Tectonic evolution of the San Juan Islands thrust system, Washington
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The Geological Society of America
Field Guide 9
2007
Tectonic evolution of the San Juan Islands thrust system, Washington
E.H. Brown
B.A. Housen
E.R. Schermer
Department of Geology, Western Washington University, Bellingham, Washington 98225, USA
ABSTRACT
The mid-Cretaceous San Juan Islands–northwest Cascades thrust system is
made up of six or more nappes that are a few kilometers or less thick, up to one
hundred kilometers in breadth, and that were derived from previously accreted
Paleozoic and Mesozoic terranes. This field trip addresses many questions regard-
ing the tectonic evolution of this structural complex, including the homeland of the
terranes and the process of post-accretionary dispersal that brought them together,
how thrusting in the San Juan Islands might have been related to coeval orogenic
activity in the neighboring Coast Plutonic Complex, and the origin of blueschist
metamorphism in the thrust system relative to subduction and nappe emplacement.
The geology of this trip has many counterparts in other outboard regions of the
Cordillera, but some aspects of the tectonic processes, as we understand them to
date, seem to be unique.
Keywords: San Juan Islands, thrust faults, terranes, blueschist metamorphism, kine-
matic analysis, paleomagnetism.
INTRODUCTION San Juan Islands–northwest Cascades thrust system are poorly
known and have been the focus of our recent work.
Rocks and structures of the San Juan Islands of northwest Many aspects of the lithology, structure, and metamorphism
Washington record a long and complex history related to Cor- are similar to the Mesozoic evolution of other parts of the Cordil-
dilleran convergent margin tectonism. The area is underlain by lera; other aspects may be unique to the San Juan Islands. The east-
the San Juan Islands–northwest Cascades thrust system, made west transect across the San Juan Islands during this field trip will
up of nappes a few kilometers or less thick and up to 100 km in highlight the different terranes juxtaposed by the thrust system,
breadth (Figs. 1, 2), thrust onto the continental margin during and structures formed before, during and after high-pressure–
mid-Cretaceous time (e.g., Misch, 1966; Brown, 1987; Bran- low-temperature (HP-LT) metamorphism. The trip builds on ear-
don et al., 1988). The nappes have an oceanic history, indicat- lier work that identified the main terranes and structures in the
ing accretion to the edge of the North American continent, but San Juan thrust system (e.g., McClellan, 1927; Danner, 1966;
they also bear clear evidence of interaction with the continen- Vance, 1975; Whetten et al., 1978; Brandon et al., 1988). Our
tal margin long preceding their emplacement in Washington. recent results on structure, metamorphism, geochronology, and
Their mid-Cretaceous arrival in Washington as thrust sheets paleomagnetism will provide a forum for discussions that bear
was likely the consequence of some type of post-accretionary on the tectonic history and correlation with other Cordilleran
fragmentation and dispersal. The timing and mechanisms of the terranes. We will compare and contrast units from the external,
accretion, dispersal and final emplacement of terranes of the unmetamorphosed parts of the thrust system to the more internal
Brown, E.H., Housen, B.A., and Schermer, E.R., 2007, Tectonic evolution of the San Juan Islands thrust system, Washington, in Stelling, P., and Tucker, D.S., eds.,
Floods, Faults, and Fire: Geological Field Trips in Washington State and Southwest British Columbia: Geological Society of America Field Guide 9, p. 143–177,
doi: 10.1130/2007.fld009(08). For permission to copy, contact editing@geosociety.org. ©2007 The Geological Society of America. All rights reserved.
143144 Brown et al.
Figure 1. Regional setting of the San Juan Islands—northwest Cas-
cades area in the northwest Cordillera. AX—Alexandria; BA—Baker
CH terrane; CC—Cache Creek terrane of Miller (1987); CH—Chugach
AX
terrane; EK—Eastern Klamath terrane; FR—Franciscan complex;
GV—Gravina belt; GVS—Great Valley sequence; H—Huntington
56
terrane; IZ—Izee terrane; MT—Methow basin; QS—Quesnellia;
13 SC–FR—Straight Creek–Fraser River fault; SF—Shoo Fly complex;
5 ST—Stikinia; WA—Wallowa terrane; WJ—Western Jurassic belt;
GV ST
WR—Wrangellia; WTrPz—Western Triassic and Paleozoic belt;
AX YT YT—Yukon-Tanana terrane. Sources: Burchfiel et al. (1992a); Gehrels
128
and Kapp (1998); Wheeler and McFeely (1991). B.C.—British
56
AK Columbia; CA—California; cc—Cache Creek belt; Cz—Cenozoic
B.C rocks and surficial deposits; ID—Idaho; mc—McCloud belt of Miller
.
WR (1987); OR—Oregon; NV—Nevada; Wash.—Washington.
CC
cc QS
COA
units that experienced subduction and HP-LT metamorphism. In
particular, we would like to consider how the geology of the area
ST
ST
mc relates to various hypotheses regarding the origin and paleogeog-
PLU
raphy of the terranes, and the evolution of deformation before,
Ya
130 during, and after emplacement in their current location.
la
TON
co
50
m
MT
fa
SC-FR fault
ul
IC
t
TECTONIC SETTING
C
O
WR
M
P
LE QS The San Juan Islands–northwest Cascades thrust system lies
X mc
at the south end of the 1500 km long Coast Plutonic Complex,
a belt of continental arc plutons and metamorphic country rock
50
MT B.C. 116 that formed from Late Jurassic to Early Cenozoic (Figs. 1, 2).
Fig. 2 Q S Wash. Outboard of the Coast Plutonic Complex and intruded by it is
CP the Insular superterrane composed of the co-joined Wrangellia
C
and Alexander terranes. Inboard of the Coast Plutonic Complex
ID are rocks of the Early Cretaceous continental margin, including
Cz the Methow stratigraphic sequence in Washington. Detritus, cur-
rent indicators and stratigraphy in the Methow sequence indicate
Columbia absence of an outboard sediment source until ca. 110 Ma (Ten-
46 nyson and Cole, 1978; Haugerud et al., 2002), thus we view the
Embayment WA
Cz locale of the Washington Cascades and San Juan Islands as an
BA
cc
OR H
ocean basin until that time. Major orogenic activity characterizes
IZ
the region from ca. 110–80 Ma, during which nappes of the San
WJ Blue Mtns.
42 Juan Islands–northwest Cascades thrust system were emplaced,
125 cc the Coast Plutonic Complex was intruded by voluminous arc
200 kilometers
z
plutons, and country rock of the complex was locally buried to
P
Tr
W
cc Klamath depths of up to 35 km (in the “Cascade crystalline core”; Figs. 2
EK 42
mc
Mountains
116 and 3) and was deformed by orogen-normal and orogen-parallel
FR CA displacements. Overlapping the waning stages of this orogenic
NV
No. pulse was development of the Nanaimo stratigraphic sequence,
Sierra bearing detritus from the San Juan Islands–northwest Cascades
cc SF
thrust system as well as from the Coast Plutonic Complex, in
GVS
mc an elongate basin extending north from the San Juan Islands. In
Eocene time the orogen was cut obliquely and displaced ~170 km
(estimates range from 90 to 190 km; e.g., Vance, 1985; Misch,
1977) by the N-S dextral, strike-slip Straight Creek–Fraser River
fault system. Restoration of the fault shows the San Juan Islands–
northwest Cascades thrust system to have lain along the southern
margin of Wrangellia and the Coast Plutonic Complex (Fig. 3)123
CPC
Q
B.C. VC CH
WA HZ
NA NORTHWEST
WR
YA T
BP
ES 121
HS TB 49
A CZ
LM NK
Mt Baker
OC CO EA
Ros
GA CH TS YA A'
s Lak
FC EA
NK
EA
e fault
LS
CH
SAN JUAN LS HH CASCADES
ISLANDS
HH
EM
D
Me
DM
lan
F
ge B
Q EM
WM CRYSTALLINE
elts
N T
PUGET
88-96 Ma
SOUND WM CORE plutons
48
30 kilometers
T CW
. fault
Straight Ck. - Fraser R
TG
CN
Windy
CN Pass
Thrust
MS
T ING
EA
A
Figure 2 (on this and following page). San Juan Islands–northwest Cascades thrust system and surroundings. Based on compilation by Brown
and Dragovich (2003) and references therein. Abbreviations given in Table 1. (A) Map. B.C.—British Columbia; WA—Washington.146 Brown et al.
A A'
Mt Baker
Lummi Twin Sisters window
Orcas Island Shuksan Chilliwack
Island Range YA thrust batholith
GA VC
NA OC FC YA
FC TS
CO LM CN EA CH EA SL
EA BP CH
HS TB ES TB NK
ES OC BP CH BP
CH CC
NK
WRANGELLIA
depositional or intrusive contact 10 km Straight
B fault contact no vertical exaggeration Creek fault
Figure 2 (continued). (B) Cross section.
in Late Cretaceous time. South of this orogenic complex is the Q
UE
Columbia Embayment, an area covered primarily by Cenozoic SN
Mid-Late ELLI
A
volcanic rocks, thought to be underlain by primitive crust, and Cretaceous MT
B.C.
Plutons
considered in some models to be a possible homeland for the WA
Ro
thrust system nappes (e.g., Davis et al., 1978; Vance et al., 1980). Jur. - s s
HZ Lake
East and south of the Columbia Embayment are accreted terranes SE Fa
E. Cret. SE ult
of the Blue Mountains, and Klamath Mountains respectively N
Zo
ne
(Fig. 1), the latter especially bearing similarities to units of the
W G Plutons CW
San Juan Islands–northwest Cascades thrust system. RA eo
rg Cascade
NG ia HL
EL N S
tr Crystalline
LI ai B.C.
STRUCTURAL STRATIGRAPHY A NA t ING Core
WA NK EA
PRC EA WPT
N
The nappe pile of the San Juan Islands–northwest Cascades NWCS HB
PACIFIC NAPPES MN
thrust system (Fig. 2) is characterized by mid to late Paleozoic
OCEAN
terranes overlain by Mesozoic terranes. The structurally lowest
MEL
component of the nappe complex is the East Sound Group in the
ANGBELTS
N
San Juan Islands and correlative Chilliwack Group in the Cas- CZ CZ
E
cades. These are island arc derived sedimentary and volcanic rocks
of Devonian–Permian age (Danner, 1966; Vance, 1975; Misch, SC-FR fault
restored
1966; Tabor et al., 2003). Calc-alkaline Devonian plutonic rocks 100 KM
presumed to be related to this arc are the Turtleback and Yellow
Aster Complexes of the San Juan Islands and Cascades, respec- Figure 3. Regional geology shown with hypothetical restoration of the
tively (Mattinson, 1972; Whetten, et al., 1978; Brandon et al., Straight Creek–Fraser River fault. (SC-FR fault) based on ~170 km of
1988; Tabor et al., 2003). This assemblage is likely related to arc displacement (e.g., Umhoefer and Schiarizza, 1996). Abbreviations:
HB—Hicks Butte inlier, HL—Harrison Lake stratigraphic sequence,
rocks that extend from California to northern British Columbia MN—Manastash Ridge inlier, PRC—Pacific Rim Complex, SE—
and mark mid-late Paleozoic convergence along the continental Settler Schist. See Table 1 for other unit abbreviations.
margin (McCloud belt of Miller, 1987).
Higher in the nappe pile, in both the San Juan Islands and
Cascades, is a disrupted section including Permian to Juras- The highest nappes in the San Juan Islands–northwest Cas-
sic ribbon chert, Permian HP-LT schist, ocean island basalt, cades thrust system are Late Jurassic rocks that include ophio-
Permian limestone bearing Tethyan fusulinids (exotic to North litic plutonic rocks, mid-oceanic-ridge basalt, ribbon chert, and
America), and other materials (Fig. 2, Table 1). In the San Juan arc-derived mudstone-sandstone. Units of these upper nappes
Islands, units are Orcas Chert, Deadman Bay Volcanics, and Gar- that we will examine include rocks in the Lopez fault zone, the
rison Schist (Brandon et al., 1988), observed on this field trip. In Constitution Formation, Fidalgo Ophiolite and Easton Meta-
the Cascades, this zone is referred to as the Bell Pass Mélange morphic Suite. These units are closely similar to terranes in the
and in addition to the above mentioned rock types includes the western Jurassic belt, Franciscan Complex and Coast Range
10 × 4 km Twin Sisters dunite slab (Tabor et al., 2003). Rocks Ophiolite of the Klamath Mountains and California Coast
and structures of this zone are similar to the “Cache Creek belt” Range (e.g., Brown and Blake, 1987; Garver, 1988; Blake and
of Miller (1987) that extends sporadically from northern British Engebretson, 1994; J.S. Miller et al., 2003).
Columbia to California and apparently represents accretionary The nappe geometry portrayed in Figure 2B and described
mélange of mainly oceanic rocks. above interprets the Cascades and San Juan Islands nappe pilesTABLE 1. KEY TO UNITS
Thrust system units
EASTON METAMORPHIC SUITE (EA)—Late Jurassic ocean floor and trench deposits, well-recrystallized Early Cretaceous blueschist.
FIDALGO COMPLEX (FC)—Late Jurassic arc-related ophiolite, minimal fabric, incipient prehnite-pumpellyite metamorphism.
CONSTITUTION FORMATION (CO)—Late Jurassic trench deposits, minimal fabric, incipient blueschist metamorphism.
LUMMI FORMATION (LM)—Late Jurassic ocean floor and trench deposits, penetrative fabric, incipient blueschist metamorphism.
LOPEZ STRUCTURAL COMPLEX (LS)—Jurassic to Early Cretaceous ocean floor and trench deposits, incipient blueschist metamorphism.
TWIN SISTERS DUNITE (TS)—Mantle-derived ultramafic tectonite.
TURTLEBACK COMPLEX (TB) and correlative YELLOW ASTER COMPLEX (YA)—early to middle Paleozoic gabbro/tonalite, and paragneiss in YA, minimal
fabric, amphibolite, greenschist and prehnite-pumpellyite facies metamorphism.
GARRISON SCHIST (GA) and correlative VEDDER COMPLEX (VC)—ocean floor deposits, Permian epidote-amphibolite and blueschist metamorphism.
ORCAS CHERT including DEADMAN BAY VOLCANICS (OC) and correlative BELL PASS MELANGE (BP)—Triassic-Jurassic chert, lesser oceanic-island
basalt in OC and BP; exotic blocks of Early Cretaceous sandstone-argillite, Twin Sisters Dunite, Yellow Aster Complex, and Vedder Complex in BP; Garrison
Schist and limestone with Permian Tethyan fusulinids in OC.
EAST SOUND GROUP (ES) and correlative CHILLIWACK GROUP including Cultus Formation (CH)—Silurian to Jurassic island arc, McCloud fauna, minimal
fabric, incipient blueschist metamorphism.
NOOKSACK FORMATION (NK)—Jurassic to Early Cretaceous island arc possibly formed contiguous with Wrangellia. Slaty fabric, incipient prehnite-
pumpellyite metamorphism.
INGALLS TECTONIC COMPLEX (ING)—Early to Late Jurassic ocean floor and forearc or backarc–related ophiolite, prehnite-pumpellyite metamorphism and
thermal aureole. Occurs east of the Straight Creek–Fraser River fault, but is correlative with the higher nappes in the thrust system.
Mélange belts
HELENA-HAYSTACK MELANGE (HH)—serpentinite matrix, blocks of graywacke, mudstone, chert, basalt-rhyolite and 150–170 Ma gabbro-tonalite.
WESTERN MELANGE BELT (WM)—scaly argillite matrix, blocks are mostly Late Jurassic–earliest Cretaceous lithic sandstone/siltstone, some 150–160 Ma
gabbro-tonalite blocks.
EASTERN MELANGE BELT (EM)—mostly meta-chert and greenstone, Devonian-Jurassic fossils, 165–190 Ma tonalite-gabbro, Permian Tethyan fusulinids.
Footwall units to the San Juan Island thrust system
HARO FORMATION and SPIEDEN GROUP (HS)—Triassic to Early Cretaceous arc-derived sedimentary rocks, zeolite facies metamorphism.
WRANGELLIA (WR)—Paleozoic arc and Triassic ocean plateau complex, microcontinent, zeolite facies metamorphism.
Cascade crystalline core, part of the Coast Plutonic Complex
TONGA FORMATION (TG)—Early Cretaceous trench deposits and arc volcaniclastic rock, greenschist and amphibolite facies.
CHIWAUKUM SCHIST (CW)—Early Cretaceous accretionary complex, Barrovian amphibolite facies metamorphism.
Overlap units
NANAIMO GP. (NA)—Late Cretaceous epicontinental marine sedimentary rock, zeolite facies.
CHUCKANUT FORMATION and related units (CN)—Eocene fluviatile sedimentary rock, virtually unmetamorphosed.148 Brown et al.
to be approximately at the same level and laterally contiguous. AGE OF THRUSTING
This is based in part on correlations of terranes between the two
regions (as shown in Fig. 2 and Table 1). The structural model The age of assembly of the nappes is uncertain because
also assumes a simple in-sequence assembly of the nappe pile. observed structures could potentially have formed during one
Because the stratigraphy is not exposed under the broad nappe of of many tectonic events, including initial accretion going back
Easton Suite between the Cascades and San Juan Islands, out-of- to the Paleozoic for the older terranes, post-accretionary ter-
sequence thrust models relating nappes in these two areas could be rane translation of at least hundreds of kilometers, emplace-
viable. Cowan and Bruhn (1992) proposed that Cascades nappes ment of nappes into the regional geologic setting of north-
lie at a higher structural level than those in the San Juan Islands. west Washington, and deformation related to the Eocene and
McGroder (1991) favors a break in continuity of nappes in the younger fold and thrust belt affecting the Nanaimo Group
hidden zone between the Cascades and San Juan Islands caused and Chuckanut Formation (England and Calon, 1991). Cer-
by out-of-sequence thrusting and folding of the nappe pile. tainly some metamorphic fabric and possibly some fault
Peripheral to the San Juan Islands nappe pile along its boundaries are inherited from events pre-dating assembly of
northwest flank are the arc-derived Late Triassic Haro Forma- nappes in their present setting (Brown et al., 2005). However,
tion, the Late Jurassic–Early Cretaceous Spieden Group, and the there is good evidence for major mid-Cretaceous assembly.
Late Cretaceous Nanaimo Group bearing detritus from the thrust This deformation is referred here to as the thrusting event.
system. These units lack evidence of HP-LT metamorphism and Nappes of the thrust system were emplaced and unroofed in
penetrative tectonite fabric, and thus are considered to be “exter- the San Juan Islands vicinity by the time of deposition of the
nal” to the thrust system (Brandon et al., 1988). Owing to the dif- Nanaimo Group (Vance, 1975); the oldest part of the Nanaimo
ferent tectonic and metamorphic histories, a fault is assumed to known to bear detritus from the thrust system is ca. 85 Ma
separate the nappe pile from the external units. This fault, named (latest Campanian-earliest Santonian; Brandon et al., 1988). A
the Haro fault, cannot be directly observed, but is inferred to dip maximum age for thrusting in the San Juan Islands is given by
under the nappe pile based on regional dips and a gravity survey fault juxtaposition of Late Aptian (112–115 Ma) fossiliferous
(Johnson et al., 1986; Palumbo and Brandon, 1990). The Haro rock with 124 Ma HP-LT metamorphic rock on Lopez Island
fault may have been reactivated during south-vergent thrusting in (one of our field trip stops). In the Cascades, a population of
the Cowichan fold and thrust belt (England and Calon, 1991). detrital zircons in the Nooksack Formation (footwall to the
The ultimate footwall to nappes of the San Juan Islands– nappes) gives a maximum depositional age of 114 Ma, and a
northwest Cascades thrust system is problematic in the San Juan large sandstone raft in the Bell Pass Mélange bears 119 Ma
Islands, but clearer in the Cascades. Based on arguments given detrital zircons (Brown and Gehrels, 2007).
above that external units underlie the San Juan Island nappes and More precise ages of thrusting are known for two localities:
observation that Wrangellia underlies Nanaimo Group units on K-Ar whole rock ages of 87 and 93 ± 3 Ma were obtained for two
Vancouver Island, one could infer that Wrangellia is basement mylonite samples from the west flank of the Twin Sisters Dunite
to the San Juan Island nappes (e.g., Cowan and Bruhn, 1992). (Armstrong in Brown, 1987). Movement on the Windy Pass
In the Cascades, evidence indicates that nappes are thrust over thrust is dated at ca. 94 Ma by relationships with U-Pb zircon-
the southern end of the Coast Plutonic Complex. In the central dated plutons that predate, postdate and are involved in thrusting
Cascades, the Ingalls Complex, a component of the San Juan (R.B. Miller et al., 2003). Thus, major displacement is broadly
Islands–northwest Cascades thrust system, is thrust over Chi- bracketed between ca. 115 and 85 Ma based on youngest terranes
waukum Schist and Mount Stuart batholith along the Windy involved and the age of rocks bearing detritus of the nappes, and
Pass Thrust (Figs. 2, 3; Miller, 1985). In the northwest Cascades, a more limited time frame is suggested to be ca. 90–95 Ma from
the relatively undeformed Jurassic-Cretaceous Nooksack Group dated rocks in two fault zones.
which underlies the nappe pile (e.g., Misch, 1966) appears to be a
southern extension of the Harrison Lake stratigraphic sequence in METAMORPHISM
the southern British Columbia Coast Plutonic Complex (Fig. 3;
Monger and Journeay, 1994). Along its western flank, the Coast Most units in the San Juan Islands–northwest Cascades
Plutonic Complex is intrusive into Wrangellia. Thus, one inter- nappe pile show effects of Cretaceous HP-LT metamorphism.
pretation for the regional structure is that Wrangellia and the The degree of recrystallization and metamorphic fabric devel-
Coast Plutonic Complex constituted a structural block in mid- opment varies greatly, even within the same units. In the Cas-
Cretaceous time that served as footwall to the San Juan Islands– cades, evidence of HP-LT metamorphism is found virtually
northwest Cascades thrust system in both the San Juan Islands in all thrust system units of Jurassic or older age. The blue-
and Cascades (e.g., Brown, 1987; McGroder, 1991; Monger and schist facies Easton Metamorphic Suite in the Cascades bears
Brown, 2008). Other interpretations place nappes of the San Juan synkinematic metamorphic minerals dated at 120–130 Ma by
Islands–northwest Cascades thrust system within, and as part of, K-Ar and Rb-Sr (Brown et al., 1982; Armstrong and Misch,
the country rock of the Coast Plutonic Complex (Monger and 1987). Rock units younger than 120 Ma (Nooksack Forma-
Journeay, 1994; Cowan and Brandon, 1994). tion and sandstone in the Bell Pass Mélange) lack definitiveTectonic evolution of the San Juan Islands thrust system, Washington 149
evidence of high-pressure metamorphism. In the San Juan TECTONIC EVOLUTION
Islands, aragonite (Fig. 4) and lawsonite are widely devel-
oped in Jurassic and older rocks that are otherwise relatively A number of features and arguments point to primary accre-
unaltered (Vance, 1968; Glassley et al., 1976). This incipient tion and residence of terranes of the San Juan Islands–northwest
HP-LT metamorphism has been considered to be related to Cascades thrust system along the continental margin prior to
mid-Cretaceous thrusting (Brandon et al., 1988; Maekawa mid-Cretaceous assembly in the present nappe pile. As Brandon
and Brown, 1991) but so far the only isotopic ages available, et al. (1988) note, the presence of detritus in sandstones from
Ar-Ar muscovite, indicate metamorphism at 124 Ma (Brown diverse sources, including metamorphic rock, chert, and silicic
et al., 2005) and ca. 137–154 Ma (Lamb, 2000), older than the arc volcanic rock (e.g., Constitution Formation) suggests prox-
emplacement phase of thrusting. imity to a “continent-like” landmass. They also note that else-
The age of blueschist metamorphism relative to thrusting where in the Cordillera correlatives of Paleozoic terranes of the
is critical to understanding the tectonics of the thrust system. San Juan Islands–northwest Cascades thrust system (e.g., East
If aragonite was formed during thrusting, burial on the order Sound Group) accreted long before the mid-Cretaceous. Addi-
of 20 km is required at the ~200 °C temperature estimated for tional arguments and evidence are provided by the: (1) the Yellow
metamorphism (Brandon et al., 1988), indicating a great thick- Aster Complex (Figs. 2 and 6; Table 1), a pre-Devonian terrane
ness of overlying nappes. An alternative concept that blueschist with links to the continent indicated by beds of quartz arenite
metamorphism in the thrust system is inherited from an event and a suite of detrital zircons that match those of the miogeo-
predating nappe emplacement may be possible for the older cline (Brown and Gehrels, 2007); and (2) Permian blueschist
terranes. However, Schermer et al. (2007) showed that HP-LT metamorphism in some units (Garrison Schist, Vedder Complex;
metamorphism lasted during several phases of brittle deforma- Armstrong et al., 1983), indicating that these rocks were involved
tion that followed juxtaposition of the internal San Juan Island in convergent margin tectonics long before thrusting in the San
nappes, including the late Aptian Richardson rocks. If all of Juan Islands–northwest Cascades system.
the HP-LT metamorphism in the San Juan Islands is related to Although terranes of the thrust system are similar to
the same subduction zone, the time span of deformation and other outboard units of the Cordillera, especially those in the
metamorphism in that subduction zone could be several tens Klamath Mountains with which they have been correlated
of millions of years (at least from 124 Ma to some time after (see below), some aspects of the thrust system are unique.
112 Ma, but likely beginning earlier). The subduction zone The stacking sequence of the San Juan Islands–northwest
model requires emplacement in the San Juan Islands vicinity Cascades thrust system is older on the bottom, younger on
after HP-LT conditions ended, and on structures that are not top, approximately reversed from that generally understood
exposed in the internal nappe pile (Schermer et al., 2007). Fig- for primary accretion, as in the Klamath Mountains where the
ure 5 summarizes various interpretations of the age of meta- oldest rocks are on top (Irwin, 1981). The duration of assem-
morphism relative to deformation. bly of the terranes is a few tens of millions of years at most,
1.0 mm
A B
Figure 4. Aragonite in the San Juan Islands. (A) Coarse aragonite from marble in the Orcas Chert unit, McGraw-Kittinger quarry, Orcas Island
(Vance, 1977, p. 194). The sample shown is a single crystal exhibiting twin lamellae on a cleavage surface that extends across the entire speci-
men. (B) Aragonite veins crossing foliation in the Constitution Formation, South Beach, San Juan Island.150 Brown et al.
SW directed
thrusting NW directed
Brown, 1987 thrusting
87-90 Ma
NW Cascades HP-LT 120-130 Ma
San Juan Is. NW directed
thrusting
Maekawa & HP-LT
Brown, 1991 penetrative cleavage
SW directed
thrusting HP-LT
Cowan &
Brandon, 1994
local penetrative
cataclasis cleavage
D1 D2
SW-NE NW
contraction thrusting
Bergh 2002 NW-SE
strike-slip
penetrative cleavage
HP-LT
penetrative
Brown et al., cleavage thrusting
2005
HP-LT 124 Ma
SW-NE contraction
NW-SE extension emplacement
Schermer in SJI
et al, 2007
penetrative
cleavage veins & brittle
faulting
HP-LT
remagnetization
Burmester
et al. 2000 remagnetization in the eastern SJI sometime during K normal chron
rotation of SJI rocks after remagnetization
140 130 120 110 100 90 80
Ma
Figure 5. Interpreted sequence of deformational and metamorphic events in the San Juan Islands
(SJI) thrust system presented in different reports. Absolute time of events is for the most part only
loosely constrained in the reports referenced here. HP-LT—high-pressure–low-temperature.
much briefer than the ~300 m.y. period of accretion that built all the Paleozoic rocks. We are not aware of anywhere else
the Klamath complex (Irwin, 1981). Cretaceous blueschist along the Cordillera that Paleozoic rocks are affected by Cre-
metamorphism in the San Juan Islands–northwest Cascades taceous blueschist metamorphism. Thus, the building process
thrust system affects not only Jurassic-Cretaceous Franciscan of the San Juan Island nappe pile is different than that under-
type rocks as in the Klamath Mountains, but also apparently stood for other parts of the Cordilleran margin.Cretaceous Rocks
Jurassic Rocks 30 177 Ma
fossil age
SPIEDEN GROUP
25
155 Ma Sentinal Island Fm.
Number
70 20
60
EASTON SUITE
Number
15
50
40 10
30 5
224 Ma
20 0
80 100 120 140 160 180 200 220 240 260 280 300
10 238 Ma
0 119
80 120 160 200 240 280 sandstone in
25
148 Ma BELL PASS
60 165
20 MELANGE
Number
50 FIDALGO COMPLEX
Number
15
40
143
30 10
233
20 5
10 237Ma 0
80 100 120 140 160 180 200 220 240 260 280 300
0
80 120 160 200 240 280 3 ss in BPM
70 1
148 Ma
60 200 600 1000 1400 1800 2200 2600
50 LUMMI FORMATION
Number
152 Ma
40 125
30
30 TONGA
FORMATION
Number
20
20
10
0
80 120 160 200 240 280 10
149 Ma
40 0
80 100 120 140 160 180 200 220 240 260 280 300
CONSTITUTION
Number
30 FORMATION
80 153 Ma
70
20
60 NOOKSACK
GROUP
Number
10 50
40
0
80 120 160 240 30
200 280
Ma 20 114 Ma
10
0
80 100 120 140 160 180 200 220 240 260 280 300
Early Paleozoic Rock Ma
25 1825
YELLOW ASTER
COMPLEX
20
15
number
2069 2321
10
2528
1404
5 3316
960
0
800 1200 1600 2000 2400 2800 3200
Ma
Figure 6. Detrital zircon age distributions in terranes of the San Juan Islands–northwest Cascades thrust system (Spieden
Group from Housen and Fanning, unpublished; other units from Brown and Gehrels, 2007).152 Brown et al.
Notwithstanding the important contributions of many previ- Islands, respectively, that they interpreted to indicate northwest-
ous studies of the San Juan Islands–northwest Cascades thrust directed thrusting. Brandon et al. (1993) disputed this conclu-
system, the homeland of the nappes and the tectonic process of sion for the San Juan Islands, suggesting that lineations mapped
their transport and emplacement remain unresolved issues. Three by Maekawa and Brown (1991) are the product of differential
published interpretations (Fig. 7) are: solution-mass-transfer, not thrusting. Cowan and Brandon (1994)
(1) An orogen-normal contractional model in which the nappes described folds and Riedel shears in the Lopez and Rosario fault
formed as continental borderland terranes that were caught zones that they interpret to indicate southwest transport of the
in a collision zone between the offshore Wrangellian micro- nappes (orogen-normal). In the eastern San Juan Islands, Lamb
continent and North America (Brandon and Cowan, 1985; (2000) reported northeast vergent (orogen-normal) isoclinal folds
Brandon et al., 1988; Rubin et al., 1990; McGroder, 1991; dated by synkinematic mica at ca. 137–154 Ma (see above) in
Burchfiel et al., 1992b; Cowan and Brandon, 1994; Monger rocks inferred to be related to the Easton Suite. Bergh (2002)
and Journeay, 1994). observed folds, stretching lineations, and shear zones in the
(2) A transcurrent-transpressional model in which the nappe ter- Lopez and Rosario fault zones supporting both orogen-normal
ranes originally accreted or were deposited south (or north?) and orogen-parallel displacement and conceived the two-stage
along the margin from their present location and then moved model described above and shown in Figures 5 and 7. Burmester
coastwise, finally stacking up in a reentrant of the continen- et al. (2000) found that many of the rocks in question have been
tal margin formed by the south end of Wrangellia (Brown, reoriented after acquiring their magnetization, which developed
1987; Maekawa and Brown, 1991; Brown and Dragovich, during or after the fabric was formed; therefore they suggested
2003; Monger and Brown, 2008). that the orientation of the fabrics cannot be used to determine
(3) A two-phase model in which terranes were first juxtaposed direction of transport in the present frame of reference. Brown
by orogen-normal thrusting along the continental margin et al. (2005) determined that fabric in blueschist tectonite of the
south of Wrangellia, and then underwent orogen-parallel Lopez fault zone predates thrusting and they suggested that much
thrusting and strike-slip faulting (Bergh, 2002). of the kinematic analysis in the San Juan Islands has been carried
Resolution of the emplacement history of the San Juan out on similar pre-thrust fabric and therefore may not be use-
Islands–northwest Cascades thrust system is central to our ful in understanding emplacement of the nappes. Gillaspy (2004)
understanding of mid-Cretaceous orogeny in the Pacific North- and Schermer et al. (2007) found that faults and extension veins
west, including: the cause of crustal thickening and Barrovian indicate a protracted period of orogen-normal shortening coupled
metamorphism in the crystalline core, the origin of the Nanaimo with orogen-parallel extension during aragonite metamorphism
basin, and the configuration of terranes along the North American that postdates thrusting, juxtaposition of the terranes, and pene-
margin in the Early Cretaceous. On a broader regional scale, the trative fabric formation. The different interpretations are summa-
San Juan Islands–northwest Cascades thrust system is relevant rized in Figure 5. To more effectively make use of these structural
to understanding evolution of the 1500-km-long Coast Plutonic observations, the challenge for future workers is to understand
Complex which extends from northwest Washington to Alaska. the age of outcrop-scale structures relative to the age of emplace-
Based on their interpretation as orogen-normal contractional ment of the nappes.
features, thrusts of the San Juan Islands and northwest Cascades
have been correlated with thrusts in northern British Columbia Regional Considerations
and Alaska and cited as evidence for a west-vergent thrust sys-
tem that extends virtually the entire length of the Coast Plutonic Another strategy for establishing nappe displacements is
Complex and has accommodated many hundreds of kilometers consideration of regional geology. Because units of the San
of mid-Cretaceous shortening between the Insular superterrane Juan Islands–northwest Cascades thrust system bear evidence
and North America (Rubin et al., 1990). of residence along the continental margin prior to emplacement
in the present day setting, direct accretion of these rocks from
Kinematics of Outcrop Scale Structures the west, the Pacific basin, seems improbable. Derivation of
the nappes from the northeast is envisaged in the contractional
One approach to understanding displacement of nappes in model of Brandon and Cowan (1985) and McGroder (1991)
the San Juan Islands–northwest Cascades thrust system is kine- which invokes a root zone for the nappes along the northeastern
matic analysis of outcrop scale structures. Such studies to date edge of the Cascade core in the approximate area of the Ross
yield somewhat disparate results (Fig. 5). Brown (1987), working Lake fault zone (Figs. 2 and 3). In this view, during Wrangellia
in the Cascades, reported a set of orogen-normal stretching linea- collision the nappes were driven to the southwest, riding over
tions in the Easton Suite coeval with 120–130 Ma blueschist min- the Cascade core and the northeastern flank of Wrangellia.
erals (see above). Younger orogen-parallel lineations were found Regional geologic features cited as supportive of this model are:
in mylonite zones separating Cascades nappes (ca. 90 Ma, see coeval crustal thickening in the Cascade core suggesting thrust
above). Smith (1988), and Maekawa and Brown (1991) mapped loading, contractional structures in the Cascade core, and inter-
orogen-parallel stretching lineations in the Cascades and San Juan pretation that the Nanaimo Group was deposited in a forelandMcGroder, 1991
Late Jurassic
98 Ma
ST WR CPC
MT QS
WR CC SK
NWCS CORE
QS terranes
94 Ma A
NWCS
CH MT
NK MT
SK QS
CPC
NWCS
terranes
Brown, 1987 90-95 Ma
Early Cretaceous MT
Plutons CORE
WR CPC QS WR
NK
NWCS
B
?
F
NWCS
F
100 km
E. to mid-Cretaceous Late Cretaceous
CPC Bergh, 2002
WR CPC
WR
F
C
area of
EA San Juan
EA
Islands
F
Lopez
fault Lopez
zone Rosario fault
Rosario fault zone
100 km fault zone
zone
Figure 7. Schematic drawings of three published models for tectonic evolution of the San Juan Islands–northwest Cascades thrust system
(NWCS). (A) Contractional model of McGroder (1991). Terranes of the thrust system were formed in a basin between Wrangellia and the con-
tinental margin. Convergence between these masses thrust the intervening terranes as nappes over the Cascade crystalline core (including the
Skagit migmatite complex) and onto the eastern edge of Wrangellia, achieving orogen-normal shortening of some 400-500 km. (B) Transcurrent
model of Brown (1987). Terranes of the San Juan Islands–northwest Cascades thrust system are interpreted to have accreted 100s of km south
of their present site and south of Wrangellia. Blueschist metamorphism and orogen-normal fabrics were recorded in the Easton Suite. Post-
accretionary displacement moved the terranes northward along the coast as a fore arc sliver, driven by dextral-oblique Farallon–North America
convergence, until they collided with a reentrant in the continental margin formed by the south end of Wrangellia. (C) Two-phase model of Bergh
(2002). Terranes of the San Juan Islands–northwest Cascades thrust system lay south of Wrangellia and developed orogen-normal contractional
structures during the D1 phase in response to high-angle Farallon–North America convergence. D2 structures include NW and SE coastwise
displacements as low-angle wedge extrusions caused by sinistral-oblique Farallon convergence. CPC— Coast Plutonic Complex; F—Farallon
plate. Other abbreviations as in Fig. 1 and Table 1.154 Brown et al.
basin caused by emplacement of San Juan Islands–northwest rocks had all been remagnetized during or after folding, and that
Cascades nappes. However, several aspects of regional geology the predominantly normal polarity of the remagnetized directions
pose problems for this interpretation. indicated to them that this remagnetization occurred during the
(1) The contractional model invokes transit of nappes of the Cretaceous Long-Normal Chron (116–83.5 Ma). The remagne-
San Juan Islands–northwest Cascades thrust system over tized directions from the San Juan Islands are scattered, how-
the Cascade crystalline core (Fig. 3) at precisely the time ever, indicating that a significant amount of rotation and/or tilt
of great magmatic arc activity in that region. No rocks occurred after this remagnetization event.
related to this arc activity are found in the San Juan Islands Paleomagnetic studies of the unmetamorphosed “external”
or Cascades, except where nappes lap onto the southern units of the San Juan Islands have more promising results. The
edge of the Cascade core in the vicinity of the Windy Pass exception is the Haro Formation; Hults and Housen (2000) have
thrust (Figs. 2 and 3). found that these rocks were also remagnetized prior to folding,
(2) Nappes of the San Juan Islands–northwest Cascades thrust despite their lack of any significant metamorphism.
system carry metamorphic aragonite acquired prior to (as The rocks of the Spieden Group have complex magnetizations,
well as after) thrusting. Aragonite has been shown experi- with the majority of these clastic rocks having poorly resolved
mentally to invert quickly to calcite outside its stability field magnetizations. Dean (2002) found three magnetic components in
at elevated temperature except under conditions of abnor- most of the Late Jurassic Spieden Bluff Formation samples, which
mally low T/P, less than 10 °C/km (Carlson and Rosenfeld, yielded an inconclusive paleomagnetic fold test. The Early Cre-
1981). Transit of the thrust system nappes over the active taceous Sentinel Island Formation has a simpler, two-component
arc would place them in a region of abnormally high T/P, magnetization in some of the rocks. Dean (2002) found that the
precluding preservation of aragonite. second-removed component from the Sentinel Island Forma-
(3) The elongate, orogen-parallel Nanaimo basin is flanked not tion passes the inclination-only paleomagnetic fold test, with the
by terranes of the San Juan Islands–northwest Cascades best-clustered inclinations occurring at 100% untilting. The mean
thrust system, but by plutonic rocks of the Coast Mountains. inclination of 64°, α95 = 7.8°, suggests an Early Cretaceous paleo-
Thrust system terranes occur south along strike from the latitude of 46° N. Comparing this direction with that expected for
Nanaimo (Figs. 2 and 3), and thus the basin is not likely a the present-day location of Spieden Island calculated from a stable
consequence of nappe loading. North America reference pole (Housen et al., 2003), a latitudinal
Many workers have envisaged a southerly origin of some translation of 1500 ± 1000 km is estimated for these rocks.
or all of the terranes of the San Juan Islands–northwest Cas- The Nanaimo Group has been the subject of extensive paleo-
cades thrust system, in the Columbia embayment, Klamath magnetic study, primarily from outcrops in the Canadian Gulf
Mountains, or California Coast Range (e.g., Davis et al., 1978; Islands (Ward et al., 1997; Enkin et al., 2001; Kim and Kodama,
Vance et al., 1980; Brown and Blake, 1987; Garver, 1988; Burch- 2004), with limited work from Orcas Island (Housen et al.,
fiel et al., 1992b). Davis et al. (1978) and Vance et al. (1980) 1998). All of these studies have found that most Nanaimo Group
proposed that the Mesozoic ophiolitic terranes of the San Juan rocks have poorly defined magnetizations (~60% “failure rate”
Islands–northwest Cascades system formed in a “pull-apart gap” reported for most sample collections). However, a significant
in southeastern Oregon and subsequently moved northward and number of samples in all of these studies (a few 100 out of ~1000
were obducted onto the continent. Geologic features cited in sup- samples collected) have well-defined magnetizations that pass a
port of the model are: (1) thrust emplacement of the Ingalls ophio- reversals or fold test. Studies of inclination error, notably Kim
lite over the south edge of the Cascade core, (2) absence from and Kodama (2004), suggest that inclination error in these sedi-
eastern Oregon and western Idaho of some continental margin ments is moderate (8–10°), and that when corrected for the paleo-
terranes that are part of the Mesozoic assemblage to the north magnetic inclinations in these rocks place the Nanaimo Basin at
and south along the Cordillera, and (3) Sr isotope ratios and seis- a paleolatitude of 41° N during Campanian-Maastrichtian time.
mic velocities indicating primitive crust underlying the Columbia Using a Late Cretaceous North American reference pole for
embayment. More recent geophysical evidence for a deep crustal comparison, a translation of 1600 ± 900 km is indicated for these
rift in the Columbia embayment is a linear break in the gravity rocks since ca. 75 Ma.
field running along the southern margin of the embayment (Riddi- Related constraints on the Late Cretaceous paleogeography
hough et al., 1986). of the San Juan Islands also come from paleofaunal data from
the Nanaimo Group rocks. Kodama and Ward (2001) argued
Paleomagnetic and Other Constraints of Paleogeography that the lack of rudistid bivalves in the otherwise well-preserved
paleofauna of the Nanaimo Group can be used to constrain the
Paleomagnetic studies of the rocks in the San Juan Islands paleolatitude of these rocks. Rudistids are tropical to subtropi-
have had mixed success in constraining their tectonic history, cal reef forming bivalves, and are common in a number of Late
with the main complication being an extensive remagnetization Cretaceous marginal basin rocks from Baja California to Central
that has affected all of the “internal” units that have experienced California. Using estimated locations of rudist-bearing basins,
high P-T metamorphism. Burmester et al. (2000) found that these and the locations of anoxic black shales (Marca Shale) that markTectonic evolution of the San Juan Islands thrust system, Washington 155
the presence of a cold-water upwelling zone along the ancient
California margin, Kodama and Ward (2001) suggested that the
0
Nanaimo Group rocks were located at or north of the location
14
of the Moreno Basin (central California, 42° N reconstructed
paleolatitude) at 75 Ma. Some additional support for this con- Ale utian -
KA
D
W
AS
straint comes from the recognition of a marine reptile fauna from
F
ra
AL
Nanaimo Group rocks on Vancouver Island, which share some
ng
provinciality with the marine reptile fauna of the Moreno Forma-
ell
tion from central California (Nicholls and Meckert, 2002).
Another set of data, detrital zircon age distributions, has also
been used to test paleogeographic constraints on the location of
the Nanaimo Group rocks. Mahoney et al. (1999) used the pres-
ence of several Archean-aged zircons to indicate that the Nanaimo
Group rocks had been located no more than 500 km south of its P-MF
present-day location, during Late Cretaceous time. Using the same
set of data, Housen and Beck (1999) compared variations in the 50 mm/yr
detrital zircon age distributions as a function of stratigraphic posi-
tion within the Nanaimo Group. They argued that variations in Yakutat
Proterozoic-aged zircons support a source of detritial zircons from terrane,
the Mazatzal and Yavapai orogens in southwest North America, and transform
that northward migration of the Nanaimo Basin during its deposi-
tion was consistent with other paleomagnetic evidence, and plate displacement 60
North
motion estimates. The analyses of Kodama and Ward (2001), and
F-QCF
Kim and Kodama (2004) also supported the conclusion of Housen
America
and Beck (1999), that the Nanaimo Group reached the “moder- plate
ate” paleolatitude of ~43° N at 75 Ma, consistent with the so-called
“Baja-BC” (Baja–British Columbia) hypothesis. Pacific
Taken together, these paleogeographic data would be most plate
consistent with the “Klamath origin” models discussed above.
Complicating this correlation, however, are the proposed ties
between the San Juan Islands rocks and Wrangellian or North
0
12
Cascades basement, as abundant paleomagnetic data from strati-
fied rocks of Wrangellia/Insular affinity (Wynne et al., 1995,
Enkin et al., 2003), or barometrically corrected plutonic rocks
(Housen et al., 2003) both indicate more southerly paleolatitudes Casc
(36 N, and 3000 ± 700 km of translation) for these units during 300 KM
mid-Cretaceous time (93–88 Ma).
ade arc
LRF
Modern Analogues? Juan CAN 50
de Fuca U.S .
plate .
Modern tectonic regimes along the western North American 8 mm/yr
margin (Fig. 8) that serve as possible analogues for emplacement
of the San Juan Islands–northwest Cascades thrust system via
coastwise movement are collision zones formed by northward
displacement of: (1) Siletzia against the south end of Wrangellia Siletzia,
(e.g., Wells et al., 1998), and (2) the Yakutat terrane against the
fore-arc displacement
southeast corner of Alaska in the Saint Elias orogen (Plafker et al.,
1994). Siletzia lies in the Cascade forearc, driven by a combina- Figure 8. Modern-day analogues of orogen-parallel thrusting in the
tion of oblique plate convergence and Basin and Range extension Pacific Northwest. F-QCF—Fairweather-Queen Charlotte fault,
(Wells et al., 1998). Seismic reflection allows identification of DF—Denali fault, LRF—Leech River fault. References: Plafker et al.
(1994), Wells et al. (1998); Bruhn et al. (2004).
Siletzian rocks under Wrangellia to depths of 15–20 km along
shallow to moderately north-dipping faults (Clowes et al., 1987).
Total northward displacement is not known, but Beck (1984)
suggested paleomagnetic discordance indicates as much as 300–156 Brown et al.
400 km. The current rate of arc-parallel transport is 6–8 mm/yr at ever, caution must be exercised to avoid a nasty fall on the slick
the northern end of the terrane (Wells and Simpson, 2001). seaweed-covered rocks that may be present. Please pay attention to
The Yakutat terrane is moving north along the Fairweather– the field trip guides as the departure time draws near, to ensure you
Queen Charlotte transform fault at 45–50 mm/yr relative to are on the vessel, and the trip can run in a safe and timely fashion.
North America (Plafker et al., 1994; Bruhn et al., 2004). At the After we have finished the Stuart Island stop, participants will re-
corner area in southern Alaska where plate interaction changes embark for a ~45 min trip to Spieden Island.
from transform to convergent, the Yakutat terrane is colliding
with the continent (Fig. 8). A north-dipping Benioff zone and the Stop 1-1. Fossil Cove, Stuart Island, Nanaimo Group
Wrangell magmatic arc in this region both testify to significant (Fig. 10)
subduction of the Yakutat terrane (and probably other materials).
The convergent zone is marked by a thin-skinned accretionary The Nanaimo Group comprises a set of 11 formations, ranging
complex of Cretaceous and younger rocks displaced northward from Turonian to Maastrichtian in age, composed of clastic marine
on gently to moderately dipping thrust faults (Bruhn et al., 2004). and deltaic sedimentary deposits (Fig. 10). The ages of these rocks
Displacements are strongly partitioned between strike-slip faults are constrained by biostratigraphy (e.g., Haggart, 1994), and mag-
and thrusts. Both analogues are characterized by low-dip thrusts netostratigraphy (Enkin et al., 2001). These rocks were deposited
accommodating margin-parallel displacement indicating that in a large marginal basin, extending ~175 km from its southern-
such structure, as possibly fits the San Juan Islands–northwest most extent in the San Juan Islands to its northernmost extent on
Cascades thrust system, is not a tectonic anomaly. Vancouver Island. The Nanaimo Group contains several elements
that are of tectonic interest. Structurally, the Nanaimo Group
FIELD TRIP GUIDE rocks (along with the Paleocene-Eocene Chuckanut Formation)
are folded as part of the Cowichan fold and thrust belt (England
The field trip guide begins at Friday Harbor, San Juan Island and Calon, 1991; see also Mustoe et al., this volume, and Blake
(Fig. 9). Before departing, be certain that you have brought along and Engebretson, this volume). One of the primary constraints on
warm clothes, raingear, and good field boots. the age of uplift and thrusting of the metamorphosed “interior”
Please do not use rock hammers or collect specimens any- domain of the San Juan Islands is the presence of metamorphosed
where on this trip unless specifically advised. sandstone clasts interpreted as being derived from the Constitution
Formation that are found in conglomerates of the Extension Forma-
DAY 1 tion of the Nanaimo Group on Orcas and Stuart Islands (Brandon
et al., 1988). On a larger scale, age distributions of detrital zircons
Day 1 is spent primarily on the terranes “external” to the San (Housen and Beck, 1999; Mahoney et al., 1999), paleomagnetism
Juan Islands thrust system. These units are the Haro Formation, (Ward et al., 1997; Housen et al., 1998; Enkin et al., 2001; Kim and
Spieden Group, and Nanaimo Group. They broadly overlap in age Kodama, 2004), and fossil assemblages (Kodama and Ward, 2001)
with rocks in the nappe pile but are distinguished by their absence have been used to evaluate possible large-scale displacements of
of, or very low-grade (zeolite facies), metamorphism, and, in the the Nanaimo Group rocks.
case of the Spieden and Nanaimo Groups, an absence of penetra- On Stuart Island, the turbidites and sandstones of the Haslam
tive tectonite fabric. These units are important to understanding the Formation, the conglomerates of the Extension Formation, and
younger portion of the tectonic history of the San Juan Islands. the sandstones and siltstones of the Pender Formation can be
The field trip will begin with a drive from Friday Harbor across found (Fig. 10). A stop at Fossil Cove, on the NW end of Stuart
San Juan Island to picturesque Roche Harbor, on the northern end Island (a boat trip of ~45 minutes), allows for examination of the
of San Juan Island. We will depart from the boat ramp at Roche bedding and sedimentary structures in these rocks, as well as the
Harbor, taking a chartered craft to Stuart and Spieden Islands. many fossils (primarily Inoceramus). Time permitting, we may
We will be landing on public access beach areas, but please note stop at a beach where the Extension Formation crops out, in order
that only the intertidal zone in these areas is considered to be pub- to examine the conglomerate clasts of this interesting unit.
lic property, and that the uplands are privately owned. Access to
Spieden Island in particular is restricted by its owner. Stop 1-2. North Shore Spieden Island, Spieden Group
(Fig. 11)
Directions and Other Instructions
Before departing on the Humpback Hauling vessel, be certain Spieden Island is one of the largest (perhaps the largest) pri-
that you have brought warm clothes and your lunch. Even if the vately owned island in the San Juan archipelago. It has a color-
weather appears to be sunny, raingear is recommended. A lifejacket ful history, most notably as “Safari Island,” when in the 1970s
(provided on the vessel) is required at all times, and please do not a group of investors purchased the island with the bright idea
forget yours on the beach. If you are prone to seasickness, please of transforming it into a private exotic game hunting reserve.
take appropriate precautions. The vessel has a landing-craft type The island was stocked with many species of exotic game ani-
ramp, so we will be able to disembark on relatively dry land. How- mals (mostly Asian and African deer, goat, sheep, and antelopeKn
75 ORCAS
70
Kn Kn ISLAND
70 65 Kn
1-1 Kn 55 84
Kn
30
STUART
ISLAND
Pe Pt
1-2
JKs SPIEDEN
ISLAND
ro fault
Ha Trh
1-3 Or
ca
s t TrJo
hrus
t
Pt TrJo
Pt
TrJo
Ro
sa
JKc rio
TrJo
Jc th
ru
st
Jc
SAN JUAN
ISLAND R
oc
h
H arbor
e
rd.
averton Val
Be le y rd
. SHAW
Friday
Harbor ISLAND
Argyle Ave
PTrd
San Juan Valley rd.
Lime Kiln Point Pg Jc
1-4
W
es
tS
i de
lt
rd. Bailer Hill rd.
fau
Cattle Point rd.
TrJo
ay
Kn = Nanaimo Group
kB
JKs = Spieden Group
JKl = Lopez Structural Complex
Buc
Jc = Constitution Formation
TrJo = Orcas Chert
Trh = Haro Formation N American
PTrd = Deadman Bay Volcanics Camp JKl
Pickett's Ln.
Cattle
Pg = Garrison Schist
2-1 Point
Pe = East Sound Group
2.0 km 2-2
Pt = Turtleback Complex
Figure 9. Map of San Juan Island and vicinity. Solid circles locate field trip stops. Sources are Brandon et al. (1988) and
Burmester et al. (2000).158 Brown et al.
74
Turn
50
Point 70
65
Kne
78 50
48 34
65
Kne 85
Knh
64
62 Prevost 28
76
80
Harbor Satellite
83 Knh 48
Fossil Kne 83 76 55 Island
78 72 72
Cove 50
81
63 58 71
Stop 1-1 Knp 65
83
64
62 71
56
Knp Knh
A 53 65 70
Nanaimo Group: 56 70 54
Knp: Pender Fm 75 55
75
Kne: Extension Fm 61 50 Reid 61
Harb
Knh: Haslam Fm or
Knp Knp
45 Kne Kne 63
Stu 50
32
art
Isla N
nd 14
22
55
26
scale 1 km
Figure 10 (on this and following page). Geology of Stuart Island, from
24
Mercier (1977). (A) Geologic map.
22
species). Needless to say, the concept of hunting exotic game in trip back to the Roche Harbor boat ramp, where the seaborne por-
the midst of an ecological paradise did not work out; the island tion of this trip will end.
reverted to Spieden Island, and the descendants of the surviving After leaving the Roche Harbor boat ramp, we will drive to
creatures can be seen cavorting around the island today. Davidson Head, parking on the shoulder of the road at the “neck”
Geologically, Spieden Island, and nearby Sentinel Island, are of the head. We will then walk northwest along the beach, exam-
the only known occurrences of the late Jurassic–early Cretaceous ining the exposures of the Haro Formation in the intertidal zone.
Spieden Group. The Spieden Group is composed of two forma- Fans of fresh oysters will be certain to notice the abundant (likely
tions, the Oxfordian-Kimmeridgian Spieden Bluff Formation, and seeded) oysters present on the Haro Formation outcrops.
the uppermost Valanginian Sentinal Island Formation (Fig. 11).
The ages of these units are constrained by biostratigraphy Directions to Stop 1-3
(McClellan, 1927, Haggart, 2000), primarily via fossils of From Roche Harbor waterfront, drive southwest on Reuben
Buchia. The rocks of both formations are clastic sediments, with Memorial Drive.
finer-grained turbidite deposits characterizing the Spieden Bluff 0.2 mi Go left on Roche Harbor Road.
Formation, and volcaniclastic-rich sandstone, mudstone, and con- 0.9 Go left (NW) on Afterglow Drive.
glomerates characterizing the Sentinel Island Formation. The rocks 1.8 Neck of Davidson Head; park on gravel shoulder on
also display some soft-sediment deformation features; some have a right side of road.
very weak anastomosing scaly cleavage, and have been folded.
Our field trip stop will be located on a wave-cut bench, Stop 1-3. Davidson Head, San Juan Island, Haro Formation
exposed at low tide, on the north shore of Spieden Island. Here
we will see outcrops of both formations, and localities that dis- The north shore of San Juan Island is home to one of the
play the locally abundant macrofossils. We will have ~30 min most geographically restricted units in the San Juan Islands—the
at this location; please follow the instructions of the trip leaders Late Triassic (Norian) Haro Formation. This unit crops out on
closely. After we re-embark, the vessel will take us on a ~40 min Davidson Head, and is a 700-m-thick mixed volcaniclastic unit.You can also read
