Late Miocene Sedimentary Evolution And Environmental Changes In Xining Basin

The Xining Basin is located on the northeastern edge of the Qinghai-Tibet Plateau,In terms of climate,it lies in the so-called monsoonal triangle zone,which is a transition zone from the Asian monsoon warm-humid climate to inland dry-cold climates and high-cold region of Tibet Plate.The succession fluvial-lacustrine stratigraphy from the Xining Basin holds an Eocene to Mid-Miocene(52–17 Ma)sedimentary record of both TP uplift and global climate change,it is the ideal place for us to study the uplift and climate change of the Tibetan Plateau.In this paper,we use paleomagnetic method establish the age framework of,Banyan and Hongyazi section,the late Miocene sediments.useing the grain size,magnetic susceptibility,carbonate content,(C,O)isotopic Paleoclimatic indicators research and discuss the Late Miocene sedimentary evolution and environmental changes in Xining Basin.Preliminary knowledge of the following:1.Magnetostratigraphy results show that the age of Banyan profile is range of8.22 Ma~6.98 Ma,the age of Fossil is 8.1 Ma.Hongyazi profile is range of 12.16Ma~6.92 Ma.The conclusion above has extended the top age of XianShuihe sedim-ent of late Miocene in Xining Basin,on the basis of previous studies.2.the rate of sediment deposition in the Xining basin has suddenly increased to 147.03 m/Ma around 8.25 Ma,the grain size,magnetic susceptibility,(C,O)isotope changes suddenly and obviously.Fossils show a warm subtropical semi-arid climate during this period.Comparison of fossil fossils shows that the Lajishan rapid uplift at around 8.1Ma.3.The sedimentary rate of Hongyazi profile reaches the maximum value of 320 m/Ma in the whole section at 7.33 Ma.Its grain size,magnetic susceptibility and(C,O)isotope change obviously.The dry cold climate of Xining basin is likely to be a global cooling and accelerated uplift of Qinghai Tibet Plateau double drive makes the sedimentary environment changed at 7.33 Ma.4.According to the indicator of sedimentary environment,the climate and environment change periodically trend of late Miocene in Xining basin: four main periods and six sedimentary stage。1).Early Late Miocene(12.16~8.25Ma): consists of stage I(12.16~11.1Ma),II(11.1~9.46Ma)and III(9.46~8.25Ma).The sediment rate is relatively stable,and the climate of Xining basin is relatively warm semi humid-humid climate.2).Middlly-Middle Latly Late Miocene(8.25~7.91Ma): Stage IV(8.25~7.91 Ma).The grain size and magnetic susceptibility of the sediments strongly fluctuated during this period,the rate of sediment deposition has suddenly increased to 147.03 m/Ma.The sediments are gravel beds of Lakeside deposition facies.The tectonic activity is stronlyg,and the climate is mainly semi-arid climate.3).Middle Latly Late Miocene(7.91~7.16Ma): Stage V(7.91~7.16Ma).The lake water level greatly fluctuates,showing a intermittent and shallow trend.Xining basin dry cold climate caused by dual drive,which is global cooling and accelerated uplift of Qinghai Tibet Plateau.4).Latly Late Miocene(7.16~6.92Ma): Stage VI(7.16~6.92Ma).The grain size of the sediments fluctuated violently during this period,the C isotope decreases rapidly and the O isotope increases rapidly.The climate temperature in this period is higher than Middle Latly Late Miocene.