The Sequence Stratigraphy and Genesis of Phosphorites of Doushantuo Formation at Baiguoyuan,Yichang,Hubei
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摘要: 新元古代陡山沱期是扬子地台重要的成磷期,鄂西白果园磷块岩就形成于当时海侵不断扩大的过程中。白果园陡山沱组位于黄陵背斜西北部,磷矿赋存于陡山沱组下部。对白果园剖面进行层序地层学研究,将南沱冰期后沉积的陡山沱组和灯影组划为一个二级层序。含有磷块岩的陡山沱组,代表了南沱冰期后强烈的海侵活动的特点,将其划分为六个三级层序,磷块岩形成于第一个三级层序内。在层序1的海侵体系域上部与高位体系域下部所反映的潮下带,砂屑磷块岩品位很高。冰期时的物理风化和陡山沱期化学风化作用将地表含磷物输入海洋,海侵引发的上升洋流携带富磷海水涌入浅水区,为磷块岩在层序1内的形成提供了物源与动力条件。初次海侵对磷矿形成十分有利,高品位的含磷层对应于最大海侵阶段。采用地球化学方法对含磷岩系进一步研究后表明,生物有机质对白果园磷块岩的形成有重要作用。大冰期过后陡山沱期新的古海洋生态系统开始构建,使生物生产率迅速恢复,为生物有机质的成矿作用提供了机遇。其有机碳含量介于盆地型磷块岩与台地型磷块岩之间,显示了泥页岩—碳酸盐岩型含磷岩系Corg含量的“过渡性”特点。由于其含磷岩系形成的环境为半局限性滞留洼地,白果园磷块岩是弱还原—弱氧化环境的沉积产物。与邻区贵州瓮安的台地型磷块岩进行对比,台地型磷块岩由于更为氧化的环境而具有低Corg和高P2O5特点。通过与早寒武世云南昆阳磷矿的对比,发现能量较高并具有丰富藻类的潮下带为磷块岩形成的优势区带,但白果园不具备昆阳磷矿的藻滩成磷环境。所以,白果园磷块岩的形成是海侵作用和生物有机质的参与等因素共同作用的结果。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 secondorder sequence.The Doushantuo Formation contained phosphorites represented a strong transgressive activity after the Nantuo ice age.It could be divided into six thirdorder 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 shalecarbonatite 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 semilimitations, the phosphorites of Baiguoyuan were resulted from weak reductionweak oxidation environment. Compared with platform phosphorite of Weng'an,platform phosphorite has the characteristic of low organic carbon content and high P2O5 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.
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Key words:
- Sinian Doushantuo Formation
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