Cyclostratigraphy Research Of The Early Cretaceous In Zhangye,NW China

The Cretaceous period is a crucial stage of the Earth's development and biological evolution.The study of Cretaceous climate change is crucial for understanding the relationship between greenhouse climate and global warming.A large number of research results have been obtained from Cretaceous marine strata based on multiple stratigraphic methods,and Cretaceous the geologic time scale has been established.China is characterized by the distribution of Cretaceous continental strata,but the degree of research is not so high.The continental sediments in northwest China are complete,but the stratigraphic ages are scattered and not unified yet.According to the Milankovitch signals recorded in strata,Cyclostratigraphy method can be used to establish astronomical time scale with resolution of 20-405 ka,which is benefit to the perfection of geological age framework.Cyclostratigraphy with color change are well-exposed in the Zhangye,which is an ideal area for the study of cyclostratigraphy.In this paper,paleoclimate proxy of the typical color strata of the Early Cretaceous Zhonggou Formation and Xiagou Formation in Zhangye are selected by means of color reflectance,environmental magnetism and geochemical methods,and it is considered that the color reflectance parameters L~*can better reflect the climate change driven by orbit.A total of 4760 samples selected at 20 cm intervals were analyzed to cyclostratigraphic analysis of the logarithmic lightness sequence using the Evolutive Time Opt analysis(e Time Opt).On this basis,405 ka calibrations was performed and combined with the age of black shale basal(114 Ma)in Zhangye colorfull hills,generating a 111.83-119.70 Ma continuous astronomical time scale,corresponding to the Early Cretaceous Aptian-Albian period.Based on this astronomical time scale,the duration of the Zhonggou Formation and Xiagou Formation were 2.1 Ma and 5.7 Ma,respectively.This result is helpful to construct the continental Cretaceous age framework in Northwest China,and improve the precision of stratigraphic division and correlation.In addition,comparing the research results of northwest continental Cretaceous strata with international marine strata can verify the reliability of the time results,and further understanding the global Cretaceous climate evolution and the coupling effect of land-sea-atmosphere system.Because of the stability of the 405 ka eccentricity period in geological history,this paper uses the 405 ka calibrations time series to analysis.According to the ratios of obliquity and short-eccentricity band powers to total power,it is concluded that the short eccentricity amplitude modulation of continental in northwest China during the Aptian-Albian period is 1.7 Ma,which is close to the results of marine strata research at the same time.During Aptian-Albian period,the theoretical short eccentricity amplitude modulation period is 1.9 Ma,which proves that there is a difference between the geological record results and the theoretical calculation results of the early Cretaceous.Studying the deep-time sedimentary records can understand the changes of the earth's orbital parameters in the past,thus constraining the chaotic solar system in the Aptian-Albian period of the early Cretaceous.