The geomorphology and surficial geology of the Koidu area, Sierra Leone

Abstract

The objectives of this study were to produce a landscape development model for a 400 km2 granitoid basin in the West African forest/savanna zone, and to test whether variations in landsurface morphology can be related to surficial geology. Each Unit Landform identified by morphological mapping was found to have a characteristic Morphofacies Type on the basis of slope angle, depth to topgravel, presence of unconformities of palaeosols and depth to saprolite. Textural and micromorphological analyses confirmed the distinctiveness of each Morphofacies Type. Variations in ‘stoneline’ petrography provided insights into contemporary morhodynamics. Three Process Domains (Residual, Colluvial and Fluvial) apparently interact across the landscape to produce a unique set of stratigraphic and material indicators for each Morphofacies Type. The presence of relict indicators, such as fluvially-rounded pebbles in the residual interfluve gravels, points to extensive landscape modification. Of the detailed study areas, Kania is classified as a ‘Saprolite-mantled Etchplain’ with extensive near-planate interfluves; whereas Yengema is a ‘Partially-stripped Etchplain’ with numerous rock outcrops and residual hills. This is partly explained by lithological differeces, but the main cause appears to be drainage modification. Yengema’s drainage was apparently captured by a regional trunk river during the early Quaternary, resulting in drainage reversal, fluvial incision and soil/saprolite stripping. Consequent extensive bedrock exhumation has made Yengema more sensitive to environmental instabilities than Kania (which escaped similar drainage disruption). Supportive evidence comes from the infilled Late-Quaternary valleyhead extensions and relatively deep profiles at Kania, versus the rocky valleyheads and shallow profiles characteristic of Yengema. 3000 years of farming and deforestation have caused soil/saprolite stripping, with 1-2m of colluvium filling valley swamps.