ABSTRACT

Lake Malawi, Africa, serves as an important modern analog for understanding depositional processes in an active rift setting. This paper explores the depositional processes and stacking patterns of coarse-grained facies across a large fan delta and associated sub-lacustrine fan system offshore of the South Rukuru River using single-channel seismic data coupled with deep-water vibracores and gravity cores. A companion paper (Wells et al. 1999) discusses the details of the depositional processes of the system inferred from core data.

Using 2900 km of single-channel seismic data, eight genetic facies were defined based on seismic reflection character, external geometry, accompanying core data, and location within the modern geomorphic system. Five sand-dominated facies include: (1) mouth-of-canyon-fan facies; (2) progradational-fan-delta facies; (3) channel-fill facies; (4) canyon-fill facies; and (5) basin-plain-fill facies. These facies occur as surficial deposits in water depths of more than 600 m, suggesting that coarse-grained sedimentation characterizes certain localities, in even the deepest parts of Lake Malawi.

Climatically driven lake-level changes influenced the internal character of these facies, their thickness, and to some extent their location within the South Rukuru study area. However, climate was generally subordinate to tectonism in controlling the temporal and spatial distribution of the facies. Tectonic controls can be categorized as: (1) orientation of the first-order border fault and intrabasinal faults; (2) syndepositional footwall uplift and rotation; and (3) temporal changes in the rate and location of maximum subsidence. These controls affect the location and stacking of the mouth-of-canyon fans, and the presence and orientation of sub-lacustrine canyons and the zone of maximum sediment accumulation, and depositional gradients.