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

Sachinath Mitra ¹, Syed Samsuddin Ahmed ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta 700 032, IN

Abstract

The beach and coastal dune sands of Cox's Bazar constitute the most important source of metallic minerals in Bangladesh. The sands are fine, well-sorted, negatively skewed and leptokurtic in their grain-size distributions. Plots of different size parameters suggest that (a) the finer the grain-size the better is the sorting; (b) the sediments are more negatively skewed as the grain-size decreases; and (c) sorting improves as the skewness changes from nearly symmetrical to negative. The dune sands are finer, better sorted and more negatively skewed than beach sands. The percentage of heavy minerals increases from 2 φ (250 μ m) to 3.5 φ (88 μ m) fraction. The light fraction consists predominantly of quartz (about 90 %), while amphibole, ilmenite, garnet, epidote, mica, pyroxene, magnetite. kyanite, tourmaline, staurolite, zircon, rutile and monazite constitute the heavy minerals. The distribution of the heavies in the dunes and beaches depend on their respective shape. The equant ones are predominant in the former than in the latter. The predominance of unstable minerals and their shapes suggest that the major source of the sediment may be in the easterly highlands of Chittagong-Arakan hills.

Keywords

Sedimentology, Beach and Dune Sands, Cox's Bazar, Bangladesh.

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Sachinath Mitra ¹, Syed Samsuddin Ahmed ²

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta-700 032, IN
2 Department of Geology and Mining, Rajshahi University, Bangladesh, BD

Abstract

Ti-poor magnetites from Cox's Bazar beach are seen to be remarkably homogeneous, chemically and optically, containingless than 2% uIvospinel. Mg, Ti, Cr, A1 and V, though in insignificant amounts, are found to be responsible in contributing to the reduction in hyperfine field of the Mossbauer spectrum, which showed linewidths of 0.18 mm/s at B-site and 0.26mm/s at A-site. Certain fraction of these magnetites manifests authigenic growth, presumably in a locally reduced condition. Intergrowth of ilmenite with titanomagnetites possibly resulted from oxidation of ulvospinel-magnetite and a regular partitioning of Cr, V, AI, Mg and Mn observed between coexisting titanomagnetite and ilmenite suggests a chemical equilibrium. The temperature and oxygen fugacity of coexisting titanomagnetite-ilmenite phases as calculated ranged from 625-850°C and 10^-15 to 10^-21 respectively. These temperatures and oxygen fugacities equilibrated in a T-fO₂, field very near to FMQ (Fayalite-Magnetite-Quartz) buffer curve, lending support to its genesis in the upper crust, more being of igneous sources rather than metamorphic.

Keywords

Mineralogy, Geochemistry, Geothermometry, Magnetite, Beach Sands, Bangladesh.

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Mrinal Kanti Roy ¹, Syed Samsuddin Ahmed ¹, Tapas Kumar Bhattacharjee ¹, Sultan Mahmud ¹, Md. Moniruzzaman ², Md. Masidul Haque ¹, Sudip Saha ¹, Md. Ismail Molla ³, Pulan Chandra Roy ¹

Affiliations
1 Department of Geology and Mining, University of Rajshahi, Rajshahi - 6205, BD
2 Atomic Energy Commission, Savar, Dhaka, BD
3 Gas Seepage Control and Drilling Project, Bangladesh Gas Field Co. Ltd., Bakhrabad, B.Baria, BD

Abstract

The Dupi Tila Formation is composed of yellow to light brown medium to very fine moderately hard to loose sandstone, siltstone, silty clay, mudstone and shale with some conglomerates with clasts of petrified wood. The lithofacies of matrix supported conglomerate, trough cross bedded conglomerate, massive sandstone, trough cross bedded sandstone, planar cross bedded sandstone, ripple cross laminated sandstone-siltstone, flaser laminated sandstone-shale, lenticular laminated sandstone-siltstone-shale, parallel laminated sandstone-siltstone, wavy laminated shale, parallel laminated blue shale, and mudstone are delineated within this formation. Based on the grain size, sedimentary structures, water depth and genesis of individual facies, facies are grouped into three types of facies associations like (i) coarse-grained conglomerate facies association in relation to tractive current deposits of alluvial fan set up at the base of litho-succession (FAC), (ii) medium to fine-grained sandstone-siltstone-mudstone facies association or facies association in relation to strong tide (FAT) characterizing the middle part of litho-succession, (iii) very fine-grained sandstone-siltstone-mudstone facies association in relation to less frequent weak tide or heterolithic facies association (FAHL) characterizing upper part of litho-succession and shallow marine facies association (FASM) composing the uppermost litho-succession. Presence of gluconite indicates that the depositional environment was shallow to deep marine. The dominant paleoflow direction during the deposition of Dupi Tila Formation was toward southeast to southwestern direction. The rivers were of braided type at the piedmont alluvial depositional set up at the lower part, which later changed to estuarine-tidal flat type environmental set up in the middle part to upper part and paleo-environment was shallow marine in the uppermost part.

Keywords

Paleoenvironment, Dupi Tila Formation, Lalmai Hills, Comilla, Bangladesh.

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