ABSTRACT

Conventional sequence-stratigraphic concepts relate the large-scale stratal architecture of sediment bodies to changes in relative sea level. This paper evaluates the relationship between stratal geometry and sea-level stand, based on a study of large, semicontinuous outcrops of Cretaceous carbonate platform strata in the Vercors (southeastern France). Multiple lines of evidence for sea-level change are combined, including 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 tongues, both over 100 m thick. The stratal geometries at the boundary between these platform tongues, including an apparent pinchout of a wedge of slope sediments, suggest the presence of a major lowstand unconformity. However, this stratal boundary does not coincide with the horizon containing the most extensive meteoric alteration. Furthermore, detailed platform-to-basin correlation shows that the wedge of slope sediments is not basin-restricted, but makes a thin drape over the platform top. The sedimentologic and diagenetic evidence suggest incipient drowning 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 prograde towards the basin and aggrade on the platform top, and which are interpreted as stacked highstand wedges. These wedges are usually topped by hardground surfaces with minor evidence for subaerial exposure. The platform-top horizon with the most extensive subaerial diagenesis and erosion correlates with a distinct but relatively thin unit of lithoclastic debris on the slope. A pronounced scour in the clinoforms of the Vercors does not correlate with a major exposure surface on the platform top.

This study shows that stratal geometries alone are a rather ambiguous guide to sea-level history. Without the accompanying sedimentologic and diagenetic evidence for sea-level change, most of the stratal architecture of the Vercors platform can be explained by either changes in accommodation or changes in carbonate production.