Development of intraformational (Oligocene–Miocene) faults in the northern North Sea: influence of remote stresses and doming of Fennoscandia

The post-rift Cretaceous sequence of the Horda Platform (eastern margin of the Viking Graben, northern North Sea) is overlain by Cenozoic silisiclastic sediments. Within the latter sequence (Hordaland Group; claystone and thinly layered sands), a system of intraformational faults with a strongly dominant NW–SE-trend are seen in a transgressive unit of late Oligocene age. Occasionally, the faults are associated with deeper pre-Tertiary structures, but generally there is no such connection. Still, this indicates that the Oligocene deformation involved reactivation of Mesozoic or even older faults. The base of the sequence is represented by an angular unconformity with a primary mean slope angle of 1.8° when the post-Oligocene tilting of the area is corrected for present. The faults have a dominant strike (NW–SE) deviating 45° with respect to the N–S-striking slope. The lowermost part of the late Oligocene sequence rests on an unstable unit where incipient clay pillows and diapirs are observed. An evolutionary model including anomalously high fluid pressure, downslope gravity sliding, gravity collapse and regional tectonic stresses is suggested to account for the origin of the faulting observed. It is likely that a high fluid pressure, associated gravity collapse and downslope gravity sliding were critical for fault initiation. Still, the orientation homogeneity and parallelism of the fault system suggests that the deformation was influenced by a remote tectonic stress system related to ridge-push, doming of Fennoscandia and the differential subsidence of the North Sea. This model contrasts to the deformation style seen further to the southwest in the North Sea, where complex fault geometries and cellular networks comprising polygonal prismatic and pyramidal forms are observed. The dominant deformation mechanism in these areas is believed to be failure due to an anomalously high fluid pressure, without the influence of remote tectonic stresses.