New interpretations on the relationships between lower Barremian calcarenites and rudistid facies in southern Vercors (South-East France)

Th is work is focused on the limestones with rudistids (Monopleuridae and Requienidae of the lower Barremian and the base of the upper Barremian, essentially) of southern Vercors. It is particularly focused on the relationships, which exist between this facies and the Barremian calcarenites of this area. The geological logs we have constructed, and our photographic panoramas show that the pattern of the deposits is repetitive and indicates, for the calcarenites, a decrease of the hydrodynamism, which can be interpreted as the result of an increase of the accommodation. Th e underlying rudistids limestones, which settle before marls, or argillaceous limestones, seem to be included in a transgressive context. It is also shown that: 1) these two deposits are synchronous since they follow each other; and 2) the rudistids limestones, in the studied area, are never protected by a barrier. So, the rudistid limestones of southern Vercors can be considered as deposits settling during a transgressive period.Original Abstract: Ce travail porte sur les calcaires a rudistes (Monopleuridae et Requienidae du Barremien inferieur et base du Barremien superieur, essentiellement) du Vercors meridional. Il s'interesse tout particulierement aux relations qui existent entre ce facies et les calcarenites barremiennes de ce secteur. Les coupes levees et les panoramas realises montrent que le motif de depot des formations concernees est repetitif et indique, pour ce qui concerne les calcarenites, une diminution de l'hydrodynamisme que l'on interprete comme due a une augmentation de l'accommodation. Les calcaires a rudistes sus-jacents, et qui se mettent en place avant des calcaires argileux ou des depots marneux, semblent donc s'inscrire dans une logique transgressive. Il apparait que ces deux types de depot sont diachrones car ils se suivent systematiquement dans le temps, et d'autre part, que les calcaires a rudistes ne sont jamais, dans le secteur considere, proteges derriere une barriere hydrodynamique. Il est donc possible de considerer les calcaires a rudistes du Vercors meridional comme etant des depots s'installant en contexte transgressif.