Geologic Interpretation of 

Reconnaissance Aeromagnetic 

Survey of Northeastern Alaska

GEOLOGICAL SURVEY BULLETIN 1271-F

Geologic Interpretation of 

Reconnaissance Aeromagnetic 

Survey of Northeastern Alaska

By WILLIAM P. BROSGE, EARL E. BRABB, and ELIZABETH R. KING

CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1271-F

Magnetic profiles indicate that northeastern Alaska may be divided into 

five areas, each of which has a 

distinctive magnetic character

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1970
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

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

Washington, D.C. 20402
CONTENTS

Page

Abstract__________._________________._______-_______--------__- Fl

Introduction.____________-_____-_.__-________-__--__-----_-------- 1

Previous aeromagnetic surveys in northeastern Alaska__._____________ 2

Present survey______--_________-___--__-___-___---_-------------_- 3

Magnetic contour map___-____-___-_-_____-___-__------_---------_- 4

Interpretation of the magnetic data_____________.__________-------. 5

Major magnetic features___________._____________-___--------- 6

Description of specific anomalies_______-__-_-____------_-_------- 8

Conclusions__._________-_____-____,________________-----_-_---_- 12

References cited___________.__________________-_____-_---__------_- 12

ILLUSTRATIONS

[Plates are in separate volume]

PLATE 1. Geologic map of northeastern Alaska interpreted from geologic and 

aeromagnetic data.

2. Aeromagnetic profiles of northeastern Alaska indicating boundaries of 

magnetic rock units shown on geologic map.

3. Aeromagnetic map of northeastern Alaska.

ni

CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGIC INTERPRETATION OF RECONNAISSANCE 

AEROMAGNETIC SURVEY OF NORTHEASTERN ALASKA

By WILLIAM P. BROSGE, EARL E. BRABB, and ELIZABETH R. KING

ABSTRACT

Aeromagnetic reconnaissance of northeastern Alaska in 1965 covered approximately 100,000 square miles, from lat 64° N. to the Arctic Ocean and from the 

Alaska-Yukon boundary to long 148°30' W. A magnetic contour map was compiled 

from data from the 1965 survey and earlier surveys. On the basis of these data, 

northeastern Alaska may be divided into five areas, each having a distinct 

magnetic character. A study of specific anomalies within these areas led to the 

tracing of such features as the Tintina fault zone, Ruby uplift, and Kobuk trench 

for more than 100 miles. Other features, such as inferred north-trending faults 

and serpentine belts, had not been identified previously by surface geologic 

methods. The probable extension of volcanic rocks beneath the Yukon Flats 

limits the size of a possible Tertiary petroleum basin. Discovery of large magnetic 

anomalies in the Yukon-Tanana Upland provides new possibilities for exploration for mineral deposits associated with ultramafic rocks.

INTRODUCTION

The geology of northeastern Alaska, like that of many other parts 

of the State, is not well known, and geophysical exploration has not 

been extensive. From 1888 to 1940, reconnaissance geological studies 

were restricted largely to areas near a few major navigable rivers. 

Modern, regional geologic quadrangle maps at a scale of 1: 250,000 or 

larger cover less than one-third of the area. The geologic work that has 

been done, however, indicates that northeastern Alaska is underlain 

by a variety of igneous, metamorphic, and sedimentary rocks of different ages. Nearly every geologic system, as well as the Precambrian, 

is represented. Most of the rocks have been complexly folded and 

faulted.

The choice of the proper geophysical techniques for exploring such 

large and poorly mapped areas is always difficult. However, widely 

spaced aeromagnetie profiles have proved useful for delineating the 

more important tectonic units and for planning more detailed surveys

El
F2 CONTRIBUTIONS TO GENERAL GEOLOGY

in such areas as the Cook Inlet area of Alaska (Grantz and others, 

1963) and the Arctic Archipelago of Canada (Gregory and others, 

1961). Even in an area where the geology was better known, widely 

spaced aeromagnetic profiles along a transcontinental strip from 

Washington, D.C., to San Francisco, Calif., proved very useful in 

indicating unsuspected gross geologic structures (Zietz and others, 

1966). Accordingly, it was decided that further experiments be made 

with such reconnaissance profiles in Alaska.

The aeromagnetic reconnaissance of northeastern Alaska was 

initiated and planned by Isidore Zietz in 1965 and was supervised in 

the field by Randolph W. Bromery. J. L. Vargo compiled the aeromagnetic data as profiles, and J. R. Kirby combined these profiles with 

earlier data obtained by Andreasen (1960a, b) to form a preliminary 

contour map. This contour map was later modified by E. R. King; she 

also provided most of the geophysical interpretation for this report. 

These magnetic data were turned over to Brosge and Brabb, who had 

done geologic mapping in the area; they provided most of the geologic 

interpretation.

The authors were helped by discussions with Isidore Zietz, Ernest 

Lathram, Andrew Griscom and D. F. Barnes. The writers are especially grateful to Barnes for his extensive technical review of the 

manuscript and for his many constructive criticisms. They are also 

grateful to A. S. MacLaren of the Geological Survey of Canada who 

sent them detailed aeromagnetic maps of the Yukon Territory (Canada Geological Survey, 1966) adjacent to the southeast corner of 

the map.

PREVIOUS AEROMAGNETIC SURVEYS IN 

NORTHEASTERN ALASKA

Some of the pioneering work in the application of aeromagnetic 

data to the solution of geologic problems was done in the Naval Petroleum Reserve and in the surrounding region just west of the area 

shown on plate 1 (see index map on pi. 1 and Woolson and others, 

1962).

Several total intensity aeromagnetic profiles, primarily over the 

Yukon Flats (the central part of the area shown on pi. 1), were 

obtained by the U.S. Geological Survey in 1954 and 1958 and were 

released for public inspection in 1960 (Andreasen, 1960a, b). Most of 

these profiles were flown north and south at a barometric elevation of 

2,500 feet, or slightly higher at the north ends; some of the profiles in 

the western part were flown at 4,000 to 8,000 feet above sea level. The 

north-south profiles are incorporated in this report and published 

here for the first time. Six northwest-trending profiles parallel to the
AEROMAGNETIC SURVEY OF NORTHEASTERN ALASKA F3

Yukon River between long 147° and 144° W. (see index map on pi. 1) 

were not included in this report. An aeromagnetie survey of a 900- 

square-mile area near Fairbanks (pi. 1; Andreasen and others, 1964), 

is the only other published aeromagnetic report for northeastern 

Alaska.

PRESENT SURVEY

The experimental aeromagnetic reconnaissance of northeastern 

Alaska was made in July and early August of 1965. It covered an 

area of approximately 100,000 square miles, extending from lat 64° N. 

to the Arctic Ocean and from the Alaska-Yukon boundary to long 

148° 30' W. (pi. 1). The flight elevation was 5,000 feet above sea level, 

except in the mountainous areas where it was slightly higher. Twentyone north-south flight lines were flown 10 miles apart except at the 

western edge of the area where the spacing was increased to 20 miles; 

and two east-west base lines were flown near the northern and southern 

boundaries of the survey. Only nine of the north-south lines extended 

to the Arctic Ocean; unfavorable weather conditions made it necessary 

to break off the other flight lines south of the Brooks Range. The locations of the flight lines are shown on plates 1 and 3.

A flux-gate magnetometer that recorded continuously was used for 

the measurements; many of the surveying and processing techniques 

were those described by Balsley (1952). However, the aircraft's equipment included some new experimental components designed for eventual digital processing of magnetic data, and position control of this 

survey was obtained by Doppler navigation as well as strip-film camera records. The published 1: 250,000-scale Alaskan topographic maps 

were used for navigation and initial data compilation, although this 

proved difficult in areas with topography as complex as the Brooks 

Range. Furthermore, just reconnaissance versions of these maps were 

available for the Eagle and Big Delta quadrangles at the southeastern 

corner of the survey area. Position accuracy was generally better than 

was needed for the 1:1,000,000 scale of the published maps and profiles, but a few position errors of 1 to even 4 miles could have occurred 

in places where Doppler equipment failures coincided with poor stripfilm camera records or inadequate maps.

The magnetic profiles are all relative to an arbitrary datum and 

include any instrumental or diurnal drift that occurred in the recording interval. Daily phone checks were made with the U.S. Coast and 

Geodetic Survey magnetic observatory at College, Alaska, and flights 

were canceled on days when magnetic disturbances larger than 50 

gammas were expected.

In the auroral zone magnetic disturbances show large variations 

within short distances (Morley, 1953; Westcott, 1960), and a few
F4 CONTRIBUTIONS TO GENERAL GEOLOGY

local variations with magnitudes as large as 100 gammas could be 

caused by ionospheric rather than geologic sources. However, almost 

all the observed anomalies correlate with the known geology or with 

anomalies on adjacent traverses and are probably geologic rather than 

transient.

The profiles from both the 1965 survey and the earlier surveys in 

1954 and 1958 are shown on plate 2 at a horizontal scale of 1:1,000,000 

corresponding to plate 1. The profiles all decrease to the south with a 

uniform gradient of 1.5 to 2 gammas per mile. This gradient, which is 

clearest on the parts of the profiles with fewest magnetic anomalies, 

such as north of lat 68° in the Brooks Kange and Arctic Costal Plain 

and in parts of the Yukon Flats, is the component of change of the 

earth's main magnetic field along the flightpath.

MAGNETIC CONTOUR MAP

The magnetic contour map (pi. 3) was compiled by John R. Kirby 

from the 1965 data augmented by several of the north-south lines from 

the earlier surveys (line 8-B, Andreasen, 1960a; lines 1, 2, 11, 36, 44, 

45, and 4Y, Andreasen, 1960b). Although the lines were too widely 

spaced for a detailed contour map, Kirby was able to delineate most 

of the larger magnetic features, particularly with the use of the earlier 

data to give a closer spacing in the more magnetic areas.

The values on each profile are relative to an arbitrary datum. The 

profiles were adjusted to this datum by means of the base lines at the 

north and south, ends of the surveyed area and by a group of six 

northwest-trending profiles flown previously. (Andreasen, 1960b; and 

index map on pi. 1.) Data from a test line flown before and 

after each day's work were also used to tie the profiles to a uniform 

datum and remove drift. Most of the supplementary profiles were 

recorded 2,500 feet lower than the 1965 data and have correspondingly 

higher amplitudes and more numerous small anomalies, but the 20- 

gamma contour interval is sufficiently large so that a coherent pattern 

is obtained. The errors in datum, as well as errors from mislocation, 

and differences caused by variations in elevation tend to elongate contours along the flight lines so that north-south features and trends are 

not as reliable as those that are transverse to the flight lines.

In general, the magnetic pattern shown by the contoured data east 

of Fairbanks and south of the Brooks Range is more reliable than that 

for the western margin of the map and in the area north of the Brooks 

Range where the data are widely spaced. This seems to be partially 

offset, however, by the much more open character of the magnetic field 

in the north.
AEROMAGNETIC SURVEY OF NORTHEASTERN ALASKA F5

The contoured data as originally drawn were strongly affected by 

the earth's field which slopes to the southwest at 3 to 4 gammas per 

mile, and it was necessary to remove this gradient in order to show 

the significant anomalies, particularly in the smoother sections of the 

map. The gradient removed was represented by linear contours trending N.25° W. so spaced that the gradient decreased from about 4-gammas per mile at the southwest corner of the map to about 3 gammas 

per mile at the northeast corner.

The magnetic contour map was compiled on sheets of the U.S. Army 

Map Service World Series (1:1,000,000 scale); it has been adjusted to 

fit the 1:1,000,000-scale base of the geologic map (pi. 1) that had been 

compiled by the U.S. Geological Survey. Because the two base maps are 

on different projections, the position of the contours on the new base 

is in error by as much as 1 mile in some places.

Some of the limitations of the reconnaissance aeromagnetic survey 

can be noted by comparing the area along the northern part of traverse 

70 on plate 3 with a small area of overlap along the eastern edge of 

plate 4 of Woolson and others (1962). A sharp, northeast-trending 

anomaly on the Woolson map in the vicinity of the lower Shaviovik 

River is only a subtle feature of unknown orientation on the present 

contour map.

INTERPRETATION OF THE MAGNETIC DATA

The magnetic profiles (pi. 2) show numerous anomalies caused by 

variations in magnetization of the rocks, principally the mafic and 

ultramafic varieties, but also some granitic and metamorphic rocks. 

This magnetization is a combination of that induced by the present 

earth's field and the remanent magnetization the latter tending to be 

largest in the mafic volcanic rocks. In some cases the direction of the 

remanent magnetization is reversed to give a negative anomaly. Several 

of these negative anomalies that cross basaltic dikes or serpentine 

bodies can be seen on profiles from the southern part of the area. Narrow steep anomalies on the magnetic profiles are caused by rocks at or 

very near the surfaces, whereas some of the broader, smoother 

anomalies are probably caused by rocks at considerable depths. Sedimentary rocks a ~/e generally nonmagnetic; consequently they produce 

relatively flat magnetic profiles.

The strike of most of the major tectonic units shown on plate 1, as 

well as many of the minor structures in the surveyed area, is approximately east-west and at right angles to the direction of the profiles. 

Many anomalies can be easily traced from profile to profile for long 

distances, but other correlations are highly speculative. The orientation 

of the profiles that are tansverse to the predominant geologic trends,

361-498 70) 2
F6 CONTRIBUTIONS TO GENERAL GEOLOGY

together with the distinctive character of many of the magnetic 

anomalies, permits reasonably accurate coverage with widely spaced 

profiles.

The geologic map (pi. 1) shows areas of exposed magnetic crystalline rocks, which are known from reconnaissance or regional geologic 

mapping, and shows the inferred extension of these rocks beneath a 

cover of surficial deposits, sedimentary rocks, and other relatively 

nonmagnetic rocks. The rock types in areas with no outcrops are mapped by correlating the pattern of the profiles with those of exposed 

rocks nearby. Some very large, broad anomalies in the vicinity of 

the Kandik basin and Tintina trench may indicate deeply buried 

crystalline basement rocks of Precambrian age, but the rocks could 

be younger intrusives.

MAJOR MAGNETIC FEATURES

The magnetic profiles indicate that northeastern Alaska may be 

divided into five broad areas each of which has a distinctive magnetic 

character. These areas are shown on plate 3 and are described below.

Area A. The eastern Brooks Range and small parts of the Arctic 

Foothills, Arctic Coastal Plain, and Porcupine Plateau geomorphic 

subdivisions (Wahrhaftig, 1965) compose area A. The magnetic profiles are generally flat, only a few minor anomalies apparently reflecting small areas of magnetic rocks at or near the surface. The profiles 

also have a few extremely broad low-amplitude undulations, which 

could represent magnetic rocks at great depth.

The published magnetic contour map of Naval Petroleum Reserve 

No. 4 and adjoining areas (Woolson and others, 1962, pi. 4) indicates 

that the area of low magnetic relief continues northwestward from 

area A to the lower Colville River, but does not continue westward 

to the Umiat area, which is characterized by closely spaced circular 

and ovoid anomalies. The area of low magnetic relief is also replaced 

eastward by more pronounced anomalies along the Alaska-Yukon 

border, as shown on profiles 87 and 88 (pi. 2).

The rocks exposed in area A are chiefly sedimentary and low-grade 

metamorphic rocks of Paleozoic age. Their low magnetic relief indicates that no major units of magnetic crystalline basement rocks occur 

at shallow depth in the area and that the sedimentary section is probably quite thick. The small size of the magnetic anomalies in area A 

suggests that detailed and well-controlled magnetic surveys would be 

needed to define its geologic structures, but present knowledge of the 

area's mineral potential does not justify such surveys.

Area B. Lying between the Yukon-Porcupine Rivers and the 

eastern Brooks Range, area B includes parts of the central Brooks
AEROMAGNETIC SURVEY OF NORTHEASTERN ALASKA F7

Eange, Porcupine Plateau, Ambler-Chandalar Eidge and Lowland, 

Kokrine-Hodzana Highlands, and Yukon Flats. Most of the area is 

characterized by highly magnetic rocks as indicated by sharp anomaly 

peaks. The eastern part of area B is underlain by volcanic and other 

mafic and felsic crystalline rocks of Mesozoic and probable Paleozoic 

age; the western part is mainly underlain by schist of presumed Paleozoic age and granitic rocks of Mesozoic age. The sharp nearly linear 

change in magnetic character between profiles 45 and 75 suggests that 

between the East Fork of the Chandalar and the Coleen Eivers, part 

of the north boundary of area B is a major fault or series of faults. 

Similar evidence also suggests that the south boundary of area B coincides at least in part with another fault. A north-trending fault is 

also believed to cut the west end of the large body of Jurassic mafic 

rocks described by Eeiser, Lanphere, and Brosge (1965) near the East 

Fork of the Chandalar Eiver. The large magnetic anomalies, together 

with the metallic mineralization already known to exist in area B, 

suggest that additional systematic geophysical surveys here would 

be geologically informative and economically profitable.

Area G. Parts of the Yukon Flats, Porcupine Plateau, Ogilvie 

Mountains, Eampart Trough, and the Yukon-Tanana Upland are 

included in area C. The magnetic profiles show moderate peaks that 

tend to be located in sharply defined areas along otherwise flat profiles. 

The area is bounded on the south by what appears to be a westward 

extension of the Tintina fault.

The area is underlain by a variety of sedimentary and mafic igneous 

rocks of Precambrian through Tertiary age, but it is largely blanketed 

by Quaternary alluvial deposits. Aeromagnetic surveys have already 

shown that extensive, shallow volcanic rocks limit the probable extent 

of petroleum-bearing rocks beneath the Yukon Flats (Zietz and others, 

1960) so that other types of geophysical investigations will be needed 

to provide better information about the few areas where sedimentary 

rocks underlie the Quaternary cover.

Area D. The Yukon-Tanana Upland constitutes area D. The magnetic anomalies are diverse and most of them cannot be traced from 

one profile to another. Most of the anomalies in this region are narrow 

and have steep gradients. Therefore, they appear to be related to the 

surface rocks. The area is underlain principally by metasedimentary 

and meta-igneous rocks of Paleozoic and Precambrian (?) age, by 

serpentine of Paleozoic(?) age, and by granitic rocks of Mesozoic 

age. These rocks are differentiated on plate 1 largely on the basis of 

surface geologic mapping. If a more detailed survey were made, it 

might better differentiate the serpentine and igneous rocks from the 

metamorphic rocks in this mineralized area.
F8 CONTRIBUTIONS TO GENERAL GEOLOGY

Area E. Small parts of the Porcupine Plateau, Yukon Flats, 

Yukon-Tanana Upland, and Ogilvie Mountains, and the extension of 

the Tintina Valley in Alaska are included in area E. The area is 

characterized by broad anomalies as much as 50 miles in width and of 

moderate amplitude. These anomalies probably indicate the presence 

of large bodies of magnetic rocks at considerable depth beneath the 

surface. Sedimentary rocks of Paleozoic and Mesozoic age underliemost of the area. However, the Tintina Valley and the Yukon Flats 

are underlain by nonmarine rocks of Late Cretaceous to late Cenozoic 

age, and the Hard Luck Creek area in the Ogilvie Mountains is underlain by the Tindir Group of late Precambrian age. Most of the Tindir 

Group consists of sedimentary rocks, but it also contains extrusive and 

intrusive mafic rocks; these are reflected on the magnetic profiles by 

steeper and greater anomalies superimposed on the broad anomalies of 

moderate amplitude.

DESCRIPTION OF SPECIFIC ANOMALIES

Specific anomalies within these five large areas are discussed in 

turn from north to south. The numbers for each feature refer to locations on the geologic map (pi. 1), the magnetic profiles (pi. 2), and the 

magnetic contour map (pi. 3), where they appear in parentheses.

1. Magnetic anomalies in the vicinity of the Komanzof uplift of 

Payne (1955) suggest that both the granitic and mafic rocks in 

that area have about the same magnetic intensity. These rocks 

intrude the metamorphic basement rocks and are apparently 

among the oldest intrusive rocks in the Brooks Range. The magnetic anomalies associated with these rocks are small, and trends 

of the individual anomalies are hard to establish because of the 

wide spacing of the profiles. However, the anomalies are probably 

significant because they persist in a narrow belt of considerable 

east-west extent. The rocks in this belt could connect with the 

northwest-striking magnetic rocks that are indicated beneath the 

Colville basin on the detailed aeromagnetic map of the Naval 

Petroleum Reserve (Woolson and others, 1962). Thus, these rocks 

in the northeastern Brooks Range may be part of the older terrane 

whose regional strike is transverse to the east-west strike of the 

younger rocks in the Brooks Range and Colville basin.

2. The north-trending magnetic high, shown by the contours on plate 

3, could be explained by poor datum control between the flight 

lines, but it could also suggest an uplift of magnetic basement 

rocks along a fracture at considerable depth.

3. Two belts of major magnetic anomalies coincide with exposures 

of granite, schist, and metamorphosed mafic rocks in the southern
AEROMAGNETIC SURVEY OF NORTHEASTERN ALASKA F9

Brooks Range (pis. 1 and 2). The northern belt probably represents the granite that may form the southern limit of the flatter 

magnetic profiles found in area A. The southern belt is characterized by sharp peaks, a few of which represent exposed metamorphosed mafic rocks. The entire southern anomaly could be due 

to similar mafic rocks at depth.

The granite, schist, and mafic rocks are replaced eastward by 

Paleozoic sedimentary rocks near the East Fork of the Chandalar 

River. There is unfortunately a wide gap in this area between 

magnetic profiles 71 and 75. Two pairs of intermediate profiles 

(72 and 73, and 14A and 14JB) do not extend far enough north 

to eliminate the possibility that some of the magnetic rocks extend 

across the unsurveyed interval beneath the sedimentary rocks. 

However, although they were flown at different elevations and 

although there may be location problems, the contrast between 

profiles 72 and 73 and adjacent profiles 14A, 71, and 75 suggests 

a gap in the magnetic rocks; this is indicated by the geologic contacts and the possible fault drawn on plate 1. However, the magnetic contour map in plate 3 shows an alternative interpretation 

in which the magnetic rocks are extended across the unsurveyed 

interval beneath the exposed Paleozoic sedimentary rocks. Similar 

interpretation problems also occur farther east between profiles 11 

and 87, where geologic mapping indicates another discontinuity. 

A linear negative anomaly that strikes east-west along the south 

boundary of the Brooks Range schist more or less coincides with 

the Ambler-Chandalar Ridge and Lowland section of Wahrhaftig 

(1965). This linear physiographic belt lies along the Cenozoic 

Kobuk tectonic trough of Payne (1955) and the north part of the 

Mesozoic Kobuk trough of Miller, Payne, and Gryc (1959). It 

has recently been described as the Kobuk trench by Grantz (1966). 

The outcrops along the anomaly consist of Paleozoic(?) and 

Mesozoic(?) phyllite, subgraywacke, chert, and volcanic rocks 

that strike generally parallel to the anomaly, but are cut by it in 

some places. These rocks extend as far east as the Middle Fork of 

the Chandalar River, where they terminate against the schist 

along a northeast-striking structural trend. However, the magnetic 

feature itself continues eastward across the schist as far as the East 

Fork of the Chandalar River where it also takes on a northeast 

trend between profiles 70 and 71.

A distinctive magnetic high, which extends northeast from granitic 

rocks in the vicinity of the Ray Mountains almost to the Old Crow 

Granite of Baadsgaard and others (1961), coincides with the Ruby 

"geanticline" of Payne (1955). Most of the rocks along the part
F10 CONTRIBUTIONS TO GENERAL GEOLOGY

of this structural feature that was crossed by magnetic profiles are 

either covered or unmapped, but presumably they are mostly 

granitic. The Euby uplift seems to cut across the strike of the 

rocks exposed on its flanks. Its east end coincides with a fault that 

Brosge and Eeiser have mapped (Grantz, 1966, U.S. Geological 

Survey, 1965, p. A103) as separating geosynclinal sequences of the 

Brooks Eange to the north from shelf sequences of the Porcupine 

River area to the south. This fault is believed to extend southwestward, separating granitic (?) rocks of the Euby uplift from 

volcanic and sedimentary rocks of the Yukon Flats area; it marks 

the approximate boundary between the magnetic patterns of areas 

B and C.

6,7. The mafic rocks in the Yukon Flats are shown on the map (pi. 1) 

to be widespread. All are grouped in a single general category 

in the absence of exposures other than the volcanics on the margins. 

Most of the magnetic anomalies could be caused by sheets of volcanic rock at a shallow depth overlying or within a sedimentary 

sequence. Two high-amplitude anomalies, one at 143°-144° (6) 

and one west of 147° (7), indicate tne presence of more massive 

bodies of possibly more magnetic rock; they may indicate source 

areas for the extrusive material.

8. The Yukon Flats area was considered a possible petroleum province in a sedimentary basin of Tertiary age by Miller, Payne, 

and Gryc (1959, p. 87-88). The aeromagnetic profiles suggest, on 

the contrary, that the Paleozoic or Mesozoic volcanic rocks that 

are exposed around the periphery of the Yukon Flats probably 

extend beneath the flats at relatively shallow depth (Zietz and 

others, 1960). A magnetic low (8, pis. 1 and 3) occupies the Porcupine Plateau just east of the flats and could represent the thickest 

section of sedimentary rocks. However, reconnaissance of the 

Black Eiver area (Brabb, 1969), indicates that the rocks in the 

vicinity of the magnetic low are probably Paleozoic or Precambrian rather than Tertiary and are outside the basin.

9. The major anomalies along the eastern border of the map between 

the Yukon and Porcupine Eivers may be separated from the volcanic terrane beneath the Yukon Flats by a major structural break 

in the Precambrian (?) basement rocks. This inferred structural 

break is shown as a fault in the geologic map (9, pi. 1). No major 

north-trending faults have yet been mapped in that area, but a 

well-developed north-trending joint or fault system has been 

observed on aerial photographs of the Yukon-Tanana Upland 

south of Eagle.

10. The Tintina fault zone is a major structural and topographic 

feature that marks the boundary between regionally metamor-
AEROMAGNETIC SURVEY OF NORTHEASTERN ALASKA Fll

phosed rocks of Paleozoic and Precambrian (?) age in the YukonTanana Upland and sedimentary and volcanic rocks of Precambrian and Phanerozoic age in the Kandik Basin (Brabb, 1962). 

Folded and faulted nonmarine clastic rocks of Cretaceous and 

Tertiary age extend along the fault zone itself.

Grantz (1966) and Eoddick (1967) postulate that there has 

been about 75 to 220 miles of right-lateral displacement along 

the Tintina fault zone since late Mesozoic time.

The Tintina fault zone lies along a pronounced magnetic gradient that decreases southward. Detailed aeromagnetic maps 

(Canada Geol. Survey, 1966) on the Canadian side of the border 

also show that the Tintina fault zone coincides with either a distinct southward magnetic gradient or with the crest of a belt of 

linear high magnetic anomalies.

11. The northwestern part of the Tintina fault zone between lines 

77 and 79 coincides with a broad deep-seated magnetic anomaly 

which seems to die out toward the northwest. This broad high has 

a distinct linear trend that almost parallels the fault zone except 

in the east where it swings north and gradually broadens. The 

form of the anomaly suggests a deep intrusive, the emplacement 

of which may have been structurally related to the fault zone. 

However, the wider eastern part of the anomaly (11, pi. 3) coincides with most of the known outcrops of the Precambrian Tindir Group and therefore may represent an uplift of Precambrian 

basement rocks beneath the Tindir Group.

The association of the magnetic high with a late Mesozoic and 

Cenozoic nonmarine sedimentary section in the Tintina Valley is 

similar to the association noted by Grantz and Zietz (1960) of 

magnetic highs with larger sedimentary basins. However, the 

width and length of the Tintina anomaly in relation to the narrower and shorter Tintina sedimentary outcrop decreases the 

similarity.

12. Several small, sharp anomalies in the Yukon-Tanana Upland 

coincide with mapped pods or masses of serpentine in two belts 

previously described by Mertie (1937). Magnetic patterns in this 

part of the map indicate that the serpentine is more extensive 

than had been mapped, and detailed aeromagnetic surveys may 

reveal additional serpentine pods. Serpentine is a mineral "target" 

because it is associated with asbestos deposits in a similar geologic 

terrane near Dawson, Yukon Territory (Green and Godwin, 1964, 

p. 19-22). The 1,100-gamma anomaly at Mount Sorensen, the 

largest recorded anomaly in the present aeromagnetic survey, and 

another large anomaly in the headwaters of the Salcha River 

should also be investigated for mineral deposits.
F12 CONTRIBUTIONS TO GENERAL GEOLOGY

CONCLUSIONS

The experiment of using widely spaced aeromagnetic profiles to 

determine gross structural features in northeastern Alaska seems to 

have been valuable. The survey was hampered by bad flying weather 

and by failures of newly installed navigation equipment on the 

aircraft. The instrumental modifications have already been made, however, and future reconnaissance surveys could be planned with sufficient controls to produce a better coverage and a generalized contour 

map of more uniform quality. Some features, such as the Tintina fault 

zone, Ruby uplift, and Kobuk trench, were traced for distances of 

more than 100 miles by characteristic magnetic anomalies, and where 

the line spacing did not exceed 10 miles, they could be traced beyond 

their known geologic exposures. Other features, such as the inferred 

north-trending faults and the serpentine belts, had not been previously 

identified by surface geologic methods. The probable extension of 

volcanic rocks beneath the Yukon Flats limits the size of a possible 

Tertiary petroleum basin and demonstrates the need for additional 

geophysical data in the parts of the flats where volcanic rocks were 

not revealed. Furthermore, the data will be useful for planning and 

stimulating future magnetic surveys in other areas. The discovery of 

large magnetic anomalies in the Mount Sorensen and Salcha River 

areas opens up new possibilities for the exploration for mineral deposits associated with ultramafic rocks and should help to stimulate 

additional detailed aeromagnetic surveys in the Yukon-Tanana 

Upland.

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AEROMAGNETIC SURVEY OP NORTHEASTERN ALASKA F13

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F14 CONTRIBUTIONS TO GENERAL GEOLOGY

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