热带地理 ›› 2019, Vol. 39 ›› Issue (2): 218-228.doi: 10.13284/j.cnki.rddl.003118

• 论文 • 上一篇    下一篇

南雄盆地始新统红层沉积环境分析

谢振龙1a,刘秀铭1,2,毛学刚1,乐志军1a,彭 超1a   

  1. (1. 福建师范大学 a. 地理科学学院,湿润亚热带山地生态国家重点实验室培育基地;b. 地理研究所,福州 350007; 2. 麦考瑞大学 环境与地理系,澳大利亚 悉尼 NSW 2109)
  • 出版日期:2019-03-05 发布日期:2019-03-05
  • 通讯作者: 刘秀铭(1956—),男,教授,博士生导师,研究方向为环境磁学与全球变化,(E-mail)xliu@fjnu.edu.cn。
  • 作者简介:谢振龙(1992—),男,江西九江人,硕士研究生,研究方向为环境磁学与全球变化,(E-mail)xiezhenlong1992@qq.com;
  • 基金资助:
    国家自然科学基金项目(41772180、U1405231)

Paleoenvironment of Eocene Red Beds in the Nanxiong Basin

Xie Zhenlong1a, Liu Xiuming1,2, Mao Xuegang1, Le Zhijun1a and Peng Chao1a   

  1. (1. a. State Key Laboratory Breeding Base of Humid Subtropical Mountain Ecology, School of Geographical Sciences, Fujian Normal University; b. Institute of Geography, Fujian Normal University, Fuzhou 350007, China; 2. Department of Environment and Geography, Macquarie University, Sydney NSW 2109, Australia)
  • Online:2019-03-05 Published:2019-03-05

摘要: 选取南雄盆地始新统红层——古城村剖面作为研究对象,结合磁性矿物和地球化学元素特征分析其沉积环境。古城村剖面上部和下部为红色,中部为青灰色,颜色及其深浅均随沉积层理/层次逐渐变化,说明颜色和沉积地层层理/层次为同期形成,为地层的原生色。磁学测量的饱和等温剩磁(SIRM)和剩磁矫顽力(Bcr)结果与红度(a*)呈现良好的正对应关系,热磁曲线结果表明红层样品以赤铁矿为主,漫反射光谱(DRS)佐证了原生赤铁矿的存在且是唯一可以检测到的磁性矿物。赤铁矿指示了当时的沉积环境为高温少雨。剖面中部存在青灰色地层,该层样品的热磁测量没有检测到磁性矿物信号,只检测到顺磁性矿物信号。高温处理后的DRS测量也表明其存在顺磁性的黏土矿物。通过地球化学元素分析显示其与标准石灰岩相比差异较大,而与中国典型黄土较为相似,可能说明青灰层并非石灰岩。剖面的上下层均为以原生赤铁矿为主的红层,代表高温强氧化环境。青灰层与上下红层之间为过渡渐变,地层较为连续且无不整合现象,化学元素分析也显示该层与上下红层、上地壳和黄土都具有较高的相似性,所以该层更可能是风积物在气候转为湿润条件下经成土作用形成的钙质淀积层。古城剖面反映了始新世时期干热氧化—湿润氧化还原—干热氧化的逐渐变化过程,对于准确理解南雄盆地此阶段古环境有重要意义。

关键词: 始新统红层, 磁性矿物, 地球化学元素, 沉积环境, 古城剖面, 南雄盆地

Abstract:

Continental red beds are widely distributed in mainland China and have long been considered to be formed in inter-mountain river and lake environments, while the red strata are considered to be secondary oxidation formations in the late stage. While this is a kind of scientific explanation and logical reasoning, there is no further concrete argument. In this study, the Eocene red-bed paleogeocline section of the Nanxiong Basin was selected as the research subject, and the paleoenvironment was analyzed in combination with the characteristics of magnetic minerals and geochemistry. The upper and lower parts of the Gucheng section are red, and the middle part is cinerous. All colors and their depth change gradually with the sedimentary strata / layers, indicating that the color was formed simultaneously with the sedimentary strata / layers, and the red of the strata is the primary color. The saturation isothermal remnant magnetic (SIRM) and remnant coercivity (Bcr) results showed a strong positive correlation with redness (a*). The thermomagnetic curves demonstrated that the red layer samples were mainly hematite with incomplete antiferromagnetism. Diffuse reflectance spectroscopy (DRS) confirmed the existence of primary hematite and was the only magnetic mineral detectable. Hematite indicates that the sedimentary environment at that time should be oxidized at high temperature. In the middle of the section, there was a cinerous layer, which was previously interpreted as marine limestone by earlier researchers. However, instead of magnetic mineral signals, paramagnetic minerals were detected in the thermomagnetic measurements. DRS measurements after high temperature treatment also showed the presence of paramagnetic clay minerals. Geochemical element analysis showed that it was quite different from standard limestone, but similar to the typical loess in China. These data indicated that the limestone was not limestone. The upper and lower layers of the profile were red beds dominated by primary hematite, which represent the high temperature and strong oxidation environment on the surface. The formation is continuous and exhibits conformity. The “marine limestone” in the red beds of the two sections is not deemed to be limestone by chemical element analysis. Furthermore there was a high similarity between the cinerous layer and the red layer on both sides, upper crust and loess. The cinerous ash layer was more likely to have been formed by soil-forming processes under conditions of weathering and wetting. The ancient city section reflected the gradual change process of dry-heat oxidation-wetting redox and dry-thermal oxidation during the Eocene, which is of great significance for understanding the paleoenvironment of Nanxiong Basin.

Key words: red bed of Eocene Series, magnetic mineral, geochemical element, sedimentary environment, Gucheng section, Nanxiong Basin