Geomorphological Evolution Characteristics Of The Yangtze River's First Bay

The Yangtze River,China's mother river,originated in the Tanggula Mountains of the Qinghai-Tibet Plateau,the Ge La Dan Dong Snow Mountain.The flow of the river suddenly changed from SSE to NNE,forming a huge V-shaped channel,commonly known as the"First Bay of the Yangtze River",at ShiGu town,in Yunnan province.In this paper,relevant field geological work has been carried out in the first bay section of the Yangtze River.The chronological samples of the corresponding sections have been taken,and the methods of OSL dating,carbon 14 dating,Trinity sedimentary system and reconstruction of ancient barrier lakes have been adopted.This paper makes a preliminary discussion on the evolution characteristics of the valley landforms in the First Bay of the Yangtze River,and obtains the following achievements and understandings.Firstly,through field GPS surveys and comparative observation of different sections of the first bay,three river terraces?T₀,T₁,T₂?,two ancient barrier lakes?L₁,L₂?,the corresponding dam-break flood accumulation??1???2??and the location of the barrier dams leading to the formation of two ancient barrier lakes are identified.It is found that the tug-of-war heights of two ancient dammed lakes?L₁ and L₂? correspond to two river terraces?T₁ and T₂?respectively,and the field geomorphological deposits also show phase change relationship.The two ancient dammed lakes are reconstructed by using the development range and elevation of lake facies deposits,and the scope of the two ancient dammed lakes is restored.The discovery of two lake facies deposits also reflects the occurrence of at least two river blockage events.Between the Longpan village and The tiger leaping gorge section,a large number of mixed deposits?thickness>100m?were found in the field survey,and the developed two-stage lake facies layers disappeared here.We believe that this section is the place where the dams causing river blockage are located,combining with the occurrence of mixed deposits?inclination EW,inclination to 40 degrees?.)Characteristics and distribution range of moraine ridges left by glacial landslides in Yulong Snow Mountain?tug-of-war is less than 200 m?,we think it is caused by glacial landslides and debris flows blocking the river.Secondly,through the chronological analysis,the age of T₀ is⁰.5ka,indicating the uplift of the Tibet Plateau in the past 500 years and the latest river terrace landform formed by river down-cutting;the age of T₁ indicates the tiger leaping gorge section,and the river terrace T₁ is formed from 8.0ka to 1.1ka years ago;the age of T₂,combined with the age of T₂ ¹⁴C,is 9.5±0.04ka,and the age of 3 OSL is¹5.80ka,¹6.07ka,¹4.18ka.It is considered that the formation time of T₂ terrain is 16 ka to 9ka years ago.The lenses of local lacustrine deposits in the back edge of the dam body of the two families are 20.50±1.23 ka,18.49±1.34 ka and 21.25±1.31 ka,indicating that the formation age of the dam is at least²0 ka.Finally,by using the relevant theories of geomorphology and sedimentology,and the Trinity sedimentary system of ancient dammed lakes,dammed dams and dam-break floods proposed in recent years,combined with the chronological test and analysis of samples,this paper indicates that at least two dammed-dam-break floods have occurred in the first bay of the Yangtze River since the last glacial peak?²5ka?.Two stages of ancient barrier lakes?L₁ and L₂?were formed accordingly,in which the ancient barrier lake L₁ formed by blocking the river extended to the vicinity of"the first bay of the Yangtze River".After the dam burst,a corresponding period of flood accumulation was formed in the large basin??1??;while the ancient barrier lake L₂ extended to Lamaluo village,nearly 50 km away.After the breakdown of the barrier dam,a corresponding flood was formed in the large basin at the exit of the tiger leaping gorge.Accumulation??2??.