Late quaternary evolution of Ashtamudi–Sasthamkotta lake systems of Kerala, south west India

Abstract

Sedimentological and palynological analysis of two borehole samples collected from the confluence of Kallada River with the Ashtamudi lake in the south west coast of India have been addressed while ascertaining the late quaternary evolution. Of the two borehole samples, the one collected from Pangod (9°03′N–76°42′E) reveals a fining upward sequence with medium to coarse sand (3.05 m) at base, organic carbon rich, silt and fine sand dominated sediments (2 m) at the middle and yellowish brown, muddy sediments (3.2 m) at the top. The middle layer embeds a suite of partly carbonized, sub-fossil logs of wet evergreen to semi-evergreen vegetation. The palynoflora of this layer reveals that the depositional site is within the tidal limit and deposition occurred under high precipitation and atmospheric humidity. The similarity in 14C dates of a wood at 5 m bgl (7490 ± 90 yrs BP) and the embedding sediments (7480 ± 80 yrs BP) indicate quick burial of the riparian vegetation. The West Kallada borehole (9°30′N–76°37′E) reveals Middle to Late Holocene sequence of clayey silt (6250 ± 110 yrs BP–3880 ± 80 yrs BP) and sand resting unconformably over greyish white, clayey sand with pebbles and granules derived from laterite provenance. Palynological analysis shows that the Holocene sedimentation took place under marine/nearly marine environment and later changed to brackish water and finally to freshwater environment. Marine transgression ∼6000 yrs BP coupled with heavy rainfall in the hinterlands was responsible for faster sedimentation in the region. The heavy mineral contents, especially opaques, garnet and sillimanite in the sediment samples of the study area as well as the bathymetric configuration of the Ashtamudi, Sasthamkotta and Chelupola lakes reiterate the fact that these lakes have been evolved from an embayment consequent to incomplete/partial silting up during Early to Middle Holocene higher sea levels and also under high rainfall of the Holocene climatic optimum of around 10,000–7000 yrs BP.