Newly Discovered Tuff in the Lower Shaximiao Formation in Yunyang, Chongqing, Southwestern China and Its Constraint on the Burial Age of the Yunyang Dinosaur Fauna

Abstract The Shaximiao Formation is one of the important Mesozoic burial horizons of dinosaur fossils in the Sichuan Basin. Although a large number of research results on the stratigraphic age of the Shaximiao Formation have been published, the age and division of the Shaximiao Formation are still controversial. To a certain extent, this affects the precise constraints on the burial age of the dinosaur fauna in Yunyang, Chongqing. In this study, the newly discovered tuff intercalation in the Lower Shaximiao Formation in Pu'an Township, Yunyang County, Chongqing City, was taken as the research object, and its lithology and formation age were determined using thin section identification, X-ray diffraction (XRD), scanning electron microscopy (SEM), and U-Pb zircon dating. The field observations and thin section microscopic characteristics of the rocks reveal that the clay rock intercalation has a typical sedimentary tuff structure, which is altered lithic tuff. The SEM imaging and XRD results indicate that it has a typical tuff mineral composition. The U-Pb zircon dating shows that the weighted average age of the newly discovered tuff is 174.8 ± 2.4 Ma, and the burial age of the dinosaur fossils is estimated to be 174.7 ± 2.4 Ma based on the sedimentation rate and the thickness of the stratum. Key words: Sichuan Basin; Shaximiao Formation; Yunyang Dinosaur Fauna; Tuff; Burial Age