Age and geographic distribution of the youngest Upper Freshwater Molasse (OSM) of eastern Switzerland

Systematic field work within the Upper Freshwater Molasse (OSM) of Eastern Switzerland (Cantons of Zurich, Thurgau and St.Gallen) led to the discovery of a fine grained breccia equivalent to the Hornli-breccia. This layer is used as a local lithological correlation horizon within the youngest Hornli Molasse. New findings of small mammals help to better constrain the age of the selected profiles, to characterize them, and to attempt correlations. A map with the distribution of the youngest OSM in Eastern Switzerland is p resented here. There are indications that N of the Schauenberg and NE of Wi l, sediment thickness decreases notably, at least in the younger parts of t he OSM. Along the line Schneitberg (ZH)-Wellenberg (TG)/Imenberg (TG)-Otten berg (TG) a depression exists with erosional relies being preserved up to a t least MN8, even below an elevation of 700 m above sea level. Faunas with an unambiguous age of MN9 have not been reported until now. If ever, MN9 would be expected in the layers at higher altitude of Ottenberg, Wellenberg, Altenberg and in the uppermost Schnebelhorn area. In the latter fossiliferous marls are extremely rare. E of the Bernrain-Amriswil region, up to now there are not enough paleontological data for or against normal faulting of a possible Bodenseegraben. The same is to be reported for the h ypothetic existence of a "Thurgraben" as proposed by previous authors.