The architectural evolution of Late Cenozoic delta systems in the southern North Sea

The Late Cenozoic of the Southern North Sea basin was characterised by high subsidence and sedimentation rates, as well as fluctuations in eustatic sea levels. Basin margin uplift led to loading of the north-west European rivers with large quantities of freshly weathered detritus, and resulted in the formation of a delta system which extended over 200,000 square kilometres, and comprises over 50 thousand cubic kilometres of sediment. Twenty-seven seismic sequence boundaries have been identified on a grid of NOPEC regional seismic lines. Interpretations have been digitised and developed into a series of maps, profiles and volumetric statistics using Zycor software, in order to provide a quantitative and qualitative description of delta architecture and evolution. The development of the delta system can be divided into four phases. Early stages of development were characterised by the deposition of thick, laterally restricted sequences in high energy, deep water conditions in the north-eastern part of the study area. During the second phase, a dramatic reduction in water depths coincided with multi-phase slumping and a reduction in wave energy. Relative sea level oscillations were basinwide, and may have been eustatic. The high subsidence rates of the Central Graben during the third phase, were accompanied by the development of a forebulge in response to sediment loading. Analysis of delta development patterns has allowed interpretation of a dynamic distributary channel complex. Thin widespread packages of sediment and the infill of local topography was associated with the final phase of delta development. During the Elsterian stage of the Middle Pleistocene, ice sheets invaded the southern North Sea, and deltaic sedimentation ceased. The Vail seismic stratigraphic methodology has been successfully used to divide up basin fill into units, upon which reconstruction of the delta system has been based. However, many of the mapped sequence boundaries are related to changes in sediment input and subsidence rates, rather than eustasy. Profiles showing sequence stacking patterns should not be used unless several are available covering all areas of the basin, since the same sequences have been observed indicating opposite polarities of relative sea level change in different parts of the basin