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

Unconformities in sedimentary successions (i.e. sequence boundaries) form i n response to the interplay between a variety of factors such as eustasy, c limate, tectonics and basin physiography. Unravelling the origin of sequenc e 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 m ost marked physical discontinuities (sequence boundaries) in the Cenozoic N orth Sea Basin: top Eocene, near-top Oligocene and the mid-Miocene unconfor mity. The Eocene/Oligocene transition is characterized by an abrupt increase in s ediment 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 al ong their clinoform breakpoints. The mid-Miocene shift in input direction w as followed by a marked increase in sediment supply to the southern and cen tral North Sea Basin. Correlation with global delta O-18 records suggests that top Eocene correla tes with a major longterm delta O-18 increase (inferred climatic cooling an d eustatic fall). Near-top Oligocene does not correlate with any major delt a 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 inc reases, 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 un conformity are parallel to the clinoform breakpoints and resemble scarps fo rmed by mass wasting. Incised valleys hare not been observed, indicating th at sea level never fell significantly below the clinoform breakpoint during the Oligocene to middle Miocene.