From glacial erosion to basin overfill: a 240 m-thick overdeepening–fill sequence in Bern, Switzerland

• Published in: Scientific drilling (Hokkaido, Japan) Vol. 30; pp. 17 - 42
• Main Authors: Schwenk, Michael A, Schläfli, Patrick, Bandou, Dimitri, Gribenski, Natacha, Douillet, Guilhem A, Schlunegger, Fritz
• Format: Journal Article
• Language: English
• Published: Gottingen Copernicus GmbH 25-02-2022; Copernicus Publications
• Subjects: Archives & records; Deformation; Density currents; Drilling; Feldspars; Glacial erosion; Glaciers; Gravel; Ice cover; Isotopes; Lacustrine sedimentation; Lakes; Lithofacies; Outcrops; Quaternary; Sand; Sediment; Sedimentary facies; Sediments; Sediments (Geology); Sequencing; Soil erosion; Topography; Alps; Switzerland; Jura Mountains
• ISSN: 1816-3459; 1816-8957; 1816-3459
• DOI: 10.5194/sd-30-17-2022

We drilled a 210 m-thick succession of Quaternary sediments and extended it 30 m upsection with information that we collected from an adjacent outcrop. In the 240 m-thick succession we identified 12 different lithofacies, grouped them into five facies assemblages, and distinguished two major sedimentary sequences. A sharp contact at 103 m depth cuts off cross-beds in sequence A and separates them from the overlying horizontal beds in sequence B. Although the lowermost facies assemblage of each sequence includes a till deposited during a period of ice cover, the two tills differ from each other. In particular, the till at the base of sequence A is dominated by large clasts derived from the underlying Molasse bedrock, whereas the till at the base of sequence B has no such Molasse components. Furthermore, the till in sequence A bears evidence of glaciotectonic deformation. Both tills are overlain by thick assemblages of subaqueous, most likely glaciolacustrine and lacustrine facies elements. The cross-bedded and steeply inclined sand, gravel, and diamictic beds of sequence A are interpreted as deposits of density currents in a subaqueous ice-contact fan system within 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 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 Marine Isotope Stage 8 (MIS 8; 300–243 ka) and MIS 7 (243–191 ka). This depositional age marks the end of one stage of overdeepening–fill in the perialpine Aare Valley near Bern.