The late Maastrichtian Rørdal Member (Denmark): a remarkable window of climatic, environmental and biotic changes at orbital time scales

Abstract

The late Maastrichtian Rørdal Member of the Møns Klint Formation is a remarkable lithological unit of the Danish Basin, characterised by chalk-marl cyclicity, standing out of pure Maastrichtian white chalk within the Chalk Group. A cyclostratigraphic analysis across this unit suggests a control by orbital precession and a strong amplitude modulation of the precession by the 405 kyr eccentricity cycle. Oxygen isotope data from bulk carbonate, benthic foraminifera and brachiopods indicate a prominent cooling commencing at the base of the unit with maximum cooling aligned with a 405 kyr amplitude modulation maximum. This unit thus represents a rare example of a climatic cooling associated with a 405 kyr insolation maximum. However, geochemical and calcareous nannofossil data support lower productivity levels across the unit, in contradiction with an expected ocean fertilization accompanying enhanced continental weathering as source of the clay material. An alternative model of deposition for this lithological unit is thus proposed via dense water cascading favored by sea level low, subsequent restriction of the Chalk Sea and associated gravity currents responsible for the resuspension of fine clastic deposits from the margins of the Baltic Shield.