TEMPORAL VARIABILITY IN SEDIMENT COMPOSITION FROM THE SIANG RIVER, EASTERN HIMALAYAN SYNTAXIS REGION

In effort to better understand the provenance and rate of evacuation of sediments from a rapidly eroding region in the eastern Himalaya, we sampled bank sediments from a major tributary of the Brahmaputra, the Siang River in Arunachal Pradesh, Northeast India during the 2003-2004 Monsoonal/Dry-Season. This tributary slices through the deepest gorge on earth through an area of exceptionally rapid exhumation between the massifs of Gyala Peri (7150 m) and Namche Barwa (7756 m), which constitute an actively rising antiform developing in Proterozoic rocks of the Indian plate that were deformed and metamorphosed in the Pleistocene. Samples were collected over the course of a monsoonal season and the following dry season. After obtaining simple mineral separates based on density, samples were examined under the microscope. Point counts of hornblende, garnet and epidote provided a useful mineralogical index of temporal variations in the sediments. In addition to mineralogical analyses, modal settling velocities were measured for each sample using a settling column. These samples show significant temporal variability in mineralogical composition in parallel with seasonal variation in water discharge and suspended sediment flux. Mineralogical variability is not explained well by variation in settling velocity mode, suggesting that grain size or density sorting is not the important process leading to this variability. Thus, the observed mineralogical variation in Siang sands reflects either discharge-dependent switching between mineralogically distinct provenance areas or variations in the fraction of sediments mobilized from heterogeneous sediments in stream channels within the provenance region.