PNW Geology Lecture 10

Introduction
Paleozoic Geology of the Pacific Northwest
Proterozoic Geology of the Pacific Northwest
Archean Geology of the Pacific Northwest
Glossary Terms

Introduction

Welcome to Week 10 of Pacific Northwest Geology. This week we finish traveling backwards in time through the geologic history of the Pacific Northwest. The topic of this week's lecture is the Paleozoic, Proterozoic, and Archean geologic history of the Pacific Northwest. Most of Washington and Oregon did not exist during those times, except for a few faraway pieces that later became parts of accreted terranes. One exception is northeastern Washington around Spokane and Newport, where rocks from some of these earlier stages of Pacific Northwest history can be found.

In the Archean eon most of the continental basement of North America formed. During the Proterozoic eon a wide, shallow body of salty water covered much of the Pacific Northwest, accumulating sediments that recorded the environments of the time. Late in the Proterozoic eon, part of the continental crust in the Pacific Northwest region was torn apart by a major rifting event, creating a new continental margin. During the Paleozoic era, shallow seas repeatedly covered this continental margin, forming widespread deposits of marine sediments.

Paleozoic Geology of the Pacific Northwest

The Paleozoic Era began approximately 545 million years ago with the Cambrian period. Throughout the Rocky Mountain and Basin and Range regions, the Cambrian period was distinguished by the great Cambrian transgression. The ocean gradually rose and covered the lowlands along the edge of the continent, slowly transgressing farther and farther inland until much of the land was beneath the sea. As the sea transgressed, it deposited layers of sand along the beach and near the shore. As the ocean grew deeper, layers of mud from rivers emptying into the sea accumulated in the quieter waters farther out from the shore. As the sea grew deeper and the shore advanced far inland, limestone deposits formed in the quiet waters far out from the shore in which no mud or sand was accumulating.

The result of such a transgression of the ocean across the land is a distinctive stratigraphic sequence consisting of a formation of sandstone overlain by a formation of shale, which is overlain by a formation of limestone. This is known as a transgressive sequence. The Cambrian transgressive sequence occurs from Mexico to Canada in the Rocky Mountain and Basin and Range states.

In northeastern Washington, the Cambrian Addy Quartzite (slightly metamorphosed sandstone) is overlain by the Maitlen Phyllite (low-grade metamorphosed shale), on top of which is the Cambrian Metaline Limestone. This sequence of Cambrian rocks is consistent with the Cambrian transgression. At the base of the Cambrian transgressive sequence is an unconformity. Beneath the unconformity is the older, eroded, Precambrian landscape that was covered by the sea and buried beneath the Cambrian layers of sediment.

Around the world, the Cambrian period is distinguished by the first widespread appearance of animals in the fossil record. All Cambrian animals were types that lived in the sea, including early relatives of clams, snails, crabs, and sea stars. In southeastern British Columbia, the Burgess Shale formed from sediments that formed where the ocean floor sloped from the edge of the continent into deeper water. The Burgess Shale contains exceptionally well-preserved fossils of a wide variety of Cambrian creatures. Many of them were arthropods, relatives of insects, crabs, and spiders, which lived on the floor of the ocean or else swam near the ocean floor.

A well-known type of arthropod that lived on the muddy bottoms of ancient seas is the trilobite, which had a helmet-like head with eyes, a segmented abdomen with many pairs of jointed legs, and a blunt tail segment. Trilobites first appeared in the Cambrian and existed through the Paleozoic period. They became extinct in the Permian period. A few trilobite fossils have been found in northeastern Washington, in sedimentary rocks that formed from deposits along the edge of the continent. Trilobite fossils are common in Cambrian and Ordovician shales in the Rocky Mountain region. Trilobites are also found in some of the accreted terranes of the Pacific Northwest, and many of these do not match the trilobite types from the same time periods as those that are from North America. This is consistent with the exotic origin of the terranes.

Transgression may be followed by regression, in which the sea becomes shallower and the edge of the sea retreats back from the interior of the continent. During a regression limestone deposits are covered by mud and then by sand, forming a regressive sequence of limestone-shale-sandstone from bottom to top. However, regressive sequences are not as commonly preserved as transgressive sequences. Once regression has occurred the land is above sea level and likely to be subjected to erosion. Erosion will tend to remove the upper sedimentary layers and erase the direct evidence of regression.

The ocean that transgressed during the Cambrian period remained over much of the continent through the Ordovician period, gradually raising or lowering its depth and covering varying amounts of the land. During the Silurian period the sea regressed and few sediments are preserved from Silurian time in the Rocky Mountain region. In the middle Devonian period another transgression occurred and marine sediments of Devonian age occur throughout much of the Rocky Mountain and Basin and Range regions.

By late Devonian time, what is now western North America had been a passive continental margin for hundreds of millions of years, gradually accumulating sedimentary layers beneath shallow seas, with occasional regressive intervals during which sediments were deposited on land or else the land was eroded. Then, during the late Devonian, a belt that includes parts of Nevada, eastern Idaho, and southwestern Montana was subjected to thrust faulting, uplift, and erosion on a large scale.

This was the Antler orogeny, which continued until the Pennsylvanian period. During the Antler orogeny, island arc and oceanic crust terranes were thrust against and accreted to the continent on the deep ocean side of the Antler belt, in what is now western Nevada, California, and southwestern Oregon. The Antler orogeny ended well before the end of the Paleozoic era and the Antler mountains were largely eroded away by the end of the Mesozoic era that followed. With its evidence of subduction and terrane accretion and its inland fold and thrust belt, the Antler orogeny was similar to the orogenies that would develop in the Pacific Northwest on an even larger scale during the Mesozoic era and continue through the Cenozoic era up to the present.

In the Mississippian period another transgression of the sea led to the deposition of a thick sequence of limestones throughout most of the Rocky Mountain and Basin and Range regions. In Idaho, Montana, and Wyoming, these Mississippian limestones are called the Madison Group. The Madison limestone is so thick, and so resistant to erosion in the arid and semi-arid climates of the Rocky Mountain states, that many of the prominent hills and ridges consist of Madison limestone. Many fossils occur in the Madison group, shells and skeletons of animals and reef-builders that flourished in warm, shallow seas.

During most of the Paleozoic era, what is now the western United States straddled the equator. Late in the Paleozoic era, during the Pennsylvanian and Permian periods, the supercontinent Pangaea assembled. During this interval much of the Rocky Mountain and Basin and Range regions were dry land, the sea having regressed at the end of the Mississippian period. Some of the Pennsylvanian and Permian formations in those regions are mudstones from soils and rivers and bays of the ocean, and some are sandstones that formed from wind-blown sand dunes marching across deserts.

Late in the Permian period the sea transgressed and deepened over parts of Idaho and adjoining Rocky Mountain states, forming a closed basin of deep, stagnant ocean water. In this basin layers of mud rich in organic debris were deposited, along with layers rich in phosphorous. This is the late Permian Phosphoria Formation of Idaho, southwestern Montana, and northwestern Wyoming. The Phosphoria Formation is an example of how marine sedimentary rocks can be an economic resource. The organic-rich shale has been a source of oil, and the phosphorous has been mined to use in manufacturing fertilizers, detergents, and other products that require phosphorous.

Finally, at the end of the Permian period about 245 million years ago, the Paleozoic era came to a close. Pangaea was a largely arid continent swept by strong winds. In sediments left over from the ocean of that time as well as in sediments deposited on Pangaea, the end of the Permian period is marked by the greatest mass extinction in the fossil record, with the majority of Permian fossil species ceasing to appear in the rock layers. This cleared the way on land for the rise of the dinosaurs in the Mesozoic era.

Proterozoic Geology of the Pacific Northwest

All of earth history before the Paleozoic era is referred to informally as the Precambrian. During the Proterozoic eon, the most recent part of the Precambrian, sediments of the Belt Supergroup were deposited on top of the Archean basement in the Pacific Northwest.

The Belt Supergroup, known in Canada as the Purcell Supergroup, is the most distinctive group of Proterozoic rocks in the Pacific Northwest. The Belt extends through the northern Rocky Mountain region into what is now Washington state near Spokane, Colville, and Newport. The Belt Supergroup is a layered sequence of sedimentary rock formations, including limestones, sandstones, siltstones, and shales. Strata of the Belt range from about 1.5 billion to about 800 million years old, from middle to late Proterozoic. Most of the rocks of the Belt Supergroup have been metamorphosed. However, in most of the Belt the metamorphism is at such a low grade that the rocks still appear to be sedimentary and are usually referred to as sedimentary.

The rocks of the Belt Supergroup preserve many detailed sedimentary structures, including mud cracks and salt crystal impressions that indicate the sediments were in shallow water that occasionally dried up. Among the other sedimentary structures in the Belt are cross-beds and ripples, which indicate that water was flowing and in some cases reveal which direction it was flowing. Glacier National Park in northern Montana consists largely of Belt Supergroup rocks with countless examples of these and other sedimentary structures.

Some formations in the Belt Supergroup contain stromatolites, which are layered fossil structures created by mats of primitive algae. As observed in modern stromatolites, the algae that create stromatolites are one-celled organisms that grow in mats on the bottom of shallow bodies of salty water. This type of algae is called cyanobacteria and apparently existed during the Proterozoic eon, as indicated by the stromatolites. There are no other signs of life in the Belt sedimentary layers, no shell or skeleton or leaf fossils, no fossil tracks, and no fossil burrows. Animals that could leave fossils of shells or skeletons did not occur until later, near the end of the Proterozoic eon, and fossils of recognizable animals such as mollusks or arthropods did not become common around the world until the Cambrian period.

Within the Belt Supergroup are several igneous sills, intrusions that squeezed between the sedimentary layers. Because the sills are largely parallel to the sediment layers, from a distance they may be mistaken for sedimentary strata. Close inspection shows that the layers are igneous rocks that, when they intruded as molten magma, contact metamorphosed (baked) the sedimentary rock immediately above and below the sill. The sills in some places cut across the sedimentary layers, showing where the magma moved through cracks and formed dikes.

In Granite Park in Glacier National Park, the magma that formed the large Purcell Sill appears to have come to the surface and formed a basalt flow, with a vesicular, irregular top of the flow and no baking of the sedimentary layer above it. This indicates that the sill to which the flow is connected intruded during the time the Belt sediments were being deposited. The sill has a radiometric age of about 1.2 billion years.

Approximately 800 million years ago the continent rifted apart. A large part of the Belt Supergroup drifted away with the missing part of the continent. A variety of geologic evidence points to the late Proterozoic rifting event. Geologic structures of that age map out as a jagged system of normal faults and grabens--rift valleys--intersecting at 120 degree angles, which is characteristic of a rifted continental margin. Layers of volcanic rock from that time along the continental margin are the types of basalt associated with the beginning of ocean-floor spreading. To the east, the Belt Supergroup tapers to thin beds that overlap older rock formations. To the west, the Belt Supergroup is abruptly truncated along faults that cut through Belt beds thousands of feet thick., Flow features in the sediments show they were derived from erosion of elevated land that was located even farther the west. This makes it clear that a large portion of the Belt Supergroup is missing.

Following the late Proterozoic rifting event the land to the west of the Belt Supergroup, including the missing piece of the Belt Supergroup itself, was removed from North America as new oceanic crust formed between the rifted continental masses. The missing piece of the Belt may now be part of Siberia in Russia, where rocks with ages and sedimentary structures similar to the Belt Supergroup occur. Once the ocean basin opened up, what was left behind in the Pacific Northwest was a passive continental margin with no plate boundary activity to interfere with gradual accumulation of sediment in broad basins and occasional erosion of elevated areas. The region continued to be a passive continental margin for hundreds of millions of years.

The Windermere Supergroup, of late Proterozoic age, lies unconformably on top of the Belt Supergroup. The Windermere Supergroup is a mixed bag of volcanic and sedimentary formations. The volcanic rocks provide evidence of continental rifting and the opening of a new ocean basin along the edge of the continent. One of the sedimentary strata is a lithified layer of what appears to be glacial till. This is consistent with more definitive evidence from other continents of a global ice age late in the Proterozoic eon.

Archean Geology of the Pacific Northwest

The Archean is the earliest stage of earth history from which rocks still exist. The oldest rocks on earth are gneisses in eastern Canada that go back to 3.9 to 4.0 billion years ago, according to the radiometric ages of the minerals in the rocks. Other evidence narrows the age of the earth down to somewhere in the range 4.5to 4.6 billion years ago. No rocks are preserved from the first 500 or 600 million years of earth history, due to a combination of factors. The whole earth was probably molten for a while after the planet formed. The early earth was subjected to frequent collisions with huge meteorites. Erosion has erased many rocks from the geologic record, and the older the rock, the more likely it has been lost to erosion. Subduction constantly recycles oceanic crust back into the mantle, and none of today's ocean floor is older than Mesozoic. Finally, the earliest continents, the nuclei around which the continents grew larger by accretion and orogeny, may not have formed until hundreds of millions of years after the earth formed.

In the Rocky Mountain region, the Laramide orogeny has uplifted blocks of deep crustal rocks into high mountain ranges where erosion has revealed them at the earth's surface. The centers of many Laramide mountain ranges contain metamorphic and plutonic rocks of Archean age, part of the basement of the North American continent.

In the Beartooth Range, a Laramide mountain range in southwestern Montana, there is an unusual body of plutonic rock known as the Stillwater Complex. The Stillwater Complex consists of mafic and ultramafic rock that formed layers as the magma intruded and crystallized. Some of the layers are rich in minerals that contain such elements as chromium and platinum, which are economically valuable. The minerals and ultramafic igneous composition of the Stillwater Complex indicate that it formed at very high temperatures, higher than the temperatures of more recent igneous intrusions. This is consistent with the interior of the earth being hotter in the Archean than it is now, with gradual cooling of the earth lowering its temperature over the eons.

The Archean Eon ended 2.5 billion years ago. By that time all the continents had formed a stable central region founded on a basement of plutonic and metamorphic rocks. This stable central region of the continent, with its basement of Precambrian rocks, is called the craton. The craton of North America underlies the Rocky Mountain and much of the Basin and Range region. It extends into northeastern Washington near Spokane. The rest of the states of Washington and Oregon have been added to North America since the Precambrian.

Return to schedule

Glossary terms that appear on this page: accreted terrane; continental basement; transgression; sandstone; shale; limestone; unconformity; regression; passive margin; orogeny; stromatolite; sill; dike; basalt; normal fault; graben; subduction; plutonic rock; mafic; ultramafic

Home Basics Focus Pages Lectures Virtual Field Sites back to top