Geochemistry of sediments of the Ganges alluvial plains: evidence of large-scale sediment recycling

Abstract

The Himalaya, an amalgamation of different lithotectonic units brought together during, and even before, the Himalayan orogeny, has been subjected to various phases of denudation since the Palaeogene. Sediments derived by erosion of these lithotectonic units in different proportions were delivered to the Himalayan foreland. The nature and proportions of sediments from various lithotectonic units delivered to the foreland varied in response to the prevailing tectonic and climatic conditions during the orogeny. The geochemistry of sediments collected from floodplain, river channels and reworked aeolian sediments from the Delhi and Moradabad areas of the Ganges plain of the foreland basin indicates that they were derived from moderately weathered sedimentary sources. Further, the results also suggest that the sediments are not subjected to any significant post-depositional chemical weathering. High tectonic instability in the source area, and the geomorphology and cycles of aggradation and degradation in fluvial regimes in the Ganges Plain do not favor much chemical weathering. Homogenization of various lithotectonic units of the Himalaya through several episodes of sedimentary reworking is shown by the rare earth elements (REE) geochemistry of the sediments. The predominant source, for the homogeneous sediments, is likely the High Himalayan Crystalline or the Tertiary sub-Himalayan sediments, again the later was derived from High Himalayan Crystalline. If so, the Ganges alluvial plain is geochemically a Quaternary extension of Tertiary sub-Himalaya. The average composition of lithologies subjected to erosion in the Himalaya appears to be better represented by the alluvial mud present in the plain.