RECENT DISCOVERIES OF PROTEROZOIC MICROFOSSILS IN SOUTH-CENTRAL BRAZIL

Twenty-two Precambrian microfossiliferous localities, seven newly repo rted here, are known from the Sao Francisco Block and associated Brasi lia fold belt of south-central Brazil, reported from outcrop and drill core samples of Meso- and Neoproterozoic cherts, carbonates, and sili ciclastics of the similar to 1170 to similar to 950 Ma old Paranoa Gro up and the similar to 950 to similar to 700 Ma old Sao Francisco Super group (composed of the Jequitai Formation and the overlying Bambui Gro up). Representatives of nearly 30 genera of Proterozoic microfossils, chiefly cyanobacteria and subsidiary acritarchs, have been reported fr om these strata, preserved mostly within carbonaceous black chert in f lat-laminated and high-relief stromatolites, and represent a wide spec trum of environments evidently ranging from nearshore lagoonal to supr atidal, intertidal, subtidal, and offshore marine settings. Contrary t o previously held views, the microfossils are generally more abundant and better preserved in the older unit (Paranoa Group) than in the you nger one (Sao Francisco Supergroup). Nevertheless, among the newly rep orted assemblages, one from the lower part of the Bambui Group stands out as probably the best preserved and most diverse microbiota known f rom any Precambrian unit of South America. Although paleobiologic stud ies of the Brazilian Proterozoic are still in a formative stage, recen t discoveries are highly promising, such as the detection in the work presented here of possible differences: (1) between the microfossil si zes in stromatolites of the Paranoa and Bambui Groups; (2) in the abun dance and size of acritarchs with respect to lithology and facies; and (3) in the microfossil assemblages preserved in morphologically disti nct stromatolites.