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The geology of the central North Sea. United Kingdom offshore regional report

R W Gatliff, P C Richards, K Smith, C C Graham, M McCormac, N J P Smith, D Long, T D J Cameron, D Evans, A G Stevenson, J Bulat and J D Ritchie with contributions by R Holmes, S Holloway and D H Jeffery

Bibliographic reference: Gatliff, R W, Richards, P C, Smith, K, Graham, C C, McCormac, M, Smith, N J P, Long, D, Cameron, T D J, Evans D, Stevenson, A G, Bulat, J, And Ritchie, J D. 1994. United Kingdom offshore regional report: the geology of the central North Sea. (London: HMSO for the British Geological Survey.)

British Geological Survey

United Kingdom Offshore Regional Report

The geology of the central North Sea

R W Gatliff, P C Richards, K Smith, C C Graham, M McCormac, N J P Smith, D Long, T D J Cameron, D Evans, A G Stevenson, J Bulat and J D Ritchie with contributions by R Holmes, S Holloway and D H Jeffery

Production of this report was funded by the Department of Energy (now incorporated into the Department of Trade and Industry) and the Natural Environment Research Council. The coastline used on many maps and diagrams in this book is based on Ordnance Survey mapping

London: HMSO, 1994. NERC copyright 1994. First published 1994. Dd 292047 C20 8/94 20249 3396/2 ISBN 0 11 884504 7

(Front cover): Unconformity at Siccar Point near St Abbs Head (Figure 1), often termed Hutton's Unconformity. Basal breccia of the Upper Old Red Sandstone dips at a low angle to the left (north); it rests on vertically inclined shaly siltstone and thin greywacke of Silurian (Llandovery) age.

(Rear cover) Index map United Kingdom Offshore Regional Reports

Foreword

In 1795 James Hutton described the hiring of a boat in order to study the cliffs of Berwickshire; during this cruise he observed the relationship between Devonian and Lower Palaeozoic rocks at Siccar Point (see front cover). From these and other observations he deduced the concept of unconformity, a landmark in the study of geology. In more recent times, the Central Graben in the central North Sea has been important in the study of sedimentary basin development; this is particularly because of its well-defined crustal thinning and its postrift evolution.

The economic importance of the report area is considerable, for it includes some of the United Kingdom's largest oilfields. The oil occurs in rocks of various ages, contrasting with areas to the north where the Jurassic System provides both the predominant reservoir and the source rocks. In the report area, the Jurassic Kimmeridge Clay Formation remains the main source rock, but deep-water fan sands of Paleocene age are particularly important reservoirs — most notably at the giant Forties Oilfield.

The British Geological Survey has carried out a reconnaissance survey of much of the UK Continental Shelf, including the central North Sea. The results of this work, financed largely by the Department of Energy (now incorporated into the Department of Trade and Industry), have been published in a series of maps at a scale of 1:250 000 (see inside back cover). BGS has also carried out studies of commercial data for the Department of Trade and Industry, and much commercial well data has now been released. This information, combined with the results of the BGS reconnaissance survey and published data, has been used in the preparation of this report. It forms part of a series of UK Offshore Regional Reports for the UK (see back cover); their production has been largely financed by the Department of Trade and Industry, whose contribution is gratefully acknowledged.

Peter J Cook, DSc Director, British Geological Survey Kingsley Dunham Centre Keyworth, Nottingham. NG12 5GG. October 1993

Acknowledgments

Chapters in this report have been produced by the following authors:

• Introduction — D Evans and A G Stevenson
• Structure — R W Gatliff, N J P Smith and J Bulat
• Pre-Permian — R W Gatliff
• Permian — P C Richards
• Triassic — P C Richards
• Jurassic — K Smith
• Cretaceous — R W Gatliff (Lower); T D J Cameron and J D Ritchie (Upper)
• Paleogene and Neogene — M McCormac, with a contribution by S Holloway (Paleocene and Eocene); D Long (Oligocene and Neogene)
• Quaternary D Evans with contributions by R Holmes, D H Jeffery and T D J Cameron
• Sea-bed sediments — C C Graham and A G Stevenson
• Economic geology — R W Gatliff and D Evans

The Offshore Regional Report Series is co-ordinated by the Marine Geology Group, and edited by D Evans and A G Stevenson.

In addition to the work of the authors, the report has drawn extensively upon the knowledge and expertise of other BGS staff, not only within the marine sphere, but also from the Land Survey, and of specialists within the field of sedimentology, biostratigraphy, cartography and publication. In particular, much use has been made in this report of palaeontological analyses carried out on borehole material by BGS biostratigraphers. The authors and editors are grateful to the following for constructive comment: M A E Browne, T P Fletcher, R Harland, K Hitchen, G K Lott, A A McMillan, G Warrington and R T R Wingfield.

Murphy Petroleum Limited and partners are thanked for permission to publish a portion of deep-seismic line M37-8410 in (Figure 15). Western Geophysical are thanked for permission to use parts of their seismic sections in (Figure 25) and (Figure 26).

Chapter 1 Introduction

The area covered by this report extends from the eastern coast of Scotland and northern England to the international median line in the North Sea where the UK sector joins Norwegian, Danish, German and Dutch waters (Figure 1). From south to north, the area extends from 55°N to 58°N, but it reaches only as far as 57°30′N to the west of 0°. This report area includes much of the Central Graben and its junction with the Viking Graben and Outer Moray Firth Basin (see (Figure 9)); it contains several important oilfields, the largest of which is Forties.

The coastline bordering the report area is generally characterised by cliffs, although lower-lying tracts are more common along the English section of coast, and there are major embayments at the Firth of Forth and Firth of Tay. An offshore nature reserve with underwater caves is to be found off St Abbs Head (Figure 1). Away from the coast, the sea bed slopes gently to an extensive and generally rather flat and sandy p atform; this largely ranges from 70 to 100 m water depth, but is cut by many unfilled incisions, notably the Devil's Hole which is over 220 m deep ((Figure 1) and see (Figure 75)). A number of banks are to be found within 50 km of the Scottish coast, and the south-east of the report area is dominated by the Dogger Bank, which forms a broad rise, much of it shallower than 40 m. In the north-east of the area, the water deepens to 150 m into the Witch Ground.

Apart from dykes and Quaternary deposits, no rocks younger than Carboniferous occur either along the coast or for some distance inland, although Mesozoic sediments are found in the coastal margins of the Moray Firth to the north, and Permian rocks form the coastal succession to the south of Newcastle, and locally to the north of the River Tyne. Late Precambrian (Dalradian) and Devonian rocks crop out to the north of the Highland Boundary Fault (Figure 2), to the south of which lies the Midland Valley Graben, where thick Upper Palaeozoic sedimentary and volcanic rocks have been preserved. To the south of the Southern Upland Fault, steeply dipping Lower Palaeozoic rocks are overlapped by Upper Palaeozoic strata, mainly of Carboniferous age, which occur widely in north-east England. The structural relationship of Lower Palaeozoic and Devonian rocks at Siccar Point in Berwickshire (see front cover), which the pioneer geologist Hutton observed from a boat, formed the basis of his deduction of unconformity.

These predominantly north-easterly trending Palaeozoic successions do not extend far offshore at the surface, for they are directly overlain by Permian rocks which have a strike that is remarkably parallel to the coastline. Above the Permian strata lies a wedge of Mesozoic, Tertiary and Quaternary strata ((Figure 3) and see (Figure 10)) that thickens eastwards towards the deeply faulted Central Graben. Although the graben has been a locus of sedimentation since the Permian, it is principally a rift of Late Jurassic age. In the south of the area, the Mesozoic succession is thinner, for the Mid North Sea High was a positive region until the Late Cretaceous or Tertiary.

The morphology of both the bathymetric deeps (Gregory, 1931), and the Dogger Bank and 'moorlog' dredged from the bank (Whitehead and Goodchild, 1909), were the subjects of early offshore studies, and shells dredged from the sea bed off Aberdeen were described in the last century (Dawson, 1866).

However, other than as a result of the seaward extension of coal mining, the area was not given much attention before the northward movement of oil-company exploration following gas discoveries in the southern North Sea in the 1960s. The first central North Sea exploration well was drilled in 1964 on the southern flank of the Mid North Sea High, but it was not until 1967 that a well was drilled to the north of this high. Since that time, a great many wells have been drilled (see (Figure 83)), mainly along the Central Graben where numerous discoveries have been made. The earliest discovery in the UK sector was in well 22/18-1 (Department of Energy, 1991), which is now the location of the Arbroath Oilfield (Figure 1). The UK's first offshore oilfield was Argyll; 11 oilfields are now in production in the report area, including the Forties, Fulmar and Auk fields.

Since the late 1960s, the British Geological Survey (BGS) has been investigating the area as part of a project to produce offshore reconnaissance maps of the UK; this project was supported largely by the Department of Energy (Fannin, 1989). Geophysical surveys employed airgun, sparker and higher-resolution seismic systems, as well as sidescan sonar, gravity meters and magnetometers. Geological sampling surveys have been carried out with vibrocorers, gravity corers and grabs, and have included the drilling of many shallow boreholes (Figure 1). Information on the BGS maps is partly derived from BGS interpretations of commercial data on behalf of the Department of Energy; although seismic data remain confidential, much borehole information has now been released.

Geological summary

The Dalradian rocks to the north of the Highland Boundary Fault are the oldest known in the area; they were tectonised during the Caledonian orogenic cycle. The Lower Palaeozoic sedimentary and volcanic rocks of the Southern Uplands were deposited on the margins of the Iapetus Ocean, the closure of which, as a result of continental collision in late Silurian times, resulted in the deformation associated with the Caledonian orogeny. Another metamorphic belt, part of the German-Polish Caledonides, lies concealed beneath the Central Graben; this is interpreted to mark the closure of the Tornquist Ocean during the late Ordovician (Cocks and Fortey, 1982).

During the Devonian Period, there was widespread molasse sedimentation as the newly formed Caledonian mountain ranges were eroded. The Midland Valley of Scotland was a major depocentre in mid- to late Silurian and Early Devonian times; rocks of this age are probably present along its offshore extension, but much of the south-central North Sea may have been a high (Ziegler, 1982), in part due to the buoyant effects of late Caledonian granites. Rocks of Mid-Devonian age are absent in the Midland Valley, but there was extensive lacustrine deposition at that time in the Moray Firth area, and marine limestones of this age were probably deposited in the south of the report area during the first phase of post-Caledonian rifting. Widespread redbed sedimentation occurred in the central North Sea during the Late Devonian, and deposition of this facies continued into earliest Carboniferous times.

Transgressions as a result of regional crustal extension in Tournaisian times brought about the development of fluviodeltaic and shallow-marine environments for much of the Carboniferous Period. Sedimentary rocks of this age are presently confined to the offshore extension of the Midland Valley into the Forth Approaches Basin, and to basins within the Mid North Sea High. Volcanic rocks are widespread onshore, but they are found in only one well offshore; coal-bearing strata are, however, abundant.

In Early Permian times, arid, continental sedimentation returned; much of the area was part of the Northern Permian Basin which lay to the north of the Mid North Sea-Rinkøbing-Fyn High (see (Figure 9)). Some authors (e.g. Glennie, 19866) have suggested that initial rifting of the Central Graben took place at this time. Rapid transgression during the Late Permian formed a basin in which evaporites and carbonates were deposited. Continental redbeds were laid down during Triassic times; both sandstone- and mudstone-dominated successions are found, and evidence for rifting is inconclusive. Sedimentation was affected by movement of the underlying Late Permian salt, a pattern repeated through much of subsequent geological time.

A marginal-marine environment was established in parts of the area during the Early Jurassic, but few strata are preserved, possibly because of domal uplift and erosion in the Early to Mid-Jurassic times, when volcanic centres became established in the Central Graben. Middle Jurassic sediments are predominantly fluviodeltaic, and are largely confined to the graben. The Late Jurassic was the time of major rifting. It was during this episode that the main elements of the North Sea graben system were formed (see (Figure 8)). Contemporaneous transgressions established a marine environment throughout the report area, leading to predominantly argillaceous sedimentation with minor limestones and turbidites. Late Jurassic organic-rich claystones are the major source of hydrocarbons in the province.

Marine conditions persisted throughout the Cretaceous Period. During the Early Cretaceous, there was widespread deposition of variably calcareous muds, and emplacement of local sands. Due to continued faulting, the greatest thickness is preserved at basin margins. Further eustatic sea-level rise resulted, during Late Cretaceous times, in the laying down of a blanket of chalk which covered the area, including the Mid North Sea High.

During the early Tertiary, deep-water mud deposition was dominant in the Central Graben, but deltaic systems that extended from the Scottish mainland, Orkney and Shetland provided sands which have become important reservoirs for hydrocarbons, notably at the Forties and Montrose oilfields. Largely argillaceous marine sedimentation continued throughout the Tertiary, latterly derived from the east. Post-rift thermal sag gave rise to a characteristic 'steer's head' basinal profile (Dewey, 1982), with sedimentation concentrated in the zone above the graben.

Through early Pleistocene times, subsidence and marine conditions continued, with filling of the basin from the south-east and west. Later in the Pleistocene, climatic deterioration caused ice sheets to invade the area, resulting in widespread glacial/glaciomarine sedimentation and both regional erosion and the local creation of deep incisions, some of which remain unfilled. Following final ice withdrawal, the Holocene sea-level rise left a cover of superficial sediments at the sea bed; these are largely sandy, but muddy in the deeper waters, and gravelly on banks. Biogenic carbonate material has accumulated throughout the Holocene, largely in the nearshore and on banks farther offshore.

Chapter 2 Structural development

Before the onset of petroleum exploration, very little was known about the structure of the central North Sea; one of the first indications of its crustal structure was the recognition of an elongate, high, gravity anomaly along the centre of the North Sea (Collette, 1960). This has since been shown to define a major sedimentary basin. McKenzie (1978) suggested that the coexistence of the basin and the gravity high could be explained in terms of a pure-shear, uniform-stretching model of basin formation. New gravity data (Donato and Tully, 1981), and a refraction-seismic experiment (Barton and Wood, 1984), provide supportive evidence for this hypothesis. Basin modelling of the subsidence history suggests that stretching occurred mainly in Late Jurassic times (e.g. Rattey and Hayward, 1993), but was preceded by a phase of Permo-Triassic crustal stretching.

This structural model of the Mesozoic and Cenozoic evolution of the central North Sea has been refined as more data have become available. Several major onshore structural features are inferred to extend into the report area, notably the north-easterly trending Southern Uplands block of Scotland, the Iapetus suture, and the westerly extension of the north-westerly trending Tornquist line. Aeromagnetic and gravity data show these important basement trends ((Figure 4) and (Figure 5)). Recent very deep seismic-reflection profiles run by BIRPS (British Institutions' Reflection Profiling Syndicate) have provided data for studying the deep crustal structure, and have allowed new insights into the Palaeozoic and later history (Freeman et al., 1988); a wide range of interpretations is, however, possible from the limited data available.

Deep crustal structure

Refraction-seismic and gravity data show the Mohorovičić Discontinuity (Moho) at the base of the crust to be at a 'normal' depth of about 30 to 32 km over much of the report area, rising to between 20 and 25 km beneath the Central Graben. The BIRDS seismic-reflection lines reveal reflectivity normally associated with the Moho at a depth of 10 to 11 s two-way travel time (TWTT) beneath the graben; this is slightly deeper than the level indicated for the Moho by gravity data (Holliger and Klemperer, 1990).

One of the main difficulties in interpreting very deep seismic-reflection profiles is the inability to test directly the origin of deep reflectors which, apart from being inaccessible, are commonly discontinuous and difficult to correlate on the wide grid of lines available. Interpretations are based on recognising zones of similar seismic character, and relating these to known geological structures in adjacent areas. In the central North Sea, there is typically, but not uniformly, a zone of unreflective middle crust, and a reflective lower crust and Moho. Locally, there are strong reflectors in the middle crust.

Beneath the Moho, mantle reflectors are noted on either side of the Central Graben (Figure 8)." data-name="images/P944906.jpg">(Figure 6); these dip away from the graben, and were assumed by Klemperer and White (1989) to relate to a Late Jurassic stretching event. Klemperer and White (1989) concluded that the pattern of subsidence in the central North Sea strongly supports coaxial stretching of the crust and lithospheric mantle, primarily controlled by pure shear. Previously, Gibbs (1987a) had suggested that the westerly dipping reflectors form part of a crustal-penetrating detachment related to asymmetric simple shearing of the lower crust during the Jurassic. The controversy between the pure- and simple-shear hypotheses has not yet been resolved, although the balance of evidence favours a symmetrical, pure-shear model of basin formation.

Both the age and origin of the reflective lower crust are equivocal; one interpretation (e.g. Reston, 1988) favours an extensional origin, with anastomosing shear zones originating from the main episode of stretching and crustal thinning during Jurassic times. Beneath the depocentres of the Permian basins, where halokinetic salt structures are preserved, reflectors are not recorded from the lower crust (Blundell et al., 1991). More general problems of imaging the lower crust are evident from profiles recorded perpendicular to the Central Graben, where apparent changes in character may be due to thickening of the overlying sedimentary sequence (Blundell et al., 1991).

Freeman et al. (1988), interpreting the NEC seismic profile adjacent to the UK coast, divided it into four broad crustal zones which they were able to relate to the adjacent onshore geology (Figure 7). In particular they recognised a zone of northerly dipping lower-crustal reflectors to the east of the Southern Uplands, where the reflection Moho is deeper than average at 12 s TWTT. They interpreted this zone as a remnant of the subduction complex associated with the Iapetus suture, which was originally predicted on faunal differences between the Laurentian and Avalonian margins of the Iapetus Ocean. Klemperer et al. (1990) extended this Iapetus suture zone upwards along strong dipping reflectors that divide zones B and C (see (Figure 7)), and showed that these reflectors strike east-north-east. They further suggested that this alignment reflects a change in the strike of the Iapetus suture to a line approximately parallel to the Mid North Sea High (Figure 8). Chadwick and Holliday (1991) proposed that these dipping reflectors image Early Devonian thrusts within Avalonian crust, and hence do not necessarily represent the suture based on faunal evidence, which would crop out farther north. Smith (1992) pointed out that the reflectors occur in an area of extensive Early and Late Carboniferous volcanism, and related them to magmatic underplating during the Carboniferous. He suggested that the east-north-east trend recognised by Klemperer et al. (1990) might be better correlated with known onshore Carboniferous structural trends than with the north-easterly Caledonian trend.

Reflective middle crust is a relatively rare feature on very deep seismic data, but is most commonly observed near basement blocks (McGeary et al., 1987). A very reflective zone of middle crust occurs beneath the Devil's Hole Horst (Klemperer and White, 1989); this zone tapers eastwards towards the Central Graben (Figure 8)." data-name="images/P944906.jpg">(Figure 6), and could be attributed to thinning of the middle and lower crust towards the graben.

An upper to mid-crustal reflector can be mapped both onshore and offshore in the Northumberland Trough (Reflector T in (Figure 7); Freeman et al., 1988); it dips south from 2 to 5 s TWTT, and may be ascribed to four possible horizons: (1) the top of Lower Palaeozoic rocks, (2) the top of the Cheviot Granite, (3) the base of a pre-Silurian accretionary wedge, or (4) a Caledonian back-thrust. The reflector is in an analogous position to ubiquitous mid-crustal reflectors present under the London–Brabant Palaeozoic basement high (Blundell et al., 1991), which characteristically dip towards reflectors interpreted as thrusts.

Structural history

Early Palaeozoic

The basement rocks of the central North Sea are thought to have formed part of the Caledonian mountains that were uplifted during early Ordovician to Mid-Devonian orogenies (Coward, 1990). The regional configuration of Early Palaeozoic plates suggests that a triple plate junction between Laurentia, Baltica and Avalonia is located either within the report area, or in the adjacent Norwegian or Danish sectors of the North Sea. The Iapetus Ocean and its smaller branch, the Tornquist Ocean, were closed during the Caledonian orogenies, and three distinct faunal provinces can be recognised. Scotland and the topmost nappes in Scandinavia are considered to have been part of an American, tropical province of Laurentia, whereas England and Wales belonged to a high-latitude, Avalonian province (Conway-Morris and Rushton, 1988). The location of the former Tornquist Ocean, whose closure predated late Ordovician times (Scotese and McKerrow, 1990), has been inferred from the distribution of the distinctive intermediate-latitude Baltoscandian province. Closure of the Iapetus Ocean, although beginning in early Ordovician times, was not completed until the late Silurian.

The zone of closure of the Iapetus Ocean is often termed the Iapetus suture, and is seen as a westerly dipping thrust in Norway (Nicholson, 1979). However, links to either the Tornquist suture or the north-north-westerly dipping reflectors that lie parallel to the Mid North Sea High remain unclear (Klemperer and White, 1989).

The northernmost chain of Caledonian mountains in the UK are the Grampian mountains, which formed on the southern margin of Laurentia (Dewey and Shackleton, 1984). Their exposed southern limit is marked by the Highland Boundary Fault, which essentially forms the northern boundary of the Midland Valley Graben, and may be traced offshore north-eastwards along strike on the aeromagnetic map (Figure 4). The fault is only a minor feature on seismic profiles, but has been traced across the North Sea (Figure 9), through the Ling Graben, to the north of Stavanger (Dikkers, 1977; Dore and Gage, 1987). Klemperer and Hurich (1990) rejected this connection on the grounds that known autochthonous 'Baltic' basement extends for several hundred kilometres to the north-west of Stavanger.

Onshore, a number of zones within the Caledonian belts have been defined on the basis of their tectonostratigraphical history; all are bounded by major faults. For example, the Southern Uplands is interpreted as an accretionary wedge (Leggett et al., 1979) related to the northwesterly directed subduction of Avalonia along the Iapetus suture. This zone is bounded to the north by the Southern Upland Fault. Several authors (e.g. Soper et al., 1987) have speculated that strike-slip displacement on major faults had caused separate blocks, termed suspect terranes, to become juxtaposed at different times. Recent interpretations (e.g. McKerrow et al., 1991) suggest that there were large-scale lateral displacements at the closure of the Iapetus Ocean, with the Southern Uplands rocks having been derived from Siberia. Low-grade Caledonian metamorphic rocks may form the basement across much of the central North Sea; evidence for their offshore continuation comes from the trends of both the Firth of Forth and the Southern Upland (comprising the Lammermuir Fault and the Cove Fault) fault systems, and the recovery of Lower Palaeozoic rocks in wells 26/14-1, 27/3-1, 27/10-1 and 30/16-5 (Figure 8).

The pre-Silurian geology of the eastern Midland Valley of Scotland is unknown. Offshore, reflectors at depths of up to 4 s TWTT dip southwards (Figure 7), and probably represent thick Lower Devonian sediments and metamorphic basement. Beneath this zone, reflections from the lower crust are weak. However, there is evidence both from the trend of aeromagnetic anomalies in the Forth Approaches Basin (Figure 4), and the occurrence of Carboniferous sedimentary rocks to the south of the Buchan Horst, that the onshore north-easterly trend of the Midland Valley continues offshore at least as far as the main Mesozoic Graben ((Figure 8) and (Figure 9)).

The eastern boundary of the British Caledonides is not well defined. Cartwright (1990), however, has postulated that the deformation front against the Scandinavian Caledonides could be represented by the westerly dipping Else 'Duplex' within the Ringkøbing-Fyn High. This is shown to be offset along several westerly to north-westerly trending major fracture zones as part of a broad zone of Tornquist shears. This zone of shears may extend along the trend of the Central Graben, which may have its origins as a transform zone during the Caledonian orogeny (Cartwright, 1990). In order to explain the change in tectonic style between the British and Scandinavian Caledonides, Coward (1990) invoked a sinistral transform-fault boundary along the Central Graben.

Early Devonian

Lower Devonian rocks can be considered to form part of the Caledonian cycle, for the Mid-Devonian was the time of final consolidation of the Caledonides. There is commonly conformity between folded Lower Devonian and Silurian rocks (Soper et al., 1987), and Early Devonian volcanism is evidence of continuing subduction.

Two Caledonian plutonic masses, the Farne and Dogger granites, have been postulated on the grounds of two low gravity anomalies on the Mid North Sea High ((Figure 5); Donato et al., 1983). These granites lie to the east of the onshore Cheviot Granite, which has been dated by Thirlwall (1988) at about 392 Ma (Early Devonian Harland et al., 1990). The existence of the Dogger Granite is further indicated by seismic evidence of sedimentary thinning across it. In the Dutch sector to the east, there is evidence in well A/17-1 (Figure 8) for another granitic mass beneath Devonian sediments. These offshore granites were probably intruded during the Early Devonian.

Lower ORS rocks, which include Silurian strata (Marshall, 1991), are of mountain-front fan and fluviatile facies. In the Midland Valley, the Highland Boundary Fault has controlled the distribution and structure of a 7500 m-thick, folded succession (Cameron and Stephenson, 1985; Coward, 1990) in a basin for which many authors favour a pull-apart tectonic setting (e.g. Bluck, 1984), but post-Silurian, strike-slip displacement is seemingly limited to only a few tens of kilometres (Thilwall, 1989).

The absence of Lower Devonian rocks to the south of the Midland Valley Graben in the central North Sea is suggested from drilling at the Auk Oilfield, on the Devil's Hole Horst, and from seismic evidence across the Dogger Granite. These sparse data may indicate that the Early Devonian was a time of erosion across much of the central North Sea, although some molasse deposition may have occurred in foreland-type basins.

Mid-Devonian to Carboniferous

More wells penetrate the Middle Devonian and Carboniferous rocks than reach older strata, and more coherent reflections are recorded from these rocks on seismic profiles. Consequently, there is an improved picture of basin tectonics for the extensional collapse of the Caledonian orogen in the central North Sea, but there remain uncertainties in basin-wide seismic correlations, and biostratigraphical control is poor. Various models of extensional collapse of the Caledonian orogen have been proposed (e.g. McClay et al., 1986), and there is disagreement about the degree of strike-slip fault control (Steel and Gloppen, 1980; Bluck, 1984). It is likely that the crust, overthickened by thrusting during the Caledonian orogeny, was reactivated along these thrust faults (McClay et al., 1986). Within the central North Sea, several different trends are identified.

Along the North Sea coast, a regionally significant feature of the Devonian succession is sedimentary overlap of the major Lower Palaeozoic highs, suggesting major subsidence within the North Sea. Marine Middle Devonian limestones at the Auk and Argyll oilfields (Pennington, 1975) represent an 'abnormal' facies by comparison with the continental-facies rocks of northern Britain and Norway. The distribution of these limestones approximately follows the north-westerly trend of the Polish-German Caledonides, and may represent a north-westerly propagating rift system which attempted to reopen the Tornquist Ocean.

North-westerly trending faults which affect Upper Palaeozoic rocks to the west of the Central Graben may also be associated with Mid-Devonian rifting. However, over the Mid North Sea High, fault alignments are more variable. A major north-easterly trending fault between the Farne and Dogger granites (Figure 8) was mapped by Jenyon et al. (1984) as having a Late Permian downthrow to the south-east. Interpretations of deeper reflectors suggest that this fault was initiated in Devonian times, and may have been reactivated as a reverse fault during the Variscan orogeny. A key control on the distribution of Devonian and Carboniferous basins on the Mid North Sea High appears to have been the positive effect of the Farne and Dogger granites; this is similar to the control exerted by the Wensleydale and Weardale granites on the locations of onshore Early Carboniferous highs (Bott, 1967).

Chadwick and Holliday (1991) have extended onshore Carboniferous growth faults a short distance offshore to the NEC profile ((Figure 7) and (Figure 8)); all the faults have a northeasterly strike, and commonly dip to the north-west. These basin-controlling Dinantian-age faults were probably originally Caledonian features, some of which formed in the hanging walls of thrusts (Chadwick and Holiday, 1991).

Dinantian extension in the Midland Valley, and to a lesser extent in the Northumberland Trough, was associated with the extrusion of large volumes of basaltic lava (Leeder, 1982). It is not clear what relationship exists between Dinantian rifting onshore and Devonian to Early Carboniferous rifting in the North Sea. The simplest interpretation is that the rift propagated north-westwards through time.

Variscan Orogeny

By Namurian times, igneous activity and graben formation had largely ceased. Regional compression associated with the Variscan orogeny began farther to the south in Devonian times, migrating northwards and ceasing by the Late Carboniferous. This compression was predominantly north–south, although some structures onshore in northern England and southern Scotland suggest east-north-easterly orientated compression. Offshore, Variscan folds are orientated approximately north–south close to the shore (BGS Tay–Forth Solid Geology sheet). Farther east, the evidence is less clear, although fold axes and basin orientation on the Mid North Sea High were probably governed to a large extent by the locations of the Caledonian granites. The Late Devonian to Early Carboniferous basin to the north of the Dogger Granite shows truncation of reflectors at the Permian basal unconformity. This is seen as evidence for folding and erosion prior to deposition of Permian strata (see (Figure 15)).

Late Carboniferous to Permian

Latest Carboniferous and Early Permian times saw major, east–west wrench movements and volcanism (Glennie, 1986b). Cartwright (1990) suggested that contemporaneous rifting along the Norwegian-Greenland Sea and the North Sea was linked by north-west-orientated shears, along the pre-existing Tornquist trend. In the central North Sea, direct evidence of Permian movement along the main graben faults is difficult to prove due to extensive Mesozoic faulting and associated halokinesis. However, the development of the Northern Permian Basin is indicative of some regional extension within the central North Sea.

Onshore evidence for extension comes from Late Carboniferous to Early Permian magmatism in northern England, southern Scotland, and the adjacent central North Sea. Aeromagnetic anomalies show the east-north-easterly offshore continuation of intrusions such as the Whin Sill (Figure 4). Earliest Permian volcanic rocks have been proved in several wells in the south-cast of the report area (e.g. well 31/26-A; (Figure 8)); these also give rise to aeromagnetic anomalies (Figure 4). Further aeromagnetic anomalies to the north of the Dogger Granite may also reflect volcanism, either at this time or later during the Jurassic.

The Northern Permian Basin was bounded to the northwest by the Highland Boundary Fault, To the north it continued along the line of the Viking Graben (Glennie, 1986a), and to the east it extended into the area of the Norwegian–Danish Basin. To the south, it was separated from the Southern Permian Basin by the Mid North Sea High. Within the Northern Permian Basin, there may have been several intrabasinal highs, such as the Forties-Montrose High, but later salt dissolution may have exaggerated the Permian effect of these predominantly Mesozoic structures.

In areas to the east, such as the Horda Platform (Figure 9) and Oslo Graben (Steel and Ryseth, 1990), evidence for Permian to Scythian (Triassic) rifting is displayed on seismic profiles. Yet in the central North Sea there is little evidence for Permian growth faulting or extension, although the Northern Permian Basin was connected to the Norwegian–Danish Basin. Cayley (1987) has invoked rifting in mid-Permian times, with minor normal faulting superimposed on a regional crustal downwarp, and Smith (1987) identified some Late Permian faulting in the vicinity of the Clyde Oilfield.

On the basis of poorly dated redbeds in wells on the Mid North Sea High, Glennie (1986b) suggested that Lower Permian (Rotliegend) sands pinch out abruptly against the northern margin of the Mid North Sea High. However, new BGS interpretations suggest that the upper section of redbeds in several wells on the Mid North Sea High might be a 'wadi' facies of the Rotliegend, rather than Old Red Sandstone. Several wells around the Auk Oilfield ((Figure 8); see Permian chapter) show that some of the thickest Rotliegend sequences lie close to the Mid North Sea High. There is, nevertheless, no evidence for a major fault along its margin, leading to a model as a passive, intrabasin high influenced by the underlying granites.

The Mid North Sea High is known to have formed only a partial barrier between the Northern and Southern Permian basins during Late Permian times. Jenyon et al. (1984) showed that a fault and associated salt-dissolution-bounded 'channel' of Zechstein evaporites extended across the high between the Farne and Dogger granites (see (Figure 26)). Smith and Taylor (1989) identified a second 'north-west' passage joining Northumberland and the Forth Approaches between the Cheviot and Farne granites. However, according to Ziegler (1990b), Zechstein salt is absent from that part of the Central Graben which separates the Mid North Sea High from the Ringkøbing-Fyn High.

Triassic

During Triassic times, rifting between Greenland and Norway continued to propagate southwards and westwards (Ziegler, 1989). Triassic rifting is evident across the Tail End and Horn grabens ((Figure 9); Ziegler, 1990b) to the east of the report area, but the amount of faulting in the UK sector of the central North Sea is difficult to quantify. The broad trends of Triassic sedimentary thickness indicate that the main depocentre lay in the Norwegian–Danish Basin, and significant Triassic extension may have been restricted to this area. The dominant control on deposition may have been thermal subsidence following initial crustal extension in Permian times. Barton and Wood (1984) suggested that there had been some extension across the Central Graben during the Permo-Triassic.

There is abundant evidence for extensive halokinesis of Zechstein salt during Triassic times, with the development of major turtle-back anticlines and rim synclines (Cayley, 1987; Johnson et al., 1986). It is possible that this relatively early salt movement was triggered by faulting, but the redistributed salt has obscured any evidence of significant Triassic growth faulting. Thickness variations in the Triassic succession across both the bounding faults of the Forth Approaches Basin (Figure 10) and the main faults of the West and East Central grabens, suggest some Triassic tectonism. The absence of correlatable markers within the Triassic section, and an overlying unconformity, complicate estimations of the extent of the faulting.

The Mid North Sea High appears to have acted as a uniform massif during Triassic times, with very little evidence of faulting. Yet there is minor thickening of the Triassic succession across the south-eastern bounding fault of the Zechstein 'channel' between the Farne and Dogger granites (Jenyon et al., 1984).

Jurassic

During the Jurassic Period, there was a change from the southerly propagation of the Greenland–Norwegian rift system to the northward propagation of the Central Atlantic sea-floor-spreading axis (Ziegler, 1989). The Late Jurassic was a time of active graben formation, the event largely responsible for the present structural form of the central North Sea (Figure 11). The degree of inheritance from older structures is uncertain, but it is likely to have followed structural trends developed during the Permo-Triassic, which themselves may have been governed by Tornquist shears, Mid-Devonian rifting, and older structures.

Early to mid-Jurassic: doming and volcanism

Prior to the Late Jurassic rifting there was a major phase of doming, and extensive volcanism associated with the Forties Volcanic Province. The doming resulted in a mid-Cimmerian unconformity that extended across the whole of the central North Sea, and whose effects may be recognised in sediments covering a much wider area (Ziegler, 1981; 1990a). Estimates of the amount of regional uplift vary considerably: Leeder (1983) suggested that it was of the order of 250 m, contrasting with a figure of 1500 to 2500 m proposed by Ziegler and Van Hoorn (1989). Smith and Ritchie (1993) proposed uplift of about 1000 m, based on the extrapolation of regional Triassic and Permian isopachs.

Ziegler (1990a) postulated that Lower Jurassic sediments, which are widely absent in the central North Sea, formerly extended across the North Sea from the UK to Norway and Denmark. He argued that doming began in Aalenian times, prior to the main eruptive phase during the Bajocian and Bathonian. Smith and Ritchie (1993) have suggested that the timing of uplift and volcanism was more complex; they recognised three Jurassic volcanic centres in the central North Sea, termed Puffin, Glenn and Fisher Bank (Figure 8), with a fourth centre, Ivanhoe, to the north in the Moray Firth. These centres are particularly clearly located from their associated aeromagnetic anomalies (Figure 4).

Dating of these volcanic rocks is poorly constrained. There is a major unconformity at their base, where the underlying rocks are usually of Triassic age or older, and onlapping younger sediments are generally Oxfordian or younger. A Bajocian to Bathonian age was generally accepted for the interbedded volcanic rocks and the deltaic sediments of the Pentland Formation (Deegan and Scull, 1977). However, a spread of radiometric dates suggests a more complex chronology for the development of the centres. The Fisher Bank centre is dated by the K-Ar method at 188 ±10 Ma (Latin et al., 1990a), which equates with a Sinemurian to Toarcian age (Harland et al., 1990). The Glenn centre is dated by the ⁴⁰Ar-³⁹Ar method as 153 ±4 Ma–Oxfordian to Kimmeridgian (Ritchie et al., 1988), and a K-Ar age of 160 Ma–Callovian (Howitt et al., 1975, revised by Ritchie et al., 1988). No dating is available from the Puffin centre, but further support for a series of different volcanic events is recognised from a K-Ar date of 178 to 180 Ma (Toarcian to Aalenian) from the Egersund volcanic complex in the Norwegian sector to the east (Furnes et al., 1982).

Smith and Ritchie (1993) showed that, prior to its formation, erosion at the Glenn centre was more severe than that at the Fisher Bank centre, for at the latter, thin, shallow-marine sediments of Sinemurian age are preserved. In addition, the high content of terrestrial plant debris in thin Hettangian to Sinemurian sequences from several wells in the UK sector of the central North Sea implies evidence for local Early Jurassic exposure. Doming and nondeposition in the central North Sea, rather than later erosion, may explain the absence of Pliensbachian to Toarcian sediments. Early eruption of lavas from the Fisher Bank centre ensured the preservation of Early Jurassic sediments; this contrasted with continued doming and erosion in the Glenn centre until eruption in Callovian times. Thus, erosion of the Fisher Bank volcanic rocks may have provided the bulk of the volcanic detritus interbedded within the Pentland Formation of the outer Moray Firth.

The Forties volcanic rocks are concentrated at a 'triple junction' between the Viking Graben, the Central Graben anc the Outer Moray Firth Basin. Sellwood and Hallam (1974), in order to explain the onshore patterns of Mid-Jurassic sedimentation, proposed a model of doming associated with the development of a mantle plume. followed by volcanism. Dixon et al. (1981) and Leeder (1983) suggested that the volcanic rocks formed as a result of upwelling of magma from the asthenosphere during continental extension. Latin et al. (1990a) contended that the geochemistry of the lavas is not compatible with the amount of crustal thinning associated with Mesozoic extension, and argued for an additional contribution to melting from a thermal perturbation in the mantle (a plume). Support for this latter view comes in the recognition that most Mesozoic extension occurred during Late Jurassic times, after the volcanic event.

Late Jurassic: rifting

Following the volcanic episode there was major crustal stretching, with widespread subsidence accompanied by the development of large-scale growth faults. Variations in the thickness of Middle Jurassic sediments within the Central Grapen indicate that the inception of growth faulting occurred prior to the main extensional event during Oxfordian to Early Cretaceous times. However, the major faults that define the Central Graben (Figure 11) were at their most active during the Late Jurassic. The faults shown on (Figure 11) offset the base of the Zechstein, although many other faults active during Jurassic times do not affect this horizon, but sole out within the Zechstein halites (Figure 10).

The base-Cretaceous unconformity above the Jurassic Kimmeridge Clay Formation has been considered by many authors (e.g. Ziegler, 1975) to represent a significant break in the sequence resulting from a discrete episode of rifting, for the strong seismic event shows considerable Lower Cretaceous onlap. It is now believed that the major rifting phase occurred during the Late Jurassic (e.g. Rattey and Hayward, 1993; Donovan et al., 1993), and that the strong seismic event marks the top of condensed, very high-gamma shales at the top of the Kimmeridge Clay Formation. The Late Jurassic rifting resulted in overall basin subsidence, and gradual sediment starvation of many of the grabens and half-grabens formed within the Central Graben (Barr, 1991). The base-Cretaceous unconformity is in fact of intra-Ryazanian age (Rawson and Riley, 1982), and marks the peak of basin starvation preceding the onset of a new phase of sediment input following uplift to the west.

Late Jurassic rifting has resulted in the development of two partially symmetrical grabens separated by a central high ((Figure 10) and (Figure 11)). In the north, the Forties-Montrose High separates the Western and Eastern Central grabens; farther south, the Josephine Ridge forms a central high. The Forties-Montrose High and Josephine Ridge are non-contigious, for they plunge respectively south and north to the Marnock Terrace (Figure 11). This terrace may represent a transfer zone, accommodating subtle changes in the interaction of conflicting north-easterly and north-westerly basement trends. Similarly, beyond UK waters, the Vigelard Ridge separates the Feda Graben and Outer Rough Basin from the Sogne Basin and Tail End Graben ((Figure 9); Gowers and Saebee, 1985). Traced south, the graben system is offset sinistrally as it bisects the Mid North Sea–Rinkøbing-Fyn High. This is manifested also in the en-echelon Auk and Argyll faults (Figure 11).

Roberts et al. (19906) have recognised an east-north-easterly orientated system of transfer faults which they considered to demonstrate orthogonal extension of the Central Graben, and possibly to reconcile the Jurassic rift trends with more north–south-orientated rifts to the north and south of the Central Graben (Figure 9). A displacement of the Danish Central Graben has been identified by Cartwright (1987), who ascribed this to transfer faults with a west-north-wat orientation. In the north, the Central Graben merges through a complex area of basins and narrow, fault-bounded highs with the Viking Graben, the Witch Ground Graben, and the Forth Approaches Basin.

Gowers and Saebee (1985) have postulated extensive strike-slip activity in the Central Graben, and suggested that much of the graben tectonics was transtensional. This contrasts with the model of Roberts et al. (1990b), who preferred orthogonal dip-slip opening, with only the Inner Moray Firth Basin controlled by significant (7 km) dextral motion on the Great Glen Fault. Sears et al. (1993) proposed that most of the extension in the Central Graben was oblique-slip, controlled by an intraplate stress field, with the major compressive stress orientated north-south.

Barton and Wood (1984) modelled both the subsidence history and crustal thinning across the central North Sea, and showed post-Early Jurassic extension of about 70 km. However, Kooi and Cloetingh (1989) indicated that the amount of crustal extension, calculated from thermal subsidence curves in the central North Sea, is likely to be overestimated as a result of late Neogene compression instigating; high rates of subsidence in the centre of the graben.

Ziegler (1983), by measuring fault heaves of the top Rotliegend/base-Zechstein seismic reflector, calculated that only 30 km of post-Rotliegend extension has occurred. Roberts et al. (1990b) pointed out that considerable errors are inherent in this method of calculation due to the masking effect of the overlying Zechstein salt. They computed an extension across the Central Graben in the range of 20 to 40 km, in the direction 075°–255°. Roberts et al. (1993) suggested that extension based on fault-heave measurement will be underestimated by about 50 per cent due to the large number of relatively small faults which are not imaged on seismic data.

Western margin of the Jurassic Basin

There is evidence that granitic masses within basement rocks (Figure 8) have maintained a degree of buoyancy during downwarping and extension (Bott et al., 1978); this appears to be reflected in the thinning of Permian, Triassic and Jurassic sediments over the Dogger Granite (Donato et al., 1983). Near the Northumberland coast, there is an east–west-trending axis or arch (Figure 2) where Upper Cretaceous rocks overstep what appears to be a thinning Triassic, Jurassic and Lower Cretaceous sequence. The Jurassic rocks thicken southwards into the Cleveland Basin, and Lower Cretaceous rocks thicken northwards into the Forth Approaches Basin. This area of sedimentary thinning is located between the Cheviot and Farne granites. The structural relationship is similar to that of the Market Weighton Axis, which lies on an easterly trend between the Market Weighton Granite (Bott et al., 1978) and the Hornsea Granite (Donato and Megson, 1990).

Early Cretaceous

Tectonic activity decreased towards the end of Jurassic times. Some authors (e.g. Thorne and Watts, 1989) do not recognise Early Cretaceous rifting, preferring instead to invoke the onset of thermal relaxation. It is difficult to precisely identify the timing of the end of rifting, due to complex salt movement in the underlying Zechstein, to relatively passive fill of previously starved grabens (Barr, 1991), and to faulting resulting from compaction of the underlying sediments. Several faults, however, do show major thickening of Lower Cretaceous sediments in their footwalls. This is particularly evident along the western margin of the Jaeren High (Figure 10) and on the south side of the Buchan Horst, where submarine-fan sands within the Sola Formation indicate active faulting (Andrews et al., 1990). A sequence of coarse-clastic sediments straddles the Jurassic–Cretaceous boundary in well 30/11b-1, providing further evidence for earliest Cretaceous faulting.

Regional seismic mapping throughout the UK sector of the Central Graben suggests that Early Cretaceous fault activity was most prevalent along east–west-orientated faults (Figure 11). This change in trend probably resulted from realignment of the extensional stress system, and represents a westward shift of rifting away from the North Sea.

Late Cretaceous

During the Late Cretaceous, following the Late Jurassic to Early Cretaceous extensional phase, subsidence was controlled by regional lithospheric cooling. Thick chalk accumulated across the Central Graben as local subsidence patterns continued to reflect the underlying Jurassic rift structure (see (Figure 45)). Relatively minor faulting continued, particularly along the major graben-bounding faults; this may have been primarily related to compaction of the underlying sediments. Continued halokinetic movements of the Zechstein may also have been responsible for local uplift and erosion, and in particular to the development of a locally recognisable intra-Campanian unconformity.

The chalk thickens steadily south-westwards from the West Central Shelf across the Mid North Sea High (see (Figure 45)). It is generally conformable with the underlying Lower Cretaceous sediments, but progressively overlaps older rocks towards the basin margins. The chalk deposited on the West Central Shelf acted as a source for allochthonous chalk deposits laid down within the Central Graben; their resedimentation was probably triggered by earthquakes (Hatton, 1986). Comparison of chalk isopachs in the southern North Sea (Cameron et al., 1992) with those from the report area appears to indicate a thickening into the Cleveland Basin, although inversion there has resulted in its subsequent removal.

The thickness of Upper Cretaceous sediments is not directly affected by the granites of the Mid North Sea High, a fact attributed to the onset of thermal relaxation. Nevertheless, continued buoyancy of the granites may explain why the central North Sea area and the Cleveland-Sole Pit Basin did not subside together until Cenozoic times. Some resistance to subsidence was still maintained across the Mid North Sea High, which may have led to faulting and uplift along the margin of the West Central Shelf.

In common with other major Jurassic basins of the southern North Sea, basin inversion of mid- to Late Cretaceous age is associated with the development of highs such as the Lindesnes Ridge within the Feda Graben in Norwegian waters ((Figure 8); D'Heur et al., 1985; Gowers and Saeboe, 1985). Such features are believed to relate to early Alpine collision of Europe and Africa (Ziegler, 1987). The lack of similar inversion in the UK sector of the central North Sea may be attributed to the offset of earlier Jurassic faults by north-easterly orientated transfer faults. Ziegler (1990a) showed line drawings of seismic profiles that depict minor reversal of Jurassic and Early Cretaceous faults in the south Central Graben, but it is difficult to isolate small-scale reversal of the major graben faults from the effects of renewed Zechstein halokinesis.

Cenozoic

Tertiary subsidence patterns were a continuation of those developed during Cretaceous times in response to lithospheric cooling. The large fault blocks of the Central Graben and the Mid North Sea High no longer acted as major controls on sedimentation. Tertiary faulting was minor, with some of the largest throws found close to the Devil's Hole Horst on the West Central Shelf (Figure 10). Minor salt movement continued throughout Tertiary times, with the development of several diapirs which penetrate into the Neogene succession.

Uplift and volcanism in the north-western part of the British Isles was associated with both Alpine tectonics (Ziegler and Van Hoorn, 1989) and the opening of the North-East Atlantic Ocean. Igneous activity within the central North Sea was restricted to ash falls and the emplacement of the Acklington Dyke Group ((Figure 4); Kirton and Donato, 1985). Along the western flanks of the Central Graben, Danian chalk was uplifted and eroded prior to the deposition of later Paleocene sediments (Knox et al., 1981). The regional uplift to the north-west provided a source for the Paleocene sands which were shed into the central North Sea. South-eastward thinning and shaling-out of these sands is apparent (Knox et al., 1981). Nevertheless, the total thickness of Tertiary rocks (Ziegler, 1975) shows a broadly symmetrical trough with over 3000 m of sediment in the centre.

The Tertiary succession across the Mid North Sea High (Ziegler and Van Hoorn, 1989) thickens in the opposite direction to that of the Upper Cretaceous (see (Figure 45)). This resulted from tectonic inversion of the Cleveland Basin area, and thermal relaxation subsidence in the Central Graben. There is no evidence of significant Oligo-Miocene inversion in either the Central Graben or on its flanks. However, inversion of the Forth Approaches and Cleveland basins probably occurred at this time, leading to erosion of the Cretaceous cover.

Kooi and Cloetingh (1989) have suggested that subsidence in the German and Dutch sectors of the central North Sea during late Neogene time was much higher than the thermally induced subsidence predicted from stretching models. They related this to compression caused by intraplate stress associated with Alpine tectonics. They further suggested that the major depositional sequences identified within the Tertiary succession may also be related to changes in the intraplate stress field, rather than to eustatic changes in sea level.

Subsidence in the central North Sea continued into Pleistocene times, with a particularly thick Quaternary succession preserved above the Central Graben. Salt movement continued, with some faulting of Quaternary sediments in the vicinity of piercement structures.

Chapter 3 Pre-Permian

Pre-Permian rocks crop out along the whole length of the UK coastline bordering the report area, but do not extend far offshore at the surface, as they become covered beneath eastward-thickening Permian, Mesozoic and Cenozoic strata (Figure 3). The pre-Permian geology therefore becomes increasingly difficult to interpret eastwards, although the major onshore structural features, including the Highland Boundary Fault, the Midland Valley Graben, the Southern Upland Fault and the Northumberland Trough (Figure 12), can be traced offshore utilising seismic data (Freeman et al., 1988; Chadwick and Holliday, 1991). (Figure 13) summarises the major Palaeozoic geological events.

Although over 400 exploration wells have been drilled in the report area, only about 50 have proved pre-Permian rocks. Many of these wells are on the Mid North Sea High, or on horsts at the Buchan, Auk, Argyll and Innes oilfields (Figure 12). In general, palaeontological control is very poor, and correlation using regional seismic data is speculative, for reflections from the deep pre-Permian events are generally weak and discontinuous. However, from the scattered well data and regional seismic mapping, a picture emerges of Devonian and Carboniferous basin formation, followed by latest Carboniferous inversion and erosion. These Upper Palaeozoic rocks overlie Lower Palaeozoic basement which has been intruded by major, late Caledonian granites. To the north of the Highland Boundary Fault, late Proterozoic Dalradian metamorphic rocks crop out on land and extend a short distance offshore at or near the sea bed.

Pre-Devonian

Beneath the Devonian, the basement is largely formed of the Lower Palaeozoic, accretionary-wedge, back-arc basin and arc-related sedimentary and volcanic rocks that are exposed in the Southern Uplands (Stone et al., 1987). These extend northeastwards beneath the central North Sea, with Dalradian rocks to the north of the Highland Boundary Fault. The Lower Palaeozoic sediments were extensively deformed during the Caledonian orogeny, which marked the closure of the Iapetus Ocean during collision between the continental masses of Laurentia, Baltica and Avalonia (Coward, 1990). Subduction at the Iapetus convergence zone (Chadwick and Holliday, 1991) began in early Ordovician times, when the Iapetus Ocean had a latitudinal width of about 5000 km (Torsvik and Trench, 1991), and final closure took place during the late Silurian. The Iapetus convergence zone, or suture, as well as the junctions between the Norwegian Caledonides, the Scottish Caledonides and the western extension of the Tornquist Zone (the Trans European Fault of the EUGENOS Working Group, 1988), all occur within the report area. However, the wide range of models proposed (cf. Cartwright, 1990; Coward, 1990) is an indication of the uncertainties regarding the deep structure and its development.

Freeman et al. (1988) and Klemperer and Hurich (1990) recognised, on several offshore deep-seismic lines, a northerly dipping reflector which is likely to be associated with the Iapetus suture. Their mapping places the suture along the Mid North Sea High, where it would intersect the westerly extension of southerly splays of the Tornquist Zone, near the Central Graben. The Central Graben may also follow a major Palaeozoic structural boundary, extending the Tornquist Zone to the north-west as a series of shears (Cartwright, 1990).

Well 30/16-5, drilled close to the crest of the Auk Ridge (Figure 12), proved schist underlying Middle Devonian limestone. The schist gave a Caradoc ⁴⁰Ar-³⁹Ar date of 453 ±5 Ma (Frost et al., 1981; Harland et al., 1990); several phases of deformation were identified, but the depositional age of the sediment is unknown. The occurrence of pre-Devonian rocks on the Auk Ridge close to the base-Cretaceous unconformity is the result of footwall uplift on the margin of the Mesozoic graben.

Two wells in the Forth Approaches Basin may also have drilled pre-Devonian rocks (Figure 12). Well 26/14-1 proved indurated greywacke and slaty mudstone lying unconformably below 80 m of ?Early Devonian sediment. The lower sequence is dated as late Silurian (oil-company log), and the lithologies are comparable with Silurian/Ordovician greywackes of the Southern Uplands. Well 26/12-1 bottomed in altered olivine-basalt dated on the company log as late Silurian to Early Devonian; similar volcanic rocks onshore in the Midland Valley are dated as Early Devonian (Cameron and Stephenson, 1985).

Lower Palaeozoic sediments occur on the Devil's Hole Horst (Figure 12), which remained an active tectonic high until early Tertiary times. Thick sequences of indurated shales, sandstones, greywackes and conglomerates were proved in wells 27/3-1 and 27/10-1. Tremadoc acritarchs of Avalonian and Baltican aspect, including Acanthodiacrodium, were found in a black shale bed within a conglomeratic section in well 27/3-1 (S G Molyneux, written communications, 1986 and 1987). Much of the section in well 27/3-1 is red and devoid of fauna; this may be an indication that the acritarchs are reworked within ?late Silurian or Devonian sediments. Seismic evidence suggests that the Devil's Hole Horst lies to the north of the Iapetus suture, and would thus be of Laurentian origin (Klemperer and Hurich, 1990), However, Chadwick and Holliday (1991) have suggested that the strong seismic reflector interpreted as the Iapetus suture may 'crop out' to the south of the suture as deduced from faunal evidence, and would therefore extend through Avalonian crust in its upper part.

Devonian

Following Caledonian orogenesis and the closure of the Iapetus Ocean in late Silurian times, the Devonian was predominantly a period of erosion of the new mountain range in north-west Europe; continental, terrestrial and lacustrine (Old Red Sandstone) sedimentation took place in a series of major depocentres. In the east of the central North Sea, a Mid-Devonian marine transgression from the Rheic Ocean to the south (Ziegler, 1982) interrupted continental redbed deposition.

The largest Old Red Sandstone basins in the north of the UK are the Orcadian Basin around the Moray Firth, and the Strathmore Basin in the Midland Valley of Scotland. In these basins, sedimentation was controlled by subsidence induced, at least in part, by wrench faulting. Bluck (1984) and Ziegler (1990a) have suggested that basin development was controlled by major sinistral fault movements, and Coward (1990) proposed that there was a major pull-apart basin along the site of the Viking Graben. However, both Rogers et al. (1989) and Thirlwall (1989) have indicated that most of the wrench motion on the Great Glen Fault occurred prior to deposition of the Old Red Sandstone.

The Midland Valley Graben extends offshore as the Forth Approaches Basin, and can be traced north-eastwards to the south of the Peterhead Ridge (Figure 12). The fault on the south-eastern side of the Peterhead Ridge may be a continuation of the Highland Boundary Fault. In the central North Sea, the amount of faulting during deposition of the Old Red Sandstone is not clear; on the southern margin of the Forth Approaches Basin, the faults bounding the Devil's Hole Horst may have been active. Support for this is available from nearby wells 21/26-1D and 28/12-1, both of which bottomed in undated, red-brown conglomerates that may be a locally derived, coarse-grained, Old Red Sandstone facies eroded from the adjacent horst (Figure 12).

Onshore, a threefold division of the Old Red Sandstone (Mykura, 1991) broadly corresponds to the Lower, Middle and Upper Devonian, although it is apparent that Silurian rocks are included in the Lower Old Red Sandstone, and that Carboniferous rocks form part of the Upper Old Red Sandstone (Paterson and Hall 1986; Marshall, 1991). A similar threefold subdivision is possible in the central North Sea. In contrast to the Midland Valley Graben, where the Middle Old Red Sandstone is missing due to contemporaneous uplift (Cameron and Stephenson, 1985), all three subdivisions are represented offshore.

Lower Devonian

Ziegler (1982), in a palaeogeographic reconstruction, showed deposition of Lower Old Red Sandstone facies in the Midland Valley of Scotland and the Northumberland Trough, with a limited extension offshore. A major marine basin existed well to the south, beyond the London–Brabant High, and the intermontane Orcadian Basin was the site of deposition of thick continental beds of the Lower Old Red Sandstone. However, the central North Sea was considered by Ziegler (1982) to have been a high during Early Devonian times. Evidence for this view is limited, but in well 30/16-5 on the Auk Ridge (Figure 12), the Middle Devonian rests directly on probable Lower Palaeozoic basement. Also, seismic evidence from the Mid North Sea High suggests that Middle Devonian limestones may lie not far above the top of the Lower Palaeozoic basement. Furthermore, granites likely to be similar to those found in the Southern Uplands, and of presumed Early Devonian age, have been defined on the Mid North Sea High (Figure 12) from gravity data (Donato et al., 1983). These may have been responsible for the existence of the Mid North Sea High as a long-standing, buoyant region until the Late Cretaceous (Donato et al., 1983), and make it unlikely that there is a thick Lower Devonian sequence on the high. To the west of the Buchan Oilfield (Figure 12), there is another late Caledonian granite, the South Halibut Granite (Andrews et al., 1990), suggesting that this region may also have been buoyant during the Early Devonian.

Thick Lower Devonian rocks are likely to occur immediately south of the offshore continuation of the Highland Boundary Fault (Figure 12), as a continuation of the Strathmore Syncline (BGS Tay-Forth Solid Geology sheet). Between the South Halibut Granite, the Auk Ridge, and the Mid North Sea High, there are only two wells which prove Lower Devonian sediments to be absent. Both are on the Devil's Hole Horst, so the possibility remains that Lower Devonian sediments occur beneath most of the central North Sea. North and west of the Auk Ridge, there is some seismic evidence for possible Lower Devonian rocks between strong reflections from the Middle Devonian limestones and a deeper, pre-Devonian reflector.

Middle Devonian

The Middle Devonian is absent in the Midland Valley and the Southern Uplands (Cameron and Stephenson, 1985), but Middle Devonian marine limestones occur in the Auk/Argyll area (Pennington, 1975). Ziegler (1982) considered the limestones to be the deposits of a marine incursion from the south that may have followed a structural feature exploited by the subsequent Mesozoic rift system.

The limestones reach a thickness of about 80 m in well 38/3-1 (Figure 14), where they contain corals, crinoids and brachiopods. Similar limestones have been proved in several other nearby wells, and although detailed correlation is speculative, the tops of the limestones are marked by major gamma-ray and sonic-log breaks.

The top of the limestones forms a very strong seismic reflector in the vicinity of the Auk and Argyll oilfields, where the age of the event is defined by spores, ostracods, and corals ((Figure 12); Pennington, 1975). The reflector can be mapped northwards beneath parts of the Central Graben, where the limestones are probably present beneath the Montrose High and the Auk Basin. To the south, they may be correlated with a strong reflector mappable over the eastern part of the Mid North Sea High, although the limestones may be absent on most of that high.

Cameron et al. (1992) suggested that a strong reflector recorded beneath the base of well 44/2-1 (immediately south of the report area) which bottomed in Famennian sandstones and shales, may be correlated with the Middle Devonian limestones. This reflector can be mapped northwards, and forms the base of an onlapping sequence of reflectors where it was tested on the margin of a basin at well 37/12-1, which terminated just above the lowest strong reflector (Figure 15). The section of well 37/12-1 proved that irregular reflectors immediately above the strongest reflector represent undated, interbedded, anhydritic shale and limestone that appear very similar to the Lower Carboniferous of well 44/2-1. The strong reflector could therefore represent either the Middle Devonian limestones or a near base-Carboniferous/Upper Devonian unconformity. However, as it can be mapped over the Dogger Granite (Figure 12), it is more likely to image an unconformity at the top of Lower Palaeozoic basement.

In well 38/3-1, limestone is overlain by 105 m of dark grey, dolomitic and carbonaceous mudstone. This succession may be equivalent to a similar thickness of red-brown clay-stone in well 30/16-5, and argillaceous, dolomitic and anhydrite successions in wells 30/24-3 and 30/25a-2 (Figure 14). The tops of these successions are placed close to the Middle to Upper Devonian boundary on the basis of ostracods and spores (company composite logs). The lithological and colour contrast between these sequences above the limestones may be due to primary depositional differences. However, the rocks in well 38/3-1 underlie particularly thick Upper Devonian sediments, whereas in the other wells the Middle Devonian is closer to the base of the Zechstein, and some secondary diagenetic effects are possible.

Upper Devonian

Most, if not all, of the central North Sea was an area of non-marine deposition during the Late Devonian, for the Upper Old Red Sandstone has been encountered in many wells. It has been proved absent in only two wells in the Forth Approaches Basin, and possibly also on the Devil's Hole Horst. In both cases, its absence may be due to subsequent erosion. Onshore in the Midland Valley and the Southern Uplands, Late Devonian fluvial systems drained eastwards towards the central North Sea and Mid North Sea High (Mykura, 1991).

In many wells, it is difficult to recognise the boundary between Old Red Sandstone and overlying red sandstones of the Lower Permian. This is particularly the case where the Lower Permian is of fluvial facies, as is found around the Argyll Oilfield (Bifani et al., 1987; Robson, 1991). Generally, the boundary is taken at a marked downhole increase in sonic velocity and a gamma-ray change to more irregular and higher values in the Old Red Sandstone (Figure 16). The maximum depositional thickness of the Upper Old Red Sandstone is unknown because of subsequent erosion and an absence of seismic reflectors at its base. However, around the Buchan Oilfield the Upper Old Red Sandstone is at least 675 m thick (Richards, 1985; 1990a), and in well 38/3-1, on the northern flank of the Mid North Sea High (Figure 12), over 1300 m of strata are probably of Late Devonian age.

Richards (1985; 1990a) has shown that around the Buchan Oilfield the overall sequence fines upwards, with an increasing number of siltstone and cornstone (palaeosol) beds towards the top. On a smaller scale, there are numerous upward-fining sand to silt cycles that are up to 10 m thick, but which are generally less than 1 m thick. The thicker units are interpreted as mobile, fluvial-channel deposits, whereas the thinner units are considered to be waning-flow, sheetflood deposits. The petrography of the sandstones suggests a granitic source; Richards (1990a) has shown that the South Halibut Granite was exposed during Devonian times, and could have formed a local source.

Upper Old Red Sandstone sediments of similar facies occur ir wells throughout the central North Sea. The sandstones are largely medium to fine grained, with more interbedded shales than around the Buchan Oilfield, suggesting that the central North Sea was a basinal area with relatively mature fluvial systems. The presence of coals in a sequence dated as Frasnian in well 38/3-1 is further evidence for a region of low relief. Gamma-ray log cycles are of the order of 10 m thick, and although many suggest upward-fining sequences, there are also irregular and apparently upward-coarsening sequences (Figure 17).

On the Mid North Sea High, the thickness of the Upper Old Red Sandstone is variable; the basins shown on (Figure 12) are based on regional interpretation of seismic data, and since dating is poor, they may include significant thicknesses of Lower Carboniferous sedimentary rocks (Figure 15). The locations of the basins may be primarily controlled by the presence of adjacent Devonian granites, which tend to form highs; indeed the high tested by well 37/12-1 may be underlain by a north-westward extension of the Dogger Granite ((Figure 12) and (Figure 15)).

Although basin formation began in Late Devonian times, sedimentation continued into the Early Carboniferous. On the southern flank of the Mid North Sea High, well 44/2-1 (Besly, 1990; Cameron et al., 1992) penetrated the upper part of the Old Red Sandstone sequence, which is dated as Early Carboniferous with Devonian at the base. Well 38/3-1 proved 1300 m of Upper Devonian (Frasnian) redbeds with thin coals, although the upper part of this sequence is not dated. Leeder and Hardman (1990) suggested that the more than 893 m-thick redbed sequence in well 37/10-1 (Figure 12) may also be Lower Carboniferous rock of Old Red Sandstone facies. At the Buchan Oilfield, deposition of Upper Old Red Sandstone facies has been shown to have persisted into Early Carboniferous times (Hill and Smith, 1979), as is the case in the Midland Valley Graben (Paterson and Hall, 1986). It is probable that similar sedimentation patterns occurred throughout the central North Sea.

Carboniferous

Although there are some thin limestones and anhydrites within the ?Late Devonian/early Tournaisian succession on the Mid North Sea High, it was in late Tournaisian times that there was a major change in facies from the predominant redbeds of the Old Red Sandstone. With the onset of regional crustal extension came overall subsidence that allowed the sea to transgress from the south, so that during much of Carboniferous time, fluvio-deltaic and shallow-marine environments prevailed.

It has been suggested that much of the Southern Uplands was above sea level during the Carboniferous (Cameron and Stephenson, 1985). Palaeogeographic reconstructions (Fraser and Gawthorpe, 1990; Leeder and Hardman, 1990) show a major northern fluvial source for the deltaic and marine sediments of the southern North Sea and northern England. The central North Sea may have been the source of these sediments by erosion of the Old Red Sandstone, but the widespread preservation of Upper Devonian sediments across the central North Sea is evidence that it may have been a relatively low-lying area of sediment transport from north to south. Some parts may have had locally higher topography, in particular the Devil's Hole Horst, and perhaps the offshore extension of the Southern Uplands. Pre-Permian uplift and erosion has restricted the preserved occurrence of Carboniferous sediments to two regions (Figure 12). The first, in the north, is an offshore extension of the Midland Valley Graben into the Forth Approaches Basin; the second, in the south, is the continuation of the Northumberland Trough and the Tweed Basin along the southern flank of the Mid North Sea High.

Forth Approaches Basin

In the Midland Valley of Scotland, a variably thick, fault-controlled, Dinantian sequence is generally overlain conformably by thinner Namurian and Westphalian sediments (Cameron and Stephenson, 1985). Beyond the coast, these are best preserved in a series of synclines which continue offshore into the Forth Approaches Basin; they include a north–south-trending Westphalian syncline that crosses the Firth of Forth (BGS Tay–Forth Solid Geology sheet). Onshore, much of the sequence is coal bearing, although marine influences reached a maximum near the end of Dinantian times and during the Namurian, with the deposition of the Lower and Upper Limestone formations. In West Lothian, a series of oil shales was deposited during the Dinantian. Volcanism was prevalent in the west of the Midland Valley, but also occurred in the Firth of Forth area. Read (1987) has contrasted the tectonics of the Midland Valley Graben with those of the Carboniferous basins of Northern England, suggesting that dextral strike-slip tectonics was an important element in the Midland Valley. The Midland Valley has also been interpreted as an extensional basin formed above art earlier, remnant, forearc basin (Leeder, 1982).

Visean to Namurian coal-bearing strata have been drilled in the Forth Approaches Basin and south of the Buchan Horst. The Carboniferous rocks may extend north-eastwards beneath the Fisher Bank Basin and continue to the north of the Jaeren High (Figure 12). Thin Carboniferous strata may be preserved locally to the south-west of the Devil's Hole Horst, across the offshore continuation of the Southern Uplands, and beneath the Auk Basin (Figure 12), but in these regions there are no wells reaching pre-Permian strata.

In the west of the report area, there are several BGS and coal-exploration boreholes (BGS Tay–Forth Solid Geology sheet) that can be used to extend the onshore stratigraphy offshore. However, away from the coast, there are too few wells to allow the detailed stratigraphy to be applied, and the entire coal-bearing sequence is combined into one unit. The Carboniferous in the Midland Valley is very variable in thickness due to synsedimentary faulting and folding (Cameron and Stephenson, 1985), and it is probable that similar controls operated offshore. It is possible that the faulted southern margin of the Buchan Horst and the Devil's Hole Horst faults were active during the Carboniferous.

The thickest section is in well 26/7-1 (Figure 12), where over 1000 m of Visean sediments were encountered. Wells 21/11-1, 21/12-2B and 21/13b-1A (Figure 12), on the south side of the Buchan Horst, all have coal-bearing successions dated as late Visean and ?early Namurian; in none of these wells has the base of the Carboniferous been reached. Similar sequences of Visean to Namurian age occur in the Moray Firth north of the Highland Boundary Fault where a total thickness for the coal-bearing sequence has been estimated as at least 1500 m (Andrews et al., 1990). This contrasts with central Scotland, where Carboniferous strata are barely preserved north of the fault.

The coal-bearing sequence consists of sandstones interbedded with grey, carbonaceous shales, and coals. The sandstones make up about half the sections, but individual beds are rarely more than 30 m thick. Coal seams reach a maximum thickness of about 3 m in well 21/12-2B (Figure 18), although there are many more coals in well 26/7-1. In the latter well, a lower, more sandy unit is differentiated from an upper unit that contains more thin coals, limestones and thicker mudstones.

In the Moray Firth, volcanic rocks of ?Visean age, similar to those found in the Midland Valley, are interbedded with the coal-bearing sequence (Andrews et al., 1990). Thin, tuffaceous, volcanic rocks occur in a thin Lower Carboniferous succession in well 21/2-1 on the Buchan Horst (Figure 12).

Mid North Sea High and its southern flank

The Northumberland Trough can be mapped offshore (Figure 12) as an asymmetrical, southerly dipping half-graben.

The formation of the trough has been linked to extension and subsidence in the hanging wall of the Iapetus convergence zone (Chadwick and Holliday, 1991). The Carboniferous thins eastwards, and over much of the Mid North Sea High it is thin or locally absent. Marine and deltaic sequences extend across the southern flank of the Mid North Sea High, where they have been proved in several wells, including 44/2-1 (Cameron et al., 1992), 36/13-1, 38/16-1, 38/18-1 and 38/25-1 (Figure 12). Carboniferous rocks are absent in well 38/29-1, which proved latest Devonian strata beneath the Permian. The sequence in well 38/16-1, which overlies the Dogger Granite, has a thickness in excess of 262 m and has been compared with the Scremerston Coal Group onshore (Leeder and Hardman, 1990).

The basins that originally developed during the Late Devonian on the Mid North Sea High continued to be active in Carboniferous times. Dinantian and Namurian sediments extend around the margins of the Northumberland Trough over the western part of the Mid North Sea High. These include the deposits of the Tweed Basin (Figure 12), but they are perhaps thinly developed over a wider area. Deposition of Carboniferous sediments along the southern flank of the Mid North Sea High was possibly affected by movement along easterly trending faults that throw down to the south (Leeder and Hardman, 1990).

In the extreme east of the report area (Figure 12), Namurian coal-bearing sediments occur in well 39/7-1, and a similar Visean to Langsettian (Westphalian A) sequence was proved in well 31/27-1 (company composite log). This is an indication that the Central Graben may have existed as a Carboniferous basin, although seismic mapping of the Middle Devonian limestone within the graben to the north suggests that such a basin would have been inverted and eroded prior to deposition of Lower Permian sandstones.

Chapter 4 Permian

Two major depocentres, termed the Southern and Northern Permian basins, developed in the North Sea during Permian times; they were separated by the east–west-trending Mid North Sea–Ringkøbing-Fyn High. Glennie (1990a) observed that the two basins and the separating high are orientated parallel to structures in the Variscan Foreland, and he attributed basin formation to north-westerly oriented transtension and associated collapse of the Variscan orogenic pile during the Early Permian.

The Northern Permian Basin covered much of the area now occupied by the central North Sea; it extended northwestwards into the Moray Firth, northwards into the northern North Sea, and eastwards as far as Scandinavia. Ziegler (1990b) suggested that there was no Early Permian link between the Southern and Northern Permian basins, as the Central Graben did not develop until Triassic times.

However, both Glennie (1990a) and Hamar et al. (1980) have proposed that rifting of the Central Graben may have started in the Early Permian, with the possible development of a connection between the two basins along this rift. There was probably a connection between the Northern and Southern Permian basins across most of the Mid North Sea–Ringkøbing-Fyn High throughout Late Permian times.

The Lower Permian succession in the Northern Permian Basin comprises dominantly red, clastic sediments deposited in a range of continental-desert, fluvial, and sabkha environments. Succeeding Upper Permian sediments were deposited following rapid transgression of the low-lying continental basin; they comprise evaporites and carbonates. A suite of Early Permian volcanic rocks is recorded in the south-eastern part of the central North Sea (Figure 19).

Lower Permian (Rotliegend)

The fluvial, aeolian and sabkha sediments of the central North Sea are essentially similar to the Rotliegend sediments of the southern North Sea (Glennie, 1990a; Cameron et al., 1992). However, the Northern Permian Basin was probably somewhat smaller than the Southern Permian Basin, and the distribution of Rotliegend sediments is therefore relatively limited in the central North Sea. Rotliegend sediments may be absent from most of the Mid North Sea High (Figure 19), and probably onlap both its southern and northern flanks; there is no evidence of major faulting.

Rotliegend sediments have been penetrated in many wells in the Central Graben, over parts of the West Central Shelf, and in the northern part of the Forth Approaches Basin. The succession is generally thickest in the area of the Auk and Clyde oilfields in the southern part of the Central Graben, where over 525 m of Lower Permian sandstones have been drilled. Around 290 m of Rotliegend strata are found over the Forties-Montrose High, but the succession thins to between 90 m and 150 m across much of the West Central Shelf and into the Forth Approaches Basin.

Two lithostratigraphical units, reflecting sandstone or claystone dominance, are recognised in the Lower Permian of the central North Sea. Sandstone-dominated successions constitute the Auk Formation; these occur throughout the Central Graben, over the southern part of the West Central Shelf, and in the southern part of the Forth Approaches Basin (Figure 19). Claystone-dominated successions are referred to the Fraserburgh Formation; these are found over the northern part of the West Central Shelf and in the northern part of the Forth Approaches Basin. These two lithostratigraphical units are broadly equivalent to the Leman Sandstone and Silverpit formations in the southern North Sea (Cameron et al., 1992), and similarly interdigitate locally.

The mechanism controlling the distribution of sand and clay deposition in the Northern Permian Basin is uncertain. The rate of subsidence in the Permian basins was probably greater than the rate of sediment supply, such that by the end of Early Permian times, the depositional surface may have been some 250 m below sea level (Smith, 1970; Ziegler, 1982; Glennie, l990a).

Auk Formation

The Auk Formation comprises very fine- to coarse-grained, reddish brown to grey, massive or partly laminated sandstones. It overlies Old Red Sandstone with marked unconformity in places, as in the type well 30/16-1 (Figure 19), although it is also recorded above coal-bearing Carboniferous successions, as in well 26/7-1 (Figure 20).

A basal conglomerate is recorded locally in the area of the Auk Oilfield; it contains clasts of quartz and schist in well 30/16-1, and quartz, quartzite and basalt clasts are present in well 30/16-8 (Figure 20). Heward (1988; 1989) noted that the conglomerates were possibly deposited in topographic hollows on the post-Old Red Sandstone unconformity surface, and Glennie (1990a) suggested that they may have been derived by erosion of adjacent fault scarps during rotational tilting of small half-grabens on the flanks of the Central Graben.

Although dominantly sandy, the Auk Formation Facies vary markedly where they are most thickly developed around the Auk Oilfield. Heward (1989) subdivided the formation above the basal conglomerate in the Auk Oilfield into four units: aeolian slipface sands; an interval with a greater dominance of wind-ripple, laminated sandsheet together with dry interdune and dune-apron sands; a unit comprising a greater proportion of wind-ripple laminated deposits; and an upper, waterlain, nonmarine, mass-flow facies representing rainfall reworking of an abandoned erg. The waterlain mass-flow deposits can be correlated with the deformed and reworked Weissliegend sandstones common at the top of the Rotliegend succession in the Southern Permian Basin.

In contrast to the succession at the Auk Oilfield, in which aeolian sandstones are overlain by waterlain deposits, Bifani et al. (1987) described the Auk Formation of the Argyll Oilfield as dominantly waterlain. Here, fluvial and desert-lake deposits are overlain by aeolian dune sandstones. Bifani et al. (1987) subdivided the Auk Formation into six environmental units comprising, from the base upwards: lake-margin dune field; desert lake with sheetfloods and isolated dunes; alluvial plain; braided wadi fan; damp interdune; and aeolian dune. Sedimentation is thought to have been influenced by syndepositional tectonics, resulting locally in significant lateral and vertical facies variability.

Bifani et al. (1987) suggested that much of the sediment at the Argyll Oilfield was derived from the adjacent Mid North Sea High; peneplanation of the high coincided with the onset of aeolian deposition from the north-west in late Early Permian times. It is probable that much of the Lower Permian sediment in the central North Sea was derived by erosion of basement rocks then exposed over the Fennoscandian Shield and Scottish Highlands. However, Robson (1991) has suggested that the sediments below the aeolian dunes could be of Devonian, rather than Permian, age; it is difficult to unequivocably differentiate Devonian and Permian sediments in such settings. Glennie (1990b) suggested that there was also extensive erosion, before later Early Permian sedimentation, of a lowermost Permian volcanic pile in the eastern parts of the North Sea. The denudation products from this thermally uplifted area may have contributed to the sediment budget in the report area.

Fraserburgh Formation

The claystone-dominated Fraserburgh Formation appears to be largely restricted to the northern Forth Approaches Basin and the northern margins of the West Central Shelf (Figure 19). It therefore separates the sand-filled troughs of the Central Graben and southern Forth Approaches Basin. It typically comprises a succession of dark grey to reddish brown, hard, dolomitic claystones with thin dolomites, sandstone stringers and anhydrite nodules, as in well 21/11-1 (Figure 20). Deposition probably occurred in dune-bordered sabkhas and desert lakes, in a setting possibly similar to the depositional environment recorded for the Silverpit Formation in the Southern Permian Basin (Glennie, 1972; Cameron et al., 1992).

Upper Permian (Zechstein)

Shales, carbonates and evaporites were deposited across the central North Sea following a rapid transgression over the desert surface at the start of the Late Permian. The Zechstein transgression was probably facilitated by a rise in global sea level, with waters flooding into the Northern Permian Basin from the north along the proto-Atlantic and northern North Sea rifts (Glen nie, 1990a). Zechstein deposits occur extensively in the report area (Figure 21); they are recognised in BGS boreholes (Evans et al., 1981) in the west, where their outcrop has been mapped (BGS Solid Geology sheets), and occur widely at depth farther east (Figure 3). Generally, the Zechstein overlaps the Rotliegend to rest on pre-Permian rocks towards the basin margins (Figure 20).

The initial deposits of the Zechstein transgression in both the Northern and Southern Permian basins are termed the Kupferschiefer (copper-shale). The Kupferschiefer of the central North Sea generally comprises 1 m of dark grey, sapropelic shale distinguished on well logs by its high gamma-ray spike (Figure 22). These shales probably formed under anoxic conditions, and were deposited as a drape over the Rotliegenc dune surfaces.

The Kupferschiefer passes abruptly upwards into carbonates, thus forming the basal section of a lowermost Zechstein clastic-carbonate-anhydrite-salt cycle. Up to six such cycles, termed Z1 to Z6, are recorded in the North Sea (Taylor, 1990). According to Taylor (1990), only cycles Z1, Z2, and an amalgamated Z3/Z4/?Z5 unit, are identifiable across much of the central North Sea.

Each idealised Zechstein cycle represents an upwards increase in salinity caused by evaporation following marine incursion of an enclosed basin. Such a cycle comprises a thin, basal, elastic unit that passes up to limestones and dolomites, followed by anhydrites, halites and highly soluble potash and magnesium salts. Not all the cycles follow this ideal profile, and many variations are observed. The carbonates have a wider area of distribution than succeeding anhydrites, and the extent of halites and soluble salts at the tops of the cycles is generally restricted to the more central parts of the basin. Three models are in part applicable to the North Sea Zechstein (Taylor, 1990): the cycles may have formed in an evaporating deep basin initially containing deep water, or in a shallow-water deep basin, or even in a shallow water, shallow-basin setting.

Although up to five Zechstein cycles can be recognised in the central North Sea, there is no formally defined subdivision of the succession on this basis. Coupled with an absence of a halite unit in many of the cycles, this means that correlation with individual cycles in the Southern Permian Basin is not always possible. The cycles described here are nonetheless informally termed Z1 to Z5. Deegan and Scull (1977) formally subdivided the Zechstein in the northern part of the central North Sea by erecting a 'Marginal Sequence' comprising a carbonate-dominated Halibut Bank Formation, and an overlying, anhydrite-dominated Turbot Bank Formation. Deegan and Scull (1977) also identified a dolomite-dominated succession at the Argyll Oilfield that they termed the Argyll Formation.

Some of the relatively thin Zechstein successions that Deegan and Scull (1977) attributed to the Halibut Bank and Turbot Bank formations in the northern part of the report area appear to correspond largely with cycles Z1 and Z2, but with Z3 also developed in places. Wells 22/11-1 and 22/18-4 (Figure 22) illustrate this correlation, with all three cycles seen in well 22/18-4, but only cycles Z1 and Z2 identified in well 22/11-1. In well 22/11-1 the two cycles comprise dolomite-anhydrite couplets, with no apparent development of basal elastic beds in any of the cycles. The succession is more complex in well 22/18-4: cycle Z1 comprises argillaceous limestone passing up to limestone; Z2 is made up of argillaceous limestone capped by an 11 m-thick anhydrite; and the 89 m-thick Z3 cycle consists of argillaceous and dolomitic limestone passing up to limestone, dolomite and a 48 m-thick anhydrite containing a thin dolomite bed.

Zechstein successions similar to those in the north of the report area are recorded around the Argyll and Duncan oilfields (Figure 21), where more than three cycles may be present in some wells. In well 30/24-15 (Figure 22), it is possible to subdivide the Zechstein into Halibut Bank and Turbot Bank formations based on overall carbonate/anhydrite content. It is also possible to recognise four cycles; Z1 comprises 16 m of dolomite overlain by a highly radioactive, possibly sapropelic shale, and passes up through anhydritic claystone to anhydrite. The second and third cycles are amalgamated; they begin with a 3 m-thick claystone and pass up to anhydrite. The fourth cycle is composed of anhydritic claystone overlain by anhydrite. A similar succession is observed in well 30/25-3, where four cycles may also be discerned (Figure 22).

It may be possible to identify a number of cycles within the dolomite-dominated Argyll Formation-type successions at the Argyll Oilfield, based on the presence of sapropelic/dolomitic claystone beds separating peritidal carbonates (Bifani. 1985). In well 30/25-1 (Figure 23), the Z1 cycle may comprise the basal Kupferschiefer, plus the succeeding 8 m of dolomite, and the high-gamma sapropelic claystone, with most of the dolomite forming the remainder of the succession. The high gamma-ray peak near the top of the succession in well 30/25-1 may, however, correlate with a similar peak defining a dolomitic claystone in nearby well 30/24-2, and may mark the base of a second Zechstein cycle.

None of the cycles described above contain either halite or the potash/magnesium-rich salts that form the upper parts of idealised Zechstein cycles. This is possibly a result of Triassic or later salt withdrawal. However, the Zechstein successions between the Auk/Argyll region and the northern part of the central North Sea contain significant thicknesses of halite. The presence of thick halite intervals in this 'basin' area (Taylor, 1990) can make it difficult to differentiate the cycles discussed above, particularly where there has been much salt mobilisation. Even where some cycles can be differentiated in areas of halite preservation, it can be difficult to assign particular halites to specific cycles. For example, in well 21/11-1 (Figure 24), the lower 192 m of halite can be assigned to the Z2 cycle, and therefore equates with the Stassfurt Halite of the Southern Permian Basin, but the remainder of the halite-bearing section could comprise cycles Z3 to Z5. A similar situation is encountered other wells, such as 21/26-1 (Figure 24), where it is possible to assign the three halite intervals, separated by carbonates and anhydrites, to cycle Z2 and an amalgamated Z3 to Z5 interval. Wells 21/11-1 and 21/26-1, in common with many others in this area of salt preservation, display a correlatable anhydrite bed at the tops of their successions.

Thick halites have also been found in the Zechstein of the Forth Approaches Basin. Well 26/7-1 (Figure 24) has halites developed in the upper part of cycle Z2, and in an amalgamated Z3 to Z5 succession. The halites and associated clasticcarbonate-anhydrite intervals identified in the Forth Approaches Basin are similar to those farther east on the West Central Shelf, such as in well 21/26-1 (Figure 24), suggesting a common origin for the cycles developed in the two areas.

The mobilisation of Zechstein halite during the Triassic and Jurassic periods has resulted in the development of many salt walls, pillows, diapirs, and zones of salt withdrawal. The orientation of the major salt walls varies (Figure 21): in the Forth Approaches Basin they have a north-easterly trend, on the flanks of the Mid North Sea High the alignment is east–west, and in the West and East Central grabens the salt walls are parallel to the Mesozoic graben-bounding faults. The orientation of some salt walls may be related to minor movements on Triassic faults, particularly in the Forth Approaches Basin and the East Central Graben. Elsewhere, salt-wall orientation may be controlled by the positions of faults at sub-Permian level. Diapirs appear to be less widespread than salt walls, and are confined to the West and East Central grabens (Figure 21).

One of the many consequences of halokinesis has been the alteration and masking of original depositional thickness trends. The accumulation of mobilised halite into diapirs and salt walls has resulted in the presence of many hundreds of metres of Zechstein in some wells, with nearly no preservation of the succession in adjacent wells. (Figure 25) illustrates an example of Zechstein thickness variation over a distance of a few kilometres, and (Figure 21) shows the maximum thickness of the Zechstein in different parts of the report area.

The Mid North Sea–Ringkøbing-Fyn High was probably in existence throughout Permian times, and may have completely separated the two basins during Early Permian times. However, much of it was covered by a shallow, carbonate sea during the Late Permian, for up to 800 m of Zechstein sediments are recorded over the high. Taylor (1990) noted the presence of anhydrites equivalent to the Werraanhydrit directly above Devonian and Carboniferous strata, respectively, in wells 38/29-1 and 38/16-1 ((Figure 21)). This suggests that at least part of the Mid North Sea High was emergent until the later stages of the Z I cycle.

Jenyon et al. (1984) suggested that during Late Permian times there were relatively deeper-water links between the Southern and Northern basins; one lay along the Central Graben where it transects the Mid North Sea High, and another along a 'channel' feature which transected the high farther west. Smith and Taylor (1989) also identified a relatively deep-water, Late Permian connection linking the two basins to the east of the Firth of Forth and Tay Estuary via the present-day Forth Approaches Basin. Thick halites were deposited in these channels through the high; Jenyon et al. (1984) and Jenyon (1988) described the edges of some of these salt-filled channels as 'salt slopes' caused by halite dissolution during the Early Cretaceous. One of these dissolution fronts (Figure 26) can be traced running roughly north–south through the south-eastern part of the report area (Figure 21). Jenyon et al. (1984) attributed the formation of such dissolution fronts to the presence of undersaturated formation waters flowing westwards within the basin.

Volcanic rocks

Tuffs, tuffaceous claystones and basaltic lavas/intrusives are interbedded with subordinate Lower Permian sandstones and shales in a few wells in the south-eastern part of the report area (Figure 19), where the Central Graben cuts through the Mid North Sea–Ringkøbing-Fyn High. One of these wells, 31/26-3 (Figure 27), displays a volcanic unit more than 180 m thick; it comprises at least 79 m of altered basaltic lava, overlain by 27 m of volcanic sandstone and poorly sorted conglomerate, capped by 73 m of tuff interbedded with 2 m-thick claystone beds. The basaltic lavas/intrusives in the six wells are purple to greenish, generally altered, and contain olivine and pyroxene crystals that in some cases fill vesicles. The tuffs are commonly reddish purple, hard, cindery, and contain glassy shards and small feldspar laths.

Five of the six wells terminate either within the volcanic succession or in an underlying, undated, sedimentary unit recorded as possibly Carboniferous on company logs. Although all are overlain by the Zechstein, it is difficult to assess the exact stratigraphical position and relative age of the volcanic unit, and no radiometric ages have been published. However, in well 31/26-1 (Figure 19), the volcanic rocks occur below a 100 m-thick succession of Rotliegend sandstone and claystone, implying extrusion some time before the end of the Early Permian. Ages assigned to the volcanic rocks on company logs are generally Late Carboniferous/Early Permian. Glennie (1990a; b) has argued that these volcanic rocks, like those in Germany, Poland and the Oslo-Bamble-Horn Graben, and the Whin Sill in Northumberland, are entirely Early Permian in age, and genetically related to rifting induced by a stress system associated with opening of the proto-Atlantic Ocean.

Minor basaltic and/or tuffaceous layers interbedded with Lower Permian sediments are also known from wells farther to the north-west (Figure 19). Well 30/24-10 has 29 m of red sandstone and claystone underlain by a 15 m-thick, altered olivine-basalt. In well 30/25-3, the uppermost 51 m of the more than 262 m-thick sandstone-dominated Rotliegend succession comprises siltstone and thin sandstones with mottled tuffs.

Wells with lamprophyres and/or basalts interbedded with Zechstein sediments have also been reported to the northwest of the main development of volcanic rocks (Figure 19). Recent re-examination of these rocks (R W O'B Knox, oral communication, 1992) has revealed that these are all intrusive in character, and were probably emplaced during Jurassic magmatism, which is known to have occurred in the same area (Smith and Ritchie, 1993).

Chapter 5 Triassic

Triassic strata occur over most of the central North Sea and crop out near the western limit of the report area. (BGS Solid Geology sheets), although they are notably absent over the eastern part of the Mid North Sea High (Figure 28). Sandstone-dominated successions, where the proportion of sandstone is greater than 50 per cent, occur preferentially in the East and West Central grabens, locally on the West Central Shelf, around the northern flank of the Forties-Montrose High, and in parts of the Fisher Bank Basin. Elsewhere, Triassic rocks are dominantly claystones.

The thickest successions of about 2200 m are in the East Central Graben (Figure 28), although the thickness there is only slightly greater than the 1800 m in the West Central Shelf. The central North Sea Triassic basin occupied approximately the same site as the Northern Permian Basin; consequently, both depositional and preserved thicknesses have been influenced by halokinetic movements, which probably started in the Early Triassic. Triassic thickness distribution may have also been controlled to some extent by extensional and rifting processes, but because of the halokinetic movements it is difficult to determine the effects on sedimentation of any fault-controlled extension during the Triassic. The overprint of later, especially Late Jurassic, faulting may have also considerably masked any effects of faulting during Triassic times.

The main axes of Triassic deposition (Figure 28) run parallel to the linear salt walls and diapirs which cross the report area (Figure 21), and similarly vary in orientation in the different structural compartments. In the Forth Approaches Basin, they have a north-easterly trend; on the flanks of the Mid North Sea High the alignment is east–west; in the West and East Central grabens, the salt walls are parallel to the graben-bounding faults, and the Triassic depocentres therefore trend to the north-west.

Evidence of major Triassic rifting is equivocal, other than adjacent to the Mid North Sea High where it is transected by the Central Graben (Skjerven et al., 1983). Hamar et al. (1980) suggested that the Central Graben may have been initiated as an area of rifting during the Early Permian, and continued as an active depocentre throughout the Triassic. Thorne and Watts (1989) proposed that there may have been a major phase of extension of the Central Graben during Late Permian/Early Triassic times, followed by thermally controlled regional subsidence without evidence of major normal faulting. This hypothesis is similar to the tectonic models derived for the northern North Sea by Badley et al. (1988), and for the Dutch central North Sea by Kooi et al. (1989). There may therefore have been some common control on the timing of early Mesozoic extension in all of these basins.

Fisher and Mudge (1990) have argued that the relatively fine-grained nature of the bulk of Triassic sediments in the Central Graben is not compatible with major rifting activity, since extensive faulting would tend to produce coarser-grained clastic sediments. However, widespread Permian halite deposits in the area somewhat precluded it being a significant source area for coarse-grained clastic material. Coarser-grained and particularly thick deposits are found to the east, in the Egersund and North Danish basins, adjacent to the uplifted Fennoscandian Shield which contributed the sediment. These were the major Triassic depocentres of the region, and were subjected to major Triassic fault-controlled subsidence following initial Permian rifting (Skjerven et al., 1983).

Because the central North Sea Triassic basin occupied the same position as the Northern Permian Basin, virtually every complete succession recorded rests on Zechstein sediments, usually evaporites. The only exception is in well 31/27-1 where the Zechstein is faulted out, and the Triassic overlies Carboniferous rocks (Figure 28).

Long, linear, Triassic synclines have been formed in zones of incomplete salt withdrawal, such as the Forth Approaches Basin, the West Central Shelf, and the northern part of the Mid North Sea High. Where there is complete salt withdrawal, major Triassic pod structures have become grounded on Zechstein carbonates; these are widespread, but are best developed in the East Central Graben. Johnson et al. (1986) described a mechanism of salt withdrawal and associated subsidence which explains many of the observed Triassic thickness variations, as well as the development of pods (see (Figure 85)). This model envisages the initial deposition of Triassic sediments as saucer-shaped, concave-up pods within halokinetically controlled lows created by salt withdrawal; as salt withdrawal continued through the Jurassic, the Triassic sections became grounded on Zechstein carbonates, and the rim synclines inverted, leaving the convex-up Triassic pods presently observed.

Glennie and Armstrong (1991) also noted that salt withdrawal from earliest Triassic times probably created topographic lows which became infilled with the deposits of extensive lake systems. They further suggested that thick sands may have accumulated along salt-wall solution synclines, demonstrating a direct control of facies distribution by salt migration. Although there is little evidence of major Triassic faulting in the central North Sea, faulting may have initiated some salt diapirism, thereby indirectly controlling Triassic deposition.

Fisher and Mudge (1990) suggested that the influence of salt structures on sedimentation diminished during the Late Triassic, by which time the Central Graben had a southwards regional tilt, with high ground developed somewhere in the region of the south Viking Graben to the north of the report area. An increase in sand progradation into the basin from Carnian times onwards may have been due to uplift of the East European Craton, and the generation of new source areas. However, it might have resulted from an increase in surface run-off as the continental landmass drifted northwards into a latitude of higher precipitation (Fisher and Mudge, 1990).

As in the other North Sea basins, the biostratigraphical resolution of Triassic lithostratigraphical units in the central North Sea is severely hampered by the sparsity of palynomorphs from the dominantly redbed succession. Most wells sampled for biostratigraphical analysis have yielded only sporadic, non-age-diagnostic palynomorphs (Lervik et al., 1989).

The earliest account of Triassic deposition in the UK central North Sea was by Brennand (1975), who described the succession as a reddish brown mudstone facies deposited in a flat, featureless, nonmarine area. Later, Deegan and Scull (1977) defined two major Triassic lithostratigraphical units: the argillaceous Smith Bank Formation which they suggested is distributed across most of the area; and the sandy Josephine Member which they considered to occur near the top of the Smith Bank Formation (Figure 29), and to be restricted to the vicinity of the Josephine Oilfield (Figure 28).

A third unit, the sandy Skagerrak Formation, was proposed by Deegan and Scull (1977) to be restricted to the Norwegian sector.

Deegan and Scull (1977) also noted that the claystonedominated Smith Bank Formation was deposited in a range of distal, continental environments, but argued that the sandstone- dominated Skagerrak Formation was deposited in coalescing and prograding alluvial fans which passed laterally to lakes that were subjected to minor marine incursions. Deegan and Scull (1977) further proposed that the Smith Bank Formation 'distal continental environments' passed laterally eascwards into the Skagerrak Formation alluvial fans.

Jakobsson et al. (1980) suggested a similar facies distribution to that envisaged by Deegan and Scull (1977), noting that most of the central North Sea is dominated by lacustrine/fluvial deposits, with alluvial-fan sediments confined to the eastern margin of the Central Graben. Jakobsson et al. (1980) described the rype section for the Triassic in the Fiskebank Sub-Basin of the Norwegian-Danish Basin (Figure 9) as a succession of repeated, upward-coarsening cycles of alluvial-fan/fluvial deposits overlying sabkha-environment sediments. Similar, relatively minor, upward-coarsening cycles are recorded in the Smith Bank and Skagerrak formations of the Central Graben.

Fisher and Mudge (1990) revised the lithostratigraphy of the central North Sea region; they argued that the units proposed by Deegan and Scull (1977) occur in the UK sector, together with two other units which they termed Bunter Sands and Gassum/Rhaetian Sands (Figure 29).

A generalised lithostratigraphical scheme for the report area can be devised largely on the basis of sand:shale ratios and relative vertical positions. Siltstone- or claystone-dominated units with subordinate sandstones can be termed Smith Bank Formation. Above the Smith Bank Formation, sandstone-dominated beds with interbedded claystones can be classified as Skagerrak Formation; clean, marine-influenced sandstones at the top of the succession can perhaps be related to the Gassum Sands; units of interbedded sandstones and claystones within the Smith Bank Formation can be termed either Josephine Member or Bunter Sands, or referred to the Smith Bank Formation.

Smith Bank Formation

The Smith Bank Formation is a red, dominantly silty clay-stone unit with some sandstones; well 15/26-1 in the outer Moray Firth is the type well for the formation (Andrews et al., 1990). The Smith Bank Formation (Figure 30) probably ranges in age from early Scythian to latest Norian, which spans much of the Triassic Period (Figure 29).

Fisher and Mudge (1990) proposed that the formation was deposited under largely lacustrine or quasimarine conditions, with some evidence of deposition in fluvial, sabkha, and coastal-plain environments. These environments may have persisted throughout the time of deposition, for continuing salt withdrawal during the Triassic led to the creation of large areas of subsidence in which standing water bodies were able to maintain themselves until salt withdrawal ceased and subsidence stopped. A third unit, the sandy Skagerrak Formation, was proposed by Deegan and Scull (1977) to be restricted to the Norwegian sector.

Deegan and Scull (1977) also noted that the claystone-dominated Smith Bank Formation was deposited in a range of distal, continental environments, but argued that the sandstone-dominated Skagerrak Formation was deposited in coalescing and prograding alluvial fans which passed laterally to lakes that were subjected to minor marine incursions. Deegan and Scull (1977) further proposed that the Smith Bank Formation 'distal continental environments' passed laterally eastwards into the Skagerrak Formation alluvial fans.

Jakobsson et al. (1980) suggested a similar facies distribution to that envisaged by Deegan and Scull (1977), noting that most of the central North Sea is dominated by lacustrine/fluvial deposits, with alluvial-fan sediments confined to the eastern margin of the Central Graben. Jakobsson et al. (1980) described the type section for the Triassic in the Fiskebank Sub-Basin of the Norwegian–Danish Basin (Figure 9) as a succession of repeated, upward-coarsening cycles of alluvial-fan/fluvial deposits overlying sabkha-environment sediments. Similar, relatively minor, upward-coarsening

Bunter Sands

Bunter Sands were recognised by Fisher and Mudge (1990) in the lowest part of the Smith Bank Formation (Figure 29). There is no definitive biostratigraphical information or observed lateral continuity of sandstones to support correlation with the Bunter Sands of the southern North Sea.

Josephine Sands

The Josephine Sands, described by Fisher and Mudge (1990) around the Josephine Oilfield, probably range in age from Anisian to late Ladinian; Deegan and Scull (1977) had identified the Josephine Member as Norian (Figure 29). They comprise brown, fine- to medium-grained sandstones interbedded with varicoloured shales, and are probably sheet-flood deposits. Similar, but not necessarily equivalent, Middle Triassic sandstones are observed locally in the east-central North Sea; for example, Anisian sandstones are recorded in well 29/1-1 (Figure 28).

Skagerrak Formation

The Skagerrak Formation comprises the dominantly sandy deposits (Figure 31) generally found in the graben areas (Figure 28). According to Fisher and Mudge (1990), the Skagerrak Formation tends to occur above an angular discordance which correlates with the Hardegsen unconformity in the southern North Sea, but it is difficult to recognise such a disconformity on seismic sections in the Central Graben. The sandstones are generally very fine to fine grained, and are interbedded with silty claystones; they are either laterally equivalent to, or overlie, the Smith Bank Formation, and range in age from Carnian to Norian (Figure 29). The formation is largely of sheetflood or minor-channel origin; the major source of elastic material is thought to have been the Fennoscandian Shield. Some evidence of upward-coarsening cycles may indicate local progradation of alluvial fans across the floodbasins, or alternatively the occasional development of more-persistent channel belts due to changes in run-off or local tectonic control.

Fisher and Mudge (1990) contended that sandstones of the Skagerralc Formation may have been preferentially deposited in subsiding lows where pods of Smith Bank Formation claystones had accumulated. However, Glennie and Armstrong (1991) have indicated that in places, the Skagerrak Formation may have accumulated preferentially along salt-solution hollows in the crests of the salt pillows at the margins of Smith Bank Formation pods.

Skagerrak Formation sandstones at the Marnock Field (Figure 28) form a reservoir for condensate; they are fine grained and feldspathic, with subordinate grey mudstones. The facies in the formation (G Strong and S A Smith, written communication, 1987) can be grouped into two facies associations: the lower is dominantly fluvial, and the upper is of fluvial to marginal-marine aspect. The facies are: parallel laminated and cross-bedded sandstones that are proximal sheet-flood and channel deposits; cross-bedded conglomerates with crude, parallel stratification that formed gravelly channel bars; grey mudstones and siltstones laid down as floodplain deposits; thin-bedded sandstones originating from distal sheet-floods; clean sandstones with faint laminae, and interpreted as estuarine-shoal sands; heterolithic deposits with wave ripples and intraclasts derived from marine-influenced distributaries; thin, heterolithic sandstones that formed in lagoon bays; and dark mudstones and siltstones with wave ripples and bioturbation, that were deposited in lagoon-bay/tidal flats.

Gassum/Rhaetian Sands

On some company logs, the term 'Marnock Formation' is inappropriately used to describe sandstones that equate lithologically and stratigraphically with the Skagerrak Formation. The term 'Marnock Formation' is also used informally in a different way on other company logs to describe a suite of marine-influenced sandstones at the top of the Triassic section in the northern part of the Josephine High. These sandstones (Figure 30) appear to be stratigraphically higher than the Skagerrak Formation and its equivalents, and are perhaps more comparable to the Gassum Sands (Figure 29), which were initially defined onshore in Denmark by Larsen (1966), but subsequently extended to the offshore area by Fisher and Mudge (1990). Bertelsen (1978) has suggested that in the Norwegian–Danish Basin, deposition of the Gassum Sands extended into Sinemurian times. Attempts to further refine the age of the possible Gassum Sands at the northern end of the Josephine High, and to determine its stratigraphical relationship with the Triassic/Jurassic Statfjord Formation in the East Shetland Basin, have proved unsuccessful. However, regional considerations suggest a possible Early Jurassic age for these sandstones.

The Gassum sediments over the northern Josephine High comprise dominantly upward-coarsening successions which have basal, wave-rippled, laminated mudstones, and wavy-laminated to structureless sandstones at their tops. The sedimentary structures are indicative of a marine/marginal-marine environment, and the presence of a cornstone conglomerate in well 30/lc-3 (Figure 30) probably resulted from erosion of a nearby pedogenic soil profile, possibly developed in a floodplain setting.

Chapter 6 Jurassic

Jurassic rocks occur widely in the central North Sea, particularly in the Central Graben, but almost all are concealed beneath a thick Cretaceous and Cenozoic cover. As a result of being overstepped by Lower Cretaceous sediments, the Jurassic sequence is largely absent from the Mesozoic outcrop in the western part of the area, where the limit of Jurassic rocks is poorly defined ((Figure 3) and (Figure 32)). A major unconformiry at base-Cretaceous level also cuts out Jurassic strata from a large part of the Jaeren High, the Forties-Montrose High, and parts of the West Central Shelf. Elsewhere, the present distribution and thickness of the Jurassic is a result of the complex interplay between the effects of Mesozoic tectonism, halokinesis of the underlying Zechstein sequence, and global sea-level change during deposition.

During the Jurassic Period, a prolonged episode of graben formation had widespread implications for the structural development and economic geology of the North Sea. An isopach map of Upper Jurassic sediments effectively outlines the most important elements of the graben system (Pegrum and Spencer 1990; Spencer and Larsen 1990). The eastern and western parts of the Central Graben are clearly defined as north-northwesterly trending features (Figure 32) symmetrically disposed around the Forties-Montrose High (Cayley, 1987). To the south of this high, the South Central Graben forms a smaller

Jurassic depocentre on a similar trend. North of the Forties–Montrose High, the Fisher Bank Basin is situated at the southeastern end of the Witch Ground Graben, where it separates the East Central and South Viking grabens. It lies at a 'triple junction' in the North Sea, for it marks the intersection of the three main arms of the Jurassic rift system (Figure 9). At the northern margin of the report area, the North Buchan Graben is a relatively minor basin which is structurally part of the Outer Moray Firth Basin (Andrews et al., 1990).

Lower Jurassic sediments are absent from most of the central North Sea, and their original distribution must be largely inferred from reconstructions of regional palaeogeography and tectonic history (Ziegler, 1990b). Sparse well data suggest that a marginal-marine environment was established in some parts of the area during earliest Jurassic times (Smith, 1987).

Middle and Upper Jurassic sediments usually rest unconformably upon deeply eroded Permian and Triassic rocks. The widespread absence of Lower Jurassic sediments is generally attributed to the development of a domal uplift at the northern margin of the central North Sea during the Mid-Jurassic, when a thick pile of basaltic lavas was erupted (Howitt et al., 1975; Woodhall and Knox, 1979). A separate volcanic centre developed on the northern flanks of the Auk Shelf (Figure 32) at the same time (Latin et al., 1990a; b).

The concentric distribution of Middle Jurassic facies in the North Sea has been used to suggest that erosion of the pre-volcanic dome supplied much of the detritus for contemporaneous deposition in graben areas flanking the uplift (Eynon, 1981). However, this interpretation has been reassessed in the northern North Sea, where Middle Jurassic clastic sediments are thought to be derived transversely from the graben flanks (Richards et al., 1988; Richards, 19906). In the central North Sea, sediments of Bajocian to Callovian age were largely laid down in a fluviodeltaic environment; the dominant facies include interdistributary siltstones and mudstones, thin alluvial and delta-top sandstones, and coals.

Middle Jurassic rocks are commonly absent from graben margins as a result of intra-Jurassic erosion. Isopachs of the Middle Jurassic succession are poorly constrained, but the available data indicate that the thickest nonvolcanic sequences were deposited in the possibly fault-controlled East Central Graben.

Radiometric age dates suggest that volcanism may have continued in the vicinity of the Forties Oilfield until latest Mid-Jurassic times (Ritchie et al., 1988), when the margins of the central North Sea began to be transgressed, probably from the north and east, by the sea which had gradually inundated the Viking Graben. In advance of the transgression, a coastal swamp extended across much of the central North Sea.

Although paralic deposition may have persisted locally, by the end of the Oxfordian a series of major transgressions had established shallow-marine environments across a large part of the report area. Most of the highly bioturbated shallow-marine sandstones which form the main Jurassic hydrocarbon reservoirs in the central North Sea were deposited at this time (Johnson et al., 1986).

To the north of the Buchan and Glenn highs (Figure 32), some paralic sequences of possible Oxfordian age are locally thickened on the downthrown sides of faults. This minor episode of tectonism marked the onset of the major structural reconfiguration which dominated the subsequent Jurassic history of the central North Sea. Although theoretical studies have suggested that a large component of crustal thinning in the central North Sea is related to Triassic or older episodes of extension (Barton and Wood, 1984), seismic interpretation indicates that the large Faults in the area are predominantly of Late Jurassic to Early Cretaceous age (Ziegler. 1990b). Sediments deposited during this major extensional phase include silty mudstones with thin, interbedded, argillaceous limestones, organic-rich claystones, and thin turbidites.

As a result of graben formation and related subsidence, most of the central North Sea has been a site of continuous marine deposition since the Late Jurassic. Deeply buried Upper Jurassic marine shales have attained maturity for hydrocarbon generation over a wide area of the graben, and form the main source of hydrocarbons.

Lower Jurassic

There is little evidence in the central North Sea to assist in the interpretation of Early Jurassic palaeogeography. Regional studies indicate that the Boreal Sea covered a large part of the North Viking Graben area throughout the Early Jurassic (Richards, 1990b), while at the same time, the Tethyan Sea extended across much of the Norwegian–Danish Basin (Ziegler, 1990a; b). The precise timing of the initial marine connection between the Boreal and Tethyan seas has not been established. In some reconstructions, the two marine basins became linked by Sinemurian times (Ziegler, 1990a; b); in others, the distribution of Lower Jurassic continental and coastal deposits has been used to infer little or no marine connection before the late Pliensbachian (Hamar et al., 1980; Richards, 1990b). In the central North Sea, the few wells which have proved Lower Jurassic sediments show that the sea began to encroach upon the eastern margin of the area during Hettangian to Sinemurian times. The Lower Jurassic sediments of the Cleveland Basin (Cameron et al., 1992) also onlap the southern flank of the Mid North Sea High, and crop out at the southern margin of the report area ((Figure 2) and (Figure 32)).

Regionally, the Early Jurassic marine transgression reached its greatest extent during the Toarcian (Vail et al., 1984; Hallam, 1988). However, the widespread absence of sediments laid down at the acme of the transgression has been used to infer the existence of a broad area of uplift centred on the northern margin of the central North Sea. Various models (Leeder, 1983; Ziegler, 19906; Smith and Ritchie, 1993) of Early Jurassic palaeogeography differ essentially in their interpretation of the origin, timing and scale of this uplift.

Lower Jurassic sediments have rarely been encountered in the UK sector of the central North Sea, even though several hundred wells have penetrated the Jurassic sequence in a wide range of tectonic settings. At the time of writing, there is no formal stratigraphical subdivision for the Lower Jurassic succession, but see Richards et al. (1993) for a revision of Jurassic stratigraphy in the North Sea. Some oil companies have extended the usage of nomenclature (Figure 33) from the Norwegian–Danish Basin and Danish Central Trough, where the sequence is divided into the Gassum and Fjerritslev formations (Deegan and Scull, 1977; Vollset and Dore, 1984; Michelsen, 1982), and this terminology has been used in this report.

Gassum Formation and Fjerritslev Formation

The Gassum Formation (Larsen, 1966; Bertelsen, 1978) is distributed throughout the Norwegian–Danish Basin, including the north-eastern margin of the Central Graben, and usually rests upon the Triassic Skagerrak Formation. In the Norwegian and Danish sectors it is predominantly composed of white or pale grey sandstones, which are generally coarse grained, and partly glauconitic. These sediments were deposited in a marginal- to shallow-marine environment during a gradual transgression at the close of the Triassic Period and at the beginning of Jurassic times. Sparse micropalaeontological evidence suggests that the Gassum Formation has a Rhaetian to Sinemurian age (Vollset and Dore, 1984), and developed in part as a marginal facies to the marine sediments of both the Vinding Formation of Late Triassic age, and the Lower Jurassic Fjerritslev Formation (Friis, 1987). According to Hamar et al. (1980), the marine transgression entered the area from the east.

In the Norwegian and Danish sectors of the North Sea, the Gassum Formation is commonly overlain by the Fjerritslev Formation, which comprises a sequence of dark grey, silty claystones deposited in an open-marine environment (Larsen, 1966; Vollset and Dore, 1984; Michelsen et al., 1987).

At the northern end of the Josephine High in well 30/1c-2A (Figure 34), a succession of 60 m of fine-grained, white sandstone with interbedded grey-green siltstone closely resembles the Gassum Formation, and rests upon Late Triassic argillaceous redbeds. These palynologically barren sandstones show bioturbation and wave-ripple structures, and were probably deposited in a marginal- to shallow-marine environment (G E Strong and S A Smith, written communication, 1988).

At the Clyde Oilfield, well 30/176-9 (Figure 32) penetrated 34 m of black shale with interbedded dolomite, of Hettangian to Sinemurian age, which may have been preserved locally as a result of dissolution of the underlying Zechstein halite (Smith, 1987). The preponderance of terrestrially derived palynomorphs and dark, angular, humic fragments in this shale indicates deposition in a nearshore environment (J B Riding, written communication, 1990). In terms of lithology, age and stratigraphical relationships, these claystones are closely comparable to the Fjerritslev Formation in the Danish sector (Michelsen, 1982).

Thin, probable Lower Jurassic sequences are also patchily preserved over the Maureen Terrace on the flank of the Fisher Bank Basin (Figure 32), where they rest upon Triassic sandstones and siltstones of the Skagerrak Formation. In well 22/56-9, less than 30 m of grey siltstone with thin interbedded sandstone and coal are assigned a late Sinemurian to Pliensbachian age on the composite log, and are overlain by a thick pile of volcanic rocks of the Rattray Formation. Nearby well 22/5b-5 (Figure 32) encountered 6 m of similar sediments of Sinemurian age.

Middle Jurassic

The Middle Jurassic rocks of the central North Sea were assigned to the Fladen Group by Deegan and Scull (1977). The group was arbitrarily subdivided into the Rattray Formation, which is composed predominantly of volcanic rocks, and the Pentland Formation, a sequence of nonmarine sediments incorporating less than 50 per cent volcanic rocks.

Rattray Formation

The Jurassic volcanic rocks of the central North Sea were discovered in 1970 when well 21/10-1 (Figure 35) encountered basic lava with interbedded agglomerate and volcaniclastic sediment in an unbottomed sequence 742 m thick beneath what is now the Forties Oilfield. Subsequent exploration established that volcanic rocks occur widely throughout the Forties region, and extend into the outer Moray Firth (Figure 35). In the first account of this volcanic province, Howitt et al. (1975) used radiometric age dating and biostratigraphy to suggest that the main eruptive episode took place during Bajocian to Bathonian times, although more recent studies suggest a younger age range (Ritchie et al., 1988).

Individual lavas are 1 to 9 m thick, and show clear signs of reddening and laterisation associated with subaerial eruption (Howitt et al., 1975). The rocks are commonly porphyritic, with abundant phenocrysts of olivine and clinopyroxene; microphenocrysts of plagioclase are also present in a matrix which generally consists of plagioclase, titanaugite, magnetite, ilmenite, apatite and rare analcime. Whole-rock geochemistry has confirmed that the rocks are predominantly undersaturated lavas with alkali olivine-basalt affinities; they include ankaramite, alkali olivine-basalt, hawaiite and mugearite (Gibb and Kanaris-Sotiriou, 1976; Fall, 1980; Fall et al., 1982).

Various models of North Sea tectonic evolution have been developed to account for this episode of Jurassic volcanism. Its location at the intersection of the three main arms of the North Sea graben system was related to the presence of an underlying mantle plume by Sellwood and Hallam (1974), who also argued that erosion of a volcanically domed area in the central North Sea provided the main source of Middle Jurassic elastic sediments in Yorkshire and the Moray Firth. This model was subsequently strengthened by a regional interpretation of Middle Jurassic facies distribution (Eynon, 1981). Furthermore, the growth and collapse of a volcanic-related uplift during Jurassic times has been persistently endorsed by Ziegler in a series of geological syntheses and palaeogeographic maps (Ziegler 1981; 1982; 1990a; 1990b; Ziegler and Van Hoorn, 1989).

An alternative proposal, which was initiated by Dixon et al. (1981) and subsequently supported by Leeder (1983), used the theoretical model of basin formation derived by McKenzie (1978) to suggest that the volcanic rocks may have developed from a passive upwelling of the asthenosphere during continental extension.

The conflict of ideas, between the active (thermal or plume) model and the passive (extensional or stretching) model of Mesozoic magmagenesis, has yet to be resolved. However, a major contribution to the debate has been made by Latin and his coworkers (Latin et al., 1990a; 19906; Latin and White, 1990; Latin, 1990; Latin and Waters, 1991), who have attempted to quantify the relationship between volcanism and extension in the North Sea by the application of theoretical studies of melt generation (McKenzie and Biclde, 1988). The broad conclusions of these studies suggest that the igneous rocks are unlikely to have been generated simply as a result of the amounts of extension which are considered geologically reasonable for the central North Sea. The composition and volume of the Jurassic volcanic rocks in the area suggest that extension must have been supplemented by the local modification of the mantle source by the addition of volatiles, and/or a small increase in the local temperature of the asthenosphere; that is, the presence of a thermal perturbation, or mantle plume. The main difference between the alternative models of North Sea rift development may lie in the size of the thermal anomaly required to initiate volcanism.

Although tectonic models of the area have become increasingly sophisticated, basic knowledge of the geological setting of North Sea volcanism has advanced little since the review by Woodhall and Knox (1979). However, Smith and Ritchie (1993) have pinpointed the location of three main Jurassic eruptive centres in the central North Sea (Figure 35). A fourth centre has been tentatively identified in the outer Moray Firth.

Glenn Volcanic Centre

The main focus of volcanic activity in the Forties region was probably located in the vicinity of well 21/36-3 (Figure 35), which has proved the thickest (1469 m) sequence of Jurassic volcanic rocks yet drilled in the central North Sea. The thickness of the sequence, the small proportion of interbedded sediments, and the occurrence of geochemically evolved lavas and intrusive rocks, are all strong indications of the proximity of a central vent (Howitt et al., 1975; Fall et al., 1982; Ritchie et al., 1988). This area is distinguished on both aeromagnetic and gravity anomaly maps ((Figure 4) and (Figure 5)), and steeply dipping seismic reflections within the volcanic sequence have been interpreted as the south-western flank of the original volcano (Smith and Ritchie, 1993). The presence of several small volcanic centres was, however, inferred from aeromagnetic data by Howitt et al. (1975). The age of the volcanic rocks is poorly constrained; well data indicate that they rest unconformably upon Triassic, Permian or pre-Permian strata, and are overlain by Oxfordian marine and paralic sediments. Using the ⁴⁰Ar-³⁹Ar radiometric method on core from well 21/36-3, Ritchie et al. (1988) obtained a best estimate for the age of this volcanic centre of 153 ± 4 Ma; this places the sequence within the Callovian (Snelling, 1985), and agrees with K-Ar whole-rock ages recalculated from Howitt et al. (1975).

Fisher Bank Volcanic Centre

Howitt et al. (1975) used aeromagnetic data to suggest that an extension of the Forties volcanic area occurs in the Fisher Bank Basin. Subsequently, Woodhall and Knox (1979) interpreted the aeromagnetic anomaly over this basin as being due to a large igneous intrusion at depth, possibly associated with a southerly thickening pile of volcanic rocks in the South Viking Graben. Using seismic-reflection profiles, Smith and Ritchie (1993) identified the location of a possible eruptive centre within the Fisher Bank Basin (Figure 35); this centre manifests itself as a structural culmination at base-Upper Jurassic level, and is almost coincident with the peak of the aeromagnetic anomaly (Figure 4). Up to 525 m of volcanic rocks have been drilled in well 22/5b-4 on the Maureen Terrace (Figure 34), and an unbottomed sequence 879 m thick is known farther north in the South Viking Graben. Generally, these volcanic rocks rest unconformably upon Triassic strata, but in places they overlie thin Lower Jurassic sediments. Coal-rich paralic sediments of Bajocian or Bathonian age overlie the volcanic sequences ((Figure 33) and (Figure 34)). Latin et al. (1990b) have published a K-Ar age of 188 ± 10 Ma (Pliensbachian–Harland et al., 1990) from a minor intrusion possibly associated with this volcanic complex. This age, and the stratigraphical relationships of the volcanic rocks, suggest that volcanism here may have been initiated during the later part of the Early Jurassic.

Puffin Volcanic Centre

Mesozoic igneous rocks were first described from the flanks of the Auk Shelf by Dixon et al. (1981), who dated two intrusions in the Permian sequence of well 29/25-1 (Figure 35) by the Ar-³⁹Ar method, and obtained an Early Cretaceous age of 138 4 Ma. Subsequently, a number of nearby wells drilled thick, mainly volcaniclastic, sequences; in well 29/14b-3 (Figure 35), 593 m of volcanic rocks and related sediments rest upon Triassic claystone. Latin et al. (1990a; b) broadly outlined the volcanic area using well data, and a possible eruptive centre has been located by Smith and Ritchie (1993) from its associated aeromagnetic anomaly on the northern flank of the Auk Shelf (Figure 35). Geochemically, the rocks from this centre are undersaturated basalts considerably enriched in potassium and incompatible trace elements. According to Latin et al. (1990a; b), they form an ultra-potassic series which must be derived from the lithospheric mantle. Seismic interpretation around well 29/14a-4 (Figure 35) shows that thick volcanic sequences are patchily preserved, commonly in small, synclinal basins which originated as salt-dissolution hollows. As a result, Jurassic volcanic successions locally rest directly upon Zechstein evaporites. The fact that the volcanic rocks are commonly overlain by late Oxfordian marine sandstones indicates that the Early Cretaceous radiometric age obtained by Dixon et al. (1981) is likely to be erroneous, given the absence of other evidence for Early Cretaceous volcanism. The local presence of volcanic detritus interbedded with paralic sediments in the Pentland Formation, suggests that volcanism at this centre probably occurred during the early Mid-Jurassic (Smith and Ritchie, 1993).

Pentland Formation (including Sgiath Formation)

The Pentland Formation is composed predominantly of siltstones and claystones, with interbedded sandstones and coals (Deegan and Scull, 1977). The formation was deposited around the flanks of the volcanic centres in paralic and fluviodeltaic environments. Equivalent strata in the adjacent parts of the Norwegian and Danish basins include the Sleipner Formation (Vollset and Dore, 1984) and the Bryne Formation (Jensen et al., 1986). In the central North Sea, the Pentland Formation rests unconformably upon Lower Jurassic, Triassic or Zechstein sediments; it is laterally equivalent to parts of the Rattray Formation, but also oversteps the volcanic sequence (Figure 33).

Recent well evidence has indicated that the nonmarine environment which was established throughout the Mid-Jurassic may have persisted in parts of the central North Sea until Oxfordian times. In the Witch Ground Graben (Figure 9), paralic sediments of Oxfordian age have been included in the Sgiath Formation, which forms part of the Humber Group and rests unconformably upon both Triassic and Middle Iurassic sediments (Harker et al., 1987). The Sgiath Formation has subsequently been subdivided into a lower, coal-rich unit, and an upper, marine unit; the latter is closely comparable to the Piper Formation (O'Driscoll et al., 1990). Lithologically, the coal-rich unit of the Sgiath Formation has much in common with the Middle Graben Shale Formation of the Danish Basin (Michelsen et al., 1987), and is difficult to distinguish from the Pentland Formation. As yet, the Pentland and Sgiath formations cannot be readily differentiated throughout the central North Sea.

In general, Middle Jurassic sequences are poorly dated, and have often been assigned Bajocian to Bathonian ages solely on the strength of relatively long-ranging palynofloras. The tectonic evolution of the area during the Mid-Jurassic can only be broadly outlined because of this lack of biostratigraphical resolution within the nonmarine sequence. The present distribution and thickness of these sediments has also been considerably affected by intra-Jurassic and pre-Cretaceous erosion (Figure 35); as a result, Middle Jurassic strata are almost entirely restricted to the main graben areas, and are largely absent ft-cm marginal shelves and intrabasinal highs. Thick sequences of the Pentland Formation are best preserved in the East Central Graben and the Fisher Bank Basin.

A major depocentre, possibly controlled by faulting, was initiated along the eastern margin of the East Central Graben during Aalenian or Bajocian times. The deepest part of this Jurassic basin has yet to be drilled, but nonmarine sediments up to 595 m thick have been penetrated on tilt-block highs at the western flank of the East Central Graben. Similar sediments were laid down over a wide area, extending beyond the East Central Graben to the southern end of the West Central Graber and the northern margin of the Auk Shelf, where they interdigitate with volcanic debris from the Puffin volcanic centre. The thinly interbedded sandstones, siltstones and shales are comparable in age, thickness and facies to the alluvial Middle Jurassic strata described by Graue et al. (1987) from the eastern side of the South Viking Graben. A coal seam more than 13 m thick is present locally near the base of the sequence, as in well 29/10-2 (Figure 34).

The nonmarine sediments contain a large amount of plant material and, as source rocks, are primarily vitrinitic and gas prone. However, the presence of volcanic rocks and volcaniclastic sediments downgrades the potential of the Middle Jurassic sequence both as a source rock and as a reservoir.

An area of Late Jurassic uplift between the Forties-Montrose and Jaeren highs separates the Middle Jurassic sediments of the Fisher Bank Basin from those of the East Central Graben and adjoining areas (Figure 35). On the Maureen Terrace, the volcanic and volcaniclastic rocks of the Fisher Bank volcanic centre are buried by Bajocian to Callovian sediments which form a southern continuation of the Pentland and Sleipner formations of the South Viking Graben (Johnson et al., 1993). An unconformity may separate these Middle Jurassic sediments from a lithologically similar overlying sequence (of ?early Oxfordian age), which is analagous to the Sgiath Formation (Figure 33).

If the Glenn volcanic centre is largely of Callovian age, as suggested by Ritchie et al. (1988), then the development of an unconformity at the top of the Pentland Formation, and the common absence of Callovian sediments, may be related to prevolcanic uplift around the centre. Such uplift would help to explain the regional distribution of the Pentland Formation (Smith and Ritchie, 1993); it is both younger and more localised than the pre-eruptive dome proposed by Ziegler (1990b), which included uplift associated with the Fisher Bank volcanic centre. The Sgiath Formation rests unconformably on the uplifted flanks of the Glenn volcanic centre, and is best interpreted as a coastal-plain or lagoonal deposit laid down as a precursor to the marine transgressions of Oxfordian times.

In most wells, the top of the Pentland (or Sgiath) Formation is clearly defined by a downhole change in palynoflora from the open-marine assemblages which characterise most of the Humber Group (Upper Jurassic), to the miospore-dominated assemblages of the underlying paralic or lagoonal facies. In siltstone sequences, this change is commonly marked by a conspicuous sonic-log break which corresponds to the lithological contrast between marine and freshwater (to brackish) deposits.

Upper Jurassic

Upper Jurassic sediments are widespread in the central North Sea, but are thickest in the graben areas adjacent to faults, where they are over 1000 m thick (Figure 36). All marine sediments laid down during the Late Jurassic in the central North Sea are referred to the Humber Group of Deegan and Scull (1977). The group is divided into two argillaceous units, the Heather and Kimmeridge Clay formations, as well as a number of locally sourced arenaceous units, of which the most important is the Fulmar Formation ((Figure 33); Johnson et al., 1986). The Oxfordian Sgiath Formation also forms part of the Humber Group (Harker et al., 1987), although lithologically, and in terms of its depositional setting, it shows greater affinity with the Pentland Formation of the Fladen Group (see previous section).

Fulmar Formation

In the earliest Late Jurassic times, much of the central North Sea was occupied by a coastal-plain or lagoonal environment in which paralic sediments were laid down. During the Oxfordian, this extensive plain was gradually inundated by the sea, and a series of transgressive and shallow-marine sandstones was deposited; an argillaceous sequence, the Heather Formation, was laid down farther offshore. In the UK sector, most of these sandstones are assigned to the Fulmar Formation, which forms the main hydrocarbon reservoir in the Fulmar, Clyde, Kittiwake and Guillemot (Gannet West) oilfields (Figure 1). The Upper Jurassic reservoir of the Duncan Oilfield is closely comparable to the Fulmar Formation, but has been given separate status as the Duncan Formation by Robson (1991).

The Fulmar and Clyde oilfields are located on a series of structural terraces alongside the Auk Shelf (Figure 36). Around the Fulmar Oilfield, the Fulmar Formation comprises 150 to 335 m of highly bioturbated, fine- to medium-grained, arkosic sandstones. These were laid down either as a prograding-shoreface sand, or as a migrating, storm-dominated, shelf sand (Johnson et al., 1986). Within the Fulmar Oilfield ((Figure 37) and (Figure 38)), the formation consists of a series of notably thick, upward-coarsening sequences which show the combined effects of fault movement and Zechstein halokinesis on the Fulmar Terrace (see (Figure 85); Johnson et al., 1986; Stockbridge and Gray, 1991). The sandstones were largely derived by the erosion of Triassic sediments from the Auk Shelf to the west, and there is consequently a change in facies across the oilfield, with thinner sandstones and poorer hydrocarbon reservoirs developed distally, towards the north-east.

In the Clyde Oilfield and its satellites (Figure 36), the Fulmar Formation ranges between 105 m and 245 m in thickness, and was laid down in a deepening basin which became increasingly starved of sediment (Stevens and Wallis, 1991). Reservoir facies are similar to those of the Fulmar Oilfield; internal variation within the main reservoir units has been related to the effects of halokinesis in the underlying Zechstein salt (Smith, 1987). In contrast, Gibbs (1984a) has argued that structural development around the Clyde Oilfield was controlled by gravitational sliding on low-angle faults which possibly detached within the Zechstein sequence. At the Fulmar (Figure 37) and Clyde oilfields, the Jurassic sequence is partially sealed on the base-Cretaceous unconformity surface, where the crests of both structures are truncated.

The Guillemot (Figure 37) and Kittiwake oilfields are located above diapiric walls of Zechstein halite on the West Central Shelf. In both fields, the distribution and thickness of the Fulmar Formation reservoir sandstones has been largely controlled by salt dissolution. The sand bodies are lenticular in section, and range up to 65 m in thickness in Kittiwake, and 79 m in Guillemot. The sands were laid down on a shallow-marine shelf as migrating sand ridges. The Fulmar Formation rests unconformably upon Triassic sediments and is overlain and sealed by Upper Jurassic clay-stones. Structural closure was generated by subsequent diapiric remobilisation of the underlying evaporite sequence (Armstrong et al., 1987; Glennie and Armstrong, 1991).

Although all these oilfields occur in the vicinity of the West Central Shelf, well evidence from elsewhere in the central North Sea suggests that analogous sequences of shallow-marine sandstones are developed in Jurassic basins across much of the area (Figure 34). The lithostratigraphical nomenclature of these sandstones has yet to be resolved; there are particular problems at the margins of the central North Sea, where identical sequences have been assigned various formational names (Boldy and Brealey, 1990; O'Driscoll et al., 1990). Bergan et al. (1989) recognised that wider correlations between locally defined units are unreliable because of a lack of data from the axial parts of the basin.

Graue et al. (1987) suggested that shallow-marine sediments in the northern North Sea form a backstepping series of discrete, progradational units which were progressively offset during southward transgression of the Viking Graben. By early Oxfordian times, the marine transgression had reached the north-central North Sea (Booze and Gustay. 1987). and overlapped the Fisher Bank volcanic centre on the Maureen Terrace. There is limited evidence that the sea may have entered the central North Sea to the south of the Glenn and Fisher Bank volcanic centres before this time. In some wells at the northern end of the East Central Graben, a marine, Callovian macrofauna has been redeposited in sediments of Volgian age, and indications of possible marine Callovian deposition have also been noted in the northern part of the West Central Graben (J B Riding, written communication, 1990). Any evidence of a former marine connection between these areas across the northern part of the Forties-Montrose High has been removed by erosion.

Regionally, biostratigraphical data from the base of the shallow-marine sandstones indicate that a major expansion of the area of marine deposition took place during mid-Oxfordian to early Kimmeridgian times. Data from the West Central Shelf suggest that shallow-marine deposition continued to extend westwards throughout the Late Jurassic. At some time during the Oxfordian, parts of the report area began to subside more rapidly, and local depocentres were initiated by movement on the main, basin-controlling faults. Thick sequences of shallow-marine sandstones continued to accumulate in these basins until subsidence outstripped sediment supply during Kimmeridgian and early Volgian times. An upward-fining sequence at the top of these sandstones (Figure 34) commonly marks the transition from a shallow-marine shelf to a more open-marine environment in which the Heather Formation was deposited. From then until the end of the Jurassic, argillaceous sedimentation was dominant in the central North Sea.

Heather Formation

The Heather Formation consists of marine, silty claystones with some interbedded limestones; it represents lower energy deposition farther offshore than the sandstones of the Fulmar Formation and its equivalents (Deegan and Scull, 1977; Harris and Fowler, 1987). In regional terms, the Heather Formation is broadly synonymous with the Haugesund Formation of the Norwegian sector (Vollset and Dore, 1984), and the Lola Formation of the adjacent Danish Central Graben (Jensen et al., 1986). In the central North Sea, the Heather Formation is largely late Oxfordian to early Kimmeridgian in age (Figure 33).

Evidence from seismic and well data indicates that the thickness of the Heather Formation in the central North Sea increases considerably towards the main graben-bounding faults, which would seem to suggest that its deposition coincided with a major episode of extension. However, Roberts et al. (1990a) have argued that these thick, Upper Jurassic, argillaceous sequences are in fact postrift sediments which accumulated in a basin of varied bathymetry. This basin topography was inherited from a previous period of tectonism, which possibly occurred very early in the Late Jurassic, or at the end of Mid-Jurassic times.

Up to 640 m of Heather Formation sediments have been proved in the West Central Graben. In many wells, the formation has been truncated by intra-Jurassic erosion related to Zechstein halokinesis or footwall uplift. On well logs, the top of the formation is marked by a rapid increase in gamma-ray response and a corresponding decrease in the sonic velocity of the claystone sequence (Figure 34). These log breaks are particularly conspicuous in wells where the highly radioactive uppermost unit of the Kimmeridge Clay Formation oversteps, and rests unconformably on, the older, less-radioactive claystones.

Kimmeridge Clay Formation

The Kimmeridge Clay Formation largely consists of dark grey to black, organic-rich shales with interbedded limestones (Deegan and Scull, 1977); the calcareous interbeds are generally thinner and more sparse than those in the underlying Heather Formation. The shales were deposited under anaerobic bottom conditions in a restricted, marine basin. Uranium adsorbed by the organic matter disseminated in the shales has given the Kimmeridge Clay Formation a characteristically high signature on gamma-ray logs (Figure 34); the topmost part of the formation is commonly the most radioactive. In the Norwegian sector, these 'hot' shales have been assigned to a separate unit, the Mandal Formation, whereas the lower part of the Kimmeridge Clay Formation is known as the Farsund Formation (Vollset and Dore, 1984). Deep burial of Jurassic rocks in the main graben areas has made the oil-prone Kimmeridge Clay Formation the most important source of hydrocarbons in the central North Sea.

The Kimmeridge Clay Formation is relatively thin in the central North Sea (Figure 34). Thicknesses of 100 to 150 m are typical for the graben areas, with slightly thicker sequences developed in those parts of the Jurassic basin overdeepened by withdrawal of the underlying Zechstein salt. The Kimmeridge Clay Formation was deposited at a time of active fault movement, and is locally absent from many tilt-block highs. Nevertheless, it is the most widespread of the Jurassic formations, and is present over large parts of the West Central Shelf where a thin representative of the highly radioactive upper unit of the shales commonly rests unconformably upon Triassic sediments or on the Fulmar Formation. Its limits are however difficult to define on seismic-reflection profiles, and well data are sparse both in this area and on the Mid North Sea High.

Thick sequences of Upper Jurassic, sandstone-dominated turbidites, analogous to those in the Moray Firth and South Viking Graben (Andrews et al., 1990, Johnson et al., 1993), have yet to be encountered within the Kimmeridge Clay Formation of the central North Sea. However, thin turbidite units have been described from the area; in the Fulmar Oilfield (Figure 38), the uppermost part of the reservoir (the Ribble Member of Johnson et al., 1986) is formed of turbiditic sands probably derived from the Auk High following its rejuvenation by Late Jurassic faulting. Elsewhere in the developing grabens, similar sand sequences were sourced by the erosion of shallow-marine sandstones from the flanks of the Jurassic basin.

It is possible that the lack of thick sandstone turbidites is partly a result of the central North Sea sequences being dominated by detritus eroded from largely argillaceous Triassic sediments at the basin margins and on intrabasinal highs, whereas Devonian sediments were a source farther north.

In the northern North Sea, coarse-grained synrift deposits accumulated as proximal sedimentary wedges in the hanging walls of the main graben faults. However, in the central North Sea, the same tectonic environment is complicated by the presence of thick, underlying sequences of Zechstein evaporites. As a result of the contemporaneous effect of diapirism in the vicinity of the graben-bounding faults in the central North Sea, coarse-grained sediments may have been redistributed towards the more central parts of the graben. Stow and Atkin (1987) have described a widespread facies of silt-laminated turbidites in the central North Sea; these were probably laid down as the distal parts of slope aprons and submarine fans. These rocks pass laterally into the fissile-laminated mudstones associated with hemipelagic to pelagic deposition at the basin centre.

The age of the Kimmeridge Clay Formation in the central North Sea ranges from mid-Kimmeridgian to Ryazanian (Figure 33). The deposition of black, organic-rich shales thus continued into the early part of the Cretaceous, when anoxic conditions were terminated by a widespread basin-flushing event which marks the base of the Cromer Knoll Group (Rawson and Riley, 1982). The lithological contrast between the Cromer Knoll Group and the Kimmeridge Clay Formation produces a high-amplitude seismic reflector which, for the purposes of regional mapping, is taken to mark the top of Jurassic rocks.

Chapter 7 Cretaceous

Lower Cretaceous

The Jurassic–Cretaceous boundary in the central North Sea occurs within the upper part of the high-gamma shales of the Kimmeridge Clay Formation, which were deposited in an anaerobic environment. However, there is commonly a major boundary within upper Ryazanian sediments that marks the end of anaerobic conditions, and the onset of deposition of an open-marine, predominantly argillaceous sequence with a variably calcareous content and several distinct sandstones. These sediments are all included in the Cromer Knoll Group of Deegan and Scull (1977), which extends to the uppermost Lower Cretaceous.

Haq et al. (1987) equated the ending of Kimmeridge Clay deposition with a sudden eustatic fall in sea level, followed by a rapid rise. Support for this interpretation is found in the recognition of a widespread unconformity on intrabasinal and basin-margin highs, with pronounced onlap of the Cromer Knoll Group on their flanks. Within the main graben areas of the central North Sea, the boundary is conformable; nevertheless, a change in facies is marked by a major geophysical log break in wells, and by a strong reflector on seismic records. Consequently, this is one of the most easily identified geological boundaries in the Mesozoic and Cenozoic successions of the North Sea. The boundary was considered by Ziegler (1975) to mark a major tectonic break, the late-Cimmerian unconformity, but sea-level change and 'flushing out' within a tectonically active environment, enhanced by a thermally related subsidence pattern, are now recognised as important factors in its development (Rawson and Riley, 1982). Indeed, it could be argued that the occurrence of a thin, very high-gamma unit of the uppermost Kimmeridge Clay Formation on highs around the Central Graben is support for pronounced sediment starvation associated with a maximum flooding event.

Cromer Knoll Group sediments are widely distributed (Figure 39), but thicknesses are very variable. The thickest sequences are found close to the major graben-bounding faults, such as those defining the margin of the Buchan Horst and the West Central Graben where they overlie thick Upper Jurassic sediments. Seismic evidence suggests a maximum preserved thickness of about 1800 m; the thickest drilled sequence is 1004 m in well 21/1a-12.

The Late Jurassic was a time of major rifting and faulting, with thick marine-clastic sequences deposited in a series of grabens and half-grabens; much of the faulting was associated with Zechstein salt mobilisation. Similar faulting and halokinesis continued through much of the Early Cretaceous, and only waned towards the end of that rime. Although in many localities thick Lower Cretaceous sediments overlie thick Upper Jurassic strata, many Jurassic faults became inactive or were less active in Early Cretaceous times, so that the Lower Cretaceous may be thin or absent above some thick Upper Jurassic sequences. Conversely, some thicker Lower Cretaceous sequences overlie relatively thin Upper Jurassic strata, as in well 30/11b-1; this well has relatively thick Hauterivian to Barremian sediments, and a very condensed Aptian to Albian section (Figure 40), highlighting locally varying rates of subsidence and/or sedimentation during Early Cretaceous times. In many cases, the variations in Lower Cretaceous thickness may he explained primarily by salt movement in response to sediment loading and tectonic activity.

The onlapping nature of Early Cretaceous sedimentation has resulted in only the latest Lower Cretaceous rocks covering some of the more-prominent highs. Elsewhere, the Lower Cretaceous is entirely absent due to nondeposition or erosion prior to Chalk Group sedimentation. On some highs, a relatively complete, but condensed, sequence is preserved. The occurrence of thin sand units within the Cromer Knoll Group can generally be related to erosion of nearby highs; it is debatable whether faulting or sea-level change was responsible for the generation of these units.

Along the western margin of the report area, Tertiary uplift has resulted in the complete erosion of Lower Cretaceous rocks. However, across a large area of the Mid North Sea High, and northwards to the Aberdeen Platform, there is a relatively uniform, argillaceous and calcareous sequence that is mainly less than 100 m thick (Figure 39). Small-scale normal faulting and minor halokinesis have caused local variations in thickness, but there is a general trend of southward thinning. The oldest Lower Cretaceous rocks may be as young as Barremian or Aptian in the south, but older sediments are preserved farther north. BGS shallow boreholes have proved grey mudstones and shales where the Lower Cretaceous crops out in the west ((Figure 39); BGS Marr Bank Solid Geology sheet). Close to the Lower Cretaceous sea-bed outcrop, Lott et al. (1985) recorded nine thin tuff bands within upper Aptian to lower Albian mudstones in BGS borehole BH81/40 (Figure 39). These mudstones contrast with the more-calcareous sequences recorded in several wells on the Aberdeen Platform and Mid North Sea High, such as wells 21/26-1D (Figure 41) and 37/10-1. Prior to the onset of Chalk Group deposition in Cenomanian times, erosion removed the entire Lower Cretaceous section along a small portion of the margin of the Northumberland Trough at the western end of the Mid North Sea High (Figure 39).

The first lithostratigraphical scheme for the Lower Cretaceous succession of the central North Sea was published by Deegan and Scull (1977). Since then, extensive exploration has resulted in a great deal of additional information, and there have been several attempts to refine the lithostratigraphy (Hesjedal and Hamar, 1983; Jensen et al; 1986; Isaksen and Tonstad, 1989; Crittenden et al., 1991), particularly in the adjacent Norwegian and Danish sectors. The Cromer Knoll Group remains the accepted term to cover all the sediments deposited between the end of deposition of the Kimmeridge Clay Formation and the onset of Chalk Group sedimentation, but several new formations and informal subdivisions have been proposed (Figure 42).

Deegan and Scull (1977) defined three formations in the Cromer Knoll Group: the Devil's Hole, Valhall and Rodby formations. Much of their original group comprised the Valhall Formation, which can now be subdivided into several widely mappable subunits that have been given formation or member status (Hesjedal and Hamar, 1983; Jensen et al., 1986), notably the Sola Formation (Hesjedal and Hamar, 1983) and Bosun Sand Member (O'Driscoll et al., 1990). Additional subdivision of the Valhall Formation into smaller units has been attempted in the Central and Witch Ground grabens (O'Driscoll et al., 1990; Bisewski, 1990; Crittenden et al., 1991). In the adjacent Norwegian sector, Isaksen and Tonstad (1989) abandoned the term Valhall Formation, and proposed several new names. In this report the original scheme of Deegan and Scull (1977) is preferred, with amendment to include the Sola Formation, Bosun Sand Member, and Leek Member (Jensen et al., 1986).

Devil's Hole Formation

This formation, at the base of the Cromer Knoll Group, comprises grey to greenish grey sandstones and sandy marls which were deposited in a shallow-marine environment. It is restricted to an area west of the Auk Ridge known as the Auk Basin (Figure 39) and, in the type well 29/25-1 (Deegan and Scull, 1977), has a thickness of about 65 m.

Early biostratigraphical studies suggested a late Ryazanian to Valanginian or earliest Hauterivian age for the formation (Deegan and Scull, 1977), but more recent dating indicates that the base of the formation may range to the Volgian (Crittenden et al., 1991) or earlier. Almost all of the Devil's Hole Formation in the type well, and the entire sequence in some other wells, such as 29/23-1 (Figure 41), may be of Volgian to early Ryazanian age. In a few wells, including well 29/24-1, the sands at the base of the formation have been dated as Kimmeridgian to Oxfordian (company logs). The Devil's Hole Formation is therefore in part the lateral equivalent of the Jurassic Kimmeridge Clay and Fulmar formations, and straddles the major late Ryazanian sequence boundary (Figure 42).

The Kimmeridge Clay Formation is absent in most wells in the Auk Basin, and as palaeontological evidence is not always available, the distribution of the Devil's Hole Formation shown in (Figure 39) includes Jurassic sands. Most of the sands of the Auk Basin are very similar to, but younger than, the Fulmar Formation immediately to the east of the Auk Ridge, where shallow-water sands of the Fulmar Formation were overlain by shales of the Kimmeridge Clay Formation in Kimmeridgian or early Volgian times (Johnson et al., 1986). However, it appears that to the west of the Auk Ridge, an environment that favoured shallow-marine sand deposition was maintained into Early Cretaceous times, albeit with several hiatuses or minor unconformities.

Sandstones with conglomerates of probable turbiditic origin occur within the Kimmeridge Clay Formation, and extend up into the Valhall Formation to the east of the northern continuation of the Auk Ridge in well 30/11b-1 (Figure 40). Thin sands also occur within the upper part of the Kimmeridge Clay Formation in well 30/17a-1 immediately east of he Auk Ridge, and in well 29/12-1 to the north-west. This suggests that the Auk Ridge remained a source of sand during latest Jurassic and earliest Cretaceous times, and could have provided the clastic input for the Devil's Hole Formation. Alternative sources for the Devil's Hole Formation sands are from erosion of the Mid North Sea High or an area farther west, but the restricted extent of the Devil's Hole Formation is an indication of derivation from a local source. Thin sands near the base of the Lower Cretaceous succession occur farther north, as in well 21/191A (Figure 39), but these are within the main grabens and are probably deep-water turbiditic sands derived from local fault scarps or emerged highs. Such sands are included within the Valhall Formation as they do not form mappable units.

To the north of the report area, thick, lowermost Cretaceous: sandstones are developed locally around horsts in the Moray Firth (Harker et al., 1987; Andrews et al., 1990); in the Danish sector on the western flanks of the Ringkøbing-Fyn High, the Vyl Formation (Figure 42) contains similar sands (Hansen and Buch, 1982; Jensen et al., 1986). All have been interpreted as submarine-fan sands related to erosion of local emergent highs, and contrast with the interpretation of the Devil's Hole Formation as a shallow-marine sand. It is possible, however, that an unrecognised change in facies occurs near the top of the Devil's Hole Formation, and the relatively thin sands above the late Ryazanian sequence boundary may also have been deposited in a deeper-water environment, contrasting with the older part of the formation.

Valhall Formation

As defined by Deegan and Scull (1977), the Valhall Formation includes all sediments of the Cromer Knoll Group except the Devil's Hole Formation sands and the relatively thin Rodby Formation at the top of the Lower Cretaceous succession (Figure 42). In this report, the top of the Valhall Formation is taken at a marked upward increase in gamma-ray response near the base of the Aptian; this marks the base of the Sola Formation ((Figure 40) and (Figure 41)). With more information, it has become possible to subdivide the Valhall Formation into several units which are traceable across much of the central North Sea.

The Valhall Formation sediments are largely soft, grey to reddish grey, calcareous mudstones and shales with marls and limestones; there are also some thin sandstones. In the graben, thicknesses vary from zero to several hundred metres. The sediments were all deposited in an open-marine environment, and several local hiatuses and unconformities have been identified (Bisewski, 1990; O'Driscoll et al., 1990; Crittenden et al., 1991).

Within the main graben, the Valhall Formation rests on the Kimmeridge Clay Formation. A thin (10 to 50 m) unit of interbedded shales and limestones with some sandstones commonly occurs at the base of thick Valhall Formation sequences (Figure 40); it is dated as late Ryazanian to early Valanginian. Sonic velocities are generally higher, and gamma-ray values lower, than in the overlying strata, but both are very variable. Some reworking of the underlying Kimmeridge Clay Formation may be responsible for the variable gamma-ray response.

The rapid rise in sea level which was responsible for terminating the hot-shale deposition of the Kimmeridge Clay Formation may have resulted in sediment sinks on the margins of the basin to the west of the Central Graben, allowing the deposition of relatively siliciclastic-free, calcareous deposits in the graben (Jensen et al., 1986). These authors recognised a similar unit at the base of the Valhall Formation in the Danish sector of the Central Graben, which they termed the Leek Member; this terminology is extended into the report area ((Figure 40) and (Figure 42)).

The top of the Leek Member may correlate with an unconformity described by O'Driscoll et al. (1990) within the lower Valanginian of the Witch Ground Graben to the north of the report area (Figure 42). The development of this unconformity may relate to a eustatic fall, and subsequent rise, in sea level (Haq et al., 1987). No seismic expression of this event has been observed in the central North Sea, where no significant sands appear to be associated with erosion in response to sea-level change, other than possibly in the upper part of the Devil's Hole Formation. In well 30/6-3 west of the Josephine High (Figure 39), there is a 10 m-thick, high-gamma shale within the lower Valanginian which may also correlate with this event, either as a primary deposit in a temporarily restricted basin, or as a result of local faulting and/or reworking of the underlying Kimmeridge Clay Formation.

Although the Leek Member is generally restricted to the upper Ryazanian and lower Valanginian at the base of thick sequences, including that in well 30/18-3 (Figure 40), in some thinner sequences a similar facies can extend up through much of the Lower Cretaceous as a thin, condensed, limestone-rich deposit. This is commonly the case in wells sited on the margins of the main graben, on intrabasin highs, or on salt swells. In some condensed sections these basal limestones may be as young as Hauterivian or Barremian/Aptian, as in well 21/26-1D (Figure 41). In Norwegian waters, Hesjedal and Hamar (1983) defined the Urvik Formation (Figure 42) to include similar condensed sequence; Isaksen and Tonstad (1989) ascribed the same unit to the Mime Formation. In the UK sector there are mudstones, marls and limestones within thin sequences, and all are included here within the Valhall Formation. As indicated by Isaksen and Tonstad (1989), age determination of these condensed deposits will assist in determining subsidence patterns across the basin.

Above the Leek Member, a smooth gamma-ray log is typical, reflecting relatively uniform sedimentation. In several wells, such as 21/15a-1 and 30/18-3 (Figure 40), there is a gradual upward decrease in gamma-ray values, which correlates with a decreasing siliciclastic content and an increase in calcium carbonate. Frandsen et al. (1987) recognised seismically downlapping lobes of sediment within the Valhall Format ion in the Danish sector of the central North Sea, and suggested that deposition of fine-grained sediments in the distal parts of submarine fans may be an important component of the Valhall Formation.

Well 30/1 lb-1 proved a different sequence at the base of the Lower Cretaceous. Here, interbedded sands, shales and conglomerates straddle the Kimmeridge Clay–Valhall Formation boundary, which is commonly characterised by a drop in the gamma-ray values and an increase in sonic velocity of the interbedded shales (Figure 40). The well is located in the graben east of the Auk Ridge, and it is believed that the coarse-grained clastics were derived as a result of faulting along the northern extension of the Auk Ridge. Similar deposits may occur along strike to the north, but there are no wells closer than 30/6-3, where they are absent (Figure 39). The unit is not included in the Devil's Hole Formation because of differences in lithology, environment of deposition, and its location within one of the main grabens.

Three thin marker beds with high gamma-ray peaks can be correlated within the Valhall Formation (Figure 42). The lower two, A and B, which are dated as Hauterivian/Barremian, are particularly clear on the western flanks of the main graben, as in well 29/23-1 (Figure 41). Marker A is commonly missing, particularly in thinner sequences where it may not have been deposited or is lost in a condensed basal unit. Marker C is the most widespread of the three gamma-ray peaks, and is commonly the only one identified on well logs in thick graben sequences (Figure 40). It occurs near the Barremian-Aptian boundary, and can reach a thickness of about 10 m, generally within the most calcareous part of the Valhall Formation where middle to upper Barremian and lowermost Aptian limestones occur. Its lithology is predominantly dark grey to black, partly brown, bituminous clay-stones dated by Crittenden et al. (1991) as intra-early Aptian, and correlated by them with the Fischiefer of north-west Germany.

Jensen et al. (1986) recorded a similar gamma-ray peak within limestones at the top of the Valhall Formation in Danish waters, although their mid-Barremian dating is slightly older than the apparent age of marker C in the UK sector. Jensen et al. (1986) included the peak, which they called the Munk Marl Bed (Figure 42), within the Tuxen Formation, which is otherwise predominantly limestone or chalk. On lithological, gamma-ray and sonic logs, there is a good correlation between the Munk Marl Bed and Marker C. However, on biostratigraphical grounds, Crittenden et al. (1991) preferred to correlate the Munk Marl Bed with an older gamma-ray peak, possibly Marker B.

Jensen and Buchardt (1987) have shown that the Munk Marl Bed has a high organic content, and was deposited under anoxic conditions. A thin volcanic ash has also been identified within the unit. Markers A, B and C are also likely to have a high organic content, and may indicate short periods of anoxicity; it is not known whether they contain volcanic material.

Isaksen and Tonstad (1989) have extended the usage of the Tuxen Formation into Norwegian waters. It is possible to apply the term in the UK sector, but the development of limestones is patchy, so that a separate formation of approximately age-equivalent limestones is not adopted here. In a few wells there are 20 to 30 m of porous limestones with hydrocarbon reservoir potential; in well 22/26a-2 (Figure 40) there are 21 m of gas-bearing limestone with porosities in excess of 23 per cent.

Marker C forms a widely mappable event which has been used to divide the Valhall Formation, as defined by Deegan and Scull (1977), into an upper and lower unit. A more favoured division places the top of the Valhall Formation at the top of the calcareous-rich unit a few metres above the marker, where there is a pronounced rise in gamma-ray values and a drop in sonic velocities (Figure 40). This event correlates with the base of the Sola Formation as defined in the Norwegian sector by Hesjedal and Hamar (1983).

Sola Formation

The Sola Formation consists predominantly of grey shales with a lower calcareous content than the Valhall Formation. It typically reaches a thickness of about 75 m in thicker sections (Figure 40), but is significantly thinner on the basin margins (Figure 41). At the base of the Sola Formation there is an increase in gamma-ray response as a result of an increase in organic content due to a partial return to anoxic bottom conditions similar to those of Late Jurassic times. The change in environment may be the result of an early Aptian eustatic fall in sea level (Haq et al., 1987); as sea level dropped, renewed erosion of basin highs and basin margins occurred, leading to some submarine-fan sand deposition. Bisewski (1990) related the erosion and formation of sands to increased faulting associated with the Austrian orogenic phase, which was linked to the onset of sea-floor spreading in the Bay of Biscay.

A relatively high gamma-ray and low sonic-velocity log pattern is typical for most of the Aptian sediments in the central North Sea (Figure 40) and (Figure 41). The Sola Formation can be traced throughout the area, corresponding closely to the up er part of the Valhall Formation as defined by Deegan and Scull (1977). The relatively high organic content compared with the rest of the Lower Cretaceous gives the unit some potential as a hydrocarbon source rock (Jensen and Buchardt, 1987), although maturity is generally likely to be too low to have allowed significant hydrocarbon generation.

In the north of the report area (Figure 39) and in the Moray Firth, a sequence of submarine-fan sands with interbedded shales correlates with the Sola Formation. Hesjed al and Hamar (1983) defined the Kopervik Formation (Figure 42) for similar sands in the Norwegian sector; this term has been used by oil companies in the UK sector, but a more favoured name is the Bosun Sand Member (Andrews et al., 1990; Bisewski, 1990; O'Driscoll et al., 1990). This member reaches a thickness of about 275 m, of which up to 150 m is sand. The sands display a low gamma-ray response, but are in many instances interbedded with the high-gamma shales of the Sola Formation, as in well 21/4-2 (Figure 40).

Farther south, only a few thin sands have been discovered which might be equivalent to the Bosun Sand Member. Typically, they occur in thin sequences where erosion of a local source of sand is likely. Well 22/24a-1 (Figure 39) contains about 10 m of Aptian/Albian sands which were probably eroded from the eastern margin of the Forties-Montrose High, where thick Triassic sands may have been exposed. Smaller highs within the main graben may have been the sources of similar deposits, such as in well 29/86-1. Isaksen and Tonstad (1989) showed that similar sands may occur around structural highs at different levels within the Cromer Knoll Group, depending on the time of transgression of individual highs, and proposed the informal term Ran Sandstone units to include all such deposits (Figure 42).

Rødby Formation

At the top of the Sola Formation, near the base of the Albian, there is a marked drop in gamma-ray values and an increase in sonic velocity. This change is typically taken as the base of the Rødby Formation, which consists of pink to red and partly grey, calcareous shales that are broadly equivalent to the Red Chalk Formation of onshore Britain and the southern North Sea (Rhys, 1974; Cameron et al., 1992). In many wells there is a rise in gamma-ray values and a fall in sonic velocity towards the middle of the formation; this can be used to divide the Rødby Formation into upper and lower units (Figure 40). The maximum thickness of the formation is over 100 m, although it is much thinner in many areas.

Throughout Albian times there was a eustatic rise in sea level, which explains the change in sedimentation from the relatively high-gamma, organic-rich shales of the Sola Formation to the more-calcareous Rock Formation. A short-lived eustatic drop in sea level close to the Albian-Cenomanian boundary (Haq et al., 1987) may have resulted in a minor unconformity at the base of the overlying Chalk Group, and in some wells the Rødby Formation is absent. The unconformity can also be related to compressional stresses related to the Alpine collision of Africa and Europe (Ziegler, 1990a).

Upper Cretaceous and Danian

During Late Cretaceous and earliest Paleocene times, the North Sea and much of mainland Britain were submerged by an inundation of warm, oxygenated waters. Only the highlands of Scotland, Norway, and an area east of the Viking Graben remained as land while relative sea level rose by as much as 600 m to its highstand during latest Maastrichtian times (Hancock and Kauffman, 1979). Water depths varied between 100 and 600 m in southern Britain (Scholle, 1974), but may have reached 1000 m in the central parts of the North Sea (Hancock and Scholle, 1975). A broadly uniform environment was established over a particularly wide area, and the thick chalk and chalk-marl sequences that were deposited in the central North Sea are similar to those which occur in southern Britain and much of north-west Europe. However, mainly argillaceous sedimentation prevailed in the North Viking Graben and west of Shetland and this facies extended into parts of the central North Sea between late Turonian and Campanian times.

Regional subsidence patterns were no longer influenced by crustal stretching beneath the Central Graben, as this had gradually diminished through Early Cretaceous times (Ziegler, 1982). Instead, lithospheric cooling caused a down-warping that allowed thick Upper Cretaceous sediments to accumulate along the central axis of the North Sea (McKenzie, 1978). Subsidence continued to be influenced locally by the pattern of grabens, half-grabens and intrabasinal highs that had been established during Jurassic times.

There are up to 1500 m of Upper Cretaceous sediments in the Erskine Basin (Figure 43), but because contemporary highs, such as the Forties-Montrose and Jaeren highs and those along the western margin of the Central Graben, were not submerged until late in the Cretaceous, they accumulated much thinner sequences. Minor movements continued on the masterfaults of the Central Graben (Hancock, 1990), and the thickness of Upper Cretaceous sediments was also affected locally by halokinesis of deeply buried Permian evaporites. The Mid North Sea High became submerged early in Cretaceous times, and no longer formed a positive intrahasinal feature.

Erosion following Tertiary uplift completely removed any Upper Cretaceous deposits over Scotland and a zone 20 to 100 km wide off its eastern shoreline (Figure 43). Eastward from its outcrop, where it has been proven by drilling (Evans et al., 1981), the chalk is buried progressively deeper beneath Tertiary and Quaternary sediments that have their maximum thickness of 3500 m near the eastern limit of the report area (Figure 3).

The Upper Cretaceous chalk of north-west Europe is a very fine-grained limestone or micrite that is composed predominantly of the skeletons of planktonic marine algae belonging to the Haptophyceae family (Black, 1953). The algal skeletons comprise low-magnesium calcite crystals (Hancock, 1990); in living forms, these are arranged in rings known as coccoliths, with between 7 and 20 coccoliths overlapping each other to form globular coccospheres. The algae were so light that they rarely sank to the sea bed, and fossil coccospheres are rarely found intact as they were the staple diet of small crustacea known as copepods. Most of the chalk was deposited as copepod faecal pellets, each containing tens of thousands of coccoliths (Neugebauer, 1974; Hancock, 1990).

The coccolith-rich chalk that accumulated by gravitational settling in a low-energy environment is an autochthonous deposit. Autochthonous chalks predominate in the Upper Cretaceous sequences of mainland Britain, where they contain widespread flint bands, hardgrounds and omission surfaces (Bromley, 1975). The hardgrounds represent hiatuses in marine sedimentation in the shallowest regions of the chalk seas (Hancock, 1975; Kennedy and Garrison, 1975). In the central North Sea, similar hardgrounds might be expected within the chalk sequences that occur over and adjacent to the contemporary highs. Surprisingly, there is no evidence on seismic sections to indicate the presence of hardgrounds in these areas.

There are rhythmic variations in the amount of terrigenous clay that is intercalated within parts of the Chalk of southern Britain (Hancock, 1975; Hancock and Scholle, 1975). Similar rhythmic variations are well developed within Cenomanian, Coniacian, Santonian and Danian autochthonous chalks in the Norwegian sector of the central North Sea (Kennedy, 1987). These occur on a decimetre to metre scale, and can he recognised despite the bioturbation that has destroyed many of their primary depositional fabrics; Kennedy (1987) has described them as a bioturbated periodite facies. Most likely, their periodicity was generated by orbital-forcing factors such as Milankovitch cycles (Kennedy, 1987). The chalk intervals are largely composed of biogenic sediment, whereas darker coloured chalky marls comprise an admixture of this with varying proportions of terrigenous clay.

On the West Central Shelf and over the Mid North Sea High, the Upper Cretaceous is mainly composed of autochthonous pelagic chalks and limestones. Bands and nodules of flint and chert are abundant in these areas, but are less common in the deeper-water sequences of the Central Graben. On the margins of the graben and on the flanks of its intrabasinal highs, the chalk was particularly susceptible to downs-lope failure. This enabled mass-transport processes such as submarine slides, slumps, debris flows and turhidites to spread large volumes of redeposited, or allochthonous, chalk across the floor of the Central Graben (Kennedy, 1987). There, thick Maastrichtian and Danian sediments comprise a complex interdigitation of primary-pelagic and redeposited chalks.

Kennedy (1987) demonstrated that the autochthonous chalks, debris flows, turbidites and slumps each generate a characteristic combination of responses on sonic, porosity, formation-density, resistivity and neutron-porosity logs. These responses can be used to deduce the chalk's depositional environment in the wells, as core is generally not available. This is particularly important for deducing the reservoir properties of the chalk. Slowly deposited, bioturbated, autochthonous chalks were dewatered prior to burial, so they had low primary porosity, and provide poor hydrocarbon reservoirs (Kennedy, 1987). Rapidly deposited, allochthonous, debris flows and slump deposits had excellent primary porosity (Taylor and Lapre, 1987). Much of this porosity is retained in the chalk reservoirs of the major oilfields in the Norwegian sector and at the Joanne Oilfield in the UK sector of the central North Sea (Figure 43).

The chalk-dominated Upper Cretaceous and Danian sediments of the central and southern North Sea were assigned to the Chalk Group by Deegan and Scull (1977) and Rhys (1974). There is no formal subdivision of the uniform chalky limestones of the Chalk Group of the southern North Sea (Rhys, 1974; Cameron et al., 1992). To the north, certain units contain a greater proportion of terrigenous detritus; these are sufficiently distinctive and widespread for Deegan and Scull (1977) to have subdivided the Chalk Group into seven formations, which are retained in this report (Figure 44). All the formations become less argillaceous towards the Mid North Sea High, and the Hod and Tor formations cannot be separated in the south (Figure 45).

Hidra Formation

The Hidra Formation is largely Cenomanian in age, although boreholes in the Erskine Basin suggest that chalk sedimentation began during late Albian times in basinal areas of the Central Graben. Outside the graben, BGS borehole BH81/40 (Figure 43) proved the base of the Hidra Formation a few metres above the Albian–Cenomanian boundary on the West Central Shelf (Lott et al., 1985). Elsewhere, there may be a minor unconformity at the base of the Chalk Group, due either to a temporary eustatic fall of sea level or uplift related to a change of intraplate stress. In most wells, the base of the Chalk Group is defined by a sharp decrease in gamma-ray response and an increase in sonic velocity relative to the underlying Lower Cretaceous shales (Figure 47)." data-name="images/P944946.jpg">(Figure 46). Locally the Chalk Group rests unconformably on Jurassic or older rocks.

In the Central Graben, the Hidra Formation is composed of hard, mainly white or grey limestones and chalks that are up to 160 m thick in the Erskine and Fisher Bank basins (Figure 47). Thin beds of grey and reddish brown mudstone occur near the base of the formation and are most abundant in the north. Pink, waxy, tuffaceous beds have been noted in a number of wells (Deegan and Scull, 1977). Seismic interpretation indicates that the formation onlaps the flanks of the Forties-Montrose (Figure 45) and Jaeren highs; these intrabasinal upland areas were not submerged until much later in the Cretaceous. If Cenomanian chalk was deposited over the highs that form the western flank of the Central Graben (Figure 47), then it was completely removed by penecontemporaneous uplift and erosion. Outside the Central Graben, the Hidra Formation is less than 35 m thick, and is composed of soft chalk that is commonly pink in colour, with thin beds of grey and green, calcareous mudstone.

Owing to the variable proportion of interbedded mudstones, the Hidra Formation has a typically uneven geophysical-log character (Figure 47)." data-name="images/P944946.jpg">(Figure 46). Most of its sediments are of bioturbated periodite facies, although there may be thin calcarenite turbidite beds in the Central Graben (Kennedy, 1987). The equivalent sediments in the southern North Sea (Cameron et al., 1992) and the Norwegian sector of the central North Sea (Isaksen and Tonstad, 1989) contain a lower proportion of mudstone.

Plenus Marl Formation

The Plenus Marl Formation was deposited during a worldwide late Cenomanian to early Turonian marine transgression that led to the temporary development of dysaerobic seabed conditions across the European continental shelf (de Graciansky et al., 1984). Its sediments are rich in planktonic foraminifera that could thrive in the oxygenated surface waters, but they also contain an impoverished benthonic fauna.

The Plenus Marl Formation is typically composed of grey or black, variably calcareous, soft mudstones, although Deegan and Scull (1977) have noted that these are locally red or green. The mudstones are micaceous, contain varying amounts of glauconite and pyrite, and have a high organic content in the darker layers. Minor calcarenites and larninated chalks were deposited by turbidity currents in the Central Graben (Kennedy, 1987). Within equivalent sediments in the Norwegian sector, oscillations in bottom conditions and benthonic activity are indicated by the presence of thin layers of burrowed chalk (Kennedy, 1987).

As with the Hidra Formation, the Plenus Marl Formation attains its greatest thickness within the basinal depocentres of the Central Graben, where it reaches 40 m. Elsewhere in the graben, the formation is less than 10 m thick, and it is absent over marginal and intrabasinal highs (Figure 45). It is about 1 m thick in the south and west of the report area, and the equivalent Black Band is less than 1 m thick in eastern England (Hart and Bigg, 1981) and the southern North Sea (Cameron et al., 1992).

The Plenus Marl Formation is indicated on wireline logs by high gamma-ray response and low interval velocity. Where it is thin, these form spikes, but where it is thicker, its base is sharply defined and its upper boundary is gradational, representing an upward transition into argillaceous chalk and mudstone (Figure 47)." data-name="images/P944946.jpg">(Figure 46). Due to its low velocity compared with the other formations of the Chalk Group, the Plenus Marl Formation generates a high-amplitude seismic reflector that can be mapped across much of the central North Sea.

Hod Formation

Of Turonian to Campanian age, the Hod Formation is composed principally of white, pale grey, or pink, microcrystalline and cryptocrystalline limestones and chalks. Beds of white, pale grey and green shale and silty marl, and beds of brown to black carbonaceous shale, are particularly common within the middle part of the formation. Some of the darker shales may record short-lived, dysaerobic, sea-bed conditions. Southwards, the Hod Formation becomes progressively less argillaceous, and it cannot be separated from the overlying chalks of the Tor Formation on the Mid North Sea High ((Figure 45) and (Figure 48)). North-westwards, the Hod Formation merges into the limestone-dominated Herring Formation, and the argillaceous Flounder Formation. The approximate limits of these formations are depicted on (Figure 47).

Mainly bioturbated, pelagic sedimentation prevailed during Turonian times, and continued until at least the mid-Campanian. Minor layers of laminated chalk, that may be the distal deposits of turbidity currents, occur within the Turonian of the Norwegian sector (Kennedy, 1987), and may also be present in the Central Graben basins of the UK sector. There was a greater input of terrigenous detritus during Coniacian and Santonian times, especially in the north; bioturbated periodite cycles have been recognised within this part of the sequence in Norwegian waters (Kennedy, 1987). Early Campanian sediments of the Hod Formation show a decrease in argillaceous content, and unlike those in the Norwegian sector, may include debris-flow and slump deposits in the graben.

A sequence of hard, thinly-bedded, Turonian limestones and chalks forms a distinctive high-velocity unit at the base of the Hod Formation ((Figure 45) and (Figure 48)); this is laterally equivalent to the Herring Formation of the northern part of the report area. These high-velocity sediments are over 100 m thick in the basinal depocentres of the Central Graben, are absent over its marginal and intrabasinal highs, and are between 25 m and 70 m thick over the Mid North Sea High.

Some wells have recorded a concentration of flint nodules within Turonian chalk in the south of the report area; it is at this level that flint bands have their maximum development in mainland Britain (Mortimore and Wood, 1983).

The late Turonian to Campanian sediments of the Hod Formation above the high-velocity unit are between 200 m and 650 m thick in the depocentres of the Central Graben. Chalk sedimentation extended over the Jaeren High, probably during the early Campanian. The Hod Formation there is up to 100 m thick; the highs that form the western margin to the Central Graben were not submerged until later in Cretaceous times.

Herring Formation

Turonian age, the Herring Formation extends from the Moray Firth into the northern part of the central North Sea (Figure 47), where it is restricted to the flanks of the Forties-Montrose High, and to the Fisher Bank Basin. The formation is up to 230 m thick, and as with the equivalent sediments at the base of the Hod Formation, its hard or very hard, thinly bedded limestones correspond with a high-velocity unit (Figure 48). Lower-velocity sediments towards the base and at the top of the formation are likely to be softer, bioturbated, pelagic chalks. Thin beds of calcareous mudstone have also been recorded.

Flounder Formation

As with the Herring Formation, the Flounder Formation is restricted to the northern part of the central North Sea (Figure 47). It is of latest Turonian to Campanian age, and its argillaceous sediments are locally 500 m thick in the Fisher Bank Basin, and up to 400 m thick on the south-western flank of the Forties-Montrose High, but are absent over its crest ((Figure 45) and (Figure 47)).

The transition from limestone-dominated Herring Formation to the argillaceous sediments at the base of the Flounder Formation is indicated on well logs by an increase in gamma-ray response and a significant decrease in velocity (Figure 48). Most of the formation is composed of pale and dark grey, calcareous, pelagic mudstones and shales that grade into, and are interbedded with, pale to dark grey and pinkish chalks and limestones. Thin beds of hard, white, fine-grained limestone have been recorded in many wells.

Tor Formation

The Tor Formation is late Campanian to Maastrichtian in age, and was deposited when a further rise in sea level submerged the Forties-Montrose High, and most of the western flank of the Central Graben, for the first time. Where the formation is absent, there has probably been post-Cretaceous erosion rather than nondeposition. Where it overlies Lower Cretaceous or older sediments on the intrabasinal and graben-marginal ridges, the Tor Formation is generally less than 150 m thick, but it is more than 250 m thick in the basinal depocentres of the Central Graben. It has its maximum thickness of 625 m in the Erskine Basin, where there is an intra-Campanian unconformity at the base of the Tor Formation; local uplift and erosion of the chalk may have been caused by mid-Campanian halokinesis of the deeply buried Permian evaporites.

There was a marked reduction in the input of terrigenous detritus from mid-Campanian times onwards, and the Tor is the least argillaceous formation of the Chalk Group. Consequently, in wells to the north of 56°N, its sediments generate a particularly even gamma-ray response with lower values than the Hod and Flounder formations (Figure 50)." data-name="images/P944949.jpg">(Figure 49). Sediments below the Tor Formation are less argillaceous to the south of 56°N, with the result that the Tor and Hod formations cannot be distinguished over the Mid North Sea High (Figure 45).

The Tor Formation is mainly composed of hard, white, pale grey, tan or pink limestone and chalk. The formation is generally homogeneous, but white, grey and beige packstones and mudstones, green shales, and thin beds of marl have been noted. One well in the Fisher Bank Basin has recorded a thin bed of sandstone within the Chalk Group; this was probably derived from the Forties-Montrose High.

Outside the Central Graben, bioturbated pelagic sediments characterise the upper Campanian and Maastrichtian, but over part of the Central Graben (Figure 45), pelagic sedimentation was punctuated by inflows of allochthonous slump-slide, debris-flow and turbidite units. These mass-flow deposits locally comprise up to 40 per cent of the Maastrichtian chalk, and individual allochthonous units are up to 35 m thick on the flanks of marginal and intrabasinal highs. Hatton (1986) has used well-log correlation of low-velocity intervals to deduce that sixteen, single or multiple, high-porosity, allochthonous-chalk intervals were deposited across this area during Maastrichtian times. Seismic activity on faults along the margins of the Central Graben may have triggered many of these events.

Ekofisk Formation

The Ekofisk Formation is of early Danian age, and has been removed by Tertiary erosion from almost all of the central North Sea outside the Central Graben (Figure 50). Up to 120 m of lower Danian sediments are preserved in the basinal depocentres of the graben, and between 10 and 60 m over its intrabasinal highs.

Most of the Ekofisk Formation is composed of hard, white, pale grey, tan and beige limestones and chalks. Thin beds of grey, calcareous mudstone and shale are particularly common within a basal low-porosity unit that, in the Norwegian sector, is informally designated the 'Ekofisk tight zone' (Isaksen and Tonstad, 1989). Above this unit, pelagic bioturbated sediments interfinger with three allochthonous chalk intervals (Hatton, 1986). The lowest of these is composed mainly of reworked Maastrichtian chalk, and the other two are composed of reworked Danian sediment. The uppermost allochthonous unit is the most extensive (Figure 50); it comprises slump deposits, debris flows and minor turbidites that are up to 35 m thick adjacent to the West Central Ridge and the Argyll Shelf (Hatton, 1986).

A marked influx of terrigenous detritus during the early Danian is indicated in many wells by an increase in gamma-ray response, and a decrease in sonic velocity (Figure 50)." data-name="images/P944949.jpg">(Figure 49). The top of the Ekofisk Formation, and of the Chalk Group, is marked by an abrupt facies transition to the shales, marls, sandstones and reworked chalks of the Montrose Group.

Chapter 8 Paleogene and Neogene

Paleogene

Paleogene sedimentary rocks are continuously distributed over much of the North Sea, and occur onshore farther south in south-eastern England and northern Europe. In the report area, they are confined to the east of an erosional limit some 100 km off the east coast of the UK (Figure 2), and reach an aggregate maximum thickness of some 1500 m.

The quiescent tectonic regime which existed during deposition of the Chalk Group ceased towards the end of the early Paleocene (Danian). Uplift above sea level of the Hebrides–Shetland axis to the north-west and west of the North Sea Basin (e.g. Zeigler, 1982) was accompanied by the outbreak of extensive volcanicity in the areas surrounding the present north-east Atlantic Ocean, including the Hebridean province. These tectonothermal events were associated with the phase of sea-floor spreading which resulted in the opening of the ocean (Bott, 1975). The effects of the volcanism are recorded in the North Sea by tephras at various levels in the Paleogene succession (Knox and Morton, 1988).

The uplift of the Hebrides–Shetland axis established a wholly new geography. An easterly to south-easterly flowing drainage system became established on the Orkney–Shetland Platform and Scottish Highlands, resulting in the reworking of the sedimentary cover of the newly emergent terrains, and the dispersal of their detritus into the North Sea Basin. In the UK sector there was little, if any, clastic input from the eastern margin of the basin early in Tertiary times.

The preserved Paleocene and Eocene rocks of the central North Sea consist of delta, shelf and basin systems. The basinal rocks comprise for the most part mudstones; these are generally hemipelagic in character, but commonly contain thick, stacked, submarine-fan sandstones. During the Oligocene, hemipelagic argillaceous deposition was dominant, although sands were laid down following sea-level fall in late Oligocene times. The delta and shelf systems consist of easterly prograding, upward-coarsening successions of mudstones, siltstones, and sandstones. There are also sandstones and mudstones which contain lignites; these appear to have been deposited in essentially subaerial environments on delta tops and coastal plains.

In the most general terms, it seems probable that the basinal, submarine-fan sandstones may have been deposited at times of low relative sea level. The prograding shelf systems may have been deposited when sea level was stable or gently falling following transgressions; the lignites may be the first expression of rising sea level in the subaerial environment (cf Stewart, 1987).

During Paleocene times, the shape of the depocentre of the North Sea Basin approximately followed that of the underlying Mesozoic graben system, for the major Paleocene submarine-fan sandstones within the central North Sea are largely confined by the underlying Mesozoic fault system (Figure 51).

However, evidence of tectonic reactivation of the graben-margin faults is, for the most part, lacking. It is thought that the shape of the depocentre was defined by a combination of increased subsidence towards the basin centre, and differential compaction of the Mesozoic sediments in the graben relative to those on the platform areas (Milton et al., 1990).

During Eocene times, a broader, more regularly shaped basin gradually became established (Figure 52). By the end of the Eocene, the Central Graben had been partially infilled at its north-western end; subsequently, mud-laden currents were directed towards the subsiding south-eastern portion that remained a deep-water zone. At the end of Eocene times, as the basin increased in width, the Central Graben overtook the Viking Graben as the main centre of Tertiary sedimentation in north-west Europe (Gramann and Kockel, in Vinken 1988).

One fault system is recognised to have been active in the central North Sea, certainly in late Eocene to Oligocene times, and conceivably earlier. This system trends north-west to south-east, and delimits the eastern margin of the Devil's Hole Horst (Figure 52). There was an upsurge of salt diapirism in the central North Sea during the late Eocene and early Oligocene; this may indicate a low but widespread level of tectonic activity. Like all post-Permian strata in the central North Sea, the distribution of Paleogene basinal sediments has been affected to some degree by halokinesis; how far the active growth of salt domes and walls controlled contemporaneous sedimentary processes is not evident.

Paleocene and Eocene depositional systems

Water depths in the centre of the North Sea Basin during the Paleocene and Eocene are disputed (Parker, 1975; Mudge and Bliss, 1983; Morton, 1982), but probably reached hundreds of metres at times. Both the shallow-marine and basinal rocks contain distinct assemblages of microfaunas; these were controlled by both water depth and the degree of water circulation in the basin (King, 1983, 1989; Mudge and Copestake, 1992). The most open-marine faunas are dominated by planktonic foraminifera; these occur in both upper Danian (lower Maureen Formation) and Ypresian (basal Horda Formation) rocks. In the basin centre, more-restricted, basinal-marine conditions are indicated by agglutinating foraminiferal biofacies. Shallow-marine strata and basinal calcareous rocks commonly contain calcareous benthonic foraminifera. Highly impoverished faunas dominated by diatoms, such as occur in the Sele Formation, may indicate highly restricted water circulation, with anoxic conditions at the sea bed, and possibly somewhat lowered salinity in the basin.

There were three depositional systems active in the area: delta, shelf and basin. Each contains contrasting lithological successions whose boundaries are by no means fixed. As illustrated in cross-section (Figure 53), the boundaries shifted laterally with time, for they were sensitive to change in relative sea level, to sites of subsidence, and to elastic sediment supply.

Delta

The zone of riverine delta construction was confined to the western margin of the report area, where the river systems draining the western sourcelands debouched into the North Sea Basin, possibly as Tay and Forth proto-river systems. This zone has now been largely removed by erosion. On two occasions, during the late Paleocene and late Eocene, low sea levels promoted the advance of the delta systems into the North Sea Basin, with the deposition of successions characterised by the presence of in-situ lignites and freshwater floras and faunas.

Shelf

Of more widespread occurrence is the shelf system which fronted the outbuilding deltas. The division between the Paleocene and Eocene freshwater/subaerial-deltaic and shallow-marine shelf environments cannot be defined on available seismic or well evidence, but their combined extent can be charted from their characteristic south-easterly facing, clinoformal expression on seismic profiles. These occupy the eastern Moray Firth Basin (Andrews et al., 1990), and extend southwards across the Forth Approaches Basin and much of the western part of the West Central Shelf ((Figure 51) and (Figure 52)).

The maximum progradation of the shelf zone in any one sequence is marked by a strong reflector that marks a frontal ramp (Figure 53); two of the most seismically conspicuous shelf-front limits are mapped on (Figure 51) and (Figure 52). These correspond to the phases of late Paleocene and late Eocene deltaic advance, and are termed by some authors (e.g. Harding et al., 1990) as shelf slopes where gradients were sufficient to generate small-scale turbidity flows and slump masses in the adjoining basin.

Basin

Although water depths fluctuated, deep-water basinal regimes were maintained in the central North Sea throughout Paleocene and Eocene times. The deeper-water basin coincided very broadly with the underlying Mesozoic Central Graben system, and was the site of hemipelagic mud deposition interspersed with, and in some areas almost overwhelmed by, thick spreads of sandy sediment which were swept into the basin by turbidity currents. Faunal and sedimentological evidence suggests that at one time in the late Paleocene, the basin was oversupplied with sediment to the point that the circulation of marine waters was disrupted, leading to local stagnation and reducing bottom conditions (Knox et al., 1981). The basin did however retain a marine character; aspects of sedimentology and the presence of truncational unconformities may indicate the work of strong, possibly tidal, bottom currents. Other factors controlling depositional processes in the basin at this time included the overlap of successive pulses of fan-sand deposition, for the topography created by the deposition on an earlier lobe tended to direct later sedimentation. In some cases, the gradient created at lobe margins may have been locally sufficient to initiate a secondary phase of turbidite flow (e.g. Knox et al., 1981).

The Basin turbidites are characterised on seismic profiles by a hummocky, mounded, or reflective seismic facies which contrasts with the planiform layering, or commonly blank appearance, of the mudstone facies. From their mapped distribution, it is clear that the turbidites have aggregated into numerous, discrete, submarine fans whose character and extent evolved with time. The early to mid-Paleocene fans, such as the Maureen and Andrew fans (Figure 51), are multilevel, areally extensive, coalescent turbidite systems; some degree of post-depositional reworking may have occurred, as many of the North Sea examples lack the internal sedimentary features expected of such depositional systems (Enjolras et al., 1986).

The later Paleocene to Eocene fans are smaller and areally separate. With the aid of high-resolution seismic data, a high degree of internal detail may be discerned. The main example in the report area is the early to mid-Eocene Tay Fan, which has a morphology conforming to the classical 'delta' geometry, with well-developed channel systems feeding overlapping depositional lobes ((Figure 52) and (Figure 53)). The source for both these turbidites and the early to mid-Paleocene fans lay in areas now coincident with the prograded shelf deposits to the west of the basin, although the turbidites that formed the Tay Fan may well have been initiated on the possibly contemporaneously active fault zone that defines the eastern margin of the Devil's Hole Horst.

The youngest, late mid-Eocene fan sands, such as the Alba Fan in the Fisher Bank Basin (Figure 52), contain linear, supposedly channel-sand bodies that have proved difficult to image seismically due to diffractive interference from an overlying, faulted, Oligocene claystone/limestone sequence. Their source has proved difficult to locate, but from work at the Alba Oilfield, Harding et al. (1990) suggested local derivation by slumping or mass flow off the contemporaneous shelf slope.

Scattered occurrences of mounded basinal sands, not attributable to discrete fan systems, are found along the eastern margin of the West Central Shelf (Figure 53). Such sands may be categorised as lowstand fans, deposited during times of low sea level when the shelf regime advanced basinward, and sea-bed conditions were sufficiently energetic to facilitate the transport of coarse-grained sediment beyond the normal range of slope-derived turbidites.

Post-Chalk Group Paleocene

The post-Chalk Group Paleocene stratigraphy of the central North Sea has been the subject of several studies. The very varied lithological character of the interval, and its rapid lateral facies changes, has encouraged local informal stratigraphies based on well data and given oilfield names. This tendency has hindered regional correlation. The main disagreements hinge on the degree of stratigraphical equivalence of laterally contiguous shelf and basinal successions. A formal lithostratigraphical nomenclature based on well-log response was achieved for the UK sector by Deegan and Scull (1977), and more recently for the Norwegian sector by Isaksen and Tonstad (1989). A revised scheme for the UK sector has been presented by Mudge and Copestake (1992). More detailed informal stratigraphies have included the palaeontologically and mineralogically based study by Knox et al. (1981), and Stewart's (1987) seismic-stratigraphy study. The latter author interpreted the areal distribution and chronological order of deposition of the Paleocene to earliest Eocene interval with particular emphasis on the response of sedimentation to relative sea level in the basin.

For the purposes of this descriptive passage, a recent revision by Knox and Holloway (1992) is used, with reference to other schemes where appropriate (Figure 54). The Knox and Holloway (1992) scheme is derived largely from those of Deegan and Scull (1977) and Mudge and Copestake (1992), and they follow many of the principles proposed by Mudge and Copestake (1992). The succession is divided into the Montrose and Moray groups, and then subdivided into formations consisting of either basinal mudstone units or pro-grading shelf systems. Within these formations are defined members, which are principally submarine-fan sandstones in the basinal units, and shelf sandstones in the shelf systems.

The thickness of the Paleocene interval in the central North Sea (Figure 55) is largely dependent on the presence or absence of submarine-fan sands. It is almost 1000 m thick in the sand-dominated area overlying the Buchan Horst, but argillaceous sediments thin south-eastwards to some 100 m or less over the Mid North Sea High.

Montrose Group

The Montrose Group commonly overlies the Ekofisk Formation of the Chalk Group; the boundary between the two groups is taken at the log break corresponding to the upward change from autochthonous chalks to the grey silty marls or reworked chalks of the Maureen Formation, which is the lower of two formations in the Montrose Group. The upper division is termed the Lista Formation.

Maureen Formation

The Maureen Formation can be subdivided into two informal units (Figure 54).

The lower unit, the Basal Marl unit, which is unit A3 of Knox et al. (1981) and unit 2A (in part) of Stewart (1987), is very variable lithologically, and was originally assigned to the Ekofisk Formation by Deegan and Scull (1977). ln the south-east of the report area, the unit is singularly well developed, and has been called the 'Burns Formation' on some composite logs. It consists dominantly of pale grey marls, but commonly contains both limestones and submarine-fan sandstones. The limestones are termed the Reworked Chalk unit (Figure 54), for they include beds of resedimented chalk stripped off high areas to the west of the Central Graben and carried into the basin centre by mass-flow processes. These limestones are commonly associated with strong seismic reflectors which can be mapped over wide areas in the central North Sea (Johnson, 1987). Biostratigraphically, the Basal Marl unit is characterised by the presence of both planktonic and benthonic foraminifera. Overlying the Basal Marl unit in the Central Graben ((Figure 51) and (Figure 54) is a sand unit, the Maureen Sandstone member, which comprises the Maureen Fan (unit 2B of Stewart, 1987). Distally, both the marly and the sandy units grade into a marly claystone facies that is often referred to as the Maureen Formation equivalent, or the Vale Formation of Isaksen and Tonstad (1989).

At the top of the Maureen Formation is a distinctive, dark grey, silty mudstone unit, up to 20 m thick. It is known as the Cenosphaera (or Cenodiscus) Shale, as it contains flood proportions of the radiolarian genus Cenosphaera. This mudstone forms an important lithological and biostratigraphical marker.

Lista Formation

The Lista Formation commonly overlies the Maureen Formation. Characteristically, it comprises up to 250 m of pale green, grey-green or dark red, blocky claystones and mudstones, which contain abundant agglutinating foraminifera that are commonly visible in drill cuttings. The mudstones locally become greener or redder and less carbonaceous upwards. The gamma-ray log records a low-level spectrum, whereas the sonic log is characterised by being peaked and erratic (Figure 56).

The Lista Formation in the central North Sea contains a major submarine fan; this was called the Andrew Fan by Deegan and Scull (1977), and is now renamed the Mey Sandstone Member (Figure 53). A typical drilled succession through the member consists of stacked, 5 to 10 m-thick sand/shale successions with rather ragged log motifs, accompanying 50 to 100 m-thick massive sand units with 'boxcar' motifs (Figure 56). The thickness of the member is very variable, and is mainly controlled by the fan's lenticular geometry; it is around 200 to 300 m thick in the Fisher Bank Basin, thinning to zero at the limits outlined in (Figure 51).

The Mey Sandstone Member can be divided into lower, middle and upper sandstones (units 3, 4 and 5 of Stewart, 1987); these correspond respectively to the Andrew, Glamis and Balmoral members of Mudge and Copestake (1992). In the north-west of the area, each unit can be assigned to individual fans, and the middle sandstone unit is characteristically tuffaceous. The upper sandstone and the tuffaceous sandstone are restricted to the north-western part of the report area, but the lower sandstone is much more widespread, reaching far into the Central Graben (Figure 51).

Undifferentiated shelf equivalents

As indicated on (Figure 53), the basinal Montrose Group passes laterally, westward, into a prograded shelf unit. This unit consists of a silt/sand succession up to 500 m thick which has not been differentiated or subdivided in any formal stratigraphical scheme.

Moray Group

In the Central Graben, the boundary between the Moray and Montrose groups is taken at the upward change from the commonly green, blocky mudrocks with abundant agglutinating foraminifera of the Lista Formation, to the dark grey, generally laminated shales of the Moray Group. This boundary is characterised by a regional seismic reflector (Rochow, 1981). The Moray Group is divided into the Dornoch, Sele and Balder formations (Figure 54).

Dornoch Formation

The Dornoch Formation consists of delta and shelf systems which originated to the north-west of the report area, and prograded eastwards over both the undifferentiated shelf equivalents of the Montrose Group, and the Lista Formation. It is up to 600 m thick, and can commonly be divided into four units: the Lower Dornoch Sandstone, Dornoch Mudstone and Upper Dornoch Sandstone are informal subdivisions, but the Beauly Member at the top is formally defined.

Seismically, the group is marked by prograding reflectors which at their eastern limit define a step or 'front' which can be traced south from the Buchan Horst to the Devil's Hole Horst (Figure 51). This is seen to represent the point of maximum advance of this delta/shelf system into the basin. This advance was accompanied by the development of emergent delta-swamp environments over a wide area of the Moray Firth and Devil's Hole Horst. The Beauly Member is a terrestrial, delta-top formation consisting of silt to coarse-grade sand, and is characterised by in-situ lignites. The member is up to 100 m thick, but thins towards the shelf margin.

Sele Formation

Basinward, the Dornoch Formation passes laterally into the Sele Formation, which, where basinal fan sandstones are absent, consists of 30 to 80 m of dark grey, laminated shales with an impoverished microfauna consisting predominantly of pyritised diatoms. The gamma-ray log response of the Sele Formation is markedly higher than that of the underlying Lista Formation, commonly with a sharp peak at, or close to, the basal contact (Figure 56). The sonic log records a smooth, low-level trace. The lower part, in which the widespread and economically important Forties Sandstone Member is developed (Figure 54), is characterised biostratigraphically by dinoflagellate cysts of the genus Apectodinium, particularly A. augustum (Harland) Lentin and Williams.

The Forties Sandstone member is a commonly massive sand unit which is up to 250 m thick (unit 7 of Stewart, 1987). It comprises a major fan complex occupying much of the Central Graben, as well as the secondary Gannet Fan on the margin of the West Central Shelf (Figure 51), which has separate provenance (Armstrong et al., 1987).

In the area immediately to the east of the Dornoch Formation shelf front ((Figure 51) and (Figure 54)), a second member, the Cromarty Sandstone Member, is developed within the Sele Formation. The Cromarty Sandstone Member is thought to have been derived from the Upper Dornoch Sandstone during a time of relatively lowered sea level, and to have been deposited against the slope of the prograding shelf system.

In the eastern Central Graben in the Norwegian sector, local sand developments occur within the Sele Formation. These may be up to 150 m thick and were given separate status as the Fiskebank Formation by Deegan and Scull (1977). The use of this term is retained.

Balder Formation

The Balder Formation is of particular interest because of its association with volcanic activity and ocean-floor spreading in the North Atlantic (Morton and Knox, 1990). The formation is usually about 50 m thick, and in the central North Sea it overlies the Sele Formation and onlaps the Dornoch Formation (Figure 54). It is subdivided into two informal units: the lower comprises laminated, brown or grey mudstones with abundant tuffs; this is the well-known Ash Marker, which normally corresponds to a strong seismic reflection. The upper unit consists of grey to brown mudstones with few tuffs; it is strongly transgressive, and in some western parts of the central North Sea rests directly on the Beauly Member of the Dornoch Formation. Biostratigraphically, the formation as a whole is characterised by the presence of the diatom Coscinodiscus, and the lower tuffaceous unit in particular by the dinoflagellate cyst Deflandrea oebisfieldensis Alberti.

In well 21/26-1 (see (Figure 59)), the formation is locally represented by a massive sand unit interpreted as a small-scale submarine-fan development derived from a basin-margin high in the Gannet area (Armstrong et al., 1987).

Regardless of lithology, the formation has a characteristic 'bell-shaped' well-log signature which affords easy correlation across the basin; it is a valuable, synchronous, stratigraphical marker. Although historically taken as the top of the Paleocene, it is of Ypresian (early Eocene) age, and marks the beginning of a major marine transgressive event and basin deepening, which peaked during mid-Eocene times.

Eocene (post-Balder Formation)

The maximum thickness of this interval occurs in the southeast of the report area (Figure 57). However, thick successions of Eocene (post-Balder Formation) strata also occur immediately to the east of the contemporaneous shelf edge, where silt- or finer-grained clastic sediment derived from the shelf augmented hemipelagic accumulation. Furthermore, the most extensive sand deposits are found within the Tay Fan, a major fan-sand development which extends across the West Central Shelf and West Central Graben (Figure 52). Other significant sand developments are associated with two slightly younger fans that impinge on the northernmost part of the area; the T-Block and Alba fans (Figure 52).

Formal stratigraphical schemes for the Eocene (Figure 54) have been presented by Deegan and Scull (1977) and Isaksen and Tonstad (1989). These classifications assigned the Eocene, Oligocene, and lower Miocene strata, to the Hordaland Group, and did not differentiate the sand-prone successions of the Eocene shelf regimes. However, two basinal sand units were formalised: the early Eocene fan-sand of the Frigg Formation (Deegan and Scull, 1977) which only occurs to the north of the report area, and the mid- to late Eocene Grid Formation (Isaksen and Tonstad, 1989) that is retained by Knox and Holloway (1992) as the Grid Sandstone Member. Knox and Holloway (1992) have divided Deegan and Scull's (1977) Hordaland Group into two: the Stronsay Group of Eocene age, and the Westray Group of Oligocene age. The Stronsay Group is divided into a basinal claystone and siltstone succession for which the term Horda Formation is applied, and a sand- and silt-prose shelf succession termed the Mousa Formation.

Historically, Eocene oilfield reservoir sands have been given informal formation status, and in some cases the terms have been inappropriately applied to sequences of similar age beyond the field area. Examples include the Tay formation, a term applied to sandy intervals of broadly early Eocene age that occur over wide areas of the central North Sea, whether or not they are part of the Tay Fan. In the most recent work of Knox and Holloway (1992), basinal sand developments are assigned member status; the Tay formation has become the lower and upper Tay Sandstone members. Other sands not associated with the Tay Fan are assigned to the Grid Sandstone Member.

However, for the purposes of this account, the Eocene is informally subdivided on the basis of electric-log responses (especially gamma-ray) and seismic stratigraphy. Four subdivisions are recognised ((Figure 54), (Figure 58) and (Figure 59)), and, to avoid confusion with other schemes, are designated in descending stratigraphical order with Roman numerals I to IV, each of which have shelf equivalents which correspond to the Mousa Formation. The four units can be identified throughout the North Sea Tertiary province in both basinal and shelf successions, and are likely to be synchronous with transgression/regression cycles.

Each unit is marked by a characteristic, commonly cyclic, gamma-ray motif (Figure 58), but unit tops are not in all cases marked by a sharp break on the gamma-ray log ((Figure 59), well 21/12-1). Picks are therefore generally taken with respect to breaks on the sonic log, which vary considerably in amplitude. This method affords correlation with seismic data, and allows recognition of the log breaks as seismic-sequence boundaries. Some are regionally persistent seismic markers, and may be unconformities. Where sandy intervals are present in the section the log signature is modified, for sand bodies have a typical low-gamma/high-sonic motif, with a slight upward-coarsening trend displayed on the gamma-ray log (Figure 59).

Eocene Unit IV

Eocene unit IV is characterised by an overall high gamma-ray level, usually with a distinct peak at the base, and a more subdued one at the top. The top is clearly marked by a sonic break (Figure 58), allowing the unit to be mapped over wide areas using seismic data. The unit, equates with unit 10 of Stewart (1987), but well evidence indicates a wider range of occurrence than that determined seismically. It is ostensibly Ypresian in age, although numerous micropalaeontological determinations place the unit top in the lower part of the Lutetian (Figure 54). The base of the unit forms the base of the Eocene section only where there is an erosional unconformity, such as on the crests of salt diapirs. Unit IV averages some 90 m in thickness, but thins rapidly where it onlaps contemporaneous highs, and only its top may be present on the shelf. Except in the Tay Fan, where it is replaced by the sand-prone lower Tay Sandstone Member, the unit is dominated by dark-coloured claystones, with the diagnostic presence of planktonic foraminifera, commonly including Globigerina Linaperta Finlay, in a characteristically reddened section at its base.

Eocene Units II and III

Eocene units II and III are similar; both have the same 'scooped' or cuspate gamma-ray log motif showing an initial drop immediately above the unit base, then a gradual upward increase ((Figure 58) and compare well 22/2-5 in (Figure 59)). Ideally this is matched to a regular upward decrease in sonic velocity above a sharp rise defining the basal break. The unit II–unit III boundary is generally represented by a well-marked log break, which corresponds to a regionally persistent seismic event. The two units together represent most of the time span of the mid-Eocene (Lutetian and Bartonian), but the lithostratigraphical boundaries do not appear to correspond to those of the stages ((Figure 54) and (Figure 58)).

Within areas of fan-sand deposition, either or both units may contain thick sand successions as typified by well 22/2-5 (Figure 59). Lutetian sands developed within unit III comprise the upper storey of the Tay Fan (Figure 53), and are assigned to the upper Tay Sandstone Member (Knox and Holloway, 199:). Lesser developments in the Fisher Bank Basin, forming the southern margin of the T-Block Fan (Figure 52), are assigned to the Grid Sandstone Member. Discontinuous Bartonian sands of unit II are largely confined to the northern part of the Central Graben and the Fisher Bank Basin; there they form the southward extension of the T-Block and Alba fans. These sands have been assigned to the Grid Sandstone Member, with the specific exception of the Alba Oilfield reservoir sands, which are termed the Nauchlan Sand Member (Knox and Holloway, 1992).

In the basin regime beyond the fan-sand developments, the units are dominated by clay- to silt-grade sequences, and are now included in the Horda Formation. The combined units are 100 to 150 m thick, with abundant, thin, calcareous or limestone bands. On the shelf, time-equivalent units of similar thickness consist of sets of stacked sands with intervenir g claystone bands, and are included in the Mousa Formation.

Eocene Unit I

Where present over the Central Graben, this uppermost Eocene unit is dominantly mudstone, and has a typically upward-coarsening gamma-ray motif. The sonic response is not generally distinctive, but a break typically developed at the top of the unit corresponds to a strong event seen on seismic profiles throughout the central North Sea. The unit is taken as being wholly late Eocene (Priabonian) in age, although early Oligocene units may be included in it locally. On the West Central Shelf (Figure 53), unit I forms a sandy, commonly glauconitic, prograded-shelf succession that is some 80 m thick and is included in the Mousa Formation by Knox and Holloway (1992). The sands thin rapidly into the Central Graben, where they are rarely more than 10 m thick, and may be absent altogether due to erosion. Where present, they are assigned to the Grid Sandstone Member.

Oligocene

During a transgressive phase in early Oligocene times, the sea reoccupied the western fringes of the North-West European Basin that had been exposed during the late Eocene. The absence of late Eocene faunal markers, combined with the presence of mid-Eocene fauna at, or close to, the top of the Eocene shales, led Knox et al. (in Vinken, 1988) to suggest the presence of a regional pre-Oligocene hiatus. This is most evident to the south of 56°N (Figure 60), where well logs indicate an unconformity at the base of the lower Oligocene, which rests directly on either middle Eocene or very thin upper Eocene deposits. The unconformity extends along the western edge of the basin, where Oligocene sediments overlap on to mid-Eocene and older sediments (Figure 63)." data-name="images/P944961.jpg">(Figure 61). There are local unconformities at this level close to salt piercement structures (Figure 60).

The report area contains the thickest sequence of Oligocene sediments recorded in north-west Europe (Vinken, 1988), locally exceeding 1000 m (Figure 60). The Oligocene sediments of the Central Graben were not differentiated by Deegan and Scull (1977), but included within the Hordaland Group. Nevertheless, informal names such as the Thurso formation (Sutter, 1980) have been applied to Oligocene strata, and they are now included in Knox and Holloway's (1992) Westray Group.

The sediments mainly comprise brownish grey mudstones that contain more organic matter than the underlying Eocene sediments. Thin stringers up to 2 m thick of yellowish brown limestone and dolomite are common, particularly towards the south-eastern end of the Central Graben. Only in the north-western corner of the report area are there coarser-grained sediments, mainly siltstones, that were laid down closer to the uplifted Scotland–Shetland Massif from which they were derived (Andrews et al., 1990). Well logs indicate gently upward-coarsening cycles, although these do not reach sand-grain size (Sutter, 1980). Both siltstones and mudstones are micaceous, and there are conspicuous beds of dark brown, organic shale. The sediments are interpreted as indicating a low-energy, marine environment. An isolated tuff of early to mid-Oligocene age recorded in well 29/10-1 (Figure 60) may correlate with ash recorded in DSDP Hole 336 from the middle Oligocene of the Voring Plateau, off mid-Norway (Sutter, 1980).

A major mid-Chattian (late Oligocene) eustatic fall in sea level (Vail et al., 1977) may have been the cause of the deposition of some arenaceous units within the predominantly argillaceous sediments of the Central Graben. One such unit, the Vade Formation of Isaksen and Tonstad (1989) in the Norwegian sector, locally acts as a gas reservoir (Ekern, 1986). It is composed of well-sorted, glauconitic, coarse-grained silts and very fine-grained sands interbedded with argillaceous siltstones. Similar increases in grain size have been identified in the upper Oligocene of the Moray Firth (Andrews et al., 1990), and in the Danish sector (Nielson, 1980; Bjorslev Nielsen et al., 1986).

Oligocene sediments were not deposited in the western part of the report area, and were reported absent at the Kittiwake Oilfield (Figure 60) by Glennie and Armstrong (1991).

Central Graben changed from a restricted, partly anoxic basin, to an oxygenated, shallower-water environment in which benthonic foraminifera were dominant over planktonic types (Gradstein and Berggren, 1981).

These changes also caused the beginnings of a large-scale disappearance of agglutinating foraminifera at about the Eocene–Oligocene boundary (Gradstein and Berggren, 1981). The agglutinating foraminifera indicate areas of deeper water with restricted circulation (King, 1983), and their disappearance was complete in all but the deepest parts of the Central Graben by the mid-Chattian eustatic sea-level fall (Berggren and Gradstein, 1981).

Neogene

Miocene

However, released well data from the oilfield area indicate increased gamma-ray values compared with the underlying Eocene Horda Formation sediments. Elsewhere these increases correlate with abundant glauconite grains and pellets, and an increase in organic content, commonly reported near the base of the Oligocene succession (Figure 58) and (Figure 59).

During the early Oligocene, the Greenland–Svalbard gap began opening, allowing greater communication between the Atlantic and Arctic oceans, and causing an influx of colder water into the North Sea. This change is evidenced by the replacement of warm-water fish by boreal forms (Gramann and Kockel in Vinken, 1988), and a drop of 12°C in North Sea bottom-water temperatures (Buchart, 1978). This cold-water influx increased oxygenation, and caused a fall in the carbonate compensation depth. This change was coincident with shallowing of the basin due to sediment infilling, so that the Miocene sediments reach over 1200 m in thickness in the vicinity of the Josephine Oilfield (Figure 62), but vary considerably in thickness over relatively short distances, perhaps due largely to salt movement both during and after deposition. The western extent of Miocene strata is uncertain due to paucity of data, but there could be two westerly protrusions which may represent minor supply from proto-Forth and proto-Tyne/Tees rivers.

The base of the Miocene occurs at up to 2000 m below sea level, and is locally unconformable (Figure 62) as the uppermost Oligocene, or more commonly the lowermost Miocene, is missing. Lower Miocene sediments are predominantly fine-grained and lithologically comparable to those of the Oligocene, and they similarly belong to the grey mudstones of the Westray Group, part of the Hordaland Group of Deegan and Scull (1977). However, well logs reveal more erratic gamma-ray values and greater variation in sonic velocity. A study of foraminifera from the Forties Oilfield indicates that early Miocene water depths were about 150 to 200 m, implying an outer-shelf environment (Sudijono, 1975).

A rapid increase in sedimentation rates in the central North Sea began at about 12 Ma, around mid-Miocene times, at the Westray-Nordland Group boundary (Bjorslev Nielsen et al., 1986). There was a contemporaneous marked increase in both subsidence and sedimentation rates in continental-margin basins around the North Atlantic, causing uplift of the basin edges in Norway and the British Isles, possibly the result of increased compression associated with a reorganisation of the directions of the Atlantic spreading system (Cloetingh et al., 1990).

The mid-Miocene was the time when Scandinavia first became a major sediment source, and a change from dominantly sandy to dominantly muddy sediments in the Moray Firth indicates diminishing sediment output from Scottish areas (Deegan in Vinken, 1988). The base of the middle Miocene is an easily recognised seismic marker which shows strong onlap at structural highs. High gamma-ray response from basal middle Miocene sediments indicates a high organic content, probably related to environmental restrictions (Kockel in Vinken, 1988).

From late Miocene times onwards, regressive, sandy deposition became more prevalent within the dominantly argillaceous facies. This was due to input from large deltaic systems from the east and south-east, as indicated by pro-grading sediment bodies (Kockel in Vinken, 1988). Deep-water conditions gave way to shallower seas as sedimentation outpaced subsidence. Agglutinating foraminifera, which during the Oligocene became restricted to only the deepest parts of the basin, finally disappeared in late Miocene times (King, 1983); this suggests complete infilling of the deepest parts of the basin, and the establishment of full water circulation.

The clay mineralogy of Miocene mudstones shows a gradual decrease in smectite, and an increase in illite and kaolinite (Nielson, 1980). This is probably due to the diminishing role of volcanism as a source of sedimentary material (Karlsson et al., 1979). The gradual reduction in smectite becomes abrupt near the base of the upper Miocene; this trend coincides with an upward change from fissile to nonfissile mudstones, and to denser sediments with a higher sonic velocity (Nielson in Vinkett, 1988). Also chlorite appears at this change, and subsequently increases in abundance, suggesting reduced intensity of weathering in the source areas due to colder climate, and higher relief as Scandinavia was uplifted (Karlsson et al., 1979; Cloetingh et al., 1990).

Pliocene

The only Tertiary sediments known onshore adjacent to the report area are the 'Buchan Gravels' in north-east Scotland.

These are probably of Pliocene age (Flett and Read, 1921), and are fluviatile and littoral in origin (McMillan and Merritt, 1980). They have been subjected to, or are derived from, deep Tertiary weathering that produced an abundant kaolinitic matrix, and have a restricted clast lithology of highly stable metaquartzite, vein quartz and flint (Hall, 1984; 1985).

Offshore, the base of the Pliocene succession lies up to 1300 m below present-day sea level in the axis of the Central Graben (Figure 63)." data-name="images/P944961.jpg">(Figure 61), where sedimentation was continuous across the Miocene–Pliocene boundary. However, west of the Central Graben, the onlapping, unconformable base of the Pliocene occurs at less than 200 m below sea level, and upper Miocene sediments are missing locally (Figure 63). Pliocene sediments locally exceed 900 m in thickness; the depocentre is farther west than that of the Miocene, with the major sediment source remaining to the east.

Evidence from wells in the north-east of the report area indicates subsidence rates of 5 to 16 cm/1000 years from the Pliocene to the present time (Cloetingh et al., 1990). Estimates of sedimentation rates indicate values of 10 cm/1000 years in the Danish and Norwegian sectors of the Central Graben (Bjørslev Nielsen et al., 1986). Sedimentation and subsidence appear to have kept pace in the UK sector, for depths deduced from foraminiferal assemblages at the Josephine Oilfield are fairly constant through the Pliocene section (Knudsen and Asbjörnsdóttir, 1991). These high subsidence and sedimentation rates have produced abnormal fluid pressures in the sedimentary column below 3000 m, and have contributed to hydrocarbon maturation.

The Pliocene sediments are a continuation of the underlying Miocene mudstones, with increased abundance of sandy mud, and local muddy sand beds up to 20 m thick. These sands are usually moderately sorted and sub- to well rounded. The base of the Pliocene is glauconite rich in the south and west, and the amount of glauconite decreases slightly upwards. Lignite is locally present in the south. The first ice-rafted debris was laid down in mid-Pliocene times, penecontemporaneously with that reported in the North Atlantic (King, 1983). During the Pliocene, sedimentation patterns became increasingly affected by glacioeustatic fluctuations of sea level.

Immediately south-east of the report area, seismically identified upper Pliocene prodeltaic units (BGS Dogger Quaternary Geology sheet) are termed the Brielle Ground and Westkapelle Ground formations. These comprise sands and muds. The base of the oldest, the Brielle Ground Formation, consists of dominantly glauconite-rich sand. These formations thin both westwards and northwards, and are believed to have been laid down by a westward-flowing Baltic river system.

Although the Pliocene-Pleistocene boundary is formally defined as 1.6 Ma (Aguirre and Pasini, 1985), in the southern North Sea it is often placed at about 2.4 Ma, when the first indications of a cold climate appeared in Dutch sequences (Zagwijn, 1992). In the northern and central parts of the North Sea, the boundary has been taken close to the top of the Olduvai palaeomagnetic event (Stoker et al., 1985); no strong reflector has been identified at this level, but a 'crenulate reflector' may be close to the Pliocene-Pleistocene boundary (Holmes, 1977; Kunst and Deze, 1985). This seismic reflector is identified by the occurrence of bright spots and hyperbolic reflectors, which suggest a concentration of boulders, and hence a time of erosion, possibly due to a marine transgression. Such a transgression may have marked a late mid-Pliocene unconformity identified elsewhere in the North Sea (Rokoengen and Ronningsland, 1983; Andrews et al., 1990).

The last common occurrence of the benthonic foraminifer Cibicides grossa Ten Dam and Reinhold has also been taken as the Pliocene–Pleistocene boundary by Feyling-Hansen et al. (1983), although King (1983) suggested that this extinction lies within the upper Pliocene. The highest occurrence of C. grossa in the Josephine Oilfield occurs at 700 m depth (Feyling-Hansen et al., 1983). A similar upper boundary has been identified both to the west in BGS borehole BH81/27 (Figure 63), and to the east in the Danish sector at the Tyra Oilfield (Knudsen and Asbjörnsdóttir, 1991). At the Josephine Oilfield, Pliocene sediments comprise sandy clays with a few comminuted shells and some red-brown ferruginous concretions. From foraminiferal evidence, Knudsen and Asbjörnsdóttir (1991) suggested that a magnetic reversal in BGS borehole BH81/34 (Figure 63) at about 287 m below sea level may be the Olduvai palaeomagnetic event, and hence the Pliocene–Pleistocene boundary; below it, the sediments comprise silty sand rich in mica and glauconite.

Chapter 9 Quaternary

The Quaternary Period is divided into the Pleistocene and Holocene epochs, the latter comprising only the last 10 000 years. Although the definition of the beginning of the Pleistocene has varied between researchers, in 1985 the International Commission on Stratigraphy formally ratified the Pliocene–Pleistocene boundary at approximately 1.64 Ma, based on the stratotype in southern Italy. This boundary is associated with several marine biostratigraphical events (Aguirre and Pasini, 1985), and coincides with the first appearance in the Mediterranean of the cold-water mollusc Araica islandica (Linn). In contrast, pollen research has identified the first major cold episode in The Netherlands at about 2.3 Ma, at the base of the Praetiglian Stage (Zagwijn, 1989; Gibbard et al., 1991). The first appearance in the North Atlantic of ice-rafted sediment at approximately 2.4 Ma (Shackleton et al., 1984) may also be considered to define the start of the Pleistocene in north-west Europe. The formal boundary, at approximately 1.64 Ma, occurs just above the top of the Olduvai (normal polarity) magnetic event, and the 2.3 Ma boundary above the top of the Gauss (normal polarity) magnetic epoch (Figure 64). Consequently, both boundaries are readily linked to palaeomagnetic stratigraphy, and have global application. In accord with recent Dutch and BGS practice (e.g. Zagwijn, 1992; Cameron et al., 1992), the Pliocene–Pleistocene boundary at approximately 2.3 Ma is used for this report.

In the areas of thickest Quaternary sediment north of 56°N (Figure 65), sedimentation was continuous across the Pliocer e–Pleistocene boundary, which is identified from foraminiferal biostratigraphy (Knudsen and Asbjornsdottir, 1991). In the southern part of the North Sea Basin, the base of the Quaternary corresponds approximately to the base of the Westkapelle Ground Formation (Cameron et al., 1992), which on seismic-reflection profiles is traceable between Britain and The Netherlands above an angular unconformity (Batson and Cameron, 1985). The Westkapelle Ground Formation is not recognised in the UK sector north of 55°N, so that in the report area successively younger formations form the base of the Pleistocene succession ((Figure 64) and (Figure 66)). In most areas west of a line approximating to its 200 m isopach (Figure 65), the Quaternary overlies Paleogene or older sediments with angular unconformity.

The Quaternary succession of the central North Sea locally exceeds 800 m in thickness (Figure 65), largely as a result of continued postrift subsidence above the Central Graben (Caston, 1977; 1979). Salt movement and faulting between 56°N and 57°30′N have, however, affected the lowermost Pleistocene strata. Cloetingh et al. (1990) invoked stress changes due to North Atlantic plate reorganisation as a mechanism by which tectonic subsidence has been much greater in the North Sea during the Pliocene and Quaternary than during earlier Cenozoic times.

The Quaternary Period has been characterised at high latitudes by climatic cycles of cold or glacial, and warmer, interglacial.. intervals. However, only since mid-Pleistocene times has the climate periodically changed to fully glacial conditions in the North Sea. The published stages and correlative chronology for these intervals vary (e.g. Zagwijn, 1989; Harland et al., 1990), but for the purposes of this report, the climatic cyclicity is correlated with Quaternary stages defined by pollen zones in The Netherlands ((Figure 64); Zagwijn, 1974; 1989). A complete record of Quaternary climatic events is preserved only in the oceanic-sediment oxygen-isotopic record, which indicates that eight major climatic cycles of approximately 90 ka duration have occurred since the start of the mid-Pleistocene (Bowen et al., 1986). During the early Pleistocene there were 22 or more cycles of greater frequency, and generally of lesser intensity, than those in the mid- and late Pleistocene (Porter, 1989).

An early, informal BGS stratigraphy for the Quaternary strata of the central North Sea (Thomson and Eden, 1977; Holmes, 1977) ascribed the top 240 m of deposits at the basin axis to the Weichselian on the basis of radiocarbon data that have subsequently been discredited. Further investigations, employing biostratigraphical, bioclimatic, palaeomagnetic and radiocarbon data (Stoker et al., 1985), demonstrated that Weichselian sediments rarely exceed 50 m in thickness, and then generally in valleys (Long and Stoker, 1986). The greater part of the basin fill is of lower and middle Pleistocene sediments. Amino-acid, thermoluminescence and C¹⁴ data have since given some support to these new strati-graphical assignations (Sejrup et al., 1987; Jensen and Knudsen, 1988; Sejrup et al., 1991; Knudsen and Sejrup, 1993), and sediments of comparable ages have been described from the adjacent Danish sector of the central North Sea (Knudsen, 1985). The surface distribution of the Quaternary formations is shown in (Figure 67). Different stratigraphies have been established to the north and south of 56°N east of 0° (BGS Quaternary Geology sheets; Stoker et al., 1985; Cameron et al., 1987), and full integration between them has yet to be achieved (Figure 64).

In the North Sea south of 55°N (Balson and Cameron, 1985; Cameron et al., 1992), lower Pleistocene sediments were deposited in a delta complex that was an extension of a large delta system originating in the Low Countries (Zagwijn and Doppert, 1978; Zagwijn, 1979, 1989; Bijlsma, 1981). The growth of the delta appears to have been largely independent of any variation in climate, and to have ceased during Cromerian Complex or early Elsterian times (Jeffery and Long, 1989). North of 56°N, prodeltaic sands and muds accumulated during the early Pleistocene (Stoker and Bent, 1985).

Although cold conditions were reported by King (1983) in studies of Pliocene deposits, and by Knudsen and Asbjornsdottir (1991) who analysed the microfauna of Praetiglian/Tiglian sediments, the earliest hints of glacier-proximal sedimentation in the north-central North Sea are preserved in BGS borehole BH81/26, just north of the report area. Here, a diamicton more than 10 m thick has been tentatively equated with marine-isotope stage 22, of Bavelian age (Sejrup et al., 1987). The oldest lodgement till, assigned to an early Cromerian Complex glaciation, is reported to have been deposited in the Forth Approaches from a grounded, tidewater ice sheet (Stoker and Bent, 1985).

The evidence indicates that before Elsterian times, shelf glaciers were not sufficiently extensive to block deposition of sediments in the delta extending from the southern North Sea. However, growth of this already decaying delta was ultimately halted by regional erosion of the delta and its source areas, as a result of Elsterian glaciation which blocked sediment pathways northward (Gibbard, 1988). Following shelf glaciation during the Elsterian, sedimentation in the central North Sea was affected by multiple regional glaciations during Saalian and Weichselian times.

Lower Pleistocene

Sediments of early Pleistocene age have been assigned markedly differing nomenclatures to the north and south of 56°N (Figure 64). The sediments comprise the marine and nonmarine parts of a northward-advancing delta complex made up of two amalgamated delta systems. Small, western deltas received sediment from Britain (Stoker and Bent, 1987), while a much larger eastern delta extending from the Low Countries received sediment from the European mainland (Bijlsma, 1981; Cameron et al., 1987).

Palaeomagnetic measurements made on samples of borehole core give predominantly reversed magnetic polarity, interpreted as belonging to the largely early Pleistocene Matuyama magnetic epoch (Stoker et al., 1983; Cameron et al. 1984). The Matuyama-Brunhes reversed-to-normal polarity transition has been identified in several boreholes within, and close to, the report area, providing a marker horizon at the top of the lower Pleistocene within the argillaceous Aberdeen Ground Formation ((Figure 64); Stoker et al., 1983; Andrews et al., 1990).

In the oldest formations of the Pleistocene in the southern North Sea, the assemblages of benthonic foraminifera are similar to those of the present-day southern North Sea and western English Channel (Cameron et al., 1984), although there is also a cold-water mollusc fauna (T Meijer, written communication, 1991) in the Crane Formation, of Tiglian age (Cameron et al., 1992; BGS Silver Well Quaternary Geology sheet). In the Outer Silver Pit Formation and its successors, and in the lower part of the Aberdeen Ground Formation, autochthonous foraminifera, which are commonly sparse, are dominated by Elphidium excavatum var. clavatum Cushman and other species which today are characteristic of boreal to high-arctic waters (Stoker et al., 1985; Knudsen and Asbjornsdottir, 1991). However, the dinoflagellate-cyst assemblages in all the early Pleistocene formations point to the influence of warm-temperate surface waters (Cameron et al., 1984; Stoker et al., 1985; Long et al., 1988; Cameron et al., 1992). The marine water-temperature record from the North Sea area therefore appears to be partly contradictory, and is incompatible with the pollen evidence for considerable vegetational change on land, that is often interpreted as a response to climate (e.g. Zagwijn, 1989).

Deltaic division

The Deltaic division (Figure 64) occurs to the south of 56°N, and is subdivided into several formations (Cameron et al., 1992) that together are laterally equivalent to the Aberdeen Ground Formation identified to the north. The geometry of those formations of the Deltaic division that predate the Yarmouth Roads Formation rends to be lenticular in the Southern North Sea Basin (Cameron et al., 1992), but is wedge-shaped in the report area where subsidence has been greater (Figure 66). Consequently, recognition of individual formations in seismic profiles becomes increasingly difficult basinwards, particularly where their internal and bounding reflectors are nearly parallel (BGS Dogger Quaternary Geology sheet). Formational subdivisions have therefore not been identified to the north of 56°N, where they are grouped into the Aberdeen Ground Formation (Figure 64).

Up to four acoustic facies and subfacies are generally developed within each of the formations of the Deltaic division. These acoustic facies correspond to systematic lithological variations (Cameron et al., 1987; 1992), representing three principal depositional environments: prodelta, delta front, and delta top. The latter is largely represented by the diachronous Yarmouth Roads Formation.

A majority of authors (Laban et al., 1984; Cameron et al., 1984, 1989a; Balson and Cameron, 1985; Stoker et al., 1985; Long et al., 1988) considered that the formations of the Deltaic division developed during high sea-level stands, and that the reflectors separating them represent basinwide unconformities developed during eustatic falls of sea level, perhaps during cold climatic intervals (Cameron et al., 1989a). Thus, the formations and their bounding reflectors have been linked respectively with the thermal maxima and minima interpreted from the pollen assemblages of lower to middle Pleistocene sediments in The Netherlands (Zagwijn, 1985).

More recent mapping in the southern North Sea and part of the report area (BGS Dogger and Silver Well Quaternary Geology sheets) has, however, suggested that the reflectors bounding the formations, although commonly persistent, are not developed basinwide, and are depositional hiatuses rather than unconformities. Furthermore, the simple architecture of the Deltaic division is in contrast to the intricacy displayed by regressive/transgressive sequences responding to rapid sea-level change (Colman and Mixon, 1988; Christie-Blick, 1991). There was steady overall regression in the UK sector of the North Sea Basin until Cromerian Complex times (Cameron et al., 1992); any transgressions would have been local, resulting from differences in the balance between subsidence and sedimentation. Both diachroneity and architecture can be explained by a simple model of overall regression through delta-top advance (Cameron et al., 1992).

Ijmuiden Ground Formation

The Ijmuiden Ground Formation, of Tiglian age but possibly extending into the earliest Eburonian, is the oldest Quaternary formation preserved in the southern part of the report area. Its full extent (Figure 68) and thickness in the report area have not been determined; it is entirely of the prodelta acoustic facies, which in BGS borehole BH89/05 in the southern North Sea (Cameron et al., 1992) consists of calcareous, bioturbated, dark grey, silty clay with thin beds of very fine-grained sand and concretions of iron monosulphide. These deposits are of the same lithofacies as the basinal sediments of the Aberdeen Ground Formation (Stoker and Bent, 1987).

Winterton Shoal, Markham’s Hole and Outer Silver Pit formations

While the Southern North Sea Basin was being filled rapidly with deltaic sediments in post-Tiglian times (Cameron et al., 1992), sediment supply in the UK sector between 54°N and 56°N continued to be so restricted that only distal, prodelta clays, and thin, very fine-grained sands were being deposited. The Winterton Shoal, Markham's Hole and Outer Silver Pit formations (Figure 68) are components of this prodeltaic succession; they range in age from Eburonian to Menapian (Figure 64).

In the southern North Sea, a delta-front acoustic facies of the Winterton Shoal Formation records the first distinct uniting of deltas from southern Britain and the Continent, and the concomitant northward swing of delta advance (Cameron et al., 1992). These changes probably occurred during Eburonian times.

The northward component in the direction of delta advance was well established by the time that the Markham's Hole Formation accumulated (Cameron et al., 1992). The prodelta acoustic facies is dominant in the report area (Figure 68), and near its western margin has been sampled in BGS borehole BH81/35, where it is at most 4 m thick. The stiff, dark grey mud has yielded a rich population of the dinoflagellate cyst Tectatodinium pellitum Wall, a southern-temperate shelf species (R Harland, written communication, 1983). The formation also appears to include a basin-marginal sandy lithofacies ((Figure 68); Gaston, 1979) of poorly sorted, pale grey, gravelly sands, which may be shallow-marine deposits derived from an ancestral Tyne-Tees river.

With deposition of the Outer Silver Pit Formation (Figure 68), north-westward advance of the delta brought greater volumes of prodeltaic muds into the central North Sea. In borehole BH81/35 these muds are not separable litho-logically or microfaunally from those of the Markham's Hole Formation. Deposition of the sandy, shallow-water lithofacies of British origin at the western margin of the basin (Caston, 1979) may have died out during accumulation of the Outer Silver Pit Formation; a possible explanation is that the mouth of the river supplying the sediment had moved to the north to be within an area affected by later Quaternary erosion.

Aurora and Batavier formations

The Aurora Formation ((Figure 64) and (Figure 68)) records the first major influx of delta-front sands into the central North Sea in Bavelian times. The strong downwarping of the basin east of 1°E and north of 55°N has resulted in a delta-front acoustic facies that is more thickly bedded than that of the older formations in the southern North Sea. The Aurora Formation has been sampled in BGS borehole BH81/35, in which it consists of 6 m of interbedded, glauconitic, fine-grained sand and grey clay, exhibiting flame and load structures.

The Batavier Formation ((Figure 64) and (Figure 68)) offers testimony to further voluminous supply of sediment from the southeast, leading to delta accretion and the first major invasion of the report area by the delta-top acoustic facies of the Yarmouth Roads Formation. The Batavier Formation is typically made up of stiff grey clay and silty clay with sand laminae. The true geographic limits of this formation have yet to be established, for to the north it is incorporated within the Aberdeen Ground Formation. The inferred occurrence of the Matuyama-Brunhes polarity transition within a few metres of the top of the formation (BGS Dogger Quaternary Geology sheet) provides a stratigraphical marker that shows that the formation extends into the Cromerian Complex Stage.

The delta-top facies of another component of the Deltaic division, the Comet Formation, and perhaps an overlying unnamed formation, has been tentatively recognised above the Batavier Formation a little to the north of 56°N within the trough of maximum subsidence. Their full extent has not been mapped, and they are not included in (Figure 64) and (Figure 66).

Yarmouth Roads Formation

In the very south of the report area, the base of the diachroncus Yarmouth Roads Formation is coeval with the top of the reversely magnetised Aurora Formation, and of probable Bavelian age ((Figure 64); Cameron et al., 1992). A little to the north of 56°N, in borehole BH81/34 (see (Figure 71)), the projected base of the Yarmouth Roads Formation may lie close to the Ma tuyama-Brunhes transition from reversed to normal magnetic polarity, which occurred somewhat later than the beginning of the Cromerian Complex Stage (Figure 64). Since the underlying formations of the Deltaic division become younger northwards, the age of the Yarmouth Roads Formation can 'le established as very largely mid-Pleistocene in the report area. The formation is therefore described with the middle Pleistocene sediments.

Aberdeen Ground Formation

North of 56°N and west of 0° longitude, lower Pleistocene sediments consist entirely of the lower part of the widesp7ead, wedge-shaped Aberdeen Ground Formation ((Figure 64), (Figure 69) and (Figure 70); Stoker et al, 1985). The formation is characterised seismically by its high-amplitude, westerly onlapping reflectors. Sediments of early Pleistocene age in the formation are essentially those below the Matuyama Brunhes magnetic reversal, and are largely confined to the east of 1°W (Figure 69), where the formation is thickest and comprises much the greater part of the Quaternary succession. Although its base is not defined, as it is not clearly identified on seismic sections in the area of maximum subsidence, the formation exceeds 150 m in thickness in the north of the report area (BGS Peterhead and Forties Quaternary Geology sheets). Kunst and Deze (1985) have reported a Quaternary thickness of 400 to 500 m around the Gannet Oilfield (Figure 65), and the Quaternary succession is similarly considered to be much thicker above the Central Graben (Figure 66).

Stoker and Bent (1987) have divided the lower Pleistocene sediments of the formation into lithofacies associations, and suggested that these were deposited in a delta-front to shallow-prodelta/open-marine setting, with a nearshore facies at the western margins of the basin. Structureless to laminated sands, interbedded with muds that include organic debris and soft-sediment deformation structures, form the delta-front lithofacies in the south-east, in which periodic wave-current reworking has been inferred. Largely argillaceous beds with thin sands and rare phosphatic bands comprise the prodelta and marine lithofacies, as in the 'type borehole' BH81/34 (Figure 71) where they are made up of interbedded sand, silt, silty clay and clay (Stoker et al., 1985). A nearer-shore environment was envisaged in the western part of the basin, where there are channel-lag deposits and laminated subtidal sands. The clays in the formation are characteristically stiff or hard, and commonly fissured.

Middle Pleistocene

Middle Pleistocene sediments (Figure 64) attain a thickness of more than 200 m in the report area (Figure 66). North of 56°N and west of 0°, styles of acoustic signature distinguish the Aberdeen Ground, Ling Bank, Fisher, and Coal Pit formations. To the south-east lie the Yarmouth Roads, Swarte Bank, Egmond Ground and Cleaver Bank formations. All these formations were deposited as climatic conditions fluctuated in character between glacial and interglacial.

Yarmouth Roads Formation

South of 56°N and east of the Greenwich Meridian, and throughout the southern North Sea, the middle Pleistocene succession consists largely of the Yarmouth Roads Formation, the uppermost, delta-top formation of the Deltaic division, whose base is strongly diachronous in the southern North Sea (Cameron et al., 1992). On the basis of its dominantly layered acoustic signature, a locally interdigitating unit at the top of the formation has been termed the Alkaid Member ((Figure 64), (Figure 66) and (Figure 71)).

A somewhat discontinuous seismic reflector separates the Yarmouth Roads Formation from lower Pleistocene formations. The reflector is coincident with a diachronous lithofacies change from fully marine to intertidal, supratidal, and terrestrial sediments. This facies change is accompanied by the reduction or replacement of populations of autochthonous, marine, dinoflagellate cysts, and calcareous microfauna (Jensen and Knudsen, 1988; Penney, 1990). It is also associated with the incoming of locally redeposited freshwater ostracods (Penney, 1990), and by increased abundance of plant remains and of pollen from dune and saltmarsh plants, as well as from trees (Cameron et al., 1984).

Numerous boreholes into the Yarmouth Roads Formation have revealed widespread, partly or wholly decalcified, marine and nonmarine sands, commonly with scattered pebbles, abundant plant debris, and peat or wood clasts (Cameron et al., 1992). Plant remains include those of the freshwater fern Azolla filiculoides Lambert, which is absent from sediments younger than Holsteinian in Britain and north-west Europe (Godwin, 1975). The sands are associated in places with intertidal sand/mud rhythmites containing beds with mud clasts derived from intraformational hardgrounds. Well-worn shell remains are commonly those of robust bivalve species. All the foregoing features strongly support the seismic interpretation, based on discontinuous and channelled reflectors, of the Yarmouth Roads Formation as a complex of delta-top sediments laid down in diverse depositional milieus (Cameron et al., 1992).

In early Cromerian Complex times, almost the whole North Sea Basin south of 56°N was a wetland complex of delta-top sediments; creation of this vast delta plain, which has been called Ur-Frisia (Cameron et al., 1992), had taken more than 1.6 Ma (Jeffery and Long, 1989). Microfloras in the middle Pleistocene part of the Yarmouth Roads Formation indicate waters that were still temperate, but cooler than flose of the early Pleistocene, perhaps signalling the cooling which preceded the Elsterian glacial period.

A basin-wide marine transgression of late Cromerian Complex age has been recognised in The Netherlands and East Anglia (West, 1980); its ultimate shoreline occupied a broadly similar geographical position to today's (Zagwijn, 1979). This transgression converted the delta plain of Ur-Frisia into a shallow sea which perhaps was little more than a few metres deep, except over the Central Graben of the North Sea Basin. With subsequent sea-level fall during the ensuing Elsterian (Anglian) Stage, a ready-made, low-relief surface was available to facilitate the spread of British and Scandinavian ice sheets.

Alkaid Member

In the area between the Dogger Bank and 56°N, the uppermost part of the Yarmouth Roads Formation has been termed the Alkaid Member on the basis of better-defined seismic bedding that is taken to be indicative of marine deposition. Sampling indicates that it comprises compact sands with abundant organic matter, and stiff marine clays that are partly pebbly (BGS Dogger Quaternary Geology sheet). One interpretation indicates that it is equivalent to the Fisher and Ling Bank formations to the north of 56°N, and of Cromerian Complex to early Elsterian age as the Alkaid Member is older than the Swarte Bank, Egmond Ground and Cleaver Bank formations beneath the central part of the Dogger Bank (BGS Dogger Quaternary Geology sheet). However, the stratigraphical position of the units is equivocal, for under the northern part of the Dogger Bank, the base of the Egmond Ground Formation cannot be continuously traced (Figure 66), and the formation cannot be identified farther to the north and west owing to erosion, thus ruling out direct seismic correlation. Therefore, the stratigraphical relationships of the Alkaid Member to the Egmond Ground and Cleaver Bank formations north of the Dogger Bank are unclear (Figure 64). Identification of equivalence is further complicated by investigations of the Fisher Formation in BGS borehole BH81/34 (Stoker et al., 1985), which indicate that it is a glaciomarine deposit, possibly of Saalian age; this age has been corroborated by the amino-acid analyses of Knudsen and Sejrup (1993) and thermoluminescence dates from BGS borehole BH81/29 (Jensen and Knudsen, 1988). The stratigraphical relationships of the Alkaid Member illustrated on (Figure 64) therefore represent an uneasy compromise that requires further work in order to establish correlations.

Aberdeen Ground Formation

The upper part of the Aberdeen Ground Formation is mid-Pleistocene in age (Figure 64). Pebbly and sandy sediments of Cromerian Complex age in this upper part of the formation contain a cold-water foraminiferal fauna, and have been interpreted as the subglacial and proglacial products of a grounded, tidewater ice sheet (Stoker and Bent, 1985; Stoker and Stoker, 1987). These sediments contain the earliest indications of fully glacial climatic conditions in the central North Sea (Stoker and Bent, 1985).

Stoker and Bent (1985) divided this unit into four interfingering lithofacies that have been sampled in several boreholes, and which define north–south-trending belts of deposition (Figure 69). A subglacial facies locally forms a mounded unit up to 3 m thick in the west; this is a very poorly sorted, gravelly, sandy mud rich in locally derived clasts, and lacking faunal remains. It is overlain by the proximal glaciomarine facies.

The proximal glaciomarine facies has a sheet-like geometry, and consists of poorly bedded, very poorly sorted, gravelly muds and muddy sands; these contain abundant abraded shell fragments. In BGS borehole BH81/36, such sediments pass upwards into centimetres-thick, interbedded rippled sands and muds that display flaser and lenticular bedding, and contain locally derived pebbles and a sparse, cold-water, benthonic, foraminiferal assemblage.

Farther east, a distal glaciomarine facies has been sampled as stiff, poorly sorted, sporadically shelly, slightly sandy and pebbly, burrowed muds. In the east of the area lies the marine facies which is also characterististic of the early Pleistocene part of the formation; it is recorded in boreholes such as BH81/34 as hard, bioturbated, olive-brown sandy mud.

Swarte Bank and Egmond Ground formations

These formations do not extend beyond the well-defined northern and western limits of the Dogger Bank ((Figure 66) and (Figure 67)). The Swarte Bank Formation is considered to be of late Elsterian to locally earliest Holsteinian age (Cameron et al., 1992), which is the age of the older portion of the Ling Bank Formation north of 56°N. The Swarte Bank Formation fills arrays of valleys (Figure 66) cut in Pleistocene and older strata (Cameron et al., 1992). Boat-shaped in plan, with an irregular thalweg, the valleys are generally considered to have been formed by subglacial meltwater under pressure (Ehlers et al., 1984; Boulton and Hindmarsh, 1987), although Wingfield (1990) has suggested a jokulhlaup origin. Because of their shape, the term 'scaphiform' has been proposed as a descriptive, nongenetic term (Cameron et al., 1992). These scaphiform valleys in the report area are shallower and less numerous than those in the southern North Sea, probably as a result of their position beneath the ice sheet, at a greater distance from the ice front which lay off of East Anglia (Cameron et al., 1992).

The Swarte Bank Formation, which is typically up to about 50 m thick (BGS Dogger Quaternary Geology sheet), consists of three members (Cameron et al., 1987, 1989b; Balson and Jeffery, 1991), none of which has been penetrated by BGS boreholes in the report area. Elsewhere, the basal member comprises stiff, grey diamictons, in places accompanied by lenses of coarse-grained glaciofluvial sand. It is overlain by the dominant and spectacularly stratified middle member, that consists of stiff, grey, unfossiliferous, glaciolacustrine clays; these pass up into similar marine clays which have yielded a benthonic foraminiferal assemblage characteristic of shallow waters where winter ice freezes fast to the bottom (D M Gregory: written communication, 1980). Where present, tae uppermost member comprises marine sediments lacking clear glacial influence.

With amelioration of the climate following the decay of the Elsterian ice sheet, rising sea level combined with continuing tectonic subsidence, led during the Holsteinian Stage to the re-establishment of a shallow sea with open-marine conditions. This allowed the widespread development of the Egmond Ground Formation, defined on shallow-seismic profiles by its persistent tabular geometry and, above all, by its conspicuous basal reflector which commonly truncates the upper parts of the Swarte Bank Formation (Figure 66). Up to 15 m thick in the central North Sea (BGS Dogger Quaternary Geology sheet), farther south the Egmond Ground Formation comprises a lithologically variable deposit of locally gravelly sands interbedded with silt and clay (Cameron et al., 1989b), and contains shallow-water faunas. The marine faunas in the southernmost parts of the basin indicate a cool-temperate sea similar to northern parts of the present-day North Sea (Nilsson, 1983; Hinsch, 1985).

Ling Bank Formation

Above the Aberdeen Ground Formation, shallow-seismic profiles exhibit complex, stacked, concave-up, commonly irregular, curved seismic reflectors with abundant diffractions. These bound local sets of inclined seismic reflectors, and are interpreted as channels and their fills. Interspersed planar reflectors vary in abundance, but much of the sequence has a rather amorphous seismic signature. These acoustic characteristics define the Ling Bank Formation, which is widely but intermittently distributed in the east and north of the report area (Figure 70), and has been sampled in BGS boreholes BH81/29 and BH81/34 ((Figure 69); Stoker et al., 1985). The formation a n be around 100 m thick in channels, which are most abundant in the north of the report area, but is commonly thinner or absent beyond the shoulders of the channels (Figure 70). Stoker et al. (1985) tentatively suggested that the formation is of Holsteinian to Saalian age, whereas an Elsterian, or possibly Cromerian Complex to Saalian age has been postulated by Knudsen and Sejrup (1993). Ansari (1992; has interpreted pollen data from BGS borehole BH81/34 as indicating a Cromerian Complex age for the Ling Bank Formation.

Borehole BH81/34, the type borehole (Stoker et al., 1985), penetrated very dark grey silt, clay, silty clay and sand within a channel sequence (Figure 71). A 2 m-thick section at a depth of about 140 m below sea bed includes a temperate dinoflagellate-cyst assemblage (Harland, 1988a), and an interglacial pollen assemblage (Griffin, 1984); this section was considered by Stoker et al. (1985) to be Holsteinian on the basis of the presence of A. filiculoides, which does not occur in sediments younger than Holsteinian (Godwin, 1975). A study of the foraminifera by Knudsen and Sejrup (1993) confirms that conditions were largely arctic during deposition of the formation, but indicates that a thicker section higher in the formation (at c. 90-81 m depth) may be the interval corresponding to the Holsteinian interglacial. The thin lower interval may therefore have been deposited during an Elsterian interglacial, or it could be of Cromerian Complex age, making the base of the formation older than indicated in (Figure 64). The benthonic foraminifera indicate regression at the top of this marine formation, attributed by Stoker et al. (1985) to cooling of the seas at the onset of Saalian glaciation in the basin.

Cleaver Bank Formation

The Cleaver Bank Formation (Figure 64), which occurs beneath the Dogger Bank, records a reversion to glacially dominated sedimentation during Saalian times (Cameron et al., 1992). In the report area, the Cleaver Bank Formation is only poorly developed, and is largely indistinguishable. both acoustically and lithologically, from the overlying Dogger Bank Formation (Figure 66). A tabular body, it is typically up to 8 m thick in the UK sector of the southern North Sea, and comprises stiff, laminated, dark grey clays with scattered angular granules of chert or chalk, and intercalations of micaceous sands. A cold, arctic-like dinoflagellate-cyst assemblage is accompanied by abundant reworked Paleogene cysts (R Harland, written communication, 1987). The formation is interpreted as a partly marine, proglacial diamicton of eastern provenance (BGS Dogger and Silver Well Quaternary Geology sheets), a concept supported by its lateral transition east of 4°E into the subglacial, Saalian, Borkumriff Formation (Joon et al., 1990).

Fisher Formation

Overlying the Ling Bank Formation is the distinctive seismic facies of the Fisher Formation; parallel subhorizontal reflectors are interrupted by generally small, intraformational channels. The formation is widely distributed north of 56°N, and is the oldest to crop out extensively there at the sea floor ((Figure 67) and (Figure 70)). It generally does not exceed 6 m in thickness, although it locally achieves 90 m. The Fisher Formation has been probed by three boreholes, including the type borehole BH81/34 (Stoker et al., 1985) where it comprises interbedded fine-grained sand and stiff to firm, dark grey mud. Scattered pebbles and granules of local rocks, including chalk, form only a small part of the sediment. Comparison of borehole BH81/29 with a sparker profile through the site shows that a major sand bed penetrated by the borehole occupies a channel within the layered mud sequence; benthonic foraminiferal assemblages have been interpreted as indicating upward-deepening waters.

In borehole BH81/34, an arctic, marine, foraminiferal fauna in the formation has been assigned a Saalian age by Knudsen and Sejrup (1993), confirming the interpretation of Stoker et al. (1985) that the Fisher Formation was deposited at that time in a glaciomarine milieu. As indicated in (Figure 64) and (Figure 71), the Fisher Formation may be partly equivalent to the Alkaid Member of the Yarmouth Roads Formation.

Coal Pit Formation

The term Coal Pit Formation has been applied to deposits which fill channels cut during Saalian times, although the formation commonly extends beyond the channels (Figure 70). Only the oldest sediments of the Coal Pit Formation are of mid-Pleistocene age (Figure 64); it has been proposed that part of the Coal Pit Formation may be as old as Elsterian or Cromerian Complex (BGS Dogger and Swallow Hole Quaternary Geology sheets). Subdivision of the formation was not however considered possible by Stoker et al., (1985), and the whole formation is here described with the upper Pleistocene deposits.

Upper Pleistocene and Holocene

During the Eemian interglacial, there was a return to higher sea levels before a further fall in Weichselian times. In the Dutch sector to the east, this led to the development of the Eem Formation, which occurs in discontinuous patches of up to 15 m of shelly sands within the remnants of tidal channels (BGS Dogger Quaternary Geology sheet). Farther north, deposition of the Coal Pit Formation may have continued.

Later, in Weichselian and Holocene times, several glacial episodes and subsequent ameliorations are recorded in a wide range of formations, several of which are laterally equivalent (Figure 64). Uplift in Scotland has left intertidal and estuarine deposits extensively preserved in the Midland Valley (Browne, 1991) and along the east coast, where they are known by a variety of names, the most famous of which is the Carse Clay (Sissons, 1967; Paterson et al., 1981). The lithologies, both offshore (Jeffery, 1991a; b) and onshore in the east of Scotland, commonly indicate deposition from, or in association with, ice sheets; upward amelioration of ambient water temperatures is confirmed by fossil assemblages (Thomson, 1978; Paterson, 1981; Paterson et al., 1981; Browne et al., 1984; Stoker, et al., 1985).

Many of the formations occupy scaphiform valleys ((Figure 67) and (Figure 70)). In the report area, these valleys may be up to 200 m deep and more than 2 km wide. Long and Stoker (1986) concluded that the majority were cut fluvially in a subaerial, periglacial landscape by jokulhlaups. However, their shape, disposition and style of fill, also appear to be consistent with a subglacial origin (Thomson, 1978).

The offshore sequences generally form evolutionary depositional patterns resulting from subglacial through mainly glaciolacustrine to glaciomarine conditions during glacier recession. Other features include submerged rock platforms of pre-late Weichselian age that have been described off the Scottish coast, both at Stonehaven and at Dunbar (Stoker and Graham, 1983; Hall, 1989), and an onshore sand horizon that extends to more than 2 m above the maximum Holocene transgression, of about 6500 years BP. This sand has been interpreted as evidence for a short-term coastal-flooding event when a tsunami wave, generated by a large submarine slide at Storegga off Norway, struck the east coast of Scotland at about 7000 years BP (Long et al., 1989).

The sediments south of 56°N and east of 0° are again related to a separate lithostratigraphy from those to the north and west (Figure 64). A number of thin Holocene formations have been identified in the extreme south-east of the report area (BGS Dogger Sea Bed Sediments sheet; Cameron et al., 1992); their distributions have not been mapped, except for the Elbow Formation, which is a brackish-marine and tidal-flat deposit that is only patchily distributed.

Coal Pit Formation

The Coal Pit Formation, of probable Saalian to Weichselian age (Stoker et al., 1985), is widely distributed ((Figure 67) and (Figure 70)), and can exceed 120 m in thickness where it fills channels that may have been eroded during or immediately after the Saalian glaciation. Its seismic signature varies from chaotic to subparallel, and includes many planar and irregular discontinuities. The uppermost levels of the Coal Pit Formation have been described as locally indistinguishable from both the Swatchway and Marr Bank formations (Stoker et al., 1985).

In type borehole BH81/37 (Stoker et al., 1985), the lower part of the Coal Pit Formation consists of interbedded, bioturbated sand and dark grey, stiff clay with shells, pebbles and wood fragments. This section contains relatively rich assemblages of dinoflagellate cysts dominated by Operculodinium centrocarpum (Deflandre and Cookson). Harland (1988b) has pointed out the resemblance of this assemblage to that of the present interglacial at the same latitude, signalling the influence of the North Atlantic current. In borehole BH75/33, immediately north of the report area, an ameliorative zone in the formation has been assigned to the Eemian Stage using palaeomagnetic data from which the Blake event may be tentatively recognised (Stoker et al., 1985).

The upper part of the Coal Pit Formation in borehole BH81/37 is dominated by stiff, shell-rich, laminated, glaciomarine clay with scattered pebbles. Micropalaeontological data suxested to Stoker et al. (1985) that much of the Coal Pit Formation is probably a glaciomarine deposit, although the upper part of the formation in borehole BH81/27 was interpreted as intertidal in origin (BGS Marr Bank Quaternary Geology sheet). The presence of the foraminifer Elphidium? ustulatum Todd in the formation in borehole BH75/33 indicated an Eemian interglacial age to Gregory and Bridge (1979).

Dogger Bank and Twente formations

Under the Dogger Bank, at about 70 m below sea level, a continuous seismic reflector defines the base of the extensive, distinctive, generally tabular, Doer Bank Formation (Figure 66). The formation is generally up to 4 m thick, and its internal reflectors are well ordered and suggestive of a water-lain body. It is composed principally of clay-rich, stratified and laminated diamictons in which the pebbles are small. The sediments are stiff to very stiff, and as a consequence of their resistance to erosion, their limits now form the extensive area of shallow water that defines the Doer Bank (see (Figure 77)). Dinoflagellate cysts in the top layers of the formation are indicative of severe, cold, open-marine conditions (R Harland, written communication, 1987) inferred to have taken place during the Weichselian (BGS Dogger Quaternary Geology sheet).

Channel-like features of uncertain genesis occur along the relatively steep northern and western limits of the Dogger Bank Formation; these 'channels' contain diamictons identical in appearance to those outside such features, and constitute the Volans Member of the formation (BGS Dogger and Swallow Hole Quaternary Geology sheets). The 'channels', which in plan are normal to the margins of the deposit, may have been formed close to a grounded ice-sheet limit. The morphology of the pre-Holocene part of the Dogger Bank (BGS Dogger Sea Bed Sediments sheet), the distribution of the various Weichselian glacigenic sediments, and the thinning of the Dogger Bank Formation to the south and east (BGS Dogger Quaternary Geology sheet), all support this hypothesis.

Overlying the Dogger Bank and other formations is the Twente Formation (Figure 64). Proved in boreholes in the Dutch sector and commonly discernible only in seismic profiles of very high resolution, it consists of periglacial aeolian sands with subordinate organic detritus resting in discontinuous patches on Weichselian and older deposits (Cameron et al., 1989b).

Swatchway Formation

The Swatchway Formation has an amorphous, or structure-less, seismic signature, with sporadic subhorizontal reflectors. It has a lithofacies of mud and sand with scattered lithic and shell fragments, and is significantly less consolidated than the Aberdeen Ground Formation (BGS Fladen Quaternary Geology sheet). The Swatchway Formation occurs only in the north of the report area ((Figure 70) and (Figure 72)), where it is generally up to 15 m thick. In the type borehole, BH75/33, situated a little to the north of the report area, the sparse, benthonic foraminiferal assemblage is dominated by the cold-water species E excavatum var. clavatum, which suggests deposition in boreal to arctic seas (Stoker et al., 1985). Perhaps more significantly, the foraminifera are accompanied by moderately abundant populations of dinoflagellate cysts; a limited spectrum is co-dominated by O. centrocarpum and Bitectatodinium tepikiense Wilson, indicating northern cool-temperate conditions without significant sea-ice cover (Harland, 1988a). The formation can be broadly assigned a late Weichselian age (Stoker et al., 1985).

Marr Bank, Wee Bankie and Bolders Bank formations

The Marr Bank Formation comprises glacigenic sediments of late Weichselian age (Stoker et al., 1985; BGS Farne and Marr Bank Quaternary Geology sheets). A sheet-like deposit generally some 10 m to 25 m thick, it is confined to the west-central part of the report area, where it crops out extensively (Figure 67) and (Figure 72). It has a characteristically flat or slightly undulating seismic reflector at its base in the west that commonly truncates any Pleistocene or older deposits below it ((Figure 73); BGS Marr Bank Quaternary Geology sheet). The reflector becomes discontinuous eastwards, rendering the formation acoustically indistinguishable from the upper part of the Coal Pit Formation into which it locally grades laterally (BGS Devil's Hole Quaternary Geology sheet). This basal reflector represents a surface of marine planation or deposition which dips from 60 m below mean sea level in the south-west to more than 100 m in the north-east, probably as a result of tectonic subsidence and isostatic adjustment (Holmes, 1977).

On sparker and boomer records, the formation has an acoustic signature that varies from amorphous to parallel bedded, or displays groups of inclined reflectors resembling the large-scale internal cross-bedding of sand bars (Stoker et al., 1985; BGS Marr Bank Quaternary Geology sheet). The formation has been sampled in a number of boreholes, and although including muddy sediments, especially towards the north-west (Holmes, 1977), it is generally formed of sands of varying grain sizes and degrees of sorting (Stoker et al., 1985). Gravelly layers contain a variety of lithic clasts of Scottish provenance, and wood fragments and clay balls occur sporadically. No dinoflagellate cysts have been recovered from the formation. A poor ostracod fauna and a moderately abundant population of foraminifera (Thomson, 1978) indicate that the shallow waters in which the formation was deposited were high-boreal to arctic in temperature, and inner shelf to estuarine in character.

Thomson (1978) and Thomson and Eden (1977) inferred a Weichselian age for the Marr Bank Formation (then termed the Marr Bank beds) because the acoustic base appeared to be at the same level as the base of the Wee Bankie Formation (Figure 73). The western edge of the Marr Bank Formation is commonly a low scarp, probably an ice-contact face, and lateral passage by interdigitation between the two formations has been argued on seismostratigraphical grounds by both Stoker et al. (1985) and Stewart (1991).

The Wee Bankie Formation (Stoker et al., 1985) occurs near the east coast of Scotland (Figure 72), where it has a sheet-like geometry but an uneven, ridged, upper surface. It is up to 40 m thick. On sparker records, point-source reflectors can give a chaotic acoustic response pattern. Its lithology is of stiff, variably matrix-dominated polymictic diamicton with some interbeds of sand, pebbly sand and silty clay; there is a lack of significant in-situ fauna or flora, although reworked biological material is common. The formation may have originated as a basal till, perhaps in part as a lodgement till, and is assumed to be of late Weichselian age by correlation both with the Marr Bank Formation and onshore tills (Goste ow and Browne, 1986; Paterson et al., 1981). The eastern boundary of the Wee Bankie Formation in the Forth Approaches could mark the maximum extent of late Weichselian grounded ice (Stewart, 1991), as inferred by Thomson and Eden (1977).

The Bolders Bank Formation forms the south-eastward extension of the Wee Bankie Formation to the south of 56°N and east of 0° (Figure 67) and (Figure 72); BGS Swallow Hole Quaternary Geology sheet). The formation is typically not more than 1 m thick, but locally attains 40 m in an over-deepened trough. It typically has a chaotic to poorly ordered internal seismic: reflector configuration, and is characterised by reddish to greyish brown, stiff, massive diamictons in which the pebble content tends to diminish eastwards.

St Abbs Formation

The St Abbs Formation (Stoker et al., 1985) occurs off the east coast of Scotland and northern England (Figure 72), largely to the west of 2°W where it is generally no more that 20 m thick; it is characteristically structureless on seismic records, with only faint, discontinuous, subparallel reflectors. It rests either on the Wee Bankie Formation or pre-Quaternary strata, and comprises soft to stiff, plastic, weakly laminated muds and silty muds with sporadic pebbles. It was deposited in glaciomarine conditions. The formation is correlated with the Errol Beds onshore on the basis of its ostracod assemblages, implying a late Weichselian age between 18 000 and 13 500 years BP, postdating the glacial maximum (Peacock, 1981; BGS Tay-Forth Quaternary Geology sheet). Deposition of the St Abbs Formation was contemporaneous with the lower parts of the Witch Ground and Forth formations (Figure 64).

Botney Cut and Hirundo formations

The Botney Cut Formation fills extensive valleys or channels up to 200 m deep immediately to the west and north of the Dogger Bank, where it crops out extensively ((Figure 66), (Figure 67) and (Figure 71)). Where such valleys have not been filled, bathymetric deeps such as the Swallow Hole remain at the sea bed (see (Figure 75)). The Botney Cut Formation can be divided into two members (BGS Swallow Hole Quaternary Geology sheet) the lower of which is acoustically amorphous and comprises stiff, reddish brown diamicton with interbedded sand--this is equivalent to part of the Wee Bankie Formation. The upper member is parallel bedded on seismic records, and is made up of soft to firm, grey or greyish brown, sandy and pebbly muds of partly glaciomarine origin. Equivalent sediments to the north of 56°N and west of 0° comprise the St Abbs Formation and much of the Forth Formation (Figure 64). A late Weichselian to Holocene age is assigned to the sediments.

In the south of the report area, the upper part of the channel fill is locally differentiated as the Hirundo Formation (Figure 67), which is separated from the underlying Botney Cut Formation by a strong seismic reflector caused by lithological change to distinctive, reddish brown, noncalcareous clays. These sediments are closely comparable in setting and lithology to the Largo Bay Member of the Forth Formation (BGS Swallow Hole Quaternary Geology sheet).

Forth Formation

The Forth Formation, of late Weichselian to Holocene age and identified to the north of 55°N and east of 0°, crops out more extensively than any other formation in the report area (Figure 67). It partly forms an extensive, blanket-like cover typically less than 20 m thick ((Figure 70) and (Figure 73)), but occurs elsewhere as channel fill which is more than 150 m thick in the Devil's Hole region. Marine, glaciomarine, fluviomarine and estuarine facies have all been recorded in the formation (Stoker et al., 1985), which is divisible into four members.

The Largo Bay and St Andrews Bay members occur in the west of the report area. The Largo Bay Member is up to 30 m thick, and characteristically exhibits subparallel seismic reflectors. It includes muds and silty muds in the Firth of Forth, which become coarser-grained and pebbly seawards. The faunal diversity of this member decreases towards the top, reflecting climatic cooling with the onset of the Loch Lomond Stadial at 11 000 to 10 000 years BP. The St Andrews Bay Member overlies the Largo Bay Member or older strata, and is up to 40 m thick. It displays oblique reflectors on shallow-seismic records, and comprises interbedded sands and clays in the west, but pebbly muds and Shelly sands farther east. The St Andrew's Bay Member in the Firth of Forth records the development of the Forth Estuary, whereas between the Firth of Tay and Peterhead, it documents the former presence of coastal sand bars created from sediment delivered by rivers flowing from Scotland (Stoker et al., 1985).

The Fitzroy and Whitethorn members (Figure 64) have been recognised mainly to the east of the Greenwich Meridian. The dominantly argillaceous sediments of the Fitzroy Member are up to 60 m thick, and are considered to have been deposited in a low-energy environment. The Whitethorn Member may include prograding reflectors, but is generally acoustically amorphous; it comprises fine-grained sands laid down during the early Holocene in a shallow sea that had a dominantly easterly tidal-current direction (Stoker et al., 1985).

Witch Ground Formation

In the extreme north of the report area, a very broad, shallow trough, termed the Witch Ground Basin (Figure 72), is partially filled by the Witch Ground Formation (Stoker et al., 1985). The formation consists of up to 25 m of soft to very soft, greenish grey to greyish brown clays and silts with some sandy beds (Long et al., 1986); it is of late Weichselian to Holocene age, and is divided into three members.

The basal, multilayered, somewhat sandy and pebbly Fladen Member is generally less than 10 m thick and was deposited in shallow, restricted, glaciomarine waters subjected to intermittent sea-ice cover. The overlying, more-homogeneous Witch Member, typically 10 m thick, records deposition in waters warmer (free from sea ice), more open and deeper than during accumulation of the earlier member (Stoker et al., 1985; Andrews et al., 1990). The Glenn Member, at the top of the formation, occurs at the sea bed; it consists of silt or fine sandy silts, usually less than 20 cm thick, but up to 2.5 m thick within shallow depressions known as pockmarks (Hovland and Judd, 1988). It was deposited in the temperate, deep-marine, modern environment due to reworking of the underlying members of the Witch Ground Formation by rising gas which formed the pockmarks (see Chapter 10).

Chapter 10 Sea-bed sediments

The sea-bed sediments of the report area comprise a veneer of unconsolidated terrigenous and biogenic deposits, generally significantly less than 1 m thick. Where they are absent, notably at or near the coast, underlying Holocene and Pleistocene deposits, or bedrock, crop out. Locally, the sediments attain a thickness of several metres, and may pass down into the underlying deposits without significant lithological change (Thomson, 1978).

The terrigenous component consists mainly of reworked Pleistocene glacigenic sediment derived as a result of subaerial and nearshore processes which began during deglaciation, and continued throughout the early Holocene transgression. Following the establishment of modern marine conditions between 7500 and 5000 years BP (Jelgersma, 1979; Owens, 1981), most of the sea bed has been starved of terrigenous sediment input. However, coastal erosion, river discharge, and local waste dumping (BGS Farne Sea Bed Sediments sheet; Thomson, 1978) continue to provide terrigenous input. The combined fluvial input of mud from the east coast of the UK is estimated to be of the order of 1 million tons per year (McCave, 1973); most is either trapped in the estuaries or is deposited in nearshore areas of muddy sediments, such as those which occur in the vicinity of the Firth of Forth and the River Tyne (Figure 74).

The biogenic component is the only identifiable Holocene product within the sediments (Owens, 1981); it consists mostly of molluscan debris, with lesser amounts of barnacles, echinoids and foraminifera. In the deep-water areas of the North Sea, biogenic production probably began during earliest Holocene times, and present conditions for carbonate production were established after 6000 years BP (Farrow et al., 1984). Throughout the Holocene, there has been a gradual, slow accumulation of carbonate material, mainly in the coastal zone and on and around the submarine banks, including the Doer Bank ((Figure 74), (Figure 75) and see (Figure 77)). This has resulted from the in-situ production and degradation of gravel-grade carbonate, and the local dispersal of derived sand-grade carbonate by bottom currents (Owens, 1981).

Detailed studies of the sediments north of 56°N and east of 2°W by Owens (1977; 1980; 1981) have revealed a dynamic sedimentary environment. Coarse-grained sediment is being eroded and dispersed from topographic highs by tidal currents working in conjunction with storm-wave-induced oscillatory currents. In some areas, relative topography is as important as absolute water depth in controlling their distribution. There are sediments which are in equilibrium with the hydraulic environment, those which have been exhumed or transported by singular events, and others in transit. The sediments are both palimpsest and relict over several timescales, and the sea bed can be classified into zones of erosion, transport and deposition.

The colour of the sediments tends to be yellowish in the nearshore and on submarine banks, indicating an oxidising environment in shallow water. Below about 70 m, the colour becomes greyish, and then greenish below about 90 m, reflecting an increasingly reducing environment with increasing water depth (Owens, 1981).

Sediment transport

The movement of sediment is controlled mainly by tidal currents; these are strongest in shallow water in the western part of the area, where the tidal stream is aligned parallel to the coastline. The maximum surface current speeds are mainly in excess of 0.5 m/s out to a distance of about 50 km offshore, and decrease eastwards to less than 0.25 m/s at the eastern margin of the report area (Figure 76). The main sand-transport paths therefore occur in the west. The directions of sediment transport are mainly parallel with the coastline, in sympathy with the directions of the main tidal currents.

In many cases, the movement of sediment is probably initiated by storm-wave-induced oscillatory currents. The maximum 50-year wave height within the area is estimated to vary from over 20 m in the south to about 28 m in the north (Draper, 1980), and the maximum orbital-current velocities acting on the sea bed as a result of such waves are estimated to be about 4.0 m/s in water depths of about 40 m in the west, and about 3.0 m/s in water depths of 80 m towards the centre of the area (Pantin, 1991).

The combination of tidal and storm-wave-induced currents has the capacity to erode sediments of up to gravel grade, and is most effective in the shallower water in the west (Owens, 1981). However, even in the central and eastern parts of the area, storm-wave-induced bottom currents capable of initiating sand transport in water depths of 100 m are probably generated on several occasions each year (Stride, 1973).

Relative topography may also influence bottom-current strength. Anomalous distribution of erosional and depositional facies in some areas, especially in the vicinity of submarine banks, does not appear to be entirely related to absolute water depth, but can be explained by the acceleration and deceleration of bottom currents on the 'exposed' and lee sides of the banks (Owens, 1981). A similar mechanism may operate within some of the narrow, bathymetric deeps in the Firth of Forth, where bottom currents, accelerated within the restricted confines of the deeps, are thought to cause scouring and prevent significant deposition of fine-grained sediment (Thomson, 1978).

Stride (1973) reported the occurrence of a bed-load parting zone off south-east Scotland, where there is net movement of sediment away from this zone both to the north and south. It is likely, however, that the zone is situated slightly farther south, off the Farne Islands, as shown in (Figure 76) (BGS Farne Sea Bed Sediments sheet). The northerly transport path is over 250 km long, and terminates in a bed-load convergence zone situated off north-east Scotland (Stride, 1973; Owens, 1981); it therefore extends along the entire east coast of Scotland in the report area, although some sand is diverted into the Firth of Forth (Stride, 1973). This convergence zone receives lithic material from beyond the northern limit of the report area, and lies at the distal end of a carbonate dispersal path which originates in the Moray Firth (Owens, 1981).

Over much of the central and eastern parts of the area, sand-transport rates are relatively low due to the decrease in tidal-current strength and the increase in water depth; movement probably only occurs during storms. The available evidence suggests that transport paths are aligned approximately north–south, becoming north-easterly in the vicinity of the Dogger Bank (Stride, 1973).

Sediment distribution

Gravelly sediment

Gravelly sediments cover about 10 per cent of the sea bed. Gravelly sand forms most of this facies, which may contain locally developed patches of gravel and sandy gravel as winnowed, lag deposits. These sediments are restricted mainly to an arcuate zone of submarine banks around the southern, western and northern margins of the report area, notably in the vicinity of the Farne Islands and on the offshore banks east of Scotland, particularly the Marr and Aberdeen banks (Figure 74). Although the gravel on the banks consists mostly of lithic clasts derived by erosion of underlying morainic material, the sediments include much biogenic material, since these banks are centres of carbonate production. The lithic clasts are mainly subangular to subrounded, and are commonly encrusted by bryozoans and serpulids (Owens, 1977).

Sandy sediment

Sediments classified as sand and slightly gravelly sand (Folk, 1954; (Figure 74)) are predominant in the report area, where they cover about 80 per cent of the sea bed. These sandy sediments occur in a wide range of water depths, from the shallow coastal zone down to about 110 m in the north, and to below 120 m in isolated deeps in the south and west.

Within the Firth of Forth, sandy sediments are restricted to water depths of less than 20 m, and on the north side of the firth they have a dark colour due to the presence of material derived from coastal coal tips (Thomson, 1978). Off north-east England, the sandy sediments in the nearshore are restricted to relatively small, isolated patches which are bordered seawards by gravelly sediments in the north, and by muddy sediments in the south and east.

As well as the widespread distribution of sandy sediments, sand (Folk, 1954) forms a significant component of other sediments in the area (Figure 74), both on topographic highs and in depressions. The sand fraction of all the sea-bed sediments generally has a mean grain size within the medium- to fine-grained range, but tends to be coarser in the vicinity of submarine banks. Conversely, very fine-grained sands occur mainly within areas of muddy sediments, although they are also found in the south-east towards the Dogger Bank (BGS Dogger Sea Bed Sediments sheet).

The carbonate content of the sand fraction in the sandy sediments is generally less than 10 per cent (Figure 77), although the very small gravel fraction in these sediments consists of over 90 per cent biogenic carbonate (Owens, 1981). In areas of gravelly sands nearshore, and on submarine banks in the west, the carbonate content of the sand is significantly higher, rising to over 80 per cent locally because of relatively high biogenic production and in-situ degradation.

The sand fraction in the muddy sediments occupying the Witch Ground Basin in the north is also relatively rich in carbonate material, exceeding 60 per cent locally (Figure 77). This enrichment is probably due to preferential accumulation of foraminiferal tests due to low hydraulic activity (Owens, 1981). It may also reflect enhanced biogenic carbonate production in the vicinity of pockmarks, which occur within these sediments (Hovland and Judd, 1988).

Muddy sediment

Muddy sediments, of which muddy sand is the most widespread, cover about 10 per cent of the sea bed in the report area (Figure 74). Muddy sand is found in a wide range of water depths, commonly in association with smaller areas of tilt more muddy sediments, or with gravelly sediments containing significant quantities of mud. Away from the coast, it is extensively found towards the east in water depths of between 90 and 100 m (BGS Devil's Hole and Forties Sea Bed Sediments sheets). In the Witch Ground Basin in the north. the sed ments grade with increasing water depth from muddy sand to sandy mud, and then mud, ((Figure 74) and (Figure 75); BGS Forties Sea Bed Sediments sheet). Muddy sand also occurs within isolated, linear deeps such as those in the vicinity of the Devil's Hole, in the Swallow Hole, and in the Gorsethorn Deep.

Significant deposits of muddy sediment occur within the coastal zone, especially between Tynemouth and the Farne Islands, where muddy sand extends as far as 50 km offshore (Figure 74), and also in the vicinity of the firths of Forth and Tay. Although there are bathymetric deeps in these areas (Figure 75) which act as traps for fine-grained sediments, the mud occurs extensively beyond the deeps, reflecting a relative abundance of mud-grade material derived from the land and/or from underlying Holocene and Pleistocene deposits.

The muddy sediments in parts of the outer region of the Firth of Forth and within the Witch Ground Basin have a relatively high carbonate content in the mud fraction, ranging from 5 to 20 per cent; the majority is probably derived from skeletal debris as a result of bioerosion. These sediments are modern equivalents of the calcareous-shale facies found in the geological record (Farrow and Fyfe, 1988).

Bedforms

In the west of the report area, regions of sandy sediments are commonly characterised by the development of bedforms such as sand ribbons, elongated sand patches, and sand waves. Farther offshore, where the hydraulic conditions are less favourable to the movement of sediment, much of the sea bed is covered by extensive sheets of generally featureless sand. Adjacent to the median line north of 56°N, sidescan-sonar records show that the otherwise featureless sand includes large areas of randomly scattered point reflectors, which may be either very small patches of gravelly sediments or boulders.

Individual sand waves and sand-wave fields generally occur in the north and west (Figure 74). North of 57ºN, large sand waves, with average wavelengths of 200 m and heights of up to 17 m, show convergent asymmetry towards the bed-load convergence zone ((Figure 76); Owens, 1981). These sand waves occur mainly in shallow-water areas near to the coast, where maximum surface tidal currents range from 0.6 to 1.3 m/s. In some cases, sand waves have smaller sand waves or megaripples superimposed upon them (Owens, 1981).

Large sand waves have also been mapped some 50 km east of north-east Scotland, where water depths are about 80 m, and the surface tidal currents range from 0.3 to 0.5 m/s. The sand waves, which are up to 8 m in height with wavelengths of between 160 and 270 m, are anomalously large for the present hydraulic conditions. It is possible that they are active only during severe storms (Owens, 1981), or were formed during a period of lower sea level.

The largest Holocene bedforms in the area are a series of relict, early Holocene tidal sand ridges situated on the East Bank to the north-west of the Dogger Bank (Figure 74). These ridges are aligned north-east to south-west, and are up to 50 km in length, 3 to 4 km in width, and have heights of between 10 and 30 m. They generally have a smooth, symmetrical profile, and do not appear to be surmounted by sand waves, suggesting that they are now moribund (Kenyon et al., 1981). Drilling through one of these tidal sand ridges has shown that it is composed of fine-grained, well-sorted, well-packed, terrigenous sand; more than one period of development is recorded by stacked, upward-coarsening sequences (Davis and Balson, 1992). Other large bedforms include tidal sandbanks inshore of the Farne Islands, and an isolated, moribund sandbank in about 80 m water depth approximately 75 km east of Whitley Bay. The latter is probably relict from early Holocene times (BGS Farne Sea Bed Sediments sheet).

Pockmarks and gas emission

Pockmarks are shallow, ovoid, sea-bed depressions which occur in densities of up to 20 per km² within the muddy sediments of the Witch Ground Basin (Figure 74). They are probably caused by the venting of either biogenic or petrogenic gases into the water column; this disturbs the fine-grained sediment, lifting it into suspension to be partly transported by bottom currents away from the site of emission, to he deposited elsewhere. Eventually, a depression is formed at the point of emission, where the surface sediment is slightly coarser due to the removal of the finest sediment. The pockmarks within the Witch Ground Basin are up to 200 m in diameter, 2 to 10 m deep, and are mainly elongated north to south in the direction of the prevailing bottom currents.

Due to the action of bacteria, synsedimentary cementation of muddy sediments can occur within pockmarks due to oxidisation of methane to produce an aragonite cement. The bacteria form the basis of local food chains, so that pockmarks tend to be associated with locally enhanced biological activity (Hovland and Judd, 1988).

It is very likely that the emission of gas also occurs in areas of coarser-grained sediments, but the nature of the sediments is not conducive to the formation of pockmarks. Seepages of gas have been observed in areas of fine- to medium-grained sand in the Norwegian sector of the central North Sea (Hovland and Judd, 1988). Here, detailed surveys indicate that methane is being vented through small, funnel-shaped depressions which are about 20 cm in diameter. Small pockmarks up to 5 in in diameter were also observed, along with patches of strongly reflective sea bed which coincide with gas-charged, sub-bottom sediments. These patches, which form slight mounds up to 0.5 m in height, are characterised by a high level of biological activity compared with the surrounding, barren, sea bed, and are littered with shells and skeletal debris. Similar areas of strongly reflective sediments overlying gas-charged sediments have been observed within the report area (Figure 78). This phenomenon may explain the occurrence of the scattered sea-bed reflectors observed on sidescan-sonar records.

Mineralogy and geochemistry

Preliminary studies of lithic fragments and heavy minerals within the sea-bed sediments indicate that there are two main assemblages, each with a distinctive distribution. The first contains material derived mainly from basic igneous rocks; it is restricted to the south of the Firth of Forth, and extends seawards from the coastline for up to 90 km. The detritus is thought to have originated from the Devonian and Carboniferous igneous rocks of central and southern Scotland and north-east England. The second assemblage is more widespread, covering most of the remaining sea bed; it contains minerals thought to be derived from the Dalradian successions of north-east Scotland.

The highest concentrations of heavy minerals, more than 3 per cent of the sand fraction, occur mainly within a few kilometres of the coast. The maximum concentrations of trace metals in the North Sea are also found in the nearshore; copper (Figure 79), lead, zinc, cadmium and mercury concentrations occur in fine-grained, muddy sediments off the major estuaries in water depths of between about 15 and 55 m (Nicholson and Moore, 1981). Farther offshore, these values decrease rapidly to very low levels, although in the north, areas of higher concentrations have been observed. The area of slightly higher copper values extending from north-east Scotland to the Witch Ground Basin (Figure 79) possibly reflects sediment derived from the Dalradian and basic/ultrabasic rocks of the east Grampians (British Geological Survey, 1991). This pattern of distribution suggests that fine-grained, metal-rich, particulate matter carried by the river systems precipitates on encountering the marine environment. Within the Firth of Forth, the highest levels of trace metals also occur within areas of fine-grained sediments, especially on the southern side of the inner estuary (Thomson, 1978).

In the north-east, measurements of the concentration of hydrocarbon gases at a depth of about 2 in below sea bed indicate a correlation with hydrocarbon source rocks at depth (Faber and Stahl, 1984). Methane is predominant, and occurs in concentrations ranging from less than 20 parts per billion up to greater than 300 parts per billion. Samples taken from sites of gas seepage indicate that although the gas consists almost entirely of methane, interstitial gases within the sediments also contain longer-chain hydrocarbons ranging from ethane to hexane (Hovland and Judd, 1988). These heavier hydrocarbons may be preferentially adsorbed by the clay minerals within the sediments.

Chapter 11 Economic geology

Oil and gas

The central North Sea is one of the major areas of hydrocarbon exploration and production on the UK Continental Shelf; there have been numerous significant discoveries, with 11 field:, in production and several more at the development or appraisal stage ((Figure 80) and (Figure 81); Department of Energy, 1991). The most important reservoirs are Paleocene submarine-fan sands, and Upper Jurassic shallow-marine sands. Most fields produce oil with some associated gas (Abbotts, 1991), and there is an increasing number of gas-condensate discoveries.

The first hydrocarbon exploration well drilled in the UK sector of the North Sea was well 38/29-1, drilled in 1964 on the southern flank of the Mid North Sea High. This heralded the start of a major phase of exploration for gas sourced from the Carboniferous and contained in the Lower Permian Rotliegend reservoirs of the Southern North Sea Basin (Cameron et al., 1992). It was not until 1967 that drilling began to the north of the Mid North Sea High. The first well, 29/23-1, tested a typical southern North Sea Rotliegend prospect beneath a salt dome to the west of the Auk Ridge (Figure 80). In the next few years, many wells tested similar prospects, but all without success, as the hoped-for Carboniferous source proved to he absent beneath much of the central North Sea. However, drilling results proved a wide range of new plays; the first oil shows were found in 1967 at well 30/18-1 within thin Paleocene sands, and by 1972, hydrocarbons had been discovered in a variety of structures, and in reservoirs ranging in age from Devonian to Eocene.

The source for the hydrocarbons was shown to be the Upper Jurassic, organic-rich shales of the Kimmeridge Clay Formation. These had been discovered in the Moray Firth as early as 1967, although the first significant thickness proved in the central North Sea was in well 21/30-1 (Figure 80), drilled in 1968. The extent of discoveries in the central North Sea closely matches the distribution of areas with preserved, mature, Kimmeridge Clay Formation (Figure 80). Discoveries outside the area of maturity can be shown to lie on migration paths leading away from the Jurassic basinal areas. It is also possible that the discoveries on the margin of the Jurassic grabens to the west of the Gannet oilfields may be sourced front locally mature Kimmeridge Clay Formation.

In the central North Sea, there has been an overall steady increase in exploration and appraisal drilling (Figure 82). The figures suggest that the central North Sea is in a less-advanced stage of exploration compared with the southern and northern North Sea, where there have been pronounced peaks in the numbers of exploration wells (Andrews et al., 1990; Cameron et al., 1992). This is a reflection of the geological complexity of the area, and the generally smaller size of hydrocarbon accumulations.

In the northern North Sea, large, tilted fault blocks developed during the major Late Jurassic to Early Cretaceous extensional phase; these became the major hydrocarbon bearing structures of that area. However, during the same phase of extension in the central North Sea, the presence of thick, mobile, Zechstein salt allowed much smaller-scale structures to form above the salt, where they were separated from the larger-scale faults affecting the pre-Zechstein rocks. In addition, syn- and postdepositional fault movement, and salt dissolution, controlled the deposition and preservation of younger reservoir units. The Zechstein salt has therefore played a major role in the generation of hydrocarbon traps.

Discoveries and prospects

Devonian

Although there is source potential in Middle Devonian rocks to the north of the central North Sea (Andrews et al., 1990), no similar source rock has been discovered within the report area. However, the Upper Old Red Sandstone forms the oldest reservoirs in both the Buchan (Edwards, 1991) and Argyll (Robson, 1991) oilfields. In the Argyll Oilfield, there is significant intergranular porosity; at Buchan, the main porosity comes from fractures, with only 5 to 7 per cent matrix porosity (Edwards, 1991). Both fields are major tilted fault blocks in which the Devonian has been lifted above the Upper Jurassic source rock.

Carboniferous

There are no discoveries within Carboniferous rocks in the central North Sea. The Carboniferous is absent over much of the region where mature Jurassic source rocks occur, and any future discoveries will probably require an alternative source to the Jurassic shales. Within the eastern part of the Mid North Sea High, there may be potential for a mature Middle to Upper Carboniferous coal-measures source; if hydrocarbon-prone sediments are to he found there, they are most likely to occur within the deeper Early Carboniferous/Late Devonian basins located between the pre-Permian granites (see Chapter 3).

Additional potential exists within the Forth Approaches Basin, where only limited exploration has occurred. Oil shows within the Zechstein of well 26/7-1 (Figure 80) suggest that hydrocarbons may be sourced from the coal-bearing deltaic sequences of the Lower Carboniferous within the basin.

Permian

The occurrence of hydrocarbons in Permian rocks is restricted to major fault blocks, where footwall uplift has resulted in raising of the Permian above, or adjacent to, the Upper Jurassic source rock.

In 1975, Argyll became the first oilfield in production on the UK Continental Shelf. The main reservoir rocks are thin Zechstein carbonates with a well-developed secondary porosity, and the underlying Rotliegend and Devonian sandstones (Robson, 1991). The structure is a major, south-westward dipping, tilted fault block on the northern flank of the Mid North Sea High. Large, Late Jurassic, graben-bounding, normal faults have downthrown the Kimmeridge Clay Formation source rock to enable the trap to be charged. The main seal is Upper Cretaceous chalk.

About 14 km to the north of Argyll, the Innes Field also produces from Permian reservoirs, although in this field the Zechstein forms a seal to the Rotliegend sandstones in a northerly dipping fault block (Robson, 1991).

The only other oilfield with production from the Permian is Auk, which has a structure very similar to that at Argyll. Production is primarily from the Zechstein and Rotliegend, but also from thin, overlying, carbonate breccias assigned to the Lower Cretaceous (Trewin and Bramwell, 1991). Upper Cretaceous chalk forms the seal. The main reservoir is a brec­ciated, laminated dolomicrite, with much of the porosity de- rived from brecciation caused by dissolution of evaporites (Trewin and Bramwell, 1991).

North of the Auk Oilfield there are no commercial Permian discoveries in the central North Sea, despite the oc­currence of several major uplifted fault blocks. A major factor may be the effect of Zechstein salt migration up the main bounding faults, forming a lateral seal between the downthrown Upper Jurassic source and the Permian reservoirs. This may explain the absence of discoveries in the Jaeren High, beneath the Forties-Montrose High, and along the western margin of the Central Graben.

Triassic

One of the first oil discoveries in the central North Sea was within Triassic sandstones in well 30/13-2. The prospect was called Josephine, from which the structurally complex ridge in the centre of the Central Graben (Figure 8) now takes its name. The sands failed to develop as a major play, and the Triassic sequence in many wells was found to be predomi­nantly shale. However, there have been several discoveries within Late Triassic fluvial sandstones of the Skagerrak Formation ((Figure 29) and (Figure 31)); these are younger than the sands in Josephine, and are most thickly developed in the East Central Graben, where the Marnock Oilfield is the most sig­nificant discovery (Figure 80). The Marnock structure is sealed at the top by an unconformity, and laterally by dip, by faulting, and possibly by lateral facies changes.

Similar sandstones occur on the western margin of the Central Graben; their distribution is apparently controlled by dissolution channels at the top of salt pillows/walls, where salt could relatively easily dissolve in fresh water during Triassic times. These sandstones have poor reservoir quality (Glennie and Armstrong, 1991), and are usually overlain by the Jurassic Fulmar Formation, which has better reservoir characteristics.

Jurassic

Upper Jurassic sandstones form major reservoirs in the area, including that at the Fulmar Oilfield, one of the ten largest fields in production in the UK sector (Department of Energy, 1991). In this field, the predominantly shallow-ma­rine sandstones of the Fulmar Formation form the main reservoir; porosities average 23 per cent, with permeability ranging between 50 and 800 mD (Stockbridge and Gray, 1991). The upper sands in the reservoir occur within the Kimmeridge Clay Formation (Figure 83), and are interpreted as deeper-water, submarine-fan sands (Johnson et al., 1986). The field occurs on a fault terrace to the east of the Auk Ridge, and it is believed that the thick sands developed in re­sponse to extensional growth faulting and movement of the underlying salt (Johnson et al; 1986). The Clyde Oilfield has a similar origin (Stevens and Wallis, 1991), and several other discoveries occur in comparable structures. The main con­trols on the size and style of structure are the relative impor­tance of salt movement, and the amount of rotational faulting; early salt movement was important for generating a thick reservoir interval within a rim-syncline at the Fulmar Oilfield ((Figure 83); Johnson et al., 1986).

In both the Fulmar and Clyde oilfields, partial erosion of the Fulmar Formation has occurred at the unconformity at the base of the Cretaceous. This not only limits the extent of the reservoir, but also means that the seal varies from Kimmeridge Clay Formation shales to Upper Cretaceous chalk (Figure 83). Smaller discoveries, that rely on pinchout of the reservoir at the unconformity to provide the seal, in­clude the Duncan and Angus oilfields in the south of the re­port area (Figure 80).

On the western flanks of the Central Graben, the Fulmar Formation is also the main reservoir in the Kittiwake Oilfield (Glennie and Armstrong, 1991), and one of the reservoirs in the Gannet complex (Armstrong et al., 1987). These sands are interpreted as shallow-marine sheets (Armstrong et al., 1987) whose distribution and thickness were controlled by synsedimentary movement of the underlying salt.

There have also been discoveries in the deeper parts of the Central Graben, in similar, but slightly older Upper Jurassic sands. These commonly overlie Middle to Upper Jurassic delta­ic sands that form a secondary reservoir. The style of structure is predominantly fault-controlled, and associated with underlying salt movement. These structures tend to be gas- or condensate-bearing because of their great depth of burial.

Cretaceous

In the outer Moray Firth, Lower Cretaceous submarine-fan sands form an important reservoir (Bisewski, 1990; O'Driscoll et al., 1990). The distribution of similar sands is limited mainly to the northern margin of the central North Sea, and there are no commercial discoveries. The reasons for the general absence of similar sands in the Central Graben are not clear. It is possible that during the Early Cretaceous the amount of sand available for erosion may have been lim­ited by restricted uplift of the surrounding graben margins compared with that which occurred in the Moray Firth.

However, the deeper parts of the graben system have not been extensively tested, and some potential for the discovery of additional sands remains.

Immediately to the east of the UK sector, Upper Cretaceous chalk forms important reservoirs at several producing fields in Norwegian waters, including Ekofisk. Although several wells, such as 30/7a-4A at the Joanne Oilfield (Figure 80), have tested hydrocarbons from within the chalk of the UK sector, there has been no commercial production. The quality of reservoir within the Chalk Group is usually poor, with low permeability; the Tor Formation forms the best reservoir as it contains a high proportion of reworked chalk. Hardman (1982) defined a set of criteria required for successful exploration within the Chalk Group. These included rapid burial of thick chalk with the development of overpressures enhanced by overlying fine-grained sediments, and the early migration of hydrocarbons to limit the amount of diagenetic compaction and cementation of the reservoir. In the UK sector, overlying Paleocene sands do not form a seal, and have proved to be an important conduit for releasing overpressured interstitial fluids. A second difference is the deeper, early burial of the Kimmeridge Clay Formation in the Norwegian sector, resulting in earlier migration of hydrocarbons.

It may be that future commercial discoveries within the Chalk Group will be restricted to locations where extensive fracturing provides good secondary porosity and permeability. This effect can be seen above major salt diapirs within the Central Graben, and the Machar discovery (Figure 80) may become the first field in the UK sector with significant production from the chalk.

On the western flank of the Central Graben, the Chalk Group is thin, and permeability so low that chalk forms the seal to several fields, including Auk and Fulmar.

Tertiary

The Forties Oilfield is by far the largest discovery in the central North Sea ((Figure 80) and (Figure 81); Wills, 1991); the structure is a broad, low-amplitude anticline caused by sediment drape over the Forties-Montrose High. The absence of a thick underlying Zechstein sequence is a major factor in the size and relative simplicity of the structure, in which the reservoir is a thick series of Paleocene submarine-fan sands. Average porosity in the field is 27 per cent, with an average permeability of 700 mD. Coarse-grained sandstones were deposited as high-density turbidites in elongate sand fairways that followed topographic lows, and have much better reservoir characteristics than the more classical turbidites deposited between the fairways (Wills, 1991).

Similar submarine-fan sands of Paleocene, and to a lesser extent Eocene age, form the reservoirs for several smaller fields. These include the Arbroath and Montrose oilfields (Crawford et al., 1991), which lie above the Forties-Montrose High to the south of the Forties Oilfield (Figure 80), and have a similar structure to the Forties Oilfield. Other discoveries occur in different styles of trap, including anticlinal structures over salt swells on the western margin of the Central Graben, as at Gannet (Armstrong et al., 1987), and closures over or against the flanks of diapirs within the main grabens, as at Lomond. With improved seismic-reflection data and increasingly detailed palaeontology, stratigraphic pinchouts of individual submarine-fan channels and lobes are becoming important exploration targets. Individual channel sands and mounds at the base of the contemporaneous depositional shelf slope to the west now form anticlinal traps as a result of differential compaction of the surrounding shales.

The major submarine-fan sands are sourced from feeder channels to the west (see Chapter 8); both sand thickness and reservoir quality decrease to the south-east. The lateral continuity of the sands towards the west has allowed oil to migrate updip in a westerly direction, so that structures formed well to the west of the region of mature Kimmeridge Clay Formation may be sourced with oil and gas that have migrated up from the Jurassic sediments, and laterally through the Tertiary sands, as at the Fyne and Guillemot oilfields.

There have been no commercial hydrocarbon discoveries in sediments younger than Eocene.

Other resources

Coal is the most important resource on land in the Midland Valley of Scotland, and a shaft sunk between the high and low water marks at Culross (Figure 1) as early as 1604 may represent the earliest 'offshore' extraction of coal (D H Land, oral communication, 1992). Farther east at the Seafield Colliery near Kirkcaldy, coal has been mined recently underneath the Firth of Forth in the north–south-trending Leven Syncline, where Namurian and Westphalian rocks extend beneath the firth (BGS Tay-Forth Solid Geology sheet). The Dysart seam here is up to 9 m thick, and there are other seams over 2 m thick (Beveridge et al., 1991). A similarly orientated syncline exists to the east of Fife Ness; BGS borehole BH74/13 (Figure 1) was drilled into this syncline and recovered Duckmantian mudstone with coal seams.

Farther south, coal-bearing Carboniferous rocks crop out at the coast north of Newcastle ((Figure 1) and (Figure 2)), where they have been extensively mined beyond the coastline. Farther offshore, several wells have recovered coal-bearing Carboniferous rocks, notably in the north-eastward extension of the Midland Valley Graben. Seams up to 3 m thick occur in well 26/7-1 (Figure 12) below 1300 m, and it is likely that very significant Carboniferous coal resources exist away from the coastal zone. Additionally, coal occurs widely in the Middle Jurassic rocks of the Central Graben; a seam 13 m thick has been drilled in well 29/10-2 (Figure 34).

Evaporites extracted from the Upper Permian (Zechstein) in north-east England include anhydrite, gypsum, halite and potassium minerals (Smith, 1974); the extensive offshore continuation of the latter outcrop is known from BGS drilling to include anhydrite and gypsum (BGS Tay-Forth and Peterhead Solid Geology sheets). Minerals from the Zechstein evaporite cycles also occur widely at depth offshore; Permian halite is very extensively distributed in the Central Graben, where it has commonly been remobilised into domes and walls.

BGS surveys and other studies have shown that sand and gravel occur extensively offshore (Figure 76), but to date little or no extraction has been attempted. An isolated example of extraction comes from the Firth of Forth, where Portobello Beach near Edinburgh was replenished with sand recovered from a nearby offshore site off Fisherrow. Rutile and zircon have been reported in Northumbrian beach sands (Gallagher, 1974), with combined concentrations of almost 5 per cent within garnetiferous heavy-mineral bands which do not exceed 1 m in thickness. Offshore, maximum concentrations of trace metals are within a few kilometres of the coast, where heavy-mineral concentrations of some 3 per cent occur.

Knowledge of sea-bed geology has been important for the installation of the several oil platforms in the report area, as well as in the construction of sea-bed pipelines transporting hydrocarbons. The sea bed is also used for dumping; sewerage from Edinburgh is regularly dumped outside the Firth of Forth by the mv Gardyloo, and there are sewerage and fly-ash dumping grounds off the River Tyne (Bamber, 1984).

References

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

References to BGS offshore maps are not given here, but an index map showing their distribution in the report area is presented inside the back cover.

ABBOTTS, I L (editor). 1991. United Kingdom oil and gas fields, 25 years commemorative volume. Memoir of the Geological Society of London, No. 14.

AGUIRRE, E, and PASINI, G. 1985. The Pliocene-Pleistocene boundary. Episodes, Vol. 8, 116–120.

ANDREWS, I J, and six others. 1990. United Kingdom offshore regional report: the geology of the Moray Firth. (London: HMSO for the British Geological Survey.)

ANSARI, M H. 1992. Stratigraphy and palaeobotany of middle Pleistocene glacial deposits in the North Sea (Abstract of PhD thesis). Quaternary Newsletter, No. 68, 17–18

ARMSTRONG, L A, TEN HAVE, A, and JOHNSON, H D. 1987. The geology of the Gannet Fields, central North Sea, UK sector. 534–548 in Petroleum geology of North West Europe. BROOKS, J, AND GLENNIE, K W (editors). (London: Graham and Trotman.)

BADLEY, M E, PRICE, J D, DAHL, C R, and AGDESTEIN, T. 1988. The structural evolution of the northern Viking Graben and its bearing upon extensional models of basin formation. Journal of the Geological Society of London, Vol. 145, 455–472.

BALSON, P S, and CAMERON, T D J. 1985. Quaternary mapping offshore East Anglia. Modern Geology, Vol. 9, 221–239.

BALSON, P S and JEFFERY, D H. 1991. The glacial sequence of the southern North Sea. 245–253 in Glacial Deposits in Great Britain and Ireland EHLERS, J, GIBBARD, P L, and ROSE, J (editors). (Rotterdam: Balkema.)

BAMBER, R N. 1984. The benthos of marine fly-ash dumping ground. Journal of the Marine Biological Association of the United Kingdom, Vol. 64, 211–226.

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1:250 000 map series

All maps have been given names (see index map opposite), but they can also be referred to by the latitude and longitude of their southwestern corner. Thus the Marr Bank sheer may also be referred to as 56°N 02°W. The list below shows the types of maps available for each sheet in the report area using the following key:

• BA Bouguer Gravity Anomaly
• A Aeromagnetic Anomaly
• S Solid Geology
• SBS Sea Bed Sediments
• SBS & Q Sea Bed Sediments and Quaternary
• Dogger (55°N 02°E) BA, A, S, Q, SBS
• Swallow Hole (55°N 0°) BA, A, S, Q, SBS
• Farne (55°N 02°W) BA, A, S, Q, SBS
• Borders (55°N 04°W) BA, A, S
• Fisher (56°N 02°E) BA, A, S, Q, SBS
• Devil's Hole (56°N 0°) BA, A, S, Q, SBS
• Marr Bank (56°N 02°W) BA, A, S, Q, SBS
• Tay–Forth (56°N 04°W) BA, A, S, Q, SBS
• Cod (57°N 02°E) BA, A (S, Q, AND SBS on Forties)
• Forties (57°N 0°) BA, A, S, Q, SBS
• Peterhead (57°N 02°W) BA, A, S, Q, SBS
• Moray–Buchan (57°N 04°W) BA, A, S, SBS & Q

Land geology maps and memoirs

The index map shows sheet numbers of published 1:250 000 and 1:50 000 scale maps.

Other BGS maps

Other smaller-scale maps produced by BGS that cover all or part of the region shown include:

• Sea Bed Sediments around the United Kingdom (south and north sheets). Scale 1:1 000 000.
• Geology of the United Kingdom, Ireland and the adjacent Continental Shelf (south and north sheets). Scale 1:1 000 000.
• Geological map of the United Kingdom, South and North. Scale 1:625 000
• Pre-Permian geology of the United Kingdom (South). Scale 1:1 000 000.
• Aeromagnetic map of Great Britain. Scale 1:625 000.
• Bouguer gravity anomaly map of the North Sea. Scale 1:100 000.
• Free-air gravity anomaly map of the North Sea. Scale 1:1 000 000.

The diagrams in this report were produced by the Cartographic Production Group, BGS, Keyvvorth under the supervision of R J Parnaby. Cartography was co-ordinated by R J Demaine and J Arbon, and reprography by S C Wilkinson.

Book production was supervised by S M Murphy. The typesetting and page make-up was carried out by A R Hutchinson.

Figures

(Front cover): Unconformity at Siccar Point near St Abbs Head (Figure 1), often termed Hutton's Unconformity. Basal breccia of the Upper Old Red Sandstone dips at a low angle to the left (north); it rests on vertically inclined shaly siltstone and thin greywacke of Silurian (Llandovery) age.

(Rear cover) Index map United Kingdom Offshore Regional Reports

(Figure 1) Location of the report area showing licence blocks, commercial wells, oilfields, BGS shallow boreholes and simplified bathymetry.

(Figure 2) Generalised solid geology of the report area and adjacent land.

(Figure 3) Generalised cross-section across part of the report area. Note Cretaceous inlier is not shown. For location and key see (Figure 2).

(Figure 4) Aeromagnetic anomaly map of the report area and adjacent areas.

(Figure 5) Bouguer gravity anomaly map of the report area and adjacent areas.

(Figure 8)." data-name="images/P944906.jpg">(Figure 6) Interpreted deep-seismic section GECO NSDP 85-05 through the Devil's Hole Horst and Central Graben. For location see (Figure 8).

(Figure 7) Interpreted line drawing of the BIRPS' NEC deep-seismic line. For location see

(Figure 8) Structural framework of the central North Sea. The main structural blocks are differentiated by colour. After Freeman et al. (1988), and Chadwick and Holliday (1991).

(Figure 9) The structural setting of the report area.

(Figure 10) Generalised cross-sections through the Central Graben and Forth Approaches Basin. For locations see (Figure 8).

(Figure 11) Major faults displacing the base of the Zechstein.

(Figure 12) Pre-Permian structural summary map.

(Figure 13) Summary of the pre-Permian history of the central North Sea.

(Figure 14) Log correlation of Middle Devonian limestones between wells. For locations see (Figure 12).

(Figure 15) Seismic-reflection profile across a Late Devonian/Early Carboniferous basin on the Mid North Sea High. For location see (Figure 12). Published courtesy of Murphy Petroleum Limited and partners.

(Figure 16) Well logs showing the Devonian-Rotliegend boundary. For locations see (Figure 12).

(Figure 17) Upper Devonian upward-coarsening and upward-fining successions; interpretations based on gamma-ray logs. For locations see (Figure 12).

(Figure 18) The Lower Carboniferous coal-bearing sequence in well 21/12-2B. For location see (Figure 12).

(Figure 19) Distribution and facies of Lower Permian sedimentary (Rotliegend) and volcanic rocks.

(Figure 20) Typical Auk and Fraserburgh formation successions. See (Figure 19) for locations.

(Figure 21) Distribution and thickness of Upper Permian (Zechstein) sediments, and the distribution of halokinetic features.

(Figure 22) Selected wells showing the subdivision of the Zechstein into Halibut Bank and Turbot Bank formations, and an alternative subdivision into Zechstein cycles. See (Figure 21) for locations.

(Figure 23) Two wells showing cyclic subdivision of dolomite dominated Zechstein successions referred to the Argyll Formation by Deegan and Scull (1977). See (Figure 21) for locations.

(Figure 24) Selected wells showing Zechstein successions with thick halites, and their subdivisions into cycles. See (Figure 21) for locations.

(Figure 25) Seismic-reflection section illustrating abrupt Zechstein thickness variation due to halokinesis See (Figure 21) for location. Data courtesy of Western Geophysical.

(Figure 26) Seismic-reflection section illustrating the north—south-trending salt-dissolution front that runs across the Mid North Sea High. See (Figure 21) for location. Data courtesy of Western Geophysical.

(Figure 27) Well 31/26-3, illustrating thick Lower Permian volcanic rocks. See (Figure 19) for location.

(Figure 28) Simplified isopach and lithological distribution map for the Triassic of the central North Sea. Many local variations in thickness, particularly over salt features, are not depicted

(Figure 29) Summary of Triassic lithostratigraphical schemes in use in the UK central North Sea.

(Figure 30) The Smith Bank Formation and Marnock/Gassum Sands in well 30/1c-3. For location see (Figure 28).

(Figure 31) The Skagerrak Formation in well 22/24-4. For location, see (Figure 28).

(Figure 32) Generalised distribution of Jurassic rocks in the central North Sea. Also shown are wells with known or inferred Lower Jurassic sediments.

(Figure 33) Summary of Jurassic lithostratigraphy in the UK sector of the central North Sea. The volcanic and volcaniclastic rocks of the Ratuay Formation are derived from more than one volcanic centre.

(Figure 34) Representative well logs of the Jurassic succession in the central North Sea. For locations see (Figure 32).

(Figure 35) Generalised distribution and thickness of Middle Jurassic paralic sediments and volcanic or volcaniclastic rocks in the central North Sea.

(Figure 36) Generalised distribution and thickness of Upper Jurassic marine sediments (Humber Group) in the central North Sea. The western limit of the Humber Group is poorly defined.

(Figure 37) Cross-sections through the Fulmar and Guillemot oilfields. Redrawn from Johnson et al. (1986) and Armstrong et al. (1987). For locations of oilfields see (Figure 36).

(Figure 38) Logs of wells from the Fulmar and Guillemot oilfields. For locations see (Figure 36).

(Figure 39) Distribution and lithology of Lower Cretaceous sediments in the central North Sea.

(Figure 40) Geophysical-log correlations of Lower Cretaceous sediments in graben/basin areas. For locations see (Figure 39).

(Figure 41) Geophysical-log correlations of Lower Cretaceous sediments in basin-margin areas. For locations see (Figure 39).

(Figure 42) Lithostratigraphical schemes for the Lower Cretaceous of the central North Sea and adjacent areas.

(Figure 43) Distribution and thickness of the Chalk Group. Salt piercement structures have not been depicted on this or subsequent Upper Cretaceous diagrams.

(Figure 44) Lithostratigraphy of the Chalk Group in the central:North Sea. Based on Deegan and Scull (1977).

(Figure 45) Diagrammatic cross-section through the Chalk Group along the line of section depicted on (Figure 43).

(Figure 47)." data-name="images/P944946.jpg">(Figure 46) The Hidra and Plenus Marl formations in well 30/7a-5. For location see (Figure 47).

(Figure 47) Distribution and thickness of the Hidra Formation, and the extent of the Herring and Flounder formations.

(Figure 48) Correlation of Turonian to Campanian sediments between selected wells. For locations see (Figure 47).

(Figure 50)." data-name="images/P944949.jpg">(Figure 49) The Tor and Ekofisk formations in well 29/5a-3 in the Erskine Basin. For location see (Figure 50).

(Figure 50) Distribution and thickness of the Ekofisk Formation in the central North Sea.

(Figure 51) Major palaeogeographic features, and facies limits, of the Paleocene of the central North Sea. Gannet Fan after Armstrong et al. (1987).

(Figure 52) Major palaeogeographic features, and facies limits, of the Eocene of the central North Sea.

(Figure 53) Diagrammatic north-west to south-east cross-section through the central North Sea, showing lateral facies variation in the Paleogene sequences.

(Figure 54) Stratigraphical nomenclature for the Paleocene and Eocene of the central North Sea.

(Figure 55) Thickness of Paleocene sediments in the central North Sea.

(Figure 56) Geophysical-log response and lithostratigraphical correlation of typical Paleocene sections in the central North Sea. For locations see (Figure 55).

(Figure 57) Thickness of the Eocene (post-Balder Formation) sediments in the central North Sea.

(Figure 58) Idealised log response defining the main stratigraphical subdivisions, and associated seismic events, within the basinal Eocene succession of the central North Sea.

(Figure 59) Geophysical-log response and lithostratigraphical correlation of typical Eocene sections in the northern part of the central North Sea. For locations see (Figure 57).

(Figure 60) Generalised isopach map of Oligocene sediments in the central North Sea, illustrating the location of salt piercement structures. After Vinken (1988).

(Figure 63)." data-name="images/P944961.jpg">(Figure 61) Cross-section through Tertiary sediments along the axis of the Central Graben (after Deegan in Vinken, 1988), with two cross-sections drawn at right angles to the Central Graben axis, illustrating post-Eocene sediments. For locations see (Figure 63).

(Figure 62) Generalised isopach map of Miocene sediments in the central North Sea. After Vinken (1988).

(Figure 63) Generalised Pliocene isopach map for the central North Sea.

(Figure 64) Quaternary stratigraphy of the central North Sea. Stages, timescales and magnetostratigraphy after Zagwijn (1979; 1985), Mankinen and Dalrymple (1979), Bowen et al. (1986), Sibrava (1986) and Levi et al. (1990). Formation nomenclature after Stoker et al. (1985) and BGS Quaternary Geology sheets.

(Figure 65) Thickness of Quaternary sediments in the central North Sea. The base of the Quaternary is not well defined where the sequence is thickest.

(Figure 66) Partly schematic section illustrating the relationships of formations, and the transition in stratigraphical usage, north and south of 56°N (east of 0°). For key to abbreviations see (Figure 64).

(Figure 67) Partly generalised surface distribution of Quaternary formations. Taken from BGS Quaternary Geology sheets.

(Figure 68) Distributions of the early Pleistocene formations of the Deltaic division. For key to abbreviations see (Figure 64).

(Figure 69) Distribution of the Aberdeen Ground Formation, and the lithofacies of early Cromerian Complex times. After Stoker and Bent (1985).

(Figure 70) General stratigraphical relationships of Quaternary formations north of 56°N. After Stoker et al. (1985).

(Figure 71) Log of BGS borehole BH81/34, and its relationship to the seismic stratigraphies which change at 56°N. The borehole is located some 500 m off line, where there is a channel (dashed) filled with Ling Bank Formation deposit. For location see (Figure 65). For key to abbreviations see (Figure 64). Log adapted from Knudsen and Sejrup (1993).

(Figure 72) The distribution of selected late Weichselian and Holocene formations. The formations do not necessarily crop out — see (Figure 67).

(Figure 73) Cross-sections illustrating the Quaternary succession in the Forth Approaches and in the Firth of Forth. For locations see (Figure 72). For key to abbreviations see (Figure 64).

(Figure 74) Sea-bed sediment and bedform distribution in the central North Sea.

(Figure 75) Bathymetry of the central North Sea.

(Figure 76) Extreme wave conditions and mean spring-tidal currents in and around the report area. Adapted from Pantin (1991).

(Figure 77) Carbonate content of the sand fraction of sea-bed sediments in the central North Sea.

(Figure 78) a) Sidescan-sonar record showing a locally strongly reflective sea bed in an area of featureless muddy sand. b) High-resolution boomer record showing that the strongly reflective sea bed corresponds ro an underlying zone of acot:stic blanking (indicative of the presence of gas) in otherwise acoustically well-layered sediments. For location see (Figure 74).

(Figure 79) The distribution of copper in sea-bed sediments around much of the UK. The data are part of a database of geochemical analyses for 38 major, minor and trace elements in approximately 10 000 sea-bed samples around the UK from the Dover Strait to the North Channel.

(Figure 80) The main hydrocarbon discoveries and oilfields of the central North Sea, showing the distribution of mature Kimmeridge Clay Formation.

(Figure 81) Summary of hydrocarbon production in the central North Sea. Data from Armstrong et al. (1987), Johnson et al. (1986). Department of Energy (1991), and papers in Abbotts (1991).

(Figure 82) Drilling history of the central North Sea. Based on Department of Energy (1991).

(Figure 83) Development of the Fulmar Oilfield structure. After Johnson et al. (1986).