Deep Weathering of Interfluves in the Basement Complex of South-Western Nigeria: Its Geomorphological and Geohydrological Implications

Abstract

This paper derives from a quantitative study of deep interfluve weathering in three humid low-relief crystalline rock areas in south-western Nigeria. The irregular contour of the basal weathering surface is clearly evidenced in each study area. Weathering depth, depth to the water table and thickness of the saturated zone in the regolith increase generally with distance from the main river channels to the major watersheds. The spatial distribution patterns are optimally described by empirical equations computed by the least-squares method. The deep-weathering episode appears to predate the evolution of the existing drainage systems whose remarkably low densities, attributable to the high infiltration capacity of the regolith, are indicative of a low rate of denudation in the interfluves. Possible future emergence of ruwares or inselbergs is discernible at several sites but the rates of regolith stripping that would permit a reliable estimation of when the actual exhumation would occur at specific sites require further research. The widespread zone of saturation in the regolith is indicative of substantial groundwater recharge in the overburden and a high success ratio for wells. The basins of decomposition are locally important as sites with exceptionally thick groundwater zone. A strong direct relationship exists between weathering depth and thickness of the saturated zone. The general spatial trend of depth to water table implies that the approximate depths for wells at specific locations in the interfluves can be reliably predicted prior to the actual digging or drilling operations. Evidence of baseflow from the regolith aquifers is convincingly provided by the dry-season flows of the main rivers which have incised to the bedrock.