Gastineau Channel Formation, a 

Composite Glaciomarine Deposit 

Near Juneau, Alaska

GEOLOGICAL SURVEY BULLETIN 1394-C

 I

<fcs.-*-*P J^

Gastineau Channel Formation, a 

Composite Glaciomarine Deposit 

Near Juneau, Alaska

By ROBERT D. MILLER

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1394-C

A description of the depositional environment 

and lithology of diamictons of late Pleistocene 

and early Holocene age*

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1973
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 73-600290

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

Washington, D.C. 20402 Price 35 cents (paper cover)

Stock Number 2401-0243P
CONTENTS

Abstract __ _ . 

Introduction . 

Definition _______.

Typical exposures __. 

Fossils ____ _ .

Environment _____.

Age 

Depositional history 

References cited -_

Page

Cl

1

2

3

9

11

13

15

20

ILLUSTRATIONS

FIGURE 1. Map showing the known distribution of the Gastineau 

Channel Formation ____________________________

2. Diagrammatic sketches showing interpretations of the 

manner of glaciomarine deposition and accumulation of 

related deposits during the three depositional facies of 

the Gastineau Channel Formation ____ __ ___ ____

Page

C8 

18

TABLES

TABLE 1. Typical exposures of Gastineau Channel Formation and results of laboratory tests of selected samples ______ ___

2. Fossils collected from selected outcrops of the three facies 

of the Gastineau Channel Formation, Juneau area, 

Alaska _________..______________________

3. Foraminifera collected from outcrops in the Juneau area, 

Alaska, and assigned to facies of the Gastineau Channel 

Formation ______..__________________

Page

C4 

10 

12

ill

CONTRIBUTIONS TO STRATIGRAPHY

GASTINEAU CHANNEL FORMATION,

A COMPOSITE GLACIOMARINE DEPOSIT NEAR

JUNEAU, ALASKA

By ROBERT D. MILLER

ABSTRACT

The Gastineau Channel Formation is a heterogeneous sequence of glaciomarine materials exposed in the Juneau area, Alaska, that consist of pebbles 

and larger clasts dispersed through a fine-grained matrix of silt and sand. 

The age of the formation is late Pleistocene and early Holocene. The formation is named after Gastineau Channel, a fiord that separates the Juneau 

mainland from Douglas Island and along which exposures of the formation 

are especially common; the type area for the formation is the low-lying land 

along Gastineau Channel on the Juneau mainland and Douglas Island.

Three facies constitute the formation; the first facies, considered to be 

generally the oldest, is a stone-rich diamicton that is a till-like mixture of 

clasts within a matrix that is predominantly silt and sand; the second is a 

gravel-rich diamicton that is very hard and dense; the third facies is a 

sand-rich diamicton, the finest grained and least dense deposit of the three 

facies. Marine mollusks, barnacles, and Foraminifera are found in all facies 

of the formation.

Coarse fragments in the finer grained matrix in all three facies apparently 

were transported outward from shores by seasonal accumulations of sea ice 

and by berg ice broken from glacieirs outside the immediate Juneau area. 

There is no evidence that large glaciers reached tidewater along Gastineau 

Channel or the other fiords for some considerable distance to the north of 

Juneau within at least the last 10,000 years.

On the basis of numerous radiocarbon determinations on shells of marine 

fauna in the three facies of the formation, the Gastineau Channel Formation 

is considered to be late Pleistocene arid early Holocene in age.

INTRODUCTION

This report results from a geologic study of surficial deposits, 

in and near the Juneau urban area, as part of an earthquake hazards study of Alaskan coastal communities. Diamictons previously 

grouped as till or marine till were found to consist of mappable 

glaciomarine deposits traceable as a continuous unit over a considerable area.

Cl
C2 CONTRIBUTIONS TO STRATIGRAPHY

DEFINITION

The Gastineau Channel Formation is here denned as that sequence of heterogeneous glaciomarine deposits of late Pleistocene 

and early Holocene age, consisting of pebbles and larger clasts dispersed through a fine-grained matrix of predominantly silt and 

sand, that is exposed in the Juneau area. Formal stratigraphic 

names have not previously been applied to unconsolidated deposits 

in the Juneau area of Alaska. Surficial deposits of various origins 

lie along the lower slopes of the mountains bounding the fiords and 

bays, and these deposits were studied in the Juneau area during 

the summers of 1965,1966,1968, and 1971.

The Gastineau Channel Formation is named after Gastineau 

Channel, a fiord that separates the Juneau mainland from Douglas 

Island and along which exposures of the formation are especially 

common. The formation is also well exposed along Montana Creek, 

a tributary of the Mendenhall River northwest of Juneau. The 

known extent of the Gastineau Channel Formation is shown in 

figure 1; the regional extent of the formation is not known, but 

apparently similar deposits are widespread in southeastern Alaska 

and southward into northwestern Washington (Armstrong and 

Brown, 1954; Easterbrook, 1963).

Three facies constitute the Gastineau Channel Formation. These 

facies are envisioned as having accumulated for the most part 

sequentially from the oldest to the youngest facies, but also as 

having accumulated in part simultaneously. The first facies, considered to be generally the oldest, is a gray to dark-bluish-gray 

stone-rich diamicton that is a till-like mixture of clasts within a 

generally compact and massive fine-grained matrix. The matrix 

consists of a heterogeneous accumulation of sand, silt, gravel, and 

clay-sized particles, in order of predominance. Cobbles and boulders, some as large as 10 feet across, are scattered through the 

deposit. Weakly developed lenticular and tabular layers or partings occur locally; shells of marine mollusks, barnacles, and Foraminifera are common. The second facies is a gray to dark-bluishgray gravel-rich diamicton that consists mostly of closely packed 

angular to subangular pebble-sized fragments in a predominantly 

sandy matrix. This facies is low in silt and clay-sized particles, but 

is very hard and dense. Boulders as large as 15 inches in diameter 

are scattered throughout the facies; tests of Foraminifera and 

broken shells of mollusks and barnacles are present in the deposit, 

but are less common than in the other two facies. The third facies 

is a gray to dark-bluish-gray sand-rich diamicton. It is a mixture
GASTINEAU CHANNEL FORMATION NEAR JUNEAU, ALASKA C3

in which the sand matrix is dominant; however, pebbles, cobbles, 

and boulders, some as large as 5 feet or more across, are scattered 

throughout the deposit,. Shells of mollusks, many of which are 

articulated, and barnacles, as well as tests of Foraminifera, are 

common. This facies differs from the other two facies in several 

ways; well-developed thin laminations or layers in the matrix are 

widespread. Plant remains in the form of individual roots and 

stems, some of which are carbonized, are common in exposures. 

In addition, this deposit is less dense than the other facies.

All three facies of the Gastineau Channel Formation are typically overlain by a thin but widespread blanket of reddish organicrich beach gravels. The presence of and thickness of the beach 

gravel depend on the local depositional environment and subsequent exposure to erosion.

TYPICAL EXPOSURES

The type area for the Gastineau Channel Formation is designated as the low-lying land on the Juneau mainland and Douglas 

Island, especially in the bluffs along Gastineau Channel, and the 

exposures along the lower part of Montana Creek valley. The 

entire formation is not exposed at any one locality; typical exposures of each facies are, however, listed in table 1 and given in 

figure 1. The detailed locations of all the exposures in the table 

are designed for use with U.S. Geological Survey topographic 

quadrangle maps, Juneau B-2, 1962 edition, and Juneau B-3, 1962 

edition. Each locality in the table, and in the text, is described by 

a prefix g, for Gastineau Channel Formation, a number for the 

facies, gl, g2, and g3, and a number for the locality, gl-1, g2-l, 

and so forth, or by an R, for radiocarbon date localities in deposits 

other than the Gastineau Channel Formation, and a number for 

the locality, R-l, R-2, and so forth.

The first facies underlies the downtown area of Juneau in addition to the selected exposures listed in table 1. Several test holes 

were cored in this facies in the downtown area, and the deepest 

hole bottomed at 91 feet below the surface. The hole penetrated 

about 38 feet of sand and gravel in an emerged delta and terminated in the till-like diamicton. Foraminifera were found throughout the sampled cores, and there was no distinctive break in 

texture or density of the material to suggest more than one age of 

deposition. This hole penetrated the thickest known section of the 

first facies more than 53 feet; elsewhere, the facies is generally 

less than 25 feet thick where it overlies exposed bedrock.
TABLE 1. Typical exposures of Gastineau Channel Formation and results 

[Bulk density determined by Proctor compaction test. U, undated; 

Location

Local- Type of A

ity Lithologic dated (y

no. Section T. (S.) R. (E.) character material B

gl-1 __ S%NW%NW% 10 41 67 __ Stony Mollusks __111,92

diamicton. (W

2 _ SE%SE% 23 40 65 __ __do ____ do __ __2 10,64

(W

3 __ SW%SW%NE% 2 40 65 ____ do ____ Peat ____ 3>39,0

(W

4 __ NW*4SE*4SEi4 16 41 67 ____ do ____ JVLoillusks .-1W,1G

(W

_ do ___ 49,150

(W
GASTINEAU CHANNEL FORMATION NEAR JUNEAU, ALASKA C5

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TABLE 1. Typical exposures of Gastineau

Location

Locality 

no. Section T. (S.)

§3-3 __ SW%NW%NW% 26 41

4 __ NE. cor. NW%SW% 26 40 

R-l __ SEtt 21 40

Channel Formation

Lithologic 

R. (E.) character

and results

Type of 

dated 

material

67 ____ do ______ do

65 _ _ do ___ do _ .

65 __ Diamicton 

layer 

within 

deltaic 

deposits. 

Diamicton 

layer. 

Sand in 

delta.

of labo

A

(y

B

U 

U

Barnacles, 812,730

in life (W-

position.

Moillusks __ 812,880

(W-

__ do _ _ 81 2.300-

(W-

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(W-
GASTINEAU CHANNEL FORMATION NEAR JUNEAU, ALASKA C7

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C8 CONTRIBUTIONS TO STRATIGRAPHY

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GASTINEAU CHANNEL FORMATION NEAR JUNEAU, ALASKA C9

The second facies is less widespread than the other two facies 

along Gastineau Channel; it occurs in isolated mounds and ridges 

at altitudes of less than 250 feet above mean sea level. Exposure 

§2-1 (table 1), in a roadcut through a small ridge north of Peterson Creek on Douglas Island best typifies the texture and density 

common to this facies. This particular deposit is so closely packed 

with rock fragments that a railroad pick bounced from the surface 

of the material with but slight penetration. The thickness of the 

facies seems to be variable, depending on local depositional environments; most exposures reveal less than 20 feet of material, 

but the base of the material has not been exposed.

The third facies crops out intermittently along both sides of 

Gastineau Channel, along Fritz Cove, Auke Bay, Lena Cove, Tee 

Harbor, and the lower reaches of Montana Creek valley near the 

confluence with the Mendenhall River. These deposits generally 

veneer preexisting surfaces that slope upward at about 10°-15° 

from the modern beaches or sea bluffs to the mountainsides. 

Scarps that are generally no more than 10 feet high commonly 

separate the sand-rich diamicton of the third facies from the 

somewhat older, higher, and more extensive stone-rich till-like 

diamicton of the first facies. The third facies is known to reach 

an altitude of at least 200 feet above mean sea level, but may extend considerably higher on the mountain slopes. One locality, 

g3-2 (fig. 1), is overlain by an emerged delta; elsewhere, most of 

the other localities lie nearer the surface beneath muskeg and thin 

beach deposits. The third facies is the thinnest of the three facies 

in the formation, and generally ranges in thickness from 4 to 12 

feet.

FOSSILS

Marine fossils occur in all three facies of the formation. Barnacle shells, Foraminifera tests, and unbroken and broken shells 

of pelecypods, gastropods, and scattered brachiopods have been 

recovered from all facies, although fragments are dominant in the 

dense deposits of the gravel-rich diamicton of the second facies of 

the Gastineau Channel Formation. Molluscan faunas collected 

from selected localities in the Juneau area are listed in table 2, 

and Foraminifera from selected sites are listed in table 3. Interpretation of the life environment of the mollusks suggests relatively 

shallow water, ranging in depth from 60 to 150 feet (W. 0. Addicott and J. W. Miller, written commun., 1972). Samples of Foraminifera collected in 1958 by the late Don J. Miller were examined
CIO CONTRIBUTIONS TO STRATIGRAPHY

and the fossils identified by Ruth Todd (table 3). Interpretations 

of fossils collected during the study in and around Juneau seemed 

to indicate a maximum water depth of about 300 feet (Ruth Todd 

and Doris Low, written commun., 1967). A more restrictive interpretation of the water depth is provided by Roberta K. Smith 

(1971, p. 692), who, as a result of her regional study of Foraminifera in southeastern Alaska and British Columbia, Canada, concludes that the water was "probably less than 30 meters [about 98 

feet] deep." On the basis of these paleontological evaluations, I 

believe that all three of the facies of the Gastineau Channel Formation probably accumulated in water that was less than 150 feet 

deep and perhaps less than 100 feet deep.

TABLE 2. Fossils collected from selected outcrops of the three facies of the 

Gastineau Channel Formation, Juneau area, Alaska

[Locality numbers prefixed with g are shown in figure 1 and the locations are described 

in table 1; numbers prefixed with M are U.S. Geological Survey Cenozoic locality numbers assigned to faunas collected from the g-numbered localities and on file with the 

U.S. Geological Survey. Identification from USGS Cenozoic loc. M2651 by W. O. Addicott 

(written commun., 1966), and J. W. Miller (written commun., 1972). Other identifications 

by W. O. Addicott] _________________ ____ ___

Localities

S 2 oo bo

s 3

Pelecypods:

Astarte alaskensis Dall __ __ _ _ __ X X _. 

Chlamys rubida hindsi

(Carpenter) __________ __ ? __ X X X X 

cf. C. rubida (Hinds)

MacNeil (1967) ____ __ __ __ X __ __ _.

Clinocardium ciliatum

(Fabricus) __________ X __ __ XXX _. 

Cyclocardia ventricosa (Gould)_ _ X X X X 

Hiatella arctica (Linne) ____ X X X X X __ X 

calcarea Gmelin ______ __ __ __ __ X X _.

Macoma brota Dall _______ ? __ __ __ __ __ X

calcarea Gmelin _______ X X __ X X __ X 

obliqua (Sowerby) [M. 

incongrua of earlier

reports] __________ X __ __ __ XXX 

Mya truncata Linne ______ __ X X X X X X 

Mytilus edulis Linne _______ X __ __ __ __ __ __.

Nuculana fossa (Baird) ____ __ 1 X ____ __ __ X X

Panomya ampla Dall _____ __ __ __ __ __ X X

Protothaca staminea (Conrad). __ __ X __ __ __ _.

Serripes groenlandicus

(Bruguiere) __________ X X __ __ XXX 

Gastropods:

Acmaea sp _____________ __ __ __ __ __ __ X

Buccinum sp ___________ __ __ __ X __ ____ _.

Coins halli Dall _________ __ __ __ X __ __ _.

Cryptonatica clausa (Broderip

and Sowerby) ________ __ X __ __ __ __ _.
GASTINEAU CHANNEL FORMATION NEAR JUNEAU, ALASKA Cll

TABLE 2. Fossils collected from selected outcrops of the three fades of the 

Gastineau Channel Formation, Juneau area, Alaska Continued

Localities

7 "? T CO CO CO

Qfl M M

a a a

Gastropods Continued:

Lepeta concentrica

(Middendorff) ________ __ __ __ X __ ____ _.

Neptunea lirata Gmelin ______ ___ ? ___ X X X X

Polinices pallidus (Broderip

and Sowerby) _________ ___ X ___ _ .

Puncturella major Ball ______ ____ ____ ___ ____ ____ ___ X

Propebela sp ____________ __ __ __ X __ __ __.

Trochotropis cancellata Hinds _ ____ ___ ___ X ____ ___ ___.

Brachiopods:

Hemithyris sp _________ ___ ___ __ __ ___ X __.

Barnacles:

Balanus cf. B. nubilus Darwin _ X ___ ___ X __ ___ __.

Balanus sp _.___________ X X __ X X X X

1 Fragments.

ENVIRONMENT

Deposits included within the Gastineau Channel Formation have 

been called by others shell-bearing boulder-clay, till, marine till, or 

terrace deposits (Twenhof el, 1952, p. 528; C. L. Sainsbury, written 

commun., 1953; Don J. Miller, written commun., 1958; and field 

notes). These earlier descriptions have related most of the glaciomarine deposits in the Juneau area to a glacial environment where 

glacial ice either filled the fiords or pieces of ice discharged from 

glaciers into Gastineau Channel in the immediate vicinity of 

Juneau; some of these deposits were considered to be marine terraces of postglacial age. The results of my field studies in and 

around Juneau led me to believe that all three facies composing 

the deposits in the Gastineau Channel Formation accumulated 

when the Gastineau Channel area and adjacent low-lying regions 

were free of fiord-filling glaciers (Miller, 1972).

That such an environment existed is based in part on the radiocarbon dates obtained for shells in emerged deltas lying along the 

mountain slopes bounding Gastineau Channel, in part on the radiocarbon dates obtained for shells in the glaciomarine materials in 

the Gastineau Channel Formation, in part on the radiocarbon dates 

obtained by others from samples of basal peat in nonmarine muskegs in mountain valleys, and in part on the interrelationship 

between the Gastineau Channel Formation and the emerged delta 

deposits in the Juneau area.
C12 CONTRIBUTIONS TO STRATIGRAPHY

TABLE 3. Foraminifera collected from outcrops in the Juneau area, Alaska, 

by Don J. Miller, in 1958, and assigned to fades of the Gastineau Channel 

Formation by Robert D. Miller

[Fossil identifications by Ruth Todd, written commun., 1959]

Localities 1

-H IH e\i M M W>

Benthonic

Astrorhizidae:

Genus? ____________ _ _ _ _ _ _ _ _ _ X

Miliolidae:

Quinqueloculina agglutinata

Cushman _ _ _ _ _ ___ ___ _ _ X _

akneriana d'Orbigny __ __ X X __ X ___ __ __ X ___

frigida Parker _____ __ X X __ ___ ___ ___ ___ ___ ___

stalkeri Loeblich and

Tappan_________ __ X X X X __ __ X __ X 

Triloculina tricarinata 

d'Orbigny _________ _ _ _ _ X _ _ _ _ _

Pyrgo lucernula (Schwager) __ __ X _ X __ ___ __ __ X

Lagenidae:

Lagena gracillima

(Sequenza) ________ __ __ ___ _-_ - _ X __ __ __

Buliminidae :

Buliminella elegantissima

(d'Orbigny) ________ _ _ _ _ _ ___ _ _ _ X

Globobulimina auriculata

(Bailey) __________ _ _ _ _ _ _ _ _ _ X

Virgulina fusiformis

(Williamson) _______ __ XXX __ __ X X X X 

Bolivina decussata Brady __ __ __ ___ ___ - X

pacifica Cushman and

McCulloch ______ _ _ _ X _ _ _ _ X X

Fissurina sp. (globular, slit 

aperture) __________ X ___ ___ _ 

Rotaliidae:

Buccella frigida (Cushman)- XXXXX __ XXXX 

Elphidiidae:

Elphidium bartletti

Cushman __________ __ __ X __ X __ X X __ X 

clayatum Cushman ___ XXXXXXXXXX 

frigidum Cashman ___ XXXXX ___ __ __ X __

Elphidiella nitida Cushman _ __ __ __ __ __ __ __ __ __ X

Anomalinidae:

Cibicides lobatulus (Walker

and Jacob) _________ __ __ __ __ X __ __ __ __ X

Dyocibicides biserialis

Cushman and Valentine_ _ _ _ _ _ _ _ _ X X

Nonionidae:

Nonion labradoricum

(Dawson) _________ _ XXX _ _ XXXX 

Nonionella turgida

Williamson, var. digitata

N0rvang __________ __ __ __ __ __ __ __ X 
GASTINEAU CHANNEL FORMATION NEAR JUNEAU, ALASKA C13

TABLE 3. Foraminifera collected from outcrops in the Juneau area, Alaska, 

by Don J. Miller, in 1958, and assigned to fades of the Gastineau Channel 

Formation by Robert D. Miller Continued

Localities 1

Benthonic Continued

Nonionidae Continued: 

Pseudononion auricula 

(Heron-Alien and 

Earland ___________ X __ X X _ __ __ __ X X

Astrononion gallowayi

Loeblich and Tappan ___ __ ___ ___ ___ ___ ___ __ ___ X _

stelligerum (d'Orbigny) _ ___ __ X _ _ _ ___

Cassidulinidae:

Cassidulina islandica

N0rvang ___________ __ X X X X __ X X __ __ 

teretis Tappan _____ X X X X _ __ X X X X 

Epistominella exigua

(Brady) __________ _ _ _ _ _ _ _ _ X _

Planktonic

Globigerinidae:

Globigerina bulloides

d'Orbigny _________ _ _ X _ _ _ _ X _ _

1 Locality numbers prefixed with g are shown in figure 1; those prefixed with f are 

USGS numbers given to collections of Don J. Miller, to which the g-numbered localities 

were matched on the basis of descriptions of fossil locations in Miller's field notebooks. Locations of the sample sites are as follows: 

gl-5 SE^NW^SE^i sec. 36, T. 41 S., R. 67 E. 

gl-6 SW^SWiiSE 1^ sec 36, T. 41 S., R. 67 E. 

g2-6 SE. cor. SE% sec. 9, T. 41 S., R. 66 E. 

g3-5 SE^SWV^SWVi sec. 2, T. 41 S., R. 66 E. 

gl-7 SWi4SW%NW% sec. 18, T. 40 S., R 65 E. 

gl-8 NE. cor. SW&NW& sec. 27, T. 41 S., R. 67 E. 

gl-9 SWy4 SE%NE}4 sec. 34, T. 39 S., R. 65 E. 

gl-10 NE. cor. SE^SE^i sec. 34, T. 39 S., R. 65 E. 

g3-6 SW^SWViNW1/; sec. 18, T. 40 S., R. 65 E. 

g3-7 SE. cor. SW*4 sec. 2, T. 41 S., R. 66 E.

AGE

Shells in the formation were collected for radiocarbon determination from the highest exposed shell-bearing deposits of diamicton, but shells were not available in amounts sufficient for 

radiocarbon determination in outcrops at altitudes believed to be 

near the upper limits of the Gastineau Channel Formation. Consequently, it can be inferred that the formation is somewhat older 

than the oldest dates obtained from the diamictons in the 

formation.
C14 CONTRIBUTIONS TO STRATIGRAPHY

Such an inference has some support from the dates obtained 

for shells within deltaic deposits. Beds within the emerged deltas 

consist of sand and gravel, for the most part, but interlayered with 

these beds are layers of shell-bearing diamicton. Several radiocarbon dates for shells in these beds are slightly older than the 

oldest date obtained from diamicton in the Gastineau Channel Formation, although other dates fall within the age range determined 

for shells in the diamicton of the formation. Dates of 12,730 ±500, 

12,880±500, and 12,300 ±350 years B.P. obtained from mollusks 

and barnacles in an emerged delta near Auke Bay, locality R-l 

(fig. 1) (W-1830, W-1831, W-1839, respectively; Marsters and 

others, 1969, p. 222-223), suggest that the oldest part of the diamicton in the formation has not yet been sampled for radiocarbon determination. An example of a somewhat younger age determination, 10,880 ±340 years B.P. (W-1829; Marsters and others, 

1969, p. 222), was obtained at locality R-2 (fig. 1) for barnacle 

fragments in the foreset beds, nearly 190 feet above mean sea 

level, within the emerged delta near the mouth of Gold Creek in 

Juneau.

Dates of shells in the diamictons in the Gastineau Channel Formation, listed in table 1, partly overlap the ages of shells in the 

deltas. Dates that range from 11,920± 1,000 years B.P. (W-2396) 

to 9,700 + 800 years B.P. (W-2393; Meyer Rubin, written commun., 

1970), lend support to the concept that the now-emerged deltas 

and the deposits of the Gastineau Channel Formation accumulated 

during the same interval.

Ages of basal nonmarine peats now about 750 feet above mean 

sea level in mountain valleys adjacent to Gastineau Channel also 

lie within the 12,000- to 9,700-years-B.P. range. Peat from the 

base of a muskeg lying in the upper part of Montana Creek valley, 

locality R-3 (fig. 1), was dated at 10,300 ±400 years B.P. (L297D; Broecker and Kulp, 1957; Heusser, 1960, p. 97), and a similar peat, also at the base of a muskeg, in the valley of Lemon 

Creek, locality R-4 (fig. 1), was dated at 10,300 ±600 years B.P. 

(L-297A; Broecker and Kulp, 1957; Heusser, 1960, p. 96-97). 

Interpretation of the pollen in the muskeg above the dated layers 

indicates that the climate during the pollen accumulation was more 

moderate than would be expected during glacial time and that ice
GASTINEAU CHANNEL FORMATION NEAR JUNEAU, ALASKA C15

was absent from the dated sites after the peats began to accumulate (Heusser, 1960, table 5, fig. 24) and did not readvance over 

that site at a later time.

That the ages of the dated basal peats in Montana Creek and 

Lemon Creek valleys lie within the age span obtained for the deltas 

and deposits of the Gastineau Channel Formation suggests that 

the lower parts of the mountain valleys were ice-free, and that it 

is likely that all the glaciers were withdrawn from the immediate 

Juneau area during the deposition of the glaciomarine deposits 

that constitute the Gastineau Channel Formation. Nevertheless, 

an effective process by which sand, pebbles, cobbles, and large 

boulders can be transported and deposited in marine waters to be 

incorporated into a fine- to medium-grained matrix material and 

which results in a regionally distributed deposit must be available 

throughout the Juneau region. Though seemingly in conflict with 

an open-water marine environment, ice seems to be the most likely 

means of transport to accomplish this feat.

DEPOSITIONAL HISTORY

Origin of glaciomarine diamictons in British Columbia and 

northwestern Washington has been discussed in the literature. 

Diamictons similar to those around Juneau are widespread at altitudes of less than 700 feet above sea level along the coasts of 

southeastern Alaska, British Columbia, and Washington. Armstrong and Brown (1954) and Easterbrook (1963), among others, 

have discussed the possible origins for these deposits. Several of 

their possible hypotheses include those of plowing of the sea floor 

by glacial ice, deposition of debris from floating ice, such as shelf 

ice, berg ice, and sea ice, and submarine landslides, slopewash, and 

turbidity currents. Shelf ice, berg ice, and sea ice were decided 

upon by Armstrong and Brown (1954, p. 358-359) as acceptable 

methods of transport and deposition, and Easterbrook (1963, p. 

1474) believes that shelf ice and berg ice were the means applicable 

to the northern Puget Sound lowland in Washington. After evaluation of these theories, I believe that sea ice, supplemented by berg 

ice, provides the means of accumulating, transporting, and mixing 

the coarse debris with the fines to produce diamicton such as that 

found in the Gastineau Channel Formation (Miller, 1972, p. 83-
C16 CONTRIBUTIONS TO STRATIGRAPHY

85). Sea ice is ice that builds outward from the shores each winter 

season; such ice could provide transport of the coarse material 

deposited by streams on the sea ice during the early spring runoff. Transport is provided as the ice floats away during spring 

breakup, and deposition occurs as the debris is dropped from the 

melting ice into the continually accumulating soft and saturated 

finer grained deposits on the bottom of the fiord.

In addition, contributions from berg ice from outside the immediate Juneau area might have supplemented the sea ice and 

helped provide the extremely coarse particles found scattered 

throughout much of the glaciomarine deposits. The presence of 

berg ice, however, requires the termination of valley glaciers in 

the region into the open waters of the fiords. The large glaciated 

valleys that reach present water level north of Juneau do not contain any geomorphic evidence on the slopes higher than 500-600 

feet above sea level that these valleys contained glaciers that 

could have been sources for berg ice 10,000 to 12,000 years ago. 

The presently active Taku and Norris Glaciers, to name but two, 

reach, or reached, the tidewaters in Taku Inlet, southeast of 

Juneau, in historical time. These and other valley glaciers outside 

the Juneau area may have been actively providing berg ice during 

accumulation of the Gastineau Channel Formation.

The hypothesis of transport of coarse debris by either sea ice 

or berg ice, or both, is not free of problems. The principal problem 

with each type of ice for transport and deposition is the seasonal 

and therefore cyclic nature of the deposition of the coarse fragments. Sea ice would probably drop its load during late spring and 

summer; subsequent deposition would tend to be relatively free of 

coarse fragments. Berg ice would probably drop its load as localized piles of coarse material, especially where the bergs capsized. 

Neither depositional situation is reflected in either the texture or 

distribution of the coarse material within the diamicton in the 

Juneau area. At the present time, however, there seems to be no 

better explanation for the presence of the coarse fragments than 

transport and deposition by sea and berg ice.

Judging from available field and laboratory data, I currently 

visualize the Gastineau Channel Formation as being composed of 

different facies that accumulated during depositional cycles; the
GASTINEAU CHANNEL FORMATION NEAR JUNEAU, ALASKA C17

fades accumulated in part simultaneously, but for the most part 

occurred sequentially from the oldest to the youngest (fig. 2). The 

oldest, or first, facies apparently began to accumulate during the 

maximum late Pleistocene depression of the land and during the 

first part of emergence of the land after the ice sheet had retreated 

and marine waters occupied the depressed fiords in this part of 

southeastern Alaska.

As the land continued to rebound and the upper parts of the 

glaciomarine deposits gradually emerged from the water, waves 

reworked the uppermost part of the material and deposited a thin 

blanket of beach gravels on the newly emerged surface. These 

beach gravels are found throughout the Juneau area overlying the 

Gastineau Channel Formation at altitudes as high as 500-600 feet 

above sea level.

About midway through the emergence, a slowing or halt of unknown duration apparently occurred. Most of the delta deposits 

accumulated at that time, and so, also, did the second facies of the 

formation, in this case apparently as near-shore barrier bars.

During the interval that permitted the deltas and barrier bars 

to accumulate, the rise in sea level apparently exceeded uplift of 

the land, and relatively quiet waters existed along the shore. Diamicton accumulated over the deltaic beds under these quiescent 

conditions in some localities. In most places, however, the third 

facies of the formation accumulated at this time in near-shore 

tidal zones. As the land uplift accelerated and exceeded the rise 

in sea level, the deltas and barrier bars were carried above sea 

level to eventually lie as emerged deposits along the mountain 

slopes. The third facies deposits were in part reworked from the 

first facies that in most places underlies the somewhat younger 

third facies.

Eventually, the land rose at least 500 feet, and locally at least 

750 feet, above present mean sea level. This emergence carried the 

first, and oldest, facies to the highest altitudes along the slopes 

bordering Gastineau Channel.

Based on radiocarbon determinations from marine shells in the 

three facies of the formation, the Gastineau Channel Formation is 

considered to be late Pleistocene and early Holocene in age.
CIS CONTRIBUTIONS TO STRATIGRAPHY

Ancestral Gastineau Channel

Earlier sea level at start of deposition and emergence, 

about 600-700 ft above modern sea levek

Sea level at a point in time during 

the deposition of the first facies 

of the Gastineau Channel formation, and during continuing emergencex

ABergs Sea ice

gt 

Modern sea lever

First facies environment

Ancestral Gastineau Channel

Sea level at a point in time 

during the deposition 

of the second facies of 

the Gastineau Channel 

formation, and during 

continuing emergence

Modern sea level

Second facies environment

FIGURE 2. Diagrammatic sketches showing interpretations of the manner of glacdomarine deposition and accumulation of related deposits 

during the three depositaonal facies of the Gastineau Channel Forma-
GASTINEAU CHANNEL FORMATION NEAR JUNEAU, ALASKA C19

Ancestral Gastineau Channel

Sea level at a point 

in time during the 

deposition of the 

third facies of the 

Gastineau Channel 

Formation, and 

during continuing

Modern sea level

Third facies environment

Present time

tion. gt, Gastineau Channel Formation, first facies; gg, Gastineau 

Channel Formation, second facies; gs, Gastineau Channel Formation, 

third facies; do, emerged delta deposits; d, diamicton layer on top of 

delta deposits; br, raised beach deposits.
C20 CONTRIBUTIONS TO STRATIGRAPHY

REFERENCES CITED

Armstrong, J. E., and Brown, W. L., 1954, Late Wisconsin marine drift and 

associated sediments of the Lower Fraser Valley, British Columbia, 

Canada: Geol. Soc. America Bull., v. 65, no. 4, p. 349-363.

Broecker, W. S., and Kulp, J. L., 1957, Lamont natural radiocarbon measurements, [Pt.] 4: Science, v. 126, no. 3287, p. 1324-1334.

Easterbrook, D. J., 1963, Late Pleistocene glacial events and relative sea-level 

changes in the northern Puget Lowland, Washington: Geol. Soc. America 

Bull., v. 74, no. 12, p. 1465-1483.

Heusser, C. J., 1960, Late-Pleistocene environments of North Pacific North 

America an elaboration of late-glacial and postglacial climatic, physiographic, and biotic changes: Am. Geog. Soc. Spec. Pub. 35, 308 p.

Marsters, Beverly, S piker, Elliott, and Rubin, Meyer, 1969, U.S. Geological 

Survey radiocarbon dates X: Radiocarbon, v. 11, no. 1, p. 210-227.

Miller, R. D., 1972, Surficial geology of the Juneau urban area and vicinity, 

Alaska, with emphasis on earthquake and other geologic hazards: U.S. 

Geol. Survey open-file report, 108 p.

Smith, R. K., 1971, Late glacial Foraminifera from southeast Alaska and 

British Columbia and a world-wide high northern latitude shallow-water 

faunal province: Archives Sci., v. 23 (1970), no. 3, p. 675-701.

Twenhofel, W. S., 1952, Recent shore-line changes along the Pacific coast of 

Alaska: Am. Jour. Sci., v. 250, no. 7, p. 523-548.

( * U.S. GOVERNMENT PRINTING OFFICE: 1973 O 543 S78/38