Contemporary sediment transfer in Himalayan glacial systems: Implications for the interpretation of the quaternary record.

This thesis presents the results of an investigation of the glacial geomorphic processes associated with sediment production and deposition in the Nanga Parbat Massif, Pakistan Himalaya. Emphasis is placed on contemporary deposition mechanisms; the nature and timing of Quaternary glaciation; and the influence of tectonic uplift on the glacial record. The field study concentrated on the extensive valley glacier systems of the Nanga Parbat Massif (8125 m.a.s.l.), at the western end of the Himalayan mountain chain. Some 60% of the surface of this region is glacierized and has experienced rapid tectonic uplift and intense denudational processes throughout the Quaternary. The glaciers of the area are very active; they have high values of mass-flux and transport considerable volumes of surficial glacial debris. Past and present glacial activity has resulted in the extensive moraines and other glacier-related deposits which are found throughout the massif. Sampling was undertaken on both contemporary and Pleistocene glacial deposits. Systematic sampling of glacier sediments of the Rakhiot Glacier to the north of the massif and the Chungphar Glacier to the south was designed to sample down-glacier trends. Transport pathways, from sources areas to the glacier snout, were mapped and samples were collected for particle size analysis and scanning electron microscopy. A similar study of the Glacier de Cheilon in the Swiss Alps was also completed so that the two glacier sediment systems could be compared. In the Nanga Parbat Massif, the Quaternary deposits of the Rupal, Astor, Rama and Rakhiot valleys were sub-divided into sedimentary units on the basis of geomorphological mapping, logging of sedimentary sections and multi-parameter relative age dating techniques. Quaternary ELAs and glacial limits were reconstructed in an attempt to assess variations in the size and distribution of glaciers in the massif. Debris transport in the supraglacial zone is an important contributor to contemporary glacial sediment production. Transport of the thick supraglacial debris results in intense comminution producing large volumes of fine sediment. This is reflected in positive down- glacier trends in sediment fining and increased particle weathering. This finding challenges the traditional view that the basal zone of traction is the only major source of fine sediment production in glacial environments. Fine sediment is also produced outside the immediate glacial zone, by intense weathering of the exposed mountain rockwalls. Important contributions from supraglacial, subglacial and non-glacial sources explain the fines-rich nature of the glacial meltwaters which drain the massif. The importance of supraglacial debris transport is a striking characteristic of the contemporary glacial depositional system, the dominant mode of glacial deposition being the dumping of supraglacial debris in ice-marginal locations. Mass movement is an important mechanism in the depositional process as is resedimentation; resulting in complex suites of ice-marginal deposits. Broadly synchronous glacier expansion in the Pleistocene resulted in a maximum lowering of the ELA of between 900 and 1000 m, with glacier termini on the east of the massif being lowered to altitudes of 2100 m and those on the north descending to join the Indus valley at 1800 m. Mountain ice caps and piedmont valley glaciers were much thicker in the Pleistocene, filling the valley systems. Present day glaciers are a remnant of these much more extensive Pleistocene valley glacier systems. Retreat during the late Pleistocene was roughly synchronous throughout the massif with glaciers becoming increasingly confined to the floors of the valleys. The present steep relief is a result of the combination of Quaternary bedrock uplift, in the form of tectonic uplift and isostatic response to glacial unloading, and over- deepening by glacial erosion during the Pleistocene. It appears that the glacial sediment systems of the Nanga Parbat massif have changed markedly over the Quaternary with large low gradient piedmont glaciers giving way to small, steep glaciers with thick supraglacial debris covers. Retreat of Pleistocene glaciers exposed large areas of bedrock to rapid subaerial weathering with the supraglacial debris component became increasingly important in the sedimentary system.