The McHugh Complex of 

South-Central Alaska

By SANDRA H. B. CLARK

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1372-D
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog No. 72-600325

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1973

For sale by the Superintendent of Documents

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CONTENTS

Page

Abstract ___ _________________________________ Dl

Introduction __________________________________ _ 1

McHugh Complex _____________________________ ... 4

Lithology _ __ _ _ _____________________ 5

Age 7

Metamorphism __ _ ___________ ___ 8

Structure _ _ _ ____________________ 8

Depositional environments ___________________ 9

Relations to adjacent rock units __________________ 10

References cited __________________________________ 10

ILLUSTRATION

Page

FIGURE 1. Map showing the generalized distribution of the McHugh 

Complex in the western Chugach Mountains near Anchorage, Alaska _____________________.___ D2

ill

CONTRIBUTIONS TO STRATIGRAPHY

THE McHUGH COMPLEX OF 

SOUTH-CENTRAL ALASKA

By SANDRA H. B. CLARK

ABSTRACT

The name McMugh Complex is herein introduced for a heterogeneous assemblage of weakly metamorphosed clastic and volcanic rocks that are well 

exposed along the northwest flank of the Chugach Mountains near Anchorage, Alaska. Two localities are designated: The type locality along the 

Seward Highway east and west of McHugh Creek and the reference locality 

between a bridge across the Knik River on the Glenn Highway and the 

Pioneer Peak area. The McHugh Complex consists of two lithologically distinct but chaotically juxtaposed rock sequence with irregular outcrop patterns a metaclastic sequence and a metavolcanic sequence. The metaclastic 

rocks, which make up the bulk of the complex, are predominantly weakly 

metamorphosed siltstone, graywacke, arkose, and conglomeratic sandstone. 

The metavolcanic rocks include massive and pillow greenstones, basaltic in 

composition and texture, associated with radiolarian metachert cherty 

argillite, and argillite. Small amounts of ultramafic rocks and marble occur 

locally. The age of the McHugh Complex is thought to be Late Jurassic and 

(or) Cretaceous.

Metamorphic minerals are indicative of zeolite to prehnite-pumpellyite 

facies metamorphism. Bedding is rarely preserved; predominant structures 

are pervasive shear fractures. A melangelike deformation is characteristic 

of parts of the complex. The metaclastic sequence is thought to have been 

derived from a Jurassic continental magmatic arc to the north and west. 

The metavolcanic sequence is thought to represent ocean floor accumulations 

that may have been tectonically juxtaposed and mixed with the clastic sequence.

INTRODUCTION

The name McHugh Complex is herein introduced for a heterogeneous assemblage of weakly metamorphosed metaclastic and 

metavolcanic rocks that are well exposed along the northwest flank 

of the Chugach Mountains in the Anchorage area (fig. 1). Capps 

(1916) and Park (1933) informally called the western part of the 

complex, that is exposed between Potter, Indian, and Eklutna, 

"undifferentiated metamorphic rocks," and the eastern part, that

Dl
D2

61° 00

Base from U.S. Geological Survey 1:250.000 

Anchorage 1966, Seward 1963

10 MILES

0 5 10 KILOMETERS 

l l I I I I_____I

FIGURE 1. Generalized distribution of the McHugh Complex
THE McHUGH COMPLEX OF SOUTH-CENTRAL ALASKA D3

149° 00' 148°

EXPLANATION

Surficial deposits"

^

§ si <» *«*

"§ IS o KKenai and Chickaloon 

^ ^ Formations

Undifferentiated rocks

Plutonic and greenschist to amphibolite fades metamorphic rocks. 

Includes two ophiolitic ultramafic 

complexes and plutonic rocks that 

may be as young as Tertiary

Contact, approximately located

Fault, approximately located; 

dotted where concealed

Thrust fault, approximately located; short dashes on upper 

plate; dotted where concealed

Locality of sample dated by 

potassium-argon method

(Small felsic to intermediate intrusive bodies are present but not 

shown)

* Most contacts between bedrock and 

surficial deposits are generalized 

from maps by H.R. Schmoll and 

Ernest Dobrovolny of the U.S. 

Geological Survey (1971)

in the western Chugach Mountains near Anchorage, Alaska.
D4 CONTRIBUTIONS TO STRATIGRAPHY

is exposed from the Girdwood district north to the Knik River, 

"volcanic tuffs." They considered the "undifferentiated metamorphic rocks" to be an early Mesozoic or Paleozoic unit lying unconformably below the Valdez(?) Group. Park (1933), in accordance 

with Capps' (1916) observations, showed that, in the Girdwood 

district, the "volcanic tuffs" discordantly overlie Cretaceous sedimentary rocks. Both Capps and Park interpreted the contact to be 

an unconformity. Capps later (1940) included the "undifferentiated metamorphic rocks" with the Jurassic Talkeetna Formation 

and showed "greenstone tuff" (the same unit as the "volcanic 

tuffs") as a younger unit tentatively assigned to the Upper 

Cretaceous.

Recent mapping of the western Chugach Mountains in the 

Anchorage area has shown that rocks of the unit described by 

Park and Capps as "undifferentiated metamorphic rocks" as well 

as the "volcanic" or "greenstone tuffs" overlie metasedimentary 

rocks of Jurassic (?) and Cretaceous age. The contact is everywhere a fault, in some places a high-angle, sharply discordant 

structure, more commonly a low-angle structure that is parallel or 

only slightly discordant to bedding and (or) cleavage in rocks 

above and below. The rocks divided by Capps (1940) into two 

map units are very similar in metamorphic grade and lithology as 

well as in their relation to the Jurassic (?) and Cretaceous metasedimentary rocks. The two units have, therefore, been mapped as 

a single complex by Clark and Bartsch (1971a, 1971b) and Clark 

(1972).

Recent work has shown that these rocks are younger than and 

lithologically different from the Lower Jurassic Talkeetna Formation described by Barnes (1962) in the Matanuska Valley area.

McHUGH COMPLEX

The McHugh Complex is here named for its type locality; exposures are easily accessible along the Seward Highway and in the 

mountains southeast and northwest of McHugh Creek (fig. 1). 

Because the locality contains a greater percentage of metavolcanic 

rocks and metaconglomeratic sandstones than most of the complex, a second locality, considered a reference locality, is designated 

in the area between a bridge across the Knik River on the Glenn 

Highway and the Pioneer Peak area (fig. 1).

The McHugh Complex characteristically forms high jagged 

ridges and peaks. Helicopter landing sites are scarce, and much of 

the area could not be traversed at the time of the field study. Except for more accessible areas near the range front and along the 

Seward Highway, field observation stations are sparse.
THE McHUGH COMPLEX OF SOUTH-CENTRAL ALASKA D5

The aerial extent of outcrops of the McHugh Complex is not yet 

known. The unit has been traced through the Anchorage C-6 and 

C-5 quadrangles north of the Knik River (fig. 1) and into the 

Anchorage C-4 quadrangle east of the border of the area mapped 

(fig. 1). South of Turnagain Arm, the McHugh Complex crops 

out in the mountains west of Hope. Rocks that are similar to the 

McHugh Complex in lithology, style of deformation, and setting 

crop out on the southwestern Kenai Peninsula east of Seldovia (fig. 

1). These rocks were mapped by Martin, Johnson, and Grant 

(1915) as a unit of "graywacke and slate with some chert, limestone and basic igneous material" and described by them as "slates 

and graywackes of the Kachemak Bay district." Examination of 

of outcrops of this unit along logging roads near and to the north 

of Jakolof Bay revealed a distinct similarity to the McHugh Complex. Additional mapping is needed to determine if this unit in the 

Seldovia area can be correlated with the McHugh Complex.

LITHOLOGY

The McHugh Complex consists predominantly of two lithologically distinct but chaotically juxtaposed rock sequences with 

irregular outcrop patterns: a metaclastic sequence and a metavolcanic sequence. The metaclastic sequence, which makes up the 

bulk of the complex, is composed predominantly of gray, graygreen, and dark-green weakly metamorphosed clastic rocks that 

include siltstone, graywacke, arkose, and conglomeratic sandstone. 

The metavolcanic sequence includes greenstones mostly of basaltic 

composition and texture, that are commonly associated with radiolarian metachert, cherty argillite, and argillite. Small amounts of 

ultramafic rocks and marble occur locally in the complex as isolated, discontinuous outcrops or lenticular masses.

Weathered outcrop faces of the metaclastic rocks are generally 

dark green, and fresh surfaces are generally either gray or dark 

green. The finer grained metasiltstones are generally dark gray or 

black in outcrop and on fresh surfaces. The metasandstones and 

conglomeratic metasandstones are massive rocks that show little 

or no evidence of bedding in most areas although layers with apparent lateral continuity of several miles have been seen at a distance and on aerial photos. Conglomeratic layers clearly define 

bedding locally, and at places grading can be seen in the conglomeratic beds. But attempts to trace the conglomeratic layers have 

been futile; layers a few feet thick have been traced no more than 

a few feet.

The metaclastic sequence of the McHugh Complex commonly 

are composed of very immature quartz-poor sedimentary rocks.
D6 CONTRIBUTIONS TO STRATIGRAPHY

Altered plagioclase is commonly the most abundant mineral, and 

clasts of pyroxene and hornblende are not uncommon. Potassium 

feldspar, present in some of the metaclastic rocks, is absent in 

most. Rock fragments are abundant and include volcanic, plutonic, sedimentary, and metamorphic rocks types. The chemical 

composition and mineralogy of the metaclastic rocks is close to 

that of quartz diorite or granodiorite. Fine, pervasive shear fractures and a cloudy appearance of feldspar that is due to incipient 

alteration are characteristic. The amount of matrix varies greatly; 

much of the matrix was probably produced by shearing and 

granulation.

Pillow structures are locally preserved in the greenstones; they 

comprise close-packed pillows, isolated pillows, and isolated pillow 

breccia. Some of the greenstones are amygdaloidal. Most of the 

greenstones form massive dark-green or reddish-green outcrops in 

which original structures are no longer recognizable. The greenstones are altered and metamorphosed, but relict textures typical 

of basaltic rocks are generally well enough preserved to be recognizable in thin section. The composition of the greenstones is 

basaltic; plagioclase and clinopyroxene are dominant minerals. 

Metachert occurs as lenses in a sheared argillitic matrix, as gray 

rhythmically bedded metachert with thin argillitic interbeds, and 

as small irregularly shaped bodies of red, green, or gray metachert 

adjacent to greenstones. In some of the chert, radiolaria can be 

distinguished as clear rounded areas in the clouded chert matrix; 

however, central cavities and traces of spines are rarely preserved. 

The only radiolaria that were well enough preserved to be identified were found in a clast of conglomeratic metasandstone (Clark 

and Bartsch, 1971b). Very fine grained pervasively sheared black 

argillite and cherty argillite are commonly associated with the 

greenstones and chert. In some areas, lenses and very irregularly 

shaped bodies of greenstone, chert, and metagraywacke are surrounded by a sheared black argillaceous matrix.

Rocks that characterize the metaclastic and metavolcanic sequences of the complex most commonly occur in the association 

described above. There are exceptions, however. Greenstones 

occur locally in some areas of predominantly metaclastic rocks, 

and metaclastic rocks (especially metasiltstone and metasandstone) are present in some of predominantly metavolcanic areas. 

In the metavolcanic part, chert and cherty argillite sometimes occur where no greenstone has been found, and some greenstone is 

not associated with chert and cherty argillite.

Conglomeratic metasandstones are very common in the type 

locality along the Seward Highway and have been seen at many
THE McHUGH COMPLEX OF SOUTH-CENTRAL ALASKA D7

places throughout the unit, but the distribution lacks a recognizable pattern. The relative positions of the metaclastic and the 

metavolcanic sequences also are inconsistent and commonly differ 

greatly on adjacent ridges. Greenstone occurs near the fault contact of the unit with the Valdez(?) Group along the Seward Highway and in many other areas. However, greenstones were absent 

near the contact in almost as many areas as they were found. No 

pattern has been observed in the distribution of potassium-feldspar-bearing rocks. The lack of consistency in patterns of distribution of rock types is characteristic of the unit.

AGE

Ages have been determined for three rocks from the McHugh 

Complex. Because of the melangelike style of deformation, the 

youngest date obtained is critical for establishing the age of the 

complex. A granitic clast from a metaconglomeratic sandstone 

(potassium-argon locality, fig. 1) yielded a potassium-argon date 

on hornblende of 146 ± 7 million years (M. A. Lanphere and 

Arthur Grantz, oral commun., 1971). Because granitic rocks in 

conglomerates commonly give slightly younger ages than the 

source terrane (Grantz and others, 1963, Detterman and others, 

1965), the clast may be slightly older than indicated by the potassium-argon determinations. The most likely source of the granitic 

clast is the arcuate belt of Jurassic (Reed and Lanphere, 1970) 

plutonic and volcanic rocks of the southern Alaska Range and Talkeetna Mountains. Because the clastic rocks must foe younger than 

their source terrane, Late Jurassic is considered to be the maximum age of the metaconglomeratic sandstone.

The ages of other rocks in the complex are indicated by fossils. 

Identifiable radiolarian of typical Mesozoic types (Helen Foreman, 

Oberlin College, Oberlin, Ohio, written commun., 1970) have been 

found in a clast from the McHugh Complex. One limestone lens 

contained fusilinids of late Paleozoic, possibly Pennsylvanian age 

(Raymond C. Douglass, written commun., 1970). The ages based 

on fossil evidence and radiometric dating indicate that the rocks 

included in the complex range from late Paleozoic to Jurassic. 

Because the age of the complex itself must be younger than any 

of its components, Late Jurassic is considered the maximum age.

The minimum age of the complex is limited by the beginning of 

accumulation of the unmetamorphosed nonmarine conglomerates, 

sandstones, siltstones, and shales in adjacent areas of the Cook 

Inlet-Susitna Lowlands and the Matanuska Valley. These sedimentary rocks in the Cook Inlet-Susitna Lowlands crop out less 

than a mile from the McHugh Complex. The age of the nearby
D8 CONTRIBUTIONS TO STRATIGRAPHY

sedimentary rocks is Oligocene (Wolfe, 1966; Wolfe and others, 

1966; Wahrhaftig and others, 1969). In the Matanuska Valley, 

conglomerates of the Chickaloon Formation were deposited in the 

area of the north flank of the Chugach Mountains beginning in 

Paleocene time (Barnes, 1962; Wolfe, 1966; Wolfe and others, 

1966). The Paleocene Chickaloon Formation continues beneath 

the surface of the Cook Inlet area as far south as the latitude 

of Anchorage (C. E. Kirschner, oral commun., 1971). It is 

therefore considered very unlikely that the McHugh Complex 

could be Tertiary. On the basis of information available, the age 

is considered to be Late Jurassic and (or) Cretaceous.

METAMORPHISM

Metamorphic assemblages of prehnite-pumpellyite fades occur 

throughout most of the McHugh Complex. The most common and 

easily recognized of these minerals, in both the metasedimentary 

and metavolcanic rocks, is prehnite. In outcrop, prehnite occurs in 

distinctive short discontinuous veinlets that are common in the 

rocks. Pumpellyite is well developed in metabasalts and in some 

metaclastic rocks from widely-scattered localities, but it is not so 

common as prehnite. Laumontite and other zeolites are conspicuous as veinlets in outcrops near the range front but are absent in 

most of the complex.

STRUCTURE

The most prominent structural elements in the McHugh Complex are thin pervasive closely spaced shear fractures that characterize rocks in the complex as seen in outcrop, in hand specimen, 

and in thin section. Two main orientations appear to be most persistent; older steeply dipping shear fractures are approximately 

parallel to regional trends; younger shear fractures are nearly 

horizontal. Many other orientations can be seen in most outcrops, 

but a somewhat consistent pattern has been deciphered in these 

two. Bedding is not recognizable or barely recognizable in most 

outcrops. In most areas where bedding is preserved, it cannot be 

traced for more than a few feet. Because small folds can only be 

seen locally, the importance of folding in the unit is not known.

A melangelike structure is visible in several areas on outcrop 

scale. Irregular detached remnants of beds and lenses occur in a 

fine-grained, pervasively sheared matrix in many outcrops of the 

metavolcanic part of the complex. In the massive metaclastic 

rocks, melangelike structures are not apparent in outcrop, but the 

pervasive shearing suggests possible melangelike deformation. 

On a larger scale, mapped sequences of the predominantly metaclastic rocks and predominantly metavolcanic rocks generally can-
THE McHUGH COMPLEX OF SOUTH-CENTRAL ALASKA D9

not be traced from one ridge to the next. In areas where the two 

main types of rocks in the unit were differentiated, the pattern is 

that of chaotically mixed fault-bounded blocks. In some areas, an 

apparent continuity of metaclastic layers for several miles, as 

seen on aerial photos and cliff faces, suggests that parts of the 

unit may not be strongly disrupted.

DEPOSITIONAL ENVIRONMENTS

The McHugh Complex is interpreted to be predominantly a mixture of two distinct rock sequences, metaclastic and metavolcanic, 

that formed in different environments. Mixing was accomplished 

at least partly, and possibly entirely, tectonically.

The metaclastic rocks, predominantly volcanic clasts, also include clasts of sedimentary, plutonic, and metamorphic rocks. The 

immaturity of the sediments and the angularity of the clasts indicate that the clasts of the metaclastic rocks must have been rapidly 

eroded and transported. The most likely source area for these 

rocks is an older arcuate mountain belt (Jones and others, 1970) 

to the north and west that includes the Alaska Range and the Talkeetna and Wrangell Mountains and extends into the northern 

part of the Chugach Mountains (Clark, 1972). Here during 

Jurassic time plutons were emplaced (Reed and Lanphere, 1970) 

and there was andesitic volcanism (Capps, 1927, Barnes, 1962) in 

parts of this older belt. The only known occurrence of blueschist 

is near Seldovia (Martin and others, 1915) in a southern extension of the older arcuate belt. That blueschist is similar to a clast 

of blueschist that was found in one metaconglomeratic sandstone 

from the McHugh Complex. Because of the similarity of clasts in 

the two areas, as well as their proximity to each other, it is 

thought that debris from the active Jurassic continental magmatic 

arc was shed rapidly into a basin to the south and east to form 

the metaclastic part of the McHugh Complex.

The metavolcanic part of the complex is indicative of an environment of deposition that contrasts sharply with that of the 

metaclastic part. The association of tholeiitic pillow basalt, finegrained sediments, and bedded cherts suggests deposition in a 

quiet marine environment that was not receiving large amounts 

of clastic material. The mixing of the oceanic metavolcanic sequences of the McHugh Complex with the metaclastic sequences 

derived from a continental magmatic arc could be explained by 

convergence of an oceanic plate and a continental plate. Changes 

in the environment of deposition followed by tectonic mixing is a 

second possible explanation.
DIG CONTRIBUTIONS TO STRATIGRAPHY

RELATIONS TO ADJACENT ROCK UNITS

The McHugh Complex is separated from rocks to the northwest 

and southeast by major faults. In the southeast most of the contact between the rocks of the McHugh Complex and the flysch 

deposits of the Valdez (?) Group is the Eagle River thrust fault, a 

low- to moderate-angle fault that thrusts the McHugh Complex 

over the Valdez (?) Group. In some places the contact is cut by 

high-angle faults, or is a high-angle fault.

Metamorphic and plutonic rocks northwest of the McHugh 

Complex that range in age from Permian to Jurassic are part of 

the older terrane that probably was the source area of the clastic 

rocks in the McHugh Complex. The older terrane is juxtaposed 

with the McHugh Complex by the Knik fault, a complex structure 

that is thought to be a high-angle fault or zone of faulting along 

most of its length.

REFERENCES CITED

Barnes, F. F., 1962, Geologic map of the lower Matanuska Valley: U.S.

Geol. Survey Misc. Geol. Inv. Map 1-359, scale 1:63,360. 

Capps, S. R., 1916, The Turnagain-Knik region: U.S. Geol. Survey Bull,

642, p. 147-194.

 1927, Geology of the upper Matanuska Valley, Alaska: U.S. Geol. 

Survey Bull. 791, 92 p.

 1940, Kenai-Chugach Mountains region, in Geology of the Alaska 

Railroad region: U.S. Geol. Survey Bull. 907, p. 51-64, 172-174.

Clark, S. H. B., 1972, Reconnaissance bedrock geologic map of the Chugach 

Mountains near Anchorage, Alaska: U.S. Geol. Survey Misc. Field Inv. 

Map MF-350, scale, 1:250,000.

Clark, S. H. B., and Bartsch, S. R., 1971a, Reconnaissance geologic map and 

geochemical analyses of stream-sediment and rock samples of the Anchorage B-6, quadrangle, Alaska: U.S. Geol. Survey open-file report, 

63 p.

 1971b, Reconnaissance geologic map and geochemical analyses of

stream-sediment and rock samples of the Anchorage B-7 quadrangle,

Alaska: U.S. Geol. Survey open-file report, 70 p. 

Detterman, R. L., Reed, B. L., and Lanphere, M. A., 1965, Jurassic plutonism

in the Cook Inlet region, Alaska: U.S. Geol. Survey Prof. Paper 525-D,

p. D16-D21. 

Grantz, Arthur, Thomas, Herman, Stern, T. W., and Sheffey, N. B., 1963,

Potassium-argon and lead-alpha ages for stratigraphically bracketed

plutonic rocks in the Talkeetna Mountains, Alaska in Geological Survey

Research 1963: U.S. Geol. Survey Prof. Paper 475-B, p. B56-B59. 

Jones, D. L., Mackevett, E. M., Jr., and Plafker, George 1970, Speculations

on late Mesozoic tectonic history of part of southern Alaska [abs]: Am.

Assoc. Petroleum Geologists Bull., v. 54, no. 12, p. 2489. 

Martin, G. C, Johnson, B L., and Grant, U. S. 1915, Geology and mineral

resources of Kenai Peninsula, Alaska: U.S. Geol. Survey Bull. 587, 243 p.
THE McHUGH COMPLEX OF SOUTH-CENTRAL ALASKA Dll

Park, C. F., Jr., 1933, The Girdwood district, Alaska: U.S. Geol. Survey

Bull. 849-G, p. 381-424.

Reed, B. L., and Lanphere, M. A., 1970, Plutonic belts of central and southern Alaska Range and Alaska Peninsula [abs.]: Am. Assoc. Petroleum

Geologists Bull., v. 54, no. 12, p. 2502. 

Wahrhaftig, Clyde, Wolfe, J. A., Leopold, E. B., and Lanphere, M. A., 1969,

The coal-bearing group in the Nenana coal field, Alaska: U. S. Geol.

Survey Bull. 1274-D, p. D9-D11. 

Wolfe, J. A., 1966, Tertiary plants from the Cook Inlet region, Alaska: U.S.

Geol. Survey Prof. Paper 398-B, p. B1-B32. 

Wolfe, J. A., Hopkins, D. M., and Leopold, E. B., 1966, Tertiary stratigraphy

and paleobotany of the Cook Inlet region, Alaska: U.S. Geol. Survey

Prof. Paper 398-A, p, A1-A29.

U.S. GOVERNMENT PRINTING OFFICE : 1973 O-490-466