Occurrence characteristics and depositional model of graptolites from the black shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation
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摘要: 在总结前人研究的基础上，综合岩石矿物、地球化学等分析测试资料对宜昌地区五峰组-龙马溪组黑色页岩层系中笔石的赋存状态及其沉积环境进行了深入研究。结果显示：五峰组-龙马溪组黑色页岩中笔石的赋存状态存在显著差异，笔石的丰度由下至上呈现由高到低的变化趋势，硅质炭质页岩中的笔石丰度最高，以杂乱堆积为主，部分可见弱定向排列特征；灰绿色泥岩与黑色硅质页岩中笔石丰度低，以碎片为主，且硅质页岩中的笔石碎片细小。笔石的分异度在纵向上也呈现出规律性变化，赫南特阶笔石分异度最低，埃隆阶与凯迪阶对应的笔石分异度高。综合研究表明，不同笔石赋存状态所对应的有机碳质量分数w(TOC)、围岩性质、氧化还原条件、古海洋环境与水动力条件均有所差异：高笔石丰度的页岩往往对应着较高的w(TOC)；石英质量分数在40%~60%的黑色页岩中笔石最为发育；笔石在弱氧化和还原环境中均能保存，但还原环境更有利于笔石的大规模富集；纵向上笔石丰度与分异度变化均受到了古环境的显著影响, 赫南特冰期造成了笔石的大量死亡和分异度的显著降低；笔石的大量混杂堆积显示悬浮沉积是主要的沉积作用方式，而定向排列的笔石应是深水底流作用的结果。基于上述页岩中笔石赋存状态差异及其沉积地质指示，深入分析了不同笔石赋存状态的沉积环境并建立了五峰组-龙马溪组沉积期的笔石堆积模式。Abstract: Based on the pervious researches, the occurrence characteristics and depositional environment of graptolites from the black shale of the upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the Yichang area are analyzed in detail through integrating mineral and geochemical data. The results show that the occurrence characteristics of graptolites from the black shale in the Wufeng and Longamxi Formations are different, with graptolites abundance decreasing gradually from the lower part to the upper part. Carbonaceous shale always has a lot of graptolites, and most of graptolites are disordered except some of them are stored regularly. The graptolites abundance in greyish-green shale and black siliceous shale is low and most of graptolites are composed of fragments smaller in the black siliceous shale. Graptolite diversification in the black shale also shows regular variations, and the graptolite diversification in the Hirnantian is much lower than that in the Aeronian and Katian. Comprehensive analysis indicates different graptolite occurrence characteristics usually have various TOC, surrounding rock properties, oxidation-deoxidation conditions, palaeoenvironment and hydrodynamic conditions. The shale with rich graptolites abundance often has high value of TOC, with their quartz content usually ranging from 40% to 60%. Graptolites can be deposited both in weak oxidation and deoxidation conditions, but the deoxidation environment is much more favorable for graptolites sedimentation and preservation. The vertical variations in graptolite diversification and graptolites abundance are obviously influenced by palaeoenvironment, and the lower graptolite diversification and massive disappearance of graptolite in the Hirnantian are caused by glaciation and sea-level changes. Disordered graptolites are the results of suspended sedimentation, while the graptolites with directional alignment indicate reformation by ocean current. Finally, based on the different graptolite occurrence characteristics and their sedimentary geological significance, their sedimentary environment is discussed in detail and the depositional model of graptolites in the Wufeng and Longamxi Formations is proposed.
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图 2 宜昌地区五峰组-龙马溪组黑色页岩层系笔石丰度特征
a.硅质炭质页岩，笔石数量大于80，面笔率约70%，龙马溪组，鄂宜页2井；b.硅质炭质页岩，笔石数量大于70，面笔率约42%，五峰组，鄂宜页3井；c.硅质炭质页岩，见少量笔石，面笔率小于5%，五峰组，鄂宜页3井；d.硅质页岩，仅见2个笔石碎片，龙马溪组，鄂宜页3井；e.灰黑色含炭页岩，笔石数量13，面笔率约7%，部分笔石破碎，龙马溪组，鄂宜页3井；f.灰绿色泥页岩，未见笔石，龙马溪组，鄂宜页2井
Figure 2. Typical photos of graptolites abundance in different lithological rocks in the Wufeng and Longmaxi Formations, Yichang area
图 3 笔石定向性特征及五峰组-龙马溪组黑色页岩笔石排列特征照片
a.笔石无定向排列模型，笔石体混杂堆积；b.笔石弱定向排列模型，存在一个主体笔石方向；c.笔石定向排列模型，基本上所有笔石体均指向同一方向；d.硅质炭质页岩，笔石无定向排列，五峰组，鄂宜页3井；e.硅质炭质页岩，笔石弱定向排列，五峰组，鄂宜页2井；f.硅质炭质页岩，笔石定向排列，五峰组，鄂宜页2井
Figure 3. Features of graptolites directionality and typical photos from the Wufeng and Longmaxi Formations, Yichang area
图 5 宜昌地区鄂宜页3井五峰组-龙马溪组黑色岩系笔石丰度与有机碳变化特征
Figure 5. The variation of graptolites abundance and TOC in black shale of the Wufeng and Longmaxi Formations from Well EYY3, Yichang area
图 6 宜昌地区五峰组-龙马溪组黑色岩系笔石丰度与有机碳含量相关性特征
Figure 6. The relationship between graptolites abundance and TOC of the black shale in the Wufeng and Longmaxi Formations, Yichang area
图 7 宜昌地区五峰组-龙马溪组黑色页岩笔石丰度与围岩矿物组分的相关性
Figure 7. The relationship between graptolites abundance and mineral content of black shale in the Wufeng and Longmaxi Formations in Yichang area
图 8 五峰组-龙马溪组黑色页岩笔石丰度与氧化还原环境的关系
Figure 8. The relationship between graptolites abundance and oxidation-deoxidation environment of black shale in the Wufeng and Longmaxi Formations
图 9 宜昌地区鄂宜页2井五峰组赫南特阶笔石丰度变化
a.大灭绝期大量笔石化石与赫南特贝共存，笔石体数量超过90个，赫南特阶Normalograptus extraordinarius带；b.大灭绝后期页岩笔石丰度较低，赫南特阶Normalograptus persculptus带顶部
Figure 9. The variation of graptolites abundance in black shale of the Hirnantian in the Wufeng Formation from Well EYY2, Yichang area
图 10 鄂宜页3井五峰组受深水底流改造的的黑色页岩镜下特征
a.黑色硅质炭质页岩，可见陆源碎屑及硅质海绵骨针定向排列，五峰组；b.黑色含粉砂质炭质页岩，可见由砂质碎屑与富有机质黏土层组成水平纹层
Figure 10. Arthroscopic characteristic of the black shale in the Wufeng Formation from Well EYY3, Yichang area
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