Where are the lake deposits? Two atypical overdeepening fills from the Rhine Glacier lobe in southwestern Germany

During the Pleistocene, the Alpine landscape and its surroundings were repeatedly reshaped by glacial cycles. Subglacial erosion carved out overdeepened basins into bedrock. Their sedimentary fillings are important archives for understanding the glacial impact on environmental transformation and the history of the Alpine region. Some of these infills are composed of multiple cycles representing separate glaciations; in typical cases, each is represented by a succession of lake deposits. While sediments of earlier glaciations are often eroded by subsequent glaciation cycles, these can be preserved in overdeepened troughs. The International Continental Scientific Drilling Program (ICDP) project “Drilling Overdeepened Alpine Valleys” (DOVE) investigates overdeepened structures on a larger scale by recovering sediment cores around the foreland of the European Alps. As part of the DOVE venture, we present two high-quality drill cores highlighting different basin generations from within the former Rhine Glacier domain of Upper Swabia in southern Germany. The first drilling ICDP 5068_6 near the village Gaisbeuren is located in the low-lying central part of the Rhine Glacier area. We subdivided the 144 m long ICDP 5068_6 core into four sedimentological lithotypes that, in seven stratigraphic units, build up the basin infill. The deposits reveal a glacially sheared bedrock horizon with overlying diamict layers, diamictic fines, and massive sands in alternating sequences. The sedimentary deposits show structureless sections attributed to melt-out, as well as presumed glacio-tectonic deformations. At the hamlet of Lichtenegg, a 76 m long core was recovered from a drill site on the Höchsten butte, targeting the correspondent small-scale Lichtenegg basin. Due to the basin's exceptional, elevated stratigraphic position underlying presumed Early Pleistocene (glacio-)fluvial gravel, it was included in the DOVE project as a bonus site. Based on petrographic and lithological features we divided the core contents into six sedimentological lithotypes and five lithostratigraphic units. The lowermost parts of the infill, which overlie the Molasse bedrock, consist of (sub-)glacial diamicts with glaciolacustrine to glaciodeltaic sediments followed by the (glacio-)fluvial gravels, with additional glacial diamicts on top. Therefore, both investigated cores differ strikingly from the above-mentioned, “typical” lacustrine-dominated sediment successions in overdeepened basins, which we attribute to a continuously ice-proximal setting during infilling. This study emphasizes the variability of overdeepening fills depending on the specific location within the basin and relative to the glacier front.