Redefinition Of Cenozoic Chronostratigraphic Framework And Reconstruction Of Paleoenvironment And Paleoelevation In The Jianchuan Basin,southeast Of Tibetan Plateau

The Cenozoic collision between the Indian and Eurasian plates is one of the most important reasons,which cause the uplift of Tibetan Plateau accompanied by the changes of global climate,regional environment and geomorphology.The mechanism of uplift of Tibetan Plateau and its response of climate changes always play important roles in the research of geological sciences,especially in quantitative paleoelevation and paleoclimate.Previous researchers most focus on the central and southwest part,compared with the southeast Tibetan Plateau.Moreover,these published paleoelevation or paleoclimate data from the Tibetan Plateau were always based on one single method by oxygen/hydrogen isotope,T?₄₇ paleotemperature-elevation model or pollen fossils coexistence approach.They do not think about enough consideration like paleontology in biological assemblages,tectonic paleogeographic background to combine with.As the southeast margin of Tibetan Plateau?SETP?is also a key region for understanding the uplift of the Tibetan Plateau,we choose a late Eocene section in Jianchuan Basin in SETP for quantitative palaeoelevation and paleoenvironment reconstruction in this research.The Jianchuan Basin is a Cenozoic strike-slip fault pull-apart basin,which bounded by the Qiaohou fault to the west and by the Jianchuan fault to the east,as well as mainly controlled by Red River fault.We measured the Jiuziyan Formation and Shuanghe Formation in the field work.Based on the deposition sequence,sedimentary facies,biostratigraphy,and zircon chronostratigraphy,we redefine the late Eocene chronostratigraphic framework in Jianchuan Basin.This research focus on oxygen isotope,clumped isotope and pollen coexistence to reconstruct the paleoelevation.Furthermore,this study use ostracods and pollen assemblages combine with stable isotope and clumped isotopic T?₄₇ thermometry to reconstruct paleotemperature and paleoenvironment quantitatively of SETP during late Eocene.Following are the main results of this study:1.Based on the analysis of deposition sequence and discuss the sedimentary facies,the late Eocene Jiuziyan Fm.and Shuanghe Fm.were mainly deposited in a lacustrineto delta system and characterized by limestone and and coal beds interstratified with sandstones.The extreme?¹³C enrichment of Jiuziyan Fm.is characteristic of diagenetic carbonates formed in association with bacterial methanogenesis.As upper Jiuziyan Fm has high a value of?¹³C,we infer that the water condition may be have low dissolved-sulfate concentration when Jiuziyan Fm deposited.The covariances between?¹³C and?¹⁸O of this section that are all<0.7,generally seems to discriminate carbonates precipitated in a warm open lake.2.According to the analysis of 244 ostracods samples,this study recognized about51 species which belongs to 18 genera.We partition these ostracods into two zones which are Cypridopsis-Heterocypris zone and Austrocypris levis-Homoeucypris jingtanensis-Cyprinotus jiangsuensis-Darwinula zone.They are similar to the Eocene biostratigraphy in Jiangsu,Jianghan,Guangdong areas as well as Gaize Basin.Among these genera,Austrocyprisand Heterocypris were known as the chara ostracods index fossils in the Eocene time in previous research.Moreover,Austrocyprisextinction could correspond to the late Eocene variation and fluctuation event which happened at³4Ma.And the range of zircon U/Pb ages of all valvanic rocks interstratified withinthe Shuanghe Fm spans between 34.7 and 37.1 Ma,we interprese the sedimentation of the Shuanghe Fm occurred during a short timelapse at³5.9±0.9Ma.Combining fossil evidence and the absolute age constraints of a weighted mean age of 36.2 Ma±0.9Ma?2??from a very fine-grained volcanic lava at the base of the Jianchuan Fm which had erupted unconformably over the Shuanghe Fm that the Jiuziyan and Shuanghe fms were deposited in the late Eocene time before³6 Ma.3.According to the ecological research of ostracods assemblages,we suggest it stands a warm lacustrine to palustrine environment,with a low but trending to a lower salinity during the late Eocene time.This inference is consistent with paleoenvironment obtained by pollen fossils.4.Based on dealing with 150 samples in lab,a total number of 85 samples have pollen fossils,26 pollen taxa were identified.Therefore,based on major changes in pollen abundance,combined with TGView 2.0.41,which has the constrained incremental sum of squares?CONISS?to define the boundaries between the most distinguishablepollenasfollowing:pollenassemblage1,Polypodiaceaesporites-Cryptomeriapollenites-Chenopodipollis,the vegetation in this stage was dominated by deciduous broadleaved forest associated shrub steppe around the basin,suggest a warm and humid climate with low elevation;pollen assemblage 2,Quercoidites-Ulmipollenites,thevegetationistemperatedeciduous coniferous-broadleaved forest,suggest a cool environment;pollen assemblage 3,Juglanspollenites-Pinuspollenites-Compositoipollenites,this stage is dominated by temperate deciduous broadleaved mixed forest,characterized by a cool and moist environment;pollen assemblage 4,this stage is still dominated by deciduous coniferous-broadleaved mixed forest,characterized by a semi-humid environment.Overall,the pollen compositions suggest the vegetation in Shuanghe Fm.was dominated by tropical-subtropical deciduous coniferous broadleaved forest,with the paleoclimate changed from warm and humid to cool and semi-humid.5.We use the published data of the altitudinal ranges of modern Tibetan vegetation equivalents of fossil pollen assemblages in the late Eocene combined with some revised according to the geographical distribution as our study area is in southwest China in using of the co-existence approach.We suggest the raw CoA altitude of 1300-2600m is the minimum altitude at this area.Employing the CA values from the late Eocene,we used this paleoelevational range to reconstruct the region's mean annual air temperature?MAAT?,arriving at a range of between 16.8°C and 21.7°C,and a mean value of 19.25±2.45°C.However,the present climate,which has a MAAT of 13.5±1.4°C,about⁶°C cooler than that of the Eocene.6.We reconstruct the isotopic value of meteoric water in using the isotopic compositions of carbonates and T?₄₇.After excluded the influence of veins by using microdrills to separate the micrite from the sparite,and recrystallization by analysis of diagenetic phases.Besides,the evaporation rates were low at late Eocene as well as rainfall was not in character by analysis of covariance between?¹³C and?¹⁸O values,we think the isotope data from our samples are still meaningful for reconstructing palaeoelevation.The average of?¹⁸O_mw?VSMOW?value of lower Jiuziyan Fm is-8.9±1.3‰?2??,and the Shuanghe Fm has an average?¹⁸O_mw?VSMOW?value of-9.5±1.1‰?2??.This study constrains an elevation range of 2.5±0.7 km?2??in lower Jiuziyan Fm.with Hoke's?2014?the Eocene model and defines an elevation range of2.9±0.6 km?2??of Shuanghe Fm.in the same model.Based on all existing results from the Jianchuan Basin,including our study of pollen and?¹⁸O_c all paleoelevation estimates are consistent with and Eocene paleoelevation of².5 km asl,similar to present day.We therefore infer that this region had begun to experience surface uplift by36.2 Ma±0.9 Ma,before the deposition of the Jianchuan Fm.In summary,we propose that surface of Northwest Yunnan present-day mean topography at the late Eocene before³6Ma with the eruption of Jianchuan volcano,which could relate to the protracted collision between India and Eurasian plates during45-34Ma or crustal thickening associated with lower crustal flow in Eocene.Thus,the Eocene Tibetan Plateau has the geomorphology of high in east but low in west area.Moreover,we do not think that middle Miocene crustal flow were responsible for the initial uplift of SETP.A global monsoon was present and robust across a range of global mean temperatures during the late Eocene,regardless of whether Tibetan Plateau existed or not.Recent researchers supported modern-like Eocene monsoonal rainfall through their climate simulations and suggest that a reinforced hydrological cycle responding to enhanced greenhouse conditions counterbalanced the negative effect of lower Tibetan relief on precipitation.Thus,our conclusion that the late Eocene Jianchuan Basin is warmer than modern.