Eocene sequence stratigraphy of the North Sea Basin

Eocene elastic sediments of the central and northern North Sea comprise five stratigraphic sequences. Oldest is the sand-dominated, basin-centered Frigg sequence, which includes submarine fan and apron deposits on the Beryl Terrace and in the Central and Viking Grabens. Middle Eocene Lower and Upper Hordaland sequences are mixed mud and sand units which prograded from the East Shetland Platform and Moray Firth and include discrete slope aprons and basin-center fans. The thin, regressive sand-prone Belton sequence marks the top of the Eocene in most of the basin, except on the eastern margin of the Shetland Platform, where the foreshelf pro grading prism of Thet sequence sandstones and siltstones was deposited. Foraminiferal biofacies analysis of a well on the Beryl Terrace indicates that four or the five sequence boundaries coincide with significant changes in the benthic and planktic foraminiferal assemblages. Cluster analysis confirms the distinct biofacies characterizing each sequence, an impoverished coarsely agglutinated fauna with radiolaria in the Frigg sequence, normal abundances or coarsely agglutinated forms in the Hordaland units, and "Velasco"-type deep-water calcareous benthics in the Upper Eocene, including planktic foraminifera in the Belton. Paleoenvironmental analysis or these biofacies shows an overall Eocene shallowing from lower slope to outer shelf. Subsidence analysis using the derived water depths indicates that the Eocene was part or a major phase of subsidence which began in the Late Paleocene and abated by Oligocene time. Overall, the Eocene constitutes a tectonosequence, or stratigraphic sequence attributable to tectonic influences. Eocene circum-North Sea tectonics include Atlantic domain seafloor-spreading to the north and west and the encroaching Alpine compression from the south. The base Eocene, top Frigg, and top Eocene all correlate with changes in seafloor spreading thought to have contributed, through intraplate stresses, to the stress regime and differential topography of the source area and basin. Latest Eocene eustatic lowering, climatic cooling, and Arctic water circulation are thought to have influenced the Thet sequence.