ABSTRACT
Stratigraphic and chronologic information
collected for Quaternary deposits in the Willamette
Valley, Oregon, provides a revised stratigraphic
framework that serves as a basis for a
1:250,000-scale map, as well as for thickness estimates
of widespread Quaternary geologic units.
We have mapped 11 separate Quaternary units
that are differentiated on the basis of stratigraphic,
topographic, pedogenic, and hydrogeologic properties.
In summation, these units reflect four distinct
episodes in the Quaternary geologic
development of the Willamette Valley:
- Fluvial sands and gravels that underlie terraces
flanking lowland margins and tributary
valleys were probably deposited
between 2.5 and 0.5 million years ago.
They are the oldest widespread surficial
Quaternary deposits in the valley. Their
present positions and preservation are
undoubtedly due to postdepositional tectonic
deformation—either by direct tectonic
uplift of valley margins, or by
regional tectonic controls on local base
level.
- Tertiary and Quaternary excavation or tectonic
lowering of the Willamette Valley
accommodated as much as 500 m (meters)
of lacustrine and fluvial fill. Beneath the
lowland floor, much of the upper 10 to 50
m of fill is Quaternary sand and gravel
deposited by braided channel systems in
subhorizontal sheets 2 to 10 m thick.
These deposits grade to gravel fans 40 to
100 m thick where major Cascade Range
rivers enter the valley and are traced farther
upstream as much thinner valley
trains of coarse gravel. The sand and
gravel deposits have ages that range from
greater than 420,000 to about 12,000 years
old. A widely distributed layer of sand and
gravel deposited at about 12 ka (kiloannum,
thousands of years before the
present) is looser and probably more permeable
than older sand and gravel. Stratigraphic
exposures and drillers’ logs
indicate that this late Pleistocene unit is
mostly between 5 and 20 m thick where it
has not been subsequently eroded by the
Willamette River and its major tributaries.
- Between 15,000 and 12,700 years ago,
dozens of floods from Glacial Lake Missoula
flowed up the Willamette Valley
from the Columbia River, depositing up
to 35 m of gravel, sand, silt, and clay.
- Subsequent to 12,000 years ago, Willamette
River sediment and flow regimes
changed significantly: the Pleistocene
braided river systems that had formed
vast plains of sand and gravel evolved
to incised and meandering rivers that are
constructing today’s fine-grained floodplains
and gravelly channel deposits. Sub-surface channel facies of this unit are
loose and unconsolidated and are highly
permeable zones of substantial groundwater
flow that is likely to be well connected
to surface flow in the Willamette
River and major tributaries. Stratigraphic
exposures and drillers’ logs indicate that
this unit is mostly between 5 and 15 m
thick.
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