Structural control of sedimentation patterns and implication for the economic potential of the East African Rift basins

Abstract

Continental rifts contain a significant proportion of world oil reserves. The Cenozoic rifts of Africa are no exception. In addition, the Africa rifts offer a potential for the accumulation of strata-bound economic resources which have not been fully investigated. Predicting the distribution of significant accumulations of metallic and non-metallic ores requires the development of depositional models that are related to basin-controlling variables such as crustal thickness, rates and degree of extension etc. Such model development relies upon observations made in areas of recent rifting, notably in East Africa. This paper offers a set of models for a spectrum of basin types that has been identified in the East African rift. They have been developed in the light of recent advances in our understanding of rift structure and development. Basins range in character from deep, narrow, structurally simple and volcanic-poor, e.g. the Tanganyika basin, through a less confined, volcanic but still simple Turkana-type basin, to the broad, shallow, volcanic-rich and structurally complex rift found in the region around Lake Baringo. Oil source rocks are accumulated in Tanganyika-type but the best reservoir rocks and seals are a product of Turkana-type settings. Turkana-type basins may also favour the accumulation of heavy minerals around lake shores and contain potentially exploitable sands and gravels. In contrast, the volcanic rocks and shallow lakes of Baringo-type rift basins encourage precipitation of economically useful evaporites (e.g. Lake Magadi) but have no oil potential.