Morphometric analysis of drainage basins subject to diverse geotectonic controls : the case of the Gamtoos and Crocodile drainage basins, South Africa

Abstract

Morphometric analysis represents a method to describe basin processes and examine basin characteristics to enable the structure, process and evolution of the landscape to be investigated. This approach has potential to yield insights into the long-term landscape evolution in South Africa. However, recent research has shown that care must be taken in the application of morphometric analyses to constrain tectonic or climatic influence in the evolution of ancient landscapes. The study aimed to evaluate the potential of morphometric properties to help better understand the process of landscape evolution using drainage basins with contrasting geotectonic histories; the Gamtoos and Crocodile basins. ASTER digital elevation data was used to extract selected areal, linear and relief morphometric properties of the study basins in Geographical Information Systems (GIS). Lithology and structural geology data were used to analyse and interpret spatial patterns of the morphometric properties investigated. The study found that the variation of morphometric properties was not systematic and consistent across the study basins. Not all the spatial variation of the selected morphometric properties was consistently explained by the geotectonic data. The role of bedrock type could only explain the variation of drainage density and texture across all study regions. Only the drainage pattern and shape of the stream courses were able to explain the impact of inherited tectonic structures in the evolution of drainage basin in the three study areas. The analysis of the linear properties of the drainage network, to some extent, gives evidence of possible large scale forcing factors in the Cape Fold Belt in the form of tectonic uplift which is absent in the Crocodile and Central Karoo regions. However, the rest of morphometric properties are not consistent with expectations. The major cause of this is most likely signal shredding or overprinting, whereby the evidence of the past geological processes is lost over time in ancient landscapes making impossible to infer the corresponding trajectory of the evolution of the given landscape. Also, the morphometric properties might be reflecting a composite effect of multiple factors and recent geomorphic events than the inherited structural fabric. The study did not find enough evidence to contradict Richardson et al., (2016) with regard to the utility of morphometric analysis in landscape evolution studies in the Cape Fold Belt. In view of the findings, this study recommends: further studies to examine the variation of all the morphometric properties at various nested scales to assess scale effects of some of the investigated morphometric properties, the use of a more extensive list of morphometric properties, especially those that have been successfully used in similar studies and the use of a combination of the GIS and remote sensmg based studies with paleogeomophological techniques, such as dating techniques of surfaces.