Ammonite biostratigraphy on the platform–slope transition between the Vercors Urgonian platform and the Vocontian Trough (S–E France)

This study presents a high-resolution biostratigraphic analysis of a nearly 30 km long platform-to-slope transect on the southern margin of the Vercors Urgonian platform, based on detailed ammonite data. The primary objective is to refine the chronostratigraphic framework of the upper Barremian Urgonian series and to reassess the timing and significance of marlstone intercalations within the carbonate succession. For this purpose, more than 100 ammonite specimens were collected along the transect at various stratigraphic levels. All standard ammonite biozones and most subzones of the upper Barremian have been confidently identified, allowing precise dating of key sedimentary and stratigraphic events. A central issue concerns the platform-to-basin correlation of Paquier’s so-called “Heteroceras marlstone”, which conflates two distinct marly intercalations of different ages under a single term. We distinguish two major marlstone complexes – (1) Ambel / Font Froide – La Béguère and (2) Pas de la Couronne / La Révolte / Lower Orbitolina Beds – that have been variously attributed to, or confused with, the Heteroceras marlstone by Paquier, his followers, and his detractors. Based on ammonite biostratigraphy, only the second complex can be confidently correlated with the Heteroceras marlstone. Together with the Upper Orbitolina Beds, these intervals are interpreted as recording three successive ingressions, the final one marking the ultimate demise of the Urgonian platform near the Barremian–Aptian boundary. This revised biostratigraphic framework clarifies long-standing stratigraphic ambiguities, strengthens regional correlations, and provides new insights into the timing and mechanisms of Lower Cretaceous carbonate platform evolution in the western Tethys.