• Sarada P Mohanty

      Articles written in Journal of Earth System Science

    • Characterization of organic carbon in black shales of the Kachchh basin, Gujarat, India

      Hema Srivastava Ajoy K Bhaumik Devleena Tiwari Sarada P Mohanty Dattatray J Patil

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      Thirty-three black shale samples from four locations on the onland Kachchh basin, western India were analyzed to characterize organic carbon (OC), thermal maturity and to determine the hydrocarbon potential of the basin. Upper Jurassic black shales from the Jhuran Formation (Dhonsa and Kodki areas) are characterized by the presence of chlorite, halloysite, high $T_{\mathrm{max}}$, low OC, low hydrogen index and high oxygen index. These parameters indicate the OC as type IV kerogen, formed in a marine environment. The rocks attained thermal maturity possibly during Deccan volcanism. Early Eocene samples of the Naredi Formation (Naliya-Narayan Sarovar Road (NNSR) and the Matanomadh areas) are rich in TOC, smectite, chlorite and framboidal pyrite, but have low $T_{\mathrm{max}}$. These indicate deposition of sediments in a reducing condition, probably in a lagoonal/marsh/swamp environment. Organic carbon of the Naredi Formation of NNSR may be considered as immature type III to IV kerogen, prone to generate coal. Core samples from the Naredi Formation of the Matanomadh area show two fold distribution in terms of kerogen. Organic carbon of the upper section is immature type III to IV kerogen, but the lower section has type II to III kerogen having potential to generate oil and gas after attaining appropriate thermal maturity.

    • Mixed carbonate–siliciclastic sedimentation in the Upper Cretaceous Nilkanth Formation, Garhwal Himalaya, India

      SHRUTI R MISHRA ADITI SHARMA PARTHA P CHAKRABORTY SARADA P MOHANTY SATISH C TRIPATHI

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      The Upper Cretaceous Nilkanth Formation awaits a process-based depositional model despite being a topic of discussion between stratigraphers, palaeobiologists and structural geologists over the last few decades. Sedimentary facies analysis of a $\sim$50 m thick section along a $\sim$2.8 km long section along Rishikesh–Tal Bidhashini in Pauri Garhwal district of Uttarakhand allowed documentation of mixed carbonate–silicicalstic facies types, dominantly consisting of sand- and pebble-sized carbonate debris mixed with siliciclastics in a proximal to distal facies tract. Ten different facies types that include matrix rich and matrix-poor shelly conglomerate, mixed clastic-carbonate wackestone, packstone, impure calcirudite and calcarenite, biomicrite and ferruginous sandstone are documented. Delineation of facies association and documentation of facies stacking pattern provide a post-Santonian mixed carbonate– siliciclastic sedimentation history of the Nilkanth Formation, deposited in the form of mass flows of varied rheology on a barred low- to moderate-gradient carbonate ramp, formed at the leading edge of the India plate before its collision with the Kohistan–Ladakh arc. Carbonate clasts comprising bivalves, crinoids, algae, bryozoan, etc., were produced in a narrow high-energy transgressive coastline and supplied across shelf along with reworked siliciclastics from clastic shoreface bar. It is argued that the reworked fossils, including the bryozoa Ceriocava Nilkanthi, present within massflows may not justify fixing of an absolute age for the formation but may definitely help in providing an age range.

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