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

Hema Achyuthan ¹

Affiliations
1 Centre for Geoscience and Engineering, Anna University, Chennai 600 025, IN

Abstract

No Abstract.

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Hema Achyuthan ¹, Chris Eastoe ²

Affiliations
1 Department of Geology, Anna University, Chennai-600025, India, IN
2 Department of Geosciences, University of Arizona, Tucson, Arizona-85721, US

Abstract

The Jarawa Creek mud volcano on Baratang Island has produced a variety of ejecta including mud containing illite, kaolinite and microfossils, volcanic glass, brine, methane gas and sulfide nodules. The nodules are of pyrite with a δ³⁴S ranging from -3.6 to 25.4‰. Sulfate leached from the mud has δ³⁴S values of -9.3 and -11.37‰. Our study reveals that the sulfate could not have been derived either from the nodules themselves or from the sea water or from connate brine. The nodules are of sedimentary origin and the most likely source of sulfate is sedimentary sulfide implying subsurface oxidation and a supply of oxidant possibly in groundwater.

Keywords

Mineralogy, Isotopic Studies, Sulfur Isotope, Pyrite, Mud Volcano, Baratang, Andaman Islands.

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Hema Achyuthan ¹, S. N. Ghate ², S. N. Rajaguru ²

Affiliations
1 Department of Geology, Anna University, Chennai 600 025, IN
2 Department of Archaeology, Deccan College, Pune 411 006, IN

Abstract

Micromorphology, XRD analyses and SEM studies of rhizoliths from Late Middle Pleistocene yellowish brown silty fluvial sediments of Yedurwadi reveal that calcium carbonate has a localized source. Silica and calcium carbonate are formed by the plant ischolar_mains through their metabolic activity creating a favourable microenvironment for the processes of SiO₂ and CaCO₃ precipitation and replacement.

Keywords

Sedimentology, Karnataka.

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Anjum Farooqui ¹, Hema Achyuthan ²

Affiliations
1 Birbal Sahni Institute of Palaeobatany, 53, University Road, Lucknow - 226 020, IN
2 Centre for Geoscience and Engineering, Anna University, Chennai - 600 090, IN

Abstract

In this paper, we present Middle to Late Holocene chronostratigraphy and palynology data analyzed from three insitu cores - sedimentary profiles retrieved along the Adyar river and the Adyar estuary zone, Chennai. These cores offered excellent cross-sections along the east - west direction. Our data reveals that the mangrove vegetation that existed until Middle Holocene (4180 ±130 Yrs. BP) became extinct since the Late Holocene. The tidal influx influenced the estuary 10 km west inland (at Saidapet) from the present shoreline until Middle Holocene providing congenial environment for mangroves. The reasons for decline in mangroves since the Late Holocene (last 2000 Yrs. BP) is attributed to the relative sea level and coastal configuration change enhanced by the anthropogenic pressure. The palynological evidences from Saidapet (Chennai) and Sulurpet (playa of the Pulicat lagoon) indicate the transgressive phase of middle Holocene in Palar Delta. Although mangroves were abundant during Middle Holocene in both the areas, it faced complete extinction or acutc degradation since the Late Holocene.

Keywords

Palynology, Estuarine Sediments, Mangroves, Late Holocene, Coastal changes, Radiocarbon dates, Adyar, Chennai, Tamil Nadu.

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M. P. Veena ¹, Hema Achyuthan ¹

Affiliations
1 Department of Geology, Anna University, Chennai - 600 025, IN

Abstract

Silica sand deposits are exposed along the East Coast, Tamil Nadu, nearly 80 km south of Chennai. The original sediment is quartz sand to feldspathic quartz sand Soil has developed over the younger deposits. Trough cross bedding is well developed in the younger sediments. Textural analyses of silica sand were carried out for compatative study.

Silica sand varies from very coarse sand (-0.25φ to 0.5φ), medium sand (1.25φ to 2φ) fine sand (2.25φ to 3φ), very fine sand (3.25φ to 4φ) and coarse silt (4.06φ to 5φ). Fine to medium sized grains are dominant in the silica sand. The silica sand is poorly sorted, fine skewed, leptokurtic to platykurtic in characteiistics. Beach and dune sediments are predominandy coarse sand with heavy minerals. The dune sediments are moderately to well sorted, fine skewed, and leptokurtic. The beach sediments are well sorted strongly coarse skewed, and platy to leptokurtic. The sphericity and roundness values of the silica sands vary from rounded to sub rounded and are very equant. The various types of surface features are 'V shaped notches, conchoidal fractures and striations The study shows that the depositional environment of silica sand is markedly different from the aeolian and beach enviionment Bivariant plots and textural analyses of the silica sands indicate that they have been primarily deposited by fiuvial action.

Keywords

Silica Sand, Textural Analyses, Bivariant Plots, SEM Studies, Fluvial Origin, Tamil Nadu.

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Hema Achyuthan ¹, Nagalakshmi Thirunavukarasu ¹

Affiliations
1 Department of Geology, Anna University, Chennai - 600 025, IN

Abstract

In this paper we present Quaternary stratigraphy of the area around Chennai based on archaeological findings on the ferricrete surface, geomorphological observations supplemented by radiocarbon dating. The coastal landscape around Chennai, Tamil Nadu, has preserved ferruginised boulder gravel deposits, ferricretes and fluvial deposits of varying thickness. The area studied is approximately 150 km east to west and 180 km north to south with a broad continental shelf towards the seaward. Several rivers enter the Bay of Bengal along its shores like the Koratallaiyar, Cooum and the Adyar. Precambrian charnockite and Upper Gondwana sandstone and shale bedrock rim the shelf margin. For the most part, the Upper Pleistocene-Holocene fluvial sediments overlie an erosion surface that has cut into older Pleistocene sediments and ferricrete surface. Incised valleys that cut into this erosion surface are up to 5-6 km wide and have a relief of at least 30 m. The largest valley is that cut by the Koratallaiyar River. Holocene sediments deposited in the incised valleys include fluvial gravels, early transgressive channel sands and floodplain silts. Older Pleistocene sediments are deposited before and during the 120-ka high stand (Marine isotope stage 5). They consist of ferricretes and ferricrete gravel formed in nearshore humid environments. Muddy and sandy clastic sediments dated to the ca. 5 ka highstand suggest that the climate was semi arid at this time with less fluvial transport. The coarsening up sequence indicates deposition by high intensity channel processes. Pedogenic mottled, clayey silt unit represents an important tectonic event when the channel was temporarily drained and sediment were sub aerially exposed. Uplift of the region has caused the local rivers to incise into the landscape, forming degradation terraces.

Keywords

Quaternary Stratigraphy, Boulder Gravel Bed, Ferricretes, Holocene Fluvial Sediments, Geoarchaeology, Chennai.

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Hema Achyuthan ¹

Affiliations
1 Anna University, Chennai, IN

Abstract

No Abstract.

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