Late Early Miocene palaeoenvironmental changes in the North Alpine Foreland Basin

The North Alpine Foreland Basin (NAFB) experienced rapidly changing palaeoenvironmental conditions at the end of the Early Miocene (middle-to-late Burdigalian, Paratethys stages Ottnangian and Karpatian), with transformation of fully marine settings into brackish and eventually freshwater environments. These changes were related to global sea-level fluctuations, climate oscillations, and tectonic processes associated with the uplift of the Alps. This study presents a new and comprehensive data set, derived from 13 boreholes and 491 core samples from the Molasse Basin of southwest Germany, that provides a significantly better understanding of late Early Miocene palaeoenvironments in the NAFB. Based on lithostratigraphy, biostratigraphy, and palaeoecological interpretation of rich fauna and flora assemblages, we show that the upper Burdigalian sediments comprise a regressive lower segment (Grimmelfingen Formation and equivalents), as indicated by the occurrence of the bivalve Rzehakia, and a transgressive upper component (Kirchberg Formation and equivalents) characterized by the advent of new fish and mollusc assemblages. Furthermore, we present a new lithostratigraphic concept for the Kirchberg Formation, which forms an important part of the Upper Brackish Molasse (OBM). Based on biostratigraphic constraints and recently published magnetostratigraphic data, it appears that the regressive segment of the upper Burdigalian OBM sediments correlates with the end of the global third-order sea-level cycle Bur 3, while the transgressive component reflects the Karpatian transgression at the beginning of sea-level cycle Bur 4. This implies that the Karpatian transgression actually affected the Molasse Basin of southern Germany and Upper Austria, whereas previous studies had considered this part of the NAFB as terrestrial during that time span. The new results are depicted in three palaeogeographic maps for the NAFB at 18 Ma, 17.5-17.1 Ma and 17 Ma, respectively.