MICROSTROMATOLITES FROM THE 2.3 G.A. TRANSVAAL SEQUENCE, SOUTH AFRICA (STROMATOLITES, MICROFOSSIL, CHERT).

Abstract

A unique assemblage of in situ microstromatolites, articulated intraclastic microstromatolites, and disarticulated stromatolites has been identified from drill cores of the 2.3 G.a. Transvaal Sequence, South Africa. These structures occur in organic-rich lenticular and nodular replacement black cherts which are associated with early diagenetic dolomite. Petrographic evidence indicates that the chert has formed via a primary carbonate and organic matrix--partial dolomitization--silicification paragenetic sequence; and that dolomitization and silicification were closely contemporaneous diagenetic events. Microstructures which resemble three dimensionally preserved microfossils are found in the majority of the silicified Transvaal cores. These fossil-like microstructures can be grouped broadly into three morphological types: (1) filaments, (2) ovoid or spheroidal forms, and (3) bacteria-like microstructures. Certain of the filamentous forms which are associated with pyrite mineral grains are clearly of abiological origin, and their formation can be explained in the context of sedimentary diagenesis and mineral paragenesis. The three dimensional association of the ovoid and bacteria-like microstructures to the microstromatolites is such as would be predicted from studies of modern cyanobacterial/microbial mat ecosystems. Hence, these microstructures are considered to be potential microfossils. The Transvaal microstromatolitic materials represent some of the smallest stromatolites yet described from either Proterozoic or Phanerozoic sedimentary rocks. Nearly all of the basic stromatolite growth forms (i.e. columnar, bulbous, nodular, and stratiform) are represented in the Transvaal assemblage. Thus, stromatolite diversity at the "basic growth form" level apparently did not evolve through geologic time. Physical and chemical environmental parameters probably controlled stromatolite morphogenesis only to the extent that they influenced the steady state balance of microstromatolite microbial communities. Indirect evidence suggests that the Transvaal microstromatolites grew via the precipitation of primary carbonate at some level within the structures and that a correlation exists between the degradation of primary producer organic carbon and the precipitation of a structurally supportive carbonate mineral matrix.