Palaeoenvironmental Reconstruction Through Granulometric Analysis of a Palaeolake Deposit at Bhikiyasain,Kumaun Lesser Himalaya

Abstract

Grain size analysis is an essential tool for classifying sedimentary environments. The main aim of the current research is to use granulometric analysis of the Bhikiysain palaeolake sequence along the Ramganga river to describe changes in the depositional environment within the lake during the late Quaternary. The granulometric analysis was conducted using a laser particle size analyser on 32 samples, collected at 10 cm intervals in a vertical palaeolake profile, at Bhikiyasain (Ramganga Basin). The results of the grain-size analysis indicate that the size distribution of the sediment is unimodal. The unimodal size distribution of the sediment suggests that the sediment was supplied via fluvial action. The Bhikiyasain Basin (29°43.106’ N; 79°15.682’ E) underwent tectonic activity around 44 ka, resulting in the ponding of the Ramganga river and the formation of palaeolake deposit. Based on grain size analysis, variation in the colour and lithofacies, the entire profile has been divided into 6 different zones (zones 1 to 6). The silt has the highest concentration in all the zones except for zones 1 and 3. Zones with high silt concentration are inferred to represent low energy depositional environments during the time of deposition. The higher amount of sand concentration in zones 1 and 3 represent higher energy depositional environment. For the whole profile, the sorting of the samples varies between 1.1 and 2.0, indicating poor sorting of the samples. The poorly sorted sediment in all six zones represents limited transportation of sediment from the catchment and also suggests that the sediment was deposited in a low energy environment. The ternary plots also signify the dominance of silt followed by sand and clay. The skewness values range from 0.1 to 0.5 which indicates that the samples are symmetrical to very finely skewed. Variability in the skewness values may be due to changes in the intensity of wind and hydrodynamic conditions of the lake. The kurtosis value ranges from 0.9-1.4, indicating the samples are platykurtic, leptokurtic and mesokurtic in nature. Variability in the kurtosis may be due to changes in the flow characteristics of the depositional medium.