HIGH-RESOLUTION SEISMIC SURVEY 

OF AN OFFSHORE AREA NEAR 

NOME, ALASKA 

High-Resolution Seismic Survey 

of an Offshore Area 

Near N orne, Alaska 

By A. R. T AGG and H. G. GREENE 

STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 759-A 

Description and interpretation of 

data from seismic records and 

bathymetric charts 

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-<ard No. 72-600367 

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

Washington, D.C. 20402 

Stock Number 2401-00288 
CONTENTS 

Page Page 

Abstract------------------------------------------------------------------------------------- A1 

1 

2 

4 

5 

5 

5 

9 

Interpretation- Continued 

Introduction _____________________________________________________________________________ _ Buried topography- subsurface structures _____________ _ A14 

15 

17 

17 

19 

19 

19 

22 

Geologic setting------------------------------------------------------------------------ Buried channels--------------------------------------------------------

Acoustical sinks-------------------------------------------------------· 

Glacial deposits·-------------------------------------------------------· 

Equipment, procedures, and methods __________________________________ _ 

Interpretation·--------------------------------------------------------------------------

Surface (sea-floor) topography------------------------------------· Faults-------------------------------------------------------------------------

FIGURE 

Beach ridges·------------------------------------------------------------· 

Fans or deltas·-----·------------------·-----·----------------------·--·-

Surface channels or depressions ___________________________ _ 14 

Bedrock _____________________________________________________________________ _ 

Summary and conclusions ______________________________________________________ . 

Selected references·------·--·--·-·---------------------------------------·----····--· 

ILLUSTRATIONS 

1. Index map of Alaska showing location of study area·-------------------------------------·-····-·-----------------------·----·-----····---·-

2. Generalized geologic map and cross section of the coastal plain at N orne .... ·-········--·--····-·-------------------------· 

3. Track chart of N orne nearshore seismic survey---------------------------------------·-----------------------------------------------------·------··· 

4-6. Maps showing: 

4. Surface and shallow subsurface features of the N orne nearshore area·-------------·------·--·----------------------

5. Deeper subsurface features of the N orne nearshore area·-----------------------------------------------·-------------·------

6. Shallower subsurface features of the N orne nearshore area-------------------------------------------------------------------

7-8. Line drawings and geologic interpretation of seismic profiles: 

Page 

A1 

3 

5 

6 

8 

10 

7. Profile A-A' across a buried stream channel, beach ridge, and acou.stical sinks·------------·---------·----- 12 

8. Profile across a fan .......... --------------------------------------·---------·--------------------·------------------·--------··-····--····--·-············· 12 

9. Map showing bottom topographic features off N orne......................................................................................... 13 

10-13. Line drawings and geologic interpretations of seismic profiles: 

10. Profile B-B' across buried channels·--·········--···············-·--····---·······--·········-·····------·--··---·-··--·-··-··-···············-- 14 

11. Profile C-C' across an ancestral channel of Snake River .... ·-----·------······-··-·-··---····-··---------·---···---------·-- 15 

12. Profile D-D' across a terminal and a recessional moraine .......... -----·--·-···-··-------------······---·····-···--···-·--···· 16 

13. Profile across the large bedrock fault zone east of Penny River........................................................... 18 

14. Map showing six offshore areas of possible concentration of placer gold and other heavy detrital minerals .................................................................... -···············--··-··-······--···-·--··-·······-·····--···-··-··---···········-········-·-·········-· 20 

III 

HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR 

NOME, ALASKA 

By A. R. TAGG and H. G. GREENE 

ABSTRACT 

A high-resolution seismic survey was made south of Seward 

Peninsula in Norton Sound, Alaska, during the summer of 

1967, to identify sites of possible offshore gold concentrations. 

Interpretation of the seismic records was confined primarily 

to the parts above the first sea-bottom multiple, generally 

less than 150 feet below the sea floor. Sediments within this 

interval consist mostly of unconsolidated Holocene sand and 

gravel covering Pleistocene glacial and marine sediments. 

Paleozoic metamorphic rocks are near or at the sea floor in 

the northwestern part of the area. 

Interpretation of bottom topography and subbottom structures disclosed many features that may be important in gold 

exploration. Bottom-surface features include at least five outwash fans, several beach ridges, and discontinuous stream 

channels. Subbottom features include many buried stream 

channels, areas of glacial drift, and at least three shallow 

faults. Also noted were acoustical sinks that have been interpreted as gravels associated with buried stream channels and 

beach ridges. In addition, six areas were delineated where 

placer gold may have been concentrated. 

INTRODUCTION 

This report describes and interprets features seen 

on high-resolution seismic-profiling records and 

bathymetric charts of an offshore area near Nome, 

Alaska, and discusses the possible significance of 

these features as placer-gold exploration sites. It is 

based upon a continuous seismic-reflection survey 

conducted in the summer of 1967 by the U.S. Department of the Interior. The. investigation was conducted 

jointly by the Geological Survey and the Bureau of 

Mines. The 1967 studies included offshore drilling at 

sites chosen on the basis of this seismic survey, sampling of the sea bottom and modern beach, seismicrefraction surveys of part of the modern beach, and 

onshore geologic studies. 

The continuous seismic profiles were interpreted 

jointly by the Geological Survey and the Bureau of 

Mines. Interpretation of the deeper reflectors, mostly 

of Tertiary age, was undertaken at the Marine Minerals Technology Center of the Bureau of Mines. This 

report describes the shallower reflectors, probably all 

of Quaternary age, that extend to depths of less than 

150 feet below the sea bottom. Information from drill 

logs and bottom samples taken in 1967 and 1968 was 

used in the interpretation of the seismic records. 

These data provide the basis for extending the coastalplain geology at Nome (Hopkins, 1967) into the 

offshore area. 

The study area is in the northwestern part of 

Norton Sound, an embayment of the northern Bering 

Sea (fig. 1) . The area extends 10 miles east and west 

of N orne, from Cape N orne to Rodney Creek, and up 

to 10 miles offshore. 

FIGURE 1.- Index map of Alaska showing location of study 

area. 

Several reconnaissance seismic-reflection studies, 

both shallow and deep, have been made of the Bering 

Sea. Moore (1964) discussed the results of a highresolution shallow seismic-reflection survey of parts 

of the Bering and Chukchi Seas. Scholl, Buffington, 

and Hopkins (1968) have made deep seismic-reflection surveys of the outer shelf areas of the Bering Sea, 

and Scholl and Hopkins (1969) extended this work to 

the northern Bering Sea. The results of a shallow 

Al 
A2 STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA 

seismic-profiling survey of the northern Bering Sea 

have been reported by Grim and McManus (1970). 

Private groups have undertaken seismic surveys of 

nearshore waters along parts of the south coast of 

Seward Peninsula, but these studies have not been 

published. 

We thank Warren Woodward and Arthur Daily 

of Shell Oil Co., Ralph Saleski, John Fagerstrom, 

and Ben Herman of the U.S. Army Corps of Engineers, and Burton Barnes, Gary Leo, Adolph Poston, 

and Dale Stevenson of the U.S. Bureau of Mines for 

assistance in various parts of the fieldwork. 

GEOLOGIC SETTING 

The N orne coastal plain and adjacent offshore 

areas are underlain by Pliocene and Pleistocene 

marine and glacial sand and gravel. A geologic map 

and a cross section of the N orne coastal plain are 

shown in figure 2. Separate deposits are recognized 

onshore and are locally known as Submarine Beach, 

Fourth Beach, Third Beach, Monroeville Beach, 

Intermediate Beach, and Second Beach. They were 

produced during six different sea-level stands in late 

Pliocene and Pleistocene time (Hopkins, in Hopkins 

and others, 1960, p. 46). Submarine Beach, Intermediate and Third Beaches;. and Second Beach are 

separated from one another by glacial drift of the 

Iron Creek (pre-Illinoian) and Nome River (Illinoian) Glaciations. Fourth Beach, Third Beach, and 

Monroeville Beach lie on bedrock beneath the N orne 

River drift. The glacial drift and marine sediment 

on the Nome coastal plain are overlain by alluvium, 

colluvium, windblown silt, and peat of Wisconsin 

and Holocene age. 

Paleozoic schist and limestone bedrock underlie 

the coastal-plain sediments and crop out north of 

them (Collier and others, 1908, p. 149; Hummel, 

1962a, b). Beach, glacial, and alluvial sediments 

occur offshore; however, substantial quantities of 

marine sand, silt, and clay are present at several 

subbottom levels. 

An older beach deposit is recognized only in the 

subbottom just offshore and west of the mouth of the 

Snake River (fig. 2). This buried beach, however, 

lies beneath water too shallow for continuous seismic 

profiling, and it was recognized as a result of compilation of borehole data and information on bedrock 

depths obtained from seismic-refraction studies 

along the beach (Greene, 1970a). This oldest shoreline deposit lies at a depth of about 70 feet below 

sea level and contains a Pliocene molluscan assemblage. It overlies bedrock with an irregular contact 

and is in turn overlain by glacial deposits (Hopkins, 

written commun., 1969). 

The next younger shoreline deposits, Inner and 

Outer Submarine Beach, lie at depths of -35 and 

-20 feet onshore at Nome, in the area between the 

lower course of the Snake River and the present 

shore. The Submarine Beach complex rests on bedrock in some places and on Pliocene ( ?) sediments in 

others. Submarine Beach has yielded pollen and 

mollusks of a late Pliocene or early Pleistocene age 

and is a product of the Beringian transgression 

(Hopkins, 1967, p. 53). 

Submarine Beach is overlain by four lithologic 

units: drift of the Iron Creek Glaciation of preIllinoian age, a layer of fossiliferous marine sand 

and gravel, till and outwash of the Nome River 

Glaciation of Illinoian age, and marine sediments of 

Second Beach of Sangamon age (fig. 2). Mollusks 

and pollen in the marine layer between the two drift 

sheets suggest that the marine layer is correlative 

with Intermediate Beach, a shoreline deposit on 

bedrock beneath N orne River drift at an approximate altitude of 20 feet (Hopkins, written commun., 

1969). Inland from Submarine Beach are three other 

shoreline deposits: Monroeville Beach at an altitude 

of 40 feet, Third Beach at an altitude of 70 feet, and 

Fourth Beach at 125 feet. All three transgress onto 

bedrock beneath the Nome River drift (MacNeil and 

others, 1943) . 

The Bering Sea, an epicontinental sea, was most 

likely emergent to late Tertiary time. Crustal warping then created the present marine Norton Basin, 

which is partly filled by a prism of Tertiary and 

Quaternary sediments having a total thickness of 

more than 6,000 feet in central Norton Sound 

(Scholl and Hopkins, 1969) . Although the bedrock 

surface beneath Norton Basin is an erosional surface, it was steepened by tectonic subsidence during 

development of the basin. The bedrock surface beneath the N orne coastal plain slopes rather smoothly 

seaward at about 25-35 feet per mile, but just offshore the gradient steepens abruptly to 130-230 feet 

per mile (U.S. Bur. Mines, unpub. data). 

Sediments that fill the Norton Basin include coalbearing strata at least as old as late Oligocene. These 

strata are exposed at the southwestern edge of the 

basin on St. Lawrence Island (J. A. Wolfe, written 

commun., 1968) but do not occur at Nome. The older 

beds, however, may be represented by some of the 

deeper seismic reflectors that appear to lap on to the 

basement reflectors several miles offshore from 

Nome (fig. 13) (Scholl and Hopkins, 1969). Paleogeographic reconstructions suggest that the Norton 

Basin was first invaded by the sea in late Miocene 

time (Hopkins, 1967, p. 454). Marine clay containing fossils of early Pliocene age was brought up 
HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME,. ALASKA A3 

64° 

30' 

300' 

200' 

100' 

SEA 

A 

LEVEL 

.c 

:;: 

~ 

165"30' 

" c: 

.c . i

: E.c 

.g~ ~ Ill 

"' j~ i£i c: 

0 

~ ~ 

~ 

VERTICAL EXAGGERATION X 10 

1{ ~ 

I 

El 

Alluvium 

m Beach gravel 

Omitted m1 map 

Peat, loess, and colluvium 

Orni.tud on map 

t:·9.(1 Outwash and alluviation gravel de· 

posited during Salmon Lake 

Glaciation 

Contact 

LJashed where inferred 

:;:: 

~ d 

~ 

rJ.l 

i :;:: 

:I 

~ ~ 

'""' 

~ 0 

§: 

!-

~ 

1000 0 1000 2000 3000 4000 FEET 

EXPLANATION 

w~M IJ:a Marine sand and gravel Estuarine sandy silt behind 

of Second Beach Second Beach shoreline 

t~qr-~·~1 Till and outwash of Nome 

River Glaciation 

Marine sand 

~ and gravel of Third 

Beach and Intermediate Beach 

Buried bedrock valleys of 

Snake and Nome Rivers 

> 

0:: 

<( 

~ 1-

~ ,0 

a: if I !-

0 I 

~ I 

~l 

No5•oo· 

.c 

u 

.. 

.. 

Ill 

i{ ~ !Qit l £ Till of Iron Creek Glaciation 

[fr}l Marine sand and clay of 

Submarine Beach 

~ Schist 

Buried gold pay streaks 

T, Third Beach; M", Monroeville Beach; 

I, Intermediate Beach; IS, Inner Subman:ne Beach; and 05, Outer Submari.ne Beach 

A' 

3{)0' 

200' 

100' 

SEA 

LEVEL 

> 

0:: 

c 

~ c 

:::> 

0 

O::> 

oo:: 

>-c 

o::z 

~~ t-1- o::C: 

,..:::> 

1-0 

u 

~ 

§ 

~ 

FIGURE 2.- Generalized geologic map and cross section of the coastal plain at Nome. (After Hopkins, in Hopkins and 

others, 1960.) 
A4 STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA 

from a depth of 240 feet below the sea bottom in one 

of the holes drilled in 1967 about 3 nautical miles off 

the mouth of the Nome River (J. A. Wolfe, written 

commun., 1968). 

Seismic-reflection profiles of the northern Bering 

Sea commonly depict an unconformity at depths 

ranging from a few tens of feet to several hundred 

feet between nearly flat-lying beds and more steeply 

dipping underlying beds (Scholl and Hopkins, 1969; 

Grim and McManus, 1970). This unconformity may 

correspond in time to the abrupt change in slope of 

the bedrock surface just offshore of Nome. If so, it 

probably indicates a slackening of the rate of subsidence of the Norton Basin during middle or late 

Pliocene time. 

Since late Cenozoic time the sea has transgressed 

beyond the present shoreline several times, producing the buried beaches which have been important 

onshore gold producers. Between some of these 

transgressions, sea level receded below the present 

level, at which time the offshore features mentioned 

in this report were produced. At least two of these 

recessions were accompanied by glacial invasions 

beyond the present shoreline -that is, Nebraskan 

(First) or Kansan (Second) and Illinoian (Third). 

Either or both of these invasions may have carried 

placer gold offshore from existing onshore lode or 

placer deposits. 

The N orne area produced some 5 million ounces 

of placer gold from 1897 through 1964, according 

to an unpublished compilation by A. H. Daily (mining engineer, Oakland, Calif., written commun., 

1968). The greater part of the production was derived from placers on the coastal plain. 

The beach placers resting on bedrock probably 

derived most of their gold from erosion of mineralized rocks adjoining the Anvil Creek fault zone. 

Submarine Beach, Intermediate Beach, Monroeville 

Beach, Third Beach, and Fourth Beach all contained 

productive placers that were richest in their western 

parts and attenuated in value eastward. Net longshore drift on the modern beach is eastward 

(Greene, 1970b), and judging from locations of 

mined areas, eastward drift was probably the predominant agent in transporting gold along the older 

shorelines as well. 

EQUIPMENT, PROCEDURES, AND METHODS 

A continuous seismic-profiling system mounted 

aboard a small tugboat was used to obtain the seismic records. The sound source was a sparker type 

having electrical storage capacitors which discharged a total of 450 joules through a multipoint 

electrode producing a fundamental frequency of 

1000 Hz (hertz). Returning seismic energy was 

recovered with a 20-foot nonpreamplified hydrophone streamer with 11 crystal elements. Both the 

electrode and the first element of the hydrophone 

cable were towed about 100 feet behind the survey 

vessel, about 15 inches beneath the water surface 

and about 12 feet apart. 

The recovered signals were filtered to pass the 

300-1300 Hz band and then amplified. A wet-paper 

recorder graphically recorded the data at a 0.25-sec 

(second) sweep rate and 1.0-sec fire rate. 

Seismic profiles were obtained along more than 

500 nautical miles of track in the N orne nearshore 

area (fig. 3). The track lines were oriented roughly 

parallel and perpendicular to the coastline to form a 

grid with approximately a 1-mile spacing. Tracks 

parallel to the coastline were approximately 20 

miles long, and most lines normal to the coast were 

5 miles long, but four lines extended 10 miles out. 

Most of the tracks normal to the coastline begin 

about 400 feet offshore. The survey was made at 

an average speed of 5.7 knots over water 10-100 

feet deep. 

Sea-bottom multiples (reverberation of seismic 

energy between the sea floor and the sea-air interface) were very strong because of the shallowness 

of the water. These multiples, because of their harmonics and internal multiples (reverberation of 

seismic energy between two subsurface reflectors), 

complicated the records and made interpretation 

difficult; however, most reflectors in this study were 

between the sea bottom and the primary or first seabottom multiple. 

All seismic reflectors picked were plotted on a 

general work map. The data were divided into surface (bottom) and subsurface (subbottom) features 

and transferred to three separate maps. Most of the 

recorded reflectors are discontinuous, and as a result 

correlation from one line to another was not successful until the final computation stage, when features 

recognized on the seismic records were related to 

bathymetric features, bottom samples, drilling results, and onshore geology. 

Depths of reflectors were calculated using velocities of 4,900 fps (feet per second) in water and 

6,000 fps in sediments; sound velocities in sediments 

at Nome have been obtained from Arne Junger 

(written commun., 1967) and from onshore seismicrefraction work (Greene, 1970a). No corrections 

were made for changes in sea -level because lunar 

tides at Nome are generally less than 1.5 feet. 

Storm-surge seiches produce greater sea-level 

changes, at times 5 feet or more above or below 

normal sea levels, but we have no data upon which 
HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSH ORE AREA NEAR NOME, ALASKA A5 

N 

1 

0 4 KILOM ETERS 

I I I 

2 NAUTICAL MILES 

• Drill hole 

FIGURE 3.- Track chart of Nome nearshore seismic survey. 

to base corrections for these effects~ and most of 

the records were obtained during periods of good 

weather. 

Bathymetric data were obtained from U.S. Coast 

and Geodetic Survey smooth sheets for the central 

part of the study area and from the seismic profiles 

elsewhere. Features observed on the sea floor offshore near N orne will be described in another report. 

INTERPRETATION 

Structures observed in the seismic records are 

divided into (1) surface and shallow subsurface 

and (2) deeper subsurface features. Surface and 

shallow subsurface features are defined here as 

those recognized on the seismic records, and they are 

expressed in the topographical relief of the sea 

floor. They may extend, as seismic reflection features, as much as 20 feet below the sea floor and 

include beach ridges, alluvial fans or deltas, and 

filled channels marked by depressions in the sea 

bottom. These surface and shallow subsurface features are shown in figure 4. Topographic expressions 

of the drift can also be seen in profile (fig. 11). 

Deeper subsurface structures are defined here as 

those features at depths generally more than 20 

feet below the sea bottom; most do not affect the 

form of the sea bottom. Shallow and deep buried 

channels, acoustical sinks, and surface expressions 

associated with buried channels, where present, are 

shown in figure 5. Terminal moraines, recessional 

moraines, faults, and outcropping bedrock are shown 

in figure 6. Moraines and acoustical sinks (soundabsorbing layers which cause a signal loss) are 

shown on both the surface and the subsurface maps 

(figs. 4, 5, and 6). 

SURFACE (SEA·FLOOR) TOPOGRAPHY 

The present-day Nome offshore topographic features are a result of processes that have acted during 

the Pleistocene and Holocene. These features consist 

of beach ridges, fans, and surface channels. 

BEACH RIDGES 

Most beach ridges are defined in the seismic 

records by a gentle convex-upward profile, steepest 

on the seaward side and nearly flat on the landward 

side. Some beach ridges are acoustically transparent, 

but others show internal stratification consisting of 

beds that dip gently seaward beneath the steepest 

part of the surface profile (fig. 7) ; a few ridges 

show bedding that dips gently landward beneath 

the backshore slope. A strong reflector that can be 

correlated with the sea floor on either side of the 
A6 STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA 

beach ridge commonly extends beneath the ridge. 

Some features interpreted as beach ridges have a 

nearly symmetrical convex profile, equally steep on 

both the seaward and the landward sides, and may 

represent barrier bars. 

All profiles of beach ridges are graded "good" or 

"poor" depending on sharpness of definition in the 

seismic record. Those rated "good" have distinct, 

sharp topographic profiles and clearly defined in-

® ___ , __ ® . · ·? .. ~-

---?--.........._ 

~' 

? 

I 

D 

D 

? ... ? 

?~--;~--::_··::::::-_~_ · -~ 

terior structure. Those rated "poor" have little 

surface expression and lack well-defined internal 

seismic characteristics. 

Limits of the beach ridges are picked from points 

on the seismic records where the internal structures 

pinch out or where a distinct change in slope occurs 

on the sea floor. The backshore of many beach ridges 

is difficult to locate because of the absence of internal 

structures and the lack of distinct change in slope. 

I 

C---c 

~ 

-----------

FIGURE 4. - Map showing surface and shallow 
B' 

HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME, ALASKA A7 

Bottom topography and samples were taken into 

account in determining widths of the beach ridges 

and their correlation from line to line. 

Distribution.- Two major beach ridges were 

mapped; one lies at a water depth of 65-70 feet 

(indicated by a circled 2 in fig. 4) and the other 

at a water depth of 75-80 feet (indicated by a 

circled 1 in fig. 4). Two minor, smaller, beach ridges 

occur at the eastern limits of the survey area; one 

® := 

at a water depth of 85-90 feet (indicated by a circled 

3 in fig. 4) and another at a water depth of 55-60 

feet (indicated by a circled 4 in fig. 4). The -85- to 

-90-foot beach ridge lies approximately 4 miles 

offshore of Hastings Creek and is about threefourths of a mile wide and at least 3 miles long; 

this beach ridge may possibly be an extension of 

the - 75- to -80-foot beach ridge. Another minor 

beach ridge (indicated by a circled 5 in fig. 4) 

~~: 

-------~ I 

~--? 

..,: .. :~.;.:.;:.:..? 

Beach ridge 

EXPLANATION 

Acoustical-sink segment crossed by seismic track 

Dashed ·where inferred, queried where doubtful. Dotted 

lim indicates beach ridge crest 

r==~.l-:: 

Interpreted correlation of acoustical sinks 

Queried where questionabU 

Interpreted correlation of buried channel 

A--A' 

Surface channel or depression crossed 

by seismic track 

Navigation station 

CD Beach ridge referred to in the text 

Cross-section location 

0 2 3 NAUTICAL MILES 

================= 

subsurface features of the Nome nearshore area. 
AS STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA 

. ,)) J:'-ke ~ Ri3!!r ~ Q,C0~Ci.~- "'~ \

"

.. • 

/ 

_../ ' 

-+ /------=---:::-~ --- \ ( I t I I 

\ ~ \ ·~ \ 

' 

.. r,.!\ 

'"'--

_____ ~ 

,--- ' . ---

r- - 1 ··to •· •••·•• ••••• '' '1

EXPLANATION 

Shallow buried-channel segment crosaed Deeply buried channel by seismic track 

Crou-da.d. ndica~• chan"Ml center 

Center crou-datk indicate• ch.a.nml center 

and ll'i<k crou-d.a.-M• indicate cAan-JUl .UU• 

Interpreted correlation 

~ of buried channel 

IJa.sh.-dot line indicaUs center of cAannel 

Acoustical-sink sejpllent crosaed 

by seismic track 

Navigation station 

Outline of high channel-density areas Surface channel or depre88ion associated 

with acoustical sink or buried channel 

Qu<Nd whore qu .. tionable 

Outline of moderate channel-density areas 

0 2 3 NAUTICAL MILES 

======== 

FIGURE 5.- Map showing deeper subsurface features of the Nome nearshore area. 

occurs at a water depth of 45-55 feet about a mile 

off the Penny River and measures about 41/2 by 112 

miles. This ridge may be an extension of the -55-

to - 60-foot ridge of the eastern part of the survey 

area. 

Stratigraphic position. -Because most of the 

beach ridges are expressed in the sea-floor topography and in the upper 20 feet of sediment, they are 

likely to be of late Pleistocene age. They probably 

represent stillstands during the Woronzofian (midWisconsin) or Krustensternian (late Wisconsin and 

Holocene) transgressions discussed by Hopkins 

(1967). 

Significance. - Beach ridges offshore are significant as possible sites of placer gold deposits, as the 

beaches onshore were mined for their placer gold 

during the Nome gold rush. Wave action concentrated a considerable amount of placer gold in the 
HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME, ALASKA A9 

&'!>'\"'' 

\ -\- &4'30' ~ 

li' 

~ ;[ 

-""- .. ...... ·.·:::: ·· · ...... ~. ···~-..... - -·-

I 

I -!?.'6' 

onshore beach ridges and may have similarly concentrated placer gold in the offshore beaches. However, the shallow beaches offshore at Nome are not 

necessarily products of erosion of bedrock in the 

study area, and any included gold probably was 

derived from reworking of till in the nearshore or 

onshore area. Therefore, the most promising areas 

for offshore placer gold deposits are where beaches 

are carved in or rest on drift. Because of the eastward longshore drift (Greene, 1970b), nearby areas 

g: 

-... " 

~ 

" 

(yt; 

~ 

f 

• 

to the east of Nome fulfill such conditions and are 

likely sites of offshore placer gold deposits. 

FANS OR DELTAS 

Some geomorphic features observed in the 

bathymetry of the N orne offshore area appear to be 

alluvial or outwash fans, or deltas. Of the five fans 

first identified from the bathymetric data, two were 

recognized seismically in the subsurface. Sedimentological data (Nelson and Hopkins, 1972) and 
AlO STUDIES ON THE MARINE GEOLOGY OF 'THE BERING SEA 

-

bathymetric data (Tagg, unpub. data) also confirmed the two fans seen on the seismic records. 

These fan or deltalike features are characterized by 

thin beds that dip gently away from the shoreline 

(fig. 8). The reflectors are few, and none are more 

than 50 feet thick. 

Dish·ibution. - The three fans observed on the 

bottom-contour map but not in the seismic records 

have an average areal extent of 31;2 miles. One fan 

FIGURE 6. - Map showing shallower subsurface 

is 2 miles west of the mouth of the Snake River (fig. 

9). The apex of this fan is not visible and is truncated at the - 40-foot contour about 1 mile offshore; 

the fan proper extends about 1 mile farther offshore, 

where its foot has been located at about the -54-foot 

contour. 

The other two fans that were not seen on the 

seismic records are approximately 41/2 miles directly 

off Nome; the inner fan appears to be superimposed 
HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME, ALASKA All 

.f 

"/ 

I 

\ 

1 

I 

I 

?'· 

/ ) 

II' 

·/' 

< 

EXPLANATION 

Terminal moraine of pre· Illinoian age 

Terminal moraine (outer drift limit) of Iron 

Creek Glaciation (pre-Illinoian) 

Dashed where i1iferred and qu.er-ied where 

quelftionable 

~-?~-.. 

Fan 

Dalh.ed where i11ferred 

Fault 

Dashed where iriferred 

1!!. 

Recessional moraine of Iron Creek Glaciation 

(pre-Illinoian) 

~--?~ 

Terminal moraine (inner drift limit) of 

Nome River Glaciation (Illinoian) 

Daslwl where in/erred and queried 

where questio'l'l.llble 

Recessional moraine of Nome River 

Glaciation (Illinoian) 

Bedrock at or near ocean bottom 

Navigation station 

0 2 3 NAUTICAL MILES 

features of the N orne nearshore area. 

over the other. The apex is not visible on the overlying fan, which is truncated near the - 72-footcontour; it extends about one-half mile farther 

offshore to the - 30-foot contour. The apex of the 

lower fan is obscured by the overlying fan; its foot 

is on the - 90-foot contour. 

Both of the seismically identified fans occur offshore of the mouth of the Nome River (figs. 6 and 

9). The first and largest fan lies a little south of 

Nome River and covers approximately 5 square 

miles. The apex of this fan is not well defined and 

appears to be truncated along the - 40-foot contour 

about 1 mile offshore; the foot of this fan extends 

about 3 miles offshore, where it pinches out near 

the - 70-foot contour. The second fan is much smaller and covers about 2 square miles south of Nome. 

The apex of this fan is also not well defined and 

appears to be truncated between the - 60- and -70-
A12 

0 

-60 

-120 

-180 

.... 

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.... 

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0 

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-60 

-120 

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STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA 

A A' 

0 

-18 

-36 

---- -------------------------------------------------------------------------------------L -55 

s 

A 

s 

4' 

6' 

2'~ 

a· 

10' 

DIP CORRECTION 

FOR VERTICAL 

EXAGGERATION 

1:4.6 

0 400 METERS 

0 1300 FEET 

HORIZONTAL SCALE 

N ~ 

A' 

0 

L&J 

.... 

L&J 

:::E 

~ 

J: 

t 

L&J 

0 

FIGURE 7.- Line drawing and geologic interpretation of seismic profile A-A' across a buried stream channel, beach ridge, 

and acoustical sinks. Location in figure 4. 

0 0 

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-120 t- -36 

til 

Q: 

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l;j -180J_ ______________________________________________ __L 

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0 

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400 METERS :::E 

~ 

:i 

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1300 FEET 

0 Buried edge of fan Fan surface 

- 120 Dl P CORRECTION 

FOR VERTICAL 

EXAGGERATION 

1:4 .6 

-18 

-36 

-55 

FIGURE 8.- Line drawing and geologic interpretation of a seismic profile 

across a fan. 

foot bottom contours, about 2112 miles offshore. The 

foot of the fan pinches out near the -70 foot contour, about 3 miles offshore. 

Stratigraphic position. - All fans or deltas in the 

Nome offshore area were probably developed during 

the Pleistocene or early Holocene. Sedimentological 

data and onshore geological evidence indicate an 

age of middle Pleistocene (Illinoian) for the large 

fan off the mouth of Nome River. A moraine of 

Illinoian age occurs behind Second Beach just on-
165"35' 

+ ~ ~- ~ 

0 lf2 

FIGURE 9. - Map showing bottom topographic features off N orne. 

EXPLANATION 

---- ....~-- ..... From bathymetric From seismic Subaerial 

soundings profiles drainage 

pattern 

Fans 

c> ~ .::z:::z@ 

Topographic Topographic Beach line high low 

cs::s:Y 

-so 

2 3 NAUTICAL MILES 

CONTOUR INTERVAL 2 FEET 

Soundings from Coast and Geodetic Survey 

Contours by A. R. Tagg 

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A14 STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA 

shore of this large fan. Second Beach (post-Illinoian, 

Sangamon) sediments in a gravel-pit exposure overlie outwash derived from this Illinoian moraine. 

And offshore bottom-sample and drill-hole data indicate that a small tongue of alluvium is immediately 

offshore of the mouth of Nome River and overlies 

a large quantity of outwash that extends for several 

miles east and south of the alluvium (Nelson and 

Hopkins, 1972) . 

B 

0 

-60 

-120 

Association. - Many of the surface channels are 

associated with acoustical sinks and buried channels. 

In the area south of Hastings Creek, surface channels are associated with both acoustical sinks and 

buried channels. Another surface channel 5-6 miles 

west of Hastings Creek is associated with an acoustical sink (fig. 5). 

Stratigraphic position. - Surface channels on the 

sea floor have sharp profiles that indicate a young, 

~ ~ 161 -180 --'---------------------------------------l--55 161 

~ N S ~ ~ 0 400 METERS ::!! 

:i 

li: 

161 

0 0 

-60 

-120 

B 

2lf.z"~ 5" 

10" 

0 0~0~ 

0 1300 FEET 

HORIZONTAL SCALE 

Bedrock 

B' :i 

0 li: 

-18 

-36 

161 

0 

N r<l '<t\OI'-

-1~~-----------------------------------L-55 

N S 

FIGURE 10.- Line drawing and geologic interpretation of seismic profile B-B' across buried channels at the 

mouth of Snake River at N orne. 

Significance.- Because fans are sites of aggradation, there is little opportunity for the repeated 

winnowing and reworking that is generally involved 

in the development of rich placer deposits. Even so, 

rivers and glaciers in the N orne region that have 

carried auriferous sand and gravel as part of their 

sediment load may have transported at least some 

of the finer gold to the fans or deltas offshore. 

SURFACE CHANNELS OR DEPRESSIONS 

Approximately 20 surface channels or depressions 

can be seen in the seismic profiles. They are characterized by a concave-upward profile in the surface 

of the sea floor (fig. 10). 

Distribution.- Most of the surface channels are 

discontinuous, or at least cannot be correlated from 

one line to another and are scattered fairly evenly 

throughout the entire survey area. Surface channels 

can be correlated in only two locations, both offshore 

of Hastings Creek (fig. 5). 

possibly Holocene, age. However, many of them are 

surface expressions of buried channels that have 

not been completely filled with Holocene sediments. 

Therefore, some surface channels may be of late 

Pleistocene age and may have formed when sea level 

was low during the Wisconsin Glaciation. 

Significance.- Surface channels are significant 

in placer exploration because they mark sites of 

fluvial deposits that may contain concentrations of 

placer gold. Especially significant to offshore mining 

are those channels that are most extensive and 

associated with acoustical sinks and buried channels; 

these in particular indicate sites of fluvial gravel 

deposits. The best prospects for placer gold deposits 

in the submerged channels offshore of the N orne 

coastal plain are those channels cut either into bedrock or through auriferous Illinoian drift. 

BURIED TOPOGRAPHY- SUBSURFACE STRUCTURES 

Subsurface structures in the N orne offshore area 

are complex and consist mainly of a discontinuous 
HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME, ALASKA A15 

Pleistocene drainage pattern, glacial deposits, and 

acoustical sinks. These buried features vary considerably in depth, indicating several stages of development. 

BURIED CHANNELS 

Most buried channels are well defined and exhibit 

characteristic features such as a concave-upward 

profile and an acoustically transparent or sometimes 

stratified internal structure (figs. 10, 11, and 12). 

Both shallow and deep channels can be identified in 

c 

0-

to extend offshore (fig. 11). The onshore and offshore channels have similar profiles that show a 

relatively shallow tributary on the east side separated by a small ridge from a deeper tributary on 

the west. 

Deep channels.- Deep channels in the Nome 

offshore area are less common than shallow channels; only about 15 deep channels were identified in 

the seismic records. Bottom depths of the deep 

channels range from 36 to 115 feet beneath the sea 

C' 

-0 

-120 J_ _____________________ ___._ -36 (/) 

m E 2lf.!" ~ HORIZONTAL SCALEW ~ DIP CORRECTION s• 0 400 METERS :::?! 

~ FOR VERTICAL ~ 

J: EXAGGERATION 0 1300 FEET 

t: 1:4.26 10• C' J: 

~ c ~ 0 0 0 

-18 

Bedrock ~120 __L_ _____________________ ____._ -36 

E W 

FIGURE 11.- Line drawing and geologic interpretation of seismic profile 

C-C', across an ancestral channel of the Snake River. 

the seismic profiles. Shallow channels have sides 

truncated at or near the sea bottom (fig. 10) ; deep 

channels are overlain by sediments of variable thickness. 

Shallow channels.- Shallow channels are plentiful in the Nome offshore area and are by far the 

dominant subbottom feature. Approximately 370 

shallow channels are shown on the seismic records. 

Depths of the bottoms of these channels range from 

12 to 150 feet beneath the sea bottom; average depth 

is 38 feet. Although most channels have a single 

symmetrically located low point, some are asymmetrical and a few have two low points divided by a 

low sill (figs. 11 and 12) . The side walls of the 

shallow channels extend nearly up to the bottom 

surface and are usually covered by a thin veneer of 

sediment. Few channels can be correlated from one 

line to another; those that can be correlated trend 

parallel to the present coastline (fig. 5). 

A buried extension of the Snake River that Greene 

( 1970a) detected in refraction seismograms onshore 

at N orne is shown by the seismic-reflection records 

bottom and average 72 feet. The top of the sides 

of these channels range in depth from 22 to 85 feet 

below the bottom and average 45 feet. 

Channel density and distribution.- Most buried 

channels in the N orne offshore area are difficult to 

correlate. To find out if the poor correlation was 

due to the seismic-line locations, a facsimile of the 

offshore seismic grid was overlaid on a map of the 

onshore area of the N orne coastal plain, and positions where lines cut streams were marked. A poorly 

correlated pattern developed that was similar to the 

random pattern offshore. Outlined areas of greatest 

stream-channel densities from this onshore grid 

corresponded to outlines of known areas of stream 

and river courses. Therefore, areas of greatest 

channel densities were similarly outlined offshore, 

and the resulting general pattern is assumed to 

represent areas of stream and river courses (fig. 

5) similar to those produced onshore. In many 

areas of high channel densities, a correlation from 

one line to another may be made with relative 

certainty. 
D 

0 

-60 

-120 

-180 

f- -240 

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HORIZONTAL SCALE 

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0 400 METERS 

0 1300 FEET 

2¥2"~ s· 

1o• 

DIP CORRECTION FOR 

VERTICAL EXAGGERATION 

1:4.26 0 o :n o:n N Cl'l '<I' 1.01'-

FIGURE 12.- Line drawing and geologic interpretation of seismic profile D-D', across a terminal and a recessional moraine. 

-36 

-55 

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-73 5 

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HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME, ALASKA A17 

One possible reason for such a proliferation of 

discontinuous channels at N orne is the geomorphic 

province in which these channels developed. The 

onshore N orne area is a shallow seaward-dipping 

coastal plain that slopes 67 feet per mile (about 

34 o). This slightly sloping plain contains an abundance of meanders, cutoffs, and oxbow lakes. Offshore, the sea floor and surficial sediments slope 

more gently at about 15 feet per mile (about 16°). 

Therefore, it is reasonable to assume that the same 

type of stream-channel development that exists onshore today existed offshore during the time the 

offshore channels were formed. The offshore subbottom drainage pattern most likely consists of 

buried meanders, cutoffs, and oxbow lakes that are 

shown as discontinuous channels in the seismic 

profiles. 

In this report, areas with a significant number of 

buried channels are separated from areas with a 

general lack of channels (fig. 5). Within the region 

of buried channels, seven areas of high channel 

densities, as described above, are outlined. The 

criteria used to define these channel areas are (1) 

relative density of channels, (2) degree of development of the channel, and (3) good channel correlation from line to line. 

Buried channels are distributed throughout most 

of the Nome offshore area. However, about 1 mile 

offshore is a mile-wide linear zone that generally 

lacks channels. And in the northwestern part of 

the area, where bedrock is closest to the surface, 

buried channels are uncommon. 

Stratigraphic position.- Most channels are buried beneath 20 feet or more of Holocene sediments 

and are not normally detected deeper than 100 feet 

beneath the sea bottom. They probably formed during the Illinoian and Wisconsin Glaciations. The 

ancestral Snake River channel is an exception, as it 

is covered by Illinoian drift and is therefore of preIllinoian age (Hopkins, written commun., 1969). 

Significance. - Buried channels are important 

sites for concentration of heavy minerals. In the 

N orne onshore area, large amounts of placer gold 

have been recovered from them. Offshore buried 

channels probably also contain quantities of detrital 

mineral deposits, especially in the nearshore area. 

ACOUSTICAL SINKS 

An area in which a weak reflected seismic signal 

is received is here called an acoustical sink (figs. 7 

and 12). The prime characteristic of an acoustical 

sink is acoustic opacity which means that most of 

the seismic energy is absorbed or scattered and little 

or no signal is returned to the surface. 

Distribution and density.- Seismic records in the 

Nome offshore area exhibit 81 acoustic sinks: 58 

in the eastern half of the survey area and 23 in the 

western half. Sinks tend to be scattered instead of 

concentrated. The highest concentration and correlativity of acoustical sinks are within the area 4 

miles off Hastings Creek (figs. 4 and 5) . Many sinks 

in the N orne offshore can be correlated between 

lines. In the eastern half of the survey area, 24 

sinks are correlated, compared with 4 in the western 

half. Some acoustical sinks, though usually identified as gravel fill in buried channels, may be gravel 

in beach ridges. Gravels are a common constituent 

in the present-day beaches of the area and have been 

noted, for instance, off Point Hope by Creager and 

McManus (1966), closer to Nome, off Port Clarence 

in the Bering Strait, by Hopkins (1967), and at 

Nome by Greene (1970a). 

Significance. - Association of the acoustical sinks 

with buried channels and beach ridges suggests that 

stream-and-wave-deposited materials absorb scatter 

of the seismic energy. Therefore, acoustical sinks are 

interpreted here to be the result of scattering seismic energy by gravels associated with buried stream 

channels and beach ridges. Because detrital deposits 

of heavy minerals, especially gold, are concentrated 

in coarse-grained stream and beach deposits, acoustical sinks are significant features that may help 

locate these deposits. 

Stratigraphic position. - The acoustical sinks are 

the same age as the Pleistocene buried stream channels and beach ridges with which they are associated. 

GLACIAL DEPOSITS 

Glacial deposits extend 3-4 miles offshore at 

Nome and from about 1 mile east of the Nome River 

to the western limits of the survey area (fig. 6). 

These deposits are the most extensive of the offshore 

deposits and cover approximately 30 square miles. 

Glacial drift is represented in the seismic records 

by highly distorted reflectors that often show hummocky internal structures and intraformational folding and faulting and that strongly reflect or scatter 

seismic energy. A very strong reflector at what 

appears to be the base of the glacial drift bends 

upward at the drift limit and is truncated by the 

ocean bottom (fig. 12). This reflector is concave 

seaward, away from the main body of the drift. Thin 

tightly folded marine beds commonly abut the outer 

edge of the drift at depth and appear to have been 

compressed by the overriding glacier (fig. 12). 

The drift is thickest, 100 feet or more, near its 

outer limit. The drift limits, shown in figure 6, 

appear to be terminal moraines (fig. 12). Seaward 

of the terminal moraines, marine sediments overlap 
A18 STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA 

the drift. Thinly bedded sediments, probably glacial 

outwash, dip gently seaward from the terminal 

moraine. Landward of the drift limits, the glacial 

deposits become thinner and are covered by a thin 

veneer ( 20 ft or less) of marine sediments. 

After glacial deposition, transgressions of the sea 

altered, reshaped, and locally destroyed glacial features. Outwash and other glaciofluvial deposits have 

distorted the original glacial structures, making 

interpretation of glacial features difficult. 

Dist·ribut,ion. - The terminal moraine that lies 

farthest offshore at Nome, here called the "outer 

drift limit," appears to continue unbroken from the 

western limit of the survey area to just offshore of 

the mouth of the Nome River, where it becomes less 

distinct and appears to die out. A moraine better 

defined on the seismic records lies seaward of the 

outer drift limit off the mouth of the N orne River 

(fig. 6). This moraine is probably an extension of 

the outer drift limit, which has been distorted or 

obliterated in the area just offshore of the town of 

N orne; the weaker moraine feature landward is 

probably a recessional moraine. The outer moraine, 

probably an extension of the outer drift limit, bends 

landward approximately 1 mile east of the mouth of 

Nome River. The moraine that bends shoreward 

just west of the mouth of the Nome River may possibly be a younger terminal moraine. Buried channels and fan deposits distort the subbottom features 

in the area offshore of the Nome River. 

Several weakly defined morainelike features in 

the western part of the survey area are probably 

N 

0 

-30 

-60 

1-

LLJ 

LLJ 

LL. -90 

~ 

:i N HORIZONTAL SCALE 1- 0 a.. 

LLJ 

Cl 

-30 

-60 

recessional moraines. These features generally have 

the same seismic characteristics as the outer drift 

limit except that relief and definition in the subbottom are subdued. Drift features in the western 

nearshore area, just inshore of the most continuous 

recessional moraine (fig. 6), appear to consist of 

a younger terminal moraine, here called "inner 

drift limit," and a recessional moraine. Recessional 

moraines between the inner and outer drift limits 

in this western part of the survey area seem to 

indicate at least three stages of glacial stability or 

readvances (fig. 6). 

Stratigr·aphic position.- Deposition of glacial 

drift in the Nome offshore area probably took place 

during the glacial advances of early to late Pleistocene time. The outer drift and its associated recessional moraines were most likely formed during 

the Iron Creek Glaciation. The inner drift must be 

younger than the outer or it would have been destroyed; hence, the inner drift and its recessional 

moraines are probably Illinoian in age and most 

likely represent the Illinoian glacial maximum. 

Data from drill holes just shoreward of the inner 

drift limit off the mouth of Penny River show two 

layers of till separated by a layer of marine sediments. These data suggest that the inner drift limit 

represents the Illinoian Glaciation and is younger 

than the outer drift limit. 

Significance. - Illinoian glaciers scoured deeply 

into highly mineralized bedrock in the interior areas 

of the Seward Peninsula and probably crossed over 

auriferous gravels during their advance to the sea. 

s 

0 

-9 

-18 

en 

£r 

LLJ 

-27 ~ 

::::!: 

0 30 60 120 FEET s ~ f---t-+--1 0 :i 0 9 18 36 METERS~ 1-

a.. DIP CORRECTION 15• LLJ 

FOR VERTICAL Cl 

EXAGGERATION 30• 45• -9 

-18 

1LFault 

Bedrock 

-90 -27 

FIGURE 13.- Line drawing and geologic interpretation of a seismic profile 

across the large bedrock fault zone east of Penny River. 
HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME, ALASKA A19 

Auriferous material eroded by the glaciers was carried away from the source areas and deposited as 

glacial drift on the coastal plain. Considerable 

amounts of this material were deposited in terminal, 

recessional, and lateral moraines. Those morainal 

features that were subject to wave and current 

action during transgressions most likely lost fine 

constituents through winnowing, and their coarse 

constituents were left as lag deposits. 

FAULTS 

Several faults are shown by the seismic records, 

but only three are defined well enough to map (fig. 

6). Two of the three faults are very small; one displaces only the sediments overlying bedrock, and the 

other both bedrock and sediments, and has a 3- to 

4-foot scarp on the ocean bottom. The third fault 

displaces bedrock and the overlying sediment and 

has an apparent displacement of about 15 feet and a 

4- to 6-foot escarpment on the sea floor (fig. 13). 

Significance. - It has not been determined what 

relation, if any, faults in the Nome offshore area 

may have to the offshore placer gold deposits. 

Mineralization may have taken place along fault 

planes in the bedrock, as in the Anvil Creek fault. 

Faults may elevate bedrock to a level where its 

surface can be worked by offshore mining equipment, or they may create natural catch basins for 

heavy minerals. 

BEDROCK 

An acoustical basement is well defined in many 

seismic records of the Nome offshore area. The 

acoustical basement at Nome represents a metamorphic bedrock that, onshore, consists of deformed 

Paleozoic schist and limestone (Collier and others, 

1908, p. 149; Hummel, 1962a, b). Bedrock was only 

mapped in areas where it cropped out on or closely 

approached the sea floor (fig. 6). 

Distribution. - Bedrock crops out in the northwestern part of the survey area. A line representing 

points where the bedrock surface is truncated at the 

sea bottom is shown in figure 6. It angles toward 

the coastline for about 5 miles eastward from the 

western limit of the survey. About 2 miles southeast 

of Penny River the bedrock surface is displaced by 

a fault. Landward of the mapped bedrock-outcrop 

line, bedrock is exposed on the sea bottom or is 

covered with a thin veneer (less than 12 ft) of 

Holocene sands and gravels. 

Significance. - Bedrock depth and surface configuration are significant in offshore placer mining 

because detrital heavy minerals often concentrate on 

bedrock surfaces, particularly in small pockets or 

depressions. Also there is some possibility of finding 

lode gold in the bedrock or of enrichment of placer 

deposits from bedrock sources. 

SUMMARY AND CONCLUSIONS 

In the Nome offshore area, subbottom features, 

most of which were formed between early and late 

Pleistocene time, consist of stream channels, glacial 

deposits, and acoustical sinks. On the bottom and 

at shallow depths below the bottom, features, most 

of which were formed during late Pleistocene and 

Holocene time, consist of beach ridges, surface 

channels, and acoustical sinks. 

The outer terminal moraine and associated recessional moraines were probably deposited by 

glaciers of the Iron Creek Glaciation and then 

eroded, reshaped, and covered by marine sediments 

during the Anvilian, Einahnuhtan, and Kotzebuan 

transgressions discussed by Hopkins (1967). During 

Illinoian time advancing glaciers of the Nome River 

Glaciation eroded older marine sediments, onshore 

and nearshore in the area west of N orne, and deposited glacial detritus that makes up the inner 

terminal moraine and associated recessional moraines. The outer and inner drift limits represent 

glacial maxima of the Iron Creek (pre-Illinoian) 

and N orne River (Illinoian) Glaciations, respectively. Also, during Illinoian time, a large outwash fan 

was formed at the terminus of a Nome River 

Glaciation ice lobe in the nearshore area east of 

Nome. The other four fans seen in the offshore 

area formed either from outwash of glacial ice lobes 

similar to the one that constructed the large Illinoian outwash fan or as deltas during the Pelukian, 

Woronzofian, or Krusensternian transgressions. 

The buried Pleistocene drainage pattern that is 

prominent in the subbottom of the Nome offshore 

area probably developed during the latter parts of 

the Illinoian and Wisconsin Glaciations, when sea 

level was lower and the area south of the present 

N orne coastal plain was exposed to erosion. Streams 

and rivers fed by glaciers nearby were near base 

level during this time, and the drainage pattern 

consequently consisted of meandering streams with 

cutoffs and oxbow lakes. Some of the channels were 

not filled by sediments of the Krusensternian transgression and appear as discontinuous surface channels on bathymetric charts. Other channels were 

partly filled by coarse detritus carried from the 

terminus of glaciers and appear as acoustical sinks 

in the seismic records. Fine-grained marine sediments covered the coarse detritus filling the channels, leaving no surface expression. 

The last major geologic event to affect the Nome 

offshore area was the Krusensternian transgression 
A20 STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA 

of late Pleistocene to Holocene time, during which 

at least three offshore beach ridges developed, 

representing sea-level stillstands. Some of the beach 

ridges may have formed as long ago as the Pelukian 

(Sangamon) transgression. In late Pleistocene time 

sea level rose to between - 90 and - 75 feet, where 

it remained long enough to form the outer beach 

ridge. In latest Pleistocene or earliest Holocene time, 

possibly during Woronzofian (mid-Wisconsin) transgressions, sea level rose to form the - 65- to -70-

foot beach ridge. The last stillstand of the Krusensternian transgression was later in the Holocene 

time and formed the inner beach ridge between 

-45 and -60 feet. 

Nelson and Hopkins (1972) have shown that the 

glacial drift offshore of N orne is generally covered 

by a thin boulder-rich lag deposit and is commonly 

auriferous. This lag deposit was formed by wave 

erosion during the Pelukian, Woronzofian, and Krusensternian transgressions. · 

FIGURE 14.- Map showing six offshore areas of possible 
HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME, ALASKA A21 

A thin veneer of marine sediments laid down 

during latest Holocene time covers bottom features 

in the N orne offshore area. This veneer consists of 

mud, sand, and gravel and, in places, ice-rafted 

boulders. The swift, predominantly eastward longshore current keeps the sediment cover thin. 

Truncation of bedrock in the western Nome nearshore area probably began in middle or late Pliocene 

time, when subsidence of Norton Basin decreased. 

However, erosion and beveling of the truncated bedEXPLANATION 

rock surface continued during the Iron Creek and 

Nome River Glaciations and throughout the many 

transgressions that affected the Nome offshore area. 

Two faults offset both the overlying sediments and 

bedrock. This indicates that the faults have had 

late Pleistocene movement. 

Many of the structures - fans, ridges, and channels- in the bottom and subbottom Nome offshore 

area may contain concentrations of detrital heavy 

minerals. Beach ridges and buried stream channels 

Beach ridge Acoustical sinks 

Surface channel 

Buried channel. correlated 

/ Buried channel 

...... ®, Outline of area and 

area number 

Moraine, sa wteeth on leading edge 

0 Y.z 2 3 NAUTICAL MILES 

concentration of placer gold and other heavy detrital minerals. 
A22 STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA 

tend to have heavy minerals concentrated by waves 

and currents. Deposits of coarse-grained sediments 

in beach ridges and buried channels are particularly 

important sites for potential placer gold deposits. 

Glacial drift deposits in the Nome offshore area 

may contain small quantities of placer gold. Glaciers 

that deposited material on the Nome coastal plain 

crossed mineralized bedrock and auriferous stream 

channels as they advanced across the Seward Peninsula toward the Bering Sea. The highest concentrations of gold should be in the marginal and 

terminal moraines, especially where gold in these 

moraines has been concentrated by wave and current 

action during marine transgressions. 

Since bedrock in the N orne area is the source of 

the placer gold, it may indicate the location of 

placer deposits offshore. In the western part of the 

N orne offshore area, bedrock crops out at the sea 

bottom and could possibly be supplying small quantities of gold that may be concentrated and deposited 

to the east by the longshore current. In addition, 

bedrock exposed elsewhere at various earlier times 

may have contributed to gold deposits. 

A map showing six offshore areas of possible concentration of placer gold and other heavy detrital 

minerals was constructed from seismic-reflection 

data (fig. 14). Criteria for delineating these areas 

are (1) the presence of more than one feature that 

may contain concentrations of placer gold (that is, 

a beach ridge and a buried channel), (2) high 

density of buried channels, having good correlation 

from one seismic track to another, (3) high density 

of acoustical sinks, associated with either beach 

ridges or buried channels, and ( 4) the presence of 

drift close to beach ridges. 

The first area is about 11/2 miles off Hastings 

Creek and contains a beach ridge and a well-correlated continuous buried channel; both of these 

features lie east of the mouth of the Nome River, 

where small amounts of placer gold may be present 

in the Illinoian terminal moraine onshore (fig. 14). 

The second area is about 3% miles off Hastings 

Creek and contains a beach ridge and correlatable 

surface and subsurface channels that are associated 

with acoustical sinks. Area three, off Nome, contains 

several buried channels and acoustical sinks ; most 

of the buried channels are correlatable (fig. 14). 

Drift limits and recessional moraines are close to 

each other in this area, and small quantities of gold 

from the glacial drift may have been concentrated 

by wave action and deposited in the beach ridge. 

Area four, which touches shore west of Nome, contains a well-correlated extension of the buried Snake 

River channel, which passes through an area onshore that is known to contain significant gold, and 

also contains a beach ridge that is close to a terminal 

and recessional moraine. Area five, about a mile 

farther offshore, contains a beach ridge associated 

with several correlatable acoustical sinks and correlatable buried channels. Area six is off the Cripple 

River and contains correlatable buried stream 

channels and a beach ridge close to terminal and 

recessional moraines. However, the most important 

reason for outlining this area is that bedrock crops 

out on the sea bottom to the east and may be supplying gold to this area. 

SELECTED REFERENCES 

Collier, J. A., Hess, F . L., Smith, P. S., an .... Brooks, A. H., 

1908, The gold placers of parts of Seward Peninsula, 

Alaska, including the N orne, Council, Kougarok, Port 

Clarence, and Goodhope precincts: U.S. Geol. Survey 

Bull. 328, 343 p. 

Creager, J. S., and McManus, D. A., 1966, Geology of the 

southeastern Chukchi Sea, in Wilimovsky, N. J., ed., 

Environment of the Cape Thompson Region, Alaska: 

U.S. Atomic Energy Comm., p. 755-786. 

Greene, H. G., 1970a, A portable refraction seismograph survey of gold placer areas near N orne, Alaska: U.S. 

Geol. Survey Bull. 1312-B, 29 p. 

___ l970b, Morphology, sedimentation, and seismic characteristics of an Arctic beach, N orne, Alaska - with 

economic significances: U.S. Geol. Survey open-file report, 139 p. 

Grim, M. S., and McManus, D. A., 1970, A shallow seismicprofiling survey of the northern Bering Sea: Marine 

Geology, v. 8, p. 293-320. 

Hopkins, D. M., ed., 1967, The Bering land bridge: Stanford, 

Calif., Stanford Univ. Press, 494 p. 

Hopkins, D. M., MacNeil, F. S., and Leopold, E. B., 1960, 

The coastal plain at N orne, Alaska - a late Cenozoic 

type section for the Bering Strait region: Internat. Geol. 

Cong., 21st, Copenhagen 1960, pt. 4, p. 46-57. 

Hummel, C. H., 1962a, Preliminary geologic map of the Nome 

C- 1 quadrangle, Seward Peninsula, Alaska: U.S. Geol. 

Survey Mineral Inv. Field Studies Map MF-247, scale 

1: 63,360. 

_ __ l962b, Preliminary geologic map of the Nome D-1 

quadrangle, Seward Peninsula, Alaska: U.S. Geol. Survey Mineral Inv. Field Studies Map MF-248, scale 

1 : 63,360. 

MacNeil, F. S., Mertie, J. B., Jr., and Pilsbry, H. A., 1943, 

Marine invertebrate faunas of the buried beaches near 

Nome, Alaska: Jour. Paleontology, v. 17, p. 69-96. 

McManus, D. A., Kelley, J. C., and Creager, J. S., 1969, 

Continental shelf sedimentation in an Arctic environment: Geol. Soc. America Bull., v. 80, p. 1961-1984. 

Moore, D. G., 1964, Acoustic-reflection reconnaissance of 

continental shelves - Eastern Bering and Chukchi Seas, 

Chap. 15 in Miller, R. L., ed., Papers in marine geology, 

Shepard Commemorative Volume : New York, Macmillan 

Co., p. 319-362. 

Nelson, C. H., and Hopkins, D. M., 1972, Sedimentary processes and distribution of particulate gold in the northern Bering Sea: U.S. Geol. Survey Prof. Paper 689. 
HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME, ALASKA A23 

Scholl, D. W., Buffington, E. C., and Hopkins, D. M., 1968, 

Geologic history of the continental margin of North 

America in the Bering Sea: Marine Geology, v. 6, 

p. 297-330. 

Scholl, D. W., and Hopkins, D. M., 1969, Newly discovered 

Cenozoic basins, Bering Sea shelf, Alaska: Am. Assoc. 

Petroleum Geologists Bull., v. 53, no. 10, p. 2067-2078. 


Tagg and Greene-HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME, ALASKA-Geological