The Cretaceous–Palaeogene boundary at Stevns Klint, Denmark: inversion tectonics or sea-floor topography?

Abstract

The Cretaceous–Palaeogene boundary interval is exposed over 12 km in the coastal cliff, Stevns Klint, Denmark. An important lowermost Danian hardground has been interpreted as an originally horizontal marine abrasion surface. Its present elevation varies, from a few metres below, to about 35 m above sea level. This relief has traditionally been considered as resulting from late or post-Danian Laramide folding. New seismic profiles offshore Stevns Klint show, however, that the Base-Chalk reflector is not folded, is remarkably planar and has a gentle northward dip. Thus, the folding hypothesis cannot be upheld. Seismic stratigraphic analysis of the Chalk Group necessitates a fundamental revision of general ideas of chalk deposition. A highly irregular sea-floor topography was formed at many levels, and includes broad valleys, ridges, channels, drifts and mounds. A system of major WNW–ESE-oriented valleys and ridges can be traced into the succession exposed in Stevns Klint and further inland where it corresponds to the relief of the Cretaceous–Palaeogene boundary. The marked topographic elements of the chalk sea floor are elongate, with a WNW orientation roughly parallel to the axis of the Danish Basin and to the Sorgenfrei–Tornquist Zone forming the NE border of the basin. The sea-floor relief undoubtedly reflects the influence of strong contour-parallel bottom currents. The Cretaceous–Palaeogene boundary succession at Stevns Klint was thus developed on an underlying sea-floor topographic relief of about 40 m. Recognition of the highly irregular, current-influenced topography of the late Cretaceous sea floor stands in marked contrast to the conventional picture of quiet pelagic deposition of the chalk.