Subglacial tunnel valleys in the Alpine foreland: an example from Bern, Switzerland

Abstract

The morphology of the Alpine and adjacent landscapes is directly related to glacial erosion and associated sediment transport. Here we report the effects of glacio-hydrologic erosion on bedrock topography in the Swiss Plateau. Specifically, we identify the presence of subsurface valleys beneath the city of Bern and discuss their genesis. Stratigraphic investigations of more than 4,000 borehole data within a 430 km 2 -large area reveal the presence of a network of [200 m-deep and 1,000 m-wide valleys. They are flat floored with steep sided walls and are filled by Quaternary glacial deposits. The central valley beneath Bern is straight and oriented towards the NNW, with valley flanks more than 20°steep. The valley bottom has an irregular undulating profile along the thalweg, with differences between sills and hollows higher than 50-100 m over a reach of 4 km length. Approximately 500 m high bedrock highlands flank the valley network. The highlands are dissected by up to 80 m-deep and 500 m-broad hanging valleys that currently drain away from the axis of the main valley. We interpret the valleys beneath the city of Bern to be a tunnel valley network which originated from subglacial erosion by melt water. The highland valleys served as proglacial meltwater paths and are hanging with respect to the trunk system, indicating that these incipient highland systems as well as the main gorge beneath Bern formed by glacial melt water under pressure.

Unravelling the moisture sources of the Alpine glaciers using tunnel valleys as constraints

“…The valleys dissecting the highlands and windgaps in the mountainous landscape possibly served as pathways for supraglacial meltwater (Dürst Stucki et al ., ). In contrast, the several tens of metres deep tunnel valleys in the Bern area, and also at other sites in the distal foreland, represent pathways of subglacial meltwater under varying pressure conditions.…”

“…We downscaled the 25‐m‐resolution bedrock topography model of Dürst Stucki et al . () to a 10‐m‐wide resolution, thereby considering an additional 4000–5000 drillings. Figure illustrates that the available data are clustered around urban areas, which adds a bias.…”

Estimating the fill thickness and bedrock topography in intermontane valleys using artificial neural networks

“…The European Alps have acquired their distinct surface morphology over the course of several glacial cycles during the Quaternary [e.g., Kelly et al, 2004]. At least in some places, sediment-filled, U-shaped valleys and overdeepenings are thought to have been repeatedly excavated and enhanced during glacial advances, followed by backfilling upon glacial recessions [Dürst-Stucki et al, 2010;Jordan, 2010;Preusser et al, 2010]. Compared to this long-lasting period of glacial sculpting and given millennial-scale Holocene erosion rates of~1 mm yr À1 [Wittmann et al, 2007], the period of deglaciation and paraglacial adjustment after the Last Glacial Maximum [e.g., Schlüchter, 1988;Reitner, 2007] did not erase the inherited glacial imprint on the landscape.…”

Late Miocene thrusting in the North Alpine foreland: Driven by a deep-seated process and shaped by the local mechanical stratigraphy

“…The Aare valley, running across the study area, is characterized by a low relief and is filled by >100 m-thick Quaternary deposits ( Fig. 4; Dürst Stucki et al, 2010). Accordingly, the structural configuration of this part of the study area is only poorly resolved.…”