PARENT MATERIAL AND PEDOGENIC PROCESSES IN SOUTH AFR ICA

ABSTRACT: The soils over about half of South Africa are developed from sedimentary ocks of the Karoo Supergroup. Some units within the Karoo strata display variations in their mineralogical composition which correlate with fluctuations inthe environment of deposition and with palaeotemperature. The former trends are reflected in the ratio of kaolinite to 2:1 layer-silicates, range from kaolinite-dominated to kaolinite-free, and are particularly evident in rocks of the Vryheid Formation, Geothermal history is preserved in various tages of smectite illitization ranging from R = 0 structures with 25 % illite to 100 % illite. Compositional trends may be significant on a regional, local or topographic scale down to a distance qual to the depth of a soil profile or even an horizon. Clay mineralogists often express the degree of pedogenesis in terms of clay mineral sequences, which describe specific phyllosilicates as interim or stable end-products of weathering. The two most common weathering sequences reported in the literature are transformation of 2:1 layer-silicates to kaolinite (Loughnan, 1969), an