Erosional susceptibility and landscape development of different lithologies

Abstract

The types and areal extent of erosion can be highly correlated with lithology. The associations between erosion, lithology and landform development have been documented in the past, however, the relationship between the these three factors has not been quantified, which is the major aim of this study. The quantified relationship between lithogy, erosion and landform development, is illustrated using morphometric mapping, shear strength and rock mass testing. These methods have also determined the wider applicability of terrain mapping, the reasons behind landform development, and the susceptibility of different regoliths to erosion. The Wairarapa region exhibits a wide variety of landforms, which have resulted from tectonic uplift and subsequent erosion. Using a morphometric mapping technique and computer analysis, clusters of discrete landforms were able to be identified from geologic, slope, soil, climatic and relief variables, predominantly drawn from topographic maps. Twelve clusters of discrete landforms were identified in the Wairarapa study area, covering 262.5 km2. They ranged from low angle alluvial plains to limestone steeplands. To validate the 'theoretical' selection of the clusters of discrete landforms, they were compared with the New Zealand Land Resource Inventory units. It was found that 91.9% of the units mapped in this study were identified on the Land Resource Inventory worksheets. This confirms the suitability of the morphometric mapping technique to the study and in the wider context of terrain evaluation. Field reconnaissance identified the twelve clusters of discrete landforms in the field. An attempt was then made to provide a physical explanation for their presence. The use of a modified rock mass strength rating system indicated that where outcrops of bed rock occurred, actual rock mass strength was the most important factor in determining slope profiles and stability, more so than denudational processes and geologic structural controls. Shear strength tests indicated that limestone and sandstone/siltstone derived regoliths were more stable than mudstone derived regoliths over a wide variety of slope angles. That is the strength parameters of the material explained, to a large extent, the landform units which develop.