Carbon isotopes of Mesoproterozoic–Neoproterozoic sequences from Southern São Francisco craton and Araçuaí Belt, Brazil : paleographic implications.

Abstract

This paper addresses the carbon isotope variations observed on Mesoproterozoic and Neoproterozoic carbonates from the southeastern part of the Sa˜o Francisco craton and Arac¸uaı´ Belt, Brazil. Carbonates were collected across sections of the Mesoproterozoic Espinhac¸o Supergroup (Rio Pardo Grande Formation) and of the Neoproterozoic units of the Sa˜o Francisco basin, including: (i) dolomites and marls of the Macau´bas Group (Domingas Formation); (ii) dolomite pebbles and carbonatic matrix of the diamictites of the Jequitaı´ Formation; (iii) limestones of the overlying Bambuı´ Group. Limestones of the Espinhac¸o Supergroup present a flat trend of positive d13CPDB values (varying betweenC1 andC2‰), while samples of the Macau´bas Group present an upward trend of decreasing carbon isotopic values (fromC0.7 toK4.0‰). The lower d13CPDB 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 d13CPDB values (K3.1 and K0.6‰). Except for carbonatic pelites placed above the diamictites, that present d13CPDB of C7.7‰, limestone samples of all the sections of the Bambuı´ Group have d13CPDB values above C8‰. The data presented here reveal significant differences between carbonates from the Espinhac¸o and Macau´bas Groups, indicating that this latter unit may be correlated with the diamictites from the Jequitaı´ Formation, as already suggested by previous stratigraphic studies. The data also reveal the absence of the low positive d13CPDB carbonates (belowC3‰) frequently present at the base of the Bambuı´ Group, thus suggesting that the deposition of this unit in the Serra do Cabral and Jequitaı´ areas took place after the regional positive d13CPDB 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 Bambuı´ Group sediments.