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ESS 305
North Cascades Metamorphism
Spring 2015
10 May
The North Cascades have a complex geologic history, many of the details of which are
still undetermined. Nonetheless, the general picture is of a series of accreted terranes that
formed elsewhere and were docked onto the side of North America over the last few hundred
million years. Most of these terranes are a mix of oceanic igneous and sedimentary rock that
have since undergone physical distortion and metamorphism during and after their accretion.
To complicate matters, many of these rocks have undergone significant erosion and
burial under their own debris and that of nearby areas, have been offset and restacked by
numerous episodes of faulting, and have been intruded by magmas and buried under volcanic
activity. Of course, this whole jumble has been deeply incised by glaciation, too.
Today we will make a single traverse across part of this mosaic of materials in order to
become acquainted with field exposures of a few of the metamorphic rocks found here.
Considerably more of the story is presented at http://geomaps.wr.usgs.gov/parks/noca/
Take I-5 north to Exit 230 and turn east onto Highway 20 (North Cascades Highway).
Turn right onto Concrete-Sauk Valley Road and stop at the intersection with South Skagit
Highway. The outcrop is at the NW corner of the intersection. As with nearly all of today's
stops, watch for traffic.
Stop 1. Darrington
What is the texture of this rock, and how does it indicate metamorphism? The protolith
of this rock was shale, which is to say deep ocean mud. Has this rock been greatly altered (highgrade metamorphism) or only slightly altered (low-grade metamorphism)? What is this rock
now called? Identify the silvery, shiny mineral. What does this indicate about the protolith?
Return along Concrete-Sauk Valley Road. Turn right onto Highway 20 and drive into
Concrete (the town, not the substance). Turn left onto "E" Avenue and right onto Main Street.
Stop at the public restrooms and note the local preference for building material.
Stop 2. Chilliwack Formation Boulders
Surrounding the restroom are a number of dark boulders with white veins. Identify their
composition and describe your evidence. What is the protolith, and what is the rock now?
Search for "O" shaped fossils, which are crinoid stems. These particular species place the rocks
in the Carboniferous, approximately 330 Ma. See sketches below.
Return to 20 eastbound. Turn left onto North Everett Avenue and bear left toward Lake
Shannon. Turn right up Baker River Road. Pass the Lower Baker Dam and bear left. Bear left
then right at the next two forks. Park at the gate and walk to the quarry.
Stop 3. Chilliwack Formation Quarry
This quarry was the source for the limestone which was used to make Portland cement,
the essential ingredient in concrete. The quarry is in a pod-like body of limestone, which finally
played out in the late 1960s. When this happened, the concrete-making era in this area ended.
The limestone unit here is part of the Chilliwack Group, a Paleozoic island-arc assemblage of
basaltic to andesitic lava flows, breccias, volcanically-derived marine sediments and limestone. It
was accreted to the continental margin in Mid-Cretaceous time, perhaps 115 million years ago,
as part of the Insular Belt terrane. This particular section is part of the Mississippian Red
Mountain Subgroup, which contains extensive limestone beds (particularly north of the
Nooksack River). These are biogenic limestones, the remains of reef-type colonies which
developed in the waters around the ancient Chilliwack Islands in Late Paleozoic times. Those
colonies consisted largely of crinoids (aka “Sea Lilies”), which are abundantly preserved in fossil
form here. Most typically seen is the “bulls-eye” pattern of the cross-section of the stalk of the
organism.
The Chilliwack Terrane dates from pre-Mid Devonian time, was subsequently
amalgamated with the Insular Belt by Late Jurassic time, was deformed in the process of
accretion, and was again deformed as it was included in the Northwest Cascades System – a
regional stack of thrust sheets developed in the obduction of the northern end of the Melange
Belt terranes. The rocks quarried here end at or near the extent of excavation, and the sloping
wall on the southwest side is rock of volcanic origins.
These rocks have been very mildly recrystallized at sub-greenschist levels of
metamorphism. Compared to the volcanic rocks of this suite, the limestone units tend to be
relatively undeformed, preserving a good fossil record of that time.
From the quarry, one gets a nice view of Mt. Baker on a clear day. Mt. Baker is a
Quaternary stratovolcano of andesitic composition. It is largely younger than 50,000 years in
age, and last erupted in 1870. Pound-for-pound, it is the most heavily glaciated volcano in the
range. A major danger is from lahars flowing down this side of the peak, into the lakes
impounded by the dams here.
Also in view here is the 9117’ peak of Mt. Shuksan, directly to the north. This peak is
comprised of greenschist and phyllite of the Shuksan Suite.
Describe the rock type and fossils found at the quarry and speculate on the depositional
environment of the protolith for this material. Also note any folding or faulting.
Return to Highway 20 and continue east. At Rockport, turn south on SR 530 (to
Darrington). The outcrop is about 3.2 miles down this road, 1.5 miles from the Rockport –
Cascade Road, about .3 miles past the Illabot Creek Road.
Stop 4. Darrington Phyllite and Shuksan Greenschist
There are two rock types on display here, including a greenschist (Shuksan Greenschist)
and a phyllite (Darrington Phyllite). These are rocks of the Shuksan Suite, part of the northern
belt of terranes which were thrust northward over this region in mid- Cretaceous time. The
greenschist is a meta-basalt of ocean-floor origins, while the phyllite presumably reflects the
pelitic suite of ocean-floor sediments which accumulated upon it. They are part of a larger
assemblage (the Northwest Cascades Group) which includes ultramafic rocks (the Twin Sisters
Dunite) and various mélange units.
The Shuksan rocks, while not particularly evident here, are locally metamorphosed at
blueschist facies, the signature of subduction-zone conditions. Given the general character of
rocks in this package, it appears that these were the components a coastal subduction zone which
was tectonically exhumed from a southern locale, likely off the modern-day coast of southern
Oregon or northern California, in Early Cretaceous time. These rocks were transported
northward on an oceanic plate, and were subsequently obducted (thrust over) the southern
margin of the accreted Insular Belt. Here, that southern margin consisted of the Chilliwack
Terrane.
The Shuksan rocks overlie the Chilliwack Terrane along a low-angle thrust fault known
as the Shuksan Thrust. In the San Juan Islands to the west, this same thrust is known as the San
Juan Thrust. To the east, its counterpart is the Windy Pass Thrust along the south side of Mt.
Stuart, east of Snoqualmie Pass. These rocks overthrust the continent perhaps 90-95 million
years ago. In Eocene time, the eastern and western portions were displaced by right-lateral
movement on the Fraser Fault. Formerly, they were coextensive.
This stop lies with a former divide between the Skagit and Sauk Rivers. As described
above (c.f. Rockport) , this divide was eroded away as a proglacial lake in the Skagit Valley
overtopped it, flowing south into the Stillaguamish drainage. During the early stages of the
various glacial advances, this served in an ice-marginal river system as the waters of the Fraser,
Nooksack and Skagit Rivers were diverted to the south.
Return to Highway 20 east. Restrooms at Rockport State Park. At milepost 109-110 park at the
pullover on the right.
Stop 5. Marblemount Meta-Quartz Diorite
This is part of a late Triassic island-arc complex, which extends for about 50 miles along
the southern boundary of the Skagit region. These were likely coextensive with the Cadwallader
Arc complex in British Columbia (now offset along the Eocene Fraser Fault). It is uncertain
whether they developed on the oceanic rocks of the Bridge River plate, but were likely part of
the Insular Belt by late Jurassic time. The rock is metamorphosed at amphibolite facies.
This stop illustrates the abrupt change in metamorphic grade across the (Eocene) Fraser
(Straight Creek) Fault, just a few hundred yards west of here. In places, the rocks are also
deformed in response to lateral shear along that feature (total, about 145 km). This is one of the
major geologic features of western Washington, offsetting all older rock belts.
This rock intruded as a diorite approximately 220 Ma, then was metamorphosed 90-100
Ma along with the rest of the Crystalline Core of the North Cascades. The greenish color reflects
the conversion of plagioclase to epidote. Chlorite fills fractures in the rock, and biotite has been
noticeably degraded. Describe the metamorphic texture of this material. Also look for dikes of
another material. What is it, and are the dikes older or younger than the metamorphism?
Stop 6. Newhalem
These are intrusive rocks of the Late Coast Range Orogen, which characterizes about
80% of the rock in the Skagit region. The mid-Cretaceous – earliest Tertiary Coast Range
Orogen originally developed with the accretion of the Insular Belt and the inception of the Coast
Range Arc. The early phase of the orogen involved crustal thickening associated with
accretionary tectonics, and large-scale intrusion to the southwest of this region between 100 and
80 Ma. Arc magmatism then migrated northeast into the region, which developed between 80
and 60 Ma. The rocks at this site have been dated at about 70 Ma, and were intruded at depths of
20 – 25 km.
Continue on 20. Beginning at milepost 115 there are four pullovers across from
exposures of the Chilliwack Batholith. Stop at either the third or fourth.
Stop 7. Chilliwack Batholith
Despite the similar name, this is not part of the Chilliwack Formation. A relatively early
pluton of the Cascade arc, part of the large Chilliwack Composite Batholith to the north. This is
known as the “Mount Despair” pluton in that group, dating from about 30 Ma. It is a biotitehornblende granodiorite. Notable is the lack of metamorphism or deformation, in sharp contrast
to the older intrusives of the Coast Range episode. Relate the timing of its intrusion to the timing
of the metamorphism of the Crystalline Core rocks.
The Straight Creek Fault, one of many in the area, is truncated by this batholith. What
does this suggest about the timing and location of intrusion?
Continue on 20. At milepost 122, look for signs of a significant landslide. Restrooms are
at Gorge Creek Overlook. Stop shortly past milepost 125. There will be a large orange
weathered patch of outcrop.
Stop 8. Skagit Crystalline Core
What is the grade of metamorphism found here? Describe and name this rock, making
note of any prominent minerals. During metamorphism, this package of sediments and intrusive
igneous rocks was buried to a depth of 30 km and has been subsequently uplifted and exposed at
the surface.
Continue on 20 and bear right over the river at Diablo, past Diablo Dam. Park at the
overlook past milepost 131.
Stop 9. Diablo Overlook
This is a fine vantage on the Diablo Lake area. Across the valley, the peaks (from left to
right) include Davis Peak (2149m / 7051’), Elephant Butte (2249m / 7380’) and Sourdough
Mountain (1816m / 6106’). All of these peaks are comprised of the Skagit Gneiss, a product of
the Late Cretaceous Coast Range Orogen. Nearly 80% of the rock here is orthogneiss,
representing the vast quantities of tonalite (quartz diorite) intruded by the Coast Range Arc.
These magmas intruded country rock largely consisting of the Napeequa (Bridge River) and
Cascade River (Cadwallader) Terranes. These rocks were metamorphosed and preserved as
“rafts” of paragneiss within the intrusive orthogneiss. Here in the Skagit region, maximum
orogenic development was likely achieved by ~75 Ma.
The outcrop across the highway is a biotite – hornblende quartz diorite orthogneiss.
While these rocks have been metamorphosed, most are largely directionless. They appear to date
from 65- 70 Ma, making them relatively young for the Skagit suite. By that date, deformation
had apparently ceased at this level. Several different orthogneisses are present here, cut by sills
and dikes of various ages. The most recent features are the large light-colored dikes which cut all
other rocks. These are pegmatite dikes of the Challis episode, and they are found over a broad
section of the Skagit region. It may be that a large Challis-age pluton intruded to underneath this
area, and these dikes radiate from its cupola (top). These dikes have been dated at about 45
million years ago.
Just west of here, Thunder Arm of Diablo Lake heads on Thunder Creek. Thunder Creek
is a large drainage with an unusual north-south orientation (most features here are oriented NW –
SE). The Thunder Creek Valley is cut along a set of north and northwest-trending faults which
control its orientation. The master fault of this set is the Thunder Creek Fault, which can be seen
cutting through the east ridge of Colonial Peak at a prominent notch. It continues north through
Thunder Lake on the other side of the hill across Diablo Lake, and across the Skagit to form the
valley of Sourdough Creek on the far slopes. The Thunder Lake Fault is an Eocene (Challis)
feature which cuts (and is therefore younger than) the 45 Ma dikes seen here. It is in turn cut by
the ~36 Ma rocks of the Chilliwack Batholith to the north. The fault is a right-lateral strike-slip
feature, likely driven by concurrent movement on the Fraser (Straight Creek) Fault to the west. It
was probably active from ~44 – 40 Ma.
The most dramatic development here in recent (geologic) time concerns the origins of the
upper Skagit drainage. Three million years ago, prior to the recent ice ages, the ancestral Skagit
River had its origins on a divide located roughly where Diablo Dam now stands. Above that
divide, what is now the upper Skagit originally drained north into Canada, down this wide, lowgradient valley. As the earliest episodes of continental glaciation commenced, a large valley
glacier descended south out of Canada, flowing (what was then) up the upper Skagit Valley. As
it did, it impounded a large lake here, much like modern day Ross Lake and Diablo Lake.
Eventually, that lake overtopped the divide and drained west into the ancestral Skagit River.
These waters cut through the divide, and cut the deep Skagit Gorge into what was originally a
headwater valley. As the glacier finally retreated, the valley floor had been lowered enough that
the upper portion now flowed south and west into the Skagit, reversing its original course.
Many of the remote summits of the upper Skagit region were the sites of fire lookouts
through the 1950s. In that last decade, some of these were manned by people like Gary Snyder,
Philip Whalen and Jack Kerouac. These were the avant-garde poets of the “beat generation,”
taking their inspiration from the rugged grandeur of this spectacular region. In time, their works
have become widely acclaimed. Gary Snyder manned the station at Sourdough Mountain.
The rocks in the roadcut on the other side of the road reflect a long a complex history of
intrusion. Structurally, this roadcut lies in the very center of the north-south trending Skagit
Antiform. The light-colored pegmatite dikes in the wall are part of the Eocene Challis suite of
magmas, which seam this region. Speculation is that these dikes radiate from a large pluton at
depth under this region. Like other plutons of the Challis suite, is uniquely K-feldspar rich in
comparison to the Coast Range or Cascade Arc rocks. Sketch the outcrop to indicate the
distribution of the pegmatite dikes, which have been dated at about ~45 Ma.
Continue on 20 and cross the bridge over John Pierce Falls.
Stop 10. Alpine Roots
The Skagit region is broadly antiformal in structure, striking to the north. At this point we are in
the center of the antiform, and thus in the deepest part of the Coast Range Orogen here. The
rocks here are spectacular banded migmatites (rocks which have the appearance of being
partially melted). These are rocks which were subjected to temperatures of over 700oC and
pressures equivalent to burial at over 25 km. They were probably not actually melted, but were
deformed and metamorphosed extensively in the presence of high temperature / high pressure
fluids like water and carbon dioxide. These events took place at the height of the Coast Range
Orogeny in this area, probably about 75 million years ago. At that time, this region would have
been the base of the thick pile of igneous rocks accumulated above the Coast Range Arc.
The rocks here are a mix of orthogneiss (meta-igneous) and paragneiss (metasedimentary or
metavolcanic) types, but a mixing of components during metamorphism has produced all manner
of intermediate forms. Much of the rock has been recrystallized as a banded gneiss, where the
felsic and mafic components have become segregated into distinct layers. The most distinctive
features here are pods of metamorphosed ultramafic rock, typically tan to green in color. The
chemical reactions between the magnesium-rich ultramafic rock and the potassium, aluminum
and silicon-rich surrounding rocks produce silvery talc, green tremolite and actinolite, tan
phlogopite (a mica) and other diagnostic minerals. The presence of ultramafic rock suggests that
the Napeequa (Bridge River) Terrane is a major paragneiss component here. Tectonic slices of
ultramafic mantle rocks are common within the Napeequa Schist.
The rocks here are migmatites, which span the boundary between igneous and
metamorphic. They formed under very high temperatures (720°C) and pressures (depths of 30
km) at the base of the collisional zone. Describe their textures and explain why only part of
these rocks melted.
Return westward on 20 to the large mass wasting event just east of Newhalem.
Stop 11. Mass Wasting
This is the site of the Afternoon Creek rockslide, one of a series of events that occurred in
November and December of 2003. Examine and describe the repairs and remediation efforts in
this area.
Return to Seattle.
To Hope, B. C .
40mi
64km
Sk
ag
Rive
C o lu m
b ia
Wa s h in
g to n
it
Chilliwack
Lake
B ri ti s h
3
r
MOUNT BAKER
WIL D E R N E S S
To Glacier
(U S F S/N PS information)
4mi
6km
MAN NIN G
P R O VI N C IA L PA R K
Mount Redoubt
8957ft
2730m
542
Hozomeen
Redoubt
Glacier
Little
FOREST
RG
E
G orge Gorge
Lake
Dam
N E W H ALE M
Newhalem
Pyramid
Creek
ic
ct
Ar
Diablo Lake Ruby
Arm
Overlook
Diablo Ruby
Mountain
Colonial Lake
Creek
Ruby
Creek
F ourth of July Pass
3501ft
1067m
)(
ee
B oston
Peak Buckner Mountain
9080ft
G oode Mountain
2768m
9206ft
B ooker Mt.
2806m
Black
)(
Warrior Mine
Cascade
Cottonwood
Pass Trail
Rainy Pass
Lake
Ann
le
Rive
Pacific Crest
National
Scenic Trail
r
Dome Peak
8595ft
2679m
Mo
u
CHELAN
NRA
d
oa
R
Val
ley
kin
Shuttle-bus Rive
r
route
S tehe
Harlequin
ai
nt
n
p
oo
H ig
GLA CIER
PEAK
Devore
Creek
Trail
WILD E R N E S S
STE H E KIN
Stehekin Landing
North Cascades
Stehekin Lodge
Golden West
Visitor Center
C astle Rock
8137ft
2480m
hw a
y
W E N AT C H E E
La
ke
s
rr y
10 Miles
LU C ERN E
il
Tra
5
ad
FOREST
Fe
10 Kilometers
N AT I O N A L
re
ho
0
5
Railro
Creek
Glacier Peak
10528ft
3209m
0
War Creek
Pass
Moore Point
H O LD E N
N or t h
r
Old Stehekin School
Buckner
Purple Mountain
Place
Purple
Point
Company
Creek
Trail
L
ve
Rainbow Falls
Weaver Point
White C huck Mountain
6935ft
2114m
Ri
LA K E
C H ELA NSAWT O O TH
WILD ERN E SS
LAKE
Tumwater
High Bridge
E arly Winter
Spires
) ( C opper Pass
)(
Twisp
Pass
Twisp Pass
Trail
Dolly Varden
Suiatt
DARRIN G T O N
Creek
Bridge Creek
Shady
530
Rainy Pass
) ( 4860ft
1481m
Liberty
B ell Mt.
Rainy
7808ft
Lake
2380m
sp
Trapper
Lake
20
Washington
Pass Overlook
i
Tw
River
Bridge
Mount F ormidable
8324ft
2537m
r)
winte
k
Boston
Glacier
Sahale
Mt.
)(
To Mazama
8mi
13km
Methow Pass
in
Cr
FOREST
se d
r
Johannesburg
Mountain
C ascade Pass
5384ft
1641m
SN O Q UALMIE
N AT I O N A L
d
(Clo
ve
k
BAKER-
Roa
FOREST
y
F orbidden
Peak
Ri
Sau
MOUNT
ite
Eldorado Peak
8672ft
2703m
Eldorado
Glacier
N AT I O N A L
E asy Pass
6562ft
2000m
SOUTH UNIT
e
scad
Ca
River
R O C K P O RT
an
MAR BLE M O U NT
Rockport
State Park
OKANOGAN
Hw
Gr
NATIONAL PARK
B ackcountry Permits
H arts Pass
en i c
Marblemount
530
Crater Mountain
8127ft
2477m
Diablo
Dam
Peak
7182ft
2189m
FOREST
(A d ministere d by
Okanogan NF)
NORTH CASCADES
C O N C RETE
it
Skag
GO
SN O Q UALMIE
)(
IT
BAKER-
N AT I O N A L
Ross Lake
Overlook
Ross Dam
DIA BL O
20
WIL D E R N E S S
) ( Holman Pass
MOUNT
Jack
Mountain
9065ft
2763m
)(
er
Bak
N AT I O N A L
ROSS LAKE
NRA
Ross
Lake
Resort
Hopkins Pass
PA S AY T E N
Devils Dome
6982ft
2128m
Sc
des
ca
Cas
rth
No
ek
SN O Q UALMIE
G
KA
North
Cascades
Visitor
Center
Cr e
BAKER-
ek
S
Bacon
MOUNT
Gorge
Creek
Falls
Cr e
Damnation Peak
5643ft
1720m
Lake
Shannon
Lake
ell
od
U pper B aker Dam
Thornton
Lakes
k
Sourdough Mountain
6106ft
1861m
Go
Mount Triumph
7271ft
2216m
B acon Peak
7067ft
2154m
ee
McMillan Spire
Mount Terror
7151ft
2484m
Mount Despair
7293ft
2223m
Cr
C astle Pass
Pacific Crest
National Scenic
Trail ) ( Woody Pass
er
E
NORTH CASCADES NATIONAL PARK
NORTH UNIT
Lake
)(
ss
NG
Mount Prophet
7579ft
2310m
Beav
RA
Mount Fury
8291ft
2527m
)(
Ro
ET
g
CK
Mount B aker
10775ft
3248m
Bi
Mount C hallenger
8248ft
2512m
PI
Mount Shuksan
9131ft
2783m
MOUNT BAKER
WIL D E R N E S S
Creek
) ( B eaver Pass
3619ft
1103m
Desolation
Peak
6102ft
1860m
)(
Creek
Beaver
20
ES
Hozomeen Lake
Heather Meadows
Visitor Center
To Information Center
at Park Headquarters
and Sedro Woolley
23mi
37km
B aker Dam
CANAD
A
U N IT E
D S TAT
Hozomeen Mt.
8068ft
2459m
Hiking trail
Ranger station
Picnic area
Unpaved road
C ampground
Wheelchair
accessible
Lake
Chelan
To Fields Point
and Chelan via
passenger ferry