Abstract Analyses of clay minerals in the Quaternary sediments of Lake Malawi and Lake Turkana in East Africa demonstrate the sensitivity of these mineralogical indicators to environmental change. The clay mineralogy of the modern sediments in hydrologically open Lake Malawi is influenced mostly by the numerous source regions, which control the detrital composition. The clays consist of smectite, kaolinite, and illite. Kaolinite is more abundant in the northern part of the lake, whereas smectite is more significant in the south. Highly crystalline nontronite also occurs locally, and may be a diagenetic product formed by reactions with lake water just above the depth of the anoxic chemocline. The relative abundance of clay minerals varies with depth in some of the cores in response to changing paleolimnological conditions, but there are significant contrasts between the northern and southern parts of the lake. Most notably, a closed-basin episode at ca. 6 to 10 ka is characterized by elevated smectite in the central and southern lake. This is consistent with a semiarid climate and elevated water alkalinity at this time. However, kaolinite is more abundant in the northern part of the lake during this period. Lake-level lowering may have allowed coarser kaolinite to be concentrated in the shallower deltaic areas of the northern lake. In Lake Turkana, smectite is the most abundant clay mineral in the sediments, reflecting the relative stability of this mineral in contact with the slightly saline (TDS = 2,500 mg/1) and highly alkaline (pH = 9.2) lake water. The Mg content of smectite in the surface sediments increases from ∼ 2% to ∼ 5% from north to south in the lake, which may indicate removal of Mg from the lake water. In the northern lake, there is little change in the mineral composition with depth in sediment cores, but in the south end of the lake, the Mg content of the smectite decreases with depth. The earlier sediments were deposited under fresh-water conditions, and the composition of the smectite reflects the changing lake-water chemistry over time. An earlier hypothesis that Mg-bearing smectite was dissolving in the sediments is not evident in the present data. However, this Mg-rich smectite probably formed prior to ca. 5 ka, when a small saline lake existed in the deeper South Basin.

Abstract Although the East African Rift (EAR) System is often cited as the archetype for models of continental rifting and break-up, its present-day kinematics remains poorly constrained. We show that the currently available GPS and earthquake slip vector data are consistent with (1) a present-day Nubia–Somalia Euler pole located between the southern tip of Africa and the Southwest Indian ridge and (2) the existence of a distinct microplate (Victoria) between the Eastern and Western rifts, rotating counter-clockwise with respect to Nubia. Geodetic and geological data also suggest the existence of a (Rovuma) microplate between the Malawi rift and the Davie ridge, possibly rotating clockwise with respect to Nubia. The data indicate that the EAR comprises at least two rigid lithospheric blocks bounded by narrow belts of seismicity (<50 km wide) marking localized deformation rather than a wide zone of quasi-continuous, pervasive deformation. On the basis of this new kinematic model and mantle flow directions interpreted from seismic anisotropy measurements, we propose that regional asthenospheric upwelling and locally focused mantle flow may influence continental deformation in East Africa.

Abstract Lacustrine diatomaceous deposits are widespread in the East African Rift System, occurring in Miocene to Holocene sedimentary units. Problems in their classification can be clarified with a common set of well-defined terms, including: diatomite, siliciclastic diatomite, subdiatomite, diatominc, and diatom-poor. Additional descriptors refer to lamination, fragmentation, and dissolution. Diatom taphonomy and assemblage alteration reflect sedimentation, interface, and analysis controls, including: dissolution, settling rates, fragmentation, and floral mixing. The spatial and temporal distribution of East African diatomaceous deposits, especially in the Kenya and Malawi Rifts, suggest specific models for the development of a variety of diatomaceous facies. The models are based on factors that control the availability of diatom nutrients, including: fluvial inputs, water depth, seasonality of rainfall, wind regimes and their influence on upwelling and overturn, and turbidity currents. In turn, these factors reflect tectonic and geomorphic setting, drainage basin geology and resultant water chemistry, and climatic setting.