Abstract: Actualistic studies of the distribution, formation, and taphonomy of vertebrate and invertebrate traces at saline (60-100 g l -1 TDS), alkaline (pH: 10.5) Lake Bogoria in the Kenya Rift Valley have revealed a diverse trace assemblage in the lake-margin sediments. Although hypersaline lakes like Lake Bogoria restrict lacustrine faunal diversity, local marginal subenvironments, including hot springs and ephemeral streams, provide favorable areas for the activities of many species of insects, mammals, birds, and reptiles. Several factors, including sediment texture and moisture content, substrate cohesion, substrate consolidation, and the presence of a food source (i.e., vegetation, microbes, animal waste, flamingo carcasses), control the distribution of traces at Lake Bogoria by influencing the behavior of vertebrates (e.g., “dirt bathing” and nest building) and invertebrates (mainly feeding and locomotion). The distribution of vertebrate traces is also controlled by the proximity to fresh water, but the relationship between invertebrate trace formation and pore-water salinity is less clear. Many characteristic features of closed-basin lakes, including frequent changes in lake level and shoreline position, the presence of thermal springs, and evolved fluid compositions resulting from evaporation, can have direct impacts on trace taphonomy. Notable taphonomic factors include efflorescent salt crystallization, which may temporarily cement substrates or destroy traces during crystal growth in the capillary fringe; substrate wetting and drying, which can induce soil crusting and the shrinking and swelling of smectitic clays, which in turn can modify trace morphology; and the presence of benthic microbial mats and biofilms, which can temporarily stabilize substrates or contribute to their early cementation by mediating carbonate precipitation. Semiarid environments, such as the Kenya Rift, are favorable settings for the early cementation of substrates by carbonates (e.g., calcite), and, during prolonged, stable dry phases, the preservation of trace fossils and their substrates by zeolites and other minerals (Mn- and Fe- oxyhydroxides).

Abstract: Lake Bogoria is a saline (100 g l –1 TDS), alkaline (pH- 10.3) meromictic lake that lies in an asymmetric half-graben in the semi-arid central Kenya Rift. The lake is fed by about 200 hot springs and ephemeral streams draining a volcanic catchment. Despite its shallow depth (11 m), the brine is stratified and stable meromixis prevails. Sedimentation along the border fault is dominated by major slope failures, many earthquake-induced, and youthful fan deltas. Small deltas on the ramp and axial platforms are slowly prograding lakeward. Within the lake, the littoral zone is dominated by siliciclastic sediments, many deposited by density flows. In the deeper, central part of the lake, sedimentation is mainly chemical-organic, with some detrital fines introduced as hypopycnal plumes. Cores spanning the last 30,000 years reveal organic muds, which formed during periods of intermediate to high lake levels and high microbial productivity, interbedded with evaporites (trona, nahcolite) that were precipitated during arid phases of low lake level. A large suite of authigenic minerals, including sodium silicates and zeolites, is present in the lake sediments. The evidence from Lake Bogoria demonstrates the complex interplay between climatic change and tectonics in a young rift lake, and confirms the important role played by hydrothermal springs in sedimentation in continental rifts.

Abstract Magadiite and cherts from the Magadi basin in southern Kenya and four other localities in the East African Rift all share distinctive compositional systematics: (1) negatively fractionated REE patterns; (2) high absolute concentrations of U, Nb, and Zr (up to 1,500 ppm versus a crustal average of 190 ppm); (3) normalized enrichments of U, Nb, and Zr relative to REE; (4) extreme fractionations of U-Th, Nb-Ta, and Zr-Hf; (5) positive Ce but negative Eu anomalies; and (6) normalized peaks at Mo, Ag, and Sb. Alkaline lake cherts composed of secondary quartz retain the trace-element patterns of the precursor, albeit at lower absolute element contents. Sublacustrine sinters at Lake Bogoria share some of the compositional features of the cherts from Magadi but lack the Ce and Eu anomalies and U-Th, Nb-Ta, and Zr-Hf fractionations. Cherts from ocean-ridge, pelagic, and continental-shelf settings are characterized by progressively larger Zr contents (≤ 170 ppm), but all are distinctly lower than in the alkaline-lake cherts, where micron-scale Zr-rich phases were identified as possible authigenic zircons. Both marine and alkaline-lake cherts share positive Ce anomalies. These result from scavenging of Ce by Fe-Mn oxyhydroxides in seawater, but from the solubility of Ce (IV) in oxidized alkaline brines for the lacustrine cherts. Europium anomalies range from small negative to positive in marine cherts, whereas alkaline-lake cherts feature large negative anomalies. These compositional systematics, which reflect the aqueous environment in which the chert precursor formed, confer a tool for interpreting the paleoenvironment of cherts in the geological record.

Abstract Driven by requests to provide carbonate analogs for subsurface hydrocarbon exploration in rift settings, we have identified and described select examples, summarized them from a carbonate perspective, and assembled them into a GIS database. The analogs (Fig. 1 ) show a spectrum of sizes, shapes and styles of deposition for lacustrine and marginal marine settings, wherein the types of carbonates inferred in the subsurface from seismic and cores (emphasis on microbialites and tufas) can be illustrated. This introductory chapter and overviews of each analog provide the basic descriptions of the analogs and their potential application. The analog examples are grouped as Early Rift Lakes , Other Lakes , and Marine Basins . The Early Rift Lake examples are all from East Africa and include: Lakes Turkana, Bogoria, Natron and Magadi, Manyara, and Tanganyika. Other Lakes includes four examples from the Western United States (Great Salt Lake and high lake level Lake Bonneville, Mono Lake and high lake level Russell Lake, Pyramid Lake and high lake level Lake Lahontan, and Searles Lake) and two from Australia (Lakes Clifton and Thetis). The Marine Basins are Shark Bay from Australia and the Red Sea. Landsat images and DEMs for each analog delineate present and past lake/basin margins, and for several examples the shorelines representing different lake levels can be compared to illustrate changes in size, shape, and configuration that may impact the presence of carbonates. A subset of the examples illustrates the location and various styles of carbonate deposition within lacustrine and marine settings. Links