Timing of the Jehol Biota from Western Liaoning and Hebei Province, China

Category Biostratigraphy
Group GSI.IR
Location International Geological Congress,oslo 2008
Author He, Huaiyu۱; Zhou, Zhonghe۲; Wang, Xiaolin۲; Zhu, Rixiang۱
Holding Date 08 September 2008

The Jehol Biota from western Liaoning and Hebei Province includes representatives of almost all of the major clades of Lower Cretaceous terrestrial and freshwater vertebrates, a wide variety of invertebrates and a diverse flora. However, the geological age and stratigraphic correlation of the Jehol Group that produces the Jehol Biota has been a long standing conteroversial issue, ranging from the Late Jurassic to the late Early Cretaceous. Despite the existing geochronological work on the volcanic rocks in this region in the last two decades, several recent datings of the tuffs interbedded in the sediments of the Jehol Group have greatly improved our understanding of the exact age of the Jehol fossil assemblages. The Jehol Group comprise, from the bottom up, Dabeigou, Yixian and Jiufotang Formations respectively. Most published ages are from the Yixain Formation. 40Ar/39Ar dating of interbedded tuff from the Jianshanggou Bed of the lower Yixian Formation in Sihetun section in Liaoning Province provided the first direct age (125.0±0.2Ma) for the fossil-bearing sediments in the middle Jehol Group. This result was confirmed by SHRIMP U-Pb date of 124.7±1.53Ma from the same bed, 124.9±1.7Ma and 122.8±1.6Ma from Huangbanjigou locality, LA-ICPMS U-Pb date of 125-122Ma from Dawangzhangzi locality and 40Ar/39Ar date of 123.54±1.53Ma from Lujiatun locality. Correlation between the Dabeigou Formation in northern Hebei and the Jehol Group in western Liaoning is controversial in the past. 40Ar/39Ar dating on tuffs interbedded in the vertebrates fossil-bearing sediments of the Dabeigou Formation in Fengning, Hebei Province, resulted in an age of 130.7±1.4Ma, which is not only generally consistent with earlier SHRIMP U-Pb ages of 133.9±2.5 Ma and 130.1±2.5 Ma for the formation obtained from two tuffaceous sandstones in the Luanping area in northern Hebei Province, but also confirmed the proposal that the Dabeigou Formation represents a lower horizon than the Yixian Formation in Liaoning Province. 40Ar/39Ar step heating analyses of K-feldspar and the SHRIMP U-Pb zircon data from tuffs interbedded within the fossil-bearing shales of the Jiufotang Formation, from the upper part of the Jehol Group indicated that the Jiufotang Formation was deposited at 120.3±0.7 Ma. In sum, a more precise estimate of the duration of the Jehol Biota for at least 11Ma from 131Ma to 120Ma (Late Hauterivian - Aptian) during the Early Cretaceous has been proposed. The Jehol Biota had experienced early, middle and late stages of evolution, corresponding to the fossil assemblages from the Dabeigou, Yixian and Jiufotang Formations, respectively.