The Study On Jiumengchong Formation Of Cambrian In Guizhou

Based on the studies of Nangao section of Danzhai, Dongdao section of Jianhe, Yangtiao Section of Majiang, and associated with the previous studies, the lithologic combination and diversification characteristics, biological assemblage characteristics, carbon isotopic geochemical characteristics and geological age of the Jiumengchong Formation in Guizhou Province are discussed herein. Research shows that Jiumenchong Formation distributed mainly in the east of Wuchuan-Weng‘an-Guiding and the west of Nanming-Jianhe-Sandu, and is deposited in the transition slope facies of Guizhou. The Jiumenchong Formation is a stratigraphical unit situated between the underlying Niutitang Formation and the overlying Bianmachong Formation which was respectively demarcated by the occurrence of black organic limestone and yellow-green carbonaceous siltstone, and consisting of black carbonaceous shale intercalated with thin to medium thick limestone in its lower part, and thin to medium thick limestone with abundant trilobite Hupeidiscus in its upper part. In general, there is a trend with decreasing of the thickness of Jiumenchong Formation and gradualy increasing of the proportion of carbonaceous shale in lower part from northwest to southeast of Guizhou. The present study documents diverse fossils from Jiumenchong Formation, including trilobite, Brachipoda, bradoriids and some problem fossils, and some new fossils such as Eohadrotreta, Palaeobolus, Meishucunella and Parahipponicharion are discovered in Nangao, Jianhe and other deep water areas except common fossils trilobite Hubeidiscus orientalis and Brachipoda Lingula. The discovery of abundant fossils in the Jiumenchong Formation can not only enrich the biological composition of Jiumenchong Formation, but also provide the information to reveal biological assemblage of deep water facies in Guizhou Province. The trilobite assemblage shows that the lower limit horizon of H. orientalis in Jiumengchong Formation could extend to Yunnanaspis-Yiliangella zone of Canglangpuian Stage, upper limit horizon can extend to Drepanuroides zone ofCanglangpuian. This means strata containing H. orientalis should be lower Canglangpian. But there is a set of black shales with thin limestone in lower part of Jiumenchong Formation under the lower limit horizon of H.orientalis, and the horizon of H.orientalis in the Yangtze shallow shelf facies of Guizhou is equivalent to the upper Qiongzhusian to lower Canglangpuian. So we speculated that the deposited age of Jiumengchong Formation should be late Qiongzhusian to early Canglangpuian. The carbon isotopic characteristic showes that the lower part of Jiumenchong Formation has a significant carbon isotope positive drift, and this drift can correspond with the middle and upper part of shuijingtuo Formation in Yichang City, Hubei Province, the upper part of Niutitang Formation to the lower part of Mingxinsi Formation in Jinsha County, Guizhou Province. Combined with the biostratigraphic study indicates the Jiumenchong Formation is equivalent to middle and upper part of shuijingtuo Formation in Hubei and lower-middle part of Mingxinsi Formation in Guizhou.