Permian and Triassic Rifting in Northwest Europe
Geological Society Special Publications Series Editor A. J. FLEET
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Permian and Triassic Rifting in Northwest Europe
Geological Society Special Publications Series Editor A. J. FLEET
GEOLOGICAL
SOCIETY
SPECIAL PUBLICATION
N O . 91
Permian and Triassic Rifting in Northwest Europe EDITED BY
S. A. R. B O L D Y Amerada Hess Ltd, London, UK
1995 Published by The Geological Society London
THE GEOLOGICAL SOCIETY The Society was founded in 1807 as The Geological Society of London and is the oldest geological society in the world. It received its Royal Charter in 1825 for the purpose of 'investigating the mineral structure of the Earth'. The Society is Britain's national society for geology with a membership of around 7500. It has countrywide coverage and approximately 1000 members reside overseas. The Society is responsible for all aspects of the geological sciences including professional matters. The Society has its own publishing house, which produces the Society's international journals, books and maps, and which acts as the European distributor for publications of the American Association of Petroleum Geologists, SEPM and the Geological Society of America. Fellowship is open to those holding a recognized honours degree in geology or cognate subject and who have at least two years' relevant postgraduate experience, or who have not less than six years' relevant experience in geology or a cognate subject. A Fellow who has not less than five years' relevant postgraduate experience in the practice of geology may apply for validation and, subject to approval, may be able to use the designatory letters C. Geol. (Chartered Geologist). Further information about the Society is available from the Membership Manager, the Geological Society, Burlington House, Piccadilly, London W1V 0JU, UK. The Society is a Registered Charity, No. 210161. Published by The Geological Society from: The Geological Society Publishing House Unit 7 Brassmill Enterprise Centre Brassmill Lane Bath BA1 3JN UK (Orders: Tel 01225 445046; Fax 01225 442836)
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Contents GLENNIE, K. W. Permian and Triassic rifting in northwest Europe
1
COWARD, M. P. Structural and tectonic setting of the Permo-Triassic basins of northwest Europe
7
CARTER, m., YELLAND, A., BRISTOW, C. & HURFORD, A. J. Thermal histories of Permian and Triassic basins in Britain derived from fission track analysis
41
SWIECICKI, T., WlLCOCKSON, P., CANHAM, A., WHELAN, G. & HOMANN, H. Dating, correlation and stratigraphy of the Triassic sediments in the West Shetlands area
57
HITCHEN, K., STOKER, M. S., EVANS, D. & BEDDOE-STEPHENS, B. Permo-Triassic sedimentary and volcanic rocks in basins to the north and west of Scotland
87
ANDERSON, T. B., PARNELL, J. & RUFFELL, A. H. Influence of basement on the geometry of Permo-Triassic basins in the northwest British Isles
103
GOLDSMITH, P. J., RICH, B. & STANDRING, J. Triassic correlation and stratigraphy in the South Central Graben, U K North Sea
123
GRIFFITHS, P. A., ALLEN, M. R., CRAIG, J., FITCHES, W. R. & WHITTINGTON, R. J. Distinction between fault and salt control of Mesozoic sedimentation on the southern margin of the Mid-North Sea High
145
CHADWICK, R. A. & EVANS, D. J. The timing and direction of Permo-Triassic extension in southern Britain
161
RUFFELL, A., COWARD, M. P & HARVEY, M. Geometry and tectonic evolution of megasequences in the Plymouth Bay Basin, English Channel
193
SHANNON, P. M. Permo-Triassic development of the Celtic Sea region, offshore Ireland
215
KEELEY, M. L. New evidence of Permo-Triassic rifting, onshore southern Ireland, and its implications for Variscan structural inheritance
239
Index
255
Contents GLENNIE, K. W. Permian and Triassic rifting in northwest Europe
1
COWARD, M. P. Structural and tectonic setting of the Permo-Triassic basins of northwest Europe
7
CARTER, m., YELLAND, A., BRISTOW, C. & HURFORD, A. J. Thermal histories of Permian and Triassic basins in Britain derived from fission track analysis
41
SWIECICKI, T., WlLCOCKSON, P., CANHAM, A., WHELAN, G. & HOMANN, H. Dating, correlation and stratigraphy of the Triassic sediments in the West Shetlands area
57
HITCHEN, K., STOKER, M. S., EVANS, D. & BEDDOE-STEPHENS, B. Permo-Triassic sedimentary and volcanic rocks in basins to the north and west of Scotland
87
ANDERSON, T. B., PARNELL, J. & RUFFELL, A. H. Influence of basement on the geometry of Permo-Triassic basins in the northwest British Isles
103
GOLDSMITH, P. J., RICH, B. & STANDRING, J. Triassic correlation and stratigraphy in the South Central Graben, U K North Sea
123
GRIFFITHS, P. A., ALLEN, M. R., CRAIG, J., FITCHES, W. R. & WHITTINGTON, R. J. Distinction between fault and salt control of Mesozoic sedimentation on the southern margin of the Mid-North Sea High
145
CHADWICK, R. A. & EVANS, D. J. The timing and direction of Permo-Triassic extension in southern Britain
161
RUFFELL, A., COWARD, M. P & HARVEY, M. Geometry and tectonic evolution of megasequences in the Plymouth Bay Basin, English Channel
193
SHANNON, P. M. Permo-Triassic development of the Celtic Sea region, offshore Ireland
215
KEELEY, M. L. New evidence of Permo-Triassic rifting, onshore southern Ireland, and its implications for Variscan structural inheritance
239
Index
255
From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe,
Geological Society Special Publication No. 91,' 1-5
Permian and Triassic rifting in northwest Europe K. W . G L E N N I E
University of Aberdeen, UK The Permo-Triassic roughly coincided with the life span of Pangaea. So far as Europe is concerned, Pangaea's creation began in the early Carboniferous (Visean) when the northward-drifting megacontinent Gondwana began to collide with the Iberian portion of the slower moving Laurussia, while Proto-Tethys was subducted beneath the southern margin of central Europe. Creation was complete by the end of the Carboniferous or early in the Permian with the final development of the Variscan orogenic belt, which trends from Brittany eastward through central Europe, and with the addition of western Siberia along the line of the Ural orogen (Ziegler 1989). Despite, or perhaps because of, its bulk, Pangaea was not a stable megacontinent. No sooner had it formed than it tried to break apart again. The disintegration of Pangaea had already started before the end of the Triassic with the westerly extension of Tethys between Iberia and Africa, though not yet underlain by oceanic crust, and, by early in the Jurassic, rifting was taking place between Africa and the Americas in the newly forming Central Atlantic Ocean (Ziegler 1988). Indeed, E-W extensional movements within a Proto-Atlantic Ocean possibly began as early as the Late Carboniferous (Haszeldine & Russell 1987), whilst mid-Permian extension is well documented in East Greenland (Surlyk et al. 1984). These extensional movements may have propagated southward to initiate fracturing in the Viking and Central Grabens of the North Sea, along which the Late Permian Zechstein Sea was to break into the subsiding Rotliegend basins, and towards the Central Atlantic, where a shallow seaway eventually developed early in the Jurassic. Thus in the northern half of Pangaea, the continued existence of the former Laurussia was already at risk in the Permian, although crustal separation in the North Atlantic, which possibly started in the Rockall Trough in the Early Cretaceous, was finally achieved along the line of the Reykjanes Ridge only in the Paleocene. These major events on the periphery of what is now Europe, separately and jointly, were factors that probably controlled a whole sequence of tectonic events within the continent, which, in turn, controlled its patterns of mostly terrestrial sedimentation. Climatically, the northward drift of Laurussia had carried NW Europe from a region of equatorial rain forest during the later Carboniferous to the latitudes of a trade wind desert, like the modern Sahara, in the Permian. The Late Permian basins of Upper Rotliegend and Zechstein deposition were arid. Rotliegend sediments are characterized by dune sand and the saline mudstones of a semipermanent desert lake, and the Zechstein by shallow-water carbonates and anhydrite and deeper-water halite. During the Triassic, however, brackish-water fluvial and lacustrine sediments occupied the basinal areas, although even here, halite horizons (e.g. Rrt, Muschelkalk and Keuper in the North Sea area) associated with local transgressions from Tethys, testify that arid conditions were never far away.
2
K.W. GLENNIE
The Variscan Orogeny seems to have been doomed to failure; it was to become a range of highlands but not a major mountain range. No sooner had it formed than it began to collapse, with the coeval development of a very widespread N W - S E and conjugate N E - S W system of fractures through it and across its northern foreland. This may have been the outcome of a right-lateral reorientation of the relative movement between the former Laurussia and Gondwana (Ziegler 1990). Some of these fractures were obviously extensional as many were associated with igneous activity concentrated around 290-295 Ma BP; this comprised dyke swarms and sills as well as tufts and basaltic lavas of the Lower Rotliegend volcanics (Dixon et al. 1981; Sorensen and Martinsen 1987). Thermal subsidence of the Permian basins of the North Sea area seems to have begun about 20 Ma after the end of the main volcanic activity and was most marked over North Germany, which was the site of the strongest Lower Rotliegend volcanism and the development of a system of associated horsts and grabens (e.g. Gast 1988). The timing and amount of rifting associated with the North Sea graben system is still a matter of some dispute. Some workers (e.g. Ziegler 1990, and others), believe that rifting of the North Sea grabens was not initiated until the Triassic. They base much of their interpretation on seismic data (e.g. failure to recognize Zechstein halite in the middle portion of the Central Graben, even though there is good evidence elsewhere of the local removal of halite by solution; Johnson et al. 1986). In the deeper parts of structurally and stratigraphically complex areas such as the Central Graben, however, it is very difficult to recognize on seismic lines all lithologies of various ages, let alone decide on that basis just when rifting began, especially if initially it had gone through both transtensional and transpressional phases of movement. Other workers, including the author (e.g. Glennie 1990a,b), consider that rifting probably began during the Early Permian. Such rifting was possibly coeval with rotation of the north-trending series of en echelon half grabens (Worcester, Cheshire Basin, etc.) as well as intra-Variscan basins such as the Western Approaches and Celtic Sea Basins. This interpretation would seem to be supported in the North Sea area by the occurrence of Lower Rotliegend volcanism in the Central, Horn and Oslo Grabens, and by the preservation of Zechstein halite within the South Viking Graben together with Rotliegend dune sands as far north as the Beryl Embayment. The Zechstein Sea is believed to have flooded the Rotliegend basins with water of boreal origin via this route (Glennie and Buller 1983) rather than through the Bakevellia Sea and around the southern edge of the Pennine uplift, for which there is no supporting evidence. Debate is still generated concerning the style and amount of North Sea extension (e.g. Gibbs 1987; Latin et al. 1990). There is general agreement that the most active phase of crustal extension took place during the Late Jurassic to Early Cretaceous time span, but there is no concensus on the relative contributions of the Triassic and earlier Jurassic Periods. Despite associated volcanic activity, a possible Permian component is usually ignored, whereas apart from the mid-Jurassic volcanics at the Moray Firth-Viking-Central Graben trilete junction, Late Jurassic extension across the Central Graben was not associated with volcanic activity. B-factors vary from worker to worker depending on the style of extension assumed and the time spans during which crustal stretching is considered to have been operative (e.g. Sclater and Celerier 1988, and associated articles).
PERMIAN AND TRIASSIC RIFTING IN NW EUROPE
3
The cross-sectional geometry of Triassic sequences within the East Shetland Basin clearly indicates that in that area extension was related to fault-block rotation, which was accentuated in the Late Jurassic. A controlling factor must have been the proximity of the North Viking Graben, which limits the eastern margin of the East Shetland Basin, but little is known about the timing or amount of extension within that graben, where Permian strata may be as much as 10 km below present sea level (Ziegler 1990). Apart from the marine Zechstein and Muschelkalk sequences, the PermoTriassic of NW Europe consists largely of arid or semiarid terrestrial sediments that are very poorly dated. Fossils generally are either absent or non-diagnostic for age. Because of a lack of faunal or floral control, the apparent age of some sedimentary sequences has been changed in recent years from Triassic to Permian on the basis of regional correlations. For instance, following interpretations in vogue during the 1930s (Sherlock 1948), no sediments of Permian age are shown in the West Midlands of England on the 1948 edition of the Geological Survey Map of Great Britain. North Sea exploration has now made it likely that at least part of this sequence is Permian in age (e.g. the Bridgnorth Sandstone: Smith et al. 1974; Karpeta 1990; Warrington et al. 1980); the 1979 edition of the same map has advanced only by designating much of the sequence as undifferentiated Permian and Triassic. Other than the radiometric ages of igneous rocks, which are still few and far between, there is an almost complete lack of dating in many of the smaller red-bed basins of presumed Permo-Triassic age in NW Europe. Germany seems to be better off in this respect, and is able to use Russian faunal stages for the Permian, controlled to a limited extent by magnetostratigraphy (Gebhardt et al. 1991). Further west, rare palynofloras are beginning to provide a little control, but in their absence, as is the case northwest of the Scottish mainland, even seismic correlation from one isolated half graben to the next is, at best, conjectural, and New Red Sandstone cannot be separated from its Devonian Old Red counterpart with any confidence. The Permo-Triassic had a time span of some 90 Ma. On the basis of radiometric dating of Westphalian lavas in Germany, it now seems likely that the PermoCarboniferous transition occurred about 300 Ma ago (Lippolt et al. 1984; Leeder 1988). Following Lower Rotliegend volcanism, much of the greater North Sea area seems to have been the site of erosion or non-deposition for up to 20 Ma or more (Saalian Unconformity: see Table 1 in Brown 1991) before Upper Rotliegend deposition began. Some areas of Late Carboniferous inversion (e.g. axis of Sole Pit Basin) were subjected to erosion down to Namurian horizons. Post-Saalian subsidence was greatest over the area of former volcanic activity in northern Germany and Poland. During a very short time span, estimated to be no more than 20 Ma, straddling the Permo-Triassic transition (early Tatarian to Scythian), rates of subsidence reached 220 m per million years (Menning 1991). Some of this subsidence may be related to the rapid crustal loading of Rotliegend basins whose surfaces were already below global sea level, first by the influx of the Zechstein Sea (250 to 300 m of water: Glennie 1990a), and then by the dense evaporites that were deposited in thicknesses of up to 3000m or more (Taylor 1990). Menning (1991), indeed, estimates the duration of Zechstein deposition at around 5 Ma, which implies that deposition proceeded locally at the rate of over 600m per million years. The
4
K.W. GLENNIE
succeeding terrestrial sedimentation slowed duration the early Triassic (Bunter), and in the areas of former maximum subsidence may have adjusted to isostatic equilibrium at about the time of the Hardegsen Unconformity. This brief preface not only indicates some of my own interests in the Permo-Triassic of N W Europe, but hopefully also highlights some of the important facts and difficulties in interpreting the structure and sedimentation patterns associated with Permo-Triassic rifting in NW Europe. It provides only armchair explanations of some important events and processes, and ignores others: more will be discussed in the succeeding pages. And perhaps a few comments will stimulate one or two readers to put their knowledge and ideas on paper. The relatively extensive reference list for such a short contribution may include some useful papers that are not mentioned elsewhere.
References BROWN, S. 1991. Stratigraphy of the oil and gas reservoirs: UK Continental Shelf. In: ABBOTTS, I. L. (ed.) United Kingdom Oil and Gas Fields, 25 years Commemorative Volume. Geological Society Memoir, 14, 9-18. DIXON, J. E., FITTON, J. G. & FROST, R. T. C. 1981. The tectonic significance of postCarboniferous igneous activity in the North Sea Basin. In: ILLING, L. V. & HOBSON. G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 121-137. GAST, R. E. 1988. Rifting im Rotliegenden Niedersachsens. Geowissenschaften, 6, 115-122. GEBHARDT, U., SCHNEIDER, J. & HOFFMANN, N. 1991. Modelle zur Stratigraphie und Beckenentwicklung im Rotliegenden der Norddeutschen Senke. Geologisches Jahrbuch A, 127, 405-427. GEOLOGICAL SURVEYOF GREAT BRITAIN, 1948. Ten-Mile Map, Sheet 2. 3rd edition, 1979. GIBBS, A. D. 1987. Deep seismic profiles in the northern North Sea. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North West Europe. Graham and Trotman, London, 1025-1028. GLENNIE, K. W. 1990a. Outline of North Sea history and structural framework. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. 3rd edition, Blackwell, Oxford, 34-77. --, 1990b. Lower Permian- Rotliegend. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. 3rd edition, Blackwell, Oxford, 120-152. & BULLER, m. T. 1983. The Permian Weissliegend of N.W. Europe: the partial deformation of aeolian dune sands caused by the Zechstein transgression. Sedimentary Geology, 35, 43-81. HASZELDINE, R. S. & RUSSELL, M. J. 1987. The late Carboniferous northern Atlantic Ocean: implications for hydrocarbon exploration from Britain to the Arctic. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 1163-1175. JOHNSON, H. D., MACKAY, T. A. & STEWART, D. J. 1986. The Fulmar Oil Field (Central North Sea): geological aspects of its discovery, appraisal and devleopment. Marine and Petroleum Geology, 3, 99-125. KARPETA, W. P. 1990. The morphology of Permian palaeodunes - a reinterpretation of the Bridgnorth Sandstone around Bridgnorth, England, in the light of modern dune studies. Sedimentary Geology, 69(1/2), 59-75. LATIN, D. M., DIXON, J. E,. FITTON, J. D. & WHITE, N. 1990. Mesozoic magmatic activity in the North Sea Basin: implications for stretching history. In: HARDMAN, R. F. P. & BROOKS, J. (eds) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geological Society Special Publication, 55, 207-227. LEEDER, M. R. 1988. Recent developments in Carboniferous geology: a critical review with implications for the British Isles and NW Europe. Proceedings Geologist's Association, 99(2), 74-100.
PERMIAN AND TRIASSIC RIFTING IN NW EUROPE
5
LIPPOLT, H. J., HESS, J. C. & BURGER, K. 1984. Isotopische Alter von pyroclastischen Sanidinen aus Kaolin-kohlensteine als Korrelationsmarken fiir das mitteleuropaische Oberkarbon. Fortschr. Geol. Rheinid. u. Westf., 32, 119-150. MENNING, M. 1991. Rapid subsidence in the Central European Basin during the initial development (Permian-Triassic boundary sequences, 258-240Ma). Zentrablatt ffir Geologie und Pala6ntologie, Stuttgart. 1, 809-824. SCLATER, J. G. & CELERIER, B. 1988. Errors in extension measurements from planar faults observed on seismic reflection lines. Basin Research, 1(4), 217-221. SHERLOCK, R. L. 1948. The Permo-Triassic Formations. Hutchinsons, London. SMITH, D. B., BRUNSTROM, R. G. W., MANNING, P. I. SIMPSON, S. & SHOTTON, F. W. 1974. P e r m i a n - a Correlation of Permian Rocks in the British Isles. Geological Society, London, Special Report, 5. SORENSEN, S. & MARTINSEN, B. B. 1987. A palaeogeographic reconstruction of the Rotliegendes deposits of the Northeastern Permian Basin. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North West Europe. Graham and Trotman, London, 497508. SURLYK, F., PIASEKI, S., ROLLE, F., STEMMERIK, L., THOMSEN, E. & WRANG, P. 1984. The Permian Basin of East Greenland. In: SPENCER, m. M. et al. (eds) Petroleum Geology of the North European Margin. Norwegian Petroleum Society, Graham and Trotman, London, 303-315. TAYLOR, J. C. M. 1990. Upper Permian - Zechstein. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. 3rd edition, Blackwell, Oxford, 153-190. WARRINGTON, G., AUDLEY-CHARLES, M. G., ELLIOTT, R. E. et al. 1980. Triassic- a Correlation of Triassic Rocks in the British Isles. Geological Society, London, Special Report, 13. ZIEGLER, P. A. 1988. Evolution of the Arctic-North Atlantic and Western Tethys. American Association Petroleum Geologists Memoir 43. 1989. Evolution of Laurussia. Kluwer, Dordrecht. -1990. Geological Atlas of Western and Central Europe. 2nd edition, Shell, The Hague.
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From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe, Geological Society Special Publication No. 91, 7-39
Structural and tectonic setting of the Permo-Triassic basins of northwest Europe M. P. C O W A R D
Geology Department, Imperial College, London SW7 2BP, UK Abstract: The Permo-Triassic basins of NW Europe formed by (i) late-orogenic
spreading of the Variscan mountain belt, eventually leading to ocean floor formation in the Mid-Atlantic, (ii) continental rifting propagating south from the Arctic and (iii) thermal subsidence following Carboniferous volcanic activity in the North Sea. Spreading values in the northernmost North Sea, west of Shetland and west and southwest of Britain, are relatively high but the sediments are dominantly continental; the Permo-Triassic rifting affected lithosphere thickened during Caledonian and Variscan orogenesis. However, subsidence associated with the Arctic rifting, together with global sea level changes, allowed marine incursions into the North Sea, producing the large Zechstein salt basins. Interference of the different basin formation mechanisms led to a complex three dimensional fault pattern and multidirectional extension across Britain and the North Sea. This fault pattern formed the basic framework for subsequent Jurassic and Cretaceous basin formation.
The Permo-Triassic marks a change in tectonic regime in N W Europe, from Palaeozoic plate accretion, producing the Caledonian and Vafiscan orogenic belts, to continental extension, generating rift basins in the Central Atlantic extending from the Caribbean to Gibraltar and the North Atlantic from north of Ireland to the Boreal Sea. The Permo-Triassic marked the time of rift initiation in many of Britain's offshore basins, including the North Sea. It was also the time of extensive sand accumulation and deposition, generating many of the reservoir rocks for Britain's oil and gas. This paper aims to give a short overview of Permo-Triassic tectonics and basin development in NW Europe, including the Permo-Triassic plate configuration and its influence on the palaeo-environment and the stratigraphic record. In particular, the paper aims to discuss the origin of the various basins, the mechanisms for rifting and subsidence and the variations in rift opening direction in time and space. Unfortunately there is a lack of regional structural and stratigraphic data dealing with this time period. This paper aims to redress this, and compiles and summarizes data and ideas developed in the keynote stratigraphic publications dealing with the Permo-Triassic of NW Europe. The paper also adds some new and possibly controversial conclusions about local and regional structures.
Pre-Permian tectonic framework Figure 1 shows a simplified map of the pre-Permian tectonic zones of N W Europe, based on Ziegler (1982) and Coward (1990).
8
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PERMO-TRIASSIC BASINS OF NW EUROPE
9
Precambrian crystalline basement The Precambrian basement of NW Europe consists of crystalline material derived from three tectonic domains, accreted together during Caledonian collision. The North Atlantic Block, which lies northwest of the Hebrides, comprises Archaean gneisses (~2900Ma) reworked extensively in the Middle Proterozoic (18001600Ma). Mid-crustal tectonic fabrics, which are mostly flat or dip gently to the SE, were generated by large-scale NW-directed thrust tectonics in the Middle Proterozoic followed by NW/SE extension. These fabrics are modified by large NW/SE trending shear zones which acted as large-scale tear faults-transfer zones during the phases of crustal thickening and subsequent extension. The crystalline basement is covered by a locally deep Late Proterozoic basin (the Torridonian) and thin Lower Palaeozoic sediments. The Scandinavian Block, or Baltica, is formed of crystalline crust generated by magmatic arc accretion during the Middle Proterozoic (Svecofennian terrain) followed by Late Proterozoic reworking and possibly plate accretion in southern Scandinavia (Sveconorwegian terrain). This latter phase of collision tectonics was associated with SE-directed overthrusting on N/S-trending mylonite belts. Lower Palaeozoic sediments occupy rift basins along the southern edge of Baltica, possibly related to continental break-up and rifting of Baltica away from some unknown continent. The Welsh-Brabant Massif forms the crystalline basement to Central England and the southern North Sea. It is a Late Precambrian magmatic arc and was essentially undeformed until the Caledonian tectonic events. However, in NW Wales there are Late Precambrian mylonites and blueschists and along the northern margin of the Brabant Massif, north of the English Channel, there is evidence of middle-lower crustal fabric development (Blundell et al. 1991), suggesting that the Massif may have formed from the accretion of several magmatic arcs. During the Early Palaeozoic the western edge of the Brabant Massif was subjected to rifting associated with arc-related magmatism and volcanicity, possibly developed above a SE-dipping subduction zone.
Caledonian plate coll&ion The Caledonian orogenic belt extends from northern Norway to the Gulf of Mexico and formed as a result of the closure of a system of oceans, generally grouped together under the name Iapetus. During the Palaeozoic, thin strips of a southern continent (Gondwana) were broken off to generate at least three elongate ocean basins. The strips moved northwest relative to Gondwana to close, sequentially, the Proto-Iapetus Ocean during the Ordovican, the Neo-Iapetus Ocean during the Silurian-Devonian and the Rheic Ocean during the Late Carboniferous. The accretion direction was generally towards the northwest, perpendicular to the strike of the belt, and the thrusts verge towards the northwest or southeast. The tectonic pattern was complicated by large-scale lateral escape structures, whereby much of NW Europe was expelled towards the northeast away from the zone of most intense collision in the central Appalachians (Coward 1990, 1993). Therefore in Britain several domains can be recognized bounded by major strike-slip faults (Fig. 2). Northwest of the Great Glen Fault, Proterozoic metasediments (Moines) were intensely foliated and metamorphosed to upper greenschist and amphibolite facies in
l0
M. COWARD 100 km t
i
Laurent ian craton Protemzoic gneissic basement
DOMAIN I
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~amm Ol~iolite
Fig. 2. Caledonian domains of Britain. The structures are dismembered by large NE/SWtrending strike-slip faults. From Coward (1990).
Silurian times and thrust to the WNW over the North Atlantic Craton. Crustal scale shear zones, related to these thrust tectonics, can be recognized on deep seismic data from the middle to lower crust and upper mantle (Brewer & Smythe 1984; Cheadle et al. 1987). During this period of thrust tectonics the region northwest of the Great Glen lay 200-1000 km further northeast relative to the rest of Britain, to be displaced in a left-lateral sense during the Devonian (Coward 1990, 1993). Hence, the Moine structures appear to be part of a major mountain belt formed by continent-continent collision between the North Atlantic and Scandinavia Cratons during the Late Ordovician to Silurian, later dismembered by Devonian strike-slip tectonics. Southeast of the Great Glen Fault, a Proterozoic basement was rifted during Late Precambrian times to form several deep basins, infilled with > 10 km of sediments and volcanics. The extensional faults were reactivated in a major compressional episode of Late Cambrian-Ordovician age, resulting in large-scale positive inversion. This compression was associated with the accretion of one or more magmatic arcs, closing the Proto-Iapetus Ocean to the northwest and opening the Neo-Iapetus Ocean to the southeast.
PERMO-TRIASSIC BASINS OF NW EUROPE
11
Closure of the Neo-Iapetus Ocean was accompanied by NW-directed subduction generating a thick Ordovician-Silurian accretionary prism in the Southern Uplands. Closure continued until the Late Silurian with the final docking of the Brabant Massif, with the accretionary prism and the production of large NW-dipping shear zones through the crust, but no large-scale obduction. The simple pattern of ocean closure between the Brabant Massif and the terrain to the northwest is not mimicked in Scandinavia, where Silurian collision was associated with SE-directed thin-skinned overthrusting and crustal shortening in the order of 500 km (Hossack & Cooper 1986).
Devonian-Carboniferous collapse of the Caledonian mountain belt and regional rifting The main crustal fabrics formed by Caledonian tectonics were modified and offset by Late Caledonian displacement on NE/SW-trending shear systems. These shears are interpreted to have resulted from the lateral ~pulsion of an England-North Sea Block to the northeast away from an Acadian indentor in the Early Devonian (Coward 1990). The block was approximately triangular in shape, and bounded by (i) the Ural Ocean to the east, (ii) the left-lateral Great Glen-Midland Valley-North Atlantic shear systems to the northwest, and (iii) the right-lateral English ChannelSouth Polish Trough shear systems (in Devonian times) and the South WalesSouthern North Sea-Polish Trough shear system (in Early Carboniferous times) to the south. Figure 3 shows a suggested plate reconstruction for Dinantian times.
Fig. 3. Simplified plate tectonic reconstruction for the Dinantian.
12
M. COWARD
Pull-apart basins developed along the major shear systems. In the northern North Sea, the West Orkney Basin-East Shetland Platform-Viking Graben formed as a large pull-apart basin in the left-lateral shear system during Devonian-Early Carboniferous times (Coward 1990, 1993). This basin stretched crust previously thickened during the Caledonian Orogeny so that although stretching values were large, the basins were non-marine and filled with terrestial or lacustrine sediments. In the English Channel and SW England, deep basins and locally oceanic crust developed in the right-lateral shear system. These basins were partly closed during early Variscan tectonics in the Devonian and Early Carboniferous. However, in the southern North Sea and East Midlands, NW-trending Carboniferous basins developed in the right-lateral shear system, associated with clockwise rotation of Caledonian crustal fragments (Coward 1993). Early Carboniferous continental escape was synchronous with regional back-arc extension associated with northward subduction of Variscan oceanic crust. NW/SEdirected extension was characteristic of the northern England and Irish basins. The S England and North Sea rift basins affected crust of normal thickness, generating marine conditions. However, the northern pull-apart basins acted as the loci of sediment transport from the Caledonian mountains into the southern basins; deltaic facies prograded southwards during the Early-Middle Carboniferous from the southern end of the proto-Viking Graben.
Variscan plate collision and the inversion o f Palaeozoic basins Variscan structures of NW Europe also formed as a result of NW-accretion of crustal blocks and magmatic arcs onto the Laurentian foreland. Closure of the NeoIapetus Ocean was associated with opening of Rheic Ocean or system of oceans to the southeast. The Variscan structures were associated with the closure of these oceans as indicated by the presence of ophiolites in the internal Variscan belt. The Trans-European Fault Zone was active at this time, forming the lateral boundary of the Variscan Bohemian Massif (Ziegler 1990). In SW Britain Variscan tectonics involved a NW-verging thin-skinned fold and thrust belt (Shackleton et al. 1982). NW/SE-trending tear faults were developed at this time; some of the larger faults, such as the Bray Fault, may have bounded blocks of different crustal age (Coward 1993). The northwest edge of the Variscan thin-skinned belt can be traced from the South Celtic Sea Basin, through north Devon and to southern England. It bounds the edge of the Carboniferous Culm Basin. However, to the north, Devonian and Carboniferous basins were locally strongly inverted, with thick-skinned basement uplifts in South Wales/Bristol Channel and the Mendips. Minor inversion occurred throughout the Carboniferous, particularly at the beginning of the Westphalian, when Variscan folding affected parts of Britain from South Wales to the Midland Valley. However, the main phase of inversion occurred during the Late Carboniferous-Early Permian, when all the Carboniferous basins were inverted and the shear sense along all the strike-slip faults was reversed.
Permo-Triassic basins By Early Permian times, the Caledonian and Variscan Oceans had fully closed and mountain ranges stretched from the northwest edge of South America, through the
PERMO-TRIASSIC BASINS OF NW EUROPE
13
Fig. 4. Simplified plate tectonic reconstruction for the Early Permian.
Fig. 5. Permian basins of NW Europe, showing the relationship between rift basins west and north of Britain and simple subsidence basins in the North Sea.
14
M. COWARD
northern Caribbean, along the Appalachian and European Variscan mountain belts, around the Black Sea and SE Caspian and along the Urals to the Arctic (Fig. 4). These mountain belts formed a topographic and climatic barrier similar to the present day Alpine-Himalayan belt, influencing structure and stratigraphy during Late Palaeozoic and Early Mesozoic times. Southeast of this mountain belt in Asia lay the Tethyan Ocean. Accretionary tectonics continued in southern Asia as a new slice of Gondwana, termed Cimmeria, broke off and moved northwards about a pole of rotation in the present day Eastern Mediterranean. The northward movement of Cimmeria closed Palaeo-Tethys and opened Neo-Tethys. Two rift systems dominated NW Europe (Figs 4 & 5). The Arctic rift system propagated towards western Ireland along the eastern edge of Greenland, reworking many of the Caledonian and Devonian structures. Rift systems also developed along the Appalachian and Variscan mountain belts. Both the Arctic and Appalachian/ Variscan rifts had approximately NW/SE extension directions, with slight interference in the western part of Britain. Permo-Carboniferous volcanic rocks occur in the North Sea and northern Germany (Fig. 6) and Triassic volcanic rocks occur in what
NW EUROPE IN EARLY PERMIAN TIMES
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PERMO-TRIASSIC BASINS OF N W EUROPE
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M. COWARD
eventually became the Central Atlantic and also in the western Tethyan-Dauphinois Basin. Salt-filled rift or thermal sag basins formed along the northern edge of the Appalachian-Variscan mountain belt, in the present-day Texas Gulf, the southern North Sea and the Peri-Caspian Basins.
Bas&s formed by collapse of the Appalachian-Variscan mountain belt Late Palaeozoic-Early Mesozoic rift basins characterize the Variscan terranes of North America and western Europe and form the precursor basins for Atlantic rifting. In the eastern USA, the basins follow trends of Appalachian structures and appear to rework both thrust and strike-slip mylonites (Fig. 7). In the western UK, deep Permo-Triassic basins formed in Cardigan Bay, the Celtic Sea and the Western Approaches (Fig. 8). These basins trend NE/SW, perpendicular to Caledonian and Variscan thrust directions and approximately parallel to the strike of Caledonian fabrics, but slightly oblique to the strike of Variscan thrusts in south Wales. The basins are cut by NW/SE-trending tear faults which link with strike-slip faults in onshore southwest England. These tear faults cannot be traced into onshore Ireland to the northwest or into France to the southeast, and must have acted as transfer systems to basin opening (Coward & Trudgill 1989).
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PERMO-TRIASSIC BASINS OF NW EUROPE
17
The extension direction was NW/SE, parallel to the tear faults and to small lateral ramps which offset the main basin-bounding faults, especially along the southeast region of the Cardigan Bay Basin. In the latter region the tear faults are steep to vertical and can be recognized on seismic data from the offset of prominent reflecting horizons and from their associated flower-like secondary faults (Coward & Trudgill 1989). Similar strike-slip faults with flower structures occur in the Cockburn Basin (Smith pers. comm. 1994). The North Celtic Sea Graben is c. 350 km long but only 50 km wide. It is bounded on the northwest by a master fault which can be traced to middle crustal levels as a prominent reflector on SWAT deep seismic data (BIRPS & ECORS 1986). The geometry of the basin changes across a major tear-fault system to the northeast into the St George's and Cardigan Bay Basins, where the master fault dips to the northwest. To the southeast the North Celtic Sea Graben is separated from the shallower, but structurally more complex, South Celtic Sea Graben by the Pembroke Ridge. The South Celtic Sea Graben also changes character to the northeast across the major Devon tear fault, into the Bristol Channel Basin. From regional interpretations of seismic and well data (Naylor & Shannon 1982; Tucker & Otter 1987; Coward & Trudgill 1989), the North Celtic Sea Graben contains a thick sequence of Permo-Triassic continental deposits and evaporites overlain by Liassic and Middle Jurassic shales and limestones. On the SWAT 4 deep seismic lines (Fig. 9), the lower crust displays prominent to gently dipping short reflectors, in contrast to the more transparent upper crust. The position of the Moho is inferred to be at the base of the zone of prominent reflectors. The Moho is flat on the seismic sections, at c. 10 km depth TWT, deepening slightly beneath the North Celtic Sea Graben. However, on a true depth scale the crust may thin slightly beneath the Graben (Cheadle et al. 1987). The lower crust, i.e. the zone of prominent reflectors, thins from c. 5 km to c. 3 km to the southeast. There is a zone of southerly dipping reflectors in the mantle beneath the North Celtic Sea Graben observed on the SWAT 4 and 5 lines. These reflectors may represent some form of mantle shear. The upper crust shows weak, moderately prominent S-dipping reflectors along the length of SWAT-4 (Fig. 9). These reflectors can also be seen on some shallow commercial seismic data and are interpreted as Variscan structures, probably thrusts and associated fabrics, which may have been reactivated during Mesozoic extension (Ruffell & Coward 1992). On deep seismic data, the S-dipping reflectors do not cross, but appear to merge with, the zone of lower crust flat reflectors. Some authors (e.g. Gibbs 1985) argue that the most prominent of the S-dipping reflectors mark the Variscan thrust fault, which was later reactivated as a major extensional fault. There is no apparent thickening of the rift-phase sediments towards the S-dipping reflectors; the sediments appear to thicken towards the southeast margin of the basin. The South Celtic Sea Graben shows a similar history to that of the North Celtic Sea Graben, with continental rift-phase sediments in the Permo-Triassic and Middle Jurassic marine shales and limestones (Kamerling 1979; Van Hoorn 1987). The basement structures can be seen in onshore SW England, where numerous extensional faults dip to the NW (Shackleton et al. 1982). On the Devon and Cornwall coasts, both brittle and ductile extensional shears occur, including major ductile low-angle detachments which rework the Variscan back-thrusts in the Rusey-Tintagel area. The ductile detachment zone is c. 1 km thick, with numerous small-scale ductile shear
18
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PERMO-TRIASSIC BASINS OF NW EUROPE
19
movement indicators. Much of this extension in Cornwall is associated with the Permo-Carboniferous granites. Granite sheets intrude several of the normal faults in the Lands End-St. Ives district, and many of the Cornish tin loades are related to the same extensional system. In the Brixham area of south Devon, wide calcite-filled fissure systems suggest the migration of fluids during this phase. Seismic refraction studies by Brooks et al. (1984) and Doody & Brooks (1986) suggest that the Cornish granites are cut by some fiat shear structures at their base in the middle crust, i.e. they are sliced across by a thrust or an extensional detachment. South of Cornwall, the deep Plymouth Bay Basin contains a sequence, more than 10 km thick, of Permo-Triassic sediments imaged on SWAT 8 and SWAT 9 deep seismic lines (e.g. Fig. 10) (BIRPS & ECORS 1986). High-amplitude reflectors halfway up the sequence can be correlated with the Exeter Volcanics of Devon, inferring that the lowermost sediments in the Basin are Late Carboniferous in age. The Plymouth Bay Basin is not fault-bounded although some interpretations show small faults at the base of the Permian. The basin overlies S-dipping reflectors, correlated with the Carrick, Lizard and Start Thrusts onshore (Day & Edward 1983; BIRPS & ECORS 1986; Harvey et al. 1994). Thus the Plymouth Bay Basin is a deep unfaulted basin, but neither the top of the reflective lower crust nor the Moho is depressed or uplifted beneath it. A simple stretching model cannot be applied to the Plymouth Bay Basin; more likely it developed above a deep crustal ramp, probably where the fiat detachments beneath the Cornish middle crust dip down to lower crustal levels. The onlap of sediments towards the northwest, as seen on the SWAT data (Fig. 10), supports this argument. The later sediments in the basin were
Fig. 10. Line drawing of part of the SWAT 9 deep seismic line through the Plymouth Bay Basin showing packages of seismic reflectors in the basin; the prominent reflectors in the middle of the sedimentary pile are interpreted to be the equivalent of the Exeter Volcanics. Some of the Variscan thrust nappes are named. From Harvey et al. (1994).
20
M. COWARD
deposited in a NW/SE-trending depocentre, parallel to faults onshore, suggesting a switch to a NW/SE strike-slip dominated environment (Harvey et al. 1994). In the Wessex Basin, sedimentation began in intermontane troughs and consisted of fan breccias interspersed with fluvial and aeolian sands (Laming 1966). The section of the Wessex Basin exposed on the south Devon coast, north of Torbay, probably displays an analogue for Permian extension in other basins offshore to the west. Along the south Devon coast braided stream deposits, sheet flood conglomerates, mud flow breccias and massive locally derived breccias can be identified. The general provenance was from the west. In the Late Permian, deposition became more widespread and the sediments finer grained, so that fluvial and aeolian sands are interspersed with thick red mudstones of playa lake origin. The Permo-Triassic boundary is generally taken to be at the base of the Budleigh Salterton Pebble Beds, which represent an extensive but temporary influx of coarse detritus from the south. Tectonically these deposits may indicate further movement on the E/W-trending, S-dipping faults which reactivate Variscan structures (Chadwick 1986). The Triassic is dominantly arenaceous in the lower part (Sherwood Sandstone Group) and mainly fluvial, but still partly aeolian and argillaceous towards the top (Mercia Mudstone Group). Locally evaporites occur in the Mercia Mudstones in southern Dorset and Somerset. The Cardigan Bay Basin is an asymmetric structure, whose main boundary faults lie on the southeast margin (Barr et al. 1981; Tucker & Arter 1987; Dobson & Whittington 1987), probably reactivating Caledonian fabrics. Thick packages of Permo-Triassic sediments thicken across the basin towards the southeast (Fig. 11), although inversion has modified the southeast margin of the basin. The basin contains > 10 km of Mesozoic sediments. The northwest part of the basin comprises SE-dipping Permo-Triassic sediments which dip towards the master bounding fault. There are several SE-dipping antithetic faults and some pinch-outs of sediment which may represent onlaps into the tilted half-grabens or small low-angle faults. In real terms the Moho is relatively flat, although in depth sections there is a small velocity pull-down beneath the thick sedimentary cover. The lower crustal reflectors are most prominent in the northwest part of the basin (as seen on SWAT 2) (Fig. 12). These structures are not cut by the basin-bounding faults. There is no evidence for a thermal subsidence phase beneath the main part of the Cardigan Bay Basin and presumably the zone of lower crustal and mantle stretching is situated elsewhere, to the northwest of Cardigan Bay. Alternatively, the lower lithospheric stretch may diffuse over a wide region so that the thermal subsidence was slight and evidence for it destroyed during Cretaceous to Tertiary inversion. Thus different models for stretching apply to adjacent basins linked by tear or transform faults. These tear faults control the distribution of Triassic extension (and subsequent inversion) and are probably reactivated Variscan shear zones. This same system bounds many of the early basins in the Atlantic and was probably also responsible for the development of Tethys to the southeast. Relaxation and extension of the Variscan thrust belt resulted in slip along major NW/SE-trending left-lateral strike-slip zones that extended across Europe as far east as the Black Sea (Ziegler 1988) (Fig. 5). The basins probably formed by late orogenic collapse. The presence of granites and volcanic rocks in SW England suggests high heat flow. Variscan subduction had ceased and so simple back-arc spreading models are not applicable. Some model of roll-back or delamination of the Variscan
21
P E R M O - T R I A S S I C BASINS OF N W E U R O P E
,
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22
M. COWARD
Fig. 12. Line drawing of SWAT 2 across the Cardigan Bay Basin (from BIRPS & ECORS 1986). Location shown on Fig. 8. subducted plate may explain the regional extension along the length of the Appalachian-Variscan mountain belt, similar to models used to explain Tertiary Basin and Range subsidence in the western USA.
Permo-Triassic of the North Sea Permian In the North Sea there were two main areas of subsidence forming the northern and southern Permian basins, separated by the Mid-North Sea-Ringkobing Fyn system of highs (Figs 5 & 13) (Glennie 1990). A small Permian basin also occurs in the Moray Firth. The early basin sediments are termed the Rotliegend, an old German miners' term for the red beds that underlie the Zechstein (Glennie 1990). Rotliegend sedimentation occurred during earliest Permian to Late Permian times; Zechstein deposition took place entirely within the Tartarian, the youngest stage of the Permian. Lower Permian volcanism (early Rotliegend) is most evident in north Germany and Poland and within the Horn-Bamble-Oslo Grabens (Fig. 6). The thickest Rotliegend sequences are preserved in northern Germany in the areas of most widespread lower Rotliegend volcanism, suggesting that thermal subsidence was important (Glennie 1990). Lower Rotliegend volcanics and sediments are preserved in the Oslo Graben-Bamble Trough-Horn Graben areas north of the Trans-European Fault Zone. Transfer zones between the Horn Graben and Bamble Trough trend NW/SE, suggesting that this was the Early Permian extension direction parallel to the Trans-European Fault Zone. Early Rotliegend igneous activity includes the intrusion of the Whin Sill in northern England and some of the dyke swarms in the Midland Valley of Scotland.
PERMO-TRIASSIC BASINS OF NW EUROPE
23
Fig. 13. Simplified map showing the limit of igneous activity and the trend of dykes in the North Sea.
In these areas, volcanism followed Westphalian inversion and pre-dated much of the Permo-Triassic extension, suggesting that the region was underlain by hot asthenosphere, possibly the edge of a NW European hot spot. The trend of the dyke swarms in the Midland Valley of Scotland and adjacent regions suggests local NE/SW extension, perpendicular to that in the Horn-Oslo Grabens. It should be noted that if the region was underlain by a hot spot, only small amounts of extension could lead to the upwelling of asthenospheric melt. Some of the Permo-Carboniferous regional uplift could be due to thermal doming, while mantle cooling could contribute to much of the subsequent Rotliegend subsidence. The Southern Permian Basin has a main depocentre trending WNW/ESE across the northern parts of west and east Germany, in which up to 1.5 km of shales and halite accumulated. The basin sediments onlap the Brabant Massif in the south and the Mid-North Sea High in the north (Fig. 14). Late Carboniferous inversion structures formed a topography which was infilled by the Stephanian Barren Red Beds and then the Rotliegend sandstones. The Brabant Massif was a positive feature throughout the Carboniferous, but the Mid-North Sea High shows evidence of broad inversion during the latest Carboniferous-Early Permian.
24
M. COWARD
Fig. 14. Schematic section through the South Permian Basin. From Fisher & Mudge (1990). The Polish Trough forms part of the southern depocentre of the Southern Permian Basin and is controlled by NW/SE-trending fault systems, with the northern margin being the Tornquist Line (Fig. 5). This NW/SE-trending rift basin, the PolishDobrodgea Rift, extends from Poland to the Black Sea. Rifting was associated with Permo-Triassic sea-floor spreading in the Black Sea (Ziegler 1988). Several models may explain the origin of the Southern Permian Basin: (i) Rifting Model. Badley et al. (1988) and Smith et al. (1993) argue that that Early Permian extension initiated the N/S-trending Viking and Central Graben systems and that the Southern Permian Basin was related to this stretching. This model is considered unlikely, as evidence for important Early Permian extension is lacking in the Southern Permian Basin. The Upper Carboniferous Barren Red Beds and the Rotliegend infill a topography resulting from Carboniferous inversion tectonics (Fig. 14). Thickness changes are a result of infilled topography rather than graben development. Permian rifting however, occurred in the Northern Permian Basin and west of Shetland. Some minor rifting events could have occurred in the north and east of the Southern Permian Basin. (ii) Flexural Basin Model. The Southern Permian Basin and its continuation into Poland and its offset to the Peri-Caspian Basin, lie on the foreland of the Variscan mountain belt. Lithospheric thickening could have produced a flexural foreland basin and account for some of the subsidence. However, the edge of the Permian Basin lies 200 km north of the main thrust front and the basin formation coincided with mountain belt erosion and uplift rather than lithospheric thickening. This model is therefore unlikely. (iii) Thermal Subsidence Model. Thermal subsidence following Carboniferous rifting and, more importantly, following Late Carboniferous volcanism could account for the large subsidence basin. Fig. 13 shows the probable dimensions of the Late Carboniferous thermal dome and the subsequent Permian subsidence basin. Similarly, models can be proposed for the origin of the Mid-North Sea High. This High post-dates Variscan inversion and foreland basin development. It overlies a region of intense Late Carboniferous intrusive activity and may represent a zone of thickened crust less liable to subsequent thermal subsidence. Wadi and dune sands accumulated on the southern flanks of the Southern Permian Basin, reflecting transport of sediment away from the Variscan source. In the Northern Permian Basin, aeolian and fluvial sediments were derived from the Caledonides (Glennie 1990). Within the Southern Permian Basin, the Permian winds
PERMO-TRIASSIC
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PALEOWIND DIRECTION Fig. 15. Palaeowind directions determined for the Rotliegend of the North Sea. From Glennie (1990). blew from a general easterly direction (Glennie 1972, 1990) (Fig. 15), suggesting that the Rotliegend was deposited in a trade-wind desert, similar to the present day Sahara. This wind pattern agrees with regional palaeomagnetically derived plate reconstructions (Glennie 1990). In the Late Permian there was a glacio-eustatic rise in sea level related to the melting of Permo-Carboniferous ice of Gondwana. Rifting in the Faeroes-East Greenland region established a seaway to the Arctic Ocean. The resulting marine transgression from the north led to the establishment of a Zechstein Sea across northern and central Europe. In the North Sea, the Zechstein basins are separated by the Mid-North Sea High. The Zechstein sequences are underlain by the organic-rich shales of the Kupferschiefer, which form a distinctive time marker and imply that the Rotliegend basins had subsided below sea level before the Zechstein transgression (Ziegler 1988). The Zechstein contains a number of carbonate-evaporite cycles reflecting glacioeustatic sea-level fluctuations, forming one of the world's saline giants. The southern margin of the Zechstein Sea was formed by the Brabant Massif, while in the north,
26
M. COWARD
embayments occupied the southern end of the Viking Graben and the Moray Firth. No salt has been recorded from the Viking Graben north of the Beryl Embayment, suggesting that this was the boundary of the basin, although to the north and northeast there must have been the seaway allowing sea-water replenishment during high-stands. Localized Permian salt basins occur in East Greenland (Surlyk et al. 1984) and in the Norwegian Tromso Basin (King 1977). Permian evaporites occur in parts of the Irish Sea and NW England basins (Jackson et al. 1987), probably fed by a seaway through to the Arctic, to the west of Scotland. Four main evaporitic cycles and a partial fifth cycle are recorded from the North Sea. Within each cycle there is a general basinward transition, from shales or continental sandstones at the basin margins, overlapped by carbonates, anhydrites and halites (Taylor 1984). The resultant pattern is a product of sea shrinking under the influence of evaporation following a single flooding episode, together with the effects of regional subsidence. There are stratigraphic variations where different amounts of subsidence occurred between or during cycles. Successive cycles are increasingly evaporitic, significant carbonates being confined to the first three cycles. Triassic
During the Triassic there was a return to terrestial conditions in the north as the basins became the sites of large desert lakes infilled by Bunter shale, rimmed by transgressive fluvial deposits. Fault activity, following the deposition of the Bunter, triggered reactive salt diapirism and resulted in local uplift and erosion of fault blocks. The rift pattern has a dominantly N/S orientation. Extension and faulting and associated subsidence occurred in the Central and Viking Grabens. Triassic sediments thicken across the faults bounding the Unst Basin, Sele High, Egersung Basin and the Horn and Horda Grabens (Figs 16 & 17) (Lervik et al. 1990). In central Europe the Triassic is characterized by the Germanic tripartite facies comprising Lower Triassic clastic sequences (Bundsandstein), followed by a Middle Triassic marine transgression during which the Muschlkalk platform carbonates were established, and finally the Upper Triassic (Keuper) playa and tidal flat deposits
Fig. 16. Simplified section across the Horda Platform and Horda Fault Zone, offshore Norway, showing thickening of Triassic sediments into W-dipping fault-bounded half-graben. From Steel & Ryseth (1990).
PERMO-TRIASSIC BASINS OF NW EUROPE
27
Fig. 17. Simplified section through the Central North Sea Graben and adjacent East North Sea High. Adapted from Glennie (1984).
(Ziegler 1988). Local volcanism and variations both in subsidence rates and sedimentary facies indicate extensional faulting of European lithosphere of variable thickness and/or thermal gradient. In the southern North Sea, the Triassic Basin occupied approximately the same position as the Southern Permian Basin. The new tectonic framework was dominated by the Central and Horn Grabens which dissected the Mid-North SeaRonkobing-Fyn High. Over 2 km of Triassic sediments accumulated in the southern part of the Central Graben. In the southern part of the North Sea, the Sole Pit, Broad Fourteens and West Netherlands Basins were all active. The Bunter of the southern North Sea is represented by the Bacton Group (Rhys 1974), representing a phase of clastic deposition with red sandstones, shales and mudstones. The lower Bunter Shale Formation reflects the large playa lake infilled with lacustrine and floodplain deposits overlying the Zechstein. Marginal clastic sediments prograded into the centre of the basin, so that overall there is a gross coarseningupwards sequence. The Bunter Sandstones reflect rejuvenation of the basin by eustatic lowering of sea level and tilting, associated with the initial stages of rifting. During deposition of the Haisborough Group, the equivalent of the Muschelkalk and Keuper, marine conditions were re-established in the basin. Sedimentation was dominantly in distal floodplain environments alternating with coastal sabkha or shallow-marine environments. The rift basins of the southern Central Graben and southern Horn Graben were distal relative to any clastic source area during deposition of the Haisborough Group. At the base of the Haisborough Group, the Dowsing Dolomite Formation has a variably developed evaporite zone, the Rot Halite. The thickness of the salt varies, possibly due to deposition in extensional-generated depressions (Fisher & Mudge 1990). Ziegler (1975) suggests that the salt may have originated from leached and reprecipitated Zechstein halites, but Holser & Wilgus (1981) cite the high bromide content of the halites as indicative of marine origin. Fisher & Mudge (1990) suggest increased marine influence eastwards and hence a marine incursion from the east.
28
M. COWARD
Salt tectonics were active during the Triassic. Small reactive diapirs are recorded in the southeast of quadrant 49 (Pritchard 1991) and extensional faulting decoupled from basement structures occurred in the post-salt section of block 48/11 (Arthur 1993). Reactive diapirs were generated at the edge of the Central Graben and Broad Fourteens Basin. Permian salt may have been exposed subaerially in passive grabens during the early stages of Haisborough Group rifting and hence could have contributed to the saline conditions during Rot Halite deposition. Much of the irregular thickness of the Triassic in areas west and east of the Central North Sea (Fig. 18) is probably due to slip of the Triassic carapace above Zechstein salt, leading to reactive diapirism of salt between Triassic thinskinned rifts, or to pillow-like buckle folding of the Trias above the salt (Stewart & Coward in press). The salt movement produced a series of essentially N/S-trending salt rises separated by depressions. The depressions formed the sites for much of the Triassic deposition. The simple divisions of the Triassic break down in the northern North Sea and Viking Graben, where Triassic sediments are dominantly red beds including alluvial fan, fluvial, aeolian, sabkha, lacustrine and shallow-marine facies. The abundance and interrelationship of the different facies were dependent on the tectonic setting. In active fault-bounded basins, alluvial fans from the fault scarps merge with playa lakes in the basin centres. In stable basins there were broad floodplains between marginal fans and the basin centres (Fisher & Mudge 1990). In areas of active salt tectonics the sediments were deposited in topographic lows or synforms between the salt rises (e.g. Smith et al. 1993). The major source of coarse clastic material was on
Fig. 18. Simplified sections across the Viking Graben (top) and Central Graben (bottom) illustrating the tectonic style. Note the apparent thickening of Triassic sediments onto Wdipping faults. The main area of Triassic subsidence is offset to the east of the zone of greatest crustal thinning. In the Central North Sea the Triassic structures are complicated by salt tectonics. From Ziegler (1982).
PERMO-TRIASSIC BASINS OF NW EUROPE
29
the eastern margin of the basin, where the Scandinavian Craton was uplifted and eroded throughout much of the Triassic. Thick sequences of coarse clastic sediments (Skagerrak Formation) accumulated in the Egersund and North Danish Basin. The extension direction and its possible variations throughout the North Sea are unknown. In the northern North Sea, the Permian structural pattern was modified by the superposition of a graben system which controlled Mesozoic sedimentation patterns. The major new structural element was the Viking Graben which transected the Northern Permian Basin and breached the Rinkobing-Fyn High. In the axial parts of the Viking Graben, Triassic sediments may exceed 3 km in thickness, although the active Triassic faults lay to the east. Most of the Triassic grabens were asymmetric with W-dipping normal faults (Figs 17 & 18). There was rapid but asymmetric subsidence in the Central and Horn Grabens, where Triassic sediments exceed 4km thickness. Beach (1987) suggested that the dominant extension direction was NW/SE and that the Central, Horn and Bamble Grabens are pull-apart structures in a right-lateral shear system containing Late Carboniferous-Early Permian-Early Triassic sediments. The main areas of
Fig. 19. Simplified location map for basins north and west of Shetland, showing the apparent continuation of the Tern-Unst system into the Fair Isle and West Shetland system. Boxed area is the Tern Basin. x-section shown on Fig. 20; y-section shown on Fig. 21. From Thomas & Coward (1995).
30
M. COWARD
extensional faulting are bounded by the Trans-European Fault Zone and the northern Tornquist Line; the Oslo Graben to the north shows relatively little Triassic subsidence (Ziegler 1982).
Permo-Triassic west of Shetland North and west of Shetland the Permo-Triassic consists of aeolian and fluvial sandstones and conglomerates and some playa evaporites deposited in large halfgrabens bounded by W-dipping normal faults (Fig. 19). The faults are part of the same set which produced Triassic half-grabens in the East Shetland Basin. The Tern Basin of the East Shetland Basin (boxed on Fig. 19) can be traced into the Unst Basin on the adjacent Shetland Platform (Fig. 19). West of Shetland the Triassic grabens are deep. The master fault to the West Shetland Basin lay on its southeast margin, but prominent antithetic faults formed along the northwest margin, on the flanks of the Rona Ridge. The Permo-Triassic sequences show important truncations suggesting an inversion episode (Fig. 20). The Westray Fault on the western margin of the Faeroes Basin reactivated a strong basement fabric, possibly a northern continuation of the Outer Isles fault zone. During the Triassic, the Judd Fault acted as a transfer zone and its trend and location may have been fixed by the presence of a basement structure. The southeast continuation of the West Shetland Basin is offset by this transfer system. The Sula Sgeir and Stack Skerry Basins form deep fault-bounded half-grabens, up to 10 km deep (4 seconds TWT on seismic sections). Two megasequences, separated by an unconformity, can be distinguished based on seismic character (Fig. 21). The lowermost sequence is thought to be Devonian-Carboniferous while the upper sequence is Permo-Triassic (Stein 1988). Erosional truncations of the syn-rift sequence suggest post-Triassic inversion. This may be part of a widespread Callovian unconformity recorded from the Faeroe Basin (Hazeldine et al. 1987) and the Hebrides Basin (Morton 1989). Booth et al. (1993) estimate that more than 1.5 km of sediment was removed by this inversion and erosion event.
Fig. 20. Simplified line drawing of a seismic line across the West Shetland Basin and Rona Ridge (location shown in Fig. 19).
PERMO-TRIASSIC
BASINS OF NW EUROPE
31
O
~ ~
. .,...~
o o
o
~ 4-a
o
h.
~
r.~
o
o o
Q
6
.'~ co I---
eq,.Q
32
M. COWARD
From borehole data, the Permo-Triassic to Lower Jurassic megasequence consists of at least three sequences, each related to a rifting event. The two lower sequences consist of fining-upwards successions of continental deposits, while the upper sequence records a transition to marine conditions during the Jurassic (Booth et al. 1993). South of the Sula Sgeir Basin, the major faults dip E or SE, but NW-dipping faults form the edge of the Rockall Trough to the SW. Thus the West Orkney Basin is a Devonian basin with E-dipping faults reactivated in the Permo(?)-Triassic. The transfer zone to these two sets of faults occurs along the southeast continuation of the Wyville-Thompson Ridge. Therefore the Judd Fault and Wyville-Thompson Ridge are fundamental structures which control the polarity of Triassic-Tertiary basins northwest of Scotland. Earle et al. (1989) map a NW/SE-trending fault which they term the Orkney-Faeroe Alignment. However, little evidence has been found for this fault from seismic data, and the change in dip of the major faults may reflect a difference in crustal anisotropy related to the distribution of Lewisian and Caledonian shear zones. The Rockall Basin contains a thick sequence of W-dipping sediments covered by Tertiary lavas. The basal sediments are considerd to be Triassic. Up to 4 km of Permo-Triassic red beds are preserved in the North Minch Basin and serve as a model for other Permo-Triassic basins in the area (Steel 1974). The Minch Basin is an asymmetric basin containing a westward-thickening fill, dominated on the west side by proximal alluvial fan deposits (Steel 1974). Repeated rejuvenation of the North Minch Basin margin led to stacking of alluvial fans, with repeated coarseningupwards sequences and to the deposition of several kilometres of sediments. During quiescent periods, scarp retreat and denudation of the hinterland led to the deposition of fining-upwards sequences containing a higher proportion of mudstones. The fans are capped by floodplain deposits which onlap from the central part of the basin. On the eastern dip slope side of the basin, the succession is very thin and consists of fining-upwards fan sequences, indicative of relatively stable conditions.
Northwest England basins During the Early Permian, isolated intermontane basins in northern England and the East Irish Sea Basin were infilled with thick aeolian sandstones (Collyhurst Sandstone). Marked facies changes and relatively slow subsidence in both the clastic Lower Permian and evaporitic Upper Permian sediments are related to a low-lying playa and a subsequent coastal sabkha environment. This subsidence could be considered to be the waning phase of the thermal subsidence which accompanied growth faulting during deposition of the Sherwood Sandstone Group (c. 1.Skm thick). Several halite deposits accompanied deposition of the Mercia Mudstone Group (>3 km). The evaporites form detachment horizons and hence the Upper Triassic stratigraphy is complicated by low-angle faulting and halokinesis. Permo-Triassic extension was dominantly NE/SW (Figs 22 & 23). Fault patterns, transfer zones and minor structures indicate NE/SW extension of 1.1 to 1.15 (Knipe et al. 1993). NE/SW-trending basin-bounding faults in the north and south of the basin had strike-slip or oblique-slip displacement. Inversion and erosion have removed much of the post-Triassic cover. The Lake District partially represents a large tilted fault block locally bounded to the west by
PERMO-TRIASSIC BASINS OF NW EUROPE
33
.) ;..o0_..... Fig. 22. Simple map of the Manx-Cheshire Permo-Triassic Basin. the Boundary Fault and to the east by the Vale of Eden Fault (Fig. 23). However, much of the domal uplift of the Lake District reflects Tertiary inversion; in the northern Lake District, Permo-Triassic sediments probably covered much of the Lake District Dome. The Dent Fault, a Carboniferous normal fault reactivated as a lateral structure bounding the Vale of Eden tilted fault block and some of the inversion structures along the Dent and other Carboniferous extensional faults, may be related to Permo-Triassic NE/SW extension. The Lake District Boundary Fault and several other NNW-trending faults die out in transfer zones or lateral tips. Much of the sediment supply into the basin came from the tip zones of the major structures. Thus the eastern part of the Irish Sea Basin was infilled from the north and south, around the tips of the Lake District and Formby-Clwyd systems. In the Irish Sea Basin, the Keys Basin depocentre preserves one of the thickest (>4km) Triassic sequences in Britain (Jackson et al. 1987). In the northern part of
34
M. C O W A R D
NORTHUMBERLANDBASIN SOLWAYFIRTH
o~ .~.:-
-
ALSTON BLOCK 20 k m
J! ~r
5.......................
ASKRIGGBLOCK
Monocllne, Inversion in S t e p h a n l i n
CLITHEROE- PENDLE FOLD SYSTEM .DEENSTERPLATFORM CLITHEROEFAULT KNOTTS FAULT
MANCHESTER
_
""
m
c__j
'
i!
CHESHIRE
_.....
PERMO-TRIAS CARBONIFEROUS BASEMENT DEPTH TO DOMINANT SEISMIC REFLECTION
Fig. 23. Map showing the main faults in the Manx-East Irish Sea basin and adjacent parts of NW England.
the basin, E-dipping faults are dominant and the sediments are generally tilted towards to west. In the southern part of the basin, W-dipping faults induce tilting towards the east. The transfer zone between these two domains has a complex tilt history, reflecting southward propagation of the Keys Fault and northward propagation of the Crosh Vusta Fault. There is some dispute over the timing of fault activity. According to Knipe et al. (1993) the faults have only minor influence on Triassic sedimentation patterns and much of the displacement probably postdated the Sherwood Sandstone. However, Jackson & Mulholland (1993) claim that 65-70% of the movement on the Keys Fault was Permo-Triassic in age. The main lateral boundary faults to the basin probably rework Carboniferous extensional basins. A thick (4-6km) Carboniferous sequence, which shows some inversion prior to Permian deposition, can be traced west from the Carboniferous Bowland Basin. The Carboniferous faults, which rework older Caledonian structures, can be traced to mid crustal depths on the WINCH deep seismic data.
PERMO-TRIASSIC BASINS OF NW EUROPE
35
Faults trending N N W dominate the structure of the Vale of Clywd and the Cheshire Basin. These structures controlled depocentre development during the Mid-Late Carboniferous as well as basin inversion during the Variscan (Williams & Eaton 1993). Some of the faults were reactivated during the Permian and Late Triassic-Early Jurassic to control Mesozoic basin evolution. Folding of the PermoTriassic sequences is thought to be related to Tertiary inversion (Williams & Eaton 1993). The Worcester Basin continues the N/S trend of the Morecambe Bay and Cheshire Basins and may reactivate a Variscan structure. This set of basins interferes with the Channel structures in the western part of the Wessex Basin, so that in South Devon both NE/SW- and NW/SE-trending faults were active, giving a chocolatetablet graben structure. According to Jackson & Mulholland (1993), the line of maximum Triassic subsidence can be traced from the East Irish Sea and Cheshire Basins, through the Worcester Graben, to the Pays de Bray Fault and the Paris Basin, that it, it makes a NW/SE-trending trough across NW Europe, approximately perpendicular to the main axes of Permo-Triassic rifting.
Discussion The major Triassic rift systems of NW Europe are shown in Fig. 24 and a global reconstruction in Fig. 25. By the Triassic, the Atlantic rift system had developed and become linked to Western Tethys. The Palaeo-Tethyan Ocean had almost fully closed, forming the Cimmerian mountain belt from Turkey to China. In NW Europe
Fig. 24. The Triassic rifts of NW Europe with suggested opening directions.
36
M. COWARD
Fig. 25. Simplified plate tectonic reconstruction for the Triassic. two extension directions dominate: (i) NW/SE along the line of the future Atlantic Ocean, and (ii) NE/SW in parts of NW Europe. Two models are suggested for these different trends: (i) The NE/SW rifting may reflect extension related to the Neo-Tethyan Ocean. The NE/SW extension in the North Sea and East Irish Sea Basin may mark the tip of rifting associated with opening of the Palaeo-Tethyan and Neo-Tethyan oceans. (ii) The NE/SW extension may reflect the bulk strain generated by the interference of the Arctic and Atlantic rift systems with slightly different opening directions. The complex pattern of faulting in areas such as Morecambe Bay-Worcester Graben may be due to this form of triple junction. The stretching direction in the North Sea varies in time and space. During the Permian, the NE/SW-trending Oslo and Horn Grabens were generated presumably by NW/SE extension parallel to the Trans-European Fault Zone. Some of the Permo-Triassic structures in the Viking Graben may have been generated at this time. Triassic extension of the Viking Graben was NW/SE, parallel to that of the Rockall-Faeroes Basins. However, during the Triassic the Central Graben presumably opened orthogonally, in a NE/SW direction, parallel to the opening of the Broad Fourteens, Sole Pit and East Irish Sea Basins. The edge of the Central Graben and the Sord and Egersund Basins in Norway follow the trend of the Lower Palaeozoic boundary of the Scandinavian Block and may be reworked Late Proterozoic-Early Palaeozoic fault systems. Certainly, the NW/SE-trending Polish Trough follows this same boundary through eastern Europe.
PERMO-TRIASSIC BASINS OF NW EUROPE
37
Thus there is a rectangular pattern of rift development in N W Europe. The Polish Trough, Central N o r t h Sea, East-Irish Sea-Worcester Graben and Cantabrian Basin in N Spain have dominantly N E / S W extension. The Atlantic rift system and the Greenland-Arctic basins extended in a NW/SE direction. The Greenland Basin propagated from the north towards the northwest part of Britain, opening along the line of the Greenland-Scandinavian Caledonides. The Atlantic rift propagated from the south along the line of the Variscan mountain belt. Both rift systems were tramlined to some extent by the basement structure, particularly the trends of the Caledonian and Variscan tear faults. Although the Caledonides and Variscides were both generated by NW/SE plate accretion, the collision episodes were separated by strike-slip tectonics and lateral expulsion of continental blocks. It is not surprising that the opening directions of the Atlantic and Arctic rift systems are sufficiently different to generate a broad triple junction across N W Europe. This tectonic framework developed during the Permo-Triassic was followed by the patterns of rifting during the Jurassic and Cretaceous. Rifts trending N/S reactivated and deepened the Viking Graben during the Middle to Late Jurassic. The Central Graben was re-rifted and deepened during the Latest Jurassic-Earliest Cretaceous. Knowledge of Permo-Triassic tectonics is therefore crucial for understanding and modelling subsequent Mesozoic and Tertiary basin evolution in N W Europe.
References
BADLEY, M. E., PRICE, J. D. RAMBECH DAHL, C. & AGDESTEIN, T. 1988. The structural evolution of the northern Viking graben and its bearing upon extensional modes of basin formation. Journal of the Geological Society, London, 145, 455-472. BARR, K. W., COLTER, V, S. & YOUNG, R. 1981. The geology of Cardigan Bay - S t George's Channel basin. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North West Europe. Heyden, London, 432-443. -1987. A regional model for linked tectonics in NW Europe. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 43-48. BIRPS & ECORS 1986. Deep seismic reflection profiling between England, France and Ireland. Journal of the Geological Society, London, 143, 45-52. BLUNDELL, D. J., HOBBS, R. W., KLEMPERER, S. L., SCOTT-ROBINSON, R., LONG, R. E., WEST, T. E. & DUIN, E. 1991. Crustal structure of the central and southern North Sea from BIRPS deep seismic reflection profiling. Journal of the Geological Society, London, 148, 445-457. BOOTH, J., SWIECICKI, T. & WILCOCKSON, P. 1993. The tectono-stratigraphy of the Solan basin, west of Shetland. In: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geological Society, London, 987-998. BROOKS, M., DOODY, J. J. & AL-RAWI, F. R. J. 1984. Major crustal reflectors beneath southwest Britain. Journal of the Geological Society, London, 141, 439-444. CHADWICK, R. A. 1986. Extensional tectonics in the Wessex Basin, southern England. Journal of the Geological Society, London, 143, 465-488. CHEADLE, M. J., MCGEARY, S., WARNER, M. R. & MATTHEWS, D. H. 1987. Extensional structures on the western UK continental shelf: a review of evidence from deep seismic profiling. In: COWARD, M. P., DEWEY, J. F. & HANCOCK, P. L. (eds) Continental Extensional Tectonics. Geological Society, London, Special Publication, 28, 445-465. COWARD, M. P. 1990. The Precambrian, Caledonian and Variscan framework to NW Europe. In: HARDMAN, R. F. P & BROOKS, J. (eds) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geological Society, London, Special Publication, 55, 10-34.
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1993. The effect of Late Caledonian and Variscan continental escape tectonics on basement structure, Paleozoic Basin kinematics and subsequent Mesozoic basin development in NW Europe. In: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe." Proceedings of the 4th Conference. Geological Society, London, 1095-1108. COWARD, M. P. & TRUDGILL, B. 1989. Basin development and basin structure of the Celtic sea basins (SW Britain). Bulletin of the Geological Society of France, (8) t.v. 423-436. DAY, G. A. & EDWARDS, J. W. F. 1983. Variscan thrusting in the basement of the English Channel and Western Approaches. Proceedings of the Ussher Society, 4, 432-436. DOBSON, M. R. & WHITTINGTON, R. J. 1987. The geology of Cardigan Bay. Proceedings of the Geological Association, 98, 331-353. DOODY, J. J. & BROOKS, M. 1986. Seismic refraction investigation of the structural setting of the Lizard and Start complexes, SW England. Journal of the Geological Society, London, 143, 135-140. EARLE, M. M., JANKOWSKI, E. J. & VANN, I. R. 1989. Structural and stratigraphic evolution of the Faeroe-Shetland Channel and Northern Rockall Trough. In: TANKARD, A. J. & BALKWILL, H. R. (eds) Extensional Tectonics and Stratigraphy of the North Atlantic Margins. American Association of Petroleum Geologists Memoir, 46, 461-469. FISHER, M. J. & MUDGE, D. C. 1990. Triassic. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. Blackwell, Oxford, 191-218. GIBBS, A. D. 1985. Basin development and hydrocarbon prospectivity. Tectonophysics, 133, 189-198. HOLSER, W. T. & WILGUS, C. K. 1981. Bromide profiles of Rowet salt, Triassic of Northern Europe, as evidence of its marine origin. Neues Jahrb Mins. GLENNIE, K. W. 1972. Permian Rotliegendes of North-West Europe interpreted in light of modern defect sedimentation studies. AAPG Bulletin, 56, 1048-1071. 1984. The structural framework and Pre-Permian history of the North Sea area. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. Blackwell, Oxford, 17-39. 1990. Rotliegend sediment distribution; a result of late Carboniferous movements. In: HARDMAN, R. F. P. & BROOKS, J. (eds) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geological Society, London, Special Publication, 55, 127-138. HARVEY, M., STEWART, S., WILKINSON, J., RUFFEL, A. & SHAIL, R. (1994). Tectonic evolution of the Plymouth Bay Basin. Proceedings of the Ussher Society, 271-278. HAZELDINE, R. S., RITCHIE, J. O. & HITCHEN, K. 1987. Seismic and well evidence for the early development of the Faeroe-Shetland Basin. Scottish Journal of Geology, 23, 283-300. HOSSACK, J. R. & COOPER, M. A. 1986. Collision tectonics in the Scandinavian Caledonides. In: COWARD, M. P. & RIES, A. C. (eds) Collision Tectonics. Geological Society, London, Special Publication, 19, 287-303. JACKSON, D. I. & MULHOLLAND, P. 1993. Tectonic and stratigraphic aspects of the East Irish Sea Basin and adjacent areas: contrasts in their post-Carboniferous structural styles. In: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geological Society, London, 791-808. , - - , JONES, S. M. & WARRINGTON, G. 1987. The geological framework of the East Irish Sea Basin. In: BROOKS, J & GLENNIE, K. (eds) Petroleum Geology of Northwest Europe. Graham & Trotman, London, 191-204. KAMERLING, P. 1979. The geology and hydrocarbon habitat of the Bristol Channel basin. Journal of Petroleum Geology, 2, 75-93. KING, R. E. 1977. North Sea joins ranks of world's major oil regions. Oil Gas Journal, 81, 35-45. KNIPE, R. J., COWAN, G. & BALENDRAN, V. S. 1993. The tectonic history of the East Irish Sea Basin with reference to the Morcambe Field. In: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe." Proceedings of the 4th Conference. Geological Society, London, 857-866. LAMING, D. J. C. 1966. Imbrication, paleocurrents and other sedimentary features in the Lower New Red Sandstones, Devonshire, England. Journal of Sedimentary Petrology, 36, 940-959.
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LERVIK, K. S., SPENCER, A. M. & WARRINGTON, G. 1990. Outline of Triassic stratigraphy and structure in the central and northern North Sea. In: COLLINSON, J. D. (ed.) Correlation in Petroleum Exploration. Norwegian Petroleum Society. Graham & Trotman, London. MORTON, N. 1989. Jurassic sequence stratigraphy in the Hebrides Basin, NW Scotland. Marine and Petroleum Geology, 6, 243-260. NAYLOR, D. & SHANNON, P. 1982. Geology of Offshore Ireland and West Britain. Graham & Trotman, London. RHYS, G. H. 1974. A proposed standard lithostratigraphic nomenclature for the southern North Sea and an outline structural nomenclature for the whole of the (UK) North Sea. Report No. 74/8. Institute of Geological Sciences, HMSO. RUFFELL, A. H. & COWARD, M. P. 1992. Basement tectonics and their relationship to Mesozoic megasequences in the Celtic Sea and Bristol Channel area. In: PARNELL, J. (ed.) Basins on the Atlantic Seaboard: Petroleum Geology, Sedimentology and Basin Evolution. Geological Society, London, Special Publication, 62, 385-394. SHACKLETON, R. M., RIES, A. C. & COWARD, M. P. 1982. An interpretation of the Variscan structures in SW England. Quarterly Journal of the Geological Society, 139, 533-541. S M I T H , R . I., HODGSON, N. & FULTON, M. 1993. Salt control on Triassic reservoir distribution, UKCS Central North Sea. In: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geological Society, London, 547-557. STEEL, R. J. 1974. New Red Sandstone floodplain and piedmont sedimentation in the hebridean province, Scotland. Journal of Sedimentary Petrology, 4, 336-357. STEWART, S. A. & COWARD, M. P. in press. A synthesis of salt tectonics in the southern North Sea, UK. Marine and Petroleum Geology. - & RYSETH, m. 1990. The Triassic-Early Jurassic succession in the northern North sea: megasequence stratigraphy and intra-Triassic tectonics. In: HARDMAN, R. F. P. & BROOKS, J. (eds) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geological Society, London, Special Publication, 55, 130. STEIN, m. M. 1988. Basement controls on Hebridean basement development, NW Scotand. Basin Research, 1, 107-119. SURLYK, F., MASECKI, S., ROLLE, F., STEMNERIK, L., THOMSEN, E. & WRANG, P. 1984. The Permian Basin of East Greenland. In: SPENCER, A. M. (ed.) Petroleum Geology of the North European Margin. Graham & Trotman, London, 303-315. TAYLOR, J. C. M. 1984. Late-Permian-Zechstein. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. Blackwell, Oxford, 61-83. THOMAS, D. W. & COWARD, M. P. 1995. Late Jurassic-Early Cretaceous inversion of the northern East Shetland Basin, northern North Sea. In: BUCHANAN, J. G. & BUCHANAN, P. G. (eds) Basin Inversion. Geological Society, London, Special Publication, 88, 275-306. TUCKER, R. M. & ARTER, G. 1987. The tectonic evolution of the North Celtic Sea and Cardigan Bay basins. Tectonophysics, 137, 291-307. VAN HOORN, B. 1987. Structural evolution, timing and tectonic style of the Sole Pit inversion. Tectonophys&s, 137, 239-284. WILLIAMS, G. D. & EATON, G. P. 1993. Stratigraphic and structural analysis of the Late Palaeozoic-Mesozoic of NE Wales and Liverpool Bay: implications for hydrocarbon prospectivity. Journal of the Geological Society, London, 150, 489-499. ZIEGLER, P. m. 1975. Geological evolution of the North Sea and its tectonic framework. American Association of Petroleum Geologists, Bulletin, 59, 1073-1097. -1982. Geological Atlas of Western and Central Europe. Shell Internationale Petroleum Maatschappij B.V., The Hague. - - - 1 9 8 8 . Evolution of the Arctic-North Atlantic and the Western Tethys. American Association of Petroleum Geologists Memoir, 43. 1990. Geological Atlas of Western and Central Europe (2nd edn). Shell International Petroleum, Maatschoppij B.V., The Hague.
From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe, Geological Society Special Publication No. 91, 41-56
Thermal histories of Permian and Triassic basins in Britain derived from fission track analysis A. CARTER, A. YELLAND 1, C. BRISTOW & A. J. H U R F O R D Research School of Geological and Geophysical Sciences, Birkbeck College and University College London, Gower Street, London WC1E 6BT, UK a(Present address." Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK) Abstract: Permian and Triassic rifling in northern Europe resulted in the
deposition and preservation of alluvial sediments within discrete graben structures throughout the UK. The sediments are mostly continental red-bed facies which contain little or no organic materials for vitrinite reflectance palaeotemperature estimation. However, the sandstones commonly contain detrital apatite from which fission track age and track length data can be readily derived, and the elusive low temperature thermal history quantified. Published and new fission track data from over 80 Permian and Triassic sandstones in Britain reveal contrasting thermal histories. Triassic rocks of northwest Scotland have seen only moderate palaeotemperatures (60-75~ reached during the Mesozoic, in contrast to the Cheshire and Irish Sea Basins which experienced greater maximum palaeotemperatures (80-110~ during the Late Cretaceous (Lewis et al. 1992b). In southwest England, maximum palaeotemperatures of 70-80~ (~95~ in the basin centre at Wytch Farm) were reached during the Lower Cretaceous. The Permian and Triassic sediments of this study indicate that maximum burial is not restricted to the Late Cretaceous and that, in the southwest especially, temperatures peaked much earlier, during the Lower Cretaceous. This implies a significant variation throughout Britain in both Mesozoic cover and its subsequent exhumation.
The predominantly continental nature of Permian and Triassic sediments means that there is virtually no organic material for vitrinite reflectance palaeotemperature estimation and hence, to date, little is known directly about the thermal histories of these sediments. Apatite fission track analysis, however, is a powerful method for constraining thermotectonic histories and is ideally suited to Permo-Triassic sandstones which are known to contain detrital apatite. Fission track data from Permian and Triassic sediments have previously been published in studies of the East Midlands (Green 1989a), Isle of Skye (Lewis et al. 1992a) and northwest England (Lewis et al. 1992b). These studies have revealed that, in Britain, the Permian and Triassic, together with most pre-Cretaceous rocks today at outcrop, reached maximum palaeotemperatures in excess of 70~ during Late Cretaceous/Early Tertiary times. This study presents results of fission track analysis from 28 additional samples collected from southwest England and the central Midlands, complementing
42
A. CARTER E T AL.
published data and providing additional insights into the regional thermal history of the U K Permian and Triassic rocks.
Permian and Triassic tectonic setting Permian and Triassic rocks outcrop across most of Britain from southern England to northern Scotland (Fig. 1) and are mostly continental clastic sediments which were deposited in a rift setting. The main phase of rifting occurred in the Triassic,
Fig. 1. Simplified map showing the Permian and Triassic outcrop and subcrop of the British Isles (after Audley-Charles (1970) and Stoneley & Selley (1986)), including the localities of the new areas examined within this paper. Black circles refer to smaller outcrops of strata.
THERMAL HISTORIES OF BASINS FROM FISSION TRACK ANALYSIS
43
under a broadly E-W extensional regime influenced by early North Atlantic rifting (Dore 1992; Srivastava & Verhoef 1992), although the Permo-Triassic basins have different orientations reflecting the influence of older tectonic lineaments. In southern England, E-W-oriented Hercynian trends are dominant, with subsidiary N-S faults. In the Midlands, N-S Malvernian trends controlled the orientation of the Worcester Graben, whilst N-E/S-W trends along Caledonian/Longmyndian structures are dominant in the Cheshire Basin (Chadwick & Evans 1995). In the Southern Uplands, Anderson et al. (1995) have shown that the Stranraer, Dumfries and Thornhill Basins are bounded by NNW-SSE trending faults, which show dipslip and oblique-slip reactivation of Caledonian strike-slip faults in response to ENE-WSW extension, perpendicular to the dominant Caledonian fabric. In the Hebrides, NE-SW-trending basins are controlled by faults which parallel Caledonian thrusts. In southern Britain, the earliest Permian sediments unconformably overlie deformed Devonian and Carboniferous rocks which were folded and uplifted in the Hercynian orogeny. Within the Variscan thrust belt, and further north in the Worcester Graben and Cheshire Basin, Carboniferous basins were inverted (Chadwick & Evans 1995). Because of the inversion, the earliest Permian sediments are preserved on the basin flanks (Leeder & Hardman 1990), whilst later Permian and Triassic sedimentation occurred in basins which reformed over the older basins (Chadwick & Evans 1995). In the Wessex Basin, early Permian calc-alkaline volcanics (the Exeter lavas) were erupted very early in the basin's history. Early activity appears to have occurred in a broad sag basin (Glennie 1990). Coward (1995) has suggested that the North Sea subsidence and volcanics result from a hot spot centred beneath the Mid-North Sea High. The sediments which accumulated in Permian and Triassic basins were derived from the Variscan mountains in the south, inverted Devonian and Carboniferous basins and unroofed Caledonian granites. In the north, the main source areas are more difficult to determine because of subsequent rifting. The most likely clastic source areas are Caledonian highlands on either side of uplifted rift flanks.
Fission track methodology and application Apatite fission track (FT) analysis is ideally suited to obtain information relating to low temperature (< 130~ thermal histories, specifically the magnitude and timing of maximum palaeotemperature. The analytical principles of the FT technique are dealt with by Hurford & Green (1982, 1983), Gleadow et al. (1986), Laslett et al. (1987), Duddy et al. (1988) and Green (1989b). Interpretation of FT data requires examination of track lengths and both mean and individual grain ages. For geologically relevant time scales, fission tracks are stable at temperatures greater than about 60~ above this temperature partial annealing (track shortening) occurs until, at temperatures of 110-125~ fission tracks are completely annealed. Because fission tracks form continuously, each track experiences a different maximum temperature, resulting in an integrated length distribution diagnostic of the sample's thermal history, characterized specifically by its mean length and standard deviation. The precise temperature required for annealing is dependent upon apatite chemistry: apatite crystals with high C1 content are more resistant to annealing than those with lower C1 contents (Green et al.
44
A. CARTER E T AL.
1989a). Rapid cooling from >ll0OC to <50~ produces a mean track length of >14.5 pm with a narrow standard deviation and an age that records the timing of this rapid cooling event. Slower cooling results in significant track shortening, reduced mean lengths, larger standard deviations, and an apparently mixed age which, probably will not be related directly to a discrete event. More detailed explanations of the temperature dependence of the annealing process are to be found in Laslett et al. (1987), Duddy et al. (1988) and Green (1989b). A primary measure of the single grain age spread within a fission track data set is given by the age dispersion value (relative standard deviation) that accompanies the central (modal) age (Galbraith 1992). A dispersion value (expressed as a percentage variation) greater than 10% indicates a spread in single grain ages about the modal age, beyond that expected from a single age population. In such cases the spread will be due either to multiple provenance ages, or to partial annealing that has exploited the compositional variation present in the sample, enhancing the existing spread in
Fig. 2. Hypothetical representation of changing single grain age distributions within a sediment in response to progressive annealing (and cooling in the lowest diagram). The horizontal axis represents increasing relative precision of single grain age; the vertical axis represents the standard error of each measurement (depicted by the 2o- error bars on selected points). The nominal stratigraphic age of the sediment is depicted by the shaded sector, the arc of which forms part of the radial age scale (Ma). Single grain ages are located by drawing a line from the origin through the data point to the radial age scale.
THERMAL HISTORIES OF BASINS FROM FISSION TRACK ANALYSIS
45
grain ages. A graphical method used to display the single grain ages and their associated errors is the radial plot of Galbraith (1990) (Fig. 2). The arcuate axis on the right represents fission track age and is scaled in Ma. Grain age precision is plotted on the x-axis, with those grains plotted closer to the arcuate axis being the more precise. Single grain ages are obtained by drawing a radius from the origin on the y-axis on the left through the crystal age to intersect the accurate scale. The usefulness of radial plots in observing the effects of increasing track annealing/ temperatures is illustrated by the three plots in Fig. 2. The upper plot shows an unannealed sample in which all grain ages are either older or equal to the stratigraphic/intrusion age. Moderate annealing is evident in the middle plot, where some grain ages are younger than the stratigraphic age, even at the 2a error level. As annealing increases, compositional effects are enhanced and the spread in single grain apparent ages widens, as measured by the age dispersion value. In the bottom plot of Fig. 2, extreme annealing removes the combined influences of compositional variation and inherited tracks, bringing the grain ages together to form a single age population with reduced age dispersion. The final stage in the interpretative procedure uses a forward modelling program with a Monte Carlo approach to predict FT age, mean length and standard deviation parameters, using the annealing algorithm of Laslett et al. (1987). Figure 5 illustrates the Monte Carlo approach used to assess the temperature history of a sample from the southwestern peninsula (see below). Fission track ages quoted in this study are central ages d:l standard error, followed by the percentage age dispersion.
Regional fission track results Southwestern peninsula The Permo-Triassic red-bed sequence in Devon and Somerset began with the deposition of fan breccias, fluvial and aeolian sands resting unconformably on folded Devonian and Carboniferous basement. Deposition was initially within local intermontane basins, becoming more extensive as the surrounding uplands were progressively denuded. Locally derived detritus is in evidence, including granite from the Cornubian batholith emplaced during the late Carboniferous. The general absence of palaeontological evidence in the Permian rocks has led to a poor stratigraphic resolution. A mid-Triassic fauna is the oldest stratigraphically useful material, although Warrington & Scrivener (1990) have confirmed a Late Permian sequence in Devon. Samples collected for FT analysis were assigned stratigraphic ages according to Warrington & Scrivener (1990). Table 1 details the resultant fission track data from the Permian and Triassic samples, whilst Fig. 3 shows the sample locations and FT central ages. The Permian and Triassic sediments sampled range in age from the Dawlish Sandstone Formation (Late Permian) to the Mercia Mudstone Formation (Middle to Late Triassic). The FT ages range from 278 ~ 14 to 191 • 10 Ma, with mean track lengths of < 14 lain. Comparisons of the FT ages with the nominal stratigraphic ages (~255-230 Ma) indicate that some sediments may have experienced minor postdepositional annealing, The radial plot in Fig. 4a from the Dawlish Sandstone Formation (Tatarian) is typical of the Permian samples. The FT age of 256 + 19 Ma
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THERMAL HISTORIES OF BASINS FROM FISSION TRACK ANALYSIS
47
Fig. 3. Simplified geological map of South Devon, Somerset and Dorset, showing sample localities and FT Central ages (Ma). (25%) is effectively identical to the stratigraphic age, although the age dispersion value of 25% indicates a spread in individual crystal ages beyond that of a single population. The most probable youngest source age in the region is given by the timing of the exhumation of Cornubian granites at ~270 Ma. Examination of the spread in measured ages shows some crystals to be younger than stratigraphic age, which suggests minor post-depositional annealing. Annealing is more obvious in the Triassic Mercia Mudstone Group (Carnian) at Weston-Super-Mare, where the FT age of 2 3 0 + 2 9 M a (29%) is within error of stratigraphic age. However, the radial plot shows a number of individual crystals with ages significantly below stratigraphic age (Fig. 4b), the youngest crystal ages being between 130 and 150 Ma. The mean track length of ~13 lam and standard deviation of ~1.5 lam are consistent with minor post-depositional annealing. By forward modelling using the annealing algorithm of Laslett et al. (1987), bestfits between predicted and measured FT age and track length parameters are found when a maximum post-depositional annealing temperature of ~80 + 5~ is used, although several solutions are available for the timing of the maximum palaeotemperature (Figs 5a & b). Additional stratigraphic information may constrain this time further. At the end of the Triassic, rifting ceased and sedimentation became marine until the late Jurassic, when conditions were brackish. Deposition of continental deposits in the Lower Cretaceous was followed by the Aptian/Albian transgression which, in Devon, led to the deposition of Upper Greensand unconformably on Permian strata. This provides an additional constraint
48
A. CARTER E T AL.
Fig. 4. (a) Plot of the Dawlish Sandstone Formation (Tartarian); despite an FT age (2564-19Ma) within error of the stratigraphic age, the plot shows some crystals below stratigraphic age, suggesting, qualitatively, that there has been post-depositional annealing. (b) Plot from the Mercia Mudstone Group, showing a similar distribution, although stratigraphically the sample is some 30 Ma younger. (c) Plot of Sherwood sandstone where all apatite ages are younger than stratigraphic age. for the FT data, indicating that the Permian and Triassic rocks were at near-surface temperatures in the Lower Cretaceous. As the Tertiary succession, estimated at some 750m (Stoneley and Selley 1986) would appear insufficient to cause the observed annealing of apatites in the Permo-Triassic samples, maximum temperatures must have been reached between 200 and 120 Ma. Accordingly, better constrained thermal modelling shows best-fits of predicted and observed data with peak temperatures of 80 4- 5~ reached between 140 and 120 Ma, with rapid cooling below 60~ coincident with the onset of the Aptian/Albian inversion. Fission track data from within the Wessex Basin are confined to two well samples from the Sherwood Sandstone Formation (Table 1). Significant post-depositional annealing is observed in both the Wytch Farm (1613 m) and Marchwood (~1700 m) samples, which have similar FT ages of 157-t-7Ma (13%) and 150 4-8 Ma (16%), with short mean lengths of 10.44 gm and 9.75 lam, respectively. Currently the samples are at depths where minor annealing is taking place, although this has had little effect in obscuring the last major annealing event, partly because the samples have only recently reached these depths. Maximum temperatures of ~ 9 5 + 5~ in lower Cretaceous times are suggested from best-fit modelling of the FT data, which would
THERMAL HISTORIES OF BASINS FROM FISSION TRACK ANALYSIS
49
Fig. 5. The procedure by which FT data undergo forward modelling using the Monte Carlo approach. In (a), 200 simulations predict FT parameters which are compared with the measured data. Best fits, seen in (b), indicate the time-temperature combinations needed to produce the measured data. Where possible, stratigraphic constraints are used to reduce the range of best fits. (e) The time-temperature paths are restricted to boxes that reflect estimated Mesozoic burial, Aptian/Albian unconformity followed by possible Tertiary burial and erosion. (d) The best fits indicate a much narrower time span during which maximum heating (,-~80 4-5~ may have taken place.
50
A. CARTER E T AL.
suggest the subsequent removal of at least 1 km of Late Jurassic-Cretaceous overburden, perhaps during the Aptian/Albian inversion. This would agree with the observation based on organic maturation indicators that some 1200m of Upper Jurassic and Lower Cretaceous beds seen in the Swanage-Kimmeridge area are missing at Wytch Farm (Selley & Stoneley 1987). Examination of the FT radial plots shows the youngest crystals to have ages of ~115-120 Ma, approximately coincident
Fig. 6. Situated above the burial history graph for the L. Lias and Kimmeridge Clay at Wytch Farm is a radial plot for the Sherwood sandstone. The spread in crystal ages is confined to the time interval between deposition and maximum burial. Furthermore, no apatites are observed younger than the cooling event that occurred during the Albian.
THERMAL HISTORIES OF BASINS FROM FISSION TRACK ANALYSIS
51
with the apparent timing of inversion and erosion. This coincidence is highlighted in Fig. 6, which shows the radial plot for the Wytch Farm data tied to the burial history advanced by Selley & Stoneley (1987). Fission track apatite data from southwest England Permian and Triassic samples today at outcrop thus indicate post-depositional annealing consistent with maximum temperatures of <80+5~ reached during the Late Cretaceous. At present-day geothermal gradients (~30~ the depth of burial would have been some 2-2.5 km, with up to 3 km towards the centre of the basin.
Midlands Early Triassic sedimentation in the central Midlands was concentrated in the northwest and western parts of the region, aided by rapid subsidence and contemporaneous rift faulting. Subsequently, sedimentation spread progressively eastwards towards the stable London Platform, accompanied by a transition from
Fig. 7. Simplified geological map of the central Midlands showing sample localities and FT central ages (Ma).
52
A. CARTER ET AL.
continental fluvial to marine littoral deposits. Outcrop samples were collected for F T analysis principally from the Sherwood Sandstone Group, but in a few cases also from the Mercia Mudstone Group. These new samples supplement those of Green (1989a) by extending the investigated area further west and south in the Worcester Graben, and linking with the Irish Sea and Cheshire Basin Triassic data reported by Lewis et al. (1992b). The sample localities and measured FT ages are shown in Fig. 7, together with the published data of Green (1989a) from the northern part of the Midlands. The new FT data and precise sample locations are presented in Table 1. In a regional FT survey of the East Midlands, Green (1989a) proposed that rocks currently at outcrop had previously been heated to temperatures between 70 and 100~ from which cooling took place in the Late Cretaceous/Early Tertiary (80-60 Ma). Heating of the Triassic rocks within this region (with one exception) was slightly more restricted by Green to temperatures of 70-90~ Our new Triassic data, detailed in Table 1 and displayed in Fig. 7, show little variation in FT age throughout the area, being typically ~ 2 1 0 + 2 0 M a (<10%), younger than the inferred sample stratigraphic ages, and thus suggesting some post-depositional annealing. Mean track lengths are ~12.5 lam with standard deviations of ~1.8 jam. Estimated palaeotemperatures are 75 + 10~ in agreement with the values proposed by Green (1989a). Examination of the radial plots shows that the youngest grain ages cluster close to 150 Ma, which provides a probable older limit for the timing of the palaeotemperature maximum. Forward modelling of the data provides best,fits of predicted and measured data, which indicate that temperatures could have peaked at any time between 150 and 70 Ma, followed by progressive cooling. N o r t h e r n E n g l a n d , C h e s h i r e a n d Irish S e a B a s i n s The fission track data of Green (1989a) and Lewis et al. (1992b) have shown that, after deposition of Triassic sediments in discrete basins, there followed a period of regional subsidence accompanied by several kilometres of burial. Green reported FT ages in 12 Triassic samples from the central and eastern Midlands that ranged from 348 4-23 to 109+ 12Ma, with mean track lengths of 12.5 and 10.1 l.tm, noting that the younger ages accompanied short mean lengths and older ages longer mean lengths. While some samples have FT ages older than their depositional ages, the length data indicate moderate post-depositional annealing temperatures between 70 and 90~ In the Vale of Eden, Cheshire and Irish Sea Basins, Lewis et al. identified severe post-depositional annealing with maximum palaeotemperatures averaging 80-110~ in some 50 Triassic sediment samples. The spread in FT age between 172-4- 18 and 50 + 4 Ma in the Vale of Eden and between 246 • 16 and 73 + 6 Ma in the Cheshire Basin directly reflects these higher temperatures. In essence, the youngest ages give the time closest to the peak of heating; although many samples have been totally reset, the reduced mean lengths of some indicate that the measured age does not provide the time of cooling, but rather is the product of a moderate exhumation. An important feature of the two previous studies in northern England and the Midlands is that virtually all rocks at outcrop have experienced post-depositional annealing requiring temperatures >70~ Triassic and older rocks from these areas are characterized by similar age and track length distributions, indicating that they have shared similar post-Triassic thermal histories. The evidence from the most
THERMAL HISTORIES OF BASINS FROM FISSION TRACK ANALYSIS
53
severely annealed samples is that temperatures peaked in the Late Cretaceous to Early Tertiary. Scotland
Rocks of Permian and Triassic age are exposed in many parts of Scotland, associated with fault-bound, intermontane basins of the New Red Sandstone continent. On the west coast of Scotland their distribution is controlled by a series of NE-SW- and NNE-SSW-trending fault systems that include the Great Glen and Minch faults. Two main areas in Scotland where Triassic rocks are exposed have been investigated previously using the FT technique: Isle of Lewis (Minch basin) and Skye and Raasay (Sea of Hebrides basin). Isle of Lewis A consistent regional picture of FT ages (west of the Moine thrust) has been documented recently by Lewis et al. (1992a), who showed FT apparent ages clustering around 300Ma. The FT age of 292 + 18 Ma (16%) for the Triassic Stornoway conglomerate from the Isle of Lewis comprises individual grain ages that range from 500 to 200 Ma. The youngest crystals infer that some post-depositional heating has taken place, and modelling both the track length and mean age for this sample indicates that temperature has not exceeded ~75~ during the Mesozoic and Cenozoic. Applying a geotherm of 30~ km -1 suggests a maximum burial depth of 2 km, which would be consistent with unpublished FT data for all of the northern Scottish highlands. Skye and Raasay Outcrop samples from the Sea of Hebrides and surrounding regions have been shown to belong to one of two distinct FT apatite age groups, either ~50 Ma or ~300 Ma (Lewis et al. 1992b). The 50 Ma group is interpreted to be the product of exposure to temperatures > 110~ derived from emplacement of the Tertiary igneous complex on Skye, thus obscuring the record of pre-Tertiary thermal history in the Triassic rocks of Skye and Raasay. However, outside the Tertiary igneous thermal influence, FT analysis of the Mid-Jurassic Bearreraig sandstone (Lewis et al. 1992a) indicates that temperatures have not been significantly above 50~ since deposition, a conclusion supported by the organic chemistry from MidJurassic shales (Thrasher 1992). These data help constrain the depth and temperature which the underlying Triassic strata have experienced: since on southern Skye and Raasay little more than ~ 3 0 0 m of Lower to Middle-Jurassic sediments were deposited, the underlying Triassic would have experienced only slight track annealing at temperatures ~60~ since deposition which, persisting with a geotherm of 30~ km -1, infers a maximum cover depth of 1.5 km.
Discussion The new and published FT data reveal that the Permian and Triassic sediments today at outcrop reached maximum temperatures at times during the Cretaceous and earliest Tertiary. In southwest England temperatures peaked at ~80~ between 140 and 120 Ma. Further north, similar temperatures are evident within the Worcester Graben, although the time at which they were reached is less clear (between 150 and 70 Ma). The timing of maximum temperature is better defined in the Cheshire and
54
A. CARTER E T AL.
Irish Sea Basins where Permian and Triassic sediments experienced greater maximum temperatures of between 80 and 110~ causing total track resetting in some cases. Such temperatures were reached around the end of the Cretaceous with cooling to below 60~ during the Tertiary. In northwest Scotland, the Triassic currently at outcrop has experienced more moderate temperatures, of the order 6075~ reached during the earliest to middle Cretaceous. Comparison of amounts of burial and subsequent exhumation between the different regions (using a geothermal gradient of 30~ -1) reveals maximum depths of _>3 km in the unexposed basin centre in the southwest, and <2 km at the basin margins. In exposed basin centres in northwest England, burial depths of 3 km or more were achieved compared with <2 km maximum burial at the basin margins, and this is similar to the levels of burial seen in the Midlands. In northwest Scotland, maximum burial depth has not exceeded ~1.5 km since the Triassic. Clearly, Permian and Triassic sediments at basin margins throughout Britain have experienced comparable kilometre-scale burial, with concomitant heating and subsequent exhumation, although the precise timing of maximum temperature differs from basin to basin. However, basin centres show significant variation in exhumational history. In the North Minch, Sea of Hebrides, and Worcester and Wessex Basins, the centres have experienced limited exhumation, the Triassic strata remaining subsurface, in contrast to much greater exhumation of the Cheshire Basin and Vale of Eden, where Triassic rocks are now exposed. FT data from the central unexposed Triassic rocks of the Wessex Basin suggest maximum temperatures and burial histories similar to the outcropping Triassic of the northwest England basins. The thermal histories of the Cheshire Basin and Vale of Eden thus appear to be anomalous within the UK. A possible explanation may lie in the Late CretaceousEarly Tertiary timing of peak temperatures in north-west England, which contrasts with the early to mid-Cretaceous timing elsewhere in Britain. One of the most favoured explanations for a K/T boundary peak in palaeotemperatures is the initiation of rifting of the northeast Atlantic and its associated magmatism - the Tertiary igneous activity. The possible thermal effects of intrusives on surrounding rocks resulting in a sharp rise in geothermal gradients, via hot mobile fluids as well as by conduction, has already been investigated using fission tracks. On Skye, the largest of the Tertiary igneous centres, Lewis et al. (1992a) found that any fission track annealing related to intrusion is confined to a 10 km radius about the centre. Elsewhere in Britain and Ireland, samples analysed adjacent to and at intervals away from Tertiary dykes indicate that any elevation in temperature associated with intrusion is local (Green 1989a; Lewis et al. 1992b; Carter unpublished data). However, this does not preclude a regional increase in geothermal gradient associated with the Tertiary igneous activity. Consideration was given to this possibility by Lewis et al. and Green, who concluded that, although hot fluids may have been responsible for the palaeotemperatures measured in their studies, the geothermal gradients remained close to 30~ km -~, similar to those of today. The palaeotemperatures seen in northwest England, equivalent to _>3 km of burial (at 30~ km-1), cannot be explained by invoking either higher geothermal gradients or hot fluids. Accordingly, the measured FT data are explained in terms of heating as a consequence of additional kilometre-scale burial which has subsequently been removed during the latest Cretaceous of Tertiary. This amounts to the removal of some 3 km of overburden from parts of the northwest England during the Tertiary,
THERMAL HISTORIES OF BASINS FROM FISSION TRACK ANALYSIS
55
and _<2 km elsewhere in Britain. Given the relatively close proximity of the regions studied, a significantly variable exhumation rate due to climate and sea-level change is unlikely, and thus the greater exhumation in northwest England must be due to a larger tectonic component than elsewhere in Britain.
Conclusions Previous regional FT studies (Green 1986; 1989a; Lewis et al. 1992b) have identified Late Cretaceous burial affecting northern and central England, followed by Tertiary exhumation. The thermal history information obtained from Permian and Triassic sediments in this study reveals that the timing of maximum burial is not restricted to the Late Cretaceous. In southwest England especially, maximum temperatures were reached significantly earlier, during the Lower Cretaceous. The Permian and Triassic of northwest England experienced greater burial and subsequent tectonically induced Tertiary exhumation than is seen elsewhere onshore in Britain. The levels of burial experienced by outcropping Permian and Triassic sediments require significant additional thicknesses of Jurassic and Cretaceous sediments. The use of the fission track method to decipher thermotectonic history is especially valuable in red-bed sequences where the lack of organic material precludes the use of standard reflectance techniques. Support for this work has been provided by a BP Research Grant, and NERC Research grants GR3/7068 and 8291. Susie Garnish and Vijay Vohora are thanked for their perseverance in extracting the elusive apatites. Certain of the data discussed in this article have been reported previously by Paul Green and Cherry Lewis.
References ANDERSON, T. B., PARNELL, J. & RUFFELL, A. H. 1995. Influence of basement on the geometry of Permo-Triassic basins in the northwest British Isles. In: GLENNIE, K. W. (ed.) Permian and Triassic Rifting in Northwest Europe. Geological Society, London, Special Publication, 91, 103-122. AUDLEY-CHARLES, M. G. 1970. Stratigraphical correlation of the Triassic rocks of t h e British Isles. Quarterly Journal of the Geological Society, London, 126, 19-47. CHADWICK, R. A. & EVANS, D. J. 1995. The timing and direction of Permo-Triassic extension in southern Britain. In: BOLDY,S. A. R. (ed.) Permian and Triassic Rifting in Northwest Europe. Geological Society, London, Special Publication, 91, 161-192. COWARD, M. P. 1995. Structural and tectonic setting of the Permo-Triassic basins of northwest Europe. In: BOLDY,S. A. R. (ed.) Permian and Triassic Rifting in Northwest Europe. Geological Society, London, Special Publication, 91, 7-40. DORE, A. G. 1992. Synoptic palaeogeography of the Northeast Atlantic Seaway: late Permian to Cretaceous. In: PARNELL,J. (ed.) Basins of the Atlantic Seaboard. Geological Society, London, Special Publication, 62, 421-446. DUDDY, I. R., GREEN, P. F. & LASLETT, G. M. 1988. Thermal annealing of fission tracks in apatite 3: variable temperature behaviour. Chemical Geology (Isotope Geoscience Section), 73, 25-38. GALBRAITH, R. F. 1990. The radial plot: graphical assessment of spread in ages. Nuclear Tracks, 17, 207-214. 1992. Statistical models for mixed ages. Abstracts of the 7th International Workshop on Fission Track Thermochronology. University of Pennsylvania, Philadelphia. GLEADOW, A. J. W., DUDDY, I. R., GREEN, P. F. & LOVERING, J. F. 1986. Confined fission track lengths in apatite- diagnostic tool for thermal history analysis. Contributions to Mineralogy and Petrology, 94, 405-415.
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GLENNIE, K. W. 1990. Rotliegend sediment distribution: a result of late Carboniferous movements. In: HARDMAN, R. F. P. & BROOKS, J. (eds) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geological Society, London, Special Publication, 55, 127-138. GREEN, P. F. 1986. On the thermotectonic evolution of Northern England: evidence from fission track analysis. Geological Magazine, 123, 493-506. 1989a. Thermal and tectonic history of the East Midlands shelf (onshore UK) and surrounding regions assessed by apatite fission track analysis. Journal of the Geological Society, London, 146, 755-773. 1989b. The relationship between track shortening and fission track age reduction in apatite: combined influences of inherent stability, annealing anisotropy, length bias and system calibration. Earth and Planetary Science Letters, 89, 335-352. , DUDDY, I. R., GLEADOW, A. J. W. & LOVERING, J. F. 1989a. Apatite fission track analysis as a palaeotemperature indicator for hydrocarbon exploration. In: NAESER, N. & MCCULLOCH, T. H. (eds) Thermal History of Sedimentary Basins: Methods and Case Histories, Springer-Verlag, New York, 181-195. HURFORD, m. J. & GREEN, P. F. 1982. A user's guide to fission track dating calibration. Earth and Planetary Science Letters, 59, 343-354. & -1983. The zeta age calibration of fission track dating. Isotope Geoscience, 1, 285-317. LASLETT, G. M., GREEN, P. F., DUDDY, I. R. & GLEADOW, A. J. W. 1987. Thermal annealing of fission tracks in apatite 2: a quantitative analysis. Chemical Geology (Isotope Geoscience Section), 65, 1-13. LEEDER, M. R. & HARDMAN, R. F. P. 1990. Carboniferous geology of the southern North Sea Basin and controls on hydrocarbon prospectivity. In: HARDMAN, R. F. P. & BROOKS, J. (eds) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geological Society, London, Special Publication, 55, 87-105. LEWIS, C. L. E., CARTER, A. & HURFORD, A. J. 1992a. Low-temperature effects of the Skye Tertiary intrusions on Mesozoic sediments in the Sea of Hebrides Basin. In: PARNELL, J. (ed.) Basins of the Atlantic Seaboard. Geological Society, London, Special Publication, 62, 175-188. , GREEN, P. F. G., CARTER, A. & HURFORD, A. 1992b. Elevated palaeotemperatures throughout northwest England: three kilometres of Tertiary erosion? Earth a;~dPlanetary Science Letters, 112, 131-145. SELLEY, R. C. & STONELEY, R. 1987. Petroleum habitat in south Dorset. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 139-145. SRIVASTAVA, S. P. & VERHOEF, J. 1992. Evolution of Mesozoic sedimentary basins around the North Central Atlantic: a preliminary plate kinematic solution. In: PARNELL, J. (ed.) Basins of the Atlantic Seaboard. Geological Society, London, Special Publication, 62, 397-420. STONELEY, R. & SELLEY, R. C. 1986. A Field Guide to the Petroleum Geology of the Wessex Basin. Department of Geology, Imperial College, London. THRASHER, J. 1992. Thermal effects of the Tertiary Cullins Intrusive complex in the Jurassic of the Hebrides; an organic geochemical study. In: PARNELL, J. (ed.) Basins of the Atlantic Seaboard. Geological Society, London, Special Publication, 62, 35-49. WARRINGTON, G. & SCRIVENER, R. C. 1990. The Permian of Devon, England. Review of Palaeobotany and Palynology, 63, 263-272. -
From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe,
Geological Society Special Publication No. 91, pp. 57-85
Dating, correlation and stratigraphy of the Triassic sediments in the West Shetlands area T . S W I E C I C K I l, P. W I L C O C K S O N l, A . C A N H A M G. WHELAN 3 & H. HOMANN 4
2,
1Amerada Hess Ltd, 33 Grosvenor Place, London SW1X 7HY, UK 2Geochem Group Ltd, Chester Street, Chester CH4 8RD, UK 3IEDS Ltd, Tetbury, Glos GL8 8RX, UK (Current address." Reservoir Research Ltd, Maryhill Road, Glasgow G20 OAB, UK) 4Atlas Wireline Services, 455 London Road, Isleworth, Middx TW7 5AB, UK Abstract: Very thick sequences, up to 25 000 ft, of Permo-Triassic sediment are preserved within the Papa and East Solan Basins, in the West Shetlands area. The active margin of this Permo-Triassic basin lay along the West Shetland Spine Fault. Due to severe erosional truncation, the position of the westerly passive margin cannot be delineated. The Triassic basin fill, referred to the Papa Group, has been proven by drilling to be at least 8000ft thick. A combination of palynological, log and sedimentological analyses have allowed the succession to be informally subdivided into lithostratigraphic units. The oldest, of earliest Triassic age, is the Otter Bank Shale Formation deposited in a coastal/alluvial plain setting. This is gradationally succeeded by the coarse-grained Otter Bank Sandstone Formation comprising sediments derived from the interdigitation of sheetflood, braidplain and aeolian environments of deposition. These represent the initial erosional products derived from the uplifted, rifted basin margin. This major phase of Early Triassic rifting is believed to have taken place during the Scythian. The succeeding Foula Sandstone Formation marks the establishment of predominantly axial braidplain systems, deposited during a period of intermittent but waning tectonic influence during Middle to Late Triassic times. The Papa Group is referred to the New Red Sandstone Supergroup. This paper details the results of investigations into the dating, correlation and stratigraphy of the Triassic sediments in the West Shetlands area. Much of the work presented is based around the results of the drilling programme undertaken by Amerada Hess Ltd and co-venturers in this region between 1990 and 1992. The study of the largely continental and often barren sediments of the Triassic basins of the N W European continental shelf has often proven difficult. By the application of a multidisciplinary approach, however, much insight can be gained into the tectonic and depositional history of these sediments.
Regional setting The study area (Fig. 1) lies to the west of the Shetland Islands, being bounded on the southeast by the West Shetlands Platform. To the northwest are developed a series of down-thrown half-grabens and intervening basement highs, the most prominent of which is the Rona Ridge (Fig. 1). To the north, a simple half graben, the West
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T. SWIECICKI ET AL.
Fig. 1. Tectonic elements of the West Shetland continental margin. Shetland Basin, is developed between the Platform and the Rona Ridge, whilst to the southwest major transfer zones trending NW-SE, perpendicular to the structural grain, have been identified (Duindam & Van Hoorn 1987; Booth et al. 1993). Here, a series of superimposed basins of various ages are present, separated by intervening highs (Fig. 2). In the main area of interest, the Papa and East Solan Basins lie between the West Shetland Platform and the Rona Ridge. As can be seen on the regional seismic section (Fig. 3) and the accompanying geoseismic interpretation (Fig. 4), the Papa Basin shows major growth into the West Shetland Spine Fault. Sediments within the basin comprise Permo-Triassic red-beds, as proven by well 205/27-1 which penetrated over 8000 feet of continental clastics before terminating in weathered basalts of probable Permian age. Along the active margin of the New Red Sandstone Basin, some 25 000 ft of sediment (equivalent to 3.6 seconds two-way time on seismic sections) may be preserved. Across the Otter Bank Fault, within the largely Cretaceous Solan Basin (Booth et al. 1993), a thin wedge of Permo-Triassic sediments is preserved beneath Jurassic and Cretaceous strata. The close similarity in stratigraphy argues strongly that, at the time of deposition, the perched sediments of the Papa Basin were contiguous with those of the East Solan Basin. Thus the term Papa Basin is used here to refer to the original extent of the Papa Basin prior to its Early Cretaceous division into the Papa (senso stricto) and East Solan Basins.
STRATIGRAPHY OF THE WEST SHETLANDS AREA
59
Fig. 2. Tectonic elements and exploration in the vicinity of the Solan and Papa Basins.
The uppermost surface of the Permo-Triassic basin-fill is marked by a major angular unconformity with around 20 ~ dip divergence between the New Red Sandstone and the overlying sediments; thus whilst there is an element of depositional thinning of the Permo-Triassic basin fill away from the active margin, this is further enhanced by significant erosion. The distinct wedge-shaped geometry that we see today is at least partially a preservational, as opposed to purely depositional, wedge. Due to this erosional attenuation it is not possible to determine where the passive margin of the former Papa Basin was located; the Rona Ridge is a candidate, although the apparent lack of sediment derived from this margin precludes a definitive judgement. During Triassic times the area lay towards the centre of the Pangean supercontinent, some 15-20 ~ north of the equator, in an area typified by a semiarid climate with seasonal rainfall (Fig. 5). The region was situated close to the line of extensional stress that in the Tertiary was to develop into the North Atlantic rift system (Ziegler 1990).
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Fig. 5. Late Triassic palaeogeography (after Ziegler (1990)).
Lithostratigraphy Recent wells drilled by Amerada Hess Ltd have been extensively cored, with a total of some 1500 ft of Triassic core having been cut. By use of log, palynological and sedimentological analyses supplemented by more esoteric techniques such as palaeocurrent studies and palaeomagnetic analysis, an informal lithostratigraphic subdivision has been erected (Fig. 6) that has proven its worth in predicting stratigraphy ahead of drilling. Whether this scheme has a chronostratigraphic significance cannot yet be established, though palaeomagnetic work may eventually shed light on this. The entire Triassic succession is referred to the Papa Group of the New Red Sandstone Supergroup. Within the Papa Group, the oldest mapable interval comprises fine-grained, often anhydritic mudstones referred to the Otter Bank Shale
STRATIGRAPHY OF THE WEST SHETLANDS AREA
63
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Formation. These are reliably dated as earliest Triassic (Griesbachian) in age. They rest, probably with minor disconformity, on poorly dated sediments of probable Permian age. The Otter Bank Shale Formation grades rapidly upwards into the coarse-clastic dominated Otter Bank Sandstone Formation, which is also considered to be of Early Triassic age. Finally, succeeding the Otter Bank Sandstone Formation, with probable disconformity, is the Foula Sandstone Formation of Middle to Late Triassic age, readily divisible into two broad coarsening-upwards parasequences. Close to the active margin, coarse-grained alluvial fan material is anticipated to have been deposited, analogous to the Stornoway Formation described by Steel (1974) from the Hebrides.
Biostratigraphy and chronostratigraphy Dating sediments of Triassic age has long proved problematical to biostratigraphers. The potential for preservation of both micro- and macrofossils is considerably reduced by the predominantly non-marine environment and extreme oxidizing conditions which prevailed during much of the Triassic. Thick Triassic sections are therefore often entirely barren of all fossil groups. Working with sparse assemblages has obvious limitations and the rare recovery of long-ranging taxa may often not help to provide age determinations. In addition, the absence of taxa cannot be used as evidence for the absence of a chronstratigraphic unit, as is sometimes possible when working with diverse and abundant assemblages. The rate of recovery of palynomorphs in the West Shetlands area is not unusual for sand-prone facies of the Triassic. The lowermost sequence, the Otter Bank Shale Formation which is predominantly claystone, has yielded rich and relatively diverse assemblages, permitting well constrained age determination. The following section is a chronostratigraphical discussion, from oldest to youngest, based on palynological analyses carried out over this section (Fig. 7). Analyses have been carried out on samples from cores and sidewall cores where available, augmented elsewhere by cuttings samples.
The Otter B a n k Shale Formation The Otter Bank Shale Formation has yielded a palynological assemblage which is extremely distinctive in character, indicating a restricted Griesbachian (earliest Triassic) age (Fig. 8). This assemblage is often completely dominated by the taxon Tympanicysta stoschiana Balme (also known as Chordecysta chalasta Foster), which often exhibits chain formation. The affinity of this form is uncertain although it appears to be most closely comparable with certain fungal cysts (Balme 1979; Visscher & Brugman 1986). In the literature it has sometimes been referred erroneously to the microforaminifera. The first downhole appearance of T. stoschiana may be marked by a few (reworked?) examples, below which a flood of specimens can comprise up to 50% or more of the assemblage. In these sections, T. stoschiana is
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67
associated with large numbers of taeniate bisaccate pollen, particularly Lunatisporites spp., Striatoabiettes spp. and Protohaploxypinus spp., together with trilete spores such as Kraeuselisporites spp. and Lundbladispora spp. All these taxa make up a characteristic assemblage which is easily identifiable. The base of this 'Griesbachian assemblage' is sharply defined; the lower part of the Otter Bank Shale Formation is often barren (confirmed where sidewall core material has been used). Thus it seems likely that the 'Griesbachian assemblage' occurs within a discrete horizon or horizons within the Otter Bank Shale Formation. It is possible that the lowest part of the Otter Bank Shale Formation is Permian in age, and some evidence for this is provided by the occurrence of the Permian taxa Vittatina sp. and Lueckisporites virkkiae Potonie and Klaus, below the Otter Bank Shale Formation. This 'Griesbachian event' is recognized worldwide, notably in East Greenland where it was first described by Balme (1979) and subsequently by Piasecki (1984), in the Barents Sea region (Hochuli et al. 1989), and in the Canadian Arctic (Utting 1985). It has also been recorded in the Alps (Visscher & Brugman 1986) whilst Foster (1979) recorded the event in Australia. The event is also present in Triassic sediments in the Central North Sea. Balme (1979) divided the East Greenland succession into a Permian 'Vittatina complex', an earliest Griesbachian 'Protohaploxypinus assemblage' and an early Griesbachian 'Taeniasporites assemblage'. The latter most closely resembles the assemblage recovered in the West Shetlands area, although Balme only recorded abundances of T. stoschiana up to 9%. This Tympanicysta stoschiana-dominated assemblage has been recorded with acritarchs in other parts of the world, e.g. East Greenland and Australia, although in the West of Shetland no marine indicators have been recovered. In East Greenland ammonites (Otoceras and Glyptophiceras) co-occurring with the Anchignathodus typicalis conodont fauna are recorded in the assemblage, and thus provide a well constrained early Griesbachian age determination for the major part of the Otter Bank Shale Formation.
The Otter Bank Sandstone Formation The age of the Otter Bank Sandstone Formation is the most poorly constrained of the three formations. ?Cyclotriletes sp. and ?Angustisulcites sp. are recorded within this unit (Fig. 8). Angustisulcites has its stratigraphic extinction in the Carnian and Cyclotriletes is know from the Middle and Early Triassic. A poorly constrained Middle to Early Triassic is therefore indicated for this unit. The presence of Densoisporites nejburgii (Schulz) within the Otter Bank Sandstone Formation indicates a possible Anisian, but more probably a Scythian age, as this form has its stratigraphic extinction point at the base of the Anisian but becomes a more common component of Scythian assemblages (Visscher & Brugman 1986).
Foula Sandstone Formation Palynological evidence from this sand-prone formation is sparse. The presence of poorly preserved examples attributable to the genus Camerosporites have been recovered, suggesting a Carnian or older age. In the upper levels of the Foula
68
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STRATIGRAPHY OF THE WEST SHETLANDS AREA
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Sandstone Formation C. secatus Leschik (Fig. 9) is recorded, whilst towards the base C. pseudoverracatus Scheuring is present. However, the preservation of these specimens is very poor and identifications are tentative and may prove to be unreliable. The presence, however, of a well preserved specimen of Echinitosporites iliacoides Schulz and Krutzsh near the base of this formation, together with Paracirculina sp., indicates a Ladinian-restricted age (Hochuli et al. 1989) and suggests that deposition of the Foula Sandstone Formation probably began in the Ladinian.
Sedimentology Sedimentological analysis of the Papa Group has been undertaken on over 1500 ft of core. The Papa Group comprises sediments deposited in a variety of continental, predominantly fluvial, environments described below.
Facies associations The inferred relationships between depositional environments (facies associations) and facies are presented in Fig. 10. Four environmentally related facies associa-tions are recognized: braided fluvial systems, dry sandflat, damp sandflat and playa systems. The most volumetrically significant are the deposits of braided fluvial systems, which are dominated by pebbly and sandy channel-fill deposits, together with subordinate abandonment fines and ephemeral channel deposits. Sequences assigned to the dry sandflat environment are dominated by dry aeolian sandsheet deposits, together with subordinate aeolian dune and damp aeolian sandsheet facies. The damp aeolian sandflat association is dominated by damp aeolian sandsheet deposits, together with subordinate dry aeolian sandsheet and playa margin deposits. The playa system is highly variable, consisting of inter-bedded playa margin and ephemeral fluvial channel/sheetflood deposits, together with playa lake, dry and damp aeolian sandsheet and very minor vegetated flood-plain deposits. Individual facies are detailed below.
Braided fluvial channel systems Sequences interpreted as the deposits of braided fluvial channels predominate in the cored intervals and are subdivided into four facies that reflect variations in depositional processes. (1) Channel-fill deposits comprise fine- to medium-grained sandstones that are often pebbly with rounded granule to small pebble-grade clasts. Pebble-grade conglomerates are rare in the Otter Bank Sandstone, although they increase in importance in the Foula Sandstone. Pebbles rarely exceed 1 inch in length, and consist of intraformational mudstone clasts or extraformational vein quartz and alkali feldspar, with subordinate plutonic igneous and metamorphic crystalline rock fragments. Two subfacies are identified on the basis of different sedimentary structures, grain size and detrital clay content. Sub-facies 1. Dune-bedded to bar-scale fluvial channel-fill units show erosive bases, with multiple internal erosion surfaces that indicate composite or compound bedforms. Pebbly channel-base lags generally overlie erosion surfaces, often showing \
70
T. SWIECICKI ET AL.
Fig. 10. Depositional environments and facies classification for the Papa Group. long-axis alignment and imbrication of clasts. Lag deposits are locally overlain by true conglomerates, but generally fine upwards into pebbly sandstones with aligned granule or pebble-strewn foresets, suggesting a gravel bar-overpassing mechanism (cf. Allen 1983). Dune to bar-scale trough cross-stratified and planar cross-stratified sets (Fig. 11), up to c. 5 ft thick, are present throughout. These indicate the presence of in-channel sinuous and straight-crested dune or bar bedforms, respectively. Asymptotic crossstratification is common, showing an upward gradation from sandy, low-angle toesets to more pebble-rich, higher-angle foresets. Water escape structures are common. They are inferred to represent the slumping of water-saturated sediment into interbar areas in response to bar emergence. Gross upward-fining sequences are recorded, up to c. l0 ft thick, interpreted as compound channel-fills. These suggest l0 ft as a minimum channel depth. Sub-facies 2. Flat-bedded fluvial channel-fill sets are characterized by sharp, erosive, commonly pebble-lagged bases and sharp tops, commonly marked by the erosive base of an overlying channel. Basal pebble lags are dominated by imbricate intraformational claystone clasts. Internal low-angle reactivation or truncation surfaces are common. Low-angle lamination or stratification is the dominant sedimentary structure, locally occurring with faint micaceous partings, reflecting lower-flow-regime plane bed accretion, possibly on interbar channel floors during episodes of bar emergence. Isolated, pebble-strewn laminae occur locally, showing pebble long-axis alignment parallel to lamination. Minor ripple cross-lamination and convolute lamination are present locally. Flat-bedded channel-fills are more argillaceous than dune-bedded varieties and of fine, rather than fine to medium, sand grade. Pebbly channel-fill deposits represent the gravel and sand fill of a perennially flowing, braided fluvial system. Channels were subject to episodic accretion,
STRATIGRAPHY OF THE WEST SHETLANDS AREA
71
Fig. 11. Core photographs of braided pebbly channel-fill sandstones: Foula Sandstone Fm. (left) and Otter Bank Sandstone Fm. (right).
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alternating with episodes of erosion and downcutting of channel bases, associated with channel and bar migration. The accretion and/or preservation of fine-grained argillaceous abandonment deposits is recorded locally. Evidence for extended periods of subaerial exposure within channels is lacking, although bar-top emergence may account for the presence of slump structures and minor aeolian deflation lags. A modern analogy for this fluvial channel type is the Donjek River, Yukon (Williams & Rust 1969; Rust 1972: Miall 1977). A perennial system is suggested, albeit one with significant variations in discharge. This accords with the observed preservation of both high (dune-bedded) and low (flat-bedded) stage deposits in what is interpreted as the depositional products of a single fluvial system. (2) Sandy channel-fill deposits range from fine- to medium-grade sand and show moderate sorting. Units are erosively based, with internal erosion surfaces suggesting compound bedforms. Thin, pebbly channel-base lags are recorded, mainly comprising intraformational claystone clasts which locally show pebble imbrication and clast alignment. Trough cross-stratification, (Fig. 12), in sets up to c. 5 ft thick, predominates over planar cross-stratification, indicating the migration of sinuous and straight-crested dune or bar bedforms, respectively. Rare mica-draped low-angle lamination reflects lower-regime plane bed accretion. Convolute lamination, water injection structures and slump deformation are locally common, particularly towards the base of the Otter Bank Sandstone. As with the pebbly channel-fills, these deformation features are interpreted as representing bedform instability during episodes of low- or falling-stage discharge. This facies records sand deposi-tion in braided fluvial channel systems with a sparse granule and pebble-grade bedload. The similarities between pebbly and sandy channel-fills suggest they were deposited in similar fluvial systems, with the variation in clast component reflecting either changes in source area or in the transporting capacity of the system. The closest modern analogy for this channel type is the South Saskatchewan River (Cant 1978). (3) Ephemeral channel/sheettlood deposits range from fine sand to pebble grade, with a mean of fine-medium sand, and sorting is poor to moderate. Granule and pebble-grade intraformational claystone clasts are common, occurring as basal lags or scattered throughout the beds. Extraformational clasts are present in places and are predominantly composed of vein quartz and alkali feldspar. Detrital clays are locally abundant, particularly near the finer-grained, upper parts of beds, where clay flasers are developed. Ephemeral channel or sheetflood deposits (Fig. 13) comprise single, erosively based upward-fining units, less than 4ft thick, usually c. 2ft thick. Low-angle lamination predominates, with frequent low-angle reactivation or truncation surfaces reflecting fluctuating upper- and lower-flow-regime plane bed accretion, with erosion or non-deposition. Lower-flow-regime plane beds are clearly identified by wispy clay drapes and micaceous partings. Subordinate trough and planar crossstratification are also developed, indicating the presence of sinuous and straightcrested dune-scale bedforms. Small-scale cross-lamination indicates ripple bedforms which often show a climbing geometry, indicating high sediment supply and deposition rates. Water escape and injection structures, together with convolute lamination, are occasionally present, suggesting rapid rates of deposition with resultant loading-induced deformation. This facies represents deposition from ephemeral fluvial flows which may have been laterally confined to broad, shallow channels (streamfloods) or effectively
STRATIGRAPHY OF THE WEST SHETLANDS AREA
73
Fig. 12. Core photographs of braided sandy channel-fill deposit and Otter Bank Sandstone Fm.
74
T. SWIECICKI E T AL.
Fig. 13. Core photographs of ephemeral channel/sheetflood (left) and channel abandonment deposits (right), Otter Bank Sandstone Fro.
STRATIGRAPHY OF THE WEST SHETLANDS AREA
75
laterally unconfined (sheetfloods). The ephemeral nature of the flows is indicated by the intimate interbedding of these deposits with aeolian successions. The sedimentary structures accord with those described from modern continental environments subject to ephemeral fluvial activity, such as Bijou and Medano Creeks (McKee et al. 1967; Langford & Bracken 1987). (4) Channel abandonment deposits (Fig. 13) comprise argillaceous, poorly to very poorly sorted, very fine sandstones, siltstones and claystones. Intraformational claystone granules and pebbles are locally present, together with rare extraformational clasts. Low-angle lamination predominates, often with clay drapes, suggesting depositon as minor floods in abandoned channels. Lower flow regime is also suggested by the presence of ripple cross-lamination. Climbing ripple crosslamination suggests locally high deposition rates. Episodic higher flow energies are indicated by granule and pebble lags, together with local dune-scale trough and planar cross-stratification, possible indicating minor flood events. Convolute lamination is present locally, as are rare simple, sand-filled subvertical burrows that indicate local colonization of the substrate. Lower bed contacts are usually sharp, rarely erosive or gradational. These deposits probably accumulated in shallow, sluggish or ponded waters remaining in channel reaches after avulsion of the active channel system or during episodes of extremely low flow.
Aeolian systems Sediments interpreted as aeolian deposits generally comprise over 20% of the cored sections and are best developed in the upper parts of the Otter Bank Sandstone Formation, but also occur within the Foula Sandstone Formation. The aeolian deposits identified are believed to have been laid down in interfluve areas subject to ephemeral inundation. Changes in water table elevation, either in response to local flooding or more regional fluctuations, would have controlled substrate mobility during aeolian activity. (1) Aeolian dune deposits comprise clean, moderate to well sorted fne- to mediumgrained sandstones. Small-scale grainfall lamination (Hunter 1977) is the dominant sedimentary structure (Fig. 14), within asymptotic sets up to c. 15 ft in thickness. Occasional wedge-shaped, coarser laminae represent grainflow/sandflow deposits (Hunter 1977; Kocurek & Dott 1981). This cross-stratification is interpreted as aeolian deposition on the lee surfaces of transverse dunes. These sets of crossstratification are locally truncated by reactivation surfaces associated with a change in set orientation. These truncations suggest complex dune forms interpreted as coalesced barchan dunes that formed local dunefields or barchanoid ridges. The most common dune forms, however, were probably isolated barchans. Commonly, the basal surface of aeolian dune sequences is represented by a deflation lag horizon or desiccated surface, particularly where the underlying deposits are playa mudstones. (2) Dry aeolian sandsheet deposits comprise fine- to medium-grained, moderately sorted sandstones. Locally poorly developed lamina-specific bimodal sorting is recognized. Single-grain-thickness lags of coarse sand, granules and pebbles form deflation lag horizons. The sandstones are generally clean, lacking detrital clay matrix or mica. Low-angle small-scale grainfall or aeolian plane bed laminations (sensu Hunter 1977) are the dominant sedimentary structure (Fig. 14). Numerous
76
T. SWIECICKI E T AL.
Fig. 14. Core photographs of damp aeolian sandstone (left) and aeolian dune deposits (right), Otter Bank Sandstone Fm.
STRATIGRAPHY OF THE WEST SHETLANDS AREA
77
low-angle reactivation or truncation surfaces erosively terminate underlying strata, but are concordant with overlying strata. Subordinate, coarser, inversely graded laminae are interpreted as subcritically climbing wind-ripple deposits (Hunter 1977; Fryberger et al. 1979, 1983). Rare wavy or undulose clay-draped adhesion laminae (Kocurek & Fielder 1982) indicate damp substrate conditions transitional with those of damp aeolian sandsheet, probably as zibars (Nielson & Kocurek 1986). The presence of adhesion structures interbedded with wind-ripple and grainfall deposits indicates a continuum between dry and damp aeolian sandsheets, which are often complexly interbedded. Conditions favourable for warm-climate aeolian sandsheet formation include a high water table and/or episodic fooding (Kocurek & Nielson 1986). The numerous interbedded ephemeral channel/sheetflood units provide ample evidence for flooding of the interfluve area. (3) Damp aeolian sandsheet deposits comprise argillaceous, fine- to mediumgrained sandstones that are very poorly sorted, due to high levels of detrital clay matrix. Clay and silt streaks or drapes are common (Fig. 15), often desiccation cracked and curled (Fig. 14). Wavy or crinkly adhesion lamination (Kocurek & Fielder 1982) is the dominant sedimentary structure. Soft-sediment deformation, mainly loading, affects sand-rich laminae. This facies probably represents deposition on flat or low-angle planar sandsheets that were damp at the sediment surface. The development of such damp substrates suggests an elevated water table and/or proximity to a source of flooding.
Lacustrine/floodplain systems Sediments interpreted as lacustrine/floodplain deposits comprise less than 10% of the cored Triassic sections. Playa margin deposits predominate over playa lake and vegetated floodplain deposits. (1) Playa margin deposits are delicately interlaminated sandstones and claystones (Fig. 15). Maximum grain size is generally fine sand, although intraformational silty claystone granules and pebbles are also present locally. Low-angle lamination is the dominant sedimentary structure, resulting from lower-flow-regime plane bed accretion, but loading has resulted in a wavy or undulose appearance. Clay drapes originate from fallout of suspended fines in standing bodies of water, although some clay drapes occur with adhesion structures, indicating the trapping of aeolian dust on a damp substrate. Aeolian deflation surfaces and lags are also present, as are desiccation cracks, rare grainfall and wind-ripple laminae, indicating episodic subaerial exposure. This facies represents predominantly subaqueous deposition dominated by distal sheetfloods, resulting in laminated and rippled sand, with settling of mud from suspension generating clay drapes. The closest modern analogy for such an environment would be a sheetwashed sandflat-playa lake setting such as the Great Sand Dunes National Monument, Colorado, and the Mojave River Wash, California (Langford 1989). (2) Playa lake deposits comprise claystones or silty claystones that form discrete units up to 3 ft thick. Small amounts of very fine to fine sand are locally admixed, resulting in a poor or very poorly sorted sediment. Siderite nodules up to 6 inches in diameter are occasionally present. Sandy, lower-flow-regime plane bed lamination
78
T. SWIECICKI E T AL.
Fig. 15. Core photographs of damp aeolian sandsheet (left) and playa margin deposits (right) Otter Bank Sandstone Fm.
STRATIGRAPHY OF THE WEST SHETLANDS AREA
79
predominates, subsequently modified by loading. Locally, starved sandy unidirectional ripples are also preserved, together with more localized symmetrical waveripple cross-lamination. Sand-filled desiccation cracks are also recorded, often showing ptygmatic folding due to subsequent compaction. Deposition occurred through fallout of fines from suspension in relatively shallow lakes. Wave ripple cross-lamination indicates wind-driven currents, developed in shallow lake waters. Episodic exposure of the lake floor is indicated by the presence of desiccation cracks, infilled by sand that was probably blown across the exposed lake floor. (3) Vegetated floodplain deposits are rare, forming less than 1% of the cored Triassic interval; they are confined to the Foula Sandstone Formation. Fine-grained argillaceous sandstones show low-angle lamination, with ripple cross-laminated tops that resemble ephemeral channel/sheetflood deposits, but differ in showing downward branching rootlets associated with carbonate nodules and brecciated horizons, representing caliche soil profiles. This suggests a degree of floodplain stability and subaerial exposure, associated with colonization by vegetation.
Evolution of the Papa Group The Otter Bank and Foula Sandstone Formations show distinctive facies associations related to different depositional settings. The Otter Bank Sandstone can be subdivided into an upper and a lower unit on this basis. The lower Otter Bank Sandstone was deposited exclusively by braided fluvial systems (Fig. 9). Virtually complete cored sequences show overall upward-coarsening profiles, reflecting the transition from ephemeral channels/sheetfloods, through small-scale sandy channelfills to larger-scale pebbly channel-fills~ This sequence is thus inferred to represent braidplain progradation into the coastal/alluvial plain setting of the Otter Bank Shale Formation. An analogous onshore sequence is identified in the St Bees Shale to St Bees Sandstone transition, which is well exposed in the coastal cliff sections and quarries at Saltom Bay, West Cumbria (Fig. 16). The upper Otter Bank Sandstone Formation comprises interbedded dry and damp sandflat, playa lake and small-scale sandy braided fluvial channels (Fig. 8). A succession of facies, from dry sandflat deposits to ephemeral fluvial deposits, overlain by playa lake deposits, is often repeated. This alternation is interpreted as recording breakthrough of ephemeral channels into a dry sandflat area associated with a low water table. As fluvial floodwaters percolated into the sediment,_the water table rose, resulting in playa lake development. Subsequent cut-off of floodwaters led to eventual lake floor desiccation with a return to dry or damp sandflat deposition. The Foula Sandstone Formation comprises two large-scale upward-coarsening braided fluvial channel systems. In the finer-grained parts of these units, dry or damp sandflat and playa environments predominate (Fig. 9). These sequences record repeated braidplain progradation into a sandflat area. The characteristics of the fluvial system suggest that it was a major system with substantial but fluctuating discharge, probably draining a large hinterland area. An axial fluvial system is proposed, although ephemeral fluvial activity in the interfluve areas may reflect lateral hanging-wall drainage in response to local storm events. The likely stages in the evolution of the Triassic basin-fill are illustrated in Fig. 17. Whether the main braided fluvial systems and the aeolian-dominated sandflats were
80
T. SWIECICKI E T AL.
Fig. 16. The St Bees Shale to St Bees Sandstone transition at Saltom Bay, Cumbria.
coeval is equivocal, but the occurrence of braided channel-fill facies in all subdivisions of the Papa Group suggests that braided fluvial activity persisted throughout, although reduced in intensity during deposition of the upper part of the Otter Bank Sandstone.
Palaeocurrent analysis Within the biostratigraphical and sedimentological framework outlined, two additional techniques have proven useful in elucidating the stratigraphic development of the Triassic Papa Group. All wells were logged using either the high resolution stratigraphic dipmeter (SHDT), formation microresistivity scanner (FMS), or the acoustic imaging (CBIL) log. Borehole imaging log data were used to analyse the flow pattern of the braided river system draining the West Shetland area by identifying current-bedding structures in the predominantly medium- to coarse-grained sediments. Such log data, when calibrated with core data, enabled the accurate measurements of bedding, cross-bedding and co-set dips over thousands of feet of section. These data, once corrected for structural dip removal and well deviation, allowed an accurate estimation of palaeocurrent direction to be determined. Such data can be displayed either in traditional tadpole/roseplot format, which illustrate the major changes in direction of braidplain/sandflats deposition or by means of a dip-vector plot (Fig. 18). This displays the azimuth (i.e. dip direction) of current-bedded features successively from the base to the top of the sedimentary succession. This format visually enhances overall trends and shifts in direction. At the break between
STRATIGRAPHY OF THE WEST SHETLANDS AREA
81
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82
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Fig. 18. Papa Group palaeocurrent analysis. the lower and upper Foula Sandstone, for instance, the easterly flowing lower Foula braidplain system is seen to be terminated by a period of alluvial floodplain deposition before being succeeded by the braidplain system of the Upper Foula Sandstone that flowed from the NE to SW. Such a break, especially where recognizable in more than one well, may be indicative of tilting of the basin in response to a tectonic event, probably attributable to a further phase of rifting.
Palaeomagnetic analyses A pilot palaeomagnetic study has been undertaken, based on a completely cored succession through the Otter Bank Sandstone and basal Foula Sandstone Formations (Fig. 19). Duplicate samples were analysed every 20ft through this interval and samples were successively demagnetized both thermally and using alternating field techniques. Using such techniques, the majority of samples displayed 'cleaned' and stable magnetic inclination values clustered around the expected Triassic value of 53 ~. Of the 54 samples measured, 47 showed sufficient directional stability on demagnetization to define a mean inclination and therefore polarity. Of these, 41 showed a reversed polarity; only two showed stable, replicable normal polarities. Deposition of the Otter Bank Sandstone and Lower Foula Sandstone took place during a predominantly reversed interval. Given the problems of poor biostratigraphic control and correlation, an agreed composite palaeomagnetic stratigraphy for the Triassic is still some way away (Turner pers. comm.). According to published
STRATIGRAPHY OF THE WEST SHETLANDS AREA
83
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information (Turner et al. 1989), these data would indicate that the sediments were post-Griesbachian in age. An Early Triassic (Dienerian to Smithian) age appears most likely for the Otter Bank Sandstone Formation, as the Upper Spathian to Anisian appears to have been dominantly normal polarity. Coupled with the biostratigraphic data, indicating a definitive Griesbachian age for the underlying Otter Bank Shale Formation and a Ladinian age for the overlying lower Foula Sandstone, this may indicate that uppermost Early Triassic (Spathian) and lowest Middle Triassic (Anisian) sediments may be absent. Such an interpretation would be consistent with the break in depositional environment and palaeocurrent direction across the Otter Bank Sandstone/Foula Sandstone boundary, with reversed polarity Ladinian sediments of the Foula Sandstone resting with probable minor disconformity on reversed polarity Early Triassic (Dienerian-Smithian) sediments of the Otter Bank Sandstone Formation.
Summary and conclusions The earliest dated Triassic sediments of the Triassic Otter Bank Shale Formation were deposited in a coastal/alluvial plain setting either during a period of tectonic quiescence or in earliest syn-rift times, when sedimentation was directed away from the rift axis. These sediments were coeval with a marine transgression that prograded rapidly southwards along the proto-North Atlantic rift system affecting East Greenland. The Otter Bank Sandstone Formation comprises erosional products derived from the uplifted, rifted margins of the Papa Basin in Early Triassic times, whilst the succeeding Foula Sandstone Formation represents sediments deposited as a result of the establishment of predominantly axial braidplain systems during a period o f intermittent but waning tectonic influence. This paper has evolved from review of the exploration potential of the West Shetlands area by Amerada Hess Ltd. We would like to thank our colleagues who contributed much to our understanding of this area, in particular Ian Roche, Steve Boldy, Richard Warren and Ian Norbury. Our thanks are also extended to Neit Meadows of Geochem and Phil Copestake of IEDS for much useful discussion. The seismic line shown is used with permission of the following partners in Amerada Hess acreage in the Solan Basin: Aran Energy Exploration Ltd; Arco British Ltd; Brabant Oil Ltd; British Borneo Petroleum; Deminex UK Oil and Gas Ltd; DSM Energy (UK) Ltd; Fina Exploration Ltd; Hardy Oil and Gas (UK) Ltd; Kerr-McGee Oil (UK) plc; Monument Exploration and Production Ltd; Neste Oy, Texaco Exploration Ltd; The Norwegian Oil Co. DNO (UK); Unocal (UK) Ltd. This work was not, however, conducted on their behalf and does not necessarily represent their technical views.
References ALLEN, J. R. L. 1983. Studies in fluviatile sedimentation: bars, bar-complexes and sandstone sheets (low-sinuosity braided streams) in the Brownstones (L. Devonian), Welsh Borders. Sedimentary Geology, 33, 237-293. BALME, B. E. 1979. Palynology of Permian-Triassic boundary beds at Kap Stosch, East Greenland. Medd Gronland, 200, 1-37. BOOTH, J. E., SWIECICKI, T. & WILCOCKSON; P. 1993. The tectono-stratigraphy of the Solan
Basin, West of Shetland. In: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe." Proceedings of the 4th Conference. Geological Society, London, 987-998.
STRATIGRAPHY OF THE WEST SHETLANDS AREA
85
CANT, D. J. 1978. Development of facies model for sandy braided river sedimentation: Comparison of the South Saskatchewan River and the Battery Point Formation. In: MIALL, A. D. (ed.) Fluvial Sedimentology. Canadian Society of Petroleum Geologists Memoir, 5, 627-639. DUINDAM, P. & VAN HOORN, B. 1987. Structural evolution of the West Shetland continental margin. In: BROOKS, J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 765-773. FOSTER, C. B. 1979. Permian plant microfossils of the Blair Athol Coal Measures, Baralaba Coal Measures, and Basal Rewan Formation of Queensland. Geol. Surv. Queensland, 372. FRYBERGER, S. G., ALBRANDT, T. S. & ANDREWS, S. 1979. Origin, sedimentary features, and significance of low-angle eolian 'sandsheet' deposits, Great Sand Dunes National Monument and vicinity, Colorado. Journal of Sedimentary Petrology, 49, 733-746. --, AL-SARI, A. M. & CLISHAM, T. J, 1983. Eolian dune, interdune, sand sheet, and siliciclastic sabkha sediments of an offshore prograding sand sea, Dhahran Area, Saudi Arabia. American Association of Petroleum Geologists Bulletin, 67, 280-312. HOCHULI, P. A., COLIN, J. P. & VIGRAN, J. O. S. 1989. Triassic Biostratigraphy of the Barents Sea Area. Correlation in Hydrocarbon Exploration. Norwegian Petroleum Society, 131-153. HUNTER, R. E. 1977. Basic types of stratification in small eolian dunes. Sedimentology, 24, 361-387. KOCUREK, G. & DOTT, R. H. JR. 1981. Distinctions and uses of stratification types in the interpretation of eolian sand. Journal of Sedimentary Petrology, 51, 579-596. -& FIELDER, C. 1982. Adhesions structures. Journal of Sedimentary Petrology, 52, 12291241. - & NIELSON, J. 1986. Conditions favourable for the formation of warm-climate eolian sand sheets. Sedimentology, 33, 795-816. LANGFORD, R. P. 1989. Fluvial-aeolian interactions: Part 1, Modern Systems. Sedimentology, 36, 1023-1035. -& BRACKEN, B. 1987. Medano Creek, Colorado, a model for upper-flow-regime fluvial deposition. Journal of Sedimentary Petrology, 57, 863-870. MCKEE, E. D., CROSBY, E. J. & BERRYHILL, H. L. 1967. Flood deposits, Bijou Creek, Colorado, June 1965. Journal of Sedimentar Petrology, 37, 829-851 MIALL, A. D. 1977. A review of the braided-river depositional environmental. Earth-Science Reviews, 13, 1-62. NIELSON, J. & KOCUREK, G. 1986. Climbing zibars of the Algodones. Sedimentary Geology, 4 8 , 1-15. PENN, I. E., HOLLIDAY, D. W., KIRBY, G. m., SOBEY, R. A., MITCHELL, W. I., HARRISON, R. K. & BECKINSALE, R. D. 1983. The Larne No 2 Borehole: discovery of a new Permian volcanic centre. Scottish Journal of Geology, 19, 333-346 PIASECKI, S. 1984. Preliminary palynostratigraphy of the Permian Lower Triassic sediments in Jameson land and Scoresby Land, East Greenland. Bulletin of the Geological Society of Denmark, 32, 139-144. RUST, B. R. 1972. Structure and process in a braided river. Sedimentology, 18, 221-245. STEEL, R. 1974. New Red Sandstone floodplain and piedmont sedimentation in the Hebridean Province, Scotland. Journal of Sedimentary Petrology, 44, 336-357. TURNER, P., TURNER, A., RASMO, A. & SOPENA, A. 1989. Palaeomagnetism of PermoTriassic rocks in the Iberian Cordillera, Spain: Acquisition of secondary and characteristic remnance. Journal of the Geological Society, London, 146, 61-76. UTTING, J. 1985. Preliminary Results of Palynological Studies of the Permian and Lowermost
Triassic Sediments, Sabine Peninsula, Melville lslands, Canadian Arctic Archipelago. Current Research, Part B. Geological Survey of Canada, Paper, 85-1B, 231-238. VISSCHER, H. & BRUGMAN, W. A. 1986. The Permian-Triassic boundary in the Southern Alps : a palynological approach. Mem. Soc. Geol. lt., 34, 121-128. WILLIAMS, P. F. & RUST, B. R. 1969. Sedimentology of a braided river. Journal of Sedimentary Petrology, 39, 649-679. ZIEGLER, P. m. 1990. Geological Atlas of Western and Central Europe. 2nd edition, Shell International Petroleum, The Hague.
From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe,
Geological Society Special Publication No. 91, pp. 87-102
Permo-Triassic sedimentary and volcanic rocks in basins to the north and west of Scotland K. H I T C H E N 1, M. S. S T O K E R 1, D. E V A N S 1 & B. B E D D O E - S T E P H E N S 2
1Marine Geology and Operations Group 2M&eralogy and Petrology Group, British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK Abstract: Red-bed sequences of known or inferred Permian and Triassic age are preserved in numerous offshore basins to the north and west of Scotland. These developed following the Late Carboniferous Variscan Orogeny, as compressive uplift was superseded by a tensional stress regime causing the fragmentation of the Variscan foldbelt and its northern foreland. Basin formation persisted through the Permian and Triassic periods, and the sediment infills reflect syn- and post-depositional fault activity. Syndepositional basins take the form of deep, fault-bounded half-grabens whose development was controlled by the extensional reactivation of earlier (Precambrian/Caledonian) thrusts and transfer faults. These basins accumulated wedge-shaped packages of sediment dipping into major basin-bounding faults. In contrast, post-depositional basins are characterized by a parallel-bedded infill which appears to reflect local preservation as a result of later faulting. These basins may be Late or post-Triassic in age. The PermoTriassic rocks were deposited during a period of major regression, with fluviatile, lacustrine and aeolian environments predominating. Principal lithologies include sandstones, that are partly conglomeratic, intercalated with mudstones and claystones. The localized development of carbonates and evaporites indicates some marine influence. Palaeontological data are commonly sparse throughout the succession and generally cannot readily differentiate the two systems. There is limited evidence for both Early Permian and Late Triassic volcanism. The Early Permian volcanism was 'within-plate' and 'continental' in character, and was probably related to the first phase of post-Variscan continental disintegration. The Late Triassic volcanism indicates a subsequent episode of synrift igneous activity west of Britain and Ireland.
At the end of the Variscan Orogeny, Britain was situated in the centre of the Pangaean supercontinent (Ziegler 1988). As the compressional orogenic forces which had built Pangaea waned, they were replaced by a tensional regime which led to the development of intracratonic basins and rifts. The most prominent of these was in the Norwegian-Greenland Sea; this rift may have extended as far south as the Faeroe-Rockall area (Ziegler 1988). Evidence of widespread Permo-Carboniferous volcanism associated with this change in tectonic setting is preserved onshore in Scotland (Francis 1991). Some Permo-Triassic basin infills are the products of syntectonic sedimentation adjacent to active fault scarps, possibly due to the reactivation of earlier Caledonian structures (Brewer & Smythe 1984). Such basins characteristically contain wedge-shaped packages of seismic reflectors that dip into
88
K. HITCHEN E T AL.
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the major basin-bounding fault, as in the West Orkney Basin (Coward & Enfield 1987). Other basins formed due to Late or post-Triassic faulting, which has preserved parallel-bedded reflector packages; the North Rona Basin is an example of this type (Kirton & Hitchen 1987). The Permo-Triassic was a time of low eustatic sea levels. Combined with the low latitudinal position of the British Isles, this gave rise to an arid continental environment typified by non-marine, elastic, red-bed sedimentation (Warrington et al. 1980). Coarse-grained deposits collected adjacent to active fault scarps (Steel & Wilson 1975) or in upland areas, whereas finer-grained, fluvial, lacustrine or aeolian sediments accumulated in areas of lower relief. Partial flooding of the Shetland Platform may have occurred briefly during the Late Permian, and widespread marine conditions were established by the Rhaetian transgression in latest Triassic times. The arid climate and oxidizing depositional environments of the Permian and Triassic were not conducive to the preservation of fauna or flora, so that biostratigraphical information is sparse. This paper takes a regional view of Permo-Triassic sedimentary and volcanic rocks north and west of Scotland, excluding the inner Hebridean, Clyde and Malin areas. The present structural setting is considered, as well as the tectonic setting in which the rocks were deposited. Evidence from onshore outcrops, British Geological Survey (BGS) offshore surveys, commercial exploration drilling, and thousands of kilometres of oil-industry deep-seismic profles indicate that rocks of presumed Permian and Triassic age are widespread on the continental shelf to the north and west of Scotland. Their presence in the adjacent Rockall Trough and Faeroe-Shetland Basin is, however, conjectural. Permian and Triassic sedimentary rocks are commonly lithologically and seismically indistinguishable from each other, and few exploration wells have penetrated the full Permo-Triassic succession before being terminated. The amount of core available for public inspection from the commercial wells is very limited, but numerous BGS shallow boreholes have penetrated the interval.
The Permo-Triassic basins Although most Permo-Triassic basins in the Hebrides-West Shetland region are orientated NNE-SSW, a major difference in their polarity occurs across the North Orkney/Wyville Thomson (NOWT) transfer zone (Stoker et al. 1993). The NOWT transfer zone is conceptually similar to the Orkney-Faeroe Alignment of Earle et al. (1989), but the NOWT transfer zone is dog-legged, whereas the Orkney-Faeroe Alignment is straight. To the south of the NOWT transfer zone, seismic reflectors dip westwards into major basin-bounding, SE-dipping faults on the northwest margins of basins that include the North Minch, North Lewis and West Orkney basins. To the north of the NOWT transfer zone, reflectors dip eastwards into NWdipping faults on the southeast margins of basins such as the North Rona and West Shetland basins. Many of the major basin-bounding faults are listric, and consequently show curved traces at outcrop (Fig. 1). The basins have been modified during Mesozoic and Cenozoic times by both extensional and compressional tectonic forces (Coward and Enfield 1987; Earle et al. 1989; Booth et al. 1993; Knott et al. 1993).
PERMO-TRIASSIC ROCKS NORTH AND WEST OF SCOTLAND
91
Basins south of the N O W T transfer zone Barra Trough This basin comprises a fault-bounded outlier within Lewisian basement (Fig. 1) (Stoker et al. 1993). BGS shallow-seismic profiles indicate folded, predominantly westerly dipping, parallel-bedded reflectors, suggesting that post-depositional faulting is responsible for preservation of these strata, which may formerly have been more widespread over the Hebrides Shelf. BGS borehole 90/16 penetrated 13 m of red, partly pebbly sandstone in stacked, upward-fining cycles. No biostratigraphical age has been obtained for the sediments. Farther west, on the outer shelf, rocks unconformably overlying Lewisian basement have been mapped as Permo-Triassic, cropping out in a thin north-south strip (James & Hitchen 1992). However, the results of subsequent BGS drilling have shown that these sediments, which underlie Paleocene basalts, are of older Paleocene age (BGS unpublished report).
W e s t F l a n n a n Basin Up to 4000m of mainly westerly dipping sediments have been postulated in this basin by gravity modelling. The basin structure is unclear due to an almost complete cover of early Tertiary lavas (Evans et al. 1989). Close to the eastern margin of the basin, BGS borehole 88/4 proved 8 m of white to grey, poorly sorted, fine- to coarsegrained and slightly pebbly, sandstone, interpreted to be of fluvial origin. This, and the bulk of the remaining sediments in the basin, are interpreted to be PermoTriassic in age, although the pre-Cenozoic succession in this and the nearby West Lewis Basin is known to include Jurassic and Cretaceous rocks (Hitchen & Stoker 1993).
Flannan Trough At its narrower southern end, the Flannan Trough (Fig. 1) is a NE-SW-orientated, westerly tilted graben that possibly contains up to 2000 m of sediments, many of which crop out at the sea bed. Samples recovered in this area are red or purple, partly micaceous, fine- to coarse-grained sandstones and siltstones (Evans et al. 1990). BGS borehole 88/8, drilled in the western part of the basin, proved reddish brown, fine-grained, micaceous sandstone with intraformational mudstone clasts. A fluvial origin has been proposed for the sandstone (BGS unpublished report). None of these sediments has been dated, but they are presumed to be Permo-Triassic. To the northeast, the trough broadens and merges into the North Lewis Basin.
N o r t h L e w i s Basin This basin is situated to the west of the Minch Fault north of Lewis (Fig. 1). Interpretation of seismic data suggests that Permo-Triassic rocks are up to 3000 m thick and are in part overlain by Lower Jurassic sediments (Stoker et al. 1993). The
92
K. HITCHEN ET AL.
basin is controlled by major eastward-dipping faults that developed in the hanging wall of the Outer Isles Thrust which is offset by the NW-trending shear of the Ness Fault Zone (Stein 1988; Evans et al. 1990). From the continuation of the onshore outcrop of the Stornoway Formation west of the Minch Fault, BGS borehole 72/32 proved 8 m of red, poorly cemented, fine-grained sandstone, possibly deposited in a floodplain environment (Fyfe et al. 1993). Southwest of the Sula Sgeir High, on the western flank of the basin, two BGS boreholes have drilled probable Permo-Triassic sediments. Borehole 78/5 proved fine-grained red sandstone with green reduction spots, interbedded with red, micaceous siltstone. Borehole 90/17 drilled 26m of fine-grained sandstone with siltstone and mudstone. The sandstone is an immature arkose comprising quartz, feldspar and some rock fragments. Cementation is limited, resulting in many friable beds with porosity from four samples ranging from 13 to 37%. This is mainly primary, intergranular porosity enhanced by leaching of accessory minerals and secondary dissolution of sodic feldspars.
West Lewis Basin
Situated to the west of the Sula Sgeir High, this basin is another outermost-shelf basin which is largely covered by early Tertiary basalts which obscure the structure. Seismic data show reflectors from an unknown thickness of Permo-Triassic to Tertiary sediments that dip northwestwards (Hitchen and Stoker 1993). BGS borehole 90/1 drilled 11 m of red-brown and grey-green, poorly sorted, variably fineto coarse-grained sandstone that is in part muddy, calcareous and pebbly. It is in part massive, but also displays cross-bedding. Carbonate nodules are common in this sandstone, which was probably deposited in a fluviatile environment (Stoker et al. 1993).
North Minch Basin
The development of the North Minch Basin, as well as the contiguous Sea of the Hebrides-Little Minch Trough (Fyfe et al. 1993), has been governed by postCaledonian reactivation of the late Proterozoic Outer Isles Thrust at the Minch Fault, against which both basins generally thicken westwards (Stein 1988). PermoTriassic rocks crop out at the sea bed around the eastern and southern margins of the North Minch Basin (Fyfe et al. 1993). According to Stein (1988, 1992), the basin contains sediments ranging from Precambrian (Torridonian) to Tertiary, which were deposited during extensional episodes. Stein (1992) estimated that over 2000m of Carboniferous and up to 3000 m of Permo-Triassic strata may be present in the deepest parts of the basin. However, the presence of Carboniferous rocks is based on circumstantial evidence, and may be less likely following the results of well 156/17-1 (Fyfe et al. 1993). This was drilled near the centre of the basin, through Lower Jurassic shales, into 1115 m of Permo-Triassic sandstone, siltstone and mudstone which are in part conglomeratic and anhydritic, and include some grey-green reduction patches in thin marl layers (Fig. 2). These strata rest directly upon subarkosic sandstone considered by the operator to be Torridonian in age.
PERMO-TRIASSIC ROCKS NORTH AND WEST OF SCOTLAND
93
156/17-1 Gammo-raylog Sonic Lo9 (API) units) (mkxmecomls/fo=t) 150 140 4.0 0 360m PABBA SHALE
c_)
~
FORIdATION ~
I~INUAL)m"U I ~ U
BEDS
!FORMATION
C.) <~: 0 m
-
.~;:.~I
~ i
i
!
200
1758m
Sondstone Idudstone Limestone
Fig. 2. Log of BP well 156/17-1 in the North Minch Basin. For location see Fig. 1. Interpretation taken from composite log.
94
K. HITCHEN ET AL.
Throughout Permian and Triassic times, syntectonic deposits probably collected in the western parts of the Sea of the Hebrides-Little Minch Trough and North Minch Basin during episodic, though as yet undated, periods of movement on the Minch Fault. On the east of Lewis, the Stornoway Formation is regarded as the onshore continuation of the sediments found in the North Minch Basin (Fyfe et al. 1993). The formation consists mainly of conglomerates and sandstones, with thin impersistent siltstones, and has been estimated to be up to 4000 m thick by Steel & Wilson (1975). The formation has been dated from palaeomagnetic evidence as Late Permian to Triassic (Storetvedt & Steel 1977). Offshore, in the western part of the North Minch Basin, deposits may typically consist of stacked, coalescing, rapidly deposited alluvial fans of medium- to coarse-grained, poorly sorted sandstones and conglomerates. In the eastern parts of the basin, alluvial floodplain deposits are more likely, derived from areas of low relief and consisting of siltstones and mudstones, partly calcareous, with reduction horizons. The two facies may interdigitate in the centre of the basin. At times corresponding to periods of fault inactivity, the finergrained facies may have predominated across the basin. West Orkney
Basin
This basin complex lies between the Solan Bank High and the Orkney Islands; it comprises a series of half-grabens with the major faults aligned NE-SW. The West Orkney Basin may have formed progressively by the development of normal faults in the hanging walls of former thrusts, including the Moine Thrust (e.g. Coward et al. 1989). Seismic-reflector packages generally thicken northwestwards into the major normal faults, indicating fault movement during deposition (Brewer & Smythe 1984; Kirton & Hitchen 1987; Enfield & Coward 1987; Coward & Enfield 1987; Coward et al. 1989; Earle et al. 1989). The deepest and most northwesterly half-graben is sometimes referred to as the Stack Skerry Basin; this has a Permo-Triassic infill, proved by well 202/19-1 (Fig. 3), whereas half-grabens nearer Orkney may have an infill composed of Devonian overlain by Permo-Triassic (Earle et al. 1989). BGS shallow drilling has indicated that Permo-Triassic rocks crop out beneath the Quaternary across most of the offshore extent of the West Orkney Basin. At least nine BGS boreholes (Fig. 1) have drilled presumed Permo-Triassic sediments comprising mainly fluviatile sandstones, siltstones and mudstones with occasional pebbly or conglomeratic horizons (Evans et al. 1981). The thickest Permo-Triassic sequence drilled north and west of Scotland is in well 202/19-1 (Fig. 3), which proved 2931 m. The basal 613 m in the well consist of mudstone and claystone, slightly sandy and anhydritic, with a few thin limestones. This division is overlain by an evaporitic unit 819 m thick, comprising further fine-grained lithologies, with halite beds up to 6 m thick and sporadic, thin limestones, sandstones and dolomites. This interval may be Late Permian in age. The succeeding 1500m consist predominantly of finegrained sandstone with subordinate mudstone, claystone and siltstone. Well 202/19-1 terminated at 3188m depth (Fig. 3), just above a series of subparallel seismic reflectors (Fig. 4). These reflectors may represent either PermoTriassic or ?Devonian sediments, or possibly Early Permian volcanic rocks extruded during the first stages of development of this half-graben. Down-dip (northwestwards) from this well, 7-8 km of Permo-Triassic sediments may be present in the deepest part of this half-graben (Fig. 4).
PERMO-TRIASSIC ROCKS NORTH AND WEST OF SCOTLAND
95
202/19-1 Gamma-ray log 0
Sonic Log
(h~ units)
tricro~oMs/footJ
150 140
40
257m
-
-
CO t~
ii===-=--
!:i,;:~:i?. ~'- :;:!L!ii~/i:
- . _ _ '. ... _---.~,
i!i!.,'~(:i
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e,
. ~
i
500
3188m
Predominantlysandstone ~
Cloystone/mudstone/siltstone
~ -1-
/~hydriticsandycloystone/mudstone above ~th interbeddedhalite Sporo~climestone
"
Tracesof Hdte
Fig. 3. Log of the Permian and Triassic section in Shell well 202/19-1 in the West Orkney Basin. For location see Fig. 1; for structural setting see Fig. 4. Interpretation taken from composite log.
96
K. H I T C H E N E T AL.
NW
SE
0 ....
.
.
.
.
-.....:.
WEST ORKNEY BASIN SOLAN BANK HIGH
0
~
WELL 202/19-I
Sea bed
norshown
~J
Fig. 4. Seismic profile, with interpretation, across the westernmost half-graben of the West Orkney Basin. For location see Fig. 1.
Basins north of the N O W T transfer zone N o r t h R o n a Basin This NE-SW-orientated basin, adjacent to the Solan Bank High (Fig. 1), contains at its southwest end up to 5000m of southeasterly dipping sediments (Stoker et al. 1993). Seismic data show that these are parallel-bedded, having been preserved by post-depositional movement on the southeast faulted margin of the basin (Kirton & Hitchen 1987). Dating of the sediments and establishment of the timing of the faulting is not possible. BGS borehole 78/7 recovered reddish brown to grey-green, ripple-laminated, sandstone and mudstone considered to have been deposited in a fluviatile environment during Permo-Triassic times. At its northeast end, the basin comprises a number of half-grabens and the PermoTriassic section is thinner; an overlying Mesozoic infill is draped by Tertiary sediments (Fig. 7 in Kirton & Hitchen 1987).
PERMO-TRIASSIC ROCKS NORTH AND WEST OF SCOTLAND
97
West Shetland Basin
The West Shetland Basin is a major NE-SW-orientated half-graben in which deposition has largely been controlled by movement on the NW-dipping, basinbounding, Shetland Spine Fault on its southeast margin (Fig. 1). Permo-Triassic rocks are absent from the northeast end of the basin, but at the southwest end a sedimentary succession 7-8 km thick is present. Devonian rocks may form part of this succession, but the Permo-Triassic component may be up to 4 km thick (Hitchen & Ritchie 1987; Duindam & van Hoorn 1987; Haszeldine et al. 1987). Wedge-shaped packages of seismic reflectors, thought to represent Permo-Triassic rocks, dip into the Shetland Spine Fault; several wells have drilled into, but not penetrated through, this succession. Close to the Shetland Spine Fault, well 205/25-1 proved 184m of Triassic sandstone and conglomerate beneath Jurassic sediments. The conglomerate is poorly sorted and includes igneous and metamorphic rock fragments, presumably derived from the adjacent West Shetland Platform. In a similar structural position, well 205/ 30-1 proved 210m of sandstone and claystone. On the northwest side of the basin, well 205/23-1 drilled 1509m into Triassic sandstone, claystone and marl beneath Upper Jurassic sediments. The sandstone probably represents a fault-related influx of sediment into an area of generally low relief. To the southwest, the deepest penetration into the Permo-Triassic in this basin was by well 205/27a-1, drilled on a SE-dipping, tilted fault block. The well proved a 1480 m thick succession, mainly sandstone, of which the basal 64 m has been dated as Scythian (lowermost Triassic), overlying 954m of presumed Permian sandstone, siltstone and conglomerate (Fig. 5) (Stoker et al. 1993). Nearby, well 205/26a-2 drilled a small horst block and proved 317 m of Triassic sandstone and mudstone.
Faeroe-Shetland Basin
No wells have proved Permo-Triassic rocks in the Faeroe-Shetland Basin east of the Westray Ridge, although their presence here has been postulated by Mudge & Rashid (1987). To the southwest of the Westray Ridge, well 204/29-1 terminated after drilling 426m into Permo-Triassic rocks beneath Upper Jurassic sediments. This well proved sandstones and finer-grained lithologies.
Igneous rocks On mainland Scotland, notably in the Midland Valley, there was extensive Carboniferous volcanism, particularly during the Vis~an and Namurian. After the relatively quiescent Westphalian and Stcphanian intervals, further intrusive and extrusive activity took place during the Early Permian in geographically widely spaced localities ranging from Duncansby Ness, in the north, to east Fife, Arran, the Midland Valley, Mauchline, Sanquhar and Thornhill in the south (Francis 1991) (Fig. 1). Early Permian lavas and tufts have also been reported by Penn et al. (1983) at Larne in Northern Ireland and alkali-olivine basalts, dated at 285 4- 5 Ma (earliest Permian), have been described from the small island of Glas Eilean between Islay and Jura (Upton et al. 1987). The nature of the latter occurrence, and the
K. HITCHEN
98
E T AL.
205/27a-1 C,ammo-ray log Sonic log (.,~PIunits) t,mmena/f~ 0 150 140 4.0 266m
~
9
<
g
"
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,
! ':
i~::!;() ,: :" .
Z
.
.
i~ ,
-
.
.
~
500
.
.--.i. J
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Conglomerate Sandstone ~ltstone /~hydritic cloystone Lavas
Fig. 5. Log of the Permian and Triassic section in Shell well 205/27a-1 in the West Shetland Basin. For location see Fig. 1. Interpretation taken from composite log.
PERMO-TRIASSIC ROCKS NORTH AND WEST OF SCOTLAND
99
geochemistry of these lavas, suggest progressively waning eruptive activity. Offshore, the existence of Permian igneous rocks is restricted to four sites: wells 210/4-1,210/ 13-1,220/26-2 and 205/27a-1. Well 210/4-1 proved 6 m of porphyritic lava within a presumed Permian sequence (Hitchen & Ritchie 1987). The lava is highly altered, fine grained and feldspathic, with pyroxene phenocrysts now pseudomorphed in carbonate and serpentine. Originally it may have been a member of an alkali basalt-trachyte series, a hawaiite or a mugearite (Dixon et al. 1981). In well 210/13-1, fragments of presumed Permian lava occur in a Triassic sandstone (Hitchen & Ritchie 1987). These fragments show varying degrees of iron oxide/hydroxide alteration, rendering some clasts virtually opaque. A reasonably common feature is the presence of a fine- to very-fine-grained felsitic groundmass, partly with a flow alignment of tiny feldspar laths and microlites. Some clasts are aphyric whereas others show variably altered sanidine and plagioclase phenocrysts, which can be clouded by fine sericitic mica or replaced by calcite. There is little evidence for the presence of mafic silicate phenocrysts, except for rare calcite or oxide pseudomorphs which may be after clinopyroxene. Quartz patches in some fragments appear to be mostly secondary, and there is no evidence for quartz phenocrysts accompanying the feldspar. The bulk of the clasts therefore appear to represent fairly acid volcanic or subvolcanic rock of dacitic or trachyandesitic affinity. Farther north, well 220/26-2 terminated after drilling approximately 5m into fine-grained, generally non-porphyritic, ?trachyandesitic Early Permian lava. Fragments of this lava all show a characteristic trachytic or subtrachytic groundmass fabric of aligned plagioclase and ?alkali-feldspar laths. Slightly larger, rounded microphenocrysts of plagioclase and augitic pyroxene occur sporadically, but most pyroxene occurs as granular, anhedral crystals scattered throughout the groundmass. Chloritic alteration, possibly replacing glass, has occurred in the interstices between feldspar laths, and there are also some traces of epidotic and sericitic alteration. The common opaque minerals appear to be mainly magnetite, but show some marginal alteration to haematite and limonite. The lava in this well is overlain in turn by tuffaceous siltstone and siltstone containing volcanic fragments. The presence of Early Permian lavas in the three above wells north-northeast of Shetland suggests that a formerly extensive cover of volcanic rocks is now preserved only in remnant patches. Well 205/27a-1 (Fig. 5) terminated after drilling approximately 1.5m into Early Permian ?trachyandesitic lava that is fine-grained, strongly vesicular and highly altered. Secondary iron oxides give the rock a red-purple colour. Abundant plagioclase microphenocrysts are present, with the laths displaying a subtrachytic texture, set in a fine-grained felsic groundmass containing numerous tiny feldspar laths. The feldspar shows strong alteration to sericite/clay. Subhedral haematite and/ or limonite pseudomorphs occur, possibly after primary oxide or mafic silicate in some cases. Seismic data show that well 205/27a-1 terminated in the top of a package of subparallel, high-amplitude reflectors that is probably derived from the lavas. This package can be traced over a considerable part of the southwestern end of the West Shetland Basin. By implication, Early Permian lavas may exist (as yet undrilled) in the West Orkney Basin (see Fig. 33 in Stoker et al. 1993) and at other localities to the north and west Scotland.
100
K. HITCHEN E T AL.
Discussion Although biostratigraphical confirmation is largely lacking, there is considerable evidence for the widespread existence of Permo-Triassic rocks to the north and west of Scotland. Similar evidence is also available for the Sea of the Hebrides, the Malin Sea, the Firth of Clyde and the North Sea to the east and northeast of Scotland (Evans et al. 1981; Fyfe et al. 1993). Indeed, Permo-Triassic deposition has been important in very many areas of the UK shelf, but the total extent of Permo-Triassic deposition to the northwest of the UK is unknown. To the west of Shetland, the presence of Permo-Triassic rocks has been inferred in the Faeroe-Shetland Basin from seismic interpretation by Mudge and Rashid (1987) and Earle et al. (1989). They showed that up to 2000m of strata may have been preserved between the intrabasinal highs such as the Flett and Mid-Faeroe Ridges. Haszeldine et al. (1987) also implied that Permo-Triassic rocks are present in this basin, by comparison with the West Shetland Basin. Reworked Permo-Triassic microfossils have been reported from the Upper Cretaceous to Danian interval in three unspecified wells (Hitchen & Ritchie 1987) and an estimated 7.7km of sedimentary and volcanic rocks exist in the basin at 61~ (Bott & Smith 1984), of which some may be Permo-Triassic. Evidence for the presence of Permo-Triassic rocks in the Rockall Trough is largely circumstantial. Palaeogeographic reconstructions by Ziegler (1988) show an intracontinental rift persisting throughout this interval, collecting mainly continental, clastic sediments, except during the Late Permian when evaporites and carbonates may also have been deposited. Approximately 5 km of sedimentary rocks occur in the central Rockall Trough (Roberts et al. 1988; Joppen & White 1990; Makris et al. 1991) although most, if not all, of this thickness is probably Cretaceous and Tertiary in age. Most sediments appear subhorizontally disposed, and seismic data from the centre of the trough show no tilted fault blocks or syntectonic depocentres. This structural style is dissimilar to that of the FaeroeShetland Basin where Jurassic and Permo-Triassic sediments are more likely to be present between intrabasinal structural highs (Mudge & Rashid 1987). However, Permo-Triassic sediments have been proved on the adjacent Hebrides Shelf (Stoker et al. 1993; Hitchen & Stoker 1993), and are inferred to exist in the northern Rockall Trough (Evans et al. 1990). From seismic refraction data, Roberts et al. (1988) also suggested that late Palaeozoic or early Mesozoic rocks might be responsible for a 5.3 k m s -~ interval velocity layer on the eastern side of the trough. In this paper we have documented only the known offshore occurrences of PermoTriassic igneous rocks in the area under consideration. The timing of this volcanic activity is almost certainly related to the Early Permian extensional tectonic phase of the proto-Rockall Trough-Faeroe-Shetland Basin-More Basin rift axis. Some activity may have occurred along N W - S E fractures, as at Glas Eilean near Islay; such fractures would be incipient transfer faults of this rift axis. Numerous such transfer faults have been recognized north and west of Shetland (Kirton & Hitchen 1987; Duindam & van Hoorn 1987; Earle et al. 1989; Rumph et al. 1993); some may have been initiated during the Early Permian, although most have more recent movements. The Late Permian and Triassic were times of volcanic quiescence, although igneous activity occurred in the vicinity of Irish wells 12/13-1A (Fig. 1) and 19/5-1,
PERMO-TRIASSIC ROCKS NORTH AND WEST OF SCOTLAND
101
which drilled ?late Triassic sandstones and conglomerates containing volcanigenic lithoclasts. In well 19/5-1 the conglomerate passes upwards into a tuffaceous sandstone (Tate & Dobson 1989). Farther southwest, in the North Porcupine Basin, well 26/22-1A proved deposits of similar age and lithology. This phase of volcanism, apparently much less extensive than that of the Early Permian, probably represents post-rift activity. We would like to thank BP Exploration for permission to include the log of well 156/17-1 (Fig. 2), and Eileen Gillespie for drawing the figures. This paper is published with the permission of the Director, British Geological Survey (NERC).
References BOOTH, J., SWIECICKI, T. & WILCOCKSON, P. 1993. The tectono-stratigraphy of the Solan Basin, west of Shetland. In: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. The Geological Society, London, 987-998. BOTT, M. H. P. & SMITH, P. J. 1984. Crustal structure of the Faeroe-Shetland Channel. Geophysical Journal of the Royal Astronomical Society, 76, 383-398. BREWER, J. A. & SMYTHE, D. K. 1984. MOIST and the continuity of crustal reflector geometry along the Caledonian-Appalachian orogen. Journal of the Geological Society of London, 141, 105-120. COWARD, M. P. & ENFIELD, M. A. 1987. The structure of the West Orkney and adjacent basins. In." BROOKS, J. & GLENNIE, K. W. (eds) Petroleum Geology of North-West Europe. Graham and Trotman, London, 687-696. & FISCHER, M. W. 1989. Devonian basins of northern Scotland: extension and ,. reversion related to late Caledonian-Variscan tectonics. In: COOPER, M. A. & WILLIAMS, G. D. (eds) Inversion Tectonics. Geological Society, London, Special Publication, 44, 275-308. DIXON, J. E., FITTON, J. G. & FROST, R. T. C. 1981. The tectonic significance of postCarboniferous igneous activity in the North Sea basin. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 121-137. DUINDAM, P. & VAN HOORN, B. 1987. Structural evolution of the West Shetland continental margin. In: BROOKS, J. & GLENNIE, K. W. (eds) Petroleum Geology of North-West Europe. Graham and Trotman, London, 765-773. EARLE, M. M., JANKOWSKI, E. J. & VANN, I. R. 1989. Structural and stratigraphic evolution of the Faeroe-Shetland Channel and northern Rockall Trough. In: TANKARD, A. J. & BALKWILL, H. R. (eds) Extensional Tectonics and Stratigraphy of the North Atlantic Margins. American Association of Petroleum Geologists Memoir, 46, 461-469. ENFIELD M. A. & COWARD, M. P. 1987. The structure of the West Orkney Basin, northern Scotland. Journal of the Geological Society of London, 144, 871-884. EVANS, D., CHESHER, J. A., DEEGAN, C. E. & FANNIN, N. G. T. 1981. The Offshore Geology of Scotland in Relation to the IGS Shallow Drilling Programme, 1970-1978. Report of the Institute of Geological Sciences, 81[12. - - , HITCHEN, K. & ABRAHAM, D. A. 1989. BGS 1:250000 offshore solid geology map series. Sheet 58~176 Geikie. --,---& ROBERTSON, S. 1990. BGS 1:250000 offshore solid geology map series. Sheet 58~176 Lewis. FRANCIS, E. H. 1991. Carboniferous-Permian igneous rocks. In: CRAIG, G. Y. (ed.) Geology of Scotland 3rd edition, The Geological Society, London, 393-420. FYFE, J. A., EVANS, D. & GRAHAM, C. C. 1993. United Kingdom Offshore Regional Report. The Geology of the Malin-Hebrides Sea Area. HMSO, London, for the British Geological Survey. HASZELDINE, R. S., RITCHIE, J. D. & HITCHEN, K. 1987. Seismic and well evidence for the early development of the Faeroe-Shetland Basin. Scottish Journal of Geology, 23, 283-300. -
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HITCHEN, K. & RITCHIE, J. D. 1987. Geological review of the West Shetland area. In: BROOKS, J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe. Graham and Trotman, London, 737-749. & STOKER, M.S. 1993. Mesozoic rocks from the Hebrides Shelf and the implications for hydrocarbon prospectivity in the northern Rockall Trough. Marine and Petroleum Geology, 10, 246-254. JAMES, J. W. C. & HITCHEN, K. 1992. BGS 1:250 000 offshore solid geology map series. Sheet 60~176 Peach. JOPPEN, M. & WHITE, R. S. 1990. The structure and subsidence of Rockall Trough from twoship seismic experiments. Journal of Geophysical Research, 95, 19 821-19 837. KIRTON, S. R. & HITCHEN, K. 1987. Timing and style of crustal extension north of the Scottish mainland. In: COWARD, M. P., DEWEY, J. & HANCOCK, P.L. (eds) Continental Extensional Tectonics. Geological Society, London, Special Publication, 28, 501-510. KNOTT, S. D., BURCHELL, M. T., JOLLEY, E. J. & FRASER, A. J. 1993. Mesozoic to Cenozoic plate reconstructions of the North Atlantic and hydrocarbon plays of the Atlantic margins. In: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. The Geological Society, London, 953-974. MAKRIS, J., GINZBERG, A., SHANNON, P., JACOB, A. W. B., BEAN, C. J. & VOGT, U. 1991. A new look at the Rockall region, offshore Ireland. Marine and Petroleum Geology, 8, 410-416. MUDGE, D. C. & RASHID, B. 1987. The geology of the Faeroe Basin area. In: BROOKS, J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe. Graham and Trotman, London, 751-763. PENN, I. E., HOLLIDAY, D.W., KIRBY, G. m. ET AL. 1983. The Larne No. 2 borehole: discovery of a new Permian volcanic centre. Scottish Journal of Geology, 19, 333-346. ROBERTS, D. G., GINZBERG, A., NUNN, K. & McQUILLIN, R. 1988. The structure of the Rockall Trough from seismic refraction and wide-angle measurements. Nature, 332, 632-635. RUMPH, B., REAVES, C. M., ORANGE, V. G. & ROBINSON, D. L. 1993. Structuring and transfer zones in the Faeroe Basin in a regional tectonic context. In: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. The Geological Society, London, 999-1009. STEEL, R. J. & WILSON, A. C. 1975. Sedimentation and tectonism (?Permo-Triassic) on the margin of the North Minch Basin, Lewis. Journal of the Geological Society of London, 131, 183-202. STEIN, A. M. 1988. Basement controls upon basin development in the Caledonian foreland, NW Scotland. Basin Research, 1, 107-119. 1992. Basin development and petroleum potential in The Minches and Sea of the Hebrides Basins. ln: PARNELL, J. (ed.) Basins on the Atlantic Seaboard." Petroleum Geology, Sedimentology and Basin Evolution. Geological Society of London Special Publication, 62, 17-20. STOKER, M. S., HITCHEN, K. & GRAHAM, C. C. 1993. United Kingdom Offshore Regional Report. The Geology of the Hebrides and West Shetland Shelves and Adjacent Deep-water Areas. HMSO, London, for the British Geological Survey. STORETVEDT, K. M. & STEEL, R. J. 1977. Palaeomagnetic evidence for the age of the Stornoway Formation. Scottish Journal of Geology, 13, 263-269. TATE, M.P. & DOBSON, M. R. 1989. Late Permian to early Mesozoic rifting and sedimentation offshore NW Ireland. Marine and Petroleum Geology, 6, 49-59. UPTON, B. G. J., FITTON, J. G. & MACINTYRE, R. M. 1987. The Glas Eilean lavas: evidence of a Lower Permian volcano-tectonic basin between Islay and Jura, Inner Hebrides. Transactions of the Royal Society of Edinburgh: Earth Sciences, 77, 289-293. WARRINGTON, G. and others 1980. A Correlation of Triassic rocks in the British Isles. Geological Society, London, Special Report, 13. ZIEGLER, P. A. 1988. Evolution of the Arctic-North Atlantic and the Western Tethys. American Association of Petroleum Geologists Memoir, 43. -
From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe,
Geological Society Special Publication No. 91, pp. 103-122
Influence of basement on the geometry of Permo-Triassie basins in the northwest British Isles T. B. A N D E R S O N , J. PARNELL & A. H. R U F F E L L
School of Geosciences, The Queen's University of Belfast, Belfast BT7 1NN, UK Abstract: The orientation and shape of Permo-Triassic basins in the northwest
British Isles are strongly influenced by the structure of the underlying basement. In the Hebridean region, NE-SW-elongated basins are controlled by reactivated Caledonian thrusts. Further south, the variable orientation of elongate basins reflects the local fabric of the Dalradian basement. In the Midland Valley of Scotland and Northern Ireland, Permo-Triassic basins have a less regular shape, as they are sited on Carboniferous basins with a weakly pronounced structural fabric: the NNW-SSE-elongation of some outcrops reflects the effects of later (Tertiary) faulting along this trend. In the Southern Uplands lower Palaeozoic basement, bedding and cleavage are tectonically aligned to produce a strong ENE-WSW high angle anisotropy, cross-cut by complicated arrays of steeply inclined faults. Crustal stretching would have occurred normal or parallel to the anisotropy: Permo-Triassic basins, typically elongate NNW-SSE, clearly formed in response to contemporary ENE-WSW stretching, producing half-graben by dip-slip reactivation of existing Caledonian fractures. There is widespread evidence for contemporaneous faulting in the origin of PermoTriassic basins in northwest Europe. In particular, structures orientated N W - S E and N E - S W are believed to have played an important role in the location and orientation of basins. Glennie (1990) suggests that convergence between the Gondwanan and Laurussian continents initiated N W - S E and conjugate N E - S W systems of graben. Within the North Sea, the Central Graben and the northern and southern Permian basins represent these orientations, respectively. To some extent, basin subsidence in the North Sea during the Permian was influenced by the location of Carboniferous extensional structures. NW-trending transfer faults in the Carboniferous help to facilitate changes of dip between adjacent half-graben (Coward 1990). Several late Palaeozoic-Mesozoic basins, orientated as above, occur in the hanging walls of pre-existing thrust planes, which were presumably reactivated extensionally during basin formation: examples in northwest Britain include the Hebridean basins in the hanging wall of the Outer Isles Thrust (Stein 1988) and in the Solway region (Hall et al. 1984; Kimbell et al. 1989). On a more local scale, many workers have identified N W - S E and N E - S W structures which delimit or influenced Permo-Triassic basins in northwest Britain. However, examination of a map of basin distribution in the region immediately shows that in some areas basins are orientated particularly along N E - S W trends whereas in other areas the preferred trend is broadly N W - S E to N - S (Fig. 1). In this account, we review Permo-Triassic basins across the inshore/onshore region of
104
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Fig. 1. Northern Britain divided into Caledonian terranes (Hutton 1987), with Permo-Triassic basins (stippled) shown in simple outline (after Naylor & Shannon 1982). Basins discussed in text: LI, Loch Indaal; LNL, Lough Neagh-Larne; M, Malin; P, Portpatrick; R, Rathlin; S, Solway; SH, Sea of the Hebrides9 Scotland and Northern Ireland from the Hebrides to the Solway. The region is divided into a series of Caledonian terranes (after Hutton 1987). Within each terrane we consider those structural aspects of the pre-Permian basement which may have helped to control basin orientation and distribution. We then give two examples in more detail, one each relating to basins with a N E - S W and a N N W - S S E trend.
Orientation of basins in Caledonian terranes The Laurentian and Grampian terranes include several distinct groups of basement rocks, including Lewisian, Moinian and Dalradian. Most of the Permo-Triassic basins within these terranes are elongate N N E - S S W to NE-SW, following the local structural grain of the basement. These basins include those in the Hebridean region (Minch and Sea of the Hebrides), in the Malin Sea, Loch Indaal and Rathlin. The Hebridean basins are westward-thickening half-graben which developed in the hanging wall of the NNE-SSW-trending Outer Isles Thrust (Stein 1988). The basins
INFLUENCE OF BASEMENT ON THE GEOMETRY OF BASINS
105
further east in the Skye region may similarly be cited upon reactivated Caledonian thrusts: the Triassic in the Heast area of Skye and a postulated Kyle Basin in the Sound of Sleat lie above the Kishorn and Moine Thrusts, respectively (Fig. 2). In Lewis, at least, thick Permo-Triassic conglomerates stacked against the Outer Isles Thrust indicate growth on the fault during sedimentation. The WINCH seismic profile shows that basins in the Grampian terrane are also westward-thickening halfgraben, and that the reflectors within the basins, the dips of which systematically increase with depth, likewise imply growth faulting, e.g. Loch Indaal (Fig. 3), after Hall e t a l . (1984). The bounding faults of the basins in the Grampian terrane were not reactivated Caledonian thrusts but originally strike-slip faults. The Rathlin Basin (=Rathlin Trough), bounded by the Foyle Fault and Tow Valley Fault, is an example, discussed further below. The Malin Sea region exhibits both NE-SW and NW-SE-trending structures (Fig. 4). Several graben and half-graben follow the Caledonoid trend (the Stanton .~.
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106
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Trough and some minor basins within the south Donegal-Malin area (Fig. 4)). However, some NW-SE faults developed in a dextral strike-slip regime during the late Carboniferous (Dobson & Whittington 1992) and were reactivated during the Mesozoic-Tertiary, in particular controlling Parma-Triassic subsidence (Evans et al. 1980). These include those bounding the Colonsay and Loch Indaal Basins. The orientation of Permo-Triassic basins in the Midland Valley of Scotland and Northern Ireland is less clearly defined than in other terranes, these basins being less markedly elongate. This probably reflects the fact that throughout much of this terrane the Caledonian basement is covered by Carboniferous rocks and thus reactivation of basement structures is dampened and not reflected so clearly in postCarboniferous sedimentation. Lower Carboniferous sedimentation along the Midland Valley was focused between the major N E - S W Highland Boundary and Southern Uplands Faults. Several workers have suggested that during the Upper Carboniferous, N E - S W trending controls on sedimentation gave way to subsidence along NNW-SSE trends (Mykura 1967; Hall & Smythe 1973; McLean 1978). Parma-Triassic outcrops in the Clyde/North Channel region trend NW-SE,
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INFLUENCE OF BASEMENT ON THE GEOMETRY OF BASINS
107
including the North Channel Basin and the Northeast Arran Trough (McLean & Deegan 1978). However, it is not clear how active the NNW-trending structures in the Midland Valley were in the Permo-Triassic; this trend is also followed by demonstrably Tertiary structures, including dyke intrusion. In Northern Ireland, outcrops of the Triassic in County Antrim are elongate along and bounded by NNW-SSE-trending faults (Fig. 5), yet the Lough Neagh-Larne Basin is elongate along the N E - S W trend of the Midland Valley. The faults were clearly active during the Tertiary as they extend up to the basalts of the Palaeocene Antrim Lava Group. Our local mapping has identified an early (?synsedimentary) phase of NE-SWtrending faulting as distinct from the later N N W - S S E trend. The geological map of southern Scotland shows a series of approximately N-S to NNW-SSE-elongate Permian basins within the Lower Palaeozoic basement (Brookfield 1978), including the Moffat, Dumfries, Thornhill and Stranraer Basins, together with the more equidimensional Lochmaben Basin. Several of the basins are faulted on their eastern margins, most clearly the Stranraer Basin (Mansfield & Kennett 1963). East of Scotland, NNW-SSE-trending CarboniferousPermian basins cut the mid-North Sea High (Armstrong 1972). On the Irish side of the North Channel, elongate basins with a similar trend occur at Strangford, County Down, Kingscourt, County Cavan and, in the North Channel itself, the Portpatrick Basin also exhibits this trend. Bounding faults occur on the east side of the
Fig. 5. Triassic outcrops on the north side of Belfast Lough, bounded by NNW-trending faults which also cut Tertiary basalt lavas (Antrirn Lava Group) and are therefore at least partially of Tertiary age.
108
T. B. ANDERSON E T AL.
Strangford (Bullerwell 1961) and Portpatrick (Hall et al. 1984) Basins. Kelling & Welsh (1970) noted that the Loch Ryan Fault, the eastern bounding fault of the Stranraer Basin, is parallel to dextral wrench faults in the Lower Palaeozoic basement. The role of structures in the Southern Uplands basement is discussed in detail below. As the southern margin of the Southern Uplands, the ENE-WSW North Solway Fault represents the limit to Dinantian sedimentation in Northern England (Ord et al. 1988; Gawthorpe et al. 1989). Following Carboniferous-Permian inversion, Permo-Triassic sedimentation was controlled by reactivation of the E N E - W S W fault trend (Leeder et al. 1989). The formation of the Solway and Northumberland Basins along this orientation reflects extensional reactivation of the Iapetus Suture and subsidence in the hanging wall (Coward 1990). Lesser N W - S E or NNW-SSEtrending faults may have been syndepositional, but had been initiated earlier, having functioned as transfer faults in the Carboniferous (Kimbell et al. 1989). An exception may have been the N N W - S S E Eden-Pennine Fault, whose orientation was influenced by rigid blocks of Caledonian granite to the west and east; this fault was subsequently the margin of N N W - S S E Permo-Triassic sedimentation along the Eden Valley. We now examine two regions in more detail. The Rathlin Basin is an example of a Permo-Triassic basin following the Caledonide trend. In contrast, the PermianTriassic basins of the Southern Uplands trend NW-SE, perpendicular to the Caledonoid trend of the basement fabric.
The Permo-Triassic Rathlin Basin in the Grampian terrane The Rathlin Basin is an elongate, roughly trapezoid-shaped basin, most of which is offshore Scotland and Northern Ireland. The southernmost part onshore Northern Ireland is largely buried beneath a cover of the Tertiary Antrim Lava Group, except at the very margin, where small exposures are seen on the topographic high of Dalradian rocks known as the Highland Border Ridge (George 1967). Other extreme marginal sites where the Permo-Triassic is exposed are in the west, on the north shore of Lough Foyle, County Donegal, in the north at the Mull of Oa, Islay, and in the east on the western coast of Kintyre (Fig. 6). Seafloor mapping shows that erosion of the Permo-Triassic has isolated the Islay and Kintyre outcrops from the main part of the basin (Evans et al. 1980). The Lough Foyle outcrop is a shallow sliver preserved on the upthrown side of the Foyle Fault, a major lineament along Lough Foyle which forms a marked aeromagnetic feature. Older maps show the Kintyre outcrop as Devonian and current published maps show the Lough Foyle outcrop as Carboniferous. However, our unpublished studies of the diagenesis and petrography of these rocks show that they are closely comparable with the PermoTriassic and are unlike the Carboniferous elsewhere in Scotland and the north of Ireland. These marginal sites have been examined for evidence of control on subsidence by basement structure. The Islay occurrence is too small to yield significant information (contact of Permo-Triassic and Dalradian in a 'sea stack'; Pringle (1952)), and the southern margin outcrops are really exposures of a sedimentary veneer deposited on the Highland Border Ridge, between the Rathlin Basin and the Larne-Lough Neagh Basin to the south (Fig. 6). We have therefore concentrated on the western and eastern outcrops. These are most pertinent as they
109
I N F L U E N C E OF BASEMENT ON THE G E O M E T R Y OF BASINS
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Fig. 6. Extent of Rathlin Basin, including regions of onshore outcrop of Permo-Triassic (note most of Permo-Triassic in Northern Ireland is beneath younger cover). Basin margins follow structural grain in Dalradian basement, as reflected by Dalradian stratigraphic boundaries. expose the basement contacts along the long NE-SW-trending (Caledonoid) margins, the Foyle and Tow Valley Faults. The broad southern margin of the basin abuts the topographic high of the Sperrin Mountains; the northern end narrows towards Knapdale where Dalradian rocks crop out at the surface. Neither margin includes a coarse, marginal facies. The rocks are predominantly red/yellow/ buff sandstone, with some pebbly bands and rare purple mudstones at Lough Foyle. It is likely that sedimentation occurred beyond the present margins, but rocks are only preserved seaward of the faults. The faults may well have been active since the Triassic and helped to preserve the Permo-Triassic rocks: the Tow Valley Fault was probably active in Tertiary times (Parnell et al. 1989). At all margins, the Permo-Triassic lies upon or against Dalradian basement. On a regional scale, the Dalradian stratigraphy highlights a series of NE-SW-trending Dalradian outcrops (Fig. 6). Detailed structural studies of the Dalradian in Donegal (e.g. Alsop & Hutton 1993) show that there have been several episodes of deformation. The most prominent folding, along a NE-SW axis, is the cause of the stratigraphic strike shown in Fig. 6. Similar studies in the southwest Highlands of Scotland, including Kintyre, also show that the Dalradian has experienced several
110
T. B. ANDERSON
E T AL.
phases of deformation, but the fold structures whose axes trend N-S to NNE-SSW, including the Islay anticline and Loch Awe syncline, are predominant (Roberts & Treagus 1977) and control the stratigraphic strike. The strike of the rocks in the eastern Inishowen Peninsula, Donegal, is 045~ ~ identical to that of the Foyle Fault. In Kintyre, the Dalradian fabric strikes 010 ~ which is parallel to the trend of the northern end of the Tow Valley Fault and the coastal outcrop of Permo-Triassic. In particular, the stratigraphic boundary between the Southern Highland and Argyll Groups highlights the strike near both the western and eastern margins. This boundary appears near both margins because of folding and faulting in between. To the south, a broad synformal structure, the Foyle Syncline, creates an approximately E-W stratigraphic strike, which is generally conformable with the southern margin of the basin. At the Lough Foyle margin, the Permo-Triassic is in faulted contact with the Dalradian, along an outcrop which is itself cut by faults normal to the margin. The Southern Highland Group Dalradian on the east side of the Inishowen Peninsula is a mix of psammites and pelites. The psammites are massively bedded, while the pelites show a prominent cleavage and have been particularly prone to folding along NE-SW axes. The published geological maps for this district (Irish sheets 11 and 12, described by Nolan & Egan (1885) and Hull e t al. (1890)) show a marked variation in the azimuths of dip in the Dalradian rocks adjacent to the fault with the PermoTriassic (Fig. 7). The general dip of the Dalradian is to the NNW, but adjacent to the fault the 'dips' are to the SSE, except in the vicinity of a transverse fault (Fig. 7). Our own field survey shows that the NNW-dipping structure is in fact cleavage dipping at up to 40 ~ and that the SSE-dipping surfaces are highly inclined and folded bedding planes. Close to the Lough Foyle coast, the Dalradian rocks are rich in pelitic layers,
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INFLUENCE OF BASEMENT ON THE GEOMETRY OF BASINS
111
which exhibit monoclinal folds (inclined west-verging fold-pairs). Faults downthrowing to the east follow the trend of the fold axes and partly take advantage of the steeply dipping beds. Ordnance Survey maps show a N E - S W line of waterfalls within a kilometre of the coast, marking the sites of monoclinal folding and faulting within the pelitic rocks. The Permo-Triassic rocks are preserved seaward of the most prominent of the faults, i.e. subsidence was controlled by lithologically related folding and faulting in the Dalradian basement. At the Kintyre margin, the actual contact between Permo-Triassic and Dalradian is exposed in very few localities, but field relationships where they are in close proximity suggest that it is both a faulted and unconformable contact. The contact is visible at the southernmost exposure of Permo-Triassic at Bellochantuy, where faults progressively downthrow the Permo-Triassic/Dalradian unconformity to the west. This contact occurs at a site where the Dalradian shows numerous faults; these are orientated parallel with small-scale monoclinal folds (Fig. 8) which are eastwardverging fold-pairs, i.e. the lower limbs are east of the fold hinges. The faults downthrow in an opposite sense to the monoclines and cut the upper limbs (Fig. 8). Rare faults cutting the Permo-Triassic sandstones (Fig. 9) are parallel to the basin margin faults at about 010 ~ The faults were healed during diagenesis and appear to be synsedimentary. Most are normal faults downthrowing basinward, but other compressional features are also evident, which may be coeval with the extensional structures. Stratigraphic sections in the vicinity of the Tow Valley Fault at the eastern margin of the Rathlin Basin (Fig. 10) suggest that there was westward downthrow on this lineament in the Permo-Triassic. To the east of the Tow Valley Fault,
Fig. 8. Photograph of fault in Dalradian basement near margin of Permo-Triassic outcrop, Bellochantuy, Kintyre. Fault cuts upper limb of monocline and is parallel to both a fold axis and to a marginal basin-bounding fault.
112
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INFLUENCE OF BASEMENT ON THE GEOMETRY OF BASINS
113
thick Carboniferous successions are preserved in two fault-bounded blocks at Ballycastle and Machrihanish, suggesting that Carboniferous deposition was not limited by a topographic high in t h i s region. Other sections show the Carboniferous missing or very thin beneath a thin (condensed) Triassic cover. If the Carboniferous was formerly widespread, these thin sequences indicate that the Carboniferous was eroded before limited Triassic deposition directly upon uplifted Dalradian east of the fault.
The Permo-Triassic basins in the Southern Uplands-Down-Longford terrane The Southern Uplands-Down-Longford terrane is remarkable amongst the collage of suspect terranes which compose the British Caledonides for its clear definition and for the relative homogeneity of lithology and structural fabric in the rocks of which it is composed. The terrane lies between the Southern Uplands Fault to the N N W and the Navan-Silvermines Fault-Solway line, commonly recognized as the 'Iapetus Suture', to the SSE (Fig. 11). Both bounding faults are characterized by large and uncertain displacements, probably including major components of sinistral strike-slip which occurred during Caledonian accretion and docking (Hutton 1987). The terrane is almost entirely composed of well-bedded Lower Palaeozoic (Upper Ordovician and Silurian) greywackes and shales, with rare interlayered volcanics. Caledonian deformation has imposed a highly distinctive structural style and fabric on these rocks. Numerous tight, upright folds, typically trending at 070 ~ are present throughout the terrane, so that bedding in the greywackes is typically vertical or subvertical. The northern limbs of these folds are almost invariably the thicker, so that the strata dominantly young towards the north; however, graptolites, the only common fossils, show that on a larger scale the rocks become consistently younger towards the south. This Southern Uplands paradox (Anderson & C a m e r o n 1979) is solved by the insertion of a number of faults dividing the terrane into a sequence of tracts, each a few kilometres across (Craig & Walton 1959; McKerrow et al. 1977). Within each tract the beds face predominantly north, but the tract-bounding faults repeatedly introduce younger rocks to the south. This structure lends itself readily to the interpretation of the terrane as an accretionary prism of fore-arc sediments, accreted on the northwestern edge of Iapetus (McKerrow et al. 1977). The accretionary or thrust deformation imposes a strong mechanical anisotropy or fabric across the whole terrane: bedding, tract-defining faults and a locally well developed slaty cleavage all strike E N E - W S W and are vertical or steeply inclined. This initial fabric is cut at various high angles by steeply inclined strike-slip faults (Anderson 1987), the dextral mode (Fig. 12) striking at 140 ~ and the sinistral mode at 010 ~ None of the faults have large displacements, but their presence is evident in almost any large outcrop. Several Devonian granodiorite plutons and associated minor intrusions in the central and western parts of the terrane constitute the only major lithological inhomogeneity. These rocks are basement to some eight Permo-Triassic basins. The Caledonian structural fabric, outlined above, controls basin geometry, and locally, on the basin margins, its distortion helps define the mechanics of extension and basin
114
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Fig. 12. Compass rose diagram of the strike directions of strike-slip faults in the lower Palaeozoic rocks of the Ards Peninsula, Southern Uplands central belt. development. The eight basins (Fig. 11) fall readily into three different groups, geographically and on the basis of their underlying structural configuration. Around the North Channel, the Strangford Basin, Portpatrick Basin and Stranraer Basin are all dearly half-graben, created by westerly downthrow and rotation against major faults on their northeastern margins. Some 100km to the west-southwest, the isolated Kingscourt Basin is a half-graben against the Kingscourt Fault to the west. Further northeast, the several basins in the Dumfries area appear more complicated; no simple half-graben arrangements can account for the outcrop shape and bed geometry of the Dumfries, Lochmaben, Moffat and Thornhill Basins. Only the Portpatrick or North Channel Basin clearly transgresses the terrane boundaries: the North Channel is obviously a major structure with a significant Permian and younger history. The remaining on-land basins are limited by the terrane boundaries or sutures. If these boundaries were not completely stitched all along their length, they would have had the effect of partitioning PermoTriassic extensional strain in the basement. The on-land basins are elongate N - S or NNW-SSE. The mean orientation of their long axes, based on present outcrop, is
116
T. B. ANDERSON E T AL.
160 ~ and the standard deviation 18 ~. This is perpendicular to the Caledonian structural fabric described above, strongly suggesting that extension is along and precisely controlled by the strong mechanical anisotropy in the basement. In the area around the North Channel, Permo-Triassic extension was almost certainly effected by the reactivation of Caledonian strike-slip faults in a normal (extensional) mode. The fault bounding the Strangford Basin follows the northeast shore of Strangford Lough on a trend of 130 ~ approximately parallel to the modal trend of Caledonian dextral faults in the local basement. Gravity modelling (Cook & Murphy 1952; Bullerwell 1961) indicates that basement is thrown down to the southwest by some 2 km. Accessible outcrop of the Permo-Triassic basin infill is geographically limited, but it appears that the northeasterly dip of bedding decreases upward, indicating that faulting and rotation were penecontemporaneous with sedimentation. The offshore Portpatrick Basin, which contains 2 - 3 k m of Carboniferous and Permo-Triassic sediments (Hall et al. 1984), terminates northeastward against a major (unnamed) fault on a trend of 150 ~ parallel to and probably determining the straight western coast of the Galloway peninsula. The parallel Stranraer Basin has a similar sediment thickness and structure (Mansfield & Kennett 1963), terminating against the Lough Ryan Fault, which trends at 140 ~ along its northeastern margin (Kelling & Welsh 1970). In both the Portpatrick and Stranraer Basins, bedding dips E or NE, but evidence of variation in dip is scarce or equivocal. In this same North Channel area, fold plunges in the lower Palaeozoic rocks are generally to the east, commonly at angles of 40 ~ (Barnes et al. 1987), yet the metamorphic grade does not decrease eastward or increase westward. The preferred model for basin formation is one of E N E - W S W crustal stretching, accommodated by the development of half-graben through the reactivation of previously existing N-S or NW-SE Caledonian fractures (Fig. 13). A system of parallel faults of westerly downthrow, with concomitant easterly rotation of all structures older than faulting, is clearly in accord with this basin geometry and also
Schematic Cross Section of H a l f - G r a b e n Basins and Basement Fold Plunges in the Western Part of the Southern Uplands Terrane
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INFLUENCE OF BASEMENT ON THE GEOMETRY OF BASINS
117
explains, at least in part, the easterly fold plunge and lack of a corresponding change in the metamorphic grade in the basement rocks beneath. The Kingscourt Basin, 100 km to the southwest, is essentially a mirror image of those just described. The basin-defining Kingscourt Fault, on the western side of the half-graben, is a N - S fracture, parallel to many of the sinistral strike-slip faults in the Lower Palaeozoic basement. Over 700m of Upper Carboniferous and PermoTriassic basin sediments dip west towards the fault (Jackson 1965). Outcrop of the Lower Palaeozoic basement around Kingscourt is sparse but, on limited data, it seems that in this area the Caledonian folds also plunge gently west. Thus at Irish Grid N911936, about 4 k m northwest of Ardee, and at N783972, 1.5kin north of Kingscourt, fold hinges and hinge-parallel lineations in the Silurian strata underlying the basin plunge between 12~ and 35 ~ WSW. In effect, not only does the basin mirror those around the North Channel but so does the basement structural fabric. The four basins in the Dumfries area are better exposed and the basin margins can be examined in many stream sections. In several of these sections, particularly on the edges of the Lochmaben and Moffat Basins, there are spectacular arrays of normal faults. It appears that, although some margins are unconformities and others are faults, invariably there is faulting and associated rotation, commonly through angles of 40 ~ or more, affecting both the basement rocks and the unconformable Permo-Triassic above. Again, the faults in the basement are commonly reactivated Caledonian factures. Some of these reactivated basement faults are not ideally orientated with respect to the new Permo-Triassic stress field and E N E - W S W extension, so that slickenside striae, indicating oblique displacements rather than straightforward down-dip normal extension, are common. Subsidiary fractures of very small displacement, some possibly Riedel fractures, commonly splay from the larger faults in apparent accommodation of extension oblique to the dip direction of the latter. Three outcrops merit description and illustration. Figure 14 shows a normal-fault array developed in red sandstone beds cropping out on the south bank of Garrel Water at NY038912, on the northern edge of the N
Normal Fault Array at NY038912 Red Permian Sandstone i n North F a c i n g B l u f f on Stream Bank
J,
ENE oT~176 ~
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~
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_~
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j,
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Fig. 14. Normal-fault array developed in Permian red sandstones exposed in a north-facing bluff on the south bank of the Garrel Water (NY038912), on the northern margin of the Lochmaben Basin. Poles to the nine fault surfaces are shown on a lower hemisphere equalarea stereogram.
118
T. B. ANDERSON E T AL.
Lochmaben Basin. Some nine faults, each throwing down to the SW, but none with displacements of more than 0.5 m, contribute toward N E - S W extension. Some 20 m upstream and to the north, the sandstones are in faulted contact with the underlying lower Palaeozoic greywackes. The greywackes have bedding inclined at 32 ~ to the NW immediately north of the fault, increasing to a much more typical orientation of 055/70NW some 150m further north, clearly indicating a strong fault-associated rotation of the ubiquitous basement fabric at the basin margin. The unconformity between the Permo-Triassic and the Silurian is superbly exposed at Mollinburn (NY058922), also on the northern edge of the Lochmaben Basin. In an exposure behind a large garage, bedded Permian sandstones oriented at 115/62SW rest with planar unconformity on the eroded edges of greywacke sandstone beds striking at 001 ~ and dipping 62~ At this locality the unconformity is again exposed, some 50 m north, on the bank of the small stream. Permian beds, orientated at 112/50SW, rest, again with planar unconformity, on inverted Lower Palaeozoic greywackes, whose bedding trends 038/58NW. Similarly oriented and inverted Lower Palaeozoic rocks continue for some way upstream. Figure 15 shows these bedding orientations plotted as poles on an equal -area stereogram. Rotating the Permian bedding to the horizontal about an axis parallel to strike, and performing a parallel rotation about the same axis and through the same angle, restores the Silurian strata to an orientation 062/86SE, very much in accord with the regional basement norm. Clearly basin-margin rotation at this locality is of 50 ~ to 60 ~ towards the southwest and affects a considerable area of the underlying basement. N
+
9
Fig. 15. Orientation data from the sub-Permian unconformity exposed as Mollinburn (NY058922), on the margin of the Lochmaben Basin, presented on a lower hemisphere equalarea stereogram. Poles to the surface of sub-Permian unconformity and parallel bedding in Permian sandstones are shown as crosses. Pole to bedding in lower Palaeozoic basement are shown as dots and the mean as an open circle. Rotation of Permian to the horizontal about an axis parallel to strike (straight arrowed line) and equal parallel rotation of lower Palaeozoic bedding mean (arrowed small circle) returns the latter to Permian orientation of 062/86SE, which is very close to the present-day regional norm.
INFLUENCE OF BASEMENT ON THE GEOMETRY OF BASINS
119
The most spectacularly exposed faulted contact occurs on the north bank of the Auchenchat Burn (NT083105), at the northern extremity of the Moffat Basin (Fig. 16). A low cliff exposes a 2 m thick bed of fine breccia, with 20 mm clasts floating in a sand matrix, resting with sharp unconformity on a contorted, weathered, red-stained basement of lower Palaeozoic shales and greywacke. The surface of the unconformity typically dips at 30 ~ to 258 ~ but with considerable variation in inclination from 18 ~ to 40 ~ indicating small-scale but significant relief on the erosion surface. The unconformity is repeatedly faulted down to the southwest on a set of parallel normal faults, with throws from a few centimetres to 2 m. Several of these faults have preserved slickenside striae, typically pitching at about 75 ~ southeastward and indicating oblique normal/sinistral movement. Numerous minor fractures splay from the larger faults and from the surface of the unconformity. Again, E N E - W S W stretching, normal faulting and rotation are superbly demonstrated.
Conclusions The distribution of Permo-Triassic basins in northwest Britain and Ireland is strongly controlled by the structure of the underlying basement. Basin orientation
0
metres
."/,o"
5
9
I
,~r~,
'.% ~ ' ~
"~ 0"o'.'"
230~
,~uo, ..~.~llip, 9 "-
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Fig. 16. Field sketch of faulted sub-Permian unconformity as exposed on the north bank of the Auchenchat Burn (NT083105) at the northern extremity of the Moffat Basin. An array of oblique-slip sinistral/normal faults, oriented 140/75SW, extend and rotate the basal Permian breccia bed.
120
T. B. ANDERSON ET AL.
varies between Caledonian terranes, but is generally either along a Caledonide trend (NE-SW) or near-normal to that trend (NNW-SSE). The two regions examined in detail exemplify both these orientations. The Rathlin Basin is bounded by major Caledonoid-trending faults, whose orientation parallels the fabric in the Dalradian basement. The faults parallel fold axes in the Dalradian and, at the western margin, downthrow takes advantage of highly inclined monoclinal fold surfaces. E N E - W S W along-strike crustal stretching, partitioned within the Southern Uplands Terrane boundaries, has produced a number of small basins, essentially by reactivation of Caledonian strike-slip faults oriented between N - S and NW-SE. In the North Channel area these are a series of relatively simple half-graben with faults on their eastern side. Rotation associated with faulting and graben formation contributes to the locally strong easterly plunge of fold axes in the basement rocks. Around Dumfries the basins are fault-controlled, but in a much more complicated style, involving oblique-slip faults and extensive normal-fault arrays. The fault reactivation involved extensive and important rotation of the basement structural fabric at the basin margins. We are grateful to the Queen's University of Belfast for support of fieldwork for this study. Skilled support was provided by G. Alexander and M. Pringle. The manuscript was improved by the comments of an anonymous reviewer.
References ALSOP, G. I. & HUTTON, D. H. W. 1993. Caledonian extension in the north Irish Dalradian: implications for the timing and activation of gravity collapse. Journal of the Geological Society, London, 150, 33-36. ANDERSON, T. B. 1987. The onset and timing of Caledonian sinistral shear in County Down. Journal of the Geological Society, London, 144, 817-825. & CAMERON, T. D. J. 1979. A structural profile of Caledonian deformation in Down. In: HARRIS, A. L., HOLLAND, C. H. & LEAKE, B. E. (eds) The Caledonides of the British Isles - Reviewed. Geological Society, London, Special Publication, 8, 263-267. ARMSTRONG, G. 1972. Review of the geology of the British continental shelf. The Mining Engineer, 131,463-481. BARNES, R. P., ANDERSON, T. B. & MCCURRY, J. A. 1987. Along-strike variation in the stratigraphical and structural profile of the Southern Uplands Central Belt in Galloway and Down. Journal of the Geological Society, London, 144, 807-816. BROOKFIELD, M. E. 1978. Revision of the stratigraphy of Permian and supposed Permian rocks of southern Scotland. Geologishe Rundschau, 67, 110-149. BULLERWELL, W. 1961. The gravity map of Northern Ireland. Irish Naturalist's Journal, 13, 254-257. COOK, A. H. & MURPHY, T. 1952. Measurements of Gravity in Ireland. Gravity Survey of Ireland North of the Line Sligo-Dundalk. Dublin Institute for Advanced Study, Geophysical Memoir, 2, Pt 4. COWARD, M. P. 1990. The Precambrian, Caledonian & Variscan framework to NW Europe. In: HARDMAN, R. F. P. & BROOKS, J. (eds) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geological Society, London, Special Publication, 55, 1-34. CRAIG, G. Y. & WALTON, E. K. 1959. Sequence and structure in the Silurian rocks of Kirkcudbrightshire. Geological Magazine, 96, 209-220. DOBSON, M. R. & WHITTINGTON, R. J. 1992. Aspects of the geology of the Malin Sea area. In: PARNELL, J. (ed.) Basins on the Atlantic Seaboard." Petroleum Geology, Sedimentology and Basin Evolution. Geological Society, London, Special Publication, 62, 291-311.
-
-
INFLUENCE OF BASEMENT ON THE GEOMETRY OF BASINS
121
EVANS, D., KENOLTY, N., DOBSON, M. R. & WHITTINGTON, R. J. 1980. The Geology of the Malin Sea. Institute of Geological Sciences, Report, 79/15. GAWTHORPE, R. L., GUTTERIDGE, P. & LEEDER, M. R. 1989. Late Devonian and Dinantian Basin Evolution in Northern England and North Wales. Yorkshire Geological Society Occasional Publication, 6, 1-24. GEORGE, T. N. 1967. Landform and structure in Ulster. Scottish Journal of Geology, 3, 413-448. GLENNIE, K. W. 1990. Rotliegend sediment distribution: a result of late Carboniferous movements. In: HARDMAN, R. F. P. & BROOKS, J. (eds) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geological Society, London, Special Publication, 55, 127-138. HALL, J. & SMYTHE, D. K. 1973. Discussion of the relation of Palaeogene ridge and basin structures of Britain to the North Atlantic. Earth and Planetary Science Letters, 19, 54-60. , BREWER, J. A., MATTHEWS, D. H. & WARNER, M. R. 1984. Crustal structure across the Caledonides from the 'WINCH' seismic reflection profile: influences on the evolution of the Midland Valley of Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 75, 97-109. HULL, E., NOLAN, J., CRUISE, R. J. & M'HENRY, A. 1890. Explanatory Memoir of Inishowen, County Donegal, to accompany sheets 1, 2, 5, 6 and 11 (in part) of the maps of the Geological Survey of Ireland. Geological Survey of Ireland, Memoir. HUTTON, D. H. W. 1987. Strike-slip terranes and a model for the evolution of the British and Irish Caledonides. Geological Magazine, 124, 405-425. JACKSON, J. S. 1965. The Upper Carboniferous (Namurian & Westphalian) of Kingscourt, Ireland. Scientific Proceedings, Royal Dublin Soc&ty, Series A, 2, 131-152. KELLING, G. & WELSH, W. 1970. The Loch Ryan Fault. Scottish Journal of Geology, 6, 266-271. KIMBELL, G. S., CHADWICK, R. A., HOLLIDAY, D. W. & WERNGREN, O. C. 1989. The structure and evolution of the Northumberland Trough from new seismic reflection data and its bearing on modes of continental extension. Journal of the Geological Society of London, 146, 775-787. LEEDER, M R., FAIRHEAD, D., LEE, A., STUART, G., CLEMMEY, H., EL-HADDAHEH, B. & GREEN, C. 1989. Sedimentary and Tectonic Evolution of the Northumberland Basin. Yorkshire Geological Society Occasional Publication, 6, 207-223. MCKERROW, W. S., LEGGETT, J. K. & EALES, M. H. 1977. Imbricate thrust model of the Southern Uplands of Scotland. Nature, 267, 237-239. MCLEAN, A. C. 1978. Evolution of fault-controlled ensialic basins in northwestern Britain. In: BOWES, D. R. & LEAKE, B. E. (eds) Crustal Evolution in Northwestern Britain and Adjacent Areas. Geological Journal Special Issue, 10, 325-346. & DEEGAN, C. E. 1978. The solid geology of the Clyde Sheet. Institute of Geological Sciences, London, Report 78/9. MANSFIELD, J. & KENNETT, P. 1963. A gravity survey of the Stranraer sedimentary basin. Proceedings Yorkshire Geological Society, 34, 139-141. MURPHY, T., YOUNG, D. G. G. & BRUCK, P. M. 1971. The post-Dalradian strata along the north-west coast of Lough Foyle, Inishowen, Co. Donegal. Proceedings of the Royal Irish Academy, Section B, 71, 171-181. MYKURA, W. 1967. The Upper Carboniferous rocks of southwest Ayrshire. Bulletin of the Geological Survey of Great Britain, 26, 23-98. NAYLOR, D. & SHANNON, P. M. 1982. Geology of Offshore Ireland and West Britain. Graham & Trotman, London. NOLAN, J. & EGAN, F. W. 1885. Explanatory Memoir to Accompany Sheet 12 and part of Sheet 6 of the Maps of the Geological Survey of Ireland, including the country around Limavady. Geological Survey of Ireland, Memoir. ORD, D. M., CLEMMEY, H. & LEEDER, M. R. 1988. Interaction between faulting and sedimentation during Dinantian extension of the Solway Basin. Journal of the Geological Society of London, 145, 249-259. PARNELL, J., SHUKLA, B. & MEIGHAN, I. G. 1989. The lignite and associated sediments of the Tertiary Lough Neagh Basin. Irish Journal of Earth Sciences, 10, 67-88.
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PRINGLE, J. 1952. On the occurrence of Permian rocks in Islay and North Kintyre. Transactions of the Geological Society of Edinburgh, 14, 297-301. ROBERTS, J. L. & TREAGUS, J. E. 1977. The Dalradian rocks of the Southwest Highlands Introduction. Scottish Journal of Geology, 13, 87-99. STEIN, A. M. 1988. Basement controls upon basin development in the Caledonian foreland. Basin Research, 1, 107-119.
From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe,
Geological Society Special Publication No. 91, pp. 123-143.
Triassic correlation and stratigraphy in the South Central Graben, UK North Sea P. J. G O L D S M I T H 1, B. R I C H 2 & J. S T A N D R I N G 2
1phillips Petroleum Company Norway, PO Box 220, 4056 Tanager, Norway 2Simon Petroleum Technology Ltd, Exploration Services, Llandudno, Gwynedd, LL30 1SA, UK Abstract: Triassic strata in the South Central Graben of the North Sea have yielded moderate to good recoveries of palynofloral assemblages and have sufficiently distinct petrophysical responses to enable the subdivision of the succession into between seven and eleven units. The Triassic succession of the Amoco well, 30/12b-3, is proposed as the type section for the Skagerrak Formation of the area. Despite lacking good biostratigraphic control, typical log character for the sequences is well displayed. The mainly Scythian (early Triassic) red mudstones of the Smith Bank Formation can locally be divided into three units, but generally only one is recognized. A new lithostratigraphic nomenclature for the middle to late Triassic Skagerrak Formation is proposed. Six members, with eight units, are recognized over UK Quadrants 29 and 30, and may possibly continue into the southern part of Quadrant 22 and into Norway Quadrant 7. The member names are partly derived from the Phillips fields in which they were first recognized. The basal Judy Sandstone Member is named after the Judy Field discovery well 30/7a-4A, and is overlain by the petrophysically distinct Julius Mudstone Member. The Joanne Sandstone Member is named after the Joanne Field well 30/7a-3, and is overlain by the Jonathan Mudstone Member, which can be further subdivided into upper, middle and lower units on log character. The uppermost members are the Josephine Sandstone Member, from the type well on the Josephine structure, 30/13-1 (Deegan & Scull 1977), which is overlain by the Joshua Mudstone Member. The upper members are rarely preserved, due to extensive early to middle Jurassic uplift and erosion.
Triassic sequences have, until recently, been considered essentially barren of diagnostic biostratigraphic material (Fisher & Mudge 1990; Warrington & IvimeyCook 1992; Lervik et al. 1989), and lithological and log correlations based on limited sections have been unreliable. With the release of wells drilled to deeper targets, a broader database has become available for review. Lithological and log correlations, based on the recognition of distinctive responses, have been considerably enhanced by significant palynofloral recoveries from cuttings and sidewall cores. These recoveries have been achieved despite the detrimental effects of using polycrystalline diamond bits, which have resulted in pulverized cuttings, and the use of oil-based muds, which bind the cuttings and which sometimes contain exotic palyniferous material. However, the use of de-oiling and deemulsifying agents and caustic detergents on cuttings samples has contributed to palynomorph recovery.
124
P. J. GOLDSMITH E T AL.
Geological setting The study area and the wells included in this interpretation are located in the southern part of the UK Central Graben (Fig. 1). Throughout the Triassic, this area was at the distal end of a continental clastic system, with sediment originating mainly from active fault zones flanking the Norwegian mainland (Steel & Ryseth 1990), but with additional source areas in the Scottish Highlands and the Fladen Ground Spur. The Auk and Hidra Highs may also have been locally active. To the south, the MidNorth Sea and Ringkobing-Fyn Highs probably flanked a southern continuation of the Central Graben, linking the Northern and Southern North Sea Basins. The study area was at the confluence of southerly flowing ephemeral fluvial systems, developed
Fig. 1. Location map showing study areas and wells correlated.
TRIASSIC STRATIGRAPHY IN THE SOUTH CENTRAL GRABEN
125
in a seasonal, monsoonal climate in the northern tropics (Robinson 1973), and on the northern margin of a shallow marine environment developed in the Southern North Sea (Ziegler 1990; Warrington & Ivimey-Cook 1992), Red mudstones of the mainly Scythian, Smith Bank Formation were deposited, possibly under continental playa conditions (Warrington & Ivimey-Cook 1992), or in an extensive hypersaline lake in which fresh water and fine sediment supply exceeded evaporation (Hodgson et al. 1992). There is no precise biostratigraphic evidence in the study area for the timing of the onset of significant sand
Fig. 2. Generalized Triassic palaeogeography (adapted from Warrington 1992; Ziegler 1990; Steel 1990).
126
P. J. GOLDSMITH E T AL.
sedimentation of the Skagerrak Formation, but it is certainly no later than Anisian. Usage of the Smith Bank Formation in this study as being mainly restricted to the Scythian within the Central Graben is a revision of the usage proposed by Deegan and Scull (1977). This new usage has been adopted by Cameron (1993) in the revision of UK Permo-Triassic stratigraphic nomenclature. The Skagerrak Formation comprises sands and thin interbedded, grey and red mudstones deposited in a channelized and sheetflood fluvial environment, with ephemeral playas and marginal lacustrine or marine reworking. Sandstones of the Skagerrak Formation, deposited during the middle and late Triassic, were interrupted by two periods of extensive mudstone deposition, in the latest Anisian to early Ladinian and late Carnian to (?) early Norian. It is suggested that these may represent periods of possible marine inundation in the Central Graben extending as far north as about 57~ (Fig. 2), although the evidence for this conclusion is circumstantial. A younger, Rhaetian, mudstone may also represent a return to possible marine conditions, prior to the more extensive early Jurassic transgression. These possible marine incursions may be a result of eustatic changes and may correlate with similar cyclic trends in the Southern North Sea, North Danish and Egersund Basins. Hodgson et al. (1992) suggest that the Smith Bank Formation sediments were deposited in topographic lows between rising salt swells and that Skagerrak Formation sheetflood sands were deposited over and between these sediment pods. Continued withdrawal and dissolution of the initially thick salt layer, together with rift-related subsidence, were sufficient in the Central Graben to accommodate this sediment influx. On the graben margins, early Triassic grounding of sediment pods, due to withdrawal of thinner salt, resulted in the formation of condensed sequences or poor preservation of post-Scythian Smith Bank Formation and of Skagerrak Formation sediments. The graben flanks were also significantly uplifted during the early to middle Jurassic, and major erosion of Triassic sediments occurred at this time. The considerable thickness variations seen in wells penetrating the Skagerrak Formation in the Central Graben can be largely attributed to the effects of differential salt withdrawal through time. The grounding of thick early Triassic pods in the South Central Graben during the middle to late Triassic resulted in the lateral migration of the Skagerrak depocentres. Further flank withdrawal and uplift in late Triassic to early Jurassic times resulted in marked angular unconformities in places.
Correlation
/
It is possible to erect a correlation between the majority of the wells in this study using the wireline log and lithological characteristics of the claystone members alone (Figs 3, 4 & 5). Although it is a 'claystone-driven' correlation, the sandstone members are clearly not intimately related to the mudstone sedimentary environment, as in other 'mudstone-driven' lithostratigraphies, such as the Lista Formation of the Palaeocene. Indeed, in the Northern Central Graben, where the claystone units are absent and biostratigraphic evidence is sparse, it is not obvious how to subdivide the massive sandstone sequences encountered. On the graben flanks, where Smith Bank Formation mudstones are predominantly preserved and there are only isolated occurrences of Skagerrak Formation, it is possible that the Smith Bank Formation may be laterally equivalent to at least the lower sandstone members of the Skagerrak Formation.
30/7,t-e
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m
TRIASSIC STRATIGRAPHY IN THE SOUTH CENTRAL GRABEN
Fig. 6. Central Graben Triassic well database.
127
128
P. J. GOLDSMITH E T AL.
Figure 6 shows the Regional Triassic well database used in this study and il',ustrates the limits of the divisible Skagerrak Formation in the South Central Graben. Figure 7 illustrates the stratigraphic relationships between the South and North Central Grabens and the graben flanks. Correlations based on gamma-ray and sonic logs alone are not suitable in the Triassic, where micaceous and feldspathic sandstones tend to have erratic, high gamma-ray responses and oxidized non-organic mudstones, sometimes with anhydrite, can have low gamma-ray responses. It is necessary to use a full suite of conventional logs. The density-neutron combination is of particular value, as is the display of computer-processed lithology and porosity interpretations, together with the logs, at a condensed vertical scale, as has been adopted in Figs 3, 4 & 5. The additional evidence from biostratigraphic dating confirms the lithological correlation and establishes a basis for extending the stratigraphic interpretation of the Triassic into hitherto uncorrelatable areas. Summaries of the main biostratigraphic dating evidence from ten wells is shown in Figs 8, 9, 10 & 11, with interpreted chronostratigraphic occurrences of Triassic lithologies, from 22 wells, shown in Fig. 12. A range chart for the Triassic microfloras occurring in these wells is shown in Fig. 13. The bulk of the palynological evidence found in the study area was first recognized in Triassic sequences in the Southern North Sea (Geiger & Hopping 1968), and continental Europe (Klaus 1964; M~idler 1964; Orlowska-Zwolinska 1983; Scheuring 1970; Van der Eem 1983; Wille 1970). Additional biostratigraphic sources utilised include Balme (1970), Fisher (1979) and Hochuli et al. (1989). Further correlation evidence comes from seismic data where, on good quality data, at least one of the lithological boundaries can be identified within the Skagerrak Formation.
WESTERNPLATFORM
SOUTHCENTRAL GRABEN
NORTHCENTRAL GRABEN
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Fig. 7. Triassic stratigrahic relationships in the Central Graben.
I.-
TRIASSIC STRATIGRAPHY IN THE SOUTH CENTRAL GRABEN
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~ 41i
-~.
IJ7,tswc)41.48 [.Loo~nlon(41)~ IJT,[swc) to 17 E,Anision(41)
j!
6,:I],(core),ll,~corel I
--. (48) 42.48 1I E.Norian to $mlfl~ion (48)
24,26,53 L,Cornion(53) to L ac::linian(53) L.Cornlon(23,51) to Lodinion(]9o40)
3,23,36.39. 124w
12668:
I
EJ.odlnlan(17.41) tp L,Aniston(iS) ] -24
1245_,,7
]
Anislon(43)
L,CorNo~ to LJ~l=JOn
-36(SWC) ] N o t i o n to L,.%-'yth~n
-IT,41(Swc) ] ELocllnlon+o k~lan
30/78-6 , v~
I.m 2,,~
UlIM ~MC PonoelTY 41 IN
l
~2e5 .ITtcore)
~ Loee~Z=
JOANNE
,e,s~ n.~ z~e G~ SONICi i~t~o~rY llS~TY NBa'tI~N 0 IS~ 14~ 40~100 0 ~2 ~ q~ -1t
JUDY
13204
17,35
$ ST
'~-
~25.~8, JOANNE
sMrrHBAN
3ST
me=
,~Io == o: JUDY I.]T25
U.J~OSSIC In c o r e '~Ul~Zo~2~ Rhoo~0k~
FAU
12326
.,vgn/7a-3 iO v~ ~0e
$ ST
.
~4,2e
J u~,~.~,~,4~ ~.
38
DEPTHS TVDSS
Fig. 8. Biostratigraphic data for wells 30/7a-7, 8, 3 & 6.
mm
,
I-
,i
} ~ont2~4m
130
P. J. GOLDSMITH E T AL.
30/7a_9 i v ~ GR SONIC lS~ 14o 1oo
I
oz~-~
!.!6 Z.~
mmlw't @12 ~ U
-~.
L. JO ~ATHAN I~ IUDS' 11804 ~
.
p-
n
)3.15.28,30 t 13.15,30 Rhoetron(or younger) to 15.26 M.Norlon(15) 26 33(core) 1 E.Norlon (33) 29(core)331 to Lad[nron (29)
IE ;!
oO.
[
|.i
,8,16,36,41,54,(swc)[misflre]" , 33,41,43,(Svc){mlsflre] E.Lodlnlon(16.41) to L.Anlsion(tT) ,33.41.(swcKmlsflre] to I E.Anision(3.41) 7.16.17.41.(swc)[misf]r e ] ~33.4T - 3.33.41.(swr e]
i
.N
!
. 9
13343d
m-
2.3.8.J2.2426.34.36.38.42.48.49.SI.53.56.(swc)
F?
j~~!. i
!J' lUUSMU[STOF ItE':_:-!i
1.3.10.23.25.36.41.47.51.(swc) } E.Lodinion (10.25.41)
~
i~ ]
,3eo2~
,i
Cornion (2.53) to Lodlnlon (38,53)
_
1.3.16.24.2636.41.42.52(swc) 1
o,
E.L ocllnion(41) to E.Anlsion(24.41}
1.3.7.8.11.2224.26.33.39.41.43.44.50.(swc){mis fTr e] I
JOANNE I~ST.
i JUDY SST
25
E.Lodlnian (41) to E.Anlsian (3.24.41)
14,51 1,3.4,T,16,1824,41,43 ] ~l,3,7.SJ6,2],24,26.33.41,42.43,SI.52.tswc) J ~1.3.1921,23.24.26.31.33.41,42.43.44.47,48.51.(swc) l 1.3.7J6,73.33.41.43.52 }
DEPTHS TVDSS
Fig. 9. Biostratigraphic data for wells 30/7a-9.
E.Lodlnion (41) M.Anlsion (431 E.L.Anislon (19)
E'L~ (41) J M.Anlsion ,o (43) M.Anlslon (431 E.L.AnIsion (191 tO E.Anlsion (23.24,41)
TRIASSIC STRATIGRAPHY IN THE SOUTH CENTRAL GRABEN
131
I REFERENCE WELL FOR[ SKAGERRAK AND I SMITHBANK FMS. ] 30/12b-2.
e
~
t,~
OR SONIC I~ 140 4~ lib
30/' 2b-3 o
~ @i.2 20~4
GR
-~,
o
~
la
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~
I
J(~S HUA.~
_
3653 --11- ~
!
1360S
~
~
- 20.29
]
- 25.29.33.40.51.52 - 33.52.53 ~ U.Ccrnzon [52~3,~
z
|" 14315 ,4427
9
~
" 53
jM.Lodfnlon
-- (531
141'91 ~
"~
I
J~l~ I U S ~ ,ST~
'
~
i
14419 ~ ~ ~ - 10,17.35 1EJ. a(llnRrd,0.l,, ~ ~'~ ~J~ J U ~ I ,o j"~ ~;~tt ." IO,lTlo,i7 =': 9. _ L'AnlslonllOJTl14534 .~L . - 10,17 ~
. I ::~: ili:I~" iT
,,,~
,.
-
?
~845
3,17}
_
E.Lodlnlon (17.40, *o t.Anlslan (17.40)
E.Lodinfan 117) to L.Anlsion (17)
"
_
rif(C)
i
Sial r
sMI HBANI
- IT(c)
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~"
17.40.4,.$2 }
i ;
r
i
i
I
- 33(c)
(c):coved OEPTHS MOSS
Fig. 10. Biostratigraphic data for wells 30/12b-2 & 3.
Nomenclature The nomenclature used in this paper differs from the scheme adopted internally by Phillips Petroleum Co. for reservoir description, where the reservoir sands and their overlying mudstone seal rocks are informally termed, respectively, the Judy, Joanne and Josephine Sandstone and Mudstone Members. Three further member names have been added to differentiate the mudstone members: Julius, Jonathan and Joshua. These allow for their separate description for stratigraphic nomenclature purposes. A formal description can be found in Cameron (1993), in which this scheme has been adopted.
132
P. J. GOLDSMITH E T A L .
30/16-6 o
~r~ ~10
,~
30/1~1
~ I.Y. LM
40 leo
0 |.2
~ 45
o
-1!
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-
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)
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r
C a r n l o n (52) to L o d l n l o n (29)
"
IT.
9 '
(C) -40 } Notion to Lodlnlon 2.23,52 } C o r n i o n (521 +o M.Anislon (Z3) - 17 cc)
m
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- 26
SMII ~BA~ K FIV - 1
(r
-zecc)
I
15 [c)
15 (c)
1213 !t
(c)=cove(:l OEPTH$ MOSS
Fig. 11. Biostratigraphic data for wells 30/16-6, 30/13-1 & 30/17a-4.
!
'
Rhoetlon tO Notion
TRIASSIC
STRATIGRAPHY
STRATIGRAPHY
IN
CHRONOSTRATIGRAPHIC
JOSHUA ~IUOSTONE MEMBER
SANDSTONE ME.E.
=
.
.
.
.
.
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.
OCCURRENCE
.
.
.
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:67.
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~
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-- . . -. - -. - -. - - - :-:
JOANNE SANDSTONE
,,-,',..;....-.4-:.:.: :':':" ;:':':
:::_
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~, ~
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-
141,~-195-
: - - - - : - ! :~::~ .o~ "~2
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m3219
~ J~JLRJI~ MUOSTONE
1350E --?8-:
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:--:-
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[With thickness ond TVDSSi
12168 :'B_,'T-:
.
133
GRABEN
....
.
,n e~--
LITHOLOGY
LOWER
...
(j
CENTRAL
:,:.:..,..... ~1367g
GI48
::.g.4"::
UPPER & "-:-:-"
|
SOUTH
=-40"'~5
. --
MUDETONE MEMBER
THE
~'*'"'" 11212 ?""
.'~.'~ ".'-', b~&.~ |~'q~:
:':':'~:':':'~:':'::~:::':" ::::::: ~ -~'~"~ 1f049 == "'*%-'~2"-"3'~r~' . . . . . ~""-:'2~'"-" . . :
r3e$2
:~;:: :~'~:::~"~;r:
~'--~ Iz?S'/-'-'-52 -'-'--~--' -'-~'-:~-'--'-'---z :::-
--
~'~':'~::~'~ :::== ~ ".'.'.''.'.'-',',-'-' SANDSTONE
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MEMBER
.::,,,.:.~o~ .:,~o
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~,~::::::: ~
-'-'-" ,'-',"
:.:.:.: :,,~.: "-'.'.' .'.'.'--'.'-',14329
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-
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.
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:.:.:-:.~::..-i~:.:-i] ':~'~": r0 '"'-'-~:.:-i.] . ,..,.. .'.'.', ,___,____ -.~0_0:
--_
-----------_--_-:--: ,213
:
, o fi,:.:!] TO ..... -__-,-3.,~::
:C-: TO
Fig. 12. Chronostratigraphic occurrences of Triassic sediments in the South Central North Sea.
Smith Bank Formation The Smith Bank Formation comprises a thick sequence of red, arenaceous mudstones and is the lower of two formations in the 'Triassic Group', renamed the Heron Group by Cameron (1993). The unit has a wide density-neutron log separation which commonly tapers upwards to the base of the overlying Judy Sandstone Member. The Formation usually has a sharp basal contact with the underlying Zechstein and a moderately sharp upper contact. Gamma-ray, sonic and resistivity logs usually display a monotonous character. The Formation displays these typical characteristics in wells 30/17a-4, 30/12b-2, 30/12b-3 and 29/14b-2, although a thick silty section with conformable upper and lower boundaries occurs near the top of the Formation in wells 30/7a-4A, 30/7a-6 and 30/7a-7, which may be a lateral equivalent of the Bunter Sandstone Formation of the Southern North Sea (Figs 4 & 14). Within the study area, the base of the Formation is seen only in wells 30/17a-4, 30/12b-3 and 29/14b-2, and even here the section present above the Top Zechstein may not be of earliest Scythian age. A number of wells in the Central North Sea which have penetrated the Zechstein below the Smith Bank Formation have, at the base of the Triassic, a section of 50-100 ft in which the mudstones are siltier, anhydritic and dolomitic. These may represent the earliest Triassic deposits preserved and may be indicative of reworking of Zechstein facies. The high degree of oxidation of the Smith Bank Formation sediments is presumed to have resulted in the destruction of any kerogen, including palynomorphs, and as such precludes biostratigraphic correlation within the study area. However, Smith Bank
134
P. J. G O L D S M I T H
E T AL.
I
i i
I
I
i
.,,-q
.B
i I
I
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-+ ,..=
+ ~ ++
.+~
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+++++m+
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i!-+i -" ++.,+ 9
"+ ---+-+ +-+ _~ i+ ++ ++++~+~ +,-++,,
~i~
+~ +_!+"
++++ Ji+-ii!. +-'+~" +_+++.-. ~ |~176176176176 .++ ,,+++
- ~t++ +:m.+++. ++-+=+~~ "+++---++ " m m + + + , +i+.+.++ ~++~,,,++moo -
+!+++++!++~+_-m++m..-.;m
~,r
<~ .~+
m.+~:|+-m+~_++~++.<++< ~+~mmm- .omm+m+ +m+<:+:++_.+,-_ ++i+++P,++,,-,+-+~+ mm
......... +~mm+ +++++++++++++ +++:+.+. <-++-++:--~ .+_--+++ t++++++_, 1+!~ +,+ .+++i+--++++++~+~ --'.+.-+ ~176 ~,
m,-i,.._-~e=_-m
~ m mm~
m m m m m m m m m ~ m m m m m + , ~ ~ + : : : .- , ~
0 q= 0
+ +m + m m+ m m
.+m [.. t~
TRIASSIC STRATIGRAPHY IN THE SOUTH CENTRAL GRABEN
135
Formation mudstones on the Western Platform of the Central Graben have previously been identified as early Scythian to Anisian, or younger, in age from sporomorphs recovered from wells 21/11-1, 21/26-1D, 29/25-1 and 30/16-2 (Brennand 1975). The few taxa recovered in the study area (e.g. from well 30/12b-2, Fig. 10) are interpreted, on the basis of their preservational style, as caved from higher intervals. The unit can be correlated on its distinctive wireline log character, its stratigraphic position and its lithological nature. The sequence is presumed to be of mainly Scythian age within the Central Graben due to its position below the Judy Sandstone, which is dated as being at least as old as Middle Anisian in its upper section in well 30/7a-9.
MY[
SOUTHERN NORTH SEA (Fisher & MuClqe)
STAGE
~0
~~ _ ~
~:.:<-:.:,:.:~.ro,0ed!! I "~176
...,......,..
RHAETIAN
CENTRAL NORTH SEA CTN~Polaar'!
CENTRAL NORTH SEA (Fisher & bludqe)
N_22
215
r~."11 ~"" " 5:-:':':~ : ~
Fluvial?
o:':':':':'" ~ ~
NORIAN
%:: '
^ ^
=.,^^ ( ^ A
^
':
^
,
I
av
Mot'trio
223
CARNIAN
?
.....
I
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^ ^
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r
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==
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~
.
.....,
i~
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/
Marine
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I
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I
Fig. 14. Correlation of Triassic stratigraphy and lithofacies in the North Sea. A comparison with Fisher & Mudge (1990).
136
P. J. GOLDSMITH ET AL.
Skagerrak Formation The Skagerrak Formation in the Central and Northern North Sea comprises a very thick stacked sequence of mainly fluvial and sheetflood deposited sandstones, with thin interbedded overbank and lacustrine facies. In the South Central Graben this sequence includes three major mudstone intervals: the Julius, Jonathan and Joshua Mudstone Members, interdigitated above the Judy, Joanne and Josephine Sandstone Members, respectively (Fig. 12). Recovery of palynomorphs in the Skagerrak is variable, as a result of drilling processes and original preservation. Palynomorph recovery in the Judy Sandstone, Julius Mudstone and the lower part of the Joanne Sandstone is locally good to abundant, and yields Anisian to Carnian ages. The remainder of the Skagerrak Formation is generally barren. In the Skagerrak, the degree of reddening becomes highly variable and colour is not diagnostic, on its own, of any particular correlation potential. In most wells there are many intervals with grey or grey-green sections. Reddening occurs at discrete horizons and may indicate periods of temporary aridity. The Julius Mudstone Member is normally grey in colour, while the Jonathan Mudstone Member and Joshua Mudstone Member are commonly red, although this is thought to be secondary. In some wells, the entire Skagerrak Formation is reddened, as at wells 30/12b-2 and 3. This probably represents deep oxidation in the latest Triassic to earliest Jurassic, when substantial erosion took place. However, the reddening in these wells may also be related to exposure during intermittent sedimentation, as suggested by the relatively condensed nature of these sections.
Judy Sandstone Member This predominantly arenaceous unit is characterized by erratic log profiles, reflecting the presence of numerous interbedded, silty claystones within the massive fluvial sandstone bodies. The basal and upper contacts are usually moderately sharp. The sandstones are predominantly fine-grained, with medium- to coarse-grained sands occurring in major channel sand bodies, occasionally with conglomeratic and erosive bases. They vary from olive-grey to yellow to pinkish grey and are feldspathic, argillaceous and often micaceous. The mudstones are variably medium grey, greenish grey, dark grey, olive-black and greyish red to red, with occasional carbonate nodules. The degree of reddening is variable. The high content of feldspar and mica results in a variable, and commonly high, gamma-ray response. The upper part of the unit is dated within the range Anisian to early Ladinian from most wells, but has an upper limit probably just younger than middle Anisian (Pelsonian) from evidence in well 30/7a-9. The lowest part of the Member is commonly barren, although the upper section, immediately below the Julius Mudstone, is frequently characterized by a microfloral assemblage including: Plaesiodictyon mosellanum, Aratrisporites granulatus, Porcel-
lispora longdonensis, Protodiploxypinus sittlerii, Protodiploxypinus doubingeri, Raistrickia 'nordica', Illinites kosankei, Illinites chitnoides, Gordonispora rota, Cyclotriletes? spp. SPT (Simon Petroleum Technology) (Figs 8, 9, 10 & 11). This assemblage indicates an age no younger than early Ladinian (Fassanian) (Fig. 13). The deepest recovered samples in the Judy Sandstone Member include forms such as
TRIASSIC STRATIGRAPHY IN THE SOUTH CENTRAL GRABEN
137
Illinites kosankei and Protodiploxypinus sittleri, which range no older than middle Anisian. The recovery of Densoisporites nejburgii from a sidewall curve in well 30/7a-9 confirms a middle Anisian age.
Julius Mudstone Member This unit, which is petrophysically very distinct, is dominated by medium-grey silty mudstones which can also be brownish-grey, greenish-grey and reddened. Minor occurrences of anhydrite have been recorded and, in disseminated form, may account for the typical log responses of this unit. The gamma-ray response has a concave bow shape and usually has a lower response than the adjacent sandstones. There is a significant density-neutron log separation with a convex bow-shaped density response, and there are also convex bow-shaped, resistivity and sonic responses. Exceptions to this occur in wells 29/8a-3, 29/14b-2, and 29/19-1A, and in what is believed to be the lateral equivalent in the Marnock area, where the gammaray response is higher than the adjacent sandstones. This may be due to a facies change towards the margins of the depositional area, where anhydrite is less common. No dating evidence has been sought in these wells and an alternative correlation is a possibility. Contacts at the top and base of the Member are moderately sharp, sometimes with a sharp upper contact. A downhole decrease in both sonic velocity and bulk density at the contact between the Julius Mudstone and the Judy Sandstone creates a significant acoustic impedance contrast and a strong black event on normal-polarity seismic data, which can be of local correlative value. The Julius Mudstone Member is characterized by the downhole appearance of abundant Middle Triassic palynofloras. An acme of the miospore Cyclotriletes? spp SPT (Fig. 13) occurs in this unit in wells 30/12b-2,3, 30/17a-4, 30/7a-7 and 30/7a-9 (Figs 8, 9, l0 & 1 l) and constitutes a reliable correlative datum. This is associated with the first downhole appearance of Protodiploxypinus sittleri of early Ladinian to middle Anisian age (Fig. 13).
Joanne Sandstone Member Like the Judy Sandstone Member, this unit comprises a sequence of interbedded sandstones, mudstones and minor siltstones. The sands are also predominantly finegrained but medium to coarse channel-fill sands are somewhat more common. They are pale grey to pale orange, feldspathic, micaceous and argillaceous, while the mudstones are silty, micaceous and medium grey, dark grey to reddish-brown in colour. Log responses are similar to those of the Judy Sandstone Member, although the higher mica and K-feldspar content results in a generally higher gamma-ray response (40 to 80 API), especially in the upper section, which may occasionally exceed 200 API. The upper and lower limits of the unit are defined by the log responses of the adjoining, usually thick, claystone units. The basal contact is sharp in the Block 30/7a wells, but more gradational elsewhere, and may represent a local unconformity. The upper contact is sharp in wells in Blocks 30/lc and 30/7a, but more gradational in wells to the south. The gamma-ray log is misleading, as lower API values occur in the overlying mudstones. The age of this unit is within the Carnian to Ladinian range, although evidence for the upper limit is less strong than
138
P. J. GOLDSMITH ET AL.
for the lower limit due to the general barrenness of the sediments. In wells 30/12b-2 and 30/16-6, the first downhole occurrence, in the upper part of the unit, of Triadispora, and of Gordonisporafossulata in well 30/16-6, indicates a Carnian age. The upper limit, from evidence in well 30/7a-9, is based on the appearance near the top of the unit of the alga Plaesiodictyon mosellanum, which has a local acme in the South Central Graben in the middle to late Carnian. The Carnian upper age limit is supported by the presence of Leschikisporis aduncus and Protodiploxypinus ornatus in well 30/12b-2, and by Illinites kosankei in well 30/16-6.
Jonathan Mudstone Member This unit comprises a thick sequence of mainly red, but also variegated, silty mudstones. The reddening is presumed to be due mostly to deep weathering after sedimentation. This Member is divisible into three units based on log character. The units can be recognized by their monotonous sonic response, wide density-neutron log separation, which is significantly wider in the lower unit, and by the two distinct declining upward gamma-ray responses, above an initial bow-shaped section, in the lower and middle units. Between the lower and middle units occurs a distinct spike of low gamma-ray, high density and neutron response, which corresponds to a dolomitic or anhydritic bed. Between the middle and upper units is a second correlatable low gamma-ray section, this time related to a sandy unit. There is a sharp contact with the overlying Josephine Sandstone Member and a gradational or sharp contact with the underlying Joanne Sandstone Member. The log responses at this lower boundary produce a moderate black event on normal-polarity seismic data, similar to that for the Judy Sandstone/Julius Mudstone boundary. These characteristic responses are well displayed in wells 30/12b-2,3, 30/lc-3,4 and 5A. The member is barren of palynomorphs in all the wells sampled.
Josephine Sandstone Member This is a sequence of clean, fine- to medium-grained, partially reddened sandstones present in wells 30/lc-3,4,5A, 30/12b-3 and 30/13-1 (the type well for the Josephine Member of Deegan & Scull (1977)). It is difficult to separate on logs from overlying middle and late Jurassic sandstones due to its characteristically low gamma-ray response. A single specimen of Ricciisporites tuberculatus from well 30/13-1 (Fig. 11) indicates an age within the Rhaetian to Norian interval (Fig. 13) for the Josephine Sandstone. In all other wells it is barren.
Joshua Mudstone Member This Member is encountered only in wells 30/7a-2, 30/12b-3 and 30/13-1 within the study area. In well 30/7a-2 it is a thin, dark grey silty mudstone. It has been dated as Rhaetian to Norian, based on the association of Ovalipollis pseudoalatus and an influx of Classopollis spp. (Fig. 13). In the other two wells it is a reddened mudstone, barren of palynomorphs. It is correlated on the basis of its stratigraphic position. The lower contact is sharp where present in wells 30/12b-2 and 30/13-1, while the upper contact is unconformable with the overlying Jurassic.
TRIASSIC STRATIGRAPHY IN THE SOUTH CENTRAL GRABEN
139
Palaeoenvironment From the palynomorphs recovered, an outline of the palaeoenvironments can be suggested. As the lower part of the Judy Sandstone Member is mostly barren, semiarid conditions may have prevailed from the Scythian. The upper part of the Julius Mudstone Member contains miospore species which includes gymnosperms (e.g. Protodiploxypinus and other bisaccate pollen) representative of a hinterland flora, as well as pteridophytes, bryophytes (e.g. Cyclotriletes? spp. SPT) and arborescent lycopods (e.g. Aratisporites) indicative of swamp conditions. These hinterlandswamp affinities, together with the evidence in core from the Judy Sandstone Member of a dominantly fluviatile depositional setting, with ephemeral playas and the presence of caliche and reddened horizons, indicate fluctuating climatic conditions, possibly in a coastal setting. The productive horizons in the Julius Mudstone Member are dominated by gymnosperms with, usually, subequal numbers of swamp-derived pteridophytes and bryophytes. The Joanne Sandstone Member shows a progressive upward increase in hinterland elements until the section becomes largely barren again, although only a few wells have sections younger than Carnian preserved, and these are commonly reddened due to late Triassic weathering and erosion. The presence of the microplankton Plaesiodictyon mosellanum, which can be found in fresh to brackish water conditions, is a useful palaeoenvironmental indicator. Where it occurs in interbedded mudstones, within the lower part of the Joanne Sandstone Member and the upper part of the Judy Sandstone Member, it indicates low energy, fluvio-lacustrine conditions, possibly with standing bodies of water formed after ephemeral flooding events. In the Julius Mudstone Member its presence and good preservation suggest either a restricted, lacustrine setting or reworking into a larger body of water in a proximal setting. Evidence for suggesting marine influences in the study area is circumstantial and includes the thickness and widespread distribution of the mudstone sequences and their inferred correlation to the marine phases of the Triassic in the Southern North Sea (Fig. 14).
Discussion Evidence from dating and correlation of the study wells shows that significant sandstone sedimentation began in the South Central Graben no later than the Anisian. Good quality channelized reservoir sands were deposited throughout the middle and late Triassic, and not only during the late Triassic, as suggested by Hodgson et al. (1992), Smith et al. (1993) and Dailly & Middleton (1993), based on dating and correlation evidence from the sands of the Marnock Field. A comparison between the released wells around the Marnock area in Block 22/24 and those in this study shows that one significant, but not major, mudstone section is present in the Skagerrak sands of that area. On log character alone it is not possible to match it unequivocally, but it may be equivalent to the Julius Mudstone Member (Fig. 12). Also in Marnock, the sands overlying this mudstone show a trend of increasing gamma-ray response response towards the top of the section. This is similar to the trend seen in the Joanne sands. This is overlain by sands with a lower gamma-ray response-the 'Marnock Formation' of Hodgson et.al, and Smith et al. This is believed to be the equivalent of the Josephine Sandstone Member.
140
P. J. GOLDSMITH E T AL.
Fisher & Mudge (1990) have suggested a correlation between the Central North Sea and the Southern North Sea Triassic. Comparison of their subdivision for the Central North Sea with that shown in this paper reveals significant differences in both chronostratigraphy and lithostratigraphy (Fig. 14) and indicates that caution must be used in purely lithostratigraphic correlations. Reservoir quality in the late Triassic Skagerrak Formation sands of the Marnock Field is very high (Hodgson et al. 1992; Smith et al. 1993). Porosities and permeabilities in the Skaggerak sands of the Judy Field are also high, but within reservoirs of both middle and late Triassic age. Hodgson et al. and Smith et al. attribute this high porosity preservation to overpressuring and to the formation of early diagenetic, iron-rich, chlorite grain coatings, which have prevented the formation of late quartz overgrowths. They further suggest that this may have occurred where Triassic pods grounded by the end of the Triassic and retained a fresh water aquifer during the late Jurassic transgression, or during more humid climatic conditions in the late Triassic. Studies on the Judy Field suggest that iron-rich chlorite grain coatings formed during early burial diagenesis within alkaline aquifers throughout the middle and late Triassic, during humid periods or in wet areas. Haematite formed at the same time under oxidizing conditions during arid periods or in arid areas. The high porosities of the Judy Field Triassic sands are attributed mainly to the initial high porosity of the clean, massive channellized sands, and secondarily to the widespread early chlorite diagenesis. This was subsequently preserved by early overpressuring during the onset of burial in the late Cretaceous, caused by fluid expulsion from the thick Jurassic mudstones adjacent to the Judy Horst.
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TRIASSIC STRATIGRAPHY IN THE SOUTH CENTRAL GRABEN
141
Poor porosities in the Triassic generally occur in more marginal sediments with a high clay content, or in areas where overpressuring conditions were less likely and therefore burial compaction and diagenesis progressed further. The poorer quality of the middle Triassic sands of the Marnock area (Hodgson et al. 1992; Smith et al. 1993) may be due to their original depositional facies. Figure 15 illustrates the poroperm characteristics of the Skagerrak Formation in the Judy Field and shows the close relationship to facies.
Dating problems Two wells revealed contradictory information. In well 30/7a-6 at around 12 330 ft (core), about 200 ft below the top Triassic surface, late Jurassic palynofloras were recovered from claystone rubble. The sandstone sections above and below contain good in-situ palynofloras of middle Triassic age, but the Jurassic assemblage is not found within the overlying formation of definite Jurassic age. It is interpreted as a fracture fill; mudstones deposited on the exposed Triassic surface during the late Jurassic transgression filled open fissures adjacent to a fault scarp, which was likely to have been developed at the time. The fissures may have developed due to gravitational instability on the underlying Smith Bank Formation mudstones. A similar setting is seen on the Inner Hebridean Island of Raasay, where fissures of the order of 200 ft deep extend into the massive Scalpa Sandstone overlying the Pabba Shales near Rudha na' Leac (Tim Pearce pers. comm.). These are orientated perpendicular to the principal stress direction and have been reactivated by postPleistocene land slippage. In well 30/7a-9, six sidewall cores, between 12 390 and 12960ft (log), produce consistent dates of early Ladinian to late or early Anisian age in a Joanne Sandstone section conclusively correlated to a dated Carnian to Ladinian age section in well 30/7a-8. Similar assemblages in well 30/7a-9 and in other wells occur within the Judy Sandstone and Julius Mudstone sections. These sidewall cores are thought to have been mis-shot.
Conclusions A new nomenclature for the Triassic sequences of the South Central Graben is proposed based on a seven-fold subdivision. Overlying the Smith Bank Formation is the Skagerrak Formation, which is now subdivided into six members. The Josephine Sandstone Member is adopted from Deegan & Scull (1977), while the Judy Sandstone Member and Joanne Sandstone Member are named after Phillips fields in UK Blocks 30/7a and 30/13. Intervening mudstone members have been termed, respectively, Julius, Jonathan and Joshua. Well 30/12b-3 displays all seven formal subdivisions as well as the three informal units of the Jonathan Mudstone Member, and is proposed as the reference well for the area. A more careful biostratigraphic evaluation of the Central North Sea Triassic wells and examination of all available well logs with computer-processed interpretations may result in a more reliable regional stratigraphic framework for the Triassic than has hitherto been possible.
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The authors would like to thank Phillips Petroleum Company United Kingdom Limited, Agip (UK) Limited and British Gas Exploration and Production Limited for permission to publish this paper. Thanks are also due to colleagues at Phillips Petroleum Company and Simon Petroleum Technology for their helpful comments, to Alun Powell, Kevin Bamford, Vicky Heritage and Sue Maguire for the draughting and manuscript and to Robert Knox and Don Cameron of the BGS for their support at all stages.
References BALME, B. E. 1970. Palynology of Permian and Triassic strata in the Salt Range and Surghar Range, West Pakistan. In: KUMMEL, B. & TEICHERT, C. (eds) Stratigraphic Boundary Problems: Permian and Triassic of West Pakistan. University of Kansas, Department of Geology, Special Publication, 4, 304-453. BRENNAND, T. P. 1975. The Triassic of the North Sea. In: WOODLAND, A. W. (ed.) Petroleum and the Continental Shelf of North West Europe. Vol. 1, Applied Science Publishers, 295-310. CAMERON, T. D. J. 1993.3: Permian and Triassic of the Central and Northern North Sea. In: KNOX, R. W. O, B. & CORDEY, W. G. (eds) Lithostratigraphic Nomenclature of the UK North Sea. British Geological Survey, Nottingham. DAILLY, P. & MIDDLETON, P. 1993. Permo-Triassic palaeogeography of the Northern North Sea and Norwegian Margin. In: Permian and Triassic Rifting in N W Europe. Conference abstract. DEEGAN, C. E. & SCULL, B. J. 1977. ,4 Proposed Standard Lithostratigraphic Nomenclature for the Central and Northern North Sea. HMSO, London, for the Institute of Geological Science, Report 77/25. FISHER, M. J. 1979. The Triassic Palynofloral Succession in the Canadian Arctic Archipelago. AASP Contributions Series 5B, Contributions of stratigraphic palynology (with emphasis on North America), vol. 2 Mesozoic palynology, 83-90. -& MUDGE, D. C. 1990. Triassic. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. Blackwell, Oxford, 191-218. GEIGER, M. E. & HOPPING, C. A. 1968. Triassic stratigraphy of the Southern North Sea Basin. Philosophical Transactions of the Royal Society of London, B, 254, 1-36. HOCHULI, P. A., COLIN, J. P. & OS VIGRAN, J. 1989. Triassic biostratigraphy of the Barents Sea area. In: COLLINSON, J. D. (ed.) Correlation in Hydrocarbon Exploration. Norwegian Petroleum Society, Graham and Trotman, London, 131-153. HODGSON, N. A., FARNSWORTH, J., & FRASER, A. J. 1992. Salt-related tectonics, sedimentation and hydrocarbon plays in the Central Graben, North Sea, UKCS. In: HARDMAN, R. F. P. (ed.) Exploration Britain: Geological lnsights for the Next Decade. Geological Society, London, Special Publication, 67, 31-63. KLAUS, W. 1964. Zur sporenstratigraphischen Einstufung von gipsfiihrenden Schichten in Bohrungen. Erdoel Zeitschrift ffir bohr-und fb'rdertechnik gewinnung aufbereitung transport. 80, 119-132. LERVIK, K. S., SPENSER m. M. & WARRINGTON, G. 1989. Outline of Triassic stratigraphy and structure in the Central and Northern North Sea. In: COLLINSON, J. D. (ed.) Correlation in Hydrocarbon Exploration. Norwegian Petroleum Society, Graham and Trotman, London, 173-189. MADLER, K. 1964. Die geologische Verbreitung von Sporen in der deutschen Trias. Beihefte zum Geologischen Jahrbuch, 65. ORLOWSKA-ZWOLINSKA, T. 1983. Palynostratigraphy of the Upper Part of Triassic Epicontinental Sediments in Poland. Prace instytutu geologieznego CIV, Warsaw. ROBINSON, P. L. 1973. Palaeoclimatology and continental drift. In: TARLING, D. H. & RUNCORN, S. K. (eds) Implications of Continental Drift to Earth Sciences 1, Academic, London, 451-476. SCHEURING, B. W. 1970. Palynologische and palynostratigraphische Untersuchungen des Keupers im B61chentunnel (Solothurner Jura). Schweizerische Palrontologische Abhandlungen, 88.
TRIASSIC STRATIGRAPHY IN THE SOUTH CENTRAL GRABEN
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SMITH, R. I., HODGSON, N. & FULTON, M. 1993. Salt control on Triassic reservoir distribution, UKCS Central North Sea. In: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geological Society, London, 547-557. STEEL, R. J. & RYSETH, A. 1990. The Triassic- early Jurassic succession in the Northern North Sea: megasequence stratigraphy and intra-Triassic tectonics. In: HARDMAN, R. P. F. & BROOKS, J. (eds) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geological Society, London, Special Publication, 55, 139-168. VAN DER EEM, J. G. L. A. 1983. Aspects of Middle and Late Triassic palynology. 6. Palynological investigations in the Ladinian and Lower Karnian of the Western Dolomites, Italy. Review of Palaeobotany and Palynology. WARRINGTON, G. & IVIMEY-COOK, H. C. 1992. Triassic. In: COPE, J. C. W., INGHAM, J. K. & RAWSON, P. F. (eds) Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoir, 13, 97-106. WILLE, W. 1970. Plaesiodictyon mosellanum n. g., n.sp., eine mehrzellige Griinalge aus dem Unteren Keuper von Luxemburg. Neues Jahrbuch ffir Geologie und Paliiontologie, Monatshefte, 5, 283-310. ZIEGLER, P. m. 1990. Geological Atlas of Western and Central Europe. 2nd edition, Shell International Petroleum, The Hague.
FromBoldy, S. A. R. (ed.), 1995, Permianand TriassicRiftingin NorthwestEurope, Geological Society Special Publication No. 91, pp. 145-159
Distinction between fault and salt control of Mesozoic sedimentation on the southern margin of the Mid-North Sea High P. A . G R I F F I T H S , W.
R. F I T C H E S
1'3 M .
R . A L L E N , 1 J. C R A I G , 2
1 & R . J. W H I T T I N G T O N
1
l Institute of Earth Studies, Llandinam Building, University of Wales, Aberystwyth, Dyfed, SY23 3DB, UK 2 L A S M O plc, 100 Liverpool Street, London E C 2 M 2BB, UK 3Midland Valley Exploration, 14 Park Circus, Glasgow, G3 6AX, UK Abstract: The North Dogger Fault Zone (NDFZ) is situated at the northern margin of the UK Southern North Sea Basin at the northern limit of the mobile Zechstein Supergroup. This fault zone underwent movement during late Triassic and probably early Jurassic times. The NDFZ is comparable with the southern part of the Dowsing Fault Zone (SDFZ), which was initiated during late Scythian times and continued to move until at least mid-Carnian times. The SDFZ is located at the southwestern edge of mobile salt. Both fault zones form part of the basin-bounding fault system, which is proposed to have been initiated as a response to the growth of salt swells in the centre of the Southern North Sea Basin. The fault formation is due to folding of the Triassic strata over the swells and overall shortening of the Triassic cover. The shortening was taken up as extension at the basin margins, corresponding to the edge of the mobile Zechstein salt.
The Southern North Sea Basin has a complex tectonic history, aspects of which have been examined by various authors (e.g. Glennie & Boegner 1981; Jenyon 1985; Van Hoorn 1987; Walker & Cooper 1987; Cameron et al. 1992). The main objective of this paper is to address one component of this history, namely the Triassic evolution of the North Dogger Fault Zone (NDFZ), and to compare it with the southern part of the Dowsing Fault Zone (SDFZ) (Fig. 1). The N D F Z is situated in the Mesozoic succession and is located at the southern edge of the North Dogger Shelf and has been named after this area (Allen et al. 1994). The northern part of the basin has been investigated by Jenyon (1985) but the N D F Z was not named. The fault zone was thought to have been initiated as a response to differential loading of the Zechstein salt in the basin which induced centrifugal salt migration towards the basin margins (Jenyon 1985). The lateral migration of the salt was halted by a combination of Zechstein facies change onto the shelf and basement faulting, which delineate the shelf zone. This migration induced the formation of a shelf-parallel Zechstein salt diapir during late Triassic or Jurassic times. The fault zone developed at the basin edge as the salt withdrew into the growing diapir. The diapir crest was then removed by 'late Kimmerian' erosion. Salt from the diapiric stem or stock was also evacuated at this time to produce a secondary salt weld in the fault zone (Jenyon 1985). The N D F Z has most recently
146
P. A. GRIFFITHS ET AL.
Fig. 1. Triassic structural elements of the Southern North Sea Basin (Allen et al. 1994). been described as a Jurassic extensional graben by Cameron et al. (1992) but its Triassic evolution has not been discussed. A different interpretation of the seismic data is presented in this paper and a model which links the peripheral fault zone with basin-wide halokinesis is developed. It is proposed that the Zechstein halokinesis in the core of the Southern North Sea Basin was triggered by early Triassic tectonics related to the development of the Hardegsen unconformity. The salt swells that were initiated, probably by movement on basement faults, began to grow and deformed the overlying Triassic rocks by a passive folding process. The shortening of the Triassic strata was largely taken up in a peripheral fault system which developed at the edge of the mobile Zechstein halite. This fault system continued to develop throughout the Triassic Period as halokinesis continued in the basin centre. The model does not invoke centrifugal salt flow from the basin centre. The salt did flow laterally, but inwards rather than towards the margins, and only locally to feed growing salt swells in the core of the basin; that is, centripetal not centrifugal movement. In this model, regional-scale differential loading from the basin centre to margins is a much less important control on salt migration than the regional tectonic stress regime and local differential loading effects, such as those produced by displacement across faults cutting the top of the Rotliegendes Group.
The North Dogger Fault Zone S e i s m i c coverage, quality, horizons a n d well data This study of the N D F Z is based on a semi-regional seismic database. The seismic lines, of 1982 vintage, were acquired and processed by Seismograph Service
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(England) Limited. They consist of NE-trending dip lines, spaced at 4 km intervals, and are intersected by NW-trending strike lines, some 16 km apart. The data quality is good throughout. An example of an interpreted migrated time section is used to illustrate the picked seismic reflectors (Fig. 2). The seismic lines were tied to nine wells in the study area. Ten horizons were interpreted, from top Chalk to base Zechstein, on the basis of their stratigraphic importance and lateral continuity. Emphasis was given to interpretation within the Triassic succession. Four seismic lines are used here to illustrate the N D F Z and the associated salt structures (Fig. 3 and Fig. 4a for location). The lines are discussed in numerical order, from east to west.
Interpretation of line 1 The lowest interpreted reflector is the top of the Rotliegendes Group. This horizon is heavily faulted and forms a conspicuous faulted step, downthrown to the southwest (Fig. 3a). The N D F Z in the Triassic rocks is situated directly above the faulted step at base Zechstein level. The N D F Z has a listric profile in the time section and appears to detach within the Zechstein salt. The Zechstein Supergroup has several noteworthy structures. Northeast of the N D F Z , the Zechstein succession comprises a sequence of non-mobile carbonates and evaporites. This stability can be interpreted as a consequence of a much-reduced thickness of the Z2 Stassffirt Halite Formation and contrasts with vigorous halokinesis in the thick basinal halites to the southwest. There is a salt withdrawal basin beneath the Triassic and Jurassic succession immediately to the southwest of the fault zone. This is a result of the lateral migration of the Zechstein evaporites. Southwest of the salt withdrawal basin, the Triassic reflectors are truncated against the Base Cretaceous unconformity (Fig. 3a). From the presence of Triassic toplap against the unconformity, it is inferred that a broad salt swell was present at the time
148
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FAULT AND SALT CONTROL OF SEDIMENTATION
149
of base-Cretaceous erosion and it is possible that the swell was present in late Triassic times. A post-Triassic salt structure is present and overprints part of the Triassic swell. The complete Triassic succession is only preserved in the hanging-wall of the N D F Z and to the southwest of the post-Triassic salt structure. This is due to erosion prior to the deposition of the Lower Cretaceous sediments. Lower Jurassic rocks in the top of the hanging-wall of the N D F Z have been penetrated by well 44/2-1. The top Dudgeon and top Haisborough horizons, and the reflectors between these two horizons, diverge towards the footwall. This divergence is interpreted as a consequence of fault movement during the deposition of the Triton Anhydritic Formation. It is not possible to determine whether growth continued into Jurassic times because only a very thin sequence of Lower Jurassic sediments is preserved. Earlier Triassic reflectors do not diverge in the hanging-wall. This is taken to indicate that the N D F Z was not active prior to late Carnian times. Isopach maps show that thickness changes denoting growth faulting are not velocity effects; the observed time thickness changes correspond to real thickness variations. The fault zone has suffered inversion during late Cretaceous and Tertiary times but this process is not discussed here.
Interpretation of line 2 This line is parallel to, and situated 16 km northwest of, line 1. It shows less prominent faulting at the top of the Rotliegendes Group and the well developed faulted step in line 1 is not as obvious (Fig. 3b). The N D F Z is situated in the post-Zechstein cover near the fault step and has developed several splay and antithetic faults within the hanging-wall. To the northwest of the fault zone the Zechstein Supergroup is immobile, with a continuous Plattendolomit reflector. In the foot-wall of the fault zone the salt withdrawal basin in more extensively developed compared to line 1 and the lateral salt movement may have formed a primary salt weld. A narrow salt pillow is inferred to the southwest of the salt withdrawal basin from the truncation of the Triassic reflectors against the base-Cretaceous unconformity. This swell is thought to have been present in late Triassic times and is a northwesterly extension of the swell observed on line 1. The post-Triassic salt structure is present and is closer to the N D F Z than the same feature on line 1. The complete Triassic succession is preserved in the hanging-wall of the fault zone. As on line 1, the top Dudgeon and top Haisborough reflectors diverge towards the foot-wall, providing another clear example of growth faulting during late Carnian to late Norian times.
Interpretation of line 3 Line 3 is located 24 km northwest of line 2. The geometry of the fault zone and of the salt features has changed considerably in this distance. The top Rotliegendes horizon is heavily faulted, with an obvious fault step into the basin (Fig. 3c). As on the other lines, the Zechstein salt to the northeast of the fault zone has not suffered any halokinesis. In contrast with lines 1 and 2, the salt withdrawal basin is not as well developed, and there is a thicker Zechstein succession in the footwall of the NDFZ. To the southwest of the salt withdrawal basin the Upper Triassic succession is truncated by the base-Cretaceous unconformity, and a broad, low amplitude salt pillow is inferred to have been present prior to the deposition of the Lower Cretaceous sediments. The swell was modified during late Cretaceous times.
150
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Interpretation of line 4 This section lies 12 km northwest of the previous line. At top Rotliegendes level faulting is not as common (Fig. 3d). To the northeast of the fault zone the Zechstein Supergroup is not mobile. As on line 3, the salt withdrawal basin beneath the N D F Z is poorly developed and only a small amount of lateral salt movement has occurred. Immediately to the southwest, a low amplitude salt swell is inferred from the folding and truncation of the Triassic strata. This swell is the northwesterly continuation of the swell seen on line 3. The post-Triassic salt diapir does not appear to correspond to, or to be developed from, the crest of an earlier pillow. The crest lines of the salt pillows are parallel with the fault zone whilst the later salt diapirs are oblique to it. The growth of the salt diapirs is not thought to have been influenced by the position of the older pillows. The N D F Z , as on line 3, comprises one main fault, a major antithetic fault and many minor faults within the hanging-wall. The hanging-wall itself has been deeply eroded and only a small portion of the top Haisborough reflector is preserved. It is therefore not possible to determine whether growth faulting occurred during late Triassic times. Compared to line 3, however, where the fault zone and salt withdrawal basin are similarly developed, it can be inferred that there was no growth faulting during late Triassic times. In summary, it is inferred from the full set of seismic lines (only four of which are presented here) that the eastern part of the N D F Z exhibits growth faulting during late Carnian to late Norian times and the development of a low amplitude salt swell to the southwest, trending approximately parallel to the fault zone. In contrast, the western half of the fault zone does not show late Triassic movement, the salt withdrawal basin is poorly developed, but a low amplitude salt swell parallel with the fault zone is still present.
152
P. A. GRIFFITHS ET AL.
Maps generated f r o m the seismic data Zechstein Supergroup time thickness map From a regional viewpoint, the Zechstein Supergroup thins gradually to the north onto the southern flank of the Mid-North Sea High. There are local thickness variations caused by halokinesis superimposed on this regional trend. The local salt thickness anomalies are of two types: diapirs and salt withdrawal basins. There are three post-Triassic salt diapirs (Fig. 4a). The westernmost diapir runs oblique to the NDFZ. The others, in the eastern half of the area, trend northwest and are laterally offset from one another. The salt withdrawal basin is most obviously developed in the footwall of the eastern half of the NDFZ. The withdrawal of the Zechstein salt from the footwall is not apparent to the west. The decrease in salt withdrawal also corresponds to an apparent decrease in late Triassic growth faulting to the west. Salt swells present at the time of late Jurassic/early Cretaceous erosion have been mapped from the truncation pattern of the Triassic rocks against the baseCretaceous unconformity. There were two late Triassic salt swells which were aligned with the fault zone. The location of the lateral offset between them corresponds to the site where there is a change in orientation of the fault zgne and to a present-day thinned area of Zechstein salt. The growth of the post-Triassic salt diapirs does not appear to have been closely controlled by the positions of the fault zone or salt swells, as all three diapirs have differing orientations compared to the trace of the fault zone and the crestal trends of the salt swells.
Triassic subcrop against the base-Cretaceous unconformity map The Triassic sediments in the study area have suffered varying amounts of erosion prior to the deposition of the Lower Cretaceous sediments. The erosion has produced a distinctive subcrop pattern of the Triassic strata against the base-Cretaceous unconformity (Fig. 4b). Upper Triassic strata were removed over a large part of the area and are only preserved in the hanging-wall of the fault zone and in the deeper parts of the basin to the southwest. This limits the areal extent of time thickness and isopach maps produced for intervals within the Triassic succession. The subcrop pattern also highlights the structural highs and lows present before and during erosion. In addition, the erosional pattern indicated the position of the late Triassic salt swells inferred from the seismic sections. Bunter Shale time thickness map The Bunter Shale Formation (lowermost Triassic) time thickness map shows a gradual thinning onto the Mid-North Sea High. The regional trend does not seem to have been affected by halokinesis or regional extension (Fig. 4c). Accordingly, it is inferred that the Bunter Shale Formation was deposited on the northern flank of a broad basin and that pre-existing topographic irregularity was minimal. The Bunter Shale Formation has been partly eroded from the western half of the area. This area of erosion corresponds to the inferred trace of the late Triassic salt swell. On the time thickness map, the Bunter Shale Formation appears to thin into the NDFZ. This apparent thinning is due to truncation of the Bunter Shale Formation on a listric fault plane (Fig. 3a). There is evidence that no growth faulting or halokinesis took place during the deposition of the Bunter Shale Formation. Similarly, from the other time thickness
FAULT AND SALT CONTROL OF SEDIMENTATION
153
maps that have been produced, the other lithological units within the Triassic succession which were deposited before late Carnian times have the same contour pattern and were unaffected by tectonism. The first signs of thickness changes occur in the late Carnian strata above the Keuper Halite Member.
Triton Anhydritic Formation time thickness map For the Triton Anhydritic Formation, a time thickness map (Fig. 5a) and an isopach map (Figs 5b & 5c) have been produced. The isopach map was made using interval velocity information derived from well data. An important point to emerge from this conversion is that both the time thickness and the isopach map show similar contour patterns and therefore the time thickness changes correspond to real thickness variations. Accordingly, the more easily constructed time thickness maps may be used to study the patterns of the thickness variations in the region. On a regional scale, the Triton Anhydritic Formation thickens gradually into the basin to the south. Imposed on this pattern is the rapid thickness change, from 200 to 300 m, into the eastern half of the N D F Z (Fig. 5c). This change is interpreted as syndepositional thickening caused by active fault movement. In the west, the Triton Anhydritic Formation exhibits a more gradual thickening. The western area also corresponds to the poorly developed salt withdrawal basin in the foot-wall, whereas in the east the Zechstein salt has been almost entirely evacuated where there is more Triassic growth (Fig. 5b). Summary of the North Dogger Fault Zone The North Dogger Fault Zone can be divided into two segments, east and west, with a northeast-trending linking fault segment between them. The time thickness maps of the Triassic intervals indicate that syndepositional fault movement did not occur before late Carnian times and extension decreased westwards. This time of initiation is confirmed by syndepositional thickening of the Triton Anhydritic Formation into the fault zone. The palaeo-salt swell in the east probably formed before or during fault movement. Movement in the western half of the fault zone, in contrast, probably began in late Triassic times and continued into early Jurassic times. The western salt swell formed during this interval. It is not possible to determine accurately when the extensional fault movement ceased in the N D F Z as only a small wedge of Jurassic strata is preserved in the hanging-wall. However, there appears to have been no fault offset of the base of the Chalk. The fault movement is considered to be a response to halokinesis within the Silver Pit Basin. The fault zone is located at the slope change in the Zechstein Basin, which was described above as a faulted step. This rapid change in slope initially influenced the deposition of the halites within the Zechstein Supergroup, particularly the Stassftirt Halite Formation. As a consequence, there was a fundamental transition from thin, immobile halite to the north of the faulted step, and thick, mobile halite to the south. The N D F Z has developed at the edge of mobile Zechstein salt as an extensional response to overall horizontal shortening of the Triassic cover as salt swells grew in the basin. The fault zone was therefore caused by late Triassic halokinesis.
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The Southern Dowsing Fault Zone The NDFZ is only a part of the fault system which bounds the Southern North Sea Basin and displays late Triassic initiation. The southern part of the Dowsing Fault Zone (SDFZ) (Fig. 1) provides an illustration of halokinetic controls on sediments deposited during early and mid-Triassic times.
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The R6t Halite Member isopach map This isopach map (Fig. 6a) of the late Scythian R6t Halite Member has been derived from well data. Regionally, the halite thickens gradually northeastwards into the Sole Pit Basin whilst to the southwest it thins and oversteps the Z2 Stassffirt Halite Formation basin margin. In addition to this regional trend, the R6t Halite displays localized thickening into parts of the SDFZ (Fig. 6b), giving a clear example of late Scythian growth faulting.
Geoseismic profile The Dowsing faults, initiated in late Scythian times, detach into the Z2 halites of the Zechstein Supergroup (Fig. 7) at the edge of the mobile Stassffirt Halite and at the limit of Zechstein halokinesis. In this respect the SDFZ mimics the N D F Z in its relationship with salt thickness. The Dowsing fault movement, however, was initiated earlier, during deposition of the R6t Halite Member, continued as the Muschelkalk Halite Member accumulated, and persisted until at least mid Carnian times.
Regional model The movement on the SDFZ is considered to have been initiated during the deposition of the R6t Halite Member during late Scythian times. This corresponds to
156
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the culmination of early Triassic tectonics and the development of the Hardegsen unconformity. This event is well known in NW Germany where four Bunter Sandstone subcycles are present. These sandstones are widely developed in areas of subsidence such as Triassic graben, and condensed on the basin margins and linear uplifted zones (Ziegler 1990). The main break in sedimentation is prior to the Hardegsen sand deposition to form the Hardegsen unconformity. The linearity of the eroded zones is considered to be a product of the local build-up of stress along wrench faults. This stress regime is thought to have been short-lived and followed by the resumption of regional subsidence (Ziegler 1990). In this context, the regional stress regime during late Scythian times is inferred to have initiated or reactivated faults affecting the base of the Zechstein Supergroup. These fault movements are the triggering mechanism for the earliest-formed salt swells developed in the Sole and Silver Pit Basins where the potentially mobile Zechstein salt and the Lower Triassic overburden are thickest. The faulting induced local differential loading effects across the salt interval and influenced the positions and trends of salt swells. The regional stress regime, however, is a major controlling factor influencing the swell orientations. As the salt swells grew, the overlying Triassic rocks were folded over the pillows (Fig. 8). With the resumption of regional subsidence, the swells continued to develop and the Triassic folds increased in amplitude. The folding led to an overall horizontal shortening of the Triassic cover. The shortening of the cover rocks in the core of the basin was accommodated by extension at the basin margins, corresponding to the edge of the mobile Zechstein salt. This extension caused the SDFZ to develop during late Scythian times and the N D F Z to develop during late Carnian to late Norian times. This model is fundamentally different from that proposed by Jenyon (1985), in that centrifugal salt flow is not invoked for the Southern North Sea Basin and that the peripheral fault system has not developed as a response to the formation of a socalled primary-edge diapir. An alternative interpretation is offered for the secondary salt weld formed by the closure of the diapiric stock (Jenyon 1985). The explanation proposed here is that the weld zone is an antithetic fault to the main North Dogger Fault (Fig. 3b) and has not acted as a conduit for salt. The only type of salt structures linked with the development of the N D F Z are the two palaeo-salt swells inferred from the Triassic truncation pattern, which developed as a response to lateral salt withdrawal from the fault zone.
158
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The Zechstein salt moved laterally on a local scale into salt swells growing nearby. The swells grew vertically in their original location and caused passive folding of the Triassic cover. It is this overall shortening that caused the faulting at the basin margins, not large-scale lateral salt movement. For both examples of the peripheral fault system, it has been inferred that the growth sequences are a result of salt movement as opposed to regional extensional tectonics. Where a thick salt sequence is present, however, it is not possible to differentiate conclusively between the two mechanisms. To determine whether regional extension is the fundamental controlling factor, and salt has moved in passive response, it is necessary to examine fault systems where the Zechstein halite is thin, as on the Zechstein shelf sequence. This work forms part of ongoing studies.
Summary Faults in the southern part of the Dowsing Fault Zone were initiated and underwent movement during late Scythian times. This event is recorded by fault-controlled thickening of the R6t Halite Member. This part of the fault zone suffered continued
FAULT AND SALT CONTROL OF SEDIMENTATION
159
extension until at least mid Carnian times. The zone is located at the southwestern edge of the mobile Zechstein halite. The eastern half of the North Dogger Fault Zone was initiated during late Carnian times at the basin margin where the fault-controlled thickening of the Triton Anhydrite Formation had taken place. The western part of the fault zone may have been initiated at the same time, but sedimentary thickening and hence growth faulting have not been detected. The fault zone is situated at the northern limit of the mobile Zechstein salt. Halokinesis, triggered by the Hardegsen tectonic event in the deepest parts of the basin, continued until at least mid-Triassic times (Fig. 1). The generation of early Triassic salt swells in the core of the basin and the passive folding of the oldest Triassic strata resulted in the lateral shortening of the cover rocks. The shortening caused faults to develop at the weakest points, where the Zechstein salt was immobile. The peripheral fault system developed along the Z2 halite edge, with continued development of the basinal salt structures. The fault system did not develop as a whole, but rather components of the fault zone evolved as they were affected by the bulk shortening throughout the Triassic. The authors wish to thank an anonymous referee for his encouraging remarks and the staff of LASMO North Sea plc for their continuing support. The seismic data are published with the permission of Seismograph Service (England) Limited. Institute of Earth Studies publication no. 385.
References ALLEN, M. R., GRIFFITHS,P. A., CRAIG, J., FITCHES,W. R. & WHITTINGTON,R. J. 1994. Halokinetic initiation of Mesozoic tectonics in the southern North Sea: a regional model. Geological Magazine, 131, 559-561. CAMERON,T. D. J., CROSBY,A., BALSON,P. S., JEFFREY,D. H., LOTT, G. K., BULAT,J. & HARRISON, D. J. 1992. United Kingdom Offshore Regional Report: the Geology of the Southern North Sea. HMSO, London, for the British Geological Survey. GLENNIE, K. W. & BOEGNER, P. L. E. 1981. Sole Pit inversion tectonics. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-west Europe. Heyden, London, 110-120. JENYON, M. K. 1985. Basin-edge diapirism and undip salt flow in the Zechstein of the Southern North Sea. American Association of Petroleum Geologists Bulletin, 69(1), 53-64. VAN HOORN, B. 1987. Structural evolution, timing and tectonic style of the Sole Pit inversion. Tectonophysics, 137, 239-284. WALKER, I. M. 8r COOPER, W. G. 1987. The structural and stratigraphic evolution of the north-east margin of the Sole Pit Basin. In: BROOKS, J. & GLENNIE, K. W. (eds) Petroleum Geology of Northwest Europe. Graham & Trotman, London, 263-275. ZIEGLER, P. A. 1990. Geological Atlas of Western and Central Europe. 2nd edition. Shell International Petroleum Maatschappij, The Hague.
From Boldy, S. A. R. (ed.), 1995, Permian and Triassic rifting in Northwest Europe,
Geological Society Special Publication No. 91, pp. 161-192
The timing and direction of Permo-Triassic extension in southern Britain R. A. C H A D W I C K & D. J. EVANS
Petroleum Geology and Basin Analysis Group, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Abstract: The Permo-Triassic sedimentary basins of southern Britain form a
roughly N-S-trending linear rift system which extends more than 400 km from the northern English Channel to the Irish Sea. The component basins of the rift system typically contain 1000-4000m of dominantly continental red-beds. The basin margins are clearly imaged by seismic reflection data, and typically comprise a major syndepositional normal fault or faults lying in the hanging-wall block of a reactivated basement thrust. In southern England, basin-controlling faults have a dominant E-W (Variscoid) trend, in the Midlands the faults have a dominant N-S (Malvernoid) trend and in northern England NE-SW (Caledonoid) basin fault trends are prominent. Thus the rift system can be structurally divided into three segments, dependent on the structural gain of the underlying basement. Because the orientations of the basin-margin faults were controlled by basement structures, they did not develop perpendicular to the extension direction and consequently suffered varying degrees of oblique-normal displacements. An analytical model is developed which relates the displacement components of a set of variably oriented faults to a single regional extension direction. Application of the model to the rift basin-margin faults indicates that faults trending N-S had displacements which most closely approximated to dipslip. Conversely, faults trending E-W suffered strongly oblique-slip displacements. This is consistent with a cumulative Permo-Triassic extension vector oriented 060~ ~ roughly ENE-WSW. Analysis of thickness changes across the basin-margin faults, and backstripped burial history plots from the basin depocentres, indicate at least two phases of crustal extension. The first occurred in late Permian times, with a second major episode in early Triassic times. They were characterized by rapid fault-controlled basin subsidence and coarse clastic sedimentation. Each was followed by a period of more gradual, regional subsidence, depositional overlap of the basin margins and a tendency towards finer-grained sedimentation. The Permo-Triassic rift system of southern Britain stretches from the English Channel to the Irish Sea. This paper will discuss aspects of the structural evolution of two of the principal constituent basins of this rift system, the Worcester and Cheshire Basins. The critical role of basement fault reactivation will be examined. This, together with a detailed study of basin-margin fault geometries, supplemented by data from the Wessex-Channel Basin to the south, will be used to estimate the cumulative Permo-Triassic extension vector. Analysis of syndepositional fault displacements, augmented by detailed basin subsidence history plots, will be used to evaluate the timing of crustal extension episodes. By earliest Permian times, Variscan compression and collision had led to final suturing and consolidation of the Pangaean supercontinent. Britain lay deep within this continental mass, straddling the northern margin of the Variscan foldbelt. The
162
R. A. CHADWICK & D. J. EVANS
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Fig. 1. Permo-Triassic regional tectonic framework of North Atlantic region. Basins discussed herein highlighted by stippling. CB, Cheshire Basin; WB, Worcester Basin; WC, WessexChannel Basin; VG, Viking Graben; FR, Faeroes Ridge; RT, Rockall Trough; RH, Rockall High; M, Moray Firth Basin; NPB, North Permian Basin; OG, Oslo Graben; MNS, MidNorth Sea High; RF, Ringkobing-Fyn High; SPB, Southern Permian Basin; EIS, East Irish Sea Basin; LBM, London-Brabant Massif; CSB, Celtic Sea basins; WA, Western Approaches Basin; PB, Paris Basin. wider aspects of Permo-Triassic basin development are described elsewhere (Coward 1995), and are here only summarized briefly (Fig. 1). Regional extension, associated with rifting to the north and south, affected the NW European region for much of the Permo-Triassic, with the location and geometry of the extensional basins which formed at this time being closely controlled by the reactivation of pre-existing basement fractures. McLean (1978) suggested a NE-SW regional tensional stressfield, whilst Chadwick et al. (1990), using plate-tectonic reconstructions and studies of basin geometry and fault kinematics, proposed that Permo-Triassic extension was oriented roughly E-W.
Morphology of the Permo-Triassic basins of southern Britain The cumulative effect of Permo-Mesozoic extension in southern Britain is shown by a three-dimensional view of the top of the pre-Permian basement surface (Fig. 2).
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The major Permo-Triassic Cheshire and Worcester Basins are clearly seen, as is part of the important Wessex-Channel Basin to the south, the latter also containing a thick post-Triassic sedimentary fill. The Permo-Triassic basins themselves (Fig. 3) constitute a complex N-S-trending rift system which stretches some 400 km, from the Lyme Bay Basin in the English Channel, northwards through the Dorset, Wardour, Pewsey, Worcester, Knowle, Stafford and Cheshire Basins, to the East Irish Sea Basin. The constituent basins of this rift system are bounded by major, syndepositional normal faults, which controlled basin development. The deepest part of the rift system lay within the Cheshire Basin, where presently preserved PermoTriassic sediment thicknesses approach 4000m, and the Worcester Basin, with
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Fig. 4. Structural cross-sections through the Worcester and Cheshire Basins (Worcester Basin section based in part on seismic data from Figs 7 and 16). over 3000 m of preserved Permo-Triassic strata (Fig. 4). Elsewhere, a 1000-2500 m thickness of preserved Permo-Triassic strata is typical. The form of the Worcester and Cheshire Basins is described in more detail below.
Worcester Basin The Worcester Basin, together with the smaller Knowle Basin, comprises a graben system whose overall structural trend is N-S, parallel to the Malvernoid structural trend of the underlying basement rocks (see below). The basins are flanked to the west by Lower Palaeozoic and Precambrian rocks of the Welsh borders, and to the east by the concealed Palaeozoic massif of the London Platform. The western margin of the Worcester Basin is marked by the East Malvern Fault (Fig. 4), a major downeast normal fault throwing the Permo-Triassic sedimentary fill against Lower Palaeozoic and Precambrian rocks of the Malvern Axis. The present-day throw on this fault, at the level of the base Permo-Triassic, is in places in excess of 2500 m. At present, the basin is roughly symmetrical in profile, but it is likely that at the end of Triassic times the basin was somewhat asymmetrical, deepest in the west, close to the East Malvern Fault. The deepest part of the present-day basin is west of the Inkberrow Fault, which together with the Weethley Fault forms the eastern margin of the basin hereabouts. Further south, the eastern margin of the basin lies further east, along the Clopton fault system (Fig. 3). This major, down-west normal fault system has present-day throws, at the level of the base Permo-Triassic, ranging from about 600 m in the north (Fig. 4), to over 1300 m further south (see below).
166
R. A. CHADWICK & D. J. EVANS
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The stratigraphy of the basin fill, summarized in Fig. 5, was proved, in part, by the Kempsey Borehole (Whittaker 1980), which penetrated a thick Permo-Triassic sequence resting unconformably on Precambrian basement. The Permian succession (Bridgnorth Sandstone Formation) is dominated by aeolian sandstones (Smith et al. 1974). The Sherwood Sandstone Group (Warrington et al. 1980) is dominated by fluvial sandstones with conglomerates in the lower part. The Mercia Mudstone Group (Warrington et al. 1980) comprises mainly mudstones with localized developments of halite, interpreted as being deposited within playa lake or inland sabkha environments. In the south of the basin, the Permo-Triassic succession is overlain by Jurassic rocks which thicken southwards into the Wessex-Channel Basin. Cheshire Basin
The Cheshire Basin, flanked to east and west by Carboniferous and Lower Palaeozoic rocks, is a faulted half-graben, roughly elliptical in plan, with a long-axis trending NE-SW (Fig. 3), parallel to the Caledonoid trend of underlying basement structures (see below). The basin is markedly asymmetrical, being deepest in the southeast (of opposite structural polarity to the Worcester Basin), close to the Wem-
PERMO-TRIASSIC EXTENSION IN SOUTHERN BRITAIN
167
Red Rock fault system which forms its southeast margin (Fig. 4). The present-day combined throw on these faults, at the level of the base Permo-Triassic, in places exceeds 3500 m. In contrast, the western margin of the basin is relatively unfaulted, forming a feather-edge characterized by depositional onlap (Colter & Barr 1975). The solid geology of the basin is poorly exposed, due to extensive Quaternary to Recent cover, but deep boreholes at Knutsford, Prees and Wilkesley have provided limited insights into the basin fill and the underlying Palaeozoic basement (Evans e t al. 1993). The preserved Permo-Triassic succession is summarized in Fig. 5. The Permian succession is predominantly sandy, with mudstones at the top. The Sherwood Sandstone Group is dominated by sandstones, whereas the Mercia Mudstone Group comprises mainly mudstones but includes two thick halite units (Pugh 1960; Wilson in press). Also present in an outlier are the Penarth Group and the Lower and Middle Lias.
Basement control of basin-margin faults The principal basin-controlling normal faults exhibit a marked variation in orientation along the length of the rift system (Fig. 6a). Thus in the south, the Lyme Bay, Dorset, Wardour and Pewsey Basins have major fault trends which are roughly E-W. Further north, in the Worcester, Knowle and Stafford Basins, major fault trends are N-S. Further north still, in the Cheshire Basin, the dominant trend of the basin-margin faults is NE-SW. These differing trends of the basin-controlling structures can be related to the different basement structural provinces upon which the rift system was superimposed
J~ " ~ ~
,'-",
~
~.,',[I =, !z'- -','r
"- _)
b
Fig. 6. (a) Principal Permo-Triassic syndepositional normal faults. (b) Basement structural provinces. C, Caledonian Foldbelt (+Devonian/Carboniferous cover in places); MM, Midlands Microcraton; V, Variscan Foldbelt; PL, Pontesford-Linley Lineament; CS, Church Stretton Fault; VF, Variscan Front. Dashed lines are interpreted basement structural trends (based upon Pharaoh et al. 1987).
168
R. A. CHADWICK & D. J. EVANS
(Fig. 6b). In southern England and Wales, pre-Permian basement comprises the Variscan foldbelt, wherein strongly deformed Palaeozoic rocks have a dominantly E-W trend and are cut by major S-dipping thrusts (Chadwick et al. 1983). The most northerly thrusts are termed the Variscan Front, which marks the northern margin of the foldbelt. North of the Variscan Front, in central England and SE Wales, the Midlands Microcraton forms a roughly triangular region composed of Lower Palaeozoic and Precambrian rocks (Smith 1987; Pharaoh et al. 1987). This region has been only slightly deformed since late Precambrian times, but locally there are important N-S-trending fracture zones that are believed to date from the late Precambrian (Pharaoh et al. 1987). These were reactivated in Variscan (late Carboniferous) times and constitute the 'Malvernoid' trend. A subsidiary NW-SE trend, the 'Charnoid' trend, may also locally influence basin geometry. The early Palaeozoic Caledonian foldbelt lies north of the Midlands Microcraton. The foldbelt abuts the northwest margin of the microcraton along the important NE-SWtrending Church Stretton Fault Zone. The NE-SW 'Caledonoid' trend forms the dominant structural grain hereabouts, with major fractures, such as the PontesfordLinley Lineament, prominent (Fig. 6b). Other trends are evident in the Caledonian foldbelt. For example, a prominent NNW-SSE trend (possibly a superimposed Carboniferous feature) appears to have influenced subsequent development of the Solway Basin (Fig. 3). Thus, from south to north along the rift system, many of the principal PermoTriassic basin-controlling normal faults have Variscoid, Malvernoid and Caledonoid trends. The mechanism which caused this close parallelism of basin and basement fault trends was revealed by detailed seismic mapping in the Wessex-Channel Basin. Here, the major basin-controlling normal faults lie within the hanging-wall blocks of underlying Variscan thrusts, and can be related directly to extensional reactivation of the thrusts leading to hanging-wall collapse (Chadwick et al. 1983; Chadwick 1986). Similar correspondence between basin-controlling normal faults and underlying reactivated basement thrusts has been noted elsewhere in the UK (e.g. Stein 1988). It is important to stress, however, that basement-influenced trends are principally a characteristic of the major basin-controlling or basin-margin faults. Smaller, intrabasin structures may not show basement-related orientations. In the Cheshire basin, for example, the dominant intrabasin fault trend is N-S or NNW-SSE, suggesting that the trend of these smaller faults was perhaps influenced more by the extension direction (see below) than by basement structural grain. Details of the relationship between the basin-margin faults and underlying reactivated basement fractures are illustrated below by means of examples from the Worcester and Cheshire Basins. Reactivation
of basement faults
Worcester Basin Seismic reflection data enable the basin-bounding normal faults to be imaged in detail. Figure 7 illustrates the East Malvern Fault, which forms the western boundary of the Worcester Basin where it abuts the Malvern Axis. The Malvern Axis is a major N-S-trending line of faults and folds dating from the Proterozoic (Pharaoh et al. 1987). In Variscan (late-Carboniferous) times, transpressive forces produced W-directed reverse faults and thrusts which crop out along the axis (Fig. 7; Brooks 1970; Barclay et al. in press).
169
PERMO-TRIASSIC EXTENSION IN SOUTHERN BRITAIN
,.s:l
.,= O
.<X
z~
O
,.,=~ +..a
170
R. A. CHADWICK & D. J. EVANS
The East Malvern Fault is parallel to and forms the eastern margin of the Malvern Axis, lying in the hanging-wall of the Variscan reverse faults which cut the Precambrian and Lower Palaeozoic rocks of the axis. This major normal fault has a roughly planar surface which dips to the east at about 45 ~. To the west of the fault, basement rocks of the Malvern Axis crop out; to the east, the base of the PermoTriassic basin fill lies at about 1.4s two-way travel time (TWTT). Taking into account the present topography of the Malvern Hills, the present-day throw on the fault, at the level of the base Permo-Triassic, is in places over 2600 m. It is likely, though it cannot be proven (due to a lack of preserved Permo-Triassic strata on the foot-wall block), that much of this displacement was syndepositional, occurring in Permo-Triassic times. It is believed that the location and geometry of the East Malvern Fault were controlled by extensional reactivation of the Malvern Axis reverse faults (Chadwick & Smith 1988). Near to the eastern margin of the Worcester Basin, the important N-S-trending Inkberrow and Weethley faults are believed to have formed by extensional reactivation of a Variscan reverse fault in the basement (Chadwick & Smith 1988). Much clearer seismic images of basement fault reactivation at this margin are available from the Clopton fault system further south. A grid of several high quality seismic profiles enables detailed mapping of a N-S-trending echelon of major down-west normal faults which mark the eastern margin of the basin hereabouts (Fig. 8). The Clopton fault system dips to the west and displaces the base of the Permo-Triassic from about 0.1 s TWTT on the London Platform to about 0.9s TWTT in the basin, a throw of some ll00m. Much smaller displacement of the basal Jurassic strata indicates that most of this throw was syndepositional, occurring in Permo-Triassic times. Lying unconformably beneath the Permo-Triassic cover, basement rocks are structurally rather complex. A seismically well layered sequence within the upper part of the basement can be identified from borehole evidence as Carboniferous strata, predominantly of Westphalian age. These beds are folded into an E-facing asymmetrical anticline which is cut by minor W-dipping reverse faults. The anticline lies in the hanging-wall of a larger W-dipping reverse fault Fw. The style of folding in its hanging-wall block and W-dipping basement reflectors suggest that Fw has a rather low dip, though steeper reverse faulting cannot be ruled out. The involvement of Westphalian strata and the lack of folding in the overlying Permo-Triassic sequence indicate a Variscan age of deformation. Seismic mapping of fault Fw reveals a regional N-S (Malvernoid) trend. From this, we interpret that Fw formed as a consequence of Variscan transpressive reactivation of an older Malvernoid structure. This E-directed Variscan reverse faulting and thrusting thus forms a mirror image of the W-directed thrusting observed along the Malvern Axis, with Variscan uplift of the intervening tract (Chadwick & Smith 1988). The Clopton fault system lies in the hanging-wall block of the Variscan thrust Fw, and is therefore interpreted as forming by hanging-wall collapse during its extensional reactivation. It is noteworthy that during its reactivation, the shallower part of the thrust remained inactive, extensional strain at depth being transferred upwards along a new, steeper normal fault cutting the hanging-wall block. This mode of thrust-fault reactivation, involving short-cut normal faulting of the hanging-wall, with little or no displacement of the thrust tip, appears to be the rule rather than the exception (e.g. Chadwick et al. 1983; Stein 1988).
PERMO-TRIASSIC
EXTENSION
IN SOUTHERN
171
BRITAIN
2 o
:= o
.3 o
=
d
o~
o +,,a
~ ,,,,,,~
172
R. A. CHADWICK & D. J. EVANS
Cheshire Basin The Permo-Triassic sedimentary fill of the Cheshire Basin dips and thickens towards the Wem-Red Rock fault system, which forms its southeast margin (Fig. 4). A seismic line reveals details of the basin-margin structure (Fig. 9). Westerly dipping reflections between 0.1 and 0.65 s TWTT are visible beneath the PermoTriassic cover, near the eastern end of the seismic line. Seismic character and regional correlation suggest that the reflections correspond to concealed Westphalian Coal Measures. Their relationship to the Carboniferous succession imaged at the extreme eastern end of the line, where Keele Formation (Barren Measures) crops out, suggests that the formations are separated by a reverse fault Fc (Fig. 9). Precise displacement of the Westphalian strate is difficult to assess because of poor velocity control, the complex structure and the uncertain seismic character matches. In the footwall of Fc, other, smaller reverse faults displace the Coal Measures and Barren Measures. These seismically imaged reverse faults are considered to be related to the many Variscan compressive structures seen at outcrop in the area. These range in size from the Potteries Syncline to the numerous small thrusts seen in coal workings in the North Staffordshire Coalfield (Taylor et al. 1963; Evans et al. 1968). The geometry of Fc is poorly constrained. Here it is interpreted as being relatively low-angle, similar to Fw (Fig. 8), but the presence of a much steeper fault cannot be ruled out. Fault Fc lies along the northeasterly continuation of the NE-SW-trending Pontesford-Linley Lineament (Fig. 6b). This important basement structure is a prominent feature on images of gravity and magnetic fields (Lee et al. 1990) and has a prolonged history of fault reactivation, possibly from Proterozoic times onwards (Wills 1978; Woodcock 1984; Soper et al. 1987). We therefore interpret the reverse fault Fc as being associated with Variscan reactivation of the Pontesford-Linley Lineament. Syndepositional normal faults lie in the hanging-wall block of Fo, the base of the Permo-Triassic succession being progressively faulted down to the west. Westwards, a thicker Permo-Triassic succession was deposited upon each fault block, so that within the basin to the west, the base of the Permo-Triassic sequence lies at more than 2 s TWTT (greater than 4000 m depth). It is likely, therefore, that in PermoTriassic times these major normal faults formed by extensional reactivation of the Variscan reverse fault Fc, its hanging-wall block collapsing to form the basin margin in a manner very similar to that described above for the Worcester Basin. As elsewhere, the uppermost segment of the thrust suffered only very minor, if any, normal displacement. On a more regional scale, it is likely that development of the southeast margin of the Cheshire Basin as a whole was controlled by reactivation of the underlying Pontesford-Linley Lineament. To summarize, there is strong evidence that basement structures, mapped both at outcrop and from seismic reflection data, show a marked degree of parallelism to overlying major basin-margin normal faults, the latter forming by extensional reactivation of the former.
Extension vector from fault geometry analysis The evidence presented above indicates that the trends of the syndepositional basincontrolling normal faults are broadly parallel to the structural trends of underlying basement features and were, therefore, largely independent of the causative extension direction. Thus the major normal faults were, in general, not pure
PERMO-TRIASSIC EXTENSION IN SOUTHERN BRITAIN
o
o
>.
~ ,,,,,~
r
173
174
R. A. CHADWICK & D. J. EVANS
dip-slip faults perpendicular to the extension direction, but rather suffered varying degrees of oblique-normal slip (transtension). We propose a new technique to analyse the type of slip suffered by a group of variably orientated faults, and from this to derive the extension direction. The basis of the technique is illustrated by a simple block model (Fig. 10). A basement block is cut by a fracture composed of four variably orientated linked segments (Fig. 10a); this is acted upon by a tensile stress of arbitrary orientation (here assumed to be NE-SW). The block undergoes plane-strain extension by separation along a system of linked extensional fault segments which follow the initial basement fracture (Fig. 10b). The sense of movement on each fault segment depends upon its orientation with respect
/
I'
/
%u"n,
,
/ h~=AEcos~=0 h2=AEcosct2=AE h3=AEcosc~
h4=AEcoso~4
Fig. 10. Simple block model of fault reactivation: (a) before extension; (b) after NE-SW directed extension.
PERMO-TRIASSIC EXTENSION IN SOUTHERN BRITAIN
175
to the extension vector. Consequently, the dip-slip component of extension (i.e. the heave) across each fault segment depends upon its orientation. Thus fault segment C), which is perpendicular to the extension, experiences pure dip-slip displacement, with a heave equal to the total extension. Conversely, fault segment (~, which is parallel to the extension direction, experiences wholly strike-slip (or transfer) displacement, with a zero heave. Fault segments (~) and (~) are oblique to the extension direction and experience dextral and sinistral transtension, respectively, with heaves less than the amount of extension. The heave on each fault segment is therefore a function of the obliquity of the fault segment with respect to the extension direction, and can be expressed as follows: heave = A E cos a
(1)
where A E = total extension and a = angle between the normal to the fault and the extension direction. This relationship can be plotted graphically on what is here termed an azimuth diagram (Fig. 11), designed to display orientation and heave data of a set of normal faults. The azimuth diagram comprises two circles with a common centre: an inner circle of radius A E and an outer circle of radius 2AE, whose circumferences define geographical azimuths. Faults with zero heave are plotted on the circumference of the outer circle. Faults whose heave is equal to the total extension (AE) are plotted on the circumference of the inner circle. Faults whose heaves are intermediate between zero and A E (i.e. heave = A E c o s a ) are plotted proportionally between the inner and outer circles. Figure 11 illustrates the scenario for NE-SW-directed extension (as in Fig. 10). Thus NW-SE-trending faults (e.g. Fig. 10, fault segment (~)) have fault-normals which are oriented
N
S 0 strike slip dip slip
~) dextral transtension 0 sinistral transtension
Fig. 11. Azimuth diagram of predicted fault heaves for NE-SW-directed extension. Numbers
denote the plotted positions of the fault segments in Fig. 10.
176
R. A. CHADWICK & D. J. EVANS
NE-SW, parallel to the extension direction. They are dip-slip normal faults with heaves equal to the total extension, and thus plot on the circumference of the inner circle. Conversely, NE-SW-trending faults (e.g. Fig. 10, fault segment (T)) have fault-normals which are oriented NW-SE, perpendicular to the extension direction. These are strike-slip (transfer) faults with zero heaves and thus plot on the circumference of the outer circle. Faults whose fault-normals are oblique to the extension direction have heaves given by Equation (1), and suffer either dextral or sinistral transtension (e.g. Fig. 10, fault segments (~) and (~), respectively). The long axis of the ellipsoid defines the direction of zero heave and is orientated NE-SW, perpendicular to the extension direction. The short axis of the ellipsoid defines the direction of maximum heave (AE) and is thus parallel to the extension direction. The Permo-Triassic basin-controlling faults of southern Britain (Fig. 6a) are, in principle, amenable to this type of analysis, though in practice a slight modification of the technique is required. In the illustrative block model (Fig. 10), the total extension across the fault system is constant (AE) and fault heave can be related directly to extension. This is not the case for the set of real faults which are of variable size, each fault being associated with a unique amount of extension which varies both within a basin and, markedly, from basin to basin. Thus the heaves of these variably sized faults will not plot meaningfully upon the azimuth diagram. To overcome this it is necessary to normalize the measured heave on each fault to the size of the fault. This was accomplished by dividing the heave of each fault by its throw to give the heave/throw ratio. In the case of real faults, the mode of deformation of their hanging-wall blocks introduces a further complicating factor, in many cases difficult to quantify. In general terms, strain distribution within the hanging-wall blocks of normal faults governs the amount of heave on the fault, relative to the causative extension (White et al. 1986). Thus, if strain within the hanging-wall block has no horizontal component of extension or shortening, then the heave of the fault will be equal to the causative extension. More commonly, however, extension of the hanging-wall block (either by antithetic minor normal faulting or some form of bulk shear) results in a fault heave less than the causative extension. Reduction of heave by extension of the hanging-wall block is likely to be most pronounced in dip-slip, rather than oblique or strike-slip faults. Thus, of the faults analysed, the heaves of those which are most nearly dip-slip will be preferentially reduced. This will tend to subdue rather than falsely enhance the theoretical heave/obliquely relationship, and (provided the latter is still discernible) can therefore be neglected. A selection of high-quality seismic profiles transecting the major Permo-Triassic normal faults was depth-converted. Wherever possible, the Permo-Triassic components of displacement were isolated for each fault. This is illustrated by a diagrammatic depth-section across the Inkberrow Fault of the Worcester Basin (Fig. 12). Fig. 12a illustrates the present-day situation. The top of pre-Permian basement lies at about 3100 m depth in the hanging-wall block and 1900 m in the foot-wall block (Fig. 4). The top of the Triassic sequence lies at about OD in the hanging-wall block and would lie at about 80 m above OD in the foot-wall block (estimated by projecting from nearby outcrops). The simplest way of isolating the Permo-Triassic components of displacement is to restore the section to an endTriassic datum by reducing the fault displacement at the top-Triassic level to zero
PERMO-TRIASSIC
EXTENSION
(a) present-day
IN SOUTHERN
BRITAIN
177
(b) restored to end-Triassic datum -
no decompaction
eroded overburden hr O.D.
TOP trlAS
t~ ..,~ _--_
J~
1000
~~~ ~MEN.1,
/:.
200O
3000
top
~s~,,/_ .... 'I ..... /
top top top top
/
Itrr
/----!:-
1 ~-"[--
I
I
t,,-- h a --,I
basement heave hB = 850m Triassic heave [IT = 60m baselnent throw tB = 1200m Triassic t h r o w tr = 80m
Permo-Triassic heave hrr = 790111 Permo-Triassic throw trr = l 1 2 0 m he~/trr = 0.71
(c) restored to end-Triassic d a t u m decompacted
-
/
i'rT
/ .... ,':_ I
I
I,---hpT.-.~l Permo-Triassic heave h~r = 850m Permo-Triassic throw trT = 1200m hrl,/tl.r = 0.71
Fig. 12. Diagrammatic depth-section across the Inkberrow Fault. (a) Present-day crosssection. (b) Cross-section restored to end-Triassic datum with no allowance for compaction. (c) Section restored to end-Triassic datum and decompacted. (Fig. 12b). The Permo-Triassic heave and throw on the fault can then be measured directly. This simple restoration takes no account of post-Triassic sediment compaction, the effects of which are illustrated as follows. A post-Triassic overburden some 1200m thick was assumed (Fig. 12a), in keeping with depth of burial estimates from the Kempsey Borehole (see below). This overburden was removed by
178
R. A. CHADWICK & D. J. EVANS
backstripping, the Permo-Triassic sequences in the foot-wall and hanging-wall blocks being allowed to decompact to their end-Triassic thicknesses (Fig. 12e). The pre-Permian basement rocks are assumed to be incompressible, so the fault is unchanged at top-basement level. Because the sequence in the hanging-wall block is thicker, removal of the overburden causes it to decompact further than the sequence in the foot-wall block, reducing the displacement on the fault at top-Triassic level to virtually zero. This indicates that the present normal displacement at top-Triassic level is due almost entirely to differential compaction effects, rather than to post-Triassic extension. In this particular case, therefore, the decompacted Permo-Triassic heave and throw are virtually identical to the present-day heave and throw of the fault at the level of top-basement. It is noteworthy also that the Permo-Triassic heave/throw ratio measured from the restoration, neglecting decompaction (Fig. 12b), is very similar to that for the fully decompacted restoration (Fig. 12c). This is because the Inkberrow Fault maintains a roughly constant dip with depth; the compaction-induced displacement at top-Triassic level merely reduced the net Permo-Triassic heave and throw in proportion, with no overall change in the heave/throw ratio. In general terms, therefore, the effects of compaction are very minor; it has a negligible effect on the displacement of faults at the top-basement surface, which is the principal measured quantity. Its greatest effect is to increase the displacement of faults at the top-Triassic level. This will affect the restoration to a top-Triassic datum. However, for planar normal faults (dominant in these basins) the throw and heave at the top-Triassic level will increase in proportion, with no change in overall heave/throw ratio. A further minor effect of compaction in the foot-wall block is to decrease slightly the dip of the fault surface within the sedimentary sequence (compare Figs 12e and 12a); again, however, this has negligible effect on the fault geometry at the top-basement level. The possible effect on fault geometry of structural tilting has also been neglected. The fact that both hanging-wall and foot-wall blocks of the faults analysed are in general subhorizontal (e.g. Figs 7, 8, 9, 16), suggests that only minor block-tilting and fault-rotation has occurred. An end-Triassic restoration, neglecting the effects of compaction and block-tilting, was therefore adopted for the analysis. For some of the faults measured, insufficient stratigraphical preservation precluded complete isolation of the Permo-Triassic displacement, so results therefore include a component of post-Triassic movement. In such cases, however, it is likely that the proportion of later movement was minor. Appropriate geometrical correction was made to non-perpendicular transects. Twenty-three faults were analysed and the results are tabulated in Table 1. Because the heave/throw ratio of a fault is identical to the tangent of its hade (the angle the fault-surface makes with the vertical), the fault data are amenable to analysis by stereographic projection. The data were therefore plotted on a Lambert Equal Area Polar Plot, specifically adapted to show heave/throw ratios by annotating the template in tangents to angles (Fig. 13a) rather than the angles themselves. The template is therefore closely analogous to the azimuth diagram described above (Fig. 11). Vertical faults with a zero heave/throw ratio (zero hade) plot on the circumference; less steep faults, with higher heave/throw ratios, plot within the circle, according to the annotated template (Fig. 13a). Plotting the heave/ throw ratio of each fault on the polar plot (Fig. 13b) shows that the fault geometries vary in a systematic manner. Fault transects from the Cheshire Basin were taken
PERMO-TRIASSIC EXTENSION IN SOUTHERN BRITAIN
179
Table 1. Displacement components of major Permo-Triassic syndepositional normal faults in southern Britain Basin
Fault name (number*)
Corrected Throw (m) heave (m)
Heave/ throw
Dip azimuth (degrees)
Cheshire Cheshire Cheshire Cheshire Worcester Worcester Worcester Worcester Worcester Worcester Worcester Worcester Worcester Worcester Worcester Wess-Chann Wess-Chann Wess-Chann Wess-Chann Wess-Chann Wess-Chann Wess-Chann Wess-Chann
Wem Fault (1) Wem Fault (2) Were Fault (3) Bridgemere Fault (4) East Malvern Fault (5) East Malvern Fault (6) Inkberrow Fault (7) Weethley Fault (8) Clopton fault system (9) Clopton fault system (10) Clopton fault system (11) Clopton fault system (12~ Clopton fault system (13) Clopton fault system (14) Clopton fault system (15) (16) (17) (18) (19) (20) (21) (22) (23)
2070t 1415J; 1361J; 3093 t 2300t 1737t 1120 700 500 864 547 652 1144 1109 1092 367 1200 696 655 548 600 900 1490t
0.73 0.59 0.71 0.74 1.24 1.07 0.70 1.14 1.74 1.29 1.22 0.72 1.08 1.21 1.27 0.65 0.63 0.69 0.55 0.52 0.68 0.80 0.42
337 325 325 313 094 100 282 270 248 270 297 316 270 270 265 157 180 170 170 179 172 177 178
1520t 828~ 969~ 2270t 2856t 1864r 782 799 870 1117 667 467 1235 1341 1389 238 755 483 362 283 409 718 625t
* See Fig. 15 for fault locations. t Includes post-Triassic displacements (normally minor). Pre-Mercia Mudstone Group displacements.
a
N
b
N
S
S 9 x 9
C H E S H I R E BASIN W O R C E S T E R BASIN WESSEX-CHANNEL BASIN
Fig. 13. (a) Lambert Equal Area Plot adapted to show heave/throw ratios by displaying tangents to angles. (b) Faults from Table 1 plotted on adapted Lambert Equal Area Plot.
180
R. A. CHADWICK & D. J. EVANS
from its NE-SW-trending southeast margin. Fault normals point roughly NW and their heave/throw ratios are all less than 1.0. Faults from the Worcester Basin trend N-S, have fault-normals that point either E or W and in general have heave/throw ratios greater than 1.0 (a notable exception to this is the Inkberrow Fault with a heave/throw ratio of 0.70). Faults from the Wessex-Channel Basin trend roughly EW, have fault-normals which point to the S and all have heave/throw ratios considerably less than 1.0. This variation in heave/throw ratio is exemplified by comparing the East Malvern Fault and the Clopton fault system (Figs 7 & 8) of the Worcester Basin, with the Bridgemere and Wem Faults from the Cheshire Basin (Fig. 9); the latter have much steeper dips. Taken as a whole, the plotted fault measurements roughly define an ellipsoid whose long axis is orientated approximately N N W - S S E (Fig. 14). Because of scatter in the measurements, the precise shape and orientation of this ellipsoid is uncertain. Statistical fitting of a great circle to the data (Rockware 1988) suggests that the bestfit ellipsoid has a short axis orientated 060~ ~ and a long axis orientated 150 ~ 330 ~. The short axis defines the fault-normal direction of faults which have the largest heave/throw ratios (typically 1.2), in other words, those faults which are most nearly dip-slip. Such faults are exemplified by the East Malvern Fault and the Clopton fault system which respectively form the western and eastern margins of the Worcester Basin (Figs 7 & 8). These faults are of moderate dip and have the form of classic planar-normal faults. Conversely, the long axis of the ellipse lies close to the fault-normal direction of faults which have the smallest heave/throw ratios (typically 0.6) and which are most nearly strike-slip. Such faults are exemplified by the Bridgemere Fault, forming the southeast margin of the cheshire Basin (Fig. 9). This generally steeply dipping fault has many characteristics of a seismic flower structure, indicative of severely oblique-slip displacement.
/
/
i'"
. \x
\\\
S * CHESHIRE BASIN x WORCESTER BASIN 9 WESSEX-CHANNEL BASIN
Fig. 14. Faults from Table 1 plotted on adapted Lambert Equal Area Plot. Dashed line denotes best-fit ellipsoid.
PERMO-TRIASSIC EXTENSION IN SOUTHERN BRITAIN
181
It is notable that no perfectly vertical faults (i.e. faults which are wholly strike-slip, with zero heave/throw ratios) were observed. Thus no faults plotted at the circumference of the polar plot. It is likely that this is because the measured heave/ throw ratios reflect fault displacements throughout Permo-Triassic times. Within this period it is most unlikely that extension was restricted to a single direction. There may well have been a spread of extension vectors, with no one fault trend experiencing either pure dip-slip or pure strike-slip motion. Thus the short axis of the ellipse indicates the cumulative Permo-Triassic extension direction, perhaps made up of several different extension vectors. We conclude that this cumulative extension direction was orientated 060~ ~ roughly ENE-WSW. This extension direction is in general agreement with palaeomagnetic and plate-tectonic reconstructions of the early break-up of Pangaea (Frei & Cox 1987; Chadwick et al. 1990). The latter workers estimated that continental extension was responsible for over 200 km of separation between N W Europe and Canada in Permo-Triassic times. Figure 15 illustrates the nature of displacement on the principal basin-controlling faults of the rift system, assuming regional extension orientated 060~ ~. Under
q
Fig. 15. The effect of extension orientated 0600-240 ~ on the constituent faults of the rift system (faults numbered as in Table 1).
182
R. A. CHADWICK & D. J. EVANS
this strain regime, the bounding faults of the Worcester Basin would have been nearly dip-slip normal faults. Those faults of the Cheshire Basin with ~Caledonoid trends would have been subjected to severely oblique-normal slip, with the southeast margin of the basin experiencing sinistral transtensional displacement, and acting as an oblique transfer system. Further north, in the Cheshire Basin, the dominant fault trend swings round to roughly N-S or NNW-SSE; these faults, more nearly perpendicular to extension, would have experienced near dip-slip displacements. In the Wessex Basin the major faults would have experienced dominantly dextral oblique-normal slip.
Timing of extension The identification of syndepositional normal faulting, together with detailed analysis of basin subsidence rates, provides a powerful means of determining the timing of extensional episodes.
Syndepositional normal faulting Figure 16a illustrates a seismic line across the Inkberrow Fault (Figs 3 & 4). There is considerable westward thickening of correlative strata across the fault. The Bridgnorth Sandstone Formation thickens westward across the fault from about 330ms TWTT in the foot-wall block to about 420ms in the hanging-wall block, indicative of some 90 ms (180 m) of syndepositional fault throw. Given that strata in the hanging-wall block have suffered more subsequent compaction (Fig. 12), the true syndepositional fault throw was somewhat greater. The Sherwood Sandstone Group thickens from an estimated 400 ms TWTT in the foot-wall block to about 670 ms in the hanging-wall block, indicative of at least 270ms (500m) of syndepositional throw. The Mercia Mudstone Group also thickens across the fault but, as discussed above, this was due in part to the differential compaction of underlying strata. Marked changes in seismic character across the fault do, however, indicate some fault control of depositional facies. The normal fault near the western end of the seismic section shows somewhat different displacement behaviour. Considerable thickening of the Bridgnorth Sandstone Formation is observed, from about 300 ms TWTT in the foot-wall block to about 450 ms in the hanging-wall block, indicative of some 300 m of syndepositional throw. Above this, however, there is only minor thickening of the Sherwood Sandstone and Mercia Mudstone Groups. In addition to the two large faults, several small normal faults displace the basement surface. These faults appear to have been active only during deposition of the Bridgnorth Sandstone Formation, as they do not displace younger strata. Figure 16b illustrates a seismic line across the Weethley Fault (Fig. 4). Here, the base of the Sherwood Sandstone Group cannot be identified with certainty on the foot-wall block. Nevertheless, the combined thickness of the Bridgnorth Sandstone Formation and Sherwood Sandstone Group increases across the fault from about 330 ms to about 680ms, indicative of more than 600m of syndepositional throw. The overlying Mercia Mudstone Group also shows evidence of thickening across the fault. There is abundant evidence of syndepositional normal faulting elsewhere in the Worcester Basin, for example across the Clopton fault system (Fig. 8), which shows dramatic thickening across the basin margin, particularly of the Sherwood
PERMO-TRIASSIC EXTENSION IN SOUTHERN BRITAIN
183
Sandstone Group. Perhaps the most important fault in the basin is the East Malvern Fault (Fig. 7); here, ironically, syndepositional movement can only be inferred, due to the lack of Permo-Triassic strata on its foot-wall block. Seismic data from the southern part of the basin also show evidence of syndepositional faulting, particularly during deposition of the Bridgnorth Sandstone Formation and Sherwood Sandstone Group (Chadwick 1985a). Seismic evidence from the Cheshire Basin indicates a pattern of syndepositional normal faulting similar to that found in the Worcester Basin. The Bridgemere Fault (Fig. 9) was responsible for considerable thickening of pre-Triassic strata (the ~ Collyhurst Sandstone and Manchester Marl Formations) and also the Sherwood Sandstone Group. The combined thickness of these units increases from about 550 ms on the foot-wall block to over 1200ms on the hanging-wall block of the fault, indicating a syndepositional throw of some 1200m. Similar, though less marked, thickening also occurs over the Wem Fault (Fig. 9). At outcrop, strata of the Sherwood Sandstone Group rest upon basement rocks in the foot-wall of the Wem Fault, the Collyhurst Sandstone Formation being absent. This strongly suggests that the fault was active during deposition of the latter. Elsewhere in the Cheshire Basin, seismic evidence of fault-controlled thickening of Permian and lower Triassic strata is ubiquitous. As in the Worcester Basin, there is also seismic evidence of significant thickening of the Mercia Mudstone Group across major normal faults, for example the Bridgemere Fault. Again, however, such thickening was due, at least in part, to the effects of differential compaction of underlying strata across the fault, rather than to any primary tectonic cause. In summary, seismic reflection data from the Worcester and Cheshire Basins provide unequivocal evidence of syndepositional normal faulting. This was particularly marked during deposition of the early part of the Permo-Triassic sequence, tending to die out upwards. In the Worcester Basin there is evidence of at least two distinct faulting episodes, an early phase which affected deposition of the Bridgnorth Sandstone Formation and a later phase affecting the Sherwood Sandstone Group.
Basin subsidence analysis Subsidence history plots for the Cheshire and Worcester Basins (Fig. 17) were constructed as described below. The youngest strata preserved in the Cheshire Basin are those of the Lower Jurassic Lias Group in the Prees outlier (Walker 1914; Poole & Whiteman 1966), close to t h e basin depocentre. Seismic, borehole and outcrop data were used to construct a composite stratigraphical sequence for the basin depocentre from Permian to early Jurassic times. The thickness of younger, eroded strata was estimated from a simple depth-of-burial study. The densities and sonic velocities of strata penetrated by boreholes in the Cheshire Basin, when compared to similar lithologies in the Wessex Basin where a much more complete sequence is preserved (Chadwick 1985b), give a guide to their maximum depth of burial. Densities of 2 . 5 2 M g m -3 for mudstones and siltstones of the Lias Group and the Brooks Mill Mudstone Formation were obtained. This indicates that between 1600 and 2200 m of overburden younger than the middle Lias has been eroded, in close agreement with similar studies from the East Irish Sea Basin, which estimated 2000 m of removed overburden (Bushell 1986; Woodward & Curtis 1987).
184
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Fig. 17. Backstripped sediment-loaded (burial history) and sediment-starved subsidence curves for the Cheshire and Worcester Basins. The age of the oldest stratigraphical unit in the Cheshire Basin, the Collyhurst Sandstone Formation, is poorly constrained due to a lack of biostratigraphical markers. The top of the formation is constrained by an estimated age of 260 M a for the overlying Manchester Marl Formation (Smith et al. 1974. 1986; Forster & Warrington 1985). The base of the Collyhurst Sandstone Formation, marking the onset of deposition in the basin, has not been accurately dated. However, Warrington & Scrivener (1990) indicate that the lowest widespread, conformable Permian strata in SW England are about 260 Ma old (scattered occurrences of older Permian strata hereabouts are separated from the younger Permian rocks by a long
PERMO-TRIASSIC EXTENSION IN SOUTHERN BRITAIN
187
period of erosion). We interpret this as indicating that widespread deposition commenced in SW England at about this time. By analogy, and bearing in mind that rifting propagated from the north (Fig. 1), we assume that deposition in the Cheshire Basin commenced somewhat earlier, say 265 Ma ago. Chronostratigraphical details of the Sherwood Sandstone Group are from Warrington et al. (1980). The stage details of the Mercia Mudstone Group in the Cheshire Basin are based upon Benton et al. (in press). The Jurassic part of the subsidence curve was calculated using the stage and zonal details in Cope et al. (1980). Chronological calibration is based upon Forster & Warrington (1985) and Hallam et al. (1985), whose dates have recently been supported by Odin & Odin (1990) and Claou6-Long et al. (1991). In the Worcester Basin, the stratigraphically most complete sequence is found in the south, where the Permo-Triassic sequence is overlain by lower and middle Jurassic strata. However, this does not correspond to the site of maximum sedimentary thicknesses. In order to construct a composite stratigraphical sequence for the basin depocentre, it was necessary to incorporate information from seismic and borehole data and data extrapolated from outcrop. The thickness of younger, eroded strata was estimated by a depth-of-burial study; rocks of the Eldersfield Mudstone Formation in the Kempsey borehole have densities of approximately 2.47Mgm -3, which indicates some 1650m of eroded overburden. At the basin depocentre, in the eastern part of the basin (Fig. 3), strata younger than those at Kempsey are preserved. Of these younger strata, some 300 m of Triassic beds and 150 m of Jurassic beds are equivalent to part of the estimated Kempsey overburden. This leaves about 1200 m of other Jurassic and younger overburden rocks. Biostratigraphical information for the Permian and early Triassic parts of the sequence is limited, chronostratigraphical calibration being based on Barclay et al. (in press) and upon the ages of assumed correlative strata in the Cheshire Basin. Chronostratigraphical data for the Mercia Mudstone Group are again based upon Warrington et al. (1980), Barclay et al. (in press) and Benton et al. (in press). Additionally, an estimate of the position of the Arden Sandstone, based upon borehole and field evidence, was made in order to further constrain subsidence during deposition of the Mercia Mudstone Group. The composite stratigraphical sequences for the Cheshire and Worcester Basins were each decompacted by a backstripping method similar to that described by Sclater & Christie (1980) to give sediment-loaded (burial history) and sediment-starved (tectonic subsidence) subsidence plots (Fig. 17). The salient features of the burial history curves of both basins are two episodes of rapid subsidence, one in late Permian and one in early Triassic times, each followed by a period of much slower subsidence. The late Permian episode of rapid subsidence is interpreted as marking the onset of crustal extension in southern Britain, being associated with widespread syndepositional normal faulting in both the Cheshire and Worcester Basins, as discussed above (Figs 8, 9, 16). In the Cheshire Basin, local breccias towards the base of the Collyhurst Sandstone Formation (Hull 1869; Taylor et al. 1963; Earp & Taylor 1986) may be indications of contemporaneous fault activity. In the lowest part of the Collyhurst Sandstone Formation, very coarse-grained and pebbly sandstones probably represent local alluvial fan deposits interdigitating with more characteristic aeolian sandstones (Thompson 1985). A conglomerate or breccia, containing clasts derived from the English Midlands, occurs at the top of the Collyhurst Sandstone Formation in the
188
R. A. CHADWICK & D. J. EVANS
northern part of the Cheshire Basin (Tonks et al. 1931; Taylor et al. 1963). This may represent a further brief extensional pulse prior to the marine incursion marked by the base of the Manchester Marl Formation. Deposition of the Manchester Marl formation was characterized by much lower subsidence rates, probably indicating a cessation of extension with the establishment of post-extensional regional subsidence. In the Worcester Basin, the Permian sequence remains arenaceous throughout, perhaps reflecting greater distance from the marine conditions to the north and closer proximity to clastic source areas to the south. In both the Cheshire and Worcester Basins, the Scythian (early Triassic) witnessed the main episode of rapid basin subsidence. This corresponded to major syndepositional normal faulting, as evidenced by dramatic thickness changes within the Sherwood Sandstone Group (Figs 8, 9, 16). In the Worcester Basin, presumed early Triassic strata attain thicknesses of 1200-1500m within the basin, but are only 200-600 m thick over the fault blocks of its eastern margin. In the Cheshire Basin, the Sherwood Sandstone Group attains a thickness of 1300-1500 m in the basin, but is only about 500 m thick over the eastern margin.The onset of this major extensional episode coincided in the Cheshire Basin with deposition of the conglomeratic Chester Pebble Beds Formation and, in the Worcester Basin, with the similar Kidderminster Formation. The former commonly overlaps the basin's eastern margin to lie directly upon Carboniferous rocks. Both of these conglomeratic units contain exotic pebbles from the English Midlands, English Channel and Brittany (Wills 1956; Steel & Thompson 1983). The southerly provenance of many of the pebbles, allied to palaeocurrent data (Thompson 1985) suggests that they were transported northwards towards a depocentre in the East Irish Sea Basin. It is likely that rifting at this time created the regional N-S conduit necessary for this depositional pattern. Above the conglomerates, the Wilmslow Sandstone Formation in the Cheshire Basin and the Wildmoor and Bromsgrove Sandstone formations in the Worcester Basin complete a wholly arenaceous, fining-upwards sequence, interpreted by Steel & Thompson (1983) as reflecting progressively decreasing relief around the basins. In the Cheshire Basin, close to the boundary of the Sherwood Sandstone and Mercia Mudstone Groups, seismic data reveal a minor through widespread unconformity (Evans et al. 1993). This corresponds to a brief episode of uplift and erosion (Fig. 17), interpreted as marking the cessation of dominantly fault-controlled subsidence and the onset of post-rift regional subsidence. Uplift associated with the unconformity is indicated by rejuvenation of source areas, supplying exotic pebbles present in the pebbly sandstones and conglomerates of the Delamere Member of the Helsby Sandstone Formation. From early to mid-Anisian times there was a marked decrease in subsidence rates in both the Worcester and Cheshire Basins, with deposition of the argillaceous Mercia Mudstone Group. There is evidence of continued normal faulting, but this can, at least partly, be ascribed to differential compaction effects (see above). Subsidence rates, though lower than during the preceding synextension phase, were initially quite high, decreasing gradually with time. It is likely that some of the late Triassic coincided with a period of post-extensional, regional subsidence. The disposition of the Sherwood Sandstone Group, largely restricted to the faultbounded basins, and the much more widespread Mercia Mudstone Group, which
PERMO-TRIASSIC EXTENSION IN SOUTHERN BRITAIN
189
transgressed the basin margins, closely resembles the classic rift basin 'steer's head' profile (Dewey 1982). The subsidence curve for the Cheshire Basin (Fig. 17) indicates that strata of the Lower Lias were deposited during a period of increased subsidence rate, which slowed during deposition of the Middle Lias. Poole & Whiteman (1966) noted that the Lower Lias proved in Wilkesley was amongst the thickest known onshore in the UK. It is likely that this period of enhanced subsidence was triggered by renewed crustal extension, Permo-Triassic basins in Britain acting as loci of subsequent Jurassic sedimentation (Holloway 1985).
Conclusions Our conclusions are summarized below. 1. In Permo-Triassic, times a N-S-trending rift system developed in southern Britain, forming a small branch of a widespread anastomosing network of rift systems which developed on the NW European Shelf in response to regional extension. 2. The constituent basins of this rift system are bounded by major syndepositional normal faults which follow closely the trends of underlying reactivated basement fractures. Thus from S to N along the rift, dominant basin fault trends are E-W (Variscoid), N-S (Malvernoid) and N E - S W (Caledonoid). 3. Analysis of fault heave/throw ratios, based upon a simple fault model, suggests that the basin-controlling normal faults suffered varying degrees of obliquenormal slip, depending on their orientation. A cumulative Permo-Triassic extension vector orientated 060o-240 ~ is proposed. 4. Analysis of syndepositional normal faulting from seismic reflection data indicates that extension occurred from late Permian through to early Triassic times. Normal faulting continued into late Triassic times, but this was due in part to the effects of differential compaction. 5. Subsidence analysis confirms and refines the seismic evidence for the timing of extension. Two phases of extension can be identified, each associated with accelerated rates of subsidence and followed by a period of more gradual regional subsidence. The extension episodes occurred in late Permian times and early in the Triassic, the latter constituting the major Permo-Triassic rifting event. We are grateful to Shell UK and Clyde Petroleum plc for their kind permission to publish seismic data from the Cheshire and Worcester basins. Special thanks are due to Jim Rayner and Steve Wilkinson for their expertise in reproducing the seismic reflection data. The work is published with the permission of the Director, British Geological Survey (NERC).
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Geological Society Special Publication No. 91, pp. 193-214.
Geometry and tectonic evolution of megasequences in the Plymouth Bay Basin, English Channel A. R U F F E L L l, M. P. C O W A R D 2 & M. H A R V E Y 2
1Department of Geology, School of Geosciences, Queen's University, Belfast, BT7 INN, UK 2Department of Geology, Imperial College, Prince Consort Road, London SW7 2BP, UK Abstract: A complete Carboniferous-Permian-Triassic succession is found within the anomalously thick (10 km to acoustic basement) Plymouth Bay Basin (PBB). The evolution of this succession provides us with the most complete tectonic/ stratigraphic history of the late Palaeozoic to Triassic interval of the North Atlantic area. Deep and commercial seismic data from the PBB reveal the existence of four prominent seismic megasequences, bounded by strong and laterally persistent reflections. Isochore mapping and seismic facies analysis of each megasequence demonstrate that depocentres changed upward through the succession in a consistent manner, from E-W to elongate, NW-SE-orientated depocentres. This change may have occurred in the late Carboniferous or early Permian. The two-fold tectonic history of the PBB explains the thickness of sedimentary fill: an early basin formed above a deep crustal ramp, developed during Variscan compression along the Lizard-Start thrusts (and others); a later NW-SE early extensional or strike-slip basin depocentre is coincident with the earlier basin centre, creating further accommodation space.
The Western Approaches are underlain by a broad crustal depression, the Western Approaches Trough (WAT), which comprises a number of tectonically discrete sedimentary basins. Some authors (e.g. Chapman 1989) have used the term Western Approaches Basin for the whole area, including all the so-called 'subbasins'. In this study, the basins are each found to have very individual geological origin and history, and thus the more rigorous definition of the basins formulated by Evans et al. (1990) is followed. This places all the basins of the English Channel between Cornwall/Devon and Brittany (and thence to the continental shelf) within the WAT. One of these is the Plymouth Bay Basin (PBB). This study forms the geographic core of an assessment of the kinematics of extension and inversion in a transect from Cardigan Bay (offshore Wales) to the Paris Basin. Our regional mapping of seismic data from the Bristol Channel/ Wessex Basin to France (Fig. 1) has shown the PBB to be one of the thickest undeformed sedimentary basins of the area. The structure and nature of the sedimentary fill of the basins of the WAT are known through a number of publications. Key works include those of the French and British Geological Survey Departments (Pomerol 1972; B R G M 1980; Evans et al. 1990); those associated with hydrocarbon exploration (Bennet et al. 1985; Chapman 1989; Evans et al. 1990); and the acquisition of deep seismic data (Bullard & Gaskell 1941; BIRPS and ECORS 1986). The basins of the WAT are mainly
194
A. RUFFELL E T AL.
Fig. 1. Location of the Plymouth Bay Basin and seismic dataset (dotted lines) used in the study (courtesy BP Western Margin Group). Fine dotted fault line indicates limit of the Western Approaches Trough. Alternative names for basins include Brittany (for Iroise); St. Mary's (for Scillies) and SW Channel (for Manche Occidental). located in the British sector of the English Channel, and are described comprehensively by Evans et al. (1990) and Chapman (1989). The PBB comprises a subsidiary basin to the broad area of crustal subsidence known as the WAT (Fig. 1). The WAT, in turn, is one of a number of linked sedimentary basins, founded on continental crust, flanking the margins of the North Atlantic Ocean (Evans et al. 1990). Many of these basins had their origin in the Permian, with an early phase of rifting and subsidence following the late Carboniferous assembly of Pangaea (Chapman 1989). Thus, the WAT is found to contain a variable succession of Permian-Triassic red-beds, marina and non-marine Jurassic, limited (and predominantly non-marine) areas of Lower Cretaceous, Upper Cretaceous and Cenozoic chalks and late Cenozoic clastics. The variable distribution of these sediments is due to the differin~ subsidence histories of the host sedimentary basins. The first indication that sedimentary rocks lay on the metamorphic basement of northern France and southern England in the Western Approaches was made I from seafloor dredges described by Crawshay (1908) and Worth (1908). The presence of a deep, undeformed sedimentary basin was confirmed by seismic refraction experiments (Bullard & Gaskell 1941; Day et al. 1956; Hersey & Whittard 1966).
MEGASEQUENCES IN THE PLYMOUTH BAY BASIN
195
Basement The nature of the basement rocks to the PBB (and WAT generally) is largely unknown, although from the available evidence, some inferences can justifiably be made. These are summarized in Ruffell (1995). Outcrops surrounding the WAT show that two types of basement terrane occur. In northern France (Brittany) the rocks of the Armorican Massif crop out. These comprise three successions: Penterivian gneisses unconformably overlain by Brioverian Series low-grade metamorphic pelites and psammites. Both the Penterivian and Brioverian were deformed in the Precambrian-Cambrian Cadomian orogeny (Bott et al. 1970). They are again unconformably overlain by the third succession of the massif, CambrianDevonian/Carboniferous metasediments. Most Armorican structural fabrics strike 120-160 ~ (Hobson & Sanderson 1983). In southwest England (Cornwall and Devon), Cornubian Massif basement rocks comprise a series of CambrianCarboniferous (mostly Devonian and Carboniferous) stacked nappes, deformed during the late Carboniferous-Permian Variscan orogeny. Major thrusts, separating the nappes, are thought to be imaged on deep seismic profiles offshore to the north and south (Day & Edwards 1983; Leveridge et al. 1984). Thrusts of this, the 'Cornubian Terrane' of Day et al. (1989), strike 030-070 ~ and are cut by pre- and post-deformational strike-slip faults of the NW-SE Armorican trend. Both the Armorican and Cornubian Massifs are intruded by granites. In Cornwall, the Cornubian Batholith is probably a late subduction-zone-generated pluton of 280-290Ma (Shackleton et al. 1982; Darbyshire & Shepherd 1985). Granite emplacement is roughly synchronous with intrusion and deposition of Permian basic volcanics found at outcrop in Devon and reported by Chapman (1989) in borehole 73/12-1 (approximately 300km southwest of the PBB). These volcanics are similar in geochemistry to subduction zone volcanics (Thorpe et al. 1986), and were dated at 281+11 Ma (Miller & Mohr 1964). New dates are closer to the proposed Carboniferous-Permian boundary (Forster & Warrington 1985), possibly some 10Ma older (291-1-6Ma). Day et al. (1989) suggested that their Cornubian 'Terrane' extends from the onshore outcrop in southwest England as far west as the Goban Spur. Although dissected by Variscan NW-SE strike-slip faults, the Cornubian Terrane has largely resisted Mesozoic-Cenozoic extensional deformation. The area may thus have assisted in inversion of the Celtic Sea basins to the north in the same way that Armorica transmitted Alpine stresses into the Brittany Basin (Coward 1990). Edmonds et al. (1975) describe Armorican-type rocks as far north as Eddystone Reef, on the northeastern margin of the PBB. Goode & Merriman (1987) describe Armorican lithologies found as deep-derived xenoliths in Cornish Devonian basalts, thus indicating a northward extension of this terrane under the WAT. Whether this indicates that the thrusts to the south of CornwallDevon signify the boundary between Armorica and Cornubia, or whether these terranes occur as mixed thrust piles beneath the whole WAT, is not clear. Deep seismic data from the WAT comprise a four-line dataset acquired by BIRPS (British Institutions Reflection Profiling Syndicate) and ECORS (Etude de la Croute Continentale et Oceanique par Reflexion et Refraction Sismique). Line drawings of profiles 8 and 9 of the SWAT (South Western Approaches Trough) traverses are shown on Fig. 2, and form an integral part of this study. The SWAT survey was later augmented by an experiment into continental-ocean crust relations by the
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Variscan tectonics of the study area The Variscan orogeny of northern Europe culminated in the late Carboniferousearly Permian with continental collision along the Variscan 'suture' (Zeigler 1982). This episode caused uplift and erosion concomitant with isolated deposition in faultcontrolled ('intermontane') basins, both in the zone of collision and in the foreland.
MEGASEQUENCES IN THE PLYMOUTH BAY BASIN
197
Variscan crustal shortening was accommodated by E-W or SW-NE-striking thrusts and NW-SE orientated strike-slip faults (Meissner et al. 1981). The known thrusts of southern England, south Wales, Belgium and Germany are thought to decouple on a mid-crustal detachment, imaged on deep seismic data (BIRPS & ECORS 1986; Meissner et al. 1981). A review of Variscan tectonics in Britain is given by Coward (1990). Late Palaeozoic compression was followed by extension, driven by continental separation and the breakup of Pangaea. Permian-Triassic crustal extension was controlled by the pre-existing Variscan trends of E-W thrusts and NW-SE transfer zones. The sedimentary record of this active tectonic period is poor. The timing of any tectonic change from Variscan compression to Permian-Triassic extension has proven problematic, as a pronounced Carboniferous-Permian/Triassic unconformity is common. Where deposition was more continues, intermontane red-beds, volcanics and minor evaporites are found throughout the late Carboniferous (Stephanian) and Permian, making age determinations difficult (Smith et al. 1974). The PBB is known from borehole evidence (Fig. 3) to contain a thick succession of late Palaeozoic and Triassic sediment. It is thus one of the few basins in the North Atlantic rift system that does not show a Palaeozoic-Mesozoic unconformity (Manspeizer 1988). The PBB provides a location where the sedimentary effects of the transition from compression to extension may be observed. The development of strike-slip and early rift basins may accompany late compression, or post-date it completely. Thus the work we present here indicates times of tectonic change in the PBB only; it is not intended as a stratigraphic standard for adjacent basins. The techniques employed here may, however, provide a model for the study of basins formed during late compression and early extension.
The Plymouth Bay Basin Before hydrocarbon exploration of the Western Approaches area began in the 1970s and early 1980s, the existence of a thick sedimentary pile in the Plymouth Bay area (Fig. 1) was not known. Shallow sampling of seabed rock exposures and shallow geophysical experiments were designed to map surface geology and to trace onshore geological features such as the Cornubian granites and thrusts of the Lizard complex (King 1954; Day et al. 1956; Hill & Vine 1965; Curry et al. 1970, 1971). The structure of the Western Approaches became better understood with the publication of sections derived from seismic data acquired by oil companies (Avedik 1975). The quality of this data and its concentration to the southwest of Plymouth Bay led Avedik (1975) to consider 4 km as the maximum thickness of post-Variscan sediment in the central and southwestern end of the WAT. Initial reports of the thickness and extent of the PBB were made from borehole records and seismic data by BRGM (1980) and BIRPS & ECORS (1986). The regional structure of basins offshore from the European continental shelf is now known through commercial exploration and the acquisition of deep seismic data. These data indicate that the Carboniferous-Permian succession is most likely to be complete in the deep basins of the WAT, an example of which is the PBB. The basin has attracted scientific study (BIRPS & ECORS 1986) because of these noteworthy features: the sediment pile is suggested to be over 10km thick, in contrast to the 3 - 6 k m of adjacent basins; this stratigraphy is thought (from
198
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MEGASEQUENCES IN THE PLYMOUTH BAY BASIN
199
borehole evidence) to be composed largely of red-beds; the basin appears from isochore mapping as circular in shape, with no obvious basin-bounding faults or inheritance of older fabric. The Carboniferous-Permian-Triassic PBB underlies the broader CretaceousTertiary Western Channel Basin (BIRPS & ECORS 1986). The age of the sedimentary fill is known from the litho- and biostratigraphy derived from borehole successions throughout the WAT and surrounding the PBB (Fig. 3). The PBB was traversed by the deep seismic profiles SWAT 8 and 9; unusually strong Carboniferous-Permian reflectors from within the sedimentary pile of the PBB were considered by BIRPS & ECORS (1986) to be lava flows from their strong magnetic signature (Fig. 2). Coward (1990) considered these same reflectors as the bounding surfaces to tectonically derived packages of sediment. From the nomenclatural hierarchy provided by Vail (1987), we may suggest, from the work of BIRPS & ECORS (1986), that these represent megasequences. An important aspect of the PBB is that the late Carboniferous-Permian and Triassic sediments were deposited in an arid continental regime (Laming 1968), where no regional baselevel (such as a seaway) is required as a control on the development of sedimentary sequences. In the absence of marine base-level, we may suggest that the accommodation space made available for deposition was controlled by lake levels or by the interplay between subsidence and uplift. The relative absence of lacustrine facies at
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200
A. R U F F E L L E T AL.
outcrop in Devon (Laming 1968), plus the suggestion that the CarboniferousPermian was a tectonically active period, adds weight to our supposition that the development of sedimentary megasequences was controlled by tectonic shifts in baselevel. Seismic data from the study area show strong reflections from what is considered to be the basement-sedimentary cover contact (Fig. 2), by virtue of the impedance contrast between metamorphic rocks of the Cornubian-Armorican terranes, and unmetamorphosed post-collision red-beds of the PBB (BIRPS & ECORS 1986). A depth to basement map of the study area (Fig. 4) indicates that a number of relationships between sediment thickness and basement structure occur that are pertinent to the evolution of the PBB. Early structure maps of the Western Approaches area (including Plymouth Bay) indicate no substantial thickness of sediment (Avedik 1975). Instead, the greatest thickness of sediments in the Western Approaches was thought to be in the very centre of the English Channel, just north of the French-English Median Line, south of the PBB. The reason for this error is summarized in Brooks et al. (1983), who show how a stricter definition of seismic velocities and new experiments by Bott et al. (1970) allowed re-interpretation of the early data from the PBB in terms of a substantial, yet still unquantified, upper crustal layer of 10 km thickness (we now know this to represent the sedimentary fill of the PBB). Without knowledge of the composition of this layer, Bott et al. (1970) accurately measured the thickness of the PBB, although they extended this modelled layer beyond the confines of the basin. The first stage in our study required regional structural mapping of the WAT to which more detailed maps of the PBB could be compared. A summary regional map is shown in Fig. 4, from which some of the underlying structural trends of the area may be observed. The WAT is orientated SW-NE, on strike with underlying thrusts of Variscan origin. Extensional reactivation of these thrusts may have caused the formation of later basins like the PBB (BIRPS & ECORS, 1986). The predominant (Fig. 4) basin orientation of the WAT (SW-NE) is cut by N W - S E trending faults. Isolated areas of increased sediment thickness occur along strike to these N W - S E faults. Such faults may have originated during Variscan (and later) strike-slip movement (Dearman 1963; Hobson & Sanderson 1983), with associated basins forming during pull-apart. Isochore mapping of sedimentary thickness to basement shows a circular outline to the PBB. In addition,the orientation of SWAT lines 8 and 9 shows a symmetrical syncline within the PBB. Here, we have mapped individual megasequences of the PBB. These show changing orientations through the evolution of the basin; such thickness variations provide evidence of a complex origin to the PBB. The basin also appears to have formed independently of any mappable faults, with the possible exception of the PBB riding 'piggy-back' on underlying Variscan thrusts (such as the Start-Dodman thrusts), the depth contours of which are inserted on Fig. 4 (from Day & Edwards 1983) for comparison of their orientation to the isochore mapping completed here.
Seismic sequences Following regional and more detailed (basin-specific) structural mapping, our analysis focused on the 10 k m + of sedimentary fill to the PBB. Although borehole
MEGASEQUENCES IN THE PLYMOUTH BAY BASIN
201
penetrations are recorded from the periphery of the PBB (allowing limited lithostratigraphic correlation), the basin depocentre is filled with largely indeterminate sediment. We have employed seismic stratigraphy in further analysis of the origin of the PBB. Our methods follow those summarized in Vail (1987), basing stratigraphic division on reflection terminations characteristic of truncation, onlap, downlap and toplap. The sedimentary fill of the PBB may be divided into four discrete, unconformitybounded packages. These megasequences (in turn) contain sequences that are not further discussed here. Although the absolute age of these megasequences is not known, from the borehole records and onshore-offshore comparison it is likely that the megasequences encompass the (?late) Carboniferous to late Triassic. This age determination is compatible with the resolution of megasequences described by Haq et al. (1987). Megasequences are tectonically derived packages of sediment bounded by regional unconformity surfaces (Vail 1987) that may be related to discrete phases in the evolution of the basin. Megasequences are commonly ascribed to the tectonic evolution of extensional sedimentary basins such as onset of rift, synrift, inversion. In the PBB, four megasequences can be mapped: these are annotated (from the base) A, B, C, D on Figs 5, 6 and 7. Isochore maps of the megasequences are presented in fig. 7. The junction of megasequences B and C comprises the unusually bright reflection considered by BIRPS & ECORS (1986) to be correlative lavas to the outcropping Exeter Traps (Thorpe et al. 1986); otherwise only indirect evidence exists as to the age of the PBB succession. BIRPS & ECORS (1986) considered the bulk of the sedimentary fill to be of post-Variscan age, by virtue of the strong, relatively undeformed reflectors above the intense compressional zone of the (Variscan) Normandy thrust sheet. A number of commercial boreholes penetrate successions on the margin of the PBB (Fig. 3), and some borehole geophysical log ties to seismic data can be made. Seabed sampling of the area during early investigation of the English Channel indicated that Permian-Triassic rocks were the youngest preserved in the Plymouth Bay area (Crawshay 1908; Hersey & Whittard 1966). Both seabed samples and deep boreholes indicate that at least the upper sequences of the PBB are of Permian-Triassic age. Coward (1990) considered each megasequence as a southward-prograding package, generated from uplift of the Cornubian Massif to the north. This uplift was generated by compression, accommodated along southward-dipping thrusts: it is noted that the LizardDodman thrusts of SWAT 9 (Fig. 6) can be matched with similarly dipping subMoho reflections to the south. The present study indicates that megasequence deposition and preservation is more complex than Coward (1990) appreciated. The lower megasequences (A and B) show thickness trends parallel to the strike of underlying Variscan thrusts, while megasequences above (C and D) are orientated NW-SE. This shift in the mapped orientation of isochores has implications for the understanding of Variscan tectonics in southern England. Megasequence
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against which lower megasequences are attenuated. On SWAT 8, the unconformity at the base of this megasequence is cut by numerous normal faults. To the south and west of the PBB this megasequence can be observed on deep and commercial seismic data as having a more uniform thickness distribution, extending west into a westerly basin of the WAT, the Scillies Basin (Fig. 1). To the east, megasequence A thins gradually from the 0.6 seconds (TWT) typical of the PBB to zero along the Start-Contentin Ridge (Smith & Curry 1975). The lithology of megasequence A is unknown as there are no definite borehole penetrations. Stacking velocities provide the best thickness estimate of megasequence A: after depth conversion an estimated 2000m occurs in the PBB depocentre (Fig. 7), with around 1500m elsewhere. Comparison of isochore maps reveals that megasequence A is thickest some 35km due south of Plymouth whilst the depocentre to the overlying successions moved south of this through time. Changing thickness distributions are also observed on SWAT line 8 (Fig. 5): under shot-point 2500 megasequence A shows its greatest thickness, with the overlying depocentre to megasequence B some 15km to the south. The southern margin of the PBB is characterized by onlap of the bright reflections at the base of megasequence A onto what is presumed to be basement. Above the strong basal reflections, weaker conformable horizons are visible on both SWAT profiles (Figs 5 & 6). Internally, megasequence A shows inclined reflections displaying downlap and toplap. These dip consistently south, indicating
207
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progradation of sediment from the north, in accord with the model proposed by Coward (1990) whereby the uplift and erosion of Cornubia provided the sediment source for the PBB.
Megasequence B The lower megasequence boundary to this unit is well defined on the margins of the PBB, where laterally persistent, bright reflections truncate the inclined beds of megasequence A below. In the centre of the PBB, this boundary becomes indistinct and apparently conformable, where the megasequence boundary may be viewed as passing into its correlative conformity (Vail 1987). The basin depocentre of megasequence B lies 30 km to the west of megasequence A below (Fig. 7), thus explaining the variation in thickness between SWAT lines 8 and 9. The upper reflections of megasequence B become strong below the B-C megasequence
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~ .
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Fig. 7. (c) Isochore map of megasequence C. boundary: the zone of bright reflections thought to be lavas by BIRPS & ECORS (1986). Should the igneous origin of these beds be proven, it may be conjectured that the incoming of strong reflections below the B-C megasequence boundary represents the initial stages of igneous activity. It is these reflections on SWAT 8 that are inclined and provide a sequence stratigraphic marker for the mapping of the upper surface of megasequence D. On SWAT 8, this succession could have a real thickness of 2000m (Fig. 7). The southerly dipping reflections apparent in megasequence A are rarely observed. This may be due to lack of seismic data in the north of the PBB. Alternatively, megasequence A appears limited in extent, infilling pre-existing topography with a high depositional gradient. In this case, inclined depositional surfaces would be common by comparison to the more even distribution of megasequence B, when depositional gradients may have been lower. Subtle, inclined reflector surfaces may be present in B, but are not imaged at the resolution available.
209
MEGASEQUENCES IN THE PLYMOUTH BAY BASIN v
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Fig. 7. (d) Isochore map of megasequence D (NW-SE strike-slip trends).
Megasequence C The lower surface of this megasequence comprises the bright reflections discussed above. Similar bright and laterally persistent reflections are seen at the C - D boundary on SWAT 9 (Fig. 6), but not on SWAT 8 (Fig. 5). Downlap onto the lower surface of megasequence C is common on all the seismic data utilized in this study, and indicates progradation from the northeastern margin of the basin towards the basin centre in all cases. Less obvious (south-dipping) downlap surfaces are observed in the south of the basin (Figs 5 & 6). This suggests a similar northerly sediment source to megasequence A for the succession of megasequence C; the depocentres for megasequences A and C are virtually coincident. Isochore mapping of megasequence C shows a complete change in the shape of the PBB from those beds below. The elongate westerly shape of megasequence A evolved into a circular basin outline during deposition of megasequence B. Coincident with the development of bright
210
A. RUFFELL ET AL.
reflectors at the B-C megasequence boundary, the PBB underwent a dramatic change in orientation from almost E - W to NE-SW. Later, during deposition of megasequence D, this orientation changes to NW-SE. The present-day orientation of megasequence C is partly due to recent seafloor erosion of the whole succession (in the northeast and southwest), as well as intra-Permian-Triassic erosion at the overlying C - D megasequence boundary. The southern extent of megasequence C is difficult to define as the C - D boundary is faulted or poorly imaged. From deep and commercial data (Figs 5 & 6) we interpret the base of megasequence D as erosive in the south of the PBB; this may be significant as an indication of the surrounding uplift of the basin. Depth conversion using stacking velocities shows the thickness of megasequence C is likely to exceed 2000m at the basin depocentre.
Megasequence D The uppermost megasequence mapped in this study is elongate along a N W - S E trend. The thickness distribution is similar to megasequence C, being thinner to the east of the basin depocentre (e.g. on SWAT 8, Fig. 5), thickening on SWAT 9 (Fig. 6), and absent on commercial data to the west. Onlap at the base of megasequence D is most clearly imaged on the northern end of SWAT 9. Internally, this megasequence shows few seismic stratigraphic markers: the present-day thickness distribution may be a product of post-depositional erosion. The lower interval velocities recorded for this succession suggest 1800 m as a maximum thickness. Above megasequence D, an unmapped succession displays downlap of a similar character to the base of megasequence A, and in a similar geographic position. Seafloor samples indicate that this highest layer is comparable to the Mercia Mudstone Group (late Triassic). The higher dip of the upper surfaces precludes them from being multiples of the same events. From seafloor exposures and from an estimate of the PBB stratigraphy, this downlap surface is thought to be of intra-Triassic age. The age of the megasequences is not known. Nearby boreholes penetrate an attenuated succession (Fig. 3), or were terminated long before basement might be reached (not surprisingly!). Thus ties from seismic data to borehole geophysical logs provide few conclusive correlation surfaces. In addition, these are lithostratigraphic only. The presence of Aylesbeare Mudstone Group sediments indicates that the succession might be similar to that exposed onshore on the Devon coast (for specific locations, see Laming (1968)), where cumulative sediment thickness is approximately 6000 m less than that estimated for the PBB (Laming 1968). In their analysis of the SWAT surveys, BIRPS & ECORS (1986) commented on the usually strong reflection derived from what is now termed the B-C megasequence boundary. In this study we have come no further in the possible identification of this/these surface(s), and follow the analysis of BIRPS & ECORS (1986), who considered the magnetic model of the reflections as consistent with basic volcanic rock within a sedimentary succession. Although strong reflections are observed above and below the B-C boundary, they are not as bright and do not provide a comparable magnetic signature (Figs 5 & 6). If reflector B-C is a lava flow,it is likely to be of similar age to the Exeter Traps (earliest Permian; Forster & Warrington (1985)). On this evidence, and from the overall stratigraphy, it would appear likely that megasequence A represents the Marldon Group (Torbay-Chelston Breccias and Watcombe Beds), plus any beds not
MEGASEQUENCES IN THE PLYMOUTH BAY BASIN
211
represented in the onshore succession. Megasequences B and C may represent the Teignhead Group (Oddicombe and Teignmouth Breccias) and late Permian Exe Group, respectively. From boreholes and seafloor exposures, megasequence D is largely composed of Triassic Budleigh Salterton Pebble Beds and ?Otter Sandstone. The lower horizons of D (conglomerates) may be similar to the upper parts of C (breccias), partly explaining the indistinct reflections from this stratigraphic level. With no complete borehole penetration of this succession, no further lithostratigraphic correlation may be made.
Tectonic controls on megasequences Isochores of megasequences A and B show an ENE-WSW trend, on strike with the Variscan fabric of the area. By contrast, megasequences C and D show a NW-SE orientation, similar to major strike-slip faults that traverse much of the north European plate (Zeigler 1982). During Mesozoic extension these strike-slip faults acted as zones of tensional transfer. Later, during Cenozoic basin inversion, the same faults were reactivated under compression (Dearman 1963; Smith & Curry 1975; Hobson & Sanderson 1983). The orientation of the lower megasequences is the same as the strike of underlying and adjacent Variscan thrusts. For the early evolution of the PBB, the model proposed by Coward (1990) may be applied. Here, the thrusts are considered to have a ramp geometry below the PBB. Localized extension over this ramp facilitates subsidence and the accommodation space required for megasequences A and B. The upper megasequences (C and D) display orientations typical of compressive to extensional strike-slip basins known onshore in the Triassic of southern England (Ruffell 1990). Whilst the change from thrust-dominated tectonics to strike-slip movement and the next episode of rifting is a gradual process, reflector B-C (the 'lava' reflector) is the most convenient horizon at which to position the change from E-W, Variscan thrust-inherited tectonics, to successive NW-SE strike-slip and extensional basin development. As far as can be deduced, this change was approximately coincident with the Carboniferous-Permian boundary. BIRPS & ECORS (1986) suggested that the bright reflections, here mapped as the boundary between megasequences B and C, may represent lavas. The nearby 'Exeter lavas' are dated at 291 + 6 M a (Thorpe et al. 1986), close to the proposed CarboniferousPermian boundary (Forster & Warrington 1985). Magnetic mapping of the 'Exeter lavas' around the type locations in Devon has revealed an extensive outcrop/subcrop to the volcanics (Cornwell et al. 1990). The onshore volcanics extend further than indicated by outcrop mapping (Cornwell et al. 1990), drawing comparison with the extensive B-C reflectors of the PBB.
Conclusion The ramp-generated basins of megasequences A and B appear to have evolved from the northerly areas in which A was deposited on a ?basement unconformity, to the more southerly and widespread deposition of B. Superimposed upon these was a second basin orientation (NW-SE), formed by either pull-apart during a second compressive phase accommodated along strike-slip faults, or by early extension. Pull-apart basin formation is likely to have occurred along strike-slip faults acting as
212
A. RUFFELL ET AL.
zones o f transfer between thrusts at depth. Compression in late Variscan times is likely to have rejuvenated sediment source terrains to the north of the PBB, providing a m e c h a n i s m whereby large volumes of clastic debris were shed into the basin. Unfortunately, the orientation and density o f the seismic data grid available in this area does not allow any further analysis o f megasequences C and D; a compressive strike-slip origin m a y be favoured on account of the N W - S E orientation, whilst an extensional origin m a y be favoured as no strike-slip offset o f the A and B megasequences was resolved. W h a t is certain is that two phases o f basin development occurred during the evolution o f the PBB. The spatial coincidence o f the two types of basin provided excessive a c c o m m o d a t i o n space, and the unusual depth o f the PBB m a y thus be explained. This work was carried out while the authors were in receipt of a BP International Research Grant 'Crustal Extension and Inversion in NW Europe' for which we are grateful. A.R. thanks Bernard Anderson, John Parnell and Graham Leslie for constructive criticism. M.H. thanks Jamie Wilkinson, Simon Stewart and John Cosgrove likewise. Richard England and John McBride of BIRPS kindly provided A.R./Queen's University Belfast with deep seismic datasets.
References AVEDIK, F. 1975. Seismic refraction survey in the Western Approaches to the English Channel: preliminary results. Philosophical Transactions of the Royal Society, London, A279, 29-39. BENNET, S., COPESTAKE P. & HOOKER, N. P. 1985. Stratigraphy of the Britoil 72/10-1A Well, Western Approaches. Proceedings of the Geologists Association, 96, 225-261. BIRPS & ECORS 1986. Deep seismic reflection profiling between England, France & Ireland. Journal of the Geological Society, London, 143, 45-52. BOTT, M. H. P., HOLDER, A. P., LONG & LUCAS A. L. 1970. Crustal structure beneath the granites of southwest England. In: NEWALL, G. & RAST, N. (eds) Mechanisms of Igneous Intrusion. Geological Journal, Special Issue, 2, 93-102. BRGM (Bureau du Recherches geologies et Minieres). 1980. Synthese Geologique du Bassin de Paris. Volume II Atlas. Memoire BRGM. BROOKS, M., MECHIE, J. & LLEWELLYN, D. J. 1983. Geophysical investigations in the Variscides of Southwest England. In: HANCOCK, P.L. (ed.) The Variscan Fold Belt in the British Isles. Adam Hilger, Bristol, 186-197. BULLARD, E. C. & GASKELL, T. F. 1941. Submarine seismic investigations. Proceedings of the Royal Society of London, 177A, 476-499. CHAPMAN, T. J. 1989. The Permian to Cretaceous structural evolution of the Western Approaches Basin (Melville sub-basin), UK. In: COOPER, M, A. & WILLIAMS, G. D. (eds) Inversion Tectonics. Geological Society, London, Special Publication, 44, 177200. CORNWELL, J. O., EDWARDS, R. A., ROYLES, C. P. & SELF, S. J. 1990. Magnetic evidence for the nature and extent of the Exeter lavas. Proceedings of the Ussher Society, 7, 242-245. COWARD, M.P. 1990. The Precambrian, Caledonian & Variscan framework to NW Europe. In: HARDMAN, R. F. P. & BROOKS, J. (eds) Tectonic Events Responsiblefor Britain's Oil & Gas Reserves. Geological Society, London, Special Publication, 55, 1-34. --& SMALLWOOD, S. 1984, An interpretation of the Variscan tectonics of SW Britain. In: HUTTON, D. H. W. & SANDERSON, D. J., (eds) Variscan Tectonics of the North Atlantic Region. Geological Society, London, Special Publication, 14, 89-102. CRAWSHAY, L. R. 1908. On rock remains in the bed of the English Channel. An account of the dredgings carried out by the SS. 'Oithona' in 1906. Journal of the Marine Biological Association of the United Kingdom, 8, 99-117.
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CURRY, D., HAMILTON, D. & SMITH, 1970. Geological and Shallow Geophysical Investigations in the Western Approaches to the English Channel. Institute of Geological Sciences, Report, 70]3. , -& -1971. Geological evolution of the western English Channel basin and its relation to the nearby continental margin. In: DELANY, F. M. (ed.) The Geology of the East Atlantic Continental Margin, Part 2, Europe. Report of the Institute of Geological Sciences No 70/14, London. DARBYSHIRE, D. P. E. & SHEPHERD, T. J. 1985. Chronology of granite magmatism and associated mineralization. SW England. Journal of the Geological Society, London, 141, 249-265. DAY, A. B., HILL, M. N., LAUGHTON, A. S. & SWALLOW, J. C. 1956. Seismic prospecting in the Western Approaches of the English Channel. Quarterly Journal of the Geological Society, 112, 1544. DAY, G. A. & EDWARDS, J. W. F. 1983. Variscan thrusting in the basement of the English Channel and Western Approaches. Proceedings of the Ussher Society, 4, 432-436. , & HILLIS, R. R. 1989. Influences of Variscan structures off southwest Britain on subsequent phases of extension, In: TANKARD, A. J. & BALKWlLL, H. R. (eds) Extensional Tectonics and Stratigraphy of the North Atlantic Margins, American Association of Petroleum Geologists, Memoir, 46, 425-432. DEARMAN, W. R. 1963. Wrench faulting in Cornwall and South Devon. Proceedings of the Geologists Association, 74, 265-287. EDMONDS, E. A., MCKEOWN, M. C. & WILLIAMS, M. 1975. British Regional Geology: South-West England. HMSO, London, for the Institute of Geological Sciences. EVANS, C. D. R., HILLIS, R. R., GATLIFF, R. W., DAY, G. A. & EDWARDS, J. W. F. 1990. The Geology of the Western English Channel and its Western Approaches. HMSO, London, for the British Geological Survey, United Kingdom Offshore Regional Report, 9. FORSTER, S. C. & WARRINGTON, G. 1985. Geochronology of the Carboniferous, Permian and Triassic. In: SNELLING, N. J. (ed.) The Chronology of the Geological Record. Geological Society of London, Memoir, 10, 99-113. GOODE, A. J. J. & MERRIMAN, R. J. 1987. Evidence of crystalline basement west of the Land's End granite, Cornwall. Proceedings of the Geologists Association, 98, 39-43. HAQ, B. U., HARDENBOL, J. & VAIL, P. R. 1987. Chronology of fluctuating sea-levels since the Triassic (250 million years ago to the present). Science, 235, 1156-1167. HERSEY, J. B. & WHITTARD, W. F. 1966. The Geology of the Western Approaches of the English Channel- V, The Continental Margin and Shelf under the South Celtic Sea. Geological Survey of Canada, Paper, 66-15, 80-106. HILL, M. N. & VINE, E. J. 1965. A preliminary magnetic survey of the Western Approaches to the English Channel. Quarterly Journal of the Geological Society, London, 121, 463~,75. HOBSON, D. M. & SANDERSON, D. J. 1983. Variscan deformation in southwest England. In: HANCOCK, P. L. (ed.) The Variscan Fold Belt in the British Isles. Adam Hilger, Bristol, 108-129. KING, W. B. R. 1954. The geological history of the English Channel. Quarterly Journal of the Geological Society of London, 110, 77-101. LAMING, D. J. C. 1968. New Red Sandstone stratigraphy in Devon and west Somerset. Proceedings of the Ussher Society, 2, 23-25. LEVERIDGE, B. E., HOLDER, M. T. & DAY, G. A. 1984. Thrust nappe tectonics in the Devonian of south Cornwall and western English Channel. In: HUTTON, D. H. W. & SANDERSON, D. J. (eds) Variscan Tectonics of the North Atlantic Region. Geological Society, London, Special Publication, 14, 103-112. MANSPEIZER, W. 1988. Triassic-Jurassic Rifting: Continental Breakup and the Origin of the Atlantic Ocean and Passive Margins. Elsevier, Amsterdam, Developments in Geotectonics, 14, 103-112. MEISSNER, R., BARELSEN, H. & MURAWSKI, H. 1981. Thin-skinned tectonics in the northern Rhenish Massif, Germany. Nature, 290, 399-401. MILLER, J. A. & MOHR, P. A. 1964. Potassium-argon measurements on the granites and some associated rocks from south-west England. Geological Journal, 4, 105-126.
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POMEROL, C. 1972. Introduction, Colloque sur la geologie de la Manche, Memoires du Bureau de Recherches Geologiques et Minieres. 79, 11-12. RUFFELL, A. 1990. Stratigraphy and structure of the Mercia Mudstone Group (Triassic) in the western part of the Wessex Basin. Proceedings of the Ussher Society, 7, 263-267. 1995. Evolution and hydrocarbon prospectivity of the Brittany Basin (Western Approaches Trough), offshore N.W. France. Marine and Petroleum Geology, in press. SHACKLETON, R. M., RIES, A. C. & COWARD, M. C. 1982. An interpretation of Variscan structures in SW England. Journal of the Geological Society, London, 139, 533-541. SMITH, m. J. & CURRY, D. 1975. The structure and geological evolution of the English Channel. Philosophical Transactions of the Royal Society, A279, 3-20. SMITH, D. B., BRUNSTROM, R. G. W., MANNING, P. I., SIMPSON, S. & SHOTTON, F. W. 1974. A correlation of Permian rocks in the British Isles. Journal of the Geological Society, London, 130, 1-45. THORPE, R. S., COSGROVE, M. E. & VAN CASTEREN, P. W. C. 1986. Rare earth element, Srand Nd isotope evidence for petrogenesis of Permian basaltic and K-rich volcanic rocks from south-west England. Mineralogical Magazine, 50, 481-490. VAIL, P. R. 1987. Seismic stratigraphy interpretation using sequence stratigraphy. Part 1: seismic stratigraphy interpretation procedure. In: BALLY, A. W. (ed.) Atlas of Seismic Stratigraphy, AAPG Studies in Geology, 27, 1-10. WORTH, R. H. 1908. The dredgings of the Marine Biological Association (1895-1906), as a contribution to the knowledge of the Geology of the English Channel. Journal of the Marine Biological Association of the United Kingdom, 8, 118-188. ZEIGLER, P. A. 1982. Geological Atlas of Western and Central Europe. Elsevier, Amsterdam.
From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe,
Geological Society Special Publication No. 91, pp. 215-237.
Permo-Triassic development of the Celtic Sea region, offshore Ireland P. M. S H A N N O N
Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland. Abstract: The Celtic Sea region, lying to the south of Ireland, contains a set of
linked post-Palaeozoic sedimentary basins. Petroleum exploration in the region has concentrated upon Cretaceous and Jurassic targets with relatively little known about the Permo-Triassic evolution. Seismic reflection profiles and available well data are used to analyse the Permo-Triassic development of the basins lying in the Irish sector of the Celtic Sea. Occasional, small, faultcontrolled Permian intermontane basins were developed in the Celtic Sea region. Rifting in Early Triassic times produced a number of fault-bounded basins, with clastic facies deposited in a fluvial-dominated arid setting. Late Triassic basins, largely controlled by post-rift thermal subsidence and consisting of coastal sabkha or supratidal flat deposits, extended beyond the limits of the Early Triassic basins. The Pembrokeshire Ridge-Labadie Bank, partially cored by Variscan granites, acted as a partial boundary between the North and South Celtic Sea Basins during Triassic times. Lower Triassic sandstone deposits are best developed in the North Celtic Sea and Fastnet Basins. Upper Triassic salt is widespread in the South Celtic Sea Basin and occurs locally along the southern margin of the North Celtic Sea Basin. Upper Triassic marls and mudstones are regionally developed throughout the basins.
The Celtic Sea contains a set of interlinked Mesozoic to Cenozoic sedimentary basins. The Fastnet, N o r t h Celtic Sea and St George's Channel Basins lie northwest of intermittent basement ridges, while the Cockburn, South Celtic Sea and Bristol Channel Basins basement lie to the southeast (Fig. 1). Boundaries between the basins are sometimes diffuse and are typically manifested by (a) differences in preserved stratigraphy, (b) changes in basin shape and orientation or (c) structurally complex fault zones. The basins vary in trend from E N E - W S W to NE-SW, reflecting the orientation of basement trends that controlled the initial location and development of the basins. Water depths in the region increase westwards, from 100 m throughout much of the Celtic Sea, to a maximum of 150 m in the Fastnet Basin. Petroleum exploration to date in the Celtic Sea Basins, which have been explored only relatively lightly, has concentrated on Upper Jurassic and Lower Cretaceous reservoir targets. The Lower Jurassic and Permo-Triassic have been neglected because of (a) the perceived likely depth of such targets throughout much of the region, (b) the hitherto poor quality of seismic data at these levels and (c) the uncertain nature and distribution of reservoirs within the Permo-Triassic. Known Triassic in the Central Irish Sea and St George's Channel Basins, and the boost of recent gas and oil discoveries at these levels in the U K sector of the Irish Sea, have sharpened attention on these levels to the southwest.
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P. M. SHANNON
Fig. 1. Location map, showing the distribution of basins (shaded) within the Celtic Sea, and the wells drilled in the Irish sector of the region. Available Irish wells that encountered Triassic strata are shown with a black star, while UK wells referred to in the text are indicated with a white star. The locations of Figs 5-10 are indicated.
Some recent publications (e.g. Shannon & MacTiernan 1993) have highlighted the significant potential of such targets in the North Celtic Sea Basin. The objective of this paper is to present a regional overview of Permo-Triassic development in the Irish sector of the Celtic Sea. In particular, the questions of the basin dynamics, regional distribution and lithofacies variations are addressed. The region examined comprises all or parts of the Fastnet, North Celtic Sea, South Celtic Sea and Cockburn Basins (Fig. 1) lying in Irish designated waters.
PERMO-TRIASSIC OF THE CELTIC SEA
217
Regional geological setting Approximately 65 wells have been drilled to date in the Irish sector of the Celtic Sea (Fig. 1). Economic basement in the region comprises Devonian and Carboniferous metasediments indurated during Variscan deformation (Tucker 1977, 1978; Colin et al. 1981; Robinson et al. 1981; Naylor & Shannon 1982; Higgs 1983). The E-W to ENE-WSW Variscan trends probably exploited older Caledonian fabrics both in the offshore (Gardiner & Sheridan 1981; Shannon 1991a,b) and in the southern part of onshore Ireland (Naylor et al. 1981). The post-Variscan strata of the region developed in response to a series of rift episodes, interspersed with periods of thermal subsidence. They reflect the interplay of the breakup of Pangaea, the development of the North Atlantic Ocean and Alpine continental suturing. Up to 10 km of post-Carboniferous strata are preserved in the basins, with the North Celtic Sea Basin containing the thickest and most complete sedimentary succession (Shannon 1991a). Variscan basement is typically overlain unconformably by a poorly dated red-bed succession of presumed Permo-Triassic age. This is followed conformably by Lower to Middle Jurassic shelf limestones, marls and mudstones with local deltaic sandstones (Robinson et al. 1981), deposited under conditions of thermal subsidence (Shannon 199 la). Upper Jurassic strata are generally thin or absent from the Fastnet, Cockburn and South Celtic Sea Basins, while a thick Upper Jurassic synrift succession of fluvial, deltaic and shallow marine sandstones, mudstones and limestones occurs in the North Celtic Sea Basin. The presence of extensive Bajocian basic igneous sills and dykes within the Fastnet Basin (Caston et al. 1981) and Bathonian-Callovian porphyritic basalts in the Goban Spur to the west (Cook 1987), is suggestive of the existence of a local Middle Jurassic hot spot which may account for the absence, through thermal buoyancy, of the postBajocian Jurassic section in the basin. Early Cretaceous extension within the Celtic Sea region was accommodated by the development of a localized rift system in the region north of the Labadie BankPembrokeshire Ridge. This resulted in the preservation of a thick, fluvial-dominated, Lower Cretaceous succession in the North Celtic Sea Basin. An attenuated Lower Cretaceous section, resulting largely from non-deposition, is preserved in the Cockburn and South Celtic Sea Basins. The Early Cretaceous regression was probably caused by northward-directed transpression produced by extension and the onset of seafloor spreading in the Bay of Biscay region. Thermal subsidence in midto Early Tertiary times was interrupted by Palaeogene basin inversion. This was significant in the North Celtic Sea Basin, less pronounced in the Cockburn and South Celtic Sea Basins, and poorly developed in the Fastnet Basin. The variations are interpreted as reflecting the distribution of transpressive stresses within the region, and are influenced by the location and reactivation of structural fabrics.
The Permo-Triassic General
distribution
A small number of recent publications have provided useful information on aspects of the structure (Tucker & Arter 1987; Petrie et al. 1989), stratigraphy (Murphy & Ainsworth 1991) and petroleum potential (Shannon & MacTiernan 1993) of the
218
P. M. SHANNON
Permo-Triassic in the Celtic Sea region. Eight of the 13 available wells that encountered pre-Rhaetian strata were located in the Fastnet Basin, with three in the margin of the North and two in the South Celtic Sea Basins. These typically recorded a red-bed succession of sandstones, mudstones and evaporites (Robinson et aL 198 l; Murphy & Ainsworth 1991; Shannon & MacTiernan 1993). Seabed outcrops of red-bed facies have been documented in a number of areas of the Celtic Sea. Delanty et al. (1981) recorded red marls, sandstones and conglomerates from four gravity cores immediately south of the northern margin of the North Celtic Sea Basin. Seismic data show them to rest upon basement which subcrops to the north, while basinwards they unconformably underlie an interpreted Upper Cretaceous sequence. The red-bed section sampled was unfossiliferous. A red-bed succession of sandstones and conglomerates, more than 200 m thick, was encountered beneath the Quaternary boulder clay in onshore boreholes located within 10km of the coast in the southeast of Ireland (Clayton et al. 1986)~ This sequence of red-beds is unfossiliferous, but rests unconformably on Upper Carboniferous strata. Elsewhere, Permo-Triassic data are poorly represented onshore in Ireland, and this remains the only known occurrence of probable Permo-Triassic rocks onshore in the southern part of Ireland (Naylor 1992). The South Celtic Sea Basin extends eastwards into the Bristol Channel Basin (Fig. 1). Along the margins of the Bristol Channel, a Triassic sequence is exposed, comprising marls and mudstones resting upon thin Triassic basal breccias and sandstones. This sequence is interpreted as marginal lacustrine facies, with local and fluvial alluvial fan deposits (Tucker 1977, 1978). It rests unconformably upon a deformed and weathered palaeotopography of deformed Devonian and Carboniferous basement. This basal Triassic succession is overlain by thick red marls and mudstones, interpreted as lacustrine or inland sea deposits of the Mercia Mudstone Group. There is no evidence in this region for any Permian deposits. The Triassic succession gives way conformably to the Lower jurassic marine strata. Murphy & Ainsworth (1991) stated that the Irish Department of Energy is planning to publish a formal lithostratigraphic nomenclature for the Celtic Sea region. They proposed an informal nomenclature for the Fastnet Basin, and described six units based upon wireline log characteristics (Fig. 2). The Permo-Triassic stratigraphy in the Celtic Sea region is broadly comparable to that in the Irish Sea basins (Jackson et al. 1987; Naylor et al. 1993), onshore England and the Southern North Sea (Warrington et al. 1980; Warrington & Ivemey-Cook 1992). As in these areas, it can be subdivided into a basal sandstone-rich succession (Sherwood Sandstone Group), a central mudstone and saliferous sequence (Mercia Mudstone Group) and a thin marl-dominant series (Penarth Group). This nomenclature (Fig. 2) was adopted in the present study at group level in the absence of an alternative formal nomenclature and because of the similarity with the adjacent areas. The Sherwood Sandstone Group was subdivided into three informal units: Lower Sandstone Member, Middle Mudstone Member and Upper Sandstone Member. These correspond to the three basal Triassic units of Murphy & Ainsworth (1991) in the Fastnet Basin. The Mercia Mudstone Group of the present study corresponds to the Triassic Claystone Unit 2 of Murphy & Ainsworth (1991). However, it contains a Saliferous Member (Fig. 2) which does not have an equivalent in the Murphy & Ainsworth (1991) stratigraphy, while the marl succession is referred to as the Keuper Marl Member. The Penarth Group is subdivided into the Lower Marl Member and
PERMO-TRIASSIC OF THE CELTIC SEA AGE
219
This Paper
Lithology Murphy& Ainsworth 1991 .....
, " , " RHAETHIANLIMESTONE
RHAETIAN
TR-6
RHAETHIANMARLUNIT T R - 5
UPPER LIMESTONE MEMBER LOWER MARL MEMBER
PENARTH EROUP
NORIAN TRIASSIC CLAYSTON E UNIT 2
CARNIAN
LADINIAN .
.
.
.
.
,
ANISIAN
ScYrHIAN
,
TRIASSIC SANDSTONE UNIT 2 TRIASSIC CLAYSTONE UNff 1 TRIASSIC SANDSTONE UNIT 1
KEUPER MARL MEMBER
TR-4
MERCIA MUDSTONE GROUP
UPPER SANDSTONE MEMBER TR-3 TR-2
MIDDLE MUDSTONE MEMBER SHERWOOD SANDSTONE MEMBER LOWER SANDSTONE MEMBER
VARISCAN BASEMENT
Fig. 2. Triassic stratigraphy adopted in the present paper. This is compared to the nomenclature of Murphy & Ainsworth (1991). The three groups are subdivided into informal members, which are variable in their thickness and extent throughout the Celtic Sea region.
the Upper Limestone Member. These correspond to the Rhaetian Marl Unit and the Rhaetian Limestone of Murphy & Ainsworth (1991). The principal emphasis in the present paper is on the red-bed (pre-Penarth) groups. Dating of the red-bed (pre-Penarth Group) sequence throughout the Celtic Sea region is poor, and firm dates have only rarely been obtained (Barr et al. 1981). The Sherwood Sandstone Group is usually barren of fossils but is commonly assumed to be of Early Triassic (Scythian to Ladinian) age. However, stratigraphically comparable successions in the onshore and offshore Irish Sea area have been more confidently dated (Warrington et al. 1980). The overlying, regionally extensive marly lithologies of the Mercia Mudstone Group in the Celtic Sea region (Figs 3 & 4) have yielded rare palynomorphs which typically indicate an Anisian-Ladinian age, while the Lower Marl Member of the Penarth Group in the Fastnet Basin has early to midRhaetian palynomorphs (Rutherford & Ainsworth 1989; Murphy & Ainsworth 1991). The Upper Limestone Member of the Penarth Group and the overlying limestones have a rich palynofauna and microfossil assemblage which yielded Late Rhaetian through to Early Jurassic ages (Ainsworth et al. 1987; Rutherford & Ainsworth 1989; Murphy & Ainsworth 1991). Therefore, because of the poorly dated nature of the red-bed facies throughout the Celtic Sea region, a Triassic age is assigned largely on the basis of (a) the stratigraphic position unconformably
220
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Fig. 4. Correlation of the available wells that encountered Triassic strata in the North and South Celtic Sea Basins. The horizontal reference line is the base of the Hettangian limestone succession (approximate base of the Jurassic). Curves are Gamma Ray (LHS on 57/9-1 with scale of 0-160 API), SP (LHS on 56/20-1, 48/30-1 and 49/29-1 with scale of 100mV), Sonic (RHS on 57/9-1 with scale of 140-40 Its ft -1) and Resistivity (RHS on 56/20-1, 48/30-1, 58/3-1 and 49/29-1 with logarithmic scale of 0.2-100 f~m). overlying Carboniferous and older basement and beneath Rhaetian and Liassic marine carbonates, and (b) the similarity to dated Triassic facies in other nearby basins (Barr et al. 1981). The lithofacies and seismic stratigraphic features of each of the basins lying in the Irish sector of the Celtic Sea are described in the following sections. An outline of the lithostratigraphy and seismic stratigraphic style for each of the basins is given.
222
P. M. SHANNON
The majority of the wells in the Celtic Sea region which encountered pre-Jurassic strata are located in the Fastnet Basin, and the quality of the seismic data in this basin is very good. Consequently the basin-by-basin description starts with this basin and then follows a clockwise direction through the other basins to include the Cockburn, South Celtic Sea and North Celtic Sea Basins. Fastnet Basin
Eight wells penetrated the interpreted Triassic succession in this basin (Fig. 3). A redbed succession (Sherwood Sandstone and Mercia Mudstone Groups) overlies deformed Variscan basement, composed of Devonian (Frasnian) conglomerates, sandstones and volcaniclastics, and Lower Carboniferous shelf limestones (Robinson et al. 1981). Basement is typically seismically transparent and capped by a slight angular unconformity. The base of the interpreted Triassic succession is generally marked by a high amplitude reflector. A small, localized package of dipping coherent reflectors has been identified beneath this reflector in one small area in the south of the basin. This is interpreted as a preserved Permian succession, unconformably overlain by the interpreted Triassic succession. The Sherwood Sandstone Group, where penetrated in the Fastnet Basin, is typically 150-200 m thick and consists of the Lower and Upper Sandstone Members separated by the Middle Mudstone Member. Although thicknesses vary somewhat between wells, the major units can be broadly correlated on lithofacies throughout the basin. Sandstones in the Sherwood Sandstone Group typically have porosities of up to 13%. However, well 55/30-1 contains sandstones with porosities of up to 22% (Robinson et al. 1981). Sandstone beds within the Lower Sandstone Member are typically 7-15m thick, have sharp bases and typically display blocky wireline log patterns. Occasional conglomerates are developed, poorly sorted with angular to subangular clasts, some of which are of reworked Carboniferous material. The sandstones and conglomerates frequently contain a calcareous cement. These sedimentary rocks are interpreted as amalgamated beds, deposited in an arid proximal alluvial wadi setting. The argillaceous nature of the sandstones is interpreted as the result of clay infiltration due to floodwater discharge through permeable sands, while the calcareous cement is likely to be an early cement deposited at the air-water interface as the water table dropped rapidly through the wadi clastics. Occasional thinner sandstones occur, displaying fining-upward gamma-ray log patterns. These are interpreted as distal alluvial fan braid bar or point bar deposits. The Middle Mudstone Member throughout the Fastnet Basin contains thin beds of hard crystalline limestone and argillaceous sandstones. These lithologies are typical of distal floodplain or inland sabkha deposition. The Upper Sandstone Member of the Sherwood Sandstone Group in the Fastnet Basin contains thinner sandstone beds than the Lower Sandstone Member. The sandstones frequently contain a. calcareous cement and are typically fine-grained. Some beds with well rounded quartz grains are recorded. The sandstones contain both fining-upward and coarsening-upward wireline log motifs. Traces of gypsum and anhydrite also occur. The sandstones are interpreted as 10w energy fluvial bars and as crevasse splays, with the interbedded argillaceous beds representing
PERMO-TRIASSIC OF THE CELTIC SEA
223
floodplain or ephemeral lacustrine deposition. The occurrence of rounded sand grains is thought to represent fluvial reworking of adjacent coeval aeolian dunes. The wireline log pattern, together with their typically thin nature, argues against these beds being primary aeolian dunes. The overall tripartite pattern of the Sherwood Sandstone Group is suggestive of an initial phase of high energy clastic deposition in an arid setting, evolving to a period of low energy deposition with increasing distance from the sediment provenance. The Upper Sandstone Member represents a period of rejuvenation with increased clastic input. While a major long term climatic change cannot be totally ruled out as an explanation for the pattern, comparison with modern depositional systems would indicate a greater likelihood of tectonic control, with the high energy facies corresponding to rift episodes. The thick red mudstones (Keuper Marl Member of the Mercia Mudstone Group) which cap the Sherwood Sandstone Group are typically calcareous, sometimes anhydritic, and contain thin sandstones and siltstones. Deposition probably took place in a coastal sabkha or supratidal flat. The Penarth Group consists of the Lower Marl Member of grey marls and an overlying Upper Limestone Member. The group is typically 30-50m thick throughout the Fastnet Basin (Fig. 3). Periods of intermittent connection to shallow water, open marine conditions are indicated (Murphy & Ainsworth 1991) in the Lower Marl Member throughout the basin. These herald the onset of a regionally extensive open marine shallow sea, which resulted in the deposition of the upper part of the Penarth Group and the overlying Liassic limestones. A tripartite seismic subdivision of the red-bed succession occurs within the basin. The lowermost seismic unit contains local areas of high amplitude, discontinuous reflectors (Fig. 5), interspersed with areas of lower amplitude, higher continuity reflectors. These are overlain by a relatively diffuse zone of low amplitude, discontinuous reflectors. The uppermost seismic unit typically consists of relatively continuous high amplitude reflectors, with laterally equivalent areas of poor reflectors where the unit cannot be distinguished from the central unit. Thickness variations sometimes occur within the succession, while seismic facies differences within the Triassic succession are observed (Fig. 5). The localized, high amplitude reflectors drilled in well 55/30-1 (Fig. 5) correspond to thick sandstones interpreted as fluvial channel sediments deposited in proximal to mid-alluvial fan wadis during Sherwood Sandstone Group times. The laterally equivalent, lower amplitude, continuous events may correspond to distal and finer-grained sheetflood sandstones. Similar seismic facies relationships have been described for the Lower Triassic Sherwood Sandstone Group of the Irish Sea (Meadows & Beach 1993). The central unit within the Triassic succession corresponds in wells to the Mercia Mudstone Group (Keuper Marl Member) of red mudstones and the thin overlying Penarth Group of marls and limestones. No well data are available in regions where the upper seismic units of moderately high amplitude reflectors are developed. It is suggested that these are likely to correspond to thin beds of evaporites and/or limestones. The dipping Triassic succession subcrops the Cretaceous along the northern margin of the Fastnet Basin. It is conformably overlain by the Jurassic. The original limits of the Triassic basin at this margin are therefore unknown. The southeastern basin margin is faulted. While Robinson et al. (1981) indicate that there is no clear
224
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PERMO-TRIASSIC OF THE CELTIC SEA
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evidence from the Fastnet Basin for Triassic rifting, they state that '... it is quite likely that earlier Caledonian lines of weakness were the sites of incipient fault movement in the Triassic'. Their fig. 4 suggests that the main basin margin fault corresponds to the Triassic basin boundary. Here the Triassic and Lower Jurassic strata rest upon basement basinwards of the main basin-bounding fault, with Lower Jurassic sediments resting on basement and subcropping to the Cretaceous on the foot-wall block.
Cockburn Basin The Cockburn Basin is undrilled and therefore ages and facies are speculative. The base of the interpreted Triassic succession in this basin is typically marked by a high amplitude reflector, similar to that seen in the Fastnet Basin (Fig. 6). Towards the northern basin edge this is locally overlain by a thin unit of continuous reflectors. The middle and upper seismic units seen in the Fastnet Basin cannot be clearly distinguished and the succession is characterized by local moderate amplitude, relatively continuous reflectors. These are clearly distinguished from the conformable, probably Lower Jurassic, seismic sequence of very continuous reflectors, similar to those of the Fastnet Basin. Towards the northern boundary of the basin a half-graben containing a package of dipping reflectors underlies the interpreted Triassic section and overlies presumed Palaeozoic basement (Fig. 6). This may represent a small Permian basin. The interpreted Triassic succession subcrops to the Base Cretaceous on both the northern and southern boundaries. It extends beyond the preserved limit of the Early Jurassic basin. The nature of the basin margins is therefore uncertain.
South Celtic Sea Basin One well (58/3-1) drilled basement in the South Celtic Sea Basin. This encountered Carboniferous culm facies, fine-grained clastics (Higgs 1983), similar to those of the North Celtic Sea Basin. Two wells (49/29-1 and 58/3-1) in the South Celtic Sea Basin encountered a thick (c. 1 kin) presumed Triassic succession (Fig. 4). The Sherwood Sandstone succession was not reached in 49/29-1 and was only poorly developed in 58/3-1. Thick salt sequences (Saliferous Member of the Mercia Mudstone Group) were found in both wells. A broadly comparable succession was recorded in well 93/6-1 in the U K sector of the basin and in the Bristol Channel Basin (Kamerling 1979). The saliferous beds are interbedded with thin red mudstones, while thick mudstones are also coeval with the Saliferous Member throughout t h e Fastnet Basin and most of the North Celtic Sea Basin. The salt reflects deposition in a topographically low, shallow water depocentre. Periods of distal clastic input into a hypersaline shallow perennial lake, alternating with periods of evaporation, are likely to have given rise to the observed facies patterns. The Penarth Group is thin in the available wells in the South Celtic Sea Basin and the distinction into the two members is typically unclear. Instead, the formation is characterized by grey marls sometimes with interbedded thin limestones (Fig. 4).
226
P. M. SHANNON
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PERMO-TRIASSIC OF THE CELTIC SEA
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The Triassic seismic package in the South Celtic Sea Basin differs significantly from that in the Fastnet and Cockburn Basins in displaying marked thickness variations. The base of the succession is marked by a strong, planar, high amplitude reflector, as in the other basins. This is overlain by a seismically transparent zone, of variable thickness. The upper part of the succession is characterized by fairly continuous, moderate amplitude reflectors, similar to those seen in the Fastnet Basin. The Triassic succession is conformably overlain by a Liassic succession characterized by a lower unit of high amplitude, high frequency, continuous reflectors (corresponding to the limestone-dominant Lower Liassic sequence), and an upper unit of lower amplitude, continuous reflectors (corresponding to marls and claystones). The pillows and swells in the central part of the basin (Fig. 7) are interpreted as thick salt deposits, comparable to those encountered in wells drilled in the basin. Occasional discontinuous, high amplitude events within the interpreted saliferous succession are interpreted as isolated carbonate rafts. Two phases of halokinesis occurred. The initial salt movement resulted in the development of growth faulting in areas of salt withdrawal during Liassic times. The lowest Liassic forms a drape across the pillows, while later Liassic strata show a complementary thickening/thinning pattern to the pillows (Fig. 7). A series of radial faults, of Cretaceous to Early Tertiary age, are interpreted as crestal pillow collapse structures. They probably resulted from freshwater flushing during basin inversion. A late stage of salt movement occurred in Tertiary times, resulting in broad domal structures (Fig. 7). The northern limit of the Triassic succession in the basin is fault-bounded (Fig. 8). The Triassic strata are conformably overlain by the Lower Jurassic succession, and subcrop the overlying attenuated Lower Cretaceous section, extending beyond the preserved margins of the Early Jurassic basin. The bounding fault is a major normal fault which has suffered minor Early Tertiary reverse movement. It is uncertain whether the preserved boundary, demonstrably of pre-Late Cretaceous age, is coincident with the original Triassic basin boundary. However, seismic refraction and gravity analysis (O'Reilly 1990) suggest that the Pembrokeshire Ridge, forming the northern (foot-wall) margin of the South Celtic Sea Basin, is underlain by unthinned crust and has not undergone any noticeable uplift or tectonism since Variscan times. This suggests that the basin margin may correspond broadly to an original Triassic faulted margin.
N o r t h Celtic S e a Basin The interpreted Permo-Triassic section has been identified and mapped throughout the Celtic Sea region, with the exception of the central part of the North Celtic Sea Basin. Here, the combined effects of the depth of the section and the presence of high velocity, Upper Cretaceous chalk on the seabed makes it difficult to interpret the preUpper Jurassic succession with any confidence. In consequence, the descriptions which follow are generally devoid of information from the deepest part of the basin. Basement drilled in the North Celtic Sea Basin is either Carboniferous (Tournaisian) culm facies, fine-grained clastics (Higgs 1983) or undated, presumed Carboniferous, deformed metasediments. Further east, in the St George's Channel Basin, basement consists of Upper Carboniferous coal-bearing fluvio-deltaic strata (Barr et al. 1981). In the western part of the North Celtic Sea Basin (e.g. well 56/20-1),
228
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the red-bed succession is subdivided into the Sherwood Sandstone Group and the overlying Mercia Mudstone Group (Fig. 4). Here, the three members of the Sherwood Sandstone Group, seen in the Fastnet Basin, can be recognized. Further east in the basin, the basal sandstone sequence is either thin or absent in the available wells. In 57/9-1, for example, a thin basal conglomerate is overlain by a mudstoneprone unit. These are interpreted as condensed Lower Sandstone and Middle Mudstone Members, respectively, while the Upper Sandstone Member of the Sherwood Sandstone Group is absent, probably through shaling out, in this marginal part of the North Celtic Sea Basin adjacent to the Labadie Bank basement ridge. However, a thick Sherwood Sandstone Group succession was drilled in well 103/2-1 at the western end of the St George's Channel Basin (Shannon & MacTiernan 1993), suggesting that sandstones may be present locally along the southern margins of the North Celtic Sea Basin. The Penarth Group is poorly defined in the available wells in the North Celtic Sea Basin. The two members that characterize the group in the Fastnet Basin cannot be identified with any certainty and the succession, where present, consists of grey marls, sometimes with interbedded thin limestones (Fig. 4). The base of the Triassic section along the southern margin of the North Celtic Sea Basin is typically marked on seismic profiles by a strong, high amplitude reflector. The seismic character of the overlying succession along the margin is variable. Along the margins of the Pembrokeshire Ridge, the section interpreted as Triassic is typified by a lower sequence of high amplitude reflectors and an upper sequence of low amplitude, relatively continuous reflectors. The section interpreted as being of Late Triassic age (probably the Mercia Mudstone Group) locally onlaps the lower sequence (Shannon & MacTiernan 1993). The lower sequence thins towards the basin edge (Fig. 9) and is absent over the Pembrokeshire Ridge, where the thin, overstepping upper sequence rests upon seismically transparent basement and subcrops to the base Greensand unconformity. The Liassic, generally conformable within the main part of the basin, rests with a slight unconformity upon the Upper Triassic strata within a number of syn-Triassic fault blocks (see Shannon & MacTiernan 1993; fig. 6). The Liassic seismic signature in this part of the basin, unlike the other parts of the region described above, is characterized by weak but relatively continuous reflectors. Further west, along the southern margin of the North Celtic Sea Basin, the Triassic seismic character comprises continuous reflectors of variable amplitude. In general, the amplitude increases and reflectors become discontinuous towards the southern margin of the basin. This is interpreted as a facies change from lacustrine/ playa mudstone deposits with thin evaporites and carbonates within the basin, to fluvial channels closer to the basin margins. The Triassic is sometimes onlapped by a Liassic sequence, a feature best developed in a back-basin north of the Pembrokeshire Ridge (see Shannon 1991a; fig. 9). This relationship suggests that Triassic strata (especially the Upper Triassic Mercia Mudstone Group) extended beyond the limits of the preserved Jurassic strata, with Late Triassic fault activity producing a northward-dipping Triassic topography which was progressively onlapped by the marine deposits of the rising Jurassic sea level. Along the southern margin of the North Celtic Sea Basin, a thick Triassic salt section (Saliferous Member of the Mercia Mudstone Group) has been proved by well 57/9-1 (Fig. 4). A number of pod-like seismic structures in the upper part of the
231
PERMO-TRIASSIC OF THE CELTIC SEA
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232
P. M. SHANNON
Triassic succession along the southern margin of the basin are interpreted as salt swells (Shannon & MacTiernan 1993). These areas of thick salt development in the southern part of the North Celtic Sea Basin are interpreted as local fault-controlled depocentres, which resulted from early tilted fault-block movement close to the southern margin of the basin. Along the northern margin of the North Celtic Sea Basin, the base of the Triassic is marked by a very strong, high amplitude, seismic reflector (Fig. 10). The succeeding sequence comprises a package of moderate to high amplitude reflectors, continuous in the upper part and with variable continuity in the lower part. The sequence thins northwards across a basinward-dipping fault (Fig. 10). The variable seismic character in the thick part of the Triassic is interpreted as an interdigitation of sheet flood sandstones with fluvial channel clastics in the Sherwood Sandstone Group. The continuous upper reflectors, seen in both the thick Triassic and in the thin section to the north of the main bounding fault, are interpreted as a predominant mudstone/marl succession of lacustrine/playa deposits, with occasional thin crevasse splay sandstones, within the Mercia Mudstone Group. To the north of the fault the Triassic succession is thin and subcrops to the surface. The area of subcrop shown on Fig. 10 lies close to the location of the red-bed facies samples described by Delanty et al. (1981). The interpreted Triassic section is overlain by a Liassic succession, characterized by continuous reflectors, which is of constant thickness across the horst (Fig. 10). This indicates that the basinward-dipping fault is largely of pre-Liassic age, and marks the fault-bounded margin of a Triassic depocentre. The thick Triassic section to the south of the fault is interpreted as comprising synrift sediments, together with the overlying thermal subsidence deposits, which extend beyond the limits of the synrift basin.
Controls on Permo-Triassic sedimentation Basement rocks have been encountered in wells in most of the Celtic Sea basins (Barr et al. 1981; Robinson et al. 1981; Higgs 1983). They are typically indurated Upper
Palaeozoic rocks. The present work, together with most previous studies, suggests that Permian strata were only locally deposited or preserved. Van Hoorn (1987) suggested, on seismic evidence, the possible local preservation of some Permian strata in the South Celtic Sea Basin. Two examples of possible Permian successions were found in the Fastnet and Cockburn Basins in the present study. These may represent Permian wadi and fluvial clastics preserved locally in an intermontane, topographically elevated environment following Variscan orogenesis and crustal thickening. Coeval continental sediments deposited in fault-bounded topographic lows north of the uplifted region are preserved in the Irish Sea (Colter & Barr 1975; Ebbern 1981). In this region the Permian is overlain by a thick Triassic section broadly comparable to that seen throughout the Celtic Sea region. While evidence for the nature and thickness of the presumed Triassic in the central parts of the Celtic Sea basins is scant, owing to the significant depth, poor well control and poor quality seismic data, it is suggested that Triassic sediments were deposited in fault-bounded basins in parts of the North Celtic Sea Basin, probably in the South Celtic Sea Basin and possibly in some of the other Celtic Sea basins. Deposition took place during a phase of regional synrift extension. Thickness
PERMO-TRIASSIC OF THE CELTIC SEA
233
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variations of pre-Jurassic age occur across faults. Illustrative cross-sections (Petrie et al. 1989) indicate increased thicknesses in the hanging-walls of faults which have a major Triassic component of movement in the North Celtic Sea, South Celtic Sea and Fastnet Basins. Because much of the structuring in the Celtic Sea region took place in post-Jurassic times, it is frequently difficult to unravel the Triassic component of structuring within the North Celtic Sea Basin. However, detailed mapping along the southern margin of the basin (Shannon & MacTiernan 1993) clearly demonstrated a phase of Triassic structuring. A series of N E - S W faults had their major phase of growth in the Triassic. The localized extensional strain resulting from prolonged Triassic synrift movements in the North Celtic Sea Basin is probably relieved by strike-slip movement along major NW faults, described from the region by various authors (e.g. Robinson et al. 1981; Petrie et ai. 1989; O'Reilly et al. 1991). Localized strain variations at shallow levels, due to focusing of deep crustal strain, have been suggested by O'Reilly (1990) and O'Reilly et al. (1991) to explain observed differences in stretching factors in the basin. No comparable Late Triassic fault movement has been documented from the other basins in the region. This may partly explain the narrower width of the Fastnet Basin adjacent to the North Celtic Sea Basin. The widespread occurrence of pre-Liassic red-bed facies suggests that Triassic sedimentation was extensive in the Celtic Sea region. The presence of thin, probably Triassic, strata beyond the limits of the main fault-bounded depocentres, and the occasional Triassic onlap onto basement at the southern margin of the North Celtic Sea Basin, are interpreted as a Late Triassic thermal subsidence response to Early Triassic synrift development. Throughout most of the Celtic Sea, the Rhaetian and Lower Jurassic strata conformably overlie the red-bed facies and mark the continuation of thermal subsidence. However, the effects of localized Late Triassic tectonism along the southern margin of the North Celtic Sea Basin are seen in the onlap of the Liassic onto the Triassic. The general occurrence of thick salt sequences within and adjacent to the South Celtic Sea Basin, but their absence elsewhere in the Celtic Sea region, is thought to be a reflection of the basin geometry. It is suggested that the Pembrokeshire RidgeLabadie Bank basement ridge was a discontinuous topographic feature throughout Triassic times. While the onlap of Triassic onto the Pembrokeshire Ridge in places is indicative of the existence of a positive topographic feature, the presence of salt locally along the southern margin of the North Celtic Sea Basin suggests that the basement barrier between the North and South Celtic Sea Basins was discontinuous. The result is the general, but incomplete, separation of the thick salt basin in the South Celtic Sea Basin from the mudstone- and marl-dominated Upper Triassic strata in the North Celtic Sea and Fastnet Basins. Local interconnections with the North Celtic Sea Basin resulted in the development of some thick salt accumulations in the hanging-walls of Triassic fault blocks along southern margins of the North Celtic Sea Basin.
Summary and conclusions 1. No Permian strata have been encountered in wells in the Celtic Sea region. Occasional, localized, fault-controlled packages of pre-Triassic strata within the region are interpreted as possible Permian successions. The general absence of
PERMO-TRIASSIC OF THE CELTIC SEA
235
Permian strata is probably due to the topographically high, subaerial arid setting of the region, immediately following Late Carboniferous-Early Permian Variscan orogenesis. 2. A sandstone rich red-bed sequence, the Sherwood Sandstone Group, poorly dated but of presumed Early Triassic age, has been proved by wells drilled in the Fastnet Basin and the southwestern part of the North Celtic Sea Basin. The basal Lower Sandstone Member consists of thick amalgamated sandstones and is interpreted as arid alluvial wadi and fluvial bar deposits. The overlying Middle Mudstone Member is interpreted as distal floodplain or inland sabkha deposits. The Upper Sandstone Member contains thin sandstones, interpreted as fluvial bars and crevasse splays. This member is thought to represent a period of rift-generated rejuvenation. Elsewhere within the Celtic Sea region, the Sherwood Sandstone Group is thin and sandstones are only locally developed. 3. The Late Triassic red mudstones (Keuper Marl Member of the Mercia Mudstone Group), encountered in virtually all wells drilled in the Celtic Sea region, are interpreted as coastal sabkha or supratidal flat deposits. Thick salt deposits (Saliferous Member), of presumed Late Triassic age, were drilled in the South Celtic Sea Basin and on the southern margin of the North Celtic Sea Basin. The Mercia Mudstone Group is overlain by the Penarth Group of marls and limestones containing faunal evidence of periods of intermittent connection to shallow, open marine conditions. The Triassic succession is overlain, generally conformably, by Lower Jurassic shelf limestones. 4. The presumed Permo-Triassic sequence was identified and mapped on the basis of its seismic character. The upper part of the Triassic sequence is typified by continuous reflectors, corresponding to the low energy, mud-prone facies. The variable character of the Triassic sequence is interpreted as being due to a more sandstone-prone lithofacies, with fluvial deposits imaged as discontinuous reflectors, and sandsheet sediments seen as continuous reflectors. 5. Lower Triassic strata were deposited in fault-controlled basins in at least some of the Celtic Sea basins. In these instances, many of the controlling synrift faults show little post-Triassic activity. Upper Triassic strata onlap beyond the limits of the Early Triassic basins and represent thermal subsidence deposition which continued into Jurassic times. In other Celtic Sea basins, the location and precise nature of the Triassic basin margins is unknown due to later tectonic modification. Thanks are due to the Petroleum Affairs Division of the Department of Transport, Energy and Communications for permission to use the well data in Figs 3 & 4 The seismic sections shown in Figs 5-8 & 10 come from a non-exclusive proprietary seismic survey by Merlin Profilers (now GECO-PRAKLA Exploration Services). Permission to use the data is gratefully acknowledged.
References AINSWORTH, N. R., O'NEILL, M., RUTHERFORD, M. M., CLAYTON,G., HORTON, N. F. &
PENNEY, R. A. 1987. Biostratigraphy of the Lower Cretaceous, Jurassic and uppermost Triassic of the North Celtic Sea and Fastnet Basins. In: BROOKS,J. & GLENNIE,K. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 611-622. BARR, K. W., COLTER, V. S. & YOUNG, R. 1981. The geology of the Cardigan Bay-St. George's Channel Basin. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden & Son Ltd., London, 432-443.
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CASTON, V. N. D., DEARNLEY, R., HARRISON, R. K., RUNDLE, C. C. & STYLES, M. T. 1981. Olivine-dolerite intrusions in the Fastnet Basin. Journal of the Geological Society of London, 138, 31-46. CLAYTON, G., SEVASTOPULO, G. D. & SLEEMAN, A. G. 1986. Carboniferous (Dinantian and Silesian) and Permo-Triassic rocks in south County Wexford, Ireland. Geological Journal, 21, 355-374. COLIN, J. P., LEHMANN, R. A. & MORGAN, B. E. 1981. Cretaceous and Late Jurassic biostratigraphy of the North Celtic Sea Basin, offshore Southern Ireland. In: NEALE, J. W. & BRASIER, M. D. (eds) Microfossils from Recent and Fossil Shelf Seas. Ellis Horwood, Chichester, 122-155. COLTER, V. S. & BARR, K. W. 1981. Recent developments in the geology of the Irish Sea and Cheshire Basins. In: WOODLAND, A. W. (ed.) Petroleum and the Continental Shelf of North West Europe. Applied Science Publishers, Essex, 61-73. COOK, D. R. 1987. The Goban Spur - exploration in a deep-water frontier basin. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North- West Europe. Graham and Trotman, London, 623-632. DELANTY, L. J., WHITTINGTON, R. J. & DOBSON, M. R. 1981. The geology of the North Celtic Sea west of 7 ~ longitude. Proceedings of the Royal Irish Academy, 81B, 37-51. EBBERN, J. 1981. The geology of the Morecambe Gas Field. In: ILLING, L. V. & HOBSON, G. D. (eds) The Geology of the Continental Shelf of North-West Europe. Heyden & Son, London, 485-493. GARDINER, P. R. R. & SHERIDAN, D. J. R. 1981. Tectonic framework of the Celtic Sea and adjacent areas with special reference to the location of the Variscan Front. Journal of Structural Geology, 3, 317-331. HIGGS, K. 1983. Palynological evidence for the Carboniferous strata in two wells drilled in the Celtic Sea area. Bulletin of the Geological Survey of Ireland, 3, 107-112. JACKSON, D. I., MULHOLLAND, P., JONES, S. M. & WARRINGTON, G. 1987. The geological framework of the East Irish Sea Basin. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North-West Europe. Graham and Trotman, London, 191-200. KAMERLING, P. 1979. The geology and hydrocarbon habitat of the Bristol Channel Basin. Journal of Petroleum Geology, 2, 75-93. MEADOWS, N. S. & BEACH, A. 1993. Structural and climatic controls on facies distribution in a mixed fluvial and aeolian reservoir: the Triassic Sherwood Sandstone in the Irish Sea. In: NORTH, C. P. & PROSSER, D. J. (eds) Characterisation of Fluvial and Aeolian Reservoirs. Geological Society, London, Special Publication, 73, 247-264. MURPHY, N. J. & AINSWORTH, N. R. 1991. Stratigraphy of the Triassic, Lower Jurassic and Middle Jurassic (Aalenian) from the Fastnet Basin, offshore south-west Ireland. Marine and Petroleum Geology, 8, 417-429. NAYLOR, D. 1992. The post-Variscan history of Ireland. In: Parnell, J. (ed.) Basins on the Atlantic Seaboard." Petroleum Geology, Sedimentology and Basin Evolution. Geological Society, London, Special Publication, 62, 255-275. -& SHANNON, P. M. 1982. The Geology of Offshore Ireland and West Britain. Graham & Trotman, London. , HAUGHEY, N., CLAYTON, G. & GRAHAM, J. R. 1993. The Kish Bank Basin, offshore Ireland. ln: PARKER, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. The Geological Society, London, 845-855. , SEVASTOPULO, G. D., SLEEMAN, m. G. & REILLY, T. A. 1981. The Variscan fold belt in Ireland. Geologie en Mijnbouw, 60, 49-66. O'REILLY, B. M. 1990. Seismic Studies in the North Celtic Sea Basin." Implications for the Development of the North Atlantic Basin Assemblage. Ph.D. thesis, National University of Ireland. , SHANNON, P. M. & VOGT, U. 1991. Seismic studies in the North Celtic Sea Basin: implications for basin development. Journal of the GeologicalSociety, London, 148, 191-195. PETRIE, S. H., BROWN, J. R., GRANGER, P. J. & LOVELL, J. P. B. 1989. Mesozoic history of the Celtic Sea Basins. In: TANKARD, m. J. & BALKWILL, H. R. (eds) Extensional Tectonics and Stratigraphy of the North Atlantic Margins. American Association of Petroleum Geologists, Memoir, 46, 433-444.
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ROBINSON, K. W., SHANNON, P. M. & YOUNG, D. G. G. 1981. The Fastnet Basin: An integrated analysis. In: ILLING, L. V. & HOBSON, G. D. (eds) The Geology of the Continental Shelf of North-West Europe. Heyden & Son, London, 444-454. RUTHERFORD, M. M. & AINSWORTH, N. R. 1989. Micropalaeontological and stratigraphical recognition of the Triassic/Jurassic boundary in the North Celtic Sea and Fastnet Basins. In: BATTEN, D. J. & KEANE, M. C. (eds) Northwest European Micropalaeontology and Palynology. Ellis Horwood, Chichester, 45-69. SHANNON, P. M. 1991a. Tectonic framework and petroleum potential of the Celtic Sea, Ireland. First Break, 9, 107-122. -1991b. The development of Irish offshore sedimentary basins. Journal of the Geological Society, London, 148, 181-189. - & MACTIERNAN, B. 1993. Triassic prospectivity in the Celtic Sea, Ireland-a case history. First Break, 11, 47-57. TUCKER, M. E. 1977. The Marginal Triassic deposits of South Wales: continental facies and palaeogeography. Geological Journal, 12, 169-188. - 1978. Triassic lacustrine sediments from South Wales: shore-zone clastics, evaporites and carbonates. In: MATTER, A. & TUCKER, M. E. (eds) Modern and Ancient Lake Sediments. International Association of Sedimentologists. Special Publication, 2, Blackwell Scientific Publications, Oxford, 205-224. TUCKER, R. M. & ARTER, G. 1987. The tectonic evolution of the North Celtic Sea and Cardigan Bay basins with special reference to basin inversion. Tectonophysics, 137, 291307. VAN HOORN, B. 1987. The South Celtic Sea-Bristol Channel Basin: origin, deformation and inversion history. Tectonophysics, 137, 309-334. WARRINGTON, G . & IVIMEY-COOK, H. C. 1992. Triassic. In: COPE, J. C. W., INGHAM, J. K. & RAWSON, P. F. (eds) Atlas of Palaeogeography and Lithofacies. The Geological Society, London, Memoir, 13, 97-106. , G., AUDLEY-CHARLES, M. G., ELLIOT, R. E. et al. 1980. A Correlation of Triassic Rocks in the British Isles. Geological Society, London, Special Report, 13.
From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe
Geological Society Special Publication No. 91, pp. 239-253
New evidence of Permo-Triassic rifting, onshore southern Ireland, and its implications for V ariscan structural inheritance M. L. K E E L E Y
Fieldco International Ltd, Brook House, Waterloo Road, Leighton Buzzard, Bedfordshire LU7 7NR, UK Abstract: Reconnaissance apatite fission track analysis data from onshore southern Ireland are used to construct a thermal history for Late Palaeozoic rocks. The data show clear evidence of post-Variscan heating. The most significant thermal event took place during the Permo-Triassic, and resulted from not less than 1 km of regional subsidence. Late Palaeozoic rocks were again uplifted and exposed during the Early Jurassic. Significant deposition did not resume until the Late Cretaceous, and was followed by uplift and inversion emanating from the North Celtic Sea Basin. Repeated post-Variscan extension and compression of the Variscan basement must therefore have taken place, making it now difficult to differentiate bonafide Variscan deformation. However, the northern extent to both Permo-Triassic and Late Cretaceous sedimentation was probably controlled by extension along a line of major Variscan thrusts in south-central Ireland - the South Ireland Lineament.
The majority of the pre-Quaternary land surface of Ireland is occupied by Upper Palaeozoic rocks of Late Devonian-Early Westphalian age. The syn- and postdepositional tectonic history of these rocks has been the subject of extensive study, especially in the southern part of Ireland, where Variscan strain is widely developed. Over the last decade, two diverse schools of thought have arisen as to the nature and indeed genesis of Irish Variscan deformation. On the one hand, Matthews et al. (1983) and Naylor et al. (1983) interpret both extensional and compressional strain in a deep-rooted orogen. However, Cooper et al. (1984, 1986), Murphy (1988), Williams et aI. (1989) and Ford et al. (1991, 1992) envisage a high degree of shortening accommodated within a complex of thrusts, that merge and flatten out towards the base of the upper crust. Neither of these interpretations takes account of, or even considers, the specific possibility that post-Variscan deformation may have made some significant, but as yet indeterminate, contribution to the strain observed within the Irish Variscides. If significant post-Variscan deformation, and in particular extension, can be demonstrated, then the case for interpretation of observed structure within the Upper Palaeozoic rocks of southern Ireland proffered by Naylor et al. (1983) will become overwhelming. In his discussion of the post-Palaeozoic tectonic history of the North Celtic Sea Basin, Roberts (1989) implied that some of the strain associated with early Paleogene inversion would have extended onshore, into the Variscan Orogen. This now poses the issue of differentiating and then quantifying b o n a f i d e Variscan strain, as distinct from post-Variscan strain.
240
M. L. KEELEY
So far as any sedimentary evidence for crustal extension or subsidence is concerned, Naylor (1992) provides a summary of the patchy records of postVariscan deposition found scattered throughout southern and central Ireland (Fig. 1). These include isolated deposits of Mid-Jurassic clays and sands (Colbond Clay, Cloyne, County Cork; Higgs and Beese 1986), Liassic marine strata (Kill o' the Grange, County Dublin; Broughan et al. 1989), and Chalk (Ballydeenlea). Mitchell (1980) also presents evidence for Tertiary deposition from isolated clay deposits in central Ireland. Wilson (1981) has reviewed the extensive and well-known Permo-Triassic rift-fill deposits of Ulster and County Louth (Kingscourt). At the other end of the island, in south County Wexford, Clayton et al. (1986) have interpreted 200m of red clastics found overlying proven Silesian rocks to be of Triassic age. These would represent a preserved secondary half-graben linking the Triassic fill of the North Celtic Sea and Central Irish Sea Basins (see Shannon 1995). The brightly coloured clays with plant remains found by S. A. R. Boldy (pers. comm. 1992) to overlie the Old Red Sandstone in the Coumaraglin valley (1.5km SW of Seefin summit) of the Monavullagh Mountains, County Waterford, may prove to be a relict pocket of
Fig. 1. Distribution of post-Variscan sediments in Ireland. Structural sketch sections in Fig. 6 are marked A-A', B-B' and C-C ~.
PERMO-TRIASSIC RIFTING, ONSHORE SOUTHERN IRELAND
241
more widespread Triassic or possibly Tertiary deposition onshore. Despite this growing body of evidence, the case onshore for significant post-Variscan deposition, crustal subsidence and uplift remains poorly constrained. Methodology
In an effort to quantify the degree to which post-Variscan sedimentary subsidence and tectonic uplift have affected southern Ireland, Keeley et al. (1993) acquired reconnaissance apatite fission track analysis data from Counties Cork, Waterford, Wexford and Kilkenny (Fig. 2). Of the 16 sites sampled, 10 yielded sufficient apatite for analysis (Keeley et al. 1933, Table 1, Fig. 1). These were taken from Late Devonian and Early Dinantian-aged fluvial and shoreface sandstones (approximate absolute age range 365-360 Ma), and one from the Brandon Hill Granite, a Late Caledonian pluton (405 Ma intrusion age; Stillman 1981). Apatite fission track analysis (AFTA) is a derived technique based upon fission track dating. It has been known for some while that apatite crystals occurring in both igneous and sedimentary rocks contain natural interstitial impurities of the radioactive isotope 238U. This isotope undergoes spontaneous fission at a rate that is independent of temperature and pressure. Fission products repel each other electrostatically, causing a track of lattice damage, approximately 14-15 l~m long, within the apatite crystal. These tracks accumulate in the crystal lattice, and have been shown to be stable over geological time at temperatures below 50~ (Laslett et al. 1987). Up to temperatures of 110-120~ these tracks shorten according to well-understood kinetics (Laslett et al. 1987, Green et al. 1989) as the lattice repairs itself. Above this temperature, tracks anneal completely. Therefore, by counting the number of tracks within a crystal, the time since the crystal last cooled through the 110~ isotherm can
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be calculated. The number and length of shorter tracks will provide information about the subsequent cooling history, as new tracks form with lengths in thermal equilibrium. Analytical methods adopted for AFTA have been described in detail by Green (1989) and Lewis et al. (1992). Recent examples of the application of AFTA to resolving geological problems associated with Mesozoic and Tertiary cooling and uplift in Britain can also be found in Green (1989) and Lewis et al. (1992), who describe standard analytical procedure. For the Irish reconnaissance AFTA samples, the methodology employed and the raw numerical results (including 'central age') are set out in Keeley et al. (1993).
Analytical results The fission track data presented by Keeley et al. (1993, Table 1) provide a consistent set of results, indicating a relatively uniform cooling history. The 'central ages' of all sedimentary apatites lie in the range 226-154 Ma. No apatite single grain ages exceed 300Ma. Mean track lengths are tightly distributed between 11.98 and 12.68 gm. There is therefore very strong evidence for complete Variscan annealing and for significant post-Variscan heating. The absence of grains with fission track ages older than 200 Ma, a long mean track length (12.65 gm) and a high proportion of long track lengths in the most westerly sample (668: Old Head of Kinsale, County Cork) provides evidence that a regional post-Variscan annealing episode occurred at about 200 Ma. However, in this and other samples, the fact that mean track lengths are all significantly less than 14 gm indicates that there has been a further, post-200 Ma heating episode.
Interpretation of data Track length distribution histograms from selected representative samples have been used to model various thermal histories, in order to achieve a 'best fit' between the observed and modelled distributions (see Fig. 3, and Keeley et aL 1993). Such modelling relies upon the kinetic relationships established by Laslett et al. (1987) and Green et aL (1989). The evidence of post-Variscan sedimentation described above was used to constrain post-Variscan thermal histories. In particular, it is known that Upper Devonian-Lower Dinantian rocks were at surface immediately prior to the deposition of thin Mid Jurassic strata (cf. Higgs and Beese 1986). The thermal maturity measured by Clayton (1989; mean vitrinite reflectance, Rm = 0.30-0.35%) in these strata at Cloyne, County Cork, indicates that here, at least, heating by a further 20-40~ has taken place subsequent to deposition, thereby placing some constraints on the degree of post-Mid-Jurassic burial. Recent studies of organic thermal maturation levels in the Upper Palaeozoic of Ireland (Clayton et aL 1989; Jones 1992) provide additional important evidence. They record a pattern of increasing maturation westward and southwestward across the island (Rm > 6%) in Counties Cork and Kerry. Such high maturity levels confirm the conclusion that all fission tracks were completely annealed during the Variscan Orogeny. Only in northeast Ireland, and a single site in southeast County
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Wexford, are low maturity levels recorded that lie below the apatite fission track annealing threshold. The AFTA results were used to model a range of possible cooling histories for the Upper Devonian-Lower Dinantian strata, beginning with the more conventional scenarios that involve negligible post-Variscan heating or cooling (Figs 3a and 3b). If it is assumed that significant post-Variscan burial and uplift have not occurred, then scenarios involving either rapid (during Early Permian) or slow (during Early Permian to Early Jurassic) post-Variscan cooling all produce far too many long fission tracks and too few short tracks, compared with observed track distributions. Further experimental modelling indicates, by a process of elimination, that the 'best fit' with observed track distributions is obtained with the following sequence of thermal events (for Upper Devonian-Lower Dinantian rocks; see Fig. 3c): Variscan heating - complete annealing of all fission tracks Post-Variscan c o o l i n g - rapid uplift and erosion during the Early Permian Late Permian-Triassic h e a t i n g - gradual burial and/or geothermal heating to
temperatures of about 80~ (partial annealing) Early Jurassic cooling - gradual uplift and erosion and/or geothermal cooling M i d - J u r a s s i c - M i d Cretaceous heating - minor burial Late Cretaceous heating-significant and rapid burial and/or geothermal heating to
temperatures of about 60~ (partial annealing) Tertiary cooling - rapid or gradual cooling through uplift and erosion of almost all
post-Variscan deposits. In interpreting such data derived from AFTA, it is not always possible to distinguish between thermal events that have resulted, on the one hand, almost entirely from sedimentary burial and, on the other hand, from increased crustal heat flux. The range of possibilities for Permo-Triassic heating will now be examined. However, certain assumptions must be made about the range of possible geothermal gradients. An upper value of, say 40~ km -1, would be commensurate with crustal stretching and rifting, such as might be expected in the North Atlantic region at this time. A lower value of 20~ km -1, a present-day measurement for onshore Ireland (Keeley et al. 1993), would be a typical low value for normal continental crust. Assumptions must also be made regarding a palaeosurface temperature; a value of 10~ is used here. With net cooling of about 70~ (maximum palaeotemperature of 80~ less surface temperature), a high geothermal gradient of 40~ -~ would necessitate about 1.75 km of Early Jurassic uplift and erosion onshore southeast Ireland. About 3.5 km of uplift and erosion would be required under a low geothermal gradient regime (20~ km-1). From Fig. 3c it is clear that, in terms of Permo-Triassic deposition, such values represent upper limits. Following post-Variscan cooling, Permo-Triassic reheating undoubtedly occurred. It could equally have been as little as 20~ as shown in Fig. 3c; this would equate with a range of burial values of 0.5-1 km. Alternatively, if the present-day degree of erosion down into the Variscan Orogen occurred pre-Permo-Triassic deposition (see below), then reheating could have been as much as 70~ Such a model variant gives as good a fit with observed track
PERMO-TRIASSIC RIFTING, ONSHORE SOUTHERN IRELAND
245
distributions as that shown in Fig. 3c. This would correspond with a range of burial values of 1.75-3.5 km, though it is unlikely that low crustal heat flow would have coexisted with 3.5 km of subsidence. To summarize, most likely estimates of PermoTriassic crustal subsidence and sedimentary deposition, whilst not definitive, lie in the region of 1-1.5 km. The AFTA evidence cited above would suggest that for a given geothermal gradient, Permo-Triassic burial would be fairly uniform across the sampled area of southeast Ireland. The one observable trend is for a westward increase in maximum Permo-Triassic palaeotemperatures, with over 90~ of net cooling indicated in the Old Head of Kinsale area. This would suggest that any contemporaneous heat flow and burial increased southwestwards. Such a conclusion has important implications for syndepositional tectonics (see below). A similar approach has been adopted in order to arrive at estimates of Mesozoic burial and Tertiary inversion (Keeley et al. 1993). Post-Mid-Jurassic burial is calculated to lie in the range 1.3-2.5 km, and gross Tertiary uplift to be as much as 2.5 km, a substantial proportion of which must have resulted from the North Celtic Sea Basin inversion event (cf. Roberts 1989). The patchy nature of the Mesozoic sedimentary record in southern Ireland hints at post-Jurassic subsidence and Tertiary uplift being roughly in balance. Independent supporting evidence for substantial Permo-Triassic burial, and subsequent Early Jurassic uplift and erosion, does exist. Background carbonateargillite Liassic sedimentation in the North Celtic Sea and Fastnet Basins was interrupted during the late Sinemurian-Pliensbachian by repeated regional influxes of shallow marine and deltaic sands (Petrie et al. 1989; Naylor 1992). These might well have resulted from Early Jurassic uplift and erosion to the north. Much more ambiguous is the widespread though patchy evidence throughout Ireland provided by reddening, karstification and peneplanation of the 'Top Carboniferous Limestone' erosion surface. Traditionally, and with good reason, this is considered to have resulted from Tertiary climatic changes and weathering (Mitchell 1980). These effects are, however, undated, and could have resulted, in part, from deep Early Permian weathering and erosion into the Variscan Orogen, and subsequent Permo-Triassic 'red bed' deposition, much as in the Bristol-Mendip region, southwest England (Savage 1977).
Implications The interpretations presented above of these reconnaissance AFTA data have important implications for many aspects of Irish geology. As has been discussed, it is very difficult to avoid the conclusion that significant Permo-Triassic deposition took place over southeast Ireland. This poses the question: what were the tectonic controls on the crustal subsidence that accommodated Permo-Triassic deposition, and subsequent inversion in the Early Jurassic? Any such subsidence would have involved the extension of the recently formed Variscan Orogen, as suggested by other workers in Britain (e.g. Ruffell et al. 1995). It is therefore reasonable to assume that any extension, particularly along a basinbounding fault, would have utilized existing (Variscan) basement fractures, lines of weakness and fabrics (Stein and Blundell 1990). Moreover, any faults active under
246
M. L. KEELEY
extension would have been prime candidates for reactivation during subsequent inversion. It is worth noting at this juncture that a good many Variscan structures in Ireland have inherited a Caledonoid trend. The present-day northern boundary to the North Celtic Sea Basin is documented by Naylor and Shannon (1982), Petrie et al. (1989) and Shannon (1995). This boundary is largely fault-controlled, and follows a Variscoid-Caledonoid trend (045o-060 ~) off the southern Ireland coast, intercepting it in southeast County Wexford (cf. Clayton et al. 1986). Any northern faulted boundary to a PermoTriassic proto-North Celtic Sea Basin is also likely to follow a Variscoid-Caledonoid trend, parallel to depositional strike, and be to the northwest of the AFTA sample sites. This fault system must also have accommodated compressional reactivation to some degree (cf. Stein and Blundell 1990). Before evaluating further candidates for such bounding faults, some further consideration must be given to the controls exercised by the Variscan basement on this activity.
The Variscan basement The Irish Variscan Orogen is characterized by regional metamorphism, Upper Palaeozoic basin inversion, and a diffuse distribution of brittle and ductile deformation; there are no Variscan granites in Ireland. Low grade metamorphism has been mapped in detail, and increases westwards and southwestwards (Clayton et al. 1989; Jones 1992). Such a pattern would seem to owe more to initial rifting of the North Atlantic (cf. Haszeldine and Russell 1987) than to plate collision processes to the southeast in central Europe (cf. Zwart and Dornsiepen 1978), whether the result of variations in orogenic exhumation or heat flow. Matthews et al. (1983) related the inversion of the Upper Palaeozoic Munster Basin to Variscan underplating processes. There is clearly a linkage with the distribution of Variscan ductile deformation in southern Ireland (tight folds with axial planar cleavage), although locally intense Variscan strain is known from central Ireland (Sevastopulo 1981; Coller 1984). Mention was made earlier of the differing interpretations of Variscan structure, centred on aspects of brittle deformation. The importance of a belt of major thrusts (Fig. 1), first recognized by Phllcox (1964), that follows the Varlscold=Caledonold 'South Ireland Lineament' (Gardiner and Sheridan 1981) is appreciated by all workers. However, there is dispute as to the attitude and hence genesis of the symmetrical strike faults that bound the synclinal limbs to basement box folds in southern and southeast Ireland9 According to Naylor et al. (1983) and Keeley (1983), inter alia, these are stratigraphically and hence lithologically bounded, inward-facing normal faults (see Fig, 4b). However, Cooper et al. (1984) and subsequent workers (e.g. Murphy 1988; Williams et al. 1989) interpret them as outward-facing thrusts (Fig. 4a). There are mechanical objections to the latter interpretation, and difficulties with the coincidence of their stratigraphical limits at outcrop. These are countered by legitimate opposition to hypothetical Variscan extensional strain, necessary for normal faulting, as proposed in the former interpretation. It is now possible, however, on the basis of AFTA work presented above, to interpret some component of brittle strain within the Variscan Orogen as p o s t - V a r i s c a n in age, thereby removing objections to the Naylor et al. model. 9
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From the above, it would seem that the most logical candidate for a bounding fault to the Permo-Triassic proto-North Celtic Sea Basin (and Cretaceous) subsidence and subsequent (Early Jurassic and Early Tertiary) inversion is represented by the line of Variscan thrusts that outcrop along the South Ireland Lineament (Fig. 1). No Mesozoic sediment is known to lie north of this lineament and south of the Kingscourt outlier of Permo-Triassic in County Louth.
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PERMO-TRIASSIC RIFTING, ONSHORE SOUTHERN IRELAND
249
This proposal would place the margin to the Permo-Triassic proto-North Celtic Sea Basin some 75km northwest of the present-day margin (Fig. 6). The Old Head of Kinsale area would thus have lain nearer the basin axis than the other AFTA sites to the east. The basin-bounding master fault system following the South Ireland Lineament would also have provided a plane of weakness for subsequent episodes of uplift, subsidence and inversion (cf. Roberts 1989), thereby forming a boundary between inactive Mesozoic central Ireland and the more active southern margin. Intrabasinal subsidence on the Triassic basin shoulder could have been accommodated by both synthetic and antithetic normal faults forming on the limbs of Variscan synclines (Fig. 6; Naylor et al. 1983; Keeley 1983). Thus the framework was set in the Triassic for the geomorphological pattern observed today, which also exercised control over modest Mesozoic deposition (Naylor 1992).
Reinterpretation of crustal evolution from Late Carboniferous times Following the above interpretations, a coherent chronological sequence of tectonic events can now be made for southeast Ireland (see Figs 5 and 6): (i)
Variscan o r o g e n y - ductile deformation (folding, axial planar cleavage); regional low grade metamorphism; discrete thrusting along the South Ireland Lineament; (ii) Orogenic e x h u m a t i o n - regional ?Early Permian uplift (>2km), cooling and erosion of the Variscan Orogen; (iii) Permo-Triassic subsidence- Triassic (and Late Permian?) formation of the wide, immature proto-North Celtic Sea Basin, extending as far north as the South Ireland Lineament with > 1 km of sedimentation. Limited secondary subsidence on the basin shoulder; (iv) Early Jurassic u p l i f t - regional uplift of southern Ireland (>1.5 km), with tilting to the south; erosion of almost all Permo-Triassic sediments, producing a clastic influx into the narrower depocentres of the mature North Celtic Sea and Fastnet Basins; (v) Mid-Jurassic-Early Cretaceous subsidence - only thin terrestrial and marginal marine deposition in southern Ireland (<0.5 km); (vi) Late Cretaceous deposition- thick chalk deposition in the North Celtic Sea Basin, thinning onto southern Ireland (>0.5 km); (vii) Early Paleogene inversion - inversion of the North Celtic Sea Basin; onshore extent as far as the South Ireland Lineament, with > 1 km of uplift; erosion of most Mesozoic sediments; (viii) Tertiary quiescence- continuing uplift onshore (>0.5 km) outweighing minor localized deposition.
A model for the behaviour of the crust through this sequence of events is illustrated in Fig. 6d. Strain in the model relies upon the inheritance of lines of weakness from the Variscan basement. The South Ireland Lineament represents the outcrop of important Variscan thrusts that are thought to rise steeply from a decollement at depths of about 7 km (Ford et al. 1991), making the epithet 'thinskinned' inapplicable. These thrusts were reactivated first under Permo-Triassic
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Fig. 6. Structural sketch sections across southeast Ireland, adapted from various sources quoted in the text, orientated N N W- S S E . For locations of sections see Fig. 1. Note that Variscan thrusts X and Y, which follow the South Ireland Lineament and Wexford Boundary Lineament of Gardiner & Sheridan (1981) respectively, were active under extension during the Permo-Triassic, and again under compression in the Early Tertiary. Thrusts merge at a decollement surface at a depth of about 7 km (Ford et al. 1991). (a) From Slieve na Muck to Power Head, County Cork. (b) From Slievenamon to Bunmahon, County Waterford. (e) Drawn from the northern part of the SWAT-4 seismic reflection profile. Note the unconformity at the top of the Permo-Triassic interval, and the inversion features at the margin of the North Celtic Sea Basin. (d) Sketch of the above sections, to illustrate the mechanics of Permo-Triassic subsidence and Early Tertiary inversion in which existing discrete Variscan thrust planes were reactivated.
PERMO-TRIASSIC RIFTING, ONSHORE SOUTHERN IRELAND
251
crustal extension and then Early Tertiary inversion. Other Variscan fabrics, such as folds and cleavage, have provided lines of weakness to secondary extension during the Permo-Triassic. This tectonic model permits the interpretation of the normal strike faults in the southern Irish Variscides as being Permo-Triassic in age (cf. Naylor et al. 1983; Keeley 1983). It also has important implications concerning the application of both brittle and ductile minor structures in the Irish Variscides to resolve Variscan strain. How can bona fide Variscan strain be differentiated from post-Variscan strain, now that both are known to be significant?
Conclusions From the results of reconnaissance apatite fission track studies in southern Ireland, presented in Keeley et al. (1993) and summarized here, the conclusion that southeast Ireland experienced significant burial during the Permo-Triassic is unavoidable. Even assuming a high heat f l u x - the result of regional crustal stretching and rifting-the Variscan basement must have been buried to depths of at least 1 km by contemporaneous deposition. This Permo-Triassic heating event was followed by other thermal events in southeast Ireland. A cooling episode occurred in the Early Jurassic and is interpreted in terms of regional uplift and erosion of the Permo-Triassic deposits. This is probably linked to an influx of sands and silts recorded in the Fastnet and North Celtic Sea Basins, interrupting Liassic sedimentation. Thus, early in its formation, the North Celtic Sea Basin was about 70km wider on its northern margin. As it matured, early in the Jurassic, it became a narrower and more vigorous trough. Further heating and cooling events are indicated in the Late Cretaceous and Early Tertiary respectively. These events are interpreted first as resulting from the accumulation of a regional blanket of chalk, and then by the extension into southeast Ireland of the Paleogene North Celtic Sea Basin inversion event, as originally suggested by Roberts (1989). Post-Variscan extensional and compressional movements must have involved the Variscan basement and the existing lines of weakness within it. Given the known distribution of Mesozoic and Tertiary deposits in southern Ireland, the South Ireland Lineament of Gardiner & Sheridan (1981) is shown to represent an important boundary to post-Variscan sedimentation. Discrete Variscan thrusts are recognized along much of its length, rising steeply from depths of about 7 km. These are the prime candidates for the primary accommodation structures during both post-Variscan extension and compression. A case is presented for interpreting other fractures within the Variscan Orogen as secondary syndepositional Permo-Triassic normal faults. From this study, it must be appreciated that contemporaneous depositional boundaries do not necessarily coincide with present-day erosional remnant boundaries. The corollary is also true, that all strain observed within an exposed orogen is not necessarily attributable to the respective orogeny. Any post-orogenic movements associated with nearby basins must have involved some reactivation of existing lines of weakness within the orogen.
252
M. L. KEELEY
I am indebted to Dr Cherry Lewis, Dr Tony Hurford of the UCL Fission Track Laboratory, and Dr Paul Green of Geotrack International, Melbourne, for their efforts in processing the AFTA samples and helping to interpret the results. Dr David Naylor (ERA, Dublin) and Professor George Sevastopulo (TCD) are to be thanked for encouraging this investigation from an early stage. Other former colleagues from TCD, particularly Dr Stephen Boldy and Gareth L1.Jones, are thanked for providing valuable information arising from their research in southern Ireland. The figures were drafted by Bob Needham of Graffixx Consultancy, Henley-on-Thames..
References BROUGHAN, F. M., NAYLOR, O. & ANSTEY, N. A. 1989. Jurassic rocks in the Kish Bank Basin. Irish Journal of Earth Sciences, 10, 99-106. CLAYTON, G. 1989. Vitrinite reflectance data from the Kinsale Harbour-Old Head of Kinsale area, and its bearing on the interpretation of the Munster Basin. Journal of the Geological Society, London, 146, 611-616. - - , HAUGHEY, N., SEVASTOPULO, G. O. & BURNETT, R. 1989. Thermal Maturation Levels in the Devonian and Carboniferous Rocks in Ireland. Geological Survey of Ireland. - - - , SEVASTOPULO, G. D. & SLEEMAN, A. G. 1986. Carboniferous (Dinantian and Silesian) and Permo-Triassic rocks in south County Wexford, Ireland. Geological Magazine, 21, 355-374. COLLER, D. W., 1984. Variscan structures in the Upper Palaeozoic rocks of west central Ireland. In: HUTTON, D. H. W. & SANDERSON, D. J. (eds) Variscan Tectonics of the North Atlantic Region. Geological Society, London, Special Publication, 14, 185-196. COOPER, M. A., COLLINS, D., FORD, M., MURPHY, F. X. & TRAYNER, P. M. 1984. Structural style, shortening estimates and the thrust front of the Irish Variscides. In: HUTTON, D. H. W. & SANDERSON, D. J. (eds) Variscan Tectonics of the North Atlantic Region. Geological Society, London, Special Publication, 14, 167-176. & O'SULLIVAN, M. 1986. Structural evolution of the Irish Variscides. Journal of the Geological Society, 143, 53-61. FORD, M., BROWN, C. & READMAN, P. 1991. Analysis and tectonic interpretation of gravity data over the Variscides of southwest Ireland. Journal of the Geological Society, 148, 137-148. --, KLEMPERER, S. L. & RYAN, P. D. 1992. Deep structure of southern Ireland: a new geological synthesis using BIRPS deep reflection profiling. Journal of the Geological Society, 149, 915-922. GARDINER, P. R. R. & SHERIDAN, D. J. R. 1981. Tectonic framework of the Celtic Sea and adjacent areas with special reference to the location of the Variscan Front. Journal of Structural Geology, 3, 317-331. GREEN, P. F. 1989. Thermal and tectonic history of the East Midlands shelf (onshore UK) and surrounding regions assessed by apatite fission track analysis. Journal of the Geological Society, 146, 755-773. --, DUDDY, I. R., GLEADOW, m. J. W. & LOVERING, J. F. 1989. Apatite fission-track analysis as a paleotemperature indicator for hydrocarbon exploration. In: NAESER, N. D. & MCCULLOH, T. H. (eds) Thermal History of Sedimentary Basins-Methods and Case Histories. Springer, 181-195. HASZELDINE, R. S. & RUSSELL, M. J. 1987. The Late Carboniferous northern Atlantic Ocean: implications for hydrocarbon exploration from Britain to the Arctic. In: BROOKS, J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe, Vol. 2. Graham and Trotman, 1163-1176. HIGGS, K. & BEESE, A. P. 1986. A Jurassic microflora from the Colbond Clay of Cloyne, County Cork. Irish Journal of Earth Sciences, 7, 99-110. JONES, G. L1. 1992. Irish Carboniferous condonts record maturation levels and the influence of tectonism, igneous activity and mineralisation. Terra Nova, 4, 238-244. KEELEY, M. L. 1983. The Carboniferous stratigraphy of the Carrick-on-Suir Syncline, southern Ireland. Journal of Earth Sciences, Royal Dublin Society, 5, 107-120.
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, LEWIS, C. L. E., SEVASTOPULO, G. D, CLAYTON, G. & BLACKMORE, R. 1993. Apatite fission track data from southeast Ireland; implications for post-Variscan burial history. Geological Magazine, 130, 171-176. LASLETT, G. M., GREEN, P. F., DUDDY, I. R. & GLEADOW, A. J. W. 1987. Thermal annealing of fission tracks in apatite- 2. A quantitative analysis. Chemical Geology (Isotope Geoscience Section), 65, 1-13. LEWIS, C. L. E., GREEN, P. F., CARTER, A. C. & HURFORD, A. J. 1992. Elevated K/T palaeotemperatures throughout northwest England: three kilometres of Tertiary erosion? Earth and Planetary Science Letters, 112, 131-145. MATTHEWS, S. C., NAYLOR, D. & SEVASTOPULO, G. D. 1983. Palaeozoic sedimentary sequence as a reflection of deep structure in southwest Ireland. Sedimentary Geology, 34, 83-95. MITCHELL, G. F. 1980. The search for the Tertiary in Ireland. Journal of Earth Sciences, Royal Dublin Society, 3, 13-33. MURPHY, F. X. 1988. Facies variations within the Waulsortian Limestone Formation of the Dungarvan Syncline, southern Ireland. Geological Magazine, 99, 205-219. NAYLOR, D. 1992. The post-Variscan history of Ireland. In: PARNELL, J. (ed.) Basins on the Atlantic Seaboard." Petroleum Geology, Sedimentology and Basin Evolution. Geological Society, London, Special Publication, 62, 255-275. -& SHANNON, P. M. 1982. The Geology of Offshore Irelandand West Britain. Graham and Trotman, London. , REILLY, T. m., SEVASTOPULO, G. D. & SLEEMAN, A. G. 1983. Stratigraphy and structure in the Irish Variscides. In: HANCOCK, P. L. (ed.) The Variscan Fold Belt in the British Isles. Adam Hilger, Bristol, 20-46. PETRIE, S. H., BROWN, J. R., GRANGER, P. J. & LOVELL, J. P. B. 1989. Mesozoic History of the Celtic Sea Basins. ln: TANKARD, A. J. & BALKWILL, H. R. (eds) Extensional Tectonics and Stratigraphy of the North Atlantic Margins. AAPG Memoir, 46, 433-444. PHILCOX, M. E. 1964. Compartment deformation near Buttevant, County Cork, Ireland, and its relation to the Variscan thrust front. Scientific Proceedings of the Royal Dublin Society, 2A, 1-11. ROBERTS, D. G. 1989. Basin inversion in and around the British Isles. In: COOPER, M. m. & WILLIAMS, G. D. (eds) Inversion Tectonics. Geological Society, London, Special Publication, 44, 131-150. RUFFELL, A., COWARD, M. P. & HARVEY, M. 1995. Geometry and tectonic evolution of megasequences in the Plymouth Bay Basin, English Channel. In: BOLDY, S. A. R. (ed.) Permian and Triassic Rifting in Northwest Europe. Geological Society, London, Special Publication, 91, 193-214. SAVAGE, R. J. G. 1977. The Mesozoic strata of the Mendip Hills. In: SAVAGE, R. J. G. (ed.) Geological Excursions in the Bristol District, University of Bristol, 85-100. SEVASTOPULO, G. D. 1981. Lower Carboniferous. In: HOLLAND, C. H. (ed.) A Geology of Ireland. Scottish Academic Press, Edinburgh, 147-172. SHANNON, P. M. 1995. Perrno-Triassic development of the Celtic Sea region, offshore Ireland. In: BOLDY, S. A. R. (ed.) Permian and Triassic Rifting in Northwest Europe. Geological Society, London, Special Publication, 91, 215-238. STEIN, A. M. & BLUNDELL, D. J. 1990. Geological inheritance and crustal dynamics of the northwest Scottish continental shelf. Tectonophysics, 173, 455-467. STILLMAN, C. J. 1981. Caledonian igneous activity. In: HOLLAND, C. H. (ed.) A Geology of Ireland. Scottish Academic Press, Edinburgh, 83-106. WILLIAMS, E. A., BAMFORD, M. L. F., COOPER, M. A. et al. 1989. Tectonic controls and sedimentary response in the Devonian-Carboniferous Munster and South Munster Basins, south-west Ireland. In: ARTHURTON, R. S., GUTTERIDGE, P. & NOLAN, S. C. (eds.) The Role of Tectonics in the Devonian and Carboniferous Sedimentation in the British Isles. Yorkshire Geological Society, Occasional Publication, 6, 123-142. WILSON, H. E. 1981. Permian and Mesozoic. In: HOLLAND, C. H. (ed.) A Geology of Ireland. Scottish Academic Press, Edinburgh, 201-213. ZWART, H. J. & DORNSIEPEN, U. F. 1978. The tectonic framework of Central and Western Europe. Geologie en Mijnbouw, 57, 627-654.
Index
Page numbers in italics refer to Figures and Tables aeolian systems (Papa Group) 75-7 dune deposits 75 sandsheet deposits 77 AFTA see apatite fission track analysis Anchignathodus typicalis 67 Angustisulcites sp. 67 Antrim Lava Group 107, 108 apatite fission track analysis (AFTA) 41-2 annealing temperatures 43 azimuth diagrams 175-6, 175, 178 in Chestiire and Irish Sea Basins 52 data from British samples 46 effect of Skye Tertiary igneous complex 53, 54 forward modelling 45, 47-8, 49, 52 Ireland geothermal gradients 244 post-Variscan heating episodes 242, 244-5, 251 sample localities 241 thermal history models 242, 243, 244-5 track distribution histograms 243 Variscan annealment 242 methodology 43-5, 241-2 Midlands post depositional annealing 52 sample localities 51, 52 Scotland 53 single grain age distributions 44-5, 44 southwest England constraints on forward modelling 47-8 depth of burial 51 post depositional annealing 45-7, 48-51 Appalachian/Variscan rift system 14, 16 Aratrisporites granulatus 136 Arctic rift system 14, 37 Arden Sandstone 186 Argyll Group (Dalradian) 109, 110 Armorican Massif 195 Atlantic rift system 16, 37, 59 Auk High 124 Aylesbeare Mudstone Group 210 azimuth diagrams 175-6, 175, 178 Bacton Group 27 Bakevellia Sea 2 Baltica (Precambrian basement) 9 Bamble Trough 22, 30 Barra Trough 91 Barren Measures 172 Barren Red Beds 23, 24 basement Cardigan Bay Basin 20 Dalradian (Rathlin Basin) 109, 110, 111 Fastnet Basin 222 North Celtic Sea Basin 17, 227 Plymouth Bay Basin 19
Precambrian 9 South Celtic Sea Basin 17-9 Southern Uplands terrane 113 structural provinces 167 Variscan (in Ireland) 246-9 depth of burial 244-5, 251 reactivated structural weaknesses 249-51, 250 Western Approaches Trough 120, 195-6, 199 basin orientations influenced by basement trends 43, 90, 103, 167-72, 245-6 Cheshire Basin 166 Laurentian and Grampian terranes 104-6, 108-13, 119-20 Midland Valley terrane 106-7 Plymouth Bay Basin 19-20 Southern Uplands terrane 107-8, 113-19, 119-20 Western Approaches Trough 200 Worcester Basin 165 basin subsidence analysis (Cheshire and Worcestshire Basins) 183-7, 186, 189 Beryl Embayment 2, 26 biostratigraphic dating Celtic Sea basins 219-21 South Central Graben 128-32, 134, 136-9 well data 129, 130, 131, 132 Bohemian Massif 12 Boreholes 12/13-1A 100 19/5-1 100 26/22-1A 101 72/32 (BGS) 92 73/12-1 195 78/5 (BGS) 92 78/7 (BGS) 96 88/4 (BGS) 91 88/8 (BGS) 91 90/1 (BGS) 92 90/16 (BGS) 91 90/17 (BGS) 92 156/17-1 (BP) 92, 93 202/19-1 (Shell) 94, 95 205/23-1 97 205/26a-2 97 205/27-1 58 205/27a-1 97, 98, 99 205/29-1 97 205/30-1 97 210/4-1 99 210/13-1 99 220/26-2 99 Bowland Basin 34 Brabant Massif 9-11, 23, 25, 162 braided fluvial channel systems (Papa Group) channel abandonment deposits 74, 75 channel-fill deposits 68-72, 71 ephemeral channel deposits 72-5, 74 sandy channel-fill deposits 72, 73 sheetflood deposits 72-5
256
INDEX
Brandon Hill Granite 241 Bray Fault 12 Bridgemere Fault 180 syndepositional movement 183-4 Bridgnorth Sandstone Formation 3, 166 syndepositional faulting 182, 183 Brioverian Series (Armorican Massif) 195 Bristol Channel Basin 17, 215, 218, 225 Brittany Basin 195 Broad Fourteens Basin 27, 28, 36 Bromsgrove Sandstone Formation 188 Brooks Mill Mudstone Formation 183 Budleigh Salterton Pebble Beds 20, 211 Bunter Sandstone Formation 27, 133 Bunter Shale Formation 26, 27, 152 time thickness map 151 burial depths Britain 51, 53, 54-5 southeast Ireland 244-5, 251 Cadomian Orogeny 195 Caledonian domains in Britain 10 Caledonian Foldbelt 167 Caledonian orogenic belt 9-11 Caledonian terranes 104 Camerosporites pseudoverracatus 68 Camerosporites secatus 67-8 Cantabrian Basin 37 Cardigan Bay Basin 16, 17, 20 seismic profile interpretations 21, 22 Carrick Thrust 19 Celtic Sea basins 2, 16, 162 Permian sedimentation 232 Permo-Triassic statigraphy 218-21 post-Triassic history 217 Triassic sedimentation 232-4 Triassic stratigraphy 219, 220, 221 Variscan basement 217 see also Cardigan Bay Basin; North Celtic Sea Basin; Pembrokeshire Ridge; South Celtic Sea Basin Central Graben 1, 2, 24, 27, 30, 36, 37 influence of basement structures 103 reactive diapirs 28 section 27, 28 stratigraphy 124-41,128 Triassic well database 127 Central Irish Sea Basin 215, 240 channel-fill deposits (Papa Group) 68-72 Cheshire Basin 2, 33, 161,163, 164, 166-7, 168 basin subsidence analysis 183-5, 188 controlled by Caledonian/Longmyndian structures 43 exhumation 54 fault geometries 178-80, 179, 180, 181, 182 maximum palaeotemperature 52, 53-4 reactivation of basement faults 35, 172 regional tectonic framework 162 seismic line across southeast margin 173 stratigraphy 166 structural cross-section 165 subsidence analysis 183-6, 186 syndepositional faulting 183-4
Chester Pebble Beds Formation 188 Chordecysta chalasta 64 chronostratigraphy of Triassic sediments (South Central North Sea) 133 Church Stretton Fault 167, 168 Cimmeria 14, 35 Classopllis spp. 138 Clopton fault system 164, 165, 170, 180 Fw reverse fault 170, 171 syndeposition 182-3 Cockburn Basin 215, 216, 225, 232 Cretaceous sediments 217 seismic line 226 strike-slip faults 17 Collyhurst Sandstone 32, 183, 183, 186 Colonsay Basin 106 compaction calculations of throw 182 in fault reactivation analysis 177-8 core photographs Foula Sandstone Formation 71 Otter Bank Sandstone Formation 71, 73, 74, 76,78 Cornubian Batholith 195 Cornubian Massif 195, 201 Cornubian Terrane 195 County Cavan Basin 107 County Down Basin 107 Crosh Vusta Fault 34 Culm Basin 12 Cyclotriletes spp. 67, I36 Dawlish Sandstone Formation 45, 48 Delamere Member (Helsby Sandstone Formation) 188 Densoisporites nejburgii 67, 137 Dent Fault 33 depositional environments 90 Papa Group 68-80, 81 South Central Graben 125-6, 139 Dorset Basin 163, 164, 167 Dowsing Dolomite Formation 27 Dowsing Fault Zone 145, 158 geoseismic section 155, 157 Dumfries Basin 43, 107, 115 East Irish Sea Basin 32, 33, 35, 36, 164, 183, 188 faults 34 regional tectonic framework 162 East Malvern Fault 164, 165, 168, 169, 170, 180 syndepositional movement 183 East Shetland Basin 3, 30 East Shetland Platform 12 East Solan Basin 58 geoseismic interpretation 61 seismic profile 60 tectonic elements 59 Echinitosporites iliacoides 68 Eden-Pennine Fault 108 Egersund Basin 26, 29, 36, 126
INDEX Eldersfield Mudstone Formation 186 ephemeral channel/sheetflood deposits 72-5 evaporite deposits 1, 3, 16, 27, 218 Mercia Mudstone Formation 20, 32, 166, 167 Saliferous Member 218, 225, 230, 235 North Celtic Sea Basin 230, 232, 234, 235 Rrt Halite (Dowsing Dolomite Formation) 27, 28 South Celtic Sea Basin 225, 227, 234, 235 Triton Anhydritic Formation 153 West Orkney Basin 94 Zechstein 25-6, 145-6, 147, 149, 152, 153, 157-9 Exe Group 211 Exeter Volcanics (Traps) Plymouth Bay Basin 19, 201,210, 211 Wessex Basin 43 extension (crustal) controlled by tear faults 20 timing 182-7 of Variscan Orogen 20, 245-6, 251 vector derived from fault reactivation analysis 172-82, 189 facies associations (Papa Group) 68 Faeroe-Shetland Basin 30, 36, 90, 97, 100 Faeroes Ridge 30, 162 Fastnet Basin 215, 216, 222-5, 249 basement 222 boundary faults 223-5 igneous rocks 217 lithofacies 222-3 sedimentation 232-4, 249, 251 seismic character 223 seismic profile 224 well correlations (Triassic) 220 fault geometry analysis 172-82 fault reactivation 30, 32, 34, 35 basement faults 106, 117, 168-72 Caledonian fabrics 20, 87 Caledonian strike-slip faults 43, 105, 116, 117 Caledonian thrusts 105 Carboniferous faults 33, 106 Cheshire and Worcester Basins 35 Clopton Fault system 170 Iapetus Suture 108 Outer Isles Thrust 92, 103 Pontesford-Linley Lineament 172 Variscan structures 20 Variscan thrusts 17, 168, 200 Worcester Basin 170 fault reactivation analysis 174-82 allowances for compaction 177-8, 182 azimuth diagrams 175-6, 175 block model 174-5, 174 heave/throw ratio 176 oblique-normal slip (transtension) 174, 175 stereographic projections 178-82, 179, 180 Fc reverse fault (Wem-Red Rock fault system) 172, 173 Fladen Ground Spur 124 Flannan Trough 91 Flett Ridge 100
257
Foula Sandstone Formation (Papa Group) 64, 67-8 depositional environments 79-80, 81 palaeocurrent analysis 82 palaeomagnetic analysis 82-4 stratigraphy, facies and log profile 66 Foyle Fault 105, 108, 109, 110 Foyle Syncline 110 Fw reverse fault (Clopton fault system) 170, 171 geothermal gradients 51, 54, 244 geothermal history models 242-5 tested against measurements 243 glacio-eustatic sea level rise (Late Permian) 25 Glas Eilean 97, 100 Goban Spur 195, 217 Gondwana 1, 2, 9, 14, 25, 103 Gordonispora fossulata 138 Gordonispora rota 136 Grampian terrane 104, 105, 108-13 granites 45, 47, 195, 241 linked to extension 19 Great Glen Fault 9-10 Greenland Basin 37 Griesbachian event 67 Haisborough Group 27, 28 Haisborough reflectors 149, 151 halokinesis 26, 28, 32, 147, 152, 159 North Dogger Fault Zone (NDFZ) 145-7, 149, 151-3, 157-8 passive folding in overlying strata 145-6, 153, 157-8, 159 South Celtic Sea Basin 227 South Central Graben 126 Southern Dowsing Fault Zone 154, 157-8 triggered by Hardegsen event 146, 159 Hardegsen tectonic event 159 Hardegsen unconformity 146, 157 heave/throw ratio (fault reactivation analysis) 176-81, 189 Hebrides Basin 30 Hebrides Shelf 91, 100 Helsby Sandstone Formation 188 Heron Group 133 Hidra High 124 Highland Border Ridge 108 Highland Boundary Fault 106 Horda Graben and Fault Zone 26 Horda Platform 26 Horn Graben 2, 22-3, 27, 30, 36 hot spots 23, 43 Fastnet Basin area 217 Iapetus Ocean closure 9 Iapetus Suture 108, 113 igneous rocks Fastnet Basin 217 Goban Spur 217 North Sea 23 north and west of Scotland (Early Permian) 97-9 Plymouth Bay Basin 201,207-8, 210
258
INDEX
Illinites chitnoides 136 lllinites kosankei 136, 137, 138 Inkberrow Fault 164, 165, 170, 176, 178, 180 syndeposition 182, 183 Irish Sea Basins maximum palaeotemperatures 52, 54 see also East Irish Sea Basin; Central Irish Sea Basin Islay anticline 110 Isle of Lewis 53 isochore maps Plymouth Bay Basin 206, 207, 208, 209 Western Approaches Trough 199 Joanne Sandstone Member (Skagerrak Fm.) 136, 137-8, 139 Jonathen Mudstone Member (Skagerrak Fm.) 136, 138 Josephine Sandstone Member (Skagerrak Fm.) 136, 138, 139 Joshua Mudstone Member (Skagerrak Fm.) 136, 138Judd Fault 30, 32 Judy Horst 140 Judy Sandstone Member (Skagerrak Fm.) 133, 135, 136-7, 139 microfloral assemblage 136 Julius Mudstone Member (Skagerrak Fm.) 136, 137, 139 Keele Formation 172 Kempsey borehole 166, 177, 186 Keuper Halite Member 153 Keys Basin 33-4 Kidderminster Formation 188 Kingscourt Basin 107, 115, 117 Kingscourt Fault 115, 117 Kishorn Thrust 105 Knowle Basin 164, 164, 165, 167 Kraeuselisporites spp. 67 Kupferschiefer 25 Labadie Bank 217, 230, 234 lacustrine/floodplain systems (Papa Group) 77-9 playa lake deposits 79 playa margin deposits 77-8 vegetated floodplain deposits 79 Lake District Boundary Fault 33 Lambert Equal Area Plots 178-82, 179, 180 Laurussia 1, 2, 103 lava flows at B-C megasequence boundary 201, 207-8, 210 Leschikisporis aduncus 138 Lias Group 183 lithofacies East Irish Sea 32 Fastnet Basin 222-3, 235 North Celtic Sea Basin 235 Northern Permian Basin 28 Permo-Triassic red-beds in southwest England 45 South Celtic Sea Basin 225 Southern Permian Basin 24-5, 27
lithofacies and stratlgraphic correlations (North Sea) 135, 139-40 Lizard Thrust 19 Lizard-Dodman Thrusts 199, 201 Loch Awe syncline 110 Loch Indaal Basin 104, 105, 106 seismic profile interpretation 106 Loch Ryan Fault 108 Lochmaben Basin 107, 115, 117, 118 stereographic reconstruction of basement orientation 118 London Platform 165, 170 London-Brabant Massif 162 Lough Neagh Basin 104 Lough Neagh-Larne Basin 107, 108 Lough Ryan Fault 116 Lower Marl Member (Penarth Group) 218, 219, 223 Lower Sandstone Member (Sherwood Sandstone) 218, 222, 230, 235 Lueckisporites virkkiae 67 Lunatisporites spp. 67 Lundbladispora spp. 67 Lyme Bay Basin 163, 164, 167 Malin Basin 104 Malvern Axis 165, 168, 169, 170 Manchester Marl Formation 183, 186 Marldon Group 210 megasequences bounded by strong reflectors 199 orientation changes 209-10 Plymouth Bay Basin 201-12 Sula Sgeir and Stack Skerry Basins 30-2, 31 tectonic controls 211 Mercia Mudstone Group 32, 166, 167, 182, 186, 188, 223 apatite fission track analysis (AFTA) 45, 47, 48, 52 dated by palynomorphs 219 Keuper Marl Member 218, 223, 235 lithofacies 235 North Celtic Sea Basin 230 Plymouth Bay Basin 210 Saliferous Member 218, 225, 230, 235 seismic character 232 South Celtic Sea Basin 218 syndepositional faulting 183 microfloral range (South Central Graben) 134 Mid-Faroe Ridge 100 Mid-North Sea High 22, 23, 24, 25, 27, 124, 152 cut by Carboniferous-Permian basins 107 cut by Central Graben 27 regional tectonic framework 162 underlying hot spot 43 Middle Mudstone Member (Sherwood Safidstone) 218, 222, 230, 235 Midlands Microcraton 167, 168 Minch Basin 104 Minch Fault 91, 92, 94 Moffat Basin 107, 115, 117, 119 Moho 17, 19, 20, 196
INDEX Moine Thrust 94, 105 Moines 9-10 Moray Firth Basin 162 More Basin 100 Morecambe Bay Basin 35, 36 Munster Basin 246 Muschelkalk Halite Member 155 NDFZ see North Dogger Fault Zone Needwood Basin 164 Neo-Iapetus Ocean 9, I0, 11, 12 Neo-Tethys 14, 36 Ness Fault Zone 92 New Red Sandstone 3, 59 New Red Sandstone Basin 58 New Red Sandstone Supergroup 62 Newark Supergroup 15 normal fault geometry differential compaction 177-8, 182 extension direction 174-6 hanging-wall block extension 176 heave function of fault segment obliquity 175 measure of total extension 175, 176 reduced by hanging-wall deformation 176 heave/throw ratios 176, 178, 179, 180 stereographic projections 178-82, 179, 180 North Atlantic Craton 10 North Celtic Sea Basin 215, 216, 217, 227-32, 239, 240 basement 17, 227 Cretaceous sediments 217 crustal evolution of southeast Ireland 249 depth of burial 244-55, 251 evaporite deposits 225 northern boundary 246 Paleogene inversion 245, 249, 251 sedimentary succession 17 sedimentation 232-4, 245, 251 seismic character 17, 230-2 seismic profiles 231, 233 interpretation 18 structural sections 250 well correlations (Triassic) 221 North Celtic Sea Graben see North Celtic Sea Basin North Central Graben 126 stratigraphic relations to South Central Graben 128 North Channel Basin see Portpatrick Basin North Danish Basin 29, 126 North Dogger Fault Zone (NDFZ) 145, 146-53, 157, 159 Bunter Shale time thickness 151, 152 halokinesis 145-6, 147, 149-59 isopach maps 153, 154, 155, 155, 156 linked to passive folding of Triassic strata 146 seismic line interpretations 146-5 l, 148 syndepositional movement 153 Triassic/Cretaceous unconformity 150, 152 Zechstein Supergroup time thickness 150, 152 North Dogger Shelf 145
259
North Lewis Basin 90, 91-2 North Minch Basin 32, 90, 92, 93, 94 exhumation 54 North Orkney/Wyville Thomson (NOWT) transfer zone 90 North Porcupine Basin 101 North Rona Basin 90, 96 North Sea carbonate-evaporite cycles (Zechstein) 25-6 Permian sedimentation 22-6 Triassic sedimentation 26-30 North Sea Basins 124 North Sea graben system 2 North Solway Fault 108 North Viking Graben 3 Northeast Arran Trough 107 Northern Permian Basin 13, 24, 30, 162 Northumberland Basin 108 NOWT transfer zone 90 Oddicombe Breccia 211 Oldtown Bay Formation 243 Orkney-Faroe Alignment 32, 90 Oslo Graben 2, 22-3, 30, 36, 162 Otter Bank Fault 58 Otter Bank Sandstone Formation (Papa Group) 64, 67, 68 depositional environments 79-80, 81 palaeomagnetic analysis 82-4 stratigraphy, facies and palaeomagnetic stratigraphy 83 Otter Bank Shale Formation (Papa Group) 62, 64-7, 79, 84 stratigraphy, facies and palaeomagnetic stratigraphy 83 Otter Sandstone 211 Outer Isles fault zone 30 Outer Isles Thrust 92, 103, 104, 105 Ovalipollis pseudoalatus 138 Palaeo-Tethys 14, 35-6 palaeoclimates 1, 59, 125 palaeocurrent analysis (Papa Group) 80-82, 82 palaeoenvironments (Skagerrak Fm.) 139 palaeogeographical reconstructions of Ireland 248 palaeogeography Late Triassic 62 Triassic 125 palaeomagnetic analysis (Papa Group) 82-4, 83 palaeotemperatures (AFTA) 41, 47-8, 51-5, 245 palaeowinds (Rotliegend) 24-5, 25 palynology Papa Group 64-8 South Central Graben 128, 129-32, 134, 136-9, 140 Pangaea 1, 59, 87, 161, 181, 197, 217 Papa Basin 58, 59 geoseismic interpretation 61 seismic profile 60 tectonic elements 59
260 Papa Group chrono- and biostratigraphy 64-8, 65 depositional environments and facies classification 68-79, 70 evolution 79-80, 81 lithostratigraphy 62-4, 63 sedimentology 68-79 stratigraphy, facies and log profile 69 see also Foula Sandstone Fm.; Otter Bank Sandstone Fm.; Otter Bank Shale Fm. Paracirulina sp. 68 Paris Basin 35, 162 Pays de Bray Fault 35 Pembroke Ridge see Pembrokeshire Ridge Pembrokeshire Ridge 17, 217, 227, 230, 234 Penarth Group 167, 218, 219, 223, 225, 230, 235 Penterivian gneisses (Armorican Massif) 195 Peri-Caspian Basin 24 Permian sedimentation Celtic Sea area 232, 234 North Sea area 22-6 Permo-Triassic distribution offshore (Scotland) 88-9 onshore (British Isles) 42, 164 red-beds onshore in southern Ireland 218 red-beds in seabed outcrops (Celtic Sea) 218 in Skye 105 Permo-Triassic extension 164 direction 181 influenced by Variscan trends 197 Pewsey Basin 163, 164, 167 Plaesiodictyon mosellanum 136, 138, 139 plate collisions Caledonian 9-11, 37 Variscan 12, 37 plate tectonic reconstructions Dinantian 11 Early Permian 13 Triassic 36 Plattendolomit 149 Plymouth Bay Basin 16, 19-20, 193, 197-2i2 basement 19, 120, 195-6, 199 borehole successions 198 lithostratigraphic correlations 210-11 megasequence A 201-7, 206, 207, 209, 211-12 megasequence B 206, 207-8, 207, 211-12 megasequence C 208, 209-10, 21 l, 212 megasequence D 209, 210-11,212 seismic profiles 202-3, 204-5 interpretation 19 seismic stratigraphy 201 two phases of basin development 211,212 volcanics 19, 201,210, 211 Polish Trough 24, 36, 37 Polish-Dobrodgea Rift 24 Pontesford-Linley Lineament 167, 168, 172 Porcellispora longdonensis 136 porosity North Lewis Basin 92 Skagerrak Formation 140 Portpatrick Basin 104, 107-8, 115, 116
INDEX post-Variscan deposition (south/central Ireland) 240, 242, 244-5 pre-Permian tectonics 7-12 Proto-Iapetus Ocean 9, 10 Proto-North Celtic Sea Basin (Permo-Triassic) 246, 247, 249 Proto-Tethys 1 Protodiploxypinus doubingeri 136 Protodiploxypinus ornatus 138 Protodiploxypinus sittleri 136, 137 Protohaploxypinus assemblage 67 Protohaploxypinus spp. 67 Raasay 53 radial plots (AFTA) 44, 45, 48, 50, 52 Raistrickia "nordica" 136 Rathlin Basin 104, 105, 108-13 basin margins parallel to basement strike 109, 110-11, 120 Carboniferous deposition on Dalradian 112, 113 Dalradian basement 109, 110, 111 Reykjanes Ridge 1 Rhaetian Limestone 219 Rhaetian Marl Unit 219 Rheic Ocean 9, 12 Ricciisporites tuberculatus 138 rift basins, Newark Supergroup 15 Ringkobing-Fyn High 22, 27, 29, 124, 162 Rockall High 162 Rockall Trough 1, 32, 36, 90, 100, 162 Rona Ridge 30, 57-8, 59 Rrt Halite Member (Dowsing Dolomite Formation) 27, 28, 155, 158 isopach map 155, 156 thickness map 156 Rotliegend palaeowind directions 24-5, 25 sedimentation 22 volcanism 22-3 Rotliegendes Group 146, 147, 149 Saalian Unconformity 3 St Bees Shale-St Bees Sandstone transition 79, 80 St George's Channel Basin 17, 215, 227, 230 Saliferous Member (Mercia Mudstone Group) 218, 225, 230, 235 salt see evaporite deposits salt diapirism 26, 28, 152 see also halokinesis Scandinavia Craton 10, 29 Scandinavian Block (Precambrian basement) 9, 36 Scillies Basin 206 Sea of the Hebrides Basin 54, 104 Sea of the Hebrides-Little Minch Trough 92, 94, 104 sedimentology (Papa Group) 68-79 seismic stratigraphy (Plymouth Bay Basin) 201 Sele High 26
INDEX sheetflood deposits 72-5 Sherwood Sandstone Group 32, 166, 167, 218, 219, 232 apatite fission track analysis 48-51, 52 Fastnet Basin 222-3 lithofacies 222-3, 235 North Celtic Sea Basin 230 South Celtic Sea Basin 225 syndepositional faulting 182, 183, 188 Shetland Platform 30, 90 Shetland Spine Fault 58, 97 Silver Pit Basin 153, 157 halokinesis 153 Silvermines Fault 113 Skagerrak Formation 126-8, 136-41 biostratigraphy 128-32, 134, 136-9 well data 129-32 derived from Scandinavian Craton 29 Joanne Sandstone Member 136, 137-8, 141 Jonathen Mudstone Member 136, 138, 141 Josephine Sandstone Member 136, 138, 141 Joshua Mudstone Member 136, 138, 141 Judy Sandstone Member 133, 135, 136-7, 141 Julius Mudstone Member 136, 137, 141 limits in South Central Graben 128 palaeoenvironments 139 porosity and permeability 140-1,140 Skye 53 Smith Bank Formation 125, 126, 133-5, 141 Solan Bank High 94, 96 Solan Basin 58 tectonic elements 59 Sole Pit Basin 3, 27, 36, 155, 157 Solway Basin 104, 164, 168 South Celtic Sea Basin 12, 17, 215, 216, 225-7, 218 basement 17-19 boundary faults 227 Cretaceous sediments 217 halokinesis 227 lithofacies 225 sedimentation 232-4 seismic character 227 seismic profile 228, 229 well correlations (Triassic) 221 South Celtic Sea Graben see South Celtic Sea Basin South Central Graben 124-41 biostratigraphy 128-32, 134, 136-9 well data 129, 130, 131, 132 stratigraphic nomenclature 131, 141 stratigraphic relations to North Central Graben 128 see also Skagerrak Formation; Smith Bank Formation South Ireland Lineament 246, 247, 249, 250, 251 reactivated thrusts 249-51,250 South Viking Graben 2 see also Viking Graben South Western Approaches Trough (SWAT), seismic profiles 195-6, 196
261
Southern Dowsing Fault Zone (SDFZ) 154-9 linked to passive folding of Triassic strata 155-8 Southern Highland Group (Dalradian) 109, 110 Southern North Sea Basin 126, 145, 146, 154, 157 structural elements 146 Southern Permian Basin 13, 23-4, 162 development models 24 schematic section 24 Southern Uplands Fault 106, 113 Southern Uplands-Down-Longford terrane 113-9, 114, 120 basins normal to Caledonide basement trends 115-6 Caledonian strike-slip faults 113, 114 reactivated in normal mode 116, 117, 120 rotation of basement fabric 116-7, 118 structural fabric 113 sporomorphs (Western Platform) 135 Stack Skerry Basin 30, 94 seismic profile 31 Stafford Basin 164, 164, 167 Stanton Trough 105-6 Start Thrust 19 Start-Contentin Ridge 206 StassfiJrt Halite Formation 147, 153, 155 stereographic projections fault reactivation analysis 178-82, 179, 180 restoring pre-Permian orientation of basement strata 118 Stornoway Formation 64, 92, 94 Strangford Basin 107, 108, 115, 116 Stranraer Basin 43, 107, 108, 115, 116 stratigraphic and lithofacies correlations (North Sea) 135, 139-40 Striatoabiettes spp. 67 subsidence analysis 183-7, 186, 189 rates 3-4, 29, 186-7 Sula Sgeir Basin 30-2 Sula Sgeir High 92 syndepositional normal faults 26, 164, 167, 182-4, 189 Cheshire Basin 172, 186 influenced by underlying basement fractures 167-72, 189 North Dogger Fault Zone 153 West Orkney Basin 94 Worcester Basin 170, 186 see also normal fault geometry syntectonic deposits 87, 94, 116 Taeniasporites assemblage 67 tear faults (transfer zones; transfer faults) 16, 17, 22, 32, 34, 100, 103 in basement 9 Celtic Sea basins 16, 16 influence on rift system orientations 37 Judd Fault 30 reactivated Variscan shear zones 20 Variscan tectonics 12, 197 tectonic sequence (southeast Ireland) 249
262
INDEX
tectonic settings Caledonian 9-11 Celtic Sea basins 217 Northern Europe 8 Permo-Triassic 12-22, 42-3, 57-9, 124-6, 162 pre-Permian 7-12 Precambrian 9 Variscan 12, 196-7, 200 Teignhead Group 211 Tern Basin 29, 30 Tethyan-Dauphinois Basin 16 thermal maturity measurements 242-4 thermal subsidence 24, 32 Celtic Sea area 20, 217, 234 North Sea 24 Rotliegend 22, 23 Thornhill Basin 43, 107, 115 time thickness maps Bunter Shale 150, 152 Triton Anhydritic Formation 153, 154-5 Zechstein Supergroup 150-1, 152 Torbay-Chelston Breccias 210 Tornquist Line 24, 30 Tow Valley Fault 105, 109, 110, 111,112 Trans-European Fault Zone 12, 22, 30, 36 transfer zones (transfer faults) see tear faults transtention (oblique-normal fault slip) 174, 175 Triassic sedimentation (Celtic Sea area) 232-4 Triassic subcrop against base-Cretaceous unconformity 150-51, 152 Triton Anhydritic Formation 149, 153 isopach map 153, 154-5 syndepositional faulting 153, 159 time thickness map 153, 154-5 Tromso Embayment 26 Tympanicysta stoschiana Balme 64 assemblage 67 Unst Basin 26, 29, 30 Upper Limestone Member (Penarth Group) 219, 223 Upper Sandstone Member (Sherwood Sandstone) 218, 222, 223, 230, 235 Vale of Eden Basin 52 exhumation 54 Vale of Eden Fault 33 Variscan deformation in Ireland 239 reactivated thrusts 249-51,250 strike faults 246, 247 Variscan Front 167, 168 Variscan Orogen 2, 12, 167, 168 in southeast Ireland 239, 245-9 Variscan tectonics 12, 196-7, 200 Viking Graben 1, 12, 24, 26, 28, 29, 36 regional tectonic framework 162 section 28 Vittatina sp. 67
volcanism Goban Spur 217 Lower Permian 22, 97 Permo-Carboniferous in North Sea 14 Scotland 87 Plymouth Bay Basin 201,207-8, 210, 211 Rotliegend 2, 3, 22-3 Wardour Basin 163, 164, 167 Watcombe Beds 210 Weethley Fault 165, 170 syndeposition 182, 183 Wells 21/11-1 135 21/26-D 135 29/8a-3 137 29/14b-2 133, 137 29/19-1A 137 29/25-1 135 30/lc-3 138 30/lc-4 138 30/lc-5A 138 30/7a-2 138 30/7a-3 129 30/7a-4A 133 30/7a-6 129, 133, 141 30/7a-7, 129, 133, 137 30/7a-8 129, 141 30/7a-9 130, 135, 136, 137, 138, 141 30/12b-2 131, 133, 135, 136, 137, 138 30/12b-3 131, 133, 136, 137, 138, 141 30/13-1 132, 138 30/16-2 135 30/16132, 138 30/17a-4 132, 133, 137 44/2-1 149 49/29-1 225 55/30-1 222, 223 56/20-1 227 57/9-1 230 58/3-1 225 93/6-1 225 Welsh-Brabant Massif (Precambrian basement) 9,11 Wem Fault 180 syndeposition 183 Wem-Red Rock fault system 164, 166-7, 172 Fc reverse fault 172, 173 Wessex-Channel Basin 35, 161,162, 163, 164, 166 apatite fission track analysis 48-51 exhumation 54 fault geometries 179, 180, 180, 182 lithofacies 20 seismic mapping 168 West Flannan Basin 91 West Lewis Basin 91, 92 West Netherlands Basin 27 West Orkney Basin 12, 32, 90, 94, 95, 98 seismic profile 96
INDEX West Shetland Basin 30, 57-8, 90, 97, 98, 99, 100 offset by Judd Fault transfer system 30 seismic profile interpretation 30 West Shetland continental margin, tectonic elements 58 West Shetland Platform 57, 97 Western Approaches Basin 2, 16, 162, 193 Western Approaches Trough (WAT) 193, 194 basement 195-6, 199 Western Channel Basin 199 Westphalian Coal Measures 172 Westray Fault 30 Westray Ridge 97 Wexford Boundary Lineament 250 Wildmoor Sandstone Formation 188 Wilmslow Sandstone Formation 188 Worcester Basin 2, 36, 161, 163, 164, 165-6, 167 basin subsidence analysis 186-7 controlled by Malvernian trends 43 exhumation 54 fault geometries 179, 180, 180, 182 Inkberrow Fault cross-section 176
263
maximum palaeotemperature 53 reactivation of basement faults 35, 168-70, 171 regional tectonic framework 162 seismic line over east margin 171 seismic line over west margin 169 stratigraphy 166 structural cross-section 165 subsidence analysis 186, 186-7 syndepositional faulting 182-3 Worcester Graben see Worcester Basin Wyvitle-Thompson Ridge 32 Z2 Stassf'tirt Halite Formation 147, 153, 155 Zechstein carbonate-evaporite cycles 25-6 deposition 22 halokinesis 146 Zechstein Basin 153 Zechstein Sea 1, 25-6 Zechstein Supergroup 133, 147, 149, 153, 155, 157 time thickness map 150, 152
Permian and Triassic Rifting in Northwest Europe edited by S.A.R. Boldy (Amerada Hess, UK) Permo-Triassic rifting in Northwest Europe was part of the tectonism which caused Pangaea to disintegrate virtually as soon as it had formed. This rifting, together with associated events elsewhere in Europe, controlled much of the predominantly terrestrial sedimentation which occurred as the continent moved from the equatorial rainbelt in Carboniferous times, through the trade wind desert zone in the Permian to somewhat less arid conditions in the Triassic. To what extent Permo-Triassic rifting influenced sedimentation in the petroleum provinces of the Central and Viking Grabens of the North Sea is still debated. Elsewhere on and around the present-day continental shelf of Northwest Europe the influence of the rifting on Permo-Triassic sedimentation is becoming clearer. The twelve chapters of this volume introduce and discuss the emerging evidence for the differing styles of Permo-Triassic rifting around Northwest Europe. A number go on to discuss how this rifting influenced sedimentation and how it related to igneous activity in some areas. 9
12 chapters
9
263 pages
9 135 illustrations 9
index
ISBN
Cover illustration: Triassic St Bees Shale to St Bees Sandstone transition, Cumbria, UK (see p.80).
1-897799-33-0
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