Characteristics And Origin Of Early Triassic Chert In Nanlinghu Formation At The Area Of Majiashan, Chaohu

Lower Triassic Nanlinghu Formation is well exposed in Majiashan area, Northwest area of Chaohu City, Anhui Province, East China, which is worldwide famous because of occurrence of the earlist ichthyosaurs. The Nanlinghu Formation is here conformably contacted with underlying nodular limestone of the Helongshan Formation and unconformably with overlying Middle Triassic dolomitic limestone of the Zhouchongcun (Dongmaanshan) Formation.Three types of siliceous sedimentary from Nanlinghu Formation were found in the studied area:chert nodules between layers、chert lenses and bedded chert within layer. By comparing chemical analysis results of chert of the Nanlinghu Formation with the same analysis results of chert of Southern California, and in reference of chemical analyses of chert of Permian Qixia Formation, the formation environment and siliceous source of chert was discussed.Further research suggests that the studied area in late Early Triassic lay in the subduction zone between Yangtze plate and the North China plate, which is very similar to Southern California now. Based on the previous studies and evidences found in the research, it is concluded that main siliceous source of the Nanlinghu Formation in the studied area was provided by volcanic activities.