被动大陆边缘页岩沉积古环境及其对有机质富集的控制: 以上扬子渝北地区下志留统龙马溪组为例^*

Passive margin shale sedimentary palaeoenvironment and their controls over organic matter enrichment: a case study of the Lower Silurian Longmaxi Formation in northern Chongqing,Upper Yangtze

摘要

目前对于被动大陆边缘沉积古环境演化的研究相对较少,特别是对上升洋流活动变化规律认识不足,导致对有机质富集主控因素认识不清。据此,以渝北地区下志留统龙马溪组为例,基于岩心/露头、矿物岩石学、有机地球化学和元素分析等资料,探讨被动大陆边缘页岩沉积古环境及其对有机质富集的控制。研究表明,区内有机质富集受保存条件、陆源输入和古生产力水平等因素控制,但不同时期主控因素存在差异: 鲁丹期(LM1-LM5沉积期),保存条件、陆源输入和古生产力水平共同控制有机质富集,TOC含量最高; 埃隆早期(LM6沉积期),盆地进入挠曲—迁移期,构造活动增强,渝北地区障壁开口打开,上升洋流活动增强,保存条件变差是造成TOC含量降低的主因; 埃隆中期(LM7沉积期),迁移作用增强,障壁开口进一步打开,上升洋流活动达到最强,保存条件变差和古生产力水平急剧升高是导致TOC含量与LM6沉积期差异不大的主因; 埃隆晚期(LM8沉积期),障壁开口向西北方向迁移,TOC含量降低,有机质富集主控因素与LM1-LM5一致; 特列奇早期(LM9沉积期),海平面降至最低,以富氧环境为主,TOC含量降至最低,保存条件是控制有机质富集的主要因素。总体而言,研究区不同时期有机质富集主控因素变化与构造活动和海平面变化引起的沉积古环境变化密切相关。

Abstract

At present,there are relatively few studies on sedimentary palaeoenvironment evolution of passive margin. In particular,due to limited understanding of the changing patterns of upwelling current activity,the main controlling factors for organic matter enrichment remain unclear. On these grounds,we investigate passive margin shale sedimentary palaeoenvironments and their controls on organic matter enrichment,based on an integrated core/outcrop,mineral petrology,organic geochemistry and elemental analysis of the Lower Silurian Longmaxi Formation in northern Chongqing. The results show that organic enrichment in the study area is controlled by preservation conditions,terrigenous input and palaeoproductivity,but the main controlling factors vary in different periods. During the Rhuddanian(LM1-LM5 depositional period),the accumulation of organic matter was controlled jointly by preservation conditions,terrigenous input and palaeoproductivity,and the TOC content was the highest. In the early Aeronian(LM6 depositional period),the basin entered the flexural-migration period of enhanced tectonic activity,the barrier in the Yubei area opened,resulting in increased upwelling current activity,and the deteriorated preservation condition was the main cause of the decrease of TOC content. During the mid Aeronian(LM7 sedimentary period),the migration effect was enhanced,with the barrier further opened and the upwelling current reaching an activity peak,and the deteriorated preservation condition and sharply increased palaeoproductivity were the main reasons for little change in TOC content compared to the LM6 deposition period. During the late Aeronian(LM8 depositional period),the barrier opening migrated to the northwest,the TOC content decreased,and the main controlling factors of organic matter enrichment were the same as those of LM1-LM5. During the early Telychian(LM9 deposition period),the sea level dropped to the lowest,the oxygen-rich environment predominated,and the TOC content decreased to the lowest. The preservation conditions were the main factors controlling the enrichment of organic matter. Overall,the changes of controlling factors of organic matter enrichment in different periods in the study area are closely related to the varying sedimentary palaeoenvironments caused by tectonic activities and sea level changes.

关键词

被动大陆边缘 / 渝北地区 / 黑色页岩 / 上升洋流 / 有机质富集主控因素

Key words

passive continental margin / northern Chongqing / black shale / upwelling current / main controls of organic matter enrichment

基金资助

*湖北省自然资源科技项目(编号: ZRZY2024KJ25)资助