辽西北票地区义县组地层层序与化石层位

辽西北票地区义县组是一套中生代火山-沉积地层.义县组火山活动具有明显的旋回性特征,主要由下部基性-中基性火山喷发-沉积旋回和上部酸性、偏碱性火山活动旋回组成.具体可划分6个岩性段,自下而上分别为底砾岩段,基性、中基性火山岩段,湖相沉积岩段,酸性、偏碱性火山岩段,上部基性火山岩段和顶部砾岩段.义县组底砾岩段和基性、中基性火山岩段的沉凝灰岩中产有丰富的以鹦鹉嘴龙为主的古脊椎动物化石.湖相沉积岩段含有大量热河生物群化石,包括著名的中华龙鸟、孔子鸟和辽宁古果等珍稀化石,该层位记录了侏罗-白垩纪界限附近重大的生物演化事件.

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(S 5) 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.