Syn-Depositional Halokinesis in the Zechstein Supergroup (Lopingian) Controls Triassic Minibasin Genesis and Location

Abstract

Salt tectonics is typically caused by the flow of mobile evaporites in response to postdepositional gravity gliding and/or differential loading by overburden sediments. This situation is considerably more complex near the margins of salt basins, where carbonate and clastic rocks may be deposited at the same time and interbedded with, more mobile, evaporite strata. In these cases, syn-depositional salt flow may occur due to density differences in the deposited lithologies, although our understanding of this process and related produces is relatively poor.We here use 3D seismic reflection and borehole data from the Devil's Hole Horst, West Central Shelf, offshore UK to understand the genesis, geometry and kinematic of intra-Zechstein Supergroup (Lopingian) minibasins and their effect on post-depositional salt deformation.Unlike much of the North Sea and other salt basins, the area is affected by only modest postdepositional, salt-related deformation, meaning it is a prime location to understand syndepositional salt-related deformation. We show that intra-basin highs are dominated by immobile, pinnacle-to-barrier-like, carbonate build-ups and anhydrite, whereas mobile halite, which flowed to form large diapirs, dominates in the deep basin. At the transition between these two main domains, a belt of intra-Zechstein minibasins occur, forming due to the subsidence of relatively dense anhydrite into underlying halite. Depending on primary halite thickness, these intra-Zechstein minibasins created topographic lows, dictating the position for nucleation and subsequent down-building of Triassic minibasins. Our study refines the original depositional model for the Zechstein Supergroup in the Central North Sea, with the results also helping us better understand the style and distribution of syn-depositional salt flow in other layered evaporitic sequences and the role intra-salt heterogeneity and related deformation may have in the associated petroleum plays.