Chronostratigraphy And Paleoenvironmental Reconstruction During Early Oligocene Of Wenshan Basin,Yunnan

Yunnan has drastically changed after the uplift of the Tibetan Plateau and global climatic change from Cenozoic.It becomes a hot issue to figure out the extent of environmental impact from the Tibetan uplift to the peripheral regions,even to the global world.To answer this question,researchers from geology and other researching area are focused on the uplifting history of the Himalayan for more than a century.Differing from the ‘hot spot' area,the southeastern edge of Tibetan,there is a lot of weaknesses in systematic researches of rebuilding the paleoenvironment in southeast Yunnan,including paleovegetation and paleoclimate.As one of the most vital biodiversity hotspots regions in the world,SE Yunnan is one of hottest place due to its unique and irreplaceable geological characteristics and climate features.This paper is working on a set of fossil-bearing strata of Wenshan basin in SE Yunnan to help us recognize the history of paleoenvironmental evolution better.The main achievements are as followed:(1)Stratum features of Huazhige Formation in Wenshan Basin: Huazhige Formation in Wenshan Basin was regarded as lacustrine-limnetic facies,which is quite similar to the famous Xiaolongtan Formation.However,it should be a lacustrine-fluvial facies.According to the sedimentary characteristics,the Huazhige Formation can be separated into 2 sedimentary cycles.Every cycle approximately begins with river and ends with lake,which represents a process from shallow wadis to lakes filling of water.(2)Stratigraphic age updating: Huazhige Formation was regarded as the synonym of Miocene Xiaolongtan Formation by the regional geological survey from the last centery.There are two insitu volcanic ash layers in bearing fossil strata of Huazhige Formation in Wenshan Basin.After the U-Pb isotope chronological test of zircons,the age of Huazhige Formation is updated to early Oligocene(30±2 and 32±1 Ma),which is much older than Xiaolongtan Formation.The ages of this formation give a positive constraint to discuss the uplift history of the Tibetan Plateau,evolution phase of paleoenvironment,and the origin time of East Asian biodiversity.(3)Palynoflora of Wenshan and paleovegetation reconstruction: sporopollenin,algae,and fungus fossils are widely founded from the Huazhige Formation in Wenshan Basin.Palynoflora of Wenshan yields 31 different types of pollen taxa,including 6gymnosperm types,22 angiosperms,and some algae and fungus.This flora mainly consists of Fagaceae,Sapindaceae,Juglandaceae,Betulaceae,Rosaceae,Pinaceae,and Cupressaceae,especially Fageceae,which dominates the advantage of the flora.Wenshan flora is a classic warm temperature – subtropics evergreen broad-leaved forest,with few limestones tropical forest elements with obvious vertical zoning characteristics.Moreover,this flora might have an earlier origin time than the early Oligocene.Encompassed within these fossil floras are more than 53 families,98 genera and 212 species,excluding the ongoing systematic study of the Wenshan flora which is emerging as one of the most diverse paleofloras in south and central China,and is globally exceptional.(4)One new Itea spice is founded: Itea L.are small trees and shrubs,widely distributed in East Asia,and most of them are evergreen spices.According to past researches,most records are pollen and from Cenozoic strata in North America and Europe.Itea leaf fossils are only be found in North America so far and the leave fossils we found in Wenshan.The fossils from Wenshan are the first leaf fossil records of East Asia and Eurasia,with classic eucamptodromous venation,scalariform tertiary veins,and irregular tooth spacing and tooth shapes.These features can help us separate Itea and other genera,and also define these fossils as a new spice of Itea – Itea polyneura.These Itea leaf fossils from Wenshan indicate that the epoch of Itea should earlier than the early Oligocene and deciduous Itea spices are likely differentiated for acclimatization later.