Weathering of the Ganga alluvial plain, northern India: implications from fluvial geochemistry of the Gomati River

In the northern part of the Indian sub-continent, the Gangaa alluvial plain (GAP.) feeds its weathering products to the Ganga-Brahmaputra River system, one of the world's largest fluvial systems. The authors present a geochemical study of the GAP weathering products transported by the Gomati River (the Ganga River tributary) to under-stand weathering processes of an alluvial plain in a humid sub-tropical climate. A total of 28 sediment samples were collected during the monsoon season and were analysed by X-ray fluorescence spectrometry for 25 major and trace elements. Bulk chemistry of the channel, flood and suspended sediments mostly consists (>90%, >80% and >75%. respectively) of three elements; Al, Si and Fe. Major element concentrations normalised with respect to upper continental crust (UCC) show strong depletion of highly mobile elements (Na, Ca) and enrichment of immobile elements Cri. Si. Silica enrichment in the sand fraction is probably caused by chemical weathering of feldspar. Mineral sorting during fluvial transportation acts as the single important factor that controls the geochemistry of these weathering products and also strongly influences major and trace element distribution in the individual sediment samples. Trace element (Ba. Cr. Cu, Nib, Ni, Pb. V and Zn) concentrations were strongly correlated with major element (Si, Al: Fe. Mn and K) concentrations indicating that the abundance of trace elements is controlled by the same processes that control the major element distribution in these sediments. The GAP weathering products were geochemically distinguished as arkose to litharenite in rock classification. Chemical mobility, normalised with respect to TiO2 in UCC. indicates that Si, Na, Zr, Ba and Sr. mainly derived from feldspar, muscovite and biotite, are lost during weathering. Iron and Zn remained immobile during weathering and were strongly adsorbed by phyllosilicates and concentrated in fine-grained sediment fractions. The chemical index of alteration indicates that the GAP has experienced chemical weathering of incipient to moderate intensity. The GAP weathering products also demonstrated a progressive incomplete alteration in the alluvial sequence made-up of the Himalayan-derived sediments. A model has been proposed to better understand weathering processess and products of the GAP in temporary storage of similar to50 ka in a humid sub-tropical climate. (C) 2004 Elsevier Ltd. All rights reserved.