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The Mount Edgecumbe Tephra Deposits, a Marker Horizon in Southeastern Alaska Near the Pleistocene-Holocene Boundary

Published online by Cambridge University Press:  20 January 2017

James R. Riehle ,
Daniel H. Mann ,
Dorothy M. Peteet ,
Daniel R. Engstrom ,
David A. Brew  and
Charles E. Meyer
James R. Riehle
Affiliation:
U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508
Daniel H. Mann
Affiliation:
Quaternary Research Center, University of Washington, Seattle, Washington 98195
Dorothy M. Peteet
Affiliation:
Goddard Institute for Space Studies, 2880 Broadway, New York, New York, 10025
Daniel R. Engstrom
Affiliation:
Limnological Research Center, University of Minnesota, Minneapolis, Minnesota 55455
David A. Brew
Affiliation:
U.S. Geological Survey MS 904, 345 Middlefield Road, Menlo Park, California 94025
Charles E. Meyer
Affiliation:
U.S. Geological Survey MS 975, 345 Middlefield Road, Menlo Park, California 94025
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Abstract

Late Pleistocene tephra deposits found from Sitka to Juneau and Lituya Bay are assigned to a source at the Mount Edgecumbe volcanic field, based on similarity of glass compositions to nearvent deposits and on thinning away from Kruzof Island. The sequence of near-vent layers is basaltic andesite and andesite at the base, rhyolite, and mixed dacite and rhyolite on top. The only breaks in the tephra sequence are two 1-mm-thick silt partings in a lake-sediment core, indicating a depositional interval from basaltic andesite to dacite of no more than about a millennium. Tephra deposits at sites >30 km from the vent are solely dacite and rhyolite and are 10,600 to 11,400 14C yr old based on interpretation of 18 radiocarbon ages, including 5 by accelerator mass spectrometry (AMS). Basaltic andesite and andesite deposits nearer the vent are as much as 12,000 yr old.

Discrepancy among radiocarbon ages of upland tephra deposits provisionally correlated as the same grainfall is resolvable within ±2 σ of analytical uncertainty. Comparison of bulk and AMS ages in one sediment core indicates a systematic bias of +600 to +1100 yr for the bulk ages; correlation of tephra deposits among upland and lacustrine sites implies an additional discrepancy of 200–400 yr between upland (relatively too young) and lacustrine ages. In any case, the Mount Edgecumbe tephra deposits are a widespread, latest Pleistocene stratigraphic marker that serves to emphasize the uncertainty in dating biogenic material from southeastern Alaska.

Type
Research Article
Information
Quaternary Research , Volume 37 , Issue 2 , March 1992 , pp. 183 - 202
Copyright
University of Washington

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