Satus Pass is a small mountain pass on the east side of the Cascades. Traversed by US 97, the pass lies between Goldendale to the south and Toppenish to the north. The ridge the pass cuts through forms the southern boundary of the Yakama Nation in this part of the region. To the south, land is largely privately owned though Brooks Memorial State Park and the Simcoe Mountain Wildlife Area are nearby.
This article was inspired by a magnitude 3.1 earthquake that struck the region on the morning of August 13, 2022. While impacts from earthquakes this small are minimal, I find geology interesting and wanted to write about it.
CASCADE MOUNTAINS
Satus Pass lies in a sort of chaotic geologic region where the east slopes of the Cascades meet the ridges that cross the Columbia Basin. The Cascades formed from volcanic activity resulting from the Juan de Fuca Plate offshore subducting, or diving, beneath the North American Plate. Mount Adams, Mount St. Helens, and the other large Cascade volcanoes can trace their origins to the plate subduction.
COLUMBIA RIVER BASALT GROUP
Closer to Satus Pass, the rocks are also volcanic but don’t originate from Cascade volcanoes. The bedrock is made up of the Columbia River Basalt Group, a massive region of basalt that erupted from vents in Eastern Washington, Eastern Oregon, and Western Idaho. Most of the material was erupted between 14 and 17 million years ago, but small eruptions may have continued as late as 6 million years ago.
Lava flows from this underlie much of the lowlands of the Inland Northwest, including Spokane and the Tri-Cities, and continued down the Columbia River’s path to reach the ocean near Astoria. Outcrops of the Columbia River Basalt Group are common in the Columbia Basin, Columbia River Gorge, and along the northern Oregon Coast. At the Tri-Cities, the basalt may be as much as three miles thick.
CREATING THE RIDGES
After the basalt erupted, the part of the North American Plate that the Pacific Northwest resides on began to rotate. In the Columbia Basin area, this has manifested as compression on the north-to-south axis (meaning Hermiston is very slowly getting closer to Moses Lake). The compression created waves in the rocks, known to geologists as synclines (the trough or low spot of the wave) and anticlines (the ridge or high spot of the wave). The Saddle Mountains and Horse Heaven Hills were created this way. Small faults are present in the rock as a result of this activity.
Rather than being a pass through the Cascade Mountains, Satus Pass runs through the west end of the Horse Heaven Hills which were created by the Simcoe Mountains Anticline. This anticline runs from near Prosser in the east to about 20 miles north of Hood River. Frequent travelers on US 97 know that to the north of the pass there is a valley followed by a smaller ridge, which is the Toppenish Ridge Anticline, before the highway gets into the Yakima Valley.
The east-to-west valley between the Simcoe Mountains and Toppenish anticlines is that area’s syncline. In many places, the Columbia River Basalt is buried beneath gravelly landslide deposits and sediment delivered by small streams like Satus Creek. The topsoil is windblown dust known as loess which is several feet deep in places. It is likely that this syncline produced the earthquake noted above which occurred 12.1 kilometers underground midway between the Horse Heaven Hills and Toppenish Ridge. Satus Creek cuts into the gravel and soil deposited in the syncline, creating its own small canyon.
Before the ridges began to rise, the Columbia River likely flowed out of the north through this region before reaching the Snake (and possibly Clearwater) rivers somewhere near Biggs Junction and continuing onward to the Pacific Ocean. In some places, like Wallula Gap and the Yakima River Canyon between Selah and Ellensburg, rivers were able to cut through the rising rock but this section rose enough that the Columbia River was rerouted toward its present course.
SIMCOE VOLCANIC FIELD
Along US 97 near the top of the pass other basalt flows can be noted. To the west of the highway the Columbia River Basalt Group is the Simcoe Mountains made up of a broad volcanic region stretching from south of Goldendale to the west of White Swan. Eruptions from the Simcoe Volcanic Field were mostly basaltic, which is the same kind of rock that makes up the underlying material. That said, some early eruptions produced rhyolite.
Recent research suggests that despite their proximity to the Cascades, the Simcoe Volcanic Field is not a Cascade volcano. Hundreds of eruptions took place between 4.2 million and 600,000 years ago but it is now considered to be extinct. Goat Rocks, an extinct Cascade volcano south of White Pass, formed and eventually died during this time frame. The Boring Lava Field in the Portland area came to life while the Simcoe area was volcanically alive and is currently believed to be dormant rather than extinct. Mount Adams completely postdates activity in the Simcoe Mountains.
While the volcanic field is fairly large, nearly two-thirds of the eruptions came from vents located between the Simcoe Mountains and Toppenish anticlines. Basalt from the Simcoe region is the same material that makes up the Columbia River Basalt Group but the volume of rock erupted here was far less. The Columbia River Basalt Group is considered to have been a flood basalt eruption which are the largest type of volcanic events known on the planet. In contrast to the hundreds of miles covered by those lava flows, the flows from the Simcoe Volcanic Field rarely made it more than ten miles from the vent that erupted them.
Most of the upward movement of the anticlines that cut through the Simcoe region occurred before it started erupting, but uplift has continued to an extent since then. The earthquake that inspired this article is likely from continued compression of the Columbia Basin. Other evidence can be seen in the volcanic deposits in the highlands. No evidence has been found to suggest that the highlands contained glaciers since it formed, including during the most recent ice age.
REFERENCE MATERIAL
Much of this article was based on the Geologic Map of the Simcoe Mountains Volcanic Field and its accompanying literature. Geologic maps from the Washington State Department of Natural Resources were also used.
GEOLOGIC MAP
The image above is a (very) simplified geologic map based largely on the resources noted above. Here is a more detailed description of some of the areas shown.
CASCADES: Volcanic deposits from Cascade volcanoes. See section above.
GORGE: Narrow canyons carved by the Columbia. Abundant basalt outcrops on the edges, gravel and sand at the bottom.
HIGHLANDS: Ridges lifted by Columbia Basin compression in Washington. Foothills of the Blue Mountains or Cascades in Oregon. In both places, loess soil over Columbia River Basalt. Some small streams have created canyons with gravelly bottoms.
PALOUSE: Loess (wind blown sediment) that in some places is over 100 feet deep. Rolling hills.
SIMCOE: Volcanic deposits from the Simcoe Volcanic Field. See section above.
VALLEYS: River, stream, and Missoula flood deposits. Largely gravel under soil with some basalt outcrops.
VOLCANOES: Active or dormant volcanoes (individually named).
