Sediment Character Of Xigeda Formation And The Paleoenvironmental Implication Since Late Cenozoic In The West Of Sichuan

There existed a set of sediment derived from river and lake on the valley of Dadu riverand Anning river since Pliocene.And the sediment imply that there was a large lake in theeast of Qingzang plateau from 4MaBP to 3MaBP.It is great significance to research the creat,growth and dry process of the age-old lake, And the same to sediment environment.The Xigeda formation located in Zhoujia ditch was selected as typical profile inAnNing river valley in this dissertation. The main conclusion was made as follows accordingto the research of sediment, magnetism stratum and granularity:First, the sediment depth of Xigeda formation was about 301.1m,and the lithologycombination character is "yellow layer"and "grey layer". The sediment started in 3.58 Maand ended in 2.58Ma, the time belonged to Gaosi positive polarity epoch.Second, Xigeda formation was divided three sediment serials from bottom to top layer.SQ1:The sediment which was made from caesious clay and fine grit means lakesediment, and then the datum plane rised, the adnit space augmented,and the Xigedaformation growed.SQ2: The sediment which consisted of caesious clay, larknd fine grit andmiddling-coarse silt imply datum plane droped, water power increased in prophase anddatum plane rised, lake water expanded in anaphase when ancient topographty planed slowlyand acceptance space gathered along lake basin.SQ3: The sediment which mainly consisted of larknd fine grit and middling-coarse siltimply datum plane dropped and the process of water-cut increased, here clay layers cut down,the depth attenuation. Then the neotectonic movement reinforced, uniform level planedisassembed by movement of uprightness and ancient lake died out slowly.