Subaerial exposure unconformities on the Vercors carbonate platform (SE France) and their sequence stratigraphic significance
Authors: Fouke Bruce W. , Everts Arnout-Jan W. , Zwart Erik W. , Schlager Wolfgang , Smalley P. C. , Weissert Helmut 
Published: April 2008

The integration of data on diagenesis and stratal geometry at the margin of the Vercors carbonate platform (SE France) shows that the most prominent break in depositional style does not coincide with the platform-top horizon exhibiting the most extensive meteoric alteration. This observation again illustrates the ambiguity of geometrical criteria to define sequence boundaries related to subaerial exposure.

Outcrops at the margin of the Cretaceous Vercors platform expose prograding to aggrading tongues of platform grainstones. Growth and lateral progradation of these platform tongues was frequently interrupted, as evidenced by the deposition of wedges of fine-grained deeper water sediments that encroached the clinoform slopes. Petrographic and geochemical analyses have been carried out at strategic bedding surfaces in order to evaluate the extent to which these breaks relate to sea-level falls and subaerial exposure. The analyses reveal evidence for minor meteoric alteration at all four of the bedding surfaces on the platform top that were studied. However, the Surface 3 bedding plane is unique in that it shows the overprinting of several events of meteoric diagenesis. Petrographic and geochemical analyses suggest that the rudist floatstones at this particular surface were diagenetically overprinted at least three times by meteoric groundwaters that dissolved skeletal grains and precipitated bladed and blocky calcite cements that exhibit bladed to blocky morphologies, low Mg, Mn and Fe abundances, depleted δ
18
O and δ
13
C signatures, and freshwater fluid inclusions. Hardground borings then cross-cut the meteoric calcite cements and biomolds, indicating that the subaerial exposure and meteoric overprinting took place prior to deposition of the overlying marine grainstones. This ensuing period of marine inundation was also accompanied by the deposition of red argillaceous internal sediments and dolomitization. The marine grainstones overlying Surface 3 contain lithoclasts with truncated dolomite rhombs at their margins, suggesting that the dolomitization at Surface 3 also relates to early stage diagenesis.

The offbank continuation of the Surface 3 bedding plane exhibits a distinct wedge of dolomitized lithoclastic debris. No such debris was found along the slope continuation of the other studied surfaces, confirming that the Surface 3 bedding plane relates to a particularly significant episode of subaerial exposure and erosion. However, within the framework of changing stratal geometries at the margin of the Vercors platform, the exposure Surface 3 represents a rather insignificant event. In contrast, the most prominent break in depositional style at the Vercors platform-margin exhibits only minor meteoric alteration along its platform-top continuation. This ‘stratigraphic mismatch’ suggests that sea-level falls exposing the platform-top exerted only a minor influence on the resulting platform stratal geometries.