AN ABSTRACT OF THE THESIS OF

GORDON EVERETT NESS(Name of Student)

for the M. S.(Degree)

in Oceanography presented on 9 February 1972(Major) (Date)

Title: THE STRUCTURE AND SEDIMENTS OF SURVEYOR

DEEP-SEA CHANN:

Abstract approved:Dr. L. D. Kulm

Surveyor Deep-Sea Channel extends for approximately 700 km

over the northern Alaskan Abyssal Plain. It originates near the base

of the continental slope opposite Dry Bay and Alsek Strath and termin-

ates in the Aleutian Trench south of Kodiak Island. East of Giacomini

Seamount, the axial gradient of the channel is in the order of 10 rn/km

and its morphology is in agreement with prediction, assuming a depo-

sitional equilibrium with channelized turbidity currents. West of

Giacomini Seamount, the axial gradient increases to values as high as

7. 5 rn/km. as the channel course turns toward the northwest and

plunges into the trench. Over this part of its length the measured

center channel relief and cross-sectional area of the channel increase,

contradicting prediction. The lower channel is found to be erosional

in nature, this effect being a response to downwarping of the northern

rim of the Pacific Plate into the Aleutian Trench.

Redacted for Privacy

The channel originated in early to middle Pliocene time coeval

with the initiation of pronounced tectonism and intense glaciation in

southeastern Alaska, At this time, the channel was located perhaps

200 km south of its present position with relation to the North

American Plate, and may have been linked with one of the fossil sea-

channels on the eastern Aleutian Abyssal Plain. Throughout its

history, the channel has not been linked with any consistent river

drainage system, its sediment source instead being the large system

of piedmont glaciers in southeastern Alaska,

The distribution of coarse sedimentary material over the

northern Gulf of Alaska strongly suggests that turbidity current acti-

vity has not been confined to only those regions close to Surveyor

Deep-Sea Channel,

THE STRUCTURE AND SEDIMENTS OFSURVEYOR DEEP-SEA CHANNEL

by

Gordon Everett Ness

A THESIS

subrriitted to

Oregon State University

in partial fulfillment ofthe requirements for the

degree of

Master of Science

June 197Z

APPROVED:

Associate Professor of Ôcanographyin charge of major

Chairman o%f Department of ceanography

Dean of Graduate School

Date thesis is presented 9 February 1972

Typed by Marjorie Hay for Gordon Everett Ness

Redacted for Privacy

Redacted for Privacy

Redacted for Privacy

ACKNOWLEDGEMENTS

First, I would like to thank my major professor, Dr. L. D.

Kuim, for his support and patience, and for allowing me the freedom

to pursue my own line of inquiry into the subject of this thesis.

Secondly, I want to thank Paul Komar, Don Heinrichs, Ted Moore,

Dick Couch, Ross Heath and Jerry van Andel for taking time with me

to discuss various aspects of this work. Dave Rea, with his

familarity of the North Pacific, has also been of great help. A

special debt of gratitude is owed to Roland von Huene of the U. S.

Geological Survey, and John Wageman and Fred Naugler of NOAA for

providing me with critically needed seismic profiles. Their thought

provoking scientific reports also constitute a good portion of the

background to this work. In that respect, I also wish to acknowledge

the work of Gary Griggs on Cascadia Deep-Sea Channel. We sailed

together on Yaloc-70 and enjoyed many a late night discussion on

channels and channel processes to my great benefit.

It is impossible to even begin to thank each of the many others

who have aided me in one way or another. Certainly the crew of the

R/V Yaquina merits any such consideration. Chief Bob Ingersoll

has helped me in many ways, from keeping seismic equipment

running to keeping my efforts in proper perspective. For similar

reasons I thank my father Oscar Ness, and an old and good friend

George Bent who suggested I read Hesse' s Magister Ludi during any

troubled moments. My office partner and shipmate, Commander

Doctor Professor John Harlett, USN, who graduated just when I

needed him most, is greatly missed. It is hereby acknowledged that

Margie Hay, who tried to make some grammatical sense out of my

scribbled sentence fragments, is entitled to one supper of her choice

at any of River City' s finer dining establishments.

This work was made possible through the financial support of

the Office of Naval Research (Contract Nc,nr N00014-67-A-0369-007)

and the National Science Foundation (Grants No. GA 1246 and GA

159Z6). A departmental assistantship and a hard working and tolerant

wife provided personal financial support.

TABLE OF CONTENTS

INTRODUCTION 1

MORPHOLOGY OF THE CHANNEL 6

Contrast in Bank Heights 11Center Channel Relief 15Cross Sectional Area 18

SUB-BOTTOM STRUCTURE 23

Character of the Seismic Reflectors 23Age of the Channel Basal Reflectors 27Longitudinal Development of the Channel 31

CHANNEL RELATED SEDIMENTS 40

CHANNEL RESPONSES TO REGIONAL TECTONISM 45

The Eastern Source of Sediments 45Trench Related Downwarping of the Lower Channel 51

CONCLUSIONS AND GEOLOGIC HISTORY 61

BIBLIOGRAPHY 65

APPENDIX 1. Location and Physiography of Channel Profiles 70

APPENDIX 2. Transport Calculations from Channel Profiles 71

APPENDIX 3. Magnetic Stratigraphy of Core Y70-4-56 72

APPENDIX 4. Alaskan Abyssal Plain Piston CoreStation Locations and Descriptions 75

LIST OF FIGURES

Figure Page

I. Trackline, drill hole and piston core locations 5

2. Bathymetric chart - northern Gulf of Alaska 7

3. Channel axial gradient and center channel relief 8

4. Normalized channel bathymetric profiles 9

5. Channel morphologic parameters 12

6. Relationship between center channel relief andaxial gradient 16

7. Channel cross sectional area 20

8. Seismic section of abyssal plain sediments 24

9. Long seismic profiles near and across the middleand lower channel 26

10. Magnetic stratigraphy of Core Y70-4-56 29

11. Upper channel seismic profiles 32

12. Middle channel seismic profiles 33

13. Lower channel seismic profiles 34

14. Occurrence of coarse material in piston cores 42

15. Transverse mercator projection of North Pacificabout rotation pole at 530 N, 53° W 52

16. Transverse mercator projection of North Pacificearly Pliocene time 57

THE STRUCTURE AND SEDIMENTS OFSURVEYOR DEEP-SEA CHANNEL

INTRODUCTION

In 1958, William Gibson of the Coast and Geodetic Survey map-

ped the course of a broad and linear depression on the seafloor of the

Gulf of Alaska. Noting its apparent gentle relief, its depth and

distance from shore, and the similarity of its orientation with respect

to the Aleutian Trench and to structural features on the adjacent

southern Alaskan landmass, Gibson suggested that its origin was

tectonic, "Marking the southeast edge of the Aleutian Trench1'

(Gibson, 1958). Two years later, in describing the morphology of the

Gulf of Alaska seafloor, Gibson again discussed the feature and reco-

gnized its sedimentary origin. He then named it Surveyor Deep-Sea

Channel. Gibson still however stressed the tectonic relationship be-

tween the channel, the geosynclinal Aleutian Trench and its seaward-

moving anticlinal rise, the channel being developed in a moving fore-

deep and confined there by the rise (Gibson, 1960). I find it

satisfying that Gibson' s almost intuitive stress on the tectonic

character of Surveyor Deep-Sea Channel seems to be justified even

in terms of the most recent evidence, which will be presented in

support of this thesis.

Gibson published his two papers immediately prior to the onset

2

the so-called geologic revolution. Only within the past decade have

the concepts implicit in the New Global Tectonics been accepted, or

for that matter even proposed. Marine geologists and geophysicists,

in particular among earth scientists, have readily accepted them. As

a result, the whole scale and direction of geologic thinking has

changed. Horizontal stresses and motions have for the most part re-

placed their vertical counterparts in relative importance, and many

broad syntheses have been accomplished through the unifying concepts

of the new tectonics.

Prior to this revolution, geosynclinal theories dominated the

study of tectonics, and Gibson understandably stated his working

hypotheses in the vocabulary of those theories. Though it may seem

trivial to some, it should be stated however that to name something

is not to know it, and the vast nomenclature that developed in the

literature of geosynclines is symptomatic of the fact that the theories

were not adequate to account for a genetic mechanism. There are

eighteen modifying prefixes to the word geosyncline listed in the 1959

English edition of the glossary 'Geologic Nomenclature" published by

the Royal Geological and Mining Society of the Netherlands

(Schieferdecker, 1959). This number is exclusive of such related

terms as fossa, tethys, backdeep, foredeep, marginal foredeep and

the "leptogeosyncline" of Trumpy (1960). Clearly, the theories were

in trouble as the number of terms must have approached the number

3

of known geosynclines. This proliferation of definitions brings to

mind the decline in the popularity of the lute when that instrument

grew to include sixteen strings. The geosynclinal theories had be-

come, like the lute, baroque.

In an article discussing the methods of the earth sciences,

Chamberlin (1904) pointed out that "not a little consists of generaliz-

ations from incomplete data, of inferences hung on chains of uncer-

tam logic, of interpretations not beyond question, of hypotheses not

fully verified, and of speculations none too substantial. A part of the

mass is true science, a part philosopy .. a part speculation, and a

part is yet unorganized material." Gibson worked within the concep-

tual framework of geosynclinal theories, and was also faced with the

problem stated by Chamberlin. A decade later, the theories have

changed but the problem of methodology remains. I think it is funda-

mental that ultimately the only proof of a geologic construct is the

measure of its synergy, that is, its capacity to accept and integrate

a wide variety of data.

This thesis will attempt a new discussion of Surveyor Deep-Sea

Channel and its relationship to the Alaskan Abyssal Plain and the

adjacent landmasses. An attempt will be made to apply certain

quantitative methods of analysis to the morphology of the channel,

and the response of the channel to tectonic influences will be investi-

gated. The study will enjoy the benefits of new evidence and a newer

conceptual framework. Nonetheless, it will depend upon an acquired

intuition and this hopefully will be its strength. The theories are

certain to change, but perhaps something of value will remain of this

effort, so that twelve years from now someone may at least find

pleasure in its reading.

The bulk of the data supporting this thesis was obtained from

the summer, 1970, cruise of the R/V Yaquina of Oregon State tJniver-

sity. Bathymetric and sub-bottom seismic profiles pertinent to this

cruise are labeled with the prefix 'TYaloc" (Figure 1). Thirteen piston

cores were also recovered during that cruise. Five excellent seismic

profiles of the western portion of Surveyor Deep-Sea Channel and the

eastern Aleutian Trench were provided the author by John Wageman

and Fred Naugler of the Pacific Oceanographic Laboratories, NOAA.

These are labeled 'Oceanographer." In addition, three short seismic

profiles of Surveyor Deep-Sea Channel in the vicinity of Giacomini

Seamount were provided by Roland von Huene of the Office of Marine

Geology and Hydrology, U. S. Geological Survey. These are

labeled "Surveyor."

r

I54 152

I I I

TRACKLINE, DRILL-HOLEAND PISTON CORE LOCATIONS

NORTHERN GULF OF ALASKA

.OPPER HUGACH FAIRWEATHER

I/\ YAKUTAT

O:4//\S -

CROSSY70-3-48

S 53DSDP SITE 80

S 178OSOP SITE Y7O-2-41

Y70-2-SURVEYOR

71 Y7O-2-4O37

0 Y70454Y70-2-35 Y70-2-39

SURVEYORCHANNEL

Y7O-4-56

I I I I

(. 153'OI4 N, 141' 41.4 W

50' 148' .146' 44' 142' .

Figure 1. Trackline, drill hole and piston core locations. UI

MORPHOLOGY OF THE CHANNEL

The course of Surveyor Deep-Sea Channel extends from the

northeastern edge of the Alaskan Abyssal Plain, near the base of the

continental slope adjacent to Yakutat Seavalley and Alsek Strath, to

the floor of the eastern Aleutian Trench immediately southeast of

Kodiak Island (Figure 2). Its length is approximately 700 km. The

general trend of the channel is parallel to the trench for over slightly

more than half its length, curving generally to the southwest. In the

vicinity of Giaconiini Seamount, the channel turns toward the north-

west and its gradient steepens as it plunges into the trench.

It is convenient to refer to three portions of the channel as

upper, middle and lower (Figures 3 and 4). In the upper portion

(profiles A and B), the relief in the center of the channel is in the

order of 100 m and the axial gradient is approximately 1.4 rn/km

(Appendix I). In the middle channel (profiles C, D, E, F), the relief

is fairly uniform at about 165 m, increasing slightly to 190 m near

profile F, and the axial gradient here decreases to an average of

about 1 rn/km. In the lower channel (profiles G, H, I, J, K), the

relief increases from about 200 in to more than 450 m. Similarly,

the axial gradient increases from about 2 rn/km to a value of 7. 5

rn/km at profile I, then decreases somewhat as the channel approach-

es the floor of the Aleutian Trench.

154 150 146 )42 I8I I I

II I I I I I I I I

BATHYMETRIC CHARTNORTHERN GULF OF ALASKASHOWING COURSE OF SURVEYOR CHANNEL

-

S. 0. AREA WEST OF 142 30 AFTER VON HUENE AND SHOR (1969),. . AREA EAST OF )42 30 NECONTOURED FROM GIBSON ((960)

WITH SUPPLEMENTARY .ALOC-7O BATHYMETRIC PROFILES

DEPTH (N METERS- LOPPER 1Y

-.j

IRIVER LOCATION OF ILLUSTRATED PROFILES

60 KAYAKICY

- ..

ISLANDBAY

. YAKUTATBAY

60

-:

'000 EAVALLEY bR YY BA:'

5858

& '

O0 2))

00 ii

f 0 KDIAK500 /iAMOUNT 00/

GIACOM IN)

AMUN)QWNN°

SEAMOUNT 56

154 152 150 I48 )46 144 142 140 138 136

Figure Z. Bathymetric chart - northern Gulf of Alaska.

DISTANCE ALONG AXIS (KM)700 600 500 400 300 200 tOO 0

I I I I I I I

C-)

m2

I-

mxz2P1I-

H

LOWER MIDDLECHANNEL CHANNEL

K

Figure 3. Channel axial gradient and center channel relief.Depths corrected from Matthews Tables (1939).

UPPERCHANNEL

3600

D

3800

40000

4200Irn

4400

rn3

4600U)

15000

0wU)

Lu

I-.

-JLu>

I-

>-4

c'J

North (Right) Bank South (Left) BankA

B

C

D

H

I

-T\\ rK\/

0.4 3O0/ V.E.30X

0.5400

0.6

0.7 500

15 JO 5 0 5 10 15

KILO MET ER S

Figure 4 Normalized channel bathymetric profiles.

10

The cross sectional area of the channel (Appendix I), measured

from the original bathymetric records (Figure 4) and corrected for

crossing angles, shows a general increase downchannel and ranges

from about 3 x 105m2 for profile A to almost 12 x 105m2 for profile

K near the Aleutian Trench floor. The areas of profiles C, D and E

are each about 5.5 x iOm2. Profiles F and G exhibit smaller cross

sectional areas, and mark a slight reversal in trend from that of an

overall increasing area along the channel length.

Bathymetric profiles of Surveyor Channel reveal a bank morpho-

logy similar to that of Cascadia Channel (Griggs and Kulm, l970a).

In the upper channel, a pronounced levee, elevated well above the

adjacent abyssal plain, occurs on the north bank (Figure 4, profile A).

The relief of the levee above the surface of the plain is approximately

40 meters. Its width is 13 kilometers. Downchannel from profile A

the north bank is concave upward and is consistently higher than the

south but no definite levees are found. Profile B suggests a very

broad levee, but the trackline did not continue far enough to the north

to determine if it is of markedly shallower depth than the bathymetry

of the plain.

The morphology of the channel can be examined in terms of

turbidity current dynamics by assuming its profile to be in deposition-

al equilibrium with large full-channel turbidity currents. As

suggested by ivlenard (1955), the levees and banks of the channel will

11

be here considered to have been constructed by periodic sheetfiow

spillover from large turbulent flows of suspended sediment moving

downchannel. Three approaches will be used to discuss the morpho-

logy of Surveyor Deep-Sea Channel as a function of turbidity current

dynamics.

Contrast in Bank Heights

Komar (1969) estimated the flow parameters of turbidity

currents for several crossings of Monterey Deep-Sea Channel by

setting up and solving an equation relating the centrifugal, Coriolis

and pressure gradient accelerations acting upon a full-channel flow

(Figure 5). Balancing these accelerations yields a simple quadratic

equation

U2 AH-r

+ 2fltJsin+J

=0

where U is the flow velocity,.fl.. is the angular velocity of the earth

about its axis, is the geographic latitude of the channel, g is the

acceleration of gravity, / is the density of the turbidity current and

, is the density of water. The terms for the radius of curvature of

the channel R, the difference in bank or levee heights on either side

of the channel A H, and the channel width W are measured from

bathymetric profiles and plan views of the channel course.

;../'

/ 'T

R

Figure 5. Channel morphologic parameters

N)

13

If, in the northern hemisphere, the right bank of a channel is

higher than the left, looking downchannel, the Coriolis acceleration

must either add to the centrifugal acceleration (a left turning channel)

or be of greater magnitude but opposite sign (a right turning channel).

For the left bank to be higher than the right, the centrifugal accelera-

tion must exceed the Coriolis acceleration and be of opposite sign (a

right turning channel). With a straight channel course, the radius of

curvature goes to infinity and the centrifugal term drops out of the

equation. The right bank should therefore be higher than the left.

These relationships of the channel course to the bank heights

were found by Komar to hold for crossings of Monterey Deep-Sea

Channel. Using measurements of H, W and R, Komar calculated

curves of U versus from the above equation. The method has

also been applied to Cascadia Deep-Sea Channel using data from

Griggs (1969) and was found to yield reasonably consistent velocities

where the radius of curvature could be unambiguously determined

(Ness, 1970). The method of solving for velocities has not been

applied to Surveyor Deep-Sea Channel since the number of normal

oriented crossings is insufficient to accurately determine the

necessary morphologic parameters. The illustrated profile,s

(Figure 4) are however consistent with prediction, the right (north)

bank being always higher than the left, particularly in the linear and

steeper upper channel where the velocity dependent Coriolis term

14

dominates. The difference between bank heights decreases down-

channel from crossing H where the channel curves to the right and the

centrifugal acceleration opposes but does not exceed the Coriolis

acceleration. Profile K (Figure 4) is from a crossing at an angle to

the channel course on the steep seaward rise of the trench. Geo-

metrically adjusting the profile to be normal to the channel course

corrects for true horizontal distances, but the artificially high bank

height difference cannot be so removed. In any case, the right bank

is higher than the left and this is consistent with the other lower

channel profiles.

It is significant that the bank height morphology of the three

deep-sea channels so examined can be accounted for without exception

using the method of Komar. In no case does a measured channel

profile contradict its predicted morphology as would, for example, a

profile of a left turning channel in the northern hemisphere with a

higher left bank. The lack of contradictory evidence strongly implies

that the Coriolis, centifugal and pressure gradient accelerations are

dominant influences in the depositional formation of deep- sea channels.

It is further significant that the method applies equally well to

both the upper and lower, leveed and non-leveed, portions of deep-

sea channels. The non-leveed bank height contrasts commonly exhi-

bited in long bathymetric profiles from middle and lower portions of

both Cascadia (Griggs and Kulm, 1970a) and Surveyor Deep-Sea

15

Channels are therefore most probably related to deposition from the

differential spiflover of channelized turbidity currents.

Center Channel Relief

Komar (1972) analyzed the relative thickness of the head and

body of channelized turbidity currents and their relationship to the

channel axial gradient and the center channel relief. The equation

expressing this relationship isI

( hh= \h ) Fr

where V is the velocity of the head, Ti is the velocity of the body, hh

is the head thickness, hb the body thickness, and Fr is the Froude

number which is in part slope dependent. Komar found that for a

Froude number of 0.75 (corresponding to an axial slope of about 2.2

mlkm), the thickness of the head and the body of a flow should be

equal (Figure 6). For lower Froude numbers (lower slopes) the body

thickness will exceed the head thickness and channel spillover will be

predominantly from the body of the flow. For higher Froude numbers

(slopes steeper than 2.2 mfkm) the head thickness will exceed the

body thickness and spillover will consist of material from the head of

the flow. Suspended material within the head is continuously derived

from the body of the flow and the net velocity of the head is limited to

that of the body.

hb

hh

14

Sifl .48

0.0001 0.0010 0.0022

BODY SPILLING HEAD SPILLING

Ic.,JIcjiO

u,IoFtOdl"flI'

CLLu)

hh i I

FROUDE NUMBER, FrFigure 6. Relationship between center channel relief and

axial gradient.

1.5

17

For any given flow, the head thickness changes less in response

to changes in slope than does the body thickness. Therefore, even at

higher Froude number values, for reasonable slopes the head thick-

ness will not greatly exceed the body thickness. A typical channel of

constantly decreasing gradient should gradually increase in relief

over its length until, presumably at extremely low slope values

typical of central abyssal plains, the gravitational driving energy of

the turbidity currents becomes negligible and deposition occurs. The

break-up of the channel into a distributary network may also enhance

deposition of the suspended sediments.

The measured axial gradients of the upper and middle portions

of Surveyor Channel are all less than 2. 2 rn/km (Appendix I), with

profile A having the steepest gradient and therefore predictably the

least relief in the center of the channel. The middle channel gradients

are more gentle, and the measured relief correspondingly increases

in response to an increasing body thickness. Downchannel from

Giacomini Searnount, the axial gradients increase to values greater

than 2. 2 rn/km and the channel relief should therefore presumably

decrease in equilibrium with head spilling conditions. Instead, the

measured values of center channel relief continue to increase from

about 200 meters to more than 400 meters (Figure 3 and Appendix I)..

Surveyor Deep-Sea Channel is unique among known channels in that

its axial gradient increases markedly over the distal portion of its

length where it plunges into the Aleutian Trench. Over this length,

its relief greatly exceeds any values predicted upon an equilibrium

between the flow parameters and the channel morphology.

Cross Sectional Area

The volume transport of a full-channel turbidity current can be

estimated from the application of an equation of the form

Q=UA

where Q is the discharge rate, U is the average velocity of the

current and A is the cross sectional area of the channel. Since the

average velocity of the current is limited to the velocity of the body

of the flow, the motion of the current in a channel of slope is

governed by a Chezy-type equation of the formI12

L't gh (1 +) Cfj

where Cf is a bottom drag coefficient and ' is the ratio of the drag

at the upper interface of the current to the bottom drag (Komar,

1971). Assuming that no dilution of the flow by entrainment occurs,

and that the drag coefficients are constant, it follows that

IU °" (hb Sifl,

)2

and therefore that

19

Q S (hb sine)2 A

In effect, where the axial gradient decreases the velocity should

decrease, resulting in an increased channel cross sectional area for

a given discharge. Values of (h sin,)2 A for the Surveyor Deep-Sea

Channel bathymetric profiles have been summarized in Appendix II.

Assuming a constant discharge Q, a predicted area for any

profile A relative to the measured area of another, A, can be deter-

mined from the relationshipI

A1[hsin$ 12

h.sinp A1 iJ

Predicted area curves so generated (Figure 7) show that measured

middle channel areas are consistent with respect to one another. The

analysis predicts the smaller area of profile F and perhaps even G.

The slope used for crossing F is the downchannel slope, that is, the

change in axial depth between crossings F and G (Figure 3). It would

be equally valid to choose the upchannel depth contrast between

crossings E and F to determine the slope at F. Curves fit through

areas C, D and E would then fall only slightly below area F, and area

G would appear to be more in equilibrium with the middle channel.

The true slope at F is probably of some intermediate value, but the

distance between crossings is too great for a finer resolution.

The measured cross sectional areas of profile A and perhaps

DISTANCE ALONG AXIS (K M)

600 500 400 300 200 100 0I I

PREDICTEDAREA CURVES

I

H

T

I

A

-I- _D B0_____.rC0 -T-0 .i 0--

-I-

Figure 7. Measured channel cross sectional area from profiles Athrough K. Vertical bars include ± 15° ambiguity incrossing angles and errors in determining depths basedupon the scale of each original record.

C,I200

400

600

800

1000

1200

C)cIoU)

m-4(1)mmCj)1r3

I-

m

C

21

profile B are smaller than the predicted areas, area A being only

about half the predicted value. Since no channel crossings were made

east of crossing A, its actual slope is unknown, and it may be much

steeper than the downchannel value chosen which might explain its

small area. An alternative explanation for this will be discussed

later.

Most important to this discussion however, is the fact that

measured lower channel cross sectional areas are dramatically out

of equilibrium with middle channel areas, being larger by as much as

a factor of three (and even more with respect to their own predicted

values).

In summary, the depositional aspects of the morphology of

Surveyor Deep-Sea Channel show expected characteristics. Leveed

banks occur in the steeper upper reaches of the channel. The

occurrence is similar to that of Cascadia Deep-Sea Channel (Griggs,

1969) and appears to be common for channels developed on steeper

slopes and on the upper reaches of deep-sea fans (Normark, 1970).

The non-leveed banks of the middle and lower channel exhibit a bank

height contrast in conformity with their depositiona]. development

from the spillover of channelized turbidity currents. The lower

channel however is distinctly dissimilar from the middle and upper

reaches of the channel with respect to both its increased cross

sectional area and its greater center channel relief. From the

22

vicinity of Giacornini Seamount to the Aleutian Trench, Surveyor

Deep-Sea Channel is not in equilibrium with its predicted model.

Seismic reflection records, which will be discussed next, strongly

suggest that the lower channel has undergone post-formational

ero sion.

23

SUB- BOTTOM STRUCTURE

Seismic reflection records taken over the northern Gulf of

Alaska reveal the ubiquitous nature of a characteristic series of sub-

bottom reflectors. They appear to be best developed near and to the

north of Surveyor Deep-Sea Channel, though some can be recognized

as far as 500 km to the south. Profile VI (Figure 8) from Yaloc 70

data, is the best detailed record in this study and most clearly

illustrates the nature of these reflectors. In general, four thick

stratified units can be recognized above acoustic basement.

Character of the Seismic Reflectors

The uppermost unit (Unit A) extends from the sediment-water

interface to a depth of 0. 2 seconds (two-way travel time). It consists

of a strong and continuous initial set of low frequency reflectors ex-

tending to a depth of about 0. 07 seconds. Between 0. 07 and 0. 2

seconds, except for a single strong reflector of 0. 14 seconds, weak

and higher frequency returns become increasingly discontinuous with

distance from the channel.

At a depth of 0. 2 seconds, a pair of strong low frequency re-

flectors, common to most of the records and only occasionally dis-

continuous, marks the upper interface of the second seismic unit

(Unit B). Below that, to a depth of 0.4 seconds, weak reflectors

show the same trend in discontinuity as those of the first seismic

unit.

25

At a depth of 0. 4 seconds a single particularly strong low fre-

quency reflector occurs within a group of several strong closely

spaced reflectors. These mark the top of the third seismic unit

(Unit C) which is about 0. 25 seconds thick. Other reflectors in this

unit are much less continuous and return higher frequencies than any

others in the section. Occasional strong returns of short horizontal

extent occur.

At a depth of 0. 6 to 0. 7 seconds, the final seismic unit (Unit D)

begins and extends downward to acoustic basement. It is character-

ized by strong, fairly continuous and somewhat deformed low fre-

quency reflectors throughout. Acoustic basement, poorly expressed

in Profile VI, is generally found at an average sub-bottom depth of

0.8 to 1. 2 seconds here and to the west of the section. A fifth

seismic unit occasionally appears ponded into basement lows below

0.8 seconds (Figure 9, Profiles I and II).

The reflectors at depths of 0. 2, 0. 4 and 0. 7 seconds in Profile

VI (Figure 8) are the most prominent throughout the region. The 0. 4

second set of reflectors, marking the boundary between seismic

units B and C, is found to have a particular importance in that it

most frequently seems to occur at or near the thaiweg (the lowest

point in a channel profile) of Surveyor Deep-Sea Channel. Its

27

determined age has a significant bearing on the origin and develop-

ment of the channel.

Age of the Channel Basal Reflectors

Seismic Profile I (Figure 9), kindly supplied by John Wageman

and Fred Naugler of NOAA, extends from the base of the landward

wall of the Eastern Aleutian Trench, southward across Surveyor

Deep-Sea Channel and terminates at the foot of Giacomini Seamount.

The profile passes closely by Sites 178 and 180 of Leg XVIII of the

Deep Sea Drilling Project (von Hueneetal., 1971). The prominent

set of reflectors at the base of the channel can be easily traced be-

neath D. S.D. P. Site 178 and into the trench. Near the drill hole, the

channel basal reflectors are found at a depth of 0. 34 seconds. The

seismic velocity of North Pacific Abyssal Plain silty turbidites, as

measured in the laboratory, is 1.634 km/sec (Hamilton, 1969).

Assuming this velocity, the channel basal reflectors lie at a depth of

275 meters. Cores recovered from the drill hole at Site 178 extend

to a basalt basement. To a measured depth of 270 meters, they

consist of middle Pliocene to Holocene gray-mud turbidites with

abundant ice-rafted debris (von Hueneetal., 1971). Below the gray

mud, the sediments recovered consist of interbedded silt and sand

turbidites, diatornaceous sediments and mud. The lithologic change

at a depth of about 270 meters is therefore seen to closely correspond

to the change in the character of the seismic reflectors at that depth.

The prominent reflectors at the base of the channel near Giacomini

Seamount may therefore be dated as middle Pliocene.

The basal reflectors can be traced on other records south and

east of Surveyor Channel into the central Alaskan Abyssal Plain.

They appear to continuously shoal in this direction. At a position of

530 01' N, 1410 41' W, a similar set of reflectors are found at a

depth of 0. 14 seconds. Assuming a seismic velocity of 1. 634 km/sec,

this corresponds to a depth of 115 meters. This location was chosen

for piston coring in an effort to obtain a long stratigraphic core for

the Alaskan Abyssal Plain in an area of presumed slow sedimentation.

A 17. 5 meter hemipelagic core was recovered. It was found to con-

sist of a medium gray silty clay with periodic thin silt laminae, each

about one or two millimeters thick. Several foraminiferal rich

layers and occasional ice- rafted pebbles are additionally found in the

core.

Oriented sections of the core were sampled for magnetic

stratigraphy at one meter intervals. The total field magnetic inclina-

tions of the undisiccated samples were measured on a 5 CPS spinner

magnetometer. To a depth of 16.5 meters, the total field magnetic

inclinations were found to be of normal polarity, with inclinations

from the horizontal of an average 65 degrees (Figure lOa). At 17. 5

meters, near the very bottom of the core, a reverse field sample of

90

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CORE Y70-456 5301O'N141041'W

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DEPTH IN CORE (METERS)Figure 10. Magnetic stratigraphy of Core Y70-4--56

29

18

30

-30 degrees was measured. The meter interval between the last

normal sample and the reversed field sample was then more closely

examined. Three additional reverse field samples were found, the

highest occurring at a depth in the core of 16. 9 meters. Several

samples were then A. C. demagnetized at successively increasing 25

oersted intervals using a 3-axis demagnetizer. At each level, their

remnant magnetic field orientation and intensity were measured. It

was determined that a 75 oersted A. C. field was sufficient to

remove secondary magnetic components. All samples were then de-

magnetized at this level. The specific intensity of most of the demag-

netized samples decreased to about half their original values

(Figure lOc).

Measured normal polarity inclinations of the demagnetized

samples (Figure lOb) decreased to an average of 40 degrees, with

only a small scatter about that value. This inclination corresponds

to a geographic latitude of only 23 ° N. The original normal polaiity

inclinations averaged slightly more than 65 degrees which corres-

ponds to a geographic latitude of 47 ° N. While the original inclina-

tions are reasonably close to values predicted from the geographic

latitude of the core location, the demagnetized inclinations greatly

deviate from predicted values. No explanation for this abnormal

result is here offered.

Both original and demagnetized inclinations indicate that a

31

paleornagnetic field reversal occurs in the core at a depth of 16.7 ±

0.2 meters. Assuming this to be the Brunhes-Matuyama Epoch

Boundary, the age of the core at this depth is 6.9 x 10 years B. P.

(Cox and Dairymple, 1967). The average sedimentation rate for the

core since that time is therefore 24. 3 ± 0. 3 meters/rn. y. Extrapo-

lating this sedimentation rate to the basal reflectors, found at a depth

of 115 meters at the core site, yields an age for that surface of

4. 75 .± 0. 06 rn. y. B. P. Using the most recent time scale of

Berggren (1972) this date is found to be within the early Pliocene.

This is reasonably consistent with the middle Pliocene age found for

the 270 meter reflectors at D. S. D. P. Site 178 (von Huene et al

1971).

The basal reflectors are therefore seen to be roughly time syn-

chronous over the 500 kilometer distance between the central Alaskan

Abyssal Plain stratigraphy core and D. S.D. P. Site 178 (Figure 2).

Calculating a sedimentation rate, using a 1. 634 km/sec seismic

velocity, for the units above the basal reflectors at Site 178 yields a

value of 56 meters/rn, y. or slightly more than twice the rate on the

central abyssal plain 500 kilometers to the southeast and distant from

Surveyor Deep-Sea Channel,

Longitudinal Development of the Channel

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35

from the original seismic records of eight channel crossings and re-

duced to the same vertical scale. The indicated horizontal scales are

not normalized with respect to the course of the channel. The channel

basal reflectors are drawn in a bolder line and are further indicated

by filled circles beside each profile. These reflectors were not re-

cognized in profile A and presumably lie below the penetration depth

of the seismic record. The sediment thickness above the basal re-

flectors is seen to increase upchannel toward the east. It is about

0.3 seconds (245 meters for a seismic velocity of 1.634 km/sec) at

profiles H and I, and about 0. S seconds (410 meters) at profile B.

The position of the thaiweg with respect to the basal reflectors also

shows a consistent variation upchannel. It is at the same depth at

profile F, and 0. 22 seconds (180 meters) above the surface at

profile B.

Upper channel profiles A and B, found to be morphologically

distinct from the rest of Surveyor Channel as previously discussed,

exhibit sub-bottom characteristics that further distinguish them

(Figure 11). Their broad, smoothly rounded banks and gentle channel

wall slopes are mirrored by reflectors that show a tendency to con-

formably dip downward toward the channel axis. The bank reflectors

approximate the idealized depositional channel profiles of Hamilton

(1967) and Normark (1970). The many small angular discordances,

particularly below the thaiweg near the Pliocene surface in profile B,

IT1

and the numerous short hummock-like reflectors at depth, especially

beneath the smaller unnamed channel in profile A (Figure 11), are

interpreted as buried channel courses and suggest an upbuilding

depositional history. Profile A is quite similar in these details to a

long profile across the upper Bengal Deep-Sea Fan (Curray and Moore,

1971). Both the main channel and the smaller perched channel in

profile A appear to have grown upward, by as much as 0. 4 seconds

(325 meters) above their now buried older courses. It is significant

that they have also migrated somewhat to the north, that is to the

right looking downchannel. This is in opposition to the left hook

suggested by Menard (1955) for channels in the Northern Hemisphere.

It also contradicts the evidence for leftward migrating channels cited

by Nelson (1968) and Hamilton (1967).

Seismic profiles of the middle and lower channel (Figures 12

and 13) show a roughly systematic downchannel variation in the

nature of the bank reflectors. In profiles C and D, the non-leveed

north bank is convex upward and the bank reflectors tend to dip into

the channel axis. In profile E a convex bank dips toward the axis but

its gentle slope is abruptly terminated at a steep channel wall. Re-

flectors in the wall appear to be truncated. Profiles F, H and I show

the same truncated reflectors with no suggestion of a convex bank.

The south bank of the middle and lower channel always shows a sharp

break at the channel wall and truncated reflectors.

37

Some of the apparent truncation is without doubt due to hype r-

bolic side returns. Normark (1970) illustrated the contrast between

bathymetric profiles taken by a ship at the surface and those taken

near the bottom by a deep-towed sounder. The difference is striking.

Changes in bottom relief are obscured and even completely masked

by hyperbolic returns on surface profiles. The effect would be

mitigated in low frequency seismic profiling by the use of hydrophone

streamers whose front to back beam widths are quite small. One can

still however expect an artificial hyperbolic lengthening of internal

bank reflectors to some unknown extent into the channel. The effect

would be more pronounced in deep water and on steep channel walls.

It would also tend to decrease the channel cross sectional areas for

deeper profiles, particularly wide-beam bathymetric profiles.

The presence of flat and truncated reflectors is often used as a

criterion for establishing the erosional nature of deep- sea channels

(Normark, 1970). It is suggested that a certain caution should be

used when applying this criterion. Fine scale seismic profiling, as

from a deep-towed vehicle, may reveal that the apparently flat and

truncated reflectors may in fact have fine scale depositional charac-

teristics near the channel wall that are masked by hyperbolic effects.

Furthermore, the transverse velocity profile of a channelized tur-

bidity current is without doubt not constant over the whole flow-

sediment interface. It is therefore easy to envision different

phenomena occurring over the channelized width of a turbidity

current, particularly when one realizes that typical channel profiles

are one to two orders of magnitude wider than they are deep. Thus,

deposition may occur at the bank crest synchronous with transport in

mid-channel. Similarly, transport may be taking place along the bank

wall while erosion is occurring in mid-channel and spillover deposi-

tion taking place on the bank. And, since the size, frequency and in-

tensity of these periodic flows is apt to be quite variable, different

flow events are quite likely to cause some variation in the kind of

process that takes place at any given point in the channel. Channel

profiles most probably reflect some sort of average effect of the

larger flows. With these qualifications in mind then, it is perhaps

valid to characterize portions of a channel as depositional, transpor-

tational or erosional. Several other criteria need to be applied.

Seismic crossings of lower Surveyor Deep-Sea Channel

(Figure 13) show hyperbolic returns from the channel walls ei±ending

to depth in the sediment beneath the channel thalweg. It has been

pointed out that measured cross-sectional areas of lower channel pro-

files are much larger than predicted values. The presence of

hyperbolae suggest that the cross-sectional areas may in reality be

even larger for these deeper crossings. The channel thaiweg itself

is found to be deeper than the Pliocene channel basal reflectors, and

the axial gradient here is steeper than the middle and upper channel

39

gradients. For all of these reasons, the lower channel as a whole is

here considered to be erosional. The obvious fill in the bottom of

channel crossing H is evidence of the variability that can and does

occur in channel related turbidity current processes. For the criteria

previously cited (upbuilding above the basal reflectors, the presence

of levees, axially dipping bank reflectors, evidence for migration and

cut and fill, low channel relief, and small cross- sectional area) the

upper channel is here considered to be dominated by depositional

processes. And for similarly consistent criteria, the regimen of the

middle channel is considered to be largely transportational.

CHANNEL RELATED SEDIMENTS

Long seismic profiles of the northern Alaskan Abyssal Plain

show that the seismic units previously described extend with fairly

uniform thickness throughout the region. Profiles I through III

(Figure 9) reveal that the entire sequence of seismic units, including

the channel related uppermost Units A and B (Figure 8) are buried be-

neath trench fill to the north. The interface between seismic Units B

and C, that is between the channel related and the prechannel sedi-

ments, is nowhere seento possess characteristics indicative of any

older buried channels. The sedimentary section described at

D. S. D. P. Site 178 indicates that the sediments making up Units C and

D are nonetheless turbidites (von Huene et al. , 1971) and therefore

are probably related to either a pre- exi sting channel system,

similar in orientation to Surveyor but not now found, or are derived

from a northern pre-trench source. The channel related sedirrnts,

comprising seismic Units A and B, recovered at D. S. D. P. Site 178

are finer grained silt turbidites in a glacial gray mud.

Thirteen piston cores were recovered from the Gulf of Alaska

during the 1970 cruise of the Oregon State University Research

Vessel Yaquina. They include three cores from the axis of Surveyor

Deep-Sea Channel, three from the channel banks, six from the

Alaskan Abyssal Plain and one from the easternmost Aleutian Trench

41

axis (Figure 1). For the purposes of this effort, the cores have only

been examined for gross textural features.

Without exception, all thirteen cores exhibit characteristics

that allow them to be classified as turbidites. A very few contain

graded sand and silt members, and the medium gray silty clay

making up the bulk of the cored material is occasionally finely laminated

(Appendix IV). Relatively thick sand members are found in the axis cores,

and occur near the top of three of the abyssal plain cores. All of the

abyssal plain and bank cores except one contain frequent fine lenses

of silt or sand (Figure 14). They also contain abundant ice- rafted

pebbles. Similarly, the occurrence of sand grains homogenously

distributed throughout silty clay members suggests transport by ice-

rafting. The overall lithology is seen to be quite similar to that

describing the upper 270 meters of sediment recovered from D. S. D. P.

Site 178 (von Huene etal., 1971), and suggests that glacial and tur-

bidity current activity have been continuous throughout the region

from Pliocene time to the present.

The textural contrasts within and between the channel axis

cores indicate a high degree of unpredictability in the downchannel

energy variation of channelized turbidity currents. The presence of

thick sand members near the tops of two of the axis cores, including

the most distal Core Y70-2-35 suggests that turbidity current

activity is presently operational in the channel Itself.

42

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Figure 14.O

ccurrence of coarse material in piston cores.

43

The spatial distribution of the three bank cores does not allow

discussion of any systematic downchannel textural variation in the

bank sediments. It is interesting however that Core Y70-2-37 con-

tains almost 850 cm of only faintly laminated silty clay. No silt or

sand lenses occur throughout that length of the core. The core was

recovered from the north bank of the lower channel. The high relief

of the lower channel may preclude coarse grained recent spillover

from all but the largest flows.

Several abyssal plain cores contain near surface sand members, again

suggesting that turbidity current activity is presently operational

even over the abyssal plain. There is the suggestion in the distribu-

tion of coarse grained materials that sand members may be more

common in areas near the channel or the continental slope than in

central plain regions. There also seems to be a similar variation in

the frequency of occurrence of coarse lenses in general. Alterna-

tively, this variation may represent a tendency toward the thickening

of individual turbidite sequences with distance from the channel or

slope. A similar trend was found to occur in Cascadia Channel

(Griggs and Kulm, 1970b).

The presence of thin sand and silt lenses even in the most

central portions of the Alaskan Abyssal Plain strongly implies that

spillover from turbidity currents is not confined to regions close to

the channel, Similarly, the ubiquitous nature of seismic Units A and

B suggest that channel related turbidites make up the entire abyssal

plain. Griggs (1969) was able to trace the sediment from less

vigorous Holocene flows to a lateral distance of 17 km away from

Cascadia Channel. Presumably, more vigorous glacial turbidites may

be capable of more intense spilling. The uniform thickness and

character of seismic Units A and B and the presence of turbidites

throughout the abyssal plain can be most reasonably explained in

terms of very broad, sheet flowing, and fine grained currents contin-

uously spilling from the channel and traveling over large expanses of

the abyssal plain. The higher velocity, more turbulent and coarser

grained portion of any flow would be confined to the channel. It is

important to stress that the channel and its related sediments, corn-

prising seismic Units A and B, seem to be related to the initiation of

glaciation onshore. Similarly, Site 183 of D. S. D. P. Leg XIX on the

Aleutian Abyssal Plain to the west, yielded cores that contain glacial

erratics to an approximate depth of 130 meters near the base of the

Pliocene (Scholletal., 1971). Site 178 of D.S.D.P. Leg XVIII

yielded a similar age for the initiation of glacial activity.

45

CHANNEL RESPONSES TO REGIONAL TECTONISM

In two aspects Surveyor Deep-Sea Channel shows a pronounced

response to regional tectonic influences, the scale of which makes it

unique among deep- sea channels. In addition to those morphological

and structural characteristics already discussed, there is evidence

that strongly suggests that the upper channel was formed near the base

of the continental slope by the coalescing of broad, sheet flowing,

turbidity currents. This unusual origin is suggested by the location

of the channel relative to regional bathymetric and tectonic features.

The erosional nature of the lower channel is seen to be consistent

with the regional late Cenozoic history of lithospheric plate inter-

actions.

The Eastern Source of Sediments

Bathymetric maps of the northern Gulf of Alaska (Menard and

Dietz, 1951; Gibson, 1960; Hurley, 1960) indicate that Surveyor Deep-

Sea Channel is not contiguous with any prominent submarine canyon.

This is unusual in that other known deep-sea and fan channels appear

to be proximally associated with canyons. Examples include

Cascadia Channel and Willapa Canyon (Griggs, 1969), Astoria Fan

Channel and Astoria Submarine Canyon (Nelson, 1968; Carlson,. 1968)

and Monterey Canyon and Monterey Deep-Sea Fan Channel (Martin and

Emery, 1967; Wilde, 1965). Chase, Menard and Mammerickx, on

their chart of the bathymetry of the North Pacific (1969), show a

prominent submarine canyon on the continental slope west of Alsek

Strath contiguous with Surveyor Deep-Sea Channel. Bathymetric and

sub-bottom seismic records from a single crossing of this area by

the R/V Yaquina (Figure 1) fail to show the feature. Negative evidence

from a single crossing in an area of high relief hardly rules out the

possible existence of such a canyon, but it is also significant that no

canyon appears to the north opposite Yakutat Seavalley. The smaller

channel shown in seismic profile A (Figure 11) is assumed to trend in

this direction. This conclusion is supported both by the location of

the channel and by the fact that its south levee is slightly higher than

its north levee. The levee relationship can be explained if the channel

course here is turning sharply to its right, allowing the centrifugal

accelerations on turbidity currents to equal or slightly exceed the

Coriolis accelerations. This argument aside the bathymetry of the

upper channel region is also unusual.

Large lobate bathymetric highs on either side of the channel

near the lower continental slope seem to confine the channel to the

broad bowl-shaped depression indicated by the 3400 and 3600 meter

bathymetric contours shown in Figure 2. Sub-bottom seismic

profiles across the lobes do not penetrate to a recognizable acoustic

basement. In fact, a definite basement is not resolved on any of the

47

seismic profiles east of Giacomini Seamount. It is therefore not

possible to determine whether the lobes have any deep structural

control. In any event, it is unusual that the upper channel is confined

to a broad bowl-shaped depression instead of being situated on a

bathymetric high or a conate fan.

The main trace of the Chugach-Fairweather Fault was shown by

Stoneley (1967) to lie just east of the coast line behind Yakutat and

Dry Bays (Figure 1). In this region it is only about 70 km inshore

east of Yakutat Bay, 30 km inshore near Dry Bay, and exits the coast

at Cross Sound at about 137° west latitude (Figure 1). Stoneley pro-

posed two periods of intense folding and thrust faulting in the Gulf of

Alaska sedimentary province, one in the late Cretaceous - early

Tertiary and another in the Plio-Pleistocene. He also found evidence

for dextral strike- slip motions along the Chugach-Fairweather Fault

system. St. Amand (1957) mapped the Queen Charlotte Island Fault

system on the Chichagof, Baranof and Queen Charlotte Islands.

McKenzie and Parker (1967), Morgan (1968) Le Pichon (1968), Isacks,

Oliver and Sykes (1968) and others have treated the Fairweather-

Queen Charlotte Island Fault system as a large ridge-trench trans-

form fault extending from the Explorer Ridge, seaward of northern

Vancouver Island, to the subduction zone landward of the Eastern

Aleutian Trench. The dextral fault system constitutes a boundary

between the North American and Pacific Plates.

Morgan (1968) has estimated the present angular rate of motion

between the North American and Pacific Plates to the 6 x 10

degrees/year about a rotation pole at 530 N., 530 W. At the rotational

equator, this corresponds to a 6. 7 cm/yr velocity for motion of the

Pacific Plate relative to North America, The latitude of the trans-

form fault system relative to the pole of rotation is about 450,

therefore the relative rate of dextral offset across the system should

be about 4.4 cm/yr. Heirtzler and others (1968) show that this sense

of plate motion has been in effect at least since anomaly 5 time about

10 m. y. B. P. The implication for Surveyor Deep-Sea Channel -is that,

since its origin at no more than 5 m. y. B. P., it has moved approxi-

mately 220 km to the north with respect to its initial sediment source

on the adjacent continent. At the time of its Pliocene origin the

upper channel would have been located southwest of Cross Sound.

Therefore, unlike other major deep-sea channels, Surveyor has not

been consistently linked to any large river drainage system that

might serve as a sediment source.

The Pacific Plate in moving north has been carrying with it the

thin slice of continental crust west of the Chugach-Fairweather-

Queen Charlotte Island Fault system. South of Icy Bay, this slice is

composed almost totally of the Mesozoic Eugeosynclinal facies of the

Yakutat group (Støneley, 1967). North of Icy Bay, the Plio-

Pleistocene Yakataga Formation consists of a lower marine mudstone

49

- siltstone - silty sandstone member and an upper conglomeratic

silty mudstone member containing glacially striated boulders up to

35 meters in diameter. Coupled with a high variability in the

measured thickness of units, the conglomeratic facies lead Stoneley

to conclude that the region began undergoing intense deformation and

glaciation by early Pliocene times.

Glacial activity in the region is substantial at present and must

have been even more intense during Plio- Pleistocene cold cycles.

Major glaciers exist throughout the region forming an almost con-

tinuous ice sheet in the mountainous terrain east of the Gulf of

Alaska. Several large piedmont glaciers extend to the present shore-

line, During the late Pleistocene, the area was completely covered

with ice (Coulter et al,, 1962), The Malaspina Glacier between

Yakutat Bay and Icy Bay is presently more than 70 km wide and 3. 5

km thick at its center, and is in rapid retreat. The neighboring

Guyot Glacier has retreated 40 km up the fjord at Icy Bay in 60 years.

Wave-cut platforms are elevated more than 1, 5 kilometers above sea

level and Stoneley suggests that the deep glacial-cut fjords were cut

below sea level during colder cycles. The broad depressions of

Yakutat Seavalley and Alsek Strath extend from Yakutat Bay and Dry

Bay across the continental shelf, Similar structures are found west

of Icy Bay (von Huneetal,, 1971), These features show both a

genetic structural control, being situated along synclinal fold axis,

1i

and a glacial modification by erosion and subsequent filling. They

constitute evidence that the glacier system extended across the entire

shelf during cold cycles.

In addition to the foregoing, the region is characterized by high

seismic activity. The area from Icy Bay to Cross Sound has experi-.

enced at least four earthquakes of greater than 7.0 magnitude in

historic time (Davis and Echols, 1962), and is marked by a high

strain release (Berg, 1964).

A consistent and unique picture emerges. In the early Pliocene

intense folding, faulting and uplift along the continental margin of the

northeastern Gulf of Alaska, coeval with the initiation of glaciation,

provided large volumes of poorly sorted coarse clastic material

along a broad front on the outer continental shelf and upper slope.

Intense earthquake activity would periodically initiate wide, massive

slumps in this material that would then move down the continental

slope and entrain large volumes of water in transit, Upon reaching

the base of the slope, this sheet flowing mass would begin to deposit

some of its suspended load and move onto the abyssal plain. Here it

would be constrained and funneled inward by pre- existing bathymetric

highs, which may represent oceanic crustal analogs of the folded

structures found on the immediately adjacent continental shelf. The

funneling would result in the development of high velocity jets within

the flow. Fan channels would be developed in the sediments by the

51

jets and the channels themselves would then coalesce, resulting in a

pattern somewhat similar to a river tributary system.

This is admittedly a very speculative suggestion, but it has

appeal in that it satisfies a wide variety of the unusual conditions

found in the study area. Further examination of the upper channel

seismic reflection records reveals that both of the channels in pro-

file A (Figure 11) are unfilled and appear active. The contact be-

tween their levee deposits implies a synchronous development. In

addition, the anomalously low measured cross sectional area of the

southernmost channel in profile A further suggests that it is only

acting as one of perhaps several synchronously active tributaries to

the main channel farther west.

Downwarping of the Lower Channel

Figure 15 is a transverse mercator projection of the northeast

Pacific. It is generated about the 53° N, 53° W pole of rotation for

the Pacific and North American Plates (Morgan, 1968). The geo-

metry of plate motions requires that active fracture zones be small

circles on a sphere about the pole of rotation. On such a projection,

the trace of active fracture zones and the directions of plate motions

should be horizontal. New crustal material is thought to be generated

along the ridge system while folding and the subduction of crustal

material occurs along the central and eastern Aleutian Trench.

- - --r%4ER OUE

.::. 4U(.T \ ', -

1

' ' N \ \ ,,(

':q7 \(O\ \ ,:), ;,' /

: )'' i,ii Ii ,' 3,ç ,'/';<i' c';/'\tl"/ ' /

\\ ,/ / DIRECTION OF PACIFIC PLATE MOTION

IAN TRE$C / / WITH RESPECT TO N. AMERICA

S-'-/ / KAMCHATKA

/ / GULF OFALASKA SEAMOUNTS AND GUYOTS

.,o \ o"'AMILIA F FAULTS AND FRACTURE ZONES

(' ç" 'ADAK F z MAGNETIC ANOMALIES

' SPREADING CENTER

Figure 15. Transverse mercator projection of North Pacific aboutrotation pole at 530 N, 530 W. Magnetic anomaly N)

pattern is after Peter etal., 1970.

53

Transform motion of the crust should occur along the offsets in the

Gorda-Juan de Fuca-Explorer Rise, the Queen Charlotte Island-

Fairweather Fault system and the western Aleutian Trench. Slip

vectors from earthquake mechanism studies (Isacks, Oliver and

Sykes, 1968; Stuader, 1968a, 1968b) support the model, showing com-

patible strike- slip motions along the faults and down thrusting. motions

below the island arc, while the western Aleutian Trench shows focal

mechanisms that are virtually tangential to its physiographic ex-

pression. The northernmost corner of the Pacific Plate is poorly

defined in that a thin slice of continental crust is moving with the

oceanic plate, and the bathymetric expression of the eastern Aleutian

Trench dies out slightly to the west. The previously mentioned folded

structures on the continental shelf however have orientations con-

sistent with compressive interaction between the plates.

Several histories have been developed for the Aleutian Trench.

Hurley (1960) suggested that the fossil turbidites found buried be-

neath pelagic sediments on the eastern Aleutian Abyssal Plain were

isolated from their northerly sediment source by a downbowing of

the Aleutian Trench only 85, 000 years B. P. This date was based

upon extrapolated sedimentation rates. Burk (1965) suggested that

the Aleutian Trench is probably as young as early Pliocene, this date

being compatible with the geology of the Alaskan Peninsula.

Hamilton (1967) estimated the formation of the trench to have

54

occurred in early to middle Tertiary time based on abyssal plain

sediment thicknesses and probable rates of deposition. Pitman and

Hayes (1968) attempted to integrate the history of the northern Gulf of

Alaska with the motion of oceanic plates as deduced from magnetic

anomaly patterns. Their diagrams suggest that the central Aleutian

Trench has been in existence since Cretaceous time and that no trench

east of Kodiak Island existed until early Pliocene. Von Huene and

Shor (1969) proposed that the eastern Aleutian Trench is younger than

the central Aleutian Trench and that it developed in Pliocene time

based on volumes of trench fill. Grow and Atwater (1970) elaborated

on the model of Pitman and Hayes and cited a history of Aleutian

Island volcanic activity and Gulf of Alaska tectonism compatible with

the motion of a now missing lithospheric plate. Their model would

seem to require the continuous existence of the eastern Aleutian

Trench since middle Tertiary time. The newest evidence applicable

to the problem of dating the eastern Aleutian Trench suggests that

subsidence off of Kodiak Island began in the middle Pleistocene (Kulm

etal., 1972). From D. S.D. P. Leg XVIII Site 180 data they found one

km of late Pleistocene fill in the eastern Aleutian Trench. Pleistocene

mudstones, appearing as acoustic basement, were also found to form

the landward wall of the trench.

Seismic Profiles II through V (Figure 9) show that the course of

lower Surveyor Deep-Sea Channel has been constant since its

55

Pliocene origin. This implies that a regional gradient to the north

has been in effect at least since that time. Evidence previously dis-

cussed indicates that steepening, with consequent erosion in the lower

channel, occurred after the channel related abyssal plain sediments

were laid down. This is compatible with the Pleistocene downbowing

of the trench suggested by Kulmetal. (1972). If, however, instead

of a simple static downbowing of the trench in Pleistocene time, we

assume a continuous northward motion of the Pacific Plate with

respect to North America since anomaly 5 time, a different possi-

bility emerges.

Figure 16 shows the early Pliocene position of the Pacific Plate

with respect to a fixed North American Plate 5 m. y. B. P. assuming

the rate of closure suggested by Morgan (1968). This corresponds to

anomaly 3 time according to the time scale of Heirtzler and others

(1968). A comparison of the Pliocene position of the Gorda- Juan de

Fuca.-Explorer Rise complex with its present position (Figure 15)

suggests very little east-west motion of the rise relative to the

Pacific Northwest in the last 5 in. y. The half rate of growthof the

rise, based on the distance between anomalies, is sufficient to allow

it to roughly maintain its distance from the coast while situated on the

moving Pacific Plate. Hypothetical extensions of the rise are mdi-

cated north of the Sila Fracture Zone. The proposed pattern is

similar to that of the East Pacific Rise in the Gulf of California, as

56

indicated on the inset to Figure 16, and follows the trace of the Queen

Charlotte Islands- Fairweather Fault system. Prior to anomaly 5

time, spreading on the Pacific Plate was east-west with respect to

North America (Pitman and Hayes, 1968). Between anomaly 3 and

anomaly 5 time (5 to 10 m.y. B. P.) the present spreading orientation

began.

The magnetic patterns north of the Aja Fracture Zone are

poorly known and it is here proposed that a north-south rise crest,

similar to that proposed by Pitman and Hayes may have existed in

the northeastern Gulf of Alaska. About 5 million years ago, this

spreading center intersected the coast of the northeast Gulf of Alaska

and began shearing and uplifting blocks of the continental margin.

Compressive stresses east of the rise segments became tensional for

those blocks underridden by the rise. The resulting isostatic uplift

and faulting is compatible with the evidence for Pliocene orogeny

cited by Stoneley (1967). The initiation of intense glaciation at this

time coupled with the orogeny resulted in large volumes of sediment

being input to the Gulf of Alaska from these eastern sources. This is

in accord with the presence of glacial erratics to a depth of Z70 meters

in the drill hole at D. S.D. P. Site 178 (von Huene etal. , 1971). At

this time, Surveyor Deep-Sea Channel began to form on the abyssal

plain which gently dipped to the northwest. Continuous northward

motion of the Pacific Plate slowly pushed the lower reaches of the

TIAN

I ( 4.%E

ADWI

Ui!

-

-'S_. - -

.N /

i( N '\, / ° (o N.'. ' ' N / '

\ l,/p. / \ ,'\ ,/ \ / /

,/- / /

/ z / / /

\ ' / DIRECTION OF PACIFIC PLATE MOTION/ No" c::::J WITH RESPECT TO N. AMERICA-

.- / GULF OF ALASKA SEAMOUNTS GLJYOTS\'_' .___\ \'-\ 'o'b0 \i3O'

FAULTS AND FRACTURE ZONES

MAGNETIC ANOMALIES- SPREADING CENTER

Figure 16. Transverse mercator projection of North Pacificearly Pliocene lime0

channel over the seaward rise of the trench and erosion began.

It is difficult to determine from the data available if the eastern

Aleutian Trench was in existence in Pliocene times, or if the site of

the present trench was simply occupied by a gentle syncline. The

fossil turbidites of the eastern Aleutian Abyssal Plain described by

Hurley (1960), Hamilton (1967) and others, are found to be associated

with buried channels having a northeast trend and described by Grim

and Naugler (1969), Mammerickx (1970), Naugler (1970) and others.

Recent research by Jones, Ewing and Truchan (1971) suggests that

turbidity current activity in the eastern Aleutian Abyssal Plain

ceased about 6,9 m. y. B. P. , the time of isolation being determined

from extrapolated sedimentation rates and seismic profiles, using a

1.70 km/sec seismic velocity for the overlying abyssal plain sedi-

ments, This seismic velocity seems high. Hamilton (1969) gives

seismic velocities for pelagic sediments in the North Pacific that

range from 1.49 to 1. S3 km/sec. This is about 10% less than the

1.70 km/sec value and would result in a younger date for the cessa-

tion of turbidite deposition.

Naugler (1970) suggests that the easternmost Aleutian Channel

represents one of the last stages of turbidite deposition on the

Aleutian Abyssal Plain, the fossil Seamap Channel being cut off

earlier as a result of being closer to the trench. He raises the

possibility that the fossil Aleutian Channel may have been linked to

59

Surveyor Channel prior to capture of the latter by the downbowing

trench. A possible early Pliocene connection between the two

channels around the central hill and seamount province is shown in

Figure 16, The curvature required is less than that exhibited by

Cascadia Channel where it encounters the Blanco Fracture Zone and

abruptly turns to the west through Cascadia Gap to reach Tufts

Abyssal Plain. The relative depths of the channels of the two

abyssal plains are compatible with such a reconstruction. The real

problem centers around the relative dates of the turbidites. Most

published evidence suggests that the youngest Aleutian Plain turbidites

are at least slightly older than the basal reflectors of Surveyor Deep-

Sea Channel, however the age of the turbidite horizons near the upper

reaches of Aleutian Channel has yet to be determined.

Jones, Ewing and Truchan (1971) report a pelagic sediment

thickness of about 120 meters near the fossil Seamap Channel on the

Aleutian Abyssal Plain. D. S.D. P. Site 183 is located only slightly to

the north of Seamap Channel, As previously mentioned, the earliest

glacial records in cores recovered from that site occur at an esti-

mated depth of 130 meters, near the base of the Pliocene, This

corresponds to an average glacial sedimentation rate of about 25 m/

a rather high figure for pelagic sedimentation, This rate is

also twice as high as any sedimentation rate found by Jones and all,

and more than three times higher than the average rate they used to

determine the time at which turbidity current activity ceased on the

Aleutian Ahyssal Plain. Certainly a case can be made for more

recent, channel related, turbidite deposition.

Several possibilities arise. If Seamap Channel was connected

to a northern source of sediments, the Aleutian Trench may have

formed here as recently as middle or even early Pliocene.

Alternatively, if Seamap Channel was connected to an eastern source,

on the Alaskan Abyssal Plain, the trench could be older and Seamap

Channel would have been isolated when its eastern portion was

carried into the trench. The possibility exists that another deep-sea

channel may have been located on the Alaskan Abyssal Plain north of

Surveyor.

The Pleistocene age determination of Kulmetal. (1972) for the

genesis of the eastern Aleutian Trench certainly represents a

youngest limit. It may have originated earlier but the high rate of

burial in the axis, coupled with continuous subduction, would cer-

tainly mask any seismic evidence for a pre- Pleistocene trench.

61

CONCLUSIONS AND GEOLOGIC HISTORY

The morphology of Surveyor Deep-Sea Channel is found for most

of its length to be in agreement with that predicted by turbidity current

dynamic models. The measured contrasts in levee or bank heights

on either side of the entire channel are consistent with an assumed

depositional equilibrium between the channel morphology and turbi-

dity currents spilling the channel banks in response to Coriolis and

centrifugal accelerations.

That part of the channel east of Giaconiini Seamount also dis-

plays variations in center channel relief and cross sectional area

that are consistent with prediction. As the axial gradient decreases

downchannel the center channel relief increases in response to

changes in the slope dependent Froude number of turbidity currents.

Similarly, the measured cross sectional area of the channel in-

creases in response to changes in the slope dependent body velocity

of turbidity currents.

For that part of Surveyor Deep-Sea Channel west of Giacomini

Seamount the axial gradient increases sharply as the channel turns

northward and plunges into the eastern Aleutian Trench. The mea-

sured center channel relief and cross sectional area of this portion

of the channel continue to increase, dramatically contradicting pre-

diction. Sub-bottom seismic reflection profiles reveal that the

62

anomalous morphology of the lower channel is due to erosion in

response to a post- depositional increase in the axial gradient.

The uniform thickness and gentle declivity of seismic units, and

the continuity of individual sets of reflectors over wide expanses of

the Alaskan Abyssal Plain suggest that channel related turbidity

current activity has been responsible for the deposition of terrigen-

ous sediments over most of the northern Gulf of Alaska. The tex-

tural characteristics of sampled sediments support this conclusion,

there being a general increase in the number and thickness of coarse

grained units with proximity to both the continental slope and

Surveyor Deep-Sea Channel.

Both the well developed geologic history of the southeastern Alaskan

landmass and the stratigraphy of a ubiquitous and characteristic set

of reflectors commonly found near the channel thalweg suggest that

Surveyor Deep-Sea Channel originated in early to middle Pliocene time

coeval with the initiation of major glaciation and pronounced tectonism

in southeastern Alaska. These two processes would provide large

volumes of poorly sorted clastic material to the outer shelf and upper

slope of the continental margin. Earthquake activity, here in an area

of extreme seismic instability, would periodically initiate massive

slumping and failure in these deposits along wide expanses of the

upper slope. The resultant broad and trubulent sediment suspensions

may have been partially funneled inward by lateral bathymetric highs

upon reaching the base of the continental slope. Here the more

coarse grained materials within the flow would be deposited and high

velocity jets would be developed near the base of the sheetfiowing

turbidity current. A system of tributary channels would then co-

alesce forming the major course of Surveyor Deep-Sea Channel. The

genesis of the channel may therefore be unrelated to turbidity current

activity within a submarine canyon. In addition, its related sediments

were obtained directly from glacial sources unmodified by river

transport.

Implications from plate tectonic theories suggest that at the

time of its initial formation, Surveyor Deep-Sea Channel was located

more than 200 kilometers to the south of its present proximal position

with respect to sediment sources and the adjacent southeastern

Alaskan landmass. In the early history of its development, it may

have been distally contiguous with one of the fossil deep-sea channels

of the eastern Aleutian Abyssal Plain.

Motion of the Pacific plate with respect to North America re-

suited in the capture of Surveyor Deep-Sea Channel by the eastern

Aleutian Trench. This event largely isolated the Aleutian Abyssal

Plain from continental sediment sources. It also caused an increase

in the axial gradient of that portion of Surveyor Deep-Sea Channel

west of Giacornini Seamount, as the northern edge of the oceanic

plate was downwarped into the Aleutian Trench. Erosion by

64

accelerated turbidity currents is thought to be responsible for the

dramatic increase in center channel relief and cross- sectional area

of the lower reaches of the present course of the channel.

65

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von Huene, R., L. D. Kuim, J. R. Duncan, J. C. Ingle, S. A. Kling,L. M. Musich, D. J. W. Piper, R. M. Pratt, H. Schrader,0. Weser and S. W. Wise, Jr. 1971. Deep Sea DrillingProject Leg XVIII: Geotimes, 16(10), p. 12-16.

Wilde, P. 1965. Monterey Deep-Sea Fan: Ph. D. Thesis, HarvardUniversity, Cambridge.

APPENDICES

APPENDIX I. Location and Physiography of Channel Profiles

Crossing Latitude LongitudeAxialDepth

CenterChannelRelief

AxialGradient Cross Sectional Area

A 58008.41 N 141°41.0' W 3707 rn 99 m 1.4 rn/km 2.98 x105m2 ± 10%

B 58°06.O' 143°02.2' 3830 117 1.3 4.58 7%

C 57°46.9' 143°53.5' 3920 168 0.8 5.74 8%

D 57°32. S 144°41. 0' 3965 165 1.2 5.39 8%

E 57°20.1' 145°25.1' 4027 168 0.9 5,65 11%

F 56033.21 146041.51 4130 190 2.0 3.50 15%

G 56°04.8' 147°14. 3' 4254 212 2.4 4.39 9%

H 55°40.9' 148019.1! 4461 242 4.9 9.76 10%

I 5504601 149°00.5' 4689 366 7.5 7.07 14%

J 55°486' 149°12.0' 4804 421 5.8 787 8%

K 55°530' 149°28.8' 4912 450 3.5 11.55 10%

L 56°lO.5' 150008.0I 5107-J

APPENDIX II. Transport Calculations from Channel

71

Profiles

h Asin p tan p (m)

1

(h sin f3 10 5m2 Q (h sinp)A

A .0014 99 .3722 2.98 1.11

B .0014 117 .4047 4.58 1.85

C .0009 168 .3888 5.74 2.23

D .0009 165 .3854 5,39 2.08

E .0009 168 .3888 5,65 2.20

F .0021 190 .6317 3,50 2.22

G .0021 212 .6672 4.39 2.93

H .0056 242 1.1641 9.76 11.36

I .0056 366 1.4316 7.07 10.12

J .0056 421 1.535 7.87 12.08

K .0056 450 1.652 11,55 19.08

72

APPENDIX III, Magnetic Stratigraphy of Core Y70-4-56

Sample Demagnetization SpecificDepth Level Intensity(cm) (oe) Declination Inclination (emu/cm3)

71- 74 0 324° 710 118

75 206 44 53

177- 179 0 53 76 22

75 186 45 11

277- 279 0 19 63 31

75 160 40 7

370- 372 0 351 67 22

75 186 39 5

459- 461 0 305 54 47

75 231 32 24

577- 579 0 289 70 23

75 210 36 12

674- 676 0 290 43 90

0 290 42 92

25 291 41 91

50 288 41 87

75 288 41 84

100 286 42 84

125 286 42 80

73

APPENDIX III (Continued)

SampleDepth(cm)

DemagnetizationLevel

(oe) Declination Inclination

SpecificIntensity

(emU/cm3)

674- 676 150 286° 43° 76

175 284 42 72

745- 747 0 304 61 95

75 236 31 53

859- 861 0 316 63 45

0 314 63 42

75 220 37 22

1051-1053 0 285 66 60

75 214 40 41

1151-1153 0 99 68 80

75 137 38 57

1258-1260 0 47 74 70

75 145 40 37

1464-1466 0 18 63 30

75 129 37 9

1567-1569 0 71 53 44

25 68 55 44

50 63 53 41

75 58 53 41

74

APPENDIX III (Continued)

Sample Demagnetization SpecificDepth Level Intensity(cm) (oe) Declination Inclination (emu/cm3)

1567-1569 100 115° 23° 26

125 126 26 21

150 109 27 24

1659-1661 0 130 73 37

0 118 75 32

75 156 40 20

1696-1698 0 230 -51 58

75 347 -44 19

1710-1712 0 355 -31 114

25 351 -30 115

50 351 -29 117

75 350 -30 117

125 343 -42 105

150 344 -44 101

1733-1735 0 240 -69 16

0 327 -45 14

75 320 -38 15

1751-1753 0 275 -34 32

0 282 -32 31

75 293 -21 32

APPENDIX IV. Alaskan Abyssal Plain Piston Core Locations and Descriptions

Core No. Latitude Longitude Length Description

Y70-2-35 55° 49.7'N 149° 10.9'W 400 cm

Y70-2-37 56° 08,5'N 147° 20.O1W 1211 cm

Y70-2-39 550 59.9'N 146° 24.3'W 1207 cm

Y70-2-40 56° 31.O'N 143° 48.9tW 988 cm

Y70-2-41 57° 10.2'N 141° 03,6'W 1151 cm

Lower channel axis core. Massive cleanmedium green and black sand throughout.

Lower channel north bank core. Massive,only occasionally laminated, medium green-gray silty clay. Two prominent sandmembers occur near the core bottom andone occurs at the very top.

Abyssal plain core south of lower channel.Medium gray silty clay with abundant siltlenses and two prominent sand membersnear top. One sand lens in lower half ofcore.

Abyssal plain core south of middle channel.Medium gray silty clay with abundant siltlenses and two prominent sand membersat top. Several closely spaced sand lensesin lower half of core.

Abyssal plain core south of upper channel.Medium gray silty clay with abundant sandlenses throughout. Upper two meters onlyfaintly laminated.

-JU.'

APPENDIX IV (Continued)

Core No. Latitude Longitude Length Description

Y70-3--48 58° 07.8'N 141° 40.9TW 537 cm

Y70-3-49 58° 10. 61N 141° 39.O'W

Y70-3-50 58° 06.1'N 141° 38.8'W

Y70-4-51 59° 05. OtN 143° 40.Z'W

590 cm

595 cm

588 cm

Y70-4-52 58° 54.6'N 143° 42.4'W 1128 cm

Upper channel axis core. Medium graysilty clay. Massive sand units up to 115 cmthick in upper half of core. 30 cm ofpebbles up to 7 mm at bottom of core.

Upper channel north bank core. Mediumgray clayey silt with abundant fine sandlenses throughout.

Upper channel south bank core. Mediumgray clayey silt with abundant fine sandlenses throughout.

Eastern Aleutian Trench axis core. Med-ium gray silty clay with occasional sandlenses.

Abyssal plain core near trench and lowercontinental slope. Medium gray silty claywith abundant sand lenses. Three sandmembers at top of core to 100 cm thick.

-J

APPENDIX IV (Continued)

Core No. Latitude Longitude Length Description

Y70-4-53 570 32.3'N 144° 43.O'W 417 cm Middle channel axis core. Massive non-laminated medium gray silty clay in upperhalf of core with several sand lenses inmiddle section. Massive fine sand membersup to 70 cm thick in lower half of core.:

Y70-4-54 56° 05. 2TN 140° 41. 3'W 1207 cm Abyssal plain core 200 km south of upperchannel. Medium gray silty clay withabundant silt lenses throughout.

Y70-4-56 53° 01.4'N 141° 41.4'W 1765 cm Central Alaskan Abyssal Plain core nearouter Baranof Fan. Medium gray mud withmany widely spaced silt lenses. Manyforam- rich zones and at least one ashlayer.

-J-J