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Tephrostratigraphy of the Turkana Basin, Northern Kenya and Southern Ethiopia

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Monday, August 31, 2009

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This is a past event.

Bereket Haileab
Carleton College
Northfield, MN

The Turkana Basin of northern Kenya and southwestern Ethiopia is well–known for its mammalian fauna of Pliocene and Pleistocene age. In addition, many important hominid fossils have been discovered in the three formations of the Turkana Basin exposed east and west of lake Turkana and in the lower Omo Valley. Much of our understanding of the stratigraphic relations between these well known Plio-Pleistocene deposits of the Shungura, Koobi Fora, and Nachukui Formations results from geochemical study of the glass fractions of tephra layers interbedded within these sediments. The gross character of a particular tephra layer may change markedly from one locality to another within a region–a tephra layer may be present as a thin air fall deposit at one locality, but as a thick channel fill at another locality. Therefore one can not use lithologic characters as basis of correlations over large areas.

Over the past thirty years more than 1500 of chemical analyses of tephra layers have been obtained from the Turkana Basin. The correlations achieved thus far between the three formations of the Turkana Basin rest on compositional characteristics of these tephra layers. These analyses have been organized into approximately 130 chemical distinct types and 12 major groups. Few tephra layers are widespread, and not all tephra layers have been found in all formations, and each formation has a number of tephra layers present only locally, which reflects localized deposition or erosion.

Tephrostratigraphic study, the use of volcanic ash and tuff beds (tephra layers) is used to arrange all of the tephra layers into a single ordered sequence, has provided isotopic ages and temporal correlations. Because the number of tuffs is so large, individual tuffs are grouped into subsets with respect to widespread ash layers that serve as markers. All tephra layers mapped up to this study are discussed here from oldest to youngest with their type locality, chemical analysis and areal distribution. Those tephra layers that were not defined previously are defined.

Hosts Alex Guth ( and Elisabet Head (

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