ABSTRACT

Submillimeter- to millimeter-sized stromatolites have been found in early carbonate-replacive black cherts from the 2.3 Ga Chuniespoort Group, Transvaal Supergroup, South Africa. This assemblage includes substrate-attached microcolumns and detached (intraclast) structures comprising nearly the full spectrum of basic macrostromatolite growth forms (i.e., columnar, bulbous, and nodular). Petrographic observations suggest a mechanism for microstromatolite morphogenesis involving the generation of primary producer biomass at the outer surfaces of the structures and the subsequent mineralization of that biomass by decomposer microbial populations with the concomitant precipitation of carbonate at successively lower levels within the microstromatolites. A carbonate precipitation mechanism i volving anaerobic respiration is thus suggested, which would constrain microstromatolite growth to a sedimentary environment which may have been relatively oxygen-poor or reducing. The Transvaal microstromatolites are interpreted as an integrating assemblage of growth forms rather than a collection of distinct end members. Conservative estimates suggest that a 1-m² plot of microstromatolites could have provided a minimum of 55 g CaCO₃/yr of allochemical sediment to the Transvaal carbonate platform--an amount which is comparable to the carbonate mud production of Thalassia epibionts from modern carbonate platform systems.