HR: 0800h
AN: PP21A-1551    [Abstracts]
TI: Correlations of Scientific Drillcores and Seismic Reflection Data from Lake Malawi, Africa
AU: * Lyons, R P
EM: rplyons@syr.edu
AF: Syracuse University Department of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse, NY 13244 United States
AU: Scholz, C A
EM: cascholz@syr.edu
AF: Syracuse University Department of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse, NY 13244 United States
AU: King, J W
EM: jking@gso.uri.edu
AF: University of Rhode Island, URI Bay Campus Box 43 South Ferry Road, Narragansett, RI 02882-1197 United States
AU: Johnson, T C
EM: tcj@umn.edu
AF: University of Minnesota at Duluth, Large Lakes Observatory, 2205 E. 5th St. Research Laboratory Building 204, Duluth, MN 55812 United States
AU: Cohen, A S
EM: acohen@geo.arizona.edu
AF: University of Arizona at Tucson, The University of Arizona Geosciences Gould-Simpson Bldg. 326, Tucson, AZ 85721 United States
AB: Lake Malawi is one of the world's largest lakes, extending ~580km along the East Africa Rift System from 9-14 degrees S. The 700m deep lake has a permanently stratified water column that is anoxic below 250m. The water budget is dominated by evaporation and direct rainfall onto its surface; accordingly lake level is highly sensitive to minor shifts in climate. Three high-resolution, high-density single-channel seismic reflection surveys were collected on Lake Malawi between 1992 and 2001. These studies revealed a series of prograding lowstand delta deposits in the North Basin located in as much as 500m of water. The four deltas are define paleo-lakeshore geography and are located up to ~20km offshore today. Over the past ~1Ma, we observe evidence for a progression of major lowstand shorelines at 500m, 350m, 500m, and 200m below modern lake level. These back-stepping deltas document lake-level changes that occurred nearly instantaneously, suggesting large volumes of water were removed or added to the lake over time frames of a few hundred to a few thousand years. In February 2005, the Lake Malawi Scientific Drilling Project recovered 623m of continuous drillcore from Lake Malawi. Toplap and downlap surfaces from the lowstand deltas are correlated to the two drill site locations, one in the North Basin, and one in the Central Basin, using lithologic and physical properties logs from the recovered drillcores. At the North Basin drill site (site #2) in 359m water depth, we observe a sharp boundary ~37m below lake floor. This boundary demonstrates a transition from well sorted medium-grained sand to finely laminated muds. It correlates to the downlap surface of the 200m lowstand observed in the seismic reflection data set. Physical property logs such as magnetic susceptibility, GRAPE density, natural gamma, and p-wave velocity are posted on the seismic data and correlated to the lowstand surfaces. Using the density and p-wave velocity logs, synthetic seismograms are generated to add synthetic traces to the data set to enhance correlations at the bandwidth of the seismic reflection data. The ongoing development of an age model will constrain the timing of the drastic changes in lake level interpreted from the seismic-drillcore correlation, and will provide a continuous record of hydrologic, limnologic and climate change in the East African Rift System over the past ~1Ma.
DE: 3025 Marine seismics (0935, 7294)
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005