Filamentous microfossils in the early proterozoic transvaal supergroup: their morphology, significance, and paleoenvironmental setting

Abstract

Well preserved filamentous microfossils (Siphonophycus transvaalensis n. sp.) are described here from the carbonate (Campbellrand Subgroup) to iron-formation (Kuruman Iron Formation) transition of the Transvaal Supergroup, South Africa, estimated to be 2.5-2.3 Ga years old. The microfossils occur in petrographic thin-sections of a core sample of carbonate-chert. They are preserved by permineralization in both chert and in sparry ferroan dolomite. Stratigraphically the fossiliferous core sample occurs as part of an upward transition from a stromatolitic dolomite and limestone sequence (Campbellrand) to the overlying iron-rich sediments of the Kuruman-Griquatown Iron Formations. The average δ13C value of the kerogen in the sample is about - 36.9%. The microfossils are filamentous, unbranched, tubular to somewhat flattened, 15–25, μm in diameter and a few to many hundred microns in length. They exhibit a coriaceous, finely granular external surface texture resulting from the presence of adhering, randomly distributed, fine mineralic (carbonate) needles. In salient morphological characteristics they are comparable to the tubular, originally polysaccharide, encompassing sheaths of extant oscillatoriacean cyanobacteria. In comparison with previously described Precambrian microfossils, these fossil filaments are unusual because of their preservation in sparry carbonate (in addition to chert), their relatively large diameter, and their coating by adhering, precipitated, carbonate needles; they appear to be among the oldest assured microfossils now known from Proterozoic-age sediments. The microfossils are interwoven, occurring in subparallel aggregates that form a stromatolitic mat-like fabric; they are considered to be of an endogenetic in situ benthic origin occurring at the proximal margin of a ‘deep shelf’ environment at the front (distal margin) of the Campbellrand carbonate platform. The water depth for this environment, at the break in slope between deep shelf and euxinic basin, is estimated to have been of the order of 40–45 m. Paleomagnetic data support our interpretation that the micro-organisms inhabited a warm water marine environment, probably at low latitudes.