Facies relationships and sedimentation in large rift lakes and implications for hydrocarbon exploration: Examples from lakes Turkana and Tanganyika

Two African lakes, Turkana and Tanganyika, illustrate a spectrum of sediments and facies architectures which can occur in large rift lacustrine basins. They demonstrate contrasts between sediments deposited in a semi-arid, moderately deep, mainly volcanic basin (Turkana) and a semi-humid, extremely deep basin without significant volcanism (Tanganyika). Turkana deposits are dominated by rapidly accumulating, organic-poor, terrigenous muds. Coarse-clastic input is in part limited by ponding in marginal subbasins due to volcanic barriers. Profundal coarse-clastic deposition is rare. Sediments contain little biogenic components except in the sediment-starved south basin. Sediments are also poor in cartonates. Tanganyika deposits are dominated by organic rich, biogenic muds. Steep fault-escarpments and uniform slopes along shelf platform and rift-axis lake margins promote the efficient transport of sands into profundal settings. Total clastic sediment supply into the basin is severely restricted by basin morphology with steep relief, backslopping, a limited drainage basin area and presettling in upstream Lake Kivu. Sediment ponding along margins is rare. Littoral carbonates are common; a consequence of low net clastic sedimentation rates. Many of the differences between facies in the two lakes can be explained by contrasts in basin margin slope and relief, volcanic activity, and climate between the two lakes. The results of this study suggest that lake deposits formed in Tanganyika-type rift basins are more appropriate exploration target for hydrocarbon exploration than those from Turkana-type basins.