The Perte-du-Rhône Formation, a new Cretaceous (Aptian-Cenomanian) lithostratigraphic unit in the Jura mountains (France and Switzerland).

20 pagesInternational audienceDuring the late Early Cretaceous, the Jura domain was occupied by a photozoan carbonate platform, summarised in the Vallorbe Formation. From the earliest Aptian to the Late Cenomanian, open marine heterozoan and detrital marl, carbonate, and sandstone (up to 40 m thick) accumulated on top of the former Urgonian platform, followed during the Late Cretaceous by pelagic chalk and limestone. At the end of the Early Cretaceous, sediment accumulation was interrupted by several emersions and consecutive drowning phases identified in the field by angular discontinuities, iron crusts, and phosphatic conglomerates. Such sedimentary succession is particularly well visible in the present Bellegarde-sur-Valserine Basin of the Ain Department (eastern France). Therefore, in agreement with the Swiss Committee of Stratigraphy, we introduced new lithostratigraphic units to cover the time from the Aptian to the Cenomanian in the Jura mountains based on the type section near the historical outcrop of the Perte-du-Rhône in the Bellegarde Basin. The new Perte-du-Rhône Formation is subdivided in a lower marly unit (Fulie Member), followed by a middle green sandstone unit with phosphatic conglomerates (Mussel Member), which is finally topped by a mica-rich sandstone unit (Poncin Member)