AEG 66th Annual Meeting
Association of Environmental & Engineering Geologists
Portland, Oregon Marriott Downtown Waterfront
September 19-24, 2023
Geology of Oregon
The spectacular landscapes of Oregon were shaped by fire and flood, earthquakes and eruptions, and the shifting and collision of ocean and continental plates. Oregon’s geologic story began several hundred million years ago and has been dominated by subduction of oceanic plates beneath the western margin of North America. Late Paleozoic to Paleocene subduction accreted exotic terranes to form the foundation. From the Eocene through today, arc volcanism has built a huge mass of volcanic rocks, capped by glaciated volcanoes of the High Cascade Range. Forearc sedimentation from the Paleocene through today built a thick wedge of marine sedimentary rocks beneath the Oregon Coast Range. In the Miocene the Yellowstone hotspot triggered the Columbia River flood basalt flows, burying half the state under thick lava layers. The geologic excitement has continued, with ongoing eruptions from dozens of volcanoes like Mt. Mazama forming Crater Lake, colossal floods from glacial Lake Missoula and pluvial Lake Bonneville, tens of thousands of landslides, and subduction megathrust earthquakes on the Cascadia Subduction Zone every few hundred years.
Oregon can be divided into the nine geologic provinces, each with outstanding geologic scenery, resulting from a combination of ancient, historical, and ongoing processes.
The City of Portland is located within the Willamette Valley province, which forms a basin between the Coast Range and the Cascades. The sediments collected into this basin record multiple Ice Age floods that originated in Montana, poured through the Cascades (via the Columbia River), and backed up in the valley before eventually draining to the Pacific Ocean.
The Cascade province is actually made up of two volcanic regions, the older, broader, and deeply eroded Western Cascades and the dominating, snow-capped peaks of the younger, more easterly volcanoes of the High Cascades, such as Mount Hood, Mount Jefferson, and the Three Sisters (North, Middle, and South Sister). Another High Cascade peak, Mount Mazama, was destroyed about 6,800 years ago by a catastrophic eruption that left a deep caldera later filled by what is now Crater Lake.
The heavily vegetated, elongated Coast Range province has a varied geologic history. Its basement was formed by a volcanic island chain that collided with North America about 50 million years ago. The ancient volcanoes form many of the scenic headlands along the coast, and the sediments that have accumulated around them contain marine fossils that help unravel the area’s complicated geologic story.
Between 14 and 16 million years ago, "fissure" volcanic eruptions in eastern Washington, eastern Oregon, and western Idaho produced enormous volumes of molten Columbia River basalt that flowed like water west into the Deschutes-Columbia Plateau province in eastern Washington and northeastern Oregon, with some lava continuing to flow as far west as the Pacific Ocean via the ancestral Columbia River valley. As the basalt cooled and congealed, it formed the columnar cliffs that dominate the landscape today. Erosions by the Columbia River has exposed a particularly spectacular sequence of these rocks in the Columbia River Gorge on Oregon’s northern boundary.
High Lava Plains
The High lava Plains province, where the City of Bend is located, has some of the most recent faulting and youngest volcanic activity in Oregon. Well-preserved in a high desert climate, volcanic features stand out about the plains.
The Blue Mountains are made up of separate "exotic terranes," areas that were created elsewhere and accreted to North America as it moved west toward the Pacific. Fossils found in this province reveal their foreign origins. Placer and lode gold mines were active here in the past, and towns such as John Day and Baker City, together with the Sumpter gold dredge, are vivid reminders of the Blue Mountains gold mining heritage.
The Klamath Mountains province consists of four north-south-trending belts of metamorphic and igneous rocks that formed in an oceanic setting and subsequently collided with the North American continent about 150 million years ago. Complexly folded and faulted rocks are bounded by belts of sparsely vegetated bands of serpentinite. Oregon Caves National Monument lies within an enormous fault-bounded block of marble. The historic gold-rush town of Jacksonville remains today as evidence of the area’s colorful gold-mining history.
Basin & Range and Owyhee Uplands
Both the Basin & Range and Owyhee Uplands lie in a region that has been stretched, or extended, almost 100 percent from its original width in the last 10 million years. Evidences of this extension are massive fault block mountains such as Steens and Hart Mountains, with intervening basins containing such features as the Alvord Desert and Lake Abert. Intense volcanic and hot spring activity over millions of years has produced fine-grained gold deposits and jasperoids that are prized by rock hounds. The Owyhee Uplands gets its strange name from a group of Hawaiian trappers who got lost and died in the region in 1818.
References and Resources
Madin, Ian P., 2009, Oregon: A Geologic History, Oregon Department of Geology and Mineral Industries Interpretive Series Map 28 [companion web page] https://www.oregongeology.org/pubs/ims/ims-028/ (accessed January 2, 2020).
Oregon Department of Geology and Mineral Industries, 2020, Geologic Sightseeing, Geologic Provinces: https://www.oregongeology.org/learnmore/GeologicSightseeing.HTM (accessed January 2, 2020).