloidal organic matter and calcium sulphate was regarded as probable and crucial to the process by Woolacott (1912, 1919b) and Trechmann (1913, 1914 and later). It was Woolacott (1912) who first applied the term 'dedolomitisation' to the replacement of the dolomite by calcite in the Concretionary Limestone, following experimental work by von Morlot (1848, quoted by Woolacott); von Morlot, followed by Sorby (1856), invoked Haidinger's Reaction (CaMg(CO₃)₂+ CaSO, = 2CaCO₃ + MgSO₄) to explain the mineralogical changes.

Little further progress on understanding of the processes involved in the calcitisation of parts of the Concretionary Limestone Formation was made until Shearman (1971) suggested that the calcite of the concretions might have formed by a very late reaction between hydrocarbon seeps and former anhydrite rocks, promoted by sulphate-reducing bacteria. In discussing this possibility, Rickard (1971) pointed out that the replacement of anhydrite by calcite can be brought about by the activities of sulphate-reducing bacteria on or just below the sea bed. The general environmental setting, with fossiliferous strata lying up-slope of the fish- and concretion-bearing slope strata, however, seems to rule out a precipitated gypsum precursor for most of the laminites, and Braithwaite (1988) found no petrographical evidence of widespread calcite-replacement of primary gypsum. Both Braithwaite and Clark (1980b, 1984) follow Woolacott in recognising calcite-replacement of dolomite as the main concretion-forming process, and both stressed the importance of the association of the replacement with calcium sulphate or sulphate-rich brines in the presence of the degradation products of organic matter such as that in the bituminous laminae. Al-Rekabi (1982) thought that the calcitisation was mainly a near-surface phenomenon, but Clark (1980b) inferred that the calcitisation took place during burial at a depth of between 300 and 1200 m. The depth at which calcitisation took place in equivalent strata in Germany and Denmark was estimated at 1600 to 2000 m and over 2000 m by P Huttell and N Stentoft respectively (personal communication, 1987).

The mechanism of formation of the prismatic calcite crystals in the Concretionary Limestone was considered by Al-Rekabi (1982), who showed that tiny dolomite rhombs of the dolomitised mud were both included and excluded as the crystals grew and became idiomorphic. Both Clark (1980b) and Al-Rekabi (1982) illustrated possible sequences leading to the formation of small calcite spherulites, but Al-Rekabi, impressed by the similarity of shape of the spherulites to those of spherulitic gypsum, also envisaged an intermediate sulphate stage.

In summary, there is evidence that some laminated parts of the Concretionary Limestone Formation may be of calcitised sulphate rock, but their existence and extent has not been conclusively demonstrated; a limited distribution of this type to lower basin-slope or basin-plain environments seems likely. Large-scale calcitisation of secondary spherulitic gypsum has also been proposed, but the absence of supporting petrographical evidence and the richly fossiliferous nature of many of the spherulitic rocks argues against such a mechanism. Why, for example, should gypsum nucleate on an aragonite or calcite invertebrate shell? And where in nature are the reticulate gypsums that logically would be the basis of the reticulate calcite concretions?

The mode of origin of calcite spheroids and spherulites in a dolomitised mud has been plausibly advanced, and could account for the dominant spherulitic and club- or rod-like concretions. However, the detailed mode of origin of the vastly complex reticulate concretions remains almost unexplored and unexplained. Centrifugal rhythmic precipitation from unevenly spaced centres, provided at least partly by fluid pathways such as bedding planes and joints, is clearly involved, but the detailed shape of the concretions and their constituent calcite crystals must depend on the interaction of a whole range of compositional and physicochemical variables. For a fuller review of the factors leading to the formation of the concretions, see Braithwaite (1988).

Roker Dolomite Formation (part of EZ2Ca)

Distribution and general characteristics

This formation, the shelf facies of the Cycle EZ2 carbonate (formerly known as Hartlepool and Roker Dolomite),crops out for about 6 km under coastal parts of Sunderland town and more widely in the south of the district (Figure 34), where it forms most (and locally probably all) of the Cycle EZ2 carbonate unit. Limited data from boreholes offshore show that it is widespread in the east of the district, but they do not allow it to be delineated satisfactorily. The diachronous basal contact with the Concretionary Limestone Formation (Figure 46) is taken at the top of the uppermost finely laminated bed or of the uppermost complex calcitic concretions (whichever is the higher), and the top of the Roker Dolomite is taken at the base of the primary evaporites of the overlying Fordon Formation or of the equivalent Seaham Residue.

At its type locality at Roker, Sunderland, the formation is more than 20 m thick (Smith et al., 1986) and it is about 45 m thick in the Seaham Borehole (Smith, 1971) and in coastal cliffs south of Seaham. Offshore, it exceeds 112 m in Offshore Borehole No. 2, just south of the district (Magraw et al., 1963), but it thins northwards as the Concretionary Limestone thickens complementarily and is absent in some north-eastern parts of the district, where all the Cycle EZ2 carbonate rocks are of Concretionary Limestone (slope) facies.

The main exposures of the Roker Dolomite Formation are in the coastal cliffs between Whitburn and Roker, and in coastal cliffs and denes north and south of Seaham; rocks attributed to this formation are also exposed in Hawthorn Quarry nearby on 1:50 000 Sheet 27 and in a number of small railway cuttings and quarries in the southern coastal belt. Cavities after sulphate are ubiquitous in surface exposures. The traditional base of the formation is exposed [NZ 4070 5960] in low cliffs at Roker Cliff Park, Sunderland (but see p.88), where it intricately interdigitates with concretion-bearing limestone, and the top is exposed beneath the Seaham Residue for several hundred metres in cliffs north of Seaham.

In the east of the district, the Roker Dolomite is known only from borehole cuttings and a few short cores taken in Offshore Boreholes Nos. 4, 11, 12, 13, 17 and 18 (some located to the south, on Sheet 27) and from rather more complete cores from Offshore Boreholes Nos. 1 and 2, both on Sheet 27 (Magraw et al., 1963; Magraw, 1975).

All the Roker Dolomite exposed on land and much of that offshore has foundered as a result of the complete or partial dissolution of the Hartlepool Anhydrite, but disruption and brecciation are generally slight, except to the south of Seaham where patchy collapse-brecciation has occurred. This difference in the response to foundering is probably related to the southwards thinning of the underlying Concretionary Limestone, which is sufficiently thick north of Seaham to have exerted a cushioning effect.

The Roker Dolomite in cliffs and shore platforms at its type locality mainly comprises sparingly fossiliferous, soft, cream, dolomitised, ooid grainstone and packstone with greatly subordinate discontinuous drapes, laminae and thin beds of soft, cream, silt-grade saccharoidal dolomite. The beds of grainstone and packstone are generally 0.03 to 0.10 m thick and, though persistent, are individually variable and locally lenticular. They feature broad, apparently primary undulation, widespread small-scale planar, tabular and trough cross-lamination, and a range of low-amplitude, symmetrical and asymmetrical wave and current ripples with wavelengths up to about 1.2 m. Minor erosion surfaces occur locally. Also present are several laterally persistent oolite beds up to 0.9 m thick that contain abundant, exceptionally large load casts, and several slightly discordant, lenticular breccias up to 0.4 m thick, composed mainly of small, partly matrix-supported, sub-angular fragments of ooidal and silt-grade dolomite. Slightly inland from the Roker cliffs, oolitic Roker Dolomite in Mere Knolls Road [NZ 4028 5980] contains relatively fewer fine-grained beds and is mainly calcitic; bed forms here are larger than those in the Roker cliffs, perhaps indicating higher energy conditions.

The Roker Dolomite exposed for about 1.8 km in coastal cliffs north of Seaham (and in intersecting denes) is generally similar to that at the type locality but lacks load casts and contains fewer bedded breccias; it does, however, feature small numbers of rip-up clasts at some levels, and possible crusts. South of Seaham, much of the formation has been diagenetically altered to a coarsely saccharoidal dolomite, but pockets of recognisable ooids survive and traces of the coarser sedimentary features such as cross-lamination are widespread. Parts of the formation in this southern area are calcitic (dedolomite), and multiple grains (grapestones) and other pisoids are present in the exteme south and in adjoining parts of the Durham district (Smith and Francis, 1967, p.146, under 'Middle Magnesian Limestone') .

Beds of uncertain age, but probably belonging either to the Roker Dolomite or to interdigitate strata between it and the Concretionary Limestone, form a much-dislocated and varied sequence in low coastal cliffs and broad shore platforms between Whitburn and Whitburn Bay. They mainly comprise thin- to thick-bedded, soft, cream, saccharoidal dolomite (some oolitic), but also include beds, lenses and patches of hard brown crystalline limestone and dolomite; some of the dolomite beds contain spheroidal calcite concretions. Bivalve moulds are present throughout, but are especially abundant in the higher parts of the sequence.

The most unusual rock type seen in these strata at Whitburn is a highly altered, oncolite-like, cellular limestone that is exposed for a few metres in a collapse zone in coastal cliffs at White Steel [NZ 4133 6192]. This rock forms beds individually 0.2 to 0.4 m thick and totalling 1 to 1.2 m, and is composed of a mass of tightly compressed ovoid grains up to 12 mm across (Plate 33). Some of the grains are hollow or only thinly calcite-lined, but the leached cores of most, as well as the inter-granular spaces, are filled with a complex range of calcite cements; geopetal fabrics are present in some. None of the grains have concentric coats and most are probably compressed bivalve shells (Dr G M Harwood, personal communication, 1988).

The cellular limestone at Whitburn lies low in the presumed Roker Dolomite sequence and is overlain by about 0.4 m of rubbly brown clay, upon which rests about 0.3 m of contorted, buff calcite laminite (Plate 34); it is immediately underlain by 0.5 to 0.8 m of mottled contorted clay rich in angular fragments of the cellular limestone, and this in turn overlies crystalline limestone. Both of the clay beds have many of the characteristics of evaporite dissolution residues, and the contorted laminite may be algal.

Judging from the scanty borehole data, the Roker Dolomite in the undersea area generally comprises sparingly fossiliferous dolomites similar to those exposed on land, but pisoids and other multiple grains are relatively more abundant than in the land exposures, and fossils are also locally more plentiful. The greater relative abundance of pisoids in the seaward parts of the Zechstein Cycle 2 grainstone belt is a common pattern in Denmark, Germany, Holland, Poland and the southern North Sea (see Smith, 1985a for summary and selected references). Replacive gypsum and anhydrite suffuse most of the formation in the eastern undersea area and also form abundant patches (up to 0.1 m across) and vein-fill.

Composition and lithology

Analyses of sulphate-free Roker Dolomite from surface exposures and offshore boreholes (Trechmann, 1914; Al Rekabi, 1982) show that it is mainly a pure or only slightly calcitic dolomite rock. The main impurities are silica (up to 2 per cent, but generally less than 1 per cent), which occurs as scattered small detrital (?wind-blown) quartz grains, and metallic oxides that together commonly form 0.1 to 0.2 per cent of the rock.

The calcitic rock exposed in Mere Knolls Road, Roker, is the only known exception to the general dolomitic composition of normally bedded parts of the formation, but collapse-brecciated lower parts of the formation have been partly to mainly dedolomitised, and the uppermost 1 to 3 m of the formation has been extensively dedolomitised where it underlies the Seaham Residue. Where exposed at Featherbed Rocks [NZ 430 499], Seaham, the dedolomitised rock is a dense cryptocrystalline limestone that has a sharp but extremely irregular base and which extends downwards for several metres alongside a complex boxwork of joints and fractures; infiltrated fines form laminar fill in former dissolution cavities near the top. North of Featherbed Rocks, the contact with the Seaham Residue is locally blurred by interdigitation, but its approximate position is marked by a discontinuous layer of large, calcitic, dogger-like concretions.

The petrography of the Roker Dolomite has not been studied in detail, but its component ooids are mainly small (0.1 to 0.5 mm) and subspherical in many surface exposures. Trechmann (1914) showed that most of the ooids are hollow, the nucleii having been dissolved and their place partly taken by sparry calcite or dolomite; the cortices of the ooids are of pure dolomite. Hollow or thinly lined ooids, similar to those of the Roker Dolomite, were studied in the Cadeby Formation of Yorkshire by Kaldi and Gidman (1982), who concluded that the dissolution of the cores of the ooids was penecontemporaneous and possibly accomplished by mixed meteoric and marine fluids. Not all ooids of the Roker Dolomite are subspherical; some of the ooid grainstones on land and many of those cored offshore are a mixture of subspherical, irregular and fusiform grains, including pellets, a range of compound grains, bioclasts (some coated), stromatolite flakes, pisoids and ?oncoids up to (exceptionally) 2 cm across.

The ooids in much of the Roker Dolomite have been greatly altered during diagenesis, and in many places (especially south of Seaham) have been almost obliterated. Such rocks are now composed mainly of coarse dolomite rhombs up to 1 mm across, and there is a strong suspicion that ooids were formerly present in many parts of the formation where they are no longer apparent or are represented by scattered ghosts. The many laminae and thin beds of silt-grade dolomite seen at outcrop, however, comprise dense mosaics of interlocking dolomite rhombs about 0.03 mm across and are almost certainly dolomitised carbonate muds; they contain a little interstitial calcite.

Calcium sulphate, preserved almost entirely offshore, is mainly gypsum; in addition to forming scattered irregular patches, it forms intersecting platy clusters, fills ooids and intergranular pores and, in places, is in the form of poikilotopic crystals up to 0.1 m across. No doubt the gypsum passes at depth into anhydrite.

Biota

The Roker Dolomite has a limited biota consisting of poorly preserved plants and a small number of species of bivalves, gastropods and ostracods; invertebrates are commonly crowded together so as to form coquinas on preferred bedding planes, but most beds contain only scattered shell fragments or appear to be barren. The plant remains occur at all levels and are mainly carbonised filaments up to a few centimetres long; they are probably algal.

Bivalves in the Roker Dolomite are mainly referable to Liebea squamosa and Schizodus obscurus, with smaller numbers of Permophorus costatus; the gastropods include Yunnania? helicina and Naticopsis minima, but many cannot be identified (Pattison in Smith, 1970b). Most of the ostracods are bairdiids and, though widely and thinly scattered, they also occur in rock-forming proportions in a few thin beds. Burrows are uncommon.

Conditions of deposition

The location of the Roker Dolomite as a broad sheet on the top of the Hartlepool Anhydrite establishes it as a basin margin deposit, and its lithology, sedimentary structures and biota are all consistent with shallow-water to peritidal accumulation under moderate- to high-energy conditions punctuated by quiet spells in which carbonate mud was able to settle. The presence of load casts indicates phases of sediment dewatering, and the inferred rip-up clasts, minor erosion surfaces and possible crusts may indicate periods of subaerial exposure and/or contemporaneous cementation. Taken together, these features point to formation of the Roker Dolomite on a wide shallow shelf, probably mainly just below and partly within the small tidal range. The lenses of fragments of oolitic and silt-grade dolomite at the type locality have many of the characteristics of subaqueous debris-flow deposits, perhaps indicating that this locality at one time lay at or near the crest of the basin-margin slope, which was inherently unstable; a measure of contemporaneous lithification is also implied. The character of the biota suggests that the salinity of the water may have been slightly higher than normal, and the presence of coquinas suggests periodic storm winnowing.

Viewed broadly, the Roker Dolomite is here interpreted (Figure 46) as the deposit of a belt of shallow water shoals that separated and acted as a barrier between the lagoon-al evaporites of the Edlington Formation to the west and the Cycle EZ2 slope and basinal rocks to the east.

Fordon Evaporite Formation (EZ2E) and Seaham Residue (EZ2E(R))

Distribution and general characteristics

The Fordon Evaporites are present only in the eastern part of the undersea area, but their former extension westwards, at least as far as the present coast, is indicated by the equivalent Seaham Residue (Smith, 1971a, 1972) and by foundering of the overlying Seaham Formation. The evaporites have been cored only in Offshore Borehole No. WM7A [NZ 5270 6290], but their presence elsewhere may be inferred tentatively from their expression on wireline logs. These data suggest a thickness of up to about 90 m, which considerably exceeds the 15 to 30 m recorded in this formation (under its former name of Lower Evaporite Group) in eastern Cleveland (Napier, 1948; Raymond, 1953; Falcon and Kent, 1960). By comparison with equivalent strata in North Yorkshire (Smith, 1974; Taylor and Colter, 1975), the Fordon Evaporites in Borehole WM7A seem atypically thin, perhaps indicating partial dissolution. The formation may thicken sharply eastwards in the extreme east of the district, and westwards it probably passed into the upper part of the Edlington Formation (formerly the Permian Middle Marls) (Smith, 1974b, 1980b, 1989; Goodall, 1987), now no longer preserved in the district.

The generalised succession of the Fordon Evaporites and immediately underlying beds in Borehole WM7A is:

Dolomite, grey, finely crystalline, very finely and evenly laminated (top of main part of Concretionary Limestone Formation)

The choice of the base of the Fordon Evaporites in this borehole is complicated by the interbedding of dolomite and anhydrite in the lowest 4.87 m of the section quoted. A case can be made for recognising an interdigitated transition to the Concretionary Limestone, but the anhydrite units low in the interbedded sequence may be secondary (diagenetic); the base is here tentatively taken at the top of the highest (0.62 m thick) laminated dolomite-anhydrite unit.

The halite is lithologically similar to that reported at many levels in the Fordon Evaporites of the type borehole in North Yorkshire (Stewart, 1963); few original fabrics have survived complex diagenesis. Some of the halite appears to have been formed by a combination of displacive and replacive growth within a mainly finely laminated anhydrite host, leading to a nodular fabric similar to that of nodular and mosaic anhydrite. Contortion in parts of the halite is emphasised by slightly to tightly folded anhydride layers (Plate 35), but is also indicated by distorted crystal fabrics in places where anhydrite is absent.

The underlying finely laminated anhydrite is noteworthy as being the possible equivalent of the 2 to 3 m Basalanhydrit of the German Zechstein sequence. Detailed studies of the Basalanhydrit in Germany enabled Richter-Bernburg (1957, 1958, 1986) to correlate individual laminae for distances of more than 200 km across the basin floor and inspired similar study of anhydrite laminites in several other evaporite basins.

The Seaham Residue, at its type locality in coastal cliffs [NZ 4303 4970] to [NZ 4300 5090] between Seaham Harbour and Featherbed Rocks, is up to 9 m thick and is a heterogeneous, strongly contorted deposit (Plate 36) (Smith, 1971). Despite the general heterogeneity and contortion, however, it is possible to recognise a general sequence at the main exposure [NZ 4302 4974] about 280 m north of the harbour:

The suspected bivalve moulds in limestone high in the sequence are not specifically identifiable, but they are of about the same size and shape as Schizodus. Possible but unidentifiable microfossils were reported by Mr R K Harrison in a thin section from this bed.

The main variation at the type locality is that the upper part of the residue thickens northwards, where it contains many large foundered blocks of the Seaham Formation and grades up into a collapse-breccia. Farther north, the residue may be traced in the cliffs for 0.5 km, displaying great lateral variation. It also formed part or all of the top of Liddle Stack [NZ 4359 4852] (now almost destroyed), 1.4 km south of the type locality, and, inland, its top is exposed [NZ 4129 5023] for a short distance in the bed of Seaton Burn.

The former thickness and composition of the Fordon Evaporites at Seaham are unknown, but comparison with the sequence in eastern Cleveland, approximately along the depositional strike, suggests that they may have been 15 to 30 m thick at Seaham and composed of interbedded anhydrite, halite and dolomite. There is no evidence of the former presence of red mudstones such as those of the partly equivalent Edlington Formation in central and western Cleveland.

Conditions of deposition

Evidence bearing on the conditions of deposition of the Fordon Evaporites is in part equivocal, and is contradictory in that some beds show evidence of basin-floor accumulation and others at the same place bear features consistent with formation in shallow-water environments. The paradox of such juxtaposition has been discussed by, amongst others, Smith (1970a, b, 1980b) and Hsu et al. (1973). These authors invoked marked sea-level oscillations as a mechanism capable of exposing (or almost exposing) large areas of the basin floor. Whilst such oscillations are now widely accepted as a likely cause of apparently anomalous juxtapositions in many basin-interior evaporites, doubts nevertheless remain because there are no modern large deep-water evaporite basins available for study. In addition to varied indications of water depth at the same place, there is evidence of lateral variation in individual strata resulting from sea-floor relief. Such relief, perhaps caused by an approach to the basin margin, probably accounts for the contrasting indications of depositional environment in Offshore Borehole No. WM7A and at Seaham.

In the Fordon Evaporites of WM7A, the main evidence for accumulation below wave base lies in the fine planar lamination in the anhydrite beds, particularly of that near the base, which is comparable with that of basin-floor Zechstein anhydrite in Germany and elsewhere. The even character of much of the lamination and the presence of carbonaceous films argues for accumulation below a stratified, possibly anoxic brine subject to periodic (?annual) cycles of high phytoplankton production and to self-induced stagnation. Such an interpretation accords well with that of the underlying Concretionary Limestone at this site. Evidence of minor downslope movement in the anhydrite laminite and of probable slumping at its base is important in implying subaqueous deposition on an unstable slope, and is in accord with similar indications in the underlying carbonate rocks. The fact that these are of Concretionary Limestone (slope facies) and not Roker Dolomite (shelf facies) suggests that the anhydrite in Borehole WM7A was formed basinward of and at least 40 m below the crest of the Cycle EZ2 basin-marginal carbonate slope.

Interpretation of the origin of the halite of WM7A Borehole is more problematical than that of the anhydrite, mainly because so little of its primary fabric survives. The abundance within it of laminae and thin beds of laminated halitic anhydrite suggests a depositional environment similar to that inferred for the thicker of the beds of anhydrite; a deep-water environment was also envisaged by Colter and Reed (1980) for the halite of the Fordon Evaporites in Atwick No. 1 Borehole. Convincing proof of deep-water accumulation of substantial bodies of halite has yet to be adduced, however, despite the formulation of several explanatory hypotheses (e.g. Schmalz, 1969), and neither shallow nor deep-water origins for the halite of the Fordon Evaporites can be excluded on the local evidence.

Evidence for shallow-water accumulation of at least part of the Fordon Evaporites comes from the character of the carbonate rocks associated and interbedded with the Seaham Residue, the depositional site of which lay perhaps 15 km landward of that at the WM7A Borehole site. At Seaham, the rock underlying the residue is a cross-laminated ooid-grainstone/packstone of shallow-water origin, and the overlying carbonate rocks of the Seaham Formation are also thought to be a relatively shallow-water deposit. The presence of flat-pebble conglomerates in some of the oolite fragments within the residue probably indicates phases of contemporaneous exposure, cementation and intertidal reworking.

Seaham Formation (EZ3Ca)

Distribution and general characteristics

The Seaham Formation (formerly Seaham Beds) is the carbonate member of Cycle 3 of the English Zechstein sequence. Its main outcrop on land in the district is in a gentle syncline on the downthrow (northern) side of the Seaham Fault, but coal exploration boreholes offshore have shown that it is extensive in the eastern undersea area (Figure 34). The Seaham Formation is exposed mainly in coastal cliffs at Seaham but is also patchily exposed inland in Seaham Dene. The whole sequence has been severely disrupted and locally brecciated by foundering caused by the dissolution of the formerly underlying Fordon Evaporites, but the brecciation is generally less severe and extensive than that in foundered Cycle EZ2 carbonate strata.

Previously classed as part of the Concretionary Limestone, the rocks exposed at Seaham were recognised as a distinct sequence, named the Seaham Beds (Smith, 1971), following discoveries in deep hydrocarbon exploration boreholes in North Yorkshire (Taylor and Fong, 1969). The type locality was nominated as the vertical walls of the docks at Seaham Harbour, the more northerly of which was excavated between 1828 and 1831 to create a sheltered anchorage inside a prominent headland. The name was subsequently changed to Seaham Formation to bring it into line with current stratigraphical practice (Smith et al., 1974).

Although highly variable in local detail, the Seaham Formation and its equivalents (the Brotherton Formation and Plattendolomit) is the most uniform of the Late Permian carbonate units. For most of its outcrop it thickens eastward at 1 to 2 m per kilometre and it carries a uniquely diagnostic assemblage of the probable alga Calcinema permiana and the bivalves Liebea squamosa and Schizodus obscurus (Plate 37).

At its type locality, the Seaham Formation is disrupted by many faults and flexures induced during foundering, but appears to be between 30 and 31.5 m thick. Its base is exposed in coastal cliffs [NZ 430 496] and [NZ 4302 4976] 155 m and 280 m north-north-west of the harbour and its top was formerly seen, but is now obscured, in collapse-pipes [NZ 4325 4951] at the eastern end of the exposures of the North Dock. Although the abundance of faults and lack of continuity hinders detailed correlation between the many fault blocks at Seaham, the general sequence is:

Except for the presence of calcite concretions, the lithology and sedimentary features of the Seaham Formation at its type locality are closely similar to those in the equivalent Brotherton Formation of Yorkshire. Thin bedding predominates in much of the sequence, with complex, slightly lenticular interbedding of fine-grained almost barren to sparingly fossiliferous limestones (Calcinema-bivalve mudstone/wackestone) and fine coquinas (Calcinema-bivalve packstones, grainstones and rudstones). Some beds are markedly graded, with abundant bioclasts in the lower part and fewer in the upper. Sedimentary features are best seen in the upper part of the sequence and include broadly lenticular bedding, abundant shallow cut-and-fill structures, low-angle planar and tabular cross-lamination and a range of mainly symmetrical low-amplitude ripples with wavelengths ranging up to 0.8 m (Plate 38). The bioclasts are preserved as moulds, with Calcinema tubes in many beds arranged in dense aligned swarms. Brief study of the orientation of the tubes revealed a strong preferential WSW–ENE to west–east alignment at some levels, but a weak (Plate 37) or apparently random orientation at others. The apparent paradox of cross-laminated and rippled carbonate mudstones is resolved by examination of thin-sections, which suggests that the mud-grade grains were probably pelleted and that the pellets were almost obliterated during diagenesis. Four analyses of limestone of the Seaham Formation at its type locality showed that only 2 to 3 per cent of dolomite is present, and 1.5 to 3.2 per cent of silica (mainly as small detrital grains) (Al-Rekabi, 1982). Cream and buff, finely saccharoidal, soft dolomite forms much of the lower part of the sequence (in addition to the basal 1.8 m) at the cliff exposure 155 m north of the harbour. Elsewhere, cream powdery dolomite occurs between some concretions in the crystalline beds, and large parts of the formation in parts of the district to the south are mainly of silt-grade dolomite (Smith and Francis, 1967).

Calcite concretions in the Seaham Formation at Seaham include early finely crystalline spheroids and boudinlike bodies, and also later more complex coarsely crystalline spherulites and rod-like structures; more than one generation of the later concretions appear to be present in parts of the rock. The concretions occur at all levels in the formation, but are most abundant slightly above the middle, where groups of mainly thin beds of fine-grained limestone coalesce into massive coarsely crystalline limestones individually up to 3 m thick. Calcite spherulites (Plate 39) are the most common and eye-catching type of concretion, and range from a few millimetres in diameter to more than 20 cm. Curved brown calcite crystals, some of which are split, characterise outer parts of the coarser of these spherulitic concretions, and few traces of primary bedding structures or fossils are preserved within them. The finely crystalline spheroids are mainly smaller than the spherulites and some are hollow and weakly concentrically layered. The boudin-like bodies are biscuit shaped and lithologically similar to unaltered host rock, of which they may be relics; they lie mainly near the stepped contacts between bedded calcite mudstones and massive crystalline concretionary limestones (Figure 48).

The origin of the complex calcite concretions in the Seaham Formation has not been separately investigated, but presumably was similar to that of concretions in the Concretionary Limestone. The pattern of the concretions appears in places to have been influenced by the presence of minor discordant fissures and this, together with the lack of evidence of differential compaction around the concretions, suggests that they were formed after lithification and fracturing of the sediment. According to Al-Rekabi (1982), dolomitisation of the rocks preceded the formation of the concretions and was attributed by him to seepage reflux.

In addition to the outcrop at Seaham Harbour, about 3.5 m of thin-bedded, nonconcretionary limestone occupy a shallow syncline at the top of the cliffs [NZ 4360 4770] about 300 m south-west of Nose's Point, Dawdon. These strata were considered by Trechmann (1925) to belong to the Upper Magnesian Limestone 'Filograna Beds' (now the Seaham Formation) and this attribution was supported (but not confirmed) during the resurvey by the discovery in them of Calcinema (formerly Filograna); no underlying residue is visible in the cliff section. Elsewhere on land, rocks of the Seaham Formation occur locally as clasts in the higher parts of collapse-breccias in Cycle EZ2 carbonate rocks as far north as Ryhope. Such clasts are generally rare, but abounded in an outlier of the Seaham Residue near the top of the former Liddle Stack [NZ 4359 4852] , a once upstanding rock on the coast some 650 m north-north-west of Nose's Point; the uppermost part of this stack may have been an outlier of the Seaham Formation.

Little is known of the Seaham Formation offshore, but cores cut just below the sea bed in Offshore Borehole Nos. 4 and 10 [NZ 4617 5811] and [NZ 4534 6007] were attributed by Magraw (1975) to an Upper Nodular Limestone that might equate with the Seaham Formation. The lithology of these cores, as described by Magraw, is consistent with a position in either the Concretionary Limestone or the Seaham Formation, but the presence in No. 4 Borehole of ostracods and Spirorbis and the absence of Calcinema (BGS internal report by J Pattison) favour the former. Pattison identified a substantial list of gastropods, bivalves and ostracods from these beds in No. 10 Borehole; thus assignment of a Concretionary Limestone age here seems unavoidable. A Seaham Formation age seems probable, however, for a rockhead sample in Offshore Borehole No. 11 [NZ 4937 5100] , in which bedding planes were reported (British Coal record) to be covered with Tubulites (= Calcinema). Calcinema, Liebea and Schizodus were abundant at some levels in 21 m of weakly brecciated, partly spherulitic limestone cored beneath thin red mudstone in a borehole [NZ 4291 5537] 1.8 km offshore near Hendon, Sunderland; these beds almost certainly are part of the Seaham Formation, despite their proximity to. exposures of thick Concretionary Limestone. Their presence at this locality is probably a result of downfaulting into the Hendon Graben.

Conditions of deposition

Deposition of the Seaham Formation followed that of the Fordon Evaporites, which had virtually completed the filling of the Zechstein Basin (Smith, 1970a, b, 1980b). Sedimentation thereafter was not influenced by strong basin-floor relief, and this is reflected in the relatively uniform lithology of the Seaham Formation and its equivalents. Sedimentary structures almost throughout are consistent with moderate-energy deposition, probably under less than 20 m of water, but the absence of evidence of emersion may exclude widespread intertidal deposition; a rather deeper-water depositional environment is envisaged by Drs G M Hanvood and A C Kendall (in Smith, 1989). The graded beds lack many of the distinguishing features of turbidites, and may be storm deposits resulting from the disturbance and resedimentation of the top few centimetres of existing deposits. Evolution to high sub-tidal, low-energy deposition may, however, be indicated by the algal laminites at the top of the succession, which, in North Yorkshire, contain nodular gypsum and are overlain by the multicyclic shallow water to sabkha deposits of the Billingham Anhydrite (Smith, 1974b, 1989).

The limited diversity, but local great abundance of the biota, must indicate unusual environmental controls, probably hypersalinity; that it should remain so constant and widespread, without diversification, throughout deposition of the formation is remarkable, but is consistent with the uniformity of the primary lithology.

Billingham Anhydrite and younger formations

The youngest late Permian strata, comprising the Billingham Anhydrite (EZ3A), Rotten Marl, Sherburn Anhydrite (EZ4A) and Roxby formations, arc present in situ only in the eastern undersea area (Figure 34); they are known almost entirely from borehole cuttings and a limited suite of wireline logs. The evaporites have been dissolved where close to the base of the drift (or the sea floor where drift is absent), leaving only thin residues. By comparison with the sequence in Cleveland, the Boulby Halite (EZ3Na) ought to be present in the eastern undersea area but has not been recorded.

The Billingham Anhydrite and Sherburn Anhydrite formations, both generally 3 to 6 m thick according to their expression on wireline logs, are represented in cuttings mainly by small angular fragments of gypsum and anhydrite, with traces of dolomite. The Billingham Anhydrite is generally the thicker of the two beds at depth, but is commonly thinner and more variable near outcrop because of its greater vulnerability to dissolution. The detailed lithology of the two formations is unknown, but is presumed to be similar to that of the same strata in Cleveland (Smith, 1974b). The thin Upgang Formation (EZ4Ca), which is widespread between the Rotten Marl and the Sherburn Anhydrite, has not been detected in the offshore area, but is presumably present there.

The Rotten Marl Formation (or Carnallitic Marl in North Yorkshire) and Roxby Formation are deposits of red and red-brown siltstone, mudstone and sandstone, which produce distinctive traces on some wireline logs. Red mudstones of the Roxby Formation were first recorded in the offshore area by Magraw (1978) and, together with those of the Rotten Marl, have since been shown to be up to 90 m thick; of this the Rotten Marl contributes 3 to 8 m. The Rotten Marl has not been cored in the district, but elsewhere it is lithologically uniform and comprises blocky siltstone and mudstone with abundant listric surfaces and veins of fibrous gypsum (Smith, 1970b, 1974b); borehole cuttings from the offshore part of the district are consistent with this lithology. Cuttings from a number of eastern offshore boreholes similarly show that the overlying Roxby Formation is lithologically comparable with that proved in its type sequence in the Boulby Mine pilot borehole, Cleveland (see Woods, 1973) and in North Yorkshire (Smith, 1974b) where it comprises thinly interbedded siltstone and mudstone with abundant veins of fibrous gypsum and, particularly in higher parts, lenses and thin beds of red-brown sandstone. This lithology is typified by a 3.6 m core taken about 55 m above the base of the Roxby Formation in Offshore Borehole No. WM7A [NZ 5270 6290], 12 km east of Whitburn. Red-brown mudstone (1.5 m+) with veins of fibrous gypsum also overlay supposed Seaham Formation limestones in a cored borehole [NZ 4291 5537] 1.8 km east of the coast at Hendon, Sunderland.

On land, these youngest strata of the English Zechstein sequence are represented by scattered fragments of red siltstone and mudstone in collapse-brecciated carbonate rocks exposed along the coast, and by red and green (reduced) brecciated mudstone and subordinate purplish grey limestone in a collapse-pipe [NZ 4325 4951] near the eastern end of the north wall of North Dock, Seaham Harbour. The limestone is strongly cellular, and in both general appearance and petrographical detail shows much evidence of having at least partly replaced a former cellular anhydrite rock. Presumably, therefore, it is a relic of one of the two anhydrite beds, its position near the base of the collapse pipe favouring the Billingham Anhydrite. Farther south, pebbles of more finely cellular limestone, almost certainly from a distinctive thin bed near the middle of the Sherburn Anhydrite, occur widely in the drift under eastern Teesside and were presumably eroded from outcrops offshore in the Sunderland (Sheet 21) and Durham (Sheet 27) districts.

Foundering and collapse of Upper Permian strata

All younger rocks of the late Permian sequence in coastal and adjoining inshore parts of the district have foundered as a result of the dissolution of underlying and interbedded evaporites, and of some carbonates. The dissolution is believed (Smith, 1972) to have been effected mainly by meteoric water during the present cycle of uplift and erosion, and caused both stratigraphical anomalies that long defied satisfactory explanation and profound structural and lithological changes in the rocks involved.

Stratigraphical and regional effects of foundering

Widespread foundering has affected all strata younger than the Hartlepool Anhydrite, the dissolution of which has lowered the Cycle EZ2 carbonate rocks by up to perhaps 150 m and brought them into juxtaposition with the Cycle EZ1 shelf-edge reef. Cycle EZ3 carbonate rocks exposed at Seaham have additionally foundered as a result of the dissolution of the Fordon Evaporites (?20–30 m), and younger strata in the eastern undersea area have presumably been similarly affected by the removal of anhydrite and salt of Cycle EZ3 and perhaps also of Cycle EZ4 anhydrite (Smith, 1985a). Apart from the foundering, the dissolution of the evaporites has led to a considerable thinning of the sequence and to back-tilting or the formation of structural benches in the main carbonate units, with a substantial widening of their outcrops (Figure 49). The primary thickness variations of the combined Cycle EZ2 carbonate rocks also probably resulted, after foundering, in a gentle NNW–SSE syncline in the Seaham Formation and overlying strata, although data on this are insufficient to justify delineation on (Figure 34). All foundered strata have been broadly flexed by the vicissitudes of the dissolution and foundering processes; these flexures are well seen in most large exposures, where they are superimposed on the broad regional structures that also affect underlying Carboniferous strata.

The most spectacular and convincing proof of strata' foundering is seen in a group of exposures [NZ 39 53] at Ryhope, where foundered Cycle EZ2 carbonate rocks lie on the inferred residue of the Hartlepool Anhydrite within 200 m of the foot of the Cycle EZ1 reef talus apron and probably at least 50 m below the former level of the reef crest. Foundering of 40 m or more is suggested here, and is comparable with that inferred at Chourdon Point [NZ 443 465], just south of the district. Foundering of Cycle EZ2 carbonate rocks of 100 to 120 m is probable in most present coastal and inshore areas, and Cycle EZ3 carbonate rocks at Seaham have probably foundered by at least 140 m and perhaps more than 160 m.

Local effects of foundering

Foundering of the carbonate rocks of Cycle EZ2 has led to widespread and intense brecciation, and similar but generally less intense brecciation has affected carbonate rocks of Cycle EZ3. The brecciation of the Cycle EZ2 rocks (under their various historic names, but generally 'Middle Magnesian Limestone') was noted by all the early writers and has been discussed more fully by Lebour (1884), Woolacott (1909, 1912, 1919b), Trechmann (1913, 1914, 1925, 1954) and Smith (1972, 1985). Early speculation that much of the brecciation might be related to the former presence of gypsum was vindicated by the discovery of thick anhydrite at Hartlepool (Marley, 1892), which led Trechmann (1913) to conclude that dissolution of evaporites was indeed the probable cause of much of the brecciation. Further confirmation came with the discovery of extensive anhydrite offshore (Magraw et al., 1963; Magraw, 1975, 1978) and with the recognition by Smith (1971, 1972) of its equivalent residue at outcrop at Trow Point, at Ryhope and at Chourdon Point (1:50000 Sheet 27). The former presence of Cycle EZ2 (Fordon) evaporites onshore at Seaham was inferred by Smith (1971) and their presence offshore is recorded here.

In common with many foundered strata, the degree of dislocation and brecciation in Cycle EZ2 carbonate rocks varies greatly from place to place and appears to be related more to the local history of collapse rather than to the thickness of the evaporite dissolved. In general, the lowest 5 to 20 m of the foundered sequence is the most intensely brecciated part and brecciation diminishes progressively upwards. For this reason, exposures of the most completely brecciated rocks are concentrated where lowest Cycle EZ2 strata crop out, mainly at Cullercoats, Marden and Tynemouth Cliff (all to the north of the district), between Trow Point and the northern end of Marsden Bay (Figure 50), in quarries south of West Boldon, in river cliffs along the Wear and its tributaries between Southwick and central Sunderland, at Ryhope Colliery and in coastal cliffs at the 'Pincushion', Ryhope. Exposures of higher Cycle EZ2 beds are mainly of gently flexed and variably fractured rocks, with scattered steep-sided walls, wedges and cones of more severely brecciated rock (Figure 51). Breccia lenses are also present locally within unbrecciated or less brecciated rock and may result from the differential dissolution of carbonate rocks or secondary sulphates. Few breccias extend up into the Roker Dolomite Formation, except in the south of the district and perhaps for a short distance to the east of the Cycle EZ1 reef, where the Concretionary Limestone is thin or absent. It is clear that, in many parts of the district, large blocks of Cycle EZ2 strata foundered gently and that upper beds, at least, generally suffered little dislocation.

Dislocation of Cycle EZ3 carbonate rocks (the Seaham Formation) mainly takes the form of abundant and fairly closely spaced minor step faults rather than brecciation. The most intense brecciation noted is in cliffs southward for about 200 m from Featherbed Rocks, Seaham, where abundant large blocks of spherulitic limestone have foundered into the upper part of the Seaham Residue.

In addition to massive fracturing and brecciation, the carbonate rocks of Cycles EZ2 and EZ3 are cut by late-stage subvertical breccia-filled pipes and fissures (Winch, 1817 and later authors). These will be described separately.

In detail, the lowest collapse-breccias of Cycle EZ2 carbonate rocks are massive, hard, grey limestones comprising angular to subangular clasts in a patchy matrix of smaller clasts and finely crystalline calcite (Plate 40); the smallest clasts, up to a few centimetres across, are most abundant near the base, but all the breccias contain both large and small rock fragments, and many of the larger fragments bear evidence of more than one phase of fracturing and recementation (Sedgwick, 1829; Woolacott, 1909, 1912). In most coastal exposures, the clasts comprise a mixture of the laminated and unlaminated rock types of the Concretionary Limestone, but fragments of oolite from interbedded grainstones are present locally. In contrast, unpublished work by G N Tester (written communication) shows that collapse-breccias exposed near the Cycle EZ1 reef at and near West Boldon are mainly of altered calcite grainstones and wackestones (presumably from the Roker Dolomite facies), and that at intermediate localities, such as the Wear Gorge at Sunderland, the breccia clasts comprise a mixture of grainstones and laminated carbonate rocks. Black and locally white chert was noted in some of the breccias by Burton (1911), and both chert and chalcedony have since been found to be common components in both matrix and clasts. Miss Tester (written communication) notes that in some places there is clear petrographical evidence that some of the silica replaced both gypsum and anhydrite.

Most of the clasts in the lowest collapse-breccias have been severely altered during diagenesis, and few primary fabrics have survived. Woolacott (1909, 1912) noted that many clasts are now more calcitic than formerly, especially in their outer parts, and ascribed this dedolomitisation to the reaction of the dolomite with calcium sulphate solutions liberated by the dissolution of gypsum or anhydrite; the former proximity of the Hartlepool Anhydrite was not then known to Woolacott. The dedolomitised collapse breccias are amongst the most resistant rocks in the Magnesian Limestone sequence and form most of the coastal headlands and many sea stacks. On exposure, matrix and clasts generally erode at about equal rates where both are calcite, but 'negative breccias' (Plate 41) are formed where the clasts are less calcitic than the matrix and are less resistant to erosion. Voids filled or partly filled with internal sediment of fine rock debris and infiltrated fines are abundant in the breccias, particularly in the lowest two metres. Some of the cavity-fill bears evidence of protracted multiphase accumulation punctuated by episodes of tilting, fracturing and folding in response to episodic settlement of the breccias. The presence of cavity-fill low in the breccias at Trow Point was first recorded by Burton (1911).

Higher parts of the collapse-breccias merge upwards into increasingly large and less disturbed blocks of strata, recognisable beds becoming more apparent as height above the base increases; in most places severe brecciaflow ceases 20 to 30 m above the level of the former evaporite. These higher breccias are only patchily dedolomitised (generally where brecciation has been most severe) and have a more varied range of clasts and matrix. Sub-vertical, slickensided fractures are abundant in the most calcitic parts, many of which coincide with faults or the margins of major differentially foundered blocks. Cementation ranges from slight in the dolomitic breccias to strong where calcite veins abound, and the patchy matrix consists of variably cemented fine rock debris and infiltrated fines.

Factors influencing the degree and style of brecciation were considered by Smith (1972), who listed three main factors: 1) the competence, composition and lithology of the original rock and the frequency of joints; 2) the amount and distribution of secondary sulphate that the rock once contained; and 3) the duration, number and nature of the collapse events to which it had been subjected. He suggested that the lowest beds were first broken up into rectangular blocks by the formation of a dense network of gypsum veins as unroofing of the underlying anhydrite proceeded, and that these blocks then fell apart and became dedolomitised as the gypsum was subsequently dissolved. Higher parts of the breccia were probably broken up by repeated collapse following the creation of dissolution voids.

Late-stage breccia-filled fissures

The subvertical breccia-filled pipes and fissures that cut the collapse-brecciated and foundered carbonate rocks of Cycles EZ1 and 2 are best exposed at the northern end of Marsden Bay ((Figure 50) and Woolacott, 1909, figs. 4, 7, 8), but they are present at varied intervals in most coastal exposures of the Concretionary Limestone Formation and also in the Seaham Formation at Seaham Harbour. They were called 'brecchia dykes' by Howse and Kirkby (1863) and 'breccia gashes' by Lebour (1884) and Woollacott (1909). Although some of these bodies grade into the surrounding rocks, most have sharp margins against adjoining bedded or brecciated country rock (Plate 42). They are circular, elongate or cruciform in plan and are filled with uncemented to strongly cemented blocky fragments from higher strata; fragments near the walls are commonly steeply inclined and voids are abundant in most parts of the breccias. In Marsden Bay, the fragments are of laminated and unlaminated Concretionary Limestone from higher in the cliff or from Concretionary Limestone strata now removed. From Ryhope southwards, the breccias cutting Cycle EZ2 rocks also include a few fragments of Cycle EZ3 rocks and of younger strata including, in at least one fissure, blocks of Sherwood Sandstone from more than 220 m higher in the succession and long since eroded away (Smith, 1972). Breccia-filled fissures cutting the Cycle EZ3 Seaham Formation at Seaham Harbour have been mentioned in the preceding section.

The breccia-filled fissures are generally regarded as the product of the last stages in the collapse of the Magnesian Limestone sequence following the dissolution of evaporites. They probably overlie former caves (themselves the site of former residual sulphate masses), the roofs of which eventually failed; the failure initiated upwards stoping, which only ended when the fissures became completely choked with debris.

Age of the foundering and brecciation

In his analysis of the timing of evaporite dissolution and of related stratal foundering, Smith (1972) concluded that major dissolution was unlikely to have taken place during burial and probably was accomplished during subsequent uplift. Presumably this uplift was initiated during Mesozoic earth movements, and the intrusion of the c.58 million years-old Hebburn Dyke into already brecciated Roker Dolomite at Whitburn (p.123) suggests that uplift (and related evaporite-dissolution) was well advanced there by mid-Paleocene time. The presence of rare fragments of Sherwood Sandstone in the youngest collapse-breccias shows that the breccias are post early Triassic and also probably indicates more than 220 m of erosion since collapse occurred. Dissolution of evaporites onshore was clearly complete before the deposition of the Late Devensian glacial deposits but may have been reactivated offshore during Pleistocene low sea-level stands. Slow hydration of anhydrite and dissolution of the evaporites and some carbonates presumably continues offshore, and is accompanied by episodic cavern collapse, fissure stoping and regional foundering.

Chapter 4 Structure

Regional setting and tectonic history

The district lies in the eastern part of the Alston Block, a broad west–east-trending belt of relatively thin Carboniferous rocks that gained rigidity and buoyancy from the massive Devonian Weardale Granite and associated basement rocks. The northern and southern margins of the block are customarily taken at hinge lines now marked by the Ninety Fathom Fault and the Butterknowle Fault respectively (both outside the district). However, Hospers and Willmore (1953), Bott and Masson-Smith (1957) and Bott (1961) showed that marked gravity gradients at the northern and southern margins of the block are a response to sharp thickness changes in Lower Carboniferous rocks at depth and could not result directly from the faults themselves.

Interpretation of geophysical evidence (Bott, 1961) suggests that the eastern margin of the Weardale Granite probably lies slightly west of the district, and that the Alston Block may die out gradually eastwards rather than have a clearly defined eastern margin. Full discussion of the evolution and origin of the block and basin (or trough) structure in north-east England is inappropriate here, but it has been considered fully elsewhere (see Bott and Johnson, 1970; Johnson, 1982, 1984; Bott et al., 1984; Leeder, 1988; Fraser and Gawthorpe, 1990).

The pattern of Caledonian dislocation in Lower Palaeozoic basement rocks of the district is unknown, but Lower Palaeozoic strata exposed elsewhere in northern England have a strong SW–NE structural trend and this probably extends beneath later rocks across most of the region (Dearman, 1980). This trend probably was inherited partly from earlier structural patterns and was undoubtedly an important influence during subsequent tectonic dislocation.

The well-known thickness changes in Lower Carbonifeorus rocks approximately along the Ninety Fathom and Butterknowle hingeline (?growth) faults were attributed by Bott et al. (1984) to sharply differential contemporaneous subsidence resulting from stretching of the lithosphere by north–south tension. This differential subsidence ceased during the Namurian and the Westphalian Coal Measures accumulated under conditions of increasingly uniform epeirogenic downwarping (Johnson, 1984). Some differential subsidence has nevertheless been invoked by Haszeldine and Anderton (1980), Fielding (1982, 1984a, b, c) and Fielding and Johnson (1986) to account for part of the local sedimentological variation in the Coal Measures of Durham and Northumberland.

The time of the ending of regional subsidence is unknown, but uplift and tectonic dislocation of the Carboniferous rocks of the district occurred during the Variscan earth movements, most of which were accomplished before the Stephanian intrusion of the Whin Sill (radiometric age about 300 million years) but which are thought to have continued to a lesser extent later (see Jones et al., 1980 for summary and full references). Hickling (1949) showed conclusively that most of the structure in the Coal Measures predates deposition of the late Early Permian Yellow Sands, confirming the unquantifled views of earlier workers.

Most of the evidence relating to latest Carboniferous and earliest Permian events was obliterated by prolonged desert erosion, but subsidence of the North Sea sedimentary basin began during this erosive phase and was probably initiated by crustal extension and thinning connected with the onset of Atlantic rifting (Russell, 1976; Russell and Smythe, 1978; Leeder, 1988). Thickness variations and evolving sedimentary patterns in the Late Permian rocks of north-east England show that the district then lay near the western margin of this basin, with gentle eastwards tilting about a hinge line probably located a few kilometres west of the district margin. Episodic subsidence probably continued throughout the Triassic and possibly also the Jurassic and Cretaceous periods, with minor breaks (and perhaps limited uplift and dislocation) at intervals throughout.

The age of the folds and faults in the Permian rocks of the district is unknown, but has generally been taken to be early Tertiary (e.g. Bott and Johnson, 1970). Bott and Johnson note that this is thought to have been a time of widespread uplift in northern England, possibly complementary to rapid Tertiary subsidence in the North Sea area and coincident with the last stages of the opening of the North Atlantic Ocean. An inferred regional eastward tilting of 1 to 2° was regarded as Tertiary by Hickling (1949) and other authors, but this must also include other significant components. Magraw (1963) presented evidence of tectonic movement postdating the intrusion of a Tertiary dyke encountered in Offshore Borehole No. 5 (off Tynemouth).

In addition to tectonic and regional Tertiary uplift, an element of differential isostatic uplift in coastal areas presumably accompanied dissolution of the Hartlepool Anhydrite (and may be continuing today), and both depression and rebound doubtless took place during the waxing and waning of the various Pleistocene ice sheets and accompanying sea-level changes.

Structure of the Carboniferous rocks

The tectonically induced structure of the Carboniferous rocks is well documented by mining record plans covering almost all land areas of the district and also substantial undersea areas. Data from these plans are summarised in (Figure 52), and are taken from (or reduced to) the level of the Maudlin Coal seam. No attempt has been made to deduce contours in the Coal Measures beyond the worked area, in the belief that available borehole information is unlikely to reveal the full complexity of the structure there. Earlier syntheses were by Hickling (1949) for all except the north-west of the district and by Armstrong and Price (1954) for the northern coastal area; summary diagrams and brief analyses were also given by Drysdale and Armstrong (1957) and Jones et al. (1980).

Although the quality of the mine plans is extremely variable and many of the older plans are sketchy, together they reveal a structural pattern dominated by an east-north-eastward dip of 30 to 40 m per km (about 2°), upon which is superimposed a number of broad open folds. By far the largest of these folds are the Boldon Syncline and the Harton Dome (or Marsden Anticline of some authors) in the north of the district, but there are many smaller domes and synclines, and a major NW–SE pericline lies 4 to 5 km north-east of Seaham Harbour (the 'Vane-Tempest structure' of Jones et al., 1980). Dips on the flanks of the folds are generally less than 4° and there are several coastal areas where dips of 1° or less are widespread. Steeper dips occur alongside (and dipping into) some of the larger faults, those on the north side of the Seaham Fault locally exceeding 10°. The Boldon Syncline passes southwards into a complex trough-faulted system of similar trend, and may be continued (though offset) in the south of the district by the Hendon Graben and associated structures.

Normal faults abound in the Coal Measures of the district and are most numerous in the core of the Boldon Syncline. They fall into three main groups, NNW–SSE, SW–NE and west–east, and in several places they form graben up to 1 km across. The deepest of these is in the offshore area east of Hendon and Ryhope, where displacements along the marginal faults exceed 100 m, but several narrow graben in the Boldon Syncline are more than 30 m deep. In general, the NNW–SSE faults are the least continuous of the three main sets and the greatest displacements are along the west–east faults. These include an estimated 300 m displacement on the Seaham Fault just offshore and an estimated 250 m (from borehole evidence interpreted by geologists of British Coal) on a west-east fault 8 km east of Marsden Bay. The SW–NE faults, though laterally continuous, generally have the least displacement. They include the Biddick Dyke (fault) and Muck Dyke (fault), both of which reverse throw at different points along their length and exceed 50 m displacement only locally ((Figure 52), inset). These faults may be parts of a single fracture, but the information from mine plans is inconclusive and they are viewed here as separate structures. The data do not show if any of the three main fault trends is younger than the others, but some, at least, have undergone two or more phases of movement.

In constructing the 1:10 560 and 1:50 000 scale maps, a hade of 20° was found to be in best overall agreement with the underground information, including intersections of faults in cored boreholes, and was used in calculating the surface and sub-Permian position of faults proved underground. The data from the older mine-plans, however, do not generally permit precise assessment of the inclination of fault planes, and hades ranging from 45° to vertical probably occur. The surface trace of the Biddick and Muck dykes (faults) was shown on the maps to switch sides at the point of displacement reversal, but this could not be verified and parts of these faults may be vertical or reversed. Work on faults in the Coal Measures of Derbyshire has shown that the vertical displacement on fault planes changes systematically around a maximum, and that many faults die out upwards (Rippon, 1985). No such study has been made of faults in the district, however, and it was assumed in constructing the maps that all faults proved underground reach the rockhead surface and have a constant throw at any one point along their length. The wide 'want' (the space between provings of the opposite sides of a fault in workings) of the Seaham Fault had previously led to speculation that this fault might have a hade of about 45°, but J Clowes (in British Coal records, 1975) noted that the hade is about 25° where the fault was intersected in undersea drivages east of Seaham. Strata on the north (downthrow) side of the fault are contorted and much fractured, with an overall dip of 45 to 50° towards the fault. Parts of the Claxheugh and St Hilda faults also have a wide want; the latter hades at about 45° where intersected in an undersea drivage off South Shields (Magraw, 1963).

Structure of the Permian rocks

Permian rocks of the district have suffered relatively little tectonic disturbance, compared with that of the Coal Measures, but their detailed structure is poorly documented and may be more complex than appears likely at first sight. An attempt to reconstruct the configuration of the Carboniferous/Permian unconformity is depicted in (Figure 53) and is based on all relevant surface and borehole information; it should be regarded as no more than a first approximation. Earlier reconstructions were by Anderson (1945) and Drysdale and Armstrong (1957), and Hickling (1949) attempted a contour map on the base of the Magnesian Limestone. Although differing greatly in detail, all these attempts agree in depicting an eastwards sheet dip of 20 to 25 m per km (about 1.5°). This dip is undoubtedly partly an expression of the primary slope of the old land surface and partly a result of Permian and post-Permian differential subsidence associated with the infilling of the North Sea depositional basins; whether it has also a small tectonic component is uncertain.

Folds on the unconformity surface are broad and open, and at least partly stem from its original topography. Reversals of dip are uncommon, the main example being the NW–SE pericline off Seaham where westward dips are doubtfully interpreted on scanty borehole evidence. Folds in the Magnesian Limestone are more abundant and generally slightly tighter than those on the unconformity; those in the Raisby Formation at least partly drape ridges in the Yellow Sands and those in the Concretionary Limestone, Roker Dolomite and Seaham Formation probably result mainly from differential foundering caused by dissolution of the Hartlepool Anhydrite and Fordon Evaporites. Except for the inferred pericline off Seaham, all the main folds in the Magnesian Limestone could be viewed as expressions of substrate irregularities or of tensional forces.

Faults at the unconformity and in Permian strata are poorly documented but generally appear to have a much smaller displacement than in the Coal Measures. A displacement ratio of 1:3 was suggested by Drysdale and Armstrong (1957) and this average figure has been used in the estimation of throws on the faults shown in (Figure 53); a ratio of 1:6 or more may be appropriate on some of the major faults, but cannot be verified. Some faults may be entirely post-Permian and therefore of equal displacment in Carboniferous and Permian strata. The displacement of the NW–SE Hendon Graben is at least 70 m in Permian strata and that on the Seaham Fault is sufficient to override the regional tilt and to sharply distort the outcrop pattern (Figure 34).

All the main faults affecting Permian strata are normal, and the planes of those cutting the Yellow Sands and the Raisby and Ford formations are steeply inclined and generally simple. In contrast, faults cutting younger Magnesian Limestone strata commonly are expressed as shatter belts a few metres wide and with several to many movement planes. Along the coast, too, there are many minor faults at the edges of large blocks of foundered strata; they extend downwards no farther than the underlying evaporite dissolution residue.

Chapter 5 Igneous rocks

The known igneous rocks of the district are all basic and intrusive, and comprise the Whin Sill and the Muck and Hebburn (or Monkton) dykes. The Ludworth Dyke may cross the extreme south-eastern corner of the district if it extends far enough, and an extension of the Tynemouth Dyke may enter the northern offshore area.

Whin Sill

The Whin Sill is intruded into Carboniferous strata ranging from Visean to Westphalian in age and has a Westphalian/Stephanian radiometric age of 301 ± 5 million years (Forster and Warrington, 1985, based on recalculation of averages quoted by earlier authors). It probably is present at depth throughout the district, but has been proved only in Harton No. 1 Borehole [NZ 396 656] at Marsden, South Shields (Ridd et al., 1970). The Harton Borehole penetrated three separate leaves of the Whin Sill, with an aggregate thickness of 89.4 m (the greatest thickness proved to date); the three leaves are 51.3 m, 4.6 m and 33.5 m thick and lie in Visean strata at depths of 943.1 m, 1172.9 m and 1372.8 m respectively (Figure 4).

Ridd and his co-workers describe the dolerite in the Harton Borehole as grey-green and finely crystalline, and quote extensively from petrographical reports by Dr R Walls, who examined thin sections taken from cuttings. Dr Walls noted that all the thin sections were of nonporphyritic quartz dolerite which was 'a good match' with Whin Sill rocks described by Holmes and Harwood (1928) from outcrops farther west and north; the rock was generally fresh and without coarsely crystalline patches. The petrography and distribution of the Whin Sill in general have been summarised by Holmes and Mockler (1931), Dunham (1970) and Randall (1980), who quote full references.

The contacts of the leaves of the Whin Sill in Harton No. 1 Borehole were not cored and precise relationships are therefore unknown. However, Jones (in Ridd et al., 1970) and Jones and Cooper (1970) reported the results of vitrinite reflectivity studies on coals encountered in the borehole and showed that the rank of coals near each of the dolerite leaves is much higher than normal for the area. Strata in which coals indicate thermal metamorphism extend from 180 m below the lower leaf to 425 m above the upper leaf of the sill.

Muck Dyke

The Muck Dyke is unusual in being mainly a gas dyke with only scattered, mainly small, pods, narrow dykes and thin sills of dolerite. It is intruded along or close to the Biddick Fault system and adjoining en echelon fractures, the width of the fracture belt being about 60 m (Armstrong and Price, 1954). The only known surface exposure of the dyke is on the north bank of the River Wear [NZ 3393 5667], near Low Barmston, where Price (1955) recorded nodules of pale grey dolerite in Westphalian sandstone a short distance above the Wear Mouth Marine Band. Underground, the Muck Dyke has been recorded as dykes up to about 0.6 m thick in the Maudlin and Hutton coals in the Washington and Hylton areas, and it feeds a sill about 0.3 m thick and up to 0.4 hectares in extent in the Hutton Seam, in workings from Hylton Colliery (Holmes and Harwood, 1928; Armstrong and Price, 1954).

The dolerite is dark grey or dark grey-green, fine grained and generally petrographically similar to that of the Whin Sill. Where it adjoins coal or other sedimentary rocks, however, the introduction of calcium carbonate and other contaminants has widely led to the alteration of the dolerite into pale grey 'white trap', as in the only known surface exposure. The dyke is one of the SW–NE to WSW–ENE-trending group commonly known as the Whin Dykes (e.g. Holmes and Harwood, 1928; Holmes and Mockler, 1931), which are thought to be genetically related to the Whin Sill and of similar age.

The Muck Dyke is associated with an unusually wide belt of thermally metamorphosed (partly cindered) coal, the rank of which was considerably increased for up to several hundred metres in both directions (Figure 6). The name of the dyke is that given by early local miners, alluding to its supposedly detrimental effect on the quality of adjoining coal, but later miners found that the higher rank coal could be marketed as semi-anthracite and thus command a higher price. Edwards and Tomlinson (1958) showed that the area of thermally altered coal dies out both south-westwards and north-eastwards, indicating the approximate extent of the gas- and magma-bearing fissures; this corresponds roughly to parts of the Biddick Fault system with the greatest vertical displacement (30 to 60 m).

Hebburn (or Monkton) Dyke

The Hebburn (or Monkton) Dyke (also called the Walker or Harton Dyke according to locality and author) trends WNW–ESE through Carboniferous strata in the northern part of the district. Though formerly exposed in quarries at Walker (south-eastern Newcastle upon Tyne) and Brockley Whins, just north of Boldon Colliery village (Winch, 1817; Howse and Kirkby, 1863), it is known mainly from intersections in mine workings. In these, it has been penetrated at various levels in at least nine places in the district and appears to be continuous at least as far as the present coastline, except for a possible gap near Monkton village where it is offset by about 200 m. The dyke is generally a single, subvertical body ranging from 4 m wide in parts of the Maudlin Coal workings from Whitburn Colliery to 15 m in Hutton workings from Harton Colliery (Armstrong and Price, 1954); where seen by Teall (1884, fig. 2) in unspecified workings from Boldon Colliery it is vertical and 13.6 m wide in the siliciclastic Coal Measures rocks, increasing to 14.9 m in the coal. Exceptionally, two closely spaced dykes were penetrated in Maudlin workings [NZ 383 619] beneath Cleadon, and a separate dyke was proved [NZ 367 624] some 150 to 170 m north of the main dyke in Maudlin workings south of Whiteleas. The Hebburn Dyke is not generally coincident with a fault, but it may follow the Cleadon Fault for some hundreds of metres east of Cleadon, and other dislocations for short distances where they cross it at a narrow angle. It was not seen during the resurvey of the Magnesian Limestone but was discovered cutting brecciated Roker Dolomite in coastal cliffs [NZ 4108 6156] at Whitburn by Mr G Fenwick of Sunderland University (personal communication 1993).

The petrology and composition of the Hebburn Dyke in coal workings at Boldon Colliery were investigated by Teall (1884) and Holmes and Harwood (1929). Teall described the interior of the dyke as an almost black, finely crystalline, nonporphyritic rock with scattered small white spherical amygdales, and this description closely matches that of Winch (1817) of the dyke rock in the former quarry near Brockley Whins. Study of thin sections revealed the rock to be a slightly porphyritic tholeiite comprising a groundmass of plagioclase and pyroxene with scattered small plagioclase and pyroxene phenocrysts (many zoned and some strongly corroded) and an abundant partially devitrified acid mesostasis; the amygdales are mainly filled with chalcedony, some with a core of calcite. Bulk analysis (Holmes and Harwood, 1929) shows silica and alumina contents of 57.09 and 13.76 per cent respectively, and gives a calculated normative mineral composition of quartz 15.13 per cent, feldspars 50.01 per cent, pyroxenes 24.55 per cent and iron ores 5.54 per cent. Teall noted that the marginal parts of the dyke are microporphyritic and greatly altered, with abundant scattered calcite.

The trend and composition of the Hebburn Dyke link it to the Mull swarm of Paleocene age (Holmes and Harwood, 1929 and later authors), although the age control is indirect and scanty.

Thermal alteration of coal adjoining the Hebburn Dyke is restricted to a relatively narrow belt of cindering (see Teall, 1884, fig. 2) and enhanced rank on each side of the dyke. Total widths of the belt thus affected range from about 10 m to about 25 m (Armstrong and Price, 1954), roughly in line with the rule-of-thumb observation that the zone of alteration in coal is commonly about three times the width of the dyke (Jones and Cooper, 1970). Alteration of other Carboniferous sedimentary rocks adjoining the Hebburn Dyke takes the form of baking and slight additional induration for a few metres.

Chapter 6 Quaternary

Quaternary drift deposits cover almost all the solid rocks of the district and reach thicknesses exceeding 60 m in parts of the Tyne and Wear valleys; generally, however, they are less than 10 m thick. The drifts are well exposed in coastal cliffs, especially at Whitburn and between Hendon and Seaham, but good permanent exposures are uncommon inland, where information on the drift conies mainly from temporary exposures and from some thousands of boreholes.

The history of research on the Quaternary drifts of northern England, including those in the district, was summarised by Beaumont (1968), who listed more than 90 key papers; these include important regional syntheses of north-east England by Howse (1864), Woolacott (1905, 1907, 1921), Raistrick (1931), Francis (1970) and Lunn (1980). More local summaries have been provided for the district by Smith (1978, 1981) and for the adjoining districts to north and south by Land (1974) and Smith and Francis (1967) respectively. Through all these works, there runs a constant awareness of the interdependence of the drift geology and the agricultural and industrial evolution of the region; this relationship is nowhere closer than in the Sunderland district.

All the main drift deposits of the district are thought to be Late Devensian and younger in age, those of direct and indirect glacial origin probably having been formed between about 20 000 and 13 000 years ago (Francis, 1970; Smith, 1981). The number and extent of glaciations before that of the Late Devensian Dimlington Stadial is not known, any deposits of such episodes, together with those of intervening warmer intervals, having been swept away or recycled. The former presence of at least one earlier ice sheet in coastal areas south of the district has been inferred from the pebble and other contents of Ipswichian and older raised beach and glacial deposits (see Smith and Francis, 1967 for summary).

The subdrift (rockhead) surface

Little is known of the pre-Quaternary evolution of the landscape of the region, although it is generally speculated that late Mesozoic or early Tertiary uplift was followed by prolonged continental erosion (Beaumont, 1970). Workers including King (1967) and Beaumont (1970) envisaged the area as initially forming part of an extensive eastwards-sloping peneplain, which according to Bott (1967), was gently domed by the differential isostatic uplift of the Weardale Granite. Removal of 2 to 3 km of Permian and younger strata was presumably accomplished during and following this uplift, the resulting detritus being transported generally eastwards into the subsiding sedimentary basins of the North Sea area.

The broad features of the landscape, resulting from erosion by eastwards-flowing consequent drainage and the varied resistance of the Carboniferous and Permian rocks, may have been established before the Quaternary era began, but the amount and pattern of Pleistocene erosion is almost unknown. All that can be said with confidence is that much of the subdrift surface in the district had evolved to its present general configuration by the onset of the Late Devensian glaciation, though even then an average of several metres of regolith and rock must have been removed from the whole region in order to provide the known thickness of Late Devensian glacial deposits. That such removal was not uniform was emphasised by Lunn (1980), and the probability that some of the major glacigenic subdrift landforms might have originated during earlier glacial episodes was discussed by Smith (1981).

Examination of rockhead contour maps such as that for urban Sunderland (Figure 54) reveals that the sub-drift surface is generally subparallel with the modern surface, though it is more mature and has a considerably greater relief. It bears a dendritic network of valleys (Figure 55), some of which probably evolved as part of normal consequent drainage patterns and some of which were modified (or perhaps originated) by proglacial or subglacial meltwaters. 'Humped' profiles distinguish some of the valleys cut or modified by glacial meltwaters; they are typified by the valley of the Proto Wear between Chester-le-Street and Sunderland. It is possible that this valley was once continuous with the valley of the Twizell Burn (west of Chester-le-Street), but was truncated by the cutting of the Team Valley. Complex patterns of drainage evolution and stream capture have been proposed by several authors (see Beaumont, 1970 for full references), but there is little unanimity, and the age and origin of the Team Valley itself remains a mystery. Most of the major valleys on the subdrift surface are graded to about 40 m below modern sea level, perhaps in response to glacial low sea-level stands.

In common with most lowland areas of north-east England, the general line of most of the main subdrift valleys is followed by a modern counterpart with sharply steeper sides but, commonly, with a shallower thalweg. Presumably as a consequence of superimposition from a drift cover, the two valley systems coincide in detail only locally and the modern valleys meander from side to side of the broader subdrift valleys and locally cut into and expose bedrock in places where the two systems diverge. Several minor streams, especially in coastal parts of the district, have no subdrift counterpart and some minor subdrift valleys have no modern counterpart. Both major and minor modern stream valleys are deeply incised in their lower courses, probably as a result of a combination of rapid downcutting as ice finally melted, low late Glacial sea levels and isostatic rebound. Steepening of the lower thalwegs of most streams, especially of the shorter coastal 'denes', probably results from Holocene coastal recession. Where the present lower Wear Valley coincides with its drift-filled precursor, it was cut entirely in drift and the valley is moderately wide; where the two valleys diverge, however, as under much of Sunderland (see (Figure 54)), the present valley was cut mainly in Magnesian Limestone and the narrowness of the resulting gorge placed the Wear at a considerable commercial disadvantage compared with wider valleys such as that of the Tyne.

In addition to its network of valleys, the subdrift surface also bears a number of benches that appear to be erosional. By far the most spectacular of these benches lies along the coast, especially between South Shields and Whitburn, where a remarkably smooth rockhead surface up to 800 m wide slopes gently seawards from a pronounced concave slope break at about 31 m above OD. This bench was apparently once wider, having been cut into by recent coastal erosion, and it is backed by a line of rock-cored slopes that may be degraded cliffs. The extent of this bench strongly suggests a marine origin and invites association with the Ipswichian marine Easington Raised Beach that lies on a rock platform at about +32 m in Shippersea Bay, just south of the district. Inland, less well-defined benches with upper limits at about 24 m and 42 m lie beneath mainly thin drift along the flanks of the Wear Lowlands, being especially well marked north-west of Offerton and around the flanks of the Cleadon and Fulwell hills. The origin of these inland benches is unknown, but they may be of lacustrine origin. Large parts of the low-lying flat ground between Wardley and Boldon Colliery hear unexpectedly thin drift and these also may represent a former planation surface.

The detail of the subdrift surface depends largely on the nature of the underlying bedrock and on whether it was scraped clean by overriding Late Devensian ice or whether some regolith survived. The survival of regolith is most common on the Magnesian Limestone of the Whitburn area, where sheets and pockets of frost-shattered limestone commonly separate in-situ rock from the lowest drift, but similar debris is also found locally inland. Generally, however, the drift/rock contact is sharp, being smoothly rounded and polished on much of the Magnesian Limestone but rather more uneven on Coal Measures rocks, where the varied lithology led to differential scouring and plucking. The local origin of much of the drift is abundantly demonstrated by the presence of trains of debris streaming away from recognisably distinctive beds, becoming smaller and less coherent with increasing distance from their source. Roches moutonées were recorded by Howse (1864) on a bare Magnesian Limestone surface at Trow Point Quarry [NZ 383 664], and there are a few records of ice-generated striae which are confined to the Magnesian Limestone ((Figure 59); summary in Smith, 1981). Ice-wedge casts are not known in bedrock within the district, but are present in Coal Measures mudstones in a disused quarry [NZ 3128 4750] just south of the district. Sharply contorted shale and sandstone at rockhead at a temporary exposure [NZ 305 621] at Wardley was probably cryoturbated. Farther east, strongly cryoturbated brash composed of a mixture of local rock fragments and erratics lies on Magnesian Limestone at a number of places (e.g. (Plate 53)) and is up to 3 m thick in coastal cliffs north and south of Souter Point, Whitburn.

Distribution of drift

The distribution and thickness of the Quaternary deposits of the district is closely linked to the broad configuration of the rockhead surface. In general, the higher parts of this surface, rather more than half the total area, bear a relatively simple drift sequence less than 8 m thick (Figure 56), and the lower parts (the broad subdrift valleys) bear the thickest and most complex drift. Some areas of thin drift are, of course, in valley bottoms, from which formerly thick drift has since been removed. From the presence of drift relics, even on the highest parts of the district, it appears that drift cover was probably once continuous, before erosion swept parts of it away. It should be noted that, for the purposes of mapping, deposits less than 0.5 m thick were not delineated and are included in drift-free areas on the published map; in some built-up areas, extensive unrecorded sheets of urban rubble (made ground) may exceed this thickness.

The relationship of drift thickness to rockhead topography and the general interrelationships of the various members of the drift sequence are shown in (Figure 57), and the overall distribution of the main drifts is shown in (Figure 58). The latter supplements the 1:50 000 Drift map of the district by showing places where laminated clay has been proved below later deposits, but it must be emphasised that this clay is known to be patchy and is not necessarily continuous between provings.

Despite the apparent simplicity of the drift, the age, mutual relationships and mode of origin of some of the younger members of the sequence remain uncertain. There are many small exposures of interbedded drift that cannot readily be related to the regional sequences and which presumably record local events of which no other traces remain.

Late Devensian deposits

Basal sand and gravel

The lowest members of the Pleistocene drifts are a varied but limited suite of sands and gravels that form thin sheets, fill minor rockhead hollows or (most commonly) occupy the lowest part of the floor of several of the deeper buried valleys; some of the rockhead hollows may be minor buried valleys. The sands and gravels are presumed to be Late Devensian in age but they are unfossiliferous and an earlier age cannot be excluded.

The sheet-forming deposits were first noted by Robson (1915) in coastal sections between Whitburn and Ryhope, where they are generally less than a metre thick and comprise a calcite-cemented mixture of subrounded pebbles and cobbles of rock types similar to those in the overlying boulder clay; comparable gravels have been noted in coastal cliffs at Seaham and in the district to the south (Smith and Francis, 1967).

The sands and gravels that fill hollows are clearly water-laid and occur sparingly both in coastal cliffs and inland; as with the sheet-forming deposits, they comprise a mixture of local and erratic pebbles, cobbles and, locally, boulders. The valley bottom gravels lie mainly below sea level in the buried valleys of the Proto-Tyne and its tributaries, and are best known from borehole records. They were up to 1.1 m thick in exploratory boreholes in the Tyne valley between Howdon and Jarrow, and were described by Armstrong and Kell (1951) as fine to medium gravel, partly clean but partly strongly iron stained, 'in beds of erratic thickness and inclination'.

Durham Lower Boulder Clay

Throughout the district and most other parts of northeast England, the lowest widespread Quaternary deposit is the Durham Lower Boulder Clay. Except where the basal sand and gravel is present, it rests directly on the planed-off surface of Carboniferous or Permian rocks which, to varying degrees, it has engulfed. The deposit is the main or only drift member on much of Gateshead Fell, the Great Lumley area and the high ground in the east of the district, but it is generally thin and locally absent in the coastal platform between South Shields and Whitburn.

The Durham Lower Boulder Clay is a tough, overconsolidated, grey or brown, sandy clay, which contains abundant subangular to subrounded pebbles and cobbles, and a few boulders; most of the boulders are of Carboniferous Limestone (many highly polished), ganister and quartz dolerite ('whin') and lie either at or near the base of the deposit. The smaller clasts include a wide variety of rock types, many from south-west Scotland, the Vale of Eden and northern parts of the Lake District and Pennines, but Beaumont (1971, 1972) showed that fragments of local Coal Measures rocks are the most abundant clasts in western and central parts of the district and that the clay minerals in the matrix were derived almost entirely from the breakdown of Coal Measures rocks. Magnesian Limestone fragments are absent in Durham Lower Boulder Clay of the Wear Lowlands, but become increasingly abundant as the clay is traced eastwards across the Permian escarpment. The orientation of clasts in the deposit is shown in (Figure 59); it suggests, in conjunction with the inferred provenance of the erratics, a generally eastwards ice movement modified by the effects of local rockhead relief.

In a few places, the upper part of the clay has been weathered to reddish brown, like its counterpart in eastern Northumberland (Eyles and Sladen, 1981), and the mainly thin Durham Lower Boulder Clay of the Cleadon and Fulwell hills is locally red-brown throughout.

Basal layers of the Durham Lower Boulder Clay are commonly unevenly and weakly laminated and contain discontinuous layers and lenses of sandy detritus and rock flour (Plate 43); in places it has been rammed deep into fissures in the underlying surface. Higher parts of the deposit, however, are generally indivisible except in the coastal belt south of Hendon, where the clay widely comprises a thick dark brown upper unit and a more stony and tougher dark grey or brown-grey lower unit (1 to 2 m) in which Magnesian Limestone clasts are dominant and which also contains abundant fragments of soft red sandstone. This subdivision was first reported by Coupland and Woolacott (1926), who noted that the upper unit is less gritty than the lower and is coarsely bedded. The lower unit is probably equivalent to the lowest unit of the Durham Lower Boulder Clay exposed in coastal cliffs south of Chourdon Point (Smith and Francis, 1967), and the presence of a discontinuity in the thick upper unit at Seaham may indicate an approach to the full four-fold subdivision of the deposit at the coast of south-east Durham. The deposit is almost free of lenses of sand, gravel and laminated clay, but its upper part is interfolded with such sediments in cliff exposures near Whitburn (Smith, 1981, fig. 6) and is complexly interbedded with sand and gravel in coastal cliffs at Ryhope. Flame structures are common at the top of the clay in the Whitburn cliff sections.

The Durham Lower Boulder Clay has long been regarded as a ground moraine deposited by an ice sheet emanating from north-west England and south-west Scotland, and this view is accepted here. The possibility that only the grey basal layer in coastal areas might be a true ground moraine or lodgement till, the remainder being what would now be termed a melt-out till (as defined by Boulton, 1970, 1972, 1976) was suggested by Coupland and Woolacott (1926).

The only known ice-wedge cast in Durham Lower Boulder Clay was seen in temporary excavations [NZ 3854 5093] at Burdon; it was sand-filled, 0.6 m deep, and overlain by 1.2 m of bedded alluvium and hillwash.

The stiffness of the Durham Lower Boulder Clay, even when wet, makes it an excellent load-bearing foundation and it stands well for limited periods in trenches and tunnels; only the presence of large erratics creates problems in most excavations. There is evidence, however, that parts of the deposit may formerly have fluidised under periglacial conditions, leading to substantial mud-flows.

Tyne–Wear Complex

The thick and varied drifts overlying Durham Lower Boulder Clay in the main buried valleys are grouped collectively under the name Tyne–Wear Complex. In places they are more than 55 m thick, but thicknesses of 5 to 15 m are more common. Deposits of this complex are widespread but patchy up to about 70 m above OD in the Tyne and Wear valleys, and the Wear Lowlands, and similar and possibly related thin drifts occur locally up to about 132 m above OD. They widely rest on the eroded top of the underlying boulder clay and, in a few places, as in parts of the coastal belt between South Shields and Whitburn and locally on the floor and flanks of the buried valleys, they rest directly on rockhead.

The sediments of the Tyne–Wear Complex in the main buried valleys generally comprise interbedded laminated silty clays and clayey silts, fine-grained sands, stony  clays and some gravels; the last are most common at the base of the sequence. Laminated clays and silts are predominant in the lower parts of the complex where it is thick, but they pass upwards by interdigitation into bedded sands; the stony clays form lenses and wedges, which mostly extend from valley margins into the laminated clays and silts. Where the Tyne–Wear Complex is thin, as in much of the area around Wardley and Boldon Colliery, the sediments are mainly of laminated clay and silt. Bedding in the deposit is generally smooth and even, with low primary dips, but local strong contortion is present in coastal cliffs at Whitburn, in sands and laminated silts inferred to belong to the Complex.

On the published Drift map the Tyne–Wear Complex is not distinguished as a stratigraphical unit. Instead, the surface lithology is shown as Laminated Clay or Glacial Sand and Gravel, as appropriate. The surface distribution of the Complex is shown in (Figure 58), however, with some indication also of its distribution beneath the Pelaw Clay.

The laminated clays of the district are barren, dark brown, plastic deposits with grey-brown to purple-brown weathering tints and with strongly discoloured columnar and blocky joints in the zone of surface weathering. Although load casts are present in a few exposures, lamination in the clays is generally smooth and even, and in large exposures can be seen to dip gently towards the axes of the main buried valleys; discontinuous to continuous films, laminae and thin beds of fine-grained micaceous sand are abundant, especially in upper parts of the complex, and commonly are current rippled. Pebbles and small cobbles are thinly scattered in some beds and comprise an erratic suite similar to that of the underlying boulder clay. Small cream to grey, pedogenic, calcareous concretions are locally present in the lower part of the zone of weathering on laminated clay.

The upwards transition from laminated clays and silts to bedded sand is generally accomplished by alternation over a vertical distance of 1 to 2 m, and lenses and thin beds of laminated clay persist in the sands for some distance above the contact. Sands of the complex are thickest and most widespread in the southern parts of Washington New Town and in the Wear Valley north and south of Chester-le-Street; in the latter area and also at Fatfield (Plate 44), the sand has been dissected into a markedly hummocky topography. Sand in the north of the district is now concentrated on the flanks of the Tyne buried valley at Felling and St Anthony's, but these occurrences are probably erosional remnants of a once continuous and more extensive sheet; the base of the sand dips gently towards the axis of the buried valley.

In detail, the sand of the Tyne–Wear Complex is mainly fine to medium grained and level bedded, coal grains being almost ubiquitous. Small-scale cross-lamination and current ripples are common in most exposures and indicate sediment transport to have been mainly northwards and eastwards. Pebbles are generally small and uncommon, except in rare thin lenses of fine gravel, and include many coal fragments in addition to a suite similar to that in the laminated clays. Coal was particularly abundant (Plate 45) in abandoned sand pits [NZ 306 537] at Fatfield, where the sand is generally coarser than in most parts of the complex.

Stony clays closely resembling the Durham Lower Boulder Clay have been proved in many boreholes through the Tyne–Wear Complex, especially in the lower Tyne Valley, and have also been seen in a few surface exposures. In the latter, as in temporary exposures seen in 1961 at the south portal of the Tyne Tunnel, they rest on erosion surfaces cut in laminated clay, and in some places the underlying clay has been torn up or distorted into drag folds. The stony clays are up to 7 m thick and appear, from their lack of continuity between closely spaced boreholes, to be lenticular; unlike the Durham Lower Boulder Clay, they are not overconsolidated and they lack a strong clast fabric.

Sediments of the Tyne–Wear Complex are present almost everywhere in the Wear Lowlands, though commonly they are only a few centimetres or millimetres thick. Across much of the low-lying area between Wardley, Hebburn and Boldon, for example, they are represented by discontinuous thin lenses of laminated clay that fill minor hollows in the surface of the boulder clay and underlie the generally flat base of the Pelaw Clay; a temporary section [NZ 2992 6143] at Fell Gate, Wardley revealed:

At the margins of the main buried valleys, as was seen in opencast coal workings [NZ 2893 5151] to [NZ 2912 5139] near Lumley Castle in 1966, the laminated clays and silts thicken sharply between diverging Durham Lower Boulder Clay and Pelaw Clay.

The deposits of the Tyne–Wear Complex pose considerable problems to foundation engineers, mainly becaue of the plasticity of the laminated clays and silts when wet. Tunnels cut in the clays can close rapidly and trenches can collapse overnight if not adequately supported. Failure of the foundations of a major electricity tower [NZ 3350 6637] at Howdon Pans was attributed to plastic flow of the underlying moist laminated clay, and deep piling or other expensive foundation measures are normally required for all heavy structures. Plastic flow of laminated clay, induced by the advance of a high mound of colliery spoil, was responsible for the formation of a pressure ridge [NZ 338 535] up to 5 m high and 15 m wide at Herrington Colliery.

The deposits of the Tyne–Wear Complex are interpreted as mainly lacustrine, the laminated clays and silts having formed under the moderately deep water of Glacial Lake Wear (Figure 60). The overlying sands are regarded as the product of waning and shallowing lacustrine conditions; in places, as at south-east Washington, they may have been deposited on deltas and valley trains, but there are no signs of ice-contact structures. The scattered stones in the laminated clays and silts are inferred to have been ice rafted, and the lenses and wedges of stony clay are interpreted as the product of periglacial mudflows of Durham Lower Boulder Clay (at least some into water). Periglacial effects, including liquifaction and plastic flow, have also been invoked to explain the involutions and other contortions in the clays, silts and sands in coastal cliffs at Whitburn (Smith, 1981).

Ryhope Sands

Coastal cliffs between The Crags [NZ 413 546] (Hendon) and a point [NZ 421 512] about 1.5 km north of Featherbed Rocks, Seaham, expose a complex sequence of sands, silts, clays and gravels that extends inland for up to one kilometre at Ryhope Colliery but generally for only a few hundred metres. These deposits rest on the deeply eroded surface of the Durham Lower Boulder Clay and, in the south, clearly occupy a shallow valley cut in the latter.

The deposits inland mainly comprise a few metres of sandy gravel, which was formerly worked in pits [NZ 409 412]; [NZ 534 531]; [NZ 416 522] around Ryhope. Pebble-cobble gravel predominated in the pits, which are now filled, but boulders up to 0.3 m across were locally abundant. Fragments of Cyprina and Littorina were recorded by Coupland and Woolacott (1926). The clasts in the gravel comprised a suite similar to that of the Durham Lower Boulder Clay, much augmented by rafts and rolled fragments of the boulder clay itself and by subrounded cobbles and boulders of unmistakably local Permian dolomites and limestones. Their lithology, foreset trends and imbrication patterns indicate eastwards and then southwards sediment transport, and link the gravels with the cutting of the impressive dry valley of Tunstall Hope at Ryhope Colliery.

Sand-filled ice-wedge casts up to 3.5 m deep were present [NZ 4165 5229] at one place in the gravel workings in the Ryhope Sands, south of Ryhope, and up to 0.6 m of laminated clay separated the gravel from the overlying brown prismatic clay here.

Sand predominates in the coastal cliffs, where gravel occurs only patchily at the base of the sequence (where it is partly cemented) and in scattered, mainly thin, lenses. Coal grains are locally abundant. Much of the sand is roughly horizontally bedded and it is up to 13 m thick in cliffs between The Crags (Hendon) and Salter-fen Dene [NZ 414 538], where it is interbedded with subordinate silt and clay. Cross-bedding trends indicate northwards sediment transport in the extreme north of the outcrop near Nanny Rain's Springs [NZ 414 543], but southwards and south-eastwards sediment transport is indicated at most other exposures, where some individual sets are up to 6 m thick and may be deltaic. Between Salterfen Dene and Ryhope, the sequence is diversified by the presence of several thick lenses and tongues of tough, grey-brown, stony clay, which strongly resemble Durham Lower Boulder Clay and may be mudflows of the latter. The sand in the cliffs, though extremely uneven in thickness as a result mainly of the relief of the eroded top of the underlying boulder clay, is generally less than 5 m thick to the south of Pincushion [NZ 420 523] and tapers to a feather edge south of Ryhope Dene.

The Ryhope Sands are interpreted (Smith, 1981) as the remains of an outwash plain and delta fan, perhaps partly lacustrine, deposited following erosion of the Durham Lower Boulder Clay by early drainage from Glacial Lake Wear. If this interpretation is correct, it implies that at one time, however briefly, all the waters of Lake Wear and of the whole Tyne and Wear drainage catchment may have discharged eastwards through Tunstall Hope.

The Ryhope Sands are not distinguished from other deposits of Glacial Sand and Gravel on the published Drift map, but their location and extent are shown in (Figure 58).

Durham Upper Boulder Clay

Red-brown stony clay up to 5 m thick lies in a number of gentle hollows in the top of the Ryhope Sands in coastal cliffs between Salterfen Rocks and the mouth of Salter-fen Dene [NZ 414 538]. It was first recorded by Coupland and Woolacott (1921), who noted that it contained a suite of northern erratics, including typical Cheviot rocks. The clay is now inaccessible and those observations cannot be confirmed in detail, but it is superficially similar to Durham Upper Boulder Clay as typically exposed in coastal cliffs south of Easington (Smith and Francis, 1961), and the two deposits may be equivalent. Probable Durham Upper Boulder Clay is also present on the north bank of the Tyne [NZ 369 690] at Tynemouth, just north of the district, showing that Cheviot and Tweed Valley ice may at one time have lain near to the present coastline and locally extended onshore. At Tynemouth, the deposit is separated from the underlying Durham Lower Boulder Clay by 0.4 to 0.7 m of pebbly sand, and is not to be confused with the reddened (weathered) upper part of the Durham Lower Boulder Clay reported by Eyles and Sladen (1981) farther north on the Northumberland coast. Occurrences of Upper Boulder Clay are not distinguished on the published Drift map from those of Lower Boulder Clay.

Red-brown boulder clay is also present in places offshore, and a 21.5 m core of it was recovered from borehole WM 7A [NZ 5270 6290], 11 km east of Whitburn; its stratigraphical affinities have not been established.

Fluvioglacial sand and gravel

In addition to the Ryhope Sands and to sands of the Tyne–Wear Complex, deposits of fluvioglacial sand and gravel are scattered unevenly throughout the district. They form isolated patches at Cleadon [NZ 38 62], Whitburn [NZ 40 62], Seaham [NZ 400 602] and Grindon [NZ 359 546] in the north and centre of the district and are relatively abundant on the Magnesian Limestone plateau between Houghton-le-Spring and Seaham. All these deposits overlie Durham Lower Boulder Clay, although in some places most or all of the latter was removed before deposition of sand and gravel commenced. On the published Drift map these deposits are grouped together with the Ryhope Sands and the sands of the Tyne–Wear Complex as Glacial Sand and Gravel.

The deposit at Cleadon is composed of up to 12 m of gravelly sand; its pebbles and cobbles comprise a suite similar to that of the Durham Lower Boulder Clay, but it also contains scattered pebbles of chalk and flint. Woolacott (1906) noted a number of minor faults in the sand and gravel here, adjoining parts of which were calcite cemented. The presence of worn marine shells in this deposit led to early claims (Howse, 1864, Woolacott, 1897 et seq.) that it was a marine beach deposit, but Trechmann's view (1952) that it is probably fluvioglacial is accepted here. The deposit at Whitburn is poorly exposed but appears to be generally similar to that at Cleadon. Partly bedded gravel of considerable thickness was also reported by Howse (1880) in a railway cutting at about 30 m above OD at Marsden, probably near to the northern end of the present quarry. The gravel rested on a 'sea-worn' surface that bears evidence of a former cover of Durham Lower Boulder Clay and was overlain by clay that would now be called Pelaw Clay. Howse noted that the gravel contained no marine shells, but had no doubt that it was a marine beach deposit; its elevation, stratigraphical relations and age clearly throw doubt on this interpretation and a relationship with the lacustrine Tyne–Wear Complex seems more likely.

The sand at Grindon (now removed) formed a slightly sinuous clay-covered ridge about 23 m high, and, although rich in coal grains, was only sparingly pebbly, except in the uppermost 3 to 5 m. A varied pebble suite was mainly of western origin but included considerable numbers of pebbles of Cheviot lavas and of local Magnesian Limestone. The deposit was strongly contorted in places, with dips of up to 70°, and featured many minor faults (Smith, 1981, fig. 8); it was interpreted by Woolacott (1905) as an esker and by Trechmann (1952) as a kame. Its content of Cheviot-derived clasts may relate the Grindon sand to the Upper Boulder Clay but the exact nature of this relationship is unknown.

Little is known about the composition and provenance of the many small patches and mounds of sand and gravel in the south of the district, the presence of most of which is inferred from their topographical expression and from indifferent exposures. The extensive and elevated hummocky tract at Burdon, however, was cut into by large workings at Warden Law [NZ 37 50] which are now degraded, but which formerly exposed 15 to 21 m of interbedded sandy and clayey pebble-cobble gravel. Clasts in the gravels were mainly of western provenance, but they also contained fragments that could have come only from the north and east; these included many fragments of Permian reef dolomite, collapse breccias, and a few of concretionary limestone and Cheviot lavas. The gentle sheet dip is towards the southwest, and cross-lamination in the sandy gravels was similarly mainly westwards. The hummocky appearance of the area between Burdon and Seaton is reminiscent of constructional morainic topography, but the exposures at Warden Law suggested that this appearance is misleading and that the sands and gravels are probably erosional relics of a westwards-sloping sheet that was formerly much more extensive.

Superficial clays

Except for recent alluvium, peat and wind-blown sand, virtually all Quaternary drifts in lowland parts of the district are topped by brown and red-brown silty clays. These clays are generally at least 0.5 m thick and commonly 1 to 2 m; they form continuous sheets in many central and northern parts of the district and extend less continuously to about 130 m above OD on the flanks of major eminences such as Gateshead Fell. In a few places, as in parts of the coastal platform between South Shields and Whitburn, superficial clay rests directly on rockhead, although even here there are residual boulders, gravels and patches of Durham Lower Boulder Clay in rockhead hollows (Howse, 1864).

The presence of the superficial clays has been recognised by most workers, and they have been accorded a number of names. This account follows Smith (1981) in recognising a widespread Pelaw Clay and a more restricted Prismatic Clay, but distinction between these requires good exposures, and delineation of their mutual boundary in the field was not attempted. Thus Pelaw Clay as shown on the published drift map may include some Prismatic Clay, or may locally consist entirely of Prismatic Clay. Where both clays are present, the Prismatic Clay invariably overlies, and is readily distinguished from, the Pelaw Clay. Both deposits give rise to soils that differ from those on the Durham Lower Boulder Clay by their lower sand and pebble content, and by their slightly red colour. All the superficial clays display strong columnar jointing in the weathered layers; joints extend to the base in places where the clay is thin.

Pelaw Clay

The Pelaw Clay of the Wear Lowlands was seen in many large temporary exposures during the resurvey of the district, which showed it to be generally 1 to 2 m thick, but locally up to 4.5 m. It is a slightly reddish brown to dark brown blocky, silty clay that contains scattered to abundant pebbles and small cobbles; it is generally unbedded, but weak bedding traces are present in places and lenses and thin beds of sand are common. The clasts comprise a suite similar to that of the Durham Lower Boulder Clay, but display no coherent preferred fabric and are much less abundant. The base of the deposit is generally sharp and flat against the underlying sands and clays, but is locally gradational against both laminated clay and Durham Lower Boulder Clay. Small, grotesque, buff and grey, calcareous concretions are locally common near the base of the weathering profile and are likely to have been formed by calcium carbonate leached from pebbles and matrix nearer to the surface. Leaching and reprecipitation of iron oxides is indicated by strong discolouration of the clay adjacent to joints.

Work by Beaumont (1972) showed that the clay mineral content of the Pelaw Clay (his Upper Wear Clay) was almost indistinguishable from that of the Durham Lower Boulder Clay, and was probably derived almost exclusively from the weathering of Carboniferous shales and mudstones.

The Pelaw Clay of the coastal platform between South Shields and Whitburn is well exposed in coastal cliffs near Whitburn Colliery and Whitburn, where it is widely 2 to 4 m thick; it is generally similar to its counterpart in the Wear Lowlands but contains, in addition, pebbles of chalk and flint (Howse, 1864, 1880). Weak bedding is locally apparent, especially in basal layers of the clay, and the contact with the grey-brown Durham Lower Boulder Clay, where present, is sharp and level.

The general weakness of the Pelaw Clay causes considerable problems in excavations, which require special support. The weakness stems from a low natural shear strength allied to the columnar jointing and to moisture retentivity in the lower part of the profile; this creates an inverted strength profile and leads rapidly to failure of the walls of unsupported excavations.

The mode of origin of the Pelaw Clay remains uncertain. Its continuity over a wide range of other deposits, including rock, shows that it is a distinct deposit, and its low strength and lack of a preferred fabric probably indicate that it is not a ground moraine. Howse (1880) recognised that the clay is younger than, and distinct from, the Durham Lower Boulder Clay and was formed by different processes, and Woolacott (1905) thought that it might have resulted from late-glacial marine reworking of the pre-existing glacial deposits; there is, however, no supporting evidence of late-glacial sea levels up to 130 m above OD. A view expressed tentatively by the writer (Smith, 1981) is that the Pelaw Clay might be the product of periglacial modification and redistribution of existing deposits, but especially of the plastic laminated clays, following the draining of Glacial Lake Wear. The processes envisaged were winter cryoturbation and slow solifluction combined with summer liquefaction, homogenisation and mass flow in a saturated surface layer with its drainage impeded by underlying permafrost.

Prismatic Clay

The Prismatic Clay derives its name from the marked columnar jointing that it shares with upper parts of the Pelaw Clay, but it differs from the latter in being dull brown and more sandy, and in containing only a few small pebbles (Plate 46). It is generally less than one metre thick, with a maximum of about 1.5 m. The deposit is unevenly and sparingly distributed inland, mainly in hollows and minor valleys, but is more extensive on the coastal platform between Hendon and Seaham, where it overlies the Ryhope Sands, has a sharp base and is weakly bedded in its lowest part. The Prismatic Clay at Ryhope passes uninterruptedly over ice-wedge casts in gravel in the Ryhope Sands and is therefore younger than the latter. A generally alluvial origin is likely, with local variations caused by creep and the incorporation of hillwash.

Both Pelaw and Prismatic clays are probably represented in thin, complex, but patchy, bedded drifts on the flanks of the Cleadon and Fulwell hills, where they generally overlie either thin Durham Lower Boulder Clay or sands and clays that may be part of the Tyne–Wear Complex.

Sand and gravel of Fulwell Hills

A deposit of sand and gravel was formerly exposed at about 43 to 45 m above sea level on the northern flank of Fulwell Hills, but has since almost been worked out. It was first discovered by Kirkby (1860), who compared it with beach gravel, and was interpreted by Howse (1864) and Woolacott (1897 et seq.) as a marine beach deposit. Full description by Kirkby included sketches of many cylindrical sand-filled pipes up to 4 m deep in the surface of the Magnesian Limestone, which have not been recorded elsewhere in the district. Kirkby also recorded the presence of patchy boulder clay beneath the sands, which was partly banked against a bed of gravel. Woolacott (1897, 1900b, 1905) noted about 3.5 m of partly cemented gravel at this locality, from which he collected rolled flints, pebbles of Cheviot lava and a few worn marine shells including Cyprina and Littorina; no bored pebbles were recorded. At the time of the resurvey (1965) all that remained of the deposit was 0 to 1.5 m of gravel composed almost entirely of subrounded to rounded clasts of local Concretionary Limestone in a sand matrix and resting on a markedly uneven Concretionary Limestone surface. The gravel was partly cemented in its lower part and yielded neither marine shells nor bored pebbles.

The view that the Fulwell Hills gravel was a marine beach deposit was based mainly on the identification of the slight topographical bench on which it lay as part of a widespread marine planation backed by a line of supposed sea cliffs and sea caves. The view cannot be sustained, however, if Kirkby's observation that it is younger than the local boulder clay is correct, because there is no evidence of post-late Devensian sea levels at 45 m above OD. A lacustrine origin seems more likely.

Complex drifts around East Herrington and Silksworth

Ground lying between about 85 and 90 m above sea level in and around East Herrington and Silksworth features a thin sequence of bedded sands, gravels and clays. These deposits overlie the eroded surface of Durham Lower Boulder Clay and are widely overlain by brown prismatic clay. They occupy broad surface hollows that open northwards and north-eastwards, and overlie a hummocky rockhead surface that features a number of confluent buried valleys.

There are few natural exposures of the drift in these areas, and precise delineation of the various deposits is hindered by the overlying clay. Many temporary exposures seen during and since the resurvey, however, revealed that the sequence is commonly 1 to 3 m thick and displays such lateral variability that even adjacent sections can be correlated only with difficulty. The complexity may be judged from the fact that, although more than 100 sections exceeding 1 m deep were examined, all displayed at least two lithological units and most displayed three or more; the following sections are typical:

Contacts in this section were conformable and the base of the sequence is probably only slightly above Durham Lower Boulder Clay

In general, the deposits at East Herrington are thinner than their more extensive counterparts at Silksworth and include fewer stoneless laminated clays and silts. Pure gravel is uncommon, but is more abundant at Silksworth than at East Herrington, and clayey gravels and gravelly clays are common in both areas. Pebbly sandy to gritty clays (other than Durham Lower Boulder Clay and Prismatic Clay) are interbedded with the other deposits in most of the sections seen, and contain an erratic suite similar to that of the Durham Lower Boulder Clay. Some, however, contain mainly or solely local Magnesian Limestone rock debris, and streaks and lenses of such debris occur locally in clays near rockhead. Although beds are mainly lenticular, bed contacts in most sections are smooth and undisturbed by cryoturbation or other influences.

The origin of the complex drifts of East Herrington and Silksworth is uncertain and it seems likely that they were affected by a range of environmental influences, including the generally low-lying location and the diverse surface of the underlying till and Magnesian Limestone. Many of the clays and sands are clearly water-laid, and the presence of smoothly laminated and stoneless clays and silts probably indicates phases of lacustrine deposition. However, the pebbly clays and clayey gravels, especially those containing mainly local debris, may have been derived from existing clays and rocks by a combination of periglacial activity, creep and mass flow, perhaps indicating a lake-marginal environment; this would accord with the sedimentary structures of some of the sands, a number of which are strongly cross-stratified and possibly deltaic. On the published drift map these varied deposits are classified as Head.

Miscellaneous Late-Glacial and Post-Glacial deposits

Mixed drifts

In addition to the complex drifts of the Herrington–Silksworth area, thin sequences of interbedded superficial drifts have been revealed by temporary exposures at various elevations on the flanks of several hills. Thus, for example, 0.9 to 1.2 m of eastwards-dipping silty sand overlie 0.6 to 1.5 m of Durham Lower Boulder Clay at about 62 m above OD on the southern margin of Marsden Quarry [NZ 4037 6393], Whitburn Colliery; and interbedded sands and clays were noted in several excavations on the eastern flanks of Gateshead Fell. Farther south, varied deposits exposed at the edges of Field House Sand Hole, Houghton-le-Spring are exemplified by a section [NZ 3555 5058] measured on the (then) eastern lip of the quarry:

Beds in this section are probably mainly of reworked local Magnesian Limestone mixed with reworked till, and appear to form part of a solifluction complex. Nearby, other exposures are similarly varied, but the most easterly (lowest, 132 m above OD) also include wedges of laminated clay and may be lake-marginal.

Similar thinly interbedded drift sequences were encountered in trenches [NZ 344 593] near Town End Farm, west Sunderland, and in gently sloping ground [NZ 3346 4963] in the western outskirts of Houghton-le-Spring. At Town End Farm, tongues up to 1.2 m thick of ?soliflucted Permian Yellow Sands locally extend downslope for more than 50 m from the Coal Measures–Yellow Sands contact, and at Houghton-le-Spring thin beds of Magnesian Limestone gravel were found to be interbedded with sand fully one kilometre downslope from the nearest Magnesian Limestone outcrop. The Houghton section is also of interest because silty clay between the gravels yielded brown and black, peaty, woody tissue; radiocarbon analysis of this tissue by Gakushuin University (Tokyo) and Isotopes Inc. (New Jersey) gave ages of 2160 ± 110 years and 3400 ± 100 years respectively. None of these patchy deposits has been shown on the published drift map.

Slopewash

Throughout the district, sheets and wedges of hillwash are widespread on slopes, overlying both rock and earlier drifts; they are generally less than 2 m thick and have not been shown separately on the 1:10 560 and 1:50 000 maps. The slopewash generally comprises a mixture of erratic and local rock debris in a matrix of dull brown, streaky clay. It is presumed to be the product of mass downslope movement, including rainwash and solifluction effects, and probably continues to accumulate today.

Alluvium

Discontinuous belts of stream alluvium are present in parts of most valleys in the district and are up to 800 m wide in the Wear Valley at Chester-le-Street; the alluvium alongside the Tyne and much of the lower Wear is overlain by made ground.

Boreholes and natural sections show that the alluvium generally comprises bedded silts and sands with scattered lenses of gravel (especially near the base), widely overlain by up to 2 m of brown silty clay with columnar joints. Even in the Wear Valley, the alluvium is mainly less than 8 m thick, and in the narrow coastal denes it is commonly only 2 or 3 m thick. No freshwater fossils have been reported.

The partly estuarine alluvium of the lower Tyne Valley is similar in composition to that of the other valleys of the district but differs in lying partly below modern sea level and in its recorded fossil content. In places it extends to 25 m below OD. Fossils reported from excavations in connection with the Tyne Tunnel (Armstrong and Kell, 1951) include unworn marine shells such as Mytilus and barnacle-encrusted Littorina; both were collected at about 13.5 m below OD from a bed of sand near the base of the postglacial deposits. Armstrong and Kell also recorded abundant plant remains and a number of unspecified vertebrate bones from silt and fine-grained sand overlying the shelly layer. Slightly farther downstream, Howse (1863) recorded plant and tree debris, and bones of deer, Bos primagenius and other vertebrates in excavations in estuarine alluvium 1.5 to 5 m below the surface at Jarrow Slake [NZ 34 65]. The alluvium of Jarrow Slake was also the source of the crystal pseudomorphs known as jarrowite, which were described by Browell (1860) and later shown to be after the calcium carbonate hexahydrate mineral ikaite (see Shearman and Smith, 1985 for full discussion). This unstable mineral is known to crystallise only at temperatures near to freezing point.

Terrace alluvium is confined to a single broad but low terrace remnant at Chester-le-Street, now covered by houses, and to narrow terrace remnants in Ryhope Dene (and its upstream equivalents) and in Dawdon Dene at Seaham; they are composed mainly of sandy gravel. The terraces may be relics of valley trains formed during the early phases of deglaciation.

Lacustrine alluvial silt and clay (other than laminated clay) fills or partly fills a number of minor hollows that are scattered over the district, but are most common in the low-lying area between Usworth and Castletown, and in the hummocky area south of Burdon.

Marine deposits

Much of the coast of the district is rugged and cliff-bound, with resistant rocks, mainly Magnesian Limestone calcitic collapse-breccias and other limestones, forming uneven shore platforms up to 1 km wide but generally 0.1 to 0.3 km. The platforms bear patchy thin coverings of pebble-cobble gravel and scattered to abundant boulders. Local Magnesian Limestone is a major constituent in all the beach gravels but they also contain abundant clasts from the glacial deposits (including many of the largest boulders such as Carboniferous limestone, ganister and Whin dolerite) and a proportion of rolled flints (mainly ships' ballast) and artefacts. The width of the coastal rock platform affords some protection to the cliffs, as at Whitburn, the name of which is a corruption of 'White Burn' in reference to the considerable expanse there of white-capped breaking storm waves.

Embayments in the coastline, mainly coincident with the outcrop of the softer dolomitic parts of the Magnesian Limestone, but also including the places where buried valleys occur, are marked mainly by stretches of beach sand up to 0.2 km wide; the main sandy beaches are at South Shields, Marsden Bay, Whitburn Bay, Roker, Ryhope and north of Seaham. Slight net southwards sediment transport is apparent on some of the beaches, but most appear to have a stable long-term sand budget; research by King (1954) on beach deposits in Marsden Bay showed that most sand movement was normal to the trend of the coast.

Peat

Patches of thin sedge peat and peaty alluvium, mainly too small to depict on the 1:50 000 map, occupy minor hollows in the drift at a number of places inland; they also form local lenses up to 0.5 m thick in the alluvium of the valleys of the rivers Tyne and Don. Peat in the Tyne alluvium was encountered in dock excavations at Jarrow Slake and in a few foundation and exploration boreholes, and peat in the Don alluvium is occasionally exposed in the banks of the stream. Peat (0.3 m) was also recorded by Howse (1863), at a site in western South Shields, beneath 4 m of alluvial clay and only slightly above the contemporary Tyne river level; at its base was found a skeleton of an Irish Elk, which Howse believed to have drifted in, like the peat, during flood conditions.

Woody peat ('submerged forest') has long been known at Seaburn in Whitburn Bay, and is exposed on the foreshore there when storms temporarily sweep away its normal covering of beach sands. Trechmann (1947) claimed that it extends for nearly 1.6 km, but Woolacott (1897) was unable to prove that it extended more than a short distance north of the mouth of Cut Throat Dene. Howse (1864) recorded the remains of alder, birch, hazel and oak in the peat at Seaburn, together with bone of Bos primigenius and antlers of two species of deer; Woolacott (1897, 1913b) added beech and noted undisturbed tree roots during another brief phase of exposure. During the resurvey, the peat (1.5 m) was exposed in temporary excavations [NZ 4051 6023] at the southern end of Whitburn Bay and was found to contain logs up to 0.25 m across. The peat in this excavation overlay about 1.2 m of stony clay (probably Lower Boulder Clay) with abundant roots, but Woolacott (1913b) noted a patchy layer of rounded stones between the till and the peat at the main beach exposure and recorded that one pebble bore a marine Serpula. The Whitburn peat has not been radiometrically dated but is probably about the same age (5000 to 9000 years BP) as similar peat at West Hartlepool (Barker and Mackay, 1961; Gaunt and Tooley, 1974). A single sample (YG 6389) analysed by Dr R Harland contained a relatively small number of pollen grains dominated by tree pollen (chiefly Ulmus, Corylus and Alnus) suggestive of temperate conditions and a Zone VII or Zone VIII age.

Landslips

Many small landslips were mapped, especially on the steep sides of several valleys, but are not shown on the published 1:50 000 map.

Blown sand

Sand blown from the adjoining beaches forms patchy sheets up to 2 m thick on the coastal cliffs at Trow Point, Marsden Bay, Whitburn Bay, Ilendon and Ryhope.

Tufa and travertine

The only patch of calcareous tufa recorded in the district is about 25 x 70 m across and up to 1.5 m thick; it coats the south-east side of the valley of Sharpley Burn [NZ 389 509], Burdon, at its confluence with Burdon Dene. Travertine forms thin discontinuous sheets coating the walls of the few caves and the many fissures in the Magnesian Limestone, and forms stalactites up to 0.30 m long in tension (cambering?) fissures in the Magnesian Limestone in an old quarry [NZ 358 508] near High Haining, Houghton-le-Spring.

Made ground

Substantial parts of the district are covered with made ground, ranging from indurated waste from chemical plants alongside the Tyne and Wear to thick accumulations of colliery waste and quarry spoil. The made ground is shown on the maps in places where its average thickness exceeds 1 m, but it was found impracticable to delineate extensive spreads of urban rubble that are commonly 1 to 2 m thick. Industrial waste was widely used in the construction of harbours and riverside wharfs, and both industrial and domestic waste were used extensively for filling valleys (some quite large, especially in the Walker and Wallsend areas of Newcastle) and quarries, and also for raising and thus reclaiming poorly drained areas. A major feature of the lower reaches of Tyneside and Wearside during the 18th and 19th centuries was the presence of many large mounds of shelly flint gravel that had been carried as ships' ballast from the beaches of Essex and elsewhere, and discharged alongside the northern rivers.

Many of the former waste heaps have now been graded and landscaped, softening the earlier image of industrial dereliction, and much waste (including burnt shale from the colliery spoil heaps and most of the flint gravels) has been removed and used by the construction industry. Some areas of restored made ground have been built on, but most are now used as public amenity areas such as parks and sportsfields, or have been returned to agricultural or industrial use.

The varied quality of made ground, and its unpredictable distribution, present considerable problems to the construction industry. It is probable that these deposits are more extensive than are shown on the various maps and that some areas of made ground remain undiscovered.

Interpretation of the Quaternary sequence

Modelling of Late Devensian ice sheets (Boulton et al., 1977, 1985) suggests that the ice covering the district emanated mainly from a major dispersion centre located over the western Southern Uplands of Scotland, where an ice dome with its broad summit at about 1800 m above OD fed ice sheets moving outwards in all directions. Ice flowing southwards and south-eastwards from this dome was obstructed and deflected eastwards by thick ice filling the Irish Sea Basin and by ice spreading out from a minor ice dome over the Lake District, and flowed rapidly eastwards so as to cover north-east England and extend southwards far down the Vale of York and the east coast. The evidence of drumlin trends, erratic trains and the orientation of till fabrics and glacial striae leaves no doubt that ice covering the district came mainly over the northern Pennines and the Tyne valley, and an ice-sheet surface at about 750 to 800 m above OD has been inferred (Boulton et al., 1985); allowing for isostatic depression, an ice thickness on the Wear Lowlands of perhaps 1000 m is implied. Early views (e.g. Howse, 1864; Kendall, 1902; Woolacott, 1907; Raistrick, 1931) that the ice streaming eastwards into the present area of the North Sea was futher obstructed by massive ice sheets extending from Scandinavia have now been abandoned in the absence of confirmatory evidence from ice-sheet modelling and from studies of Pleistocene deposits beneath the North Sea (Cameron et al., 1987).

The ice sheet modelling studies do not recognise a separate ice dome over the eastern Southern Uplands and Cheviot Hills, but a case for an ice cap there was summarised by Clapperton (1970) and is tentatively accepted here. The evidence suggests to the writer that the ice of this local dome accumulated to a critical thickness only after the outward flow from the main dome had become established, and Cheviot ice was initially deflected mainly eastwards down the Tweed Valley by the more powerful western ice. Subsequently, however, waning of the western ice allowed the younger Tweed–Cheviot ice to surge south-eastwards into the newly vacated area of the North Sea, its western edge lying close to, and locally crossing, the present coastline.

In this scenario, the Durham Lower Boulder Clay and associated sands and gravels have generally been accepted as the varied product of the advancing and then downwasting western ice, and the red-brown Upper Boulder Clay and its equivalents have been regarded by many (e.g. Woolacott, 1907, 1921; Smythe, 1912; Raistrick, 1931; Smith and Francis, 1967; Smith, 1981) as the ground moraine resulting from the south-eastwards advance and then downwasting of the Tweed–Cheviot ice. The combination of westwards-retreating western ice and south-eastwards-advancing northern ice led to blockage of valleys carrying meltwaters eastwards, and to the formation of a string of ice-dammed lakes in eastern Northumberland and Durham (Smythe, 1908, 1912; Woolacott, 1921; Raistrick, 1931; Smith and Francis, 1967). The sands and clays of the Tyne–Wear Complex are envisaged as the deposits of one of the largest of these bodies, Glacial Lake Wear (Figure 60), which at one time overflowed south-eastwards so as to cut Tunstall Hope and deposit the Ryhope Sands. The highest sediments of the Tyne–Wear Complex type, at about 132 m above OD, indicate the level to which the lake may have extended, but it probably stood at different levels as outlets opened and closed; temporary stands may have led to the cutting of benches on the Durham Lower Boulder Clay of the Wear Valley such as that at about 43 to 45 m on the flanks of the Cleadon and Fulwell hills. Permafrost doubtless affected exposed glacial and lacustrine sediments around the lake margins at these times and may have led to the interdigitation there of lacustrine and inferred mass-flow deposits; the ice wedge at Burdon may date from this phase.

The age and affinities of the various fluvioglacial sands and gravels (other than the Ryhope Sands) is problematical but they overlie Durham Lower Boulder Clay and their content of Cheviot pebbles links them to the Upper Boulder Clay. However, the elevation at which the Warden Law gravels lie (up to 197 m above OD), and their inferred former sheet-like form and greater extent, can only be accounted for by postulating the subsequent removal of vast quantities of gravel and perhaps of Upper Boulder Clay. Similarly, the contorted sand at Grindon, if correctly identified as a kame or esker related to the Upper Boulder Clay, can only be satisfactorily explained if Upper Boulder Clay ice had advanced inland at Sunderland for several kilometres farther than appears likely from other evidence. An alternative view (Smythe, 1912) that the Warden Law and Grindon deposits were formed on the eastern margin of the retreating western ice is difficult to reconcile with their content of Cheviot pebbles, unless it can be shown that these could have been derived from western ice. In this connection, an important observation by Howse (1864) was that some Cheviot pebbles might have been picked up by Tyne Valley (= western) ice from the valley of the River Reedwater.

The last events of the Late Devensian glaciation in the district were the stagnation and melting of the Tweed–Cheviot ice and the consequent drainage of ice-dammed lakes. This led to the creation of a new drainage system, partly superimposed on the earlier system as lacustrine deposits were cut through, and graded to a low sea level; the late-glacial gorges of the lower river valleys are the expression of a combination of this phase of rapid downcutting and one of isostatic rebound. The mobile Pelaw Clay and some prismatic clay may have begun to be formed during this episode of deglaciation, perhaps by periglacial processes acting on the newly exposed lake depoists, and such features as the ice-wedges (?polygons) in the Ryhope Sands may have been initiated at the same time.

The chronology of subsequent late-glacial and post-glacial (Flandrian) events and their sedimentary expression is uncertain, but the presence of marine shells near the base of the Tyne alluvium at Jarrow shows that sea-level recovery initially outstripped isostatic rebound and that the sea penetrated some way into the Tyne valley before alluvial and estuarine sedimentation was reestablished as the sea withdrew. A marine transgression is also indicated by the inundation of the coastal peats, including that in Whitburn Bay, and pulsed interactions between sea-level recovery, isostatic rebound and coastal recession is likely to be the cause of terraces in the Wear and other valleys. The latest events, probably mainly post-glacial, were the deposition of alluvium that filled the lower parts of valleys previously graded to well below modern sea level but subsequently shortened by the Flandrian transgression and coastal recession, and the deposition of modern beach and windblown sand.

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.

ABBOTT, G. 1903. The cellular Magnesian Limestone of Durham. Quarterly Journal of the Geological Society of London, Vol. 59, 51–52

ABBOTT, G. 1907. Concretions. South-eastern Naturalist, 1–7.

ABBOTT, G. 1914. Discoid limestones which simulate organic characters. The Pioneer, 1–8.

AL-REKARBI, Y. S. 1982. Petrography, porosity, permeability and geochemistry of the Upper Magnesian limestone of N E England. Unpublished PhD thesis, University of Dundee.

ANDERSON, W. 1945. On the chloride waters of Great Britain. Geological Magazine, Vol. 82, 267–273.

ANDERSON, W. 1945. Water supply from underground sources. Geological Survey Wartime Pamphlet, No. 19, Part III, Supplement [18 pp.] .

ANDERSON, W, and DUNHAM, K C. 1953. Reddened beds in the Coal Measures beneath the Permian of Durham and south Northumberland. Proceedings of the Yorkshire Geological Society, Vol. 29, 21–32.

APLIN, G. 1981. Diagenesis of the Zechstein Reef Complex of N E England. Institute of Geological Sciences Stable Isotope Report, No. 66.

APLIN, G. 1985. Diagenesis in the Zechstein main reef complex, N E England. Unpublished PhD thesis, University of Nottingham.

ARMSTRONG, G, and KELL, J. 1951. The Tyne Tunnel. Geological Magazine, Vol. 8, 357–361.

APLIN, G, and PRICE, R H. 1954. The Coal Measures of north-east Durham. Transactions of the Institution of Mining Engineers, Vol. 113, 973–997, and Vol. 114, 83–86 and 111–114.

BABCOCK, J A. 1977. Calcareous algae, organic boundstones, and the genesis of the Upper Capitan Limestone, West Texas and New Mexico. 3–44 in 1977 Field Conference Guidebook. HILLMAN, M E, and MAZZULLO, S J (editors). Society of Economic Paleontologists and Mineralogists, Publication No. 77–16.

BARKER, H, and MACKEY, J. 1961. British Museum natural radiocarbon measurements III. Sample description C. Great Britain BM-80. West Hartlepool submerged forest. Radiocarbon, Vol. 3, 41–42.

BEAUMONT, P. 1968. A history of glacial research in Northern England from 1860 to the present day. University of Durham Occasional Paper, No. 9.21. pp.

BEAUMONT, P. 1970. Geomorphology. 25–45 in Durham County and City with Teesside. DEWDNEY, J C (editor). (Newcastle upon Tyne: British Association for the Advancement of Science.)

BEAUMONT, P. 1971. Stone orientation and stone count data from the Lower Till sheet, eastern Durham. Proceedings of the Yorkshire Geological Society, Vol. 38, 343–360.

BEAUMONT, P. 1972. Clay mineralogy of glacial clays in eastern Durham, England. 83–108 in Research methods in Pleistocene geomorphology. YATSU, E, and FALCONER, A (editors). (University of Guelph: Geographical Publication, No. 2.)

BEERBOWER, J R. 1964. Cyclothems and cyclic depositional mechanisms in alluvial plain sedimentation. 31–41 in Symposium on Cyclic Sedimentation. MERRIAM, D F (editor). Bulletin of the State Geological Survey of Kansas, Vol. 169, 1.

BELL, J, HOLDEN, J, PETTIGREW, T H, and SEDNLAN, K W. 1979. The Marl Slate and Basal Permian Breccia at Middridge, Co. Durham. Proceedings of the Yorkshire Geological Society, Vol. 42, 439–460.

BOTT, M H P. 1961. A gravity survey off the coast of northeast England. Proceedings of the Yorkshire Geological Society, Vol. 33, 1–20.

BOTT, M H P. 1967. Geophysical investigations of the northern Pennine basement rocks. Proceedings of the Yorkshire Geological Society, Vol. 36, 139–168.

BOTT, M H P, and JOHNSON, G A L. 1970. Structure. 10–22 in The geology of Durham County. HICKLING, G (editor). Transactions of the Natural History Society of Northumberland, Durham and Newcastle upon Tyne, Vol. 41.

BOTT, M H P, and MASSON-SMITH, D. 1957. The geological interpretation of a gravity survey of the Alston Block and the Durham Coalfield. Quarterly journal of the Geological Society of London, Vol. 113, 93–117.

BOTT, M H P. SWINBURN, P M, and LONG, P E. 1984. Deep structure and origin of the Northumberland and Stainmore troughs. Proceedings of the Yorkshire Geological Society, Vol. 44, 479–495.

BOULTON, G S. 1970. The deposition of subglacial and melt-out tills at the margins of certain Svalbard glaciers. Journal of Glaciology, Vol. 9, 231–245.

BOULTON, G S. 1972. Modern Arctic glaciers as depositional models for former ice sheets. Journal of the Geological Society, London, Vol. 128, 361–393.

BOULTON, G S. 1976. A genetic classification of tills and criteria for distinguishing tills of different origin. Geographia, Vol. 12, 65–80.

BOULTON, G S, JONES, A S, CLAYTON, K M, and KENNING, M J. 1977. A British ice-sheet model and patterns of glacial erosion and deposition in Britain. 232–245 in British Quaternary studies, recent advances. SHOTTON, F W (editor). (Oxford: Clarendon Press.)

BOULTON, G S. SMITH, G D, JONES, A S, and NEWSOME, J. 1985. Glacial geology and glaciology of the last mid-latitude ice sheets. Journal of the Geological Society of London, Vol. 142, 447–474.

BRAITHWAITE, C J R. 1988. Calcitisation and compaction in the Upper Permian Concretionary Limestone and Scaham formations of north-east England. Proceedings of the Yorkshire Geological Society, Vol. 47, 33–45.

BRAUCH, W. 1923. Verbreitung and Bau der deutschen Zechstein-Riffbildungen. Geologisches Archiv, Vol. 2, 100–187.

BRONGERSMA-SANDERS, M. 1966. Metals of Kupferschiefer supplied by normal sea water. Geologische Rundschau, Vol. 55, 365–375.

BRONGERSMA-SANDERS, M. 1968. On the geographical association of strata-bound ore deposits with evaporites. Mineralium Deposita, Vol. 3, 286–291.

BRONGERSMA-SANDERS, M. 1969. Permian wind and the occurrence of fish and metals in the Kupferschiefer and Marl Slate. Proceedings of the 15th Inter-University Congress (1967), University of Leicester, 61–71.

BRONGERSMA-SANDERS, M. 1972. Hydrological conditions leading to the development of bituminous sediments in the pre-evaporite phase. 19–21 in Geology of saline deposits. RICHTER-BERNBURG, G (editor). (Paris: UNESCO.)

BROWELL, E J J. 1860. Description and analysis of an undescribed mineral from Jarrow Slake. Transactions of the Tyneside Naturalists' Field Club, Vol. 5, 103–104.

BROWELL, E J J, and KIRKBY, J W. 1866. On the chemical composition of various beds of the Magnesian Limestone and associated Permian rocks of Durham. Natural History Transactions of Northumberland and Durham, Vol. 1, 204–230.

BURGESS, I C. 1965. The Permo-Triassic rocks around Kirkby Stephen, Westmorland. Proceedings of the Yorkshire Geological Society, Vol. 35, 91–101.

BURTON, R C. 1911. On the occurrence of beds of the Yellow Sands and marl in the Magnesian Limestone of Durham. Geological Magazine, Vol. 8, 299–306.

BYERS, C W. 1977. Biofacics patterns in euxinic basins: a general model. 5–17 in Deep water carbonate environments. COOK, H E, and ENOS, P (editors). Special Publication of the Society of Economic Paleontologists and Mineralogists, No. 25.

CALVER, M A. 1968a. Distribution of Westphalian marine faunas in northern England and adjoining areas. Proceedings of the Yorkshire Geological Society, Vol. 37, 1–72.

CALVER, M A. 1968b. Coal Measures invertebrate faunas. 144–177 in Coal and coal-bearing strata. MURCHISON. D G, and WESTOLL, T S (editors). (Edinburgh: Oliver & Boyd.)

CAMERON, T D J, STOKER, M S, and LONG, D. 1987. The history of Quaternary sedimentation in the UK sector of the North Sea Basin. Journal of the Geological Society of London, Vol. 144, 45–58.

CARD, G W. 1892. On the flexibility of rocks; with special reference to the Flexible Limestone of Durham. Geological Magazine, Vol. 9, 117–124.

CARRUTHERS, R G. 1930. 69 in Summary of progress of the Geological Survey of Great Britain for 1929.

CARTER, R M. 1975. A discussion and classification of subaqueous mass-transport with particular application to grain-flow, slurry-flow and fluxoturbidites. Earth Science Reviews, Vol. 11, 145–177.

CLAPPERION, C. 1970. The evidence for a Cheviot ice cap. Transactions of the Institute of British Geographers, Vol. 50, 115–127.

CLARK, D N. 1980a. The sedimentology of the Zechstein 2 carbonate formation of Eastern Drenthe, the Netherlands. 131–165 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, II, and PERYT, T M (editors). Contributions to Sedimentology. (Stuttgart: Schweizerbart'sche Verlagsbuchhandlung.)

CLARK, D N. 1980b. The diagenesis of Zechstein carbonate sediments. 167–203 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H, and PERYF, T M (editors). Contributions to Sedimentology. (Stuttgart: Schweizerbart'sche Verlagsbuchhandlung.)

CLARK, D N. 1984. The Zechstein of N W Europe. 151–170 in Carbonate geology (course notes). Open University.

CLARK, D N, and TALLBACKA, L. 1980. The Zechstein deposits of southern Denmark. 205–231 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H, and PERYT, T M (editors). Contributions to Sedimentology. (Stuttgart: Schweizerbart'sche Verlagsbuchhandlung.)

CLARKE, A M, CHAMBERS, R E, ALLONBY, R H, and MAGRAW, D. 1961. A marine geophysical survey of the undersea coalfields of Northumberland, Cumberland and Durham. Mining Engineer, Vol. 15, 197–215.

CLARKE, R F A. 1965. British Permian saccate and monosulcate miosporcs. Palaeontology, Vol. 8, 322–354.

COLTER, V S, and REED, G E. 1980. Zechstein 2 Fordon Evaporites of the Atwick No. 1 Borehole, surrounding areas of N E England and the adjacent southern North Sea. 115–129 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H, and PERYF, T M (editors). Contributions to Sedimentology. (Stuttgart: Schweizerbart'sche Verlagsbuchhandlung.)

COUPLAND, G, and WOOLACOTT, D. 1926. The superficial deposits near Sunderland and the Quaternary sequence in East Durham. Geological Magazine, Vol. 63, 1–12.

DAGLISH, J, and FORSTER, G B. 1864. On the Magnesian Limestone of Durham. Transactions of the North of England Institute of Mining and Mechanical Engineers, Vol. 13, 205–213.

DAVIES, J H, and TRUEMAN, A E. 1927. A revision of the non-marine lamellibranchs of the Coal Measures, and a discussion of their zonal sequence. Quarterly Journal of the Geological Society of London, Vol. 83, 210–259.

DAVIES, W, and REES, W J. 1944. British resources of steel moulding sands, Part 5. The Permian Yellow Sands of Durham and Yorkshire. Journal of the Iron and Steel Institute, No. II for 1943, 104–111.

DEAN, W E, DAVIES, G R, and ANDERSON, R Y. 1975. Sedimentological significance of nodular and laminated anhydrite. Geology, Vol. 3, 367–372.

DEANS, T. 1950. The Kupferschiefer and associated lead-zinc mineralisation in the Permian of Silesia, Germany and England. Report of the XVII I International Geological Congress, London, pt. 7, 340–352.

DEARMAN, W R. 1980. Caledonian movements. 78–79 in The geology of north-east England. ROBSON, D A (editor). Special Publication of the Natural History Society of Northumbria.

DEMAISON, G J, and MOORE, G T. 1980. Anoxic environments and oil source bed genesis. Bulletin of the American Association of Petroleum Geologists, Vol. 64, 1179–1209.

DRYSDALE, W S, and ARMSTRONG, G. 1957. Offshore boring. Transactions of the Institution of Mining Engineers, Vol. 117, 263–288.

DUNHAM, A C. 1970. Whin Sills and dykes. 92–100 in The geology of Durham County. HICKLING, G (editor). Transactions of the Natural History Society of Northumberland, Durham and Newcastle upon Tyne, Vol. 41.

DUNHAM, K C. 1948. A contribution to the petrology of the Permian evaporite deposits of north-eastern England. Proceedings of the Yorkshire Geological Society, Vol. 27, 217–227.

DUNHAM, K C. 1960. Syngenetic and diagenetic mineralisation in Yorkshire. Proceedings of the Yorkshire Geological Society, Vol. 32, 229–284.

DUNHAM, K C. 1961. Black shale, oil and sulphide ore. Advancement of Science, Vol. 18, 284–299.

DUNN, T C (editor). 1980. The Magnesian Limestone of Durham County. (Houghton-le-Spring: Durham County Conservation Trust)

EASTWOOD, T. 1935. British regional geology: Northern England. (London: HMSO.)

EDWARDS, A H. 1963. The characteristics and quality of the seams at Westoe Colliery. Mining Engineer, Vol. 38, 117–129.

EDWARDS, A H, and TOMLINSON, T S. 1958. A survey of 16 low volatile coals in north-east and south-east Durham. Transactions of the Institution of Mining Engineers, Vol. 117, 49–78.

EDMUNDS, W M. 1975. Geochemistry of brines in the Coal Measures of northeast England. Transactions of the Institution of Mining and Metallurgy, B 39–52.

EVANS, S E. 1982. The gliding reptiles of the Upper Permian. Zoological Journal of the Linnean Society, Vol. 76, 97–123.

EYLES, V A, and SLADEN, J A. 1981. Stratigraphy and geotechnical properties of weathered lodgement till in Northumberland. Quarterly Journal of Engineering Geology, Vol. 14, 129–142.

FALCON, N L, and KENT, P E. 1960. Geological results of petroleum exploration in Britain 1945–1957. Memoir of the Geological Society of London, No. 2.

FIELDING, C R. 1982. Sedimentology and stratigraphy of the Durham Coal Measures, and comparisons with other British coalfields. Unpublished PhD thesis, University of Durham.

FIELDING, C R. 1984a. A coal depositional model for the Durham Coal Measures of N E England. Journal of the Geological Society, London, Vol. 141, 919–931.

FIELDING, C R. 1984b. Upper delta plain lacustrinc and fluviolacustrine facies from the Westphalian of the Durham coalfield, N E England. Sedimentology, Vol. 31, 547–567.

FIELDING, C R. 1984c. 'S' or 'Z' shaped coal seam splits in the Coal Measures of County Durham. Proceedings of the Yorkshire Geological Society, Vol. 45, 85–89.

FIELDING, C R. 1986. The anatomy of a coal seam split, Durham coalfield, northeast England. Geological Journal, Vol. 21, 45–57.

FIELDING, C R, and JOHNSON, GAL.  1987. Sedimentary structures associated with extensional fault movement from the Westphalian of N E England. 511–516 in Continental extensional tectonics. COWARD, M P, and DEWEY, J F (editors). Special Publication of the Geological Society of London, No. 28.

FORD, M. 1927. Shaft-sinking at Washington, Co. Durham. Transactions of the Institution of Mining Engineers, Vol. 74, 20–32.

FORSTER, S C, and WARRINGTON, G. 1985. Geochronology of the Carboniferous, Permian and Triassic. 99–113 in The geochronology of the geological record. SNELLING, N J (editor). Memoir of the Geological Society of London, No. 10.

FOWLER, A. 1943. On fluorite and other minerals in Lower Permian rocks of south Durham. Geological Magazine, Vol. 80, 41–51.

FOWLER, A. 1944. A deep bore in the Cleveland Hills. Geological Magazine, Vol. 81, 193–206.

FOWLER, A. 1957. Minerals in the Permian and Trias of north-east England. Proceedings of the Geologists' Association, Vol. 67, 251–265.

FRANCIS, E A. 1970. Quaternary. 134–152 in The geology of Durham County. HICKLING, G (editor). Transactions of the Natural History Society of Northumberland, Durham and .Newcastle upon Tyne, Vol. 41.

FRASER, A J, and GAWTHORPE, R L. 1990. Tectono-stratigraphic development and hydrocarbon habitat of the Carboniferous in northern England. 49–86 in Tectonic events responsible for Britain's oil and gas reserves. HARDMAN, R F P, and BROOKS, J (editors). Special Publication of the Geological Society of London, No. 55.

FUCHTBAUER, H, and PERYT, T M (editors). 1980. The Zechstein Basin with emphasis on carbonate sequences. Contributions to Sedimentology. (Stuttgart: Schweizerbaresche Verlagsbuchhandlung.)

FUZESY, L M. 1980. Origin of nodular limestones, calcium sulphates and dolomites in the Lower Magnesian Limestone in the neighbourhood of Selby, Yorkshire, England. 35–44 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H and PERYT, T M (editors). Contributions to Sedimentology, 9. (Stuttgart: Schweizerbart'sche Verlagsbuchhandlung.)

GARWOOD, E J. 1891. On the origin and mode of formation of the concretions in the Magnesian Limestone of Durham. Geological Magazine, Vol. 8, 433–440.

GAUNT, G D, and TOOLEY, M I 1974. Evidence for Flandrian sea-level changes in the Humber estuary and adjacent areas. Bulletin of the Geological Survey of Great Britain, No. 48, 25–41.

GEINITZ, 1861. Der Dyas or Permian Formation in England. Translation by E J Browell and J W Kirkby. Transactions of the Manchester Geological Society, Vol. 4, 1864, 120–146.

GEORGE, T N, JOHNSON, G A L, MITCHELL, M, PRENTICE, J E, RAMSBOTTOM, W H C, SEVASTOPOULO, G D, and WILSON, R B. 1976. A correlation of Dinantian rocks in the British Isles. Special Report of the Geological Society of London, No. 7.

GIBBONS, M J. 1978. The geochemistry of sabkha and related deposits. Unpublished PhD thesis, University of Newcastle upon Tyne.

GIBBONS, M J. 1983. The depositional environment and petroleum geochemistry of the Marl Slate/Kupferschiefer. Geological Society Newsletter, Vol. 12, 20–21 (abs.).

GLENNIE, K W. 1970. Desert sedimentary environments. Developments in. Sedimentology, Vol. 14.

GLENNIE, K W. 1982. Early Permian (Rotliegendes) palaeowinds of the North Sea. Sedimentary Geology, Vol. 34, 245–265.

GLENNIE, K W. 1983. Lower Permian Rotliegend desert sedimentation in the North Sea area. 521–541 in Eolian sediments and processes. BROOKFIELD, M E, and AHLBRANDT, T S (editors). Developments in Sedimentology, No. 38. (Amsterdam: Elsevier.)

GLENNIE, K W. 1984. Early Permian-Rotliegend. 41–61 in Introduction to the petroleum geology of the North Sea. GLENNIE, K W (editor). (Oxford: Blackwell.)

GLENNIE, K W, and BOEGNER. 1981. Sole Pit inversion tectonics. 110–120 in The petroleum geology of the continental shelf of N Europe. ILLING, L V, and HOBSON, G D (editors). (London: Hayden.)

GLENNIE, K W, and BULLER, A T. 1983. The Permian Weissliegendes of N W Europe: the partial deformation of aeolian dune sands caused by the Zechstein transgression. Sedimentary Geology, Vol. 35, 43–81.

GLENNIE, K W. MUDD, G C, and NAGTEGAL, P J C. 1978. Depositional environment and diagenesis of Permian Rotliegendes sandstones in Leman Bank and Sole Pit areas of the UK southern North Sea. Journal of the Geological Society of London, Vol. 135, 25–34.

GOODALL, I G. 1987. An investigation of the facies and diagenesis of the Edlington Formation (Upper Permian) of Tees-side. Unpublished PhD thesis, University of Reading.

HARWOOD, G M. 1980. Calcitised anhydrite and associated sulphides in the English Zcchstein first cycle carbonate (EZ1 Ca). 61–72 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, II, and PERT, T M (editors). Contributions to Sedimentology, 9. (Stuttgart: Schweizerhart'sche Verlagsbuchhandlung.)

HARWOOD, G M. 1981. Controls of mineralisation in the Cadeby Formation (Lower Magnesian Limestone). Unpublished PhD thesis, Open University.

HARWOOD, G M. 1983. The application of cathodoluminescence in relative dating of barium mineralisation in the Lower Magnesian Limestone (Upper Permian), United Kingdom. Economic Geology, Vol. 78, 1022–1027.

HARWOOD, G M. 1986. The diagenetic history of Cadeby Formation carbonate (EZ1 Ca), Upper Permian, eastern England. 75–86 in The English Zechstein and related topics. HARWOOD, G M, and SMITH, D B (editors). Special Publication of the Geological Society of London, No. 22.

HARWOOD, G M, and COLEMAN, M L. 1983. Isotopic evidence for UK Upper Permian mineralisation by bacterial reduction of evaporites. Nature, Vol. 301, 597–599.

HARWOOD, G M, and SMITH, D B (editors). 1986. The English Zechstein and related topics. Special Publication of the Geological Society of London, No. 22.

HARWOOD, G M, and SMITH, F W. 1986. Mineralisation in Upper Permian carbonates at outcrop in eastern England. 103–111 in The English Zechstein and related topics. HARWOOD, G M, and SMITH D B (editors). Special Publication of the Geological Society of London, No. 22.

HASELHURST, S R. 1911. Some phenomena in the Permian of Northumberland and their relationship to sections in Durham and Cumberland. Proceedings of the University of Durham Philosophical Society, Vol. 4, 15–24.

HASZELDINE, R S. 1981. Westphalian B coalfield sedimentation in N E England and its depositional setting. Unpublished PhD thesis, University of Glasgow.

HASZELDINE, R S. 1983a. Descending tabular cross-bed sets and bounding surfaces from a fluvial channel in the Upper Carboniferous coalfield of north-east England. Special Publication, International Association of Sedimentologists, Vol. 6, 449–456.

HASZELDINE, R S. 1983b. Fluvial bars reconstructed from a deep, straight channel, Upper Carboniferous coalfield of north-east England. Journal of Sedimentary Petrology, Vol. 53, 1233–1247.

HASZELDINE, R S. 1984a. Muddy deltas in freshwater lakes, and tcctonism in the Upper Carboniferous coalfield of N E England. Sedimentology, Vol. 31, 811–822.

HASZELDINE, R S. 1984b. Carboniferous North Atlantic palaeogeography: stratigraphic evidence for rifting, not megashear or subduction. Geological Magazine, Vol. 5, 443–463.

HASZELDINE, R S, and ANDERTON, R. 1980. A braidplain facies model for the Westphalian B Coal Measures of north-east England. Nature, London, Vol. 284, No. 5751, 51–53.

HEMINGWAY, J E. 1968. Sedimentology of coal-bearing strata. 43–69 in Coal and coal-bearing strata. MURCHISON, D G, and WESTOLL, T S (editors). (Edinburgh: Oliver & Boyd.)

HERRMANN, A G, and RICHTER-BERNBURG, G. 1955. Fruhdiagenetische Storungen der Schichtung and Lagerung im Werra-Anhydrit (Zcchstein 1) am Sudwest. Harz. Zeitschrift der deutschen geologischen Gesellschaft., Vol. 105, 689–702.

HICKLING, G, and HOLMES, A. 1931. The brecciation of the Permian rocks. Proceedings of the Geologists' Association, Vol. 42, 252–255.

HIRST, D M, and DUNHAM, K C. 1963. Chemistry and petrography of the Marl Slate of S E Durham, England. Economic Geology, Vol. 58, 912–940.

HIRST, D M, and SMITH, F W. 1974. Controls of barite mineralisation in the Lower Magnesian Limestone of the Ferryhill area, County Durham. Transactions of the Institution of Mining and Metallurgy, B49-B56.

HITE, R J. 1966. Shelf carbonate sedimentation controlled by salinity in the Paradox Basin, southeast Utah. 48–66 in Third symposium on salt. RAU, J L, and DELLW1G, L F (editors). (Cleveland: Northern Ohio Geological Society.)

HODGE, M B. 1932. The Permian Yellow Sands of north-east England. Proceedings of the University of Durham Philosophical Society, Vol. 8, 410–458.

HOLLIDAY, D W, BURGESS, I C, and FROST, D V. 1975. A re-correlation of the Yoredale limestones (Upper Visean) of the Alston Block with those of the Northumberland Trough. Proceedings of the Yorkshire Geological Society, Vol. 40, 319–334.

HOLLINGWORTH, N T J. 1987. Palaeoecology of the Upper Permian Zechstein Cycle 1 reef in north-east England. Unpublished PhD thesis, University of Durham.

HOLLINGWORTH, N T J, and TUCKER, M E. 1987. The Upper Permian (Zechstein) Tunstall Reef of north-east England: palaeoecology and early diagencsis. 25–51 in Lecture Notes in Earth Sciences. No. 10, The Zechstein Facies in Europe. PERYI, T M (editor). (Berlin: Springer-Verlag.)

HOLLINGWORTH, N T J, and PETTIGREW, T H. 1988. Zechstein reef fossils and their palaeoecology. London, Palaeontological Association.

HOLMES, A. 1931. Concretionary and oolitic structures of the Permian rocks. Contributions to the geology of Northumberland and Durham. Proceedings of the Geologists' Association, Vol. 42, 255–259.

HOLMES, A, and HARWOOD, H F. 1928. The age and composition of the Whin Sill and the related dykes of the north of England. Mineralogical Magazine, Vol. 21, 493–542.

HOLMES, A. 1929. The tholeiite dykes of the north of England. Mineralogical Magazine, Vol. 22, 1–52.

HOLMES, A, and MOCKLER, G S. 1931. Late Carboniferous and Tertiary intrusions. Contributions to the geology of Northumberland and Durham. Proceedings of the Geologists' Association, Vol. 42, 261–271.

HOLTEDAHL, O. 1921. On the occurrence of structures like Walcott's Algonkian algae in the Permian of England. American Journal of Science, (5),Vol. 1, 195–206.

HOPKINS, W. 1927. Further modifications of the correlation of the coal-seams of the Northumberland and Durham coalfield. Transactions of the Institution of Mining Engineers, Vol. 74, 221–241.

HOPKINS, W. 1928. The distribution of mussel bands of the Northumberland and Durham Coalfield. Proceedings of the University of Durham Philosophical Society, Vol. 8, 1–14.

HOPKINS, W. 1929. The distribution and sequence of the non-marine lamellibranchs in the Coal Measures of Northumberland and Durham. Transactions of the Institution of Mining Engineers, Vol. 78, 126–144.

HOPKINS, W. 1930. A revision of the Upper Carboniferous non-marine lamellibranchs of Northumberland and Durham and a record of their sequence. Transactions of the Institution of Mining Engineers, Vol. 80, 101–110.

HOPKINS, W. 1933. Impoverished areas of the Maudlin and Busty seams in the Durham coalfield. Transactions of the Institution of Mining Engineers, Vol. 85, 212–222.

HOPKINS, W. 1934. Lingula horizons in the Coal Measures of Northumberland and Durham. Geological Magazine, Vol. 71, 183–189.

HOPKINS, W. 1960. Hopkins Shell Bed. 183 in Lexique Stratigraphique International, Vol. 1, Europe. TROTTER, F M (Recorder), Upper Carboniferous.

HOPKINS, W, and PHILIPSON, A J. 1947. A record of the fossil shells and plants from the 'J' sinking, Washington, Co. Durham. Transactions of the Institution of Mining Engineers, Vol. 106, 273–284.

HOWSE, R. 1848. A catalogue of the fossils of the Permian system of the counties of Northumberland and Durham. Transactions of the Tyneside Naturalists' Field Club, Vol. 1, 219–264.

HOWSE, R. 1858. Notes on the Permian system of the counties of Northumberland and Durham. Annals and Magazine of Natural History, (2), Vol. 19, 33–52, 304–312 and 463–473.

HOWSE, R. 1863. Notes on the fossil remains of some recent and extinct mammalia found in the counties of Northumberland and Durham. Transactions of the Tyneside Naturalists' Field Club, Vol. 5, 111–121.

HOPKINS, W. 1864. On the glaciation of the counties of Durham and Northumberland. Transactions of the North of England Institute of Mining and Mechanical Engineers, Vol. 13, 169–185.

HOWSE, R. 1880. Note on the discovery in 1836–1837 of a fossil fish (Acrolepis kirkbyi n.s.) in the Upper division of the Magnesian Limestone at Marsden. Natural History Transactions of Northumberland, Durham and Newcastle-upon-Tyne, Vol. 7, 171–173.

HOWSE, R. 1880. Old sea caves and a sea beach at Whitburn. Natural History Transactions of Northumberland, Durham and Newcastle-upon-Tyne, Vol. 7, 361–365.

HOWSE, R. 1890. Catalogue of the local fossils in the Museum of Natural History. Natural History Transactions of Northumberland, Durham and Newcastle-upon-Tyne, Vol. 11, 227–288.

HOWSE, R, and KIRKBY, J W. 1863. A synopsis of the geology of Durham and part of Northumberland. Transactions of the 'Tyneside Naturalists' Field Club, 33 pp.

HSU, K J, RYAN, W, and CITA, M B. 1973. Late Miocene desiccation of the Mediterranean evaporite. Nature, London, Vol. 242, 240–244.

HULL, J H. 1968. The Namurian stages of north-eastern England. Proceedings of the Yorkshire Geological Society, Vol. 36, 297–308.

HUTTON, W. 1831. Notes on the New Red Sandstone in the county of Durham, below the Magnesian Limestone. Transactions of the Natural History Society of Northumberland, Durham and Newcastle-upon-Tyne, Vol. 1, 60–74.

JOHNSON, G A L. 1967. Basement control of Carboniferous sedimentation in northern England. Proceedings of the Yorkshire Geological Society, Vol. 36, 175–194.

JOHNSON, G A L. 1970. Carboniferous. 23–42 in The geology of Durham County. HICKLING, G (editor). Transactions of the Natural History Society of Northumberland, Durham and Newcastle upon Tyne, Vol. 41.

JOHNSON, G A L. 1980. Carboniferous, Dinantian and Namurian rocks. 11–23 in The geology of north-east England. ROBSON, D A (editor). Special Publication of the Natural History Society of Northumbria.

JOHNSON, G A L. 1982. Geographical change in Britain during the Carboniferous Period. Proceedings of the Yorkshire Geological Society, Vol. 44, 181–203.

JOHNSON, G A L. 1984. Subsidence and sedimentation in the Northumberland Trough. Proceedings of the Yorkshire Geological Society, Vol. 45, 71–83.

JONES, J M, and COOPER, B S. 1970. Coal. 43–65 in The geology of Durham County. HICKLING, G (editor). Transactions of the Natural History Society of Northumberland, Durham and Newcastle upon Tyne, Vol. 41.

JONES, J M and MACRAW, D. 1980. Carboniferous Westphalian (Coal Measures) rocks. 23–36 in The geology of north-east England. ROBSON, D A (editor). Special Publication of the Natural History Society of Northumbria.

JONES, J M and MACRAW, D. ROBSON, D A, and SMITH, F W. 1980. Movements at the end of the Carboniferous period. 79–85 in The geology of north-east England. ROBSON, D A (editor). Special Publication of the Natural History Society of Northumbria.

JONES, K, and HJRST, D M. 1972. The distribution of barium, lead and zinc in the Lower and Middle Magnesian limestone of County Durham, Great Britain. Chemical Geology, Vol. 10, 223–236.

KALDI, J. 1980. The origin of nodular structures in the Lower Magnesian Limestone (Permian) of Yorkshire, England. 45–60 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H, and PERYT, T M (editors). Contributions to Sedimentology, 9. (Stuttgart: Schweizerbart’sche Verlagsbuchhandlung.)

KALDI, J, and GIDMAN, J. 1982. Early diagenetic cements: examples from the Permian Lower Magnesian Limestone of England and the Pleistocene carbonates of the Bahamas. Journal of Sedimentary Petrology, Vol. 52, 1073–1085.

KELLETT, J G. 1927. A petrological investigation of the Coal Measures sediments of Durham. Proceedings of the University of Durham Philosophical Society, Vol. 7, 208–232.

KENDALL, P F. 1902. On a system of glacier lakes in the Cleveland Hills. Quarterly Journal of the Geological Society of London, Vol. 58, 471–571.

KERKMANN, K. 1969. Rife and algenbanke in Zechstein von Thuringia. Ereiberger Earschungshefte, Vol. C252, 9–52.

KIDSTON, R. 1922. List of fossil plants from the Upper Carboniferous rocks of the Northumberland and Durham Coalfield and their bearing on the age of the coalfield. Summary of progress of the Geological Survey of Great Britain for 1921, 129–145.

KING, C A M. 1954. The relationship between wave incidence, wind direction and beach changes at Marsden Bay, County Durham. Transactions and Papers of the Institute of British Geographers, Publication 19, 12–23.

KING, C A M. 1967. An introduction to trend surface analysis. Bulletin of quantitative data fur geographers, University of Nottingham, Geography Department, Nottingham, No. 12.

KING, W. 1848. A catalogue of the organic remains of the Permian rocks of Northumberland and Durham. (Newcastle upon Tyne: privately printed.)

KING, W. 1850. A monograph of the Permian fossils of England. (London: The Palaeontographical Society.)

KIRKBY, J W. 1858. On Permian entomostraca from the Shell Limestone of Durham. Transactions of the Tyneside Naturalists' Field Club, No. 4, 122–171.

KIRKBY, J W. 1860. On the occurrence of Lingula credneri Geinitz in the Coal Measures of Durham, and on the claim of the Permian rocks to be entitled a system. Quarterly journal of the Geological Society of London, Vol. 16, 412–421.

KIRKBY, J W. 1860. The occurrence of "sand-pipes" in the Magnesian limestone of Durham. The Geologist, Vol. 3, 293–298, 329–336.

KIRKBY, J W. 1860. On the Permian Chitonidae. Transactions of the Tyneside Naturalists' Field Club, No. 4, 238–263.

KIRKBY, J W. 1863. Fossil fish in Magnesian Limestone at Fulwell Hill. Transactions of the Tyneside Naturalists' Field Club, Vol. 5, 248.

KIRKBY, J W. 1864a. On the occurrence of fossils in the highest beds of the Durham Coal Measures. Transactions of the Tyneside Naturalists' Field Club, Vol. 6, 220–225.

KIRKBY, J W. 1864b. On some fossils from the Lower Magnesian Limestone of Sunderland. Transactions of the Tyneside Naturalists' Field Club, Vol. 6, 212–220.

KIRKBY, J W. 1864c. On some remains of fish and plants from the 'Upper Limestone' of the Permian series of Durham. Quarterly Journal of the Geological Society of London, Vol. 20, 345–358. Republished 1867 in Natural History Transactions of Northumberland and Durham, Vol. 1, 64–83.

KIRKBY, J W. 1866. On the fossils of the Marl Slate and Lower Magnesian limestone. Natural History Transactions of Northumberland and Durham, Vol. 1, 184–200.

KITSON, D C. 1982. Stratigraphical relationships, morphology and diagenesis of the Hesleden Dene Algal Biostrome. Unpublished MSc thesis, University of Reading.

KRINSLEY, D H, and SMITH, D B. 1981. A selective SEM study of grains from the Permian Yellow Sands of north-east England. Proceedings of the Geologists' Association, Vol. 92, 189–196.

LAND, D H. 1974. Geology of the Tynemouth District. Memoir of the Geological Survey of Great Britain, Sheet 15.

LEBOUR, G A. 1884. On the breccia-gashes of the Durham Coast and some recent earth-shakes at Sunderland. Transactions of the Institution of Mining and Mechanical Engineers, Vol. 33, 165–177.

LEBOUR, G A. 1902. The Marl Slate and Yellow Sands of Northumberland and Durham. Transactions of the Institution of Mining Engineers, Vol. 24, 370–391.

LEE, M R. 1990. The sedimentology and diagenesis of the Raisby Formation (EZI carbonate), northern England. Unpublished PhD thesis, University of Newcastle upon Tyne.

LEE, M R, and HARWOOD, G M. 1989. Dolomite calcitisation and cement zonation related to uplift of the Raisby Formation (Zechstein), northeast England. Sedimentology, Vol. 65, 285–305.

LEEDER, M R. 1988. Recent developments in Carboniferous geology: a critical review with implications for the British Isles and N W Europe. Proceedings of the Geologists' Association, Vol. 99, 73–100.

LEWIS, K B. 1971. Slumping on a continental slope inclined at 1–4°. Sedimentology, Vol. 16, 97–110.

LOGAN, A. 1962. A revision of the palaeontology of the Permian Limestones of County Durham. Unpublished PhD thesis, King's College, University of Durham.

LOTZE, F. 1934. Der gang der Salzahscheidung im innersten Teil des deutschen Zechsteinbeckens Gebiet von Lehrte u. Hannover. Kali, Vol. 28, 237.

LOVE, L G. 1962. Biogenic primary sulphide of the Permian Kupferschiefer and Marl Slate. Economic Geology, Vol. 57, 350–366.

LOVELL, J P B. 1977. The British Isles through geological time; a northward drift. (London: Allen and Unwin.)

LUNN, A G. 1980. Quaternary. 48–60 in The geology of north-east England. ROBSON, D (editor). Special Publication of the Natural History Society of Northumbria.

MAGARITZ, M, and SCHULZE, K H. 1980. Carbon isotope anomaly of the Permian period. 269–277 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H, and PERYT, T M (editors). Contributions to Sedimentology, 9. (Stuttgart: Schweizerbart'sche Verlagsbuchhandlung.)

MAGARITZ, M, and TURNER, P. 1982. Carbon cycle changes of the Zechstein Sea: isotopic transition zone in the Marl Slate. Nature, London, Vol. 297, 389–390.

MAGARITZ, M, and TURNER, P, and KADING, K-C. 1981. Carbon isotope change at the base of the Upper Permian Zechstein sequence. Geological Journal, Vol. 16, 243–254.

MAGDEFRAU, K. 1933. Zur entstehung der mitteldeutschen Zechsteinriffe. Zentralblatt fur Mineralogie, Geologic and Palliontologie, Vol. 11, 621–624.

MAGRAW, D. 1963. The importance of geology in the planning of undersea workings at Westoe Colliery, South Shields. Transactions of the Institution of Mining Engineers, Vol. 38, 101–117.

MAGRAW, D. 1975. Permian of the offshore and coastal region of Durham and Northumberland. Journal of the Geological Society of London, Vol. 131, 397–414.

MAGRAW, D. 1978. New boreholes into Permian beds off Northumberland and Durham. Proceedings of the Yorkshire Geological Society, Vol. 42, 157–183.

MAGRAW, D. CLARKE, A M, and SMITH, D B. 1963. The stratigraphy and structure of part of the south-east Durham coalfield. Proceedings of the Yorkshire Geological Society, Vol. 34, 153–208.

MARLED, J. 1892. On the Cleveland and south Durham salt industry. Transactions of the North of England Institute of Mining and Mechanical Engineers, Vol. 39, 91–125.

MILLS, D A C, and HULL, J H. 1976. Geology of the country around Barnard Castle. Memoir of the Geological Survey of Great Britain, Sheet 32.

NAPIER, E. 1948. The lower anhydrite seams of the Tees area. Proceedings of the Yorkshire Geological, Society, Vol. 27, 210–216.

NATIONAL COAL BOARD. 1959–1961. Durham Coalfield seam maps; Busty, Harvey, Hutton. (London.)

OELSNER, O. 1959. Bemerkungen zur Herkunft der Metalle im Kupferschiefer. Freiberger Forschungshefie, Vol. 58, 106–113.

OPDYKE, N. 1961. The palaeoclimatological significance of desert sandstones. 45–60 in Descriptive palaeoclimatology. NAIRN, A E M (editor). (London: Interscience Publishers.)

PATTISON, J. 1970. A review of the marine fossils from the Upper Permian rocks of Northern Ireland and north-west England. Bulletin of the Geological Survey of Great Britain, No. 32, 123–165.

PATTISON, J. 1978. Permian. 187–193 in The ecology of fossils. MCKERROW, W S (editor). (London: Duckworth.)

PATTISON, J. SMITH, D B, and WARRINGTON, G. 1973. A review of Late Permian and Early Triassic biostratigraphy in the British Isles. 220–260 in The Permian and Triassic systems and their mutual boundary. LOGAN, A, and HILLS, L V (editors). Memoir of the Canadian Society of Petroleum Geologists, No. 2.

PERYT, T M. 1986. Zechstein Stromaria (= Archaeolithoporella) cement reefs in Thuringia. Notes Jahrbuch fur Geologie und Paldontologie, Monatshefte, Vol. 5, 307–316.

PERYT, T M, and PIATKOWSKI, T S. 1977a. Stromatolites from the Zechstein Limestone (Upper Permian) of Poland. 124–135 in Fossil algae, recent research and developments. FLUGEL, E (editor). (Berlin: Springer-Verlag.)

PERYT, T M. 1977b. Algal-vadose pisoliths in the Zechstein Limestone (Upper Permian) of northern Poland. Sedimentary Geology, Vol. 19, 275–286.

PETTIGREW, T H. 1979. A gliding reptile from the Upper Permian of north-east England. Nature, London, Vol. 281, 297–298.

PETTIGREW, T H. 1980. Geology. 6–26 in The Magnesian Limestone of Durham County. DUNN, T C (editor). (Houghton-le-Spring: Durham County Conservation Trust.)

PETTIGREW, T H. 1985. Palaeoekologia Angielskiego Cechsztynu. Przeglad Geologiczny, Vol. 4, 202–204.

PISERA, A, and ZAWIDZKA, K. 1981. Archaeolithoporella from Upper Permian reef limestones of the Northern Caucasus. Bulletin de l'Academie Polonaise des Sciences, Serie des Sciences de la Terre, Vol. 29, 233–238.

POLLARD, J E. 1966. A non-marine ostracod fauna from the Coal Measures of Durham and Northumberland. Palaeontology, Vol. 9, 667–697.

POLLARD, J E. 1969. Three ostracod-mussel bands in the Coal Measures (Westphalian) of Northumberland and Durham. Proceedings of the Yorkshire Geological Society, Vol. 37, 239–276.

POMPECKJ, I W., 1914. Das Meer des Kupferschiefers. Branca-Festschidt, 444–494.

POMPECKJ, I W., 1920. Kupfcrschiefer und Kupferschiefermeer. Zeitschrift der deutschen geologischen Gesellschaft, Vol. 72, 329–339.

POTONIE, R. 1938. Die Nomenklatur der UnterwasserAblagerungen. Preussische Geologische Landesanstalt Jahrbiicher, Vol. 58, 426–438.

POTTS, T. 1892. Sunderland: a history of the town, port, trade and commerce. 326 pp (Sunderland: Williams.)

PRICE, R H. 1955. 42 in Summary of progress of the Geological Survey of Great Britain for 1954.

PRYOR, W A. 1971. Petrology of the Permian Yellow Sands of northeastern England and their North Sea Basin equivalents. Sedimentary Geology, Vol. 6, 221–254.

RAISTRICK, A. 1931. Glaciation. 281–291 in Contributions to the geology of Northumberland and Durham. Proceedings of the Geologists' Association, Vol. 42.

RAMSBOTTOM, W H C, CALVER, M A, EAGAR, R M C, HODSON, F, HOLLIDAY, D W, STUBBLEFIELD, C J, and WILSON, R B. 1978. A correlation of Silesian rocks in the British Isles. Special Report of the Geological .Society of London, No. 10.

RANDALL, B A O. 1980. The Great Whin Sill and its associated dyke suite (Permo-Carboniferous). 67–75 in The geology of north-east England. ROBSON, D A (editor). Special Publication of the Natural History Society of Northumbria.

RAYMOND, L. R. 1953. Some geological results from the exploration for potash in north-east Yorkshire. Quarterly Journal of the Geological Society of London, Vol. 108, 283–310.

RAYMOND, L. R. 1962. The petrology of the Lower Magnesian Limestone of north-east Yorkshire and south-east Durham. Quarterly Journal of the Geological Society of London, Vol. 118, 39–64.

RICHARDSON, G. 1983. Geological notes and local details for 1:10 000 sheets NZ26NW, NE, SW and SE (Newcastle upon Tyne and Gateshead). (Keyworth, Nottingham: British Geological Survey.)

RICHARDSON, G, and FRANCIS, E H. 1971. Fragmental clayrock (FCR) in coal-bearing sequences in Scotland and north-east England. Proceedings of the Yorkshire Geological Society, Vol. 38, 229–260.

RICKARD, D T. 1971. Discussion on comments by D J Shearman in introducing paper by BEALES, F W, and ONASICK, E P. Transactions of the Institution of Mining and Metallurgy, Vol. 80, B52-B53.

RICHTER-BERNBURG, G. 1955. Stratigraphische Gliederung des deutschen Zechsteins. Zeitschrill der deutschen geologischen Gesellschaft, Vol. 105, 843–854.

RICHTER-BERNBURG, G. 1957. Isochrone Warven im Anhydrit des Zechstien 2. Geologisches Jahrbuch, Vol. 74, 601–610.

RICHTER-BERNBURG, G. 1958. Die Korrelierung isochroner Warven im Anhydrit des Zechstein 2 (Zweiter Beitrag). Geologisches Jahrbuch, Vol. 75, 629–646.

RICHTER-BERNBURG, G. 1986. Zechstein 1 and 2 anhydrites: facts and problems of sedimentation. 157–163 in The English Zechstein and related topics. HARWOOD, G M, and SMITH, D B (editors). Special Publication of the Geological Society of London, No. 22.

RIDD, M F, WALKER, D B, and JONES, J M. 1970. A deep borehole at Harton on the margin of the Northumbrian Trough. Proceedings of the Yorkshire Geological Society, Vol. 38, 75–103.

RIPPON, J H. 1985. Contoured patterns of the throw and hade of normal faults in the Coal Measures (Westphalian) of northeast Derbyshire. Proceedings of the Yorkshire Geological Society, Vol. 45, 147–161.

ROBSON, D A (editor). 1980. The geology of north-east England. Special Publication of the Natural History Society of Northumbria.

ROBSON, S. 1915. A conglomerate under the boulder clay of the Sunderland District. Proceedings of the University of Durham Philosophical Society, Vol. 5, 155–156.

RUSSELL, M J. 1976. A possible Lower Permian age for the onset of ocean floor spreading in the northern North Atlantic. Scottish Journal of Geology, Vol. 12, 315–323.

RUSSELL, M  J, and SMYTHE, D K. 1978. Evidence for an early Permian oceanic rift in the northern north Atlantic. 173–179 in Petrology and geochemistry of continental rifts. NEUMANN, E R, and RAMBERG, 1 B (editors). (Dordrecht: Reidel Publishing Co.)

SCHMALZ, R F. 1969. Deep-water evaporite deposition: a genetic model. Bulletin of the American Association of Petroleum Geologists, Vol. 53, 798–823.

SCHREIBER, B C, FRIEDMAN, G M, and DECIMA, A. 1976. Depositional environments of Upper Miocene (Messinian) evaporite deposits of the Sicilian Basin. Sedimentology, Vol. 23, 729–760.

SCHWEITZLER, H-J. 1986. The land flora of the English and German Zechstein sequences. 31–54 in The English Zechstein and related topics. HARWOOD, G M, and SMITH, D B (editors). Special Publication of the Geological Society of London, No. 22.

SCOTT, A C. 1978. Sedimentological and ecological control of Westphalian B plant assemblages from West Yorkshire. Proceedings of the Yorkshire Geological Society, Vol. 41, 461–508.

SCOTT, A C. 1979. The ecology of Coal Measures floras from northern Britain. Proceedings of the Geologists' Association, Vol. 90, 97–115.

SEDGWICK, A. 1829. On the geological relations and internal structure of the Magnesian Limestone, and the lower portions of the New Red Sandstone Series in their range through Nottinghamshire, Derbyshire, Yorkshire and Durham, to the southern extremity of Northumberland. Transactions of the Geological Society of London, (2), Vol. 3, 37–124.

SHEARMAN, D J. 1971. Introduction to paper by BEAUS, F W, and ONASICK, E P. Transactions of the Institution of Mining and Metallurgy, Vol. 80, B50-B52.

SHEARMAN, D J. MOSSOP, G, DUNSMORE, H, and MARTIN, M. 1972. Origin of gypsum veins by hydraulic fracture. Transactions of the Institution of Mining and Metallurgy, Vol. 81, B149-B155.

SHEARMAN, D J, and & SMITH, A J. 1985. Ikaite, the parent mineral of jarrowite-type pscudomorphs. Proceedings of the Geologists Association, Vol. 96, 305–314.

SMITH, A H V. 1962. The palaeoecology of Carboniferous peats based on the miospores and petrography of bituminous coals. Proceedings of the Yorkshire Geological Society, Vol. 33, 423–474.

SMITH, A H V. 1963. Palaeoecology of Carboniferous peats. 57–85 in Palaeoclimatology. NAIRN, A E M (editor). (London.)

SMITH, A H V, and BUTTERWORTH, M. 1967. Miospores in the coal seams of the Carboniferous of Great Britain. Special Paper in Palaeontology, Vol. 1, 1–324

SMITH, D B. 1958. Some observations on the Magnesian limestone reefs of north-eastern Durham. Bulletin of the Geological Survey of Great Britain, No. 15, 71–84.

SMITH, D B. 1960. 36 in Summary of progress of the Geological Survey of Great Britain for 1959.

SMITH, D B. 1968. The Hampole Beds—a significant marker in the Lower Magnesian Limestone of Yorkshire, Derbyshire and Nottinghamshire. Proceedings of the Yorkshire Geological Society, Vol. 36, 463–477.

SMITH, D B. 1970a. The palaeogeography of the British Zechstein. 20–23 in Third Symposium on salt. RAI:, J L, and DELLIVIG, L (editors). (Cleveland: North Ohio Geological Society.)

SMITH, D B. 1970b. The Permian and Trias. 66–91 in The geology of Durham County. HICKLING, G (editor). Transactions of the Natural History Society of Northumberland, Durham and Newcastle upon Tyne, Vol. 41.

SMITH, D B. 1970c. Submarine slumping and sliding in the Lower Magnesian Limestone of Northumberland and Durham. Proceedings of the Yorkshire Geological Society, Vol. 38, 1–36.

SMITH, D B. 1971. The stratigraphy of the Upper Magnesian Limestone in Durham-a revision based on the Institute's Seaham Borehole. Report of the Institute of Geological Sciences, No. 71/3.

SMITH, D B. 1972. Foundered strata, collapse-breccias and subsidence features of the English Zechstein. 255–269 in Geology of saline deposits. RICHTER-BERNBURG, G (editor). (Paris: UNESCO.)

SMITH, D B. 1974a. The stratigraphy and sedimentology of Permian rocks in North Yorkshire. Journal of Earth Sciences (Leeds), Vol. 8, 365–386.

SMITH, D B. 1974b. Permian. 115–144 in The geology and mineral resources of Yorkshire. RAYNER, D H, and HEMINGWAY, J E (editors). (Leeds: Yorkshire Geological Society.)

SMITH, D B. 1978. 1:50 000 geological sheet 21 (Sunderland), including explanation. Geological Survey of Great Britain.

SMITH, D B. 1979. Rapid marine transgressions of the Upper Permian Zechstein Sea. Journal of the Geological Society, London, Vol. 136, 155–156.

SMITH, D B. 1980a. Permian and Triassic rocks. 36–48 in The geology of north-east England. ROBSON, D A (editor). Special Publication of the Natural History Society of Northumbria.

SMITH, D B. 1980b. The evolution of the English Zechstein basin. 7–34 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H, and PERYT, T M (editors). Contributions to Sedimentology, 9. (Stuttgart: Schweizerbart'sche Verlagsbuchhandlung.)

SMITH, D B. 1980c. The shelf-edge reef of the Middle Magnesian Limestone (English Zechstein Cycle 1) of north-eastern England-a summary. 3–5 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H, and PERYF, T M (editors). Contributions to Sedimentology. (Stuttgart: Schweizerbart'sche Verlagsbuchhandlung.)

SMITH, D B. 1981. The Quaternary geology of the Sunderland district. 146–167 in The Quaternary in Britain. NEALE, J W, and FLENLEY, J (editors). (Oxford: Pergamon Press Ltd.)

SMITH, D B. 1981a. The Magnesian Limestone (Upper Permian) reef complex of north-eastern England. 161–186 in European fossil reef models. TOOMEY, D F (editor). Special Publication of the Society of Economic Paleontologists and Mineralogists, No. 30.

SMITH, D B. 1981b. Bryozoan-algal patch reefs in the Upper Permian Lower Magnesian Limestone of Yorkshire, north-east England. 187–202 in European fossil reef models. TOOMEY, D F (editor). Special Publication of the Society of Economic Paleontologists and Mineralogists, No. 30.

SMITH, D B. 1984. Red Basal Permian Sands; a new discovery in NCB boreholes off Sunderland, north-east England. Proceedings of the Yorkshire Geological Society, Vol. 44, 497–500.

SMITH, D B. 1985a. Gravitational movements in Zechstein carbonate rocks in north-east England. Fl-F11 in The role of evaporites in hydrocarbon exploration. TAYLOR, J C M (editor). (London: JAPEC course notes No. 39.)

SMITH, D B. 1985b. Zechstein reefs and associated facies. El-E23 in The role of evaporites in hydrocarbon exploration. TAYLOR, J C M (editor). (London: JAPEC course notes No. 39).

SMITH, D B. 1986. The Trow Point Bed—a deposit of Upper Permian marine oncoids, peloids and columnar stromatolites in the Zechstein of north-east England. 113–125 in The English Zechstein and related topics. HARWOOD, G M, and SMITH, D B (editors). Special Publication of the Geological Society of London, No. 22.

SMITH, D B. 1989. The Late Permian Palaeogeography of north-east England. Proceedings of the Yorkshire Geological Society, Vol. 47, 285–312.

SMITH, D B. 1994. Marine Permian of England (Geological Conservation Review, Vol. 8). Joint Nature Conservation Committee. (London: Chapman and Hall.)

SMITH, D B. BRUNSTROM, R G W, MANNING, P I, SIMPSON, S, and SHOTTON, F W. 1974. A correlation of the Permian rocks in the British Isles. Special Report of the Geological Society of London, No. 5.

SMITH, D B, and FRANCIS, E A. 1967. Geology of the country between Durham and West Hartlepool. Memoir of the Geological Survey of Great Britain, Sheet 27.

SMITH, D B. HARWOOD, G M, PATTISON, J, and PETTIGREW, T. 1986. A revised nomenclature for Upper Permian strata in eastern England. 9–17 in The English Zechstein and related topics. HARWOOD, G M, and SMITH, D B (editors). Special Publication of the Geological Society of London, No. 22.

SMITH, D B, and PRAY, L C. 1977. Laminar and nodose encrustations in the Middle Magnesian Limestone (Zechstein) reef of northeast England. Reef Newsletter, No. 4, 27.

SMYTHE, J A. 1908. The glacial phenomena of the country between the Tyne and the Wansbeck. Transactions of the Natural History Society of Northumberland, Durham and Newcastle-upon-Tyne, Vol. 3, 79–109.

SMYTHE, J A. 1912. Glacial geology of Northumberland. Transactions of the Natural History Society of Northumberland, Durham and Newcastle-upon-Tyne, Vol. 4, 86–116.

SMYTHE, J A. 1922. Minerals of the North Country: barium minerals. Vasculum, Vol. 8, 113–116.

SORBY, H C. 1856. On the Magnesian Limestone having been formed by the alteration of an ordinary calcareous deposit. Report of the British Association, (2), 77.

STEELE, R P. 1981. Aeolian sands and sandstone. Unpublished PhD thesis, University of Durham.

STEELE, R P. 1983. Longitudinal draa in the Permian Yellow Sands of north-east England. 543–552 in Eolian sediments and processes. BROOKFIELD, M E, and AHLBRANDT, T S (editors). Developments in Sedimentology, Vol. 38.

STEWART, F H. 1963. The Permian Lower Evaporites of Fordon in Yorkshire. Proceedings of the Yorkshire Geological Society, Vol. 34, 1–44.

STOBBS, J T. 1905. The occurrence of Anthracomya phillipsi in the Durham Coalfield. Geological Magazine, Vol. 2, 506–507.

STONELEY, HILDA M M. 1958. The Upper Permian flora of England. Bulletin of the British Museum (Natural History). Geology, Vol. 3, 295–337.

STRAKHOV, N M. 1962. Principles of lithogenesis. Translated by P Fitzsimonns, 1970. xii + 577 pp. (Edinburgh: Oliver and Boyd.)

SWEENEY, M, TURNER, P, and VAUGHAN, D J. 1987. The Marl Slate: a model for the precipitation of calcite, dolomite and sulphides in a newly-formed anoxic sea. Sedimentology, Vol. 34, 31–48.

TARR, W A. 1933. Origin of the concretionary structures of the Magnesian Limestone at Sunderland, England. Journal of Geology, Vol. 41, 268–287.

TAYLOR, B J et al., 1971. (4th edition) Northern England . British regional geology: (London: HMSO.)

TAYLOR, J C M. 1980. Origin of the Werraanhydrit of the UK Southern North Sea—a reappraisal. 91–113 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H, and PERYT, T M (editors). Contributions to Sedimentology, 9. (Stuttgart: Schweizerbart'sche Verlagsbuchhandlung.)

TAYLOR, J C M. 1984. Late Permian-Zechstein. 61–83 in Introduction to the petroleum geology of the North Sea. GLENNIE, K (editor). (Oxford: Blackwell.)

TAYLOR, J C M, and COLTER, V S. 1975. Zechstein of the British Sector of the southern North Sea Basin. 249–263 in Petroleum of the Continental Shelf of North-West Europe, Vol. 1. WOODLAND, A W (editor). (Barking: Applied Science Publishers.)

TAYLOR, J C M, and FONG, G. 1969. Correlation of Upper Permian strata in east Yorkshire and Durham. Nature, London, Vol. 224, 173–175.

TEALL, J J H. 1884. Petrological notes on some north of England dykes. Quarterly Journal of the Geological Society of London, Vol. 40, 209–247.

TONKS, L H. 1939. Marine and freshwater fossils from the Middle Coal Measures of Ryhope, Co. Durham. Bulletin of the Geological Survey of Great Britain, No. 1, 1–7.

TRECHMANN, C T. 1913. On a mass of anhydrite in the Magnesian Limestone at Hartlepool and on the Permian of south-eastern Durham. Quarterly journal of the Geological Society of London, Vol. 69, 184–218.

TRECHMANN, C T. 1914. On the lithology and composition of Durham Magnesian Limestones. Quarterly journal of the Geological Society of London, Vol. 70, 232–265.

TRECHMANN, C T. 1921. Some remarkably preserved brachiopods from the I.ower Magnesian limestone of Durham. Geological Magazine, Vol. 58, 538–543.

TRECHMANN, C T. 1925. The Permian Formation in Durham. Proceedings of the Geologists' Association, Vol. 36, 135–145.

TRECHMANN, C T. 1931. The Permian. 246–252 in Contributions to the geology of Northumberland and Durham. Proceedings of the Geologists' Association, Vol. 42.

TRECHMANN, C T. 1945. On some new Permian fossils from the Magnesian Limestone near Sunderland. Quarterly Journal of the Geological Society of London, Vol. 100, 333–354.

TRECHMANN, C T. 1947. The submerged forest beds of the Durham coast. Proceedings of the Yorkshire Geological Society, Vol. 27, 23–32.

TRECHMANN, C T. 1952. On the Pleistocene of East Durham. Proceedings of the Yorkshire Geological Society, Vol. 28, 164–179.

TRECHMANN, C T. 1954. Thrusting and other movements in the Durham Permian. Geological Magazine, Vol. 91, 193–208.

TRECHMANN, C T, and WOOLACOTT, D. 1919. On the highest coal-measures or zone of Anthracomya phillipsii in the Durham Coalfield. Geological Magazine, Vol. 6, 203–211.

TRUEMAN, A, and WEIR, J. 1946. A monograph of British Carboniferous non-marine lamellibranchiata. (London: The Palaeontographical Society.)

TUCKER, M E, and HOLLINGWORTH, N T J. 1986. The Upper Permian reef complex (EZ1) of north east. England: diagenesis in a marine to evaporitic setting. 270–290 in Reef diagenesis. SCHROEDER, H, and PURSER, B H (editors). (Berlin: Springer-Verlag.)

TURNER, P, and MAGARITZ, M. 1986. Chemical and isotopic studies of a core of Marl Slate from N E England: influence of freshwater influx into the Zechstein Sea. 19–29 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H, and PERYT, T M (editors). Contributions to Sedimentology, 9. (Stuttgart: Schweizerbart'sche Verlagsbuchhandlung.)

TURNER, P, and VAUGHAN, D J. 1977. Evidence of rapid changes in the Permian geomagnetic field during the Zechstein marine transgression. Nature, London, Vol. 270, 593–594.

TURNER, P, and WHITEHOUSE, K I. 1978. Dolomitisation and mineralization of the Marl Slate (N E England). Mineralium Deposita, Vol. 13, 245–258.

VAUGHAN, D J, and TURNER, P. 1980. Diagenesis, magnetization and mineralization of the Marl Slate. 73–90 in The Zechstein Basin with emphasis on carbonate sequences. FUCHTBAUER, H, and PERYT, T M (editors). Contributions to Sedimentology, 9. (Stuttgart: Schweizerbart'sche Vcrlagsbuchhandlung.)

WALKER, T R. 1967. Formation of red beds in modern and ancient deserts. Bulletin of the Geological Society of America, Vol. 78, 353–368.

WALKER, T R. 1976. Diagenetic origin of continental red beds. 240–282 in The continental Permian in west, central and south Europe. FALKE, H (editor). NATO AV, Series C, Vol. 22.

WALKER, T R. WAUGH, B, and CROWE, A J. 1978. Diagenesis of first-cycle desert alluvium, of Cenozoic age, southwestern United States and northwestern Mexico. Bulletin of the Geological Society of America, Vol. 89, 19–32.

WAUGH, B. 1978. Authigenic K-feldspar in British PermoTriassic sandstones. Journal of the Geological Society of London, Vol. 135, 51–56.

WESTOLL, T S. 1941a. The age of certain Permian fish-bearing strata. Geological Magazine, Vol. 78, 37–44.

WESTOLL, T S. 1941b. The Permian fishes Darypterus and Lekanichthys. Proceedings of the Zoological Society of London, Vol. 111, 39–58.

WESTOLL, T S. 1943. Mineralisation of Permian Rocks of South Durham. Geological Magazine, Vol. 80, 119–120.

WILLS, L J. 1951. A palaeogeographical atlas of the British Isles and adjacent parts of Europe. (London: Blackie & Son.)

WILSON, E. 1881. The Permian Formation in the north-east of England. Midland Naturalist, Vol. 4, 97–101, 121–124, 187–191, 201–208.

WINCH, N J. 1817. Observations on the geology of Northumberland and Durham. Transactions of the Geological Society of London, Vol. 4, 1–101.

WOODS, P J E. 1973. Potash exploration in Yorkshire: Boulby mine pilot borehole. Transactions of the Institution of Mining and NIetallurgy, Vol. 82, B99-B106.

WOODWARD, H. 1918. Fossil arthropods from the Carboniferous rocks of Cape Breton, Nova Scotia; and from the Upper Coal-measures, Sunderland, England. Geological Magazine, Vol. 5, 462–471.

WOOLACOTT, D. 1897. Geology of north-east Durham. (Sunderland: Hills and Co.)

WOOLACOTT, D. 1900a. On the boulder clay, raised beaches and associated phenomena in East Durham. Proceedings of the University of Durham Philosophical Society, Vol. 1, 247–258.

WOOLACOTT, D. 1900b. On a portion of a raised beach on the Fulwell Hills, near Sunderland. Natural History Transactions of Northumberland, Durham and Newcastle-upon-Tyne, Vol. 8, 165–171.

WOOLACOTT, D. 1903. An explanation of the Claxheugh section, Co. Durham. Natural History Transactions of Northumberland, Durham and Newcastle-upon-Tyne, Vol. 14, 211–221.

WOOLACOTT, D. 1905. The landslip at Claxheugh, Co. Durham. Transactions of the Natural History Society of Northumberland, Durham and Newcastle-upon-Tyne, Vol. 1, 434–436.

WOOLACOTT, D. 1905. The superficial deposits and pre-glacial valleys of the Northumberland and Durham coalfields. Quarterly Journal of the Geological Society of London, Vol. 61, 64–95.

WOOLACOTT, D. 1906. On an exposure of the 100 feet raised beach at Cleadon. Proceedings of the University ofDurham Philosophical Society, Vol. 2, 243–246.

WOOLACOTT, D. 1907. The origin and influence of the chief physical features of Northumberland and Durham. Geographical Journal, Vol. 30, 36–54.

WOOLACOTT, D. 1909. A case of thrust and crush-brecciation in the Magnesian Limestone of County Durham. Proceedings of the University of Durham Philosophical Society, Mem. No. 1.

WOOLACOTT, D. 1912. The stratigraphy and tectonics of the Permian of Durham (northern area). Proceedings of the University of Durham Philosophical Society, Vol. 4, 241–331.

WOOLACOTT, D. 1913a. The geology of north-east Durham and south-east Northumberland. Proceedings of the Geologists' Association, Vol. 24, 87–107.

WOOLACOTT, D. 1913b. Report of an excursion to Sunderland and Tynemouth. Proceedings of the Geologists' Association, Vol. 24, 108–114.

WOOLACOTT, D. 1918. On sections in the Lower Permian rocks at Claxheugh and Down Hill, Co. Durham. Transactions of the Natural History Society of Northumberland, Durham and Newcastle-upon-Tyne, Vol. 5, 155–162.

WOOLACOTT, D. 1919a. On borings at Cotefield Close and Sheraton, Co. Durham (Permian and Coal Measures). Geological Magazine, Vol. 6, 163–170.

WOOLACOTT, D. 1919b. The Magnesian Limestone of Durham. Geological Magazine, Vol. 6, 452–465, 485–498.

WOOLACOTT, D. 1921. The interglacial problem and the glacial and post glacial sequence in Northumberland and Durham. Geological Magazine, Vol. 58, 26–32, 60–69.

YARDLEY, M J. 1984. Cross-bedding in the Permian Yellow Sands of County Durham. Proceedings of the Yorkshire Geological Society, Vol. 45, 11–18.

Fossil index

• Acanthocladia
• Acentrophorus
• A. varians
• Acrolepis
• A. sedgwickii
• Agathammina
• alder
• Alnus
• Ammmodiscus
• Ammonema
• Anthracoceras cf. hindi
• 'Anthracomya cf. phillipsii'
• Anthraconaia
• A. cf. pulchella
• A. cf. salteri
• A. modiolaris
• A. rubida
• A. sp. cf cymbula
• A. sp. nov.
• Anthraconauta phillipsii
• Anthracopupa cf. britannica
• Anthracosia
• A. aff. caledonica
• A. aff. disjuncta
• A. aff. ovum
• A. aff. phyrygiana
• A. aff. planitumida
• A. aff. suhrecta
• A. beaniana
• A. caledonica
• A. cf. acutella
• A. cf. aquilina
• A. cf. barkeri
• A. cf. beaniana
• A. cf. caledonica
• A. cf. lateralis
• A. cf. nitida
• A. cf. phrygiana
• A. cf. regularis
• A. cf. simulans
• A. concinna
• A. ovum
• A. phrygiana
• A vsimulans
• A. sp. cf. aquilina
• A. sp. cf. fulva
• A. sp. intermed. disjuncts/lateralis
• A. sp. nov. cf. aquilinoides
• Anthracosphaerium aff. propinquum
• A. cf. exiguum
• A. cf. propinquum
• A. cf. turgidum
• A. radiatum
• Archaeolithoporella
• Astartella
• A. vallisneriana
• A. Aviculopecten cf. gentilis
• bairdiids
• Bakevellia
• B. binneyi
• B. cf. ceratophaga
• beech
• Belinurus trechmanni
• birch
• bivalves marine Carboniferous
• marine Permian
• non-marine
• Bos primagenius
• brachiopods
• chonctoid
• bryozoans
• ramose and pinnate
• Calamites
• Calcinema
• permiana
• Callipteris
• Carbonicola
• C. aff. communis
• C. aff. cristagalli
• C. declivis
• C. pseudorobusta
• Carbonita
• C. cf. faintlina
• C. humilis
• Cleiothyridina
• Coelacanthus
• Coeloslylina
• Coelurosauravus
• conodonts platformed
• Corylus
• crinoids
• crustaceans
• Curvirimula
• C. trapeziforma
• C. subovata
• Cyprina
• deer
• Dielasma
• D. elongatum
• Donaldina
• Dorypterus
• Dunbarella macgregori
• Dyscritella
• Elk, Irish
• entomostraca
• Estheria
• eurypterids
• Fenestella
• ferns
• Filograna
• fish
• fish debris
• fish remains
• fish scales
• foraminifera
• fucoids
• gastropods
• Geinitzina
• Geisina arcuata
• Geisina? subarcuata
• Glomospira
• goniatite spat
• goniatites
• hazel
• Hilbollina sp.
• Hindeodella
• Horridonia
• H. horrida
• Hyperarnmina
• insect remains
• insects
• lanassa
• Kingopora
• Lachrymula pringlei
• land plants
• Leaia
• Lepidoderma
• Liebea
• L. squamosa
• Lingula
• Lingula mytilloides
• Lioestheria vinti
• Littorina
• lycopods
• marine shells
• Mourlonia
• mussels
• Mytilus
• Naiadites
• N aff. obliquus
• N aff. productus
• Nalatus
• Nangustus
• N. cf. daviesi
• N cf. elongalus
• N cf. flexuosus
• N cf. obliquus
• N cf. productus
• N cf. subtruncatus
• N. obliquus
• N quadratus
• N triangularis
• Naticopsis
• N. minima
• nautiloids
• Neocalamites
• Neochonetes
• N.? davidsoni
• oak
• Orbiculoidea
• Orthothrix
• ostracods
• Palaeoniscum
• Parachites?
• Parallelodon cf. sernicostatus
• Pennophorus
• P. costatus
• Pernopeclen carboniferus
• Planotites
• P. aff. ophthalmoides
• P. ophthalmoides
• plant remains
• plants
• Platysomus
• Posidonia sulcata
• Protorosaurus
• Pseudoctenis
• Pseudontortotis
• P. speluncaria
• Pseudovollzia
• pteridosperms
• Pterospirifer
• Pygoperus
• Rectocornuspira?
• reptiles
• Rugosochonetes skipseyi
• Schizodus
• S. obscurus
• Serpula
• Sphenobaiera
• sphenopsids
• Spiriferellina
• Spirorbis
• Stenoscisma
• Straparolus sp. nov.
• Streblochondria
• Stromaria
• Strophalosia
• S. morrisiana
• Synocladia
• Taeniopteris
• tree roots
• Tubulites
• Ullmannia
• Ulmus
• vertebrate bones
• Wodnika
• worms
• serpulid

Geology of the country around Sunderland

The district described in this memoir comprises Wearside and parts of south Tyneside, and is dominated by the built-up area which extends from Sunderland north-westwards to Wallsend. The predominantly urban landscape is offset by an attractive coastline along which fine exposures of the Permian Magnesian Limestone are seen in spectacular cliffs, stacks and natural arches.

The district is the type section of the marine Permian succession in the United Kingdom. In addition to the Magnesian Limestone, it contains evaporites which, on land, have been dissolved away, giving rise to widespread collapse structures.

Carboniferous Coal Measures underlie the whole district and formed the basis of the coal mining industry, of which only a few small opencast workings survive.

Almost all the district is covered by glacial and periglacial deposits of Late Devensian age, which are locally complexly interbedded.

This memoir will be useful for planners and civil engineers involved in urban renewal, who are faced by problems of ground instability caused by old coal workings and the complex distribution of drift deposits.

Figures, plates and tables

Figures

(Figure 1) Main physical features and settlements of the district.

(Figure 2) Simplified geological map of the district. Since this diagram was prepared, the Hebburn Dyke has been shown to reach the surface in the area covered by the Magnesian Limestone.

(Figure 3) Principal shafts and boreholes in the Sunderland district.1 Wallsend Colliery C Pit; 2 Percy Main Colliery, Percy Pit; 3 St Hilda Colliery; 4 Westoe Colliery, New Shaft; 5 Westoe Colliery, Crown Shaft; 6 Offshore Borehole W7; 7 Offshore Borehole W8; 8 Wallsend Colliery, G Pit; 9 New Pit, Howdon Dock; 10 Wallsend Colliery, A Pit; 11 Walker Colliery surface borehole; 12 Chapter Main Colliery (Templetown Old Pit); 13 Harton No.1 Borehole; 14 Offshore Borehole W5A; 15 Wallsend Colliery, East Pit; 16 Hebburn Colliery, C Pit; 17 Hebburn Colliery, A Pit; 18 Hebburn Colliery, B Pit; 19 Jarrow Colliery; 20 Offshore Borehole WM5; 21 Lawson's Main Colliery; 22 Rising Sun Colliery No.1 Borehole; 23 Walker Colliery, Ann Pit; 24 Restoration (or High) Pit, 25 Walker Colliery, Jane Pit; 26 Walker Colliery, King Pit; 27 Harton Colliery, Old Pit; 28 Harton Colliery, New Pit; 29 Cleadon Pumping Station Well; 30 Whitburn Colliery; 31 Tyne Main Colliery, William Pit; 32 Nightingale Pits; 33 Monkton Pit; 34 Felling Colliery, Venture Pit; 35 Boldon Colliery; 36 Offshore Borehole WM6; 37 Offshore Borehole WM7; 38 Offshore Borehole WM7A; 39 Felling Colliery, Engine Pit; 40 Wardley Colliery; 41 Offshore Borehole WM13; 42 Heworth Colliery; 43 Follonsby or Follingsby Colliery; 44 Fulwell Pumping Station Well; 45 Offshore Borehole No.10; 46 Offshore Borehole WM8; 47 Offshore Borehole WM2; 48 BGS Down Hill Borehole; 49 Carley Hill Pumping Station Well; 50 Stormont Main Colliery, King Pit; 51 Springwell Colliery; 52 Springwell surface boreholes; 53 Usworth Colliery; 54 South Moor Farm Borehole; 55 Hylton Colliery; 56 Wearmouth Colliery; 57 Offshore Borehole No.4; 58 Offshore Borehole WM3; 59 Vale Pit; 60 Washington Colliery, J and F Pits; 61 Washington Colliery, B Pit; 62 Washington Colliery, I Pit; 63 Offshore Borehole WM3; 64 Washington Colliery, Glebe Pit; 65 Hylton Borehole; 66 Sunderland Hospital Well; 67 Offshore Borehole No.8; 68 Wash Houses Pit; 69 Humbledon Hill Pumping Station Well; 70 Offshore Borehole VT9; 71 Biddick Colliery; 72 Harraton Colliery; 73 Chartershaugh Colliery; 74 Penshaw Colliery, Whitfield Pit; 75 Herrington Colliery; 76 Silksworth Colliery; 77 Ryhope Colliery; 78 Offshore Borehole No.18; 79 Offshore Borehole VT 10; 80 Offshore Borehole VT 11; 81 Penshaw Colliery, E Pit; 82 Newbottle Colliery, Dorothea Pit; 83 Ryhope Pumping Station Well; 84 Offshore Borehole VT3; 85 Offshore Borehole D8; 86 Lumley Colliery, Ninth Pit; 87 Bournmoor Colliery, C Pit; 88 Newbottle Colliery, Success Pit; 89 Newbottle Colliery, Margaret Pit; 90 Stony Gate Pumping Station Well; 91 Burdon Pumping Station Well; 92 Offshore Borehole VT 1; 93 Offshore Borehole VT 2; 94 Offshore Borehole D4; 95 Offshore Borehole Dl; 96 Lumley Colliery Sixth Pit; 97 Bournmoor (Lambton) Colliery D Pit; 98 Houghton Pit; 99 Vane Tempest Colliery and BGS Seaham Borehole; 100 Offshore Borehole VT 8; 101 Offshore Borehole No.11; 102 Offshore Borehole D2; 103 Lumley New Winning; 104 Lumley Colliery, Fourth Pit; 105 Lumley Colliery, Fifth Pit; 106 Murton Colliery surface borehole, 1965; 107 Seaham Colliery; 108 Seaham Pumping Station Well; 109 Offshore Borehole D7; 110 Offshore Borehole D6; 111 Offshore Borehole D3; 112 Offshore Borehole D5A; 113 Lumley Colliery Eighth Pit; 114 Offshore Borehole No.12; 115 Rainton Colliery, Hunter's House Pit; 116 Rainton Colliery, Plain Pit; 117 Rainton Colliery, North Pit; 118 Rainton Colliery, Nicholson's Pit; 119 Eppleton Colliery; 120 Dawdon Colliery.

(Figure 4) Sequence proved below Coal Measures in the Harton Borehole, South Shields. Slightly modified from Ridd et al., 1970, and reproduced by kind permission of the Yorkshire Geological Society.

(Figure 5) Westphalian sequence in the district. The miospore zones are after Smith and Butterworth, 1967.

(Figure 6) Distribution of volatile matter (by percentage) in the Top Busty Coal, showing regional variation and the local effects adjoining the Muck Dyke intrusion.

(Figure 7) Sections of Lower Coal Measures strata below the Victoria Coal.

(Figure 8) Sections of strata between the Brockwell and Busty coals.

(Figure 9) Details of the Busty coals.

(Figure 10) Sections of strata between the Top Busty and Harvey coals (see (Figure 9) for line of section).

(Figure 11) Details of the Harvey Coal.

(Figure 12) Sections of strata between the Harvey Coal and the Harvey Marine Band (see (Figure 11) for line of section).

(Figure 14) for line of section)." data-name="images/P935963.jpg">(Figure 13) Sections of strata between the Harvey Marine Band and the Hutton Coal (see (Figure 14) for line of section).

(Figure 14) Distribution of thick sandstone below the Hutton Coal.

(Figure 15) Details of the Hutton Coal.

(Figure 16) Sections of strata between the Hutton and Brass Thill coals (see (Figure 15) for line of section).

(Figure 17) Details of the Brass Thill Coal.

(Figure 18) Sections of strata between the Brass Thill and Maudlin coals (see (Figure 17) for line of section).

(Figure 19) Details of the Low Main Coal.

(Figure 20) Details of the Maudlin Coal.

(Figure 21) Sections of strata between the Maudlin and Main coals (see (Figure 20) for line of section).

(Figure 22) Details of the Main Coal.

(Figure 23) Sections of strata between the Main and High Main coals (see (Figure 22) for line of section.)

(Figure 24) Details of the Five-Quarter Coal.

(Figure 25) Isopachytes of the Metal Coal (metres) showing the location of workings (generalised). Line symbols as in (Figure 24).

(Figure 26) Details of the High Main Coal.

(Figure 27) Sections of strata between the High Main and Ryhope Five-Quarter coals (see (Figure 26) for line of section).

(Figure 28) Details of the Ryhope Five-Quarter Coal.

(Figure 29) Sections of strata between the Ryhope Five-Quarter Coal and the base of the Ryhope Marine Band (see (Figure 28) for line of section).

(Figure 30) Sections of strata between the Ryhope and Wear Mouth marine bands (see (Figure 28) for line of section).

(Figure 31) Strata between the Wear Mouth and Down Hill marine bands in the Down Hill Borehole.

(Figure 32) Distribution of Upper Coal Measures in the Boldon Syncline.

(Figure 33) Upper Coal Measures proved in the Down Hill Borehole.

(Figure 34) Distribution of Permian strata in the Sunderland district and adjoining parts of the Tynemouth and Durham districts. On the 1:50 000 geological map the Raisby Formation is named Lower Magnesian Limestone and the Ford Formation is named Middle Magnesian Limestone.

(Figure 35) Stratigraphical relationships of the Permian strata in the district, before dissolution of interbedded evaporites. The section is about 15km long. On the 1:50 000 geological map the Raisby Formation is named Lower Magnesian Limestone and the Ford Formation is named Middle Magnesian Limestone.

(Figure 36) Orientation of cross-bedding foreset dips in the Yellow Sands in County Durham and adjoining areas; mean of many measurements. Reproduced from Steele (1981) by permission of the author.

(Figure 37) Thickness of Yellow Sands and Marl Slate in the Sunderland district and parts of adjoining districts.

(Figure 38) Distribution and thickness of the Raisby Formation and proven occurrences of the Trow Point Bed. The Trow Point Bed may not have been recognised in some boreholes in which it was penetrated.

(Figure 39) Cross-section of the shelf-edge reef of the Ford Formation showing the distribution of subfacies. Adapted from Smith (1981a, fig. 3).

(Figure 40) Section through reef dolomite of the Ford Formation. As seen in 1973 (now mainly covered) in road cutting [NZ 3788 5532] to [NZ 3801 5535] on north side of Humbledon Hill, Sunderland, showing masses of autochthonous partly encrusted bryozoan boundstone (M) and roughly bedded shelly detrital rubble; other parts of the rock here comprise varied mixtures of autochthonous rock and detritus. Note the hints of differential compaction beneath the boundstone masses and of slight eastwards progradation in the rubble sheets. The floor of the cutting slopes ENE and the reef has been tectonically tilted by about 2° in the same direction.

(Figure 41) Mutual relationships of facies of the Trow Point Bed and adjoining strata. Adapted from Smith (1986, fig. 3).

(Figure 42) Approximate distribution and present thickness of the Hartlepool Anhydrite (EZ1A).

(Plate 26))." data-name="images/P936014.jpg">(Figure 43) Representative sections of the Hartlepool Anhydrite (EZ1A). The uppermost anhydrite in W15 Borehole is secondary (see (Plate 26)).

(Figure 44) Laccolithic intrusion of flow-banded dissolution residue (stippled) of the Hartlepool Anhydrite into overlying collapse-breccia of Cycle EZ2 carbonate. Bedded off-reef beds of the Ford Formation were exposed beneath the residue (here 1 to 3 m thick) near the section illustrated. North side of railway cutting [NZ 400 537], Ryhope (now filled).

(Figure 45) Section in very fine-grained dolomite of the mid-slope facies of the Concretionary Limestone Formation.Showing glide-planes (G), truncation surfaces (T) and slump contortion in unlaminated beds; undisturbed lowest and middle beds (L) are of finely laminated hemipelagic/pelagic dolomite. East side of Cleadon Pumping Station [NZ 3870 6360], South Shields.

(Figure 46) Inferred depositional environments and stratigraphical relationships of the Concretionary Limestone and Roker Dolomite formations in north-east England.  An oscillating pycnocline probably impinged high on the basin-margin slope, separating deep anoxic basin-floor waters from a thinner oxic surface layer: a dysaerobic layer may also have been present. The Roker Dolomite is inferred to have passed west-south-westwards into the Edlington Formation (top left).

(Figure 47) Concretionary Limestone Formation. Lenses and tongues of white blocky calcite (shown dark) in finely-laminated limestone transected by upwards-radiate coarse prismatic calcite; note the pseudo-ripples caused by distortion of laminae by the growth of low-angle calcite prisms. West part of Southwick Quarry [NZ 382 592], just above the Flexible Limestone.

(Figure 48) Seaham Formation. Stepped contact between bedded calcite mudstones and massive crystalline limestone containing spherulitic concretions (cross-hatched), with boudin-like bodies of finely crystalline calcite (stippled) at contact. Cliffs, north side of Seaham Harbour [NZ 4323 4950].

(Figure 49) Schematic section showing the stratigraphical effect of inshore dissolution of evaporites in the Permian sequence of north-east England; compare with (Figure 35).

(Figure 50) Foundered strata of the lower part of the Concretionary Limestone Formation.  Showing massive dedolomitised collapse-breccias sharply overlain by slightly to severely collapse-brecciated dolomite and limestone; late-stage collapse-pipes cut the latter. The residue of the Hartlepool Anhydrite probably lies 2 to 5 m below the lowest rocks shown. Cliffs at Velvet Beds, north end of Marsden Bay, South Shields.

(Figure 51) Foundered strata of the middle part of the Concretionary Limestone Formation. Showing bedded limestone and dolomite passing into semi-breccias and breccias and cut by late-stage collapse-pipes and fissures (C); parts of the most severely brecciated rocks have been dedolomitised. The top of the residue of the Hartlepool Anhydrite is probably about 45 m below the lowest rocks shown. Cliffs at north end of Smugglers' Cove, Marsden, South Shields. These strata are more disturbed than most beds at this level.

(Figure 52) Structure of the Carboniferous rocks shown by contours drawn on the base of the Maudlin Coal. Inset map shows the main structural features, including parts of faults with a displacement of 50 m or more.

(Figure 53) Contours on the Carboniferous/Permian unconformity, showing faults thought to have a displacement of 8 m or more.

(Figure 54) Rockhead contours at Sunderland, showing areas of solid rock at surface and the position of the main buried valleys.

(Figure 55) Drift-filled (buried) valleys of the Sunderland district. Approximate rockhead levels in metres above or below Ordnance Datum. Based on Smith (1981, fig. 2).

(Figure 56) Distribution of drift less than 8 m thick (stippled).

(Figure 57) Stratigraphical relationships of the main Quaternary deposits of the district. Redrawn from Smith (1981, fig. 5) with permission of Pergamon Books Ltd.

(Figure 58) Distribution of the main Quaternary deposits of the district (Prismatic Clay omitted). Redrawn from Smith (1981, fig. 4), with permission of Pergamon Books Ltd.

(Figure 59) Clast orientation in the Durham Lower Boulder Clay, partly after Beaumont (1971). Also shown are the trends of glacial striae on the Magnesian Limestone (after Beaumont, unpublished) and the location of ice-wedge casts in drift. Redrawn from Smith (1981, fig. 3), with permission of Pergamon Books Ltd.

(Figure 60) Aproximate shape of Glacial Lake Wear (stippled) during the inferred 43 m stand. The cutting of Tunstall Hope would have been initiated when the lake stood at perhaps 90 m above OD. Reproduced from Smith (1981, fig. 7), with permission of Pergamon Books Ltd.

Plates

(Front cover) Cover photograph Stack and natural arch formed of Permian Concretionary Limestone, Whitburn Colliery.

(Plate 1) Unconformity of Yellow Sands on reddened sandstone of the Upper Coal Measures; hammer (0.3 m) rests on contact. Low cliff [NZ 3575 5761] on north bank of River Wear, Castletown, Sunderland.

(Plate 2) Marl Slate (black, at top) overlying bioturbated Yellow Sands; scale bar 1 cm. Offshore Borehole W9B [NZ 5062 5705], depth 186.1 m.

(Plate 3) Small-scale sedimentary and/or diagenetic (stylolitic) structures in fine-grained (mud to silt-grade) dolomite of the Raisby Formation; scale 1 cm. Offshore Borehole W12B [NZ 5598 6598], depth 340.5 m.

(Plate 4) Oblique joints in thin-bedded dolomite of the Raisby Formation, terminating upwards against the base of a debris-flow (arrowed); the field of view is about 5 m across. The Trow Point Bed (dark grey in photo) and the residue of the Hartlepool Anhydrite are visible at the top. Coastal cliffs [NZ 3886 6629], 100 m north-north-west of northern tip of Frenchman's Bay, South Shields.

(Plate 5) Raisby Formation: proximal turbidite or debris-flow of ill-sorted matrix-supported clasts of mud-grade to silt-grade dolomite; note the general imbrication (indicating transport from right), large over-riding slabs near base and graded bed (pale grey in photo) at top. Old quarry [NZ 3378 5485] at east end of Dawson's Plantation, Penshaw.

(Plate 6) Raisby Formation: supposed turbidite of subaqueously reworked aeolian Yellow Sands resting on scoured surface of silt-grade dolomite ; scale 1 cm. Offshore Borehole B8 [NZ 4100 8700], depth 142.6m.

(Plate 7) Raisby Formation: thinly interbedded laminated and unlaminated fine-grained ?turbiditic dolomite; many of the darker (bituminous) laminae are modified by low-amplitude concordant stylolites; core 15 cm long. Seaham Borehole [NZ 4258 5031], depth 115.2 m.

(Plate 8) Trails and burrows on bedding surface of silt-grade dolomite of the Raisby Formation; coin (top left) is 28 mm across. Rock shore-platform, Graham's Sand [NZ 3845 6657] , Trow Point, South Shields.

(Plate 9) Compressed pisoids in argillaceous carbonaceous ooidal dolomite, near top of Raisby Formation; scale 1 cm. Offshore Borehole W12B [NZ 5600 6598], depth 336.7 m.

(Plate 10) Complex of slide blocks (olistoliths) of thin-bedded dolomite of backreef beds of the Ford Formation just west of the reef/ backreef contact; the field of view is about 15 m across. South side of abandoned railway cutting [NZ 3618 5734] , South Hylton, Sunderland.

(Plate 11) Ford Formation: thickly encrusted bryozoans in dolomitised boundstone of patch-reef; field of view about 2 cm across. The encrustations resemble Stromaria. From 1.5 m bed on south side of Gilleylaw Plantation Quarry [NZ 3755 5370], Silksworth, Sunderland.

(Plate 12) Ford Formation: dolomitised bryozoan boundstone of reef-core, showing mud-grade to silt-grade infill (blotchy) and patchy dolomite microspar (pale grey); field of view is roughly 1.2 cm across. Road cutting [NZ 3595 5863] near Hylton Castle, Sunderland. (Open University photo, reproduced by permission).

(Plate 13) Ford Formation: thickly encrusted bryozoans in dolomitised boundstone of reef-core; field of view roughly 1.2 cm across. Same locality as (Plate 12). (Open University photo, reproduced by permission).

(Plate 14) A reconstruction of the palaeocommunity of the core facies of the shelf-edge reef of the Ford Formation, showing domination by framework of sessile filter-feeders (mainly species of Fenestella (centre), Synocladia (bottom left) and Acanthocladia (bottom right); contemporaneous shelly benthos detritus and draped ?microbial sheets (here shown in substrate) fill niches around and between the main frame-builders. Reproduced, with permission of the publishers, from Hollingworth and Tucker (1987), fig.10, in Lecture Notes in Earth Sciences, Peryt, T M (editor). (Springer-Verlag; Berlin.)

(Plate 15) Ford Formation: coated clast of boundstone embedded in thick laminar fill of subvertical tension gash in high reef-core rock near the reef-crest; scale 15 cm. North side of crag [NZ 3915 5450] , Maiden Paps, Sunderland.

(Plate 16) Ford Formation: ill-sorted reef talus, comprising large tumbled blocks of dolomitised algal-bryozoan boundstone (bottom left and top right) and basinward-dipping sheets of finer detritus; hammer 0.3 m. Rock face [NZ 3968 5381] at southern end of Tunstall Hills, Sunderland.

(Plate 17) Ford Formation: Trow Point Bed, showing ooids and oncoids in a weakly laminated matrix of carbonaceous mud-grade to silt-grade dolomite; field of view 2.5 cm across. Coastal cliffs [NZ 3890 6618] in middle of north-west side of Frenchman's Bay, South Shields.

(Plate 18) Radial arrays of columnar stromatolites of the Trow Point Bed, resting on the dedolomitised top of the Raisby Formation and overlain by collapse-brecciated Cycle EZ2 limestone; scale 15 cm. South side of sea stack [NZ 3849 6650], Graham's Sand, Trow Point, South Shields.

(Plate 19) Typical mosaic Hartlepool Anhydrite, showing 'chickenwire' mesh of finely crystalline dolomite; scale 1 cm. Offshore Borehole B8 [NZ 4100 8700], depth 123.4 m.

(Plate 20) Stellate selenite radiating from dolomite stringers (i.e. fluid conduits) near base of Hartlepool Anhydrite; coin diameter 17 mm. Offshore Borehole B8 [NZ 4100 8700], depth 130.7 m.

(Plate 21) Residue of Hartlepool Anhydrite (mid grey in photo) on dedolomitised Trow Point Bed (pale grey) and overlain by dedolomitised Cycle EZ2 collapse-breccia; scale 15 cm. Coastal cliffs [NZ 3841 6660] at south side of Trow Point, South Shields.

(Plate 22) Finely laminated dolomite of the lower slope subfacies of the Concretionary Limestone Formation, showing drag folds caused by submarine creep or sliding, and cavities after former sulphate; scale 1 cm. Offshore Borehole B8 [NZ 4100 8700] , depth 99.7 m.

(Plate 23) Inverse-graded dolomite of the middle or lower slope subfacies of the Concretionary Limestone Formation, showing concentrations of flaky organic matter; scale 1 cm. Offshore Borehole B9A [NZ 4002 8997], depth 64.1 m.

(Plate 24) Large slump fold and other disturbances in thin-bedded fine-grained limestone of the upper slope subfacies of the Concretionary Limestone Formation. Hammer bottom right. Coastal cliffs [NZ 4133 6297], Whitburn Colliery.

(Plate 25) Sharp slump contortion in partly laminated dolomite (dark grey) and limestone (pale grey) of the Concretionary Limestone Formation; note the ostracods and bivalve; scale 1 cm. Offshore Borehole B8 [NZ 4100 8700], depth 78.9 m.

(Plate 26) Part of anhydrite bed in the lower slope subfacies of the Concretionary Limestone Formation; the anhydrite ranges from discontinuous layers of small coalescent nodules to tight enteroliths and lies in a replaced and displaced matrix of fine-grained slightly bituminous (?sapropelic) dolomite; scale 1 cm. Offshore Borehole W15 [NZ 4809 7622], depth 169 m.

(Plate 27) Calcitic concretions from the Concretionary Limestone Formation, illustrating some of the types: A Subvertical calcite lobes in mainly laminated calcite mudstone; note the radial calcite selvedges and the slight distortion of laminae. Loose specimen in wall [NZ 4073 5905], Roker, Sunderland, almost certainly from Fulwell, Southwick, or Carley Hill Quarry. Coin 2.5 cm across. B. Calcite spherulites in the Concretionary Limestone Formation; coin 2.3 cm. Top of east face of Marsden Hall Quarry [NZ 3964 6455], South Shields. C. Bizarre calcite concretion featuring mutually intersecting radial and concentric elements centred on steeply inclined joints; cream fine-grained dolomite occupies-many interstices. Note the partial preservation of fine primary lamination. Same locality as A. Field of view 28.5 cm wide. D. Coarse curved radial calcite crystals, many splitting centrifugally, passing into lobes similar to those in A; note the traces of primary fine horizontal lamination. Loose specimen in wall [NZ 4057 5920] in Roker Park, Sunderland, almost certainly from Fulwell Quarry, Sunderland. Coin 2.3 cm across. E.  Exceptionally coarse radial/concentric ('reticulate') calcite concretions with small amounts of interstitial fine-grained dolomite; most of the radial elements are single or compound calcite crystals, splitting centrifugally. North-east corner [NZ 3881 5955] of disused quarry at Carley Hill, Sunderland. Field of view 17 cm wide. F. Downward-pointing calcite lobes in mainly unlaminated fine-grained dolomite; note the radial calcite selvedges (as in A), small lenticular (displacive) anhydrite patches (white) and the basal cavity (black), possibly after halite. Offshore Borehole W15 [NZ 4809 7622], depth 177.23 In at top. Field of view 7 cm wide. G. Radial arrays of calcite lobes alongside steeply inclined joints and fractures in near-horizontal Concretionary Limestone; most of the lobes are single calcite crystals, splitting centrifugally. Same locality as E. Scale 15 cm.

(Plate 28) Displacive calcite (?after calcium sulphate) in finely laminated slightly bituminous limestone of the lower slope subfacies of the Concretionary Limestone Formation; scale 1 cm. Offshore Borehole B8 [NZ 4100 8700], depth 64.4 m.

(Plate 29) Tepee-like expansion structure in finely laminated limestone of the lower slope subfacies of the Concretionary Limestone Formation. Scale 1 cm. Offshore Borehole B8 [NZ 4100 8700], depth 90 m.

(Plate 30) Collapse-brecciation in thin-bedded limestones, caused by dissolution of dolomite beds; cliff is about 10 m high. Concretionary Limestone, south side of Lizard Point [NZ 4099 6421], Marsden.

(Plate 31) Partial stylolitic brecciation of laminated finely crystalline Concretionary Limestone. Note the calcite lenses (white) and voids, and evidence of volume loss. Offshore Borehole No.3 [NZ 4619 5812] , depth 174 m. Field of view 11.5 cm across.

(Plate 32) Partial stylolitic brecciation in calcite laminite of the lower slope subfacies of the Concretionary Limestone, also showing incipient radial calcite concretions with some gypsum, areas of non-calcitised dolomite (dark grey), invasive vein of fibrous gypsum (grey, top centre) and partly displacive lenses and tongues of blocky to prismatic void-lining calcite (white); scale 1 cm. Offshore Borehole W15 [NZ 4809 7622], depth 161.5 m.

(Plate 33) Limestone coquina formed of compressed bivalves and other grains; note the leached cores and partial geopetal cavity-fill. Presumed Roker Dolomite Formation, coastal cliffs at east tip of White Steel [NZ 4133 6192], Whitburn. From peel kindly supplied and interpreted by Dr G M Harwood. Scale 1 cm.

(Plate 34) Presumed Roker Dolomite: ?algal-laminated limestone (bindstone) on mushy clayey rubble (?evaporite dissolution residue) on cellular limestone (?calcitised compressed coquina, see (Plate 33)); scale 15 cm. Locality as for (Plate 33).

(Plate 35) Contorted lamina of finely crystalline anhydrite in flow-lineated halite of the Fordon Evaporites; coin 2 cm. Offshore Borehole WM7A [NZ 5270 6290], depth 188m.

(Plate 36) Lower part of the Seaham Residue at its type locality [NZ 4302 4974], Seaham, showing strong contortion; base of dislocated median bed of limestone at top.

(Plate 37) Seaham Formation: tubes of Calcinema permiana and scattered casts of bivalves (Liebea squamosa and Schizodus obscurus); scale 1 cm. West side of Red Acre Point [NZ 4327 4952] ,Seaham Harbour.

(Plate 38) Sedimentary structures in thin-bedded calcite mudstones of the Seaham Formation; coin 2.3 cm. North wall of North Dock [NZ 4323 4950], Seaham Harbour.

(Plate 39) Coarse calcite spherulites in limestone of the Seaham Formation; coin 2.3 cm. Coastal cliff [NZ 4316 4952] immediately north of the harbour, Seaham.

(Plate 40) Concretionary Limestone Formation: dense, dedolomitised collapse-breccia comprising clasts of dolomite mudstone (dark grey) in a matrix of calcite; coin 2.5 cm. Coastal cliffs, Trow Point [3840 66681, South Shields. Reproduced from Smith, 1972, fig.5.1 by permission of UNESCO.

(Plate 41) Floundered Concretionary Limestone Formation: 'negative breccia', a variety of collapse-breccia caused by the surface removal of mainly tabular clasts of laminated dolomite from a calcitic matrix; scale 15 cm. Northern tip of Trow Point [NZ 3837 6671], South Shields.

(Plate 42) Cream partly brecciated dolomite of the Concretionary Limestone Formation sharply overlying massive dedolomitised collapse-breccia (pale grey) and cut by breccia-pipe filled with angular blocks of grey and brown crystalline limestone (dark grey in photo). Coastal cliffs [NZ 3972 6560], northern end of Marsden Bay, South Shields. See also (Figure 50).

(Plate 43) Basal part of Durham Lower Boulder Clay resting on thin rubbly gravel of angular fragments of local Magnesian Limestone on distorted (?cryoturbated) Magnesian Limestone debris. Coastal cliffs [NZ 4212 5124] about 400 m north of Hough Foot Plantation, Seaham.

(Plate 44) Worm Hill [NZ 310 541], Fatfield, Washington; a low hill carved from a 12 m sheet of ?lacustrine sand of the Tyne–Wear Complex. Worm Hill (not Penshaw Hill) is the reputed scene of the legend of the Lambton Worm. The Permian escarpment (tree-covered) forms the skyline on right.

(Plate 45) Coal pebbles in cross-laminated sand of the Tyne–Wear Complex. Abandoned sand pit [NZ 3058 5367], Fatfield, Washington.

(Plate 46) Prismatic Clay on sand of the Ryhope Sands. East side [NZ 4163 5231] of former workings (now filled), Ryhope.

Tables

(Table 1) Characteristics of strata of the varied facies of the Coal Measures of County Durham (slightly modified from Fielding, 1984a and published by kind permission of the Geological Society of London)

(Table 2) Classification of Permian strata in the Sunderland district and approximate correlation with the Permian sequence in the Southern North Sea and adjoining areas