• Introduction
• Holocene epoch
• Pleistocene epoch
• Pliocene epoch
• Miocene epoch
• Oligocene epoch
• Eocene epoch
• Paleocene epoch
• Glossary Terms

Related Basics Pages:

• Depositional Environments
• Determining Absolute Ages in Geology
• Geologic Structures
• Stratigraphy & Determining Relative Ages

Related Focus Pages:

• #2--Geologic Timeline of the Pacific Northwest
• #3--Changing Climates, Landscapes and Life Forms of the Pacific Northwest
• #9--Geologic Structures of the Pacific Northwest

Introduction

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 look at the Cenozoic Era, which began about 66 million years ago. The Cenozoic Era consists of the Tertiary and Quaternary periods. The Tertiary period comprises the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs. The Quaternary period is made up of the Pleistocene and Holocene epochs. We are living in the Holocene epoch of the Quaternary period.

Holocene epoch

Many large mammal species that existed during the preceding Pleistocene epoch became extinct as the species Homo sapiens became established in the region. All the geological activity you see around you today represents that Holocene geology of the Northwest. Volcanic activity in the Cascade Range, continued uplift of the Coast Ranges, earthquakes related to subduction of the Juan de Fuca Plate, and erosion and deposition of sediments along rivers and shorelines are some of the prominent geologic activities of the modern Pacific Northwest.

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.

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-no Mt. Rainier, no Mt. St. Helens. 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. For example, 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 through 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.

Miocene epoch

The Miocene epoch is most noteworthy for the eruption of the Columbia River Basalt Group. These basalt flows began erupting about 18 million years ago from fissures in southeastern Washington and northeastern Oregon, mainly in the northern portion of the Blue-Wallowa Mountains. Most of the volume of the Columbia River Basalts had erupted by 16 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 lava. 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 gingko 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 hot spot, the Yellowstone hot spot. A hot spot 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 hot spot located beneath oceanic crust.

Where a hot spot 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 as large volumes as volcanic ash and form calderas. There is evidence in the volcanic rocks of the Yellowstone hot spot 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 hot spot beneath the relatively thick North American continental crust may have led to the eruption of the Columbia River Basalts. Eruption of the basalts would 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. In Oregon, earlier arc volcanism occurred more to the west. In Washington the earlier Cascade volcanism was largely in the same location as the modern volcanoes are located. Prior to the Miocene, however, the Cascade volcanoes were close to the edge of the ocean, because the Coast Ranges to the west had not yet risen above the surface of the water.

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 Blakeley Formation, in the Seattle 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, approximately where it is located now in Washington, about 35 million years ago.

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. A variety of ancestors of the modern horse have been identified among the fossils in these sedimentary strata in eastern Oregon, mainly in the John Day Formation.

Deposits of volcanic ash, interbedded with sedimentary strata of Oligocene age, are also found in the Rocky Mountain region. This seems too far east of the Cascades for the Cascade Volcanic Arc to have been the source. Local volcanic centers were probably active east of the Cascade Range during the Oligocene epoch, erupting felsic ash and pumice.

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 one big exception is plutons and volcanic rocks produced by the Cascade Volcanic Arc, which began roughly 35 to 37 million years ago.

Eocene epoch

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 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, 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 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, and the Pioneer complex in south central Idaho. 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.

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.

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 they get buried deeply enough into the upper crust of the Earth, those sediments get compacted, cemented, and as a result lithified, becoming sedimentary rock.

Sand gets 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 (and this repeats a lesson from earlier in the quarter), the erosional sediment source of akosic sand, and the arkose (arkosic sandstone) into which it lithifies, is mountains, not very far away, consisting largely of uplifted granite and gneiss undergoing erosion. 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:

• peat when it is just below the level of the water and the mud in the ground, very near the Earth's surface
• lignite when it is buried a bit deeper, becoming darker and a more dense,
• bituminous coal when it is buried even deeper and becomes pretty much black and more recognizable as coal, though still fairly dirty with other elements and in some cases with plant fossils still quite visible and well-preserved
• anthracite when really squeezed and slightly heated; anthracite is black shiny, high-grade coal with most of the plant fossils destroyed.

That coal-forming plant material, if it goes beyond the sedimentary rock stage of the rock cycle and gets deep and hot and pressurized enough to be metamorphosed, may turn into graphite, a mineral made of carbon. However, a lot of the carbon would have been volatalized away by then, turned into gas and lost from the rock cycle, or transferred through chemical reactions to other rocks undergoing metamorphism. But we are not focusing on metamorphic rocks here.

Sedimentary rocks, and the clues they contain to what was happening in the neighborhood -- happening in Earth's surface environments, including what the living things may have been up to -- are the focus while we consider those Eocene depositional basins of the Pacific Northwest.

The sediments and fossils of the Puget Group reveal much information about the Eocene environment in the Puget Sound area. The coal formed in lushly vegetated, swampy areas near the coast, fed by rivers that deposited thick sequences of sand, silt, and mud along the river channels. Coal is the buried and lithified remains of woody, leafy plants. After being buried and heated in the sedimentary strata of the crust, the plants are reduced to mainly carbon, and thus become coal. The Bellingham, Seattle, and Cle Elum areas used to have thriving coal-producing mines. Coal is still being mined from Puget Group sediments south of Seattle, by heavy machinery in open pit mines.

The sandstone in the Puget Group contains lots of feldspar, biotite, and muscovite along with quartz. The Cascade Mountains contain no 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. These Eocene rivers did not have to pass through the Columbia Gorge to get to the coast. The Cascade Range was not yet the barrier that it is today.

Volcanism in the Northwest during the Eocene 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 alkalic 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; flood basalt; colonnade; entablature; joint; vesicle; felsic; hot spot; magma; tectonic plate; mafic; pluton; strike-slip fault; graben; metamorphic core complex; metamorphic rock; gneiss; schist; plutonic rock; detachment fault; sandstone; coal; feldspar; biotite; muscovite; quartz; asthenosphere