Cyclostratigraphy And Floating Astronomical Time Scale For The Lower Cretaceous Of Shahezi Formation In Songliao Basin,Northeastern China

The extreme greenhouse climate of mid-Cretaceous evolved from the Early Cretaceous gradually,but people could only concerntrate on marine records to study climate changes.Comparing to marine environment,terrestrial environment is more sensitive to paleoclimate changing,but because of imperfection of continental records,people don't know the real paleoclimate and paleoenvironment changing process of the Early Cretaceous.Songliao Basin is the largest continental lake basin in the world,which was deposited a continuous terrestrial strata.The project of Continental Scientific Drilling started in 2014,it will provide a good chance to obtain a continuous core.It also provide an ideal chance to investigate the evolution of ancient lake and its response to global climate changing.However,because of the high-resolution stratigraphic framework of Songliao basin has not been established,it leads to a negative impact on recording major geological events comparing to marine geological record for Songke 2 borehole(SK-2).Songliao Basin was mainly deposited black mudstone,shale and argillaceous siltstone in Shahezi formation of Early Cretaceous,it is also one of the objective strata of SK2.This research is based on the theory of stratigraphy,sedimentology,paleontology and cyclostratigraphy and utilized 836 meters of the drilling and logging data of Songshen 4 well,which is an adjacent well of SK-2,to do cyclostratigraphic study,which is geophysical data,on Shahezi formation combine with the results of previous study of Songliao Basin.The methods include removing mean,removing trend,pre-whitening,spectral analysis,wavelet analysis,Fourier transform and filtering analysis.The results reveal that Songshen 4 well recorded a series of meter-scale sedimentary cycles in the Shahezi Formation,they are 28?14.4m,7.7?5.5m,2.8?1.7m and 1.3?1.2m.The ratios of cycle wavelengths are?20:5:2:1.These meter-scale cycles are interpreted as Milankovitch cycles of 405 ka and 100 ka eccentricity,37 ka obliquity and 20 ka precession.A floating astronomical time scale(ATS)is established by filtered 100 ka short eccentricity cycles.The Shahezi Formation recorded a total of 30.5 long eccentricity cycles and 123.2 short eccentricity cycles.Based on this information,we conclude that duration of the Shahezi Formation is 12.32 Myr.The estimated sedimentation rates have 2.3 Ma and 0.8 Ma cycles.