Luminescence dating of late Quaternary fluvial sediments in the Rapti Basin, north-central Gangetic plains

There is evidence, in parts of the southern Gangetic plain, of river incision and terrace formation, although the elevation ranges across the plains are small relative to the seasonal variation of stage in the main rivers. In the northern parts of the plain, there is interaction between the longitudinal rivers and lateral influx of water and sediment in rivers crossing fans such as the Gandhak and the Kosi, and the inter-fan areas are locations where aggradation and avulsion are dominant. Thus, the fluvial history of the Ganges and its tributaries, and the relationship of that history to environmental changes including climate change, tectonic history and variation in relative sea level, is likely to be very complex. Unravelling the history will require systematic dating of fluvial sediments, with luminescence methods (TL and OSL) being of significance given the absence of datable organics, and the ambiguities associated with dating carbonates (shells, kankar). This preliminary exercise involves applying these methods to test hypotheses about the ages of fluvial surfaces in the case of the lower Rapti River. TL dates are likely to be unreliable given the water turbidity and lack of zeroing, but OSL dates on both modern and recent sediments provide useful information on the ages of fluvial sediments in the region. The evidence is consistent with the view that the Rapti has had a post-glacial history of aggradation and avulsion, with shifts in its position and changes in locations of deposition therefore not being directly related to allogenic environmental changes.