Facies and ichnology of an Upper Cretaceous chalk contourite drift complex, eastern Denmark, and the validity of contourite facies models

摘要

The Upper Cretaceous chalk of the Danish Basin has been interpreted as a major contourite complex on the basis of high-resolution seismic data. The sea floor had a pronounced topography with kilometrewide ridges and valleys up to almost 200 m deep, interpreted to have been formed by contour-parallel bottom currents. Only few ancient contourite systems have been recognized, mainly based on sedimentary facies and only rarely on architecture and morphology. Two cored boreholes, 345 and 443.3 m deep through the Danish chalk contourite complex, offer a unique possibility to compare seismic and sedimentary facies. The contourite chalk is completely bioturbated except for thin intraclast conglomerates and a few thin levels, showing possible primary lamination. In terms of lithology and trace fossils the contourite chalk is similar to horizontally bedded pelagic chalk uninfluenced by bottom currents. Published contourite models cannot normally be used for the chalk because of the very fine grain size, generally complete bioturbation, and lack of any vertical trends in grain size on a millimetre to centimetre scale. It is thus only rarely possible to document the influence of bottom currents on the basis of facies analysis alone and this can be inferred only by architectural analysis of seismic-scale outcrops.