Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

V. N. Vasudev ¹, S. M. Naqvi ², M. Shukla ³, B. Uday Raj ²

Affiliations
1 Department of Mines and Geology, Race Course Road, Bangalore, IN
2 National Geophysical Research Institute, Hyderabad, IN
3 Birbal Sahni Institute of Palaeobotany, Lucknow, IN

Abstract

Stromatolites of stratifera type showing development of pseudocolumnar structures occur both in chert as well as dolomites near the Kumsi Manganese Mines in the Shimoga schist belt. This report ot stromatolites in the Shimoga belt further confirms widespread biogenic activity prior to 2.6 Ga in the Dharwar Craton.

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B. Kumar ¹, S. Das Sharma ¹, M. Shukla ², M. Sharma ²

Affiliations
1 National Geophysicai Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Birbal Sahni Institute of Palaeobotany, Lucknow - 226 007, IN

Abstract

Carbon isotope records on carbonates from the Bhima basin are characterized by ¹³C enriched values (up to 4‰ PDB). These data are integrated with the available palaeontological records to suggest a Terminal Proterozoic (post - Varanger) age for the Bhima sediments. The carbon isotope compositions of Katamadevarhalli and Shahabad carbonates analysed in the present study exhibit two distinct trends. While the Katamadevarhalli carbonates show a positive trend, those from Shahabod display a negative trend from bottom to top, probably indicating long-term temporal trend of carbon isotope values.

Keywords

Chronostratigraphy, Carbonates, Carbon and Oxygen Isotopes, Bhima Basin, Terminal Proterozoic, Karnataka.

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D. J. Patil ¹, Das Sharma ¹, B. Kumar ¹, A. M. Dayal ¹, M. Shukla ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Birbal Sahni Institute of Palaeobotany, Lucknow - 226 007, IN

Abstract

Carbon isotopic compositions of carbonate rocks from Narji and Koilkuntla Formations belonging to the Kurnool Group range frorn -1.5 to +2.2%0 and +3.1 to +4.3%0 respectively. 13C values on coexisting organic carbon are around -26%0. ⁸⁷Sr/⁸⁶Sr ratios of limestones from Koilkuntla Formation (0.7086 - 0.7104) exhibit less variation than those belonging to Narji limestones (0.7073 - 0.7132). Bivariant plots of ⁸⁷Sr/⁸⁶Sr and δ¹³C against Mn/Sr and δ¹⁸O show open system diagenetic trends for the carbonate rocks of Narji Formation. Similar plots for Koilkuntla Formation do not reveal any definite alteration trends. Comparison of carbon isotope data coupled with the least altered ⁸⁷Sr/⁸⁶Sr values of Narji and Koilkuntla limestones shows similarity in the isotopic ratios exhibited by other worldwide Late Neoproterozoic carbonate successions. This finding is corroborated by available age information on the underlying Cuddapah Supergroup of rocks as well as palaeontological evidences on the rock formations of Kurnool Group.

Keywords

Carbonate Rocks, Carbon and oxygen isotopes, ⁸⁷Sr/⁸⁶Sr, Kurnool Group, Andhra Pradesh.

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