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

Abstract

Large lakes are attractive research sites because they provide ideal spatial scales for understanding the complex relationships between facies distributions 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 Lake Malawi, east Africa, where river discharge is high, offshore gradients are steep, and gravity-driven processes are common. The coring program was tied to a simultaneous seismic investigation, which is discussed in a companion paper (Soreghan et al. 1999). All facies, except the hemipelagic drape, contain widespread evidence of gravity-driven sedimentation processes, with turbidity currents being responsible for most of the sediment transport and deposition. Multiple fining-up turbidite units dominate the sandy facies of the canyons, channels, mouth-of-canyon fans, and basin-plain fill environments, and thin distal turbidites are present in cores through most of the muddier facies. Angular gravel and coarse sand are common not only throughout the canyons of the sloping relay ramp region but also as far offshore as the main axial channel and its depocenter, the basin-plain fill facies. Turbidites range in thickness from 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 commonly 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 systems. Vertical excursions in lake level of 100 m over only a few hundred years may lessen the distinctions between highstand and lowstand deposits. The border-fault setting allows coarse sediment to be delivered to the steep offshore, even during periods of lake high-stand. However, during periods of falling and low lake level it is likely that downcutting and erosion of highstand deltas increase the magnitude and intensity of turbidite deposition in the mouth-of-canyon fan and in the basin-plain fill environments.