The Aare main overdeepening on the northern margin of the European Alps: basins, riegels, and slot canyons

Abstract. This work summarizes the results of an interdisciplinary project where we aimed to explore the origin of overdeepenings through a combination of a gravimetry survey, drillings, and dating. To this end, we focused on the Bern area, Switzerland, situated on the northern margin of the European Alps. This area experienced multiple advances of piedmont glaciers during the Quaternary glaciations, resulting in the carving of the main overdeepening of the Aare River valley (referred to as the Aare main overdeepening). This bedrock depression is tens of kilometres long and up to several hundreds of metres to a few kilometres wide. We found that, in the Bern area, the Aare main overdeepening is made up of two > 200 m deep troughs that are separated by a ca. 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 a ca. 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. In contrast, the Quaternary suite comprises a Middle Pleistocene to Holocene succession of unconsolidated glacio-lacustrine gravel, sand, and mud. A synthesis of published gravimetry data revealed that the upstream stoss side of the bedrock riegel is ca. 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. Apparently, the slot 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. It appears that such a situation has repeatedly occurred since the Middle Pleistocene Transition approximately 800 ka, when large and erosive piedmont glaciers several hundreds of metres thick 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, riegels, and slot canyons on the plateau situated on the northern margin of the Alps.