| 兰州盆地渐新世沉积物岩石磁学性质探究 |
| |
| 引用本文： | 张鹏,敖红,安芷生,王千锁.兰州盆地渐新世沉积物岩石磁学性质探究[J].地球物理学报,2015,58(7):2445-2459. |
| |
| 作者姓名： | 张鹏 敖红 安芷生 王千锁 |
| |
| 作者单位： | 1. 中国科学院地球环境研究所, 黄土与第四纪地质国家重点实验室, 西安 710061;2. 中国科学院大学, 北京 100049;3. 南京师范大学地理科学学院, 江苏省环境演变与生态建设重点实验室, 南京 210046 |
| |
| 基金项目： | 国家重点基础研究计划(2013CB956402),国家自然科学基金项目(41174057, 41290253)和黄土与第四纪地质国家重点实验室开放基金共同资助. |
| |
| 摘 要： | 我国西北地区出露大量富含哺乳动物化石的"第三纪红层",探明这些地层的岩石磁学特征对进一步开展磁性地层、环境磁学和古气候学研究具有重要意义.本文对青藏高原东北缘兰州盆地渐新世地层进行了详细的岩石磁学研究,分别确定了咸水河组下部砂岩和泥岩中磁性矿物含量、种类及其在加热过程中的转变;并结合X射线衍射,漫反射光谱和粒度分析等非磁学手段,揭示了磁性矿物种类与沉积环境的关系.结果表明,剖面底部和顶部的砂岩为河道沉积,磁性矿物含量较低,主要为高矫顽力的赤铁矿和针铁矿;中部泥岩为湖相沉积,指示了一个相对稳定的静水沉积环境,其磁性矿物含量比砂岩要高,不仅含有高矫顽力的赤铁矿和针铁矿,也含有低矫顽力的磁铁矿.在岩石磁学和环境磁学结果的基础上,结合野外考察和古生物证据,我们推测兰州盆地在晚渐新世为半湿润半干旱的气候条件,这为赤铁矿的生成提供了有利条件,导致盆地中富含赤铁矿的红色泥岩广泛发育.
|
| 关 键 词： | 岩石磁学 环境磁学 兰州盆地 第三纪红层 |
| 收稿时间： | 2015-02-18 |
Rock magnetism properties of Oligocene sediments in the Lanzhou Baisn |
| |
| ZHANG Peng,AO Hong,AN Zhi-Sheng,WANG Qian-Suo.Rock magnetism properties of Oligocene sediments in the Lanzhou Baisn[J].Chinese Journal of Geophysics,2015,58(7):2445-2459. |
| |
| Authors: | ZHANG Peng AO Hong AN Zhi-Sheng WANG Qian-Suo |
| |
| Institution: | 1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Key Laboratory of Environmental Evolution and Ecological Construct of Jiangsu Province, College of Geography Science, Nanjing Normal University, Nanjing 210046, China |
| |
| Abstract: | The Lanzhou Basin, located in the convergence zone of monsoon region of southeast China and arid region of northwest China, has the potential to reveal the onset time of aridification in Central Asia and the evolutionary history of East Asia monsoon. Its well developed mammalian fossil-bearing Cenozoic sequences provide valuable material to understand the chronology of the strata and the environmental evolution of the NE Tibetan Plateau. Here we present detailed rock magnetic results from the Duitinggou section in the Lanzhou Basin to explore the relationship between the magnetic minerals and sedimentary environment.Environmental magnetic parameters, e.g., low field magnetic susceptibility (χlf), percentage of frequency dependent magnetic susceptibility (χfd%), susceptibility of the anhysteretic remanent (χARM), saturation isothermal remanent magnetization (SIRM) and high coercivity (hard) remanent magnetization (HIRM*) were measured to identify the content of the magnetic minerals and the variation of these magnetic minerals in different lithologies. Rock magnetic measurements (e.g., temperature dependence of magnetic susceptibility (χ-T), isothermal remanent magnetization (IRM) and hysteresis loops) were carried out to explore the type of the magnetic minerals. Moreover, non-magnetic methods (e.g., X-ray diffraction (XRD) and diffuse reflection spectrum (DRS)) were employed to discriminate the magnetic minerals. The grain-size analysis helps to distinguish different sedimentary environment.The values of χlf, χfd%, χARM, SIRM and HIRM* of the sandstone are much lower than that of mudstone, which indicates the relatively low concentration of the total magnetic minerals in the sandstone. The χfd% is less than 5% and varies with the content of the magnetic minerals, suggesting the low concentration of the superparamagnetic particles (SP) in the section. The sharp drop of χ at 500~600 ℃ in the heating curves indicates the presence of magnetite. All samples have an increased χ during cooling after heated to 700 ℃, which may result from the neoformation of the fine-grained magnetic minerals. Furthermore, the XRD results show that chlorite provides iron source (Fe2+) or reducing agent for newly formed fine-grained ferrimagnetic minerals, which leads to increased χ. The component analyses of IRM acquisition curves suggest that both low-coercivity component and high-coercivity component contribute to the acquired IRM, in which the high-coercivity component accounts for more than 69%. Consistent with these rock magnetic results, the hysteresis loops of the mudstone samples is wasp-waisted and not closed at 800 mT, which indicates that both high and low coercivity components exist in the mudstone. But the hysteresis loops of the sandstone are thin and less wasp-waisted, indicating the dominance of high coercivity component. The second derivative spectra of DRS display a higher content of hematite and goethite in the mudstone than in the sandstone. The grain-size parameters imply a fluvial sedimentary environment for the sandstone and lacustrine facies for the mudstone.The concentration of magnetic minerals in the sandstone is low, and dominated by high-coercivity component (goethite and hematite). The magnetic minerals in the sandstone is related to a fluvial reduction condition, which are dissolved or converted to weak magnetic minerals, leading to the low concentration of magnetic minerals. The mudstone contains more magnetic minerals, including low coercivity (magnetite) and high-coercivity (hematite and goethite) component. We speculate that the mudstone is lacustrine facies according to the field investigation and grain-size analysis. The semiarid environment in the Lanzhou Basin gives rise to the formation of the hematite in the surface soil and enhances the erosion of the surface soil (with extremely weak pedogenesis), thus providing the lake with detrital deposit rich in hematite. |
| |
| Keywords: | Rock magnetism Environmental magnetism Lanzhou Basin Tertiary red beds |
| 本文献已被 CNKI 等数据库收录！ |
| 点击此处可从《地球物理学报》浏览原始摘要信息 |
| 点击此处可从《地球物理学报》下载免费的PDF全文 |
|
