Permian and Triassic Rifting in Northwest Europe, Geological Society Special Publication No

Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021 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. GLENNIE 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. Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021 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 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 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). 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 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 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.

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