喀斯特峡谷河流下切速率研究——以北盘江尼珠河大峡谷为例

Study on river downcutting rate in karst canyon:A case study of Nizhu River Grand Canyon in Beipan River

西南喀斯特地区的深切峡谷中普遍缺少阶地和阶地沉积物,导致对其地貌演化过程的认识受到限制。在喀斯特峡谷两侧的洞穴中,有很多受河流作用的影响而保存了河流相沉积,这样的洞穴可以等同于河流阶地。在北盘江上游的尼珠河大峡谷,由河流侧蚀形成的倒钱洞(26°22'52″N,104°40'38″E)堆积了厚层的河流砾石层,本研究对砾石层上覆钙板进行了铀系测年。结果显示:砾石层脱离河流水位的年代为距今89.2 ka,此后河流下切了85 m,下切速率达到0.95 m/ka,该结果是北盘江河流下切速率的首次报道。本研究还认为,受山区峡谷地形的影响,短时性洪水位可以升高数米,并能保留相关沉积。但古洪水位不能等同于河流开始下切的位置,古洪水位的年代也不能算作河流下切的开始时间,应以砾石层的高度和形成年代来计算下切速率更为合理。

The lack of terraces and terrace sediments in deep valleys in karst areas of Southwest China restricts the understanding of geomorphological evolution.In the caves on both sides of the karst gorge,there are many river facies deposits preserved by the influence of the river.Such caves can be equivalent to river terraces.In the Nizhu River Grand Canyon in the upper reaches of Beipan River,the Daoqian cave formed by river lateral erosion has accumulated a thick layer of river gravel(26°22'52″N,104°40'38″E).We have carried out U-series dating on the gravel layer covered with dripstone floor.The results show that the gravel layer was abandoned at 89.2 ka ago,and the river has been downcutting for 85 m since then,and the rate of river incision reaches 0.95 m/ka.This result is the first report on the river incision rate of Beipan River.It is also considered that the temporary flood level can be increased by several meters and the relevant sediments can be retained due to the influence of the mountain canyon topography.However,the ancient flood level can not be equal to the position where the river begins to downcutting,and the age of the ancient flood level can not be regarded as the start time of the river downcutting.It is more reasonable to calculate the rate of river incision by the height and formation age of the gravel layer.