四川盆地南部志留系龙马溪组富有机质页岩沉积环境的元素地球化学判别指标

Geochemistry Evaluation Index of Redox-sensitive Elements for Depositional Environments of Silurian Longmaxi Organic-rich Shale in the South of Sichuan Basin

氧化还原敏感元素是确定古海洋水体氧化还原环境的重要指标,常用的环境判别指标有U/Th,V/Sr,V/Cr,Ni/Co,V/(V+Ni),δU,Ce/Ce*,Eu/Eu*等元素比值。对四川盆地南部宁203井志留系龙马溪组页岩沉积环境的元素地球化学研究结果表明,龙马溪组底部富有机质页岩沉积于缺氧环境,上部为氧化环境。有机碳含量与V/Cr和Ni/Co呈正相关关系,且生物生产力相关元素富集,说明有机质保存与缺氧环境有关。认为在对页岩沉积水体氧化还原条件进行研究时,应将地球化学指标与沉积和古生物特征综合起来得出科学解释。

Redox-sensitive elements are the important indicators to determine the redox conditions of ancient ocean environments, and the trace and rare earth element ratios, such as V/Sr, V/Cr, Ni/Co, V/（V＋Ni）, U/Th, δU, Ce/Ce＊and Eu/Eu＊ratios, are commonly used as the indices of oxic and/or anoxic depositional environments. The analytic samples are selected from Silurian Longmaxi shale of Well Ning-203 in the south of Sichuan Basin, which is representative of regionally-distributed Longmaxi shale. The analysis of element geochemistry shows that the shale in the downmost part of Longmaxi Formation is organic-rich, which indicates the anoxic depositional environment and the shale in the upper part indicates the oxic environment. The relationships of values with V/Cr or Ni/Co ratios are all positively correlated, and enrichment of biological productivity-related elements reveals that preservation of organic-rich shale is related to anoxic environment. It is suggested that geochemical indices should be combined with sedimentary and paleontological features in studying the redox conditions of ancient sedimentary environments of shale.