福建长乐屏山风化坑与河流壶穴的成因及其证据

风化坑和壶穴是一种常见的岩石坑穴地形,其形成原因和形态完全不同,但容易被误解和混淆.结合对福建长乐三溪河地区这两种坑穴地形的实地考察,本文从地貌学,沉积学,岩石化学和矿物学等各个方面对两者的形成过程、影响因素进行了论证和对比.研究表明：风化坑形成于岩石面积水的风化作用,壶穴则是由河流的旋转水流对河床产生的侵蚀作用形成;山顶风化坑的发育与河流流水作用没有关系,河流壶穴的形成与河谷的发育过程有关,河谷中的风化坑只能在河流深切后,流水不再作用到的河床部位发育;只要条件合适,风化坑或壶穴随时可以生成;风化坑与河流壶穴的形态,坑内沉积物的磨圆度、粒度特征等反映出各自不同的形成过程.风化坑内碎屑与周边岩石的化学蚀变指数CIA 值的差异反映了风化坑的化学风化成因;风化坑内碎屑与周边岩石石英长石比例的差异说明风化坑是矿物差异风化的结果;用CIA 值和英长比均无法区分河流壶穴和风化坑中的碎屑颗粒,但两者化学元素迁移特征的差别反映了风化坑的风化作用和河流壶穴的流水搬运作用的成因差别;风化坑的风化程度达不到当地风化壳的风化程度,但不同气候带风化坑碎屑的CIA 值能反映不同气候带风化作用的强度差异.

Evidences for the formation of the weathering pits and the stream potholes at Changle,Fujian Province of China

In Sanxi river area of Changle of Fujian province, there are some pits developed in granitic rock on the top of mountains and the surface of river beds. Such pits known as mountain top glacial potholes or river valley glacial potholes have been considered to be typical glacial potholes, which were used to support the evidence that the Quaternary glacier has been extended to the area of Fujian Province, China. The field investigation for these pits we made in March 2011 shows that the pits are weathering pits or river potholes, instead of glacial potholes. In this paper, the origin and formation process, as well as the related influencing factors, of the weathering pits and the river potholes developed in this area were studied and compared from the aspects of geomorphology, sedimentology, petrochemistry and mineralogy. The studies show that the formation of the river potholes is due to the mechanical erosion of stream water while that of the weathering pits on the top of mountains has nothing to do with river water but is the water standing in rock surface depression that results in chemical weathering. The weathering pits may also be found in river valleys, but can be only developed in the positions where the river water could not be reached after a river incision. The potholes and the weathering pits are morphologically distinct with particles of different roundness and grain sizes, denoting the different formation processes generated by chemical weathering or mechanical erosion. The different CIA values and the different quartz to feldspar ratios between the particles in the weathering pits and the rock where the pits were formed could be used to indicate the results of chemical weathering and mineral differential weathering. The river potholes also have different CIA values or quartz to feldspar ratios between the particles and the rocks as the weathering pits, and then the value and the ratio could not be used to distinguish the potholes from the weathering pits. However, the river potholes have a chemical element migration between the rocks and the particles, which is very different from that of the weathering pits, reflecting the processes of the particle transportation by river water in the potholes and the chemical weathering in the weathering pits.