灵台黄土-红粘土序列的磁性地层及粒度记录

MAGNETOSTRATIGRAPHY AND GRAIN SIZE RECORD OF A THICK RED CLAY-LOESS SEQUENCE AT LINGTAI, THE CHINESE LOESS PLATEAU

甘肃灵台县任家坡黄土－红粘土剖面的磁性地层学研究表明；中国北方连续的风成堆积可下推到705MdB．P。第三纪红粘土的粒度组成从上到下变化很小，与黄土－古土壤序列的粒度大幅度变化形成强烈的反差，意味着晚第三纪时期的气候总体上要比第四纪时期稳定。在705～2．6MaB．P．的红粘土沉积中，可分出的110～115个土壤B层与钙质结核层的组合，初步表明在这个时期有约110～115次气候旋回。

In the Chinese Loess Plateau, complete loess deposits are generally underlain bya set of reddish clay-silt deposits which are informally named as the red clayformation. Paleomagnetic measurements of different sections in varying details allindicate that the loess-red clay contact is located around the Gauss / Matuyamapaleomagnetic reversal, i.e. at about 2.6 Ma B.P. Detailed analyses of grain size andgeochemistry of red clay samples suggest that the red clay may be also of eolianorigin like the loess. To date, the magnetostrahgraphy of four red clay sections hasbeen established. They are situated respectively in Xifeng, Baoji, Xi'an and Jiaxian.Most of the magnetostratigraphic results suggest that the red clay was deposited duringthe Pliocene. But the sections are relatively thin, ranging form 30m to 65m inthickness. So the older red clay deposits remain to be sought. Recently, we found anew loess-red clay section in which the red clay has a thickness of about 130m,representing the thickest one so far found in the Loess Plateau. In this paper, thegrain size record and magnetosrtatigraphy of this section are reported.The new section is located at Lingtai(107° 30' 33' E, 35° 00' 75' N)in the middlePart of the Loess Plateau. The loess-red clay deposit at Lingtai is about 305m thick,of which the upper 175m deposit is the Pleistocene loess. In the loess deposits, eachof the paleosol horizons identified in the Baoji section can be readily traced in thefield, indicating the completeness of the loess at Lingtai. The red clay deposits belowthe loess are comprised essentially by reddish soil horizons and carbonate nodulehorizons. In the field, we recognized about 1 10-- 1 15 nodule horizons. The color ofthe soil horizons in the red clay is much redder than that of the paleosols within thePleistocene loess. Their thickness ranges from about 30cm to 200cm. In most cases,the differentiation of soil horizons is indistinct. In other words, the A and C horizonsfor the soils are unrecognizable in the field. This is one of the major differencesbetween the red clay and the overlying loess. So most of the soils in the red claycan be designated as a B(Bt or Bw) horizon. In .a pedogenetic point of view, theentire red clay sequence can be regarded as a thick soil complex. ms obviouslyresults from the slow deposition of 'parent materials'(atmospheric dust) during thedevelopment of the soils.In the Lingai section, we collected 625 fully orientated samples at an interval of15-25cm from the entire red clay sequence. In the loess sequence, paleomageticsamples were also taken from horizons where paleomagnetic reversals are anticipatedto occur according to previous remanence measurements in other loess sections.Remanence data are obtained alter alternative demagnetization, using at 20 or 25mT.In the loess deposits, we obtained a reversal sequence almost the same as that inother loess sections. In the red clay sequence, remanence data obtained suggest thatthe. eolian red clay at Lingtai began to accumulate at about 7.05MaB.P.This study extends eolian deposits in north-central China back to 7.05MaB.P. Asa red clay sequence thicker than 70m in the Loess Plateau is first found, a problemthus raised is whether or not the Lingtai section represents the oldest wind-blowndeposits in the Plateau during the Late Cenozoic. In the field, we observed that thelower part of the Lingtai red clay section is much less weathered than the upper andmiddle parts, as indicated by a less reddish color and less clay skins. This implies adrier climate during the latest Miocene than that during the Pliocene. Studies of theeolian sediments in Northwest Pacific Ocean have shown that aridification innorthwestem China was much earner than the age of 7.05MaB.P. So it is possible forthe Loess Plateau to receive dust deposition in an earner stage and it should be keptin mind in future researches to seek for older wind-blown deposits.The grain size record composed of 8 850 samples exhibits that in the entire redclay sequence, the particle size of all the samples does not show sign