青藏高原东部黄河切开若尔盖湖盆的沉积证据与年代研究

Sedimentary evidence and age of the Zoige Basin dissected by the Yellow River in eastern Tibetan Plateau

对青藏高原东部若尔盖盆地内外实地考察,在玛曲瓶颈段黄河二级阶地前沿陡坎,发现了含有古深湖相和古河床河漫滩相地层序列的典型沉积剖面。通过沉积物粒度特征分析和光释光(optically stimulated luminescence,OSL)测年,研究结果表明:(1)剖面下部淡蓝灰色古深湖相沉积层为深水厌氧环境下形成的湖相沉积物,而覆盖其上的杂色卵石层夹淡黄橙色透镜状沙层则是古河床河漫滩相沉积物,指示了强动力流水作用过程。这2组地层的不整合接触关系,是黄河切开若尔盖湖盆导致古湖水外泄的直接证据。(2)剖面古深湖相沉积层顶部和古滨浅湖相沉积层底部的OSL测年结果表明,古黄河在37 ka BP切开若尔盖湖盆,导致湖水外泄,35 ka BP湖水变浅消失,黄河沟通了若尔盖盆地水系。(3)晚更新世东昆仑大断裂强烈的新构造运动和37 ka温暖湿润气候的综合影响使得尚处于玛曲断陷谷地草原的古黄河源溯源侵蚀加剧,由西向东在玛曲城南瓶颈段切开了若尔盖古湖盆,导致古湖水外泄,从而沟通了若尔盖湖盆的水系,使之成为黄河源。该研究结果对于深入理解青藏高原东北部河湖水系演变及黄河水系的形成具有重要的科学意义。

Through extensive and detailed field investigation inside and outside the Zoige Basin in eastern Tibet Plateau,a typical sedimentary profile containing the stratigraphic sequence of paleolake facies and paleoriverbed floodplain facies was found in the front of the second terrace of the Yellow River in the Maqu section at the outlet of the Zoige Basin.Through grain size characteristic analysis and Optically Stimulated Luminescence(OSL)dating,we found that:1)The pale-blue-grey paleolake sedimentary layer in the profile indicates that the lacustrine sediment formed in the deep-water anoxic environment,while the mottled pebble layer covered by the pale-yellow-orange lenticular sand layer was a typical fluvial floodplain sediment,which indicates the process of strong dynamic flow.The unconformable contact relationship between the two layers indicates that the profile is a typical sedimentary evidence for studying the Zoige Basin dissected by the Yellow River.2)The OSL dating results of the top boundary of the pale-blue-gray paleolake sedimentary layer and the bottom boundary of the paleoshore-shallow lake sedimentary layer show that the Yellow River dissected the Zoige Basin at 37 ka BP,resulting in the leakage of lake water.At 35 ka BP,the lake water became shallow and disappeared,and then the Yellow River connected the water system of the Zoige Basin.3)The strong neotectonic movement of the East Kunlun fault in late Pleistocene and the warm and humid climate of the Tibet Plateau in 37 ka resulted in the aggravation of the headward erosion of the paleo-Yellow River source in the rift valley grassland,and the Zoige Basin was dissected by the paleo-Yellow River from the west to the east in the Maqu bottleneck reach,resulting in the discharge of the paleolake water through the Yellow River,thus connecting the water system of the Zoige Basin and making it the source of the Yellow River.The study results have important scientific significance for an in-depth understanding of the evolution of river-lake water system and the formation of Yellow River water system in northeastern Tibet Plateau.