Stratigraphy and palaeoenvironment of the dinosaur-bearing Upper Cretaceous Iren Dabasu Formation, Inner Mongolia, People's Republic of China

New field observations and sedimentological analyses of the dinosaur-bearing Upper Cretaceous Iren Dabasu Formation in the Iren Nor region of Inner Mongolia (People's Republic of China) have led to a better understanding of its palaeoenvironment. The fluvial deposits represent a braided river that, due to the large amount of fines, does not fit the classical model for braided rivers. The study area is divided into two parts: in the northern part, sediments of the main channel belt of the ancient braided river system are exposed along a dry river valley on the northern edge of the Iren Nor salt lake, while in the southern part, comprising all other studied exposures, different facies of the ancient floodplain are represented, including minor channels, temporary ponds, and palaeosols. The difference between the northern and southern parts is also reflected in the fossil content; only the southern exposures have yielded dinosaur remains. The ancient braided river had a broad, vegetated floodplain populated by a diverse dinosaur fauna. Four species of charophytes are described and illustrated from the Iren Nor site, together with eight species of ostracods, one of which (Cypridea irennorensis sp. nov.) is new. Contrary to the vertebrate data, both groups of microfossils indicate a latest Cretaceous age (Campanian-Maastrichtian) for the Iren Dabasu Formation, and suggest a possible correlation with the Nemegt Formation, which would allow the age estimation to be refined to latest Campanian-Early Maastrichtian.