AN EARLIER PLEISTOCENE GLACIATION WAS FOUND IN BODUIZANGBU VALLEY, SOUTHEASTERN TIBET

Southeastern Tibet display high mountains and deep valleys in geomorphology and is the most wet- warm area in Tibet Plateau because of Bengal bay vapor influence through the Great Gorge of Yarlung Zangbo River. As a center of glaciers, ancient glaciations have been studied in past decades. Two Pleistocene glaciations Guxiang and Baiyu were suggested 40 years ago. They have been dated to MIS-6 and MIS-4-2 glaciations using CRN and OSL methods.

Recent years we found some evidences for an earlier glaciation also in the Boduizangbo River Valley. The evidences are mainly two higher moraine platforms high 870m (3770m asl) and 730m (3550m asl) above river level on the north side of the valley. The moraine platforms are higher 540m, 490m and 440m above the Guxiang lateral moraine summits respectively and keep reverse slope. The moraine above Nitong village keeps 8m reverse slope, implying a thickness of the moraine at least 8m. The moraines contain big granite boulders in both places. Some of granite boulders on the platform opposite Baiyu Village reach 3m in diameter. These granite boulders and tills deposited on underlying limestone bed rock. We think that these moraines should be trace remains of an older glaciation. We tentatively use ESR method dated these moraines to about 490ka. Since then the valley has been cut down about 800m, possibly signifying a violent uplift of southeastern Tibet Plateau during late Quaternary.