Carbon isotopes of Mesoproterozoic-Neoproterozoic sequences from Southern Sao Francisco craton and Araquai Belt, Brazil: Paleographic implications

This paper addresses the carbon isotope variations observed on Mesoproterozoic and Neoproterozoic carbonates from the southeastern part of the Sao Francisco craton and Aracuai Belt, Brazil. Carbonates were collected across sections of the Mesoproterozoic Espinhaco Supergroup (Rio Pardo Grande Formation) and of the Neoproterozoic units of the Sao Francisco basin, including: (i) dolomites and marls of the Macaubas Group (Domingas Formation); (ii) dolomite pebbles and carbonatic matrix of the diamictites of the Jequitai Formation; (iii) limestones of the overlying Bambui Group. Limestones of the Espinhaco Supergroup present a flat trend of positive delta(13)C(PDB) values (varying between +1 and +2parts per thousand), while samples of the Macaubas Group present an upward trend of decreasing carbon isotopic values (from +0.7 to -4.0parts per thousand). The lower delta(13)C(PDB) values of this latter unit were obtained on the upper part of the section. Dolostone pebbles and carbonates in the matrix of the diamictite also present negative delta(13)C(PDB) values (-3.1 and -0.6parts per thousand). Except for carbonatic pelites placed above the diamictites, that present delta(13)C(PDB) of +7.7parts per thousand, limestone samples of all the sections of the Bambui Group have delta(13)C(PDB) values above +8parts per thousand. The data presented here reveal significant differences between carbonates from the Espinhaco and Macaubas Groups, indicating that this latter unit may be correlated with the diamictites from the Jequitai Formation, as already suggested by previous stratigraphic studies. The data also reveal the absence of the low positive delta(13)C(PDB) carbonates (below +3parts per thousand) frequently present at the base of the Bambui Group, thus suggesting that the deposition of this unit in the Serra do Cabral and Jequitai areas took place after the regional positive delta(13)C(PDB) excursion observed in other parts of the basin. Hence, it is proposed that these areas were paleo-highs during the deposition of the lower portion of the Bambui Group sediments. (C) 2004 Elsevier Ltd. All rights reserved.