Reconstruction of redox conditions during deposition of organic-rich shales of the Upper Triassic Yanchang Formation, Ordos Basin, China

The concentration and distribution of organic matter, phosphate nodules, and framboidal pyrite preserved in shales are controlled by organic matter decomposition and the cycles of phosphorus, sulfur, and iron, which, in turn, are controlled by bottom-water redox conditions. The organic-rich shale in the 7th member of the Upper Triassic Yanchang Formation (Ch7) contains high organic carbon concentrations and abundant phosphate nodules and pyrite framboids, providing an opportunity to better understand the redox conditions at the time of deposition. Framboidal pyrite in the Ch7 organic-rich shales are composed of octahedral, pyritohedral, and spherulitic microcrystals with uniform crystal size. SEM photographs and statistical data suggest that pyrite framboids have large mean diameters (10.0 to 18.1 μm) and standard deviations (2.8 to 5.9 μm). This may indicate that the organic-rich shale was deposited under oxic-suboxic bottom-water conditions. The Corg:P ratios of the organic-rich shale exhibit considerable variation, ranging from 7.2 to 216, but the majority of them (83.2%) belong to the lower-intermediate range (< 130), suggesting that the redox conditions were predominantly oxic-suboxic and intermittently anoxic. The Corg:P ratios also show periodic vertical variation, which suggests that redox conditions fluctuated between oxic-suboxic and anoxic. Additional support for the trends based on Corg:P ratios is provided by DOPT values, which show similar vertical variation and redox patterns. All evidence shows that the redox conditions during deposition of the Ch7 organic-rich shale was predominantly oxic-suboxic accompanied by intermittent anoxic environments. The result of this study will be helpful in better understanding the formation mechanism of the Ch7 organic-rich shale.