CORRELATION OF SEISMIC REFLECTION AND SEDIMENT DRILLCORE DATA FROM THE LAKE MALAWI RIFT BASIN, EAST AFRICA

Lake Malawi is one of the world's largest lakes, extending ~580km along the East Africa Rift System, from~9 to ~14 degrees S. The water column of the 700m deep lake is permanently stratified and 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. Detailed studies of these data reveal a series of prograding lowstand delta deposits in the North Basin located in as much as 500m of water. The discovery of the ancient clinoforms is used to develop a lake-wide stratigraphic framework and reconstruct the lake-level history of the entire basin. These back-stepping lowstand deltas are located within four distinct stratigraphic packages and define paleo-lake-level and lakeshore morphology. They are also correlated to high-amplitude events in the central basin and lowstand deltas in the southern basin of the lake. A regular pattern of high-amplitude events followed by extended periods of seismically transparent sediments are present in the Central Basin. The high-amplitude events correlate to the lowstand events in the North Basin. In the South Basin, a lowstand event is present in the upper ~200ms of sediment. This event correlates to the youngest of lowstands from the North Basin. In order to constrain the timing of the lake level events identified from the seismic reflection data, drill cores were acquired from two sites in 2005. The sediment lithostratigraphy is correlated to the seismic refelction data using whole-core and discretely-sampled geophysical, geochemical, and digital imagery data sets. A detailed age model, developed using radiocarbon, luminescence, Be-10 and paleomagnetic ages constrains 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 ~1.5 Ma.