辽宁西部晚中生代地层研究之进展及存在问题——辽宁西部晚中生代地层和鱼群研究之四

辽宁西部晚中生代地层的综合层序为:义县组、九佛堂组、沙海组/张老公屯组、阜新组和孙家湾组.中西部朝阳—喀左—建昌以及凌源—三十家子盆地的义县组大致相当于东部阜新—义县—锦州盆地的狭义义县组及其底部的北票尖山含化石沉积夹层;中西部的九佛堂组则可与东部的金刚山、吐呼鲁和九佛堂三个岩组相对比;张老公屯组火山岩在层序上大部分可能位于九佛堂组和阜新组之间.以北票鲟、中华弓鳍鱼和狼鳍鱼为代表的鱼类化石普遍见于中国北方热河群及其相当岩群的下部河湖相地层中,以粒鳞鱼、固阳鱼和副狼鳍鱼为代表的鱼类化石多见于上部的扇三角洲和湖沼相含煤地层中.热河群及相当岩群时代的确定以及陆相侏罗—白垩系界线的厘定受制于目前地层和生物化石的研究程度.从鱼群的面貌看,义县—九佛堂组、沙海—阜新组及相当地层的时代可能分别为晚侏罗世晚期—早白垩世早期和早白垩世中期.

NEW ADVANCES IN THE LATE MESOZOIC STRATIGRAPHIC RESEARCH OF WESTERN LIAONING, CHINA

The study of the Late Mesozoic deposits (generally called "Jehol Group" except for the uppermost Sunjiawan Formation) of western Liaoning was initiated early in this century, but the stratigraphic sequence of the Jehol Group, its correlation with the corresponding non-marine strata in northern China and the age assignment have long been controversial issues. The present paper is to deal with these problems mainly based on the data of fossil fishes.The Late Mesozoic lithostratigraphic succession in western Liaoning is adapted as follows (Table 1): the Yixian (excluding the former Jingangshan Beds and Tuhulu Beds), Jiufotang, Shahai/Zhanglaogongtun, Fuxin and Sunjiawan formations in ascending order, and certain irregular or diachronous phenomena of lithostratigraphic nomerclature are recognized among the separate basins in this region, e.g., the current Yixian Formation in the western Lingyuan-Sanshijiazi and middle Chaoyang-Kazuo-Jianchang basins can only becorrelated with the Yixian volcanic rocks and its base sedimentary horizon in the eastern Fuxin-Yixian-Jinzhou Basin, but the Jiufotang Formation in the western and middle basins is equivalent to the Jingangshan and Tuhulu beds plus the Jiufotang Formation in the eastern basin after the litho and biostratigraphic principles. The igneous rocks over the Yixian Formation has long been ignored or misplaced, and its reasonable position in the sequence should be between the Jiufotang and Fuxin formations.The Jehol Group can be coincidentally correlated with the corresponding strata in other regions or basins of northern China on the basis of fossil fishes (Table 2). The lower part (Yixian and Jiufotang formations) of the Jehol Group and its equivalents in northern China were generally originated in fluviolacustrine environment while the upper part (Shahai and Fuxin formations) deposited in deltaic and lacustroswamp facies.The Lycoptera and Kuyangichthys ichthyofaunas (Table 3) show a Late Jurassic and Early Cretaceous aspect, not a few forms are closely related with, or quite similar to fishes found in the Upper Jurassic of Kazakhstan in central Asia, in the Purbeck-Wealden of western Europe, and in the Lower Cretaceous of Kitakyushu in Japan. Based on the evidence of the fish faunae, together with the data afforded by other fossil groups, event stratigraphy and radiometric dating etc., the age of the Jehol Group and its corresponding strata in northern China is assigned to late Late Jurassic-mid Early Cretaceous (about Portlandian-Barremian).