Geochronology of the Omo Group, Turkana Basin, east Africa
Ian McDougall1 and Francis H. Brown2
1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA
The Omo-Turkana Basin of northern Kenya and southern Ethiopia developed in the northern Kenya Rift about 4.3 Ma ago in the Early Pliocene. Nearly 800 m of sediments, included within the Omo Group, crop out in the basin. Numerous rhyolitic tuffs in the sequence have facilitated not only secure correlations between the individual formations of the Omo Group but they have also provided material for precise 40Ar/39Ar age measurements. Ages have been determined on alkali feldspar crystals from pumice clasts within tuffs in the lower part of the Omo Group up to the level of the KBS Tuff, which has previously been dated at 1.87 ± 0.02 Ma. The results from 17 stratigraphic levels encompassing the 2.4 Ma time interval from the base of the group to the KBS Tuff provide a numerical time framework for the geological history of the lower part of the Pliocene sequence. The new ages, which have a precision in the order of one percent, are all consistent with the stratigraphic order, providing confidence that they accurately record the age of the individual volcanic eruptions, with deposition of the tuffs and pumices occurring shortly thereafter. These new ages provide a precise and accurate time scale for the lower part of the sequence in the whole of the Omo-Turkana Basin, and with earlier published results on the upper part, we now have over 30 dated volcanic eruptions recorded within the sequence.
These results are particularly significant as they provide age estimates for the many hominid and other vertebrate fossils recovered from the sequences in the Omo-Turkana Basin, so that individual fossils often can be assigned an age to better than 50 or 100 thousand years. The importance of these results is that the evolutionary history of hominids is constrained directly by age determinations rather than by assumptions as to the evolutionary stage, and also enables comparisons to be made throughout a much wider area, including in areas outside the Omo-Turkana Basin. In addition, some of the tuffs also are recognized in deep sea sedimentary cores from the Gulf of Aden and the Arabian Sea, enabling correlations into the marine sedimentary record to be made with confidence. In turn, this provides the means of correlating the climatological record in the deep sea sediments with that found in the sediments of the Omo-Turkana Basin which is located well within the African continent. The sedimentological record in the Turkana Basin reflects the climatic changes associated with the Milankovitch cycles, which have a profound effect on the intensity of the monsoonal activity most notably in the Ethiopian Highlands.
Figure 1. Tuff outcrop on skyline within the Omo Group sequence, Ileret area, Turkana Basin