Cyclostratigraphy Of Pingliang Formation Of Ordovician In The Qishan Section In SW Ordos,North China

The Ordovician is an important period of the tectonic-climate environment and life evolution during the Paleozoic,and it is also the most prosperous‘Greenhouse Earth'period of the Earth.During the Ordovician,the global tectonic movement has been gradually intensified,and the Early Paleozoic plate tectonics and paleo geographic pattern have been preliminarily formed.The climatic environment has undergone significantly and frequently changed,which was in the greenhouse environment with the marine life radiated greatly during the Early through Middle Ordovician followed by the ice age and the mass extinct in the late Late Ordovician.There is a direct causal relationship or indirect relationship between these events,but the specific process of collaborative evolution is still unclear,one of the main reasons is the lack of high-precision geological time scale.In the past two decades,the accuracy of Cenozoic and Mesozoic geological ages has reached 0.02 Ma and 0.4 Ma respectively by means of cyclostratigraphy and other methods.However,the accuracy of Paleozoic,especially the early Paleozoic,is still very low.Due to the climate change of greenhouse and ice age in Ordovician,the signal of astronomical orbit cycle preserved in sedimentary strata is more obvious than that in Cambrian and Silurian strata,which is an ideal time to study high-resolution geochronology by means of cyclostratigraphy.Chinese scholars have made great progress in the study of Ordovician cyclostratigraphy.They have basically completed the study of Ordovician cyclostratigraphy of typical sections in South China,but the study of North China is still relatively small.The southwest margin of Ordos Basin is a well-developed and widely exposed area of Ordovician in the north of China.A lot of researches have been carried out on the rock association,sedimentary characteristics,biostratigraphy and zircon U-Pb chronology of volcanic ash intercalation,which laid a good foundation for the study of high-resolution cyclostratigraphy.In this paper,the high-density magnetic susceptibility and chroma samples are collected and measured in the upper part of the Ordovician Pingliang Formation of Qishan section in the southwest margin of Ordos Basin by using the method of cyclostratigraphy.Combined with the U-Pb age of zircon and division of stratigraphic age,the astronomical age scale of the sampling section is established.The main conclusions are as follows.1)The Pingliang Formation in the Qishan section is about 600m thick,which composed largely of siltstone of deep-water slope facies,intercalated with coarse sandstone,silty mudstone and mudstone,with obvious sedimentary rhythm in the upper and lower part of the section.The individual sedimentary cycle layer is 0.3-2.4m thick,generally 1.2-2m thick,which consist of the light grayish green,light grayish yellow,light grayish brown sandstone,siltstone,and silty mudstone.The thickness of single layer of rock is 2-30cm.Parallel bedding was well developed,and with contour current structure in some intervals.26 sedimentary cyclicities were observed in the Upper Section.2)At the top?36.5m thick?and bottom?30.3m thick?of Pingliang group,the magnetization and chroma of 3715 samples were measured at 2-10 cm intervals.The?lf value of the upper part?a-stage of Pingliang formation sampling interval 2cm,1493 samples?is 4-16×10^-8m³/kg,the average value is 10.55×10^-8m³/kg,and the?lf value of the lower part?b-stage of Pingliang formation sampling interval 5cm,516samples?is 3.57-19.4×10^-8m³/kg,the average value is 12.7×10^-8m³/kg.The variation of the upper and lower magnetic susceptibility values is of one order of magnitude,and the periodic fluctuation of the values with the thickness is more obvious.The results of chroma measurement in the samples of 4.15-28.15m of a-stage showed that the lightness?L*?was between 60-75,with an average value of 68.83;the redness?a*?was between 0-5,with an average value of 2.39;the yellowness?b*?was between7.5-20,with an average value of 15.16.The chromaticity values fluctuate periodically on the section,and L*is negatively correlated with a*and L*is positively correlated with b*and a*is positively correlated with b*reflecting that the rock is in the deep-water environment and has not been disturbed by obvious tectonic activities.Using correlation coefficient method to evaluate the deposition rate of the magnetic susceptibility data,it is found that the deposition rate of the upper part and the lower part is approximately the same,and the variation is between 1.6-2.6cm/kyr.All of these show that the magnetic susceptibility and chromaticity can be used to study the astronomical orbit cycle signal of the section.3)The analysis of time series shows that there are 2.34m and 1.65m,0.58m,0.35m and 0.3m cycles in the a-stage of the susceptibility,which correspond to the short eccentricity,obliquity and precession periods respectively.Using the short eccentricity of 135kyr to carry out the depth-time conversion of the data,the astronomical cycles of 135kyr,86kyr,31kyr,19kyr and 17kyr are identified,including16 short eccentricity cycles and the calculated floating astronomical time scale lasts about 2.16 Ma.There are 1.88m and 1.25m,0.47m,0.28m and 0.23m thick cycles in the b-stage.After the depth time conversion of 135kyr cycle with short eccentricity,the cycles of 133kyr,87kyr,36kyr,20kyr and 17kyr are identified.There are also 16cycles with short eccentricity,and the floating astronomical age scale is 2.128Ma.4)According to the zircon U-Pb dating results of porphyry at the top of Pingliang Formation and the determination of the boundary age of Majiagou formation and Pingliang Formation,the sampling age of the a-stage is about 455.875-453.715Ma,and that of the b-stage is 458.4-456.272Ma.The Pingliang Formation in Qishan section is 458.4-453.7Ma,which is located in the Sandbian stage,this result further clarifies the geological age of the Pingliang formation.