The interplay between true eustatic sea-level changes, tectonics, and climatic changes: what is the dominating factor in sequence formation of the Upper Oligocene-Miocene succession in the eastern North Sea Basin, Denmark?

The Upper Oligocene-Miocene succession of the eastern North Sea is subdivided into six sequences on the basis of an integrated study of outcrops, boreholes, and seismic data. All available data and methodologies/criteria for the definition of sequences have been used. Datings of the sequences are based on palynology, micropalaeontology, and macropalaeontolgy, which give them a high confidence. The six sequences defined in this study do not correlate to the eight marked changes in true eustatic sea-level variation as indicated from oxygen isotope curves nor to the nine glacial maxima that have been recognised within the Miocene. The development of the individual sequences and the stacking pattern of the sequences is dependent both on true eustatic sea-level changes, relative sea-level changes (tectonics), and sediment supply. For example, a true eustatic sea-level fall as well as uplift may result in major progradation of a siliciclastic wedge. In the Miocene succession studied here, a distinct progradation in the lower Aquitanian was the result of a true eustatic sea-level fall. This resulted in deposition of a clean and widespread sand-rich clastic wedge. Marked progradation associated with an uplift of the hinterland in the Langhian resulted in sand-rich deposits alternating with coals. The interbedded coal seams were the result of a rising ground water table within the basin due partly to stable or even rising sea level and partly to local subsidence around faults. Similarly, a major transgression may be a result of a true sea-level rise or accelerated subsidence of a basin. In the case of the eastern North Sea Basin, accelerated subsidence was responsible for the major transgression in the Serravallian and Tortonian times, which is a period with a general climatic cooling. The frequency of true eustatic sea-level changes and relative sea-level changes (tectonic pulses) in the Miocene is of the order of 2.9-2.0 to 4 m.y., respectively. Both these intervals are within the time range of the studied sequences, which span 2-4 m.y. Consequently, but not surprisingly, neither of the two processes responsible for changes in relative sea level can be solely attributed to glaciations nor to tectonics. The climatic changes during the Late Oligocene-Miocene, which changed from cool temperate to subtropical with several reversals, did not influence the sediment yield.