Transcript Overview of Bedrock and Surficial Geology of the Pacific Northwest Prepared By: Patrick Stephenson, ES 473 Environmental Geology, Spring 2009 Abstract Constrained by.

Overview of Bedrock and Surficial Geology of the Pacific Northwest
Prepared By: Patrick Stephenson, ES 473 Environmental Geology, Spring 2009
Abstract
Constrained 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 Lowland
Created 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.
Figure 7. Puget geology section (Borden)
Figure 3. Overhead view of
Willamette Valley (Gannett)
Figure 4. Generalized geologic section west to east (Gannett)
Olympic
Mountains
“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
Cascade
Mountains
Geology of the Willamette Valley
Western 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.
Coast Range
Mountains
Figure 1. Physiographic area of Willamette-Puget Lowland region
Puget Lowland Geology
Washington’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.
Conclusion
Situated 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 Cited
Blunt, 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.
Figure 2. Cross-section view of
subduction zone
(Lillie, Robert J.)
Phanerozoic
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.
Figure 6. Geologic Timescale
(Geologic History)
Troost, Kathy G. et al. "Geology of Seattle and the Seattle area, Washington." The Geological Society
America Reviews in Engineering Geology XX(2008): 1-35. Print.
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27 May 2009 <http://www.ohs.org/exhibits/traveling-exhibits/150-million-years/theassembly-continuescascadia-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>.