Magnetostratigraphy of the Shungura and Usno Formations, southwestern Ethiopia: new data and comprehensive reanalysis

Abstract

Summary. Six hundred new samples are added to those we previously reported from the Shungura and Usno Formations of southwestern Ethiopia, and the entire data set is reviewed. Limited mineralogical study of sands indicates maghemite and hematite as carriers of remanence. After AF cleaning the remanent directions are well polarized into normal and reversed groups, with greater scatter in reversed directions. The time-averaged magnetization axis corresponds to a ‘far-sided right-hand’ virtual pole, a pattern previously found by Wilson. Intrasite dispersion is commonly large but better for the new samples, which we attribute to field technique. Polarity zonation is accomplished by statistical smoothing on stratigraphic plots of angle from dipole axis. Magnetozones are correlated with the standard polarity-chronologic intervals using K-A dates at nine stratigraphic levels. The main change from our previous interpretation is that the reversed zone in the Basal Member of the Shungura Formation is correlated with the top of the Gilbert Epoch rather than with the Mammoth event of the Gauss Epoch. Thus the Basal Member and Member A are somewhat older than we previously believed, but age assignments for major fossil localities are unaffected. The inferred age ranges for the Shungura and Usno Formations are 3.5 to 0.8 Myr and 3.7 to 2.8 Myr respectively. The present analysis yields five zones of normal polarity in the lower Matuyama Reversed Epoch, that is, between the Matuyama/Gauss transition and the base of the Olduvai. All were found both in the previous data and in the new samples taken at different locations. For three of the five zones an unbroken series of normal samples extends over several lithologies, so a geomagnetic event is indicated.