The New Magnetostratigraphic Framework for the Lower Miocene (Burdigalian) in the North Alpine Foreland Basin

There is still a considerable controversy about the Oligocene-Miocene chronostratigraphic correlation within the Paratethys domain. Especially in the German and Swiss Molasse basins a robust correlation framework is missing for the late Early Miocene, represented by the lithostratigraphic units of the Upper Marine Molasse (OMM), Upper Brackish Molasse (Kirchberg and Grimmelfingen Fm) and (older) Upper Freshwater Molasse (OSM). Even in several recent papers there is a misfit of about 1 Ma in the chronostratigraphic interpretations between the Swiss and German Molasse Basins concerning the base of the Upper Freshwater Molasse. Despite the existence of a huge number of biostratigraphic studies, magnetostratigraphic constraints are rare. In 2013, Reichenbacher et al. proposed an alternative correlation of the Upper Brackish and Upper Freshwater Molasse in the North Alpine Foreland Basin, which would solve the present controversy. This study was based on an integrated magneto-biostratigraphic study on four sections and three boreholes. However, the data coverage, especially of the three drill cores, was not sufficient to unambiguously justify the modification in the chronostratigraphy. We enlarged the magnetostratigraphic framework by resampling these drill cores in high resolution and sampled eight additional cores. We obtained magnetostratigraphic results from a total of 530 individual samples, which were analysed using alternating field and thermal demagnetization techniques. Additionally, we performed detailed rock magnetic analysis and identified magnetite as the main carrier of the magnetic remanence. Less abundant hematite and greigite are present in the studied material. Basically, we confirmed the results of Reichenbacher et. al (2013), where the Upper Freshwater Molasse (OSM) is almost entirely of normal polarity and the underlying lower Kirchberg Fm and the upper part of the Grimmelfingen Fm are of reverse polarity. However, we found evidence for an additional short normal polarity interval at the top of the Grimmelfingen Fm. Based on these results and adding biostratigraphic arguments we conclude that the Kirchberg Fm represent the rather short reverse polarity interval C5Cn.2r and the main hiatus at the base of the Grimmelfingen Fm is located within chron C5Cr. By this, the beginning of the German and Swiss OSM are perfectly coeval and start at about 16.5 Ma. Reichenbacher, B., Krijgsman, W. & 14 co-authors (2013). An alternative magnetostratigraphic framework for the Lower Miocene (Burdigalian / Ottnangian, Karpatian) in the North Alpine Foreland Basin. Swiss Journal of Geosciences 106, 309–334.