Paleodepositional conditions and organic matter accumulation mechanisms in the Upper Ordovician-lower Silurian Wufeng-Longmaxi shales, Middle Yangtze region, South China

Abstract

The deposition of black shales has been widely attributed to primary productivity, preservation controlled by redox conditions, and detrital dilution controlled by sedimentation rates. It is difficult to distinguish the relative importance of productivity and redox conditions in organic matter accumulation. On the Upper Ordovician-Lower Silurian Wufeng-Longmaxi black shales, we combined petrographic features, major and trace elemental proxies, and organic carbon isotopic analysis to determine the relative importance of productivity versus anoxia on organic matter accumulation and the evolution of factors controlling organic matter accumulation across the Ordovician-Silurian boundary in the Middle Yangtze region. Our research also sheds light on the temporal variations in redox conditions that caused the Late Ordovician mass extinction.Productivity, redox conditions, detrital dilution, and basin restriction changes over time as the Wufeng-Longmaxi Formations were deposited (from upper Katian to Rhuddanian stage). Based on the stratigraphic profiles of productivity (biogenic Si and P/Al), redox conditions (Mo/Al, U/Th ratios, and DOPT values), detrital fraction flux (Al2O3), and the cross-plots of biogenic Si versus Co*Mn values, our findings indicate that organic matter accumulation was a combined effect of productivity and anoxia; however, the elevated productivity caused by upwelling that introduced plenty of nutrients to the oceans was the direct factor driving organic matter accumulation. Previous research found a simultaneous variation in stratigraphic profiles of productivity and redox proxies; however, in our datasets, we found an offset between productivity geochemical proxies (biogenic Si content) and redox conditions (Mo/Al). Biogenic Si profile peaks occur stratigraphically beneath Mo/Al profile peaks. Anoxia does not coexist with increased paleoproductivity across the Ordovician-Silurian boundary in the Middle Yangtze region, and redox conditions may be disrupted by local geological events.