Print this pageDevelopment of a Modern Sublacustrine Fan System, Lake Malawi, Africa: Implications for Exploration of Deep-water Sands in Rift Basins
SCHOLZ, C. A., M. J. SOREGHAN, and J. T. WELLS
Lake Malawi is a classic example of a tropical lacustrine rift basin. New data collected near the South Rukuru River fan-delta and sublacustrine fan reveal substantial deposits of coarse-grained sediments in a variety of water depths. These data include approx. 2000 km of seismic data, and 20 vibrocores (265-677m water depth).
The South Rukuru is one of seven major rivers which enter Lake Malawi, but is the only large drainage on a border fault margin. This structurally complex and high-relief depositional setting is dominated by onshore rift mountains and the primary basin border fault which forms the lake shoreline. Offshore, a major synthetic boundary fault forms a 75 m-high submerged escarpment. Between the two faults is a 10x30 km subaqueous relay ramp, which receives a considerable fraction of the river-derived sediment load. The relay ramp is severely deformed by a series of oblique, NE-striking faults. Normal and oblique motion along these structures, combined with uplift of their associated footwall blocks has controlled the location and orientation of sublacustrine canyons on the relay ramp.
The most prospective deep-water facies are base of slope and base of canyon fans, and canyon and channel fill facies. The deep-water coarse-grained sediments were deposited by high-density turbidity flows and include gravel deposits more than 20 km from the river mouth. The complex facies stacking pattern is primarily a result of deformation on the relay ramp and border faults, but is overprinted by high-amplitude (>300 m) climatically-driven lake level shifts.