Organic Geochemistry of the Early Jurassic Oil Shale from the Shuanghu Area in Northern Tibet and the Early Toarcian Oceanic Anoxic Event
Organic Geochemistry of the Early Jurassic Oil Shale from the Shuanghu Area in Northern Tibet and the Early Toarcian Oceanic Anoxic Event
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DOI：
10.1111/j.1755-6724.2005.tb00904.x
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发表时间：
2005-06
期刊：
影响因子：
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通讯作者：
Chen Lan;Yin Haisheng;Hu Rui-zhong;Zhong Hong;Zou Yan-rong
中科院分区：
文献类型：
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作者：
Chen Lan;Yin Haisheng;Hu Rui-zhong;Zhong Hong;Zou Yan-rong
摘要本文介绍了藏北北方双湖地区新的地质地球化学资料，记录了早托阿西期的海洋缺氧事件。双湖地区的地层序列主要由灰色至深色交替的油页岩、泥灰岩和泥岩组成。在双湖油页岩剖面的顶部发现了菊石层，其主要为早期Toarcian时代，大致位于Harplocearas falciferrum带内。因此，双湖油页岩地层可与广泛分布于欧洲陆表海的早托阿西期黑色页岩进行对比，后者记录了一次大洋剥蚀事件。双湖地区沉积有机质以富含有机碳的层状页岩为主，有机碳含量高，为1.8%~ 26.1%。碳同位素曲线显示干酪根的δ 13 C值（δ 13 C干酪根）在−26.22至−23.53‰ PDB之间波动，正偏移接近2.17‰，尽管明显较小，但也可能与欧洲其他早期托阿西期海洋缺氧事件（OAE）有关。有机碳/氮比值在6 ~ 43之间，C/N比值曲线与δ 13 Ckerosen值曲线一致。生物组合的特点是底栖生物的稀缺和石灰质超微化石（球石）的水华，揭示了高生物生产力的表层水和底栖动物在底层水的海洋缺氧事件的不利环境。根据有机地球化学和生物组合特征，认为碳同位素漂移是由海平面和生产力的变化引起的，黑色页岩沉积，特别是油页岩沉积与颗石的繁盛和高生产力有关。
Abstract This paper presents new geological and geochemical data from the Shuanghu area in northern Tibet, which recorded the Early Toarcian Oceanic Anoxic Event. The stratigraphic succession in the Shuanghu area consists mostly of grey to dark‐colored alternating oil shales, marls and mudstones. Ammonite beds are found at the top of the Shuanghu oil shale section, which are principally of early Toarcian age, roughly within the Harplocearas falciferrum Zone. Therefore, the oil shale strata at Shuanghu can be correlated with early Toarcian black shales distributing extensively in the European epicontinental seas that contain the records of an Oceanic Anoxic Event. Sedimentary organic matter of laminated shale anomalously rich in organic carbon across the Shuanghu area is characterized by high organic carbon contents, ranging from 1.8% to 26.1%. The carbon isotope curve displays the δ13C values of the kerogen (δ13Ckerogen) fluctuating from −26.22 to −23.53‰ PDB with a positive excursion close to 2.17‰, which, albeit significantly smaller, may also have been associated with other Early Toarcian Oceanic Anoxic Events (OAEs) in Europe. The organic atomic C/N ratios range between 6 and 43, and the curve of C/N ratios is consistent with that of the δ13Ckerosen values. The biological assemblage, characterized by scarcity of benthic organisms and bloom of calcareous nannofossils (coccoliths), reveals high biological productivity in the surface water and an unfavorable environment for the benthic fauna in the bottom water during the Oceanic Anoxic Event. On the basis of organic geochemistry and characteristics of the biological assemblage, this study suggests that the carbon‐isotope excursion is caused by the changes of sea level and productivity, and that the black shale deposition, especially oil shales, is related to the bloom and high productivity of coccoliths.