Lacustrine, Spring, and Wetland Depositional Systems in Rift Basins
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.
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Continental rift basins have long been of interest to sedimentologists. Of all the terrestrial settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. These records are a product of a complex interplay between several factors that include geological structure and tectonic activity, volcanism, climate and its temporal variability, hydrology, biology and time. Sedimentation in Continental Rifts is a timely update on this exciting interdisciplinary field and presents new approaches and insights into tectonic and structural controls of sedimentation. Other topics included are lacustrine and fluviatile depositional environments and some lesser-known settings, such as springs, wetlands, and paleosols. Several papers consider the behavior of silica in rift lakes, particularly the roles of microorganisms in silica precipitation, whereas others examine the paleoenvironmental importance of freshwater carbonates. The contents of the volume show that sedimentological research in rift basins has progressed beyond basic facies description and general models, and is now focused on understanding the integrative effects of physical, chemical and biological processes in rifts.