Absolute Age Determination of One of the Oldest Quaternary(?) Glacial Deposit (Bunthang Sequence) in the Tibetan Plateau Using Radioactive Decay of Cosmogenic 10 Be and 26 Al, the Central Karakoram, Pakistan: Implication for Paleoenvironment and Tectonics

Absolute age of the deposition of 1.3 ㎞-thick Bunthang sequence within the Skardu intermontane basin of the Central Karakoram was determined using radioactive decay of cosmogenic 10 Be and 26 Al burial dating. The Bunthang sequence deposited around 2.65 Ma, which is the oldest glaciation in the region. The timing of deposition of the Bunthang sequence is consistent with the previous suggestion that the basin filling took place between Brunhess and Matuyama chrons. Four major sedimentary facies interfinger within the Bunthang sequence: glacial diamict, lacustrine, fluvial and lacustrine facies upward. This sedimentary distinctiveness and the lack of evidence on the faults for alternative pull-apart basin model around the Bunthang sequence, suggest that the depressional basin was formed by deep subglacial erosion during the extensive Bunthang Glacial Stage and subsequently the sediments underlain by basal diamict, was quickly deposited by proglacial and paraglacial processes. Temporary ponding of the Indus River due to tectonic uplift in the downstream or blockage by mass movements might make the basin filling more possible. The hypothesis that the single ice sheet developed on the Tibetan Plateau during the global last glacial cycle should be refuted by the existence of the older extensive Bunthang glacier. Furthermore, the extensive glaciation during the early Quaternary (and thus progressive decrease in extent with time) suggests that there may have been significant uplift of the Pamir to the west and Himalaya to the south, which would have reduced the penetration of westerlies and Indian summer monsoon and hence moisture supply to the region.