Boundary definition of Wufeng Formation and Longmaxi Formation in well DD1 and sedimentary environment evolution of Northeastern Yunnan

Although the Upper Ordovician Linxiang Formation marl in DD1 well is covered with thick silty mudstone, there is a lack of graptolite fossil, and no Hirnantia limestone marker stratum of the Guanyinqiao Member. Therefore, it is difficult to determine the boundary between Ordovician System and Silurian System by biostratigraphic and petrostratigraphic methods. The analysis of trace elements and REE of silty mudstone in the well DD1 shows that Th, Sr, Y, Th/U, Ce_n, Eu_n, δCe and δEu curves at the middle part of silty mudstone (1288.3m) present sharp abnormal peaks with significantly higher ΣREE content of silty mudstone (1288.3m) than its upper and lower silty mudstone and great right slope of the REE distribution curve. By comparing the elemental geochemistry characteristics of the well DD1 with other O-S boundary in the Yangtze Region, it can be concluded that there exists Guanyinqiao Member of Wufeng Formation in the well DD1, and the thickness of this marker layer is also less than 1 m. The geochemical redox discriminant graph shows that the sedimentary water medium mainly was in weak oxidation environment from the Wufeng period to the early Longmaxi period, and its oxidation was the strongest in the Guanyinqiao period. According to the analysis of sedimentary markers, the evolution process of the sedimentary facies at the O-S transition is as follows:subtidal → intertidal → supertidal → intertidal; in the Guanyinqiao period, the water was the shallowest, and there existed supertidal deposits.