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Journal of the Geological Society

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

¹ Research School of Earth Sciences, The Australian National University, Canberra, A.C.T. 0200, Australia (e-mail: Ian.McDougall@anu.edu.au)
² Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112-0112, USA

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 ⁴⁰Ar/³⁹Ar 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.

This article has been cited by other articles:

				D. J. Lowe Globalization of tephrochronology: new views from Australasia Progress in Physical Geography, June 1, 2008; 32(3): 311 - 335. [PDF]

				I. McDOUGALL and F. H. BROWN Geochronology of the pre-KBS Tuff sequence, Omo Group, Turkana Basin Journal of the Geological Society, March 1, 2008; 165(2): 549 - 562. [Abstract] [Full Text] [PDF]

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