Touchet Formation
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The Touchet Formation (also referred to as Touchet beds) consists of large quantities of gravel and fine sediment which overlays several thousand feet of volcanic basalt of the Columbia River Basalt Group. These Touchet beds are often covered by windblown loess soils which was deposited later.[1][2]
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[edit] Naming of the Touchet Formation
The type locality for the Touchet Formation was originally observed at the confluence of the Touchet River with the Walla Walla River by Flint in 1938. Once the phenomena was correctly identified, numerous other examples were also found through the basin of the former Lake Lewis.[3]
[edit] Creation of the Touchet Formation
The Touchet beds were deposited during the various flood events (e.g., Missoula Floods) that occurred around 16,000 years ago. During the floods, flow through the Wallula Gap was slow enough such that water pooled in a temporary lake, Lake Lewis. Lake Lewis backflooded up the Yakima, Walla Walla, Touchet and Tucannon River Valleys. In these relatively calm arms of the lake, the slack waters were thick with suspended materials eroded from the scablands above. Some of the suspended materials settled out, creating thick Touchet Formation layers or rhythmites which are found throughout these valleys. Typically the heaviest materials settled out first, followed by finer materials, providing a layer with a graduated particle composition.[3]
The side valleys were protected from the violent currents of the main channel; as a result the flood strata laid down by earlier floods was not eroded away by subsequent floods, but was buried and preserved. Other sediments and materials such as volcanic ash from eruptions of Mount St. Helens are interspersed between layers. They serve to date the various Missoula Floods; in 1980, R. B. Waitt studied the Touchet Formation in the wall of the Burlingame Canyon west of Walla Walla, Washington, where he counted at least 41 flood deposit layers. These floods could only occur when glacial Lake Missoula existed, which estimates at 15,300 to 12,700 years ago. Mt. St. Helens "S" ash fell in eastern Washington about 13,000 years B.C.E. and is ten rhythmites below the top of the Touchet beds.[4][5]More recent studies have used carbon dating to establish the ages of deposition for the various layers.
[edit] Locations where Touchet beds are evident
[edit] Touchet river valley
The Gardena Terrace rhythmites can be found most of the way to Walla Walla, Washington. They are extensively exposed at Burlingame Canyon, just southwest of the town of Touchet. [2]
[edit] Walla Walla river valley
[edit] Yakima river valley
The Toppenish Basin and Benton Basin of the lower Yakima river both have extensive Touchet beds.[6]
[edit] Tucannon river valley
[edit] References
- ^ Walla Walla Level 1 Watershed Assessment.
- ^ a b Bjornstad, Bruce (2006). On the Trail of the Ice Age Floods: A Geological Guide to the Mid-Columbia Basin. Keokee Books; San Point, Idaho. ISBN 978-1-879628-27-4.
- ^ a b Carson, Robert J. and Pogue, Kevin R. (1996). Flood Basalts and Glacier Floods:Roadside Geology of Parts of Walla Walla, Franklin, and Columbia Counties, Washington. Washington State Department of Natural Resources (Washington Division of Geology and Earth Resources Information Circular 90). ISBN none.
- ^ Waitt, R.B. (1980). About Forty Last-glacial Lake Missoula Jökulhlaups through Southern Washington. Jour. Geology, v. 88, p. 653-679. ISBN none.
- ^ Waitt, R.B. (1985). Case for Periodic, Colossal Jökulhlaups from Pleistocene Glacial Lake Missoula. Geol. Soc. America Bull., v. 96, p. 1271-1286.. ISBN none.
- ^ USGS publication
