雅鲁藏布江中游杰德秀古湖的发现及其意义

在雅鲁藏布江中游山南宽谷段发现了一套湖相沉积地层，形成时代为晚更新世晚期，为冰川阻江形成的堰塞湖沉积物。野外调查发现该套湖相地层在桑日县、乃东县、扎囊县等地均有出露，综合研究认为其为一个东起桑日县扎巴村、西至贡嘎机场附近的大型古堰塞湖泊，面积达700多km2。通过区域调查，该堰塞湖形成的湖相沉积地层在贡嘎县杰德秀镇出露最厚，厚10余m，主要由粉砂质黏土层、细砂层组成，水平纹层等湖相沉积特征明显，本研究称之为杰德秀古湖。杰德秀剖面顶部14C日历校正年龄为15 680～15 105 aBP，属于末次盛冰期。对比林芝古湖、格嘎古湖和松宗古湖等雅鲁藏布江下游堰塞湖的沉积物特征和形成时间，认为杰德秀古湖发育于末次盛冰期。沉积相特征和湖水库容量模拟分析表明，该堰塞湖发育时期，雅鲁藏布江中游山南宽谷段并未完全封闭，为一个湖水面积巨大的过水湖或吞吐湖，类似于现代过水水库。桑日县扎巴村雅鲁藏布江河谷两岸分布有冰碛物，其与雅鲁藏布江中游山南宽谷段的湖相沉积地层的时空关系指示杰德秀古湖与冰川阻塞河道有关，杰德秀古湖是由于冰川阻江形成的冰川堰塞湖。研究结果也进一步说明，雅鲁藏布江现代水系形成以前，其中游不存在面积巨大的众多古湖泊，雅鲁藏布江也不是溯源侵蚀疏干多个古湖泊而形成的，在古湖泊形成之前，雅鲁藏布江已经贯通。雅鲁藏布江流域内的古湖泊是由于气候变化、新构造运动或地震活动等原因造成河道堵塞形成的堰塞湖。

Discovery and Significance of the Lacustrine Sedimentation in the Middle Reach of Yarlung Zangbo River in Tibetan Plateau

The authors recognized a barrier lake in the middle reach of the Yarlung Zangbo River, which is formed by the glacier blocking the river during the late stage of Late Pleistocene. The field investigation finds that the lacustrine strata are in the Sangri County, Nêdong County, Zhanang County and other places along the middle reach of Yarlung Zangbo River, showing that the lake is a large paleaolake with an area of more than 700 km2 from Zaba village near Sangri County (east) to Gonggar airport (west), iethe Jiedexiu paleaolake. The lacustrine sedimentary section is typically a continuous silty clay layer and has horizontal bedding structures; the thickness of the section in Jiedexiu town of Gonggar County is the largest and more than 10 meters; the AMS 14C age of the top of the section is 15,680-15,105 aBP, belonging to the Last Glacial Maximum (LGM) period. Combined with the formation age and sedimentary characteristics of the lacustrine strata in Linzhi, Songzong, Gega paleaolake and other areas along the lower reach of the Yarlung Zangbo River, the results reveal that the Jiedexiu paleaolake develops during the LGM period; the characteristics of sediments and storage capacity of paleaolake show that the Jiedexiu paleaolake is not completely blocked during developing period, but a large scale lake of taking in and sending out waters, liking a modern reservoir. The spatial characteristics of moraine, which is found in the south bank of the Yarlung Zangbo River, and lacustrine sediments in the board valley of the river, indicate that the Jiedexiu paleaolake is formed from glaciers blocking the Yarlung Zangbo River during the LGM period. This paper also demonstrates that the Yarlung Zangbo River is an antecedent river, it isnt formed by headwater erosion of many paleaolake in the region, but it exists there before the formation of the Jiedexiu and other paleaolakes. Paleaolakes, such as Jiedexiu paleaolake in the Yarlung Zangbo River basin, are formed by blocking river valley caused by climate changes, neotectonic movement, and seismic activity and so on.