Tectonically controlled landscape evolution and its relation to the lithology, hydrology and hydrogeology of weathered crystalline rock in Uganda

摘要

Deeply weathered landscapes unaffected by Pleistocene glaciation or aeolian erosion are common to low latitude regions of Africa, Asia, South America and Australia. These terrains evolve through deep weathering and stripping, which are effected by the movement of meteoric water. Current models of landscape evolution involving eustatically and climatically controlled cycles of deep weathering and stripping, are untested by field evidence and fail to explain the development of key landforms within deeply weathered landscapes. Furthermore, a lack of detailed field study in these terrains limits knowledge of the geomorphic processes implied by evolutionary models. In this thesis, a tectonic model is proposed in Uganda on the basis of geotectonic, climatic, sedimentological and chronological evidence. Cycles of stripping are controlled by episodes of tectonic uplift while cycles of deep weathering occur during periods of prolonged tectonic quiescence. The tectonic model readily explains the development of weathered landforms and is well supported by field study.;Lithological, hydrological and hydrogeological studies were conducted in two basins that feature contrasting evolutionary paths and different contemporary geomorphic processes. Deep weathering of the landsurface in central Uganda since mid-Mocene is indicated by the stratigraphy, texture and mineralogy of the weathered mantle. Continued deep weathering is suggested by monsoonally derived recharge events that greatly exceed runoff. Transmission of the recharge flux occurs primarily via a regional aquifer in the weathered mantle but also by a less conductive aquifer in the underlying fractured bedrock. Stripping of the landsurface in southwestern Uganda since the mid-Pleistocene is reflected by truncated weathered profiles. Contemporary stripping is shown to occur by monsoonally generated runoff events that annually exceed groundwater recharge. Minimal recharge is transmitted by a poorly transmissive aquifer in the bedrock and by a highly localised aquifer in the weathered mantle. Integration of observed geomorphic processes (deep weathering and stripping) within a model of long-term landscape evolution enables, for the first time, a coherent understanding of the interplay among landscape evolution, and the lithology, hydrology and hydrogeology of deeply weathered landscapes.