Proterozoic microfossils from subsurface siliciclastic rocks of the São Francisco Craton, south-central Brazil

Compressed organic-walled microfossils attributable to five filamentous taxa, and ten coccoidal eight, (taxa originally spheroidal forms and two originally ellipsoidal, colonial forms), all having long time-ranges in the Proterozoic, have been identified in fine-grained siliciclastic sediments in cores from four bore holes cutting subsurface equivalents of the Mesoproterozoic Conselheiro Mata Group and Neoproterozoic Bambuı́ Group on the São Francisco Craton in south-central Brazil. This is the first documented report of organic-walled microfossils from subsurface Proterozoic rocks of Brazil, including the first reference to occurrences of microfossils in the Conselheiro Mata Group and the Serra de Santa Helena, Lagoa do Jacaré, and Serra da Saudade Formations of the Bambuı́ Group. Surprisingly, microfossils in the Conselheiro Mata Group are better preserved, more diverse, and more abundant than in the younger, extremely depauperate assemblages of the Bambuı́ Group, possibly reflecting contrasting biostratinomic and paleoclimatic conditions. The Conselheiro Mata assemblages are dominated by fragments of filamentous microbial mats and small to medium-sized (5200 mm), unornamented, simple acritarchs, none of which yet provides conclusive biostratigraphical evidence as to the age of the microfossiliferous successions. The morphologic simplicity, relatively small size, and low diversity of the microfossils in the Conselheiro Mata Group, however, are consistent with the Mesoproterozoic age attributed to this unit. Additionally, the relative proportions of planktic (sphaeromorphs) versus benthic (tubular filaments) microfossils are suggestive of fluctuations in sea level along part of the most fossiliferous bore hole. © 2000 Elsevier Science B.V. All rights reserved.