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Late Neogene volcanics and interbedded palaeosols near Mount Kenya

By
W. C. Mahaney
W. C. Mahaney
Department of Geography, 4700 Keele St., N. York, Ontario, M3J 1P3, Canada and Quaternary Surveys, 26 Thornhill Ave., Thornhill, ON L4J 1J4, Canada
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René W. Barendregt
René W. Barendregt
University of Lethbridge, Department of Geography, Lethbridge, Alberta, T1K 3M4, Canada
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Mike Villeneuve
Mike Villeneuve
Continental Geoscience Division, Geological Survey of Canada, Ottawa, ON, K1A 0E8, Canada
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Jaroslav Dostal
Jaroslav Dostal
Saint Mary's University, Department of Geology, Halifax, Nova Scotia, 83H3C3, Canada
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T. S. Hamilton
T. S. Hamilton
Camosun College, Department of Chemistry and Geosciences, Lansdowne Campus, 3100 Foul Bay Rd., Victoria, British Columbia, V8P 5J2, Canada
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Michael W. Milner
Michael W. Milner
Department of Geography, 4700 Keele St., N. York, Ontario, M3J 1P3, Canada and Quaternary Surveys, 26 Thornhill Ave., Thornhill, ON L4J 1J4, Canada
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Published:
January 01, 2011

Abstract

Two lava flows with interbedded palaeosols outcrop c. 40 km SW of Mount Kenya, near the Amboni River north of Mweiga, Kenya along the Nyeri/Thompson Falls Road, at 0°18′S; 37°48′E. These flows, overlain by loess, are principally trachyandesite and form the base of the Mount Kenya Volcanic Series which, in the early literature, is described as being of probable Miocene/Pliocene age. Here we report 39Ar/40Ar dates (c. 5.2–5.5 Ma) and reversed magnetizations which establish a Latest Miocene to Earliest Pliocene age for these flows. Weathering characteristics of palaeosols interbedded with the lavas indicate generally dry climatic conditions during the Late Miocene, punctuated with humid events during the Pliocene and Quaternary. These Late Miocene–Quaternary palaeosols depict a relatively long and complex weathering history, followed by loess deposition. The palaeosols appear to have been episodically deflated, initially in phase with the deposition of lavas when surfaces were devoid of vegetation and later during periods of climatic deterioration when wind systems intensified. Such weathering histories within palaeosol profiles are also documented on nearby Mount Kenya, where well-weathered lower palaeosol horizons developed on Matuyama-age tills are overlain by much younger less-weathered horizons developed on Brunhes-age loess. The geochronology of Late Miocene lavas reported here provides maximum ages for weathering histories of palaeosols formed in a xeric tropical highland climate.

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Geological Society, London, Special Publications

The Formation and Evolution of Africa: A Synopsis of 3.8 Ga of Earth History

D. J. J. Van Hinsbergen
D. J. J. Van Hinsbergen
University of Oslo, Norway
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S. J. H. Buiter
S. J. H. Buiter
Geological Survey of Norway
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T. H. Torsvik
T. H. Torsvik
University of Oslo
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C. Gaina
C. Gaina
Geological Survey of Norway
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S. J. Webb
S. J. Webb
University of the Witwatersrand, South Africa
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Geological Society of London
Volume
357
ISBN electronic:
9781862396050
Publication date:
January 01, 2011

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