Lithological evidence for the evolution of weathered mantles in Uganda by tectonically controlled cycles of deep weathering and stripping

Landscape evolution in terrains that have been unaffected by glacial or aeolian erosion, occurs by cycles of deep weathering and stripping. Several factors have been proposed to control these cycles including sea level, climate and tectonics. In this study, a tectonic model of landscape evolution recently developed for Uganda, was tested by detailed study of the weathered mantle. The study involved an analysis of weathered-mantle stratigraphy, texture, mineralogy and elemental geochemistry, and was conducted in two catchments of central and southwestern Uganda which feature contrasting geomorphic processes (deep weathering and stripping) and different (post mid-Miocene) evolutionary histories. Application of graphical (sedimentological) logs to weathered-mantle stratigraphy is introduced in this work and represents an improvement upon more subjective classification schemes employed previously. The stratigraphy, texture, mineralogy and elemental geochemistry of the weathered mantle support cycles of deep weathering and stripping predicted by the tectonic model of landscape evolution. In central Uganda, deep weathering of the landsurface as a result of tectonic quiescence since the mid-Miocene is indicated by thick (30 m) weathered profiles that result from prolonged chemical denudation. This is evident from the progressive alteration of crystalline rock from relatively unweathered bedrock fragments at the base of the profile to a ferricrete crust at the top. Lithological evidence indicates that the weathered mantle has developed in situ and that the water table separates environments of different weathering intensity. In southwestern Uganda, stripping of the landsurface following mid-Pleistocene uplift is reflected by thinner, truncated weathered profiles that are not capped by ferricrete and are composed primarily of coarser, less weathered material.