Carbon and oxygen isotope profiles across Meso-Neoproterozoic limestones from central Brazil: Bambui and Paranoa groups

We present carbon and oxygen isotopic data on carbonates along three profil es of the Neoproterozoic Bambui group in central Brazil. This unit covers a n area of more than 300 000 km(2) and comprises carbonate-silicliclastic se diments at the base that grade into siliciclastic sediments towards the top . The Bambui group overlies by unconformity the Paranoa group, which consis ts mostly of siltstone, quartzite and minor limestone. The data presented h ere improve the stratigraphic correlation within the Bambui basin and show that it evolved in an environment significantly different from that of the Paranoa basin. Our data show large fluctuations of delta C-13(PDB) in limes tones from the Bambui Group (from + 0.8 to + 13.5 parts per thousand) in al l the three studied areas. Some of these fluctuations represent stratigraph ic markers that can be used as a chronostratigraphic tool within a basin sc ale. This observation is relevant considering the lack of fossil record and other stratigraphic markers in Neoproterozoic sequences. We also present t he first isotopic profiles along the Paranoa-Bambui transition, which shows that the delta C-13(PDB) values grade from + 1.0 parts per thousand in the Paranoa group, to + 2.6 parts per thousand in the lower portion of the Bam bui group, increasing up to + 12 parts per thousand in the upper part of th is unit. Based on our carbon isotope data, as well as other geological, min eralogical and Nd isotope studies, we argue that the sediments of the Paran oa group were deposited on an open platform that was fully connected to the ocean. On the other hand, the sediments of the Bambui group were deposited in an epicontinental sea and during a tectonic inversion in a foreland bas in at about 790-600 Ma. This unit displays an increased amount of elastic s ediments upwards. We argue that the high carbon isotope values observed in limestones and marlstones from the Bambui group are correlated to worldwide high carbon isotope values reported for the Neoproterozoic. However, we al so point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic com position of the Bambui carbonates. (C) 2000 Elsevier Science B.V. All right s reserved.