• Ashok Sahni

      Articles written in Journal of Earth System Science

    • Palynostratigraphy and depositional environment of Vastan Lignite Mine (Early Eocene), Gujarat, western India

      M R Rao Ashok Sahni R S Rana Poonam Verma

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      Early Eocene sedimentary successions of south Asia, are marked by the development of extensive fossilbearing, lignite-rich sediments prior to the collision of India with Asia and provide data on contemporary equatorial faunal and vegetational assemblages. One such productive locality in western India is the Vastan Lignite Mine representing approximately a 54–52 Ma sequence dated by the presence of benthic zone marker species, Nummulites burdigalensis burdigalensis. The present study on Vastan Lignite Mine succession is based on the spore-pollen and dinoflagellate cyst assemblages and documents contemporary vegetational changes. 86 genera and 105 species belonging to algal remains (including dinoflagellate cysts), fungal remains, pteridophytic spores and angiospermous pollen grains have been recorded. On the basis of first appearance, acme and decline of palynotaxa, three cenozones have been recognized and broadly reflect changing palaeodepositional environments. These are in ascending stratigraphic order (i) Proxapertites Spp. Cenozone, (ii) Operculodinium centrocarpum Cenozone and (iii) Spinizonocolpites Spp. Cenozone. The basal sequence is lagoonal, palm-dominated and overlain by more open marine conditions with dinoflagellate cysts and at the top, mangrove elements are dominant. The succession has also provided a unique record of fish, lizards, snakes, and mammals.

    • Nature and composition of interbedded marine basaltic pumice in the ~52–50 Ma Vastan lignite sequence, western India: Implication for Early Eocene MORB volcanism offshore Arabian Sea

      Sarajit Sensarma Hukam Singh R S Rana Debajyoti Paul Ashok Sahni

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      The recognition of pyroclasts preserved in sedimentary environments far from its source is uncommon. We here describe occurrences of several centimetres-thick discontinuous basaltic pumice lenses occurring within the Early Eocene Vastan lignite mine sedimentary sequence, western India at two different levels –one at ~5 m and the other at 10 m above a biostratigraphically constrained 52 Ma old marker level postdating the Deccan Volcanism. These sections have received global attention as they record mammalian and plant radiations. We infer the repetitive occurrence of pumice have been sourced from a ~52–50Ma MORB related to sea-floor spreading in the western Arabian Sea, most plausibly along the Carlsberg Ridge. Pyroclasts have skeletal plagioclase with horsetail morphologies ± pyroxene ± Fe–Ti oxide euhedral crystals, and typically comprise of circular polymodal (radii ≤10 to ≥30 μm), non-coalescing microvesicles (>40–60%). The pumice have undergone considerable syngenetic alteration during oceanic transport and post-burial digenesis, and are a composite mixture of Fe–Mn-rich clay and hydrated alteredbasaltic glass (palagonite). The Fe–Mn-rich clay is extremely low in SiO₂, Al₂ O₂, TiO₂ , MgO, alkalies and REE, but very high in Fe₂O₃, MnO, P, Ba, Sr contents, and palagonitization involved significant loss of SiO₂, Al₂O₃, MgO and variable gain in Fe₂O₃, TiO₂, Ni, V, Zr, Zn and REE. Bubble initiationto growth in the ascending basaltic magma (liquidus ~1200–1250◦C) may have occured in ~3 hr. Shortdistance transport, non-connected vesicles, deposition in inner shelf to more confined lagoonal condition in the Early Eocene and quick burial helped preservation of the pumice in Vastan. Early Eocene Arabian Sea volcanism thus might have been an additional source to marginal sediments along the passive margin of western India.

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