AbstractConstrained by the Cascade Mountain Range to the east and Oregon

Coast Range to the west, the Willamette River basin represents a forearc depositional environment in the Cascadia subduction zone. Sediment deposition varies through time depending on the morphology of the river system, punctuated by localized volcanism. Long-term Tertiary and Quaternary depositional processes have resulted in complex stratigraphy, with interbedded sedimentary and volcanic strata.

Similar to the Willamette Valley, the Puget Lowland receives drainage from the Washington Cascade Mountains, Washington Coast Range, and Olympic Mountains on the north end of the Olympic Peninsula. The Puget Lowlands form a narrow gap between the confining mountain ranges and is underlain by a wide variety of materials, including poorly sorted fluvial deposits, lahar deposits, and formations of glacial till and outwash. The combination of older consolidated bedrock at depth, and overlying surficial sediments, establishes the geologic framework upon which to assess the potential for seismic-related hazards in western Oregon and Washington.

General Geologic Area and Setting of Willamette-Puget LowlandCreated by the collision between the Pacific and North American plates, the

Cascade, Olympic, and Coast Range Mountains constrain a narrow strip of land that is the Willamette Valley and Puget Lowland. This region was quickly settled by the first white settlers for the abundant natural resources and access to navigable water ways for shipping and trade. Due to no previous written history, the hazards associated with this region had gone unnoticed until recent work done by geological scientists uncovered evidence of large scale geologic events that could pose risks to rural and urban population centers.

Geology of the Willamette ValleyWestern Oregon is an active continental margin where the Pacific plate is

obliquely subducting beneath the North American Continent, creating a deep forearc basin east of the subduction zone. Marine volcanic rocks that make up the basement material of the Coast Range were formed during the Eocene and in some areas exceed 10,000 ft in thickness. Middle Eocene to early Oligocene marine sandstone, siltstone, claystone, and shale were deposited on top of the Eocene volcanic rocks, with thicknesses exceeding 23,000 ft. Due to uplift and deformation of rock layers during subduction and accretion, these sedimentary rocks are now the basement material for the Willamette Valley. (Gannett et al.) Post accretion volcanic processes formed the mountains of the high Cascades and during formation thin lenses of volcanic material had been deposited along the eastern edge of the valley. Erosion of the high Cascades has created gravel rich depositional layers, which exhibit both high permeability and porosity, making excellent aquifers for agricultural and municipal use.

ConclusionSituated within mere miles from one another; the Willamette Valley

and Puget Lowland are both principal examples of convergent zone forearc basins. Bordered by subduction zone volcanism on the eastern margin and accretionary prism coastal mountains to the west. Throughout formation these lowlands have accumulated copious amounts of rich soils that have made the Willamette Valley one of most flourishing agricultural areas in the western United States. Moreover, during uplift and erosion of the Western and High Cascades, considerable amounts of fluvial gravels were deposited throughout the eastern portion of the lowlands. These processes created ideal conditions for plentiful groundwater aquifer systems to support extensive agricultural growth, and provide abundant municipal water supplies for growing urban centers.

References CitedBlunt, David J. et al. Washington. Washington Division of Geology and Earth Resources.Chronology of Pleistocene Sediments in the Puget Lowland, Washington. Olympia, Washington: GPO, 1987. Print.

Yeats, Robert S. et al. "Tectonics of the Willamette Valley, Oregon." Assessing Earthquake Hazards and Reducing Risk in the Pacific Northwest (1996): 183-222. Print.

Gannett, Marshall W. et al. United States. United States Geological Survey.Geologic Framework of the Willamette Lowland Aquifer System, Oregon and Washington. United States Government Printing Office, Washinton: GPO, 1998. Print.

Borden, Richard K. et al. Washington. Washington Division of Geology and Earth Resources.Late Pleistocene Stratigraphy in the South-Central Puget Lowland, Pierce County, Washington. Olympia, Washington: Publications: Washington Division of Geology and Earth Resources, 2001. Print.

O'Connor, Jim E. et al. "Origin, Extent, and Thickness of Quaternary Geologic Units in the Willamette Valley, Oregon." Quaternary Deposits in the Willamette Valley (2001): 1-52. Print.

Troost, Kathy G. et al. "Geology of Seattle and the Seattle area, Washington." The Geological Society of America Reviews in Engineering Geology XX(2008): 1-35. Print.

Lillie, Robert J.. The Assembly Continues: Cascadia Subduction Zone. 2008. The Oregon Historical Society. 27 May 2009 <http://www.ohs.org/exhibits/traveling-exhibits/150-million-years/the- assembly-continues-cascadia-subduction-zone.cfm>.

"Geologic History of Southern California." USGS: Western Earth Surface Processes Team. 26 May 2006. United States Geological Survey. 27 May 2009 <http://geomaps.wr.usgs.gov/socal/geology/geologic_history/index.html>.

Overview of Bedrock and Surficial Geology of the Pacific NorthwestPrepared By: Patrick Stephenson, ES 473 Environmental Geology, Spring 2009

Figure 1. Physiographic area of Willamette-Puget Lowland region

Coast RangeMountains

OlympicMountains

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Figure 2. Cross-section view of subduction zone (Lillie, Robert J.)

Figure 3. Overhead view ofWillamette Valley (Gannett)

Figure 4. Generalized geologic section west to east (Gannett)

“Figure 82. Structural cross section between Corvallis and Lebanon, Oreg., showing channel and overbank facies of unnamed fluvial sedimentary deposits, high-terracegravels, late Pleistocene outwash deposits of the Rowland Formation, and catastrophic flood deposits of the Willamette Formation. Data are from water wells, engineering bore holes, and petroleum exploration wells.” (Yeats)

Figure 5. Mid Willamette Valley cross section

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Figure 6. Geologic Timescale(Geologic History)

Puget Lowland GeologyWashington’s Puget Lowland in many ways mirrors

Oregon’s Willamette Valley in geologic structure and material due in large part to their proximity to one another and the convergent margin. Yet, one notable variance is glaciations of the Puget Sound during the late Pleistocene (300,000-10,000 yr B.P.). Work had been done to try and make regional correlations between stratigraphic layers, but had been hampered by limited outcrops, complexity of the sequences, and unreliable dating techniques for Pleistocene sediments older than about 45,000 yr B.P. (Borden et al.) Originally thought to be two glacial units separated by one non-glacial unit was later revised to be four glacial units separated by three non-glacial units. “Glacial sedimentary units are dominated by ice-contact facies such as till, glacialacustrine facies, and high-energy glaciofluvial facies.” Glacial drifts are most easily identified by glacial till which lacks sorting and bedding, has matrix support of the clasts, and relatively dense nature make it easier to distinguish from most non-glacial deposits except lahars.

Figure 7. Puget geology section (Borden)