A genetic link between synsedimentary tectonics-expelled fluids, microbial sulfate reduction and cone-in-cone structures

The late Jurassic (Tithonian) marlstones of the Boulonnais area (English Channel, France) contains diagenetic carbonate beds and nodules. Some nodules exhibit cone-in-cone structures on their lower face. We studied such nodules using various techniques of imaging and chemical (major and trace-elements) and isotopic analyses (Ccarb, Corg, O and S stable isotopes). We interpret the cone-in-cone to be the end product of carbonate-nodule formation during early diagenesis. The diagenetic carbonate precipitation was induced by microbial activity (bacteria and(?) archeae) fueled by upward-migrating fluids. Fluid expulsion was itself triggered by synsedimentary fault movements. Under such circumstances, cone-in-cone structures can form during early diagenesis as the result of bacterially-mediated sulfate reduction, possibly coupled to sulfate-dependent anaerobic oxidation of methane. This genetical scheme may be transposed to many other settings.