Characterization of two phases of encrustment in a sequence of ferrallitic soils in South Cameroon and its effects on landscape evolution

In a sequence of ferrallitic soils developed on gneiss in South Cameroon, two indurated domains (surfaces) are described. The upstream domain is characterized by alumino-ferruginous indurated materials, enriched in iron with gibbsite present. The downstream domain is characterized by ferruginous indurated materials with numerous quartz grains. Structural, mineralogical and geochemical analyses carried out on these materials reveal that the two indurated domains result from two phases of encrustment that took place in sequence. The upstream domain is the result of a long and complex evolution that probably began with the formation of aluminous cuirass (hard, thick ironstone). The second (most recent) phase of encrustment, the downstream indurated domain, was formed by absolute accumulation of iron resulting from the degradation of the upstream indurated materials. The evolution of the two indurated domains seems to be linked and contributes to the lowering of the relief.