The Mid-Pleistocene landscape history of the Lower Aare Valley with emphasis on subglacial overdeepening

Gegg, Lukas (2021). The Mid-Pleistocene landscape history of the Lower Aare Valley with emphasis on subglacial overdeepening. (Dissertation, University of Bern, Faculty of Science)

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Throughout the most recent part of geological history, glaciers repeatedly built up in the Alps and advanced into the mountain foreland, episodically covering the majority of Switzerland and neighbouring regions with ice. This had severe geomorphic impacts that include the subglacial erosion of overdeepenings. These closed basins are not only a potential source of geohazards, but also contain valuable resources and archives of past environments, and therefore deserve our closer attention. However, our knowledge of i) the erosional mechanisms and the subglacial conditions that lead to the formation of overdeepenings, and ii) of the number, the timing, and the extent of Alpine glaciations, is still very limited.

The present thesis is centred around four scientific boreholes in the Lower Aare Valley in northern Switzerland and addresses the above-mentioned uncertainties. The study area hosts the overdeepened Gebenstorf-Stilli Trough, whose subsurface morphology is the first major focus. It is constrained by borehole and geophysical data that together reveal a complex trough shape controlled by the particular local bedrock architecture comprising rocks of varying subglacial erodibility. The results further highlight the important role of basal water in overdeepening erosion, which is further corroborated in a second case study. There, surficial brecciation of the walls of a paleokarst network in limestone underlying the overdeepening is presented. Following detailed macro- and microscopic analysis, it is interpreted as the result of subglacial hydrofracturing, and thus illustrates the extreme water pressures below the glacier ice.

Finally, the focus is shifted towards the sedimentary archives of the Gebenstorf-Stilli Trough and its surrounding. Based on a multi-method sedimentological approach, the diverse Quaternary deposits are characterised and their depositional history is reconstructed. Several phases of glacial and glaciofluvial reactivation of the major drainage pathways as well as contributions from confluent glaciers are identified. An integration of luminescence data indicates that overdeepening erosion in the Lower Aare Valley dates back to MIS 10 or earlier, and that the local sedimentary record spans large parts of the Middle and Late Pleistocene.

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