Holocene changes in fluvial geomorphology, depositional environments, and evolution of coastal wetlands—A multiproxy study from Southwest India

The Kuttanad Kole Wetlands (KKW) in Southern Vembanad basin, SW India is a unique wetland system, where paddy cultivation is being carried out 1.0–2.0 m below mean sea level. The fertile soil in the KKW in its upper, eastern part (upper Kuttanad) is formed essentially from deposition of fluvial sediments from the hinterland rivers. The wetlands host one of the largest subfossil shell deposits in India. The present study has been undertaken to use the spatial distribution and geochronology of the lime shell deposits along with multiparametric study of the borehole data collected through various projects to decode the depositional processes and coastal evolution of this significant wetland system. Except the top 2.0–3.0 m riverine sediments, the rest of the core sediments is of lagoonal nature with occasional presence of molluscan shells. The top riverine sediments are yellowish brown with heavy minerals derived from charnockite provenance. However, the heavy mineral suite in the lagoonal sediments of early–middle-Holocene age is dominated by sillimanite—an indicator mineral of the khondalite that occur in areas south of the Achankovil Shear Zone. This shows that, during early–middle-Holocene, the KKW was not influenced by the hinterland rivers draining the charnockite provenance, instead was influenced by sediments derived from the Kerala Khondalite Belt. This together with the occurrence of subfossil shells of Villoritta sp. in the lagoonal sediments with radiocarbon dates of 3.0–4.0 k yr BP reiterate that the hinterland rivers that drain the charnockite provenance joined the KKW only during late-Holocene. Based on the study a fourfold evolutionary model for KKW has been proposed and discussed.