Magnetic Stratigraphy Study In Panxian Area Of Guizhou And The Exploration Of Establishing The Standard Magnetic Polarity Sequence Of The Early Triassic

The Triassic in China is widely distributed,and the sea and continental strata are simultaneously developed.In terms of space,the southern part of China is marine sedimentary,and the north is continental sedimentary.In terms of time,the sedimentary strata begin to transition from marine sediments to continental sediments.The Triassic in most parts of China is characterized by locality and complexity.In particular,the sedimentary strata in southern China showed obvious dichotomy during this period,accepting marine sediments,firstly,and then accepting continental sediments.The Triassic sedimentary strata in China also recorded a complex and rich history of geological evolution.Therefore,the study of the Triassic strata in China is of great significance for clarifying the temporal and spatial framework of its strata,and understanding the geological evolution history during its period.Predecessors have carried out a lot of research on the Triassic strata in China,at present there is no relatively complete and unified temporal space-time framework.Based on this background,this paper conducts a detailed magnetic stratigraphic study on the drilling hole of RYZ13 continuous drilling core in Panxian area,Guizhou,and obtains the age limit of the formation in the area.This provides magnetic stratigraphy evidence for the establishment of the early Triassic standard magnetic polarity sequence and the time-space framework of the Triassic strata in China,as well as the geological evolution of Chinese Triassic period.In this paper,716 samples(including Yongningzhen Formation and Feixianguan Formation)were obtained from the drilling hole RYZ13 continuous drilling core,and detailed magnetic stratigraphic researches were carried out.From the results of rock magnetism,it can be seen that the original sedimentary structure of the sample has not been damaged;The main magnetic bearing mineral of the first type of samples is magnetite,and the main magnetic bearing mineral of the second type of samples is hematite.The obtained paleomagnetic data has a normal average magnetic tilt angle of Is=38.18°,a reversed average magnetic tilt angle Is=-48.73°,and the absolute difference between the normal and reversed polar magnetic dip angles is between 0°-20°.Therefore,the data passed Mc Fadden(1990)reverse test.Based on the reliable data and stratigraphic data,the lithology sequence,magnetic dip sequence and magnetic polarity sequence of the RYZ13 are established respectively.The actual situation of the stratum is compared with the international standard magnetic polarity sequence.The age limit of the borehole core stratumal also is limited,and the following conclusions are drawn: the age of the bottom boundary of the Feixianguan Formation is 252.16 Ma,the boundary age of the Yongningzhen Formation and the Feixianguan Formation is 250 Ma,and the age of the top boundary of the Yongningzhen Formation is 247.06Ma;the deposition rate of Yongningzhen Formation can reach 181m/Ma,and the deposition rate of Feixianguan Formation can reach 221m/Ma.At the same time,the results of this study have very important practical reference significance for the determination of the age of the Induan bottom boundary,the age of the Olenekian bottom boundary and the Anisian bottom boundary age.