New single crystal 40 Ar/ 39 Ar ages improve time scale for deposition of the Omo Group, Omo–Turkana Basin, East Africa

Journal of the Geological Society - Tập 169 Số 2 - Trang 213-226 - 2012
Ian McDougall1,2, Francis H. Brown3, Paulo Vasconcelos2, B. E. Cohen2, David S. Thiede2, Michael J. Buchanan3
1Research School of Earth Sciences, The Australian National University, ACT 0200, Australia
2School of Earth Sciences, The University of Queensland, QLD 4072, Australia
3Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112-0112, USA

Tóm tắt

Six tuffaceous beds within the Omo Group of the Omo–Turkana Basin have been dated using the 40 Ar/ 39 Ar single crystal total fusion method on anorthoclase, yielding eruption ages. The Omo Group constitutes up to 800 m of subaerially exposed sediments surrounding Lake Turkana within the East African Rift system in northern Kenya and southern Ethiopia. Rhyolitic explosive eruptions produced tuffs and pumice clasts that are considered to have been deposited shortly after eruption. The new age data on feldspars from the pumice clasts range from 4.02 ± 0.04 Ma for the Naibar Tuff of the Koobi Fora Formation to 1.53 ± 0.02 Ma for Tuff K of the Shungura Formation. The Orange Tuff in the KBS Member of the Koobi Fora Formation was dated at 1.76 ± 0.03 Ma, providing good control in this part of the sequence where formerly there was a >200 ka gap. Data are consistent with earlier measurements and significantly improve age resolution within the Omo Group, which has yielded many vertebrate fossils, including hominin fossils comprising a number of species. We suggest new age estimates for a limited number of hominin specimens. Supplementary material: Eleven tables and nine figures are available at www.geolsoc.org.uk/SUP18506 .

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Journal of the Geological Society

Tập 169 Số 2

213-226

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