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The Northumberland-Solway Basin and adjacent areas

By R.A. Chadwick, D.W. Holliday, and S. Holloway. Contributor, D.J.D. Lawrence

Bibliographical reference: Chadwick, R A, Holliday, D W, Holloway, S, And Hulbert, A G. 1995. The Northumberland–Solway basin and adjacent areas. Subsurface memoir of the British Geological Survey.

The Northumberland–Solway basin and adjacent areas. Subsurface memoir of the British Geological Survey.

London: HMSO 1995. First published 1995. NERC copyright 1995. ISBN 0 11 884501 2. Printed in the UK for HMSO Dd 292044 C8 03/95

• Authors
• R A Chadwick, MA, MSc British Geological Survey Keyworth
• D W Holliday, MA, PhD, CGeol British Geological Survey Keyworth
• S Holloway, BA, PhD A G Hulbert British Geological Survey Keyworth
• Contributor
• D J D Lawrence, BSc British Geological Survey Edinburgh

The full range of Survey publications is available through the Sales Desks at Keyworth and at Murchison House, Edinburgh, and in the BGS London Information Office in the Natural History Museum (Earth Galleries). The adjacent bookshop stocks the more popular books for sale over the counter. Most BGS books and reports can be bought from HMSO and through HMSO agents and retailers. Maps are listed in the BGS Map Catalogue, and can be bought together with books and reports through BGS-approved stockists and agents as well as direct from BGS.

(Front cover) Cover photograph North-facing scarp face of the Whin Sill, intruded into rocks of the Upper Liddesdale Group, at Crag Lough, Northumberland. These rocks form part of the fill of the Northumberland Trough and dip gently to the south, towards the Stublick Fault. The high ground to the south (top left) forms part of the Alston Block. (BGS photograph L1513)

(Rear cover)

(Frontispiece) Perspective view of the top surface of the Caledonian basement rocks in the region, viewed from the west.

(Succession) Summary of stratigraphy, geological and tectonic events.

Other publications of the Survey dealing with this and adjoining districts

Books

• Memoirs
• See Appendix 2
• British regional geology
• Northern England (4th edition) The south of Scotland (3rd edition)

Maps

• 1:1 000 000
• Geological map of the United Kingdom, Ireland and the continental shelf, Solid, 1991
• 1:625 000
• Geological map of Great Britain, Sheet 1, Solid, 3rd edition, 1979
• Bouguer anomaly map of the British Isles, Northern Sheet 1981
• Aeromagnetic map of Great Britain, North Sheet 1972
• 1:250 000
• Borders, Solid Geology, 1986
• Farne, Solid Geology, 1988
• Lake District, Solid Geology, 1980
• Tyne-Tees, Solid Geology, 1981
• 1:50 000 and 1:63 360 See Appendix 2

Acknowledgements–Notes

The interpretation of the seismic data, and the production of the structure contour and preserved thickness maps, have been carried out by Mr R A Chadwick, Dr S Holloway and Mr A G Hulbert. Geological support was provided by Dr D W Holliday, who has prepared the palaeogeographical maps and written much of the accompanying text, with contributions from Mr Chadwick and Dr Holloway. The structure contour maps of the Northumberland and Durham Coalfield (Figure 28) and (Figure 29) were compiled by Mr D J D Lawrence. The computing to produce the 3-D perspective views (Figure 6) and (Figure 7) and (Map 1), (Map 2), (Map 3), (Map 4), (Map 5), (Map 6), (Map 7), (Map 8), (Map 9), (Map 10), (Map 11), (Map 12), (Map 13), (Map 14), (Map 15), (Map 16) was undertaken by Mr Hulbert with assistance from Ms B Birch. The book has been compiled by Dr Holliday and edited by Drs A Whittaker and A A Jackson.

Arco British Ltd., BP Petroleum Development Ltd., Edinburgh Oil & Gas plc (and partners Aran Energy, Maxus Energy and Tullow Exploration), Hurricane International Ltd, and Enterprise Oil plc are thanked for permission to display seismic data. Approval to publish details of the Longhorsley Borehole has been given by AmBrit Resources Ltd. Permission to include information from the Westnewton and Brafferton boreholes has been given by Enterprise Oil plc. All of the above companies, together with Amoco (UK) Exploration Company, Fina Exploration Ltd and Pentex Oil Ltd are also thanked for their permission to publish the structure contour maps. Details of the Acklington Station and Becklees boreholes are included by permission of British Coal Corporation. Mr J Ward of Edinburgh Oil and Gas is particularly thanked for much useful discussion, and for information relating to the Easton Borehole. The preserved thickness and palaeogeographical maps were reviewed by Mr I C Burgess.

Throughout this book the word 'region' means the area shown on (Map 1), (Map 2), (Map 3), (Map 4), (Map 5), (Map 6), (Map 7), (Map 8), (Map 9), (Map 10), (Map 11), (Map 12), (Map 13), (Map 14), (Map 15), (Map 16) . The word 'district' refers to the area of a specified 1:50 000 geological sheet.

Preface

Traditional methods of geological surveying are not always reliable indicators of subsurface structure. Nor is any deep borehole necessarily a good guide to what might be found at depth at other nearby localities. To overcome such difficulties, seismic reflection profiling methods have been developed and advanced in recent decades. Such exploration methods are of particular importance to the oil industry, and the application of these and other basin analysis techniques to the sedimentary basins of the United Kingdom has significantly increased the economic resources of the nation, and greatly enhanced knowledge of the structure and geological evolution of both the land area and of the adjoining continental shelf.

Many recent maps and memoirs of the British Geological Survey have made use of seismic reflection profiles, but generally only in a limited way. This book is based on exhaustive use of such data, and aims to present, in an atlas format, a concise review of the tectonic and sedimentary history of the Carboniferous rocks of the Northumberland–Solway Basin and adjacent areas. It forms a sequel to a previously published British Geological Survey atlas of the onshore Mesozoic basins of England and Wales and is the first in a series relating to Upper Palaeozoic basins.

Whereas traditional British Geological Survey 1:50 000 maps and memoirs have been principally concerned with the surface and near-surface geology of relatively small areas, this account is essentially regional in scope, dealing particularly with the deeper, concealed parts of the Carboniferous succession and associated structures not considered in the earlier publications. The results of the study are largely contained in the accompanying 1:625 000 scale structure contour, preserved thickness and subcrop maps and associated palaeogeographical maps. The accompanying written account is intended both as a regional review, as an explanation and partial amplification of these maps, and as a summary of basin evolution and tectonic history.

The Carboniferous rocks of northern England and adjacent parts of southern Scotland have long had considerable economic importance. In the past, they have spawned a wide range of extractive industries, notably those of coal, ironstone and a variety of mineral deposits. Many of these deposits are either near exhaustion, or the requirement for the raw material is currently in decline. More recently, the region has been investigated as a possible source of oil and gas and of geothermal energy. Immediate exploitation of any such resources seems unlikely, but the potential of the region as a future supplier of 'hot dry rock' (HDR) geothermal energy has been demonstrated. This book, therefore, can be seen as providing an up-to-date summary and distillation of information obtained during the search for, and extraction of, mineral and energy resources from the region as traditional extractive activities come to a close. It will also serve as a basic reference source when economic conditions change, new commodities are required, or new geological ideas prevail, when the region will once again become the subject of detailed subsurface evaluation.

Peter J Cook, DSc Director. British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham. NG12 5GG 16 November 1994

The Northumberland-Solway Basin and adjacent areas–summary

During the exploration of northern England for oil and gas in the 1980s, numerous seismic reflection surveys were carried out. These provide the principal means of investigating the geological structure of the Carboniferous rocks below the levels that can be reliably predicted from outcrops, mine workings and most boreholes. In this account such data have been integrated with geological information to produce a series of structure-contour and preserved-thickness maps, at a scale of 1:625 000, illustrating the evolution and present-day structure of the Northumberland–Solway Basin and adjacent areas. These maps are accompanied by a short descriptive text, and a set of palaeogeographical maps, which together provide a review of basin evolution.

Subsidence of the Carboniferous basins was initiated in late Devonian or early Carboniferous times, heralding a major period of crustal extension and rifting, and was characterised by sharp changes in stratigraphical thickness over the faults controlling the basin margins. Up to 5000 m of strata accumulated locally during this time. Although some syndepositional faulting continued into Westphalian times, from the early Asbian onward, subsidence in general was controlled by regional thermal relaxation effects and spread to interbasin areas. Up to 2100 m of strata were deposited at this time.

Short periods of compression affected some areas during Carboniferous times, but more significant transpressive movements and basin inversion occurred towards the end of the Carboniferous. Locally, basin-fill strata were strongly folded and some of the earlier normal faults were partially reversed. Deposition ceased with regional uplift, and widespread erosion ensued. A period dominated by extensional faulting in the Mesozoic gave way in Cenozoic times to further basin inversion, regional uplift and erosion.

Chapter 1 Introduction

This book presents in an atlas format a concise review of the tectonic and sedimentary history of the Carboniferous rocks of the Northumberland Trough, its westerly continuation, largely beneath Permo-Triassic cover, the Solway Basin, and adjacent areas, including the Lake District and Alston Blocks and the northern part of the Stainmore Trough (Figure 1), (Figure 2) and (Figure 3). It forms a sequel to the previously published subsurface geological atlas of the Mesozoic basins of England and Wales (Whittaker, 1985).

The surface and near-surface geology of the region has been described in numerous British Geological Survey 1:50 000 maps and memoirs (Appendices 1 and 2). This account is more regional in approach and examines particularly the deeper, concealed parts of the Carboniferous succession not considered in the earlier publications. The results of the study are largely contained in the accompanying 1:625 000 scale structure contour, preserved thickness and subcrop maps ((Map 1), (Map 2), (Map 3), (Map 4), (Map 5), (Map 6), (Map 7), (Map 8), (Map 9), (Map 10), (Map 11), (Map 12), (Map 13), (Map 14), (Map 15), (Map 16) ), and associated lithofacies and palaeoenvironmental (palaeogeographical) maps (Figure 15), (Figure 16), (Figure 17) (Figure 18), (Figure 20), (Figure 22), (Figure 23), (Figure 25) to (Figure 26), and (Figure 31) which form the bulk of this work. The accompanying written account is intended both as a regional review and as an explanation and partial amplification of these maps and figures. A brief summary account of this work has been published by Chadwick et al. (1993a).

The region includes remote mountainous and moorland countryside as well as some of the most heavily urbanised areas in Britain. The main urban centres are located in Tyne and Wear and in Cleveland (Figure 1), formerly important centres of the coal, ship-building and iron industries. At present, important petrochemical complexes are located in Cleveland. Outside the urban areas, much of the lower-lying land is rural. The higher ground of the Pennines, Cheviot Hills and Southern Uplands comprises open moorland, and the Lake District has a more rugged mountainous aspect.

The Carboniferous rocks of the region have considerable economic importance and have yielded coal, ore minerals, building stone, aggregate, limestone, ironstone and ceramic materials. Prior to the 1980s, only limited investigation of the concealed geology was possible, mainly through coal exploration and extraction, and from scientific drilling and potential field geophysical surveys. More recently the region has been actively explored for hydrocarbons (Figure 4). This has provided an extensive network of commercial seismic reflection profiles, and some new deep boreholes, which for the first time have allowed accurate elucidation of the detailed structure of the Carboniferous strata beneath much of northern England. Significant new data have also become available from geothermal exploration. This account relies heavily on these data, and the boundaries of the study region have been determined in part by the availability of these new (seismic) geophysical surveys.

Good quality seismic data have been acquired in many parts of the Northumberland Trough, Solway Basin, Alston Block and northern Stainmore Trough where Carboniferous rocks are exposed at the surface, or occur beneath a thin drift cover. In such areas the entire Upper Palaeozoic basin fill is generally imaged by the seismic method. However, data quality commonly deteriorates in areas where Permo-Triassic rocks crop out, and also in areas of thick drift. For example, in the central and western parts of the Solway Basin, the deeper parts of the Upper Palaeozoic succession are not well-imaged and seismic horizons locally have been ghosted-in to tie with reflectors visible in the eastern part of the basin. Data quality is also poor at the northern end of the Vale of Eden Basin, perhaps because of the southward thickening of the Penrith Sandstone. All depth and isopach contours shown in the Vale of Eden Basin are inferred from limited borehole and outcrop information, and from the analysis of gravity data (Collar in Arthurton and Wadge, 1981).

Stratigraphical calibration of the seismic data is provided by the scatter of deep boreholes over the region. These are augmented by surface exposures, except in the centre of the Solway Basin, which has an extensive cover of younger, Permo-Triassic rocks forming the Carlisle Basin. The very thick early Carboniferous deposits which form the deeper part of the Lower Border Group in the Northumberland–Solway Basin, and their equivalents in the Stainmore Trough, are neither penetrated by boreholes nor are satisfactorily exposed at surface.

Summary of previous research

As a result of centuries of mining the ore deposits of the Northern Pennines and the coalfields of Northumberland, Durham and Cumbria, much was already known of the Carboniferous rocks of the region as modern geology emerged in the early 19th century (e.g. Forster, 1809, 1821; Thompson, 1814; Winch, 1817; Phillips, 1836). At this time, the stratigraphical succession had already been described in some detail, and regional changes in thickness and facies identified. A number of major fault lines were recognised, as well as the presence of the sub-Carboniferous and sub-Permo-Triassic unconformities.

In the second half of the century, the efforts of the early pioneers were consolidated. Much new data came to light during this time, particularly from the completion of the 6-inch primary mapping by the Geological Survey (Appendix 1). Thus, by the early 20th century, a large store of information had already been accumulated. Major contributions on the Carboniferous succession, its subdivision and correlation include those of Tate (1867), Lebour (1875), Miller (1887), Smith (1910, 1912) and Garwood (1913, 1931). The importance of a number of major faults, dividing the region into blocks and basins, was recognised (Kendall, 1911; Marr, 1921; Versey, 1927; Trotter and Hollingworth, 1928). These are the Stublick–Ninety Fathom echelon, the Pennine and the Close House–Lunedale–Butterknowle faults. On the blocks the succession is relatively thin, incomplete and largely undeformed; in the basins it is much thicker, more complete and relatively more deformed.

In the succeeding period, the region has continued to be of great interest to researchers, and much of its area has been remapped at the 6-inch or 1:10 000 scale by the Geological Survey (Appendix 2). The development of specialist branches of geology is reflected in the present-day research undertaken in the region. Thus, considerable advances in understanding of the evolution of the region have come from the application of new techniques in geophysics, micropalaeontology, sedimentology and structural geology. The most recent reviews are by Taylor et al. (1971), Robson (1980), Johnson (1984), Leeder et al. (1989) and Chadwick et al. (1993a).

Continued advances have been made in defining and subdividing the Carboniferous succession throughout the region (Figure 5). In most areas lithostratigraphical subdivision follows Geological Survey usage as outlined on the relevant 1:50 000 maps and accompanying memoirs (Appendix 2). However, in some areas the lithostratigraphical scheme is less than ideal because of rapidly alternating lithologies, major lateral facies changes, the lack of suitable, easily recognisable markers and the use of inappropriate biostratigraphical criteria. Biostratigraphical subdivision and correlation, despite the application of the additional micropalaeontological methods, continues to present problems. This is, in part, a result of restricted faunal development in marginal marine areas, and the occurrence of facies-dependent faunas. Significant advances include the studies of Rayner (1953), Westoll et al. (1955), Johnson (1959), Johnson et al. (1962), Johnson and Dunham (1963), Hull (1968), Neves et al. (1973), George et al. (1976), Owens et al. (1977), Mitchell et al. (1978), Ramsbottom et al. (1978), Conil et al. (1980) and Armstrong and Purnell (1987). Much remains to be achieved in the field of regional correlation and detailed palaeontological study is still required in many areas.

The first major application of geophysical techniques in the region came from the study of gravity data by Bott and Masson Smith (1957a). This supported the view of Dunham (1934), based on the zonation of ore minerals, that there was a concealed granitic batholith beneath the Northern Pennines. The presence of this intrusion was later confirmed by drilling (Dunham et al., 1965). Following this early success, geophysical surveys (gravity, magnetic, seismic and magnetotelluric) have played an increasingly important role in the study of the Carboniferous rocks of the region and of its crustal structure (e.g. Bott and Masson Smith, 195713; Bott 1961, 1967, 1974: Bott et al., 1984, 1985; Bamford et al., 1971; Beamish, 1986; Lee, 1982, 1986; Evans et al., 1988). Significant insights into regional deep structure have come from nearby offshore deep seismic profiles produced by the British Institutions Reflection Profiling Syndicate (BIRPS) (Beamish and Smythe, 1986; Freeman et al., 1988). Important accounts to make use of seismic reflection data in the study of the Carboniferous basins of the region are those of Kimbell et al. (1989), Fraser and Gawthorpe (1990), Chadwick and Holliday (1991), Collier (1991), Smith (1992) and Chadwick et al. (1993a).

General reviews of the structure of the Carboniferous rocks of the region include those by Dunham (1948, 1990), Holmgren (1974), Robson (1980), Grayson and Oldham (1987) and Collier (1989b). Notable local, detailed studies have been published by Shotton (1935), Turner (1935), Robson (1954), Shiells (1964), Ord et al. (1988) and Underhill et al. (1988). The geophysical studies, already mentioned, also provide insights into the structure of the deeper subsurface. A number of authors have applied modern ideas of sedimentary basin evolution (notably those of McKenzie, 1978) to the region; in particular, Leeder and McMahon (1988) and Kimbell et al. (1989) have attempted to analyse the subsidence history of the Northumberland Trough in the light of these theories.

The nature and origin of the sedimentary successions have been the subject of much research and controversy. The cyclicity of the sequence, and the relative effects of eustatic sea-level change, local and regional tectonics, and sedimentary processes, have led to much discussion (e.g. Dunham, 1950; Moore, 1959; Johnson, 1967; Bott and Johnson, 1967; Leeder and Strudwick, 1987; Maynard and Leeder, 1992). Much regional and local sedimentological work remains to be carried out, but important contributors to date include Robson (1956), Burgess and Harrison (1967), Leeder (1973, 1974a, 1987), Elliott (1975); Hodgson (1978), Heward (1981), Haszeldine (1984), Fielding (1984a), Turner and Munro (1987), Smith and Holliday (1991) and Reynolds (1992).

The hydrocarbon prospectivity of the region has been reviewed by Scott and Colter (1987) and Chadwick et al. (1993a). The organic content of rocks in the region, and the light it throws on thermal history and the petroleum geology of the basin, has received attention in recent years (e.g. Creaney, 1980; Burnett, 1987). The possibility of deep groundwaters in the Fell Sandstone Group proving to be a source of low enthalpy geothermal energy was considered by Cradock-Hartopp and Holliday (1984) and Holliday (1986). The hot dry rock (HDR) potential of the region was reviewed by Evans et al. (1988) and by the British Geological Survey (1988).

Despite the increasing specialisation of much of the geological research in the region, the inter-relationship of deep structure, tectonics, stratigraphy and sedimentation is widely acknowledged. Recently, there has been a tendency for studies to adopt a multidisciplinary approach, for example Leeder et al. (1989), Kimbell et al. (1989), as is the case with this account.

Outline of geological history

The Lower Palaeozoic and early Devonian rocks of northern England and southern Scotland, which comprise the basement of the late Devonian and Carboniferous rocks described in this account, provide evidence of the closure of a major ocean, the Iapetus Ocean, as a result of subduction and the subsequent collision of the palaeo-North American (Laurentian) and palaeo-European (Avalonian) continents. This region of complex structure, termed the Iapetus Convergence Zone, formed during the later stages of convergence and collision in late Silurian and early Devonian times. Crustal thickening at this time led to a major phase of granite-batholith intrusion. Subsequently, throughout much of Devonian time, the region formed part of the Old Red Sandstone continent and was dominantly an area of erosion.

A period of crustal extension, perhaps beginning locally in late Devonian times, became widespread in the early Dinantian. The Maryport–Stublick–Ninety Fathom fault system (Figure 3), which is the main structure controlling sedimentation in the Northumberland–Solway Basin, and other large synsedimentary faults, appear to be inherited from reactivated, pre-existing basement structures. Many of these are orientated east-north-east, but a north-easterly or north-north-easterly trend is also locally significant, especially in the west. The areas which formed structural highs during early Carboniferous times, the Lake District, Alston and Cheviot blocks and the Southern Uplands, contain large, rigid, low density granitic intrusions in the pre-Carboniferous basement.

Early synextensional ( 'rift') strata, of Courceyan to Chadian age, are largely restricted to the fault-bounded basinal areas. The main depocentres were adjacent to the Maryport–Stublick–Ninety Fathom and the Closehouse–Lunedale–Butterknowle fault systems (Figure 3). Local thickening against other syndepositional faults is common. For the most part, adjacent block areas received little or restricted sedimentary cover during this time. Younger synextensional rocks, of Arundian to Holkerian age, gradually spread more widely as the extensional phase gave way in post-Holkerian times to a period of regional thermal relaxation subsidence and sedimentation which was characterised by only limited normal faulting. Crustal compression or dextral transpres sion during the Variscan Orogeny, led to uplift and basin inversion, which terminated sedimentation in late Westphalian times as the region once more became an area of widespread erosion.

Much of the Carboniferous sequence of the region comprises alternations of shallow marine, coastal and fluviodeltaic deposits. Marine influences were more marked in the west and south-west, whereas a persistent source of clastic sediment lay to the north-east. From late Namurian times onwards, marine influence on sedimentation was generally limited. Local clastic, sediment sources were of major significance only in the early synextensional phase of sedimentation. The maximum depositional thickness of strata in the basins is difficult to estimate because of the subsequent erosion of much of the Upper Carboniferous sequence. Up to about 5000 m of Dinantian and 1000–2000 km (compacted thicknesses) of Silesian strata are likely to have been deposited in the major depocentres.

Following the Variscan Orogeny, erosion stripped away much of the Silesian cover of the region. In Permian times, sedimentation recommenced in the Vale of Eden and more locally in the Carlisle–Solway area (Carlisle Basin). In late Permian, Triassic and early Jurassic times, sedimentation became more widespread and probably eventually covered the whole region. Later Mesozoic structural and sedimentary history is not clear, although there is evidence of normal and possibly strike-slip faulting in the Carlisle Basin since early Jurassic times. It is probable that a cover of late Cretaceous Chalk may have existed, but the former presence of other formations is uncertain. Similarly the nature and age of post-early Jurassic structures cannot be determined with accuracy. The region underwent widespread uplift and erosion during Cenozoic times, when most of the post-Carboniferous cover was removed and further erosion of Carboniferous rocks took place. Over a number of structural highs, including the Lake District, Southern Uplands, and parts of the Alston Block, all Carboniferous rocks have been eroded away and basement rocks crop out at the surface.

The present-day structure of the region is graphically illustrated by 3-D perspective views of the top of the Caledonian basement surface (Frontispiece, (Figure 6) and (Figure 7).

Chapter 2 Lower Palaeozoic

The Late Devonian to Carboniferous rocks of the region rest with marked angular unconformity on, and largely conceal, a basement of Lower Palaeozoic and Early Devonian sedimentary, volcanic and intrusive rocks. These basement rocks were deformed and metamorphosed during a number of compressional tectonic episodes which traditionally have been referred to as the Caledonian Orogeny. A detailed account of the origin and evolution of these older rocks is beyond the scope of this study, but a brief account is included here because basement structure is of such fundamental importance to the initiation and evolution of the Northumberland–Solway and Stainmore basins.

Lake District and Alston Block

Ordovician and Silurian rocks, ranging in age from Tremadoc to Přídolí, are exposed in the Lake District (British Geological Survey, in press), and in the adjacent Cross Fell and Teesdale inliers (Burgess and Holliday, 1979; Arthurton and Wadge, 1981) (Figure 2). Elsewhere in the region, rocks of the same age have been proved in only a few scattered boreholes which penetrate the Upper Palaeozoic cover. The main Lower Palaeozoic rock types are shale and greywacke. Limestone is less common and is principally of late Ordovician age. Volcanic rocks are well developed, particularly in the mid-Ordovician Borrowdale and Eycott Volcanic groups. In the Lake District and the Alston Block, major intrusions, mainly of granitic composition, cut the Lower Palaeozoic rocks (Figure 3). They range from mid-Ordovician to early Devonian in age (Firman, 1978a; Brown et al., 1985; Rundle, 1992). The presence of a major granitic intrusion, the Weardale Granite, beneath the Carboniferous cover of the Alston Block has been confirmed by the Rookhope Borehole, although its three-dimensional form is interpreted mainly from gravity data (Dunham et al., 1965; Bott, 1967). The exposed granites of the Lake District also appear to form only a small part of a much larger, partially concealed pluton (Bott, 1974; Lee, 1986). A number of these intrusions, e.g. Shap, Skiddaw and Weardale, are high heat production (HHP) granites and are associated, at the present time, with higher than average surface heat flow (Lee et al., 1987; Rollin, 1987).

The pre-Upper Palaeozoic basement rocks which crop out in the Lake District have a complex tectonic history not yet fully understood. Little is known of the concealed basement rocks of the Alston Block. The main deformational phase and the formation of the cleavage appears to have occurred in early Devonian (Acadian) times (Soper et al., 1987). The trend of the cleavage has an arcuate form. In the west it is orientated north-east or east-north-east, but towards the east the trend gradually changes first to east–west and then to east-south-east. A number of major east-north-easterly trending faults cut the rocks of the Lake District (Webb and Cooper, 1988; Cooper et al., 1988; Lee, 1989; Cooper and Molyneux, 1990). These zones of dislocation were active both during the period of early Palaeozoic deposition and subsequently as the loci of sinistral strike-slip movement.

Southern Uplands

Lower Palaeozoic shales and greywackes of the Southern Uplands form the basement beneath the northern margin of the Northumberland–Solway Basin. The overall structure is a south-easterly directed, imbricate thrust stack. Deformation of these rocks was diachronous, ending in mid-Silurian times. The dominant orientation of the cleavage and the major faults is east-north-east to west-south-west. The geological structure of this area, a source of controversy since the 19th century, is still a matter of debate (McKerrow, 1987). An accretionary prism model for these rocks is widely accepted (Leggett et al., 1979; 1983), but alternatives, such as the back-arc to foreland basin thrust model of Stone et al. (1987), have recently received much support.

By early Devonian times, the Southern Uplands rocks had been uplifted and deeply eroded. Early Devonian clastic rocks, the Lower Old Red Sandstone, are preserved locally, and contemporaneous lavas and pyroclastic rocks of intermediate to acid composition are preserved in the Cheviot Hills (Robson, 1976). A period of widespread intrusion of granitic magma also occurred at this time in the Southern Uplands, locally cutting the Lower Old Red Sandstone volcanic rocks of the Cheviot Hills. Analysis of gravity data suggests that these granites are largely concealed and still only partially unroofed (Dawson et al., 1977; Lagios and Hipkin, 1979; Lee, 1982). None of these granites is known to have high heat production nor to be associated with higher than normal heat flow (Lee et al., 1987; Rollin, 1987).

Iapetus Convergence Zone

The Lower Palaeozoic and Early Devonian rocks of northern England and adjacent parts of southern Scotland are thought to provide evidence of the closure of a major ocean, the Iapetus Ocean, as a result of the convergence and subsequent collision of the palaeo-North American and palaeo-European continents of Laurentia and Avalonia respectively. The area of collision and attendant terrane accretion at the margins of these two continents is termed the Iapetus Convergence Zone, and is largely concealed by the younger rocks of the Northumberland–Solway Basin (Phillips et al., 1976; McKerrow and Soper, 1989; Chadwick and Holliday, 1991). The precise location of the boundary between Laurentian and Avalonian continental crust, the Iapetus Suture, is uncertain. From faunal evidence, it is thought to lie to the south of the outcrops of Lower Palaeozoic rocks in the Southern Uplands and Cheviot Hills, which were deposited on the southern margin of Laurentia, and to the north of the Lower Palaeozoic rocks of the Lake District and the largely concealed Pennine blocks, which were laid down on the northern margin of Avalonia (Cocks and Fortey, 1982; Fortey et al., 1989). The systematic variation in cleavage and fold transection angles in the Lake District has been interpreted in terms of transpressive (oblique-reverse) strains associated with the northward movement of the Midlands Microcraton, which may have acted as a rigid indenter during accretion of Avalonia onto the Laurentian margin.

Investigations of the deep crustal structure of the Iapetus Convergence Zone, from seismic reflection and magnetotelluric data, have identified an east-north-easterly trending zone which dips at about 25° towards the north. Several authors have equated this zone with the putative location of the Iapetus Suture (Hall et al., 1984; Beamish, 1986; Beamish and Smythe, 1986; Freeman et al., 1988; Klemperer, 1989), but alternative explanations are possible (e.g. Soper et al., 1992). At lower crustal levels, this boundary clearly separates two seismically distinct areas, and may correspond to the position of the crustal suture (Freeman et al., 1988; Chadwick and Holliday, 1991). However, the updip projection of this boundary towards the present land surface suggests that at midcrustal level and, more specifically, at upper crustal level, it lies wholly within Avalonian crust (Beamish and Smythe, 1986; McKerrow and Soper, 1989). Chadwick and Holliday (1991) have suggested that, at these higher levels, the geophysical boundary departs from the line of the suture and marks the position of a major late Caledonian (Acadian) crustal shear zone (Figure 8). This structure may be manifest at outcrop as the east-north-easterly trending Causey Pike Thrust (Figure 3), mapped in the northern part of the Lake District (Cooper et al., 1988; Webb and Cooper, 1988; Cooper and Molyneux, 1990), and which exhibits both thrusting and strike-slip displacement.

Influence of basement structure on Carboniferous basin development

Carboniferous sedimentation was greatly influenced by the presence of east-north-east-trending faults and large granitic intrusions within the basement rocks. Several of the faults have a long history of movement, but, during the Acadian orogenic event in early Devonian times, many acted as transpressive, mainly sinistral, structures. In early Carboniferous times, these faults are believed to have been reactivated in response to a new period of crustal extension (Leeder, 1982; Kimbell et al., 1989).

The most important east-north-easterly trending basement structure, the Acadian crustal shear zone ((Figure 8), see above), has been identified from geophysical studies and has been related to the Iapetus Suture. It is thought that the Maryport–Stublick–Ninety Fathom fault system, which forms the southern margin of the Northumberland–Solway Basin and controlled its development, formed in the early Carboniferous by extensional reactivation of this crustal structure, in its hanging-wall block (Figure 8) (Chadwick and Holliday, 1991). Smith (1992) has related the onset of early Carboniferous basaltic volcanism, and the intrusion of the late Carboniferous–early Permian tholeiitic Whin Sill Suite, to basement structural control within the Iapetus Convergence Zone.

The Closehouse–Lunedale–Butterknowle fault system (Figure 3) appears to have much in common with the Stublick and Ninety Fathom faults. It forms the faulted northern margin of the Stainmore Trough and its syndepositional throw equals or even exceeds that of the other fault system. It seems probable that this major extensional structure also developed in response to reactivation of a significant crustal structure, perhaps an easterly continuation of one of the recently recognised lineaments in the central and southern Lake District identified by Lee (1989). However, the presently available seismic data reveal little of any such relationship.

Numerous other easterly or east-north-easterly trending syndepositional faults in the region with smaller throws may also be related to basement features. Similarly, the north-easterly or northwards-trending syndepositional faults, which were inverted to form structures such as the Bewcastle Anticline, are believed also to be related to basement fractures. An alternative view, that these more northerly trending lines of weakness originated in Carboniferous times as transfer faults (synextensional, strike-slip faults), is thought to be unlikely because of their variable orientation.

The Alston, Lake District and Southern Uplands blocks, which contain large basement granitic intrusions, acted as structural highs which resisted subsidence throughout much of the synextensional phase of sedimentation, but were buried during later regional subsidence (Leeder, 1982). This is probably due in part to the rigidity of the batholiths and perhaps also to buoyancy effects. The Cheviot Granite is an exception to this, in that it did not form a marked high during the early Carboniferous extensional phase of deposition. The radiogenic Lake District and Weardale granites provided an important heat source throughout the deposition and subsequent history of the Carboniferous rocks. Although all of the Carboniferous structural highs in the region can be related to basement granites, studies elsewhere, in the East Midlands and in parts of north-west England (Plant and Jones, 1989), show that this is not necessarily a general rule.

Chapter 3 Late Devonian-Early Carboniferous: synextensional phase of basin development

It is now widely believed that the Carboniferous basins of northern England formed in a crustal stress field having a dominant horizontal component of north–south tension (Leeder, 1982; Kimbell et al., 1989; Fraser and Gawthorpe, 1990). Carboniferous basin formation in Britain is thought to have resulted from back-arc extension north of a collision-type orogenic belt in the Iberia–Amorica–Massif Central region, which developed during northwards-directed subduction of the Rheic Ocean (Leeder, 1982, 1988; Fraser and Gawthorpe, 1990). Several authors have suggested that extension was accompanied by a significant component of dextral strike-slip displacement (Dewey, 1982; Arthurton, 1984), but within the region, evidence for this is equivocal.

Studies of the subsidence history of the Northumberland Trough have shown that fault-controlled early to mid-Dinantian subsidence was much more rapid than the subsequent late Dinantian to Westphalian regional subsidence, the rate of which declined approximately exponentially with time (Leeder and McMahon, 1988; Kimbell et al., 1989). This type of subsidence history, where a rapid extensional or 'rift' phase is followed by a more gradual postextensional or 'sag' phase, is common to many sedimentary basins and was attributed by McKenzie (1978) to the process of uniform lithospheric extension. McKenzie's hypothesis states that instantaneous horizontal extension of the lithosphere by a factor 13, causes thinning of the lithospheric crust and mantle by a factor up. Within the upper crust, fault-bounded sedimentary basins develop and, at depth, elevation of the lithospheric isotherms gives rise to a positive thermal anomaly. The crustal thinning causes an initial, isostatic subsidence, equal to the average depth of the faulted basins. Subsequently the lithospheric thermal anomaly decays, giving a further, time-dependent, thermal relaxation subsidence. The postextensional thermal subsidence is of a regional nature, with unfaulted sedimentary sequences overlying the rifted basins and commonly overlapping their faulted margins to give a characteristic 'steer's head' cross-sectional profile (Dewey, 1982) (Figure 9). More detailed discussion and analysis of the subsidence history of the Northumberland–Solway Basin is given in Leeder and McMahon (1988) and Kimbell et al. (1989).

In the Northumberland–Solway Basin, most of the synsedimentary normal faulting accompanied deposition of the Lower and Middle Border Groups, of Courceyan to Holkerian age, but minor normal faulting continued thereafter, probably into Westphalian times (Figure 10) and (Figure 11). However, younger Dinantian formations (Figure 5) show an increasing overprint of thermal relaxation processes, with strata tending both to onlap onto the blocks and to thicken towards the basin centre (Figure 11) and (Figure 12). Similar relationships are apparent in the Stainmore Trough (Figure 13). The lack of borehole control, combined with this thermal overprinting in basins which were probably still actively extending, renders it inappropriate to define precisely the end of 'synextensional' subsidence and the onset of 'postextensional' thermal subsidence. To facilitate description and analysis, we have followed Kimbell et al. (1989) and have defined synextensional subsidence as encompassing deposition of the Lower and Middle Border groups and their lateral equivalents (Figure 5), together with underlying beds of the Upper Old Red Sandstone and early Carboniferous basic volcanic rocks, where these are present. Younger strata of Asbian to Westphalian age, have been ascribed to the postextensional thermal subsidence phase.

Kimbell et al. (1989) interpreted the synextensional phase in the Northumberland Trough as comprising essentially one continuous, uninterrupted period of subsidence, resulting from basin-margin faulting, the rate of which gradually reduced with time. This is probably an oversimplification. Fraser and Gawthorpe (1990) have suggested that Dinantian extension in the Stainmore Trough, and elsewhere in northern and central England, was pulsed. The present work in the Stainmore Trough and Northumberland Trough broadly supports this view. However, as discussed below, the magnitude and perhaps the timing of each pulse, appears to have varied significantly from basin to basin.

Within the basins studied here these tectonic pulses caused no breaks in sedimentation or consistently defined sequence boundaries, with a few local exceptions noted below. Similarly, no sequence boundaries resulting from regional sea^-level changes have been mapped. This is a consequence of the dominantly clastic, fluviodeltaic, shallow-water sedimentation in the basins, which was continually in equilibrium with subsidence. Minor fluctuations in sea level, which were numerous, produced effects on a scale below the resolution of the seismic data employed.

Syndepositional faulting

The extensional phase of basin evolution was characterised by the close association between sedimentation and widespread contemporaneous normal faulting. This is revealed on the seismic data by sharp changes in thickness across well^-defined lines of faulting (e.g. (Figure 10) (Figure 11), (Figure 12), (Figure 13). At outcrop, evidence of syndepositional tectonic activity is common (Leeder, 1987). Most of the faults trend east to west or east-north-east to west-south-west, with an important, subsidiary set near the northern margin of the Solway Basin which trend north-north-east to south-south-west. The faults are roughly planar, at least to the base of the sedimentary fill, with little evidence of rollover geometry. Any suggestion that the faults are listric, becoming subhorizontal decollement surfaces within the Carboniferous basin fill (Leeder et al., 1989; Collier, 1989a), can be largely discounted on the basis of the seismic data. Many of these faults have undergone renewed movement, since the end of the extensional phase, with normal or reverse displacements, commonly associated with strike^-slip, mainly dextral, movement (Holmgren, 1974; Jones et al., 1980; Collier, 1989b).

Basin-bounding fault systems

There are two, major, basin^-bounding fault systems within the region. The southern margin of the Northumberland^–Solway Basin is bounded by a normal fault^-echelon downthrowing to the north, the Maryport–Stublick–Ninety Fathom system (Figure 10) and (Figure 11) (Kimbell et al., 1989). This is generally readily identified on the seismic data except on the southern margin of the Solway Basin, where it can be mapped with confidence only locally because of limitations imposed by the distribution and poor quality of the seismic profiles. The maximum throw of the fault system, at the level of the top of the Caledonian basement, is up to 5000 m (Map 1). The suggestion that the Northumberland and Solway basins have differing structural polarity, in the sense that evolution of the latter may have been principally controlled by movements on its northern margin (Barrett 1988; Leeder et al., 1989), has not been supported by the present work. Similarly, the northern margin of the Stainmore Trough is bounded by the Closehouse–Lunedale–Butterknowle normal fault system, which downthrows to the south (Figure 13) (Collier, 1991). The maximum throw of this fault system, at the level of the top of Caledonian basement, is up to 5000 m (Map 1).

The seismic data suggest that the faults in both systems dip at moderate angles (45–60º). Strata do not everywhere thicken towards the faults since the dominant style of extension was by planar 'keystone' faulting with only minor tilting of the hanging-wall blocks (Figure 10 (Figure 11), (Figure 12), (Figure 13).

The relatively great thickness of the rocks deposited during the extensional phase is a measure of the large net dip-slip component of displacement of these faults. In places these basin-margin faults appear to comprise single fault surfaces, but more commonly the total displacement is shared between a number of parallel or en-echelon faults (Kimbell et al., 1989; Collier, 1991), linked together by complex systems of relay or transfer ramps (Peacock and Sanderson, 1991). Locally, true transfer faults (Gibbs, 1984) appear to be developed, particularly along the southern margin of the Solway Basin, offsetting the Maryport Fault, but these have not been explicitly imaged by the seismic profiles. For the most part, the early Dinantian rocks are bounded by these fault systems although, commonly, the younger strata overlap onto the footwall block, gradually widening the basin with time (Figure 10) (Figure 11), (Figure 12), (Figure 13).

Antithetic fault-systems

The northern margin of the Northumberland–Solway Basin is taken at a system of en-echelon synsedimentary dislocations including the Waterbeck, Gilnockie, Featherwood and Alwinton faults (Figure 3). These normal faults are continued to the west of the region by the North Solway Fault (Deegan, 1973). They are essentially planar to the base of the sedimentary fill. Fault-plane dips are somewhat variable but are generally around 60°. The synextensional displacement on these southward-downthrowing structures is locally in excess of 1000 m. They are thought to be important basin-forming faults, subsidiary and antithetic to the Maryport–Stublick–Ninety Fathom faults in the Northumberland Trough but, together with these, defining a complex and roughly symmetrical graben in the Solway Basin (Figure 12)b . Towards the east, in the vicinity of the Cheviot Hills, the syndepositional throw of this fault system diminishes. Near the Northumberland coast, the system may continue as the Cragend–Chartners and Hauxley faults, but the net syndepositional throw both there and offshore (Chadwick and Holliday, 1991) is relatively small. In the east, the overall shape of the basin more closely approaches a half-graben with a poorly defined boundary between the basin and the Cheviot Block to the north (Figure 11) and (Figure 12)a. It is in this area that the Cheviot Block carries a significant cover of synextensional strata belonging to the Lower and Middle Border groups.

Evidence of synextensional movement along most of the length of the North Solway Fault, and of related contemporary erosion of its footwall block, is seen in rocks which are well exposed on the northern shore of the Solway Firth beyond the western margin of the region (Deegan, 1973; Ord et al., 1988). However, farther east, Lower Border Group rocks onlap the footwall, and the proportion of locally derived erosion products is much reduced (Craig, 1956). Within the region, there is no seismic reflection evidence of footwall erosion products within the Lower Border Group in the hanging-wall of the en-echelon Waterbeck Fault, and Lower Border Group rocks again onlap the footwall. The extent of footwall erosion farther east towards the Cheviot Hills is not clear, because of the presence of younger cover rocks. If present, it is likely to be restricted to the early part of the extensional phase.

In addition to the major faults considered above, a number of parallel or subparallel subsidiary lines of syndepositional displacement are revealed by the seismic data in the Northumberland Trough (Figure 11) and (Figure 12). These are normal, mainly planar faults, dipping at around 60°. Maximum syndepositional throws, up to 300 m, are probably largely confined to the Lower Border Group. The footwall blocks of these faults form a series of linear intrabasinal highs. Contemporary erosion of these blocks was probably restricted to the very early part of basin evolution.

In contrast to the Northumberland and Solway basins, the floor of the Stainmore Trough, where mapped, is remarkably flat with litee evidence of significant antithetic normal faulting (Figure 13).

Oblique faults

The faults described above generally follow the easterly or east-tiorth-easterly axial strike of the basins. However, in the Northumberland–Solway Basin a number of significant syndepositional dislocations lie oblique to this dominant trend. These are orientated variously from north-east to north-north-west and almost without exception are associated with considerable postdepositional, Variscan, structural inversion and reversal of movement (Figure 14), considered in more detail in Chapter 5.

The Back Burn Fault is a normal subplanar structure dipping at about 60° and downthrowing to the east (Figure 3) and (Figure 14); seismic profiles indicate syndepositional displacement in excess of 1000 m. Syndepositional downto-the-east normal faulting also occurred beneath the Bewcastle Anticline (Figure 12)c, along the line of the Goat Island-Lyne Thrust (Figure 32) and the East Christianbury Fault (Figure 3). The footwall blocks of these and related structures define a series of north-north-easterly trending intrabasinal highs at the western end of the Northumberland Trough and near the northern margin of the Solway Basin (Map 1). The magnitude of early synextensional displacements suggests that initially these highs may have been exposed and subjected to contemporary erosion, providing a local clastic sediment supply. However, evidence from the exposed rocks of the Bewcastle Anticline indicates that by Chadian times they were sourced remotely, from the north-east (Leeder, 1974a), suggesting that the intrabasinal highs had by then been submerged.

The north-easterly trending Lemmington Anticline (Figure 3), in the north-east of the region, has not been investigated by seismic surveys. However, it also is inferred to be related to reversal of a former syndepositional normal fault, as are the north-north-westerly trending Holborn and Berwick monoclines, located beyond the northern margin of the region. These structures are considered in more detail in Chapter 5.

Perhaps the most prominent of the oblique faults is the north-north-westerly trending Pennine Fault. Because of the lack of seismic data, syndepositional movement on this structure can only be inferred, but displacement of this type was probably much less than on the Maryport–Stublick–Ninety Fathom and ClosehouseLunedale–Butterknowle faults. A subsidiary basin, the Vale of Eden Basin, may have formed in its hanging-wall block, separating the Lake District and Alston blocks, during early Dinantian times (Figure 3). Outcrop data indicate widespread erosion of the Alston Block which formed the footwall during the synextensional phase of deposition (Burgess and Holliday, 1979; Arthurton and Wadge, 1981). The fault may have acted as a major transfer structure between the Solway and Vale of Eden basins to the west and the Northumberland Trough and Alston Block to the east. Exposures in Upper Teesdale suggest that there was no Dinantian synsedimentary movement on the subparallel Burtreeford Disturbance, on the Alston Block (Burgess and Holliday, 1979).

Synextensional rocks (Courceyan To Holkerian)

The synextensional rocks of the region are largely confined to the basinal areas, and are thickest close to the major basin bounding-faults. The maximum thickness near the Stublick–Ninety Fathom faults is between 2500 and 4000 m, possibly up to 5000 m close to the Maryport Fault, and around 3500 m adjacent to the Closehouse–Lunedale–Butterknowle system. These rocks are largely absent from the structural highs, and only relatively small thicknesses, up to 200 m, of beds from the later part of the extensional phase normally occur around the margins of the Alston and Lake District blocks. More complete, but still relatively thin, synextensional sequences occur on the eastern part of the Cheviot Block and on the main intrabasin highs.

The Northumberland–Solway Basin

Upper Old Red Sandstone

Red sandstones, siltstones and conglomerates, up to 200 m thick, and ascribed to the Upper Old Red Sandstone, have a narrow, discontinuous outcrop on the northern margin of the Solway Basin (Figure 2). This is contiguous with the much wider outcrop, in the Scottish Border area to the west of the Cheviot Hills, which extends beyond the northern margin of the region. The age of these beds is uncertain. They may be as old as late Devonian (House et al., 1977), but an early Carboniferous age for at least the upper part of the division is more probable (Lumsden et al., 1967; George et al., 1976; Greig, 1988). The beds are of fluviatile origin, showing stream flow to the north-east. They are believed to have been deposited in a hot and semiarid environment by a system of interior drainage (Leeder, 1973).

The Upper Old Red Sandstone is thought to be relatively thin throughout the basin. It crops out within the area of seismic coverage only to the north of the Water-beck Fault, and has not been proved by any deep borehole. There is not, therefore, unequivocal evidence of the subsurface distribution of the Upper Old Red Sandstone as it is not readily distinguishable on the seismic reflection data from younger formations. However, as there is a general overlap of older strata towards the northern margin of the basin, it seems probable that, at least in the west, the Upper Old Red Sandstone thickens from outcrop into the subsurface towards the active syndepositional faults. In central and eastern areas, little or no Upper Old Red Sandstone is present near the northern margin of the basin, and the presence of these beds in the subsurface is much less certain. However, overlap of beds towards the north is very markedly developed in this part of the basin (Figure 11), and the presence of Upper Old Red Sandstone in the subsurface towards the Stublick–Ninety Fathom faults cannot be discounted. As it cannot be separated seismically from younger beds using the seismic reflection data, any Upper Old Red Sandstone present has been included in the thickness map with the overlying strata of the Lower Border Group (Map 2).

Leeder (1974b) has proposed that the eruption of the early Carboniferous Kelso/Birrenswark lavas marked the initiation of the Northumberland–Solway Basin, and that the underlying Upper Old Red Sandstone is a prerift deposit. However, the distribution of the latter seems to have been controlled by faults which were also active during later sedimentation. Beyond the northern margin of the region, the location and deposition of the Upper Old Red Sandstone, as well as of the Kelso Lavas and the Cementstones in the Tweed Basin, are related to syndepositional displacement on the Pressen–Flodden–Ford Fault System (Chadwick and Holliday, 1991). It seems likely, therefore, that the Upper Old Red Sandstone was deposited in a series of small, linked basins, the precursors of the Northumberland–Solway and Tweed basins. These basins, with internal drainage and cut off from the sea, formed as a result of a period of crustal extension of limited magnitude in late Devonian (Fammenian) to early Carboniferous (Courceyan) times. A lengthy cessation of rifting, prior to the eruption of the early Carboniferous volcanic rocks and the beginning of more widespread synextensional sedimentation, is indicated by the regional development of palaeosols at the top of the Upper Old Red Sandstone (Leeder, 1976).

Younger strata of Old Red Sandstone facies occur in a number of areas, notably on the Cheviot Block and around the Lake District and the Alston Block. These are basin margin deposits, ranging up to Asbian in age (George et al., 1976). They were derived from basement which was locally exposed, commonly as a result of foot-wall uplift. The age of the rocks of Upper Old Red Sandstone facies in the Vale of Eden, Ravenstonedale and the Stainmore Trough is uncertain. Some of the red beds thereabouts are demonstrably of Carboniferous age, interfingering with rocks ranging from Courceyan to Holkerian age, but the age of others is unclear and some are commonly ascribed a Devonian age (Capewell, 1955; Wadge, 1978; Burgess and Holliday, 1979; Arthurton and Wadge, 1981).

Early Carboniferous volcanic rocks

Upper Old Red Sandstone sedimentation was followed, along the Scottish Border, by the eruption of basaltic lavas. These are known as the Birrenswark Lavas in the Solway Basin (Leeder, 1974b; Lumsden et al., 1967; Smith 1992), and the Kelso Lavas in the Tweed Basin and the country west of the Cheviot Hills (e.g. Greig, 1988). Similar lavas, of comparable age, also occur on the northern margin of the Lake District around Cocker-mouth (Eastwood et al., 1968). The eruption of these lavas was probably related to tensional fracturing along the main hinge lines at the time of initiation of the main basin margin faulting.

According to Leeder (1974b) the lavas are localised in occurrence, and probably do not extend far into the subsurface beneath the Northumberland–Solway Basin. The seismic data provide little conclusive insight into this question but are not entirely consistent with Leeder's view. Kimbell et al. (1989) have suggested that a more widespread development of the lavas could be associated with high amplitude seismic reflection events close to the base of the basin fill. However, no such reflections can be consistently identified or related to known volcanic outcrops.

Lower Border Group

The Lower Border Group (Cementstone Group) crops out extensively along the northern margin of the Northumberland–Solway Basin (Fowler, 1936, 1966; Nairn, 1956; Lumsden et al., 1967; Day, 1970). The group is also brought to the surface or to the sub-Permian incrop, within the central parts of the basin, by late Carboniferous folding. The Lower Border Group forms the main part of the synextensional sedimentary fill (Figure 10) (Figure 11), (Figure 12), with a maximum thickness of more than 4000 m close against the Maryport–Stublick–Ninety Fathom faults (Map 2) and (Map 4). The group is characterised by interbedded sandstones, shales, limestones and, in the subsurface, anhydrite. A Courceyan to Chadian age for these beds is indicated by the limited biostratigraphical information (Johnson, 1980; Armstrong and Purnell, 1987).

Substantial regional extension was initiated in the Northumberland–Solway Basin in Courceyan times, as indicated by the Birrenswark, Kelso and Cockermouth lavas. The Stainmore Trough probably also originated at this time. The initial extension-induced subsidence, which appears to have been very rapid and uninterrupted, resulted in marked palaeogeographical changes, destroying the internal drainage system and allowing the sea episodic access to the newly formed or rejuvenated basins. The maximum rate of basin subsidence and sedimentation occurred during Courceyan to Chadian times, when the lower part of the Lower Border Group (up to the top of the Lynebank Beds, (Figure 5) was deposited (Map 2). The lower part of the Lower Border Group is characterised by a large variation in thickness, up to 3500 m, related to syndepositional normal faulting. The local occurrence of basaltic lavas and tuffs indicates a continuing association of extensional faulting and volcanism. A contemporaneous, Courceyan to Chadian period of rifting has been recognised in a number of basins elsewhere in northern and central England, recorded as the seismic sequence EC1 of Ebdon et al. (1990) and Fraser and Gawthorpe (1990). The relative magnitude of subsidence and accompanying sedimentation related to this phase of rifting appears to vary considerably from basin to basin. Elsewhere, for example, the Widmerpool Gulf of the East Midlands, subsidence at this time appears to have been much less than in the basins considered in this account (Fraser and Gawthorpe, 1990).

Fault-related thickness changes, generally less than 800 m, across the subsidiary east-north-easterly trending intrabasinal faults, were much diminished in the upper part of the Lower Border Group, Bewcastle, Main Algal and Cambeck beds, (Figure 5) of Chadian to Arundian age (Map 4). There is no clear evidence that sedimentation spread far onto the footwall blocks at this time, and the main basin-margin faults are likely to have remained active. However, the possibility exists that extensional faulting ceased and that significant thicknesses of beds, equivalent to upper part of the Lower Border Group, were removed from the blocks at a time of later footwall uplift and erosion. This possible slowdown in rifting could correspond to a lull of comparable age recognised in other basins, the EC2 seismic sequence of Ebdon et al. (1990) and Fraser and Gawthorpe (1990).

The nature of the lower (Courceyan) parts of the Lower Border Group remains poorly understood. Great thicknesses of strata are indicated by the seismic data, but these have not been penetrated by boreholes, and only thin, basin-margin equivalents are seen at outcrop.

The higher parts of the group, locally present in surface exposures, comprise alternations of sandstone, silt-stone, mudstone, limestone and dolomite. Their sedimentology has been studied in some detail (Leeder, 1974a, 1975a, 1975b; Scott, 1986). Although evidence for the former presence of limited amounts of evaporite minerals had been detected, the Easton Borehole, drilled in 1990, proved in the subsurface a substantial thickness of anhydrite rocks, of probable sabkha origin, which have been removed at outcrop by groundwater dissolution. Sedimentary breccias, such as those exposed in the Bewcastle Anticline (Day, 1970), probably mark the former sites of these evaporite beds (J Ward, oral communication, 1990). The evaporites occur in the Bewcastle and Lynebank Beds, and in strata below those currently exposed. Reflectors corresponding to packages of anhydrite-rich rocks are prominent on seismic reflection profiles, particularly from the centre of the basin; but appear to die out to the west, south and east.

The siliciclastic rocks, of alluvial, fluviatile, lacustrine and deltaic origin, were derived from two principal sources (Leeder, 1974) (Figure 15) and (Figure 16). The Southern Uplands provided a local sediment source for the Whita fluviodeltaic system, and a similar local source from the Alston and Lake District blocks is likely, but is still unproven. A large river system flowing from the north-east provided a major siliciclastic input into the central part of the basin, which, in the upper part of the group, became the dominant sediment source as relief around the basin margins was subdued and the area of Lower Border Group sedimentation gradually increased.

The depositional environments of the carbonate rocks of the group range from low energy, shallow marine and tidal flat to lacustrine (Leeder, 1975a, 1975b). Indicators of deposition in open-marine, near normal salinities, decrease from south-west to north-east along the axis of the basin. The character of the evaporite and carbonate rocks suggests that the climate was hot and arid during the deposition of the Lower Border Group (Leeder et al., 1989). Wetter conditions, however, must have prevailed in the source region of the major north-easterly derived river system. Evaporites are absent from the Cambeck Beds at the top of the Group (Figure 5), and carbonate beds are generally less numerous. This may indicate somewhat reduced aridity at this time.

The supply of sediment always kept pace with the rapid rate of subsidence of the basin. The marked cyclic nature of the strata is indicative of repeated fluctuations of relative sea level. Following periods of marine transgression and sedimentation, fluviodeltaic siliciclastic sediments built out along the basin axis from the east, and locally from the basin margins. As sea level again rose, these were forced to retreat and were overlain by supra-tidal anhydrite and tidal-flat and subtidal carbonates.

Middle Border and Fell Sandstone groups

The Middle Border Group and its equivalents crop out on the northern margin of the basin and in central areas around the Bewcastle Anticline (Fowler, 1936; Craig, 1956; Nairn, 1965; Lumsden et al., 1967; Day, 1970). They also subcrop beneath the Permo-Triassic rocks of the Carlisle Basin (Map 16). Their maximum thickness is around 600–800 m (Map 6), and their age is Arundian to Holkerian.

Crustal extension is believed to have continued during the deposition of the Middle Border Group, but the rate of subsidence was considerably less than in the lower part of the Lower Border Group. It is likely that any earlier footwall-block rocks on the Alston Block and northern margins of the Lake District would have been extensively eroded at this time. Rifting of similar age is widely recognised in other basins of northern and central England, recorded as the seismic sequence EC3 of Ebdon et al. (1990) and Fraser and Gawthorpe (1990). Elsewhere, for example the Widmerpool Gulf of the East Midlands, the rate of subsidence and related sedimentation was commonly much greater than that observed in the Northumberland–Solway Basin (cf. Fraser and Gawthorpe, 1990).

During the Holkerian, there was a further diminution of fault activity, with significant sedimentation spreading onto the footwall blocks, which has been preserved as the Orton Group of the Alston Block and the 7th Limestone of the Lake District (Figure 5). This period of reduced extensional faulting probably corresponds to the early part of a quiescent period, seismic sequence EC4, recognised by Ebdon et al. (1990) and Fraser and Gawthorpe (1990).

Although the influence of syndepositional faults on basin subsidence was still marked at this time, the Middle Border Group, and its equivalents, tend to thicken towards the basin centre (Map 6), indicating the increasingly important contribution of thermal relaxation effects to basin subsidence. One result of this is that the area of sediment deposition spread more widely onto the margins of the Alston and Lake District blocks, particularly during Holkerian times, and that the importance of local sediment sources diminished during this period.

The Middle Border and Fell Sandstone groups (Figure 5) (George et al., 1976; Johnson, 1980) exhibit marked regional changes of facies (Figure 17) and (Figure 18). In the east, the Fell Sandstone Group iS largely arenaceous, up to medium grained, with only sparse interbeds of red mudstone. The group was laid down by braided rivers flowing from the north-east (Robson, 1956; Hodgson, 1978; Heward, 1981; Turner and Munro, 1987). It passes westwards into the Middle Border Group of the central part of the basin, a mixed fluviodeltaic and shallow-marine succession, comprising generally finer-grained sandstones, siltstones, mudstones and thin limestones, deposited under generally hot and semiarid conditions (Leeder et al., 1989; Smith and Holliday, 1991) (Figure 18). Farther west, from the evidence of the West-newton Borehole, these beds become increasingly marine, with an increase in carbonate content and only limited siliciclastic fluviodeltaic input. On the Lake District and Alston blocks to the south, the equivalent Orton Group and the 7th Limestone are almost totally marine in origin (Mitchell et al., 1978) (Figure 18).

The Middle Border Group and Fell Sandstone record a major increase in siliciclastic sediment supply from the north-east perhaps caused by a more significant fall in relative sea level. The rate of sediment supply continued to keep pace with subsidence. The Middle Border Group shows repeated alternations of fluviodeltaic and shallow marine facies caused by fluctuations in sea level. A period of greater aridity is indicated by muddy floodplain deposits with calcretes in central Northumberland (Smith and Holliday, 1991).

At the northern margin of the basin, basaltic volcanic rocks occur locally within the Middle Border Group. Their most prominent development, the Glencartholm Volcanic Beds, occurs at the top of the group and continues into the lower part of the Upper Border Group (Lumsden et al., 1967; Day, 1970). The eruption of these volcanic rocks is suggestive of localised extensional fault movement on the basin's northern margin. This increased tectonic activity at the Middle/Upper Border group boundary (Holkerian/Asbian) appears to have been accompanied by uplift of the Lake District and Alston Blocks (and perhaps also of the Cheviot Block), where early Asbian rocks are absent. Smith and Holliday (1991) have reported a change from fluvial-dominated deltaic sedimentation in the Middle Border Group to more tidally influenced deposition in the lower part of the Upper Border Group, which they related to this contemporary tectonic activity. This phase of rifting was not recognised in the seismo-stratigraphical analyses of Ebdon et al. (1990) and Fraser and Gawthorpe (1990) although early and mid-Asbian strata are commonly absent on block areas elsewhere in northern and central England.

The Vale of Eden Basin and the Stainmore Trough

During the synextensional phase of subsidence, the Northumberland–Solway Basin was at times connected to the Stainmore Trough by way of the Vale of Eden (Figure 3). This might have been as a result of transtensional normal faulting on the Pennine Faults. No seismic reflection surveys have been carried out in the Vale of Eden, and the synextensional strata are largely concealed by younger Carboniferous and Permo-Triassic rocks except on the upper part of the hanging-wall ramp to the east of the Lake District (Mitchell et al., 1978). Contemporaneous strata on and adjacent to the Alston Block, which forms the footwall block of the basin, are exposed along the Pennine escarpment (Burgess and Holliday, 1979; Arthurton and Wadge, 1981). By way of contrast, the rock sequences in the eastern part of the Stainmore Trough have been investigated by seismic surveys, but the only confirmatory detail comes from the limited penetration of these rocks by the Seal Sands and Brafferton boreholes (Collier, 1991).

The synextensional rocks of the Vale of Eden Basin, the Ravenstonedale and Orton groups, are dominantly carbonates of shallow water origin. Younger rocks overlap older beds towards the Lake District. The thickness in Ravenstonedale is around 900 m (Mitchell et al., 1978) and around 500 m in the transitional zone to the Alston Block (Burgess and Holliday, 1979). Locally derived fluvial and alluvial fan red beds, including the Basement Beds and Shap Conglomerate, mainly comprising conglomerates, breccias and sandstones, occur towards the base of the Carboniferous succession. These were derived from contemporary exposures of Lake District rocks (Capewell, 1955; Holliday et al., 1979) or, near the Pennine Faults, from the Alston Block (Burgess and Harrison, 1967; Burgess and Holliday, 1979). They interfinger with beds ranging in age from Courceyan to Holkerian. Some other, older red beds of uncertain age, including the Polygenetic and Mell Fell conglomerates (Capewell, 1955; Wadge, 1978; Arthurton and Wadge, 1981), are also regarded here as early Carboniferous (Courceyan) synextensional deposits, although they have been ascribed a Devonian age by some workers. A regional influx of sand from the north-east into the marine area (Ashfell Sandstone) is broadly synchronous with the Fell Sandstone Group (Figure 5).

Eastwards, in the Stainmore Trough, up to 3800 m of synextensional rocks are present south of the Closehouse–Lunedale–Butterknowle faults (Collier, 1991) (Figure 13); (Map 2), (Map 4) and (Map 6). The extensional evolution of the Stainmore Trough appears to have followed a similar pattern to that of the Northumberland Trough (compare (Figure 10) and (Figure 11) with (Figure 13). However, there are a number of uncertainties in correlation of seismic reflection events between the two basins. Thus, the interpretations of the seismic data presented in this account differ in a number of significant details from those illustrated by Fraser and Gawthorpe (1990), and are closer to the interpretations of Collier (1991).

There are insufficient data to recognise precisely the equivalents of the three stratigraphical divisions of the Border Group. From the limited well information, and by comparing seismic character, the boundary between the equivalents of the lower (Lynebank Beds) and the upper (post-Lynebank Beds) part of the Lower Border Group, that is between the EC1 and EC2 seismic sequences, has been placed at higher levels in the seismic profiles than by Fraser and Gawthorpe (1990). In the Stainmore Trough the beds forming EC1 were deposited during a period of significant hanging-wall tilting, and their top is marked by a slight angular unconformity which forms a locally prominent sequence boundary. The younger Dinantian seismic sequences identified by Fraser and Gawthorpe (1990) seem to relate more to supposed synchroneity of structural events with other basins which cannot be supported by local evidence. Their EC5 seismic sequence (late Asbian–early Brigantian) is more likely to be late Holkerian to early Asbian in age (cf. Collier, 1991). Similarly, the presence of a relatively thick sequence of preHolkerian strata, postulated by Fraser and Gawthorpe (1990) on the footwall-block of the basin (the Alston Block), is not supported by the Roddymoor Borehole (Johnson, 1980) or by the seismic data to which it connects (Collier, 1991).

From the limited borehole data, the nature of the deeper synextensional rocks of the Stainmore Trough is largely unknown. Siliciclastic rocks with limestones of Arundian and Holkerian age were proved in the Seal Sands Borehole (Figure 19), and may be typical of much of the higher part of the synextensional basin fill. The main sediment source, as in Northumberland, probably lay to the north-east. The extent of any clastic input from the Alston Block, in the area adjacent to the Closehouse– Lunedale–Butterknowle faults, is not known, but may have been substantial, at least during early synextensional (Courceyan) times.

Chapter 4 Late Dinantian-Silesian: postextensional phase of basin development

From early Asbian times onwards, extensional faulting at the basin margins was much reduced, and sedimentation gradually encroached onto the previously emergent footwall-blocks. This process was completed by late Asbian times with local exceptions on the northern margins of the Lake District, where the earliest rocks overlying the Caledonian basement are of Brigantian age. From Asbian times onwards, patterns of sedimentation had became more uniform across the region. Lateral thickness changes are generally gradual and are less related to faulting than in the older rocks. Such variations that do exist reflect more the effects of compaction on the underlying sedimentary pile, which was thinner on the blocks than in the basins, than the influence of synsedimentary faulting. By mid-Lansettian to early Bolsovian times there was little or no locally influenced variation in facies or thickness of the deposits.

One of the characteristic features of postextensional sedimentation, according to the McKenzie (1978) model, is that each lithological division thickens towards the basin centre rather than towards the faulted basin margins. However, as noted previously, the basins considered here depart from this simple model in that minor extensional faulting continued into the 'postextensional' phase. The seismic reflection data indicate that limited extensional faulting continued until at least Namurian times, with the Upper Border, Liddesdale and Stainmore groups showing local, fault-related thickening, particularly at the main basin-bounding Maryport– Stublick–Ninety Fathom and Closehouse–Lunedale– Butterknowle faults (Figure 10), (Figure 11), (Figure 12),to (Figure 13). The maximum synsedimentary throw in the three groups is 800, 500 and 400 m respectively. Thereafter, significant syndepositional displacement cannot be resolved using the seismic data, but sporadic normal faulting during Westphalian times has been inferred from sedimentological investigations by Fielding and Johnson (1987) and Collier (1989a).

In a number of other Carboniferous basins of northern and central England, which were not continuously supplied by abundant sediment from fluviodeltaic sources, the onset of regional thermal subsidence led to widespread marine transgression and the establishment of relatively deep-water conditions, even over early Dinantian block areas (Leeder, 1982; Smith and Smith, 1989). Within the region, periodic higher sea levels are implied by the thick and widespread limestones of midAsbian to late Arnsbergian age. However, the input of sediment continued to match subsidence and only rocks of fluviodeltaic and shallow-marine shelf origin were deposited.

Local synsedimentary folding, especially in the Solway Basin, but also elsewhere, led to sediment thinning over actively growing anticlines, and the development of at least one significant unconformity. Such movements were especially important during the late Namurian and again in late Westphalian times. They form early, embryonic phases of the Variscan Orogeny, the main phases of which brought about the cessation of deposition and ushered in a period of uplift, erosion and igneous intrusion (Whin Sill and associated dykes). During this time, many of the strata laid down during the postextensional phase were removed. The areas most strongly affected were parts of the Solway Basin, but other depocentres, including parts of the Northumberland Trough and the Stainmore Trough, also underwent varying degrees of uplift and erosion at this time. Former block areas, the Lake District, Southern Uplands, Cheviot Block and western parts of the Alston Block, were also significantly eroded. Near complete sequences are preserved only in the coalfields on the eastern and western margins of the region. In the Durham Coalfield, the late Dinantian to Silesian sequence is about 1000 m thick, compared with up to about 1350 m in the eastern part of the Northumberland Coalfield. The maximum thickness of equivalent beds to the west of the region, in the Solway Basin (Canonbie Coalfield extension) is up to 2100 m, including some late Westphalian rocks which were deposited contemporaneously with early Variscan folding.

Upper Border Group

The Upper Border Group and equivalent strata of early Asbian age, crop out widely in the western and central parts of the Northumberland Trough, and along the northern margin of the Solway Basin (Miller, 1887; Trotter and Hollingworth, 1932; Fowler, 1936, 1966; Craig, 1956; Nairn, 1956; Lumsden et al., 1967; Day, 1970; Frost and Holliday, 1980). They also subcrop beneath the Permo-Triassic rocks of the Solway Basin (Map 16), and in most other basinal areas the group is concealed beneath younger Carboniferous rocks. The group is up to 800 m thick (Map 8). Volcanic activity appears to have been restricted to the early part of the Upper Border Group (Glencartholm Volcanic Beds).

From field evidence and limited palynological data, Land (in Day, 1970) and Frost and Holliday (1980) (see also Johnson, 1984) suggested that there was a major northerly thickening of the upper part of the Upper Border Group over the Antonstown Fault, from about 300 m to about 600 m. However, the seismic reflection data provide no support for such a thickness change, and it is concluded that the field workers were misled by inadequate biostratigraphical data and by significant lateral facies changes which led to the loss, or lack of recognition, of key marker beds. It would appear from the seismic evidence that the Lewisburn Beds of the North Tyne area (Fowler, 1966) must span most of the Upper Border Group, and not just the upper part as thought by Frost and Holliday (1980).

On the Alston and Lake District blocks, rocks of early Asbian age are commonly missing. Relatively thin sequences, a few tens of metres thick, of siliciclastic rocks and limestones equivalent to the upper part of the Upper Border Group, occur locally on the margins and flanks of the Alston Block, e.g. in the Harton Borehole (Frost and Holliday, 1980). No beds of this age are known on the northern margin of the Lake District, and, if present around Ravenstonedale and the eastern margin of the Lake District, they are represented by a thickness of only a few tens of metres of limestone. To the north, on the Cheviot Block, biostratigraphical evidence indicates that the Scremerston Coal Group, which is up to 100 m thick, can be correlated with the upper part of the Upper Border Group and the lower part of the Lower Liddesdale Group ((Figure 5); George et al., 1976). The presence of rocks equivalent to the lower part of the Upper Border Group cannot be proved, and, if present, must be extremely thin (c.20 m). Alternatively, there may be a break in sedimentation, between beds of the Fell Sandstone and the Scremerston Coal groups, comparable to that between beds of similar age on the Alston Block.

The Upper Border Group comprises a laterally and vertically variable sequence of sandstones, siltstones, mudstones, limestones and coals (Figure 19). Little sedimentological work on the group and its equivalents has been published other than the summary accounts of Frost and Holliday (1980) and Leeder et al. (1989). Locally sourced, clastic rocks derived from eroding fault-scarps, such as the North Solway Fault beyond the western margin of the region (Deegan, 1973; Ord et al., 1988), are restricted in extent. Although there was some erosion of the emergent blocks at this time, there is little evidence that these were major sediment sources. It seems more likely that they were emergent areas of subdued relief. A detailed study of the lower part of the Upper Border Group in the Stonehaugh Borehole of central Northumberland (Smith and Holliday, 1991) has suggested that there was strong tidal influence on sedimentation during its deposition, possibly as a consequence of increased tidal range in the narrow seaway occupying the Northumberland Trough between the islands over the Southern Uplands and Alston blocks.

As with earlier sequences of the region, the deposition of the Upper Border Group and equivalent strata was dominated by the conflicting influences of marine incursion from the west and south-west, and the influx of fluviodeltaic siliciclastic sediment from the east and north-east (Leeder et al., 1989) (Figure 20). As a result, the strata are markedly cyclic with rapid alternations of marine and fluviodeltaic deposits. The faunas of the limestones together with the flora and sedimentological character of the siliciclastic rocks suggest that the climate at this time was warm and humid. Marine influence throughout the deposition of the Upper Border Group was greater than in the underlying Middle Border Group. The muddy and sandy nature and relative thinness of the limestone beds in central and eastern Northumberland, together with the character and thickness of the overlying prograding fluviodeltaic sequences, suggest that sea depths generally were not great, probably no more than 10 m. Only in the upper part of the group are greater sea-water depths indicated (15–20 m) from the character of the rocks (Frost and Holliday, 1980). Whether this rise in sea level, which continued into the late Asbian–Brigantian Liddesdale Group, is indicative of eustatic sea-level change, or was the result of regional thermal subsidence, is not yet clear (Collier, 1991).

The equivalents of the Upper Border Group do not crop out in the Stainmore Trough, but they are thought to have been proved in the Seal Sands (Figure 19) and Brafferton boreholes. Because of the lack of detailed biostratigraphical control in these wells, the precise equivalents of the Upper Border Group are difficult to identify positively. They are believed to correlate with a sequence up to 700 m thick (Map 8) of alternating shales, sandstones and thin limestones, proved beneath the Alston Group in the boreholes.

Liddesdale and Alston groups

The Liddesdale and Alston groups crop out, or subcrop beneath younger rocks, widely throughout the region. They are the subject of an extensive literature (e.g. Wells, 1957; Johnson, 1959; Johnson and Dunham, 1963; Frost, 1969; Rowley, 1969; Burgess and Mitchell, 1976), including most Geological Survey memoirs relating to the region (Appendix 2). The beds are over 800 m thick in the Solway Basin and Northumberland Trough and over 1200 m in the Stainmore Trough (Map 10). They are of late Asbian to Brigantian age (George et al., 1976) and are typically of Yoredale facies, a cyclothemic sequence comprising limestone, shale, sandstone, coal (Forster, 1809; Phillips, 1836; Hudson, 1924; Dunham, 1948; 1950; Moore, 1959; Wells, 1960; Bott and Johnson, 1967; Elliott, 1974; Leeder and Strudwick, 1987; Reynolds, 1992) (Figure 21).

Although similar cyclothems occur in both older and younger Carboniferous strata, they reach their maximum development in this part of the succession. Perhaps the most distinctive features of Yoredale cyclothems, compared with other cyclic sequences, are their greater thickness and the occurrence of thick, laterally persistent, relatively pure limestones with well-developed, abundant open-marine faunas. These features allow many cyclothems to be individually mapped at surface and traced laterally with some confidence over thousands of square kilometres (e.g. Holliday et al., 1975; Burgess and Mitchell, 1976; George et al., 1976; Johnson, 1980) (Figure 21). The maximum depth of sea water at this time was probably around 30–40 m. In the west and south-west the fluviodeltaic input to these cyclothems is generally restricted and commonly reduced to a thin shale parting (Burgess and Mitchell, 1976; Mitchell et al., 1978). However, towards the north-east the cyclothems are made up of greater proportions of fluviodeltaic rocks with much reduced basal marine members (Figure 22) and (Figure 23). The average thickness of the cyclothems is around 30 m, which with their variable lithology, renders them below the limits of seismic resolution.

The oldest rocks of Yoredale facies are restricted to the basin areas. However, in late Asbian times, the combined effects of the higher sea levels and regional ther mal subsidence led to a major marine transgression at the level of the base of the Melmerby Scar Limestone, and equivalents the Denton Mill and Dun limestones. This transgression covered, perhaps for the first time, all of the Alston Block, much of the Lake District Block, and formed the first significant, fully marine incursion onto the Cheviot Block (Figure 22). From late Asbian to early Pendleian times, Yoredale-style sedimentation was characteristic of the whole region.

Stainmore Group

Little study has been made of the Namurian and early Westphalian rocks of the region which are, in the main, poorly exposed and proved by few complete, cored boreholes. Biostratigraphical evidence is commonly incomplete or inadequate and the upper part of the sequence is generally poorly fossiliferous. As a result, there has been controversy and confusion over the classification of these rocks, and over the presence or absence of regional or local unconformities and non-sequences within and above them. The seismic reflection data have thrown some new light on these controversies, but are unable to solve all of the problems.

The proposal that strata between the Great Limestone and the Subcrenatum Marine Band should be termed the Stainmore Group (Burgess and Holliday, 1979) is followed here. The Hensingham Group of Cumbria is broadly equivalent (Mitchell, et al., 1978). The deposition of these rocks spans virtually the whole of Namurian times (Hull, 1968; Ramsbottom et al., 1978). These strata were formerly present everywhere in the region, but outcrops are now restricted mainly to the margins of the coalfields and the high ground of the Alston Block (Eastwood, 1930; Trotter and Hollingworth, 1932; Fowler, 1936; Dunham, 1948, 1990; Reading, 1957; Johnson and Dunham, 1963; Owens and Burgess, 1965; Lumsden et al., 1967; Eastwood et al., 1968; Mills and Hull, 1968, 1976; Farmer and Jones, 1969; Day, 1970; Burgess and Holliday, 1979; Frost and Holliday, 1980; Arthurton and Wadge, 1981; Dunham and Wilson, 1985; Holliday and Pattison, 1990). The thickness of the group ranges from 50 m in West Cumbria to over 800 m in the Northumberland–Solway Basin and over 1000 m in the Stainmore Trough (Map 12).

The group can be subdivided on broad lithological grounds into two divisions, more or less corresponding to the Upper Limestone Group and 'Millstone Grit' of earlier classifications (Figure 5) (Hull, 1968). Regionally, there appears to be a gradational and diachronous boundary between these two divisions (Hull, 1968; Holliday and Pattison, 1990), although the boundary can generally be drawn in strata of early Kinderscoutian age (Figure 24). Towards the west, this subdivision is less clearly identified in the Hensingham Group, in which fewer limestones have been recognised in the lower division and the upper division is less sandy.

The lower division, with the Great Limestone at the base, contains numerous widely traceable limestones and marine beds (Figure 24) and (Figure 25), and is similar to the Yoredale facies of the Liddesdale and Alston groups. However, apart from the Great Limestone itself, other limestones and marine beds are relatively thin, except in the south (Ramsbottom et al., 1978). In the south-east of the region, beds and replacement bodies of chert are closely associated with the limestones (Wells, 1955; Hey, 1956). Thick channel sandstones occur at several levels in the sequence cutting out significant thicknesses of underlying strata (Dunham, 1990; Holliday and Pattison, 1990; Hodge and Dunham, 1991).

The upper division is characterised (Figure 24) and (Figure 26) by the widespread occurrence of sandstones, which are commonly coarse grained and pebbly. Although these sandstones rest on erosion surfaces, there is little evidence for deep erosion of earlier strata, and they appear to be sheet deposits from braided rivers, rather than channel in-fills. Despite earlier views to the contrary, the sandstones do not form continuous beds over the whole region and, although the concept of a First Grit, Second Grit etc. retains some local value both in the field and in correlation (Ramsbottom et al., 1978; Holliday and Pattison, 1990), in many sections and boreholes finer-grained rocks, siltstones and seatearths, predominate. Marine beds are restricted to only a few horizons and contain limited faunas mainly of little biostratigraphical value.

During Namurian times the main sediment supply was still derived from northerly and north-easterly sources and marine influences increased towards the south. The distribution of facies broadly follows these trends (Fig ures 25 and 26), but there are some anomalies. For example, marine strata contain a high proportion of limestone in the eastern parts of the Northumberland and Stainmore troughs, but, around the Lake District, siliciclastic marine beds become more significant. This may indicate an additional north-westerly sediment source. Detailed sedimentological studies of these rocks include the work of Elliott (1974, 1975, 1976), Percival (1983, 1986) and Brenner and Martinsen (1990). The continuing interaction between marine influence and siliciclastic input in the subsiding basin led to frequent alternation of marine and nonmarine strata. The reasons for this are debated, but a eustatic origin for the more widespread cycles seems probable. Read and Forsyth (1989, 1991) and Read (1991) have attempted to analyse the Namurian successions of the Midland Valley of Scotland and of the Central Pennines in the light of eustatically controlled sequence stratigraphy. It may prove possible to apply similar ideas within the region, but the scale of the sequences is beneath the resolution of the seismic reflection data utilised here.

The dominant structural controls on sedimentation during Namurian times were regional thermal relaxation subsidence and the effects of differential compaction over former block and basin areas. However, both extensional and compressive movements were locally of significance, but the seismic data do not allow assessment of the precise timing and sequence of these tectonic events. Only very minor normal displacements can be discerned on the Stublick–Ninety Fathom fault system, but rather more movement took place on the Lunedale–Butterknowle faults with some thickening of strata into the Stainmore Trough (Map 12). Evidence of minor basin shortening and syndepositional folding, perhaps associated with dextral fault displacement, is found in the hanging-wall block of the Lunedale–Butterknowle Fault system, and more especially on the Carlisle Anticline (Figure 27). In the former area, the strata thinned over the growing anticline, but the rate of uplift of the anticlinal crest appears never to have exceeded the rate of ongoing regional subsidence, and no unconformity has been detected either on the seismic data or from field studies (Mills and Hull, 1976). However, a period of erosion of the Carlisle Anticline, close to the Namurian–Westphalian boundary, can be inferred from the seismic data (Figure 27). This unconformity is probably the same as that exposed nearby in the Canonbie Coalfield (Lumsden et al., 1967; Picken, 1988) and appears to be related to contemporary strike-slip displacement on the Gilnockie Fault. An unconformity at a similar level has also been inferred on the northern margin of the Lake District, most notably in the Crosby Anticline in the hanging-wall block of the Maryport Fault (Eastwood, 1930) and in the Cockermouth district (Eastwood et al., 1968). An unconformity at about this stratigraphical level is also visible on seismic data on the western flank of the Solway Syncline. These various lines of evidence indicate a significant period of dextral transpression, affecting much of the region, most particularly in and around the Solway Basin.

The general pattern of Namurian sedimentation and tectonics in the Northumberland–Solway Basin follows, in subdued form, that of the Midland Valley of Scotland, as described by Read (1981, 1988, 1989). There, a similar regime of regional thermal relaxation subsidence was accompanied by periodic pulses of tension and transpression notably from the late Arnsbergian onwards. A period of late Arnsbergian channelling, also found in Northumberland (Holliday and Pattison, 1990), the change from delta-dominated sedimentation (cf. the lower division of the Stainmore Group) to river-domi nated sedimentation (cf. the upper division of the Stain-more Group) in the early Kinderscoutian, and the occurrence of an unconformity between Namurian and Langsettian strata in the Douglas Coalfield (cf. Carlisle Anticline) are related by Read (1989) to these events and to uplift of the Scottish Highlands sediment source area. The fact that these sedimentary and tectonic features can be recognised over such a wide area is a measure of their significance. They were probably early pulses of the Variscan Orogeny whose culmination had such profound effects on the region later in Carboniferous times.

Coal Measures

Westphalian Coal Measures crop out in the west of the region, in the Canonbie and Cumbrian coalfields (Eastwood, 1930; Lumsden et al., 1967; Eastwood et al., 1968; Day, 1970; Arthurton and Wadge, 1981; Picken, 1988; Barnes et al., 1988; Young and Armstrong, 1989), and in the east, the Northumberland and Durham Coalfield (Fowler, 1936; Smith and Francis, 1967; Land, 1974; Jones and Magraw, 1980; Smith, 1994) (Figure 28) and (Figure 29). Smaller outliers occur along the line of the Stublick Fault (Trotter and Hollingworth, 1932) and in the Stainmore Trough (Owens and Burgess, 1965; Burgess and Holliday, 1979).

In the west, the Coal Measures are mostly concealed in the Solway Syncline between the Cumbrian and Canonbie coalfields, and the subcrop continues around the northern margin of the Lake District into the Vale of Eden (map 16). The maximum thickness of strata believed to occur in the centre of the Solway Syncline, is greater than 1600 m (Map 14). Here, the rocks are commonly reddened to a depth of more than 100 m beneath the sub-Permo-Triassic erosion surface, and the coals removed, as a result of post-Carboniferous weathering, locally hindering the elucidation of the nature and sequence of the beds (Mitchell et al., 1978). The Coal Measures which occur east of the Pennines are also partly concealed by Permian and Triassic rocks (Map 16). Offshore of Tynemouth the maximum thickness is locally up to 830 m. The zone of sub-Permian reddening in the east is generally of lesser significance, being up to 15 m thick (Anderson and Dunham, 1953).

The Coal Measures are of similar facies throughout the region (Figure 30) and (Figure 31). Most of the Langsettian to early Bolsovian strata comprise an alternating succession of grey mudstone, siltstone, sandstone and coal, the deposits of a coastal or deltaic plain that extended over much of Britain (Haszeldine, 1983, 1984; Fielding, 1984a, 1984b, 1986). The dominant structural controls on sedimentation appear to have been continued regional thermal relaxation, and compaction-assisted subsidence. The available seismic data do not provide any evidence for contemporaneous fault activity within the region. However, detailed examination of the rocks points to some subdued influence on sedimentation by normal faulting (Fielding and Johnson, 1987; Collier, 1989a).

Beds of Late Bolsovian to Westphalian D age, comprising sandstone and shale, are now mainly restricted to the Canonbie Coalfield and its concealed extension the Solway Syncline. They are largely fluvial in origin, and dominantly red in colour, indicating deposition in a hot, arid environment. Coals are uncommon in this part of the succession. Interpretation of the seismic data (Figure 27) suggests that these red beds were deposited contemporaneously with the early phases of development of the syncline, and were perhaps derived, in part, from the complementary Carlisle Anticline. This is consistent with the views of other workers that the late Carboniferous red beds of much of Britain were laid down in basins that formed between, and were partially sourced from, actively developing Variscan anticlines (Besly, 1988; Smith and Smith, 1989; Leeder and Hardman, 1990). According to Picken (1988), borehole data suggest that there is at least one unconformity in these red measures in the Canonbie Coalfield. Clear, unambiguous evidence for this is not provided by the seismic data, and it is possible that some or all of the missing strata in the boreholes may be explained by normal faulting.

Chapter 5 Variscan structures and basin inversion

In southern Britain, and on the continent of Europe, a series of deformational events, collectively termed the Variscan Orogeny, took place during Devonian and Carboniferous times. These culminated in late Carboniferous times with the uplift of a mountain fold belt which extended from Belgium and northern France, across southern England and Wales into Ireland. A zone of northerly directed thrusts, emplaced in late Carboniferous times, which roughly marks the northern limit of regional Variscan deformation in southern England and Wales, is known as the Variscan Front. Northern England lies to the north of the Variscan Front, upon the Variscan Foreland. Here, Variscan deformation was much less pervasive, being largely restricted to the reactivation of localised pre-existing lines of weakness, and associated with varying degrees of basin inversion. The main Variscan movements affecting the region postdate deposition of the Westphalian rocks and predate deposition of the Permian strata which rest unconformably on the Carboniferous beds. However, as noted in the previous chapter, earlier minor phases of compressional deformation locally influenced patterns of Silesian sedimentation. The quartz-dolerite Whin Sill and associated dykes were intruded late in the Variscan Orogeny, postdating the main folding episode.

The effects of the Variscan Orogeny are well displayed in many parts of the region (Figure 32), and are described in the British Geological Survey memoirs (Appendices 1 and 2), with more general reviews and accounts by Robson (1954, 1977), Holmgren (1974) and Jones et al. (1980). The account by Shiells (1964) of the structure of north-east Northumberland, combines a wealth of detailed local information with a wide-ranging regional review, and is particularly relevant to the region. In this account, previously available information is reviewed briefly and emphasis is given to information derived from the study of seismic reflection data, particularly in the Solway Basin where the structure of the Carboniferous rocks is largely concealed by younger strata.

Solway Basin

The major Variscan structures of the region lie within the Solway Basin, which appears to have been more strongly inverted than the Northumberland and Stain-more troughs. The Carboniferous rocks of the Solway Basin are folded into a major north-north-easterly trending syncline, the Solway Syncline and two complementary anticlines, including the Carlisle Anticline (Figure 12)b, (Figure 27) and (Figure 32). The latter feature, clearly distinguished by the pre-Permian subcrop pattern (Map 16), is associated with the removal by erosion of much of the Carboniferous sequence along its axis e.g. (Map 4), (Map 5), (Map 6), (Map7) and (Map 8).

In addition, two north-north-easterly trending linear inversion structures lie in the northern part of the Solway Basin. The Bewcastle Anticline (Figure 32) is essentially an asymmetrical anticline with a steep north-west-facing limb. It is cut obliquely by the Goat Island–Lyne Thrust (Figure 12)c, which dips towards the south-east and downthrows to the north-west. This thrust was formed by reversal of a major Dinantian synsedimentary normal fault, the East Christianbury Fault, which dips and downthrows towards the south-east. The Back Burn Monocline is a similar subparallel structure formed by reverse displacement of the underlying Back Burn Fault (Figure 14), a Dinantian synsedimentary normal fault which dips and throws down to the south-east.

The Brackenhill Fault (Figure 32), (Figure 33) and (Figure 34) is a complex west-dipping Variscan reverse fault which has undergone post-Permo-Triassic normal dip-slip displacement to produce its present day normal throw at outcrop (Figure 12)b. The fault trends north-east to south-west across the basin and forms the eastern margin of the Carlisle Anticline (e.g. (Map 5), (Map 6), (Map7) and (Map 8)). From the seismic reflection data it appears to be terminated, or is cut by, the Lowling Fault. The Brackenhill Fault is associated with several small antithetic faults and synthetic thrust faults too small to represent on the maps. On some of the seismic lines the fault can be interpreted as a 'fishtail' structure (Figure 34), similar to some of the small-scale structures seen in outcrop in north-east Northumberland (Shiells, 1964). The fault cannot be detected on the seismic reflection sections in the deeper parts of the Lower Border Group, where weak anhydrite layers, similar to those found in the Easton Borehole, may have acted as slip planes distributing its throw laterally, and making its position at the base of the Upper Palaeozoic basin fill difficult to resolve.

Many sections across the Carlisle Anticline cannot be structurally balanced. This suggests that across the Brackenhill Fault a significant component of strike-slip movement accompanied the more obvious easterly directed thrusting (Figure 34). However, the sense of strike-slip displacement is not clear from the seismic data. It is possible that this fault could represent a Variscan transfer zone which either crosses the Southern Uplands west of the Cheviot Block or is contiguous with the shear zone described by Shiells (1964) to the south-west of the massif.

The Hilltop Fault, which runs roughly parallel to the Back Burn Fault, and defines the eastern margin of the Canonbie Coalfield (Map 14), was inferred from outcrop distribution (see Day, 1970). Picken (1988) noted that it was not visible on British Coal seismic profiles; this may be because the fault is reversed, and hades to the southeast in the same direction as the adjacent Back Burn reversed fault. A further possibility is that a thin strip of vertical strata belonging to the Stainmore Group separates the coalfield and the vertical Liddesdale Group rocks, negating the requirement for a fault (Day, 1970). The seismic data do not resolve this uncertainty, and the fault is mapped as a small, downthrowing to the west, reversed fault which coalesces at depth with the Back Burn Fault (e.g. (Map 7), (Map 9), (Map 11), (Map 13).

On a more regional scale, partial reversal of the Maryport Fault (cf. Chadwick et al., 1993b) accompanied major inversion of the southern part of the Solway Basin with regional upwarp and erosion. Evidence of this is provided by the significant angular unconformity at the base of Permo-Triassic strata (Figure 12)b; (Map 16). In the south-eastern part of the basin, the Maryport Fault shows reverse displacement at shallow depths, and a considerable net downthrow to the south with Coal Measures faulted against Liddesdale Group strata at the basin margin (Figure 12)b; (Map 11), (Map 13) and (Map 16). This reverse throw passes downwards into a large normal, downto-the-north displacement at greater depths.

Elsewhere in the basin, the main Dinantian syndepositional faults have east-north-easterly trending anticlines in their hanging walls. The Crosby Anticline (Eastwood, 1930) appears to be a rollover into the Maryport Fault which may have been tightened by Variscan compression. A similar structure is the east-north-easterly trending asymmetrical anticline formed against the adjacent Bank End Fault, near the Westnewton Borehole. The seismic data suggest that this fold is mainly a Variscan compressional structure, at least down to the level of the Upper Border Group. In the south-western part of the Solway Basin a low amplitude east-facing monocline is inferred from the seismic data just east of the Westnewton Borehole. Seismic resolution is insufficient to discern whether this structure resulted from the reactivation of an underlying fault.

Northumberland Trough and Cheviot Block

The Northumberland Trough shows less evidence of inversion than the Solway Basin, and lacks structures on the scale of the Solway Syncline and the Carlisle Anticline. The arcuate shape of the Coal Measures outcrop to the north of the Alston Block (Figure 28); (Map 13) indicates regional, though rather gentle, upwarp of the basin. A number of broad, open folds were formed which from field evidence can be inferred to predate the intrusion of the Whin Sill or are pre-Permian (i.e. Variscan) in age. Superimposed upon this broad folding, a number of smaller inversion structures can be related to reversal of discrete faults.

Reversal of the faults on the southern margin of the Northumberland Trough is indicated by a minor reverse fault which splays off the Ninety Fathom Fault (Figure 10). Associated minor anticlines occur along the line of the Ninety Fathom Fault in its hanging-wall block. The distribution of these anticlines is suggestive of dextral transpression rather than true dip-slip compressive motion (Jones et al., 1980).

In the central part of the Northumberland Trough, the important Antonstown–Sweethope fault system (Figure 3) shows signs of minor inversion. At shallow depths the faults oppose each other as a synthetic–antithetic pair and the intervening strata are folded into a gentle anticline of probable compressional or transpressional origin (Figure 35). Similar gentle anticlinal flexuring can be observed in the hanging wall of the en-echelon Stakeford Fault.

Three large Variscan inversion structures occur in north-east Northumberland (Shiells, 1964). Only the most southerly of these asymmetrical anticlines, the north-north-easterly trending, west-facing, Lemmington Anticline (Figure 32), is situated within the region, close to the northern margin of the basin, on the southern flank of the Cheviot Block. None of the seismic profiles crosses this structure. The Lemmington Anticline appears to have formed by reverse movement of the associated Bolton Fault, which probably had a significant early Carboniferous syndepositional normal displacement downthrowing to the east. The Bolton Fault passes south-westwards into the Swindon Fault (Shiells, 1964), a normal fault trending east-north-east, and downthrowing to the north. Locally this fault has complex reversed faulting in its hanging-wall block, indicative of transpressive movements (Figure 36).

Vale of Eden Basin and Alston Block

Variscan inversion structures occur to the south of the Northumberland–Solway Basin, both at the eastern margin of the Vale of Eden Basin and on the Alston Block. The seismic reflection data provide no new evidence relating to these structures. The north-north-westerly trending Pennine Fault (Figure 32) forms the boundary between the Vale of Eden Basin and the Alston Block (Shotton, 1935; Burgess and Holliday, 1979; Arthurton and Wadge, 1981). This has a long complex history spanning the period since at least the early Devonian and the intrusion of the Weardale Granite. During late Carboniferous times, an easterly facing monocline was formed, in association with easterly directed thrusts, reversing an early Carboniferous fault downthrowing to the west. On the Alston Block, the Burtreeford Disturbance (Burgess and Holliday, 1979; Jones et al., 1980; Dunham, 1990) is another east-facing faulted monocline, but this does not appear to overlie a pre-existing fault. In places it has a vertical displacement of about 150 m. To the east of the region, in the offshore area of the Durham Coalfield, the Vane Tempest Structure is a north-north-westerly orientated asymmetrical anticline with a steep south-western flank (Jones et al., 1980).

Stainmore Trough

The Stainmore Trough appears to have been fairly weakly inverted. Minor anticlines and local small reverse faults (Map 9), (Map 11), (Map 13) occur close to the ClosehouseLunedale–Butterknowle Fault in its hanging-wall block, and indicate Variscan compression or transpression. A typical example of these inversion structures is exposed in the Closehouse barytes mine situated near the northern margin of the Stainmore Trough (Burgess and Holliday, 1978; Dunham, 1990). Namurian sequences are thinner over the crests of some of these anticlines, indicative of earlier compressional movements, which were probably synchronous with those in the Solway Basin. A broad upwarp, culminating close to the southern edge of the region, indicates more regional, but very gentle, basin inversion.

Igneous intrusions (Whin Sill Suite)

The quartz-dolerite Whin Sill and associated dykes were intruded towards the end of Variscan deformation, mainly in the eastern part of the region (Randall, 1980a; Francis 1982; Smith 1992). The youngest rocks intruded are Duckmantian in age, and weathered quartz-dolerite pebbles are known from Permian rocks in the Vale of Eden. Isotopic age determinations using the potassium-argon method suggest that the age of the sill complex is around 295 Ma (Fitch and Miller, 1967; Wadge et al., 1972). The intrusions postdate the main phases of Variscan inversion and the formation of the Holborn Anticline, the Pennine Fault and the Burtreeford Disturbance (Shiells, 1964; Jones et al., 1980). However, they are older than the mineralisation of the Alston Block, which affects the Sill itself. Locally it is possible to identify the sills on seismic sections, but there are insufficient supporting borehole data to allow unequivocal mapping of these over large areas. The dykes are not clearly imaged on the seismic profiles.

The origin of the structures

Shiells (1964), Jones et al. (1980) and Bernard et al. (1990) agree that the orientations of most of the inversion structures are consistent with regional east-northeast–west-south-west- or east–west-oriented bulk compression, though a component of simple-shear cannot be ruled out. Thus, pre-existing north-north-easterly or north-north-westerly trending faults underwent strong, predominantly dip-slip reversal, whereas east–west-trending faults experienced dominantly strike-slip displacement with little reversal. This is exemplified by the Solway Basin, where the north-north-easterly trending intrabasin faults were reversed preferentially compared with the east–west-trending basin margin faults. On a smaller scale, orientation-dependent fault reversal is displayed by the Bolton–Swindon Fault System discussed above. The north-north-easterly trending Bolton Fault was strongly reversed, probably with dominantly dip-slip displacements. The east-north-easterly trending Swindon Fault was only weakly reversed, complex hanging-wall faulting being suggestive of oblique reversal and probable dextral transpression (Figure 36).

Where the compressive stress field reactivated obliquely oriented structures, severe localised strike-slip and oblique-slip strain patterns are common. Thus, Jones et al. (1980) have suggested that subsidiary folding along many of the easterly or east-north-easterly trending faults, such as the Hauxley, Stakeford, Ninety Fathom and Butterknowle faults (Figure 3), would be consistent with a dextral shift.

According to Bernard et al. (1990), there is evidence from surface exposures of a separate, later Variscan, compressive phase orientated north–south, contempora neous with the emplacement of the Variscan Front thrusts in southern England. North–south compression in the region is indicated by partial reversal of the major east–west-trending basin bounding faults. However, with the exception of parts of the Maryport Fault ((Figure 12)b; Chadwick et al., 1993b), reversal of these structures was very minor, indicating that north–south compression was subsidiary to the dominant east–west shortening. Even where reversal of the Maryport Fault can be demonstrated, this lies close to a probable left-stepping offset of the fault zone caused by the Pennine Fault (Map 11), where localised uplift would also be expected during east–west compression.

It is important to emphasise that where there is now no post-Carboniferous cover, the Variscan age of the reversed basin margin structures cannot always be proved conclusively, and they may be partially or even wholly of younger age; for example, they may be related to Alpine (Cenozoic) basin inversion.

The age of the Whin Sill and its associated dykes, relative to the later north–south orientated phase of inversion, is unclear. Nor is it generally agreed whether the intrusions occurred during a time of compression, or whether they are indicative of a separate phase of crustal tension. Holmes and Harwood (1928) and Shiells (1964) suggested that the Holy Island and High Green dyke echelons, were intruded into shear zones to the north and south of the Cheviot Block respectively, towards the end of the east–west compressional movements. This allowed the magma to be emplaced into tension gashes within the shear zones and to form the dyke echelons. The orientation of these dykes indicates dextral displacement south of the Cheviot Block on the High Green dyke echelon, which probably followed the line of the Swindon and Cragend–Chartners fault complex (Figure 32). The orientation of dykes within the St Oswald's Chapel Dyke echelon is also indicative of dextral movement. An alternative view is that intrusion of the Whin Sill and the dyke echelons occurred during a post-compression, but pre-Permian, period of tension which was orientated between north-west–south-east and northnorth-west–south-south-east (Anderson, 1951).

Chapter 6 Post-Variscan events

A detailed account of the post-Carboniferous rocks of the region (Figure 37) and (Figure 38) is beyond the scope of this present study. They are the remnants of a once more extensive cover which was uplifted and eroded during Cenozoic times. The Permian and Triassic red-bed rocks of the Durham Coalfield and of the Dumfries, Carlisle and Vale of Eden basins overlie a locally complex sub-crop of Carboniferous rocks (Map 15) and (Map 16). The relatively smooth, peneplained basal Permian surface imaged on seismic reflection profiles in the Carlisle Basin (Figure 27) and (Figure 33), indicates a long period of erosion following the Variscan movements. The Permo-Triassic rocks in the east of the region have been reviewed by D B Smith (1980; 1994), and are not considered further (Figure 38). Those in the west of the region, the Carlisle Basin and Vale of Eden, were reviewed by Arthurton et al. (1978) (see also Jackson et al., 1987). The Lower Jurassic rocks west of Carlisle are described by IvimeyCook et al. (in preparation). The thickness and extent of any younger Mesozoic rocks, formerly present but now eroded away, is unknown. Apatite fission track studies have suggested that this former cover may have been in excess of 2000 m thick in some parts of the region (Green, 1986; Lewis et al., 1992).

Permian and early Mesozoic sedimentation

Deposition is thought to have recommenced during Permian times principally in three areas; the Carlisle Basin (which for the most part unconformably overlies the older Carboniferous rocks of the Solway Basin), the Vale of Eden Basin and the Dumfries Basin. The seismic reflection data indicate that the Vale of Eden and Carlisle depocentres were not connected at this time, or that they were linked only by a thin veneer of strata, and that the Dumfries and Carlisle basins were not connected onshore, but could have been contiguous offshore. The first rocks to be deposited in these areas were those of the Permian Appleby Group, and comprise aeolian and fluvial sandstones of the Penrith Sandstone, and associated basin-margin continental breccias (Brockram) (Figure 37). They are believed to be up to about 400 m thick in the Vale of Eden. The Penrith Sandstone does not crop out in the Carlisle Basin, being overlapped by the upper part of the St Bees Shales. It has been proved in the Silloth No. 1 Borehole, close to the onshore Carlisle Basin depocentre, where it is 379 m thick (Figure 38).

The Penrith Sandstone is succeeded diachronously by the Cumbrian Coast Group comprising the Eden Shales and the partially equivalent St Bees Shales. These are up to 200 m thick in the Vale of Eden, but are much reduced in the Carlisle Basin. The St Bees Shales are less than 50 m thick in the Silloth No. 1 Borehole, where the equivalents of most of the Eden Shales are of fluvial Penrith Sandstone facies (Figure 37) and (Figure 38). The shales are succeeded and overlapped by the mainly fluvial Sherwood Sandstone Group, which is up to 500 m thick. A more-or-less uniform sedimentary environment was established over much of the region at this time. Younger Triassic strata, (the Mercia Mudstone and Penarth groups) and the marine strata of the early Jurassic Lias Group are restricted in occurrence to the central parts of the Carlisle Basin.

Permian subsidence in the United Kingdom and adjacent continental shelf is thought to be a consequence of continental rifting (Holloway, 1985; Glennie, 1986). However, the only fault in the region which may show evidence of Permian syndepositional displacement on the seismic reflection profiles is that to the east of the Silloth No. 1 Borehole. Elsewhere, there is as yet no conclusive evidence of synsedimentary faulting in the onshore part of the Carlisle Basin during deposition of the Penrith Sandstone. On the seismic profiles, the Penrith Sandstone can be seen simply to pinch out up dip to the north, east and south of the Carlisle Basin depocentre, the Southern Uplands and the Lake District blocks acting as structural highs and probably remaining subaerial. The Permo-Triassic rocks of the Vale of Eden are believed to form a half-graben controlled by the Pennine Fault, which downthrows to the west (Burgess and Holliday, 1979; Arthurton and Wadge, 1981), but there is no seismic reflection or borehole information to confirm the equivocal outcrop evidence.

Similarly, there is little evidence from the seismic reflection profiles for the nature of the structural control on late Permian to early Jurassic sedimentation in the region. It is likely that the Southern Uplands and Lake District blocks remained as structurally high areas, initially subaerially exposed, but from early to mid-Triassic times onwards receiving variable amounts of sediment. No unequivocal local evidence of syndepositional faulting can be observed, and the general impression is of a gradual expansion of the depositional area consistent with a period of postextensional regional subsidence. However, in the offshore extension of the Carlisle Basin, to the west of the region, evidence of Mesozoic synsedimentary displacement on the continuation of the Maryport Fault has been presented by Chadwick et al. (1993b), and evidence of syndepositional faulting has been reported in the East Irish Sea Basin by Jackson et al. (1987).

Post-early Jurassic faulting

Evidence of faulting, which probably postdates the early Jurassic, Lias Group, is widespread in the Carlisle Basin (Map 15). Dating of these displacements is problematical because of the limited age range of the cover rocks and their present restricted areal extent.

Several north-westerly trending faults occur in the area west of the Brackenhill Thrust, particularly in the south-west of the Carlisle Basin (Map 15). A north-westerly trending fault also defines the western margin of the Lias Group outlier (BGS, 1992) that occurs in the centre of the Carlisle Basin, indicating a post-early Jurassic age for these faults. The seismic reflection profiles and recently acquired confidential boreholes suggest that the mapping of the Liassic outlier requires considerable revision from that shown on the Carlisle 1:50 000 Geological Sheet 17 and described by Dixon et al. (1926).

Some of the north-westerly trending post-early Jurassic faults appear to swing towards an east–west trend in the south of the basin, approximately parallel to the basin-bounding faults of the Carboniferous Solway Basin, some of which, e.g. the Maryport and Bank End faults, and their antithetic structures, e.g. the Lowling Fault, have been reactivated in post-Triassic, presumably post-early Jurassic times (cf. Chadwick et al., 1993b). These faults form an easterly to east-north-easterly trending group.

Another group of faults to show post-Triassic (probably post-early Jurassic) movement is the Brackenhill Thrust and its associated minor antithetic faults, which trend approximately north-east. Down-to-the-north-west normal movement (backslip) took place on the Brackenhill Thrust (Figure 34), and associated down-to-the-south-east normal movement occurred on its antithetic structures.

The fact that the majority of the north-westerly trending extensional faults lie to the west of the Brackenhill Fault suggests that this structure could have acted as a transfer fault, allowing post-early Jurassic extension to have been confined to or concentrated within the Carlisle–Solway area rather than across the Northumberland–Solway Basin as a whole. The presence or absence of faults which postdate the early Jurassic cannot be proved in most of the Carboniferous rocks of the Northumberland Trough, due to the restricted extent of younger cover rocks. Post-Variscan normal faulting occurred along the Stublick–Ninety Fathom and Butterknowle faults, which downthrow Permo-Triassic rocks to the north and south respectively, in the same sense as the early Carboniferous basin-controlling normal faults.

Cenozoic basin inversion and uplift

Minor inversion of the Carlisle Basin may have occurred in early Cenozoic times and again in mid-Cenozoic times, the latter associated with Alpine compression (Chadwick, 1985; Chadwick et al, 1993b). Evidence for this is equivocal, however, as such movements are commonly difficult to distinguish from the earlier Variscan compression. Faulting in early Cenozoic times is indicated by the intrusion of east-south-easterly trending tholeiitic dyke echelons (Holmes and Harwood, 1929; Randall, 1980b). According to Robson and Smith (1980) these intrusions formed at a time of north-south compression and are accompanied by sinistral strike-slip displacements. Inferred deformation of the Base Permo-Triassic reflector near the Brackenhill Fault and, to a lesser extent the Backburn Fault (Figure 33) and (Figure 34), may be due to Alpine compression or transpression.

Eastward tilting and uplift of the region occurred in Cenozoic times, as a consequence of thermal doming associated with the opening of the North Atlantic Ocean to the west and subsidence of the North Sea Basin to the east (Chadwick, 1985). This tilting probably continues to the present day and may be associated with regional uplift, commonly localised along pre-existing faults. This is impressively displayed at the northern margin of the Lake District and at the northern, southern and western margins of the Alston Block where the Pennine Fault has a present-day topographical fault scarp which locally exceeds 500 m. Considerable uplift of the Lake District can also be inferred from the probable thickness of Permian and Mesozoic rocks (c.1500 m) believed to have formerly extended over the area prior to Cenozoic erosion (Trotter, 1929; Taylor et al., 1971; Holliday, 1993). Uplift of similar magnitude is supported by apatite fission track studies (Green, 1986; Lewis et al., 1992). Other, apparently young, fault scarps seen within and bounding the Cheviot Hills also offer evidence of Cenozoic vertical movements (Robson and Smith, 1980).

Chapter 7 Economic geology

The presence of coal throughout much of the Carboniferous sequence, together with significant occurrences of ironstone, galena, sphalerite, barytes and evaporite minerals, has been both a major stimulus to the economy and prosperity of the region and an important spur to geological research and exploration.

In recent years the acquisition of seismic reflection profiles and the drilling of several deep hydrocarbon exploration wells has provided new insights into the geology of the region, and thrown new light on its economic potential. These aspects of the subsurface geology are discussed in this chapter. More detailed local accounts of the economic geology of the region can be found in the published Geological Survey memoirs (Appendix 2).

Coal

Despite the worldwide fame of the region as a producer of coal, there is now only limited extraction, and working is restricted to some opencast operations, and a few private mines. Most of the coal mined in the past came from the Westphalian Coal Measures, though numerous seams in older Carboniferous rocks once supported local industries. The long-established coalfields of Northumberland, Durham and West Cumbria are now close to exhaustion, but the possibility of unexploited areas of Coal Measures, largely concealed by Permo-Triassic rocks, in the Vale of Eden (Arthurton and Wadge, 1981) and around the Solway Firth to the south of the Canonbie Coalfield (Picken, 1988) has been recognised for some time (Map 16). No further light is shed upon the coal potential of the Vale of Eden by this study, but much new information is now to hand concerning the Carlisle–Solway area.

The sub-Permian geological map of Smith (1985) shows a synclinal area of Coal Measures in the Solway Basin, projecting some 20 km south of Canonbie, beneath Permo-Triassic cover. Picken (1988) suggested that the Coal Measures might have only a limited continuation south of Canonbie, although commercial deep seismic data traded by British Coal, at that time at a preliminary state of interpretation, indicated that the Westphalian subcrop could be much more extensive. In the light of the new work, the Coal Measures are now thought to extend considerably farther to the south-west in the core of the Solway Syncline (Map 13) and (Map 14), and a concealed subcrop of Westphalian strata probably connects the Canonbie and West Cumbrian coalfields (Map 16)

In view of Picken's (1988) suggestion that economically exploitable coal deposits still remain in the Canonbie Coalfield, the potential of this much larger area of subcrop could be very great, although there are a num ber of mitigating factors. It will probably not be possible to exploit all of the subcrop area because of the thick cover of red Upper Coal Measures and Permo-Triassic strata. Exploratory boreholes just north of the Lake District (Eastwood, 1930) proved few thick coals in the beds beneath the Permo-Triassic rocks, suggesting that coal quality and quantity reduces northwards in this area. The extent of sub-Permo-Triassic reddening and weathering of the Coal Measures, and the effects of these on the coal seams, is another unknown factor. Nevertheless, large areas of Lower and Middle Coal Measures, probably with workable bituminous coals, appear to be present at depths shallower than the working limit of c.1200 m. Additional drilling beneath the Permo-Triassic cover to the west and north-west of Carlisle is required if the full potential of these coal-bearing rocks is to be assessed.

Further study of the Vale of Eden is required to determine the full extent and potential of Coal Measures strata in that area. Westphalian rocks have been proved at outcrop and in boreholes but details are sparse (Arthurton and Wadge, 1981). Few coal seams have been proved and the strata are extensively reddened. A substantial area of Coal Measures subcrop has been inferred, partially from the results of the analysis of gravity data (Collar in Arthurton and Wadge, 1981; Smith, 1985). However, the gravity analysis is far from conclusive and other confirmatory work is required. In particular, some seismic reflection data should be acquired, followed, if justified, by exploratory drilling.

Hydrocarbons

The hydrocarbon prospectivity of the Carboniferous rocks of northern England has been studied in some detail by Fraser et al. (1990). Scott and Colter (1987) have published a more comprehensive discussion of prospects within the Northumberland–Solway Basin. Both of these studies were based essentially on data available prior to the main phase of hydrocarbon exploration in the region (1984–1991).

The close association and alternation of shallow marine and fluviodeltaic deposits, in a basin of extensional origin, commonly provides a suitable environment for the generation and entrapment of hydrocarbons. Oil shows have been recorded in several boreholes around the Canonbie Coalfield and near the northern margin of the Solway Basin, including the Archerbeck (Lumsden and Wilson, 1961) and Becklees boreholes. Gas occurs commonly in mine workings throughout the region, and has been encountered in small quantities in many boreholes. Bitumen has been found at a number of localities, and is locally associated with mineral veins (Creaney et al., 1980; Dunham, 1990). These observations confirm that, at least locally, both oil and gas have been generated within the region.

Because there are so few deep boreholes, and with so much of the thick, early synextensional sequence poorly understood, a full evaluation of the source rock potential of the region has not yet been carried out. Many of the argillaceous rocks are highly carbonaceous; coals are prominent in the sequence from the level of the Upper Border Group upwards. Beds of oil shale occur locally in both the Border and Liddesdale groups, and thick marine mudstones occur sporadically in all parts of the succession with the exception of the Coal Measures.

Total Organic Carbon (TOC) analyses have been carried out on samples from a number of British Geological Survey and other boreholes (e.g. Archerbeck, Stonehaugh, Ferneyrigg, Throckley, Allenheads No.1 and Roddymoor), revealing values of organic carbon commonly in excess of 1 per cent and, in several cases, greater than 5 per cent. Organic geochemical studies by a number of workers have shown that the organic matter is dominantly gas prone. Scattered occurrences of sapropelic or oil-prone, algal-rich rocks have been detected in samples from many of the boreholes and also at outcrop, but as yet there is little evidence to indicate either their extent or volume. Thus, whilst the geochemical data clearly demonstrate the substantial gas-generating potential of the region, its ability to source large quantities of liquid hydrocarbons remains to be proved.

Samples from many boreholes show a steady increase in maturity with depth, except where the sequence has been intruded by the Whin Sill, i.e. on the Alston Block and in the eastern and central parts of the Northumberland Trough. On the Alston Block, the rocks are largely mature to over-mature as a result of the combined effects of the high regional heatflow, associated with the basement high heat production (HHP) granite, and the thermal metamorphism resulting from the injection of the Whin Sill (Ridd et al., 1970; Creaney, 1980). In the basins, most of the strata examined are in the oil-generating window (Creaney et al., 1980; Scott and Colter, 1987; Burnett, 1987). However, in the deeper parts of some boreholes and more locally at outcrop, or where the rocks have been influenced by the Whin Sill, mature or overmature values have been recorded in basin strata (Jones and Creaney, 1977). There is strong evidence in most parts of the region that present day levels of organic maturity are close to those attained at about the time of the Variscan deformation, following the intrusion of the Whin Sill (Creaney, 1980; Scott and Colter, 1987), and that the peak period of hydrocarbon generation was in late Westphalian times. In much of the region, the depth of post-Variscan erosion was greater than the thickness of the subsequent cover of Permian and Mesozoic rocks, suggesting that there has not been any significant hydrocarbon generation since Variscan times. The main area where Carboniferous rocks are likely to have been deeper in Mesozoic (end-Cretaceous) times, than at the end of the Carboniferous, is in the Solway Syncline. This area was also probably unaffected by the intrusion of the Whin Sill. Unfortunately, even in the Sol way Syncline, burial was probably insufficient to have brought the Coal Measures into the gas-generating zone, though deeper sources of oil or gas could have been rejuvenated. In eastern coastal areas, there may also have been a relatively thick younger cover, but so great were the thermal effects of the Whin Sill intrusion, that all subsequent oil-generating potential was effectively eliminated (Ridd et al., 1970; Jones and Creaney, 1977).

Although, the reservoir characteristics of Carboniferous rocks in Britain are in general poor (Holliday, 1986), the experience of the East Midlands oilfields shows that porosity and permeability values in some sandstones are locally adequate for hydrocarbon entrapment (Fraser et al., 1990). The late Namurian to early Westphalian sandstones of the region have many similar characteristics to the reservoir rocks of the same age in the East Midlands, and it seems probable that porosity and permeability values of up to 15 per cent and 50 mD respectively may be found in the Silesian sandstones of the region. It is unfortunate, therefore, that these rocks have been preserved in only relatively small areas. Even more local in occurrence are the red sandstones of the Upper Coal Measures found in the Canonbie Coalfield and the Solway Syncline. The Fell Sandstones and the Middle Border Group are the only sandstone-dominated divisions of the Dinantian. These have excellent reservoir properties near to outcrop, but the results of the Longhorsley Borehole indicate that at depth the sandstones are very tight and compact, with porosity values less than 5 per cent. Similar, generally poor, reservoir characteristics were also encountered in the Middle Border Group of the Stonehaugh Borehole, although here a few levels between 400 and 600 m exhibited better porosity and permeability, with values of 10–15 per cent and 10–80 mD (Cradock-Hartopp and Holliday, 1984). Elsewhere in the sequence the occurrence and distribution of sandstones is locally variable, with the best reservoir characteristics being found in channel sandstones, which are difficult to predict and locate in the subsurface. Therefore, from present information, it must be concluded that there is a lack of well-established, easily predictable, reservoir rocks of Carboniferous age within the region.

A number of closed anticlinal structures occur in the basins and at their margins. Several of these have been drilled with disappointing results. The structures are mostly Variscan in age, with some later modification, generally in the form of extensional faulting. Thus, for the most part, the structures probably formed during Variscan basin inversion and, therefore, just postdate the main period of hydrocarbon generation in the region. In a few instances the folds were initiated during sedimentation and are in part earlier than the end-Carboniferous deformation. Thus, pre-Variscan hydrocarbon accumulations may well have formed in synsedimentary folds, but whether they could have survived the subsequent phases of deformation is uncertain. The only area where post-Variscan generation could be expected is in the Solway Basin, where younger cover rocks are thickest, and where a number of major Variscan structures occur.

While it seems likely that the Carboniferous argillaceous beds could adequately seal most of the closed structures, this remains unproved. The effects of the numerous faults crossing many of these closures is also unclear.

Overall, prospects of finding economic hydrocarbons in the region are not encouraging. The apparently largely gas-prone basin-fill and the generally poor reservoir characteristics are particularly disappointing. Hydrocarbons, including some sizeable quantities of oil, have probably been generated here, but much was likely to have been lost during the deformation and uplift associated with the Variscan Orogeny and the intrusion of the Whin Sill. For hydrocarbons generated during Carboniferous times to have survived to the present day, their trapping structures must have remained sealed throughout the Variscan deformation and subsequent periods of tectonism. Only in the Solway Basin does it seem possible that significant renewed hydrocarbon generation in Carboniferous rocks could have taken place in Mesozoic times. Any closures on the flanks of the Solway Syncline could provide attractive gas prospects. More generally, in adjacent offshore areas, where the Permo-Triassic rocks are thicker, prospects may improve, particularly under the Solway Firth. In the east, the presence of the Whin Sill, and its attendant thermal effects, restrict the likelihood of any post-Variscan hydrocarbon generation off the Northumberland coast.

Coal-bed methane

In contrast to the above slightly pessimistic assessment of conventional hydrocarbon resources, the region has potential for the production of coal-bed methane, that is adsorbed methane which can be produced directly from coal seams, rather than free methane produced from porosity in conventional reservoir rocks (Glover et al., 1993).

The Northumberland and Durham coalfields are largely worked out onshore. The coals have a relatively low methane content (mean CH4 = 1.3 m³ tonne⁻¹) (Greedy, 1991), and resources are likely to be restricted in amount. A similar assessment can be made of the largely exhausted West Cumbrian Coalfield although methane levels are relatively high there; 7.5 m³ tonne⁻¹ (Greedy, 1986).

The Canonbie Coalfield is known to produce high methane yields compared with many other British coal-bearing areas (mean CH4 = 6.3 m³ tonne^-−1) (Greedy, 1991). The area of the coalfield south of the Rowanburn Fault has not been mined, and the same sequence of thick coals that occurs in the Middle Coal Measures of the mined area has been proved over at least part of the downdip continuation (Picken, 1988). Additionally, coal seams occur in the Lower Goal Measures, which are particularly well developed in the deeper, central parts of the concealed coalfield. As the Coal Measures are thought to be continuous beneath the Solway Firth, between the Canonbie and Cumberland coalfields and into the Vale of Eden (Map 16), they may represent a considerable, as yet unexploited, coal-bed methane resource. However, no grey, coal-bearing measures have yet been proved in the central part of the Vale of Eden, north of the Stainmore Coalfield. The Coal Measures outliers along the line of the Stublick Fault have little coal-bed methane potential as they are shallowly buried and have been extensively mined. Further prospects could also exist in the older coal-bearing rocks of the Stainmore, Liddesdale, Alston, Upper Border and Scremerston Coal groups.

Geothermal energy

Many parts of the region, notably the Lake District and the Alston blocks, are areas with heat-flow values and geothermal gradients significantly in excess of the respective national averages (Rollin, 1987). Because of this, the region has figured prominently in UK geothermal energy assessment programmes (Downing and Gray, 1986; British Geological Survey, 1988). Two main methods of heat extraction from the rocks of the region have been considered, 'hot dry rock' and low-enthalpy' systems.

In a hot dry rock system, cold water from the surface is pumped down a borehole into hot, impermeable but fractured rocks at depth. There, it is warmed and then, through a second borehole, pumped to the surface where its newly acquired heat can be used for space heating or, if the temperature exceeds 200°C, for direct electricity generation. The radiogenic high-heatproduction Shap, Skiddaw and, most particularly, Weardale granites have been the main targets within the region for this kind of system. Full reviews of these studies are given elsewhere (Lee, 1986; Evans et al., 1988). The present work provides no new relevant details.

Low-enthalpy systems aim to extract directly the hot groundwater held in the pore spaces in deep sedimentary aquifers. The review of Holliday (1986) concluded that Carboniferous and older rocks are generally impermeable at the necessary depths (about 2000 m) and have little potential as a source of low-enthalpy energy. There are some possible local exceptions, such as the Middle Border and Fell Sandstone groups of Northumberland, which because of their proven potential as shallow aquifers have been considered as a possible source of geothermal energy (Cradock-Hartopp and Holliday, 1984). The depths and thicknesses of these beds are now much better constrained (Map 6) and (Map 7), but new data, from the Longhorsley and Easton boreholes, has shown that the porosity of the sandstones at depth is generally very low (about 5 per cent). As noted previously, in the discussion of potential hydrocarbon reservoir rocks, the Carboniferous strata of the region are essentially tight and compact, and the sandstone bodies discontinuous and lenticular. It must be concluded that they have little foreseeable potential as a source of low-enthalpy geothermal energy.

Underground storage/disposal of fluids

Storage or disposal of fluids, such as toxic waste or carbon dioxide, in closed and sealed structures formed in deep aquifers, has been considered in a number of areas of the country. The Seal Sands Borehole, the deepest borehole in onshore Britain, located in the Stainmore Trough a few kilometres beyond the south-eastern margin of the region, was drilled for such a purpose. The potential of the region for this kind of use is extremely poor in view of the limited number of closures so far demonstrated, their restricted extent, and, as detailed above, the generally poor porosity and permeability characteristics of the putative host rocks.

Evaporites

The discovery of numerous beds of anhydrite in the Easton Borehole suggests that evaporitic rocks of early Dinantian age are common below the level of the Bewcastle Beds (Lower Border Group) throughout much of the Northumberland Trough. Indeed, their presence may have contributed to the structural complexity of the Bewcastle Anticline and the other inversion structures of the Solway Basin. It is unlikely that these evaporitic beds will form a significant economic resource in the foreseeable future, particularly while so little is known of their nature and extent. However, there are a number of interesting possibilities which might lead to future exploitation of these rocks.

Where the beds of anhydrite come close to the surface, as in the Bewcastle Anticline, they have been dissolved out. However, within a few hundred metres of the surface, there is probably a zone in which the anhydrite is being converted to more valuable gypsum by contact with groundwater. There is currently no information with which to quantify or evaluate any such occurrences.

Gypsum-anhydrite rocks of similar, early Carboniferous, age are known in several. other areas of the British Isles. More saline deposits, including halite or potash minerals have not been discovered, but are found in economic quantities in the Maritime Provinces of eastern Canada which, prior to the opening of the Atlantic Ocean, was once geographically much closer (Bradley, 1982). It is doubtful whether any such rocks would be at mineable depths in Cumbria or Northumberland, but their presence would be of considerable significance, particularly as a putative source of mineral-bearing fluids.

Mineral deposits

Full details of the mineral deposits of the Alston Block and of parts of the Northumberland Trough are given by Dunham (1990). The deposits of the Lake District have been reviewed by Firman (1978b).

The main worked mineral deposits within the Carboniferous rocks of the region are principally on the Alston Block (F W Smith, 1980; Dunham, 1990), with only localised occurrences being found to the north of the Lake District (Cooper et al., 1991, 1992) and in the Northumberland Trough (Smith, 1923; Bateson et al., 1983). They are essentially vein infills, locally with associated replacement bodies, mainly late Carboniferous to early Permian in age, and belong to the Pennine-style of mineralisation (Plant and Jones, 1989). They formed during a period of tensional crustal stress after the intrusion of the Whin Sill, prior to or contemporaneously with early Permian rifting. The mineralising fluids appear to have been saline brines derived from the de-watering of sediments in the adjacent Carboniferous basins (Sawkins, 1966; Solomon et al., 1971; Dunham, 1988). Therefore, the discovery, in the Easton Borehole, that a significant proportion of the early synextensional rocks in the Northumberland Trough are evaporites, is potentially of some significance in the understanding of northern England metallogenesis. It seems likely that these rocks could prove to have been one of the principal sources of the mineralising fluids.

Much of the mineralisation in central Ireland, which occurs in the westerly continuation of the Northumberland–Solway Basin, is of a different type–the so-called Irish-style of mineralisation (Andrew et al., 1986; Plant and Jones, 1989). These deposits are of syngenetic origin, being deposited during the early synextensional stage of basin evolution. Syndepositional faults acted as channels for the mineralising fluids and largely controlled the location of the deposits. No mineral occurrences of this kind are known within the region. The structure contour and isopach maps ((Map 1), (Map 2), (Map 3), (Map 4), (Map 5), (Map 6), (Map 7), (Map 8), (Map 9), (Map 10), (Map 11), (Map 12), (Map 13), (Map 14), (Map 15), (Map 16) ) should allow exploration targets to be located with more precision. In Ireland, a number of important deposits are thought to be located where the main syndepositional fault-trend is cut or offset by oblique faults. A similar situation occurs in a number of areas of the Solway Basin where north-easterly or north-north-easterly trending, syndepositional, possibly transfer faults are thought to intersect the main basin-bounding, easterly or east-north-easterly trending structures. The possibility of Irish-style mineralisation being present hereabouts is strengthened by the deposition of evaporites at the likely period of mineralisation. Evaporitic brines provide a powerful mechanism for the dissolution and transport of metals such as Pb, Zn and Cu. For these reasons, further exploration of the margins of the Solway Basin for Irish-style mineral deposits is justified.

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.

ANDERSON, F W. 1951. The dynamics of faulting and dyke formation with applications to Britain. (Edinburgh: Oliver and Boyd.)

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.

ANDREW, C J, CROWE, R W A, FINLAY, S, PENNELL, W M, and PYNE, J F (editors). 1986. Geology and genesis of mineral deposits in Ireland. (Dublin: Irish Association for Economic Geology.)

ARMSTRONG, H A, and PURNELL, M A. 1987. Dinantian conodont biostratigraphy of the Northumberland Trough. Journal of Micropalaeontology, Vol. 6, 97–112.

ARTHURTON, R S. 1984. The Ribblesdale fold belt, NW England-a Dinantian-early Namurian dextral shear zone. 131–138 in Variscan tectonics of the North Atlantic region. HUTTON, D H W, and SANDERSON, D J (editors). Special Publication of the Geological Society of London, Vol. 14.

ARTHURTON, R S. BURGESS, I C, and HOLLIDAY, D W. 1978. Chapter 13 Permian and Triassic. 189–206 in The geology of the Lake District. MOSELEY, F (editor). (Leeds: Yorkshire Geological Society.)

ARTHURTON, R S. and WADGE, A J. 1981. Geology of the country around Penrith. Memoir of the Geological Survey of Great Britain, Sheet 24 (England and Wales).

AVELINE, W T, and HUGHES, T McK. 1888. The geology of the country around Kendal, Sedbergh, Bowness and Tebay (2nd edition, revised STRAHAN, A) (revisor). Memoir of the Geological Survey of Great Britain.

BAMFORD, D, NUNN, K, PRODEHL, C, and JACOB, B. 1971. Crustal structure of Northern Britain. Geophysical Journal of the Royal Astronomical Society, Vol. 54, 43–60.

BARNES, R P, YOUNG, B, FROST, D V, and LAND, D H. 1988. Geology of Workington and Maryport. British Geological Survey Technical Report, No. WA/88/3.

BARRETT, P A. 1988. Early Carboniferous of the Solway Basin: a tectonostratigraphic model and its bearing on hydrocarbon potential. Marine and Petroleum Geology, Vol. 5, 271–281.

BATESON, J H, EVANS, A D, and JOHNSON, C C. 1983. Mineral reconnaissance in the Northumberland Trough. Mineral Reconnaissance Report, Institute of Geological Sciences, No. 62.

BEAMISH, D. 1986. Deep crustal geoelectric structure beneath the Northumberland Basin. Geophysical Journal of the Royal Astronomical Society, Vol. 84, 619–640.

BEAMISH, D. and SMYFHE, D K 1986. Geophysical images of the deep crust: the Iapetus Suture. Journal of the Geological Society of London, Vol. 143, 489–497.

BERNARD, F, MASCLE, A, LE GALL, B, DOLIGEZ, B, and Rossi, T. 1990. Palaeo-stress fields in the Variscan foreland during the Carboniferous from microstructural analysis in the British Isles. Tectonophysics, Vol. 177, 1–13.

BESLY, B M. 1988. Palaeogeographic implications of late Westphalian to early Permian red-beds, central England. 201–221 in Sedimentation in a synorogenic basin complex-the Upper Carboniferous of northwest Europe. BESLY, B M, and KELLING, G (editors). (Glasgow: Blackie.)

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

BOTT, M H P. 1961b. Geological interpretation of magnetic anomalies over the Askrigg Block. Quarterly Journal of the Geological Society, London, Vol. 117, 481–495.

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. 1974. The geological interpretation of a gravity survey of the English Lake District and Vale of Eden. Journal of the Geological Society of London, Vol. 130, 309–331.

BOTT, M H P. and JOHNSON, G A L. 1967. The controlling mechanism of Carboniferous cyclic sedimentation. Quarterly Journal of the Geological Society of London, Vol. 122, 421–442.

BOTT, M H P. LONG, R E, GREEN, A S P, LEWIS, A H J, SINHA, M C, and STEVENSON, D I. 1985. Crustal structure south of the Iapetus suture beneath northern England. Nature, London, Vol. 314, 724–727.

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

BOTT, M H P. and MASSON SMITH, D. 1957b. Interpretation of a vertical field magnetic survey in north-east England. Quarterly Journal of the Geological Society of London, Vol. 113, 119–136.

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

BRADLEY, D C. 1982. Subsidence in Late Paleozoic basins in the northeastern Appalachians. Tectonics, Vol. 1, 107–123.

BRENNER, R L, and MARTINSEN, O J. 1990. The Fossil Sandstone-a shallow marine sand wave complex in the Namurian of Cumbria and North Yorkshire, England. Proceedings of the Yorkshire Geological Society, Vol. 48, 149–162.

BRITISH GEOLOGICAL SURVEY. 1988. Geothermal energy in the United Kingdom: review of the British Geological Survey's Programme 1984–1987. (Keyworth, Nottingham: British Geological Survey.)

BRITISH GEOLOGICAL SURVEY. 1991. Geological map of the United Kingdom, Ireland and the adjacent continental shelf: South. Solid 1:1 000 000. (Keyworth, Nottingham: British Geological Survey.)

BRITISH GEOLOGICAL SURVEY. In press. Regional geochemical atlas: Lake District. (Keyworth, Nottingham: British Geological Survey.)

BROWN, G C, FRANCIS, E H, KENNAN, P, and STILLMAN, C J. 1985. Caledonian igneous rocks of Britain and Ireland. 1–15 in The nature and timing of orogenic activity in the Caledonian rocks of the British Isles. HARRIS, A L (editor). Memoir of the Geological Society, London, No. 9.

BURGESS, I C, and HARRISON, R K. 1967. Carboniferous Basement Beds in the Roman Fell district, Westmorland. Proceedings of the Yorkshire Geological Society, Vol. 36, 203–225.

BURGESS, I C HOLLIDAY, D W. 1979. Geology of the country around Brough-under-Stainmore. Memoir of the Geological Survey of Great Britain, Sheet No. 31 (England and Wales).

BURGESS, I C MITCHELL, M. 1976. Visean lower Yoredale limestones on the Alston and Askrigg blocks, and the base of the D2 Zone in northern England. Proceedings of the Yorkshire Geological Society, Vol. 40, 613–630.

BURNETT, R D. 1987. Regional maturation patterns for late Visean (Carboniferous, Dinantian) rocks of northern England based on mapping of conodont colour. Irish Journal of Earth Sciences, Vol. 8, 165–185.

CAPEWELL, J G. 1955. The post-Silurian pre-marine Carboniferous sedimentary rocks of the eastern side of the English Lake District. Quarterly Journal of the Geological Society of London, Vol. 111, 23–46.

CARRUTHERS, R G, BURNETT, G A, and ANDERSON, W. 1930. The geology of the Alnwick district. Memoir of the Geological Survey of Great Britain, Sheet 6 (England and Wales).

CARRUTHERS, R G, BURNETT, G A, and ANDERSON, W. 1932. The geology of the Cheviot Hills. Memoir of the Geological Survey of Great Britain, Sheets 3 and 5 (England and Wales).

CHADWICK, R A. 1985. Cenozoic sedimentation, subsidence and tectonic inversion. 61–63 in Atlas of onshore sedimentary basins in England and Wales. WHITTAKER, A (editor). (Glasgow: Blackie and Son Ltd.)

CHADWICK, R A. EVANS, D J, and HOLLIDAY, D W. 1993b. The Maryport Fault: the post-Caledonian tectonic history of southern Britain in microcosm. Journal of the Geological Society of London, Vol. 150, 247–250.

CHADWICK, R A. HOLLIDAY, D W. 1991. Deep crustal structure and Carboniferous basin development within the Iapetus Convergence Zone, northern England. Journal of the Geological Society of London, Vol. 148, 41–53.

CHADWICK, R A. HOLLOWAY, S, and HULBERT, A G. 1993a. The evolution and hydrocarbon potential of the Northumberland-Solway Basin. 717–726 in Geology of Northwest Europe: Proceedings of the 4th Conference, PARKER, j R (editor).

CLOUGH, C T. 1888. The geology of the Cheviot Hills. Memoir of the Geological Survey of Great Britain.

CLOUGH, C T. 1889. The geology of Plashetts and Kielder. Memoir of the Geological Survey of Great Britain.

COCKS, L R M, and FORTEY, R A. 1982. Faunal evidence for oceanic separations in the Palaeozoic of Britain. Journal of the Geological Society of London, Vol. 139, 465–478.

COLLIER, R E Lt. 1989a. Modelling the role of differential compaction and tectonics upon Westphalian facies architecture in the Northumberland Basin. 189–199 in The role of tectonics in Devonian and Carboniferous sedimentation in the British Isles. ARTHURTON, R S, GUTTERIDGE, P, and NOLAN, S C (editors). (Leeds: Yorkshire Geological Society.)

COLLIER, R E Lt. 1989b. Tectonic evolution of the Northumberland Basin; the effects of renewed extension upon an inverted extensional basin. Journal of the Geological Society of London, Vol. 146, 981–989.

COLLIER, R E Lt. 1991. The Lower Carboniferous Stainmore Basin, N England: extensional basin tectonics and sedimentation. Journal of the Geological Society of London, Vol. 148, 379–390.

CONIL, R, LONGERSTAEY, P J, and RAMSBOTTOM, W H C. 1980. Materiaux pour l'etude micropaleontoligique du Dinantien de Grande-Bretagne. Memoirs de l'Institut Geologique de l'Universite de Louvain, No. 30, 1–187.

COOPER, A H, and MOLYNEAUX, S G. 1990. The age and correlation of the Skiddaw Group (early Ordovician) sediments in the Cross Fell Inlier (northern England). Geological Magazine, Vol. 127, 147–157.

COOPER, D C, LEE, M K, FORTEY, N J, COOPER, A H, RUNDLE, C C, WEBB, B C, and ALLEN, P M. 1988. The Crummock Water aureole: a zone of metasomatism and source of ore metals in the English Lake District. Journal of the Geological Society of London, Vol. 145, 523–540.

COOPER, D C, CAMERON, D G, YOUNG, B, CHACKSFIELD, B C, and CORNWELL, J D. 1992. Mineral exploration in the Cockermouth Area, Cumbria. Part 2: follow-up surveys. British Geological Survey Technical Report, WF/92/3.

COOPER, D C, CAMERON, D G, YOUNG, B, CHACKSFIELD, B C, and CORNWELL, J D, and BLAND, D J. 1991. Mineral exploration in the Cockermouth Area, Cumbria. Part 1: regional surveys. British Geological Survey Technical Report, WF/91/4.

CRADOCK-HARTOPP, M A, and HOLLIDAY, D W. 1984. The Fell Sandstone Group of Northumberland. Investigation of the geothermal potential of the UK (Keyworth, Nottingham: British Geological Survey.)

CRAIG, C Y. 1956. The Lower Carboniferous outlier of Kirkbean, Kirkudbrightshire. Transactions of the Geological Society of Glasgow, Vol. 22, 113–132.

CREANEY, S. 1980. Petrographic texture and vitrinite reflectance variation on the Alston Block, north-east England. Proceedings of the Yorkshire Geological Society, Vol. 42, 553–580.

CREANEY, S. JONES, J M, HOLLIDAY, D W, and ROBSON, P. 1980. The occurrence of bitumen in the Great Limestone around Matfen, Northumberland-its characterisation and possible genesis. Proceedings of the Yorkshire Geological Society, Vol. 43, 69–79.

CREEDY, D P. 1991. An introduction to geological aspects of methane occurrence and control in British deep coal mines. Quarterly Journal of Engineering Geology, Vol. 24, 209–220.

DAKYNS, J R, TIDDERMAN, R H, and GOODCHILD, J G. 1897. The geology of the country between Appleby, Ullswater and Haweswater. Memoir of the Geological Survey of Great Britain.

DAKYNS, J R, RUSSEL, R, CLOUGH, C T, and STRAHAN, A. 1891. The geology of the country around Mallerstang; with parts of Wensleydale, Swaledale and Arkendale. Memoir of the Geological Survey of Great Britain.

DAWSON, J, FLOYD, J D, PHILIP, P R, BURLEY, A J, ALLSOP, J M, BENNETT, J R P, MARSDEN, G R, LEAKS, R C, and BROWN, M J. 1977. A mineral reconnaissance survey of the Doon-Glenkens area, south-west Scotland. Report Mineral Reconnaissance Programme, Institute of Geological Sciences, 18.

DAY, J B W. 1970. Geology of the country around Bewcastle. Memoir of the Geological Survey of Great Britain, Sheet 12 (England and Wales).

DEEGAN, C E. 1973. Tectonic control of sedimentation at the margin of a Carboniferous depositional basin in Kirkudbrightshire. Scottish Journal of Geology, Vol. 9, 1–28.

DEWEY, J F. 1982. Plate tectonics and the evolution of the British Isles. Journal of the Geological Society of London, Vol. 139, 371–412.

DIXON, E E L, MADEN, J, TROFFER, F M, HOLLINGWORTH, S E, and TONKS, L H. 1926. The geology of the Carlisle, Longtown and Silloth district. Memoir of the Geological Survey of Great Britain, Sheets 11, 16 and 17 (England and Wales.)

DOWNING, R A, and GRAY, D A (editors). 1986. Geothermal energy—the potential in the United Kingdom. (London: HMSO for British Geological Survey.)

DUNHAM, K C. 1934. The genesis of the North Pennine ore deposits. Quarterly Journal of the Geological Society of London, Vol. 90, 689–720.

DUNHAM, K C. 1948. Geology of the Northern Pennine Orefield, Vol. 1 Tyne to Stainmore. Memoir of the Geological Survey of Great Britain (First Edition).

DUNHAM, K C. 1950. Lower Carboniferous sedimentation in the Northern Pennines (England). Report of the 18th Session of the International Geological Congress, Great Britain, 1948, Vol. 4, 46–63.

DUNHAM, K C. 1988. Pennine mineralisation in depth. Proceedings of the Yorkshire Geological Society, Vol. 47, 1–12.

DUNHAM, K C. 1990. Geology of the Northern Pennine Orefield, Vol. 1 Tyne to Stainmore. Memoir of the Geological Survey of Great Britain (2nd edition.)

DUNHAM, K C. DUNHAM, A C, HODGE, B L, and JOHNSON, G A L. 1965. Granite beneath Visean sediments with mineralisation at Rookhope, north Pennines. Quarterly Journal of the Geological Society of London, Vol. 121, 383–417.

DUNHAM, K C. WILSON, A A. 1985. Geology of the Northern Pennine Orefield, Vol. 2 Stainmore to Craven. Memoir of the Geological Survey of Great Britain.

EASTWOOD, T. 1930. The geology of the Maryport District. Memoir of the Geological Survey of Great Britain, Sheet 22 (England and Wales).

EASTWOOD, T. DIXON, E E L, HOLLINGWORTH, S E, and SMITH, B. 1937. The geology of the Whitehaven and Workington district. Memoir of the Geological Survey of Great Britain, Sheet 28 (England and Wales).

EASTWOOD, T. HOLLINGWORTH, S E, ROSE, W C C, and TROTTER, F M. 1968. Geology of the country around Cockermouth and Caldbeck. Memoir of the Geological Survey of Great Britain, Sheet 23 (England and Wales).

EBDON, C C, FRASER, A J, HIGGINS, A C, MITCHENER, B C, and STRANK, A R E. 1990. The Dinantian stratigraphy of the East Midlands: a seismostratigraphic approach. Journal of the Geological Society of London, Vol. 147, 519–536.

ELLIOTT, T. 1974. Abandonment facies of high-constructive lobate deltas, with an example from the Yoredale Series. Proceedings of the Geologists' Association, Vol. 85, 359–365.

ELLIOTT, T. 1975. The sedimentary history of a delta lobe from a Yoredale (Carboniferous) cyclothem. Proceedings of the Yorkshire Geological Society, Vol. 40, 505–536.

ELLIOTT, T. 1976. Sedimentary sequences from the Upper Limestone Group of Northumberland. Scottish Journal of Geology, Vol. 12, 115–124.

EVANS, C J, KIMBELL, G S, and ROLLIN, K E. 1988. Hot dry rock potential in urban areas. Investigation of the geothermal potential of the UK (Keyworth, Nottingham: British Geological Survey.)

FARMER, N, and JONES, J M. 1969. The Carboniferous Namurian rocks of the coast section from Howick Bay to Foston Hall, Northumberland. Transactions of the Natural History Society of Northumbria, Vol. 17, 1–27.

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

FIELDING, C R. 1984b. Upper delta plain, lacustrine and fluvio-lacustrine facies from the Westphalian of the Durham Coalfield, NE England. Sedimentology, Vol. 31, 547–567.

FIELDING, C R. 1986. Fluvial channel and overbank deposits from the Westphalian of the Durham Coalfield, NE England. Sedimentology, Vol. 33, 119–140.

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

FIRMAN, R J. 1978a. Chapter 10. Intrusions. 146–163 in The geology of the Lake District. MOSELEY, F. (editor). (Leeds: Yorkshire Geological Society.)

FIRMAN, R J. 1978b. Chapter 15. Epigenetic mineralisation. 226–241 in The geology of the Lake District. MOSELEY, F (editor). (Leeds: Yorkshire Geological Society.)

FITCH, F J, and MILLER, J A. 1967. The age of the Whin Sill. Geological Journal, Vol. 5, 233–250.

FORSTER, W. 1809. A treatise on a section of strata from Newcastle-upon Tyne to the mountain of Cross Fell in Cumberland; with remarks on mineral veins in general. (1st edition). (Alston: W.Forster.)

FORSTER, W. 1821. A treatise on a section of strata from Newcastle-upon Tyne to the mountain of Cross Fell in Cumberland; with remarks on mineral veins in general. (2nd edition). (Alston: W. Forster.)

FORTEY, R A, OwENS, R M, and RUSHTON, A W A. 1989. The palaeogeographic position of the Lake District in the early Ordovician. Geological Magazine, Vol. 126, 9–17.

FOWLER, A. 1936. The geology of the country around Rothbury, Amble and Ashington. Memoir of the Geological Survey of Great Britain, Sheets 9 and 10 (England and Wales).

FOWLER, A. 1966. The stratigraphy of the North Tyne basin around Kielder and Falstone, Northumberland. Bulletin of the Geological Survey of Great Britain, No. 24, 57–104.

FRANCIS, E H. 1982. Magma and sediment-I: Emplacement mechanism of late Carboniferous tholeiite sills in northern England. Journal of the Geological Society of London, Vol. 139, 1–20.

FRASER, A J, and GAWTHORPE, R L. 1990. Tectonostratigraphic 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.

FRASER, A J, NASH, D F, STEELE, R P, and EBDON, C C. 1990. A regional assessment of the intra-Carboniferous play of northern England. 417–440 in Classic petroleum provinces. BROOKS, J (editor). Special Publication of the Geological Society of London, No. 50.

FREEMAN, B, KLEMOERER, S L, and HOBBS, R W. 1988. The deep structure of northern England and the Iapetus Suture zone from BIRPS deep seismic reflection profiles. Journal of the Geological Society of London, Vol. 145, 727–740.

FROST, D V. 1969. The Lower Limestone Group (Vise^-an) of the Otterburn district, Northumberland. Proceedings of the Yorkshire Geological Society, Vol. 37, 277–309.

FROST, D V. In press. Geology of the country around Northallerton. Memoir of the Geological Survey of Great Britain, Sheet 42 (England and Wales).

FROST, D V. and HOLLIDAY, D W. 1980. Geology of the country around Bellingham. Memoir of the Geological Survey of Great Britain, Sheet 13 (England and Wales).

GARWOOD, E J. 1913. The Lower Carboniferous succession in the north-west of England. Quarterly Journal of the Geological Society of London, Vol. 68, 449–596.

GARWOOD, E J. 1931. The Tuedian Beds of northern Cumberland and Roxburghshire, east of the Liddel Water. Quarterly Journal of the Geological Society of London, Vol. 87, 97–159.

GEORGE, T N, JOHNSON, G A L, MITCHELL, M, PRENTICE, J E, RAMSBOTTOM, W H C, SEVASTOPULO, 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.

GIBBS, A D. 1984. Structural evolution of extensional basin margins. Journal of the Geological Society of London, Vol. 141, 609–620.

GLENNIE, K W. 1986. Early Permian-Rotliegend. 63–85 in Introduction to the petroleum geology of the North Sea (2nd edition). GLENNIE, K W (editor). (Oxford: Blackwell Scientific Publications.)

GLOVER, B W, HOLLOWAY, S, and YOUNG, S P. 1993. An evaluation of coalbed methane potential in Great Britain. British Geological Survey Technical Report, WA/93/24.

GRAYSON, R F, and OLDHAM, L. 1987. A new structural framework for the northern British Dinantian as a basis for oil, gas and mineral exploration. 33–59 in European Dinantian environments. MILLER, J, ADAMS, A E, and WRIGHT, V P (editors). (Chichester: J. Wiley and Sons Ltd.)

GREEN, P F. 1986. On the thermo-tectonic evolution of Northern England: evidence from fission track analysis. Geological Magazine, Vol. 123, 493–506.

GREIG, D C. 1988. Geology of the Eyemouth district. Memoir of the Geological Survey of Great Britain, Sheet 34 (Scotland).

HALL, J, BREWER, J A, MATTHEWS, D H, and WARNER, M R. 1984. Crustal structure across the Caledonides from the "WINCH" seismic reflection profile: Influences on the evolution of the Midland Valley of Scotland. Transactions of the Royal Society of Edinburgh, Vol. 75, 97–109.

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

HASZELDINE, R S. 1984. Muddy deltas in freshwater lakes, and tectonism in the Upper Carboniferous Coalfield of NE England. Sedimentology, Vol. 31, 811–822.

HEWARD, A P. 1981. Lower Carboniferous fluvial facies in Northumberland and the Borders. 19–28 in Field guide to modern and ancient fluvial systems in Britain and Spain. ELLIOT, T (editor). (Keele: University of Keele.)

HEY, R W. 1956. Cherts and limestones from the Crow Series near Richmond, Yorkshire. Proceedings of the Yorkshire Geological Society, Vol. 30, 289–299.

HODGE, B L, and DUNHAM, K C. 1991. Clastics, coals, palaeodistributaries and mineralisation in the Namurian Great Cyclothem, Northern Pennines and Northumberland Trough. Proceedings of the Yorkshire Geological Society, Vol. 48, 323–337.

HODGSON, A V. 1978. Braided river bedforms and related sedimentary structures in the Fell Sandstone Group (Lower Carboniferous) of north Northumberland. Proceedings of the Yorkshire Geological Society, Vol. 41, 509–532.

HOLLIDAY, D W. 1986. Devonian and Carboniferous basins. 84–110 in Geothermal energy-the potential in the United Kingdom. DOWNING, R A, and GRAY, D A (editors). (London: HMSO for British Geological Survey.)

HOLLIDAY, D W. 1993. Mesozoic cover over Northern England: interpretation of apatite fission track data. Journal of the Geological Society of London, Vol 150, 657–660.

HOLLIDAY, D W. BURGESS, I C, and FROST, D V. 1975. A recorrelation 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.

HOLLIDAY, D W. NEVES, R, and OWENS, B. 1979. Stratigraphy and palynology of early Dinantian (Carboniferous) strata in shallow boreholes near Ravenstonedale, Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 42, 343–356.

HOLLIDAY, D W. and PATTISON, J. 1990. Carboniferous geology of Corbridge and Prudhoe: Geological notes and local details for sheets NZO6 and the eastern parts of NY96NE and SE (Northumberland). British Geological Survey Technical Report, WA/90/13.

HOLLOWAY, S. 1985. The Permian. 26–30 in Atlas of onshore sedimentary basins in England and Wales: post-Carboniferous tectonics and stratigraphy. WHirrAKER, A (editor). (Glasgow: Blackie and Son.)

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, 495–542.

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

HOLMES, T V. 1899. The geology of the country around Carlisle. Memoir of the Geological Survey of Great Britain.

HOLMGREN, D A. 1974. Major fracture patterns in the British Isles. Proceedings of the 1st Conference on New Basement Tectonics, Utah Geological Association Special Publication, Vol. 5, 263–278.

HOUSE, M R, RICHARDSON, J B, CHALONER, W G, ALLEN, J R L, HOLLAND, C H, and WESTOLL, T S. 1977. A correlation of Devonian rocks in the British Isles. Special Report of the Geological Society of London, No. 8.

HUDSON, R G S. 1924. On the rhythmic succession of the Yoredale Series in Wensleydale. Proceedings of the Yorkshire Geological Society, Vol. 20, 125–135.

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

JACKSON, D I, MULHOLLAND, P, JONES, S M, and WARRINGTON, G. 1987. The geological framework of the East Irish Sea Basin. 191–203 in Petroleum geology of north west Europe, Vol. 1. BROOKS, J, and GLENNIE, K W (editors). (London: Graham and Trotman.)

JOHNSON, G A L. 1959. The Carboniferous stratigraphy of the Roman Wall district in western Northumberland. Proceedings of the Yorkshire Geological Society, Vol. 32, 83–130.

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. 1980. Carboniferous Dinantian and Namurian rocks. 11–23 in The geology of north east England. ROBSON, D A (editor). (Newcastle upon Tyne: Natural History Society of Northumbria.)

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

JOHNSON, G A L. and DUNHAM, K C. 1963. The geology of Moor House. Monographs of the Nature Conservancy, No. 2.

JOHNSON, G A L. HODGE, B L, and FAIRBAIRN, R A. 1962. The base of the Namurian and of the Millstone Grit in north-eastern England. Proceedings of the Yorkshire Geological Society, Vol. 33,341–362.

JONES, J M. and CREANEY, S. 1977. Optical character of thermally metamorphosed coals of northern England. Journal of Microscopy, Vol. 109,105–118.

JONES, J M. and MAGRAW, D. 1980. Carboniferous Westphalian (Coal Measures) rocks. 22–36 in The geology of north east England. ROBSON, D A (editor). (Newcastle upon Tyne: Natural History Society of Northumbria.)

JONES, J M. and MAGRAW, 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). (Newcastle upon Tyne: Natural History Society of Northumbria.)

KENDALL, P F. 1911. The geology of the district around Settle and Harrogate. Proceedings of the Geologists' Association, Vol. 22, 27–60.

KIMBELL, G S, CHADWICK, R A, HOLLIDAY, D W, and WERNGREN, O C. 1989. The structure and evolution of the Northumberland Trough from new seismic reflection data and its bearing on modes of continental extension. Journal of the Geological Society of London, Vol. 146,775–787.

KLEMPERER, S L. 1989. Seismic reflection evidence for the location of the Iapetus Suture west of Ireland. Journal of the Geological Society of London, Vol. 146,409–412.

LAGIOS, E, and HIPKIN, R G. 1979. The Tweeddale Granite-a newly discovered batholith in the Southern Uplands. Nature, London, Vol. 280,672–675.

LAND, D H. 1974. Geology of the Tynemouth district. Memoir of the Geological Survey of Great Britain, Sheet 15 (England and Wales).

LEBOUR, G A. 1875. On the limits of the Yoredale series of northern England. Geological Magazine, Vol. 12,539.

LEE, G W. 1924. On the faunal sequence of the Carboniferous rocks met in the Roddymoor Boring, Co. Durham. 146–149 in Summary of Progress for 1923. GEOLOGICAL SURVEY OF GREAT BRITAIN. (London: Her Majesty's Stationery Office.)

LEE, M K. 1982. The regional geophysics of the Cheviot area. Report of the Institute of Geological Sciences, No. ENPU 82–2.

LEE, M K. 1986. Hot dry rock. 21–41 in Geothermal energy - the potential in the United Kingdom. DOWNING, R A, and GRAY, D A (editors). (London: HMSO for British Geological Survey.)

LEE, M K. 1989. Upper crustal structure of the Lake District from modelling and image processing of potential field data. British Geological Survey Technical Report , WK/89/1.

LEE, M K. BROWN, G C, WEBB, P C, WHEILDON, J, and ROLLIN, K E. 1987. Heat flow, heat production and thermo-tectonic setting in mainland UK. Journal of the Geological Society of London, Vol. 144,35–42.

LEEDER, M R. 1973. Sedimentology and palaeogeography of the Upper Old Red Sandstone in the Scottish Border Basin. Scottish Journal of Geology, Vol. 9,117–144.

LEEDER, M R. 1974a. Lower Border Group (Tournaisian) fluvio-deltaic sedimentation and palaeogeography of the Northumberland Basin. Proceedings of the Yorkshire Geological Society, Vol. 40, 129–180.

LEEDER, M R. 1974b. The origin of the Northumberland Basin. Scottish Journal of Geology, Vol. 10,283–296.

LEEDER, M R. 1975a. Lower Border Group (Tournaisian) limestones from the Northumberland Basin. Scottish Journal of Geology, Vol. 11,151–167.

LEEDER, M R. 1975b. Lower Border Group (Tournaisian) stromatolites from the Northumberland Basin. Scottish Journal of Geology, Vol. 11,207–226.

LEEDER, M R. 1976. Palaeogeographic significance of pedogenic carbonates in the topmost Upper Old Red Sandstone of the Scottish Border basin. Geological Journal, Vol. 11,21–28.

LEEDER, M R. 1982. Upper Palaeozoic basins of the British IslesCaledonide inheritance versus Hercynian plate margin processes. Journal of the Geological Society of London, Vol. 139, 479–491.

LEEDER, M R. 1987. Sediment deformation structures and the palaeotectonic analysis of sedimentary basins, with a case-study from the Carboniferous of northern England. 137–146 in Deformation of sediments and sedimentary rocks. JONES, M E, and PRESTON, R M F (editors). Special Publication of the Geological Society of London, No. 29.

LEEDER, M R. FAIRHEAD, D, LEE, A, STUART, G, CLEMMEY, H, AL-HADDEH, B, and GREEN, C. 1989. Sedimentary and tectonic evolution of the Northumberland Basin. 207–223 in The role of tectonics in Devonian and Carboniferous sedimentation in the British Isles. ARTHURTON, R S, GUTTERIDGE, P, and NOLAN, S C (editors). (Leeds: Yorkshire Geological Society.)

LEEDER, M R. and HARDMAN, M. 1990. Carboniferous geology of the Southern North Sea Basin and controls on hydrocarbon prospectivity. 87–105 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.

LEEDER, M R. and MCMAHON, A H. 1988. Upper Carboniferous (Silesian) basin subsidence in northern Britain. 43–52 in Sedimentation in a synorogenic basin-the Upper Carboniferous of northwest Europe. BESLEY, B M, and KELLING, G (editors). (Glasgow: Blackie and Son Ltd.)

LEEDER, M R. and STRUDWICK, A S. 1987. Delta-marine interactions: a discussion of sedimentary models for Yoredale-type cyclicity in the Dinantian of northern England. 115–130 in European Dinantian environments. MILLER, j, ADAMS, A E, and WRIGHT, V P (editors). (Chichester: John Wiley and Sons.)

LEGGETT, J K, MCKERROW, W S, and EALES, M H. 1979. The Southern Uplands of Scotland: a Lower Palaeozoic accretionary prism. Journal of the Geological Society of London, Vol. 136,755–770.

LEGGETT, J K, MCKERROW, W S and SOPER, N J. 1983. A model for the crustal evolution of southern Scotland. Tectonics, Vol. 2,187–210.

LEWIS, C L E, GREEN, P F, CARTER, A, and HURFORD, A J. 1992. Elevated K/T palaeotemperatures throughout northwest England: three kilometres of Tertiary erosion? Earth and Planetary Science Letters, Vol. 112,131–145.

LUMSDEN, G I, TULLOCH, W, HOWELLS, M F, and DAVIES, A. 1967. The geology of the neighbourhood of Langholm. Memoir of the Geological Survey of Great Britain, Sheet 10 (Scotland).

LUMSDEN, G I and WILSON, R B. 1961. The stratigraphy of the Archerbeck Borehole, Canonbie, Dumfriesshire. Bulletin of the Geological Survey of Great Britain, No. 18,1–89.

MARK, J E. 1921. The rigidity of north-west Yorkshire. Naturalist, Hull, 63–72.

MAYNARD, J R, and LEEDER, M R. 1992. On the periodicity and magnitude of Late Carboniferous glacio-eustatic sea-level changes. Journal of the Geological Society of London, Vol. 149, 303–311.

MCKENZIE, D P. 1978. Some remarks on the development of sedimentary basins. Earth and Planetary Science Letters, Vol. 40, 25–32.

MCKERROW, W S. 1987. The Southern Uplands Controversy. Journal of the Geological Society of London, Vol. 144, 735–736.

MCKERROW, W S. and SOPER, N J. 1989. The Iapetus Suture in the British Isles. Geological Magazine, Vol. 126, 1–8.

MILLER, H. 1887. The geology of the country between Otterburn and Elsdon. Memoir of the Geological Survey of Great Britain.

MILLS, D A C, and HULL, J H. 1968. The Geological Survey borehole at Woodland, Co. Durham. Bulletin of the Geological Survey of Great Britain, No. 28, 1–37.

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 (England and Wales).

MITCHELL, M, TAYLOR, B J, and RAMSBOTTOM, W H C. 1978. Chapter 12. Carboniferous. 168–188 in The geology of the Lake District. MOSELEY, F (editor). (Leeds: Yorkshire Geological Society.)

MOORE, D. 1959. Role of deltas in the formation of some British Lower Carboniferous cyclothems. Journal of Geology, Vol. 67, 522–539.

NAIRN, A E M. 1956. The Lower Carboniferous rocks between the rivers Esk and Annan, Dumfriesshire. Transactions of the Geological Society of Glasgow, Vol. 22, 80–93.

NEVES, R, GUEINN, K J, CLAYTON, G, IONNIDES, N, and KRUSZEWSKA, K. 1973. Palynological correlations within the Lower Carboniferous of Scotland and northern England. Transactions of the Royal Society of Edinburgh, Vol. 69, 23–70.

ORD, D M, CLEMMEY, H, and LEEDER, M R. 1988. Interaction between faulting and sedimentation during Dinantian extension of the Solway basin, S W Scotland. Journal of the Geological Society of London, Vol. 145, 249–259.

OWENS, B, and BURGESS, I C. 1965. The stratigraphy and palynology of the Upper Carboniferous outlier of Stainmore, Westmorland. Bulletin of the Geological Survey of Great Britain, No. 23, 17–44.

OWENS, B, NEVES, R, GUEINN, K J, MISHELL, D R F, SABRY, H S M Z, and WILLIAMS, J E. 1977. Palynological division of the Namurian of northern England and Scotland. Proceedings of the Yorkshire Geological Society, Vol. 41, 381–398.

PEACH, B N, and HORNE, J. 1903. The Canonbie Coalfield: its geological structure and relations to the Carboniferous rocks of the north of England and central Scotland. Transactions of the Royal Society of Edinburgh, Vol. 40, 835–877.

PEACOCK, D C P, and SANDERSON, D J. 1991. Displacements, segment linkage and relay ramps in normal fault zones. Journal of Structural Geology, Vol. 13, 721–733.

PERCIVAL, C J. 1983. The Firestone Sill Ganister, Namurian, Northern England-the A2 horizon of a podzolic palaeosol. Sedimentary Geology, Vol. 36, 41–49.

PERCIVAL, C J. 1986. Palaeosols containing an albic horizon: examples from the Upper Carboniferous of Northern England. 87–111 in Palaeosols their recognition and interpretation. WRIGHT, V P (editor). (Oxford: Blackwells.)

PHILLIPS, J. 1836. Illustrations of the geology of Yorkshire. Part II, The Mountain Limestone District. (London.)

PHILLIPS, W E A, STILLMAN, C J, and MURPHY, T. 1976. A Caledonian plate tectonic model. Journal of the Geological Society of London, Vol. 132, 779–609.

PICKEN, G S. 1988. The concealed coalfield at Canonbie: an interpretation based on boreholes and seismic surveys. Scottish Journal of Geology, Vol. 24, 61–71.

PLANT, J A, and JONES, D G (editors). 1989. Metallogenic models and exploration criteria for buried carbonate-hosted ore deposits-a multidisciplinary study in eastern England. (London and Nottingham: Institution of Mining and Metallurgy and British Geological Survey.)

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. 1980a. The Great Whin Sill and its associated dyke suite. 67–74 in The geology of north-east England. ROBSON, D A (editor). (Newcastle upon Tyne: Natural History Society of Northumbria.)

RANDALL, B A O. 1980b. The Tertiary dykes. 75–77 in The geology of northeast England. ROBSON, D A (editor). (Newcastle upon Tyne: Natural History Society of Northumbria.)

RAYNER, D H. 1953. The Lower Carboniferous rocks in the north of England: a review. Proceedings of the Yorkshire Geological Society, Vol. 28, 231–315.

READ, W A. 1981. Facies breaks in the Scottish Passage Group and their possible correlation with the Mississippian-Pennsylvanian hiatus. Scottish Journal of Geology, Vol. 17, 295–300.

READ, W A. 1988. Controls on Silesian sedimentation in the Midland Valley of Scotland. 222–241 in Sedimentation in a synorogenic basin-the Upper Carboniferous of northwest Europe. BESLEY, B M, and KELLING, G (editors). (Glasgow: Blackie and Son Ltd.)

READ, W A. 1989. The interplay of sedimentation, volcanicity and tectonics in the Passage Group (Arnsbergian, E2 to Westphalian A) in the Midland Valley of Scotland. 143–152 in The role of tectonics in Devonian and Carboniferous sedimentation in the British Isles. ARTHURTON, R S, GUTTERIDGE, P, and NOLAN, S C (editors). (Leeds: Yorkshire Geological Society.)

READ, W A. 1991. The Millstone Grit (Namurian) of the southern Pennines viewed in the light of eustatically controlled sequence stratigraphy. Geological Journal, Vol. 26, 157–165.

READ, W A. and FORSYTH, I. 1989. Allocycles and autocycles in the upper part of the Limestone Coal Group (Pendleian El) in the Glasgow-Stirling region of the Midland Valley of Scotland. Geological Journal, Vol. 24, 121–137.

READ, W A. 1991. Allocycles in the upper part of the Limestone Coal Group (Pendleian El) of the Glasgow-Stirling region viewed in the light of sequence stratigraphy. Geological Journal, Vol. 26, 85–89.

READING, H G. 1957. The stratigraphy and structure of the Cotherstone Syncline. Quarterly Journal of the Geological Society of London, Vol. 113, 27–56.

REYNOLDS, A D. 1992. Storm, wave and tide-dominated sedimentation in the Dinantian Middle Limestone Group, Northumbrian Basin. Proceedings of the Yorkshire Geological Society, Vol. 49, 135–148.

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.

ROBSON, D A. 1954. A preliminary account of the geological structure of north Northumberland and the Borders. Proceedings of the University of Durham Philosophical Society, Vol. 12, 1–9.

ROBSON, D A. 1956. A sedimentary study of the Fell Sandstones of the Coquet valley, Northumberland. Quarterly Journal of the Geological Society of London, Vol. 112, 241–262.

ROBSON, D A. 1976. A guide to the geology of the Cheviot Hills. Transactions of the Natural History Society of Northumbria, Vol. 43, 1–23.

ROBSON, D A. 1977. The structural history of the Cheviot and adjacent regions. Scottish Journal of Geology, Vol. 13, 255–262.

ROBSON, D A. (editor). 1980. The geology of north east England. (Newcastle upon Tyne: Natural History Society of Northumbria.)

ROBSON, D A. and SMITH, F W. 1980. Post-Hercynian movements. 86 in The geology of north-east England. ROBSON, D A (editor). (Newcastle upon Tyne: Natural History Society of Northumbria.)

ROLLIN, K E. 1987. Catalogue of geothermal data for the land area of the United Kingdom. Third revision: April 1987. Investigation of the geothermal potential of the UK (Keyworth, Nottingham: British Geological Survey.)

ROWLEY, C R. 1969. The stratigraphy of the Carboniferous Middle Limestone Group of west Edenside, Westmorland. Proceedings of the Yorkshire Geological Society, Vol. 37, 329–350.

RUNDLE, C C. 1992. Review and assessment of isotopic ages from the English Lake District. British Geological Survey Technical Report, WA/92/38.

SAWKINS, F J. 1966. Ore genesis in the North Pennine Orefield, in the light of fluid inclusion studies. Economic Geology, Vol. 61, 385–401.

SCOTT, J, and COLTER, V S. 1987. Geological aspects of current onshore Great Britain exploration plays. 95–107 in Petroleum geology of north west Europe. BROOKS, J, and GLENNIE, KW (editors). (London: Graham and Trotman.)

SCOTT, W B. 1986. Nodular carbonates in the Lower Carboniferous, Cementstone Group of the Tweed Embayment, Berwickshire: evidence for a former sulphate evaporite facies. Scottish Journal of Geology, Vol. 22, 325–345.

SHIELLS, K A G. 1964. The geological structure of northeast Northumberland. Transactions of the Royal Society of Edinburgh, Vol. 65, 449–484.

SHOTTON, F W. 1935. The stratigraphy and tectonics of the Cross Fell Inlier. Quarterly Journal of the Geological Society of London, Vol. 91, 639–700.

SIMPSON, J B. 1904. The probability of finding workable seams of coal in the Carboniferous Limestone or Bernician Formation, beneath the regular Coal-Measures of Northumberland and Durham, with an account of a recent deep boring in Chopwell Woods, below the Brockwell seam. Transactions of the Institution of Mining Engineers, Vol. 24, 549–571.

SMITH, B. 1921. On borings for coal near Maryport, Cumberland. 85–91 in Summary of Progress for 1920. GEOLOGICAL SURVEY OF GREAT BRITAIN. (London: HMSO.)

SMITH, D B. 1980. Permian and Triassic rocks. 36–48 in The geology of north-east England. ROBSON, D A (editor). (Newcastle upon Tyne: Natural History Society of Northumbria.)

SMITH, D B. 1994. Geology of the country around Sunderland. Memoir of the Geological Survey of Great Britain, Sheet 21 (England and Wales).

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 (England and Wales).

SMITH, F W. 1980. The mineralization of the Alston Block. 89–96 in The geology of north east England. ROBSON, D A (editor). (Newcastle upon Tyne: Natural History Society of Northumbria.)

SMITH, I F. 1986. Mesozoic basins. 42–83 in Geothermal energy-the potential in the United Kingdom. DOWNING, R A, and GRAY, D A (editors). (London: HMSO for British Geological Survey.)

SMITH, K. 1992. Carboniferous magmatism in the Iapetus convergence zone: evidence from deep seismic reflection profiles. Journal of the Geological Society of London, Vol. 149, 907–914.

SMITH, K. and SMITH, N J P. 1989. Deep geology. 53–64 in Metallogenic models and exploration criteria for buried carbonate-hosted ore deposits-a multidisciplinary study in eastern England. PLANT, J A, and JoNEs, D G (editors). (London and Nottingham: Institution of Mining and Metallurgy and British Geological Survey.)

SMITH, N J P. 1985. Map 1: Pre-Permian geology of the United Kingdom (South). (Keyworth, Nottingham: British Geological Survey.)

SMITH, S. 1910. The faunal succession of the Upper Bernician. Transactions of the Natural History Society of Northumberland, New Series 3, 16–150.

SMITH, S. 1912. The Carboniferous Limestone formation of the north of England. (Newcastle upon Tyne: North of England Institution of Mining and Mechanical Engineers.)

SMITH, S. 1923. Lead and zinc ores of Northumberland and Alston Moor. Economic Memoir of the Geological Survey of Great Britain.

SMITH, S A, and HOLLIDAY, D W. 1991. The sedimentology of the Middle and Upper Border groups (Visean) in the Stonehaugh Borehole, Northumberland. Proceedings of the Yorkshire Geological Society, Vol. 48, 435–446.

SOLOMON, M, RAFTER, T A, and DUNHAM, K C. 1971. Sulphur and oxygen isotope studies in the northern Pennines in relation to ore genesis. Transactions of the Institution of Mining and Metallurgy, Vol. B80, 259–275.

SOPER, N J, ENGLAND, R W, SNYDER, D B, and RYAN, P D. 1992. The Iapetus suture zone in England, Scotland and eastern Ireland: a reconciliation of geological and deep seismic data. Journal of the Geological Society of London, Vol. 149, 697–700.

SOPER, N J, WEBB, B C, and WOODCOCK, N H. 1987. Late Caledonian (Acadian) transpression in north-west England: timing, geometry and tectonic significance. Proceedings of the Yorkshire Geological Society, Vol. 46, 175–192.

STONE, P, FLOYD, J D, BARNES, R P, and LINTERN, B C. 1987. A sequential back-arc and foreland basin thrust duplex model for the Southern Uplands of Scotland. Journal of the Geological Society of London, Vol. 144, 753–764.

TATE, G. 1867. The geology of the district traversed by the Roman Wall. Appendix in The Roman Wall (3rd edition). BRUCE, J C. (London.)

TAYLOR, B J, BURGESS, I C, LAND, D H, MILLS, D A C, SMITH, D B, and WARREN, P T. 1971. British regional geology; northern England (4th edition). (London: HMSO for Institute of Geological Sciences.)

THOMPSON, T. 1814. A geognostical sketch of the counties of Northumberland, Durham and part of Cumberland. Annals of Philosophy, Series 1, Vol. 4, 337.

TROTTER, F M. 1929. The Tertiary uplift and resultant drainage of the Alston Block and adjacent areas. Proceedings of the Yorkshire Geological Society, Vol. 21, 161–180.

TROTTER, F M. and HOLLINGWORTH, S E. 1928. The Alston Block. Geological Magazine, Vol. 65, 433–448.

TROTTER, F M. and HOLLINGWORTH, S E. 1932. The geology of the Brampton district. Memoir of the Geological Survey of Great Britain, Sheet 18 (England and Wales).

TROTTER, F M. and HOLLINGWORTH, S E. EASTWOOD, T, and ROSE, W C C. 1937. Gosforth district. Memoir of the Geological Survey of Great Britain, Sheet 37 (England and Wales).

TURNER, B R, and MUNRO, M. 1987. Channel formation and migration by mass-flow processes in the Lower Carboniferous fluviatile Fell Sandstone Group, northeast England. Sedimentology, Vol. 34, 1107–1122.

TURNER, J S. 1935. Structural geology of Stainmore, Westmorland, and notes on the late Palaeozoic (late Variscan) tectonics of the north of England. Proceedings of the Geologists' Association, Vol. 46, 121–151.

UNDERHILL, J R, GAYER, R A, WOODCOCK, N H, DONNELLY, R, JOLLEY, E J, and STIMPSON, I G. 1988. The Dent Fault System, northern England-reinterpreted as a major oblique-slip fault zone. Journal of the Geological Society of London, Vol. 145, 303–316.

VERSEY, H C. 1927. Post-Carboniferous movements in the Northumbrian fault-block. Proceedings of the Yorkshire Geological Society, Vol. 21, 1–16.

WADGE, A J. 1978. Chapter 11. Devonian. 164–167 in The geology of the Lake District. MOSELEY, F (editor). (Leeds: Yorkshire Geological Society.)

WADGE, A J. HARRISON, R K, and SNELLING, N J. 1972. Olivine-dolerite intrusions near Melmerby, Cumberland, and their age determination by the potassium-argon method. Proceedings of the Yorkshire Geological Society, Vol. 39, 59–70.

WEBB, B C, and COOPER, A H. 1988. Slump folds and gravity slide structures in a Lower Palaeozoic marginal basin sequence (the Skiddaw Group), NW England. Journal of Structural Geology, Vol. 10, 463–472.

WELLS, A J. 1955. The development of chert between the Main and Crow Limestones in North Yorkshire. Proceedings of the Yorkshire Geological Society, Vol. 30, 177–196.

WELLS, A J. 1957. The stratigraphy and structure of the Middleton Tyas-Sleightholme Anticline, North Yorkshire. Proceedings of the Geologists' Association, Vol. 68, 231–254.

WELLS, A J. 1960. Cyclic sedimentation: a review. Geological Magazine, Vol. 97, 389–403.

WESTOLL, T S, ROBSON, D A, and GREEN, R. 1955. A guide to the geology of the district around Alnwick, Northumberland. Proceedings of the Yorkshire Geological Society, Vol. 30, 61–100.

WHITTAKER, A (editor). 1985. Atlas of onshore sedimentary basins in England and Wales. (Glasgow: Blackie and Son Ltd.)

WINCH, N J. 1817. Observations on the geology of Northumberland and Durham. Transactions of the Geological Society of London, Vol. 4, 1–101.

WOOLACOTT, D. 1923. A boring at Roddymoor Colliery, near Crook, Co. Durham. Geological Magazine, Vol. 60, 50–62.

YOUNG, B, and ARMSTRONG, M. 1989. The applied geological mapping of the Dearham and Gilcrux area, Cumbria. British Geological Survey Technical Report, WA/98/70.

Appendix 1 Geological Survey maps and memoirs published following the primary geological survey

England

Appendix 2 Geological Survey maps and memoirs published following the 20th century resurvey (Figure 39)

Appendix 3 Generalised records of selected boreholes

Outline records for the boreholes judged to be of particular significance in the elucidation of the subsurface geology of the region are given below. The list is selective and does not include all of the deep boreholes. The depths to the main strati-graphical boundaries are the interpretation of the authors of this book, and differ in some instances from previously published accounts or from the interpretation on composite logs supplied to the British Geological Survey. Except for those indicated as currently held commercial-in-confidence, more detail of these boreholes can be found in Geological Survey records and in the publications cited.

(CB) denotes a cored borehole; (OHB) denotes an open-holed borehole with limited spot coring; (OHB/CB) denotes a borehole open-holed in its upper part only and then cored to final depth.

Acklington Station (CB)

Allenheads No. 1 (CB)

Archerbeck (CB)

Bankend (CB)

Barrock Park (CB)

Becklees (OHB/CB)

Blue Dial (CB)

Brafferton (OHB)

Chopwell (CB)

Easton (OHB)

Ferneyrigg (CB)

Harton (OHB)

Longhorsley (OHB)

Roddymoor (CB)

Rookhope (CB)

Rowanbtunhead

Seal Sands (OHB)

Silloth No. lA (OHB)

Stonehaugh (CB)

Throckley (CB)

Westnewton (OHB)

Whitley Bay (OHB)

Woodland (CB)

Figures, plates, tables and maps

Figures

(Figure 1) Location map of region with generalised topography.

(Figure 2) Simplified geological map of the region.

(Figure 3) Principal subsurface structures influencing Carboniferous sedimentation in the region.

(Figure 4) Areas licensed in the period 1980–1992 for hydrocarbon exploration and location of hydrocarbon exploration wells and other important boreholes.

(Figure 5) Summary of Carboniferous stratigraphy of the region. CL Cockermouth Lavas; GV Glencartholme Volcanics; MSL Melmerby Scar Limestone; PGB Pinskey Gill Beds; SC Shap Conglomerate

(Figure 6) Perspective view of the top of Caledonian basement rocks in the region, viewed from the south-west (true-scale).

(Figure 7) Perspective view of the top of Caledonian basement rocks in the region, viewed from the north-west (true-scale).

(Figure 5) Summary of Carboniferous stratigraphy of the region. CL Cockermouth Lavas; GV Glencartholme Volcanics; MSL Melmerby Scar Limestone; PGB Pinskey Gill Beds; SC Shap Conglomerate

(Figure 8) Relationship between the Northumberland Trough and the inferred late-Acadian crustal shear zone.

(Figure 9) Development of a sedimentary basin by initial fault-controlled synextensional subsidence followed by regional postextensional subsidence, giving a 'steer's head' profile.

(Figure 10) Seismic reflection profile across the Stublick–Ninety Fathom fault system, which forms the southern margin of the Northumberland Trough. Note the minor Variscan reverse fault splaying off the Ninety Fathom Fault. Interpretation of Weardale Granite after Kimbell et al., 1989. SF Stublick Fault; NFF Ninety Fathom Fault. For location see (Figure 3).

(Figure 12) a. Cross-section through the Northumberland Trough, the Alston Block and the northern part of the Stainmore Trough (vertical exaggeration x 2.5). b. Cross-section through the Solway and Vale of Eden basins (vertical exaggeration x 2.5). c. Cross-section through the Northumberland Trough and the Bewcastle Anticline (vertical exaggeration x 2.5). BBF Back Burn Fault GLT Goat Island-Lyne Thrust. BF Butterknowle Fault; MF Maryport Fault; BHF Brackenhill Fault; NFF Ninety Fathom Fault; ECF East Christianbury Fault; SF Stublick Fault; GF Gilnockie Fault; SwF Sweethope Fault; BA Bewcastle Anticline

(Figure 13) Seismic reflection profile across the Butterknowle Fault (BF), which forms the northern margin of the Stainmore Trough.

(Figure 14) Seismic reflection profile across the Back Burn Fault (north-east Solway Basin), showing Variscan reversal of a Dinantian syndepositional normal fault. BBF Back Burn Fault; HoF Hogwash Fault; RF Rowanburn Fault; HiF Hilltop Fault.

(Figure 15) Courceyan palaeoenvironments and lithofacies in the region: Lower Border Group (lower part) and equivalents. Arrows denote general direction of sediment transport.

(Figure 16) Chadian palaeoenvironments and lithofacies in the region: Lower Border Group (upper part) and equivalents.

(Figure 17) Arundian palaeoenvironments and lithofacies in the region: Middle Border Group (lower part) and equivalents.

(Figure 18) Holkerian palaeoenvironments and lithofacies in the region: Middle Border Group (upper part) and equivalents.

(Figure 19) Correlation of Middle and Upper Border Group strata and inferred equivalents.

(Figure 20) Early Asbian palaeoenvironments and lithofacies in the region: Upper Border Group and equivalents.

(Figure 21) Correlation of Liddesdale and Alston group strata.

(Figure 22) Late Asbian palaeoenvironments and lithofacies in the region: Lower Liddesdale and Alston groups.

(Figure 23) Brigantian palaeoenvironments and lithofacies in the region: Upper Liddesdale and Alston groups.

(Figure 24) Correlation of Stainmore Group strata with approximate positions of stage boundaries. SMB Subcrenatum Marine Band WSB Woodland Shell Beds; DFS Dipton Foot Shell Beds; WL Whitehouse Limestone; BTL Barrock Top Limestone; STL Styford Limestone; NL Newton Limestone; GrL Grindstone Limestone; RGL Reighton Gill Limestone; TL Thornbrough Limestone; UFL Upper Felltop Limestone; PHL Pike Hill Limestone; CSB Coalcleugh Shell Bed; CL Corbridge Limestone; LFL Lower Felltop Limestone; BDL Belsay Dene Limestone; RSB Rookhope Shell Beds; ASB Aydon Shell Beds; GR Gamma-ray log; RES; Resistivity log; BRCS Borehole compensated sonic log; C&A Chokierian Et Alportian; CL Crag Limestone; OL Oakwood Limestone; LTL Little Limestone SBB Snope Burn Band GL Great Limestone

(Figure 25) Early Namurian palaeoenvironments and lithofacies in the region: Stainmore Group (lower part).

(Figure 26) Late Namurian palaeoenvironments and lithofacies in the region: Stainmore Group (upper part).

(Figure 27) Seismic reflection profile showing syndepositional thinning of the Stainmore Group (Namurian) on to the flank of the Carlisle Anticline; note also the angular unconformity at the base of Permo-Triassic strata.

(Figure 28) Base Lower Coal Measures of the Northumberland and Durham Coalfield – depth contours in metres; based on extrapolation from seam contours.

(Figure 29) Base Middle Coal Measures of the Northumberland and Durham Coalfield – depth contours in metres; based on extrapolation from seam contours.

(Figure 30) Correlation of Coal Measures in the Harton Borehole.

(Figure 31) Langsettian–Bolsovian palaeoenvironments and lithofacies in the region: Lower and Middle Coal Measures and main late Westphalian inversion structures.

(Figure 32) Main Variscan structural elements of northern England.

(Figure 33) Seismic reflection profile across the Back Burn and Brackenhill faults. Note the Variscan reverse displacement on both faults; note also folding of the Permo-Triassic sequence suggesting later (?Cenozoic) reversal of the faults, particularly the Brackenhill Fault. BBF Back Burn Fault; BF Brackenhill Fault.

(Figure 34) Seismic reflection profile across the Brackenhill and associated low-angle faults (probably detaching on to Lower Border Group evaporites); note the 'fishtail structure' suggestive of oblique-slip displacements; note also how Permo-Triassic extensional reactivation of the Brackenhill Fault has switched its end-Carboniferous reverse displacement into a small net normal throw. BF Brackenhill Fault.

(Figure 35) Seismic reflection profile illustrating probable Variscan upwarp of the hanging-wall block between the Antonstown (AF) and Sweethope (SwF) faults.

(Figure 36) Complex reverse faulting in the hanging-wall block of the Swindon Fault (SwiF), indicative of probable Variscan oblique compression.

(Figure 37) Summary of the Permian to Jurassic stratigraphy of the Carlisle Basin.

(Figure 38) Permo-Triassic stratigraphy in the Silloth 1A and Seal Sands boreholes.

(Figure 39) Map showing boundaries of the 1:50,000 geological sheets of the region.

Plates

(Front cover) Cover photograph North-facing scarp face of the Whin Sill, intruded into rocks of the Upper Liddesdale Group, at Crag Lough, Northumberland. These rocks form part of the fill of the Northumberland Trough and dip gently to the south, towards the Stublick Fault. The high ground to the south (top left) forms part of the Alston Block. (BGS photograph L1513)

(Rear cover)

(Frontispiece) Perspective view of the top surface of the Caledonian basement rocks in the region, viewed from the west.

(Succession) Summary of stratigraphy, geological and tectonic events.

Maps

(Map 1) Top Caledonian Basement — depth contours.

(Map 2) Lower Border Group — preserved thickness of lower part (below top Lynebank Beds).

(Map 3) Lower Border Group — depth contours at top of Lynebank Beds.

(Map 4) Lower Border Group — preserved thickness of upper part (above top Lynebank Beds).

(Map 5) Base Middle Border/Orton Group — depth contours.

(Map 6) Middle Border/Orton Group — preserved thickness.

(Map 7) Base Upper Border Group — depth contours.

(Map 8) Upper Border Group — preserved thickness.

(Map 9) Base Liddesdale/Alston Group — depth contours.

(Map 10) Liddesdale/Alston Group — preserved thickness.

(Map 11) Base Stainmore Group — depth contours.

(Map 12) Stainmore Group — preserved thickness.

(Map 13) Base Coal Measures — depth contours.

(Map 14) Coal Measures — preserved thickness.

(Map 15) Base Permo-Triassic beds — depth contours.

(Map 16) Subcrop beneath Permo-Triassic cover rocks.