Macaubas Group Research Articles

Radiochronological age and correlation of proterozoic sediments in Brazil

A review of available Rb—Sr and K—Ar datings obtained on sedimentary sequences, metamorphosed or not, interbedded volcanics and cross-cutting intrusives of the Precambrian of Brazil yields the following conclusions: 1.(1) The Roraima and Rio Fresco Formations, resting upon the Amazonian craton, have been affected by the Trans-Amazonian orogeny and are of Lower Proterozoic age.2.(2) The Beneficiente Group, in the same region, seems to be of Middle Proterozoic, Lower Riphean age.3.(3) Upon the Sao Francisco craton, and upon the Lower Proterozoic Rio dos Remedios complex, the Paraguaçu Group might be of middle Riphean age and the glacial sequences, Macaubas Group and Bebedouro Formation, date back to between 950 and 1000 Ma.4.(4) The age of the Bambui and Una Groups, in the same region, remains undetermined. It might be either Upper Riphean, or Upper Riphean and Vendian.5.(5) The molassic series associated with the Brazilian orogeny are dated back to between the Proterozoic—Phanerozoic boundary and the Ordovician. (1) The Roraima and Rio Fresco Formations, resting upon the Amazonian craton, have been affected by the Trans-Amazonian orogeny and are of Lower Proterozoic age. (2) The Beneficiente Group, in the same region, seems to be of Middle Proterozoic, Lower Riphean age. (3) Upon the Sao Francisco craton, and upon the Lower Proterozoic Rio dos Remedios complex, the Paraguaçu Group might be of middle Riphean age and the glacial sequences, Macaubas Group and Bebedouro Formation, date back to between 950 and 1000 Ma. (4) The age of the Bambui and Una Groups, in the same region, remains undetermined. It might be either Upper Riphean, or Upper Riphean and Vendian. (5) The molassic series associated with the Brazilian orogeny are dated back to between the Proterozoic—Phanerozoic boundary and the Ordovician.

O Cráton do São Francisco

This paper is an attempt to justify the original concept of the Sao Francisco Craton and to define its limits. This Craton covered almost the entire State of Bahia and parts of the near Minas Gerais, Goias, Pernambuco and Sergipe States during the Brasiliano Cycle. The authol characterizes the Aracuai folded belt whose structures correspond to the Macaubas Group and are disposed on the southern and southeastern borders of the Craton. The nature of the limits of the Craton with the adjacent folded belts is discussed. A proposal to establish these limits along evident marginal faults with .justified inferences between them is submitted.

A GLACIAÇÃO PROTEROZÓICA NO CENTRO-NORTE DE MINAS GERAIS

Metasediments of the proterozoic Macaubas Group (Sao Francisco Supergroup) occur in great distribution and thickness (about 1 000 m) in the Upper Jequitinhonha Area. Other occurrences in Minas Gerais are found in narrow belts around the Serra do Cabral and at the western border of the Serra do Espinhaco. In the Jequitinhonha Area the sequence can be subdivided into the following lithostratigraphic units (from base to top): basal quartzites and conglomerates, pebbly metagreywackes, an alternation (sometimes graphite and manganese bearing) of phyllites, metasiltstones and quartzites, fine to medium grained quartzites and finally green schists. The metagreywackes, which were identified as tillites, show transitions of facies towards the southeast from continental tillites over aquatillites to glacial-marine sediments. Other criteria favouring a proterozoic glaciation are: grooved and polished pavements, one esker, one varved slate, and striated and facetted pebbles in flat iron form and with gouge marks. Stratigraphic, faciologic and paleostructural relationships not yet studied in detail, indicate an interfingering between the metasediments of the Bambui and the Macaubas.

Proterozoic glaciations in eastern Brazil: A review

Earlier publications on proterozoic glaciations in eastern Brazil are reviewed and revised, based on recent investigations in the Serra do EspinhaCo. The former “Lavras Series”, often cited as an example of an infracambrian glaciation, comprises units of different age. The older units (Sopa conglomerate in Minas Gerais, “Lavras Series” sensu strictu in Bahia) belong to the middle to early proterozoic Minas Supergroup. They show no indications of glaciation. The widespread and in part glacial MacaUbas Group belongs to the basal SAo Francisco Supergroup. According to stromatolites the latter has a minimum age of 950 m. y. The MacaUbas Group might have an age of more than 1.650 m. y. An infracambrian glaciation is unknown from eastern Brazil.