Middle triassic muschelkalk in northern Switzerland – stratigraphy, depositional history and synsedimentary tectonics

In Middle Triassic times, up to 200 m-thick evaporites and up to 90 m-thick shallow-marine carbonates accumulated during the Middle and Upper Muschelkalk, respectively, in northern Switzerland over a period of 3.4 Myr. These sediments were deposited under arid conditions in the southernmost part of the epicontinental Central European Basin. Deposition was mainly controlled by the inflow of sea water from the Tethyan realm in the south and a subsidence pulse during Anisian times forming the accommodation space. Because of their marginal position in the basin, direct and detailed correlation of Muschelkalk sediments of northern Switzerland with equivalent strata in interior parts of the basin is not straightforward. However, facies analysis, high-resolution regional correlations of gamma-ray logs as well as sequence stratigraphic considerations provide a detailed stratigraphic framework of the Middle Muschelkalk sediments in the study area and form the base to correlate and to compare them with equivalents in more interior parts of the basin and to analyze apparent variations in sediment thickness in detail. Locally, dissolution was recognized that could be dated to Middle Triassic times and that was likely related to subsurface fluid flow along deeper seated faults. In combination with regional thickness variation, the dissolution phenomena suggest a tectonically active depositional setting in the Middle Triassic. Differential subsidence during the Middle and Upper Muschelkalk in the study area led to the formation of several depocenters, bounded by pre-existing Paleozoic faults, which became reactivated during the Middle Triassic under an E–W extensional stress regime. During Middle Muschelkalk times, short-term local increases in accommodation space were rapidly compensated by high sedimentation rates and decreasing tectonic subsidence during Upper Muschelkalk times led to laterally rather uniform facies, which can be correlated over the entire study area. These facies were merged into five lithostratigraphic members and five beds of the newly introduced Schinznach Formation.