Stratigraphy and Structure 

of the Ekokpuk Creek Area, 

North-Central Brooks Range, Alaska 
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Stratigraphy and Structure 

of the Ekokpuk Creek Area, 

North-Central Brooks Range, Alaska 

, 

By WILLIS H. NELSON and BELA CSEJTEY, JR. 

U.S. GEOLOGICAL SURVEY BULLETIN 1848 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication 

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1990 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

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Library of Congress Cataloging-in-Publication Data 

Nelson, Willis H. (Willis Howard), 192oStratigraphy and structure of the Ekokpuk Creek area, north-central 

Brooks Range, Alaska I by Willis H. Nelson and Bela Csejtey, Jr. 

p. em. - (U.S. Geological Survey bulletin; 1848) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1848 

1. Geology, Stratigraphic-Mississippian. 2. Geology-Aiaska-Ekokpuk 

Creek Region. 3. Geology, Structurai-Aiaska-Ekokpuk Creek Region. 

4. Chert-Aiaska-Ekokpuk Creek Region. 5. Limestone-Aiaska-Ekokpuk 

Creek Region. I. Csejtey, Bela, 1934- . II. Title. Ill. Series. 

QE75.B9 no. 1848 557.3 s-dc20 89-600289 

[QE672] [551.7'51'097987] CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 1 

Description and interpretation of stratigraphic units 1 

Kanayut Conglomerate 1 

Kayak Shale 2 

Lisburne Group 2 

Kuna Formation 2 

Wachsmuth and Alapah(?) Limestones, undivided 3 

Siksikpuk Formation 3 

Otuk Formation 6 

Sedimentary rocks of Cretaceous age 6 

Quaternary deposits 6 

Structure 6 

Conclusions 8 

References cited 8 

FIGURES 

1. Index map to Chandler Lake 1 o by 3° quadrangle and location of study area 

(area of fig. 2) 2 

2. Geologic map and cross sections of part of Ekokpuk Creek area 4 

Contents Ill 

Stratigraphy and Structure of the Ekokpuk Creek Area, 

North-Central Brooks Range, Alaska 

By Willis H. Nelson and Bela Csejtey, Jr. 

Abstract 

Sedimentary rocks of Devonian to Cretaceous age in 

the vicinity of Ekokpuk Creek in the north-central Brooks 

Range are deformed and displaced northward by northwardverging tight folds and thrust faults. Here the proximity of 

black chert to limestone, both of the Lisburne Group of 

Mississippian age, is probably the result of original deposition rather than the result of tectonic juxtaposition as has 

been previously postulated. 

INTRODUCTION 

A small area along Ekokpuk Creek in the northcentral Brooks Range of northern Alaska, in the southwest corner of the Chandler Lake 1:250,000 quadrangle 

(fig. 1 ), is of particular interest because it contains two 

contrasting coeval facies of the Mississippian Lisburne 

Group- black chert and limestone-adjacent to one 

another. The proximity of these two facies has been 

attributed to tectonic juxtaposition by displacement 

along low-angle northward-directed thrust faults, which 

are common in the area. However, the distribution of 

these two facies as shown by our mapping a short 

distance south of the high point along cross section A-A' 

(fig. 2) strongly suggests that these two facies were 

originally deposited close to each other. The Ekokpuk 

Creek area is also of interest because it contains the 

southernmost known exposures of Cretaceous rocks in 

the central Brooks Range and because these Cretaceous 

rocks are the youngest rocks known to be involved in the 

low-angle imbricate faulting common in the area. 

Fieldwork in the 35-km2 study area (fig. 2) was 

done during June 1985 and included 13 days of foot 

traverses with limited helicopter support. The work was 

part of the U.S. Geological Survey's assessment of the 

mineral resource potential of the Chandler Lake and 

Killik River 1:250,000-scale quadrangles. 

Manuscript approved for publication, August 9, 1989. 

Acknowledgments 

Oxygen isotope data cited in this report were 

determined by Teresa Presser in the laboratory of Ivan 

Barnes (U.S. Geological Survey). J. Thomas Dutro, Jr. 

(U.S. Geological Survey), accompanied us in the field for 

four days and helped with geologic mapping and identified fossils from limestones of the Mississippian Lisburne 

Group and from shell-rich beds of Cretaceous age; also, 

Katherine M. Reed (U.S. Geological Survey) identified 

radiolarians from the Otuk Formation. 

DESCRIPTION AND INTERPRETATION OF 

STRATIGRAPHIC UNITS 

Kanayut Conglomerate 

In the Ekokpuk Creek area, the Kanayut Conglomerate consists mostly of fine- to medium-grained quartzite with subordinate interbedded shale and siltstone. The 

quartzite beds are faintly laminated and several centimeters thick and are composed of monocrystalline quartz 

fragments (80-85 percent), chert fragments (15-20 percent), and small amounts of detrital siltstone and argillaceous rock. The originally irregular boundaries 

between quartz grains have been flattened by local transfer of silica so that the grains now form a mosaic with 

only a small amount of intervening matrix. Although 

phantom grain boundaries are common in quartzite, 

none were recognized in thin sections that we examined. 

The mechanism and environment of deposition of 

the Kanayut were studied by Moore and Nilsen (1984), 

who concluded that these rocks represent dominantly 

fluvial deposition. They also reported that this unit is 

Late Devonian and Early Mississippian(?) in overall age 

(Nilsen and Moore, 1984). 

The Kanayut Conglomerate was named by Bowsher and Dutro (1957); they reported that the formation 

is approximately 670 m thick about 75 km east-northeast 

of the area of figure 2. 

Description and Interpretation of Stratigraphic Units 
Kayak Shale 

The Kayak Shale is mostly very dark gray silty 

shale, siltstone, and impure sandstone, in decreasing 

order of abundance. In the Ekokpuk Creek area, the 

formation also contains a few layers of limestone, each of 

which is a few to several tens of meters thick. Some of 

these layers, which are described below, are integral parts 

of the formation. Others are slivers of limestone of the 

Lisburne Group that have been interlayered with Kayak 

strata by faulting; the layer of light -gray limestone that 

153° 150° 

~· ~------------------------------~ 

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PACIFIC OCEAN 

Figure 1. Index map to Chandler Lake 1° by 3° quadrangle 

and location of study area (area of fig. 2). 

crosses the ridge near the north end of cross section 

C-C' is such a sliver. 

The basal part of the formation in the vicinity of 

Ekokpuk Creek is mostly fine-grained, impure, ironstained argillaceous sandstone. Some of these rocks 

contain scattered carbonized plant debris. Beds in the 

basal part of the formation range from a few millimeters 

to a few centimeters in thickness. Much of the bedding in 

this part of the formation is irregular, apparently the 

result of small-scale scour-and-fill action during deposition. 

The middle two-thirds of the formation is dominantly very dark gray fissile shale that locally is somewhat 

silty. 

The upper one-fifth of the formation is very dark to 

dark-gray shale, silty shale, siltstone, and fine-grained 

sandstone. This part of the formation includes a few 

layers of limestone, which are a meter to a few meters in 

thickness. This limestone is composed mostly of crinoid 

columnals along with other fossil detritus. It is medium 

gray when fresh and weathers to a yellowish brown due to 

iron oxide, which is confined largely to the matrix. 

No definitive fossils were found in the Kayak Shale 

in the Ekokpuk Creek area; however, Nilsen and Moore 

(1984, p. 62) reported that elsewhere the Kayak contains 

a varied assemblage of fossils of Early Mississippian (late 

Kinderhookian) age. 

It was not possible to determine the thickness of 

the formation because of poor exposures and intense 

folding and faulting. The cross sections (fig. 2) suggest 

that the Kayak Shale is 150 to 400 m thick; at its type 

locality, about 75 km east-northeast of Ekokpuk Creek, 

the Kayak is about 290 m thick (Bowsher and Dutro, 

1957). 

Lisburne Group 

Most of the Lisburne Group in the Ekokpuk Creek 

area is composed of black chert of the Kuna Formation, 

with a small amount of medium-gray bioclastic carbonate 

rock of the undivided Wachsmuth and Alapah(?) Limestones. 

Kuna Formation 

The Kuna Formation in the Ekokpuk Creek area is 

almost entirely very dark gray, nearly black, vitreous 

chert. White quartz veins of variable trend and thickness 

are spaced every few centimeters throughout the chert. 

These veins are as much as a centimeter thick and clearly 

seem to be fractures that have been filled by quartz 

derived from the chert. In addition to chert, the formation includes a few layers of very dark to dark-gray 

argillaceous siltstone and a subordinate amount of 

medium-gray, fine-grained limestone. Most of the lime2 Stratigraphy and Structure of the Ekokpuk Creek Area, North-Central Brooks Range 
stone occurs as scattered discontinuous blebs in the 

chert. The short axes of these blebs are parallel to one 

another and normal to bedding, in the few places where 

faint bedding can be recognized. Individual blebs are as 

much as 2 em in their short dimension and as much as 

several centimeters in their other dimensions. 

Although some of the chert of the Kuna Formation 

at Ekokpuk Creek was probably limestone that was 

replaced by silica, we believe that most of the chert in the 

Kuna at Ekokpuk Creek was deposited as primary chert. 

This is in accord with Armstrong and Mamet's interpretation (1978, p. 356 and 358) that the chert-rich part of 

the section at Till Creek, about 27 km southeast of 

Ekokpuk Creek, are starved-basin deep-water deposits. 

Also, oxygen isotope compositions indicate that most of 

the chert is primary. The o180 values for four specimens 

of chert (sample localities N6, N25A-C, fig. 1) from this 

unit were determined to be +27.5, +29.5, +28.0, and 

+ 26.8 per mil; these compositions fall within the range 

+24 to +34 per mil that Savin (1973) cited as being 

characteristic of deep-water cherts. 

The Kuna Formation was named by Mull and 

others (1982) for a sequence of black carbonaceous 

shale, black chert, and fine-grained limestone and dolomite that many previous workers had informally called 

the "black Lisburne." The designated type section is 

about 200 km east of Ekokpuk Creek along the upper 

Kuna River in the Howard Pass quadrangle. Mull and 

others (1982) designated the section of black chert that 

we describe here to be a reference section for the 

formation. 

Another section, along Till Creek and described by 

Armstrong and Mamet (1978), as well as the type section 

(Mull and others, 1982) includes more carbonate and 

argillaceous rocks than the section at Ekokpuk Creek. 

Till Creek is about 25 km east-southeast of the area of 

figure 2, in the Wiseman quadrangle. Armstrong and 

Mamet (1978, p. 338-339 and fig. 7) show all the 

above-mentioned sections separated from one another 

by faults, thus obscuring their original spatial relations. 

Wachsmuth and Alapah(?) Limestones, Undivided 

The rocks that are herein identified as the undivided Wachsmuth and Alapah(?) Limestones of the Lisburne Group in the vicinity of Ekokpuk Creek are 

coarse-grained, medium-gray echinoderm-bryozoanbrachiopod packstones, which have indistinct irregular 

bedding with individual layers that range from a few tens 

to several tens of centimeters in thickness. Bands of these 

limestones occur at two places in association with rocks 

of the Kayak Shale within the area of figure 2. 

One of these bands of limestone that is several tens 

of meters thick occurs near the contact between the 

Kayak Shale and Kuna Formation along the line of cross 

section A-A', figure 2. The undivided Wachsmuth and 

Alapah(?) Limestones are lateral equivalents of the 

Kuna Formation, and we believe that this band of calcareous rocks at the Kayak-Kuna contact is a tongue of 

limestone of the undivided Lisburne Group and that it 

was deposited as a wedge in the basal part of the Kuna 

Formation. 

The other band of limestone, which is about 20 m 

wide, crops out within the area underlain by the Kayak 

Shale near the north end of cross section C-C' (fig. 2). 

We believe that this was emplaced in the Kayak by 

faulting. 

J.T. Dutro, Jr. (oral commun., 1984), identified the 

spirifer genera Brachythyris sp. and Logani sp. or Ovatia 

sp. in the field from the band of calcareous rocks along 

cross section A-A'. These fossils are indicative of Early 

Mississippian age and identify these rocks as the 

Wachsmuth Limestone of the Lisburne Group. Armstrong and Mamet (1978) interpreted the rocks of these 

formations north of Ekokpuk Creek to be shallowmarine and slope-facies limestones. 

Siksikpuk Formation 

The Siksikpuk Formation in the vicinity of Ekokpuk Creek consists predominantly of uniformly finegrained, faintly laminated siltstone. Although mostly 

light-neutral-gray, locally this siltstone is dark to very 

dark gray, and some of it is brown. The siltstone's typical 

color and texture give it the appearance of fine-grained, 

felsic pyroclastic rocks. However, oxygen isotope data 

suggest that this siltstone was deposited in deep cold 

water. Oxygen isotope, o

180, values from two siltstones 

(sample localities N5 and N17, fig 2) are +22.31 and 

+ 24.6 per mil. These values are less than or barely 

overlap the range of o180 values cited by Savin (1973) as 

characteristic of marine cherts ( + 24 to + 34 per mil), 

are greater than the range of values cited for pelagic 

sediments ( + 16 to + 18 per mil), and are much greater 

than the range cited as typical of igneous rocks ( + 9 to 

+ 10 per mil). Savin and Epstein (1970) report oxygen 

isotope ratios between + 11.5 and + 28.5 per mil for 

carbonate-free fractions of 27 ocean core samples. 

Although atypical of the Siksikpuk Formation, the 

siltstone near Ekokpuk Creek has herein been assigned 

to that formation on the basis of its stratigraphic position 

between the Kuna and Otuk Formations and its resemblance to some of the rocks that make up a minor part of 

the Siksikpuk Formation along the mountain front to the 

north. 

In the Ekokpuk Creek area, the Siksikpuk is about 

100 m thick as determined by using the average dip 

across outcrop bands whose widths were scaled from the 

map (fig. 2). This approximation may differ by as much as 

10 m from the true thickness because the topography on 

the base map (fig. 2) is not detailed enough to determine 

an accurate outcrop width and because the exposures are 

Description and Interpretation of Stratigraphic Units 3 
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4 Stratigraphy and Structure of the Ekokpuk Creek Area, North-Central Brooks Range 

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~ Surficial deposits (Quaternary) 

~Morainal deposits (Quaternary) 

Sedimentary rocks (Cretaceous) 

t]Q:j Otuk Formation (Triassic) 

r: :p~:: I Siksikpuk Formation (Permian) 

Lisburne Group (Mississipplan}-ln this 

area consists of: 

(\(,¥.~.?\j Kuna Formation 

Wachsmuth and Alapah(?) Limestones, undivided 

~ Kayak Shale (Mississlpplan}--Locally in-

~ eludes limestone (Is) 

1· ~ Dk ·I Kanayut Conglomerate (Lower Mississippian? and Upper Devonian) 

EXPLANATION 

N5 

• 

s 

A' 

B' 

Contact-Long dashed where approximately located; short dashed where 

more indefinite; dotted where concealed 

Fault-Dashed where approximately located; dotted where concealed 

Thrust fault-Dashed where approximately located; dotted where concealed. On map, sawteeth on upper plate; in cross sections, arrows 

indicate direction of relative movement 

Sample locality cited In text 

Figure 2. Geologic map and cross sections of part of Ekokpuk Creek area. Base from U.S. Geological Survey, 1:63,360, 

Chandler Lake (A-5), 1986. Queries on cross sections indicate unknown unit. 

Description and Interpretation of Stratigraphic Units 5 
too scattered to accurately determine variations in dip 

that result from internal structure. 

The Siksikpuk Formation was named by Patton 

{1957, p. 41) for exposures along Tiglukpuk Creek, a 

tributary of the Siksikpuk River about 33 km northeast of 

Ekokpuk Creek. Helen Duncan and J. Thomas Dutro, Jr. 

(see Brosge and others, 1962, p. 2194), reported that 

fossils collected by Patton and Tailleur {1964, p. 

429-430) at the type locality were of definite Permian, 

probable Early Permian, age. Mull and others {1982, p. 

356-357) gave a brief description of the formation. 

Otuk Formation 

The Otuk Formation is very distinctive and easily 

recognized. It is made up of evenly bedded, fine-grained 

limestone, siltstone, mudstone, and minor chert. Most of 

the rock is dark to very dark neutral gray where fresh and 

generally somewhat lighter colored where weathered. 

Some of the weathered rock is neutral gray but most of it 

is a characteristic yellow and locally orange of varying 

intensity. Pelecypod fossils of the genera Monotis and 

Halobia are characteristic of the Otuk and abundant on 

many of the bedding planes: Few sections of the formation that are more than a few meters thick are devoid of 

bedding planes bearing these fossils. The formation is 

estimated to be several tens of meters in thickness, on the 

basis of cross section C-C' (fig. 2). 

Radiolarians from chert of the Otuk Formation 

(station N7, just east of cross section B-B', fig. 2), 

identified as Staurodoras (Eliluvia) sp. cf. cocholeata 

(Nakaseko and Nishimura, 1979), and Pseudostylosphaera (Archaeospongop1Unum) sp. cf. he Iicata (Nakaseko and Nishimura, 1979), are indicative of an early 

Late Triassic (Karnian) age. 

The Otuk Formation was named by Mull and 

others {1982) for exposures along Otuk Creek within the 

area of the Killik River 1° by 3° quadrangle about 145 km 

west-northwest of Ekokpuk Creek. 

Sedimentary Rocks of Cretaceous Age 

The rocks of Early Cretaceous age within the map 

area (fig. 2) are dark-gray shale, shaly siltstone, sandstone, and a few thin, possibly discontinuous beds of 

limestone composed predominantly of fossil debris. 

These rocks are exposed only in small, widely scattered 

outcrops. Bedding is obscure but can locally be discerned 

by differences in the abundance of fine-sand and silt 

fractions from layer to layer. Where they can be distinguished, beds range from a few millimeters to a few 

centimeters in thickness. Much of the observed bedding 

is parallel and rhythmic, which suggests that the sediments were deposited in fairly deep water. 

The thin-bedded limestone is the most distinctive 

of the Cretaceous sedimentary rocks that form this unit; 

it is composed largely of clam shells <;emented by calcareous algae. Considerable reddish-brown, interstitial argillaceous material gives this limestone its distinctive 

color. These shell-rich beds were termed "coquinoid 

limestone" by Patton {1956), who first described such 

rocks from the Tiglukpuk Creek area at the mountain 

front about 40 km northeast of the area of figure 2. 

Patton (1956, p. 215) proposed the name Tiglukpuk 

Formation for an assemblage of rocks that includes the 

shell-rich beds. This formation name was abandoned 

after Imlay and Detterman (1973, p. 14) determined that 

the rocks at the type locality were disrupted by faulting. 

J.T. Dutro, Jr. (oral commun., 1984), identified the 

clam Buchia sublaevis of Cretaceous age in the field as a 

dominant component of the shell-rich beds in this unit. 

These rocks seem to be virtually identical to those at the 

Tiglukpuk Creek locality from which Jones and GrantZ 

(1964) collected Buchia sublaevis of Early Cretaceous 

(Valanginian) age. 

Quaternary Deposits 

Unconsolidated sand, gravel, silt, and clay cover 

much of the area of figure 2. In the valleys, most of the 

unconsolidated surficial deposits consist of alluvium and 

alluvial outwash. The steeper slopes are mantled with 

colluvium. Moraines are shown separately from the other 

surficial deposits on the map (fig. 2). 

STRUCTURE 

Because of the relatively small size of the Ekokpuk 

Creek study area, it is useful to view the structures of that 

area within the context of the surrounding Brooks Range. 

According to most interpretations of the regional structure of the Brooks Range, considerable shortening has 

taken place as a result of imbricate thrusting and associated folding (see, for instance, Chapman and others, 

1964; Brosge and others, 1979; Kelley, 1984; and Mayfield and others, 1984). Detailed mapping of the structure of the area around Ekokpuk Creek is sparse; Porter 

(1966) reported low-angle faults and overturned folds in 

the vicinity of Anaktuvuk Pass, about 40 km east of the 

Ekokpuk Creek area. The authors, accompanied by John 

Kelley of the U.S. Geological Survey, have seen as many 

as three repetitions of limestone strata of the Lisburne 

Group in imbricated fault slices along the front of the 

Brooks Range north of Ekokpuk Creek. 

The structure of the Ekokpuk Creek area (fig. 2) is 

characterized by a central east-west-trending band of 

deformed rocks sandwiched between thrust slices of 

rocks of Devonian age. The deformation evident in this 

central band of rocks can be attributed to relative northward movement of the overthrust mass of the Kanayut 

6 Stratigraphy and Structure of the Ekokpuk Creek Area, North-Central Brooks Range 
Conglomerate in the southern part of the Ekokpuk 

Creek area. Although stratigraphically the oldest rocks in 

the map area, the Devonian and Mississippian(?) strata 

of the Kanayut to the north of the area of figure 2 

structurally overlie other Devonian rocks owing to thrust 

faulting. Still farther north, thrust slices of the Kanayut 

overlie rocks of Mississippian age. 

The amount of displacement between the two 

major masses of the Kanayut Conglomerate cannot be 

determined. Much of the resulting deformation is represented by crumpling and thrust faulting within the Kayak 

Shale; evidence of this includes locally exposed tight 

folds, pencil-shaped prisms of rock (formed by the intersection of bedding- and axial-plane cleavage), and fault 

slivers of both younger and older formations within the 

Kayak Shale. These fault slices, in the northern and 

western parts of the map area (fig. 2), include a sliver of 

limestone of the Lisburne Group and several slivers of 

quartzite of the Kanayut Conglomerate. 

Although the Kayak Shale near the north edge of 

the map area (fig. 2) is in normal s~ratigraphic position 

above the Kanayut Conglomerate, faulting is likely to 

have occurred at or near their mutual contact. Even 

though this contact can be located to within a few meters 

along most of its length within the map area, whether or 

not it is a fault contact unfortunately could not be 

determined because the rocks along it are extensively 

broken by frost action. However, the strong deformation 

of the Kayak Shale here and the notable contrast in 

competence between the Kanayut Conglomerate and the 

Kayak Shale suggest that this is a fault zone. 

Thin sections of four specimens of quartzite of the 

Kanayut Conglomerate from the northern part of the 

area of figure 2 show no evidence of small-scale deformation such as strain shadows in quartz. This absence of 

evidence of deformation suggests that the quartzite in the 

Kanayut was so competent that deformation in it was 

confined to narrow zones or was transferred to the 

nearby, less competent rocks of the Kayak Shale. 

The general structure of the area between the 

bands of the Kanayut Conglomerate is characterized by 

asymmetrical folds disrupted by reverse faults. The faults 

and axial planes of the folds trend east -west and dip 

steeply to the south. Several closely spaced faults trend 

east-west through the middle of the area of figure 2; 

weaknesses along these faults probably localized the 

valley along which they lie. Most of these faults juxtapose 

older rocks over younger rocks, which is a common 

relationship along overthrust faults. It is noteworthy that 

along at least two of these faults younger rocks overlie 

older rocks. The superposition of younger over older 

rock along overthrust faults can be explained in a number 

of ways. An explanation that we favor is that before 

faulting ceased the rocks were folded in such a way that 

younger rocks in what was to become the upper plate 

were in positions to be cut and transported upward along 

the fault to positions adjacent to older rocks in the lower 

plate. An example of these relations is near the center of 

cross section A-A' (fig. 2), where the Otuk Formation 

overlies the Kuna Formation. Another explanation is that 

normal movement along some of the overthrust faults or 

newly developed faults parallel to the overthrust faults 

downdropped younger, upper plate rocks until they were 

adjacent to older rocks of lower plates; such faulting 

could have occurred during an episode of tension after 

relaxation of the compression that had led to the overthrusting. The normal fault that seems to offset the rocks 

of the Siksikpuk Formation a few meters near the north 

end of cross section C-C' probably was formed in this 

way. A third explanation for overthrust faults that juxtapose younger rocks over older rocks is that they are 

simply bedding-plane detachment faults. Because such 

faults do not disturb the normal stratigraphic sequence, 

they are difficult to detect. 

In the north half of the area of figure 2, several 

synclines are delineated by the chert of the Kuna Formation. This folding may be the result of drag along several 

northward-verging faults that separate the synclines. 

These folds plunge gently to the west, with the result that 

the southernmost of the two folds at the north end of 

cross section B-B' projects above the surface along cross 

section A-A'. To the west along the line of cross section 

C-C' the two folds north of Ekokpuk Creek merge to 

form a single, broad synclinal fold with a small anticline 

in the middle. 

A recumbent syncline occurs in the low ridge near 

the middle of line of cross section B-B' (fig. 2). The core 

of this syncline is composed of rocks of the Otuk Formation, which crop out sporadically. This syncline continues 

westward to the middle of the line of cross section C-C'; 

for about half of the distance westward from the line of 

cross section B-B' toward cross section C-C', the south 

limb of this syncline is delineated by an obscure band of 

chert of the Kuna Formation. This syncline can be traced 

west of cross section C-C' by discontinuous outcrops of 

chert of the Kuna Formation along its south limb to its 

continuation in the center of the western quarter of the 

area of figure 2. This syncline is interpreted to continue 

eastward beneath surficial deposits to the area along the 

line of cross section A-A'. 

Outcrops in the southeastern quadrant of the area 

of figure 2 are so small and scattered that no unique 

interpretation of the structure there is possible. However, folds and faults similar to those north of Ekokpuk 

Creek could explain the distribution of the rocks exposed 

there; the structural interpretations shown along the 

south half of the line of cross section A-A' are based on 

analogies with the structure in the north half of the area 

of figure 2. 

Rocks of the Kanayut Conglomerate at the south 

edge of the area of figure 2 seem to be thrust over the 

rocks to the north. The thrust contact is concealed by 

Structure 7 
surficial deposits over most of its length within the map 

area. It can be located to within a few meters along about 

3 km of its length in the western part of the area of figure 

2; however, even here most of the outcrops are frost riven 

and no outcrops of the actual contact were found. 

We believe that most of the faults in the area are 

south-dipping thrusts that become less steeply dipping 

downward and merge with a major sole fault. This basal 

thrust has not been recognized in the field, but we believe 

that it comes to the surface in the north band of the 

Kayak Shale at or near its contact with the quartzite of 

the Kanayut Conglomerate. 

Another interesting aspect of the geology of the 

Ekokpuk Creek area is the presence of Lower Cretaceous rocks, which, according to C.G. Mull (written 

commun., 1986), are not only the most southerly Cretaceous rocks that are known to occur in fault slices in the 

north-central Brooks Range but also the youngest rocks 

known to be involved in thrusting in the Brooks Range. 

CONCLUSIONS 

The pattern of northward-verging tight folds and 

thrust faults in the Ekokpuk Creek area is in harmony 

with the structural style in nearby parts of the northcentral Brooks Range. The close proximity of Mississippian chert in the Ekokpuk Creek area to coeval limestone 

in adjacent areas has been suggested to be the result of 

large-scale differential lateral tectonic transport of rocks 

from contrasting sedimentary environments. However, 

our mapping suggests that this proximity is due to the two 

facies' being deposited close to each other. Cretaceous 

rocks are the youngest rocks involved in the deformation 

in this part of the Brooks Range, and the Cretaceous 

rocks at Ekokpuk Creek are the most southerly Cretaceous rocks known to be involved in the deformation 

there. 

REFERENCES CITED 

Armstrong, A.K., and Mamet, B.L., 1978, Microfacies of the 

Carboniferous Lisburne Group, Endicott Mountains, arctic Alaska, in Stelck, C.R., and Chatterton, B.D.E, eds., 

Western and arctic Canadian biostratigraphy: Geological 

Association of Canada Special Paper 18, p. 333-394. 

Bowsher, A.L., and Dutro, J.T., Jr., 1957, The Paleozoic section 

in the Shainin Lake area, central Brooks Range, Alaska: 

U.S. Geological Smvey Professional Paper 303-A, p. 1-39. 

Brosge, W.P., Dutro, J.T., Jr., Mangus, M.D., and Reiser, H.N., 

1962, Paleozoic sequence in eastern Brooks Range, Alaska: American Association ofPetroleum Geologists Bulletin, v. 46, no. 12, p. 2174-2198. 

Brosge, W.P., Reiser, H.N., Dutro, J.T., Jr., and Nilsen, T.H., 

1979, Geological map of Devonian rocks in parts of the 

Chandler Lake and Killik River quadrangles, Alaska: U.S. 

Geological Survey Open-File Report 79-1224, 1 sheet, 

scale 1:200,000. 

Chapman, R.M., Detterman, R.L., and Mangus, M.D., 1964, 

Geology of the Killik-Etivluk Rivers region, Alaska: U.S. 

Geological Survey Professional Paper 303-F, p. 325-407. 

Imlay, R.W., and Detterman, R.L., 1973, Jurassic paleobiogeography of Alaska: U.S. Geological Survey Professional 

Paper 801, 34 p. 

Jones, D.L., and Grantz, Arthur, 1964, Stratigraphy and structural significance of Cretaceous fossils from Tigluk:puk 

Formation, northern Alaska: American Association of 

Petroleum Geologists Bulletin, v. 48, no. 9, p. 1462-1474. 

Kelley, J .S., 1984, Geologic map and sections of a portion of the 

Chandler Lake A-1 and A-2 quadrangles, Alaska: U.S. 

Geological Survey Open-File Report 84-555,1 sheet, scale 

1:63,360. 

Mayfield, C.F., Curtis, S.M., Ellersieck, Inyo, and Tailleur, I.L., 

1984, Reconnaissance geology of southeastern Misheguk 

Mountain quadrangle, Alaska: U.S. Geological Survey 

Miscellaneous Investigations Series Map I-1503, 2 sheets, 

scale 1:63,360. 

Moore, T.E., and Nilsen, T.H., 1984, Regional variations in 

fluvial Upper Devonian and Lower Mississippian(?) Kanayut Conglomerate, Brooks Range, Alaska: Sedimentary 

Geology, v. 38, p. 465-497. 

Mull, C.G., Tailleur, I.L., Mayfield, C.F., Ellersieck, Inyo, and 

Curtis, S.M., 1982, New upper Paleozoic and lower Mesozoic stratigraphic units, central and western Brooks 

Range, Alaska: American Association of Petroleum Geologists Bulletin, v. 66, no. 3, p. 348-362. 

Nakaseko, Kojiro, and Nishimura, Akiko, 1979, Upper Triassic 

Radiolaria from southwestern Japan: Osaka University, 

College of General Education, Science Reports, v. 28, no. 

2, p. 61-109. 

Nilsen, T.H., and Moore, T.D., 1984, Stratigraphic nomenclature for the Upper Devonian and Lower Mississippian(?) 

Kanayut Conglomerate, Brooks Range, Alaska: U.S. Geological Survey Bulletin 1529-A, 63 p. 

Patton, W.W., Jr., 1956, New formation of Jurassic age, in Gryc, 

George, and others, Mesozoic sequence in Colville River 

region, northern Alaska: American Association of Petroleum Geologists Bulletin, v. 40, no. 2, p. 213-218. 

---1957, A new upper Paleozoic formation, central Brooks 

Range, Alaska: U.S. Geological Survey Professional Paper 

303-B, p. 41-45. 

Patton, W.W., Jr., and Tailleur, I.L., 1964, Geology of the 

Killik-Itkillik region, Alaska: U.S. Geological Survey Professional Paper 303-G, p. 409-500. 

Porter, S.C., 1966, Stratigraphy and deformation of Paleozoic 

section at Anaktuvuk Pass, central Brooks Range, Alaska: 

American Association of Petroleum Geologists Bulletin, v. 

50, no.S,p. 952-980. 

Savin, S.M., 1973, Oxygen and hydrogen isotope studies of 

minerals in ocean sediments, in Symposium on Hydrogeochemistry and Biogeochemistry, Tokyo, Japan, 1970, Proceedings: Tokyo, Japan, v. 1, p. 372-391. 

Savin, S.M., and Epstein, Samuel, 1970, The oxygen and 

hydrogen isotope geochemistry of ocean sediments and 

shales: Geochimica et Cosmochimica Acta, v. 34, p. 43-63. 

8 Stratigraphy and Structure of the Ekokpuk Creek Area, North-Central Brooks Range 

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