Shaped and filled by the Rhine Glacier: the overdeepened Tannwald Basin in southwestern Germany

The Alpine region was shaped by repeated glaciations during the Quaternary, which led to the formation of overdeepened valleys and basins. These features today, hidden below the present-day land surface, host multiple stacked and nested glacial sequences and offer valuable insight into the environmental history and geomorphological evolution of the region. The project Drilling Overdeepened Alpine Valleys (DOVE) of the International Continental Scientific Drilling Program (ICDP) is dedicated to investigating such overdeepened structures around the Alps. Within DOVE, we here focus on the Tannwald Basin in southern Germany. Situated distally within the area formerly occupied by the Rhine Glacier piedmont lobe; it was shaped by multiple glaciations, yet it is located outside the Last Glacial Maximum (LGM) ice extent. Previous seismic imaging and the presence of interglacial pollen sequences indicate a multi-phase infill history. The complex sedimentary architecture observed in a newly drilled core allows for comparison with seismic data and lithological evidence from other sites. On the basis of a lithofacies model that introduces 17 lithotypes, we propose that the basin fill is composed of three lithostratigraphic units that reflect the glacial history of the basin. After the erosion of the Tannwald Basin, a cold-climate, stacked basin-infill sequence recorded sedimentation of two glacial advances, before it was covered by LGM outwash. The sedimentary record includes an extensive basal glacial shear zone with deformed bedrock and several overlying diamict horizons. Further upcore, deformation structures underscore the role of gravitational processes as well as profound glaciotectonics, deforming the sediment deep within the subsurface. While the sedimentary record indicates a rather rapid infill of the depression, further age constraints and detailed investigations of ice-contact sediments will clarify open questions regarding the temporal classification of the deposits.