Geochemistry of carbonate formations of the Chhattisgarh Supergroup, central India: implications for Mesoproterozoic global events

Abstract

The Chhattisgarh Supergroup is one of the major Proterozoic marine sedimentary sequences of India. It consists of largely undeformed and unmetamorphosed siliciclastic, volcaniclastic, and carbonate formations deposited in two sub-basins, Hirri and Bharadwar, separated by an Archean greenstone belt. In spite of its apparent importance for Mesoproterozoic oceanic records, very few geochemical studies have been carried in the basin. Here, we present results of our high resolution geochemical and C–O–Sr isotopic studies in two carbonate formations of the supergroup: the Charmuria and the Chandi. We observe elevated δ13C values increasing from 2.6‰ to 3.6‰ in these formations, which is consistent with the globally reported late Mesoproterozoic values. Such consistently positive δ13C values are attributed to increased organic carbon burial in the basin margins during the deposition of these carbonates. Based on the principles of δ13C isotope stratigraphy, we suggest a depositional age between 1.0 and 1.2 Ga for these carbonates which form the upper part of the supergroup. The lowest 87Sr/86Sr ratios obtained from the Charmuria and Chandi formations, 0.70723 and 0.70816, respectively, are more radiogenic than the contemporaneous seawater, suggesting that the Sr isotopic system of the formations are altered. Based on the similarity in the δ13C values, we stratigraphically correlate the carbonate formations of the Raipur Group in both the Hirri and Bharadwar sub-basins. We also present a compilation of available δ13C and 87Sr/86Sr records from all the Proterozoic sedimentary successions of India and compare it with the global datasets. We find that while the Indian basins possess records of the Bitter Springs and Shuram δ13C anomalies, they lack evidence for the other major global events of the Proterozoic.