Carbonate rocks and related facies with vestiges of biomarkers: Clues to redox conditions in the Mesoproterozoic ocean |
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| Institution: | 1. Geological Studies Unit, Indian Statistical Institute, 203 Barrackpore Trunk Road, Kolkata 700108, India;2. Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantock''s Close, BS8 1TS Bristol, United Kingdom;3. Polish Geological Institute, Rakowiecka 4, 00-975 Warszawa, Poland;4. Cabot Institute, University of Bristol, BS8 1UJ, Bristol, United Kingdom;5. School of Earth Sciences, University of Bristol, BS8 1RJ Bristol, United Kingdom;1. School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lillybank Gardens, G12 8QQ Glasgow, UK;2. Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK;3. Department of Geosciences, Eberhard Karls University Tübingen, Wilhelmstrasse 56, 72074 Tübingen, Germany;4. School of Geosciences, Meston Building, King''s College, University of Aberdeen, UK;1. Hiroshima University, Higashi-Hiroshima, Japan;2. University of Delhi, New Delhi, India;3. National Institute for Polar Research, Tokyo, Japan;4. National Science Museum, Tsukuba, Japan;5. Pondicherry University, Pondicherry, India;1. Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantock''s Close, BS8 1TS, United Kingdom;2. Polish Geological Institute, Rakowiecka 4, 00-975 Warszawa, Poland;3. Cabot Institute, University of Bristol, Cantock''s Close, BS8 1TS, United Kingdom;4. Department of Earth Sciences, University of Bristol, BS8 1RJ, United Kingdom;5. Dipartimento di Biologia, Ecologia e Science della Terra, Università della Calabria, Via P. Bucci Cubo 15b, 87036 Rende, Italy;1. Tectonics, Resources and Exploration (TRaX), Department of Earth Sciences, University of Adelaide, SA 5005, Australia;2. Geological Studies Unit, Indian Statistical Institute, Kolkata 700108, India;3. Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA, Australia;4. Department of Earth and Planetary Sciences/Geotop, McGill University, Montréal H3A 0E8, Canada;5. Tectonics, Resources and Exploration (TRaX), Adelaide Microscopy, University of Adelaide, SA 5005, Australia |
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| Abstract: | The Raipur Group of the Chattisgarh Basin preserves two major Late Mesoproterozoic carbonate platforms. The lower platform is about 490-m thick, separated from the upper platform (~ 670 m thick) by a 500-m thick calcareous shale. Carbonate strata cover almost 40% of the Chattisgarh Basin outcrop and represent two major platform types: a) a non-stromatolitic ramp (the Charmuria/Sarangarh Limestone) and b) a platform developed chiefly in the intertidal to shallow subtidal environment with prolific growth of stromatolites (the Chandi/Saradih Limestone). The first platform consists primarily of the black Timarlaga limestone that is locally replaced by early diagenetic dolomite. This carbonate platform experienced strong storm waves and was subsequently drowned by a major transgression, during which extensive black limestone–marl rhythmite was deposited, followed by deposition of the Gunderdehi Shale. The carbonate factory was later re-established with development of an extensive stromatolite-dominated Charmuria/Sarangarh platform that ranged from restricted embayment to open-marine conditions. Sea-level change played a major role in controlling the broad facies pattern and platform evolution. The δ13C signatures of the Chattisgarh limestones, falling within a relatively narrow range (0 to + 4‰) are typical for Upper Mesoproterozoic carbonate rocks. δ18O values, however, have a greater range (− 5.7 to − 13.3‰) indicating significant diagenetic alteration of some samples. Likely dysoxic or anoxic conditions prevailed during deposition of the black Timarlaga limestone and well-oxygenated conditions during deposition of the Gunderdehi Shale and Saradih/Chandi stromatolite. The lack of 17β,21α (moretanes) and high Tmax values suggest mature organic matter in the non-stromatolitic ramp. A paucity of diagnostic eukaryotic steroids indicates that algae were rare in the Chattisgarh Basin. A high content of hopanes supports a generally bacterially-dominated Proterozoic ocean in which various stromatolites flourished. |
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