A Preliminary Study On The Landform And Environmental Records Of Baoshan Basin In Southwest Yunnan

The baoshan basin is located in the southeastern margin of the Qinghai-Tibet plateau.At the same time,the climate environment is affected by the southwest monsoon,which is also a good place to carry out ancient environmental changes.The lacustrine platform sequence of baoshan basin and the terrace sequence of donghe river were established through field observation.In the laboratory,a 418m-long borehole core obtained from the drilling work in the southeast of baoshan basin was tested and analyzed by means of magnetic susceptibility,colorimetry and other paleo-environmental substitution indexes.The paleo-environmental evolution process was preliminarily established by combining with the comparison of lithological characteristics of the core.On this basis,the response of fluvial and lacustrine physiognomy development to tectonic movement and the comparison between environmental change and regional and global environmental evolution are discussed.There are three levels of terraces in the east river section of baoshan basin T1,T2 and T3 terraces are stacked terraces,and the tug-of-war height of t1-t3 terraces is 3.5m,9m and 14m respectively.The terrace form is seriously damaged and T3 terrace is less reserved.There are also quaternary lacustrine platforms in baoshan basin.The T4 platform is 1680m above sea level,about 1600m long,about 400mm wide on average,and 35m high in tug-of-war.T5 platform is about 994m long,417m wide,1725m above sea level and 80m high in tug-of-war.T6 platform is about 1600m long and 500m wide,with an altitude of 1768m and a tug of war height of 123m.T7 platform is about 640m long and 300m wide,with an elevation of 1820m and a tug-of war height of 175m.The form of the platform is relatively complete.The main sedimentary types of the platform are lacustrine sand layer,clay layer and relatively thin gravel layer.Based on the analysis of the susceptibility and chromaticity of paleoenvironment,the environmental evolution of baoshan basin is divided into four stages.The first stage:8-6.23 Ma B.P.,in which the magnetic susceptibility value is the lowest and the brightness L is also the lowest,and the degree of redness a and degree of yellowness b are at high values,indicating that the precipitation is relatively large and the temperature is relatively high at this time,and the climate turns into a warm and humid type.The second stage was 6.23-3.8 Ma B.P.during this period,the magnetic susceptibility changed little,the brightness L decreased slightly,and the redness a and yellowness b decreased slightly,representing a slight decrease in precipitation and a slight decrease in temperature.Generally,it was not as warm and humid as in the previous period.The third stage:3.8-3.46 Ma B.P.during this period,the magnetic susceptibility changed little,the brightness L increased slightly,and the redness a and yellowness b showed small decreases,which indicated that the precipitation decreased,the temperature decreased,and the climate was relatively dry and cold.The fourth stage:3.46-3.4Ma B.P.during this period,the variation of magnetic susceptibility was small,the brightness L was significantly increased,the redness a and yellowness b were increased,the precipitation decreased,the lake water level further decreased,the temperature increased,and the climate was relatively warm and dry.On the whole,in the process of climate change in baoshan basin,the overall temperature drops and the overall humidity gradually decreases.The comparison between the southwest monsoon recorded by boreholes in baoshan basin and the records of the northeast Indian Ocean confirms that the southwest monsoon is weakened within 8-3.4 Ma.The change of monsoon environment was revealed by comparing the evolution of south Asian monsoon recorded by baoshan borehole with the records of dust particle size,deposition rate and ocean oxygen isotope recorded by the late Cenozoic in north China.