Transcript No Slide Title - Western Oregon University

Quaternary Geology and Geomorphology of the Willamette Basin, Western Oregon
Prepared by:
Ian Macnab
ES 473 Environmental Geology
ABSTRACT
Northern Willamette Valley
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.
• Regionally too thin and fine grained to serve as a major groundwater source, but
instead act as a buffer, limiting the infiltration of agricultural leachate (Iverson,
2002). Boulder fans provide an important localized groundwater source in the
northern valley (O’Connor et al., 2001).
Stayton Subbasin
Fig. 2. (A) Cross sections of subsurface
geology of Willamette Valley. (B) Stratigraphic
framework for Willamette Valley. (O’Connor et
al., 2001)
Columbia River Basalt Group
Marine sedimentary rocks and
Cascade Range rocks
• Major source of groundwater in the valley. Ribbon-like nature of gravel bars can
make locating productive portions of the deposit difficult
B
SUMMARY
1. The sedimentary record provides evidence
of the Quaternary evolution of Willamette
Valley from a high energy, braided channel
system to the lower energy meandering
system present today.
• Commonly only a few meters thick, but in some locations the deposits can be as
thick as 150 m.
• Tectonic deformation and uplift has isolated terraces from further deposition.
• Not regionally important as a source for groundwater due to elevation above the
regional water table.
PLEISTOCENE SAND AND GRAVEL OF THE WILLAMETTE RIVER AND
MAJOR CASCADE RANGE TRIBUTARIES (Qg1, Qg2)
• Flanked by Tertiary marine sedimentary
and volcanic rocks in the Coast Range
to the west and Tertiary and Quaternary
volcanic and volcaniclastic rocks in the
Cascade Range (Orr and Orr, 2000).
• The valley has accumulated up to 400 m
of Neogene and 100 m of Quaternary fill
from the surrounding uplands and the
Columbia River Basin (O’Connor et al.,
2001).
Fig. 1. Generalized geology of the Willamette Valley. Dark line outlines valley
margin (Hagardy)
• 5 to 15 m thick with low to no cementation and compaction
• 2.5 to .5 Ma deeply weathered fluvial sands and gravels underlying terraces
flanking lowland margins and valleys.
• Columbia River Basalts indicate valley
has been a topographic low since at
least 15 Ma. Uplift of the Coast and
Cascade Ranges further defined the
basin (O’Connor et al., 2001).
Alluvium and basin-fill sediment
• 12 ka to present deposits anastomosing and meandering river channel gravel bars
and flanking sand/silt mantled floodplains.
WEATHERED TERRACE GRAVELS (QTg)
• Located in northwestern Oregon as the
southern end of a structural depression
stretching north to British Columbia
(Reckendorf, 1993) (Fig. 1 and 3B).
Missoula flood deposits
HOLOCENE FLOODPLAIN DEPOSITS OF THE WILLAMETTE RIVER AND
MAJOR TRIBUTARIES (Qalc)
A
WILLAMETTE VALLEY GEOGRAPHIC
AND GEOLOGIC SETTING
Eugene
• Sediment grades from coarse boulder and gravel fans in the northern valley to finer
silt and clay in the south.
Southern Willamette Valley
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.
Salem
• 15 to 12.7 ka gravel, sand, silt, and clay from dozens of cataclysmic Glacial Lake
Missoula floods (Fig. 3B).
• Up to 35 m thick blanketing most of the lowland of the Willamette Valley to an
altitude of 120 m above sea level.
INTRODUCTION
Portland
MISSOULA FLOOD DEPOSITS (Qfc, Qff1, Qff2)
•
Widely distributed braided channel alluvium dated from >420 to 12 ka
•
10 to 50 m thick deposits grade from finer grain, thin beds near the Coast
Range to coarser gravels interfingered with 40 to 100 m thick gravel fans
emanating from the mouths of Western Cascade drainage basins (Fig. 3A).
•
The deposits are divided into 2 major sedimentary units by dozens of Missoula
Flood Deposits.
•
Strata generally saturated and a principle groundwater source in the Willamette
Valley. Deposited prior to the floods, unit Qg2 is noticeably more compacted
and cemented than the post flood deposits (Qg1) making the older unit a less
productive aquifer than the younger unit.
References Cited
140
Haggerty, R., Influence of the Missoula Floods on Willamette Valley Groundwater: Unpublished
report, Oregon State University
60
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
Surficial deposits in the Willamette can be divided into 5 major sedimentary units
reflecting four distinct episodes in the Quaternary geologic development of the
Willamette Valley (Fig. 2A and 2B) (O’Connor et al., 2001).
•
•
Fig. 4. Pleistocene deposits
exposed on the east bank of the
Willamette Valley at river mile 102.5
(O’Connor et. al., 2001)
100
QUATERNARY DEPOSITS
Weathered Terrace Gravels (QTg)
Pleistocene Sand and Gravel of the Willamette River and Major Cascade
Range Tributaries (Qg1, Qg2)
Missoula Flood Deposits (Qfc, Qff1, Qff2)
Holocene Floodplain Deposits of the Willamette River and Major Tributaries
(Qalc)
3. Information on the extent and distribution
of the aquifer materials can be used to
estimate amounts of groundwater
available. This data can then be used to
regulate groundwater withdrawal for
sustainable use.
20
300
•
•
2. The unique characteristics of each distinct
sedimentary unit directly impacts its
hydrologic properties. Large clast sizes
combined with low to moderate
cementation and compaction rates allows
both the Pleistocene braided channel
deposits and the Holocene floodplain
deposits to be important regional aquifers.
Although not an important aquifer, fine
grain Missoula flood deposits act as a filter
to limit infiltration of groundwater
pollutants.
B
A
Fig. 3. (A) Thicknesses of coarse Pleistocene
alluvium deposits (contours interval 40 ft). (B)
Glacial Lake Missoula and flood impacted areas.
(Hagardy)
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