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High-resolution stratigraphy and 40Ar/39Ar geochronology of the Ileret Tuff Complex, Omo-Turkana Basin

Ashley Savelkouls , Hayden Dalton, and David Phillips

Abstract

The Omo-Turkana Basin, spanning northern Kenya and southern Ethiopia, is a region of significant paleoanthropological importance. The Okote Member (1.6--1.4 Ma) is notable for its fossiliferous sediments, including well-preserved hominid fossils, interbedded with volcanic ash (tuff) layers. These tuff layers serve as temporal markers in a dynamic sedimentary environment; precise dating of these layers has been crucial for constructing a detailed stratigraphy of the Okote Member. The Ileret Tuff Complex offers a unique opportunity for developing high-resolution stratigraphy for key archaeological sites (e.g. FxJj14E, which contains fossil footprints from multiple hominid species) in the Ileret region. Previous 40Ar/39Ar dating analyses of these tuffs lacked the resolution required to differentiate the ages of closely spaced eruptions. The age uncertainties are further compounded by reworking of sequences by post-eruption fluvial processes. This study presents a high-resolution tephrostratigraphic model for the Ileret Tuff Complex by employing a combined approach of field observations, geochemical fingerprinting and high precision (< 0.1%) 40Ar/39Ar geochronology. Statistical approaches to geochemical fingerprinting, such as principal component analysis (PCA), allow for identification and correlation of volcanic tuffs and their associated pumice clasts. Utilising a Bayesian approach, preliminary 40Ar/39Ar age estimations suggest eruptions at 1.5262 ± 0.0018 (2σ) Ma for the base (Lower Ileret Tuff) and 1.5156 ± 0.0009 (2σ) Ma (Lower Koobi Fora Tuff) for the top of the of the Ileret Tuff Complex. These revised ages and geochemical signatures have important implications for the ages of key hominid fossil sites and our understanding of volcanic behaviour in the region.