Substantial amounts of authigenic clay minerals can accumulate in terrestrial mudstones where the following conditions are met: surface or pore waters are alkaline, aqueous silica activity is high, at least some dissolved magnesium (Mg) is present, and detrital input is relatively low. Availability of Al- or Fe-rich detrital clays likely leads to Mg-rich smectite formation, whereas sepiolite or kerolite is favored in environments with no detrital substrates. Surface waters of the Ngorongoro Crater, Tanzania, provide a good example of alteration of incoming detrital clay minerals into authigenic sediments with strong partitioning of Mg into silicate rather than carbonate phases. Quaternary deposits of Olduvai Gorge (Tanzania) and the Olorgesailie Basin (Kenya) provide end members for comparing processes in highly saline and alkaline settings (Olduvai) versus those in diatomaceous fresher water environments (Olorgesailie). Authigenic clays from around the world suggest that illitization and octahedral alterations are decoupled, and therefore indicative of different processes, emphasizing the need to supplement basal layer X-ray diffraction analyses with analyses of hkl reflections and geochemistry of purified phases. In general, authigenic clay minerals are more common in underfilled lake basins, usually associated with evaporitic basins with siliceous input from volcaniclastics or hydrothermal discharge.

Abstract Wetlands are continental depositional environments and ecosystems that range between ephemerally wet to fully aquatic habitats, and, thus, the character of a wetland soil is directly related to the position of the water table over seasonal and longer timescales. The sediment and paleosol records of wetlands are products of a unique setting that can be both exposed to the atmosphere and water-saturated at the same time. Wetlands tend to occupy low-gradient portions of the landscape in places where the phreatic zone is at least ephemerally exposed at the surface, and hydrophytic vegetation has an opportunity to colonize. Groundwater-fed wetlands are an end member of a continuum of waterlogged environments and are associated with localized groundwater discharge (GWD); e.g., springs and seeps that can sustain permanent saturation. Research has tended to follow one of two parallel tracks: sedimentology or pedology. An objective of this paper is to bring these two separate lines of inquiry closer together. The signature of wetland pedogenesis includes redoximorphic features, enhanced hydrolytic alteration or dissolution of soluble phases, and preservation of biotic indicators of wetland habitats. Histosols (peats) and other hydric soils (indicated by gley color and reduced minerals like pyrite and siderite) are common in sites with a permanently high water table and anaerobic conditions. Illuvial clays, in contrast, record episodes in which wetlands dry out and drainage improves sufficiently for these features to form. A case study from Holocene-age Loboi Swamp, Kenya, illustrates the importance of integrating field observations and laboratory analyses. Wetland conditions were observed through thin section micromorphology, mineralogy, bulk geochemistry, and macro- and microfossils. The record of Loboi Swamp is characterized by the juxtaposition of features indicating episodes of soil saturation alternating with those indicating desiccation. In order to extract the most information recorded in groundwater-fed wetlands, soils and sediments should be studied as part of the larger spatial and climatic frameworks in which they occur.

Abstract 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.