广东雷州半岛火山凝灰岩河流壶穴的形成机制

FORMATION MECHANISM OF VOLCANIC TUFF RIVER POTHOLES IN LEIZHOU PENINSULA,GUANGDONG PROVINCE

河流壶穴是一种常见的河床微地貌,是基岩河床下切的方式之一。壶穴的形成受岩性、气候和动力条件等诸多因素影响,形成机制较为复杂。本研究以广东雷州半岛火山凝灰岩河流壶穴为研究对象,对壶穴的岩性特征、形态特征、空间分布特征等展开研究,分析火山凝灰岩河流壶穴的形成机制及地貌过程。结果表明:1)雷州半岛火山凝灰岩壶穴数量多,规模大,形状以椭圆形和浅型为主;受岩性和跌水坡度影响,与其他岩性的壶穴相比,火山凝灰岩中极深型壶穴数量所占比例最大;2)壶穴口径-深度关系表明,随着壶穴的发育,深度的增长速度会快于口径,但深度的增长有一定限度;3)壶穴长轴走向与岩石节理走向仅有两个方向一致,但不是主要方向,表明壶穴发育与节理关系较小;4)研究区出露的凝灰岩河床同时存在壶穴和火山弹的点分布。空间分布格局上,最近邻指数表明二者均为集聚分布模式;双变量K函数表明壶穴分布点在火山弹分布点附近表现出集聚特性,即火山弹的分布模式对壶穴的分布产生影响。火山凝灰岩壶穴的发育与凝灰岩中夹杂的火山弹有关。部分壶穴与火山弹的空间位置重合,凝灰岩中的火山弹被流水冲走后留下的凹坑为壶穴的形成提供条件,同时还可能影响壶穴的形状和发育过程。

The pothole is a common micro-landform of riverbed,which is one of the ways of bedrock riverbed incision.The formation of potholes is affected by many factors,such as lithology,climate,dynamic conditions and so on,and the formation mechanism is complex.In order to analyze the formation mechanism and geomorphic process of volcanic tuff river pothole,we investigated 760 tuff river potholes in Longmen River(20°42’7″N,109°56’30″E),Leizhou Peninsula of Guangdong Province.In this study,we investigated the morphological indicators of potholes(such as long axis of caliber,short axis,depth,etc.),lithologic characteristics and spatial location,and compared and analyzed the morphological characteristics of potholes of different lithology.In addition,the relationship between potholes and volcanic bombs in tuff is studied and discussed by spatial analysis.On the basis of our analyses,the following conclusions are drawn:(1)The volcanic tuff potholes in Leizhou Peninsula are large in quantity and scale,and their shape is mainly oval and shallow,accounting for 58.97%and41.82%,respectively.Due to the influence of lithology and riverbed slope,the number of extremely deep potholes in volcanic tuff accounts for the largest proportion(8.7%)compared with other lithology potholes.(2)The pothole diameter-depth relationship shows that with the development of pothole,the depth will grow faster than the pothole diameter,but the depth will be limited when it grows to a certain depth.(3)The plane shape of pothole is mostly round or nearly round;There are only two directions(285°~315°and 75°~90°)consistent with the direction of rock joint,but they are not the main direction,indicating that pothole development is less affected by joint.(4)Both potholes and volcanic bombs are developed in the exposed tuff riverbed in the study area.In the spatial distribution pattern,the nearest neighbor indexes of potholes and volcanic bombs are 0.67 and 0.47,respectively,indicating that both potholes and volcanic bombs are clustered.Meanwhile,the Cross-K function shows that potholes show clustering characteristics in the area near the distribution points of volcanic bombs.That is,the distribution pattern of volcanic bombs affects the distribution of potholes.The development of volcanic tuff potholes is related to volcanic bombs in the tuff.The space position of some potholes coincides with that of volcanic bombs.Potholes form in the pits left by volcanic bombs in the tuff that have been washed away by running water.In addition,volcanic bombs may affect the shape and development of potholes.