Crustal Deformation at the Leading Edge

of the Oregon Coast Range Block, Offshore

Washington (Columbia River to Hoh River)

Professional Paper 1661–A

U.S. Department of the Interior

U.S. Geological Survey

USGS
Crustal Deformation at the Leading Edge

of the Oregon Coast Range Block, Offshore

Washington (Columbia River to Hoh River)

By Patricia A. McCrory, David S. Foster, William W. Danforth, and Michael R. Hamer

Professional Paper 1661–A

U.S. Department of the Interior

U.S. Geological Survey

Crustal-block motions complicate the slip rates of seismotectonic elements 

of the Cascadia subduction boundary

Earthquake Hazards of the Pacific Northwest Coastal and Marine Regions

Robert Kayen, Editor
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply endorsement 

by the U.S. Government.

United States Government Printing Office: 2002

For additional copies please contact:

USGS Information Services

Box 25286

Denver, CO 80225

This report and any updates to it are available online at

http://geopubs.wr.usgs.gov/prof-paper/pp1661-A/

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Text edited by Peter Stauffer

Layout and design by Sara Boore

Manuscript approved for publication, December 31, 2001

 

Library of Congress Cataloging-in-Publication Data

Crustal deformation at the leading edge of the Oregon Coast Range block, offshore

Washington (Columbia River to Hoh River) / by Patricia A. McCrory ... [et al.].

 p. cm. -- (Earthquake hazards of the Pacific Northwest coastal and marine regions)

 (Professional paper ; 1661-A)

 Includes bibliographical references.

 1. Geology, Structural--Oregon--Coast Range. 2. Geology, Stratigraphic--Quaternary. 3.

 Faults (Geology)--Oregon--Coast Range. 4. Plate tectonics--Oregon--Coast Range. I.

 McCrory, Patricia Alison. II. Series. III. U.S. Geological Survey professional paper ;

 1661-A.

 QE156.C63 C78 2002

 551.8’09795--dc21 2002042604
Contents

Abstract.........................................................................................................................................................................................

Introduction...................................................................................................................................................................................

Tectonic setting......................................................................................................................................................................

Kinematic model of Coast Range Block motion....................................................................................................................

Location of block boundary in coastal Washington...............................................................................................................

Evidence for contraction at block boundary...........................................................................................................................

Temporal constraints on Quaternary structural activity.................................................................................................................

Cenozoic stratigraphic sequence of coastal Washington........................................................................................................

 Siletz terrane...................................................................................................................................................................

 Olympic subduction complex and Hoh rock assemblage...............................................................................................

 Neogene overlap strata....................................................................................................................................................

 Quaternary sediments......................................................................................................................................................

General constraints on offshore contraction estimates..................................................................................................................

Data acquisition......................................................................................................................................................................

Sea-floor exposure of fault traces and fold axes....................................................................................................................

Correlation of subsurface folded and faulted strata...............................................................................................................

Deformation of Quaternary unconformities...........................................................................................................................

Contraction estimates for offshore faults and folds.......................................................................................................................

North-south domain: Columbia River to Grayland................................................................................................................

 Age of late Pleistocene datum.........................................................................................................................................

 Slip rate of fault zone in Willapa Bay.............................................................................................................................

East-west domain: Grays Harbor to Cape Elizabeth..............................................................................................................

 “Grays Harbor” fault zone..............................................................................................................................................

 “Saddle Hill” fault zone..................................................................................................................................................

 Structures in the Point Grenville area.............................................................................................................................

 Contraction estimate.......................................................................................................................................................

Northern domain: Cape Elizabeth and north..........................................................................................................................

Tectonic implications of crustal shortening...................................................................................................................................

Strain partitioning near the block boundary...........................................................................................................................

Seismic hazard implications of Quaternary tectonism..................................................................................................................

Conclusions...................................................................................................................................................................................

Acknowledgments.........................................................................................................................................................................

References.....................................................................................................................................................................................

Appendix 1. M197WO—The 7-14 July, 1997, cruise on NOAA ship RV McArthur..................................................................

Appendix 2. M398WO—The 25 June-4 July, 1998, cruise on NOAA ship RV McArthur..........................................................

Appendix 3. Previous cruises in the area studied.........................................................................................................................

Plates [in pocket]

1. Maps showing Quaternary structures in the southern Washington Continental Shelf and adjacent coastal areas

2. Seismic-reflection profiles and sidescan-sonar images from the southern Washington Continental Shelf

Figures

 1. Generalized map of the Cascadia subduction system showing tectonic plates and major basement units.......................

2. Generalized east-west cross section across the central Cascadia subduction margin showing inferred configuration

 of major basement units....................................................................................................................................................

3. Map showing location of onshore faults and anticlinal folds in coastal Washington that correspond to those

 mapped offshore................................................................................................................................................................

4. Generalized stratigraphic column showing tectonostratigraphic units in the study area..................................................

5. Photograph showing exposure of Siletz basaltic rocks near Grays Harbor.......................................................................

6. Photograph and diagram showing subduction-complex (Hoh mélange) rocks in fault contact with Quinault strata 

near Cape Elizabeth...........................................................................................................................................................

7. Photographs and diagrams showing marine onlap strata onshore adjacent to the study area...........................................

8. Photographs and diagrams showing lower Pleistocene sediments onshore adjacent to the study area............................

01

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9. Graph of marine isotopic stages based on benthic foraminiferal data from ODP Site 677 in the equatorial

 east Pacific Ocean..............................................................................................................................................................

10. Photograph and diagram showing MIS-2 glacial gravels overlying tilted lower Pleistocene sands at the Quinault River............

11. Photographs and diagrams showing MIS-5 marine terrace sediments onshore adjacent to the study area......................

12. Map showing trackline locations and grab-sample sites for the 1997 and 1998 RV McArthur and RV Corliss cruises.........

13. Map showing the locations of drill-core sites in the Willapa Bay and Grays Harbor area and

 RV Wecoma gravity-core sites on the mid shelf................................................................................................................

14. Map showing trackline locations for the 1967 to 1976 research cruises that collected seismic-reflection data

 used in this study...............................................................................................................................................................

15. Examples of seismic-reflection and sidescan-sonar data used to map structures offshore Washington...........................

16. Composite lithostratigraphic column and seismic-reflection profiles in Willapa Bay......................................................

17. Photograph and diagram showing 0.8-m vertical offset of lower Pleistocene strata in the bluffs of

 northwestern Long Island in Willapa Bay.........................................................................................................................

18. Seismic-reflection profiles of the “Saddle Hill” fault zone 4 km and 8 km west of the coastline, showing

 displacement of upper Quaternary strata...........................................................................................................................

19. Onshore and offshore profiles, parallel to the coastline, used to calculate shortening of Quaternary strata

 accommodated in folds.....................................................................................................................................................

20. Generalized north-south block diagram showing inferred configuration of Oregon Coast Range and

 Olympic Mountains blocks...............................................................................................................................................

21. Schematic kinematic diagram of basement block movements and stratal shortening in the study area...........................

Tables

1. Grab-sample log, cruise of RV McArthur, June 25-July 4, 1997......................................................................................

2. Grab-sample log, cruise of RV Wecoma, March 16-30, 1998...........................................................................................

3. Core log, cruise of RV Wecoma, March 16-30, 1998.......................................................................................................

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26


Crustal Deformation at the Leading Edge

of the Oregon Coast Range Block, Offshore Washington

(Columbia River to Hoh River)

Abstract

Determination of slip rates for the seismotectonic elements 

that make up the Cascadia subduction boundary is complicated 

by crustal-block motion within the forearc. In particular, geodetic measurements (McCaffrey and others, 2000) indicate that 

the Oregon Coast Range block is translating north-northwest 

at 6-10 mm/yr relative to stable North America. Differential 

motion of the Oregon Coast Range block with respect to other 

crustal blocks that compose the forearc results in a complex 

pattern of faults and folds with orientations highly oblique to 

those expected from subduction-driven contraction.

The leading (northwestern) edge of the Oregon Coast 

Range block traverses coastal Washington, where it abuts subduction-complex rocks of the Olympic Mountains to the north. 

Block kinematics predicts north-northwest-directed contraction 

where the boundary trends east-northeast near Grays Harbor, 

Washington. Crustal deformation observed near Grays Harbor 

is consistent with north-northwestward motion of the Oregon 

Coast Range block. Deformation is localized within the more 

ductile subduction-complex rocks of the Olympic coast rather 

than the more rigid basaltic rocks that underlie the Oregon 

Coast Range block. The boundary near Grays Harbor is relatively discrete. Seismic-reflection and sidescan-sonar data 

image several faults and folds that trend east-northeast on the 

inner continental shelf between Grays Harbor and Cape Elizabeth, Washington, across a zone 40 km wide from south to 

north. In contrast, in Puget Sound to the east, where the Oregon 

Coast Range block abuts the relatively rigid pre-Tertiary rocks 

of Vancouver Island, relative motion is distributed across a zone 

more than 200 km wide from south to north.

An estimate of the contraction rate across the transverse 

coastal zone can be derived from the degree of folding and 

faulting of Quaternary strata. Constraints on the contraction 

rate come from two datums: (1) the older datum (Raft datum) is 

an unconformity at the base of a widespread, glacially derived 

deposit, and (2) the younger datum (Langley datum) is a widespread unconformity marking the base of upper Pleistocene 

sediments (<150 ka). Shortening of the older, ca. 900-600-ka 

Raft datum is ~2.0 km, corresponding to a shortening rate of 

2.2-3.3 mm/yr.

Quaternary folds range in length from 3 to 15 km and 

have amplitudes as great as 160 m. Of the seven primary 

faults and associated folds imaged in this coastal zone, the 

southernmost structure appears to be the most important. 

This structure marks the boundary between the Oregon Coast 

Range block and the Olympic Mountains block, vertically 

offsets the late Pleistocene Langley datum by more than 40 m, 

and displaces the sea floor several meters. Other faults in this 

zone displace the late Pleistocene datum 2-10 m.

Block kinematics predicts transpressional shear where 

the block boundary trends north-northwest through Willapa 

Bay, Washington. Crustal deformation observed in Willapa 

Bay documents a component of contraction in an east-northeast direction. Lateral slip, if any, on mapped faults remains 

undetected. In contrast to the kinematic model, this portion 

of the boundary exhibits the geometry of an east-dipping 

thrust fault.

Faults mapped with both new and earlier seismic-reflection 

data are consistent with a thrust geometry. These data depict a 

3-km-wide, 30-km-long fault zone trending north-northwest 

through Willapa Bay. However, the main strand shows a sense 

of vergence opposite to that of the inferred boundary thrust. 

The main fault strand vertically displaces a ca. 20-ka erosional 

surface beneath the bay 10-12 m, west side up. A vertical displacement rate of 0.5±0.2 mm/yr has been calculated across the 

entire fault zone. This “Willapa Bay” fault zone may include 

a backthrust above the block boundary. Alternatively, the fault 

geometry may reflect lateral juxtaposition of bedrock slivers. 

No shallow trace (<90 m) has been identified for the postulated 

block boundary fault. Nonetheless, the recency and magnitude 

of movement on the “Willapa Bay” fault zone implies activity 

on the postulated main thrust to the west.

These observations provide the first constraints on rates 

of Quaternary structural activity in coastal Washington. Furthermore, they suggest that this region may accommodate as 

much as half of the differential motion between the Oregon 

Coast Range block and stable North America. The remaining 

differential motion may be accommodated in the Olympic 

Mountains, which are uplifting as much as ~3.2 mm/yr 

as indicated by geodetic measurements (Savage and others, 

1991), or as broad-scale buckling of the Oregon Coast Range 

basement beneath Grays Harbor and Willapa Bay.

Introduction

The Washington Continental Shelf lies within the Cascadia subduction margin and continues to be shaped by plateboundary processes far from the deformation front. A major 

By Patricia A. McCrory1, David S. Foster2, William W. Danforth2, and Michael R. Hamer1

1

Menlo Park, California

2

Woods Hole, Massachusetts

Introduction
Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

crustal block boundary in the subduction margin traverses the 

study area and continues onshore to mark the southern edge 

of the Olympic Mountains (fig. 1). Structural evidence for 

differential motion across this block boundary has been documented both onshore (McCrory, 1996, 1997) and offshore 

(Wolf and others, 1997, 1998). Sidescan-sonar and high-resolution seismic-reflection data were collected offshore in 1997 

and 1998 on the RV McArthur (Foster and others, 1999a, b; 

McCrory and others, in press a, b) and RV Corliss (Cross 

and others, 1998, 1999; Twichell and others, 2000). These 

data image a structural high that extends 40 kilometers from 

Grays Harbor northward to Cape Elizabeth, Washington, and 

is at least 35 kilometers wide from the coast to the mid 

shelf. This high encompasses a zone of faults and tight folds, 

S

S'

Z'

Z

100 KILOMETERS

0 100 MILES

0

Juan de Fuca

Plate

Pacific

Plate

North America

Plate

Ju

a

n

de

Fu ca

Ridge

Blanco transform

G

ord

a

Ridg

e

Mendocino transform

SAF

Area of

plates 1A and 1B

OCR

CV

OM

KM

Df

Mtj

Pacific Ocean

Vancouver Island

Cape Flattery

Puget

Sound

132° 128° 124°W

40°

44°

48°N
Active faults in the nearshore area have been understudied because of the difficulty of collecting marine seismic and 

sidescan data in shallow water and the difficulty of quantifying fault-slip histories. However, nearshore faults can represent a significant seismic hazard owing to their proximity to 

populated areas and their shallow rupture depths as compared 

to the subduction plate boundary. In southern Washington, 

many nearshore faults extend onshore, so the full length of 

these faults needs to be determined in order to calculate 

expected earthquake magnitudes. In fact, the ability to link 

structures across the shoreline makes the Olympic coast an 

ideal area in which to recognize and quantify recent structural 

activity in the nearshore zone and to develop methodologies 

suited to this challenging zone.

Tectonic Setting

The 1,100-km-long Cascadia subduction boundary 

accommodates relative motion between the subducting Juan 

de Fuca oceanic plate and the North America continental 

plate across a 300- km-wide region from the trench to the 

volcanic arc (fig. 1). Relative motion is directed northeastward 

and varies from about 23 mm/yr at the southern end of the 

Cascadia subduction zone to 40 mm/yr at the northern end 

(Wilson, 1993). Along the southern half of the plate boundary, 

relative motion is oblique to the trend of the trench axis. This 

obliquity may drive translation of crustal blocks in the forearc 

region (Wells and others, 1998). Along the northern half of 

the boundary, where the study area is located, relative motion 

is fairly orthogonal. 

The Cascadia Continental Slope is primarily composed 

of weakly consolidated material that has been scraped off the 

subducting Juan de Fuca Plate and accreted to the margin 

over the past 2 m.y. (Barnard, 1978). The Continental Shelf 

and onshore forearc regions are underlain by an amalgamation 

of terranes that were accreted during Cenozoic subduction. 

The Siletz terrane, a major forearc terrane, is composed of 

Paleogene oceanic basalt and overlying marine strata (Wells 

and others, 1984, Tréhu and others, 1994; Snavely and Wells, 

1996). This terrane underpins the relatively rigid Oregon 

Coast Range (OCR) block, which is more than 500 km long 

and 200 km wide (Wells and others, 1998) and occupies 

the central portion of the forearc (fig. 1). Differential motion 

between the OCR block and adjacent forearc blocks creates 

regional structural complexities within the overall subduction 

regime.

Ongoing north-south shortening in western Washington is 

attributed to northward translation of the Oregon Coast Range 

block relative to stable North America, nominally represented 

by pre-Tertiary basement rocks of southern British Columbia 

(Snavely and Wells, 1996). Beneath Puget Sound this shortening occurs within the relatively thin (15-20 km thick) Siletz 

basement (Tréhu and others, 1994; Stanley and others, 1999) 

and overlying strata where a series of rhomboidal basins 

and intervening basement highs provide striking aeromagnetic 

images (Blakely and others, 1999). Here, approximately 4 

mm/yr of contraction is distributed across at least six faults 

which accommodates north-northwest-directed crustal shortening along the block boundary. The geophysical data reveal 

numerous sites where fault scarps, anticlinal folds, or diapirs 

breach the sea floor. Little was previously known about the 

age of most recent movement on these structures or the rate 

at which they deform (Grim and Bennett, 1969; Wagner and 

others, 1986; Palmer and Lingley, 1989).

Figure 1. Generalized map of the Cascadia subduction system showing 

tectonic plates and major basement units (modified from Jennings and 

others, 1977; Tipper and others, 1981; Hyndman, 1995; Snavely and Wells, 

1996; McCrory, 1996; Wells and others, 1998). The Cascadia subduction 

margin extends from the thrust fault along the trench axis to the volcanic 

arc, approximately 300 km to the east. Black arrows denote modern motion 

vectors of the oceanic plates, and of the Oregon Coast Range (OCR) forearc 

block, with respect to a fixed North America Plate, in mm/yr (Wilson, 

1993; D. S. Wilson, written commun., 2001). North-northwest motion of 

northern California relative to a rigid North America requires a component 

of shortening along the Oregon-Washington coast, in part concentrated 

along the northern boundary of the OCR block. Cross section S-S’ is shown 

in figure 2; cross section Z-Z’ is shown in figure 20.

Tectonic plates of oceanic crust in

Pacific Ocean

Deformation front or active

accretionary prism

Olympic Mountains block, underlain by

accreted subduction-complex

basement, including the Franciscan

and Olympic subduction complexes

Oregon Coast Range block, underlain by

the Siletz basaltic terrane

Klamath Mountains terrane and other

pre-Cenozoic basement terranes

Cascade Range volcanic rocks

Active Cascade volcanic centers

Oceanic spreading ridge

Transform or strike-slip fault

Thrust fault, sawteeth on upper plate

Rate of plate motion

East-west profile shown in figure 2

North-south profile shown in figure 20

Mendocino triple junction

San Andreas Fault

EXPLANATION

SAF

OCR

CV

OM

KM

Df

Mtj

S S'

Z Z'

0 20 40 mm/yr

Introduction
Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

(Gower and others, 1985; Johnson and others, 1996, 1999; 

Wells and Johnson, 2000). In coastal Washington the Siletz 

terrane is thicker (~30 km thick, Parsons and others, 1999) 

than under Puget Sound and abuts the significantly more 

ductile subduction-complex terrane that underpins the Olympic Peninsula (fig. 2) rather than pre-Tertiary basement. Velocity models based on wide-angle refraction data suggest a 

rhomboidal structural configuration for Siletz rocks at midcrustal depths in coastal Washington (Parsons and others, 

1999), with lows under Willapa Bay and Grays Harbor and an 

intervening high. This configuration is similar to that observed 

in the Puget Sound region, but with significantly less apparent 

relief and no direct evidence of recent structural movement 

associated with basement irregularities.

Kinematic Model of Coast Range Block Motion

Cenozoic reconstruction of the western continental margin 

of North America based on the kinematic model of Wilson 

and others (in press) assigns relative motion to several crustal 

blocks that compose the continental margin. Specifically, this 

model refines the late Cenozoic Pacific-Juan de Fuca-North 

America velocity vectors by considering Oregon Coast Range 

motion relative to North America. The kinematic model indicates that the Oregon Coast Range (OCR) block has moved 

about 150 km northward in concert with the Sierra NevadaCalifornia Coast Ranges block relative to stable North America 

over the past 15 m.y. at an average rate of 10 mm/yr, similar to 

calculations by Atwater and Stock (1998).

We presume that northward translation of the block over 

the past 15 m.y. has been converted to convergence in the 

upper crust at the northern or leading edge of the Coast 

Range block. In coastal Washington, we propose that this 

convergence is primarily accommodated by a combination 

of east-northeast-striking thrust faults and north-northweststriking strike-slip faults.

The kinematic models of Wells and others (1998) and 

Wells and Simpson (2001) for modern block motion within the 

Cascadia forearc defines a pole of rotation for the OCR block 

in central Washington based on very long baseline interferometry (VLBI) and paleomagnetic data. This model predicts 

northward motion of about 8 mm/yr in coastal Washington. 

Recent global positioning system (GPS) measurements indicate a rate of about 6-10 mm/yr (McCaffrey and others, 

2000) for the same region. Northward translation of the OCR 

block during the Quaternary results in convergence directed 

340° between this block and the Olympic Mountains block 

in southwestern Washington. We attribute crustal deformation 

observed between the Columbia River and the Queets River 

(46° to 47°30’N.) (fig. 3) primarily to this differential motion.

Location of Block Boundary in Coastal Washington

The OCR block boundary in coastal Washington is characterized by juxtaposition of basaltic crust of the Siletz terrane against marine sedimentary and volcanic rocks and 

mélange of the Olympic subduction complex. Aeromagnetic 

data depict a sharp contrast in the upper crust where 

magnetic basaltic basement is juxtaposed against the mélange 

basement (Finn and others, 1998; R. J. Blakely, written 

commun., 2000). The boundary inferred from aeromagnetic 

data trends north-south in southwestern Washington, paralleling the coastline between the Columbia River and Grays 

Harbor, and then swings eastward around the Olympic Mountains. A similar pattern is seen in Bouger gravity data, where 

the strong gradient tracks the boundary between the Olympic 

subduction complex and the more dense basalt basement of 

the Crescent Formation (Finn and others, 1998; R. J. Blakely, 

written commun., 2000).

The P-wave velocity model calculated by Parsons and 

others (1999) suggests that the boundary is an eastward-dipping thrust fault beneath the Willapa Bay-Grays Harbor area. 

Subsurface lithostratigraphies from petroleum exploration 

wells (Rau and McFarland, 1982; Snavely and Wagner, 1982) 

also suggest a dipping contact in the upper 2 km beneath Willapa Bay and Grays Harbor—OCR basaltic rocks are there 

underlain by fractured and sheared sedimentary rock interpreted to be subduction-complex mélange (fig. 3). In contrast, 

only subduction-complex rocks are found beneath overlap 

strata in wells north of the boundary, and only OCR basaltic 

rocks are present in wells south of the boundary.

Evidence for Contraction at Block Boundary

The 6-10 mm/yr of relative motion between the OCR 

and North America provides a contraction budget of about 

8 mm/yr in coastal Washington. The distribution and orientation of faults and folds onshore to the east of the study area 

suggest that direct convergence occurs between the OCR and 

Olympic Mountains (OM) and is expressed in part as permanent crustal contraction within the subduction-complex rocks. 

In the Grays Harbor-Moclips River area (47° to 47°15’N.), 

Quaternary thrust faults and folds trend east-northeast (fig. 3), 

indicating northward-directed crustal shortening (McCrory, 

1996, 1997).

This deformation pattern is consistent with the contemporary north-south compressional stress field observed in 

petroleum exploration wells in the area. Magee and Zoback 

(1992) infer a N.12°W. principal compressive stress direction in the Shell Sampson Johns No. 1-15 well beneath 

Saddle Hill on the basis of borehole-breakout data (fig. 3, 

site N). Magee and Zoback (1992) infer stress directions of 

N.05°W. and N.14°E. in the Shell Luse No. 1-23 well (fig. 3, 

site M) and the Shell Grays Harbor No. 1-15 well (figure 3, 

site O), respectively.

The few recorded earthquakes in the study area are typically quite deep (>30 kilometers, fig. 2), at depths corresponding to the subducted Juan de Fuca slab. Crustal seismicity is 

sparse in the subduction complex in the coastal region (fig. 2), 

even though P-wave (compressional) velocities of ~4.5-6.0 

km/s at 5-15 kilometers depth (Parsons and others, 1999) are 

within the range where crustal seismicity is observed elsewhere 

along the subduction margin (McCrory, 2000). A small cluster 
Figure 2. Generalized east-west cross section across the central Cascadia subduction margin showing inferred configuration of major basement units (mo

no vertical exaggeration. See figure 1 for location and explanation of patterns. The location of the Juan de Fuca Plate beneath the margin is constrained by (1999). Paired arrows denote relative movement on Cascadia megathrust and relative movement on block boundary. Circled numbers identify potential seWashington crustal sources: (2) interplate, megathrust sources; (3) lower plate, Juan de Fuca sources. Red dots show ambient seismicity 1963-2001 in a 140line (data derived from a search of the Council of the National Seismic System Earthquake Catalog, online at http://quake.geo.berkeley.edu/cnss/cnss-cataloccurs within the subducted oceanic slab, but a small cluster occurs along the boundary fault.

Introduction

Earthquakes, 1963-200146° - 47.25° latitude>1 km depth, M>1

126°

125°

124°

profileZZ'

Coastline

WillapaBayfaultzone

E

Juan de Fuca plate

Cascadia

accretionary prism

Olympic Mountains

block

DEPTH,INKILOMETERS

S

West 

10203040

Overlap strata

Subduction

Complex

Oceanic sediments 0

0 20 40 60 80 100

2

3

1

Juan de Fuca Plate

Mantle

Crust

DISTANCE, IN KILOMETERS 

120 140 160 180 200 220 
Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

Willapa

Bay

Grays

Harbor

Point Grenville

Cape Elizabeth

Cape

Shoalwater

Cape

Disappointment

Quinault

R

iver

Copali s

River

Moclips River

Raft R iver

Humptulips

River

Q ueets River

Cheh al is River

Willapa

River

Colu mbia Riv er

Long Beach

Peninsula

Hoh River

Grayland

Langley Hill

Saddle Hill

Duck Creek ridge

Whale Creek

Wreck Creek ridge

P

i

a

n

le

C

o

o

s

p

e

a

is

li

m

s

i

R

c

i

s

v

i

e

te

r m

of

a

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rs

t

h

wa

th

te

a

r

t

(

la

1

c

9

k

9

s

2)

* a megathrust signature

Bathymetric contours generated from NOAA-NOS hydrographic soundings.

Bathymetric contour - 10 m Projection - UTM ZONE 10

Datum - NAD83

5 0

5 0

10

10

10 KILOMETERS

10 MILES

46°30'

47°00'

47°30'N

R

Q

P

N

M

K L

J

E

F

G

C

D

B

A

124°30' 124°00'W

I

H

O
Temporal Constraints on Quaternary 

Structural Activity

Cenozoic Stratigraphic Sequence of Coastal 

Washington 

Two major basement units, the Siletz terrane and the 

Olympic subduction complex, underlie the study area. Both 

units are separated from overlying Miocene to Pliocene marine 

strata (Rau, 1970; Wagner and others, 1986; Palmer and Lingley, 1989) by a regional unconformity of late middle Miocene 

age (fig. 4) (Rau and McFarland, 1982; Kulm and Fowler, 

1974; Snavely, 1987; McNeill and others, 2000). Locally, 

of earthquakes does occur along the block boundary beneath 

Willapa Bay. Given the low to moderate rate of slip postulated 

for individual faults (<1 mm/yr) in the region, this sparsity of 

ambient seismicity is not particularly significant.

Present-day displacement inferred from geodetic measurements indicates that the coastal region deviates from the general model of interseismic contraction expected from a locked 

megathrust. The velocity measured at the Point Grenville site, 

north of the block boundary, is more northerly than that at 

coastal sites south of the boundary (Svarc and others, in press). 

Also, the east-west Grays Harbor transect indicates greater 

than expected uplift in coastal Washington (Savage and others, 

1991; Svarc and others, 1999). These anomalies may result 

from superimposed effects of deep and shallow strain fields 

from the megathrust and upper plate structures, respectively.

≤ 20

≤ 150

≤ 780

≤ 1,800

unspecified

Strike-slip

Dip-slip (normal or

high angle)—bar and ball

on downthrown side

Dip-slip (thrust or

low angle)—sawteeth

on upper plate

Age of most recent structural

movement, in ka

Fault (age indicated by color

code at right)

Anticline

Olympic Mountains basement

Oregon Coast Range basement

Oregon Coast Range rocks

above Olympic Mountains basement

Petroleum well site

Direction of maximum horizontal stress

Eocene volcanic rock

Inferred boundary of Oregon Coast Range block

EXPLANATION

Figure 3. Map showing location of onshore faults and anticlinal folds in coastal Washington (from 

Walsh and others, 1987; Wells, 1989; McCrory, 1996, 1997) that correspond to those mapped offshore. 

Location of exploratory petroleum wells from Rau and McFarland (1982), Palmer and Lingley (1989), and 

Magee and Zoback (1992); stress orientations from Magee and Zoback (1992). Location for Shell Luse 

No. 1-23 well (site M) has been corrected (moved south) as compared with Magee and Zoback (1992) 

and McCrory (1996). Site N corresponds to three wells, from south to north: Shell Sampson Johns No. 

1-15, Shell Sampson Johns No. 2-15, and Union State No. 3. Note proximity of the Atwater paleoseismic 

site to the thrust fault just to south.

Temporal Constraints on Quaternary Structural Activity
Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

++

++++++++

0 Ma

10

20

30

40

50

60

TER

TIA

R

Y

Oligocene Miocene Pliocene Eocene Paleocene

QUAT.

Hiatus

Hiatus

Hiatus

Hiatus

0 Ma

6

5

4

3

2

NW SE NW SE

HOH

ROCK

ASSEM

OLYMPIC

SUBDUCTION

COMPLEX

SILETZ

TERRANE

QUINAULT FM

upper member

QUINAULT

FORMATION

lower

member

middle

member

MONTESANO

FM

Saddle Mtn basalt

ASTORIA

FM

LINCOLN CK

FM

HUMPTULIPS

FM

Grande Ronde

basalt

lower Quaternary beds

Raft Datum

Langley Datum

MIS 6 beds

MIS 2 beds

MIS 5 strata

Quinault Datum

AGE LITHOLOGY

Figure 4. Generalized stratigraphic column showing tectonostratigraphic units in the study area (modified from Palmer 

and Lingley, 1989), with expanded column of upper part (Pliocene-Quaternary). Northwest edge of column is in the 

vicinity of the Queets River. Southeast edge is in the vicinity of the Columbia River. Siletz terrane is the principal basement 

rock for the Oregon Coast Range block. Olympic subduction complex and Hoh rock assemblage are the principal 

basement rocks for the Olympic Mountains block. Age for the base of the Quinault Formation (ca. 6.5 Ma) is based on 

biostratigraphy of Rau (1970). Age for the top of the Quinault Formation (ca. 2 Ma) is based on biostratigraphy of Ingle 

(1967). Dashed division of the Quinault Formation into informal members is schematic and based on lithostratigraphic data 

of McCrory (unpub. data, 1989). The Quinault datum that separates the Quinault Formation from the underlying Hoh rock 

assemblage is well exposed at several locations along the coast. The Raft datum that separates the Quinault Formation 

from overlying lower Pleistocene gravels is poorly exposed at the coast just north of the Raft River. The Langley datum that 

separates lower Pleistocene gravels from upper Pleistocene gravel or sand deposits is exposed at a number of locations 

along the topographic ridges discussed in text. See text for descriptions of the stratigraphic units.
Olympic Subduction Complex and Hoh Rock Assemblage

Basement rocks in much of coastal and nearshore Washington are composed of Oligocene to Miocene sedimentary 

and volcanic rocks scraped off the subducting Juan de Fuca 

Plate and accreted to the subduction margin by late Miocene 

time, ca. 10 Ma (Rau, 1973, 1975; Tabor and Cady, 1978; 

Snavely, 1987). Pervasively deformed rocks of the subduction 

complex (fig. 6) are divided into two major units onshore, a 

middle Eocene to lower Oligocene Olympic subduction complex (Tabor and Cady, 1978; Brandon and Vance, 1992) that 

underlies the Olympic Mountains and the middle Miocene 

Hoh rock assemblage (fig. 6) (Rau, 1975; Orange and others, 

1993) that is exposed in coastal Washington.

The subduction complex is poorly exposed onshore adjacent to the study area, but petroleum wells drilled along the 

coast north of Grays Harbor penetrated subduction-complex 

basement beneath middle Miocene to upper Pliocene overlap 

strata (fig. 3) (Rau and McFarland 1982; Palmer and Lingley, 

1989). Equivalent rocks occur on the shelf as inferred from 

offshore well stratigraphies (Snavely and Wagner, 1982; Rau 

and McFarland, 1982; Palmer and Lingley, 1989) and correlation with onshore rocks. The subduction complex is inferred 

to generally young seaward, with the rocks underlying the 

these basement rocks and overlap strata are unconformably 

overlain by shallow marine or nonmarine Quaternary sediments (Grim and Bennett, 1969; Palmer and Lingley, 1989).

Siletz Terrane

Basement rocks in much of coastal and nearshore Oregon 

are assigned to the Siletz terrane (Wells and others, 1984, Tréhu 

and others, 1994; Snavely and Wells, 1996), a terrane that 

accreted to the North America subduction margin by Eocene 

time, ca. 50 Ma. The Siletz terrane consists of Paleogene oceanic basalt (fig. 5) and overlying forearc strata. The basaltic 

terrane is quite thick in Oregon and southernmost Washington, 

where it behaves as a rigid block. The terrane thins northward 

in the Puget Sound region of Washington, where it is warped 

and broken into a series of subblocks (Brocher and others, 2001; 

ten Brink and others, 1999). Overlying Miocene and younger 

strata show little evidence of deformation away from the subblock boundaries (Johnson and others, 1996, 1999). In coastal 

Washington, the upper surface of the Siletz basement has relief; 

however, available data are not sufficient to discern whether this 

relief is derived from structural activity or, in turn, whether any 

postulated structures might be active.

Figure 5. Exposure of Siletz basaltic rocks onshore near Grays Harbor, adjacent to the study area.

Temporal Constraints on Quaternary Structural Activity
10 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

Hoh rocks

Quinault

Formation

U D

Quinault River

PACIFIC

OCEAN

fault

Figure 6. Subduction-complex (Hoh mélange) rocks in fault contact with Quinault strata at the mouth of the Quinault River, near 

Cape Elizabeth. See plate 1A for location and direction of view.
11

Continental Slope having been accreted during the past 2 m.y. 

(Barnard, 1978).

Onshore, the older Olympic subduction-complex rocks 

(Brandon and Vance, 1992) override the younger Hoh rock 

assemblage along a major thrust system (Snavely and Kvenvolden, 1989). This configuration is observed in Rayonier No. 

1-A well in the Grays Harbor area (fig. 3, site P) (Rau and 

McFarland, 1982), where mélange rocks of late Eocene to Oligocene age overlie mélange rocks dated as early Miocene (on 

the basis of benthic foraminiferal fauna) at a depth of 1.85 km.

Neogene Overlap Strata

Moderately deformed marine strata of middle Miocene 

to Pliocene age are divided into two units onshore adjacent to 

the study area, the middle-upper Miocene Montesano Formation and the upper Miocene-Pliocene Quinault Formation (fig. 

4) (Rau and McFarland, 1982; Palmer and Lingley, 1989). 

These two units are separated by a 15°-20° angular unconformity in the Grays Harbor area (Palmer and Lingley, 1989) 

based on well data and seismic-reflection profiles. The age of 

the Montesano Formation is constrained by radiometrically 

dated volcanic rocks, the Pomona Member of the Saddle 

Mountain Basalt (ca. 12 Ma), which intrude basal Montesano 

strata in the Grays Harbor area (Walsh and others, 1987).

The slope to inner shelf lithofacies of the Montesano 

Formation are poorly exposed onshore (Fowler, 1965). Petroleum well data indicate that the Montesano Formation is as 

much as 775 m thick (Rau and McFarland, 1982; Palmer and 

Lingley, 1989). Well and nearshore seismic-reflection profiles 

indicate that Montesano strata thin markedly northward from 

Grays Harbor (Rau and McFarland, 1982; Palmer and Lingley, 1989). These strata are not observed in the subsurface 

north of the Copalis River (fig. 3). The Montesano Formation 

thins rapidly to the south as well; Montesano strata are not 

observed south of Shell Minard No. 1-34 well (fig. 3, site K) 

on the northern spit of Grays Harbor. Strata equivalent to the 

Montesano Formation are present offshore in the Tidelands 

State No. 2 well, 3 kilometers southwest of Saddle Hill (fig. 3, 

site L) (Rau and McFarland, 1982; Palmer and Lingley, 1989). 

The lithostratigraphy in this well is used to identify and map 

Montesano-equivalent strata on the shelf (Palmer and Lingley, 

1989).

The upper slope to deltaic lithofacies of the Quinault 

Formation are exposed onshore from Point Grenville north to 

the Raft River (Horn, 1969; Rau, 1970, 1975) and penetrated 

in petroleum wells from Long Beach Peninsula north to Cape 

Elizabeth (Rau and McFarland, 1982). Well data indicate that 

the Quinault Formation reaches at least 620 m in thickness. 

Planktonic foraminiferal fauna (Ingle, 1967, 1976) constrain 

the upper Quinault Formation (Horn, 1969; Rau, 1970), 

exposed in the sea cliffs between Point Grenville and Cape 

Elizabeth, to Pliocene age (ca. 5-2 Ma). The Quinault Formation in the Cape Elizabeth area is composed of interbedded 

conglomerate and sandstone (Horn, 1969; Rau, 1970). The 

conglomerate beds are highly resistant to erosion and display 

a distinctive ribbed pattern where they form the shore plat1 METER

Figure 7. Marine onlap strata onshore adjacent to the study area. See plate 1A for locations and directions of views. A, Quinault 

Formation, lower turbidite member, near the Raft River. B, Quinault Formation, middle siltstone member, near Duck Creek. C, Quinault 

Formation, upper conglomerate member, near Cape Elizabeth. D, Quinault Formation, upper conglomerate member, near Duck Creek. 

Note the bedrock ribs extending seaward.

A B

Temporal Constraints on Quaternary Structural Activity
12 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

terrace deposit

Quinault Formation

bedrock rib

bedrock rib

C

Figure 7. Marine onlap strata onshore adjacent to the study area. See plate 1A for locations and directions of views. A, Quinault 

Formation, lower turbidite member, near the Raft River. B, Quinault Formation, middle siltstone member, near Duck Creek. C, 

Quinault Formation, upper conglomerate member, near Cape Elizabeth. D, Quinault Formation, upper conglomerate member, 

near Duck Creek. Note the bedrock ribs extending seaward.—Continued
13

Cape Elizabeth

Quinault Formation

terrace deposits

bedrock ribs

D

Figure 7.—Continued

Temporal Constraints on Quaternary Structural Activity
14 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

form (fig. 7). Folded strata on the inner shelf form ribs that 

jut from the sea floor in similar fashion and may represent 

equivalent facies offshore.

Mud diapirs intrude the overlap strata both onshore (Rau 

and Grocock, 1974; Rau, 1987; Orange and Campbell, 1997) 

and offshore (Grim and Bennett, 1969; Wagner and others, 

1986). The diapirs are commonly found along fault traces 

that apparently provide conduits for overpressured material 

in the underlying subduction complex to rise through the 

overburden. Some diapirs breach the sea floor and appear to 

be venting fluids into the water column; one forms a small 

mud volcano on the sea floor (see plate 1A).

Quaternary Sediments

Quaternary deposits can be divided into four main 

units, three glacially derived deposits and one marine-terrace 

deposit, each bounded by erosional unconformities. A coarsegrained sedimentary unit overlies Quinault strata and underlies upper Pleistocene marine-terrace and glacial deposits 

onshore between Grays Harbor and Whale Creek. A similar 

unit, in terms of stratigraphic position, clast composition, and 

degree of weathering, occurs in the Willapa Bay area. The 

contact between the Quinault Formation and this gravel unit 

is poorly exposed in the sea cliffs near the Raft River. Well 

logs and nearshore seismic-reflection profiles indicate that an 

angular unconformity (Palmer and Lingley, 1989) separates 

equivalent strata offshore. Deformation of this unit ranges 

from mild to strong; strata are flat-lying to severely tilted 

(fig. 8). This coarse-grained unit has not been dated directly 

and is tentatively assigned an early Pleistocene age (Walsh 

and others, 1987; McCrory, 1997) on the basis of its relative 

position above Pliocene Quinault strata and beneath upper 

Pleistocene glacial sediments (Moore, 1965).

Onshore exposures of lower Pleistocene strata reveal a 

range of fluvial and alluvial lithofacies, including poorly sorted 

gravel lenses, cross-bedded sands and gravels, and minor clay 

beds (fig. 8) that are interpreted to be glacially derived (Moore, 

1965; Pringle, 1986). The distribution of mapped soils that 

correspond to this unit (Pringle, 1986) suggests that lower Pleistocene deposits are widely distributed between Willapa Bay 

and Point Grenville. Gravels containing clasts derived from the 

Olympic Mountains are observed as far south as Long Island 

in Willapa Bay. Because of its widespread distribution, this 

unit is tentatively correlated with the major glacio-eustatic lowstand associated with either MIS-22 or MIS-16 (marine-isotope 

stage), ca. 0.9 or 0.6 Ma (fig. 9), respectively (Shackleton and 

others, 1990; Williams and others, 1997). This correlation is 

based on the assumption that a strong, long-lived glacial period 

was required to develop and preserve the widespread deposit.

Subsurface well data (Rau and McFarland, 1982) and field 

investigations (McCrory, 1997) indicate that the lower Pleistocene (?) sands and gravels vary widely in thickness onshore, 

with the unit ranging up to about 200 m thick. An equivalent 

gravel unit is inferred offshore between Grays Harbor and 

Cape Elizabeth on the basis of bottom samples (Venkatarathnam, 1969; this paper) and sidescan-sonar data (Twichell and 

others, 2000; this paper). However, these gravels may in fact 

represent an amalgamation of several glacio-fluvial units of differing ages deposited during a series of lowstands. In this case, 

reworking during subsequent sea-level transgressions likely has 

obscured distinct units on the inner shelf. Nearshore seismicreflection profiles and well data indicate that lower Pleistocene 

strata together with overlying upper Pleistocene sediments 

reach 300 m in thickness (Palmer and Lingley, 1989), with the 

upper Pleistocene sediments never more than about 40 m thick 

(Herb, 2000). Offshore, the strata thin and pinch out over structural highs (Palmer and Lingley, 1989).

Two distinct depositional units derived from episodes of 

alpine glaciation overlie the lower Pleistocene strata with an 

angular unconformity onshore. These two glacial units mainly 

consist of till lithofacies bounded by moraines and flanked 

on the west by outwash lacustrine, fluvial, and alluvial lithofacies (fig. 10) (Moore, 1965) and are locally separated by a 

marine sand and cobble lithofacies (fig. 11). This intervening 

marine unit accumulated during late Pleistocene interglacial 

conditions (MIS-5, ca. 125 to 83 ka; Thackray and Pazzaglia, 

1994; McCrory, 1996). An equivalent unit likely occurs offshore between a widespread late Pleistocene erosional surface 

and a Holocene shell lag found in gravity cores (R. L. Phillips, 

unpub. data, 1998).

The younger, more restricted glacial unit correlates with 

the major MIS-2 glacial advance (ca. 20 to 15 ka) on the basis 

of radiocarbon dates (Moore, 1965). Similar radiocarbon ages 

have been reported for young glacial deposits in the Queets 

River area to the north (Thackray and Pazzaglia, 1994). The 

older, more widespread glacial unit is tentatively assigned to 

the penultimate major glacial advance (MIS 6, ca. 155 to 

150 ka) on the basis of correlation with the global sea-level 

curve (fig. 9) (Chappell, 1983; Shackleton and others, 1990; 

Williams and others, 1997). This correlation is again based 

on the assumption that a strong, long-lived glacial period was 

required to develop and preserve the widespread deposit.

The outwash facies appear truncated at the coastline, 

suggesting that they originally extended further west across 

an emergent shelf. Minor intervening glacial advances are 

recorded as discontinuous sets of glacially derived deposits 

along the Quinault River (Moore, 1965).

General Constraints on Offshore 

Contraction Estimates

Constraints on the location of Quaternary structures and 

on the recency and magnitude of activity come from four 

types of geologic and geophysical data. (1) Seismic-reflection 

profiles allow the mapping of sea-floor offset or warping and 

the correlation of stratigraphic packages between individual 

tracklines. The profiles also are used to detect growth faulting or folding and to map the depth and extent of subsurface 

unconformities. (2) Sidescan-sonar swaths are used to detect 

sea-floor offsets and to map the orientation of faults and folds 

expressed on the sea floor. When sea-floor reflective character 

is matched with bottom samples to determine sea-floor sedi-
15

erosional contact

slope wash

flat lying

terrace deposits

slope wash

steeply dipping

lower Pleistocene strata

Figure 8. Lower Pleistocene sediments onshore adjacent to the study area. See plate 1A for locations and directions of view. A, 

Tilted lower Pleistocene gravels near Whale Creek overlain by flat-lying MIS-2? gravels. B, Tilted lower Pleistocene sand and gravel 

deposits, near Wreck Creek; exposure is about 3 m high. C, Faulted lower Pleistocene gravels on Langley Hill.

A

General Constraints on Offshore Contraction Estimates
steeply dipping

lower Pleistocene strata

road

16 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

Figure 8. Lower Pleistocene sediments onshore adjacent to the study area. See plate 1A for locations and directions of view. A, 

Tilted lower Pleistocene gravels near Whale Creek overlain by flat-lying MIS-2? gravels. B, Tilted lower Pleistocene sand and gravel 

deposits, near Wreck Creek; exposure is about 3 m high. C, Faulted lower Pleistocene gravels on Langley Hill—Continued.

B
16 cm

faults

17

Figure 8.—Continued

C

General Constraints on Offshore Contraction Estimates
18 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

ment or rock type, the age of deformed strata can be estimated 

as well. (3) Well and borehole data are used to determine 

the age and thickness of stratigraphic units depicted in the 

seismic-reflection profiles. (4) Dated fossil or wood material 

and ashes from sediment cores and bottom samples are used 

to estimate rate of sediment accumulation and age of unconformities depicted in the seismic-reflection profiles. Together 

these data sets allow the mapping of structures and datums and 

provide age constraints on the deformed stratigraphic units.

Data Acquisition

The locations of geologic structures mapped on the inner 

continental shelf between the Columbia River and the Hoh 

River represent a significant revision to previous mapping 

efforts (for example, Grim and Bennett, 1969; Wagner and 

others, 1986; Wolf and others, 1997). This revision is primarily 

based on new data collected during research cruises in 1997 and 

1998 aboard the National Oceanic and Atmospheric Administration ship RV McArthur and the Washington Department of Fish 

and Wildlife vessel RV Corliss (fig. 12). The 1997 McArthur

cruise (M197WO; see appendix 1) collected approximately 400 

km of high-resolution seismic-reflection profiles (Foster and 

others, 2001) and sidescan-sonar swaths (McCrory and others, 

in press b) in a grid pattern with a 5-km spacing. These data 

were collected on the Continental Shelf between Grays Harbor 

and the Raft River in water depths ranging from 20 to 100 m. In 

addition, the cruise collected seven sea-floor sediment samples 

in water depths ranging from 28 to 70 m (table 1) along an 

east-west transect offshore Cape Elizabeth using a Shipek grab 

sampler. The companion 1997 Corliss cruise collected about 

500 km of high-resolution seismic-reflection profiles (Cross and 

others, 1998) and sidescan-sonar swaths (Twichell and others, 

2000) in the nearshore area between the Columbia River and 

Point Grenville, in a similar grid pattern (fig. 12). Corliss water 

depths ranged from 10 to 100 m.

The 1998 McArthur cruise (M398WO; see appendix 2) 

collected about 370 km of high-resolution seismic-reflection 

profiles (Foster and others, 1999a, b) and sidescan-sonar swaths 

(McCrory and others, in press a) between Grays Harbor and 

the Hoh River, in a grid pattern with either a 3-km or 5-km 

spacing (fig. 12). Water depths ranged from 12 to 78 m. The 

1998 Corliss cruise collected about 130 km of high-resolution 

seismic-reflection profiles (Cross and others, 1999) between 

the Columbia River and Grays Harbor in water depths ranging 

from 8 to 70 m. The Corliss cruise also collected about 160 km 

of sidescan-sonar swaths (Twichell and others, 2000) in water 

depths ranging from 8 to 40 m. In addition, 95 bottom grab 

samples were collected on this cruise along east-west transects 

in water depths ranging from 8 to 70 m, using a Van Veen grab 

sampler (fig. 12). A 1998 Wecoma cruise (W198WO) collected 

15 grab samples (table 2) in water depths ranging from 4 to 172 

m (G. Dunhill, unpub. data, 1998) using a Shipek grab sampler 

(fig. 12) and 10 gravity cores (16-151 cm long; table 3) in water 

depths ranging from 33 to 132 m (fig. 13). Shell materials from 

the cores have been radiocarbon dated (R. L. Phillips, unpub. 

data, 1999) and range in age up to 2.4 ka.

Accuracy in trackline location for the new data sets is 

excellent, within 10 m. Sidescan-sonar data were collected in 

375-m-wide swaths with 19-cm pixel resolution on the McArthur and 200-m swaths with 10-cm pixel resolution on the 

3

4

5

6

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

AGE, IN MILLIONS OF YEARS BEFORE PRESENT

d18 OX

Y

G

E

N, IN

‰

1 3 5

a

c

e

7 9 11 13 15 17 19 21 23

2 4 6 8 10 12 14 16 18 20 22

Chow Chow

glaciation

Humptulips

glaciation

interglaciation

lower Quaternary

glaciation ?

Figure 9. Graph of marine isotopic stages (MIS) (from Shakleton and others, 1990) based on benthic foraminiferal data from ODP Site 677 

in the equatorial east Pacific Ocean. Oxygen-isotope data record both ocean temperature and ocean volume and are commonly employed 

as a proxy for sea-level change. Numbered minima and maxima refer to specific cool and warm marine isotopic stages, respectively; 

letters refer to specific warm substages during MIS 5. This plot shows the inferred correlation between Pleistocene glacial units and 

major glacio-eustatic sea-level lowstands (blue). Graph also shows correlation between Pleistocene interglacial marine-terrace units and 

major MIS-5 sea-level highstands (red).
19

faults

terrace deposits erosional contact

2 METERS

Figure 10. Exposure of MIS-2 glacial gravels overlying tilted lower Pleistocene sands at the Quinault River. Note 

the fault in lower Pleistocene strata that offsets their upper contact and continues into upper Pleistocene (MIS-2) 

deposits. See plate 1A for location and direction of view.

General Constraints on Offshore Contraction Estimates
20 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

fault

upper Pleistocene strata

slope wash

Figure 11. Exposures of MIS-5 marine-terrace sediments onshore adjacent to the study area. See plate 1A for locations and 

directions of view. A, Faulted upper Pleistocene marine-terrace deposits near Raft River. B, Upper Pleistocene marine-terrace 

sands and pholad-bored outcrop of Hoh rocks near Hogsback. The borings are diagnostic of an intertidal environment and confirm 

the marine origin of the terrace platform. C, Upper? Pleistocene bioturbated marine muds on Langley ridge. D, Faulted upper? 

Pleistocene estuarine channel sands, near Cape Shoalwater.

A
21

pholad-borings at

wave-cut surface

in bedrock outcrop

of Hoh rocks

wood in terrace deposit

terrace sands

slope wash

16 cm

Figure 11—Continued.

B

General Constraints on Offshore Contraction Estimates
22 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

mud

sand

gravel

worm borings

slope wash

5 cm

Figure 11. Exposures of MIS-5 marine-terrace sediments onshore adjacent to the study area. See plate 1A for locations and 

directions of view. A, Faulted upper Pleistocene marine-terrace deposits near Raft River. B, Upper Pleistocene marine-terrace 

sands and pholad-bored outcrop of Hoh rocks near Hogsback. The borings are diagnostic of an intertidal environment and confirm 

the marine origin of the terrace platform. C, Upper? Pleistocene bioturbated marine muds on Langley ridge. D, Faulted upper? 

Pleistocene estuarine channel sands, near Cape Shoalwater—Continued.

C
23

fault

faults?

MIS-5? terrace deposits

slope wash

Figure 11—Continued.

D

General Constraints on Offshore Contraction Estimates
24 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

M1-12

M1-01

M1-02

M1-03

M1-04

M1-05

M1-7A

M1-09

M1-10

M1-14

M1-16

M1-18

M1-07

M1-3A

M1-2A M1-06

M1-9A

M1-08

M1-11

M1-11 M1-11

M1-13 M1-13

M1-13

M1-17

M1-15

C1-12

C1-47 C1-47 C1-42

C1-39

C1-45

C1-40

C1-13

C1-39 C1-39

C1-30 C1-30

C1-46

C1-37

C1-36

C1-28

C1-33

C1-31

C1-26

C1-25

C1-32 C1-35

C1-27 C1-38 C1-38

C1-01

C1-01 C1-01

C1-04 C1-08

C1-02 C1-10

C1-06

C1-03

C1-05

C1-07

C1-09

C1-11

C1-43

C1-41

C1-29

C1-34

M3-25 M3-16

M3-23

M3-21

M3-19

M3-20 M3-24

M3-22

M3-17

M3-17 M3-17

M3-17

M3-36

M3-35

M3-34

M3-30

M3-32 M3-37

M3-28

M3-33

M3-31

M3-29

M3-27

M3-18

M3-26

C198-32

C198-30

C198-31

C198-32

Bathymetric contours generated from NOAA-NOS hydrographic soundings.

Bathymetric contour - 10 m Projection - UTM ZONE 10

Datum - NAD83

5 0

5 0

10

10

10 KILOMETERS

10 MILES

C198WO site

W198WO site

M197WO site

Grab sample sites

C198WO

M398WO

C197WO

M197WO

EXPLANATION

Geophysical tracklines

Willapa

Bay

Grays

Harbor

Point Grenville

Cape

Elizabeth

Cape

Shoalwater

Cape

Disappointment

C ol u mbia River

Quinault Rive r

Copali s

Rive r

Moclips River

Raft River

Q ueets River

124°30' 124°00'W

46°30'

47°00'

47°30'N

Figure 12. Trackline locations for the 1997 and 1998 RV McArthur and 1997 and 1998 RV Corliss cruises. Also 

shown are the locations of grab samples collected during the 1997 McArthur, 1998 Corliss, and 1998 RV Wecoma

(W198WO) cruises.
25

Time Latitude Longitude Water Sample

JD/GMT N. W. Depth Number

(m)

191/1115 47° 21.53’ 124° 34.01’ 70.0 SH-1

191/1149 47° 21.45’ 124° 31.18’ 51.4 SH-2

191/1218 47° 21.56’ 124° 27.36’ 39.7 SH-3*

191/1223 47° 21.61’ 124° 24.49’ 40.2 SH-4

191/1251 47° 21.26’ 124° 25.32’ 29.3 SH-5*

191/1257 47°’21.27’ 124° 25.52’ 30.3 SH-6

191/1325 47° 21.44’ 124° 23.27’ 27.5 SH-7

Table 1. Grab-sample log, cruise of RV McArthur, June 25-July 4, 1997.

[Sampler: Shipek grab sampler. * means no sample recovered. JD is Julian Day; GMT is Greenwich Mean Time.]

Table 2. Grab-sample log, cruise of RV Wecoma, March 16-30, 1998.

[Sampler: Shipek grab sampler. See URL: walrus.wr.usgs.gov/docs/infobank/ for links to RV Wecoma cruise data]

Date/Time Latitude Longitude Water Station Sample Description

(local) N. W. Depth Number Number

(m)

16 Mar/1930 45° 44.99' 123° 59.95' 40 1 G- 1 Clean sand, with many sand dollars

17 Mar/1249 45° 44.98' 124° 02.97' 65 2 G- 2 Fine- to medium-grained sand;

poorly sorted; mafics and quartz

18 Mar/1249 45° 45.02' 124° 08.20' 92 3 G- 3 Fine-grained sand; mafics and quartz

18 Mar/0215 45° 45.01' 124° 13.80' 125 4 G- 4 Silty, fine-grained sand; silt is greenish;

subrounded mafics, rounded quartz

(more mafics than quartz: 70/30%)

18 Mar/0400 45° 45.10' 124° 28.20' 172 5 G- 5 Clay with >5% quartz

19 Mar/1747 45° 45.01' 124° 07.72' 14 6 G- 6 Medium- to coarse-grained sand;

muscovite, biotite, angular olivine,

angular quartz, many mafics

19 Mar/1840 46° 15.05' 124° 10.71' 33 7 G- 7 Fine-grained sand; muscovite,

biotite, clear mineral?

20 Mar/1244 46° 14.89' 124° 14.06' 65 8 G- 8 Silty sand with abundant organic

material (shells, tube worms, etc.)

20 Mar/2153 46° 15.02' 124° 20.66' 133 9 G- 9 No recovery

21 Mar/1030 46° 45.00' 124° 19.62' 62 10 G-10 Silt and fine-grained sand;

muscovite, plant material

22 Mar/0351 46° 45.08' 124° 15.24' 50 11 G-11 Silty fine-grained sand; muscovite,

many worms

22 Mar/0512 46° 45.02' 124° 12.29' 40 12 G-12 No recovery

22 Mar/0630 46° 45.10' 124° 10.98' 29 13 G-13 Clean fine-grained sand; abundant

mica; subrounded mafics, feldspar,

quartz

24 Mar/1435 47° 21.67' 124° 42.47' 112 16 G-15 Success on second try; silty clay

24 Mar/1601 47° 23.44' 124° 35.89' 74 17 G-16 100% mud

24 Mar/1803 47° 24.60' 124° 29.96 39 18 G-17 100% gravel

24 Mar/1849 47° 25.21' 124° 27.17' 26 19 G-18 No recovery

24 Mar/1944 47° 25.55' 124° 25.88' 23 20 G-19 No recovery

24 Mar/0954 47° 52.25' 124° 46.09' 58 21 G-20 Medium- to fine-grained sand; quartz,

mafics, small clams and organic debris

General Constraints on Offshore Contraction Estimates
26 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

Corliss. These data have been integrated as a mosaic at 2-m 

pixel resolution (P.A. McCrory and W.W. Danforth, unpub. 

data, 1999). The boomer and sparker data can resolve features 

as small as 3 m within the upper 200 m beneath the sea floor. 

Resolution of the accompanying bathymetric or chirp data is 

less than 1 m with as much as 15 m of penetration.

These new data are supplemented with a sparse set of 

single-channel seismic-reflection data collected during four 

USGS and one University of Washington research cruises 

between 1967 and 1976 (fig. 14; see appendix 3). The quality 

of these older data range from good to poor owing to variations 

in the acoustic sources and receivers and the sea state. Accuracy 

in trackline location for these earlier cruises was within 500 m.

Sea-Floor Exposure of Fault Traces and Fold Axes

The seismic-reflection data provide an image of the subsurface geometry of faults and folds offshore in the upper 100-500 m 

of strata. The grid spacing allows reconnaissance coverage of shallow features and, in general, is adequate to map structures from 

one track to the next. McArthur line 98-27 crosses within 0.7 km 

of Tidelands State No. 2 well (fig. 3, site L; fig. 15A) and is used 

to correlate between onshore and offshore strata. Corliss line 97-

47 crosses within 0.5 kilometer of P-0155-1 well (fig. 3, site G); 

Corliss line 97-43 crosses within 1.7 km and McArthur line 98-36 

crosses within 3.5 km of this well. All three lines are used to identify the early Pleistocene Raft datum (fig. 4). The sidescan-sonar 

data depict the orientation of faults and folds exposed on the sea 

floor (fig. 15B). These data proved essential for constraining the 

strike and length of emergent structures. Faults on the Continental 

Shelf are generally considered active if they offset the sea floor, as 

sea-level rise during the past 20 k.y. should have eroded any prior 

fault scarps flat. However, this designation proves ambiguous in 

areas where the surficial strata are older than Quaternary and the 

possibility of relict pre-Quaternary morphology exists.

Sediment recovered from the grab samples and gravity 

cores is used both to verify the interpretation of sidescan-sonar 

facies (Twichell and others, 2000) and to provide sedimentation rates for surficial deposits (R. L. Phillips, unpub. data, 

2000). The sea floor displays a typical westward progression 

of beach, nearshore, and mid-shelf facies except for the structural high beneath the inner shelf north of Grays Harbor. This 

structural high is emergent on the sea floor at its northern end, 

where Neogene and lower Pleistocene strata are exposed, and 

is covered by a thin gravel layer at its southern end.

Correlation of Subsurface Folded and Faulted Strata

The orientation of geologic structures mapped in the study 

area varies considerably (Wagner and others, 1986; this study). 

These structures deform strata of inferred Pliocene and Pleistocene age (Grim and Bennett, 1969; Nittrouer, 1978; Wagner 

and others, 1986). The new marine data image a series of faults 

and associated anticlinal folds separated by broad synclinal 

folds. Structures in the central portion of the study area trend 

east-northeast across the shelf and project onshore between 

Grays Harbor and Cape Elizabeth. In general, strata can be 

correlated between folds by matching the character and pattern 

of the seismic reflectors. Palmer and Lingley (1989) used 

formation thickness logged in exploratory petroleum wells 

offshore (fig. 3, sites F, G, L, and Q) and several coastal wells 

onshore (Rau and McFarland, 1982) to correlate with seismic 

reflectors and thus identify and map offshore stratigraphic 

units. Many of the strata exposed on the sea floor and in 

the shallow subsurface in the axes of the anticlines are likely 

equivalent to the Pliocene Quinault strata exposed onshore 

between Cape Elizabeth and Point Grenville (figs. 7, 15B), 

and present in the subsurface south of Point Grenville. Strata 

exposed on the sea floor and in the upper subsurface of synclinal axes include younger units, likely equivalent to the upper 

Quaternary fill in the stream valleys onshore.

The control points on plate 1A represent locations where 

deformed strata have been identified from seismic-reflection 

data, largely from high-resolution boomer or sparker profiles, 

or sidescan-sonar data. These deformation features are principally offset reflectors interpreted to result from shallow faulting 

or tilted and folded reflectors interpreted to result from crustal 

tectonic processes. The high vertical exaggeration of the highDate/Time Latitude Longitude Water Core Station Sample

(local) N. W. Depth Length Number Number

(m) (m)

20 Mar/1600 46° 14.92' 124° 14.02' 65 1.35 8 C-1

20 Mar/2126 46° 15.02' 124° 20.66' 33 0.16 9 C-2

22 Mar/0925 46° 45.00' 124° 25.40' 84 1.51 14 C-3

23 Mar/1458 46° 44.96' 124° 39.90' 132 1.15 15 C-4

29 Mar/0719 46° 59.94' 124° 32.01' 75 0.47 22 C-5

29 Mar/0954 46° 49.98' 124° 35.96' 106 0.96 23 C-6

29 Mar/1208 46° 49.93' 124° 25.89' 75 0.8 24 C-7

29 Mar/1509 46° 35.81' 124° 22.14' 81 0.95 25 C-8

29 Mar/1756 46° 24.01' 124° 17.04' 63 0.61 26 C-9

29 Mar/2047 46° 08.02' 124° 12.99' 85 0.52 27 C-10

Table 3. Core log, cruise of RV Wecoma, March 16-30, 1998.

[Sampler: gravity corer, weight 973 lbs. See URL: walrus.wr.usgs.gov/docs/infobank/ for links to RV Wecoma cruise data]
27

60 55

53

27

5

5

6 40

40

32 32

13 10

37

37

33 36

60

34

34

4.6

4.0

7.0

8.5

13.6 7.5

15.2

9.0

27.6 15.2

15.2

0.2

0.6

1.0

1.5

0.8

0.5

1.2

1.0

1.4

15.4

15.3

22.1

18.4

18.3

Willapa

Bay

Grays

Harbor

Point Grenville

Cape Elizabeth

Cape

Shoalwater

Long Beach

Peninsula

Cape

Disappointment C ol u mbia River

Quinault Rive r

Copali s

Rive r

Moclips River

Raft River

Q ueets River

Bathymetric contours generated from NOAA-NOS hydrographic soundings.

Bathymetric contour - 10 m Projection - UTM ZONE 10

Datum - NAD83

5 0

5 0

10

10

10 KILOMETERS

10 MILES

USGS-PSU drill core

Smith et al. drill core

W198WO gravity core

PSU-GHC drill core

CORE SITES (Numbers indicate length of each core in meters.)

124°30' 124°00'W

46°30'

47°00'

47°30'N

Figure 13. Locations of drill-core sites in the Willapa Bay and Grays Harbor areas (from Peterson and Phipps, 

1992; Smith and others, 1999; and Herb, 2000) and 1998 RV Wecoma (W198WO) gravity-core sites (G. Dunhill, 

unpub. data, 1998) on the mid shelf.

General Constraints on Offshore Contraction Estimates
28 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

O10-30

O10-24

O10-24

O10-25

O10-26

O10-27

O10-28

O10-29

O10-30

O10-30

O10-20

O10-21

O10-22

O10-23

010-16

O10-15

O10-14

O10-13

O10-12

O10-11 O10-11

O10-12 O10-1

O

9

10-17

O10-17

L3-16

L3-18

L3-14

L3-12

L3-11

L3-10

L3-10

L3-13

L3-17

L5-25

L5-23

L5-19

L5-18

L5-17

L5-16

L5-24

L11-08

L11-09

L11-10

Willapa

Bay

Grays

Harbor

Point Grenville

Cape Elizabeth

Cape

Shoalwater

Grayland

Cape

Disappointment

Long Beach

Peninsula

C ol u mbia River

Quinault Rive r

Copali s

Rive r

Moclips River

Raft River

Q ueets River

G185WO

L1180WF

L577WO

L376WO

O174WO

O1067WO

GEOPHYSICAL

TRACKLINES

Bathymetric contours generated from NOAA-NOS hydrographic soundings.

Bathymetric contour - 10 m Projection - UTM ZONE 10

Datum - NAD83

5 0

5 0

10

10

10 KILOMETERS

10 MILES

124°30' 124°00'W

46°30'

47°00'

47°30'N

Figure 14. Trackline locations for the 1967 to 1976 research cruises that collected seismic-reflection data used in this 

study. See appendix 3 for description of cruise data. Subsurface information in Grays Harbor used in this study was 

derived from the 1995 research cruise G185WO (see Peterson and Phipps, 1992).
29

Figure 15. Examples of seismic-reflection and sidescan-sonar data used to 

map structures offshore Washington. Green dotted lines denote stratigraphic 

reflectors, red dotted line denotes late Pleistocene unconformity surface, 

blue dashed lines denote inferred faults, yellow dashed line denotes first 

sea-floor multiple. See plate 1A for locations of the images. A, Tidelands 

State No. 2 well stratigraphy (see fig. 3, site L) (from Rau and McFarland, 

1982) is superimposed on a McArthur seismic-reflection offshore profile that 

crosses within 700 m of the well. Plot depicts correlation between seismic 

stratigraphy and lithofacies. The Quinault Formation designation of Rau 

and McFarland (1982) for this well was reinterpreted by Palmer and Lingley 

(1989), who separate the Quinault Formation into Montesano and Quinault 

Formations at an observed angular unconformity. Palmer and Lingley (1989) 

assign the overlap strata in this well to the Montesano Formation only. See 

figure 18A for a larger version of the profile. B, Corliss seismic-reflection 

profile and corresponding sidescan-sonar image (375-m swath width) 

that depict the rugged relief of strata equivalent to the upper Quinault 

lithofacies onshore. C, McArthur seismic-reflection profile that shows 

inferred lowstand and highstand deposits and bounding unconformities 

within inferred upper Quaternary strata.

A

resolution profiles makes it difficult to determine fault dips with 

any accuracy. In general, all fault dips greater than 30° appear as 

vertical structures. Observation of deformation is also hampered 

by sea-floor multiples overprinting the seismic reflectors. This 

problem is most evident in shallow water, such as west of Long 

Beach Peninsula, or where the sea floor is highly reflective, such 

as in Willapa River (underlain by the Crescent basalt).

Deformation of Quaternary Unconformities

The seismic-reflection data are adequate for identifying and 

mapping the erosional surface at the base of inferred upper 

Pleistocene sediments offshore (fig. 15C). We contoured the 

highest widespread unconformity beneath the sea floor using 

present sea level as the reference datum. Seismic velocity was 

converted to depth using an average speed of 1,500 m/s (twoway travel time). The contour map depicts a gently dipping 

surface beneath the inner shelf (plate 1B) that has been modified 

by erosional and tectonic processes. Between Willapa Bay and 

Grays Harbor, this surface forms a flat bench beneath the midshelf at a depth of 70-85 m. The older data (1976 and 1977 Lee 

cruises) depict a deeper bench at a depth of 115-125 m beneath 

the outer shelf to the west, separated from the mid-shelf bench 

by a linear scarp (plate 1B). This morphology is similar to marine 

terraces onshore that form benches cut during sequential glacioeustatic highstands. We infer that the offshore benches represent 

similar erosional platforms cut during glacio-eustatic lowstands.

From Grays Harbor past the northern end of the study 

area (Hoh River), the erosional unconformity has an irregular surface morphology that extends from near the present 

coastline out to a depth of about 70 m (Wolf and others, 1997; 

this study). Available seismic-reflection profiles and sidescansonar images depict wide areas of rocky outcrop throughout 

this region. The lack of a Holocene sediment cover may result 

from recent uplift (Palmer and Lingley, 1989), as this region 

has undergone intense folding and faulting compared with 

adjacent areas to the west and south. Filled paleochannels 

cross the region of rocky outcrop (plate 1B), suggesting that 

streams have cut through it during sea-level lowstands.

The unconformity changes character both east to west 

General Constraints on Offshore Contraction Estimates

0.1

0

0.2

TWO-WAY TRAVEL TIME, IN SECONDS

M398 - LINE 27 SPARKER

NW SE

Vertical exaggeration x 13.3

0

150

450

300

600

750

1050

1350

900

1200

1500

DEPTH, IN METERS

8000 9000 10000

SHOT NUMBER

Montesano Formation (Palmer and Lingley, 1989)

(Rau and McFarland, 1982)

Fault?

100'
30 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

Figure 15. Examples of seismic-reflection and sidescan-sonar data used to map structures offshore Washington. Green dotted lines denote stratigraphic 

reflectors, red dotted line denotes late Pleistocene unconformity surface, blue dashed lines denote inferred faults, yellow dashed line denotes first sea-floor 

multiple. See plate 1A for locations of the images. A, Tidelands State No. 2 well stratigraphy (see fig. 3, site L) (from Rau and McFarland, 1982) is superimposed 

on a McArthur seismic-reflection offshore profile that crosses within 700 m of the well. Plot depicts correlation between seismic stratigraphy and lithofacies. 

The Quinault Formation designation of Rau and McFarland (1982) for this well was reinterpreted by Palmer and Lingley (1989), who separate the Quinault 

Formation into Montesano and Quinault Formations at an observed angular unconformity. Palmer and Lingley (1989) assign the overlap strata in this well 

to the Montesano Formation only. See figure 18A for a larger version of the profile. B, Corliss seismic-reflection profile and corresponding sidescan-sonar 

image (375-m swath width) that depict the rugged relief of strata equivalent to the upper Quinault lithofacies onshore. C, McArthur seismic-reflection profile 

that shows inferred lowstand and highstand deposits and bounding unconformities within inferred upper Quaternary strata—Continued.

B

across the shelf and south to north along the coast. Variations 

along the shelf primarily reflect differing sediment supplies 

and differing capacities of coastal streams to distribute sediment. Discussion of these variations is beyond the scope of 

this paper. Variations across the shelf suggest that the surface 

is a composite of multiple erosional episodes, and thus may 

vary in age (see discussion below). In general the outer shelf 

appears to be a region of sediment accumulation and the 

inner shelf a region of sediment erosion and reworking. This 

configuration may be explained by tilting of the Continental 

Shelf, with the outer shelf subsiding and the inner shelf rising.

Contraction Estimates for Offshore Faults 

and Folds

The study area can be divided into three regions on the basis 

of the orientation of geologic structures. South of Grays Harbor 

(46°53’N.), folds and faults trend north-south, roughly parallel 

to the western margin of the Coast Range block. Between Grays 

Harbor and Cape Elizabeth (47°21’N.), structures trend eastwest, subparallel to an eastward step in the block boundary. The 

region north of Cape Elizabeth is defined by a shift in structural 

orientation back to north-south, typical of subduction-related 

deformation elsewhere in the accretionary complex.

North-South Domain: Columbia River to Grayland

Between the Columbia River and Grayland, the boundary 

between the Oregon Coast Range (OCR) and Olympic Mountains (OM) blocks is marked by a north-south fault zone. The 

boundary parallels the coastline and apparently dips about 

25° eastward (fig. 2) (Snavely and Wagner, 1982; Parsons and 

others, 1999). On the basis of block kinematics, the northsouth-trending boundary on the west side of the OCR block 

TW

O-WAY TRAVELTIME, IN SECONDS 0.12

0.10

0.08

0.06

0.04

0.02

0.0

90

60

30

0

DEPTH, IN METERS

C197 - LINE 5 BOOMER

NW SE

Vertical exaggeration x 16.7 at 1,500 m/s 1 KILOMETER

bedrock ribs

7000 6000 5000 4000

7000 6000 5000 4000

SHOT NUMBER

0°

5°

10°

20°

V. E.

1:16.7
1000

M398 - LINE 21 SPA

RKER

W

E

Vertical exaggeration x 10.0 at 1,500 m/s 1 KILOMETER

Prograding low-stand? strata

Flat-lying high-stand? strata

0

75

150

0

0.10.2

500 1500 2000 2500 3000 3500 4000 4500 5000 5500

SRETE M NI , HTPED

SDNOCES NI , E MI T LEVART YA W- OWT

SHOT NUMBER

0°

5°

10°

20°

V.E.1:10.0

31

C

Figure 15—Continued.

is expected to accommodate predominantly dextral shear. 

However, the subsurface geometry of the boundary suggests 

thrust movement. From well data and wide-angle seismic 

data, the boundary can be extrapolated to the surface just west 

of the coastline. This location has not been confirmed, as no 

subsurface trace has been identified in the available highresolution seismic-reflection data. Locating the trace of this 

fault is complicated by sea-floor multiples overprinting and 

obscuring the erosional unconformity in water depths < 20 

m. In Willapa Bay to the east, we have mapped a 3-km-wide 

fault zone in the upper plate of the dipping boundary (plate 

1A). This multi-strand fault zone elevates upper Quaternary 

strata on its western side and lowers strata on its eastern side 

to form a horst-graben pair (plate 1A). If the horst is elevated 

from thrust faulting, then the central fault strand would represent a back thrust in the upper plate of the east-dipping block 

boundary. Alternatively, the fault strands may accommodate 

transpressional slip and the graben may represent an elongate 

sag between fault strands. A back thrust would intersect the 

boundary thrust no deeper than 7 km, implying that such a 

structure terminates at too shallow a depth to be independently seismogenic. Steeply to gently east-dipping transpressional faults would intersect the boundary thrust much deeper. 

In either case, the recency and magnitude of movement on 

these fault strands imply activity on the postulated main thrust 

to the west.

Single-channel seismic-reflection profiles in Willapa Bay 

(Hill and others, 1981; Cross and others, 1999) depict a 

30-km-long fault zone that trends north-northwest through the 

bay and may continue another 10+ km northwestward across 

the inner shelf (plate 1A). The fault zone vertically offsets 

a widespread buried unconformity beneath Willapa Bay (fig. 

16). This buried surface has a surprisingly complex morphology that depicts areas of roughness, small paleochannels, and 

closed depressions (plate 1B). Rough paleorelief occurs where 

older strata have been elevated in a structural ridge to form the 

buried erosional surface on the western side of the fault zone. 

The rough patches, which are 25-35 m below sea level and 15 

m below the bay floor, together form a linear trend that parallels the fault zone,. To the north, the structural ridge abuts a 

north-south-trending topographic high near Cape Shoalwater, 

and to the south it projects toward Long Island, a northnorthwest-trending topographic high underpinned by an anticlinal fold (Walsh and others, 1987; Wells, 1989) that juts 

into Willapa Bay. The closed depressions east of the buried 

ridge and the minor channels that breach the ridge are reminiscent of geomorphic features associated with young strikeslip faults onshore. A number of north-northwest-trending 

Neogene structures have been mapped south and east of Willapa Bay (Wells, 1989). Some of these structures project into 

the bay and closely align with individual fault strands we 

have mapped beneath the bay. Confirmation of these potential 

connections will require further work.

The erosional surface shoals eastward, rising from about 

25 m below sea level on the shelf west of Willapa Bay to 14 

m below sea level on the east side of Willapa Bay (plate 1B). 

Upper Pleistocene strata are exposed in some modern tidal 

channels and in the bluffs that bound the east side of Willapa 

Bay (Clifton and others, 1989; Kennedy and others, 1982; R. L. 

Contraction Estimates for Offshore Faults and Folds
32 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore WashingtonBedded sand Cross-bedded sand Silt and mud O174 - LINE 078 SINGLE CHAN

10

Vertical exaggeration x 4.2 at 1,500 m/s

W

TIME, IN MINUTES

1132 1130

erosional surface

erosion

fault zone

601020304050

39.2530.72

7.591.75

AGE FROM

RADIOCARBON

DATES, IN ka

DEPTH,

IN METERS

0

N

S

0°

5°

10°20°

V.E.1:4.2

Figure 16. Composite lithostratigraphic column and seismic-reflection profiles in Willapa Bay. See plate 1

A for locations of the seismic-reflection image

column from borehole studies (Smith and others, 1999; Herb, 2000), with radiocarbon dates indicating that the erosional surface (dotted red line) formlocations of borehole sites. 

B, C, Seismic-reflection profiles showing offset ca. 20-ka erosional surface across “Willapa Bay” fault zone in central Willa

Bay (

C). Red dotted line denotes unconformity; blue dashed lines denote inferred faults, yellow dashed line denotes first sea-floor multiple.

A

B
33

0°

5°

10°20°

V.E.1:12.5

erosional surface

E

W

erosional surface

fault zone

fault

vessel noise

bend in trackline

075150

C19

8 - LIN

E

3

2

S

PA

R

K

E

R

SRETE M NI , HTPED

SDNOCES NI , E MI T LEVART YA W- OWT

Vertical exaggeration x 12.5 at 1,500 m/s

1 KILOMETER

"Wilapa Bay" fault zone

SHOT NUMBER

Figure 16—Continued

C

Phillips, oral commun, 1998). The upper Pleistocene strata may 

correlate with the unit beneath the eastward-shoaling surface.

Age of Late Pleistocene Datum

The age of the erosional surface beneath Willapa Bay 

(plate 1B) is constrained by radiocarbon dates from shell 

and wood material recovered from boreholes that penetrate 

Long Beach Peninsula (fig. 16A) (Smith and others, 1999; 

Herb, 2000). In particular, dates from ~36 m below and ~8 m 

above the erosional surface indicate that the surface formed 

between 30.7 and 7.6 ka. The surface is likely a ravinement or 

lowstand surface, judging from the paleochannels associated 

with it, which would restrict its age to the MIS-2 glacial 

period, ca. 20 ka. Alternatively, the surface could have been 

cut during the subsequent rise in sea level that would have 

flooded this portion of the shelf starting about 9 ka. Peterson 

and Phipps (1992) mapped a similar surface at similar depths 

beneath Grays Harbor, where marine sediment overlying the 

erosional surface is radiometrically dated at ca. 7.5 ka, suggesting that the two estuarine surfaces are correlative.

The highest widespread unconformity mapped beneath the 

Continental Shelf (Nittrouer, 1976; Wolf and others, 1997; 

Twichell and others, 2000) was likely cut during major late 

Pleistocene sea-level transgressions and modified by stream 

erosion during sea-level lowstands. The nearshore portion of 

this surface may be a continuation of the prominent unconformity mapped in Willapa Bay, as well as of the one previously 

mapped in Grays Harbor (Peterson and Phipps, 1992). The 

paleoshelf surface projects eastward into both estuaries (plate 

1B) at elevations within a few meters of those beneath the 

estuaries (all range from 20 to 30 m below sea level).

Radiocarbon dates from gravity cores recently collected 

at mid-shelf depths offshore Willapa Bay (fig. 13) place the 

base of Holocene deposits at 4-7 m below the sea floor in 

60-80 m of water (R.L. Phillips, unpub. data, 1999) and 

25-40 m above the mapped unconformity. Extrapolating constant rates of sedimentation calculated from the dated cores 

(0.4-0.7 mm/yr) suggests that the surface formed about 70 

ka in the mid-shelf region. If, however, sediment accumulation occurred mainly during submerged conditions, then this 

surface could be approximately twice as old or have been cut 

about 140 ka—during the MIS-5 sea-level transgression. If 

this is correct, then the surface beneath Willapa Bay clearly 

represents a much younger surface.

The disparity in age estimates for the erosional surface 

suggests that a simple model of the surface representing a 

single transgressional event is unlikely. In fact, the surface 

mapped on the inner shelf onlaps a buried mid-shelf high 

and then separates into a series of three unconformity-bound 

stratigraphic sequences beneath the outer shelf. This geometry suggests that the outer shelf is a region of sediment accumulation (owing perhaps to downward tilting or subsidence), 

where successive cycles of sea-level fluctuations are preserved as a stack of unconformity-bounded strata. Apparently 

the inner shelf, with a single unconformity, is a region of sediment reworking and removal (owing perhaps to upward tilting 

Contraction Estimates for Offshore Faults and Folds
34 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

or uplift), where subsequent sea-level cycles remove or significantly modify previous ones. In this scenario the mid-shelf 

serves as a hinge-line where successive transgressions are 

preserved as lag deposits, without, however, angular discordance, making them difficult to discern. Because of these complexities and the general lack of direct age control, we assign a 

broad late Pleistocene age (<150 ka) to the surface in the outer 

and mid-shelf regions and a latest Pleistocene age (<20 ka) to 

the surface in the inner shelf and estuarine regions.

Slip Rate of Fault Zone in Willapa Bay

The fault zone in Willapa Bay is composed of multiple 

fault strands with both west-side-up and east-side-up sense 

of movement. Evidence for significant late Pleistocene movement on this fault zone is observed both onshore adjacent 

to Willapa Bay and beneath the bay floor. For example, in 

southern Willapa Bay, the fault zone projects onshore on the 

eastern side of Long Island, where a thrust fault offsets upper 

Pleistocene deposits 2 m. On the northwest side of the island, 

a vertical fault offsets Pleistocene deposits 0.8 m, west side 

up (fig. 17).

The fault zone aligns with a series of depressions in 

the erosional surface, reminiscent of sag ponds aligned 

with strike-slip faults. For example, in central Willapa Bay, 

a fault strand that vertically offsets the erosional surface 

about 5 m bounds the southwest side of an elongate depression (Wolf and others, 1998). The parallel alignment of the 

depression and the fault suggest that fault displacement may 

have partially controlled development of the depression. 

Four kilometers to the north, a fault strand, observed on 

several seismic-reflection profiles, offsets the unconformity 

between 4 and 8 m, west side up (fig. 16B). If the fault zone 

has a component of lateral shear, then these depressions 

may reflect differential slip between strands, causing a 

graben to form.

The largest cumulative offsets and widest zone of faulting are observed in northern Willapa Bay. Here, the buried 

unconformity and underlying strata are vertically offset 10-12 

m, west side up, across a narrow zone (Wolf and others, 

1998). However, another fault strand 500 m to the west offsets 

the unconformity 4-5 m, west side down. A Corliss trackline 

crosses the fault zone at an oblique angle, and the seismicreflection data depict a multistrand fault zone about 2 km 

wide (fig. 16C). This zone projects onshore at Cape Shoalwater. A quarry near Cape Shoalwater exposes offset estuarine deposits along the northern projection of the fault zone 

(fig. 11D). These are minor thrust faults with a cumulative 

displacement of about 1 m in cross-bedded sands overlying 

the marine-terrace platform. Lateral slip, if any, is undetected. 

The Willapa Bay data yield a maximum vertical offset rate of 

~10 m per 20 k.y. or 0.5 mm/yr.

A fault mapped 4 km west of Cape Shoalwater may be 

an en echelon continuation of the “Willapa Bay” fault zone. A 

fault mapped 11 km west of Cape Shoalwater vertically displaces the ravinement or transgressive surface about 8 m. This 

scarp is buried by about 12 m of post-ravinement sediment. If 

the surface has an age of 20 ka, this age yields an offset rate 

of 0.4 mm/yr. If this surface is ca. 140 ka, then the rate is 0.06 

mm/yr. This structure may correlate with a thrust fault and 

associated anticline mapped by McNeill and others (1998) 

from industry multichannel data about 5 km further west.

A number of faults and folds underlie the shelf north of 

Willapa Bay, adjacent to the Grayland area. In general the 

structures are buried fairly deep, often beyond reach of the 

high-resolution seismic-reflection data. New seismic data do 

not image faults or folds in the upper 90 m of strata adjacent 

to Willapa Bay. The older, deeper penetration data image 

faults and folds beneath the shelf that trend north-south, parallel to the block boundary. These structures typically do not 

deform strata above the late Pleistocene unconformity and 

thus may not have been active in late Quaternary time.

East-West Domain: Grays Harbor to Cape Elizabeth

Near the entrance to Grays Harbor, structures abruptly 

shift from a north-south orientation to an east-northeast 

orientation over a 6-km-wide zone (plate 1A). The eastnortheast-trending folds and faults are clearly contractional 

structures both offshore (plate 2) and onshore (McCrory, 

1996), and they characterize the deformation between 

Grays Harbor and Cape Elizabeth. The change in structural 

orientation is marked by a major fault that cuts across the 

shelf and projects into northern Grays Harbor (plate 1A). 

Onshore, the multistrand fault appears to mark the boundary 

between the Oregon Coast Range and Olympic Mountains 

blocks as judged by basement affinities in petroleum wells 

drilled on either side of the fault zone (fig. 3). The Sunshine Minard No.1 well (fig. 3, site J), on the south side of 

the fault, bottoms in Eocene basalt, and the Shell Minard 

No. 1-34 well (fig. 3, site K), 4 km to the north, bottoms in 

Hoh mélange (Rau and McFarland, 1982). The projected 

block boundary offshore coincides with the bounding fault 

mapped with new seismic-reflection and sidescan-sonar 

data. In addition, recently collected multibeam data image 

a trough that extends to within 3 km of the coastline (G. R. 

Gelfenbaum, unpub. data, 1999) along trend with the fault, 

suggesting recent disruption of the sea floor.

The “Grays Harbor” fault and associated anticline mark 

the southern end of a bedrock high that extends 60 km from 

Grays Harbor northward to the Raft River (plate 1B). Nearshore, the high is either barren of sediment, covered with 

relict, oxide-coated gravels, or crossed by small channels 

filled with unconsolidated sediment (plate 2G). Seaward from 

about 60-m water depth, the high is commonly onlapped by 

rippled sediment. The lack of significant sediment cover may 

result from ongoing uplift associated with underlying structures. Uplift would disrupt modern sediment transport (Wolf 

and others, 1997; Twichell and others, 2000) by isolating the 

region from both northward-transported sediment from the 

Columbia River and westward-transported sediment from 

local coastal streams.

The bedrock high coincides with the mapped unconformity in the central region (plate 1B). This unconformity sur-
35

fault

lower Pleistocene strata

Figure 17. Bluffs on northwestern Long Island in Willapa Bay, showing 0.8-m vertical offset of lower Pleistocene strata. See plate 

1A for location and direction of view.

Contraction Estimates for Offshore Faults and Folds
36 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

face has been modified by erosion. In particular, three filled 

channels cut through the bedrock surface: one aligns with the 

entrance to Grays Harbor, another projects toward Copalis 

River, and the third projects toward Moclips River. In detail, 

the morphology of the surface shows some correlation with 

underlying structures. Anticlinal folds tend to underpin areas 

of emergent bedrock, and synclinal folds tend to align with 

paleochannels. This correlation suggests that the complex 

morphology is in part controlled by Quaternary structural 

activity.

“Grays Harbor” Fault Zone

 Seismic-reflection profiles depict the “Grays Harbor” 

fault as a 2.5-km-wide zone of reverse faults and back thrusts 

within a broad anticlinal fold with a wavelength of 15 km. 

The upward extent of fault strands varies from profile to 

profile. On some profiles, fault strands appear to die out in 

lower Pleistocene strata. On other profiles, individual fault 

strands breach and offset the sea floor. Twenty km west of 

Grays Harbor, the fault vertically displaces the sea floor 12 

m over a 1.7-km-wide zone (plate 2J, K). Two kilometers 

west of Grays Harbor, the fault vertically offsets the late 

Pleistocene erosional surface by more than 40 m, and one of 

the fault strands displaces the sea floor a few meters. Owing 

to uncertainties about the age of the unconformity near shore 

(150-20 ka), a vertical displacement rate is not calculated for 

this offset.

“Saddle Hill” Fault Zone

About 4 km north of the “Grays Harbor” fault, another 

multistrand fault crosses the shelf and continues onshore. This 

fault bounds a filled channel that breaches the bedrock high. 

Moore (1965) documented thrust faults within Pleistocene 

gravels beneath Saddle Hill (McCrory 1996), where the fault 

projects onshore. Petroleum well Tidelands State No. 2 (fig. 

3, site L) was drilled into a fault strand about 3 km offshore. 

Rau and McFarland (1982) document thrust faulting within 

Olympic subduction rocks penetrated in this well using a 

biostratigraphic age reversal. McArthur lines 98-27 and 98-29 

cross near the well location. These seismic-reflection profiles 

depict a pair of faults about 1 kilometer apart on either side 

of an anticline (fig. 18).

Montesano sandstone lithofacies (fig. 4) abruptly thin 

north of Saddle Hill between the Union State No. 3 and Shell 

Sampson Johns No. 2-15 wells (fig. 3, both site M), suggesting substantial fault displacement (Rau and McFarland, 1982; 

Palmer and Lingley, 1989). Palmer and Lingley (1989) propose that the “Saddle Hill” fault accommodates translational 

motion as well as compressional motion, on the basis of this 

mismatch in sandstone thickness across the fault zone. Given 

the evidence for thrust faulting discussed above, the facies 

mismatch may instead result from sediment-dispersal barriers that formed by growth faulting concurrent with sediment 

accumulation in the Miocene.

Structures in the Point Grenville Area

On the inner shelf between Saddle Hill and Point Grenville, additional faults and folds are well imaged in seismicreflection profiles and sidescan-sonar swaths. These structures 

range in length from about 5 to 20 km. Available data are not 

sufficient to map the longer structures further west than the 

mid-shelf region, about 25 km west of the coastline. A series 

of synclinal folds with wavelengths of 3-6 km (plate 2A-C) 

can be mapped from the mid shelf to the coastline. Several 

synclinal axes project towards stream valleys onshore, suggesting a structural basis for the location of Connor Creek, 

Copalis River, Boone Creek, Joe Creek, and Moclips River 

(plate 1A). Additionally, several intervening anticlinal folds 

project towards structural ridges previously mapped onshore 

(McCrory, 1996, 1997). McCrory (1996, 1997) documented 

thrust faults and folds in Quaternary strata that underpin the 

onshore ridges just south of Copalis River, Joe Creek, and 

Moclips River. One large anticlinal structure, imaged on 

Corliss line 97-01 between Copalis River and Boone Creek 

(fig. 19), is not emergent onshore.

The structural pattern changes adjacent to Point Grenville. Here structures are more closely spaced, with wavelengths of 0.5-1 kilometer, and appear to curve around 

Point Grenville (plate 1A). These curved folds and faults are 

bounded on the north by a large anticlinal fold (plate 2E) 

that is emergent on the sea floor. Wagner and others (1986) 

mapped the anticline as diapiric because of the lack of internal 

reflectors on seismic-reflection profiles. This interpretation is 

supported by additional seismic-reflection profiles collected in 

this study and by the incoherent mélange material exposed on 

the sea floor in sidescan-sonar images of the fold core. The 

fault-bounded diapir exposed in the adjacent sea cliffs at Point 

Grenville (Rau and Grocock, 1974; Rau, 1975) may be an 

onshore extension of this structure.

Faults and folds to the northwest of the tightly folded area 

trend northeast-southwest, consistent with structural orientations 

to the south. One of these faults displaces the sea floor (Wagner 

and others, 1986; this study) and projects onshore near Cape 

Elizabeth where the MIS-5a terrace datum jumps from 24 to 

51 m in elevation (West and McCrumb, 1988, McCrory, 1996), 

reaching there its highest elevation along the Washington coast. 

A 26-m offset of the ca. 83-ka marine terrace platform (MIS-5a) 

yields a rate of vertical displacement of 0.3 mm/yr.

Contraction Estimate

The onshore-offshore structural correlations are here used 

as a basis for estimating Quaternary deformation. To obtain a 

first-order approximation of the rate of shortening across this 

40-km-wide region, we assume that the topographic ridges 

onshore reflect Quaternary shortening that correlates with 

the erosionally truncated anticlines offshore. This assumption ignores the effects of erosion that would tend to subdue 

relief. All the topographic relief is assigned a structural origin 

analogous to the anticlinal folds preserved offshore. The presence of estuarine deposits on the crest of Langely Hill at 67-m 
0°

5°

10°

20°

V.E.

1:10.0

0°

5°

10°

20°

V.E.

1:13.3

M398 - LINE 27 SPARKER

0.0

0.1

T

0.2 WO-WAY TRAVEL TIME, IN SECONDS TWO-WAY TRAVEL TIME, IN SECONDS

0.0

0.1

0.2

0

75

150

DEPTH, IN METERS

NW SE

NW SE

0

75

150

DEPTH, IN METERS

Vertical exaggeration x 13.3 at 1,500 m/s

Vertical exaggeration x 10.0 at 1,500 m/s

1 KILOMETER

1 KILOMETER

SHOT NUMBER

SHOT NUMBER

8000 9000 10000

5000 4000 3000

M398 - LINE 29 SPARKER

37

Figure 18. Seismic-reflection profiles of the “Saddle Hill” fault zone at 4 km (A) and 8 km (B) west of the coastline, showing displacement of 

upper Quaternary strata. See plate 1A for locations of the images. Green dotted lines denote stratigraphic reflectors, red dotted line denotes late 

Pleistocene unconformity surface, yellow dashed line denotes first sea-floor multiple, blue dashed lines denote inferred faults. Profile A crosses 

within 700 m of Tidelands State No. 2 well.

A

B

elevation (fig. 11C) lends support to this assumption. The 

ridges are cored by lower Pleistocene deposits as indicated by 

well stratigraphies (fig. 19; Rau and McFarland, 1982), and a 

maximum early Pleistocene age is assigned to the deformation. We also assume that the unconformity surface preserved 

in synclinal folds offshore is equivalent to the surface buried 

in the stream valleys onshore. The offshore surface is younger 

than the onshore one, and at the south end of the profile likely 

represents the late Pleistocene datum mapped in plate 1A.

The combined amplitude of the folds ranges up to 160 m, 

and their wavelengths vary from 15 km in the south to 3 km in 

the north. We have measured the length of the geomorphic surface onshore and of the synclinal folds offshore (fig. 19) from 

Grays Harbor to Point Grenville following the shape of the 

folds. The offshore measurement only includes synclinal folds 

imaged on Corliss line 97-01. A 3.5-km gap across the strike of 

the “Saddle Hill” fault zone was caused by equipment malfunction during the cruise. In addition, the shortening estimate does 

not include the offset terrace north of Point Grenville, which is 

not completely imaged on the Corliss line.

The composite datum suggests a decrease in line length 

of about 2.0 km (fig. 19). If the age of the surface is 0.9 Ma, 

the shortening rate is about 2.2 mm/yr; if its age is 0.6 Ma, 

then the rate is 3.3 mm/yr. If the folding reflects movement on 

buried thrust faults offshore analogous to those mapped in the 

core of the anticlinal ridges onshore, then the cumulative slip 

rates assigned to such faults would be considerably higher. As 

a first approximation, this region adjacent to the block boundary appears to accommodate as much as half the available 

6-10 mm/yr convergence at the surface.

Contraction Estimates for Offshore Faults and Folds
38 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

Copalis River

Connor Creek

Joe Creek

Boone Creek

Moclips River

Wreck Creek

Saddle Hill

Joe Creek ridge

Moclips ridge

Langley Hill

Wreck Creek ridge

Vertical exaggeration x 50 at 1,500 m/s

ELEVATION, IN METERS

40 35 30 25 20 15 10 5 -0 

90

120

6030

0

NN

TWO-WAY TRAVEL TIME, IN SECONDS

0.0

0.040.080.12

40 35 30 25 20 15 10 5 0 

C97 LINE 1.1-1.5 BOOMER

O L

M

*

DISTANCE FROM NORTHERN GRAYS HARBOR, IN KILOMETERSDISTANCE FROM NORTHERN GRAYS HARBOR, IN KILOMETERS

Site of upliftedmarine depositsshown in figure 11C

7 6 5 4 3 2 

Figure 19. Profiles parallel to coastline used to calculate shortening of Quaternary strata. See plate 1

A for locations of the profiles. 

A, Onshore geomorp

shortening (Raft datum) accommodated in anticlinal folds. Labeled well symbols denote locations of coastal wells with respect to topography. See figulines beneath well symbols denote thickness of Quaternary strata in these wells (from Rau and McFarland, 1982). 

B, Offshore seismic-reflection profile u

approximately equivalent to Langley datum) accommodated in synclinal folds (a portion of this profile is also shown in plate 2

A). Circled numbers identi

faults depicted in profile. Note the decreasing wavelength of folds from south to north. 

A

B
39

Seven primary anticlinal folds and associated faults are 

imaged on Corliss line 97-01 (fig. 19). The western extent of 

these transverse structures is not well documented. Some are 

more than 35 km long and others appear to die out after 10-15 

km. The morphology of the unconformity—with contours jutting 

to the west— suggests that the fault zone may extend another 10 

km westward. The wavelength of the folds decreases northward: 

the fold associated with the bounding fault is about 15 km wide, 

whereas further north the fold wavelength is as short as 3 km. 

This configuration suggests the presence of a northward-shoaling 

detachment fault in the subsurface below the folds on the basis of 

fault-bend fold models. Detailed consideration of this inference is 

beyond the scope of this paper. Interestingly, the onshore ridges 

are higher in elevation in the north, where the fold wavelength 

is shorter, suggesting that these northern structures are growing 

in size along trend from offshore to onshore. The lack of surface 

relief in the south may result from stream erosion or from the 

structures diminishing rapidly eastward. Available data are not 

sufficient to assign slip rates to specific structures. For preliminary 

hazard assessments, therefore, the shortening rate of 2.2-3.3 mm/yr 

is spread evenly across this zone of seven primary structures.

Northern Domain: Cape Elizabeth and North

North of Cape Elizabeth, structures shift progressively 

from an east-northeast orientation to a north-northwest orientation (plate 1A) over a 6-km-wide zone. The erosional surface in this region forms a gently westward-dipping surface 

down to a depth of about 85 m (plate 1B). Below 85 m, the 

surface dips more steeply and is buried by a thickening wedge 

of sediment. A narrow filled channel cuts across the surface 

and projects eastward toward the mouth of Queets River. A 

large apron of prograded strata flanks the channel (fig. 15C) 

and likely represents glacial outwash deposits derived from 

the Queets River drainage. In general, the structures underlying the northern region are not expressed in the morphology 

of the erosional surface. However, isolated patches of emergent bedrock do occur along some structures.

Many faults mapped in the northern region do not offset 

or deform the late Pleistocene unconformity. However, some 

faults do show evidence of ongoing contraction consistent with 

subduction-driven processes. A nearshore fault just north of 

Cape Elizabeth projects onshore to join with the late Pleistocene structure beneath Duck Creek ridge (McCrory, 1996). A 

fault and associated anticline that deform the sea floor offshore 

from Whale Creek (plate 1A) project toward faulted upper 

Pleistocene deposits (McCrory and others, 1996) exposed in 

the sea cliffs south of the Raft River (fig. 11A).

Faults on the Continental Shelf north of the Raft River do 

not generally project onshore; rather, they tend to subparallel 

the coastline. Some of these faults disrupt the late Pleistocene 

erosional unconformity and offset the sea floor. For example, 

a fault about 20 km west of the Queets River (plate 2I, shot 

point 9000) disrupts the sea floor and underpins localized 

bedrock outcrops (plate 1A, B). This evidence of late Quaternary structural activity in the northern region implies continued subduction-related contraction far from the deformation 

front, perhaps driven by interplate coupling. Slip-rate estimates, however, are not possible with the available data.

Tectonic Implications of Crustal Shortening

Differential motion across the central Cascadia forearc 

in coastal Washington results in a complex pattern of folds 

and faults within the continental-shelf and onshore portions of 

the accretionary margin. Northward translation of the Oregon 

Coast Range (OCR) block is driven either by a margin-parallel component of oblique convergence between the Juan de 

Fuca and North America Plates or by broad northwest-southeast shearing between the Pacific and North America Plates 

across the Juan de Fuca Plate (fig. 20) (see, for example, 

England and Wells, 1991; Walcott, 1993). In either scenario, 

the OCR block acts as an indentor against which the relatively 

weak accretionary rocks of coastal Washington deform.

On a more regional scale, the pre-Cenozoic terrane composing southern British Columbia acts as a buttress at the 

northern boundary of the Olympic Mountains (OM) block. 

Uplift rates in the northern Olympic Mountains, ~3.2 mm/yr 

(Savage and others, 1991), are the highest along the Cascadia 

subduction margin, except for the region of the Mendocino 

triple junction. Brandon and Vance (1992) document ongoing 

unroofing of the subduction complex in the Olympic Mountains 

on the basis of fission-track studies, and attribute this unroofing 

to uplift driven by underplating of subducted material. Savage 

and others (1991) attribute the uplift to elastic strain accumulation above a locked megathrust. Models that account for elastic 

strain accumulation leave significant positive residuals (greater 

than predicted uplift) in the Grays Harbor area (Svarc and 

others, 1999). These residuals may represent shortening in the 

upper plate of the megathrust and, in particular, may represent 

shortening from forearc block convergence. Alternatively, the 

residuals may indicate a zone where the subduction fault is 

locked that is locally wider than previously modeled.

Strain Partitioning near the Block Boundary

Subduction-complex rocks beneath the study area were 

accreted to the Washington continental margin during Cenozoic time. Their primary structural fabric is expected to reflect 

east-northeast-directed convergence associated with subduction processes. Folds that trend north-northwest in the northern portion of the study area are attributed to this regime. Too 

few shallow structures are mapped in the southern portion of 

the study area to adequately assess their kinematic origins. 

We attribute the faults beneath Willapa Bay to forearc-block 

motion. However, in the absence of observations of inferred 

strike-slip motion, a subduction regime cannot be ruled out. 

Quaternary structural orientations in the central portion 

of the study area denote north-south shortening. These structural orientations are skewed relative to mapped structures in 

the surrounding region (Wagner and others, 1986; this study) 

that reflect the subduction regime (sensu stricto). In detail, the 

skewed structures show a progressive clockwise shift in oriTectonic Implications of Crustal Shortening
40 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

entation from south to north. The southernmost structures—in 

the Copalis River area—trend east-northeast, and the northern 

ones—in the Wreck Creek area—trend west-northwest (plate 

1A). Farther north, thrust fault and fold trends abruptly shift to 

a north-northwest orientation (McCrory, 1996).

The progressive shift in structural orientation suggests 

that the study area serves as a transition zone between northsouth-directed compression in the south and east-west-directed 

compression in the north. This shift occurs adjacent to a major 

boundary between crustal blocks of widely differing rheology. 

Paleomagnetic and geodetic data indicate that the relatively 

rigid southern block, the OCR block, is actively translating northward along the Cascadia margin (Wells and others, 

1998; McCaffrey and others, 2000; Wells and Simpson, 2001). 

Observed geologic shortening discussed above occurs primarily 

in the relatively ductile subduction complex just north and west 

of the block boundary (fig. 1). The proximity of the shortening 

to the block boundary suggests that these structures accommodate ongoing convergence between the OCR and OM blocks.

If a third to a half of available contraction is consumed 

across this zone of transverse structures, the question remains 

as to where the remainder is consumed. The broad-scale 

warping of Siletz basement on the southern side of the block 

boundary is one likely sink. Wide-angle seismic data indicate 

relative basement lows beneath Grays Harbor and Willapa 

Bay, with an intervening basement high (Parsons and others, 

1999). This may be inherited relief or could represent broadscale buckling with a 25-km wavelength. Geodetically derived 

uplift rates (Savage and others, 1991) indicate slow uplift 

beneath Willapa Bay (0.4 mm/yr) in support of continued 

O P

N

M

K

L

J

E

H

I

C

D

B

A

EXPLANATION

Juan de Fuca oceanic slab

Olympic Mountains block, underlain by

accreted subduction-complex basement, including the Olympic subduction complex

Oregon Coast Range block, underlain by

the Siletz basaltic terrane

Thrust or high-angle dip-slip fault, arrows show relative motion

Direction of maximum horizontal stress

Petroleum well site, Olympic Mountains basement

Petroleum well site, Oregon Coast Range basement

Petroleum well site, Oregon Coast Range rocks

above Olympic Mountains basement

crust

mantle

Oregon Coast Range

block

Olympic Mountains

block

Juan de Fuca plate

2

1

3

10

0

20

30

40

124°15' 124°00' 123°45'W

46°30'

46°15

46

'

°45'

47°00'

47°15'

47°30'

47°45'N

N

S

Z

Z'

GHfz

KILOMETERS

WBfz

Figure 20. Generalized north-south block diagram at no vertical exaggeration showing inferred configuration of Oregon Coast Range and 

Olympic Mountains blocks. See figure 1 for location of profile Z-Z’: Circled numbers identify potential seismic sources: (1) upper plate, interblock 

sources; (2) interplate, megathrust sources; (3) lower plate, Juan de Fuca sources. The location of the Juan de Fuca Plate beneath the margin is 

constrained by a velocity model of Parsons and others (1999). Heavy paired arrows denote relative movement on Cascadia megathrust between 

the crustal blocks and the Juan de Fuca slab; heavy opposed arrows denote relative movement between crustal blocks. WBfz indicates “Willapa 

Bay” fault zone, and GHfz indicates ”Grays Harbor” fault zone.
41

downwarping relative to the surrounding area. Some contraction may be distributed as general uplift in the Olympic Mountains and some may account for the high geodetically derived 

uplift rates near the northern block boundary in the Clallam 

area (fig. 21).

Seismic Hazard Implications of Quaternary Tectonism

The boundary between the OCR and OM blocks is characterized by northeast-trending thrust faults and associated 

anticlines developed primarily in subduction-complex basement and overlying strata. The location and orientation of 

these structures indicate ongoing north-south contraction and 

suggest relative convergence between these basement blocks 

in coastal and nearshore Washington. A historic record of 

felt earthquakes (1884, April 1887, 1891, 1909) and tsunamis (May 1887, 1920) exists for the Grays Harbor area (L.J. 

Workman, written commun., 1994), although the record is too 

sparse to locate the source of the older events. Some of the 

earthquakes likely occurred in the subducted Juan de Fuca 

slab (for example, the 1909 and 1920 events), as did the 1999 

MW 5.9 Satsop earthquake. The 1887 tsunami is intriguing 

48°N

46°

126° 124° 122°W

0 5 mm/yr

Rate of plate motion

?

OCR

OCR

OCR

OM

OM

OM

Juan de Fuca

Plate

"fixed

North Am

Plat

Total OCR motion

relative to fixed

North America

Component

consumed in

Grays HarborWillapa Bay

contraction

Component

consumed in

Clallam-Olympic

Mountains

uplift

Figure 21. Schematic kinematic diagram of block movements and stratal shortening in the study area. See figure 1 for explanation of patterns and 

symbols. Long black arrow shows velocity vector for Oregon Coast Range block relative to fixed North America. Opposed black arrows indicate 

consumption of translational motion as geologic contraction in the Grays Harbor-Willapa Bay area and the Olympic Peninsula. Opposed gray 

arrows in the Clallam Bay-Olympic Mountains area indicate where remaining strain may be consumed in geologic contraction.

Seismic Hazard Implications of Quaternary Tectonism
42 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

because it occurred two weeks after a locally felt earthquake 

and may represent delayed slumping triggered by a local, 

shallow offshore source of crustal seismicity.

If the northeast-trending structures in the study area are 

seismogenic, they contribute to the regional earthquake hazard 

and may be represented in the paleoseismic record. In fact, 

paleoseismic evidence of fault slip beneath the structural ridge 

south of the Copalis River may be present in Holocene marsh 

strata exposed along the river. Copalis marsh strata contain 

three stratigraphic reversals that Atwater (1992) infers to 

reflect coseismic subsidence during megathrust earthquakes. 

Atwater (1992) identifies an additional seismic event, ca. 1.1

ka (fig. 3), from liquefaction evidence alone—this event lacks 

a stratigraphic reversal to indicate subsidence. The structure 

beneath Langley Hill, 5 km south of the Copalis River, could 

be the source for this seismic event. In this scenario, subsidence would not be expected on the Copalis River because it is 

located on the upper plate of the thrust fault.

Holocene marshes in southern Washington estuaries may 

contain a record of both megathrust and crustal seismic events, 

as has been proposed for the southern and central Oregon coast 

(Goldfinger and others, 1992; Nelson and Personius, 1996). 

The marshes that fringe the estuaries typically preserve only the 

last 3-4 k.y. of the Holocene stratigraphic record (Atwater and 

others, 1995; Atwater and Hemphill-Haley, 1997). Even though 

recurrence intervals for faults with slip rates less than 1 mm/yr 

are typically measured in thousands of years, such an event 

could fall within the available stratigraphic record.

The estimated 2.2-3.3 mm/yr of contraction in coastal 

Washington could indicate as much as 5-7 mm/yr of cumulative fault slip spread across seven thrust faults (each dipping 

30°). In this scenario, each fault would have an average slip 

rate between 0.7 and 1.0 mm/yr depending on the age of the 

early Pleistocene Raft datum. If the 2.5-m fault slip measured 

at the Langley quarry (McCrory, 1996) is a typical surface 

rupture, then a fault with a slip rate of 1.0 mm/yr would 

require ~2.5 k.y. to accumulate enough elastic strain to trigger 

a comparable event. If the slip rate is 0.7 mm/yr, ~3.6 k.y. 

would be the average interval between rupture events. In the 

scenario described above, the tidal marsh stratigraphies in 

Copalis River, Grays Harbor, and Willapa Bay would likely 

record no more than one such crustal seismic event.

The 10-12 m fault offset of the ca. 20-ka surface beneath 

Willapa Bay likely required multiple rupture events. Such slip 

events would be accompanied by uplift of western Willapa 

Bay and subsidence of the eastern bay. The main fault strand 

may terminate in the boundary fault, inferred to be about 7 

km beneath Willapa Bay (fig. 20). In this case, the rupture 

area would be small and generally above the seismogenic 

zone. Nonetheless, recent movement on this fault and ambient 

seismicity (fig.2) point to the potential for recent movement 

on the boundary fault as well. Borehole studies map out a 

late Pleistocene bedrock high beneath Long Beach Peninsula 

(Herb, 2000) (plate 1), consistent with its location on the 

upper plate of the boundary thrust. Alternatively, this bedrock 

high may delineate a northern continuation of the Chinook 

Fault (Wells, 1989) south of Willapa Bay (fig. 3) and of the 

anticlinal structure that underpins Long Island.

Distinguishing crustal or upper-plate fault slip events 

from megathrust slip events in the paleoseismic record 

requires documentation of differential movement. A crustal 

slip event should produce local areas of uplift juxtaposed 

against areas of subsidence or no detectable change. Ideally, 

radiometrically dated sites would be sufficient to document 

uplift of the upper plate of a crustal fault and subsidence of 

the lower plate. However, documenting uplift alone, or a pattern of subsidence inconsistent with a megathrust signature 

may be adequate to demonstrate local slip events.

Conclusions

The Cascadia subduction boundary comprises several 

seismotectonic elements. The primary element is defined by 

subduction of the Juan de Fuca Plate beneath the continental 

margin. In the central portion of the subduction zone, relative 

convergence is about 40 mm/yr and is expected to generate 

great earthquakes with recurrence intervals measured in hundreds of years. A secondary seismotectonic element results 

from relative motion within the Cascadia forearc. The Oregon 

Coast Range block in the Cascadia forearc translates northward with rates that range from about 4 mm/yr in the Puget 

Sound region to about 8 mm/yr in coastal Washington. Faults 

that accommodate relative motion of this block with respect to 

neighboring blocks are expected to generate moderate to large 

earthquakes with recurrence intervals measured in thousands 

of years. Even with the much smaller expected magnitudes 

and the much longer recurrence intervals as compared to 

expected megathrust earthquakes, these faults pose significant 

seismic hazard owing to their close proximity to populated 

regions.

Marine investigations along the southern Washington 

coast document intense Quaternary tectonic activity at the 

leading (northwestern) edge of the Oregon Coast Range 

block. Permanent shortening is primarily localized within the 

relatively ductile rocks of the Olympic Mountains block, on 

the north side of the block boundary. Here folds range in 

length from 3 to 15 km and have amplitudes as great as 160 

m. The wavelength of inferred buckling of the Oregon Coast 

Range block adjacent to its leading edge is about 25 km; 

the amplitude is not discernible at the surface except for the 

general correlation with topographic relief.

We have mapped a zone of faults and folds, 40 km wide 

from south to north, that are oriented transverse to the Cascadia subduction fabric. This zone appears to convert 2.2-3.3 

mm/yr (as much as half) of the Oregon Coast Range translation into permanent crustal shortening. We correlate several 

faults mapped offshore with previously mapped onshore faults 

that displace Quaternary deposits. Some of the offshore faults 

displace the sea floor as much as 12 m, indicating Holocene 

movement. A late Quaternary fault bisects Willapa Bay above 

the Oregon Coast Range boundary. This fault offsets a ca. 20-

ka datum 10-12 m, yielding a vertical offset rate of 0.5 mm/yr. 

Fault geometry suggests that some strands of the “Willapa 

Bay” fault zone may terminate at shallow depths above the 

main thrust boundary whereas others may extend far deeper.
43

Acknowledgements

We thank the Olympic Coast National Marine Sanctuary 

for support of U. S. Geological Survey research within sanctuary waters. We thank Captains Morris and Timmons and the 

crew of the National Oceanic and Atmospheric Administration 

ship McArthur for their cooperation in conducting our geophysical surveys. We thank Thomas O’Brien for his unrivaled 

data acquisition skills. We are indebted to William Schwab 

and Steve Wallace for their unwavering support of the marine 

investigations. We appreciate the cooperation of Guy Gelfenbaum and David Twichell in conducting companion cruises 

on the Washington Department of Fish and Wildlife vessel 

Corliss. We thank Steve Wolf for his rigorous compilation and 

interpretation of preexisting seismic-reflection data. We also 

acknowledge the invaluable processing support of Scott Bruce, 

Jon Childs, and Ray Sliter, and Scott Harrison’s energetic participation in the 1997 McArthur cruise. Reviews by Thomas 

Brocher, Tom Parsons, Peter Stauffer, and Ray Wells improved 

the paper.

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46 Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington

Appendix 1. M197WO—The July 7-14, 1997, 

Cruise on NOAA Ship RV McArthur

See URL: walrus.wr.usgs.gov/docs/infobank/ for links to 

cruise data. See Foster and others (2001) for seismic-reflection data archives and McCrory and others (in press a) for 

sidescan-sonar data archive.

USGS Scientific and Technical Personnel

Patricia McCrory, Menlo Park, California

Thomas O’Brien, Woods Hole, Massachusetts

David Foster, Woods Hole, Massachusetts

Scott Harrison, Saint Petersburg, Florida

Scientific Equipment

Navigation—USGS differential GPS navigation was logged 

using an Ashtech system with USGS YoNav navigation software. 

McArthur GPS navigation was logged on a Magnovox system.

Bathymetry—The pipe-mounted bathymetric system had a 

3.5-kHz acoustic source (transducer). Seismic data were received 

on an ORE system and recorded digitally on a MudSeis acquisition system. Analog data were recorded on an EPC graphic 

recording system.

Sidescan sonar—The EdgeTech sidescan system had dual 

100- and 500-kHz acoustic sources. Only the 100-kHz source 

was recorded on this cruise. Slant range from the ship to the sonar 

towfish was provided by a Benthos 7000 system. Reflectivity data 

were recorded with a Triton-Elics ISIS digital acquisition system 

and processed onboard using USGS XSonar software.

Boomer—The dual-plate Geopulse system with a 1-kHz 

EG&G acoustic source was towed on a surface catamaran. 

Seismic data were received on an ORE system and recorded 

digitally on a MudSeis acquisition system using a Benthos 

hydrophone streamer. The seismic data were processed 

onboard using Seismic-UNIX software. Analog data were 

recorded on an EPC graphic recording system.

Sediment sampler—A Shipek grab sampler was used to 

collect bottom samples.

Cruise Instructions

The cruise instructions called for eight east-west survey lines 

and three north-south cross lines in the Grays Harbor-Queets 

River, Washington, area. In general, the nearshore portions of 

tracklines were truncated at 30-m water depth to avoid tangling 

the sidescan tow fish in crab-pot strings. The sidescan fish was 

pulled onboard during nearshore north-south transects. During the 

last trackline (Line 18) on the final night of data collection, the 

sidescan fish did become tangled in a crab-pot line and was rendered inoperable. Because of the shortened tracklines and favorable weather conditions, three additional lines were completed. In 

total, geophysical data were collected along 400 km of trackline 

(fig. 12). In addition, seven sea-floor sediment samples (table 1) 

were collected during an east-west Conductivity-TemperatureDepth (CTD) transect offshore Cape Elizabeth.

YoNav, the differential global positioning system (DGPS) 

navigation software used in the geophysical lab, had a bug that 

prevented real-time display of the ship’s location. However, 

the DGPS navigation was logged with the sidescan-sonar data 

and used to plot these sidescan swaths. The McArthur GPS 

navigation system was used for boomer trackline locations. 

The DGPS navigation located tracklines with an accuracy of 

5-10 m; the GPS system had an accuracy of ~20 m. The DGPS 

was used for the sidescan navigation, and the NOAA GPS was 

merged with the boomer shotpoint files by correlating time 

between data sets. The bathymetric system was pipe-mounted 

on the port side of the ship and, even in moderate sea conditions, yielded noisy data owing to motion of the ship. The 

sidescan system was towed about 20 m above the sea floor 

using a USGS winch from the aft A-frame. This configuration 

yielded a 375-m swath width and a resolution of 19 cm. The 

seismic-reflection (boomer) system consisted of two plates 

towed on a catamaran from the starboard McArthur winch and 

A-frame. Bottom grab samples were collected in 30 to 70 m 

of water using the starboard McArthur winch and A-frame.

Appendix 2. M398WO—The June 25 - July 4, 

1998, Cruise on NOAA Ship RV McArthur.

See URL: walrus.wr.usgs.gov/docs/infobank/ for links to 

cruise data. See Foster and others (1999a, b) for bathymetry 

and seismic-reflection data archives and McCrory and others 

(in press b) for sidescan-sonar data archive. 

USGS Scientific and Technical Personnel

Patricia McCrory, Menlo Park, California

Thomas O’Brien, Woods Hole, Massachusetts

David Foster, Woods Hole, Massachusetts

William Danforth, Woods Hole, Massachusetts

Scientific Equipment

Navigation—USGS differential GPS navigation was logged 

using an Ashtech system with Capt’n navigation software. 

Bathymetry—The chirp subbottom profiling system, with 

a 2-7-kHz acoustic source, was mounted on a DataSonics 

sonar tow fish. The chirp data were recorded with the sidescan-sonar data on a Triton-Elics ISIS digital data acquisition 

station and processed onboard using Seismic-UNIX software.

Sidescan sonar—The DataSonics SIS-1000 sidescansonar system had a swept FM (chirp) acoustic source in the 

100-kHz band. Slant-range distances from the ship to towfish 

were provided by a Benthos 7000 system. The reflectivity 

data were recorded using a Triton-Elics ISIS digital acquisition system and processed onboard using USGS XSonar 

software.
47

Sparker seismic reflection—The sparker system, with 

a 0.8-kHz acoustic source, was constructed by K. F. Parolski (USGS, Woods Hole) and towed from the fantail of the 

ship. The seismic data were recorded digitally on a MudSeis 

acquisition system using a Benthos hydrophone streamer and 

processed onboard using ProcessSEGY software.

Boomer seismic reflection—A single-plate Seismitec 

system with a 1-kHz acoustic source and a 500-J Applied 

Acoustics power supply were towed on a surface catamaran. The seismic data were received on a Geopulse system, 

digitally recorded on a MudSeis acquisition system using a 

Benthos hydrophone streamer, and processed onboard using 

Seismic-UNIX software.

Cruise Instructions

Cruise instructions called for six east-west survey lines 

and three north-south cross lines in the Cape Flattery, Washington, area. A few hours of sonar and chirp data are missing 

from the first north-south trackline (Line 5), owing to repair 

of an intermittent malfunction in the towfish. On the second 

day of data collection, we switched from the boomer seismicreflection system to the sparker system to increase subbottom 

penetration from less than 50 m to greater than 100 m, and the 

sparker system was deployed for the remainder of the cruise. 

The east-west lines were shortened to reduce time, and we 

were able to complete all but the westernmost north-south line. 

Cruise instructions called for four east-west survey lines and 

two north-south cross lines in the Queets River, Washington, 

area as a northward continuation of tracklines collected during 

the 1997 McArthur cruise. All planned tracks were completed except the westernmost north-south trackline. Cruise 

instruction called for eight northwest-trending survey lines 

in the Copalis River area as a more detailed site survey of an 

extensive region of bedrock outcrop observed during the 1997 

McArthur cruise. These lines were in a region of dense crabpot strings, and care was taken to maneuver the ship through 

these obstacles during daylight hours. Nonetheless, the towfish 

became entangled with crab-pot lines four times during our 

first day in the Copalis study area. We quickly learned how to 

avoid the pots. To reduce time, only six northwest-trending 

lines were run in this area and an additional east-west line was 

added—from 20 km offshore Grays Harbor into the harbor. 

The sonar was not deployed on this final trackline (Line 37). 

In total, geophysical data were collected along 545 km of 

trackline in water depths ranging from 15 to 170 m (fig. 12).

The DGPS (differential global positioning system) 

navigation located tracklines with an accuracy of 5-10 m. 

The sidescan-sonar system was towed about 20 m above the 

sea floor using a USGS winch from the aft A-frame. This 

configuration yielded a 375-m swath width and a resolution 

of 19 cm. The chirp system recorded seismic-reflection data 

with a resolution of 10-20 cm in the upper 5-15 m of strata 

beneath the sea floor. The sparker system consisted of 2 electrodes in a wire cage towed from the starboard corner of the 

fantail. The lower frequency sparker system recorded subbottom data with a resolution of 2-4 m in the upper 150-200 m 

below the sea floor.

Appendix 3. Previous Cruises in the Area 

Studied

See URL: walrus.wr.usgs.gov/docs/infobank/ for links to 

cruise data.

O1067WO—1967 Cruise on UW Ship RV Oceaneer

Oceaneer seismic data (fig. 13) were recorded at a 1/2-

second scan rate (200-fathom or 365.8-m scale) on an analog 

(paper) system. Penetration depth for the Oceaneer sparker 

system was about 150 m, with a nominal resolution of 3-4 

m at the sea floor and 2-m resolution in the subsurface. Data 

quality is good. 

O174WO—1974 Cruise on Vessel Oomiak

Oomiak seismic data (fig. 13) were recorded at a 1/4-

second scan rate (187.5-m scale) on an analog (paper) system 

(Hill and others, 1981). Penetration depth for the Oomiak

Uniboom system was about 50 m, with nominal resolution 

of 1-2 m at the bay floor and 1 m or less resolution in the 

subsurface. Oomiak data quality ranged from average to poor.

L376WO; L577WO; L1180WF—1976, 1977, and 

1980 Cruises on RV S.P. Lee

S.P. Lee seismic data (fig. 13) were recorded at 1/2-

second scan rate (375-m scale) on both analog (paper) and 

digital systems. Penetration depth for the S.P. Lee Uniboom 

system was about 75 m, with a resolution of 1-2 m at the 

sea floor and 1 m or less in the subsurface. Data quality is 

average.

Appendix 3. Previous Cruises in the Area Studied