ALBERT

Description: The concept of sequence-stratigraphy uses the stratal geometries at the margins of carbonate platforms to define depositional sequences and systems tracts. The aim of this study was to research if prograding, purely aggrading and retrograding phases of a Cretaceous carbonate platform showed differences in the composition and facies type of the slope sediments. The Vercors Plateau in SE France provides excellent outcrops 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 were thin-sectioned and point-counted, using point-count groups characterizing palaeoenvironments along the platform-to-basin transect. The composition logs as well as the numerical analysis of the point-count data, both show a clear relationship between grain composition and stratal geometry. The prograding and purely aggrading intervals are similar in composition, as they are both relatively coarse grained, and enriched 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 observations suggest that the retreating intervals represent incipient drownings of the platform. However, rather than distinctly separated groups the different phases of platform development form a continuous range of variation between high platform input and high basinal input. Compared to the prograding units the purely aggrading intervals are relatively rich in peloids, which may be suggestive of relatively low-energy 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, aggradation or retreat of the platform.