( 

Gravity Survey of 

Epicentral Region 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 543-C 

THE ALASKA EARTHQUAKE, MARCH 27,1964: 

REGIONAL EFFECTS 

Gravity Survey and 

Regional Geology of the 

Prince William Sound 

Epicentral Region, - 

Alaska 

I 

I 

By J. E. CASE, D. F. BARNES, GEORGE PLAFKER, and S. L. ROBBINS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 543-C 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1966 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402-Price 20 cents (paper cover) 
THE 

ALASKA EARTHQUAKE 

SERIES 

The U.S. Geological Survey is publishing the results 

of investigations of the earthquake in a series of six 

Professional Papers. Professional Paper 543 describes 

the regional effects of the earthquake. Other Professional Papers describe the effects of the earthquake on 

communities; the effects on hydrology; the effects on 

transportation, communications, and utilities; and the 

history of the field investigations and reconstruction 

effort. 

CONTENTS 

Page 

Abstract ---..----_----------__-- C1 Stratigraphy-Continued 

Introduction -______------------- 1 Unconsolidated deposits- -- - - - 

Stratigraphy .................... 1 Rock densities -------__.---- 

Valdez Group -___-----_----- 4 Structure .____--_--_-______-..-_ 

Orca Group ----.--------_-- 4 Gravity ----___.-----______-_--- 

Granitic rocks ------------__ 4 General features of the gravity 

map---2----------------- 

ILLUSTRATIONS 

FIGURES 

Page Page 

Gravity-Continued 

C5 Regional gravity gradient - - - - C9 

5 Port Gravina gravity low- _ - _ 9 

5 Prince William Sound gravity 

6 high_--_______----------- 10 

References cited- _ _. - - -_- --- - --- 11 

8 

Page 

1. Generalized geologic map of the Prince William Sound region- _ - _ _ _ _ - - - - C2 

2. Simple Bouguer anomaly map, Prince William Sound region ______----__ 7 

3. Gravity anomalies across Prince William Sound ---_-.-_-___-_--------- 8 

4. Interpretation of the gravity anomaly over Knight Island .__-_______--- 10 

TABLE 

Page 

1. Densities of Prince William Sound rock units ......................... C5 

THE ALASKA EARTHQUAKE, MARCH 27, 1964: REGIONAL EFFECTS 

GRAVITY SURVEY AND REGIONAL GEOLOGY OF THE PRINCE 

WILLIAM SOUND EPICENTRAL REGION, ALASKA 

By J. E. Case, D. F. Barnes, George Plafker, and S. L. Robbins 

Sedimentary and volcanic rocks of 

Mesozoic and early Tertiary age form a 

roughly arcuate pattern in and around 

Prince William Sound, the epicentral 

region of the Alaska earthquake of 1964. 

These rocks include the Valdez Group, a 

predominantly slate and graywacke sequence of Jurassic and Cretaceous age, 

and the Orca Group, a younger sequence 

of early Tertiary age. The Orca consists 

of a lower unit of dense-average 2.87 

g per cm3 (grams per cubic centimeter)- 

pillow basalt and greenstone intercalated 

with sedimentary rocks and an upper 

unit of lithologically variable sandstone 

interbedded with siltstone or argillite. 

Densities of the clastic rocks in both the 

Valdez and Orca Groups average about 

2.69 g per cm3. Granitic rocks of relatively low density (2.62 g per cm3) cut the 

Valdez and Orca Groups at several 

localities. 

Both the Valdez and the Orca Groups 

Following the Alaska earthquake of March 27, 1964, the U.S. 

Geological Survey made an intensive investigation of the effects of 

the earthquake in the Prince 

William Sound region where the 

epicenter was located. Concurrent 

- 

ABSTRACT 

were complexly folded and extensively 

faulted during at least three major 

episodes of deformation: an early period of 

Cretaceous or early Tertiary orogeny, 

a second orogeny that probably culminated in late Eocene or early Oligocene 

time and was accompanied or closely 

followed by emplacement of granitic 

batholiths, and a third episode of deformation that began in late Cenozoic time and 

continued intermittently to the present. 

About 500 gravity stations were established in the Prince William Sound region 

in conjunction with postearthquake geologic investigations. Simple Bouguer 

anomaly contours trend approximately 

parallel to the arcuate geologic structure 

around the sound. Bouguer anomalies 

decrease northward from +40 mgal 

(milligals) at the southwestern end of 

Montague Island to -70 mgal at College 

and Harriman Fiords. Most of this 

change may be interpreted as a regional 

INTRODUCTION 

investigations included studies of 

the regional geology, changes of 

land level, faulting, and effects of 

waves on shorelines which were 

summarized by Plafker and Mayo 

(1965). At the same time, this 

gravity survey, mentioned briefly 

STRATIGRAPHY 

gradient caused by thickening of the 

continental crust. Superiniposed on the 

gradient is a prominent gravity high of as 

much as 65 mgal that extends from 

Elrington Island on the southwest, across 

Knight and Glacier Islands to the Ellamar 

feninsula and Valdez on the northeast. 

This high coincides with the wide belt of 

greenstone and pillow basalt of the Orca 

Group and largely reflects the high 

density of these volcanic rocks. A large 

low in the east-central part of the sound is 

inferred to have a composite origin, and 

results from the combined effects of lorn,- 

density sedimentary and granitic rocks. 

The Prince William Sound gravity 

high extends southwest-northeast without 

major horizontal offset for more than 100 

miles. Thus the belt of volcanic rocks 

causing the high constitutes a major 

virtually continuous, geologic element o 

south-central Alaska. 

by Barnes and Allen (1965), was 

conducted in order to establish the 

regional geologic and tectonic setting of the earthquake. Geologic 

sections of this report were prepared 

by Plafker and Case, the gravity sections by Case, Barnes, and Robbine. 

The geologic map of Prince compiled by Grant and Higgins L. R. Mayo, and J. E. Case in 1964. 

William Sound (fig. 1) has been (1910) and Moffit (1954). Struc- Some geologic details on Knight 

generalized and modified from pub- tural data were gained from recon- Island are based on recent mapping 

lished small-scale geologic maps naissance studies by George Plafker, by Richter (1965). The ages of the 
ALASKA EARTHQUAKE, MARCH 27, 19f34 

3." 0 10 20 30 MILES 

.. IL9IIl I I 

Geology compiled from Moffit (1954) and Grant and 

Higgins (1910) supplemented with reconnaissance 

mapping by George Plafker, L. R. Mayo, and 

J. E. Case during 1964 

1.-Generalized geologic map of the 
cinAvrl'Y SURVEY, REGIONAL GEOLOGY, PRINCE WILLIAM SOUND EPICENTRAL REGION 

EXPLANATION 

I Qu / 

Unconsolidated deposits 

Granitic rocks 

I Granite and quartz diorite in stocks, 

I dikes, and sills that intrude Valdez 

-L ' and Orca Groups r: I 

Sedimentary rocks of Orca Group 

Slightly metamorphosed and complexly 

folded marine sandstone and darkgray to black or reddish-brown hard 

siltstone and argillite. Includes minor 

amounts of conglomerate, limestone, 

and basaltic lava flows. Contains 

plant material of Tertiary age and 

nondiagnostic marine megafossils 

Volcanic rocks of Orca Group 

Gently to moderately folded, greenishblack slightly altered basaltic flows 

(greenstone) that commonly exhibit 

pillow structures. Intertongues with 

massive conglomerate and breccia and 

thin-bedded argillite and sandstone. 

Contains marine megafossils of probable middle to late Eocene age i 

I Valdez Group 1 Tightly folded and metamorphosed 

~r~arine graywacke and dark-gray to I black slate containing minor amounts 

I of argillite and conglomerate 

c 4x7 ,T9 

Glacier 

Contact 

&shed where approximately located; 

dotted where inferred 

---.-.-- U 

Fault 

Dashed where approximately located; 

dotted where inferred. U, upthrown 

side 

90 

~Gicline Overturned anticline 

Minor folds 

Shozving trace of axial plane, plunge of 

axis, and dip of limbs 

2 -F * 

Upright Overturned Vertical 90 on up n$e 

Generalized strike and dip of beds 

A 

45 

Strike and dip of foliation 

Prince William Sound region, Alaska. 
ALASKA EARTHQUAKE, MARCH 27, 1964 

Valdez and Orca Groups have been 

revised by reinterpretation of the 

collections from the Valdez Group 

and by studies of a new fossil 

collection from the Orca Group 

(Plafker and MacNeil, 1966). 

The geologic units in Prince 

William Sound shown on figure 1 

include: (1) t,he Valdez Group, a 

sequence of eugeosynclinal clastic 

rocks of Jurassic(?) and Cretaceous 

age; (2) the Orca Group, a sequence 

of early Tertiary age which can be 

subdivided, in a general way, into a 

lower predoininantly volcanic unit 

and an upper predominantly sedimentary unit; (3) bodies of granitic 

rocks which intrude both the Valdez 

and Orca Groups; and (4) unconsolidated continental and marine 

deposits of Quaternary age. 

VALDEZ GROUP 

The Valdez Group includes the 

oldest bedded rocks exposed in the 

Prince William Sound region. As 

defined by Schrader (1900, p. 408), 

the Valdez is a lithologically monotonous sequence of slate, argillite, 

and metamorphosed graywacke that 

is exposed along the northern and 

western shores of the sound and 

also underlies much of the adjacent 

Chugach and Kenai Mountains. 

The sequence is characterized by 

thin to thick beds of light-gray or 

tan-weathering poorly sorted sandstone of graywacke type which is 

rhythmically interbedded with thinbedded, finely laminated dark-gray 

to black argillite and slate. In the 

vicinity of granitic intrusive rocks, 

the slate and argillite are locally 

altered to phyllite and the graywacke to semischist. Thin units of 

stretched pebble or cobble conglomerate and lense3 of altered basaltic 

flows or intrusives occur locally 

within the sequense. 

The sedimentary sequence assigned to the Valdez Group is 

probably tens of thouqands of feet 

t,hick. The thickness cannot be 

accurately measured because the 

sequence lacks key beds and the 

units are duplicated and interrupted 

by complex folding and faulting. 

Determinative fossils from the 

Valdez Group, and probably equivalent sequences elsewhere along the 

margin of the Gulf of Alaska, include Inoceramus and Aucella that 

indicate a Jurassic(?) and Cretaceous age for the Valdez (D. L. Jones, 

oral commun., Nov. 24, 1965). 

ORCA GROUP 

The predominantly volcanic unit 

that forms the lower part of the 

Orca Group crops out in a discontinuous belt 52 miles long and up to 

12 miles wide that trends from the 

mainland east of Valdez Arm southwestward through Glacier, Lone, 

Knight, Bainbridge, Evans, and 

Elrington Islands (fig. I). The unit 

consists principally of slightly 

altered greenish-black dense basaltic 

lava flows, pillow lavas, flow breccias, and coarser textured diabase 

intrusives which have collectively 

been termed "greenstone." R/Iost 

of the flows are characterized by 

strikingly well formed pillow structures. The volcanic rocks intertongue with thick beds of conglomeratic argillite, conglomerate, darkgray siltstone or argillite, and 

graywacke sandstone. The thickness of the unit differs markedly 

along the length of the outcrop belt. 

Along the southeast shore of Valdez 

Arm it is several thousand feet 

thick (Capps and Johnson, 1915, 

p. 45). The greatest thickness of 

the unit probably occurs on and near 

Knight Island. The volcanic rock; 

dip 40"80° W. across most of an 

outcrop belt as much as 11 miles 

wide. The breadth and dips indicate an apparent thickness of 

about 40,000 feet, but it is entirely 

possible that there has been significant repetition by folding and 

faulting. Marine megafossils collected during the 1964 reconnaissance survey indicate that the 

volcanic unit is of early Tertiary 

(probably middle to late Eocene) 

age (Plafker and MacNeil, 1966). 

The predoininantly sedimentary 

unit that con~prises the upper part 

of the Orca Group includes the 

bedded rocks that occur especially 

on the eastern and southern shores 

of the sound and on the islands of 

the central part of the sound (fig. 1). 

In its gross aspects the unit is 

distinguished from the Valdez 

Group by a somewhat more variable 

lithology and a generally lower degree of metamorphism. The sequence consists mainly of thin to 

thick beds of graywacke sandstone, 

but also includes light-colored arkosic, carbonaceous, tuffaceous, calcareous and conglomeratic sandstones. The sandstone commonly 

is interbedded with dark-gray to 

black or reddish-brown dense hard 

siltstone or argillite which locally 

contains abundant carbonized plant 

remains. Minor anlounts of lightgray-weathering limestone occur as 

thin beds and concretions within the 

siltstone or argillite. Tabular and 

lenticular masses of basaltic lavas 

are locally interbedded with the 

clastic rocks, most notably on 

Hinchinbrook Island and the adjacent, mainland to the east. The 

unit is at least several thousand feet 

thick, but complex folding and 

faulting preclude a reliable measurement. 

The only fossils diagnostic of age 

t,hat have been obtained from the 

sedimentary unit are Alnus pollen 

that indicates a post-hfesozoic age 

(W. R. Evitt, written commun., 

Nov. 10, 1964). The unit overlie;, 

and probably is in part interbeddej 

with, the volcanic sequence of the 

Orca Group froin which lower 

Tertiary fo-sils have been collected; 

GRANITIC ROCKS 

Granitic rocks intrude the Valdez 

Group at numerous localities in 

the northwestern part of Prince 

William Sound, and a single large 

intrusive mass cuts the Orca Group 
GRAVITY SURVEY, REGIONAL GEOLOGY, PRINCE WILLIAM SOUND EPICENTRAL REGION c5 

on the mainland in the eastern part 

of the sound. The intrusive rocks 

consist mainly of pale-pink to 

pinkish-gray porphyritic biotite 

granite and light-gray hornblendebiotite quartz diorite. The granite 

body that cuts the Orca Group on 

the mainland in the eastern part of 

the sound can be no older than the 

rocks it intrudes (early Tertiary) 

and may be slightly younger. The 

remaining granitic bodies that intrude the Valdez Group are lithologically similar and probably belong to the same epoch of intrusion, 

although the available data do not 

preclude a pre-Orca age. 

UNCONSOLIDATED 

DEPOSITS 

of the Prince William Sound region 

are overlain with marked angular 

unconforinity by nearly flat-lying 

fluvial, glacial, and marine deposits 

of gravel, sand, mud, and till. The 

various types of these uncoasdi;s+ 

dated deposits are not differentiated 

on figure 1. 

ROCK DENSITIES 

The densities of approxilnately 

50 hand specinlens representing 

the major lithologic units (except 

the unconsolidated sediments) were 

measured by conlparing their weight 

in air and water (table I). No 

effort was made to determine the 

effect of porosity on the measured 

densities, but tests of the influence 

of porosity on densities of similar 

The Mesozoic and Tertiary rocks rocks froin other parts of Alaska 

TABLE 1.-Densities of Prince William Sound rock units 

Rock unit 

Granite __--- _ - - -. - - - - - - - - - - - - - - - - - - - - - 8 2.58 2.69 2.62 

Sedimentary rocks of Crca Group -------- 2.69 

Volcanic rocks of Orca Grouo- ------- .-- I I: 1 : 1 : 1 2.81 

Specimens 

Sedimentary rocks of ~aldei Group___ _ _ _ 1 10 1 2.64 1 2.74 1 2.69 

Densities (g per cm3) 

Minimum Maximum Average i 

The Valdez Group was intensely 

folded and faulted during a major 

period of orogeny which is tentatively placed in the time interval 

from Early Cretaceous to early 

Tertiary. The strike of bedding 

planes and the trend of fold axes 

are allnost east-west in the region 

east of Valdez Arm, and the trend 

swings abruptly southwestward in 

the region west of the arm. Folds 

are characteristically tightly appressed and overturned toward the 

south; drag folds and minor thrust 

faults are common. 

A second major period of orogeny 

that probably culminated in late 

STRUCTURE 

Eocene or early Oligocene time 

(Plafker and RSacNeil, 1966) complexly folded and faulted the Orca 

Group and intensified the deformation of older rocks. It was accompanied by, or closely followed by, 

emplacenient of granitic batholiths. 

The strike of bedding planes and 

fold axes approximately parallels 

the structural trends of the older 

rocks, but they are not,ably divergent at many localities. The folds 

range from open to tightly appressed and locally are overturned 

both towards the north and south. 

They are of small amplitude and 

lateral extent and are complicated 

have shown that the effect is 

negligible. 

The heaviest rocks in the assemblage are volcanic rocks of the 

Orca Group and the lightest are the 

granitic intrusive rocks. The sampling is not sufficiently representatjive to establish definitely the 

lack of density contrast between 

the Orca sedimentary rocks and the 

older, more metamorphosed Valdez 

sedimentary rocks. However, seven 

slate sainples from the combined 

groups had an average density of 

2.74 g per cm3 (grains per cubic 

centimeter), whereas 17 argillite 

and graywacke samples averaged 

2.67 g per cn13. Furthermore, four 

metagraywacke samples from the 

Valdez Group averaged 2.69 against 

2.66 for the four less-metamorphosed 

graywacke sainples from the Orca 

Group. Thus, overall densities of 

the two forinations would depend 

on the relative abundance of these 

rock types. No saniples were 

obtained froill the Quaternary deposits which probably have an 

average density of about 2 g per 

cm3. 

by intricate drag folding and minor 

thrust faults. 

The illost recent period of deformat,ion, which has continued 

internlittently since late Cenozoic 

time, is attested by the uplift of the 

adjacent Chugach and Kenai Mountains, by the frequent earthquakes 

in the region, and by recent surface 

faulting. 

All the rocks of the Prince 

William Sound region are intensely 

fractured and complexly faulted. 

The major lineaments show up on 

aerial photographs as conspicuous 

linear depressions and have been 

delineated in photogeological studit s 
ALASKA EARTHQUAKE, MARCH 27, 1964 

by Condon and Cass (1958) and 

Condon (1965). Two preexisting 

faults that displace sedimentary 

rocks of the Orca Group on Montague Island (fig. 1) were reactivated during the March 27 earthquake (Plafker, 1965). The only 

faults thus far recognized in the 

region that juxtapose lithologically 

distinctive sequences are in the 

Valdez Arm area (fig. 1) where rocks 

of the Valdez Group have been 

thrust from the northeast over the 

younger rocks of the Orca Group 

(Capps and Johnson, 1915, p. 62- 

63). Topographic and geologic 

evidence suggests the presence of 

major faults at other places where 

contact relationships are obscured 

by the lack of key beds, the presence 

of brecciation and shearing associated with minor folding and faulting, and the extensive areas covered 

by water, unconsolidated deposits, 

or vegetation. According to Moffit 

(1964, p. 275), there are no known 

depositional contacts between the 

Valdez and Orca Groups anywhere 

in Prince William Sound. 

At least seven major shear zones 

as much as 1,000 feet wide transect 

the area mapped by Richter (1965, 

p. 14) on Knight Island. Richter 

reports that there is no good evidence to indicate the sense or motion 

GRAVITY 

along the shear zones, but leftlateral strike-slip movement of as 

much as half a mile is weakly 

suggested by the supposed dislocation of a coarse-grained gabbro 

body along one of the zones. 

On Knight Island the thick 

sequence of volcanic rocks dips 

westward beneath probably older 

rocks of the Valdez Group which 

crop out west of Knight Island 

Passage. Here, the structural relationships suggest the possibility 

that the two units may be separated 

by a westward-dipping overthrust 

concealed beneath Knight Island 

Passage. 

Gravity was measured at approximately 500 stations within the 

sound (fig. 2). Most of the measurements were made with LaCoste 

and Romberg portable geodetic 

meter (2-17, but approximately 75 

measurements were made with a 

World-Wide meter. The measurements are based on absolute 

gravity values of 981,954.90 mgal 

beneath bench mark "A71" at 

Cordova Post Office, 982,008.00 

mgal at bench mark "H11" in 

Valdez, and 981,921.30 mgal at 

bench mark "L31" in Anchorage 

that were established by at least 

three postquake ties to the North 

American Standardization Pendulum station (Woollard and Rose, 

1963) at Fairbanks. These values 

differ by as much as 0.5 mgal from 

previous values measured near the 

stations established by Thiel and 

others (1958) prior to the earthquake. These deviations reflect 

small differences in location, adjustments in the North American 

gravity network, and changes 

caused by the Alaska earthquake 

(Barnes, 1966). During the survey, 

float-plane flights were used to 

establish 10 subsidiary base stations 

within the sound. Most of the 

remaining measurements were made 

at shoreline points reached by 

small-boat traverses. Base stations 

were occupied at intervals of 14 

days. A few inland stations were 

reached by automobile or aircraft 

trips. The drift and calibration 

characteristics of meter G-17 are 

such that the accuracy of 95 percent 

of observed gravity values should be 

better than 0.2 mgal. Contouring 

in the water-covered parts of 

Prince William Sound was helped 

by surface-ship gravity measurements made by the U.S. Coast and 

Geodetic Survey ship Surveyor. 

W. H. Bastian and L. R. Mayo 

assisted in the collection of the 

gravity data, and L. J. LaPointe 

made some of the computations. 

Most of the gravity stations were 

located on beaches or rocks along 

the shoreline. Elevation control 

was obtained by measuring the 

height of the station above water 

level and making a tide correction 

to obtain an elevation relative to 

mean sea level. A few stations were 

occupied at tidal bench marks and 

a few at geodetic bench marks 

established prior to the earthquake. 

Elevations at bench marks with 

respect to mean sea level had 

changed, owing to the tectonic 

deformation that accompanied the 

earthquake, so corrections to postearthquake mean sea level were 

required for these stations. Elevations of all shoreline stations are 

probably correct to within 3 feet, 

and the elevations of the few stations not on the seacoast should be 

correct to within 20 feet. Location 

control was provided by topographic maps of the Geological 

Survey, scale 1 : 63,360. Simple 

Bouguer anomalies were computed 

with a reduction factor corresponding to a density of 2.67 g per cm2. 

Simple Bouguer anomaly values 

are probably accurate to within 

about 1 mgal. Terrain corrections 

were made for a few stations close 

to the line A-A' (fig. 3) and ranged 

from 0.2 to 13.5 mgal for terrain 

through zone 0 of Hayford-Bowie 

(Swick, 1942). 
GktAVITY SURVEY, REGIONAL GEOLOGY, PRINCE WILLIAM SOUND EPICENTRAL REGION 

1 0 10 20 30 MILES 

Gravity station 

2.-Simple Bouguer anomaly map, Prince William Sound region, Alaska. 
ALASKA EARTHQUAKE, MARCH 27,1964 

3.-Gravity anomalies and profile across Prince William Sound. 

KENAl PENINSULA PORT KNIGHT 

A BLACKSTONE COCHRANE NELLIE ISLAND Knight Green Montague A' 

4000' BA Y SEA LEVELL- JUAN PASSAGE Island Island 

GENERAL FEATURES OF 

THE GRAVITY MAP 

The simple Bouguer anomaly 

contours indicate a general trend 

from positive values over the 

Alaskan Continental Shelf to negative values on the mainland. Values 

are as high as +40 mgal at the 

southwestern part of Montague 

Island and decrease to - 70 mgal at 

College and Harriman Fiords (fig. 

2). Contours are crudely arcuate 

around Prince Willian Sound, 

roughly paralleling the dominant 

trend of the geologic structure. 

Superimposed on this regional pattern are the arcuate Prince William 

Sound gravity high, which is largest 

over Knight Island, and the broad 

gravity low covering Port Gravina 

and the southeastern part of the 

sound. The high coincides with a 

belt of greenstone and pillow basalt 

and is inferred to be caused by 

these rocks. The low is probably 

caused by a thick accun~ulation of 

recent sedimentary deposit.^, a granitic intrusive rock, and, in some 

areas, terrain effects. In part the 

low is an "apparent" low due to the 

combined effect of the high to the 

- 

northwest and the regional gradient. 

NO other major correlations between 

the pattern of gravity anomalies 

and the mapped geologic features 

are apparent at this stage of interpretation. 

The new data provide a more 

complete gravity map of the Prince 

William Sound area than was 

available from the limited number 

of measurements made by Thiel and 

others (1959) which served as the 

basis for earlier Alaskan gravity 

maps (Woollard and others, 1960, 

fig. 2; Ivanhoe, 1961). These maps 

showed the high gravity values in 

the southern part of the sound and 

the low values in the Chugach 

Mountains. However, neither the 

magnitude nor the estent of the 

Prince William Sound high to the 

north and northeast was recognized, although Woollard and others 

correctly interpreted the anomaly as 

being caused by mafic rock (1960, 

p. 1029). Furthermore, we have 

measured lower gravity values in 

the Chugach Mountains than did 

Thiel's group, so the gradient on 

the northwest side of Prince William 

Sound high is steeper, and causes a 

- 

much larger gravity difference than 

had been previously indicated by 

Woollard and others (1960). Between Knight Island and Harriman 

Fiord there is a total change of 135 

mgal in 40 miles, in contrast to the 

change of about 60 mgal shown by 

Woollard and others. Between 

College Fiord and Kiniklik (near a 

low part of the Prince William 

Sound positive anomaly) there is a 

change of 70 mgal in 20 miles, in 

contrast to the change of about 50 

mgal shown by Woollard and others. 

The zero isobase separating the 

areas of subsidence and uplift 

during the Alaska earthquake of 

March 27, 1964 (Plafker, 1965, fig. 

2; Plafker and Mayo, 1965, fig. 8) 

approximately coincides with the 

steepest part of the gradient along 

the northwest side of the sound and 

locally parallels the gravity contours. This relationship could be 

purely coincidental, or it could be 

that the gradient represents, at 

least in part, a fault or hinge line at 

depth along which movement occurred during the earthquake. 

Press and Jackson (1965, p. 867) 

have postulated a near-vertical 

Island 

8000~ 

16,000'-r 

24,000'- 

32,000'-I: 

40,000'- Location of profile shown in figure 2 I I I 1 I I 

-- 

Greenstone .L 

0 10 20MILES 

- 

- 

- 

- 

- 
GRAVITY SURVEY, R .EGIONAL GEOLOGY, PRINCE WILLIAM SOUND EPICENTRAL REGION c9 

fault which lies within 15 kilometers 

of the surface beneath the zero 

isobase and extends to a depth of 

100-200 km. However, such a 

fault could explain only a part of 

the gradient on the north side of 

Knight Island, and its postulated 

configuration appears to be contrary 

to most of the available geologic 

and seismologic data (Plafker, 1965, 

p. 1686). 

The large steep gradient on the 

south side of the island suggests 

that the relatively dense greenstones 

are the real cause of the Knight 

Island anomaly. Such interpretation, based on observed geology and 

measured density variations, seems 

preferable in view of the data 

presently available, and it forms 

the basis of the remaining discussion. However, future data may 

show that some of the Prince 

William Sound anomalies are caused 

by tectonic features that have not 

yet been recognized. 

REGIONAL GRAVITY 

GRADIENT 

In order to delineate the regional 

gravity field, which is presumed to 

reflect variations in crustal thickness and density, the local anomalies 

due to the greenstone belt must be 

removed by analytical or graphical 

methods. Conversely, in order to 

isolate the anomaly caused by the 

greenstone belt, a regional gravity 

gradient must be removed. Obviously, a unique solution in removal 

of either the local anomaly or the 

regional is impossible, but reasonable approximations can be made 

for both the regional and local 

anomalies. 

Relationships between local geology, simple Bouguer anomaly, 

and complete Bouguer anomaly 

along a profile extending from 

southeast of Montague Island to the 

head of Passage Canal are shown in 

figure 3. The complete Bouguer 

anomaly profile is approximate: 

values at stations near the line of 

pr~file for which terrain corrections 

were made were projected to the 

line of profile. The complete 

Bouguer anomaly profile shows a 

rsgional gravity gradient that is 

nearly linear or slightly convex 

upward. For the following analysis, it is assumed that the regional 

gravity gradient is determined by 

the complete Bouguer gravity values 

at nlontague Island and at the 

Kenai Peninsula. The assumed 

regional gradient between these 

points is shown by the long-dashed 

line (fig. 3). Data obtained from 

the U.S. Coast and Geodetic Survey 

ship Surveyor on the Continental 

Shelf and by Thiel and others (1959) 

at Middleton Island, about 50 

miles southeast of Montague Island, 

suggest that approximately the 

same regional gradient extends to 

the continental slope. The gradient 

may be assumed to represent a 

gradual increase in crustal thickness from the continental margin to 

the northeast side of the sound. 

Woollard and others (1960) 

showed a similar gravity gradient 

and inferred increase in crustal 

thickness from about 24 km at 

Middleton Island to 33 km at 

Cordova and 37 km under the 

Chugach Mountains on their northsouth profile through Middleton 

Island, Cordova, and Valdez. However, from reinterpretation of seismic data obtained by Tatel and 

Tuve (1956) from shots fired in 

College Fiord, Woollard and others 

believe that the crust is thicker 

beneath Prince William Sound than 

the gravity data suggested. They 

derived a crustal thickness in Prince 

William Sound of about 50 km. 

The traveltime curves show high 

velocities east of Valdez, suggestive 

of mafic rocks where our new data 

show a possible extension of the 

Prince William Sound gravity high. 

Analysis of the new gravity data 

indicates that the regional gravity 

gradient is reasonably consistent 

with a change in crustal thickness 

from about 50 km near College 

Fiord (Woollard and others, 1960) 

to about 20 km near the continental 

margin, as measured by Shor (1962) 

southeast of Kodiak. 

PORT GRAVINA 

GRAVITY LOW 

The broad gravity low in the 

southeastern part of the Prince 

William Sound is best developed at 

the eastern end of Port Gravina. 

There it is in part related to the 

granitic pluton that extends from 

Port Gravina eastward into the 

Chugach Mountains (fig. I), although the outcrop area is small 

in comparison to the area of the 

gravity anomaly. However, the 

anomaly probably also reflects a 

thick accumulation of poorly consolidated sediments in the central 

and eastern parts of the sound. Its 

apparent extension into Montague 

Strait is influenced by terrain 

effects. Sample terrain corrections 

for two stations along the steep 

shoreline of the strait showed that 

the complete Bouguer anomalies at 

these stations are as much as 10 

mgal higher than the simple 

Bouguer values contoured on the 

map. Corrections of this magnitude 

would tend to straighten the +15 

and +20-mgal contours in Montague Strait and to eliminate much 

but not all of the tonguelike low 

shown by the contours on figure 2. 

The gravity low is also an apparent 

low imposed by the Prince William 

Sound high on the regional gravity 

gradient. 

The low which would remain in 

Montague Strait after terrain corrections and much of the low which 

occupies the central and southeastern part of the sound in part 

represent thick, poorly consolidated 

glacial and marine sedimentary 

deposits. The average water depths 

in the central and eastern parts of 

the sound are shallow, about 300- 

800 feet, whereas water depths of 

1,000-2,000 feet are common in the 
ALASKA EARTHQUAKE, MARCH 27, 1964 

northwestern part of the sound and 

in some of the narrower passages 

and fiords. If the sound were once 

glaciated to a depth equivalent to 

the present water depths in the 

fiords, poorly consolidated material 

as much as 1,000-1,500 feet thick 

may be present at the site of the 

gravity low. However, sparker 

data obtained from the U.S. Coast 

and Geodetic Survey ship Surveyor 

(R. C. Malloy, oral commun., 1964, 

1965) suggest that 500 feet of sediment may be the maximum accumulation in most of the southeastern part of the sound. This 

thickness would account for not 

more than 10 mgal of the gravity 

low. 

The main cause of the gravity low 

in the southeastern part of the 

sound is thus presumed to be the 

granitic rocks which crop out near 

Port Gravina and which probably 

extend westward beneath the cover 

of sedimentary rocks. Although 

the one sample obtained from the 

Port Gravina pluton had a density 

as high as the average of the Valdez 

and Orca sedimentary rocks, samples from plutons on the opposite 

side of the sound indicate that the 

average density of the Prince 

William Sound granite is about 

0.07 g per cm3 less than the densities of the Valdez and Orca sedimentary rocks. The absence of 

gravity lows over the intrusive 

bodies on the northwestern side of 

the sound suggests that they are 

smaller and that the Port Gravina 

intrusive is larger and possibly less 

dense. However, terrain corrections 

and precise contouring might also 

show small negative anomalies associated with the intrusive rocks on 

the northwest. 

PRINCE WILLIAM SOUND 

GRAVITY HIGH 

When the assumed regional 

Bouguer anomaly values are subtracted from the observed Bouguer 

anomaly profile, a residual positive 

anomaly of as much as 67 mgal can 

be isolated over the Knight Island 

greenstone (fig. 4). The residual 

anomaly is asymmetrical: the gradient on the southeast flank is much 

steeper than on the northwest flank 

of the anomaly. The steepness of 

the gradient on the southeast flank 

indicates that most of the relatively 

dense rock mass causing the anomaly probably lies at shallow depthwithin the upper 10 km of the 

crust. 

The greenstones and pillow basalts in general dip steeply. Richter 

(1965) has postulated that the belt 

of greenstone cut by shear zones 

may represent a complex anticlinal 

structure whose axis follows the 

approximate centerline of Knight 

Island. The breadth of exposures 

on Knight Island is about 8 miles, 

or 42,000 feet, and the sequence 

probably extends downward at 

least an equivalent amount if the 

steep dips are maintained at depth. 

Available density measurements 

for rocks in Prince William Sound 

indicate that the average density of 

the greenstones is greater than that 

of the graywacke, slate, and granite 

with which the greenstones are in 

contact (table 1). The average 

density of the greenstone is 2.87 g 

per cm3, the slate and graywacke is 

2.69 g per cm3, and the granitic 

rock is 2.60 g per cm3. Thus the 

density contrast between greenstone and the adjacent rocks is 

about 0.14.2 g per cm3. 

A simplified model of the greenstone mass on Knight Island has a 

computed gravitational effect which 

approximates the residual anomaly 

(fig. 4). In construction of the 

model and computation of its 

gravitational effects, the following 

KENAl PENINSULA PORT 

A BLACKSTONE COCHRANE NELLIE &~~~~ Knight Green Montague A' 

4000' JUAN PASSAGE Island Island Island SEA LEVELd -- /- -- - - --- r 

4.-Interpretation of the gravity anomaly over Knight Island. Ap is the density contrast. 

8000~ 

16,000~ 

24,000~ 

32,000'- 

Graywacke, slate, and argillite 

Graywacke, slate, argillite, 

and granitic(?) rocks 10 

40,000'- Location of profile shown in figure 2 
GRAVITY SURVEY, REGIONAL GEOLOGY, PRINCE WILLIAM SOUND EPICENTRAL REGION c11 

assumptions were made: (1) the 

source of the anomaly is twodimensional, that is, its length is 

great with respect to its breadth; 

(2) the average density contrast of 

the greenstone mass is about +0.2 

g per cm3; (3) the width of the 

anomalous mass at the surface is 

about 40,000 feet; (4) it extends 

downward to a depth of about 

40,000 feet below sea level, and (5) 

the contacts of the anomalous mass 

dip northwest, and the northwestern contact dips more gently 

than the southeastern. Thus the 

model represents a mass whose 

breadth increases with depth. This 

configuration is consistent with 

Richter's hypothesis that the volcanic rocks form a major anticline. 

Gravitational effects of the sirnplified model were computed by the 

method described by Hubbert 

(1948). The computed effects approximately coincide with the residual anomaly, both in total amplitude and in gradient. Slight 

changes in density contrast or in 

dimensions of the mass would produce an exact match, but these 

changes are not warranted because 

the dimensional assumptions and 

the regional gradient are more 

uncertain. The significant facts 

are: (I) a large greenstone mass is 

the most probable cause of the 

Knight Island anomaly; (2) its 

density is slightly greater than that 

of the adjacent rocks; (3) the mass 

probably extends to great depth, 

40,000 feet or more below sea level; 

and (4) the gravity data indicate 

that faults may border both flanks 

of the Knight Island greenstone. 

The gravity highs form a continuous belt from the greenstone outcrops on Elrington and Latouche 

Islands, through the area of maximum width of outcrop on Knight 

Island, to another high over the 

greenstone outcrops on Glacier Island, and then across Valdez Arm 

to the outcrops on Ellamar Peninsula. No large greenstone outcrops 

have been mapped northeast of the 

Ellamar outcrops, and a Bouguer 

gravity measurement of - 1.3 mgal 

at the southeast end of Silver Lake 

suggests that the greenstones there 

are deeper and smaller. However, 

uncompleted surveys show positive 

gravity values 10 miles east of 

Barnes, D. F., 1966, Gravity changes 

during the Alaskan earthquake: Jour. 

Geophys. Research, v. 71, no. 2, 

p. 451-456. 

Barnes, D. F., and Allen, R. V., 1965, 

Progress report on Alaskan gravity 

surveys [abs.]: Alaskan Sci. Conf., 

15th, College, Alaska, 1964, Proc., 

p. 165-166. 

Capps, S. R., and Johnson, B. L., 1915, 

The Ellamar district, Alaska: U.S. 

Gcol. 'urvey Bull. 605, 125 p. 

Condon, W. H., 1965, Map of eastern 

Prince William Sound area, Alaska, 

showing fracture traces inferred from 

aerial photographs: U.S. Geol. Survey 

Misc. Geol. Inv. Map 1-453, scale 

1:125,000. 

Condon, W. H., and Cass, J. T., 1958, 

Map of a part of the Prince William 

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Sound area, Alaska, showing linear 

geologic features as shown on aerial 

photographs: U.S. Geol. Survey Misc. 

Geol. Inv. Map 1-273, scale 1:12,5,000. 

Grant, U. S., and Higgins, D. F., 1910, 

Reconnaissance of the geology and 

mineral resources of Prince William 

Sound, Alaska: U.S. Geol. Survey 

Bull. 443, 89 p. 

Hubbert, M. K., 1948, A line-integral 

method of computing the gravimetric 

effect of two-dimensional masses: 

Geophysics, v. 13, no. 2, p. 215-225. 

Ivanhoe, L. F., 1961, Bouguer gravity 

map of Alaska [abs.]: Am. Assoc. 

Petroleum Geologists Bull., v. 45, 

no. 1, p. 128-129. 

Moffit, F. H., 1954, Geology of the Prince 

William Sound region, Alaska: U.S. 

Geol. Survey Bull. 989-E, p. 225-310. 

Valdez on the Richardson Highway, 

and 40-50 miles east of Valdez 

along the Tasnuna River up to its 

junction with the Copper River 

which could be continuous with the 

Prince William Sound high. If so, 

the incomplete data thus suggest 

that the belt of gravity highs may 

extend for 150 miles from Elrington 

Island at the coast to a point on 

the Copper River, where it is more 

than halfway across the Chugach 

Mountains. The computed profile 

(fig. 4) crosses the greenstone belt 

where the gravity anomaly associated with it is largest. Although the magnitude of the high 

varies along the belt, the fact that 

there are no major horizontal offsets 

of the anomaly northeast of Elrington Island suggests that no 

major post-Eocene faults having 

strike-slip displacement measureable in tens of miles obliquely cut the 

belt of gravity highs. The gravity 

survey has thus shown that the 

greenstone belt, which may be part 

of an ancient volcanic arc, constitutes one of the more continuous and 

significant geologic units along the 

Gulf of Alaska coastline. 

Plafker, George, 1965, Tectonic deformation associated with the 1964 Alaska 

earthquake: Science, v. 148, no. 3678, 

p. 1675-1687. 

Plafker, George, and MacNeil, F. S., 

1966, Stratigraphic significance of 

Tertiary fossils from the Orca Group 

(Mesozoic), Prince William Sound, 

Alaska: U.S. Geol. Survey Prof. 

Paper 550-B, p. B62-B68. 

Plafker, George, and Mayo, L. R., 1965, 

Tectonic deformation, subaqueous 

slides, and destructive waves associated with the Alaskan March 27, 

1964, earthquake-an interim geologic evaluation: U.S. Geol. Survey 

open-file report,, 21 p. 

Press, Frank, and Jackson, David, 1965, 

Alaskan earthquake, 27 March 1964 

-Vertical extent of faulting and 
ALASKA EARTHQUAKE, MARCH 27, 1964 

elastic strain energy release: Science, 

v. 147, no. 3660, p. 867-868. 

Richter, I). H., 1965, Geology and mineral 

deposits of central Knight Island, 

Prince William Sound, Alaska: Alaska 

Div. Mines and Minerals Geol. 

Rept. 16, 37 p. 

Schrader, I?. C., 1900, A reconnaissance of 

a part of Prince William Sound and 

the Copper River district, Alaska, in 

1898: U.S. Geol. Survey Ann. Rept. 

20, pt. 7, p. 341-423. 

Schrader, F. C., and Spencer, A. C., 1901, 

The geology and mineral resources of 

a portion of the Copper River district, Alaska: U.S. Geol. Survey 

Spec. Pub., 94 p. 

Shor, G. G., Jr., 1962, Seismic refraction 

studies off the coast of Alaska 1956- 

1957: Seismol. Soc. America Bull., v. 

52, no. 1, p. 37-57. 

Swick, C. H., 1942, Pendulum gravity 

measurenlents and isostatic reductions: U.S. Coast and Geod. Survey 

Spec. Pub. 232, 82 p. 

Tatel, H. E., and Tuve, M. A., 1956, The 

earth's crust, seismic studies: Carnegie Inst., Washingt.on, Year Book 

1955-1956, p. 81-85. 

Thiel, Edward, Ostenso, N. A., Ronini, 

W. E., and Woollard, G. P., 1958, 

Gravity measurements in Alaska: 

Woods Hole Oceanogr. Inst. Tech. 

Rept. Ref. 58-54, 104 p. 

1959, Gravity measurements in 

Alaska: Arctic, v. 12, no. 2, p. 67-76. 

IVoollard, G. P., Ostenso, N. A., and 

Thiel, Edward, 1960, Gravity anomalies, crustal structure, and geology in 

Alaska: Jour. Geophys. Research, 

v. 65, no. 3, p. 1021-1037. 

Woollard, G. P., and Rose, J. C., 1963, 

Interna.tiona1 gravity measurements: 

Wisconsin Univ. Geophys. and Polar 

Research Center, 518 p. 

* U.S. GOVERNMENT PRINTING OFFICE: 1966--213-780