Recent discoveries of Proterozoic microfossils in south-central Brazil

Twenty-two Precambrian microfossiliferous localities, seven newly reported here, are known from the Sao Francisco Block and associated Brasilia fold belt of south-central Brazil, reported from outcrop and drill core samples of Meso- and Neoproterozoic cherts, carbonates, and siliciclastics of the similar to 1170 to similar to 950 Ma old Paranoa Group and the similar to 950 to similar to 700 Ma old Sao Francisco Supergroup (composed of the Jequitai Formation and the overlying Bambui Group). Representatives of nearly 30 genera of Proterozoic microfossils, chiefly cyanobacteria and subsidiary acritarchs, have been reported from these strata, preserved mostly within carbonaceous black chert in flat-laminated and high-relief stromatolites, and represent a wide spectrum of environments evidently ranging from nearshore lagoonal to supratidal, intertidal, subtidal, and offshore marine settings. Contrary to previously held views, the microfossils are generally more abundant and better preserved in the older unit (Paranoa Group) than in the younger one (Sao Francisco Supergroup). Nevertheless, among the newly reported assemblages, one from the lower part of the Bambui Group stands out as probably the best preserved and most diverse microbiota known from any Precambrian unit of South America. Although paleobiologic studies of the Brazilian Proterozoic are still in a formative stage, recent discoveries are highly promising, such as the detection in the work presented here of possible differences: (1) between the microfossil sizes in stromatolites of the Paranoa and Bambui Groups; (2) in the abundance and size of acritarchs with respect to lithology and facies; and (3) in the microfossil assemblages preserved in morphologically distinct stromatolites.