| 摘要: |
| 兰州段黄河阶地发育，其沉积物汇集了来自上游各盆地的剥蚀物，记录了位于青藏高原东北缘第四纪中晚期以来构造地貌演化史。分析获得T0—T9阶地60—90 μm石英光释光灵敏度和快、中、慢组分的时序变化特征：老阶地（T7—T9）快组分占比最少，新阶地（T0—T3）快组分占比最多，中间阶地（T4—T6）快组分占比介于新、老阶地之间；中间阶地各样片投点较分散，而T0—T3、T7—T9这些新、老阶地系列各样片组成比较集中。可能反映了上游黄河由于下切作用和溯源侵蚀物源发生变化的过程。分析新老阶地释光信号变化趋势与阶地基座地层的构造沉积史发现：上游黄河未贯穿前，各新生代盆地经历阶段性构造稳定期的慢速泥沙堆积和构造抬升期的快速砾石堆积，导致其中的石英经历不同的剥蚀和搬运历史，释光信号亮化过程发生分异。早期黄河先侵蚀搬运历史复杂的盆地沉积物，很可能是颗粒组分释光信号分散的主控因素。在T3阶地增强且集中的释光信号与黄河下切至各新生代盆地下伏基岩同步出现。研究探索了光释光信号灵敏度-快组分分散度作为指示沉积物物源构造-搬运历史多样性趋势的新指标。 |
| 关键词: 兰州 黄河阶地 光释光信号 物源 |
| DOI：10.7515/JEE222090 |
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| 基金项目:国家自然科学基金项目（40802039）；黄土与第四纪地质国家重点实验室开放基金（SKLLQG1840） |
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| Variations in quartz OSL signal of Yellow River terraces, Lanzhou and preliminary inference to provenance |
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GONG Fangyuan1, 2, YANG Lirong1, 2, 3*, SHI Yuanbo1, 2
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1. Department of Geology, Northwest University, Xi’an 710069, China
2. State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China
3. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
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| Abstract: |
| Background, aim, and scope The terrace sediments in the Lanzhou section of the Yellow River reveal significant information regarding the tectonic, geomorphological, and climate evolution of the Qinghai-Xizang Plateau and the northern margin of the upper reaches of the Yellow River during the middle and late Quaternary. Sediments resulting from denudation processes in the upper Yellow River Basin, which flows through the Qinghai-Xizang Plateau and its northern margin, have accumulated in the Lanzhou section, influencing both the middle and lower reaches of the Yellow River. The Lanzhou terraces, located in the upper Yellow River, are well developed and preserved, making them important areas of scientific interest. This study investigates the link between the OSL sensitivity characteristics of 60—90 μm quartz particles in the T0—T9 terraces of Lanzhou and their association with upstream provenance. Materials and methods Sediment samples were collected from the T0—T9 terraces along the Lanzhou section of the Yellow River. With the exception of the first terrace, samples were from the riverbed facies gravel layer or the floodplain facies sand across the sequence, ranging from the modern floodplain (T0) to the oldest terrace (T9). The 60—90 μm particle size, which is distributed across all three sedimentary units and is abundant in river sediments, was selected for the study of quartz luminescence sensitivity. Results The results indicated that the proportion of fast components is higher in the younger terraces compared to the older ones. Additionally, the ternary diagram shows that the F—M—S pattern for samples from T4—T6 terraces is relatively scattered, while the composition of T0—T3 and T7—T9 terraces, representing the younger and older terraces, respectively, is more concentrated. Discussion Based on the inferred source changes revealed by the previous study of the cosmogenic burial ages of the terraced gravel, as the Yellow River cuts down, the provenance in its upper reaches shifts. The geological context of this source shift, which influences the OSL signal, is rooted in the alternating tectonic stability (slow accumulation) and tectonic uplift (rapid accumulation) during the formation of the Cenozoic basin in the upper reaches of the Yellow River. These tectonic processes led to different erosion and transportation histories for the quartz grains, which may affect the “lighting” of the OSL signal in the sand and gravel sediments. During the early stages, when the Yellow River transported sand and/or gravel from the basin sediments, the OSL signal exhibited a diverse composition with the fast component. However, as the Cenozoic basin was incised by the Yellow River, starting from the T3 terrace, the underlying bedrock became the primary source of the river’s sediments, potentially leading to an increase and concentration of fast component. Conclusions Resulting from phased downcutting and retrogressive erosion, the upper reaches of the Yellow River initially eroded Cenozoic basin sediments (T7—T9). As downcutting and retrogressive erosion progressed, the underlying metamorphic bedrock was incorporated (T4—T6). By the time the river reached the T3 terrace, the overlying basins had been fully cut through, and the underlying bedrock became the main source of the river’s sediments. Recommendations and perspectives This study demonstrates the potential application of OSL signals to trace provenance changes in the upstream Yellow River. |
| Key words: Lanzhou Yellow River Terrace OSL signal provenance |
