K-Ar and 40Ar/39Ar dating of the hominid-bearing Pliocene-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya

Abstract

In the Koobi Fora region, east of Lake Turkana, northern Kenya, there occurs a sequence ∼500 m thick of lacustrine, fluvial, and deltaic sediments that contains abundant vertebrate fossils, including hominids, as well as stone tools. Rhyolitic tuffs within the sedimentary sequence have facilitated stratigraphic mapping. Some of the tuffs contain pumice clasts, from which anorthoclase phenocrysts have been separated, providing ideal material for K-Ar and 40 Ar/ 39 Ar dating. Seven tuffs have been dated, generally yielding concordant ages on multiple samples from each tuff. Results are consistent with the stratigraphic sequence. The 40 Ar/ 39 Ar age spectra give nearly ideal flat patterns, indicating that the feldspars have remained undisturbed since crystallization and cooling, with no evidence of thermal overprinting. As the pumice clasts are considered to have been deposited very soon after their eruption, the measured ages provide a close approximation to that of deposition of the tuffs. The Moiti Tuff, ∼30 m above the base of the sequence, has a maximum age of 4.10 ± 0.07 Ma. The Toroto Tuff, some 70 m higher in the sequence, is very securely dated at 3.32 ± 0.02 Ma. Stratigraphically higher tuffs and their ages include the Ninikaa Tuff, 3.06 ± 0.03 Ma; the KBS Tuff, 1.88 ± 0.02 Ma; the Malbe Tuff, 1.86 ± 0.02 Ma; the Chari Tuff, 1.39 ± 0.02 Ma, and the Silbo Tuff, 0.74 ± 0.01 Ma, near the top of the sequence. The geochronology and geological data indicate at least three hiatuses in the sequence, each on the order of 0.5 to 0.7 Ma duration. Conventional K-Ar age measurements on basalts from the basin margin suggest that deposition of the Koobi Fora Formation began no earlier than ∼4.3 Ma ago in the early Pliocene. These results provide a well-documented numerical time framework for the sedimentary sequence in the Koobi Fora region and for the hominids and other fossils contained therein. Most of the hominid fossils, including forms assigned to the Homo lineage and a coexisting Australopithecus lineage, occur within sediments deposited in the basin over the interval from ∼2.0 to 1.4 Ma ago in the late Pliocene and early Pleistocene.