Abundant microbial mats from the Mesoarchean Moodies Group (Barberton Greenstone Belt, South Africa, ca. 3.2 Ga) are densely interbedded with coarse-grained and gravelly sandstones in a nearly mud-free setting. They apparently grew in marginal marine and possibly subaerial coastal and tidal environments. Characteristic sedimentary structures include anastomosing bedding, gas or fluid escape structures, sand volcanoes, biomat doming, patchy silicified microstromatolites, and microbial sand-chip conglomerates. They indicate rapid growth of mechanically tough microbial mats, possibly aided by early seafloor silicification, in a high-energy, high-sedimentation-rate environment.

The observations expand the knowledge of the habitat of Archean microbial mats and life on early Earth. If tidal environments were more widespread in the Archean than today, biomats in extensive intertidal settings may have contributed to immobilize large volumes of sand.

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