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Chemistry and Mineralogy of Some Plio-Pleistocene Tuffs from the Shungura Formation, Southwest Ethiopia

Published online by Cambridge University Press:  20 January 2017

A. M. Martz  and
F. H. Brown
A. M. Martz
Affiliation:
Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112
F. H. Brown
Affiliation:
Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112
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Abstract

The Shungura Formation of southwestern Ethiopia has yielded many tens of thousands of vertebrate fossils including hominids and microvertebrates, and in addition has also yielded fossil wood, pollen, and invertebrates. Widespread tuffs have made subdivision and detailed mapping of the formation possible, have provided material for potassium-argon dating, and have allowed direct lithostratigraphic correlation with the Koobi Fora Formation in northern Kenya. The basis for correlation between the two formations is the distinctive chemistry of the tuffs, but systematic chemical variation within some tuffs invalidates some statistical correlation techniques. Here chemical analysis of glass separates and minerals from tuffs of the Shungura and Usno Formations are presented which may allow further ties to be established when data become available on other tuffs of the Koobi Fora Formation. The tuffs consist primarily of glass, but also contain phenocrysts of anorthoclase, hedenbergitic pyroxene, sodic amphibole, ilmenite, titanomagnetite, chevkinite, quartz, zircon, and rarely orthopyroxene and plagioclase. The glasses show evidence of alkali loss during hydration, and are not now peralkaline, although it is likely that they were initially. The source volcanoes were most likely situated within the Ethiopian rift valley, or on its margins.

Type
Original Articles
Information
Quaternary Research , Volume 16 , Issue 2 , September 1981 , pp. 240 - 257
Copyright
University of Washington

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