Cretaceous syn-sedimentary faulting in the Wildhorn Nappe (SW Switzerland)

During Cretaceous time, the area of the future Helvetic nappes (Central Alps, south-western Switzerland) was part of a large ramp-type carbonate depositional system on the European margin, in which the area of the Wildhorn Nappe was transitional to the more distal and relatively deeper Ultrahelvetic basin. The Wildhorn Nappe includes an Upper Cretaceous succession bearing clear evidence for syn-sedimentary normal faulting, such as syn-sedimentary geometries related to well oriented NE-striking faults, sedimentary dykes, lateral variations in the thickness and facies of formations, anomalous and discordant contacts corresponding to palaeo-escarpments, and slump folds. Four stages of syn-sedimentary fault activity have been recognized. (1) Post-Cenomanian disruption and exhumation of the Schrattenkalk platform related to distributed normal faulting, which contributed to the initiation of karst erosion on topographic highs and sedimentation in topographic lows. (2) Turonian-Santonian marine transgression accompanied by localized normal faulting, creating growth-fault structures, differential subsidence and slope instability. A transition from distributed to more localized faulting is observed, related to a final stage in the evolution of the Cretaceous extensional process. (3) Early Maastrichtian faulting. The facies and thickness of subsequent sediments reflect a passive adaption to the pre-existing topography of the sea floor, established during the earlier tectonic movements. (4) Post-Maastrichtian north-directed tilt and erosion. In the Wildhorn Nappe, palaeo-fault activity most probably ended in the Early Maastrichtian rather than continuing into the Eocene. Until now, the regional importance and magnitude of Late Cretaceous extension has not been recognized in the Helvetic domain. This widespread event may be related to post-breakup extensional tectonics along the European margin or, alternatively but less likely, to lateral gravitational collapse of the margin.