108 EUROPE/Permian to Recent Evolution accompanied by a low level of volcanic activity with the exception of the Bay of Biscay, in the evolution of which wrench faulting compensating for crustal extension in the Atlantic domain played an important role Areas not affected by rifting continued to subside during the Triassic and Early Jurassic in response to thermal relaxation of the lithosphere, accounting in conjunction with eustatically rising sea-levels for a progressive overstepping of basin margins During the Early Triassic, continental to lacustrine conditions prevailed in the evolving grabens and thermal sag basins of WCE, in which the ‘Bunter’ redbeds were deposited Clastics were shed into these basins from adjacent Variscan and Caledonian highs, as well as from Fennoscandia During the late Early Triassic, the Tethys Seas ingressed the continuously subsiding Southern Permian Basin via the Polish Trough, giving rise to the deposition of carbonates in Poland and distal halites in northern Germany and the Southern North Sea (Figure 5) As during the Middle Triassic, the trend of highs that had separated the Northern and Southern Permian basins became gradually overstepped and with this these basins coalesced, thus forming the composite North-west European Basin The Tethys Seas advanced further into this continuously subsiding basin complex via the Polish Trough as well as via the Burgundy Trough and the Trier and Hessian depressions, establishing a broad neritic basin in which the ‘Muschelkalk’ carbonates, evaporites, and halites were deposited (Figure 6) Furthermore, intermittent marine transgressions advanced from the Tethyan shelves via the Bay of Biscay rift into the grabens of the Western Shelves By contrast, continental conditions continued to prevail in the grabens of the Central and Northern North Sea and the North-western Shelf With the beginning of the Late Triassic (see Mesozoic: Triassic), clastic influx from Fennoscandia and eastern sources increased, causing the replacement of the carbonate-dominated Muschelkalk depositional regime by the evaporitic ‘Keuper’ red-beds containing halites Whilst the Polish seaway, which had linked the Tethys and the North-west European Basin, was closed, intermittent marine transgressions advanced through the Burgundy Trough into the evolving Paris Basin and the continuously subsiding North-west European Basin, as well as through the Bay of Biscay rift into the grabens of the Western Shelves However, continental conditions persisted in the grabens of the Central and Northern North Sea and the North-western Shelf Only during the Rhaetian did the Arctic Seas start to advance southwards into the rifted basins of the North-western Shelf, whilst neritic conditions were established in the broad North-west European Basin (Figure 7) The Triassic series attains thicknesses of up to km in the grabens of the Western Shelves, the North Sea, and in the Polish Trough, and up to km in the grabens of the North-western Shelf In the Northern and Southern Permian Basins, the diapirism of Permian salts commenced during the Triassic, and accounted for local subsidence anomalies Jurassic In conjunction with continued rifting activity and cyclically rising sea-levels, the Arctic and Tethys Seas linked up during the Rhaetian–Hettangian, via the rift systems of the North-western and Western shelves and the continuously subsiding North-west European Basin (see Mesozoic: Jurassic) In the open marine, shale-dominated North-west European Basin, which occupied much of the Southern and Central North Sea, Denmark and Germany (Figure 7), the Belemnitidae (see Fossil Invertebrates: Cephalopods (Other Than Ammonites)) developed during the Hettangian and Sinemurian Persisting clastic influx from the East-European Platform allowed only for temporary marine incursions via the Polish Trough Similarly, fluvio-deltaic conditions prevailed in the grabens of the Northern North Sea until the end-Hettangian to Early Sinemurian when neritic conditions were also established in these basins By Late Simemurian times, this facilitated a broad faunal exchange between the Boreal and Tethyan realms and the dispersal of the Belemnitidae In response to rising sea-levels and continued crustal extension, open marine conditions were established in the Central Atlantic during the Simemurian, permitting Tethyan faunas to reach the Pacific by Pliensbachian times In basins which were dominated by the warmer Atlantic and Tethyan waters, carbonates and shales were deposited, whilst shales prevailed in the North-west European Basin, which was dominated by the cooler Arctic waters During the Early Jurassic, repeated stagnant water stratification gave rise to the deposition of organic-rich shales, forming important oil source-rocks, for example, in the Paris Basin and the southern parts of the North-west European Basin During the Late Aalenian–Early Bajocian, the Arctic seas became separated from the Tethys and the Central Atlantic in conjunction with the uplift of a large arch in the Central North Sea from which clastics were shed into the adjacent continuously subsiding basins (Figure 8) Uplift of this arch was associated with major volcanism that may be related to the impingement of a short-lived mantle plume Open marine communications between the Arctic and the Tethys–Atlantic seas were re-opened