摘要
成都平原地区的红土剖面一般由3部分构成,自上而下分别为成都粘土层、网纹红土层、河流相砂砾石层。文章对位于成都双流的胜利红土剖面样品的粒度、石英颗粒表面形态以及稀土元素分布模式进行了系统分析,并将研究结果与甘孜地区的典型黄土样品进行了对比。结果表明,红土剖面中的成都粘土、网纹红土层是以具有风成特性的细颗粒物质为主,并且其粒度分布及粒度参数特征与甘孜地区典型风成黄土样品非常相似,而与该剖面中下伏的河流相样品有很大不同。成都粘土及网纹红土样品的石英颗粒与甘孜黄土及北方黄土一样,大多数都呈不规则的棱角状、次棱角状。红土堆积的稀土元素分布模式与甘孜黄土也非常相似。所有这些证据都表明成都平原红土剖面中的成都粘土及网纹红土层属于风成成因。本次分析结果结合以前的研究表明,第四纪中期整个长江流域的环境状况发生了很大变化,主要表现为冰期时植被覆盖率的大幅度降低,而青藏高原在此时期的快速隆升可能是形成该区环境变化的直接原因。
The Red Earth Formation is an important pedo-stratigraphical unit in Southern China. This formation is among the best Quaternary climatic records in the tropical and subtropical China. The red earth sections in the Chengdu Plain mostly consist of the uppermost Chengdu Clay, the middle vermiculated red soil （VRS） and the underlying fluvial deposits.
In this study, grain size, morphology of quartz grains and rare-earth element （ REE ） geochemistry were analyzed for the samples taken from the Shengli section at Shuangliu County. The results were compared with the typical Quaternary loess from Ganzi County of west Sichuan Province. Grain-size distribution was determined with a Malvern Mastersizer 2000 particle analyzer with a precision less than 1% . Scanning electronic microscopy （SEM） analyses and REE measurements were carried out in the Analysis and Measurement Center, China University of Geosciences, Wuhan.
The grain size analytical results indicate that the Chengdu Clay and VRS in the Shengli section consist of fine uniform particles. The distributions are bimodal, with the dominant grain-size fraction of 5 - 50μm, and the 〉 63μm fraction occupies only a small amount. These are essentially similar to the grain-size distributions of the Ganzi loess samples, but differ from the muhimodal patterns of the fluvial deposits underlying the VRS in this section. The grain size parameters of the Chengdu Clay and VRS are also similar to those of the Ganzi loess samples. The mean grain-sizes of the Chengdu Clay and VRS are about 7.26 （6.8μm） and 7.0φ （7.8μm） respectively, close to those of the Ganzi loess samples, but differ from the fluvial deposits underlying the VRS. The mean square deviation values of the Chengdu Clay, VRS and Ganzi loess are similar, which are smaller than the fluvial samples, indicating a much better sorting and relatively stable depositional dynamics. The Chengdu Clay, VRS and Ganzi loess samples are positively skewed, but the fluvial samples underlying the VRS are negatively skewed. The kurtosis values of the Chengdu Clay and VRS samples are close to the Ganzi loess, which are bigger than those of the fluvial samples. SEM observations show that a majority of the quartz grains from the Chengdu Clay and VRS have irregular and angular shapes and many are characterized by sharp edges. These grain-morphology features are highly similar to those of the Quaternary eolian deposits in West Sichuan Province and Northern China, which are considered as characteristic of eolian dust deposits. The REE patterns of the Chengdu Clay, VRS and Ganzi loess samples also show extreme homogeneity, suggesting the same origin of these deposits.
The results in this paper combined with the previous studies indicate that the climatic condition along the Yangtze River had a change in the Middle Quaternary, marked by the decreased vegetation cover in the glacial periods. This climatic change may be the effect of the Tibet Plateau uplift.
出处
《第四纪研究》
CAS
CSCD
北大核心
2007年第2期286-294,共9页
Quaternary Sciences
基金
中国地质调查局地调项目(批准号:1212010541401)
国家自然科学基金项目(批准号:40472088)资助
关键词
成都平原
红土堆积
成因
Chengdu Plain, Red Earth Formation, origin
