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Abstract: Sensitivity of Microsampled versus Bulk Rock 87Sr/86Sr Chemostratigraphy in Recording Basin-to-Platform Fluid Migration, Early Cretaceous Vercors Carbonate Platform, France

FOUKE, BRUCE W., Department of Geology, University of Illinois, 245 Natural History Building, 1301 W. Green Street, Urbana, IL 61801, USA: WOLFGANG SCHLAGER and MAURICE G.M. VANDAMME, Institute for Earth Sciences, Free University, Deboelelaan 1085, 1081 HV Amsterdam, The Netherlands

Summary

High-resolution 87Sr/86Sr chemostratigraphy and diagenetic covariation modelling along basin-to-platform transects in Barremian limestones exposed at the southern margin of the Vercors carbonate platform, indicate that connate fluids derived from compaction and dewatering of the Vocontian Trough migrated into the platform top lithologies. These basin-derived waters selectively reacted with terebratulid brachiopod shells, significantly altering their isotopic composition with respect to Sr, C and O. Conversely, the geochemical fingerprint of these basinal fluids was not captured in coeval bulk rock analyses, which primarily record Cretaceous marine isotopic signatures.

The Montagnette outcrop exhibits two stacked 70 m-thick prograding platform tongues that are laterally interbedded with marly basinal deposits. Physically continuous master bedding planes extending undisrupted from the basin onto the ramped platform, create a chronostratigraphic framework within which: 1) a detailed paragenetic sequence was established, revealing a complex series of carbonate dissolution, precipitation, and fracturing events; 2) over one hundred bulk rock and microsampled 87Sr/86Sr, d18O and d13C analyses were done to characterize the major events of the paragenetic sequence; and 3) results were compared between age equivalent shallow-water and deep-water facies. The bulk rock analyses in both the basin and platform sections show no significant variations in 87Sr/86Sr (0.707523 to 0.707555) from published Barremian seawater ratios. Conversely, the platformward microsampled shell material exhibits large variations in 87Sr/86Sr (0.706910 to 0.707450), indicating diagenetic alteration along stratiform conduits. The low 87Sr/86Sr composition of the platform brachiopods indicates that the diagenetic Sr was derived from Jurassic sediments in the Vocontian trough.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah