海相优质页岩的形成环境及发育模式——以中扬子地区秭归新滩剖面五峰—龙马溪组页岩为例

运用地球化学和岩石学方法,从古盐度、古气候、古生产力、海域封闭性以及氧化还原条件等方面,分析中扬子地区秭归新滩剖面上奥陶统五峰组—下志留统龙马溪组海相优质页岩的形成环境,并探讨了优质页岩发育的控制因素及模式。结果表明:中扬子地区五峰—龙马溪组优质页岩以硅质页岩为主,主要发育在五峰组至埃隆阶底部;优质页岩主要形成于温暖潮湿、低盐度—正常盐度、弱封闭—半封闭的缺氧环境中,并且具有较高的古生产力;海相优质页岩的发育是构造活动、海平面升降以及陆源—生物硅质供应等多因素综合作用的结果,而在埃隆期之后,随着扬子板块的持续向西北推进,海域面积不断萎缩,生物硅质供应降低,陆源碎屑供应迅速增加,优质页岩不再发育。最后,总结了海相优质页岩的发育模式。

Forming environment and development model of high-quality marine shale: A case of the Wufeng-Longmaxi Formations shale in Zigui-Xintan section in the Middle Yangtze region

This paper analyzes the forming environment of high-quality marine shale in the Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Zigui-Xitan section in the Middle Yangtze region from the aspects of paleo-salinity,palaeoclimate,paleo-productivity,sea area closure and REDOX conditions by using geochemical and petrologic methods,and discusses the controlling factors and development model of high-quality marine shale. The results show that: The high-quality shale of the Wufeng-Longmaxi Formations in the Middle Yangtze region is mainly siliceous shale,which is mainly developed from the Wufeng Formation to the bottom of Aeronian;High-quality shale is mainly formed in the anoxic environment with warm and humid palaeoclimate,low salinity to normal salinity and weakly enclosed to semi-enclosed conditions,and it has a high paleo-productivity. The development of marine high-quality shale is a comprehensive result of multiple factors including tectonic activities,sea-level change and terrigenous-biological siliceous supply,and After Aeronian,the Yangtze plate continuously moved northwestward and sea area reduced,causing a rapid decrease of autogenous siliceous materials and increase of terrigenous clasts. Consequently,high-quality marine shale no longer existed. Finally,the development model of high-quality marine shale is summarized.