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1Syracuse University, Syracuse, NY
2University of Oklahoma, Norman, OK

Abstract: Tropical Sublacustrine Fans: Seismic Data Examples from Lakes Tanganyika and Malawi, East African Rift

Sublacustrine fans are important oil and gas reservoirs in lacustrine rift basins in many localities around the world. Such deepwater coarse sediment facies are particularly important as exploration targets because they are commonly positioned adjacent to deep-basin, organic-rich hemipelagic source rocks. Sublacustrine fan systems are distinct from submarine fans for several reasons including- spatial scale; their close proximity to subaerial drainage systems; the confined and closed nature of lacustrine basins; and the unique behavior of sediment-laden gravity flows in freshwater. High-quality seismic images recovered from modern fan systems in Lakes Malawi and Tanganyika in the East African Rift reveal the details of several "tropical" end member sublacustrine fans.

Over 4000 line km of intermediate-resolution (small Previous HitairNext Hit Previous HitgunTop source) seismic data were acquired over sublacustrine fans in Lakes Malawi and Tanganyika between 1992 and 1997. Much is revealed of their geometry and stacking patterns by comparing these dense grids of 2D seismic images to, and correlating with coincident basin-scale multichannel seismic profiles, 1 kHz high-resolution seismic data, side-scan sonar imagery, and more than 50 sediment cores.

First-order geometries and fan development are controlled by large-scale rift architecture and extensional deformation. Unlike high-latitude lake systems which tend to be hydrologically overfilled and have stable lake levels, tropical lake basins are prone to marked shifts in lake level. In Lakes Tanganyika and Malawi high-amplitude (>400 m) and high-frequency (kYr) shifts in water level markedly impact the fans' sediment delivery systems. Fan stacking patterns are controlled mainly by high-amplitude, climatically-driven lake level shifts, which appear to cycle at Milankovitch frequencies.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas