Magnetostratigraphy of the Koobi Fora Formation, Lake Turkana, Kenya

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Publication Type Journal Article
School or College College of Mines & Earth Sciences
Department Geology & Geophysics
Creator Brown, Francis Harold
Other Author Hillhouse, J. W.; Cerling, T. E.
Title Magnetostratigraphy of the Koobi Fora Formation, Lake Turkana, Kenya
Date 1986
Description The Koobi Fora Formation, a Pliocene and Pleistocene sequence of sedimentary deposits northeast of Lake Turkana, has yielded numerous fossils and stone artifacts of early hominids. Stratigraphic correlation of the hominid-bearing deposits throughout the Turkana region was established primarily by the chemistry and isotopic ages of volcanic tuffs and complemented by magnetostratigraphic studies. We have reinterpreted previously published magnetostratigraphy from the upper part of the Koobi Fora Formation because the original stratigraphy and dating of tuffs have been revised. In our reinterpretation we include previously unpublished data from the uppermost part of the formation. The upper magnetozones correlate with parts of the Brunhes Normal-Polarity Chron and Matuyama Re versed-Polarity Chron (about 0.6-0.85 Ma) and are separated from the magnetozones of the upper part of the Matuyama (2.0-1.25 Ma) by a disconformity. The Olduvai Normal-Polarity Subchron is represented within the Matuyama, but the lower part of the Matuyama (2.4-2.0 Ma) is missing due to an erosional disconformity. We have also determined magnetozones in the lower part of the Koobi Fora Formation, which had not been sampled for paleomagnetism during the earlier studies. Our time calibration of the magnetozones is made possible by isotopic dating of several tuffs and by chemical correlation of Koobi Fora tuffs with dated tuffs in the Shungura Formation of southern Ethiopia. The tephra correlations are corroborated by the excellent concordance between the magnetostratigraphies of the Koobi Fora and Shungura formations. The lower part of the Koobi Fora spans the interval from about 4 Ma to 2.4 Ma, within the Gilbert Re versed-Polarity and Gauss Normal-Polarity chrons. Rock magnetic studies indicate that detrital magnetite carries most of the stable remanence, although hematite contributes to the remanence as indicated by thermal demagnetization. The hematite, which presumably formed by postdepositional oxidation of original iron oxide grains, carries chemical remanent magnetization (CRM) that diminishes the sharpness of polarity zone boundaries. The CRM accumulated continuously within the uppermost 10 m of sediment below the land surface, so that the CRM reinforces the depositional remanent magnetization within thick magnetozones but obscures magnetozones having durations of roughly less than 70,000 years in sections where the sedimentation rate was approximately 15 cm/1000 years.
Type Text
Publisher American Geophysical Union (AGU)
Volume 91
Issue B11
First Page 581
Last Page 511
Language eng
Bibliographic Citation Hillhouse, J. W., Cerling, T. E., & Brown, F. H. (1986). Magnetostratigraphy of the Koobi Fora Formation, Lake Turkana, Kenya. Journal of Geophysical Research, 91(B11), 11,581-11,595.
Rights Management Hillhouse, J. W., Cerling, T. E., & Brown, F. H., © Magnetostratigraphy of the Koobi Fora Formation, Lake Turkana, Kenya, Journal of Geophysical Research, 91(B11), 11,581-11,595, 1986 American Geophysical Union. Reproduced/modified by permission of American Geophysical Union.
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Format Extent 13,443,824 bytes
Identifier ir-main,16888
ARK ark:/87278/s6v41cch
Setname ir_uspace
Date Created 2012-06-13
Date Modified 2013-02-05
ID 703439
Reference URL https://collections.lib.utah.edu/ark:/87278/s6v41cch
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