THE LITHOSTRATIGRAPHIC CHARACTERISTICS OF YIXIAN FORMATION IN BEIPIAO-YIXIAN AREA,LIAONING PROVINCE

The Yixian formation in Beipiao-Yixian area, Liaoning Province includes a series of volcanic-sedimentary strata with volcanic rocks more than 80%, involving four volcanic subcycles.The first subcycle is characterized by basic-alkalescent basic and intermediate-basic volcanism;the second is marked by acid and intermediate-basic volcanism;the third shows intermediate-basic volcanism;and the fourth is noted for acid-intermediate volcanism.The volcanic-sedimentary strata of Yixian formation include at least 7 sedimentary intercalations, which may be further merged into four beds, i.e.the basal conglomerate bed, the sedimentary interbed in lower volcanic rocks, the middle sedimentary bed of lacustrine facies, and the upper sedimentary bed of fluvial and lacustrine facies.The organic fossils of sedimentary strata are concentrated mainly in three beds.By summing up the cyclicity, rock association and occurrence of volcanic rocks with their overlying and underlying contact relations and forming environment, and concerning the research results of isotope chronology and biostratigraphy, the Yixian formation of the working area may be subdivided into four members.The isotopic dating data show that the Yixian formation should be Early Cretaceous in age, formed 135-120 Ma ago, and the sedimentary beds of lacustrine facies, containing precious fossils, such as Sinosauropteryx, Confuciusornis, Archaefructus liaoningensis etc., formed 125-127 Ma ago.