The role of lithology. Parent material controls on badland development

In this chapter, current knowledge on badland materials is reviewed. It is shown that most of the research on badlands in the past has focused on determining the type and amount of weathering and crust formation, distinguishing between sites prone to runoff generation and site where infiltration predominates, explaining the initiation and development of different landforms on badland slopes, and the influence they are subjected by the effect of the vegetation cover and human activities. More recent research on materials shows that badlands mostly develop in fine-grained and/or marly dispersive sediments, with variable portions of clay minerals, in a wide range of climate regimes and slope characteristics, although less frequently in very humid landscapes because of their suppression by vegetation and the leaching of dispersive elements. Key material properties that influence weathering are grain size, amount and type of clay minerals and physico-chemical characteristics such as EC, pH, available cations, SAR and the so-called diagnostic ‘site signatures’ formed from the ratios: SAR/EC, SAR/pH, SAR/grain size. Besides the listed properties, future research studying the content of OM in badlands could emphasize that material is important, not just because of its influence on erodibility of sediments, but also for understanding the influence of released organic carbon during sediment weathering on global carbon cycle. Once exposed to surface conditions, materials in badlands undergo weathering processes which leads to development of weathering profiles. The typical weathering profile which develops on badland materials consists of crust, subsurface and unweathered material. The structure and thickness of the weathering profile is a function of parent material properties, climate and slope characteristics. Over time, rego- lith formed on erodible and dispersive bedrock materials tends to stabilize, again as a function of slope and climate conditions. This is not permanent because prolonged precipitation can cause decrease in surface strength and runoff. Vegetation also has a distinct role in stabilizing clayey sediments due to lowered dispersivity in the upper horizon of badland material, even though such stabilization is often not permanent and erosion processes reoccur cyclically. According to the results presented in this chapter it can be concluded that factors and properties of badland materials play a key role in the development of different geomorphic processes and related landforms in badland areas, and that clarifying further the complexity of their interplay in differing climatic and topographic settings remains an interesting challenge for geomorphologists working in these fascinating terrains.