金沙江巧家—蒙姑段的阶地发育与河谷地貌演化

Terraces development and their implications for valley evolution of the Jinsha River from Qiaojia to Menggu

金沙江水系演化与河谷发育问题长期以来是地质地貌学界关注的重大问题,目前仍存在较大争议。河流阶地及其相关沉积是河谷发育过程的产物,可以提供河谷发育的时代与形式等诸多信息。金沙江在巧家—蒙姑段河谷中,葫芦口附近发育和保存了8级基座阶地,结合光释光和电子自旋共振测年方法,依据古气候资料,推断T6~T1的下切时间分别对应于深海氧同位素(MIS)的36/35、34/33、24/23、20/19、14/13和4/3阶段,即气候由冷至暖的转型期。青岗坝附近则发育了5级由堰塞湖相沉积组成的堆积型阶地,指示了中更新世以来该段河谷在下切过程中经历了频繁的滑坡堵江堰塞,发育形式以"下切—滑坡—堰塞—堆积—下切"过程为主。此外,河流的平均下切速率自0.82 Ma以来由此前的0.56 mm/a下降至0.19 mm/a,表明中更新世以来频繁发生的堵江堰塞事件严重抑制了该段河谷的下切作用。综合流域内河流阶地序列及相关沉积的研究,金沙江下游段现代河谷的形成时代不晚于早更新世。

The valley evolution of the Jinsha River is a research hotspot in the geomorphology,but there is a big dispute on the formation time and the evolution process of the river valley.Fluvial terraces and the correlative sediments contain abundant information about the fluvial valley evolution.Through field investigation combined with Optically Stimulated Luminescence(OSL)and Electron Spin Resonance(ESR)dating,we confirmed that 8 fluvial strath terraces were formed,and then preserved,along the course of the Jinsha River near Hulukou.The formation ages of T6-T1 are 1190±154 ka,1079±118 ka,942±92 ka,816±48 ka,544±82 ka,and 41±5 ka,respectively,corresponding well with the transition period of MIS.Near the Qinggangba,5 terraces(T5-T1)composed of the dammed lake sediments are preserved on the hillslope,with ages of 629 ka,88 ka,71 ka,49 ka and 20 ka,respectively,indicating that the Jinsha River valley from Qiaojia to Menggu has been repeatedly dammed by the landslide deposits since the mid-Pleistocene epoch.The valley evolution defers to the model of"cut-landslide-dammed-aggradation-cut".We calculated the incision rate based on the formation age and the height of the strath terraces near Hulukou,and found that the incision rate can be separated into two parts:before 0.82 Ma,the incision rate is 0.56 mm/a;from 0.82 Ma to now,it descended to 0.19 mm/a.We think this transition is induced by the frequent landslide events,which can bring a large amount of sediments to the river valley,and then impound a lake and accumulate sediments upstream,inhibiting the river incision.Synthesized studies of the fluvial terraces indicate that the formation of the modern valley of the Jinsha River may have begun in the late Early Pleistocene.