Relationships between karst and tectonics: case-study of the cave system north of Lake Thun (Bern, Switzerland)

Abstract

The cave system is situated north of Lake Thun, in the Helvetic border chain. The overall geology is simple: the slightly dipping (15–25° towards the southeast) strata are interrupted by a NE–SW trending normal fault with a throw of 150 m in the NE and about 500 m in the SW. Since a part of the region is covered by flysch, the caves are the only way to observe the geological setting of the underlying Cretaceous and Eocene series. We show that observations in caves may yield valuable information about the onset of the tectonic movements: in particular, observations in the Bärenkluft region clearly demonstrate that the beginning of prealpine extension had already begun in the Upper Cretaceous, and that this normal fault has been inverted later during Alpine compression. We also illustrate the influence of tectonic stress and strain upon karstification. The Alpine tectonic phases, with alternating compression and extension, contributed to the development of different karstogenetic levels. Tectonic strains opened and possibly closed some fractures, allowing (or preventing) water to flow through parts of the karst massive. The structural setting, defining the overall geometry of the limestone bed, played an important role in the development of the various phases of the system. Most of the conduits appear to belong to old, deep phreatic systems. Tectonics is only one of a number of factors controlling karstification. Together with lithology, it represents the geological control. Geomorphological factors (mainly spring and catchment positions, but also erosion of the flysch cover), as well as bioclimatical factors (quantity and physico-chemical characteristics of water), and hydrodynamics and transport processes can play a significant role on the genesis of karst systems.