Metamorphic origin of anastomosing and wavy laminas overprinting putative microbial deposits from the 3.22 Ga Moodies Group (Barberton Greenstone Belt)

Anastomosing branching and wavy laminae in quartz-rich sandstones of the 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB), South Africa, have been extensively described as fossilized microbial mats developed in terrestrial to marine transitional environments. Petrological and geochemical characteristics of a ~ 350 m thick Moodies succession in the Saddleback Syncline in the central BGB were analyzed to reconstruct the post-depositional history of the sediments and to better constrain the origin of the laminae. The studied sandstones are composed mainly of quartz, potassium feldspar, and chert clastic grains with various proportions of microquartz and sericite cements. In coastal floodplain and inter- to supra-tidal environments, quiescent periods with low current velocity allowed the repeated deposition of thin (<5 mm thick) and fine-grained laminae with clay matrix and potentially organic matter. In contrast, subtidal settings under consistently high-energy conditions led to the deposition of coarse- to medium-sized sands without fine-grained lamina. Intergrain areas in those coarse sediments were infilled with a microquartz cement during burial diagenesis. Due to the presence of clay matrix, the fine-grained laminae had a reduced porosity that prevented the microquartz cementation within the laminae. This original clay matrix was later replaced with sericite cements during metamorphism, forming the anastomosing and wavy laminae observed at the hand-specimen scale in the studied sandstones. Microscopic dark areas within the laminae are composed mostly of opaque minerals with scarce carbonaceous material. The scarcity of carbonaceous material and the fluctuating energy settings, potentially inauspicious for the continuous development and preservation of biomats, are not in favor of a flourishing Paleoarchean microbial life on the Moodies sediments.