GEOLOGY 313

Columbia River Flood Basalt Field Trip Report

Notes and observations on our excursion through a part of the basalt flood province

Max Barnett5/16/2016

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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 3South 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 waves tell of a low energy environment with water moving back 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.

Stop 12

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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.

Potato Hill Volcanics, Deary, Stop 13

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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.