13 Catastrophic Glacial Outburst Floods in the Pacific Northwest

Monday, 13 May 2002
1:30 PM - 5:30 PM

PRESERVATION OF A 14, 570 YEAR-OLD COLUMBIAN MAMMOTH IN TOUCHET BEDS NEAR MOXEE CITY (YAKIMA, CO.) WASHINGTON

Paper number: 13-8
Program: THEME SESSIONS
Time: 3:05 PM – 3:20 PM

Authors

Abstract

Remains of a Columbian Mammoth, Mammuthus columbi, (complete tusk, molar and bone fragments) have been found in Touchet Bed deposits near Moxee City, Washington. This location is at the upper extent of Missoula flood deposits in the Yakima Valley. Analysis of a 2 meter deep trench revealed four predominantly fine-grained beds which average 50 cm thick above older alluvial fan deposits. The beds are cut by numerous clastic dikes up to 8 cm wide. The deposits contain clasts of basalt and crystalline rocks foreign to south-central Washington, ranging from sand-sized up to 25 cm in length. The mammoth tusk produced a 14C date of 14,570±50 years, an age consistent with the inferred stratigraphic position of the Touchet Beds below the 13,000 year old Mt. St. Helens "S" tephra couplet.

The Mammoth tusk age is consistent with the hypothesis that earlier Missoula floods were larger and corresponding slackwater deposits more extensive than later ones. Previous observations in the Lower Yakima River valley support the idea of progressively smaller floods. There are 11 post-Mt. St. Helens "S" Touchet Beds near Mabton and 5 such layers 35 km upstream near Zillah. At this site, 35 km farther upstream, there are no Touchet Beds exposed above the tephra layer.