黔北地区奥陶纪—志留纪之交黑色页岩地球化学特征及意义——以贵州道真道页1井为例

The geochemical characteristics and significance of the Ordovician—Silurian black shale in northern Guizhou——A case study of the Well Daoye-1 in Daozhen County

奥陶纪—志留纪之交是华南被动大陆边缘盆地向前陆盆地演化的重要转折时期,形成广泛分布的五峰组—龙马溪组黑色页岩。为进一步探讨该套黑色页岩的形成背景,以黔北地区道页1井为例,开展沉积序列和地球化学研究。研究结果表明:五峰组及龙马溪组底部主要为硅质碳质页岩,夹多层斑脱岩,U/Th为0.2~2.90,V/Cr为1.18~14.34,Ni/Co为2.31~11.59,TOC为0.68%~5.91%,平均3.76%,稀土配分曲线为平坦型,δEu以弱正异常为主、个别为弱负异常;龙马溪上部及新滩组主要由泥岩及钙质泥岩组成,U/Th为0.16~0.23,V/Cr为0.88~1.79,Ni/Co为1.77~3.91,TOC为0.1%~0.84%,δEu以明显负异常为主,表现出低斜率右倾型稀土配分曲线。以上特征表明五峰组及龙马溪组底部形成于间歇性缺氧的还原环境,龙马溪组上部及新滩组为富氧环境。结合斑脱岩的分布特征综合分析认为,五峰组及龙马溪组底部页岩是汇聚背景下前陆盆地早期沉积的产物,强烈火山作用给大陆边缘海带来营养元素,刺激初级生产力的增加,造成水体迅速缺氧,可能是造成同期海底缺氧的主要诱因。

The Ordovician—Silurian boundary was an important transitional period in the evolution from passive continental margin basin to foreland basin in South China,and the Wufeng—Longmaxi black shale was widely distributed.sedimentary sequence and geochemical study were carried out for the Well Daoye-1 as an example in this study.The results show that the Wufeng Formation and the bottom of the Longmaxi Formation mainly consist of siliceous carbonaceous shale and bentonite,with the U/Th ratio is 0.2~2.90,the V/Cr value is 1.18~14.34,the Ni/Co is 2.31~11.59,and the TOC is 0.68%~5.91%.The REE patterns curve is flat.TheδEu showed weak positive anomalies and individual weak negative anomalies.The upper part of the Longmaxi Formation and Xintan Formation mainly consist of mudstone and calcareous mudstone,with the U/Th ratio is 0.16~0.23,the V/Cr is 0.88~1.79,the Ni/Co is 1.77~3.91,the TOC is 0.1%~0.84%.TheδEu showed obviously negative anomaly,and the REE patterns curve is dip to right.Such features indicate that the Wufeng Formation and the bottom of the Longmaxi Formation formed in the reductive environment with intermittent hypoxia,the upper part of the Longmaxi Formation and the Xintan Formation mainly formed in the oxygen-enriched environment.Based on the comprehensive analysis,the distribution characteristics of bentonite,which indicated that the shale of the Wufeng Formation and at the bottom of the Longmaxi Formation formed at the early period of the foreland basin in convergence background,strong volcanism brings nutrients to the mainland marginal sea,stimulates the increase of primary production,and causes rapid hypoxia in deep water,which may be the main cause of hypoxia in the bottom of the ocean during the same period.