Methane-Derived Carbonates Formation Triggered by the Latest Albian Anoxia in Northwestern Tunisia Basins

Abstract

AbstractThe organic-rich beds of Mouelha Member outcropping in northwestern Tunisia include authigenic carbonates typified by soft-sediment deformation (slumps) and doughnuts shaped concretions. Planktic foraminiferal biostratigraphy indicates a late Albian age (buxtorfi Subzone). The carbonate bodies display veins and fractures partially filled with impsonite (bitumen). Microfacies analyses of these bodies reveal multiple generations of carbonate cements, pyrite, and clotted pelmicrite. Scanning electron microscope (SEM) analysis performed on clotted micrite shows the presence of pyrite spheroids and micrometer-sized filamentous structures. Oxygen isotopes of micrite revealed slightly depleted values ranging from −3.92 to −0.6‰ PDB, similar to normal marine values. Depleted values of δ13C (ranging between −36.88 and −11.63‰ PDB) for these carbonates suggested a methane-derived carbon source and its anaerobic oxidation. In addition, the 13C moderate depletion suggests thermogenic methane or a petroleum source. These results provide unequivocal evidence that these carbonates are cold seeps related. The cold seep features indicate that microbial communities have used ascending methane fluids and contributed to the precipitation of authigenic carbonates. We suggest that contemporaneous volcanic intrusions into the basin may have triggered the release of thermogenic methane flux as cold seeps in the sedimentary basin. The accumulation of methane and other released gases in the water column is triggered by the presence of anoxic and sluggish waters. Therefore, they may be responsible for the upwelling of deep anoxic waters which contribute to the projection of carbonate bodies (e.g., chimney-like shape), indicating that authigenic carbonate precipitation is induced by anaerobic oxidation of methane.KeywordsNorthwestern TunisiaLatest AlbianCold seepsAuthigenic carbonatesAnaerobic oxidation of methaneAnoxic waters