High-resolution seismic profiles, side-scan sonar records, and sediment cores collected from Lake Malawi have been analyzed to determine the nature of sedimentation in a modern rift lake. More than 4500 m of sediment have accumulated in the deepest basin in the northernmost part of the lake. If the modern sedimentation rate of 1 mm/yr is representative of most of the lake's history, then the deepest basin may, when compaction is accounted for, be on the order of 26 Ma. Although it is an open-basin lake at present, it has several times in the past been a closed-basin lake in response to drier climate. Lake level has been 100-150 m lower than present at least three times in the last 10 k.y.

The distribution of modern sediment is quite complex. Little or no deposition occurs in most regions shallower than 100 m due to storm-generated surface-wave activity. Gravitational transport of sediment by creep, debris flows, slumping, and turbidity currents is common, particularly off deltas and border faults. Diatom-rich clays occur in the deep basins far removed from major terrigenous input. These typically have organic carbon concentrations of 3-6 wt. %. Laminated sediments are common in Lake Malawi, and their frequency increases with water depth. Neither the abundance of organic carbon nor frequency of laminations increases abruptly below the depth of the chemocline. This probably results from the rise and fall of the chemocline as well as lake level in response to climatic cha ges. Although source rock potential of Lake Malawi as a future petroleum resource is high, the reservoir rock potential has yet to be demonstrated.