Deltaic sedimentation in a modern rift lake

Five of the largest deltas of Lake Malawi in Africa were surveyed to compare and contrast bathymetry, bedform distribution, and sedimentary facies using high-resolution seismic reflection profiling, side-scan sonar, and sediment coring. Striking variability in delta morphology and depositional facies was found, depending primarily on tectonic setting but also influenced strongly by lake processes and the significant past variability in lake level resulting from climatic change. Deltas on border faults and accommodation zones have much steeper offshore gradients than those on shoaling margins, resulting in narrow sandy shelves, abundant mass wasting, and sand bypass to the deep basins as turbidites, either confined to turbidity channels or spread across sand ramps. At the axial margin of the lake, where the trend of faults is orthogonal to the shoreline, deltaic facies are dominated by mud, mass wasting is common, and numerous turbidite channels coalesce into a single major, fault-controlled channel. In contrast, deltas on shoaling margins have broad, sandy shelves that are strongly influenced by coastal currents, which rework the sands into well-developed sand wave fields. Turbidites are not common in this setting, and the slope regions are covered with poorly sorted sandy muds with little evidence for downslope transport.