A HOLOCENE SOIL-CHRONOASSOCIATION OF THE GANGETIC PLAINS BETWEEN RAMGANGA AND RAPTI RIVERS

Abstract

Fourteen soil-geomorphic units have been delineated from the Gangetic plains between the Ramganga and Rapti rivers. The units delineated are based on remote sensing studies and field checks. One additional unit immediately east of the study area, described earlier by Mohindra et al. (in press), and named here as the Lower Deoha/Ganga-Ghaghra unit also has been included. On the basis of degree of soil profile development, these units have been grouped into five members (QGH1 to QGH5, QGH5 being the oldest) of a soil chronoassociation. The member QGH1 (age < 500 yrs. B.P.) includes soils of floodplains of the Ghaghra and Rapti, Kosi-Gola Piedmont, Gholia-Dhobania-Bhambhar Piedmont and Young Sihali-Mohan-Kandra Piedmont. QGH2 member (age > 500 yrs. B.P.) comprises soils of the Upper Kosi-Gola Plain, Old Ghaghra Plain and Old Sihali-Mohan-Kandra Piedmont. Soils of the Lower Kosi-Gola° Plain, Upper Rapti-Ghaghra Interfluve, Upper Deoha/Ganga-Ghaghra Interfluve and Old Ghaghra Bar are included in QGH3 member (age > 2500 yrs. B.P.). The QGH4 member (age > 6500 yrs. B.P.) includes soils of the Lower Deoha/Ganga-Ghaghra Interfluve. Soils Of the Middle Deoha/Ganga-Ghaghra Interfluve and Lower Rapti-Ghaghra Interfluve are included in the QGH5 member (age 9000 to 11000 yrs. B.P.). Pedological studies suggest that conditions favourable for formation of pedogenic calcrete in the Gangetic Plains prevailed from 11000 to 6500 yrs. B.P. Amelioration of climate and improved drainage resulted in large scale to complete removal of calcrete from soil profile in some areas or its dissolution and reprecipitation in lower horizons in other areas. Neotectonics seems to have played a significant role in evolution of morphology and soils of the study area. It determined areas of active sedimentation and pedogenesis (in upland areas). It led to tilting and sagging of large blocks. Tectonic slopes/faults determined direction of courses of large rivers.