Sedimentary Environment Pattern of Yangtz Plate in Wufeng Age of Late Ordovician

Strata of Wufeng age are widely distributed in Yangtze Plate,and our sedimental types can be recognized from east to west, i.e., Yushan, Yuqia.Yichang and Hanyuan type. Yichang type is characterized by graptolite black shales Wufeng Formation Two different opinions about the sedimental environment of Wufeng Formation have been arguing in recent years: i.e., shallow sea deposits or deep sea deposits. Based on the paleontology, petrogrphic characteristics,especialy the analyses of the rare earth elements, the authors concluded that the graptolite black shales of Wufeng Formation were deposited in a epicontinemal shallow restricted basin. Yushan type consists mainly of micritic limestone, bird-eye limestone and shales bearing abundant algae, corals, brachiopods, trilobites. This type constructs some isolated carbonate reef platformes staning on the southeast margin of the restricted basin. Between the restricted basin and carbonate reef platformes, there is a depression trough filled by shallow water flysch deposits called Yuqian Formation or Changwu Formation. Hanyuan type developed in the west part of Yangtze Plate and consists of carbonate rocks and black shales, representing another type of reef carbonate platform and ramp.