Print this pageControls on the Distribution of Coarse-Grained Facies within a Sub-Lacustrine Fan System, Lake Malawi, Africa
SOREGHAN, MICHAEL J., and CHRISTOPHER A. SCHOLZ
Lake Malawi, one of the large lakes within the East African Rift System,
serves as an important analog for explorationists working in syn-rift lacustrine
basins. We have used recently collected single and multi-fold seismic data
coupled with 20 deep water vibracores to develop models of stacking patterns of
the coarse-grained facies across a large fan-delta and associated sub-lacustrine
fan system that developed along a major basin-bounding fault.
Four genetic facies were defined by seismic reflection character, external geometry, core data, and location within the modern geomorphic system. The base of slope fan facies (approximately 100 m thick) occurs at the base of an offshore synthetic boundary fault and is characterized by low amplitude to chaotic internal reflectors and an upper convex surface in three dimensions. The progradational fan- delta facies (approximately 50 m thick) occurs on the submerged footwall block of the synthetic fault and is characterized by downlapping basal reflections but otherwise poor continuity, low-amplitude internal reflectors. The channel/canyon fill facies (up to approximately 120 m thick) fills deeply scoured surfaces in the submerged footwall block and is characterized by high amplitude, discontinuous reflectors. Finally, the basin-plain fill facies (approximately 120 m thick), characterized by moderate amplitude, discontinuous divergent reflectors occurs in the zone of maximum sediment accumulation offshore of the main basin-bounding fault. The base of slope fan and progradational fan-delta facies form the best potential hydrocarbon reservoirs based on size and geometry of the deposits and sand to shale ratios observed in cores that sampled these facies.
Climatically driven lake level changes impacted the internal character of these facies within the Lake Malawi fan system, but were subordinate to tectonism in controlling the temporal and spatial distributions of these facies.