Abstract
The geometry of glacial overdeepenings on the Swiss Plateau close to Bern was inferred through a combination of gravity data with a 3D gravity modelling software. The target overdeepenings have depths between 155 and > 270 m and widths between 860 and 2400 m. The models show incisions characterized by U-shaped cross-sectional geometries and steep to over-steepened lateral flanks. Existing stratigraphic data reveals that the overdeepenings were formed and then filled during at least two glacial stages, which occurred during the Last Glacial Maximum (LGM) within the Marine Isotope Stage (MIS) 2, and possibly MIS 6 or before. The U-shaped cross-sectional geometries point towards glacial erosion as the main driver for the shaping of the overdeepenings. The combination of the geometries with stratigraphic data suggests that the MIS 6 (or older) glaciers deeply carved the bedrock, whereas the LGM ice sheet only widened the existing valleys but did not further deepen them. We relate this pattern to the different ice thicknesses, where a thicker MIS 6 ice was likely more powerful for wearing down the bedrock than a thinner LGM glacier. Gravity data in combination with forward modelling thus offers robust information on the development of a landscape formed through glaciers.
Highlights
The geometry of glacial overdeepenings on the Swiss Plateau close to Bern was inferred through a combination of gravity data with a 3D gravity modelling software
Because the sedimentary fill inhibits any direct observations of their geometry, interpretations about their formation have been subject to controversial debates in the literature[16]
Mapping showed that the Belpberg comprises a suite of Burdigalian Upper Marine Molasse (UMM) sandstones with conglomerate interbeds[22] (Fig. 2a)
Summary
The geometry of glacial overdeepenings on the Swiss Plateau close to Bern was inferred through a combination of gravity data with a 3D gravity modelling software. Multiple geophysical methods including seismic, geoelectric, and gravimetric surveys have been employed to disclose such geometries underneath overdeepening fills[18–20] These surveys have been complemented with the results of shallow drillings, which yielded a complex picture of these o verdeepenings[12], yet with a relatively low resolution of their lateral flanks and their bases. This is the case for the Bern region, situated in the Swiss Plateau on the northern margin of the Alps (Fig. 1). We conducted a gravity survey for the area south of Bern (Fig. 1) to reconstruct the cross-sectional geometric details of the lateral flanks and the bottom of the target overdeepenings. We compiled and re-interpreted stratigraphic data of the overdeepening fills to propose a scenario of how the overdeepenings were formed within the framework of Alpine glaciations
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