40Ar/39Ar dating of the Kapthurin Formation, Baringo, Kenya

Abstract

The40Ar/39Ar radiometric dating technique has been applied to tuffs and lavas of the Kapthurin Formation in the Tugen Hills, Kenya Rift Valley. Two variants of the40Ar/39Ar technique, single-crystal total fusion (SCTF) and laser incremental heating (LIH) have been employed to date five marker horizons within the formation: near the base, the Kasurein Basalt at 0·61±0·04Ma; the Pumice Tuff at 0·543±0·004Ma; the Upper Kasurein Basalt at 0·552±0·015Ma; the Grey Tuff at 0·509±0·009Ma; and within the upper part of the formation, the Bedded Tuff at 0·284±0·012Ma. The new, precise radiometric age determination for the Pumice Tuff also provides an age for the widespread Lake Baringo Trachyte, since the Pumice Tuff is the early pyroclastic phase of this voluminous trachyte eruption.These results establish the age of fossil hominids KNM-BK 63-67 and KNM-BK 8518 at approximately 0·510–0·512Ma, a significant finding given that few Middle Pleistocene hominids are radiometrically dated. The Kapthurin hominids are thus the near contemporaries of those from Bodo, Ethiopia and Tanzania. A flake and core industry from lacustrine sediments in the lower part of the formation is constrained by new dates of 0·55–0·52Ma, a period during which the Acheulian industry, characterized by handaxes, is known throughout East Africa. Points, typical of the Middle Stone Age (MSA), are found in Kapthurin Formation sediments now shown to date to between 0·509±0·009Ma and 0·284±0·012Ma. This date exceeds previous estimates for the age of the MSA elsewhere in East Africa by 49ka, and establishes the age of Acheulian to MSA transition for the region. Evidence of the use of the Levallois technique for the manufacture of both small flakes and biface preforms, the systematic production of blades, and the use and processing of red ochre also occurs in this interval. The presence of blades and red ochre at this depth is important as blades signify a high degree of technical competence and red ochre suggests symbolic behavior.