Newly discovered sedimentary structures produced by ancient microbial mats in Early Archean sandstones of the 3.2 Ga Moodies Group, South Africa, differ fundamentally in appearance and genesis from Early Archean stromatolites and bacterial cell fossils preserved in chert. Wrinkle structures, desiccation cracks, and roll-up structures record the previous existence of microbial mats that effectively stabilized sediment on the earliest known siliciclastic tidal flats. In thin-section, the sedimentary structures reveal carpet-like, laminated fabrics characteristic of microbial mats. Negative ␦ 13 C isotope ratios of ؊20.1 to ؊21.5 ؎ 0.2‰ are consistent with a biological origin for the carbon preserved in laminae. The biogenicity of the sedimentary structures in the Moodies Group is substantiated by comparative studies on identical mat-related features from similar tidal habitats throughout Earth history, including the present day. This study suggests that siliciclastic tidal-flat settings have been the habitat of thriving microbial ecosystems for at least 3.2 billion years. Independently of controversial silicified microfossils and stromatolites, the newly detected microbially induced sedimentary structures in sandstone support the presence of bacterial life in the Early Archean.

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