Cyclostratigraphy Of The Lower Crteacous Huanhe Formation In The Southwestern Ordos Basin,China

The cyclostratigraphy provides powerful tool for the stratigraphic correlation and the establishment of astronomical time scale.The key and difficult issues are the recognition of cyclic signals and its reproducibility,as well as establishment of the astronomical time scale in continental sediments.The Ordos Basin,central China,preserved relatively continuous Mesozoic fluvial and lacustrine succession,provides an ideal region for the study of cyclostratigraphyin the terrestrial sediments.The Lower Cretaceous Zhidan Groupin the southwestern Ordos basin,consists of Yijun Formation,Luohe Formation,and Huanhe Formation,with a thickness between 420.36 m and 1135.22 m of fluvial-lacustrine and desert clastic sediments,is the ideal material for the study of terrestrial sedimentary cyclostratigraphy.In this study,the lithologic feature and natural gamma logs of the Lower Cretaceous Huanhe Formation from 25 boreholes in 5 coal exploration areas in the southwestern Ordos Basin are analyzed to study the sedimentary facies.Then astronomical orbital theory used on nature gamma values from 7 cores to identify the astronomical orbital forcing and setup an astronomical time scalefor the Huanhe Formation.The main conclusions are as follows:1.The Huanhe Formation is a set of light gray-green,gray,light grey-red fine clastic rocks,with a thickness of 149.41 to 718.42 m,and gradually thin from northwest to southeast in the studied area.The sedimentary environment is delta and lacustrine,and further it is divided into the delta front,shore and shallow lake,and semi-deep lake sub-facies.2.The cyclostratigraphy analysis for natural gamma value of 7 wells showed that the Huanhe Formation deposited was obviously controlled by the astronomical orbital forcing.The precession,obliquity and the eccentricity period were identified in 5 wells(4401,NK613,NK301,2801,701),but only precession and obliquityperiod were identified in the 2 wells(Z504,N701).Moreover,the cyclicthickness(corresponding to the theoretical cycle length)recognized in different wells are varied,i.e.,the precession period of 1.04~2.04 m(16 ka~22 ka),the obliquity period of 2.14~5.31 m(28 ka~57 ka),and the short eccentricity Periods of 6.01~13.20 m(100 ka~136 ka)and long eccentricity periods of 19.96~41.66 m(400 ka),respectively.These differences may be caused by deposition rates in different sedimentary environments.3.The floating astronomical time scales of the well 4401(delta frontal sub-facies)includes 11.5 long eccentricity periodsof 400 ka,34.5 short eccentricity periods of 135 ka,81 obliquity periods of 57 ka,and 209 precession periods of 22 ka,this yield a floating astronomy time scale of 4.6 Myr and the averaged accumulation rate of 13.41cm/ka.The well NK613(shore shallow lake sub-facies)includes 11.5 long eccentricity periodsof 400 ka,80 obliquity periods of 57 ka,209 precession periods of 22 ka,this yield a floating astronomy time scale of 4.6 Myr and the averaged accumulation rate of 5.91cm/ka.The well N701(delta frontal subfacies)is the most pronounced with anobliquity period of 49 ka.The obliquity curve serves as a target curve for establishing the astronomical timescale,and it encompasses 51.5 obliquity periods of 49 ka and 113 precession periodsof 22 ka,this yield a floating astronomy time scale of2.5 Myr,and averagedaccumulation rate of 6.97cm/ka.Thewellsinwestern and central area recorded 11.5 long eccentricity periods of 400 ka,showing a well cyclic periodicity records and reproducibility.However,N701 well in the east,without of long-short eccentricity period,probably caused by various lithology,or other factors which need further to be studied.Anyway,the geologic time estimated by the obliquity period is shorter in the east than in the middle and west,this probably suggests deposition hiatus in the well N701.