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The Middle Archean Moodies Group (ca. 3.22 Ga), Barberton Greenstone Belt, South Africa, exposes one of the world’s oldest ecosystems. It includes kerogen-rich laminae and thin chert bands interbedded with coarse-grained and gravelly sandstones. The strata record a medium-energy, tidal coastal environment. Analyses of the microscopic structure and chemical composition of the chert bands through petrographic microscopy, Raman microspectroscopy, laser-induced breakdown spectroscopy (LIBS) analyses, C isotopes, and scanning electron microscope (SEM) photography of macerated material, supported by textural observations of hand samples, suggest that these laminae represent variably compressed and early-silicified microbial mats.

Internal wavy laminations, amorphous carbon composition, and negative δ13C values strongly imply a biogenic origin. Complete HF maceration of chert bands revealed polygonal cell structures in a formerly extracellular polymeric substance matrix. The tuft- and dome-micromorphology of the laminations resembles that of recent photosynthetic filament-dominated microbial mats. Facies interpretations indicate that microbial mats extensively colonized subtidal to intertidal Archean siliciclastic coastlines.

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