Geology, soils and saprolites of the Swaziland Middleveld

Abstract

Quartz-diorite, diorite and granodiorite of Archaic ages are the most common rock types in the Swaziland Middleveld. The composition of the regolith cover is dominated by soil-saprolite complexes. The genetic and mineralogical gradient within the soil-saprolite complexes and the grade and spatial distribution of denudation of the land surface are important characteristics for evaluating the vulnerability to erosion and the site properties of eroded areas in land use planning. The saprolites were formed by intensive chemical weathering under warm and humid climatic conditions dating from Lower Cretaceous times. The saprolitisation process can be considered polygenetic, subdividing the saprolites into a near-surface oxidation zone and an underlying reduction zone caused by the geochemical environment. Assuming isovolumetric weathering, the mass losses of the upper saprolite zones, obtained as pore volume, amount to between 41.1 and 54.0%, depending on the mineral composition of the parent rock. Even in the lower saprolite zones plagioclases were transformed into white pseudomorphs of kaolinite. The weathering of feldspars, micas and amphiboles and the amount of Fe-oxides increase towards the surface. The clay fraction of the saprolites is dominated by kaolinite, with small amounts of illite and smectite. The latter reaches a maximum in the middle saprolite zones which coincidentally show a distinctly lower amphibole content. Therefore, smectite is formed as an intermediate phase and is not stable at higher weathering intensities in the upper saprolite zones which are characterized by desilification and removal of bases. Typical soils of the Middleveld are Ferralsols associated with Cambisols and Acrisols. Most of the recent soils have developed from colluvial sediments characterized by a multi-layer structure often marked by embedded stone lines. A strict genetic dependence between the parent rocks and the overlying soils has not been established. The different sediments which form the allochthonous part lead to compound horizons. The micromorphological investigations indicate a former Luvisol-Acrisol phase with deep-penetrating illuviation during the polygenetic soil formation. The mineral composition of the soils is dominated by quartz and kaolinite accompanied by small amounts of illite. Gibbsite is present in the uppermost soil horizons and can be interpreted as indicative of the recent phase of ferrallitization.