| 引用本文: | 刘秀铭，吉金平，章涛，陈家胜，Ali Rashid Tabrez，Siroos Jafari.2021.湖南古丈奥陶纪红石林地层特征与石灰岩沉积环境分析[J].地球环境学报,12(1):1-18 |
| LIU Xiuming, JI Jinping, ZHANG Tao, CHEN Jiasheng, Ali Rashid Tabrez, Siroos Jafari.2021.Analysis on sedimentary environment of the Red Stone Forest strata in Guzhang, Hunan Province[J].Journal of Earth Environment,12(1):1-18 |
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| 湖南古丈奥陶纪红石林地层特征与石灰岩沉积环境分析 |
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刘秀铭，吉金平，章涛，陈家胜，Ali Rashid Tabrez，Siroos Jafari
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1.福建师范大学 地理科学学院，福建省湿润亚热带山地生态重点实验室——省部共建国家重点实验室培育基
地，福州 35007
2. Department of Earth and Environmental Sciences, Macquarie University, Sydney NSW 2109, Australia
3.临沂艺术学校，临沂 276000
4. COMSATS University of Islamabad, Islamic Republic of Pakistan
5. Soil Science Department, Agricultural Sciences and Natural Resources, University of Khuzestan, Molla Sani-Khouzestan, Iran
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| 摘要: |
| 湘西古丈红石林国家地质公园出露着奥陶系牯牛潭组红色石灰岩地层，含角石化石，被认为是海相红层。野外观察地层红色深浅与其层理/层次变化关系判断为近于同期形成，属原生色。水生化石存在红层中，前者反映水下缺氧条件，而红层指示着氧化环境，两种指示尖锐对立，意味着可能至少一方存在认识偏差。采样分析结果表明，赤铁矿和针铁矿是其中可以被检测到的磁性矿物；地层碳酸钙含量为20%—24%，粒度以粉砂为主显示双峰与风积物高度相似而与海相红层差别大；因此，红石林地层可能不是海相化学沉积，而是强氧化条件下形成的以粉砂为主的红色钙质胶结碎屑沉积物。红石林地层稀土元素标准配分模式与风积物类似，与海相红层差别甚大；红层中存在同一层位红色和灰色共存的网纹构造，指示着同期氧化与还原条件共存状态；红层中发育着网纹状瘤状钙结核既顺层也垂直层面同时发展，这些现象在海相环境均难以形成，似乎只有地表成土环境才能得以发生。表明红石林地层可能是风积物为物源，在地表成土过程形成的沉积序列。红石林沉积环境可能与澳洲干旱区现代临时性湖积碳酸盐环境较为接近：短暂积水不仅留下动物脚印、海相化石和泥裂，同时将半干旱条件下碳酸盐胶结粉砂沉积物变成现代泥灰岩-石灰岩，实质上是钙质胶结粉砂岩，在地表氧化成为红色。因此，古丈红石林地层可能是一套记录着奥陶纪地球冷暖变化的海陆边缘相风积粉砂粒级为主的钙质胶结碎屑沉积物地层。 |
| 关键词: 古丈红石林 石灰岩 赤铁矿 奥陶纪 粒度 稀土元素 沉积环境 |
| DOI：10.7515/JEE202030 |
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| 基金项目:国家自然科学基金项目（41772180，41210002） |
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| Analysis on sedimentary environment of the Red Stone Forest strata in Guzhang, Hunan Province |
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LIU Xiuming, JI Jinping, ZHANG Tao, CHEN Jiasheng, Ali Rashid Tabrez, Siroos Jafari
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1. State Key Laboratory of Subtropical Mountain Ecology (Funded by Ministry of Science and Technology and Fujian Province), School of Geographical Sciences, Fujian Normal University, Fuzhou 350007, China
2. Department of Earth and Environmental Sciences, Macquarie University, Sydney NSW 2109, Australia
3. Linyi Art School, Linyi 276000, China
4. COMSATS University of Islamabad, Islamic Republic of Pakistan
5. Soil Science Department, Agricultural Sciences and Natural Resources, University of Khuzestan, Molla Sani-Khouzestan, Iran
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| Abstract: |
| Background, aim, and scope There is Ordovician reddish carbonate deposit located in the northwest of Guzhang County of Hunan Province, which contains marine fossils (such as Sinoceras chinense) is weathered to Karst feature of landscape known as Red Stone Forest (RSF). This reddish Ordovician carbonate is generally interpreted as marine limestone formed in shallow water environment, someone calls such limestone as marine red beds. However, their red color indicating that these sediments were forming oxidation environment, while aquatic fossils and limestone both reflect the hypoxia conditions under water. So these two redox conditions of implication are sharp opposition, implying likely at least one recognizing is misunderstood. The aim of this study is to provide an example that some of red limestone is a sediment of calcareous cemented silt—sand particles, which is mainly happening on/or close the Earth surface. Materials and methods Observation carefully in field for relation of color change with bedding. If reddish color gradually varying bedding, such red color is original, or it forms during similar period of bedding formation. Environmental Magnetism technique are used to identify type of magnetic minerals; The calcium carbonate contents are analyzed; Particle distribution, pattern of Rare Earth Elements (REE) are carried out. Results Hematite can be clearly detected, or reddish color is contributed by hematite; Goethite may present as well but less certain, they are respectively ferric oxide and hydroxide. The red beds change their red color gradually from light red to dark red with bedding/horizons, indicating that the reddish color is the original color (similar period of formation as bedding/horizon) rather than secondary. The calcium carbonate content of the RSF is between 20%—24%, and the particle size is mainly silt, which shows that the bimodal peaks are similar to the eolian deposits, but different from the marine red layer. Therefore, the strata of the Red Stone Forest is unlikely a limestone of marine chemical deposition, but actually a clastic sediment of red calcium cemented silt formed under strong oxidation conditions. The standard distribution model of rare earth elements in the RSF is similar to the eolian deposits, but different from the marine red beds. Discussion In red beds of the RSF, red and gray reticulated structures coexist (like red and white reticulated laterite), indicating the coexistence of different oxidation and reduction conditions at the same time. In addition, there are reticulated tuberculous calcium nodules that develop simultaneously along the bedding and on the vertical plane. These phenomena are difficult to explain by the marine environment, which seems to occur only in the surface soil-forming oxidation environment. It indicates that the RSF strata may be a sedimentary sequence formed by the eolian silt in the process of land surface soil formation. Its sedimentary environment may be related to the modern temporary lacustrine carbonate environment in arid areas is relatively close to Australia: a brief water not only leave a marine fossils, animal footprints and mud cracks, at the same time, the carbonate cemented silt sediment under semi-arid conditions was transformed into modern marl-limestone, which is essentially calcareous cemented silt sediment and oxidized to red on the surface. Conclusions Therefore, the Guzhang RSF strata may be a set of calcareous cementitious clastic sediments mainly composed of eolian silt grains in the marginal facies of sea and land, which is of great scientific value and significance to the study of the history of the earth’s climate change. Recommendations and perspectives As red limestone is often reported, geologists is worth paying more attention on its sedimentary environment. |
| Key words: red beds red limestone hematite Ordovician particle size Rare Earth Elements (REE) sedimentation environment |
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