Carbon and oxygen isotope profiles across Meso-Neoproterozoic limestones from central Brazil: Bambuı́ and Paranoá groups

We present carbon and oxygen isotopic data on carbonates along three profiles of the Neoproterozoic Bambuı́ group in central Brazil. This unit covers an area of more than 300 000 km and comprises carbonate–silicliclastic sediments at the base that grade into siliciclastic sediments towards the top. The Bambuı́ group overlies by unconformity the Paranoá group, which consists mostly of siltstone, quartzite and minor limestone. The data presented here improve the stratigraphic correlation within the Bambuı́ basin and show that it evolved in an environment significantly different from that of the Paranoá basin. Our data show large fluctuations of dCPDB in limestones from the Bambuı́ Group (from +0.8 to +13.5‰) in all the three studied areas. Some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale. This observation is relevant considering the lack of fossil record and other stratigraphic markers in Neoproterozoic sequences. We also present the first isotopic profiles along the Paranoá–Bambuı́ transition, which shows that the dCPDB values grade from +1.0‰ in the Paranoá group, to +2.6‰ in the lower portion of the Bambuı́ group, increasing up to +12‰ in the upper part of this unit. Based on our carbon isotope data, as well as other geological, mineralogical and Nd isotope studies, we argue that the sediments of the Paranoá group were deposited on an open platform that was fully connected to the ocean. On the other hand, the sediments of the Bambuı́ group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790–600 Ma. This unit displays an increased amount of clastic sediments upwards. We argue that the high carbon isotope values observed in limestones and marlstones from the Bambuı́ group are correlated to worldwide high carbon isotope values reported for the Neoproterozoic. However, we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the Bambuı́ carbonates. © 2000 Elsevier Science B.V. All rights reserved.