金沙江堰塞湖细粒碎屑沉积物元素地球化学特征及其对河流演化的指示

堰塞湖沉积物是研究水系演化和区域构造活动的关键对象。本文对金沙江奔子栏-巧家河段新生代堰塞湖、下伏河流相及现代河滩沉积物细粒泥沙(0.063 mm),进行元素地球化学特征分析,揭示各堰塞湖物源变化情况,并探讨对金沙江水系演化的指示意义。研究发现:1.萨赛登、奔子栏、石鼓、涛源和金塘等堰塞湖与现代河滩沉积物的元素地球化学特征相同,物源一致,泥沙均主要来源于金沙江; 2.下伏于昔格达组的格里坪河流砾石层与下游拉鲊昔格达组剖面的元素地球化学特征与其它堰塞湖相同,泥沙来源于金沙江;而受支流雅砻江来沙影响,昔格达村和炳草岗等处昔格达组的微量元素特征不同于格里坪下伏河流相和拉鲊剖面及其它堰塞湖沉积; 3.奔子栏-巧家河段各堰塞湖、下伏河流相及现代河滩物源的一致性表明,这些堰塞湖形成前,东流的金沙江已经形成,并且物源至今没有发生过重大变化。

The Elemental Geochemistry of Fine-grained Sediments in Dammed Lakes of the Jinsha River and Its Indication to the River Evolution

Dammed lake deposition is a key for understanding the drainage evolution of the Jinsha River and regional tectonic activities. The elemental geochemistry of fine-grained (<0.063 mm) sediment samples from Cenozoic dammed lakes, underlying fluvial facies and modern banks of the Jinsha River between Benzilan and Qiaojia was analyzed to reveal the provenance change of each dammed lake, and to briefly discuss the indicative significance to drainage evolution of the Jinsha River. The three key findings of this study are: 1. The elemental geochemistry of sediments from modern banks and dammed lakes such as Sasaideng, Benzilan, Shigu, Taoyuan and Jintang are the same, indicating they have the same provenance of the Jinsha River; 2. The elemental geochemistry of the fluvial gravel beds beneath the Xigeda Formation at Geliping and the Xigeda Formation at the downstream Lazha are the same as those of other dammed lakes, indicating the sediment all comes from the Jinsha River; the trace element geochemical characteristics of the Xigeda Formation at Xigeda Village and Bingcaogang are different from those of underlying fluvial facies at Geliping and Lazha and other dammed lake deposits as a result of affection by sediment from the Yalong River; 3. The consistency of the provenance of dammed lakes, underlying fluvial facies and modern banks between Benzilan and Qiaojia indicates that the east-flowing Jinsha River had already been formed before the formation of these dammed lakes, and their provenance has not changed significantly so far.