Mapping of the Maastrichtian-Danian boundary in the coastal area of Køge Bugt by gamma- and resistivity logging

Abstract

The Maastrichtian-Danian boundary surface has been mapped by geophysical logging of water supply borewells in the coastal area of Koge Bugt south of København. The transition from bryozoan limestone of Danian (earliest Palaeogene) age with a variable apparent resistivity between 100 and 250 Ohmm to chalk of Maastrichtian (latest Cretaceous) age with a relatively stable apparent resistivity in the range 60-80 Ohmm is easily detected on most resistivity logs. Gamma logs show a characteristic anomaly with two peaks in the uppermost Maastrichtian chalk approximately 8-10 meters below the Maastrichtian-Danian boundary. This gamma anomaly is caused by a layer of marl, which is found persistently in the whole area between Kobenhavn and Koge. The marl is probably equivalent to the Kj~lby Gaard Marl, known from outcrops of uppermost Maastrichtian chalk in northwest Jylland. A preliminary contour map has been prepared displaying the elevation of the boundary surface between the Danian limestone and Maastrichtian chalk below. This map shows that the limestones are weakly folded with structures very similar to those observed in the outcrops of Stevns Klint and also detected by recent seismic studies of the limestone basement in 0resund. The structures can be interpreted without introducing major vertical displacements along faults.