辽西地区义县组岩相古地理特征

Characteristics of lithofacies palaeography of Yixian Formation in western Liaoning

义县组广泛发育于辽西地区晚中生代盆地中,为一套以陆相火山喷发为主,夹多层沉积岩层的火山--沉积岩系。其沉积层中富含热河生物群化石。通过岩相古地理和火山岩相等研究方法,把义县组自下而上分为4个岩性段,各段的岩相特征为:一段主要为河流相,由复成分细砾岩组成;二段湖泊相广泛分布,形成了一套含珍稀化石的湖相沉积层;三段以河流相为主;四段为湖泊相、河流相和冲积扇相,均小面积发育。4个岩性段均不同程度发育火山岩相(包括各火山亚相)。与土城子期对比,义县期的古地貌不会高于土城子时期,古水流主要来自火山盆地坳陷湖的四周。

Yixian Formation,widely exposed in Late Mesozoic basin of western Liaoning Province,is mainly a series of volcanic-sedimentary formation including some sedimentary intercalation,and bearing aboundant Jeho Biota fossils.According to research method of lithofacies paleogeography and volcanics facies,the Yixian Formation can be subdivided into 4 members,and their lithofacies are as follows：the first subcycle composed of polymictic conglomerate beds is chiefly developed in river facies.Lacustrine facies is broadly distributed in the second subcycle,formimg a set of lake sediments with rare fossils.The third one showed mainly river facies.The fourth member of Yixian Formation in mainly consists of lake facies,river facies and alluvial fan facies.They developed in small area.All of the lithologic members are composed of volcanics facies（including volcanics subfacies）.Through comparing with Tuchengzi age,the ancient relief of Yixian age could not be higher than the former,and paleocurrent is mainly from volcanic basin around downwarped lake.Finally,the ancient geographic environment on different sediment is restored.The study can offer important reference for regional stratigraphic correlation.