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Abstract: Evolution of the Lake Previous HitVictoriaNext Hit Basin, East Africa

SCHOLZ, CHRISTOPHER A., Rosenstiel School of Marine and Atmospheric Science, University of Miami

Lake Previous HitVictoriaTop is the largest lake in Africa by area, and is one of the most biologically productive lakes on earth. It formed in the Cenozoic as a tectonic sag basin between the eastern and western branches of the East African rift valley. Although more than 250 km across, it is only 69 m deep at its deepest point. Holocene sediments recovered from the lake have TOC values of more than 15% and Hydrogen Index values in excess of 1000, and the lake is considered a modern analog for the Green River Formation.

Several acoustic data sets were integrated to assess the basin history. These include deep refraction data, intermediate -resolution multichannel reflection data, single-channel high-resolution airgun seismic data, and high-resolution echosounder data.

Seismic data provide evidence of major lake level fluctuations in the mid-late Quaternary. At least five depositional sequences are identified in the upper 40-50 m of the sedimentary section, and are defined based on stratal relationships and marked variations in acoustic character. The uppermost unconformity identified in the seismic data correlates with a soil horizon observed in sediment cores, suggesting complete desiccation of the lake near the end of the Pleistocene. The lowermost sequence observed on single-channel data is thickest on the far western margin of the lake, and thins dramatically to the east, the site of the modern depocenter. The timing of deposition of the older sequences and of the shift of the basin depocenter from west to east is unconstrained, but likely coincided with the uplift of the western rift shoulder that occurred during the mid-late Pleistocene. Unlike the extremely smooth, flat lake floor, the basement underlying the lacustrine sediments at 200-600 ms sub-bottom has considerable relief, this topography was probably produced by an ancient drainage network.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah