STRATAL PATTERNS, SEDIMENT COMPOSITION, AND SEQUENCE STRATIGRAPHY AT THE MARGIN OF THE VERCORS CARBONATE PLATFORM (LOWER CRETACEOUS, SE FRANCE)

Conventional sequence-stratigaphic concepts relate the large-scale str atal architecture of sediment bodies to changes in relative sea level. This paper evaluates the relationship between stratal geometry and se alevel stand, based on a study of large, semicontinuous outcrops of Cr etaceous carbonate platform strata in the Vercors (southeastern France ). Multiple lines of evidence for sea-level change are combined, inclu ding stratal geometry, detailed quantitative microfacies analyses, and diagenetic patterns at platform-top hardground surfaces. The studied outcrops of the Cirque d'Archiane show two main prograding platform to ngues, both over 100 m thick. The stratal geometries at the boundary b etween these platform tongues, including an apparent pinchout of a wed ge of slope sediments, suggest the presence of a major lowstand unconf ormity. However, this stratal boundary does not coincide with the hori zon containing the most extensive meteoric alteration. Furthermore, de tailed platform-to-basin correlation shows that the wedge of slope sed iments is not basin-restricted, but makes a thin drape over the platfo rm top. The sedimentologic and diagenetic evidence suggest incipient d rowning of the platform at the boundary between the two main platform tongues, preceded by a minor exposure event only. Internally, the main platform tongues consist of smaller (10-30 m thick) units that progra de towards the basin and aggrade on the platform top, and which are in terpreted as stacked highstand wedges. These wedges are usually topped by hardground surfaces with minor evidence for subaerial exposure. Th e platform-top horizon with the most extensive subaerial diagenesis an d erosion correlates with a distinct but relatively thin unit of litho clastic debris on the slope. A pronounced scour in the clinoforms of t he Vercors does not correlate with a major exposure surface on the pla tform top. This study shows that stratal geometries alone are a rather ambiguous guide to sea-level history. Without the accompanying sedime ntologic and diagenetic evidence for sea-level change, most of the str atal architecture of the Vercors platform can be explained by either c hanges in accommodation or changes in carbonate production.