The influence of fault array evolution on synrift sedimentation patterns: Controls on deposition in the Strathspey-Brent-Statfjord half graben, northern North Sea

Abstract

The dispersal and deposition of sediments in a rift basin are controlled by the sediment supply and the generation of accommodation space; hence, for the facies mosaic and depositional architecture of synrift sediments to be understood, both these variables must be constrained. Subsurface data sets, comprising three-dimensional (3-D) seismic and well data, provide the opportunity to quantify the rates of sediment supply and accommodation generation for the duration of the extensional event and over an area of regional extent. In this article, we address the controls on synrift sedimentation through detailed analysis of a high-resolution subsurface data set from the Late Jurassic northern North Sea rift basin. The sedimentation history in the study basin comprises four discrete stages intimately linked to the growth of the normal fault population. The earliest stage of rifting is characterized by a distributed fault population comprising a large number of faults with low slip rates. Sediment supply outpaced tectonic subsidence at this time, and rising eustatic sea level was the primary control on sedimentation. As rifting progressed, strain was localized onto a smaller number of active structures with higher displacement rates. The basin developed a grabenlike geometry, and the lateral propagation and linkage of fault strands controlled accommodation generation. Although the basin was flooded, the rate of sediment supply remained high and largely kept pace with the rate of tectonic subsidence. During the final two stages, the fully linked, half-graben bounding fault was the only active structure in the basin; the rate of sediment supply at this time was greatly exceeded by the rate of tectonic subsidence, and the basin became underfilled. Significantly, the final stage of the sedimentation history is characterized by large-scale fault interactions that changed the fault-controlled basin floor topography; hence, modified sediment dispersal and deposition. We conclude that sediment dynamics and facies distribution in a rift can only be understood in the context of the coevally active fault population. As the faults active at the close of a rift event are very different in location and character to those active during the initiation of rifting, this further emphasizes the need to integrate structural, sedimentary, and stratigraphic studies in rift basins.