Stratigraphy and palaeoecology of the Upper Marine Molasse (OMM) of the central Swiss Plateau

Abstract

The lithostratigraphic framework of the Upper Marine Molasse (OMM) in Switzerland is represented by two concepts. One was established in the central-northern Molasse Basin largely based on informal lithostratigraphic units without biostratigraphic control. The other concept was developed in the southern basin part and resulted in the distinction of the Lucerne Formation and the St. Gallen Formation based on detailed lithofacies analysis. This study aims at linking both concepts at map scale 1:25,000 for a potentially basin-wide correlation of the existing lithostratigraphic units. Based on lithostratigraphic analyses, mammal biostratigraphy and on the palaeoecological reconstruction of marine selachian faunas throughout the OMM of the central Swiss Plateau, we were able to allocate this part of the OMM into the well-established Lucerne Formation and St. Gallen Formation (Middle–Late Burdigalian, mammal units MN3a to MN4b). We further define two new lithostratigraphic units: The member Safenwil-Muschelsandstein (Lucerne Formation), and the bed Staffelbach-Grobsandstein (St. Gallen Formation). The Safenwil-Muschelsandstein denominates the well-known marker horizon of shell sandstone deposits, rich in fragments of irregular echinoids (scutella sp.), near the top of the Lucerne Formation. The Staffelbach-Grobsandstein represents a coarse-grained, often pebbly sandstone or even a fine-grained conglomerate at the base of the St. Gallen Formation. In the study area, the distinction between the Lucerne Formation and the St. Gallen Formation is best documented in the composition of the remnant marine fauna (especially teeth of selachii). Throughout the Lucerne Formation, the marine fauna is dominated by shallow marine faunal elements. In contrast, the Staffelbach-Grobsandstein reveals for the first time a large number of faunal elements of the open sea, which is interpreted as an indication of the marine transgression of the St. Gallen Formation. Furthermore, we present a unique palaeoecological study on marine faunas (mostly sharks) throughout the entire OMM sequence of the central Swiss Plateau.