Demarcation Of The Boundary Between Yingcheng Formation And Denglouku Formation Of Lower Cretaceous In The North Of Songliao Basin And Paleoclimate Change

The purpose of Continental Scientific Drilling in Songliao basin is to drill through the continental Cretaceous system,obtain continuous and complete core data,and conduct related paleoclimate and paleoenvironment research.SK2 is the deepest scientific drilling in China.From April 13,2014 to March 18,2018,the drilling depth reached 7018 m,the total length of core obtained was 4134.81 m,and the core recovery rate was 96.61%.From top to bottom,the core of Denglouku Formation,Yingcheng Formation,Shahezi Formation,Huoshiling Formation and basin basement are obtained continuously.Based on the ultra long continuous coring data of SK2,this paper studies the core of the second member of Denglouku formation of SK2 at centimeter level.Focusing on the boundary between Yingcheng Formation and Denglouku Formation in Songliao Basin,a new demarcation scheme is proposed.The paleoclimate change from Yingcheng Formation to Denglouku Formation is restored by using the methods of sandstone composition analysis and stratum color analysis.Mainly gained the following understandings:(1)SK2 has obtained 105.05 m of core of the second member of Denglouku Formation,with core recovery of 99.1% and core diameter is 214 mm.The second member of Denglouku formation is composed of sedimentary rocks,mainly including 9 rock types: fine conglomerate,coarse sandstone with gravel,coarse sandstone,medium sandstone,fine sandstone,siltstone,argillaceous siltstone,silty mudstone and mudstone.Braided river facies,abandoned channel and channel sub-facies are identified,which are further divided into three sedimentary micro-facies,namely channel siltation,sand bar and mid-channel bar.Through the analysis of core basic data,it is preliminarily found that the sedimentary environment of the second member of Denglouku Formation is shallow water environment with strong hydrodynamic force,and the gradual rise of temperature leads to the transition from reduction environment to oxidation environment,but the overall temperature is suitable for biological survival activities.(2)The LA-ICP-MS U-Pb dating results of the second member of Denglouku formation in SK2 show that the age range of detrital zircons is 102-326 Ma,the youngest peak ages indicate that the lightest main sediment source of the second member of Denglouku Formation comes from the vicinity of 113 Ma,and the large-scale sedimentary period is in Albian.The youngest zircon age is 102±4Ma,which can represent the lower sedimentary limit of the second member of the Denglouku Formation.The formation time of the formation is not earlier than 102 Ma,which belongs to the late Albian period.(3)Based on the analysis of the previous research results in Songliao Basin and the core data obtained from SK2,the comprehensive histogram of Yingcheng Formation Denglouku Formation of SK2 is established.According to the core characteristics of Yingcheng Formation and Denglouku Formation,the interpretation of connecting well seismic profile in SK2 area,the distribution law of conglomerates in yingsi formation of Xujiaweizi area,conglomerates in Denglouku formation of Songnan and the fourth member of Yingcheng formation of SK2.According to the correlation of conglomerates and LA-ICP-MS U-Pb dating of tuff at the top of the fourth member of Yingcheng of SK2,the conglomerates of the fourth member of Yingcheng acquired by SK2 should be the missing the first mumber of Denglouku Formation.The boundary between Yingcheng Formation and Denglouku Formation is determined between the volcanic rocks of Yingcheng Formation and this set of conglomerates.The depth of SK2 is 3102.37 m.(4)By using the methods of sandstone composition analysis and stratum color analysis,the process of paleoclimate change in the period from Yingcheng Formation to Denglouku Formation of the lower Cretaceous in the north of Songliao basin can be divided into three stages: the first stage is Yingcheng Formation,with moderate temperature,warm and humid climate;the second stage is the first mumber of Denglouku Formation and the early and middle stages of the second mumber of Denglouku Formation,with gradually rising temperature,from the half with medium / high temperature The humid climate changes into a semi-arid climate with high / high temperature,during which a short dry climate occurs,which is the transitional period of climate change between Yingcheng Formation and Denglouku Formation;the third stage is the later stage of the second mumber of Denglouku Formation,with high and stable temperature,which is a tropical semi-arid climate.