SUBSURFACE FLUVIAL DEPOSITIONAL ENVIRONMENT CHARACTERIZATION USING SEDIMENTOLOGICAL AND GEOPHYSICAL LOGS: A STUDY FROM THE ANCIENT CITY OF VARANASI, CENTRAL GANGETIC BASIN, INDIA

The Indian city of Varanasi is located in the Central Gangetic Basin, adjacent to the River Ganges. The subsurface of the city is comprised of fluvial deposits originating from the River Ganges through modern and recent history. The present study reviews the geological controls on the evolution of River Ganges in Varanasi and covers a detailed understanding of the recent Quaternary fluvial depositional environments through the investigation of Gangetic sediments that constitute the subsurface of Varanasi. Sediment samples were collected at 1.5-3 m intervals up to a depth of 50 m during the drilling program and combined with geophysical borehole (gamma) logging and physical properties of the sediments to characterize the lithofacies present. The lateral and vertical variation of lithofacies and associated depositional environments have been visualized through correlation of boreholes along selected transects.

The results show that laterally and vertically stacked floodplain deposits form the majority of the shallow sub-surface. However, channel sand deposits are encountered in some boreholes at deeper depths and their lateral extension is limited to the flood plain muddy sediments. The central part of the city is characterized by thicker floodplain deposits, which may reflect the establishment of a more stable interfluve region. The gamma counts of the sub-surface sediments also vary along the transects and showing a decreasing trend towards the central part of the city.

The variation of sediment sequences especially the presence of discontinuities may reflect possible allogenic and autogenic factors which probably controlled the Late Quaternary deposition environment in Central Gangetic Basin. These factors are likely river morphological changes (braided to meandering), or a combination of neotectonic influences and climatic fluctuations. The detailed information of the sedimentary depositional environment and fluvial setting thus plays a key role in the understanding of the evolution and future sustainability of ancient cities such as Varanasi in the Gangetic Basin.