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

R. R. Nair ¹, N. H. Hashimi ¹, M. V. Shankaranarayana Guptha ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa-403004, IN

Abstract

Dredging on the western continental shelf of India has shown that the most common rock types outcropping on the irregular topography of the outer shelf (70-90 m) are algal limestones and shelly limestones containing superficial ooids and pelletoids, while those on the upper continental slope (130-180 m) are algal bryozoan limestones. The limestones have a radiocarbon age ranging between 9,000 and 11,000 years. Depositional environment on the continental shelf during the Holocene appears to be characterised by shallow waters in association with lagoon-barrier beach environment.

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G. C. Bhattacharya ¹, N. H. Hashimi ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa-403004, IN

Abstract

The programme for computation of grain size data on the DCM desk calculators (MOSCAL-1080 PS and MICROSYSTEM 1101) is described. In these programmes the weight percentages or sample weights are used to compute standard statistical parameters by the method of moments. Details are given on data preparation and operation of the programme. A flow chart and the programme are given along with a test problem.

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N. H. Hashimi ¹, N. V. Ambre ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa 403004, IN

Abstract

Gypsum crystals have been found in the inner shelf silty clay/clayey silt off the Maharashtra Coast between Vengurla and Bombay. Generally these occur as euhedral single or twinned crystals of selenite. Very often shells are found embedded within the crystals. The possible conditions of formation of the crystals arc discussed and it is suggested that the crystals are non-evaporitic in origin.

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R. M. Kidwai ¹, R. R. Nair ¹, N. H. Hashimi ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa 403004, IN

Abstract

Fifty-eight sediment samples from the outer continental shelf between Vengurla and Mangalore were analysed for heavy mineral content. The heavy minerals consist of principally opaques, hornblende, epidote, garnet, sillimanite, hypersthene and zircon, with minor amounts of andalusite, augite, kyanite, monazite, muscovite and tremolite-actinolite. This mineral assemblage and the distribution pattern define four heavy mineral provinces on the continental shelf from Vengurla to Mangalore. Further, the mineral assemblage suggests derivation from mixed metamorphic and igneous terrain with a predominant contribution from metamorphic rocks.

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N. H. Hashimi ¹, R. R. Nair ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa 403004, IN

Abstract

The continental shelf in this area is about 80 km wide and the edge of the shelf varies between 90 and 120m water depth. Geologically modern fine-grained clastic sediments occur between the water depths of 15 to 50m corresponding to a distance of about 40 km from the coast. Beyond 50 m to the shelf edge are calcareous sands. Noncarbonate components of these deep water sands are essentially quartz, many of which are heavily iron-stained and are attributed to the presence of rich iron formation on the land. The carbonate components in the outer shelf sediments are mainly foraminifers, pelecypods, gastropods, pteropods, and ooids, while ooids and pteropods are absent on the inner shelf. X-ray mineralogy of the silt fraction of the sediments shows the presence of quartz, calcite, aragonite and orthoclase as major minerals. Some of the sedimentary processes effecting the distribution of these sediments are discussed.

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V. Purnachandra Rao ¹, R. R. Nair ¹, N. H. Hashimi ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa 403 004, IN

Abstract

Seventy-five sediment samples collected from the Kerala continental shelf and slope during the 17th and 71st Cruises of RV Gaveshani were analysed by X-ray diffraction for clay mineral content. The distribution of total clay < 4μ fraction) indicates low percentages < 10%) on the outer shelf and high percentages on the inner shelf (20 to 50%) and upper continental slope (11 to 47%). The clay minerals are composed of montmorillonite (smectite), kaolinite, illite and gibbsite and the nonclay minerals are quartz and feldspar. Kaolinite and gibbsite have low values on the outer shelf and high values on the inner shelf and slope, while illite increases in the offshore direction. However, montmorillonite behaves differently from others in having high values on the outer shelf and low values on the inner shelf and slope. These variations in the clay mineral distribution have been attributed to differences in the energy of the various environments and due to size segregation of the minerals.

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N. H. Hashimi ¹, R. Nigam ¹, R. R. Nair ¹, G. Rajagopalan ²

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa-403 004, IN
2 Birbal Sahni Institute of Palaeobotany, Lucknow-226 007, IN

Abstract

A new Holocene curve is generated for the western Indian continental margin. While constructing this curve careful selection of the dates were made by giving due considerations to the genetic characteristics of the dated material. This new curve shows a low at 100 m depth around 14,500 years BP and a rise to 80.m depth around 12,000 years BP showing a rate of ˜10m/1000 years which was followed by astillstand for about 2,000 years. From 10,000 to 7,000 years it rose at a very high rate (˜20 m/1000 years). Beyond 7,000 years BP it showed minor fluctuations.

Keywords

Holocene, Sea Level Fluctuation, Sea Level Curve, Arabian Sea, West Coast of India.

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N. H. Hashimi ¹, M. C. Pathak ¹, P. Jauhari ¹, R. R. Nair ¹, A. K. Sharma ², S. S. Bhakuni ², M. K. S. Bisht ², K. S. Valdiya ²

Affiliations
1 National Institute of Oceanography, Goa - 403 004, IN
2 Department of Geology, Kumaun University, Nainital - 263 002, IN

Abstract

The Naini lake is a product of rotational movernent on a NW-SE trending Nainital Fault, quite after the establishment of the drainage of a mature stream named Balia Nala. Detailed bathymetric study, permits division of this crescent-shaped lake into two subbasins separated by an approximately 100 m wide transverse underwater ridge, 7m to 20m below the lake surface. The southeastern and northwestern parts, 11 to 25 m and 4 to 27 m deep, respectively, are filled with unconsolidated sediments, 15 m thick on an average. The semiconsolidated sedimentary layers locally exhibit slumping an slopes and deformation in channel beds. These features together with thetransverse Central Ridge, the deep channel cutting through it, the small scarps and terraces or step-like features on the slopes of the narrow V-shaped undemater valley in the southeastern part and its thick filling in parts due to blockade resulting presumably from fault movements, and the existence of deformed and displaced acoustic reflectors are interpreted as indicating quite recent movements along the Nainital Fault. Hitherto unidentified pinnacle-like objects penetrating through or perched atop sediments, are suggestive of either tectonic displacement and squeezing or of the hazard of mass movements which have occurred. A number of small but rapidly growing deltas along shores indicate accelerated pace of sediment input, resulting from quickened erosion in the catchment area.

Keywords

Naini Lake, Neotectonism, Bathymetric Study, Kumaun Himalaya.

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Rajiv Nigam ¹, N. H. Hashimi ¹

Affiliations
1 Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa - 403 004, IN

Abstract

No Abstract.

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