Palaeogeography And Paleoclimate Of Yunyang Dinosaur Fossil Fauna Of The Shaximiao Formation In Chongqing

The Sichuan Basin is rich in dinosaur fossils,In 2015,the newly discovered dinosaur fossils in Pu’an Township,Yunyang County,Chongqing were mainly distributed in the lower section of Xintiangou Formation and Shaximiao Formation in the Middle Jurassic.The discovery of this fossil group further extends the geographical distribution range of dinosaur fossils in the Sichuan Basin.At present,the research of Yunyang Puan dinosaur fossils is still in its infancy.On the one hand,it is urgent to carry out the characteristic analysis of dinosaur fossil group;on the other hand,based on the research hotspot of global change in recent years,the study of palaeoenvironment and paleoclimate restoration and law of environmental evolution from the pu’an dinosaur fossils in Yunyang is also an important supplement to the study of global change.Through the analysis of the distribution and aggregation of dinosaur fossils,sedimentary and burial characteristics,petrographic stratigraphic characteristics and geochemical characteristics of existing strata,it is important to study the palaeoenvironment and paleoclimate where dinosaurs lived and buried,which is of great significance to reveal the evolution law of geological environment..This paper takes the dinosaur fossil group and buried strata in the lower section of shaxiimiao Formation as the research object.Based on the field geological surveys,using petrology,paleontology,sedimentology,geochemistry and other research methods to discuss the characteristics of dinosaur fossil fauna and the characteristics of sedimentary environment and paleoclimatic in the Shaximiao Formation in Pu’an,Yunyang,which will provide help for dinosaur evolution research,support for fossil deposit burial mode,and provide scientific basis for subsequent paleo-ecosystem reconstruction.The main results and understandingare as follow:(1)Chongqing Yun Yang Shaximiao dinosaur fauna is abundant in variety.The vertebrates includes 2class,3 orders,8 families,named 4 new species(including 2 new genuses and species).It is the important supplement of Shunosaurus-Omeisaurus fauna in zigong,and shows that dinosaurs is flourish in the middle and later period of middle Jurassic in the sichuan basin.(2)The concentrated burial site of the Shaximiao Formation dinosaur fossils in Yunyang,Chongqing,has a low degree of fossil association,which mixed size,morphology and a large number of juvenileindividual bone fossils.At the same time,the dominant arrangement direction of fossil bones is obviously different from the main direction of paleo-current in Sichuan Basin during this period.Itindicates that the dinosaurs were transpoerted and buried in close distance,after the sudden and accidental death.The results of chemical analysis of dinosaur bone fossilsand surrounding rocks indicate show thatthe abnormal element content may be caused by the surrounding volcanic activity or/and arid climate environment,which reflects that the dinosaur lived in a relatively arid environment with a semi-arid to semi-humid dry and hot climate accompanied by the surrounding volcanic activity.(3)By Studying the lithologic characteristics,grain-size distribution and sedimentary structure,the lower section of Shaximiao Formation is sedimentary by river phase,the purple argillaceous siltstone(containing calcareous nodules)represents the geological record of flooding in the geological history period.In the lower sction of Shaximiao Formation,the calcareous nodules of exposure origin,the characteristics of fossil production and the geochemical characteristics of clastic rocks were exposed on a large scale.It shows that the lower section of Shaximiao Formation is in a relatively arid oxidativeenvironment.(4)According to the analysis of filling sequence and style in Sichuan Basin,it shows that there was no great change in filling sequence and style between the middle Jurassic Xintiangou Formation and Shaximiao Formation in Yunyang area.The lower section of Xintiangou Formation in Yunyang area is in a transitional sedimentary environment,and it isdelta sedimentary environment.The lower section of Shaximiao Formation in Yunyang area is dominated by fluvial sedimentary environment.According to the analysis of palaeogeography and palaeoclimate,the distribution of dinosaur fossils in Sichuan Basin is constrained by palaeoclimate and palaeoclimate.(5)Integrated application of element geochemistry and isotope geochemical methods,the Yun Yang Puan Shaximiao Formation of fine grained clastic rocks through the main elements,trace elements and Sr and Li isotope analysis research,think the Middle Jurassic Yun Yang Shaximiao Formation of dinosaurs lived and buried in a relatively arid environment,its ancient climate is semi-arid and semi-humid climate.Sr and Li isotopes have important roles in the paleoclimatic inversion of terrestrial finegrained clastic rocks.Compared with other conventional geochemical indicators,Sr and Li isotopes have good consistency and can be used as paleoclimatic proxies of terrestrial clastic rocks.