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

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This paper addresses the potential role of regional-scale salt dissolution as a mechanism responsible for salt-related deformation. This analysis is based 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 makes use of a regional 2D seismic grid tied to five key exploration wells. This 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 provides a template and key datums for reconstruction of the adjacent zone of deformation. Structural styles in this zone include simple salt-cored anticlines, salt walls, salt diapirs, detachment faults and, unusually for the North Sea, intra-Zechstein salt-cored folds (pre-Triassic). Detailed mapping was carried out in UK Block 26/10, and this shows that the dominant structural trend is NE–SW, suggesting that basement tilting influenced structural development. The stratal geometry of the numerous Triassic mini-basins is used 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 thereof): (1) deformation through downbuilding during a phase of regional subsidence, or (2) flow induced by regional salt dissolution. Volumetric arguments necessary to discriminate between these two mechanisms are inconclusive due to later deformation of the Top Zechstein datum. Selective preservation 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 subjacent dissolution possibly resulting from low salinity groundwater circulating in the pre-Zechstein aquifers.