New geochronology of the Lower Cretaceous in the Luanping Basin, northern Hebei: Age constraints on the development of early Jehol Biota

The Valanginian and Hauterivian stages of the Early Cretaceous are an important time interval for the coevolution of key vertebrate taxa and flowering angiosperm, as well as for the diversification of dinosaurs and early birds on continents. Among these, the Early Cretaceous Jehol Biota in northern China provides the most valuable fossil evidence. Lower Cretaceous terrestrial strata in the Luanping Basin, northern China, is notable for its continuous sedimentary sequence and preservation of the early Jehol Biota, thus providing crucial evidence for probing into terrestrial ecosystem and paleoclimatic change. In this study, we present a chronology for the Lower Cretaceous Dabeigou and Dadianzi formations based on SIMS U-Pb zircon analyses and previous biostratigraphy and lithostratigraphy from the Zhangjiagou section in the Luanping Basin, northern China. The lowest Dabeigou Formation yields age of 134.1 +/- 1.0 Ma, and the upper Dadianzi Formation is constrained to 127.8 Ma, respectively. Previous geochronologic, biostratigraphic studies, and our new geochronology show that the Dabeigou and Dadianzi formations in the Luanping Basin are coeval with the Huajiying Formation in the Sichakou basins, representing the early Jehol Biota evolution, and thus are undoubtedly deposited earlier than the Yixian Formation in western Liaoning. Our findings, combining with the fossil evidence and geochemistry data show that the early evolution of the early stage of the Jehol Biota is constrained to similar to 132 Ma, characterized by a positive carbon isotope excursion (CIE) and increased biodiversity. The cooler temperature than the underlying sediments attribute to the primary productivity and thus helps the Jehol Biota to thrive.