【作者】 王建明；

【导师】 王建力；

【作者基本信息】 西南大学 ， 第四纪地质学， 2009， 硕士

【摘要】 中国第四纪黄土是在干旱的气候条件下由风力作用带到其它地区堆积的沉积物质,其分布的广泛性、沉积时代的连续性及对全球环境变化事件与古气候演变的敏感性,对于重建第四纪古环境研究中具有重大的意义,还直接关系到农业生产与工程建设等社会经济的发展。我国在黄土研究领域取得了令世人瞩目的成果,开创了全球第四纪环境研究的新天地,使得中国黄土成为全球第四纪环境演化的三大信息载体之一。但作为我国第四纪黄土沉积物中的一种独特类型,长江下游的下蜀黄土、长江三角洲地区以及长江三峡地区的风尘沉积及其次生产物的研究水平与北方黄土的相比,还有较大差距。这与前者空间分布较分散的客观事实有重要关系,而影响其研究水平最重要的因素是我国南方湿热气候条件对原始风尘沉积物巨大的改造作用。本文的研究地点是长江三峡的巫山地区。在长江南北两岸的山地与江边确定了2个代表性剖面,即望天坪1号点（WTP1）与巫山师范学校3号点（WS3）,共采集第四纪沉积物样品41个,基岩样品1个。对第四纪沉积物样品进行自然风干并研磨,分别制样后进行粒度与磁化率测试等实验。粒度是沉积物最基本的物理特征,粒度分析是分析沉积环境、搬运过程和搬运机制的重要手段之一。粒度的基本特征不仅可以作为沉积物分类命名的定量依据,而且是追溯沉积物沉积环境的重要证据。粒度实验与分析表明,巫山第四纪沉积物主要由5～50um粉砂组成,WS3与WTP1剖面平均占65.37%、53.44%,为众数粒组;＞50um砂粒平均含量分别占15.54%、0.79%;＜5um粘粒平均含量分别占19.09%、45.78%。从砂粒（＞50μm）、粉砂（5～50μm）和粘土（＜50μm）三大组分来看,WS3与WTP1第四纪沉积物全剖面均以粉砂粒级为主,平均含量分别占65.37%、53.44%,均为粘质粉砂土类型,与北方黄土呈较明显延续关系。WS3剖面的MΦ值介于4.89～5.64Φ之间,平均5.29Φ,与频率分布曲线主峰偏向粗端的情况一致;偏度SKΦ主要介于0.76～0.78之间,平均0.78,全剖面粒度标准离差δΦ变化于3.23～4.19之间,平均3.66,说明整个剖面粒度组成极不均一,也反映物质来源可能较复杂。WTP1剖面的MΦ基本介于6.11～7.230之间,平均6.88Φ,偏度SKΦ主要为-2.75～3.14,半均-0.81,呈负偏态,略有粗尾。标准离差δΦ变化于1.43～1.54之间,平均1.46,粒度组成非常均一,显示了单一的搬运营力。其Kd值也证实了这一点,Kd值主要变化于0.37～1.37之间,平均0.65,波动不大。频率分布曲线为单峰,粒度参数具有较高的一致性特征。综上分析可知巫山第四纪沉积物具有较明显的风成特征。质量磁化率（x）是样品磁性强弱的反映。WS3第四纪沉积物全剖面的质量磁化率平均值为9.184(10^-8m³·kg^-1),介于7.310～10.857之间,波动较大。WTP1第四纪沉积物全剖面的质量磁化率波动于0.965～2.349之间,相对较小,平均值为1.687(10^-8m³·kg^-1),不到WS3的1/5,比较反常,可能与二者磁性增加机制差异较大有关。总体而言,两剖面的磁化率均偏低,可能表明两地的风化成土作用并不十分明显。频率磁化率(x_fd)可反映超顺磁性矿物的存在与否。WS3剖面频率磁化率介于-0.427～0.524之间,平均为0.064;WTP1剖面频率磁化率在-2.507～0.376之间波动,平均-0.386。二者x_fd＜2.0%,表明两剖面均基本没有SP颗粒;前者x_fd偏正而后者偏负表明前者物质组成相对粗,粒度实验结果也证明了这一点。“磁滞”现象是铁磁性物质区别于抗磁性和顺磁性物质的重要特征之一。从磁滞回线形态看,WS3剖面样品整体上显得扁而胖,WTP1剖面样品则高而瘦,表明前者的主要磁性矿物为赤铁矿,后者的主要磁性矿物为磁铁矿。对WTP1剖面基岩样品（WTP1-br）的磁滞测试发现,“回线”几乎成一条直线,反映其不含磁性物质,基本否定了巫山第四纪沉积物主要为风化成土的观点,与粒度反映出的风成特征较一致。综上分析,巫山第四纪沉积物的粒度与磁化率特征均表明其与北方黄土在组成上有相似之处,有一定继承性。但山地样品的磁化率相对低山的异常偏低现象,佰得进一步研究。更多还原

【Abstract】 Chinese Quaternary loess is accumulated sedimentary materials blown from one area to other areas by the wind.Owing to its wide distribution,the continuity of deposition and its sensitivity to global environmental changes and paleo-climate evolution,an investigation into it will be of great significance for the study of paleo-environment,as well as for the agricultural production and engineering projects,among others.So far,the study of loess in China has been fruitful,which enables China to be one of the top three media of recording evolutions of the Quaternary environments.Besides,it pioneered a new path of research in this field.However,as a unique type of Chinese quaternary sediment,the loess in the lower reaches of Yangtze River,and the dust deposition with its by-products in the Delta Region of the river and in the Three Gorges region are studied much less and less fruitful when compared with the accomplishments made in other regions.The disparity can be ascribed to the following reasons:one is that the loess in the former regions is much denser than the latter regions.Another factor is that the humid climate condition in the Southern regions has a great impact on the transformation of the primitive dust deposition.The studied region is located in the Wushan regions of the Three Gorges of Yangtze River.And the 41 samples of Quaternary Deposit as well as 1 sample of bedrock are taken from WTP1 and WS3 sites in Wushan Chongqing province.The samples are then dried naturally and grinded for Grain-size test and magnetism test. Grain-size is the fundamental physical features of sediment.And grain-size analysis is an important means to study the sediment environment,and the process and mechanism of shipping as well.The basic feature of grain-size can not only count as the quantitative basis for the classification and naming of sediments,but also as a clue to trace what the environment was like in their formation.We arrive at the following conclusion based on relevant experiment and analysis of the results:According to the grain-size analyses of 41 samples of the Quaternary Deposit from WTP1 and WS3 sites in Wushan Chongqing province,based on minute comparison of the grain-size features with typical loess,the genesis of Wushan Quaternary Deposit is discussed.The results show that:Wushan Quaternary Deposit mainly consists of 5～50um particles.The average percentage of the grain-size distribution accounts for 53.44%,65.37%respectively in WTP1 and WS3 sites.And 5～50um particle group is the "dominant group" of Wushan Quaternary Deposit.The average content of＞50μm particles and＜5um particles account for 0.79%,15.54% and 45.78%,19.09%respectively.The grain-size frequency distribution presents single-peak mode.And the particle parameters are almostly coincident.It can be concluded that Wushan Quaternary Deposit is also the atmospheric dust sediment based on grain-size elementary analyses.The quality of magnetic susceptibility（x） is a reflection of the strength of magnetism.The average magnetic susceptibility of WS3 Quaternary Deposit of the whole profile accounts for the 9.184(10^-8m³·kg^-1),ranging between 7.310～10.857.The magnetic susceptibility of WTP1 Quaternary Deposit of the whole profile fluctuates between 0.965～2.349, with a mean of 1.687(10^-8m³·kg^-1),less than one-fifth of WS3.The contrast may be caused by the two different magnetic mechanism.On the whole,the magnetic susceptibility of the two profiles is low,reflecting that the role of weathering is not obvious.Frequency magnetic susceptibility(x_fd) is used to prove the existence of the super-paramagnetic minerals.According to the result,WS3 profiles frequency magnetic susceptibility is between -0.427～0.524,with an average of 0.064,and all sections x_fd＜2.0%,indicates that the fundamental particles do not exist in the profile.Frequency magnetic susceptibility at WTP1 profile is fluctuated between -2.507～0.376,with an average of -0.386,the whole profile being x_fd＜2.0%,indicates that there are basically no SP particles in the profiles,the discovery that the grain-size WS3 profiles are more coarse than the granularity WTP1 profile well illustrates the point."Hysteresis" phenomenon is one of the important characteristics which distinguish anti-ferromagnetic materials from magnetic materials.In view of the form of the hysteresis loop,the WS3 profile Samples are higher and thinner than the WTP1 ones,indicating that the former magnetic mineral is mainly composed of hematite,the latter magnetite.The hysteresis test of the bedrock sample through WTP1 profile（WTP1-br） indicates that "back line" has the shape of almost a straight line,reflecting that it basically does not contain magnetic substances.This result is in good consistency with the characteristics of Grain size.Consequently,the hypothesis that the Wushan Quaternary Deposit was formed by wind transport is disconfirmed.As is discussed above,the characteristics of grain-size and magnetic susceptibility of Wushan Quaternary Deposit have shown little effect of weathering,which exhibits certain similarities with the composition of the North.But it appears to be abnormal that the magnetic susceptibility of the samples taken on mountain is high while that of the samples taken below mountain is low,which is worth further study.更多还原