粤北丹霞山地区河流阶地光释光测年研究

    OSL Dating of the River Terraces in the Danxiashan Area of Northern Guangdong Province

    • 摘要:   在丹霞地貌区发育较多的河流阶地,但迄今对其研究相对较少,尤其缺乏对相关河流阶地形成年代的研究。河流阶地记录了河流下切过程和环境变迁信息,开展阶地年代学研究有助于理解丹霞地貌演化历程,重建河流演化历史,以及探讨气候变化响应机制。粤北丹霞山地区的锦江和灵溪河(山间溪流)河流阶地剖面中均发育有河流“二元结构”沉积特征,光释光测年结果显示,锦江T0、T1、T2阶地以及灵溪河T1阶地分别形成于0.41±0.08 ka、70.14±6.08 ka、300.64±14.66 ka及18.26±3.22 ka。锦江T1和T2阶地下切时间分别对应深海氧同位素5至4阶段(MIS5-MIS4)转型期和MIS9-MIS8阶段转型期。这表明在冷期-暖期转型阶段,锦江的下切能力显著超越沉积通量,从而使得前期的河漫滩沉积物与下伏宽谷面形成了阶地。而灵溪河T1阶地形成于MIS2期间,这可能与山间溪流的特殊性质(河谷较窄)以及季节性径流的影响有关。
        根据阶地砾石层和阶地面高度与年龄的关系,我们估算出锦江在~300 ka至~70 ka和~70 ka至今两个时间段的最小下切速率分别为(0.05±0.00)mm/a和(0.22±0.02)mm/a。下切速率显著加快的原因主要有两点:一是河流侵蚀与加积作用交替,长时间下切导致速率减缓;二是切割冲洪积物速率快于切割基岩。这些下切速率数据为丹霞地貌演化过程的恢复和重建提供了重要依据。

       

      Abstract:   In the research on the Danxia landform, the study of river terraces, which record the process of landform evolution, is relatively scarce, especially the lack of research on the formation age of related river terraces. River terraces contain information on the river incision process and environmental changes, and conducting terrace chronology studies can help reconstruct the history of river evolution, and further explore the response to climate change.
        In the Danxiashan area of northern Guangdong, the authors found typical sedimentary characteristics in the river terrace profiles of the Jinjiang River and Lingxi River (a mountain stream), and carried out optically stimulated luminescence (OSL) dating. The dating results show that the Jinjiang T0, T1, T2 terraces and the Lingxi River T1 terrace were formed at 0.41 ± 0.08 ka, 70.14 ± 6.08 ka, 300.64 ± 14.66 ka, and 18.26 ± 3.22 ka, respectively. The incision times of the Jinjiang T1 and T2 terraces correspond to the transition stages of Marine Isotope Stage 5-4 (MIS5-MIS4) and MIS9-MIS8, respectively. This indicates that during the transition from cold to warm periods, the incision capacity of the Jinjiang River significantly exceeded the sedimentation flux, leading to the formation of the earlier floodplain sediments and the underlying wide valley floor into terraces. The Lingxi River T1 terrace was formed during the MIS2 period, which may be related to the characteristics of the mountain stream (narrower valley) and the influence of seasonal runoff.
        Based on the relationship between the gravel layer and the height and age of terrace surface, we have estimated that the minimum incision rates of the Jinjiang River during the two time periods of ~300 ka to ~70 ka and ~70 ka to the present are (0.05±0.00) mm/a and (0.22±0.02) mm/a respectively. There are two main reasons for the significant acceleration of the incision rate: first, the alternating effects of river erosion and accumulation, which leads to a slowdown in the incision rate over time; second, the rate of cutting through alluvial-pluvial deposits is faster than that of cutting through bedrock. These incision rate data provide important evidence for the restoration and reconstruction of the evolution process of Danxia landform.

       

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