Basin-to-platform chemostratigraphy and diagenesis of the Early Cretaceous Vercors Carbonate Platform, SE France

Cathodoluminescence petrography has been combined with bulk-rock and micro-sampled ⁸⁷Sr/⁸⁶Sr, δ¹⁸O and δ¹³C analyses to reconstruct the diagenetic history of Lower Cretaceous (Barremian) limestones exposed at the southern margin of the Vercors carbonate platform in southeastern France. Stratigraphic and geographic trends in these analyses, coupled with mass balance water-rock interaction modeling, have been used to constrain the chemical composition, source, and relative timing of the groundwaters responsible for diagenetic alteration. This study was completed on the Montagnette outcrop, which is composed of two stacked 70-m-thick units of prograding platform-margin limestones that interfinger with marly basinal sediments. The Montagnette exposure is exceptional, in that it is a single complete outcrop of basin-to-platform limestones that are preserved in their original lateral depositional continuity. Cathodoluminescence petrography indicates that the limestones have experienced a complex sequence of carbonate dissolution, precipitation, and fracturing events. Over one hundred bulk-rock and micro-sampled ⁸⁷Sr/⁸⁶Sr, δ¹⁸ δ¹³C analyses were completed to characterize each major event of deposition and diagenesis. Bulk-rock analyses from the Montagnette basin-to-platform transect exhibited no significant variations in ⁸⁷Sr/⁸⁶Sr (0.707523 to 0.707555). These values are consistent with estimated Barremian seawater ⁸⁷Sr/⁸⁶Sr ratios. In contrast, micro-sampled shell material in the platform margin and platform top deposits exhibit large variations in ⁸⁷Sr/⁸⁶Sr (0.706910 to 0.707450). The least radiogenic Sr (lowest ⁸⁷Sr/⁸⁶Sr) occurs in shell material located toward the platform, while the more seawater-like ratios occur in sections without shells that are located toward the basin. This pattern implies that selective diagenetic alteration of the shell material took place after basinal brines flowed up into the platform and spread out laterally. The Sr in the diagenetic waters was probably derived from water-rock interaction with Jurassic basinal sediments in the Vocontian Trough. This conclusion is based on: (1) the extremely low ⁸⁷Sr/⁸⁶Sr composition of shell material in the platform limestone; (2) modeled co-variation trends in ⁸⁷Sr/⁸⁶Sr, δ¹³C, and δ¹⁸O; and (3) the absence of another nearby source of less-radiogenic Sr in the Vercors region.