金沙江龙街段晚更新世以来的阶地发育与河谷地貌演化

Terraces Development and Their Implications for Valley Evolution of the Jinsha River Since Late Pleistocene near Longjie,Yunnan

河流阶地是研究现代河谷形成发育的重要地貌标志。通过野外考察,发现在元谋龙街附近金沙江发育了5级河流阶地,其中T1~T4以龙街组湖相沉积为基座,T5以基岩为基座。电子自旋共振测年结合前人研究结果表明T1、T2、T3、T4和T5分别形成于(18±1.7)ka、(23±1.4)ka、(26±2.4)ka、(29±1.4)ka和(78±12)ka,标志着晚更新世以来金沙江在该区进行了5次下切过程和1次强烈的加积过程。晚更新世以来由于滑坡堵江事件的扰动,河谷的发育形式以"下切—滑坡—堰塞—堆积—下切"过程为主。

The drainage evolution and valley development of the Jinsha River is an important issue constantly concerned by researchers in geology and geomorphology.Despite hundreds of years of research,there is a big dispute on the formation time and the evolution process of the fluvial valley.Fluvial terraces are very important geomorphic markers for studying the formation and evolution of the fluvial valley.Through field investigation combined with Electron Spin Resonance(ESR)dating,we confirmed that 5 fluvial terraces were formed,and then preserved,along the course of the Jinsha River near the Longjie,which are all strath terraces.Among them,T5 developed on the base rock,with an age of(78±12)ka;all T4~T1 developed on the lacustrine sediments,named Longjie Group by Chinese,with an age of(29±1.4)ka,(26±2.4)ka,(23±1.4)ka,(18±1.7)ka,respectively.Compared with the global and regional climate change history,the terraces are all the result of the river responding to the climate change.T5 formed at MIS 5/4,and T4~T1 formed at the period of regional climate fluctuation.The relationship of terraces and the Longjie Formation,combined with sedimentary characteristics analysis demonstrate that the Longjie Formation is landslide dammed lake sediment.The landslide and blocking events.seriously influenced the valley evolution,inhibiting the river incising,and making the valley evolution defer to the mode of"cut-landside-damming-fill-cut"in the period of Late Pleistocene.Synthesized studies of the terraces and the correlative sediments indicate that the formation of the Jinsha River valley may have begun in the late Early Pleistocene.