40Ar/39Ar 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 40Ar/39Ar 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 40Ar/39Ar 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 similar to0(.)5Ma 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 similar to11 cm/ka, compared to 21-23 cm/ka at Tabarin. (C) 2002 Academic Press.