GEOARCHAEOLOGY OF THE PLEISTOCENE WASIRIYA BEDS, RUSINGA ISLAND, KENYA
Recent detailed stratigraphic work in the Wasiriya Beds has focused on the Nyamita locality, where > 31 m of sediment are exposed along an ~1 km transect. At Nyamita in situ and surface MSA artifacts and abundant vertebrate fossils have been documented and collected. Based on radiocarbon dates on gastropod shells, the Nyamita deposits are at least 43 ka, and possibly considerably older. The Wasiriya Beds at Nyamita are composed of sands, silty clays, conglomerate lenses, tuffaceous silts, and at least two cold spring carbonate (tufa) deposits. Exposed tuffaceous silt blocks contain fossil gastropods, weak ped structure, palesol carbonates and root casts. Several well-defined tuffaceous marker beds permit lateral correlation in the field and by geochemical compositional analyses by electron microprobe.
This study found no evidence of Pleistocene lacustrine facies, but instead indicates a complex paleoenvironment of alternating landscape stability and high-energy fluvial conditions, with Pleistocene topography similar to the modern situation: steep gulleys and sediment seasonally shed from local highlands ~2 km away. Further, these observations suggest that lake level during the deposition of the Wasiriya Beds was likely considerably lower than at present. The tufas imply a locally high water table and spring activity suggests some areas on Rusinga were wetter than the broader region. Abundant pedogenic carbonate nodules in the paleosols suggest a seasonal environment with periods of local aridity. Detailed stratigraphic control provides the foundation for a landscape-scale approach to paleoenvironmental and behavioral variability, and is the basis for our ongoing analyses using isotopic, ecological, and archaeological approaches to understand the behaviors of early Homo sapiens in the Lake Victoria region.