Abstract: Seven river terraces are recognized based on detailed field investigation in the Heiyukou area, Shanxi-Shaanxi Gorge of the Yellow River, of which the highest T7 and T6 terraces are covered by red clay. The lithology and grain size of pebbles on different terraces along the river are studied. It is found that the lithology of pebbles on the sixth and seventh terrace is similar to that of lower Yellow River terraces and floodplain in the Heiyukou area, but different from that of Weifen River, a tributary of the Yellow River running from east to west. The limestone pebbles, which are dominant pebbles in Baode area, not only become less in amount, but become smaller in grain size from the upstream to the downstream. This indicates that the pebbles of Late Tertiary are the relic of an ancient river running from north to south. Magnetostratigraphy study shows that the bottom age of the strata, 70 m fine sands in lower part and red clay in the upper part, covering on the pebbles of the T7 terrace is about 6.8 Ma, and the bottom age of the red clay covering on the T6 terrace is about 3.3 Ma. This indicates that the ancient river formed at least before 6.8 Ma in this area, and the age of T6 (3.3 Ma) matches well with Phase A of Qinghai-Xizang Movement. There is about 160 m incision in this area after Phase A of Qinghai-Xizang Movement. The average incision rate is more than five times faster than that before Phase A of Qinghai-Xizang Movement.

Key words: river terrace, red clay, paleomagnetism, Shanxi-Shaanxi Gorge, Yellow River

摘要： 野外考察发现山西黑峪口地区存在七级阶地序列，其中上覆红粘土的T6、T7阶地为晚第三纪阶地。对该区阶地上砾石的岩性和粒径进行了研究，岩性对比表明，T6、T7阶地上砾石的岩性与黄河低阶地上砾石的岩性相似，而与东西向的蔚汾河阶地岩性差别较大；上下游之间对比发现，灰岩砾石岩性由保德地区占主导地位到克虎地区退居次要地位，灰岩砾石的粒径也从上游向下游变小，说明该区晚第三纪阶地并非东西向河流形成的，而是古黄河或古南北向水系的产物。T7阶地上覆盖厚度70 m以上的粉、细砂层和晚第三纪红土层，磁性地层研究表明，该层的底部时代在6.8 Ma左右，T6阶地上覆红粘土的底部年龄在3.3 Ma左右，山陕峡谷由北向南流的古河流在6.8 Ma之前已经形成，T6阶地的形成时代与青藏运动A幕相对应。青藏运动A幕以来河流在该区下切了160 m，平均下切速率约为青藏运动A幕以前平均下切速率的5倍以上。

关键词: 河流阶地, 红土, 古地磁, 山陕峡谷, 黄河