Browsing by Author "Mishra, Diwakar"

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    High energy transgressive deposits from the Late Jurassic of Wagad, Eastern Kachchh, India
    (Elsevier, 2008) Mishra, Diwakar
    The whole sedimentary succession (ca 600 m thick) of Wagad area ranging in age from Callovian to Early Kimmeridgian has been divided in to three Formations namely Washtawa, Kanthkot and Gamdau in ascending order. Prograding Kanthkot Formation was frequently interrupted by transgressions. Field and petrographic investigations revealed that the Kanthkot Formation represents three fossiliferous marker beds corresponding to Transgressive sequence I; Transgressive sequence II and Transgressive sequence III. These transgressive sequences are composed of two lithounits: medium to coarse grained/gritty, graded to massive, sheetlike, fossiliferous calcareous sandstone (storm lag unit I) and fossiliferous mudrocks (swell lag unit II). The thickness of the unit I varies from 5 to 75 cm and contains mostly convexly oriented shell fragments and whole shell of Pelecypods, Cephalopods and Brachiopods. Unit II (5-15 cm) is distinguished by sheetlike, massive or laminated, yellowish colour, soft fossiliferous mudrocks. This unit is intercalated with moderately bioturbated sandy siltstone. Unit I is dominant over Unit II in the sequences. Study suggests that the transgressive units were deposited close to wave base by high energy storm flows followed by low energy marine swells during transgression. The intense storms played a major role in the distribution of siliciclastics and nonclastic materials. Storms are evidenced by the occurrence of two distinctly different types of units (storm lags and swell lags). High energy levels are characterized by sand dominated sequence, abundance of reworked sediment particles, high proportion broken shells with convex up orientation and erosional and sharp nature of basal contacts of units together with well preserved bioclasts. Sudden short term changes from high to low energy during transgression are explained by the occurrence of medium to coarse grained siliciclastics interbedded with moderately bioturbated mudrocks. Moderately bedded individual strata, high content of coarse clastics along with polished granule size quartz and abundance of comminuted shells indicate a significant change in depositional setting, possibly closure approach of the source of terrigenous fraction or source uplift. Synrift sedimentation in the present study is documented by an abundance of coarse clastics and an over all aggradational nature of transgressive sequences.
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    Lithofacies and depositional dynamics of golden Oolite (Bathonian), Kachchh Mainland, Gujarat (India)
    (Elsevier, 2006) Mishra, Diwakar; Tiwari, R.N.
    The Golden Oolite Member of the Patcham Formation consisting of 84 m thick alternate sequence of limestones and mudstone are well exposed in the Jhura Dome, Kachchh Mainland, Gujarat. Petrographic study of limestones reveals four types of microfacies: oolitic fossiliferous grainstone (A1); fossiliferous intraclastic grainstone (A2); sandy fossiliferous grainstone (A3); and pebbly fossiliferous grainstone (A4). The microfacies normally form microfacies assemblages with calcareous mudstone (B1) and are stacked vertically in ascending order as A1–A3, A1–B1, A4–B1 and A2–B1. The assemblage (A1–A3) is characterised by interbedding of moderately to thickly bedded, hard and compact, golden coloured oolitic fossiliferous grainstone and sandy fossiliferous grainstone exhibiting small scale low angle planar cross beddings. It contains well preserved bioclast. Assemlage (A1–B1) is distinguished by rhythmic alternations of earthy, concretionary calcareous mudstone and moderately to very thickly bedded golden coloured oolitic fossiliferous grainstone showing ripple bedding, abundant bioclast and reworked intraclasts, whereas assemblage (A4–B1) exhibits rhythmic alternations of bioturbated, earthy, concretionary calcareous mudstone and moderately to thickly bedded pebbly fossiliferous grainstone. Assemblage (A2–B1) is characterised by earthy,bioturbated,calcareous mudstone containing thin uneven beds of fossiliferous intraclastic grainstone having micritic intraclast and microfossils. The study of lithofacies suggests two main depositional processes for the formation of golden oolite: (1) The high energy physical sedimentation from current flows during transgression characterized by irregular to sharp nature of basal contact of each cycle, abundance of well preserved bioclasts and reworked intraclasts and large scale ripple bedding; (2) Settling of fines from suspension during fair-weather period as distinguished by homogenous fine grained interbeds of mudstone in the sequence. The transition of facies from A1–A3 to A2–B1 marks deepening upward event during Bathonian period from shallow inner shelf to calcareous mud dominated outer shelf. The energy condition was very high during deposition of the lower and middle part (A1–A3 and A1–B1 assemblage) whereas low to very low as revealed by abundance of bioturbated calcareous mudstone (B1) with episodic interruption of moderate to high energy storm event depositing A4 (pebbly fossiliferous grainstone) microfacies during the upper part (A4–B1 and A2–B1 assemblage) of the sequence.
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    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.
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    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.
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    Sedimentology and paleocurrent study of the early triassic rocks in the Ruhuhu basin, SW Tanzania
    (College of Natural and Applied Sciences, University of Dar es Salaam, 2020) Kiwango, Audax Syprian; Mishra, Diwakar
    The sedimentary succession ranging in age from Permian to Early Triassic in Ruhuhu basin was subdivided into eight informal lithostratigraphic units, identified by the symbols from K1 to K8. Manda Formation (K8) belongs to Early Triassic age and comprises two Members, i.e., Lower Kingori Member and Upper Lifua Member (150-200 m thickness). The implications of present study relate to inter and intra basinal correlations which may provide regional depositional framework from a mass of local details. Present investigation connoting the lithofacies studies in conjunction with palaeocurrent and grain size analysis of the early Triassic strata aims at interpreting the depositional environment of Lifua Member. Based on the present study, five lithofacies have been identified, namely (i) Massive matrix supported paraconglomerate (Gmm), (ii) Massive sandstone (Sm), (iii) Parallel-horizontal laminated sandstone (Sh), (iv) Planar cross-bedded sandstone (Sp) and (v) Fine silt, mud and clay (Fl). Sandstone facies (Sm, Sh, and Sp) exhibit normal grading and unimodal palaeocurrent direction. Grain size analysis indicated that the sandstones were moderately sorted, finely skewed, mesokurtic and most of the grains were silty-sand. Bivariate scatter plots suggest that the Lifua Member sandstone is of riverine environment. Lithofacies, palaeocurrent and grain size studies suggest fluvial environment dominated by sand channel deposits.
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    Sedimentology, sequence stratigraphy and syn-rift model of younger part of Washtawa formation and early part of Kanthkot formation, Wagad, Kachchh basin, Gujarat
    (Springer Nature, 2009) Mishra, Diwakar; Biswas, S. K.
    The 600 m thick prograding sedimentary succession of Wagad ranging in age from Callovian to Early Kimmeridgian has been divided into three formations namely, Washtawa, Kanthkot and Gamdau. Present study is confined to younger part of the Washtawa Formation and early part of the Kanthkot Formation exposed around Kanthkot, Washtawa, Chitrod and Rapar. The depositional architecture and sedimentation processes of these deposits have been studied applying sequence stratigraphic context. Facies studies have led to identification of five upward stacking facies associations (A, B, C, D, and E) which reflect that deposition was controlled by one single transgressive — regressive cycle. The transgressive deposit is characterized by fining and thinning upward succession of facies consisting of two facies associations: (1) Association A: medium — to coarse-grained calcareous sandstone — mudrocks alternations (2) Association B: fine-grained calcareous sandstone — mudrocks alternations. The top of this association marks maximum flooding surface as identified by bioturbational fabrics and abundance of deep marine fauna (ammonites). Association A is interpreted as high energy transgressive deposit deposited during relative sea level rise. Whereas, facies association B indicates its deposition in low energy marine environment deposited during stand-still period with low supply of sediments. Regressive sedimentary package has been divided into three facies associations consisting of: (1) Association C: gypsiferous mudstone-siltstone/fine sandstone (2) Association D: laminated, medium-grained sandstone — siltstone (3) Association E: well laminated (coarse and fine mode) sandstone interbedded with coarse grained sandstone with trough cross stratification. Regressive succession of facies association C, D and E is interpreted as wave dominated shoreface, foreshore to backshore and dune environment respectively. Sequence stratigraphic concepts have been applied to subdivide these deposits into two genetic sequences: (i) the lower carbonate dominated (25 m) transgressive deposits (TST) include facies association A and B and the upper thick (75m) regressive deposits (HST) include facies association C, D and E. The two sequences are separated by maximum flooding surface (MFS) identified by sudden shift in facies association from B to C. The transgressive facies association A and B represent the sediments deposited during the syn-rift climax followed by regressive sediments comprising association C, D and E deposited during late syn-rift stage.