Lagerstätte fossils from the Lower Cretaceous Yixian Formation of the Pingquan Basin, North China: stratigraphical correlation and palaeoenvironmental implications

The Lower Cretaceous Yixian Formation in northeast China mainly contains the Jehol Biota. Notable investigations of the area have led to the discovery of feathered dinosaurs, angiosperms and numerous other exceptionally well-preserved fossils. The Yixian Formation is mostly composed of basalts interbedded with siliciclastic sediments that can be divided into several beds (from lower to upper layers, the Jianshangou Bed, Dawangzhangzi Bed and Jinggangshan Bed). However, the stratigraphy and fossils of the Yixian Formation in the northern Hebei Province are less understood than those of the western Liaoning. This study carried out a sectional excavation on a lacustrine deposit outcrop of the Yixian Formation from the Pingquan Basin in northern Hebei (the Shimen section). This fieldwork resulted in a detailed lithostratigraphical description of 52 layers, including three abundant fossil layers found within a fine-grained clastic deposit that is 15.48 m thick. A preliminary palaeontological record was also created from primary observations and from information from previous investigations. Representative fossils included the lizard Jeholacerta formosa, fish Lecoptera sp. tadpole shrimp Chenops yixianensis conchostracans Eosestheria, dragonfly Samarura and the palaeobotanical conifers Liaoningocladus boii and Schizolepis. Comparing the lithostratigraphy and biostratigraphy of this section with other Lower Cretaceous sections in the western Liaoning, the lacustrine strata of the Shimen section correlate to the lower part of the Yixian Formation's Dawangzhangzi Bed. Based on the analysis of the characteristics of the sediment and the typical types of the plants from this section, it is suggested that the Pingquan Basin is a small volcanic-sediment basin, in temperate palaeoclimate during its deposition period.