High-resolution stratigraphy from the continental record of the Middle Miocene Northern Alpine Foreland Basin of Switzerland

We present a new chronology for the Middle Miocene continental record of the Northern Alpine Foreland Basin of Switzerland. The investigated sediments belong to the Upper Freshwater Molasse (OSM) and reach a maximum thickness of similar to 900 m. The sediments consist mainly of alluvial fan and fluvial deposits, but lake and pond environments are occasionally also documented. The investigated sections contain highly fossiliferous horizons with very diverse and characteristc faunas. The integration of mammal-based biostratigraphy, magnetostratigraphy and radiometric age data allow an excellent high-resolution chronostratigraphic framework for the OSM deposits to be established. Approximately 40,000 mammal teeth from screenwash collections derived from more than 60 localities form the basis of our study. Thirteen characteristic local mammal assemblages within the OSM have been recognized, covering the European Neogene mammal (MN) units from MN 4 to the base of MN 9 (ca. 17.5-11 Ma). Since nearly all key-species of the European MN-reference localities are recorded in the research area, they enable a reliable correlation to the MN-scheme. The biozonation proposed here is constrained by magnetostratigraphic sections, radiometrically dated volcanic ash layers and other local and regional stratigraphic marker beds. The European reference localities of MN 4 to MN 7+8 are correlated into the local faunal succession presented here according to the MN-zonation proposed by MEIN (1975, 1989). As a result, the distribution of the reference localities within the chronologic time scale is very asymmetric. The reference localities Sansan (France, MN 6), Steinheim (Germany, MN 7), and Anwil (Switzerland, MN 8) probably occurred within only 0.5 Myr, whereas the reference localities Pontlevoy (France, MN 5) and Sansan are separated by approx. 1.4 Myr. Since nearly all key-species of the European MN-reference localities are recorded within our research area, we aim to establish precise time brackets and/or boundaries between the European MN-units valid for Central Europe. We also attempt a correlation to other well-established local mammal zonations in Spain and southern Germany. Both the Swiss and Spanish mammal zonations are well elaborated and magnetostratigraphically constrained. Our biostratigraphic data presented here agree well with those of Germany (Bavaria) and Austria from the eastern part of the Molasse basin. The numerical ages of the MN-zones, however, differ considerably by ca. 0.5-1.0 Myr from the Spanish records. Until now, differences in the timing of the MN-zones have been explained by strong differences in the local faunal succession (provincialism) and/or different interpretation of MN-zones. It is suggested, however, that major faunal events used for the MN-zonation (e. g. the first occurrence date, FOD, of modern cricetids) are highly diachronous from Central to Western Europe (Spanish bioprovince). The diachrony of the FOD of modern cricetids is probably caused by their east-west immigration, whereas the diachronous extinction of the eomyid genus Ligerimys appears to have been caused by a local environmental and climatic change toward a more open and less humid habitat.