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From Boldy, S. A. R. (ed.), 1995, and Rifting in Northwest , Geological Society Special Publication No. 91,' 1-5

Permian and Triassic rifting in northwest Europe

K. W. GLENNIE

University of Aberdeen, UK

The Permo-Triassic roughly coincided with the life span of . So far as Europe is concerned, Pangaea's creation began in the early (Visean) when the northward-drifting megacontinent 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 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 , and, by early in the , rifting was taking place between Africa and the Americas in the newly forming Central (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 (Surlyk et al. 1984). These extensional movements may have propagated southward to initiate fracturing in the Viking and Central of the , 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 , 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 , 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. Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021

2 K.W. GLENNIE

The Variscan seems to have been doomed to failure; it was to become a range of highlands but not a major . No sooner had it formed than it began to collapse, with the coeval development of a very widespread NW-SE and conjugate NE-SW 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 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 , 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 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). Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021

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 Permo- Triassic 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 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 on the 1948 edition of the Geological Survey Map of . 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 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 Permo- Carboniferous 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 (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 Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021

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 NW 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.) 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 post- Carboniferous igneous activity in the North Sea Basin. In: ILLING, L. V. & HOBSON. G. D. (eds) Petroleum 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 SURVEY OF 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. 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 and NW Europe. Proceedings Geologist's Association, 99(2), 74-100. Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021

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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. Permian-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, 497- 508. 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.