Geochemical characteristics and organic matter accumulation of Late Ordovician shale in the Upper Yangtze Platform, South China

Abstract The Ordovician shale (Wufeng Formation) in the Upper Yangtze Platform is now considered a successful development interval, independent of the Longmaxi Formation. However, accumulation mechanisms of organic matter (OM) within the Wufeng shale are rarely discussed and remain highly controversial. Petrological and geochemical characteristics were analyzed on 25 Wufeng samples from well X1 located in the Upper Yangtze Platform, and their implications for the OM accumulation mechanisms are further discussed. The Wufeng Formation is divided into three intervals based on graptolite biozones (named WF2, WF3, and WF4). Geochemical redox proxies (Mo, U, V, Ni, Cr, Co) suggest that WF2 and WF4 were deposited in oxic–suboxic conditions, whereas WF3 was deposited in suboxic–anoxic with episodes of euxinic conditions. Geochemical productivity proxies (Ba xs , P, Si xs ) indicate high paleoproductivity when WF2 and WF3 were deposited, which may have been controlled by upwelling as suggested by the high ratio of Cd/Mo. WF2 (averaged 1.42% TOC) is not as organic-rich because of poor preservation conditions and terrestrial dilution. High paleoproductivity and suitable preservation conditions were the controlling factors for the organic-rich WF3 interval (averaged 3.00% TOC). However, low paleoproductivity and poor preservation conditions caused low accumulation of OM in WF4 (averaged 0.82% TOC). OM accumulation within the Wufeng Formation reflects the interaction between the variations in redox conditions and paleoproductivity. The sedimentary environment during Wufeng deposition was mainly controlled by sea level, which, in turn, was controlled by global climate and global and regional tectonics.