Impact of paleoenvironment, organic paleoproductivity, and clastic dilution on the formation of organic-rich shales: a case study about the Ordovician-Silurian black shales, southeastern Chongqing, South China

摘要: Three triggering factors, including preservation, organic paleoproductivity, and clastic dilution, for the formation of black shale successions during Ordovician-Silurian transition have been debated many years. Preserving conditions are controlled by redox microbial activities which close relationship with salinity. Five outcrop sites Wufeng-Longmaxi shales located in southeastern area Chongqing analyzed their elemental geochemistry, total carbon, clay minerals. Clay minerals boron used to reconstruct paleosalinity (S p) bottom water this period. Depending on minerals’ specific combining pattern, calculation S p is modified. The results show that concentration ranges from 1.73 16.12 ‰, an average value 9.68 ‰. This shows increasing trend Late Ordovician Early Silurian, indicating oligohaline-mesohaline water. Desalinization global ocean was caused melting continental glaciers. negative correlation carbon (TOC) content, since decomposition rates improved high V/(V + Ni) V/Cr ratios paleoredox seawater transition, suggest oxygen levels increased Silurian landward as a whole. Correlation between TOC content reveals anoxic favorable accumulation matter. Ba (Ba/Al)terr calculate biogenic Ba, order paleoproductivity. no apparent implies paleoproductivity does not solely control Si/Al Ti/Al considered indicators input. Our data imply input quite low distal basin around boundary, consistent maximum TOC. In proximal basin, dilutes enrichment We may conclude complex, non-linear interactions among three factors most likely lead High production organic-rich shale, whereas organic-poor shale. Paleoenvironment Yangtze Platform reconstructed. A combination multiple significantly affects shales.