MAGNETOSTRATIGRAPHY AND GRAIN SIZE RECORD OF A THICK RED CLAY-LOESS SEQUENCE AT LINGTAI, THE CHINESE LOESS PLATEAU
灵台黄土-红粘土序列的磁性地层及粒度记录

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