松宗古湖——藏东南帕隆藏布江末次盛冰期发育的一个冰川堰塞湖

SONGZONG LAKE:AN ICE-DAMMED LAKE OF LAST GLACIAL MAXIMUM IN PURLUNG TSANGPO RIVER,SOUTHEAST TIBET

青藏高原东南部帕隆藏布江松宗地区晚更新世期间发生了一次可以识别的支谷冰川阻江形成冰川堰塞湖———松宗古湖的事件。松宗滑坡处的湖相沉积剖面厚度≥88m,其中厚达18.33m并具湖相沉积代表性的连续粉质粘土层底部和顶部的光释光年龄分别为22.5±3.3kaB.P.和16.1±1.7kaB.P.,属末次盛冰期。松宗滑坡处河谷两侧冰碛台地与湖相沉积的接触关系和空间特征指示该湖相沉积与末次盛冰期董曲支谷冰川阻塞帕隆藏布江有关。湖相地层的剖面沉积特征揭示出这个冰川堰塞湖可能贯穿于整个末次盛冰期,但整个帕隆藏布并没有形成统一的山谷冰川。

An distinguashable ice-dammed lake formed during the Late Pleistocene at the Purlung Tsangpo River, a main downstream tributary of the Yarlung Tsangpo River, in the Songzong Basin of southeast Tibet has been recognized. The lacustrine deposit, more than 88 meters thick measured by Total Station （TCA2003）, is disclosed after the Songzong landslide. The successional silty clay in the profile is 18.33 meters thick, clearly suggesting the formation of a lake. The Optical Simulated Luminescence （OSL） ages at the bottom and top of the silty clay layer are 22.5 ± 3.3kaB. P., 16. 1 ± 1.7kaB. P., respectively, indicating that the lacustrine deposits were formed during the Last Glacial Maximum ranging from 25kaB. P. to 15kaB. P. It was discovered by field geological survey that the sediments were deposited in the outside of the moraine terrace of the Songzong Basin. Particularly, the high and large moraine terrace is outcropped in the front of the Dongqu Valley, a big tributary ravine at the south bank of the river. The Dongqu Valley upstream is one of the modern glacier centers of southeast Tibet. The special characteristics of the lacustrine sediment and the ancient lake line in the Songzong Basin indicate that the lacustrine sediment is related to the blocking of the Purlung Tsangpo River by the glacier in Last Glacial Maximum from Dongqu Valley. Moreover, the depositing velocity of the lacustrine reaches to 2.29 -2.82mm/a, more quickly than other close lakes in Tibet. Evidence also shows that the Songzong ice-dammed lake might run through the Last Glacial Maximum.