The Late Toarcian to Early Aalenian Opalinuston-Formation in Southern Germany – a sequence stratigraphic approach

The Opalinuston-Formation in Southern Germany comprises a thick sequence of Late Toarcian to Early Aalenian claystones. In parts of Bavaria and Baden-Württemberg, the formation has the potential for hosting high-level radioactive waste. However, in contrast to Switzerland where the Opalinuston-Formation has been studied in detail, there is a general lack of coherent geological data in Germany. We present new data from the research project "Sequence Stratigraphy of the Aalenian in Southern Germany" (SEPIA) on the geological variability of the Opalinuston-Formation based on four drill cores, each of which penetrated through the entire formation, and parts of the over- and underlying rock strata. In order to investigate different depositional environments of the Opalinuston-Formation on a basin scale, the drill cores obtained constitute a transect through the entire Southern German basin in the Middle Jurassic, from the eastern edge of the Black Forest in the Wutach area to the western boundary of the Bohemian Massif in Upper Franconia. Cores were examined in high resolution for their petrophysical properties and element distribution using non-destructive analytical tools such as XRF core scanner and multi-sensor core logger. Furthermore, one sample per core meter was taken for a precise biostratigraphic classification of the rocks and a geochemical-mineralogical and sedimentological characterization. Initial results show that the Opalinuston-Formation in Southern Germany, which appears often homogeneous in macroscopic terms, exhibits notable variability. Particularly with regard to the element distribution (especially Si/Al ratio), a distinct cyclicity becomes visible, which is interpreted as changes in grain size due to relative sea-level fluctuations during deposition. In summary, our preliminary results highlight the geological variability of the Opalinuston-Formation in Southern Germany and allow for a discussion of the possibilities of a sequence stratigraphic approach in order to finally identify the most suitable parts of sub-areas in claystone formations, elsewhere.