Tephrostratigraphy of the Bedded Tuff Member (Kapthurin Formation, Kenya) and the nature of archaeological change in the later middle Pleistocene

Correlation of volcaniclastic deposits of the Bedded Tuff Member (K4) of the Kapthurin Formation (Kenya) provides the means to assess the nature of archaeological change during the later middle Pleistocene, a time period critical to human evolution but poorly represented at other African localities. Field stratigraphic evidence, and petrographic and electron microprobe geochemical analyses of volcanic glass and phenocrysts, define eight subdivisions of K4 tephra. These include a succession of deposits from a local volcanic source that erupted intermittently, as well as other tuffs likely from different sources outside the Baringo basin. Upper portions of the Bedded Tuff Member date to ∼235,000 yr. The Bedded Tuff Member is underlain by sediments that include the Grey Tuff, dated to 509,000 ± 9000 yr. The tephrostratigraphic framework defined here is used to place Acheulian and Middle Stone Age (MSA) archaeological sites in chronological order. Results show the persistence of Acheulian large cutting tool manufacture after the advent of points, considered an MSA artifact type. Two assemblages from the site of Koimilot record the appearance at ∼200,000–250,000 yr of a variety of Levallois flake production methods, an integral if incompletely understood feature of the MSA, here likely derived from local technological antecedents. Combined evidence from the tools and flake production methods suggest an incremental and mosaic pattern of change in hominin adaptive strategies during the Acheulian–MSA transition.