New Knowledge on Yixian Formation
义县组研究的新认识

The Yixian Formation in western Liaoning Province comprises a set of Mesozoic continental volcanic-sedimentary rocks, which mainly consist of intermediate-basic volcanic lava, volcanic fragmental rock and lacustrine (including a little fluviatile) sedimentary rocks. The formation can be divided into four lithologic members based on the regularity of volcanic activity. From bottom to top, they are the first membei-the basal conglomerate and basic to intermediate basic volcanic member, the second member-the lacustrine sedimentary rock (or called the rare fossil layer) and acid-intermediate volcanic lithologic member, the third member-the basic, intermediate-basic, meta-alkaline volcanic member, and the fourth member-the conglomerate (including sedimentary volcanic breccia) and acid-intermediate, meta-alkaline volcanic member. There are 6 fossil layers in Yixian Formation. The basal conglomerate and basic, intermediate-basic volcanic sedimentary tuff in the first member contains abundant ancient vertebra fossils dominated by Psittacasaurus. The lacustrine sedimentary rock in the second member contains a large number of Jehol Biota fossils including the famous Sinosauropteryxprima, Confuciusornis sauctus and Archaefructus liaoningensis, thus serving as the chief fossils-bearing layer in Yixian Formation. Moreover, all the lacustrine fossil layers are synchronous. The age of Yixian Formation can be assigned to Early Cretaceous.