title: The Bümpliz trough sedimentary succession: a key to unravel the history of Mid-Pleistocene ice cover and lake formation in the overdeepened Aare Valley (Bern, CH)
Creators: name: Schwenk, Michael Alfred
Publisher: name: Universität Bern
Year: 2022
Subjects: subject: 550 Earth sciences &amp; geology

description: During the Quaternary glacial–interglacial cycles, glaciers repeatedly advanced from the Alps into their surrounding lowlands. These glaciers left an imprint on the Alpine landscape that is reminiscent of the mass and thickness of the once present ice. The immense impact of glacial erosion is documented by the formation of overdeepened bedrock troughs in and around the Alps. The prevalent landforms and deposits found in the present-day landscape were formed during the last glaciation, and, therefore, the last glacial cycle is well understood. However, the recognition of the number and the timing of older Alpine glaciations in the surficial geological record is at odds with the globally recognized periods of cooling and ice buildup. In this context, archives of older glacial–interglacial cycles could have been preserved in overdeepened valleys in and around the Alps and where thick sequences of Quaternary sediments accumulated.
In the present thesis I explore the glaciation history of the Middle Aare Valley based on a 211.5 m deep scientific drilling that was sunk into the sedimentary infill of such an overdeepened trough. In the Bern area, where the overdeepened trough is located, the advances of the Aare Galcier during the past 300 kyr have apparently been accompanied by the formation of a paleo-lake that had reached out onto the Swiss Plateau. However, the conditions under which this lake had formed have not been well explored yet. In the first part of the thesis I report the results of the sedimentological analysis conducted on the recovered deposits. A depositional model is developed that comprises two glacial–limnic sequences distinguished in the sedimentary succession. This model outlines the formation of till during two glacial advances into the investigated overdeepened trough and, after the recession of the ice, the sediment accumulation in a lake. Eventually, the basin was filled and fluvial deposits accumulated thereafter. This transition from a limnic into a fluvial setting is used to infer a regional paleo-base level that is probably tied to the paleo-lake referred to above. Furthermore, a minimum age for the encountered sediment is derived from feldspar luminescence measurements (250–300 ka). The second part of the thesis revolves around a provenance analysis. Potentially, the sediment in the Bern area could have been supplied by two major glaciers, the Valais Glacier and the Aare Glacier. I employ the sediment bulk chemical composition to fingerprint four source rock units in the central Swiss Alps, where both glaciers originated. These fingerprints are then used to determine the most probable source of the sediment in the core. The results of this provenance analysis show that the Aare Glacier had dominated the Bern area during the formation of the investigated sedimentary suite. In the third chapter, the depositional age of the sedimentary sequence is investigated through isochron burial dating based on cosmogenic 10Be and 26Al. This is the first application of cosmogenic nuclide dating on sediments recovered from depths > 80 m in an overdeepened trough around the Alps. The obtained nuclide concentrations are rather low and yet the nuclide ratios are exceptionally high, reaching up to 19. Nevertheless, a burial age of the lower half of the recovered sedimentary suite is calculated. The age of 320±250 ka is in good agreement with the previously reported luminescence age, and both ages indicate that the entire sequence was probably deposited during a single glaciation with two distinct glaical advances.