The Magnetostratigraphy Study On The Early Miocene Sediments In Xining Basin, Qinghai Province

The early Miocene sediments deposited very well in Xining basin that located in junction zone among eastern humid monsoon areas, northwestern inland arid areas, alpine cold regions of Tibet plateau. It remained famous Xiejia mammal fauna. Xiejia profile that mammalian fossils were collected is also the Xiejia Stage stratotype profile. We can get the accurate age of Xiejia mammal fauna and Xiejia Stage by the magnetostratigraphy study on xiejia profile. And it was very important to the study of mammal evolution process in northern china especially in western china, Neogene Stage establishment, study of the uplift process of Tibet plateau.Paleomagnetic measurements demonstrate that Xiejia profile records 18 normal and 17 reversed polarity zones. These polarity zones corresponded very well with polarity zones between C5ACn and C6Cn.1n in Cande 95 Time scale. We use inter— interposition method to calculate the age of profile. The age of the upper stratigraphy was 14.05 Ma, the age of the lower stratigraphy was 23.41 Ma, the span of the whole profile was 9.31 Ma.The paleomagnetism age of the xiejia mammal fauna is about 18Ma. It dates as late early Miocene and is approximately equivalent to early MN4.The geological age of the Xiejia Formation is 15.68—22.93Ma. It was dated between late time of middle Miocene and late time of lower Miocene. So, the upper sediments of the Xiejia Formation deposited in middle Miocene. The upper sediments of the Mahalagou Formation deposited in lower Miocene.Great paleoenvironment change of the basins that located in northeast region of Tibet plateau is closely related to the second uplift of Tibet plateau and the formation of Asia monsoon. This conclusion was found by the analysis and correlation of magnetstratigraphy, biostratigraphy,lithohorizon in xining basin, lanzhou basin and linxia basin. Owning to the depositing features in these basins, the Tibet plateau northeastern stratium area should be classified by stratigraphy classification work in the future.