| 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 TOC 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. |
