HIGH-RESOLUTION STRATIGRAPHY AND 40Ar/ 39Ar GEOCHRONOLOGY OF THE OKOTE TUFF COMPLEX

Precise dating of volcaniclastic marker beds has been essential to developing a detailed stratigraphy of the Koobi Fora Formation, Omo-Turkana Basin. The Okote Member is known for its fossiliferous sediments (including well-preserved hominid fossils) interbedded within volcanic tuff layers. Research to determine the eruptive ages of these tuffs has utilised ⁴⁰Ar/³⁹Ar geochronology and tuff geochemistry (tephrochronology). However, more recent developments in geochronology allow for increased precision when determining the ages of eruptive events.

The Okote Tuff Complex outcrops throughout the Turkana Basin, including in the Koobi Fora, Nachukui and Shungura Formations. In the Koobi Fora region, the Complex hosts a series of closely spaced tuffs, primarily outcropping in the south-west of Sibiloi National Park. These tuffs provide key time markers for archaeological localities. Previous ⁴⁰Ar/³⁹Ar dating analyses of these tuffs (via feldspar crystals from entrained pumices) lacked the resolution required to differentiate the ages of closely spaced eruptions. The uncertainties are further compounded by the complexity of the sediments due to reworking by post-eruption fluvial processes.

Here we report revised ages and stratigraphy for the Lower Koobi Fora, Koobi Fora and Karari Blue tuffs using geochemical fingerprinting combined with precise (~1-2 ka resolution) ⁴⁰Ar/³⁹Ar geochronology. Discrete tuffs and their associated pumice clasts were identified using major element geochemistry of glasses and ⁴⁰Ar/³⁹Ar single crystal ages were obtained from anorthoclase grains extracted from the pumices using a high-resolution ARGUSVI/+ multicollector mass spectrometer. Several statistical methods have been applied to the ⁴⁰Ar/³⁹Ar data estimate eruption ages, yielding reproducible, precise ages. Preliminary ages for Lower Koobi Fora and Koobi Fora tuffs are 1.5150 ± 0.0010 Ma and 1.4940 ± 0.0010 Ma, respectively. A combined approach of geochemistry and high-precision ⁴⁰Ar/³⁹Ar geochronology resolves key complexities in the stratigraphy of the Okote Tuff Complex.