Overdeepenings in the Bern region, Switzerland: Understanding their formation processes using 3D gravity forward modelling

Over the last 2.6 million years, during the Pleistocene, numerous glacial and interglacial periods have succeeded each other. This resulted in a repeated sculpting the bedrock due to the action of glaciers. The newly formed or reshaped valleys were then filled with lakes and sediments. Beneath these valleys, deep and steep structures can be found. These troughs are called tunnel valleys or overdeepenings. The mechanisms behind their formation are poorly understood as today the bedrock of formerly glaciated landscapes is covered by hundreds of meters of sediments and/or water.

The aim of this thesis is to investigate the mechanisms that led to the formation of tunnel valleys in the Swiss foreland near the city of Bern. To this end, I combine information about the density contrast between the Quaternary fill and the bedrock together with borehole data and the results of gravity surveys along 11 profiles across the Aare valley. This data is then designed to be the input of PRISMA, one of the routine of Gravi3D, a 3D forward modelling software which I developed for this project.

In the first part of this thesis, I present the proof of concept of our method, with its application for two overdeepened valleys to the South of Bern: the Gürbe and Aare valleys. I show how gravity data combined with density contrast and depth information allowed us to derive a U-shaped cross-sectional geometry for these two valleys. A central part of this proof of concept is the application of the PRISMA routine, which allowed to forward model the measured gravity signal, thereby disclosing the cross-sectional geometries outlined above.

The second part of the thesis focuses on the improvement made to the method and to the workflow including steps from the field to the model.

The third part and core of my thesis focuses on the application of the now refined method and on the investigation of the geometry of the Aare valley's overdeepening network. This is documented with 10 gravity profiles. The results indicate an evolution of the geometry, from upstream of the city of Bern to downstream of it, evolving from an asymmetrical U-shaped overdeepened valley to a trough with an upper U-shaped section and a V-shaped lower part. Interestingly, within the city of Bern, the U-shaped profiles disclose the occurrence of one or multiple inner gorges of a few tens of metres in width but of hundreds of meters in depth. These inner gorges occur beneath the overdeepenings. Such features and the V-shaped geometries are clear evidence for water-based erosion that would have taken place either before or during the formation of the overdeepenings. This indicates that downstream and within the city of Bern, features of water-based erosion have been preserved, while upstream recent glacial processes likely overprinted such traces.

Finally, in the last chapter of this thesis, I explore the possibilities and options to further explore the complex geometry of the overdeepening network and the possible application of BGPoly (Bouguer Gravity Polygons), which is the second routine of Gravi3D.