During Pliensbachian–Bajocian times northern Europe, including Britain, was covered by an epeiric sea. Sediments formed include clays, sandstones, limestones and ironstones, usually cyclically arranged; different facies were developed synchronously in different areas. There is very little evidence, by way of slumps or turbidites, that redeposition processes were active. The sequence clay, sandstone, limestone/ ironstone is here interpreted as representative of a shallowing. Thus, at any one time, despite differing bathymetric zones, bottom slopes were apparently subdued enough to ensure that sediment displacement did not generally take place.
To reconcile the concept of Jurassic ‘basins’ and ‘swells’ with the subdued bottom slopes of the north European epeiric sea, we suggest that these structural elements were characterized by great and negligible subsidence respectively, but that sedimentation was always rapid enough to maintain a roughly level sea floor. We relate the Mendip, London Platform and Dorset coast ‘swells’ to early Jurassic positive fault motions in the basement. The Market Weighton ‘swell’ is ascribed to relative buoyant rise of a salt pillow or granitic body whose movement was probably triggered by the same motions. These, presumably extensional, tectonics were probably the driving force behind the formation of the whole north European epeiric sea and must be related, in turn, to the opening of the oceanic central Atlantic and Alpine–Mediterranean Tethys.