Highstand deltas in Lake Malawi, East Africa: environments of deposition and processes of sedimentation

Deposits from the modern highstand lacustrine deltas in Lake Malawi offer an excellent opportunity to test models of deltaic sedimentation in a tectonically active setting and to examine variations in sand-body geometry along the axis of a large lake that has significant gradients in physical processes. A coring project in five of the largest deltas in the lake was initiated to describe the shallow-water environments of deposition and processes of sedimentation. Fine sand to coarse sand was characteristic in distributary mouth bar, beach, spit, point bar, and mid-channel shoal sediments. Finer sediments were common in natural levee, delta-front swale, and channel-fill deposits. Fining-upward units, displayed in the shoaling margin and accommodation zone deltas, were probably single, waning-flood deposits. The less common coarsening-upward sequences, always within a few kilometers of the shoreline, represent typical progradation of distributary mouth bar over distal bar and prodelta sediments. The fact that deltas switch depositional sites episodically at decade time scales and form sandy, two-dimensional meander belts may indicate that a typical three-dimensional geometry consists of overlapping, stacked deltaic sequences. -from Authors