Review of North Sea Basin Development

Home , History of the North Sea, Permian Basin (Europe)

Review of North Sea Basin development

P. E. KENT

CONTENTS Introduction . . . 436 Older Palaeozoic 436 Devonian 436 Carboniferous 439 5 Permian 44x 6 Trias . AA d 7 Jurassic . . . 447 8 Late Jurassic/early Cretaceous movements 45 ° 9 Cretaceous .... 45x IO Tertiary .... 454 II Mesozoic and Tertiary Volcanics 455 Mechanism of North Sea subsidence 457 I3 Appendix History of local positive structures (a) Brent . . 459 (b) Frigg 460 (c) Piper 46x (d) Forties. 463 (e) Auk 463 (f) Argyll . 464 (g) Ekofisk 465 I4 References . . 467 SUMMARY The large amount of data now available on minor faulting only, but in the south inversion the North Sea stratigraphy, and on the history of the troughs took place in late Cretaceous of the positive structures which form the sites and early Tertiary times; this may have had a of the oilfields, provides a basis for a review compressional (orogenic) cause. of basin development. The Tertiary basin developed as a single, The area was initially occupied by two relatively simple synclinal subsidence of the inter-cratonic basins: the Northern Basin whole North Sea area, centred on the main rift had a Devonian ancestor; the Southern Basin system but showing an absence of fault-control. dates back at least to the Carboniferous. Interruptions in deposition on the individual During the Permian and Trias broad inter- structural high areas show wide correspondence cratonic subsidence continued in the south, in the North Sea. Except for a major late but rifting developed across the separating Triassic movement limited to the Netherlands Mid North Sea High and in the Northern and adjoining areas they show also close Basin. Rifting (taphrogenic) control of sub- relation to those in Bdtain. The most notable sidence became widely dominant through the are early middle Jurassic, early upper Jurassic Jurassic and lower Cretaceous, dying away in and end Jurassic/lower Cretaceous. These the upper Cretaceous, with major subsidence produced erosional phases even in the deeper in the Viking Graben in the Northern Basin parts of the basin. They were entirely of and in a series of narrow troughs in the south. epeirogenic type. No opening of the crust was associated with Halokinetic movements of Permian salt the central rifting: deep penetrations have produced many local complications (especially encountered Devonian or Precambdan in the in the Southern North Sea Basin) with con- rift floor. For the most part the upper Cretac- sequent structural and stratigraphical anom- eous sagged into the earlier depressions with aries from the middle Trias onwards. J1 geol. Soc. Lond. vol. xjx , x975. pp. 435-468. x8 figs. Printed in Northern Ireland. 1

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i. Introduction

SIX MONTHS AGO I had the responsibility of presenting the Keynote Address to the symposium on "Petroleum and the Continental Shelf of North West Europe--the Geology and the Environment," jointly organised by this Society, with the Institute of Petroleum, the Petroleum Exploration Society of Great Britain and the Institute of Geological Sciences. The proceedings of that symposium are due to be published shortly after the date of this Address, but the large amount of new information released has provided an opportunity for a synthesis on the development of the basin and the relation of North Sea history to the classic British geological sequence (Woodland 1975). Others have recently provided broad-brush pictures relating the overall relationship of Northwest Europe to continental plate movements in the light of hypothetical concepts of stress systems. In contrast, the intention here is to deal with North Sea basin development as a problem of palaeogeography and epeiro- genie controls in a lesser area--but still one measuring a thousand miles north- south by four hundred miles east-west.

2. Precambrian and older Palaeozoic According to Sorgenfrei (I969 p. 168), Precambrian rocks have been found at depth in north Jutland and on the Ringkobing-Fyn high. The former at least resemble those of the Fenno--Scandinavian shield, and appear to have been marginal to the Caledonian geosyncline to the north west. Cambro-Silurian rocks occur in southern Scotland and northern England, in the Hardanger and Oslo regions of Norway, and at depth in the southern flanks of the Ringkobing-Fyn high. A continuous inter-cratonic basin extending across the southern North Sea towards the occurrences in the English South Midlands is assumed and is supported by a solitary identification of Silurian beneath the Permian of the Dogger Bank area. Elsewhere the pre-Devonian has not been recorded. The northern North Sea conceals the presumed link between the Caledonian chains of Scotland and Norway, but the pre-Devonian rocks are mostly beyond practical reach of the drill. There are nevertheless rare cases of such penetrations, and these include a report of "Caledonian Basement" rocks beneath the Mesozoic northeast of the Highlands in the Cormorant field, showing that the Scottish structural block continues north-eastwards at least as far as the floor of the Viking Graben system. 3. Devonian The Devonian saw the establishment of the framework of resistant blocks which controlled British basinal development throughout the Mesozoic and Tertiary. At this time the Caledonian orogenic belt was fragmented into the separate massifs of Wales, East Anglia-Brabant, the northern Pennines, Southern Uplands and Highlands, between which the smaller downwarps of the British Isles developed (Kent 1975). Only Cornubia apparently remained to be developed later

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by the Hercynian orogeny. On the other side of the North Sea, stable blocks of the Ringkzbing-Fyn high beneath Denmark and the Fenno-Scandian massif stood in the same relationship, framing the easterly and northeasterly tides of the North Sea basins. A Devonian history of subsidence is not yet documented within the intervening

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basins; it is partly inferred from the histories of the stable blocks but in the case of more westerly basins (Irish Sea) has been deduced also from gravity data by Bott and others. There is nevertheless at least a partial congruence of the northern North Sea Basin with the Orcadian basin which has its western margin in north- eastern Scotland and the Moray Firth, but data in the easterly part are scanty. Old Red Sandstone rocks have been touched in a few wells in the northern North Sea, including one within the Viking Graben. The most important section is the penetration in the Argyll Field (Pennington z975) , where well 3o/24-3

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/131/5/435/4885037/gsjgs.131.5.0435.pdf by guest on 27 September 2021 Review of North Sea Basin development 439 penetrated about 800 feet (24° m) of upper Devonian in Old Red Sandstone facies upon 382 feet (93 m) (incomplete) of marine middle Devonian. The upper Devonian is described as uniformly dipping anhydritic siltstones, fine grained sandstones and shales, with Frasnian spores. The middle Devonian consisted of shales and limy dolomites containing ostracods, overlying "a thick fossiliferous limestone containing both rugose and tabulate corals"-a reefal tendency-proved to an incomplete thickness of 112 feet (34 m). The Devonian was penetrated in the upper part of a tilted fault block, dipping at xo ° and overlain without discordance by Rotliegendes sediments. This northerly discovery of marine middle Devonian is so far unique. Connection with the nearest marine areas in Belgium and southern England may well have lain within the southerly North Sea area, suggesting that the basinal downwarp there had already begun. Further resolution of relationships must depend largely on interpretation of deep seismic data. 4-Carboniferous Exploration in the southern North Sea has confirmed the long standing assumption that the Carboniferous basins of northern England were, and still are, continuous with their deeply buried counterparts in the Netherlands and Northwest Germany.

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Armstrong (i972 , fig. 6) showed the extension of British coalfields off the east coast, and Eames (I975) has plotted the pre-Permian subcrops across the North Sea. Eames showed that an elongate NW-SE simple syncline with an axial belt of Westphalian C/D flanks the north side of the Wales-Brabant massif, and that north of this is an irregular outcrop with the same general trend of earlier Coal Measures (Westphalian A, B), with Westphalian C/D and Stephanian further north representing the western end of the main North German basin. He has represented this large Carboniferous outcrop as having an east-west northern limit between the Cheviots/Southern Uplands and the Ringk~bing-Fyn high. This is in line with the earlier view of Armstrong, who moreover quoted evidence that the North Sea High is crossed by minor fault troughs of Coal Measures, trending NNW-SSE, analogous to those crossing the Southern Uplands (I972 p. 47I). Since the Carboniferous basin was developed between this northerly

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barrier and the Wales-Brabant massif (depositionaUy as well as structurally, if the offshore high continued the Southern Uplands) it was clearly a genetic precursor of the Southern North Sea Permian Basin. The outcrop trends based on well sections mapped by Eames agree with seismic data in showing that the Southern North Sea basin is almost entirely floored by gently folded Carboniferous. This was an area of foreland folding, sheltered from the Hercynian orogeny by the Wales-Brabant massif. In general the Permian provides the lowest rocks of interest for hydrocarbon entrapment and there have been few deep penetrations into the Carboniferous. The Gulf well 53/lO-I however penetrated nearly 3ooo ft of normal Coal Measures with numerous coals east of Yarmouth (Kent & Walmsley 197o p. 169), and there is an unpublished record of penetration of basinal lower Carboniferous off the Yorkshire coast. There is no reason to suppose that the range of facies and stratigraphic development in the southern North Sea differs significantly from that known on land in Britain, but it would be of great interest to know whether, for example, the deep Mesozoic troughs (depocentres) were developed over lower Carboniferous basins. During the upper Carboniferous Northern England, the Southern North Sea, the Netherlands and north German plain were dominated by the Millstone Grit and Coal Measure delta. In Britain this appears to have been continuous and was apparently little affected by the Pennine blocks, although the Southern Uplands Welsh massif and East Anglia-Brabant massif limited subsidence. This great spread of deltaic material requires a correspondingly large source area, and it is hard to escape the conclusion that at this period the Northern North Sea basin was much smaller. There is a record of middle Carboniferous in the Piper area, and there was a limited easterly extension of the Midland Valley into the Firth Approaches Basin, but a large part of the Caledonian mountain belt possibly including part of the Devonian downwarped areas and land areas further north must have constituted a land area sufficiently large to contribute the many thousands ofcubic miles of northerly derived deltaic sediments which accumulated in the upper Carboniferous basin.

5. Permian The circumstance that the lower Permian Rotliegendes sandstone provides the main gas reservoir for the southern North Sea, and that the overlying Zechstein evaporites provide the cap rock and are the source of complicated halokinetie structures in the overburden has encouraged the publication of data on this system (Brunstrom & Walmsley 1969, Kent & Walmsley 197 o, Glennie 1972, Blanche 1973, Taylor & Colter 1975). The greater part of the North Sea area with adjoining land areas underwent subsidence during the Permian, sedimentation being related to the Northern and Southern Basins largely separated by the Mid-North Sea High and Ringkobing-Fyn High. Rotliegendes. The Rotliegendes is essentially a continental formation, deposited in a basin extending from the Pennines across the Southern North Sea into the Low

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Countries and northern Germany, transgressive across eroded upper Carbon- iferous. The southern and western marginal belt of this basin is characterised by high porosity dune sands (the Yellow Sands of Durham, the Leman Sand of the North Sea) which grade basinwards into the less well sorted waterlaid sands, and via a sand/shale alternation into clays with anhydrite and salt in the centre of the basin~the Haselgebirge facies (Glennie I972 ). The dune sand belt measures up to 200 km. wide in the Southern North Sea; it is locally flanked on the landward side by fluviatile beds (as along the Norfolk coast) and this alternative marginal facies becomes dominant over much of the Netherlands and central Germany. The northern edge of the southern basin, flanking the Mid-North Sea High and the Ringkobing-Fyn High, has no comparable sand belt. In the Northern North Sea basin identification of the lower Permian is less certain (with the possibility of confusion with Old Red Sandstone) and it is less well documented, but the same pattern of facies distribution appears to be developed--a marginal belt with dune sand in the south western quadrant, grad- ing via a mixed series, seen in the Argyll field, into a poor, rather shaly anhydritie development further north, as in the Piper field. The occurrence of this northerly dune sand belt supports the concept that sand distribution was controlled by a prevailing easterly wind blowing across the Permian desert basin, the sand being arrested in the marginal parts of each North Sea basin. There are also cases of thick dune sand development further north, in the area east of the Moray Firth, perhaps related to a topographical rise at the edge of the East Shetland shelf. It appears that there were alternating periods of flooding (playa lakes) when fluviatile and lacustrine sediments were dominant, and periods when the area dried out and barchan dunes were widely developed. A phase of igneous activity occurred during this period. Extrusion analogous to the volcanics of the Scottish Midland Valley, of the Oslo graben, of Germany and Devonshire is indicated by two flows of highly altered basalt in the Argyll field (Pennington I975) and by local occurrences in the far north. (It is perhaps significant that the isolated occurrence NE of the Shetlands is developed in the WSW-ENE trending East Shetland Basin, a trend parallel to the Midland Valley). Volcanoes were active at this time along the flank of the Ringkobing-Fyn high (Rasmussen I974), and locally on the Mid-North Sea high. In the southern North Sea the Rotliegendes is distributed as a sheet sand/shale. In the northern English basins and in the Oslo graben margins are fault controlled (end Carboniferous or early Permian faulting), and in the Argyll field the angular truncation of thick Rotliegendes by Zechstein shows that intra-Permian faulting was occurring in the northern North Sea also. The taphrogenic control of sedimentation seen in the Mesozoic had thus begun in the lower Permian.

Zechstein. Over most of the southern North Sea the Zechstein is developed on the North German pattern. The sequence began with the marine transgression of" the Kupferschiefer, which is one of the most widely persistent geological horizons known, and this was followed by four cycles of dolomite/anhydrite/hahte/potash deposition, totalling c. Iooo m. The vertical sequence of evaporitic rocks is

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/131/5/435/4885037/gsjgs.131.5.0435.pdf by guest on 27 September 2021 Review of North Sea Basin development 443 paralleled by a lateral passage on the margins of the basin, so that halite (for example) tends to grade landwards into anhydrite and reefal dolomite (Taylor & Colter x975). The marginal facies, in which the four cycles are virtually completely represented by dolomite, is developed in the outcrop belts in England and Germany. The Zechstein basin is continuous through a contemporary rift separating the Mid-North Sea High and the Ringkobing-Fyn High (the East Dogger Bank Graben of Sorgenfrei i969, fig. 7) into a comparable broad evaporite basin extending from the outer Firth Approaches Basin eastwards across the Danish Basin and Danish Embayment, and northwards into the Ekofisk area, a distribution which demonstrates that the central basin and graben system of the Northern North Sea Basin was developed by Zechstein times. Further north still the Zechstein is represented mainly by dolomite, probably reefal in the Argyll field, commonly associated with anhydrite. This facies extends into the Moray Firth basin and into the Viking Graben at least as far as 6o°N (east of the Orkneys). It is a matter of speculation as to whether there was an upper Permian marine connection still further north to an open sea beyond.

6. Trias Subsidence continued to be related to a double basin system partly separated by the east-west barrier of the Mid-North Sea High/Ringkobing-Fyn High through the early Mesozoic, but taphrogenic control became more marked in the north. In the North Sea generally the Trias follows the Permian with perfect conformity whenever both are present. The marine influence ceased but the end of the Palaeozoic corresponded with a particularly peaceful interlude in the history of basin subsidence. An excellent account of Trias stratigraphy and dating in the North Sea has recently been published by Brennand (i975) and the following account is largely based on his work. As he has shown, the southern North Sea basin is probably unique in having a complete Triassic succession. The relative ease of correlation of the sheet-like succession of cyclically deposited sediments there in fact gives a misleading impression of regional Triassic history, for in marginal areas and in the north the sequence is basally defective over highs, had a median erosional episode (the Hardegsen unconformity--which is no more than a disconformity in the south) and later beds have locally been removed either by pre-Rhaetie uplift and erosion, or by the later stages of angular unconformity which developed in the middle and early upper Jurassic. Because of the fineness and deep redness of the Triassic sediments in the North Sea, Brennand has explained the characteristic arrangement of sandy early Trias ("Bunter" of the old terminology) followed by fine grained later Trias ("Keuper") as due to uplift of the edges of the Permian basin with progressive erosion of previously deposited Permian. This concept is somewhat difficult to accept, firstly because of the conformity within the basin, secondly because in England (at least) Permian shorelines are well known and both Bunter and Keuper transgressed far beyond them. The alternative concept is that the shrinkage

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of the end-Permian sea permitted clastic shoreline material (much of it derived from a red-weathered landscape) to be carried by fluviatile or aeolian agencies much further into the basins than formerly, so that only in the centre of the North Sea, for example, was a fine grained and saliferous lower Trias developed. The process was reversed with the expansion of the Keuper lake; the coarse marginal deposits were precipitated much further back from the central areas, although the latter still continued to provide the main locus for the salt deposition. In the U.K. sector of the southern North Sea a joint Industry/IGS Committee has proposed a new rock terminology to meet the problem that the British strata fail to correspond precisely to the standard German succession (Rhys I974). These new lithostratigraphic terms apply to the Southern North Sea and the Anglo-Dutch basin. Further north the facies is different: the names are mostly inapplicable and the dating is less certain. There was only one marine transgression before the Rhaeficmthat of the Muschelkalk. It penetrated the English land area far enough west to leave

FIo. 5" Trias basins. Thicknesses are in metres (modified from T. P. Brennand t975).

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Lingula in the Waterstones in Nottinghamshire, but it has not been detected in the more northerly basins. In the southern North Sea the first haloldnetic movement of the Permian salt began at this time, affecting depositional thicknesses of Muschelkalk and Keuper over rising salt plugs (Brunstrom & Walmsley 1969, p. 879). The Mid-North Sea High and Ringkobing-Fyn high formed a partial barrier between the northern and southern basins in the Trias. The latter was cut by deep rifts such as the Horn Graben, with lOOO-2OOO metres of Trias but without a later history of subsidence, which is closely analogous to the troughs which cross the Southern Uplands massif further west. In the Forth Approaches Basin and the westerly Central Graben the Permian is succeeded by red mudstones comparable with Bunter Shale (basal Bunter). This has an erosional upper limit beneath Jurassic, and upper Trias has not been proved, but seismic evidence of thickening to a total of c. Iooo metres suggests that it may be present in synelines. North Danish waters coincide with a deep east-west basin (the Danish Embay- ment), believed to be fault controlled, the axis of which swings concentrically with the coast of southern Norway northwards into the Egersund Basin*. The fill of 12ooo ft. (36oo m) or more is mainly elastics, with sands and silts predominating; evaporites are reported to be absent. It is not yet known whether this is a marginal facies of the whole upper Trias, or whether an important part of the later beds has been removed by later Jurassic erosion. In the Moray Firth basin also the Trias facies is a marginal one, with a domin- ance of sandstones particularly in the lower part. The aeolian reptile-bearing Elgin Sandstone is part of this sequence. Wells on structural highs have penetrated nearly 500 metres of Trias offshore, but seismic evidence indicates that this is considerably below the maximum locally present. In the Viking Graben the Trias again attains great thicknesses: 6000 ft (incomplete) has been drilled in the Brent field. Scanty pollen evidence suggests that a large part of this is late Triassic, probably Rhaetic. The sequence shows a rapid alternation of sandstones, siltstones and mudstones without evaporites. This degree of subsidence indicates that the Viking Graben basin was in existence during the Trias, and that it was active as a taphrogenic unit in controlling depositional thicknesses. The taphrogenic control of Triassic subsidence in the Northern North Sea finds its analogy in the Cheshire Basin and Worcester Graben, as well as in westerly offshore basins. In part subsidence was controlled by faulting; in part by monocline formation; both may be symptomatic of crustal attenuation affecting weak belts between the residual Caledonian massifs. 7" Jurassic Northern and Southern basins continued to develop largely separately during the Jurassic. Rifting was of major importance in both areas but the later history of the rifts was different, with inversion characteristic in the south.

* Southern Stavanger Basin. Nomenclature is not formalized in this area.

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Southern North Sea. During the Jurassic the southern North Sea and north-west Germany were characterised by linear basins and rifts which developed con- temporaneously, so that thick deposits occurred in the rifts while the sediments were thin and often subsequently removed from the flanking high blocks. The centres of the rifts themselves were subsequently deeply eroded following post- Jurassic inversion. This mechanism has been demonstrated in detail by Heybroek (I975) for the Dutch Offshore section of the Central Graben and its flanking highs. Deposition in that area was continuous from lower Permian through the Lias, the latter being in a dominantly argillaceous facies with siltstone and thin sandstone

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intervals, as in eastern England. Lower Jurassic deposition was followed by extensive regression and then by renewed transgression of mainly lacustrine Aalenian and early Bajocian (totalling I oo m in the rift) and by marine middle Jurassic elastics ending in Callovian. After a regression, upper Jurassic transgression began with paralic sediments (including sandstones and coals) followed by a rather more marine facies. Thickness of the upper Jurassic exceeds t6oo m in the trough, probably a maximum for the Southern North Sea. The lower Cretaceous again began with regression, and deposition was interrupted after the Barremian; the next strata are Albian-Apfian marls followed by upper Cretaceous chalk. Overall in this area there is a considerable lithological resem- blance to the sequence in Yorkshire and Lincolnshire. It is believed that sediments up to the end of the Lias were widely deposited in the southern North Sea, but that block movements in Aalenian, late Callovian, late upper Jurassic and late lower Cretaceous led to removal of the accumulations except in the Graben area. The sequence was further reduced by the inversion of the southerly Central Graben and other troughs (as in comparable Jurassic rifts in north-west Germany), so that the upper Cretaceous chalk locally transgresses across i5oo-2ooo metres of previously deposited sediments. The regional structure was additionally complicated by halokinetic salt-plug intrusion. The history of other fault-and-trough systems in the south has not been worked out in comparable detail. A close analogy is however provided by the Sole Pit basin bounding the offshore edge of the East Midlands shelf. There a thick Permo- Triassic sequence was succeeded by more than 2ooo feet (6oo m.) of Lias, and by middle/upper Jurassic which is not fully elucidated but in which upper Oxfordian rocks rest more or less directly on Lias on the edge of the trough. Halokinetic towage began early (mid-Trias) and invaded the edge of the shelf (Brunstrom & Walmsley I969, fig. 5), and this was followed by a Cretaceous phase of inversion which gave rise to broad uplift of the graben area further discussed in later para- graphs. Northern North Sea. The Jurassic of the Northern North Sea was regarded as a secondary prospect for hydrocarbons before exploration began, but thanks to W. E. East Midland Shelf Sole Pit Intermediate Dutch Schill Trough Block Offshore Grund Dowsing Graben High Fault

~~ ~ ~ " ".-." :': : ." : Tertiary "..'.'..'..'.'''. --~'~'L:2h-~//- ~ ~;..., ,/,,, .".-:. "' ~~=~~

Not to scale Tertiary Upper Cretaceous u Lower Cretaceous M Jurassic FIo. 8. Section illustrating stratigxaphic L relationships and inversion structures in Trias the southern North Sea (diagrammatic).

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excellent reservoir developments in an estuarine/deltaic facies it has now proved to be the most important objective. Thick sands are developed particularly in the early upper Jurassic (Oxfordian, as at Brora) ; in the middle Jurassic, analogous to the Estuarine/Deltaic development of Yorkshire, and in the Rhaeto-Liassic, as in the basin-edge development of southern Denmark. Penetrations of the series are almost entirely limited to high fault blocks within the Viking Graben, but lithology indicates that the reservoirs are sheet sands and the available sections are believed to be broadly typical. Sellwood (z974) has suggested that the middle Jurassic sands were largely derived from erosion of late Palaeozoic sandstones on the Mid-North Sea High and adjoining area, but available evidence is against thick Devonian or Carboni- ferous there. The Highlands and the far north appear to be the most likely source. The lowest sand is known in the Brent and Statfjord fields. It is 58o feet (177 m.) thick at Brent, dated as Rhaetic and lower Lias, massive and apparently non- marine. Above is a 2oo feet (6o m) marine shale interval (provisionally named the Dunlin Shale) dated as Pliensbachian to Aalenian. The succeeding Brent sand is an estuarine/deltaic development dated as Bajocian-Bathonian, 8oo feet (24o m) thick, and this is overlain without angular unconformity by upper Jurassic (late Oxfordian/Kimmeridge/Portlandian) shale. Broadly similar sequences are reported to be present at the Hutton and Ninian fields. Major Faults v--Limit of main volcanic spread • "" • Area with>75% Lava /~s,, Magnetic intensity """ maximal A- --" VIKING i~ V----. ,,~

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Further south, off the Moray Firth, the lower part of the Jurassic is occupied by the volcanic development of the Forties/Piper area, where lavas and tufts built up centrally above sea level, and interdigitated marginally with marine shales during the Bajocian and Bathonian. In the Piper oilfield the volcanic sequence on the edge of this centre, locally reduced by erosion on the Piper horst, is overlain by late Oxfordian Piper Sand, which transgresses within the limits of the fault block across a thin non-marine Callovian shale and the middle Jurassic volcanic member on to Trias. The Piper Sand is a massive sand I 15-36o feet (35-1oo m) thick, with shale partings. This is essentially a shore-line development, the environment being interpreted as a high-energy beach bar complex, alternating with organic rich marine shales in which the late Oxfordian ammonite Amoeboceras prionodes (S. Buck.) is reported. The Piper Sand may be regarded as the distal equivalent of the sandy Oxfordian with coals known at Brora on the mainland coast. Still further basinwards thin sands in the Argyll field and elsewhere may be the deeper water lithological representatives, but there is insufficient information available to trace the overall geometry of the upper Jurassic estuarine/marine sand development beyond the comment that it appears to be widespread over the Moray Firth embayment of the North Sea basin. The later Jurassic Kimmeridge Shale appears to extend over the whole basinal region with little change of facies. It represents a fairly sudden deepening of water, since it overlies the late Oxfordian shallow water sands of the Piper field (as on the Scottish coast) and (together with a discontinuous Upper Oxfor- dian element) transgresses the topographically irregular surface of the eroded middle Jurassic fault blocks, for example at the Brent, Auk and Argyll fields. This deepening is not special to the North Sea with its central rift system: it is equally marked in Yorkshire and in Dorset, where the relatively deep water Kimmeridge follows the very shallow water Corallian reefal limestones. In the North Sea the lithological change locally came a little later than the beginning of the Kimmeridgian stage, and this also finds an analogy on land, in Lincolnshire, where there is a Kimmeridgian sand (Elsham Sandstone, up to IO m thick) developed within the basal zones. A very minor increase in depth of water could have ended the bioclasfic accumulation phase, and associated marginal transgression would have limited the spread of sand across the basin. The broad picture in the later Oxfordian appears to be one of widespread shallow water and local erosion of uplifted blocks followed by the beginning of a general deepening which was sufficiently rapid to permit survival of a topography measured in hundreds of metres, perhaps related to accentuated tensional "necking" of the North Sea crust which continued through the Kimmeridgian stage into the lower Cretaceous. It was a deepening coinciding with a significant period of tectonic stability in north-west Europe. The Portlandian equivalent is not widely recognised in the North Sea although it is recorded from Brent and nearby fields, and there is no recorded development of a Purbeck facies away from southern England and west Germany, but this part of the succession is not well known, the beds being frequently eliminated by the early Cretaceous erosional phase.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/131/5/435/4885037/gsjgs.131.5.0435.pdf by guest on 27 September 2021 45 ° P, E. Kent 8. Late Jurassic early Cretaceous ("late-Kimmerian") movements As the present writer has observed elsewhere (I975), Stille did the science a disservice in his codification of hypothetical orogenic phases. Apart from the inappropriateness of some of the terminology ("Subhercynian" for a Cretaceous episode is the outstanding case) there is considerable doubt about whether the movements are in most cases orogenic in origin, particularly in the Carboniferous (Rayner I967, Ramsbottom i973) , and there is a much broader spread of movement than the scheme implies in the Mesozoic. "Early Kimmerian" and "late Kimmerian" are convenient shorthand but are imprecise terms in relation to the known geology. Much of the Mesozoic differential movement in the North Sea was broadly continuous. This is most clearly seen in the salt plug areas, where halokinetic displacement starting during the Triassic, has produced attenuation and unconformities in all the Mesozoic units and (generally less strongly) in the Tertiary. Additionally, deposition of the thick sequences in the northern central rift system appears to be related to fault controlled subsidence which (being related to deep seated ductile flow) was also probably nearly continuous. Movement of the individual fault blocks was however episodic, and provides an increasingly sharply defined understanding of the history of basin development. The Piper field shows transgression across an angular unconformity by upper Oxfordian rocks, and this seems to be a relatively widespread relationship. In the Brent field the bulk of the transgressive upper Jurassic shales is recorded as Kimmeridge, but Bowen (I 975) shows an Oxfordian element in his diagram (fig. I I), and the truncating sand is Oxfordian in the Piper field (Williams et al. 1975). Both truncation and overlap on irregular topography are involved in most cases, and with spot sampling of deeply buried sequences it is difficult to be sure of the date of the oldest element in an overlap situation (as well as that of the youngest beds overstepped). These and other cases however show that the main incident of uplift and erosion was post-Bathonian/pre-late Oxfordian. There was possibly a minor further movement in the early Kimmeridge before general quiescence permitted the widespread deposition of the deeper water Kimmeridgian and Portlandian shales. In the southern North Sea it appears that Neocomian stages are involved in the broad Cretaceous overlap, implying transgression after an end-Jurassic movement, but in the north the evidence most widely available is of Aptian/Albian rocks truncating earlier Cretaceous and Jurassic together, as at Piper. Whenever the two phases can be compared the known amplitude of movement at this time was less than that of the late Oxfordian. Locally further block movement with unconformities took place well into the upper Cretaceous as shown by the sub- Coniacian unconformity at Piper. None of these movements appears to be compressional: they are all either halokinetic in origin or related to differential movement of tensional fault blocks. The only indication of a compressional incident is provided by the end-Cretaceous inversion of the southerly fault basins, described below, which has been ascribed to an early phase of the Alpine orogeny by Heybroek (I975).

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with basal angular unconformities over positive structures throughout the North Seamfor example at Brent, Forties, Piper, Argyll, Indefatigable and in the offshore Dutch Graben. Both upper and lower Cretaceous take part in this transgression, although the latter is discontinuous. The upper Cretaceous itself is almost universally found but is locally overlapped by Tertiary, as on the Auk high, and over halokinetic structures in the southeastern North Sea.

Lower Cretaceous. No connected account of North Sea lower Cretaceous is yet available; it tends to be a minor element in the stratigraphic column and the relationship of the constituent stages is complicated. Argillaceous rocks are dominant and dating depends on the micro-faunas: the series appears to be entirely marine. In the southern North Sea the Neoeomian stages are widespread, and the sequence may be locally complete. In Danish waters deposition of deep water shale was continuous from the Jurassic onwards in the basins, but only the Albian-Aptian transgressed over the Ringkobing-Fyn high. The base is commonly transgressive across Jurassic onto Trias, with basal sandstone or shallow-water red beds locally developed. In the Dutch Graben a northward overlap from Valanginian to Hauterivian is recognised (Heybroek 1975) and there is a regional break after the Barremian. Aptian shales follow. The Albian is in the condensed Red Chalk facies in the southeast, in the "English Basin". In the northern North Sea there is less information about dating but it appears that the strongly transgressive lower Cretaceous is largely Aptian-Albian. This postdates the major faulting, as on the Brent, Piper and Auk structures, and in consequence lower Cretaceous may rest on rocks from Kimmeridge down to Trias and even Devonian. Thicknesses however increase considerably in the structural troughs, and the earlier phases of deposition (at least) were controlled either by the final fault movements (as also at the Hewitt gasfield in the south) or by sagging along the same fines (Howitt 1974 fig. 2; Hancock & Scholle 1975, fig. 2). On some larger structures tilting and faulting continued into the upper Cretaceous, but in the centre of the basin (as at Frigg) the lower Cretaceous and later rocks are only affected by mild thickness changes over the faulted Jurassic structures. Volcanism is stated to have occurred in the Maim[lower Cretaceous at various localities---off Stavanger, and "where the Fair Isle-Elbe trend crosses the Viking Graben" (W. H. Ziegler, 1974 verbal presentation). In the Netherlands a volcanic neck at Zuidval is reported to be end-Jurassic and transgressed by a Valanginian sand (Cottencon et al. 1975 in press). These effusions do not appear to have had any widespread effect.

Upper Cretaceous. Over most of the North Sea area the upper Cretaceous is developed as Chalk, much of it, as Hancock & Scholle have observed (1975 in press) diagenetically hardened to a limestone, as in East Yorkshire. Northwards in the Viking Graben, it passes by interdigitation into marl and then shale, and in the distal part into silt which may be lOOO m or more thick (Howitt x974,

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Hancock & SchoUe I975). According to Byrd (I975) the North Sea Chalk was deposited in moderately deep quiet water, probably at some hundreds of metres depth. The development of the shale as a broad tongue coincident with the Viking Graben flanked by Chalk on the shelves to east and west, as W. H. Ziegler indicated at the November conference, strongly indicates that the shale is a deeper water facies than the Chalk, being located in the area of maximum subsidence, perhaps analogous to the modern "Tongue of the Ocean" in the Bahamas. Hancock & Scholle (i975) have recorded that Cenomanian and Turonian are commonly absent (although the Coniacian is thick) and have discussed the possibility that the basin floor may have been below carbonate compensation depth, but conclude that this is unlikely since the same feature is found in the Danish shelf area. Late movement of the subsidiary faults seems a possible explanation within the Graben. The Viking Graben, with the Central Graben further south, had a further influence on Chalk deposition, in that the rifts largely limited the continuing deposition of Chalk through the Danian into lower Tertiary in the northern and southern parts of the North Sea basin respectively (Dunn et al. I973 fig. 8). In the Ekofisk area contemporary subsidence of the central graben system is indicated by slump deposits of Danian age containing Maestrichtian and Danian blocks (Hancock & Scholle i975). The overall thickness of the Chalk itself likewise shows a close relation to the pattern of the early Mesozoic troughs, reaching 3ooo ft (9oo m) in the central graben system and on the flanks of inversions in the south and thinning to less than a third of this thickness elsewhere, as on the Kingkobing-Fyn high. Much more local thinning is found over halokinetic structures in southerly areas. On some of the more active blocks, such as Piper, located on the intersection of the Moray Firth trends with the Viking Graben, local differential block movement continued well into the upper Cretaceous, and faulting took place into the Campanian (Williams et al. I975). This appears to be an exceptional case. Cretaceous basin development in the North Sea was thus transitional between that of the sharply fault-controlled Jurassic and Permo-Trias, and the broad gentle synclinal subsidence of the Tertiary. The taphrogenic control of the earlier Mesozoic lasted into the lower Cretaceous, but from the Cenomanian onwards the thickness changes were tess abrupt, related to differential subsidence without major faulting. The Cretaceous ended with a median deeper water facies in the centre of a relatively simple basin, as in the overlying Tertiary with which it is there continuous. In the south an important feature of the later Cretaceous and Palaeocene was the development of inversion movements over some of the deeper troughs. This is a feature widely recognised in southern Germany (P. A. Ziegler I974, fig. 15) ; it is regarded as responsible for the uplift of the Weald and mid-Channel block, with the Cleveland uplift in Yorkshire as a probable analogue (Kent I973 p. t6). (The monoclinal fold system of Dorset and the Isle of Wight also shows the relationships characteristic of inversion, with a Tertiary basin lying

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outside (north of) the full Jurassic development, but since it involves Eocene and probably Oligocene it must have been mid-Tertiary in its final development.) In the North Sea basin the history of movement is well documented in the Dutch Central graben, described above, where the centre of the trough suffered uplift so that late upper Cretaceous rests locally on horizons as low as middle Jurassic, representing a vertical movement of I5oo metres in the central parts, and the Sole Pit trough is a parallel case with movement mainly in the Turonlan and Senonian, renewed in the Maestrichtian (Hancock & Scholle I975). Uplift approaching 2ooo m. has been indicated from Bunter palaeotemperature studies by Marie (i 975) in the Sole Pit area. Comparable features have not been recorded hitherto in the northern North Sea; inversion seems to be a characteristic of more southerly areas only. If the mechanism was isostatic recovery it would have been expected in the north also, and P. A. Ziegler (I974) has ascribed these inversions to late Cretaceous/early Tertiary Alpine orogenic pressure, the troughs "acting like shear pins in the otherwise rigid platforms. Once inverted these basins become largely inactive." o. Tertiary The dominant characteristic of the Tertiary is the relative simplicity ofthe tectonic controls of sedimentation, so that the whole series was deposited in a single unfaulted subsiding basin, aligned north-south in the northern North Sea, swinging somewhat to the southeast (in line with the underlying fault trends) in the south and into the Netherlands. In the central North Sea the Tertiary begins with Danian Chalk, distinguishable from the Cretaceous by its nannofossils, which provides the main reservoir for the Ekofisk group of oil-fields in SE. Norwegian waters and for the Dan field of offshore Denmark (Dunn et al. I973). In general this is a dense chalk, but fair porosity is developed in the oilfields of the Ekofisk area, which are partly structural, partly stratigraphical traps with the constituent coccoliths forming an open grid without interstitial filling (Dunn et al. t973, fig. I2, Childs & Reed I975). This porosity development appears to be irregular but local, partly coincident with uplift over deep seated salt plugs, which also controlled minor erosion of the underlying Cretaceous and localised fracturing. Locally there is a development of "melange," a breccio-conglomerate of Danian to Palaeocene age which originated as slumps from submarine fault scarps and which slid as much as 20 km down the basin slopes (Hancock & Scholle I975). The Danian measures I3o ft (40 m) at the Dan field, I7o ft (52 m) at Montrose and 45 °ft (I4o m) at Ekofisk. In post-Danian times elastics were the dominant sediments, and it was the prospect of sand development in the long Tertiary column indicated by seismic survey that led to initial interest in the northern North Sea basin. In general terms Tertiary deposition was of marine elastics, predominantly fine grained but with a major sand element in the Palaeocene and Pliocene, particularly in the western side of the basin. The sands are partly of metamorphic elastics but include also rounded grains suggesting that the Permo-Trias was subject to erosion on the basin margins. Discussions of the depositional conditions

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of the Palaeocene sands have been published by several people (Fowler I975, Childs & Reed 1975) but remain controversial: they are variously regarded as offshore bars, lobate deltas or turbidites. This problem should be resolved when it is possible to integrate the data on a regional basis. Intercalated shales may contain few or no marine fossils and are suggestive of a brackish lagoonal environment. The Eocene and later Tertiary rocks commonly form a monotonous shale sequence, with minor streaks of limestone and dolomite in Eocene and Oligocene. Distinctive lithological markers of red or varicoloured shale assist correlation of this part of the sequence. Deposition appears to have been continuous from Eocene through to Pliocene in the central part of the basin, with progressive shallowing of the basin and development of lenticular sands in the Neogene. Pyroclastics are present in minor quantity in the Tertiary sequence. The most widespread and distinctive is the Palaeocene Ash Marker, which is a ioo ft thick alternation of tufts, fine sandstones and shales immediately beneath the Eocene. This unit covers much of the North Sea, as well as outcropping in Denmark (Fig. I o). It may be related to a volcanic centre in the Skaggerak, or alternatively to longer distance transport from the Hebridean province. Less notable tufts are recognised in the Eocene (as in the Forties area). The total thickness of the Tertiary reaches 3500 m in the deepest part of the Northern Basin near Ek0fisk (Heybroek et al. 1967). In the neighbourhood of the Auk oilfield the total is around 2200-2700 m (750o-90o0 ft). In the Argyll field the Tertiary subdivisions (beneath 175o ft (500 m) of Pleistocene) are given as measuring 2ooo ft (61o m) for the Pliocene, 15oo ft (45 ° m) for the Miocene, 13oo ft (40o m) for the Oligocene, 14oo ft (43 ° m) for the (lower) Eocene and 400 ft (12o m) for the Palaeocene. Montrose is comparable but with Palaeocene about 1200 ft (36o m). In the Forties area thicknesses are: Pleistocene and Recent 4oom, Pliocene 56om, Miocene 280 m, Oligocene 38om, Eocene 320 m, Palaeocene 57 °m. According to W. H. Ziegler (i 975) the Mesozoic shelves are overlain by shallow water Tertiary, whereas the central thick development is in a deep water facies. From the geometry of the Tertiary basin one would not expect any sharp delimita- tion between these facies, but it is clear that the central part was subsiding faster throughout most of the Tertiary and subsidence was at times more rapid than the sedimentary fill. In the Pliocene the basin was finally filled to the point where a shallow water facies developed across the basin. There is no evidence of differential faulted subsidence of the central North Sea rift system during the Tertiary, and in this respect it differs from, and may be entirely unrelated to, the Eocene-Oligocene Rhone-Rhine graben system (P. A. Ziegler 1974). The contemporary analogues of that rift movement are to be found in the Oligocene fault basins of western Britain, not in the North Sea.

I I. Mesozoic and Tertiary volcanics The main volcanic episode in the North Sea was related to a middle Jurassic centre situated in and near the Forties oilfield, where the complex fractures of

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the Great Glen-Moray Firth fault system cut the north-south graben (Fig. 9)- This has been documented by Howitt et al. (1975). The central area of this important complex appears to have had several vents, giving rise to a series of basaltic lavas, agglomerates, tufts and volcanic sandstones aggregating c. IOOO m thick, and covering an area 13o × IOO km (about half the size of Wales). The proportion of lava varies from 73 % in central sections to a minor element in the peripheral areas. The thickness variation of elastics suggests distribution by a southerly wind. Fine to medium tufts are dominant among the pyro- and epi-clastics. Ash is absent. Lateritisation and boles show that the series was in part subaerially accumulated. The volcanic series is accurately dated since it overlies Trias and Zechstein, is interbedded locally with fossiliferous Bathonian sediments and is overlain by Oxfordian. The absence of ash in this Bathonian episode, with the indication of a southerly wind, suggests that the contemporary early Bathonian volcanoclastic Fullers Earth of southern England relates to a different centre, perhaps located in the Western Approaches. A later Jurassic volcanic episode is represented by tuff layers in the Kimmeridge shale of the Brent field; this may be related to a volcanic centre of about this date which is indicated in south-eastern Norwegian waters (probably in Block 25/IO), but no account of its tectonic setting has however yet been published. Local volcanism occurred at about this time at Zuidval in the northern Netherlands, where a volcanic neck cutting older Mesozoic and Kimmeridgian produced breccias of Purbeck date and is overlain (truncated) by a Valanginian sandstone containing igneous detritus. The Fullers Earth of the lower Greensand (Aptian) in southern England is somewhat later than this, and was probably derived from volcanoes in the south. Later Cretaceous volcanicity is indicated by bentonite/montmoriUonite bands in the earlier Chalk of eastern England and the southern North Sea; the locus of extrusion for these is not yet known. In the lower Tertiary volcanic activity was widespread, although it still formed a minor proportion of North Sea sediments (Jacqud & Thouvenin I975). The first episode, occurring in the early Palaeocene (Thanetian) is localised and may represent a minor recrudescence of the middle Jurassic centre, for it appears to be limited to the Forties-Piper area. The most important tuff development was somewhat later, pre-dating the upper Palaeocene, extending over much of the North Sea and outcropping in Denmark and Germany; this may be partly related to a centre in the Skaggerak and--in the case of the mid- and western North Sea area--to the Hebridean volcanic centres, for it is reasonably certain that wind directions were consistently westerly or northwesterly. Other less important tuff intercalations can be traced in the later Eocene and are also thought to be related to the western Scottish province. None of these volcanic occurrences bulked large in relation to the North Sea sedimentary fill, and none of them was sufficiently long lasting to suggest a continuing simple stress pattern disrupting the crust. The author is not aware of field evidence supporting the existence of a belt of Cretaceous/Tertiary intrusions extending across the southern North Sea from

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I2. Mechanism of North Sea subsidence

Overall the North Sea has been a tensional area characterised by negative

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Fro. xo. Distribution of p~t-C,~bonii'e~m volcanic rocks.

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movement since the Caledonian orogeny. In the Devonian the known subsidence was at least partly coincidental with the northern Mesozoic basin; in the Carbon- iferous subsidence coincided with the southern Permo-Triassic basin, which then formed part of the broad northern European miogeosynclinal trough. From early Permian onwards both sections subsided simultaneously, although remaining partially separated until the upper Cretaceous. Basin edges over this period were commonly monoclinal or faulted, and fault-control of subsidence with local contemporary erosion of horsts is known in the Permian, Trias, Jurassic and lower Cretaceous. In the late lower and upper Cretaceous, faulting in the central parts gave place to monoclinal sagging as a preliminary to the late Cretaceous and Tertiary unfaulted simple subsidence. Thicknesses of post-Carboniferous formations total 3-5000 m in the deeper parts of the southern North Sea basin, and (on seismic evidence) up to 8000 m in the central rift in the northern North Sea. If Carboniferous and post-Caledon- ian Devonian sediments are taken into account subsidence must total some I o km centrally, and was the resultant of movements which were regionally continuously downwards, except for marginal uplift contemporary with the Hercynian orogeny further south. Of the surrounding massifs only the Scottish Highlands and Norway appear to have undergone complementary steady uplift and erosion, but other blocks (East Anglia, Southern Uplands, Mid North Sea High, Ringkobing-Fyn High) remained static near sea level with oscillations measured in hundreds of metres only (Kent i975). If subsidence of the basin had been due to local effects it would require more effective matching of positive and negative displacements; the circumstance that the negative movements were much more extensive areaUy and vertically than the uplifts is most adequately explained by subsidence due to regional attenuation of the crust, presumably related to the opening of the Atlantic. (It remains unexplained why the southerly massifs have remained so long in effective equilib- rium.) The earlier phases--Devonian, Carboniferous and to some degree Permian--were dominantly related to slow plastic yield without fracture; during the Trias and (more particularly) Jurassic brittle fracture occurred, possibly related to a more rapid attenuation phase. Subsidence, still on a large scale, without fracturing was resumed in the late Cretaceous and Tertiary, presumably because during the true oceanic opening (with its magnetic stripes), the stress system has changed to a less sharply localised creep. Only the end Cretaceous-early Tertiary inversion structures of the south can be regarded as possible indications of a short period of orogenic stress. The North Sea basin thus shows, in a daily increasing amount of detail, a history of subsidence which is closely similar to that of other so-called "inactive" (Atlantic-type) coasts world wide. It shows the inception of broad basins in the Permian; taphrogenic control ofsubsidence through the early and middle Mesozoc and a sharp resumption of simple inter-cratonic basin subsidence in the upper Cretaceous which has lasted throughout the Tertiary to the present. The factor still needing an explanation is the near simultaneity of these events on a world-wide basis, and particularly the lower Cretaceous ending of the phase of major faulting,

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of the widespread erosion]fault blocks and the inception of simple unfractured subsidence at that time. x3. Appendix

HISTORY OF LOCAL POSITIVE STRUCTURES IN THE NORTHERN SEA For good reasons in hydrocarbon exploration, interest and activity ccntrcs on positive structures which provide traps---commonly traditional anticlines but (particularly in the North Sea) fault blocks also. Being structurally high they provide dccpcr stratigraphicalpenetration than would bc obtained in the basins, and in consequence of their importance these arc the areas for which most data arc available on stratigraphy and structure. Unconformities and breaks in deposition arc not only at a maximum on these structures but they arc dated and documented in some detail. This section deals brieflywith several of these structures, as providing local types for the interpretation of structural development. They arc described in order from south to north (Fig. I).

(a) Brent Field Brent is one of the largest of the North Sea oilfields, and is structurally typical of at least eleven eases of tilted, meridionaUy elongate blocks of Jurassic, deeply buried beneath gently arched unconformable Cretaceous (Fig. II). It is situated mainly in Block 2II[29, 88 miles (14o kin) east of Unst in the Shetlands, within the main Viking Graben.

W. S.L,. .E. i i

TERTIARY

i UPPE~

Lower Cretaceou~_~ ~"~

•Jurassic .~ 1'Lower Jurassic F xo. x x. Composite section of middle Jurassic fault-block oilficld in northern North Sea (Brcnt area).

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The deepest strata recorded are continental Trias (bedded shale and thin sandstones), which have been proved to an incomplete thickness of 6000 ft. The Triassic red beds are succeeded by a 580 ft sand group (the Statfjord Sand) possibly ranging from Rhaetic through early Lias; this is overlain by 200 ft of lower Jurassic shale and 75 ° ft of middle Jurassic sand with coals (Brent Sand). Both reservoirs are oil bearing. An irregular denuded surface of the middle Jurassic is directly overlain by upper Jurassic Shale (Kimmeridgian and Portlandian, with a local thin upper Oxfordian element) and both are truncated by lower Cretaceous (Albian-Aptian) (Bowen z975, Blair z975 fig. 2.). The sub-crops of the truncated Jurassic formations run north-south parallel to the boundary fault. The unconformable cover of upper Cretaceous is developed in the shale facies which is characteristic of the Viking Graben area, indicating that although faulting had ceased deposition took place in deeper water coinciding with the underlying Jurassic trough. Here the enormous thickness of Trias implies contemporaneous subsidence of the main Viking Graben system. The subsidiary Brent fracture system developed in the early upper Jurassic and early lower Cretaceous before movement ceased; it suffered no later displacement of importance. The amplitude of this fault-controlled local structure is quoted as 6oo0 ft. (b) Frigg Gasfizld The Frigg discovery is located well north in the northern basin (Norway Block 25/z 2o5 km (z3o miles) east of Lerwick in the Shetlands) and will produce from major lensing sands in the lower Eocene (Fig. z 2).

WEST EAST

S.L. S.L.

L A T E R T E R T I A R Y

7000"~R~Fd *'-" '"" " . ;".-. s""AN, D.'."'~ : • .~ L, EO~ PALAEOCENE " '""':~- • /

LOWER CRETACEOUS ..,,. 16700" ,,s" ~ U. JUR. u. JUR. U. JURa.._

Fxo. x~. Diagrammatic section through FHgggas field.

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This structure provides a specimen of relationships within the Viking Graben, as far as they are known down to the deepest penetration at z6,7oo ft--a depth only sufficient to reach the Jurassic. The Tertiary, upper and lower Cretaceous are thick and the sequence apparently continuous. The main structural break is located beneath the lower Cretaceous, coinciding with a period of block faulting and erosion with an amplitude of c. zooo m (Blair I975 fig. 3). (c) Piper Field The Piper oilfield (Block t5/z 7 on the outer Moray Firth (200 km (z3o miles) E of Wick) described by J. J. Williams et al. (x975) is a tilted fault- blocks tructure in which hydrocarbons are trapped in upper Jurassic sands, with an oil column in excess of Iioo ft (Fig. x3). This is the northernmost reported occurrence of Carboniferous, dated as "middle Carboniferous." It is overlain by 4oo ft of upper Permian evaporites, mainly thin bedded dolomite, and this by Triassic red shale which thins over a NE--SW axis across the block. The junction with the Jurassic is unconformable. The transgressive Jurassic is non-marine, dated from ostracods as Callovian; it includes a volcanic intercalation. This also was partly eroded before deposition of the Oxfordian Piper Sand, which partly rests on middle Jurassic volcanics, and was sealed by Kimmeridge and overlapping lower Cretaceous shale. This sequence was block faulted, tilted and eroded partly during, but mainly post, Aptian-Albian, and was transgressed by

WEST EAST S.L. I $.L. I ( I I T E R T I A R Y

CRETACEOUS..-I ~ -8000 " ~60"00 t~ ...,~..~,-__ ~'60; ".•..'~.~"-"-~...-0000 " co. -~, ~ -~ i : L L~ ~~~.""'7;;" ; ':'/.:-~' ~ ...... ~ ~- k~~~:~".--.. •. ~Kim.

T.o.97oo ~ ,I "r.o]793" ,0,1z0,=TAtSALE r [VE,TICAt x 5] ~ ~IASTRIASSIC • 0 1 2MILES "%.

Fzo. z3. Cross section of Piper oilfield.

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o U J Z <

..,J >

< 0 t °N 6 ' 0

I i r

;ee~i s,O00 ~i. ,d. ~ ,,i , ," , , , T , ? i , I "F I

$eJ:Je L~ S,O00

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upper Cretaceous. Faulting continued on a lesser scale, particularly in the Campanian, but after this sedimentation was affected only by supra-tenuous thinning over the block. Piper thus demonstrates a long history of block faulting, beginning at least as early as lower Jurassic, reaching a maximum in the Albian-Ceno- manian and dying away in the early upper Cretaceous. (d) Forties Oilfield The Forties oil reservoir is a gentle east-west anticline developed at the lower Tertiary level and dying out gently upwards. This structurc has a length of z 5 km, breadth 8 km, and amplitude of 2oo m. The Tertiary rocks conformably overlic Chalk, possibly with a minor representative of lower Cretaceous, the base of which is unconformably transgressive across strongly faulted Jurassic rocks (Thomas et al. 1974). As with the fault-block structures of more northerly areas, strong faulting which may be largely intra-Jurassic ended in the lower Cretaceous, and later displacement was by arching without fracture (Fig. I4). The structure differs from most others in the region in that the Jurassic is mainly volcanic, and the Forties area lies on the southern side of a Bathonian volcanic complex, located on the intersection of the north-south central graben fault system with the NE-SW to E-W faulting of the Moray Firth (Howitt et al. 1975). (e) Auk Oilfield The Auk oilfield (Block 3o/I6, 3oo km (I9o miles) east of Dundee) is situated in the outer part of the Forth Approaches Basin, with a Rotliegendes reservoir on a structural horst immediately west of the Central North Sea Graben (Fig. 15). It provides a sample of history of movements in the southerly part of the northern North Sea (Brennand & Van Veen I975). The sequence on this high block is markedly defective, with Tertiary overlapping thin upper Cretaceous locally on to Rotliegendes sand and shale. There are unconformities also beneath the upper Cretaceous, lower Cretac- eous and Trias, presumably also (not documented) beneath the Permian. Uplift of this part of the main Grabcn edge has thus gone on throughout the

W. E.

Te r t i a ry

U. Cret~ L. Cret...... :. _ ... Trias ~~'J~'~ / ~'~~-'~.C ret. Zechstein-~'-' " . ~,/ ~/~.,~ ..... Rot. ~ ~/J" \- \ L Cret. ? O.R.S. Jurassic

Fxo. 15. Cross section of Auk oilficld.

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Mesozoic. Thick Jurassic on the downthrown block to the east indicates that the early Cretaceous movement was of major importance as elsewhere, but information on relative displacements and the sequence in the adjoining major trough is not yet available. (~ Argyll Field The Argyll oilfield (Block 3o/24, 430 km (235 miles) east of Edinburgh), described by Pennington (I975) , is highly informative in relation to North Sea basin development. It is located on the western lip of the main graben, and as a result of the uplift brings relatively old beds within reach of the drill. (Fig. x6). The oil bearing structure is a gently warped NE--SW subsidiary horst, cut off in the north-east by the main graben. Drilling proved that the upper Cretaceous transgresses onto Trias, that this overlies Zechstein, in turn truncating lower Permian Rotliegendes, which rests on Old Red Sandstone with a 9oo ft marine middle Devonian dolomite member (dated by its ostracod fauna). The last mentioned is a remarkable discovery, some 5o0 miles from the nearest marine Devonian, perhaps (as discussed above) implying a marine tongue occupying an early North Sea depression.

SOUTHWEST NORTH EAST MID-NORTH SEA GRABEN SL. ZL.

T E R T I A R Y (8,500 FT. )

U. CR~ L • CRET~~-~.'~'/'/~f L I U ~CRET ju RAsSICsslc 1~<~ -_~..> -~Ijl-2~_/.-~// ~ I " r" / \ •

Fxo. I6. Cross section of Argyll oilfield.

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It is evident from the stratigraphical relationships that this part of the graben edge was activated during the Permian, with an uplift of several thousand feet as measured on the unconformity. Uplift was renewed post- Jurassic/pre-lower Cretaceous, and again post-lower/pre-upper Cretaceous (attested by unconformities) with a final early Tertiary movement which was small in comparison to the earlier displacements.

(g) Ekofisk Area Ekofisk and the associated oil and gas fields lie in Norwegian waters (Nor- wegian Block 214, 385 km (24° miles) east of Dundee). This contrasts with the Auk and Argyll fields in being in the central trough, and within the area of the northern Permian evaporite basin, in which halokinetic movement of evaporites has controlled structural style (Byrd i975). According to Condreay, quoted by Blair (x975 fig. 4) early Permian deposition in the area was related to fault block movements, with the Rofliegendes thickening eastwards into trapdoor subsidences and absent on the highs (Fig. I7). The tilted blocks were buried by Zechstein evaporites and by earlier Mesozoic without significant deformation. (This interpretation is presumably based on seismic interpretation, the deep relationships are not in fact known from drilling at Ekofisk). Haloldnetie movement began in the lower Cretaceous, with differential flowage related to the buried fault blocks, and continued through the upper Cretaceous and into the Tertiary. Porous Danian Chalk was deposited and a fracture system developed over the growing highs. The concept is thus that this part of the North Sea basin was developed by the early Permian; that (as is independently indicated by the distribution of Permian evaporites) most of the central rift system has a pre-Triassie

W. $.l. E. ~N"" . • :y.".

UPPER CRETACEOUS

~ .~'~',.",~ ~ ~ ~'~../'~..~TRIAS SIC -~.-----'~. ~,~...~. LOWER CRETACEOUS ~ -I I~ .~,,,~ + ---%-,,,, ~._.- j ~ ~-~ ~<..>,,-- + -- :. ---" .. + ..

4- "° ° 4" ÷

Fxo. x7. Interpretative section of Ekofisk area.

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0 I0 20 Kms. I I ,I Fzo. z 8A. Seismic section across Northern North Sea from East Shetland to Viking Graben, showing tram- gression of upper Cretaceous and Tertiary on the block. The progradation in the Tertiary is steepened by the exaggerated vertical scale (about ,o times). S N

:'~':.--I -~'~"'--"~~~~"~.L!..~|. !.~! 3_~L~"~-~-~,~,=,~"~~'~,.",L'~-,~ "~-'~-~ -

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• .7...... ~,.::%'"~'~,,~'~.'."::.~.~ ~ :.~J;'.' '.E~~ ~=',~.,.~.~ ~ ~,~'~..,7,;~,'.~'-I. '' ~ ,.~' =~--.-..~,-.~.~t"J,'.~ ~,~.~'_-:~ ,~i ~'i:;',~':; ' ~ ~'"':~.,~'~" :;~...v.~.v~,~ ...... '" "'":".~" ' .: , '- '";;:" ''" "~"~....-.- ...... "" .', "'":-.~,.~,,"~.- ...... C'" ~'~"" ~'~;~.- : L.'-~,~4~"." , . ,-," :;".".~ ~ ~ ~. ~.:~,.~'~,~pp~_-T.,!~~ ~;~.;~ ~-----'~'~,,~ "~' ~'-*::~..-~t~. ,~ ,-, ~r.~...~'~.~, "~ ~i~..~~"' ~:~ ".,."~'... ~~ -, :, '~.:.,,."~"["..., ~'~ .,~' '" .'~.~,, "'~ ~'~.~ • ""'. '"..... '" ~ ", '-- :'.r,• "~ ',,..~..,.,. • '-, ' : ,.a, ' ' . ' .~..~,.;.' " ,~, ,,....-"~' ~,,~, ,.' ' ~.:"~ .~,-.,~,,:.~,'-'1~"~ ' - :~.~;,;r,i~' ' ,. ~.,~" ,~ ~,," ~',".',<,., ,.-,: ~ ...... ' "" ' =, ' ~.~"t~ ~ ~;.~, " =.. ,'.",,.~"-:.~,~,~,,,, -,' " .,,: ,,~" ~:G', ~' ~'1.,~ ,~,'..~ I.," ',' .,,, *;~-.----." ~ -r, ~, " . :~:"...... ; '..';". ".• -..., " •~;-. "L ;.~','.-- ' ~,~ .. : % ,.-, ,' ..:~,.,': ',~:-~.'~"~ .... '" :~""~,L~',~.~,~,,,,', . - t~ [,~ ",'t .~,!~i~ "- .~.,i:,~'.~ ~,"L-'-':'~:.-'~",, "' ,' ,;~m~'.., ,~,a;.C.~".'u,,, r~ '" -~ ~.~:~:.,'~i~... :~ ~,;~.'."",'"..'~.~,~., " ~. - ~,::"' ' ""'~,,,~ ~ ~ "'*" ,--,. !", ,,':' "~" ~ ~'~,.l:~,'!.,,' ~+.,,~ .-'.','~;.,~.~., "~' .. ,. , ..,, ...... - ", , . .,,, .. .,. ., • ..,. , . , ~ , ~ ... h . .. ~ - ,. . ,. , .,,,, .,,,., ~ ~ _., . ,.;.:..':."'.=:,,,; :..,, ;.',,-", ,':::::,.,,,, ,' .~:,;;.',,.':,:::, ; :;:::'.:i:,..~,~:,:..r ' ,~!: ~';~::~L~'~,~;,~..~ ~,~'~,,~ .~:;:.! ~'.':':~,~'~:.-,:-;'"~:~,~G:,~ ~"~';'£.~'." ~:,,'.:m;;.'~.,C.: ~'.:- ;,~'.:~' ,;,; .":,.'.':: :~..TO,~':-~,:~.",;:':" ."~, ~,;',~..',, ;" '., :':::,"~,:;.':.:: ,,;;:':,:. : %'.].~'":';.~. ::,;: ~T.,'.~::,,' ":"~,::',,:'~.'~ ,," £':~', ~:~"~,I'.':'~',~' ~,~;~, ;..',r~; i,,i ,",~,!,..~[~'*j~l~,!:;e~,~;~'~,;,:~t..,',~!~.,::,,"T'.~G':.:,;;;;:: ;;;., ,~ .,]'~,.,'1,,,~.~",~ ' ~;: :~:::": ;;: :~:'"~ ' "' :~":]":" ',.::":~. :T, :':; '::,': ,',;;;; ;; :::: "::::,,:;; ." ":'~':::':"-.,,,;".,":;;--::1 : ,:.:: ,~,~,;~',;."; ,i~ :';',~i,~,",;;;i;,';~', ;[[;";: ~';'$~:.";;.;,~i,.,~'" :;~', ~,; ,,;' ;:,"- :' '~,"Y,~ '= ;;', ~;'.",~,~:, '" :::~ ::: :"'~'~ ...... ::""; .':.'t"-~ ''~''' ';~','~ .... : '.':'"' ":' '~":" ':;" ":' ; ;'" ":":;':':'"' '~,;:-.'."~;~'~' ~', ~.":,;,~';~,;:~;;;'i;~;":;;~ ;;i!' , ;,~ ,~: ~5', ..... ,~',~'~-~,.:.,-."'.[::::','!;:~,',~",~ ~ :"~:.:,":;,'! :~,,:: ~ .':': ~ ~;: ~ Z;':,:;~ ':,:'~ ~'~ :~7.".,~"~'.': .. ,, - ,.: ...... • .",;".. ':: ; ,,-,..'.,:~ ~.,' ,!, ~,'.,:.';; ,'::,::, ,:.',~,,".;~.,:,~ ,aL ~,;,;~;.'%.;','~,.~'t~'r~[[4.~,~, 'r. ;~I:,~: L,,~",~L~. ' [::.F.'::.':... ~ .~'-"~Y.~/' ';' '." ~,,';'**,:;.':~ :: ,u:~,~;,:~,,,',J,=,~, ~,~, ,,,,:::. ,. ,~i;';,:.;":" '"',, '~',',:"' ."J"""

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0 ! 2 Kms. I I I B. Seismic section in Moray Firth Basin illustrating pre-upper Cretaceous fault structures.

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history (although this is still conjectural for the Viking Graben), and that fault blocks within the central North Sea graben were actively moving during the lower Permian.

ACKNOWLEDGEMENTS. Again the writer has to express his thanks to P. J. Walmsley, Frank Howitt, J. B. Butler and D. A. L. Jenkins of BP for reading the draft paper and for monitoring the de- scriptions in the light of their current work on the North Sea, and to J. J. Pennington of Hamilton Bros. for valuable discussions of regional problems.

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Received 3 ° April x975; address delivered 25 April i975.

SIR PETER KENT, Natural Environment Research Council, Alhambra House, 27-33 Charing Cross Road, London WC2H oAX.

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