Constraints on the accumulation of organic matter in Upper Ordovician-lower Silurian black shales from the Lower Yangtze region, South China
Constraints on the accumulation of organic matter in Upper Ordovician-lower Silurian black shales from the Lower Yangtze region, South China
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DOI：
10.1016/j.marpetgeo.2020.104544
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发表时间：
2020-06
影响因子：
4.2
通讯作者：
Shengchao Yang;Wenxuan Hu;S. Yao;Xiaolin Wang;Wei He;Yin Wang;Feng Zhu;Funing Sun
中科院分区：
文献类型：
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作者：
Shengchao Yang;Wenxuan Hu;S. Yao;Xiaolin Wang;Wei He;Yin Wang;Feng Zhu;Funing Sun
晚奥陶世—早志留世是一个重要的地质事件多发时期。在O/S转换时期，富有机质黑色页岩广泛沉积，成为重要的非常规能源。古生产力、碎屑输入和氧化还原条件是控制有机质聚集的三个主要因素。虽然每个因素的影响相对清楚，但这些因素的复杂影响仍然知之甚少。具体而言，这些因素的演变及其对O/S转变过程中OM积累的影响尚未得到系统的研究。本文通过笔石化石鉴定、锆石U-Pb测年和井序钻井剖面地球化学分析，探讨了下扬子地区O/S过渡黑色页岩OM成藏的制约因素。结果表明：(1)西1岩心层序受O/S边界笔石带WF2 (447.62 Ma) - LM5 （441.57 Ma）的约束，上卡天（WF2 - WF3）沉积速率减小，下鲁达尼统沉积速率增大；（ii）互补的火山输入（Katian和Rhuddanian）和上升流事件（Hirnantian）对古生产力有很强的控制作用；（iii）华南造山作用控制了碎屑输入和氧化还原条件；(4)下扬子地区的OM聚集经历了3个O/S转换阶段。在第1阶段（WF2-WF3），强烈的火山作用促进了生产力，而在氧气环境中不利于有机质的保存。在第2阶段（WF4-LM3），上升流和火山活动带来了高产能，为OM的富集提供了最佳配置。在第3阶段（LM4及以后），由于地面稀释和不利的保存条件，保存了罕见的OM。最终，在上升流、火山作用和氧化还原条件等控制因素的有效协调下，WF4-LM3成为突出的富有机质生物带。
The Late Ordovician to early Silurian (O/S) was an important period in which many significant geological events occurred. During the O/S transition, organic-rich black shales were widely deposited and became important unconventional energy sources. Paleoproductivity, detrital input, and redox conditions are the three major factors controlling organic matter (OM) accumulation. Although the effect of each factor is relatively clear, the complex influences of these factors remain poorly understood. Specifically, the evolution of these factors and the corresponding influence on the accumulation of OM during the O/S transition has not yet been systematically investigated. This paper investigates the constraints on OM accumulation in O/S transition black shales from the Lower Yangtze region, based on graptolite fossil identification, zircon U–Pb dating, and geochemical analyses of a well sequenced drilling profile. The results show that (i) the SY-1 core sequence was well constrained from graptolite zones WF2 (447.62 Ma) to LM5 (441.57 Ma) across the O/S boundary, with decreasing sedimentation rates (SRs) in the upper Katian (WF2 to WF3) and increasing SRs in the lower Rhuddanian; (ii) complementary volcanic inputs (Katian and Rhuddanian) and upwelling events (Hirnantian) exerted strong control over paleoproductivity; (iii) orogenic processes in South China controlled detrital inputs and redox conditions; (iv) OM accumulation in the Lower Yangtze area across the O/S transition occurred in three stages. In stage 1 (WF2–WF3), strong volcanism boosted productivity, whereas OM preservation was not favored in the oxic environment. In stage 2 (WF4–LM3), upwelling and volcanism resulted in high productivity and provided the optimal configurations for OM enrichment. In stage 3 (LM4 and later), rare OM was preserved due to terrestrial dilution and unfavorable preservation conditions. Eventually, WF4–LM3 became the prominent OM-rich biozones due to the effective coordination of the controlling factors, namely upwelling, volcanism, and redox conditions.