Synsedimentary collapse on a carbonate platform margin (lower Barremian, southern Vercors, SE France)

A large area of synsedimentary collapse covering about 25 km2 has been identified within lower Barremian carbonate platform strata of the southern Vercors (Vocontian Basin, SE France). New observations of the prograding/ aggrading strata of the Cirque d’Archiane and the Glandasse Plateau reveal the presence of a disturbed zone, showing tilted and imbricated blocks as well as slump features. Measurements and analysis on these synsedimentary distorted strata suggest that they compose a slide which slipped on the Barremian slope. The occurence of two coarse bioclastic grainstone beds on the eastern side of the Archiane Valley, in a distal hemipelagic context, suggests that this instability generated a tsunami that reworked proximal bioclastic material, previously transported out to the hemipelagic domain at the same time as the slide, rearranging it under the influence of tractive currents. The idea of high frequency sea-level variations to explain these strata is unlikely. They do not exhibit univocal sedimentologic evidence (such as bundles or herring-bone stratifications) that could indicate sea-level fluctuations. Furthermore, no evidence of major subaerial exposure on the platform top has been reported, neither acceleration of downward migration of platform facies is observed on the slope. It seems more appropriate, considering the palaeogeographical setting of this area, that these grainstone beds represent bioclastic deposits controlled by tractive currents. The origin of the slide could either be sediment loading or tectonics compounding sediment loading. These two events (slide and bioclastic beds) are proposed to be genetically linked, as well as with numerous tectonic activity evidence already reported at other sites in the region. The events observed in this work do not allow classical biostratigraphy dating methods to be applied. The various structures observed in these disturbed zones show that the material slipped in a direction wholly consistent with the unstable setting of the margin of the Vocontian Basin in Lower Cretaceous times.