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

Abstract

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