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Magnetic Polarity Stratigraphy of the Middle Pleistocene (Ensenadan) Tarija Formation of Southern Bolivia

Published online by Cambridge University Press:  20 January 2017

Bruce J. MacFadden ,
Oscar Siles ,
Peter Zeitler ,
Noye M. Johnson  and
Kenneth E. Campbell Jr.
Bruce J. MacFadden
Affiliation:
Florida State Museum, University of Florida, Gainesville, Florida 32611 USA
Oscar Siles
Affiliation:
Servicio Geologico de Bolivia, La Paz, Bolivia
Peter Zeitler
Affiliation:
Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 USA
Noye M. Johnson
Affiliation:
Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 USA
Kenneth E. Campbell Jr.
Affiliation:
Natural History of Museum of Los Angeles County, Los Angeles, California 90007 USA
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Abstract

The Tarija Formation of southern Bolivia, which is well known for its classic vertebrate faunas, is of prime importance in understanding of the chronology of the Ensenadan Land Mammal Age. This formation consists of well-exposed and relatively fossiliferous sections of clays, clayey silts, sands, gravels, and tuffs which were deposited in a predominately fluviatile regime in a Pleistocene structural basin. Four stratigraphic sections, each measuring 110 m or less, were studied to establish a magnetic polarity stratigraphy. Paleomagnetic samples were collected from the finer-grained sediments at 100 sites spaced at stratigraphic intervals of 5 m or less. All paleomagnetic specimens were demagnetized in alternating fields of least 250 oersteds (oe). Some specimens were also thermally demagnetized at 200°C or more. Of the 100 sites, 77 were ultimately used to determine the magnetic polarity zonation. Based on the four sections sampled, the Tarija Formation spans a time interval from about 1 my to about 0.7 my B.P. or perhaps younger. The lower half of the composite section is of reversed polarity punctuated by a short normal event. This sequence probably represents the late Matuyama chron with the Jaramillo subchron. The upper part of the section is of normal polarity and represents early Brunhes time. A tuffaceous unit 43 m above the Brunhes-Matuyama boundary yielded a fission track (zircon) age of 0.7 ± 0.2 by B.P. These data indicate that the classic Tarija fauna is middle Pleistocene Ensendan in age.

Type
Original Articles
Information
Quaternary Research , Volume 19 , Issue 2 , March 1983 , pp. 172 - 187
Copyright
University of Washington

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