OTHERWISE INACCESSIBLE: RECOVERING 208.5 M OF MID-PLEISTOCENE GLACIAL DEPOSITS THROUGH DRILLING

Within the reach of mountain glaciers, Earth’s surface is dominated by Holocene and Late Pleistocene glacial deposits and landforms, which were formed during the last and the penultimate glaciation. Sedimentary records from Mid-Pleistocene glaciations are more likely preserved in glacial overdeepenings. Overdeepenings are local bedrock troughs with their thalweg below the current fluvial base level and they offered accommodation space after glacier retreat. In the Swiss Alps, such overdeepenings preserve material that has been deposited at least since the Marine Isotope Stage 11 (MIS 11; 424–374 ka). We drilled into a terminal branch of such an overdeepening situated in the northern Alpine foreland (Aare overdeepening near Bern, central Switzerland) to better understand the glaciation history and the landscape evolution in this region. Our drilling reached the bedrock and thereby recovered the complete sedimentary sequence. Within the 208.5 m-long sedimentary core, we identified twelve lithofacies, grouped them into five facies assemblages, and distinguished two major depositional sequences A (lower) and B (upper).

Either sequence starts with a basal subglacial traction till, thus recording a phase of ice cover. The till sections are overlain by thick assemblages of subaqueous, most likely glaciolacustrine and lacustrine facies elements. The glacial till is enriched in material that is derived through erosion of the surrounding Molasse sandstones that constitute the bedrock in the region. We interpret the (glacio)lacustrine facies of sequence A as deposits of an ice-contact fan setting. In contrast, the (glacio)lacustrine facies of sequence B are considered to record a less energetic prodelta environment that gradually developed into a delta plain. Towards the top, sequence B evolves into a fluvial system recorded in sequence C, when large sediment fluxes of a possibly advancing glacier resulted in a widespread cover of the region by a thick gravel unit.

Feldspar luminescence dating on two samples from a sand layer at the top of sequence B provided uncorrected ages of 250.3±80.2 and 251.3±59.8 ka. The combination of these ages with lithostratigraphic correlations of sedimentary sequences encountered in neighboring scientific drillings suggests that sequence B was deposited between MIS 8 (300–243 ka) and MIS 7 (243–191 ka). This depositional age marks the end of a major stage of an overdeepening–fill in the Bern area in central Switzerland.