Study On Middle Jurassic Dinosaur Fossils And Their Sedimentary Environment From Lower Shaximiao Formation,Yunyang,Chongqing

Yunyang dinosaur fauna in Chongqing was discovered in 2015,located in Laojun village,Yunyang County,northeast of Chongqing,by the Modaxi River,a tributary of the Yangtze River.It is a world-class dinosaur fauna and is featured by long geological time span,wide and dense in fossil distribution,and rich in fossil diversity.The Yunyang dinosaur fauna is a major discovery in the field of dinosaur research in China even in the world.And it has very high scientific value as well as the development and utilization value.Based on the previous studies,this paper mainly focuses on the study of the dinosaur fossils and the sedimentary environment of the lower Shaximiao formation of the Middle Jurassic,and introduces the excavation process and fossil assemblage of Yunyang dinosaur fauna.A cladistic analysis is carried out on a complicated and well-preserved sauropod dinosaur cervical vertebra,which represents an Omeisaurus sp..Based on the analysis of the lithological properties,grain size distribution,and sedimentary structures of the fossil-bearing strata,the sedimentary environment is studied and the sedimentary facies is classified.The results are as follows:1.The dinosaur fauna in the Middle Jurassic Shaximiao Formation is equal to the Shunosaurus fauna,belonging to the early Middle Jurassic.The dinosaur fossils belong to sauropods,theropods,ornithopods,stegosaurs,as well as the other fossils,such as mammal-like reptiles,and bivalves.Among them,the sauropod fossils are predominated.2.A well-preserved sauropod cervical vertebra is studied and identified as an Omeisaurus sp..3.The lower Shaximiao Formation is measured in this study.The stratigraphic sequence is mainly composed of a set of purple-red,grayish-yellow-green mudstone and siltstone,along with sandstone(partially containing gravel),which implicates that there was a lake rich in water and high in energy in the early stage,whereas the lake was less water and lower energy in the middle and late stages.4.The lower Shaximiao Formation mainly includes two sedimentary facies that are lake delta facies and lacustrine facies respectively.The lake delta facies can be further subdivided as delta front subfacies including the subaqueous distributary channel microfacies and subaqueous interdistributary bay microfacies.Simultaneously,the lacustrine facies can be subdivided into shallow lake and bar microfacies and shallow lake mud microfacies,which harbors most dinosaur fossils.