Synthesis of a chrono- and biostratigraphical framework for the Lower Cretaceous of Jiuquan, NW China: Implications for major evolutionary events

Abstract

With a depositional history spanning the early Aptian to early Albian (~19 mys), the Lower Cretaceous Xinminpu Group in Jiuquan, NW China provides detailed record of an inland ecosystem responding to global climatic and environmental changes. The Xinminpu Group exhibits the most typical elements of the Jehol Biota found outside NE China, a highly diverse avifauna and a mid-Cretaceous angiosperm flora. The absence of a comprehensive stratigraphical framework for this area however limits the degree to which the unit can be interpreted relative to coeval sections and the geological records in genera. Recent fossil discoveries and radio-isotopic age data can provide a more detailed biostratigraphical and chronostratigraphical framework for Lower Cretaceous units around Jiuquan. The data complied here indicate that the Chijinpu Formation correlates with the middle-upper Yixian and lower Jiufotang formations of NE China making it early Aptian in age, and the Xiagou Formation correlates with the upper Jiufotang and Shahai formations making it late early Aptian-early Albian in age. This research further reports a novel zircon U-Pb age constraint of 112.4 ± 0.3 Ma for a basalt sample from the lowermost Zhonggou Formation, which is supported to be correlated with the Fuxin Formation of early Albian age. The Chijinpu Formation contains typical elements of the Jehol Biota, i.e. the Eosestheria-Lycoptera-Ephemeropsis trisetalis assemblage, confirming the interpretation that the Jehol Biota had reached Jiuquan during its second evolutionary stage. The Xiagou Formation contains the amphibious avifauna appearing just above OAE1a, indicating a close link between the early Ornithuromorpha evolution and early Aptian global climate changes. The Zhonggou Formation in Jiuquan records the first appearance of an angiosperm flora above OAE1b. Depositional indicators suggest a humid climate with a gradual shift to arid conditions during this time. This review paper laid foundation for the systematic study of the well-preserved Early Cretaceous ecosystem in Jiuquan, allowing to understand correlations between inland and marine environments as recorded in sediments from this critical period.