CHEMOSTRATIGRAPHY OF THE LOWER CRETACEOUS DINOSAUR-BEARING XIAGOU AND ZHONGGOU FORMATIONS, YUJINGZI BASIN, NORTHWEST CHINA

The Early Cretaceous of northwest China has yielded abundant vertebrates, invertebrates, and plant fossils from numerous intermontane basins. Developing a chronostratigraphic context for these important fossil finds is important to understanding the development of modern terrestrial ecosystems, the evolution of dinosaurs, and the Cretaceous greenhouse climate. This study utilizes carbon isotope chemostratigraphy of the fossil-rich Xinminpu Group in the Yujingzi Basin in northwest Gansu Province. Lithostratigraphic descriptions defined three facies. The lowest is a tan, coarse arkosic sandstone, over-lain by gray to variegated mudstones, sandstones, and thin limestones, which transition to red sandstones, conglomerates, and mudstones. Depositional environment interpretation of these lithofacies assisted in refining 450 samples of bulk sedimentary organic carbon as well as charcoal samples to develop a composite curve to correlate to existing carbon isotope curves from the region and to better constrained Cretaceous sections globally. The carbon isotope curve is correlated based on the broad positive excursion associated with carbon isotope fluctuations of the Paquier Episode that spans the late Aptian to early Albian. Based on this correlation, the middle gray mudstone facies, which contains a diverse fauna including theropods, sauropods, and ornithopods, as well as turtles and invertebrates, is placed within the upper Aptian. The upper red sandstone facies, which contains therizinosaurs and the ceratopsian dinosaur Auroraceratops, is placed in the lower Albian.