Geochemistry of sediments of the Peninsular rivers of the Ganga basin and its implication to weathering, sedimentary processes and provenance

Abstract

Abstract Geochemistry of the Himalayan rivers has been a major point of contention in sedimentary geochemistry because of its global impact on ocean chemistry and climate. Among the Himalayan rivers, the Ganga and its Himalayan catchment have been studied extensively. However, the Peninsular rivers constituting a significant part of the Ganga basin have not received much attention, heretofore. In the present geochemical study of the Peninsular river's sediments, we have found that the initial chemical weathering of rocks in the catchment produces texturally, mineralogically and geochemically diverse sediments and fluvial processes further segregate them into the geochemically contrasting coarse channel and fine-grained suspended sediments. The overbank sediments possess an intermediate texture and geochemistry, but more towards the suspended sediments. The sediment geochemistry data indicates that the Peninsular river's sediments show an increase in the weathering intensity from the west (Banas river) to the east (Son river) along the climate gradient, i.e., from the arid-semiarid to the sub-tropical condition. The higher chemical index of alteration (CIA) values of the Peninsular river's sediments than the Yamuna and Ganga river sediments reflect the control of weatherable lithology in the tectonically stable, low relief and subtropical climate condition. The Yamuna and Ganga river sediments with lower CIA values show more physical weathering in the tectonically active Himalaya. The suspended and overbank sediments of the Yamuna and Ganga rivers show an increase in their CIA values after the confluence of the Peninsular rivers, indicating the contribution of more weathered materials by the Peninsular rivers. It has been found that there is a geochemical split between the channel and suspended sediments of the Peninsular rivers for sharing different sources, i.e., the dominant contribution of felsic sources to the channel and mafic sources to the suspended sediments. The weathering geochemistry, trace element systematics, and REE patterns suggest that the dominant sources of the channel sediments of the Peninsular rivers are the felsic crystallines and sedimentary lithologies of the Aravalli range, Bundelkhand and Chhotanagpur granite and gneisses, and Vindhyan sandstones. Whereas, the mafic lithologies such as the Deccan traps and mafic components of the felsic lithologies appear to be the major sources of the suspended sediments. Within the range of intermediate composition, the overbank sediment chemistry reflects more contribution of the mafic sources except for the Ken which shows a higher contribution of the felsic sources. The geochemical split between the channel and suspended sediments of the Peninsular river's sediments for the different provenances indicates differential weathering of the Deccan basalts, Bundelkhand crystalline, and Vindhyan sedimentary rocks; and also the hydrodynamic control of the Peninsular rivers during erosion, transport, and deposition. However, the Himalayan rivers do not show such contrast between the channel and suspended sediments because of the higher physical weathering and sediment mixing in the high gradient topography of the Himalayan catchment.