Processes of sedimentation on a lacustrine border-fault margin: Interpretation of cores from Lake Malawi, East Africa

Large lakes are attractive research sites because they provide ideal spatia l scales for understanding the complex relationships between facies distrib utions and processes of sedimentation. In this paper, we describe deepwater facies and associated environments of deposition, based primarily on cores , from the tectonically active South Rukuru sublacustrine fan system of Lak e Malawi, east Africa, where river discharge is high, offshore gradients ar e steep, and gravity-driven processes are common. The coring program was ti ed to a simultaneous seismic investigation, which is discussed in a compani on paper (Soreghan et al, 1999). All facies, except the hemipelagic drape, contain widespread evidence of gr avity-driven sedimentation processes, with turbidity currents being respons ible for most of the sediment transport and deposition, Multiple fining-up turbidite units dominate the sandy facies of the canyons, channels, mouth-o f-canyon fans, and basin-plain fill environments, and thin distal turbidite s are present in cores through most of the muddier facies, Angular gravel a nd coarse sand are com mon not only throughout the canyons of the sloping r elay ramp region but also as far offshore as the main axial channel and its depocenter, the basin plain fill facies, Turbidites range in thickness fro m 0.25 to 2.0 m, are usually stacked, and can be recognized by well-defined Bouma sequences. A hemipelagic drape of highly laminated to structureless mud, which forms an uneven blanket over much of the lake bottom, is commonl y interspersed with turbidites, Because lake levels can change rapidly as a result of climatic variations, the rate of sediment mobilization is extremely fast relative to marine syst ems. Vertical excursions in lake level of 100 m over only a few hundred yea rs may lessen the distinctions between highstand and lowstand deposits. The border-fault setting allows coarse sediment to be delivered to the steep o ffshore, even during periods of lake highstand, However, during periods of falling and low lake level it is likely that downcutting and erosion of hig hstand deltas increase the magnitude and intensity of turbidite deposition in the mouth-of-canyon fan and in the basin-plain fill environments.