Salt dissolution and salt-related deformation of the Forth Approaches Basin, UK North Sea

This paper addresses the potential role of regional-scale salt dissolution as a mechanism responsible for salt-related deformation. This analysis is b ased on an investigation of the Late Permian to Triassic succession of the Forth Approaches Basin, on the NW margin of the North Permian Basin and mak es use of a regional 2D seismic grid tied to five key exploration wells. Th is basin is ideal for this study because it consists of a deformed central region with a NE-SW trending axis, and an undeformed periphery which provid es a template and key datums for reconstruction of the adjacent zone of def ormation. Structural styles in this zone include simple salt-cored anticlin es, salt walls, salt diapirs, detachment faults and, unusually for the Nort h Sea, intra-Zechstein salt-cored folds (pre-Triassic). Detailed mapping wa s carried out in UK Block 26/10, and this shows that the dominant structura l trend is NE-SW, suggesting that basement tilting influenced structural de velopment. The stratal. geometry of the numerous Triassic mini-basins is us ed to constrain the way in which accommodation space in the mini-basins was created by depletion of mobile salt. The spatial pattern of salt depletion can be-accounted for by two contrasting mechanisms (or some combination th ereof): (1) deformation through downbuilding during a phase of regional sub sidence, or (2) flow induced by regional salt dissolution. Volumetric argum ents necessary to discriminate between these two mechanisms are inconclusiv e due to later deformation of the Top Zechstein datum. Selective preservati on of Z3 and Z4 cycles of the Zechstein over large parts of the Block 26/10 study area suggests that much of the salt depletion process preferentially removed Z2 halite. This observation is compatible with a process of subjac ent dissolution possibly resulting from low salinity groundwater circulatin g in the pre-Zechstein aquifers. (C) 2001 Elsevier Science Ltd. All rights reserved.