92 / 2022-01-24 16:53:36

上扬子地区五峰组-龙马溪页岩的沉积环境和有机质富集机理

Paleoenvironment,Shale,Upper Yangtze region,Geochemistry,Organic matter accumulation

Abstract: Through macro and micro lithology identification, X-ray diffraction analysis, major and trace element analysis, the petrological and geochemical characteristics of the Wufeng-Longmaxi Formations in Upper Yangtze region were analyzed to reconstruct the sedimentary environment and explore the mechanism of organic matter enrichment. Research shows that the Wufeng-Longmaxi Formations mainly developed four lithofacies: siliceous shale, silty shale, argillaceous shale, and shell-bearing lime mudstone/muddy limestone. The average TOC content of Wufeng Formation is 2.86%. The lower part of the Longmaxi Formation has the highest TOC content, with an average of 4.22%, and the middle and upper TOC content is lower, with an average of 1.76%. Inorganic geochemical characteristics indicate that the terrigenous influx in the Wufeng period was moderate, and the strong restricted anoxic-euxinic conditions were developed; the terrigenous influx in the early Longmaxi period was low, and gradually increased in the middle and late, and the sedimentary environment also changed from anoxic-euxinic to oxic. During the Ordovician-Silurian (except for Hirnantian), the basin had high primary productivity in general. Based on the research results of petrology and geochemistry, build models of organic matter accumulation. In the Late Ordovician, volcanic eruptions brought a lot of nutrients and caused the prosperity of primary productivity, and the severely restricted basin gradually became anoxic, both of which were conducive to the formation and preservation of organic matter. However, regression caused an increase in terrigenous influx, which diluted the organic matter content. During the Rhuddanian period, the melting of glaciers triggered continuous transgressions, and the deepening of the water body resulted in anoxic condition. The basin was far away from the provenance, leading to the reduction of terrigenous influx. Upwelling and volcanic events made surface water eutrophication again, and lower organisms flourished. Low terrigenous influx, high primary productivity, and anoxic-euxinic environment all contribute to the accumulation of organic matter in the lower Longmaxi Formation. At the end of Rhuddanian, regression occurred again, terrigenous influx increased, and the ocean was oxidized, which were the main reasons for the low organic matter content in the middle and upper Longmaxi Formation.



摘  要：通过宏微观岩性鉴定、全岩X衍射定量分析和主微量元素测试，分析了上扬子地区五峰组-龙马溪组岩石学和地球化学特征，以重建当时的沉积环境和探讨有机质富集机理。研究表明，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/泥质灰岩四类岩相。五峰组TOC平均含量2.86%。龙马溪组下部TOC含量最高，平均4.22%，中上部TOC含量偏低，平均1.76%。无机地球化学特征表明，五峰期陆源输入中等，海盆发育强受限的缺氧-静水条件；龙马溪早期陆源输入较低，在中后期逐渐增多，沉积水体也由缺氧-静水转变为含氧条件。奥陶-志留转折期（除赫南特期），表层水总体具有较高的初级生产力。基于岩石学和地球化学研究成果，构建有机质积累模式。晚奥陶世，火山喷发带来大量营养物质引起初级生产力的繁盛，强受限程度的海盆逐渐缺氧，两者利于有机质形成和保存。但海退造成陆源输入增多，一定程度上稀释了有机质含量。早志留世鲁丹期，冰川消融引发持续海侵，水体加深产生缺氧条件，海盆远离物源区，陆源输入量降低。上升流和火山事件使表层水再次富营养化，低等生物借此勃发。低陆源输入量、高初级生产力、缺氧-静水环境共同促成了龙马溪组下部有机质的富集。鲁丹末期海平面再次下降，陆源输入增多，水体氧化，是龙马溪组中上部有机质含量较低的主要原因。