Carbonate platform stratal geometries and the question of subaerial exposure

Integrated petrographic, geochemical and fluid inclusion analyses indicate that the most prominent exposure unconformity at the southern margin of the Vercors carbonate platform in southeastern France, does not coincide with the major break in depositional geometry. Therefore, changes in sediment supply and dispersal strongly influenced the resulting stratal geometries, and prevent direct reconstructions of sea level from the stratal patterns. Platform-to-basin transects exposed in the Cirque d'Archiane contain two prograding grainstone tongues separated by a wedge of finer-grained slope sediments that thin toward the platform. Diagenetic analyses have been done at several strategic platform-top bedding planes to determine the extent to which these sedimentologic breaks represent subaerial exposure. While many bedding planes contain some evidence of meteoric alteration, one of these bedding surfaces is unique in that it exhibits several generations of overprinted meteoric calcite cements (bladed to blocky morphology, precipitation in biomolds, extinct to zoned cathodoluminescence, low Mg and δ¹⁸O, variable δ¹³C, freshwater fluid inclusions) that are cross-cut by marine borings. Eroded dolomitized clasts derived from this exposure surface occur in the uppermost portion of the finer-grained wedge. This combined evidence indicates that a significant episode of subaerial exposure took place at this horizon prior to deposition of the overlying platform-top sediments. The most significant change in depositional style, and therefore the most likely position for a sequence boundary defined by stratal geometry, is the top of the lower grainstone tongue. However, the intensely altered exposure surface and its basinward equivalent in the wedge lie 50 m above the top of the lower grainstone tongue. Therefore, the surface containing the most extensive record of meteoric diagenesis does not stratigraphically coincide with the surface representing the most significant lateral shift in depositional facies.