Precise 40 Ar/ 39 Ar geochronology for the upper Koobi Fora Formation, Turkana Basin, northern Kenya

Abstract

The Plio-Pleistocene Koobi Fora Formation, about 560 m thick, crops out east of Lake Turkana and is part of the much larger depositional system of the Omo–Turkana Basin. The upper half of the Koobi Fora Formation from just below the KBS Tuff to above the Chari Tuff is particularly notable for its wealth of hominid fossils and archaeological sites. Silicic tuffaceous horizons have provided the basis for stratigraphic subdivision and correlation. Pumice clasts within the tuffs contain anorthoclase phenocrysts, ideal for 40 Ar/ 39 Ar single-crystal dating. Feldspars from pumice clasts in about 15 tephra within the stratigraphic interval from the KBS Tuff to the Silbo Tuff have yielded precise ages that allow much finer definition of the numerical time framework for the sedimentary sequence between the KBS Tuff (1.869 ± 0.021 Ma) and the Chari Tuff (1.383 ± 0.028 Ma) and to yet higher in the sequence to the SilboTuff (0.751 ± 0.022 Ma). These results provide a precise and accurate time scale for the upper part of the sequence in the whole of the Omo–Turkana Basin. A number of these tuffs are recognized elsewhere in East Africa; thus ages determined at Koobi Fora also apply to the wider region.