20. 20. 100. 20. 20. 140. 100. 180. 60. 60. 300. 20. 20. 100. 140. 60. 20. 20. 60. 140. 220. Missoula flood deposits. Quaternary Geology and Geomorphology of the Willamette Basin, Western Oregon. Alluvium and basin-fill sediment. Prepared by: Ian Macnab
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Missoula flood deposits
Quaternary Geology and Geomorphology of the Willamette Basin, Western Oregon
Alluvium and basin-fill sediment
ES 473 Environmental Geology
Columbia River Basalt Group
Marine sedimentary rocks and
Cascade Range rocks
The Willamette Valley of northwestern Oregon is associated with widespread Quaternary-age sedimentary deposits. Understanding the nature of the deposits not only provides a history of Quaternary depositional environments in the valley, but also information on surficial materials that form important regional aquifer systems.
The oldest of the five major Quaternary-age sedimentary units are deeply weathered, 2.5-0.5 Ma fluvial sands and gravels flanking the valley margins. Tectonic deformation isolated these terraces from further deposition. Subsequent lowering of the valley resulted in up to 500 m of fill. The upper 10 - 50 m is comprised of braided channel alluvium that interfingers with 40 - 100 m thick fans emanating from the mouths of Western Cascade drainage basins. Dividing these gravels are dozens of 15 – 12.7 ka Glacial Lake Missoula flood deposits. These strata are up to 35 m thick and are composed of gravel, sand, silt, and clay. During the past 12,000 years, the braided channel system that dominated the depositional environment of the Late Pleistocene evolved into the anastomosing and meandering system present today.
Northern Willamette Valley
Southern Willamette Valley
Fig. 2. (A) Cross sections of subsurface geology of Willamette Valley. (B) Stratigraphic framework for Willamette Valley. (O’Connor et al., 2001)
The Willamette Valley contains widespread Quaternary-age sedimentary deposits. Understanding the origin, distribution, nature, and thickness of these surficial materials is key to understanding Quaternary depositional environments. More importantly, understanding these deposits provides information to aid in the management of the important regional aquifer systems the materials form.
Fig. 4. Pleistocene deposits exposed on the east bank of the Willamette Valley at river mile 102.5 (O’Connor et. al., 2001)
Haggerty, R., Influence of the Missoula Floods on Willamette Valley Groundwater: Unpublished report, Oregon State University
Iverson, J., 2002, Investigation of the hydraulic, physical, and chemical buffering capacity of Missoula Flood Deposits for water quality and supply in the Willamette Valley of Oregon: Dissertation retrieved online [http://hdl.handle.net/ 1957/6601] on May 15, 2008
O’Connor, J. E., Sarna-Wojcicki, A., Wozniak, K. C., Polette, D. J., Fleck, R. J., 2001, Origin, Extent, and Thickness of Quaternary Geologic Units in the Willamette Valley, Oregon: US Geological Survey professional paper 1620, p.1-52
Orr, E. L., Orr, W. N., 2000, Geology of Oregon: Kendall/Hunt Publishing Company, 254 p.
Reckendorf, F., 1993, Geomorphology, Stratigraphy, and Soil Interpretations, Willamette Valley, Oregon: Proceedings of the Eighth International Soil Management Workshop, p. 178-200
Fig. 1. Generalized geology of the Willamette Valley. Dark line outlines valley margin (Hagardy)
Fig. 3. (A) Thicknesses of coarse Pleistocene alluvium deposits (contours interval 40 ft). (B) Glacial Lake Missoula and flood impacted areas. (Hagardy)