Chemosynthetic microbialites in the Devonian carbonate mounds of Hamar Laghdad (Anti-Atlas, Morocco)

Abstract

Veins cutting across the Devonian conical mounds of the Hamar Laghdad Ridge (eastern Anti-Atlas, Morocco) were the carriers of geofluids. Although not completely determined, the composition of these fluids seems related to hydrothermal and seepage fluxes that were the primary contributors to the establishment of sub-seafloor (cryptic) environments linking deeper fluid sources to the seafloor. The optical and scanning electron microscope investigation of the laminated carbonate infill of these veins has revealed that they consist of stromatolite- and oncolite-like fabrics. In the Early and Middle Devonian mounds, the laminated carbonates contain morphologies that are attributable to microbially mediated processes that in turn suggest a direct biological contribution to the deposition of the veins infill. Microbial evidence include rod-shaped and cocci clusters embedded in amorphous membranes, iron-rich filaments organized as mat-like and biodictyon filamentous aggregates, or isolated filaments, microbial clotted textures and peloids. Moreover, the oncoids from the veins of the Middle Devonian (Hollard) mound have been interpreted as the mineral replacement of crypto-microorganisms as Frutexites. Other microbial evidence are biominerals and include carbonate with spheroids and pyrite framboids, in which typifying hydrocarbon seep environments. The Hollard Mound, which is the only one with a well-documented origin by hydrocarbon seepage, shares a number of geomicrobiological evidence with other Early Devonian mounds. These evidence, which are supplemented by δ 13C-depleted data, suggest that the infill of the veins system in the Hamar Laghdad mounds would be the product of chemotrophic/chemosynthetic microbial communities that are adapted to crypto-habitats sustained by hydrocarbon (and other components) fluid fluxes.