Abstract:
The Jaradag Fawn Limestone Formation (∼1.6 Ga, Semri Group, Vindhyan Supergroup, India) contains abundant microfossils and precipitates in early diagenetic (bedded and stromatolitic) cherts. The silicified carbonate precipitates and associated micritic event laminae were formed at or near the sediment–water interface and presumably lithified very rapidly. The precipitates, deposited inorganically without active participation of cyanobacterial mats, can be grouped into three morphological categories: radial-fibrous fans, microlaminated stratiform laminae and poorly differentiated laminated stratiform laminae. A diverse assemblage of cyanobacteria is preserved both within precipitates where preservation is excellent, and in other synsedimentary textures. Precipitates and microfossil assemblages from four Mesoproterozoic formations of Siberia are described and compared with those from Jaradag Fawn Limestone. Comparison of the Indian and the Siberian precipitates and associated microfossil assemblages reveals an almost identical pattern for contemporaneous Mesoproterozoic carbonate precipitates and microbiotas in cherts. The microbial assemblages, almost exclusively composed of the remnants of cyanobacteria, are dominated by entophysalidacean members and short trichomes, and can be termed as “typical Mesoproterozoic microbiotas”. Co-occurrence of these microbiotas and precipitates is probably related to the depositional environment of the Mesoproterozoic tidal flats, with high carbonate saturation. Neoproterozoic depositional conditions changed drastically, as did the composition of microbial communities.
