Abstract:

Based on field work, four Quaternary terraces are found according to overlapped loess sequences at Pantang-Heiyukou area and its surroundings in northern Chinese Shanxi-Shaanxi Gorge. These four terraces, which are 12, 50, 80 and 130 m above present the Yellow River level respectively, are all rock-seated terraces. The thicknesses of overlapped loess from the lowest terrace to the highest Quaternary terrace are about 20, 33, 37 and 43 m respectively. ESR dating samples were collected from river sediments on T2, T3 and T4. ESR dating result reveals that the formation ages of these three terraces are 0.609, 0.876 and 0.97±0.107 Ma in turn from T2 to T4. In order to corroborate the dating results, powder samples and oriented samples were collected from sections on T3 and T4. Paleomagnetic result shows that section bottom on T3 does not reach the boundary of B/M (Brunhes/Matuyama), which means a formation age younger than 0.78 Ma. This age is younger than that gotten from ESR dating. The bottom age of T4 section reaches an age of 1.07 Ma, which is a bit older than ESR dating result but within its range. The formation ages of the terraces are in accord with other terraces in upper and lower reach of the Yellow River such as Lanzhou and Sanmenxia which suggests river terraces controlled by tectonic movement exist universally in the domain of the Yellow River. A Late Tertiary section was resampled and measured as an improvement to former works. The result is used to discuss the incision rate of different terraces in this region.

Key words: river terraces, the Yellow River, formation age

摘要：

野外考察发现, 陕西省神木县盘塘至山西兴县黑峪口黄河两岸发育四级第四纪河流阶地, 阶地前缘基座出露, 与现今黄河河面高差分别为12, 50, 80 和130 m。四级阶地上覆盖厚度不等的黄土层, 其厚度在T1阶地上最大约为20 m, 在T2阶地上约为33 m, 在T3阶地上不同区域黄土厚度不一, 最厚处约为37 m, 在T4阶地上黄土层厚度最大, 约为43 m。分别对T2阶地、T3阶地和T4阶地上河流相砂层进行电子自旋共振(ESR)测年样品采集。ESR测年结果显示, T2阶地的形成时代约为 0.609 Ma, T3阶地的形成时代约为0.876 Ma, T4阶地的形成时代为0.97±0.107 Ma。为了与测年结果相互印证, 对T3和T4阶地上的黄土进行磁化率和古地磁样品的采集。古地磁结果显示, T3阶地上的黄土底层未能到达B/M界限, 说明黄土层底部年龄不超过0.78 Ma, 比河流相砂层的ESR 测年结果更年轻。T4阶地黄土底部年龄约为1.07 Ma, 与ESR测年结果的差别在ESR 测年的误差范围内。四级阶地在形成年代上与兰州以及三门峡地区的阶地可以对比, 说明构造运动控制的河流阶地在黄河中上游地区普遍存在。作为对之前工作的补充, 对区域内一级第三纪阶地重新进行古地磁采样和测试, 并结合四级第四纪阶地, 对黄河在该区的侵蚀速率进行探讨。

关键词: 河流阶地, 黄河, 形成年代