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Pleistocene glaciers, lakes, and floods in north-central Washington State

By
Richard B. Waitt
Richard B. Waitt
U.S. Geological Survey, 1300 SE Cardinal Ct. #100, Vancouver, Washington 98683, USA
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Published:
January 01, 2017
Publication history
24 July 2017

ABSTRACT

The Methow, Chelan, Wenatchee, and other terrane blocks accreted in late Mesozoic to Eocene times. Methow valley is excavated in an exotic terrane of folded Mesozoic sedimentary and volcanic rocks faulted between crystalline blocks. Repeated floods of Columbia River Basalt ca. 16 Ma drowned a backarc basin to the southeast.

Cirques, arêtes, and U-shaped hanging troughs brand the Methow, Skagit, and Chelan headwaters. The late Wisconsin Cordilleran ice sheet beveled the alpine topography and deposited drift. Cordilleran ice flowed into the heads of Methow tributaries and overflowed from Skagit tributaries to greatly augment Chelan trough’s glacier. Joined Okanogan and Methow ice flowed down Columbia valley and up lower Chelan trough. This tongue met the ice-sheet tongue flowing southeast down Chelan valley. Successively lower ice-marginal channels and kame terraces show that the ice sheet withered away largely by downwasting.

Immense late Wisconsin floods from glacial Lake Missoula occasionally swept the Chelan-Vantage reach of Columbia valley by different routes. The earliest debacles, nearly 19,000 cal yr B.P. (= 19.0 k.y.), raged 335 m deep down Columbia valley and built high Pangborn bar at Wenatchee. As Cordilleran ice blocked the northwest of Columbia valley, several giant floods descended Moses Coulee and backflooded up the Columbia. As advancing ice then blocked Moses Coulee, Grand Coulee to Quincy basin became the westmost floodway. From Quincy basin many Missoula floods back-flowed 50 km upvalley past Wenatchee 18–15.5 k.y. ago. Receding ice dammed glacial Lake Columbia centuries more—till it burst ~15 k.y. ago. After Glacier Peak ashfall ~13.6 k.y. ago, smaller great flood(s) swept down the Columbia from glacial Lake Kootenay in British Columbia. A cache of huge fluted Clovis points had been laid atop Pangborn bar (East Wenatchee) after the Glacier Peak ashfall. Clovis people came two and a half millennia after the last small Missoula flood, two millennia after the glacial Lake Columbia flood.

This timing by radiocarbon methods is under review by newer exposure dating—10Be, 26Al, and 36Cl methods.

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GSA Field Guides

From the Puget Lowland to East of the Cascade Range: Geologic Excursions in the Pacific Northwest

Ralph A. Haugerud
Ralph A. Haugerud
U.S. Geological Survey c/o Department of Earth and Space Sciences University of Washington, Box 351310 Seattle, Washington 98195, USA
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Harvey M. Kelsey
Harvey M. Kelsey
Geology Department Humboldt State University 1 Harpst Street Arcata, California 95521, USA
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Geological Society of America
Volume
49
ISBN electronic:
9780813756493
Publication date:
January 01, 2017

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