Abstract

The term “microbially induced sedimentary structures” (MISSs) refers to a category of microbialites that occur predominantly in clastic deposits. MISSs are formed by the interaction of microbial mats at the substrate with sediment erosion, deposition, transportation, or deformation. Five classes of MISS have been described based on the dominant microbial activity that governs or causes the formation of the structures.

This paper describes MISSs from the Early Devonian siliciclastic deposits of the Gara Djebilet Formation (southern Tindouf Basin, Algeria). The observed wrinkle structures, gas domes, laminated leveling structures, and mat chips occur in four lithologically different intervals of the Pragian Upper Member. The structures have been found mainly on the upper surfaces of sandstone beds and are interpreted as the fossilized remains of microbial mats, which are rarely and poorly preserved in iron-rich levels (ironstone). The most abundant MISSs occur in association with sedimentary structures that indicate, at least temporarily, emergent conditions and mixed low to high sedimentation rates. This supports the interpretation of a shallow marine setting, mostly in a shallow wave-dominated environment with moderate hydraulic energy, where periods of rapid sedimentation are interrupted by periods of sediment starvation. Sponge pore fabrics and gas domes are interpreted to reflect biostabilization related to gas accumulations immediately beneath a microbial mat or gas exchange between deposits and water or atmosphere in the lower supratidal zone.

The co-occurrence of microbial structures (MISSs and siliciclastic domal stromatolites) and siliciclastic strata in large areas extends our knowledge of the capability of benthic microbial organisms to settle in sand-dominated, highly stressed, shallow marine environments.