CLINOFORM COMPOSITION AND MARGIN GEOMETRIES OF A LOWER CRETACEOUS CARBONATE PLATFORM (VERCORS, SE FRANCE)

The concept of sequence-stratigraphy uses the stratal geometries at th e margins of carbonate platforms to define depositional sequences and systems tracts. The aim of this study was to research if prograding, p urely aggrading and retrograding phases of a Cretaceous carbonate plat form showed differences in the composition and facies type of the slop e sediments. The Vercors Plateau in SE France provides excellent outcr ops to study this relationship. Continuous exposure of the platform-to -basin transition allowed direct examination of the margin geometries. Five successions were measured and sampled in great detail. Samples w ere thin-sectioned and point-counted, using point-count groups charact erizing palaeoenvironments along the platform-to-basin transect. The c omposition logs as well as the numerical analysis of the point-count d ata, both show a clear relationship between grain composition and stra tal geometry. The prograding and purely aggrading intervals are simila r in composition, as they are both relatively coarse grained, and enri ched in platform biota and limeclasts. Retreating units are relatively fine grained and rich in basinal grains (small benthic foraminifers, sponge spicules), non-carbonate grains and embedding material. These o bservations suggest that the retreating intervals represent incipient drownings of the platform. However, rather than distinctly separated g roups the different phases of platform development form a continuous r ange of variation between high platform input and high basinal input. Compared to the prograding units the purely aggrading intervals are re latively rich in peloids, which may be suggestive of relatively low-en ergy conditions on the platform during aggradation. The compositional analysis also revealed significant variation in the frequency of ooids , but these variations showed no relationship with the progradation, a ggradation or retreat of the platform.