Lacustrine tempestite litho- and biofacies in the Lower Cretaceous Yixian Formation, Beipiao, western Liaoning, northeast China

The Lower Cretaceous Jianshangou Member, the basal unit of the Yixian Formation at Sihetun, Beipiao, western Liaoning, northeast China, comprises volcanic tuffs and is well-known for its diverse fossils, the Jehol Biota. Quiet lacustrine environments prevailed during this time interval. Within the lacustrine facies there is a hummocky cross-stratified, gravelly silty sandstone 0.2–0.6 m thick containing a high concentration of bivalve shells. It is characterized by the dominance of two species: Sphaerium anderssoni (Grabau) and Arguniella cf. ventricosa (Kolesnikov). The bivalves form a dense concentration of broken and sorted remains. The lithological, biotic and taphonomic features of the deposit thus differ from the units below and above it in which the sediment is finer-grained and well bedded, the fossil taxa are more diverse, autochthonous and better preserved. We suggest that the coarser sediments and the bivalve concentration were formed by strong currents that winnowed away finer material. The hummocky cross-stratification suggests a tempestite origin for this bed.