This document provides a summary of stops on a geology field trip through parts of Washington and Idaho to observe evidence of the Columbia River flood basalts. Over 14 stops, the group saw a variety of igneous rocks and structures including dikes, sills, folds, faults, and different types of volcanic rocks. Detailed observations and photos were recorded at each stop of features such as columnar joints, xenoliths, and interactions between the basalts and underlying sedimentary layers. The trip covered examples of the multiple flows that deposited over 17 million years.

1. GEOLOGY 313
Columbia River Flood
Basalt Field Trip
Report
Notes and observations on our excursion through a part of the basalt flood
province
Max Barnett
5/16/2016

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On Saturday, May 7, 2016 our Geology 313 class went on a field trip to observe outcrops and
other evidence of the Columbia River flood basalts. We made a total of fourteen stops and saw
geologic structures such as cooling fractures, dikes, sills, an anticline and a syncline, baked
layers at layers of intruded magma, a reverse fault, and a few types of igneous rocks. The trip
took us from Cheney, WA, south to Lewiston, and back to Cheney through Deary, ID. The
following report contains pictures and observations of these stops.
Granite Point, Stop 1
This very old granitic intrusion
took place about 90-100 million
years ago. This outcrop is located
on the Snake River in SE
Washington. The outcrop is seen
on both sides of the river. The
overlying Columbia River flood
basalt layers are approximately 6-
17 million years old. The layers of
basalt are clearly seen overtop the
granite and are indicative of multiple flows. In the picture to the left there is a mixture of
basaltic and granitic xenoliths. Specifically, in this picture above, it is called a leucocratic
enclave.
The granite is clearly deformed in many
ways. In this picture to the right there is a
basaltic dike cutting through the granite
and altering the granite on contact. A
closer inspection will show the burnt
edges where the basalt and granite meet.
This tells us the granite was in place
before the basalt intruded. There are
deformed and undeformed pegmatites in
the granite veins.
The picture to the left shows more deformation to
the granite. There is some sort of fabric to the
texture which may tell us this deformation took
place after it had cooled. Notice how the grains

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seem to be somewhat aligned. We determined this granite to be subsolvus because it has
yielded two feldspars. Experiments have been done to determine this to be metaluminous
granite, meaning they lack certain minerals, such as sodic amphiboles, pyroxenes, and
aluminosilicate minerals. This granite is approximately 50% K-spar and plagioclase, 20% biotite,
10% hornblende, and 20% quartz.
Lewiston Grade, above Lewiston Basin, Stop 2
The Wilbur Creek Member of the Saddle Creek
Formation is the youngest member seen here on
the Lewiston grade. The Lewiston syncline is visible
along with other anticlines and synclines at
different angles to the Lewiston syncline. Also in
this area there were approximately 300 individual
lava flows. These date back over 3 million years
ago. In the picture below a reverse fault is shown
in an outcrop alongside the Lewiston grade.
The grade can be seen from the
side where we were standing on
the left. This is above the Lewiston
Basin and is what drains into the
basin. Notice how erosion has
shaped the way this slope takes
form. There are rolling hills and
some dendritic pathways for the
drainage of water.
The picture above was taken overlooking
the Lewiston basin. Afar off is the Waha
escarpment, created by normal faulting, is
visible.

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Saddle Mountain Formation Syncline, Stop 3
We drove down a steep
grade, from one outcrop to
the next. Although, when
we got to the base of the
hill we saw the same
outcrop as we did before
our descent. This is
possible because we were
in a syncline, like we were
driving on the inside of a
dish. The Saddle Mountain
Formation is the youngest
member in the Columbia River Basalt groups at 6 million years old. This picture above shows an
outcrop at the base of the syncline we drove down. It represents the lower monumental group
of the Saddle Mountain Formation. The rock consists of an aphanitic texture that is lacking in
phenocrysts. The composition is pyroxene, Ca-rich plagioclase, and a little olivine.
Just a few steps away from the location of the photo at the top of the page surprised us with
some unusual mixing of both sedimentary and igneous rocks. The lava flow had met up with a
river bed. Judging by the size and shape of the rocks we knew it was from a moderate to high
energy environment. There are well-rounded cobbles of sedimentary rock located in a clast-
supported matrix.

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Intersection of Rte 12 and Rte 3, Stop 4
In this basaltic outcrop we saw boulder-sized rocks of the Imnaha Formation, which is the
oldest formation at 17 million years old. At a closer glance we found darker colored rocks with
phenocrysts that ranged in 1-2cm wide. The rocks are dark colored and contain olivine and
calcic plagioclase. Some of the olivine has been altered to iddingsite. This is the location where
the CRB-17 thin section in lab originates from.
The photograph to the left is a
sample of the porphyroaphanitic
basalt found at this stop. The
plagioclase phenocrysts are large
enough to determine using a hand
lens.
The photograph to the right shows the outcrop on the
side of a slope. Boulders of basalt containing
plagioclase phenocrysts are being eroded and forming
a talus.
Stop 5
Across the street we saw the Priest
Rapids Member overlaying the
Grande Ronde with some
hummocky features created by
landslides. The photograph to the
left is a sketch of the outcrop, not
to scale.

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Weigh Station. Rte 3
South of Julianna Conventional Park, Stop 6
It's important to note at this location the intrusion
of basalt on top of a sedimentary layer. The
sedimentary layer contains many fossils of aquatic
plant life and other possible aquatic life located in
shales and brittle sandstones.
The baked layer separates the sedimentary layers
from the Grande Ronde Formation. Located in the
sedimentary layer is found lithified sandstone with
lyzengane banding. These bands are rich in iron-
oxide. Also, notice the columns above the baked
layer in the photograph to the left.
symmetrical
lithified wavestell
of a low energy
environmentwith
watermovingback
and forth.

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Stop 7
Not far down the road we came upon some very fine-grained sedimentary layers that were
creating an aquitard and a stream was present. There was possibly a spring present and the
aquitard created a pathway for the water to flow and exit into a stream. A few plant fossils
were found by students.

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Grande Ronde Near Kendrick, Stop 8
This massive outcrop of basalt of the Grande Ronde is about 100 meters thick from the road
running by it to the top. The basalt columns are about 0.5 meters wide and are five- or six-sided
and form because of the process of movement in cooling. These are called cooling fractures and
are fine-grained and contain no phenocrysts. These columns are oriented in many different
angles. Some are nearly horizontal in the center of the dome-shaped outcrop. This is thought to
have taken place in a valley where basaltic lava flows were moving through a valley at a very
slow speed, or the ground somehow collapsed beneath the flow as it was cooling very slowly.
Notice how the columns are almost laying down
horizontally. They start out almost vertically and
then shift to an almost horizontal orientation
dipping away from the place the photo was
taken.

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Stop 9
Moving through the town of Kendrick we observed invasive basalt that had burrowed into
sedimentary rock. This basalt cut through sedimentary layers and pushed up the laminated
sand and silt layers. Graded bedding is present in the sedimentary layers from pebble sized
clasts to very fine-grained varves.
Sandy Interbed, Stop 10
At this stop the stratigraphy of the basalt flow is
very apparent. The soft sandstone below shows
possible crossbeds, which would be indicative of
an aeolian-depositional environment. The
noticeable colonnade layer under the entablature
layer with the baked layer separating the
sandstone and colonnades.
In the photograph to the left the
darker colored basaltic sill is shown
intruding the sedimentary layer on
top.

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Mile 13, North of Kendrick, Stop 11
The circular section that engulfs the rock
hammer seen in the photograph above is
an example of a pillow basalt. Pillow
basalts form when hot lava meets a lake
or other body of water. The lava cools
quickly, leaving it fine-grained with a
polagonite ring around the exterior of the
rock. The polagonite is an altered glass
formed when hot lava is rapidly cooled.
Also found in this area are dendrites,
which are magnesium-oxide formations
on the sides of the rock that have been
formed long after their cooling. They are
called dendrites because of their branch-
like form. The sketch to the right shows
the approximate shape of the dendrites.

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Stop 12
This section that has been eroded
away gives a great example of the
stratigraphy of these ancient
interaction between basalt and
sedimentary rock. Found here are
some grey clasts in the sedimentary
layer. These probably came from
another parent rock nearby. The
Priest Rapids layer contains
paleosols, which are fossilized soils
that have been covered by the flood
basalts and are lithified and preserve
the ancient environment in which it
was deposited. These paleosols
might contain fossils and
sedimentary structures that would
describe the environment of
deposition.

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Potato Hill Volcanics, Deary, Stop 13
These volcanic rocks differ from the Columbia River Basalt group. They are Eocene in
age and have massive xenoliths included in them that range from 6-10cm. These xenoliths are
more felsic than the encasing mafic basalt. They are composed of dacites, rhyolites, and
quartzite and show assimilation, which is contamination of the basalt flow by the country rock.
They were picked up along the way during their journey of being placed, so this is not an
outcrop. These rocks were boulder-sized and are very dense. They have been dated to be
Eocene in age, making these older than the Columbia River Basalt Group.
The picture above shows the size of the xenoliths found in these boulders alongside a
dirt road off of Highway 3.

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Rte 9, North of Deary, Stop 14
In this location there is plenty of unconformities in the orientation of the schist-bearing layers.
There is an igneous and metamorphic contact where pegmatite and granite is found next to a
pelitic schist layer. The schist is foliated because of metamorphism and directed pressure
during its metamorphism. Books of muscovite are found near the pegmatite layer and are 0.5-
2cm wide. The pelitic schist layer is approximately 60% micas and 40% argillite with garnet as
an accessory mineral. In the photograph above, we see vertical beds of the schist. The granite
layers consist of approximately 40% potassium feldspar, 40% quartz, and 20% muscovite. The
sketch below shows the orientation of the granite and schist layers.

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We saw many rock types and structures that show the effect the Columbia River flood basalts
had on the region, from the baked layers on top of sedimentary layers, to the intrusive basalt
that lifted up a granitic layer, to the assimilated xenoliths in transported boulders. The flood
basalts contributed much to the shape of the region with the amount of lava that covered the
surface along with the intrusive dikes and sills that altered the country rock by contact
metamorphism. Many of the flood basalt features can be seen in outcrops alongside the road
from Cheney, WA through Lewiston, ID and Deary, ID.