U.S. GEOLOGICAL SURVEY CIRCULAR 1087 

The Alaska Mineral Resource Assessment 

Program-Background Information to 

Accompany Geologic and Mineral-Resource 

Maps of the Valdez Quadrangle, 

South-Central Alaska 

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The Alaska Mineral Resource Assessment ProgramBackground Information to Accompany Geologic and 

Mineral-Resource Maps of the Valdez Quadrangle, 

South-Central Alaska 

By G.R. WINKLER, R.j. GOLDFARB, W.J. PICKTHORN, and 

GEORGE PLAFKER 

U.S. GEOLOGICAL SURVEY CIRCULAR 1087 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U. S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1992 • 

Free on application to 

Book and Open-File Report Sales 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

The Alaska mineral resource assessment program : background information to 

accompany geologic and mineral-resource maps of the Valdez Quadrangle, southcentral Alaska I by G.R. Winkler ... [et al.). 

p. em.- (U.S. Geological SuiVey circular; 1 087) 

Includes bibliographical references. 

Supt. of Docs. no.: 119.4/2:1087 

1. Geology-Alaska-Valdez Region. 2. Mines and mineral resourcesAlaska-Valdez Region. 3. Alaska Mineral Resource Assessment 

Program. I. Winker, Gary R. II. Series. 

QE84.V36A42 1992 92-11017 

557 .98'3-dc20 CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Purpose and scope 1 

Geography and access 1 

Summary of mineral and energy production and exploration 4 

Copper 5 

Gold 6 

Chromium 6 

Geologic investigations 6 

Early investigations 6 

Recentinvestigations 7 

Geochemical investigations 9 

Geophysical investigations 10 

Geologic and tectonic summary 11 

Wrangellia terrane 11 

Peninsular terrane 13 

Chugach terrane 15 

Prince William terrane 18 

Yakutat terrane 19 

Potential mineral resources 19 

References cited 19 

FIGURES 

1-3. Maps showing: 

1. Selected geographic features of the Valdez quadrangle 3 

2. Generalized geology of the Valdez quadrangle 12 

3. Geologic terranes of the southern Alaska region 14 

TABLE 

1. Geologic, geochemical, geophysical, and mineral-resource maps of the Valdez 1 °X3° 

quadrangle, Alaska 2 

Contents Ill 

The Alaska Mineral Resource Assessment ProgramBackground Information to Accompany Geologic and 

Mineral-Resource Maps of the Valdez Quadrangle, 

South-Central Alaska 

By G.R. Winkler, R.j. Goldfarb, W.j. Pickthorn, and George Plafker 

Abstract 

The Valdez 1 °X3° quadrangle in south-central Alaska 

contains potentially significant resources of gold, chromium, 

copper, and possibly a few other commodities. This report 

summarizes recent results of integrated geological, geochemical, and geophysical field and laboratory studies conducted by 

the U.S. Geological Survey that were designed to provide an 

assessment of the mineral resources of the quadrangle. The 

results have been published principally as maps and data 

releases in U.S. Geological Survey Open-File Reports and Miscellaneous Field Studies maps, which are cited herein and provide background information for the assessment. This report 

also cites numerous additional references for published reports 

and maps that describe the mineral resources and the geologic 

and tectonic setting of the Valdez quadrangle. 

INTRODUCTION 

Purpose and Scope 

Recent U.S. Geological Survey field and laboratory studies of the geologic setting of mineral and energy deposits in 

the Valdez l 0 X3° quadrangle in south-central Alaska were 

conducted as part of the Alaska Mineral Resource Assessment Program (AMRAP), mineral surveys of public lands 

within Chugach National Forest, and the Trans-Alaska 

Crustal Transect (TACT) study of the deep crustal structure 

of southern Alaska. The overall objective of the geologic, 

geochemical, and geophysical investigations was to provide 

modem mineral-resource information as potential input for 

land-use planning and minerals-policy development and to 

Manuscript approved for publication April 3, 1992. 

guide minerals exploration. The studies also aimed to 

enhance the geologic knowledge of an area that had received 

few recent earth-science investigations. These studies were 

completed in 1988; however, they incorporate much fundamental geologic information that was obtained in the 1950's, 

1960's, and 1970's during studies of the Copper River Basin 

area, the Gulf of Alaska Tertiary province, the epicentral 

region of the 1964 Alaska earthquake, the major fault systems of southern Alaska, and the mineral resources of the 

contiguous McCarthy 1 °X3° quadrangle to the east. A list of 

maps and reports that provide the primary mineral-resource 

data for the Valdez quadrangle is given in table 1. Other 

sources of information on the geology and mineral deposits 

of the quadrangle are cited in the text and listed in the accompanying references. 

Geography and Access 

The Valdez quadrangle includes about 18,000 km2 of 

south-central Alaska (fig. 1). The quadrangle is bounded by 

the 61 o and 62° parallels and by the 144° and 147° meridians. The quadrangle includes, from north to south, the southwest end of the scenic Wrangell Mountains dominated by the 

broad shield volcano of Mount Wrangell, part of the lowlands of the Copper River Basin drained by the Copper River 

and numerous tributaries, and the rugged, glacier-clad crest 

of the Chugach Mountains indented on the southwest by the 

fiord of Port Valdez. Mount Wrangell is still mildly active, 

and periodically small ash eruptions darken snowfields near 

the summit crater. 

Near the village of Chitina at the southern margin of the 

Copper River Basin, the Chitina River joins the Copper 

River from the east to form Alaska's third most voluminous 

drainage (fig. 1). Below the confluence, the Copper River 

flows south across the Chugach Mountains through a deep 

Introduction 
Table1. Geologic, geochemical, geophysical, and mineral-resource maps of the Valdez 1°X3" quadrangle, Alaska 

Reference Subject 

Cobb and Matson (1972) ............ Location, identification, and reported commodities of known lode and 

placer deposits. 

Cobb (1979) ............ Compilation of references on mineral-resource occurrences. 

U.S. Geological Survey (1979) ............ Aeromagnetic survey. 

Winkler, Miller, MacKevett, and Holloway (1981) ............ Characteristics and geologic controls of mineral-resource occurrences. 

Winkler, Silberman, and others (1981) ............ Bedrock geology and K-Ar geochronology. 

Williams and Johnson (1981) ............ Surficial geology. 

LeCompte (1981) ............ Interpretation of satellite imagety. 

Miller and others (1982) ............ Geochemical survey; analytical results and sample locality maps. 

Case, Burns, and Winkler (1986) ............ Aeromagnetic map and interpretation. 

Goldfarb and others (in press) ............ Geochemical interpretation; maps and tabulations of areas with 

geochemical anomalies for metals. 

Pickthorn and others (in press) ............ Mineral-resource assessment; delineation and characterization of 

areas having potential for undiscovered resources of precious, base, 

and ferrous metals. 

canyon to be joined at the southern boundary of the quadrangle by the Tasnuna and Bremner Rivers. Other major tributaries, such as the Klutina and Tonsina Rivers, originate in 

glaciers near the crest of the Chugach Mountains and flow 

northward into the Copper River Basin. Where these rivers 

emerge from the mountains, they flow into large lakes 

impounded by terminal moraines that mark the maximum 

advance of former valley glaciers. The glaciers have receded 

tens of kilometers southward to leave remnants near the crest 

of the Chugach Mountains. All of these rivers have large seasonal flows and carry enormous loads of sediment, which are 

being deposited as broad alluvial plains encroaching upon 

the lakes or transported to the turbid Copper River to be carried to its coastal delta in the Cordova quadrangle to the 

south. South of the crest of the Chugach Mountains, the 

Lowe River and drainages from the Valdez Glacier also 

carry a large load of sediment, which is deposited as a steepfronted delta in deep water where the rivers meet tidewater 

in Port Valdez. 

Topographic relief in the quadrangle exceeds 4,250 m, 

from sea level in Port Valdez to the crater rim of Mount 

Wrangell. Most ridges in the Chugach and Wrangell Mountains are rugged and steep sided, having been quarried by 

and submerged beneath glaciers until recently. The two 

mountain ranges are separated by broad open lowlands along 

the Copper and Chitina Rivers. 

The earliest published descriptions of the geography and 

geology of parts of the Valdez quadrangle resulted from 

expeditions in 1885 and 1891 (Allen, 1887; Hayes, 1892) 

that generally followed the Copper and Chitina Rivers. The 

discovery of rich placer gold deposits on the Klondike River 

in the Yukon Territory of Canada prompted more thorough 

exploration as well as a stampede of fortune hunters. At least 

4,000 people landed at Valdez during the winter of 

1897-1898 with the intention of being among the ftrst to 

2 Alaska Mineral Resource Assessment Program-Valdez Quadrangle 

reach the gold ftelds in the spring of 1898 (Schrader, 1900); 

most lacked any experience in pioneering or mining. A few 

remained to prospect the local drainages for placer gold or 

the Prince William Sound region for lodes of copper; however, about 3,000 stampeders endured severe hardships to 

pioneer routes from the coast to the interior that soon became 

trails or roads. Today' s Richardson Highway from Valdez to 

Copper Center follows what was the most common route. In 

1899 and 1900, bonanza copper lodes were discovered near 

Kennicottl Glacier in the southern Wrangell Mountains. 

Between 1905 and 1911, the Copper River & Northwestern 

Railway was constructed more than 200 km from Cordova 

up the Copper River Canyon to Chitina from which it 

extended approximately another 100 km eastward to the 

mines in the McCarthy quadrangle. Between 1911 and 1938, 

more than 1.2 billion pounds of copper and 9 million troy 

ounces of silver from the Kennecott mines (Douglass, in 

MacKevett and others, 1977) were carried by the railroad 

through Chitina to shipping docks on tidewater at Cordova. 

Production ceased in 1938, and most of the rails were salvaged for scrap me~ during World War II. The roadbed 

later was regraded between Chitina and McCarthy, and in the 

late 1960's a new bridge was constructed across the Copper 

River; in the summer months, the road now is maintained for 

passenger vehicles. An unmaintained spur road extends 

northward from the former transfer point of Strelna following a preexisting trail to mineral prospects on the Kotsina 

River; it becomes virtually impassable, however, within a 

short distance from Strelna. From Chitina, an early trail 

called the Edgerton Cutoff now has been widened and paved 

1

The company formed to develop the copper properties took its name 

from the nearby glacier, which had been named for a pioneer surveyor in the 

region, but somehow (probably inadvertently), an "e" was substituted for an 

"i" in the spelling of Keilllecott Copper Corporation. 
61° 

00' 

0 5 10 15 20 25 MILES 

I I I I I I I I I I I 

0 5 10 15 20 25 KILOMETERS 

Figure 1. Selected geographic features of the Valdez quadrangle. Heavy stippling indicates tidewater or lakes; light shading indicates glaciers or permanent ice fields. 

and extends northwestward to reach the Richardson Highway near the historic Tonsina roadhouse. A short segment of 

the Glenn Highway runs through the northwest comer of the 

Valdez quadrangle. It follows one of the earliest trails from 

upper Cook Inlet to the interior of Alaska and is the most 

direct route for highway travel between the metropolis of 

Anchorage and Glennallen. The southern end of the TransAlaska Pipeline System (TAPS) follows the Richardson 

Highway closely across most of the Valdez quadrangle. During its construction in the mid-1970's, large camps were 

maintained near the old road houses of Wortmanns, Ptarmigan, Tiekel, and Tonsina; the only remaining TAPS facilities, however, are the southernmost pump station (no. 12) at 

which the oil is heated and speeded for its ascent over 

Thompson Pass on the final leg of its 1,250-kmjourney from 

Prudhoe Bay to the pipeline terminus and marine shipping 

facility on the south side of Port Valdez. 

Prior to the 1964 Alaska earthquake, the town of Valdez 

was located on the composite delta formed by the Lowe and 

Valdez Glacier Rivers. Following catastrophic submarine 

sliding of the seaward part of the delta during the earthquake, 

much of the settlement that remained standing was inundated 

by slide-generated waves. The town subsequently was 

rebuilt in its present location behind bedrock headlands 

along the north shore of Port Valdez. When it became a shipping and staging area during construction of the southern end 

of the TAPS, Valdez burgeoned to become a city; it now is 

Alaska's 14th largest city and has a population (1990) of 

4,635. Valdez has year-round harbor facilities. lt is the base 

for a large fleet of fishing and pleasure vessels, a port of call 

for ferries on the Alaska Marine Highway system, and headquarters for U.S. Coast Guard facilities that monitor shipping 

in Prince William Sound and oil tanker traffic to and from 

the TAPS marine facility near the head of Port Valdez. 

Valdez also is served by an all-weather airport with daily ser- . 

vice. 

Copper Center is the only other town in the quadrangle; 

its 1990 population was 449. Most residents are Alaska 

Introduction 3 
Natives of the Copper River Athabascan tribe; the headquarters for the AHTNA, Inc., Regional Native Corporation is 

located a few kilometers north of Copper Center. Population 

elsewhere in the quadrangle is sparse and fluctuates seasonally. Permanent settlements are located at Chitina, Lower 

Tonsina, and Kenney Lake on the Edgerton Highway, 

numerous homesteads and hay farms are in the surrounding 

area, and seasonal cabins or lodges are on the shores of 

several outlying lakes. The transient population increases 

markedly during the summer months. Copper Center and 

Chitina have long graded runways and frequent seasonal air 

service; several shorter landing strips provide air access to 

scattered places in the quadrangle. 

All of the Valdez quadrangle east of the Copper River is 

included within the Wrangell-Saint Elias National Park and 

Preserve, which was created in December 1980 by enactment of the Alaska National Interest Lands Conservation Act 

(ANILCA). The Park headquarters is a few kilometers from 

Copper Center just north of the quadrangle boundary. 

Acknowledgments.~Many colleagues participated in the 

earth-science investigations of the Valdez quadrangle that 

are summarized in this circular. We gratefully acknowledge 

their important contributions, many of which are cited in the 

accompanying list of references. In particular, the detailed 

geologic mapping and stratigraphic studies of E.M. MacKevett, Jr., and his coworkers in the McCarthy quadrangle and 

the regional studies of George Plafker and his coworkers of 

the Border Ranges fault system and other major faults in 

southern Alaska provided the vital framework on which subsequent AMRAP and TACT investigations were built. Colleagues who made important contributions to the AMRAP 

geologic, geochemical, and geophysical studies in the quadrangle include D.P. Barnes, D.L. Campbell, J.E. Case, C.L. 

Connor, Arthur Grantz, C.D. Holloway, K.M. Johnson, J.R. 

LeCompte, R.J. Miller, R.L. Morin, W.J. Nokleberg, R.M. 

O'Leary, M.L. Silberman, S.C. Smith, S.J. Sutley, R.B. 

Tripp, R.G. Tysdal, andJ.R. Williams of the U.S. Geological 

Survey; L.E. Bums and G.H. Pessel of the State of Alaska 

Division of Geological and Geophysical Surveys; T.L. Hudson, ARCO Alaska, Inc.; and R.J. Newberry and W.K. 

Wallace, University of Alaska-Fairbanks. 

SUMMARY OF MINERAL AND ENERGY 

PRODUCTION AND EXPLORATION 

Lode mines in the Valdez quadrangle produced gold, 

copper, and silver, and probably lead and zinc, in the first 

three decades of the 20th century, principally in the Port 

Valdez and Tonsina districts between 1910 and 1929. The 

characteristics and probable geologic controls of known 

mines, prospects, and occurrences in the quadrangles are 

summarized by Winkler, Miller, MacKevett, and Holloway 

(1981). During the gold rush to the Klondike and exploration 

and development of copper mines near McCarthy in the 

4 Alaska Mineral Resource Assessment Program-Valdez Quadrangle 

Wrangell Mountains and near Latouche and Ellamar in 

Prince William Sound, prospecting for gold and copper was 

widespread; many claims were located and some were developed throughout the quadrangle area. Dilapidated structures 

in the Chugach Mountains on Columbia and Shoup Glaciers, 

above Mineral Creek, east of Valdez Glacier, Solomon 

Gulch, between the Tiekel and Tonsina Rivers, between 

Taral and Canyon Creeks, between Marshall Mountain and 

Klutina Lake, and on Barnette Creek, in the Wrangell Mountains on the upper reaches of Elliott Creek and the Kotsina 

River, and along the route of the Copper River & Northwestem Railroad are vestiges of this era. No properties outside 

the Port Valdez and Tonsina districts are known to have 

produced metals, with the possible exception of minor gold 

production from small lodes west of Chitina in the late 

1970's and early 1980's. Placer gold production from the 

Valdez quadrangle apparently has been very minor, although 

most drainages near known auriferous lodes have showings. 

Bulldozer trenches that explore iron-stained zones in 

calc-schist and greenschist on the Chetaslina River in the 

Wrangell Mountains are of uncertain vintage but may date 

from the late 1960's or early 1970's. The trenches expose 

small veins containing pyrite, chalcopyrite, and barite; samples contain trace to minor amounts of detectable gold. In the 

mid-1970's, active development began on the Opal claims 

on the upland surface between Liberty and Fivemile Creeks 

west of Chitina (Winkler, Miller, MacKevett, and Holloway, 

1981). The claims encompass numerous widely spaced 

quartz veins that cut semischistose rocks of the McHugh 

Complex and Eocene tonalite. The veins contain arsenopyrite, pyrite, galena, and sphalerite and have anomalous values of gold and silver. The claims currently are active, and 

there probably has been minor production of precious 

metals. 

Exploratory drilling for petroleum was conducted during 

the 1960's in the southwestern part of Copper River Basin 

just north of the quadrangle boundary (Alaska Geological 

Society, 1970). The test wells bottomed between 850 and 

2,700 min Jurassic or Cretaceous marine rocks, testing possible structural and stratigraphic traps in the Mesozoic section. The wells penetrated as much as 600 m of Tertiary 

nonmarine coal-bearing shale, sandstone, and conglomerate 

correlative with the Chickaloon Formation, 1,500-1,900 m 

of Lower and Upper Cretaceous rocks correlative with the 

Nelchina Limestone and Matanuska Formation, and as much 

as 1,200 m of Middle and Upper Jurassic rocks correlative 

with the Chinitna and Naknek Formations and the Tuxedni 

Group in the nearby Nelchina area and the Kotsina Conglomerate in the Wrangell Mountains (Arthur Grantz, U.S. 

Geological Survey, oral common., 1991). At least two wells 

reached the Talkeetna Formation. Apparently no encouraging shows were encountered in the Mesozoic rocks. The 

wells also tested without success the possibility that petroliferous Tertiary nonmarine sequences that are productive in 

upper Cook Inlet might be present in the area. 
Heat flux at the summit crater of Mount Wrangell 

increased notably during the 1970's (Benson and Motyka, 

1979), causing melting of the summit ice cap and seasonal 

exposure of bare ground on the north slope of the crater. 

Intermittent ash falls darkened the summit area. Although 

the volume of the magma chamber under Mount Wrangell is 

estimated to be approximately 50 kJn3 (Smith and Shaw, 

1975), its potential use as a large-scale source of geothermal 

power is limited by its lofty elevation and its remoteness 

from potential power demand. Areas of abnormally high 

heat flux are not known at lower elevations on the west flank 

of the volcano, where development would be less hazardous 

and costs less prohibitive. 

Recent exploration activity in the quadrangle has been 

quite limited and has focused mainly on the search for massive sulfide deposits, precious metal veins or lodes, and 

chromium, nickel, or platinum-group elements hosted in layered mafic-ultramafic igneous complexes. No major discoveries have been announced; however, several discoveries, 

including some resulting from our own investigations, may 

be significant, and they warrant additional detailed evaluation. 

Copper 

Although the use of native copper implements by interior 

and coastal Alaska Native peoples had been known for more 

than a hundred years, their provenance was not revealed until 

the late summer of 1899 when prospectors were led to occurrences in the McCarthy area by an informant of Chief 

Nikolai, head of the Taral band of the Copper River Indians. 

Claims also were staked in upper Elliott Creek in 1899. Prospecting soon focused on the Nikolai Greenstone-Chitistone 

Limestone contact near which most occurrences were 

located. The subsequent discovery of the Bonanza lode in the 

summer of 1900 in the McCarthy quadrangle intensified 

prospecting throughout south-central Alaska for Kennecotttype deposits. Most occurrences that crop out near the contact were discovered between 1900 and 1902 (Mendenhall 

and Schrader, 1903). For example, prospects in the mountains east ofTaral were located in 1901 (Moffit, 1912, 1914). 

The practical access that was provided by completion of the 

railroad in April 1911 prompted another two-year spurt in 

prospecting during which several additional properties were 

discovered. Within the northern part of the Valdez quadrangle, however, most copper occurrences are in veins of limited extent or associated small pods, disseminations, or 

surface coatings (Winkler, Miller, MacKevett, and Holloway, 1981 ). Most are in or near fractures or faults and exhibit 

hydrothermally altered wallrock. MacKevett (1976) inferred 

that similar deposits in the McCarthy quadrangle were produced by hydrothermal process related to Late Jurassic or 

Tertiary plutonism, an inference that probably is equally 

warranted for deposits in the Valdez quadrangle. Few, if any, 

of the deposits in the Valdez quadrangle display features of 

sabkha remobilization that typify the largest and richest 

lodes of the Kennecott type (Armstrong and MacKevett, 

1976, 1982). None of the numerous deposits on Elliott Creek 

or the Kotsina or Cheshnina Rivers in the Wrangell Mountains or the lodes in the Chugach Mountains on the uplands 

between Taral Creek and Divide Creek were able to compete 

with the phenomenally rich Kennecott lodes, which during 

26 years of shipments averaged about 13 percent copper 

(Douglass, in MacKevett and others, 1977). 

Active prospecting for submarine volcanogenic deposits 

of copper began in the Prince William Sound region in 1897 

with the staking of claims in the Ellamar area just south of 

the Valdez quadrangle boundary (Grant and Higgins, 1910). 

The Midas mine in Solomon Gulch near Valdez was discovered in 1901, and the first shipment of ore occurred in 1912 

(Johnson, 1915). Between 1912 and 1919, the two lodes at 

the mine produced more than 500 tons of copper (Moffit and 

Fellows, 1951); ore shipments also contained between 0.25 

and 0.42 oz/ton silver and 0.05 and 0.062 oz/ton gold (Rose, 

1965). Ore remains underground. Jansons and others (1984) 

estimated additional reserves of 62,000 tons of 1.6 percent 

copper. Prospects south of the divide in Jack Bay apparently 

are much smaller and lower grade (Johnson, 1919). Prospects several kilometers to the east near Sulphide Gulch 

(Johnson, 1916) probably have greater tonnage but are significantly lower grade (Rose, 1965). Additional occurrences 

at Wortmanns and Tsina Glaciers (Winkler, Miller, 

MacKevett, and Holloway, 1981) also probably are large, 

low-grade deposits. Many of the larger deposits, such as 

those of the Midas mine, are hosted by sedimentary rocks, 

indicating that they are of the Besshi-type of volcanogenic 

massive sulfide deposit (Nelson and Koski, 1987; Crowe and 

others, in press). 

During the regional prospecting induced by the 

Kennecott finds, massive copper-nickel sulfide lenses were 

discovered about 1907 near Spirit Mountain in peridotite 

dikes intruding schist, gneiss, and marble. The prospect was 

abandoned in 1917 (Overbeck, 1918). Because of the strategic importance of nickel during World War II, the prospect 

was reevaluated in 1942 and found to contain minor amounts 

of cobalt, as well as the nickel and copper (Kingston and 

Miller, 1945). Estimated reserves included 6,500 short tons 

of material grading from 0. 22 to 7. 61 percent nickel and 0.12 

to 1.56 percent copper; however, the relative inaccessibility 

of the prospect and its small tonnage and low grade were 

deemed to make it subeconomic, even under the favorable 

price conditions prevailing at that time (Kingston and Miller, 

1945). Additional investigations in the late 1960's (Herreid, 

1970) and late 1980's (Foley and others, 1989) demonstrated 

that the massive sulfide lenses also contained minor values 

of silver and platinum-group elements (chiefly palladium), 

but there has been no subsequent development activity. 

Summary of Mineral and Energy Production and Exploration S 
Gold 

Pay gravels had been known on many of the streams near 

Valdez as early as 1894, when minor sluicing was underway 

on Mineral Creek (Schrader, 1900), and nearby quartz veins 

were staked as early as 1897. Valdez principally was 

regarded, however, as a starting point for the interior until 

the Cliff mine was located in 1906 and began to produce 

gold in 1910 (Johnson, 1915). Prompted by its success, in 

1909 and 1910 prospectors located numerous additional 

occurrences in a large area extending from Valdez Glacier 

west to Columbia Glacier (Brooks, 1912). Additional deposits that soon became productive, including the CameronJohnson on Shoup Glacier and the Rose Johnson and the 

Ramsey-Rutherford east of Valdez Glacier, were discovered 

in 1911; by 1914, nine mills of various types were in operation in the productive area north of Port Valdez, which came 

to be included in the Port Valdez district. During its lifetime, 

more than 61,000 ounces of gold were produced from lode 

deposits in the district (Jansons and others, 1984). 

Gold-bearing quartz veins south of Port Valdez are 

included in the Jack Bay district. The area was thoroughly 

prospected between 1910 and 1915, but workings are small 

and no gold is known to have been produced; however, significant unrecorded gold production from the copper ores of 

the Midas mine is likely. 

Many of the streams draining northward into the Copper 

River Basin or eastward into the Copper River had placer 

activity around the tum of the century before any workable 

quartz veins were discovered (Rohn, 1900; Schrader, 1900; 

Brooks, 1914); however, only the gravels on Quartz and Fall 

Creeks are known to have produced small amounts of gold 

(Moffit, 1918, 1935). Jansons and others (1984) evaluated 

the placer gold potential of many of the tributaries of the 

Lowe and Tasnuna Rivers along the southern edge of the 

Valdez quadrangle. They found significant showings below 

Marshall Glacier and sites having moderate placer potential 

along Bench Creek (a tributary of the Lowe River at the 

mouth of Heiden Canyon) and Cleave Creek (a tributary of 

the Copper River upstream from the mouth of the Tasnuna 

River) and in the alluvial flats of the Tasnuna River between 

its junction with the Copper River and the Schwan Glacier. 

The development of the Cliff mine near Valdez stimulated extensive prospecting for lode sources of gold along 

the route of the Richardson Highway beginning in 1911. 

Most of the gold-bearing quartz veins near Tiekel in the 

Tonsina district were discovered within a few years thereafter (Moffit, 1918). Only properties on the east side of Hurtle 

Creek and near the head of Boulder Creek are known to have 

produced gold, although there also was extensive development of claims near Stuart Creek in the 1920's (Moffit, 

1935). 

Gold placers were discovered on the Little Bremner 

River in 1901 and probably produced small quantities of 

gold in the first two decades of the 20th century (Moffit, 

6 Alaska Mineral Resource Assessment Program-Valdez Quadrangle 

1912, 1914). No lode occurrences are known to be present in 

the drainage. 

In the Wrangell Mountains, gold-bearing quartz veins 

cutting mafic volcanic rocks and gabbro along Benito Creek 

were discovered in 1913 (Moffit and Mertie, 1923); they 

have been prospected intermittently to the present time, but 

there has been no development. 

Chromium 

Chromite was discovered at Bernard Mountain near Tonsina in 1954, and by 1956 claims had been extended more 

than 5 km to the northeast to include exposures of layered 

ultramafic rocks at so-called "Sheep Hill" west of Dust 

Creek (Foley and Barker, 1985). U.S. Government incentive 

prices under the Minerals Stockpile Program were halted in 

1956 before the deposits could be developed, and the claims 

were allowed to lapse soon thereafter. From time to time in 

the ensuing years, small-scale investigations have been 

renewed in the Bernard Mountain area, but no major activity 

has occurred. The U.S. Bureau of Mines has identified 

resources in three deposits at Bernard Mountain that exceed 

343,000 tons Cr20 3; one deposit at Sheep Hill is estimated 

to contain 26,000 tons Cr20 3 (Foley and Barker, 1985). 

Chromian spinels in dunite and peridotite at these locations 

are anomalous in platinum-group elements and in clinopyroxenite also contain anomalous gold (Foley and Barker, 

1987). Nearby, magmatic-sulfide prospects in layered gabbroic rocks near the benchmark "Scarp" contain as much as 

5 percent pyrite and lesser marcasite and chalcopyrite (Newberry, 1986). Near the head of Barnette Creek in the western 

part of the Valdez quadrangle, small test pits of uncertain 

vintage also have been excavated on stringers of chromite in 

sheared ultramafic rocks (Winkler, Miller, MacKevett, and 

Holloway, 1981), and disseminated and irregular masses of 

chalcopyrite, pyrrhotite, and lesser pentlandite are present in 

layered gabbronorite at several locations between the 

Nelchina and Tazlina Glaciers (Newberry, 1986; G.R. 

Winkler, unpublished data); however, these occurrences are 

remote and very limited in extent. Other bodies of serpentinized ultramafic rocks emplaced near or along the Border 

Ranges and Second Lake fault zones along the north flank of 

the Chugach Mountains are essentially unexplored. 

GEOLOGIC INVESTIGATIONS 

Early Investigations 

The earliest published geologic observations from the 

Valdez quadrangle are incidental products of two pioneering 

U.S. Army expeditions into the largely unexplored interior 

of Alaska. The first expedition, under the direction of Lieutenant H.T. Allen, in 1885 explored the main valleys of the 
Copper and Chitina Rivers on the way to the interior (Allen, 

1887). The second, which was led by Lieutenant Frederick 

Schwatka and included geologist C.W. Hayes, in 1891 

descended the Chitina and Copper Rivers on the way from 

the interior (Hayes, 1892). In 1898, F.C. Schrader of the 

Geological Survey was assigned to the Copper River military explorations under Captain W.R. Abercrombie; 

Schrader ( 1900) made extensive observations on a trip that 

began at Valdez, reached the Copper River Basin by way of 

Valdez and Klutina Glaciers, and returned to Valdez via the 

Copper, Tasnuna, and Lowe Rivers. The Harriman Alaska 

Expedition visited Prince William Sound in 1899 and provided detailed descriptions of Columbia Glacier for the first 

time (Gilbert, 1904). Oscar Rohn's geologic circuit of the 

Wrangell Mountains from Valdez in 1899 (Rohn, 1900) led 

to the beginning of systematic U.S. Geological Survey investigations in the area in 1900. Schrader and Spencer (1901) 

conducted the frrst detailed geologic studies of parts of 

Prince William Sound, the Copper River Basin, and the 

southwestern Wrangell Mountains. Mendenhall and 

Schrader (1903) and Mendenhall (1905) provided the first 

surveys of the western part of the copper belt in the Wrangell 

Mountains, and detailed studies of the Kotsina-Kuskulana 

area were continued by F.H. Moffit and his coworkers 

(Moffit and Maddren, 1909; Moffit and Capps, 1911; Moffit, 

1918; Moffit and Mertie, 1923). Grant (1906, 1909), Moffit 

(1908), Grant and Higgins (1909), Brooks (1912, 1914), and 

Capps and Johnson (1913, 1915) provided early reports on 

copper and gold mining and prospecting in Prince William 

Sound during the period of greatest activity. Grant and Higgins (1910) provided a regional summary of the geologic 

knowledge of Prince William Sound that was gained during 

the first decade of investigations, and Johnson (1915, 1916, 

1919) continued detailed studies of copper and gold occurrences in the northeastern part of the sound. During this same 

period, F.H. Moffit also investigated the geologic setting of 

copper and gold resources in the Chugach Mountains, first 

east of the Copper River (Moffit, 1912, 1914) and then west 

of the river in the Tonsina district (Moffit, 1918, 1935). In 

1917, Overbeck (1918) completed a brief examination of the 

copper-nickel prospects near Spirit Mountain east of the 

Copper River. 

Following these pioneering geological studies, the focus 

of most investigations shifted to other regions of Alaska until 

the 1940's and 1950's, when some activity resumed in southem Alaska. The resumption was prompted largely by cutoffs 

or limitations in supplies of strategic commodities prior to 

and during World War II and the Korean War. Investigations 

of the copper-nickel prospects near Spirit Mountain were 

renewed during 1942 (Kingston and Miller, 1945), and the 

copper deposits of the Prince William Sound region were 

reevaluated in 1943 (Moffit and Fellows, 1951). F.H. Moffit's culminating report on Prince William Sound (Moffit, 

1954) provides a wealth of basic information on the geologic 

setting of the region. In the early 1950's, the U.S. Government was purchasing chromite concentrates at incentive 

prices under the Mineral Stockpile Program. The U.S. 

Bureau of Mines conducted several evaluations of chromite 

occurrences in southern Alaska in the late 1940's and 

1950's, including extensive assays and mineral dressing 

tests of samples in 1957 from the chromite deposits near 

Tonsina (Pittman, Wells in Foley and Barker, 1985). 

Recent Investigations 

As part of a program of petroleum investigations in 

southern Alaska (Miller and others, 1959), the U.S. Geological Survey began stratigraphic and structural studies in the 

Nelchina area in the 1950's. The petroleum potential of 

Mesozoic rocks on the west shore of Cook Inlet was well 

known, but the correlation of these rocks with Mesozoic 

sequences that are exposed widely in the eastern Talkeetna 

Mountains and western Copper River Basin was virtually 

unstudied. Through detailed geologic mapping and biostratigraphic studies, Arthur Grantz and numerous colleagues 

were able to correlate Jurassic sequences in the two regions 

(summarized in Grantz, 1965, and Imlay and Detterman, 

1973). The Cretaceous rocks in the eastern Talkeetna Mountains, however, differ from coeval rocks on the west shore of 

Cook Inlet and are assigned to the Matanuska Formation 

(summarized by Grantz, 1964 ). During subsequent reconnaissance work in the southwestern Wrangell Mountains, 

Grantz and others (1966) described Upper Jurassic(?) and 

Lower Cretaceous rocks near Chitina that are temporal 

equivalents of the Nelchina rocks. 

Also in the 1950's, the U.S. Geological Survey began 

studies of the thick and complexly intertonguing upper Tertiary and Quaternary deposits of the Copper River Basin, 

work that continued with interruptions until the early 1980's. 

The focus of the early work was to distinguish and define 

local and regional glacial deposits that capped the mountains 

and filled much of the basin in late Wisconsin time (Nichols, 

1965; Ferrians and Nichols, 1965) and also filled Port Valdez (Williams and Coulter, 1980). At the outset, it was 

apparent that the Copper River Basin contains an array of 

unconsolidated deposits including lacustrine deposits of glacial Lake Atna (Ferrians and Schmoll, 1957; Nichols, 1965) 

and lava flows and volcaniclastic debris flows from the 

Wrangell Mountains that are interbedded with glacial deposits (Nichols and Yehle, 1969; Yehle and Nichols, 1980). The 

lake persisted during retreat of the glaciers into the mountains, as shown by lake sediments in the mountain valleys 

and locally prominent shorelines (Ferrians and Nichols, 

1965). Apparently the waters were impounded by a solid glacier dam in the Copper River canyon south of the quadrangle 

that was breached before 9,000 years ago. 

Studies in the region also focused on geologic and hydrologic hazards. Hydrologic hazards (Post and Mayo, 1971) 

include flooding by high runoff, glacier outburst floods, and 

Geologic Investigations 7 
icings in winter. The sudden release of water stored within or 

adjacent to glaciers has damaged roads and bridges on 

Tazlina and Tsina Rivers and Sheep Creek near Valdez. 

Winter icings on the Klutina River have inundated the 

village of Copper Center and are common on the Copper 

River and some of its tributaries (Williams and Johnson, 

1981). 

Geologic hazards include the effects of permafrost, volcanic activity, and earthquakes. Permafrost is widespread 

north of the Tsina River in the Chugach Mountains (Ferrians 

and Nichols, 1965; Ferrians and others, 1969; Williams, 

1970) and, in unconsolidated deposits, poses an engineering 

problem during construction. Descriptions of the engineering properties of the unconsolidated ·units in the Copper 

River Basin (Nichols and Yehle, 1969) and along the Richardson Highway in the Chugach Mountains (Coulter and 

Coulter, 1961, 1962) provided important information during 

construction of highways and public facilities in the region. 

Concerns with ground stability became paramount during 

construction of the Trans-Alaska Pipeline System in the 

1970's. Disturbance of the thermal regime in fine-grained 

sediments, particularly the glaciolacustrine silt and clay that 

is common near the surface in much of the Copper River 

Basin (Ferrians, 1971), thaws the ice-rich permafrost, leading to differential settlement. As a result, the Trans-Alaska 

Pipeline was elevated over most of these sensitive deposits 

and is consistently buried for only the southernmost 69 km 

from near Ptarmigan to its terminus at Fort Liscum. 

Mount Wrangell is the only active volcano in the 

Wrangell Mountains. The summit caldera includes three craters that have active fumaroles (Benson and Motyka, 1979), 

and ash darkens the summit snowfield from time to time. 

Personnel of the University of Alaska Geophysical Institute 

have been studying glacier-volcano interaction on Mount 

Wrangell since 1953 and have reported a major increase in 

heat flux at the summit since 1970 (Benson and Motyka, 

1979); however, activity apparently has been limited to the 

caldera since pre-late Wisconsin time (Williams and 

Johnson, 1981). Pre-late Wisconsin lava flows and volcaniclastic debris flows from Mount Wrangell are interbedded 

with unconsolidated deposits in nearby areas of the Copper 

River Basin and extend as far west as Lower Tonsina and 

south as far as Chitina (Yehle and Nichols, 1980). A recurrence of larger eruptions such as these, although unlikely, 

could spread devastation beyond the Copper and Chitina 

Rivers, and showers of volcanic ash, depending on wind 

direction at the time, might extend over large areas of the 

Valdez quadrangle. The most likely hazards of a small eruption would be local flooding and mud flows that would be 

confined to canyons on the flanks of the volcano. 

U.S. Geological Survey investigations in the Valdez 

quadrangle and throughout southern Alaska were accelerated in response to the 1964 Alaska earthquake, the epicenter 

of which was about 40 km west of the southwest corner of 

the quadrangle. Evaluations of the aftereffects of this 

8 Alaska Mineral Resource Assessment Program-Valdez Quadrangle 

extremely powerful tectonic event include detailed studies in 

the Port Valdez (Coulter and Migliaccio, 1966) and Copper 

River Basin (Ferrians, 1966) areas, as well as regional syntheses (for example, Case and others, 1966; Plafker, 1969). 

Ground-motion values for a Richter magnitude 8.5 earthquake were used to formulate design criteria for the TransAlaska Pipeline from Valdez to Willow Lake, and values for 

a Richter magnitude 7.0 earthquake were used from Willow 

Lake northward to the quadrangle boundary (Page and others, 1972). The potential earthquake hazards posed by activity on major faults in the region led to detailed studies of the 

Border Ranges fault (MacKevett and Plafker, 1974; Plafker, 

Nokleberg, and Lull, 1985) and the Contact fault system 

(Plafker and Lanphere, 1974; Winkler and Plafker, 1975, 

1981; Plafker and others, 1977, 1986), as well as regional 

studies of major fault systems throughout southern Alaska 

(for example, Plafker and others, 1977). Conjunctive 

regional studies of the plutonic rocks of southern Alaska also 

were completed (Hudson, 1979, 1983). 

In the early 1970's, thesis studies were carried out in the 

Tonsina area by graduate students at the University of 

Alaska. Hoffman (1974) described the geologic setting for 

the layered ultramafic and mafic rocks at Bernard Mountain, 

and Metz (1976) described blue-amphibole-bearing schistose rocks west of Chitina, the first discovery of blueschists 

in the quadrangle. 

During the 1970's and 1980's, the U.S. Geological Survey conducted interdisciplinary mineral-resource assessments of several nearby 1°X3° quadrangles: the McCarthy 

quadrangle on the east (Singer and MacKevett, 1976; MacKevett, 1978); the Seward and Blying Sound quadrangles on 

the southwest (Tysdal and Case, 1979, 1982); the Cordova 

and Middleton Island quadrangles on the south (Winkler and 

Plafker, 1981, 1992; Goldfarb and others, in press); and the 

Anchorage quadrangle on the west (Madden-McGuire and 

others, in press; Winkler, in press). These investigations 

under the auspices of the Alaska Mineral Resources Assessment Program (AMRAP) provide detailed information on 

the geology and mineral resources of geologic terranes that 

extend directly into the Valdez quadrangle as well as a 

wealth of basic earth-science information. 

For the AMRAP investigations summarized herein, a 

series of reports was prepared that provides information on 

the geology, geochemistry, geophysics, telegeology, and 

known and speculated mineral resources of the Valdez quadrangle (table 1). The reports provide multidisciplinary, contemporary mineral-resource information as potential input 

for land-use planning and minerals-policy development and 

as guidance for minerals exploration. The reports also 

enhance the geologic knowledge of a region that had 

received few recent earth-science investigations. 

In the early 1980's, a regional study of the potential for 

undiscovered mineral resources in the Chugach National 

Forest was completed (Jansons and others, 1984; Nelson and 

others, 1984). Additional geological mapping (Nelson and 
others, 1985) and geochemical sampling (Goldfarb and 

others, 1984) in the southern part of the Valdez quadrangle 

provide much better delineation and characterization of 

several areas that are likely to contain base-and preciousmetal resources in massive-sulfide and vein deposits. In the 

mid-1980's, multidisciplinary geophysical investigations of 

the deep crustal structure of south-central Alaska were conducted along the southern part of the TACT corridor. 

Detailed surface geologic studies were carried out in support 

of the geophysical studies (Plafker, Nokleberg, and Lull, 

1985), including mapping of five 15-minute quadrangles in 

the Valdez quadrangle (Plafker, Lull, and others, 1989) and 

a variety of structural, geochemical, and isotopic studies near 

the transect. Plafker, Nokleberg, and Lull (1989) and Nokleberg and others (1989) described the lithologic and st.~ctural 

features and the tectonic evolution of the four lithostratigraphic terranes that make up the Valdez quadrangle. 

GEOCHEMICAL INVESTIGATIONS 

In a detailed study of massive-sulfide occurrences southeast of Port Valdez, Rose (1965) reported chemical analyses 

for about 18 sulfide-bearing rock samples and about 80 

stream-sediment samples. The rock samples contained 

anomalous amounts of copper, gold, zinc, and silver, several 

of the stream-sediment samples contained anomalous 

amounts of copper, and two panned-concentrate samples 

contained visible gold. Jasper (1967) collected 102 streamsediment samples for chemical analysis and pannedconcentrate samples for microscopic examination from sites 

close to the Richardson Highway between Valdez and Tonsina and the Edgerton Highway between Lower Tonsina and 

Chitina. The resulting. data indicate nothing of economic 

interest. Herreid (1970) conducted detailed stream-sediment, 

soil, and rock-chip sampling in the vicinity of the Spirit 

Mountain nickel-copper prospect and examined the prospect 

in detail. In the 1980's, the U.S. Bureau of Mines conducted 

regional investigations of mafic and ultramafic complexes 

along the Border Ranges fault in southern Alaska, collecting 

bulk samples that were concentrated and assayed for cobalt, 

copper, chromium, gold, nickel, palladium, platinum, and 

silver (Foley and Barker, 1985; Foley and others, 1987, 

1989). 

During the 1970's, 457 stream-sediment, 500 lake-sediment, and 1,205 water samples were collected in the Valdez 

quadrangle for the National Uranium Resource Evaluation 

(NURE) program of the U.S. Department of Energy (Sharp 

and Hill, 1978; D'Andrea and others, 1981; Los Alamos 

National Laboratory, 1983). Many sample sites were clustered in areas of easy access; remote parts of the quadrangle 

were not sampled nor were heavy-mineral-concentrate samples collected. A reconnaissance stream-sediment geochemical survey was conducted throughout the Valdez quadrangle 

in 1978 and 1979 as part of the AMRAP study. Chemical 

analyses for the composite stream-sediment and heavy-mineral-concentrate samples that were collected are reported in 

Miller and others (1982). In the early 1980's, streamsediment and concentrate samples were collected at an 

additional 98 sites along the southern edge of the Valdez 

quadrangle (Goldfarb and others, 1984). Sutley and others 

(1990) collected additional stream-sediment and heavymineral-concentrate samples in the vicinity of Bernard 

Mountain. The AMRAP and NURE geochemical data were 

evaluated by Goldfarb and others (in press), who included an 

extensive R-mode factor analysis of chemical associations, 

described 31 areas of geochemical anomalies in detail, and 

interpreted sources for the anomalous values. 

Two-thirds of the anomalous areas defined by Goldfarb 

and others (in press) are within the Chugach terrane, which 

is dominated by greenschist-grade metasedimentary rocks. 

Anomalous concentrations of gold, silver, arsenic, and many 

base metals in both sediment and concentrate samples indicate a high geochemical favorability for the presence of 

mesothermal, precious- and base-metal-bearing quartz veins 

(Goldfarb and others, in press). The largest of these areas 

covers about 750 km2 between Columbia Glacier and the 

Lowe River and includes the Port Valdez mining district. 

Reports by Pickthorn (1982), Pickthorn and Silberman 

(1984), Goldfarb and others (1986), and Goldfarb (1989) 

describe the geologic setting and chemical and isotopic characteristics of mineral deposits within this area. Anomalies 

for iron, cobalt, nickel, and copper in many samples from the 

south-central margin of the Valdez quadrangle between 

Solomon Gulch and Schwan Glacier indicate the upstream 

presence of disseminated or massive sulfide occurrences. 

These volcanogenic occurrences are spatially associated 

with mafic volcanic rocks within the Chugach terrane (Winkler, Miller, MacKevett, and Holloway, 1981; Crowe and 

others, in press; Goldfarb and others, in press). Concentrate 

samples containing anomalous amounts of manganese characterize the area between Bench and Marshall Glaciers and 

the southeast comer of the quadrangle. The anomalies most 

likely are derived from erosion of recognized manganese 

enrichments in phyllite and schist of the Chugach terrane in 

upstream areas (Goldfarb and others, in press). 

Stream-sediment samples containing consistently anomalous amounts of cobalt and copper and commonly 

anomalous amounts of silver, chromium, iron, nickel, lead, 

and zinc are derived from rocks of the Wrangellia terrane 

south of the Chitina River. The anomalies define geochemically favorable ground for (1) both disseminated and 

massive volcanogenic or Kennecott-type copper-sulfidedominant mineral occurrences associated with upper Paleozoic and Triassic mafic volcanic rocks and Triassic 

carbonate sedimentary rocks and (2) magmatic cobalt- and 

nickel-bearing sulfide minerals in ultramafic bodies. North 

of the Chitina River, samples from the Wrangellia terrane are 

anomalous in copper, boron, vanadium, antimony, barium, 

zinc, and molybdenum (Goldfarb and others, in press). Some 

Geochemical Investigations 9 
of the anomalies reflect known volcanogenic copper occurrences; others may indicate the presence of weakly mineralized porphyry systems in Jurassic plutons of the Chitina 

Valley batholith (Winkler, Miller, MacKevett, and Holloway, 1981). 

Silicified, pyrite-rich volcanic rocks of the Talkeetna 

Formation crop out at Heavenly Ridge in the northwestern 

part of the Valdez quadrangle, where they are intruded by 

numerous felsic plugs and dikes (Winkler, Miller, 

MacKevett, and Holloway, 1981). Several similar occurrences are nearby in the Anchorage quadrangle and at 

Willow Mountain along the Richardson Highway, both areas 

where felsic plutons have not been identified. According to 

Newberry (1986), the alteration is probably syngenetic with 

the volcanic activity. 

A large region likely to contain chromite lenses in layered ultramafic and mafic rocks of the Peninsular terrane is 

defined around Bernard and Dust Creeks by stream sediments containing anomalous amounts of chromium, iron, 

nickel, magnesium, and vanadium. Samples collected along 

the Border Ranges fault between Barnette and Klanelneechena Creeks contain anomalous levels of a similar suite of elements as well as copper. A few samples containing 

anomalous amounts of gold, silver, and boron suggest that 

precious-metal-bearing occurrences may be present locally 

in drainages tributary to Klanelneechena Creek (Goldfarb 

and others, in press). 

GEOPHYSICAL INVESTIGATIONS 

An aeromagnetic survey was flown over the northern part 

of the Valdez quadrangle in 1954 and 1955 (Andreason and 

others, 1958) in connection with a topical study of the Copper River Basin. In 1978, an aeromagnetic survey was flown 

over the remainder of the quadrangle to provide regional 

data for the AMRAP study (U.S. Geological Survey, 1979). 

Prior interpretations of aeromagnetic maps to the northeast 

in the Nabesna quadrangle (Griscom, 1975), to the east in the 

McCarthy quadrangle (Case and MacKevett, 1976), to the 

northwest in the Talkeetna Mountains quadrangle (Csejtey 

and Griscom, 1978), and to the southwest in the Seward and 

Blying Sound quadrangles (Case and others, 1979) aided the 

interpretation of data in the Valdez quadrangle. For the survey of the Valdez quadrangle, Case, Burns, and Winkler 

(1986) spliced the contoured data sets of the two aeromagnetic surveys, provided measurements of magnetic susceptibilities for samples of the main rock units, interpreted the 

combined aeromagnetic maps in terms of the units causing 

geophysical anomalies, and illustrated five aeromagnetic 

profiles and interpetative cross sectional models. Burns 

(1982) made a detailed interpretation of aeromagnetic data 

over the northern Chugach Mountains in the northwestern 

part of the quadrangle. The most conspicuous aeromagnetic 

anomalies of the Valdez quadrangle, attaining amplitudes of 

several thousand gammas, are over the layered mafic rocks 

of the Nelchina River Gabbronorite (Plafker, Nokleberg, and 

Lull, 1989). Locally, these rocks contain 10-15 percent 

modal magnetite and ilmenite. This aeromagnetic feature 

was called the Northern Chugach Mountains anomaly by 

Andreason and others (1964). It also has a relative positive 

gravity of 20-30 milligals (mGals) (Barnes, 1977). Five 

north-south magnetic profiles modeled across the 

gabbronorite (Bums, 1982) indicate a general northerly dip 

to the body and a steep truncation on its south margin. The 

aeromagnetic anomaly in the northern Chugach Mountains 

extends discontinuously westward across the Anchorage 

quadrangle to the vicinity of Palmer, where it turns southward (Burns and others, 1991) to connect with the Knik Arm 

anomaly of Grantz and others (1963). Fisher (1981) and 

Case, Fisher, and others (1986) have proposed linkage with 

a similar aeromagnetic anomaly extending southwest along 

the continental shelf and the west sides of Afognak and 

Kodiak Islands, where layered mafic plutonic rocks are 

exposed. If the inference is correct, this gabbroic body constitutes a major geologic feature of southern Alaska. Layered 

rocks of the Tonsina ultramafic-mafic sequence of Plafker, 

Nokleberg, and Lull (1989) and the fault-bounded layered 

mafic body on Klanelneechena Creek also have pronounced 

positive magnetic expressions; the Tonsina ultramafic-mafic 

sequence coincides with a general gravity high of about +20 

to +30 mGals (Barnes, 1977). 

In the northeastern part of the quadrangle, the magnetic 

anomaly field is heterogeneous and includes numerous highamplitude, steep-gradient, positive anomalies separated by 

low-amplitude negative anomalies (Case, Bums, and 

Winkler, 1986). Most of the positive anomalies probably are 

of composite origin and are caused by volcanic rocks of the 

Wrangell Lava and by Jurassic intermediate to mafic plutons 

of the Chitina Valley batholith. Some anomalies are caused 

by the Triassic Nikolai Greenstone, gabbroic plutons 

mapped as part of the Skolai Group (Winkler, Silberman, 

and others, 1981 ), and mafic metaplutonic and ultramafic 

rocks in the Haley Creek metamorphic assemblage of 

Plafker, Nokleberg, and Lull (1989). In the northwestern part 

of the Valdez quadrangle, isolated high-amplitude anomalies 

are over volcanic rocks of the Talkeetna Formation, similar 

to the main area of Talkeetna outcrops in the Anchorage 

quadrangle to the west (Burns and others, 1991 ). South of the 

Border Ranges fault, magnetic anomalies are few, small, and 

subdued, indicating a great depth to magnetic basement 

(Case, Bums, and Winkler, 1986). The few small positive 

anomalies are caused by mafic volcanic rocks in the 

McHugh Complex and the Valdez and Orca Groups. The 

gravity field is equally subdued, with a broad low on the 

order of -20 mGals between Klutina Lake and Chitina 

(Barnes, 1977), where rocks of the Valdez Group are 

intruded by abundant felsic dikes and sills. 

10 Alaska Mineral Resource Assessment Program-Valdez Quadrangle 
In the mid-1980's, geophysical investigations of the deep 

crustal structure of southern Alaska were conducted along 

the southern part of the Trans-Alaska Crustal Transect (Page 

and others, 1986). The combined geologic and geophysical 

data indicate that the upper crust beneath the Valdez quadrangle consists of rootless sheets less than 10 km thick 

formed of rocks of the Chugach, Wrangellia, and Peninsular 

terranes. The sheets are bounded on the south by northdipping thrust faults that sole into shallow, almost horizontal 

low-velocity zones (Page and others, 1986). Puis and Plafker 

(1991) inferred that undulating reflectors in the upper crust 

at depths between about 6 and 10 km are tonalitic plutons, 

which fed swarms of dikes and sills that are exposed at the 

surface directly above (Winkler, Silberman, and others, 

1981). Subparallel, north-dipping, strong seismic reflectors 

between about 8 and 24 km depth in the Chugach Mountains 

have been interpreted (Puis and Plafker, 1991) as underplated oceanic crust and possibly upper mantle. Studies of 

earthquake seismicity (Page and others, 1989), seismic 

reflection (Fisher and others, 1989) and refraction (Puis and 

others, 1991), and potential fields (Campbell, 1991) yielded 

abundant data on the deep crust that show a gently inclined 

north-dipping Benioff zone deepening from about 25 km 

near the southern boundary of the quadrangle to about 40 km 

near Tonsina. P-wave velocities are about 5.9-7.4 km/sec in 

the lower crust beneath the Chugach Mountains and about 

7.8 krnlsec in the upper mantle (Puis and others, 1991). 

GEOLOGIC AND TECTONIC SUMMARY 

The Valdez quadrangle encompasses four distinctive 

fault-bounded lithotectonic terranes of contrasting stratigraphy, age, and structural style (Winkler, Silberman, and others, 1981; Plafker, Nokleberg, and Lull, 1989). From north 

to south, they are the Wrangellia, Peninsular, Chugach, and 

Prince William terranes (figs. 2, 3) (Jones and others, 1987). 

Rocks of the Yakutat terrane (outboard of the other four to 

the south) are not exposed in the Valdez quadrangle, but 

impingement of the Yakutat terrane against and beneath the 

continental margin is manifested by arc volcanism in the 

Wrangell Mountains (Plafker, 1987). 

Wrangellia Terrane 

The Wrangellia terrane occupies the northeastern part of 

the Valdez quadrangle and is bounded on the south by the 

Border Ranges fault and on the west by the Tarat fault (fig. 

2). The Taral fault is not exposed north of the Chitina River; 

its position beneath unconsolidated deposits of the Copper 

River Basin is inferred from aeromagnetic data (Case, Bums, 

and Winkler, 1986) to connect with the West Fork fault in 

the Gulkana quadrangle (Nokleberg and others, 1986). The 

Wrangellia terrane is divided into two distinct domains separated by the Chitina fault system (Gardner and others, 

1986): (1) a southern domain consisting of penetratively 

deformed greenschist to lower amphibolite facies metasedimentary, metavolcanic, and metaplutonic rocks and (2) a 

northern domain consisting of the typical Triassic sequence 

of the Wrangellia terrane (hereinafter referred to as the type 

Wrangellia sequence or terrane) (Jones and others, 1977; see 

also, Plafker, Nokleberg, and Lull, 1989). Metamorphic 

rocks south of the Chitina fault system include Pennsylvanian and Jurassic metaplutonic rocks (Aleinikoff and others, 

1988) and Permian and older metasedimentary and metavolcanic rocks of the southern Wrangellia terrane margin 

(Plafker, Nokleberg, and Lull, 1989). The metasedimentary 

rocks include marble containing diagnostic Early Pennsylvanian conodonts (Plafker, Harris, and Reed, 1985). Based on 

detailed work between the Richardson Highway and the 

Copper River and reconnaissance work in the Dadina River 

area, Plafker, Nokleberg, and Lull (1989) redefined these 

rocks as the Haley Creek metamorphic assemblage, including the Uranitina River metaplutonic unit and the Strelna 

Metamorphics. These rocks are penetratively deformed into 

elongate map patterns (Winkler, Silberman, and others, 

1981; Plafker, Lull, and others, 1989) and contain zones of 

large ductile strike-slip shear of unknown sense (Pavlis and 

Crouse, 1989). They are similar in composition, however, to 

parts of the type Wrangellia sequence north of the fault. 

Locally, the southern Wrangellia terrane margin includes the 

Triassic Nikolai Greenstone and the Chitistone and Nizina 

Limestones, the most characteristic rocks of the type 

Wrangellia terrane north of the fault. Rocks north of the 

Chitina fault system include upper Paleozoic mafic volcaniclastic and flow rocks correlative with the Station Creek Formation of the Skolai Group (Smith and MacKevett, 1970). In 

nearby areas, these rocks and associated lithologically variable plutonic rocks define the Pennsylvanian and Early Permian Skolai magmatic arc that constitutes the basement 

sequence of the Wrangellia terrane (Bond, 1973; Richter and 

Jones, 1973; MacKevett, 1978; Platker, Nokleberg, and 

Lull, 1989). The arc rocks are overlain successively by 

Lower Permian shallow-marine rocks (correlative with the 

Hasen Creek Formation; Smith and MacKevett, 1970), Middle and (or) Upper Triassic rift-fill tholeiitic basalt (the 

Nikolai Greenstone), and Upper Triassic and Lower Jurassic 

shallow-marine evaporitic, calcareous, and argillaceous 

strata (Chitistone and Nizina Limestones and McCarthy Formation). Late Jurassic tonalitic plutons of the Chitina Valley 

batholith (MacKevett, 1978) intrude both the type Wrangellia terrane and the southern Wrangellia terrane margin 

(Winkler, Silberman, and others, 1981); however, in the 

southern Wrangellia terrane margin, these plutons are more 

abundant, more mafic, and generally foliated, indicating a 

deeper level of exposure (Plafker, Nokleberg, and Lull, 

1989). 

Geologic and Tectonic Summary 11 
Surficial deposits (Quaternary) 

Wrangellia terrane-Divided into: 

Type Wrangellia terrane 

Southern Wrangellia terrane margin 

Peninsular terrane 

0 

I 

0 

I 

50 MILES 

I 

50 KILOMETERS 

EXPLANATION 

YYYYYYYY 

y ...... y ......... 

YYYYYYYY 

...... ..,. ...... ..,. ... ......... y y y ...... 

YYYYYYY 

.'·: .. ·. 

Chugach terrane-Divided into: 

Valdez Group (Cretaceous)-

Consists of: 

Metasedamentary rocks 

Metabasalt 

McHugh Complex (Mesozoic) 

Schist of Liberty Creek 

(Jurassic) 

12 Alaska Mineral Resource Assessment Program-Valdez Quadrangle 

Prince William terrane 

Granitic rocks (Eocene) 

Lake 

Dry well 

Fault, sawteeth on upper plate, dashed 

where queried or concealed 
Figure 2 (facing page). Generalized geology of the Valdez 

quadrangle including major geologic units and their lithostratigraphic terrane affinities, selected structural and geographic 

features, and the names of adjacent 1 :250,000-scale quadrangles. BRF, Border Ranges fault; IF, Iceberg fault; KF, Klutina 

fault; SlF, Second lake fault; TF, Tazlina fault. Modified from 

Plafker , Nokleberg, and lull (1989). 

Plutonic rocks of the Chitina Valley batholith and metaplutonic rocks of the Uranatina River metaplutonic unit are 

part of a belt of Late Jurassic plutons that extends at least 

from Chichagof Island in southeastern Alaska to Tonsina 

(Hudson, 1983). These plutons define the Chitina magmatic 

arc (Plafker, Nokleberg, and Lull, 1989); late stages of their 

intrusion overlap onset of major regional orogeny in the Late 

Jurassic that culminated in the Early Cretaceous with the initial development of the Chitina and Border Ranges fault systems (MacKevett and Plafker, 1974; MacKevett, 1978). 

In the Valdez quadrangle, synorogenic deposits include 

very thick and coarse, local wedges of the nonmarine Middle 

or Upper Jurassic Kotsina Conglomerate that are overlain 

with probable unconformity by Upper Jurassic(?) and Lower 

Cretaceous marginal-marine sedimentary rocks. These Neocomian and older(?) rocks include unnamed rocks in the 

Cheshnina and Chetaslina River canyons (Grantz and others, 

1966) and the Berg Creek Formation (MacKevett and others, 

1978). 

The Chitina fault is completely concealed in the Valdez 

quadrangle but is well exposed to the east in the McCarthy 

quadrangle (MacKevett, 1978; Gardner and others, 1986). 

Apparently, Early Cretaceous uplift between the southwestdipping thrust faults and related folds of the Chitina fault 

system and the Border Ranges fault has exposed deeper levels of the basement sequence for the Wrangellia terrane in 

the southern Wrangellia terrane margin. Furthermore, the 

position of deep-seated Jurassic plutons at the Border 

Ranges fault indicates that a substantial segment of the 

southern Wrangellia terrane margin has been tectonically 

removed since the Late Jurassic (MacKevett, 1978; Plafker, 

Nokleberg, and Lull, 1989). 

Peninsular Terrane 

The Peninsular terrane occupies the north-central and 

northwestern parts of the Valdez quadrangle and is bounded 

on the south by the Border Ranges fault and on the east by 

the Taral fault. The Peninsular terrane includes both intrusive and extrusive phases of a Late Triassic(?) through Middle Jurassic intraoceanic magmatic arc (Barker and Grantz, 

1982; Bums, 1985) and Cretaceous marine sedimentary 

strata deposited in fore-arc basins. To the west in the 

Anchorage quadrangle, a Middle Jurassic marine fore-arc 

sequence also is present. In the Valdez quadrangle, the extrusive phase of the arc is represented by the Talkeetna Formation and consists dominantly of bedded andesitic and basaltic 

tuff, tuff breccia, flow rocks, shallow intrusive rocks, and 

volcanogenic marine sedimentary rocks (Winkler, Silberman, and others, 1981). The Cretaceous rocks are represented by the Matanuska Formation, a transgressive marine 

sequence that deepened through time from shallow shoalwater deposition in its lower part to deposition on the distal 

reaches of a deep-sea fan in its upper part (Grantz, 1965). 

The Peninsular terrane generally dips northward, thereby 

exposing deeper structural levels, the crystalline roots of the 

early Mesozoic Talkeetna magmatic arc, along its southern 

edge. The crystalline roots of the arc in the Valdez quadrangle are composed principally of layered gabbronorite, peridotite, and dunite, the Border Ranges ultramafic-mafic 

assemblage of Plafker, Nokleberg, and Lull (1989) (or the 

Border Ranges ultramafic and mafic complex of Bums, 

1985). The assemblage has two subdivisions: the deeper 

level Tonsina ultramafic-mafic sequence (Plafker, Nokleberg, and Lull, 1989; see also Coleman and Bums, 1985, and 

DeBari and Coleman, 1989), consisting of a distinctive 

cumulate sequence of dunite, peridotite, and garnet-bearing 

gabbro, and the shallower level Nelchina River Gabbronorite (Plafker, Nokleberg, and Lull, 1989; Bums, in 

press). The Tonsina ultramafic-mafic sequence is exposed 

only in low hills and stream cuts from just west of the 

Richardson Highway to the east bank of the Copper River 

(Winkler, Silberman, and others, 1981). Probable correlative 

ultramafic-mafic sequences, the Wolverine Complex of 

Carden and Decker (1977) and Eklutna Complex of Bums 

(1985), are present along the northern margin of the Border 

Ranges fault system to the west in the Anchorage quadrangle 

(Clark, 1972a; Burns, 1985; Winkler, 1990). The Nelchina 

River Gabbronorite is widely exposed from just east of the 

Richardson Highway westward along the southern margin of 

the Peninsular terrane to the vicinity of Palmer in the 

Anchorage quadrangle (Winkler, Miller, Silberman, and others, 1981; Winkler, Silberman, and others, 1981; Bums, 

1985; Winkler, 1990). The Nelchina River Gabbronorite and 

its cover rocks, the Talkeetna Formation, are locally exposed 

and produce prominent, almost continuous _geophysical 

anomalies southwestward to the Kodiak Island area and the 

Alaska Peninsula, an additional 600 km, that define the full 

extent of the Talkeetna arc. 

West of the Valdez quadrangle, the southern margin of 

the Peninsular terrane includes a multiply deformed metamorphic assemblage of probable Paleozoic protoliths, the 

informally named Knik River schist terrane (Carden and 

Decker, 1977), which was basement for part of the Talkeetna 

Geologic and Tectonic Summary 13 
Cenozoic volcanic rocks-Star shows 

location of andesitic volcano 

Terranes 

Yakutat 

Prince William 

Chugach 

Peninsular-Wrangellia (composite) 

Alexander 

Ghost Rocks 

Undifferentiated 

PACIFIC PLATE 

50 IOOMILES ( O<==:=:::i50e...""""'='=JIO::::O=====I50KILOMETERS 

Submarine contours in meters \ / 

0 

EXPLANATION 

• • • 

. _____,_ 

BRFS 

CFS 

AM 

CSFS 

RMF 

WF 

KIZ 

PZ 

CMFS 

FF 

~' 

/ 

Fault-Dashed where approximately located, dotted where 

concealed, queried where inferred 

Thrust-Sawteeth on upper plate, Open teeth indicate major terrane or plate boundary 

Strike slip-Arrows indicate relative horizontal motion 

Oblique thrust-Sawteeth on upper plate, arrows indicate 

relative horizontal motion 

Border Ranges fault system 

Contact fault system 

Aleutian megathrust 

Chugach-St.Eiias fault system 

Ragged Mountain fault 

Wingham fault 

Kayak Island zone 

Pamplona zone 

Castle Mountain fault system 

Fairweather fault 

Figure 3. Geologic terranes of the southern Alaska region. Selected faults and boundaries of 1 °X3° quadrangles are also shown. 

A, Anchorage; BG, Bering Glacier; BS, Blying Sound; C, Cordova; G, Gulkana; IB, Icy Bay;· M, McCarthy; Ml, Middleton Island; 

N, Nabesna; S, Seward; TM, Talkeetna Mountains; V, Valdez (heavy outline). Modified from Plafker (1987). 

14 Alaska Mineral Resource Assessment Program-Valdez Quadrangle 
arc (Pavlis, 1983). Near Anchorage, carbonate rocks in the 

assemblage contain Permian Tethyian fusulinids (Clark, 

1972b). The basement assemblage, and Early and Middle 

Jurassic plutons that intrude it, contains anastomosing zones 

as wide as a kilometer of distributed ductile and brittle shear 

of unknown sense that Pavlis (1982) related to Early Cretaceous underthrusting on the Border Ranges fault. 

The bedded and intrusive sequences of the Wrangellia 

and the Peninsular terranes differ in several characteristics 

(Plafker, Nokleberg, and Lull, 1989). The Peninsular terrane 

lacks the distinctive Middle Triassic to Lower Jurassic riftfill basalt and intertidal to shallow-marine sedimentary 

sequence of the Wrangellia terrane, and the Wrangellia terrane lacks the characteristic Late Triassic and Early Jurassic 

arc-related volcaniclastic and plutonic rocks of the Talkeetna 

arc. The intrusive histories of the two terranes contrast markedly as well; the distinctive Late Jurassic tonalitic plutons of 

the Wrangellia terrane and the Early to Middle Jurassic ultramafic-mafic suite of the southern Peninsular terrane both terminate abruptly at the Taral fault, the terrane boundary 

(Winkler, Silberman, and others, 1981). 

Although the Wrangellia and Peninsular terranes differ in 

detail in the geology of their basement rocks, their intrusive 

sequences, and their bedded cover rocks, they apparently 

have been juxtaposed since at least the Middle Jurassic. Middle Jurassic and younger clastic sections exposed in the 

southwestern Wrangell Mountains and the Nelchina area of 

the western part of Copper River Basin are probably overlap 

assemblages (Csejtey and others, 1978). Plafker, Nokleberg, 

and Lull (1989) suggested that the abrupt termination of the 

Talkeetna arc along the Taral fault may have originated dur- · 

ing a total of 600-1 ,000 km of sinistral strike-slip offset that 

truncated a margin of the Wrangellia terrane in British 

Columbia where parts of the Queen Charlotte and Vancouver Islands are composed of coeval arc rocks. During the 

larger dislocation, about 40 km of relative overthrusting 

occurred along the Taral fault to juxtapose a flap of the 

Wrangellia terrane over the Peninsular terrane (Nokleberg 

and others, 1989). If this reconstruction is correct, then the 

Talkeetna Formation and correlative rocks in British Columbia also constitute an overlap assemblage that linked the 

Wrangellia and Peninsular terranes by Late Triassic time 

(Plafker, Nokleberg, and Lull, 1989). Furthermore, the 

occurrence in both terranes of coeval Permian carbonate 

rocks suggests that their amalgamation may have occurred 

even earlier during the late Paleozoic. 

The large sinistral displacement between the Wrangellia 

and Peninsular terranes must have occurred between the Late 

Jurassic and middle Early Cretaceous because Late Jurassic 

plutons are sheared but overlapping Valanginian to Hauterivian sedimentary rocks, the Berg Creek Formation and 

Nelchina Limestone, respectively, are apparently unaffected. The displacement also must have occurred prior to 

suturing of the amalgamated Wrangellia and Peninsular terranes (Talkeetna superterrane of Csejtey and St. Aubin, 

1981) to North America by the mid-Cretaceous. 

Chugach Terrane 

The Chugach terrane (Berg and others, 1972) occupies 

most of the southern half of the Valdez quadrangle; it is the 

central part of a continuous belt of accreted deep-marine sedimentary and mafic volcanic rocks approximately 2,000 km 

long and 60-100 km wide in the coastal mountains of southern Alaska (Plafker and others, 1977). From north to south, 

the Chugach terrane consists of three major fault-bounded 

sequences: (1) Jurassic or older greenschist- and transitional 

blueschist-facies rocks; (2) Upper Triassic to mid-Cretaceous polygenetic broken formation and melange of the 

McHugh Complex; and (3) Upper Cretaceous volcaniclastic 

flysch and oceanic basaltic rocks of the Valdez Group, which 

comprise the major part of the terrane in the quadrangle 

(Winkler, Silberman, and others, 1981). Rocks of the 

Chugach terrane record intermittent accretion and deformation beginning in the Early Jurassic and continuing into the 

latest Cretaceous or earliest Tertiary. The rocks are variably 

metamorphosed: rocks of the prehnite-pumpellyite, greenschist, blueschist, and lowest amphibolite facies are present, 

but rocks of the lower greenschist facies are the most widespread. The terrane was extensively intruded and thermally 

metamorphosed by plutons and dikes about 50 Ma. 

Along its northern margin, the Chugach terrane is juxtaposed against the Wrangellia and Peninsular terranes along 

the Border Ranges fault system. In different areas of the 

quadrangle, the schist of Liberty Creek, the McHugh Complex, or the Valdez Group are juxtaposed against the northern terranes. At its southern margin, the Chugach terrane is 

juxtaposed against the Paleogene sequence of the Prince 

William terrane along the Contact fault. 

The Border Ranges fault, as mapped in the Valdez quadrangle, is a composite feature. According to MacKevett and 

Plafker (1974), it originated as a subduction thrust along the 

trailing edge of a microcontinent·(including the Wrangellia 

and Peninsular terranes) that collided with the North American continental margin in mid-Cretaceous time (Csejtey and 

others, 1982; Monger and others, 1982; Pavlis, 1982). 

Subduction-related Early(?) Jurassic metamorphism of the 

schist of Liberty Creek was attributed by Nokleberg and others (1989) to an earliest episode of subduction under an 

"unknown backstop" to the north along a proto-Border 

Ranges fault. Alternatively, Roeske (1986) suggested that 

Geologic and Tectonic Summary 15 
the schist formed in a distant subduction zone and was distributed subsequently along the Border Ranges fault by 

strike-slip displacement. Vestiges of this contact are present 

as near-vertical broad shear zones in ultramafic rocks of the 

Tonsina ultramafic-mafic sequence along the Border Ranges 

fault between the Second Lake and Taral faults. In the central part of the Valdez quadrangle, belts of the McHugh 

Complex apparently were accreted successively beneath the 

Peninsular terrane along the Border Ranges, Klutina, and 

Iceberg faults, apparently principally in the Late Jurassic and 

Early Cretaceous but possibly beginning as early as the Late 

Triassic or Early Jurassic (Winkler, Silberman, and others, 

1981). The main episodes of melange formation are roughly 

coeval with successive magmatic events in the adjacent Talkeetna, Chitina, and Chisana arcs (Plafker, Nokleberg, and 

Lull, 1989). Syntectonic intrusion of Late Jurassic plutons of 

the Uranatina River metaplutonic unit in the Valdez quadrangle (Pavlis and Crouse, 1989) and Early Cretaceous 

tonalite-trondhjemite plutons in the Anchorage quadrangle 

(Pavlis and others, 1988) into the Border Ranges fault zone 

have been inferred to date episodes of strike-slip and thrust 

activity along the Border Ranges and related faults. Accretion of the Valdez Group in the latest Cretaceous or earliest 

Tertiary beneath both the Peninsular and Wrangellia terranes 

reactivated the Border Ranges fault as a megathrust. Vestiges of this contact are preserved beneath several klippen 

consisting of rocks of the southern Wrangellia terrane margin between Tonsina Lake and the Taral fault. The sinuous 

trace of the Border Ranges fault between the Copper and 

Tonsina Rivers results from folding and faulting of the thrust 

plate during accretion of terranes to the south along the Contact fault (Nokleberg and others, 1989). Much of the present 

trace of the Border Ranges fault in the Valdez and adjacent 

quadrangles is steeply dipping to vertical and resulted from 

Eocene reactivation of the system as an oblique-slip boundary (Little and Naeser, 1989). 

The schist of Liberty Creek forms an outcrop belt 28 km 

long and 13 km wide along the northern margin of the 

Chugach terrane between Dust Creek and the Copper River 

(Winkler, Silberman, and others, 1981; Plafker, Lull, and 

others, 1989). The unit consists of greenschist and minor 

crossite-bearing blueschist containing rare lawsonite. The 

unit is intensely ductilely deformed, and its original stratigraphic thickness is unknown (Nokleberg and others, 1989). 

Its structural thickness is at least 5 km. In the two places 

where it is exposed, the northern contact of the schist of Liberty Creek is marked by a broad, almost vertical shear zone 

of schistose serpentine and strongly deformed ultramafic 

rocks of the Tonsina ultramafic-mafic sequence (Plafker, 

Lull, and others, 1989). Along the southern boundary of the 

unit, the schist of Liberty Creek is juxtaposed against the 

McHugh Complex along an east-west-trending, almost vertical zone of strongly sheared country rocks, serpentinite, 

pyroxenite, and gabbro. The southern boundary of this zone 

is the Second Lake fault ofMetz (1976). The lithology, relict 

textures, and geochemistry of the schist of Liberty Creek 

indicate that it is an oceanic assemblage of basalt flow rocks, 

breccia, tuff, and minor argillaceous and calcareous sedimentary rocks. Its protolith age is unknown, but its position 

in the accretionary complex indicates only that it likely is 

older than the less metamorphosed Upper Triassic to midCretaceous McHugh Complex against which it is juxtaposed. Plafker, Nokleberg, and Lull (1989) correlated the 

schist of Liberty Creek with dated blueschist and greenschist 

of the schist of Iceberg Lake in the western part of the Valdez 

quadrangle (Winkler, Miller, Case, and others, 1981). The 

schistose rocks near Iceberg Lake have Early to Middle 

Jurassic metamorphic ages (Sisson and Onstott, 1986; 

Plafker, Nokleberg, and Lull, 1989) and are in close association with paleontologically dated Upper Triassic oceanic 

rocks along the south side of the Border Ranges fault (Winkler, Silberman, and others, 1981). This association may 

indicate that the schistose rocks are a more deeply subducted 

part of the Upper Triassic oceanic assemblage (Plafker, Nokleberg, and Lull, 1989). The metamorphic age of the schist 

of Liberty Creek has been difficult to determine because of 

its very fine grain size. The Early Cretaceous K-Ar ages 

(Winkler, Silberman, and others, 1981; Plafker, Nokleberg, 

and Lull, 1989) were probably reset, perhaps during accretion of the McHugh Complex to the south. 

The McHugh Complex crops out in an irregular belt 3-37 

km wide between the Border Ranges or Second Lake fault on 

the north, the Tazlina fault on the south, and the Taral fault 

on the east (Winkler, Silberman, and others, 1981; Plafker, 

Lull, and others, 1989). The McHugh Complex consists of 

pervasively disrupted and regionally metamorphosed broken 

formation and melange. Its maximum structural thickness is 

estimated to be about 20 km (Plafker, Nokleberg, and Lull, 

1989). The broken formation originally was mainly tholeiitic 

basalt and related fragmental volcanic rocks, argillite or tuffaceous argillite, radiolarian chert, argillaceous siltstone and 

sandstone, and carbonate rocks. The lithology and geochemistry of the McHugh indicates it originated as an assemblage 

of oceanic volcanic rocks and pelagic sediments mixed with 

lesser amounts of siliciclastic sediments derived from an 

active continental arc (Plafker, Nokleberg, and Lull, 1989). 

Stratal disruption in the McHugh records a complex history 

of superimposed processes. Wispy intercalations and disharmonic folding record either submarine slumping or tectonic 

deformation of unlithified sediments, or both. Postconsolidation brittle disruption is pervasive and randomly oriented 

and varies in scale from microscopic fractures to large-scale 

mixing of rock types in complex fault networks. Broad zones 

of intense faulting lack any stratal continuity, and widespread faulting of unknown offset has juxtaposed contrasting 

rocks. Disrupted brittle phacoids at all scales in sheared 

argillite and tuff impart a characteristic blocks-in-matrix 

appearance to these melangelike parts of the unit. Large, 

coherent, exotic blocks of layered gabbro and other plutonic 

rocks were derived from the Peninsular or Wrangellia ter16 Alaska Mineral Resource Assessment Program-Valdez Quadrangle 
ranes. The roughly circular klippe more than 10 km in circumference that occupies much of the drainage of 

Klanelneechena Creek between Tazlina and Nelchina Glaciers may be the largest of these exotic blocks derived from 

the microcontinental block to the north; however, several 

blocks of mafic plutonic rocks as large as 1 km are included 

in McHugh melange between the Border Ranges and Klutina 

faults. Large, exotic blocks of schistose rocks and marble 

also are present in the McHugh (Winkler, Miller, and Case, 

1981; Winkler, Silberman, and others, 1981). The schist of 

Iceberg Lake forms a discrete faulted slab 40 km long and as 

wide as 4 km enclosed within melange marked by conspicuous elongated aligned blocks of marble as long as 2 km; 

other smaller blocks of schist and marble are particularly 

conspicuous along the Iceberg and Klutina faults. 

The best age for the McHugh Complex is provided by 

radiolarians, which are relatively common in chert in the 

melange matrix. In the western part of the Valdez quadrangle, the McHugh contains radiolarian assemblages of Late 

Triassic to mid-Cretaceous ages, the inferred depositional 

age span for the unit (Winkler, Silberman, and others, 1981). 

In general, the older samples are near the northern margin of 

the unit, and the youngest are near the southern margin. This 

age distribution is interpreted to indicate progressive accretion of slices of the McHugh Complex from north to south, 

and the structural style is interpreted to indicate disruption at 

a convergent plate margin. The age of these accretionary episodes is not known, but accretion must have been completed 

before accretion of the Upper Cretaceous Valdez Group that 

is outboard (south) of the McHugh. 

The Valdez Group crops out in a belt more than 50 km 

wide that encompasses the crest of the Chugach Mountains. 

The Valdez is emplaced beneath rocks of the McHugh Complex along the Tazlina fault, a broadly folded thrust that dips 

moderately to the north (Winkler, Silberman, and others, 

1981). The Tazlina fault occupies a comparable structural 

position to the Eagle River fault in the Anchorage quadrangle (Clark, 1972b; Winkler, 1990). Immediately beneath the 

Tazlina fault, surfaces of intense deformation in the Valdez 

are defined by isoclines and by metasandstone boudins that 

are elongated subparallel with the main fault surface. Rocks 

of the overlying McHugh Complex do not show a similar 

increase in deformation above the fault. Between Tonsina 

Lake and the Taral fault, the Valdez is surmounted by a thin, 

folded and faulted thrust sheet and outlying klippen of rocks 

of the southern Wrangellia terrane margin (Plafker, Lull, and 

others, 1989). Maximum structural thickness of the sheet is 

estimated to be 1-2 km. East of the Taral fault, the Valdez 

Group is separated from the southern Wrangellia terrane 

margin on the north by a steeply north dipping trace of the 

Border Ranges fault The southern limit of the Valdez Group 

is the Contact fault along which rocks of the Prince William 

terrane are thrust relatively beneath the Chugach terrane 

(Plafker and others, 1977; Nokleberg and others, 1989). 

The Valdez Group consists of a thick, monotonous 

sequence of strongly deformed volcanogenic flysch and 

subordinate tholeiitic basalt flows and tuffs. The rocks are 

foliated, tightly folded, and commonly overturned to the 

south, and they are imbricated along numerous steeply north 

dipping faults. The volcanic rocks are not widespread in the 

Valdez quadrangle but extend southward into the adjacent 

Cordova quadrangle where they are much more abundant 

(Winkler and Plafker, 1981, 1992). Although the volcanic 

rocks are rarely more than a few tens of meters thick, they 

persist for more than 20 km along strike and are conspicuous 

in a broad east-west zone from east of the Copper River to 

near Hogback Glacier (Winkler, Silberman, and others, 

1981). The aggregate stratigraphic thickness of the Valdez 

Group is not known, but at least several kilometers of strata 

are present; the structural thickness of the unit, based on geophysical data, is slightly less than 20 km (Plafker, 

Nokleberg, and Lull, 1989). 

In most places in the Valdez quadrangle, structural features of the Valdez Group contrast markedly with those in 

the McHugh Complex: the Valdez is characterized by penetrative fabrics that record two major deformational events 

(Nokleberg and others, 1989). The first event produced tight 

isoclinal folds and transposition of fold hinges and is manifested by a pervasive planar or slaty cleavage that is generally parallel with bedding. This event is related to accretion 

of the Valdez Group against the continental margin and 

resulted in a series of imbricated coherent stratigraphic 

sequences bounded by shear zones or thrusts and only local 

preservation of fold hinges. The age of the frrst event is not 

constrained well but is believed to be latest Cretaceous to 

earliest Tertiary, prior to accretion of the Prince William terrane along the southern margin of the Chugach terrane. 

Underplating of a structural thickness of almost 20 km of the 

Valdez Group resulted in vertical uplift along the Border 

Ranges fault sufficient to expose the deepest metamorphosed levels of the accretionary prism by no later than 50 

million years ago. At least 3 km of uplift within 15 million 

years of deposition of the Valdez Group probably occurred 

to the west in the Anchorage quadrangle (Little and Naeser, 

1989). The second deformational event produced upright, 

open to tight folding of earlier structures and is manifested 

by one or more generations of crenulation cleavage in pelitic 

rocks, generally at high angles to earlier planar cleavage. At 

deeper structural levels, this second event produced intense 

ductile deformation and flattening. The second event probably is related to regional metamorphism and intrusion of plutons and dikes into the southern part of the Chugach terrane 

in the middle Eocene and again in the Oligocene, concurrent 

with deformation and intrusion in the Prince William terrane 

to the south. 

Although internal primary sedimentary structures are 

well preserved in many of the coarser grained sedimentary 

rocks, the Valdez Group is regionally metamorphosed to 

lower greenschist facies. Fluid inclusion studies of metamorphic segregation veins near Tiekel indicate minimum temperatures of metamorphism of 230°C-300°C (Sisson and 

others, 1989); crosscutting quartz veins near Valdez have 

Geologic and Tectonic Summary 17 
minimum trapping temperatures of 21 0°C-280°C and minimum trapping pressures of 1.5 kbar (Goldfarb and others, 

1986). Metamorphic grade increases ~along strike to the 

southeast and culminates in the adjacent Bering Glacier 

quadrangle in a schist and gneiss crystalline complex containing sillimanite-grade migmatitic rocks (Hudson and 

Plafker, 1982). The metamorphic grade decreases westward 

to lowermost greenschist facies in the adjacent Anchorage 

quadrangle (Winkler, 1990). Superimposition of progressive 

metamorphism across regional structural trends within the 

Valdez Group indicates that metamorphism occurred after 

deformation and accretion against the continental margin. A 

minimum age for metamorphism of the Valdez Group is 

given by K-Ar ages of 53.5-47 .6±1.6 Ma (Winkler and 

Plafker, 1981; Winkler, Silberman, and others, 1981). The 

higher grade metamorphic rocks represent deeply buried 

parts of the accretionary prism, and their development is 

believed to be related to emplacement of early Eocene anatectic granitic plutons in the region (Hudson and others, 

1979) that culminated about 52±3 Ma. 

The voluminous flysch of the Valdez Group probably 

accumulated in a continental-margin trench adjacent to an 

Andean-type arc (Monger and others, 1982; Nilsen and 

Zuffa, 1982; Plafker, Nokleberg, and Lull, 1989). Regional 

petrologic and sedimentologic data (Hollister, 1979; Zuffa 

and others, 1980; Nilsen and Zuffa, 1982; Dumoulin, 1987) 

indicate that the bulk of this sediment was derived from an 

evolved arc in the region of southeastern Alaska and coastal 

British Columbia, transported northwestward along the continental margin, and deposited as elongated fans mostly 

within a marginal trench. The mafic volcanic rocks of the 

Valdez Group are interpreted to represent oceanic crust upon 

which the clastic materials were deposited (Plafker, Nokleberg, and Lull, 1989; Lull and Plafker, 1990). 

Prince William Terrane 

In the southwest comer of the Valdez quadrangle, the 

Chugach terrane is separated from the Prince William terrane by the steeply north-dipping Contact fault (Plafker and 

others, 1977; Winkler, Silberman, and others, 1981). The 

fault trace in the Valdez quadrangle is a small part of a major 

early Tertiary terrane boundary that extends from near 

Mount Saint Elias to the continental shelf south of Kodiak 

Island (Plafker and others, 1977, 1986). The Prince William 

terrane consists primarily of the very thick and complexly 

deformed Orca Group, an accreted deep-sea fan complex 

interbedded with tholeiitic basalt and minor pelagic deposits 

(Winkler, 1976). Lenses of conglomerate in the Orca in the 

Sawmill Bay area are thought to represent feeder channels 

that distributed finer grained sediment to mid-fan depositional lobes where the great bulk of Orca flysch was deposited (Winkler and Tysdal, 1977). The sediment was derived 

mainly from the Coast Mountains of southeastern Alaska 

and British Columbia during the principal phase of plutonism and uplift in the Paleocene to middle Eocene 

(Hollister, 1979) and deposited as submarine fans on the 

Kula oceanic plate as it moved relatively northwestward. 

Interbedded basalt resulted from magma leakage as sea-floor 

spreading brought the Kula-Farallon Ridge near the continental margin. Widespread middle Eocene anatexis, intrusion, and low-pressure high-temperature metamorphism in 

the southern Wrangellia terrane margin and the Peninsular, 

Chugach, and Prince William terranes (Hudson and others, 

1979; Winkler, Silberman, and others, 1981; Hudson and 

Plafker, 1982; Sisson and others, 1989) manifest subduction 

of the still active ridge in the eastern Gulf of Alaska 

Intrusion of early middle Eocene plutons across both the 

Contact and Border Ranges faults indicates that there has 

been little or no younger horizontal displacement along the 

faults (Winkler and Plafker, 1981). 

The stratigraphic thickness of the Orca Group in the 

region is unknown but may be as much as 6-10 km (Winkler 

and Plafker, 1981); the structural thickness is much greater, 

perhaps as much as 20 km. 

Successive accretion of wedges of flysch and basalt produced complex deformation of the Orca Group. Intense folding and faulting in both the upper and lower plates of the 

Contact fault resulted in parallel structures in both the 

Valdez and Orca Groups adjacent to the fault. At depth in the 

region, the Contact fault marks an abrupt change in deep 

crustal structure manifested by a sharp increase in seismic 

velocities (Fuis and Plafker, 1991 ). 

The structural effects of underplating the Orca Group are 

not well understood in the Valdez quadrangle. They are complicated by the initial development of the southern Alaska 

orocline at about this time during rapid oblique subduction 

of Pacific oceanic crust beneath the Alaskan continental 

margin (Engebretson and others, 1986). Bends in the traces 

of the Border Ranges, Contact, and Castle Mountain faults, 

deflections in regional metamorphic and structural fabrics of 

Paleogene age in the Chugach and Prince William terranes, 

and paleomagnetic data indicating early Tertiary counterclockwise rotation of southwestern Alaska have been 

adduced as evidence for the orocline (summarized in 

Plafker, 1987, Little and Naeser, 1989, and Nokleberg and 

others, 1989). To the west in the Anchorage quadrangle, successive episodes of accretion in the Eocene are recorded by 

renewed normal offset on the Border Ranges fault and 

reverse-slip uplift on the Castle Mountain fault, followed by 

dextral oblique-slip of several tens of kilometers on the 

Border Ranges and Castle Mountain faults (Fuchs, 1980; 

Little, 1990). The obliquity of offset accommodated simultaneous accretionary thickening and oroclinal bending of the 

continental margin and created wrench faults, en echelon 

18 Alaska Mineral Resource Assessment Program-Valdez Quadrangle 
folds, fault-bend folds, and local pull-apart basins along the 

Border R~ges fault in the Anchorage quadrangle (Little, 

1990), none of which are known in the Valdez quadrangle. 

Yakutat Terrane 

The Y ak.utat terrane is a composite terrane along the 

northern Gulf of Alaska continental margin that is composed 

mostly of relatively buoyant crustal rocks (Plafker, 1987). 

Rocks of the Yakutat terrane are not exposed in the Valdez 

quadrangle; in fact, the nearest exposures are approximately 

65 km to the south in the Cordova quadrangle (Winkler and 

Plafker, 1981, 1992). The effects of the terrane's accretion 

are manifested, however, in the Valdez quadrangle by volcanism in the Wrangell Mountains and by renewed rapid uplift 

of the Chugach Mountains and deep erosion along the 

Border Ranges fault indicated by apatite cooling ages of 

22-16 Ma for samples from the Chugach terrane (Little and 

Naeser, 1989). By Miocene time, the Yakutat terrane 

became coupled to the Pacific plate along the FairweatherQueen Charlotte dextral transform fault and was displaced 

northwestward to impinge against the continental margin 

between approximately Mount Saint Elias and the Copper 

River (Plafker, 1987). Northward displacement of the terrane is inferred to coincide roughly with the duration of arc 

volcanism in the Wrangell Mountains from about 20 Ma to 

the present (Plafker, 1987). At least 2,000 m of andesitic 

flows, tephra, hypabyssal plugs and dikes, and minor volcanogenic sedimentary rocks in the northeast part of the 

Valdez quadrangle issued principally from the shield volcano of Mount Wrangell and may represent only a part of the 

volcanic pile that is Quaternary in age. 

POTENTIAL MINERAL RESOURCES 

Potentially significant undiscovered resources of precious, base, and ferrous metals may be present in several 

areas of the Valdez quadrangle near known occurrences 

(Winkler, Miller, MacKevett, and Holloway, 1981; Pickthorn and others, in press). North of the Border Ranges fault 

system, gold, silver, copper, zinc, and lead are present in disseminations and veins in late Paleozoic metavolcanic and 

metaplutonic rocks in several places in the southwestern 

Wrangell Mountains and the northern Chugach Mountains; 

in disseminations and veins in Triassic subaerial volcanic 

rocks near Elliott Creek in the Wrangell Mountains; in 

replacements and veins in Triassic and Jurassic marine sedimentary rocks near Elliott Creek in the Wrangell Mountains; 

and in veins and breccia zones in Jurassic volcanic rocks at 

Willow Mountain and Heavenly Ridge in the northern 

Chugach Mountains. Molybdenum is present in veins and 

disseminations in Jurassic plutonic rocks along the Chitina 

River west of the mouth of the Kuskulana River. East of the 

Copper River between Spirit Mountain and Summit Lake, 

ultramafic sills enriched in copper and nickel and minor silver, gold, and platinum-group elements intrude the late Paleozoic metamorphic sequence. Between the Richardson 

Highway and the Copper River at Bernard Mountain, Dust 

Mountain, and Sheep Hill, layered rocks in the Tonsina 

ultramafic-mafic sequence contain numerous lenses and disseminations of chromite and local enrichments in platinumgroup elements and gold. Near the head of Barnette Creek, 

ultramafic rocks in a broad shear zone along the southern 

boundary of the Nelchina River Gabbronorite also contain 

lenses of chromite, none of which are known to be large. In 

several places between Tazlina Lake and the western boundary of the Valdez quadrangle, layered rocks in the Nelchina 

River Gabbronorite contain elevated values of copper and 

nickel. Sulfide minerals have been noted locally and may be 

disseminated through much of the layered pluton; in places, 

their presence is marked by weak copper staining. 

South of the Border Ranges fault system in the Chugach 

Mountains, gold, silver, copper, zinc, lead, and antimony are 

present in massive sulfide deposits and veins hosted by Cretaceous and Paleogene sedimentary and volcanic rocks and 

in veins hosted by felsic dikes and stocks that cut these 

sequences. Manganese is present in veins in schistose rocks 

along Liberty Creek northwest of Chitina. Placer gold may 

be present in small alluvial deposits underlain by rocks of the 

Valdez Group near known auriferous lodes and in alluvial 

sands within the canyons of the Copper, Bremner, Tasnuna, 

and Lowe Rivers and their tributaries. 

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Kingston, Jack, and Miller, D.J., 1945, Nickel-copper prospect near 

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References Cited 25 



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volumes, publications listing and index) and as a set of mtcrofiche. 

Supplements for 1982, 1983, 1984, 1985, 1986, and for sub~­

quent years since the last permanent catalog may be purchased by mall 

and over the counter in paperback book form. 

State catalogs, "List of U.S. Geological Survey Geologic and Water-Supply Reports and Maps For (State)," may be purchased by mail and 

over the counter in paperback booklet form only. 

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet 

form only. 

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" is available free of charge by mail or may be 

obtained over the counter in paperback booklet form only. Those wishing 

a free subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 National Center, Reston, VA 22092. 

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and publications.