Sandstone Composition of the 

Valdez and Orca Groups, 

Prince William Sound, Alaska 

Sandstone Composition of the 

Valdez and Orca Groups, 

Prince William Sound, Alaska 

By JULIE A. DUMOULIN 

U.S. GEOLOGICAL SURVEY BULLETIN 1774 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1987 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Dumoulin, julie A. 

Sandstone composition of the Valdez and Orca Groups. 

Prince William Sound, Alaska. 

U.S. Geological Survey Bulletin 1774 

Bibliography 

1. Sandstone-Alaska-Prince William Sound Region. 2. 

Geology. Stratigraphic-Cretaceous. 3. Geology, 

Stratigraphic-Tertiary. 4. Valdez Group (Alaska) 5. Orca 

Group (Alaska). J. Title. II. Series. 

QE75.B9 No. 1774 622 s 87-600307 

[QE471.25.525] [551.5'31 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 2 

General geology 2 

Valdez Group and correlative rocks 2 

Orca Group and correlative rocks 5 

The Contact fault system 5 

Previous petrographic studies 7 

Methodology 8 

Results of studies of Valdez Group samples 8 

West, central, and east Valdez areas 8 

Differences between west Valdez- and central Valdez-area samples 10 

Provenance 12 

Anomalous Valdez Group samples 13 

Results of studies of Orca Group samples 14 

Eastern exposures of the Orca Group 14 

Western exposures of the Orca Group 19 

Anomalous Orca Group samples 22 

Comparison of results of studies of the Orca Group with previous work 25 

Tectonic models and the placement of the Valdez-Orca boundary 25 

Evidence for the northward displacement of the Valdez and Orca Groups 26 

Petrographic constraints on the significance and location of the Valdez-Orca boundary 27 

The existence of a terrane boundary 27 

Alternative locations of a terrane boundary 27 

Johnstone Bay fault 27 

Bainbridge fault 28 

Montague Strait fault 28 

Placer River fault 28 

Imbricate faulting along terrane boundary 28 

Summary 28 

Areas that have anomalous sandstone petrography Mount Denson area 29 

Areas spatially associated with rna fie rocks 29 

Resurrection Peninsula area 29 

Knight Island area 30 

Ellamar area 30 

Montague Island and Ragged Mountain areas 30 

Petrography 31 

Age 31 

Metamorphic-grade relationships 33 

Structural style 34 

Sedimentary facies 34 

Conclusions 34 

References cited 34 

FIGURES 

1. Generalized geologic-tectonic setting of the Prince William Sound area, Alaska. 

Terrane boundaries 2 

2. Geologic map of the Prince William Sound area 3 

3. Turbidites of the Valdez Group in the Port Wells area, western Prince William 

Sound 4 

4. Fossil localities and location of Valdez Group-Orca Group contact in western 

Prince William Sound as delineated ·by previous workers 6 

Ill 
5. Location of petrographic areas and samples point-counted for this study 7 

6. Modal analyses of sandstone of the Valdez Group from the west Valdez, east 

Valdez, and central Valdez areas 10 

7. Photomicrographs in transmitted light of thin sections of sandstones from the 

Valdez Group 11 

8. Modal analyses of sandstone from the Valdez Group and (at least partly) correlative 

lithostratigraphic units 12 

9. Modal analyses of sandstone of the Valdez Group from the Mount Denson and 

Resurrection Peninsula areas compared with analyses of sandstone of the Orca 

Group from the Port Gravina-Cordova area 15 

10. Modal analyses of sandstone of the Valdez Group from the Mount Denson and 

Resurrection Peninsula areas compared with those from the west Valdez, 

central Valdez, and east Valdez areas 16 

11. Modal analyses of sandstone of the Orca Group from the Port Gravina and Cordova 

areas compared with analyses of sandstone of the Orca Group from the 

Hinchinbrook Island area 17 

12. Photomicrographs in transmitted light of thin sections of sandstones from the Orca 

Group 18 

13. Modal analyses of sandstone of the Orca Group from the Sargent Icefield and 

Chenega areas compared with analyses of the Ghost Rocks Formation 20 

14. Comparison of sandstone modal analyses from the Valdez and Orca Groups 21 

15. Modal analyses of sandstone of the Orca Group from the Ellamar and Knight Island 

areas 23 

16. Comparison of modal analyses of sandstone from the Knight Island-Ellamar 

(combined), Montague Island, and Ragged Mountain areas with those from the 

Port Gravina-Cordova area 24 

17. Modal analyses for sandstone of the Orca Group from the Montague Island and 

Ragged Mountain areas compared with analyses from the Zodiac fan and from 

the Sitkalidak Formation 32 

18. Ratio of volcanic clasts to total lithic fragments in Zodiac fan sediment, Sitkalidak 

Formation sandstone, and Orca Group sandstone from the Montague Island and 

Ragged Mountain areas 33 

TABLES 

1. Composition of sandstone from the west Valdez, east Valdez, and central Valdez 

areas 9 

2. Geochemical analyses of selected Valdez and Orca Group sandstone samples 13 

3-5. Composition of sandstones: 

3. From the Valdez Group: Mount Denson and Resurrection Peninsula 

areas 14 

4. From the Orca Group: Cordova and Port Gravina areas 16 

5. From the Orca Group: Sargent Icefield and Chenega areas 19 

6. Statistical summary of first-degree trend surface for point-count data 21 

7-8. Composition of sandstones: 

7. From the Orca Group: Knight Island and Ella mar areas 22 

8. From the Orca Group: Montague Island and Ragged Mountain areas 31 

I.V 
Sandstone Composition of the Valdez and Orca Groups, 

Prince William Sound, Alaska 

By Julie A. Dumoulin 

ABSTRACT 

The contact between the Upper Cretaceous 

Valdez Group and the Paleocene and Eocene Orca 

Group has been interpreted as a major boundary 

between the Chugach and the Prince William tectonic 

terranes. Petrographic analyses of sandstone present 

in the Prince William Sound area, however, reveal no 

sharp compositional break across this contact. 

In the western part of the Prince William Sound 

area, compositions of both groups chan6e 

systematically from west to east. No abrupt change 

occurs at the Contact fault system, which juxtaposes 

the two groups; ternary plots and discriminant analysis 

cannot distinguish sandstone of the easternmost part 

of the Valdez Group (east of the Placer River fault) 

from that of the westernmost part of the Orca Group 

(Sargent Icefield area). 

Valdez Group sandstone west of the Placer River 

fault contains abundant volcanic lithic clasts, and 

lathwork textures predominate over felsitic textures; 

mean Q-F-L is 15.5-35-49.5. East of the Placer River 

fault, sandstone of both groups becomes progressively 

richer eastward in quartz, potassium feldspar, and 

plagioclase, and progressively poorer eastward in 

lathwork-textured volcanic clasts. Trend-surface 

analysis indicates that these changes are statistically 

significant at the 99 percent confidence level (95 

percent level for plagioclase). The composition of the 

eastern part of the Orca Group (Cordova-Port Gravina 

area) is more nearly homogeneous; average Q-F-L is 

27-57-16, and lithic clasts are mostly volcanic. The QF-L and Qp-Lv-Ls plots of point-count data indicate a 

dominantly magmatic-arc provenance for sandstones 

of both groups, with continued erosion from the 

volcanic cover to the plutonic roots of the arc through 

time. 

The petrographic data for these sandstones are 

best explained by assuming that there is no terrane 

boundary in western Prince William Sound, and that 

the Valdez and Orca Groups form a single flysch 

sequence derived from one progressively unroofing 

source in Late Cretaceous and Paleogene time. The 

magmatic arc of the Peninsular terrane represents one 

possible source. If a major terrane boundary does exist 

in western Prince William Sound, sandstone 

petrographic data indicate that it is most likely to lie 

along the Bainbridge fault, 50 km east of the Contact 

fault. The Bainbridge fault marks the most striking 

petrographic discontinuity found in this study-it 

separates the quEtrtz-rich rocks of the Knight Island 

area (Q-F-L = 51-31-18) from the more feldspathic 

rocks of the Chenega area (Q-F-L = 27-51-22). 

Several regions of anomalous sandstone 

composition are delineated by this study. Sandstone of 

the Mount Denson area, currently assigned to the 

Valdez Group, contains significantly more quartz and 

feldspar clasts than do other sandstones of the Valdez 

Group, and is modally similar to Orca Group sandstone 

(mean Q-F-L 28-60-12). Sandstone spatially 

associated with volcanic rocks--in the Resurrection 

Peninsula area (currently assigned to the Valdez 

Group) and in the Knight Island and Ellamar areas 

(currently assigned to the Orca Group)-is relatively 

rich in quartz and feldspar. The Qp-Lv-Ls plots 

indicate the presence of an important subductioncomplex component in the provenance of these rocks, 

but their anomalous compositions may also reflect 

barrier effects of local volcanic edifices. Sandstone of 

the Montague Island and Ragged Mountain areas, 

currently assigned to the Orca Group, is rich in quartz 

and sedimentary lithic clasts (Montague area mean QF-L = 41.5-34.5-24). These rocks show similarities in 

petrography age, metamorphic grade, structural style, 

and sedimentary facies, to the Sitkalidak Formation, 

400 km to the west, as well as petrographic 

similarities to the sediments of the Zodiac fan. 

INTRODUCTION 

D.L. Jones and his coworkers have suggested that 

the contact between the Upper Cretaceous Valdez 

Group and the Paleocene and Eocene Orca Group 

represents a major terrane boundary. For example, 

Jones and others (1981) and Silberling and Jones (1984) 

used this contact as the boundary between the 

Chugach and Prince William terranes (fig. 1). 

However, the placement of the Valdez-Orca contact 

has long been a source of controversy. This is 

especially true in western Prince William Sound, where 

the regional strike of the two groups is parallel and 

fossil data are sparse. Preliminary petrographic 

investigations (Mitchell, 1979; Winkler, 1976) 

suggested that the sandstone composition of the 

Valdez Group and its lateral equivalents is distinct 

from that of the Orca Group. The present study was 

undertaken in order to characterize the sandstone 

composition of the two groups throughout the Prince 

William Sound area and particularly to test whether a 

Introduction 1 
significant change in sandstone composition may help 

define the placement of the contact between the 

Valdez and Orca Groups. 

ACKNOWLEDGMENTS 

This study was part of a survey of the geology 

and mineral resources of the Chugach National Forest, 

conducted under the auspices of the second Roadless 

Area Review and Evaluation (RARE II). I particularly 

acknowledge the many important contributions of my 

Chugach RARE II colleagues S.W. Nelson and M.L. 

Miller, U.S. Geological Survey. I also thank R.J. 

Goldfarb, U.S. Geological Survey, for his statistical 

analysis of the sandstone point-count data, and R.G. 

Tysdal and S.H.B. Clark, both of the U.S. Geological 

Survey, for the loan of thin sections from western 

Prince William Sound. 

ALASKA 

0 

60 

GENERAL GEOLOGY 

Valdez Group and Correlative Rocks 

The Valdez Group comprises part of a belt of 

Cretaceous and Jurassic accretionary rocks more than 

2000 km long and as much as 100 km wide that extends 

along the Alaska coastal margin from Baranof Island 

on the east to the Sanak and Shumagin Islands on the 

west (Plafker and others, 1977) (fig. 1). It is composed 

primarily of flysch deposits with volumetrically minor 

intercalations of mafic intrusive and volcanic rocks 

(figs. 2 and 3). 

This belt has been called the Chugach terrane 

(Jones and others, 1981), and it is separated from the 

Peninsular terrane to the north by the Border Ranges 

fault system. It consists of a discontinuous-landward 

melange facies and a continuous-seaward flysch facies. 

140 ° 

GULF OF ALASKA 

0 100 200 

~-__,__-~ kilometers 

EXPLANATION Terranes Terrane-bounding faults 

A Alexander P Peninsular W Wrangellia BRF Border Ranges fault 

C Chugach PW Prince William Y Yakutat CF Contact fault 

Figure L Generalized geologic-tectonic setting of the Prince William Sound area, 

Maska. Terrane boundaries generalized from Jones and others, 1981; Silberling and 

Jones, 1984. 

2 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, AJaska 
In the Prince William Sound area, the melange 

facies is known as the McHugh Complex. Generally 

correlative rocks to the west are the Uyak Complex 

(Kodiak area) and the Seldovia Bay Complex (southern 

Kenai Peninsula); to the east, generally correlative 

rocks are the melange facies of the Yakutat Group 

and, in the Chichagof Island area, the Kelp Bay 

Group. Paleontologic ages of the rna trix of the 

melange facies range from Jurassic to Late 

Cretaceous (Nelson and others, 1986); paleontologic 

and radiometric ages of slabs and blocks in the 

melange range from late Paleozoic to Late 

Cretaceous. (See summary in Winkler and others, 

1984.) 

The melange facies is separated from the flysch 

facies by the Eagle River and related faults. Valdez 

Group is the name assigned to the flysch facies that 

extends from the southern margin of the Kenai 

Peninsula eastward to Chichagof Island (fig. 1). 

Correlative rocks to the southwest are the Kodiak and 

Lake George 

'-· " I ,.__" I 

\ ! 6 I To s 

(j ( 

60° (' 

Shumagin Formations. Rocks in southeastern Alaska 

that are probably correlative include the Sitka 

Graywacke, particularly the Ford Arm unit of Decker 

(1980), and part of the Yakutat Group. 

The flysch facies of the Chugach terrane consists 

of slope, fan, and basin-plain deposits and has a 

dominantly magmatic-arc provenance (Zuffa and 

others, 1980). Flysch deposition was apparently 

confined to a trench in which sediment transport was 

dominantly westward (Nilsen and Zuffa, 1982). 

In general, the Valdez Group is folded and 

deformed, and it is metamorphosed to grades ranging 

from laumontite to amphibolite (or hornblende 

hornfels) facies (Nelson and others, 1985). 

Laumontite- to mid-greenschist-facies rocks dominate 

in the western part of the study area; the eastern part 

contains mainly mid- to upper-greenschist facies, with 

the development of amphibolite facies locally north of 

Port Fidalgo and in a larger area east of the Copper 

River (Hudson and Plafker, 1982; Goldfarb and others, 

0 25 50 KILOMETERS Tov 

EXPLANATION 

Sedimentary rocks lTertiaryl--Consist of [::G:J the undivided Yakataga (part), Redwood, 

Paul Creek, Tokun, Kulth ieth , and 

Stillwater Fo r mations 

Intrusive rocks IOi igo cene and Eocenel--

Consist of undivided granite , 

granodiorite, diorite , and gabbro 

Orca Group (Eocene and Paleocenel--

Divided into: 

Sedimentary rocks 

Mafic volcanic and intrusive rocks 

Valdez Group ( Upper Cretaceousl--

Divided into: 

~ ~ Sedimentary rocks 

~ Mafic volcanic and intrusive rocks 

.A, __ 

RARE II study area boundary 

Contact 

Fault - Dashed where 

concealed or inferred 

Normal or strike slip 

Thrust or high angle reverse, 

sawteeth on upper plate 

Figure 2. Geologic map of the Prince William Sound area, simplified and slightly modified from Nelson and 

others (1985). See figure 1 for location. 

General Geology 3 
Figure 3. Turbidites of the Valdez Group in the Port Wells area, western Prince William 

sound. Width of outcrop is about 3_m. Photograph by Marti L. Miller, U.S. Geological 

Survey. 

4 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
1986; Sisson and others, 1986). The Late Cretaceous 

age of the group is based on a few widely scattered 

Campanian(?) and Maestrichtian pelecypods (Jones and 

Clark, 1973). 

Orca Group and Correlative Rocks 

The Orca Group also consists predominantly of 

flysch deposits, with subordinate intercalations of 

mafic intrusive and volcanic rocks. It too is part of an 

accretionary belt, of Paleogene age, that extends 

through Prince William Sound to the Kodiak Islands to 

the west and probably underlies much of the 

contiguous continental shelf (Plafker 1969, 1971; 

Tysdal and Case, 1979). Laterally equivalent rocks in 

the Kodiak-Sitkalidak Islands are the Paleocene Ghost 

Rocks Formation of Moore and others (1983) and the 

Eocene Sitkalidak Formation of Moore and Allwardt 

(1980}. This accretionary belt is considered by Jones 

and others (1981) to constitute the Prince William 

terrane, separated from the Chugach terrane to the 

north by the Contact fault system. 

The Orca Group in Prince William Sound 

represents middle (and possibly inner) submarine-fan 

deposits (Winkler, 1976; Nelson and others, 1985). 

Correlative rocks to the west include abyssal-plain, 

slope, slope-basin, or forearc-basin deposits (Moore 

and others, 1983). 

Like the Valdez Group, the Orca Group has been 

folded and deformed and has reached metamorphic 

grades of laumontite to upper-greenschist facies, with 

local development of amphibolite (or hornblende 

hornfels) facies (Nelson and others, 1985; M.L. Miller, 

written commun., 1985). The lowest grade rocks form 

a northeast-southwest-trending belt that includes 

Montague and Hinchinbrook Islands; metamorphic 

grade increases to the north and the northwest. In 

general, the Orca Group shows prehnite-pumpellyite to 

lower-greenschist facies, with the highest grade rocks 

occuring in three situations: (1) as aureoles around 

plutons, (2) near faults, and (3) on a more regional 

scale, east of the Copper River (Goldfarb and others, 

1986; M.L. Miller, written commun., 1985). 

Sparse microfossil and megafossil assemblages 

have been recovered from the Orca Group (Plafker and 

others, 1985a and b). Northern exposures of the group 

are intruded by a belt of granitic, granodioritic, and 

tonalitic plutons; K/ Ar ages on mineral separates from 

these plutons range from 50.5-53.5 ± 1.6 Ma (Plafker 

and Lanphere, 197 4). Plafker and others (1985b) 

maintained that the combined paleontologic and 

radiometric data constrain the age of the Orca Group 

to the late Paleocene through the early middle 

Eocene. However, "the lower age range of the unit is 

not known" (Plafker and others, 1985b, p. 74) and no 

data exist that would preclude an early Paleocene age 

for parts of the Orca Group. The upper age limit of 

the Orca Group is interpreted by Plafker and others 

(1985b) to be the emplacement age of the late early to 

earliest middle Eocene plutons, but these plutons do 

not intrude southeastern exposures of the Orca Group 

(Montague and Hichinbrook Islands, Ragged 

Mountain). Futhermore, these southeastern exposures 

contain microfaunas of middle and late Eocene age 

(Plafker and others, 1985a) (fig. 4). Therefore, in this 

report, the age of the Orca Group is considered to be 

Paleocene through Eocene. 

The Contact Fault System 

The Orca Group is separated from the Valdez 

Group to the north by the Contact fault, which 

includes the Bagley, Gravina, and Landlock fault 

segments in the Cordova quadrangle (Winkler and 

Plafker, 1981) (fig. 5). Both the Orca and Valdez 

Groups are cut by numerous large-scale faults. 

Because of the similarity of the lithology and, locally, 

the metamorphic grade of the two groups, and the 

paucity of fossils obtained from either group, the 

placement of the boundary between them has been 

difficult to locate. This is particularly true in the 

western part of Prince William Sound, where the 

regional strike of the two groups is parallel. 

Early workers originally assigned to the Orca 

Group rocks of the islands east of Bainbridge and 

Knight Island Passages, the rocks of Naked and Glacier 

Islands, and rocks of the mainland adjacent to Orca 

Bay, Port Gravina, and Port Fidalgo (Grant and 

Higgins, 1910) (fig. 4). Moffit (1954) included the 

rocks of eastern Chenega Island in the Orca Group, but 

he assigned the rocks of western Bainbridge Island and 

the west side of Valdez Arm to the Valdez Group. He 

stated, however, that "the Orca Group *** cannot yet 

be clearly differentiated from the Valdez Group" (p. 

248) and "the longer the work has gone on, the less 

certain appears to be the distinction" between the two 

groups (p. 240). 

At the time of Moffit's work, available fossil 

data fostered the belief that the two groups were of 

the same (Mesozoic) age; with this and the lithologic 

similarity of the two groups in mind, Moffit (1954) 

pointed out the desirability of discontinuing the use of 

the two names. After the discovery of crabs and 

pelecypods of Paleogene age north of Galena Bay 

(Plafker and MacNeil, 1966), the idea that the two 

sequences were separated on the basis of age was 

revived. Plafker and others (1977) placed the OrcaValdez contact in western Prince William Sound about 

25 km west of the boundary Moffit had used (fig. 4)-

close to what is now called the Johnstone Bay faultseparating rocks assigned a Cretaceous age (Valdez 

Group) from those assigned a Paleogene age (Orca 

Group). 

Tysdal and Case (1979) placed the contact 

another 25 km to the west, claiming that they were 

not able to distinguish a stratigraphic break along the 

trend Plafker and others (1977) used as the contact. 

The contact delineated by Tysdal and Case is now 

called the Contact fault. Although Tysdal and Case 

(1979) stated that in the northern part of the Seward 

quadrangle this structure is a major fault zone (with a 

trace of sheared and fractured rock 2-4 km wide), they 

added that "the intensity of deformation along the 

fault zone decreases from north to south" (p. 10). 

South of Wells Passage, rocks of the same 

metamorphic grade "occur on both sides of the fault 

zone, and the exact location of the fault is difficult to 

define" (Tysdal and Case, 1979, p. 10). 

Thus, the area in western Prince William Sound 

where rocks have been variously assigned to both the 

General Geology 5 
Orca and Valdez Groups by previous workers 

encompasses a belt about 50 km wide between the 

Contact fault of Tysdal and Case (1979) and the trend 

of the Bainbridge and Knight Island Passages. This 

large area has so far yielded no age-specific fossil 

assemblages, so the rocks cannot be assigned to either 

the Orca or Valdez Groups on this basis. The most 

easterly fossil collected in this part of the Valdez 

Group is a pelecypod of Late Cretaceous age found in 

float near Point Cochrane (fig. 4), along the 

southwestern shore of Port Wells. The most westerly 

fossil assemblage in the Orca Group consists of poorly 

preserved foraminifers from Bear Point, identified as 

probably Tertiary in age (Tysdal and Case, 1979). 

Lithologically, the area between the Contact 

fault of Tydsal and Case (1979) and the trend of the 

Bainbridge and Knight Island Passages is, in large part, 

distinct from adjacent rocks of either the Orca or the 

Lake George ;fo 

D 

Valdez Groups. Much of the area consists of a belt of 

massive sandstone referred to by Helwig and Emmet 

(1981) as the Blying Sound belt. Earlier workers 

(Moffit, 1954; Tysdal and Case, 1979) also recognized 

the distinctive character of these rocks. On trend 

with this belt of massive sandstone in the Unakwik 

Inlet area (particularly in the vicinity of Miners Lake 

and Long Bay) are extensive chaotic deposits (probably 

formed through mass slumping) and one of the thickest 

sequences of conglomerate known in the Orca Groupmore than 300 m of matrix-supported conglomerate 

that has subordinate intervals of massive sandstone 

and thinly interbedded sandstone and mudstone. These 

facies associations indicate inner-fan deposition 

(Nelson and others, 1985). 

Also, metamorphic grade does not change along 

the Con tact fault in western Prince William Sound. 

Along the northern part of the Sargent Icefield and 

147° 

EXPLANATION 

Fossil Localities Type 

Location of Valdez GroupOrca Group contact 

0 25 50 ......._ __ __._ ___ .J KILOMETERS 

Age 

o Late Cretaceous 

<D Tertiary (probable! 

0 Paleocene and 

(orJ Eocene 

C Coccolith 

d Dinoflagellate 

Foraminifer 

m Megafossil 

o Pelecypod 

r Radiolarian 

S Silicoflagellate 

Grant and Higgins, 1910 

Moffit, 1954 

Plafker and others, 1977 

-+- Tysdal and Case, 1979 

Figure 4. Fqssillocalities and location of Valdez Group-Orca Group contact in western Prince William Sound 

as delineated by previous workers. Fossil localities marked * were collected in float. Bbp, Bainbridge 

Passage; BI, Bainbridge Island; BP, Bear Point; CI, Chenega Island; GB, Galena Bay; ffi, Icy Bay; KIP, Knight 

Island Passage; PC, Point Cochrane; PNJ, Port Nellie Juan; UI, Unakwik Inlet. See figure 1 for location. 

6 Sandstone Composition of the Valdez and Orca Groups, Pl'ince William Sound, Alaska 
between Port Nellie Juan and Passage Canal, rocks of 

the prehnite-pumpellyite facies are present on both 

sides of the fault. Farther south, rocks of lower- and 

mid-greenschist facies crop out both east and west of 

the fault (Goldfarb and others, 1986; M.L. Miller, 

written commun., 1985). 

Thus, there is no paleontologic litho logic, 

metamorphic, or structural criterion that allows easy 

placement of the Orca-Valdez contact in western 

Prince William Sound. Reconnaissance studies of the 

sandstone of both groups, however, indicated that 

sandstone composition might aid in the determination 

of the location of the Orca-Valdez boundary. 

PREVIOUS PETROGRAPHIC STUDIES 

The petrography of the sandstone of the Valdez 

Group in Prince William Sound has previously been 

studied only in a limited area around the Hope-Sunrise 

mining district (Mitchell, 1979). Petrographic data for 

150° 

• 

• 

.. 

.. .i' 

• 

Peninsula 

area 

'(:5····· 

correlative rocks to the west of Prince William Sound 

have been published for the Shumagin Formation (Burk, 

1965; Moore, 1973) and the Kodiak Formation 

(Connelly, 1978; Winkler, in Nilsen and Moore, 1979). 

Petrography of the (at least in part) correlative Sitka 

Graywacke to the east of Prince William Sound was 

studied by Decker (1980). The study by Zuffa and 

others (1980) is an overview of the rock-fragment 

petrography for sandstone throughout the Chugach 

terrane. 

The major petrographic study of the Orca Group 

was the investigation of the Hinchinbrook Island area 

by Winkler (1976). McGlasson (1976) provided a few 

point counts for sandstone of the Orca Group on 

Knight Island. The petrography of correlative rocks to 

the west (the Sitkalidak and Ghost Rocks Formations) 

was discussed by Moore and others (1983) and Winkler 

(in Nilsen and Moore, 1979). 

- These previous studies indicated that the 

sandstones of the Orca and Valdez Groups may be 

petrographically distinguishable. The mean 

147° 

East Valdez area 

0 25 50 '--------L----J kilometers 

EXPLANATION 

Fault - Dashed where concealed or inferred 

--- Normal or strike slip 

..L •• Thrust or high angle reverse, 

sawteeth on upper plate 

Petrographic areas and sample localities 

Valdez Group 

e West Valdez 

o Central Valdez 

0 East Valdez 

@ Resurrection 

Peninsula 

* Mount Denson 

Orca Group 

• Sargent lcefield 

o Chenega 

(]) Port Gravina 

+ Cordova 

il Knight Island 

{r Fllamar 

A Montague Island 

b. Ragged Mountain 

Figure 5. Location of petrographic areas and samples point-counted for this study. JBF, Jack Bay fault; LF, 

Landlock fault; RMF, Ragged Mountain fault; RRF, Rude River fault. See figure 1 for location. 

Previous Petrographic Studies 7 
composition of the Valdez samples studied by Mitchell 

(1979) is Q (total monocrystalline and polycrystalline 

quartz) = 20, F (total feldspar) = 24, and L (total lithic 

clasts) = 56, whereas that of the Orca samples studied 

by Winkler (1976) is Q = 43, F = 46, and L = 11. 

This study seeks to test whether this striking 

compositional difference characterizes the Orca and 

Valdez Group sandstones throughout the Prince 

William Sound area. That is, is there a sharp change in 

the composition of the sandstone of the Prince William 

Sound area, and, if so, does it coincide with the 

current placement of the contact between the Valdez 

and Orca Groups? Dumoulin (1984) and Winkler and 

others (1984) presented the preliminary results of this 

study. 

METHODOLOGY 

The present investigation sought to obtain as 

wide a distribution of samples as possible in the study 

area. However, in certain areas, metamorphism and 

(or) deformation obliterated too much of the original 

fabric and composition to allow determination of 

sedimentary modes and provenance. Thus, few 

samples collected east of longitude 147° and north of 

the Contact fault system were suitable for point 

counts. 

Further constraints on sample selection were 

based on grain size and the percentage of matrix. 

Because the compositional mode of different size 

fractions may differ significantly (Odum, 1975; 

Ingersoll and others, 1984), medium-grained samples 

(samples having an average grain size between 0.25 

and 0.5 mm) were used to facilitate comparison with 

the results of previous studies, as well as with the 

provenance fields delineated by Dickinson and Suczek 

(1979). Some point counts of coarser and finer grained 

sandstones were performed on samples from areas 

where medium-grained rocks were not available; 

however, these point counts, although mentioned in the 

discussion, are not included in the final point-count 

tabulations. Samples composed of more than 25 

percent matrix were also excluded; such samples have 

probably undergone too much alteration to allow 

determination of original modes (Dickinson and 

Suczek, 1979; Ingersoll and Suczek, 1984). 

A total of 950 samples from the Valdez and Orca 

Groups were examined for this study; 111 of the leastaltered samples were chosen for point-count analysis. 

Detrital grains were counted at 200 magnification 

using a 1-mm-grid spacing; all point-counted thin 

sections were stained with sodium cobaltinitrate to 

facilitate identification of potassium feldspar. 

Classification of counted grains in general followed 

the conventions of Dickinson (1970). However, the 

Gazzi-Dickinson method (Ingersoll and others, 1984) of 

assigning sand-sized crystals and grains in larger 

fragments to the crystal or grain category, rather than 

to the larger grain or fragment category, was not 

used. Although this technique seems to reduce 

variation of composition with grain size (Ingersoll and 

others, 1984), it was not used by previous workers 

(Decker, 1980; Winkler, 1976), with whose results I 

wished to compare my findings. Most lithic clasts in 

the rocks of this study are sufficiently fine grained 

that use of the Gazzi-Dickinson technique does not 

significantly alter the final modes. Four hundred 

framework grains (that is, all grains except silt, clay, 

other matrix, and cement) were counted for each 

sample. 

To facilitate statistical analysis and comparison 

with previous work, the point counts are grouped 

according to geographic areas within the Prince 

William Sound region; these areas are separated by 

major faults and (or) geographic breaks, such as large 

bodies of water. Samples from the Valdez Group are 

subdivided into five categories; those from the Orca 

Group are separated into eight categories (fig. 5). 

R.J. Goldfarb (written commun., 1985) carried 

out a statistical analysis of the Prince William Sound 

point-count data. Trend-surface analysis was used to 

look for spatial trends in 50 samples collected from 

western Prince William Sound. Planar (first-degree) 

surfaces were fitted to the data using least-squares 

methodology. Discriminant-function analysis was 

performed on the entire data base of this study in 

order to identify which petrographic variables best 

discriminate between selected geographic areas. The 

computer program DISCRIMINANT of the Statistical 

Package for the Social Sciences System (Nie and 

others, 1975) was used. 

RESULTS OF STUDIES OF VALDEZ GROUP SAMPLES 

West, Central, and East Valdez Areas 

The majority of Valdez Group samples suitable 

for point-count study came from the western part of 

Prince William Sound, that is, west of longitude 147°. 

In this report, the area east of this line (but north of 

the Contact fault system) is designated the east 

Valdez area (fig. 5). West of this line, the central 

Valdez area extends north and west of the Contact 

fault to the Placer River fault; the area west of the 

Placer River fault is the west Valdez area. Samples 

collected from the two subareas in these geographic 

divisions have anomalous compositions relative to the 

majority of Valdez Group samples; these subareas will 

be treated separately. The area south of the unnamed 

fault that separates predominantly mafic volcanic 

rocks of the Resurrection Peninsula from 

predominantly sedimentary rocks to the north (Nelson 

and others, 1985) is designated the Resurrection 

Peninsula area. The area between the Jack Bay fault 

to the north and the Landlock Bay segment of the 

Contact fault system to the south is the Mount Denson 

area (fig. 5). 

The results of point-count studies of samples 

from the main part of the Valdez Group appear in 

table 1 and figure 6. In general, samples collected 

from the west, central, and east Valdez areas are quite 

similar. All are subquartzose, and most are 

feldspatholithic or lithofeldspathic (following the 

usage of Crook, 1960)-that is, they contain less than 

75 percent detrital quartz, and the ratio of feldspar to 

lithic grains is between 1:3 and 3:1. Samples are very 

poorly sorted to moderately well sorted (based on 

visual comparison with the scale in Pettijohn and 

8 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, AJaska 
others, 1973); most contain 10-20 percent matrix. This 

matrix consists of a complex mixture of orthomatrix 

and epimatrix, as defined by Dickinson (1970)-that is, 

recrystallized detrital clays intergrown with 

phyllosilicate minerals, epidote, and sphene that were 

introduced primarily during diagenesis. In some 

samples, pseudomatrix (deformed interstitial 

sedimentary lithic fragments) is also present. Onethird of the point-counted samples contain trace 

amounts (less than 1 percent) of calcite and (or) 

zeolite cement; one sample contains 8 percent patchy 

calcite cement. 

West, central, and east Valdez-area samples 

contain relatively low percentages of detrital 

quartzose grains-between 6 and 30 percent of the 

total framework grains (fig. 7a and b). (Unless 

otherwise noted, all clast percentages in the following 

discussion are percentages of framework grains.) Most 

of this quartz is monocrystalline, and the grains show 

both straight and undulatory extinction. Some of the 

quartz grains have features attributable to a volcanic 

provenance (bipyramidal form and resorbed margins), 

but more common are rounded to angular grains that 

exhibit slightly undulatory extinction-such grains may 

be derived from plutonic or metamorphic source rocks 

(Folk, 1974). Chert-to-total-quartz ratios (C/Q) in 

these samples are low, in most instances less than 

0.1. Some of the rare chert grains noted contain 

recrystallized radiolarians. 

Feldspar contents are moderate and range from 

23 to 45 percent. Plagioclase-to-total-feldspar ratios 

(P/F) are high (0.68-0.99); many of the plagioclase 

grains are twinned, and some are zoned. Calcite and 

sericite alteration is extensive in a minority of the 

plagioclase grains. Twelve samples contain no 

potassium feldspar. A few samples contain 9-12 

percent potassium feldspar, but most contain 2-5 

percent. Trace amounts of microcline are present in a 

few samples. 

Lithic-clast content of the Valdez Group samples 

is moderate to high (28-66 percent). Volcanic-lithicto-total-lithic-clast ratio (V /L) is varied but fairly 

high; it ranges from 0.1 to 0.93, but is 0.5 or greater in 

most samples. Most of the volcanic lithic clasts have 

felsitic or lathwork textures; only a few microlitic 

grains were observed. Felsitic grains constitute as 

much as 16 percent of the total framework-grain 

content; lathwork grains reach a high of 31 percent in 

a sample from the west Valdez area. Subordinate but 

notable amounts of metamorphic (as much as 22 

percent) and sedimentary (as much as 16 percent) 

lithic clasts occur in the Valdez Group samples 

studied. Mudstone and silty mudstone are the main 

types of sedimentary lithic clasts; fine-grained 

metasedimentary rocks (phyllite and slate) dominate 

the metamorphic-clast suite. 

Heavy minerals are locally abundant. Pyroxene 

and amphibole are present in about half the Valdez 

Group samples and constitute more than 4 percent of a 

few samples. Other heavy minerals observed include 

garnet, allanite, sphene, zircon, and tourmaline. Micas 

(mostly biotite, minor muscovite, and chlorite) 

constitute less than 3 percent in most samples, but 

reach 10 percent in a few. 

Table !.--Composition of sandstone of the Valdez Group from the west Valdez (n=lS), east Valdez (n=2), 

and central Valdez (n=l2) areas 

[Hope-Sunrise district data (n=l9) from Mitchell (1979). The first twelve categories represent percentage of total framework grains, except for matrix, which represents percentage of the whole rock. 

Quartzose grains (total), feldspar (total) and lithic fragments (total) equal 100 percent. All means 

and standard deviations represent percent, except for the last three categories, which are ratios. 

See figure 6 for graphic presentation of data, n.d., no data] 

West Valdez 

West Valdez East Valdez Central Valdez area (Hopearea area area Sunrise 

district) 

Mean Standard l-ie an Standard Mean Standard Mean 

deviation deviation deviation 

Monocrystalline quartz 10.8 4.1 8.5 3.2 19.3 3.4 13.4 

Polycrystalline quartz 2.4 1.5 2.3 1.8 4.3 1.9 2.0 

Chert 1.5 2.0 2.8 2.8 0.9 0.8 0.7 

Plagioclase 29.3 8.0 40.1 5.5 29.4 4.8 19.5 

Potassium feldspar 1.9 2.4 0 0 3.9 3.8 1.8 

Volcanic lithic fragments 32.1 13.6 26.9 0.53 11.2 5.7 31.2 

Sedimentary lithic fragments 8.3 4.2 6.4 6.2 8.5 3.1 12.8 

Metamorphic lithic fragments 4.7 3.6 4.3 4.2 10.8 5.1 10.8 

Plutonic lithic fragments 1.5 1.1 0.9 0.5 1.7 1.5 n.d. 

l-1atr ix and cement 14.8 4.6 n .a 7.4 17.3 4.9 12 

Heavy minerals 1.9 1.5 1.8 2.5 1.8 1.1 n.d. 

t-1ica and chlorite 4.2 2.1 5.3 1.4 6.4 2.2 1.3 

Quartzose grains (total) 15.6 6.7 14.5 7.8 26.5 3.5 20 

Feldspar (total) 34.9 8.5 44.5 7.8 38.5 4.5 24 

Lithic fragments (total) 49.5 9.9 41.0 0 35.0 3.7 56 

Chert/Total quartz 0.08 0.08 0.17 0.11 0.03 0.03 0.34 

Plagioclase/Total feldspar 0.89 0.08 0.98 0.1 0.83 0.12 0.82 

Volcanic lithic fragments/ 

Total lithic fragments 0.67 0.18 0.7 0.05 0.35 0.17 0.58 

Results of Studies of Valdez Group Samples 9 
Differences between West Valdez- and Central ValdezArea Samples 

area samples fit well with the west Valdez samples and 

are grouped with them in the following discussion. 

Trend-surface analysis shows an increase in quartz, 

potassiu:n feldspar, and plagioclase and a decrease in 

lathwork-textured volcanic clasts in samples from the 

central Valdez area compared to those from the west 

Notable differences exist in Q-F-L modes and 

lithic-clast composition between the west Valdez- and 

the central Valdez-area samples; the two east Valdeza 

Continental block 

provenance 

am 

F Magmatic arc provenance L F Magmatic arc provenance 

am ap 

Lt 

Increasing ratio 

plutonic/volcanic 

Collision orogen 

sources 

components 

p 

AREA MEAN 

• West Valdez w 

Q9 East Valdez e 

0 Central Valdez c 

+ Dickinson and Suczek, 1979 

K Lv Arc orogen sources 

EXPLANATION 

CLAST TYPE 

a Total quartzose grains Lv 

am Monocrystalline quartz Ls 

ap Polycrystalline quartz F 

L Total lithic grains P 

Lt Total lithic grains and K 

polycrystalline quartz 

------ Provenance fields+ 

Volcanic lithic grains 

Sedimentary lithic grains 

Total feldspar 

Plagioclase 

Potassium feldspar 

La 

Figure 6. Modal analyses of sandstone of the Valdez Group from the west Valdez, east 

Valdez, and central Valdez areas. 

10 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
Valdez area. This trend is significant at the 99-

percent confidence level (95-percent confidence level 

for plagioclase) and will be discussed at length below. 

Discriminant analysis shows that quartz, silty 

A 

B 

mudstone, and metasedimentary lithic clasts are 

significantly more abundant in central Valdez-area 

samples. Furthermore, lathwork grains are more 

abundant than felsitic grains in most west Valdez-area 

c 

D 

Figure 7. Photomicrographs in transmitted light of thin sections of sandstones from the Valdez Group. Chi, 

chlorite; K, potassium feldspar; Ls, sedimentary lithic clast; Lv, volcanic lithic clast; P, plagioclase; Q, 

quartz. A and B, Sandstone samples from the west Valdez area contain abundant lithic volcanic clasts and 

lesser plagioclase, sedimentary lithic clasts, and chlorite. Sandstone samples from the Resurrection 

Peninsula, C, and the Mount Denson area, D, contain few lithic clasts but abundant quartz and feldspar. 

Results of Studies of Valdez GfOUP Samples 11 
samples, but felsitic grains dominate or are subequal 

to lathwork grains in most central Valdez-area 

samples. 

Provenance 

Ternary plots of framework-grain modes and 

detailed analyses of the lithic-clast suite suggest an 

arc-orogen provenance for Valdez Group sandstone in 

the Prince William Sound area. This conclusion agrees 

with that arrived at by previous studies of the Valdez 

Group (Mitchell, 1979) and correlative rocks (Moore, 

1973; Winkler, in Nilsen and Moore, 1979; Decker, 

1980; Zuffa and others, 1980) (fig. 8). 

Dominance of volcanic clasts over other lithic 

types, local abundance of pyriboles, and low 

percentages of quartzose grains all point to an 

important volcanic source for Valdez Group 

sedimentary rocks. Based on the compositional 

interpretations of volcanic lithic textures proposed by 

Dickinson (1970) and others, the dominance of felsitic 

and lathwork grains in these samples indicates a 

bimodal volcanic suite of rhyolitic/dacitic and basaltic 

composition. In general, this agrees with the findings 

of Decker (1980) and Mitchell (1979), although they 

found felsitic grains to be more abundant than 

lathwork grains, whereas the samples in the present 

study contained mostly subequal or lathwork-dominant 

populations. Moore (1973) and Zuffa and others (1980) 

interpreted the volcanic detritus in Chugach terrane 

strata as primarily andesitic. Winkler (p. 24, in Nilsen 

and Moore, 1979) described the volcanic clasts of the 

correlative Kodiak Formation as "apparently *** 

andesitic or basaltic" in composition; textures are 

dominantly lathwork (G.R. Winkler, U.S. Geological 

Survey, written commun., 1985). 

Some of this apparent disagreement in 

interpretation may be a result of differences in 

nomenclature. Moore (1973) reported that volcanic 

clasts observed in his study were dominantly 

pilotaxitic-a texture that would have been included in 

the lath work category of this study. Zuffa and others 

(1980) did not use a lathwork category, and they 

interpreted felsitic textures as indicative of an acidto-intermediate composition. It is also likely that the 

composition of the volcanic detritus contributed to the 

Chugach terrane varied through time and across the 

geographic extent of the depositional area. 

Evidence of varied proportions of mafic · and 

felsic volcanic-lithic fragments in west Valdez-area 

sandstone is provided by chemical analyses of four 

point-counted samples (table 2, samples 1-4). All four 

samples contain more than 18 percent volcanic lithic 

grains. Two samples, in which clasts with lathwork 

textures predominate over those with felsitic textures, 

have low concentrations of Si0 2 (59 and 61 percent) 

and relatively high concentrations of TI02 (0.87 and 

0.86 percent), Fe2o3 and MgO. Two samples, in which 

felsitic textures predominate over lathwork textures, 

contain considerably more Si02 (72 and 71 percent) and 

less Ti02, Fe2o;3, and MgO. 

A subordmate plutonic source for Valdez Group 

sandstone is indicated by the presence of sericitized 

feldspar, microcline, mica, and plutonic-rock 

fragments. Another subordinate, but notable, 

component of the Valdez Group and its correlative 

rock units is sedimentary lithic grains. Moore (1973), 

Winkler (in Nilsen and Moore, 1979) and Decker (1980) 

interpreted these fragments as of intrabasinal origin, 

although Decker also identified detritus in the Sitka 

a 

EXPLANATION 

UNIT 

Valdez Group 

West and east 

Valdez areas 

Central Valdez area 

Hope-Sunrise area 

Shumagin Formation 

Kodiak Formation 

Sitka Graywacke 

Ford Arm unit 

Chugach terrane flysch 

facies 

FIELD MEAN 

w 

c 

H 

s 

K 

FA 

Figure 8. Modal analyses of sandstone 

from the Valdez qroup and (at least 

partly) correlative lithostratigraphic 

units. Q, total quartzose grains; F, total 

feldspar; L, total lithic grains; Qp, 

polycrystalline quartz; Lv, volcanic lithic 

grains; Ls, sedimentary lithic grains. 

12 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
Graywacke as having been recycled from the 

Cretaceous Kelp Bay Group melange facies. Zuffa and 

others (1980), however, emphasize that grains from a 

broad spectrum of sedimentary rock types are 

present. Samples from the Prince William Sound area 

also show such a spectrum, ranging from mudstone 

clasts apparently deformed while still unlithified to 

angular clasts of sandstone and metasedimentary 

fragments, such as quartz-muscovite schist and 

phyllite. The majority of these clasts are of medium 

grain size, unlike the large "intraclasts" described by 

Decker (1980). This variety of clast types implies an 

extrabasinal origin for some of the grains-for 

example, a previously accreted subduction complex. 

Trend-surface analyses and ternary plots of 

Valdez Group samples from the Prince William Sound 

area support the interpretation of Zuffa and others 

(1980) that the primary source for sedimentary rocks 

of the Chugach terrane was an evolved arc with an 

increasing degree of dissection to the east. On the 

provenance diagrams of Dickinson and Suczek (1979), 

samples from both the west and central Valdez areas 

plot within the arc-orogen field, but central Valdezarea samples plot nearer the plutonic end of that field 

(fig. 6). The diagram that shows the ratio of 

polycrystalline quartzose grains (Qp) to total volcanic 

lithic grains (Lv) to total sedimentary lithic grains (Ls) 

shows that the central Valdez-area rocks contain a 

higher percentage of Ls in most samples than do the 

west Valdez-area rocks; furthermore, a distinct 

subpopulation of central Valdez-area rocks contains Ls 

clasts as the dominant lithic constituent. 

Anomalous Valdez Group Samples 

Samples from the Resurrection Peninsula in 

western Prince William Sound and the Mount Denson 

area in eastern Prince William Sound have anomalous 

compositions when compared to other Valdez Group 

samples (table 3; figs. 9 and 10). These rocks are 

dominantly feldspathic and have 12-21 percent matrix 

of the type previously described for typical Valdez 

Group sandstone. A few samples contain trace 

amounts of calcite cement. Quartz content is low (24-

34 percent) as in other Valdez Group samples, but 

feldspar content is high (42-67 percent) and lithic 

content is low (7-18 percent) (figs. 7c and d). 

Mount Denson-area samples have moderate to 

high V /L ratios (0.4-0.65); felsitic textures are 

dominant in the volcanic clasts. Clasts of mudstone 

and schist are also abundant. Potassium feldspar 

content ranges from 7 to 17 percent. 

Resurrection Peninsula samples also contain 

notable amounts of potassium feldspar (8-10 percent), 

but sedimentary and metamorphic lithic clasts are 

more abundant than volcanic lithic clasts. 

On framework mode diagrams that show ratios of 

Q-F-L and of monocrystalline quartzose grains (Qm) to 

feldspar (F) to total lithic and polycrystalline 

quartzose grains (Lt), the samples from Mount Denson 

and the Resurrection Peninsula plot in the area 

between the magmatic arc and continental block fields 

defined by Dickinson and Suczek (1979), and show no 

overlap with the west, central, and east Valdez-area 

samples. On Qp-Lv-Ls diagrams, a predominantly arcorogen provenance is suggested for the Mount Denson 

samples; Resurrection Peninsula samples show a wide 

scatter but are closest to the subduction-complex 

provenance. 

The mafic complex of the Resurrection Peninsula 

was originally thought to intrude Valdez Group flysch 

(Tysdal and others, 1977), but Nelson and others (1985) 

mapped the contact as a thrust fault. If the latter 

interpretation is correct, the Resurrection Peninsula 

samples in this study (which represent minor amounts 

of sediment intercalated with the mafic complex south 

of this thrust fault) may have had a different source 

than the main body of the Valdez Group. On the other 

hand, sedimentary rocks of Resurrection Peninsula 

may have had the same general provenance as other 

Valdez Group sandstones, but, as suggested by Winkler 

(1976) for Orca Group sandstones on Hinchinbrook 

Island, the mafic rocks may have formed temporary 

barriers that restricted normal sediment influx and 

allowed deposition of strata having these anomalous 

Table 2.--Geochemical analyses of selected sandstone samples of the Valdez and Orca Groups 

Si02 

Al203 Fe2o3 

MgO 

CaO 

Na2o 

K~O 

T~0 2 P205 

r.mo Total 

Valdez Group Orca Group 

Lathwork textures dominant 

(west Valdez 

area) 

1 

60.7 58.6 

15.8 15.4 

7.3 7.4 

3.7 5.0 

3.2 4.0 

4.0 3.5 

1.3 2.2 

0.87 0.86 

0.23 0.22 

0.1 0.12 

97.2 97.3 

Felsitic textures dominant 

(west Valdez 

area) 

Quartz- Volcanirich clastic (Ragged 

(Ellamar Mountain 

area) area) 

Locality Sample Number 

6 

72.3 70.9 85.9 47.4 

12.6 12.8 6.9 13.6 

4.7 4.9 1.4 14.2 

1.5 1.8 0.9 5.6 

1.0 1.0 0.5 a.a 

3.1 3.6 2.6 2.5 

1.5 1.1 0.4 0.8 

0.69 0.63 0.41 2.5 

0.18 0.18 0.05 0.2 

0.09 0.06 0.02 0.2 

97.7 97.0 99.0 95.8 

Results of Studies of Valdez Group Samples 13 
compositions. Dickinson (1970) suggested that 

reworking of volcanic sandstones could concentrate 

feldspar, and the Resurrection Peninsula samples are, 

overall, better sorted than other Valdez Group 

samples. They are also at the lower end of the 

medium grain-size range. Several workers have 

suggested that, in some sandstones, decreasing grain 

size may lead to increasing feldspar content at the 

expense of lithic grains (Ingersoll and others, 1984). 

However, the difference between feldspar content in 

typical and anomalous Valdez Group sandstones, 

expecially when compared to the slight difference in 

grain size between these two groups, seems too large 

to be explained completely by such a process. 

Mount Denson-area samples are not particularly 

fine grained, and they are poorly to very poorly 

sorted. As presently mapped, the sedimentary rocks in 

this area have no spatially associated age-correlative 

volcanic rocks that could have provided barriers to 

sediment accumulation. Discriminant analysis and 

ternary plots show a complete overlap between Mount 

Denson-area sample composition and the composition 

of those Orca Group samples immediately southeast of 

the Mount Denson area. The implications of these 

findings will be discussed at length below. 

RESULTS OF STUDIES OF ORCA GROUP SAMPLES 

The samples from the Orca Group also were 

separated into geographic areas before petrographic 

and statistical analysis (fig. 5). The area east of the 

Contact fault and west of the Johnstone Bay fault is 

here called the Sargent Icefield area. The Chenega 

area extends east from the Johnstone Bay fault to the 

Bainbridge fault of Helwig and Emmet (1981). The 

Knight Island area includes all the islands west of the 

Montague Strait fault; the area east of this fault and 

west of long 14 7° W. is the Montague area. The 

Ragged Mountain area is defined by the Ragged 

Mountain fault on the east and the change from 

bedrock to Qua ternary cover on the west. All other 

samples east of longitude 147° are included in the Port 

Gravina-Cordova area, with the exception of samples 

near the Ellamar mining district, which are treated 

separately. 

Eastem Exposures of the Orca Group 

The Port Gravina-Cordova area includes the 

shoreline of Orca Bay (figs. 4 and 5), the area that 

Moffit (1954) designated as the type locality of the 

Orca Group. In the present study, samples collected 

north of the Rude River fault (Port Gravina area) were 

analyzed separately from those collected south of the 

fault (Cordova area). However, the samples from both 

areas were found to be so similar that they are treated 

as a single group in the following discussion (table 4, 

fig. ll). Most of the samples are feldspathic and 

contain 10-15 percent matrix. As described earlier for 

Valdez Group samples, the matrix in these (and all 

other) Orca Group samples consists primarily of clay 

that is, at least in part, recrystallized, phyllosilicate 

minerals, epidote, and sphene; epimatrix, orthomatrix, 

and the pseudomatrix of Dickinson (1979) cannot easily 

be distinguished and were not formally separated 

during point counts. About a third of the samples 

contain localized cement-mostly calcite and, to lesser 

degree, zeolite (laumontite). Samples range from 

moderately well sorted to very poorly sorted. 

Table 3.--Composition of sandstone of the Valdez Group from the Mount Denson 

(n=S) and Resurrection Peninsula (n=3) areas 

~ee table 1 for further explanation of parameters. See figure 9 for graphic 

presentation of data] 

Mount Denson Resurrection 

area Peninsula 

area 

Mean Standard Mean Standard 

deviation deviation 

Monocrystalline quartz 22.6 2.3 23.4 3.9 

Polycrystalline quartz 2.9 1.4 3.8 2.5 

Chert 0.5 0.3 0.7 0.5 

Plagioclase 46.4 7.2 42.3 9.1 

Potassium feldspar 9.4 4.1 13.3 0.7 

Volcanic lithic fragments 6.2 2.8 2.5 0.7 

Sedimentary lithic fragments 1.8 o.8 3.3 4.1 

Metamorphic lithic fragments 2.8 1.3 5.9 6.1 

Plutonic lithic fragments 0.8 0.6 0.7 0.7 

Matrix and cement 18.3 2.2 18.2 4.4 

Heavy minerals 2.4 0.5 3.3 2.1 

Micas and chlorite 3.7 2.2 3.6 3.2 

Quartzose grains (total) 27.7 2.4 30.3 8.1 

Feldspar (total) 60.1 5.7 56.4 12.1 

Lithic fragments (total) 12.3 4.7 13.3 6.0 

Chert/Total quartz 0.02 0.01 0.02 0.02 

Plagioclase/Total feldspar 0.82 0.07 0.79 0.05 

Volcanic lithic fragments/ 

Total lithic fragments 0.53 0.11 0.25 0.18 

14 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
Quartz contents are low to moderate (9-32 

percent) (fig. 12a). Grains with both straight and 

undulous extinction are present. Most grains are 

monocrystalline and of relatively nondescript plutonic 

or metamorphic type. C/Q ratios are low (less than 

0.05); most samples contain less than 1 percent chert. 

Feldspar content is high (44-63 percent) and is 

mostly plagioclase. (P/F ratios range from 0. 71 to 

0.96.) About 20 percent of the plagioclase grains are 

obviously twinned, and some grains are normally 

zoned. Potassium feldspar is present in all samples 

and constitutes 5-10 percent of most. One-third of the 

Q 

Continental block 

provenance 

Qm 

F Magmatic arc provenance L F Magmatic arc provenance lt 

Increasing ratio of 

plutonic/volcanic 

components 

p 

AREA 

* Mt. Denson 

® Resurrection 

Peninsula 

Port GravinaCordova 

Qm 

MEAN 

d 

p 

+ Dickinson and Suczek, 1979 

Qp 

Collision orogen 

sources 

K Lv Arc orogen sources 

EXPLANATION 

CLAST TYPE 

Q Total quartzose grains 

Qm Monocrystalline quartz 

Q p Polycrystalline quartz 

L Total lithic grains 

Lt Total lithic grains and 

polycrystalline quartz 

------ Provenance fields+ 

Lv Volcanic lithic grains 

Ls Sedimentary lithic grains 

F Total feldspar 

P Plagioclase 

K Potassium feldspar 

Ls 

Figure 9. Modal analyses of sandstone of the Valdez Group from the Mount Denson and 

Resurrection Peninsula areas compared with analyses of sandstone of the Orca Group 

from the Port Gravina-Cordova area. 

Results of Studies of Orca Group Samples 15 
a Op 

F L 

EXPLANATION 

AREA FIELD MEAN 

Mt. Denson d 

Resurrection Peninsula 

West and east Valdez w 

Central Valdez c 

Figure 10. Modal analyses of sandstone of the Valdez Group from the Mount Denson and 

Resurrection Peninsula areas compared with those from the west Valdez, central Valdez, 

and east Valdez areas. Q, total quartzose grains; F, total feldspar; L, total lithic grains; 

Qp, polycrystalline quartz; Lv, volcanic lithic grains; Ls, sedimentary lithic grains. 

Table 4.--Composition of sandstone of the qrca Group from the Cordova (n=ll) and Port 

Gravina (n=S) areas, eastern Prince William Sound 

[Hinchinbrook area data (n=59) from Winkler (1976). See table 1 for further explanation 

of parameters. See figure 11 for graphic presentation of data. n.d., no data] 

Cordova area Port Gravina Hinchinbrook 

area area 

Mean Standard Mean Standard Mean Standard 

deviation deviation deviation 

Monocrystalline quartz 22.1 2.8 21.6 3.0 n.d. n.d. 

Polycrystalline quartz 2.4 1.1 2.8 1.5 n.d. n.d. 

Chert 0.6 0.5 0.4 0.5 n.d. n.d. 

Plagioclase 42.5 4.0 45.0 7.9 n.d. n.d. 

Potassium feldspar 9.1 3.3 9.6 3.2 n.d. n.d. 

Volcanic lithic fragments 6.1 2.3 5.7 2.4 n.d. n.d. 

Sedimentary lithic fragments 3.6 1.9 2.7 1.2 n.d. n.d. 

Metamorphic lithic fragments 3.5 1.3 3.0 2.5 n.d. n.d. 

Plutonic lithic fragments 2.5 1.2 2.5 1.3 n.d. n.d. 

Matrix and cement 12.1 2.1 13.5 3.8 n.d. n.d. 

Heavy minerals 2.6 1.1 1.6 0.6 n.d. n.d. 

Micas and chlorite 4.2 1.7 4.3 1.9 n.d. n.d. 

Quartzose grains (total) 26.9 3.3 26.4 4.0 43.1 7.2 

Feldspar (total) 56.4 4.7 58.8 6.6 46.0 9.5 

Lithic fragments (total) 16.7 4.3 14.8 4.0 10.9 7.9 

Chert/Total quartz 0.02 0.02 0.02 0.02 0.12 0.08 

Plagioclase/Total feldspar 0.81 0.06 0.81 0.06 0.92 0.5 

Volcanic lithic fragments/ 

Total lithic fragments 0.38 o.o8 0.38 0.18 0.53 0.27 

16 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, AJaska 
samples contain minor amounts of microcline. 

Lithic content is low (7-25 percent); V /L ratios 

are varied (0.17-0.64). Volcanic lithic clasts possess 

mainly felsitic or lathwork textures and constitute as 

much as 10 percent of some samples. Volcanic clasts 

are particularly abundant in sandstone from the Sheep 

Bay region; conglomeratic intervals here contain 

numerous pebbles of felsic porphyry and tuff (Nelson 

and others, 1985). 

Sedimentary lithic clasts (especially mudstone 

and silty mudstone) and metamorphic lithic clasts 

(mostly schist, phyllite, and slate) are .important 

Q 

Continental block 

provenanc.e 

Om 

'Recycled orogen 

provenance 

' \ 

... '....... '"' ........ \ 

'·,,,\\, 

,/'\, 

F Magmatic arc provenance L F Magmatic arc provenance Lt 

Increasing ratio of 

plutonic/volcanic 

components 

p 

Qm 

AREA MEAN 

CD 

+ 

Port Gravina } 

Cordova 

Hinchinbrook 

+ Dickinson and Suczek, 1979 

p 

h 

Q.p 

Collision orogen 

sources 

(J) 

(J) 

/----:r ~1---~,' 

- + + ft-t- ~\ (J)(J) ++\+-

K Lv Arc orogen sources 

EXPLANATION 

CLAST TYPE 

Q Total quartzose grains 

Qm Monocrystalline quartz 

Q p Polycrystalline quartz 

L Total lithic grains 

Lt Total lithic grains and 

polycrystalline quartz 

Provenance fields+ 

Lv Volcanic lithic grains 

Ls Sedimentary lithic grains 

F Total feldspar 

P Plagioclase 

K Potassium feldspar 

Ls 

Figure 11. Modal analyses of sandstone of the Orca Group from the Port Gravina and 

Cordova areas compared with analyses of sandstone of the Orca Group from the 

Hinchinbrook Island area (Winkler, 1976). 

Results of Studies of Orca Group Samples 17 
A c 

B D 

Figure 12. Photomicrographs in transmitted light of thin sections of sandstones from the 

Orca Group. B, biotite; C, chert; H, hornblende; K, potassium feldspar; Ls, sedimentary 

lithic clast; Lv, volcanic lithic clast; P, plagioclase; Py, pyroxene; Q, quartz. ~ 

sandstone samples from eastern exposures of the Orca Group tPort Gravina-Cordova 

area) contain abundant feldspar, and lithic clasts are mostly volcanic. ..!!, sandstone 

samples from western exposures of the Orca Group (Sargent Icefield area) are similar to 

samples of the Valdez Group from the west and central Valdez areas; they contain 

abundant heavy minerals indicative of a volcanic provenance • ..Q, sandstone samples from 

the Ellamar area contain abundant quartz and chert clasts and are better sorted than 

most Orca Group sandstones. Q, sandstone samples from the Montague Island area 

contain abundant quartz and sedimentary lithic clasts as well as locally abundant biotite. 

18 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
subsidiary constituents. Quartz-feldspar symplectite 

grains and plutonic clasts are a minor but notable 

component of all samples; coarse-grained sandstone 

north of Port Fidalgo contains as much as 25 percent 

plutonic fragments. 

Micas constitute 2-4 percent of the composition 

of most Port Gravina-Cordova samples. Garnet is 

generally present; other heavy minerals observed 

include pyroxene, amphibole, pumpellyite, zircon, 

epidote, allanite, and tourmaline. Several samples 

slightly too coarse grained to be included in this study 

contain as much as 3 percent pyriboles. 

Comparison of the ternary plots of Port GravinaCordova samples with the provenance fields delineated 

by Dickinson and Suczek (1979) indicates that the 

primary source for these Orca Group strata was a 

deeply dissected volcanic arc. Like the Mount Denson 

samples, they plot at the junction of the continentalblock and arc-orogen fields on Q-F-L and Qm-F-Lt 

diagrams. On the Qp-Lv-Ls diagram, an arc 

provenance is also indicated, with important 

contributions from a subduction-complex source. 

These results agree with the interpretations 

suggested by the lithic-clast and heavy-mineral suites 

described above. The locally abundant pyriboles and 

volcanic lithic clasts in Port Gravina-Cordova samples 

may in part reflect an intrabasinal provenanceerosion and reworking of Orca Group volcanic rocks 

that crop out extensively in parts of the Port GravinaCordova area. However, these volcanic rocks are 

mafic in composition and could not have provided a 

source for felsic clasts like those from the Sheep Bay 

area. Uneroded volcanic cover in less deeply dissected 

parts of the hypothetical arc source may have provided 

both felsic and mafic volcanic debris to the Port 

Gravina-Cordova rocks. 

Westem Exposures of the Orca Group 

Orca Group samples from western Prince William 

Sound have important compositional differences from 

the eastern-area samples just described (table 5, fig. 

13). 

The rocks of the Sargent Icefield area (fig. 12b) 

are moderately well sorted to very poorly sorted; 

matrix content ranges from 7 to 20 percent, but is 8 

to 15 percent in most samples. One-third of the 

samples contain patchy calcite cement. 

The quartz content (16-26 percent) and C/Q ratio 

(0-0.1) of these rocks are low. Feldspar content is 

moderate (31-56 percent) and consists mostly of 

plagioclase (P/F 0. 75-0.95). Potassium feldspar 

content ranges from 1 to 13 percent, but more 

typically ranges from 7 to 10 percent. Lithic content 

is low to moderate (18-39 percent) and V/L ratios are 

varied (0.19-0.62). Total volcanic lithic-clast content 

reaches 11 percent in some samples; felsite grains are 

somewhat more abundant than lathwork grains in most 

samples. Sedimentary lithic grains (particularly 

mudstone and siltstone) and metamorphic lithic grains 

(phyllite and schist) typically constitute 3-8 percent 

each. Most samples contain 1-5 percent micas. 

Pyroxene and hornblende are locally abundant (7 

percent of one sample, but less than 3 percent of 

most); other heavy minerals noted include garnet, 

sphene, allanite, epidote, zircon, and tourmaline. 

Discriminant analysis indicates that these 

samples fall between the fields delineated by typical 

Valdez Group samples (from the west, central, and 

east Valdez areas) and typical Orca Group samples 

(from the Port Gravina-Cordova area). Sandstone 

samples from the Sargent Icefield area contain more 

potassium feldspar and mica than most Valdez Group 

Table 5.--Composition of sandstone of the Orca Group from the Sargent Icefield 

(n=l5) and Chenega (n=8) areas, western Prince William Sound 

[Ghost Rocks Formation data (n=8) from Winkler (in Nilsen and Moore, 1979). See 

table 1 for further explanation of parameters. See figure 13 for graphic 

presentation of data. n.d., no data] 

Monocrystalline quartz 

Polycrystalline quartz 

Chert 

P lag ioc lase 

Potassium feldspar 

Volcanic lithic fragments 

Sedimentary lithic fragments 

Metamorphic lithic fragments 

Plutonic lithic fragments 

Matrix and cement 

Heavy minerals 

Micas and chlorite 

Quartzose grains (total) 

Feldspar (total) 

Lithic fragments (total) 

Chert/Total quartz 

Plagioclase/Total feldspar 

Volcanic lithic grains/ 

Total lithic grains 

Sargent Icefield 

area 

Mean Standard 

deviation 

17.6 3.2 

3.1 1.0 

o.8 0.7 

35.8 6.0 

7.5 3.6 

11.4 4.2 

7.5 4.2 

5.7 3.5 

2.3 1.6 

13.5 4.3 

1.7 1.3 

5.9 2.6 

23.4 4.1 

47.7 8.3 

28.9 7.2 

0.03 0.03 

0.81 0.06 

0.44 0.14 

Chenega 

area 

Mean Standard 

deviation 

20.9 3.1 

3.3 1.3 

0.8 0.7 

38.2 8.7 

8.8 3.6 

11.5 4.7 

3.7 2.4 

4.5 3.7 

1.7 1.2 

16.8 3.8 

1.9 0.9 

3.8 1.3 

26.7 4.9 

50.8 9.1 

22.5 8.6 

0.03 0.03 

0.79 0.09 

0.56 0.21 

Ghost Rocks 

Formation 

Mean 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

21.0 ' 

37 .o / 

42.0 

0.08 

0.92 

0.95 

Results of Studies of Orca Group Samples 19 
samples, but they contain more felsic, lathwork, and 

metasedimentary clasts than the Port GravinaCordova samples. On the provenance diagrams of 

Dickinson and Suczek (1979), Sargent Icefield samples 

coincide roughly with the fields delineated by central 

Valdez-area samples; the Sargent lcefield plot extends 

somewhat farther toward the plutonic end of the arcorogen field on the Q-F-L and Qm-F-Lt diagrams. On 

the Qp-Lv-Ls diagram, Sargent Icefield samples, like 

central Valdez-area samples, show some sedimentary 

input from subduction-complex sources .in addition to a 

primary arc-orogen input. 

Chenega-area samples are, in general, quite 

similar to Port Gravina-Cordova samples and have 

Q 

.Continental block 

provenance 

Qm 

F Magmatic arc provenance l F Magmatic arc provenance lt 

Increasing ratio o 

plutonic/volcanic 

components 

p 

Qm 

AREA MEAN 

• 

0 

Sargent lcefield 

Chenega 

Ghost Rocks 

Formation 

+ Dickinson and Suczek, 1979 

s 

c 

g 

Qp 

Collision orogen 

sources 

' ' 

•co 1• \11 •• •o'\•\\\''· . • ~00 s. \ ·. 

. . \ ',,_ 

K Lv Arc orogen sources Ls 

EXPLANATION 

.CLAST TYPE 

Q Total quartzose grains 

Qm Monocrystalline quartz 

Q p Polycrystalline quartz 

L Total lithic grains 

Lt Total lithic grains and 

polycrystalline quartz 

------ Provenance fields+ 

Lv Volcanic lithic grains 

Ls Sedimentary lithic grains 

F Total feldspar 

P Plagioclase 

K Potassium feldspar 

Figure 13. Modal analyses of sandstone of the Orca Group from the Sargent Icefield and 

Chenega areas compared with analyses (from Winkler, in Nilsen and Moore, 1979) of the 

Ghost Rocks Formation of Moore and others (1983). 

20 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
lower lithic and higher feldspar contents than do 

Sargent Icefield samples. The lithic suite of almost all 

Chenega sandstones is predominantly volcanic, and 

felsitic textures are slightly more abundant than 

lathwork textures. 

Trend-surface analysis in western Prince William 

Sound indicates that the west-to-east compositional 

changes observed in Valdez Group sa:nples continue 

Q 

across the Con tact fault into the Orca Group. Thus, 

the petrographic analyses of this study (fig. 14) reveal 

no sharp break in sandstone composition at the 

Contact fault, which, as presently mapped, juxtaposes 

the Valdez and Orca Groups. Compositions in both 

groups become progressively richer eastward in quartz, 

potassium feldspar, and plagioclase, and poorer 

eastward in lathwork volcanic clasts (table 6). TrendQp 

EXPLANATION 

UNIT 

Valdez Group 

West and east 

Valdez areas 

Central Valdez area 

Orca Group 

Sargent lcefield area 

Chenega area 

Port GravinaCordova area 

FIELD MEAN 

w 

c 

s 

h 

p 

Figure 14. Comparison of sandstone modal analyses from the Valdez and Orca Groups. 

Q, total quartzose grains; F, total feldspar; L, total lithic grains; Qp, polycrystalline 

quartz; Lv, volcanic lithic grains; Ls, sedimentary lithic grains. 

Table G.--Statistical summary of first-degree trend surfaces for point-count 

data 

[x-coordinate is longitude, Y-coordinate is latitude, + = significant at the 

given level, 0 = not significant at the given level] 

Potassium 

feldspar 

Quartz 

Lathwork 

Percentage of Variance 

Accounted for by 

Linear Trend 

36.1 

21.6 

volcanic fragments 31.3 

Plagioclase 13.5 

Significance Direction of 

Level of Trend Increase 

Trend 

0.05 0.01 

X Y X Y 

+ + + + 

+ 0 + 0 

+ 0 + 0 

+ + 0 0 

3E 

E 

w 

SE 

Results of Studies of Orca Group Samples 21 
surface analysis indicates that these compositional 

changes are statistically significant. Although the 

percentage fit does not exceed about 30 percent of the 

total variance for any of these components, the trends 

are significant at the 0.01 level for three of the 

components (and at the 0.05 level for plagioclase). 

Thus, these trends could occur by chance less than one 

time in twenty. Ternary plots and discriminant 

analysis cannot distinguish sandstone in the central 

Valdez area from Orca Group sandstones in the 

Sargent Icefield area. The Q-F-L and Qp-Lv-Ls plots 

of point-count data indicate a dominantly magmaticarc provenance for both groups, with progressive 

unroofing from the volcanic cover to the plutonic roots 

of the arc through time. 

Anomalous Orca Group Samples 

Samples from the Knight Island area and the 

Ellamar area (on trend with, but to the northeast of, 

Knight Island) have the most quartz-rich compositions 

of any rocks examined in this study (table 7, fig. 15). 

Most of the sandstones are moderately well to poorly 

sorted; a few are very poorly sorted. The Ellamar 

samples are particularly well sorted compared to most 

Prince William Sound-area sandstones, and they are at 

the finer end of the medium-grained size range (fig. 

12c). 

Knight Island and Ellamar samples are quartz 

rich (43-72 percent), lithic poor (8-25 percent), and 

have moderate feldspar contents (17-41 percent). The 

C/Q ratio of these samples is low (0.04-0.15), but total 

chert contents are higher in these rocks than in any 

other Prince William Sound-area samples (as much as 

11 percent). Geochemical analyses of several pointcounted samples collected from the Knight-Ellamar 

(combined) area confirm the quartz-rich nature of 

these rocks; one sample from the Ellamar area 

contains 86 percent Si02 (table 2, sample 5). 

Potassium feldspar is absent in five of the samples, but 

constitutes as much as 15 percent of the others. The 

V /L ratios are generally low (0.14-0.59). Notable lithic 

clasts include mudstone, metasedimentary grains, and 

felsitic and lathwork volcanic grains. Mica (mostly 

muscovite and biotite) constitutes 5-7 percent of most 

samples, but reaches 20 percent in a sample collected 

from Naked Island. 

McGlasson (1976) performed point counts on four 

sandstone samples from Knight Island and obtained 

similar results. In general, he found quartz contents to 

be greater .than 40 percent, lithic clast-content less 

than 30 percent, and chert, polycrystalline quartz, and 

detrital phyllosilicate grains quite abundant. 

Knight-Ellamar samples do not fit well into any 

of the provenance fields delineated by Dickinson and 

Suczek (1979) for Q-F-L and Qm-F-Lt diagrams, but 

they coincide best with the recycled-orogen 

provenance. On the Qp-Lv-Ls diagram, these samples 

fit the subduction-complex source field quite well. 

Compared to data presented by Valloni and Maynard 

(1981) for modern deep-sea sands, the best match is 

with sands from a trailing-edge margin supplied by a 

collision orogen. 

The fields defined by the Knight-Ellamar samples 

on the Q-F-L and Qp-Lv-Ls diagrams do not overlap 

the fields defined by the Port Gravina-Cordova-area 

samples (fig. 16). Sandstones of the Knight Island and 

Ellamar areas are spatially associated with apparently 

contemporaneous volcanic rocks so, as postulated for 

Resurrection Peninsula samples, their anomalous 

compositions may have resulted (at least in part) from 

sediment blockage by volcanic barriers. On the other 

hand, Knight-Ellamar sedimentary rocks may have had 

Table ?.--Composition of sandstone of the Orca Group from the Knight Island 

(n=lO) and Ellamar (n=3) areas 

(see table 1 for further discussion of parameters. See figure 15 for graphic 

presentation of data) 

Knight Island Ellamar 

area area 

Mean Standard Mean Standard 

deviation deviation 

Monocrystalline quartz 36.4 8.2 49.2 7.3 

Polycrystalline quartz 5.7 3.2 5.3 0.9 

Chert 4.3 2.7 3.7 0.5 

Plagioclase 23.4 4.6 23.1 8.6 

Potassium feldspar 3.5 3.6 0.8 1.3 

Volcanic lithic fragments 5.2 1.9 4.1 1.9 

Sedimentary lithic fragments 4.5 3.7 5.8 4.4 

Metamorphic lithic fragments 6.1 2.1 3.6 0.5 

Plutonic lithic fragments 0.8 0.9 0.7 0.4 

Matrix and cement 14.9 3.1 13.2 6.6 

Heavy minerals 1.1 0.9 0.8 0.7 

Micas and chlorite 8.1 6.2 2.3 2.2 

Quartzose grains (total) 50.8 9.7 -60.2 8.4 

Feldspar (total) 30.8 7.3 25.4 9.2 

Lithic (total) 18.4 5.5 14.4 4.5 

Chert/Total quartz 0.09 0.03 0.07 0.02 

Plagioclase/Total feldspar 0.85 0.12 0.93 0.06 

Volcanic lithic fragments/ 

Total lithic fragments 0.32 0.11 0.32 0.24 

22 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
a different provenance than the Orca Group sandstones 

in other parts of the Prince William Sound area-a 

provenance in which highly evolved arc-orogen sources 

and subduction complexes were more important than 

volcanic-rich, less-evolved orogen complexes. 

of the Ragged Mountain area (hereafter RM) are also 

compositionally distinguishable from sandstone in the 

type locality of the Orca Group (table 8, figs. 16 and 

17). Samples are mostly lithofeldspathic and 

moderately well sorted, but some are poorly to very 

poorly sorted. Quartz contents are moderate (M, 29-

43 percent; RM, 22-35 percent) and C/Q ratios are 

The sandstone of the Montague Island. area 

(hereafter M for brevity) and, to a lesser extent, that 

a 

Continental block 

provenance 

am 

F Magmatic arc provenance L F Magmatic arc provenance 

Qm ap 

Lt 

Increasing ratio of 

plutonic/volcanic Collision orogen 

components 

p 

AREA MEAN 

* * 

Ellamar 

Knight Island 

Knight Island* 

* Data from McGlasson, 1976 

+ Dickinson and Suczek, 1979 

e 

k 

km 

sources I 

I 

I 

Subduction camp~ \ 

sources Ji.. • , 

. .\ 

~· \)} 1}\ 

---------- --+ -~\ :_\\\',,, 

K Lv Arc orogen sources 

EXPLANATION 

CLAST TYPE 

a Total quartzose grains 

am Monocrystalline quartz 

a p Polycrystalline quartz 

L Total lithic grains 

Lt Total lithic grains and 

polycrystalline quartz 

------ Provenance fields+ 

Lv Volcanic lithic grains 

Ls Sedimentary lithic grains 

F Total feldspar 

P Plagioclase 

K Potassium feldspar 

Ls 

Figure 15. Modal analyses of sandstone of the Orca Group from the Ellamar and Knight 

Island areas. 

Results of Studies of Orca Group Samples 23 
relatively low (M, 0.06-0.23; RM, 0.03-0.09), although 

values for the Montague Island samples are the highest 

counted for this study, and total chert content is 8 

percent in some Montague Island samples (fig. 12d). 

Feldspar contents are moderate (M, 24-40 percent; 

RM, 32-50 percent), and maximum potassium feldspar 

contents are about 15 percent in both groups. Lithic 

content is low (M, 13-26 percent; RM, 14-32 percent), 

and the V/L ratio varies but is typically 0.2-0.3. 

Volcanic lithic clasts constitute a maximum of 10 

percent in both groups, with felsitic and lathwork 

clasts most abundant. A conglomerate 5 km west of 

Ragged Mountain, near Martin River Slough, contains 

more than 75 percent felsic tuff and porphyry clasts, 

and a coarse-grained sandstone from this area contains 

24 percent felsic volcanic clasts. 

At Ragged Mountain mafic volcanic clasts are 

abundant in coarse-grained sandstone interbedded with 

volcanic rocks (units Tots and Tosv of Nelson and 

others, 1985). Similar sandstone is present on Hawkins 

and Hinchinbrook Islands (unit Tosv), and south of 

Miles Glacier (unit Tots). (These rocks are not 

included in the point-count data base of this study 

because of their coarse grain size.) Samples contain as 

much as 90 percent predominantly lathwork-textured 

volcanic lithic clasts, as well as fossiliferous 

calcareous clasts not noted in any other sandstone 

examined in this study. A chemical analysis of one 

Q 

L 

such sample from Ragged Mountain contains high 

levels of Ti02 (2.5 percent), Fe2o3 (14.2 percent), and 

MgO (5.6 percent), and relatively low Si02 (47 percent) 

(table 2, sample 6). These volcaniclastic sandstones 

may have been produced in large part by intrabasinal 

erosion of Orca Group mafic volcanic rocks. 

Sedimentary lithic clasts (mainly mudstone and 

siltstone) are the dominant lithic grains in most 

Montague Island and Ragged Mountain samples, but 

metamorphic lithic grains (phyllite, schist-notably 

biotite schist) are also abundant, particularly in 

samples from westem Montague Island. 

Mica (biotite and lesser white mica) is 

particularly abundant in the Montague Island and 

Ragged Mountain samples. Eight Montague Island 

samples contain more than 8 percent mica (as much as 

16 percent in one sample). The mica grains are 

distinguished by irregular extinction and kinking, · 

probably a result of sediment compaction (Moore, 

1973). Most samples contain garnet in the heavymineral suite; a few of the Ragged Mountain samples 

contain as much as 3.5 percent pyroxene and 

amphibole. Other heavy minerals are epidote, sphene, 

and zircon. 

Like the Knight-Ellamar samples, Ragged 

Mountain and, especially, Montague Island samples 

define fields on the Q-F-L and Qp-Lv-Ls diagrams that 

overlap only slightly with fields defined by Port 

Qp 

EXPLANATION 

AREA 

Knight lsland-EIIamar 

Montague Island 

Ragged Mountain 

Port Gravina-Cordova 

FIELD MEAN 

ke 

m 

p 

Figure 16. Comparison of modal analyses of sandstone from the Knight lsland-Ellamar 

(combined), Montague Island, and Ragged Mountain areas with those from the Port 

Gravina-Cordova area. Q, total quartzose grains; F, total feldspar; L, total lithic grains; 

Qp, polycrystalline quartz; Lv, volcanic lithic grains; Ls, sedimentary lithic grains. 

24 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
Gravina-Cordova samples (fig. 16). Compared to 

provenance fields de linea ted by Dickinson and Suczek 

(1979), Montague Island-Ragged Mountain samples fall 

between the arc-orogen, continental-block, and 

collision-orogen fields on the Q-F-L diagram and fall 

within the subduction-complex field on the Qp-Lv-Ls 

diagram (fig. 17). Discriminant analysis shows that 

Montague Island samples contain more quartz, biotite, 

and sedimentary and metasedimentary lithic clasts 

than do Port Gravina-Cordova samples. 

Comparison of Results of Studies of the Orca Group 

with Previous Work 

The major previous study of Orca Group 

sandstone petrography is that of Winkler (1976). He 

examined 59 samples, collected primarily from 

Hinchinbrook Island, but including a few samples 

collected from adjacent islands and the mainland. On 

the Q-F...:.L diagram, his results encompass the fields 

delineated by both the Port Gravina-Cordova and the 

Montague Island-Ragged Mountain samples of this 

study (figs. II and 17; table 4). Winkler determined a 

mean Q-F-L of 43-46-11 for his samples-slightly more 

quartz-rich than the Montague Island samples of this 

study, and slightly poorer in lithic components than the 

Port Gravina-Cordova samples. The C/Q ratio is close. 

to that determined for the Montague Island area, and 

the P /F and V /L ratios are higher than those 

determined for the Montague Island, Ragged Mountain, 

or Port Gravina-Cordova areas. 

Comparison of Winkler's (1976) findings to those 

of this study is hampered by the fact that only about 

one-quarter of his samples are medium grained-the 

rest are fine and very fine grained. Other geologic 

considerations (discussed below) indicate that rocks of 

Hinchinbrook Island may have more similarities with 

those of the Montague Island-Ragged Mountain areas 

than with those of the Port Gravina-Cordova region. 

As mentioned above, the results of McGlasson's point 

counts of Knight Island samples agree fairly well with 

the findings of the present study in that area. 

Petrographic information about rocks correlative 

with the Orca Group is available from the Ghost Rocks 

Formation (Winkler, in Nilsen and Moore, 1979) and the 

Sitkalidak Formation(Winkler, in Nilsen and Moore, 

1979; Moore and others, 1983). -Eight samples were 

studied from the Ghost Rocks Formation; the mean QF-L (21-37-42) is most like that of the samples from 

the Sargent Ice field area of this study. The Ghost 

Rocks samples are, however, more lithic rich in 

general, and more volcanic lithic rich in particular 

(V /L = 0.95) (table 5). Samples collected from the 

Sitkalidak Formation show striking similarities to 

those of the Montague Island and Ragged Mountain 

areas, as will be discussed more fully below. 

TECTONIC MODELS AND THE PLACEMENT OF THE 

VALDEz-oRCA BOUNDARY 

One goal of this study was to test whether the 

petrographic distinction that previous workers 

suggested existed between sandstones of the Valdez 

and Orca Groups could be shown to exist throughout 

the Prince William Sound area, and thus could help to 

resolve the problem of placement of the contact 

between the two groups. The results of this study 

indicate that there is no petrographic break in 

sandstone composition across the Contact fault as 

presently mapped. 

A second goal was to test the concept that the 

Valdez and Orca Groups comprise separate 

tectonostratigraphic terranes. To what extent can 

sandstone-composition analyses for the two groups 

constrain the tectonic scenarios evoked to explain 

their present structual settings? 

Jones and others (1981, p. 1) defined a 

tectonostratigraphic terrane as "a fault-bounded 

geologic entity, usually of regional extent, that is 

characterized by a distinctive stratigraphic sequence 

or rock assemblage that differs markedly from those 

of nearby partly or entirely coeval neighbors." 

Terrane analysis must consider the total record of 

sedimentation and structural history up until the time 

of accretion. 

Some previous workers treated the Valdez and 

Orca Groups (and their correlative sequences) as 

terranes in the sense described above-that is, thick 

accretionary wedges derived from different sources 

and accreted to the continent in two discrete 

episodes. For example, Plafker and others (1977) 

stated that the Chugach terrane (Valdez Group and 

correlative units) was accreted to the continental 

margin along the Border Ranges fault system by latest 

Cretaceous time (fig. 1). Consequently, the Contact 

fault system is interpreted as a more southerly plate 

boundary against which the Prince William terrane 

(Orca Group and correlative units) was accreted during 

early Tertiary time (Winkler and Plafker, 1975, 1981). 

Various sources have been proposed for the 

protoliths of the Chugach terrane. Zuffa and others 

(1980) suggested that at least some of the detritus 

comprising Chugach terrane flysch may have been 

derived from older, previously accreted terranes, such 

as Wrangellia, or from the melange part of the 

Chugach terrane-for example, the McHugh or Uyak 

Complexes. Decker (1980) agreed, stating that the 

Kelp Bay Group (a southeastern correlative of the 

McHugh Complex) may have been a source for some of 

the felsitic material deposited in the Sitka 

Graywacke. He also considered a basalt-rhyolite belt 

of Triassic age in the Alexander terrane as a possible 

source, but concluded that it was probably 

volumetrically incapable of supplying the entire 

Chugach terrane. 

Nilsen and Zuffa (1982) suggested that the 

Chugach terrane and adjacent terranes to the north, 

including the Peninsular terrane and Wrangellia, 

formed a single "megaterrane" that moved north as a 

unit. They proposed derivation of Chugach terrane 

flysch from the magmatic arc and adjacent shallow 

marine forearc basin of the Peninsular terrane. Pavlis 

(1982) suggested that the mid-Cretaceous GravinaNutzotin magmatic arc in the Talkeetna 

"superterrane" (a composite of Wrangellia and the 

Peninsular and Alexander terranes) was a source and 

accounted for the present mismatch of nearly 1,000-

km between this source and the distribution of 

Chugach terrane flysch by later large-scale 

Tectonic Models and the Placement of the Valdez-orca Boundary 25 
intraterrane faulting. Winkler and Plafker (1981) 

agreed that Chugach terrane strata may have been 

originally deposited far to the south, then tectonically 

transported northward, but they were unable to 

identify a source terrane. 

A commonly cited source for the Orca Group 

(Prince William terrane) is the plutonic and high-grade 

metamorphic complex of the Coast Mountains of 

British Columbia and southeastern Alaska. First 

suggested by Hollister (1979), the idea was adopted by 

Helwig and Emmet (1981) and Winkler and Plafker 

(1981). However, as Helwig and Emmet pointed out, a 

possible flaw in this argument is an apparent mismatch 

of the age of the Orca protolith with its proposed 

source. The maximum uplift of the coastal 

metamorphic and plutonic belt occurred 47-52 Ma 

(Hollister, 1979). This is slightly younger than the age 

of the Orca Group if the age (50.5-53.5 ± 1.6 Ma) of 

the plutonic belt that intrudes the northern parts of 

the Orca is taken as a minimum age limit for the 

entire group. 

Evidence for the Northward Displacement of the 

Valdez and Orca Groups 

Paleomagnetic data may provide further 

constraints on possible source areas for the Valdez and 

Orca Groups. Most of the paleomagnetic studies 

published to date support the idea that both groups 

were originally deposited south of their present 

positions relative to North America, but how far south 

and during what period northward movement occurred 

remain uncertain. 

Gromme and Hillhouse (1981) examined two 

gabbro plutons in the Chugach terrane of probable 

Paleogene age in the Fairweather Range in 

southeastern Alaska, as well as sheeted mafic dikes 

and pillow basalt of the Resurrection Peninsula. The 

mafic complex of the Resurrection Peninsula is of 

uncertain age, however, and is in probable fault 

contact with the sedimentary rocks of the Valdez 

Group. Gromme and Hillhouse concluded that the 

entire Chugach terrane has moved 25°-26° northward 

and that this movement took place between 20 and 41 

Ma. This would require a minimum rate of crustalplate motion slightly higher than the upper limit of 

contemporary absolute plate velocities. 

Stone and others (1982) presented paleomagnetic 

data from sedimentary rocks in the western part of the 

Chugach terrane (the Shumagin and Kodiak 

Formations). Their results indicate that at 70 Ma the 

paleolatitude of the Chugach terrance was between 

6° and 32° N., and that the terrane reached its 

present position at some time in the late Tertiary 

through consistent northward movement from Late 

Cretaceous time until the present. 

Both of these scenarios seem to conflict with a 

Late Cretaceous or early Paleocene time of 

emplacement for the Chugach terrane, unless the 

terrane were emplaced against the continental margin 

far to the south and then moved northward by dextral 

transform faulting. Cowan (1982) suggests that such a 

mechanism has displaced parts of the Chugach terrane 

as much as 1,100 km northward since early Tertiary 

time. 

Paleomagnetic evidence from the Prince William 

terrane also indicates a southern origin. Stone and 

others (1982) obtained paleomagnetic data from 

sedimentary rocks of the Sitkalidak and Ghost Rocks 

Formations. Thev postulated a paleolatitude for these 

rocks between 21 b to 28° N. at about 40-50 Ma. 

Plumley and others (1983) presented results of a 

detailed paleomagnetic study of a series of volcanic 

flows interlayered with sedimentary rocks of the Ghost 

Rocks Formation. They concluded that, since 62 Ma, 

these rocks have been transported 25° north from a 

paleolatitude of 40.3° N. ± 6.2°. Moore and others 

(1983), using paleomagnetic data and other geologic 

constraints, suggest that the Prince William terrane 

was emplaced near its present latitude by 45 Ma. 

Another paleomagnetic study of the Prince 

William terrane (Hillhouse and Gromme, 1977) 

examined the sheeted dikes and pillow basalts of 

Knight Island. The results of the investigation were 

equivocal, but provided no compelling evidence for 

long-distance transport of the rocks of the Orca 

Group. Plumley and others (1983), however, proposed 

that the equivocal results may reflect the incomplete 

elimination of secondary magnetic components. The 

only other paleomagnetic data available from the 

Prince William terrane are the preliminary results of 

Plumley and Plafker (1985) from Galena Bay in the 

central Prince William Sound area. These results 

(discussed below) indicate that as much as 40° of 

northward movement has occurred since the Paleogene 

for at least part of the Prince William terrane. 

Paleontologic data also indicates that the Prince 

William terrane may have undergone significant 

northward transport. The sparse foraminiferal 

assemblages of Paleocene and early to middle Eocene 

age from the Orca Group contain some tropical to 

subtropical faunal elements (Plafker and others, 

1985a). However, the basal core sample (a claystone 

interpreted to be a stratigraphic unit of the Prince 

William terrane) obtained from the Middleton Island 

borehole contains a late middle Eocene, low-diversity 

microfossil assemblage indicative of a cold-water 

environment. These findings indicate that the Prince 

William terrane had reached high latitudes (50° ± 5° 

N.) by 40-42 Ma (Keller and others, 1984). 

Plumley and others (1983) summarized the 

results of paleomagnetic studies from lower Tertiary 

rocks of the Peninsular terrane. Although the 

uncertainties are large, these authors determined a 

mean paleolatitude of 37° N. at about 57 Ma for these 

samples. 

Thus, most available paleomagnetic data from 

the Peninsular, Chugach, and Prince William terranes 

support the idea of considerable northward movement 

since early Tertiary time. The interpretations of 

Plumley and others (1983) further indicate that all 

three terranes were at about the same paleolatitude 

during early Tertiary time; these authors argued that 

at least the Chugach and Prince William terranes 

appear to have been effectively tied together by early 

Paleocene time. Other authors have proposed that the 

Peninsular and Chugach terranes have been linked 

since the Cretaceous (Pavlis, 1982; Stone and others, 

1982). 

26 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
Petrographic Constraints on the Significance and 

Location of the Valdez-orca Boundary 

Can petrographic data from Prince William 

Sound place any constraints on these tectonic 

scenarios? The petrographic similarity of sandstone 

from the westernmost part of the Orca Group (the 

Sargent Icefield area) and the adjacent part of the 

Valdez Group (the central Valdez area) argue against 

the existence of a major terrane boundary at the 

Contact fault in western Prince William Sound. In 

addition to their petrographic similarity, these rocks 

show no differences in metamorphic history and no 

fossil evidence for an age difference, as discussed 

above. 

Similar congruence in sandstone composition is 

evident across the Chugach-Prince William terrane 

boundary on Kodiak Island (fig. 1). Winkler (in Nilsen 

and Moore, 1979) studied samples from the Kodiak and 

Ghost Rocks Formations (Valdez and Orca Group 

correlatives, respectively) and found a complete 

overlap in the composition of samples from the two 

formations. Both groups of samples were relatively 

rich in volcanic lithic clasts (Kodiak Formation, mean 

Q-F-L= 24-32-44, V/L = 0.96; Ghost Rocks Formation, 

mean Q-F-L= 21-37-42, V/L = 0.95). 

Three possible conclusions follow from this 

petrographic similarity across the Contact fault. (1) 

There is no major terrane boundary between rocks 

presently assigned to the Chugach and Prince William 

terranes; both the Valdez and Orca Groups were 

derived from a single evolving source. (2) There is a 

major terrane boundary, but, at least in the western 

part of Prince William Sound, it is not located along 

the Contact fault. If this terrane boundary is to 

separate Cretaceous from Paleogene rocks, it would 

have to fall somewhere between the Contact fault and 

the Orca-Valdez contact as mapped by Grant and 

Higgins (1910) and Moffit (1954) (fig. 4). (3) There is a 

terrane boundary along the Contact fault; petrographic 

similarities on both sides of this terrane boundary, as 

well as indications of derivation from a continuously 

evolving source across this boundary, reflect 

interleaving of Valdez and Orca Group sandstones as 

imbricate fault slices, or are fortuitous or artifacts of 

insufficient data-set size. 

The Existence of a Terrane Boundary 

I believe the petrographic data best supports the 

first conclusion; that is, there is no terrane boundary 

(in the sense of a break that separates discrete 

packages of rocks that have distinct geologic histories) 

between the Valdez and Orca Groups in western Prince 

William Sound. Rather, the Valdez and Orca Groups 

constitute a single package, derived episodically or 

continuously from a single evolving arc orogen during 

Late Cretaceous into Eocene time. 

The tectonic model that best fits this scenario is 

an early linkage of the Peninsular terrane and the 

Chugach-Prince William (composite) terrane, the 

Peninsular terrane being the source of Chugach-Prince 

William terrane protoliths. The sediment source for 

the Valdez and Orca Groups would not be affected by 

subsequent movement of the megaterrane because the 

terranes would be linked prior to the Late Cretaceous; 

sediment generation could occur during northward 

migration. Alternatively, sediment generation could 

have occurred prior to accretion, perhaps with later 

intraterrane dismemberment through large-scale 

strike-slip faulting, as suggested by Pavlis (1982). 

If, however, the Peninsular and Chugach-Prince 

William terranes were not linked throughout 

Cretaceous and early Tertiary time, then the source of 

the parts of the Valdez and Orca Groups deposited 

prior to 62 Ma must lie somewhere along the North 

American Pacific coast-between the current latitudes 

of southern Washington and southern California, if one 

accepts the paleomagnetic constraints of Plumley and 

others (1983). (This set of circumstances would 

exclude the plutonic-metamorphic complex of British 

Columbia and southeastern Alaska as a possible source 

of Chugach-Prince William protoliths.) One drawback 

of this scenario is that if the Chugach-Prince William 

terrane were moving throughout its depositional 

history (Late Cretaceous into Eocene), but its source 

were not attached to it (as in the first scenario), one 

would expect to see greater diversity and less 

systematic distribution of sandstone compositions than 

is observed in the Valdez-Orca package. 

Alternative Locations of a Terrane Boundary 

The petrographic data from this study could also 

support the second conclusion-that a major terrane 

boundary does exist, but it is not located along the 

Contact fault in western Prince William Sound. Four 

possible alternative locations of this boundary will be 

considered: the Johnstone Bay fault of Tysdal and 

Case (1979), the Bainbridge fault of Helwig and Emmet 

(1981), the Montague Strait fault of Tysdal and Case 

(1979), and the Placer River fault (Tysdal and Case, 

1979). 

Johnstone Bay Fault 

Adopting the Johnstone Bay fault as the terrane 

boundary (Plafker and others, 1977) would have the 

following positive consequences: (1) It would include 

the gold-bearing rocks south of Sargent Icefield 

(presently assigned to the Orca Group) in the Valdez 

Group, which contains all other similar gold-bearing 

rocks presently known (Nelson and others, 1984); (2) It 

would place the only major area of mid-greenschist 

facies rocks known in the Orca Group of western 

Prince William Sound in the same terrane as adjacent 

mid-greenschist facies rocks of the Valdez Group (M.L. 

Miller, written commun., 1985). 

However, according to Tysdal and Case (1979, p. 

10), the Johnstone Bay fault is a minor structure 

marked by "a zone of breccia a few tens of meters 

wide." Although Condon and Cass (1958) suggested 

that, on the basis of aerial photo interpretation, the 

fault was connected with a smaller fault north of Icy 

Bay, Tysdal and Case could not confirm this 

connection. Such a minor discontinuous structure does 

not seem likely to represent a terrane boundary. In 

addition, although this boundary would group the most 

lithic-rich of the Orca Group samples with the lithicTectonic Models and the Placement of the Valdez-orca Boundary 27 
rich Valdez Group, no distinct petrographic boundary 

can be drawn between the sandstone of the Sargent 

Icefield area and that of the adjacent Chenega area. 

Bainbridge Fault 

A second candidate for the terrane boundary is 

the Bainbridge fault of Helwig and Emmet (1981). This 

would be similar to the Valdez-Orca contact proposed 

by Moffit (1954). The Bainbridge fault is described as 

a westward-dipping thrust fault that places massive 

sandstone on top of cataclastically deformed thinner 

bedded rocks (Helwig and Emmet, 1981). Adoption of 

this boundary has the following advantages: (1) It 

coincides approximately with the mafic belt that runs 

from Elrington Island, through Knight Island, and north 

to Glacier Island; (2) It separates distinct petrographic 

assemblages-the quartz-rich rocks of the Knight 

Island area and the more feldspatholithic rocks of the 

Chenega area; (3) Use of this fault as the terrane 

boundary would put all the plutonic rocks on the west 

and northwest side of the boundary-that is, in the 

Valdez Group. Using the Bainbridge fault as a 

terrane boundary would eliminate any conflict in 

western Prince William Sound between fossil 

assemblages of late middle and late Eocene age 

reported from localities in southern parts of the Orca 

Group (Plafker and others, 1985a and b) and the 50.5-

53.5 ± 1.60-Ma age of the oldest group of plutons. 

Montague Strait Fault 

A third possible terrane boundary is the 

Montague Strait fault. As discussed further below, 

this fault separates rocks of lower metamorphic grade 

to the east from slightly higher grade rocks to the 

west, but the difference in grade is not striking. Also, 

the petrographic distinction between sandstone of the 

Knight Island area and that of the Montague Island 

area is not sharp. 

Placer River Fault 

The fourth possible terrane boundary is the 

Placer River fault. Because both Late Cretaceous 

pelecypod assemblages that have been found between 

it and the Contact fault to the east are float 

occurrences, the rocks in this area may be thought of 

as of uncertain age. However, geologists mapping in 

the Anchorage quadrangle (Winkler, written commun., 

1985) have found Late Cretaceous pelecypods in place 

east of Lake George. This is east of the projected 

trace of the Placer River fault, but only by about 5 

km. The trace of this fault is marked by a shear zone 

50-150 m wide, and rocks adjacent to the fault have a 

well-developed schistose fabric (Tysdal and Case, 

1979). Although rocks west of the Placer River fault 

(west Valdez area) are richer in lithic grains 

(particularly volcanic lithic grains) than are rocks east 

of the fault (central Valdez area), this petrographic 

distinction is not as sharp as that marked by the 

Bainbridge fault (tables 1, 5, and 7; figs. 6, 13, and 

15). Thus, if a boundary between the Orca and Valdez 

Groups is to be selected to reflect petrographic 

distinctions, the Bainbridge fault of Helwig and Emmet 

(1981) seems to be the best choice. 

Imbricate Faulting along the Terrane Boundary 

If the Contact fault is a major terrane break, the 

petrographic similarity of rocks on either side of the 

fault may be the result of the presence of interleaved 

slices of Valdez and Orca Group sandstones. This 

interleaving would be reflected in the rocks of the 

Sargent Icefield, and perhaps the Chenega, areas, 

where the petrographic characteristics are 

intermediate between those of rocks in the western 

and central Valdez areas and rocks at the type locality 

of the Orca Group (Port Gravina-Cordova area). 

According to Winkler and others (1984), the Eagle 

River fault, which separates the McHugh Complex 

from the Valdez Group in the Seward and Anchorage 

quadrangles, is an example of such a zone of imbricate 

slivers of two units. 

The reconnaissance nature of studies that have 

been conducted in the Sargent Icefield area and the 

difficult terrain mean that an imbrication zone 

marking the Orca-Valdez contact in this area cannot 

be ruled out. As described above, however, rocks of 

the Sargent Icefield area represent an internally 

coherent and distinctive lithologic belt of massive 

sandstone and, to the north, slump deposits and 

conglomerate (Helwig and Emmet, 1981; Nelson and 

others, 1985). These lithologies are distinct from the 

dominantly thin-bedded and shale-rich facies that are 

present to the west in the central Valdez area and to 

the east in the Chenega area. Thus, the lithologic 

continuity of the Sargent Icefield area argues against 

explaining its transitional petrographic character as a 

result of tectonic mixing. 

Summary 

The results of petrographic studies of the 

sandstone in the western part of Prince William Sound 

are best explained by one of two possible tectonic 

reconstructions. (1) There is no terrane boundary in 

western Prince William Sound, and the Valdez and 

Orca Groups might best be thought of as a single 

flysch sequence derived from one continuously 

evolving source during Late Cretaceous, Paleocene, 

and Eocene time. The magmatic arc of the Peninsular 

terrane represents one possible source. (2) The 

Bainbridge fault of Helwig and Emmet (1981) separates 

the Cretaceous (and lower Paleogene ?) age lithic-rich 

sedimentary rocks of the Chugach terrane from the 

Paleogene, relatively quartz- and feldspar-rich 

sedimentary rocks of the Prince William terrane. 

Protoliths of the Chugach terrane were derived 

primarily from a volcanic arc; protoliths of the Prince 

William terrane were derived, at least in part, from 

the more evolved roots of an arc. The plutonicmetamorphic complex of the Coast Mountains may 

represent this source, particularly if the Orca Group 

is, at least in part, slightly younger than the intrusive 

belt (50.5-53.5 Ma) in Prince William Sound, a 

possibility that will be discussed further below. 

28 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
AREAS mAT HAVE ANOMALOUS SANDSTONE 

PETROGRAPHY 

Mount Denson Area 

The Mount Denson area, in the northwestern part 

of the Cordova quadrangle (figs. 4 and 5), is underlain 

by rocks presently assigned to the Valdez Group. The 

composition of sandstone in this area is anomalous 

relative to that of sandstones in other areas where the 

Valdez Group is present. 

As described above, framework-grain mode 

diagrams and discriminant analyses show no overlap in 

composition between the Mount Denson-area samples 

and samples from the west, central, or east Valdez 

areas, but they do show a complete overlap with 

samples from the adjacent Port Gravina-Cordova area, 

the type locality of the Orca Group. No sandstone 

samples collected from north of the Jack Bay fault 

(fig. 5) were available for analysis during the course of 

this study, so the petrographic character of these 

rocks is not known. 

The petrographic character of the Mount Denson 

samples has several possible explanations. Like the 

congruence in sandstone composition on both sides of 

the Contact fault in the western Prince William Sound 

area, this may be evidence that in the eastern part of 

the sound as well, the Contact fault does not separate 

two discrete packages of rock with distinct sources 

and geologic histories. Instead, both the Orca and 

Valdez Groups may be part of a single package faulted 

and deformed since its deposition. A second possibility 

is that the actual contact between the Orca and 

Valdez Groups is not the Landlock strand of the 

Contact fault, as presently mapped (Winkler and 

Plafker, 1981; Nelson and others, 1985), but lies 

further north-perhaps along the Jack Bay fault. 

The existence of the Landlock fault (fig. 5) was 

first proposed by Capps and Johnson (1915, p. 33), who 

described it as "inconspicuous, for it runs parallel to 

the bedding of the slates and graywackes of the Valdez 

Group and also lies parallel to the surface of the 

geeenstone of the Orca Group***giving the rocks a 

deceptive appearance of stratigraphic conformity." 

They believed, however, that "there can be little 

question that the fault exists", since it juxtaposes 

younger rocks with those "of much greater age" (p. 33). 

No fossils have been found in the Mount Denson 

area to confirm or refute the supposition that these 

rocks are older than adjacent rocks southwest of the 

Landlock fault. In the absence of paleontologic data, 

metamorphic-grade differences have been used to 

argue that Mount Denson-area rocks are older than 

rocks to the south (Capps and Johnson, 1915). 

However, no significant grade difference exists 

between rocks in the northern part of the Mount 

Denson area (including the samples point-counted for 

this study) and those southwest of the Landlock fault; 

rocks in both areas are of prehnite-pumpellyite to low 

greenschist facies (M.L. Miller, written commun., 

1985). Higher-grade (amphibolite facies) rocks occur 

southeast of Mount Denson, but these rocks reflect low 

pressure, high-temperature metamorphism, possibly 

produced by a buried pluton (M.L. Miller, written 

commun., 1986), and need not be older than adjacent 

lower-grade rocks. 

Winkler and Plafker (1975) adopted the Landlock 

fault as a segment of the Contact fault system, which 

they interpreted as a 11major early Tertiary plate 

boundary." However, the trace of the Landlock 

segment varies from sinuous to linear, and its shallow 

dip (about 45° N.) is anomalous for the Contact fault 

system (Winkler and Plafker, 1981). It also represents 

a distinct southward jog in the regional trace of the 

Contact fault system. 

Adoption of the Jack Bay fault as the contact 

between the Orca and Valdez Groups would have 

several advantages. It would eliminate the southward 

jog in the master Contact fault system, and it would 

keep rocks of essentially equivalent petrographic 

character together in a single group (the Orca 

Group). Furthermore, the Jack Bay fault has a steeper 

dip, more in keeping with the rest of the Contact fault 

system. 

Areas Spatially Associated with Mafic Rocks 

Other rocks in the Prince William Sound area 

that have anomalous compositions are spatially 

associated with mafic rocks. These rocks include 

sandstones currently assigned to the Valdez Group (in 

the Resurrection Peninsula area) and sandstones 

currently assigned to the Orca Group (in the Knight 

Island and Ellamar areas). 

Resurrection Peninsula Area 

The relation between the mafic complex of the 

Resurrection Peninsula and the spatially associated 

sedimentary rocks has been interpreted in various 

ways. Tysdal and others (1977) stated that the mafic 

igneous rocks of the peninsula intrude and are 

interbedded with flysch typical of the Valdez Group. 

Nelson and others (1985) mapped the relation as a 

thrust fault that places flysch above interbedded 

pillow basalt, broken pillow breccia, and minor 

amounts of siltstone and argillite. Even if the mafic 

complex-sedimentary rock contact is interpreted as a 

thrust fault, the two units could still be 

contemporaneous but have suffered post-formational 

faulting. Without age control, the stratigraphic 

relation of the mafic complex and the adjacent 

sedimentary rocks remains equivocal. 

The anomalous composition of the sandstone of 

the Resurrection Peninsula area relative to other 

Valdez Group sandstones has several possible 

explanations. If the sandstone samples collected from 

the Resurrection Peninsula are considered to be part 

of the mafic complex and in fault contact with typical 

Valdez Group flysch, they may have had a separate 

source and different subsequent history from that of 

the flysch, or they may have a different composition 

as a result of barrier effects produced by local 

volcanic edifices. If the sandstone of the Resurrection 

Peninsula is a part of the Valdez Group-flysch 

package, two interpretations might be advanced to 

explain its anomalous composition. As mentioned 

above, the finer grain size and slightly better sorting 

Areas that have Anomalous Sandstone Petrography 29 
of these samples may indicate that sedimentary 

processes (such as reworking and winnowing) have 

altered the original composition. Alternatively, local 

volcanic centers may have greatly influenced Valdez 

Group sedimentation, as suggested by Mitchell (1979). 

Quartzofeldspathic sandstones-like those reported 

from the southwestern Kenai Peninsula by Cowan and 

Boss (1978) and those of the Resurrection Peninsulacould have accumulated in areas distal to these 

volcanic centers. 

Knight Island Area 

The Knight Island samples of this study are also 

in an equivocal tectonic relation to the adjacent mafic 

complex. McGlasson (1976) suggested that the 

sedimentary rocks are in tectonic contact with the 

gabbro on the east side of Knight Island and are 

interbedded with the pillow-basalt sequence on the 

west side of the island. Tysdal and Case (1979) 

indicated that the mafic rocks are in depositional or 

intrusive contact with the sedimentary rocks on both 

sides of the island. Nelson and others (1984), however, 

considered the contact to be a structural one 

throughout its extent. 

Striking lithologic similarities between the mafic 

complexes of Resurrection Peninsula and Knight 

Island, as well as the lack of conclusive age 

determinations for either complex, allow one to 

speculate that the two sequences were originally part 

of a single mafic complex. Whole-rock greenstone KAr ages may not support this idea (Winkler and others, 

1984). These ages, considered useful in dating thermal 

events related to accretionary or collisional processes, 

indicate that the ages of the rocks of the Resurrection 

Peninsula were reset at about 47-50 Ma, 10-14 million 

years earlier than the rocks of Knight Island. 

However, this resetting may be only partial, so the 

radiometric data do not resolve the question of the 

relative ages of the two mafic complexes, or even the 

question of their time of emplacement. 

The petrographic character of the sandstones of 

Knight Island and Resurrection Peninsula also indicates 

the differences between the two sequences, although, 

as detailed above, the sandstone now spatially 

associated with the mafic complexes may not be 

cogenetic with these complexes. Although the 

sandstone from both areas is relatively lithic poor, the 

Knight Island samples contain more quartz and less 

feldspar than do the Resurrection Peninsula samples. 

Ellamar Area 

There is nothing equivocal about the association 

of the Ellamar-area sandstone with the adjacent 

volcanic rocks. All three Ellamar samples are from 

sandstone lenses 5-7 m thick that are clearly 

interbedded with pillowed and massive basalts. Mafic 

rocks are volumetrically dominant in the Ellamar area, 

comprising 75-92 percent of two sections measured by 

Nelson and others (1985). 

The sandstones of the Ellamar area, like those of 

the Resurrection Peninsula, are at the lower end of the 

medium grain-size classification, and are also the 

best-sorted samples of any examined in this study. 

Knight Island samples, however, are neither 

particularly well sorted nor fine grained. 

The Ellamar and Knight Island-area rocks may be 

part of a single package of sedimentary and mafic 

rocks distinct from the rest of the Orca Group. Nelson 

and others (1984) described an arcuate gravity and 

aeromagnetic high that trends northeastward from 

Elrington Island through La touche and Knight Islands, 

runs eastward through Glacier Island and the Ellamar 

area, and then continues east across the Contact fault 

to the mountains south of the Tasnuna River. This 

gravity and aeromagnetic pattern roughly coincides 

with a belt of mafic volcanic rocks. Sandstone 

intercalated with this volcanic belt has a composition 

distinct from that of sandstone to the northwest and 

southeast and could constitute a separate petrographic 

province. The composition could reflect either a 

distinct provenance or the blocking effect of volcanic 

edifices. The particularly quartz-rich nature of the 

Ellamar samples, along with their finer grain size and 

well-sorted texture, may indicate concentration 

through reworking of an already quartz-rich sediment. 

One line of evidence that supports the theory 

that the Ellamar rocks, at least, are part of a package 

separate from the rest of the Orca Group is the 

paleomagnetic data from the Galena Bay area 

(Plumley and Plafker, 1985), 3-10 km north of the 

Ellamar sandstone-sample localities. The 

paleomagnetic data, obtained from pillow-lava flows 

interbedded with sedimentary rock units, indicate an 

inclination far shallower than expected and may imply 

a goleward translation of these rocks of as much as 

40 since Paleogene time. In contrast, the Prince 

William terrane is thought to have had at most 25° of 

northward translation (Plumley and others, 1983). The 

volcanic rocks iri the Ellamar area are also distinctive 

in that they lack the positive magnetic anomaly 

characteristic of other volcanic rocks of Prince 

William Sound; this may be the result of metamorphic 

alteration (Winkler and Plafker, 1981), but could also 

indicate that the Ellamar volcanic rocks represent a 

thin fault slice of no great vertical extent. 

Montague Island and Ragged Mountain Areas 

Orca Group sandstones from the Montague Island 

area and, to a lesser degree, from the Ragged 

Mountain area, are in many respects similar to 

sandstones of the Sitkalidak Formation that crop ~t 

along strike to the southwest in the Sitkalidak Islaltd 

area (fig. 1). Stewart (1976) suggested that the 

Sitkalidak Formation is the proximal equivalent of 

sediments that comprise the Zodiac fan, located about 

400 km southwest of Sitkalidak Island on the Aleutian 

abyssal plain. Winkler (in Nilsen and Moore, 1979) 

disagreed, asserting that the Sitkalidak sandstones are 

too rich in volcanic detritus to correlate well with the 

Zodiac sands. Moore and others (1983) presented more 

extensive petrographic data from the Sitkalidak 

Formation and argued that, particularly on the basis of 

30 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, Alaska 
Q-F-L and V/L ratios, the Sitkalidak sandstones do 

correlate well with Zodiac fan sands. Sandstones from 

the Montague Island and Ragged Mountain areas are 

similar in composition and age to Sitkalidak sandstones 

and Zodiac fan sands. In addition, the rocks of the 

Montague Island-Ragged Mountain-area and the 

Sitkalidak Formation fall within similar sedimentary 

and metamorphic facies, and possess similar structural 

styles. 

Petrography 

Table 8 and figures 17 and 18 show modal data, 

Q-F-L plots, and V /L ratios for sandstone from the 

Montague Island and Ragged Mountain areas compared 

to data from the Sitkalidak Formation and the Zodiac 

fan. Using the criteria of Moore and others (1983), the 

Montague Island and Ragged Mountain samples 

correlate well with the Sitkalidak and Zodiac fan 

samples. In fact, Montague Island sandstone correlates 

even more closely with the Zodiac fan sediments than 

do the Sitkalidak sandstones. The Montague Island and 

Ragged Mountain samples have an average grain size 

of about 0.3 mm, slightly coarser than in the Zodiac 

samples of Stewart (1976) or the Sitkalidak samples of 

Moore and others (1983), which are fine grained 

(average grain size between 0.15 and 0.25 mm). 

Compositional variation due to such modest 

differences in grain size is, however, unlikely to be 

extensive or to invalidate the general petrographic 

similarities between the Montague Island, Ragged 

Mountain, Sitkalidak Formation, and Zodiac fan 

samples. 

The age of the Sitkalidak Formation is thought 

by Moore and Allwardt (1980) to be Eocene. The 

Sitkalidak is not intruded by the plutons (56-64 Ma) 

that intrude the adjacent Ghost Rocks Formation and 

is overlain by the nonmarine Sitkinak Formation of 

Oligocene age. The most reliable age-diagnostic fossil 

known from the Sitkalidak is a crab that occurs most 

abundantly in rocks of late Eocene age (Moore and 

Allwardt, 1980) in western Washington. 

The Zodiac fan sediments contain a sparse 

nannoflora of late early, middle, and late Eocene age 

(Stewart, 1976). 

Sparse fossil data indicate that the southeastern 

parts of the Orca Group are younger than the rest of 

the group, and may be younger than the plutonic belt 

that intrudes the more northern Orca Group 

exposures. Fossil localities at eastern Montague 

Island, the southern part of the Ragged Mountain area, 

and Hinchinbrook Island (fig. 4) have yielded 

radiolarians, coccoliths, dinoflagellates, and 

foraminifers thought to be of middle middle through 

late Eocene age (Plafker and others, 1985a). Fossils 

found elsewhere in the Orca Group are of Paleocene to 

earliest middle Eocene age (Tysdal and Case, 1979; 

Plafker and others, 1985a,b). 

Some of the middle and late Eocene fossils occur 

adjacent to or in the same samples as Paleocene or 

early Eocene forms. Plafker and others (1985b) 

suggested that the younger age assignments reflect 

either misidentification of poorly preserved, sparse 

assemblages or problems with the age ranges of 

certain fossils. However, at least some of the 

Table a.--Composition of sandstone of the Orca Group from the Montague Island (n=l2} and 

Ragged Mountain (n=7) areas 

[sitkalidak Formation data (n=27) from 11oore and others (1983). See table 1 for 

explanations of parameters. See figure 17 for graphic presentation of data] 

Montague Island Ragged Si tkalidak 

area Mountain Formation 

area 

i1ean Standard Mean 3tandard Mean Standard 

deviation deviation deviation 

Monocrystalline quartz 26.2 5.2 24.6 5.1 17.4 3.6 

Polycrystalline quartz 5.9 1.8 2.0 0.7 5.8 3.9 

Chert 4.2 1.8 1.6 0.6 0.9 1.1 

Plagioclase 24.3 4.6 33.6 6.7 19.1 5.4 

Potassium Feldspar 4.9 4.4 8.6 4.0 3.2 1.6 

Volcanic lithic fragments 6.6 2.0 6.5 2.7 11.1 4.6 

Sedimentary lithic fragments 7.9 3.4 11.0 5.4 12.2 4.6 

!1etamorphic lithic fragments 4.7 1.8 3.1 2.4 3.5 3.0 

Plutonic lithic fragments 1.1 0.8 1.6 1.0 2.8 1.4 

Matrix and Cement 13.4 4.2 15.2 4.4 20.9 4.0 

Heavy minerals 1.3 0.6 2.0 0.9 0.6 0.9 

Micas and chlorite 11.4 5.0 3.2 1.9 1.9 1.2 

Quartzose grains (total) 41.5 4.8 30.2 4.8 31.6 6.3 

Feldspar (total) 34.6 6.1 46.0 8.2 29.7 10.1 

Lithics (total) 23.9 5.7 23.8 6.4 38.6 9.3 

Chert/Total quartz 0.12 0.05 0.06 0.02 0.04 0.04 

Plagioclase/Total feldspar 0.81 0.14 0.78 0.09 0.9 0.1 

Volcanic Lithic fragments/ 

Total Lithic fragments 0.32 0.11 0.02 0.13 0.4 0.1 

Areas that have Anomalous Sandstone Petrography 31 
limestone layers that have yielded the mixed-age 

assemblages are carbonate turbidites, (Dumoulin and 

Miller, 1984); in these rocks, a mixed fauna may 

simply represent erosion of pelagic layers of different 

ages, and the age of the youngest fossil assemblage 

probably represents a maximum depositional age. 

Q 

The middle and late Eocene fossil ages barely 

overlap or entirely postdate the emplacement age 

Continental block 

provenance 

Qm 

F Magmatic arc provenance L F Magmatic arc provenance Lt 

Increasing ratio of 

plutonic/volcanic 

components 

p 

Qm 

AREA MEAN 

.... Montague Island m 

!::. Ragged Mountain 

Zodiac Fan 

Sitkalidak s 

Formation 

+ Dickinson and Suczek, 1979 

Qp 

Collision orogen 

sources I 

I 

I 

Subduction comple~ \, 

sources \ \ 

AA \ 

A\ 

AA A \, 

"lAA\ 

,-----------\A A ~\ 

11• ' 

11 \r 11\ 11 \ 11 ', I 11 ',,',, 

K Lv Arc orogen sources 

EXPLANATION 

CLAST TYPE 

Q Total quartzose grains 

Qm Monocrystalline quartz 

Qp Polycrystalline quartz 

L Total lithic grains 

Lt Total lithic grains and 

polycrystalline quartz 

------ Provenance fields+ 

Lv Volcanic lithic grains 

Ls Sedimentary lithic grains 

F Total feldspar 

P Plagioclase 

K Potassium feldspar 

', 

Ls 

Figure 17. - Modal analyses for sandstone of the Orca Group from the Montague Island 

and Ragged Mountain areas compared with analyses from the Zodiac fan (Stewart, 1976) 

and from the Sitkalidak Formation of Moore and Allwardt (1980), (Moore and others, 

1983). 

32 Sandstone Composition of the Valdez and Orca Groups, Prince William Sound, A1aska 
range (50.5-53.5 ± 1.6 Ma) of the plutons that intrude 

the northern parts of the Orca Group. No plutons 

intrude rocks on Hinchinbrook or Montague Islands; a 

few small undated granite plugs intrude northwestern 

Ragged :viountain. The Rude River pluton, which 

intrudes the Orca Group northeast of, and apparently 

on strike with, the rocks of Hinchinbrook Island, is of 

late early or early middle Eocene age (Plafker and 

others, 1985b). However, the Rude River pluton is 

separated by more than 60 km and several laterally 

extensive faults from the nearest middle middle 

Eocene or younger fossil locality. 

In summary, fossil data indicate that the rocks of 

Ragged Mountain and Hinchinbrook and Montague 

Islands are younger than the rest of the Orca Group. 

They may be younger than the early to earliest middle 

Eocene plutonic belt, but regional geologic relations 

complicate this interpretation. However, even if all 

the Orca Group were restricted to the early middle 

Eocene or older, present age constraints would allow 

its correlation, at least in part, with the Sitkalidak 

Formation and Zodiac fan sediments. 

Metamorphic-Grade Relationships 

The Montague Island-Ragged Mountain sandstone 

and Sitkalikdak Formation sandstone show similarities 

in their metamorphic grade relative to adjacent 

rocks. Tysdal and Case (1979) state that rocks east of 

the Montague Strait fault reach prehnite-pumpellyite 

and (or) zeolite grade, in contrast to the lowergreenschist-facies conditions west of the fault. 

However, rocks of Montague Island examined for this 

study appear to be mostly zeolite facies or lower, with 

local laumontite as veins and patchy alteration of 

feldspar. Only a few samples contain minor amounts 

of secondary pumpellyite (M.L. Miller, written 

commun., 1985). Prehnite-pumpellyite assemblages 

are extensively developed in the volcanic rocks of 

Hinchinbrook Island and Ragged Mountain, and it may 

be that well-developed prehnite-pumpellyite 

assemblages do not occur on Montague Island because 

of a lack of suitable host rocks (volcanic rocks or 

volcanic lithic-rich sedimentary rocks). 

Vitrinite-reflectance values obtained from 

Montague, Green and Hinchinbrook Islands range from 

0.8 to 1.27 (Helwig and Emmet, 1981) indicating 

minimum temperatures of about 150°-200°C (Bostick, 

1979). Values of 1.2 and 1.9 obtained from two 

samples from Point Martin, south of Ragged Mountain 

(Mull and Nelson, 1986) indicate minimum 

temperatures of 200°-250°C (Bostick, 1979). The 

transition from zeolite to prehnite-pumpellyite facies 

is poorly constrained, but Turner (1981) suggested that 

it occurs at about 200°-230°C. 

Vitrinite-reflectance values from the Sitkalidak 

Formation are slightly lower than those reported from 

the Montague Island and Ragged Mountain areas; they 

range from 0.55 to 0.61 and, along with porosity data, 

have been interpreted as defining a peak temperature 

of 100°-125°C, given an effective burial time of 5-20 

m.y. (Moore and Allwardt, 1980). A minimum 

temperature of 125°C is obtained using the method of 

Bostick (1979). Laumontite occurs in some Sitkalidak 

sandstone samples (Moore and Allwardt, 1980). 

The vitrinite-reflectance values from the 

Sitkalidak Formation are considerably less than those 

obtained from the Ghost Rocks Formation. The Ghost 

Rocks values range from 2.15 to 2.95 (Moore and 

others, 1983) and suggest a maximum temperature of 

225°-250°C, given an effective burial time of 5-10 

m.y. Vitrinite reflectance data are not available for 

the northern and western parts of the Orca Group, but 

metamorphic-assemblage data indicate that, in 

general, metamorphic-grade increases to the north and 

west of the Montague and Ragged Mountain areas 

(M.L. Miller, written commun., 1985; Goldfarb and 

others, 1986). 

Thus, the rocks of Montague and Hinchinbrook 

Islands and Ragged Mountain, like those of the 

Sitkalidak Formation, seem to have a somewhat lower 

metamorphic grade than the rocks directly adjacent to 

them. However, the rocks of the Orca Group have 

apparently reached slightly higher minimum 

temperatures than the Sitkalidak Formation. 

RATIO OF VOLCANIC LITHIC FRAGMENTS 

TO TOTAL LITHIC FRAGMENTS (V/U 

0.1 0.2 0.3 0.4 I I 

-

-

0.5 

ZODIAC FAN 

SITKALIDAK FORMATION 

of Moore and Allwardt (1980) 

ORCA GROUP 

Montague Island area 

ORCA GROUP 

Ragged Mountain area 

Figure 18. Ratio of volcanic clasts to total lithic fragments in Zodiac fan sediment 

(Stewart, 1976), Sitkalidak Formation sandstone (Moore and others,1983), and Orca Group 

sandstone from the Montague Island and Ragged Mountain areas. 

Areas that have Anomalous Sandstone Petrography 33 
Structural Style 

The structural style of the Orca Group on 

Montague and Hinchinbrook Islands is similar to that 

observed in the Sitkalidak Formation. Helwig and 

Emmet (1981) documented landward structural 

vergence in Montague-Hinchinbrook rocks and 

attributed its formation to obductive offscraping of a 

thick submarine-fan sequence deposited on a slowly 

descending plate decoupled on its landward side. 

Moore and Allwardt (1980) used a similar explanation 

for the extensive landward vergence in the eastern 

(more deformed) parts of the Sitkalidak Formation. 

Sedimentary Facies 

Finally, sedimentary facies, known only from 

reconnaissance mapping of Montague Island and from 

more detailed mapping on Hinchinbrook Island 

(Winkler, 1976), are similar to those described from 

the Sitkalidak Formation by Nilsen and Moore (1979) 

and Moore and Allwardt (1980). In general, the less 

deformed parts of the Sitkalidak Formation consist of 

thin, relatively shale-rich, graded beds with local 

occurences of thick, massive sandstone. These 

sequences are interpreted as having been deposited on 

an outer fan, lobe fringe, and basin plain by Nilsen and 

Moore (1979). Moore and Allwardt (1980) suggested 

that this facies association constitutes a slope-basin or 

slope-apron deposit. Elsewhere, particularly in its 

more deformed parts, the Sitkalidak Formation 

consists of thicker bedded and coarser grained deposits 

(sandstone beds commonly 1-10 m thick) with local 

pebbly sandstone and conglomerate. These rocks are 

interpreted to be trench-filling inner- and middle-fan 

deposits (Moore and Allwardt, 1980). 

The rocks of Montague and Hinchinbrook Islands 

consist largely of facies C and D turbidites (as defined 

by Mutti and Ricci-Lucci, 1978) in which sandstone and 

shale contents are roughly equal or shale is dominant. 

Local slurried sandstone beds with scattered argillite 

fragments are present; such beds were described by 

Nilsen and Moore (1979) in the Sitkalidak Formation 

and are typical of interlobe and lobe-fringe 

sequences. Calcite-cemented turbidites and 

calcareous concretions, also described from the 

Sitkalidak Formation, are common in the Montague 

and Hinchinbrook areas, especially in thinner bedded 

and more shale-rich sequences. Abundant coalified 

plant debris, noted in sandstone beds from the 

Sitkalidak Formation, is present in the MontagueHinchinbrook area as well. 

Coarser grained, thicker bedded sandstone 

sequences also occur in the Montague-Hinchinbrook 

area; bed thicknesses exceed several meters, 

sandstone:shale ratios reach 20/1, and intervals of 

conglomerate and pebbly mudstone with channelized 

bases occur locally. Such deposits are likely to 

represent inner- or middle-fan sequences. 

CONCLUSIONS 

Petrographic data from sandstones in the Prince 

William Sound area show no abrupt change across the 

Contact fault, which juxtaposes the Upper Cretaceous 

Valdez Group and the Paleocene and Eocene Orca 

Group, and has also been suggested to represent the 

boundary between the Chugach and Prince William 

terranes. In the western part of Prince William Sound, 

sandstone compositions in both groups change 

systematically from west to east; however, graphical 

and statistical- analyses cannot discriminate sandstone 

of the easternmost exposures of the Valdez Group 

(central Valdez area) from that of the westernmost 

exposures of the Orca Group (Sargent Icefield area). 

Sandstone of both groups becomes gradationally richer 

eastward in quartz, potassium feldspar, and 

plagioclase, and gradationally poorer eastward in 

lathwork-textured volcanic clasts. In the eastern part 

of Prince William Sound, sandstone of the Valdez 

Group north of the Landlock segment of the Contact 

fault (Mount Denson area) cannot be differentiated 

from sandstone of the Orca Group south of this fault 

(Port Gravina-Cordova area). 

The compositional data are best explained by 

considering the Valdez and Orca Groups as a single 

flysch sequence derived primarily from one 

continuously unroofing source-a magmatic arc 

progressively eroded from its volcanic cover to its 

plutonic roots through time. If a terrane boundary 

does exist in the Prince William Sound area, and if 

such a boundary coincides with a petrographic 

discontinuity, it may lie along the Jack Bay fault in 

eastern Prince William Sound, or perhaps farther 

north. In western Prince William Sound, the boundary 

is most likely to lie along the Bainbridge fault, 50 km 

east of the Contact fault. The Bainbridge fault 

separates quartz-rich rocks of the Knight Island area 

on the east from more feldspatholithic rocks of the 

Chenega area on the west. 

The southeastern exposures of the Orca Group 

(Montague Island and Ragged Mountain areas, and 

perhaps the Hinchinbrook Island area as well) may 

constitute a distinct sequence of rocks, perhaps 

younger than the rest of the Orca Group. Rocks of the 

Montague Island area, in particular, show striking 

similarities to the Eocene Sitkalidak Formation of 

Moore and Allwardt (1980), 400 km to the west, as well 

as to the sediments of the Zodiac fan of the Aleutian 

abyssal plain. 

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