Radiocarbon Dating and Holocene Episodes of Alluvial Sedimentation in the Koratallaiyar-Cooum River Basin, Chennai, South India

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.