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

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, δ18O and 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.