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Active Tectonics of the Devils Mountain 

Fault and Related Structures, Northern 

Puget Lowland and Eastern Strait of 

Juan de Fuca Region, Pacific Northwest

U.S. Geological Survey Professional Paper 1643

U.S. Department of the Interior 

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Active Tectonics of the Devils Mountain Fault and 

Related Structures, Northern Puget Lowland and 

Eastern Strait of Juan de Fuca Region, 

Pacific Northwest

By Samuel Y. Johnson, Shawn V. Dadisman, David C. Mosher, 

Richard J. Blakely, and Jonathan R. Childs

U.S. Geological Survey Professional Paper 1643

Prepared in cooperation with the 

Geological Survey of Canada

U.S. Department of the Interior 

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat Director

Version 1.0

First printing 2001

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center 

Denver, CO 80225

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

Library of Congress Cataloging-in-Publication Data

Active tectonics of the Devils Mountain Fault and related structures, northern Puget 

Lowland and eastern Strait of Juan de Fuca region. Pacific Northwest / 

by Samuel Y. Johnson ... [et al.].

p. cm. - (U.S. Geological Survey professional paper; 1643) 

Includes bibliographical references.

1. Faults (Geology)—Juan de Fuca, Strait of, Region (B.C. and Wash.) 

2. Geology—Juan de Fuca, Strait of, Region (B.C. and Wash.). 

I. Johnson, Samuel Y. II. Series.

QE606.5.J83 A37 2002

551.8'09797—dc21 2001033439

ISBN=0-607-97457-5
Contents

Abstract................................................................................................................................................. 1

Introduction........................................................................................................................................... 1

Acknowledgments............................................................................................................................... 1

Geologic Setting................................................................................................................................... 2

Seismic-Reflection Surveys—Data Acquisition and Processing................................................ 6

Stratigraphy and Seismic Stratigraphy............................................................................................ 7

Pre-Tertiary Basement Rocks................................................................................................... 7

Tertiary Sedimentary Rocks...................................................................................................... 7

Uppermost Pliocene(?) to Pleistocene Strata........................................................................ 7

Onshore Stratigraphy........................................................................................................ 7

Offshore Stratigraphy........................................................................................................ 9

Identification and Age of the Base of the Uppermost Pliocene(?)

to Pleistocene Section...................................................................................................... 9

Uppermost Pleistocene and Holocene Strata........................................................................ 9

Seismic-Reflection Profiles................................................................................................................ 9

U.S. Geological Survey Line 176............................................................................................... 11

U.S. Geological Survey Line 177............................................................................................... 11

U.S. Geological Survey Line 167-168....................................................................................... 11

U.S. Geological Survey Line 166............................................................................................... 12

Industry Line 1.............................................................................................................................. 13

U.S. Geological Survey Line 165............................................................................................... 13

U.S. Geological Survey Line 164............................................................................................... 13

U.S. Geological Survey Line 162............................................................................................... 13

U.S. Geological Survey Line 158............................................................................................... 14

Industry Line 2.............................................................................................................................. 14

Geological Survey of Canada Line 15...................................................................................... 14

SHIPS LineJDF4.................................................^ 14

Industry Line 3.............................................................................................................................. 14

Geological Survey of Canada Line 37...................................................................................... 15

Geological Survey of Canada Line 35...................................................................................... 15

Evidence for Faulting Onshore, Whidbey and Camano Islands................................................... 15

Devils Mountain Fault................................................................................................................. 17

Location............................................................................................................................... 17

Outcrops.............................................................................................................................. 20

Subsurface Data................................................................................................................. 21

Strawberry Point Fault............................................................................................................... 22

West Coast of Whidbey Island......................................................................................... 22

Central Whidbey Island..................................................................................................... 23

East Coast of Whidbey Island.......................................................................................... 23

Utsalady Point Fault.................................................................................................................... 30

West Coast of Whidbey Island......................................................................................... 30

Central and Eastern Whidbey Island .............................................................................. 30

Camano Island.................................................................................................................... 30

Fault Mapping, Style, and Rates of Deformation ............................................................................ 32

Devils Mountain Fault................................................................................................................. 32

Strawberry Point Fault................................................................................................................ 37

Utsalady Point Fault.................................................................................................................... 39

Discussion.................................................................................................................................... 39

Earthquake Hazards............................................................................................................................. 40

Earthquake Occurrence............................................................................................................. 40

Significance for Earthquake Hazards...................................................................................... 40

Conclusions........................................................................................................................................... 41

References Cited.................................................................................................................................. 42
Plates

1. Maps showing tracklines, and U.S. Geological Survey, Geological Survey of 

Canada, and industry seismic-reflection and aeromagnetic profiles, eastern 

Strait of Juan de Fuca area, Pacific Northwest, figures 5 and 8-17

2. U.S. Geological Survey, Geological Survey of Canada, and industry 

seismic-reflection profiles, eastern Strait of Juan de Fuca area, 

Pacific Northwest, figures 18-26

Figures

1. Schematic geologic map of northwestern Washington and southwestern 

British Columbia showing Puget Lowland and flanking Cascade Range and 

Olympic Mountains.............................................................................................................. 3

2. Map showing structural geology of northeastern Strait of Juan de Fuca area........ 4

3. Shaded relief map of the aeromagnetic anomaly map, eastern Strait of

Juan de Fuca-northern Puget Lowland region............................................................... 5

4. Side-looking airborne radar image of portion of northwestern Washington............. 6

Note: Figures 5 and 8-17 are on plate 1; figures 18-26 are on plate 2

5. Maps showing location of tracklines for seismic-reflection profiles in 

eastern Strait of Juan de Fuca area

6. Diagram showing stratigraphy and age of geologic units occurring in

northern Whidbey Island area ........................................................................................... 8

7. Map showing depth to base of uppermost Pliocene(?) to Pleistocene strata,

eastern Strait of Juan de Fuca region .............................................................................. 10

8. U.S. Geological Survey Line 176, seismic-reflection profile and aeromagnetic 

profile, Skagit Bay

9. U.S. Geological Survey Line 177, seismic-reflection profile and aeromagnetic 

profile, Saratoga Passage

10. U.S. Geological Survey Line 167 and 168, seismic-reflection profile and 

aeromagnetic profile, eastern Strait of Juan de Fuca

11. U.S. Geological Survey Line 168, seismic-reflection profile (geopulse source), 

eastern Strait of Juan de Fuca

12. U.S. Geological Survey Line 168, seismic-reflection profile (geopulse source), 

eastern Strait of Juan de Fuca, south of segment in figure 11

13. U.S. Geological Survey Line 166, seismic-reflection profile and aeromagnetic 

profile, eastern Strait of Juan de Fuca

14. Industry Line 1, seismic-reflection profile and aeromagnetic profile, eastern 

Strait of Juan de Fuca

15. U.S. Geological Survey Line 165, seismic-reflection profile and aeromagnetic 

profile, Rosario Strait, eastern Strait of Juan de Fuca

16. U.S. Geological Survey Line 164, seismic-reflection profile and 

aeromagnetic profile, eastern Strait of Juan de Fuca

17. U.S. Geological Survey Line 164, seismic-reflection profile (geopulse source), 

eastern Strait of Juan de Fuca

18. U.S. Geological Survey Line 162, seismic-reflection profile and aeromagnetic 

profile, eastern Strait of Juan de Fuca

19. U.S. Geological Survey Line 162, seismic-reflection profile (geopulse source), 

eastern Strait of Juan de Fuca

20. U.S. Geological Survey Line 158, seismic-reflection profile, and aeromagnetic 

profile, eastern Strait of Juan de Fuca

21. Industry Line 2, seismic-reflection profile and aeromagnetic profile, eastern 

Strait of Juan de Fuca

22. Geological Survey of Canada Line 15, seismic-reflection profile and aeromagnetic profile, eastern Strait of Juan de Fuca

IV
23. SHIPS Line JDF4, seismic-reflection profile and aeromagnetic profile, eastern 

Strait of Juan de Fuca

24. Industry Line 3, seismic-reflection profile and aeromagnetic profile, eastern 

Strait of Juan de Fuca

25. Geological Survey of Canada Line 37, seismic-reflection profile and aeromagnetic profile, eastern Strait of Juan de Fuca

26. Geological Survey of Canada Line 35, seismic-reflection profile and aeromagnetic 

profile, eastern Strait of Juan de Fuca

27. Map of northern Whidbey Island area showing locations of key outcrop 

localities, radiocarbon dates, nearby seismic-reflection profiles, and 

faults and folds that deform Qaternary deposits............................................................ 16

28. Map of northern Whidbey Island area showing locations of water wells used 

in making stratigraphic correlation diagrams, nearby seismic-reflection 

profiles, and faults and folds that deform Quaternary deposits................................... 17

29. Photograph showing bluff outcrops of Quaternary strata at locality c

of figure 27............................................................................................................................. 20

30. Interpretive east-west stratigraphic correlation diagrams north and south of

Devils Mountain fault.......................................................................................................... 24

31. Photographs showing deformed Quaternary units at several

Strawberry Point localities shown in figure 27............................................................... 26

32. Interpretive stratigraphic correlation diagram parallel to coast of Strawberry

Point crossing Strawberry Point fault zone .................................................................... 28

33. Interpretive stratigraphic correlation diagram crossing Utsalady Point

fault zone, northwestern Whidbey Island........................................................................ 32

34. Line drawing of deformed Quaternary strata exposed on Utsalady Point,

Camano Island...................................................................................................................... 34

35. Photographs of deformation and facies in upper Pleistocene deposits in

Utsalady Point fault zone at Utsalady Point, Camano Island........................................ 35

36. Photographs showing faults exposed in a vegetated roadcut southeast of

fault exposed at Utsalady Point......................................................................................... 36

37. Interpretive stratigraphic correlation diagram crossing part of Utsalady

Point fault zone, northwestern Camano Island............................................................... 37

38. Map of northern Whidbey Island area showing location of faults and folds 

that deform Quaternary deposits, inferred rate of vertical fault-slip, and 

shortening rates................................................................................................................... 38

Tables

1. Information on water wells used in constructing stratigraphic correlation

diagrams................................................................................................................................ 18

2. Radiocarbon data for samples from northern Whidbey and Camano Islands.......... 21

Active Tectonics of the Devils Mountain Fault and 

Related Structures, Northern Puget Lowland and Eastern 

Strait of Juan de Fuca Region, Pacific Northwest

£y Samuel Y. Johnson, Shawn V. Dadisman, David C. Mosher,1 Richard J. Blakely, and Jonathan R. Childs

Abstract

Information from marine high-resolution and conventional 

seismic-reflection surveys, aeromagnetic mapping, coastal exposures of Pleistocene strata, and lithologic logs of water wells is 

used to assess the active tectonics of the northern Puget Lowland 

and eastern Strait of Juan de Fuca region of the Pacific Northwest. These data indicate that the Devils Mountain fault and the 

newly recognized Strawberry Point and Utsalady Point faults are 

active structures and represent potential earthquake sources.

The north-dipping (45°-75°) Devils Mountain fault extends 

westward for more than 125 kilometers from the Cascade Range 

foothills to Vancouver Island. The Devils Mountain fault is 

bounded by northwest-trending en-echelon folds and faults, a 

pattern that strongly suggests it is a left-lateral, oblique-slip, 

transpressional "master fault." Aeromagnetic anomalies coincide with both the trace of the Devils Mountain fault and en-echelon structures. Quaternary strata are deformed on nearly all 

crossing seismic-reflection profiles, and onshore subsurface data 

indicate offset of upper Pleistocene strata.

The west-northwest-trending, subvertical Strawberry Point 

fault cuts across northern Whidbey Island and has a minimum 

length of about 25 kilometers. On the west coast of Whidbey 

Island and in the Strait of Juan de Fuca, the fault has south-sideup offset and forms the northern boundary of an uplift of preTertiary basement rock. Exposures and subsurface logs of upper 

Pleistocene strata from Strawberry Point on the east coast of 

Whidbey Island indicate that the fault bifurcates into a 2-kilometer-wide zone as it crosses Whidbey Island. Each of the four 

fault splays within this zone has apparent north-side-up offset, 

and upper Pleistocene strata between the faults exhibit considerable shortening (dips as steep as 45°). The vertical fault trace, 

reversal of offset along strike, and evidence for associated contractional deformation suggest that the Strawberry Point fault is 

an oblique-slip, transpressional fault.

The northwest-trending, subvertical Utsalady Point fault 

similarly cuts across northern Whidbey Island and has a minimum length of 25 kilometers. It forms the southern margin of 

the pre-Tertiary basement horst block on the west coast of 

Whidbey Island, where it has north-side-up offset. Offshore 

seismic-reflection data from east of Whidbey Island indicate 

that it also bifurcates eastward into a broad (1.5 kilometer) 

zone of several splays. Onshore outcrops and subsurface logs 

from Camano Island indicate a probable reversal of offset (to 

south side up) along the zone and display both faulting and 

folding (dips as steep as 24°) in upper Pleistocene strata. As

'Geological Survey of Canada, P.O. Box 1006, Cartmouth, NS CANADA B2Y 4A2.

with the Strawberry Point fault, the vertical fault trace(s), 

reversal of offset, and evidence for associated contractional 

deformation suggest that the Utsalady Point fault is an obliqueslip, transpressional fault.

Collectively, the Devils Mountain, Strawberry Point, and 

Utsalady Point faults represent a complex, distributed, transpressional deformation zone. The cumulative slip rate on three main 

faults of this zone probably exceeds 0.5 millimeter/year and 

could be much larger. This new information on fault location, 

length, and slip rate should be incorporated in regional seismic 

hazard assessments.

Introduction

Seismic hazard assessments depend on understanding the 

locations, lengths, and slip rates of active faults. Obtaining this 

information is difficult in the Puget Lowland of Washington 

State, where most landforms were generated during the last glaciation and are quite young (=12—20 ka), and vegetation covers 

much of the landscape. As a result, the earthquake hazard posed 

by many significant faults in this region is poorly understood. 

Several potentially significant faults occur in the northern Puget 

Lowland and eastern Strait of Juan de Fuca region (Gower and 

others, 1985). These include the southern Whidbey Island fault, 

the Devils Mountain fault, and associated structures (Gower and 

others, 1985; Johnson and others, 1996).

Johnson and others (1996) integrated seismic-reflection 

data, well logs, and information from outcrops to document 

Quaternary deformation along the southern Whidbey Island 

fault. Our purpose in this report is to present comparable new 

geologic and geophysical information and synthesis that demonstrate Quaternary activity on the Devils Mountain fault and two 

newly defined structures, the Strawberry Point and Utsalady 

Point faults. These faults are the major components of an 

oblique-slip transpressional deformation zone that extends westward for more than 125 kilometers from the Cascade Range 

foothills across the eastern Strait of Juan de Fuca.

Acknowledgments

For the U.S. Geological Survey seismic-reflection surveys, 

we thank Guy Cochrane, Kevin O'Toole, Larry Kooker, Fred 

Payne, Curtis Lind, and the captain and crew of the Robert Gray 

for help in data collection, and John Miller and William 

Stephenson for help in data processing. For the Geological Survey of Canada seismic-reflection surveys, we thank the captain,

1
crew, and scientific staff of the CCGS John P. Tully on cruises 

PGC96006 and PGC97007. The U.S. Geological Survey 

research cruises were supported by the Coastal and Marine Geology Program and the Earthquake Hazards Reduction Program 

(EHRP). Geological Survey of Canada surveys were funded in 

part by a U.S. Geological Survey EHRP grant to David C. 

Mosher. We thank Michael Fisher for organizing the Seismic 

Hazard Investigations in Puget Sound (SHIPS) marine seismicreflection survey and providing access to the SHIPS data base. 

We are grateful to Mobil and ARCO for providing industry seismic-reflection data. Douglas Kelly and Myrtle Jones provided 

important water-well data and insights, and Jon Cox drafted 

early versions of several stratigraphic correlation diagrams. 

David Dethier, Donald Easterbrook, James Yount, Joseph 

Dragovich, and David Norman provided important insights on 

the glacial geology of northern Whidbey Island. Constructive 

reviews were provided by Thomas Brocher, David Dethier, 

Donald Easterbrook, Joseph Dragovich, Derek Booth, Kathy 

Troost, John Oldow, and David Harding. We benefited from 

stimulating discussions with these reviewers and with Robert 

Bucknam, James Evans, Carol Finn, Art Frankel, Ralph 

Haugerud, Robert Karlin, Harvey Kelsey, Daniel Muhs, Alan 

Nelson, Thomas Pratt, Rowland Tabor, Timothy Walsh, Craig 

Weaver, Ray Wells, and John Whetten.

Geologic Setting

Western Washington and southwestern British Columbia lie 

within the structurally complex continental margin of the Pacific 

Northwest (fig. 1). Oblique convergence of the Juan de Fuca 

plate below North America (Engebretson and others, 1985), 

along with larger-scale shearing of the Pacific Plate against 

North America, provides the driving force for crustal faulting 

and deformation. (See, for example, Wang, 1996; Wells and others, 1998; Stanley and others, 1999.) The Washington Coast 

Range and Olympic Mountains represent part of a forearc sliver 

that is moving northward relative to the Cascade Range and is 

buttressed to the north by pre-Tertiary basement in the San Juan 

Islands and on Vancouver Island. The boundary between these 

two terranes is apparently a distributed deformation zone that 

extends through Washington's Puget Lowland (Johnson and others, 1996). The active Seattle fault (Bucknam and others, 1992; 

Johnson and others, 1994, 1999; Nelson and others, 1999) and 

southern Whidbey Island fault (Johnson and others, 1996) occur 

within this distributed zone. Both the Seattle fault and the southern Whidbey Island fault are associated with significant aeromagnetic and gravity anomalies (for example, Finn and others, 

1991; Blakely, Parsons, and others, 1999; Blakely, Wells, and 

Weaver, 1999), marking the southern boundary of the Seattle 

basin and the southwest boundary of the Everett basin (fig. 1), 

respectively.

Johnson and others (1996) have described the southern 

Whidbey Island fault (fig. 1) as a broad (6-11 km) transpressional zone comprising three main splays, within which the local 

late Quaternary uplift rate is at least 0.6 mm/yr. This zone represents part of the boundary of the forearc sliver, juxtaposing 

Eocene marine basaltic basement on the south and southwest

with heterogeneous pre-Tertiary basement on the northeast. This 

crustal boundary is buried in the Puget Lowland but is exposed 

on southern Vancouver Island as the Leech River fault (fig. 1; 

Muller, 1983; Clowes and others, 1987).

The Devils Mountain fault (Hobbs and Pecora, 1941; 

Loveseth, 1975; Tabor, 1994) forms the northern boundary of the 

Tertiary to Quaternary Everett basin (Johnson and others, 1996) 

and is associated with an alignment of aeromagnetic anomalies 

that extend more than 100 km from the Cascade Range foothills 

to Vancouver Island (figs. 2, 3). At its east end, the Devils 

Mountain fault merges with the north-trending Darrington fault 

zone (Tabor, 1994). In the Cascade Range foothills, the fault 

forms a prominent «30-km-long topographic lineament aligned 

with Lake Cavanaugh (fig. 4). Farther west, the Devils Mountain fault trace extends through Quaternary deposits of the 

Skagit River delta and Whidbey Island and into the eastern Strait 

of Juan de Fuca. Based on interpretation of petroleum-industry 

seismic-reflection data, Johnson and others (1996) suggested 

that the fault continues its westerly trend across the strait (fig. 1). 

Previously, Whetten and others (1988) and Tabor (1994) suggested that to the west of Whidbey Island, the trace of the Devils 

Mountain fault extends northwest into an exposed fault zone that 

juxtaposes disparate pre-Tertiary rocks in the southern San Juan 

Islands.

Loveseth (1975) suggested left-lateral slip on the Devils 

Mountain fault based on its onland linearity, juxtaposition of 

upper Eocene to lower Oligocene continental and marine strata, 

and horizontal slickensides (Hobbs and Pecora, 1941) in the 

fault zone. Whetten (1978) suggested as much as 60 km of leftlateral slip. Tabor (1994) inferred that the Devils Mountain fault 

was continuous with the north-trending Darrington fault zone to 

the east (fig. 1) and suggested that pre-34 Ma right-lateral slip 

was probable, based on fault fabric and offset rock units. Evans 

and Ristow (1994), who also regarded the Devils Mountain and 

Darrington faults as one continuous structure, argued for left-lateral slip on the basis of orientations of folds in rocks straddling 

the fault. Johnson and others (1996) noted that on seismicreflection data from the eastern Strait of Juan de Fuca, the Devils 

Mountain fault is locally imaged as a north-dipping reverse fault.

Oligocene rocks are clearly deformed along the Devils 

Mountain fault zone, but an upper age limit for faulting has not 

yet been determined. Loveseth (1975) and Adair and others 

(1989) did not report deformation in upper Pleistocene strata 

along the fault in the Cascade Range foothills, but both noted the 

youthful appearance of the topographic lineament coinciding 

with the structure (fig. 4). Naugler and others (1996) first 

reported Quaternary displacement on the Devils Mountain fault 

on the basis of high-resolution seismic-reflection data from Lake 

Cavanaugh (figs. 1, 4) that reveal both faulting and folding of 

inferred upper Pleistocene to Holocene lacustrine sediments. In 

a regional study of latest Pleistocene glaciomarine deposition, 

Dethier and others (1995, their fig. 7) noted that the Devils 

Mountain fault coincides with a significant (-10 m) inflection 

point on a graph of the maximum altitude of glaciomarine drift.

South of the Devils Mountain fault, Gower and others 

(1985) proposed the presence of another potentially active structure, the northern Whidbey Island fault, based on interpretation

Devils Mountain Fault and Related Structures, Pacific Northwest
49

124° 122°

48°

47 C

^*^: * ^ 

•,-.Vancouver Island :

50 KILOMETERS

Surficial deposits 

(Quaternary)

Sedimentary rocks 

(Neogene to Paleogene)

EXPLANATION

Cascade Range igneous rocks 

(Eocene and younger) and 

Columbia River Basalt 

Group (Miocene)

Crescent Formation and 

other Eocene volcanic rocks

Basement rocks 

(pre-Tertiary)

Figure 1. Schematic geology of northwestern Washington and southwestern British Columbia showing Puget Lowland and flanking Cascade Range 

and Olympic Mountains, and southern Vancouver Island. B, Bellingham; E, Everett; 0, Olympia; S, Seattle; T, Tacoma; V, Victoria. Heavy line, fault; 

dashed where inferred, sawteeth on upper plate of thrust fault. Triangle, modern Cascade volcano. DAF, Darrington fault; DMF, Devils Mountain 

fault; EB, Everett basin; GP, Glacier Peak; HC, Hood Canal; LC, Lake Cavanaugh; LRF, Leech River fault; MB, Mount Baker; SB, Seattle basin; SCF, 

Straight Creek fault; SF, Seattle fault; SJ, San Juan Islands; SJF, San Juan fault; SWF, southern Whidbey Island fault. Geology from maps and compilations of Tabor and Cady (1978), Washington Public Power Supply System (1981), Gower and others (1985), Walsh and others (1987), Tabor and 

others (1988,1993), Whetten and others (1988), Yount and Gower (1991), Tabor (1994), Johnson and others (1996,1999,2001).

of gravity and magnetic anomalies (MacLeod and others, 1977), 

and on faulting of Quaternary sediments exposed at Strawberry 

Point on northeastern Whidbey Island (fig. 2). Our offshore and 

onland investigations, including recent high-resolution 

aeromagnetic surveys (Blakely, Wells, and Weaver, 1999; 

Blakely and Lowe, 2001), reveal two faults in the vicinity of the 

inferred northern Whidbey Island fault. Neither of these faults

coincides with the trace of the northern Whidbey Island fault as 

inferred by Gower and others (1985), so we do not use that name 

in this report and recommend that it be abandoned. Instead, we 

refer to the two structures as the Strawberry Point fault and the 

Utsalady Point fault, named for sites (fig. 2) where the faults 

have deformed upper Pleistocene deposits exposed in coastal 

bluffs.

Geologic Setting
30

2.I

a. 

to

(D CO

•oSin

§2r£LN_ _>1S|A I - HMF ' RMF" —- \

v ^ Juan de Fuca

10 KILOMETERS EXPLANATION

Quaternary deposits

Pre-Quaternary bedrock

Inferred fault— 

Queried where extent uncertain

—/-—.. Anticline 

"/—— Syncline

Locations of shallow (< 30 km) earthquakes, magnitude (M) > 2.5 

D M = 5.0 D M = 4.0-4.4 n M = 3.0-3.4 

H M = 4.5-4.9 n M = 3.5-3.9 n M = 2.5-2.9

Figure 2. Structural geology of northeastern Strait of Juan de Fuca area (fig. 1), based on interpretation of seismic-reflection (figs. 8-26), aeromagnetic (fig. 3), outcrop (fig. 27), and subsurface data (fig. 28). Onshore faults (except on Whidbey and Camano Islands) are based on mapping of Whetten and others (1988). Not all faults and folds deform Quaternary deposits. Earthquake locations 

and magnitudes are from the Pacific Northwest Seismograph Network. DMF, Devils Mountain fault; Fl, Fidalgo Island; G, Goat Island; I, Ika Island; LI, Lopez Island; RP, Rocky Point; SJI, San Juan 

Island; SP, Strawberry Point; SPF, Strawberry Point fault; SWF, southern Whidbey Island fault; UP, Utsalady Point; UPF, Utsalady Point fault.
123°15' 123C 122°45' 122°30'

CD£_e <ao'GO3

48° 

30'

nT

25 KILOMETERS

Figure3. Shaded relief map of the aeromagnetic anomaly map, eastern Strait of Juan de Fuca-northern Puget Lowland region (fig. 1). Illumination is from the northeast. Heavy 

white lines, coastlines. Light white lines, faults and folds of figure 2. Vertical white line on west part of diagram, boundary between two different magnetic surveys. Cl, Camano 

Island; Fl, Fidalgo Island; LI, Lopez Island; RP, Rocky Point; SJF, Strait of Juan de Fuca; SD, Skagit River delta; SJI, San Juan Island; VI, Vancouver Island; Wl, Whidbey Island.

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Figure 4. SLAR (side-looking airborne radar) image of portion of northwestern Washington (fig. 1). White arrows show topographic lineament associated with Devils Mountain fault. Middle arrow also shows location of elongate Lake Cavanaugh (LC), which occurs along the lineament. Cl, Camano Island; DMF, Devils Mountain fault; SD, Skagit River delta; Wl, Whidbey Island. Area of image shown in figure 1.

Seismic-Reflection Surveys—Data Acquisition 

and Processing

High-resolution seismic-reflection surveys (Mosher and 

Simpkin, 1999) have been shown to be important tools in interpretation of active faults (for example, Barnes and Audru, 1999; 

Johnson and others, 1999). Our investigation of active structures 

in the northern Puget Lowland and eastern Strait of Juan de Fuca 

relies heavily on a network of multichannel seismic-reflection 

data collected in 1995 by the U.S. Geological Survey (USGS) 

and in 1996 by the Geological Survey of Canada (G.S.C.). Collectively, we acquired several hundred kilometers of data in the 

eastern Strait of Juan de Fuca and Skagit Bay (fig. 5, on pi. 1), 

crossing the Devils Mountain fault 15 times and the Strawberry 

Point and Utsalady Point faults 6 times.

For the 1995 USGS survey (fig. 5A), the seismic source 

consisted of two 0.655-L (liter) (40 in.3) airguns fired at 12.5-m 

intervals. Data were digitally recorded for 2 s using a 24-channel 

(6.25-m group interval; 150-m active length) streamer. Resulting common mid-point (CMP) stacked data are six-fold and 

have a 3.125-m CMP spacing. Profiles were located using GPS 

satellite navigation with an accuracy of ±100 m. Relative 

locations from fix to fix are accurate to within a few meters. 

These data were deconvolved and filtered before and after stack, 

then time-migrated using a smoothed velocity function. Deconvolution was only partly effective in suppressing water-bottom 

multiples.

For the 1996 G.S.C. survey (fig. 5C), the seismic source 

consisted of a 0.655-L (40 in.3) sleeve airgun fired at 16-m intervals, and data were digitally recorded for 1 s using a 24-channel 

(8-m group interval, 192-m active length) streamer. GPS

Devils Mountain Fault and Related Structures, Pacific Northwest
satellite navigation with an accuracy of ±20 m was used to locate 

profiles. CMP stacked data are four-fold and have a 4-m CMP 

spacing.

Both USGS and G.S.C. data are typically of high quality in 

the upper approximately 0.5-0.7 s and degrade significantly with 

greater depth. On both the USGS and G.S.C. cruises (fig. 5A, C), 

higher frequency seismic-reflection data were collected concurrently with the airgun data. The USGS used a geopulse boomer 

source and recorded data for 500 ms on a single-channel 

streamer. The G.S.C. used a Huntec hydrosonde deep tow 

boomer system and recorded data for 250 ms on a single "internal" hydrophone and a short external streamer trailing behind the 

towed source. The quality of these higher frequency data is 

highly variable due to local nonreflective and (or) gassy sub-bottom sediments. Maximum penetration is about 150-200 ms 

(120-160 m).

The U.S. Geological Survey, the Geological Survey of Canada, and several academic institutions collaborated in 1998 on a 

deeper multichannel seismic-reflection survey, "Seismic Hazard 

Investigation of Puget Sound" (SHIPS; Fisher and others, 1999), 

that traversed the eastern Strait of Juan de Fuca (fig. 5B). These 

data were collected using a 79.3-L (4,838 in.3 ) airgun array and 

recorded on a 2.4-km-long, 96-channel streamer. To enhance the 

shallow (<10 km) geologic section for the purposes of this 

report, the near-offset 48 channels of these data were stacked 

(24-fold, 12-m CMP) and velocity migrated. We also examined 

several proprietary industry seismic-reflection data (3 to 5 s 

records) profiles from the eastern Strait of Juan de Fuca (fig. 5fi), 

most of which were collected in the late 1960's or early 1970's 

with airgun sources of various sizes, when the area was considered a frontier for petroleum exploration. Examples of these 

industry data included in this report have been stacked but not 

migrated.

Stratigraphy and Seismic Stratigraphy

Several distinct stratigraphic units occur onshore in this 

portion of the northern Puget Lowland (fig. 6). Four different 

units, distinguished on the basis of stratigraphic position and 

seismic stratigraphic facies (for example, Sangree and Widmier, 

1977; Stoker and others, 1997), are imaged on offshore seismicreflection data. These include pre-Tertiary basement rocks, Tertiary sedimentary rocks, uppermost Pliocene(?) to Pleistocene 

strata, and uppermost Pleistocene to Holocene strata.

Pre-Tertiary Basement Rocks

Pre-Tertiary bedrock consisting of the Jurassic Fidalgo 

Complex, which includes ophiolite and tectonically mixed, variably metamorphosed, sedimentary, volcanic, and plutonic rock, 

is exposed north of the Devils Mountain fault in the Skagit River 

delta, on northern Whidbey Island and Fidalgo Island, and in the 

San Juan Islands (Whetten and others, 1988; Tabor, 1994; fig. 2). 

Ultramafic and mafic rocks of the Fidalgo ophiolite form notable 

magnetic highs beneath the Skagit River delta, northernmost 

Whidbey Island, Fidalgo Island, and the eastern San Juan Islands 

(figs. 2, 3; Whetten and others, 1980). In contrast, Mesozoic

sedimentary rocks commonly form subdued aeromagnetic 

anomalies, such as on southern San Juan Island and southwestern Lopez Island (fig. 3). One small outcrop of pre-Tertiary 

rocks is exposed south of the Devils Mountain fault on Whidbey 

Island at Rocky Point (fig. 2). Pre-Tertiary bedrock, consisting 

of the Mesozoic Leech River complex and Paleozoic to Mesozoic volcanic, plutonic, and metamorphic rocks, also underlies 

southern Vancouver Island north of the Leech River fault (fig. 1; 

Roddick and others, 1979). Pre-Tertiary rocks are generally nonreflective and represent acoustic basement on seismic-reflection 

profiles.

Tertiary Sedimentary Rocks

Tertiary sedimentary rocks crop out adjacent to the Devils 

Mountain fault east of northern Whidbey Island within the 

Skagit River delta and in the Cascade Range foothills (Whetten 

and others, 1988; fig. 1). Units include the Eocene, nonmarine 

Chuckanut Formation (Johnson, 1984a; Evans and Ristow, 

1994) and the upper Eocene to lower Oligocene marine to marginal marine rocks of Bulson Creek (Marcus, 1980) (fig. 6). 

These rocks are cut by numerous west- and northwest-trending 

faults and are gently to steeply folded and locally overturned 

(Whetten and others, 1988; Dragovich and others, 2000). On 

industry seismic-reflection profiles, Tertiary strata are characterized by relatively continuous, high-amplitude, parallel to subparallel, moderate frequency reflections and can thereby generally 

be distinguished from nonreflective pre-Tertiary basement.

Uppermost Pliocene!?) to Pleistocene Strata

Onshore Stratigraphy

Uppermost Pliocene(?) to Pleistocene deposits of the Puget 

Lowland comprise a stratigraphically complex basin fill of glacial and interglacial deposits that are locally as thick as 1,100 m 

(Yount and others, 1985; Jones, 1996). Easterbrook (1994a, b) 

described six glacial drift units, three of which are exposed on 

Whidbey Island (fig. 6). In the northern Puget Lowland, glacial 

drift typically consists of till, outwash, and glaciomarine deposits. Interglacial deposits are typically fluvial and deltaic, including peat.

The Double Bluff Drift is the oldest Pleistocene unit recognized on Whidbey Island (Easterbrook, 1968, 1994b). It does 

not crop out on the northern Whidbey Island area of this investigation, where it is inferred to occur below sea level on the basis 

of stratigraphic correlation diagrams derived from water-well 

logs. Deposition of the Double Bluff Drift is thought to have 

occurred between about 250 and 130 ka (Blunt and others, 1987; 

Easterbrook, 1994b) and probably coincides approximately with 

marine isotope stage 6 (for example, Shackleton and others, 

1983; McManus and others, 1994; Kukla and others, 1997).

The interglacial alluvial- and delta-plain deposits of the 

Whidbey Formation (Easterbrook and others, 1967; Easterbrook, 1968, 1969; Heusser and Heusser, 1981) overlie the 

Double Bluff Drift on Whidbey Island. The Whidbey Formation is locally more than 60 m thick and is widespread in the

Stratigraphy and Seismic Stratigraphy 7
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Figure 6. Stratigraphy and age of geologic units occurring in northern 

Whidbey Island area (figs. 1,3). Ages for bedrock units based on 

Johnson (1984a), Whetten and others (1988), and Rau and Johnson 

(1999). Ages for Quaternary units based mainly on Blunt and others 

(1987), Berger and Easterbrook (1993), Easterbrook(1994a,b), Porter and 

Swanson (1998), and Troost (1999).

central and northern Puget Lowland (Easterbrook, 1968, 

1994b). Amino-acid analyses of shells and wood at 13 localities suggest an age of 100±20 ka for the Whidbey Formation; 

four thermoluminescence ages range from 102±38 to 151±43 

ka (Blunt and others, 1987; Berger and Easterbrook, 1993). 

These dates indicate a correlation with interglacial marine isotope stage 5 (for example, Shackleton and others, 1983; 

McManus and others, 1994; Kukla and others, 1997), and pollen data indicate internal climatic fluctuations occurred during 

its deposition (Heusser and Heusser, 1981; T.A. Ager, written 

commun., 2000). We therefore infer that the top of the Whidbey Formation (where it is not significantly eroded) has an age 

corresponding to the end of this stage (and not the age of a 

stage 5 substage), about 80 ka. Our investigations suggest that

the top of the Whidbey Formation can be recognized and 

traced in the northern Whidbey Island area using water-well 

logs (see "Evidence for Faulting Onshore, Whidbey and 

Camano Islands"), and therefore provides a potential marker 

for estimating both amounts and rates of recent deformation.

The Possession Drift discontinuously overlies the Whidbey Formation (fig. 6). At its type locality on southern Whidbey Island, it thins laterally through a distance of about 400 m 

from a thickness of 25 m to an unconformity between the 

Whidbey Formation and younger sediments (Easterbrook, 

1968). Stratigraphic correlation diagrams from northern Whidbey Island based on water-well logs (see "Evidence for Faulting Onshore, Whidbey and Camano Islands") suggest a 

similarly variable occurrence and thickness for this unit. 

Radiocarbon dates from the Possession Drift yield infinite ages 

(>~45,000 14C yr B.P.); amino acid analyses of marine shells in 

Possession Drift at several localities suggest an age of about 

90-75 ka (Blunt and others, 1987). Based on these ages and 

those from underlying and overlying strata, the Possession 

Drift appears correlative with marine isotope stage 4 (for 

example, Shackleton and others, 1983; McManus and others, 

1994) and was probably deposited between about 80 and 60 ka.

The informally named nonglacial alluvial and deltaic 

Olympia beds locally overlie Possession Drift or older deposits 

in the central and northern Puget Lowland (fig. 6). Outcrop 

and water-well data (see "Evidence for Faulting Onshore, 

Whidbey and Camano Islands") indicate that these strata have 

variable thickness (as much as ~25 m) and are locally absent on 

northern Whidbey Island. To the northeast in the Swinomish 

Island-Skagit River delta area, Dragovich and Grisamer (1998) 

and Dragovich and others (1998) suggested that Olympia beds 

are as thick as 50 m based on interpretations of subsurface data. 

Olympia beds were probably deposited between about 60 and 

17 ka (Fulton and others, 1976; Blunt and others, 1987; 

Clague, 1994; Easterbrook, 1994b; Porter and Swanson, 1998; 

Troost, 1999). Hansen and Easterbrook (1974) and Easterbrook (1976) described a thin sequence of inferred glacial 

deposits within the Olympia beds at Strawberry Point (fig. 2), 

which they considered evidence for the -34,000^0,000 14C yr 

B.P. "Oak Harbor stade of the Possession glaciatlon." Fulton 

and others (1976) and Clague (1978) argued, however, that an 

ice advance at this time was unlikely because of a lack of evidence to the north in southern British Columbia for correlative 

ice occupation.

Strata associated with the Fraser glaciation on northern 

Whidbey Island include advance outwash of the Esperance 

Sand Member of the Vashon Drift, the main body of the Vashon Drift, and recessional deposits of the Everson Drift (Easterbrook, 1968). These units have variable thickness but 

collectively are commonly 50-100 m thick or more. Radiocarbon dating suggests that this Stratigraphic "package" in the 

northern Whidbey Island area was deposited between about 

17,000 and 12,000 14C yr B.P. (Easterbrook, 1968, 1969; Blunt 

and others, 1987; Dethier and others, 1995; Porter and Swanson, 1998). Hicock and Armstrong (1981) presented evidence 

for a =21,000 14C yr B.P. pre-Vashon, early Fraser till-bearing 

unit (Coquitlam Drift) to the north in southern British 

Columbia, but ice from this early Fraser stade apparently did 

not reach the northern Whidbey Island area.

8 Devils Mountain Fault and Related Structures, Pacific Northwest
Offshore Stratigraphy

Two distinct seismic units occur above pre-Tertiary or Tertiary "basement" in the eastern Strait of Juan de Fuca. On both 

industry and higher resolution seismic-reflection data, the lower 

of these two units consists of seismic facies typical of glacial 

deposits (Davies and others, 1997). Characteristics include discontinuous, variable-amplitude, parallel, divergent, and hummocky reflections, with common internal truncation, onlap, and 

offlap of reflections. Based on this seismic facies and on stratigraphic position, this seismic unit is inferred to comprise uppermost Pliocene(?) to Pleistocene deposits (excluding uppermost 

Pleistocene postglacial deposits). Given the physiography of the 

eastern Strait of Juan de Fuca, it is likely that much of this unit 

consists of recessional glaciomarine drift. Present-day sealioor 

morphology is largely governed by these drift deposits (Hewitt 

and Mosher, 2001; Mosher and others, 2001). We did not recognize any internal sequences within the inferred uppermost 

Pliocene(?) to Pleistocene section that could be traced across the 

region and might correlate with eustatic fluctuations associated 

with multiple glacial and nonglacial intervals. We attribute this 

lack of internal stratigraphy to irregular and large-scale glacial 

erosion and deposition.

Identification and Age of the Base of the Uppermost Pliocene(?) to 

Pleistocene Section

The base of the inferred uppermost Pliocene(?) to Pleistocene seismic unit is typically most distinct on conventional 

industry seismic-reflection data (fig. 5B), where it is recognized 

on the basis of contrasts in seismic facies (Johnson and others, 

1996, 1999). As described in the preceding paragraph, these 

mainly Quaternary strata are generally characterized by low- to 

moderate-amplitude, discontinuous to continuous, irregular 

hummocky, divergent, and parallel reflections, with common 

internal truncation, onlap, and offlap. In contrast, pre-Tertiary 

rocks are nonreflective, and reflections from underlying Tertiary 

strata have higher amplitude, are more continuous, and are typically parallel to subparallel. Where Tertiary rocks are folded, 

this contact is generally an angular unconformity that may pass 

laterally into a disconformity.

On high-resolution seismic-reflection profiles (fig. 5A, C), 

this seismic-unit contact and the contrast in seismic facies 

between older "basement" and mainly Quaternary strata are generally less distinct. For these profiles, the location of the contact 

is generally based on projection from nearby conventional industry profiles (fig. 5B) or on the basis of locally distinct unconformities and onlapping surfaces. Once the contact at the base of 

the mainly Quaternary section is identified at one or more locations on individual high-resolution profiles, it can generally be 

traced across the profile based on reflection continuity. Complete regional coverage is accomplished by thus iteratively combining the industry and high-resolution data. Figure 7 is a 

contour map based on these data that shows the depth to the base 

of the uppermost Pliocene(?) to Pleistocene section in the northeastern Strait of Juan de Fuca.

Knowing the age of the base of the uppermost Pliocene(?) 

to Pliocene section in offshore data is important because it

provides a potential marker for estimating rates of deformation. Determining this age is, however, problematic. No boreholes have penetrated the submerged section, and multiple 

pulses of deep subglacial scour and subsequent filling in offshore areas (for example, Booth, 1994) suggest that the age of 

the surface may vary locally and that correlation with adjacent 

dated units on land is untenable. For this investigation, we infer 

that this surface has a maximum age of ~2 Ma, coinciding with 

the age of the first glaciation for which there is evidence in the 

Puget Lowland (Easterbrook, 1994a, b), slightly older than the 

Pliocene-Pleistocene boundary (Gradstein and Ogg, 1996). 

However, because of repeated deep glacial erosion, we think it 

probable that the oldest deposits in this unit imaged on many 

seismic-reflection profiles are much younger.

Uppermost Pleistocene and Holocene Strata

Variable-amplitude, parallel, and continuous reflections that 

fill in local basins bounded by Pleistocene bathymetric highs 

characterize the uppermost seismic unit recognized in the eastern Strait of Juan de Fuca. East of Whidbey Island, these sediments are inferred to be clay and silt derived from the Skagit 

River (fig. 2). In the eastern Strait of Juan de Fuca where there 

are no major inputs of postglacial terrestrial sediment, the uppermost Pleistocene to Holocene basin fill probably consists largely 

of glacial recessional deposits variably reworked by strong tidal 

currents.

Seismic-Reflection Profiles

A series of seismic-reflection profiles extending from east 

to west, across the eastern Strait of Juan de Fuca and through 

Skagit Bay and Saratoga Passage (figs. 2, 5) document the structural style of the Devils Mountain fault and related structures. 

Selected lines from this data base are shown in figures 8-26 (pis. 

I, 2). Velocities used to estimate Quaternary unit thickness and 

dip, and vertical exaggeration, are based on analysis of well logs 

and seismic-reflection data from the region (Johnson and others, 

1994, 1996; Pratt and others, 1997; Brocher and Ruebel, 1998): 

uppermost Pleistocene and Holocene strata—1,600 m/s; uppermost Pliocene(?) to Pleistocene strata—1,800 m/s. All high-resolution airgun and industry profiles are plotted at about the same 

horizontal distance scale and vertical time scale in order to facilitate comparison, and both interpreted and uninterpreted sections 

are included. To simplify the discussion, the uppermost 

Pliocene(?) to Pleistocene section and the overlying postglacial 

uppermost Pleistocene and Holocene section are in some places 

collectively referred to as "Quaternary."

Faults on seismic-reflection profiles (figs. 8-26) are recognized on the basis of truncated reflections and abrupt changes 

in reflection dip or seismic facies, characterized by reflection 

amplitude, frequency, continuity, and geometry (Sangree and 

Widmier, 1977; Stoker and others, 1997). Fault dip is generally defined in the upper part of a seismic-reflection profile 

where the record is best, then projected downdip to the base of 

the profile. Undeformed beds are characterized by flat reflections and no abrupt changes in dip or seismic facies. Faults are

Seismic-Reflection Profiles 9
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10 Devils Mountain Fault and Related Structures, Pacific Northwest
also numbered on the profiles so that text and figures can be 

easily compared. This numbering scheme is specific to each 

profile; faults with the same number on different profiles are 

generally not the same structure.

Aeromagnetic profiles parallel to seismic-reflection tracklines are also shown in figures 8-26. For practical purposes, 

these profiles have a relatively compressed vertical scale. Thus, 

they highlight only the largest anomalies; lower amplitude 

anomalies are more clearly displayed in the figure 3 map.

Seismic-reflection and aeromagnetic data (figs. 3, 5, 8-26) 

were used together to map the faults and folds across the eastern 

Strait of Juan de Fuca region shown in figure 2. However, not 

every structure in figure 2 is shown on every crossing seismicreflection profile displayed in figures 8-26. All of the structures 

shown in figure 2 are inferred to cut or warp pre-Tertiary or Tertiary "basement," but many do not obviously deform Quaternary 

deposits. In some cases, these older structures are deeper than 

the level imaged by shallower high-resolution seismic-reflection 

profiles (fig. 5A, C), and mapping their trends is based entirely 

on deeper industry data (fig. 5B). Conversely, in places the 

deeper industry data lack the resolution to image structures in 

the upper 1 km, and mapping of faults and folds is based mostly 

on the shallower high-resolution profiles.

U.S. Geological Survey Line 176

Line 176 (fig. 8) trends northwest and extends through 

Skagit Bay on the east side of northern Whidbey Island (fig. 

5A). On this profile, the combination of shallow water (<50 m), 

shallow pre-Tertiary basement (inferred from nearby outcrops), 

and proximity to land has resulted in a seismic record with 

numerous water-bottom multiples, reverberations, and diffractions. Despite this "noise," numerous faults can be identified on 

the profile on the basis of truncated and warped reflections, and 

juxtaposition of panels with disparate seismic facies. Among 

these faults, the Devils Mountain fault zone is identified based 

on extension of bedrock faults mapped onland by Whetten and 

others (1988) 1-2 km east of the profile. The Devils Mountain 

fault is imaged as a 2-km-wide zone of four subvertical structures. The juxtaposition of disparate seismic facies is largest 

along the southern fault (6) in the Devils Mountain zone, which 

correlates with an aeromagnetic low (figs. 3, 8A) and is inferred 

to be the main splay. Both this structure and the fault immediately to the north (7) truncate reflections in the Quaternary section within about 20-30 m (20-30 ms) of the seafloor. At least 

three other faults are imaged north of the Devils Mountain zone 

(10, 11, 12), cutting an aeromagnetic high formed by rocks of 

the Fidalgo ophiolite (Whetten and others, 1988). Of these, 

only the northernmost fault (12) appears to offset the Quaternary section.

South of the Devils Mountain fault zone, a variably thick 

(100-250 m; 110-275 ms) uppermost Pliocene(?) to Pleistocene 

section overlies nonreflective pre-Quaternary strata and is overlain by a relatively thin (<135 m; 150 ms) uppermost Pleistocene 

to Holocene (postglacial) section that is largely obscured by 

water-bottom multiples. At least one fault (5) that extends up 

into the lower part of the uppermost Pliocene(?) to Pleistocene

section occurs in the Everett basin between the Devils Mountain 

zone and a zone of faults offshore of Strawberry Point.

The structural zone offshore from Strawberry Point consists of at least four subvertical faults (1, 2, 3, 4) that span a 

distance of 3 km along Line 176, about 2 km perpendicular to 

the inferred strike of the zone (fig. 2). Although water-bottom 

multiples are prominent in the upper part of the record in this 

zone, all four structures appear to propagate upward into the 

Quaternary section and within about 45-90 m (50-100 ms) of 

the seafloor. Fault 1 juxtaposes discontinuous, moderateamplitude, warped reflections (on the southeast) and a largely 

nonreflective domain, probably cored by uplifted bedrock, to 

the northeast. Faults 2 and 3 truncate low-amplitude reflections, most obvious between =300 and 400 ms. Fault 4 juxtaposes disparate domains of subhorizontal reflections. Overall, 

uplift of basement rock within and north of the fault zone is 

suggested by the juxtaposition across fault 1 of relatively nonreflective material to the north and more reflective material to 

the southeast. No obvious aeromagnetic anomaly is associated with this fault zone along the profile (figs. 3, 8A).

U.S. Geological Survey Line 177

Line 177 (fig. 9) trends northeast and extends through 

Saratoga Passage between eastern Whidbey Island and northern 

Camano Island (fig. 5A). On this line, a 2.3 km-wide fault zone 

(2 km normal to the inferred fault trend; fig. 2) lies offshore 

from Utsalady Point. This zone comprises five faults (1 to 5), 

all of which extend upward into at least the lower part of the 

Quaternary section. The faults warp, tilt, and truncate reflections, but vertical offset does not appear to be significant across 

individual faults within the zone or across the fault zone as a 

whole. Northeast and southwest of this zone, subhorizontal to 

locally hummocky reflections characterize the Quaternary strata 

of the Everett basin. Southwest of the zone, significant erosional relief occurs at the base of the inferred postglacial uppermost Pleistocene to Holocene section. The fault zone occurs 

along the southwest margin of a gentle aeromagnetic high (figs. 

3, 9A).

U.S. Geological Survey Line 167-168

Line 167-168 (figs. 10, 11, 12) trends north and extends 

from the eastern Strait of Juan de Fuca into southern Rosario 

Strait, from about 1 to 5 km west of Whidbey Island (fig. 5A). 

On the airgun profile (fig. 10), the Devils Mountain fault is 

imaged as a zone of three structures (faults 4, 5, and 6) about 

1.5 km wide that correlate with a gentle aeromagnetic high (figs. 

3, 10A). The northernmost structure (6) is subvertical and is 

inferred to offset the Tertiary-Quaternary contact about 100-120 

m (110-130 ms). This fault juxtaposes a panel of gently south 

dipping, moderate-amplitude continuous reflections on the 

north and more discontinuous, variable-amplitude, higher frequency reflections on the south. The fault is overlain by a break 

in slope in the seafloor, and higher resolution geopulse data (fig. 

11) show a contrast in the reflectivity of postglacial (uppermost

Seismic-Reflection Profiles 11
Pleistocene to Holocene) sediments coinciding with this break. 

This contrast indicates juxtaposition of sediments with different 

physical properties, such as grain size, bedding, and (or) gasfilled porosity (Hovland and Judd, 1988; Fader, 1997). Thus, the 

fault may project to the surface. The geometry of the break in 

slope (south side up) is opposite to the inferred sense of vertical 

slip on the fault at depth, suggesting a possible reversal of slip. 

Alternatively, uplift could have exposed more erodible sediments on the uplifted northern block, leading to topographic 

inversion.

The two southern faults in the Devils Mountain fault zone 

on this profile (4, 5) are subvertical, truncate subhorizontal Quaternary reflections, juxtapose domains of different reflection 

properties, and propagate upward into at least the lower part of 

the postglacial uppermost Pleistocene to Holocene section. The 

amount of vertical separation on the base of the Quaternary 

across these two structures appears minimal. The juxtaposition 

of different seismic facies with relatively small vertical offset on 

both these faults and on fault 4 to the north suggests out-ofplane (strike-slip) movement on these structures.

Two kilometers north of the Devils Mountain fault zone, 

another steep north-dipping fault (7) truncates reflections on the 

south-dipping limb of a gentle anticline. The fault appears to 

project into the lowest part of the uppermost Pliocene(?) to 

Pleistocene section, and Quaternary strata on the south-dipping 

limb of the anticline dip south and thin toward the fold axis, indicating deposition during fold growth. The north limb of the fold 

is also cut by a steep fault (8) that does not appear to extend into 

the inferred Quaternary section. Northwest-trending structures 

that occur north of fault 8 in Rosario Strait (fig. 3) mainly affect 

basement rocks and are not clearly imaged on line 167-168 (fig. 

10). The locations of these structures are based on industry data, 

including profiles collected by Puget Power (1979).

Three faults (1,2, and 3) occur south of the Devils Mountain fault zone. Based on their westerly traces determined from 

seismic-reflection data farther west (figs. 10, 12, 13, 14, 15) and 

on the pattern of aeromagnetic anomalies (fig. 3), these structures are correlated and inferred to be continuous with the faults 

on the east side of Whidbey Island offshore of Strawberry Point 

and Utsalady Point (figs. 8, 9), locations for which these faults 

are here named. The subvertical Strawberry Point fault (3) juxtaposes north-dipping to subhorizontal reflections representing 

Tertiary and Quaternary strata on the north (largely obscured by 

multiples) and seismically opaque, uplifted basement rocks and 

a thin overlying sedimentary section including a small (~ 850 m 

wide) postglacial basin on the south. The uppermost Pliocene(?) 

to Pleistocene section north of and adjacent to the fault dips 

north about 10°, and vertical separation on the base of this unit 

across the fault is inferred to be about 80 m (=90 ms). The companion geopulse profile (fig. 12) shows disrupted reflections in 

the upper =10 ms along the fault, but the underlying record is 

nonreflective.

The subvertical Utsalady Point fault forms the southern 

margin of the uplift bounded on the north by the Strawberry 

Point fault and is imaged by at least two splays (1 and 2; fig. 

10). The northern splay (within the uplift, fault 2) warps and 

truncates reflections, juxtaposing reflective and nonreflective 

zones. The southern splay juxtaposes the uplifted block and

subhorizontal reflections in the Everett basin to the south. The 

faults project upward into at least the lower part of the uppermost Pliocene(?) to Pleistocene section, and vertical offset on 

the base of this unit across these two splays is about 200-225 m 

(220-250 ms). The companion geopulse profile (fig. 12) shows 

these two faults truncating reflections in the inferred uppermost 

Pleistocene to Holocene section to within 5-10 m of the seafloor. Between the Strawberry Point and Utsalady Point faults, 

the geopulse profile (fig. 12) also displays another subvertical 

fault that cuts and warps reflections within both the uppermost Pliocene(?) to Pleistocene and the postglacial uppermost 

Pleistocene to Holocene sections, to within about 15 m of the 

seafloor.

The shallow basement between the Strawberry Point and 

Utsalady Point faults is inferred to consist of pre-Tertiary sedimentary basement rock because of its nonreflective character 

and because pre-Tertiary sedimentary rock is exposed 1 km to 

the east at Rocky Point on the west coast of Whidbey Island (fig. 

2). The Rocky Point exposures of these basement rocks have 

surprisingly low bulk magnetic susceptibility (see "Evidence for 

Onshore Faulting, Whidbey and Camano Islands") and form a 

prominent aeromagnetic anomaly that extends eastward and provides an important constraint for projection of the Strawberry 

Point and Utsalady Point faults across Whidbey Island.

U.S. Geological Survey Line 166

Line 166 (fig. 13) trends north and extends from the eastern 

Strait of Juan de Fuca into southern Rosario Strait, about 6 km 

west of Whidbey Island (fig. 5A). On this profile, the Devils 

Mountain fault is imaged as two splays ( 4 and 5 ) that dip 

steeply (=60°-80°) to the north. The southern splay (4) truncates south-dipping reflections within the uppermost Pliocene(?) 

to Pleistocene section on the south limb of a hanging-wall anticline, as well as nearly horizontal reflections in the Everett basin 

to the south. The northern splay (5) truncates reflections within 

the Tertiary section on the south-dipping limb of this fold, but 

does not obviously break into the Quaternary section. Dips in 

uppermost Pliocene(?) to Pleistocene beds on the south and 

north flanks of this fold are about 11° and 7°, respectively. 

There appears to be about 200-225 m (220-250 ms) of structural 

relief on the base of the uppermost Pliocene(?) to Pleistocene 

from the crest of the hanging-wall anticline across the fault zone 

to the floor of the Everett basin. This inference and estimates of 

offset on the Devils Mountain fault on this profile are speculative 

because reflections in the Everett basin are partly obscured by 

water-bottom multiples. These multiples also partly mask hummocky reflections in the uppermost Pliocene(?) to Pleistocene 

section south of the Devils Mountain fault, which either could be 

depositional in origin (such as moraines) or could represent disruption by faulting or folding. A small aeromagnetic anomaly is 

associated with the Devils Mountain fault on this profile (figs. 3, 

13A).

Farther south, the Strawberry Point (3) and Utsalady Point 

(2) faults are imaged as subvertical structures that bound a nonreflective horst block and truncate moderate- to high-amplitude, subparallel, continuous to discontinuous reflections in the

12 Devils Mountain Fault and Related Structures, Pacific Northwest
Everett basin. This horst block coincides with an aeromagnetic 

low (figs. 3, 13A) and is inferred to consist of pre-Tertiary 

sedimentary rock overlain by a thin uppermost Pleistocene to 

Holocene (postglacial) section. Juxtaposition of this horst 

block and Everett basin Tertiary and Quaternary strata requires 

significant uplift. This seismic profile suggests about 150-200 

m (165—220 ms) of vertical separation on the base of the uppermost Pliocene(?) to Pleistocene section across the Strawberry 

Point fault, and about 250-300 m (275-330 ms) across the 

Utsalady Point fault.

Industry Line 1

Industry Line 1 (fig. 14) is subparallel to U.S. Geological 

Survey Line 166 (fig. 5A, B) and provides a slightly deeper view 

of structure in this area. The Devils Mountain fault is not well 

imaged, but it appears to juxtapose relatively nonreflective material north of the fault and a section characterized by warped discontinuous reflections to the south (best seen at =0.7 s). As on 

Line 166 (fig. 13), the Strawberry Point and Utsalady Point 

faults appear as subvertical faults bounding a generally nonreflective uplift that correlates with an aeromagnetic low (fig. 

14A). The uplift has relief above the seafloor and is onlapped or 

draped by Quaternary strata, which are warped across the axis of 

the uplift.

U.S. Geological Survey Line 165

Line 165 (fig. 15) trends west, extending partly across the 

opening to Rosario Strait, between Lopez and Fidalgo Islands 

(fig. 5A). This line images a gentle asymmetric anticline with a 

subvertical fault or faults along its axis, coinciding with a bathymetric rise and east-facing scarp on the seafloor. The fault propagates upward into the lower part of the uppermost Pliocene(?) 

to Pleistocene section, and these strata are involved in the folding. Because water-bottom multiples obscure much of the 

uppermost part of the record, the amount of deformation in the 

upper part of the Quaternary section is unclear.

U.S. Geological Survey Line 164

Line 164 (figs. 16, 17) trends north and extends from the 

eastern Strait of Juan de Fuca into southern Rosario Strait (fig. 

5A). On the airgun profile (fig. 16), the contact between the 

inferred Tertiary and uppermost Pliocene(?) to Pleistocene sections is characterized by an upward decrease in frequency, 

amplitude, and continuity of reflections. This contact is offset 

about 45-60 m (50-65 ms) across the steeply (=70°-75°) northdipping Devils Mountain fault (3), which correlates with a gentle aeromagnetic high (figs. 3, 16A). The offset contact is most 

clear where disparate seismic facies are juxtaposed between 

about 100 and 300 ms. The companion higher frequency seismic-reflection profile (fig. 17) similarly shows an abrupt vertical 

termination of reflections at this location, extending up to within 

10 m of the surface. This abrupt termination likely indicates a 

significant contrast in the physical properties of the sediment,

such as grain size, stratification, or gas-filled porosity (Hovland 

and Judd, 1988; Fader, 1997). The steep fault dips and the contrasts in reflection properties across the fault on both images is 

consistent with out-of-plane (strike-slip) displacements.

Farther south on Line 164 (fig. 16), two faults are mapped 

as the Strawberry Point (2) and Utsalady Point (1) faults. Both 

structures are imaged as high-angle faults that truncate subhorizontal reflections, but only fault 1 appears to continue upward 

into the lower part of the uppermost Pliocene(?) to Pleistocene 

section. These faults are not recognized on seismic-reflection 

profiles to the west (for example, figs. 18, 19, pi. 2), and their 

more subtle seismic and aeromagnetic character (figs. 3, 16A) 

provides an indication that they are dying out (fig. 2). South of 

these faults, the inferred Tertiary section is warped into a gentle 

anticline and syncline.

U.S. Geological Survey Line 162

Line 162 (figs. 18, 19) is a north-trending line south of 

Lopez Island (fig. 5A). The airgun line (fig. 18) shows three 

faults (1, 2, 3) in a 1,200-m-wide zone representing the Devils 

Mountain fault. The faults dip 60°-80° to the north in the upper 

=300-500 m. On reflections within the Quaternary section 

across 1, structural relief amounts to about 150-200 m (160-220 

ms), and we attribute the bathymetric relief (50-60 m) within the 

fault zone to Quaternary uplift. All three faults extend upward 

into the lower part of the inferred uppermost Pliocene(?) to 

Pleistocene section. The southern strand (1) juxtaposes relatively flat lying beds in the Everett basin and a panel of southdipping beds in the fault zone. The southerly dips extend 

upward into the uppermost Pleistocene to Holocene section, 

indicating that some structural and topographic growth has 

occurred in the last =13,000 years. The middle fault (2) 

juxtaposes the domain of south-dipping beds to the south with a 

zone of less reflective north-dipping beds and represents a 

faulted anticline axis. The northern fault (3) occurs within the 

north limb of this fold, truncates and warps reflections on its 

trace, and forms a boundary between the less reflective horizon 

to the south and a panel of more continuous higher amplitude 

reflections to the north. Dips north of this fault are about 10°- 

15° in the uppermost Tertiary section and about 5°—8° in the 

middle part of the uppermost Pliocene(?) to Pleistocene section. A small positive aeromagnetic anomaly is associated with 

the fault zone on this profile (figs. 3, ISA).

The companion higher frequency profile (fig. 19) provides 

additional details from the uppermost part of the section. On 

this profile, the inferred postglacial uppermost Pleistocene to 

Holocene section is represented by relatively continuous, highamplitude, high-frequency, parallel reflections; the underlying 

Pleistocene section is characterized by nonreflective or lowamplitude, discontinuous reflections. The southern strand of the 

Devils Mountain fault (1 in fig. 18) appears as two splays (1 A, 

1 B). These faults truncate and warp reflections in the upper part 

of the uppermost Pliocene(?) to Pleistocene section (at =170 ms) 

and warp but do not obviously cut the uppermost Pleistocene to 

Holocene section. Fault 1 B forms the boundary between parallel reflections on the south and a region of warped reflections

Seismic-Reflection Profiles 13
(dips as much as 8°) and diffractions on the north. The middle 

fault (2) is not obvious—its location in figure 19 is based on the 

companion airgun profile (fig. 18). The inferred postglacial 

uppermost Pleistocene to Holocene section between faults 2 and 

3 to the north is very gently warped. Fault 3 truncates and warps 

the upper part of the uppermost Pliocene(?) to Pleistocene section and cuts the lower and middle parts of the inferred postglacial section to within about 6-7 m of the seafloor (above the 

stratigraphic level shown in fig. 18).

U.S. Geological Survey Line 158

North-trending Line 158 (fig. 20) is located south of southwestern Lopez Island (fig. 5A). The line shows three faults (1,2, 

3) dipping about 45° to 60° to the north (above -450 ms). This 

1,200-m-wide zone of faults is located about 1,000 m north of 

the projected westerly trace of the Devils Mountain fault, where 

no obvious fault is imaged in the upper -400 m (-450 ms). 

Industry Line 2 (fig. 21A, see next section), located 1.5 km to the 

west of line 158 (fig. 5), reveals "blind" faulting below about 1 

km along the projected fault trace, the basis for mapping the 

fault in this area in figure 2. Faults 1,2, and 3 all extend upward 

into the uppermost Pliocene(?) to Pleistocene section, and we 

infer about 50-100 m (55-110 ms) of structural relief on the 

base of this unit within the fault zone. Faults 1 and 2 bound a 

panel of steep north-dipping reflections in basement (inferred 

Tertiary sedimentary rocks). Fault 3 juxtaposes north- and 

south-dipping panels and represents a faulted anticline axis. Farther north, the basement and at least the lower part of the uppermost Pliocene(?) to Pleistocene section are warped into a series 

of gentle anticlines and synclines that have wavelengths of 1-2 

km. South of the projection of the Devils Mountain fault, the 

contact at the base of the uppermost Pliocene(?) to Pleistocene 

section in the Everett basin is picked mainly on the basis of 

nearby industry data and is nearly flat. Figure 3 shows that the 

Devils Mountain fault zone and the fold belt on its north flank 

correlate with low-amplitude aeromagnetic anomalies.

Industry Line 2

North-trending Industry Line 2 (fig. 21) lies about 1 km 

west of U.S. Geological Survey Line 158 (fig. 5A, B) and provides a deeper image of structure south of Lopez Island. In contrast to the shallower data of figure 20, this profile shows two 

north-dipping faults (1,2) along the projected westerly trace of 

the Devils Mountain fault. Each occurs below about 0.5 s, 

deeper than the level imaged in figure 20, and neither of these 

faults obviously disrupts the Quaternary section. Faults 1 and 2 

form the structural boundary between subhorizontal beds in the 

Everett basin to the south and gently folded Tertiary and Quaternary strata in an uplifted block to the north. This broad boundary is overlain by a gentle aeromagnetic high (figs. 3, 21A). The 

folds on the uplifted northern block are cut by at least one northdipping thrust fault (3) overlain by a hanging-wall anticline. 

This folded terrane coincides with the southeast portion of a 

band of aeromagnetic anomalies that extend northwest along the 

west coast of San Juan Island (fig. 3).

Geological Survey of Canada Line 15

Geological Survey of Canada Line 15 (fig. 22) extends 

southwest of southern Lopez Island into the eastern Strait of 

Juan de Fuca (fig. 5C). The southernmost fault identified on 

this profile (1) lies on strike with the westerly projection of the 

Devils Mountain fault and is inferred to be its main strand. This 

structure dips about 50°-55° to the north and juxtaposes subhorizontal reflections in Tertiary and uppermost Pliocene(?) to 

Pleistocene strata (to the south) and a panel of north-dipping 

reflections in correlative beds to the north. The north-dipping 

reflections are in turn truncated by another fault (2) about 700 

m north of the main strand. Both faults extend upward into the 

lower part of the uppermost Pliocene(?) to Pleistocene section, 

and fault 1 appears to offset the base of the postglacial uppermost Pleistocene to Holocene section. South of fault 1, the contact at the base of the uppermost Pliocene(?) to Pleistocene 

section is a locally angular unconformity.

About 3,600 m north of the inferred main strand of the 

Devils Mountain fault, another north-dipping fault (3) truncates the south limb of an asymmetric anticline. The fault 

extends upward into the Quaternary section, and about 100 m 

(110 ms) of structural relief is present on the inferred base of 

the uppermost Pliocene(?) to Pleistocene section across the 

structure. North of this fault, inferred Tertiary and lower Quaternary strata are gently folded and Tertiary strata are cut by at 

least one fault (4).

Ships Line JDF4

SHIPS Line JDF4 (fig. 23) extends south from offshore 

southern San Juan Island (fig. 5B). This profile images the Devils Mountain fault as a north-dipping (~45°-55°?) blind structure 

that truncates and juxtaposes different seismic facies in inferred 

pre-Quaternary strata. This fault forms the core of an asymmetric anticline that coincides with an aeromagnetic high (figs. 3, 

23A). The prominent reflection at the base of the uppermost 

Pliocene(?) to Pleistocene section is folded above the fault, but 

not ruptured. The dip on the base of this unit on the south limb 

of the fold is -11°, and about 400^50 m (450-500 ms) of 

inferred structural relief is present on this reflection from the 

crest of the fold north of the blind fault to the floor of the basin 

south of the blind fault. Reflections in inferred uppermost 

Pliocene(?) to Pleistocene strata on the south-dipping limb of the 

fold dip south and converge toward the anticline axis, suggesting 

that they were deposited during fold growth. The aeromagnetic 

high at the south end of the profile probably reflects a change in 

the lithology of basement rocks, possibly associated with the 

northwest extent of the southern Whidbey Island fault (Johnson 

and others, 1996).

Industry Line 3

Industry Line 3 (fig. 24) extends south from Rosario Strait 

into the eastern Strait of Juan de Fuca (fig. 5C). This profile 

images the Devils Mountain fault as a north-dipping structure 

(=60°-70°) that truncates the south limb of an asymmetric

14 Devils Mountain Fault and Related Structures, Pacific Northwest
anticline. Quaternary beds are gently folded (dips as much as 

10°) on the flanks of the anticline but do not appear to be truncated by the fault. The anticline is overlain by an aeromagnetic high (figs. 3, 244) bounded on the south by a steep, 

west-trending gradient that correlates with the Devils Mountain fault and on the north by a steep, north west-trending gradient that correlates with a belt of northwest-trending folds.

Geological Survey of Canada Line 37

Geological Survey of Canada Line 37 (fig. 25) extends 

southwest from offshore of San Juan Island into the eastern 

Strait of Juan de Fuca (fig. 5C). On this profile, the Devils 

Mountain fault is a north-dipping blind fault (1) overlain by an 

asymmetric hanging-wall anticline (fault-propagation fold of 

Suppe and Medwedeff, 1990). Reflections from uppermost 

Pliocene(?) to Pleistocene beds above the fault on the south-dipping limb of this anticline appear continuous, dip about 7°, and 

shallow upward. There is approximately 180 m of structural 

relief on the base of this unit from the crest of this anticline into 

the flanking footwall basin. Following the methods of 

Schneider and others (1996, their fig. 10), the horizontal component of fold growth was determined to be about 135 m, and dip 

on the blind fault is inferred to be about 53°. Lowest uppermost 

Pliocene(?) to Pleistocene beds also appear to be very gently 

warped in the syncline and anticline that occur within 2 km 

north of the Devils Mountain fault, but they flatten upwards. For 

nearly 6 km north of the Devils Mountain fault, the Quaternary 

section has a relatively constant thickness of about 200 m (220 

ms). Farther north, the basement appears irregular and faulted 

(faults 2,3, and others?) and the thickness of the Quaternary 

section is highly variable.

The uplift on the north side of the Devils Mountain fault 

forms a prominent aeromagnetic high bounded by west- and 

northwest-trending gradients on its south and north sides, 

respectively (figs. 3, 25A). The steep west-trending gradient is 

associated with the Devils Mountain fault. The steep northwest-trending gradient occurs within folded terrane north of 

the Devils Mountain fault, does not correlate with any feature 

on the shallow seismic-reflection data, and therefore must have 

a deeper source. The aeromagnetic high at the south end of the 

profile similarly has a deeper source and correlates with a significant contrast in basement lithology (Johnson and others, 

1996).

Geological Survey of Canada Line 35

Geological Survey of Canada Line 35 (fig. 26) extends 

southwest from about 4 km east of Vancouver Island into the 

eastern Strait of Juan de Fuca (fig. 5C). On this line, our 

favored interpretation (Interpretation 1, fig. 26A) is that the 

Devils Mountain fault (1) continues its west trend as a northdipping (~45°-55°) structure that offsets the contact between 

pre-Tertiary rock and uppermost Pliocene(?) to Pleistocene 

deposits (marked by a set of two high-amplitude reflections) 

about 250 m (280 ms). Alternatively (Interpretation 2; fig. 

265) and less likely, the Devils Mountain fault is a blind

structure on this profile (as in the nearest profiles to the east, 

figs. 23—25), and the north-dipping limb of the fold is poorly 

imaged on this profile. In this second scenario, Quaternary 

structural relief is about 500 m (550 ms), Quaternary shortening is about 450-500 m, and fault dip is 45°-50°.

Immediately north of the fault in the hanging wall (Interpretation 1), the pre-Tertiary basement surface dips gently south 

for about 1,200 m and is draped by uppermost Pleistocene to 

Holocene (postglacial) beds. Farther north, the pre-Tertiary 

basement surface is irregular, appears faulted (2 and 3), and is 

variably overlain by both uppermost Pliocene(?) to Pleistocene 

and uppermost Pleistocene to Holocene strata. Fault 3 coincides 

with a steep, continuous, northwest-trending aeromagnetic gradient (figs. 3, 26A ) and probably represents a significant splay of 

the Devils Mountain fault.

South of the Devils Mountain fault, uppermost Pliocene(?) 

to Pleistocene sediment is relatively flat and fills the irregular 

faulted and folded surface at the top of highly magnetic basement (figs. 3, 26A). The prominent anomaly formed by this 

magnetic basement extends southeast onto the northeastern 

Quimper Peninsula (Blakely and others, 1999b; Blakely and 

Lowe, 2001) where it can clearly be correlated with Eocene 

basaltic rocks of the Crescent Formation. Thus, the Devils 

Mountain fault in this location corresponds to a major crustal 

boundary between pre-Tertiary basement rocks and younger Tertiary basement rocks (Johnson, 1984b; Johnson and others, 

1996).

Evidence for Faulting Onshore, Whidbey and 

Camano Islands

The approximate locations of the Devils Mountain, 

Strawberry Point, and Utsalady Point faults on Whidbey Island 

are inferred based on projections from offshore seismic-reflection data (figs. 8-26), aeromagnetic data (fig. 3), and onshore 

geologic mapping in the Skagit delta (Whetten and others, 

1988). With this information, we examined exposures (fig. 

27) and subsurface data (fig. 28, table 1) to more precisely 

locate faults and constrain fault histories, lengths, and slip 

rates. Additionally, virtually all coastal-bluff exposures on 

Whidbey Island and on the northeastern Olympic Peninsula 

have been examined for this and previous investigations 

(Johnson and others, 1996, among others) to provide a framework for recognizing structural deformation in Quaternary 

strata.

Subsurface information comes from lithologic logs of 

water wells (fig. 28; table 1). Ground water is the primary 

source of water used on Whidbey and Camano Islands, and 

ground-water resources have been studied extensively (for 

example, Anderson, 1968; Cline and others, 1982; Jones, 1985; 

Sapik and others, 1988), with a focus on aquifer properties, 

subsurface stratigraphy, and saltwater intrusion. For this study, 

we used a data base, maintained by the Island County Health 

Department, that includes logs for more than 3,500 water 

wells. Stratigraphic interpretation of these logs can be problematic because (1) most wells are quite shallow and were 

logged by non-geologists, and (2) outcrops of Quaternary strata

Evidence for Faulting Onshore, Whidbey and Camano Islands 15
122°45'

--^f^^^^h — -2

Outcrop locality

Line of stratigraphic diagram

73,595 ybp 14C date (yr B.P.)

— ——— Offshore seismic-reflection 

profile trackline (fig. 5A)

-f Strike and dip of bedding 

Horizontal bedding

Inferred Quaternary fold

Inferred Quaternary fault—Queried where extent 

uncertain; U, upthrown side; D, downthrown side

Figure 27. Northern Whidbey Island area showing locations of key outcrop localities described in text (designated by letters and triangles), radiocarbon dates (table 2), nearby seismic-reflection profiles (pi. 1), and faults and folds that deform Quaternary deposits. All dates are described in 

table 2 except the one obtained at g, which is described in Pessl and others (1989). Stratigraphic data from many outcrops were used in constructing 

stratigraphic correlation diagrams (figs. 30,32,33,37). Circled numbers refer to faults that cut Strawberry Point, as described in text. Dark-shaded 

areas are underlain by pre-Quaternary bedrock; light-shaded areas are underlain by Quaternary deposits. DMF, Devils Mountain fault; G, Goat Island; I, Ika Island; MP, Maylor Point; PP, Polnell Point; S, Swantown; SPF, Strawberry Point fault; UP, Utsalady Point; UPF, Utsalady Point fault.

throughout the Puget Lowland reveal that substantial erosional 

relief can occur within and at the upper and lower contacts of 

glacially influenced depositional units (fig. 6). Despite these 

potential problems, the water-well data base can have great 

value in stratigraphic investigations when (1) the lithologic logs 

are carefully selected based on quality of logging, (2) a large 

proportion of deeper wells (>50 m) which penetrate several

stratigraphic units are used; (3) the number of wells used for 

correlation on stratigraphic diagrams is large enough to mitigate problems associated with variable erosional relief and with 

poor logging in a few wells, and (4) information from boreholes can be correlated with nearby outcrops (fig. 27). Table 1 

is a list of wells (including depth and location) used to draw 

stratigraphic correlation diagrams for this investigation.

16 Devils Mountain Fault and Related Structures, Pacific Northwest
122°45'

Location of water wells (table 1)

Line of stratigraphic diagram

Offshore seismic-reflection 

profile trackline (fig. 5A)

— - - — Inferred Quaternary fold

— — Inferred Quaternary fault—Queried where extent 

uncertain; U, upthrown side; D, downthrown side

Figure 28. Northern Whidbey Island area showing locations of water wells (designated by numbers and dots) used in making stratigraphic correlation diagrams (table 1; figs. 30,32,33,37) and faults and folds that deform Quaternary deposits. Circled numbers refer to faults that cut Strawberry 

Point, as described in text. Dark-shaded areas are underlain by pre-Quaternary bedrock; light-shaded areas are underlain by Quaternary deposits. 

DMF, Devils Mountain fault; G, Goat Island; I, Ika Island; MP, Maylor Point; PP, Polnell Point; S, Swantown; SPF, Strawberry Point fault; UP, Utsalady 

Point; UPF, Utsalady Point fault.

Devils Mountain Fault

Location

The Devils Mountain fault zone is not exposed on Whidbey 

Island. Based on projection of seismic-reflection data (figs. 8, 

10), the southern fault of the zone (inferred to be the most active

strand and the northern margin of the Everett basin) extends 

through an area characterized by large landslides and variable 

exposure on Whidbey Island's east coast and a marshy lowland 

on the island's west coast. Inland, the projected "basin-margin" 

fault strand is covered by vegetation and surficial deposits developed mainly on Vashon Drift (Pessl and others, 1989). No obvious topographic lineament is associated with the fault where it

Evidence for Faulting Onshore, Whidbey and Camano Islands 17
Table 1. Information on water wells used in constructing stratigraphic correlation 

diagrams (figs. 30,32,33,37).

[Well locations are shown in figure 28. Location column shows township/range, section, 

quarter section. Latitude and longitude are from digital data base maintained by the Island 

County Health Department. For wells not included in this data base, the location within 1/16 

of the section is designated (Anderson, 1968)]

Figure 30A:

Well#

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

Location

T33N/R1E, sec. 2, NW 1/4

T33N/R1E, sec. 2, NE 1/4

T33N/R1E, sec. 1, NW 1/4

T33N/R1E, sec. l,SWl/4

T33N/R1E, sec. l,NWl/4

T33N/R1E, sec. 1,NE 1/4

T33N/R1E, sec. l,NEl/4

T33N/R1E, sec. l,NEl/4

T33N/R1E, sec. l,NEl/4

T33N/R1E, sec. l,NEl/4

T33N/R2E, sec. 6, NW 1/4

T33N/R2E, sec. 5, NW 1/4

T33N/R2E, sec. 6, NE 1/4

T33N/R2E, sec. 6, NE 1/4

T33N/R2E, sec. 5, NW 1/4

T33N/R2E, sec. 5, NW 1/4

T33N/R2E, sec. 5, SW 1/4

T33N/R2E, sec. 5, SE 1/4

T33N/R2E, sec. 5, SW 1/4

Lat(°)

48.379

48.378

48.379

48.376

48.379

48.379

48.379

48.380

48.378

48.378

48.381

48.382

48.378

48.382

48.382

48.382

48.383

48.375

48.374

Figure 30B:

Well#

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

Location

T33N/R1E, sec. 3, SE 1/4

T33N/R1E, sec. 11, NW 1/4

T33N/R1E, sec. 11, NW 1/4

T33N/R1E, sec. 11, NW 1/4

T33N/R1E, sec. 2, SE 1/4

T33N/R1E, sec. 2, SE 1/4

T33N/R1E, sec. 1LNE1/4

T33N/R1E, sec. ll,SWl/4

T33N/R1E, sec. ll,NEl/4

T33N/R1E, sec. 1, SW 1/4

T33N/R1E, sec. 12, SW 1/4

T33N/R1E, sec. 12, NE 1/4

T33N/R1E, sec. 12, SE 1/4

T33N/R1E, sec. 12, NE 1/4

T33N/R2E, sec. 7, NW 1/4

T33N/R2E, sec. 7D, NW 1/4

T33N/R2E, sec. 6, SW 1/4

T33N/R2E, sec. 6, NE 1/4

T33N/R2E, sec. 6, SE 1/4

T33N/R2E, sec. 6, SW 1/4

T33N/R2E, sec. 6, SW 1/4

T33N/R2E, sec. 6, SW 1/4

T33N/R2E, sec. 7, SW 1/4

T33N/R2E, sec. 7, NE 1/4

T33N/R2E, sec. 7, SW 1/4

T33N/R2E, sec. 7, SE 1/4

T33N/R2E, sec. 7, SE 1/4

Lat(°)

48.377

48.369

48.369

48.368

48.368

48.373

48.364

48.357

48.366

48.371

48.359

48.362

48.361

48.364

48.364

48.367

48.371

48.374

48.371

48.371

48.371

48.360

48.365

48.360

48.362

48.360

crosses Whidbey Island, in contrast to the conspicuous trace of 

the fault in the Cascade Range foothills to the east (fig. 4).

The projection of the southern "basin-margin" fault in the 

Devils Mountain fault zone across Whidbey Island is on strike 

with a strand of the Devils Mountain fault mapped in the Skagit 

delta to the east, north of Goat and Ika Islands in Skagit Bay 

(figs. 2, 27; Whetten and others, 1988). Goat and Ika Islands are 

underlain by bedrock, however, and clearly lie north of the

Longf0) 

122.649 

122.645 

122.634 

122.630 

122.628 

122.623 

122.622 

122.620 

122.617 

122.617 

122.613 

122.611 

122.600 

122.600 

122.589

122.584

122.585

122.583

122.581

LongC0) 

122.664 

122.654 

122.654 

122.650 

122.642 

122.646 

122.643 

122.639 

122.637 

122.633 

122.630 

122.619 

122.618 

122.615 

122.611

122.611

122.609

122.607

122.608

122.604

122.600

122.606

122.598

122.602

122.595

122.597

Depth in feet (m)

195 (59.4)

286 (87.2)

165 (50.3)

152 (46.3)

205 (62.5)

196 (59.7)

196 (59.7)

205 (62.5) 

238 (72.5)

223 (68.0)

272 (83.0)

287 (87.5)

254 (77.4)

200(61.0)

233 (71.0)

189 (57.6)

160 (48.8)

284 (86.6)

282 (86.0)

Depth in feet (m)

109 (33.2)

95 (29.0)

158 (48.2)

152 (46.3)

243 (74.1) 

214 (65.2)

213 (64.9)

92 (28.0)

246 (75.0)

140 (42.7)

48 (14.6)

155 (47.2)

118(36.0)

108 (32.9)

139 (42.4)

182 (55.5)

216 (65.8)

261 (79.6)

238 (72.5)

267(81.4)

274 (83.5)

236 (71.9)

177 (54.0)

249 (75.9)

76 (23.1)

39(11.9)

180(54.9)

northern margin of the Everett basin. Moreover, an aeromagnetic anomaly that defines the basin margin (fig. 3) in Skagit 

Bay lies just south of these islands, on the southern margin of a 

small, prominent, shallow (<350 m), high-amplitude, northwesttrending aeromagnetic anomaly interpreted as mafic pre-Tertiary 

rock. Thus, the "basin-margin" fault appears to step south about 

1,000 m in Skagit Bay. This step probably coincides with the 

steep gradient on the northeast margin of the high-amplitude

18 Devils Mountain Fault and Related Structures, Pacific Northwest
Table 1—Continued. Information on water wells used in constructing stratigraphic correlation diagrams (figs. 30,32,33,37).

[Well locations are shown in figure 28. Location column shows township/range, section, 

quarter section. Latitude and longitude are from digital data base maintained by the Island 

County Health Department. For wells not included in this data base, the location within 1/16 

of the section is designated (Anderson, 1968)]

Figure 32:

Well#

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

Location

T33N/R2E, sec. 21, SE 1/4

T33N/R2E, sec. 21, SE 1/4

T33N/R2E, sec. 22, SW 1/4

T33N/R2E, sec. 22, SE 1/4

T33N/R2E, sec. 22, SE 1/4

T33N/R2E, sec. 22, SE 1/4

T33N/R2E, sec. 27, NW 1/4

T33N/R2E, sec. 27, NE 1/4

T33N/R2E, sec. 27, NE 1/4

T33N/R2E, sec. 27, NE 1/4

T33N/R2E, sec. 26, SE 1/4

T33N/R2E, sec. 26, NW 1/4

T33N/R2E, sec. 26, NW 1/4

T33N/R2E, sec. 26, NE 1/4

T33N/R2E, sec. 26, SE 1/4

T33N/R2E, sec. 35, NE 1/4

T33N/R2E, sec. 36, NW 1/4

T33N/R2E, sec. 36, NW 1/4

T33N/R2E, sec. 36, SW 1/4

T33N/R2E, sec. 35, SE 1/4

T33N/R2E, sec. 35, SE 1/4

T32N/R2E, sec. 2, NE 1/4

T32N/R2E, sec. 2, NE 1/4

T32N/R2E, sec. 2, NE 1/4

T32N/R2E, sec. 2, NW 1/4

T32N/R2E, sec. 2, NW 1/4

T32N/R2E, sec. 2, NW 1/4

T32N/R2E, sec. 2, SW 1/4

T32N/R2E, sec. 2, SW 1/4

Latn

48.331

48.331

48.332

48.327

48.327

48.324

48.324

48.324

48.322

48.320

48.320

48.321

48.320

48.320

48.315

48.310

48.306

48.306

48.303

48.299

48.297

48.295

48.295

48.295

48.295

48.295

48.291

48.288

48.285

Longn

122.552

122.552

122.547

122.535

122.530

122.528

122.541

122.536

122.535

122.530

122.525

122.519

122.519

122.515

122.510

122.508

122.504

122.504

122.505

122.510

122.510

122.510

122.510

122.510

122.520

122.520

122.520

122.520

122.560

Depth in feet (rn)

198 (60.4)

221 (67.4)

175 (53.3)

180(54.9)

157 (47.9)

145 (44.2)

208 (63.4)

176 (53.6)

183 (55.8)

182 (55.5)

262 (79.9)

419 (127.7)

149 (45.4)

261 (79.6)

160 (48.8)

141 (43.0)

420 (128.0)

260 (79.3)

300 (91 .4)

180 (54.9)

201 (61.3)

256 (78.0)

191 (58.2)

185 (56.4)

196(59.7)

138(42.1)

300(91.4)

190 (57.9)

163 (49.7)

Figure 33:

Well#

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

Location

T33N/R1E, sec. 26, NW 1/4

T33N/R1E, sec. 22, SE 1/4

T33N/R1E, sec. 27, NE 1/4

T33N/R1E, sec. 27, NW 1/4

T33N/R1E, sec. 27, NE 1/4

T33N/R1E, sec. 27, NE 1/4

T33N/R1E, sec. 28, NE 1/4

T33N/R1E, sec. 28, NE 1/4

T33N/R1E, sec. 27, SE 1/4

T33N/R1E, sec. 27, SW 1/4

T33N/R1E, sec. 27, SW 1/4

T33N/R1E, sec. 34, NW 1/4

T33N/R1E, sec. 34, NW 1/4

T33N/R1E, sec. 28, SE 1/4

T33N/R1E, sec. 28L, SW 1/4

T33N/R1E, sec. 33, NE 1/4

T33N/R1E, sec. 33, NW 1/4

T33N/R1E, sec. 33, NW 1/4

T33N/R1E, sec. 33, NW 1/4

T33N/R1E, sec. 33, SW 1/4

T33N/R1E, sec. 33, SE 1/4

T33N/R1E, sec. 32, SE 1/4

T33N/R1E, sec. 32, NE 1/4

T33N/R1E, sec. 32, SE 1/4

T32N/R1E, sec. 5, NW 1/4

Latn

48.326

48.329

48.325

48.324

48.321

48.321

48.324

48.321

48.314

48.318

48.317

48.309

48.309

48.313

48.310

48.310

48.307

48.306

48.305

48.299

48.303

48.306

48.299

48.298

LongC0)

122.650

122.660

122.670

122.670

122.670

122.670

122.680

122.690

122.660

122.670

122.680

122.673

122.675

122.686

122.687

122.693

122.698

122.698

122.697

122.687

122.703

122.709

122.704

122.715

Depth in feet (m)

682 (207.9)

534 (162.8)

159 (48.5)

200 (61.0)

250 (76.2)

315 (96.0)

366(111.6)

261 (79.6)

208 (63.4)

269 (82.0)

240 (73.2)

183 (55.8)

174 (53.0)

193 (58.8)

194(59.1)

134 (40.8)

162(49.4)

286 (87.2)

287 (87.5)

320 (97.5)

245 (74.7)

280 (85.3)

375(114.3)

249 (75.9)

1000 (304.8)

Evidence for Faulting Onshore, Whidbey and Camano Islands 19
Table 1—Continued. Information on water wells used in constructing stratigraphic 

correlation diagrams (figs. 30,32,33,37).

[Well locations are shown in figure 28. Location column shows township/range, section, 

quarter section. Latitude and longitude are from digital data base maintained by the Island 

County Health Department. For wells not included in this data base, the location within 1/16 

of the section is designated (Anderson, 1968)]

Deep test well:

Well# Location Lat(°) Long(°) Depth in feet (m) 

101 T32N/R1E, sec. 12, NW 1/4 48.280 122.629 1933 (589.2)

Figure 37:

Wellfl

102

103

104

105

106

107

108

109

110

111

112

Location

T32N/R2E, sec. 24, NW 1/4

T32N/R2E, sec. 14, SE 1/4

T32N/R2E, sec. 23, NE 1/4

T32N/R2E, sec. 23, NE 1/4

T32N/R2E, sec. 23, NE 1/4

T32N/R2E, sec. 23, NW 1/4

T32N/R2E, sec. 23, SW 1/4

T32N/R2E, sec. 23, SW 1/4

T32N/R2E, sec. 23, SW 1/4

T32N/R2E, sec. 23, SW 1/4

T32N/R2E, sec. 23, SW 1/4

Lat(0)

48.254

48.255

48.253

48.253

48.252

48.249

48.247

48.246

48.245

48.246

48.244

Long(°)

122.500

122.503

122.506

122.510

122.512

122.515

122.515

122.517

122.516

122.519

122.518

Depth in feet (m) 

213(64.9) 

187(57.0) 

150 (45.7) 

280 (85.3) 

400(121.9) 

320 (97.5) 

340 (103.6) 

370(112.8) 

182(55.5) 

158 (48.2) 

145 (44.2)

aeromagnetic anomaly, and may represent a transfer zone or tear 

fault in the fault system. In this scenario, the inferred mafic rock 

is a basement block uplifted within this transfer zone.

Outcrops

Pleistocene strata are exposed near the fault along the east 

coast of Whidbey Island. Locality a (fig. 27), south of the fault 

trace, consists of three outcrops exposed within 100 m in coastal 

bluffs or on the beach at low tide. Beds consisting of sand, 

organic-rich mud, and gravel strike N. 25°-35° E. and dip 25°- 

36° NW. Detrital wood yielded an infinite radiocarbon date 

(>43,510 ' 4C yr B.P.; SJ-00-7; table 2). About 200 m farther 

north (b in fig. 27), similar sand and gravel facies exposed in a 

landslide scarp strike 70° and dip 17° NW.

About 300 m north of the inferred fault trace (c in fig. 27), 

Quaternary strata exposed in the upper part of steep coastal 

bluffs are subhorizontal (fig. 29). The lowest beds at this locality 

consist of interbedded fine to coarse sand (plane-bedded and 

crossbedded), laminated silty mud, and peat. Based on their sedimentology, these strata are interpreted as interglacial alluvialplain deposits. The peat yielded radiocarbon dates of >48,480 

14C yr B.P. (SJ-98-17; table 2) and >40,130 14C yr B.P. (SJ-00-1; 

table 2). These strata, and the peat-bearing strata exposed at a, 

are correlated with the Whidbey Formation, the oldest known 

interglacial alluvial-plain deposits recognized on Whidbey 

Island and in the northern Puget Lowland (Easterbrook, 1994a, 

b). The correlation is based on three factors:

1. The lithology and sedimentology of these strata are identical to those of well-documented Whidbey Formation localities 

elsewhere on Whidbey Island. (See, for example, Easterbrook, 

1968, 1994b; Stoffel, 1981.)

2. Subsurface data (fig. 30, see next section) from this 

local northern Whidbey Island area show that strata occurring at 

similar elevations and with similar lithology are overlain by two

Figure 29. Bluff outcrops of Quaternary strata at locality c (fig. 27). 

Annotation shows units (fig. 6) and contacts. 0, Olympia beds; P, 

Possession Drift; W, Whidbey Formation. Two infinite radiocarbon 

dates from organic matter in the Whidbey Formation were obtained 

at this locality (table 2, fig. 27).

20 Devils Mountain Fault and Related Structures, Pacific Northwest
Table 2. Radiocarbon data for samples from northern Whidbey and Camano Islands.

[Laboratory data are from Beta Analytic Inc.]

Sample 

Number

SJ-98-17 

SJ-98-14

Radiocarbon 

laboratory 

Number

120288 

120287

Field 

locality 

(fig. 27)

c 

h

Radiocarbon 

age( 14CyrB.P.)

>48,480 

1,750±50

13C/12 C (o/oo)

-25.6

-25.8

Calibrated age 

(calendar year, 

95.4% confidence)

NA1 

A.D. 210-420

Material dated

Peat 

Wood in peat

Comment

Whidbey Formation north of 

Devils Mountain fault.

At Rocky Point between

SJ-99-2 134854

SJ-99-1A 132512

SJ-99-1B 141690

SJ-00-1 144555

SJ-00-7 144556

SJ-00-11 144557

SJ-00-12 144558

SJ-00-13 144559

SJ-00-14 144554

SJ-00-48 147710

m >44,000 -27.9

r 21,100±150 -24.9

r 15,190±220

c >40,130 -28.2

a >43,510 -26.9

/ >44,320 -25.6

/ 29,270+340 -27.6

k 25.770±310 -28.4

n 44.740±910 -28.9

/ 37,190±550

NA1

NA1 

-23.6 17,435-18,985 B.P.

NA1 

NA1 

NA1

NA1 

NA1 

NA1

NA1

Strawberry Point and 

Utsalady Point faults.

Peat Whidbey Formation in

Strawberry Point fault zone at 

Strawberry Point.

Organic laminae Nonglacial Olympia beds at 

Utsalady Point.

Organic laminae Nonglacial Olympia beds at 

Utsalady Point.

Peat Whidbey Formation north of 

Devils Mountain fault.

Wood in peat Whidbey Formation south of 

Devils Mountain fault.

Peat Whidbey Formation south of

Strawberry Point fault zone at 

Strawberry Point.

Peat Olympia beds south of

Strawberry Point fault zone at 

Strawberry Point.

Peat Olympia beds south of

Strawberry Point fault zone at 

Strawberry Point.

Detrital wood Whidbey Formation in

Strawberry Point fault zone 

at Strawberry Point; young age 

suggests probable contamination.

Detrital wood Basal Olympia beds south of

Strawberry Point fault zone 

_____________at Strawberry Point.________

'NA, not applicable.

units of interbedded "hardpan" (the term drillers use for dense 

clay-rich diamict), gravel, and sand interpreted as glacial drift. 

The Whidbey Formation predates both the Possession and 

Vashon Drifts, whereas the younger nonglacial Olympia beds 

predate only the Vashon Drift (fig. 6). At c, the Whidbey Formation is overlain by crudely stratified gravel assigned to the 

Possession Drift and interbedded sand (parallel bedded and 

crossbedded) and silt assigned to the nonglacial Olympia beds 

(fig. 6).

3. Peat from these strata is too old (> 48,480 14C yr B.P.) to 

yield reliable radiocarbon ages (table 2), consistent with previous dating results from the Whidbey Formation (Blunt and others, 1987). In addition to the dates reported here, J.D. 

Dragovich (written comrnun., 2000) obtained four radiocarbon 

dates from the same unit at four localities within 2.5 km north of 

location c (fig. 27) along Whidbey Island's northeast coast. Two 

dates from wood samples similarly yielded infinite ages 

(>43,790 14C and >37,100 14C yr B.P), whereas an organic silty 

sand sample yielded a finite age of 41,380±2,150 14C yr B.P. and 

organic material in silt yielded a finite age of 40,590±530 14C yr

B.P. These two finite ages from organic sediment are very close 

to the practical limit of radiocarbon dating, and they may have 

been contaminated by minute amounts of younger carbon.

Subsurface Data

East-west stratigraphic correlation diagrams on the north 

and south sides of the inferred trace of the Devils Mountain fault 

on northern Whidbey Island (locations, figs. 27, 28) are shown 

in figure 30. The diagram north of the fault includes 19 wells 

within 1.4 km of the inferred fault trace and one surface outcrop 

(figs. 29, 30A). Stratigraphic units inferred on the correlation 

diagram include pre-Whidbey Formation Pleistocene deposits, 

the Whidbey Formation, Possession Drift, Olympia beds, Esperance Sand Member, main body of Vashon Drift, and Everson 

Drift (fig. 6). Lateral stratigraphic variation across this diagram 

probably reflects irregular scour and fill associated with the Possession and Vashon glaciations, and possible faulting within the 

block north of the Devils Mountain fault (fig. 8A), but it may

Evidence for Faulting Onshore, Whidbey and Camano Islands 21
also reflect incorrect stratigraphic correlation. Interpretations 

on the correlation diagram south of the Devils Mountain fault 

(fig. 305), which includes data from 27 wells and one outcrop 

proximal to the inferred fault trace, suggest greater lateral stratigraphic continuity.

The stratigraphic correlation diagrams (fig. 3QA, 5) were 

generated to provide constraints on vertical offset across the 

Devils Mountain fault. In this regard, lateral variability and 

potentially uncertain correlation of units above the Whidbey 

Formation make their utility as deformation markers problematic. The top of the interglacial Whidbey Formation is inferred 

to provide the most useful datum for the following reasons:

1. The top of the Whidbey Formation is a distinct contact 

in both outcrops (for example, Easterbrook, 1968) and subsurface data (fig. 30), generally characterized by an abrupt upward 

transition from interbedded sand, silt, clay, and peat to overlying 

interbedded gravel, sand, and "hardpan."

2. The Whidbey Formation is relatively thick and widespread on Whidbey Island (Easterbrook, 1968), and is present in 

every well that reaches its stratigraphic level. This occurrence 

reflects its deposition during a period of relatively high sea level 

(marine isotope stage 5; Muhs and others, 1994; Pillans and others, 1998), a condition which generates significant stratigraphic 

"accommodation space" (Jervey, 1988; Posamentier and others, 

1988) in which sediments can accumulate and have high potential for preservation in the stratigraphic record. In contrast, 

younger (marine isotope stage 3) nonglacial Olympia beds 

(fig. 6) are absent (due to nondeposition or erosion) in several 

wells and in most outcrops on Whidbey Island. Sea level at this 

time was much lower than at present and in stage 5 (Pillans and 

others, 1998), significantly limiting both stratigraphic accommodation space and preservation potential.

3. As an alluvial-plain deposit, the top of the Whidbey Formation was a nearly horizontal surface across northern Whidbey 

Island following deposition. At the end of Whidbey deposition, 

paleo-relief on this surface in local areas like this small part of 

northern Whidbey Island would have been at most a few meters, 

reflecting erosion and deposition associated with fluvial channels and levees. Modern alluvial-delta plains of the Puget Lowland (such as the Skagit delta a few kilometers east of Whidbey 

Island) provide a useful analog.

4. In both correlation diagrams, the top of the Whidbey 

Formation is interpreted as a gently undulating though relatively 

horizontal surface; thus erosion into the Whidbey Formation by 

advance outwash facies of the Possession Drift appears to have 

been fairly limited and (or) uniform across this small area of 

northern Whidbey Island. Widespread preservation of this relatively horizontal surface in areas not within fault zones on northern Whidbey Island is documented by two stratigraphic 

diagrams in Easterbrook (1968; his fig. 15 and plate 1-5) from 

the coastal bluffs south of Swantown (d1 and 62 in fig. 27). 

These correlation diagrams show the top of the Whidbey 

Formation as a nearly horizontal surface for 4.6 km (elevation of 

17m) and 3.5 km (elevation of 10 m) of continuous exposure. 

However, elsewhere (such as fin fig. 27), there is locally as 

much as 20 m of erosional relief on the top of the Whidbey 

Formation.

On the northern correlation diagram (fig. 30A), our 

interpretation is that the top of the Whidbey Formation was

penetrated by 14 wells for which it has a maximum elevation 

of 38.4 m and a mean elevation of 26.6±5.8 m. On the southern correlation diagram (fig. 305), our interpretation is that the 

top of the Whidbey Formation was penetrated by 16 wells for 

which it has a maximum elevation of 26.9 m and a mean elevation of 14.7±6.0 m. The elevation south of the Devils 

Mountain fault is similar to that observed in outcrops south of 

Swantown (d1 in fig. 27; Easterbrook, 1968), whereas this 

contact is much higher north of the Devils Mountain fault. 

The difference between the maximum and mean elevations on 

the two correlation diagrams is 11.5 m and 11.9 m, respectively. The mean and standard deviation of the sum and difference of the mean depths (Spiegel, 1961) is 11.9 m±8.3 m. If 

one assumes that the amount of erosional relief on this datum 

is similar north and south of the projected fault trace, then 

these values provide a rough estimate for the amount of vertical separation along the Devils Mountain fault since deposition of the top of the Whidbey Formation.

Strawberry Point Fault

West Coast of Whidbey Island

There are no exposures where the trace of the Strawberry 

Point fault projects onto the west coast of Whidbey Island, about 

1 km north of Rocky Point (figs. 27, 28). North of the projected 

trace, no exposures exist for several kilometers. Within about 1 

km south of the inferred fault trace, Pleistocene glacial deposits 

occur in discontinuous bluff exposures, and pre-Tertiary bedrock and Holocene peat crop out at beach level at Rocky Point 

(g in fig. 27). The bedrock consists of Upper Cretaceous 

graywacke, argillite, greenstone, and chert (Whetten and others, 

1988) and is extensively faulted and foliated. These rocks have 

surprisingly low bulk magnetic susceptibility—0.53xlO~3 (SIU) 

based on 30 in-place measurements—and form a distinct and 

continuous west-northwest-trending aeromagnetic low over 

Rocky Point (fig. 3). In contrast, Quaternary deposits that crop 

out adjacent to bedrock exposures yield susceptibilities that are 

roughly an order of magnitude higher.

Pleistocene strata (2 to 15-20 m thick) that overlie the bedrock at g (fig. 27) consist of horizontal to very gently dipping 

(0°-2°) Everson Drift (fig. 6) that yielded a 14C date on shells of 

13,595±145 14C yr B.P. (Pessl and others, 1989). Older Pleistocene strata, more than 450 m thick elsewhere on northern 

Whidbey Island (as in well 101, fig. 28, table 1), are absent at 

this locality.

A woody peat is exposed south of the inferred fault trace at 

locality h (fig. 27) for about 200 m along the beach at very low 

tides (<40 cm below mean low low water). A fragment of tree 

branch (2 cm diameter) from this peat yielded a 14C date of 

1,750+/-50 14C yr B.P. (SJ-98-14; table 2). This peat includes 

rhizomes of either seacoast bulrush (Scirpus maritimus) or hardstem bulrush (Scirpus acutus), both of which generally occur in 

the middle and upper parts of brackish or salt marshes (elevation 

>tide of 100 cm) (Cooke, 1997). The presence of these rhizomes is consistent with a relative sea-level rise of about 1.5 to 3 

m since about 1.7 ka at this locality. This rise supports regional 

investigations (for example, Clague and others, 1982; Eronen

22 Devils Mountain Fault and Related Structures, Pacific Northwest
and others, 1987) that suggest rates of sea-level rise of 1 mm/yr 

or less for the last few thousand years in Puget Sound and southern British Columbia.

There are an insufficient number of deep water wells flanking the Strawberry Point fault in this area to allow construction 

of well-constrained stratigraphic correlation diagrams either parallel to (as in fig. 30) or across the fault in order to constrain fault 

slip.

Central Whidbey Island

Seismic-reflection data (figs. 8, 10, 13) suggest that the 

Strawberry Point fault changes from a discrete fault into a broad 

fault zone as it crosses Whidbey Island from west to east (figs. 2, 

27, 28). On central Whidbey Island, the trace of this zone is covered by vegetation and surficial deposits developed mainly on 

Vashon Drift and recessional outwash (Pessl and others, 1989). 

There are no obvious topographic lineaments associated with the 

fault zone on existing topographic maps in this area. Furthermore, the number of deep water wells flanking the Strawberry 

Point fault in this area is insufficient to construct stratigraphic 

correlation diagrams either parallel to or across the fault in order 

to constrain fault location or slip. The location of the fault zone 

and its components on central Whidbey Island is thus somewhat 

speculative, and is based primarily on aeromagnetic data (fig. 3; 

Blakely and Lowe, 2001).

East Coast of Whidbey Island

Seismic-reflection data (fig. 8) suggest that the Strawberry 

Point fault is a zone comprising several splays along Strawberry 

Point on the east coast of Whidbey Island. Within this zone, 

there is significant deformation of upper Pleistocene strata discontinuously exposed in coastal bluffs (fig. 31). There are also a 

significant number of lithologic logs from water wells adjacent 

to the coastline (table 1, fig. 28). Together, the outcrops, lithologic logs, and seismic-reflection profiles provide the data 

needed for a stratigraphic correlation diagram across Strawberry 

Point that provides some constraints on fault-zone interpretation 

(fig. 32).

Easterbrook (1968), Hansen and Easterbrook (1974), and 

Blunt and others (1987) have described aspects of the stratigraphy and palynology of the nonglacial Olympia beds exposed at 

Strawberry Point, and reported dates ranging from 22,700 to 

47,600 14C yr B.P. The unit consists mainly of mud and peat but 

also includes coarse sand, gravel, and redeposited peat that may 

have been deposited in Skagit River floodplains and paleochannels, graded to a lower sea level. Younger deposits (Esperance 

Sand Member and main body of Vashon Drift) locally overlie 

these strata in the coastal bluffs (fig. 31 A). The nonglacial 

Olympia beds are underlain by a variably thick unit of diamict, 

gravel, and sand assigned to the Possession Drift, in turn underlain by a fine-grained unit consisting of interbedded ripple- and 

plane-laminated sand, silty mud, and peat. This lower finegrained unit occurs in the axis of a gentle anticline on the southeast coast of Strawberry Point and in faulted and folded strata on

the northeast coast. These older strata are assigned to the Whidbey Formation because (1) radiocarbon dates yield infinite ages 

(see next paragraph and table 2), (2) they underlie the well-dated 

Olympia beds and Possession Drift (fig. 6), (3) lithologies are 

similar to those described for the Whidbey Formation at nearby 

Whidbey Island localities (for example, Easterbrook, 1968), and 

(4) no similar nonglacial units older than the Whidbey Formation 

have been identified in this part of the Puget Lowland. Pebbly 

diamict that underlies the Whidbey Formation in Strawberry 

Point water wells is considered pre-Whidbey Formation glacial 

drift (figs. 6, 32).

Figures 27 and 28 show where the four faults identified on 

seismic-reflection data (fig. 8) are projected to intersect the 

coastline. South of fault 1, the exposed Pleistocene section (fig. 

31 A) is folded in a gentle anticline with the Whidbey Formation 

in its core at sea level, an interpretation supported by well data 

(fig. 32). Outcrops of Whidbey Formation and Possession Drift 

on the south limb of the fold dip 5°, and outcrops of Possession 

Drift and nonglacial Olympia beds on the north limb of the fold 

(/' in fig. 27) dip 7°. Peat in the Whidbey Formation exposed at 

beach level at / yielded a radiocarbon date of >44,320 14C yr B.P. 

Both low-tide beach exposures and the figure 32 correlation diagram suggest that dips on the north limb of the fold may be 

steeper than 7° in the Whidbey Formation and older strata, but 

this interpretation is based on outcrop with minimal stratigraphic 

context and projection of only one well (No. 65), and is thus tentative. The section exposed at sea level on the north limb of the 

fold extends upward from the Whidbey Formation through thin 

Possession Drift into well-dated, nonglacial, peat-bearing Olympia beds (Hansen and Easterbrook, 1974) and is about 30 m 

thick, providing a minimum estimate for the amount of vertical 

fold growth since the top of the Whidbey Formation was deposited. For this investigation, we obtained radiocarbon dates of 

37,190±550 14C yr B.P. at /, 29,270±340 14C yr B.P. at/, and 

25,770±310 14C yr B.P. at kfrom the Olympia beds (fig. 27). 

Exposures of the Olympia beds are flat-lying and nearly continuous north of the gentle fold and south of the projected trace of 

fault 1 (from /to k in fig. 27).

The projected traces of both faults 1 and 2 are covered by 

vegetation. However, between these faults, flat-lying massive 

diamict and crudely stratified gravel and sand crop out extensively (II in fig. 27) in low coastal bluffs, with a maximum 

exposed thickness of 5-6 m. These strata have an inferred glacial origin based on their texture, and underlie a fine-grained 

interval in a nearby water well (64, figs. 28, 32; table 1). In the 

simplest interpretation, the strata in these outcrops are Possession Drift and the overlying fine-grained strata correlate with the 

Olympia beds. In this scenario (hypothesis 1), vertical offset on 

fault 1 has raised the basal contact of the Olympia beds about 

3-9 m on the north side of the fault. The vertical separation on 

the top of the older Whidbey Formation based on well logs (figs. 

28, 32), also south side down, is about 10-15 m. Alternatively, 

the coarse-grained section that occurs at sea level between faults 

1 and 2 could underlie the Whidbey Formation. In this less 

likely scenario, south-side-down vertical separation on fault 1 

increases to about 40-60 m.

Fault 2 coincides with a significant dip change in coastal 

outcrops. The last exposure south of the fault (12, -20 m south 

of the fault shown in figs. 27 and 28) consists of flat-lying,

Evidence for Faulting Onshore, Whidbey and Camano Islands 23
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(outcrop) 50 ^~i~'

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>40,130yrB.P.

EXPLANATION

glacial drift and interglacial PS Graveldeposits E23 Sand and gravel P Clay/gravel _ 25

„ - _ _ _ _ _ m Sand Q Hardpan _ _ ______ .... ......... .......... _ _.

H Clay

V.E. -16 ^Sand/clay

i:s-.;l Hardpan, sandy

fel Hardpan, gravelly

C/3 

DC

f^rl Peat interbeds — — Inferred stratigraphic

contact
5km

41

WEST

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Everson Drift and ^sk""""

200 

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Everson Drift and 26 /;

Vashon Drift

,100 

—

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^

24

23 

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|X /%p '.:y. : /

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EXPLANATION M 

•

B^l Gravel

H Sand and gravel g Clay/gravel 

S

. 

_ _ 

Bl Sand

^ n Clau

3 pre-Whidbey Formation (55 c "j/ i 

..,.,.. i • i EFH csand/clay 2 glacial drift and mterglacial VF«16 i=-i , , | deposits SPeatinterbeds

B)30> 

R

LOJ Hardpan 

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— — Inferred stratigraphic • 

contact

(outcrop) 

> r^i LJJ

,. .feU ... —— n LU

14C =

> 43,51 OyrB.P.

— -25

Figure 30. Interpretive east-west stratigraphic correlation diagrams north (A) and south (B) of the Devils Mountain fault. Information is from outcrops (fig. 27, lettered sections) and lithologic logs from water wells (fig. 28 and table 1, numbered sections), northern Whidbey Island.
26 Devils Mountain Fault and Related Structures, Pacific Northwest
Figure 31 (above and facing page). Deformation of upper Pleistocene strata at Strawberry Point. A, 

Gently dipping (shown by white dashed lines) glacial and interglacial deposits on the north flank of a 

gentle anticline exposed south of fault 1 at /, Strawberry Point (fig. 27). Interbedded sand and mud at 

the base of the bluff are assigned to the Whidbey Formation (W); organic matter in these beds yielded 

an infinite radiocarbon date (table 2, fig. 27). These strata are overlain by a thin diamict assigned to 

the Possession Drift (P), in turn overlain by mud, sand, and gravel assigned to the nonglacial Olympia 

beds (0). Detrital peat at the base of the Olympia at this locality yielded a date of 37,190± 550 14C yr B.P. 

The Olympia is overlain by Esperance Sand Member (E) and main body of Vashon Drift (V). 6, Gravel 

and sand assigned to the Possession Drift cut by a thrust fault (arrows) with displacement of 1-2 m, 

approximately 40 m south of m (fig. 27). C, Whidbey Formation strata dipping -45° NW., locality m on 

Strawberry Point. Beds consist of mud, sand, and woody peat (white-weathering bed above shovel). 

Peat at this locality yielded an infinite radiocarbon date. Shovel handle is divided into 10 cm increments. D, Exposure of Whidbey(?) Formation sand and mud at locality o on Strawberry Point (fig. 27). 

Beds strike north, dip 28° W., and are cut by subvertical fractures (see arrow in upper right). E, Exposure of highly fractured friable sand of the Whidbey(?) Formation at locality p on Strawberry Point (fig. 

27). Box shows approximate location of F. F, Close-up of outcrop at p (see E), showing highly fractured 

friable sand.

Evidence for Faulting Onshore, Whidbey and Camano Islands 27
100-

50-

!<rjm-

-50-

-100-

SOUTH overlap

N. 30° E. N. 30° W. •

= 5.000 ft

1,000m Everson Drift and 

Vashon Drift (V)

m I / 

\=i

74

Whidbey 

Formation (W)

= pre-Whidbey Formation glacial 

drift and mterglacial deposits 25,770 

yr B,P,

Everson Drift and 

Vashon Drift (V)

62

64 63

5/

EsperahceSartd

Olympia 

beds(O)

Possession 

Drift (P)?

Whidbey 

Formation (W)

pre-Whidbey F6rmation;glacial 

drift and interglacial deposits

anticline fault 1 fault 2

Figure 32. Interpretive stratigraphic correlation diagram parallel to coast of Strawberry Point crossing Strawberry Point fault zone, using 

outcrop data (figs. 27,31; lettered sections) and lithologic logs from water wells (table 1, fig. 28; numbered sections), northern Whidbey Island. 

Faults queried where uncertain; barbs show sense of relative fault offset. Note overlap between the two panels of this figure.

crudely stratified sand and gravel assigned to the Possession 

Drift. These strata are cut by a thrust fault that strikes N. 77° E., 

dips 45° NW., and has apparent displacement of 1-2 m (fig.

The first exposures north of fault 2 (m 7 in figs. 27, 32) consist of a continuous 33-m-thick section of sand, silt, and peat 

assigned to the Whidbey Formation that strike N. 35° E., and dip 

42°-48° NW. (fig. 3 1C). Peat from these strata yielded a radiocarbon date of >44,000 14C yr B.P. (fig. 27, table 2). The section 

is cut by numerous fractures that generally trend northeast (10°— 

50°) and dip steeply (>70°) to the southeast. Offsets of 1-15 cm 

are common on these fractures, and range up to 100 cm. These 

Whidbey Formation strata are overlain by a 10-m-thick covered 

interval, then a continuous 60-m-thick section of sand and gravel 

assigned to the Possession Drift. These beds have the same 

structural attitude as the underlying Whidbey Formation and 

form part of one continuous northwest-dipping stratigraphic section that is more than 100 m thick and extends from the base of 

the coastal bluff to elevations of 15—20 m on the bluff face. Farther north between faults 2 and 3, sand and gravel outcrops (m2 

in fig. 32; not shown in fig. 27) have similar structural attitude, 

appear continuous with the underlying section, and are also correlated with the Possession Drift. The thickness of strata

between faults 2 and 3 is not accurately portrayed in figure 32, 

which plots the distribution of strata relative to the elevation at 

which they occur.

The outcrops on opposite sides of the projection of fault 2 

suggest a minimum of about 10m of north-side-up vertical separation along the structure. Determining a more precise value 

for this separation is problematic for three reasons: (1) Uncertainty exists in the correlation of the section that occurs between 

faults 1 and 2; (2) Well data are lacking in the critical area just 

south of the fault (between wells 61 and 62 in fig. 32), thus the 

thickness of Possession Drift and the elevation of the top of the 

Whidbey Formation immediately south of fault 2 are unknown; 

(3) Projecting outcrop dips into the subsurface cannot be done 

with certainty. Cumulative vertical separation on faults 1 and 2 

on the top of the Whidbey Formation thus appears to be a minimum of 20-25 m.

The projected trace of fault 3 is also covered. Three outcrops of Pleistocene strata occur between the inferred projections of faults 3 and 4 on Strawberry Point (figs. 27, 32). At 

the southern locality (n 1 in figs. 27, 32), a =4-m-thick section 

of interbedded sand and mud dips 24° NW. These strata are 

correlated with the Whidbey Formation based on lithology and 

sedimentology, and because they are at the same elevation as a

28 Devils Mountain Fault and Related Structures, Pacific Northwest
overlap

53

....

1—— 12 •>. VM

— —— i.

N

J

1

\

61

^

=rf=

\

m7 X\

roi m2 -

"N\ ^

H x.3

H C _ -?

> 44,000

yr B.P.

\ Whidbey

\ Formation

\

\

\

S. outcrop i 

— ^"dips 

45" N

Everson Drift and

Vashon Drift

59

i

P

?

1

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L1V

? ^

r.

60

j=?

j

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s=

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"\

outcrop dips

IV) pS, l-x

•~~'- — —,'-; <<• F 

E ^S

58

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m

•

^

^

^

•

•

!

^ 

I

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g Formatio

= i

1 1 =

= i

^-^

55 p

- L- 54 igj

xyy ;|S

x Ip-

§! : ° =

X'M

o^l

p

~""~~--~. p

t==

fj=:

3\

n(W)

. , prfe-Whidbey Formation glacial

drift and interglacial deposits

1 [

fault 2 faults 

———— Strawberry Point fault zone -

Everson Drift and 

Vashon Drift (V)

50

"""EsperanceSaftd

52 ^51

i^^ Whidbey? 

'plssession? Formation (W) 

Drift (P)

NORTH

48 47

EXPLANATION

2°Z Gravel B Clay/gravel 

r^-l Sand and gravel E Hardpan 

[3 Sand j2] Hardpan, sandy 

d Clay [73 Hardpan, gravelly 

J5| Sand/clay E Peat interbeds

fault 4

400

300

200

100

Oft

-100

-200

-300

-400

very thick (as much as 100 m) subsurface section of mainly 

clay and silt penetrated by nearby water wells (figs. 27, 28). 

The nonglacial Olympia beds are not known to reach comparable thickness anywhere in the region (for example, Dragovich 

and Grisamer, 1998; D.J. Easterbrook, written commun., 2000). 

A small sample of detrital wood from these strata yielded a 

radiocarbon date of 44,740±910 14C yr B.P., an age very close 

to the practical limit of radiocarbon dating. The sample could 

have been contaminated by minute amounts of younger carbon, and the date is here considered to be "infinite." Forty 

meters to the north (n2 in fig. 32; not shown in fig. 27) across a 

covered interval, a 5-m-thick section of sand and gravel 

assigned to the Possession Drift, based on lithology and stratigraphic position, has a similar structural attitude. Thus, the 

Whidbey-Possession stratigraphic contact, exposed in the structural domain between faults 2 and 3, appears to be repeated 

between faults 3 and 4.

Determining the amount of vertical separation across fault 

3 is problematic because of extensive cover between coastal 

exposures and the presence of just one useful well in the domain 

between faults 2 and 3. If one assumes that the contact between 

the Whidbey and Possession is repeated and that the structural 

dips in coastal exposures are continuous through the noncovered 

areas, then the amount of vertical separation needed to produce 

the repeated contact is about 80-120 m.

Outcrop data suggest that the section might be repeated not 

only across fault 3 but also between faults 3 and 4 by a fault not

apparent on the seismic-reflection data. About 10 m of fractured, west-dipping (28°) sand and mud tentatively interpreted 

as Whidbey Formation is exposed at locality O (figs. 27, 3 ID); 

alternatively, these strata could have been deposited during the 

Olympia nonglacial interval and thus form the upper part of a 

continuous stratigraphic section, extending from the Whidbey 

Formation to Olympia beds, in the southern part of the domain 

between faults 3 and 4. The beds at o are cut by west-northwest-striking (=70°) subvertical fractures.

Outcrop locality p occurs along the projected trace of 

fault 4 (fig. 27) and is the last bluff exposure on the northeast 

coast of Strawberry Point. The outcrop consists of 4-5 m of 

highly fractured friable sand (fig. 3 IE, F) overlain by pebbly 

diamict. Fractures have variable northwest to northeast strikes 

(50° to 340°), dip 30°-90°, are lined with clay, and commonly displace stratigraphic markers 2-10 cm. These strata 

are tentatively interpreted as uppermost Whidbey Formation 

overlain by Possession Drift. Well data suggest possible minor 

north-side-up slip on fault 4 (-10 m) following deposition of 

the Whidbey Formation, but the amount is poorly constrained.

We interpret the faulting and folding within the outcrops at 

Strawberry Point to be tectonic in origin. This structural zone 

coincides precisely with the fault zone recognized a few hundred meters offshore on seismic-reflection data (figs. 5A, 8, 27). 

Similarly deformed Pleistocene strata are extremely rare in the 

Puget Lowland (among others, Gower and others, 1985; S.Y. 

Johnson, unpub. data, 1994-2000). Deformation by landsliding

Evidence for Faulting Onshore, Whidbey and Camano Islands 29
can be ruled out for several reasons: (1) Dipping sections are 

continuous vertically for as much as 100 m and laterally to the 

limits of exposure—they have not been disrupted or broken 

apart by gravity sliding, as is the case for many small recent 

landslides in the Strawberry Point area. (2) Dipping sections 

cross major stratigraphic contacts between mud- and sand-dominated units. These unit boundaries typically form the failure 

planes for Puget Lowland landslides. (3) The strike of dipping 

sections is perpendicular to the trend of the modern coastal 

bluffs, not parallel to the bluff free face as would be expected 

for latest Pleistocene or Holocene landslides. (4) Topography 

along Strawberry Point indicates the presence of many small 

landslides derived from the upper part of the bluff, but nothing 

remotely similar to the size of the observed dipping sections. 

Also, the dipping section between faults 2 and 3 extends to the 

top of the bluff; hence no source for a possible massive landslide exists in that location.

The coincidence with the offshore fault zone and the rare 

occurrence of deformed Pleistocene strata elsewhere in the 

Puget Lowland argue against a glaciotectonic origin (for example, Aber and others, 1989; Hart and Boulton, 1991; Hart and 

Roberts, 1994) for deformation in Strawberry Point outcrops. 

Beds are not deformed in the thin-skinned fold-and-thrust style 

that characterizes glaciotectonic deformation, and no obvious 

glaciotectonic landforms (such as composite ridges or push 

moraines) occur at Strawberry Point. Moreover, typical glaciotectonic features such as listric faults, small-scale folds, chevron 

folds, tectonic pods and boudins, augens, and pressure shadows 

are absent; secondary faults and fractures are generally subvertical and not low angle; and tectonic fabric is not widely distributed throughout the zone.

Utsalady Point Fault

West Coast of Whidbey Island

The Utsalady Point fault projects onto Whidbey Island's 

west coast across a sandy beach and lowland south of Rocky 

Point (figs. 27, 28), an area of no exposures. The closest exposures north of the fault are at Rocky Point (g in fig. 27), where 

pre-Tertiary basement rock is overlain by Everson Drift (fig. 6; 

Pessl and others, 1989). Easterbrook (1968) described the closest coastal bluff exposures to the south, 4 km away along bluffs 

south of Swantown (dl in fig. 27). These exposures consist of 

the Whidbey Formation overlain by Esperance Sand Member 

with no intervening Possession Drift or nonglacial Olympia 

beds (fig. 6).

Nearby offshore seismic-reflection profiles (figs. 10, 13) 

indicate about 200-300 m of down-to-the-south vertical separation on the base of the uppermost Pliocene(?) and Pleistocene section across the Utsalady Point fault. On land, this 

contact is at an elevation of a few meters above to 50 m below 

sea level north of the fault (fig. 33). No wells south of the 

fault (as deep as 300 m) encountered basement. The thicker 

Pleistocene section south of the fault appears to consist almost 

entirely of pre-Whidbey Formation glacial and interglacial

deposits (fig. 33). The geometry of the Utsalady Point fault 

on offshore seismic-reflection data strongly suggests that the 

noted onshore relief on the base-of-uppermost Pliocene(?) to 

Pleistocene surface is created by a near-vertical fault bounding two subhorizontal surfaces and not by a gentle subsurface 

slope between Rocky Point and well 100 (figs. 28, 33). The 

top of basement also deepens to about 160 m about 1 km 

north of the Utsalady Point fault, indicating a possible southside-up fault splay in this area, consistent with offshore seismic-reflection data (fig. 10).

Using the top of the interglacial Whidbey Formation as a 

younger datum (discussed in Devils Mountain fault subsurface 

data) for determining possible offset on the Utsalady Point fault 

is problematic on the figure 33 correlation diagram because of 

the apparent absence of Possession Drift on this part of Whidbey Island. In these wells, the top of the Whidbey Formation 

may be in contact with lithologically similar strata of the nonglacial Olympia beds or with the Esperance Sand Member as in 

the Swantown coastal bluff exposures (d1) at the south end of 

the profile. In either case, the top of the Whidbey Formation is 

not an obvious surface on lithologic logs of wells adjacent to the 

fault. Well data are thus consistent with Quaternary offset on 

the Utsalady Point fault in this area but do not provide the data 

needed to estimate the amount of post-Whidbey Formation 

offset.

Central and Eastern Whidbey Island

Across central and eastern Whidbey Island, the Utsalady 

Point fault is covered by vegetation and by surficial deposits 

developed mainly on Vashon Drift and recessional outwash 

(Pessl and others, 1989). No obvious topographic lineament is 

associated with the fault zone on existing topographic maps, 

and there is an insufficient number of deep water wells flanking 

the fault to construct stratigraphic correlation diagrams either 

parallel to or across the fault that might constrain location and 

slip. Thus, the location of the Utsalady Point fault as it crosses 

and possibly bifurcates on central and eastern Whidbey Island is 

speculative. Constraints on the location shown in figures 2, 27, 

and 28 include (1) the locations of faults on offshore seismicreflection profiles (figs. 9, 10, 13), (2) weak aeromagnetic 

anomalies (fig. 3), (3) one deep test well south of the fault on 

the Maylor Peninsula (No. 101, fig. 28, table 1), which encountered bedrock at an elevation of about 475 m below sea level, 

and (4) a 500-m-long continuous exposure of flat-lying, locally 

fractured but unfaulted glacial and interglacial strata on the 

southeast coast of Polnell Point (q in fig. 27).

Camano Island

One splay in the central part of the Utsalady Point fault 

zone is exposed in the coastal bluffs at Utsalady Point on 

Camano Island (rin fig. 27). Strata are exposed almost continuously for 250 m along the coast and consist of three principal 

lithologies (figs. 34, 35): (1) laminated silty mud and sand with 

rare pebbles and organic detritus; (2) fine to medium sand and

30 Devils Mountain Fault and Related Structures, Pacific Northwest
less common silty mud in which primary bedding has been 

largely destroyed by convolute stratification, pillar structures, 

and other soft sediment deformation features; and (3) lenses and 

wedges of gravel, pebbles, cobbles, and boulders. The geometries of units and the mix of lithologies suggest variable depositional environments within channels, floodplains, and possibly 

lakes of an alluvial plain, and postdepositional mixing of sandy 

strata by processes associated with liquefaction. Sparse finegrained organic matter disseminated within sandy silt beds a few 

meters apart yielded radiocarbon dates of 21,100±150 and 

15,190±220 14C yr B.P. (SJ-99-1A, IB; table 2). The difference 

between the two dates is larger than expected given their close 

stratigraphic position. Both samples are low in carbon, and it is 

possible that the age of one or both samples is too young due to 

minor contamination.

These dates place the strata within the latter part of the 

Olympia nonglacial interval (fig. 6), and they indicate that these 

strata are younger than the Olympia beds exposed across 

Saratoga Passage on the southeast coast of Strawberry Point (fig. 

27, table 2; Hansen and Easterbrook, 1974). In the Tacoma area 

of southern Puget Sound, Troost (1999) reported radiocarbon 

dates as young as 15,110 14C yr B.P. for Olympia beds. The 

older of the two ages from Utsalady Point is similar to many 

reported for the Coquitlam Drift, a pre-Vashon glacial formation 

exposed about 100 km to the north near Vancouver, B.C. (Hicock and Armstrong, 1981), and to the pre-Vashon Evans Creek 

Drift, deposited by valley glaciers extending from the Cascade 

Range to the edge of the Puget Lowland (Armstrong and others, 

1965).

Beds at the Utsalady Point locality are deformed into a 

broad northwest-trending fold. Structural dip ranges from 24° 

(figs. 34, 35) on the northeast limb of the fold to 10° on the 

southwest limb. The fold axis is cut by a vertical east-southeast 

(120°) trending fault (figs. 33, 345, C). This fault truncates 

irregular pods and wedges of massive sand and pebbles in the 

middle of an =8-m-wide zone in which primary bedding has 

been destroyed by faulting and soft-sediment deformation. The 

fault is characterized by a 20-30 cm wide zone of highly sheared 

sediment, which contains both rotated and broken clasts. Eight 

fractures adjacent to the master fault have a mean strike and dip 

of 127° and 84° SW., respectively.

Vertical offset on this fault appears to be less than a few 

meters based on the projected elevation of the contact between 

silty mud and sand facies on opposite sides of the mixed zone 

adjacent to the fault. The pebbly pods on the east side of the 

fault have probably been intruded from a source bed lower in the 

section. The source for the pebble wedge on the west side of the 

fault is not obvious; it may also have been intruded from lower 

in the section (requiring transport from out of the plane of the 

exposure), or alternatively, it could be the depositional fill of a 

large fissure that formed along the fault. Water-well data (fig. 

37) show no nearby tills at this elevation, so formation and filling of this pebble wedge by nontectonic subglacial processes is 

unlikely.

In the bluff outcrops about 50 to 150 m west of this fault 

on the west limb of the fold, a 6-m-thick sandy horizon displays large-scale soft-sediment deformation (fig. 35D). Features include numerous dikes and pillar structures, convolute

stratification, ball-and-pillow structure, and intrusion by large 

dikes of sandy silt to gravel that extend up through as much as 

13 m of section. Subvertical fractures with a mean strike of 

305° (/?=16) are common on the west limb of the fold.

As with the Pleistocene strata exposed at Strawberry Point 

(see p. 23-30, Strawberry Point fault, east coast of Whidbey 

Island), we infer that the faulting and folding of beds at Utsalady 

Point is tectonic in origin. The coincidence with the fault zone 

recognized offshore in Saratoga Passage is striking (fig. 9), and 

such deformation of Pleistocene strata in the Puget Lowland is 

extremely rare (Gower and others, 1985; S.Y. Johnson, impub. 

data, 1994-2000). Landsliding or glaciotectonic origins can be 

ruled out for most of the same reasons cited for the Strawberry 

Point fault.

Two faults are exposed in a vegetated roadcut 900 m 

southeast (128°) of the fault exposed at Utsalady Point (s in fig. 

27). Strata cut by these faults consist of sand and gravel and 

are inferred to be Vashon glaciofluvial advance deposits, correlative with the Esperance. One fault (fig. 36A) is subvertical, 

strikes 95°, juxtaposes massive gravel (on the south) and planestratified sand (on the north), and has minimum vertical displacement of 150 cm. The second fault (fig. 365) occurs 15m 

to the southeast, strikes 0° and dips 67° E., juxtaposes massive 

gravel (on the east) and stratified sand (on the west), and similarly has minimum vertical displacement of 150 cm. These 

exposed faults indicate southeastward continuation of the 

Utsalady Point fault zone, consistent with the trend of aeromagnetic anomalies (fig. 3).

Figure 37 shows a stratigraphic correlation diagram across 

part of the Utsalady Point fault zone based on lithologic logs 

from water wells, with the bluff exposure and dates for the 

Olympia beds at Utsalady Point (figs. 27, 34, 35; location r) providing important stratigraphic control. The outcrop section at r 

correlates with a fine-grained interval in the subsurface that is 

much thicker than any known section of Olympia beds from this 

part of the Puget Lowland. This suggests that the Possession 

Drift is missing from this local area and that Olympia beds 

directly overlie the lithologically similar interglacial deposits of 

the Whidbey Formation. Because of this lithologic similarity, 

there is no basis for inferring the Whidbey-Olympia contact in 

the subsurface near this locality, and no obvious stratigraphic 

marker is present to constrain vertical offset on the fault exposed 

at Utsalady Point.

The southwestern splay in the Utsalady Point fault zone 

projects onto the correlation diagram between wells 106 and 

107 (figs. 28, 37). The stratigraphy of wells southwest of this 

projected fault differs from those to the northeast in several 

ways. (1) Pre-Whidbey Formation glacial drift is inferred to be 

present at the bottom of one deep well on the southwest side of 

the fault but was not encountered at similar depths by wells 

northeast of the fault. (2) The Possession Drift is inferred to 

be present in five of six wells southwest of the fault; the 

Possession is apparently absent in the five wells northeast of 

the fault. (3) West of the fault, the elevation of the inferred top 

of the Whidbey Formation is more than 15m; east of the fault 

it is below sea level at outcrop locality r (figs. 27, 34, 35, 37). 

(4) The top of the Olympia beds in six wells southwest of the 

fault has a mean and maximum elevation of 56.7±7.9 m and

Evidence for Faulting Onshore, Whidbey and Camano Islands 31
SOUTHWEST overlap

50-

Om -

-50 -

-100 -

-150 -

-200-

-250 -

-300 -I

Everson Drift and 

Vashon Drift (V)

pre-Whidbey Formation glacial 

drift and interglacial deposits

Figure 33. Interpretive stratigraphic correlation diagram crossing Utsalady Point fault zone northwestern Whidbey 

Island (figs. 27,28). Correlation diagram is based on a few outcrops (fig. 27; lettered sections) and lithologic logs from 

water wells (table 1, fig. 28; numbered sections). Faults dashed where infer red, queried where extent uncertain; barbs 

show direction of relative fault movement. Note overlap between the two panels of this figure.

66.4 m in contrast to 26.9±3.8 m and 30.8 m for three wells 

northeast of the fault. This difference is much less, only 11.3 

m, in the two wells closest to and on opposite sides of this fault 

(106 and 107). These contrasts cumulatively suggest that the 

southwest side of the fault has moved up relative to the northeast side. This is opposite to the sense of throw along the 

Utsalady Point fault on western and central Whidbey Island. 

Vertical offset on this fault is not obvious on the nearby (~2.5 

km) offshore seismic-reflection profile in Saratoga Passage 

(figs. 5, 9).

Fault Mapping, Style, and Rates of Deformation

Devils Mountain Fault

Offshore seismic-reflection and onshore geologic data indicate that the Devils Mountain fault is continuous from the Cascade Range foothills (Whetten and others, 1988; Tabor and 

others, 1988; Tabor, 1994) across northern Whidbey Island and 

into the eastern Strait of Juan de Fuca, a minimum distance of 

approximately 125 km (fig. 1). The Devils Mountain fault was

32 Devils Mountain Fault and Related Structures, Pacific Northwest
overlap NORTHEAST

6km

CO

Q9

X

X

„„ _

/

s

^

m

9

V

/

/^,.

91

l£^

•

s

/

/

90

^;

SSSc

^

mrt

*\

E \

\

\

^v

-

=^

r^

^

a

=

S

^^-

/

/

~t

C^P°

•PfO1;

•

^

L:d

/

""

87

m

^

s

rt

^

li —

Whidbey Formation ^

,_ _

81

FF] Everson Drift and

— ?

-v

and Olympia(?) beds

\

85 84 14 C _ 1 80 Vashon Drift (V)

m % <^ 13,595 yrB.p. •; ^ 7g

? m =i H V 82 v .? r;'c ra n

H \^ II 83 gg .,/W. 79 §S- '7r8

P ^'^fn g 5 x"--*""""" ; ; ^ ./|S^\-:'H~--""""^

;•:• •-'••' = 1 •- R3 j ::. ". Esperance :, -.^/ ^ m Whidbey

= = ty ^^'•V^ -^?— r-^fe/ ^ = - and

H == s/H\B^ — —- — Jll^m : Olympia(?)

^ = m lm M^~— ^—?^ beds

v / B|B 1 ^^-^

n / ~L3| = •

?- -/ ——— ^' s- •

Whidbey -§ •

Formation H

and Olympia(?) | ,.,u -ju c *• . -i 

, . n pre-Whidbey Formation glacial beds pre-Quaternary *, .,, . . . . ... •.. M , . , " drift and mterglacial deposits bedrock -^ M

/

\

S

^

s

—

=

^

i

^^

B

•

S

,--/e

\

\

—

\

J\

M.

! \ 

i \

s

s 

i

g

^

S

*TM^T

=

=

g

i

s

-

pre-Quaternary H 

bedrock

EXPLANATION

^> KS3 Gravel |^3 Clay/gravel

hC J Sand and qravel U,.7] Hardpan

ITT! Sand |?7| Hardpan, sandy

JB| Clay L _ 1 Hardpan, gravelly

B3 Sand/clay H Pre-Quaternary bedrock

flr-l Peat interbeds

.!, V.E. - 8

300

_„_____ 200

— 100

Oft

-100

-200

-300

-400

Utsalady Point fault

-600

... .700

-800

-900

-1000

recognized on all but one of 14 crossing high-resolution seismicreflection profiles and all deeper industry profiles, and it appears 

to cut and (or) fold inferred Quaternary reflections on virtually 

every profile (figs. 8-26). The dip of the upper 1-2 km of the 

fault plane varies from about 45° to 75° (61°±10° for crossing 

seismic profiles) to the north with the steepest dips to the east in 

profiles flanking Whidbey Island. The pattern of juxtaposed 

reflections on some profiles also suggests out-of-plane (strikeslip) offsets.

For its entire length, the fault coincides closely with the 

southern margin of a variable-amplitude aeromagnetic anomaly 

(fig. 3). The variability in amplitude is inferred to correlate 

mainly with the lithology of basement rocks, being strongest 

where pre-Tertiary igneous basement occurs and weakest where

the basement consists of pre-Tertiary and Tertiary sedimentary 

rocks (fig. 6). The fault also roughly corresponds with a change 

in the depth to the base of uppermost Pliocene(?) to Pleistocene 

strata (fig. 7).

Seismic-reflection and aeromagnetic data east of Whidbey 

Island in Skagit Bay (figs. 2, 3, 8) suggest at least one tear fault 

or transfer zone along the Devils Mountain fault that could represent a segment boundary. A second segment boundary could 

be present in the eastern Strait of Juan de Fuca south of southeastern San Juan Island. In this area, the westernmost USGS 

high-resolution seismic-reflection profile (fig. 5A, west of the 

area of fig. 20) did not image a fault zone on strike with the Devils Mountain zone to the east and west. Although a nearby 

industry profile (fig. 21) did image a fault along the strike of the

Fault Mapping, Style, and Rates of Deformation 33
EAST 10m WEST

fig. 35A

21,100±150ybp 

15,190±220ybp

EXPLANATION

Vegetation and 

slope wash

Modern beach

Massive pebbles and gravel

Mixed sand and mud; local 

convolute stratification

Massive sand; 

uncommon pebbles

Laminated silty mud

Fracture 

Fault

Figure 34. Line drawing (traced from photo mosaic) of deformed Quaternary strata exposed at location r (fig. 27), Utsalady 

Point, Camano Island.

Devils Mountain fault, this structure does not appear to deform 

the Quaternary section and may be an inactive strand. Other 

possible segment boundaries or gaps may be present along the 

fault trace but are presently unrecognized.

North of the Devils Mountain fault in the eastern Strait 

of Juan de Fuca, seismic-reflection data indicate three zones 

of northwest-trending folds and faults (fig. 2). Mapping 

these structures is limited by the incomplete coverage by 

seismic-reflection profiles (fig. 5); however, their trends correspond closely with aeromagnetic anomalies (fig. 3). Figure 2 

shows their minimum lengths and extent. The eastern zone 

occurs at the south end of Rosario Strait and coincides 

closely with a high-amplitude aeromagnetic anomaly (fig. 3) 

that forms the southern boundary of a Jurassic ophiolite 

(Whetten and others, 1980). These faults and folds are well 

imaged on seismic-reflection data collected by Puget Power 

(1979). The middle zone of north west-trending faults and 

folds occurs south of western Lopez Island and correlates 

with low-amplitude aeromagnetic anomalies that extend 

northwest, parallel to the southwest coast of San Juan Island 

(figs. 2, 3). The gradients of these magnetic anomalies indicate sources deeper than the section imaged by the seismic 

data, but it appears that the structure of these deeper sources 

is controlling the structural pattern of the upper 1 km. The 

western zone occurs at the mouth of Haro Strait and coincides closely with moderate- to high-amplitude aeromagnetic 

anomalies (figs. 2, 3). Many of the northwest-trending

structures in these three zones cut and (or) fold reflections in 

inferred Quaternary strata. Thus, these faults and folds 

appear to be Quaternary en-echelon structures active at the 

same time as the Devils Mountain fault and not older, inactive features cut by a younger Devils Mountain fault.

The Devils Mountain fault probably continues west of our 

data base, passing a few kilometers south of the city of Victoria 

and merging to the west with the Leech River and (or) the San 

Juan fault on southern Vancouver Island (Muller, 1983). At 

present, however, neither the Leech River nor San Juan fault has 

been recognized as an active or potentially active fault. An alternative is that much or all of the deformation along the fault is 

transferred to the presumed en-echelon northwest-trending 

structural zone that coincides with the prominent aeromagnetic 

anomaly in Haro Strait (figs. 2,3) which extends onto Vancouver 

Island north of Victoria. Geological Survey of Canada seismicreflection Line 35 (fig. 26) is important in distinguishing 

between these two end-member alternatives. This line crosses 

the Devils Mountain fault south of the northwest-trending 

structural zone in Haro Strait and shows significant deformation 

of the base of the uppermost Pliocene(?) to Pleistocene section 

on the west-trending fault trace.

The map-view geometry of the Devils Mountain fault and 

associated structures resembles that described for many left-lateral, oblique-slip fault zones (for example, Christie-Blick and 

Biddle, 1985), with the Devils Mountain fault acting as the 

"master fault" within a broadly distributed zone of deformation.

34 Devils Mountain Fault and Related Structures, Pacific Northwest
cp 

V) -2CD 0)

oCD

Figure 35. Deformation and facies in upper Pleistocene deposits in Utsalady Point fault zone atUtsalady Point, 

Camano Island (rin fig. 27; fig. 34). A, Folded (24° dip) and fractured =21-ka 

laminated sandy silt facies. Staff is 1.5 m long. B, Fault (dashed white line) 

cutting silty mud, sand, and gravel (line 

drawing in fig. 34). C, Shear fabric 

along fault shown in B. Bands on 

shovel handle are 10 cm wide. D, 

Large irregular sandy silt intrusion (SI, 

dashed white line shows boundaries) and "bed" of sand, silt, and gravel mixed by extensive liquefaction and 

soft-sediment deformation (LB).
Figure 36. Faults (A, B) exposed in a vegetated roadcut 900 m southeast of fault exposed at Utsalady Point (s in fig. 27), cutting inferred 

Vashon glaciofluvial advance deposits. The faults, which juxtapose well-stratified sand and massive gravel, are 15 m apart and occur within 

the Utsalady Point fault zone.

This map geometry notably contrasts with the expected structural pattern across a zone of normal contraction, a setting in 

which secondary faults and folds would have trends similar to 

that of a master fault. Based on the distinctive map pattern, we 

believe that the Devils Mountain fault is a transpressional, 

oblique-slip fault, characterized by both north-south shortening 

and left-lateral slip.

The vertical component of slip on the Devils Mountain 

fault can be constrained with onshore subsurface data (fig. 30) 

from Whidbey Island, assuming that stratigraphic interpretation 

of the top of the Whidbey Formation is correct and that similar 

amounts of postdepositional modification of this surface have 

occurred on each side of the fault. Given the interpretation in 

figure 30, the vertical separation on the top of the Whidbey Formation (80 ka; fig. 6) across the fault is about 1 1.9±8.3 m. This 

offset requires between about 4 and 24 m of fault offset (12-14 

m for the mean), assuming (based on nearby seismic-reflection 

profiles) the fault dips 60°-75° in this area. The result is a late 

Pleistocene vertical slip rate of about 0.05-0.30 mm/yr (0.15- 

0.18 mm/yr for the mean) and a shortening rate of 0.01-0.15 

mm/yr (fig. 38).

Glaciomarine features provide another possible tool for 

constraining fault offset. Dethier and others (1995) noted a =10 

m inflection point (up to the north) coinciding with the Devils 

Mountain fault on a graph showing elevation of a ~13 ka glaciomarine surface representing the maximum altitude of the 

marine limit. These data are consistent with the north-side-up 

offset on the Devils Mountain fault determined in this investigation. At present, however, data points for such glaciomarine 

features are too widely spaced to provide valid constraints for

the Devils Mountain fault and other local structures. New highresolution topographic imaging tools such as airborne laser terrane mapping (for example, Kitsap County Public Utility District, 1998; Bucknam and others, 1999; Nelson and others, 

1999; Harding and Berghoff, 2000) may provide the best opportunity for the type of detailed investigations needed to use glacial and glaciomarine landforms in studies of active faults in 

this region.

The vertical component of slip on the Devils Mountain 

fault can also be estimated using offshore high-resolution seismic-reflection data. The drawback to this approach is that 

multiple cycles of widespread Quaternary glacial erosion and 

deposition in the eastern Strait of Juan de Fuca have affected 

both bedrock and Pleistocene deposits. As a result, the age of 

the lowest preserved uppermost Pliocene(?) to Pleistocene 

deposits on any one profile is unknown, and correlation of 

Quaternary units across the fault zone is either difficult or 

impossible. The most conservative approach is to use the base 

of the uppermost Pliocene(?) to Pleistocene section as a datum 

and to assign this surface a maximum age of 2 Ma (see section, "Identification and Age of the Base of the Uppermost 

Pliocene(?) to Pleistocene Section"). Using this datum on six 

profiles (figs. 8, 10, 13, 16, 20, 26) yielded minimum Quaternary slip rates varying from 0.03 to 0.13 mm/yr (fig. 38). We 

applied the methods of Schneider and others (1996, their fig. 

10) to Geological Survey of Canada Line 37 (fig. 25), a special case in which a fault-propagation fold is imaged above the 

locally blind Devils Mountain fault. The vertical and horizontal components of fold growth are 180 m and 135 m, respectively, and inferred uppermost Pliocene(?) to Pleistocene fault

36 Devils Mountain Fault and Related Structures, Pacific Northwest
SOUTHWEST NORTHEAST

2 km

110

200 -•

100

o 

!<0ft

-100

-200

12

il\=U

H

• o

|/=^

==

JS

—— sJ -t

^

•

g

fFF

^

^

^L

0 C

^L

\

\

— —

=

==

rr

H

IP

°<

z

— *

y

\ 109 Everson Drift and Vashon Drift (V)

^T\>^ 1.°-8

5E

"^

\

\

7

X

Possession

Drift(P)

Whidbey

Formation (W)

'-

?. =

5

H

•

•

^=

•

•

^

H

^

^4

_L\_

^

,/ x'

/

0 /

/

-Y

\

\\

\

.

11

/

i

SaS ^^^

H

B

^ 

==

^

=?

X

\

^

B

•

5

^

\

\

S5

v 107 104

\V\» 

\\

\\

\\

\

I

\v

\

l\

i \

X

\

5

s^

^ 105

-"— ?

P-0-9

X

\

X

\

Oo°(

Jj

I

•

•

^^

g

=

H

' \si

- —— :

• . —— '

P C '

W

111 v

— — 1 c -

pre-Whidbey Formation

glacial drift

^

^

='-

-IL

y

1

•

[

•

n

H=

^

•

u

^

~=

1=L

^^L

~~^_

/

1

[.E.

\ ;

A

^;\ v

^ \

^\ \

^

^^

m

jjj

•

B

H

=

B

•

^'^-'^

'^

=j^

-^r

F^

^

=?;

s

Esperance Sand

Member (S) 

, 102

/

/ _ ^103. .

' /

Olympia

beds(O)

? ?

Whidbey

Formation (W)

^

li

Bj

•

B

==

r

H

i

i

_L

EXPLANATION

H5P Gravel

ap

B

^

==

^

g

B

B

^=

*

TJ7

-^

14 C

C3 Sand and gravel Figs. 34,35

ESJSand

9 HS Clay

fault

SOUthweSt Splay, ^ Sand/clay V.E. - 1 2

Utsalady Point p| Clay/gravel

fault zone MHardpan

— 75

— 50

— 25

— Om

— -25

C = 21,100 yrB.P. 

15,190yrB.P.

— -50

Figure 37. Interpretive stratigraphic correlation diagram crossing part of Utsalady Point fault zone at Utsalady Point, northwestern Camano Island. Correlation diagram is based on lithologic logs from water wells (table 1, fig. 28; numbered sections) and one 

outcrop (r in fig. 27; figs. 34,35). Faults dashed where inferred, queried where extent uncertain; barbs show sense of relative offset.

displacement is 225 m on a plane dipping 53°. Using 2 Ma as 

the base of the Quaternary section yields an approximate minimum vertical slip rate of about 0.11 mm/yr.

The longer term minimum slip rates determined from seismic-reflection data are thus consistent with the shorter term, 

approximate, late Pleistocene vertical-slip rate (0.05-0.30 mm/ 

year) estimated from onshore subsurface data (fig. 38). We 

emphasize that this rate reflects only the vertical component of 

slip on a structure we infer to be an oblique-slip fault.

Strawberry Point Fault

Seismic-reflection data and local onshore geology suggest 

that the west-trending Strawberry Point fault is continuous from 

the eastern Strait of Juan de Fuca to at least Skagit Bay, a minimum distance of 22 km (figs. 2, 27). It occurs as one discrete 

subvertical structure with its south side up along western Whidbey Island and in the easternmost Strait of Juan de Fuca (figs.

10, 13), where it forms the northern boundary of a relatively 

nonmagnetic basement block (fig. 3). The pre-Tertiary sedimentary rocks that form the core of this block are exposed at Rocky 

Point on the west coast of Whidbey Island (fig. 2). We infer 

about 80-200 m of vertical relief on the base of the uppermost 

Pliocene(?) to Pleistocene section along the Strawberry Point 

fault on two seismic-reflection profiles (figs. 10, 13) west of 

Whidbey Island. Assuming a maximum age of 2 Ma for this 

contact (see section, "Identification and Age of the Base of the 

Uppermost Pliocene(?) to Pleistocene Section"), this relationship yields a minimum vertical slip rate for the Quaternary of 

0.04-0.10 mm/yr (fig. 36). The occurrence of late Holocene 

(=1.7 ka) bulrush rhizomes at sea level on the south side of the 

fault at Rocky Point (fig. 27; table 2) provides a horizontal 

datum that rules out south-side-up vertical offset of more than a 

few meters on the Strawberry Point fault in the last 1,750 years. 

Onshore and offshore Strawberry Point on eastern Whidbey 

Island, deformation is distributed on several subvertical splays 

and gentle to moderate folds in a ~2-km-wide zone. Inferred

Fault Mapping, Style, and Rates of Deformation 37
122°45'

48° 

15'

Strait of 

Juan de 

Fuca

3 KILOMETERS4- EXPLANATION

Inferred Quaternary fold

——— — Inferred Quaternary fault—Queried where extent 

uncertain; U, upthrown side; D, downthrown side

10.17-0.201 Inferred rate of vertical fault slip 

(S^TbTblT) Inferred rate of shortening

Figure 38. Northern Whidbey Island area showing locations of faults and folds that deform Quaternary deposits, inferred rate (commonly a minimum) of vertical fault-slip (mm/yr), and shortening rates on the Devils Mountain fault (DMF), Strawberry Point fault (SPF), and 

Utsalady Point fault (UPF). Queried rates are based on meager data and are highly speculative, and each fault probably has a significant 

component of lateral slip for which slip-rate information is not available. Dark-shaded areas are underlain by pre-Quaternary bedrock 

(fig. 6). Light-shaded areas are underlain by Quaternary deposits. MP, Maylor Point; PP, Polnell Point; S, Swantown; UP, Utsalady Point.

stratigraphic offsets (fig. 32) suggest that the four main subvertical splays have north-side-up displacement, opposite to the 

sense of vertical fault slip on the Strawberry Point fault west of 

Whidbey Island. Using the top of the Whidbey Formation as a 

datum, the fold south of fault 1 (fig. 27) has a late Pleistocene 

uplift rate of about 0.38 mm/year (fig. 37). The shortening rate 

required to produce this uplift is about 0.09 mm/yr (method of 

Rockwell and others, 1988). The cumulative minimum latest 

Quaternary (80 ka to present) vertical slip rate for faults 1 and 2 

is inferred to be about 0.25 mm/yr given the favored, more conservative stratigraphic correlation. The folded strata between

faults 2 and 3 require an uplift rate of 0.88 mm/yr and a shortening rate of 0.19 mm/yr.

The amount of vertical slip on faults 3 and 4 is not well 

constrained. Outcrops of upper Pleistocene strata locally dip 

24° to 48° (fig. 27), making projections of stratigraphic surfaces 

tenuous. Making the speculative assumption that dips and 

stratigraphic contacts are continuous yields vertical slip rates as 

high as 1.0-1.5 mm/yr on fault 3 and about 0.1 mm/yr on fault 

4. Shortening rates recorded by folded strata between faults 3 

and 4 are likewise difficult to estimate because of poor outcrops; 

our poorly constrained best estimate is =0.08 mm/yr.

38 Devils Mountain Fault and Related Structures, Pacific Northwest
The cumulative vertical slip rate on the four splays of the 

Strawberry Point fault at Strawberry Point is thus inferred to be 

a minimum of about 0.25 mm/yr, but larger rates are probable. The cumulative shortening rate recorded by folded strata 

within and adjacent to the fault zone is a minimum of about 

0.35 mm/yr.

Folding in outcrops within and adjacent to the Strawberry 

Point fault zone at Strawberry Point indicate contractional deformation that, together with the reversal of the sense of vertical 

offset in the zone across Whidbey Island, strongly suggests the 

Strawberry Point fault is an oblique-slip transpressional fault 

(Christie-Blick and Biddle, 1985). Regional fault geometry (see 

"Fault Mapping, Style, and Rates of Deformation"—"Devils 

Mountain Fault") suggests left-lateral displacement, but we have 

not identified piercing points that could be used to quantify the 

sense, amount, and rate of inferred lateral slip.

Utsalady Point Fault

Seismic-reflection profiles and local onshore geology suggest that the northwest-trending Utsalady Point fault is continuous from the eastern Strait of Juan de Fuca across Whidbey 

Island, Saratoga Passage, and Camano Island, a minimum distance of 28 km (fig. 2). East of Whidbey Island, the projection of 

the Utsalady Point fault zone coincides with the southern margin 

of a low-amplitude aeromagnetic high (fig. 3; Blakely and others, 1999b). This anomaly extends at least 25 km southeast of 

Utsalady Point, suggesting that the fault may have a length of 50 

km or more.

The Utsalady Point fault occurs as one discrete subvertical 

structure with north-side-up offset west of Whidbey Island (figs. 

2, 10, 13, 27, 28). We infer about 200-300 m of vertical relief 

across the fault at the base of the Quaternary section on two seismic-reflection profiles (figs. 10,13) in this area. Assuming a maximum age of 2 Ma for this contact (see section, "Identification and 

Age of the Base of the Uppermost Pliocene(?) to Pleistocene Section"), this displacement yields a minimum vertical slip rate for 

the Quaternary of about 0.10-0.15 mm/yr (fig. 37). Onshore 

western Whidbey Island, subsurface data (fig. 33) indicate more 

than 300 m of relief on the base of the Quaternary, yielding a minimum vertical slip rate of 0.15 mm/yr since about 2 Ma.

Seismic-reflection data from Saratoga Passage (fig. 9) east 

of Whidbey Island represent the fault as a broad (-1.5 km) zone 

comprising several splays, on which significant vertical offset on 

the base of the Quaternary is not obvious. Camano Island 

onshore data (figs. 34-37) suggest that the southernmost splay in 

the zone has north-side-down offset, and therefore that the sense 

of vertical displacement changes along the strike of the fault. 

This along-strike variation, together with the evidence for contractional deformation in the fault zone (folding of -21 ka beds; 

figs. 34, 35), suggests that the Utsalady Point fault is also an 

oblique-slip transpressional structure (Christie-Blick and Biddle, 

1985). As discussed (see "Fault Mapping, Style and Rates of 

Deformation"—"Devils Mountain Fault"), regional fault geometry suggests left-lateral displacement, but we know of no piercing points that can be used to quantify the sense, amount, and 

rate of inferred lateral slip.

Subsurface data from Camano Island (fig. 37) do not provide sufficient information on the top of the Whidbey Formation 

for it to be an effective marker in constraining vertical offset 

along the Utsalady Point fault. As noted previously ("Evidence 

for Faulting Onshore, Whidbey and Camano Islands"), the top of 

the Olympia beds is significantly higher on the south side of the 

southern splay in the zone, but its elevation appears to drop to 

the north approaching this splay, and this datum is only llm 

higher on the south than to the north in the two wells (106, 107) 

closest to the splay. Moreover, seismic-reflection data (fig. 9) do 

not show obvious south-side-up offset, and considerable erosional relief occurs on the top of the Olympia beds on Whidbey 

Island and elsewhere in the Puget Lowland (Troost, 1999, 

among others). Regardless of uncertainties concerning stratigraphic correlation and vertical slip rate in the Camano Island 

region, outcrops at Utsalady Point indicate significant post-20 ka 

deformation in the Utsalady Point fault zone. The folded strata 

at Utsalady Point record a minimum uplift rate of 0.81 mm/yr 

and a shortening rate of 0.17 mm/yr.

Discussion

The geometry of structures in the eastern Strait of Juan de 

Fuca-northern Puget Lowland region suggests that the Devils 

Mountain fault is a transpressional, left-lateral, oblique-slip 

"master fault," and that the Strawberry Point fault and Utsalady 

Point faults are probably en-echelon structures. This deformation may be driven by stress associated with convergence along 

the continental margin. GPS data from this region (Dragert and 

Hyndman, 1995; Khazaradze and others, 1999) indicate that this 

stress produces significant east-northeast-directed crustal strain 

subparallel to the plate convergence vector. The larger part of 

this strain is thought to be elastic, recoverable following a large 

plate-boundary earthquake. A smaller component might result 

in inelastic crustal faulting and folding. In this setting, left-lateral slip would be favored on east-west-trending structures such 

as the Devils Mountain fault.

Although there are no obvious piercing points in available 

data that can be used to quantify the amount and rate of lateral 

slip across this zone, regional geology provides possible constraints. Whetten (1978) suggested about 60 km of left-lateral 

slip on the Devils Mountain fault based on offset of the Trafton 

sequence, a distinctive Mesozoic melange unit (part of the eastern melange belt of Tabor, 1994), using piercing points in the 

San Juan Islands north of the fault and Cascade Range foothills 

south of the fault. This potential piercing point is suspect 

because many workers (such as Whetten and others, 1980; Brandon and others, 1988; Tabor, 1994) have shown that the Trafton 

unit occurs within a stack of thrust sheets that might have had 

greater lateral extent than present exposures demonstrate. Also, 

this unit could occur farther west than present exposures reveal 

on the south side of the fault in the subsurface of the Everett 

basin. This possible piercing point does, however, probably 

reveal the maximum amount of left-lateral slip possible across 

the zone.

The first obvious record of movement along the Devils 

Mountain fault dates from about 40 Ma, when coarse (clasts as

Fault Mapping, Style, and Rates of Deformation 39
large as 1 m) alluvial-fan conglomerate of the rocks of Bulson 

Creek was deposited adjacent to the fault in the Everett basin 

(Marcus, 1980; Whetten and others, 1988). These conglomerates include silicic volcanic clasts that are petrographically similar to Eocene ash-flow tuff units that crop out north of the fault, 

apparently limiting the amount of post-40 Ma left-lateral slip to 

less than about 30 km. Making the highly questionable assumptions that (1) the rate of left-lateral slip has been constant over 

the last 40 m.y. and (2) 30 km is the maximum amount of left 

slip possible across the structure yields a maximum lateral slip 

rate of 0.75 mm/yr over this time period. There are no similar 

questionable constraints for the possible amount of lateral slip 

across the Strawberry Point and Utsalady Point faults.

Vertical slip rates on the Strawberry Point and Utsalady 

Point faults appear comparable to or higher than those on the 

Devils Mountain fault (fig. 37), the structure that regional fault 

geometry suggests has been the "master fault" within the system. These rates and the evidence for post-80 ka deformation 

from outcrops at Strawberry Point and Utsalady Point (figs. 31, 

35) may indicate that fault kinematics have changed or are in a 

transitional period in which local deformation has most recently 

been concentrated on the Strawberry Point and Utsalady Point 

faults rather than the Devils Mountain fault. Such changes are 

common within complex strike-slip fault systems (ChristieBlick and Biddle, 1985) and are not surprising. The map geometry of the Strawberry Point and Utsalady Point faults also 

indicates that they may be connected at depth and represent 

components of a large transpressional flower structure (Harding 

and others, 1983; Christie-Blick and Biddle, 1985).

Given the evidence for left-lateral slip on the Devils Mountain, Strawberry Point, and Utsalady Point faults, we believe that 

deformation along the northwest-trending southern Whidbey 

Island fault to the south in the eastern Strait of Juan de Fuca 

region (fig. 1) is mainly contractional. This interpretation is consistent with seismic-reflection data from the eastern Strait of 

Juan de Fuca presented in Johnson and others (1996, their fig. 5), 

who showed this structure as a zone of thrust faults. Johnson 

and others (1996) previously suggested a component of right-lateral slip on the southern Whidbey Island fault because it forms a 

part of the crustal boundary on the east and northeast flank of the 

north-moving Coast Range block. Right-lateral slip on this 

crustal boundary is required farther to the south.

Determining the rate of lateral slip across the structural 

zone comprising the Devils Mountain, Strawberry Point, and 

Utsalady Point faults is thus an important problem left unresolved by this investigation. The difficulty in constraining lateral 

slip across this structural zone in large part reflects the recent 

glacial and post-glacial (=20 ka) age of the landscape. Landforms of tectonic origin (such as scarps, pressure ridges, terraces, or offset drainages) have been eroded or filled in by 

repeated pulses of glacial erosion and deposition so that, to a 

large degree, only landforms generated during or after the last 

glaciation are evident. Given the rates described previously and 

the relatively friable nature of many glacial deposits, post-glacial 

landforms may be subtle and have limited preservation potential.

New high-resolution topographic and bathymetric imaging tools may provide the best opportunity for identification 

of tectonic landforms in this landscape. For example, Bucknam and others (1999) and Nelson and others (1999) used a

high-resolution airborne laser terrane mapping survey to identify a post-glacial scarp in a dense forest in the Seattle fault 

zone. New multibeam bathymetric surveys from offshore 

California (Bohannon and others, 1998, among others) have 

shown many tectonic features and offsets associated with 

strike-slip faulting in the seafloor, and such surveys could 

have similar value in the eastern Strait of Juan de Fuca.

Earthquake Hazards

Earthquake Occurrence

The historic earthquake record and recorded seismicity support the interpretation that this region is seismically active. The 

historic record extends back to about 1850 but is of variable 

quality until about 1929 (Ludwin and Qamar, 1995); accurate 

recording of regional earthquakes began in 1969 when the 

Pacific Northwest Seismograph Network was installed. It is 

noteworthy that Victoria (fig. 1) felt a cluster of at least 14 earthquakes between 1859 and 1870 with two having a Mercalli Magnitude Index of VI. The source of these earthquakes is unknown, 

but they might be associated with the nearby Devils Mountain 

fault.

Recorded crustal earthquakes in the study area with magnitude (M) larger than 2.5 are shown in figure 2. Between Haro 

Strait and Lake Cavanaugh (figs. 1, 2), these earthquakes and 

smaller events have clustered at depths of 12-20 km along a 50° 

N. dip projection from the surface trace of the Devils Mountain 

fault (Stanley and others, 1999). If these events are associated 

with the Devils Mountain fault, then its north dip imaged by 

seismic-reflection data decreases slightly with depth. It seems 

equally likely, however, that these events are associated with the 

northwest-trending folds and faults here considered to be related 

to the Devils Mountain fault.

Although fewer earthquakes have been recorded south of 

the Devils Mountain fault in the study area, at least four events 

(M>2.5) may correlate with the Utsalady Point or Strawberry 

Point faults (fig. 2). One M=5.0 event (1/26/1957) occurred 

beneath Oak Harbor within the Utsalady Point fault zone; a 

second M=5.0 event (1/28/1945) has been located near a projection of the Utsalady Point fault zone a few kilometers east 

of Camano Island. Relevant smaller events include a M=3.3 

(11/1/2000) earthquake that occurred adjacent to the Utsalady 

Point zone at Strawberry Point and a M=3.5 event (3/13/1992) 

that occurred in the eastern Strait of Juan de Fuca close to the 

location where the Strawberry Point and Utsalady Point faults 

converge and terminate.

Significance for Earthquake Hazards

Geologic and geophysical data indicate that the Devils 

Mountain fault and the newly recognized Strawberry Point and 

Utsalady Point faults are active structures that represent potential 

earthquake sources. Critical source parameters that contribute to 

earthquake hazard assessment include fault location, length, slip 

rate, and recurrence interval of large earthquakes (Wells and 

Coppersmith, 1994; Frankel and others, 1996). Figures 2, 27,

40 Devils Mountain Fault and Related Structures, Pacific Northwest
and 28 show the locations of these faults in the eastern Strait of 

Juan de Fuca and northern Puget Lowland. Large earthquakes 

generated in this source zone could affect broad areas and large 

populations in the Puget and Fraser Lowlands and on southern 

Vancouver Island. Urban centers proximal to the fault zone 

include Victoria (<5 km from the potential earthquake source), 

Everett (40 km), Bellingham (45 km), Seattle (75 km), and Vancouver (75 km) (fig. 1). Moreover, significant parts of this 

region are underlain by large deltas and sedimentary basins, 

areas that are particularly prone to strong ground motion.

Using the relationship between rupture length and moment 

magnitude (M) defined by Wells and Coppersmith (1994) and 

assuming that the entire mapped length of the Devils Mountain 

fault (=125 km) could rupture in one event, an earthquake of 

M=7.5 could be generated on this structure. This investigation 

recognized two possible segment boundaries on the Devils 

Mountain fault that might limit rupture during a large earthquake; however, no data are available to test this segmentation 

hypothesis.

Based on their minimum lengths (-22-28 km), earthquakes of M=6.7 could occur on the Strawberry Point and 

Utsalady Point faults. Each of these faults breaks into several 

splays east of Whidbey Island (figs. 27, 28), but this bifurcation is unlikely to limit earthquake rupture—it should be 

assumed that one or more of these splays merges with a single 

trace of each fault at depth. The Strawberry Point and Utsalady Point faults might also connect at depth, a scenario in 

which surface deformation could occur along each fault in the 

same earthquake.

Because incorporating slip rates on faults is an important 

component of seismic hazard assessment and mapping (Frankel 

and others, 1996, among others), we presented and explained 

our best information for each fault (fig. 38). We consider these 

variably speculative rates as starting points and expect that they 

will be refined and modified in the future as more data accumulate. As stressed previously ("Fault Mapping, Style, and Rates 

of Deformation"), determining slip rates across the zone is 

problematic because (1) data constrain only the vertical component of slip on oblique-slip faults, and (2) both the sense of vertical slip and the vertical slip rate vary along the strike of the 

Strawberry Point and Utsalady Point faults. Our best estimate 

for vertical slip on the Devils Mountain fault is 0.05-0.31 mm/ 

yr (fig. 38). We suspect that the lateral component of slip may 

be larger, but probably no more than about 0.75 mm/yr based on 

regional geologic considerations. We think reasonable minimum vertical slip rates for both the Strawberry Point and Utsalady Point faults are about 0.2 mm/yr and that field evidence for 

contractional deformation suggests an additional significant lateral component of slip. Thus, the cumulative slip rate on the 

three main faults of this complex northern Puget Lowland structural zone probably exceeds 0.5 mm/yr and could be much 

larger. By comparison, Johnson and others (1999) suggested 

about 0.7-1.1 mm/yr of slip on faults of the Seattle fault zone.

Paleoseismological investigations of the Devils Mountain 

fault, Strawberry Point, and Utsalady Point faults are lacking; 

hence no information exists on earthquake recurrence interval. 

Williams and Hutch inson (2000) reported evidence for four late 

Holocene tsunami deposits from a marsh near Swantown on the 

northwest coast of Whidbey Island (fig. 27). The overlap in age

between the two youngest deposits (1,160-1,350 and 1,400- 

1,700 14C yr B.P.) and inferred great earthquakes at the Cascadia 

plate boundary, about 250 km to the west, suggests that they 

were emplaced by tsunamis from this source area. The two older 

layers (1,810-2,060 and 1,830-2,020 14C yr B.P.) do not correlate with plate boundary events and may be products of earthquakes on local faults in the eastern Strait of Juan de Fuca, such 

as the Devils Mountain, Strawberry Point, and Utsalady Point 

faults. In another investigation, Bourgeois and Johnson (2001) 

found evidence of three liquefaction events in post A.D. 900 deltaic deposits from the Snohomish River delta, 35 km south of the 

Devils Mountain fault. One of these events has been correlated 

with a strong earthquake on the Seattle fault, but the others have 

not been correlated with known events; the Devils Mountain, 

Strawberry Point, and Utsalady Point faults are nearby possible 

earthquake sources. Thorson (1996) speculated that earthquake 

cycles and recurrence intervals for Puget Lowland faults may 

have been perturbed during glacial loading and rebound, but this 

hypothesis cannot be tested with available data for any fault in 

the Puget Lowland.

Conclusions

Marine seismic-reflection surveys, aeromagnetic mapping, 

coastal exposures of Pleistocene strata, and lithologic logs of 

water wells provide important information for assessing the 

active tectonics of the northern Puget Lowland and eastern Strait 

of Juan de Fuca region of the Pacific Northwest. These data 

indicate that the Devils Mountain fault and the newly recognized 

Strawberry Point and Utsalady Point faults are active structures 

and represent potential earthquake sources.

The north-dipping (45°-75°) Devils Mountain fault extends 

westward for more than 125 km from the Cascade Range foothills to Vancouver Island, where it may merge with the Leech 

River and (or) San Juan faults. The Devils Mountain fault is 

bounded by northwest-trending en-echelon folds and faults, a 

map pattern strongly suggesting that it is a left-lateral, obliqueslip, transpressional "master fault." Aeromagnetic anomalies 

coincide with both the trace of the Devils Mountain fault and enechelon structures. Quaternary strata are deformed on nearly all 

seismic-reflection profiles, and onshore subsurface data suggest 

offset of upper Pleistocene strata.

The west-northwest-trending, subvertical Strawberry Point 

fault cuts across northern Whidbey Island and has a minimum 

length of 22 km. On the west coast of Whidbey Island and in the 

Strait of Juan de Fuca, the fault has south-side-up offset and 

forms the northern boundary of an uplift of pre-Tertiary basement rock. The fault bifurcates into a 2-km-wide zone as it 

crosses Whidbey Island. A stratigraphic correlation diagram 

based on outcrops and water-well logs suggests that each of the 

four fault splays within this zone has apparent north-side-up offset. Outcrops at Strawberry Point also indicate considerable 

shortening (dips as steep as 45°) in upper Pleistocene strata 

between the faults. The vertical fault trace, reversal of offset, 

and evidence for associated contractional deformation indicate 

that the Strawberry Point fault is also an oblique-slip, transpressional fault.

Conclusions 41
The northwest-trending, subvertical Utsalady Point fault 

similarly cuts across northern Whidbey Island and has a minimum length of 28 km. It forms the southern margin of the 

uplifted pre-Tertiary basement block on the west coast of Whidbey Island, where it has north-side-up offset. Offshore seismicreflection data from east of Whidbey Island indicate that it also 

bifurcates into a broad (1.5 km) zone of several splays to the 

east. Onshore outcrops and subsurface logs from Camano Island 

indicate a probable reversal of offset (to south side up) along the 

zone; they also reveal both faulting and folding (dips as steep as 

24°) in upper Pleistocene strata. As with the Strawberry Point 

fault, the vertical fault trace(s), reversal of offset, and evidence 

for associated contractional deformation suggest that the Utsalady Point fault is an oblique-slip, transpressional fault.

Collectively, the Devils Mountain, Strawberry Point, and 

Utsalady Point faults represent an active complex, distributed, 

transpressional deformation zone. The cumulative slip rate on 

three main faults of this zone probably exceeds 0.5 mm/yr and 

could be much larger.

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