Siliciclastic biolaminites indicative of widespread microbial mats in the Neoproterozoic Nama Group of Namibia

Siliciclastic biolaminites are thinly laminated sediments resulting from the interaction of epibenthic microbial mat growth and sediment supply. A thick succession of ancient siliciclastic biolaminites consisting of alternating thin layers of siltstone and sericitic argillite is reported here from the Ediacaran Vingerbreek Member (Schwarzrand Subgroup) of the Nama Group in Namibia. The succession was laid down in a low-energy, intertidal environment, behind a sheltering sand barrier or shoal. Intercalated are sandstone beds and bodies originating from storm events or representing tidal channel fills. The participation of microbial mats in the accretion of the siliciclastic biolaminites is indicated by mat growth and destruction structures, like ‘elephant skin’, circular cracks with involute margins, and sand clasts, and by microstructures like ‘coated grains’ and isolated silt-sized grains ‘trapped’ in mat-derived sericitic argillite. Typical features of the biolaminites are isolated, tepee-like, antiformal structures and isolated upturned packages of the laminated sediment, overlain by undisturbed layers. By comparison with modern examples, these structures are interpreted as upturned margins of major shrinkage cracks developed once in thick microbial mats, partly involving underlying biolaminated sediment.Microbial mats inducing accretion of siliciclastic biolaminites act as barriers and retain sediments, locally leading to sizeable sedimentary successions, in the intertidal zone. This may be expected particularly in the Precambrian where mats were widespread due to the absence of predators.