The Acheulian to Middle Stone Age transition: Tephrostratigraphic context for archaeological change in the Kapthurin Formation, Kenya

The disappearance of the Acheulian Industrial Complex from the African archaeological record between 150,000–300,000 years ago is an important, but poorly understood phenomenon. The replacement of the Acheulian by diverse stone tool industries of the Middle Stone Age (MSA) signals profound hominid adaptive changes that led to the appearance of modern humans in Africa prior to their dispersal and global colonization. A succession of Acheulian and MSA sites occur in the Middle Silts & Gravels and Bedded Tuff members of the Kapthurin Formation, Kenya. I address the nature and timing of the Acheulian-MSA transition in the Kapthurin Formation by (1) constructing a comprehensive stratigraphic framework of volcanic ash (tephra), allowing chronological ordering of known archaeological sites, and (2) discovery and excavation of Koimilot (GnJh-74), a MSA site with two stratified lithic assemblages. Tephrostratigraphic correlation shows that Koimilot is the youngest presently known site from the Kapthurin Formation. ^ Correlation of Bedded Tuff Member tephra is by field stratigraphic and textural observations combined with 272 geochemical analyses by electron microprobe of 53 samples from 20 archaeological and geological locales. The Bedded Tuff Member preserves a succession of basaltic followed by trachytic eruptions from a single volcanic source that underwent progressive chemical evolution, the latest stages of which date to ∼235,000 year ago. Stratigraphic position and geochemical composition define eight sub-divisions of the Bedded Tuff Member. Tephrostratigraphic correlation demonstrates the interstratification of Acheulian and MSA sites, and thus erodes any notions of a simple or gradual shift from the Acheulian to the MSA. ^ Excavations at Koimilot (GnJh-74) produced two stratified assemblages formed in a distal alluvial fan environment. The older Locus 1 preserves multiple episodes of local raw material procurement and the production of flakes from radial cores using Levallois and other methods. At the younger Locus 2, artifacts occur as lag deposits within stream channels, but include characteristic large (∼10 cm) Levallois points and elongated flakes. Comparison of Koimilot with other, older Kapthurin Formation sites suggests that although the nature of the transition is complex, sites with MSA lithic technology reflect an elaboration of methods derived from the local Acheulian.