Geochemical Survey: A Tool for Subsurface Geological Mapping in Alluvial Terrain

Abstract

The regional geochemical mapping was carried out in the Indo-Gangetic alluvial plain of the Western and Central India which was previously classified as Varanasi alluvium, a polycyclic sequence of large alluvial fans transported by perennial Himalayan Rivers. The results based on the geochemical dispersion pattern of major and minor oxides as well as trace elements conclude that there are four major factors determining the dispersion patterns of the variables which are: (1) the underlying lithology with a strike of NE-SW, (2) transport of sediments along the general slope of the study area towards south and south-west as reflected in the Digital Elevation Model, (3) formation of Reh and (4) anthro-pogenic influence. The present study focusing on the effect of the underlying geology on the geochemical signature of the study area infers that the geochemical distribution pattern of certain variables clearly follows the trend of the Delhi fold belt. It can be interpreted that the subsurface lithology belonging to Delhi Supergroup is present at a depth of 500m which is considerably shallower, imprinting its geochemical signature on the sediments. The presence of minor outcrops of quartzite belonging to the Delhi Supergroup with the NE-SW strike is also documented in the study area. Based on this study we propose to reclassify the Varanasi Alluvium sediments as equivalent to Banda Alluvium. The study also demonstrates the utility of geochemical mapping as a tool to map the subsurface geology of an alluvial terrain. The results of this study are also corroborated by the regional scale gravity survey.