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

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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V. Purnachandra Rao ¹

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

Abstract

Pseudo-gypsum crystals have been found in the coarse fraction of the sediments from the inner continental shelf off Machalipatnam Bay. They range in size from 3 to 7 mm and are elongate arid lenticular in shape. Bassanite and calcite are pseudomorphs after gypsum. The origin of the pseudo-gypsum has been discussed and it is suggested that these are indicators of late Pleistocene climatic aridity. The bay sediments deposited during late Pleistocene were subsequently eroded by waves and currents due to sea level changes and the coarse fraction of the sediments containing pseudogypsum were transported and deposited in the inner continental shelf.

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V. Purnachandra Rao ¹, Ch. M. Rao ¹, A. Mascarenhas ¹, K. Mohan Rao ², N. P. C. Reddy ², H. C. Das ³

Affiliations
1 National Institute of Oceanography, Dona Paula 403 004, Goa, IN
2 National Institute of Oceanography, Regional Centre, 52, Kirlampudi, Layout, Visakhapatnam 530 023, IN
3 Oil India Limited, Duliajan, Assam 786 602, IN

Abstract

Sedimentological and geochemical investigations of the sediments in a core from the eastern continental margin of India, at a water depth of 1200m, revealed two distinct sediment types. The Late Pleistocene sediments are greyish-black in colour and consist of mud turbidites in the lower part and dolostone breccia in the upper part. They also consist of illite and chlorite-rich clay minerals with high organic carbon and sulfur. Organic matter is of dispersed nature. These are similar to black shales and are apparently formed by rapid burial of terrigenous organic matter by turbidites with intermittent reducing conditions during lowered sealeveIs. Dolostone fragments appear to have been transported by mass movement processes during the terminal pleistocene. The Holocene sediments: are moderate yellowish-brown in colour and with detrital vivianite nodules at the base. These sediments are clayey with mont-morillonite-rich, illite-poor clay minerals, lower organic carbon and sulfur contents and ferruginised pyrite grains, deposited under oxidising conditions. The marked changes in the sedimentary environments are attributed to climate and sealevel changes during Pleistocene and Holocene.

Keywords

Environment, Pleistocene, Holocene, Black Shales, Off Penner River, Continental Margin.

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V. Purnachandra Rao ¹, M. Thamban ¹

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

Abstract

The calcareous deposits on the western continental shelf of India, off Bhatkal (water depths 50 to 58 m) occur as crusts, sheets, cylinders and reddish brown mudstones. The outer layers of the crusts are micrite-dominated and beneath this layer detrital/carbonate sands with thin heavy mineral laminations are found. Textural studies indicate that the detrital sands are derived from dunes. Drusy calcite and neomorphic calcite cements are associated with the sands. Sheet deposits contain coarse calcareous component-dominated layer within micrite layers and abundant micrite cements. These are similar to dune associated calcretes.

Cylinders are rhizoliths and show different stages of ischolar_main calcification. Circular bodies. about 0.15 mm to 0.70 mm diameter, interpreted as vascular cylinders of the ischolar_main tissues are typical. Radial fibrous calcite and spar calcite with inclusions indicate neomorphic cements. Reddish brown mudstones contain calcified ischolar_main-hair sheaths, micrite glaebules and reworked pollen suggesting that these are indurated soils. X-ray diffraction studies indicate the presence of ferroan calcite, quartz, pyrite and dolomite in rhizoliths and ferroan calcite, goethite and quartz in mudstones. Mudstones and some rhizoliths are Fe-rich and some other rhizoliths are Fe-poor but enriched with Mg and Mn. The particulate matter in the calcareous deposits were initially at the proximity of the coast and cemented by metastable calcites during the ultimate Pleistocene interglacial sea-level stands on the shelf. Pedogenic cementation processes overprinted and developed them into eolianites and paleosols during the subsequent Late Pleistocene sea-level regression. The compositional differences of these deposits were apparently controlled by type of sediments and associated sedimentary environments.

Keywords

Sedimentology, Calcretes, Rhizoliths, Paleosols, Continental Shelf.

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Pratima M. Kessarkar ¹, V. Purnachandra Rao ¹

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

Abstract

The texture, organic carbon (OC), CaCO₃ and Rock-Eval parameters of the sediments from two gravity cores collected at depths below the oxygen minimum zone (OMZ) of the southwestern margin of India are presented and compared the results with those within the OMZ. Clayey silt/silty clays are the characteristic sediments. The OC in the core top sediments between Cape Comorin and Mangalore is higher below the OMZ than those from the OMZ. However, it is higher within the OMZ than those below the OMZ in the sediments between Mangalore and Goa. The down-Core variations of OC are identical in these cores. In both the cores, relatively high OC content and low sedimentation rates correspond to the intervals of late Holocene and Last Glacial Maximum (LGM) and, low OC and high sedimentation rates to the early Holocene sediments. The CaCO₃ follows sand content in a core off Cape Comorin, with low values at ,the core top, increase marginally in the early Holocene and LGM and then decrease in the late Pleistocene sediments. The CaCO₃ values in a core off Mangalore are higher in the intervals of the late Holocene and early deglaciation than in early Holocene and LGM intervals. Rock-Eval parameters distinguish the sources of organic matter only at high OC concentrations.

The high OC during the LGM may be related to the productivity, associated with convective mixing occurring during the NE monsoon. The low OC/CaCO₃ and high clay content during the early Holocene may be the consequences of the intensified SW monsoon that results in stronger near-Surface stratification leading to low productivity. High OC and low CaCO₃ during the late Holocene suggest increased productivity and early diagenesis in the near surface sediments. We suggest that the variations in productivity and downslope transport of sediment controlled the OC enrichment.

Keywords

Organic Carbon, Sediment Cores, Productivity, Late Quaternary, SW Margin of India.

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A. Anil Kumar ¹, V. Gopinathan ¹, K. Sreemathi Gopinath ¹, V. Purnachandra Rao ²

Affiliations
1 Department of Geology, Government College, Vidyanagar, Kasaragod, Kerala - 671 121, IN
2 National Institute of Oceanography, Dona Paula, Goa - 403 004, IN

Abstract

Pink and grey granites occur as minor, more or less circular bodies at Angadimogar and Kumbdaje areas of Kasaragod district, covering about 10 km². These granites are emplaced within the charnockites and gneisses during the Late Precambrian. These granites arc coarse grained and traversed by anastomising patches of aplite. They are in sharp contact will the country rock. The Angadimogar pluton was carlier considered a syenitic body. Normative mineralogy and petroghaphy of the Angadimogar and Kulnbdaje plutons show high content of silica and other salic minerals and confirm that they are in fact granites. The major oxide chemistry of the plutons is similar to the major granitic plutons of South India. Thus, contrary to the earlier views, we suggest that these bodies are granites.

Keywords

Nomenclature, Granites, Syenites, Angadimogar, Kumbdaje, Kerala.

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V. Purnachandra Rao ¹

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

Abstract

No Abstract.

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