Temporal and spatial changes of microfossil associations and ichnofacies in the Austrian marine Miocene.

Research project P 13743 Miocene microfossil associations and ichofacies Johann HOHENEGGER 28.06.1999 The geodynamic history of the marine Eastern Alpine Miocene is characterized by a complex system of basins, their deepening, shallowing and opening of seaways. Basin development is connected during the Early Micoene with the evolution of the Alpine Carpathian foredeep, while in the Middle Miocene intramountain basins were developed in the Alpine Carpathian nappe system and sedimentation occurred mainly in the down faulted Vienna and Styrian Basins. Restriction of these basins from the open sea started in the Sarmatian leading to brackish conditions distinguished by endemic faunas in the Pannonian. This geodynarnic history is reflected in bottom facies, which is pictured in benthic microfossil assemblages and ichnofossils beside mollusc faunas. Spatial differences of the sea-bottom that reflect water depth effect changes in microfossil assemblages as well as trace fossils. Beside water depth, which is a typical complex environmental gradient composed of the single factors water energy, light transparency, and hydrostatic pressure, changes in salinity are mainly caused by restrictions of parts or the whole basins from open seas. Salinity and oxygen content again is best reflected in benthic microfossil assemblages and ichnofossils, which proves the value of community analyses for the reconstruction of paleoenvironments. Planktic organisms like globigerinid foraminifers and nannofossils are important for stratigaphic determination of the sediments. Beside this important role, they can be used determining physical and chemical properties of water masses, e.g. temperature, surface currents, upwelling, alkalinity etc. The subject of this study will be the interaction between marine current systems, sediment distribution, ichnofacies, and microfossil assemblages, and the influence of geodynamic processes in the Alpine-Carpathian thrust belt.

The geodynamic history of the marine Eastern Alpine Miocene is characterized by a complex system of basins, their deepening, shallowing and opening of seaways. Basin development is connected during the Early Miocene with the evolution of the Alpine Carpathian foredeep, while in the Middle Miocene intra-mountain basins were developed in the Alpine Carpathian nappe system and sedimentation occurred mainly in the Vienna and Styrian Basins. Restriction of these basins from the open sea started in the Sarmatian leading to brackish conditions distinguished by endemic faunas in the Pannonian. The project concentrated on two subjects, the development of the microfauna (especially Foraminifera), nannoflora (calcareous microfossils) and the distribution of ichnofossils (lebensspuren) during the marine Miocene in Austria and adjacent countries (Slovakia, Slovenia). Lower Miocene outcrops and sections from the Egerian to the Ottnangian were mapped and sampled, but the main work focused on the transition from the Lower to the Middle Miocene (Karpatian - Badenian). Karpatian sediments were studied in the Molasse (Northern Lower Austria) and the Styrian Basin, where many outcrops and drillings could be investigated. The main result was the detection of cooler conditions in the Karpatian compared to the following Lower Badenian, which is expressed in planktic and benthic foraminifers, coccoliths (nannoflora), and supported by stable isotope investigations done in a separate project of the project bundle `Changes in Eastern Alpine Miocene Ecosystems, related to geodynamic events`. A new method of paleodepth estimation based on depth ranges of species was developed and could be applied in regions, where common paleodepth estimation methods like the plankton/benthos-ratio failed. The cooperation between different projects in the cluster was successful, because large sedimentation gaps that are not mirrored in the profils could be detected in the uppermost Karpatian and lowermost Badenian of the Styrian Basin due to the sudden changes in the microfaunal and nannofloral content, but primarily calibrated by magnetostratigraphic dating. These results become fundamental for the new edition of the volume on the Karpatian. Cooperation with project members working on mollusks resulted in various publications about the Lower Badenian, especially investigations in the Molasse Basin of Lower Austria. The comprehensive results about the so-called Grund Formation, containing sedimentology, ichnology, nannoflora, foraminifers, ostracods, bivalves, gastropods, and vertebrates, will be published soon as special issue in an international, peer-reviewed journal.