The Cenozoic fill of the North Sea Basin (UK sector 56-62° N), a seismic stratigraphic study with emphasis on Paleogene massflow deposits

Abstract

Seismic stratigraphic study techniques allow to recognize several Cenozoic sedimentary cycles in the central and northern part of the North Sea area (UK sector), and to deduce their depositional history. Large-scale sedimentation patterns are illustrated with emphasis on Paleogene massflow deposits, forming important hydrocarbon-bearing reservoirs. Five regional unconformities form the bases of five depositional sequences (DS). These sequences can be subdivided into systems tracts in which seismic lithofacies units are outlined, calibrated by 76 wells. The base of DS-1 (Paleocene-Early Eocene) reflects the change from predominantly pelagic carbonates to clastic deposition. Within DS-1, prograding slope systems are present. Fan-delta systems supplied clastics to the shelf, whilst in the coastal area prograding deltas, swampy lakes and barrier complexes existed. Base-of-slope sand-prone sediments were laid down as slope-front fills, in submarine fan mounds and in parallel-bedded basinfloor deposits. Massflow sedimentation is dominant in the deeper parts of the basin. Axial basinfloor transport is indicated by the mound geometry and abnormal tichkness of the bottom sets. A volcanic pulse is expressed by a volcano-clastic seismic marker. In DS-2 (Eocene-Early Oligocene) a slope system fringed the western margin of the basin. Massflow sedimentation continued; sources and depocentres are correlatable to those active in the Paleocene. Subordinate input sources existed in the Norwegian sector. The base of DS-3 (Late Oligocene-Middle Miocene) is onlapped by fine-grained marine deposits. Sediment thickness increases basinwards. Rapid sedimentary loading of underlying shales prevented proper dewatering, causing plastic deformation and under-compaction. In DS-4 (Late Miocene-Early Pleistocene) sediment input from the east increased. DS-5 (Quaternary) shows that sediment supply from the south-southwest became more significant.