川东北地区晚埃迪卡拉纪灯影期海水氧化还原环境重建

晚埃迪卡拉纪全球海洋发生了大面积的缺氧,海洋化学结构呈现明显的非均质性,直接影响了埃迪卡拉型生物的演化与分布。四川盆地发育完整的晚埃迪卡拉系地层,以灯影组巨厚层碳酸盐岩沉积为代表。但是对于该套巨厚层碳酸盐岩沉积时的古海水氧化还原性质备受争议。为了解决这一问题,对川东北地区鹿池剖面的灯影组地层开展了系统的沉积学和稀土元素地球化学分析。该地区灯影组的岩石类型主要为泥微晶白云岩、黏连白云岩、叠层/层纹白云岩,以及溶蚀白云岩,沉积环境为开阔碳酸盐岩台地相。地球化学数据结果显示灯影组碳酸盐岩普遍具有较低的稀土总量(∑REE+Y值为0.4～3.3μg/g)、较低的Mn/Sr值(0.2～2.8)和较高的Fe含量(55.9～1 772.6μg/g)。灯影组的REE+Y配分曲线(经页岩标准化)可划分为四个阶段,且Ce异常指示该地区经历了弱氧化到弱还原再到缺氧状态,表明埃迪卡拉纪晚期海洋浅部水体也发生了缺氧现象。

Redox Conditions of the Late Ediacaran Dengying Period in Northeastern Sichuan,China

Marine anoxia pervasively occurred in the late Ediacaran period, displaying obvious heterogeneity in redox stratification and ocean chemical structure. Such redox conditions directly affected the evolution and distribution of Ediacaran organisms. The Sichuan Basin of the upper Yangtze Platform records complete Ediacaran successions, which are mainly represented by a suit of thick carbonate deposits （the Dengying Formation）. However, the redox conditions of these carbonates remain controversial. In order to constrain the seawater chemistry and marine redox in the later Ediacaran, we carried out a detailed sedimentological and geochemical analysis of the Dengying Formation at the Luchi outcrop, northeastern Sichuan Basin. The Dengying Formation is mainly composed of mud-microcrystalline dolostone,bonded dolostone, stromatolitic/laminated dolostone, algae-laminated dolostone, and breccias, suggesting deposition from a carbonate platform. Geochemical data suggest that the Dengying Formation has generally low total rare earth element content （∑REE+Y; ranging from 0.4 to 3.3 μg/g） and Mn/Sr values （ranging from 0.2 to 2.8） but high Fe content （ranging from 55.9 to 1 772.6 μg/g）. The REE+Y patterns （Post-Archean Average Shale （PAAS） - normalized） of the Dengying Formation can be divided into four stages, and the Ce anomaly patterns indicate that the seawater of the northeastern Sichuan Basin was transitioned from weak oxic to weak anoxic and ultimately became anoxic during the deposition of the Dengying Formation, indicating that the shallow water may be anoxic in the late Ediacaran period.