Sedimentology and facies analysis of Mesoarchaean stromatolitic carbonate rocks of the Pongola Supergroup, South Africa

The c. 3.0 Ga old Chobeni Formation (Nsuze Group, Pongola Supergroup) contains several carbonate successions interpreted to have been deposited in a tide-dominated, shallow-marine environment. Facies consist of mixed siliciclastic and carbonate rocks and include subtidal cross-bedded sandstone and oncolitic dolostone, intertidal wave-rippled oodolarenite, and supratidal wrinkly-laminated dolarenite-dololutite. Sedimentary facies are partially arranged in a shallowing-upward pattern reflecting cyclic variations of Mesoarchaean sea level. Microbialites occur in a variety of sedimentary sub-environments and include laminated as well as structureless, lenticular-domal types. Differences in microbialite morphology, macrofabric, and size suggest potentially distinct assemblages of microorganisms responding to different physicochemical and environmental conditions. Microbialite textures are consistent with in situ carbonate precipitation and to a lesser extent by trapping and binding of sediment. Despite their great age, the Pongola carbonates are compositionally and texturally very similar to much younger carbonate successions preserved in the geological record.