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.

Efficacy of bedrock erosion by subglacial water flow

“…Glasser and Bennett, 2004), often called Nye channels (N-channels; Weertman, 1972) and kilometre-scale tunnel valleys (e.g. Ó Cofaigh, 1996;Glasser and Bennett, 2004;Jørgensen and Sandersen, 2006;Dürst Stucki et al, 2010;Kehew et al, 2012), and more recently its role has been invoked as a necessary mechanism in the carving and deepening of inner gorges (Dürst Stucki et al, 2012;Jansen et al, 2014).…”

TOPO-EUROPE: Understanding of the coupling between the deep Earth and continental topography

“…10). Detailed studies of the Alpine foreland showed a similar impact of glaciation with deep gorges eroded by sub-glacial fluids beneath the ice sheets from the Alps (Dürst Stucki et al, 2010Stucki et al, , 2012.…”

10Be‐derived assessment of accelerated erosion in a glacially conditioned inner gorge, Entlebuch, Central Alps of Switzerland

“…Korup and Schlunegger (2007) and Montgomery and Korup (2011) suggest that inner gorges throughout the Alps would have been successively preserved and deepened throughout multiple glacial/interglacial cycles. Furthermore, Duerst- Stucki et al (2010; found that subglacial meltwater under pressure created a tunnel valley network under the Aar glacier in the vicinity of Berne, Switzerland. They also used a similar mechanism to explain the formation of inner gorges that cut across conglomerate beds with low erodibilities.…”

“…The erosional work of glaciers also result in the deposition of large amounts of unconsolidated glacial till, which comprises a very wide range of clast sizes. Fluvial processes acting either during glaciations (as subglacial meltwater, Duerst-Stucki et al, 2010 or upon glacial retreat (Montgomery and Korup, 2011) lead to incision into the glacially-sculpted topography. This occurs primarily by the propagation of transient erosional waves (testified by pervasive knickzones), frequently resulting in the formation of inner gorges (Whipple, 2009), where incision interacts with hillslope processes (Gallen et al, 2011), and by the evacuation of unconsolidated glaciogenic material Van den Berg and Schlunegger, 2012).…”