Sedimentological and palynological constraints on the basal Triassic sequence in Central Switzerland

In Central Switzerland, Mesozoic sedimentation began after erosion and peneplainisation of the Hercynian relief and late Paleozoic continental deposition in SW-NE striking pull-apart basins. The first Triassic sedimentary sequence overlaying a weathered crystalline basement consists of a relatively thin (<10 m), lithologically highly variable unit with coarse-grained siliciclastic deposits at the base, grading into a mixed sandstone/shale-dolomite sequence followed by well-bedded dolomites with chert nodules.Sedimentary texture analyses and petrological investigations revealed four different sedimentary units starting at the base with a regolith unit that represents the weathered crystalline basement. It is overlain by terrestrial plain deposits, followed by mixed siliciclastic-carbonaceous sediments and a sequence of dolomites, deposited between the supralittoral and eulittoral zones of a tidal flat (Mels-Formation), and the eulittoral to sublittoral zones of a carbonate tidal flat environment (Röti-Dolomit), respectively.Palynological data from four localities in Central Switzerland indicate a heterochronous early Anisian age (Aegean – Bithynian/Pelsonian) for the supra- to eulittoral mixed siliciclastic-carbonaceous sediments. These new biostratigraphic ages suggest that the first Triassic marine transgression in Central Switzerland is time equivalent with those of the basal Wellendolomit in Northern Switzerland but slightly older than in the Germanic Basin. Consequently, Central Switzerland was located at this time at the northern shoreline of the Tethys and not on the southern limit of the Germanic Basin.