Methane-derived carbonate formation triggered by the latest Albian anoxia in northwestern Tunisia basins

Abstract

The late Albian exposures of northwestern Tunisia include authigenic carbonates typified by soft-sediment deformation (slumps) and doughnut-shaped concretions. Planktic foraminiferal biostratigraphy indicates a late Albian age (buxtorfi subzone). The carbonate bodies display veins and fractures partially filled with impsonite (bitumen). The microfacies analysis revealed multiple generations of fibrous and sparry calcite cements, pyrite, and clotted pelmicrite. Scanning electron microscope (SEM) analysis performed on clotted micrite shows the presence of pyrite spheroids and micrometer-sized calcified filamentous structures attributed to anaerobic methanotrophs. Depleted values of δ13C (ranging between − 36.88 and − 11.63‰ PDB) for these carbonates, suggested a thermogenic methane-derived carbon source with a contribution of crude oil to the carbon pool. The petrography and stable isotopes data provide unequivocal evidence that these carbonates are cold seeps–related. The cold seep features indicate that microbial communities have used ascending hydrocarbons and methane fluids and contributed to the precipitation of authigenic carbonates. The paleotemperature calculated from oxygen isotopes of early diagenetic micrite is indicative of parent fluids warmer (68 °C) than those of late Albian seawaters (~ 27 °C). We suggest that hydrothermal fluids linked to volcanic intrusions in the organic-rich lower Fahdene Formation beds may have triggered the release of thermogenic methane flux as cold seeps in the sedimentary basin. The accumulation of methane with other released gases in the water column is triggered by anoxic and stagnant water masses. The sluggish methane flux associated with the upwelling of deep anoxic waters contributes to the carbonate precipitation and the projection of carbonate bodies (e.g., chimney-like shape), induced by microbially mediated anaerobic oxidation of methane.