Interaction between bottom currents and slope failure in the Late Cretaceous of the southern Danish Central Graben, North Sea

Abstract

The NW European Chalk Group was deposited in a deep epicontinental sea traditionally conceived as a quiet depositional setting that was only affected by redeposition along structural highs. The ‘chalk sea’ was, however, locally affected by powerful bottom currents during some periods. Three-dimensional seismic reflection data from the southern Central Graben show abundant intra-chalk morphological elements concentrated at two stratigraphic levels within the Turonian–Campanian sequence. Both levels are topped by extensive unconformities. The geometry of the elements produced by alongslope processes is interpreted as caused by SE-directed bottom currents flowing between the Bo-Jens and Adda ridges. The highest current intensities led to the formation of the two unconformities, judged by the occurrence of the largest current-formed channels and drifts at these two stratigraphic levels. This sea-floor topography was locally modified by large-scale downslope mass transport. Stratigraphical coincidence of the largest slumps and bottom-current erosion suggests a coupling between downslope and alongslope processes. Current erosion impinged upon the slopes during periods of highest current speeds, destabilizing the slope sediments and triggering slumping. The smoothing of the bathymetry following local relaxation of the tectonic inversion accounted for the coeval deactivation of alongslope and downslope processes during the succeeding Maastrichtian deposition.