Browsing by Author "Tiwari, R. N."

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Microfacies analysis of transgressive condensed sequence: a study from the Oxfordian of Kachchh Basin, Gujarat
    (Geological Society of India, 2007) Tiwari, R. N.; Mishra, Diwakar
    1-9m thick Dhosa Oolite Member of Chan Formation exposed in Kachchh Mainland, western India, represents a condensed transgressive sequence of the Oxfordtan age. It is composed of hard and compact, fossilifeious, sandy/conglomeratic oolitic limestone interbedded with friable, thinly bedded calcareous siltstone/fine sandstone Field&petrographic studies revealed six microfacies namely, (i)sandy/pebbly ironstone(A1), (ii)conglomeratic fossililerous ironstone (A2), (iii)conglomeratic fossiliferous oolitic packstone(B1), (iv) sandy oolitic packstone (B2), (v) siltstone (C1), (vi) fine grained sandstone (C2). These microfacies are grouped in to three microfacies assemblages namely (i)sandy oolitic Packstone-Siltstone assemblage (B2-C1), (ii) Siltstone-Fine grained sandstone assemblage (C1 - C2) and (iii) conglomeratic fossiliferous oolitic packstone - fine grained sandstone assemblage (C2-B1). The study of microfacies suggests that the deposition of the Dhosa Oolite Member took place in an open marine shallow shelf environment with clear proximal-to'distal trend from southeast to northwest. Two transgressive pulses were responsible for its formation. An earlier slow and frequently interrupted transgression together with subsidence is lecordedby abrupt change in clastic grain size, mixing of two lithologies and abundance of siliciclastics in each microfacies Later, rapid transgression is suggested by the extremely reduced thickness, erosive to irregular contacts, development of intraformatjonal conglomerates, multi phase reworked sediments, an increase in the abundance of carbonates and ammonites and negligible sediment input Lateral vanation in the microfacies and their assemblages together with variable extent of marine mega fauna (ammonites) from southeast to northwest marks the depositional slope ot the basin towards northwest during studied interval. The mixed lithologies in this condensed sequence together with the bioturbated nature of sediments suggest that the rate of sedimentation was extremely slow.
  • Thumbnail Image
    Item
    Provenance study of siliciclastic sediments, Jhura Dome, Kachchh, Gujarat
    (Geological Society of India, 2005) Mishra, Diwakar; Tiwari, R. N.
    Provenance of the siliciclastic rocks of Patcharn and Chari Formations (Bathonian to Oxfordian) has been ascertained by means of heavy mineral studies. The study shows abundance of transparent heavies like garnet, zircon, tourmaline, staurofite, rutile, hornblende, andalusite, kyanite, anatase and epidote, in decreasing order, and constitute 26.47 per cent of total heavies. The opaque group is represented mainly by goethite and limonite, which constitute 73.45 per cent. The statistical data of heavies reveals that garnet, zircon and tourmaline are more or less uniform throughout the stratigraphic column and constitute 43.92 per cent, 22.27 per cent and 19.99 per cent respectively. The average percentage of staurolite and rutile is 5.40 and 4.52. The percentage of anatase, kyanite, hornblende, anddusite and epidote is very less. On the basis of distribution pattern of heavy minerals in stratigraphic column, heavies are grouped into two distinct assemblages i.e. (i) Garnet-Staurolite-Hornblende-Kyanite-Epidotaes semblage, (ii) prismatic and rounded to subrounded grains of Zircon-Tourmaline-Rutile assemblage. The presence of heavy mineral assemblages in the stratigraphic column reveals that the sediments of Patcham and Chari Formations have been derived mainly from two 1ithologicalIy different Precambrian terrains; one is dominated by metamorphic rocks and the other is igneous (acid and basic), besides a little contribution of sedimentary source. Source rocks were situated close to basin of deposition in the noflheast and east i.e. Aravalli range and north and northwest dominated by Granite-Syenite suite belonging to Nagar-Parkar Massif. The variable ZTR index indicates manifestation of relief and climatic change in the source area.