晋东南王宁R红土剖面的磁性研究及其形成年代和环境

山西省榆社县王宁剖面的古地磁测试结果表明，Ｒ红土始自高斯正极性带顶部，向上跨越包括奥杜威极性亚带在内的松山负极性带下半部。这一结果揭示了老Ｒ红土的存在，对于确定Ｒ红土的层位和沉积年代提供了重要依据。Ｒ红土磁化率值的变化与土壤化程度呈正相关性，反映了形成期内气候波动的特征。在它所跨越的１Ｍａ间，Ｒ红土记录了９次明显的气候波动。这种由古土壤层和Ｒ红土层更迭及磁化率峰谷交替表现出来的气候波动韵律与天文韵律中０．１Ｍａ周期相近。且红土的磁化率各向异性度很小，线理、面理同等发育，粘滞剩磁很强，以及阻挡温度谱等主要磁学性质都与原生黄土系列相近，意味着二者成因的相似性。

STUDY OF MAGNETISM ON WANGNING RED LOAM SECTIONIN YUSHE BASIN, SE SHANXI PROVINCE: ITS AGE ANDSEDIMENTARY ENVIRONMENT

The Red Loam is distributed in the Zhuozhang River valley in southeastern Shanxi Province. Neogene and Quaternary fruvial and lacustine sediments and earthyaccumulation are preserved in the basin with a thickness of more than 500m. Theunconformity of relation between Red Loam and fluvio-lacustine deposits of YusheGroup can be observed in different sections. It can be also seen at the rim of thebasin that Red Loam, containing debris of bedrock, overlays directly on the bedrockof Triassic and gradually dissappeared not far away. Red Loam is a set of red siltydeposition with homogeneous blocky texture, containing several layers of paleosols,overlain by Lishi Loess and Malan Loess. Red Loam differs from loess with itsdeeper colour, more argillized and containing ferro-manganoan nodules which do notappear in loess.It has been reported by the previous study that the polarity of remanent magnetization was normal for Red Loam and concluded that Red Loam was depositedduring Brunhes Chron. The authors have noted in the field work that Red Loamin different outcrops were not deposited in the same period. Based on the deposition thickness, bands of paleosol and the relation with the overlying and underlyinglayers, the Red Loam in the basin can be divided into old Red Loam and new RedLoam. Between them there is a sedimentation interrupt. The Wangning sectionstudied in this paper belongs to the old Red Loam. It is 23.9m in thickness, tiltlyoverlain by Malan Loess and Lishi Loess. All the earthy strata, totally 31.4m thick,overlays unconformably on the medium-fine sand layer of the middle part of UpperPliocene Zhangcun Formation.Sampling has been done two times. One hundred and three horizons have beencollected. The remanent magnetization measuements show that the polarity of allloess samples are normal and undoubtedly the loess was deposited during Brunhes. Tworeversed polarity zones and two normal zones were recorded in the Red Loam. Thereversed polality zones corresponded to Matuyama and the two normal zones wereseparately assigned to Olduvai and the top of Gauss. From this, the average sedimentation rate of Red Loam in Wangning section could be estimated as 0.23m/14 aand its time span is about one million years. The erosion unconformity on the topof Red Loam in this section could be calculated as 1.5Ma later, which just concideswith the period of the end of the fluvial-lacustrine sedimentation of Yushe Group.The common background of these two geological incidents was that the basin waschanging from subsidence to uplifting, resulting in erosion strengthened and the interruption of sedimentation sequences generally occured in this basin. The accumulation of Red Loam in Wangning section started at about 2.5Ma. This was an important geological incident in this area and could be considered as an indication ofthe altration from warm and wet to cold and dry. This conclusion coincides withthe climatic history of the area determined by Palynologic study, and also coincideswith the period of the begining of loess deposition in the Loess Plateau.The magnetic susceptibility and the pedogenetic level are of possitive correlation, and the rhythm of megnetic susceptibility of Red Loam are comparable withthe cycle of astronomical rhythm during 1ast 100 000 years. This suggests that thepaleosol-parent material sequence in Red Loam and its corresponding peak-valley sequence of magnetic susceptibility have recorded information of climatic during Pleistocene. By the comparision of the magnetic polarity sequence and magnetic susceptibility curves of different sections in combination with the strata sequence and comparison of paleosols, it is possible to early out climatic stratigraphic comparison onlower Pleistocene in a larger scale in Loess Plateau. It could also provide beneficial information to understand the environmental changes during Early Pleistocene.The minimum axes of magnetic susceptibility ellipsoid of Red Loam clusteredin the vertical direction, but not as much concentrated as underwater sediments did.There was no dominant direction in the