U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1776–D

Studies by the U.S. Geological Survey in Alaska, 2008–2009

Location and Extent of Tertiary Structures in Cook Inlet 

Basin, Alaska, and Mantle Dynamics that Focus 

Deformation and Subsidence
Front Cover

Oil platforms in Cook Inlet mark the crest of subsurface anticlines. 

This view is from Granite Point looking to the southwest down Cook 

Inlet. Illiamna Volcano lies in the far distance on the right.
Location and Extent of Tertiary Structures 

in Cook Inlet Basin, Alaska, and Mantle 

Dynamics that Focus Deformation and 

Subsidence

By Peter J. Haeussler and Richard W. Saltus

Professional Paper 1776–D

U.S. Department of the Interior

U.S. Geological Survey 

Studies by the U.S. Geological Survey in Alaska, 2008–2009
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

This report and any updates to it are available online at:

http://pubs.usgs.gov/pp/1776/d

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment:

World Wide Web: http://www.usgs.gov/

Telephone: 1-888-ASK-USGS

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.

Although this report is in the public domain, it may contain copyrighted materials that 

are noted in the text. Permission to reproduce those items must be secured from 

the individual copyright owners.

Library of Congress Cataloging-in-Publication Data

Suggested citation:

Haeussler, P.J., and Saltus, R.W., 2011, Location and extent of Tertiary structures in Cook Inlet Basin, Alaska, and 

mantle dynamics that focus deformation and subsidence, in Dumoulin, J.A., and Galloway, J.P., eds., Studies 

by the U.S. Geological Survey in Alaska 2008–2009: U.S. Geological Survey Professional Paper 1776–D, p. 26.
iii

Contents

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

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

Geologic Overview.........................................................................................................................................1

Data Sources..................................................................................................................................................4

Tertiary Structures in Cook Inlet..................................................................................................................8

Faults.....................................................................................................................................................10

Castle Mountain Fault ...............................................................................................................10

Wishbone Hill Faults..................................................................................................................10

Lake Clark Fault..........................................................................................................................10

Bruin Bay Fault...........................................................................................................................10

Border Ranges Fault..................................................................................................................11

Little Susitna River Escarpment ..............................................................................................11

Capps Glacier Fault ...................................................................................................................12

Folds......................................................................................................................................................12

Castle Mountain Fault Anticline ..............................................................................................12

Big Lake North............................................................................................................................12

Pitman .........................................................................................................................................12

Wasilla State No. 1—Needham ..............................................................................................12

Fish Creek—Knik Arm No. 2.....................................................................................................12

Lorraine—Knik Arm State No. 1..............................................................................................12

Middle Lake Unit No. 1—Alaska Gulf No. 1...........................................................................12

Figure Eight Unit No. 1...............................................................................................................13

Bell Island....................................................................................................................................13

Stump Island...............................................................................................................................13

Ivan River.....................................................................................................................................13

Lewis River—Pretty Creek.......................................................................................................13

Beluga River................................................................................................................................13

North Cook Inlet—SRS .............................................................................................................13

Moquawkie .................................................................................................................................13

Nicolai Creek ..............................................................................................................................14

Granite Point...............................................................................................................................14

North Trading Bay......................................................................................................................14

Trading Bay Unit—McArthur River Field...............................................................................14

West McArtur River...................................................................................................................14

McArthur River—Redoubt Shoal............................................................................................14

Middle Ground Shoal ................................................................................................................15

Beaver Creek..............................................................................................................................15

Swanson River............................................................................................................................15

Kenai Lowlands..........................................................................................................................15

Turnagain Arm............................................................................................................................15

Pincher Creek.............................................................................................................................16

Swan Lake...................................................................................................................................16

West Fork ....................................................................................................................................16
iv

Sterling.........................................................................................................................................16

Kenai—Cannery Loop...............................................................................................................16

Kasilof .........................................................................................................................................16

Falls Creek...................................................................................................................................16

Deep Creek—Happy Valley......................................................................................................16

Kalgin Island South....................................................................................................................16

Starichkof (Cosmopolitan)........................................................................................................17

North Fork—Nikolaevsk ...........................................................................................................17

Iniskin .........................................................................................................................................17

Chinitna Point West...................................................................................................................17

Chinitna Point East.....................................................................................................................17

Hawk............................................................................................................................................17

East of Raven..............................................................................................................................18

West of LCI COST No. 1.............................................................................................................18

Starichkof Southwest................................................................................................................18

Kachemak....................................................................................................................................18

Seldovia Arch .............................................................................................................................18

Augustine East............................................................................................................................18

Augustine Far East.....................................................................................................................18

Lower Cook Inlet .................................................................................................................................18

Discussion.....................................................................................................................................................18

Concluding Remarks....................................................................................................................................22

Acknowledgments.......................................................................................................................................22

References....................................................................................................................................................22

Figures

1. A, Location of Tertiary anticlines and faults in Cook Inlet Basin, Alaska; 

 B, Seismic-reflection profiles used for the interpretations in this report..........................2

2. Three views of the stratigraphic column for Tertiary Cook Inlet Basin sediments..........5

3. Magnetic maps of the Cook Inlet region, with faults and folds shown as on 

 figure 1A, listed in table 1, and discussed in the text............................................................6

4. Detailed two-dimensional magnetic model along part of seismic line 285-1, 

 Cook Inlet Basin, Alaska............................................................................................................8

5. Maps showing the relationship of the Cook Inlet Basin to the subducted

 Yakutatterrane slab, Alaska ....................................................................................................20

6. Map showing Yakutat slab and its inferred effect on mantle fluid flow ..........................21

Tables

1. Anticlines in the Cook Inlet Basin, Alaska...............................................................................9
Abstract 

This report is a new compilation of the location and 

extent of folds and faults in Cook Inlet Basin, Alaska. Data 

sources are previously published maps, well locations, and 

seismic-reflection data. We also utilize interpretation of new 

aeromagnetic data and some proprietary seismic-reflection 

data. Some structures are remarkably well displayed on 

frequency-filtered aeromagnetic maps, which are a useful tool 

for constraining the length of some structures. Most anticlines 

in and around the basin have at least shows of oil or gas, and 

some structures are considered to be seismically active. The 

new map better displays the pattern of faulting and folding. 

Deformation is greatest in upper Cook Inlet, where structures 

are oriented slightly counterclockwise of the basin bounding 

faults. The north ends of these structures bend to the northeast, 

which gives a pattern consistent with right-transpressional 

deformation. 

Subduction of the buoyant Yakutat microplate likely 

caused deformation to be focused preferentially in upper Cook 

Inlet. The upper Cook Inlet region has both the highest degree 

of shortening and the deepest part of the Neogene basin. This 

forearc region has a long-wavelength magnetic high, a large 

isostatic gravity low, high conductivity in the lower mantle, 

low p-wave velocity (Vp ), and a high p-wave to shear-wave 

velocity ratio (Vp /Vs). These data suggest that fluids in the 

mantle wedge caused serpentinization of mafic rocks, which 

may, at least in part, contribute to the long-wavelength 

magnetic anomaly. This area lies adjacent to the subducting 

and buoyant Yakutat microplate slab. We suggest the buoyant 

Yakutat slab acts much like a squeegee to focus mantle-wedge 

fluid flow at the margins of the buoyant slab. Such lateral flow 

is consistent with observed shear-wave splitting directions. 

The additional fluid in the adjacent mantle wedge reduces the 

wedge viscosity and allows greater corner flow. This results in 

focused subsidence, deformation, and gravity anomalies in the 

forearc region. 

Introduction

The Cook Inlet basin is the longest-producing oil and 

gas province in Alaska. The discovery well was drilled in 

1957 in the Swanson River field. All known traps of oil and 

gas in the basin are contraction-related features (for example, 

Kirschner and Lyon, 1973; Boss and others, 1976; Magoon 

and Claypool, 1981) and many of these are fault-cored folds 

(Haeussler and others, 2000; Bruhn and Haeussler, 2006). 

Therefore, information about the location and extent of the 

anticlines in Cook Inlet is critical for assessing the oil and 

gas resources in the region. Moreover, stratigraphic evidence 

indicates at least some folds are growing (Haeussler and 

others, 2000; Bruhn and Haeussler, 2006). Thus, knowledge of 

the location and extent of the structures also aids in assessing 

the earthquake hazard in the region. The purpose of this 

report is to provide an updated map of the Tertiary anticlines 

in the basin and the significant faults around the margins 

of the basin, and to try and understand the distribution of 

deformation in the basin. 

The structural style and the tightness of the folds are 

variable. All folds imaged clearly on available seismicreflection data have faults associated with, or coring, the 

structures. However, not all of the mapped folds are oil and 

gas traps, as drilling data demonstrate, and not all of the 

mapped structures are seismically active. 

Geologic Overview

Cook Inlet basin is a northeast-trending forearc basin 

located between the Chugach and Kenai Mountains to the 

southeast, and the Alaska Range and Aleutian volcanic arc 

to the northwest (Dickinson and Seely, 1979) (fig. 1). The 

subducting Pacific plate is 50-60 km beneath the center of the 

basin. Three major fault zones define the basin margins—the 

Castle Mountain fault on the north, the Bruin Bay fault to the 

northwest, and the Border Ranges fault along the southeast 

side (Grantz, 1966; Pavlis, 1982). Most rocks in the Chugach 

and Kenai Mountains are part of a vast Mesozoic and early 

Tertiary accretionary complex that rims the southern Alaska 

Location and Extent of Tertiary Structures in Cook Inlet 

Basin, Alaska, and Mantle Dynamics that Focus 

Deformation and Subsidence

By Peter J. Haeussler1 and Richard W. Saltus2

1

U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508.

2

U.S. Geological Survey, Denver Federal Center, MS964, Denver, CO 

80225.

Studies by the U.S. Geological Survey in Alaska, 2008–2009

Edited by Julie A. Dumoulin and John P. Galloway

U.S. Geological Survey Professional Paper 1776–D
2 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

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F

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F

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F F F

F

F

F

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F

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3

10

9

8

F

7

5

4

14

26

19

12

27 18

23

25

17

16

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13

30

24

35

32

33

36

B

C

D

E

6

1

20

21 22

28

29

31

34

37

42

41

38

39

45

51

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46

49

50 54

53

52

55

40

43 44

48

G

11

154°W 152°W 150°W

59°N

60°N

61°N

Explanation

bedrock

unconsolidated

!( wells - surface

!( wells - bottom

F anticlines-certain

F anticlines-approx

F anticlines-inferred

S homoclines

fault - certain

fault - queried

# volcanoes

0 15 30 60 Kilometers

Swanson River

Cook Inlet

Kenai Mountains

Alaska Range

Aleutian Range

Castle Mountain fault

Border Ranges fault

Bruin Bay fault

Lake Clark fault

Chugach

Mountains

West Foreland

Wishbone Hill

Anchorage

Homer

Seldovia

Middle

Ground

Shoal

North Cook Inlet

Beluga River

Spurr

Volcano

Beluga River 

fold

Trading

Bay Chuitna

River

Susitna River

Little Susitna River

Kenai Lowlands

#

#

#

#

Kenai

Lewis River 

No.1 well

LCI COST

No.1 well

Redoubt 

Volcano

Illiamna

Volcano

Augustine

Volcano

Douglas

Volcano

Fourpeaked

Volcano

Hayes

Volcano Hatcher Pass

Kalgin

Island

#

Nikiski

A

Figure 1. A, Location of Tertiary anticlines and faults in Cook Inlet Basin, Alaska. Faults are lettered and folds are numbered and refer 

to descriptions in the text and in table 1. Synclines are not mapped. All folds and faults are concealed by surficial deposits, but the 

map legend uses exposed fold symbols for greater clarity. B, Seismic-reflection profiles used for the interpretations in this report. Only 

previously published lines are labeled with their number. The data source of the profile is listed in front of the line number as follows: 

FM, Fisher and Magoon (1978); HBP, ARCO data published in Haeussler, Bruhn, and Pratt (2000); BH, ARCO data published in Bruhn and 

Haeussler (2006); NP, not published ARCO data; WG, Western Geophysical data in Fisher and others (2008, 2010). 
Geologic Overview 3

Figure 1.—Continued. 

#

#

#

#

#

#

#

HPB 285-1

HBP 203-140

HBP BL93-04

HBP 203-210CD

BH 203-290

FM 757

FM 756

FM 754

FM 752

FM 755

FM 753

FM 751

154°W 152°W 150°W

59°N

60°N

61°N

Explanation

bedrock

unconsolidated

faults

# volcanos

seismic lines

HBP (ARCO)

BH (ARCO)

NP (ARCO)

FM

WG

0 15 30 60 Kilometers

Cook Inlet

Kenai Mountains

Alaska Range

Aleutian Range

Castle Mountain fault

Border Ranges fault

Bruin Bay fault

Lake Clark fault

Chugach

Mountains

Anchorage

Homer

Seldovia

Spurr

Volcano

Kenai Lowlands

#

#

#

#

Kenai

Redoubt 

Volcano

Illiamna

Volcano

Augustine

Volcano

Douglas

Volcano

Fourpeaked

Volcano

Hayes

Volcano

seismic line 

on Figure 4

B
4 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

margin, and rocks of much of the Alaska Range are either 

arc-derived volcanic or plutonic rocks (Magoon and others, 

1976; Plafker and others, 1994). The Tertiary basin fill 

unconformably overlies both the accretionary complex rocks 

to the southeast and the volcanic or plutonic rocks to the 

northwest (Calderwood and Fackler, 1972). Triassic through 

Upper Cretaceous stratified rocks are inferred to underlie the 

Tertiary basin fill. These stratified rocks are thought to be 

related to those that crop out northwest of the Inlet (Magoon 

and Egbert, 1986).

Tertiary deformation of Cook Inlet basin started 

between Eocene and early Oligocene time (Barnes and 

Payne, 1956; Clardy, 1974; Stamatakos and others, 1988; 

Little and Naeser, 1989; Little, 1990). Deformation in 

upper Cook Inlet is transpressional and resulted in folds, 

faults, and eroded horst blocks. Adjacent grabens filled 

with synfaulted fluvial deposits, and deformation continued 

until late Miocene to Holocene time (Kirschner and Lyon, 

1973; Calderwood and Fackler, 1972; Haeussler and others, 

2000). Most deformation appears to be Pliocene to Recent 

(Haeussler and others, 2000; Bruhn and Haeussler, 2006) 

and resulted in folds that are doubly plunging, asymmetric, 

and discontinuous. Kirschner and Lyon (1973) first inferred 

a genetic link between regional strike-slip faulting and 

folding after observing that fold hinge lines curve towards 

the strike of the right-lateral Castle Mountain fault at the 

northeastern end of the basin. Bruhn and Haeussler (2006) 

proposed that the basin is being squeezed between the Bruin 

Bay and Border Ranges faults by right-lateral faulting along 

the Castle Mountain fault. This geometry results in righttranspressional deformation within the basin.

The Tertiary strata in Cook Inlet were deposited 

predominantly by alluvial processes, but locally with tidal 

influence (Calderwood and Fackler, 1972; Flores and 

Stricker, 1993; Flores and others, 1994, 1997, 1999). Most 

sediment was derived from erosion of the Alaska Range 

and Chugach Mountains (Kirschner and Lyon, 1973). The 

youngest sediments in the basin are unnamed Quaternary 

deposits, which consist of both glacial and alluvial sediments 

(fig. 2; Schmoll and others, 1984). The thickness of the 

Quaternary section is undetermined, partly because no 

exposures reveal the entire Quaternary section, and partly 

because oil companies, which have spent the most effort 

examining the strata of Cook Inlet, have focused on older 

oil-bearing strata. As a result, the Quaternary sediments 

commonly are lumped with the uppermost part of the 

underlying Sterling Formation, which is Pliocene (ReininkSmith, 1995; Dallegge, 2002; Dallegge and Layer, 2004). 

The Sterling, Beluga, and Tyonek Formations all consist 

of varying amounts of sandstone, siltstone, shale, and coal, 

and they likely have interfingering relationships (Swenson, 

1997). Dallegge (2002) concluded that the Sterling and 

Beluga Formations are, at least in part, time equivalent 

based upon 40Ar/39Ar dating of tephras. Both the older ‘layercake’ and the newer time-transgressive views of Cook Inlet 

stratigraphy are shown in figure 2. 

Data Sources

Some structures in Cook Inlet are much better known 

than others, the best-documented are those with drilling 

related to oil and gas exploration and production. Structural 

anticlines are the traps for every known oil and gas field in 

Cook Inlet (for example, Magoon and Claypool, 1981). The 

Alaska Oil and Gas Conservation Commission (AOGCC) 

publishes an Annual Report on the status of production from 

individual fields, and some of these reports contain isopach 

maps showing the location and extent of a particular structure. 

The annual report is available online at http://doa.alaska.gov/

ogc/annual/annindex.html (last accessed January 25, 2011). 

However, the structure contour maps, provided to the AOGCC 

by the field operators, rarely change from year to year. Thus, 

it does not matter which version of a recent Annual Report 

is examined—they all have essentially the same data. We 

used the 2002 AOGCC report. The compilation of Magoon 

and others (1976) remains a seminal work on the geology of 

Cook Inlet, and they had access to some unpublished industry 

data. Some structures have been illustrated by Fisher and 

Magoon (1978), Winkler (1992), Haeussler and others (2000), 

Bruhn and Haeussler (2006), and Fisher and others (2008, 

2010), however, other structures are poorly documented. For 

example, a few of the structures on the map by Magoon and 

others (1976) apparently were defined by linear trends of 

dry and abandoned wildcat wells. In some cases, only one or 

two wells define the location of a structure, and there is no 

publically available corroborating evidence for its existence. 

Fisher and Magoon (1978) collected and analyzed seismicreflection data from a coarse grid in lower Cook Inlet. (The 

term “lower Cook Inlet” is used for the region southwest of 

West Foreland (fig. 1), and the term “upper Cook Inlet” is used 

for the region northeast of West Foreland). Fisher and others 

(2008, 2010) interpreted some additional seismic-reflection 

data from the west side of lower Cook Inlet. Where their 

new maps overlap those of Fisher and Magoon (1978), they 

made some minor changes in their interpretation of the region 

northeast of Augustine Island. 

We also utilize a high-resolution aeromagnetic dataset 

(fig. 3; Saltus and others, 2001; United States Geological 

Survey, 2002). High-frequency filtered aeromagnetic data 

image some folds well, because some strata are sufficiently 

magnetic to create anomalies, particularly in the upper Sterling 

Formation and Quaternary section. When these strata were 

folded, they produced linear anomalies along or parallel to 

the limbs of folds (Saltus and others, 2001). One example of 

a fold with flanking magnetic anomalies is a syncline adjacent 

to the Beluga River fold (fig. 4). Modeling indicates that one 

or more of the horizons imaged on seismic reflection are more 

magnetic, which produces magnetic anomalies where the 

relatively magnetic bed(s) are up-turned and truncated on the 

limbs of the fold. The Middle Ground Shoal, Granite Point, 

North Cook Inlet, and Beluga River anticlines, for example, 

are imaged by the aeromagnetic data (fig. 3 folds 22, 17, 14, 

and 13, respectively). Other folds are not imaged with the 
Data Sources 5

aeromagnetic data, such as the Kenai and Sterling fields (fig. 

3 folds 31 and 30, respectively). Therefore, the absence of 

an aeromagnetic signature does not necessarily mean that a 

fold is absent, but rather that the magnetic strata that would 

produce magnetic anomalies are absent. The position of the 

magnetic marker bed(s) relative to the crest of the anticline 

as well as the thickness and magnetic intensity of the bed(s), 

also will cause variation in the geometry and amplitude of the 

magnetic anomalies associated with an individual structure.

Although the source of magnetic layering within the 

Sterling Formation and Quaternary section has not been 

studied sufficiently, outcrop measurements of magnetic 

susceptibility in these units (Altstatt and others, 2002; 

Saltus and Haeussler, 2004) show values consistent with the 

aeromagnetic anomalies produced. It is our judgment that 

these layers are the result of magnetite concentration in highenergy deposits. Bruhn and Neuffer (unpublished report 2002) 

found magnetite, hematite, and pyrrhotite in two subsurface 

samples of the Tyonek Formation. They also attempted to 

determine the magnetic carrier of one of the samples through 

thermal demagnetization of an isothermal remanence. The 

sample had only 13 percent of its initial remanence at 351°C, 

which suggests that pyrrhotite is a likely carrier of some 

of the remanence. Above this temperature, the sample was 

chemically altered and increased its remanent intensity. 

Nonetheless, the present-day beach sand at Nikiski, for 

example, is very dark, magnetic (values generally greater than 

5×10-3 SI), and contains magnetite. This observation indicates 

COOK

INLET

?

West Foreland

Formation

Hemlock

Conglomerate

Tyonek Fm.

Beluga Fm.

Sterling Fm.

Bell Is. s/s

Swenson, 1997

Unnamed

unnamed - till and 

alluvium

ERA

System

Time

Ma Stage Series

COOK INLET

Lower Upper

Max

Thickness

meters

Clamgulchian

Homerian

Seldovian

Angoonian

Unnamed

Kummerian

Ravenian

Fultonian

Franklinian

Unnamed

PLIOCENE MIOCENE

QUATERNARY

OLIGOCENE EOCENE PALEO- CENE

Sterling

Fm.

Beluga

Fm.

Tyonek

Fm.

Hemlock

Conglomerate

1. Bell Island 

Sandstone

2. Tsadaka Fm.

West

Foreland

Fm.

Wishbone

Fm.

Arkose

Ridge Fm.

Chickaloon Fm.

1000

450

2135

1525

1850

K E N A I G R O U P

?

Pleistocene

Holocene

T E R T I A R Y

C E N O Z O I C

60

55

50

45

40

35

25

20

30

5

10

15

4

2

1

3

Reinink-Smith, 1995; Fisher and Magoon, 1978

unnamed - till and 

alluvium

PLIOCENE MIOCENE

OLIGOCENE EOCENE PALEO- CENE Pleistocene Holocene Series

Sterling

Formation

Beluga Formation

Tyonek Formation

Hemlock

Conglomerate

Early Pliocene

Homerian Stage

Upper half of Miocene

Late early Miocene

or middle Miocene

Seldovian Stage

Early Miocene

Late Oligocene

Clamgulchian Stage

? ?

? ?

? ?

unconformity

unconformity

unconformity

unnamed alluvium

and glacial deposits

KENAI

PENINSULA Stage

Older Tertiary

Rocks

Flores and Stricker, 1993

erosion or nondeposition

Figure 2.

NEOGENE

NEOGENE

Figure 2. Three views of the stratigraphic column for Tertiary Cook Inlet Basin sediments, from Dallegge and Layer (2004). 40Ar/ 39Ar dating by Dallegge (2002) and Dallegge and Layer (2004) support the time-transgressive interpretation of Cook Inlet stratigraphy of Swenson 

(1997).
6 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

S

S

F

F

F

F

F

F

F

F

F

F

F

F F

F

F

F

F

F

F

F F F

F

F

F

F

F F

F F

F

F

F

F F

F

F

F

F

F F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F F

F

F

F

F

#

#

#

#

#

#

#

154°W 152°W 150°W

59°N

60°N

61°N

Explanation

faults

# volcanos

F anticlines-certain

F anticlines-approx

F anticlines-inferred

S homoclines

0 15 30 60 Kilometers

-465 .0 -340 .0 -285 .0 -255 .0 -235 .0 -220 .0 -200 .0 -190 .0 -175 .0 -165 .0 -155 .0 -140 .0 -120 .0 -90.0 -40.0 10.0 40.0 75.0 110 .0 185 .0 365 .0

Totalfield Magnetic Intensity nT

Kenai Mountains

Alaska Range

Aleutian Range

Castle Mountain fault

Border Ranges fault

Bruin Bay fault

Lake Clark fault

Chugach

Mountains

Anchorage

Homer

Seldovia

Spurr

Volcano

#

#

#

#

Redoubt Kenai

Volcano

Illiamna

Volcano

Augustine

Volcano

Douglas

Volcano

Fourpeaked

Volcano

Hayes

Volcano

A

Figure 3. Magnetic maps of the Cook Inlet region, with faults and folds shown as on figure 1A, listed in table 1, and discussed in the text. 

High resolution data from Saltus and others (2001) and U.S. Geological Survey (2002). A, Total field magnetic intensity map. Low resolution regional data from Saltus and Simmons (1997). B, Band-pass filtered magnetic intensity map for region of high resolution data. Data 

is the “bandpass filter 1” from Saltus and others (2001). 
Data Sources 7

S

S

F

F

F

F

F

F

F

F

F

F

F

F F

F

F

F

F

F

F

F F F

F

F

F

F

F F

F F

F

F

F

F F

F

F

F

F

F F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F F

F

F

F

F

#

#

#

#

#

#

#

154°W 152°W 150°W

59°N

60°N

61°N

Explanation

faults

# volcanos

F anticlines-certain

F anticlines-approx

F anticlines-inferred

S homoclines

0 25 50 12.5 Kilometers

-68.1 -23.0 -11.6 -7.1 -5.7 -4.8 -4.0 -3.4 -2.9 -2.4 -2.0 -1.6 -1.3 -1.0 -0.7 -0.4 -0.1 0.1 0.4 0.7 1.0 1.4 1.7 2.1 2.6 3.1 3.8 4.6 5.7 7.1 9.3 12.7 18.9 30.6 64.0

Bandpass filtered magnetic intensit y nT

Kenai Mountains

Alaska Range

Aleutian Range

Castle Mountain fault

Border Ranges fault

Bruin Bay fault

Lake Clark fault

Chugach

Mountains

Anchorage

Homer

Seldovia

Spurr

Volcano

#

#

#

#

Redoubt Kenai

Volcano

Illiamna

Volcano

Augustine

Volcano

Douglas

Volcano

Fourpeaked

Volcano

Hayes

Volcano

B

Figure 3.—Continued.
8 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

that magnetite is a common magnetic mineral in the Tertiary 

sediments, and it likely is responsible for most of the induced 

magnetization.

Three seismic-reflection datasets constrain our 

interpretation (fig. 1). Our work focused on upper Cook Inlet, 

an area for which little previous work has been published. 

We examined seismic-reflection profiles donated to the U.S. 

Geological Survey and University of Utah by ARCO Alaska, 

Inc. Some of these profiles were published in Haeussler and 

others (2000), and there is a line drawing of another profile 

in Bruhn and Haeussler (2006). Additional unpublished lines 

were used to guide interpretation. Fisher and Magoon (1978) 

first interpreted and published seismic data from lower Cook 

Inlet. Recently, Fisher and others (2008, 2010) interpreted and 

published another dataset from lower Cook Inlet. These papers 

are focused on deformation of the western part of lower Cook 

Inlet. Where there are differences between the interpretations 

of Fisher and Magoon (1978) and Fisher and others (2008, 

2010), we adopt the latter. The high-resolution aeromagnetic 

survey overlaps the northernmost lines of their seismicreflection profiles, but is not particularly helpful in discerning 

structures. Therefore, in the absence of additional data we 

use their interpretations almost without modification for the 

southern part of the study area. 

Tertiary Structures in Cook Inlet

A summary of the characteristics of Tertiary structures 

in Cook Inlet is shown on figure 1 and listed in table 1. 

Structures also are shown on aeromagnetic maps in figure 3. 

In the following section, we discuss aspects of each structure 

that fail to fit neatly or concisely into the data table. In 

particular, we discuss and evaluate the quality of the data that 

document the lesser-known structures. In general, the more 

poorly defined structures are inferred by tracing anticline 

crests between the locations of wildcat wells. In the absence 

of any other data this may be reasonable, but one can be much 

Figure 4.

0

1,000

2,000

0

DEPTH, IN METERS

DISTANCE, IN METERS

5,000

Figure 4. Detailed two-dimensional magnetic model along part of seismic line 285-1, Cook Inlet Basin, Alaska, 

from Saltus and others (2001). Seismic line location is shown on figure 1B. The magnetic layers generally follow the seismic layering and have a susceptibility of 19 × 10-3 SI. This shows that prominent high-frequency 

magnetic anomalies can be a product of folding. 
Tertiary Structures in Cook Inlet 9

Table 1. Anticlines in the Cook Inlet Basin, Alaska.

[AOGCC, Alaska Oil and Gas Conservation Commission; DOG, Alaska Department of Oil and Gas]

Name of structure and label on 

Figure 1A

Length, in 

Kilometers Evidence for structure Comment on shape of structure Data source

Confidence of 

existence of 

structure

(1) Castle Mountain fault anticline 34 seismic, aeromag, wildcat wells cored by steeply dipping faults, 4-km wide Saltus and others, 2001; Haeussler and others 2000; 

this study

certain

(2) Big Lake North 18 seismic very broad antiform this study certain

(3) Pitman 63 seismic, four wildcat wells very broad antiform Kirschner and Lyon, 1973; Magoon and others, 

1976; this study

certain

(4)Wasilla St. No. 1-Needham 46 three wildcat wells unknown Magoon and others, 1976 poor

(5) Fish Creek - Knik Arm No. 2 16 aeromag, two wildcat wells unknown this study poor

(6) Lorraine-Knik Arm State No. 1 18 two wildcat wells unknown Magoon and others, 1976; this study poor

(7) Middle Lake Unit No. 1-Alaska 

Gulf No. 1

23 four wildcat well, trend inferred from aeromag unknown Magoon and others, 1976; this study fair

(8) Figure Eight Unit No. 1 10 one wildcat well, trend inferred from aeromag unknown Magoon and others, 1976 poor

(9) Bell Island 18 one wildcat well, trend inferred from aeromag unknown Magoon and others, 1976 poor

(10) Stump Lake 7 gas field, trend inferred from aeromag doubly plunging elongate dome AOGCC; this study certain

(11) Ivan River( 11 gas field, trend inferred from aeromag doubly plunging antiform AOGCC; this study certain

(12) Lewis River 13 gas field, trend inferred from aeromag antiform between two thrusts AOGCC; this study certain

(13) Beluga River 22 gas field, seismic, aeromag doubly plunging curving fault-cored fold AOGCC; Haeussler and others, 2000; Saltus and 

others, 2001; this study

certain

(14) North Cook Inlet-SRS 40 gas field, seismic, aeromag fault-cored fold; limb dip to 70° AOGCC; Haeussler and others, 2000; Saltus and 

others, 2001; this study

certain

(15) Moquawkie 12 oil field unknown AOGCC fair

(16) Nicolai Creek 7 gas field unknown AOGCC good

(17) Granite Point 15 oil field fault-cored fold; limb dip to 90° AOGCC; Haeussler and others, 2000; Saltus and 

others, 2001; this study

certain

(18) North Trading Bay 3 oil field unknown AOGCC certain

(19) Trading Bay 9 oil field, seismic doubly plunging curving assymetric 

anticline; one cross fault

AOGCC; this study certain

(20) West McArthur River 18 oil field; seismic fault-cored fold AOGCC; this study certain

(21) McArthur River-Redoubt Shoal 27 oil field, seismic, aeromag doubly plunging broad antiform, cross faults AOGCC; this study certain

(22) Middle Ground Shoal 44 oil field, seismic, aeromag fault-cored fold AOGCC; this study certain

(23) Beaver Creek 14 gas field, trend inferred from aeromag smaller anticline on margin of Swanson 

River fold, tight syncline in between

AGOCC; Magoon and others, 1976; this study certain

(24) Swanson River 40 oil field, aeromag anticline with numerous cross faults AGOCC; Magoon and others, 1976; this study certain

(25) Kenai Lowlands 85 aeromag linear long E-dipping monocline(?) this study fair

(26) Turnagain Arm 9 one wildcat well possible fold on basement high Hartman and others, 1972; this study poor

(27) Pincher Creek 15 unknown unknown Magoon and others, 1976 poor

(28) Swan Lake 17 one wildcat well unknown Magoon and others, 1976 poor

(29) West Fork 11 gas field unknown AOGCC; Magoon and others, 1976 certain

(30) Sterling 10 gas field probably a broad anticline AOGCC; Magoon and others, 1976 certain

(31) Kenai-Cannery Loop 21 gas field, weak aeromag anomalies almost symmetric anticline AOGCC; DOG; Magoon and others, 1976; this 

study

certain

(32) Kasilof` 9 four wells, weak aeromag anomalies unknown Magoon and others, 1976; this study certain

(33) Falls Creek-Ninilchik 27 13 wells, aeromag, DOG unit map likely has three anticlinal crests AOGCC; Magoon and others, 1976; this study certain

(34) Deep Creek-Happy Valley 12 8 wells probable left step between Deep Creek and 

Happy Valley areas

Magoon and others, 1976 fair

(35) Kalgin Island South 49 aeromag linear, seismic east-dipping monocline this study fair

(36) Starichkof (Cosmopolitan) 50 gas wells, seismic, aeromag, DOG unit map broad anticline Fisher and Magoon, 1978; this study certain

(37) North Fork-Nikolaevsk 15 5 wells, bedrock geology, DOG unit map likely has two anticlinal crests AOGCC; DOG; Magoon and others, 1976 certain

(38) Iniskin 15 seismic, aeromag, wells broad fold Fisher and Magoon, 1978 certain

(39) Chinitna Point West 40 seismic broad fold Fisher and others, 2010 certain

(40) Chinitna Point East 52 seismic broad fold Fisher and Magoon, 1978; Fisher and others, 2010 certain

(41) Hawk 40 seismic broad fold Fisher and Magoon, 1978; Fisher and others, 2010 certain

(42) East of Raven 25 seismic unknown Fisher and others, 2010 certain

(43) West of LCI COST No. 1 10 seismic unknown Fisher and others, 2010 certain

(44) Starichkof Southwest 17 seismic, aeromag unknown Fisher and Magoon, 1978; Fisher and others, 2010; 

this study

certain

(45) Kachemak 38 seismic, aeromag, one wildcat well unknown Fisher and Magoon, 1978; Fisher and others, 2010; 

this study

certain

(46) Seldovia Arch 68 seismic very broad uplift perpendicular to other 

structures

Fisher and Magoon, 1978 certain

(47) Augustine East 16 seismic unknown Fisher and others, 2010 certain

(48) Augustine Far East 18 seismic unknown Fisher and others, 2010 certain

(45) Lower Cook Inlet North 11 seismic unknown Fisher and Magoon, 1978 certain

(49) LCI Middle 28 seismic broad fold Fisher and Magoon, 1978 certain

(50) Loon 40 seismic broad fold Fisher and Magoon, 1978 certain

(51) Southeast Loon 17 seismic unknown Fisher and Magoon, 1978 certain

(52) Teal 28 seismic broad fold, tighter than most in the lower 

Inlet

Fisher and Magoon, 1978 certain

(53) Shaw Island 16 seismic unknown Fisher and Magoon, 1978 certain

(54) Kamishak Bay 12 seismic unknown Fisher and Magoon, 1978 certain

(55) Sukoi Bay 9 onshore geology broad anticline Magoon and others, 1976 certain
10 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

more confident in the presence of a structure if it also is 

observed on seismic-reflection profiles and (or) aeromagnetic 

data. Aeromagnetic images of structures provide important 

tools for tracing structures on a regional scale, in contrast 

to a seismic-reflection profile that provides detailed 

cross-sectional information. We use published names for 

structures, where available, and we assign informal names 

for unnamed structures using either well names or local 

geographic features. The following discussion proceeds in 

the order the structures are listed on table 1, which is from 

north to south. Only anticlines are discussed in the section on 

folds. In addition, this report is not meant to be a complete 

description of each structure, but rather is intended to show 

the interpreted location and length. The casual reader may 

wish to skip this description section and proceed to the 

Discussion. 

Faults

All faults are shown on figures 1 and 3 and are identified 

by a letter.

Castle Mountain Fault 

The Castle Mountain fault (fault A, figs. 1, 3) is 

recognized in bedrock in the Talkeetna Mountains (Detterman 

and others, 1976a; Fuchs, 1980). The western part of the 

fault has a Holocene fault scarp (Detterman and others, 

1974; Haeussler, 1998; Willis and others, 2007). The fault 

has a history extending back at least 47 million years (Parry 

and others, 2001). Perhaps more individual studies have 

been published on the Castle Mountain fault than any other 

structure in the basin because of ease of access to the fault 

and exposures in the Talkeetna Mountains (Parry and others, 

2001; Bruhn and others, 2000; Bunds, 2001; Haeussler and 

others, 2000, 2002). South-facing Holocene scarps define 

the active fault trace from the area near Houston westward 

to the Susitna River. Discontinuous scarps define the fault 

between Houston and the road to Hatcher Pass, but no faultline scarps have been identified east of the Hatcher Pass 

Road. The extent of the fault was mapped by Detterman and 

others (1974, 1976a), Clardy (1974), and Haeussler (1998). 

Historical earthquakes in 1984 (Lahr and others, 1986) and 

1996 had right-lateral offset along the part of the fault without 

a scarp. The fault dips steeply to the north and has had both 

right-lateral and north-side-up dip-slip offset (Detterman and 

others, 1974; Haeussler and others, 2002). At its western end, 

the Castle Mountain fault connects with the Lake Clark fault 

(Detterman and others, 1976b). Haeussler and Saltus (2005) 

infer 26 km of right-lateral offset on the Lake Clark fault in 

the last ~35 Ma, based on offset aeromagnetic features, and 

a similar or greater amount of offset on the Castle Mountain 

fault also would have occurred. Willis and others (2007) 

found an outwash channel right-laterally offset 36 m across 

the fault. They infer an age of 11,300 to 13,470 calendar 

years B.P. for the feature, which yields a modern slip rate of 

0.27-0.32 cm/yr Additional studies on the Castle Mountain 

fault were published by Detterman and others (1975), Bruhn 

(1979), Bruhn and Pavlis (1981), Fuchs (1980), Bunds 

(2001), and Parry and others (2001).

Wishbone Hill Faults

At Wishbone Hill (faults B, figs. 1, 3), a syncline was 

elevated above the valley floor, possibly by shortening of the 

rocks beneath one or both sides of the fold (Barnes and Payne, 

1956; Bunds, 2001). This fold is the most easily seen and 

visited in the entire basin. The fold appears to have formed 

due to shortening across the northeastern corner of the basin. 

When the syncline is viewed looking westward, it provides a 

down-plunge view of the basin strata. At Wishbone Hill (faults 

B, figs. 1, 3), north-south striking cross faults cut a syncline 

(Bunds, 2001). Similar cross faults have been recognized or 

inferred in the subsurface of many of the folds in Cook Inlet, 

perhaps most notably in the Swanson River field, as well as 

the McArthur River, Trading Bay, and Redoubt Shoal fields. 

Lake Clark Fault

The Lake Clark fault (fault C, figs. 1, 3) is along 

strike from, and connects with, the Castle Mountain fault 

near the Beluga River. The Lake Clark fault cuts through 

Tertiary granitic rocks in the region south of Spurr Volcano. 

Haeussler and Saltus (2005) found that the regional trends 

of aeromagnetic anomalies are offset about 26 km across the 

Lake Clark fault, and they infer that ~35 Ma intrusives are 

responsible for the magnetic anomalies. Schmoll and Yehle 

(1987) found that a small part of the Lake Clark fault, which 

they referred to as the Lone Ridge fault (located near the 

“3” in fig. 1), has a prominent geomorphic scarp, which they 

suggest is evidence of an active fault trace. They also suggest 

that a sequence of Quaternary moraines is offset across the 

fault. Reger and Koehler (2009) reassessed the evidence for 

offset moraines. They concluded the moraines were offset 25 

m vertically (south-side down), and they inferred the moraines 

likely are marine isotope stage 4 or 6 age (that is, about 65 

kya or 150 kya, respectively (Martinson and others, 1987)) . 

Like previous workers (Plafker and others, 1975), Reger and 

Koehler (2009) found no evidence of Holocene offset along 

the fault. Because the Castle Mountain fault has evidence 

of Holocene offset and the Lake Clark fault does not, this 

relationship suggests that deformation is being transferred on 

to Cook Inlet folds and faults south of the Lake Clark-Castle 

Mountain fault junction (Bruhn and Haeussler, 2006).

Bruin Bay Fault

The Bruin Bay fault (fault D, figs. 1, 3) flanks the 

northwestern edge of Cook Inlet and joins the Castle 

Mountain-Lake Clark fault system near the Beluga River. 

The fault is a southeast-directed thrust fault in outcrop along 
Tertiary Structures in Cook Inlet 11

the Beluga and Chuitna Rivers (Magoon and others, 1976). It 

is inferred to run beneath Trading Bay, and to the southwest 

it lies near the western margin of Cook Inlet. Near Redoubt 

Volcano the fault is found again in outcrop, where it offsets 

Jurassic rocks (Detterman and Hartsock, 1966). A seismicreflection profile in the vicinity of the Beluga gas field shows 

that it is a steeply west-dipping fault (Haeussler and others, 

2000). On a structure-contour map of the Trading Bay oil 

field, provided by the operator to the AOGCC, a “west Trading 

Bay fault” was mapped adjacent to the field on the northwest 

side. This fault is probably the Bruin Bay fault, as mapped 

by Magoon and others (1976). Southwest of southern Cook 

Inlet the Bruin Bay fault is intruded by a 26-27 Ma pluton 

(Magoon and others, 1976). The fact that the Bruin Bay fault 

cuts Beluga Formation sediments near its junction with the 

Castle Mountain fault indicates the oldest age of movement on 

the northern part of the fault is younger than when movement 

ceased in lower Cook Inlet. No Holocene fault scarps lie along 

the fault (Detterman and others, 1975).

Border Ranges Fault

The Border Ranges fault (fault E, figs. 1, 3) juxtaposes 

rocks of the accretionary complex of the Chugach and Kenai 

Mountains from the backstop rocks of the Peninsular terrane, 

which mostly consist of Jurassic volcanic and intrusive 

rocks (for example, Pavlis, 1982; Plafker and others, 1994). 

In the Matanuska Valley the fault has right-lateral offset of 

Tertiary age (Little and Naeser, 1989; Little, 1990), but there, 

and further to the southwest, it must have been a normal or 

normal-oblique fault in Tertiary time. We depict the fault as a 

normal fault, which we suggest was the last phase of motion 

along the fault. Three lines of evidence support this inference. 

First, Mesozoic rocks of the Kenai and Chugach Mountains 

are topographically above the Tertiary sediments of the Cook 

Inlet basin. There must have been northwest-side-down 

movement in Tertiary time. Second, aeromagnetic data do 

not support interpretation as a thrust fault. The accretionary 

complex rocks along the east side of the fault are almost 

nonmagnetic. The Mesozoic metavolcanic rocks (for example, 

Detterman and Hartsock, 1966), just to the east of the fault, are 

slightly magnetic (Alstatt and others, 2002). However, a strong 

magnetic anomaly lies just west of the trace of the Border 

Ranges fault in bedrock, which is likely related to Jurassic 

mafic intrusions (for example, Burns, 1982). In the Anchorage 

area, the eastern contact of the flanking aeromagnetic high 

is very linear, which indicates that it is a fault contact. Third, 

southeast of Anchorage, the flanking aeromagnetic high lies 

west of the surface trace of the Border Ranges fault. This 

is probably caused by there being some thickness of the 

nonmagnetic Peninsular terrane rocks lying between the east 

side of the anomaly and the surface trace of the Border Ranges 

fault. The relationships can be explained by one west dipping 

normal fault or two normal faults, or both. Therefore, there 

is no evidence that the accretionary complex was thrust over 

the inferred Jurassic intrusive rocks. The timing of the faulting 

is not clear. Evidence for normal faulting was not found on 

seismic-reflection profiles to the southeast of the study area 

across the Border Ranges fault (for example, Mackevitt and 

Plafker, 1974, Pavlis, 1982), and there is no evidence for 

Holocene faulting along the Border Ranges fault (Haeussler 

and Anderson, 1997).

The aeromagnetic data are useful for constraining the 

trace of the Border Ranges fault on the Kenai Peninsula 

where no outcrops of the Peninsular terrane rocks are 

found. As discussed above, in the Anchorage area and to 

the northeast, the Border Ranges fault lies on the east, or 

southeast side of the magnetic high that correlates with 

Jurassic intermediate to mafic intrusive rocks. To the 

southwest of Anchorage, the Border Ranges fault is not 

constrained by outcrops, until the western tip of the Kenai 

Peninsula near Seldovia, where the fault juxtaposes different 

Mesozoic rock units (for example, Bradley and others 

1999; Hawley, 1992). Outcrop relationships near Seldovia 

reveal Cook Inlet basin Tertiary sediments on both sides of 

the Border Ranges fault, and depositionally on top of the 

Mesozoic accretionary prism rocks (Martin and others, 1915; 

Bradley and others, 1999), but the relationships do not clarify 

the fault’s sense of slip. Limited aeromagnetic coverage 

in this region indicates an aeromagnetic high lies on the 

northeast side of the fault—as in the Anchorage area. On the 

basis of these observations, it appears reasonable to infer the 

location of the Border Ranges fault in the subsurface as lying 

on the southeast side of the prominent aeromagnetic anomaly 

(fig. 3). If so, the Tertiary Cook Inlet Basin sediments lie on 

either side of the inferred fault trace (Bradley and others, 

1999). Beluga Formation outcrops just west of Homer show 

no evidence of its presence.

From the Anchorage area toward Homer, the magnetic 

high along the Border Ranges fault diminishes in amplitude. 

The reason for this is not certain, but it seems likely that it 

is related to a decreased size or an absence of the Jurassic 

intermediate to mafic intrusives. 

Little Susitna River Escarpment

This escarpment is a 15-km-long linear feature that lies east 

of the Little Susitna River, (feature F, figs. 1, 3). The escarpment 

is unusually long and linear, and it is consistently east-side-up 

and about 3 m high. These characteristics are consistent with 

the surface expression of an active fault trace. Alternatively, the 

escarpment may be related to downcutting of the Little Susitna 

River during late Quaternary or early Holocene deglaciation. 

There is another curving escarpment on the other (west) side 

of the Little Susitna River, which has a similar height and 

geomorphic position. However, it is arcuate, indicating it is 

related to fluvial processes, and it appears to be a terrace riser. 

This relationship indicates the Little Susitna River escarpment 

also is a terrace riser and not an active fault scarp. We, thus, 

favor the fluvial origin for the scarp, but mention it here for 

completeness. There is no evidence for a fault at the north end 

of the scarp on poor-quality seismic-reflection profiles.
12 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

Capps Glacier Fault

A fault between Mesozoic and Tertiary rocks east of 

Spurr Volcano was first mapped by Barnes (1966) and then 

included in a compilation by Magoon and others (1976) 

(feature G, figs. 1, 3). Gillis and others (2009) considered the 

fault a significant basin-bounding fault, with some component 

of right-lateral slip. The fault juxtaposes Paleocene intrusions 

and Eocene West Foreland Formation. Thus, the fault is 

Eocene or younger. The western extension of the fault is 

covered by Holocene volcanic deposits from Spurr Volcano. 

Lower Cook Inlet Faults

Fisher and Magoon (1978) mapped a number of thrust 

faults associated with folds in lower Cook Inlet, and minor 

changes to these interpretations appear in Fisher and others 

(2008, 2010). These faults are shown on figures 1 and 3. The 

only dated section nearby is in the COST well, which has 

Eocene strata at a depth of 413 m beneath the surface (Turner, 

1986), thus, the faults are constrained only to be active after 

Eocene time.

Folds

In contrast to faults, folds are numbered on figures 1 and 

3. All folds are concealed by surficial deposits, but the map 

legend uses exposed fold symbols for greater clarity.

Castle Mountain Fault Anticline

A 4-km-wide anticline on the north side of the Castle 

Mountain fault was noted by Haeussler and others (2000) 

on a seismic-reflection profile (feature 1, figs. 1, 3). They 

interpreted this anticline as being cored by several steeply 

dipping faults. Wildcat wells drilled on the crest of this feature 

penetrate Miocene Tyonek Formation sediments, and thus, 

the anticline is younger than Miocene. An aeromagnetic high 

parallels the surface trace of the Castle Mountain fault in the 

region near Houston, and this high correlates with the location 

of the anticline as shown by the seismic-reflection profile in 

Haeussler and others (2000). The aeromagnetic high may be 

related to the uplift of more magnetic basement rocks in the 

core of the anticline. Therefore, we infer the lateral extent 

of the anticline is, at minimum, the same as the extent of the 

aeromagnetic anomaly. It is possible that this anticline extends 

to the southwest and is responsible for the large westward 

bend of the Susitna River just north of the Castle Mountain 

fault, as first hypothesized by Kelly (1961). 

Big Lake North

This feature is based on a broad antiform observed on a 

seismic-reflection profile (Haeussler and others, 2000), with 

one well drilled on it (feature 2, figs. 1, 3). The lateral extent 

and orientation are suggested by a weak aeromagnetic high 

extending between this feature and the Castle Mountain fault. 

Pitman

A long curving anticline was mapped by Magoon and 

others (1976) and apparently was inferred by tracing a line 

through four wildcat wells (feature 3, figs. 1, 3). We divide 

this anticline into the Pitman fold and the Middle Lake Unit 

No. 1 fold (feature 7), based on weak trends in high-frequency 

filtered aeromagnetic data. These trends do not parallel the 

geometry of the fold axes in Magoon and others (1976). 

Kirschner and Lyon (1973) also portrayed this feature as two 

folds, similar to our interpretation. This structure is a broad 

anticline on seismic-reflection profiles. We extend the structure 

east of the north side of the Wishbone Hill syncline (fig. 1), 

based on the aeromagnetic expression.

Wasilla State No. 1—Needham

This anticline was mapped by Magoon and others (1976) 

and is suggested by three aligned wildcat wells, all of which 

were dry holes (feature 4, figs. 1, 3). There is no indication of 

a fold on the aeromagnetic data, and we do not have seismicreflection data that cross its inferred location. The southern 

part of the fault parallels a large aeromagnetic anomaly at the 

southeastern boundary of the Cook Inlet Basin, which locally 

appears to be related to Jurassic intrusive rocks (for example, 

Saltus and others, 2001). The extent of this structure is the 

same as that mapped by Magoon and others (1976), except we 

extend the structure east of the south side of the Wishbone Hill 

syncline (fig. 1).

Fish Creek—Knik Arm No. 2

This small structure is indicated by two wildcat wells 

and by a northeast trend of aeromagnetic anomalies, which 

parallels other structures to the north (feature 5, figs. 1, 3). 

The inferred trend of the structure does not cross any seismicreflection profiles to which we have access. 

Lorraine—Knik Arm State No. 1

This anticline was mapped on Magoon and others (1976) 

compilation between three aligned wildcat wells (feature 

6, figs. 1, 3). Seismic reflection data show no indication of 

an anticline near the northernmost well, and therefore, we 

reduced the length of the structure from that mapped by 

Magoon and others (1976). There is no indication of a fold on 

the aeromagnetic data. 

Middle Lake Unit No. 1—Alaska Gulf No. 1

This structure is suggested by four aligned wildcat 

wells, by a broad anticline on an unpublished seismic line, 
Tertiary Structures in Cook Inlet 13

and by vague trends in the aeromagnetic data (feature 

7, figs. 1, 3). The Middle Lake Unit No. 1 well and the 

Alaska Gulf No. 1 well are on either end of the four aligned 

wildcat wells. An east-west trending seismic-reflection 

profile shows thinning of Tertiary strata near the inferred 

fold axis. Another seismic-reflection profile stops at the 

inferred trace of the fold and does not reveal its presence. 

Thus, it is difficult to verify the existence of this structure. 

There is a northeast-southwest trending feature in the 

aeromagnetic data beneath the inferred fold. Elsewhere 

in the basin, these aeromagnetic features typically are on 

the flanks of folds. Nonetheless, we use the trend of the 

aeromagnetic anomalies to guide the orientation and length 

of this structure.

Figure Eight Unit No. 1

The only evidence for this structure is the Figure Eight 

Unit No. 1 well, which was a dry hole (feature 8, figs. 1, 

3). We find no evidence for a fold on an east-west seismicreflection profile 5 km north of the Figure Eight well. We use 

northeast-trending aeromagnetic anomalies in this region to 

infer the orientation of the feature. 

Bell Island

The Bell Island structure was mapped by Magoon and 

others (1976) based on the presence of the Bell Island well 

(feature 9, figs. 1, 3). The inferred orientation of the structure 

parallels trends in the high-frequency filtered aeromagnetic 

data, but it is not expressed like the Beluga fold (feature 13). 

We are uncertain of the presence of a structure here. 

Stump Lake

The Stump Lake gas field was discovered in 1978 and 

according to a structure contour map provided by the field 

operator for the AOGCC annual summary the structure is 

about 5 km long (feature 10, figs. 1, 3). High-frequency 

filtered aeromagnetic anomalies trend north-northeasterly 

in this region and suggest the structure may be around 7 km 

long. 

Ivan River

The Ivan River gas field was discovered in 1966 and, 

according to a structure contour map provided by the field 

operator for the AOGCC annual summary, the structure is 

about 7.5 km long (feature 11, figs. 1, 3). The wells in the 

field are flanked by moderate-intensity aeromagnetic highs, 

which trend north-northeast, parallel to the flanks of the 

fold. We infer that the structure extends for the length of the 

flanking aeromagnetic anomalies. To the north, the East Lewis 

River No. 1 well lies along the same trend, and we infer it is 

positioned on the same fold (fig. 1).

Lewis River—Pretty Creek

The Lewis River gas field was discovered in 1975 and 

occupies a north-south trending anticline (feature 12, figs. 1, 

3). According to a structure contour map provided by the field 

operator for the AOGCC annual summary, the structure is 

about 6.7 km long. High-frequency aeromagnetic anomalies, 

which flank producing wells along the structure, indicate the 

Lewis River fold is roughly 6 km longer and extends southsouthwestward. The southern end of this structure, and the 

flanking aeromagnetic anomalies, connect with the Pretty 

Creek field. A lack of wells between the two fields suggests 

there is a saddle between the fields. This structure also is along 

the trend of the Beluga River field, which suggests the master 

faults coring the structures are connected at depth. 

Beluga River

The Beluga River gas field was discovered in 1962 and, 

according to a structure contour map provided to the AOGCC 

by the field operator, the structure is about 12 km long 

(feature 13, figs. 1, 3). We extend the structure to the south 

based on the trend of prominent flanking magnetic anomalies 

originating from magnetic strata probably in the upper part of 

the Sterling Formation (Saltus and others, 2001). A seismicreflection profile across the fold indicates it is a fault-cored 

fold and that growth strata were deposited along its northwest 

side (Haeussler and others, 2000). 

North Cook Inlet—SRS

The North Cook Inlet structure and the SRS structure 

have previously have been considered separately, but the 

aeromagnetic data indicate they constitute one structure 

(feature 14, figs. 1, 3). The northern North Cook Inlet gas field 

was discovered in 1962. A seismic-reflection profile across 

the northern part of the structure shows that it is a fault-cored 

fold (Haeussler and others, 2000). North-northeast trending 

high-frequency aeromagnetic anomalies flank the fold axis. 

These anomalies nearly converge to the north-northeast of the 

structure, which suggests that the fold plunges to the north. 

To the south-southwest of the North Cook Inlet structure, the 

magnetic anomalies also flank the SRS fold. Therefore, the 

magnetic data indicate the SRS and the North Cook Inlet folds 

constitute one longer fold. The anomalies constrict slightly 

between the two named folds where the North Cook Inlet and 

SRS fields lie, which suggests that there is a structurally lower 

saddle between two highs. No producing oil or gas wells lie 

between the two fields, therefore, the fields likely are anticlinal 

crests with a saddle in between. 

Moquawkie

The Moquawkie field was discovered in 1965 (feature 

15, figs. 1, 3). No structure contour maps of the field 
14 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

were provided to the AOGCC. We infer an anticline with 

a north-northeasterly trend based on regional trends of 

folds and the presence of an anticline along the Chuitna 

River (Magoon and others, 1976). Aurora Gas geologists 

report that the Moquawkie structure is about 12 km long, 

is westward vergent, and is a single structure with three 

anticlinal culminations —the Moquawkie field, the Lone 

Creek structure to the north, and the Kaloa structure to the 

south. There is no clear signature of the structure on the 

aeromagnetic data, and we have no seismic-reflection profile 

that crosses it. 

Nicolai Creek 

The Nicolai Creek gas field was discovered in 1966 

(feature 16, figs. 1, 3). No structure contour maps of the field 

were provided to the AOGCC. We infer there is an anticline 

with a north-northeasterly trend based on the trend of nearby 

structures and on the presence of wildcat wells to the south of 

the field, which also define its length. This structure lacks an 

aeromagnetic signature.

Granite Point

The Granite Point oil field was discovered in 1965 and 

is defined clearly by the location of wells along the crest of 

the anticline (feature 17, figs. 1, 3). According to a structure 

contour map provided to the AOGCC by the field operator, the 

fold is about 10 km long. However, we infer the structure is 

about 5 km longer based on dry holes along strike, which we 

infer were drilled on the same structure. Because the Granite 

Point and Middle Ground Shoal fields are along the same 

trend, they may be connected at depth. The structure is poorly 

imaged on aeromagnetic maps. The Albert Kaloa gas field, 

discovered in 1968, consisted of one abandoned well without 

production. It is located to the northwest of the Granite Point 

field. We suggest the gas shows found in this well are related 

to the Granite Point fold, and that it is a continuous structure. 

Fold crests in this part of the inlet typically are 3.5-6 km 

apart. If there is a separate anticline at the Kaloa field, the 

fold crests would be only 2 km apart. Moreover, west-dipping 

Quaternary(?) beds in the vicinity of the field also do not 

indicate there is a separate structure (Haeussler and others, 

2000). 

North Trading Bay

The North Trading Bay oil field is an outlier of the 

Trading Bay oil field (feature 19) and, according to a structure 

contour map provided to the AOGCC by the field operator, 

the structure is oriented north-south and is about 3-km long 

(feature 18, figs. 1, 3). The distribution of wells suggests that 

the North Trading Bay fold is a distinct structure from, and en 

echelon with, the Trading Bay fold. The structure is absent on 

the aeromagnetic maps. 

Trading Bay Unit—McArthur River Field

The Trading Bay Unit or McArthur River field was 

discovered in 1965, and it has wells scattered over the broad 

crest of the anticline (feature 19, figs. 1, 3). This anticline has 

the largest area of fold closure of any in Cook Inlet. According 

to a structure contour map provided by the field operator to the 

AOGCC, the structure has a triangular shape and lies between 

faults on both its northwestern and southeastern flanks. We 

mapped these faults on figure 1 as shown in the AOGCC 

Annual Report. The structure continues to the southwest 

because it is imaged on a seismic-reflection profile, and 

because there is a southwestward linear distribution of wells. 

The structure is not observed on the aeromagnetic data. 

West McArthur River

The north end of this fold is indicated by the linear 

trend of nine wells drilled along it (feature 20, figs. 1, 3). The 

structure is absent on the high-frequency filtered aeromagnetic 

data. However, numerous strong northwest-trending anomalies 

in this area probably obscure the signal from a small structure 

the size of the West McArthur River. To the south of the trend 

of the wells, we image a fold on a seismic-reflection profile to 

the south of West Foreland, which indicates the fold extends 

more than 9 km to the south. Along strike to the north of 

this fold lies the Trading Bay structure, which may indicate 

the two folds are related. However, the lack of wells drilled 

between the two structures, and the southward curve of the 

southernmost wells in the Trading Bay field indicate these 

probably are separate structures.

McArthur River—Redoubt Shoal

The McArthur River field was discovered in 1965 and is 

perhaps one of the widest in Cook Inlet, with producing wells 

distributed over an area 4 km wide (feature 21, figs. 1, 3). It 

is also one of the longer structures. According to a structure 

contour map provided to the AOGCC by the field operator, 

the fold is about 18 km long. The Redoubt Shoal structure 

lies along trend to the south. A structure contour map of the 

field provided to the AOGCC by the field operator shows a 

north-northeast trending structure. However, we do not place 

much confidence in the map because the fold trend is based on 

alignment of only three wells, and the contour pattern is unlike 

any other structure in the inlet. The high-frequency filtered 

aeromagnetic data indicate a well-defined linear anomaly 

to the east of the field, along the west side of the Middle 

Ground Shoal structure. A seismic-reflection profile across 

the south end of the Redoubt Shoal anticline is along strike 

from McArthur River field location to the north. Therefore, 

it appears the McArthur River fold and the Redoubt Shoal 

fold are part of the same long structure. However, because 

producing wells are spread over a much wider area at the 

north end of the feature, it suggests there may be significant 

structural changes between the north and south ends.
Tertiary Structures in Cook Inlet 15

Middle Ground Shoal

The Middle Ground Shoal oil field was discovered in 

1962 and is probably the largest, tightest fold in Cook Inlet 

(Boss and others, 1976; Bishop, 1982) (feature 22, figs. 1, 3). 

It has an unusual geometry in that it is a west-vergent faultcored fold above a west-dipping master fault (see Haeussler 

and others, 2000). Beds in the west limb of the fold are 

reported by Bishop (1982) to be vertical. Wells are distributed 

along a 25 km length, but the fold is observed on a seismicreflection profile 6 km to the south of the southernmost 

wells and, thus, extends further south. A low-amplitude fold 

with flanking east- and west-vergent faults is located along 

a seismic profile 20 km south of the southernmost wells, 

which indicates the structure continues further south, and the 

tightness of the fold decreases dramatically south of the region 

with oil production. Flanking high-frequency aeromagnetic 

anomalies also indicate the structure extends south of the 

producing wells. 

Beaver Creek 

The Beaver Creek gas field was discovered in 1967 

and, according to a structure contour map provided to the 

AOGCC by the field operator, the structure is about 7 km long 

(feature 23, figs. 1, 3). However, on a map of high-frequency 

filtered aeromagnetic data a pair of prominent ~60-km-long 

northeast-trending anomalies lie 8 km west of the field. 

Proprietary seismic-reflection data show that these anomalies 

originate from west-dipping strata in the Sterling Formation 

or overlying Quaternary sediments along the west flank of 

the Beaver Creek fold. Proprietary seismic-reflection data 

show the fold has a higher amplitude in the vicinity of the 

Beaver Creek field, and it decreases in amplitude to the north, 

where fold closure ceases. The seismic-reflection data show 

that the fold extends at least as far north as the middle of the 

Swanson River field. Based on the extent of the aeromagnetic 

anomalies, we infer it extends a few kilometers farther. The 

syncline between the Beaver Creek fold and the southern 

extension of the Swanson River fold is tight in contrast to the 

broader anticline crests. Thus, the Beaver Creek fold appears 

to be a parasitic fold on the larger Swanson River structure. 

Swanson River

In 1957 the Swanson River oil field was the first oil 

discovery in the Cook Inlet region (feature 24, figs. 1, 3). 

The location and extent of the field is defined by numerous 

wells along the crest of the structure. A structure contour 

map provided by the operator to the AOGCC indicates there 

is at least a 15-km-long length of fold closure; and flanking 

aeromagnetic anomalies, which likely originate from magnetic 

strata in the Sterling Formation, suggest it has a much longer 

extent. On this basis, we infer the structure extends about 

16 km further north. The Birch Hill gas field is part of the 

same structure. The magnetic anomaly on the east side of 

the structure is nearly 100 km long (see Kenai Lowlands 

structure). The Swanson River fold likely extends ~9 km 

further south and includes the Wolf Lake field. There is a 

broad anticlinal culmination on an east-west seismic-reflection 

profile through the Beaver Creek structure. The Sterling Gas 

field (feature 30) is along strike 12 km to the south, and it may 

be part of the same structure, but without supporting evidence, 

we leave these as two separate structures. 

Kenai Lowlands 

We infer the presence of the Kenai lowlands structure 

from aeromagnetic data, and it may reflect the deeper structure 

of the lowlands (feature 25, figs. 1, 3). High-frequency filtered 

aeromagnetic data show several very long linear features in 

the Kenai lowlands (fig. 3). Two features are located between 

Kenai and Nikiski, and two others lie along the west side of 

the Swanson River fold. From examination of proprietary 

seismic-reflection data, we conclude that the Kenai-Nikiski 

aeromagnetic anomalies arise from west-dipping magnetic 

strata within the Sterling Formation. We have not examined 

seismic-reflection data across the magnetic anomaly on the 

east side of Swanson River, but we infer it also is related 

to tilted strata. The long length of the anomalies and the 

structural position of the feature suggest it may be similar to 

the Kalgin Island South structure (feature 35), which is imaged 

on seismic-reflection profiles as homoclinally dipping strata. 

Because the Swanson River fold is flanked by these anomalies, 

we infer that the anomaly on the east side of Swanson River is 

related to west-dipping strata. If this interpretation is correct, 

then there is a structure as long as these flanking anomalies. 

The lack of a mirror image of the anomalies indicates the 

structure is a monocline. If so, this broad flexure is narrow to 

the north, where the tighter fold of the Swanson River field 

is located, and it is diffuse with multiple folds on the limb 

to the south, where the Beaver Creek (23), West Fork (29), 

Sterling (30), and Kenai-Cannery Loop (31) folds lie. Without 

seismic-reflection data we are unable to further evaluate this 

hypothesis. 

Turnagain Arm

The presence of a structure near the mouth of Turnagain 

Arm is enigmatic (feature 26, figs. 1, 3). Hartman and others 

(1972) mapped west-side-down faults in this region based on 

correlations of units between oil and gas wells (see Border 

Ranges fault discussion). The Turnagain Arm Unit No. 1 

wildcat well at the entrance to Turnagain Arm indicates that at 

least one oil industry geologist thought there was a structure 

here. On a poor-quality seismic reflection line that runs from 

Turnagain Arm to the west-northwest, the Tertiary sediments 

are imaged on the top of the seismic basement. The top of 

the basement and the sediments thin onto the area where the 

wildcat well was located. We show an inferred anticline on our 

map, but remain unconvinced as to the presence or nature of 

the structure at this location. 
16 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

Pincher Creek

There is less evidence for the Pincher Creek structure 

than any other in Cook Inlet. It was shown on the map 

of Magoon and others (1976) (feature 27, figs. 1, 3). No 

exploratory wells have been drilled on the feature. A strong 

aeromagnetic anomaly lies beneath its inferred location, but it 

likely reflects the margin of Jurassic intrusive rocks along the 

Border Ranges fault (Saltus and others, 2001). 

Swan Lake

The Swan Lake structure was mapped by Magoon and 

others (1976), and we infer its presence was deduced from 

a dry wildcat well (feature 28, figs. 1, 3). There is a strong 

aeromagnetic anomaly beneath its inferred location, which 

likely reflects the margin of Jurassic intrusives and the Border 

Ranges fault (Saltus and others, 2001). 

West Fork

The West Fork gas field was discovered in 1960 (feature 

29, figs. 1, 3). No structure-contour map was provided to 

the AOGCC by the operator. We infer the field has a northnortheast trend based on the distribution of the discovery well 

and several other wildcat wells, and the regional trend of the 

adjacent Kenai Lowlands aeromagnetic anomaly.

Sterling

The Sterling gas field was discovered in 1961 and has 

two closely spaced wells (feature 30, figs. 1, 3). According 

to a structure-contour map provided to the AOGCC by the 

operator, the fold has a north-northwest trend. We infer 

the fold has a north-northeast trend similar to all the better 

documented folds nearby, as well as the trend of the Kenai 

Lowlands (feature 25) magnetic anomaly. There is a weak 

flanking aeromagnetic anomaly about 10 km long, which we 

infer is the length of the structure. 

Kenai—Cannery Loop

The Kenai gas field was discovered in 1959 and, 

according to a structure contour map provided to the AOGCC 

by the operator, the field trends north-south (feature 31, figs. 

1, 3). The Cannery Loop field is 3 km along strike to the north 

and, based on the trend of the long aeromagnetic anomalies 

on the west side of the structure, it trends north-northeasterly. 

Proprietary seismic-reflection data indicate the structure fails 

to extend as far north as the Beaver Creek field (feature 23). 

Kasilof

The Kasilof fold was mapped by Magoon and others 

(1976) as part of a longer structure, which we divide, on 

the basis of the aeromagnetic signature, into the Kasilof 

(feature 32, figs. 1, 3) and the Falls Creek-Ninilchik (feature 

33) structures. The fold is inferred from evidence from 

several wildcat wells. A high-frequency aeromagnetic 

anomaly west of the wells trends north-south, as does the 

nearby Kenai field, and thus we infer this fold also trends 

north-south for approximately the length of the magnetic 

anomaly. We are unable to connect the Kasilof fold with the 

Falls Creek-Ninilchik fold based on the inferred north-south 

trend of the fold, and because the northeast end of the Falls 

Creek-Ninilchik fold is well defined by well locations and 

aeromagnetic data. Furthermore, the fold axis lies east of the 

trend of the Kasilof fold. 

Falls Creek—Ninilchik

The Falls Creek-Ninilchik fold was mapped by Magoon 

and others (1976) as part of a larger structure, which we divide 

into the Falls Creek-Ninilchik (feature 33, figs. 1, 3) and Kasilof 

(feature 32) structures. The fold can be inferred from wells 

drilled along its length, and Buthman and Smith (2002) used 

seismic and gravity data to map the feature. It also is imaged 

clearly on the high-frequency filtered aeromagnetic data. 

Anomalies flank the length of the fold and define its extent. The 

aeromagnetic data fails to show that the fold connects with the 

Kasilof structure, as previously mapped. Buthman and Smith 

(2002) also mapped and identified on seismic-reflection profiles 

four normal cross faults along the shoreline of Cook Inlet. The 

Alaska Department of Oil and Gas (DOG) divides this structure 

into three pools for their unitization scheme. As they had access 

to proprietary oil industry data, it seems likely that this fold has 

three anticlinal crests with saddles in between. 

Deep Creek—Happy Valley

The Deep Creek and Happy Valley fold was first 

mapped by Magoon and others (1976) (feature 34, figs. 1, 

3). Eight wells were drilled along this inferred structure. The 

Deep Creek field operator presumably supplied sufficient 

information to DOG so that the field was unitized and 

considered a structurally distinct field, but not structurally 

distinct from the Happy Valley pool. The Happy Valley 

pool has a limited area close to the Happy Valley wells. It is 

notable that the alignment of the Deep Creek wells is slightly 

offset from the alignment of the Happy Valley wells, which 

suggests there is a small stepover, or jog, in the fold between 

the two groups of wells. The feature is poorly imaged on the 

aeromagnetic maps, but northeast-trending anomalies lie to 

the east and north. We modify the location and extent of the 

structure as mapped by Magoon and others (1976) based on 

the drilled wells and the DOG unit and pool map.

Kalgin Island South

A structure is inferred from a prominent linear 

aeromagnetic anomaly south of Kalgin Island (feature 35, figs. 
Tertiary Structures in Cook Inlet 17

1, 3). We infer it is a monocline where flat strata in the center 

of Cook Inlet are tilted southeastward at the west margin of the 

basin. We infer this monocline for the following reasons: (1) a 

proprietary seismic-reflection profile that crosses the south end 

of this anomaly at a low angle indicates the anomaly is related 

to east-dipping strata in a monocline at the west margin of the 

basin; (2) it is not a fold because it does not have mirror-image 

magnetic anomalies; (3) this magnetic anomaly is located near 

the basin margin, and thus, it is a logical location for tilted 

beds; and (4) Fisher and Magoon (1978) show a seismicreflection profile to the southwest of this magnetic feature. 

The profile lacks a fold in the appropriate place, but there is 

a southeast-dipping monocline along the northwest margin of 

the basin. In conclusion, the evidence points to the presence of 

a monocline. There could be an adjacent fault associated with 

the relative uplift of the northwest side of the basin. 

Starichkof (Cosmopolitan)

The Starichkof fold, also referred to as the Cosmopolitan 

unit (feature 36, figs. 1, 3), is inferred from (1) wells drilled at 

the north end of the feature, (2) recent wells drilled westward 

into the structure from near Anchor Point, (3) flanking highfrequency aeromagnetic anomalies, and (4) seismic-reflection 

data (Fisher and Magoon, 1978). The DOG unitized the 

Cosmopolitan field at the north end of the fold. Thus, the 

DOG must have been presented with evidence for an anticlinal 

crest in the area near the exploratory wells. Nonetheless, the 

aeromagnetic anomalies and seismic data indicate it extends 

another 30 km to the southwest. Based on these data, it is one 

of the longer and better defined folds in lower Cook Inlet. 

North Fork—Nikolaevsk

This structure was first mapped by Magoon and others 

(1976) probably because the Sterling Formation crops out 

along the crest of this knob and because several wildcat wells 

lie along it (feature 37, figs. 1, 3). The AOGCC lists this 

as a shut-in gas field. The DOG divided this field into two 

units (the North Fork Unit and the Nikolaevsk Unit), which 

indicates that oil companies could demonstrate the presence 

of two antiformal closures along the fold, with a saddle in 

between. Weak magnetic anomalies parallel the inferred 

structure at the southwest end and to its east. The anomalies 

along the east side continue northward, which indicates that 

the fold extends further north than previously mapped. This 

fold is along strike of the Kachemak fold (feature 45) to the 

southwest, which was observed on seismic-reflection data by 

Fisher and Magoon (1978).

Iniskin 

This fold was mapped by Fisher and Magoon (1978) 

from seismic-reflection data (feature 38, figs. 1, 3). There is a 

prominent magnetic anomaly 4.5 km to the southeast on the 

high-frequency filtered aeromagnetic data that is parallel to 

the axis of the fold as inferred by Fisher and Magoon (1978). 

On this basis, we infer that the trend of the axis of the fold was 

mapped correctly by Fisher and Magoon (1978). However, 

northeast of Fisher and Magoon’s (1978) seismic-reflection 

profile, three exploratory wells were drilled slightly east of 

the axis of the structure. This may indicate that oil industry 

geologists working with proprietary seismic-reflection data 

found the axis of the fold to lie east of the location mapped by 

Fisher and Magoon (1978). If we drew the fold axis between 

the location mapped by Fisher and Magoon (1978) and the 

wells, the resultant line would fail to parallel the flanking 

aeromagnetic anomaly. Therefore, we leave the trend and 

length of the fold the same as that mapped by Fisher and 

Magoon (1978). 

Chinitna Point West

This structure was mapped by Fisher and others (2010) 

using seismic-reflection data (feature 39, figs. 1, 3). The 

mapped sigmoidal shape is unusual and is inconsistent with 

the structural style of the region. Evidence for the fold is 

absent on aeromagnetic data. It seems possible that instead 

of the unusual sigmoidal shape, there could be a stepover 

between the northeastern and southwestern parts of the fold. 

Chinitna Point East

This structure was mapped by Fisher and Magoon (1978) 

using seismic-reflection data (feature 40, figs. 1, 3). Fisher 

and others (2010) map this fold as having a longer length and 

different orientation, which we use for our compilation. We 

find no evidence for the fold on aeromagnetic data. 

Hawk

This structure was mapped by Fisher and Magoon (1978) 

using seismic-reflection data (feature 41, figs. 1, 3). Fisher 

and others (2008, 2010) map the length and shape differently. 

Evidence for the fold is absent on aeromagnetic data. Fisher 

and others (2008) show the Hawk OCS well was drilled in 

a syncline west of the crest of this fold. There appears to be 

a link between Augustine Volcano and the Hawk structure. 

Fisher and others (2008) noticed there was a swarm of 

seismicity in the area centered around the Hawk well, which 

started about 8 months before the 2006 eruption of Augustine 

Volcano. The swarm was elongate in a west-northwesterly 

direction, it was about 25 km long and about 10 km wide. 

Earthquake hypocenters were between 10-30 km. They also 

noticed a correlation between seismicity in this area and 

an increase in seismicity at Augustine Volcano, about 25 

km to the southwest. After the volcano erupted, seismicity 

in this area subsided. The volcanic swarm was oriented 

west-northwest, and there are earthquake focal mechanisms 

consistent with right-lateral slip along a west-northwest 
18 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

trending structure. The orientation of this structure is nearly 

perpendicular to the folds in the region, however, the swarm 

structure is much deeper than the structures imaged on 

seismic-reflection data. 

East of Raven

This fold was mapped by Fisher and others (2010) 

(feature 42, figs. 1, 3). Fisher and Magoon (1978) mapped a 

fold about 2.5 km to the east, but its location and length are 

substantially different than those of the structure in the later 

paper. The LCI COST No. 1 well (Magoon, 1986) was drilled 

in a syncline to the east.

West of LCI COST No. 1

This fold was mapped by Fisher and others (2010) 

(feature 43, figs. 1, 3). Fisher and Magoon (1978) mapped a 

fold roughly on strike to the northwest, but it is absent in the 

later work. Evidence for the fold is absent on aeromagnetic 

data. The LCI COST No. 1 well (Magoon, 1986) was drilled 

about 1 km east of the north end of the structure. Presumably 

it was drilled in a syncline. 

Starichkof Southwest

This structure was mapped by Fisher and Magoon (1978) 

and by Fisher and others (2010) (feature 44, figs. 1, 3) as the 

southern end of the Starichkof (feature 36) fold. We interpret 

trends in aeromagnetic anomalies as indicating this is en 

echelon to the Starichkof fold, although it is located along 

trend. 

Kachemak

The southern extent of the Kachemak fold was mapped 

by Fisher and Magoon (1978) using seismic-reflection data, 

and it also is on the map by Fisher and others (2010) (feature 

45, figs. 1, 3). At its northern end, the Kachemak No. 1 well 

was drilled on the inferred fold crest. The fold trace follows 

the trend of a prominent high-frequency aeromagnetic 

anomaly. 

Seldovia Arch

The Seldovia arch is a well-known feature in lower Cook 

Inlet. The arch was first mapped by Magoon and others (1976) 

and Fisher and Magoon (1978) (feature 46, figs. 1, 3). This 

broad feature is mapped using seismic-reflection data. It lacks 

an aeromagnetic signature. It is the only structure in lower 

Cook Inlet that runs nearly perpendicular to most folds and 

faults. Haeussler and others (2000) speculate that the Seldovia 

arch formed to accommodate southwestward extrusion of the 

forearc. Vitrinite reflectance studies indicate that maximum 

burial of sediments in Cook Inlet post-date development of 

the arch (Johnsson and others, 1993). Fission-track studies 

of the nearby LCI Cost Well (Murphy and Clough, 1995) 

indicate little uplift. Nonetheless, reflectors within the Sterling 

Formation on seismic-reflection profiles are truncated as they 

approach the arch from the northeast. Thus, the simplest way 

to explain both observations is that Tertiary sediments pinch 

out onto the arch, and there also has been some uplift on the 

arch in Pliocene or younger time. 

Augustine East

This fold was mapped by Fisher and others (2010) using 

seismic-reflection data (feature 47, figs. 1, 3). It lies between 

the seismic lines of Fisher and Magoon (1978). 

Augustine Far East

This fold was mapped by Fisher and others (2010) using 

seismic-reflection data (feature 48, figs. 1, 3). It lies between 

the seismic lines used by Fisher and Magoon (1978), and, 

thus, apparently was missed in the older dataset. The structural 

interpretation in the area of the Augustine East and Far East 

folds is significantly different than that of Fisher and Magoon 

(1978). 

Lower Cook Inlet Structures

Structures numbered 49-55 (figs. 1, 3) all were mapped 

by Fisher and Magoon (1978) using seismic-reflection data, 

and also are shown on the map by Fisher and others (2010). 

All these structures are broad. A test well was drilled west of 

the Loon (structure 50) and two wells were drilled west of the 

Teal (structure 52). It seems odd that the exploratory wells are 

not on the crest of the folds as mapped by Fisher and Magoon 

(1978) and Fisher and others (2008, 2010). However, the 

seismic line (line 21) shown in Fisher and others (2008) has 

numerous faults and folds in the area, and apparently, only the 

largest were mapped. The Sukoi Bay fold (feature 55) was 

mapped onshore as a broad fold in West Foreland Formation 

(Magoon and others, 1976).

Discussion

This revised catalog of structures in Cook Inlet is useful 

for considering their relationship to the regional tectonic 

setting. The folds in the northeastern corner of the basin 

lie almost parallel to the basin bounding faults. The folds 

along the northwestern margin of the basin (north of West 

Foreland and south of the Susitna River) all lie at a small 

angle counterclockwise to the basin bounding Bruin Bay fault. 

Moreover, many of these folds turn more northeasterly at their 

northern limit. This structural geometry is consistent with 

the right-lateral transpressional deformation across the basin 

(that is, shortening with a component of right-lateral shear) 
Discussion 19

(Haeussler and others, 2000; Bruhn and Haeussler, 2006). 

Although the basic structural geometry can be seen on the map 

by Magoon and others (1976), the new map is far more robust. 

From the Kasilof field (32, fig. 1) and northward, the 

folds generally trend more northerly and show a much greater 

degree of contraction than those to the south. The Beluga 

River (13), Swanson River (24), North Cook Inlet-SRS (14), 

Granite Point (17), and Middle Ground Shoal (22) all show 

significant shortening across the structures, as previously 

documented by numerous workers (see summary in Haeussler 

and others, 2000). These structures contrast with those in 

southern Cook Inlet, south of Kalgin Island, which show only 

a small amount of shortening (Fisher and Magoon, 1978; 

Fisher and others, 2008). In addition, this region in northern 

Cook Inlet with relatively tight folds correlates with the 

Neogene depocenter of the Cook Inlet Basin, as well as a large 

low on an isostatic residual gravity map (fig. 5, Shellenbaum 

and others, 2010; Saltus and others 2007). 

Subduction of part of the Yakutat microplate (Plafker, 

1987) had a profound influence on the Neogene subsidence 

and deformational history of the Cook Inlet Basin. To 

summarize, the Yakutat collision caused counter-clockwise 

rotation of the southern Alaska block (Lahr and Plafker, 1980), 

which resulted in right-transpressional deformation across 

upper Cook Inlet. In this paper, we infer that a shallowereastward geometry of the subducting slab resulted in an 

unusually hydrated and serpentinized mantle wedge with 

lateral variations in heat flow beneath Cook Inlet. These 

conditions enhanced corner flow in the mantle wedge (in 

the region between the subducting slab and the base of the 

continental crust under Cook Inlet), which caused dynamic 

subsidence. In the following paragraphs we discuss these 

factors. 

The Yakutat microplate collision is resulting in counterclockwise rotation of the southern Alaska block and righttranspressional deformation in the Cook Inlet region (figs. 5B, 

6). Deformation across interior Alaska appears to be driven 

by the Yakutat collision (for example, Haeussler, 2008). 

The Denali fault system is the longest individual structure 

accommodating this collision (fig. 6). It is an active rightlateral fault, and it caused the 2002 M7.9 earthquake. As 

a consequence of this deformation, the region south of the 

Denali fault is rotating counter-clockwise (Lahr and Plafker, 

1980). The western edge of the rotating southern Alaska block 

is not well defined, but right-transpressional deformation in 

the Cook Inlet region is consistent in geometry and timing 

with the deformation being driven by the Yakutat collision 

(Haeussler and others, 2000; Bruhn and Haeussler, 2006; 

Haeussler, 2008). 

The Yakutat microplate is both colliding into and 

subducting beneath the southern Alaska margin, a process 

occurring for approximately the last 20-25 Ma (Plafker, 1987) 

(fig. 5B). The subducting Yakutat slab likely consists of a 

24-27-km-thick (Christeson and others, 2010) package of 

mafic rocks, which are more buoyant and likely less water-rich 

than typical oceanic crust (Plafker, 1987; Eberhart-Phillips 

and others, 2006). The Yakutat slab is contiguous with typical 

oceanic crust that lies to the southwest (fig. 5B). EberhartPhillips and others (2006) mapped the three-dimensional 

extent of the Yakutat slab with seismic tomography and 

analysis of Vp and Vp /Vs ratios, and they found the edge of 

the subducted slab lies at the northern end of the Cook Inlet 

Basin (fig. 5B). Evidence for the buoyancy of the Yakutat slab 

can be seen in the distance between the trench and the 100-

km contour of the slab, which the arc volcanoes commonly 

lie above (see figure 6). In southern Cook Inlet, where typical 

oceanic crust is being subducted, the distance between the 

trench and 100-km contour is 315–350 km, which is still 

large from a global perspective. North of Cook Inlet, where 

the Yakutat slab is being subducted, the distance is more than 

500 km. Moreover, a notable gap in modern volcanic-arc 

volcanism above the subducting Yakutat slab indicates the 

presence of unusual conditions that prevent arc volcanism. 

The edge of the Yakutat slab marks the edge of a 

serpentinized mantle wedge beneath Cook Inlet (YSE, fig. 

5). To the southwest of the Yakutat slab edge, there is a longwavelength magnetic high, and a gravity low that extends 

to the southwest for the length of Cook Inlet (Saltus and 

others, 1999, 2001.) (Southwest of Cook Inlet, there are no 

magnetic data). Large magnetic highs and gravity lows are 

typical of many continental forearc basins and likely reflect 

partial serpentinization of the mantle (Blakely and others, 

2005). A two-dimensional forward model of the gravity and 

magnetic data from upper Cook Inlet indicates the longwavelength magnetic high and the gravity low are consistent 

with serpentinization of the upper mantle (Saltus and others, 

2001; Blakely and others, 2005). In addition, a thermal model 

of the southern Alaska margin (Oleskevitch and others, 1999) 

indicates that temperatures in the upper mantle wedge are 

significantly cooler than the Curie temperature of magnetite. 

Thus, it is reasonable to infer a source for the magnetic 

anomaly in the upper mantle wedge at depths of 20-30 km 

(Saltus and others, 2001). Magnetotelluric data from this same 

area (Green, 2003) also show high conductivities at depth, 

which are consistent with serpentinization of mafic rocks, 

or mafic rocks with zones of high porosity. Lastly, EberhartPhillips and others (2006) found that the mantle wedge in the 

Cook Inlet region has relatively low Vp and high Vp / Vs, which 

indicates fluids in the wedge. All of these data are consistent 

with a hydrated mantle wedge and the Cook Inlet magnetic 

and gravity anomalies arising from a serpentinized part of the 

upper mantle (for example, Saltus and others, 2001; Blakely 

and others, 2005).

Previously, Saltus and others (1999) termed the magnetic 

high beneath Cook Inlet, the “south Alaska deep magnetic 

high” (fig. 5C). They extended the feature to the northeast, 

where the long wavelength magnetic anomaly continues. 

Here, we argue the character and the sources of the anomalies 

change across the Yakutat slab edge. Northeast of the Yakutat 

slab edge, there is a greater medium- to high-frequency 

content of the south Alaska deep magnetic high. Moreover, the 

source of the anomaly is likely the Talkeetna batholith, which 
20 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

S

S

F F

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0 20 40 60 80 100 10 Kilometers

-90.0 -60.0 -30.0 0.0 20.0 40.0 60.0 80.0

Isostatic Residual Gravity mGal

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0 10 20 30 40 50 5 Kilometers

Depth to base 

of Tertiary

0

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Figure 5.

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-280.4 -146.5 -107.0 -79.9 -58.9 -41.0 -27.0 -12.9 4.7 22.5 49.4 91.1 147.5 284.7

Totalfield magnetics nT

YSE

YSE

YSE

YSE

TB TB

C

150 km

100 km

50 km

Southern Alaska Deep Magnetic High

Figure 5. Maps showing the relationship of the Cook Inlet Basin to the subducted Yakutat terrane slab, Alaska. Dashed gray line is 

the Yakutat Slab Edge (YSE), from Eberhart-Phillips and others (2006). A, map of depth to Tertiary basement in the Cook Inlet Basin 

from Shellenbaum and others (2010). Symbols are those used in figure 1. B, Map showing the location of the subducting Yakutat slab 

from the tomography of Eberhart-Phillips and others (2006). C, Merged total field magnetic intensity map. Faults and folds are shown 

only for area of figure 1. Symbology is the same as figure 1. Grey contours to Benioff zone seismicity from Eberhart-Phillips and others (2006). Outline of southern Alaska Deep Magnetic High (Saltus and others, 1999) shown with white lines. TB, Talkeetna batholith. 

D, Isostatic residual gravity map. Faults and folds are shown only for the area of figure 1. Symbology is the same as figure 1. TB, 

Talkeetna batholith. 
Discussion 21

consists of a vast area of Jurassic to early Tertiary intrusives 

(Winkler, 1992). If the south Alaska deep magnetic high was 

sourced entirely from the mantle wedge, it should follow the 

contours of the subducting slab, however, northeast of the 

Yakutat slab edge it does not. Instead, the south Alaska deep 

magnetic high crosses the 50-km contour at the Yakutat slab 

edge and is above a much shallower part of the slab in the 

Talkeetna batholith region (fig. 5C).

The serpentinized mantle wedge beneath upper Cook 

Inlet indicates a strongly hydrated mantle wedge, which 

likely had profound effects on crustal rheology. Geochemical, 

petrologic, and seismological observations from subduction 

zones around the world indicate that water in the mantle 

wedge can reduce the viscosity of the mantle by several orders 

of magnitude (Billen and Gurnis, 2001). The presence of a 

low Vp and high Vp /Vs mantle wedge beneath upper Cook Inlet 

(see figs. 6F and 7F in Eberhart-Phillips and others, 2006), 

southwest of the Yakutat slab edge, indicates the presence of 

fluids in the wedge. The mantle wedge above the Yakutat slab, 

northeast of the Yakutat slab edge, has relatively high Vp and 

low Vp /Vs (Eberhart-Phillips and others, 2006), indicating the 

absence of fluid and probably less alteration to serpentinite. 

Rossi and others (2006) concluded there could be up to 15 

percent serpentinization in the region <80 km deep where 

the wedge is cold. Stachnik and others (2004) examined 

the attenuation tomography of the mantle wedge above the 

Yakutat slab. They found that the wedge is highly attenuating 

where the slab is more than 80 km deep, but less attenuating 

Alaska Yukon

160° 158° 156° 154° 152° 150° 148° 146° 144° 142° 140° 138° 136°

66°

64°

62°

60°

58°

56°

150 km

100 km

50 km

50 km

Pacific Plate

PAC/ YAK?

YAKUTAT SLAB

North American

Plate

TF

DF

FF

QCF

Aleutian Megathrust

PAC-NAM

YAK-NAM

SAB

CIB

shear-wave 

splitting 

directions

inferred 

direction of 

fluid flow 

above slab

Area of focused 

subsidence and 

deformation, and 

inferred lowviscosity mantle 

Figure 6. Map showing Yakutat slab and its inferred effect on mantle fluid flow. Figure modified from Eberhart-Phillips 

and others (2006). Mantle shear-wave splitting directions above subducting slab from Christensen and Abers (2010). 

These directions likely reflect the long axes of olivine in the mantle and are thus considered to indicate mantle flow 

azimuth. Contours to top of subducting slab are shown in green, active volcanoes are shown as triangles. DF, Denali 

fault; TF, Totschunda fault, FF, Fairweather fault; QCF, Queen Charlotte fault; SAB, southern Alaska block.
22 Location and Extent of Tertiary Structures in Cook Inlet Basin, Alaska, and Mantle Dynamics

than other wedges where less than 80 km deep. They argued 

this shallower mantle-wedge is 100-150°C cooler than other 

mantle wedges and suggested there may be less corner flow 

of the mantle in this region, but higher flow above the deeper 

part of the slab. Jadamec and Billen (2010) used a threedimensional buoyancy-driven model of the southern Alaska 

slab and found that mantle-flow velocity around the slab, 

including the region studied by Stachnik and others (2004), 

may be up to 10 times the driving plate velocity. There are no 

attenuation tomography studies to the southwest of the Yakutat 

slab edge, however, the Vp and Vp / Vs tomography (EberhartPhillips and others, 2006) are consistent with a wedge that 

has more fluids and lower viscosity. Billen and Gurnis (2001) 

concluded that a low-viscosity wedge has a dramatic influence 

on the force balance in a subduction zone and leads to an 

observable signal in the topography, gravity, and geoid. Such a 

signal is seen in Cook Inlet. 

The gravity data from upper Cook Inlet are consistent 

with a dynamic component of basin subsidence. Topography 

is near sea level in the region of thick Neogene sediments 

in Cook Inlet Basin, and this same area also has a large, 

negative isostatic residual-gravity anomaly (fig. 5D). Part of 

this anomaly likely is due to the serpentinized upper mantle. 

However, the south Alaska deep magnetic high extends to 

southern Cook Inlet (fig. 5C), but the isostatic residual-gravity 

anomaly does not (fig. 5D). All of Cook Inlet lies at or near 

sea level, but the gravity anomaly map differs from north to 

south. Thus, the upper Cook Inlet region is out of isostatic 

balance; it is too low for the observed gravity anomaly, and 

the basin is being pulled downward from beneath. Lowdensity mantle material is required beneath upper Cook Inlet. 

To reiterate our previous inferences, this low-density mantle 

would be higher in fluids, lower in viscosity, and contain more 

serpentinite. 

We speculate that the buoyant Yakutat slab acts like a 

squeegee to focus subducted fluids into the adjacent mantle 

wedge in the upper Cook Inlet region (fig. 6). Shear-wave 

splitting directions in the mantle above the subducting 

Yakutat slab (Christensen and others, 2003; Eberhart-Phillips 

and others, 2006; Christensen and Abers, 2010; see also the 

compilation in Jadamec and Billen, 2010) are oriented parallel 

to the strike of the slab, which suggests lateral flow away from 

the nose of the shallow slab. The same parallel-to-the-strikeof-the-slab flow directions were found in the recent numerical 

modeling work of Jadamec and Billen (2010). This flow may 

be like the bow wave of a blunt-nosed boat, where the water 

flow initially is almost perpendicular to the direction the boat 

is moving but later is parallel to the sides of the vessel. In the 

case of southern Alaska, the fluid flow would be focused away 

from the shallow dip region of the Yakutat slab, toward the 

upper Cook Inlet area, where there is a thicker mantle wedge 

and where oceanic slab dip is steeper. Mantle flow, although 

roughly parallel to the strike of the slab, also would have a 

downward toroidal component of corner flow in the mantle 

wedge to induce dynamic subsidence of the Cook Inlet Basin. 

As a result, there is hydration of the mantle wedge, a decrease 

in wedge viscosity, serpentinization, increased Neogene 

subsidence and deformation, and the associated isostatic 

residual-gravity low. 

Concluding Remarks

The uncertainties in the identification and mapping of 

fold and fault structures combined with the imprecision in 

dating of movement indicate that much work remains to be 

done in quantifying both oil and gas potential and seismic 

hazards for the Cook Inlet. Aeromagnetic-data interpretation, 

particularly when tied to quality reflection seismic data and 

well control, offers great promise for mapping the full extent 

of many structures. Systematic collection of high-resolution 

seismic data with accompanying detailed bathymetry and 

gravity data, would greatly assist in understanding and 

estimating resources and hazards in Cook Inlet. Lastly, a 

broadband receiver function experiment, like the BEAAR 

study (Stachnik and others, 2004; Veenstra and others, 2006), 

across both sides of upper and lower Cook Inlet, would help 

us understand the characteristics of the mantle that drive 

deformation and subsidence in the region. 

Acknowledgments

We thank Dwight Bradley, Ron Bruhn, Bob Swenson, 

and Les Magoon for thoughtful comments on the writing of 

the manuscript, as well as insightful technical reviews and 

reality checks, which resulted in a substantially improved 

manuscript.

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Produced in Menlo Park, California

Manuscript approved for publication, April 6, 2011

Text edited by Tracey Suzuki

Layout and design by Stephen L. Scott
Dumoulin and Galloway, eds.—Studies by the U.S. Geological Survey in Alaska, 2008–2009—Professional Paper 1776–D