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MPC-03 Precambrian isotope chemostratigraphy

C, O and Sr isotope study of Shahabad limestone of Bhima basin, Southern India

Nagarajan Ramasamy, SASTRA University (India)
Nagendra Raghavendra, Anna University (India)
Sial Alcides Nobrega, Federal University of Pernambuco (Brazil)
Ellam Rob M, Scottish Universities Environmental Research Centre (United Kingdom)
 

A carbonate dominated shallow marine basin named Bhima Basin consists of five formations (in ascending order, namely Rabbanpalli (clastic) Formation, Shahabad (limestone) Formation, Halkal (shale) Formation, Katamadevarhalli (limestone) Formation and Harwal (shale) Formation) are widely distributed in northern Karnataka of southern India. In which Shahabad Formation has been selected for the study of secular variation in C, O and Sr isotope composition of marine micritic limestones. δ¹³C values are varying in wide range between +3.50‰PDB to -1.38‰PDB. Likewise, the δ¹⁸O values are recorded range between -5.76‰PDB to 12.93‰PDB. ⁸⁷Sr/⁸⁶Sr ratio for these limestone samples varying between 0.70699 and 0.7117. Higher burial rate of organic matter could have been responsible for the positive δ^{13C values which has confirmed for Neoproterozoic successions. Most of the samples show the δ18O values very similar to the average Proterozoic carbonate values. Albeit most of the samples are retaining the original isotopic signature, one forth of the samples show very high negative values > 10‰PDB, which clearly shows the diagenetic alteration on this limestones. Same time, 3/4th of the samples have Mn/Sr values much less than 1 and no definite trend between δ13C and δ18O is suggesting that water rock ratios are sufficient to alter the δ18O , but not enough alter δ13C during diagenesis. The diagenetic alteration is also confirmed by petrography study and also by higher 87Sr/86Sr ratios. The Primary Positive δ13C values coupled with low 87Sr/86Sr ratio suggest that these limestones have been deposited during late Neoproterozoic (Vendian) to Cambrian and which are comparable with other Proterozoic basins of India as well as global basins. Unfortunately this study is concentrated on surface samples of Shahabad Formation; hence we could not concentrate on Chemostratigraphic aspects. A detailed narrow sampling study (on lithosections) in near future can able to discuss about the PC-C boundary on this basin.}