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ABSTRACT

High resolution seismic data, multichannel seismic data and sediment cores were used to examine the Songwe Sequence, the uppermost of four depositional sequences identifiable on multichannel seismic data from Lake 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 length of the 560 km long lake, and is concentrated in three main depocentres. 14C age dates from sediment piston cores are extrapolated to provide an age estimate of about 78 000 yr bp for the oldest sediments within the Songwe Sequence. In the North and Central bathymetric basins of the lake, high resolution seismic data indicate a dynamic depositional 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 pelagic and hemipelagic sedimentation. In many areas the Songwe Sequence is underlain by a pronounced angular unconformity, suggestive of a significant, prolonged, low lake stage prior to deposition of the sequence. Seismic reflectors within the Songwe Sequence can be correlated to younger low lake stages identified from sediment core data. Major late Quaternary low lake level stages in Lake Malawi, interpreted from features identified in the seismic data and sediment core analyses, are tentatively interpreted at 6000 to 10 000 yr bp, 28 000 to >40 000 yr bp, and prior to 78 000 yr bp. Budget calculations indicate mean sediment concentrations from catchment runoff during the period of deposition of the Songwe Sequence to be about 190 mg 1−1, comparable to estimates of modern rainy season discharges from the major river systems. Erosion rates within the drainage basin are estimated to be higher than the African average by a factor of three or more, probably due to the high relief within the Lake Malawi catchment.