Controls of sedimentation in the Malawi rift valley, Central Africa

Abstract

The Malawi Rift is the southermost classic rift valley trough in the Cenozoic African Rift Valley System. Much of its floor is flooded by Lake Malawi which is approximately 570 km long, 60 km across and up to 500 m deep. This rift basin is a potentially useful analogue of many economically important ancient sedimentary troughs. Rift-valley structures are extremely important in controlling both the distribution of sediments in the basin as a whole and also details of sedimentation on a local scale. On the basin-wide scale an important association between the type of rift boundary structure and amount of rift floor subsidence is recognised. This association appears to apply to several section of the African Rift System. Pre-rift topography is major control on the position of major rivers, and their deltas or turbidite fans, along the basin margin. The area of pre-rift drainage network captured in the course of rift movements is a significant factor in the hydrologic budget of the basin. Climate is, however, the dominant control on the hydrologic budget and at present the basin is flooded to overflow level. Prevailing winds are important in beach ridge and back-beach aeolian dune development. Climatic conditions produce thermal stratification of the lake, with an oxic epilimnion down to 250 m depth overlying an anoxic hypolimnion. The oxic bottom waters in the shallower parts of the lake appear to promote iron precipitation in the sediments.