Interpretations of the magnetostratigraphy of the Hadar hominid site, Ethiopia

Abstract

As part of an investigation to determine the age of the hominidbearing strata of the Hadar Formation of Ethiopia we have determined the magnetostratigraphy of two separate sections of the strata. Two major zones of reversed polarity occur within the predominantly normally magnetized sequence, one at the top and the other in the middle. There are difficulties with the absolute age determinations of volcanic units within the strata which result in uncertainties in correlating the Hadar magnetostratigraphy with the standard magnetic reversal time scale. The age of the upper part of the section is established by the BKT-2 tuff at ∼2.8 Myr (refs 1–4). The K–Ar dating of the Kadada Moumou basalt which lies in the middle of the sequence yields variable results ranging from 3.1 to 3.6 Myr (refs 1–4). Regional geochemical5 and bio-stratigraphical6 studies suggest the younger age for the Kadada Moumou basalt. We show here that this is also the age that results in the most consistent interpretation of the observed magneto-stratigraphy of the formation, which is that the Hadar Formation was deposited during the Gauss Normal epoch and that the two reversals represent the Kaena and Mammoth events within that epoch.