A Study On The Sequence And Formation Of River Terraces In Jinjiangjie Of Jinshajiang River

Jinshajiang is the upstream of the longest river in China,Yangtze river. To study the river terraces and formed age which could help us to understand the evolution history of the regional river system and the new tectonic movement. The segment of Jinjiangjie was studied in this paper, and it’s a segment which occupied an important position but lack of strong research. Sedimentary characteristics research and the age dating methods of ESR, OSL,It has established terrace sequence which was relatively complete and marked with accurate age, and restored the evolution history of the segment of Jinshajiang middle-downstream through the field investigation, Meanwhile, the vertical movement since Pleistocene in Chenghai fault has been discussed by using the principles of terrace sequence and formed age.The segment of Jinjiangjie of Jinshajiang reached the upper and lower positions of Chenghai fault. The study indicated that it had formed six terraces at least on the upper river valley of Chenghai fault, and it relatively showed from the lower to upper: T1, as the accumulation terrace, the height was15 m from the river, age was 0.046±0.002Ma; T2, as accumulation terrace, the height was 32 m from the river, age was 0.096±0.010Ma; T3, as the accumulation terrace, the height was 66 m from the river,age was 0.490±0.045Ma; T4, as the accumulation terrace, the height was 89 m from the river, age was 0.538±0.050Ma; T5, as the accumulation terrace,the height was 101 m from the river,age was 0.648±0.064Ma; T6, as the substrate terrace, the height was 118 m from the river, age was 0.707±0.070 Ma. Also, it had formed six terraces at least on the lower river valley of Chenghai fault: T1, as the accumulation terrace, the height was 23 m from the river,terrace age was 0.093Ma; T2, as the substrate terrace, the height was 76 m from the river, age was 0.308Ma; T3, as the substrate terrace, the height was 239 m from the river, age was 0.557±0.050Ma; T4, as the substrate terrace, the height was 249 m from the river, age was 0.734±0.070Ma; T5, as the substrate terrace, the height was 295 m from the river, age was 1.139±0.100Ma; T6, as the substrate terrace, the height was 312 m from the river, age was 1.179±0.110 Ma. Totally, it had 11 period terraces on the segment of Jinjiangjie which depends on the age of formation.The movement rate of undercut of the upper river valley of Chenghai fault is 0.147mm/a which has been calculated according to the terraces age,and the movement rate of undercut of the lower river valley is 0.246mm/a, the mean vertical movement rate of Chenghai fault was 0.099mm/a since the middle Pleistocene.The contemporary Jinshajaing water system forn after altitude of 2000 m terrain surface,before the Paleo-Jinshajiang is south water system, later was been divered by east flow Yangtze so that form the pattern of modern water system according to the theory of cycle of erosion landform simulation indicate. The form time of altitude of 2000 m terrain surface according to the minimum incised rate which is 0.147mm/a calculate, the contemporary Jinshajaing water system form time is 5.5Ma. The time is 3.3Ma if according to the maximum incised rate which is 0.246mm/a calculate.So the contemporary Jinshajaing water system formed time do not early at the end of the Miocene, may be in Pliocene.Jinshajiang valley water system evolution history has experienced three stages according to the sedimentary characteristics of terraces,age and langscape simulation result speculation: ①River diversion wide valley form stage, at the end of the Miocene to Pliocene, the Paleo-Jinshajiang changed the direction to east, and the contemporary Jinshajaing formed. The typical topography was wide valley, but the Jinshajiang terraces didn’t form at this time. ②Gorge form stage, the denudation plane was disintegrate, Jinshajiang narrow valley was formed, and four period terraces were formed by the river erosion and corrosion from Pliocene to 0.65 Ma. The ecological environment started to transfer to dry-hot valley. ③Dry-hot Valleys form stage, Jinshajiang was deeply cut, seven period terraces were formed, the foehn effect was obvious, the dry-hot valley was formed from 0.65 Ma to present. Meanwhile, the loess was accumulated which adapted to the condition of the dry-hot weather, and the valley vegetation was evolved to the savanna.