Reconnaissance Geology of the 

Northern Yukon-Koyukuk Province, 

Alaska 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 774-A 

Reconnaissance Geology of the 

Northern Yukon-Koyukuk Province, 

Alaska 

By WILLIAM W. PATTON, JR. 

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 774-A 

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

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 73-600056 

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

Washington, D.C. 20402 

Stock No. 2401-00299 
CONTENTS 

Page 

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

Introduction ----------------------------------------------------------.. ·-------------------- 1 

General setting ---------------------------------------------------------------------------- 2 

Stratigraphy --------------------------------------------------------···------------------··· 2 

Devonian and older(?) rocks -------------------------------------------- 2 

Pelitic schists and carbonate rocks -------------------------- 2 

Pelitic schists ------------------------------------------------------ 2 

Carbonate rocks -------------------------------------------------- 2 

Permian to Jurassic rocks ------------------------------------------------ 5 

Mafic volcanic and intrusive rocks -------------------------- 5 

Lower Cretaceous (Neocomian) rocks---------------------------- 7 

Andesitic volcanic rocks-------------------------------------------- 7 

Lower and Upper Cretaceous rocks -------------------------------- 9 

Graywacke, mudstone, sandstone, and coal .......... 9 

Volcanic graywacke and mudstone------------------ 9 

Calcareous graywacke and mudstone -------------- 9 

Sandstone, siltstone, shale, and coal ---------------- 10 

Marginal trough deposits ---------------------------------- 11 

Upper Cretaceous and lower Tertiary rocks ....... ----------- 11 

Felsic volcanic rocks -------------------------------------------------- 11 

Tertiary rocks---------------------------------------------------------------------- 11 

Nonmarine coal-bearing deposits---------------------------- 11 

Mangoak River -----------------------------------------........... 11 

Stratigraphy---continued 

Tertiary rocks-Continued 

Page 

Nonmarine coal-bearing deposits-Continued 

Elephant Point---------------------------------------------------- All 

Norton Sound ------------------------------------------------------ 11 

Upper Tertiary(?) and Quaternary rocks ---------------------- 11 

Basalt ---------------------------------------------------------------------------- 11 

Intrusive rocks ---------------------------------------------------------------------------- 12 

Cretaceous rocks ------------------------------------------------------------------ 12 

Granitic rocks -------------------------------------------------------------- 12 

Structure -------------------------------------------------------------------------------------- 12 

Mid-Cretaceous basins ------------------·----------------------------------- 12 

Cenozoic basins ---------------------------------------------------------·-------- 13 

Kobuk-Selawik Lowlands ------------------------------·--------· 14 

Koyukuk Flats ------------------------------------------------------------ 14 

Mesozoic and early Cenozoic structural trends ······------ 14 

Brooks Range trends -------------------------------------------------- 14 

Lisburne-Seward trends ........ ------------------·----------------·-- 14 

Yukon trends -------------------------------------------------------------- 15 

Faults ----------------------------------------------------------·------------------------- 15 

Kaltag fault ------------------------------------------------------------------ 15 

Kobuk fault zone -------------------------------------------------------- 16 

References cited -------------------------------------------------------------------------- 16 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of Alaska, showing outline of Yukon-Koyukuk province and report area-------------------------------····-------------- A1 

2. Reconnaissance geologic map of northern Yukon-Koyukuk province ---------------------------------------------------------------------------- 3 

3. Map showing major structural features in northern Yukon-Koyukuk province ---------------------------------------------------------- 4 

4. Characteristic aeromagnetic profiles and diagrammatic cross section across northern Yukon-Koyukuk province.. 5 

5. Correlation chart of Mesozoic and Cenozoic rocks in northern Yukon-Koyukuk province and Colville basin........ 6 

6. Chart showing major stratigraphic units in northern Yukon-Koyukuk province and its borderlands-------------------- 7 

7. Map showing Cretaceous depositional basins and present-day distribution of major stratigraphic units 

in northern Yukon-Koyukuk province--------------·----------------------------------------------------------------------------------------------------------- 8 

8. Map showing occurrences of Tertiary nonmarine coal-bearing deposits in northern Yukon-Koyukuk province.... 12 

9. Map showing mid-Cretaceous basins-----------------------·-···········-------------------------------------------------------------------------------------------------- 13 

10. Map showing Mesozoic and early Cenozoic structural trends in northern and western Alaska---------------------------------- 15 

III 

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

RECONNAISSANCE GEOLOGY OF THE NORTHERN 

YUKON-KOYUKUK PROVINCE, ALASKA 

By WILLIAM w. PATTON, JR. 

ABSTRACT 

The northern part of the Yukon-Koyukuk province is a broad 

wedge-shaped depression of Cretaceous and Tertiary volcanic 

and sedimentary rocks bordered on the west, north, and southeast by a metamorphic complex, chiefly of Paleozoic age. The 

province is not a simple sedimentary basin, as formerly believed, 

but a highly mobile tract subjected to repeated volcanism and 

plutonism during Cretaceous and Tertiary times. The oldest 

rocks in the province are a thick sequence of marine andesitic 

volcanic rocks of earliest Cretaceous (Neocomian) age. These 

are overlain by terrigenous sedimentary rocks, largely of Early 

and Late Cretaceous (Albian and Cenomanian) age, which 

were deposited in two deep troughs separated by a broad volcanic and plutonic high. The sedimentary sequence, which 

locally may be as much as 25,000 feet thick, is made up chiefly 

of marine volcanic graywacke and mudstone but includes coalbearing paralic deposits in the upper part. Subaerial felsic 

volcanic rocks of Late Cretaceous and early Tertiary age are 

widespread in the eastern part of the province, and two broad 

basalt lava fields of late Cenozoic age occur in the western part 

of the province. 

Three structural trends are recognizable among the intensely 

deformed rocks of the northern Yukon-Koyukuk province and 

its borderlands: East-west along the northern margin and in 

the adjoining Brooks Range, north-south in the western part 

of the province and on the Seward Peninsula, and northeastsouthwest along the Yukon and Koyukuk valleys. The province 

is transected between Ruby and Unalakleet by the Kaltag fault, 

a major strike-slip fault with 40-80 miles of probable rightlateral offset. Major structural discontinuities occur along the 

Kobuk fault zone at the northern edge of the province, but 

evidence for large-scale strike-slip displacement is inconclusive. 

INTRODUCTION 

The Yukon-Koyukuk province is a broad tract of 

Cretaceous and Tertiary rocks that stretches across 

west-central and southwestern Alaska from the Brooks 

Range to the Yukon River delta (fig. 1). This vast 

region, covering more than one-sixth the total land area 

of Alaska, has been classed as a possible petroleum 

province (Miller and others, 1959) and is of continuing 

interest in the search for new sources of oil and gas. 

This report is concerned with the northern part of 

the province, where, since 1958, the U.S. Geological 

Survey has been conducting a program of reconnaissance-scale geologic mapping (fig.1). Much of the area 

has now been visited either by helicopter or riverboat, 

and, although information is still sketchy in many 

places, the broad outlines of the surface geology are 

known. Subsurface data, however, are limited to a small 

number of reconnaissance aeromagnetic profiles flown 

across the province by the U.S. Geological Survey and 

the records of a single 12,000-foot test well near Nulato 

on the Yukon River drilled by Paul Benedum and 

Associates in 1960. 

Many colleagues in the U.S. Geological Survey have 

contributed to the mapping program in the northern 

Yukon-Koyukuk province since its inception in 1958, in 

particular, T. P. Miller, 1963-70, A. R. Tagg, 1959-

FIGURE I.-Index map of Alaska, showing outline of YukonKoyukuk province and report area (shaded). 

Al 
A2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

1963, and J. M. Hoare, I. L. Tailleur, and Bond Taber, 

1958. 

GENERAL SETTING 

The rocks of the northern Yukon-Koyukuk province 

are composed chiefly of volcanogenic sedimentary and 

andesitic volcanic rocks of Early and early Late Cretaceous age (figs. 2, 3, 4, and 7). Locally this assemblage 

is overlain by Late Cretaceous and Cenozoic subaerial 

volcanic rocks and intruded by Cretaceous granitic 

rocks. Bordering the province on three sides are narrow 

bands of mafic volcanic and intrusive rocks of late 

Paleozoic or early Mesozoic age and broad crystalline 

complexes of schist and carbonate rocks of Paleozoic 

and possibly Precambrian age. 

Early workers (Payne, 1955; Miller and others, 1959) 

envisaged the Yukon-Koyukuk province as a simple 

Mesozoic and Cenozoic sedimentary basin or "geosyncline" comparable in tectonic history and style to the 

Colville basin on the Arctic Slope. However, as mapping 

of the Yukon-Koyukuk province has progressed, it has 

become apparent that these two broad structural 

depressions differ markedly (fig. 5). The Colville basin 

developed as a single large sedimentary trough steadily 

filled through most of Mesozoic·and early Tertiary with 

detritus shed from the site of the present Brooks Range. 

The only igneous activity of consequence occurred in 

the Jurassic with the intrusion and extrusion of diabase 

and basalt along the disturbed belt of the western and 

central Arctic Foothills. In contrast, the Yukon-Koyukuk province was a highly mobile tract subjected to 

repeated volcanism through Mesozoic and Cenozoic 

times and to widespread plutonism in the Cretaceous. 

Sedimentation, though locally voluminous, was confined for the most part to a single short cataclysmic 

episode in mid-Cretaceous (Albian and Cenomanian) 

time. The sediments were not deposited in a single 

large basin covering the entire province but rather were 

distributed in smaller troughs that were separated by 

tectonically and volcanically active highs. Most of 

these sediments are composed of first- and second-cycle 

volcanic debris derived from these highs and from volcanic terranes that rim the province. 

The mapping suggests that the Yukon-Koyukuk 

province is more aptly described as a volcanic or volcanogenic province rather than a sedimentary basin. 

STRATIGRAPHY 

The major stratigraphic units exposed in the northern 

Yukon-Koyukuk province and its borderlands range in 

age from Devonian and possibly older to Quaternary 

(fig. 6). Formal stratigraphic names were used locally 

for several of these units by earlier workers (fig. 6), but 

as none of these names seem to have provincewide 

application, they are not employed in this report. 

DEVONIAN AND OLDER(?) ROCKS 

PELITIC SCHISTS AND CARBONATE ROCKS 

A broad crystalline complex made up largely of 

metasedimentary rocks borders the northern YukonKoyukuk province on three sides. These metasedimentary rocks, chiefly pelitic schist and carbonate 

rocks, are widely exposed in the southern Brooks 

Range, Kokrines-Hodzana Highlands, and Kaiyuh 

Mountains, and on the eastern Seward Peninsula (fig. 

2). Scattered exposures along the southeast shore of 

Kotzebue Sound suggest that they may also extend 

from the Seward Peninsula northward beneath Kotzebue Sound and the Kobuk-Selawik Lowlands to connect with similar metamorphic rocks at the southwest 

end of the Brooks Range. The metasedimentary rocks 

may also underlie the eastern part of Norton Sound, as 

indicated by their presence at the tip of Cape Denbigh 

and by their characteristic signature on magnetic profiles that cross the Sound (Scholl and Hopkins, 1969, 

p. 2076; King, 1961, p. 725). The metasedimentary 

rocks do not crop out south of the Kaltag fault on the 

west side of the province but may lie buried beneath 

the surficial deposits of the Yukon delta. 

PELITIC SCHISTS 

Most of the metasedimentary rocks are pelitic schists 

including quartz-mica schist, mica schist, phyllite, 

semischist, and fine-grained quartzite with local intercalations of calc schist, quartzo-feldspathic schist, and 

greenschist. In the Kokrines-Hodzana Highlands the 

schists are extensively intruded by Cretaceous granitic 

rocks and widely altered to andalusite-cordierite hornfels. All the schists are pervasively intruded by small 

bodies of altered diabase and gabbro of late Paleozoic 

or early Mesozoic age. 

Because schists have not been studied in detail anywhere around the perimeter of the Yukon-Koyukuk 

province, their stratigraphy and internal structure are 

poorly understood. Some schists are clearly interlayered or interfingered with carbonate rocks and can 

be dated as mid-Paleozoic on fossil evidence (Patton 

and others, 1968). But in most places the stratigraphic 

relations are unclear, and there is no evidence upon 

which to base an age assignment. Whole-rock Rb-Sr 

dates of 700 m.y. (million years) that were obtained 

by Sainsbury and others (1970) from possibly correlative metamorphic rocks on the central Seward Peninsula suggest that some of these schists may be as old 

as Precambrian. 

CARBONATE ROCKS 

Limestone and dolomite in massive sections as much 

as several thousand feet thick are widely exposed in 
RECONNAISSANCE GEOLOGY OF THE NORTHERN YUKON-KOYUKUK PROVINCE 

Base from U.S. Geological Survey 

1:2,500,000, 1954 

Surficial 

deposits 

• Basalt 

NWE?Ifl 

f-elsic volcanic 

rocks 

}

,i;: 

<!<! 

:::JZ 

ocr: LU 

f-

} 

' >-

<!UJ a: r-:::Jo::=: 

LU0ZfCI:LU<!CI: 

UU LU 

fGranitic rocks 

Contact 

EXPLANATION 

Graywacke, mudstone, 

sandstone , and coal 

Andesitic volcanic 

rocks 

-.--

Fault 

~ Mafic volcanic ~ and intrusive rocks }~ ~ ~ ~ ~ 0 JPb, basalt, !{abbro, and chert LU :::J 

~ JPu, ultramafic rocks o.. _ ...., 

<! ~ 

u ~ ~ }~ffi zo O_i 

Pelitic schist and > 0 

carbonate rocks LU o Dz 

<! 

Arrows indicate direction of 

horizontal movement 

Thrust fault 

Barbs on upper plate 

FIGURE 2.-Reconnaissance geologic map of northern Yukon-Koyukuk province. Sections are shown in figure 4. 

A3 
A4 SHORTER CONTRIBUTIONS TO GENERAL GEOLQGY 

NORTON 

SOUND 

Base from U .S. Geological Survey 

1:2,500,000, 1954 

Graniti c rocks 

and 

Metasedimentary 

volcanic • rocks 

~ wO a:W 

uu <( 

1-

}

' Cfl 

w::> 

a:o Yw 

wu 

a:<( 

0..1-

160" 156" 

EXPLANATION 

Contact 

Dashed whl'fe inferred 

Anticlinal axis 

Showing din~ct o11 uf pluni(C 

Synclinal ax is 

Silowill/i dircctiu11 uf pllmgc 

Fault 

Dashed " ·here i11ji•rrcd 

0 

\5 2 " 

20 40 60 80 100 MILES 

siri"k~- slip fault 

Arrows show dirretion uf horizontal mm•emellt. Dashed where illferred 

Thrust fault 

Barbs un upper platr; dashed 

where i11[errcd 

Trends in metamorphic rocks 

of Brooks Range 

FIGURE 3.-Major structural features in northern Yukon-Koyukuk province" Sections are shown in figure 4 . 

the Brooks Range and Kaiyuh Mountains and on the 

Seward Peninsula. These rocks are everywhere intensely 

deformed, and in many places are completely recrystallized to coarse marbles. Contacts between the carbonate rocks and the pelitic schists are generally 

faulted, and many of the carbonate masses may be 

allochthonous. Evidence for large-scale thrusting of the 

carbonates has been found in the western Brooks 
RECONNAISSANCE GEOLOGY OF THE NORTHERN YUKON-KOYUKUK PROVINCE A5 

Arbitrary datum; flight lines are approximately parallel to contours of 

earth's magnetic field (U.S. Coast and 

Geod. Survey, 1965). Aeromagnetic 

profiles from Dempsey, Meuschke, and 

Andreasen ( 1957 a,b) 

A' AL--------

GAMMAS 

:t 

200~ 0 

6000' 

SEA LEVEL 

6000' 

12,000' 

18,000' 

C QTb 

B 

c· 

2~000'~------------~~--------------------------------~ 

0 20 40 60 MILES 

FIGURE 4.-Characteristic aeromagnetic profiles (A-A', B-B') and diagrammatic cross 

section (C-c') across northern Yukon-Koyukuk province. (See figs. 2 and 3 for 

location of profile and section lines and for explanation of letter symbols. Inferred 

faults not shown in figure 2.) 

Range (Tailleur and others, 1967) and on the Seward 

Peninsula (Sainsbury, 1969). 

The carbonate rocks appear to be chiefly Devonian 

in age but locally may include rocks as old as Silurian 

and Ordovician. Small collections of corals and mollusks of Devonian and possibly Silurian age have been 

reported from several areas bordering the northern 

Yukon-Koyukuk province, including the eastern Seward 

Peninsula (Smith and Eakin, 1911, p. 47-50), the 

southern Brooks Range (Patton and others, 1968), and 

the northern Kuskokwim Mountains (Mertie and Harrington, 1924, p. 21). 

PERMIAN TO JURASSIC ROCKS 

MAFIC VOLCANIC AND INTRUSIVE ROCKS 

Mafic volcanic and intrusive rocks of late Paleozoic 

and possibly early Mesozoic age are extensively exposed 

along the margins of the Yukon-Koyukuk province; 

they form a narrow band that can be traced for more 

than 500 miles along the northern and southeastern 

border of the province. The rocks are widely distributed 

as small intrusive bodies within the adjoining metamorphic terrane (fig. 2). In the Kaiyuh M~untains the 

mafic rocks lie in a broad syncline flanked on the east 

and west by metamorphic rocks. The unit, which 

appears to constitute a typical ophiolite suite (Steinmann, 1926), is composed of pillow basalt, diabase, 

gabbro, radiolarian chert, serpentinized periodotite and 

dunite, and slate. All the volcanic and intrusive rocks 

are altered, in various degrees, to a characteristic 

"greenstone" mineral assemblage of chlorite, epidote, 

and sodic plagioclase; locally some of the fine-grained 

volcanic rocks have a faint foliation. The ultramafic 

rocks occur in bodies ranging in size from tiny fault 

blocks a few feet across to huge tabular masses covering as much as 20 square miles in area (Patton and 

Miller, 1970). Only the largest of these ultramafic 

bodies are shown on the geologic map (fig. 2). 

The mafic volcanic and intrusive rocks clearly are 

older than the Cretaceous sedimentary and volcanic 

rocks and probably are younger than the Devonian and 

older ( ?) metamorphic rocks, which they locally appear 

to intrude. In various places around the rim of the 

province, the mafic rocks have been assigned provisional ages ranging from Devonian to Jurassic (Eakin, 

1916; Patton and Miller, 1966, 1970; Fritts, 1970). 

However, none of the evidence for these assignments is 

completely convincing. To date the only firm evidence 

for an age is a single collection of Permian fossils (B. L. 

Mamet, oral commun., 1970; J. T. Dutro, Jr., oral 

commun., 1971) discovered in 1970 in a sequence of 

mafic volcanic rocks near Bettles (Patton and Miller, 

in U.S. Geological Survey, 1971). 

Elsewhere in northern and western Alaska, similar 

assemblages of mafic rocks have yielded potassiumargon and fossil ages ranging from Permian to Jurassic. 

Tailleur (1970) reported a Jurassic potassium-argon 

age from mafic rocks in the western part of the Brooks 

Range. In the northern part of the range, Jurassic 

fossils have been recovered from an assemblage of mafic 
A6 

~ 

0 

N 

0 

2 

w 

(.J 

(.J 

0 

N 

0 

Cl,) 

w 

:E 

Quaternary 

Tertiary 

Cretaceous 

Jurassic 

Triassic 

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

Pliocene 

Miocene 

Oligocene 

Eocene 

Paleocene 

Upper 

Lower 

Sedimentary rocks 

NORTHERN 

YUKON-KOYUKUK 

PROVINCE 

f I 

EXPLANATION 

Volcanic and hypapyssal 

intrusive rocks 

Plutonic rocks 

COLVILLE BASIN 

FIGURE 5.-Correlation of Mesozoic and Cenozoic rocks in northern Yukon-Koyukuk province and Colville basin. 

volcanic rocks and radiolarian cherts (Patton and Tailleur, 1964, p. 481). In the Yukon River valley, about 

50 miles southeast of the province, mafic volcanic rocks 

belonging to the Rampart Group have been dated by 

fossils as probably Permian, and gabbro that intrudes 

the volcanic rocks has been dated by the potassiumargon method as Triassic (Brosge and others, 1969). 

The weight of evidence suggests that the mafic volcanic and intrusive rocks of the Yukon-Koyukuk province are late Paleozoic (Permian) and possibly early 

Mesozoic (Triassic and Jurassic) in age. The fossilbearing volcanic rocks near Bettles can be dated as 

Permian, but until additional evidence is found, it 

would seem unwise to conclude that all the diverse rock 
RECONNAISSANCE GEOLOGY OF THE NORTHERN YUKON-KOYUKUK PROVINCE A7 

.-----------------~-------------------------------------------.---------------------------. 

PERIOD 

OR 

EPOCH 

Quaternat"y 

Quaternary 

and 

late Tertiary(?) 

Early Tertiary 

and 

Late Cretaceous 

Late Cretaceous 

Late Cretaceous 

and 

Early Cretaceous 

(Cenomanian 

and Albian) 

STRATIGRAPHIC UNITS 

Glacial drift, waterlaid and windblown silt deposits 

<Sand. gravef, clay, coM / felsic volcanic rocks 

1------------,-'> Quartz conglomerate, sandstone, shale, 

and coal (nonmarine marginal trough deposits) 

Calcareous 

graywacke and 

mudstone in western 

part of province 

Shallow marine and 

nonmarine sandstone, 

shale, conglomerate, 

tuff, and coal in eastern part of province 

LOCAL 

STRATIGRAPHIC NAMES 

Kaltllt.nd 

NuiMc» Formations of Yukon 

River Valley 

l :I-... C ICIII ~ 2i ~! o c en • .xO a: 

::"t1Sc 

Sit:] 

,¥-'20:::~ 

Ungalik Con- ~ .! Z > 

1-----------------:7 < and mudstone glomente in 

Volcanic graywacke, conglomeratecn 

1 

.E "1:.1; 

/ Norton Bay .na 

~------------~ ~--------------------------~----------~~--------~--~ 

Early Cretaceous 

(Neocomian) Andesitic volcanic rocks Koyultu6t Group in Koyukuk River region 

Jurassic t•o 

Permian 

Pillow basalt, diabase, gabbro, serpentinized peridotite and 

dtmite, chert and slate 

Devonian 

and 

older(?) 

Pelitic schist and carbonate rocks 

Carbonm rocks included in Baird Group 

in sou1hem Brooks Range 

FIGURE 6.-Major stratigraphic units in northern Yukon-Koyukuk province and its borderlands. 

types which compose this ophiolitelike assemblage are 

exactly the same age. 

LOWER CRETACEOUS (NEOCOMIAN) ROCKS 

ANDESIDC VOLCANIC ROCKS 

The base of the-Cretaceous sequence in the northern 

Yukon-Koyukuk province is made up of marine andesitic volcanic rocks that appear to underlie nearly the 

entire province (fig. 7). The andesitic rocks are widely 

exposed on a broad uplift that extends eastward across 

the province from the Seward Peninsula and Kotzebue 

Sound to the Koyukuk valley (fig. 2). Aeromagnetic 

profiles suggest that they also underlie large parts of 

the Koyukuk Flats and Kobuk-Selawik Lowlands 

(Zietz and others, 1959; Patton and Miller, 1968; Patton and others, 1968). In the Koyukuk River region 

these andesitic rocks have been referred to the Koyukuk Group (Schrader, 1904; Smith, 1913; Martin, 

1926; Imlay and Reeside, 1954) (fig. 6). 

The andesitic volcanic assemblage is composed predominantly of volcaniclastic rocks, but it also contains 

porphyritic pyroxene andesite flows and hypabyssal 

rocks. The volcaniclastic rocks include crystal and 

lithic tuffs, massive breccias, agglomerates and conglomerates, and tuffaceous graywacke and mudstone. 

Lenticular masses as much as 300 feet thick of shelly 

limestone with abundant Buchia are intercalated in 

the volcanic sequence in the upper Koyukuk and 

Kobuk valleys. Along the Hogatza plutonic belt (fig. 

3), broad areas of volcanic rocks have been propylitically altered to a uniformly hard pale-green hornfels 

composed largely of chlorite, epidote, calcite, and sodic 

plagioclase. 
AB SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

164° 680 

164° 680 

/ 

I 

I 

I 

I 

I 

156° 

'T,_G ,._,..~.-

156° 

Early Cretaceous 

(Neocomian) 

(Albian and Cenomanian) 

EXPLANATION 

Nonmarine 

-

marginal trough 

deposits of quartz conglomerate, sandstone, tuffs, 

and coal 

Nearshore and nonmarine cal- - Nearshore and nonmarine - careous graywacke, mudstone sandst-Qne, mudstone, and, 

and conglomerate coal 

Marine volcanic graywacke 

mudstone and conglomerate 

Marine andesitic volcanic 

assemblage, chiefly volcaniclastic rocks 

.. 

Approximate outline of 

Yukon- Koyukuk province 

Probable direction of 

sediment transport 

0 50 100 

156° 

150 MILES 

Early Cretaceous 

(Albian) 

Late Cretaceous 

Present 

FIGURE 7.-Cretaceous depositional basins and present-day distribution of major stratigraphic units in northern YukonKoyukuk province. Cretaceous depositional basins not palinspastically restored for offset along Kaltag fault. 
RECONNAISSANCE GEOLOGY OF THE NORTHERN YUKON-KOYUKUK PROVINCE A9 

The total thickness of the andesitic volcanic rocks 

is not known because the base of the assemblage is not 

exposed. Along the northern and southeastern margins 

of the province, the volcanic rocks are overlapped by 

clastic sedimentary deposits of late Early and Late 

Cretaceous age; and on the Seward Peninsula at the 

western margin of the province, the volcanic rocks 

appear to be in thrust-fault contact with metasedimentary rocks of Paleozoic and older age. An estimated 5,000 feet of andesitic volcanic rocks is exposed 

along the Koyukuk River below Hughes, but the full 

thickness of the assembage may be several times this. 

Marine fossils have been found in the andesitic volcanic rocks at 25 widely scattered localities in the 

northern Yukon-Koyukuk province. Earliest Cretaceous (Neocomian) species of Buchia are common to 

nearly all the collections. Five potassium-argon dates 

obtained from flows and hypabyssal intrusives also 

have yielded Early Cretaceous ages ranging from 

134+5 m.y. to 117+4.3 m.y. (Patton and Miller, 1966; 

Patton, 1967; Patton and others, 1968; Geol. Soc. 

London, 1964). 

Although most of the andesitic volcanic rocks are 

earliest Cretaceous (Neocomian) in age, the unit 

locally includes some younger crystal and lithic tuff 

beds that seem to be laterally gradational with sedimentary rocks of mid-Cretaceous (Albian and Cenomanian) age. 

LOWER AND UPPER CRETACEOUS ROCKS 

GRAYWACKE, MUDSTONE, SANDSTONE, AND COAL 

Terrigenous sedimentary rocks of Cretaceous age 

underlie about half of the northern Yukon-Koyukuk 

province (fig. 2). Most of these rocks were deposited 

during a relatively short interval in mid-Cretaceous 

(Albian and Cenomanian) time and are characterized 

throughout the province by wide variations in thickness 

and lithology. Owing to their "poured in" nature and 

lack of sorting, these strata are not amenable to rigorous stratigraphic treatment and classification. Earlier 

workers referred these beds to the Bergman Group 

(Schrader, 1904; Martin, 1926; Imlay and Reeside, 

1954) in the upper Koyukuk-Kobuk region and to the 

Shaktolik Group (Smith and Eakin, 1911; Martin, 

1926; Imlay and Reeside, 1954) in the Yukon RiverNorton Sound region (fig. 6). However, the lithologic 

character and areal limits of the two groups as presently 

defined are so vague and generalized that the names 

appear to have little practical value and are hereby 

abandoned. 

In this report the Cretaceous sedimentary rocks are 

subdivided into four lithologic units which, though 

broadly gradational, can be mapped in a gross way. 

Probable areas of deposition and present-day distribution of these units are shown in figure 7. 

VOLCANIC GRAYWACKE AND MUDSTONE 

Marine turbidites, chiefly volcanic graywacke and 

mudstone, accumulated in enormous thicknesses during late Early Cretaceous (Albian) time. These rocks 

are widely distributed in the northeastern part of the 

province and along the drainage divide between the 

Yukon River and Norton Sound. The turbidites appear 

to have been deposited in two basins that were separated by a broad arcuate high of Neocomian andesitic 

volcanic rocks extending from Kotzebue Sound and 

the Seward Peninsula to the Koyukuk River valley 

(fig. 7). Typically these rocks form a monotonously 

uniform, rhythmically interbedded succession with few 

distinctive horizon markers. At the margins of the 

province they contain masses of polymict conglomerate 

as much as 5,000 feet thick, and along the borders of 

the volcanic high they are intercalated with crystal and 

lithic tuffs. Field and aeromagnetic data suggest that 

these volcanic graywacke and mudstone beds have an 

aggregate thickness of more than 20,000 feet along the 

Yukon River-Norton Sound divide (Gates and others, 

1968). 

Modal grain counts show that the volcanic graywackes are composed, on the average, of about 40 percent volcanic rock fragments and feldspar (chiefly 

plagioclase), 35 percent argillaceous matrix, and less 

than 10 percent quartz (Patton and Miller, 1966; Patton, 1967). The volcanic debris appears to have been 

derived in part from the late Paleozoic and early Mesozoic mafic volcanic rocks that rim the province, and 

in part from earliest Cretaceous andesitic volcanic 

rocks within the province (fig. 7). 

Fossils in the volcanic graywacke and mudstone unit 

are confined, for the most part, to isolated occurrences 

of ammonites. Collections from 18 separate localities in 

the northern part of the province include Grantziceras, 

Lemuroceras, Cleoniceras, Beudanticeras, Arcthoplites, 

and Gastroplites, wh.ich date these rocks as late Early 

Cretaceous (Albian). 

CALCAREOUS GRAYWACKE AND MUDSTONE 

In the western part of the province the volcanic 

graywacke and mudstone turbidites are overlain gradationally by an assemblage of shallow-water calcareous 

graywacke and mudstone, possibly as much as 5,000 

feet thick. These calcareous strata appear to have been 

deposited in a broad trough that extended along the 

western margin of the province from the Kobuk River 

to beyond the south edge of the report area (fig. 7). 

The strata coarsen westward across the trough and at 

the west edge of the province grade into nonmarine 
A10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

conglomeratic coal-bearing deposits that lap onto the 

adjoining volcanic and metamorphic terrane. Several 

small outlying masses of the coal-bearing deposits are 

preserved in local structural depressions within the 

metamorphic terrane of the eastern Seward Peninsula 

(fig. 2). 

The calcareous graywacke and mudstone are composed of from 30--80 percent carbonate (Patton, 1967; 

Patton and Miller, 1968), much of which appears to be 

detrital and derived from Paleozoic limestone and dolomite of the Seward Peninsula and western Brooks 

Range (fig. 7). Massive pebble-cobble conglomerates 

made up almost entirely of limestone and dolomite 

clasts occur along the west edge of the province on the 

Seward Peninsula and in the Waring Mountains. 

The calcareous graywacke and mudstone assemblage 

is tentatively assigned a late Early and an early Late 

Cretaceous age. Fossils in these strata are notably 

scarce, but several small collections of shallow-water 

mollusks of probable late Early Cretaceous (Albian) 

age have been found in the lower part of the unit in the 

Waring Mountains (Patton and Miller, 1968), and 

samples collected from the coal-bearing facies on the 

Seward Peninsula have yielded pollen of both Early 

and Late Cretaceous ages (E. B. Leopold, written 

commun., 1963). In the Waring Mountains, this 

assemblage is overlain by nonmarine marginal trough 

deposits which have been dated by potassium-argon 

methods at 84 m.y. (Patton and Miller, 1968). 

SANDSTONE, SILTSTONE, SHALE, AND COAL 

In the southeastern part of the province, the volcanic 

graywacke and mudstone unit is succeeded by shallow 

marine and nonmarine beds of sandstone, siltstone, 

shale, and coal which appear to be broadly correlative 

with the calcareous graywacke and mudstone unit (fig. 

6). These coal-bearing beds, which have an aggregate 

thickness of at least 10,000 feet (Patton, 1966), consist of a regressive sequence that grades upward from 

marine shale and sandstone into nonmarine shale, siltstone, sandstone, and coal. Winnowed strand-line sandstone and quartz conglomerate occur locally in the zone 

of interfingering marine and nonmarine beds. This unit 

is generally distinguished from the other mid-Cretaceous sedimentary units by better sorting and by a 

higher percentage of quartz and other resistant rock 

and mineral detritus. 

The coal-bearing beds appear to have been deposited 

in a broad shallow trough that extended along the 

eastern margin of the province from the north edge of 

the Koyukuk Flats to beyond the south edge of the 

report area (fig. 7). On the west they rest on volcanic 

graywacke and mudstone, but eastward they appear to 

overlap onto andesitic volcanic rocks. The probable 

source of these deposits was the metamorphic terrane 

in the Kaiyuh Mountains and Kokrines-Hodzana Highlands bordering the southeastern margin of the province (figs. 2 and 7). Some ash-fall tuffs were intercalated 

with the deposits at the north end of the depositional 

trough, presumably derived from the broad volcanic 

complex that lay to the north. 

The exposures of this coal-bearing sequence along 

the Yukon River have been described in detail by several workers (Martin, 1926; Hollick, 1930; Patton and 

Bickel, 1956a). Martin (1926, p. 400) assigned the 

marine beds in the lower part of the sequence to the 

Nulato Formation and the nonmarine beds in the upper 

part to the Kaltag Formation (fig. 6). He described a 

third unit, the Melozi Formation, which he supposed 

represented a nonmarine tongue beneath the Nulato 

but which more recent evidence (Patton and Bickel, 

1956a; Bell, 1963, p. 10) suggests· is a marginal facies 

of the Kaltag. Owing to wide variations in lithology and 

poor exposure, it has not been possible to map the 

Nulato and Kaltag Formations beyond their type areas. 

The marine strata in the lower part of the sequence 

contain abundant shallow-water mollusks of late Early 

Cretaceous (Albian) age including Inoceramus altifluminus McLearn and Gastroplites kingi McLeam. 

The contact with the overlying nonmarine beds appears 

to be diachronous, however, for Turrilites acutus Passy 

of early Late Cretaceous (Cenomanian) age (D. L. 

Jones, written commun., 1968) was discovered recently 

in the upper part of the marine beds on the Anvik 

River near the south edge of the map area. The marine 

strata, including the Nulato Formation of the Yukon 

River area, must therefore be considered as both Early 

and Late Cretaceous in age rather than entirely Early 

Cretaceous as suggested by Imlay and Reeside (1954). 

The nonmarine beds, including the Kaltag Formation, contain abundant plant fossils, but their exact age 

remains in dispute. Hollick (1930) first described the 

flora in detail and assigned these beds to the Late Cretaceous. Later, the age was revised by Imlay and Reeside (1954, p. 237) to late Early Cretaceous (Albian), 

chiefly on the basis of the close association of these 

beds with the mollusk-bearing marine beds of the 

Nulato Formation. Subsequently, however, this Early 

Cretaceous age assignment was questioned by Bell 

( 1963), who pointed out the strong floral correlation 

between the Kaltag Formation and the Late Cretaceous 

(Cenomanian) Dunvegan Formation of western Canada. The recent discovery of the Cenomanian mollusk 

Turrilites acutus Passy in the underlying marine beds 

on the Anvik River seems to provide firm evidence that 

at least some of the plant-bearing beds included in this 

unit are Late Cretaceous in age. 
RECONNAISSANCE GEOLOGY OF THE NORTHERN YUKON-KOYUKUK PROVINCE All 

MARGINAL TROUGH DEPOSITS 

A narrow band of nonmarine quartz conglomerate 

has been traced for nearly 450 miles along the northern 

and southeastern margin of the Yukon-Koyukuk 

province (fig. 7). Small patches of these conglomerates 

are found along northeast-trending faults in the vicinity 

of·Hughes on the Koyukuk River. In addition to quartz 

conglomerate, these molasselike deposits contain minor 

amounts of quartz sandstone, shale, thin bituminous 

coal beds, and ash-fall tuffs. They rest uncomformably 

upon Albian and Cenomanian sedimentary deposits 

within the province and lap onto the pre-Cretaceous 

igneous and metamorphic rocks that rim the province. They appear to have accumulated in a narrow 

fault-bounded trench bordering a highland source terrane composed of Devonian and older metamorphic 

rocks (fig. 7). Along the lower Kobuk River, these 

deposits appear to be at least 3,000 feet thick (Patton 

and Miller, 1968). · 

Poorly preserved plant fossils of probable Cretaceous 

age have been collected from these deposits at several 

localities, and biotite from an interbedded ash-fall tuff 

has been dated by potassium-argon methods as 83.4 

+2.2 m.y. (middle Late Cretaceous) Patton and Miller, 1968). 

UPPER CRETACEOUS AND LOWER TERTIARY ROCKS 

FELSIC VOLCANIC ROCKS 

Felsic extrusive and hypabyssal rocks, chiefly latites 

and rhyolites, are widespread in the eastern part of the 

province (fig. 2). Subaerial flows and tuffs, as much as 

2,000 feet thick, occur in gently to moderately folded 

synclines that rest unconformably upon andesitic volcanic rocks of earliest Cretaceous age and upon sedimentary rocks of mid-Cretaceous age. Locally in the 

northeastern part of the province, these volcanics interfinger with marginal trough quartz conglomerate 

deposits of Late Cretaceous age. Swarms of small sills, 

dikes, and pluglike bodies of latite and rhyolite pervasively intrude older Cretaceous volcanic and sedimentary rocks along a 60-mile-wide belt bordering the 

southeastern margin of the province. Many of these 

hypabyssal bodies appear to be alined along northeasttrending faults. 

The felsic volcanic rocks range in age from Late Cretaceous to early Tertiary. A potassium-argon age of 85 

m.y. (Late Cretaceous) was obtained from a crystal 

tuff in the upper Selawik River region (Patton and 

others, 1968), and an age of 58 m.y. (early Tertiary) 

from a felsic flow in the Koyukuk River valley. A pollen 

flora of early Tertiary age (E. B. Leopold, written 

commun., 1966) was recovered from terrestrial sediments interbedded with the volcanics on the Yukon 

River. 

TERTIARY ROCKS 

NONMARINE COAL-BEARING DEPOSITS 

Small deposits of poorly consolidated nonmarine 

coal-bearing beds of Teritary age have been found at 

three widely scattered localities in the northern YukonKoyukuk province (fig. 8). All three deposits appear to 

be of limited extent and confined to small structural or 

topographic basins. 

MANGOAK RIVER 

Moderately dipping beds of gravel and sand are 

exposed in a 30-foot-high stream bluff on the Mangoak 

River at the south edge of the Kobuk-Selawik Lowlands (Patton and Miller, 1968). The deposits appear 

to lie in an east-trending fault zone along the north 

front of the Selawik Hills. Lignitic coal, found in float 

at the base of the bluff, contains an abundant Tertiary 

pollen flora (E. B. Leopold, written commun., 1970). 

ELEPHANT POINT 

A 2-foot seam of lignite in a beach bluff of silt near 

Elephant Point on Kotzebue Sound was reported by 

Quackenbush in 1909. This locality was revisited by 

R. J. Janda and D. S. McCulloch (oral commun.) in 

1961 and although the exposure of coal was not found, 

float of coal containing a pollen flora of early Tertiary 

age (E. B. Leopold, oral commun., 1963) was collected 

from the beach nearby. 

NORTON SOUND 

Clays containing lignitic coal were discovered in a 

badly slumped beach bluff 10 miles south of Unalakleet on the shore of Norton Sound. Samples of the coal 

yielded an abundant pollen flora of early Teritary age 

(E. B. Leopold, written commun., 1966). 

UPPER TERTIARY(?) AND QUATERNARY ROCKS 

BASALT 

Nearly flat-lying flows of olivine basalt cover more 

than 2,000 square miles of the province in the Buckland River area southeast of Kotzebue Sound and 

along the south shore of Norton Sound near St. 

Michael. The basalt appears to have been extruded 

over a terrain of moderate relief and locally is as much 

as 500 feet thick. The flows are deeply incised by Holocene drainages and thickly mantled by late Pleistocene 

wind-blown silts and glacial drift. Most of the flows are 

believed to be early Pleistocene and probably Pliocene 
A12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

156° 148° 

RANGE \ 

------- ---Bettles 

__ ARCTIC CIRCLE \\ --------------------------- lf0 ·set 

• MANGOAK RIVER \l~ 

SEWARD 

PENINSULA 

ELEPHANT 

I POINT 

\ 

\ 

\ 

\ 

' 

0 

EXPLANATION 

Approximate outline of 

Yukon-Koyukuk province 

50 100 MILES 

FIGURE B.-Occurrences of Tertiary nonmarine coal-bearing deposits in northern Yukon-Koyukuk province. 

in age, although some fresh unmodified cinder cones 

and flows that are clearly Holocene in age occur in both 

the Buckland River and St. Michaels areas. 

INTRUSIVE ROCKS 

CRETACEOUS ROCKS 

GRANITIC ROCKS 

The Hogatza belt of granitic plutons extends for a 

distance of 225 miles across the north-central part of 

the province from the Seward Peninsula to Koyukuk 

River valley (fig. 3). Sixteen separate plutons that 

range in area from 3 to 350 square miles have been 

mapped along this belt (Miller, 1970; Miller and others, 

1966; Patton, 1967). The plutons, which have been 

dated by the potassium-argon method at 11 selected 

localities, can be divided into a syntectonic mid-Cretaceous (,-~100 m.y.) suite in the western and central 

parts of the belt and a late tectonic or posttectonic 

Late Cretaceous (,_,go m.y.) suite in the eastern part. 

The mid-Cretaceous suite, composed of monzonite, 

syenite, quartz monzonite, and minor alkaline subsilicic 

rocks, intrudes earliest Cretaceous andesitic volcanic 

rocks and is stratigraphically bracketed by the Albian 

and Cenomanian sedimentary deposits. The Late Cretaceous suite, chiefly of quartz monzonite and granodiorite, intrudes both the andesitic volcanic rocks and 

the sedimentary rocks and appears to be related spatially and temporally to the Late Cretaceous terrestrial 

felsic volcanic rocks. 

Intrusives of quartz monzonite and granodiorite are 

also widespread in the crystalline compex of the Kokrines-Hodzana Highlands. Two potassium-argon determinations from these plutons have yielded mid-Cretaceous (,-~100 m.y.) ages (Brosge and Reiser, 1964). 

STRUCTURE 

:MID-CRETACEOUS BASINS 

The mid-Cretaceous sedimentary rocks occur in two 

vaguely defined structural depressions: the Lower 

Yukon basin, which extends in a broad band alcng the 

west edge of the province from the latitude of Kotzebue 

Sound southward to the Yukon delta, and the KobukKoyukuk basin, which occupies a V -shaped area along 

the northern and southeastern margins of the province 
RECONNAISSANCE GEOLOGY OF THE NORTHERN YUKON-KOYUKUK PROVINCE A13 

SEWARD 

PENINSULA 

1~· ~a· 

R A N G E 

0 

EXPLANATION 

Approximate outline of 

Yukon - Koyukuk province 

50 100 MILES 

FIGURE 9.-Mid-Cretaceous basins. 

(fig. 9). The two basins are separated by a broad structural high of Early Cretaceous volcanic rocks that 

trends eastward from Kotzebue Sound to the Koyukuk 

River and then southward beneath the Koyukuk Flats. 

Near the Kaltag fault, the high narrows and the basins 

converge. South of the Kaltag fault, the two basins 

appear to be offset to the southwest about 60-80 miles 

(fig. 9). 

Aeromagnetic data indicate that the Lower Yukon 

basin may be 15,000-25,000 feet deep (Zietz and 

others, 1959). Magnetic profiles over the basin are 

characteristically smooth and free of large-amplitude 

anomalies suggesting that the underlying Early Cretaceous volcanic rocks probably are buried at great 

depth (fig. 4). Nulato No. 1, a stratigraphic test hole 

located on the east side of the basin, was drilled to a 

depth of 12,000 feet entirely in Cretaceous sedimentary 

rocks (fig. 9). 

Few geophysical data are available for the V-shaped 

Kobuk-Koyukuk basin. Surface mapping suggests that 

the north limb may be filled with more than 10,000 feet 

of volcanic graywacke and mudstone and nonmarine 

marginal trough deposits. The sedimentary section 

along the southeast limb, however, seems to be substantially thinner, probably less than 5,000 feet thick. 

Aeromagnetic profiles that cross the southeast limb 

east of the Koyukuk Flats are typified by steep gradient and high-amplitude anomalies reflecting in part the 

thinness of the Cretaceous sequence and in part of the 

local occurrence of Late Cretaceous and early Tertiary 

felsic volcanic and hypabyssal rocks (fig. 4) . 

CENOZOIC BASINS 

The northern Yukon-Koyukuk province includes two 

broad lowland areas, the Koyukuk Flats and the 

Kobuk-Selawik Lowlands, which are thickly blanketed 

by Quaternary silts and glacial deposits. Payne ( 1955) 

and Miller, Payne, Gryc (1959) have suggested th.at 

these two areas, as well as other large interior and 

coastal lowlands of central Alaska, may represent Cenozoic structural depressions that contain thick sections 

of Tertiary strata. The following assessment of the subsurface character of the Kobuk-Selawik Lowlands and 

Koyukuk Flats is based upon scattered aeromagnetic 
A14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

profiles across the lowlands and recent bedrock mapping around their margins. 

KOBUK-SELAWIK LOWLANDS 

It seems unlikely that Cretaceous or Tertiary sedimentary rocks are present in significant thicknesses 

beneath the Quaternary surficial deposits in the eastern part of the Kobuk-Selawik Lowlands. Intrusive 

and volcanic rocks crop out near the south edge of Selawik Lake, and aeromagnetic profiles along the north 

side of the lake show steep-gradient, large-amplitude 

anomalies which suggest that igneous rocks are at shallow depth (Andreasen, 1960; Dempsey and others, 

1957c). Aeromagnetic profiles over the Kobuk River 

delta in the western part of the lowland are relatively 

smooth and free of marked magnetic anomalies (Miller 

and Anderson, 1969). This smoothness probably reflects the presence of a thick wedge of Quaternary fill 

beneath the delta, but the possibility that the delta and 

adjoining parts of Kotzebue Sound may also be underlain by Cretaceous or Tertiary sedimentary rocks cannot be ruled out. Recent marine geophysical surveys 

suggest that as much as 3 km of moderately deformed 

strata of Cenozoic and possibly Late Cretaceous age 

underlies large parts of the adjoining Chukchi Sea 

basin ( Grantz and others, 1970). 

KOYUKUK FLATS 

Aeromagnetic profiles across the Koyukuk Flats, 

characterized by anomalies of large amplitude and steep 

gradient, indicate the presence of highly magnetic rocks 

at shallow depths (fig. 4). Surface mapping around the 

margin of the flats suggests that the Quaternary alluvial deposits are probably underlain by andesitic volcanic rocks of Early Cretaceous age (fig. 2). The flats 

do not appear to contain any substantial thickness of 

Cenozoic sedimentary strata, except along the Kaltag 

fault where evidence of recent sinking (Patton and 

Hoare, 1968) and a gravity low (Barnes, 1969) indicate 

the presence of a fault trench filled with late Cenozoic 

deposits. 

MESOZOIC AND EARLY CENOZOIC 

STRUCTURAL TRENDS 

All pre-Tertiary rocks in the northern Yukon-Koyukuk province are intensely deformed and are characterized by steep dips, tight folds, and closely spaced highangle faults. (See, for example, Patton and Bickel, 

1956a, b; Bickel and Patton, 1957; Patton, 1966, 1967.) 

Some broad regional flexures are shown in figure 3, but 

it should be emphasized that these larger features are 

everywhere complicated by small, tightly appressed 

folds and innumerable small faults. 

Fold and fault trends along the northern margin of 

the province roughly parallel the east-west grain of the 

Brooks Range. The broad structural high along the 

Hogatza plutonic belt also has a general east-west 

alinement (fig. 3). 

In the western part of the province and adjoining 

parts of the Seward Peninsula, fold trends are predominantly north-south and northeast-southwest (fig. 3). 

The Devonian and older ( ?) metamorphic rocks of the 

Seward Peninsula appear to be thrust eastward onto 

Early Cretaceous andesitic volcanic rocks, and folds 

along the western margin of the province in the midCretaceous sedimentary rocks are overturned to the 

east. North of the Kaltag fault, fold trends are bent to 

the southwest, possibly by large-scale fault drag. South 

of the fault, a southwest regional grain persists as far 

as the Yukon delta. 

Folds and faults in the eastern part of the province 

have a strong northeast-southwest orientation parallel 

to the southeast margin of the province. The east-west 

grain of the Brooks Range and Hogatza plutonic belt 

are deflected to the northeast in the northeastern apex 

of the province (fig. 3). 

An attempt to fit the complex pattern of folds and 

faults in the northern Yukon-Koyukuk province into 

the regional structural framework of northern and western Alaska is shown in figure 10. Trend lines have been 

taken largely from the tectonic map of North America 

(King, 1969). 

BROOKS RANGE TRENDS 

The oldest folding is represented by the east-west 

Brooks Range trends, which affected nearly all of northern Alaska and the north edge of the Yukon-Koyukuk 

province. Deformation along these lines began in Late 

Jurassic with the rise of the ancestral Brooks Range 

and continued through Cretaceous (Patton and Tailleur, 1964, p. 493). In the northern Yukon-Koyukuk 

province, orogenic activity in Cretaceous time was 

manifest in the plutonic and volcanic activity along the 

Hogatza plutonic belt and in the east-west folding in 

the Kobuk and upper Koyukuk region. 

LISBURNE-SEWARD TRENDS 

The north-south trends along the western part of the 

province seem to be part of a broad slightly arcuate 

fold belt that stretches from Cape Lisburne southward 

through the Seward Peninsula and western YukonKoyukuk province to the Yukon River delta. A strong 

eastward -directed compression is reflected in the eastward thrusting of Paleozoic onto Mesozoic strata at 

Cape Lisburne (Campbell, 1967) and on the eastern 

Seward Peninsula and in the westward dip of axial fold 

planes in the Cretaceous strata in the western part of 

the Yukon-Koyukuk province. Deflection and over-
RECONNAISSANCE GEOLOGY OF THE NORTHERN YUKON-KOYUKUK PROVINCE A15 

180° 

O<f.o 

OQ 0 

0 

I 

I 

I 

200 MILES 

160° 140° 

oF 

Taken largely from King, 1969 

FIGURE 10.-Mesozoic and early Cenozoic structural trends in northern and western Alaska. Northern part of Yukon-Koyukuk 

province is shaded. 

thrusting of east-west Brooks Range trends clearly indicate that these Lisburne-Seward trends are younger 

than Brooks Range trends, probably Late Cretaceous 

and early Tertiary. 

YUKON TRENDS 

Northeast-trending folds and faults along the southeastern margin of the province appear to be part of a 

broad band of northeast-trending structures that extend from the lower Yukon River through east-central 

Alaska into the eastern Brooks Range. To the southwest these trends merge with, and become indistinguishable from, the Lisburne-Seward trends. To the 

northeast they appear to deflect east-west Brooks 

Range trends and die out in a conspicuous right-angle 

bend of the Brooks Range fold belt. The Yukon trends 

are clearly younger than the Brooks Range trends, but 

their relation to the Lisburne-Seward trends is uncertain. 

FAULTS 

KALTAG FAULT 

The Kaltag fault has been traced for 275 miles across 

west-central Alaska from Norton Sound to near the 

mouth of the Tanana River (Patton and Hoare, 1968). 

Recent marine geophysical studies by Scholl and others 

( 1970) suggest that it may continue another 200 miles 

southwestward across the Bering Sea shelf. The Kaltag 
A16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

fault transects the Yukon-Koyukuk province between 

Ruby and Unalakleet and appears to offset all major 

lithologic and structural trends (figs. 2 and 3). Province boundaries and geologic trends within the province 

appear to be displaced 40 to 80 miles right laterally 

(Patton and Hoare, 1968). The structural grain north 

of the fault in the western part of the province bends 

southwestward and intersects the fault at a small angle 

(fig. 3). This southwestward bend is accompanied by 

a marked increase in deformation of mid-Cretaceous 

strata and may represent large-scale fault drag. The 

major lateral displacement of Cretaceous and early 

Tertiary trends seems to have occurred before middle 

Tertiary time, but some movement has continued into 

Holocene time, as evidenced by local drainage offsets 

of as much as 1.5 miles (Patton and Hoare, 1968). 

KOBUK FAULT ZONE 

The Kobuk fault zone extends along the northern 

border of the province for 300 miles and may continue 

an additional150 miles eastward to the Porcupine lineament (figs. 3 and 10). This zone of faulting, as much 

as 20 miles wide, is made up of numerous closely spaced 

high-angle fractures, only a few of which are shown in 

figure 3. The Kobuk trench, a narrow linear topographic depression which can be traced along the south 

edge of the Brooks Range from middle reaches of the 

Kobuk River eastward to the Yukon Flats, lies within 

the fault zone and appears to be a fault-controlled 

feature. 

It has been suggested by Grantz (1966) that the 

Kobuk fault zone represents a major strike-slip fault, 

possibly the offset extension of the Tintina fault. Although no clear-cut evidence of lateral offset has been 

found, some large-scale horizontal dislocation seems 

required by major structural discontinuities across the 

fault zone. For example, along the lower Kobuk River, 

trends in the Brooks Range bend sharply southward to 

intersect the fault zone at nearly right angles (fig. 3). 

On the south side these trends are opposed by eastwest and northeast-southwest trends in the Waring 

Mountains. Farther east in the region of the central 

Kobuk River, a broad band of northwest-trending folds 

in the Cretaceous sedimentary rocks on the south side 

of the fault is cut off and opposed on the north side by 

east-west trends in the metamorphic rocks of the 

Brooks Range. 

The age of the major faulting is thought to be latest 

Cretaceous or Tertiary, but some Quaternary movementis indicated by local rupturing of Pleistocene glacial drift (Patton and Miller, 1966). 

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Andreasen, G. E., 1960, Total intensity aeromagnetic profiles of 

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Barnes, D. F., 1969, Progress on a gravity map of Alaska: EOS, 

Am. Geophys. Union Trans., v. 50, no. 10, p. 550-552. 

Bell, W. A., 1963, Upper Cretaceous of the Dunvegan, Bad 

Heart, and Milk River Formations of western Canada: 

Canada Geol. Survey Bull. 94, 76 p. 

Bickel, R. S., and Patton, W. W., Jr., 1957, Preliminary geologic 

map of the Nulato and Kateel Rivers area, Alaska: U.S. 

Geol. Survey Misc. Geol. Inv. Map I-249, scale 1=125,000. 

Brosge, W. P., Lanphere, M.A., Reiser, H. N., and Chapman, 

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Campbell, R. H., 1967, Areal geology in the vicinity of the 

Chariot site, Lisburne Peninsula, northwestern Alaska: 

U.S. Geol. Survey Prof. Paper 395, 71 p. 

Dempsey, W. J., Meuschke, J. L., and Andreasen, G. E., 1957a, 

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Alaska: U.S. Geol. Survey open-file report, 1 sheet. 

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---1957c, Total intensity aeromagnetic profiles of west 

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Eakin, H. M., 1918, The Cosna-Nowitna region, Alaska: U.S. 

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Geological Society of London, 1964, The Phanerozoic timescale: Geol. Soc. London Quart. Jour., v. 120, supp., p. 

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Grantz, Arthur, 1966, Strike-slip faults in Alaska: U.S. Geol. 

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Grantz, Arthur, Wolf, S. C., Breslau, Lloyd, Johnson, T. C., 

and Hanna, W. F., 1970, Reconnaissance geology of the 

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Geologists, p. F1-F28. 

Hollick, Arthur, 1930, The Upper Cretaceous floras of Alaska: 

U.S. Geol. Survey Prof. Paper 159,123 p. 

Imlay, R. W., and Reeside, J. B., Jr., 1954, Correlation of Cretaceous formations of Greenland and Alaska: Geol. Soc. 

America Bull., v. 65, no. 3, p. 223-246. 

King, E. R., 1961, An aeromagnetic profile from Anchorage to 

Nome, Alaska: Geophysics, v. 26, no. 6, p. 716-726. 

King, P. B., compiler, 1969, Tectonic map of North America: 

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RECONNAISSANCE GEOLOGY OF THE NORTHERN YUKON-KOYUKUK PROVINCE A17 

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