Reconstruction of Alpine Cenozoic paleorelief through the analysis of caves at Siebenhengste (BE, Switzerland)

The cave region of Siebenhengste, situated north of Lake Thun (Switzerland), contains one of the most important cave systems in the world, which extends from 500 to 2000 m a.s.l. It has a complex multiphase history. The recognized speleogenetic phases are related to spring level and to old valley floors. The six most recent phases were investigated in St. Beatus cave and Barenschacht. They suggest a progressive Quarternary Aare valley incision to 890, 805, 760, 700, 660, and 558 m a.s.l. that is confirmed by statistical analysis of small caves. U/Th-datings of flowstone allowed a timing of the valley deepening phases: the valley bottom was at 760 m already before 350 ka, the one at 700 m was active between 235 and 160 ka. The cave morphology in the upper part of the cave system was coupled with sedimentological observations. This combination leads to the hypothesis that the uppermost (oldest) cave parts were already created in the Miocene, during and after the last deposition of the Molasse. Ideas about the evolution of the paleorelief suggest that today's Aare valley is a product of glacial erosion, and that the old Aare valley shifted its position several times between the Miocene and today.