Abstract: Evolution of the Lake Victoria Basin, East Africa
SCHOLZ, CHRISTOPHER A., Rosenstiel School of Marine and Atmospheric Science, University of Miami
Lake Victoria 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