40Ar/(39)Ar dating of Chemeron Formation strata encompassing the site of hominid KNM-BC 1, Tugen Hills, Kenya.

A fossil hominid temporal bone (KNM-BC 1) from surface exposures at Baringo Paleontological Research Project site BPRP#2 in the Chemeron Formation outcropping in a tributary drainage of the Kapthurin River west of Lake Baringo, Kenya has been attributed to Homo sp. indet. K-feldspar phenocrysts from lapilli tuffs bracketing the inferred fossiliferous horizon yield single-crystal(40)Ar/(39)Ar ages of 2.456+/-0.006 and 2.393+/-0.013 Ma. These age determinations are supported by stratigraphically consistent ages on higher tuff horizons and from nearby sections. In addition, new(40)Ar/(39)Ar ages on tuffaceous units near the base and top of the formation along the Kapthurin River yield 3.19+/-0.03 and 1.60+/-0.05 Ma respectively. The base of the formation along the Kapthurin River is thus approximately 0.5 Ma younger than the uppermost Chemeron Formation strata exposed at Tabarin, 23 km to the north-northwest. The upper half of the formation along the Kapthurin River was deposited at an average rate of approximately 11 cm/ka, compared to 21-23 cm/ka at Tabarin.