雅鲁藏布江中游日喀则—约居段古堰塞湖重建研究

The middle Yarlung Tsangpo is consists of broad valleys, alternating with gorges. The gorges and its surrounding mountain ranges are characterized by deep incision, narrow river gorges and threshold hillslopes. During the geological period the gorges possibly dammed, and a series of dammed palaeo-lake sedimentary terraces is scattered within the river valley. However, the age, the genesis and location of the dam, and the process of dam failure are remain controversial. In this project, we will focus on the dammed palaeo-lake sediment located in the broad Xigazê valley and the Dazhuka-Yueju gorge in the middle Yarlung Tsangpo, and the sediments at the outlet of its tributaries in the gorge. Based on investigation of the river valley landforms, characteristics of sediments, Radix sp. shell fossils, and geochronology study, we will (i) reconstruct the sedimentary sequence of the Lhainxiang and 4744-km sequence, and recover the palaeo-hydrogeochemistry and palaeo-hydrological information; (ii) confirm the genesis and location of the dam; (iii) date the formation and failure age of the dammed palaeo-lake; and (iv) reveal the process of dam failure. Finally, we will reconstruct the evolution of the dammed palaeo-lake in the study area, and discuss its relationship with the dammed palaeo-lakes at downstream. This study is very important for understanding of the long-term dynamic evolution of the fluvial systems, and the design of hydraulic engineering in the Yarlung Tsangpo.

雅鲁藏布江中游宽谷与峡谷相间分布，峡谷段河谷深切，两侧山体高耸陡峭。河谷阶地中保存着古湖相沉积，指示着该地区在地质历史时期被堵塞并形成堰塞湖。但古堰塞湖的发育年代、形成原因、堵江位置、坝体高度及其溃决过程等问题仍不清楚。拟开展的研究将在前人和申请人已有工作的基础上，以雅鲁藏布江中游的日喀则宽谷和大竹卡—约居峡谷及两侧支沟为重点调查区域，以古堰塞湖沉积和沟口沉积物为重点研究对象。基于河谷地貌、沉积物特征、萝卜螺壳体化石和地质年代学研究，建立联乡剖面和4744-km剖面的沉积序列，重建堰塞湖古水化学和古水文特征；确认古堰塞湖的成因及坝体位置；进一步精确限定古堰塞湖开始和消亡的时间；分析古堰塞湖的溃决过程。最终，重建研究区古堰塞湖的演化过程，探讨其与下游古堰塞湖的联系。拟开展的工作对理解雅鲁藏布江河流地貌的发育过程有重要意义，同时也有益于该流域水利工程设计和建设。

雅鲁藏布江中游山高谷深，冰川发育，新构造运动活跃，堵江事件多发。因此，在雅鲁藏布江中游的河流阶地中，多处出露着古堰塞湖沉积。本项目主要以日喀则—约居河段为研究区，以古堰塞湖沉积阶地、支沟二级阶地和峡谷出口沉积物为研究对象。根据沉积物地貌特征、地质年代学、物理化学指标分析和软体动物壳体地球化学研究等，重建该河段古堰塞湖：①综合分析8处古堰塞湖沉积剖面，将阶地发育过程分为：古堰塞湖前沉积（河流相或者风尘堆积）、古堰塞湖早期沉积（厚砂层）、古堰塞湖沉积（亚砂土及砂层）、溃决后沉积物（砂和卵砾石层）和顶部沉积（黄土及坡积物）。②基于联乡剖面萝卜螺化石壳体化石重建的古水体δ18O、EC和Sr/Ca值，比现代雅鲁藏布江重得多，表现为过水湖特征，即堰塞湖。③联乡剖面沉积物表明，古堰塞湖发育于35.0~13.2 cal. ka BP，应为逐渐消亡。④在大竹卡—约居出口的南岸沟谷上部主要为冰碛物，同时在沟内发现泥石流堆积，判断为沟头冰缘湖泊溃决，冲刷沟内冰碛物等风化物质进入雅鲁藏布江，形成堵江事件。⑤古堰塞湖海拔为3811 m，长达185 km，体积为22.55 km3，沉积物和水体分别占11.56 km3和10.99 km3。⑥雅鲁藏布江中游在晚更新世应发育3个相互独立的古堰塞湖，各河段古堰塞湖的消亡时间相对较为接近（约13 ka）。⑦古堰塞湖发育时期，雅鲁藏布江中游与全球气候变化较一致，但也存在一定的差异性。本研究有利于理解雅鲁藏布江中游河流地貌演化及与古环境的联系。