LATE QUATERNARY SEQUENCE STRATIGRAPHY OF LAKE MALAWI (NYASA), AFRICA

High resolution seismic data, multichannel seismic data and sediment c ores were used to examine the Songwe Sequence, the uppermost of four d epositional sequences identifiable on multichannel seismic data from L ake Malawi (Nyasa). The sequence has a maximum thickness of about 115 m in two areas of the lake, but is typically less than 70 m thick over most of the basin. The sequence is distributed along the entire lengt h of the 560 km long lake, and is concentrated in three main depocentr es. C-14 age dates from sediment piston cores are extrapolated to prov ide an age estimate of about 78 000 yr BP for the oldest sediments wit hin the Songwe Sequence. In the North and Central bathymetric basins o f the lake, high resolution seismic data indicate a dynamic deposition al environment, dominated by turbidity and mass flow deposits. Seismic data from the southern basin show acoustically transparent sediments with relatively low amplitude internal reflections, indicative of pela gic and hemipelagic sedimentation. In many areas the Songwe Sequence i s underlain by a pronounced angular unconformity, suggestive of a sign ificant, prolonged, low lake stage prior to deposition of the sequence . Seismic reflectors within the Songwe Sequence can be correlated to y ounger low lake stages identified from sediment core data. Major late Quaternary low lake level stages in Lake Malawi, interpreted from feat ures identified in the seismic data and sediment core analyses, are te ntatively interpreted at 6000 to 10 000 yr BP, 28 000 to > 40 000 yr B P, and prior to 78 000 yr BP. Budget calculations indicate mean sedime nt concentrations from catchment runoff during the period of depositio n of the Songwe Sequence to be about 190 mg l-1, comparable to estimat es of modem rainy season discharges from the major river systems. Eros ion rates within the drainage basin are estimated to be higher than th e African average by a factor of three or more, probably due to the hi gh relief within the Lake Malawi catchment.