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Chapter 1:
Evolution of the Ancestral
North American Margin
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Chapter 1:
Evolution of the Ancestral
North American Margin
2700 - 200 Million Years Ago
While the assembly of the Pacific Northwest region did not commence until Mid-Jurassic time, the continental margin
on which it was assembled has a history which extends back into relative antiquity. That history is the prologue to the
assembly of the Pacific Northwest, and its consideration provides an important aspect of context for that story.
This is a history which spans over four billion years, the early part of which we know very little about. We can trace
the origins of the oldest local rocks of North America back about 2.7 billion years, and the origins of the modern continent back about 200 million years. In between, the record we have is a fragmentary one at best. We have a record for
a the period between 1500 and 1400 million years ago, perhaps a scrap from about 1000 million years ago, a section
from 750 to perhaps 650 million years ago, and a record from between 570 and 350 million years ago. Beyond this,
there is little to the local record for this immense span of pre-Jurassic time.
Despite this fragmentary record, it is possible to piece together a brief history of this vast period based on our broader
understanding of regional and global geologic evolution. Much of that story is structured around the master cycle
of plate tectonics – the Wilson (a.k.a. “supercontinent”) cycle. In this cycle, the inevitable collision of continental
masses destroys the oceanic trenches (subduction zones) which separate them, and eventually leads to the creation of
a single large continental landmass. These large “supercontinents” are then broken up by rift zones (spreading centers)
which once again scatter the continental fragments, in a continuing cycle. At least four supercontinents are recognized
over the last two billion years: Columbia and Rodina in the Mid Proterozoic, Pannotia in the late Proterozoic, and
Pangea in the early Mesozoic. Most of the known record for this vast period of time reflects on this continuing master
cycle of plate tectonics.
For the Paleozoic record we can also consider a different cycle – the cycle of rising and falling sea levels, to structure
our observations. Most local rocks of this age are parts of continental-scale deposits which reflect these continuing
eustatic cycles. While our record over this era is incomplete at best, we can draw some inferences from the surrounding provinces to gain a regional perspective on events over these times.
This is not a story about the Pacific Northwest. It is a story about how the ancestral continental margin developed prior to the assembly of this province. Striving not to venture far into the Rocky Mountain province to the east, and not
knowing with any certainty what lies beneath the thick cover of the Columbia River Basalts to the south, this story is
largely about northeastern Washington and southeastern British Columbia. These are properly considered the westernmost provinces of the Rocky Mountain Belt. An understanding of how these provinces evolved will be an important
foundation for interpreting the subsequent course of events in the assembly of the Pacific Northwest.
Figure 1-1 (cover page) Selkirk Mountains, looking southwest from Salmo Mountain, east of Metaline Falls
Figure 1-2 (left) East from Salmo Mountain, looking across the Idaho panhandle into ancestral North America.
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Basement Rocks:
Rocks of the North American Craton
The history of the Earth can be traced back to something like 4.6 billion years ago, with the basic structure of a core,
mantle, aesthenosphere and lithosphere probably dating from about 3.8 billion years ago. From this early date, some
form of plate tectonics was probably in effect. In this early setting the continents were small and relatively thin accumulations of intermediate plutonic rocks, amounting to somewhere between 10 and 40% of the modern continental
exposure. Relatively little is known of this early era in Earth history, and few rocks survive from this period.
This setting came to an end at the conclusion of the Archean Era, in an event known as the Kenoran Orogeny. Between 2.7 and 2.5 billion years ago, in this poorly-understood event, much of the granitic-type rock of the modern
continents was accumulated. This resulted in a great thickening of the continental masses, and the establishment of the
modern plate tectonic regimes. This was about a billion years before the earliest proto-North American continent was
assembled, and over two billion years before the modern North American continent came into being.
These old rocks make up the crystalline “basement” of the continent here, which probably extends west for at least
300 km beneath the accreted terranes of the Pacific Northwest. To the best of our knowledge, these rocks are not
exposed anywhere in Washington State. While they are presumed to underlie its eastern margins, they are not exposed
anywhere in our region.
Just across the border into Idaho however, it is possible to see these ancient rocks. Here, the 2.65 billion year-old
Pend Oreille Gneiss outcrops as a window on the ancient cratonic basement of the continent. These rocks are part
Figure 1-3 The Pend Oreille Gneiss, ancient rocks of the North American craton.
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Figure 1-4 The Pend Oreille Gneiss
of the Hearne-Wyoming Terrane, one of the older fragments of
continental rock in North America. This rock is a biotite gneiss,
derived from a granitic (tonalite) protolith. It has been extensively
deformed, a testament to innumerable collisional events in its history. As we shall consider later, some of those more recent events
reflect the assembly of the regions to the west.
By tracing the recorded history of early collisional events, it
appears that these early continental masses began to aggregate together between 1.9 and 1.8 billion years ago. By 1.6 billion years
ago the elements of an early supercontinent, known as Columbia,
had been assembled. The supercontinent takes its name from the
Columbia basin, because (the far eastern margins of) this region
offers some of the best evidence for its amalgamation. The final
assembly of this early continent may be recorded in the 1.57 bilFigure 1-5 The supercontinent of Columbia, assembled by 1.6 billion
years ago. Laurentia represents the ancestral North American continent.
At this time, it was positioned 180 degrees from its present orientation,
and located in the southern hemisphere.
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West Africa
South America
West Antarctica
Asia
Greenland
Laurentia
India
Australia
Siberia
Europe
Figure 1-6 The 1.56 billion-year-old Laclede Gneiss, a record from the final assembly of the Columbia Supercontinent.
Note the prominent garnets.
lion year old Laclede Gneiss, which adjoins the older Pend Oreille Gneiss. Over much of Idaho and Montana there is
evidence for a metamorphic event between 1.5 – 1.6 billion years ago.
East of the accreted terranes of the Pacific Northwest, these gneissic rocks underlie the great thicknesses of continental sediments which would accumulate in the Mid and Late Proterozoic Era, and the sediments which accumulated on
the ancestral continental margin in Cambrian and later times. More recently, as the Pacific Northwest has accumulated, its accreted terranes have overthrust these older rocks to the east. While not exposed on the surface, these cratonic
rocks are thought to extend at least as far west as the modern Okanogan Valley. East of this point, they are the deepest
basement rocks of the modern Pacific Northwest.
Europe
Asia
Laurentia
South
America
Figure 1-7 (Left)
The supercontinent of Columbia (rotated 180 degrees)
showing the initial spreading
centers associated with its
breakup.
Figure 1-8 (Right)
The supercontinent of Columbia (rotated 180 degrees)
showing development of the
Belt-Purcell basin as a failed
rift zone (aulacogen).
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Europe
Laurentia
Belt - Purcell
Rift Basin
South
America
The Ancient Supercontinent of Columbia
Rocks of the Belt-Purcell Supergroup
The ancient supercontinent of Columbia appears to have been assembled between 1.8 and 1.6 billion years ago, and
started to break up about 1.5 billion years ago. The breakup of such supercontinents occurs by the development of rift
zones, spreading centers which develop within the continent. These features usually develop as three spreading centers converging on a central point. Two of these eventually align to form a linear zone which forms the center of a new
ocean basin, driving the continental masses apart. The third rift zone develops to some degree, but eventually fails to
mature. Such failed rift zones are known as aulacogens (al-‘ock-o-jenz).
About 1.5 billion years ago, it appears that continental masses on the (now) north side of Columbia (Baltica, Siberia?)
were rifted off the ancient supercontinent as it started to break up. As that rift zone developed, the third “arm” of that
system, eventually doomed to become an aulacogen, cut through the very heart of Laurentia. Aligned broadly northsouth, the center of this zone cuts through what is now Montana, extending some 3500 km from Northern Canada
to the Southwestern United States. About a billion and a half years ago, the newly-assembled continent of Laurentia
started to pull apart along this zone. While this effort was ultimately destined to failure, the record of this event is
preserved across a broad swath of modern North America. That record is held in a group of rocks known in the United
States as the Belt Supergroup (after the Belt Mountains in Montana), and in Canada as the Purcell Supergroup (after
the Purcell Mountains of southeastern British Columbia). These rocks were derived from sediments accumulated in
this vast rift basin, in the middle of the ancient continent of Laurentia.
The Belt-Purcell Supergroup is a discontinuous continental-scale belt of silt, sand, conglomerate and limestone, averaging about 300 km in width. With the exception of the limestones, these were sediments weathered and eroded from
the original cratonic rocks of Laurentia. Beyond their lateral scale, the truly remarkable aspect of these rocks is the
vertical extent to which they accumulated. In places, they achieve an astonishing thickness of nearly 18 km (10 miles).
This represents the (minimum) depth of the rift basin which developed here.
Belt Purcell
Rift Basin
Australia
Figure 1-9 (Right)
Part of the former supercontinent of Columbia (rotated 180 degrees) showing
the Belt-Purcell basin as a
failed rift zone (aulacogen).
Depictions of this ancient
supercontinent are speculative at best. Compare with
figure 1-8 (left)
Laurentia
India
Asia
West Antarctica
5
The “classic” Belt-Purcell sequence, as
typified by the rocks in Idaho and Montana, is found in Washington east of the
town of the Colville Valley. There it
includes at least ten different formations,
in a normal progression, extending east
toward Idaho. To the west of the valley a
different set of Belt-Purcell rocks, the Deer
Trail Group, makes up much of the Huckleberry Mountain range. The Deer Trail
group correlates with the upper portion
of the Belt-Purcell sequence to the east,
although there are significant differences
between them. The two groups are separated by a series of thrust faults which follow
the Colville Valley, along which there has
been considerable east-west contraction.
Originally, these two groups were likely
separated by considerably more distance.
Alberta
British Columbia
Montana
Washington
Oregon
Idaho
East of the Colville Valley, the “classic” Belt-Purcell sequence starts out with
British Columbia
west
Washington
Idaho / Montana
north
Mt. Nelson
Dutch Creek
Si Yeh
Garnet Ranges
Roosville
Stensgar
Gateway
Sheppard
McHale
Mt. Shields
Sheppard
Van Creek
Edna*
Snowslip
McNamara
Kitchener
Togo
Creston
Aldrich
Helena
Empire
St. Regis
Revette
Burke
Prichard
Figure 1-10 (Top) Map showing the regional distribution of the Belt-Purcell Supergroup.
Figure 1-11 (Bottom) Regional correlation chart for the Belt-Purcell Supergroup. The Deer Trail group lies west of the Colville
valley, and has been displaced from a locale further west. *The Edna Formation has been subdivided into the
Chamokane Creek and Detroit-Wabash Formations
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a thick section of interbedded
quartzite, siltite and argillite
known as the Prichard Formation. North of the border, this is
known as the Aldrich Formation.
The Prichard, which exceeds 4
km in thickness here, displays the
characteristic record of deepwater deposition. The bedding is
centimeter scale, parallel-planar
in aspect, and typically displays
graded bedding which progresses
from coarse to fine-grained
sediments. This pattern is characteristic of submarine landslide
(turbidite flow) deposits, which
usually accumulate at the base of
the continental slope. In this case
however, it was the bottom of a
vast continental rift zone, a linear
seaway which nearly split the
ancestral continent of Laurentia.
Above the Prichard Formation,
the patterns of deposition typically
reflect a shallow-water setting.
This suggests that the Prichard
formation accumulated faster than
the rate of subsidence, largely
filling the early rift basin. The 2.4
km-thick Ravalli Group, comprising the Burke, Revette and St.
Regis Formations, lies above the
Prichard Formation. Correlative
rocks to the north of the border
comprise the Creston Formation.
Figure 1-12 (Above) Rocks of the Prichard Formation, east of Chewelah. These are fine-grained, deepwater rocks, some of which display the graded bedding
characteristic of turbidite deposits. These sediments
accumulated on the bottom of the rift zone which split
the Laurentian continent. On exposure, these rocks often stain to brilliant colors as the iron in them oxidizes.
Figure 1-13 (Right) Detail from the exposure above.
Belt rocks frequently preserve bedding features in
exquisite detail. This is owed in part to the lack of
bioturbation, as there were no organisms capable of
disturbing these sediments at that date. By this circumstance, very fine features have often been preserved.
7
These rocks consist of variable
proportions of siltite, quartzite and
argillite. They also preserve ripple
marks in the Burke Formation,
and mud chip inclusions in the St.
Regis Formation, along with other
shallow-water features.
Above the Ravalli Group is about
150 meters of siltite, quartzite, and
argillite in the Empire Formation,
topped by an 800 meter section
of the Wallace Group. The Empire Formation correlates with the
Kitchener Formation in British
Columbia. The Wallace Group
consists of carbonate-bearing siltite
and quartzite, and is a widespread
unit in Idaho. There, it is a 5 kmthick black to gray formation of
Figure 1-14 (Above) The Burke
Formation, along the north shore of
Deer Lake.
Figure 1-15 (Right) The Revette
Formation, along the north shore of
Deer Lake
Figure 1-16 (Below Right) The St. Regis Formation, along the north shore
of Deer Lake
Figure 1-17 (Below) St. Regis sample
showing prominent purple mud chips.
8
thinly laminated sediments, some
of which have dolomite or limestone horizons. The preservation
of these bedding features is often
quite spectacular, owing to a lack
of bioturbation in the rocks. In
our area, the rocks contain beds
of dolomite, which weather to a
tan color. These are interspersed
with the more typical fine-grained
sediments, in which soft-sediment
deformation is common. These
rocks accumulate to only about
750 meters of strata.
Above the Wallace Group are a
set of formations typical of the
Missoula group in Idaho and
Montana. These include about
1400 meters of argillite and siltite
in the Snowslip Formation, 500
meters of dolomite known as the
Shepard Formation, about the
same thickness of argillite, siltite
and dolomite in the Mount Shields
Formation, and about 150 meters
of siltite, argillite and dolomite
in the Bonner Formation. These
rocks appear to be correlative with
the Si Yeh and Dutch Creek Formations in British Columbia. They all
reflect shallow-water depositional
settings, with the Mount Shields
and Bonner Formations preserving ripple marks, mud chips, mud
cracks, and salt casts. Stromatolite
Figure 1-18 (Above Right) Mudcrack patterns in rocks
of the Mount Shields Formation, east of Chewelah.
Mudcrack features are common in rocks of the Missoula
Group, and reflect deposition in a shallow-water
transient - marine setting.
Figure 1-19 (Right) A modern-day stromatolite. These
are bioclastic communities of algae which capture and
cement grains of sediment into their structure. Stomatolites are limited to very stable, shallow-water settings.
9
Stromatolites and the Length of the
Precambrian Day
Stromatolites are among the earliest forms of colonial life, and have persisted into modern times. They consist of algal cells in a colonial setting,
and can grow only in stable, shallow-water conditions. They accumulate
by a two-stage process. As they grow vertically in the day, grains of
sand accumulate between the individual algal cells. At night, they grow
a new layer of horizontal cells (laminae) which serve to permanently
trap that sand. By this process they develop as round, somewhat mushroom-shaped bio-clastic communities. Good modern examples grow in
Shark Bay Australia. Good fossilized specimens appear in the Belt rocks
in Glacier National Park. Locally, fossil specimens can be found in the
Detroit-Wabash Formation of the Deer Trail Group.
An interesting feature of stromatolites is called heliotropism. Like most
plants, they grow toward the sun, the location of which changes cyclically over the course of the year. The result is that the vertically-growing
algae grow in a helical spiral, with one complete spiral equaling one
calendar year. The intriguing aspect is that stromatolites also produce
one layer of horizontal laminae every night, so the number of laminae
per spiral equals the number of days in a year. Studying stromatolites
from 850 Ma rocks in Australia, it has been determined that the year at
that time was about 435 days long. This means that a day was about 20
hours long, and that the speed of the Earth’s rotation has been slowing
over time. No such studies have been done on the stromatolites of the
Belt-Purcell Supergroup, but we would expect that the day in that time
was something like 17 to 18 hours long.
10
(Above) Modern-day stromatolites,
Shark Bay, Australia
(Below) Fossil stromatolites in rocks
of the Belt-Purcell supergroup, Glacier
National Park
fossils, the remains of ancient bioclastic communities, can be found in
many of these rocks. The persistence
of shallow-water conditions amidst
this volume of accumulation suggests
that the rate of subsidence was largely
matched by the rate of deposition.
West of the Colville Valley, the rocks
of the Deer Trail Group appear to
correlate broadly with those of the
Missoula Group. The base of the Deer
Trail section is up to 2 km of medium to dark gray argillite and green
to gray siltite of the Togo Formation, most of which has been highly
deformed and phyllitized to some
degree. Bedding in this formation
ranges from submillimeter to about 10
cm. Above the Togo Formation are
the rocks of the Chamokane Creek
Formation, consisting of carbonatebearing and noncarbonate-bearing
British Columbia
Deer
Trail
Washington
Group
Colville
Spokane
Upper Belt Rocks
Lower Belt Rocks
Figure 1-20 (upper Right) Map of the Belt-Purcell rocks in
northern Washington and Idaho, showing the location of the
Deer Trail Group.
Figure 1-21 (Right) The Togo Formation, on the Hunters
- Springdale Road. Most examples are more deformed than
this.
Figure 1-22 (Below) Rocks of the Detroit-Wabash Formation, on the Hunters - Springdale Road. The large round
features to the right of the hammer are the fossilized remains
of stromatolites, an indicator of stable, shallow-water conditions.
11
Bonners
Ferry
Idaho
Montana
Figure 1-23 (Above) mm-scale bedding in the
McHale Formation. Dime for scale. Bedding
preservation reflects the lack of bioturbation
in these rocks
Figure 1-24 (Right) The McHale Formation,
at an outcrop along the Hunters-Springdale
Road.
quartzite and siltite, interbedded with dolomite and argillite. This unit is about 600 meters thick, with individual beds
varying from 5 to 15 cm. Above the Chamokane Creek Formation is a unit known as the Wabash - Detroit Formation (the formations in this group are all named after mine sites). The Wabash - Detroit Formation is about 250 meters
thick, and consists largely of dolomite, with subordinate argillite, quartzite and siltite. Stromatolite fossils have been
found in this unit, an indication of stable, shallow-water settings. The Chamokane Creek and Wabash-Detroit Formations were formerly known as the Edna Formation.
The two uppermost formations in the Deer Trail group are the McHale and Stensgar Formations. The McHale Formation is almost entirely argillite, and is known as the McHale Slate. The lower third of this is a gray argillite with
lighter colored laminae, ranging in thickness from submillimeter to about 3 cm. The upper portion is a greenish-gray
to pale lavender argillite, with indistinct bedding features. The uppermost unit in the Deer Trail group is the Stensgar
Formation, which is almost entirely dolomite. Most of these are white, tan or pink in color, weathering to tan or gray
in appearance. The upper part of the formation contains evaporite minerals and the imprints of algal mats, reflecting a very shallow to transitional marine setting. The Stensgar dolomite has historically been an important economic
resource, mined for the production of magnesite.
Figure 1-25 (Left) The Stensgar Dolomite, at an outcrop south of Colville. Hammer provides scale.
Figure 1-26 (Below) A sample of the Stensgar Dolomite,
showing distinctive cubic salt-crystal casts which form
in an evaporite setting. Dime provides scale.
12
Figure 1-27 (Right) Darkcolored dikes cutting rocks
of the Prichard Formation
east of Chewelah. These
dikes are rift volcanics, and
reflect the origins of the BeltPurcell Basin as an oceanic
spreading center.
Further to the east these
dikes become more common,
and are often tens of meters
in scale. In our area, they
are typically meter-scale
intrusions.
The formations of the Deer Trail Group are broadly correlative
with rocks to the east, but would appear to be a shallower-water
facies from a more marginal setting along the rift basin. Formerly
separated by a greater distance, the Deer Trail and other Belt-Purcell rocks were juxtaposed as the continental margin was collapsed at a later date.
Figure 1-28 (Below) A sample of Purcell dike rock.
These rocks are gabbroic to basaltic in character,
and have been mildly metamorphosed in the course
of later events. Dime provides scale
These rocks are key to understanding the rift nature
The formations of the Belt-Purcell Supergroup probably date
from 1.5 to 1.4 billion years old, in round numbers. These rocks of the Belt-Purcell basin, and are frequently overlooked by those specializing in sedimentary rocks.
have been studied by generations of geologists, but the setting
which they accumulated in has only been recognized in the last
few decades. The rift-basin setting of these rocks is revealed by
the numerous basaltic dikes which cut them, dikes which emanated from the spreading center which developed under this region. As that rift zone developed as an alucogen, subsidence and
deposition ceased as the spreading center expired. While we don’t
know how long the rocks of the Belt-Purcell Supergroup accumulated, alucogens typically have a lifespan of tens of millions
of years. It would seem unlikely that it would exceed a hundred
million years. By modern standards, this alone would represent a
truly remarkable span of deposition.
If the youngest of the Belt-Purcell rocks are something like 1.4
billion years old, we have no depositional record for the next
several hundred million years. Over this period, continental fragments continued to collide and amalgamate around the ancient
continent of Laurentia. By something like 1.1 billion years ago,
that process had produced the next known supercontinent, known
as Rodinia.
13
Figure 1-29 The supercontinent of
Rodinia, one billion years ago.
West
Africa
Baltica
Amazonia
Siberia
Laurentia
Australia
East Antarctica
Congo
India
The Ancient Supercontinent of Rodinia
Rocks of the Buffalo Hump Formation
The ancient supercontinent of Rodinia – a Russian term for “homeland”, was finally amalgamated between 1300 and
1100 million years ago. It apparently persisted until about 725 million years ago, a remarkable span of some four hundred million years. If this is the case, this would easily make it the most long-lived of any of the supercontinents.
For this vast period of time, we know very little about the conditions in ancient Rodinia. We do know that even
the most god-forsaken reaches of modern Siberia would be immeasurably more hospitable. The atmosphere at that
time had only about 5% of the present level of oxygen, and was largely nitrogen and carbon dioxide. Life hadn’t yet
progressed beyond single-celled organisms, and even simple land plants were still half a billion years in the future.
Without vegetation to anchor sediments, seasonal dust storms may have raged across the supercontinent. If interpretations from the more recent supercontinents are accurate, much of the land may have experienced a desert climate.
We do know that, well to the east, there was an early attempt to break up the newly-formed supercontinent. A rift
zone, running from the modern-day Great Lakes southwest through Kansas, developed just before 1 billion years ago.
The Keweenawan flood basalts erupted along this rift, and nearly 12 kilometers of mafic plutonic rock (gabbro) crystallized at depth along this zone. Sediments deposited above the basalt flows are brightly-colored red sandstones and
shales, generally considered to be river and lake deposits.
We may have a small scrap of Rodinian rock, in an isolated section of meta-sediment known as the Buffalo Hump
Formation. While many have considered this formation to be part of the Deer Trail Group, it lies unconformably on
the Stensgar Dolomite, and is in considerable contrast to it. The Buffalo Hump Formation is largely a quartzite in its
lower section, becoming more argillitic toward the top. Dolomitic rocks are absent. Many of the quartzites are thick14
Figure 1-30 (Above) Rocks of the Buffalo Hump Formation, at
an outcrop south of Chewelah. These may be rocks which accumulated on the ancient supercontinent of Rodinia. These rocks
are quartzites, and the material is probably re-worked sediments
from the older Belt-Purcell rocks. Hammer provides scale.
bedded and very coarse grained, while the argillites tend
to be highly deformed and phyllitic. In less-deformed
exposures, bedding in the argillites is typically laminated,
with evidence for soft-sediment deformation. Only about
500 meters of this formation are preserved, and only in
scattered outcrops. Some have suggested an age of about
1050 for this unit, and have pointed out that these appear
to be recycled sediments of the Belt-Purcell rocks. As
noted, others have argued that they bear a strong resemblance to certain Belt-Purcell lithologies, and may be the
uppermost (preserved) section of that supergroup.
If these are rocks of Rodinia, they don’t tell us much
about the setting over this period. Whatever conditions
persisted on this corner of the ancient supercontinent,
that record has largely been lost to the forces of time. Our
window on history does not open again until about 750
million years ago, just before the breakup of the supercontinent.
15
Figure 1-31 (Above) Hand-sample of the Buffalo Hump Formation, from the outcrop above. Dime provides scale.
Figure 1-32 The breakup of Rodinia,
about 750 million years ago. Diagram
shows spreading centers, and location of
the new Windermere Coast.
Baltica
West
Africa
Amazonia
Siberia
Laurentia
Windermere
Coast
Australia
East
Antarctica
Congo
The Breakup of Rodinia
Rocks of theWindermere Group
When the supercontinent of Rodinia started to break up, it was an event of considerable local significance. The rift
zone which developed in this process ran roughly though what is now southeast Idaho, Eastern Washington, northeast
into the Purcell Mountains of Canada, and then northwest along the western flank of the modern Rocky Mountains.
As this rift zone grew to form an ocean basin, it established the ancestral western coastline of North America. It was
upon this ancestral coastline that the accreted terranes of the Pacific Northwest would eventually be emplaced. The
displaced continental fragment to the west, by our best evidence, is now a part of eastern Australia.
The earliest regional record we have for this period starts out at about 750 million years ago, about the date that rifting
began. The rocks preserved from this period extend in a band heading northeast through the northeast corner of Washington State, into southeastern British Columbia for a total distance of about 250 km. Further north, an equivalent
group of rocks appears in the Mackenzie Mountains of northern Canada. Collectively, they appear to represent rocks
of a new continental margin. They are known as the Windermere Group (Windermere Supergroup in Canada). The
name taken from a small town in southeastern British Columbia.
The Windermere Group is preserved in two areas in Washington State, in the “magnesite belt” on Huckleberry Mountain to the east of Chewelah, and in the “Salmo-Priest” area east of Metaline Falls, in the northeast corner of the state.
They overlie the rocks of the Belt-Purcell Supergroup along an unconformity, and the various formations have been
juxtaposed along northeast-trending faults. These rocks continue at least 100 km north into southeast British Columbia, but with the exception of one formation, they start to thin out rapidly north of the border. Further north, they are
known as the Horsethief Creek Group. These rocks have all been metamorphosed in later events.
16
North America
Windermere Group
Belt-Purcell
Supergroup
Intermontane
Belt
Banff
Calgary
Canada
United States
Figure 1-33 (left) Regional distribution of the Windermere Group
rocks. Note that rocks of this age extend south all the way into California. These rocks illustrate the outline of the coastline formed in the
breakup of Rodinia.
Figure 1-34 (Above) Windermere Group rocks in southern British
Columbia, and their relationship to rocks of the Belt-Purcell Supergroup. The purple line represents the boundary between rocks of the
North American continent and those of the accreted terranes (here, the
Intermontane Belt) to the west.
The Windermere Group in our area consists of four formations, a lower (meta) conglomerate, a (meta) volcanic section, a (meta) clastic section and an upper (meta) quartzose section. In Washington, the conglomerate section is known
as the Huckleberry, or Shedroof conglomerate. In southeast British Columbia, it was originally named as the Toby
Formation. The metavolcanic unit in Washington is known as the Huckleberry, or Leola Volcanics. In southeast British Columbia, it was originally named the Irene Volcanic Formation. The metaclastic section is widely known as the
Monk Formation, while the upper quartzite section is known as the Three Sisters Formation. We will refer to these as
the Toby, the Irene, the Monk and the Three Sisters Formations.
17
Figure 1-35 (Above) An outcrop of the Toby Formation, on Sullivan Creek east of Metaline Falls. Arrow points to a conspicuous band of cobbles, a common feature in the Toby Formation.
Figure 1-36 (Right) A sample of Toby conglomerate. Pebbles
are common belt lithologies, largely quartzite and dolomite.
Note how the pebbles have been stretched. Dime provides scale.
The character of the Toby and Irene Formations was an
early indication that this was a rifted-margin setting. The
Toby Formation is largely a conglomerate, consisting of
clasts of Belt-Purcell Supergroup rocks supported in a
matrix of sandy siltite and argillite. In the Salmo-Priest
area, it is about 2 km thick. Sorting is poor in all materials,
with conglomerate clasts ranging from pebbles to boulders.
Some sections appear completely devoid of any sense of
stratification, a characteristic of what is called a diamictite.
Diamictites form in landslide settings, as would be expected on a rifting margin. The Irene Volcanics are largely
basalt dikes and flows, typical of a divergent margin setting. From a relatively early date, researchers recognized
this as a rift setting.
18
Icebergs
Grounded or Floating Ice
Graded Sandstone Facies
Diamictite Facies
Siltstone-Argillite
(Dropstone) Facies
Figure 1-37 (Above) Depositional setting for
the Toby Formation, showing different sedimentary facies. Deposition was in a marine
basin forming in a rift zone within the supercontinent of Rodinia. Adapted from Marmo
and Ojakangas, 1984
Figure 1-38 (Right) The Irene Formation, at
an outcrop south of the town of Addy. The rock
is a greenstone, the metamorphosed equivalent
of basalt flows. Lighter-colored patches may
represent pillow structures in this submarine
flow. Locally, this is known as the Huckleberry
Volcanics. Hammer provides scale.
While the Irene Volcanics are a unique
aspect to this area, researchers soon started
to notice that diamictites of this age were
found around the world, on virtually every
continent. While this combination does
reflect a rifting margin in this area, the
diamictites appear to be the product of a
different process. The other common origin for such an accumulation is by glacial
deposition, which appears to have been
the case here. These rocks accumulated in
what appears to have been a world-wide
episode of continental glaciation in the
Late Proterozoic. This episode is widely
known as the Varangian Glaciation, but
also as the Rapitan glacial episode in
Canada. While this was not the first episode of glaciation on the planet, it is the
19
first for which we have abundant evidence. The conglomerate clasts in the Toby Formation appear to have been glacial
dropstones, carried in the ice as it flowed over the continent, and dropping out as they flowed over the early rift basin.
The Irene Volcanics start to appear in the upper half of the Toby Formation as discontinuous dikes and layers of
greenstone, the metamorphosed equivalent of basalt flows. These rocks increase in proportion toward the top of the
Toby Formation, until they form a continuous greenstone layer. In the Salmo-Priest area, this accumulates to about
1500 meters of strata. The lower flows are largely massive features, but do display pillow structures characteristic of
submarine eruption. The upper portions contain more tuffaceous and volcaniclastic material. These rocks are clearly
rift volcanics, the product of a spreading center which developed in the heart of Rodinia. By the time the Irene Volcanics were erupting above the Toby Formation, this rift basin had apparently matured to the point where it had become
a marine setting.
Figure 1-39 (Left) Deformed intraformational limestones, in the upper portion of the Monk Formation.
Limestones are largely limited to the upper portion of
the Monk Formation. Brecciated and deformed sections
like this make for spectacular exposures. This outcrop is
at the head of Sullivan Creek, east of Metaline Falls.
Figure 1-40 (Below) The Monk Formation, a typical exposure of the dark-gray carbonaceous argillite section.
This outcrop is north of Colville, and is about ten meters
tall. The white accumulation at the base of the outcrop
is snow
20
Figure 1-41 (Right) The Monk
Formation. This is an exposure in the
central portion of the formation. Some
of the beds in this section show graded bedding, suggesting deposition on
a submarine slope. This outcrop is on
lower Sullivan Creek, east of Metaline
Falls.
Figure 1-42 (Below) Deformed Monk
limestone, from the same outcrop as
pictured in Figure 1-39. These rocks
reflect multiple episodes of deformation
Overlying the Irene Volcanics is
the Monk Formation, the most
variable member of the Windermere Group. It is also the only
local member which does not thin
out rapidly north of the Canadian
border. In the Salmo-Priest area it
is primarily a dark-gray carbonaceous argillite, about 1200 meters
of which are preserved. The middle
part of the Monk here contains
a thick (50 – 100m) section of
diamictite, with a graded, upwardfining aspect. This suggests that it
was probably the product of a large
mudflow or slump, perhaps on the
lower part of a submarine slope.
It includes some very large (10’s
of meters) blocks of argillite. The
upper portion of the formation here
is also largely an argillite, and contains sections of planar-laminated
argillites with occasional limestone
beds. Some of these rocks have been extensively deformed, particularly the upper limestone beds. Elsewhere, the Monk Formation
is largely a conglomerate and sedimentary breccia (a conglomerate
made up of angular fragments), including some very large (100’s of
meters)-sized blocks of local dolomite. These lithologies are what
one would expect on a rapidly evolving rift margin, characterized
by large-scale block faulting and landslides on the submarine slope
of the continent.
The uppermost unit of the Windermere Group in our area appears
to be the Three Sisters Formation. In places the relationship
between the Monk and the Three Sisters appears to be gradational
in aspect, supporting such an interpretation. The formation takes
its name from the Three Sisters Peaks south of Nelson, British
21
Figures 1-43, 1-44 Rocks of the Three Sisters Formation, on the top of Salmo Mountain, east of Metaline Falls. These rocks are
quartzites with sections of conglomerate, as
shown to the right. These sediments were
likely derived from the weathering of BeltPurcell rocks.
Columbia, but only extends about 45
km north of the border. In Washington
State, it is preserved in the Salmo-Priest
area, in the northeast corner of the state.
Here it is about 2 km thick, and consists
of quartzite, conglomeratic quartzite,
conglomerate and phyllite. The lower
700 m is largely a laminated to massively bedded phyllitic argillite. This is
topped by a 1200 m section of multihued, thin to thick-bedded quartzite,
22
Figure 1-45 (Right) Regional correlation chart
for rocks of the Windermere Group. Numbers
represent millions of
years ago.
NW Canada
SE Canada
Liab
Reno
Hamill
Inqua
conglomeratic quartzite
and conglomerate. The
coarse clastic rocks in
this formation appear
to have been deposited
by persistent current
activity in a shallow
marine environment.
This contrasts sharply
with the deposits of
the underlying Monk
Formation, and would
appear to represent a
mature shoreline setting along the newlyformed continental
margin.
Washington
Risky
Blueflower
Gametrail
Quartzite
Ranges
Metaline
Maitlen
Addy
Sub - Sauk Unconformity
Sheepbed
Ice Brook Glacials
Ice Brook
Keele
Twitya
Horsethief
Creek
Shezal
Toby
Sayunei
Coates Lake
Three
Sisters
Monk
Irene
Toby
Rapitan Glacials
Little Dal
The sequence of rocks in the Windermere Group here - from early basin deposits to marine rift volcanics to an actively evolving continental slope and finally a mature shoreline setting, would seem to reflect the complete cycle of
continental rifting and margin development. The fact that similar rocks are found in eastern Australia (the Sturtian
Glacial deposits and associated rocks) supports an interpretation that these likely represent the other side of the rift
basin at this time.
At the same time however, the rocks in the MacKenzie Mountains of northern Canada paint a somewhat more complex picture. The equivalent of the Toby and Sturtian (Varangian) glacial deposits are known here as the Rapitan
Group, dating from around 750 million years ago. In both Australian and northern Canadian rocks however, a second
glacial episode is indicated at about 600 million years ago. These are known as the Marinoan Glaciation of Australia,
and the Ice Brook Glaciation in northern Canada. These rocks are topped by two clastic-carbonate “grand cycles”
which extend to the end of the Proterozoic. It would appear that rocks representing this upper glacial sequence and the
two clastic-carbonate “grand cycles” which cap them are not present in our area, at least as far north as the Horsethief
Creek Group. Based on thicknesses in the MacKenzie Mountains area, this represents over 2 km of strata which is
missing from over the Three Sisters Formation. This missing strata (the depth of which increases to the southwest) is
known as the “sub-Sauk unconformity”, or broadly as the “sub-Cambrian” unconformity.
It would be difficult to overstate the importance of the transition represented by these rocks, on both a local and global
scale. On a local scale, these rocks represent the transition from the mid-continental setting of the past billion years
into a new continental margin setting. It was on this margin that the accreted terranes of the Pacific Northwest would
eventually be emplaced. On a global scale, the transition was equally as profound. Prior to this time, dating from an
early horizon of perhaps 3.8 billion years ago, life was still a single-celled proposition. Perhaps in response to the
pressures of the Late Proterozoic glaciations, this period saw the first development of multicellular life, leading into
the veritable “explosion” of life which characterized the early Paleozoic. In both respects this was a very critical
period of transition, setting the stage for the further evolution of this region.
23
Mining the Ancestral Margin
Mineral production has been an important economic activity in northeastern Washington since the arrival of
European settlers. Most of these industries have been based in rocks of the Belt-Purcell Supergroup, and those
of the Sauk Sequence. Most of these have been in carbonate rocks, particularly in the various dolostone (dolomite) formations.
Lead, zinc and silver mining have been a mainstay in the Metaline Falls district for over a hundred years. It
was a leading producer of lead for bullets in World War I, but expanding foreign supplies and a volatile market
for this metal have left it a marginal enterprise here. The zinc and silver in these rocks has proven more profitable over time, and the district remains the states most important producer of these metals. Formerly processed
at a smelter in Northport, most of this is now transported to the smelter in Trail, B.C.
The most extensive mines here have been in dolomitic rocks of the Deer Trail Group (principally, the Stensgar
Dolomite), for the extraction of magnesite. This mineral is essential for making high-grade steel, and is valuable for making refractory (furnace) linings. When the supply of magnesite from Austria was cut off in World
War I, the mines around Chewelah grew to world-class status. In 1916 they were the leading producer, producing 700 tons per day. Much of this was cast into refractory bricks. The plant closed in 1968, when the fifty-year
moratorium on imports from Austria expired.
The other major center for magnesite production was in the town of Addy. The Addy quartzite had been mined
for the production of ferrosilicon, an important refractory material and an important component in magnesium
smelting. A magnesium smelter was later constructed here, capable of producing 45,000 tons per year. Eventually however, expanding global economies caught up with the plant. In the end, the cost of extracting these
minerals from local sources was not competitive in the world market. After spending years importing ferrosilicon from Norway and magnesite from China, the plant closed in 2001.
(Right) Smelter at
Northport, circa 1910.
Image courtesy of the
Stevens County Historical Society.
24
Deposition on the Passive Margin
Rocks of the Sauk and Tippecanoe Sequences
The rifting episode which broke up the supercontinent of Rodinia scattered the continents, but not for a lengthy period. By 600 million years ago, evidence suggests that they had joined back together again, this time as the supercontinent of Pannotia (Greek, meaning “all southern” – as it developed in the southern hemisphere). In this assembly, the
western margin of what would become North America remained a coastal setting. Pannotia was a relatively transient
supercontinent, and did not persist for very long. By earliest Paleozoic time (~540 Ma) it had been fragmented by
intracontinental rifting, this time in regions well removed from what is now the Pacific Northwest.
The continental fragments of Pannotia were destined to amalgamate again, starting at about 350 million years ago, to
form the next supercontinent - known as Pangea. Over this 200 million year cycle between Pannotia and Pangea, the
region along the northwestern edge of North America was characterized as a passive margin. This is the arrangement
now enjoyed along the Atlantic coast, where the expanding oceanic plate in the North Atlantic is driving the North
American Continent westward. From a tectonic standpoint, it is a relatively quiescent setting.
Along this passive margin, sediments accumulated from sources in the continental interior. Owing to this relatively
quiescent tectonic setting, they accumulated as a laterally-continuous, continental-scale set of features. Those features
Figure 1-46 (Above) A roadcut throuth the Metaline Formation, south of Metaline Falls. The Metaline is a carbonate formation
of limestone and dolomite, dating from Cambrian time. These sediments accumulated along a passive continental margin.
25
British Columbia
Figure 1-47 (Right) Map
showing the distribution
of lower Paleozoic rocks,
relative to the Windermere
and Belt-Purcell Supergroup
rocks. Not included are rocks
of the Covada Group, and
the younger rocks above it.
Those rocks were displaced
from an original locale further to the west.
Figure 1-48 (Below Right)
Local members of the Sauk
and Tippecanoe sequences.
The Tippecanoe is only partially preserved in our area.
Bonners
Ferry
Colville
Montana
Washington
Spokane
Early Paleozoic Rocks
Windermere Group
Belt-Purcell Supergroup
reflect long periods of regionally-extensive deposition, bound
by regionally-extensive periods of erosion (unconformities) in
the record. In our area, two such ‘unconformity bound sequences’ characterize most of the Paleozoic record. These include the
(Cambrian) Sauk Sequence, and the (Ordovician) Tippecanoe
Sequence. These are local representatives of continental-scale
features. The distinctly Native American nomenclature avoids
confusion with elements of the geologic time scale.
Along this passive margin setting, these two sequences accumulated in response to rising sea levels (marine transgression),
and are therefore known as “transgressive sequences.” The
bounding unconformities reflect periods of marine regression
(falling sea levels), where conditions of erosion dominate. Such
an unconformity exists at the base of the Sauk sequence, between the Windermere Group and the Sauk rocks. As discussed
above, this unconformity appears to eliminate about 2 km of
strata between exposures in northern and southern Canada.
Locally, this feature can be seen between the Salmo-Priest and
Huckleberry Mountain exposures of the Windermere Group.
In the Huckleberry Mountain area, the Monk and Three Sisters
Formations were eliminated before the deposition of the Sauk
Sequence.
26
Idaho
Ledbetter
Formation
Tippecanoe
Sequence
Metaline
Formation
Maitlen
Formation
Addy
Formation
Sauk
Sequence
The Sauk sequence spans the period form 570 to 488 million years ago, making it Cambrian in age. It consists of
three formations: a quartzite (meta-quartzose sandstone) formation reflecting transitional or shallow-water marine
deposition, an argillite (meta-siltstone) formation reflecting near-shore deposition, and finally a carbonate formation
reflecting accumulation in deeper waters. This vertical progression of rocks normally distributed on a lateral basis is a
reflection of deepening waters, in a period of marine transgression. This is a classic example of what is called “Walthers Law.”
In a pattern that unfortunately follows throughout the remainder of this work, the various formations of the Sauk Sequence have different names in different areas, and particularly across the international boundary. The lower quartzite
formation is known as the Addy and Gypsy Formations in northeastern Washington, and as the Reno and Quartzite
Ranges Formations in British Columbia. The central argillite formation is known as the Maitlen Formation in Washington, and the Liab Formation in British Columbia. The upper carbonate formation is known as the Old Dominion
and Metaline (sic) Formations in Washington, and the Nelway Formation in Canada. We will refer to them here as the
Addy, Maitlen and Metaline Formations.
The Addy Formation takes its name from a small town in northeastern Washington, between Chewelah and Colville.
It is a quartzite unit, and is Early Cambrian in age, by fossil evidence. Trilobite fossils have been found in the Addy
area, some being a species unique to that locale. Ripple marks, load casts, trace fossils and other features, along with
compatible fossil species, suggest a shallow-water marine or transitional environment for the Addy Quartzite. It is preserved in several fault-bound slices in a belt along Huckleberry Mountain, in the area north of Chewelah, and in the
Salmo-Priest area around Metaline Falls. It is on the order of 1500 m thick, and is used commercially as a raw material for glassmaking. In the past, it has been used to produce ferrosilicon, formerly used extensively in steel production.
Figure 1-49 (Below) The Addy Formation, outside the town of Addy (lower right). These rocks are largely quartzite. Trilobite fossils have been found near this location (see Figure 1-50), the oldest definitive shelled fossils in Washington.
27
Figure 1-50 (Right) The
depositional setting of the
Addy Formation, adapted
from Lindsey et al. 1994. The
setting here changed as sea
levels rose, initiating deposition of the Maitlen Formation.
Figure 1-51 (Below Right)
Trilobite fossils from the
Addy Formation. These are
a variety called Nevadella
Addyensis, a species first
discovered here. These are
the oldest definitive fossils in
Washington State.
This sample is courtesy of
the Dave Morgan Family, of
Sumas Washington.
The fossil record in the Addy Formation stands in dramatic
contrast to earlier deposits. Life, which shows up in rocks as
old as 3.9 billion years, progressed little until Windermere time,
remaining as one-celled organisms. Perhaps in response to
pressures brought by the Late Proterozoic glacial episodes, the
diversity and complexity of life in this planet literally exploded
at the dawn of the Paleozoic. No where is this better displayed
than in the Mt. Stephen Formation near Banff, Alberta. This
is the locale of the world-famous Burgess Shale, the world’s
premier site for Cambrian-age fossils. Preserved here are various genera of worm-like creatures, three extinct sub-classes of
crustaceans, plus a variety of arthopods including exquisitelydetailed trilobites. Included are the earliest known representative of the cordates – our most distant ancestors. Over one
hundred fifty different species have been recovered from this
world-class site.
The southern equivalent of the Mt. Stephen Formation in Washington State is the Maitlen Formation, which conformably
overlies the Addy Formation. The Maitlen is largely an argillite
formation, a weakly-metamorphosed section of meta-silt and
sand. In some places, it has a phyllitic texture. It is less than
1 km thick, and is of more limited exposure than the Addy or
Metaline Formations. In the Sullivan Lake area above Metaline
(sic) Falls, the Addy Quartzite can be seen grading into the
28
Figure 1-52 (Left) A hand
- sample of the Addy Quartzite, showing the purple color
characteristic of the middle
part of the formation.
Figure 1-53 (Below) The
Maitlen Formation, near
Sullivan Lake. Here, light
- colored layers of Addy
Quartzite grade into argillite
of the Maitlen Formation.
Maitlen Argillite in alternating layers. The equivalent of the Maitlen Formation in southeast British Columbia is the
Liab Formation.
Above the Maitlen lies the Metaline Formation, taking its (misspelled) name from the town of Metaline Falls in
northeast Washington. This unit is on the order of 1000-1600 meters in thickness. The Metaline is a carbonate formation, a mixture of limestone and dolomite. It is an economically important deposit, a source for lead and zinc mines in
Metaline Falls, Salmo and the Northport-Colville districts. It has also been mined as a source of magnesium carbonate
(dolomite) and for limestone to make Portland Cement. The Metaline limestones have yielded some exquisite trilobite
fossils.
29
Figure 1-54 (Above Left) The Metaline Formation, south of Metaline Falls. Person gives scale
Figure 1-55 and 1-56 (Right) Trilobite fossils from the Metaline Formation. Top specimen is Chancia, bottom is Kootensia. Fossils courtesy of the Dave Morgan Family, Sumas WA.
To the north, rocks of Cambrian to Ordovician age are preserved in the Lardeau Group in southeastern British Columbia. The Lardeau consists of pelagic and quartzose to feldspathic clastic sedimentary rocks, along with submarine
basalt flows. These basaltic rocks, along with similar flows found in the upper portions of the Covada Group, suggest that these formations continued to accumulate into Devonian time, when arc magmatism developed along the
continental margin here. Notably, the Lardeau Group was deformed and metamorphosed prior to accumulation of the
(middle to upper Mississippian) Milford Group.
Deposition in the Sauk Sequence ceased by Early Ordovician time, and a period of widespread erosion associated
with a fall in sea-level (regression) ensued. This produced the unconformity seen between the Sauk and the overlying
Tippecanoe Sequence, which is Ordovician in age.
30
The Burgess Shale: A World-Class Fossil Site
Back in the 1880’s, while laying track for the Canadian
Pacific Railway over the Rocky Mountains, railway
workers noticed trilobite fossils along the west side
of the crest. These were first reported in 1886 by R.G.
McConnell, of the Geologic Survey of Canada. These
reports came to the attention of Dr. Charles Walcott,
who was serving as the Director of the US Geologic
Survey at the time. An avid student of the Cambrian, he
came north in 1907 to the Banff and Lake Louise area
to investigate, and published a paper on those fossils in
1908.
Walcott found the region appealing for field work,
served by the Canadian Pacific Railway and featuring accommodations at the renowned Chateau Lake
Louise. Now serving as the Director of the Smithsonian Institution, he returned the following year to start
mapping the area, and late in that season discovered a
very productive fossil bed in the Mt. Stephen Formation. It was too late in the year to investigate further,
but he returned in the following summer and traced the
bed high up onto the slopes of Mt. Wapta, above the
town of Field, British Columbia. There, he made one
of the most important fossil discoveries of all time, in
the fine-grained sedimentary rocks of the Burgess Shale
Member.
In these fine-grained rocks were the carbonaceous
imprints of an astonishing array of Cambrian fauna,
preserved in absolutely exquisite detail. Over a hundred
and fifty species were found, including at least eight
known phyla of animals, and at least as many which
were unknown. Most species (40%) were arthropods,
but also included sponges, coelenterates, echinoderms,
mollusks and annelids. It was an incredibly diverse
range of fossils in an absolutely perfect state of preservation. It was one of the great fossil locations of the
world, and a premier world-class site for Cambrian
fossils. In 1981 it was designated as the 86th UNESCO
World Heritage Site.
Among the creatures preserved in this remarkable
locale were small elongate animals which are currently believed to be the earliest members of the Chordate
Family. Chordates are animals which at some point in their
development have a “notochord”, including a dorsal nerve
31
chord. Humans are modern members of that family, and this
represents our most distant known ancestor, some 505 million years ago.
Walcott, who was essentially a self-taught high-school
dropout, went on to become the world’s leading authority on the Cambrian. He served as the Chief of the US
Geological Survey, the Secretary of the Smithsonian
Institution, and was instrumental in establishing the
Carnegie Insitute in Washington D.C. Some 65,000
specimens from the Burgess site were collected and
stored at the Smithsonian Institution. The Walcott
Collection of Burgess fossils is housed in Cambridge,
England.
As the sea level again rose
in the Mid-Ordovician, deposition resumed with the
Tippecanoe sequence. The
limited members of this
sequence preserved in the
Pacific Northwest include
the Ledbetter Formation
in northeast Washington,
and the equivalent Active
Formation in southern
British Columbia. The
Ledbetter is a calcareous
shale and slate, typically
black in color. Common
in these shales are the
imprints of graptolites,
a small colonial marine
organism with a branchshaped structure. The
class ranges in age from
Mid-Cambrian to Carboniferous, and local varieties
have been dated at Late
Ordovician in age. These
Ordovician graptolitebearing shales and slates
are part of a continental
- scale deposit. They
accumulated in a largely
shallow-water setting during the Tippecanoe marine
transgression.
Figure 1-57 (Above) An outcrop of
the Ledbetter slate, along the Hunters
- Cedonia Road.
Figure 1-58 (Left) A sample of Ledbetter Slate, showing the imprints of
graptolites. Graptolites are branchshaped free-floating creatures, and
they accumulate on the ocean floor
when they die. As fossils, they are
good age-diagnostic indicators. The
varieties in the Ledbetter Slate date
from Late Ordovician time.
32
Figure 1-59 (Right) Map showing the distribution of Tippecanoe
- age deposits in the western US
and southern Canada. The black
shale deposits of this sequence
are found in a broad belt across
the continent. Because they commonly bear graptolite fossils, they
are often called the “graptolite
facies”. Areas in white are where
these rocks have been removed by
erosion.
Figure 1-60 (Below Right) The
Covada Group, near Covada on
the Colville Reservation. The
rocks here are argillites, cut by
numerous dikes of basaltic and
felsic volcanics, the former of
which are older.
Quartz
Sands
Graptolitic
Shales
Carbonate
Rocks
WA
OR
Shale
Further to the west, the (meta) sedimentary rocks of the Covada Group date from Ordovician time. The Covada
Group outcrops along both sides of the Columbia River south of Kettle Falls, including a large section on the Colville
Reservation. In contrast to the Ledbetter Formation, the Covada is largely a deep-water assemblage. It consists of
chert, black slate, greywacke sandstone and minor limestone. Sections of submarine basalt (greenstone) likely reflect
magmatism of the Kootenai Arc.
Like the Deer Trail Group
in its relationship to the
Belt-Purcell rocks to the
east, the Covada Group
is a more distal facies to
the Ledbetter Formation
than is suggested by its
current position. At the
same time, some of the
quartzose sediments in this
group were derived from
continental rocks – suggesting that it was not
completely removed from
those sources. Parts of the
Covada Group appear to
be turbidite deposits, a mix
of sediment that typically
accumulates on the continental slope. This may
represent the depositional
setting for much of this
section.
33
The Covada Group is
unconformably overlain
by unnamed clastic and
carbonaceous rocks which
extend into the Carboniferous. This package is
equivalent to the Eagle
Bay Assemblage in southeastern British Columbia.
That assemblage includes
clastic metasediments and
carbonate rocks, along
with Devonian through
Mississippian felsic to
intermediate metavolcanics. Both the Covada and
Eagle Bay Assemblages
are pericratonic deposits,
accumulated on the distal
margin of the continent.
Figure 1-61 (Above) Tilted strata of the
Covada Group, above Hunters on the Hunters - Springdale Road. These are largely
fine - grained mudstones, metamorphosed to
slate. . These rocks lie within the Kootenay
Deformed Belt, and their attitude is a product
of multi-phase deformation.
Figure 1-62 (Right) Map showing the regional distribution of the Covada Group and
the Eagle Bay Assemblage. Also shown are
the location of the Lardeau Group. and the
suite of Devonian Plutons which intruded this
region.
Figure 1-63 (Opposite Page) Geologic map
of the northeastern corner of Washington
(from the Washington State Geologic Map
series, Department of Natural Resources).
Rocks west of the Covada Group are part of
the Intermontane Belt.
Devonian
Plutons
Lardeau
Group
Eagle Bay Assemblage
British Columbia
Washington
Covada Group
34
Idaho
Rocks of the Covada Group and younger cover
Rocks of the Sauk and Tippecanoe Sequences
Rocks of the Windermere Group
Rocks of the Belt-Purcell Supergroup
35
The Evolution of A Convergent Margin
Rocks of the Kootenai Arc
The passive margin setting which had its origins in the Windermere rifting episode came to an end in Mid-Devonian
time, after a reign of some 400 million years. The circumstances surrounding this change are not entirely clear. This is
about the time that the megacontinents of Laurentia and Gondwana collided, closing the Iapetus Ocean basin. As this
situation developed, something like 350 million years ago, it forced changes in the global plate-tectonic organization.
By this circumstance, the west coast of North America developed into a convergent margin setting at about this date.
A mild episode of continental contraction accompanied the establishment of this convergent margin, an episode
known as the Antler Orogeny. In this episode rocks along the western margin of the continent were thrust east over
younger rocks, and were locally folded in the process. This produced a relatively low-scale uplift called the “Antler
Mountains,” which extended south from Northern Idaho. While the Antler Mountains do not appear to have extended
into Canada, there is evidence for deformation of this age in northwestern British Columbia, marked by prominent accumulations of conglomeratic marine sediments.
Further west, rocks in a belt extending north from northeast Washington into southeast British Columbia were deformed in this event, contracting along an east-west axis. While it is difficult to estimate how much deformation
accompanied this event (this region has been deformed multiple times, particularly in the mid-Cretaceous), some of it
clearly dates from this episode. This belt has been known historically as the “Kootenay Arc” – a term which reflects
its arc-like shape. This name was assigned prior to the adoption of our modern lexicon of plate tectonics (wherein the
term “arc” has come to designate a subduction-generated magmatic complex), and it is thus a rather confusing title in
Figure 1-64 (Above) The Flowery Trail Pluton, east of Chewelah. This is a late Triassic pluton of the Kootenai Arc.
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modern use. A more appropriate term, provisionally adopted here (for reasons detailed below), would be the “Kootenay Deformed Belt”.
The key to understanding this deformational episode may lie in a series of plutons of Late Devonian to Early Carboniferous age in southeastern British Columbia, which mark the inception of a mid-Paleozoic magmatic arc along
the continental margin here (see figure 1-62). This contractional event appears to have accompanied the process of
establishing a new subduction zone along the continental margin. Given this circumstance, it is somewhat surprising
the modest degree of deformation which this represented. It was a diminutive event compared to the tectonism experienced in later episodes.
This arc regime persisted sporadically over the next 170 million years, into Mid Jurassic time. While none of the
earlier plutons of this suite are exposed in Washington, the (Late Triassic) Flowery Trail Pluton east of Chewelah is a
good local example of these rocks. While this arc feature has never been formally named in the professional literature,
some authors have taken to referring to it as the “Kootenay Arc” – adding to the nomenclatural confusion here. Following on the liberties taken to the nomenclature above, we will provisionally adopt that older term (previously used
in a structural sense) for this feature. We shall provisionally refer to this (magmatic arc) feature as the “Kootenai Arc”
– adopting the Canadian spelling to reduce the confusion.
The tectonics of an active margin, particularly the later tectonics of terrane accretion, have not favored the preservation of Late Paleozoic rocks here. This is particularly true south of the international border. These are limited to small
fault-bound slices of upper Devonian and Mississippian to Permian dolomite and limestone, preserved on the flanks
of the Deer Trail Belt in the Huckleberry Mountain area, west of the Belt Supergroup rocks along the east side of the
Colville Valley, and west of the Ordo-Cambrian rocks north of Colville. Some of these rocks preserve a fossil record
of brachiopods, conodonts and crinoid species, suggesting a relatively shallow marine setting on a continental shelf.
All in all however, the record is too sparse to offer any detail on the paleogeographic setting.
To the north, the Milford Group in southeastern British Columbia provides our most significant window on this time.
The Milford is largely Mississippian in age, but may be as young as Permian. It consists of middle to upper Mississippian limestone, overlain by sandstone, phyllite, chert and conglomerate. This is in turn overlain by upper Mississippian to lower Pennsylvanian andesitic (pillow) lavas and tuffs - eruptions and deposits of the Kootenai Arc.
The fact that these all are marine rocks illustrates that this region remained a marine setting through most of the Paleozoic. Further to the east, Late Devonian, Early Pennsylvanian and Early Permian times were marked by sea-level regressions which exposed the continent. Throughout this period, this region appears to have remained a marine setting.
Toward the end of the Paleozoic Era, the continental fragments dispersed in the breakup of Pannotia again came
together, this time to form the supercontinent of Pangea. This process was largely completed by the end of Permian
time. In this new setting, the future western margin of North America remained a coastline.
Figure 1-65 (Immediate Right)
Devonian limestone from above
Hunters, showing brachiopods and
other fossils. Quarter gives scale
Figure 1-66 (Far Right) Mississippian - age limestone from near
Springdale. Arrows point to conodont fossils, which are age-indicative. Dime gives scale.
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Figure 1-67 The supercontinent of
Pangea, Early Triassic time.
North
America
Panthalassa
Ocean
Tethys Sea
Africa
South
America
The Supercontinent of Pangea
The Dawn of the Last Cycle
Pangea was the first supercontinent to enjoy terrestrial vegetation, forests, land animals and a whole lot more. In
places, it was a lush and verdant setting, in a period popularly known as the “age of the reptiles.” This was a huge
evolutionary leap from the desolate sterile settings of Columbia, Rodinia, and Pannotia, reflecting the “explosion” of
life which was seen over Paleozoic time. The future Pacific Northwest was at this time still a marine setting, with the
coastline some distance to the east. At this date, the region was at a tropical to paratropical latitude. We are largely left
to imagine the abundance of life which lived in these Late Paleozoic waters, along the western margin of Pangea.
At the end of the Permian Period, all of that nearly came to an end. In what was probably the greatest extinction event
in the history of the planet, nearly 90% of all species on Earth disappeared in the transition from the Paleozoic to the
Mesozoic Era. The events which precipitated this extinction are the subject of considerable debate, but it is clear that
life on Earth barely squeaked through the event.
Pangea persisted through the great extinction of the Permo-Triassic, into the Mesozoic Era. The supercontinent continued to hold together through the Triassic Period, adding the last minor pieces to its accumulated mass. By the time this
period was coming to an end, some thirty five million years after that event, life had rebounded around the planet. By
this date, early relatives of the dinosaurs were roaming the world.
At the end of the Triassic, thermodynamics finally caught up with the Pangean supercontinent. As it entered the Jurassic period, it was starting to be broken up along a series of intracontinental rift zones. One set of those zones would
eventually go on to become the modern Atlantic Ocean. This breakup initiated the most recent cycle, wherein the
Pacific Northwest has its origins.
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Figure 1-68 (Right) Along
Alladin Creek, east of Northport. Rocks here include
Paleozoic strata.
Summary:
Evolution of the Ancestral North American Margin
The ancestral western margin of North America developed over an almost unfathomable amount of time, stretching
back over two and a half billion years into the past. On a large scale, the course of its evolution has largely reflected
the cyclic assembly and breakup of a series of supercontinents, a pattern known as the Wilson cycle. That course
reflects the assembly and breakup of the supercontinents of Columbia, Rodinia and Pannotia, through the assembly of
Pangea.
The local record for this vast period of time is at best a fragmentary one. We have an isolated outcrop of the ancestral
gneiss which comprises the basement to the region, a thick package of sediments dating from the breakup of Columbia, perhaps a thin scrap of rocks accumulated on Rodinia, and a partial section of the strata accumulated in the
breakup of Rodinia. We have a relatively complete record for the early Paleozoic, and evidence for the initiation of
a convergent margin and arc magmatism in mid-Paleozoic time. From this fragmentary record, we can consider the
course of events over this vast period of time in only the most general of terms.
However fragmentary that record might be, it is in many ways a graphic illustration on some of the most fundamental
of Earth processes. The rocks here include ancestral gneisses from the early history of the planet, a truly phenomenal accumulation of sediments deposited in an alaucogenic setting, a remarkable group of formations reflecting the
process of continental rifting, and a classic succession of formations which illustrate the effects of eustatic changes in
a passive-margin setting. Toward the end of this span we see evidence for the deformation which accompanied the
establishment of a convergent margin here, and for the arc-magmatism which that relationship supported. In the end, it
is a very illustrative view on these fundamental Earth processes.
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The change to the face of the Earth over this span of time is certainly the most graphic known. Those changes came
slowly in the vast expanse of the Proterozoic, limited largely to the geologic cycles outlined above. The ancestral
supercontinents of Columbia, Rodinia and Pannotia were sterile and desolate settings, distinguished entirely by their
physical geology and the prevailing climatic conditions. This setting changed radically starting about 600 million
years ago, as life took a series of exponential leaps in complexity and adaptability. Within just a few hundred million
years, life became the most conspicuous characteristic on the surface of the planet. By the time Pangea was fully assembled, dinosaurs ruled its breadth. This was a period of truly remarkable transformation for the Earth.
Through all the varied events which contributed to its construction, we close this chapter with the future continent of
North America as part of the supercontinent of Pangea, something like 200 million years ago. The breakup of Pangea,
which involved the opening of the Atlantic Ocean as North America was driven westward, marks the beginning of the
most recent Wilson cycle. It is within this cycle that the Pacific Northwest has its origins.
Figure 1-69 (Above) Cottonwood Creek, southeast of Chewelah.
Chapter 1: The Ancestral North American Margin
Evolution of the Pacific Northwest, © J. Figge 2009
Published by the Northwest Geological Institute, Seattle
Available on-line at www.northwestgeology.com
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