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, overlain 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. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Suarez, M. B., T. Milder, P. Nan, C. A. Suarez, H. You, D. Li, and P. Dodson 2019. Chemostratigraphy of the Lower Cretaceous dinosaur-bearing Xiagou and Zhonggou formations, Yujingzi Basin, northwest China; pp. 12–21 in Hailu You, Peter Dodson, and Eric Morschhauser (eds.), Auroraceratops rugosus (Ornithischia, Ceratopsia) from the Early Cretaceous of northwestern Gansu Province, China. Society of Vertebrate Paleontology Memoir 18. Journal of Vertebrate Paleontology 38(Supplement). DOI: 10.1080/02724634.2017.1510412.中国西北早白垩世众多山间盆地产出了大量脊椎动物、无脊椎动物与植物化石。为这批重要化石发现构建年代地层学背景，对于解析现代陆地生态系统的形成演化、恐龙的演化历程以及白垩纪温室气候的特征均具有重要价值。本研究以甘肃西北部榆井子盆地（Yujingzi Basin）内富含化石的新民堡群（Xinminpu Group）为研究对象，开展碳同位素化学地层学（carbon isotope chemostratigraphy）分析。岩石地层学描述识别出三套岩相（facies）：最底部为黄褐色粗长石砂岩（coarse arkosic sandstone），其上覆层为灰色至杂色泥岩、砂岩与薄层灰岩，向上逐渐过渡为红色砂岩、砾岩（conglomerates）与泥岩。基于上述岩相的沉积环境解释（depositional environment interpretation），本研究对450件bulk沉积有机碳（bulk sedimentary organic carbon）样品与炭屑样品（charcoal samples）开展处理分析，构建了综合碳同位素曲线，以与区域内已有的碳同位素曲线（carbon isotope curves）进行对比，并更精准地约束全球范围内的白垩纪地层序列。该碳同位素曲线基于横跨阿普第晚期至阿尔布早期的帕基尔幕（Paquier Episode）对应的显著碳同位素正偏移事件完成对比标定。基于此对比结果，产出包括兽脚类恐龙（theropods）、蜥脚类恐龙（sauropods）、鸟脚类恐龙（ornithopods）以及龟类与无脊椎动物在内的多样化石群的中部灰色泥岩岩相，被归入阿普第阶上部；而产出镰刀龙类（therizinosaurs）与角龙类（ceratopsian）恐龙黎明角龙（Auroraceratops）的上部红色砂岩岩相，则被归入阿尔布阶下部。 补充数据——本文可免费获取补充材料，网址为www.tandfonline.com/UJVP 本文引用格式：Suarez, M. B., T. Milder, P. Nan, C. A. Suarez, H. You, D. Li, and P. Dodson 2019. 中国西北榆井子盆地早白垩世含恐龙的下沟组与中沟组化学地层学；载于Hailu You, Peter Dodson, and Eric Morschhauser (eds.), 《中国甘肃西北部早白垩世黎明角龙（Auroraceratops rugosus，鸟臀目，角龙亚目）》，《脊椎动物古生物学会回忆录》18，《脊椎动物古生物学杂志》38(Supplement)，第12–21页。DOI: 10.1080/02724634.2017.1510412.