Articles written in Journal of Earth System Science

    • Evidence of the Permian marginal marine sedimentation recorded in sub-surface drill cores, Lower Gondwana successions, southern India


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      Evidence of marine influences within the Permian Barakar Formation of the Pranhita–Godavari (P–G) Basin and its age equivalent, the Kommugudem Formation of the Krishna–Godavari (K–G) Basin, are previously investigated from the outcrop studies. The present work carefully documents the signatures of tidal and wave influences from the Early Permian rocks, solely based on excellently preserved subsurface drill core samples from both the basins. Tidalites, represented by laterally accreted tidal bundles, tidal rhythmites, tidal beddings, oppositely directed strata bundles, and double mud drapes, are preserved within the sandstone–mudstone heterolithic rocks, signifying deposition predominantly in an upper subtidal–intertidal setting. Association of wave-generated structures with the tidalites are indicative of open marine waves, interacting with the tides in the upper subtidal to intertidal region. Records of such tidal and wave processes unambiguously point to the significant marine influence within the continental setup during the Lower Gondwana sedimentation in both the P–G and the K–G basins during the Early Permian time, which indicates a regional encroachment of the sea onto the land.


      $\bullet$ A unique approach to study the tidal and wave signatures within the Early Permian sediments from the Pranhita–Godavari Basin and the Krishna–Godavari Basin using subsurface drill core samples.

      $\bullet$ The presence of tidal features such as tidal bundles, tidal rhythmites, tidal beddings, with occurrence of the spring–neap tidal cyclicity within the thinly bedded sandstone–mudstone heterolithic rocks signify sedimentation in upper subtidal-intertidal settings.

      $\bullet$ The intercalation of tidal deposits with combined flow and wave-generated structures indicates an open tidal flat setting.

      $\bullet$Such features unambiguously point to the presence of marine influence within the continental riftogenic basins during the Early Permian time.

    • Origin and significance of glauconites in a sequence stratigraphic perspective: Sylhet Formation, Assam and Assam–Arakan Basin, India


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      The paper documents occurrence, origin and significance of glauconite-rich intervals at certain specific stratigraphic levels within Early to Middle Eocene Sylhet Formation of Assam and Assam–Arakan Basin. The microfacies with which glauconite is associated are fossiliferous silty shale, glauconitic sandstone, lime mudstone–wackestone and wackestone–packstone. The glauconites occur in three forms, viz., glauconite pellets, infillings within bioclasts and cements (i.e., non-pellet stringers and blebs), representing a complete spectrum of maturity with presence of four types, nascent, slightly evolved, evolved and highly evolved varieties. The elliptical, immatured pellets occur in glauconitic sandstone and exhibit yellowish-green to grass green colour, while relatively matured pellets are dark brownish green to brown, irregular, broken and mostly associated with shale. The Sylhet glauconites indicate deposition in a tideaffected marginal marine to inner-shelf carbonate ramp with low sedimentation rate, and normal marine salinity and represent an overall fining-up 2nd order TST. High-resolution sequence stratigraphic analysis allows to subdivide the 2nd order TST into 3rd order systems tracts: (i) a basal TST, represented by shale/limestone, (ii) an HST incorporating sandstone, limestone and shale, and (iii) a TST at the top. The intervening 3rd order HST is further subdivided into 4th order parasequence sets separated by marine flooding surfaces. Electrolog correlation reveals that the ubiquitous presence of glauconite-rich horizons corresponds to marine flooding events (MFE) of 3rd and 4th order sea-level changes. The glauconites associated with MFE result in a condensed section with minimum sediment supply and provide a key parameter to build the sequence stratigraphic architecture. The occurrences of green clays may possibly owe their origin due to migration of K$^+$/Fe$^{2+}$ ions from seawater or surrounding sediments to a given reducing microsystem centered on organic debris. The sources of K and Fe are seawater and detrital minerals, respectively, and are achieved by the diffusion-nucleation-crystal growth process.

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