Relative sea-level changes and organic matter enrichment in the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in the Central Yangtze area, China

Based on the sedimentological and geochemical analysis of total organic carbon (TOC) and major and trace elements in the marine Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in Yichang, Central Yangtze area, paleoredox conditions (indicated by U/Th, V/Cr, Ni/Co, TOC-TS-Fe diagram, and Mo-U covariations), paleoproductivity (indicated by Ba-bio, Ba/Al, P/Ti, Ni/Al, Cu/Al, and Zn/Al), terrestrial detrital influx (indicated by Ti and Al), paleoclimate (indicated by chemical index of alteration and Rb/Sr), and upwelling (indicated by Co x Mn) are revealed within the framework of sea-level changes. The influences of these factors on organic matter enrichment are also discussed. The results identify four third-order sequence boundaries in the Wufeng-Longmaxi Formations; each sequence can be divided into one transgressive systems tract (TST) and one regressive systems tract (RST) according to sea-level changes. The high-TOC interval mainly developed in TST1 (average 3.44%) and TST2 (average 3.68%). The analysis of geochemical indicators shows that during the TST1 depositional stage, the climate was dry and cold, the bottom water was in a stable anoxic condition accompanied by rising sea level, and the seawater had moderate-high paleoproductivity. In the RST1 stage, extensive sea-level fall occurred, and the water was anoxic, with decreased productivity and weakened organic matter accumulation. In the TST2 stage, the sea level rise rapidly, extensive anoxicity and high productivity of the water produced the most ideal environment for organic matter enrichment and the highest TOC. In the RST2 stage, sea level slowly fell, the water column gradually became a stable oxidized environment, productivity was low, the terrestrial detrital influx increased and tended to stabilize, and the climate gradually changed to warm and humid conditions, forming relatively organic-poor shale. The upwelling mainly developed during the period of TST1 to TST2, and ceased in the RST2. The correlation analysis between TOC and the above indicators and the Co x Mn versus Cd/Mo diagram show that TOC is chiefly controlled by paleoredox, paleoproductivity, climatic conditions, and terrestrial detrital influx, indicating that preservation conditions are the crucial factor. Sea-level change played a key role in preservation conditions. In the transgression stages, sea-level rise led to the enhancement of reducing water, which controlled the organic matter enrichment in shale.