金沙江寨子村古堰塞湖沉积年代研究

近年来，金沙江金江街寨子村古堰塞湖沉积受到学术界的日益关注。总体来说存在两种认识，一种认为这套沉积物是早更新世昔格达古湖沉积（昔格达组）的一部分，它可以反映古金沙江的袭夺演化历史，另一种观点认为它是一套堰塞湖沉积，具有沿河分布的特点，且存在残存的堵江堰塞滑坡坝体。然而这些认识目前都缺少年代学数据的支撑。为此，我们采用光释光（OSL）测年技术，选择地层界线出露相对完整的沿江小学剖面（26°14'54.034″N，100°25'32.114″E）系统地进行了地层学、年代学研究。结果发现，寨子村古堰塞湖沉积整体覆盖在金沙江第二级阶地上，在靠近残余堰塞坝体处厚度较大，逐渐向上游尖灭。其顶部OSL年龄为93 ka B.P.，底部为110 ka B.P.。堰塞湖沉积之上有约2 m厚的黄土堆积，其下部发育一层古土壤，古土壤顶部OSL年代为73 ka B.P.，表明寨子村古堰塞湖沉积形成于晚更新世，不是先前推测的早更新世，与学术界广泛认为的昔格达组不属于同一时代地层，因此难以记录早更新世古金沙江的袭夺演化历史。

The age of Zhaizicun ancient dammed lake sediments in Jinsha River

In recent years, the Zhaizicun ancient dammed lake sediments in the Jiniangjie reach of Jinsha River have increasely attracted attention in academia. In general, there are two types of understanding about the sediments at the present moment. One view holds that the Zhaizicun lacustrine sediments are part of Xigeda ancient lake sediments (Xigeda Formation)and they can be used to reconstruct the river-capturing history of Jinsha River (the upper reaches of the Yangtze River)in Early Pleistocene. The other view argues that the formation of the lacustrine sediment, which have the characteristics of distribution along the Jinsha River, is attributed to the blockage of Jinsha River by a massive landslide, furthermore the landslide dams causing the river to be blocked also can been found in the downstream of the lacustrine sediments. However, these hypotheses lack accurate chronology data support currently. Here we repot the latest stratigraphy and chronology research results from Yanjiangxiaoxue section (26°14'54.034″N, 100°25'32.114″E), where the top and bottom of the lacustrine strata expose completely, on the basis of optical stimulated luminescence (OSL)measurements data. The ages of the top and the bottom of the Zhaizicun ancient dammed lake sediments, which overlie the second terrace of Jinsha River and are gradually thinner from landslide dams toward the upstream, are 93 ka B.P. and 110 ka B.P. respectively. About 2 m of loess cover the lacustrine sediment, the age of the top of the paleosoil developing in the bottom of the loess is 73 ka B.P. This new evidence indicates that Zhaizicun ancient dammed lake sediments deposited not in Early Pleistocene but in Later Pleistocene and they should not be correlated with Xigeda Formation, whose age accepted widely in academia is Early Pleistocene. Therefore, the capturing history of Jinsha River in Early Pleistocene cannot been recorded by Zhaizicun ancient dammed lake sediments.