Characterisation of Ottnangian (middle Burdigalian) palaeoenvironments in the North Alpine Foreland Basin using benthic foraminifera—A review of the Upper Marine Molasse of southern Germany

Abstract

Abstract Considerable facies changes and varying foraminiferal assemblages have been described from the lower Miocene sediments of the Upper Marine Molasse in the North Alpine Foreland Basin of southern Germany, and have resulted in the distinction of a “Western Molasse” and “Eastern Molasse”. However, the causes and processes leading to these local differences have largely remained unexplored to date. The aim of this study is to explore possible explanations in order to better understand the palaeogeographic, stratigraphic and environmental setting. The study focuses on the Ottnangian (middle Burdigalian) segment of the Upper Marine Molasse in southern Germany, and includes a review of previously published data, together with new data from the borehole Stockhausen which is located between the areas of the Eastern and Western Molasse. Methods include quantitative analysis of benthic foraminiferal assemblages, qualitative studies of planktonic foraminifera and calcareous nannoplankton, as well as palaeoenvironmental considerations based on the actualistic approach. The results of these studies demonstrate that the lower and middle Ottnangian Upper Marine Molasse include a single transgressive–regressive sedimentary cycle with gradual changes of foraminiferal assemblages in the Eastern Molasse. On the other hand, two sedimentation cycles are recognizable in the Western Molasse and in the borehole Stockhausen where the lower and middle Ottnangian sediments are separated from one another by an unconformity and display an abrupt change in lithofacies and foraminiferal content. The benthic foraminiferal assemblages from the area of the Western Molasse are indicative of a rather shallow sea, but indicative of a much deeper facies in the basin area of the Eastern Molasse. It can therefore be concluded that the shallow sea to the west was more sensitive to minor changes in sea level than the area of the Eastern Molasse, either because of global sea level variation or as a result of regional tectonics. This means that the occurrence of two sedimentary cycles in the Western Molasse, and absence of the second cycle in the Eastern Molasse, may be the result of shallower water depths and a lower subsidence rate in the area of the Western Molasse. Another interesting result from our studies relates to the sea-level highstand and maximum extension of the Upper Marine Molasse Sea that, based on our studies, has occurred during the early Ottnangian, rather than during the middle Ottnangian as suggested previously. During this highstand, the area of the Eastern Molasse displays a differentiation into a deep neritic basin facies characterised by high organic flux and moderate oxygen depletion at times, and a marginal marine facies with well oxygenated environments. The latter facies remained poorly understood until recently. The entire region of the South German Molasse Basin was affected by a regression during the middle Ottnangian. This phase is characterised by unstable environmental conditions in the Eastern Molasse, i.e. salinity fluctuations and temporarily strong oxygen depletion of bottom waters. In the Western Molasse, life conditions became even worse and, as a result, a drastic decline in marine life is observable.