Geometry of overdeepenings obtained through three-dimensional gravity modelling

We investigated the formation mechanism of tunnel valleys, by producing 3D models of bedrock topography using gravimetry. We obtained the cross-sectional geometry of tunnel valleys in the Swiss foreland, near Bern. The combination of information about the densities of the sedimentary fill and of the bedrock together with borehole data and gravity surveys along profiles across the valleys served as input for our 3D gravity modelling software, Prisma. This finally allowed us to model the gravity effect of the Quaternary fill of the overdeepenings and to produce cross-sectional geometries of the overdeepenings. We focused on two sections situated in the Gürbe valley and in the Aare valley. We determined a density of 2’500 kg/m3 for the Upper Marine Molasse bedrock, and with Prisma we obtained a bulk density of kg/m3 for the Quaternary infill. Our gravity surveys across the valleys yielded a maximum residual anomaly of -2.9 mGal for the Gürbe valley and -4.1 mGal for the Aare valley. The application of our Prisma model showed that these anomalies can be explained by Quaternary suites with a thickness of 160 m and 235 m for the infill of the Gürbe and Aare valleys, respectively. The high-resolution information about the cross-sectional geometry of the tunnel valley flanks, from the application of Prisma, allowed us to infer a two-step formation process of the overdeepened trough.  A first glaciation, during MIS 6 or before, would have deepened the trough. And a second glaciation, during the Last Glacial Maximum  (MIS 2), would have widened the valleys. We explain this pattern by the differences between the ice thicknesses of the LGM and MIS 6 glaciers and by the relatively low erodibility of the Upper Marine Molasse bedrock. The Molasse units indeed comprise tender and porous sandstones and offer a lower erosional resistance than the Quaternary infill, which consists of cohesive and thus competent glacio-lacustrine marls. This probably offered ideal conditions for the thick and thus erosive MIS 6 glaciers to erode deeply into the Molasse bedrock. In contrast, the lacustrine fill of this trough possibly prevented the thinner and thus less erosive LGM or MIS 2 glaciers to further incise the bedrock. The consequence was that erosion of the LGM glaciers mainly occurred on the lateral sides, thereby resulting in a widening of the tunnel valleys. Finally, we apply this approach to the remaining gravity profiles, to create a 3D model of the geometry of the overdeepening network near Bern.