Factors controlling organic matter accumulation in the Upper Ordovician Wufeng shale from upper Yangtze Platform

A series of experiments,including total organic carbon ( TOC),mineral compositions,major and trace ele- ments test,were performed on twenty-five shale samples in the Upper Ordovician Wufeng Formation from upper Yan- gtze Platform. This study analyzed the characterizations of major and trace elements distribution and their implications for sedimentary environment,and the factors controlling organic matter accumulation were further discussed. The re- sults showed that late Katian through Hirnantian Wufeng shale could be divided into three intervals ( named WF1, WF2,and WF3) according to TOC content. WF1 and WF3 contained similar TOC content,whereas WF2 contained the most abundant organic matter (OM). Geochemical redox proxies of Mo and U indicated that the redox conditions were dominated by oxic to suboxic when WF1 and WF3 deposited and the depositional environment for WF2 were mainly suboxic to anoxic. The reducibility of bottom water gradually increased from WF1 to WF2 and then decreased from WF2 to WF3. All the three intervals were characterized by high content of Ba and biogenic Si,indicating a high paleo- productivity,which might be controlled by the upwelling evidenced by the higher ratio of Cd / Mo. The concentration of Al and Ti were relatively stable through the Wufeng Formation,indicating a period characterized by stable sea-level fluctuations. The TOC content of WF1 had positive relationships with EFMo and EFU,whereas the TOC content had negative relationships with Al and Ti,indicating that the favorable redox conditions made WF1 OM preserved. The TOC content of WF2 had positive relationships with biogenic Si,indicating that the high paleoproductivity made WF1 OM accumulate. Both the paleoproductivity and redox conditions had effect on WF3 OM accumulation,because both EFMo and EFU and biogenic Si content had relationships with the TOC content in WF3.