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M. S. Srinivasan ¹, B. K. Chatterjee ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi, IN

Abstract

Carbonate rocks are widespread on the Andaman-Nicobar Islands, northern Indian ocean, ranging in age from Early Miocene to Pleistocene. Lithologically they range from deep water to shallow watcr calcarenites.

Thin section analysis of the limestones has revealed the following major lithofacies: lithic foraminiferal-algal grainstone-packstone; lithic foraminiferal-algal wackstone-mudstone; volcanoclastic packstone-wackstone/radiolarian cherts; foraminiferal packstone; coralgal boundstone and coarse lithic foraminiferal-algal-molluscan packstone grainstone. They exhibit sequential changes in their fabrics, biogenic and clastic constituents. Depths of deposition ranged from high energy, shallow neritic (lithic foraminiferal- algal grainstone-packstone facies) at the begnining of Early Miocene to progressively lower bathyal marked successively by lithic foraminiferal-algal wackstone-mudstone, volcanoclastic packstone-wackstone and radiolarian chert facies. The sudden influx of glass shards and tephras in thc later Early Miocene suggests that progressive deepening of the basin occurred contemporaneously with explosive volcanism in neighboring areas. During this interval of basinal subsidence, the limestones received abundant argillaceous material from subaerial erosion of nearby volcanic terrain. Gradual reduction in the frequency of glass shards and pumice from Middle Miocene (Ongeian Stage) onwards indicates culmination of volcanic eruptions and basinal stability. Gradual shallowing to subtidal and patch reef (Coralgal facies) environments began to form from the Middle Miocene onwards at Little Andaman, Havelock, Nicholson and other islands. However, deep basinal conditions prevailed during the Late Miocene to Early Pliocene at Neill, Sir Hugh Rose and Car Nicobar Islands. These different conditions may be attributed to tectonism in the Andaman sea region which occurred during the close of Middle Miocene. Thereafter, younger limestone facies of Plio-Pleistocene age, reflect a shoaling upwards to probable intertidal conditions due to rapid infilling and vertical uplift of the basin floor.

No hydrocarbons have been observed in the limestone sequences. However, the regional stratigraphic and petrographic studies reveal the presence of potential reservoir rocks in the Ongeian-Havelockian limestones of Middle Miocene age on the Andaman-Nicobar Islands.

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S. K. Ghosh ¹, B. K. Chatterjee ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi-22I 005, IN

Abstract

Siliciclastic rocks around Surgutaria viIIage are mature to sub-mature quartzarenites to quartz-wackes, Quartz-wackes are produced due to extensive readjustment of sericite-chert matrix during phyllomorphic stage of development in the arenite frame work. Various statistical parameters, and current structures clearly reveal that the deposition of siliciclastics took place in a stable shallow marine to tidal flat environment.

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N. Pandey ¹, B. K. Chatterjee ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi 221005, IN

Abstract

The Precambrian Banded Iron Formation of Gandhamardan hill, Keonjhar district, Orissa, belongs to Noamundi Group. It is restricted largely to oxide facies of Lake Superior type. The sulphide facies in association with black chert is present in small amount. It is almost unmetamorphosed and exhibits many depositional and penecontemporaneous deformation sedimentary structures. A preliminary field and petrographic examination reveals that iron formation was initially deposited at least in major part as hematite-magnetite-chert rock in a near-shore epicontinental environment partially restricted from free-circulation of sea water. Iron and silica were precipitated almost contemporaneously largely due to physico-chemical changes in sea water. The presence of siliceous stromatolitic laminations and alga-like spheroid Archaeospheroides barbertonis similar to that of Fig Tree cherts, however, suggests biogenic activity during the deposition of the Banded Iron Formation.

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Sumit K. Ghosh ¹, B. K. Chatterjee ²

Affiliations
1 Wadia Institute of Himalayan Geology, 33, General Mahadev Singh Road, Dehra Dun 248001, IN
2 Department of Geology, Banaras Hmdu University, Varanasi 221 005, IN

Abstract

The early Proterozoic Kolhan sihciclastrc rocks of the Kolhan Group (c. 2100 Ma) exhibit extensive-development of thickly bedded sandstone, impersistent conglomerate and siltstone. Six major lithofacies were recognised, viz, granular lag (GLA), granular sandstone (GSD), sheet sandstone (SSD). parallel laminated sandstone (PLSD), rippled sandstone (RSD) and thinly laminated siltstone-sandstone (TLSD). The vertical and lateral arrangement of lithofacies with their general finmg upward sequence is interpreted to reflect superimposition of retrograding shoreline on the earlier, prograding humid alluvial fan-sandflat complex and can be comparable with the present day storm-induced sedimentation on the NW Atlantic shelf.

Keywords

Palaeoenvironment, Petrology (Sedimentary), Early Proterozoic, Kolhan, Keonjhar, Orissa.

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