• SILADITYA SENGUPTA

      Articles written in Journal of Earth System Science

    • Interpretation of stratigraphy and structure of the Neoarchaean Dharwar Supergroup of rocks in Chitradurga area, Dharwar Craton

      ABHINABA ROY H M RAMACHANDRA SILADITYA SENGUPTA

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      The Neoarchaean Dharwar Supergroup of rocks in the Chitradurga area unconformably overlie the Mesoarchaean Peninsular Gneissic Complex in the west and are tectonically juxtaposed with Javagondanahalli Schist Belt in the east. The rocks of the supergroup have been divided into older Bababudan and younger Chitradurga Groups. We support the recent division of the Bababudan rocks into a lower conglomerate–sandstone facies association and an upper sandstone–mudstone facies association indicating tidal flat depositional environment. The Talya Conglomerate sequence at the base of the Chitradurga Group is inferred to represent a fault-controlled debris flow deposit. The basin opens out to the east where sedimentation and volcanism took place on an uneven basement surface. The Vanivilas and Ingaldhal Formations likely represent contemporaneous and overlapping sequences indicative of facies variation in space. The KM Kere Conglomerate at the base of the Hiriyur Formation represents a facies series comprising a sequence of volcanic–pyroclastic–volcaniclastic–epiclast association. We propose a four-fold stratigraphic classification with introduction of a new ‘Kantaramanahalli Formation’, placed above the Vanivilas and Ingaldhal Formations and below the Hiriyur Formation. The interpretation of multiple deformed nature of Dharwar Supergroup of rocks and the dominance of the second deformation ($D_{2}$) is supported. The initial irregularities on basement surface and the $F_{1}$ folds have significant role in fold superposition and outcrop patterns. The intra- and interformational ductile shear zones have dominant sinistral transcurrent component. Structural studies are conclusive of simple shear ($D_{2b}$ ) superposed on intense pure shear ($D_{2a}$) indicative of an overall transpressional type of horizontal tectonics.

    • Mesoproterozoic orogeny along the eastern boundary of Aravalli Craton, northwestern India: A structural and geochronological study of Hindoli–Jahazpur Group of rocks

      SILADITYA SENGUPTA KRISHNAPRIYA BASAK

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      Two linear NE–SW trending belts of platformal orthoquartzite-carbonate sequence, namely, the East Jahazpur Belt (EJB) and the West Jahazpur Belt (WJB), within a vast tract of slate–argillite–meta greywacke and subordinate meta volcanics of Hindoli Group (HG) together occur sandwiched between Vindhyan sediments to the east and Banded Gneissic Complex-II (BGC-II) to the west, at the eastern boundary of Aravalli Craton, Rajasthan, India. A NE–SW trending crustal-scale transpressive shear separates this low-grade supracrustal sequence from the gneisses of older Mangalwar Complex (BGC-II) to the west and the Great Boundary Fault (GBF) restricts it in the east against the Vindhyan rocks. A lensoid body of strongly deformed and mylonitised Jahazpur Granite (JG), is the basement for the supracrustal rocks and occupies the core of a SE verging D1-antiformal nappe deBned by the EJB (Jahazpur Group-I) rocks. This large scale D1 thrust nappe structure in the EJB is manifested by stacking of early recumbent folds in the carbonate and quartzite units and a prominent zone of brecciation and ferruginisation all along the eastern margin of EJB. In the WJB (Jahazpur Group-II), D1 is represented by large scale isoclinal reclined fold geometry, likely to be influenced by the major transpressive shear and refolding at its western margin. D2 deformation phase, variably superposed over the D1 structures, together controls the NE–SW disposition of Hindoli and Jahazpur rocks. Based on the structural patterns and lithostratigraphic characters, we propose a revised stratigraphy of the Hindoli–Jahazpur rocks. Whole rock Rb–Sr dating of the strongly deformed and mylonitised Jahazpur Granite constrained the age of the tectono-metamorphic event that affected these rocks. Two suites of mylonitised Jahazpur Granite from two different localities yielded ages of 1423 ± 52 and 1393 ± 33 Ma, using Sector 54 TIMS. The tectonism that resulted in folding, thrust nappe, shearing and mylonitisation involving basement JG of Neoarchean age and the overlying Paleoproterozoic Jahazpur–Hindoli supracrustal sequences, therefore, represents a Mesoproterozoic (${\sim}$1.4 Ga) orogeny that is correlatable with the breaking-up of Columbian Supercontinent along the eastern margin of the Aravalli craton.

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