Overdeepenings in the Swiss plateau: U-shaped geometries underlain by inner gorges

Abstract

We investigated the mechanisms leading to the formation of tunnel valleys in the Swiss foreland near Bern. We proceeded through producing 3D maps of the bedrock topography based on drillhole information and a new gravimetric survey combined with modelling. In this context, the combination of information about the densities of the sedimentary fill and of the bedrock, together with published borehole data and the results of gravity surveys along 11 profiles across the valleys, served as input for the application of our 3D gravity modelling software referred to as PRISMA. This ultimately allowed us to model the gravity effect of the Quaternary fill of the overdeepenings and to produce cross-sectional geometries of these troughs. The results show that 2–3 km upstream of the city of Bern, the overdeepenings are approximately 3 km wide. They are characterized by steep to oversteepened lateral flanks and a wide flat base, which we consider as a U-shaped cross-sectional geometry. There, the maximum residual gravity anomaly ranges between − 3 to − 4 mGal for the Aare valley, which is the main overdeepening of the region. Modelling shows that this corresponds to a depression, which reaches a depth of c. 300 m a.s.l. Farther downstream approaching Bern, the erosional trough narrows by c. 1 km, and the base gets shallower by c. 100 m as revealed by drillings. This is supported by the results of our gravity surveys, which disclose a lower maximum gravity effect of c. − 0.8 to − 1.3 mGal. Interestingly, in the Bern city area, these shallow troughs with maximum gravity anomalies ranging from − 1.4 to − 1.8 mGal are underlain by one or multiple inner gorges, which are at least 100 m deep (based on drilling information) and only a few tens of meters wide (disclosed by gravity modelling). At the downstream end of the Bern area, we observe that the trough widens from 2 km at the northern border of Bern to c. 4 km approximately 2 km farther downstream, while the bottom still reaches c. 300 to 200 m a.s.l. Our gravity survey implies that this change is associated with an increase in the maximum residual anomaly, reaching values of − 2.5 mGal. Interestingly, the overdeepening’s cross-sectional geometry in this area has steeply dipping flanks converging to a narrow base, which we consider as V-shaped. We attribute this shape to erosion by water either underneath or at the snout of a glacier, forming a gorge. This narrow bedrock depression was subsequently widened by glacial carving. In this context, strong glacial erosion upstream of the Bern area appears to have overprinted these traces. In contrast, beneath the city of Bern and farther downstream these V-shaped features have been preserved. Available chronological data suggest that the formation of this gorge occurred prior to MIS 8 and possibly during the aftermath of one of the largest glaciations when large fluxes of meltwater resulted in the fluvial carving into the bedrock.

Overdeepening or tunnel valley of the Aare glacier on the northern margin of the European Alps: Basins, riegels, and slot canyons

“…Whereas such information yielded highly resolved spatial information on the bedrock geometry, particularly on its shallower <50 m-deep part (Reber and Schlunegger, 2016), reconstructions of the details for the deeper and thus central part of the Aare main overdeepening have been thwarted because of a lack of drilling information at that time. Here, we benefit from the results of a recent gravity survey conducted in the city area of Bern and information of new drillings >50 m deep (Bandou et al, 2023; Figure 3c). In particular, Bandou et al (2023) measured the Bouguer gravity anomalies along 10 sections (black lines in Figure 3c).…”

“…Here, we benefit from the results of a recent gravity survey conducted in the city area of Bern and information of new drillings >50 m deep (Bandou et al, 2023; Figure 3c). In particular, Bandou et al (2023) measured the Bouguer gravity anomalies along 10 sections (black lines in Figure 3c). The obtained values were then subtracted from the regional gravity field yielding a residual gravity anomaly value at each site where gravity data was collected.…”

“…These features were used as key information for invoking dissection 75 by meltwater as an important erosional mechanism (Carter and Anderson, 2006;Steinemann et al, 76 2021). Although bedrock swells or riegels were reported as common features in overdeepenings (Gegg 77 and Preusser, 2023), the occurrence of inner gorges or slot canyons (Figure 1) have only recently been 78 disclosed (Bandou et al, 2023). It is the scope of this work to document such structures in an 79 overdeepening and to discuss their importance for our understanding of how such depressions were immediately following a full glacial period during which a piedmont glacier, which extended far into the foreland, started to melt.…”

“…Here, we summarize the results of an interdisciplinary project where we aimed at exploring the origin of tunnel valleys or overdeepenings using a combination of data collected through a gravimetry survey (Bandou et al, 2022(Bandou et al, , 2023, drillings (Reber and Schlunegger, 2016;Schwenk et al, 2022a, b) and dating (Schläfli et al, 2021;Schwenk et al, 2022a). We focus our study on the Bern area situated on the northern margin of the European Alps (Figure 3a).…”

“…We focus our study on the Bern area situated on the northern margin of the European Alps (Figure 3a). For this region, we draw a map of the bedrock structure combining the results of a gravimetry survey in the region (Bandou et al, 2023) with information obtained through drilling. This map shows that an overdeepened trough or a tunnel valley system, referred to as the Aare main overdeepening (Schwenk et al, 2022), is made up of two basins separated by a bedrock riegel, which itself is cut by one or multiple slot canyons.…”