- Cretaceous Geology of the Pacific Northwest
- Jurassic Geology of the Pacific Northwest
- Triassic Geology of the Pacific Northwest
- Glossary Terms
Welcome to Week 9 of Pacific Northwest Geology. During this week we continue pushing further back into the geologic history of the Pacific Northwest. The topic of this week's lecture is the Mesozoic geologic history of the Pacific Northwest.
The Mesozoic era began 252 million years ago, following the assembly of the supercontinent Pangaea and the greatest mass extinction the world has ever known. That set the stage for the beginning of the Triassic period, the first period of the Mesozoic era. The Jurassic period, which followed the Triassic, is especially important in Pacific Northwest geologic history for the beginning of subduction, orogeny, and terrane accretion on a large scale, a process that has continued up to the present. The Jurassic period was followed by the last of the three Mesozoic time periods, the Cretaceous.
The Cretaceous period is best known for its evidence of hot, humid climates, its dinosaur fossils, and the extinction of dinosaurs and most other species on earth at the end of the Cretaceous. The final demise of the dinosaurs apparently happened at the same time the earth was impacted by a large meteorite. Dinosaur fossils, and evidence of the end of the Cretaceous period, have been uncovered in several places in the Northwest, including Montana. So far, however, no dinosaur fossils have been found in Washington State.
During the Cretaceous period many terranes, including the entire Insular Superterrane, collided with and accreted to the Pacific Northwest region. Also during Cretaceous time, many terranes apparently moved northward along the edge of the continent. Terranes that were already attached to the region underwent clockwise rotation. This rotation may have occurred because the terranes that rotated were caught between the continent to the east and other terranes to the west that were pushing north along the coastal margin.
The Sevier, Laramide, and North Cascades-British Columbia Coast Range orogenies all reached their peak rate of structural deformation and rock cycle processing during the middle to late Cretaceous period. According to paleomagnetic evidence, the Farallon Plate subducted rapidly beneath the continental margin during that time. Some studies of the mechanics of plate convergence suggest that the angle of subduction of the Farallon Plate would have decreased through the Cretaceous. as the plate subducted more rapidly. As a result, by the end of the Cretaceous period, the plate may have been scraping along the base of the continental crust far inland from the subduction zone. This would account for the eruption of subduction-type volcanic rocks, including andesite, far inland from the coast, in states such as Montana and Wyoming.
The Laramide orogeny involved deep crustal rocks in the Rocky Mountains region and the Sevier orogeny involved layers of sedimentary rock in the upper crust. Geologists hypothesize that rapid, shallow-angle subduction of the oceanic plate beneath the continent combined with large-scale collision of terranes along the leading edge of the continent was responsible for these orogenies. The oceanic plate scraping beneath the base of the continent may have pushed up blocks of the lower crust to create the Laramide ranges and rapid accretion of large terranes may have compressed the upper crust and caused the Sevier orogeny.
In the middle to late Cretaceous, rocks of the North Cascades crystalline core were deeply buried, underwent extensive regional metamorphism, and were intruded by large plutons. Multiple intrusions formed batholiths, bodies of intrusive igneous rock large enough to encompass more than 100 square kilometers on a map.
In the Coast Range of British Columbia, in central Idaho, and in the Sierra Nevada Mountains of California, the middle to late Cretaceous was a time when enormous amounts of magma intruded the crust, forming three of the largest batholiths in the world, the Coast Range, Idaho and Sierra Nevada batholiths. The rapid rate of subduction of the Farallon plate may have been responsible, at least indirectly for such large volumes of magma. The granodiorites and other plutonic rocks of the Cordilleran batholiths formed as fluids released by the subducting plate generated molten rock from the mantle, which rose and melted the overlying deep continental crust.
By late in the Cretaceous period, the terranes of the North Cascades and San Juan Islands had accreted to North America. Sediments of the Nanaimo Group were deposited across several of these terranes in the northern San Juan Islands. Because the Nanaimo Formation overlaps these terranes, it provides a minimum age of when the San Juan Island terranes had accreted. They must have accreted prior to deposition of the Nanaimo sediments late in the Cretaceous period.
Some rocks in the San Juan Islands terranes are early Cretaceous in age. The presence of late Cretaceous fossils in the overlapping Nanaimo Group narrows the time of accretion to between about 110 and 85 million years ago.
There has been debate among geologists about how the San Juan Island and North Cascades terranes accreted. Did they get shoved eastward into the edge of the continent by the Farallon Plate, and caught up and squeezed soon afterward by accretion of the Insular Terrane? Or did they get shoved northward along the coast of the continent by strike-slip faulting and oblique convergence of oceanic plate(s) with North America, until they were jammed into an indentation in the edge of the continent?
The shear zones of the San Juan Islands, including those illustrated in the San Juan Island Virtual Field Site, are seen by one group of geologists as being due to eastward-directed thrust faulting. Eastward-directed accretion would be consistent with normal subduction shoving the terranes directly into the edge of the continent. Another group of geologists has argued that the shear zones contain structures that are consistent with being shoved toward the north rather than toward the east. This north-directed shoving would support the idea of oblique subduction forcing terranes northward along the edge of the continent.
This debate about the San Juan Islands and North Cascades terranes being shoved eastward from the ocean or shoved northward along the edge of the continent is linked with paleomagnetism in the rocks. In some of the terranes and intrusions of the San Juan Islands and North Cascades the paleomagnetism indicates that the terranes originated far to the south. If so, they must have come a long ways north by by the end Eocene epoch at the end of the Neogene period. It is unclear how the terranes could have moved so far north during the late Cretaceous to mid-Tertiary time interval. The strike-slip faults that have been mapped, such as the Straight Creek fault, do not seem to add up to enough northward motion to explain the paleomagnetism. Geologists continue to study this problem.
On the east side of the North Cascades, the rocks of the Methow Valley record accretion of island arc and oceanic crust terranes during the middle and late Cretaceous. After an island arc accreted and sediments accumulated in ocean water along the edge of the continent, thrusting, more accretion, and uplift of terranes to the west created a more confined basin that filled with sediment above sea level. This basin became a site of rivers and swampy areas during the Late Cretaceous period. The layers of sediment deposited in the basin were thrust-faulted and folded during and after the late Cretaceous as more terranes accreted to the west.
During the Cretaceous period many species of dinosaurs flourished in the landscapes of what is now the Rocky Mountains region. Rivers meandering through the Cretaceous landscape provided wetlands lush with plants to eat for the herbivores, and places to hide while stalking prey for the carnivores. The Cretaceous strata in the high plains and along the edges of the Rocky Mountain ranges in Montana and Wyoming have yielded many well-preserved dinosaur fossils, even nests and eggs in some cases. One type of dinosaur fossil found in the Rocky Mountain region is the well-known species Tyrannosaurus rex, which came into existence during the late Cretaceous period. Tyrannosaurus rex may have been among the dinosaur species that still existed when the sky was darkened by the debris of a gigantic meteorite impact, drawing the curtain on the Cretaceous period and the Mesozoic era.
A variety of geologic evidence indicates that during most of the Cretaceous period climates were much warmer in many parts of the world than they are now. This is sometimes called the Cretaceous "hothouse" climate.
East of the Rocky Mountains region, the central portion of the North American continent was inundated by a shallow inland sea during much of the Cretaceous period. Within the Pacific Northwest, the inland sea of the Cretaceous period reached into the Rocky Mountain region in Montana. Dinosaurs inhabited the landscape that bordered the inland sea.
By the very end of the Cretaceous period the inland sea had retreated far to the east, out of the Pacific Northwest. Later, after the Mesozoic era, by Eocene time the inland sea had advanced one last time and then finished its last retreat from the whole North American continent.
The Cretaceous in the Pacific Northwest was a time of rapid growth. Most of the terranes and several of the largest batholiths were added during this period shifting the shore of the western ocean from to western Washington and Oregon.
The Cretaceous period began with roughly a third of what is now Washington state present and part of North America, extending west of what is now the Idaho border. With the accretion of multiple terranes, by the end of the Cretaceous period the continental edge extended to approximately two thirds of the way across the state. The ocean coast was roughly along the west side of the present-day Cascade mountains near where Puget Sound is now. The Olympic Peninsula and Willapa Hills were not yet present.
By early Jurassic time the previously passive coastal region of the Pacific Northwest became an active margin--a site of subduction, volcanism, igneous intrusion, regional metamorphism, and terrane accretion. This change from passive to active was spurred by the North American continent rifting and breaking free of the rest of Pangaea and moving westward toward the Pacific Ocean basin.
The earliest indications of subduction in the Jurassic are confined to a small area known as the Kootenai Arc (sometimes spelled Kootenay). Located northwest of Spokane, the Kootenai Arc records an early interval of intrusion, volcanism, and folding of rocks along the Jurassic edge of the continent. Sedimentary rocks from the continental shelf were shoved onto the edge of the continent during the development of the deformed, thrust faulted, steeply folded Kootenai Arc.
Next came the Intermontane Superterrane, which added much of the rocks of the Okanogan Highlands to north central Washington and south central British Columbia. The accreted terranes of the Intermontane group consist mainly of island arc and ocean floor rocks, going back to Paleozoic in age.
After each stage of terrane or superterrane accretion, the associated subduction zone shifted farther west. However, the continental margin was not a straight line. The southern half of Washington and the northern half of Oregon seem to have been a major indentation or large bay along the edge of the continent. This is referred to as the Columbia Embayment. The embayment continued to exist through the Jurassic and Cretaceous periods before it was filled with volcanic and sedimentary rocks. The Columbia River Basalts now cover much of the Columbia Embayment.
East of the Rocky Mountains region during the Jurassic, the inland sea deepened and spread widely forming what is known as the Sundance Sea. The Sundance Sea extended as far west as eastern Idaho. Many fossils of marine creatures have been recovered from sediments of the Sundance Sea, just as many Jurassic dinosaur fossils have been found in the sediments deposited on the lands that bordered the sea.
By Late Jurassic time the Sundance Sea had retreated eastward from the Pacific Northwest and the sediments of the Morrison Formation were deposited widely in the Rocky Mountain states, including Idaho and Montana. The Morrison Formation accumulated in river valleys and basins of a semi-arid landscape. In terms of large size and high quality of preservation, the Morrison Formation contains some of the most famous dinosaur fossils ever discovered. The layers of the Morrison Formation were caught up in the Sevier orogeny during the subsequent Cretaceous period, causing its strata to be extensively faulted and folded.
At the beginning of the Triassic period most species of animals of the preceding Permian period, both on land and in the oceans, had become extinct. Most of Oregon and about two-thirds Washington state did not yet yet exist, except as island arcs and sections of oceanic crust far away from the continent, exotic terranes of the future that were still in place on their native grounds. The crust of the Pacific Northwest that did exist was a coastal region of the supercontinent Pangaea.
Parts of the portion of Pangea that ultimately became the Pacific Northwest were under ocean water during the Triassic. Mud, silica, lime, and sand accumulated in the various environments of this Triassic ocean floor and became lithified into shale, chert, limestone, and sandstone. Some of the shale is rich in phosphorous, which is used in making fertilizer and other industrial products. After cheaper sources of phosphorous in other countries are depleted, the Phosphoria Formation of Idaho, Montana and Wyoming may eventually be mined.
As the Triassic period unfolded, those parts of the Northwest that were above sea level accumulated sand, mud and other sediments. These Triassic sediments point to arid and semi-arid, wind-blown landscapes, across which sand dunes slowly marched and rivers emptied into evaporating lakes. Most of the sediments that accumulated on these Triassic landscapes of Pangaea are stained red by iron oxides, the same minerals that make rust red. Triassic red beds are found in many parts of the world.
The widespread red beds of the Triassic seem to have formed from the climate of Pangaea (relatively warm and dry with strong, steady winds) the geography of Pangaea (which was a giant continent - a supercontinent - with most of its land far inland from the open ocean), and possibly from the chemistry of the atmosphere at the time (it may have had a higher concentration of carbon dioxide, for example).
Fossils of the earliest dinosaurs appear in rocks of early to middle Triassic age. Fossils of the earliest known forms of mammals also occur in Triassic rocks. Compared to dinosaurs during the Mesozoic era, mammals were small creatures, a minor component of the land animals of the time.
By the end of the Triassic period, approximately 40 million years after the period began, the great continent of Pangaea was entering its final stages of rifting and dismemberment, a process that had begun by middle Triassic time. North America was part of the northern section of Pangaea. By the end of the Triassic or early in the Jurassic, the complex sequence of rifting and drifting that broke Pangaea apart was far enough along that the North American continent began drifting west on its own, subducting an oceanic plate beneath it as it moved. Those distant island arcs and sections of oceanic crust were now on a conveyor belt, which was moving them inexorably toward a new home on the edge of the North American continent.
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Glossary terms that appear on this page: Pangaea; subduction; orogeny; accreted terrane; regional metamorphism; batholith; magma; granodiorite; plutonic rock; mantle; thrust fault; paleomagnetism; island arc; passive margin; active margin; shale; chert; limestone; sandstone
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes