HIGHLY C-13-ENRICHED CARBONATE AND ORGANIC-MATTER IN THE NEOPROTEROZOIC SEDIMENTS OF THE BAMBUI GROUP, BRAZIL

Paleontological and radiometric dating methods (Rb-Sr, K-Ar and Pb-Pb) suggest Neoproterozoic ages for sedimentary carbonate rocks of the Ba mbui Group in the southern part of the Sao Francisco Basin of central Brazil. The delta(13)C values for carbonates fall within two ranges, - 2 to +2 parts per thousand and +10 to +16 parts per thousand. For both groups coexisting organic carbon is depleted in C-13 by 28 +/- 2 part s per thousand relative to the carbonates. Similar to numerous Neoprot erozoic examples worldwide, secular variations in delta(13)C in the Ba mbui Group show a significant positive excursion beginning with values near 0 parts per thousand in carbonates immediately above a diamictit e and rising to values as high as +16 parts per thousand upsection. Th e Sr isotope data for carbonates from the Bambui Group agree with the age inferred from the secular variation in oceanic delta(13)C values. The striking feature in this study is that both the carbonate and tota l organic carbon are considerably more enriched in C-13 than previousl y reported for Neoproterozoic carbonates elsewhere. It is not clear if these unusual delta(13)C values are caused by primary of secondary pr ocesses.