Marine ingressions in the Oligocene lacustrine environment of the Limagne rift (French Massif central) : halophilous organisms and strontium isotopes; dating by mammals

In the Oligocene lacustrine deposits of the French Massif central (Sainte-M arguerite section), newly discovered fossils afford more accurately defined biostratigraphy and palaeoenvironmental conditions. Previously known palyn ological and dinocysts frame and new results on calcareous nannofossils, Ch arophyta, Foraminifera, Gastropoda, Ostracoda, mammals and geochemical resu lts lead to a more precise calibration of the section, and to a better unde rstanding 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 simult aneous drops of Sr-87/Sr-86 ratios of chalky laminites toward Oligocene mar ine values were recognized; the palaeogeographic interpretation for these c oncentrations, 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.