Shelf sediments of the Ganges delta with special emphasis on the mineralogy of the western part,Bay of Bengal,Indian Ocean |
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| Authors: | Tapas Kumar Mallik |
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| Institution: | Offshore Mineral Exploration and Marine Geology Division, Geological Survey of India, Calcutta-700016 India |
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| Abstract: | Shelf sediments at the mouth of the Hooghly River which forms the western part of the Ganges—Brahmaputra delta consist of sands, silts and clays and their various admixtures. The bulk of the sediments consists of moderately sorted fine sand to very fine sand. There is variation in lithology in the vertical and horizontal directions, as revealed from the study of the grab and core samples collected in this area, suggesting growth of the delta in different stages. In most of the samples three populations have been recognised. The grain-size variation has resulted from mixing of a number of different grain-size populations. A detailed study of the heavy minerals indicates that the suite consists of hornblende, tremolite/actinolite, opaques, zircon, pyroxenes (ortho-and clino-), garnet, sillimanite, chlorite, muscovite, biotite, epidote, monazite, kyanite, staurolite, riebeckite, carbonates and glauconite. The assemblage and the distribution patterns suggest two distinct mixed igneous and metamorphic sources for the sediments—the Himalayas to the north, mainly drained by the Ganges—Brahmaputra and their tributaries, and the peninsular shield drained by the Dhamra and other easterly-flowing rivers debouching into the Bay of Bengal. The mineralogy of the core samples in the top and bottom layers does not differ, indicating that the source area remained the same during the time of deposition of the sediments. The mineralogy of the sediments in this area, when compared with the mineralogy of the Deep-Sea Drilling Project site in the Bengal fan situated very far to the south, shows similarity because the sediments of the Ganges are carried to the deep-sea. Depending on the mineral assemblage, the area has been divided into four distinct zones: (1) Hooghly River province, consisting of hornblende, tremolite/actinolite, epidote and garnet; (2) a mixed province characterised by epidote, monazite, zircon, kyanite, staurolite, hornblende, tremolite/actinolite, biotite; (3) Dhamra River province characterised by opaques, sillimanite and orthopyroxene; (4) an offshore province comprising muscovite, chlorite and pyroxene. ILlite and kaolinite are the principal clay minerals in the sediments. The mineralogical, grain size and the lithologic studies of the sediments from the core samples suggest a southward to south-southwestward direction of dispersals of the sediments in the eastern part of the area. |
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