邙山黄土地层再研究

邙山黄土位于郑州西北方向约25km的黄河南岸，以赵下峪剖面最为典型。在野外地层观察的基础上，结合磁性地层、磁化率和粒度测量结果，对赵下峪剖面进行了重新划分。该剖面总厚度为172m，出露地层为全新世古土壤S0到第11层古土壤S11。磁性地层测量结果显示，B／M界限位于L8底部(149．3m深处)，与黄土高原内部及南部极性转换带的位置基本一致，证明了野外地层划分结果的正确性。剖面磁化率和粒度曲线与黄土高原其他剖面的对比研究进一步证实了本次地层划分的准确性。赵下峪剖面97m以上的地层包括S2、L2、S1、L1和SO，其中97～87m之间的古土壤层为S2，而不是前人认为的S1。赵下峪剖面与黄土高原其他剖面对比还发现，邙山黄土自S2发育以来，粒度和沉积速率同时变大，是由于黄河在此时期贯通三门峡段基岩山地，古三门湖水外泄，在三门峡以东形成冲积扇和三角洲，成为邙山黄土沉积的新物源。

RESTUDY ON THE STRATIGRAPHY OF MANGSHAN LOESS

The Zhaoxiayu loess section in Mangshan Yuan which is located 25 km to the west of Zhengzhou is typical of the loess stratigraphy. On the basis of the observation of the section in field, together with magnetostratigraphic study measurements of magnetic susceptibility and grain size, the stratigraphy of Zhaoxiayu loess section has been re-established. The Zhaoxiayu loess section is 171 m in thickness and composed of 11 loess layers interbedded with 12 palaeosols. The Brunhes/Matuyama boundary was found to be located in the lower part of L8, slightly different from many other loess sections,but confirming the field stratigraphic classification. The result of magnetic and grain size measurements,compared with other loess sections in the Loess Plateau, also testified the field stratigraphic classification. The upper 97 m of the section contains S2,L2,S1,L1 and S0. The palaeosol between 97 and 87 m is S2, which is considered S1 by some geologists previously. In addition, contrasted with other sections, the top 97 m has relatively low susceptibility values, high sedimentation rate and much coarser grain-size distribution. The change in lithological characters implies that there must have been a major addition of sediment source ever since the deposition of S2, apart from the deserts that act as a common source for most of the loess deposits in central China. This sediment source is believed to be the nearby Yellow River flood plain, and the palaeo-delta lying at the east mouth of the Sanmen Gorge as well.