SITE FORMATION OF THE LATE AND POTENTIALLY MIDDLE PLEISTOCENE FARRE ARCHAEOLOGICAL SITE, CHALBI BASIN, NORTHERN KENYA

The dispersal of Homo sapiens across and out of Africa during the Late Pleistocene (130 –10 ka) is a major event in the history of our species. These important events are recorded within sedimentary deposits of East African basins. The Late Pleistocene site of Farre is located within the Chalbi basin of northern Kenya, and is an open-air site containing thousands of artifacts and fossils. This study attempts to reconstruct the depositional and weathering environments of Farre through comparison with modern analogues in the Chalbi basin that include proximal, medial and distal alluvial fan settings, dunes and playa.

Today, the Farre site is exposed on an interdune setting along a proximal alluvial fan. Optically-stimulated luminescence (OSL) ages from Farre site sediments show that the site is likely older than 56 ka, which exceeds the maximum age for OSL dates on quartz in the basin. The the overlying sediment and soil are ~22 ka. The nearby Menengai Tuff (36 ka) is at a similar elevation with respect to Farre and is found in comparable stratigraphy and provides support that the site has a Late Pleistocene component. The ratio TiO2/ZrO2, an indicator of parent material uniformity and provenance, was compared between Farre and the modern-analogue sites. Farre has a mean and standard deviation TiO2/ZrO2 of 12.11±1.39, which is most similar to the modern quartz-rich distal fan analogue, 11.79% ± 2.03. The Chemical Proxy of Alteration (CPA) and Weathering Index (WI), were also calculated for Farre and each modern-analogue. The mean CPA and WI values of the artifact-bearing layers at Farre are 77.72% and 9.28% respectively, similar to modern medial fan soils, having values of 74.25% and 12.57%. Although Farre sediment is similar to that found on a quartz-rich distal fan, the CPA and WI data suggest that the weathering pathways differ. One possible explanation is that Farre was situated on a distal fan during occupation. As the alluvial fan prograded and buried the site, Farre layers were subjected to weathering by means of overprinting in a medial or proximal fan setting.