• M G Thakkar

      Articles written in Journal of Earth System Science

    • Response of a dryland fluvial system to climate–tectonic perturbations during the Late Quaternary: Evidence from Rukmawati River basin, Kachchh, western India

      Archana Das Falguni Bhattacharya B K Rastogi Gaurav Chauhan Mamata Ngangom M G Thakkar

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      Dryland rivers, dominated by short-lived, localised and highly variable flow due to discrete precipitation events, have characteristic preservation potential, which serves as suitable archives towards understanding the climate–tectonic coupling. In the present study, we have investigated the fluvial records of a major, southerly-draining river – the Rukmawati River in the dryland terrain of southern Kachchh, in western India. The sediment records along the bedrock rivers of Kachchh register imprints of the Indian summer monsoon (ISM), which is the major source of moisture to the fluvial system in western India. The Rukmawati River originates from the Katrol Hill Range in the north and flows towards the south, into the Gulf of Kachchh. The field stratigraphy, sedimentology, along with the optical chronology suggeststhat a braided-meandering system existed during 37 ka period due to an overall strengthened monsoon. A gradual decline in the monsoon strength with fluctuation facilitated the development of a braided channel system between 20 and 15 ka. A renewed phase of strengthened monsoon with seasonality after around 15 ka which persisted until around 11 ka, is implicated in the development of floodplain sequences. Two zones of relatively high bedrock uplift are identified based on the geomorphometry and morphology of the fluvial landform. These zones are located in the vicinity of the North Katrol Hill Fault (NKHF) and South Katrol Hill Fault (SKHF). Geomorphic expression of high bedrock uplift is manifested by the development of beveled bedrock prior to or around 20 ka during weak monsoon. The study suggests that the terrain in the vicinity of NKHF and SKHF is uplifting at around 0.8 and >0.3 mm/a, respectively. Simultaneously, the incision in the Rukmawati River basin, post 11 ka, is ascribed to have occurred due to lowered sea level during the LGM and early Holocene period.

    • Nepheline syenite and related rocks at Meruda Takkar hill, northern Kachchh: Neoproterozoic Malani basement or Mesozoic alkaline magmatism?


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      An extremely small but significant exposure of possibly Precambrian or Pre-Deccan Mesozoic plutonic rocks spanning 200 $\times$ 90 m in size with 15 m elevation, remotely located amidst vast salt flats of the Great Rann of Kachchh received attention when it was first reported by Biswas in 1968; however, systematic petrological and petrographical studies remained undone. Therefore, in the present study, we addressed complete geological accounts to establish its possible origin. Our detailed geological study indicates that the complex is made up of nepheline and aegirine bearing alkali feldspar syenite with numerous fine-to-medium grained felsic dykes. It lays two viewpoints: (1) it has affinity with Trans Aravalli Belt anorogenic felsic magmatism, i.e., Precambrian and (2) it is genetically related to the pre-Deccan Mesozoic plutonic intrusives like Nir Wandh in Pachham Island and Mundwara and Sarnu–Dandali complexes in Rajasthan. The nearest felsic magmatic suite is an exposure of gray and pink granites of Nagar Parkar of Neoproterozoic age. Presence of undersaturated minerals and xenoliths of diorites within syenite parent body may suggest partial melting of lower crust probably during Rodinia supercontinent rifting. However, presence of alkali-rich minerals, alkali feldspars, kaersutite, aegirine, apatite and rutile needles and the complex being part of the Kachchh rift basin suggests its genetic resemblance with Pre-Deccan Mesozoic plutonic events. The radiometric age will confirm the age of this plutonic massif.


      Based on the field geological and petrographical studies of Meruda Takkar syenitic complex, two viewpoints have been raised:

      $\bullet$ The alkali feldspar syenite complex of Meruda has close affinity with the anorogenic felsic igneous suites contemporaneous with Malani Igneous Suite and slightly younger than post-Delhi Erinpura granites, i.e., Neoproterozoic intraplate magmatism chronologically equivalent to the splitting of Rodinia supercontinent,

      $\bullet$ The alkali feldspar syenites of Meruda Takkar have genetic and mineralogical resemblance with the pre-Deccan trap Mesozoic plutonic activities allied to what is prevailing at alkaline complexes of Mundwara and Sarnu–Dandali and in the northern Island belt alkaline intrusives of Nir Wandh, Kuran, Kaladungar and Sadhara sills and dykes.

      Radiometric age determination and geochemical analysis of the Meruda syenite are needed to establish its age and origin.

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