THE STRATIGRAPHIC SEQUENCE AND FOSSIL-BEARING HORIZON OF THE YIXIAN FORMATION IN WESTERN LIAONING,CHINA

The strata of Yixian formation are mainly of Mesozoic volcanic-sedimentary rocks.In accordance with the new geological surveying and mapping in l:50 000 scale, the lithostratigraphic units of Yixian formation in Beipiao area are basically established.The feature of volcanic cycle is obvious in Yixian formation.It consists of six members,which, from bottom to top,are:(1)basal polymictic conglomerate;(2)basic and intermediate-basic volcanic rocks,including four subcycles of volcanic activities;(3)rocks of lacustrine facies(S 5);(4)acid and alkalescence volcanic rocks;(5)upper basic volcanic rocks;and (6)top conglomerate.The fossil vertebrates of Psittacosaurus are found in the basal polymictic conglomgate and the sedimentary tuff among the second member of basic and intermediate-basic volcanic rocks.It is confirmed that the Jehol Biota of Yixian formation, including the reptile Sinosauropterxy, angiosperms Archaefructus etc.,occurs just in the sedimentary strata of lacustrine facies(S₅) between the 4th and 5th basic and intermediate volcanic rock horizon.The important event of biological evolution are recorded just around the boundary between Jurassic and Cretaceous from this horizon.The research data from fossil spore-pollen and fossil plants show a phenomenon that after the arid weather of Jurassic Tuchengzi Age,the weather in Yixian Age became warm and moist.Also,by the latest paleontological study,the paleoecological environment of Jehol Biota in this area is discussed.