Organic petrology and geochemistry of mudstones from the lower Shahejie Formation, in the Tanggu area of eastern China: evidence for the presence of an ancient saline lake

IF 1.3 4区 地球科学 Q2 GEOLOGY
L. Li, G. Yao, M. Cai, Y. Liu
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引用次数: 2

Abstract

Mudstones in the lower Shahejie Formation, in the Tanggu area of the Huanghua Depression, have been found to contain analcime and ankerite. Hydrothermal sedimentation has been invoked to explain the origin of these two minerals, raising the question of whether hydrothermal activity occurred at a sufficient scale to significantly raise the salinity of the depositional environment. We applied a suite of organic petrological and geochemical methods to directly address this question. Maceral composition, kerogen type, and the distribution of n-alkanes, hopanes, and steranes indicate that the organic matter contained in these mudstones and dolomitic mudstones is mainly derived from algae and bacteria. The dominant acritarch genera, C31R/C30 hopane ratio, gammacerane index, Pr/Ph ratio, and the relationship between Pr/n-C17 and Ph/n-C18 suggest that the mudstones and dolomitic mudstones were deposited in an anoxic, saline lacustrine environment. Tmax, biomarker maturity indices, the thermal alteration index (TAI) and acritarch alteration index (AAI), and vitrinite reflectance all indicate that the organic matter is at an immature to early mature stage. The estimated maximum paleotemperature is close to the present-day burial temperature, and much lower than the homogenization temperature of the analcime veins. Combined with the absence of unresolved complex mixtures on the n-alkane pattern, this suggests that hydrothermal activity had a negligible impact on the salinity and alkalinity of the depositional lake.
中国东部塘沽地区沙河街组下段泥岩的有机岩石学和地球化学:古代盐湖存在的证据
黄骅坳陷塘沽地区下沙街组泥岩中发现含有方沸石和铁白云石。热液沉积被用来解释这两种矿物的起源,这就提出了一个问题,即热液活动的发生规模是否足以显著提高沉积环境的盐度。我们应用了一套有机岩石学和地球化学方法来直接解决这个问题。宏观组成、干酪根类型以及正构烷烃、hopanes和steranes的分布表明,这些泥岩和白云质泥岩中所含的有机物主要来源于藻类和细菌。优势的顶生岩属,C31R/C30 hopane比值,伽玛组分指数,Pr/Ph比值,以及Pr/n-C17和Ph/n-C18之间的关系表明,泥岩和白云质泥岩沉积在缺氧的盐湖环境中。Tmax、生物标志物成熟度指数、热蚀变指数(TAI)和近热蚀变指标(AAI)以及镜质组反射率均表明有机质处于未成熟至早熟阶段。估计的最高古温度接近现今的埋藏温度,远低于方沸石矿脉的均质温度。再加上正构烷烃模式中不存在未解决的复杂混合物,这表明热液活动对沉积湖的盐度和碱度的影响可以忽略不计。
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来源期刊
Geologica Acta
Geologica Acta 地学-地质学
CiteScore
2.50
自引率
6.70%
发文量
13
审稿时长
>12 weeks
期刊介绍: - Relevant conceptual developments in any area of the Earth Sciences. - Studies presenting regional synthesis. - Thematic issues or monographic volumes presenting the results from one or more research groups. - Short papers reflecting interesting results or works in progress. - Contributions and results from Research Projects, Workshops, Symposiums, Congresses and any relevant scientific activity related to Earth Sciences. - Geologica Acta aims to stimulate rapid diffusion of results and efficient exchange of ideas between the widespread communities of Earth Science researchers (with special emphasis on Latinamerica, the Caribbean, Europe, the Mediterranean
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