Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

S. K. Baksi ¹, B. S. Venkatachala ²

Affiliations
1 Indian Statistical Institute, Calcutta, IN
2 Oil & Natural Gas Commission, Dehradun, IN

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B. S. Venkatachala

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B. S. Venkatachala ¹

Affiliations
1 Institute of Petroleum Exploration, Oil & Natural Gas Commission, Dehra Dun, IN

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No Abstract.

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V. V. Sastri ¹, A. T. R. Raju ¹, R. N. Sinha ¹, B. S. Venkatachala ¹, R. K. Banerji ¹

Affiliations
1 Institute of Petroleum Exploration, Kaulagarh Road, Dehra Dun, IN

Abstract

Outcrops ranging in age from Late Jurassic to Recent are located nearer the western margin of the basin. The eastern parts of the basin are covered by alluvium and contain a relatively thicker and more complete sedimentary sequence. The exposed and subsurface sedimentary sequence are adequately fossiliferous and several faunal as well as palynological zones are established.

The basin was formed during Late Jurassic by sagging of a part of the Indian shield, mainly along the dominant NE-SW Eastern Ghat trend. The basin was well differentiated into depressions and ridges formed owing to taphrogenic fragmentation and block faulting along the dominant basement trends during the onset of Late Jurassic. Initial sedimentation was under non-marine environment. The first marine transgression occurred during the close of Late Jurassic. Marine environment of sedimentation continued till Cretaceous, although through a series of minor transgressions and regressions. The depocentres were mainly due west.

A major regression occurred during the close of Cretaceous. The basin underwent an easterly tilt and the depocentres shifted due east prior to marine transgression during the beginning of Tertiary. The differentiation of the basin into depressions and ridges was almost lost during Eocene. Sedimentation during the Tertiary period was marked by repeated transgressions and regressions as is evidenced by biofacies, isopachs, and several sedimentation breaks which are relatively more pronounced in the western parts of the basin.

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B. S. Venkatachala ¹

Affiliations
1 Palynology Laboratory, Institute of Petroleum Exploration, Oil & Natural Gas Commission, Dehradun, IN

Abstract

Palynological investigations carried out during recent years from the Coastal Gondwana sediments have necessitated rethinking on earlier age assignments made on the basis of mega plant fossils.

The Sriperumbudur Beds, Raghavapuram and Vemavaram Shales, Sivaganga Beds and Athgarh Sandstones were dated as Late Jurassic on the basis of Ptilophyllum flora occurring in them and the absence of Weichselia and Onychiopsis. Foraminiferal evidences ascribed an Early Cretaceous age. This discrepancy between plant and animal fossil evidences was attributed to differential evolutionary rates. An Early Cretaceous age assignment is now favoured on the basis of the occurrence of important taxa of stratigraphic value such as: Impardecispora purverulentus, and/or I. apiverrucata, Crybelosporites stylosus, Polycingulatisporites reduncus, Ischyosporites crateris, Aequitriradites spinulosus, Microcachryidites antarcticus, and Staplinisporites caminus.

The plant megafossil assemblage, though distinct; might not be representative or complete. The age cannot be assigned on negative evidences such as the absence of certain fossils. The frequently mentioned discrepancy in age inferred from faunal and floral evidences appears a fallacy. The incompleteness of the evidence, either faunal or floral, is a reality and should be ascribed to our inability to find forms rather than their actual absence.

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B. S. Venkatachala ¹

Affiliations
1 Institute of Petroleum Exploration, Dehra Dun, IN

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No Abstract.

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S. M. Naqvi ¹, B. S. Venkatachala ², Manoj Shukla ², B. Kumar ¹, R. Natarajan ¹, Mukund Sharma ²

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN
2 Birbal Sahni lnstitute of Palaeobotany, Lucknow 226007, IN

Abstract

Silicified cyanobacterial remains are recorded from the black cherts interbedded in the Donimalai Formation of the Archaean Sandur Schist Belt and amorphous organic matter in the stromatolites underlying Deogiri Formation of the same belt. The presence of elemental carbon at the fringes of the cyanobacteria is confirmed by microprobe analysis. Carbon (δ¹³C= -1.4 ± 0.5% vs PDB) and oxygen (δ¹⁸O= + 10.6±0.5% vs SMOW) isotopic ratios and the presence of cyanobacteria extend the existence of photosynthetic activity into the Archaean.

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N. C. Mehrotra , B. S. Venkatachala , P. N. Kapoor

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N. C. Mehrotra , B. S. Venkatachala , P. N. Kapoor

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N. C. Mehrotra , B. S. Venkatachala , S. N. Swamy , P. N. Kapoor

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Naresh C. Mehrotra ¹, B. S. Venkatachala ¹, P. N. Kapoor ²

Affiliations
1 Birbal Sahni Institute of Palaeobotany, 53, University Road, Lucknow - 226 007, IN
2 KDMIPE, ONGC, Dehradun - 248 195, IN

Abstract

This paper presents an updated account of palynostratigraphic, source rock evaluation and palaeoenvironment data on Western Offshore and Krishna Godavari (K-G) Basins. Forty three dinoflagellate biohorizons have been identified in the Tertiary subsurface of Mumbai Offshore, facilitating a high resolution correlation of Oil bearing horizons. In K-G Basin integrated palynostratigraphic data and the composite general lithologies for the Mesozoic and Cenozoic have been presented in a tabular form; Eighty two dinoflagellate biohorizons have been identified. A very fine dinoflagellate based stratigraphic resolution has been achieved at most levels making it useful in geological modelling for hydrocarbon exploration in both Western Offshore and K-G Basin.

Useful information on matured organic matter facies distribution in Panna Formation (Western Offshore) and Cretaceous of K-G Basin has been summarized. Recently, potential source rock facies have been identified in Panna Formation in Western Offshore.

Limitations of present studies and future areas of focus have been outlined.

Keywords

Palynology, Hydrocarbon Exploration, Western Offshore, K-G Basin.

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B. S. Venkatachala ¹, Ashok Kumar ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehradun - 248 001, IN

Abstract

Vaishnodevi Limestone is a distinctive landmark succession in the Jammu Himalaya. Significant biotas have been identified from a well exposed chert outcrop near Bidda and shale near Muttal. The chert biota is dominated by mat forming crustose, coccoid, chroococcacean cyanobacteria that include Eoentophysalis belcherensisand Palaeopleurocapsa wopfnerii. The other coccoid taxa are Myxococcoides grandis, Gloeodiniopsis lamillosa, Eogloeocapsa avzyanica, Sphaerophycus cf., S. medium, Eosynechococccus moorei, E. grantiis, Siphonophycus kestron, S. robustum, Oscillatoriopsis sp., Circumvaginalis elongatus, Archaeoellipsoides major, and A. minor. Stratigraphically this is the oldest biota thus far known from the Vaishnodevi Limestone and is assigned a palaeoproterozoic-Mesoproterozoic age.

The Muttal biota is composed of Siphonophycus kestron, S. robustum, Oscillatoriopsis sp., Calyptothrix spp., Eosynechococcus medius, Archaeotrichion contortum, Archaeollipsoides bactriformis, A. grandis, A. obesus, Arctacellularia ellipsoidea, Nostochomorpha prisca, Paleonostocalia irregularia, Leiosphaeridia laminariala, Leiosphaeridia sp., Lophosphaeridum spp., Micrhystridium spp., Sphaerocongregus sp., Obruchevella sp., Chuaria sp., Kildinosphaera sp., Spiromorpha indet. and vase shaped microfossils (VSMs). The Muttal biota is from the topmost shale unit of Vaishnodevi Limestone and is assigned a Neoproterozoic age.

It is concluded that the age of the Vaishnodevi Limestone spans the entire Proterozoic and deposited in a supratidal to intertidal and subtidal environment within a shallow shelf regime.

Keywords

Palaeontology, Palaeoenvironment, Vaishnodevi Limestone, Jammu, Himalaya.

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B. S. Venkatachala ¹

Affiliations
1 Dehradun, IN

Abstract

No Abstract.

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N. C. Mehrotra , B. S. Venkatachala , S. N. Swamy , P. N. Kapoor

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No Abstract.

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