Zhang Jia-Fu, Qiu Wei-Li, Hu Gang, Zhou Li-Ping
MOE Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.
Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China.
Methods Protoc. 2020 Feb 20;3(1):17. doi: 10.3390/mps3010017.
Dating fluvial terraces has long been a challenge for geologists and geomorphologists, because terrace straths and treads are not usually directly dated. In this study, the formation ages of the Yellow River terraces in the Baode area in China were determined by dating fluvial deposits overlying bedrock straths using optically stimulated luminescence (OSL) dating techniques. Seven terraces (from the lowest terrace T1 to the highest terrace T7) in the study area were recognized, and they are characterized by thick fluvial terrace deposits overlaid by loess sediments. Twenty-five samples from nine terrace sections were dated to about 2-200 ka. The OSL ages (120-190 ka) of the fluvial samples from higher terraces (T3-T6) seem to be reliable based on their luminescence properties and stratigraphic consistency, but the geomorphologic and stratigraphic evidence show that these ages should be underestimated, because they are generally similar to those of the samples from the lower terrace (T2). The formation ages of the terrace straths and treads for the T1 terrace were deduced to be about 44 ka and 36 ka, respectively, based on the deposition rates of the fluvial terrace deposits, and the T2 terrace has the same strath and tread formation age of about 135 ka. The incision rate was calculated to be about 0.35 mm/ka for the past 135 ka, and the uplift rate pattern suggests that the Ordos Plateau behaves as a rigid block. Based on our previous investigations on the Yellow River terraces and the results in this study, we consider that the formation ages of terrace straths and treads calculated using deposition rates of terrace fluvial sediments can overcome problems associated with age underestimation or overestimation of strath or fill terraces based on the single age of one fluvial terrace sample. The implication is that, for accurate dating of terrace formation, terrace sections should be systematically sampled and dated.
对地质学家和地貌学家来说，确定河流阶地的年代一直是一项挑战，因为阶地的阶面和阶地地面通常无法直接测定年代。在本研究中，利用光释光（OSL）测年技术对中国保德地区黄河阶地上覆基岩阶面的河流沉积物进行测年，从而确定了黄河阶地的形成年龄。研究区域内识别出了七个阶地（从最低的T1阶地到最高的T7阶地），其特征是厚层河流阶地沉积物之上覆盖着黄土沉积物。对九个阶地剖面的25个样本进行测年，年代约为2 - 200 ka。根据其发光特性和地层一致性，较高阶地（T3 - T6）河流样本的OSL年龄（120 - 190 ka）似乎是可靠的，但地貌和地层证据表明这些年龄可能被低估了，因为它们总体上与较低阶地（T2）样本的年龄相似。根据河流阶地沉积物的沉积速率推断，T1阶地的阶面和阶地地面形成年龄分别约为44 ka和36 ka，T2阶地的阶面和阶地地面形成年龄相同，约为135 ka。计算得出过去135 ka的下切速率约为0.35 mm/ka，隆升速率模式表明鄂尔多斯高原表现为一个刚性地块。基于我们之前对黄河阶地的研究以及本研究结果，我们认为利用阶地河流沉积物沉积速率计算得出的阶面和阶地地面形成年龄，可以克服基于单个河流阶地样本年龄对阶面或堆积阶地年龄低估或高估的相关问题。这意味着，为了准确测定阶地形成年代，应对阶地剖面进行系统采样和测年。
