A Discussion on Quaternary Glaciation in the Bogda Region, Tian Shan
Zheng Benxing and Wang Cunnian (Lanzhou Institute of Glaciology and Cryopedology, Academia Sinica)
Abundant traces of ancient glaciation exist in the periphery of the contemporary glaciers in the region of Mt. Bogda. At the upper reaches of the rivers, there are double U-shaped valleys, cirques and a lot of end moraines in the lower U-shaped valley. At the lower reaches of Sangong river and Sigong river on the northern slope, there are low glaciofluvial terraces, middle and high morainic terraces, outwash fans near the outlets and alluvial-puvial plains outside the outlets. It has been shown by our field investigation, analyses of grain size, rock and clay mineral, C14 date, and spore-pollen that, during Quaternary, at least three glaciations and one Neoglaciation in Holocene took place in this region, that is, the high moraine platform of Tiantai Glaciation, the middle moraine terrace of Tianshan Glaciation, the Linchang (Forest Station) end moraine and Luweibaliang moraine of Tianchi Glaciation, as well as the Xiaohaizi end moraine of Tianfeng Neoglaciation in Holocene. During the Early Pleistocene, the climate became cold for many times, there may be small glacier, but no glacial deposit, only cemented conglomerate in river facies at the pediment zone had been found. In the Tiantai Glaciation of Middle Pleistocene, the wide-tail valley glaciers 30 km long developed on the northern slope, and the valley glaciers of more than 10 km long on the southern slope. Because of the strong uplifting of the mountains at the end of Middle Pleistocene, the mountain body enlarged and river valleys deepened, the original pediment moraine was uplifted to the altitude of 1100-1300 m, as high as the top of the present lower mountains. In Late Pleistocene, quite large and long valley glaciers developed, the termini of which moved at least down to 1200 m a. s. 1. on the northern slope, even reaching the vicinity of outlets. In the Last Glaciation, which can be divided into two subglaciations, the glaciers moved to the middle mountain zone. In Neoglaciation of Holocene, small valley glaciers several kilometres long developed, with their termini at: 3200-3400 m a. s. 1..