Occurrence of major sequence stratigraphic boundaries in relation to basin development in Cenozoic deposits of the southeastern North Sea

Abstract

The post-Danian Cenozoic succession in the southeastern North Sea is subdivided into seven informal “major sequence stratigraphic units”, comprising twenty-one sequences. The boundaries of the units are easily recognized on seismic sections over the entire study area. The North Sea is an epicontinental basin. It underwent considerable changes during the Cenozoic as regards subsidence rates, direction of sediment transport, and position of depocentres. The six unit boundaries are interpreted to reflect some of the major events in the development of the basin. The boundary between units 1 and 2 (earliest Eocene) is associated with a regional hiatus followed by a large relative sea-level rise, whereas the boundary between units 2 and 3 (Middle Eocene) coincides with a shift in the direction of sediment transport from north to west. The boundary between units 3 and 4 (Eocene-Oligocene transition) marks an important change in the depositional environment of the North Sea with a shift in the direction of sediment transport from west to mainly northeast. In the northeastern part of the study area, thick prograding deposits of unit 4 are situated on distal deposits of units 1, 2 and 3. The transition between units 4 and 5 (latest Oligocene) shows a pronounced basinward shift in onlap, and it is interpreted to reflect a major fall in the relative sea level. The boundary between units 6 and 7 (Middle Miocene) is identified on a shift from a prograding reflection pattern to an aggrading pattern. The lower part of unit 7 is interpreted to represent a starved depositional environment, related to a pronounced increase in subsidence rate. The North Sea sequences are correlated with the Danish onshore deposits. The sequence boundaries generally coincide with an unconformity in the Danish deposits. A glauconitic horizon characterises the initial deposits overlying each of the unconformities.