泾川晚第三纪红粘土的磁性地层及其与灵台剖面的对比

本文对甘肃泾川的一套325m厚的黄土-红粘土序列进行了沉积学和磁性地层学研 究。泾川剖面红粘土序列的粘度特征表明，其与上覆黄土一样同为风成成因。古地磁测量结果确定该剖面的底界年龄约为8．0MaB．P．。根据野外观察，可以将泾川剖面红粘土序列从顶到底划分为6套地层。与泾川剖面相比，灵台红粘土序列保存了上部的5套地层。泾川和灵台两个红粘土剖面的地层对比表明，中国北方晚中新世以来的红粘土沉积是基本连续的。

COMPLETENESS AND CONTINUITY OF THE LATE TERTIARY RED CLAY SEQUENCE IN NORTHERN CHINA:EVIDENCE FROM THE CORRELATION OF MAGNETOSTRATIGRAPHY AND PEDOSTRATIGRAPHY BETWEEN JINGCHUAN AND LINGTAI

A 325-m-thick red clay-loess sequence was discovered recently at Jingchuan(35°17′ 30″N, 107°22′ 05″E), which is located in the middle part of the Chinese LoessPlateau. It consists of a complete Pleistocene loess-paleosol sequence with a thicknessof about 199m and a 126m Tertiary red clay deposits. In the loess deposits, each ofthe paleosol units identified in the Baoji type section can be readily traced in the field, indicating the completeness of the loess sequence at Jingchuan. The red clay depositsunderlying the loess are essentially composed of reddish soil horizons and carbonatenodule horizons. The red clay sequence at Jingchuan can be subdivided into six unitsfrom the top to the bottom according to the pedogenic characteristics. Relativelybetter-developed soil structure is seen in the first and third units. However, only thethird unit can be regarded as the best-developed soil complex. We collected 510 orientated samples at an interval of 25～30cm from the top ofS32 to the bottom of the red clay sequence in the Jingchuan section. The magneticremanence was measured with a 2G three-axis cryogenic magnetometer in the Instituteof Geology and Geophysics, Chinese Academy of Sciences. Remanence data areobtained after thermal demagnetization. In most cases, a thermal demagnetization at300℃ to 450℃ leads to an unambiguous polarity assignment. The remanence dataobtained show that the basal age of the Jingchuan section is about 8.0MaB. P. Themagnetostratigraphy of the red clay sequence yields linear sedimentation rates whichvary from about 1.93cm/ 103a during the interval of 8.0～4.2MaB. P., to about 3.27cm/103a between 4.2MaB. P. and 2.6MaB.P. Grain size analysis of closely spaced samples exhibits that in the entire red claysequence, the median grain size are centered at about 3～8μm with the sand fraction(> 63μm) being negligible, and all the curves indicating changes in different grain size parameters have a similar pattern. The characteristics of the grain size distributionsuggest that the red clay sequence at Jingchuan is also wind-blown in origin, thusextending the eolian record in the Loess Plateau to about 8.0 MaB. P. Another eolian red clay sequence at Lingtai with a thickness of 130m and abasal age of 7.0MaB. P. has been well studied by some Chinese geologists. The redclay sequence at Lingtai is subdivided into five units on the basis of pedogeniccharacteristics. Each unit of the red clay sequence at Lingtai has similar magneticsusceptibility record and pedogenic characteristics to the corresponding unit of the redclay sequence at Jingchuan. The correlation of pedostrafigraphy of the two red claysections testifies to the continuity of the Tertiary red clay sequence in the LoessPlateau. The verification of completeness and continuity of red clay deposits in the LoessPlateau is of great significance in paleoclimafic studies. In recent years, the history ofEast Asian summer monsoon recorded by the red clay and the transporting agency ofthe red clay have attracted much attention. Ding et al.15] proposed that the strongestEast Asian summer monsoon over the past 7.0 MaB. P. occurred between 3.9 and5.5MaB. P. on the basis of pedogenic characteristics of the red clay at Lingtai, whichis strongly supported by the pedogenic characteristics of Jingchuan red clay sequence.Thus the East Asian summer monsoon has experienced non-linear evolution since theLate Miocene, and the strongest summer monsoon occurred in the Pliocene other thanthe Late Pleistocene. As to the transporting agency of the red clay, some authorssuggested that the East Asian winter monsoon had played a significant role intransporting the dust from north to south in the Loess Plateau during the Neogene.The Jingchuan section is located about 45km north of the Lingtai section. Neither thegrain size nor the sedimentation rate shows a southward decrease from Jingchuan toLingtai, which is consistent with the record constructed by Ding et al. 24] that nodistinct decrease in grain size along the 400kn-long north-south red clay transectfrom Jingbian to Baoji, which is situated in the northernmost and southernmost of theLoess Plateau, respectively. Therefore, it may be inferred that the red clay materialscould have been transported basically by the westerlies although the west-east changesof the red clay particles in the Loess Plateau remain to be investigated.