GEOLOGY, SOILS AND SAPROLITES OF THE SWAZILAND MIDDLEVELD

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 a nd mineralogical gradient within the soil-saprolite complexes and the grade and spatial distribution of denudation of the land surface are i mportant characteristics for evaluating the vulnerability to erosion a nd the site properties of eroded areas in land use planning. The sapro lites were formed by intensive chemical weathering under warm and humi d climatic conditions dating from Lower Cretaceous times. The saprolit isation process can be considered polygenetic, subdividing the saproli tes into a near-surface oxidation zone and an underlying reduction zon e caused by the geochemical environment. Assuming isovolumetric weathe ring, the mass losses of the upper saprolite zones, obtained as pore v olume, amount to between 41.1 and 54.0%, depending on the mineral comp osition of the parent rock. Even in the lower saprolite zones plagiocl ases were transformed into white pseudomorphs of kaolinite. The weathe ring of feldspars, micas and amphiboles and the amount of Fe-oxides in crease towards the surface. The clay fraction of the saprolites is dom inated by kaolinite, with small amounts of illite and smectite. The la tter reaches a maximum in the middle saprolite zones which coincidenta lly 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 d esilification and removal of bases. Typical soils of the Middleveld ar e Ferralsols associated with Cambisols and Acrisols. Most of the recen t soils have developed from colluvial sediments characterized by a mul ti-layer structure often marked by embedded stone lines. A strict gene tic dependence between the parent rocks and the overlying soils has no t been established. The different sediments which form the allochthono us part lead to compound horizons. The micromorphological investigatio ns indicate a former Luvisol-Acrisol phase with deep-penetrating illuv iation 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 ferrallit ization. (C) 1997 Elsevier Science B.V.