Radiolarian siliceous rocks and palaeoenvironmental reconstruction for the Upper Ordovician Wufeng Formation in the middle-upper Yangtze area

Abundant radiolarian fossils were discovered in the siliceous rocks from the Wufeng Formation in Zhuazhuayan,Leibo,Sichuan.This discovery indicates that the silica-rich sea water once entered into the Yangtze craton in response to the second phase of marine trangressions during the Wufengian of the Late Ordovician,resulting in the deposition of radiolarian siliceous sediments.It also implys that there once existed the channels connected the Bayan Har Sea with the Xikang-Yunnan old land and the central Sichuan palaeo-uplift on the western margin of the Yangtze craton.The palaeoenvironments are reconstructed in this study for the middle-upper Yangtze area during the Wufengian of the Late Ordovician on the basis of depositional sequences,regional correlation,lithologic associations and stratigraphic contacts in the Wufeng Formation in the Zhuazhuayan section,Leibo,Sichuan.