The northern coast of the Ottnangian (middle Burdigalian, early Miocene) Molasse Sea in Germany: sediments, foraminiferal assemblages and biostratigraphy

Abstract

This study provides new data on the litho- and biostratigraphy of the middle Burdigalian (Ottnangian) marine sediments at or close to the former northern coastline of the German Molasse Basin, based on cores from three boreholes (Burgau, Hamlar 2 and Lutzingen) and two outcrops (Haunsheim and Dattenhausen). Methods include quantitative analysis of benthic foraminiferal assemblages (90 samples), studies of planktonic foraminifers and ostracods and investigations of lithofacies. The data indicate that the transgressive–regressive marine sediments of the Upper Marine Molasse (OMM) at Burgau and Hamlar 2 can be subdivided into the well-known lower and middle Ottnangian sedimentation cycles, with the first cycle being represented by the “OMM-Basisschichten” and Kalkofen Formation and the second by the Baltringen and Steinhofe Formations. We show for the first time that also the northernmost marginal-marine OMM facies (Lutzingen, Haunsheim and Dattenhausen) can be correlated with the lower and middle Ottnangian sedimentation cycles. Consequently, our results do not support the presence of a previously suggested third sedimentation cycle within the OMM sediments. Our micropaleontological data reveal regionally diverse depositional environments, reflecting different water depths and co-varying environmental variables. Significant differences in abundance, diversity and species composition of the benthic foraminiferal assemblages demonstrate marginal-marine facies for Lutzingen, Haunsheim and Dattenhausen, nearshore facies for Hamlar 2 and shallow marine basin facies for Burgau. The characteristic lower Ottnangian benthic foraminiferal species exhibit restricted ecological tolerances. Hence, the absence or scarcity of these species in nearshore to marginal-marine deposits has no stratigraphic significance. The presence of two sedimentation cycles in the western German Molasse Basin alone may have resulted from the interplay of regional tectonics and basin development that have led to different rates of subsidence and sedimentation in the western and eastern sectors of the German Molasse Basin.