LAUSR

ABSTRACT Over‐deepened basins exist throughout the Alpine realm. Improving our knowledge on these basins is of high social relevance, since these areas are often well‐populated and they possess, for instance, unusual hydrological settings. Nonetheless, geophysical and sedimentological investigations of over‐deepened basins are rare. We analyse the sedimentary succession of such a basin, the Tannwald Basin, through geological interpretation of seismic reflection profiles. The basin is located approximately 60 km north of the European Alps. It was incised into Tertiary molasse sediments by the Rhine Glacier and later filled by glacial, fluvial, and lacustrine deposits of 250 m thickness. The Leibniz Institute for Applied Geophysics acquired a grid of five high‐resolution seismic reflection lines that imaged till the deepest parts of the Tannwald Basin. The seismic profiles, processed to a pre‐stack depth migration level, allow a detailed geological interpretation that is calibrated with the help of a nearby borehole. We determine the structure and the seismic facies of the sediment succession in the basin and presume the following hypothesis of the evolution of the basin: sub‐glacial erosion comprises the excavation of the over‐deepened basin as well as detachment of large fragments of molasse material. These molasse slabs were deposited within the basin in a layer of basal till that graded upwards in water‐lain till and fine‐grained deposits. During the last two glaciations, the basinal structure became buried by till sequences and glacio‐fluvial sediments.