UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

PRELIMINARY GEOLOGIC MAP AND STRUCTURAL SETTING 

OF THE KATAKURUK DOLOMITE IN THE S ADLEROCHTT MOUNTAINS,

NORTHEASTERN ALASKA

by 

Chester T. Wrucke,* John S. Kelley,^ and Augustus K. Armstrong^

Open-File Report 89-123

This report is preliminary and has not been reviewed for conformity with U.S. Geological

Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for

descriptive purposes only and does not imply endorsement by the USGS.

1989

U.S. Geological Survey, Menlo Park, California 

U.S. Geological Survey, Anchorage, Alaska 

U.S. Geological Survey, Socorro, New Mexico
PRELIMINARY GEOLOGIC MAP AND STRUCTURAL SETTING OF THE KATAKTURUK 

DOLOMITE IN THE S ADLEROCHTT MOUNTAINS, NORTHEASTERN ALASKA

by

Chester T. Wrucke, John S. Kelley, and Augustus K. Armstrong 

INTRODUCTION

The Katakturuk Dolomite is a potential reservoir rock under the coastal plain of the Arctic 

National Wildlife Refuge (ANWR). The coastal plain is an area of limited bedrock exposure and 

possibly large potential for oil and gas resources (U.S. Fish and Wildlife Service, 1986; Dolton 

and others, 1987; Bird and others, 1987) and for which considerable geologic information is 

needed for an assessment of the petroleum potential. This report presents tentative conclusions on 

the stratigraphy and structural setting of the Katakturuk Dolomite and associated rocks in the 

Sadlerochit Mountains where the Katakturuk is widely exposed and may have a structural setting 

similar to that under the coastal plain of ANWR.

The Sadlerochit Mountains occupy a prominent geographic and structural position as the 

northernmost mountain range in ANWR. It is one of three prominent east-trending ranges that lie 

north of the Franklin and Romanzof Mountains of the northeastern Brooks Range. The easttrending anticline underlying the Sadlerochit Mountains continues down plunge under the coastal 

plain, and progressive northwest truncation along regional unconformities exposed on the north 

flank of the Sadlerochit Mountains probably continues northward under coastal plain cover. The 

Katakturuk Dolomite, if present below the unconformities under the coastal plain, would have 

potential for oil and gas in large detached anticlines involving pre-Mississippian rocks (Kelley and 

Molenaar, 1985; Kelley and Poland, 1987, p. 259, fig. 203).

This investigation focuses on three issues. These are (1) the degree and nature of 

deformation of the Katakturuk Dolomite, (2) whether or not the Katakturuk is repeated by 

imbricate faults, and (3) the lateral extent of significant detachment along the pre-Mississippian 

unconformity at the top of the Katakturuk.

Previous geologic work in the Sadlerochit Mountains includes a measured section of the 

Katakturuk and the original definition of the formation by Dutro (1970), a preliminary geologic 

map of the range by Reiser and others (1970), a colored geologic map by Bader and Bird (1986) of 

part of northeastern Alaska showing the principal geologic features and structural setting of the 

Sadlerochit Mountains, and reports on the structural framework of the the coastal plain and 

adjacent bedrock areas by Kelley and Poland (1987) and Leiggi (1987). A detailed geologic map 

of the Sadlerochit and Shublik Mountains is by Robinson and others (1989). Clough and others 

(1987), Clough and others (1988), and Blodgett and others (1986a, b) reported on the stratigraphy 

and sedimentology of the Katakturuk Dolomite.

STRATIGRAPHY

PROTEROZOIC ROCKS

Unnamed Rocks

Quartzite, siltstone, argillite, limestone, and basalt are the oldest rocks in the map area. 

They are probably Precambrian in age as no fossils have been found in them, and they underlie the 

Katakturuk Dolomite, which also is probably Precambrian in age. The contact with the Katakturuk 

may originally have been an angular unconformity, but breccia at the base of the overlying 

Katakturuk and other structural evidence presented later indicates that the contact has had 

movement
The basalt in the pre-Katakturuk rocks possibly is correlative with rocks in the Shublik 

Mountains that Moore (1987) referred to informally as the Mount Copleston volcanic rocks. He 

reported that a sample of the basalt in the Sadlerochit Mountains has similar mineralogy and 

composition as the rock in the Shublik Mountains. According to Moore (1987), the basalt in the 

Shublik Mountains preserves original textures and some original minerals but contains 

metamorphic minerals of the low greenschist facies. Our observations reveal that the preKatakturuk basalt in the Sadlerochit Mountains also is weakly metamorphosed.

Reiser and others (1970) considered the quartzite and argillite, and by inference the 

siltstone, as belonging to the Neruokpuk Formation and the limestone as a separate unit of 

uncertain stratigraphic position. They designated these units as Paleozoic(?) in age. They showed 

the basalt as a separate unit of pre-Mississippian and younger(?) age. Robinson and others (1989) 

show the pre-Katakturuk rocks in the Sadlerochit Mountains as Neruokpuk.

The Neruokpuk is considered to be in part Precambrian but to have rocks that may be as 

young as Devonian (Sable, 1977). Dutro and others (1972) concluded that the Neruokpuk may 

contain several stratigraphic sequences, all of uncertain ages and correlation. In view of these 

uncertainties and the difficulties previous workers in the Sadlerochit Mountains had in applying a 

name anrfage to them, the rocks below the Katakturuk are here designated as unnamed and are 

considered as Proterozoic in age and probably correlative with some units of the Neruokpuk 

Formation.

Katakturuk Dolomite

The Katakturuk Dolomite was defined by Dutro (1970) from exposures in the Shublik 

Mountains, immediately south of the Sadlerochit Mountains. Dutro found the Katakturuk in the 

Shublik Mountains to be mostly dolomite that formed on a marine shelf and that it is at least 1,040 

m thick. He divided the formation into nine informal members. In the Sadlerochit Mountains, the 

Katakturuk also is mostly dolomite but is about 2,500 m thick.

By far the most extensive exposures of the Katakturuk are found in the Sadlerochit 

Mountains where the formation crops out east-west along strike for 55 km, nearly the entire length 

of the range, and for an average width of about 3.5 km. In the present study, the formation has 

been divided into three informal members that were mapped for a distance of about 35 km in the 

central part of the range and that almost certainly can be traced to the west end of the range 

(Robinson and others, 1989). However, these members were not recognized east of the Nularvik 

River in this reconnaissance examination, owing to abundant breccia and extensive cover of 

solifluction debris. In general, the Katakturuk in the Sadlerochit Mountains has considerable 

lateral and stratigraphic continuity and, except near the east end of the range, is relatively little 

deformed.

The upper contact of the Katakturuk outside the Sadlerochit Mountains is an unconformity 

beneath Cambrian and younger rocks (Blodgett and others, 1986a; Sable, 1977). In the 

Sadlerochit Mountains, the basal beds of the overlying Lisburne Group are brecciated, and they 

cross strata of the Nanook Limestone and the Katakturuk in angular discordancy.

No fossils have been found in the Katakturuk Dolomite in the Sadlerochit Mountains. The 

formation is considered to be Proterozoic in age because it underlies the Nanook Limestone, whose 

lowest strata in the Shublik Mountains are considered to be Late Proterozoic or Early Cambrian in 

age (Blodgett and others, 1986a).

ORDOVICIAN ROCKS 

Nanook Limestone

The Nanook Limestone crops out in the Sadlerochit Mountains from about 10 km west of 

Katakturuk River to the east end of the range (Robinson and others, 1989), but in this study these 

rocks were mapped with the underlying Katakturuk Dolomite. Beds of the Nanook are mostly 

light- to medium-gray limestone and dolomite that in outcrop resemble beds of the Katakturule
Only the upper part of the Nanook occurs in the Sadlerocht Mountains, whereas lower as well as 

upper parts of the formation are found to the south in the Shublick Mountains (Blodgett and others, 

1986a; Sable, 1977). These relations suggest that the Nanook lapped onto a Katakturuk high in 

the Sadlerochit Mountains. In the Sadlerochit Mountains, the Nanook rests disconformably on the 

Katakturuk whereas the contact with the overlying Lisburne Group is an angular unconformity.

Studies of the Nanook Limestone in the Shublik Mountains show that the formation may be 

in part of Late Proterozoic age and that it contains fossils as old as Late Cambrian in age and as 

young as late Early or early Middle Devonian in age (Blodgett and others, 1986a; Robinson and 

others, 1989). The only fossils that have been found in the Nanook in the Sadlerochit Mountains 

are of Ordovician age (James Blodgett, oral commun., 1989). The Nanook in the Sadlerochit 

Mountains could have Cambrian and Devonian strata, but it is not likely to have beds of 

Proterozoic age.

PENNS YLVANIAN AND MISSISSIPPIAN ROCKS 

Lisburne Group

Brosge and others (1962) extended the name Lisburne Group to carbonate rocks in the 

eastern Brooks Range and included in it the Lower Mississippian Wachsmuth Limestone, the 

Upper Mississippian Alapah Limestone, and the then newly named Pennsylvanian and Permian(?) 

Wahoo Limestone. The Wachsmuth has not been recognized in the Sadlerochit Mountains. The 

Wahoo in the area of this report contains beds of very latest Mississippian age but primarily 

contains beds of Morrowan (Early Pennsylvanian) and Atokan (Middle Pennsylvanian) age and 

does not contain strata of Permian age (Mamet and Armstrong, 1984). Studies of the Lisburne 

Group in the Sadlerochit Mountains include an investigation of the paleoecology and rugose 

colonial corals by Armstrong (1973) and a paper on diagenesis and stratigraphy of the group by 

Wood and Armstrong (1975). Armstrong and Mamet (1975,1977,1978) studied the 

Carboniferous stratigraphy of the northeastern Brooks Range and included descriptions, 

illustrations, and details of the distribution and zonation of the microfossils in the Lisburne Group.

Included here with the Lisburne Group is the type locality of the Mississippian Itkilyariak 

Formation (Mull and Mangus, 1972), which rests on Proterozoic rocks and is overlain by and 

grades into the Alapah Limestone of the Lisburne. The type section is in the NW1/4, Sec. 7, T.3 

N., R. 31 E. (incorrectly identified by Mull and Mangus), near the east end of the Sadlerochit 

Mountains. According to Mull and Mangus, the formation at the type locality consists of reddishbrown to maroon limestone, sandstone, conglomerate, and breccia interbedded with greenish-gray 

shale and sandstone. At this locality, the formation is about 45 m thick and crops out for a distance 

of 800 m. It is the only locality of the formation that has been recognized in the area.

TRIASSIC AND PERMIAN ROCKS 

Sadlerochit Group

The Sadlerochit Group unconformably overlies the Lisburne Group and consists of the 

Upper Permian Echooka Formation and the Lower Triassic Ivishak Formation (Detterman and 

others, 1975). Rocks of the Sadlerochit Group crop out above strata of the Lisburne Group the 

entire length of the Sadlerochit Mountains and are the youngest Paleozoic rocks in the area.

INTRUSIVE ROCKS

Jurassic gabbro forms a sill in the unnamed Proterozoic clastic rocks east and west of 

Itkilyariak Creek near the east end of the map area. Mafic intrusive rocks are rare in the 

northeastern Brooks Range. Sable (1977) reported mafic dikes in the Devonian(?) Okpilak 

batholith, centered about 40 km southeast of the east end of the Sadlerochit Mountains. Unlike the 

sill in the Sadlerochit Mountains, these dikes are mineralogically altered and schistose.
STRUCTURAL GEOLOGY

STRUCTURAL SETTING

The rocks of the Sadlerochit Mountains lie in a complex stack of northeast- to east-trending 

imbricate thrust sheets and northerly verging folds that extends from well within the Brooks Range 

northward to the range front and under the Arctic coastal plain. Kelley and Poland (1987) and 

Leiggi (1987) discussed the structural geology of the coastal plain and adjacent bedrock areas, and 

Bader and Bird (1986) show the distribution of rocks and structures in the northeastern Brooks 

Range, including the Sadlerochit Mountains.

The Weller thrust of Kelley and Poland (1987), a major thrust fault that extends east-west 

the length of the Sadlerochit Mountains, emplaced the unmapped Proterozoic rocks, the Katakturuk 

Dolomite, the Nanook Limestone, and the overlying Mississippian to Triassic rocks into their 

present structural position in the upper plate. Lower plate rocks range in age from Mississippian to 

Paleocene. The thrust dips 50 degrees south, approximately parallel to the attitude of the overlying 

Proterozoic strata. At depth the thrust merges with a regional decollement Near the east end of 

the range, the surface expression of the fault dies in a north-vergent anticline in which the 

Mississippian, Permain, and Triassic strata are folded into the thrust plane. Thrust transport was 

north-northwest (Kelley and Poland, 1987).

THE NANOOK-KATAKTURUK STRUCTURAL BLOCK

The Katakturuk Dolomite together with the overlying Nanook Limestone acted as a 

structural unit, the Nanook-Kataktunik structural block, during thrust faulting in the Sadlerochit 

Mountains. The principal structure of the Nanook-Katakturuk block is a south-dipping homocline 

exposed along the axial part of the Sadlerochit Mountains. Dips in the homocline are 20 to 60 

degrees, with a mean value of 30 degrees. A conspicuous feature of the Katakturuk in the 

homoclinal block is a high degree of structural and stratigraphic continuity. Beds in certain thin 

stratigraphic intervals can be traced for hundreds of meters (Robinson and others, 1989).

Conjugate high-angle faults cut the Nanook-Katakturuk block and the Mississippian to 

Triassic strata. Most of these faults strike approximately north 30 degrees east and have the 

greatest offset and length of all the faults. Faults that strike approximately north 45 degrees west 

are fewer in number, shorter, and probably of smaller offset than the faults that strike northeast

Northeast-trending faults in the conjugate set have consistent apparent offsets. The offsets 

suggest either dip-slip motion, in which southeast blocks are down-thrown, or left-lateral strikeslip motion. The greatest amount of apparent offset along individual faults typically is along the 

northeastern segments nearest their apparent intersection with the Weller thrust and becomes 

progressively less along southwestern segments. Apparent offset diminishes and the faults 

disappear in the upper part of the block and the overlying strata.

Northwest-trending faults in the conjugate set have poorly represented and conflicting 

offsets. In only 2 localities do northwest-trending faults offset mapped units. East and adjacent to 

the upper Nularvik River, a northwest-trending fault in sec. 14 and 23, T. 3 N., R. 28 E., has an 

apparent dip-slip offset with the down-thrown block on the southwest, and in sec. 7 and 16, T. 3 

N., R. 29 E., and sec. 12 T. 3 N., R. 28 E., a northwest-trending fault has an apparent dip-slip 

offset with the down-thrown block on the northeast.

Cross-cutting relations between northwest- and northeast-striking faults appear systematic. 

Northwest-striking faults terminate against northeast-trending faults, except for an equivocal case 

in SW/4, sec. 18, T. 3 N., R. 29 E., where trucation relations are not clear. Northeast-striking 

faults do not appear to cut and offset northwest-striking faults; clear continuations of northweststriking faults are not apparent across northeast-striking faults.

The conjugate fault system cutting the Nanook-Katakturuk block and overlying strata 

appears to terminate against the Weller thrust Even faults that have relatively large offsets in the 

Katakturuk show no apparent displacement at the Weller thrust or of the Mississippian to Triassic
strata below the thrust Additionally, the conjugate fault system does not clearly involve the 

Precambrian clastic and metaclastic rocks that underlie the Katakturuk.

The Katakturuk Dolomite in the Sadlerochit Mountains is pervasively fractured and locally 

brecciated. Fractures are conspicuously present in thin section, hand specimen, and outcrops. 

Most fractures are filled to some degree by carbonate cement Slip between breccia clasts probably 

involved small amounts of rotation. Despite pervasive fractures, the Katakturuk Dolomite exhibits 

the high degree of structural and stratigraphic continuity previously noted.

Some fracture zones in the Katakturuk Dolomite are associated with the high-angle faults 

and with bedding plane shears. Breccia locally occurs along the conjugate faults and is well 

developed in shaly beds at the top of the lower member.

Disruption of the Katakturuk Dolomite is more intense in the eastern part of the Sadlerochit 

Mountains than in the western part. West of the Katakturuk River, the Katakturuk is structurally 

simple, is not extensively fractured, and mappable conjugate faults were not identified East of the 

Katakturuk River, small unmappable normal faults, bedding plane shears, and breccia zones are 

numerous. From the Nularvik River east to the last exposures in the Sadlerochit Mountains, the 

Katakturuk Dolomite is shattered and locally brecciated and faulted. Bedrock outcrops are 

extensively covered with solifluction debris, probably reflecting the degree of fracturing of the 

dolomite. Mapping of units identified farther west cannot be done with confidence. Marker beds 

indicate that the map units recognized to the west are present, but poor bedrock exposure severely 

limits mapping the units with precision.

Upper and Lower Contacts of the Nanook-Katakturuk Structural Block

The Nanook-Katakturuk block is in fault and likely depositional contact with adjacent rocks 

in the Sadlerochit Mountains. Fault contacts resulted from thrust-ramping of the block over much 

younger strata. The Nanook-Kataktutruk block is at least locally structurally detached along 

unconformities with overlying and possibly underlying strata. Leiggi (1987, p. 753, fig. 4) 

inferred a thrust fault at the base of the Katakturuk in the Sadlerochit Mountains. Additionally, 

locally prominent breccia zones and shears resulting from small scale thrust-detachment or shear 

related to flexural slip folding has modified depositional relations.

The contact between the Nanook-Katakturuk block and the underlying Proterozoic clastic 

rocks probably is a fault West of the Nularvik River, the conjugate faults mapped in the block 

terminate at the Weller thrust, but east of the river, where the unnamed Proterozoic clastic rocks 

intervene between the Weller thrust and the Katakturuk Dolomite, the conjugate faults terminate 

along the base of the Katakturuk. These relations, as discussed earlier, suggest that the contact of 

the Katakturuk with the underlying rocks is a thrust fault. Locally the unnamed Proterozoic rocks 

are sheared, and quartzite beds are broken by low-angle faults. Basal beds of the Katakturuk were 

examined incompletely to determine whether shearing along the contact is local or pervasive. 

These beds of the Katakturuk are not concordant with underlying clastic rocks.

The depositional contact between the Nanook-Katakturuk block and the overlying 

Mississippian strata is probably only locally faulted in the Sadlerochit Mountains. Kelley and 

Poland (1987) recognized a detachment thrust, the Sunset thrust, within a widespread 

Mississippian shale where the shale is present and along a regional unconformity at the base of 

Mississippian strata where the shale is not present They based their interpretation on prominent 

detachment folds in Mississippian carbonate strata in the Franklin Mountains where the shale is 

present, and they tentatively identified imbricate faults involving pre-Mississippian strata that 

possibly merge with a detachment along the base of Mississippian strata in the eastern Shublik 

Mountains and detachment folds in the eastern Sadlerochit Mountains. The results of the present 

study and that of Kelley and Molenaar (in press) infer that the Sunset thrust is probably restricted 

to the eastern Sadlerochit Mountains and has little displacement
Folds involving Mississippian to Triassic strata detached from pre-Mississippian rocks 

occur in the eastern and southern Sadlerochit Mountains. Strongly north- to north-northwest 

vergent and locally overturned concentric folds in Mississippian and Pennsylvanian carbonate 

strata and Permian and Triassic clastic strata crop out east of the Nularvik River on the North flank 

of the Sadlerochit Mountains. A near-overturned north-vergent anticline in part cut by the Weller 

thrust crops out at the east end of the Sadlerochit Mountains (Kelley and Molenaar, in press; Reiser 

and others, 1970); rocks in the core of the fold are part of the regional homocline in the range and 

apparently are not commensurately folded into the core of the anticline (Kelley and Poland, 1987). 

Anticlines and synclines in Mississippian to Triassic strata are recognized as far west as the upper 

Katakturuk River on the south flank of the Sadlerochit Mountains.

Breccia and shears are locally evident along the contact between the Nanook-Katakturuk 

structural block and overlying Mississippian carbonate strata (Kelley and Poland, 1987). 

Elsewhere, the contact is clearly exposed and exhibits no evidence of structural disruption (Imm 

and Watts, 1987; Wallace, Meigs, and others, 1987). In areas where the Kayak Shale is absent, 

Robinson (1987) reported clasts in the basal Lisburne Group derived from rocks underlying the 

Lisburne Group in the Sadlerochit Mountains, strongly inferring that at least locally detachment is 

not coincident with the contact between Mississippian and older rocks.

DISCUSSION

Detachments Involving the Nanook-Katakturuk Structural Block

There appear to be three thrust faults that separate the Nanook-Katakturuk structural block 

from other rocks in the Sadlerochit Mountains. The Nanook-Katakturuk block and underlying 

Proterozoic rocks have been ramped up from unexposed substrate into their present position above 

the Weller thrust. Additionally, the Nanook-Katakturuk block appears to be in fault contact with 

the underlying unnamed Proterozoic clastic rocks. Detachment also locally occurs between the 

Nanook-Katakturuk block and overlying Mississippian rocks.

The Weller thrust ramps upward from a decollement below the Katakturuk Dolomite and 

emplaces an anticline cored by Katakturuk Dolomite over strata (not shown on the map) as young 

as Paleocene in age. The decollement below the Katakturuk Dolomite is not exposed, although the 

probable detachment between the Katakturuk Dolomite and underlying Proterozoic clastic rocks 

could be representative of the decollement

Detachment between the Katakturuk Dolomite and underlying Proterozoic rocks is inferable 

from field relations. High-angle conjugate faults appear to terminate along the contact between the 

Katakturuk Dolomite and the unnamed Proterozoic clastic rocks as they terminate elsewhere along 

the Weller thrust. Continuations of the conjugate faults are not readily apparent in the Proterozoic 

rocks, inferring either truncation of the conjugate faults or merger of conjugate tear faults with a 

thrust fault.

Detachment along the regional unconformity at the base of the Mississippian section on the 

north flank of the Sadlerochit Mountains probably is restricted to the area east of the Nularvik 

River. Along the south flank of the Sadlerochit Mountains, Mississippian strata are probably 

detached from the Nanook-Katakturuk block as far west as the upper Katakturuk River.

Anticlines and synclines in Mississippian to Triassic strata inferably detached from preMississippian rocks crop out on the north flank of the Sadlerochit Mountains east of the Nularvik 

River. The folds are strongly vergent to overturned to the north and are concentric. Where deeply 

eroded, then* limbs are tightly appressed. Their concentric style infers that they terminate at depth, 

presumably at the unconformity at the base of the Mississippian strata.

The strike of folds in Mississippian to Triassic strata in the northeastern Sadlerochit 

Mountains further infers detachment along the unconformity at the base of Mississippian rocks. 

For example, the western segment of the Itkilyariak anticline (Kelley and Poland, 1987) trends 

east-northeast, perpendicular to the regional transport direction rather than to faults that cut pre-
Mississippian rocks, such as the Weller thrust North of Weller Creek the segment of a syncline 

not involved in drag folding along the Weller thrust also trends east-northeast Folds involving 

Mississippian to Triassic strata attached to pre-Mississippian rocks would most likely have strikes 

parallel to the strike of pre-Mississippian rocks and the faults that cut the pre-Mississippian rocks, 

namely east rather than east-northeast.

Contrasts in folding style between rocks above and below the unconformity at the base of 

the Mississippian section infer detachment in the eastern Sadlerochit Mountains. The north-vergent 

and nearly overturned anticline in Mississippian to Triassic strata above the Weller thrust involves 

incommensurately folded Proterozoic rocks (Kelley and Poland, 1987, p. 260, fig. 204). The 

apparent lack of continuity in rotation across the unconformity indicates at least limited slip 

between the concentrically folded Mississippian to Triassic strata and the discordant underlying 

Proterozoic rocks.

Folds that probably reflect detachment along the unconformity at the base of the 

Mississippian section occur as far west as the upper Katakturuk River on the south flank of the 

Sadlerochit Mountains. The folds are strongly asymmetrical, north-vergent, concentric anticlines 

and synclines in Mississippian to Triassic strata, similar to folds elsewhere in strata that are 

probably detached from pre-Mississippian rocks. Additionally, the folds strike south-southwest, 

suggesting that they are detached from pre-Mississippian rocks that have an east-trending structural 

grain.

No unanimity of opinion exists on the area! limits of detachment of Carboniferous strata in 

the northeastern Brooks Range. In some of the first reports of detachment tectonics in the 

northeastern Brooks Range, workers (Kelley and Molenaar, 1985; Leiggi and Russell, 1985; 

Rattey, 1985) recognized widespread detachment of Carboniferous strata. Kelley and Molenaar 

(1985) reported widespread detachment along the unconformity at the base of the Pennsylvanian to 

Mississippian Lisburne Group in the Sadlerochit and Shublik Mountains, which Kelley and Poland 

(1987) subsequently referred to as the Sunset thrust. Leiggi (1987) recognized a progressive 

northward decrease in structural shortening of the Lisburne Group and estimated 41% and 15% 

shortening, respectively in the Franklin and Sadlerochit Mountains. Robinson (1987), suggested 

that the Lisburne Group depositionally overlies pre-Mississippian rocks in the Sadlerochit 

Mountains and that the Kayak Shale, regionally present below the Lisburne Group elsewhere in 

northeastern Alaska, is absent by nondeposition as suggested by Armstrong and Mamet (1978, 

1975). Wallace, Hanks, and others (1987) and Wallace, Meigs, and others (1987) suggested that 

detachment at the base of the Lisburne Group is restricted to areas where the Kayak Shale is 

present south of the Sadlerochit Mountains and that detachment of the Lisburne Group extends no 

farther north than the Shublik Mountains.

The present work offers evidence of structural detachment, albeit with unknown but likely 

small magnitude displacement, in the eastern and southern Sadlerochit Mountains. Folds in 

Mississippian to Triassic strata detached from underlying pre-Mississippian rocks indicate 

detachment beyond the original distribution of Kayak Shale. Locally the detachment may not be 

confined to the unconformity. It is probable that detachment along the unconformity at the base of 

the Mississippian strata, that is, along the Sunset thrust of Kelley and Poland (1987), dies out in 

the Sadlerochit Mountains. This conclusion is based (1) on evidence that clasts of preMississippian rocks along the base of the Lisburne Group indicate that only minimal transport has 

occurred across that detachment in the eastern Sadlerochit Mountains, and (2) on the undisturbed 

contacts at the base of the Lisburne Group in the western Sadlerochit Mountains.

Detachment along the unconformity at the base of the Mississippian strata in the eastern and 

southern Sadlerochit Mountains possibly approximates the limit of regional detachment in the 

context of the regional work of Leiggi (1987), Wallace, Hanks, and others (1987) and Wallace, 

Meigs, and others (1987). The apparent northern limit of this detachment in the Sadlerochit 

Mountains trends west-southwest across the central part of the range. The trend is orthogonal to 

the regional transport direction of Kelley and Poland (1987) and to slickensides observed by them 

in footwall cuts in the Marsh Creek thrust sheet along upper Marsh Creek.
CONCLUSIONS

The Katakturuk Dolomite is exposed in a homocline in an erosionally breached doubly 

plunging east-striking anticline that extends the length of the Sadlerochit Mountains. The anticline 

is underlain by the Weller thrust that dies out in the eastern Sadlerochit Mountains and extends the 

length of the range. The anticline is one of a family of fault-floored anticlines superimposed on a 

broad anticlinorium that plunges both east and west under the coastal plain and dips under the 

coastal plain to the north.

The Nanook Limestone and Katakturuk Dolomite make up a south-dipping homoclinal 

block largely detached from underlying rocks and at least partially detached from unconformably 

overlying strata. The homocline is discordant with overlying more steeply dipping Mississippian 

to Triassic strata in the south limb of an anticline. The Weller thrust emplaces the NanookKatakturuk block and parts of the leading limb of the anticline on strata as young as Paleocene in 

age along the foot of the western range front In the eastern part of the range, the Weller thrust 

ramped Katakturuk Dolomite and underlying Proterozoic clastic rocks, likely detached from the 

Katakturuk Dolomite, against Mississippian to Triassic strata. Apparent dip-slip displacement 

across the Weller thrust progressively diminishes eastward and dies out into a strongly northvergent anticline at the east end of the Sadlerochit Mountains. In the eastern and southeastern parts 

of the Sadlerochit Mountains, the Nanook-Katakturuk block probably is detached from overlying 

Mississippian and younger strata along a regional unconformity; the detachment ceases to be 

recognizable in the western Sadlerochit Mountains.. A conjugate set of high-angle faults, probably 

tear faults contemporaneous with the Weller thrust and an unnamed fault between the Katakturuk 

Dolomite and underlying Proterozoic clastic and metaclastic rocks, cut the Nanook-Katakturuk 

structural block and overlying Mississippian to Triassic strata.
REFERENCES

Armstrong, A.K., 1973, Pennsylvanian carbonates, paleoecology, and colonial corals, north

flank, eastern Brooks Range. Arctic Alaska: U.S. Geological Survey Professional Paper

747, 19 p. 

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10
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