Research Of Paleoenvironment Process During The Eocene In Yilan Area Of Yi-Shu Graben

Predicting the future global warming trend and its eco-environmental effects have become an important topic in the scientific community.The Eocene is the most recent typical extreme greenhouse climate period in the process of earth evolution,and its environment and climate evolution characteristics play an important role in our better understanding of greenhouse climate change.The Yilan area is located in the south-central part of Heilongjiang Province of China.Eocene strata deposited continuously in this area completely record the paleoclimate information of this period.This paper takes continuous cores as the research object,and defines the characteristics of the sedimentary environment based on the core lithology,sedimentary structure,and the logging curve shape,and then takes some accurate samples on this basis,analyses the evolution process of Eocene paleoclimate environment in Yilan area through the statistical analysis of detrital composition,magnetic susceptibility and chromaticity index of the samples finally.The results shows that:Through comprehensive analysis of core lithology,sedimentary structure and structure,and logging curve shape,it is believed that the core is divided into first-grade cycles three times from bottom to top-the first time develops underwater fan facies,the second developes lakes,nearshore subaqueous fans and fan delta facies,and the third developes lakes and fan delta facies.The water body undergoes a change process of deeper-deeper-deeper-shallower-deeper-shallower with the cycle.The content of detrital components(quartz,feldspar,detritus,single crystal quartz,polycrystalline quartz,heterobase)was counted by Gazzi-Dickinson method under microscope.The results show that there is an obvious climate fluctuation stage in this core,of which from 797 to 900.2m the weathering erosion effect was gradually strengthened,the F/Q increased from high to low,Q% and component maturity increased,and the climate gradually changed from relatively dry and cool to hot and humid,from 432.4 to 797 m,the weathering erosion effect was weakened,the F/Q increased,Q% and component maturity gradually decreased,and the climate gradually decreased from hot and humid,and from 131.75 to 432.4m,with weak weathering erosion,high F/Q,low Q% low component maturity,the climate was relatively dry and cool.The core magnetic susceptibility and chromaticity indicators show that in 797-900.2m,low-frequency magnetic susceptibility and brightness gradually decrease,frequency magnetic susceptibility,redness and yellowness values gradually increase,and the climate gradually changes from relatively dry and cool to hot and humid;in 432.4-797 m,the low-frequency magnetic susceptibility and brightness range from low to high,the frequency magnetic susceptibility,redness and yellowness values from high to low,and the climate gradually decreases from heat and humidity;in 131.75-432.4m,low-frequency magnetic susceptibility and brightness are high,frequency magnetic susceptibility and redness are low,yellowness is high due to chlorite,and the climate is relatively dry and cool.Based on the ordered clustering of core debris components,magnetic susceptibility,and chromaticity,combined with the analysis of sedimentary characteristics,it is believed that Eocene climate evolution in Yilan area can be divided into 3 stages: in Early Eocene,the climate gradually changed from relatively dry and cool to humid and hot,mainly moist and hot.During Middle Eocene,the climate was generally hot and humid,but it showed a downward trend in the later period.In Late Eocene,the climate was generally relatively dry and cool,but the fluctuation was relatively obvious.The regional correlation of climate shows that the Eocene climate change process in Yilan area is in good agreement with Chinese mainland and global climate change.