Bioevents and palaeoenvironmental changes in carbonate platforms: The record of Barremian “Urgonian” limestones of SE France
Abstract Biostratigraphic studies of Barremian platform carbonates from SE France, and nearby regions indicate the Last Occurrence (LO) horizons of some common shallow water taxa. Firstly two dasycladale algae: Piriferella paucicalcarea and Salpingoporella genevensis , which have their first occurrence in the upper Hauterivian disappear in the lower Barremian, at the Nicklesia pulchella – Kotetishvilia compressissima transition. Secondly a rudist bivalve: the genus Agriopleura , which has its first occurrence in the upper Hauterivian has its LO in the lower upper Barremian, within the Gerardhtia sartousiana zone. The specific stratigraphic levels of the above bioevents recorded in platform carbonates may be used for dating corresponding ammonite free-successions. Stratigraphic implications are constrained by the biogeographical extent of the key species which is essentially the Western and Central European margin of the Mediterranean Tethys. The joint LO of P. paucicalcarea and S. genevensis are inferred to have been linked with oceanographic and climatic changes, i.e. a cooling event. Platform carbonates were interrupted by a drowning event represented by a Maximum Flooding Surface or Transgressive surface. Shallow water Orbitolinidae record significant extinctions and deep water ammonites show a significant turnover. Similarly, modifications in ammonite faunas, bottom currents, temperature changes, i.e. cooling, and a major extinction among the family Orbitolinidae coincided with the LO of Agriopleura . Platform perturbations, including exposure, possibly a sequence boundary, and basin margin instability, both with a tectonic background, were associated with this event. The foregoing events and the linkage of palaeobiogeographic changes, that is the migration of the key taxa towards southern latitudes, in correspondence with the LO bioevents indicate that thermal changes, including seasonality, appear to have been the prominent controlling factor.