Enhanced Continental Weathering and Intense Upwelling Drove the Deposition of Organic-Rich Shales in the Late Permian Dalong Formation, South China
2025
Yin Gong | Yiming Li | Peng Yang | Meng Xiang | Zhou Zhou | Zhongquan Zhang | Xing Niu | Xiangrong Yang
Marine black shales are important to geologists, because they are not only potential sources and reservoir rocks for shale gas/oil, but also, their deposition could influence the climatic and oceanic environments. Here, a detailed study of the shales in the Dalong Formation in South China was conducted to understand the changes in continental weathering and upwelling and their influences on organic matter accumulation in the late Permian. The results revealed that the deposition of the Dalong and Daye Formations could be divided into five stages, with the highest TOC values (>2%) being observed in stages 2 and 4, intermediate TOCs (~1% to 2%) being observed in stages 1 and 3, and the lowest TOC values (<1%) being observed in stage 5. This study attributed the enhanced organic matter accumulation in stages 2 and 4 to enhanced continental weathering (high CIA values and δ<sup>26</sup>Mg values) and intense upwelling (high Mo/TOC ratios and low δ<sup>13</sup>C<sub>org</sub> and Co<sub>EF</sub> × Mn<sub>EF</sub> values), both of which contributed to high primary productivity and increased anoxia of the bottom waters, further leading to the accumulation of organic matter. Overall, both enhanced continental weathering and upwelling contributed to the development of anoxia, even euxinia, of the seawater and further triggered an end-Permian mass extinction (EPME).
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