【摘要】 <正> 中国黄土高原第四纪黄土堆积的地层划分,以及与第三纪的界限问题,长期以来存在着不同的意见.在地层划分问题上许多人认为黄土地层剖面中第一层褐土型古土壤以上的黄土层为晚更新世堆积;并且认为黄土堆积最早始于距今100万年以前.多年来,我们在黄土高原进行了大量的野外调查研究,对比了不同地点的黄土地层剖面,对黄土高原的黄土地层问题提出了一些新的认识.本文利用三个典型地层剖面的综合研究资料(图1),对黄土高原的第四纪黄土堆积地层划分及其下限问题加以论述.这三个剖面分布于第四纪地质史上有不同发展特点的地貌区内.平凉附近的剖面位更多还原

【Abstract】 Since the last several years, large amounts of investigation and research work have been carried out on the Loess Plateau in China. In this paper, based on the Lithological and stratigraphical study on three selected loess profiles, the author discussed the problems of stratigraphic subdivision of loess deposits, and the lower boundary of the Quaternary system as well. The three profiles selected from three regions with different geomorphological features are: 1) The profile near the Pingliang County, Gansu Province, is on the "loess yuan" (the socalled "yuan" is a table-like elevated land surroued by deep-cutting valleys) in front of the Liupanshan mntains; 2) The Xifeng Profile is in the vicinity of the Xifeng County, Gansu Province within one of the largest "loess yuan", it belongs to the central part of an ancient erosinal basin; 3) Tne profile near the wuqi County, Shanxi Province, is in a "loess mao" area (the socalled "mao" is loess hills with deep-cutting valleys) which was a lacustrine basin in the Early Pleistocene. Studies of lithological characteristics and paleomagnetic measurements for the three profiles permit us to consider that: the so-called "Malan Loess" (that is the first loess layer distributed in the Loess Plateau) belongs to the Late Pleistocene. And the begining of the deposition of all the loess on the Plateau in northwestern China were found to be before the Reunion event of Matuyama eph, about 2.4 m. y. B. P.The thicknesses of the three profiles are 173, 170 and 195 m, respectively.According to the analysis of the grain size of loess components in the three profiles, some conclusions can be made as follows:As the amount of silt fraction in loess deposits in the three profiles varies very slightly, the amount of the fraction of fine-sand grains (0.05 mm) in the loess deposits, will be taken as the determinative factor of the farction of lithological characteristics of the loess.Several sedimentary cyclothems, which represent the variation of the compo-sing granular fraction in loess, are found in all these profiles.Analysis of the mineral constituents of loess deposits in the three profiles indicates that the heavy minerals contained in the loess are of about twenty six kinds, occasionally only ten kinds or more of heavy minerals are found in some of the loess beds, in which, epidote, hornblende, hematite, limonite, garnet and zircon are the most abundant. The light minerals found mainly include quartz, feldspar and calcite. Other kinds of minerals are rareAll the three profiles are enriched in hematite and limonite, especially the profiles of Pingliang and Wuqi. This is probably because the pre-Quaternary formations in these regions contain higher quantities of hematite and limonite.The amount of main mineral constituents varies with depth in different geological times. Therefore, the vertical variation of main mineral constituents in the loess profiles may be taken as the indicator of the stratigraphic subdivis-ion of the loess series.The amounts of quartz and feldspar in loess deposits of the three profiles increase with depth from upper to lower, and larger amounts of these two minerals are found in the Tertiary beds rather than in the Quaternary loess in these area.Among the relative amounts of calcite in the three profiles, the Pingliang profile appears to be the highest, the Xifeng profile comes second, and the Wuqi profile the third. This indicats that the loess deposits in different areas were formed in different palaeogeographic environments.The study of the palaeomagnetic stratigraphy of the three profiles was also carried out. A magnetic anomaly, which is considered to be the Laschamp excursion of the Brunhes epoch, was revealed by the mesurement of the samples collected from the upper part of the first loess bed (that is the so-called "Malan loess") in the top of the Pingliang profile. Another remarkable polarity anomaly, has been recorded in the second loess bed underlying the first fossil soil layer, which is considered to be the Blake excursion. At t更多还原