Succession des faunes d'Ammonitesdu Sinémurien et du Pliensbachien dans les Préalpes médianes de Suisse romande (Vaud et Fribourg)

Abstract

The biostratigraphy and paleontology of the Sinemurian and Pliensbachian ammonite faunas from the Median Nappe (Alpine Front Ranges, Vaud and Fribourg, Switzerland) allow us to recognize 15 biostratigraphical horizons. Correlations between these and the Middle European standard zonation are easy. The Upper Sinemurian faunal associations from the studied area are very close with those from the eastern eart of the Median Nappe (Langenegggrat, Thun area, Switzerland), and from the southern part (Chablais, France) and klippes of Savoy, (France). Similar comparisons are not get possible for Lower Sinemurian and Pliensbachian because ammonites of this age are only known within the area studied in the present paper. As in the Langenegggrat (Berner Prealps) and in Chablais (Savoy) our ammonite fauna are usually associated with condensed or reduced sedimentation. Nevertheless, it is still possible to obtain accurate biostratigraphical data. Apart a new Tethyan Asteroceras, A. meridionalis sp. nov., and rare Pliensbachian Phyllocerataceae, the fauna is composed of either endemic North-West European taxa, or ubiquitous species. So, from a paleobiogeographical point of view, the studied ammonites show the strongest affinities with the Middle European faunas. The scarcity of clearly Mediterranean species emphasizes this feature. This statement suggests that a physiographical and/or ecological context controled a major faunal discontinuity (North-West European versus Tethyan fauna) and, this individualized the future internal alpine zone (Briançonnais and Piémont areas) during the Lias. Moreover one new species, Asteroceras meridionalis is descibed here.