Development of coarse-grained facies in lacustrine rift basins: Examples from East Africa

Detailed studies of high-resolution and multifold seismic reflection data from the two largest East African rift lakes, Malawi and Tanganyika, reveal a complex suite of coarse-grained depositional facies. These facies occur within specific regions of the controlling half grabens that compose the rift lakes. Sand-prone environments include subaqueous channels and small drowned fluvial complexes. Channel systems range from large erosional canyons to deep-water turbidite channel-levee systems. Lowstand and highstand deltas of axial and shoaling-side rivers are volumetrically important coarse-grained facies. Fan deltas develop along the base of major border faults during lake lowstands; subaqueous talus fan deposits occur along the base of the border faults during lake highstands. Lowstand deltas are the best-preserved progradational facies in these rift lakes. In addition to simple tectonic control, drastic tectonically or climatically induced lake-level change significantly regulates the production of coarse-grained lacustrine synrift deposits.