Soil clay mineralogy as a key to understanding planation and formation of fluvial terraces in the South African Lowveld

- The lithology of the Letaba valley is reflected in the assemblage of clay minerals.
- Local sediment sourcing is a vital contributor to formation of the Letaba terraces.
- Clay mineralogy of the Letaba terraces contradicts the Lowveld pediplane theory.

Clay mineralogy of the fluvial plains reflects the complexity of geological setting, material weathering and transport. Clay minerals produced in the weathering process are closely associated with the parent rock and serve as an indicator of the material sources. Soil clay mineralogy of South African river valleys is insufficiently studied and this research can make a meaningful contribution towards advancing the understanding the history of planation surfaces in Africa and their evolution.Here we examine the valley of the Letaba river - one of the many Limpopo tributaries originating in the Drakensberg escarpment and flowing through the basement complex of the Lowveld geomorphic province. Nine soil profiles were selected from a total of 37 profiles represented by Ustifluvents, Haplustepts, and Haplustalfs for mineralogical study of the < 1 μm fraction using XRD. Rock samples collected from soil profiles and from soil surface were studied in thin sections by optical microscopy to determine the sources of clay mineral associations. The study of rock samples indicates that indeed these rocks are the main sources of clay minerals of the soils associated with respective lithology.The upstream section of the study area is dominated by kaolinite associated with the general dominance of granite and granitic gneiss lithology in the upper reaches of the river. The occurrence of mafic (dolerite) dikes along the river course immediately shows in soils as presence of smectite trail in the clay fraction. As the river cuts through the greenstone belt the clay mineralogy of the fluvial terraces immediately reflects this change as an increase in talc and chlorite proportions and the appearance of vermiculite. The traces of talc originating from greenstones are present throughout the valley within the middle reaches of the river and may be showing the contribution of small streams and tributaries transporting the material from the water divide to the valley of the Letaba.The changes in soil clay mineralogy of alluvial soils across the lithological boundaries indicate the prevalence of locally-sources material over long-distance-transported sediment in aggradation of these terraces. The analysis of clay mineralogy in fluvial environments may be a good indicator of local sediment source and short distance transport contribution towards formation of fluvial terraces in the valley of the Letaba river system. This contradicts the pediplane theory of Lowveld planation and emphasizes the role of localized erosion/sedimentation processes.