A normal magnetostratigraphic polarity interval in the Main Ethiopian Rift at 1.6 Ma: Implications for Acheulean and Homo erectus chronology

Abstract

The timing of Homo erectus and Acheulean emergence in the Early Pleistocene is important to the understanding of the evolution of the genus Homo but is currently insufficiently resolved. Relevant chronologies are often based on a combination of radioisotopic dates and other age indicators such as magnetostratigraphy and biochronology. Here, we report a newly recognized normal magnetozone at ∼1.6 Ma in the Konso Formation, southern Main Ethiopian Rift, Ethiopia. This magnetozone occurs at one of the Konso Fm localities, KGA19, that spans the >1.75- to <1.45-Ma time period otherwise ill-represented in the Formation. We describe the lithostratigraphy and tephrostratigraphy of the KGA19 sedimentary sequence and demonstrate that the Konso Fm Kayle Tuff-1 and the KGA19 Bench Tuff are correlative to the Turkana Basin Orange and Morutot tuffs, respectively. Paleomagnetic analyses of the western sector of KGA19 revealed a normal polarity interval at ∼1.6 Ma, with its age based on ⁴⁰Ar/³⁹Ar dates and tephrostratigraphic correlations. This suggests that the KGA19 normal magnetozone represents the Gilsá excursion otherwise documented in limited marine cores, Chinese Loess sequences, and few lava flows. A review of these records and sediment accumulation rates of fossil-bearing sequences of eastern Africa suggests that short events such as the Gilsá excursion would be detected if sampling resolution is sufficiently high relative to sedimentation rates. In light of these findings and considerations, evaluation of the Melka Kunture magnetostratigraphy suggests that the age of the Garba IV H. erectus and Acheulean is ∼1.6 Ma, not >1.95 to ∼2 Ma.