A Magnetostratigraphic Study Of Yangjiashan Fauna In The South Of Linxia Basin

Linxia basin is located on the northeastern edge of the Tibetan Plateau and belongs to the boundary between the eastern monsoon region,the northwest arid region,and the cold and arid region of the Tibetan Plateau.It is a semi-humid climate to semi-arid climate transition zone in the marginal region of the monsoon.River-lake and aeolian sedimentary sequences with a thickness of up to 1600 m developed in the basin on the late Cenozoic.Abundant mammalian fossils are buried in the sedimentary sequence.Tens of thousands of fossils have been unearthed to date,which is one of the most complete and most abundant fossils in the Cenozoic in the world.Unique regional location,complete sedimentary sequence and abundant mammalian fossil output are for exploring a series of scientific issues(the uplift of the Tibetan Plateau and its environmental effects,the formation of the East Asian monsoon circulation system,the inland aridification of Asia and the evolution of living systems,etc.)Provide a good research vehicle.Since the 1980 s,scholars have divided the strata of the Linxia Basin in detail from the fields of biostratigraphy,paleomagnetic geochronology,and rock stratigraphy,and accurately dated them.However,the stratigraphic division of the Linxia Basin is based on the results of paleobiologists ' biostratigraphy on the one hand,and on the basis of the results of paleomagnetic scholars ' magnetic stratigraphy on the other hand,resulting in no unified opinion on stratigraphic division.The research status and existing problems of the basin have given us a scientific understanding of the Linxia Basin 's geological evolution history and paleontological age.Therefore,this paper selects the Dongjiawan-Lanjiashan section,which is rich in mammalian fossils,has a high degree of research on fossils,a clear output horizon,and a good profile.The Yangjiashan fauna and related sedimentary sequences in the upper strata of the formation establish a precise age framework.The study of the magnetic fabric shows that the sedimentary magnetic fabric of the study profile is normal magnetic fabric,and the deposition sequence is not affected by the later regional tectonic movement.The main paleocurrent direction of the formation period of the Yangjiashan fauna fossil layer is NW-SE,indicating that the upstream of the river is the Jishi mountains.The river that originated in the Jishishan Mountains flows from the northwest to the southeast along the western Qinling Mountains.During the short period of the storm and flood period,the debris flow in the western Qinling area has been mixed with gravel and a large number of mammal remains and bones.The sedimentary dynamics decreased sharply when it flowed northward into the valley and merged into the river trunk road before the mountain.Eventually,it accumulated in the Dongjiawan area at the low depression at that time,resulting in the accumulation and burial of mammal fossils.The Lowrie 3D IRM technique demagnetization curve of a representative sample of Dongjiawan-Lanjiashan shows that the magnetic minerals in the study profile are mainly hematite and goethite with high coercivity,and magnetite and maghemite with low coercivity.magnetite and hematite are the main magnetic bearing minerals.The results of magnetic stratigraphic studies show that the Dongjiawan-Lanjiashan section recorded 27 magnetozones,including 14 with normal polarity and 13 with reverse polarity.Constrained by mammalian populations and repeated comparisons,it was found that the magnetozones of the study section can be well compared with GPTS(2012).According to the sedimentation rate and extrapolation method,the magnetic age at the top of the Dongjiawan-Lanjiashan section is ~6.166 Ma,and the magnetic age at the bottom is ~10.523 Ma.The age span of the entire section is 4.357 Ma.The age of Yangjiashan fauna is6.247-6.3Ma,which is Baodean.