Incursions marines dans l'environnement lacustre du rift oligocene de Limagne (Massif Central, France); apport des organismes halophiles et des isotopes du strontium; datation par les mammiferes

In the Oligocene lacustrine deposits of the French Massif central (Sainte-Marguerite section), newly discovered fossils afford more accurately defined biostratigraphy and palaeoenvironmental conditions. Previously known palynological and dinocysts frame and new results on calcareous nannofossils, Charophyta, Foraminifera, Gastropoda, Ostracoda, mammals and geochemical results lead to a more precise calibration of the section, and to a better understanding of the palaeoenvironment and palaeogeography. The water chemistry fluctuated from freshwater to brackish conditions but direct relationships with the open sea were certainly difficult; however levels where Potamides lamarckii coincide with foraminifera, coccoliths and dinocysts, and simultaneous drops of ⁸⁷ Sr/ ⁸⁶ Sr ratios of chalky laminites toward Oligocene marine values were recognized; the palaeogeographic interpretation for these concentrations, based on bio- and geochemical data, leads to the reinforced assumption of seaways between the different neighbouring marine basins and the Clermont basin due to tectonic and/or transgressive events.