EVOLUTION OF THE NW MARGIN OF THE NORTH PERMIAN BASIN, UK NORTH-SEA

Mapping of depositional zones within the Zechstein Supergroup using se mi-regional 2D and 3D seismic data constrains the geological evolution of the northwestern margin of the North Permian Basin in UK offshore Quadrants 14, 15, 19, 20 and 21. Four depositional zones are identifie d on the basis of mappable seismic facies, which are assumed to vary w ith halite content. Well control characterizes the most reflective sei smic facies as an interbedded carbonate and anhydrite 'shelf' successi on which grades laterally through elastic-rich sequences to the least reflective, a thick halite basinal equivalent. The trends of the depos itional zones illustrate the geometry of the North Permian Basin and m ay have been locally controlled by basement lineaments. Within the Nor th Permian Basin a N-S-trending axis (Devil's Hole High) characterized by thick reflective facies separates the Central North Sea from the F orth Approaches Basin. Basement fault blocks control facies type and t hickness along the northwest margin of the North Permian Basin in the Moray Firth area. Permian lilt of the Zechstein shelf related to regio nal flexure played a role in precipitating gravity-driven tectonics wi thin 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 fol ded zones of interbedded halite, carbonate, sulphate and clastics. The 'rafts' initially slid downdip towards the main halite basin then the ir evolution became dominated by differential subsidence of the interb eds which forced salt layers into adjacent passive diapirs. This episo de predates deposition of the Triassic Smith Bank shales which rest up on the Zechstein with local angular unconformity. The spatial distribu tion of Zechstein facies influenced the tectonostratigraphy of the Tri assic, Jurassic and Cretaceous systems. Boundaries between Zechstein f acies correspond to the limit of Triassic halokinesis on the West Cent ral Shelf and the limit of detachment of Jurassic and lower Cretaceous thin-skinned tectonics in Moray Firth half graben.