北票—义县地区义县组岩石地层特征

北票-义县地区的义县组地层是一套火山-沉积地层,其中火山岩地层占80%以上,具有四个火山作用亚旋回,第一亚旋回以基性-偏碱的基性-中基性火山作用为特征,第二亚旋回以中基性-酸性的火山作用为特征,第三亚旋回以基性-中性火山作用为特征,第四亚旋回以中酸性火山作用为特征;义县组火山-沉积地层包含至少7个沉积夹层,可以进一步归纳为四套沉积层,即底砾岩层、下部火山岩中的沉积夹层、中部湖相沉积层和上部河湖相沉积层,沉积层中生物化石主要集中在3个主要层位.综合火山岩的旋回性特征,岩石组合、岩石产出特征,不同岩石组合的上、下接触关系和岩石形成环境的分析结果,结合同位素年代学及生物地层学的研究成果,区内义县组被划分成了4个岩性段.同位素年龄数据显示义县组时代为早白垩世,其形成年龄介于135~120Ma之间,含中华龙鸟、孔子鸟和辽宁古果等珍稀化石的湖相沉积层是在125~127Ma之间形成的.

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.