The Sequence Stratigraphy and Genesis of Phosphorites of Doushantuo Formation at Baiguoyuan,Yichang,Hubei

Key words: 
• Sinian Doushantuo Formation

Abstract: Neoproterozoic Doushantuo Period was very important epoch of phosphorite formation in Yangtze Platform.And phosphorite of Baiguoyuan in the western Hubei was formed by the transgression process' expanding. The Doushantuo Formation of Baiguoyuan was located at the northwestern part of the Huangling anticline,and phosphorite was deposited in the lower part of the Doushantuo Formation.With undertaking the sequence stratigraphic study on the Baiguoyuan section,the Doushantuo Formation and the Dengying Formation which were formed after the Nantuo ice age could be grouped into one secondorder sequence.The Doushantuo Formation contained phosphorites represented a strong transgressive activity after the Nantuo ice age.It could be divided into six thirdorder sequences,and the phosphorite was formed in the first sequence, within which the late transgressive system tract and the early highstand system tract corresponded to subtidal zone,and arenite phosphorite had very high grade.The physical weathering during glacial epoch and chemical weathering during the Doushantuo Period produced the phosphoric material that was input into ocean.The phosphoric seawater was transported to shallow water areas with upwelling caused by transgression, providing the source and power conditions for phosphorite's deposition in the first sequence.The initial transgression was very favour of phosphorite's formation,high grade phosphoria formation corresponds to the maximum transgression.With the application of geochemistry,studying on phosphatic series, the results indicates that organic matter play an important part in the formation of phosphorite.After the great glaciation period, new ecosystem of Doushantuo Period was established,the biological productivity recovered rapidly, providing opportunity for biomineralization.The shalecarbonatite phosphatic series' organic carbon content is between basin phosphorite and platform phosphorite's content, intergrade is characteristic of its.Because the environment was retention basin of semilimitations, the phosphorites of Baiguoyuan were resulted from weak reductionweak oxidation environment. Compared with platform phosphorite of Weng'an,platform phosphorite has the characteristic of low organic carbon content and high P2O5 content as a result of more oxidative environment.Compared with Kunyang phosphorite deposits of early Cambrian located in Yunnan, we could conclude that subtidal zone was favorable area for phosphogenesis,which had high energy and abundant algaes.But phosphorite in Baiguoyuan didn't have the environment of algal beach which belonged to phosphorite deposits in Kunyang.So the phosphorite in Baiguoyuan generated from transgression,biomineralization and so on.