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Chapter 2
The Omenica Episode
New Lands Along an Old Coast
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Chapter 2
The Omenica Episode
New Lands Along and Old Coast
180 - 120 Million Years Ago
The assembly of the Pacific Northwest began with the breakup of
the supercontinent of Pangea in Mid-Jurassic time, about 200 million years ago. Since that time, between 500 and 700 kilometers
of new terrain has been added to the western edge of the continent, representing most of Washington State, British Columbia
and Alaska. These new lands are largely ocean-floor and volcanic
island rocks, brought here by the processes of plate tectonics, and
added (accreted) to the edge of the continent. This is the major
process by which this region has been assembled over time, an
ongoing process over the last 200 million years.
The Omenica (“ah-men-eek’-ah”) Episode was the initial episode
in the development of the Pacific Northwest. This episode commenced about 180 million years ago with the accretion of the first
in a succession of volcanic island-arc chains, this one known as
the Intermontane Belt. The Intermontane Belt is a large composite
“megaterrane” of several different island groups, amalgamated
prior to their accretion on the margin of the continent. The accretion of this large terrane belt resulted in the extinction of the
Kootenai Arc, which had been in place since Mid-Devonian time.
In its place, a new magmatic arc developed just to the west, supported by subduction along the western margin of this newly-accreted terrane. That continental arc is known as the Omenica Arc,
the namesake for the Omenica Episode.
The Omenica Episode was initiated with the accretion of the Intermontane Belt, and persisted until the next large terrane belt - the Insular Belt, was accreted in mid-Cretaceous time. The accreted Intermontane Belt became the site of
an Andean-type margin, which supported the Omenica Arc through Jurassic time. That Andean-type margin appears
to have given way to a Japan-type margin in Early Cretaceous time, with the arrival of the Insular Belt in an offshore
position. This resulted in the extinction of the Omenica Arc, and initiated a period of relative stability along the Intermontane margin. That period of relative stability represents a mature phase to the Omenica Episode, which ended with
the resumption of accretionary tectonics in mid-Cretaceous time.
Figure 2-1 (Cover) Sinlahekin River, south of Loomis
Figure 2-2 (Left) Kruger Mountain Valley, northwest of Oroville
Figure 2-3 (Above) Columbia River from the Silver Creek Road, south of Covada.
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The Dawn of the Pacific Northwest
North America Moves West
For an uncertain span of time prior to its accretion, the Intermontane island
chain probably existed as an active volcanic island belt no more than 300
– 500 km west of the continental margin. Given the relative paucity of
Early Jurassic magmatism in the continental margin, it may well have occupied a Japan-type marginal setting over that period of time.
Ancestral
North
America
This may have provided a relatively stable setting here over the final years
of the supercontinent of Pangea. As the supercontinent began to break up
however, it had the effect of driving the North American Continent (now
properly identified as such) westward. As North America moved westward
it overrode the intervening oceanic plate, eventually to collide with the Intermontane Belt. This, more so than the eastward movement of the oceanic
plate, was apparently responsible for the accretion of the Intermontane Belt.
That westward movement of the continent is an integral part of the equation
for the evolution of the Pacific Northwest over the entire course of its history. While we will largely treat it as a constant, against which the changing
configuration of oceanic plates will play, it is the overall defining characteristic of this entire story. It marks the dawn of the Pacific Northwest 200
million years ago, as it will mark its end several hundred million years in
the future. This is the backdrop against which this entire course of geologic
history plays out.
Figure 2-4 (Left) Lower Methow Valley, northwest of Winthrop
Figure 2-5 (Above) Bolster Road, north of Chesaw
Figure 2-6 (Above Right) Map showing the general location of the Intermontane Belt
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The
Intermontane
Belt
The Intermontane Belt
The First Megaterrane
The basement rocks of the Intermontane
Belt are the remains of a series of volcanic island-arc terranes, along with the
oceanic crust on which they developed.
That oceanic crust dates from Mid to
Late Paleozoic time, while the island-arc
complexes are from Permian to Early
Jurassic in age. The inception of this
magmatic arc in Permian time suggests
that its development may have been tied
to the final amalgamation of the Pangean
Supercontinent.
As originally recognized in the late
1970’s and early 1980’s, the Intermontane Belt is a composite “megaterrane”
consisting of two volcanic island-arc terranes and two associated belts of intervening oceanic rocks. The two islandic
terranes are the Stikine Terrane (Stikina) to the north and the Quesnel Terrane
(Quesnellia) to the south. Between these
two terranes are sandwiched oceanic
rocks of the Cache Creek Terrane, the
remains of what was once an intervening
ocean basin. This assemblage takes its
name from the fact that it now occupies
the inter-mountain region between the
Rockies and the Coast Range in British
Columbia.
Cache Creek
Terrane
Quesnel
Terrane
Stikine
Terrane
Figure 2-7 (Above) Map of the Intermontane Belt, showing the
locations of the Stikine, Cache Creek and Quesnel Terranes. The
Stikine and Quesnel Terranes are two volcanic island complexes,
while the Cache Creek Terrane is the remains of an oceanic
basin which used to intervene between them.
Figure 2-8 (Left) Pond, east of Conconully..
Figure 2-9 (Right) Omak Lake, southeast of Omak.
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The Southern Intermontane Belt
Rocks of the Quesnellian Archipelago
The Quesnel (“K’nel”) Terrane, also known as Quesnellia, takes its title from a small town of that name in north-central British Columbia. It has its origins as a volcanic island-arc chain, in an archipelago perhaps dating from as early
as Permian time. This means that this chain of volcanic islands may have developed for over one hundred million
years before arriving on our shores in Mid-Jurassic time.
The oldest rocks of the Quesnel Terrane are those of the oceanic plate on which it developed. Those oceanic-plate
rocks date from Mid to Late Paleozoic time, and came to intervene between the island group and North America.
Originally the Slide Mountain Terrane, a somewhat generic term for a narrow strip of oceanic rock which now intervenes between Quesnellia and the older rocks of the continental margin, was interpreted as the remains of this ocean
basin. These rocks do include minor ultramafic rocks and mafic volcanics, but those components are now interpreted
as part of an accretionary wedge which developed above the continental subduction zone. Scattered outcrops of these
rocks occur in Washington. More common here are the continental-slope rocks of the Covada Group and its younger
cover. Despite this changing interpretation, we still refer to that intervening ocean basin as the Slide Mountain Ocean.
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Figure 2-10 (Right) Generalized stratigraphic column
for the Intermontane Belt.
(Nomenclature modified from
Cheney, 1994)
Rossland
Group
Nicola
Group
Hall Formation pelite
Elise Formation volcanics
Archibald Formation arkose siltstone
Brooklyn Formation Limestone
Mafic volcanic rocks, argillites
Chert and Limestone Breccia
Attwood Group
Argillites, volcanics, limestone
Figure 2-11 (Below) Greenstone of the Knob Hill Group.
Along the Similkameen River,
west of Oroville. The Knob
Hill Group is an ophiolite
Knob Hill Group
Ophiolite Complex: ultramafic rocks,
mafic igneous rocks, chert
Oceanic-plate rocks of Carboniferous to Permian age occur as fault-bound sections within Quesnellia. Here, they
include the distinctive ophiolite suite of gabbro and basalt, with occasional tectonic slices of ultramafic rock, and the
associated cover of pelitic (mudstone) sediments. The nomenclature of these rocks varies considerably between areas,
but can be organized into two groups. The ophiolite sequence is known as the Knob Hill Group, named for a mine
outside of the town of Republic. In places, this is also known as the (lower) Palmer Mountain Greenstone (metagabbro) and the (upper) Kobau Formation of greenstone (metabasalt) and chert. Locally, it has also been known as the
Thompson Group in British Columbia. Collectively, they may have a thickness on the order of 5 km.
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The pelite cover which
mantles the Knob Hill
Group goes by the name
of the Attwood Group. It
takes its name from a small
community in the Okanogan Highlands. It is largely
an assemblage of pelagic
(ocean-floor) mudstones
and minor limestones. In
places, this is also known
as the Anarchist Group,
consisting of the Spectacle
and Bullfrog Mountain
Formation. Other local appellations include the Mt.
Roberts, Flagstaff Mountain, Harpers Ranch and
Shoemaker Formations.
They consist of (now
metamorphosed) pelites
with varying amounts of
limestone, and with rare
sections of intermediate
volcanic rocks. In the Loomis area in north-central
Washington, the Attwood
(Anarchist) Group is over
4 km thick.
Just when the earliest volcanic islands erupted on this oceanic plate is uncertain, but the presence of intermediate
volcanic rocks in the Attwood Group sediments suggests that early island-arcs may have developed in Permian time.
Those early island edifices were largely eroded away by the Early Triassic period, as they lie beneath a major unconformity of that age.
Figure 2-12 (Above) Metasedimentary rocks of
the Attwood Group, south of Chesaw. These are
metapelites, originally deposited as ocean-floor
mud on top of the Knob Hill Ophiolite.
Figure 2-13 (Left) Detail from the outcrop
above. Note siliceous (light colored) layers in
the sediment.
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Grand
Forks
British Columbia
Washington
Colville
Republic
Rossland Group
Nicola Group
Omak
Attwood / Knob Hill
Attwood Group
Knob Hill Group
Orodo-Carboniferous
including Covada Group
North American Rocks
The major volcanic island-arc rocks of the Quesnel Terrane reflect a sequence of two episodes of island-arc magmatism here. The first of these is known as the Nicola Arc, and it dates from Mid to Late Triassic time. The second,
known as the Rossland Arc, dates from the Early to Mid Jurassic Period. Both of these were likely supported by the
same subduction zone, and reflect on its episodic character. These two arc complexes, with their associated clastic and
carbonate cover, made up most of the Quesnellian Archipelago.
Figure 2-14 (Above) Map of the southern end of Quesnellia,
showing the distribution of the major rock groups. (Adapted
from Cheney, 1996).
Figure 2-15 (Left) Breccia (sharpstone) conglomerate at the
base of the Nicola Group. At an outcrop just south of Danville. Most of these clasts are chert and limestone, presumably rocks of the Attwood Group. These may be part of an
explosion breccia. Pipe gives scale.,
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The rocks of the Nicola Arc complex are known as the Nicola (nick’-ola) Group, named for a valley and native tribe
in central British Columbia. The Nicola Group shows a logical progression in its development. The basal layer is a
sharpstone (breccia) conglomerate of chert fragments, material from the Attwood Group of sediments. This may have
been an explosion breccia, marking the initial eruption of the Nicola Volcanics. This basal conglomerate is overlain by
mafic volcanic rocks (now, greenstones) of the Nicola Arc. As the early Quesnel Islands developed to relatively shallow depths, they became fringed with reef-type structures, the remains of which are now a thick section of limestone,
dolomite and minor clastic sediments. This section, which overlies the Nicola Volcanics, is known as the Brooklyn
Formation. In places, it appears to be nearly 2 km in thickness. Macrofossils of Late Triassic age have been found in
the Brooklyn limestones.
Figure 2-16 (Above)
Limestone of the Brooklyn
Formation. Pine Creek Road,
northwest of Riverside. Late
Triassic fossils have been
found in these rocks. Hammer provides scale.
Figure 2-17 (Right) Clastic
rocks of the Archibald Formation, near Tonasket.
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The Nicola Group ranges in age from Middle to
Late Triassic time. Unconformably overlying these
rocks is a younger sequence of Early to Mid-Jurassic age. This assemblage developed around
the Rossland Arc, and is known as the Rossland
Group. Rossland is a small town in south-central
British Columbia.
The earliest member of this group is the Archibald
Formation, which is largely an arkosic siltstone.
These sediments may have been derived from the
uplift and erosion of deeper plutonic elements
of the earlier Nicola Arc. Fossils in this formation support an Early Jurassic age. Overlying the
Archibald Formation is the Elise Formation of
mafic to intermediate, non-marine volcanic rocks
(now, greenstones) of the Rossland Arc. At depth, a
corresponding suite of intermediate plutonic rocks
(e.g. Kruger Mountain, Osoyoos, Kamloops, Loomis and Similkameen Plutons) was intruded. The
intermediate chemistry of these rocks reflects the
maturation of the island-arc complex. At this date,
the Quesnel Terrane was clearly an island archipelago, rising above the surface of the ocean. The
uppermost unit of the Rossland Group is the Hall Formation, which is largely a meta-pelite (mudstone). It unconformably overlies the volcanic rocks of the Elise Formation. This unit is up to 1400 m thick, and yields fossils dating
it as young as Mid-Jurassic, about 180 million years ago.
Figure 2-18 (Above) Greenstone (metabasalt) of the
Elise Formation, part of the
Rossland Group. Rounded
shapes may be relict pillows.
Hammer provides scale.
At an outcrop south of Danville.
Figure 2-19 (Left) Felsic
plutonic rocks of the Loomis
Pluton. At an outcrop west
of Loomis. These rocks reflect the growing maturity of
this volcanic island complex.
Figure 2-20 (Above Right)
Shankers Bend, on the
Similkameen River, west of
Oroville.
52
The Stikine Terrane
The Cache Creek Connection
The Stikine (stih-keen’) Terrane (Stikinia, named for the Stikine River in northwest British Columbia) is a typical
collection of Late Paleozoic to Mid-Jurassic island-arc rocks, not at all unlike Quesnellia to the south. It consists of a
succession of island-arc complexes, built on oceanic crust of Mid Paleozoic to Mid Jurassic age. That oceanic crust is
known as the Cache Creek Terrane, the remains of which lie between the northern half of Quesnellia and the Stikine
Terrane to the west. The Cache Creek Terrane is an ophiolite complex, and is interpreted as the remains of an ocean
basin which once intervened between the two terranes.
The Stikine Terrane consists of at least two successive island-arc sequences, the nomenclature of which varies considerably in the literature. The earliest major arc sequence exposed here is Late Triassic in age, and is known as the Takla
Arc. The Early to Mid-Jurassic arc sequence is known as the Hazelton Arc. The remains of older arc complexes likely
exist under these rocks. A belt of Carboniferous to Permian limestone, some 450 km long, occurs along the northwest
side of the terrane, suggesting intervals of relative quiescence in the early evolution of these islands.
Numerous researchers have suggested that the Takla and Hazelton Arcs may have been coextensive with the Nicola
and Rossland Arcs of Quesnellia, but those relationships are not clear. There is evidence for Late Triassic deformation
across much of Stikinia, the significance of which remains uncertain. Some have suggested that this may reflect the
amalgamation of Stikinia with Quesnellia. The youngest fossils in the Cache Creek Terrane however, are Mid-Jurassic
in age, arguing that the final closure of that intervening basin did not happen until that time or later.
53
In the “classic” synthesis of regional terrane assembly (dating from the late 1970’s and early 1980’s), the Stikine and
Quesnel Terranes, along with their oceanic components, were amalgamated as the composite Intermontane Belt prior
to its accretion along the margin of the continent. If the various arc complexes across these terranes are interpreted
as coextensive, then these terranes were amalgamated by Late Triassic time. In this scenario, the final collapse of the
intervening Cache Creek ocean basin did not occur until the Intermontane Belt was accreted. Beyond those associated
with that accretionary event, there is no record of later collisional tectonics over the Omenica Episode.
More recently, a number of researchers have come to question this “classic” scenario. It has been suggested that perhaps Stikinia is part of the Insular Belt, which was accreted in mid-Cretaceous time. There are Late Triassic and Early
Jurassic arc sequences in the Insular belt which could be correlative with the Takla and Hazelton arcs. In this interpretation, the Cache Creek Terrane is the remains of the ocean basin which closed in the mid-Cretaceous accretion of the
Insular Belt.
While this is an issue which we will re-visit in the next chapter, we will tentatively accept the more “classic” version
at this point. As will be developed later, many factors still support that original interpretation.
The Early Jurassic Margin of North America
The Southern Boundary of the Pacific Northwest
If the oldest island groups of the Intermontane Belt date from Permian time, it would be difficult to speculate on
where those early features developed. In Quesnellia, those early islands had been almost entirely eroded away by
the time the Nicola Arc commenced in Late Triassic time. How far west of the continental margin the Nicola Group
accumulated remains uncertain. Nicola magmatism occurred at the same time as plutons of the Kootenai Arc were
being intruded (e.g. the Flowery Trail Pluton east of Chewelah), evidence that the intervening oceanic basin was being
consumed at this time. .
Pacific
Northwest
Region
Intermontane
Islands
Transform Fault
Figure 2-21 (Left)
The contrast in preaccretionary settings between
the Pacific Northwest and
the Sierrra-Klamath Region
to the south. Both settings
feature active island-arc
belts offshore, and an active
continental arc.
The contrast between these
two settings requires that a
transform fault separated the
two sections of oceanic plate.
Sierra - Klamath
Region
54
Figure 2-22 (Right)
Diagram illustrating the
inferred contrast in accretionary settings between the
Pacific Northwest and the
Sierra-Klamath region to the
south.
Omenica Orogeny
Accretion in the Pacific
Northwest resulted in the
development of an Andean
- type margin and the limited
Omenica Orogen. To the
south, accretion resulted in
the development of a collisional margin, and the widespread Nevadan Orogeny.
Nevadan Orogeny
Significantly, the Rossland Arc appears to have been an active feature at the time the Intermontane Belt was accreted.
This circumstance is similar to that seen further south in the accreted terranes of the Sierra-Klamath Region, with the
accretion of an active island-arc belt . In that setting however, it is thought that the accretion of the island belt happened as the intervening ocean basin was consumed along both of its margins, in a “bipolar” subduction arrangement.
As this terrane unit was accreted, the continental margin became a collisional setting as both subduction zones were
eliminated. This resulted in an episode of widespread deformation, known as the “Nevadan” Orogeny. That deformation extended well back into the continent, as extended as far north as central Idaho.
By contrast, the accretion of the Intermontane Belt (as detailed below) resulted in deformation no further east than the
Kootenay Deformed Belt, much less than is evidenced to the south. This abrupt contrast in accretionary styles suggests that a different set of plate relationships were in effect here, one which did not result in a collisional setting. In
this arrangement, which parallels that seen in the Philippine Islands, the Rossland Arc was apparently supported by
an east-dipping subduction zone, just as along the continental margin. As the Intermontane Belt was accreted, that
outboard subduction zone (formerly supporting the Rossland Arc) became the subduction zone along the continental
margin. This circumstance avoids the collisional setting seen to the south.
This contrast in plate relationships between the Pacific Northwest and the Sierra-Klamath Belt requires a major
transform boundary within the oceanic plate adjoining the continental margin here. In modern aspect, that transform
boundary would fall along an east-west trend in the general vicinity of the Washington-Oregon border. Provisionally,
we might call this the “Columbia Transform.” Thus from this early date, as recognized by researchers for decades,
there is a fundamental difference in the course of geologic evolution between regions north and south of this latitude.
From this earliest date of inception, the Pacific Northwest has been a distinct and unique geologic province from
regions to the south.
55
Life on the Jurassic Intermontane Margin
At the time the Intermontane Belt was accreted, world sea levels were at a high point. Large portions of the continent were
covered in shallow epiric seas, and the uplifted belt of the newly-accreted continental margin probably rose as a set of
islands over these shallow waters. Given the dynamics of accretion and the subsequent development of the Omenica Arc,
these may have been fairly rugged islands over Jurassic time. We haven’t preserved any definitive Jurassic sedimentary
rocks here, so we don’t have much of a record of surface events over this period, Without that record, we can only speculate on what kinds of life might have called these islands home.
We do know that the dominant forms of vegetation at this time were gymnosperms, cycads, ginkgos and confiers. Given
the relatively warm climate and the ubitquitous character of plant life, it is a reasonable assumption that these islands were
covered in a rich tapestry of Jurassic vegetation. Cycads and seed ferns were probably wide-spread, supported by a wet
maritime climate. There seems little doubt that the seas around the island teemed with ichthyosaurs and plesiosaurs, along
with a spectrum of marine life. Along the shorelines, early crocodiles, frogs and other amphibians likely made their homes.
In the canopy above, the earliest forms of birds may have rested by night.
But were there dinosaurs? Sauropods and Stegosaurs as depicted above? Brachiosaurs and other huge species? Given the
relatively remote setting of these islands, one would be tempted to suggest not. We suspect that most of these species were
not accomplished long-distance swimmers. On the other hand, we have also been increasingly amazed by the geographic
range in which these creatures are found, so it would be imprudent to dismiss the possibility. Given a paucity of sedimentary rocks from this period, it is a question we may never be able to answer with any certainty. None the less, it is an
interesting thought to consider.
Whether there were dinosaurs here in the Cretaceous Period is another matter. We have Cretaceous non-marine sediments
in the Methow region. It has long been speculated that dinosaur fossils might be lurking in these rocks. It will be interesting to see what the future reveals of the past here.
Image above: Jurassic scene, from a painting in the Smithsonian Collection
56
Attwood Group
Knob Hill Group
The Accretion of the Intermontane Belt
The Dawn of the Omenica Episode
By the time that the Pangean Supercontinent started to break up about 200 million years ago, the Intermontane Islands
may have been no more than 300 – 500 km west of the continental margin. Given the relative paucity of Mid-Jurassic intrusions in the Kootenai Arc, and the relatively robust character of the Rossland Arc, it would appear that much
of the oceanic plate motion was being taken up on the outboard subduction zone at that time. As the supercontinent
of Pangea started to break up, it started to drive North America to the west. More so than the eastward advance of the
Intermontane Belt, the westward advance of North America was probably responsible for closing the Slide Mountain
Basin.
As the Intermontane Belt first approached the continental margin, outboard slices of that margin, principally the
rocks of the Covada Group and its younger cover, were thrust eastward over the continent. As the continental margin
reached the thick accumulation of the Quesnel Islands, it started to deform them. The Chesaw Thrust Fault, which
Figure 2-23 (Above)
The Chesaw Thrust Fault, east
of Palmer Lake, near Loomis.
The fault places the Knob Hill
Group over the Attwood Group
Figure 2-24 (Right) Setting
of the Intermontane Islands
prior to accretion, showing the
location of the Slide Mountain
Ocean Basin.
North America
Slide Mountain
Ocean
Intermontane
Islands
Rossland Arc
57
Kootenai Arc
Intermontane Islands
North America
Kootenay Deformed Belt
Intermontane Belt
Figure 2-25 (Top) and 2-26 (Bottom)
Diagram illustrating the thrusting and stacking of continental sequences in the accretion of the Intermontane Belt. The stack of
steeply-dipping continental rocks is the Kootenay Deformed Belt.
places rocks of the Knob Hill Group over younger rocks of the Attwood Group, probably marks the inception of this
regime (see figure 2-14) . Dating from 190 – 200 ma, this fault shows a pattern of overthrusting to the east, and is
probably the earliest evidence that accretion was underway.
As the continent continued to advance, the rocks of Quesnellia were obducted eastward across the edge of the continent, behind the rocks of the Covada Group, along with distal facies of the of the Sauk and Tippecanoe sequences, the
Windermere Group, and the Belt-Purcell Supergroup. On low-angle thrust faults, these rocks may have been thrust as
much as 200 km across the continental margin. At the same time, deeper and progressively thicker sections of rock
were forced into the subduction zone, until movement was no longer possible. With that, subduction ceased and the
Kootenay Arc became extinct.
Movement on the outboard plate was still accommodated by the subduction zone to the west, which had supported
the Rossland Arc. On accretion, that island-arc became the new continental arc regime, known as the Omenica Arc.
This happened between 180 and 170 million years ago, in Mid-Jurassic time. With these events the Omenica Episode
began, and the assembly of the Pacific Northwest was underway.
The contact between the rocks of Quesnellia and those of the Covada Group is a thrust fault which trends northeast,
cutting across the Columbia River at Kettle Falls. Sections of continental rock were thrust east as far as modern-day
Chewelah, stacking them in a repeating homoclinal sequence. In this process outboard rocks on the continental margin
(the Covada Group and younger rocks), along with distal members of the Sauk and Tippecanoe sequences, the Windermere Group and the Belt – Purcell Supergroup (the Deer Trail Group) were thrust east against more proximal rocks.
The result was a ~50 km wide belt of repeating, vertically-inclined strata which comprise the Kootenay Deformed
Belt.
Figue 2-27 (Right) (Modified) map of northeastern Washington (DRN state geologic map) showing location of the Kootenay
Deformed Belt.
58
Kootenay
Deformed
Belt
Rocks of the Covada Group, and younger cover
Rocks of the Sauk and Tippecanoe Sequences
Map Area
Rocks of the Windermere Group
Rocks of the Belt-Purcell Supergroup
Figure 2-28 Steeply dipping strata of the Covada Group, east of Hunters on the Hunters-Springdale Road
This 40-50 km – wide package may contain the remnants of up to 200 km of the continental margin, delaminated from
the craton and thrust to the east. The easternmost fault in this repeating structure is the Jump-Off-Joe Fault, which
parallels the southern Colville Valley on a north-south strike. This illustrates that deformation did not extend far back
into the continent, as was the case in regions south of here. While this pattern overprinted the effects of an earlier episode of deformation (dating from Devonian time), it is the major characteristic of the region known as the Kootenay
Deformed Belt.
East-directed overthrusting by Quesnellian rocks occurred between 187 and 173 Ma, followed by an episode of westdirected back-thrusting between 173 and 165 Ma. In this event, the continent appears to have “rebounded” from compression, thrusting continental rocks of the Ledbetter Slate and the Sauk Sequence to the east, back over the accreted
rocks of the Covada group and its younger cover.
60
Kuskanax
Batholith
North
America
Figure 2-29 (Right)
Map showing the distribution of Omenica Arc Plutons
in Washington and southern
British Columbia. Numerous smaller stocks are not
illustrated. As a group, these
plutons straddle the Intermontane - North American
boundary. More plutons are
found to the north.
Intermontane Belt
Penticton
Nelson
Batholith
British Columbia
Washington
Toats Coulee
Pluton
Republic
Trail
Metaline Falls
Idaho
Colville
Omak
Blue Goat
Pluton
Lane Mountain
Pluton
The Omenica Arc
The Heart of the Omenica Orogen
As the Intermontane Belt was accreted to the continent, the (islandic) Rossland Arc was effectively transformed into a
continental arc. That arc is known as the Omenica Arc, after the Omenica Mountains of north-central British Columbia. Beyond the initial deformation associated with the accretion of the Intermontane Belt, this magmatic arc would be
the dominant regional feature for the next forty to fifty million years.
The Omenica Arc produced a belt of plutonic rocks originally known in Canada as the “Omenica Crystalline Belt.”
They extend from northeast British Columbia south into north-central Washington, centering on the “suture zone”
between the Intermontane Belt and the old continental margin. Most of these Plutons range in age from 180 – 140 million years old, and include the Nelson and Kuskanax Plutons in southeastern British Columbia, and the Lane Mountain, Blue Goat and Toats Coulee Plutons in Washington State, to name but a few.
The geochemistry of the Omenica plutons reflects their rise through the continental crust over which the Intermontane
Belt was obducted. Geochemists can discern this by the concentrations of certain “rare earth” elements in those rocks.
To a degree, those plutons may have served to “weld” the accreted terrane to the old continental margin. To the south,
as a new subduction zone was established at the end of the Late Jurassic “Nevadan Orogeny”, the Omenica arc spread
south as far as modern-day Nevada. Late Jurassic magmatism of this regime was an important element in the development of the Sierra Nevada Ranges in California and Nevada.
Most of the Omenica plutons were intruded during a period of very rapid plate movement which persisted until about
140 million years ago, at the end of the Jurassic Period. In this episode, plutons of the Omenica Arc were intruding,
and contributing to, a thickened “welt” which had developed just back of the continental margin. This feature had its
61
Omenica Arc
Volcano
Intermontane Belt
Omenica Arc
Pluton
Bridge River Plate
Rocks of the Kootenay
Deformed Belt
North American
Plate
origins in the accretion of the Intermontane
Belt, and was subsequently supported by the
underlying Omenica Arc. The combination of
magmatism, burial and convergent tectonics
which characterized this setting coincided to
produce a regional-scale orogenic belt along
this zone, known in Canada as the Omenica
Orogen. In Washington State, this event has
been called the “Columbian Orogeny” in
decades past.
The overall extent of crustal thickening, deformation and regional metamorphism associated with the Omenica Orogeny is difficult to
discern. Parts of the region were previously
deformed and intruded by Kootenai Arc plutons, all of it was later intruded, deformed and
metamorphosed as part of the (mid Cretaceous
– Early Tertiary) Coast Range Orogen, and
was later intruded by magmas of the (Eocene)
Figure 2-30 (Above) Diagramatic representation of
the Omenica Orogen, illustrating the Omenica Arc
and the Omenica Arc plutons.
Figure 2-31 (Right) The Lane Mountain Pluton,
an Omenica Arc intrusion. At Waitts Lake, west of
Chewelah. Note K-feldspar megacrysts.
Figure 2-32 (Far Right) The Toats Coulee Pluton,
an Omenica Arc intrustion. West of Loomis.
62
Challis Arc. Research suggests that the Coast Range Orogeny probably effected the deepest burial and highest grade
of metamorphism in the area.
We do know that a substantial orogenic belt developed in the Omenica episode. Deformed rocks of Quesnellia were
cut by plutons of the Omenica Arc, so considerable deformation preceded the widespread development of continental arc magmatism. Regional deformation, intrusion and metamorphism of the Omenica Orogeny represents the first
major installment on this multi-phase orogenic belt, subsequently affected by both the Coast Range and Challis Arc
regimes. Trying to decipher the relative and quantitative effects of these episodes is a challenging task. There is little
doubt however, that the Omenica Belt was to some degree characterized by all of these aspects, and qualified as an
orogenic setting.
Over that course of events, the rocks of the Intermontane Belt and the older continental margin have been intruded,
deformed and metamorphosed. Most of the intrusive and metamorphic effects of the Omenica Orogen were apparently
concentrated in the first half of the Omenica Episode, between 170 and 140 million years ago (Mid to Late Jurassic time). This is thought to have been a period of relatively rapid plate convergence, perhaps three times the rate we
currently experience in this area. Plate velocities dropped sharply after about 140 Ma, and the region appears to have
experienced a lull in magmatism between 135 and 125 million years ago. Over that later period, events appear to have
been characterized by uplift and erosion.
At surface levels, the Omenica arc raised a vigorous chain of volcanoes behind the newly-accreted margin. Those volcanoes shed abundant volcanic detritus into the flanking basins, particularly on the oceanic side to the west. Along the
margin of the continent, those volcanic sediments accumulated to great depths in a coastal marine basin. producing a
thick section of sedimentary rocks. These rocks are known as the Methow Group in Washington, and the Tyaughton
Group in British Columbia.
63
Deposition on the Leading Edge
Rocks of the Methow - Tyaughton Basin
As the Intermontane Belt was accreted, it probably remained an island setting. Much of western North America was
a shallow marine setting over the Mid to Late Jurassic, part of what was known as the Zuni transgression. Omenica
Arc magmatism clearly raised a chain of volcanoes along the island belt, eroding to supply sediment to both the east
and west. The sediment which spread west accumulated along the edge of the continent, in what was likely a fore-arc
basin setting. Those deposits are known in Washington as the Methow (met’-how) Group, and in British Columbia
as the Tyaughton (tie-ought’-ton) Group. Together, they mark the western coastline in Late Jurassic and Early Cretaceous time.
The rocks of the Methow-Tyaughton Group accumulated on a section of oceanic-crust rocks (basalt, gabbro, ultramafics) known as the Spider Peak Formation. These represent the “trailing edge” of the Intermontane Belt, which
subsequently became the “leading edge” of the North American continent. The earliest sediments in the Tyaughton
sequence date from Early Jurassic time, and are known as the Ladner Group. The Ladner is locally subdivided into
the (Early Jurassic) Boston Bar Formation, and the (Early to Mid Jurassic) Dewdney Creek Formation. The Boston
Bar Formation consists largely of fine-grained marine clastic rocks and volcanic sandstones, with minor conglomerates. Much of the rock is a thinly bedded siltstone and black shale, commonly containing woody debris. Andesitic
lavas occur in the Dewdney Creek Group, a product of submarine eruptions.
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The important observation on the
age of the Ladner Group is that
this sequence accumulated in large
part before the Intermontane Belt
was accreted to the continent.
These are not sediments of the
Omenica arc, they are sediments
of the Rossland Arc. These are
Quesnellian rocks, rather than
North American rocks.
Tyaughton
Basin
Fraser
Fault
Intermontane
Belt
In the Methow region to the
south, rocks of the Newby Group
have long been considered correlative with the Ladner Group. It
too consists of two members, the
(lower) Twisp Formation and the
(upper) Buck Mountain Formation. The Twisp Formation is
a complexly deformed unit of
thinly bedded black argillites and
volcanically-derived sandstones.
Unfortunately, it does not contain
any age-diagnostic fossils. It outcrops around the town of Twisp,
Washington, and unconformably
underlies the Buck Mountain
Formation. The Buck Mountain
Formation consists of nearly 3
km of andesite flows and breccias, volcanicallyderived sandstones, siltstone and shale, and some
thick conglomerate beds. Unlike the upper Ladner
(Dewdney Creek) rocks however, the fossil record
of the Buck Mountain Formation yields largely
Early Cretaceous species.
Methow Basin
Vancouver
British Columbia
Washington
Seattle
Figure 2-33 (Left) The upper Methow Valley, above Mazama. This
is a classic glacially-carved trough.
Figure 2-34 (Upper Right) Map showing distribution of the rocks
of the Tyaughton and Methow Basins. These were a coextensive
marine basin along the western shore of the Intermontane Belt,
separated in Eocene time along the Fraser Fault.
Figure 2-35 (Right) The Twisp Formation, near Twisp, WA. These
are thin-bedded argillites and volcanically-derived sandstones, and
have been subject to extensive deformation. Unfortunately, this unit
does not contain any age-diagnostic fossils. The affinities of these
rocks remain uncertain.
65
Paleocurrent
UK
90
Pasayten
Group
LK
Jackass Mtn.
Group
Thunder Lake
Group
100
150
J
Ladner
Group
200
110
Midnight
Peak
Subaerial Andesitic Volcanics
East
Red Fluvial Clastics
Winthrop
Cross-bedded Arkose
Sandstone
West
Virginian
Ridge
Chert - Lithic Sandstone,
Conglomerate, Shale.
East
Harts
Pass
Massive Tabular Arkose
Sandstone
West
Panther CR.
Granitoid and Volcanic
Clast Conglomerate
West
Goat Cr.
Figure 2-36 (Above)
Stratigraphic column for the upper strata of the Methow Basin (right), and relationships with rocks of the Tyaughton
Basin (left).
If the Buck Mountain Formation is Early Cretaceous in age, then the Newby “Group” may not be correlative with
the Ladner Group. The age of the Twisp Formation could be Early to Mid Jurassic, but it could also be Late Jurassic
in age. This would make it time-equivalent with the Thunder Lake Group, which unconformably overlies the Ladner Group in Manning Park, British Columbia. The age of the Twisp Formation remains an unresolved issue in the
Methow sequence. We will again consider the Early Cretaceous Buck Mountain Formation, in the following chapter.
The Buck Mountain and Thunder Lake Group are unconformably overlain by an Early Cretaceous assemblage of
marine sandstones, siltstones, shales, argillites and conglomerates. In British Columbia, this is known as the Jackass
Mountain Group, while in Washington State it includes the Goat Creek, Panther Creek and Harts Pass Formations. In
the Methow region, the Goat Creek Formation is a coarse to fine-grained, well-bedded marine arkose sandstone and
black argillite. To the north of the border, it is a dark-gray arkose sandstone and gray siltstone.
Rocks younger than the Goat Creek Formation in Washington, including the upper portion of the Jackass Mountain
Group in British Columbia, were actually deposited during the (~120 - 58 ma) Coast Range Episode. We discuss
these rocks here because their contrast with the older rocks is graphic evidence for changes in the Bridge River Basin
to the west. Those changes actually reflect the inception of the Coast Range Episode, the subject of the following
chapter.
66
Figure 2-37 (Right)
The Buck Mountain Formation, Chewuch River. These
rocks are andesite flows
and breccias. Elsewhere,
sedimentary beds in this unit
have yielded fossils of Early
Cretaceous age.
The Panther Creek Formation overlies the Goat Creek Formation, and is a distinctive marine conglomerate formation. The conglomerate clasts are granitic and volcanic cobbles.Granitic cobbles first appear in the upper portion of
the Buck Mountain Formation, and are abundant by Panther Creek (early mid Cretaceous) time. The evolution from
volcanic to granitic cobbles and the parallel maturing of sandstone compositions (from volcanic-lithic to quartzofeldspathic) likely reflects erosion into the core rocks of the Omenica Arc. By Early Cretaceous time, the Omenica
Arc was apparently no longer an active feature. The sediments of the upper Buck Mountain, Goat Creek and Panther
Creek reflect deep erosion of the Omenica highlands.
Figure 2-38 (Right)
The Panther Creek Formation, east of Banker Pass
(east of Mazama). The
conglomerate clasts here are
largely plutonic and volcanic species, reflecting erosion into the deeper plutonic
roots of the Omenica arc.
This is part of the Jackass
Mountain Group.
67
Figure 2-39 (Above)
Fine-grained sediments of the Harts Pass Formation, near
Harts Pass. This is an exceptionally thick formation, on the
order of 2.5 kilometers. These fine-grained sediments would
appear to reflect an extended period of relative quiesence along
the Intermontane Coast.
Figure 2-40 (Right)
Fossils in the Slate Peak Member of the Virginian Ridge Formation, on Slate Peak. These fossils are clams and snails of a
shallow-marine habit, accompanied by the shells of belemnites
and other free-swimming species. These are very rich fossil
beds, affording a good venue on Early Cretaceous marine life.
Quarter provides scale.
68
Figure 2-41 (Right)
Ripple marks in rocks of the
Slate Peak Member of the
Viriginian Ridge Formation.
These are large ripples,
with a wavelength of about
10 cm., and are broadly
symetrical. They are unequivocal evidence that this
was the shoreline in Early
Cretaceous time.
The Harts Pass Formation conformably overlies the Panther Creek Formation. This is a shallow-water marine formation, composed of thick-bedded sandstones and thin-bedded siltstone and shale, in about equal proportions. The
sandstones are quartzo-feldspathic and weather to a white or buff color. The finer - grained rocks are typically gray
to black in color. The formation is fossiliferous, and has yielded clam, snail and ammonite species, among others.
At least part of the formation was deposited in a transitional setting, as evidenced by ripple marks in the rocks. The
formation is something on the order of 2400m (8000’) thick, and is regionally extensive across the western end of the
Methow basin. Compared to the older units, it reflects a shallower marine setting.
Unconformably overlying the Harts Pass Formation is the mid-Cretaceous (Albian-Aptian) Virginian Ridge Formation. This is the earliest formation of a mid to Late Cretaceous assemblage known on both sides of the border as the
Figure 2-42 (Right)
Angular unconformity
(red line) between the
Harts Pass and Virginian
Ridge Formations, along
Rattlesnake Creek north
of Mazama. The Virginian
Ridge Formation is below,
the prominent outcrop
reflecting the conglomerate in the basal unit of the
formation. The rocks of the
Harts Pass Formation are
above (in a geographic, not
stratigraphic sense).The
prominent ribs evidence
more resistant beds.
Harts Pass Formation
Virginian Ridge Formation
69
Figure 2-43 (Right)
The Patterson Lake Conglomerate member of the Virginian
Ridge Formation, from an
outcrop along Patterson Lake,
west of Winthrop. The clasts
here are largely volcanic and
sedimentary varieties.
Figure 2-44 (Below)
The Devils Pass Member of
the Virginian Ridge Formation. Note chert clast at
lower right. From an outcrop
along the Twisp River, west of
Twisp.
Pasayten Group. The Virginian Ridge Formation can be divided into three members. The lowest of these is a conglomerate formation known as the Patterson Lake Conglomerate. This is a marine to non-marine formation which
features conglomerate clasts primarily of sedimentary and volcanic rock. In places, some of the clasts are over a meter
across. Arkosic sandstones and red to black mudstones are secondary components in this package. The size of the
boulders in this conglomerate layer has inclined some researchers to suggest that it reflects syn-depositional faulting of the basin. The Patterson Lake Conglomerate Member is topped by the Slate Peak Member. This is a shallow
marine to non-marine section of sandstone and shale. Locally, marine sections are abundantly fossiliferous, containing
the fossilized remains of clams, snails, belemnites and other shallow-water denizens.
70
Figure 2-45 (Above)
The Winthrop Formation, at an outcrop
along the Twisp River, west of Twisp.
Note the distinctive coal bed.These
are arkose sandstones and siltstones,
reflecting a terrestrial setting. Much of
Winthrop has bedding that is more massive than this.
Figure 2-46 (Right)
Fossils in the Winthrop Formation, at
the outcrop above. Here as elsewhere,
fossils are not particularly well preserved in this unit. These are clearly terrestrial species, but their identification
would be a challenge.
71
Mazama
Late Cretaceous
Winthrop
Midnight Peak Formation
Pipestone Canyon Formation
Winthrop Formation
Winthrop / Virginian Ridge Undifferentiated
Virginian Ridge Formation
Harts Pass Formation
Early Cretaceous
Buck Mountain Formation
Jurassic (?)
Twisp
Twisp Formation
Figure 2-47 (Above)
Map of the Methow region in Washington. Area map in upper right corner shows location. Red line shows location of State
Route 20.
The eastern boundary of the Methow region is the Pasayten Fault, which borders high-grade rocks of the Okanogan Batholith
to the east. The western boundary to the province is the Ross Lake Fault Zone, The Ross Lake Fault is a terrane-bounding fault
with the rocks of the Insular Belt to the west.
72
The most important member of the Virginian Ridge Formation is the
upper section known as the Devils Pass Member. This is a marine
conglomerate member of conspicuous composition. Many of the clasts
in this unit are chert pebbles, derived from an oceanic source. Of equal
significance, paleocurrent indicators in this member reflect the origin of
these clasts as being from the west. After perhaps 80 million years of
receiving sediment from the east, this represents a significant reversal
of that trend. The only way that oceanic sediments could flow east onto
the continental shelf here would be if the oceanic basin to the west was
being uplifted. This uplift heralds the impending accretion of the next
“megaterrane” unit, the Insular Belt, and reflects the collapse of the
intervening ocean basin.
Conformably overlying the Virginian Ridge Formation is the Winthrop
Formation, a nearly white, massive non-marine arkosic sandstone with
beds of light-gray shale and occasional coal. These sandstones vary
from massive to parallel-laminated, to locally cross-bedded. Large
Figure 2-48 (Above) An outcrop of the Winthrop Sandstone, illustrating its typical white color and massive appearance. On State
Highway 20, west of Mazama.
Figure 2-49 (Above Right) An outcrop of the Winthrop Sandstone, along the Methow River north of Mazama. This is a particularly well-bedded section.
73
trough cross-beds are a characteristic of the formation. The upper portion of this unit has minor volcanic flows, tuffs
and volcaniclastic sandstones. The non-marine character of this formation is the culmination of an uplift process ongoing since Harts Pass time. That uplift reflects the collapse of the Bridge River Basin, and the inception of the Coast
Range Episode. These subjects are covered in the following chapter.
Figure 2-50 (Above)
The Methow high country,
east of Winthrop. This is
a very popular district for
recreation, favored by a dry
climate and a spectacular
setting.
Figure 2-51 (Right)
Steeply dipping strata of the
Methow Group, seen on the
summit of Tatie Peak. View
from Slate Peak, looking
south.
Peak to the right is Golden
Horn Mountain, The distinctive summits of “The
Needles” lie to the left.
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Republic, Washington
A Town that Gold Built
If there was only one town in Washington State which could claim to be a mining town, it would be the town of Republic, in Ferry County. The town has its origins in 1896, when at the behest of prospectors, the Federal Government compelled the Colville Tribes to sell off the northern half of their reservation for settlement. Within hours of the opening of
the region, claims were staked at the Knob Hill prospect outside what is now Republic. Over the years that mine yielded
some 2.5 million ounces of gold, and three times that much silver. It was a value over a hundred times what the government paid for the entire region.
Gold in the Republic area is concentrated in quartz veins associated with the Challis intrusive episode. These are epithermal deposits, a system of intrusion which has features in common with hot springs. Elsewhere in the region gold is more
commonly found in skarn (carbonate) deposits, as illustrated in the Crown Jewel prospect on Buckhorn Mountain. These
typically develop in the Brooklyn Limestone. The vein deposits around Republic were easy to locate because they are
resistant to weathering, and their identification required no geologic acumen. Thousands flocked to the region in the late
1890’s, scratching at every quartz vein in the valley.
Originally known as “Eureka”, the town of Republic was formally platted in 1900. The name of Eureka was already
taken by another town, so they adopted the name Republic from the nearby Republic Mine. By 1902 the railroads
finally reached the town, which by this date had a population of several thousand. Large-scale development of the Knob
Hill prospect began in 1910, eventually sinking a shaft some 1700 feet. In 1937-40 a 500 ton per day cyanide mill was
installed, which was a marked improvement over smelting the crude ore off site. The mine operated into the 1970’s, and
served as a major employer in the region. Operations in the adjoining Golden Promise mine continued until 1995.
After considerable delays in meeting environmental concerns, the Crown Jewel Mine on Buckhorn Mountain opened in
2008, the only active mining operation in the county at this time. None the less, considerable development continues on a
number of prospects, and mining will certainly be part of the history of the region well into the future.
While the decline of mining has certainly hurt the town of Republic (population about 1,000), it remains the only major
town in the region, and has made a concerted effort to diversify its economic base. The town itself has the authentic
(albeit tidy) look of a mining town, and is a popular attraction in itself. It is in a popular region with sportspersons, and
offers a unique and quite beautiful landscape for tourists. The Stonerose Fossil Site (see page 196) is a popular attraction, as is the Prospector Days Festival (second weekend in June) and the state Open Fiddle competition in Mid-August.
For lodging in town, the author is pleased to recommend the Prospector Inn, which has accomodated him graciously over
the years.
Image: Republic, from the north. Image from Wikipedia
Insular Belt
Islands
Bridge River Basin
North
America
Gambier
Arc
Bridge River Plate
Intermontane Belt
The Arrival of the Insular Belt
The Evolution of a Japan-Type Margin
The Harts Pass, Virginian Ridge and Winthrop formations reflect uplift of the continental margin, as well as uplift in
the oceanic (Bridge River) basin to the west. As detailed in the following chapter, these were events associated with
the collapse of the Bridge River Basin and the accretion of the Insular Belt, the next “megaterrane” to be accreted. As
with the Intermontane Belt, this begs the question when this new parcel of real-estate first appeared on the scene. As
with the Intermontane Belt, there is a body of evidence to suggest that it may have occupied a “Japan-Type” margin
here over the Early Cretaceous, after the Omenica Arc had apparently ended its tenure.
Much of this evidence is based on a concordance of Early Cretaceous sediment types seen along the eastern side of
the Insular Belt (e.g. the Gambier Group) and in the upper sections of the Tyaughton sequence (e.g. the Brew Group).
These similarities, along with the concurrent lack of magmatism in the Omenica Arc, support an interpretation that
these sediments were deposited in a restricted basin between the Insular Belt and the continental margin. This ocean
Figure 2-52 (Above)
Digramatic representation of a Japan-type margin, with the Insular Belt lying some 500 km offshore. Note that the inboard
subduction zone has become inactive, and the Bridge River Plate has become affixed to North America.
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basin is known as the Bridge River Basin, and it may have been on the order of 400 – 600 km wide. These aspects
will be considered in greater detail in the following chapter.
The point of consideration here is that it appears that the Omenica Episode, while starting out as an Andean-Type
margin over Mid to Late Jurassic time, may have evolved into a Japan-Type margin as the Insular Belt arrived in
Early Cretaceous time. This happened as ocean plate velocities declined rapidly, which likely had some effect on this
development. Judging by the apparent extinction date on the Omenica Arc, this may have happened by ~135 million
years ago. There are no known regional tectonic events associated with the development, which simply reflects the
transfer of all ocean-plate movement to the outboard subduction zone.
The relatively quiescent setting of the Methow Basin over this period may be reflected in the thick, regionally – extensive, fine-grained deposits of the Harts Pass Formation, which accumulated to nearly two and a half kilometers
of strata. These rocks would appear to represent an extended period of deposition in a relatively stable environment.
This quiescent setting came to an end as the Bridge River Basin, which intervened between the Intermontane Belt and
the Insular Islands offshore, began to collapse. As noted earlier, this event is reflected in the rocks of the Virginian
Ridge Formation.
Figure 2-53 (Below) Dead Horse Point, on the Harts Pass Road north of Mazama. This is the western end of a historically
problematic section of mountain road engineering. This was originally a mining road, dating from the late 1890’s. The rocks are
of the Virginian Ridge Formation.
77
Summary:
The Omenica Episode
The Omenica Episode, the first episode in the geologic evolution of the Pacific Northwest, was initiated in Mid-Jurassic time with the accretion of the Intermontane Belt. Accretion of that belt of islandic rocks occurred as North America began moving westward with the opening of the Atlantic Ocean, as the supercontinent of Pangea began to break
up in Mid-Jurassic time. For an uncertain span of time prior to that event, the Intermontane island chain probably
existed as an active volcanic arc no more than 300 – 500 km west of the continental margin. Given the relative paucity
of Early Jurassic magmatism in the continental margin, it may well have occupied a Japan-type marginal setting over
that period of time.
The westward-moving North American Continent collided with the Intermontane Belt between 190 and 170 million
years ago, in Mid-Jurassic time. The collisional tectonics associated with that accretionary event, along with the concurrent evolution of a new continental arc regime, culminated in the development of an orogenic belt (the Omenica
Belt) along the “suture” zone between the ancestral continent and this newly-accreted terrane.
Figure 2-54 (Above) Steeply dipping rocks of the Winthrop and Viriginian Ridge Formations, looking north along the Pacific
Crest from Slate Peak. Summit in the shadow is Pasayten Peak.
Figure 2-55 (Right) On the Sinlahekin River, south of Loomis
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Figure 2-56 (Above)
Bothen Creek Valley and Campbell Lake, southeast of Winthrop.
This orogen was established by 165 Ma, and likely persisted through about 140 Ma - the active span of the Omenica
Arc. While the region remained a largely islandic setting over this period, evidence does support the existence of a
large contiguous volcanic island belt here. Volcanic detritus from this active arc complex accumulated along the western shelf of the islands, as the sediments of the Methow-Tyaughton Group. The maturity of those sediments, along
with the lack of Early Cretaceous magmatism in the Omenica Arc, suggests that this orogenic cycle was concluded by
Early Cretaceous time.
By that date, the continental margin may have evolved into a Japan-Type setting, with the Insular Belt positioned
perhaps 400-600 km to the west. Between them lay the Bridge River Ocean. This may have been a relatively stable
relationship for 10 -15 million years, creating a relatively quiescent setting along the Methow-Tyaughton coast. The
collapse of that basin, as foreshadowed in the rocks of the Pasayten Group, was a protracted event which spanned a
period from 120 to 100 million years ago. Because those events are intimately tied to the accretion of the Insular Belt
and the development of the Coast Range Orogen, they are covered in the following chapter.
With this in mind, we somewhat arbitrarily close the door on the Omenica Episode at about 120 million years ago, in
mid-Cretaceous time. From this date forward, the accretion of the Insular Belt dominates the immediate and extended
course of events here. With these developments, we move into the Coast Range Episode.
Chapter 2: The Omenica Episode
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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