Photos of Pacific Coast, Cascades, Columbia Plateau
Geology of the Pacific Northwest

Lecture 8--The Cenozoic era in the Pacific Northwest

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Welcome to Week 8 of Pacific Northwest Geology. During this week and the remaining weeks of the quarter, we will be delving into the geologic history of the Pacific Northwest, working from the present back through geologic time.

This week we'll travel backward in time through the Cenozoic era, which began 66 million years ago. Take a look at the geologic time table to see how the era breaks down into periods and epochs.

The Paleogene and Neogene periods were formerly considered a single period of geologic time, the Tertiary period. You will still find many references to the Tertiary period in the geology literature because the system of geologic time did not break up the Tertiary period into the Paleogene and Neogene periods until recently.

We are living today in the Holocene epoch of the Quaternary period of the Cenozoic era.

Holocene epoch

All the geological activity you see around you today represents the Holocene geology of the Pacific Northwest. The region is at an active margin of the North American continent, with a subduction zone and recently uplifted areas of crust. As a result, the land is like a bees' nest, constantly humming with geological activity.

Some of the prominent geologic activities of the modern Pacific Northwest are volcanic activity in the Cascade Range and Yellowstone area, continued uplift of the Coast Ranges, earthquakes related to subduction of the Juan de Fuca Plate, erosion and deposition of sediments along rivers and shorelines, and uplift of mountains along faults in the Pacific Northwest interior.

The Holocene epoch began when the climate warmed, the giant continental ice sheets retreated into Canada and wasted away, and extensive alpine glaciers shrank into a smaller number of glaciers higher up in the mountains. Many large mammal species that existed during the preceding, colder Pleistocene epoch became extinct as the species Homo sapiens became established in the region.

To read more about the Holocene geology of the Pacific Northwest, go to Lecture 2.

Pleistocene epoch

During the Pleistocene ice ages many of the mountains of the Northwest were covered with alpine glaciers. Continental ice sheets covered the northern parts of Washington, Idaho and Montana. Near the end of the Pleistocene epoch the glacial Lake Missoula outburst floods formed the Channeled Scablands of eastern Washington.

Volcanoes were active in the Cascades, and massive eruptions from the caldera in Yellowstone National Park spread tephra far beyond the Northwest.

To read more about the Pleistocene geology of the Pacific Northwest, go to Lecture 2.

Pliocene epoch

In broad outline, the landscapes of the Pliocene epoch were much like they are today in the Pacific Northwest. The Cascade Range, Coast Ranges, and Rocky Mountains were already present. However, at the beginning of the Pliocene epoch much of the uplift and erosion that shaped these mountain ranges into their modern form had not yet occurred. The Columbia Plateau existed, but the Channeled Scablands did not yet exist. A forearc basin existed between the Cascade Volcanic Arc and the Coast Ranges, but Puget Sound did not yet exist.

None of the modern composite cones of the Cascade Range yet existed, although the Cascade Volcanic Arc, driven by plate tectonic subduction, had been around since late Eocene or early Oligocene time, since about 40 million years earlier. Volcanoes come and go, build up and erode away, in a few million years. In the Cascade Range back then, there were earlier, unnamed volcanoes, but there was no Mount Rainier or Mount Saint Helens yet. Volcanic activity in the Cascade Range during the Pliocene emanated from an earlier generation of volcanoes. One of these Pliocene volcanoes of the Cascade Range was in the Goat Rocks area, southeast of Mt. Rainer. The Goat Rocks volcano has long been inactive. So much of it has eroded away that it has lost its conical shape.

During the Pliocene, the Columbia River Basalts that had erupted during the preceding Miocene epoch continued to undergo folding and thrust faulting in the Yakima Fold Belt. As the ridges of the Yakima Fold Belt grew higher and longer, the Columbia River responded by shifting its course.

The widespread sand and gravel beds laid down during the late Pliocene epoch by the shifting Columbia River, and the deposits that formed in the lakes of its drainage system, remain today as the Ringold Formation. In the Hanford Basin portion of the Columbia River the eroding Ringold Formation has revealed fossils of a variety of fishes and mammals that lived in the Pliocene Columbia River drainage, most of them now extinct.

Much of the uplift of the modern Cascade Range took place during the Pliocene epoch. Along the eastern edge of the Cascade Range, the basalt flows of the Columbia River group are raised up high on the flanks of the range. This can be seen in the Mission Ridge area south of Wenatchee -- the same flows that are thousands of feet lower on the Columbia Plateau to the east rise up at an angle along the ridge. It can be assumed that the surfaces of the basalt layers, which originated as liquid lavas, were originally very close to horizontal. The fact that some layers are now tilted and raised thousands of feet along the eastern flanks of the Cascade Range gives an idea of how much the range has risen since the Miocene, when they erupted.

The same is true of mountain ranges in the Blue-Wallowa Mountains and Basin and Range landscape regions-much of the rise of those mountain ranges occurred during the Pliocene epoch. The extension of the crust, normal faulting, and development of basins in the Basin and Range, which began during the preceding Miocene epoch, continued to occur at a rapid rate during the Pliocene epoch.

As the Rocky Mountains continued to rise and erode throughout the Pliocene epoch, large volumes of sediment were eroded and washed down into adjacent lowlands. This created a widespread layer of gravel across the landscapes that sloped down from the Rockies. Evidence from preserved soil layers and plant and animal fossils indicates that there was an interval during the Pliocene when the climate in the Rocky Mountain region was warmer and more arid than it is now.

The Basin and Range was undergoing crustal extension and bimodal volcanism during the Pliocene epoch, much like it does today. The Yellowstone hotspot was erupting from an earlier caldera, the outline of which can be traced to the west of Yellowstone National Park.

Miocene epoch

The Miocene epoch is most noteworthy for the eruption of the Columbia River Basalt Group. These basalt flows began erupting about 17 million years ago from fissures in southeastern Oregon, and over time the main fissure sources of the basalt moved northward to northeastern Oregon and nearby parts of southeastern Washington, in and north of the Blue-Wallowa Mountains. Most of the volume of the Columbia River Basalts had erupted by 14 million years ago. The last small basalt eruption occurred in the Columbia Plateau of eastern Washington near the end of the Miocene epoch, about 5 million years ago.

In total, the Columbia River Basalts covered 200,000 square kilometers of land with about 3,000,000 cubic kilometers of basalt. The size and scope of these lava flows is difficult to imagine. No lava flows on the scale of flood basalts have occurred in more recent geologic time.

A typical Columbia River Basalt flow has a lower layer of regular, vertical columns known as the colonnade and an upper layer of irregular joints and cracks known as the entablature. These columns, joints, and cracks develop because the basalt contracts to a smaller volume as it turns from liquid to solid. The columns build from the base upward as the bottom of the flow slowly loses heat into the stable ground beneath. The irregular cracks and joints of the entablature develop from the top down as the top of the flow rapidly loses heat to the unstable air above.

The very base of a Columbia River Basalt flow may have low-grade opal, basaltic glass, and palagonite where the hot lava encountered wet soil. Opal is an amorphous mixture of silica and water. Glass forms from lava chilling and solidifying too quickly to form minerals. Palagonite is an orange-colored mixture of clays with other iron-bearing minerals that forms from the breakdown of basaltic glass.

If a basalt flow enters into a lake, the lava chills quickly and bunches up into pillow-like shapes known as pillow basalt. In some places it can be seen that Columbia River Basalts flowed into lakes, forming layers of pillow basalt that sloped down and out into the lake. The basalt pillows tend to have glassy rinds, basaltic cores, and palagonite mixed with basaltic glass between the pillows.

The very tops of Columbia River Basalt flows commonly have vesicles, holes in the basalt that represent rising bubbles of gas that became part of the solidified flow. The volcanic gas has long since leaked out, but the vesicles remain.

There were long intervals of time, up to millions of years, between flows of Columbia River Basalt. During these intervals, lakes and streams formed on the landscape, volcanic eruptions from the Cascade Range distributed ash and other volcanic debris onto the western portion of the Columbia Plateau, and plants and animals re-populated the area. Near Spokane the Latah Creek formation consists of sediments laid down in lakes and streams during the Miocene epoch, with finely preserved plant fossils. At Vantage in central Washington, a layer of wet soil, wetland, and forest was buried beneath a Columbia River Basalt flow. The logs of the trees that were buried became petrified-the wood was replaced by silica as groundwater, which contained silica dissolved from the volcanic rocks, slowly seeped through.

Among the species of trees that grew in the Vantage area during the Miocene were ginkgo trees, which subsequently became extinct in North America. Gingko Petrified Forest State Park at Vantage displays examples of petrified wood from the buried Miocene forest. It appears from the types of plant fossils that the climate in the area was warmer during the winters, and wetter throughout the year, than it is now. Perhaps the Cascade Range was not such a high and continuous mountain range at that time in the Miocene. If so, moister air and more moderate winter temperatures from the Pacific Ocean would have flowed into the Vantage area.

Starting about 18 million years ago, large-volume, caldera-forming eruptions of felsic ash began occurring in southeastern Oregon, southwestern Idaho and the adjacent part of Nevada. The locations of these large-volume eruptive centers, and the calderas that resulted, gradually shifted eastward and northeastward through the rest of the Miocene and Pliocene epochs, reaching the Yellowstone region during the Pleistocene epoch, where high heat flow and the potential for volcanic activity continues today.

This chain of younger and younger calderas and ash flows from southeastern Oregon across southern Idaho to Yellowstone is probably the track of the North American continent across a hotspot, the Yellowstone hotspot. A hotspot is a plume of heat and magma that rises from deep in the earth, deeper than the tectonic plates. The Hawaiian Islands, for example, have been formed on the Pacific Plate as it has tracked across a hotspot located beneath oceanic crust.

Where a hotspot is located beneath continental crust, the mafic (basaltic) magma from the mantle may not erupt all the way through the crust. Instead, the magma may pond within the continental crust and cause the crust to melt into felsic magma, which would then erupt explosively, forming calderas and emitting large volumes of volcanic ash. There is evidence in the volcanic rocks of the Yellowstone hotspot track that mafic magmas from the mantle have been involved in causing the crust to melt and form felsic magma. It is also possible that the start-up of the Yellowstone hotspot beneath the relatively thick North American continental crust may have led to the eruption of the Columbia River Basalts. Eruption of the basalts seem to have occurred from a region of the crust that was undergoing enough tension to crack apart and let the mafic magma through to the surface.

Out along the coast, the Eocene to Miocene sediments and pillow basalts of the Coast Ranges, which formed on the ocean floor, were shoved beneath the edge of the continent during the Miocene. This thrusting of slices of oceanic crust beneath the edge of North America began the process of faulting, uplift and erosion that has formed the modern Coast Ranges.

The Cascade Volcanic Arc was established in its present location during the Miocene Epoch. Tephra and lahars form the Cascades interfinger with Miocene-age Columbia River basalt flows in the border zone between the Columbia Plateau and the Cascade Range in central Washington state.

Oligocene epoch

During much of the Oligocene epoch, the coastal region west of the Cascade Volcanic Arc was covered by the ocean, and was the site of sand, gravel, silt, and mud deposited in bays, estuaries, and deltas. The Olympic Mountains and Coast Ranges had not yet uplifted to their present forms. The Blakeley Formation, in the Seattle-Bremerton area of the Puget Sound region, is an example of an Oligocene sedimentary formation that records coastal deposition. The Blakeley Formation contains fossils of clams, snails, tree branches, and leaves, which show how close it was to the shore. Layers of volcanic pumice and ash in the Blakeley Formation may represent the beginning of the Cascade Volcanic Arc about 35 million years ago, approximately where it is located now in Washington.

In Oregon during the Oligocene, sediments accumulated in bays along the coast and in river estuaries and deltas. The Oligocene volcanic activity of the Cascade arc in Oregon was located west of the modern High Cascades of Oregon. This older, eroded, inactive belt of arc-type volcanic rocks in Oregon is called the Western Cascades.

In eastern Oregon during the Oligocene epoch, layers of volcanic ash accumulated far to the east of the Cascade arc. Layers of sediment from streams and lakes, which include fossils of Oligocene mammals, lie between the Oligocene ash layers in eastern Oregon. Various species related to the modern horse have been identified among the fossils in these sedimentary strata in eastern Oregon, mainly in the John Day Formation.

In the North Cascades of Washington, the Straight Creek Fault may have been active during the early Oligocene epoch. The Straight Creek Fault cuts and offsets the Eocene Chuckanut Formation. This demonstrates that the fault must have formed after the Eocene epoch when the Chuckanut Formation originated. The Straight Creek Fault is cut off and ends at the Chilliwack Batholith, which demonstrates that fault motion must have stopped by the time the Chilliwack Batholith formed. The Chilliwack Batholith actually comprises several plutons that range in age from Oligocene to Pliocene. Because the Chilliwack Batholith cuts off the Straight Creek Fault, and the batholith itself is not faulted, it seems clear that by sometime during the Oligocene epoch the Straight Creek fault had ceased its strike-slip fault motion.

The Straight Creek fault forms the western boundary of the North Cascades Crystalline Core. In the North Cascades, and in the western North Cascades west of the Straight Creek Fault, most of the rocks are older than Oligocene. The big exception is plutons and volcanic rocks produced by the Cascade Volcanic Arc, which began roughly 40 million years ago, either late in the Eocene epoch or early in the Oligocene epoch.

Eocene epoch

The Eocene epoch was a time of major change in the Pacific Northwest. Part way into the Eocene epoch, the last major terrane, the Siletzia terrane, accreted to the coast. The plate boundary along the coast underwent transform (sideways) tectonics rather than normal subduction tectonics. Unusual volcanic rocks erupted throughout much of the Northwest, to as far inland as as the east side of the Rocky Mountains. Huge volumes of sandstone were deposited into tectonic (fault-bounded) basins as the various types of mountain ranges in the area underwent uplift and erosion. The last major coal deposits were formed from swampy areas along rivers feeding sediment to the coast. The climate was warm and humid, as this was many millions of years before the Earth cooled into its more recent ice ages and glaciations. Biological evolution, occurring millions of years after the dinosaurs had become extinct, produced fascinating new forms of life. Some world-class examples of certain types of mammals and flowering plants were preserved in fossils from the Pacific Northwest, ancestors to the modern species we are familiar with today.

Table of Eocene Pacific Northwest Geology



Possible Cause

Example Rock Formations
(not a complete listing)

Entire Eocene epoch

Biological evolution of plants and animals during the Eocene epoch gave rise to families that in some cases are ancestral to what we see in today's world. For some lineages of mammals and flowering plants, the Pacific Northwest's Eocene rocks contain key fossils.

Changing environments across a variety of flourishing ecosystems, with warm and wet climates during much of the Eocene epoch, made a template for the evolution of ancestors of life forms such as hoofed animals that included small multi-toed ancestral horses. Also first appearing in the Eocene were direct ancestors of some modern flowering plants including certain types of trees and the rose family. (The first flowering plants had appeared much earlier, in the Mesozoic era.) Clarno Formation (eastern Oregon). Chuckanut Formation (near Bellingham, WA). Cowlitz Formation (southwest Washington). Tyee Formation (western Oregon). Chumstick Formation (near Wenatchee, WA). Klondike Mountain Formation (Republic, WA).
Early Eocene Eocene "thermal optimum," the last widely warm global climate, reached its peak warmth about 57 Ma. An inland sea reached the center of North America. Palm-tree-like plants grew in what is now Washington. Swamps that became coal deposits were lush in several parts of the Pacific Northwest. Unclear. One thing is likely: The warming caused release of CO2 that had been sequestered within shallow seafloor deposits, greatly enhancing the atmospheric greenhouse effect. Swauk Formation (near Cle Elum, WA) and several other Eocene sedimentary rock formations across the Pacific Northwest that consist mainly sandstone, along with some coal beds, shale, local conglomerate, and less common volcanic layers such as tuff (solidified ash).
Early Eocene Erosion of volcanic arc led to erosion of granitic basement rock and deposition of arkosic sandstones Uplift and erosion of volcanic arc removed much of the volcanic cover, exposed underlying intrusions to erosion. Swauk Formation (near Cle Elum, WA). Chuckanut Formation (Bellingham, WA).
Early Eocene Granitic and metamorphic rock from Idaho and interior British Columbia contributed sediment to western Washington and Oregon coasts. Modern Cascadia subduction zone not yet established, so rivers could flow more directly from the Rockies to the coast. Mountains in the western Pacific Northwest were discontinuous tectonic uplifts more than continuous volcanic arcs, though there may have been localized volcanic arc activity at times. Swauk Formation (near Cle Elum, WA). Chuckanut Formation (Bellingham, WA).
Middle Eocene Tectonic stress patterns changed from compression to a combination of transform and tension. Tectonic plate(s) offshore switched to dominantly transform (moving sideways to the north) along the edge of the PNW coast; the Kula Plate may have been present offshore for awhile, instead of the Farallon Plate. sequence of change in stress seen in Swauk Formation, Teanaway Formation/Teanaway Volcanics, and Chumstick Formation in central Washington state; grabens opened up in other parts of PNW as well
most of Eocene epoch

Metamorphic core complexes formed in north-central and northeastern Washington, south-central British Columbia, Idaho, Utah, and western Montana. Metamorphic core complexes are dome-like zones of soft, hot, metamorphic and intrusive (granitic) rock that welled up rapidly toward the surface from deep in the crust along detachment faults.

The deep crust may have undergone extreme heating after previous subduction volcanism followed by detachment and removal of the underlying oceanic plate letting hot mantle asthenosphere rise up and heat the base of the crust even hotter. At the same time, tectonic stress across the area changed from compression to extension. All this may have been related to accretion of Siletzia, transform plate motion occurring along the coast, and the subducting plate foundering (breaking apart) beneath the region. Each metamorphic core complex (which used to be called a "gneiss dome") contains its own array of named granitic intrusions and metamorphic rock formations.

Examples of metamorphic core complexes (MCCs) include the Okanogan MCC and Kettle MCC in north-central Washington (near the border with Canada), the Spokane MCC, the Shuswap and Valhalla MCCs in south-central British Columbia, the Bitterroot MCC in western Montana, and the Pioneer Range MCC in southern Idaho, among others.
Late Eocene Grabens and half-grabens ("wrench basins," "pull-apart basins") opened up in several places in the Pacific Northwest. Tectonic plate(s) offshore continued to be switched to dominantly transform (moving sideways to the north) along the edge of the PNW coast. The Kula Plate may have been present offshore for awhile, instead of the Farallon Plate. Chumstick Formation (in Chiwaukum Graben near Wenatchee, WA); Klondike Mountain Formation in Republic Graben near Republic, WA; other rocks deposited in partly tectonic wrench basins or extensional basins in Eocene Pacific Northwest include Chuckanut Formation (around Bellingham, WA), Pipestone Canyon Formation (near Winthrop, WA), and Clarno Formation (eastern Oregon).
Late Eocene

End of unusual Challis volcanics igneous event. Transition to modern Cascadia subduction zone.

Earlier in the Eocene epoch, the Siletzia Terrane had accreted to the coast, blocked the earlier plate boundary, and ruptured the previously subducted portion of the Farallon Plate from beneath the area. It may be that late in the Eocene epoch, subduction of the remaining, offshore piece of the Farallon Plate started up along the coast, initiating the modern Cascadia subduction zone. Ohanapecosh Formation (in central and south Washington State Cascades volcanic zone).


Compared with today, the geology of the Pacific Northwest was very different during the Eocene epoch. The Cascade Volcanic Arc did not exist, at least not in a form similar to its modern location and elevation. The Cascade Mountains themselves did not exist, although there were some volcanoes and patches of mountains in parts of what is now the Cascade Range. The Coast Ranges, including the Olympic Mountains, did not exist. Volcanic activity in the Pacific Northwest was scattered during the Eocene epoch from the coast to the Rockies, with centers of eruption in north central Washington, central Idaho, eastern Oregon, scattered locations in Montana, and northwestern Wyoming.

Geologic structures indicate that the crust of the Pacific Northwest was subjected to shear and tension during the Eocene epoch. The Straight Creek fault was active as a strike-slip fault by the end of the epoch, with rocks to the west of the fault moving northward. The town of Wenatchee is located in the Chiwaukum graben, which was pulled apart during the Eocene and simultaneously filled with clastic sediments from streams that drained the surrounding hills and mountains.

In the Republic and Toroda Creek portions of the Okanogan Highlands landscape region, grabens formed during the Eocene. As they formed, the grabens became receptacles for the Sanpoil Volcanics that were erupting at that time as well as related sediments that contained volcanic ash.

The Klondike Mountain Formation is one of the volcaniclastic sedimentary formations in the Republic graben. It formed mainly as layers of sediment on the bottoms of lakes. The Klondike Formation contains fossils of plants, insects, and fish of Eocene age. Based on the plant fossils, it seems likely that the climate was warmer in the Okanogan Highlands region during the Eocene epoch than it is now, especially during winters.

Several metamorphic core complexes were formed in the Pacific Northwest during the Eocene epoch, including the Okanogan and Kettle complexes in north central and northeastern Washington, the Bitterroot complex on the border of Idaho and Montana, the Pioneer complex in south central Idaho, and the Albion complex on the Idaho border with Utah.

Metamorphic core complexes are cored by high-grade metamorphic rock such as gneiss or schist, along with plutonic rock, all of which forms deep in the crust. Development of a metamorphic core complex involves layers of rock, which had been on top of the gneiss, schist, and granite, sliding off to the side along detachment faults. As surface rocks slide off to the side, the metamorphic core lifts upward to the earth's surface. This process takes place where the stress in the crust is one of tension, or stretching apart.

In central and western Washington during the Eocene epoch, deposits of sediments that were the precursors to sandstone, coal, and other sedimentary rocks accumulated in basins. The Chumstick Formation is one of these Eocene formations. The Chumstick Formation accumulated in the previously mentioned Chiwaukum Graben.

To the southwest of the Chiwaukum Graben more precursors of sandstone, coal, and other sedimentary rocks were deposited in the Swauk basin, eventually becoming the Swauk Formation. The Swauk Formation is older than the Chumstick Formation. The Swauk goes back to the early Eocene epoch, the Chumstick came later, and formed during a middle to late stage of the Eocene epoch.

During a short geologic interval of perhaps two million years, the Swauk basin where the Swauk formation sediments had been accumulating, stopped subsiding and receiving large amounts of sediment. During this relatively short interval of time, the previously deposited layers of Swauk sedimentary rock were tilted, folded, and their upper parts eroded, and the Swauk formation was buried beneath a large batch of unusual volcanic rocks, known as the Teanaway formation.

There are small amounts of sedimentary layers in the Teanaway formation, and the sediment is rich in volcanic rock detritus recently eroded from the Teanaway volcanoes that were active at that time, but most of the Teanaway formation is volcanic rock, including basalt flows, basaltic volcanic breccia, and small bodies of rhyolite.

The basaltic Teanaway eruptions were fed by underground intrusions that forced their way up through cracks in the Swauk Formation and other pre-existing bedrock in the area. Magma that did not erupt to the surface solidified in these widened underground cracks and became the Teanaway dikes. You can see the Teanaway dikes in roadcuts along Highway 97 going over Blewett Pass in north central Washington, each dike cutting at a steep angle through beds of Swauk formation sedimentary rock.

Along Highway 2 farther north and east in central Washington, in Corbaley Canyon and Pine Canyon between the towns of Orondo and Waterville, are many more dikes of the same mid-Eocene age as the Teanaway dikes farther south. These dikes also tend to be oriented in the same direction across the landscape, suggesting the crust across the entire area was subjected to the same type of transtensional tectonic stress while magma was forcing its way up from the mantle and deep crust underneath it. In Corbaley and Pine Canyon, the dikes cut through metamorphic and plutonic bedrock, an exposed outlier of the North Cascades crystalline core.

Whereas the Teanaway dikes near Blewett Pass are nearly all basaltic, the Eocene-age dikes in Corbaley and Pine Canyons include many felsic (rhyolitic) and some intermediate (andesitic) dikes, along with mafic (basaltic) ones.

These Eocene rocks in central Washington record a disruption (uplift, folding, tilting, and erosion) of the Swauk basin and Swauk formation, followed immediately by a rapid but short-lived volcanic event, the Teanaway dikes and volcanic rocks, which lasted just a couple of million years or even less in that area. All of this was quickly followed by the Chiwaukum graben cracking open, expanding, and filling with sediments at the same time as its bounding faults were active and the basin kept pulling apart.

These amazing Eocene events, when the crust spread apart into grabens and pull-apart basins, metamorphic core complexes rose to the surface, and non-subduction zone volcanism spread across the region, are now interpreted as being due to a huge terrane accretion and change in plate tectonic interactions at the continental margin along the coast to the west.

It was at this time that Siletzia accreted to the edge of the continent. Siletzia is an accreted terrane that contains, along with some seafloor sedimentary rocks, huge piles of pillow basalt and lava flows that had built up into an oceanic plateau and basaltic islands in the ocean. The Siletzia terrane, named for the Siletz River in the Oregon Coast Range, forms much of the Olympic Mountains and coast ranges of Washington and Oregon, In the state of Washington Siletzia includes the Crescent Formation in the Olympic Peninsula and Willapa Hills.

The accretion of Siletzia to the leading edge of the continent apparently jammed the subduction zone and temporarily shut down the previous plate boundary, which may have partly been a subduction zone with a volcanic arc.

During this interval of time in the Eocene epoch, following accretion of Siletzia, the divergent plate boundary between the Farallon Plate and the Kula Plate to its north may have impinged on the continental margin. This turned the plate boundary system along the coast of the Pacific Northwest into a transform plate boundary, or nearly a sideways-sliding transform plate boundary with also some convergence (pushing in against the continent) but without subduction.

This partly transform (sideways sliding) plate boundary removed part of the Siletzia terrane and slid it north to Alaska. This tectonic process may have caused tectonic stress that varied from transpression (sideways shear plus some convergence, or pushing together) to transtension (sideways shearing plus some tension, or pulling apart). It was during this time that affected the crust across central and northern Washington wrenched apart to form the Chiwaukum graben, the Republic graben, and the Toroda Creek graben.

As for the Teanaway volcanics, they may have been produced by the previous subduction zone getting shut off by the accretion of Siletzia along the edge of the continent, causing the subducting plate to tear apart and let hot mantle up beneath the Pacific Northwest, making the mantle rock melt and rise up as magma.

In addition, there may have been a hotspot causing the oceanic islands of Siletzia to build up from more than the usual oceanic crust volcanic eruptions, and some effects from this hotspot may have passed beneath the area, adding more heat and magma to the mix. The Teanaway volcanics would be the result of this tear in the subducted plate, and possibly a hotspot, passing by beneath the area. The effects of this torn subducted plate, letting hot mantle up to melt, make magma, and spawn volcanoes, may have continued as far east as Montana and Wyoming, causing the unusual inland volcanism of the northern Rocky Mountains during the Eocene epoch.

As for the possible hotspot that caused the build-up of oceanic islands that formed Siletzia, whether it ended up becoming what is now the Yellowstone hotspot is an open question. Some geologists think so, but it is hard to reconstruct exactly how the hotspot would have tracked, in space and time, between the coast in the Eocene and Yellowstone now. This will need further research to be determined. In the meantime, we need to continue through the Eocene epoch in the Pacific Northwest.

West of the Cascades, in the Puget Sound area, there are several Eocene sedimentary formations that consist mainly of sandstone and also contain coal beds. These Eocene, sandstone-dominated formations of the Puget Sound region are known collectively as the Puget Group. The Puget Group includes the Renton Formation near Seattle and the Chuckanut Formation near Bellingham.

Near Olympia and Centralia are the abandoned Wilkeson sandstone quarry and the currently active Tenino sandstone quarry, which have supplied building stones and decorative stones (carved into pretty shapes, usually put on buildings as part of their decor) for many historic buildings in Washington state and even other states going back to when quarrying and sandstone construction was most active, the late 1800s through early 1900s. These sandstones, at the Wilkeson and Tenino quarry, are also part of the Eocene-age Puget Group.

Of course, sandstone and coal do not start out as sandstone and coal. Sedimentary rocks do not start out as sedimentary rocks.

As you will recall from your rock cycle, sedimentary rocks originate as sediments of certain types, deposited in certain environments of the earth's surface realm. Then, if the sediments get buried deeply enough into the upper crust of the earth, they are compacted, cemented, and lithified, becoming sedimentary rock.

Sand becomes lithified into sandstone. Certain types of sand and sandstone tell you how far away the source uplands (mountains or other high-elevation zones) were, and the rock types those mountains comprised.

For example, nearby mountains, consisting largely of uplifted granite and gneiss undergoing erosion is the erosional sediment source of arkosic sand, and the arkose (arkosic sandstone) into which it lithifies. Arkosic sediment grains include quartz, but also lots of feldspar and some other minerals typical of granite or gneiss, such as biotite, hornblende, or muscovite.

Coal originates from a mass of dead plants that grew in a swampy or boggy environment. All those dead plants piled up thickly in the ground and were quickly buried beneath water and mud so they did not oxidize and decompose. On its way to getting buried deeper and deeper and turning into the rock known as coal, this dead plant material goes through several stages:

The sediments and fossils of the Puget Group reveal much information about the Eocene environment in the Puget Sound area. The coal formed in wet, lushly vegetated, swampy areas near the coast, fed by rivers that deposited thick sequences of sand, silt, and mud along the river channels. The Bellingham, Bellevue (east of Seattle), Cle Elum (east of Snoqualmie Pass) Black Diamond (east of Tacoma) and Centralia (south of Olympia) areas used to have thriving coal-producing mines.

Coal was still being mined from Puget Group sediments south of Seattle, by heavy machinery in open pit mines, up until the early 2000s, but the last working coal mine in Washington state has shut down.

Much of the sediment that became the Puget Group was deposited in large river deltas and estuaries, where rivers gently (at a low gradient) entered bays at the edge of the ocean and spread their sand and silt out into embayments along the edge of the ocean that got deeper offshore. The weight of the sediment caused the crust to sink a little, and the sediment itself compacted and partly de-watered as it got buried, making room for the deltas and bays to take in more sediment layers on top of the preceding ones, building up huge thicknesses of sand and silt and occasionally (from energetic flood discharges) rounded gravel layers.

In southwestern Washington, mostly to the west of Centralia and Longview and part way out to the present-day coast, there were intervals of volcanic eruptions during the Eocene epoch, which in some cases interfinger with the Eocene sandstone layers. The volcanic rock formations include the Grays Harbor volcanics. These unusual fore-arc volcanics, or else continental margin volcanics erupted when there was no subduction zone, are probably due to the accretion of Siletzia, the tearing apart of the Farallon plate from the depths of the subduction zone, upwelling of the mantle in place of the foundered oceanic plate, and also impingement, quite possibly, of a hotspot, the hotspot that had caused all the eruptions that built up into the volcanic plateau and ocean islands of Siletzia, which may be the same as what is not the Yellowstone hotspot, when it first came beneath the continental crust of North America.

The sediments can also tell us about the shape of the earth at that time, including the distribution and nature of the mountains, rivers, lakes, and coastlines. The sandstone in the Puget Group contains lots of feldspar, biotite, and muscovite along with quartz. The Cascade Mountains contain few rocks with significant amounts of muscovite in them. However, the Idaho Batholith and several batholiths in the Okanogan Highlands of south central British Columbia do contain a large amount of muscovite. It seems likely that the rivers draining into what is now western Washington were originating in Idaho or central British Columbia during the Eocene epoch.

Although rivers may have drained to the coast from Idaho and interior British Columbia, the sedimentary rocks and their related geologic structures tell us that there once were localized masses of mountains in parts of what is now the Cascade Range. The arkosic sediments were formed from erosion of mountains of granitic and metamorphic rock, and in some places the size and shape of the sediment grains and the structures of the sedimentary beds tell us that eroding mountain slopes were very close by. This includes sediments in the Chuckanut formation near Bellingham, the Chiwaukum basin near Wenatchee, and the Swauk basin near Cle Elum. The nearby mountains were probably massifs (mountainous, uplifted masses) formed along major fault zones driven by plate tectonic interactions. The types of localized mountain ranges that existed in central-to-western Washington and Oregon back then may have included block mountain ranges, fold and thrust belts, or transverse mountain ranges, the particular type of mountain range depending on where and when in the Eocene the block of mountains were located.

Volcanism in the Northwest during the Eocene epoch was distributed in a widespread array. There were small amounts of Eocene intrusion and eruption in the Chiwaukum graben and central Cascades, the moderately large Sanpoil volcanic center near Republic in north central to northeastern Washington, and the very large Challis volcanic zone in central Idaho.

Also during the Eocene epoch, several batholiths or large intrusions formed, including the Golden Horn Batholith on the eastern edge of the North Cascades Crystalline Core. The Golden Horn Batholith plugged the Ross Lake Fault Zone, a strike-slip and oblique-slip fault zone that was active along the eastern margin of the North Cascades Crystalline Core during the interval from Late Cretaceous to Eocene time, prior to the formation of the Golden Horn Batholith. The Golden Horn Batholith is unusual in its chemistry and mineralogy. It contains a lot of potassium and sodium, more so than other intrusions in the Cascade Range.

In Montana, all the way east onto the High Plains, other unusual volcanic rocks erupted during the Eocene epoch. These rocks had even higher amounts of sodium and potassium than the Golden Horn Batholith. These volcanic rocks are different from igneous rocks that typically erupt from composite cones alongside subduction zones. However, during the Eocene, a typical volcanic arc and subduction zone seem to have been missing from the Pacific Northwest.

It may be that the Kula plate was moving northward along the coastal margin of the Pacific Northwest during the Eocene. This might explain the shearing and pulling apart of the crust that occurred during this time. It may also be possible that the rapid subduction of the Farallon Plate which had taken place before the Eocene epoch had led to a detached portion of the subducting plate sinking beneath the continental crust. The asthenosphere rising up to replace the sinking remnant of the Farallon plate could have been the source of the unusual volcanic rocks. We may not be able to prove or disprove these hypotheses, but the distinctive rocks and structures of the Eocene remind us of how different it was around the Pacific Northwest then.

Paleocene epoch

Terranes that accreted during the preceding Cretaceous period finished being pushed and shoved into place in the Paleocene epoch. Strike-slip faulting seems to have taken place during the Paleocene, bringing terranes northward along the margin of the continent. Several large faults in the North Cascades region show evidence of strike-slip faulting during the Paleocene epoch. One of these is the Ross Lake fault. Another is the Pasayten fault, on the east side of the Methow Valley near Twisp in north central Washington.

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Glossary terms that appear on this page: alpine glacier; ice sheet; thrust fault; basalt; normal fault; fissure; bimodal volcanism; flood basalt; colonnade; entablature; opal; palagonite; joint; vesicle; felsic; heat flow; hotspot; magma; tectonic plate; mafic; pluton; strike-slip fault; graben; clastic; metamorphic core complex; metamorphic rock; gneiss; schist; plutonic rock; detachment fault; sandstone; coal; feldspar; biotite; muscovite; quartz; asthenosphere

Geology of the Pacific Northwest
Lecture #8
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 7/26/13