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Structure and evolution of the East Midlands region of the Pennine Basin

By T C Pharaoh C J Vincent M S Bentham A G Hulbert C N Waters N J P Smith. Contributors J Williams C Cortreel K L Kirk B C Chacksfield.

Bibliographical reference: Pharaoh, T C, Vincent, C J, Bentham, M S, Hulbert, A G, Waters, C N, and Smith, N J. 2011. Structure and evolution of the East Midlands region of the Pennine Basin. Subsurface memoir of the British Geological Survey.

British Geological Survey

T C Pharaoh C J Vincent M S Bentham A G Hulbert C N Waters N J P Smith.

Structure and evolution of the East Midlands region of the Pennine Basin Subsurface memoir

Keyworth, Nottingham: British Geological Survey 2011. © NERC copyright 2011 First published 2011

(Front cover) Cover photograph. Statue of the ‘Oil Patch Warrior’ by Jay O’Melia at Duke’s Wood. The bronze statue commemorates the contribution of the Noble Drilling Company of Ardmore, Oklahoma, in developing Britain’s first oilfield, at Eakring–Duke’s Wood, in 1943. An identical statue is located in Ardmore. (Photographer: Tim Pharaoh; P756089).

(Rear cover)

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Preface

On account of its rich resources of coal and hydrocarbons, the subsurface of the East Midlands region has been intensively explored by boreholes and seismic surveys. This memoir is based on an exhaustive use of such data, acquired during seven decades of exploration, and aims to present a concise review of the tectonic and sedimentary history of the Carboniferous rocks of the East Midlands region. It is the fourth in a series of subsurface memoirs relating to Upper Palaeozoic basins, and forms a sequel to a previously published account of the adjacent south-west Pennine Basin (Smith et al., 2005). It will serve as a basic reference source when economic conditions change or new commodities are required, or when new exploration models and exploitation technologies are developed, which once again result in new interest in the region by the exploration industry.

This account is regional in scope, dealing particularly with the deeper, concealed parts of the Carboniferous and older succession and associated structures, not considered in earlier publications on the Permian to Recent evolution. The results of the study are largely contained in the accompanying 1:625 000 scale structure contour and preserved thickness 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.

John N Ludden, BSc, PhD.  Executive Director. British Geological Survey Kingsley Dunham Centre Keyworth,Nottingham NG12 5GG

Acknowledgements

The interpretation of the seismic data, and the production of the structure contour maps was carried out by Dr T C Pharaoh, Ms C J Vincent and Mrs M S Bentham. These personnel also prepared the written account in conjunction with Mr N J P Smith. Mr A G Hulbert was responsible for the final computer generation and production of the maps, including the calculation and presentation of the thickness maps. Dr C N Waters contributed significantly to the account of Carboniferous stratigraphy and carried out the scientific editing. Dr J N Carney and Mr K Ambrose are thanked for their helpful internal reviews.

The United Kingdom Onshore Geophysical Library (UKOGL) is the trustee of the UK onshore exploration seismic database. Lynx Information Systems Ltd manage the archive and release data on behalf of the Library. The seismic data shown in this memoir can be obtained from UKOGL (www.ukogl.org.uk).

Notes

The word 'region' refers to the area of the map shown in (Figure 1).

Chapter 1 Introduction

(Geological succession)

This report presents a review of the tectonic and sedimentary history of the eastern part of the English Midlands (hereafter referred to as the region) emphasising the structure and evolution of the Carboniferous basins. It is the fourth in the Subsurface Memoir series of the British Geological Survey, following the Northumberland–Solway Basin (Chadwick et al., 1995), the Craven Basin (Kirby et al., 2000) and south-west Pennine Basin (Smith et al., 2005), and forms a sequel to a previously published subsurface study of the Mesozoic basins of England and Wales (British Geological Survey, 1985a).

The region includes areas of high urban population density, contrasting with pastoral countryside and fenland (Figure 1). The cities of Nottingham, Leicester, Lincoln and Peterborough are the principal conurbations, but much of north Nottinghamshire and South Yorkshire bear the legacy of industrialisation following development of the concealed Yorkshire–Nottinghamshire coalfield. The geology of the region (Figure 2) is superficially simple. Carboniferous strata of the south-eastern part of the Pennine Basin dip eastward beneath a cover of Permian and Mesozoic strata. This description belies the complex structure and stratigraphy of the Carboniferous basins however; rapid stratigraphical variation occurs where the Carboniferous strata onlap onto the Anglo–Brabant Massif in the south of the region. Early Palaeozoic strata have very limited outcrop, and are mainly known from deep boreholes. Deformation of the rocks occurred during a number of tectonic episodes, producing structures that control their nature and distribution. The surface and near-surface geology have been described in the British Geological Survey 1:50 000 Series maps and memoirs (Appendix 1). This account examines the deeper parts of the Permian–Mesozoic and, in particular, the Carboniferous successions, in greater detail than in earlier publications. Over 2000 seismic reflection profiles have been interpreted, and some 500 deep boreholes studied during the course of this work, and the results are synthesised in the accompanying 1:625 000 scale structure contour and isopach maps (Appendix 3) and associated text figures. This account provides both a regional review and an explanation of these maps and figures.

The Carboniferous rocks of the region have long been of economic significance. Coal has been worked for centuries. Prior to the 1960s, only limited investigation of the concealed geology was possible, mainly through coal exploration and extraction, and oil exploration. The National Coal Board (NCB) established a sophisticated system for coal seam identification and correlation using geochemical methods. This was combined with biostratigraphical information from marine bands to yield a very detailed lithostratigraphical correlation of the Coal Measures. The region has experienced several phases of hydrocarbon exploration: immediately following the First World War; just before and after the Second World War, when drilling was mainly on the edges of structural highs recognised from gravity anomalies; in the 1950s and 1960s, with the widespread application of seismic techniques; in the 1970s and 1980s, using 2D reflection seismic data of increasingly high fold; and since 1995, using 3D reflection seismic. The 2D data are variable in fold of stack (12 to 60) and data quality (poor to good). The acquisition of the hydrocarbon exploration seismic data, in combination with Coal Authority high resolution seismic data over more localised areas, has resulted in an extensive network of seismic reflection profiles (Figure 3), except across the Anglo–Brabant Massif (Figure 4) and in the main conurbations. This account relies heavily on these data. The 3D data are now becoming available through release, but were not extensively used during the mapping programme. A few examples of these data are however included as figures. All phases of exploration have involved the drilling of deep boreholes which, together with surface exposures, provide stratigraphical calibration.

Depth conversion of interpreted seismic data to produce the structural maps was complicated by a number of factors, including the variable age of the data and the quantity and quality of the data. Horizons mapped from the seismic data in two-way-travel-time (TWTT) were converted to depth using velocity functions based on depths and times in key boreholes. Particular attention was paid to the depth conversion of the early Carboniferous interval, due to the known variation of Tournaisian and Visean lithologies (and thus interval velocities) at crop and in boreholes in surrounding basins (i.e. platform carbonates and basinal mudstone facies).

Summary of previous research

The Geological Survey surveyed and published maps on a scale of one inch to one mile (1:63 360) for virtually all of the region between 1844 and 1880. In Derbyshire, mapping commenced at the southern end of the Pennines in 1850, and extended into the adjacent parts of Nottinghamshire and Lincolnshire. The results of these coalfield surveys were summarised in memoirs by Aveline (1879) and Lamplugh and colleagues (1909, 1910, 1911). Between 1886 and 1903, several other volumes appeared by Jukes-Browne and Dalton (1885), Fox-Strangways (1903, 1905), Whitaker and Jukes-Browne (1899), Ussher et al. (1888) and colleagues describing the geology of parts of Lincolnshire, Northamptonshire and Leicestershire. Subsequently, various economic memoirs were published dealing with gypsum, iron ore, coal and refractory minerals (e.g. Gibson et al., 1908, 1913; Mitchell and Stubblefield, 1941a). Also significant in one of the driest parts of the country, this region is underlain by excellent aquifers, as described in memoirs on water supply by Lamplugh et al. (1914) and Ineson (1953). The first maps to show the distribution of drift cover were published in 1871. Rapid progress in mining led to an equally rapid increase in knowledge and revision of older maps between 1903 and 1909. Several special memoirs have also been published, including the Concealed Coalfield of Yorkshire and Nottinghamshire (first published in 1913, with subsequent editions appearing in 1926 and 1951). Other workers, such as Sorby, were active in the Yorkshire area at this time. The works of Wills (1951, 1956) greatly improved knowledge of the concealed coalfields and the Triassic system in the region. Post-war memoirs once again reflected the economic significance of the recently nationalised coal industry, as well as the developing oil province. Memoirs by Eden et al (1957), Edwards (1967) and Smith et al. (1973) are remarkable for their detail of information on the Coal Measures sequence and for their borehole correlations. The geology of the East Midlands (Sylvester-Bradley and Ford, 1968) contains numerous review papers, particularly on the exposed geology.

Another important new direction for survey lay in the previously neglected Fen country (e.g. Gallois et al., 1988), with numerous provisional 1:50 000 sheets issued in the 1990s. Recent years have seen a number of new memoirs and sheet descriptions for the central part of the region published by Carney, Ambrose* and colleagues (1995, 1997, 1998*, 1999, 2000*, 2001, 2002, 2003, 2004, 2007). In addition, the region has been at the front of pioneering work in 3D modelling and visualisation (e.g. Bridge et al., 1999; Jones et al., 2005; Sheppard, 2005a).

The drilling of numerous deep boreholes and the application of geophysical techniques of increasing sophistication and precision, in the search for hydrocarbons, revolutionised knowledge of the concealed subsurface of the region. Initially drilling was focussed in areas with a known history of mine oil seeps; this led to the discovery of the Eakring–Duke's Wood oilfield in 1939, and Egmanton, Plungar, Langar and Bothamsall fields a few years later (Falcon and Kent, 1960). In 1936–7 some of the principal tectonic elements were elucidated using gravity and magnetic surveys, and subsequently refined by early seismic refraction and reflection surveys. Exploration focussed on exposed Variscan inversion anticlines, for example at Ironville and Calow, just west of the region. Deep boreholes demonstrated the presence of early Carboniferous structures such as the Edale Gulf, Widmerpool Gulf and Gainsborough Trough (Lees and Tait, 1946; Falcon and Kent, 1960; Kent, 1966; Howitt and Brunstrom, 1966; Kent, 1975; George et al., 1976; Strank, 1987) and identified the principal 'fairway' of reservoir sands in the northern and western part of the region. Gravity and magnetic surveys (e.g. Lee, 1988) outlined the complex distribution of early Carboniferous basins in the northern parts of the region.

Seismic reflection data have revealed details of subsurface structures such as faults, folds, structural highs and basins (Figure 4). This new knowledge has convincingly demonstrated that the Palaeozoic and Mesozoic history of the region was controlled by the reactivation of major lines of basement weakness and development of syndepositional normal faults in the overlying cover. By the early 1980s these data were of sufficient quality to image the detailed structure of the early Carboniferous extensional basins (e.g. Smith et al., 1985; Gutteridge, 1987, 1991; Ebdon et al., 1990; Fraser et al., 1990; Fraser and Gawthorpe, 1990; Corfield et al., 1996; Rees and Wilson, 1998). The behaviour of the Carboniferous syndepositional faults and their relationship to the Variscan folding is well documented (e.g. Fraser et al., 1990; Fraser and Gawthorpe, 1990; Corfield et al. 1996). The faults are reactivated in compression, producing hanging-wall inversion anticlines. Interpretation of geophysical logs from boreholes has greatly aided stratigraphical correlation (Knowles, 1964; Spears, 1964; Whittaker et al., 1985; Bridge et al., 1998; Rees and Wilson, 1998; Powell et al., 2000).

In the 19th and 20th century, considerable advances were made in knowledge of the biostratigraphy and sedimentology of the Carboniferous succession. Namurian rocks of the Central Pennine Basin were the subject of ground-breaking sedimentological (Sorby, 1859) and biostratigraphical studies (Bisat, 1924, 1928). It was recognised that the Millstone Grit was provenanced in Greenland and Scandinavia (Gilligan, 1920), and that tectonic uplift had controlled sediment supply to the basin. Subsequent studies provided increasing stratigraphical and sedimentological resolution (e.g. Allen, 1960). Shackleton (1962) concluded that the Rough Rock was deposited by a host of small rivers and occasional flash floods. The Namurian of northern England was the subject of a pioneering sedimentological review by Reading (1964), and later work by Jones (1981) and Collinson (1988), which demonstrated the progressive infill of the Central Pennines Basin. Several authors have published on the sedimentology of particular intervals within the region (Walker, 1966; Trewin and Holdsworth, 1973; Collinson and Banks, 1975; Collinson et al., 1977; McCabe, 1977; Chisholm, 1981; Bristow, 1988; Maynard, 1992). Collinson (1988) and Steele (1988) provided the most recent reviews of the stratigraphy and sedimentology of Namurian rocks, with Holdsworth and Collinson (1988) reviewing the concepts and nature of the cyclicity associated with the Millstone Grit and its deposition.

Much of the early work on Westphalian successions was concerned with stratigraphy and palaeontology; sedimentological research did not commence until the late 1960s. Since the Second World War, the subdivision and correlation of the British 'Coal Measures' has relied heavily upon the identification of marine band faunas (e.g. Stubblefield and Trotter, 1957; Ramsbottom et al., 1978). It has been suggested that the bases of the Westphalian A, B and C stages should be defined by stratotypes in the Pennines, two of which lie close to the margins of this region: the base Westphalian B at Duckmanton, Derbyshire and base Westphalian C near Bolsover, Derbyshire. Despite early work on the coalfields of the region, relatively little sedimentological research on the Westphalian rocks was carried out until the pioneering work of Elliot (1968a, b). Subsequent work suggested deposition was in a delta plain setting that included lakes, lacustrine deltas, fluvial systems, distributaries, overbank and crevasse splay environments (Guion, 1971, 1978, 1984, 1987; Guion and Fielding, 1988). The region lay within the 'Pennine Province' (Trueman, 1954; Wills, 1948, 1951, 1956; Calver, 1968; Ramsbottom et al., 1978) which was bounded by landmasses forming significant palaeogeographical elements throughout the Carboniferous, both to the north (Southern Uplands Massif), and the south (Anglo–Brabant Massif or 'St George's Land'). Exploration by the British Coal Corporation has defined the southern boundary of the Pennine Basin in more detail. Study of Westphalian A–C sand body orientations and palaeo-slope analysis has led to important new understandings of sand body connectivity and the resultant implications to basin fill (Rippon, 1996; McGlenn and Rippon, 2005). During the 1980s and 1990s, mapping by BGS in the coalfields of the region led to a greater understanding of the Variscan Orogeny and its timing and effect on sedimentation (e.g. Waters et al., 1994). These authors have demonstrated how the early effects of the orogeny were felt in the region from late Westphalian B times, and that these movements greatly influenced sedimentary facies and distribution.

There has been much debate about the cause of cyclicity in the Carboniferous rocks of northern England. Bott and Johnson (1967) favoured tectonic control as the main driving force for sea level changes. Other authors emphasised the importance of eustatic sea level oscillations (Ramsbottom, 1973, 1974, 1977; Maynard and Leeder, 1992; Holdsworth and Collinson, 1988). The application of sequence stratigraphical principles (Fraser and Gawthorpe, 1990; Fraser et al., 1990; Read, 1991; Maynard, 1992; Martinsen, 1993) has gone some way to resolve these debates. The long exploration campaign by British Petroleum in the region contributed enormously to the development of these concepts. More recently, the widespread marine transgressions controlling the cyclicity of Namurian–Westphalian strata are increasingly being seen as glacioeustatic in origin (Martinsen et al., 1995), with mounting evidence for significant glaciation in Gondwana during the Carboniferous (Heckel, 1986; Veevers and Powell, 1987).

Much of the Permian to Mesozoic history of the region is closely associated with the evolution of the southern North Sea Basin (Cameron et al., 1992). Aside from the Quaternary deposits, the youngest preserved sedimentary rocks in the region are the Chalk of the Lincolnshire Wolds. The Mesozoic and Cenozoic evolution of northern England is poorly understood and has been the subject of much controversy; in particular the thickness and nature of any former cover over the Pennines. A Lake District–Pennine Island, emergent for much or all of Mesozoic time was proposed by Wills (1951) and supported by Kent (1974), Ziegler (1982) and Cope et al. (1992). However, Trotter (1929) calculated that up to 2100 m of Mesozoic rocks, in addition to an unknown amount of Carboniferous strata, had been eroded from blocks in northern England; this view was supported by Eastwood (1935), Taylor et al. (1971) and Arthurton et al. (1978). Depth of burial and eroded overburden studies, based on well log and apatite fission track data, now provide support for the former presence of kilometre-scale cover successions, subsequently removed by post-Cretaceous uplift and erosion (Green, 1986; Lewis et al., 1992; Chadwick et al., 1994; Green, 2005).

Chapter 2 Precambrian basement

The Pre-Carboniferous basement of the region comprises complexly structured Precambrian (Neoproterozoic) rocks, unconformably overlain by Lower Palaeozoic rocks, both being deformed during the polyphase Caledonian Orogeny. This chapter describes the Precambrian rocks of the region. Although the nearest crop is in Charnwood Forest, just beyond the western edge of the region, these strata are encountered in a few deep boreholes (Figure 5). The seismic character of much of the Precambrian succession is rather uniform due to the lack of compositional contrast. At outcrop the rocks may show layering, but good reflectivity is only likely to be generated by gently dipping intrusive contacts or shear zones.

Early plate tectonic models for the Precambrian of southern Britain (e.g. Dewey, 1969; Baker, 1971; Wood, 1974; Barber and Max, 1979; Thorpe et al., 1984) inferred late Proterozoic subduction of oceanic lithosphere south-eastward beneath Wales, and assumed all components were part of one contemporaneous system. The possibility of transcurrent displacement was ignored. The introduction of high precision U-Pb zircon and Ar-Ar mineral dating techniques enabled the history of this destructive margin to be established (Tucker and Pharaoh, 1991; Noble et al., 1993). Analysis of Precambrian rocks at outcrop, and of gravity and magnetic potential fields, led to the proposal that while comparable to the Avalonian successions of the type area in Newfoundland, they belong to a series of 'terranes' with distinct histories and geochemical signatures (Pharaoh et al., 1987b; Pharaoh and Carney, 2000). They are separated by important structural lineaments (Figure 5), many reactivated during subsequent orogenic and rifting episodes, which played a significant role during the Palaeozoic, Mesozoic and Cenozoic structural evolution of the region (Pharaoh and Gibbons, 1994).

Two of the five proposed Precambrian terranes in southern Britain, namely the Charnwood and Fenland terranes (Pharaoh et al., 1987a, b; Pharaoh and Carney, 2000), are believed to underlie the region. They were accreted to the South American margin of the Proto-Gondwana Supercontinent, while it was located at low southerly latitude in latest Neoproterozoic times (Cocks and Torsvik, 2006). Together they form the Avalon Superterrane (Gibbons, 1990; Gibbons and Murphy, 1995; Gibbons and Horák, 1996) or Composite Terrane (Keppie, 1985). Transcurrent movement along the terrane boundaries is thought to have been significant during the terminal phases of Neoproterozoic subduction, about 570 to 540 Ma, when oblique subduction gave way to the development of a sinistral transcurrent system (e.g. Gibbons, 1987, 1990; Gibbons and Horák, 1996; Nance and Murphy, 1996). The youngest Precambrian contribution to the crust is the Warren House group of the Malvern Hills, interpreted as marine marginal basin crust (Pharaoh et al., 1987a) formed at 566 ± 2 Ma (Tucker and Pharaoh, 1991). Magmatism within the ensialic Uriconian marginal basin persisted until 560 ± 1 Ma (Ercall Granophyre). It is likely that the Precambrian craton was stabilised soon after this, probably following the folding of the Longmyndian Supergroup.

Terranes

Charnwood Terrane

The Charnwood Terrane forms the eastern half of the Midlands Microcraton (Figure 5). The key surface outcrops of this terrane lie just to the west of the region, in Charnwood Forest and in the Nuneaton area (Pharaoh and Carney, 2000; Smith et al. 2005). It is likely that the part of this terrane, which lies within the Charnwood Boundary Fault System (Smith et al., 2005), extends into the extreme south-western corner of the region (Figure 5). In Charnwood Forest, just to south-west of the region, the Charnian Supergroup is over 3000 m thick. It comprises well-bedded volcaniclastic rocks deposited in moderately deep water, representing the fill of a marine marginal basin located adjacent to a calc-alkaline volcanic arc. Pyroclastic fragments are present in these volcaniclastic strata (Carney, 2000), but primary pyroclastic units (e.g. welded ashflows) have not been recognised. Instead there are slump breccias and debris flows at numerous horizons, which testify to an unstable tectonic environment. Domes of dacite and andesite were emplaced into wet sediment at Bardon and Whitwick. Numerous bodies of diorite, some granophyric (North and South Charnwood Diorites), representing a more evolved, high-K calc-alkaline suite (Pharaoh et al., 1987b) were emplaced late in the magmatic episode. Equivalent rocks in the Nuneaton area were dated at 603 ± 2 Ma using the U-Pb zircon method (Tucker and Pharaoh, 1991), suggesting that most of the Charnian strata, including those bearing Ediacara-type fossil assemblages, are older than about 603 Ma, although this has been disputed (Compston et al., 2002). The magmatic arc itself is not exposed however. Sedimentological criteria suggest it lay to the north-west (Moseley and Ford, 1985). Geophysical criteria suggest it lay to the south-west (Pharaoh et al., 1991). The geochemical signature of the magmatic rocks is very distinctive, clearly distinguishing them both from other penecontemporaneous Precambrian magmatic suites (e.g. the Fenland suite), and later Ordovician calc-alkaline arc magmatic rocks (Pharaoh et al., 1991). The trace element and isotopic evidence indicate that the Charnian magmatic arc was founded on immature continental crust (Pharaoh et al., 1987b), unlike the Fenland arc.

Recently it has been demonstrated that epiclastic strata of the Brand Group, the uppermost level of the stratigraphy in the exposed Charnwood Massif, contain the trace fossil Teichichnus sp, indicating an early Cambrian age (Bland, 1994). These strata, which share the same greenschist facies metamorphism and cleavage as the Charnian Supergroup, must lie unconformably upon the volcanic part of the succession. The cleavage in both volcaniclastic and epiclastic sequences has been dated at about 410 Ma by the Ar-Ar method (Carney et al., 2008), close to, but not quite as young as the best estimate for Acadian (latest Caledonian) deformation (Sherlock et al., 2003). The cleavage strikes west-north-west and transects the axial planes of the north-west trending fold axial planes (Carney et al., 2008). Thus the cleavage, the greenschist metamorphism and probably also the folding must be latest Silurian, rather than Precambrian age as previously supposed.

The extent of the Charnian rocks eastward beneath the Hathern Shelf is unknown. Cleaved rocks proved by the Sproxton and Foston boreholes were previously correlated with the Charnian Supergroup (e.g. Kent, 1967) but geochemical evidence has cast doubt on this interpretation. For these occurrences an Early Palaeozoic age is now regarded as more likely (Pharaoh et al., 1991).

Fenland Terrane

The Fenland Terrane lies north-east of the Charnwood Boundary Fault System (Pharaoh and Carney, 2000; Smith et al., 2005), and forms the unexposed Precambrian basement to at least part of the concealed Caledonides of eastern England (Pharaoh et al., 1987a). Ignimbritic ashflow tuffs of felsic composition (Dearnley, 1966) were encountered in the Glinton 1, Orton and Oxendon Hall boreholes (Figure 5). These have yielded precise U-Pb zircon ages of 616 ± 6 and 612 ± 21 Ma (Noble et al., 1993).

Although comparable in age to the Charnian magmatic rocks, they are petrographically and geochemically distinct, with ∈Nd isotopic ratios indicating the involvement of mature continental crust only in the Fenland suite (Noble et al., 1993). The petrographical, geochemical and isotopic signatures of the latter are closer to that of the ignimbritic tuffs of the contemporaneous Arfon Group of North Wales, and the two suites may be correlative. The occurrence of the known examples of the Fenland suite is structurally controlled by lineaments within the concealed Caledonides: the Charnwood/Fenland terrane boundary, and the Glinton Thrust (Chapter 3).

Chapter 3 Early Palaeozoic to Mid Devonian: orogenic basement

Lower Palaeozoic rocks crop out at very few places in the region. However, numerous boreholes (mostly for water) in the south of the region show that the basement of the Anglo–Brabant Massif, subcropping over about half of the region, is mainly composed of such rocks. They are also widely encountered in numerous deep exploration boreholes throughout the region (Figure 6). They belong to two main tectonic regimes: the Midlands Microcraton, in the south-western extremity of the region; and the concealed Caledonides of eastern England (Pharaoh et al., 1987a; British Geological Survey, 1996), over the majority of the region.

Plate tectonic setting and structural evolution

The Early Palaeozoic geological evolution of the region was dominated by the creation and destruction of the Iapetus Ocean, (Dewey, 1982; Soper and Hutton, 1984) and in particular, the part of the ocean basin known as the Tornquist Sea (Cocks and Fortey, 1982). Iapetus began to open in late Neoproterozoic times as the supercontinent of Pannotia (Dalziel, 1997) broke up, as reflected by rifting and volcanism in Scandinavia and the East European Platform (Pharaoh et al., 2006), generating the smaller continents of Laurentia, Baltica and Gondwana. Stages in the subsequent closure of the ocean (Figure 7) are marked by the several deformation phases of the Caledonian Orogeny and the progressive consolidation of the Caledonian Orogen, which traditionally includes the Early Palaeozoic areas of the British Isles and Scandinavia, together with adjacent areas of Svalbard, Greenland, Ireland and the northern Appalachians. In Cambrian and Ordovician times, several small microcontinents were rifted away from the northern margin of Gondwana and drifted northwards to be sequentially accreted to Baltica and Laurentia (Figure 7)b, c, d. Each phase of accretion resulted in the closure of small ocean basins, the formation of crustal sutures, and collisions between arcs, mobile terranes and adjacent continental margins of Laurentia and Baltica (McKerrow and Soper, 1989; MacNiocaill et al., 1997; van Staal et al., 1998; Cocks and Torsvik, 2006). The 'Caledonian Orogeny' has therefore been redefined (McKerrow et al., 2000) and restricted to phases of tectonism within, and on the borders of the Iapetus Ocean, from Cambrian to Mid Devonian times. The phases were often localised and in many instances diachronous, spanning a time interval of some 200 Ma. They include the Penobscottian (Tremadoc), Taconian and Shelveian

(Mid and Late Ordovician), Scandian (late Silurian), Acadian (Early Devonian), Ligerian (Mid Devonian) and Bretonian (latest Devonian) phases (van Staal et al., 1998; Ziegler, 1990), although not all are represented in the region. Final closure of the Iapetus Ocean completed the amalgamation of Laurussia (or 'Old Red Sandstone Continent').

Cambrian to Early Ordovician rifting

In Cambrian times, the region formed part of the Gondwana Supercontinent, located at high (>60°S) palaeolatitudes, on the southern margin of the Iapetus Ocean. In southern Britain, evidence for rifting of the supercontinent in earliest Cambrian times is found in the deposition of the basal Cambrian clastic succession (Brasier et al., 1978; Dalziel, 1997). This was followed by deposition of generally fine-grained strata across a gently subsiding shelf region. Differentiation into fault controlled basins and highs probably occurred in mid Cambrian times, with accelerating rates of subsidence in the late Cambrian (Smith and Rushton, 1993). During the Early Ordovician (Tremadoc), there was a dramatic reorganisation of the plate tectonic configuration. An elongate piece of crust broke away from the edge of Gondwana (Figure 7)b to form the Avalonia Terrane (Ziegler, 1990), of which the region occupied the eastern part. It drifted rapidly northwards from high southerly latitudes to around 30°S (Figure 7)d by Late Ordovician times (Torsvik and Trench, 1991; Trench and Torsvik, 1992; Trench et al., 1992). This was achieved by subduction of the Iapetus Ocean along the northern margin of Avalonia (slab-pull and roll-back), reflected in the late Tremadoc onset of subduction-related calc-alkaline magmatism in north Wales (Kokelaar et al., 1984), and by the opening of the Rheic Ocean (ridge-push) between Avalonia and Gondwana (Cocks and Fortey, 1982). These events are reflected in significant changes in subsidence rate in the basins of southern Britain (Smith and Rushton, 1993; Prigmore et al., 1997).

Mid to Late Ordovician magmatism and hydrothermal activity

Ongoing subduction in Mid Ordovician times (Figure 7)c led to a lithospheric thermal anomaly and buoyancy of the volcanic arc and its retro-arc margin. The region appears to have straddled the boundary between these two domains. The Midland Microcraton occupied the buoyant backarc region during Ordovician arc magmatism, as indicated by the near absence (due to non-deposition or erosion) of Ordovician strata, emplacement of numerous minor intrusions of generally lamprophyric affinity (Thorpe et al., 1993; Pharaoh et al., 1993) and a strong thermal overprint on sedimentary fabrics in Cambro-Tremadocian strata (Merriman et al., 1993). Llanvirn strata were encountered at Great Paxton and Wyboston, but Caradoc sedimentary strata are absent. They are replaced by volcanic and volcaniclastic rocks of equivalent age. Felsic ashflow tuff at North Creake has been dated at 449±13 Ma using the zircon U-Pb method (Noble et al., 1993). Several other occurrences of volcanic rock are of suspected Ordovician age (see below), but have not so far been dated using the robust and precise U-Pb method. The subvolcanic microgranite intrusion proved by Claxby 1 Borehole has been dated at 457 ± 20 Ma by this method (Noble et al., 1993), and may represent a hypabyssal offshoot of the batholith postulated beneath The Wash (Pharaoh et al., 1997). Several plutonic intrusions belonging to a consanguineous, calcalkaline series are found in the region (Pharaoh et al., 1993). These include the exposed South Leicester Diorite Suite and Mountsorrel Granodiorite, dated at 452 ± 10 Ma (Pidgeon and Aftalion, 1978; Noble et al., 1993); the granodiorites proved by Rempstone and Kirby Lane boreholes, and the diorite sill proved by Warboys Borehole, just beyond the south-eastern margin of the region. The hydrothermal effects associated with this magmatism may have persisted into earliest Silurian times (see below).

Mid Ordovician calc-alkaline arc magmatism related to these plate movements can be traced from northern England through eastern England to Belgium (Fitton and Hughes, 1970; Pharaoh et al., 1993; Van Grootel et al., 1997) and is attributed to closure of a small ocean basin, the Tornquist Sea (Cocks and Fortey, 1982), which lay between Avalonia and Baltica (Pharaoh et al., 1993; 1995; 1999). However, there is still controversy regarding the polarity of this subduction; most models invoke a south-east-dipping subduction zone in north-west Wales to account for the Leinster–Lake District arc.

Shelveian (Late Ordovician) deformation phase

The Late Ordovician Shelveian (Ashgill) deformation phase (Toghill, 1992) resulted from the 'soft collision' (docking) of Avalonia with Baltica to produce Balonia, following closure of the Tornquist Sea (Pharaoh et al., 1995; Torsvik, 1998). This oblique collision involved significant dextral transcurrent shear (Torsvik and Trench, 1991; Trench and Torsvik, 1992). Deformation was intense in the mid North Sea region (Frost et al., 1981; Pharaoh et al., 1995) where prograde greenschist–amphibolite metamorphism is dated at 450–425 Ma by the 40Ar-39Ar method (Frost et al., 1981). To the west of the region it led to the reactivation of the south-west-trending Menai Straits and Welsh Borderland lineaments, in particular uplift along the Tywi Lineament/Anticline (Woodcock, 1984a, b; Woodcock and Gibbons, 1988; Lynas, 1988; Arthurton et al., 1988; Toghill, 1992). Localised folding and faulting of Cambrian–Tremadoc strata (the 'Hartshill Event') associated with dextral transpression of probable Late Ordovician age is also recognised near Nuneaton (Carney et al., 1992; Bridge et al., 1998), just to west of the region. Shelveian deformation is not recognised within the region, and is likely to have been restricted to lineaments with a complex history of reactivation and therefore overprinted. The overall north-west to south-east structural trend of the basement in the region, has more to do with the evolution of the Tornquist-related Caledonides, than the exposed Caledonide terranes of western Britain (e.g. in Wales and northern England). The Midlands Microcraton was uplifted during this phase, with associated thrust faulting along the north-west and north-east margins (Smith, 1987). During a later phase of reactivation, during the Variscan Orogeny, the north-eastward dipping Thringstone Fault emplaced Tremadoc rocks over Westphalian to the south-west (Butterley and Mitchell, 1946). Farther south-east, Charnian rocks are probably emplaced over Carboniferous strata along this fault. This is considered to be the probable deformation boundary of the concealed Caledonides (Pharaoh, 1999; Smith et al., 2005).

Silurian basin development

Subduction of the Iapetus Ocean north-westward beneath Laurentia continued throughout Llandovery and Wenlock times (Leggett et al., 1979). During this time, two deep water basins became established, the Welsh Basin in the west and the Anglian Basin in the east. The wedge-shaped Midlands Microcraton now formed a platform with episodic development of carbonates, as well displayed in the classic Silurian sequences of the Welsh Borderland (Smith et al., 2005). The latter contrast strongly with the turbidites deposited in the rapidly subsiding deep water basins of Wales, northern England and East Anglia (Woodcock and Pharaoh, 1993; Verniers et al., 2002), areas which would subsequently become the Acadian 'slate belts' (Soper et al., 1987).

Acadian deformation phase (late Silurian–Mid Devonian)

The final episode of the Caledonian Orogeny was the Acadian deformation phase (McKerrow, 1988; McKerrow et al., 2000) in latest Silurian to Mid Devonian times. The deep water turbidite basins in Wales and East Anglia (Woodcock and Pharaoh, 1993) shallowed and their final Downtonian–Dittonian strata were deposited under terrestrial braidplain conditions. Possibly beginning in latest Silurian times (Carney et al., 2008), these basins were inverted. The cause of the deformation is uncertain; it may have been the early impingement of part of the Armorican Terrane Assemblage onto the Avalonian part of Laurussia (Soper et al., 1987); or possibly due to low-angle (Andinotype) subduction of Rheic Ocean lithosphere northward beneath Laurussia (Woodcock et al., 2007). Whatever the cause, the tectonic consequences for southern Britain were severe.

The triangle-shaped Midlands Microcraton was driven northwards, indenting the accretionary mosaic resulting from the soft collision of Laurentia, Baltica and Avalonia some 30 Ma previously (Figure 7)c. Acadian deformation was most intense in the former deep water basins lying either side of the Midlands Microcraton, in Wales, northern and eastern England (Turner, 1949; Soper et al., 1987; Pharaoh et al., 1987a; Woodcock, 1991; Woodcock and Pharaoh, 1993; Smith et al., 2005). Here, thick turbidite and hemipelagite sequences suffered strong folding, cleavage formation and metamorphic recrystallisation. The crustal lineaments on the northwestern flank of the microcraton were reactivated during sinistral transpression, and folds with anticlockwise-transecting cleavage developed in the 'slate belts', for example in Wales and the Lake District (Soper et al., 1987).

The crust of the former deep water basins was thickened and melts of lower crustal material were emplaced as a number of Early Devonian granite complexes. On the north-eastern flank of the microcraton, north-west-trending lineaments such as the inferred Charnwood Boundary Fault Zone and the Dowsing–South Hewett Fault Zone, are inferred to have experienced dextral transpression (Pharaoh et al., 1995). These basement structural trends were probably established during the Ordovician closure of the Tornquist Sea (Pharaoh et al., 1995), and suffered reactivation and tightening during the Acadian Phase. While the microcraton suffered only slight deformation and is gently metamorphosed at diagenetic–low anchizonal grade (Merriman et al., 1993), a strong cleavage or schistosity is developed in the Charnian Supergroup and Brand Group at outcrop (Carney et al., 2008) and in the deep boreholes within the concealed Caledonides. Uplift and erosion in Mid Devonian times was followed by renewed deposition in late Givetian times (e.g. in the London region).

Lower Palaeozoic rocks of the region

Cambrian and Ordovician sedimentary and volcanic rocks

Fossiliferous Cambrian–Tremadoc strata (Stockingford Shales) are known from a few cuttings, and several shallow boreholes drilled in the late 19th century in the vicinity of Leicester (Harrison, 1885; Fox-Strangways, 1903; Le Bas, 1968): Elmesthorpe, Bosworth Wharf, Kingshill Spinney, Lodge Farm No. 2, Crown Hills, Knighton Fields and Cowpastures boreholes (Figure 6). Hornfelsed metasedimentary rocks cropping out in the aureole of Mountsorrel intrusion at Brazil Wood have been correlated with the Stockingford Shales (Harrison, 1885; Le Bas, 1968).

The Eakring 146 Borehole (Figure 6) proved 84 m of grey phyllitic mudstones and subordinate sandstone, beneath a coarse clastic sequence of latest Devonian or earliest Carboniferous age (Edwards, 1967). Phosphatic fragments in thin section from 2200 m depth, possibly derived from trilobites and an Acrotreta-like brachiopod, led C J Stubblefield (in Edwards, 1967) to infer an Early Palaeozoic, possibly Cambrian age for these strata. The core is affected by steep bedding dips, veining and slickensides and was metamorphosed under high anchizonal conditions (Pharaoh et al., 1987a; Merriman et al., 1993). Thin (<10 m thick) flows of rather altered andesite and dacite lava are intercalated with the sedimentary rocks at 2204 m and 2240 m depth. They are geochemically similar to lavas in the Cox's Walk Borehole on the Foston High, which proved 243 m of andesite, dacite and rhyolite lavas (Pharaoh et al., 1991), unconformably overlain by Tournaisian–Visean strata. Despite extensive alteration, primary igneous textures, for example amygdales, flowage textures and zoning in plagioclase, are well preserved (Berridge et al., 1999). The lavas exhibit a calc-alkaline fractionation trend. A Rb-Sr isochron age of 466 ± 11 Ma, Mid Ordovician on the Gradstein et al., 2005 timescale, was interpreted as the age of eruption, given the lack of penetrative deformation (Pharaoh et al., 1991). This interpretation requires confirmation from U-Pb mineral dating, which has demonstrated the ease with which the Sr isotopic system can be reset by low grade metamorphism (Noble et al., 1993).

North Creake Borehole, in Norfolk, proved a felsic ashflow tuff (Pharaoh et al., 1991), dated by Noble et al. (1993) at 449 ± 13 Ma. Felsic and intermediate volcanic rocks were also proved at Great Osgrove Wood, Hollowell, Sproxton, and Upwood, as described by Pharaoh et al. (1991). These have not yet yielded precise radiometric ages, but an Ordovician age is suspected (Pharaoh et al., 1993). Green tuffs were encountered by Beckering 1 Borehole on the north-western flank of the Stixwould High (Figure 6). Lithic sandstones proved by the Bardney 1, Gas Council Stamford 2, 10 and Wittering 1 boreholes are associated with the volcanic proving, and may also be of Ordovician age (Pharaoh et al., 1991).

The Ironville 5 Borehole, a little west of the region, proved 95 m of weakly metamorphosed, grey, bioturbated sandstones, siltstones and mudstones that yielded an acritarch flora of Early Ordovician, Tremadoc or Arenig age (Molyneux, 1991). Argillaceous layers are affected by an intense, penetrative cleavage postdating a bedding-parallel mica fabric, possibly developed during diagenesis and/or burial metamorphism (Pharaoh et al., 1987a). Mica crystallinity values indicate high anchizonal metamorphic conditions (Merriman et al., 1993). Numerous highly altered sheets of spessartitic lamprophyre, up to 5 m thick, intrude the metasedimentary strata. In thin section they contain pseudomorphs after pyroxene or amphibole, and probably also after olivine. They belong to the Midlands Minor Intrusive Suite (Pharaoh et al., 1993) that commonly intrudes Cambro-Ordovician strata in the Midlands (Thorpe et al., 1993). This subduction related magmatic suite (Pharaoh et al., 1993) may have been emplaced at various times from the Tremadoc onwards; the only reliable radiometric date, obtained using the U-Pb method on badelleyite, gives an earliest Silurian age of about 442 Ma (Noble et al., 1993). Halton Holegate 1 Borehole (Figure 6) is believed to have entered metasedimentary basement at 1604 m depth interpreted as a faulted contact (Figure 24). Inspection of the cuttings suggests that an upper unit of lithic sandstones (greywackes) and interbedded slates, about 115 m thick, overlies 240 m of slates (to TD). A poorly preserved acritarch flora including Acanthodiacrodium spp. indicates an Ordovician (tentatively Early Ordovician) age (S G Molyneux, written communication, 1989). The geophysical log (high density, gamma, high sonic velocity) signatures of the metasediments are rather similar to the Ordovician greywackes and slates proved by the Burmah offshore well 47/29A-1, located in The Wash about 30 km to east of Halton Holegate (Figure 6).

The Upwood Borehole proved Ordovician; Great Paxton Borehole, uncleaved mudstones, siltstones and thin sandstones of Llanvirn age (Stubblefield, 1967; Skevington, 1973; Rushton and Hughes, 1981; Jenkins, 1983) with 60–70° dips; Huntingdon Borehole proved grey siltstones and mudstones yielding a Llanvirn biostratigraphical age (Strachan in Williams et al., 1972), and Wyboston Borehole proved steeply dipping, grey, silty mudstones assigned a Tremadoc age (Stubblefield, 1967; Bulman and Rushton, 1973). All these occurrences lie just off the eastern edge of the microcraton (Molyneux, 1991).

Undated quartzite–phyllite association (?Cambrian)

In Lincolnshire and Norfolk, a large number of boreholes enter moderately deformed unfossiliferous metasedimentary rocks of uncertain age and affinity. In the most part these are known only from cuttings or at best, short cored sections. None has yielded a reliable biostratigraphical age. The boreholes considered here include (in alphabetical order) Bardney 1, Beckering 1, Galley Hill, Gas Council GH10, Gas Council Gst10, Grove 3, Hunstanton, Lexham 1, Nocton 1, Sibsey 1, South Creake 1, Spalding 1, Stixwould 1, Welton 1, Wiggenhall 1 and Wisbech 1, all of which contain unfossiliferous, very hard, quartzitic sandstones (or 'quartz-rich metamorphic rocks') with variable amounts of phyllite, or phyllites alone. A Palaeozoic, specifically Cambrian age, has been inferred for some of these quartzites by analogy with the quartzites at Nuneaton (BP unpublished well composite logs for Nocton, Bardney, Stixwould etc). The core samples from boreholes at Galley Hill, Nocton, South Creake and Stixwould are quartzitic sandstones usually preserving clastic grain shapes but with matrix phyllosilicates (white mica±chlorite) forming coarse (crystallites >0.1 mm) books and flakes in strong preferred orientation. The dip of bedding ranges from 5–90°, with a predominance of steep dips (>50°). Hunstanton, Lexham, Spalding and Wisbech boreholes entered similar rocks according to their cuttings log descriptions. Grove 3 and Welton 1 boreholes penetrated chloritic phyllites (associated with quartzite at Grove 3) and from cuttings descriptions, Sibsey 1 appears to have encountered a similar lithology, possibly containing garnet and amphibole in addition to the ubiquitous chlorite and epidote (drilling log). Although the terminal lithology at Bardney was described as a green-grey quartzite, and compared with Nocton (BP Completion Report; Kent, 1967), the lithology is actually a greywacke containing intraclasts of grey siltstone and mudstone. The latter contain a weak upright cleavage while the clastic matrix appears uncleaved. A similar lithology is believed to form the pre-Westphalian basement penetrated by Beckering 1 Borehole (unpublished British Gas Well Composite Log). This has a sonic velocity (Vp) of 5800 ms-1. The GH10 borehole lies close to the subcrop of proven Early Palaeozoic (Cambrian–Tremadoc) rocks at the edge of the Midland Microcraton.

Foston 1 Borehole proved 134 m of well cleaved purple and olive green metasiltstones, unconformably overlain by Tournaisian–Visean strata (Berridge et al., 1999). These 'slates' were previously correlated (Kent, 1967) with the highest unit of the Charnian Supergroup (Precambrian) of Charnwood, but an Early Palaeozoic age for the protolith is equally likely (Berridge et al. 1999). A penetrative slaty cleavage is crosscut by a crenulation cleavage. A white mica crystallinity value of 0.22° 2θ indicates greenschist facies metamorphic conditions (Pharaoh et al., 1987a).

Without biostratigraphical dating, the affinity and correlation of these rocks remains speculative. One possibility is that they are correlative with the Brand Group (Cambrian) of Charnwood Forest, which contains similar rock types; the lithological similarity (degree of metamorphism excepted) with the Cambrian quartzites at Nuneaton (Wills and Shotton, 1934; Brasier et al., 1978) noted by BP has already been mentioned; they could be correlatives of the lithologically similar 'Bedded Succession' (South Stack Series, New Harbour Group) of the Mona Complex in Anglesey, now believed to have a Cambrian age (Collins and Buchan, 2004); or they could be counterparts of the lithologically similar Tubize Formation of the Brabant Massif dated by acritarchs and Oldhamia assemblages to latest Neoproterozoic and Cambrian (Vanguestaine, 1973; De Vos et al., 1993). All of these correlations favour a Cambrian age for the association.

Ordovician plutonic rocks

Ordovician plutonic rocks crop out at Mountsorrel, near Leicester, where they intrude supposed Stockingford Shales (Cambrian–Tremadoc). The age of emplacement was recalculated by Noble et al. (1993) at 452 ± 10 Ma. They are also found in a series of fossil 'inselbergs' in the Triassic landscape of South Leicestershire, known as the South Leicestershire Diorites. The diorite (Elmesthorpe), tonalite (Stoney Stanton, Croft, Enderby) and microtonalite (Narborough) 'bodies' are now regarded as belonging to one differentiated tabular sheet-like intrusion (Le Bas, 1968; 1972) for which there is seismic evidence in the Leicester district (Allsop, 1987; N J P Smith, pers. comm, 2009). The Rempstone 1 Borehole proved 82 m of granodiorite in the footwall of the Normanton Hills–Hoton Fault, on the southern margin of the Widmerpool Half-graben (Pharaoh et al., 1993). The Rempstone Granodiorite is xenolithic and less altered than that proved by the Kirby Lane Borehole (Le Bas, 1972), near Melton Mowbray, and is petrographically similar to the Mountsorrel Granodiorite. All the above intrusions belong to a co-magmatic series of small calc-alkaline plutons in the East Midlands basement (Le Bas, 1972; 1982) that were generated above a subduction zone in Mid Ordovician (Caradoc) times (Pharaoh et al., 1993).

Undated hornblende-quartz-diorite and 'granite' were also encountered by the Desford (Stocks House) and Cottage Homes (Countesthorpe) boreholes near Leicester. According to Le Bas (1968) these have affinities to the Barrow Hill Diorite, a component of the South Leicester Diorite Suite. Although the putative Wash Batholith (see below) itself remains undrilled, a borehole at Claxby Pluckacre (Claxby 1), near Horncastle (Figure 6), proved a sodic microgranite at 1321.31 m TVDSS, unconformably overlain by Westphalian strata (Pharaoh et al., 1997). Granophyric texture suggested that the Moorby Microgranite is likely a hypabyssal intrusion. The emplacement of the microgranite was dated at 457 ± 20 Ma (Mid Ordovician) using the zircon U-Pb method (Noble et al., 1993). The geochemical and isotopic composition of the microgranite suggest geochemical similarity to the sodic granite proved at Wensleydale (Pharaoh et al., 1997).

Hydrothermal alteration

The Galley Hill Borehole, located to the south-east of Grantham (Figure 6), penetrated 61 m of metasedimentary rocks underlying the Sherwood Sandstone Group (Triassic) at 560 m drilled depth. They comprise coarse siltstones and fine-grained sandstones, generally pale brownish-grey or greenish-grey in colour, of uncertain biostratigraphical age. A weak, non-penetrative fabric of fracture or locally, crenulation type, is present. Original sedimentary layering shows a highly variable dip. Tight folds of sedimentary layering are interpreted to have a synsedimentary origin. Irregular zones of rapid transition from pale to dark in red, mauve and purple shades, discordant to the bedding lamination, are interpreted as 'diffusion fronts' resulting from hydrothermal alteration. At a number of levels dark tourmalinised spots (<1 cm) and veins are present, parallel to the fracture cleavage. Thin sections reveal a granoblastic texture, indicating that the sedimentary protolith recrystallised as a quartzo-feldspathic hornfels. Ragged, anhedral porphyroblasts of biotite, up to 500 µm, are largely retrograded to chlorite and opaque minerals. White mica occurs both as fine- grained crystals in an inherited matrix between the other minerals, and as a part of the granoblastic fabric. The granoblastic fabric is cut by networks of fractures on a spacing of a few millimetres. Some of these contain a fill of white mica, others contain tourmaline, but the two minerals are not observed in close association. Tourmaline veining can be associated with quartz veinlets up to 2 mm wide, transecting the granoblastic fabric. Electron microprobe analysis indicates that the tourmalines belong to the alkali group and are of a schorlitic nature. Average Fe/(Fe+Mg) and Na/(Na+Ca) ratios are 0.60 and 0.90, respectively. Intermediate compositions of tourmaline between the schorl and dravite end-members are typical for tourmaline generated by alteration of (meta)pelites (Henry and Guidotti, 1985). White mica, hand-picked from the samples, yielded a near-plateau age of 435.6 ± 1.6 Ma by the Ar-Ar method (A Boven, unpublished, pers. comm, 2005). If the observed mineralisations are indeed associated with the late hydrothermal stages of an intrusion, then the emplacement is probably older than the measured age of 435.6 Ma, either latest Ordovician or earliest Llandovery age.

Silurian

No clear evidence has been found for the continuation into the region of the platformal Silurian strata inferred to underlie the Derby High, and possibly the western part of the Hathern Shelf (Smith et al., 2005). The borehole provings at Lakenheath 1, of possible Pridoli age (Bassett et al., 1982), and Eriswell, likely represent late shallowing up successions within the Anglian Basin (Woodcock and Pharaoh, 1993), which largely lies to the east of the region, in East Anglia and the southern North Sea.

Structure of the Caledonide Basement

Geophysical potential field evidence

In the absence of any significant outcrop of Caledonian basement within the region, geophysical (gravity and magnetic) potential field data provide useful information on structural trends in the basement of the Anglo–Brabant Massif (Cornwell and Walker, 1989; Lee et al., 1990; 1991; 1993; Busby et al., 1993; Busby et al., 2006). Gravity lineaments (Figure 9) have a north-westerly trend, parallel to thrust structures known at crop, such as the Thringstone Fault, and structures known from seismic data, such as the Eakring–Denton–Glinton linked thrust system (see below). By contrast the magnetic lineaments, with a west-north-westerly trend (Figure 10) are orientated obliquely to the gravity lineaments (Cornwell and Walker, 1989). Wills (1978) interpreted these as Precambrian basement ridges. This relationship might be reflecting the occurrence of two generations of structural trends within the basement, i.e. late Neoproterozoic (Avalonian/Cadomian) and Early Palaeozoic (Shelveian and/or Acadian), which are inferred to have affected the region (see above). The recent recognition that much of the deformation within the Precambrian sequence is actually of early Acadian age means that an alternative hypothesis is equally viable; that the trend of the gravity lineaments is reflecting the orientation of major block-bounding thrust structures; while the magnetic lineaments denote the orientation of magnetic features (Fe-rich lavas, intrusions and metasedimentary rocks) internal to the major blocks. This is backed up by the field observations supporting a regime of dextral transpression during Acadian deformation (Carney et al., 2008). Thus both geophysical trends may have developed contemporaneously, within the Acadian transpressional regime.

The Wash Batholith and other putative intrusions

Gravity and seismic refraction evidence for the possible presence of a large batholithic complex beneath The Wash was reviewed by Chroston et al. (1987). Unlike some of the crustal blocks of northern Britain, for example at Wensleydale and Weardale (Dunham, 1974), this prominent negative gravity anomaly has not yet been drilled. Combined gravity and magnetic (GRAVMAG) modelling suggests that a cupola of this putative granite batholith may approach to within a kilometre of the surface in the south-western corner of The Wash, near Boston (Figure 8). Magnetic anomalies are located at the margins of the discrete gravity lows, and support the interpretation of the latter as intrusions. It is likely that the batholith underpins much of the Anglo–Brabant Massif (Boston High), a persistent feature since Devonian times. Indeed its presence may be the reason for the very existence of the high itself. Evidence for the persistent buoyancy of the high will be presented in later chapters. The GRAVMAG modelling indicates no direct linkage with the large negative anomaly associated with the putative Wash Batholith (Pharaoh et al., 1997), but is permissive of an offshoot from the latter, intruding metasedimentary rocks of the type proven by Sibsey and Halton Holegate boreholes (Figure 8).

Seismic evidence

Evidence from seismic data also suggests a north-west to south-east trend for structures within the Caledonian basement. The Glinton Thrust has been mapped using seismic data (Pharaoh, in Chadwick and Evans, 2006) as a north-west-striking, north-east-dipping thrust within the Anglo–Brabant Massif, of presumed Acadian age (Figure 11). Its mapped trace in the subsurface lies parallel to a major gravity lineament, a little further north-east (Figure 9). The inferred thrust brings Precambrian basement of Fenland Terrane type (proved by Glinton 1 Borehole) onto probable Ordovician rocks (e.g. Wittering 1, Upwood and Great Paxton boreholes). The thrust projects north-westward towards the Denton and Eakring Fault Systems, which can be mapped continuously in the subsurface for a distance of some 60 km (Figure 8). The whole linked system is likely to be at least 100 km long. The history of Late Palaeozoic and Mesozoic reactivation of this ancient lineament is reviewed in Chapters 4, 7 and 8).

Summary

To conclude, although the pre-Carboniferous basement of the region is known from a relatively small number of boreholes, it is clear that it is largely composed of Early Palaeozoic metasedimentary rocks, with subordinate calcalkaline igneous rocks. The metasedimentary rocks (e.g. Ironville 5, Eakring 146, Foston 1, Welton 1, Grove 3, Halton Holegate 1, Stixwould 1, Bardney 1, Hunstanton, Wiggenhall 1 and Sibsey 1 boreholes) are of proven or probable Cambro-Ordovician age, and were cleaved and metamorphosed under high anchizonal to greenschist facies conditions during the Acadian Phase of the Caledonian Orogeny (Pharaoh et al., 1987a; Merriman et al., 1993). The igneous rocks include lavas and tuffs (e.g. North Creake, Cox's Walk, Great Osgrove Wood boreholes) occasionally interbedded with the sedimentary strata (e.g. Eakring 146, Beckering 1 boreholes), minor intrusions (e.g. Ironville 5, Claxby 1 boreholes) and plutonic rocks (e.g. Rempstone 1, Kirby Lane, Warboys boreholes).

The geochemistry of the magmatic lithologies supports their generation in a calc-alkaline arc located above a subduction zone, possibly caused by the underflow of ocean crust from the Iapetus Ocean and Tornquist Sea southward and westward beneath the microcontinent of Eastern Avalonia (Pharaoh et al., 1993). Furthermore the available isotopic evidence for a Mid Ordovician age, combined with other petrographical and geochemical attributes, suggests broad contemporaneity with Ordovician volcanism in Wales, the Lake District and Belgium (Pharaoh et al., 1993; Noble et al., 1993). The Caledonian structural grain is inferred from geophysical potential field data (e.g. Lee et al., 1991) to trend north-west to south-east, reflecting the trend of the inferred boundary between the concealed Caledonide fold/thrust belt of eastern England and the Midlands Microcraton (Pharaoh et al., 1987a; Smith, 1987), and also the trend of the Thor (Tornquist) Suture separating Avalonia and Baltica (Berthelsen, 1998; Pharaoh, 1999).

Chapter 4 Late Devonian and Carboniferous basin development

Two main Late Palaeozoic depocentres, controlled by major syndepositional faults, are present in the region; the Widmerpool Half-graben, in the south, and the Gainsborough Half-graben, in the north (Figure 4), (Figure 12). Shallow water facies are developed on intervening platforms and marginal shelves, the East Midlands Platform of Fraser et al. (1990). These major depositional elements were recognised from geophysical surveys and deep boreholes early on in the exploration of the region, although they were initially explained in terms of 'troughs' and 'gulfs' (Falcon and Kent, 1960; Kent, 1968) and only later as extensional basins (Leeder, 1976; 1987; 1988; Grayson and Oldham, 1987). The southern half of the region (Figure 12) comprises the northern edge of the Anglo–Brabant Massif, part of an area of residual topography stretching across southern Britain from Wales to London and then Belgium following the Caledonian Orogeny, also referred to as St George's Land (George, 1957). The massif varied in area and significance with time. All of the Carboniferous sequences exhibit onlap onto its northern flank.

The evolution of the Carboniferous basin complex of the East Midlands is now much better understood as a result of detailed seismostratigraphical studies of reprocessed high quality seismic reflection data (Smith et al., 1985; Ebdon et al., 1990; Fraser et al., 1990; Fraser and Gawthorpe, 2003). These studies have demonstrated that Tournaisian–Visean sedimentation in this region is characterised by alternating phases of rift-related tectonism and quiescence (Figure 13). In the Widmerpool Half-graben, six sequence units (EC1–6) are recognised within the Tournaisian–Visean (early Carboniferous) syn-rift phase, and two sequences (LC1–2) within a late Brigantian to Bolsovian post-rift phase (Figure 13). Sequences EC1, 3 and 5 show wedge-shaped geometries thickening onto the basin-bounding fault and thinning up the hanging-wall dip slope (Figure 14), (Figure 15). Associated phenomena include strong onlap, coarse clastic wedges, drowning of hangingwall carbonate margins and footwall uplift (Ebdon et al., 1990). They are interpreted as tectonically driven sequences arising from periods of lithospheric extension. Similar sequences are observed from seismic data in the Gainsborough Half-graben, although the third (EC5) rift pulse is not well represented. Intervening sequences, EC2, 4 and 6, reflect the development of prograding carbonate ramps, typically thinning towards basin-marginal faults. They are interpreted as stillstand or regressive sequence tracts deposited during tectonically quiescent periods.

Following EC6 in late Brigantian times, there was a minor inversion event along faults with a north-north-west to south-south-east trend (Ebdon et al. 1990) prior to deposition of sequence LC1, controlled by thermal subsidence. Subsequent Carboniferous strata of this post-rift phase were deposited in basins in which the effect of extensional faulting is minimal, and the effects of residual topography and differential, compaction related subsidence more significant. For this reason, the more general term 'basin' is used in this later, post-rift phase. The initiation of the Eakring inversion anticline and other comparable structures dates from this time. Further structural elements are described for the first time here, mostly in the east of the region, outwith the licence areas formerly held by BP (Figure 4).

Principal structural features

Anglo–Brabant Massif

The massif occupies the south-eastern half of the region, extending from Charnwood to The Wash (Figure 4). It was consolidated in Mid Devonian times, during the Acadian deformation phase, following a rather complex history of amalgamation during Ordovician times (Chapter 3). Several subprovinces, for example the Charnwood–Sproxton, Sibsey and Boston highs, are recognised in the literature (e.g. Fraser and Gawthorpe, 2003). Most of the Late Palaeozoic strata exhibit a degree of onlap onto the massif.

Charnwood–Sproxton High

This high occupies the northern edge of the Anglo–Brabant Massif. Precambrian and Early Palaeozoic basement crops out in nearby Charnwood Forest and at Mountsorrel. Carboniferous strata (Tournaisian–Visean) locally overstep onto the high (Carney et al., 2001), but are generally absent. The south-western margin of the Charnwood Massif lies at the Thringstone Fault, in the south-western extremity of the region. The CHARM seismic line (Maguire, 1987) imaged the Thringstone Fault as a north-east-dipping zone (Figure 16), confirming the observations in Merry Lees Drift (Butterley and Mitchell, 1946). The fault, which also demarcates the north-eastern side of the Leicestershire Coalfield, is interpreted as a major Variscan inversion structure (Smith et al., 1985). The eastward extension of the Charnwood High is referred to as the Sproxton High by Fraser and Gawthorpe (2003).

Hathern Shelf and associated faults

The Hathern Shelf lies between the Sileby Fault, at the northern edge of the Charnwood–Sproxton High, and the Normanton–Hoton Hills Fault (Figure 16), forming a narrow structural shelf at the edge of the Widmerpool Half-graben. During deposition of sequence EC1 in Late Devonian–Chadian times, the Sileby Fault was the major basin-bounding fault of the Widmerpool Half-graben system (Fraser and Gawthorpe, 2003), as shown by the relatively thick, anhydrite-bearing Tournaisian strata (Hathern Anhydrite Formation) present on the shelf, and proved by the Hathern 1 Borehole (Falcon and Kent, 1960). Subsequently the rate of subsidence slowed. Extensive footwall uplift and erosion occurred during the EC3 (Chadian–Holkerian) rift pulse (Ebdon et al., 1990), reflected in prominent intra-Visean angular unconformities in quarries near Breedon, just west of the region (Ambrose and Carney, 1997b). Here the Milldale Limestone Formation (early Chadian) is unconformably overlain by the Cloud Hill Dolostone Formation (?Holkerian–early Asbian). A prominent angular unconformity is visible in seismic data crossing the shelf (Figure 16), (Figure 17).

Widmerpool Half-graben and associated faults

The Normanton Hills–Hoton Fault System (Figures 16), (Figure 17) formed a major Tournaisian–Visean syndepositional extensional fault system controlling the southern boundary of the Widmerpool Half-graben, and the northern edge of the Hathern Shelf (Ebdon et al., 1990; Carney et al., 2001). The fault system extends into the region from the southern outskirts of Derby. At the western edge of the region its downthrow to the north is at least 3000 m at the base Carboniferous level, although it was greater prior to Variscan inversion. The mappable length of the fault is about 30 km within the region. Eastward beyond Asfordby it develops a number of strands with reduced throws (Figure 18), and eventually dies out to the south of the Foston High. Here extension was taken up by the Barkston Fault, offset to the northern side of the high (Figure 12). The Normanton Hills–Hoton Fault was first mapped in the Mesozoic cover (Fox-Strangeways, 1905), representing an extensional reactivation of the Tournaisian–Visean and Variscan faulting. At depth, the fault apparently passes into a more gently dipping detachment surface (Fraser and Gawthorpe, 2003).

Seismic data indicate that over 3500 m of Tournaisian–Visean strata are present at the basin depocentre, in the vicinity of the Variscan inversion axis. As boreholes provide only a very limited penetration of this sequence, evidence for the tectonic and stratigraphical history of the half-graben is necessarily derived from seismic reflection data. Ebdon et al. (1990) and Fraser et al. (1990) recognised wedge-shaped geometries (Figure 14) in sequences EC1, 3 and 5. These thicken onto the basin-bounding fault and thin up the hanging-wall dip slope, and are interpreted as tectonically driven sequences arising from periods of lithospheric extension. Seismic sequences EC3 and EC5 show marked thickening onto the Normanton Hills–Hoton Fault, and rather less on the Sileby Fault, indicating that the former had superceded the latter as the principal basin-controlling fault by Arundian times. They were deposited during episodes of active rifting, accompanied by footwall rotation and volcanic activity along the Cinderhill–Foss Bridge Fault System (Figure 14), (Figure 19). Intervening sequences, EC2, 4 and 6 comprise carbonate ramps rimming the adjacent shelf, prograding south-westward into the basin and feeding calciturbidites in the same direction. The most rapid phase of subsidence of the half-graben was during sequences EC1–3 (Courceyan to early Holkerian) when up to 3 km of strata accumulated. Sequences EC4–6 (Holkerian to Brigantian) comprise a maximum of 1 km of strata. The contrast with the Nottingham Platform (Figure 17), for example the sequence proved by Ironville 5 Borehole just west of the region (Figure 35a), (Figure 35b), is notable. Here, on the uplifted shoulder of the rift, sequences EC1–3 are half the thickness of those of EC4–6, reflecting the rapid extension in the early history of the rift. According to Ebdon et al. (1990) the earliest volcanic rocks form part of sequence EC5, but eruption continued into EC6.

The hinge line between the half-graben and the Nottingham Platform lies about 1 km north of the Cinderhill–Foss Bridge Fault System, just south of the Cropwell Butler boreholes (Figure 17), (Figure 19). The fault system, which comprises 2 or 3 parallel strands, with downthrow to south-west, is mapped from surface exposure and mine records. It formed a hinge zone at the top of the hanging-wall dip slope of the half-graben, subsequently reactivated by Permo-Triassic extension. Seismic sections indicate that the hinge zone formed a linear focus for episodic Tournaisian–Visean volcanism, extending from Matlock (Lower and Upper Millers Dale lavas and Tissington Volcanic Member) to the Vale of Belvoir. The Ironville 5 Borehole proved 27 m of tuffs of Brigantian age, while Strelley 1 Borehole, also just west of the region, encountered a 372 m thick lens of lavas and tuffs of comparable age in the vicinity of the Cinderhill Fault (Figure 19). Seismic reflection data suggest that these Brigantian volcanic rocks continue as a belt along the Cinderhill Fault into the region, as well as being present adjacent to the Harlequin Fault. The North-east Leicestershire Coalfield (Figure 18) lies at the eastern end of the Widmerpool Half-graben, to west of the Denton Fault. Tournaisian–Visean strata were encountered in Bottesford 4, Redmile 1, Plungar 8A, Woolsthorpe Bridge and Harston 1 boreholes. Plungar 8A Borehole proved the fullest sequence, 422 m of strata ranging in age from Chadian (and ?late Courceyan) to Arundian and possibly younger (Riley, 1992). Namurian strata (124 m thick in Plungar 8A) and Westphalian strata (470 m thick in Bottesford 4) are much thicker than in the neighbouring Sleaford Half-graben and Foston High.

Following sequence EC6 in late Brigantian times, there was a minor inversion event along faults with a north-north-west to south-south-east trend (Ebdon et al., 1990) prior to deposition of sequences LC1–2 (Namurian–Westphalian), controlled by thermal subsidence. The initiation of the Eakring inversion anticline and other comparable structures dates from this time. Sequence LC1 (late Brigantian–Pendleian) is thickest just to north of the Normanton Hills–Hoton Fault. According to Fraser and Gawthorpe (2003) this provides evidence for pre-Namurian inversion, but it could equally be due to post-Brigantian differential compaction subsidence. Sequence LC1a (late Brigantian–late Pendleian) comprises distal prodelta mudstones of the advancing delta system which had already filled the basins of northern England by this time (Fraser et al., 1990). The depocentre of this sequence remained within the half-graben, but was displaced northward with respect to the Tournaisian–Visean depocentres. During sequence LC1b (late Pendleian–Alportian) the Widmerpool Half-graben was starved of sediment. This may reflect large scale switching of delta lobes in the southern North Sea region, or climatic changes in the hinterland to the north, affecting the rate of sediment supply (Fraser and Gawthorpe, 1990). Sequence LC1c (Kinderscoutian–late Langsettian) is characterised by the continued and widespread progradation of the Kinderscout, Ashover, Chatsworth and Crawshaw delta systems into the Widmerpool Half-graben. The Ashover Delta entered the half-graben longitudinally in a north-westerly direction following the inherited Tournaisian–Visean structural grain. Lower delta plain conditions pertained over much of northern England at the end of LC1 (Guion and Fielding, 1988). Another episode of volcanism produced localised volcanic piles, particularly thick below the Vale of Belvoir (Burgess, 1982; Kirton, 1984; Carney et al., 2003). Volcanism had ceased by late Langsettian times, however (Kirton, 1984). During the deposition of sequence LC2 (late Langsettian–Bolsovian), upper delta plain coal swamp conditions were established over most of northern England.

Nottingham Platform

The pre-Carboniferous basement (undrilled, except by Ironville 5 Borehole, just to the west of the region) lies at a relatively uniform depth of about 1250 m below OD (Figure 17). Seismic reflection data indicate that Tournaisian–Visean strata are relatively thin across the platform. The thickness increases from about 200 m in the north-east, adjacent to the Eakring–Foston Fault System, to about 350 m just north of the Cinderhill–Foss Bridge Fault System. Boreholes within the region penetrate only the uppermost few tens of metres of the late Tournaisian–Visean sequence. To the west of the region, the Ironville 5 Borehole proved 621 m of late Tournaisian–Visean strata overlying Early Palaeozoic basement. Courceyan strata are absent, and Chadian (dolomitic limestones) and Arundian strata are relatively thin (about 200 m) compared to the Holkerian–Brigantian sequence (>400 m). Plungar 8A Borehole proved 486 m of strata (unbottomed), ranging in age from Chadian (and ?late Courceyan) to Arundian

and possibly younger (Riley, 1992). See Carney et al. (2003) and Howard et al. (2009) for more detailed borehole correlations. The north-westward extension of the platform is exposed in the Derbyshire Dome (Smith et al., 1985). The north-eastern boundary of the platform is the Eakring–Foston–Denton Fault System, beyond which lies the Newark Low and Sleaford Half-graben (Figure 4). The Nottingham Platform was not as strongly affected by Variscan inversion and pre-Permian erosion as the Widmerpool Half-graben, and has preserved the younger Carboniferous strata of the Nottinghamshire Coalfield.

Welbeck Low

According to Gutteridge (1987), inversion at the end of EC6 led to the creation of late Brigantian intrashelf basins, such as the Welbeck Low, with counterparts in the exposed Derbyshire carbonate platform. The low is framed by the Anston–Manton, Ladybrook and Eakring inversion (Figure 49) anticlines on its north, south and eastern margins (Ebdon et al., 1990).

Edale and Alport basins

The Edale Basin is smaller and shallower than the Widmerpool and Gainsborough basins, and lies almost entirely to the west of the region. It is controlled by a major east-trending fault (Bakewell Fault) at the northern margin of the Derbyshire carbonate platform (Smith et al., 1985). The latter extends into the region as the Ladybrook–Mansfield Fault system. The Alport Basin lies to north of the Edale Basin (Smith et al., 2005), and is separated from it by the Eyam Tilt Block. Its eastern extremity, transitional with the Gainsborough Half-graben, lies at the north-western limit of the region (Figure 4). The basin is controlled by the east-trending Alport Fault, which has a downthrow to the north of up to 4000 m.

Foston High

The Foston High occupies the footwall of the Barkston Fault (Figure 20). Visean strata were penetrated by two boreholes. Cox's Walk Borehole proved 58 m of Holkerian strata (Riley, 1992), unconformable upon the pre-Carboniferous basement. The absence of Courceyan to Asbian strata is either due to the persistence of the Foston High throughout early Carboniferous times, or to pre-Holkerian footwall uplift resulting in erosion of the early Tournaisian–Visean sequence. Seismic data in the vicinity of Great Osgrove Wood Borehole, on the southern flank of the high (Figure 21), suggests the presence there of a much thicker Tournaisian–Visean sequence than on the north-western flank e.g. (Figure 18). These strata form a wedge, thinning towards the top of the high (Figure 20). This indicates either that the Foston block was here acting as a distinct tilt block during Tournaisian–Visean times, identified as the Witham tilt block; or that the Widmerpool Half-graben extended even further eastward in Tournaisian–Visean times, before being truncated during Variscan inversion by the north-north-west-trending Denton Fault.

Foston 1 Borehole proved about 20 m of Tournaisian–Visean strata unconformable upon the Caledonian basement. This is the only borehole in this district in which Namurian and Westphalian strata are completely absent. The presence of thin Namurian strata upon most of the high may indicate that pre-Westphalian (?Namurian) inversion of the high was not as severe as in the Sleaford Half-graben where Namurian strata are absent.

Sleaford Half-graben and associated faults

The Sleaford Half-graben lies in the hanging wall of the Barkston Fault (Figure 20). Seismic reflection lines oriented north–south across the Foston High and Sleaford Low demonstrate early Dinantian syndepositional throw on the Barkston Fault, with marked thinning of the sequence northward onto the hangingwall dip slope. (Figure 20) clearly demonstrates the effect of fault-controlled subsidence in Tournaisian–Visean times (with maximum depositional thickness against the Barkston Fault in the south). The interpretation presented infers that other subsidiary faults within the half-graben may also exhibit Tournaisian–Visean syndepositional throw. The interpretation is based on the tracing of the 'Arundian Shale Marker' horizon recognised by BP southward from Bassingham 1 Borehole, and indicates that the most dramatic thickening onto the Barkston Fault (hence syndepositional movement) occurred in ?Courceyan to Chadian times. However, less pronounced thickening is also visible in the Arundian–Holkerian sequence.

Brigantian strata are apparently only preserved in the south of the Sleaford Low, in Gables Farm, Hurn Corner and Westfield Lane boreholes, close to the Barkston Fault. This, together with the absence of Namurian strata in the low, suggests a widespread inversion of the Sleaford Low in pre-Westphalian times. Bassingham 1 Borehole penetrated 478 m of Visean strata, ranging in age from late Chadian to Asbian (Riley, 1992), overlain by 191 m of Langsettian–Bolsovian strata. Regional seismic interpretation indicates the presence of a further 400–500 m of strata (probably Tournaisian) overlying the basement in the vicinity of Bassingham 1 Borehole. Seismic reflection data indicate that the Tournaisian–Visean sequence thickens southward onto the plane of the Barkston Fault (Figure 20)(b), where at about 2500 m thick, the sequence is three times thicker than that at Bassingham. Unfortunately none of the boreholes in this area penetrates deeply into the Tournaisian–Visean sequence. Gables Farm, Hurn Corner and Westfield Lane boreholes prove Brigantian strata preserved high in the hanging wall of the Barkston Fault. To the north, Namurian erosion has cut down to strata of early Asbian age at Mareham Grange Borehole and Holkerian age at Burton Lodge Borehole.

Namurian strata are absent (or very thin) in the western part of the Sleaford Half-graben, and absent in the east. In the east of the district, a significant intra-Westphalian break of mid Bolsovian age is recognised (Riley, 1992). In Burton Lodge Borehole, strata of the Etruria Formation (late Bolsovian) rest unconformably upon early Westphalian strata, strata of Duckmantian age being absent. The Sleaford Half-graben may be regarded as a small scale analogue of the Widmerpool Half-graben, offset to north-east of the Foston High. They are separated by the Eakring–Denton Reverse Fault which probably reflects Carboniferous reactivation of a thrust fault or shear zone within the Caledonian basement. Within the Grantham district, Stenwith and Bennington G1 boreholes proved over 200 m of Westphalian strata.

Newark Low

Seismic reflection data indicate the presence of at least 2 km of Tournaisian–Visean strata in the hanging wall of the Eakring Fault, for which a Tournaisian–Visean syndepositional movement history is inferred (Figure 22). The area represents the north-westward continuation of the Sleaford Half-graben.

Coningsby Half-graben and associated faults

The Coningsby Half-graben is contiguous with the Sleaford Half-graben (Figure 4) and (Figure 23). While the latter is controlled by a major syndepositional fault (Barkston Fault) against the Anglo–Brabant Massif on its southern flank, and exhibits a hanging-wall dip slope rising gradually northwards towards the Nocton High (Figure 20), the Coningsby Half-graben shows the opposite polarity. The displacement on the Sibsey Fault decreases eastward, and seismic data suggest the presence of a tilt block dipping northwards, controlled by a syndepositional fault (Halton Holegate Fault) at the southern edge of the Stixwould High (Figure 24). The interpretation of Halton Holegate 1, drilled by Enterprise plc, is complicated by the lack of a terminal core and abundant caving during drilling. However, examination of the cuttings suggests that the borehole penetrated a pre-Namurian sequence, before entering basement. Geophysical log signatures of these strata are similar to those proved by Coningsby 1 Borehole (Figure 23), which are of possible Tournaisian age. Seismic data suggest that the contact is faulted, and that Halton Holegate 1 ended in the footwall of the Halton Holegate Fault (Figure 24), on the Stixwould High. Unfortunately the seismic data in this part of the region, with a thickening Mesozoic and significantly, Permian cover, are badly affected by multiples here so the base of the graben is not well imaged. However, (Figure 23) shows a better quality seismic line located in an intermediate position, and appears to cross the transition between these two half-graben systems, south-east of the Nocton High. (Figure 25) shows the narrow, northeastern re-entrant of the half-graben. Seismic coverage is poor onshore north of The Wash, and the limited data available indicate that the half-graben is narrow (about 10 km wide) and may not extend far into the North Sea.

Boston High

Gravity potential field data (Figure 9) show a prominent low in this area, which forms the north-eastward continuation of the Anglo–Brabant Massif, the northern margin of which is here defined by the Sibsey Fault. It has been suggested that the high (and indeed, most of the area of The Wash) is underlain by an inferred granite batholithic complex (Figure 8), previously referred to as the Wash Batholith (Chroston et al., 1987). This hypothesis remains to be confirmed by drilling. However, the Claxby 1 Borehole (Pharaoh et al., 1997) may have encountered a hypabyssal component of this postulated complex within the Stixwould High (see below). The felsic ignimbrite proved by North Creake Borehole to south of The Wash (Kent, 1967; Pharaoh et al., 1991) has yielded a U-Pb zircon age of 449 ± 13 Ma (Noble et al., 1993), and may represent an eruptive component of this same magmatic complex. Metasedimentary rocks, presumably of early Palaeozoic age, overlain by thin Westphalian strata, were proved by Sibsey 1 Borehole, on the crest of the high.

Stixwould High

The Stixwould High is a rather prominent east–west trending feature, extending for at least 50 km across the region, from Lincoln to the North Sea (Figure 4). Two boreholes (Claxby 1, Beckering 1) demonstrate that Westphalian strata directly overlie the pre-Carboniferous basement on this high, and that Namurian strata, if present at all, are extremely thin and restricted to the latest Namurian. The sodic microgranite proved by Claxby 1 at 1321 m depth, has been interpreted as a possible hypabyssal offshoot from the Wash Batholith (Pharaoh et al., 1997). The emplacement of the Moorby Microgranite has been dated at 457 ± 20 Ma by the U-Pb zircon method (Noble et al., 1993). This is identical within the errors to the zircon age of the felsic ignimbrite at North Creake, so a Late Ordovician age for the whole of this inferred magmatic complex is most likely. Halton Holegate 1, Stixwould 1, Bardney 1 and Beckering 1 boreholes, also proved well-cleaved metasedimentary rocks on this high. Their age is uncertain, but probably Ordovician or older (Pharaoh et al., 1989).

Nocton High

The Nocton High was originally identified by BP on the basis of gravity evidence (Falcon and Kent, 1960). It is a distinctive north-trending structural element, almost unique in a region with dominant west-north-westerly grain. Seismic data indicate that only the eastern flank (against the Coningsby Half-graben and Lincoln Platform) is strongly faulted (Figure 25), (Figure26). The southern flank dips southwards, eventually passing into the dip slope of the Sleaford Half-graben (Figure 23).

The western flank (Figure 25) is less strongly faulted and transitional to the Newark Low. Namurian strata are thin or absent, possibly due to Namurian inversion, and if Westphalian strata were deposited they were removed following Variscan inversion on a north-north-westerly trend.

Lincoln Platform

This platformal area (Figure 26), (Figure 27) lies at the south-eastern margin of the Gainsborough Half-graben, separated from the Askern–Spital High by the continuation of the Askern–Spital Fault Zone. It includes important oilfields such as Welton and Nettleham which are developed in a north-north-west-trending belt of Variscan inversion anticlines marginal to the Nocton High (Figure 52). Here,

Namurian strata are thin or absent, and the most important reservoir comprises an amalgamated basal Westphalian sand sequence, equivalent to the Rough Rock, Crawshaw and sub-Alton sandstones away from the platform.

Gainsborough Half-graben and associated faults

On its north-eastern side this basin is controlled by the Askern–Spital Fault (Figure 28), with major Tournaisian–Visean downthrow to the south-west. Thus its polarity is opposite to that of the Widmerpool Half-graben. According to Fraser and Gawthorpe (2003), there is a fourfold increase in Tournaisian–Visean thickness across the fault. Downthrow at the base Carboniferous level exceeds 2500 m. Early Carboniferous seismic sequence geometries are very similar to those of the Widmerpool Half-graben, thickening onto the Askern–Spital Fault, but with opposite polarity. The southern bounding fault of the half-graben (EC1) was initially the Beckingham Fault. At the onset of EC3 rifting, fault control shifted to the Clarborough Fault, some 7 km further south (Fraser and Gawthorpe, 2003). The thickening of the Namurian sequence into the basin has been ascribed to the infill of pre-Namurian bathymetry, perhaps as much as 300 m at the depocentre (Steele, 1988); differential compaction and localised tectonic activity. In late Namurian times, mild reactivation of extensional faulting occurred, particularly in the eastern part of the basin, for example the north-east-trending Scampton Fault (Fraser and Gawthorpe, 2003). The result was a strong footwall unconformity and thickened sandstone (Rough Rock equivalent) on the hanging wall. Evidence for mid–late Duckmantian tectonism is found in the West Firsby area, adjacent to the Askern–Spital Fault, represented by major erosion at the level of the Wooley Edge Rock (Aitken et al., 1999). This may represent a precursor of Variscan compression. Although the Namurian–Westphalian strata were not significantly uplifted and eroded, a number of inversion features developed parallel to the Askern–Spital and Nettleham Faults, forming the Corringham, Glentworth, Hemswell and West Firsby oilfields. Another belt of small oilfields extends along the southern margin of the half-graben, e.g. the Scampton North, Torksey (Figure 27) and South Leverton oilfields. The largest accumulations of all were at Beckingham and Gainsborough, on a ridge in the centre of the half-graben (Figure 29).

Askern–Spital High and Fault

The lineament of the South Craven Fault to Askern–Spital Fault is one of the most important Carboniferous structural elements in northern England, for it stretches for 90 km from the southern edge of the Askrigg Block at Ingleton to the Stixwould High (Kirby et al., 2000). Only the southern third of the structure, with a north-west-trend, lies in the region, where it defines the north-eastern edge of the Gainsborough Half-graben and the Lincoln Platform (Figure 4). The Beckering 1 Borehole lies at the transition between the Askern–Spital and Stixwould highs (Figure 4). It demonstrates that Tournaisian–Visean strata are absent at this point (unpublished Gas council well composite log).

South Humberside Platform

The southernmost part of this platform lies east of the Askern–Spital Fault (Figure 28), at the northern extremity of the region. The pre-Carboniferous basement lies deeper than at the edge of the Askern–Spital High, and is covered by Tournaisian–Bolsovian strata which thicken northwards towards the Humber estuary. These include Namurian strata, which reappear to north of Horncastle (e.g. Kelstern 1 Borehole). The Saltfleetby Gasfield (Hodge, 2003), the largest gasfield discovered in the onshore UK, and the Keddington Oilfield lie in the middle of the platform, just beyond the northern limit of the region.

Chapter 5 Syn-rift stratigraphy (Late Devonian–Brigantian)

Late Devonian

Following the Acadian deformation phase, the palaeo-continent of Laurussia (Ziegler, 1990), also known as the 'Old Red Continent', was stabilised and subjected to a long period of subaerial erosion with intermittent marine transgression. Palaeomagnetic data indicate that the region lay just south of the Equator at this time (Figure 7)f. Upper Devonian strata of continental red-bed facies, for example the Redhouse Sandstone Formation of the Ashbourne region (Chisholm et al., 1988; Waters et al., 2007) and the Calke Abbey Sandstone Formation of the Loughborough district (Carney et al., 2001), were deposited in intermontane basins, but have no counterpart at crop in the region reviewed here. Two subsurface occurrences of red arenaceous strata may be representative of this facies, and of possible Late Devonian or Tournaisian age. In the Melton Mowbray area, 124 m of the Scalford Sandstone Formation (Carney et al., 2004) was proved in the Scalford 1 Borehole, from 945.8 m to 1069.54 m depth (TD), in likely tectonic contact with overlying mudstone and thin limestone of the Widmerpool Formation (Brigantian). The strata comprise brown and green, fine- to medium-grained calcareous sandstones, brown, red and purple siltstones and grey mudstones, and may represent an early syn-rift fluviatile or fan–delta association (Waters et al., 2009), containing detritus fed from escarpments formed by the initiation of rifting along the Hoton–Normanton Hills or Sileby faults and occupying the eastern part of the Hathern Shelf. In the Ollerton area, Eakring 146 Borehole proved 522 m of reddish-brown conglomerates and sandstones (Lees and Taitt, 1946; Edwards, 1967) between 1672 and 2194.6 m depth, lithologically similar to the Scalford Sandstone Formation. Apparent dips of between 50 and 70° were observed in the core. The clasts include quartz, quartzite, siltstone, phyllite and igneous rocks, all known from the pre-Carboniferous basement in the region (Chapter 3) and possibly eroded from nearby fault scarps. The overlying 417 m of strata at Eakring comprise similar facies interbedded with limestones which have yielded Chadian and Arundian fossils (Edwards, 1967), reflecting the persistence of faulting into Arundian times. The age of the strata in both of these subsurface provings is uncertain; it is assumed to be Late Devonian or possibly Tournaisian.

Tournaisian–Visean

During early Carboniferous times, lithospheric extension, crustal stretching and basin subsidence strongly affected the region (Gawthorpe et al., 1989; Gawthorpe and Leeder, 2000). Several pulses of rifting have been recognised (Figure 30), accompanied by cycles of transgression and regression (Fraser and Gawthorpe, 2003). The region lay close to the Equator (Figure 7)f, with a climate varying from arid to monsoonal (Besly, 1988).

The very limited outcrop of Tournaisian and Visean strata in the region means that a thorough overview of its early Carboniferous evolution can be only obtained by studies of:

1.      The 800 or more boreholes drilled for hydrocarbon exploration in the region, many of which reach the Visean but few of which penetrate it completely. This is particularly true in the Gainsborough Trough, thus knowledge of the Tournaisian and Visean stratigraphy is biased towards the Widmerpool Half-graben.

2.      The dense network of 2D seismic reflection data, totalling over 3000 km line length, usually recorded to 3s TWTT, and ranging from low-fold, generally poor quality (early 80s vintage) to higher fold and better quality (late 80s/early 90s vintage).

3.      Knowledge of relationships in the nearest large area of Carboniferous exposure, the Peak District. These are therefore described in some detail in the following sections.

As elsewhere in northern Britain, shallow water carbonates accumulated on highs (e.g. Gawthorpe, 1987; Gutteridge, 1987, 1991; Fraser et al., 1990; Gawthorpe and Gutteridge, 1990; Chadwick et al., 1995; Fraser and Gawthorpe, 2003; Kirby et al., 2000; Smith et al., 2005), which were progressively inundated during Tournaisian and Visean times (Strank, 1987). Thicker, more complete successions accumulated in deeper water basinal areas controlled by major syndepositional faults. In the south and east, stratigraphical complexities result from the interaction of rifting, subsidence and eustatic variation at the margin of the Anglo–Brabant Massif (Worssam and Old, 1988; Carney et al., 2002; 2004). In the eastern part of the region, a major erosional break is present, and Namurian strata are generally absent. Westphalian strata overstep across Visean strata, locally resting directly upon pre-Carboniferous basement (Stixwould and Wolds Highs). Given the near-equatorial position of the region at this time, it is likely that the pre-Westphalian erosion surface on the highs has significant local relief and is strongly karstified, as recognised elsewhere in southern Britain (Walkden, 1987). The irregular nature of the pre-Westphalian surface observed at Welton (e.g. Rothwell and Quinn, 1987) and Nettleham supports this contention.

These relationships are illustrated by a series of maps showing the changing palaeogeography and facies of the region (Figure 31), (Figure 32), (Figure 33), (Figure 34). The strata are overlapped onto the Anglo–Brabant Massif by sequences of Namurian and Westphalian age.

Courceyan–Chadian

Tournaisian strata are known from only 6 boreholes in the region, and are thus poorly documented. The isopach map presented (Map 2) suggests a maximum thickness of 1200 m in the hanging wall of the Hoton–Normanton Hills Fault, and 900 m in the hanging wall of the Barkston Fault. The Hathern Anhydrite Formation, previously referred to as the Hathern Anhydrite Series (Llewellyn and Stabbins, 1970) and redefined as a formation by Carney et al. (2001), was proved by Hathern 1 Borehole (Figure 35a), (Figure 35b) between 538 m and 635 m depth (TD). It comprises bedded and nodular anhydrite, interbedded with grey dolostone, mid-to-dark grey mudstone or calcareous mudstone, and massive to nodular limestone. The evaporites were deposited within a sabkha environment reflecting intermittent marine incursion, restricted circulation and episodic desiccation immediately following the initial phase of rifting. Miospores of the CM Zone indicate a Tournaisian age (Llewellyn et al., 1969; Llewellyn and Stabbins, 1970). The top of the formation is truncated by a major unconformity affecting the strata of the Hathern Shelf (Figure 16), (Figure 17), (Figure 35a). The base (unproven) is probably an angular unconformity resting upon pre-Carboniferous rocks. The thicknesses derived from the seismic interpretation indicate that elements such as the Hathern Shelf, Widmerpool and Gainsborough half-grabens were not yet distinct at this time (Strank, 1987; Fraser and Gawthorpe, 2003), but only became differentiated as extension and subsidence continued through Chadian times. The Hathern Anhydrite Formation is lithologically and chronostratigraphically comparable to the Middleton Dale Anhydrite Formation of the Derbyshire High, proved by the Eyam Borehole (Dunham, 1973; Strank, 1985), and the Rue Hill Dolomite proved by the Caldon Low Borehole (Aitkenhead and Chisholm, 1982; Chisholm et al. 1988). However, lateral continuity between the Derbyshire High and the present region has not been demonstrated. Anhydrites, possibly belonging to the Hathern Anhydrite Formation, are overlain by mudstone, siltstone and thin limestone beds of the Long Eaton Formation in the Long Eaton 1 Borehole, penetrating the much thicker succession in the Widmerpool Half-graben (Figure 35a).

Other subsurface provings within the region include Grove 3 Borehole, between 2765 m and 2896 m depth, and Welton 1 Borehole, between 2350 m and 2530 m depth (Figure 35b). In both occurrences, the BP composite logs describe extrusive volcanic rocks comprising up to one third of the lowest part of the sequence. Study of the cuttings shows that they are highly chloritic, reflecting a significant detrital input from the underlying phyllitic basement (Pharaoh et al., 1987a), rather than a contemporary volcanic source. The upper two-thirds of the succession comprises thin dark carbonaceous limestones, dolomites with relict ooids, dark pyritic siltstones and mudstones. Bedded anhydrite has not been reported. The Courceyan (Tournaisian) strata are attributed to seismostratigraphical sequence EC1 by Fraser et al. (1990) and Fraser and Gawthorpe (2003). Seismic reflectors thicken towards the Sileby Fault (Figure 16), the major basin-bounding fault at this time, and show that the sequence was deposited during a phase of extension and normal faulting, during which the Widmerpool Half-graben and other major basins, were initiated.

The bulk of the post-evaporitic Visean strata are of open marine, platform and ramp carbonate facies assigned to the Carboniferous Limestone Supergroup of onshore Great Britain (Waters et al., 2007; Waters et al., 2009). The platform carbonates were deposited on geographically isolated horsts or tilt block highs, each with distinct group names. Within the region reviewed here, carbonate developments in platformal regions (East Midlands Platform and Hathern Shelf) are referred to as the Peak Limestone Group, with a type area in the Peak District of Derbyshire. The lithostratigraphy of the group is shown in (Figure 36), adapted from Aitkenhead and Chisholm (1982) and Waters et al. (2009). A distinction is made between comparatively massive and dolomitic, shallower water limestones of the northern part of the Peak District inlier (Buxton to Matlock), referred to the Woo Dale Limestone Formation, and limestones with mud mounds deposited in perhaps 220–280 m of water (Bridges and Chapman, 1988) in the south-western part of the inlier (Dovedale and Manifold Valley), referred to the Milldale Limestone Formation. On the Nottingham Shelf, known only from subsurface information, the Milldale Limestone Formation is overlain, possibly unconformably, by the Belvoir and Plungar Limestone Formations (Carney et al., 2001; 2004). Strata deposited in basins with even greater depth, principally mudstones and limestone turbidites, are referred to as the Craven Group, with a type area in the Craven Basin (Waters et al., 2007; Waters et al., 2009). The principal representatives in the region are the Long Eaton and Widmerpool formations. The lithostratigraphy of the group, which extends up into the early Namurian, is shown on (Figure 36).

In the subsurface of the East Midlands, deep boreholes encounter platform carbonates comparable to the Milldale Limestone Formation in a number of places. In the Hathern 1 Borehole, limestones interpreted as the Milldale Limestone Formation, overlying the Hathern Anhydrite Formation, represent a lateral continuation of the former onto the East Midlands Platform. The main exposures are at the quarries of Breedon-on-the-Hill and Cloud Hill, where the formation is at least 380 m and 130 m thick, respectively (Carney et al., 2001), and almost completely dolomitised. The formation comprises both the typical deeper water 'Waulsortian' mud mound subfacies, and a fine- to coarse-grained, bioclastic, ooidal and peloidal grainstone subfacies, deposited within a high-energy, storm-dominated shelf environment (Carney et al., 2001). Foraminiferal assemblages at Cloud Hill Quarry (Cf4a1 subzone) and the ammonoid Fascipericyclus fasiculatus found at Breedon-on-the-Hill Quarry, both indicate an early Chadian (Tournaisian) age (Carney et al., 2001). A prominent unconformity is present at the top of the Milldale Limestone Formation, beneath the Cloud Hill Dolostone Formation (Asbian), with strata of Arundian and Holkerian age largely absent.

A prominent unconformity at this level in the seismic data across the Hathern Shelf (Figure 16) is attributed to severe footwall erosion during the rapid extension of cycle EC3 (Arundian).

Typical mud mounds of 'Waulsortian type' have been described from near the top of the thick (>500 m) Chadian sequence in Egmanton 68 Borehole, Grove 3 Borehole and possibly Welton 1 Borehole (Strank, 1987). These structures, which have a 'reefal' appearance on seismic data, are composed of fossiliferous, massive micrite with common spar-filled cavities. The inter-reef facies consists of well-bedded crinoidal biosparite and subordinate dark grey, cherty micrite. These facies may have been deposited in water up to 280 m deep (Bridges and Chapman, 1988). The facies has also been recognised from outcrop on the Hathern

Shelf just to west of the region. The Waulsortian reef in Breedon-on-the-Hill Quarry shows up to 100 m of unbedded dolostone, overlying about 280 m of well-bedded inter-reef facies (Ambrose and Carney, 1997b; Carney et al., 2001); and Cloud Hill Quarry exposes 130 m of well-bedded inter-reef facies with limestones, commonly bituminous clay, shaly mudstone and siltstone.

Thinner Chadian sequences were proved in more platformal locations by Ironville 5 and Bardney 1 boreholes, and may, like the Woo Dale Limestone Formation, have been deposited in shallower water. Massive dolostones were proved by Grove 3 Borehole, and a chlorite-rich clastic sequence at Welton. Other deep boreholes, for example Plungar 8A, Bassingham 1, Coningsby 1 and possibly Strelley 1 (Fraser and Gawthorpe, 2003), did not penetrate the base of the sequence. The Belvoir Limestone Formation (Carney et al. 2004) described from the Plungar 8A Borehole comprises a brown to white, dolomitic, locally ooidal limestone up to 120 m thick, with rare thin volcanic beds of 'green ash' which overlies dolomitic limestones of the Milldale Limestone Formation. A late Chadian age is assumed for the uppermost part of the formation, as samples from above 1149 m in the type locality yielded bilaminar Koninckopora, with no archaediscids present (Riley, 1992). It was deposited in shallow to moderate depth waters on a carbonate ramp. This formation may occupy other parts of the Nottingham Shelf.

The Long Eaton Formation, comprising a monotonous sequence of dark grey or brown calcareous mudstones and siltstones, with thin beds of bioclastic limestone, graded calcisiltite, packstone and grainstone (description from borehole chip samples) interpreted as carbonate turbidites, was deposited in basinal environments, in both the Widmerpool and Gainsborough half-grabens, and probably also in the Sleaford Half-graben. It has been described from the Long Eaton 1 Borehole, just to the west of the region, from 2747 m to 880 m depth (Brandon, 1996), and Ratcliffe-on-Soar Borehole, from 1832 m to 1157 m depth (Carney and Cooper, 1997; Carney et al., 2001). The formation overlies nodular anhydrite and interbedded dolostone in the Long Eaton 1 Borehole (Figure 35a), (Figure 35b), possibly a lateral equivalent of the Hathern Anhydrite Formation. Seismic interpretations suggest that the Long Eaton Formation thickens south-westwards within the Widmerpool Half-graben to a maximum of 4000 m. Deposition of these strata was probably by sediment gravity flow mechanisms. Microfaunas show the formation ranges in age from early Chadian to Asbian. However, the apparent absence of late Chadian and Arundian strata in the Long Eaton 1 Borehole and at Breedon Cloud Hill Quarry suggests the presence of a depositional hiatus within the formation (Riley, 1997). This internal unconformity, if it exists, may relate to a late Holkerian phase of basin inversion, also seen on the margin of the Anglo–Brabant Massif (Fraser and Gawthorpe, 1990).

The Chadian strata are attributed to seismic sequence EC2 (Fraser et al., 1990; Fraser and Gawthorpe, 2003), which is characterised by high-amplitude, laterally continuous reflectors in the basin centre which diverge and thicken along the hanging-wall dip slope, where downlapping hummocky clinoforms are identified. The latter are believed to represent aggradational to progradational carbonate ramps and rimmed shelves developing during a tectonically quiescent phase.

Arundian–Holkerian

A further phase of extension commencing in early Arundian times resulted in drowning of the Chadian carbonate platforms, including the distinctive mud mound-facies, as tilt blocks subsided and sea level rise outpaced carbonate production (Fraser et al., 1990). A major phase of transgression resulted in progressive inundation of the Anglo–Brabant Massif and other highs, reaching its maximum in Holkerian times (Strank, 1987; Fraser and Gawthorpe, 1990). The principal exception was in the footwall regions of the major rifts, for example the Hathern Shelf, which experienced strong uplift and severe erosion at this time, as noted above. Carbonate ramps and rimmed shelves developed in late Holkerian times during a still-stand phase, and in the Peak District, thin coals and birdseye structures testify to regression and local emergence. The isopach map (Map 5) suggests the presence of up to 2000 m of these strata in the Widmerpool Half-graben, and rather less (maximum of 1000 m) in the Sleaford Half-graben.

According to Strank (1987) at least ten boreholes have proved Arundian strata, seven penetrating a complete succession. The Arundian and Holkerian substages represent a major phase of sea level rise during which highs were progressively inundated. The Woo Dale region remained land until mid–late Arundian times, while by contrast, 520 m of strata are present at Eyam. Typically only 80–100 m of dark grey, argillaceous limestones are present across most of the region. Occasionally, for example at Welton 1, Biscathorpe 1 and Coningsby 1 boreholes, these are interbedded with grey green siltstone and white quartzitic sandstone. In part this is a tectonic effect due to rapid differential subsidence across tilt blocks. Partial dolomitisation of Arundian shelf limestone is widespread. In several boreholes, for example at Grove 3, Egmanton 68, Bassingham 1, Welton 1, Coningsby 1 and Bardney 1, the uppermost unit of the Arundian, about 35 m thick, is a grey pyritic shale or siltstone, as reflected in the gamma ray log (Figure 35a), (Figure 35b). This 'Arundian Shale Member' provides a useful marker horizon. It may reflect the maximum depth of the transgression.

A critical issue is the thickness present in the basinal successions, for example in the Widmerpool Half-graben. According to Riley (pers comm, 1997), Arundian strata are absent in the Long Eaton 1 Borehole. If this is true, then a major break, possibly tectonically controlled, must have affected deposition in the basinal areas too, and the greater part of the Long Eaton Formation is of Holkerian age. This interpretation is not consistent with the monotonous character of the log suite, which suggests an unbroken succession of bioclastic turbidites, or the sequence model presented by Fraser and Gawthorpe (2003), which suggests deposition in the basinal areas was unaffected by such tectonic disturbance. In (Figure 35a), the only obvious break, at 1911 m depth, is interpreted as a possible occurrence of the Arundian Shale Member. This distinctive horizon is recognised in boreholes penetrating both shallow and deep water sequences (Figure 35a), (Figure 35b), and may represent a phase of sediment starvation at the peak of the transgression.

A prominent unconformity is present at the top of the Milldale Limestone Formation on the Hathern Shelf, with Arundian and Holkerian strata largely absent (Figure 36). The unconformity is overlain by the Cloud Hill Dolostone Formation (Asbian). In Plungar 8A Borehole, the Belvoir Limestone Formation (Chadian) is in turn overlain by a grey, pebbly sandstone present at the base of the overlying Plungar Limestone Formation. Most of the 199 m thick formation comprises dolomitic limestones with two thinner sandstones in a 15 m thickness of strata. White to buff, crystalline limestones, ooidal in places, occur above the youngest sandstone. The formation is interpreted as a carbonate ramp or platform lithological association. Riley (1992) noted that faunal remains are sparse for this unit, with a late Arundian age regarded as tentative. This conflicts with the macrofauna obtained from 140 m below the top of the unit in the adjacent Plungar 8 Borehole, which includes the brachiopod Gigantoproductus, indicative of a Brigantian age.

Thirty boreholes have proved Holkerian strata in the region (Strank, 1987), with 15 encountering a complete succession. The sequence is thickest in the Long Eaton 1 (850 m), Grove 3 (300 m) and Egmanton 68 boreholes, but thins eastward and is of the order of 100–150 m (e.g. Biscathorpe 1), reflecting increasing pre-Westphalian eastward erosion of the Tournaisian–Visean sequence. Ratcliffe-on-Soar 1 penetrated a monotonous series of calcareous, carbonaceous and pyritic dark grey mudstones, thinly interbedded with grey dolomitic limestones of late Holkerian age. At Grove 3, the limestones are mottled, buff and brown. Micropalaeontological studies have shown that the Nocton, Foston and Stixwould Highs were finally inundated by the transgression at this time. Restricted faunal assemblages in those localities reflect a more nearshore environment than in the Woo Dale area, where assemblages are more diverse (Strank, 1987). Thick dolostones are present at Eyam and Woo Dale, to the west of the region. In Eakring 146 Borehole, conglomerates and sandstones are interbedded with limestones as late as Arundian (BP Egmanton 68 Completion Report) or Holkerian age (Edwards, 1967), suggesting that tectonically active fault scarps persisted until this time. The Arundian and Holkerian substages correspond to the seismic stratigraphical sequence EC3. This is characterised by a wedge-shaped, thickening to fault geometry, and a weakly reflective signature which indicates the monotonous nature of the basinal sequences (Fraser et al., 1990). The sequence suggests rejuvenation of the major basin-bounding faults during strong extension.

Asbian

Latest Holkerian to mid Asbian times represent a phase of stillstand or regression following the extensive transgression of Arundian to mid Holkerian times. However a shelf province, fringed by apron reefs (Peak Limestone Group), and an off-shelf deep water facies (Craven Group), were by now clearly differentiated (Waters et al., 2007; Waters et al., 2009). Asbian strata may have been deposited widely across the northern margin of the Anglo–Brabant Massif but have been removed by subsequent erosion (Chisholm et al., 1988).

The shelf province is well exposed in the Peak District and includes the Bee Low Limestone Formation (including the Chee Tor Rock, Lower Miller's Dale Lava, Miller's Dale Limestone and Ravensdale Tuff members). The carbonates are typically thickly bedded calc-arenites, cyclically developed in sediment-starved shallow water areas, rimmed by reefs. The lavas are mostly olivine basalts, usually amygdaloidal and highly weathered. Numerous K-bentonite horizons represent ashfall from distant sources. Asbian shelf-edge reefs are described by Strank (1987). In the concealed East Midlands Platform, the Asbian strata have a fairly uniform thickness of around 200 m, for example in the Grove 3, Ironville 5, Eakring 146 and Egmanton 68 boreholes. The limestones are predominantly pale grey, thick-bedded bioclastic limestones comparable to the Bee Low limestones at crop in Derbyshire. Further east, regional uplift and erosion in post-Asbian, probably Namurian times, resulted in Asbian strata forming the subcrop over a wide swathe in the central–northern part of the region (Figure 37). As a result, the Asbian sequence in boreholes such as Bassingham 1, Welton 1 and Bardney 1 is heavily eroded (Figure 35a), (Figure 35b). At Coningsby 1, only 30 m is present (Figure 35a), (Figure 35b), and further east, Asbian strata are inferred to be entirely absent.

The sequence is thicker in the basinal areas such as the Widmerpool Half-graben, for example in the Long Eaton 1 Borehole (350 m), Ratcliffe-on-Soar 1 Borehole (290 m) and Duffield 1 Borehole. The Lockington Limestone Formation, proved in the Long Eaton 1 Borehole between 880 m and 695 m depth (Brandon, 1996), and the Ratcliffe-on-Soar 1 Borehole between 1157 m and 1300 m depth (Carney and Cooper, 1997; Carney et al., 2001), is restricted to the subsurface of the Widmerpool Half-graben. The formation comprises units of pale brown, argillaceous to sandy limestone turbidites, between 15 and 30 m thick. These are normally graded with sharp bases, interbedded with thinner sequences of pyritic and carbonaceous mudstone and thin sandstone beds, and are interpreted as deep water turbidite systems. The formation has a serrated wireline signature, contrasted to that of the more monotonous underlying and overlying formations comprising hemipelagic lime mudstones and calciturbidites. Thick basinal Asbian sequences were also proved in the Alport and Edale boreholes, in the Edale Gulf to west of the region (Smith et al., 2005). Asbian strata have not been penetrated in the deep Gainsborough Trough, only at Grove 3, on a ridge at its south-western margin.

On the Hathern Shelf, for example in the quarry at Breedon, the Cloud Hill Dolostone Formation is time-equivalent to the Lockington Limestone Formation. The unit was initially defined as the Cloud Hill Dolomite Formation (Monteleone, 1973) and later redefined as the Cloud Hill Dolostone Formation (Ambrose and Carney, 1997b). At the type section, in Cloud Hill Quarry, the formation is at least 125 m thick, displaying basal unconformity and mud-mound reef dolostone (Ambrose and Carney, 1997b). The formation comprises grey, pale buff and reddish grey, fine- to coarse-crystalline, thin- to very thickly bedded dolostones, with shaly mudstone or clay partings and beds. Mud-mound reef dolostones are typically buff to grey, massive, fine-grained and fossiliferous, with common brachiopods, crinoids, corals, gastropods, nautiloids and ammonoids. The latter indicate a late Asbian age for the upper part of the formation (Ambrose and Carney, 1997b). A ?Holkerian to Asbian age for the lower part of the formation is derived from palynological determinations from bioclastic grainstones of the Cloud Wood Member. The formation is comparable in age and depositional environment with the Hopedale and Bee Low Limestone formations of the Peak District. The latter comprises very thick beds of pale grey biosparite and biopelsparite calcarenites, with pedogenic crusts and palaeokarstic surfaces, commonly overlain by thin, red-brown and grey-green bentonites.

The Asbian substage corresponds to most of seismic sequence EC4 recognised by Fraser et al. (1990) and Fraser and Gawthorpe (2003). High amplitude reflectors within the basins indicate the presence of high-velocity limestone units within an otherwise monotonous succession of mudstones. Steeply dipping clinoforms identified at the basin margins are inferred to represent the progradation and aggradation of carbonate platforms. The sequence is interpreted to have developed during a phase of limited tectonic activity and low subsidence rate which favoured the basinward progradation of carbonate platforms.

Late Asbian–Brigantian

The third and final pulse of rifting, initiated in late Asbian times and continuing into early Brigantian times, produced a smaller amount of subsidence than the second pulse, but did allow emplacement of mantle-derived basaltic magmas along some of the basin-bounding faults. These effects were more severe in the Widmerpool Half-graben than in the Gainsborough Half-graben (Fraser and Gawthorpe, 1990). In the former, up to 600 m of Brigantian strata are present at the centre of the basin, rather than at the Hoton–Normanton Hills Fault. Renewed fault block rotation resulted in the development of a further unconformity in the footwall of the fault system. Subaerial and subaqueous volcanic rocks were erupted from a number of centres localised on the Cinderhill–Foss Bridge Fault System (Figure 38).

In mid Brigantian times, a marine regression led to a further basinward migration of platform carbonate systems. In latest Brigantian times, a period of inversion affected the whole region, most strongly in the east, where severe erosion affected Asbian–Brigantian strata (Figure 37). Encroachment of deltaic systems carrying abundant clastic material from the north brought Brigantian carbonate sedimentation to an end. The clear differentiation between Asbian shelf and basinal facies, continued into the Brigantian, although the fringing apron reefs disappeared and only isolated knoll reefs remained. Eruptive volcanism resulted in locally complex stratigraphical relationships. Localised tectonic uplift generated disconformities, and subaerial palaeokarstic dissolution hollows are common, particularly in Derbyshire (Walkden, 1974) where such surfaces are locally associated with subaerial and subaqueous basaltic lava flows and tuffs (Walkden, 1977; Waters, 2003). Brigantian strata occupy the subcrop in the north-western part of the region (Figure 37), the eastern limit of this corresponding to a line between Long Clawson and Glentworth mapped by Strank (1987, fig 9.8). To the east, they were either not deposited, or more likely eroded in a phase of pre-Namurian uplift (Strank, 1987; Gutteridge, 1987). As a result, Namurian strata in the west, and Westphalian strata in the east overstep onto limestones of Brigantian, Asbian or Holkerian age (Strank, 1987, fig. 9.9). According to Strank (1987), the top of the Brigantian is marked almost everywhere by an unconformity, and the thickness of Brigantian strata preserved is highly variable. It is difficult to define the exact base of the Brigantian over most of the East Midlands Platform because of the presence of 'diagnostic' Asbian marker fossils in the basal 20 m or so of Brigantian limestone (Strank, 1987).

The Monsal Dale Limestone Formation of the Peak District comprises a heterogeneous succession of thin- to thick-bedded limestones with cherts, and dark grey, thin-bedded cherty limestones with argillaceous partings (Waters et al., 2009), deposited in shallow water with emergence surfaces. Shelf margins were eroded and the earliest Monsal Dale limestones rest on the remnants of Asbian apron reefs (Aitkenhead et al., 1985). Shelf margin bioclastic grainstone shoals in the northern Derbyshire shelf fed a submarine fan in the Edale Gulf, that passed up into a mudstone-dominated succession. The formation includes a complex stratigraphy of basaltic lavas and tuffs: the Upper Miller's Dale Lava of the Castleton–Buxton–Tideswell region; the Winstermoor Lava, Lower and Upper Matlock Lava and Shothouse Spring Tuff members in the Matlock–Wirksworth region; the 293 m thick Fallgate Volcanic Formation of the Ashover area (Aitkenhead and Chisholm, 1982); the Shacklow Wood Lava and Lees Bottom Lava members of the Alport–Bakewell–Taddington Dale region; and the Cressbrook Dale Lava and Litton Tuff members of the Eyam–Longstone–Litton region. Several eruptive centres, including Matlock and Miller's Dale are recognised, but the largely subaqueous flows cannot be correlated between them (Aitkenhead et al., 1985). The majority of the volcanism is of tholeiitic composition (MacDonald et al., 1984). The overlying Eyam Limestone Formation comprises thin-bedded, cherty bioclastic limestone with mudstone intercalations. Small reef-knolls composed of massive biomicrite were deposited in comparatively shallow water with periodic emergence.

The Ticknall Limestone Formation was named by Monteleone (1973) and further defined by Ambrose and Carney (1997a). It is exposed in Cloud Hill Quarry and in disused limestone quarries at Ticknall, south Derbyshire, on the Hathern Shelf to west of the region. The BGS Ticknall Borehole (Ambrose and Carney, 1997a) proved 55 m of buff to pale grey, fine- to medium-grained, massive- to thinly bedded dolostone and limestones, with shaly mudstone interbeds, common palaeosols and karstic surfaces with green clayey drapes, and bioclastic grainstones and calcaeous mudstones where undolomitised. Some sandy dolostones and dolomitic sandstones are present at the base of the formation. Palaeosols and karstic surfaces with green clayey drapes are typical of the formation, indicative of periodic emergence. Foraminiferal determinations and macrofossils are consistent with a Brigantian age (Riley, pers. comm, 1997). This supports faunal lists for the outcrops at Ticknall and Calke Park (Parsons, 1918; Monteleone, 1973). The Ticknall Limestone overlies strata of early Asbian age. The nature of this junction is unclear, possibly being a fault or a karstic unconformity (K Ambrose, written communication, 2009). Brigantian strata have been encountered in over a hundred boreholes in the region. They typically comprise alternations of pale grey biosparites and grey to dark grey biomicrites, comparable to the Monsal Dale limestones. It is likely that the patch reefs recognised in Derbyshire also extend to the subsurface.

The Widmerpool Formation was first described from the BGS Duffield Borehole, Derbyshire, from 1047.5 to 414.2 m (Aitkenhead, 1977) and named after a borehole near Widmerpool (Falcon and Kent, 1960). The formation was formally defined by Aitkenhead and Chisholm (1982) with revision by Waters et al. (2009). The geographical extent of the formation has been expanded to include Dovedale and the Manifold Valley, including the former Mixon Limestone Shales (>200 m thick), for which ammonoids indicate a late Asbian age (Chisholm et al., 1988). The top of the formation is taken at the base of the Namurian Cravenoceras leion Biozone. It has subsequently been recognised in the Long Eaton 1 Borehole, in Ratcliffe-on-Soar 1 Borehole where it is 741 m thick, and several other boreholes in the Widmerpool Half-graben and Hathern Shelf, where it is somewhat thinner (110–130 m). The formation comprises dark to pale brown, green or grey, calcareous and locally carbonaceous, pyritic, fissile mudstone, thinly interbedded with turbidite, argillaceous, cherty limestone, and thin beds of quartzose siltstone, calcareous or quartzose sandstone and tuff. The proportion of limestone to mudstone increases northwards, with the formation passing laterally into the proximal turbidite deposit of the Ecton Limestone Formation of Dovedale, and subsequently into the Hopedale Limestone Formation of the Derbyshire shelf-edge (Figure 36). The Widmerpool Formation also locally overlies platform carbonate shelf sequences such as the Milldale Limestone Formation, for example on the Hathern Shelf, where it reflects late Brigantian drowning of the shelf.

Lavas and hyaloclastites are common in the Widmerpool Formation. The Ratcliffe Volcanic Member in the Ratcliffe-on-Soar Borehole, 124 m thick (between 507 m and 631 m depth), comprises five tuff beds intercalated with mudstone (Carney and Cooper, 1997; Carney et al., 2001). Cuttings samples indicate a sequence of olive green to brown silty carbonaceous mudstones and limestones with beds of bluish grey, pyritous tuff, interpreted as submarine volcanic ashfall deposits. The member is correlated with similar tuffaceous beds of P2 age in the Duffield Borehole (Aitkenhead, 1977) and the Tissington Volcanic Member of the Ashbourne district (Chisholm et al., 1988). A volcanic complex developed along the Cinderhill Fault, and penetrated by Strelley 1 Borehole, was active in Asbian–Brigantian times.

The late Asbian to early Brigantian strata correspond to seismic sequence EC5. They show similar thickening to fault, wedge-shaped geometry to those of sequences EC1 and EC3, although of reduced magnitude. Internally the sequence consists of low amplitude, high frequency, laterally continuous events that progressively onlap onto the hanging-wall dip slope. During this active rifting phase, footwall rotation was accompanied by uplift and erosion. By the end of EC5, shallow water conditions were established across the Widmerpool Half-graben. The mid Brigantian strata are attributed to seismic sequence EC6. The base of the sequence exhibits downlap onto EC5 along the hanging-wall dip slope, compatible with a basinward prograding regressive sequence tract. A series of complex clinoforms have been identified on the hanging-wall dip slope, and large mounded features on the Hathern Shelf (Fraser and Gawthorpe, 2003).

Chapter 6 Post-rift stratigraphy (Pendleian–Westphalian D)

Syn-rift extension and subsidence was replaced in late Brigantian time by a regime of regional, thermally driven subsidence (Leeder, 1982; Fraser et al., 1990; Fraser and Gawthorpe, 2003) and the depositional patterns of the post-rift Namurian–Westphalian sequence are markedly different (Fraser and Gawthorpe, 1990; Fraser et al., 1990; Chadwick et al., 1995; Kirby et al., 2000). The Visean-filled half-graben structures were superceded by less well defined and less complex basins, and the terminology used in the figures and text of this chapter reflects this. Changing climate and renewed uplift of source areas resulted in marked changes in lithofacies, compared to the Visean (Besly, 1988; Cope et al., 1992b). Deposition took place within the Pennine Basin, between the Anglo–Brabant Massif in the south and the Southern Uplands Massif of Scotland, in the north (Collinson, 1988; Martinsen et al., 1995). Pronounced end-Visean basin floor relief was progressively infilled by siliciclastic sedimentary rocks, with sediment supply exceeding subsidence rates (Guion and Fielding, 1988). Namurian and Westphalian strata dominantly comprise alternations of sandstone and mudstone, with subordinate coal and seatearth, and were deposited in a major fluviodeltaic system developed across the whole of northern England. Subsidence was not rapid in the west of the region. Waters et al. (2007) recognised the following megafacies:

  1. A fluviodeltaic ('Millstone Grit') facies extended across central England during the Namurian to early Westphalian. The main succession has a distant northerly provenance, although deposits of similar facies, but more locally derived, occur on the margins of the Anglo–Brabant Massif.

2.      A fluviodeltaic ('Coal Measures') facies extended across central England and North Wales, and beyond the Anglo–Brabant Massif in South Wales, Bristol, and Kent during the Westphalian.

3.      An alluvial ('Barren Measures') facies occurs as two subfacies: A 'Red-bed' subfacies comprising locally derived sediments, located in central England, North Wales and the southern fringes of the Anglo–Brabant Massif, was deposited during the Westphalian and Stephanian; the 'Pennant' subfacies comprises sediments of distant southerly provenance, present south of the massif, is predominantly of Bolsovian to Cantabrian age, but is locally present in the southern parts of the Pennine Basin, where it is of Asturian (Westphalian D) age.

During Pendleian times (Figure 39) fluviodeltaic ('Millstone Grit') successions started to accumulate at the northern margin of the Pennine Basin, with thick turbidite-fronted delta successions. By Marsdenian times the deltas had prograded across and largely infilled the central part of the basin, resulting in a predominance of relatively thin, sheet-like deltaic sandstones and well-developed delta-top deposits. The emergent Anglo–Brabant Massif sourced more localised fluviodeltaic sequences which prograded short distances north and south of the massif. The Namurian progradational succession, formed during falling or low relative sea level, was punctuated by marine transgressions during highstands of sea level. The latter are marked by goniatite- and bivalve-bearing marine bands which occur throughout the succession and record periodic transgressions across the delta system. These markers are valuable for the following reasons:

They are essentially isochronous, representing nearly instantaneous marine flooding events, whereas most of the deltaic facies are strongly diachronous (Figure 40).

1.      Their higher levels of radioactivity enable them to be readily distinguished from deltaic mudstones using the gamma-ray geophysical log (Knowles, 1964; Spears, 1964; Whittaker et al., 1985).

2.      The above characteristics facilitate correlation in outcrop and boreholes over wide areas.

3.      The strongly cyclic patterns of sedimentation in these strata are traditionally known as 'cyclothems'. The average duration of a cyclothem in the Namurian Millstone Grit Group is estimated at approximately 180 000 years (Maynard and Leeder, 1992).

In late Namurian to early Westphalian, delta progradation led to a transition to upper delta plain, coal swamp conditions, which covered most of northern England by late Langsettian to late Bolsovian times (Guion and Fielding, 1988). Cyclothems are also present in the Westphalian strata, but coal and soil horizons are more abundant and marine incursions are less common than in the Namurian strata. Although 'Coal Measures' cyclic successions are lithologically similar, two basinal areas were separated by the Anglo–Brabant Massif. From Bolsovian to Cantabrian (Stephanian) times fluvial sediments ('Pennant Measures') derived from the northward propagating Variscan thrust sheets were deposited across much of southern England and South Wales. Meanwhile, on the flanks of the Anglo–Brabant Massif, reddened alluvial deposits accumulated. During Asturian (Westphalian D) times the 'Pennant Measures' breached the massif and for a short period were deposited within the Pennine Basin.

At the beginning of the Namurian, the region lay in equatorial latitudes (Scotese et al., 1979; Smith et al., 1981; Cope et al., 1992b), with a humid climate, but by the late Westphalian semi-arid conditions prevailed. It has been suggested that the northerly migrating Variscan deformation front contributed to this climatic change. Deposition of the red beds of the Warwickshire Group has been attributed to the development of a rain shadow in the lee of the emerging Variscan mountains to the south (Besly, 1983; 1988).

Namurian

Namurian strata are not exposed in the region, but occupy an extensive area of the concealed subsurface in the north-western part of the region, where they are in stratigraphical continuity with the eastern flank of the Derbyshire Dome (Figure 41). Up to 1000 m may be preserved in the Widmerpool Basin, and 800 m in those parts of the Edale and Gainsborough basins which lie within the region (Map 5). Considerable variations in thickness occur, partly due to the infilling of remnant Visean topography, and differential compaction, but also as a result of syndepositional faulting. The sequence thins southwards onto the Anglo–Brabant Massif, and eastwards onto the Nocton High (Figure 41), (Map 5), where Namurian strata are overlapped by Westphalian strata. The overlap is particularly strong in the east, where Westphalian strata rest unconformably upon Visean strata or pre-Carboniferous basement.

A late Brigantian to earliest Namurian inversion event uplifted basin margins and led to intra-Carboniferous erosion (Strank, 1987; Coward, 1990; Fraser and Gawthorpe, 1990; Fraser et al., 1990). The principal evidence for this is the variable preservation of Brigantian strata in the west where the sequence is most complete (Strank, 1987) and their entire removal as a result of uplift in the east (Figure 37). It has been suggested that the inversion may reflect tectonism associated with nappe emplacement to south of the region (Sellwood and Thomas, 1986; Fraser et al., 1990), although the persistent buoyancy of this part of the Anglo–Brabant Massif may also have contributed.

The morphology of the Namurian basins resulted from a combination of factors including postextensional regional thermal relaxation subsidence, inherited relief due to incomplete infilling of sediment-starved Tournaisian–Visean rifts, local active faulting along basin-margin faults, distance from the sediment source and differential compaction of underlying platform and basinal rocks. Submerged highs acted as barriers, preventing the turbidites from reaching certain parts of the basin (Waters et al., 2007); for example in Derbyshire, the Ashover Grit delta (Marsdenian) with a source in the north-east, was deflected around an intrabasinal high, arriving in Staffordshire via south-east Nottinghamshire (Jones, 1980; Jones and Chisholm, 1997). Basinal muds of the Namurian part of the Craven Group (Waters et al., 2007) blanket the irregular surface of the Visean carbonate platforms, the thickest and oldest developments occurring in the basinal areas, such as the Craven Basin (outwith the region), Gainsborough, Edale and Widmerpool basins. In these areas, up to 900 m of Millstone Grit deltaic facies are present. These thicknesses compare with the 1225 m recorded at Wharfedale (Ramsbottom et al., 1978) in the northern part of the Central Pennine Basin.

The Bowland Shale Formation (Waters et al., 2007; Waters et al., 2009), formerly known as the Edale Shales, is a dark grey calcareous mudstone with thin turbiditic sandstones and disseminated pyrite. It is the youngest formation of the Craven Group found in these areas. The base of the formation is taken at the base of the Cravenoceras leion Marine Band. The top of the formation is taken at the base of the Millstone Grit Group, a highly diachronous boundary that is considerably younger in the southern part of the region (Figure 40). In the Duffield Borehole, to the west of the region, 166 m (incomplete) are present ranging from Pendleian to Chokierian age.

The Bowland Shale Formation passes laterally southwards into the siliciclastic sandstone-dominated Morridge Formation of the northern margin of the Anglo–Brabant Massif. Backstripping of regional seismic data suggests a water depth of about 300 m in the Widmerpool Basin, an interpretation supported by deep water conodont faunas (Fraser and Gawthorpe, 2003). Thinner and younger successions occur above the Visean highs. The most significant provenance lay far to the north of the region, where a major river system draining southwards from Greenland and Scandinavia supplied vast quantities of feldspathic detritus (Drewery et al., 1987; Cliff et al., 1991; Evans et al., 2001; Morton and Witham, 2002). Northerly derived deltas reached the Midland Valley of Scotland and northern England during late Visean times, and repeatedly prograded southwards across the Pennine Basin during the Namurian, eventually overwhelming the basin and subordinate western and southern sources in late Namurian times (Collinson, 1988; Guion and Fielding, 1988). Three main progradational phases, during the Pendleian, Kinderscoutian and Marsdenian substages, are recognised, each with depocentres progressively farther south (Kirby et al., 2000). The dominant lithology is arkosic or subarkosic arenite (formerly referred to as 'grit') developed in cyclic, upward-coarsening sequences, associated with grey siltstone, mudstone with subordinate coal and seatearth. Marine bands represent transgressive events, typically comprising dark grey to black, calcareous, shaly mudstones between 0.5 m and 15 m thick.

A second, more local source, probably the Anglo–Brabant Massif, supplied some sediment into the basin in the south of the region including protoquartzitic sands (Holdsworth, 1963; Trewin and Holdsworth, 1973; Aitkenhead, 1977; Aitkenhead et al., 1985; Chisholm et al., 1988; Fulton and Williams, 1988; Martinsen et al., 1995; Hallsworth et al., 2000). These sediments mirror the northerly sourced Millstone Grit in having shallow water sheet-like deltaic facies in the south, passing northward into deeper water, turbidite-fronted lobate deltas. The distinctive ammonoid fauna and widespread correlation of the marine bands makes them of primary stratigraphical importance, and a total of 46 ammonoid-bearing marine bands are recognised (Waters and Davies, 2006). Each of the seven Namurian substage successions within the Millstone Grit Group has recently been assigned a distinct formation name (Waters et al., 2009), with the exception of the thin, often mudstone-dominated successions of the Chokierian and Alportian, which have been joined to form a single formation. The newly named units are: Pendleton Formation (Pendleian), Silsden Formation (Arnsbergian), Samlesbury Formation (Chokierian to Alportian), Hebden Formation (Kinderscoutian), Marsden Formation (Marsdenian) and Rossendale Formation (Yeadonian). In addition, a new name, Morridge Formation (Pendleian to Marsdenian), has been introduced specifically to contain protoquartzite arenite strata, restricted to the Goyt Trough of Staffordshire and the Widmerpool Half-graben, with a provenance in the Anglo–Brabant Massif (Trewin and Holdsworth, 1973), as distinct from northerly-sourced, quartzofeldspathic strata of the main part of the Millstone Grit Group.

Six basic lithofacies associations are recognised in the Central Pennine Basin (Collinson, 1988): basinal mudstone, basinal turbidite, turbidite-fronted deltas, sheet deltas, elongate deltas and sheet-like channel sandstones. Turbidite-fronted deltas constitute the dominant basin-fill succession. The best examples occur within the Pendleian succession of the northern part of the basin (Baines, in Collinson, 1988), the Kinderscoutian succession of the central parts (Walker, 1966; Collinson, 1969; McCabe, 1978) and in the Marsdenian succession of the southern margins (Mayhew, 1967; Chisholm, 1977; Jones, 1980; Jones and Chisholm, 1997). Sedimentation was dominated by basinal mudstone (Craven Group) until mid Namurian (Kinderscoutian, and locally Marsdenian) times. The mudstone was succeeded first by turbidite-fronted and then by shallow water deltas (Millstone Grit Group) as the main river systems progressively prograded into the area from the north (Walker, 1966; Collinson, 1988). However, across the region, the picture is complicated by the influence of the Anglo–Brabant Massif, which acted as a sediment source for smaller northerly prograding deltas (Morridge Formation).

Two main types of delta subfacies are recognised in the Central Pennine sub-basin (Waters et al., 2007):

1.      Deep water deltaic sequences commonly several hundred metres thick, as exemplified by the Lower Kinderscout Grit (570 m) and Ashover grits (360 m). These facies comprise a lower turbidite sequence, a middle upwards-coarsening succession dominantly of siltstone, and an upper part of erosive channels, filled with pebbly coarse-grained sandstones (Walker, 1966; Collinson, 1969; McCabe, 1978). These represent deltas fed by large distributaries in which coarse sands were transported in feeder channels, bypassing the delta slope to be deposited in a delta-front apron of coalescing turbidite lobes.

2.      Shallow water deltaic sequences commonly tens of metres thick, comprising a lower succession coarsening upwards from mudstone to sandstone, overlain by laterally extensive sandstones and commonly capped by seatearths and thin coals. These deltas lacked significant transport of sediment by turbidity currents. The lower part of the succession is dominated by mouth-bar deposits that are overlain by distributary sands. These are mostly sheet-like and laterally extensive, as exemplified by the Chatsworth Grit and Rough Rock. However, elongate deltas are locally developed.

The strongly cyclical nature of the alternating sandstone, siltstone and marine bands, and rapid facies variation, contrasting with the widespread persistence of individual marine horizons, has been recognised since the early 20th century. Wright et al. (1927) were amongst the first to describe the development of a Millstone Grit cycle in terms of depositional setting and relative sea level changes. Subsequently, a eustatic origin, associated with changes in sea level, was invoked (Ramsbottom, 1977). Cyclicity has been related to short-period glacioeustatic sea level changes, with regressions and transgressions attributed to the fall and rise of sea level respectively. In recent years the technique of sequence stratigraphy has found widespread application to the Namurian successions (Read, 1991; Martinsen, 1993; Collinson et al., 1992; Maynard, 1992). Fraser et al. (1990) assigned the Brigantian–Namurian–earliest Westphalian Succession to their late Carboniferous sequence LC1 (Figure 13). The Craven Group strata are assigned to LC1a and LC1b. The Millstone Grit Group strata are assigned to LC1c. Church and Gawthorpe (1997) recognised ten high frequency (fourth-order) depositional sequences within the Marsdenian–Yeadonian succession of the Widmerpool Basin. The dominant control on sequence thickness variation and tracts is a longer period (third-order) cycle of sea level rise and fall. This is in contrast to the view of Martinsen et al. (1995) who concluded that the major cycles of deposition show no systematic architecture and suggested instead that they result from major avulsive shifts of sediment depocentres.

Pendleian (E1)

Strata of Pendleian age are proved in several boreholes. The nearest outcrop lies to the west of the region, on the flanks of the Derbyshire Dome. The youngest formation of the Craven Group, the Bowland Shale Formation (formerly the Edale Shale) is a dark grey mudstone deposited from suspension in quiet, relatively deep water in the Edale and Widmerpool basins. Fraser et al. (1990) recorded 80–200 m of Craven Group equivalent strata in the Gainsborough Basin. These basins were largely unaffected by coarser clastic detritus and the mudstones were banked against topographical slopes in the Visean limestone at basin margins (Broadhurst and Simpson, 1967). The presence of a condensed sequence of mudstone above the Visean platform in Derbyshire, suggests that mud also draped the upstanding platform areas (Ramsbottom et al., 1962).

Unfossiliferous mudstone accumulated when the water was brackish or fresh, interrupted by marine bands reflecting higher salinities and connection to the open ocean (Martinsen et al., 1995). Boreholes indicate that this condensed facies occupies a large part of the Pendleian subcrop (Figure 42)(e.g. 10 m in Strelley 1 Borehole). Much of the Edale Basin within the region is believed to be dominated by distal turbidites (Fraser and Gawthorpe, 2003) which lay on a prodelta slope fronting deep water deltas (Steele, 1988) lying to north of the region, for example in Bradford, where the Pendleton Formation is 600 m thick. By contrast, the Bowland Shale at crop in the Edale valley (Jackson, 1923), and proved in the Alport and Edale boreholes, comprises up to 150 m of mudstone (Stevenson and Gaunt, 1971). This contrast is believed to reflect the more energetic currents in the deeper basins compared to the intrabasinal slopes and highs, which acted as barriers to turbidity currents (Trewin and Holdsworth, 1973; Collinson, 1988).

The southern part of the subcrop, in the Widmerpool Basin, is dominated by proximal turbidites (Fraser and Gawthorpe, 2003), now defined as part of the Morridge Formation (Waters et al., 2009). In the Staffordshire Basin, the succession is dominated by mudstone and siltstone, but contains subordinate graded protoquartzitic sandstone (e.g. the Minn Sandstones) and interbedded calcareous siltstone (Trewin and Holdsworth, 1973). It has a provenance in the Anglo–Brabant Massif to the south. Clasts were carried into the basin by turbidity currents flowing from deltas flanking the massif (Trewin and Holdsworth, 1973; Aitkenhead et al., 1985; Bolton, in Collinson, 1988; Rees and Wilson, 1998).

Over 170 m of graded turbiditic sandstones, siltstones and mudstones are present in the Duffield 1 Borehole, west of the region (Aitkenhead 1977; Ramsbottom et al., 1978; Chisholm et al., 1988). They have subsequently been identified in boreholes at Ilkeston 1 (135 m), and Kinoulton 1, Long Clawson 2, Old Dalby 1 and Rempstone 1, where about 80 m are present (Figure 43a). At Rempstone, a producing oilfield is reservoired in this package of strata, informally referred to in BP borehole composite logs and completion reports as the Rempstone Formation (Pendleian–early Arnsbergian). The formation comprises clean, rather structureless quartz-arenites (reservoir sands) up to 14 m thick, possibly emplaced as grainflows; thin units of matrix-supported conglomerate containing disrupted clasts and folded beds of sandstone, and locally derived clasts of granite (Carney et al., 2004), are interpreted as debris flows; the latter are interbedded with dark grey to brown carbonaceous mudstones and siltstones. The quartz-arenites are inferred to have been sourced from the Anglo–Brabant Massif, while the debris flow units were derived from adjacent fault scarps.

Arnsbergian (E2)

In early Arnsbergian times, the turbidite-fronted deep water deltas of northern England began to prograde southwards into the region, reflected in increasing amounts of sandstone in the geophysical log signature (Figure 43b). The Silsden Formation (Waters et al., 2009) represents the first precursor of the Millstone Grit Group to appear in the region, referred to as Delta 2 by Steele (1988). It comprises siltstones and thin turbidite sandstones (prodelta slope), coarsening up into pebbly feldspathic sandstones (delta front). Thickness varies from 160 m in Scaftworth 2 (just north of the region) to 35 m in Glentworth 1, all considerably less than the 400 m present in Bradford.

Deposition of basinal mud with distal turbidites continued in the central part of the region, for example the Edale Basin. At crop in Derbyshire the mudstone sequence is up to 100 m thick (Stevenson and Gaunt, 1971), and ranges in age from Arnsbergian to early Kinderscoutian (Edwards and Trotter, 1954). In the subsurface (e.g. the Chesterfield area) deposition of mud persisted until the Marsdenian. In the Widmerpool Basin, deposition of the Morridge Formation continued into early Arnsbergian times (Figure 43a). The mudstone succession contains thin protoquartzitic proximal sandstone turbidites, siltstones and limestones. These become more distal away from the Anglo–Brabant Massif and thin against the former carbonate shelf highs. Duffield 1 Borehole penetrated 175 m of this sequence, and a comparable thickness is present in Kinoulton 1, Long Clawson 2, Old Dalby 1 and Rempstone 1 (Figure 43a). In Kinoulton 1 and Long Clawson 2, a thick sill overlain by tuff is present at the mid Arnsbergian level, according to the composite logs. The tuff suggests the presence of a localised volcanic centre. Mudstones with Arnsbergian ammonoids have been proved in the borehole at Hathern (Edwards and Trotter, 1954).

Chokierian to Alportian (H1 and H2)

Rocks of this age are generally very thin or absent in the region. Elsewhere in northern England this substage coincides with a non-sequence or unconformity (Ramsbottom et al., 1978). During the Chokierian and Alportian, clastic input into the Pennine Basin appears to have been considerably reduced. The succession is condensed with some hemipelagic marine mudstones. Fraser et al. (1990) identified the Chokierian mudstones as a potential hydrocarbon source rock in the Gainsborough Trough. Sandstones of the Millstone Grit Group (Samlesbury Formation) appear earlier in the eastern part of the subcrop (e.g. in Gainsborough 2, Normanby 1 and Glentworth 1) than further west (e.g. Walkeringham 1 and Scaftworth 2), where the Chokierian sequence is muddy and sands are not well developed until Alportian times. The sandstones are only slightly thinner (60 m) than their correlatives in the Bradford area (Brocka Bank Grit), and may reflect delta progradation from the eastern flank of the basin, rather than from the north. Many hydrocarbon wells in the north of the region encountered thick sandstones attributed to the 'Alport Sandstone', most notably at Walkeringham 1, where 85 m are present (Figure 43b), and in the Glentworth and Corringham fields where these strata are 50 m thick. These belong to Delta 3 of Steele (1988), which is greater than 250 m thick in the Doncaster area. The Chokierian–Alportian sequence attenuates markedly eastwards, from 80 m in Apleyhead 1, to 35 m in Farleys Wood 4.

In the Widmerpool Basin, this part of the Namurian sequence is often poorly dated and thin, typically 90 m or less for both substages. The sequence here forms the upper part of the Bowland Shale Formation (Craven Group), deposition of which continued until Kinderscoutian times. The lithologies are similar to those at crop in west Derbyshire and Staffordshire, namely a succession of mudstone with thin protoquartzitic sandstones, usually turbiditic (Aitkenhead et al. 1985).

Kinderscoutian (R1)

Kinderscoutian strata are found throughout the Namurian subcrop, and are more extensive than those of the preceding substages because they represent a transgressive sequence tract and are of dominantly coarse clastic facies. They equate to the Hebden Formation of Waters et al. (2009). The northerly sourced deltas prograded rapidly southwards in mid Kinderscoutian times in response to a massive influx of feldspathic sand from the northerly source. They encroached upon and smothered the relatively thin Pendleian to early Kinderscoutian successions. The lowest sandstones are usually turbiditic, with a distribution strongly influenced by basin floor relief. They were succeeded by fluviatile deposits as the basin was infilled. In the Peak District, strata of this age (Mam Tor Sandstone) are up to 450 m thick, but thin rapidly eastwards in the subsurface to 230 m at Bramley Moor 1, 130 m at Ranskill 1, and less than 30 m at West Firsby 2. The eastward thinning pattern is seen in both Namurian well transects (Figure 43a), (Figure 43b). By contrast, the variation from north to south is rather less, so that the thickness present in Walkeringham 1 is comparable to that in Kinoulton 1. This reflects the rapid and uniform southward progradation of the Kinderscout delta system along the axis of the regional depocentre, and the lower rate of subsidence (and or compaction) at its eastern margin.

The thin-bedded distal turbidites of the Mam Tor Sandstone (Allen, 1960) pass up into the very thick-bedded, more erosional turbidites of the Shale Grit (Walker, 1966). There is evidence for strong bathymetric control on currents and clastic supply. The overlying Grindslow Shales represent delta slope deposits and the Lower Kinderscout Grit, fluvial distributary channels (Collinson, 1969; McCabe, 1978; Hampson, 1997). The general southward progradation of deltas (Deltas 4 and 5 of Steele, 1988) resulted in the youngest sandstone, the Upper Kinderscout Grit, extending farthest south. Heavy mineral studies have demonstrated the northerly provenance of the Kinderscout Grit (Chisholm and Hallsworth, 2005). Deep water deltas are much rarer after Kinderscoutian times, suggesting that the Kinderscoutian deltas may have 'levelled the playing field'.

In the Widmerpool Basin (Figure 44), a number of relatively thin (typically less than 10 m thick) sheets of sandstone are developed within a predominantly muddy early Kinderscoutian succession. Little is known about the petrography of these sands. Their physical separation from the Millstone Grit deltas by intervening basinal mudstones (Figure 40) suggests that they were not provenanced by the former, but may represent correlatives of the southerly provenanced protoquartzitic sandstones attributed to the Morridge Formation in Staffordshire (Carney et al., 2004; Waters et al., 2007, 2009).

Marsdenian (R2)

During Marsdenian times, shallow water deltas dominated, and by the late Marsdenian they extended across the entire Pennine Basin to reach the southern limit of subcrop (Figure 41), (Figure 45). The Marsdenian successions are equivalent to the Marsden Formation of Waters et al. (2009). The substage commences with a major transgression associated with the Bilinguites gracilis Marine Band. Subsequent cycles are associated with the progradation of both linear and sheet-like deltas, principally in the north of the region. Mud deposition with only rare coarser intercalations continued until R2b times, when the main northern delta began to supply feldspathic clastic material into the region. The first significant sandstone of northern provenance to enter the region is the Ashover Grit, deposited during the final cycle of R2b age. This sandstone was deposited in a turbidite-fronted delta, but the main fluvial sandbody at the top of the cycle is elongate and interpreted as an incised valley fill (Jones and Chisholm, 1997). The locus of sedimentation lay in the north of the region (i.e. the Gainsborough Basin; Steele, 1988) and at crop in Derbyshire, where >300m may be present, at least in part a result of localised growth faulting (Chisholm, 1977). Synsedimentary growth-faulting (Bristow, 1988) may reflect gravitational collapse of the large scale deltaic successions (Chisholm 1977, 1981).

Subsequent deltas are laterally persistent sheet sandbodies. The last major feldspathic sandstones of R2c age are generally referred to as the Chatsworth Grit (Aitkenhead et al., 1985; Chisholm et al., 1988; Waters et al., 2008), and are up to 50 m thick at outcrop. Waters et al. (2008) have shown that the Chatsworth Grit occupies an east–west trending incised valley which represents the culmination of sedimentation of distinct sandbodies during a falling stage. The Chatsworth Grit fills a channel that formed at the low point of a lowstand. Both the Ashover and Chatsworth Grits are the low-sinuousity channel deposits of rivers flowing from the east or north-east. Heavy mineral studies have demonstrated the northerly provenance of the Ashover Grit and Chatsworth Grit (Chisholm and Hallsworth, 2005).

In the Widmerpool Basin the turbidite-fronted deep water deltaic successions of the Ashover Grit show a palaeocurrent towards the north-west. Their petrography indicates a typical northern feldspathic source however (Jones and Chisholm, 1997). These systems appear to have flowed north-westwards along the axis of the Widmerpool Basin to reach the Staffordshire Basin. According to Church and Gawthorpe (1994) and Jones and Chisholm (1997), the Ashover Grit represents a lowstand systems tract infilling incised valleys following a fall in sea level. The depth of the incised valley increases from 34 m at Kinoulton 1 to 60 m at Ilkeston 1. The trend of the palaeovalleys is west-north-west to east-south-east, and perhaps indicates that major faults, for example the Cinderhill Fault, exerted some control (Church and Gawthorpe, 1994).

The borehole transects (Figure 43a), (Figure 43b) reveal a similar pattern of thickness variation to that seen in Kinderscoutian times, both for the substage as a whole and for the individual grits. The substage thickness is greatest in the north-west (e.g. north Derbyshire, 450 m, Bramley Moor 1, 215 m, and Anston 1, 175 m) thinning rapidly eastwards (e.g. High Marnham 1, 40 m and Hemswell 1, 25 m). In the Widmerpool Basin, between 165 m (Ilkeston 1) and 60 m (Belvoir 1) are present (Figure 43a), (Figure 43b). Here, the Ashover Grit thins eastward from 20 m at Wilds Bridge 1 and Cropwell Bishop 1, to 12 m at Granby 1, and 5 m at Bottesford 4. The Chatsworth Grit thins from 37 m to 7 m in the same transect. These transects very clearly show the effect of differential subsidence and/or compaction across the basin.

Yeadonian (G1)

The Yeadonian succession (Rossendale Formation) comprises a lower argillaceous unit and an upper sandstone unit, the Rough Rock, which indicates a transition from transgressive marine to fluviatile conditions. Early Yeadonian strata comprise a thick succession of dark shales associated with two widespread marine transgressions, marked by the Cancelloceras cancellatum and Cancelloceras cumbriense marine bands. The upper part of the succession is dominated by the sheet sandstone of the Rough Rock. The latter is the youngest and most widespread Namurian sandstone in the Central Pennine Basin, comprising major multistorey, fluvial sheet sandstones up to 45 m thick, which are laterally persistent, medium to very coarse grained, subarkosic and pebbly (Shackleton, 1962; Bristow, 1988). It was deposited from braided river channels generally flowing towards the south-west (Bristow, 1988), but quite variable across the basin (e.g. in Staffordshire). The Rough Rock is thin or absent across much of the Peak District. One braided stream prograded southwards to Sheffield, and laterally into the Gainsborough Basin (Bristow, 1988). Steele (1988) indicated the presence of elongate north-east to south-west channel deposits extending across the Glentworth/Corringham area to South Leverton. Further south, fine-grained sandstones and siltstones were deposited in a prodelta or lacustrine environment. Braided rivers carrying sediment derived from the Anglo–Brabant Massif flowed westwards along the axis of the Widmerpool Basin. According to Church and Gawthorpe (1994), the distribution of the Rough Rock in boreholes in the Widmerpool Basin suggests that it infills incised valleys up to 32 m deep, in places (e.g. Saxondale 1, Cropwell Butler 1 and 2), locally excising the Chatsworth Grit. The braided rivers appear to have flowed into a brackish area of standing water in the basin centre. Heavy mineral studies have demonstrated a northerly provenance (Cliff et al., 1991) with intermixing with a southerly source in the proximity of the Anglo–Brabant Massif (Trewin and Holdsworth, 1973; Glover et al., 1996; Chisholm and Hallsworth, 2005). By late Yeadonian times, the Central Pennine Basin had been completely filled by the southward-prograding delta front.

Westphalian

Across most of the Pennine Basin, Namurian strata are conformably overlain by Westphalian rocks of the Pennine Coal Measures Group (Waters et al., 2007; 2009). At the southern margin of the basin, Visean and Namurian strata are locally overlapped and the Pennine Coal Measures Group oversteps pre-Carboniferous strata of the Anglo–Brabant Massif. The massif had a subdued relief and supplied minimal amounts of sediment to the north. On the eastern margin of the basin, Westphalian (and possibly latest Namurian) strata, including a prominent amalgamated 'Basal Sandstone' sequence, rest unconformably upon pre-Carboniferous basement (Beckering 1 and Claxby 1 boreholes) or an irregular surface of eroded and likely karstified Carboniferous Limestone (Welton Oilfield).

Up to 1500 m of the group are present in the region, rather less than the 2500 m present at the basin depocentre near Manchester. Westphalian strata of the Pennine Basin are now distributed within a number of geographically separate areas that include the South Staffordshire, North Staffordshire, Warwickshire, Coalbrookdale, Lancashire and Nottinghamshire–Yorkshire coalfields. As a result, there has been a proliferation of local names and lithostratigraphical schemes, reflecting variable rates of basin subsidence, uplift and sediment flux in each coalfield (Ramsbottom et al., 1978). Within the region, these strata outcrop in the Nottinghamshire–Yorkshire Coalfield and subcrop over a large part of the subsurface (Figure 41), (Map 8).

The sedimentology of the Pennine Coal Measures has been described by Fielding (1984a, b, 1986) and Guion and Fielding (1988). The strata comprise cyclical successions (cyclothems) of mudstone, siltstone, sandstone, seatearth (palaeosol) and coal deposited in upper and lower delta plain environments, crossed by major distributary channel complexes 2–20 km wide. Compared with other areas of Britain, the Pennine Lower and Middle Coal Measures contain a relatively small percentage of sandstone (around 20 per cent), Allocyclic events (e.g. change in climate, subsidence rate, local tectonism and glacioeustacy) and autocyclic controls (sediment compaction and sedimentary processes), influenced the sedimentation pattern. Tectonic controls were locally important, particularly from late Bolsovian times onwards.

The base of a typical Coal Measures cyclothem is marked by grey, well-bedded mudstones. Dark grey, carbonaceous and fissile mudstones form useful marker horizons, particularly when associated with a marine fauna (marine bands), and can be identified in geophysical logs by their high gamma values. Ironstones are common in the mudstones, comprising nodules and thin layers of siderite. The mudstones typically grade up into laminated and bioturbated siltstones and then pale grey sandstones with ripple lamination, cross-bedding and low-angle planar lamination, typically of sub-greywacke composition.

The coarsest sandstones typically formed in major distributary channels, and are up to 20 m thick. These have the best reservoir potential but comprise less than 1 per cent of the sequence. Thin sandstones with erosive bases were deposited in minor channels, separated by extensive lakes where mud and cannel coal accumulated. Lacustrine deltas and crevasse-splay deposits, forming localised coarsening-upward and fining-upward successions, respectively, result from the breaching of the channel banks. Most are rather thin, interbedded with siltstones, and have poor porosity and permeability, except where oil was emplaced early (Hawkins, 1978). These strata are commonly overlain by palaeosols, either ganisters (leached sandstones) or seatearths, including kaolinitic fireclays, with numerous rootlet horizons. Coal formed from peat accumulating on the floor of equatorial rain forests in extensive swamps. Marine bands comprising dark grey to black mudstone, extend for thousands of kilometres, and reflect flooding of the delta plain during periodic sea level rises (Calver, 1968; Ramsbottom et al., 1978).

There is a general decrease in the number and thickness of marine bands up the sequence, and increase in the importance of coals and seatearths from the Millstone Grit Group into the overlying Pennine Coal Measures Group. These reflect less numerous marine incursions into the delta-top environment with time. Cyclic sequences in the Pennine Coal Measures Group are thinner and more numerous than in the underlying Millstone Grit Group. The Warwickshire Group (formerly part of the Upper Coal Measures, Barren or Red Measures) ranges in age from Bolsovian to Autunian, and is attributed to the inversion megasequence of Fraser et al. (1990) and Fraser and Gawthorpe (1990) (Figure 13). The boundary between the two groups is more or less conformable in many parts of the basin. However, an unconformity is locally present on the margins, for example in Lincolnshire. This study follows Smith et al. (2005) in referring to this discontinuity as the Symon Unconformity.

The variations in thickness of the Westphalian strata in the region broadly supports a 'thermal sag' model (Leeder, 1982; Fraser and Gawthorpe, 2003). Strata of Namurian to Bolsovian age onlap southwards and thin towards the Anglo–Brabant Massif (Fulton and Williams, 1988). To the north, the Pennine Coal Measures Group (Langsettian to Bolsovian) becomes thicker and individual coal seams split. In a depocentre centred in south Lancashire, between Manchester and Stoke (Trueman, 1947; Fraser et al., 1990), the maximum preserved thickness of the group may be up to 2500 m (Smith et al., 1984). However, by mid Duckmantian times this simple picture was complicated by the first effects of the progressive northward migration of the Variscan deformation front, and later Westphalian depositional systems were increasingly affected by faulting and uplift. The simple 'thermal sag' model does not account for many features evident in central England over the southern part of the Pennine Basin (Besly, 1988; Waters et al., 1994). The Warwickshire Group records the initial basinwide Variscan inversion events within the Variscan Foreland, culminating in Asturian (Westphalian D) times with the onset of the second and strongest compressional pulse of the Variscan Orogeny. Within the basin, there is evidence for local control of deposition by remnant topography, but the important effects of synsedimentary folding and faulting are well documented (Fielding, 1984a; Fielding and Johnson, 1987; Guion and Fielding, 1988; Read, 1988; Waters et al., 1994).

The Pennine Coal Measures Group is divided into 3 formations (Figure 39): Pennine Lower Coal Measures (Langsettian), Pennine Middle Coal Measures (Duckmantian–Bolsovian) and Pennine Upper Coal Measures (Bolsovian–Asturian) formations (Waters et al., 2007; 2009). The latter is subdivided into four members in the East Pennine Coalfield, in ascending order: Ackworth, Brierley, Hemsworth and Badsworth members.

Pennine Coal Measures Group

The base of the Pennine Coal Measures Group is defined at the base of the Subcrenatum Marine Band, or where this is absent or unidentified, at the lowest coal-bearing sequence (Waters et al., 2009). The grey Pennine Coal Measures Group extends up to the base of the lowest overlying red-bed formation of the Warwickshire Group (Table 1), and in the region is up to 1500 m thick. The boundary between the Pennine Lower and Middle Coal Measures Formation is the Vanderbeckei Marine Band. The top of the Cambriense Marine Band marks the boundary between the Pennine Middle and Upper Coal Measures Formations.

The Pennine Lower Coal Measures Formation (Langsettian) is thickest (over 500 m) in the west of the region. Of these strata, 520 m are present in Shirebrook West 1 Borehole (Figure 46b). The sequence thins southward, onto the Anglo–Brabant Massif, and eastwards, onto the Nocton and Stixwould Highs (Figure 47). As a result, 200 m or less are preserved in the Lincoln district, for example 155 m of strata in Welton 1 Borehole and 100 m in Coningsby 1. Major sheet and distributary channel sands, the Crawshaw Sandstone, Wingfield Flags and Kilburn Rock, are widely distributed through the region, and are important hydrocarbon reservoirs. The Crawshaw Sandstone rests upon, and locally erodes the Subcrenatum Marine Band. It is considerably thicker in the Edale Gulf, and palaeocurrent data indicate that the delta lobes were deflected to flow westwards towards the basin depocentre (Fielding 1984b; Guion and Fielding, 1988). It is thin or absent in the north (Figure 46a). In contrast, the majority of Westphalian A–C sand bodies are thicker in the north then in the south (Figure 46b). From the base of the Subcrenatum Marine Band to the 80 Yard Coal (Figure 48) the cyclothems usually have a marine band at the base and a palaeosol at the top, and include micaceous sandstones. The Sub-Alton Sandstone, underlying the Listeri Marine Band (the main reservoir at Bothamsall Oilfield) is generally thinner and less widely distributed, and is absent in the southern part of the subcrop (Widmerpool Basin). In this region, part (in places, all) of the Coal Measures strata are replaced by contemporaneous volcanic rocks of the Saltby Volcanic Formation (Carney et al., 2004). The development of this eruptive centre (Figure 38) in Langsettian times prevented the access of the delta distributaries into this part of the region.

Several thin sandstones are present immediately overlying the Listeri Marine Band. In the Lincoln district, all or several of the above sandstones are locally amalgamated into a unit informally known by BP as the 'Basal Sands'. These are clean with a good porosity (16–18 per cent) and form an important reservoir up to 40 m thick in Welton 1 Borehole . The Amaliae Marine Band forms the regional seal for these amalgamated sandstones and is a convenient horizon for seismic mapping.

Between the 80 Yard and the basinwide Kilburn (Better Bed) Coal (Figure 48), coal seams are thin and rare, only the palaeosol beneath the Kilburn Coal is well developed, and marine band faunas are very restricted. The middle part of the formation includes micaceous sandstones sourced from the north, and weakly micaceous sandstones sourced from the west (Chisholm, 1990; Chisholm et al., 1996; Hallsworth and Chisholm, 2000). Between the Kilburn Coal and the base of the Vanderbeckei Marine Band, there is a thick succession of laterally impersistent cyclothems lacking true marine bands and with thick coals. The Wingfield Flags and Kilburn Rock, underlying the Kilburn Coal (Figure 48), and the Parkgate Rock and Deep Soft Rock, when clean, may have reservoir potential. The early Langsettian sandstones are considered to have the same northerly source as the underlying Millstone Grit (Chisholm et al., 1996). The Deep Hard Coal is an important coal in this sequence and was exploited briefly at Asfordby Mine, where working was severely complicated by the presence of intrusive centres and sills (Figure 38). The contemporaneous nature of volcanism has been demonstrated at Plungar, at the eastern end of the Widmerpool Basin, where boreholes demonstrate a succession dominated by abundant alkali basalt lavas and tuffs, instead of coal-bearing strata (Carney et al., 2004).

Subsidence rates were low along the southern margin of the Pennine Basin resulting in a relatively few thick seams (Waters et al., 1994). Northwards, towards the basin depocentre, subsidence rates are greater and seams split. The cyclothems are typically upward-coarsening lake-fills with the principal driving force being continuous subsidence. The sandstones in the upper part of the formation, in contrast, appear to be sourced from the west (Hallsworth and Chisholm, 2000; Chisholm and Hallsworth, 2005). From mid Langsettian to Duckmantian times, there was a transition away from lower delta plain environments characteristic of early Langsettian, to upper delta plain.

The Pennine Middle Coal Measures Formation (Duckmantian–Bolsovian) is thickest (up to 440 m) in the west of the region, and can be broadly divided into two unnamed members (Aitkenhead et al., 2002). From the base of the Vanderbeckei Marine Band to the base of the Maltby Marine Band (Figure 48) the succession is similar to the upper member of the Pennine Lower Coal Measures Formation. It accumulated in an upper delta plain environment with rare large sandstone-filled distributary channels, for example the Eagle Sandstone, an important reservoir locally up to 40 m thick, in the Gainsborough–Beckingham Oilfield. It too represents an amalgamated stack of distributary channels, with production coming from different 'shoestring' sands in each borehole. The provenance of these sandstones lay to the west (Hallsworth and Chisholm, 2000; Chisholm and Hallsworth, 2005). The Top Hard Coal is the correlative of the Barnsley Coal of South Yorkshire, and is often up to 3 m thick in the northern part of the region. The upper part of the formation, between the Maltby Marine Band (late Duckmantian) and the top of the Cambriense Marine Band (Bolsovian), displays a return to derivation from the north as well as the start of a new clastic influx, this time from the east and south-east (Hallsworth and Chisholm, 2000; Chisholm and Hallsworth, 2005). The Abdy–Brinsley Rock and Oaks Rock are well developed in the north of the region (Figure 46a) and thin to south and east. They are absent in the south and west (Figure 46b). There was also a possible return to lower delta plain environments at this time. Marine bands are common at the bases of cyclothems and coals are thin.

The Pennine Upper Coal Measures Formation comprises interbedded grey mudstone, siltstone and pale grey sandstone, with thin coal seams. Marine bands are absent, but beds containing estheriids are common. The formation is characterised by the presence of nonmarine bivalves of the Anthraconauta phillipsi and Anthraconauta tenuis biozones, of late Bolsovian and Asturian (Westphalian D) age, respectively. The deposits accumulated in a delta-top environment with large distributary channels, similar to that described for the Pennine Lower Coal Measures Formation. The sandstones are mainly derived from the south-east (Hallsworth and Chisholm, 2000; Chisholm and Hallsworth, 2005). Marine bands are absent indicating that marine incursions did not extend across the delta top.

Up to five major sandstones underlie the unconformity at the base of the Etruria Formation (Warwickshire Group). These include the Ackworth Rock, up to 40 m thick with an erosive base; the even thicker Wickersley Rock, and the Ravenfield Rock (Figure 48). The Bolsovian sequence is also thick at Biscathorpe (>250 m), where three further sand bodies are present (Figure 46a). In Welton 1, these sandstones have a porosity of about 15 per cent. They are thinner and less well developed in the west. In the completion logs of BP hydrocarbon wells, the sandstones above the Mexborough Rock are frequently referred to as sandstones 'F', 'G' and 'H' (Figure 46a).

The stratigraphical equivalents of these Bolsovian strata in the southern North Sea are the lower Schooner Formation (Cameron et al., 1992) or the upper Cleaver Formation (Moscariello, 2003). Detailed studies of the petrography and zircon age spectra of these strata by Morton et al. (2005) suggest a provenance dominated by tourmaline-rich granitic terrane, probably of Variscan affinity, and most likely the Saxo–Thuringian Zone of Germany. A south-easterly provenance by way of the Silver Pit region is inferred (Morton et al., 2005). The thickness variation observed in the East Midlands region is compatible with such a source. By contrast, both earlier (Caister and Westoe coal formations) and later (Ketch Formation) sandstones exhibit the same polygenetic, northern metamorphic source as the majority of the Namurian sandstones. The higher parts of the Pennine Coal Measures Group are frequently stained red as a result of oxidation beneath the nearby sub-Permian unconformity, a phenomenon occurring lower in the succession towards the east and south-east. 'Red Measures' have been described from the boreholes Coningsby 1, Claxby 1, Nocton and elsewhere, but an analysis of the borehole geophysical logs suggests that these represent reddened strata of the Middle and Upper Pennine Coal Measures Formations, proximal to contemporary highs, and not 'red beds' deposited unconformably upon the 'grey measures'.

Warwickshire Group

The Warwickshire Group (Powell et al., 2000; Waters et al., 2007) is named from the Warwickshire Coalfield, where up to 1225 m of multicoloured mudstone, siltstone, sandstone and conglomerate, with only minor coal development, overlie the Pennine Coal Measures Group. These are of late Bolsovian to Stephanian age (Besly, 1988), and have historically been known as the Upper or Barren Measures because of the scarcity of coals. The red beds have undergone oxidation at, or close to, the time of deposition, as opposed to those that have been reddened by later weathering, such as the sub-Permian weathering of Carboniferous strata in east Lincolnshire.

The lower boundary of the group is highly diachronous, with primary red beds forming earlier on the margins of the Pennine Basin (Besly, 1988), most notably in South Staffordshire. The Symon Unconformity (p.71), separates the Pennine Coal Measures Group and the overlying Etruria Formation in the type area of the Coalbrookdale Coalfield (Scott, 1861; Clarke, 1901; Hamblin and Coppack, 1995). Analysis of the borehole geophysical logs (Figure 46a) in the Farnsfield area, to the west of the Eakring–Foston Fault, shows that the Symon Unconformity cuts down eastwards to approach the level of the Cambriense Marine Band (Edwards, 1967; Fraser and Gawthorpe, 2003; Carney et al., 2004). Here, Variscan inversion is presumed to have commenced as early as Bolsovian times. Seismic reflection data from the Welton area (Figure 49) suggest that a similar relationship pertains thereabouts, although reflection multiples from the Permian strata cause a masking effect at this stratigraphical level.

The Etruria Formation (formerly the Etruria Marl) comprises red, purple, brown, ochreous, green, grey and commonly mottled mudstone with subordinate lenticular greenish grey lithic sandstones and conglomerates, referred to as 'espleys'. The formation has relatively local source areas within the Anglo–Brabant Massif.

Up to 60 m are present in Cotmoor Lane and Fiskerton 1 boreholes (Figure 46a). Brunified alluvial/gley, semi-gley and ferralitic palaeosols are present (Glover et al., 1993). Up to 6 m of volcaniclastic material described from Barkestone Bridge Borehole may have been sourced from contemporary eruptive centres in the West Midlands (Glover et al., 1993) or from erosion and reworking of more local materials (Carney et al., 2004). The formation is interpreted as a well-drained, alluvial floodplain facies association with palaeosols, and sandstones representing single channel fill or multilateral or multistorey channel fills (Glover et al., 1993).

The Etruria Formation is unconformably overlain by the Halesowen Formation, comprising grey-green, micaceous sandstone (lithic arenite), and grey-green mudstone, with thin coals and Spirorbis limestone beds, with local intraformational conglomerate. The lithic arenites are equivalent to those of the Pennant Sandstone Formation and sourced from the developing Variscan fold-thrust belt much further south (Glover et al., 1996; Morton et al., 2005). Strata may be locally reddened. Up to 80 m are present in the Parkhill 1 and Fiskerton 1 boreholes, preserved beneath the Permo-Trias unconformity (Figure 46b). In the type area of Staffordshire, three upward-fining cycles are interpreted as representing the sequential transition from fluvial to floodplain-dominated sedimentation (Waters et al., 2009). The presence of thin coals and lacustrine (Spirorbis) limestones indicate a relatively high water table with development of swamps (Glover and Powell, 1996). Outside the region, for example in Staffordshire, the Halesowen Formation rests unconformably on Pennine Coal Measures or older rocks in anticlinal and marginal areas, and on the Etruria Formation in synclines (Smith et al., 2005). The unconformable base (Powell et al. 2000) may indicate a separate pulse of orogenic movements, or deformation continuing throughout deposition of the Etruria Formation (Waters et al., 1994). Later red-bed facies, attributed to the Salop and Clent formations, have not been identified. A later tectonic event resulted in the Clent Unconformity in the South Staffordshire Coalfield. These earth movements provide the first widespread evidence of early Variscan inversion events. The migration northwards of the Variscan deformation front culminated in the main phase of the Variscan Orogeny in Asturian (Westphalian D) to Stephanian times. Uplifted fault blocks occupying the sites of what are now Triassic basins at Knowle and Hinckley were the principal sources of coarse detritus (Wills, 1956; Besly, 1988). On seismic profiles the Warwickshire Group is characterised by a package of reflectors that are generally of lower amplitude and poorer continuity than that of the Pennine Coal Measures Group.

Chapter 7 Variscan structures and basin inversion

The region lay in the foreland of the Variscan Orogeny, which completed the accretion of the crust of central Europe, about 290 Ma (Franke, 2000; Pharaoh et al, 2006; Kroner et al., 2008). Within the region, tectonic activity was focussed on the reactivation of major crustal faults and lineaments initiated during Neoproterozoic and early Palaeozoic times.

Palaeomagnetic evidence (Tait et al., 2000) supports independent motion of terranes within the Armorican Terrane Assemblage (Figure 7)e until at least Late Devonian times (about 370 Ma). These events were accompanied by the destruction of the Rheic Ocean, lying between Gondwana and Laurussia, and the birth of its short-lived descendent, the Rheno–Hercynian Ocean (Franke et al., 1995). In early Carboniferous times, widespread high-pressure metamorphism in the Variscan internides (Bretonian Event) records rapid crustal thickening following closure of the Massif Central Ocean and collision with Iberia (Ziegler, 1990; Franke, 2006). Progressive closure of the ocean basins within the Armorican Terrane Assemblage resulted in an east–west trending mountain chain in central Europe from late Namurian times. Foreland basin development to the north of the Variscan deformation front began in the South Wales Coalfield in early Namurian times (Gayer and Jones, 1989). The Pennine Basin of eastern England was located about 500 km north of the Variscan Front at this time. During the Westphalian, pulses of compressional deformation began to affect the foreland, which experienced flexural subsidence. North-west- and north-north-west- directed thrusts have been described in the Variscan massifs of the Ardennes, Rhenish Massif and Cornubia (south-west England) (Coward and Smallwood, 1984; Coward, 1993). Seismic data confirm the continuity of the Variscan Front between these areas, passing through southern England just south of London (Whittaker and Chadwick, 1983). These latest Carboniferous movements completed the construction of the supercontinent known as Pangaea.

Initially the pulses of deformation were weak, causing minor warping and localised incision, as exemplified by the base of the Woolley Edge Rock in late Duckmantian times (Aitken et al., 1999). By late Bolsovian/ Asturian times, northward propagation of the thin-skinned Variscan fold-thrust belt onto the foreland had brought the Variscan Front to its final position in southern England, about 200 km south of the region. Original early Carboniferous extensional faults were reactivated. The style and magnitude of inversion depends on the orientation of these faults, and that of the basement faults which underlay them. In the region, the most prominent inversion anticlines are associated with north-westerly trending structures such as the Eakring Anticline and the western part of the

Widmerpool Half-graben. As these structures grew their crests were eroded, and clastic sediment was fed into small basins such as the Farnsfield Trough (Figure 46b). Increasing aridity, due to the drift of the region towards the northern desert belt, as well as the development of a rain-shadow in the lee of the Variscan mountains, resulted in the deposition of primary red-bed strata of the Warwickshire Group (late Bolsovian–Asturian). Geophysical log correlations (Figure 46b) demonstrate the presence of an unconformity, the Symon Unconformity, at the base of this sequence. In places up to 500 m of earlier Westphalian strata may have been removed (Fraser and Gawthorpe, 2003). Percolation of oxidising waters below the land surface resulted in widespread reddening of primary grey Westphalian strata, creating secondary red beds. The climax of inversion occurred in late Asturian–Stephanian times, leading to widespread uplift of the foreland and extensive erosion (Besly, 1988; Corfield et al., 1996). This produced long wavelength inversion anticlines on the hanging wall blocks of the major north-west-trending syndepositional faults, such as the Normanton Hills–Hoton Fault. By contrast, north-north-west-trending faults such as the Eakring–Denton Reverse Fault and Ironville Thrust (just west of the region), show less Visean syndepositional growth and are characterised by shallower detachments and tight inversion anticlines adjacent to the faults (e.g. at Eakring and Welton). Such structures constitute the majority of hydrocarbon-bearing traps in the East Midlands (Fraser and Gawthorpe, 2003).

Principal inversion structures

Thringstone Fault

The Thringstone Fault lies on the south-west flank of the Charnwood High, separating it from the Leicestershire Coalfield. It extends into the south-western extremity of this region (Figure 5). The CHARM seismic line (Maguire, 1987) imaged the fault as a north-east-dipping zone, confirming the observations in Merry Lees Drift (Butterley and Mitchell, 1946). The fault is interpreted as a major Variscan inversion structure (Smith et al., 2005), although it may have an earlier, Caledonian history (Chapter 3).

Eakring Fault and Anticline

The Eakring Fault (Figure 50) shows Visean syndepositional growth (Smith et al., 1985), with the thickness of the succession doubling across the structure. It displays a reverse throw of 250 m at the Namurian and Westphalian level, a consequence of Variscan inversion, which was concentrated on faults with a north-westerly trend (Figure 49). The Eakring–Duke's Wood Anticline is a periclinal structure with a north-north-west-trend, extending between Eakring and Rolleston, in the hanging wall of the Eakring Fault. The base Namurian horizon was uplifted by about 450 m along the crest of the fold relative to the Nottingham Platform, 1.5 km away. About 250 m of the uplift was achieved by reversal of the fault, the rest by growth of the inversion anticline. Subsequently, Pennine Middle and Upper Coal Measures were eroded from the crest of the fold prior to deposition of Permian strata. The base of Permo-Triassic strata is offset by a normal throw of about 60 m. According to Corfield et al. (1996), the development of periclinal, frequently en-échelon folds such as Eakring, Welton and Calow, involved a significant component of sinistral strike-slip on north-trending structural elements, analogous to the north-trending Lask Edge Fault, at the western edge of the North Staffordshire Basin. The Ecton Anticline, for example, has the geometry of a positive flower structure (Corfield, 1991). The Eakring Anticline does not have such a geometry and appears to result from the reactivation of a structure in the pre-Carboniferous basement. Wills (1956) postulated that the major phase of folding here occurred in late Bolsovian times. This inference is supported by the presence of strata (Etruria Formation) of this age overlying the Symon Unconformity in the adjacent Farnsfield Trough (Howard et al., 2009).

Denton Fault and Foston High

The Denton Fault delimits the western edge of the Foston High (Figure 18). It represents a continuation of the Eakring lineament south-eastwards, and ultimately links to the Glinton Thrust (Figure 8) (Chadwick and Evans, 2006). The fault effectively represents a transfer zone between the Widmerpool and Sleaford half-grabens, offset along it. Seismic data indicate that the fault dips to the north-east beneath the high (Figure 18). Pre-Carboniferous basement of the high is uplifted 400 ms TWTT (equivalent to about 500 m uplift) compared to the regional level to the west. This value is comparable to that seen further north along the fault system at Eakring.

At its junction with the Barkston Fault, the Denton Fault has the appearance of a spectacular thrust structure (Figure 18). Traced southward, the effect of thickening down the dip slope of the Witham tilt block results in the fault having a monoclinal appearance, with smaller apparent displacement of Carboniferous strata. It appears to truncate the eastern end of the Widmerpool Half-graben.

Widmerpool Anticline

Variscan compression led to the development of a major inversion anticline (Figure 51) along the axis of the Widmerpool Half-graben, possibly localised on an earlier roll-over anticline, and drilled by the Ratcliffe-on-Soar 1 and Long Eaton 1 boreholes, where the Namurian–Westphalian strata were largely or entirely removed prior to deposition of the Permo-Trias strata. Using the criteria of Corfield et al. (1996) the Widmerpool Half-graben has suffered moderate inversion, with removal of much of the post-rift sequence.

Rempstone Anticline

Variscan compression focussed on the Normanton Hills–Hoton Fault, a zone with strong contrast in competency between the basement high on the southern side and the Widmerpool Half-graben to the north. The result was a more localised fold than the Widmerpool Anticline to the north, from which this structure is distinct. In (Figure 54), while most of the fold development was clearly of Variscan age, it is possible to see from the gentle warping of the overlying Permo-Triassic strata that a small Alpine component of strain may also be present, as reflected by the surface geology (Carney et al., 2004).

Hathern Shelf

Significant further uplift of the Hathern Shelf during Variscan inversion led to erosion of post-rift LC1/2 strata between Rempstone 1 and Kirby Lane boreholes (Fraser and Gawthorpe, 2003).

Sleaford Half-graben

Mild inversion of the Sleaford Low preceded deposition of sequence LC1. Namurian strata are absent in the eastern part of the half-graben, and Westphalian strata are rather thin. Variscan inversion produced long wavelength inversion anticlines in the hanging-wall blocks of these faults. Further uplift and erosion of the Sleaford Half-graben occurred in mid Bolsovian times, prior to deposition of the Etruria Formation (late Bolsovian) and as a result of Variscan (end Westphalian to Stephanian) orogenic movements.

Nocton High

The north-trending Nocton High remained a positive feature through late Carboniferous times, so that only a thin Namurian–Westphalian sequence accumulated. Seismic data indicate that strong Variscan uplift caused even these to be eroded from the crest of the high (Figure 25). The high exerted a strong buttressing effect, and a north-west to south-east trending inversion anticline is developed along much of its north-east flank (Nettleham field and North Greetwell and Branston structures; (Figure 52) and (Figure 53).

Stixwould High

Several boreholes in this part of the region (Bardney 1, Stixwould 1, Claxby 1) proved red beds (so called 'Barren Measures') overlying Langsettian–Duckmantian strata. As described earlier, there is some doubt whether these represent 'primary red beds' of the Warwickshire Group, rather than secondarily reddened grey Coal Measures. These were not eroded during the main Asturian/Stephanian phase of inversion.

Ladybrook Fault and Mansfield Anticline

The east-trending Ladybrook Fault exhibits a prominent Variscan inversion anticline (Mansfield Anticline), developed on a probable depositional roll-over anticline at the margin of the Welbeck Low.

Welton Anticline

The Welton oilfield is the largest of the East Midland accumulations, and the second largest onshore discovery. It produced 5000 barrels of oil per day (bopd) at peak, declining to around 1200 bopd in 2008. It is reservoired in basal-Westphalian sands folded into a broad anticlinal structure (Figure 52), with marginal reverse faulting, and buttressed by the Askern–Spital High to the east. Namurian strata are absent, and the Westphalian strata are inferred to lie with angular unconformity upon an eroded (and possibly karstified) Visean sequence (Lincoln Platform), which had been tilted and eroded during a pre-Westphalian phase of inversion (Fraser and Gawthorpe, 2003).

Gainsborough Half-graben

This basin suffered only mild inversion (Corfield et al., 1996), since much of its post-rift fill is retained. Several inversion anticlines show condensed LC2 stratigraphy over their crests (Fraser and Gawthorpe, 2003), indicating intra-Westphalian pulses of inversion. The Scampton North and West Firsby oilfields are developed in the same basal Westphalian sand reservoir as Welton, but rather thicker (up to 150 m). The structures are inversion anticlines produced by deformation against the bounding faults of the half-graben (e.g. the Scampton Fault and the Askern–Spital Fault), which as at Rempstone in the Widmerpool Half-graben, exhibit a strong buttressing effect.

Chapter 8 Post-Variscan structure and stratigraphy

As a result of the highly variable degree of Variscan inversion throughout the region, Permo-Triassic strata transgress across all the Carboniferous stratigraphical units, to rest upon Early Palaeozoic and even Precambrian rocks on the Anglo–Brabant Massif (Figure 49). By Permian times, the Carboniferous structural elements were mostly no longer recognisable. The region now lay on the East Midlands Shelf or East England Platform (BGS, 1985b), marginal to the southern North Sea Basin.

Permian–Cenozoic structural evolution

The average regional dip of the Permo-Triassic strata is 1.25° eastwards, locally steeper in the vicinity of major faults. Some of the Variscan faults were reactivated and show minor amounts of normal movement affecting Permo-Triassic strata (Map 9). A north-west to south-east-trending fault pattern dominates, cross-cut by a smaller number of south-west to north-east structures. Fault displacement is typically normal with throws reaching in excess of 100 m. The most significant displacements occur on the Normanton Hills–Hoton Fault, which displaces the Lias by 95 m (Figure 54) (Brandon, 1996; Carney et al., 2001); the Sileby Fault; the Harlequin Fault (locally 40 m downthrow to north); the Cinderhill–Foss Bridge Fault System (up to 35 m downthrow to south-west); the Eakring–Foston Fault (up to 60 m downthrow of base Permo-Trias to north-east) (Figure 50); and the Barkston Fault, displaces the base Permian and base Jurassic strata by up to 40 m (Figure 20). Another east-trending fault, lying about 8 km north of the Barkston Fault, with no apparent surface expression, shows a southerly normal downthrow of up to 30 m at the base Permian level. Some faults with a north-westerly orientation show minor throw of Permo-Triassic strata, but do not affect Jurassic strata. Most of the faults affecting Jurassic strata have a more east-west orientation. The precise age of these movements is poorly constrained however. An antithetic fault to the Normanton Hills–Hoton Fault at Rempstone was reactivated in post-Triassic times (Figure 15).

Permian

Following the culminating phase of the Variscan Orogeny, in latest Westphalian–Stephanian times, the crust of north–central Europe was intruded by alkaline and calc-alkaline magmas (Wilson et al., 2004). These resulted from a combination of backarc extension within the Paleotethys Ocean, transtension and regional thermal destabilization (Stampfli and Borel, 2002; Scheck-Wenderoth et al., 2008). This led to extensional collapse of the orogen and localised rifting (in the Worcester Graben) to west of the region. North-west directed extension is inferred from Permian normal faulting in southern Britain (Chadwick and Evans, 1995). After thermal doming and significant erosion, the crust underwent regional subsidence following decay of the lithospheric thermal anomaly. With subsidence exceeding sedimentation rates, a vast land-locked depression developed, stretching from eastern England to Poland, which was eventually flooded by ocean water in late Permian times. The region lay on the western flank of this depression, which would eventually develop into the Southern Permian Basin (Ziegler, 1990). Rotliegend strata are absent on the Anglo–Brabant Massif, which must have remained a source area for the Rotliegend strata of the North Sea and equivalent-age strata in the Worcester Graben. In late Permian times rifting in the Norwegian–Greenland Sea (Torsvik et al., 2002) opened a seaway linking the Southern North Sea to the Arctic Ocean (Coward et al., 2003; Ziegler, 1990). A worldwide glacioeustatic sea level rise in late Permian times led to marine incursion from the north and to the development of the hypersaline Zechstein Sea across much of northern Central Europe. Evaporites were deposited in the region.

Triassic

The next attempt to rift Pangaea began in the Arctic–North Atlantic and propagated southwards into north–central Europe through Triassic times, culminating during Mid to Late Triassic times (Scheck-Wenderoth et al., 2008). Minor normal faulting is observed along predominantly north-trending faults to the west of the region (Smith et al., 2005). East–west extension is reflected by the accelerated subsidence of north-trending troughs such as the Worcester Graben and Needwood Basin to west of the region, and the Central Graben and Sole Pit Basin, in the offshore to the east. In the region itself, no such Triassic rift structures are recognised however, and the East Midlands Shelf formed a slowly subsiding platform marginal to the Southern North Sea Basin.

Jurassic

Westward propagation of the Neotethyan spreading axis and accelerating rifting in the Central Atlantic culminated during the Early Jurassic in crustal separation between Gondwana and Laurussia (Stampfli and Borel, 2002). By Early Jurassic times, the region lay at about 35°N and the climate had turned more humid. In the Mid Jurassic, rifting accelerated in the North Sea rift system following the development and subsequent subsidence of a plume-related thermal dome affecting the lithosphere of the central North Sea, the so-called 'mid Cimmerian event' (Ziegler, 1990; Underhill and Partington, 1993). During Late Jurassic times deposition was restricted to mainly west–east-trending depocentres such as the Weald Basin, developed on a Variscan template, well to the south of the region (BGS, 1985b; Hansen et al., 2002), and the Cleveland Basin, well to the north. Once again, the region occupied a relatively stable, slowly subsiding platform between more rapidly subsiding basins. The thin succession was much affected by eustatic influences. The London–Brabant Massif formed a clastic provenance for most of Mid and Late Jurassic times (Cope et al., 1980).

Cretaceous

The spreading axis of the proto-Atlantic propagated northward, reaching the Labrador Sea between North America and Greenland in Early Cretaceous times (Stampfli and Borel, 2002). Following the mid Cretaceous opening of the North Atlantic Ocean, extensional stress was focussed between Greenland and Norway, and tectonic activity abated throughout the region. The Late Cimmerian Unconformity is most marked in the basin-marginal and shelf areas (Rawson and Riley, 1982; Ziegler, 1981; 1982; 1990). In Late Cretaceous times, the region along with most of north central Europe, was subjected to a regime of regional thermal subsidence. In addition, sea level rose to a level 100–200 m higher than the present as a consequence of high global sea floor spreading rates, flooding the margins of the continents. The Chalk was deposited in a vast shallow marine basin. Basins within the Tethys Ocean began to close in Late Cretaceous times. The developing Alpine orogenic belt propagated westward from the Austro-Carpathian area toward the Alpine Tethys, reaching the western Alps in Late Cretaceous times (Stampfli and Borel, 2002). This convergence induced compressive stresses to the lithosphere of the Alpine foreland, resulting in basin inversion. The effect of these events on the region are poorly understood at present.

Cenozoic

Closure of Tethys was completed in the Cenozoic with the progressive development of the Alpine–Carpathian Orogen. The region lay in the foreland of the latter and was affected by a number of pulses of inversion throughout this time. The final, Savian (end Oligocene/ Early Miocene) pulse, caused significant uplift in the Sole Pit (Glennie and Boegner, 1981), Weald and Cleveland basins (BGS, 1985b) outside the region. Apatite fission track studies indicate that Carboniferous strata in the southern Pennines cooled from a peak palaeo-temperature of about 80°C in early Palaeogene times (Green, 2005), compatible with the removal of 1–2 km of cover during the Cenozoic. Another significant effect was the regional tilting eastward of 1.5°. This was sufficient to cause some hydrocarbon traps to spill, and others to begin filling, for example at Hatfield Moors (Ward et al., 2003) and Saltfleetby (Hodge, 2003). It is likely that traps within the region may have been likewise affected.

Stratigraphy

The principal Permian and Mesozoic formations are identified in (Figure 55).

Permian

The basal Permian strata comprise thin and discontinuous sandstones (Yellow Sands Formation) and breccias (Basal Permian Breccia). The former originated as aeolian dunes, and range in thickness from 0–20 m, to over 50 m. At depth they are bluish-grey in colour, but are oxidised to yellow toward the crop. The Basal Permian Breccia, up to 5 m thick and interpreted as a residual piedmont gravel, comprising locally derived bedrock, is generally restricted to the south of the region. The Marl Slate Formation, well developed further north, is absent in the region. The Cadeby Formation of depositional cycle EZ1 (formerly 'Lower Magnesian Limestone') varies in thickness from 0 m in the south, to over 130 m in the north, before thinning to a few metres in the north-east. The formation varies from a thin dolomitic breccia and sandstone in the south, to a calcareous mudstone facies (formerly 'Lower Permian Marl') in the north. The marl facies is overlain by dolostone with subordinate mudstone. These strata are interpreted as the deposits of a littoral to open-shelf transition, lying behind a shelf-edge reef (Figure 56) which lay in the north-east part of the region (Smith, 1989). Following the identification of the reef front from borehole data, summarised in (Figure 56), seismic evidence in the form of north-eastward-downlapping clinoforms of reef talus was identified on seismic data along the reef front e.g. (Figures 57), (Figure 58, (Figure 59). The Hayton Anhydrite and Kirkham Abbey Formation are restricted to the north-east of the reef front. The Edlington Formation (EZ2), formerly 'Middle Permian Marl', also shows considerable thickness variation, from 0 m in the south to over 70 m in the north. In the south, the formation is sandy, and in the Nottingham district is represented by aeolian and fluviatile sandstones of the lower division of the Lenton Sandstone (Howard et al., 2009). Northward, the Edlington Formation passes into calcareous siltstone and mudstone. The content of evaporitic minerals also increases to the north. The Brotherton Formation (EZ3), formerly 'Upper Magnesian Limestone', comprises thinly bedded dolostone and dolomitic limestone with bituminous mudstone partings. It thickens from 0 m in the south to over 45 m in the north-east, and is overlapped by the Sherwood Sandstone Group in the south-central part of the region. The Roxby Formation (formerly 'Upper Permian Marl'), a sequence of calcareous and gypsiferous mudstones, ranges in thickness from 0 m in the south to over 80 m in the north. Evaporites occur in two main units; the Billingham Anhydrite (cycle EZ3) at the base, only developed in the north-east extremity of the region (Smith, 1989), and the Sherburn

Anhydrite (cycle EZ4) in the upper part. The region lay marginal to the main evaporite basin to the north-east, and halites were not deposited.

Triassic

The Sherwood Sandstone Group is a red sandstone with locally abundant pebble beds. Subordinate red mudstone and siltstone is common in the upper part where the sandstone-dominated succession grades into the Mercia Mudstone Group. In the south of the region, the group rests unconformably upon Carboniferous and Precambrian strata. The pebble content decreases northward, more distally from the provenance in the Variscan mountains of Armorica. The group gradually increases in thickness across the region from a minimum of about 60 m in the south-west to over 370 m in the north-east. The overlying Mercia Mudstone Group comprises red-brown, structureless mudstones and siltstones, thin beds of green-grey dolomitic siltstone ('skerries'), and subordinate beds of laminated mudstone. These deposits of a flat, low-lying desert, were interspersed with two fluvial intervals depositing the Cotgrave Sandstone Member and Arden Sandstone Formation. Much of the group is gypsiferous, particularly the upper part. It is conformably overlain by the Penarth Group. Halite deposits occur in some areas but are absent in this region.

Jurassic

Lower Jurassic rocks underlie the eastern edge of the Trent Valley lowlands and the Vale of Belvoir, occupying part of the River Trent basin, flowing northward to the Humber Estuary. They are argillaceous strata laid down during a long period of stable marine conditions. The Lower Lias is a mudstone succession with interbedded minor limestones. Ferruginous strata are also present, for example the 'Plungar Ironstone' and Frodingham Ironstone, the latter extensively mined at Scunthorpe, to north of the region. The Lincoln Edge extends from Grantham to Lincoln and the Humber Estuary. The lower slope of the edge scarp is formed by the Whitby Mudstone (formerly Upper Lias), an essentially argillaceous sequence, overlain by Middle Jurassic rocks which form the edge itself. The sequence includes the Marlstone Rock Formation, a chamositic ooidal limestone up to 9 m thick in the southern part of the region, formerly mined in the Leicestershire ironstone field north-west of Melton Mowbray; the Northampton Sand (ironstone), and fine sandstones and mudstones of the Grantham Formation (formerly the Lower Estuarine Series). The latter is absent in the Lincoln area but thickens northwards. The Lincolnshire Limestone is thickest (up to 25 m) in the Grantham district. The clastic, nonmarine strata of the Rutland Formation (formerly Upper Estuarine Beds) overlie the Lincolnshire Limestone with a low angular discordance, possibly a consequence of mid Cimmerian uplift and erosion in the offshore (Underhill and Partington, 1993). The Great Oolite Group, comprising the bivalve-rich Blisworth Limestone (7–8 m), the marine to brackish mudstones of the Blisworth Clay (7–8 m) and thin Cornbrash, lies to west of the Lincoln Clay Vale, the basin of the River Witham flowing south-eastward into The Wash via the South Lincolnshire Fens. Clay sedimentation dominated during the deposition of the Oxford (up to 80 m), Ampthill (up to 65 m), and Kimmeridge (up to 130 m in The Wash) clay formations. The Spilsby Sandstone is latest Jurassic in part. The Jurassic sequence contains a number of organic-rich mudstones (Charmouth Mudstone, Whitby Mudstone, Oxford Clay, Kimmeridge Clay) which have proven hydrocarbon source-rock potential outside the region; they are all immature within the region however.

Cretaceous

Lower Cretaceous rocks are absent over the Market Weighton High, north of the Humber, but thicken southwards from Market Rasen. The Spilsby Sandstone is overlain by the Claxby Ironstone, Tealby Formation and Fulletby Beds. The Late Cimmerian Unconformity is not well developed in the region, but the phosphatic Carstone Sands were deposited in irregular hollows during a subsequent marine transgression. The overlying Red Chalk is more widely developed. The White Chalk forms the Lincolnshire Wolds which extend into the NE corner of the region.

Cenozoic

Cenozoic rocks are absent from the region. Holliday (1999) has presented evidence for the erosion of about 1200 m of latest Cretaceous–early Cenozoic sediments.

Chapter 9 Subsurface economic resources

The region is rich in mineral resources of coal and hydrocarbons. Coal has been mined here for hundreds of years. The first significant onshore UK oilfield at Eakring–Duke's Wood was discovered immediately before the outbreak of the Second World War. Various other strata-bound mineral deposits in the Mesozoic succession have been worked over the centuries.

Coal

The region lies within the Nottinghamshire–Yorkshire Coalfield which extends along the length of the eastern flank of the Pennines, from Sheffield to Nottingham (Eden et al., 1957; Smith et al., 1967; Frost et al., 1979), into north-east Leicestershire (Spink, 1965), eastwards to the Lincolnshire coast, and beyond into the southern North Sea (Smith, 1985). Until the 16th century, coal was worked from outcrop or from shallow 'bell pits', and therefore restricted to the western edge of the region. During the 17th century, new mining techniques, firstly 'pillar and stall' and subsequently 'longwall' mining were introduced and enabled the rich resources of the concealed coalfield to be accessed. In the late 19th to early 20th century over 400 mines were active. Some of the collieries had a long working life, for example Babbington (Cinderhill) which worked from 1842–1988. The colliery at Asfordby, near Melton Mowbray, was opened in 1985 as a 'superpit' to exploit the thick Deep Hard seam beneath the Vale of Belvoir, but closed in 1997 as a result of mining problems. These included more faulting than anticipated, and the presence of troublesome plugs and sills of volcanic rock. The only collieries working in the region at present (April 2009) are Welbeck, Thoresby, Manton and Harworth, in North Nottinghamshire, and Rossington and Maltby, in South Yorkshire. In addition, production is obtained from a number of opencast pits (e.g. Rainworth) near Mansfield. The Westphalian successions are the standard for the coalfields of the Pennine Basin (Ramsbottom et al., 1978). The succession dips eastward beneath Permian and younger rocks, and has been described by Edwards (1951, 1967) and Smith et al. (1967). Boreholes prove that it is generally absent between the exposed Nottinghamshire Coalfield and the Leicestershire Coalfield, to the south-west of the region, where Permo-Triassic strata overlie Namurian rocks (Smith, 1985). Within the coalfield, up to 1500 m of grey Pennine Coal Measures Group strata rest conformably on Namurian strata, and are overlain by red and varicoloured mudstone and sandstone of the Warwickshire Group. An unconformity is recognised at the base of the Warwickshire Group (Edwards, 1951). Rocks of Langsettian to Bolsovian age contain up to 100 horizons at which coals are developed. The seams exploited range from the Blackshale/Ashgate seams in the Pennine Lower Coal Measures Formation, up to the High Main Seam of the Pennine Middle Coal Measures Formation. Further information on the seams worked is presented by Edwards (1967) and Howard et al. (2009). Lateral facies changes throughout the basin cause significant variations in coal seam thickness and abundance (Sheppard, 2005b). In particular, significant amalgamation of both coal seams and sandstone bodies is observed approaching the southern and eastern margins of the basin. The exposed Coal Measures have been worked extensively with both old workings and modern opencast pits. The concealed part of the coalfield is worked less towards the east. Intrusive and extrusive igneous rocks are found locally in Langsettian strata of the coalfield and have a detrimental effect on coalbed methane prospectivity (Creedy, 1988; Glover et al., 1993). Coal Measures strata are absent from the Widmerpool Half-graben (due to Variscan inversion and erosion) and the highs of the Anglo–Brabant Massif, in the south of the region. They are thin in the Sleaford Half-graben and over the Stixwould High, because of the persistent buoyancy effect of the Wash Batholith.

Coalbed and abandoned mine methane

Coal has the potential to trap large amounts of methane, up to 25 m3 per tonne (Creedy, 1991; 1999). Coalbed methane can be recovered by: drainage in working mines (coalbed methane, CMM); by extraction from abandoned coal mines (abandoned mine methane, AMM); and by production from unmined coal using surface boreholes (virgin coalbed methane, VCBM) (Ayers et al., 1993; Holloway et al., 2005). Most of the working mines identified above have CMM capture and utilisation procedures, for electrical power generation, for example. Typically this involves capturing the methane from unworked seams above and beneath working longwall faces. This enables mining to proceed more safely and quickly. With the advent of coal mine closures in the Yorkshire and Nottinghamshire coalfields Alkane, GreenPark, StrataGas and Warwick have taken up licences to extract AMM at former collieries, for example at Bentinck, Steetley, Shirebrook, Markham, Warsop and Whitwell. Groundwater rising through the old workings can have a negative impact on this resource. Exploratory drilling for VCBM has yet to take place, unlike in the coalfields of western Britain (Creedy, 1988; 1991; Holloway et al., 2005). This is because the average seam methane content is relatively low. Low seam permeability is also a significant problem. There is considerable scope for underground gasification provided environmental pitfalls such as subsidence and interaction with aquifers, can be avoided. It has the potential to recover perhaps 45 per cent of the calorific value of the coal, compared to the CBM recovery of typically <1 per cent (Giffard, 1923; Kent, 1954; Holloway et al., 2005).

Hydrocarbons

Seepages of oil, tar and bitumen have been known for centuries in the Carboniferous Limestone of Derbyshire, west of the region, and from coal mines such as Bevercotes Colliery, close to the Bothamsall Oilfield (Giffard, 1923; Kent, 1954). Active exploration for oil began after the First World War, resulting in the discovery at Hardstoft in 1919 (west of the district), but it was the discovery of the Eakring–Duke's Wood, Caunton and Kelham Hills oilfields in the years 1939–1943 that stimulated exploration (Lees and Taitt, 1946; Huxley, 1983; Kent, 1985).

Table 2 Hydrocarbon discoveries, fields and production in the region (Source: DECC website).

Field Date Company discovery Oil/gas RROP oil

(Mt)1

 Gas (bcm)2 Last prod.

Prod. oil to 2010 (Mt)

Prod. gas to 2010 (bcm)

 Operator
Eakring 1939 BP o 1973 0.91
Kelham Hills 1941 BP o 1965 0.29
Caunton 1943 BP o 1965 0.04
Plungar 1953 BP o 1980
Egmanton 1955 BP o
Bothamsall 1958 BP o
Corringham 1958 BP o
Gainsborough 1959 BP o
South Leverton 1960 BP o
Apleyhead 1960 BP o 1979 0.003
Glentworth 1961 BP o
Nocton 1943 BP o 1945 negligible
Welton 1981 BP o 2.209 2.32 0.15 Star
Nettleham 1983 BP o 0.212 0.2 Star
Langar 1957 BP o 1959 negligible
Rempstone 1985 BP o 0.23 0.03 0.056 Star
Beckingham 1964 BP o
Beckingham West 1985 BP o 0.01 0.022 0.023 Star
Crosby Warren 1986 RTZ og 0.062 0.007 0.089 0.16 Europa
East Glentworth 1987 BP o 0.05 0.028 0.12 Star
Farley's Wood 1983 BP o 0.033 2008 0.031 0.003 OPS
Kirklington 2 1985 BP o 0.01 0.004 minor Egdon
Long Clawson 1986 BP o 0.2 0.161 0.057 Star
Scampton 1985 BP o 0.021 0.009 minor Star
Scampton North 1985 BP o 0.212 0.247 0.01 Star
Stainton 1986 BP o 0.025 0.026 0.0007 Star
West Firsby 1988 ENT o 0.27 0.19 0.005 Europa
Whisby 1985 BP o 0.12 2005 0.046 minor Blackland
Fiskerton Airfield 1997 CIR o 0.3 2002 0.049 minor Cirque
Torksey 1963 BP o 0.007
Saltfleetby 1986 CAN go 2.52 1.634 Wingas
Keddington 1997 CAN o 0.17 0.023 0.07 Egdon
Cold Hanworth 1997 CAN o 0.1535 0.097 minor Star
Newton-on-Trent 1998 ALT o 0.025 0.003 Blackland
Hemswell 1 1984 BP o No production
Cropwell Butler 2 1984 BP o No production
Kinoulton 1 1986 BP o No production
Belvoir 1 1986 BP o No production
Beckering 1990 BG o No production
Reepham 1998 Cirque o No production

Hydrocarbon systems

The hydrocarbon system for oil essentially comprises Namurian source rocks, Namurian–Westphalian reservoirs, and intraformational seals and traps, typically inversion anticlines formed during the Variscan Orogeny. Productive hydrocarbons occur exclusively within Carboniferous strata in the East Midlands, and shows are almost wholly restricted to these strata. Within the region, 31 fields have been discovered, (Figure 60), (Table 2) in upper Carboniferous reservoirs, in a swathe extending 110 kilometres (70 miles) from near Loughborough to the Lincolnshire coast. Two other fields (Corringham and Saltfleetby) lie just outside the region. Source rocks are also present in late Visean shales (Bowland Shale Formation, Widmerpool Formation) and limestones (e.g. Bee Low Limestone Formation).

The sub-basins probably contain Visean mudstones (e.g. Long Eaton 1 Borehole), and some limestones at outcrop may also be classed as source rocks (e.g. Milldale Limestone Formation). The thick early Namurian prodelta mudstones (Bowland Shale Formation and equivalents) have been identified as the principal source (Fraser et al. 1990). Ewbank et al. (1995) distinguished an early Namurian mudstone source in the Edale Sub-basin, typified by type II kerogen, for the bitumens in the South Pennine orefield. Probably two different oil sources are represented. The most abundant carbon isotope (13C) of the oil is the δC–30‰, correlated with the prodelta mudstones (Fraser et al. 1990). An isotopically heavier oil (−27‰) is found in Caunton and Kelham Hills fields and the Newark discovery (Fraser et al. 1990), all more distant from the basin sources, which the authors interpret as being derived from marine bands and interdistributary mudstones. Similar oils are found at Parkhill and Cherry Willingham in the East Midlands.

Kirby et al. (1987) provided evidence for Carboniferous generation of hydrocarbons, particularly in the west of the basin (west of the main oilfields). Fraser et al. (1990) published data for marginally mature Coal Measures in the Bardney 1 Borehole, which is located east of the main oilfields, near the basin margin, some distance from the Namurian source kitchen. Their maturity profile was interpreted as showing no change in gradient at the end Carboniferous Variscan unconformity. However this may be incorrect, since nearly all wells show that maturity gradients are higher in the Carboniferous, with the exception of the Warwickshire Group, and it is possible to interpret different rates of maturity increase above and below the Variscan unconformity in most wells (Figure 61). This increase can be attributed to both erosion of Carboniferous strata, after the uplift caused by the Variscan Orogeny, and higher geothermal gradients in Carboniferous times.

The Carboniferous Pennine Basin was buried sufficiently deep, and with high geothermal gradients, to allow generation of hydrocarbons in late Carboniferous times (Kirby et al., 1987). Inversion of the basin along the Pennine Uplift may have prevented more source rocks becoming mature, and led to erosion of upper Carboniferous strata. The effect of later subsidence toward the North Sea basin, in Permian and Mesozoic times, has been to extend the area of generation eastwards. The combined effects of late Carboniferous uplift and Mesozoic subsidence produced a synclinal area between the Pennine Uplift and the Anglo–Brabant Massif, west of the oilfield swathe, where hydrocarbons continued to generate. This produced two probable migration directions: to the south-east towards the East Midlands oilfields and to the west (Hardstoft). The relative immaturity of the section near Eakring Oilfield shows that migration from the north-west was required. The Tournaisian–Visean sub-basins (Gainsborough, Widmerpool, Edale) are tongues of generating potential extending south-east toward the southern basin margin. Subsequently, the tilting of the region towards the southern North Sea basin has reversed the migration direction; the Saltfleetby gasfield may have been sourced from Westphalian sources there.

The main reservoirs in the oilfields are Westphalian and Namurian sandstones. Small quantities of oil have also been produced from the top of the Carboniferous Limestone in a few fields (Hardstoft, Eakring, Duke's Wood, Plungar, Nocton) and lesser amounts proved in a number of boreholes (e.g. Bingham, Nettleham 2), in every case in fractures at the top of the limestone, immediately below the contact with the overlying rocks. The potential of the Carboniferous Limestone as a hydrocarbon reservoir has not been realised. Apart from zones of dolomitisation, intergranular flow is negligible. A search for fissure systems, either of tectonic, diagenetic or palaeokarst origin, is required to determine whether significant hydrocarbon reservoirs exist. The potential might be greater in the west and east of the region where the typical East Midlands province upper Carboniferous reservoirs are absent. The Namurian fluviodeltaic and turbiditic sandstones of the Millstone Grit contain significant quantities of hydrocarbons in a number of oilfields (e.g. Eakring–Duke's Wood, Gainsborough–Beckingham, Bothamsall). It was thought unlikely that the turbiditic facies could form a good reservoir, until Rempstone Oilfield was discovered in 1985. Here cleaner grain-flow sands are present.

Poor reservoir properties have generally been confirmed by over a hundred drill-stem tests and many core analyses. Improvement in reservoir properties is caused by fracturing, which can be recognised in core and from log analysis (Evans and Brereton, 1990). The Westphalian Coal Measures sandstones form the major reservoirs of the East Midlands Oilfields, notably in such fields as Eakring–Duke's Wood, Gainsborough, Beckingham and Welton. The Pennine Lower and Middle Coal Measures of the East Midlands contain about 20 per cent sandstone. The coarsest sandstones occur in channel fills, form narrow and generally thin, mostly discrete meandering bodies, and account for less than 1 per cent of the total sequence. Porosity retention is greatest in the coarsest sandstones in the axial regions of the channels and is least in the fine-grained sandstones, where quartz cementation predominates. Thicker and more extensive sandstone bodies with greater porosity and permeability are found in the Warwickshire Group. These sandstones generally have the best reservoir characteristics of Carboniferous rocks in the region, but no major hydrocarbon shows have yet been recorded from such strata. A section of exceptionally high porosity and permeability is found in the Westphalian Basal Sandstone at Welton 2 Borehole, where the permeability determined from core analysis averages 400 mD, the porosity averages 19.5 per cent and the calculated transmissivity is 4.9 Dm. The effect of burial on the porosity of Namurian to early Westphalian channel sandstones was shown by Fraser et al. (1990), with a decrease of values from about 18 per cent at 2000 m to 10 per cent at about 2500 m.

Fine-grained, argillaceous sediments, forming effective intraformational seals to hydrocarbon accumulations, are abundant in the upper Carboniferous rocks. The early Namurian shales form a seal to the limited oil occurrences at the top of the Carboniferous Limestone. The generally argillaceous nature of the Westphalian Pennine Lower and Middle Coal Measures largely prevents escape both from the sandstones at the top of the Millstone Grit and the base of the Coal Measures, and from the more isolated sandstone bodies in the Pennine Lower and Middle Coal Measures. Indeed, it is likely that these argillaceous beds have prevented migration into the higher sandy Warwickshire Group and, except in a few instances where pre-Permian erosion cuts deepest, into overlying Permian and Mesozoic strata. Evidence from the Gainsborough and Beckingham oilfields indicates that faults are presently sealed and there, at least, do not form active pathways for migration. All known commercial hydrocarbon accumulations in the East Midlands province are held in anticlinal structures, but the traps also comprise stratigraphical and sedimentological elements. The main period of formation of these structures was at the end of the Carboniferous, but many probably were initiated in early Carboniferous times. All structures have been modified by post-Permian movements and regional tilting towards the east and north-east. Oil generation within the region probably lasted from latest Carboniferous to Cretaceous times, and the Variscan closed structures were in place into which this oil could migrate. In the Gainsborough Oilfield, many of the producing sandstone reservoirs are either discontinuous, lenticular bodies (shoestring sands) which do not occur everywhere on the structure (Coal Measures) or, if more sheet-like, are laterally variable in facies (Millstone Grit). Boreholes drilled on the highest point of closures were often dry, although the structure was productive down dip. The fields and discoveries lie on a north-east to south-west trend that extends from near Loughborough to Welton and Saltfleetby (Figure 60): between the latter two fields Westphalian Coal Measures overlap the Carboniferous Limestone and Namurian strata are absent. Although these finds are, at least in part, structurally controlled, the overlap by permeable Coal Measures Basal Sandstones against the tight limestone is one of the key trapping elements. Petroleum generated from all the source rocks of the early Namurian shales (the principal source rock), other shales and late Namurian marine bands, would have found its simplest migration up-dip to the overlapping Westphalian Basal Sandstone reservoir.

Oil generated in the East Midlands during the Carboniferous, prior to the end-Carboniferous Variscan Orogeny, would have migrated up-dip towards the surrounding shelf areas. This generally easterly migration would have been partially reversed following Variscan uplift of the Pennines inversion axis, particularly in the western half of the East Midlands. Some oil and gas in the more intense structures in the east of the East Midlands province (Carboniferous subcrop area) could have survived the tilting and reversal of migration pathways and have been protected from erosion. In the Bothamsall Oilfield, Hawkins (1978) indicated that there had been two phases of migration. The first occurred during shallow to moderate burial of the Carboniferous strata, and a second phase followed the folding (trap formation) at the end of the Carboniferous. Migration during Mesozoic times, under an easterly thickening Permian–Cretaceous wedge may have caused some westerly migration, but as the thinning in Carboniferous strata was towards the south-east it is more likely that migration continued in this direction, beneath the Variscan unconformity. Very few occurrences of hydrocarbons (almost certainly sourced from Carboniferous rocks) have been reported in Permian and younger rocks. The most notable of these is a sizeable oil show at Nocton in basal Permian sands ('Yellow Sands').

A smaller gas hydrocarbon system exists based on increased maturity to the north and east. This includes Saltfleetby Gasfield (Candecca, Morrison Middlefield Resources Ltd, then Roc Oil, now Wingas), which is the UK's largest onshore gasfield, with reserves of 73 (billion cubic feet) bcf and 2mmbbl of natural gas liquids (DTI website, 1999). This was possibly sourced from Westphalian source rocks on the margin of the Southern North Sea Basin. Roc stated that, at the time of the Wingas purchase in 2004, 54 bcf of gas had been produced since 1999. No gas was produced in 2007 (DECC website, 2008).

Hydrocarbon field descriptions

In the account which follows, fields are reviewed in order of discovery, illustrating the evolution of hydrocarbon play concepts within the region.

Eakring

The first major oil discovery was made at Eakring in 1939, closely followed by the satellite structure of Duke's Wood and nearby closures at Caunton and Kelham Hills. These traps can now be seen as culminating on a broad anticlinal ridge that brings Pennine Middle Coal Measures, and locally Pennine Lower Coal Measures, to subcrop beneath the Permo-Triassic cover (Smith, 1985). A very full account of these oilfields was provided by Lees and Taitt (1946) with additional details by Falcon and Kent (1960) and Edwards (1967). The Eakring–Duke's Wood oilfield comprises a north-north-west trending anticline with a reverse-faulted western limb (Figure 50). The main reservoir, originally interpreted as the upper Namurian Rough Rock, is now identified as the Crawshaw Sandstone (Langsettian), but production was also obtained in stratigraphically higher and lower sandstones and locally from the top of the Carboniferous Limestone (Storey and Nash, 1993). Analytical results indicate considerable differences between the crude oil in the four wartime fields, and lesser differences between reservoirs within individual fields. Both fields are now abandoned, final production figures being Eakring 2 104 630 barrels and Duke's Wood 4 413 410 barrels. The specific gravity of the oil varies from 0.837–0.853 at Eakring and 0.857–0.866 at Duke's Wood (Lees and Taitt, 1946). The oils contain a high wax content (Southwell, 1945). The gas saturation pressure is low, at about 400 psi (BP, 1962) and the original reservoir pressures were about 1000 psi (Dickie and Adcock, 1954). Brief production was achieved from the Carboniferous Limestone at one well, at a rate of 50 tons/day and two wells were producing from this reservoir (Lees and Taitt, 1946). Secondary recovery of oil began in 1948 (Dickie and Adcock, 1954). The geothermal gradient at Eakring is significantly higher than at both Duke's Wood and Kelham Hills (Southwell, 1945). In 1920, a borehole at Kelham produced oil at a rate of 5–8 gallons/day, following which it was predicted that oilfields would be discovered in the East Midlands (Dalton, 1918). The Kelham Hills field was discovered in 1941 and secondary recovery began in 1951 (Dickie and Adcock, 1954). The field produced a total of 2 077 880 barrels of oil. Specific gravity of its oils varies from 0.88 at 643 m (BOD) to 0.892 at 631 m (BOD).The gas saturation pressure is very low, at 100 psi (BP, 1962).

Caunton field

The Caunton field produced 268 940 barrels of oil, from a basal Coal Measures sandstone and the Namurian Ashover Grit. Secondary recovery of oil began in 1954. A coal exploration borehole in 1915 found gas in a fault near the eventual 1985 BP Kirklington discovery well.

Plungar Oilfield

The Plungar Oilfield lies on the north-east margin of the Widmerpool Basin. The field's structure is a broad, gently faulted dome, about 1.5 km across, with a steeper southern edge (Warman et al., 1956; Falcon and Kent, 1960). Production has been obtained from sandstones in both the Namurian and Westphalian strata, and locally from a 3–7 m oil column at the top of the underlying Carboniferous Limestone. Up to twelve sandstones are productive, but generally only two of these produce in any one well. The sandstones vary from less than 1 m thick to nearly 10 m. Permeability varies from 0.1–1000 mD, but is generally in the range 5–50 mD (Warman et al., 1956). Different sandstones have different oil–water contacts and contain oil of different character and specific gravity. Within the field the specific gravity ranges from 0.85–0.9. Gas saturations vary, although all are very undersaturated (Warman et al., 1956). The gas saturation pressure is very low at 100 psi (BP, 1962). Production, now ceased, totalled 306 070 barrels.

Egmanton Oilfield

The Egmanton Oilfield lies in a gentle west-north-west to east-south-east-trending anticline, cut by several faults parallel to the fold axis and is 4.8 km long and 0.8 km wide (Falcon and Kent, 1960). Oil occurs in several sandstones in the Pennine Lower Coal Measures and Millstone Grit Group. Again the main producer is the lower Westphalian Crawshaw Sandstone (Edwards, 1967) with some production from lower levels. Production totalled 3 327 270 barrels and the field is now exhausted. Experimental tertiary recovery methods were tried here with some success (Gair et al., 1980). A seismic profile across the field was published by Fraser and Gawthorpe (1990), showing Visean growth across the structure, similar to the Eakring field. The gas saturation pressure is low, at about 400 psi (BP, 1962).

Bothamsall Oilfield

The Bothamsall Oilfield, a roughly circular structure of radius around 1 km, has been the subject of a detailed study by Hawkins (1978). Production here is from two sandstones, the Sub-Alton (Langsettian), and the Rough Rock (upper Namurian), which is locally connected to the Crawshaw Sandstone (basal Langsettian). Hawkins (1978) documented the diagenetic modifications and reservoir properties (porosity and permeability) affected by the early emplacement of oil. Production to the end of 1982 totalled 2 564 070 barrels (Huxley, 1983). Tertiary recovery methods using surfactants have been tested here in recent years, but the field is now exhausted.

There are few available details for the Corringham Oilfield, in a small fault-bounded anticline. Oil has been found at several levels in the Pennine Lower Coal Measures and in the Millstone Grit Group, three of which have given production. Total production to the end of 1982 was 361 520 barrels (Huxley, 1983).

Gainsborough–Beckingham Oilfield

A notable feature of the Gainsborough–Beckingham Oilfield is the large number of oil-and gas-bearing sandstones, ranging in age from Namurian to Duckmantian. However, because of faulting and sedimentological variation, not all are present or hydrocarbon-bearing in every well drilled. Much of the production of 3 584 710 barrels at Gainsborough and 2 493 720 barrels at Beckingham, to the end of 1982 (Huxley, 1983), has come from the 'Eagle' Sandstone of Duckmantian age. A contour map of the Top Hard Coal horizon was published by Brunstrom (1966). Recoverable reserves were assessed at 13 mmbbl of oil and 6.5 bcf of associated gas (Fraser and Gawthorpe, 2003).

Beckingham West

Beckingham West lies west of the former mining licence awarded to exploit the original Beckingham field. This field was discovered in 1985 by BP and produces from one well (Beckingham 37; DTI, 1999). East Glentworth was discovered by BP in 1987, testing about 5 bopd from the Mexborough Rock (Bolsovian) and also from the Namurian Chatsworth Grit. Production began in 1993.

Farley's Wood Oilfield

The first exploration well on the Farley's Wood Oilfield was drilled in the 1940s and two subsequent wells failed because the thick oil-soaked sandstones were too impermeable for production (Falcon and Kent, 1960). Farley's Wood 4 well tested 1000 bopd, declining to 200 bopd after hydraulic fracturing. Onshore Oilfield Services Ltd operate the licence including this field. The South Leverton Oilfield is in a shallow east–west trending anticline. Oil occurs at two levels in the Namurian Millstone Grit Group and one in the Westphalian Pennine Lower Coal Measures, but the only producing sandstone is the upper Namurian Rough Rock. Production to the end of 1982 was 402 970 barrels. The Glentworth Oilfield is part of a major anticlinal area into which the Hemswell and Spital boreholes were also drilled. Field production amounted to 211 550 barrels by the end of 1982 (Huxley, 1983). Shows were found throughout much of the upper Carboniferous, but the production is limited to a sandstone near the top of the Westphalian Pennine Middle Coal Measures. Minor production (less than 100 000 barrels) has been obtained at Nocton and Apleyhead. Nocton 2 well produced briefly from the Carboniferous Limestone, initially at 3 tons/day, declining as water production increased (Lees and Taitt, 1946).

Welton Oilfield

Welton Oilfield was discovered in 1981, on a northwest to south-east trending anticline, and produced from a 68 m thick Westphalian Basal Sandstone, interpreted as a series of channel fills, which were abandoned progressively southwards across the field (Rothwell and Quinn, 1987). The Welton field overlies thick limestones on the Lincoln Platform to south-east of the Gainsborough Basin (Figure 11); a seismic profile (Fraser and Gawthorpe, 1990) shows Visean growth on the fault controlling its position. The oil is a sour crude (Fraser et al., 1990) and recoverable reserves of oil are predicted at over 20 mmbbl (Fraser and Gawthorpe, 1990). Production began in 1984 at over 3000 bopd. Star Energy now owns the field.

Scampton Oilfield

BP discovered the Scampton Oilfield in 1985 with the Westphalian Basal Sandstone reservoir testing 350 bopd. Production ceased in 1988. Scampton North was discovered by BP in 1985, testing 550 bopd from the basal Westphalian Basal Sandstone and the upper Namurian Rough Rock. Stainton Oilfield was discovered by BP in 1984, with 60 bopd tested from the Westphalian Basal Sandstone, which overlies thin late Namurian strata at this location. Enterprise discovered the West Firsby Oilfield in 1987, testing the Westphalian Basal Sandstone at 864 bopd, with an oil–water contact within underlying Namurian strata, which are more than 166 m thick (base Namurian is below well TD). Whisby was discovered in 1985 by BP, testing 147 bopd from a thin Westphalian Basal Sandstone. Production began in 1990. Fiskerton was discovered in 1998 by Cirque and is about 2 km south-east of Welton oilfield. Production began in 1998, and monthly output is available on the Department of Energy and Climate Change (DECC) website.

The Nettleham 1 Borehole

The Nettleham 1 Borehole (Figure 53) tested the Westphalian Basal Sandstone at 1060 bopd and a probable karstic top to the underlying Carboniferous Limestone flowed at 12.9 bopd. A lower zone of interest in Nettleham 2 Borehole could not be tested due to the large borehole diameter.

Torksey field

Torksey field produced about 50 mbbl oil from 1963 until the 1980s (Huxley, 1983). Torksey 4 Borehole tested the lower Westphalian Crawshaw Sandstone and the Namurian Chatsworth Grit. From the logs it appears that a deeper Namurian sandstone (probably Kinderscout Grit) should have been tested also. Cold Hanworth 1 well was drilled in 1986 by Enterprise; Candecca made the field discovery with a second well. Star Energy now owns the field, which began production in 1998. Reserves of 0.1535 m tonnes were initially present and production began in 1998.

Discoveries on the northern margin of the Widmerpool Half-graben

Following Plungar, several discoveries were made on the northern margin of the Widmerpool Half-graben. Langar 1 Borehole tested 198 bopd from near the top of the Namurian Millstone Grit Group, and subsequent boreholes had oil shows and produced water; production lasted only two years. Belvoir 1 Borehole tested 27 bopd from the Ashover Grit, but no production was obtained from the Namurian Chatsworth Grit. Long Clawson and Kinoulton boreholes are nearer the basin depocentre. The first Long Clawson borehole was drilled in 1943: the second well tested 70 bopd from near the top of the Millstone Grit Group in 1986. Long Clawson field began production in 1991. The Kinoulton discovery well tested oil at 9 bopd from the lower Westphalian Crawshaw Sandstone, but is now capped.

Rempstone field

The Rempstone field, found by BP in 1983, was the first discovery in the south of the Widmerpool Half-graben and is also remarkable for having an early Namurian reservoir (Pendleian age turbidites of the Morridge Formation). The structure is within the hanging wall, but close to the crop of the surface Normanton Hills Fault. The reservoir tested 21 bopd of 34.2° API oil in Rempstone 1 and similar amounts in 2z Borehole. Subsequent nearby exploration wells tested different traps and were unsuccessful.

Saltfleetby Gasfield

Morrison Middlefield Resources Ltd submitted a plan to develop the Saltfleetby Gasfield, discovered by Candecca in 1986. The proposal was to drill two more wells and produce 33 mmcfd of gas and 1500 bpd of natural gas liquids (Anon, 1999). The field was then taken over by Roc Oil (UK) Ltd and production began in 1999. This is the UK's largest onshore gasfield, with estimated recoverable reserves of 73 bcf gas. Reserves of 0.025 m tonnes were originally present (DECC website). The DECC website lists Hemswell, Everton, Cropwell Butler 2, Reepham, Belvoir and Kinoulton as significant discoveries. Hemswell Borehole, north of East Glentworth, tested 10 bopd from the Parkgate, Tupton and Deep Hard sandstones. Everton Borehole, located farther west than the Beckingham field, tested 3 mmcfd gas and 20 bpd of condensate from Alportian (Namurian) sandstones.

Gas storage

Wingas (Wintershall/Gazprom), the current owner of the Saltfleetby gasfield, has submitted a planning application to store 700 million cubic metres (1.5 bcf) of gas. It is possible that other producing hydrocarbon fields within the region, could be used for gas storage once production has ceased. The absence of Zechstein salt within the region means that gas storage in artificial caverns is not feasible.

Geothermal energy

Two potential geothermal resources exist in the region: low enthalpy resources from deep aquifers with temperatures >40°C, and the hot dry rock resource from shallow basement (especially granitic) rocks. The former has been reviewed by Downing and Gray (1986) and Rollin et al. (1995). The main reservoir is the Sherwood Sandstone Group, which has a mean porosity of 23.6 per cent (measured on cores) and mean permeability of 2166 mD (Rollin et al., 1995). The resource potential was investigated by a deep borehole at Cleethorpes, just to north-east of the region (Kirby et al., 1984). Porosity and permeability values decrease with depth, and to the north, where the group is more shaley. The geothermal heat in place (H0) is of the order of 80 x 1018J, with a total identified resource of around 12 x 1018J, for this part of the Eastern England Basin. The hot dry rock resource potential of the metamorphic basement was investigated by a borehole at Morley Quarry, in Charnwood, just outside the SW corner of the region. This demonstrated a heat flow around the national average. The low radiogenic element inventory of the Charnian sequence means that the regional heat flow is not enhanced by the presence of high heat-production components. The potential resource from inferred granite intrusions within the putative Wash Batholith has not so far been investigated, despite the fact that some of these bodies lie at a depth of around 1 km (Chroston et al., 1987). The microgranite proved by the Claxby 1 Borehole was of sodic and low radiogenic element composition, and Late Ordovician age (Pharaoh et al., 1997), suggesting that high heat production (HHP)-type granites of Devonian age may not be present. Thus the hot dry rock resource of the region is not considered to be very significant.

Evaporites

Gypsum (hydrous calcium sulphate) has been worked since at least the 16th century from various parts of the Mercia Mudstone Group in the southern parts of the region (Sherlock and Hollingworth, 1938). It is now mined only from the Cropwell Bishop Formation around Barrow upon Soar, and quarried at Kilvington and Orston. The style of mineralisation varies from formation to formation, and includes cross-cutting and bedding-concordant veins, lenses and nodules. Much of the gypsum is used for the manufacture of plasterboard.

Mineral deposits

Sedimentary iron ores were once worked from the Lias Group (Lower Jurassic). The group is a clay succession with interbedded minor limestones and ferruginous strata (e.g. the 'Plungar Ironstone'). The Frodingham Ironstone was extensively mined at Scunthorpe, to the north of the region. The Middle Jurassic sequence includes the Northampton Sand (ironstone) and the Marlstone Rock, the latter a chamositic ooidal limestone up to 9 m thick in the southern part of the region, formerly mined in the Leicestershire ironstone field. The Claxby Ironstone is of Early Cretaceous age.

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Appendix 1 BGS memoirs and other publications of the region

This appendix contains a list of the British Geological Survey 1:50 000 sheet Memoirs, Sheet Descriptions and Sheet Explanations for the region, which have been used as reference sources throughout this book. BGS maps and other publications are available from the Sales Desk, British Geological Survey, Keyworth NG12 5GG (Telephone 0115 936 3241; Fax 0115 936 3488; e-mail sales@ bgs.ac.uk; Internet address http://www.nkw.ac.uk/bgs/ home.html), and at the BGS Edinburgh office (Telephone 0131 667 1000), or through the BGS London Office, Natural History Museum, Cromwell Road, London SW7 5BD or through Stationery Office stockists and all good booksellers. A catalogue of maps and literature is available on request and can be found on the BGS website.

BGS memoirs, descriptions and explanations

(Map unnumbered) BGS memoirs, descriptions and explanations are listed by sheet number, and the latest edition of the 1:50 000 map is shown.

+ Out of print. These may be purchased from BGS libraries as black and white photocopies.

Other BGS publications relevant to the region

Cameron, TD J, Crosby, A, Balson, P S, Jeffery, D H, Lott, G K, Bulat, J, and Harrison, D J. 1992. United Kingdom offshore regional report: the geology of the southern North Sea. (London: HMSO for the British Geological Survey.)

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

Busby, J P, Walker, A SD, and Rollin, KE. 2006. Regional Geophysics of South-east England. Version 1.0. on CD-ROM (Keyworth, Nottingham: British Geological Survey.)

Recommended small-scale maps:

Sheet No Date (Map) Date (memoir) Title Memoir author
100 1974 1957 Sheffield Eden, R A, Stevenson, I P, and Edwards, W N
101 1967 1973 East Retford Smith, E G
102P 1999 Market Rasen Sumbler, M G
103P 1999 Louth Sumbler, M G
104P 1996 Mablethorpe Gallois, R W
112 1967 1963 Chesterfield, Matlock and Mansfield Smith, E G, Rhys, G H, and Eden, R A
113 1966 1967 Ollerton Edwards, W N
114 1973 Lincoln Allen, P M
115P 1995 Horncastle Berridge, N G, and Pharaoh, T C
116P 1996 Skegness Gallois, R W, and Chisholm, J I
125 1979 1972 Derby Frost, D V, and Smart, J G O
126 1996 1910 Nottingham Lamplugh, G W
127 1996 1999 Grantham Berridge, N G
128P 1995 Boston Hamblin, R J O
128 1997 The Wash Gallois, R W, and Smith, C P
130 2008 2008 Wells-next-the-sea Sheet Description and Sheet Explanation Moorlock, B S P Et Al.
141 2002 2001 Loughborough Sheet Description and  Explanation Carney, J
142 2002 2004 Melton Mowbray Description and Sheet Explanation

Carney, J N, Ambrose, K, and Brandon, A
143 1971 Bourne Not Available
144 1992 Spalding Horton, A, and Aldiss, D T
145,129 (part) 1994 1978 King's Lynn and The Wash Gallois, R W
146P 1999 Fakenham Hamblin, R J O, and Moorlock, B S P
155 1982 1988 Coalville Worssam, B C, and Old, R
156 2008 2007 Leicester Carney, J
157 1978 Stamford Not Available
158 1984 Peterborough Wyatt, R J
159 1995 Wisbech Gallois, R W
160P 1999 Swaffham Hamblin, R J O, Moorlock, B S P, and Smith, M
169 1998 1994 Coventry and Nuneaton Bridge, D M et al
170 1969 1968 Market Harborough Poole, E G, Williams, B J, and Hains, B A
171 2002 2005 Kettering Herbert, C
172P 1995 Ramsey Chisholm, J I
173 1980 1988 Ely Gallois, R W
174 In prep. Thetford

Appendix 2 Summary of principal boreholes in the region

Outline records for key boreholes are given below. The list is a representative sample from the very large number of deep boreholes in the region. The full records are held in the National Geological Records Centre, British Geological Survey, Keyworth NG12 5GG (Telephone 0115 936 3109; Fax 0115 936 3276).

All depths are given in metres below Kelly Bushing or other reference level. The depths to the main stratigraphical boundaries is the interpretation of the authors of this book, and may differ in some instances from previously published accounts or from the interptetation on composite logs supplied to the British Geological Survey. Except for those indicated as currently held 'commercial in confidence', more details of these boreholes can be found in British Geological Survey records and in the publication cited.

The BGS reference number is a unique number assigned to each borehole. The first two letters of this reference refer to the 100 km grid square. The next two numbers define the 10 km grid square within this area (easting, northing). The following two letters define within which quarter the borehole lies (e.g. NW, SE) and the final figure is the number of that borehole within that quarter assigned on a sequential basis.

Apley 1

Apley 1
BGS record number (TF17NW/2)
National Grid reference [TF 10147 75104]
Reference level (m) 18.7
Drilled by BP
Base Jurassic 412
Base Triassic 903
Base Permian 1100
Base Bolsovian 1299
Base Middle Coal Measures 474
Base Lower Coal Measures 1623
Base Millstone Grit 1640
Terminal depth in Carboniferous Limestone 1715

Bardney 1

Bardney 1
BGS record number (TF16NW/26)
National Grid reference [TF 11915 68617]
Reference level (m) 5.8
Drilled by BP
Base Jurassic 383
Base Permo-Triassic 1052
Base Bolsovian 1235
Base Middle Coal Measures 1348
Base Lower Coal Measures 1525
Top Carboniferous Limestone 1525
Base Carboniferous Limestone 1860
Terminal depth in Early Palaeozoic 1896

Bassingham 1

Bassingham 1
BGS record number (SK96SW/16)
National Grid reference [SK 92080 60598]
Reference level (m) 16.6
Drilled by BP
Base Permo-Triassic 732
Base Middle Coal Measures 793
Base Lower Coal Measures 923
Top Carboniferous Limestone 923
Terminal depth in Carboniferous Limestone 1402

Beckering 1

BGS record number (TF18SW/13)
National Grid reference [TF 10396 80252]
Reference level (m) 27.8
Drilled by GC
Base Jurassic 441
Base Permo-Triassic 1183
Base Bolsovian 1359
Base Middle Coal Measures 1525
Base Lower Coal Measures 1673
Terminal depth in pre-Carboniferous 1699

Beckingham 1

Beckingham 1
BGS record number (SK79SE/4)
National Grid reference [SK 79204 90351]
Reference level (m) 4.8
Drilled by BP
Base Permo-Triassic 655
Base Upper Coal Measures 678
Base Middle Coal Measures 1020
Base Lower Coal Measures 1394
Terminal depth in Millstone Grit 1680

Biscathorpe 1

Biscathorpe 1
BGS record number (TF28SW/11)
National Grid reference [TF 23050 83714]
Reference level (m) 87.5
Drilled by BP
Base Jurassic 676
Base Permo-Triassic 1458
Base Bolsovian 1711
Base Middle Coal Measures 1849
Base Lower Coal Measures 1981
Top Carboniferous Limestone 1981
Terminal depth in Carboniferous Limestone 2075

Caledonian Farm 1

Caledonian Farm 1
BGS record number (SK86NE/31)
National Grid reference [SK 89513 66043]
Reference level (m) 17.3
Drilled by BP
Base Permo-Triassic 727
Base Bolsovian 774
Base Middle Coal Measures 877
Base Lower Coal Measures 1001
Top Carboniferous Limestone 1001
Terminal depth in Carboniferous Limestone 1040

Cherry Willingham 1

Cherry Willingham 1
BGS record number (TF07SW/49)
National Grid reference [TF 04165 73273]
Reference level (m) 4.1
Drilled by BP
Base Jurassic 315
Base Triassic 822
Base Permian 1001
Base Bolsovian 1205
Base Middle Coal Measures 1367
Base Lower Coal Measures 1512
Top Carboniferous Limestone 1512
Terminal depth in Carboniferous Limestone 1586

Clarborough 1

Clarborough 1
BGS record number (SK78SW/30)
National Grid reference [SK 73841 83583]
Reference level (m) 67.3
Drilled by BP
Base Permo-Triassic 529
Base Upper Coal Measures 676
Base Middle Coal Measures 1031
Base Lower Coal Measures 1318
Terminal depth in Millstone Grit 1684

Claxby 1

Claxby 1
BGS record number (TF26SE/16)
National Grid reference [TF 29813 64283]
Reference level (m) 54.3
Drilled by CANDECCA
Base Cretaceous 90
Base Jurassic 553
Base Triassic 1102
Base Permo-Triassic 1320
Base Warwickshire Group 1375
Base Lower Coal Measures 1482
Terminal depth in Ordovician microgranite 1542

Cold Hanworth 1

Cold Hanworth 1
BGS record number (TF08SE/3)
National Grid reference [TF 05391 82955]
Reference level (m) 17.1
Drilled by ENTERPRISE
Base Permo-Triassic 1105
Base Bolsovian 1283
Base Middle Coal Measures 1500
Base Lower Coal Measures 1640
Base Millstone Grit 1668
Terminal depth in Carboniferous Limestone 1762

Colston Bassett North 1

Colston Bassett North 1
BGS record number (SK73SW/2)
National Grid reference [SK 71000 33810]
Reference level (m) 34.1
Drilled by BP
Base Permo-Triassic 338
Base Upper Coal Measures 360
Base Middle Coal Measures 550
Base Lower Coal Measures 881
Base Millstone Grit 1306
Terminal depth in Carboniferous Limestone 1307

Coningsby 1

Coningsby 1
BGS record number (TF25SW/18)
National Grid reference [TF 24141 53567]
Reference level (m) 7.2
Drilled by CANDECCA
Base Jurassic 467
Base Permo-Triassic 1011
Base Lower Coal Measures 1210
Top Carboniferous Limestone 1210
Terminal depth in Carboniferous Limestone 1538

Cotmoor Lane

Cotmoor Lane
BGS record number (SK65SE/14)
National Grid reference [SK 66148 52346]
Reference level (m) 102.5
Drilled by BC
Base Permo-Triassic 283
Base Warwickshire Group 488
Base Upper Coal Measures 551
Base Middle Coal Measures 785
Terminal depth in Millstone Grit 1003

Cox's Walk 1

Cox's Walk 1
BGS record number (SK83NW/10)
National Grid reference [SK 84120 38080]
Reference level (m) 56
Drilled by BC
Base Permo-Triassic 474
Base Millstone Grit 495
Base Carboniferous Limestone 560
Terminal depth in ?Ordovician volcanics 801

Cropwell Bishop 1

Cropwell Bishop 1
BGS record number (SK63NE/11)
National Grid reference [SK 68780 38120]
Reference level (m) 39.3
Drilled by BP
Base Lower Coal Measures 913
Terminal depth in Millstone Grit 1120

Cropwell Butler 2

Cropwell Butler 2
BGS record number (SK63NE/73)
National Grid reference [SK 67976 38240]
Reference level (m) 59.1
Drilled by BP
Base Permo-Triassic 279
Base Upper Coal Measures 295
Base Middle Coal Measures 505
Base Lower Coal Measures 801
Terminal depth in Millstone Grit 967

Dunholme 1

Dunholme 1
BGS record number (TF07NE/21)
National Grid reference [TF 00853 79195]
Reference level (m) 35.9
Drilled by BP
Base Permo-Triassic 992
Base Bolsovian 1166
Base Middle Coal Measures 1395
Base Lower Coal Measures 1610
Terminal depth in Millstone Grit 1651

Eakring 146

Eakring 146
BGS record number (SK65NE/68)
National Grid reference [SK 68080 59480]
Reference level (m) 104.2
Drilled by BP (D'Arcy)
Base Permo-Triassic 391
Base Lower Coal Measures 622
Top Carboniferous Limestone 778
Top Early Palaeozoic 2195
Terminal depth in Early Palaeozoic 2279

Egmanton 68

Egmanton 68
BGS record number (SK76NE/73)
National Grid reference [SK 75780 68220]
Reference level (m) 38.4
Drilled by BP
Base Permo-Triassic 500
Base Middle Coal Measures 747
Base Lower Coal Measures 1032
Base Millstone Grit 1165
Top Carboniferous Limestone 1165
Terminal depth in Carboniferous Limestone 2163

Farleys Wood 4

Farleys Wood 4
BGS record number (SK77SW/40)
National Grid reference [SK 70500 71940]
Reference level (m) 37
Drilled by BP
Base Lower Coal Measures 967
Base Millstone Grit 1175
Terminal depth in Carboniferous Limestone 1200

Fenton 1

Fenton 1
BGS record number (SK87NW/82)
National Grid reference [SK 83491 77084]
Reference level (m) 50
Drilled by BP
Base Permo-Triassic 682
Base Upper Coal Measures 881
Base Middle Coal Measures 1142
Terminal depth in Lower Coal Measures 1241

Fiskerton 1

Fiskerton 1
BGS record number (SK74NW/10)
National Grid reference [SK 73546 49826]
Reference level (m) 14.4
Drilled by BP
Base Permo-Triassic 338
Base Warwickshire Group 478
Base Upper Coal Measures 512
Base Middle Coal Measures 685
Base Lower Coal Measures 939
Terminal depth in Millstone Grit 1580

Gate Burton 1

Gate Burton 1
BGS record number (SK88SW/45)
National Grid reference [SK 83100 84000]
Reference level (m) 17.2
Drilled by Unknown
Base Permo-Triassic 721
Base Upper Coal Measures 909
Base Middle Coal Measures 1212
Terminal depth in Lower Coal Measures 1298

Glentworth 1

Glentworth 1
BGS record number (SK98NW/1)
National Grid reference [SK 93130 88070]
Reference level (m) 24.8
Drilled by BP
Base Permo-Triassic 942
Base Upper Coal Measures 1033
Base Middle Coal Measures 1346
Base Lower Coal Measures 1573
Base Millstone Grit 1942
Terminal depth in Carboniferous Limestone 2900

Grove 3

Grove 3
BGS record number (SK78SE/30)
National Grid reference [SK 76273 81342]
Reference level (m) 64.1
Drilled by BP
Base Permo-Triassic 741
Base Bolsovian 838
Base Middle Coal Measures 1045
Base Lower Coal Measures 1379
Base Millstone Grit 1625
Base Carboniferous Limestone 2896
Terminal depth in ?Early 2933
Palaeozoic phyllites

Halton Holegate 1

Halton Holegate 1
BGS record number (TF46NW/18)
National Grid reference [TF 43059 65239]
Reference level (m) 15.2
Drilled by ENTERPRISE
Base Jurassic 604
Base Triassic 1083
Base Permian 1308
Base Middle Coal Measures 1334
Base Lower Coal Measures 1384
Top Carboniferous Limestone 1384
Top Pre-Carboniferous 1718
Terminal depth in Ordovician slates 1965

Hathern 1

Hathern 1 (SK52SW/3)
National Grid reference [SK 51580 24160]
Reference level (m) 49.1
Drilled by BP (D'Arcy)
Base Permo-Triassic 141
Base Millstone Grit 278
Terminal depth in Carboniferous Limestone 635

Hemswell 1

Hemswell 1
BGS record number (SK98NE/8)
National Grid reference [SK 95433 89788]
Reference level (m) 53.1
Drilled by BP
Base Permo-Triassic 1039
Base Upper Coal Measures 1134
Base Middle Coal Measures 1359
Base Lower Coal Measures 1537
Base Millstone Grit 1648
Terminal depth in Carboniferous Limestone 1669

High Marnham 1

High Marnham 1
BGS record number (SK87SW/4)
National Grid reference [SK 80930 70280]
Reference level (m) 10.4
Drilled by BP
Base Permo-Triassic 586
Base Bolsovian 692
Base Middle Coal Measures 873
Base Lower Coal Measures 1070
Base Millstone Grit 1143
Terminal depth in Carboniferous Limestone 1158

Kinoulton 1

Kinoulton 1
BGS record number (SK63SE/33)
National Grid reference [SK 69220 30110]
Reference level (m) 44.9
Drilled by BP
Base Jurassic 37
Base Permo-Triassic 329
Base Middle Coal Measures 408
Base Lower Coal Measures 704
Base Millstone Grit 1338
Terminal depth in Carboniferous Limestone 1485

Langar 6

Langar 6
BGS record number (SK73NW/9)
National Grid reference [SK 70890 36130]
Reference level (m) 29.4
Drilled by BP
Base Lower Coal Measures 820
Base Millstone Grit 954
Terminal depth in Carboniferous Limestone 823

Long Clawson 2

Long Clawson 2
BGS record number (SK72NW/13)
National Grid reference [SK 72450 25660]
Reference level (m) 124.9
Drilled by BP
Base Jurassic 218
Base Permo-Triassic 492
Base Middle Coal Measures 539
Base Lower Coal Measures 819
Base Millstone Grit 1372
Terminal depth in Carboniferous Limestone 1450

Mansfield 2

Mansfield 2
BGS record number (SK55NE/33)
National Grid reference [SK 55500 59050]
Reference level (m) 134.1
Drilled by BP
Base Permo-Triassic 105
Base Lower Coal Measures 879
Base Millstone Grit 1326
Terminal depth in Carboniferous Limestone 1369

Manton 1

Manton 1
BGS record number (SK67NW/41)
National Grid reference [SK 61350 79470]
Reference level (m) 34.1
Drilled by BP
Base Lower Coal Measures 1129
Base Millstone Grit 1571
Terminal depth in Carboniferous Limestone 1590

Nettleham 1

Nettleham 1
BGS record number (TF07SW/36)
National Grid reference [TF 00530 74630]
Reference level (m) 39.6
Drilled by BP
Base Jurassic 302
Base Permo-Triassic 961
Base Bolsovian 1130
Base Middle Coal Measures 1330
Base Lower Coal Measures 1420
Top Carboniferous Limestone 1420
Terminal depth in Carboniferous Limestone 1480

North Greetwell 1

North Greetwell 1
BGS record number (TF07SW/50)
National Grid reference [TF 01158 72921]
Reference level (m) 38.4
Drilled by BP
Base Jurassic 305
Base Triassic 799
Base Permo-Triassic 959
Base Warwickshire Group 1016
Base Bolsovian 1076
Base Middle Coal Measures 1176
Base Lower Coal Measures 1327
Top Carboniferous Limestone 1327
Terminal depth in Carboniferous Limestone 1351

Old Dalby 1

Old Dalby 1
BGS record number (SK62SE/14)
National Grid reference [SK 68140 23700]
Reference level (m) 98.5
Drilled by BP
Base Jurassic 167
Base Permo-Triassic 442
Base Lower Coal Measures 777
Base Millstone Grit 1295
Terminal depth in Carboniferous Limestone 1500

Parkhill 1

Parkhill 1
BGS record number (SK75SW/23)
National Grid reference [SK 70442 52854]
Reference level (m) 60.8
Drilled by BP
Base Permo-Triassic 330
Base Warwickshire Group 459
Base Upper Coal Measures 600
Base Middle Coal Measures 759
Base Lower Coal Measures 1042
Terminal depth in Millstone Grit 1245

Plungar 8A

Plungar 8A
BGS record number (SK73SE/27)
National Grid reference [SK 77440 33370]
Reference level (m) 59.4
Drilled by BP
Base Permo-Triassic 440
Base Middle Coal Measures 589
Base Lower Coal Measures 867
Base Millstone Grit 930
Terminal depth in Carboniferous Limestone 1418

Rainworth 1

Rainworth 1
BGS record number (SK66SW/79)
National Grid reference [SK 60710 61420]
Reference level (m) 96.1
Drilled by BP
Base Lower Coal Measures 1024
Base Millstone Grit 1384
Terminal depth in Carboniferous 2200
Limestone

Ranskill 1

Ranskill 1
BGS record number (SK68NW/19)
National Grid reference [SK 64230 88140]
Reference level (m) 16.5
Drilled by Unknown
Base Permo-Triassic 226
Base Upper Coal Measures 477
Base Middle Coal Measures 928
Base Lower Coal Measures 1327
Terminal depth in Millstone Grit 1729

Ratcliffe-on-Soar 1

Ratcliffe-on-Soar 1
BGS record number (SK52NW/70)
National Grid reference [SK 50820 29130]
Reference level (m) 38
Drilled by BP
Base Permo-Triassic 250
Base Namurian 385
Terminal depth in Carboniferous Limestone 1840

Rempstone 1

Rempstone 1
BGS record number (SK52SE/39)
National Grid reference [SK 58200 24959]
Reference level (m) 83.5
Drilled by BP
Base Permo-Triassic 273
Base Namurian 800
Top Carboniferous Limestone 800
Top Pre-Carboniferous (faulted) 1131
Terminal depth in Ordovician granodiorite 1213

Rufford 1

Rufford 1
BGS record number (SK66SW/74)
National Grid reference [SK 64718 62200]
Reference level (m) 69.9
Drilled by BP
Base Permo-Triassic 250
Base Upper Coal Measures 250
Base Middle Coal Measures 571
Base Lower Coal Measures 868
Base Millstone Grit 1155
Terminal depth in Carboniferous Limestone 1210

Saundby 1

Saundby 1
BGS record number (SK78NE/33A)
National Grid reference [SK 79522 89125]
Reference level (m) 9
Drilled by BP
Base Permo-Triassic 704
Base Bolsovian 912
Base Middle Coal Measures 1201
Terminal depth in Lower Coal Measures 1229

Scalford 1

Scalford 1
BGS record number (SK72SE/30)
National Grid reference [SK 77454 22987]
Reference level (m) 100.6
Drilled by ARAN
Base Jurassic 240
Base Permo-Triassic 494
Base Middle Coal Measures 516
Base Lower Coal Measures 646
Base Millstone Grit 946
Terminal depth in Devonian 1070

Screveton 1

Screveton 1
BGS record number (SK74SW/1)
National Grid reference [SK 73080 43490]
Reference level (m) 25.8
Drilled by BP
Base Permo-Triassic 330
Base Warwickshire Group 397
Base Bolsovian 457
Base Middle Coal Measures 595
Base Lower Coal Measures 928
Terminal depth in Millstone Grit 1104

Shirebrook West 1

BGS record number

(SK56NW/66)
National Grid reference [SK 50410 66920]
Reference level (m) 148.8
Drilled by BP
Base Permo-Triassic 85
Base Bolsovian 239
Base Middle Coal Measures 682
Terminal depth in Lower Coal Measures 1198

South Leverton 2

South Leverton 2
BGS record number (SK78SE/1)
National Grid reference [SK 79330 80400]
Reference level (m) 11.4
Drilled by BP
Base Permo-Triassic 595
Base Bolsovian 814
Base Lower Coal Measures 1280
Terminal depth in Millstone Grit 1562

Stainton A1

Stainton A1
BGS record number (TF07NE/21)
National Grid reference [TF 06276 78509]
Reference level (m) 20
Drilled by BP
Base Permo-Triassic 1074
Base Bolsovian 1242
Base Middle Coal Measures 1419
Base Lower Coal Measures 1582
Top Carboniferous Limestone 1582
Terminal depth in Carboniferous 1620
Limestone

Strelley 1

Strelley 1
BGS record number (SK54SW/560)
National Grid reference [SK 50520 42960]
Reference level (m) 133.1
Drilled by BP
Base Permo-Triassic 19
Base Middle Coal Measures 170
Base Lower Coal Measures 598
Base Millstone Grit 931
Terminal depth in Carboniferous Limestone 1450

Torksey 4

Torksey 4
BGS record number (SK87NE/16)
National Grid reference [SK 85065 79222]
Reference level (m) 14.4
Drilled by BP
Base Permo-Triassic 723
Base Upper Coal Measures 836
Base Middle Coal Measures 1125
Base Lower Coal Measures 1452
Base Millstone Grit 1823
Terminal depth in Carboniferous 1849
Limestone

Ulceby Cross 1

Ulceby Cross 1
BGS record number (TF47SW/15)
National Grid reference [TF 41400 73830]
Reference level (m) 95
Drilled by CANDECCA
Base Permian 1564
Base Bolsovian 1661
Terminal depth in Coal Measures 1778

Welton 1

Welton 1
BGS record number (TF07NW/14)
National Grid reference [TF 03612 76807]
Reference level (m) 22.3
Drilled by BP
Base Jurassic 323
Base Triassic 827
Base Permian 996
Base Warwickshire Group 1075
Base Bolsovian 1151
Base Middle Coal Measures 1333
Base Lower Coal Measures 1489
Top Carboniferous Limestone 1489
Base Carboniferous Limestone 2530
Terminal depth in ?EP 2559

Whisby 3

Whisby 3
BGS record number (SK86NE/39)
National Grid reference [SK 88439 69444]
Reference level (m) 16.9
Drilled by BP
Base Permo-Triassic 696
Base Bolsovian 757
Base Middle Coal Measures 865
Base Lower Coal Measures 979
Base Millstone Grit 990
Terminal depth in Carboniferous 1045
Limestone

Appendix 3 Structural contour and isopach maps

(Map 1) Depth to top of Caledonide basement.

(Map 2) Preserved thickness of Tournaisian–Visean strata.

(Map 3) Depth to top Tournasian.

(Map 4) Depth to top Visean.

(Map 5) Preserved thickness of Namurian strata.

(Map 6) Depth to top Namurian (Subcrenatum Marine Band).

(Map 7) Depth to top Hard Coal.

(Map 8) Preserved thickness of Productive Coal Measures.

(Map 9) Depth to base Permo-Mesozoic strata ('Variscan Unconformity').

(Map 10) Preserved thickness of Permo-Triassic strata.

(Map 11) Depth to top Triassic.

Figures and tables

(Figure 1) Location and topography of the region.

(Figure 2) Simplified geological map of the region.

(Figure 3) Summary of seismic data.

(Figure 4) Principal Carboniferous synrift tectonic elements.

(Figure 5) Sketch map of Precambrian outcrop, subsurface provings, structures and terranes.

(Figure 6) Caledonide subcrop and basement provings.

(Figure 7) Palaeogeographical reconstruction of key terranes during Palaeozoic times.

(Figure 8) Map synthesising information on 'Caledonian' basement structure and lithology.

(Figure 9) Bouguer gravity anomaly map, with lineaments, key boreholes and top-basement fault structure map.

(Figure 10) Reduced to pole magnetic anomaly map, with lineaments, key boreholes and top-basement fault structure map.

(Figure 11) Seismic reflection profile across the Anglo–Brabant Massif near Wisbech.

(Figure 12) Location of seismic reflection profiles and transects.

(Figure 13) Carboniferous seismic sequence stratigraphy.

(Figure 14) Schematic diagram showing Carboniferous seismic sequence stratigraphy of the Widmerpool Half-graben.

(Figure 15) Seismic transect across the Hathern Shelf, Widmerpool Half-graben to Nottingham Platform.

(Figure 16) Seismic transect across the Charnwood–Sproxton High, Sileby Fault, Hathern Shelf, Hoton–Normanton Hills Fault and Widmerpool Half-graben.

(Figure 17) Seismic reflection profile of the transition from the Widmerpool Half-graben to the Nottingham Platform and the Newark Low.

(Figure 18) Seismic reflection profile across the tail end of the Widmerpool Half-graben onto the Foston High.

(Figure 19) Seismic reflection profile across the Cinderhill Fault.

(Figure 20) Seismic transect across the Sproxton High, the eastern end of the Widmerpool Half-graben, Witham tilt-block, western part of Sleaford Half-graben and the Nocton High.

(Figure 21) Detail of seismic profile at the base of the Witham tilt-block.

(Figure 22) Seismic transect across the Nottingham Platform, Eakring Fault, inverted Newark Low and Egmanton Fault.

(Figure 23) Seismic reflection profile across the Sleaford Half-graben and Stixwould High.

(Figure 24) Seismic reflection profile across the Sleaford Half-graben.

(Figure 25) Seismic transect across the Nocton High in the south.

(Figure 26) Seismic transect across the Nocton High in the north.

(Figure 27) Seismic profile showing the Welton Oilfield, northern faulted margin of the Lincoln Platform and the Gainsborough Trough.

(Figure 28) Seismic reflection profile across the Gainsborough Trough and Askern–Spital Fault.

(Figure 29) Seismic reflection profile crossing the Newark Low and the Gainsborough Trough to Grove 3 Borehole.

(Figure 30) Detailed Tournaisian–Visean sequence stratigraphy.

(Figure 31) Latest Devonian to Courceyan (EC1) palaeogeography.

(Figure 32) Chadian (EC2) palaeogeography.

(Figure 33) Mid Asbian (EC4) palaeogeography.

(Figure 34) Early–mid Brigantian (EC6) palaeogeography.

(Figure 35) Borehole correlation diagram for Tournaisian–Visean strata.

(Figure 36) Detailed Tournaisian–Visean chrono- and lithostratigraphy.

(Figure 37) Map showing pre-Namurian/Westphalian subcrop of Tournaisian–Visean strata and key boreholes.

(Figure 38) Distribution of Carboniferous volcanic rocks.

(Figure 39) Generalised Namurian and Westphalian chrono- and lithostratigraphy.

(Figure 40) Facies variation in Namurian strata (after Collinson, 1988).

(Figure 41) Map showing the pre-Westphalian subcrop of Namurian strata and location of key boreholes.

(Figure 42) Late Pendleian–early Arnsbergian (E1–2) palaeo- geography.

(Figure 43)a Borehole correlation diagram for Namurian strata.

(Figure 43)b Borehole correlation diagram for Namurian strata.

(Figure 44) Early Kinderscoutian (R1) palaeogeography.

(Figure 45) Late Marsdenian (R2) palaeogeography.

(Figure 46a) Borehole correlation diagram for Westphalian strata.

(Figure 46b) Borehole correlation diagram for Westphalian strata.

(Figure 47) Map showing pre-Permian subcrop and key boreholes.

(Figure 48) Detailed chrono- and lithostratigraphy for West- phalian strata in the East Midlands and southern North Sea.

(Figure 49) Map showing pre-Permian subcrop and principal Variscan inversion structures.

(Figure 50) Seismic reflection profile across the Eakring Fault.

(Figure 51) Seismic transect across the Widmerpool Half- graben.

(Figure 52) Arbitrary line from 3D seismic time volume, through the Nettleham, Welton and Stainton oilfields.

(Figure 53) Detail of (Figure 52) in the vicinity of the Nettleham oilfield.

(Figure 54) Detail of seismic profile across the Rempstone Oilfield.

(Figure 55) Permian–Mesozoic chrono- and lithostratigraphy.

(Figure 56) Permian Z1b palaeogeography.

(Figure 57) Borehole correlation for Permian strata in the north- eastern part of the region.

(Figure 58) Composite 2D seismic profile to show morphology of the Z1b reef front.

(Figure 59) Arbitrary line from 3D seismic profile passing through the Biscathorpe 1 and Kelstern 1 boreholes.

(Figure 60) Hydrocarbon fields and current exploration licences in the region.

(Figure 61) Hydrocarbon maturation indices in the Bardney Borehole.

Tables

(Geological succession)  Geological succession

(Table 1) Carboniferous chronostratigraphy and timescale.

(Table 2) Hydrocarbon discoveries, fields and production in the region.

Maps

(Map 1) Depth to top of Caledonide basement.

(Map 2) Preserved thickness of Tournaisian–Visean strata.

(Map 3) Depth to top Tournasian.

(Map 4) Depth to top Visean.

(Map 5) Preserved thickness of Namurian strata.

(Map 6) Depth to top Namurian (Subcrenatum Marine Band).

(Map 7) Depth to top Hard Coal.

(Map 8) Preserved thickness of Productive Coal Measures.

(Map 9) Depth to base Permo-Mesozoic strata ('Variscan Unconformity').

(Map 10) Preserved thickness of Permo-Triassic strata.

(Map 11) Depth to top Triassic.

(Map unnumbered). BGS memoirs, descriptions and explanations are listed by sheet number, and the latest edition of the 1:50 000 map is shown.