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Abstract

Seismic-reflection data from Lakes Malawi and Tanganyika in the western branch of the east African rift system reveal a variety of coarse-grained depositional facies. These facies include fan deltas and slope aprons adjacent to border faults, deep-water sublacustrine fans and channel systems, lowstand deltas, and an array of clastic and carbonate littoral deposits. Each is located in specific areas within half-grabens and develops at specific times within the cycle of lake-level variation. Rift lakes in tropical settings are highly sensitive to level fluctuations. High-amplitude and high-frequency lake-level variations may cause the resulting depositional sequence and facies architecture to be more complex than on passive margins. Controls on sequence development, such as sediment supply and lake-level variation, may be more closely coupled than on passive margins. Progradational clinoform depositional packages are uncommon in these basins probably because of the small size of catchments relative to lake surface areas and because of high slope gradients on the basin margins. Erosional truncation surfaces are more readily observed in these seismic data than are downlap surfaces.

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