Washington State: Land of Geologic Complexity

View fullsize

Geologic map of the state of Washington with physiographic provinces labeled. Washington Department of Natural Resources.

Based on the February 14, 2020 lecture by Dr. Marli Miller, Department of Earth Sciences, University of Oregon and her book co-authored with Dr. Darrel Cowan.

by Carol Hasenberg

GSOC members were delighted to welcome back Dr. Marli Miller to speak to the society about her latest book, Roadside Geology of Washington Second Edition. Miller co-wrote the book with her former PhD. Thesis advisor, Dr. Darrel Cowan of the University of Washington, whose experience and perspective in understanding the geological framework of the state was invaluable. In her opening remarks, she said that her favorite thing to say about writing these (roadside geology) books is that she learns so much.

In her lecture Miller broke down the geological history of Washington into a series of events, which define the physiographic provinces of the state, because it is the bedrock of a region that influences the outward appearance. The original western boundary, the Laurentian margin, is now on the eastern margin of Washington, and these ancient rocks are highly deformed and metamorphosed to varying degrees. A series of accreted terranes, starting with the Quesnellia terrane accreted in the Jurassic, and the latest Siletzia, which stretches from southern Oregon to southwestern British Columbia, and which accreted 50 million years ago, comprise the basement for the remaining part of the state. Atop and within these lie volcanic and sedimentary bodies that covered or erupted through the basement rocks. Stitching plutons were igneous masses that erupted as a result of terranes accreting to each other or to the continent.

The Columbia Basin Province, bordered on the north and west by the Columbia River, was inundated during the Miocene by vast floods of basalt, and later in the Pleistocene by the waters of the enormous Ice Age Floods. Glaciation from the Cordilleran Ice Sheet in the north and Puget Sound area and mountain glaciers in the North Cascades and Olympic Mountains heavily marked the state’s geography, and also deposited unsorted glacial till in vast areas, wind-blown loess and glacial outwash features.

All these events created a number of physiographic provinces within the state: The Okanogan Highlands province in the northeastern corner, home to the oldest rocks in the state, preserves the longest geologic history. To the south of this lies the vast Columbia Basin, land of flood basalts and channeled scablands. The western Coast Range Province contains the Willapa Hills to the south and the Olympic Peninsula to the north, the latter one of the finest examples of an accretionary wedge complex that one can find anywhere. Directly to the east of the Coast Range lies the Puget Lowland, consisting of Puget Sound and the flat country adjacent to its east and south shores, a forearc basin heavily scoured by the Puget Lobe of the Pleistocene Cordilleran Ice Sheet. To the east of this is the volcanic arc of the Cascadian Subduction Zone, and it is divided into two provinces, the Southern Cascades, purely the expression of the volcanic arc with Mt. Rainier, Mt. Adams, and Mt. St. Helens as its prominent peaks; and the Northern Cascades, an area under so much compressive stress that the basement rocks have been uplifted to dominate the scenery, and with a couple of arc volcanoes, Mt. Baker and Glacier Peak, thrown in for good measure.

Roadside Geology of Oregon

By Miller, Marli B.

Buy on Amazon

After describing the basic physiographic provinces of Washington, Miller described the geology of Washington as characterized by complexities. She therefore treated the audience to a series of vignettes which illustrate the complex nature of Washington geology.

One of Miller’s favorite topics throughout the lecture was the Siletz Terrane, or Siletzia, a large body which forms the basement rock from southern Oregon to the south of Vancouver Island. This terrane thus forms the basement in the Coast Range Province of Washington, and the rocks from this volcanic island arc are known as the Crescent Formation here, from the Crescent Lake type location in the Olympic Peninsula. And excellent examples of pillow basalts from the Crescent Formation can be viewed as one drives along Hurricane Ridge Road in the Olympics, the first vignette described by Miller.

Not a part of the Siletz Terrane, but an oceanic accretionary wedge, the core of the Olympic Mountains is composed of slightly metamorphosed sedimentary rocks. The sediments were thrust under the Siletz Terrane and are highly folded and deformed, then uplifted starting around 16 million years ago and exposed by erosion. Amongst the most interesting are the mélanges of the Hoh assemblage on the western coast of the Olympics, where the layers are mixed into a jumble. The sea stacks of Ruby Beach are erosional remnants of this mixture.

Miller then proceeded across the northern section of Washington on state highway 20, and her first stop was Anacortes on Fidalgo Island, which is almost entirely composed of the Fidalgo Ophiolite, a slice of late Jurassic oceanic crust and early Cretaceous sediments that were accreted approximately 100 million years ago. The origins of the San Juan-Cascade Nappes sequence of terranes which contains the ophiolite are somewhat mysterious, and they may have been transported a distance by tectonic forces.

Heading to the east on highway 20 all the way to the Okanogan Highlands, Miller introduced another vignette. One current theory about the belts of metamorphosed rock in the drive between Tonasket to Colville is that the Mississipian to Jurassic metavolcanic and metasedimentary rocks of the Quesnellia island arc were thrust over the highly metamorphosed Proterozoic to late Paleozoic rocks that represent the western margin of the continent Laurentia at the breakup of Pangea. The actual boundary between the accreted terranes and the old continent is somewhat confused by all the faulting and folding. On the highway traveling from west to east, one passes the Okanogan core complexes containing mylonites, or fine-grained gneisses which form in fault zones. One also passes through the Toroda Creek and Republic grabens which formed by crustal extension during the Eocene, when Siletzia was accreting and the subduction zone was temporarily shut down. There are also outcrops of glacial till, a much more recent remnant of the Cordilleran Ice Sheet.

Miller then proceeded south into the Columbia Basin, where Columbia River Basalt can be found everywhere. An estimated 52,000 cubic miles poured out of the faults in eastern Oregon and Washington, mostly between 17 and 14 million years ago. The basalt flowed down the ancient Columbia River to the sea and we see this basalt at several prominent areas on the Oregon coast, including Cape Lookout, Neakanie Mountain, Saddle Mountain and Hug Point.

Miller then touched briefly on the rotation of the Pacific Northwest and the faulting and folding in the Columbia River Basalt which has resulted. The Yakima Fold Belt is an example of this. The Columbia River Gorge runs just north of the trough of a syncline in this fold belt. Thus, the rock layers dip gently to the south along a cross section through the gorge. This produces an asymmetry to the profile and can be clearly seen from vantage points such as Crown Point or Cape Horn. In addition, below the Columbia River Basalt (which makes the steep cliffs on the Oregon side of the river) lies the Miocene Eagle Creek Formation and the Oligocene Ohanapecosh Formation, deposits of volcanic material from the ancestral arc volcanoes. The weathered clay top of the Ohanapecosh makes a perfect plane for sliding. The Washington side of the gorge is therefore highly susceptible to landslides. At 600 years, the Bonneville Landslide complex is the youngest and is believed to be responsible for the Bridge of the Gods legend.

Although the Columbia River is very ancient and has existed for at least 15 million years, its current dramatic look was enhanced about 18,000 to 15,000 years ago by a series of between 40 and 100 Ice Age Floods that tore down its length and reached depths of hundreds of feet. Most of these floods had their origin in Glacial Lake Missoula, which formed as a result of an ice dam created by a lobe of the Cordilleran Ice Sheet. As they tore across the Columbia Basin, they also created the channeled scablands and other erosional and depositional features.

These quick vignettes presented by Miller merely hint at a state with the most complex and interesting geologic history and variety. For more explorations you may want to read their excellent book on Washington Geology. It features Miller’s own superb photography, color cross sections and geological maps which were produced with the assistance of Washington’s Department of Natural Resources geological mapping division.

Additional Reading

Marli B. Miller and Darrel S. Cowan, Roadside Geology of Washington, Second Edition, Mountain Press Publishing Company, Missoula, Montana, 2017.

E.H. Brown, B.A. Housen, E.R. Schermer, “Tectonic evolution of the San Juan Islands thrust system,” The Geological Society of America Field Guide 9, 2007,  Washington Department of Geology, Western Washington University, Bellingham, Washington 98225, USA.

---