Morphology and origin of major Cenozoic sequence boundaries in the eastern North Sea Basin: Top Eocene, near top Oligocene and the mid-Miocene unconformity

Unconformities in sedimentary successions (i.e. sequence boundaries) form in response to the interplay between a variety of factors such as eustasy, climate, tectonics and basin physiography. Unravelling the origin of sequence boundaries is thus one of the most pertinent questions in the analysis of sedimentary basins. We address this question by focusing on three of the most marked physical discontinuities (sequence boundaries) in the Cenozoic North Sea Basin: top Eocene, near-top Oligocene and the mid-Miocene unconformity.

The Eocene/Oligocene transition is characterized by an abrupt increase in sediment supply from southern Norway and by minor erosion of the basin floor. The near-top Oligocene and the mid-Miocene unconformity are characterized by major changes in sediment input directions and by widespread erosion along their clinoform breakpoints. The mid-Miocene shift in input direction was followed by a marked increase in sediment supply to the southern and central North Sea Basin.

Correlation with global delta O-18 records suggests that top Eocene correlates with a major longterm delta O-18 increase (inferred climatic cooling and eustatic fall). Near-top Oligocene does not correlate with any major delta O-18 events, while the mid-Miocene unconformity correlates with a gradual decrease followed by a major long-term increase in delta O-18 values. The abrupt increases in sediment supply in post-Eocene and post-middle Miocene time correlate with similar changes worldwide and with major delta O-18 increases, suggesting a global control (i.e. climate and eustasy) of the post-Eocene sedimentation in the North Sea Basin.

Erosional features observed at near-top Oligocene and at the mid-Miocene unconformity are parallel to the clinoform breakpoints and resemble scarps formed by mass wasting. Incised valleys hare not been observed, indicating that sea level never fell significantly below the clinoform breakpoint during the Oligocene to middle Miocene.