沉积环境对页岩中有机质富集的约束——以蜀南地区五峰组—龙马溪组为例

Constraints of Sedimentary Environment on Organic Matter Accumulation in Shale:A case study of the Wufeng-Longmaxi Formations in the southern Sichuan Basin

记录了蜀南地区五峰组—龙马溪组岩石学和地球化学资料,以恢复古环境和探讨有机质富集机理。研究表明,蜀南地区五峰组—龙马溪组主要发育硅质页岩、粉砂质页岩、黏土质页岩和含介壳灰质泥岩/泥质灰岩。五峰组平均TOC为3.36%,龙马溪组下部TOC最高,平均为3.99%,中上部TOC偏低,平均为1.22%。古生产力指示元素(Ba_(生物)、Cu/Al、Ni/Al、Si_(过量))表明,在奥陶—志留转折期的大部分时间(除赫南特期),蜀南海域表层水中具有较高的初级生产力。陆源输入指示元素(Al、Zr、Zr/Al)表明,五峰期陆源输入中等且变化较大,龙马溪早期为持续低值,但在中后沉积期逐渐增多。氧化还原敏感微量元素(Mo/Al、U/Al、V/Al)及Mo-TOC的相关性表明,五峰期海洋发育强受限的缺氧—静水环境,龙马溪早期水体保持缺氧—静水条件,但受限程度减弱,中后期海洋演化为氧化环境。晚奥陶世,蜀南海域发育有利的缺氧—静水环境和高古生产力,但较多的陆源输入一定程度上削弱了有机质富集。早志留世,海平面升降引起多个环境参数协同变化,使连续沉积的页岩地层中有机质富集程度存在显著差异。

The petrological and geochemical data of the Wufeng-Longmaxi Formations in the southern Sichuan Basin were recorded to restore the paleoenvironment and explore the mechanism of organic matter accumulation.The study shows that the Wufeng-Longmaxi Formations in the southern Sichuan Basin mainly developed siliceous shale,silty shale,argillaceous shale,and shell-bearing lime mudstone/muddy limestone.The average total organic carbon(TOC)of the Wufeng Formation is 3.36%,the TOC of the lower Longmaxi Formation is the highest,with an average of 3.99%,and the TOC of the middle and upper parts is low,with an average of 1.22%.Paleoproductivity proxies(Ba_(bio),Cu/Al,Ni/Al,and Si_(excess))indicate that during most of the Ordovician-Silurian period(except for the Hirnan-tian period),the surface water in the southern Sichuan Basin had a high primary productivity.The terrigenous influx proxies(Al,Zr,and Zr/Al)indicate that the terrigenous influx in the Wufeng period was moderate and varied great-ly.The early Longmaxi period had a continuous low value,but it gradually increased in the middle and late periods.The correlation between redox-sensitive trace elements(Mo/Al,U/Al,and V/Al)and Mo-TOC shows that,during the Wufeng period,the ocean developed a strong restricted anoxic-euxinic environment.The water mass in the early stage of the Longmaxi maintained anoxic-euxinic conditions,the level of restriction was weakened,and the ocean evolved into an oxic environment in the middle and late periods.In the Late Ordovician,a favorable anoxic-euxinic environment and high productivity developed in the ocean,but the large amount of terrigenous influx weakened the organic matter accumulation.In the Early Silurian,sea level fluctuations caused multiple environmental parameters to coordinately change,which resulted in significant differences in the concentration of organic matter.