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义县组是一套中生代火山-沉积地层,以其丰富的热河生物群(包括珍稀鸟类等)化石闻名于世。长期以来,人们对义县组的地层层序、化石层位、地层厚度以及地质时代均有不同认识。通过1∶5万区域地质调查和填图,笔者分析了义县组的火山作用和火山活动规律。认为义县组主要由下部基性—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,并对义县组进行了分段。发现了义县组的一系列火山机构,恢复了当时的古地貌和古地理特征;指出义县组的各化石层位分别处于同时期的两大古湖泊的不同部位,可以大致相互对比。论述了义县组的基底、盆地性质和地层厚度。义县组的时代有可能是早白垩世,义县组的底界可能是辽西地区侏罗系—白垩系的界限。
The Yixian Formation is a set of Mesozoic volcanic-sedimentary formations known for its rich fossil Jehol biota (including rare birds, etc.). For a long time, people have different understanding of stratigraphic sequence, fossil horizon, stratum thickness and geologic time in Yixian Formation. Through 1: 50000 regional geological survey and mapping, the author analyzes the volcanism and volcanic activity of Yixian Formation. The Yixian Formation is mainly composed of the lower basic-Meso-base volcanic eruption-sedimentary cycle and upper acidic and alkaline alkalic volcanic cycles, and the Yixian Formation is segmented. A series of volcanic structures were found in the Yixian Formation to restore the paleogeomorphology and palaeogeographical features of that time. It is pointed out that the fossil beds of the Yixian Formation are respectively located in different parts of two ancient lakes in the same period and can be roughly compared with each other. The paper discusses the basement, basin character and formation thickness of Yixian Formation. The age of Yixian Formation may be Early Cretaceous. The bottom boundary of Yixian Formation may be the Jurassic-Cretaceous boundary in western Liaoning.