Ramasamy NAGARAJAN¹, John S. ARMSTRONG-ALTRIN², Alcides N. SIAL³, Raghavendra NAGENDRA⁴ & Rob. M. ELLAM^5
1Curtin University, Department of Applied Geology, School of Engineering and Science, CDT 250, Miri, 98009,
Sarawak, Malaysia, e-mail: nagarajan@curtin.edu.my
²Universidad Nacional Autónoma de México, Unidad Académica de Procesos Oceánicos y Costeros, Instituto de Ciencias del Mar y Limnología, Circuito Exterior s/n, 04510, México D.F., México, e-mail: john_arms@yahoo.com
³Universidade Federal de Pernambuco, Departmento de Geologia, Nucleo de Estudos Geoquímicos e Laboratório de Isótopos Estáveis (NEG - LABISE), Caixa Posta 7852, 50670-000 Recife, PE, Brazil.
⁴Anna University, Department of Applied Geology, Chennai, 600 025, India 5Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride, G750 QF, UK

CARBON, OXYGEN, AND STRONTIUM ISOTOPE GEOCHEMISTRY OF THE PROTEROZOIC CARBONATE ROCKS, BHIMA BASIN, SOUTH INDIA: IMPLICATION FOR DIAGENESIS

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A carbonate dominated shallow marine Proterozoic Bhima basin consists of carbonates and clastic rocks, and is widely distributed in the northern part of the Karnataka state, south India. Limestones of the Shahabad Formation have been selected for this study and secular variations in C, O, and Sr isotope compositions are reported. ¹³C values are varying from +3.50 to -1.38‰_PDB. Similarly, the  ¹⁸O values recorded a range between -5.76 and -12.93‰_PDB ⁸⁷Sr/⁸⁶Sr ratio for these limestones is varying between 0.70699 and 0.7117. Higher burial rate of organic matter could have been responsible for the positive¹³C values, which is common in the Proterozoic carbonate rocks. The ¹⁸O values in most of the samples are similar to the average value of the Proterozoic carbonate rocks. Albeit some of the samples are modified in their original isotopic signature, which shows high negative ¹⁸O values < -10‰_PDB, Mn/Sr ratios > 1.5 and a negative trend between ¹⁸O and ⁸⁷Sr/⁸⁶Sr, suggesting that the ¹⁸O and ⁸⁷Sr/⁸⁶Sr isotopic signatures of Bhima carbonates have been altered by diagenesis. The post depositional diagenetic alteration is also supported by the petrographic characters of these carbonates.