From shoreface to riverbed – Facies evolution in Burdigalian deposits of the North Alpine Foreland Basin

During the Burdigalian the North Alpine Foreland Basin, as part of the central and western Paratethys, underwent various paleogeographic and paleoenvironmental changes which led to the deposition of different sedimentary facies. For instance, the Ottnangian regional stratigraphic stage (18.2 – 17.3 Ma) was characterized by a major transgression in the beginning, with the deposition of several littoral facies along the northern coastline of the North Alpine Foreland Basin. Successively, the late Ottnangian documents the final retreat of the Paratethys Sea from the North Alpine Foreland Basin and the deposition of widely distributed fluviatile units. The outcrop Neustift in southeast Germany bears many different siliciclastic facies and shows the transition of the Ottnangian marine realm towards a riverine-deltaic environment. Despite the exceptional size and complexity of the succession, this outcrop is only poorly studied. Anyhow, this section gives unique potential for the understanding of the facies evolution during the terminal Ottnangian. In total a 70 m long and 15 m high sedimentary log was recorded together with several micropaleontological samples (1 kg each) for the reconstruction of the depositional environment and stratigraphic positioning. The micropaleontological samples yielded 164 genera of benthic foraminifera and 144 species of ostracods. Moreover, these deposits are extremely rich in macrofossils (elasmobranchia, mollusca, brachiopoda, echinoidea) which are also poorly studied. We found out, that the lowermost segment of the section belongs to the marine “Littoral Facies of Holzbach-Höch”, which deposited directly on top of a transgressive layer on the crystalline basement. Several of the observed ostracod species are new to these deposits. Large-scale cross-bedding structures show that this shallow marine environment was affected by strong tidal currents along the rocky shoreline. The fine-grained sediments with wavy and lenticular bedding on top of the littoral deposits show an ongoing transgression, with neritic foraminiferal assemblages and bioturbation. They were assigned by biostratigraphy to the uppermost Neuhofen Formation. Finally, the marine deposition is replaced by a large-scale deltaic system, resulting into the deposition of the fluviatile Ortenburg gravel. This preliminary study should draw some more attention to this unique outcrop in the North Alpine Foreland Basin and the potential for further studies in the realm of sedimentology and paleontology.