Potassium-argon dating of Bed I, Olduvai Gorge, 1961–1972

Abstract Potassium-argon dating of Bed I in Olduvai Gorge was done or supervised by G.H. Curtis and J.F. Evernden in the Department of Geology and Geophysics at the University of California at Berkeley. This work proceeded in three phases, the first of which was the dating of a small number of tuff samples by Evernden and Curtis. The dates ranged from 1.57 to 1.89 Ma, averaging 1.73 Ma, for tuffs interstratified with hominid-bearing deposits. This dating was controversial because of the long span indicated for the Pleistocene and for the antiquity of early tool-making hominids. Koenigswald et al. (1962) obtained a date of 1.3 ± 0.1 Ma for a sample of basalt from below the dated tuffs of Bed I and argued that the dates from tuffs were in error. Evernden and Curtis then dated the basalts at ⩾ 4.4 Ma, which was revised to 1.92 Ma, supporting the earlier dates on tuffs. The stratigraphy of Bed I was worked out over the period of dating, and some dates that made no sense early in the stratigraphic work fell into place later. The second phase of dating was primarily intended to check the earlier date of ∼1.75 Ma for hominid remains and stone artifacts of Bed I. This work was based on a large number of tuff samples and yielded a wide scatter of ages. Contamination by older materials was recognized in some samples and inferred in others, and criteria were established for accepting and rejecting dates. Using these criteria, 15 dates were considered as valid from the total of 41 dates obtained from tuffs above the basaltic lavas. The mean age of the valid dates was 1.74 ± 0.05 Ma, confirming the antiquity of the early hominids. The 15 valid dates were, however, difficult to interpret in terms of the stratigraphy, and further selection was made among the valid dates to estimate a time span of 350,000 years for the interval between the basalts (1.92 ± 0.06 Ma) and the highest tuff dated (1.57 ± 0.05 Ma). The third phase involved dating samples from previously undated horizons and re-evaluating all work done previously on Bed I samples. The highest priority for new dating was Tuff IF, at the top of Bed I, but the results were unsatisfactory, apparently because of undetected contamination by older volcanic materials. Remeasurement of the earlier mass spectograms led to changes in some of the published dates, and the re-evaluated dates, except for those of Tuff IB, still showed a wide scatter, indicating that most of the sampled tuffs were unsuited for accurate KAr dating. Only Tuff IB, with a mean age of 1.786 ± 0.029 Ma, gave results with a precision comparable to laboratory standards. The re-evaluated Bed I dates showed no trend toward younger ages upward, suggesting a relatively short time span for Bed I. Recent 40 Ar 39 Ar dates based on analyses of single crystals mark a great improvement over the KAr dating of Bed I. Dates on all of the marker tuffs fall in the correct stratigraphic order, and the age span of Bed I is ∼0.27 Ma, from 2.02 to 1.75 Ma. Components yielding two or more ages were identified in several tuffs, thus accounting for some and perhaps most of the variability in the KAr dates.