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

Partha Pratim Chakraborty ¹, Tapan Pal ¹, Tanay Dutta Gupta ¹, Kalyan Surya Gupta ¹

Affiliations
1 Geological Survey of India, Calcutta, IN

Abstract

Detailed facies analysis in Eocene Mithakhari Group reveals eight different lithofacies of wide ranging paleogeographic significance. These facies are i) disorganised matrix-supported conglomerate, ii) graded matrix-supported conglomerate, iii) graded pebbly sandstone, iv) massive and thick-bedded sandstone, v) plane laminated and cross-stratified sandstone, vi) interbedded sandstone and mudstone, vii) massive to faintly laminated shale and viii) interbedded shale and coal; which are grouped into-five different facies associations (FA). viz. Subaerial alluvial plain (FAl), Shallow water, wave dominated shelf (FA2), Delta slope (FA3), Prodelta slope (FA4) and Submarine fan (FA5).

Lying unconformably on oceanic basement (ophiolite slices?) these sediments constitute short truncated successions. Litholog measurement in three isolated sections viz. Kaushalyanagar. Sagwannala and Rangat-Nimbutala reveal widely varying facies succession pattern. Frequent facies change, predominance of massflow deposits, signatures of synsedimentary basinal disturbance and wide paleogeographic variation indicate sedimentation in small isolated basins in an immature trench-slope setting. Profusion of progradational depositional cycles and evidences of emergence in the studied sections provide indication of frontal accretion and tectonic shoaling in the filling history of these basins.

Keywords

Sedimentology, Depositional Environment, Accretion, Eocene, Middle Andaman.

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Partha Pratim Chakraborty ¹, Soumen Paul ¹, Amlan Das ¹

Affiliations
1 Department of Applied Geology, Indian School of Mines, Dhanbad - 826 004, IN

Abstract

Preliminary study through process-Related facies analysis in late Paleoproterozoic-Early Mesoproterozoic Mungra Sandstone Formation of Kolhan Group, the least studied stratigraphy in Singhbhum Geology, reveals two major facies types, viz, hummocky sandstone bodies (below fair weather wave base) and planar and crossstratified sandstones (above fair weather wave base). From contact relations between facies types and facies succession development, a lower shoreface environment is inferred. An E-WW paleoshoreline orientation is suggested for the Mungra Sandstone Sea.

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Partha Pratim Chakraborty ¹, Subir Sarkar ²

Affiliations
1 Department of Applied Geology, Indian School of Mines, Dhanbad - 826 004, IN
2 Department of Geological Sciences, Jadavpur University, Kolkata - 700 032, IN

Abstract

Detailed facies analysis in late Proterozoic Rewa Shale reveals three physically distinctive facies that differ in their paleogeography. Between shoreface and outer shelf the facies types are distributed as (I) shale with thin siltstone/Fine sandstone interbeds of outer shelf, (II) shale interbedded with HCS bearing sheet sandstone of inner shelf within storm wave base and (III) sandstone interbedded with silty shale of distal shoreface paleogeography. High resolution facies succession analysis reveals three different orders of depositional cyclicity, the lowest of which may be correlated with third order depositional 'sequence'. Thin (av 3 5 cm) discrete levels of emergence with their invariable associations and sharp transitions from inner shelf to shoreface facies mark the subaerial discontinuities, which punctuate the apparently monotonous. Rewa Shale succession Far from the coastline on a siliciclastic ramp that lacks shelf slope break, these features, even though cryptic, can provide important clues for sea level falls in shale dominated successions.

Keywords

Depositional Cyclicity Emergence, Sequence, Rewa Shale, Son Valley, Central India.

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Partha Pratim Chakraborty ¹, Basab Mukhopadhyay ²

Affiliations
1 Department of Applied Geology, Indian School of Mines, Dhanbad - 826 004, IN
2 Geodata Division, Central Headquarters, Geological Survey of India, Kolkata - 700 016, IN

Abstract

Two sections of Oligocene Andaman Flysch fan that belong to two different facies associations viz inner fan and mid fan lobe are studied for their soft-sediment deformation (SSD) features Seismic origin is proposed for these features in view of their lateral persistency and stratigraphic selectivity Detailed field measurements of these turbidite sections at Corbyn's Cove, South Andaman (259 m thick) and Kahpur North Andaman (1397 m thick) recorded 28 and 18 nos of SSD layers respectively These layers were compared with historical earthquake records of last 86 years (I e 1914 to 2000) around the studied sections and quantitatively assessed for (l) long-term sedimentation rate and (n) estimation of environment-specific sedimentation rate, if any Active delta shedding from growing orogenic front of Himalayas is believed to be responsible for high sedimentation rate (217 m/lOOOyr at Cove and 58 m/lOOOyr at Kahpur) recorded from this fan system The observed difference in sedimentation rate between the two studied sections is interpreted to be paleogeography driven.

Keywords

Soft-Sediment Deformation, Rate of Sedimentation, Palaeoseismology, Oligocene, Andaman Flysch Group, Andaman Islands.

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Tapan Pal ¹, Partha Pratim Chakraborty ², R. N. Ghosh ¹

Affiliations
1 Geological Survey of India, Opn: WB-SK-AN, ER, Geological Survey of India, 5th floor, North Building, Bhubignan Bhawan, DK-6 Block, Salt Lake, Kolkata - 700 091, IN
2 Geology Department, Indian School of Mines, Dhanbad, IN

Abstract

Chromite placers occurring in Rutland coast of Andaman and Nicobar Islands, India, show PGE incidence. Serpentinised dunite containing thin stringers/lenses of chromite occurring adjacent to placer was possibly the source rock of the chromite placers. Chemical analysis of these chromite placers show PGE elements incidence in the range: Ru <10-56 ppb, Ir <10-12 ppb, Rh <10-14 ppb, Pd <10-136 ppb, Pt <10-28 ppb. Normalization of these PGE values against CI-chondrite indicates enrichment of Ir and Ru and depletion of Rh and Pt, a typical character of ophiolitic chromite. Except few Pd values, the PGE values depict highly fractionated PGE distribution pattern. High Nig-olivine (Fo_92-90)and highly fractionated PGE pattern may suggest derivation of the source rock from boninitic melt. Ru-Ir-Os bearing sulphides or alloys were crystallised at early magmatic stage, Pt, Pd were remobilized during hydrothermal activity and concentrated in serpentine.

Keywords

Chromite Placers, PGE Incidence, S-Undersaturation, Boninite, Rultand, Andamans.

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