Department of Geology
Eastern Washington University

Recent Masters theses in Geology

DESCRIPTION AND INTERPRETATION OF LATE QUATERNARY SEDIMENTS
IN THE ROCKY REACH OF THE COLUMBIA RIVER VALLEY,
DOUGLAS COUNTY, WASHINGTON

Stan Gough

Abstract

Late Quaternary sediments at two locations in the Columbia River Valley of
eastern Washington contain three airfall beds (thin, intermediate and thick
GP) of Glacier Peak tephra that appear to be correlative with layer B of
Porter (1978).  Radiocarbon dates bracketing the three tephra beds provide
a lower limiting date of 11,630 ± 80 yr B.P. and upper limiting dates of
11,500 ± 60 yr B.P. and 11,350 ± 60 yr B.P.  Electron microprobe analyses
of samples from the thin and thick GP beds confirm the source vent as
Glacier Peak volcano.  At the Chelan Fall locality reworked lapilli tephra
clasts, termed the early tephra, were deposited above glacial outwash
gravels and prior to the thin, intermediate, and thick GP tephra beds as
well as charcoal dated to 11,630 ± 80 yr B.P.  Two electron microprobe
analyses of the early tephra suggesting strongest correlation with one of
layers Y, W or J of Mount St. Helens appear to be erroneous.  Stratigraphic
and grain-size data support identification of Glacier Peak as the source
vent for the early tephra.  A late, relatively small glacial outburst flood
flowed down the Columbia Valley depositing about 5 m of sands at the Orondo
locality, subsequent to the deposition of the thin, intermediate, and thick
GP tephra beds and charcoal dated to 11,350 ± 60 yr B.P.  This outburst
flood, which postdates the final draining of Lake Missoula, was
contemporaneous with the earliest known human occupation of eastern
Washington.  Evidence of Clovis phase human occupation may be present,
although deeply buried, on valley bottom landforms such as alluvial fans in
the Rocky Reach of the Columbia Valley.