CARBONATE PLATFORM STRATAL GEOMETRIES AND THE QUESTION OF SUBAERIAL EXPOSURE

Integrated petrographic, geochemical and fluid inclusion analyses indi cate that the most prominent exposure unconformity at the southern mar gin of the Vercors carbonate platform in southeastern France, does not coincide with the major break in depositional geometry. Therefore, ch anges in sediment supply and dispersal strongly influenced the resulti ng 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 plat form. Diagenetic analyses have been done at several strategic platform -top bedding planes to determine the extent to which these sedimentolo gic breaks represent subaerial exposure. While many bedding planes con tain some evidence of meteoric alteration, one of these bedding surfac es is unique in that it exhibits several generations of overprinted me teoric calcite cements (bladed to blocky morphology, precipitation in biomolds, extinct to zoned cathodoluminescence, low Mg and delta(18)O, variable delta(13)C, freshwater fluid inclusions) that are cross-cut by marine borings. Eroded dolomitized clasts derived from this exposur e surface occur in the uppermost portion of the finer-grained wedge. T his combined evidence indicates that a significant episode of subaeria l exposure took place at this horizon prior to deposition of the overl ying platform-top sediments. The most significant change in deposition al style, and therefore the most likely position for a sequence bounda ry defined by stratal geometry, is the top of the lower grainstone ton gue. However, the intensely altered exposure surface and its basinward equivalent in the wedge lie 50 m above the top of the lower grainston e tongue. Therefore, the surface containing the most extensive record of meteoric diagenesis does not stratigraphically coincide with the su rface representing the most significant lateral shift in depositional facies.