2016年2期

峡东地区上埃迪卡拉系岩石、生物、层序和碳同位素地层 及其年代学意义

发布时间:2016-09-05阅读次数:

【期数】2016-2

【作者】陈孝红,周 鹏,张保民,危 凯,张 淼

【机构】中国地质调查局武汉地质调查中心,湖北,武汉430205

【关键词】灯影组、稳定碳同位素、埃迪卡拉动物,管状动物、埃迪卡拉系-寒武系界线

【中文摘要】本文系统研究了峡东灯影峡上埃迪卡拉系陡山沱组上部至灯影组石板滩段及其区内与之同期不同相区(或古地理部位)地层岩石、生物、层序和碳同位素地层,结果发现峡东埃迪卡拉系陡山沱组-灯影组界线附近地层横向上存在台地相、高陡边缘台地边缘相和盆地相多种沉积相类型,灯影组的底界和内部划分明显受沉积相的制约,以白云岩出现为标志的灯影组底界是一个明显的穿时面。与灯影组中部存在的两次海进-海退旋回相对应不仅发生了两次δ13C的上升和下降,而且发生了两次重要的生物辐射事件,在石板滩段下部和上部分别出现了具有全球广泛对比意义的叶状埃迪卡拉动物和管状Cloudiniids动物。据此可以将峡东灯影组划分出两个可以进行全球对比的年代地层单位四溪阶和龙灯溪阶。由于龙灯溪阶上部,继Cloudiniids消失之后,小壳化石出现之前发生了δ13C的震荡下降,并在灯影组白马沱段底部出现了最小δ13C达到-12‰的碳同位素强烈负偏离。因此,生物和碳同位素组成变化特点及其全球对比显示长江三峡地区寒武系的底界应大致与灯影峡剖面上灯影组石板滩段与白马沱段界线一致。

【英文摘要】Litho-, bio-, sequence- and carbon isotope chemo-stratigraphy of the Upper Ediacaran ranging from upper part of the Doushantuo Formation to the Sibantan member of the Dengying Formation in eastern Yangtze Gorges were studied in this paper. Conclusions show that there are different kinds of sedimentary facies, which included such as platform, slope with high steep margin and basin facies, in the upper part of the Doushantuo Formation and lower part of the Dengying Formation in the eastern Yangtze Gorges. The subdivision of the basal boundary of the Dengying Formation and the members of the Dengying Formation are obviously restricted by sedimentary facies. The base boundary marked by occurrence of dolostone for the Dengying Formation is not an isochronal boundary. Correlated with two transgression - regression cycles, twice increasing - decreasing trend of δ13C value and twice important biodiversity events marked by occurrence of vendobiontan and tublar fossils were happened separately in the lower and the upper part of the Shibantan member of the Dengying Formation in Dengying Gorges section. Consequence, two regional chronostratigraphic units( Sixi and Longdengxi stages) would be recognized in the Shibantan member of the Dengying Formtion in the eastern Yangtze Gorges. The decreasing of δ13C value between the disappearance of the Cloudiniids fossils at the upper part of the Shibantan member and the appearance of the small shelly fossils at the upper part of the Baimatuo member led a very negativeδ13C value of -12‰ at the base of the Baimatuo member of the Dengying Formation. The basal boundary of the Cambrian in the eastern Yangtze Gorges should be correlated with the occurrence of this negative interval, i.e the boundary between the Shibantan and Baimatuo members based on the changes of the fossil assemblages andδ13C and their correlations with other contemporaneity strata in the world.

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