Role of intrusion-induced salt mobility in controlling the formation of the enigmatic ‘Silverpit Crater’, UK Southern North Sea

Seismic interpretation of well-calibrated, overlapping, high-fidelity, 3D seismic data volumes has shown that many of the features advanced in support of a meteorite impact origin for the enigmatic ‘Silverpit Crater’ (SC) are, in fact, not only widespread in the Southern North Sea (SNS) region but also are of the same age. The mapping leads to a comprehensive alternative explanation for the formation of the SC in which Zechstein Supergroup evaporite mobility (halokinesis) controlled the development and evolution of several WNW–ESE-striking Cenozoic folds, including a buried salt withdrawal syncline in the axis of which lies the SC. The results demonstrate that the removal of structural support resulting from halokinetic outflow at depth promoted syn-sedimentary growth of the synclines and brittle deformation and collapse in the overburden. Significantly, it can now be shown that the WNW–ESE-striking synclines are exactly coincident with Palaeogene igneous dyke intrusions. The nature and shape of fault patterns that affect the Chalk Group all appear to be dictated by the form of their transecting intrusions, the associated post-depositional diagenetic alteration and structural failure resulting from the geometry of their respective underlying salt withdrawal syncline. In the case of the circular SC, the spectacular brittle ring-faulting is interpreted to result from its location above a zone of cylindrical salt evacuation, the flow from which was probably induced by the intrusion of a pipe-like igneous body. As well as explaining the genesis of the SC, the results also aid our understanding of Triassic prospectivity in the Silverpit area. Intrusion-induced fracturing provides a mechanism by which gas migrated up section from sub-salt (Carboniferous) source rocks through the thick Rotliegend Group Silverpit Claystone Formation, Zechstein Supergroup evaporite and Lower Triassic, Bunter Shale Formation seals. The prospective, gas-prone Bunter Sandstone Formation reservoirs lie in anticlinal closures above Zechstein Supergroup salt-cored pillows formed during the same phase of Early Cenozoic halokinesis that caused salt withdrawal beneath the SC.