• PRANAB DAS

      Articles written in Journal of Earth System Science

    • Understanding genesis of iron oxide concretions present in Dhandraul (Vindhyan) Sandstone: Implications in formation of Martian hematite spherules

      PRAKASH JHA PRANAB DAS DWIJESH RAY

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      The iron oxide concretions of Shankargarh (Allahabad), India belongs to Dhandraul Sandstone of Vindhyan Supergroup. Petrography of concretions shows abundant quartz grains embedded within the iron oxide cementation. XRD analysis of the concretion shows diagnostic peaks for quartz, hematite, and goethite. The A–CN–K and A–CNK–FM ternary diagrams drawn for concretion and host rock bulk composition clearly indicate the interaction of concretions rock with iron-bearing diagenetic fluids. A negative Ce anomaly, lower Th/U ratio, and enrichment of redox responsive trace element (e.g., vanadium) indicate concretion formation is redox-controlled. The concretions show Fe enrichment and Si depletion as compared to the host sandstone. The mass balance calculations indicate that the total Fe$_{2}$O$_{3}$ in the ferruginous sandstone system is 17.63 wt%. The iron mobilization and recycling in the sandstone pore spaces have formed concretions with Fe$_{2}$O$_{3}$ (25–35% by volume). The sandstone volume required to produce a 6 mm diameter iron oxide concretion is 1807.83 mm$^{3}$. The Fe laminae and random red colouration patterns in Dhandraul sandstone are consistent with the movement of iron-enriched fluid through pores and spaces. These iron oxide concretions have similarities with the hematite spherules discovered in the Burn Formation, Meridiani Planum, Mars.

      $\bf{Highlights}$

      $\bullet$ Random red colouration pattern, iron laminae, the role of advective and diffusive processes during the formation of the iron oxide concretions present in Dhandraul sandstone are discussed in detail.

      $\bullet$ Mineralogical and textural study of the Shankargarh iron oxide concretions using petrography and XRD.

      $\bullet$ Fe bearing paleofluid circulation, redox processes, and elemental mobility (enrichment and depletion of elements) are discussed in detail using whole-rock geochemistry of Shankargarh iron oxide concretions and associated sandstone.

      $\bullet$ The similarities and difference between Shankargarh iron oxide concretions and Martian hematite spherules.

    • Reflectance spectroscopic and geochemical characteristics of hydrocarbon microseepage-induced sediments from Assam–Arakan Fold Belt, India: Implications to hydrocarbon exploration

      SANTOSH GARAIN DEBASHIS MITRA PRANAB DAS

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      Characterizations of hydrocarbon microseepage-induced surface sediments are essential to utilize remote sensing and geochemistry as effective petroleum exploration tools. However, only few researchers have attempted spectroscopic and geochemical characterizations of microseepage-affected sediments and that too are focussed on arid or semi-arid climatic regions. This article aims to delineate the spectral, mineralogical and geochemical characteristics of the microseepage-affected sediments in Assam–Arakan Fold Belt (AAFB), NE India, dominated by intense precipitation. The analytical approach followed consists of diffuse reflectance spectroscopy, X-ray diffraction (XRD), X-ray Fluorescence (XRF), and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analyses. ReCectance spectroscopy reveals that microseepage-induced sediments are enriched in illitic clay and deficient in goethite than the unaffected regions. XRD studies support the presence of illitic clays in the sediments. Major element analyses indicate higher Al$_2$O$_3$ and K$_2$O and lower Na$_2$O and SiO$_2$ in the altered sediments. Trace element patterns point out enrichment of V, Cu, Zn, Ga, Zr, and Mo and depletion of Li, Cr, Co, Ni, Rb, Sr, Sc, and Y in the microseepage-affected sediments. Normalized rare earth element (REE) patterns are similar for both the altered and unaltered sediments, but the former are deficient in REE contents. Though based on limited sample analysis, the study indicates microseepage affected and unaffected sediments differ in spectral and geochemical characters. This may help in strengthening the microseepage model for hydrocarbons in high precipitation areas and thereby may help in reducing exploration risks in the basin.

      $\bf{Highlights}$

      $\bullet$ The study uses spectroscopy and geochemistry to characterize hydrocarbon microseepage-induced sediments.

      $\bullet$ Spectroscopic study indicates abundance of clay minerals and deficiency of ferric iron in microseepage-affected sediments.

      $\bullet$ Geochemical analysis shows minor compositional differences between microseepage-affected and unaffected sediments in terms of major and trace element distribution patterns.

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