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

P. M. Mohan ¹, K. Shephard ¹, N. Angusamy ¹, M. Suresh Gandhi ¹, G. V. Rajamanickam ¹

Affiliations
1 Department of Industries and Earth Sciences, Tamil University, Thanjavur - 613 005, IN

Abstract

Quaternary landforms between Vedaranyam and Rameshwaram on the east coast of India show a wide range from inland sand dune, flood plain, estuary, delta, cusp, beach, coastal dune, barrier ridge, mud flat, tidal creek to mangrove. The study area has been classified into inland, beach and offshore regions to evaluate the variation in the distribution of grain size and the depositional environments. 147 sediment samples were collected using hand augur along the beach (53) and inland (37) environments, and by Van Veen grab in the offshore area (57). Grain size analysis indicates that the area may have extended approximately 25 km towards inland from the present-day low water mark. In those days, the coast must have developed by almost similar processes as acting at present in the nearshore environment, and the sea level oscillation may have been significant in the past. Further, based on the present day rate of sedimentation, it is suggested that the study area may have developed within the last 1,50,000 years B.P.

Keywords

Quaternary Sedimentation, Coastal Geomorphology, Palk Strait, Tamil Nadu.

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P. M. Mohan ¹, G. Victor Rajamanickam ¹

Affiliations
1 Department of Industries and Earth Sciences, Tamil University, Thanjavur - 613 005, Tamil Nadu, IN

Abstract

Along the east coast of India between Chennai and Pondicherry, 163 sediment samples were collected from different sub-environments of the beach representing low water mark (LWM), foreshore (FS), berm crest (BM), backshore (BS), dune (DU), and beach ridge or strandline (SL), upto 1m depth, at fifteen stations, spaced at a distance of approximately 10 km interval, Out of four size ranges (-420 to +350μ, -210 to +177μ, -105 to +88μ and the fraction consisting of the highest percentage in the sediment - first mode), the +88μ size range shows rich concentration of heavies (8.26 to 96.6 %). The heavy mineral content in the bulk sediment samples ranges from 0.55 to 22.95 %. This concentration may be due to the winnowing by wave action and selective sorting processes. Erosion exposes one of the stations in the beach ridge, up to 4 m depth from the surface and shows a higher concentration of heavy minerals. This information, along with analytical results of the stations from the surface upto one metre depth, indicating heavy mineral concentration of 8%, suggests the possibility of buried placer deposits at about 2 to 4 m depth along the beach ridge. Based on this prognostication, a random station was selected i.e. near Mahabalipuram and manual boreholes were drilled. These boreholes have confirmed the existence of buried placer deposits having concentration of 8 to 88 % of heavy minerals around 1.5 to 4 m depth of beach ridge. R mode and Q mode factor analyses indicate the possible environmental parameters prevailing at the time of deposition as well as under the present conditions in the stations.

Keywords

Strandlines, Factor Analysis, Economic Geology, Buried Placer, Chennai, Pondicherry, East Coast, Tamil Nadu.

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P. M. Mohan ¹, G. Victor Rajamanickam ¹

Affiliations
1 Department of Industries and Earth Sciences, Tamil University, Thanjavur-613005, IN

Abstract

Twenty five sediment samples, from seven traverses, were collected from the southwestern continental shelf of India. A variation in the organic matter from zero to 1 1.26% and carbonate from 7 to 96% is noticed. The stations located off the river mouth areas show high amount of organic matter and low amount of carbonate content, whereas outershelf sediments display high amount of organic matter and carbonate contents. However, in between these two zones, at the depth of 35 to 65 m, the concentration of organic matter and carbonate is found to have a considerable decrease in amount. It is inferred that this zone might be a paleo beach environment admixtured with present day marine environment.

Keywords

Organic Matter, Carbonate, Pleistocene Shoreline, Continental Shelf, Marine Geology.

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P. M. Mohan ¹

Affiliations
1 Department of Industries and Earth Sciences, Tamil University, Thanjavur - 613005, IN

Abstract

Studies on the sediments of Parangipettai (Porto Novo) beach and its surrounding environments (Vellar river, estuary and nearshore environment) reveal that all the environments show texturally different characters but similarity in heavy mineral assemblages with varying percentage of occurrence. The river sediments are coarser (1.48 φ) and moderately sorted than estuarine sediments which are finer (4.22φ) and poorly sorted. The beach sediment; fall in the size range 2.25φ to 2.61φ in the southern region and 2.56φ to 2.71φ in the northern region, and are well to moderately sorted. In nearshore, sediments of the northern region are finer (3.50φ) and poorly sorted than those of the southern region, which are coarser (2.98φ) and moderately sorted. Sediments of river, bermcrest and backshore of southern beach and bermcrest of northern beach represent near symmetrical skewness and the other remaining beach environments have coarse skewness. Nearshore and estuarine sediments register fine and strongly fine skewed nature. In all the environments, the sediments are leptokurtic except those at low water mark of southern region where they are platykurtic. These variations are due to the physico-chemical conditions, impact of tidal action and longshore movements on the different environments. Conspicuous changes are noticed in the individual heavy mineral concentrations. Based on the mineral assemblage in the riverine environment, it is inferred that the main so∼rces for these minerals are the rock types present in the catchment area of the river. The enrichment of some heavy minerals in the beach and nearshore environments suggest that the increment may be due to the addition of these minerals from some other sources like paleo sediments and the present day offshore sediments.

Keywords

Sedimentology, Parangipettai Beach, Tamil Nadu.

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P. M. Mohan ¹, G. Victor Rajamanickam ¹

Affiliations
1 Department of Industries and Earth Sciences, Tamil University, Thanjavur-613005, Tamil Nadu, IN

Abstract

Between north of Pondicherry and Ennore (12°03' to 13°12' N - 79° 53' to 80° 15' E) two geomorphic units such as beach and beach ridges and their associated units (Low Water Mark (LWM). Foreshore (FS). Berm (BM). Backshore (BS), Dune (DU) and Beach Ridge (SL) are selected for the present study to understand the micro level variation in the depositional environment. The samples mostly show medium to fine size, well sorted to moderately sorted, very coarse skewed to fine skewed and platykurtic to leptokurtic nature. Graphic mean, standard deviation, skewness and kurtosis confirm that the sediments are of typical beach origin. Frequency curves indicate that the dune samples represent unimodal and others show bimodal to poly modal source of influence. Based on bivariant plots the samples of dune, berm, backshore, foreshore and low water mark with strandline deposits are classified.

Keywords

Sedimentology, Depositional Environments, Grain Size, Tamil Nadu.

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