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Roy, K. K.
- Tertiary Sedimentation in the Andaman-Nicobar Geosyncline
Authors
1 Geological Survey of India, Calcutta, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 9, No 1 (1968), Pagination: 32-39Abstract
Andaman orthogeosyncline is continuous with Assam-Burma in the north and Indonesian geosyncline in the south. Andaman-Nicobar group of islands constituting the non-volcanic outer arc of the East Indian Orogen, has a more or less continuous history of Sedimentation throughout the Tertiary era commencing from sometime in the Upper Cretaceous right on to the Recent. The sedimentation gradually changed character with the tectonic development of the geosyncline through geologic time.
Euxinic sediments consisting of black pyritous shales with minor amount or dark impure limestone and gritty sandstones represent the earliest episode of geosynclinal sedimentation in the restricted basins formed by submarine ridges and valleys due to emplacement of ophiolites.
These ophiolites were subaerially denuded to some extent; and during Palaeocene-Lower Eocene, algal limestone and conglomerate and subsequently more finer fractions such as tuffaceous basic wacks and serpentinous and chloritic clay beds followed. Subsidence of the geosyncline which commenced with sedimentation of the Euxinic beds produced a uniform configuration of the geosyncline during Middle Eocene (Kirthar). Andaman flysch consisting of altetnatcly bedded graywackes, siltstone and grey illitic clay beds were deposited throughout the length and breadth in this regular geosynclinal basin till Upper Oligocene when a major orogenic movement overcame sedimentation.
Shallower basins formed within and in the outer margins of the rising emhryonic island arc were filled up during Lower Miocene with foraminifcrallimcstone and shale, and radiolarian and foraminiferal chalk beds.
Middle Miocene witnessed another major orogenic movement which brought out the present configuration of the island arc. Fossiliferous clays and limestones were deposited in peripheral basins during the Pliocene-Pleistocene which are partly raised during quite recent times.
- Geophysical Approach for Delineation of Shallow Crustal Structure along Borgaon-Sanwer Transect, Madhya Pradesh
Authors
1 Central Geophysics Division, Geological Survey of India, Kolkata - 700 016, IN
2 Department of Geology and Geophysics, IIT, Kharagpur - 721 302, IN
3 Department of Geological Sciences, Jadavpur University, Kolkata - 700 032, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 60, No 2 (2002), Pagination: 173-181Abstract
Geoelectrical survey using direct current Schlumberger and equatorial dipole-dipole resistivity sounding techniques and collinear dipole-dipole traversing have been carried out along Borgaon-Sanwer profile in Madhya Pradesh. The area is covered by Deccan basalts overlying the Gondwana, Vindhyan and Bijawar sediments and the basement of Bundelkhand granite. The traverse, having a length of about 160 km, cuts across the Narmada river near Omkareswar. 2D model of 1D interpretation of Schlumberger and equatorial dipole sounding data reveals that the thickness of Deccan Trap varies from a few tens of metres to more than one km. Bundelkhand granite marks the high resistivity basement. Formations of intermediate to low resistivity are the Gondwana, Vindhyan, Bijawar and Quaternary sediments. A fault at Chhaigaon-Makhan has been delineated from the plot of resistivity pseudo-section, which is also corroborated by the findings of spectral analysis of magnetic data. 2D modelling of magnetic data has also confirmed the presence of the fault. The interpretation has also brought out the variable thickness of Deccan Trap between Borgaon and Deshgaon of 200 m to 900 m.Interpretation of the sounding curves in general reveals high resistivity Bijawar rocks occurring at shallow depth of less than one km in the area south of the Narmada river, whereas to the north of the river the Bijawar occurs at a greater depth. It may, therefore, be inferred that the course of the Narmada river is coincident with a postulated fault plane. The general findings of resistivity survey are mostly compatible with the interpretation of the gravity survey conducted earlier in the area. Besides these, some additional geological structures are also delineated.