Research School of Earth Sciences
Chronological control of Plio-Pleistocene strata in the Omo-Turkana Basin, Ethiopia and Kenya
Ian McDougall1 and Frank Brown2
1 Research School of Earth Sciences, Australian National
University, Canberra, ACT 0200, Australia
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, with deposition occurring over an area as much as 400 km (N-S) by 70 km (E-W), centred on Lake Turkana. Nearly 800 m of sediments, mainly sands, silts and clays deposited in fluvial, deltaic and lacustrine environments, comprise the Omo Group. Numerous rhyolitic tuffs in the sequence not only have facilitated secure correlations between the formations of the Omo Group but also have provided material for precise 40Ar/39Ar age measurements on single crystals of alkali feldspar separated from pumice clasts within the tuffs. Ages are now available on over 30 stratigraphic levels, all of which are consistent with their relative stratigraphic order (McDougall & Brown, 2006, 2008).
The new ages, which have a precision of the order of 1% (standard deviation of the population), based on pooling of many single crystal ages, are all consistent with the stratigraphic order, providing confidence that they accurately record the timing of the volcanic eruptions, with deposition of the tuffs and pumices occurring shortly thereafter. Thus we now have a robust numerical time framework for the depositional history of the Omo-Turkana Basin.
The majority of the ages lie between 4.2 and 0.75 Ma and have been measured on samples from the three main formations mapped in this large area, with correlations made between sequences on the basis of the distinctive geochemistry of the individual tuffs. The sequence is famous for the very large number of hominin and other vertebrate fossils that have been recovered from it, providing an important record of evolution. It is through the geochronological measurements that we are able to date individual fossils, often to significantly better than 0.1 Ma, through stratigraphic correlations of their position relative to known tuffaceous beds. This has provided an accurate time scale that is independent of assumptions as to the direction and rate of vertebrate evolution. In some cases we are able to correlate the depositional history in the region with paleoclimatic indicators in deep sea cores, related to Milankovitch cycles and the astronomical time scale.
This has been successfully accomplished in relation to the younger Kibish Formation, where correlations have been made with sapropel deposition in the Mediterranean Sea some 3000 km to the northwest (McDougall et al., 2008). With increased precision of the ages, potentially possible using the new generation of multiple collector mass spectrometers for argon isotopic analysis, controls on deposition in the Omo-Turkana Basin related to paleoclimatic factors will become correlateable with the more detailed records in the deep sea cores.
McDougall I, Brown FH (2006) Precise 40Ar/39Ar geochronology for the upper Koobi Fora Formation, Turkana Basin, northern Kenya. Journal of the Geological Society, London 163: 205-220
McDougall I, Brown FH (2008) Geochronology of the pre-KBS Tuff sequence, Omo Group, Turkana Basin. Journal of the Geological Society, London 165: 549-562
McDougall I, Brown FH, Fleagle JG (2008) Sapropels and the age of hominins Omo I and II, Kibish, Ethiopia. Journal of Human Evolution 55: 409-420