Episodic dysoxia during Late Cretaceous cyclic chalk-marl deposition – Evidence from framboidal pyrite distribution in the upper Maastrichtian Rørdal Mb., Danish Basin

This study investigates the use of pyrite framboid data to increase insight in sea floor environmental conditions during deposition of chalk-marl cycles in the Upper Cretaceous of the Danish Basin, NW Europe. Framboid data have been used successfully to differentiate stages of oxygen reduction in siliciclastic mud rocks, and the data presented here indicate that they can be used to identify temporary events of oxygen deficiency, otherwise hidden by the intense bioturbation of the chalk and marl. The study is based on investigation of 23 samples from the 30 m thick chalk-marl alternations in the upper Maastrichtian Rørdal Member in the Dalbyover-1 core, Denmark. Framboid pyrite data are integrated with sulphur isotope data, observations of sedimentary facies, bioturbation and macro fossils in core, and well log data. The study reveals that framboids are completely absent in the chalk units and present in all marl layers. Their size and size distribution are indicative of different stages of dysoxia during marl deposition. The dysoxic events were probably short-lived so successive generations of burrowing animals destroyed any primary depositional signals, like e.g. lamination.