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

Amitava Bandyopadhyay ¹

Affiliations
1 Marine wing, Geological Survey of India, 63, N.S.C. Bose Road, Calcutta - 700 040, IN

Abstract

A plug-like body of alkali olivine basalt occurs within the lower member of Wagad Formation near Khandek emplaced along a regional fault. Field relations indicate that the plug is post-Mesozoic, and late-to-post-tectonic in relation to the Late Mesozoic tectonic acitvity in the area. Plagioclase, clinopyroxene (titanaugite with 2V_z = 51°-55°) and olivine occur as major phenocrysts and augite, plagioclase, interstitial nepheline and palagonite are present in the groundmass. The rock contains nepheline both in the mode and in the norm. Lower Zr/Nb ratio is indicati ve of a mantle plume type origin for the alkalic magma. The fault apparently served as a conduit for the magma. AhotspotormantIe plume type origin for this alkali basalt has been suggested.

Keywords

Olivine Basalt, Plug, Petrology, Hot Spot, Kuchchh, Gujarat.

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Amitava Bandyopadhyay ¹

Affiliations
1 Geological Survey of India, A-2, B-wing, Rajaji Bhawan, Basant Nagar, Chennai- 600 090, IN

Abstract

Wagad Formation of eastern Kachchh, partly equivalent to the Bhuj Formation (Mesozoic) of the Mainland, exhibits a continuous sedimentation starting at the base with ferruginous sandstone interlaminated with shale (lower member) grading upward into an essentially subarkosic sandstone with interbedded kaolinitic shale (upper member). The top of the upper member is partly lateritised. The sandstone - shale interbedded sequence is found to repeat in several cycles and each of the cycle is capped by thick ferruginous sandstone.

Cross- bedded sandstone and subarkose with silty shale indicates a deltaic basin with distributaries flowing northwest and southwest. Thickening of sediments towards centre and west from other three directions indicates gradual deepening of the basin westward. Transitional sedimentation from ferruginous to subarkosic has experienced storm wave action for a short period as evident from hummocky stratification at a few places. Sedimentary structures in the area reflect initiation of Mesozoic sequence with a transgressive phase of deposition in a near shore tidal environment. Seasonal fluxes of sediment supply and maximum evaporation at times, as evident from gypsum in the sediment, have also been recorded. Sediments, its structures and freshwater plant remains denote a deltaic nonrnarine environment of deposition during upper Gondwana (Late Jurassic to lower Cretaceous (?)) and at least a part of the upper member sediments represent laterally migrating channel bar deposits.

Culmination of Mesozoic sedimentation is indicated by vertical tectonic movement in the entire Kachchh. This is responsible for formation of anticlinal fold,with broad crest that closes near Adesar (23°33'N : 70°59'E) in regional scale and a number of domes, half-domes and doubIy plunging anticlines intimately associated with high angle normal faults in local scale in the area. The Adesar anticline plunges at a low angle towards 60°and the axial trace of the fold trends NESW to ENE-WSW. The area is traversed by a number of high angle normal faults and transverse faults. One volcanic plug of alkali olivine basalt occurs within lower member of the Wagad Formation near Khandek. Field relations and petrology of this rock indicate a hot-spot or mantle-plume type origin of this post-Mesozoic alkali basalt. Other intrusives are either olivine basalt or olivine gabbro. The marine water transgresses after a long period of non-deposition under a calm environmental condition of a shallow basin at the time of lower Miocene. Bivalve fossils are found within rnicaceous sandstone and mottled claystone of Khari Nadi Formation. Following another hiatus pelletoid calcareous sandstone, equivalent to miliolitic limestone of western Kachchh of Miliolite Formation gets deposited. Long tabular current bedding shows constant eastward wind direction. Pelletoid sands, deposited in the area now forming the Great Rann wherc a near shore environment was present during the Pleistocene period, are blown eastward and get precipitated wherever any natural barrier was encountered.

Keywords

Palaeoenvironment, Sedimentation, Tectonics, Kachchh, Gujarat.

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Amitava Bandyopadhyay ¹

Affiliations
1 Geological Survey of India, 29, Jawaharlal Nehru Road, Kolkata - 700 016, IN

Abstract

Three sediment cores (116 to 125 cm long) collected from outer continental shelf from water depths of 120 to 150 m off Ganges-Brahmaputra mouth in the Bay of Bengal show oolitic sand horizons at subsurface levels around 25, 50 and 100 cm, alternating with highly assorted terrigenous clayey silt. Sediment texture, mineral and biogenic assemblage indicate near shore reefal environment for the oolitic sands. Green grains in oolitic sediments have been identified as phyllite V of verdine facies. It is inferred that the verdine-bearing oolitic sands are deposited during early Holocene transgressive phase.

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