Geomorphologische Beobachtungen an Inselbergen im südlichen Zimbabwe

Abstract

The study area is located in southern Zimbabwe. The greatest part of the area comprises surfaces of low relief dipping to the south east. Long, narrow ridges extending in a NE-SW direction are the typical landforms of the greenstone terrains. Due to the structural and lithological control several rock-dependent summit levels have formed. Landscapes with low relief, however, are to be found on the gneissic and granitic basement. Most inselbergs are associated with microcline-rich granitoid rocks. On these rocks, inselbergs are larger and more closely spaced than on the less potassic counterparts. Apart from areas of foliated gneissic rocks, there is usually no difference in soil depth on the various granitoid rocks of the basement. Thin sections of fresh and weathered rock showed that, plagioclase splits more readily even when the rocks are only slightly weathered. The difference in the rate of weathering between the more potassic granitoid rocks and the plagioclase-rich rocks seems to be more pronounced at lesser soil depth. On the basis of field observations it is suggested that sheetwash and splash are the dominant hillslope processes. Soils on the pediments are usually no deeper than 0.8 - 1.5 m. The upper 10 - 15 cm of the soil profile are well sorted. The pediments appear to evolve in relation to a slowly incising river or a stable local base level. Quantitative morphometric analysis shows significant relationships between the width of pediments, the gradient of rivers and the relative relief of the inselbergs. Therefore the change of the slope form is related to changes in the width and gradient of the pediments.