• AJAB SINGH

      Articles written in Journal of Earth System Science

    • Model showing influence of tectonics and energy condition of the fluvial environment on facies architecture: A case history of Quaternary alluvial deposits of Purna basin, central India

      ASHOK K SRIVASTAVA MANGLA N BANSOD AJAB SINGH VIVEK M KALE

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      Present work includes identification of lithofacies and reconstruction of depositional environments including controls of tectonics, and energy condition of the medium on sedimentation pattern of alluvial deposits of Purna basin, central India. The basin exhibits good development of dominantly areno-argillaceous sediments along with restricted occurrences of boulder-pebbly lithounits, covering together an area of 6,522 km$^{2}$. These sediments are also characterized by preservation of various calcrete morphotypes, vertebrate remains and Youngest Toba Tuff (YTT) ash in certain stratigraphic units. The work is based on detailed field and lithological studies of 20 river-cut sections exposed along the course of Purna river, representing entire alluvial deposits in both vertical and lateral profiles. Three lithofacies associations have been identified, i.e., (1) gravelly facies association (FA-1), (2) sandy facies association (FA-2) and, (3) silty-clayey facies association (FA-3). These associations consist of total 10 lithofacies distributed as: (i) matrix supported massive gravelly (Gmm) facies, (ii) matrix supported gravelly (Gmg) facies, (iii) horizontally stratified gravelly (Gh) facies, (iv) planar stratified gravelly (Gp) facies (FA-1), (v) horizontal sandy (Sh) facies, (vi) planar cross stratified sandy (Sp) facies, (vii) low angle cross bedded sandy (Sl) facies, (viii) massive sandy (Sm) facies (FA-2), (ix) laminated sandy (Fl) facies, and (x) palaeosol (P) facies (FA-3). Various architectural elements have also been identified. Based on lateral and vertical profiling of the lithofacies architecture, it is interpreted that gravel dominated facies in the mountainous region of the basin area are deposited by medium to high energy debris flow,whereas sandy and silty-clayey facies in the plain area are because of saltation and suspension modes of deposition under medium to low energy condition of depositing medium. The tectonic and topographical controls are well reflected on the sedimentation pattern and the same has been illustrated by proposing a model. Discussions along with the model have also been made on the progressive development of bazada or, pediment zone along the northern margin of the basin marked by the foothills of Satpura mountain ranges.

    • Geochemistry and constrained $^{40}$Ar/$^{39}$Ar dating of Youngest Toba Tuff glass shards, Purna alluvial basin, Central India

      ASHOK K SRIVASTAVA AJAB SINGH

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      Geochemistry and $^{40}$Ar/$^{39}$Ar dating of glass shards of Youngest Toba Tuff ash from two localities, i.e., Gandhigram (G-2) and Hudki (H-1) of the Purna alluvial basin, Central India have been carried out. Major oxides, determined by EPMA, exhibit SiO$_{2}$ (78.4–79.6 wt%), Al$_{2}$O$_{3}$ (11.7–13.0 wt%), Na$_{2}$O (2.0–2.6 wt%) and K$_{2}$ O (3.4–4.4 wt%) and minor and trace elements, analyzed by ICP-MS, reveal higher contents of LREEs, i.e., La (26.52–27.15 ppm), Ce (50.25–51.26 ppm), Pr (5.02–5.24 ppm), Nd (20.1–21.31 ppm), Sm (3.1–3.36 ppm) and lower values of HREEs, i.e., Tb (0.5–0.57 ppm), Ho (0.5–0.6 ppm), Tm (0.28–0.35 ppm), Yb (2.68–2.94 ppm), Lu (0.45–0.48 ppm) along with prominent dip of Eu. Trace elements show high content of Rb (188.6–200.6 ppm), Sr (37.79–48.14 ppm), Ba (347.9–375.6 ppm) and low Ga (8.90–9.67 ppm), Sc (2.33–3.09 ppm), Nb (10.24–11.6 ppm). The chemical composition of glass shards is similar to the Youngest Toba Tuff (YTT) ash reported from Indian peninsula and Malaysia including Toba Caldera. The $^{40}$Ar/$^{39}$Ar dates show plateau ages of 0.79 $\pm$ 0.02 Ma (790 $\pm$  0.02 ka) and 0.77 $\pm$  0.05 Ma (770 $\pm$  0.05 ka) for ash of Gandhigram and Hudki localities. These dates seem to be geologically meaningless and are not acceptable as the YTT ash is universally considered to be of ca. 75 ka age based on geochemistry, fission track and $^{40}$Ar/$^{39}$Ar dating of ash minerals. Therefore, the dates presently generated through $^{40}$Ar/$^{39}$Ar technique on glass shards are being discarded considering mobile behaviour of Ar isotopes, alkalis in the melts and glass shards due to isotopic fractionation and post-depositional processes.

      $\bf{Highlights}$

      $\bullet$ Glass shards geochemistry of Toba ash from Purna basin, India suggest its compositional similarities with the YTT ash of several river basins of India, Malaysia, ocean basins and source area of Toba Caldera, Indonesia.

      $\bullet$ $^{40}$Ar/$^{39}$Ar ages of 790 $\pm$  0.02 ka and 770 $\pm$  0.05 ka, on glass shards, are exceptionally high as compared to YTT, therefore, unrealistic and unacceptable, hence, rejected. Literature survey shows that $^{40}$Ar/$^{39}$Ar dating results of volcanic glass shards in particular are influenced by magmatic and post-depositional processes.

      $\bullet$ Sedimentological attributes and lithofacies architecture of ash-bearing successions reveal fluvio-lacustrine set-up of deposition for the ash.

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