New age constraints on the Yixian Formation and its implications for the Jehol Biota

Abstract

The fossils of the Jehol Biota are magnificent, exquisitely preserved and extraordinarily diverse. Since formal study of the Jehol Biota began in the 1990s, abundant terrestrial plant and animal fossils, have been discovered from the Dabeigou Formation, the Yixian Formation and the Jiufotang Formation (and their correlative strata) in Northeastern China. Although strata interpreted to be correlative with the classic Jehol localities occur throughout Eastern China, as well as in Korea, Japan, Mongolia, and Siberia, most fossils of the Jehol Biota have been discovered from classic outcrops of the 225-4000 meter-thick Yixian Formation in Western Liaoning Province. Fossils from the Yixian Formation provide unique insight into Cretaceous terrestrial ecosystems and their influence on the emergence or radiation of angiosperms, insects, dinosaurs, mammals and birds. Despite voluminous research on the Jehol fossils, critical uncertainties remain. Based on assemblages of vertebrate fossils, the Yixian Formation is generally divided into the Lujiatun Unit, Lower Lava Unit, Jianshangou Unit, Upper Lava Unit, Dawangzhangzi Unit and Jingangshan Unit (from bottom to top). This stratigraphic column for the fossil-rich Yixian Formation has been widely accepted in the past three decades. However, previously reported age data obtained through a range of methods and applied to different units of the Yixian Formation do not consistently integrate with this commonly accepted stratigraphic interpretation. In this study, I summarize previous age data for the Yixian Formation and recalibrate previously reported 40Ar/39Ar ages by using new age interpretations for the same standard. I also present new high-precision 40Ar/39Ar ages of 125.8 ± 1.0 Ma and 126.0 ± 0.8 Ma for two basaltic samples from the Lujiatun Unit. This study refutes the widely accepted stratigraphic correlations for the Yixian Formation. The age result indicates that the Lujiatun Unit was deposited contemporaneously with the Jianshangou Unit. This study provides significant clues for reconstructing sedimentary environment for the fossil-rich Yixian deposits. The results will further strengthen the regional correlations and chronostratigraphic framework of Cretaceous nonmarine strata in East Asia and resolve fundamental biological questions concerning dinosaurian evolution and paleobiogeography. The research may thus have fundamental implications for biologic evolution and the geologic circumstances that influenced major cladogenetic events.