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

T. Nagalakshmi ¹, H. Achyuthan ¹

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

Abstract

The near coastal areas of the landscape around Chennai, Tamil Nadu, preserve younger fluvial deposits than the ferricretes, which are of the late Neogene-Pleistocene age. Two major rivers meandering parallel to one another drain in this area. They are the Koratallaiyar, and the Cooum. The interfluves are less dynamic and preserve an older record Satellite imagery and aerial photographs have been used in this study for understanding the large floodplain areas. Investigations from remote sensing data, borehole lithologs, and field observations In the Koratallaiyar and Coourn Basin, indicate that channel migration through Cut-Off, overbank flooding and structural control are the most important factors controlling the floodplain structure in the study area. Many older and Palaeo-Levees exist in the form of edges (2-3 m high) in the Koratallaiyar floodplain. Well developed river terrace points to late Pleistocene early Holocene sedimentation Presently the river is incising in its own sediments.

Organic-Rich sediments and peat have been dated by radiocarbon method to understand the phases of sedimentation processes. The Korataitalyar and the Cooum River deposits can be divided into three groups based on radiocarbon calibrated ages 9,710-8,430 yrs BP, 5,910-5,490 yrs BP and -900 yrs BP. Rates of net sediment accumulation calculated using these radiocarbon dates, span a period of the order 10²-10³ yrs BP. Change in gram size and process of sedimentation relates to structural configuration of the basin, minor changes in winter monsoon climate and varying magnitude of flood events.

Keywords

Alluvial Sections, Palaeo-Levees, Net Sedimentation, Radiocarbon Dates, Winter Monsoon Climate.

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R. K. Ganjoo ¹, Ravi Kant Verma ², H. Achyuthan ²

Affiliations
1 PG Department of Geology, University of Jammu, Jammu - 180 006, IN
2 Centre for Geoscience and Engineering, Anna University, Chennai - 600 025, IN

Abstract

The Lower Pleistocene palaeosols of the Upper Siwaliks of Jammu, J&K State are formed on the alluvium, though negligibly reworked and redistributed by the fluvial processes and, yet preserve the evidences of pedogenic processes. The clayey siltstones (Bt horizons) preserve the evidences of shrink-swell disruptions in the form of slickensides and striated b-fabrics. Illuvial accumulation of translocated clay in the form of clay coatings is well preserved. The field characters substantiated by micromorphological investigations suggest warm humid to semi-arid climatic conditions for the formation of semi-arid alfisols under the influence of periodic wetting and drying attributed to monsoonal seasonal climate.

Keywords

Lower Pleistocene, Palaeosols, Alfisols, Seasonality, Siwalik, Jammu, Jammu and Kashmir.

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H. Achyuthan ¹, O. Flora ², M. Braida ², Navin Shankar ¹, B. Stenni Stenni ²

Affiliations
1 Department of Geology, Anna University, Chennai - 600 025, IN
2 Dipartimento di Scienze Geologiche, Ambientali e'Marine, Universita degli Studi di Trieste, IT

Abstract

In this paper we discuss the limitation of radiocarbon dates on the pedogenic calcic nodules formed in situ within the vertisols in the upland region of Coimbatore, Tamil Nadu. The radiocarbon ages were obtained using lowlevel scintillation counters and the dates range from ∼24 to 31 ¹⁴C kyrs BP. The ages correlate with the marine isotope stage of Late MIS3. However, since the calcic nodules are pedogenised and formed in an open system, the ¹⁴C ages should only be considered as estimates and not absolute ages because of the possibility of open-system behaviour with respect to carbon. Thus, we express caution in the interpretation of these and other radiocarbon ages obtained on pedogenic carbonate nodules. Multiple sub-mm size subsamples could provide more reliable age estimates.

Keywords

Calcic Nodules, Radiocarbon Ages, Vertisols, Late MIS3 Period, Coimbatore, Tamil Nadu.

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