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1 Department of Geology, Imperial College, London SW7 2BP, UK
2 Amerada Hess Ltd, 33 Grosvenor Place, London SW1X 7HY, UK (e-mail: sa.stewart{at}amhess.co.uk)
Mapping of depositional zones within the Zechstein Supergroup using semi-regional 2D and 3Dseismic data constrains the geological evolution of the northwestern margin of the North Permian Basinin UK offshore Quadrants 14, 15, 19, 20 and 21. Four depositional zones are identified on the basis of mappable seismic facies, which are assumed to vary with halite content. Well control characterizes the most reflective seismic facies as an interbedded carbonate and anhydrite ‘shelf’ succession which grades laterally through clastic-rich sequences to the least reflective, a thick halite basinal equivalent. The trendsof the depositional zones illustrate the geometry of the North Permian Basin and may have been locally controlled by basement lineaments. Within the North Permian Basin a N–S-trending axis (Devil’s Hole High) characterized by thick reflective facies separates the Central North Sea from the Forth Approaches Basin. Basement fault blocks control facies type and thickness along the northwest margin of the North Permian Basin in the Moray Firth area. Permian tilt of the Zechstein shelf related to regional flexure played a role in precipitating gravity-driven tectonics within the seismically reflective facies on the east flank of the Devil’s Hole High. This Permian phase of salt tectonics resulted in numerous intra-Zechstein structures which can be interpreted as rafted and folded zones of interbedded halite, carbonate, sulphate and clastics. The ‘rafts’ initially slid downdip towards the main halite basin then their evolution became dominated by differential subsidence of the interbeds which forced salt layers into adjacent passive diapirs. This episode predates deposition of the Triassic Smith Bank shales which rest upon the Zechstein with local angular unconformity. The spatial distribution of Zechstein facies influenced the tectonostratigraphy of the Triassic, Jurassic and Cretaceous systems. Boundaries between Zechstein facies correspond to the limit of Triassic halokinesis on the West Central Shelf and the limit of detachment of Jurassic and lower Cretaceous thin-skinned tectonics in Moray Firth half graben.
Key Words: Moray Firth • North Sea • Zechstein • salt tectonics • evaporites
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