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 Continental rift basins have long been of interest to sedimen-tologists. Of all the terrestrial depositional 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. These include geological structure and tectonic activity, volcanism, climate and its temporal variability, hydrology, hydrogeology, biology, and time (Fig. 1 ). The lithological records in rifts, which are naturally dominated by fluvial and lacustrine deposits, have become increasingly prominent in recent years because of their potential for studying long-term climatic changes and for testing hypotheses of orbital forcing (e.g., Olsen, 1986 ; Johnson and Odada, 1996 ). More recently, the continuing quest for the paleontological and cultural records of human origins that are entombed in the sedimentary rocks of the East African rift has raised further questions on the tempo of climatic change, changing paleolandscapes, and the environmental stresses that might have affected human evolution ( Vrba et al., 1995 ; Andrews and Banham, 1999 ). Rapid burial of thick sedimentary fills, high geothermal gradients, and consequent early maturation of lacustrine organic matter, much of which is sapropelic, have made rift basins attractive targets for petroleum exploration (e.g., Robbins, 1983 ; Katz, 1990 ; Lambiase, 1990 , 1995 ). Indeed, most of our understanding of rift-basin geometry has resulted from seismic profiling sponsored by oil companies ( Scholz et al., 1990 ; 1996 ;