Abstract
After the late Cryogenian glaciation the central region of Brazil was the site of extensive deposition of platformal carbonates of the Araras Group. This group includes a basal cap carbonate sequence succeeded by transgressive, deep platform deposits of bituminous lime mudstone and shale. Facies and stratigraphic data combined with carbon and oxygen isotopic analyses of the most complete section of the transgressive deposits, exposed in the Guia syncline, were used to evaluate the depositional paleoenvironment and to test the correlation of these deposits along the belt and with other units worldwide. The studied succession consists of 150 m thick tabular beds of black to grey lime mudstone and shale with predominantly negative delta13C PDB values around -2.5 to -1‰ . The delta13C PDB profile of Guia syncline shows a clear correlation with the upper portion of Guia Formation in the Cáceres region, about 200 km to the southwest. The delta13C PDB profile of the Araras Group is comparable with delta13C PDB profiles of Ediacaran units of the southern Paraguay Belt, western Canada, and the Congo and Kalahari cratons. Moreover, facies distribution, stratigraphy and the carbon isotopic profile of the Araras Group match the middle Tsumeb Subgroup in Namibia, which reinforces the Ediacaran age assigned to the Araras Group.
Highlights
The transition zone between the southern margin of the Amazon Craton and the northern portion of the Paraguay Belt is an extensive thrust-and-fold belt formed during the final stages of Brasiliano-Pan-African orogenesis, as a result of the convergence and collision of the Amazônia (West), São Francisco-Congo (East) and Paraná (South) blocks (Almeida 1984, Basei and Brito Neves 1992, Alkmim et al 2001)
After a severe glaciation that finished at ca. 630 Ma this region was the site of extensive deposition of platformal carbonates of the Araras Group, exposed along the southern border of the Amazon Craton and at the northern Paraguay Belt (Fig. 1)
The sedimentary succession in the Guia syncline consists of tabular beds of black to grey lime mudstone and shale 150 m in thickness (Fig. 4)
Summary
The transition zone between the southern margin of the Amazon Craton and the northern portion of the Paraguay Belt is an extensive thrust-and-fold belt formed during the final stages of Brasiliano-Pan-African orogenesis (ca. 540-500 Ma, Almeida 1984, Trompette 2000, Alvarenga et al 2000), as a result of the convergence and collision of the Amazônia (West), São Francisco-Congo (East) and Paraná (South) blocks (Almeida 1984, Basei and Brito Neves 1992, Alkmim et al 2001). 630 Ma this region was the site of extensive deposition of platformal carbonates of the Araras Group, exposed along the southern border of the Amazon Craton and at the northern Paraguay Belt (Fig. 1). The laterally extensive Araras Group is interpreted as platform carbonate deposits, as suggested by its homogeneous, predominantly deep-water lithofacies exposed on the craton and in the Paraguay Belt. The age of the Araras Group has been inferred in part on the basis of lithostratigraphic correlation with carbonate rocks of the Corumbá Group, which crop out about 500 km south of the study area This correlation is speculative because facies, sedimentary environments, chemostratigraphy of carbonates and fossils are different (P.C. Boggiani, unpublished data, Gaucher et al 2003, Alvarenga et al 2004, Figueiredo et al 2006). The uncertainties of isotope measurements were 0.1% for carbon and 0.2% for oxygen, based on multiple analyses of an internal laboratory standard (BSC, Borborema skarn calcite)
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