Lithostratigraphy and depositional history of the Middle Triassic Dowsing Dolomitic Formation of the southern North Sea and adjoining areas

• Main Author: Southworth, Christopher John
• Published: University of Oxford 1987
• Subjects: 551; Geology of southern North Sea
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.233098

Middle Triassic deposition in Northwest Europe occurred in the enclosed Germanic Basin, which extended from its connection with Tethys in eastern Poland to eastern England. A detailed lithostratigraphic subdivision of the middle Triassic Dowsing Dolomitic Formation of the U.K. Southern North Sea and subsurface eastern England is presented, based on petrophysical logs, well cuttings and limited core. This subsurface lithostratigraphy is linked to the established outcrop lithostratigraphy of eastern England by means of a surface gamma-ray profile at Kirton Brickworks, Nottinghamshire. The outcrop lithostratigraphy is thereby accurately correlated with that of the basin centre in north Germany. The major tectonic elements active during middle Triassic deposition in the Southern North Sea area are identified, including previously unreported differential subsidence in an extension of the Dutch Broad Fourteens Basin in the U.K. Sector. The basinwide Hardegsen unconformity marks the base of the middle Triassic. A transgressive phase resulted first in the deposition of lacustrine pro-delta mudstones towards the basin centre, followed by the deposition of a more extensive thin marine mudstone which formed the base of the middle Triassic over most of the U.K. Southern North Sea area. Two regressive cycles followed. Each started with deposition of the cyclic, hypersaline-marine halites (the Main and Upper Röt Halites), followed by sabkhas, then playa mudflat/lagoonal deposits. At the start of the Muschelkalk deposition, restricted marine conditions were established in the Southern North Sea area. These were followed by a further regressive sequence of hypersaline-marine halites (the Muschelkalk Halite), sabkhas, and finally playa mudstones. The establishment of continental sedimentation across the whole of the Germanic Basin marked the end of the middle Triassic. The cyclicity within the Germanic Basin correlates with the published coastal onlap charts, indicating its origin is eustatic fluctuations in Tethys.