• Partha Pratim Chakraborty

      Articles written in Journal of Earth System Science

    • Outcrop signatures of relative sea level fall on a siliciclastic shelf: Examples from the Rewa Group of Proterozoic Vindhyan basin

      Partha Pratim Chakraborty

      More Details Abstract Fulltext PDF

      The Rewa Group of the Vindhyan Supergroup in the Son valley begins with a thick (∼200m) dominantly shaly, shelfal succession, occurring between the Dhandraul Formation of the Kaimur Group (fluvial sandstone) below and Drammondganj Formation of the Rewa Group (marginal marine sandstone) above. Such a stratigraphic disposition indicates a sharp rise in relative sea level at the onset of Rewa sedimentation, inducing a shelfal depth to the Vindhyan basin. However, a number of wedge-shaped, sandstone/conglomerate bodies (maximum thickness 23.5 m) occur at multiple stratigraphic levels within the aforesaid deeper water shale succession, which appear to be of much shallow water origin representing regressive deposits. Though these bodies do not define a single physically continuous unit, either vertically or laterally, they are still designated by a single term ‘Asan Sandstone’ in the literature. On the other hand, the encasing shelfal shales are termed as Panna and Jhiri Shales, in accordance with their occurrence below or above the so-called ‘Asan Sandstone’. The present study reveals that in different sections spread over the Son valley, there are several discrete regressive wedges occurring vertically, and their depositional environment is also variable, ranging between braided fluvial, shoreface fan and braid delta. The features common to most of the regressive coarser clastic bodies are:

      •invariable presence of deeper water, shelfal shale below (Panna or Jhiri);

      •the underlying shale at places shows signatures of emergence at the top;

      •laterally impersistent, wedge-like geometry; and

      •presence of granular transgressive lags at the top. These coarser clastic wedges record several episodes of regressive deposition during short-term falling stage or lowstand of relative sea level at the early phase of Rewa sedimentation. Each individual phase of regressive deposition was, however, followed by flooding and resumption of shelf mud deposition.

    • 1420 Ma diabasic intrusives from the Mesoproterozoic Singhora Group, Chhattisgarh Supergroup, India: Implications towards non-plume intrusive activity

      Priyabrata Das Kaushik Das Partha Pratim Chakraborty S Balakrishnan

      More Details Abstract Fulltext PDF

      Besides offering significant clues towards tracking the geochemical evolution of the mantle and architectural reconstruction of different ‘supercontinent’, geochronological and geochemical appraisal of igneous inputs are also important to bracket the depositional time frame of any lithopackage, particularly, the unfossiliferous sedimentary successions. The present study deals with diabasic intrusive within Mesoproterozoic Saraipalli Formation, which is an argillaceous constituent present at the basal part of nearly 400 m thick four-tiered unmetamorphosed but deformed sedimentary succession of Singhora Group, Chhattisgarh Supergroup, central India. The SE–NW trending intrusive comprises mainly of plagioclase and augite together with minor orthopyroxene, biotite and opaque minerals. Though some plagioclase laths are partially sericitized, the ophitic-to-subophitic texture of the rock is well preserved. Major and trace element geochemical data indicate that this intrusive is basalt-to-basaltic andesite in character and of subalkaline basalt affinity. Multi-element plot shows overall LILE-enrichment and enrichment of Pb and slight depletion of Nb and P, coupled with moderate La/Nb and Th/Nb ratios. Zr, Y and Nb ternary diagrams plot in the fields of within plate basalt. Selected HFSE ratios indicate a non-plume source with crustal assimilation/sediment mixing. Sm–Nd and Rb–Sr isotope data show that the intrusive has Srinitial and Ndinitial of 0.709377–0.706672 and 0.510919–0.510815, respectively. Positive 𝜀tNd [t = 1420 Ma] values (+0.3 to + 2.3) indicate depleted isotopic nature of their protolith. The calculated $T_{DM}$ age is 1.7–1.9 Ga. The mineral-whole rock isochron data (Sm–Nd systematics) of the intrusive implies an emplacement age of ca. 1420 Ma. Considering synchronous terrain boundary shear zone development in Bastar craton on the southeastern part of the Singhora basin, mafic magmatism in Eastern Ghats and large-scale basic intrusion in Sausar mobile belt, a major tectono-thermal event around 1400 Ma is surmised that affected eastern Indian craton. Moreover, geochronology of a bedded porcellanite unit (ca. 1500 Ma) at the base and a discordant basic intrusive (ca. 1420 Ma) allowed a unique opportunity to qualitatively offer an upper bound of time bracket for the deposition of Saraipalli Formation, i.e., ∼80 Ma.

    • Nodular features from Proterozoic Sonia Sandstone, Jodhpur Group, Rajasthan: A litho-biotectonic perspective

      Arvind Singh Vikash Anand Prabhas Pandey Partha Pratim Chakraborty

      More Details Abstract Fulltext PDF

      The Sonia Sandstone of Proterozoic Jodhpur Group, Marwar Supergroup, exposed around the Sursagar dam area of Jodhpur town, Rajasthan exposes two varieties of nodular features, often spectacular in shape and size. On the basis of mode of occurrence (intra- or interbed) and stratal involvement (single or multiple) the features are classified as Type I and II. From granulometric and microscopic (optical and scanning electron) studies carried out on sandstones from the nodules and their host sandstones, geochemical analysis (SEM-EDAX) of intragranular cement present within Type I nodules, and appreciation of control of associated fracture system within Type II nodules, it is proposed that the two types of nodules vary in their formative mechanism and stage of formation. While Type I nodules are identified as product of processes operative at the early diagenetic, pre-lithification stage, the Type II nodules are undoubtedly the result of post-lithification origin triggered by formation of fracture system. Here we propose generation of vapour pressure (not exceeding the overlying hydrostatic pressure) by decay of thin, laterally impersistent organic mat as the causal factor for intrabed nodule (Type I) formation, which forced rarefication of local grain packing \tetit {vis-a-vis} early diagenetic silica cementation. The study warrants necessity of more studies on nodules to understand possible roles of organic matter and bedtransgressive fracture systems in their formation, going beyond the generalised secondary mineralization hypothesis.

  • Journal of Earth System Science | News

© 2017-2019 Indian Academy of Sciences, Bengaluru.