Purbeck beds (Late Jurassic) in the Phare de Chassiron section (Île d'Oléron, NW Aquitaine Basin, France): refined age-assignment and long-term depositional sequences.
Résumé
In western european basins, lowering of relative sea-level during the Late Jurassic-Early Cretaceous led to the widespread deposition in numerous basins of so-called Purbeck facies. Age-assignment and longdistance correlations are difficult to establish in such shallow marine to continental deposits, where ammonites are generally lacking. This paper aims to present new biostratigraphical and sedimentological data from the Phare de Chassiron section, located at the western part of the Aquitaine Basin (SW France), where Purbeck beds crop out. Marine bands, interfingered within the Purbeck beds, allow refining the age-assignment of these beds using dinoflagellate cysts, calcareous nannofossils and magnetostratigraphy. Most of the Purbeck beds appear to have a Tithonian age. Uppermost beds of the section, although not precisely dated, are not younger than the Early Berriasian. Field sedimentology, palynofacies and trace-element data (strontium) on carbonates, allow depositional environments to be characterized and a sequence-stratigraphy scheme to be established. This section provides an unique, continuous record, within the Aquitaine Basin, of the final exposure, during the Tithonian, of the former Late Kimmeridgian open-marine platform. The Phare de Chassiron may be considered as one of the most important exposures of Purbeck facies of Tithonian age in Europe.