Sedimentary Processes and Lithofacies in Lake-Margin Groundwater-Fed Wetlands in East Africa
Groundwater-fed wetlands are common features of many lake margins, and they may leave characteristic deposits that record hydrologic conditions at their time of formation. Geomorphic and sedimentologic observations of wetland complexes on the margins of Lakes Eyasi, Makat (Ngorongoro Crater), and Natron, Tanzania, show that clastic, chemical, and biological processes in the wetlands produce discernible lithofacies variation. Five distinct sets of lithofacies are identified: sands and gravelly sands (S and Sg), silty to sandy muds (Ms), bioturbated muds (Mb), alkaline muds (Ma), and organic-rich rootmats, peat, and clay (R, P, Co). Several wetland depositional subenvironments can be defined on the basis of lithofacies associations, including drainage channels (facies S, Sg, Mb, Ms), marshes (R, P, Co, Mb), hippopotamus flats (Co, Mb, Ms), pools (S, Ms, Mb), and alkaline flats (Ma, Co). Sediment supply, aqueous geochemistry, activity of large mammals, and composition and distribution of vegetation are the primary controls on sedimentation. Sediments in these subenvironments are generally thin (< 1 m), but they are thicker where large mammals like Hippopotamus amphibius generate deep (> 1.5 m) bioturbation zones. Recognizable lithofacies are found up to ∼ 1 km basinward from the sources of spring waters, with down-gradient sediment fining. Very strong evaporatively driven geochemical gradients are seen as well, with increasing salinity and alkalinity downstream and laterally. Wetland sediments are laterally restricted to only a few tens of meters, and they abruptly interfinger with sediments from adjacent fluviolacustrine environments. In all three basins, these Recent deposits overlie a regressive disconformity or hiatus that signifies a drop in lake level, recording the onset of regional aridity during the late Holocene. Such wetland sediments can be important paleohydrological indicators, generally indicating a fluctuation that led to subaerial exposure and colonization of lake flats by wetland vegetation.
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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.