Glacial erosion and subsequent shallowing-up sequence, evidence for two glacial advances into the overdeepened Aare Valley, Switzerland

A scientific drilling was conducted into a bedrock trough in Bern-Bümpliz, a branch of the Aare Valley overdeepening (Switzerland). It is the first scientific drilling in the Bern area that reached the bedrock. We analyzed the 208.5 m-thick succession of Quaternary sediments recovered in this scientific drilling and present the sedimentological results of the campaign. In the retrieved sediments 12 different lithofacies were identified, which were grouped into 5 facies assemblages, and 2 major sedimentary sequences (A = lower, B = upper), which transition into a minor sequence C. Generally, the sedimentary successions of sequences A and B are similar. The lowermost facies assemblage of each sequence consists of a till that was deposited during a period of ice cover. However, the two tills differ from each other. In particular, while the till at the base of sequence A is dominated by large clasts derived from the underlying Molasse bedrock, the till at the base of sequence B has no such Molasse components. Furthermore, the till in sequence A bears evidence for glaciotectonic deformations. Both tills are overlain by thick facies assemblages of subaqueous, most likely glaciolacustrine and lacustrine sediments. Sequence A is characterized by cross-bedded and steeply inclined sand, gravel and diamictic beds which we interpret as deposits of density currents in a subaqueous ice-contact fan system in a proglacial lake. In contrast, the lacustrine sediments in sequence B are considered to record a less energetic environment where the material was most likely deposited in a prodelta setting that gradually developed into a delta plain. Towards the top, sequence B evolves into the fluvial system of sequence C, where large sediment fluxes of a possibly advancing glacier resulted in a widespread cover of the region by a thick gravel unit. Additionally, feldspar luminescence dating was performed on two samples from a sand layer at the top of sequence B. The dating in combination with lithostratigraphic correlations with the sequences encountered in the neighboring scientific drillings to the north (Meikirch) and south of Bern (Thalgut) suggests that sequence B was deposited during Marine Isotope Stage 8 (MIS 8; 300–243 ka).