鄂西地区上二叠乐平统大隆组硅质岩成因及有机质富集机理

晚二叠世是古生代环境、生物演化的关键时期,发育广泛的富有机质沉积,并成为南方重要的烃源岩层位。为了解该时期的富有机质堆积机理,我们选择了鄂西台内盆地晚二叠世大隆组富有机质硅质岩(TOC平均值为5.8%,峰值为18%)作为研究对象。硅质岩中常量元素Al/Fe均值为0.79,Mn/Ti均值为0.21,显示为大陆边缘环境下的生物成因,即非热水成因。∑REE值与Al2O3含量有着良好的正相关关系,表明当时陆源物质的输入对硅质岩的形成有很大影响。微量元素U/Th、V/(V+Ni)均说明硅质岩沉积环境为缺氧水体环境。通过对草莓状黄铁矿粒径的统计表明,大多数(64%～89%)的草莓状黄铁矿粒径小于5μm,反映缺氧甚至硫化的环境。而且,U、V、Mo的富集以及Ni、Cu元素的亏损说明水体的缺氧主要是由于海水分层,水体循环受阻造成的。TOC与氧化还原参数的变化步调基本一致,两者有着较好的正相关关系,而与生产力指标Ba/Al相关性很差,说明有机质富集主要是由水体的缺氧而不是初级生产力控制的。

Origin of bedded chert and organic matter accumulation in the Dalong Formation of Upper Permian in western Hubei Province.

Widespread organic-rich sediments,regarded as the important source rocks,were deposited in South China during the Late Permian,a key period marked by fundamental environmental changes and biotic turnovers.This study focus on the organic-rich siliceous rocks in the Dalong Formation deposited in an intrashelf basin in western Hubei,to explore the mechanisms of silica precipitation and organic accumulation using pyrite morphology and multiple geochemical proxies.The relative high Al/Fe ratios(average 0.79)and low Mn/Ti ratios(average 0.21)favor a biogenic-dominant origin for silica precipitation,instead of a hydrothermal origin,in the intrashelf basin.The ∑REEs are positively correlated with Al2O3 contents,suggesting that influx of terrigenous material played an important role on the formation of the chert.Relatively high ratios of U/Th(>1.25) and V/(V+Ni)(>0.60) in the chert suggest an anoxic water column in which silica precipitated.Measurement of framboid pyrite size reveals that most of the pyrite framboids(64%～89%) are less than 5μm in diameter,suggesting an extremely anoxic,even euxinic environment.Moreover,the enrichments of U,V and Mo and depletion of Ni and Cu suggest that oceanic anoxia resulted from oceanic stratification due to sluggish water circulation.No correlation between TOC contents and Ba/Al values suggests organic accumulation was mainly controlled by the redox condition of water column,rather than by primary productivity.