Preprints
https://doi.org/10.5194/egusphere-2024-683
https://doi.org/10.5194/egusphere-2024-683
14 May 2024
 | 14 May 2024
Status: this preprint is open for discussion.

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

Fritz Schlunegger, Edi Kissling, Dimitri Tibo Bandou, Guilhem Amin Douillet, David Mair, Urs Marti, Regina Reber, Patrick Fabian Schläfli, and Michael Alfred Schwenk

Abstract. This work summarizes the results of an interdisciplinary project where we aimed to explore the origin of overdeepenings or tunnel valleys through a combination of a gravimetry survey, drillings, dating and a synthesis of previously published work. To this end, we focused on the Bern area, Switzerland, situated on the northern margin of the European Alps. In this region, multiple advances of piedmont glaciers during the Quaternary glaciations resulted in the carving of the main overdeepening of the Aare River valley (referred to as Aare main overdeepening). This bedrock depression is tens of km long and up to several hundreds of meters to a few kilometers wide. We found that in the Bern area, this main overdeepening is made up of two >200 m-deep troughs that are separated by a c. 5 km-long and up to 150 m-high transverse rocky ridge, interpreted as a riegel. The basins and the riegel are overlain by a >200 m- and 100 m-thick succession of Quaternary sediments, respectively. The bedrock itself is made up of a Late Oligocene to Early Miocene suite of consolidated clastic deposits, which are part of the Molasse foreland basin, whereas the Quaternary suite comprises a middle Pleistocene to Holocene succession of glacio-lacustrine gravel, sand and mud. A synthesis of published gravimetry data revealed that the upstream stoss side of the bedrock riegel is c. 50 % flatter than the downstream lee side. In addition, information from >100 deep drillings reaching depths >50 m suggests that the bedrock riegel is dissected by an anastomosing network of slot canyons. We propose that these canyons established the hydrological connection between the upstream and downstream basins during their formation. Based on published modelling results, we interpret that the riegels and canyons were formed through incision of subglacial meltwater during a glacier’s decay state, when large volumes of meltwater were released. Such a situation has repeatedly occurred since the Middle Pleistocene Transition approximately 800 ka ago, when large and erosive piedmont glaciers began to advance far into the foreland. This resulted in the deep carving of the inner-Alpine valleys, and additionally in the formation of overdeepenings on the plateau on the northern margin of the Alps.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
Fritz Schlunegger, Edi Kissling, Dimitri Tibo Bandou, Guilhem Amin Douillet, David Mair, Urs Marti, Regina Reber, Patrick Fabian Schläfli, and Michael Alfred Schwenk

Status: open (until 25 Jun 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
Fritz Schlunegger, Edi Kissling, Dimitri Tibo Bandou, Guilhem Amin Douillet, David Mair, Urs Marti, Regina Reber, Patrick Fabian Schläfli, and Michael Alfred Schwenk
Fritz Schlunegger, Edi Kissling, Dimitri Tibo Bandou, Guilhem Amin Douillet, David Mair, Urs Marti, Regina Reber, Patrick Fabian Schläfli, and Michael Alfred Schwenk

Viewed

Total article views: 121 (including HTML, PDF, and XML)
HTML PDF XML Total BibTeX EndNote
78 40 3 121 1 1
  • HTML: 78
  • PDF: 40
  • XML: 3
  • Total: 121
  • BibTeX: 1
  • EndNote: 1
Views and downloads (calculated since 14 May 2024)
Cumulative views and downloads (calculated since 14 May 2024)

Viewed (geographical distribution)

Total article views: 116 (including HTML, PDF, and XML) Thereof 116 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
1
 
 
 
 
Latest update: 19 May 2024
Download
Short summary
Overdeepenings are bedrock depressions filled with sediment. We combine the results of a gravity survey with drilling data to explore the morphology of such a depression beneath the city of Bern. We find that the target overdeepening comprises two basins >200 m deep. They are separated by a bedrock riegel that itself is cut by narrow canyons up to 150 m deep. We postulate that these structures formed underneath a glacier, where erosion by subglacial meltwater caused the formation of the canyons.