The Skælskør structure in eastern Denmark – wrench-related anticline or primary Late Cretaceous seafloor topography?

Abstract

Sorgenfrei (1951) identified a number of NW–SE oriented highs in the Upper Cretaceous – Danian Chalk Group in eastern Denmark, including the Skælskør structure and interpreted them as anticlinal folds formed by wrenching along what today is known as the Ringkøbing-Fyn High. Recent reflection seismic studies of the Chalk Group in Øresund and Kattegat have shown that similar highs actually represent topographic highs on the Late Cretaceous – Danian seafloor formed by strong contourparallel bottom currents. Reflection seismic data collected over the Skælskør structure in order to test the hypothesis of Sorgenfrei show that the Base Chalk reflection is relatively flat with only very minor changes in inclination and cut by only a few minor faults. The structure is situated along the northern margin of a high with roots in a narrow basement block, projecting towards the northwest from the Ringkøbing Fyn High into the Danish Basin. The elevated position is maintained due to reduced subsidence as compared with the Danish Basin north of the high. The hypothesis of wrench tectonics as origin can be refuted. The seismic data show that the upper part of the Chalk Group is characterised by irregular mounded reflections, interpreted as representing contourite drifts, mounds and channels formed by strong, mainly late Maastrichtian bottom currents. The Skælskør structure of Sorgenfrei is thus in reality a Late Cretaceous topographic seafloor high formed by a combination of differential subsidence complemented by topographic features on the seafloor created by bottom currents in the late Maastrichtian.