• Enigmatic molar-tooth structures (MTS) from Mesoproterozoic Deoban limestone, NW-Lesser Himalaya: Evidence for microbial decay and in-situ precipitation

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    • Keywords

       

      Molar-tooth structure; Mesoproterozoic; Lesser Himalaya; Carbon isotope; microbial decay; wave loading.

    • Abstract

       

      Molar-tooth structures (MTS) are enigmatic, micro-crystalline calcite-filled fissures, confined in Proterozoic carbonates. Here we present petrography, carbon isotope, total organic carbon (TOC) and morphological attributes in the context of interpreted palaeoenvironment to understand its development in the Mesoproterozoic carbonates of Lesser Himalaya. Lack of any detrital infill, uniform crystal size and gradational contact with host limestone indicate rapid calcite precipitation in fluid-filled cracks. Reworking of MT as intraclast, folding and offset of MT ribbons supports for early formation before significant lithification. Moderate TOC (0.1–0.9) is possibly due to organic matter preservation under sub-oxic to slightly anoxic/dysoxic conditions. Storm generated bedforms indicate deposition in between fair weather- and storm wave base. Average 1.4 per thousand depletion of $\delta^{13}$ C in MT relative to host limestone, presence of relict microbial laminae along the margin of the MT cracks and storm generated bedforms at outcrop scale indicates that the cracks might have formed by the combined effect of degassing of CO$_{2}$ generated during the microbial oxidation of organic matter and wave loading by storm. Precipitation of microcrystalline calcite within the cracks may have been triggered by alkalinity generated by the mixing of the outflowing CO$_{2}$ with seawater.

      $\bf{Highlights}$

      $\bullet$ Molar-tooth structures from Mesoproterozoic carbonates, lesser Himalaya.

      $\bullet$ Carbon and oxygen isotopic study from the molar-tooth structures.

      $\bullet$ Depletion in carbon isotope value in molar-tooth compared to host limestone.

      $\bullet$ CO$_{2}$ degassing combined with wave loading generated cracks.

      $\bullet$ Increased alkalinity due to outflow and mixing promoted precipitation.

    • Author Affiliations

       

      SUBHOJIT SAHA1 SOM DUTT1 RASIKH BARKAT2 3

      1. Sedimentology Division, Wadia Institute of Himalayan Geology, Dehradun 248 001, India.
      2. Department of Geology, University of Delhi, Delhi 110 007, India.
      3. Centre of Excellence for Energy Studies, Oil India Limited, Guwahati 781 022, India.
    • Dates

       
  • Journal of Earth System Science | News

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