THE PRECIOUS FOSSIL-BEARING BEDS OF YIXIAN FORMATION IN BEIPIAO-YIXIAN AREA: THIER AGES AND CORRELATION

The Yixian formation in Beipiao-Yixian area, composed mainly of volcano-sedimentary strata, contains four volcanic subcycles.The organic fossils, which formed mainly in the periods of the 1st, 2nd and 4th subcycles, are found in three beds.The precious fossil assemblage represented by Sinosuaropteryx, Confuciusornis and Archaefructus liaoningensis in the 2nd member of Yixian formation and formed during the period of 125 to 127 Ma.The pillow lavas, quenched rocks and high frequency sequence strata all indicate that the precious fossil-bearing beds (the 2nd member of Yixian formation) of Beipiao-Yixian area were situated in a relatively deep lake environment, to show the lake expansive system tract.The precious fossil-bearing beds of both Beipiao and Yixian areas are basically equivalent and correlatable.The Yixian formation was formed during the period of 135 to 120 Ma, i.e.Early Cretaceous in age.Many evidences have revealed that the boundary between Jurassic and Cretaceous should be set in the basal boundary of Yixian formation.