Étude des Couches supérieures à orbitolines / Marnes à Exogyra aquila sur la marge Nord-Ouest de la Téthys, sur les plate-formes provençale, vivaroise, dauphiné, jurasienne, et helvétique, à l’Aptien inférieur : Sédimentologie, stratigraphie, géochimie, et biochronologie.

The end of the Urgonian carbonate platform is marked by the sudden disappearance of an entire oligotrophic ecosystem dominated by rudist, coral and stromatoporoids. On the Ardèche platform (SE France), this crisis is characterized by a regional discontinuity, polyphased, called DFU or DRBeS according to diverse authors. Petrographic and sedimentologic analyses carried out on this boundary and overlying "Exogyra aquila Marls" (Upper Orbitolina beds, Lower Aptian) have permitted to distinguish a karstified surface, whose cavities are commonly filled with continental sediments. Over this surface, is directly superimposed a deep-subtidal hardground, characterized by microbial mats (Stromatolites) that developed on internal platform, whose formation is probably linked to the Aptian transgression. Biostratigraphy by ammonites and orbitolinids allows to date this surface to the Forbesi Zone pro parte. This latter formed during a very short period, and indicates that brief eustatic variations of great amplitude must have occurred early in the Aptian, probably related to large-scaled climatic changes that significantly modified

sediment and nutrient inputs.

The overlying "Exogyra aquila Marls" Formation, extremely well preserved on the Ardèche platform, can be structured into four members that constitute a shallowing-upwards sequence mainly comprised of hemipelagic, shaly limestones and echinoderm-rich limestones. During their deposition, an extensive tectonic activity took place, modifying the local depositional pattern and creating fluctuations within the shaly-calcareous sediments. Thanks to the abundance of ammonites, these sediments were easily dated from Forbesi and Deshayesi Zones.

The "Black Marls" Formation follows the E. aquila Marls, with a new transgression brutally interrupted by the highly erosive regional discontinuity DRBeT. The result of this erosion is a basal phosphatic breccia named Pélican Horizon, marking the base of this formation. The DRBeT discontinuity, as well as the “Horizon du Pélican”, were dated to be of the late Dufrenoyi subzone. This limit shows a significant erosion in the inner platform, along with large dissolved and recrystallized areas whose origin remains unclear.