Biostratigraphic And Chronostratigrahic Sequence Between Middle Miocene And Early Pliocnen From The Xunhua Basin On The Northestern Tibetan Plateau And Its Paleoclimatatic Evolution

The average elevation of the Tibetan Plateau is more than 4000 metres above the sea level and over an area of 2500 000km²,and is the highest and largest plateau on earth and is called as“roof of the world”.The surface uplift(hereafter abbreviated to the“uplift”)of the Tibetan Plateau was one of the most important tectonic events in the Cenozoic,which is driven entirely by the collision and northward movement of India plate.Becaues of persistent collision between Indian plate and Eurasian plate since the Cenozoic Era,it resulted in Northward extension of the Tibetan Plateau and the retreated from east to west about the Neo-Tethys Ocean from the South Tibet and Southwest of the Tarim Basin.At the meantime,it also exerted profound influence for the global and regional climatic variation during the Cenozoic Era.As a consequence,the formation and upliftof and its environmental effects are hotspot of research about the field of geoscience at home and abroad all the time.The Xunhua Basin is a typical faulted basin in the Northeastern margin of the Tibetan Plateau during the Cenozoic,and its formation and evolution must have been affected by the uplift of the Tibetan Plateau.Meanwhile,it is also located in the intersection zone of the East Asian monsoon,the India monsoon and the westerly jet,and it is very sensitive to global climate change.It contains consecutive sequence sediments which is mainly consist of Cenozoic fluvial-lacustrine deposit that keeps a nearly integral document for tectonic deformation and uplift,denudation of weathering history,and environmental and climatatic variation in the Northeastern Tibetan Plateau.In consequence,this study area becomes an appropriate region to explore the course of interaction and mechanism between the uplift of Plateau and climate changes.The Xigou section,located in the southeastern Xunhua Basin,has well-exposed Miocene to Pliocene strata with an overall thickness of more than 600m,deposited without significant hiatuses.Many fossil mammals were collected at the base of the Liushu Formation within a 1-m-thick interval at 378–379m,including Hipparion platyodus Selve,Chilotherium sp.,Gazella gaudryi Schlosser,Gazella paotehensis,Gazella dorcadoides,Gazella sp.,and Cervocerus sp.Micropalaeontological samples(including sporopollen and ostracoda),clay mineral samples and organic geochemical isotope samples of sedimentary rocks were collected systematically from bottom to top in the Xigou section.Stable carbon and oxygen isotopic analyses of our newly discovered fossil mammal teeth in the Xunhua Basin can provide not only useful proxies for reconstructing paleodiets and paleoecology but also a baseline for regional comparison with well-studied Hipparion faunas from the Linxia Basin with the goal of better understanding the history of regional environmental and ecosystem changes on the northeastern Tibetan Plateau;According to palynological records,organic carbon isotope(?¹³C),total organic carbon(TOC)and C/N ratio,the evolution of paleovegetation from middle Miocene to Early Pliocene in Xunhua Basin were determined;Combining with the palynological records,the change of clay mineral content and carbon isotope content in organic matter in sediments between Middle Miocene and Early Pliocene from the Xigou section of Xunhua Basin,the climate evolution history of the region was recovered,and the driving mechanism of climate change was discussed by comparing the climate evolution process of the Middle Miocene to early Pliocene with the global climate change and uplift of the Tibetan Plateau.Based on the aforementioned research,main conclusions are summarized as following:1.Based on the latest high-density and high-resolution magnetostraitigraphy studies in the Xigou section from the Xunhua Basin,we established a detailed chronostratigrahic sequence from middle Miocene to early Pliocene by combining with regional comparison of the Ostracode fossil assemblage and Hiapparion fauna.In addition,the widely used stratigraphic units of the Dongxiang Formation,Liushu Formation,Hewangjia Formation were formed at 14.6-9.6Ma,9.6-7.3Ma and 7.3-5Ma from bottom to top,respectively.2.Upper Miocene sediments in the Xunhua Basin of the northeastern Tibetan Plateau yielded a variety of mammal fossils dating to?9Ma,including Hipparion platyodus Selve,Chilotherium sp.,Gazella gaudryi Schlosser,Gazella paotehensis,Gazella dorcadoides,Gazella sp.,and Cervocerus sp.Stable carbon and oxygen isotope compositions of the fossil tooth enamel allowed significant inferences regarding the paleoenvironment of the Xunhua Basin at?9Ma.First,enamel?¹³C data indicate that the Hipparion fauna mostly fed on C₃ plants even though the local ecosystem contained a certain amount of C₄ grasses.Integrated with assemblage characteristics of Hipparion Fauna,we infer that the Xunhua Basin at?9Ma contained a mixed forest-grassland ecosystem.Second,the influence of the Asian Summer Monsoon was stronger at?9Ma than today,as reflected by paleo-meteoric water?¹⁸O(as reconstructed from minimum fossil enamel?¹⁸O)lower than that of modern precipitation in the region.Third,higher?¹³C and?¹⁸O values for fossil tooth enamel from the Xunhua Basin relative to contemporaneous fossils in the neighboring Linxia Basin indicate a relatively drier climate in the former basin,which can be attributed to development of an orogenic rainshadow following uplift of the Jishi Shan.3.The oroganic carbon isotope(?¹³C)in the middle Miocene to Early Pliocene sediments of the Xunhua Basin varies from-28.45‰to-22.67‰.Combined with the C/N ratio,it indicates that the Xunhua Basin is mainly derived from terrestrial C₃ plants or there may be a very small amount of aquatic plants,which indicate that C₄ plants had not expanded in the Xunhua Basin or the proportion of C₄ plants wasvery small before5Ma.According to the variation characteristics of?¹³C_org curves of the sediments in the Xunhua Basin,it can be divided into three stages:during 14.6-12.5Ma period,the?¹³C_org values range from-26.87‰to-22.67‰.During the 12.5-8Ma period,?¹³C_orgvalues range from-28.45‰to-24.04‰,with an average value of-25.78‰,showing a decreasing trend.During the 8-5Ma period,?¹³C_org values range from-27.49‰to-22.94‰,with an average value of-24.61‰,indicating an increasing trend.Based on the global CO₂ and temperature changes from the Middle Miocene to the Early pliocene,the variation of organic carbon isotope values at 12.5Ma and 8Ma is mainly controlled by the change of climate between dry and wet.4.Micro-morphology,relative content and V(Ch+I)/V(Kao+S)value of clay minerals in the Xigou Section from Xunhua Basin were investigated using scanning electron microscopy(SEM)and X-ray diffraction(XRD).The results of mineralogical analysis indicate that illite content is the highest(ranging from 45.9‰to 70.2‰,mean59.3‰),followed by semectite(ranging from 10.5‰to 30.4‰,mean 18.2‰),and chlorite and kaolinite content were 12.3‰and 10.2‰,respectively in the Xigou Section.X-ray diffraction data show that all the clay minerals were detrital formation and less influenced by diagenesis.And the source area is close to the study area,which belongs to near-source transport.The relative content and(Ch+I)/(Kao+S)value of clay minerals in the Xunhua Basin can effectively reflect paleoclimate change in the study area.5.According to sporologic evidence in Xigou section,the sporologic diagram from bottom to top can be subdivided into three pollen zones and paleovegetation and paleoclimate are reconstructed.Zone?(14.0-12.5Ma)Ephedripites–Nitraridites–Chenopodipollis–Quercoidites,represented mixed shrub-steppe vegetation with dry and cold climate;Zone?(12.5-8.0Ma)Pinaceae–Betulaepollenites–Ephedripites–Chenopodipollis–Graminidites,which showed that the type of paleovegetation and paleoclimate changed,the paleovegetation was mainly consis of forest-grass setting with warm and humid environment;Zone?(8.0-5.0Ma)Ephedripites–Nitrariadites–Chenopodipollis,indicated a desert steppe setting with cold and arid climate.The climatic parameters were obtained by applying the Coexistence Approch based on the sporopollen zones.During the period 14.6-12.5Ma,the paleoaltitude is 2400-2600m,the mean annual temperature(MAT)is 7.8-14.9?,the mean annual precipitation(MAP)is466-1031mm.During the period 12.5-8Ma,the paleoaltitude is 2400-2800m,the mean annual temperature(MAT)is 13.3-14.9?,the mean annual precipitation(MAP)is798-1031mm.During the period 8-5Ma,the paleoaltitude is 2400-4000m,the mean annual temperature(MAT)is 2.8-14.9?,the mean annual precipitation(MAP)is403-1031mm.6.Combined with the content variation of herb-bushwood,coniferous forest,broad leaved forest in palynological assemblage,clay minerals,oroganic carbon isotope,sedimentary rates of sedimentary sequence between Middle Miocene and Early Pliocene in the Xigou Section from Xunhua Basin and?¹⁸O isotope record from the global deep-sea,the palaeoclimate evolution in Xunhua Basin can be divided into 3 stages as follows:?,relative cold and dry period(14.6-12.5Ma);?,relative warm and humid period(12.5-8Ma);?,cold and dry period(8-5Ma).Based on regional geological evidences,the climate cooling and drying event between 14.6Ma and 12.5Ma corresponds temporally to global cooling;relatively warmer and wetter conditions between 12.5Ma and 8Ma are considered to be connected with orographic interception around the Xunhua Basin.The shift toward more arid and cold conditions between 8Ma and 5Ma was probably driven by ulift of the Tibetan Plateau.