UNITED STATES DEPARWNT OF THE INTERIOR GEOLOGICAL SURVEY

GEOLOGY OF THE KUKPOWRUK-NUKA RIVERS REGION,

NORTHWESTERN ALASKA

E. G. Sable, J. T. Dutro, Jr., M. D. Mangus, and R. H. Morris

Open-File Report

81-1078

This report is preliminary and has not been edited or reviewed for conformity to Geological Survey standards and nomenclature.

Page

Ceology . -con t i nurd ...... lntruduct ion 1 Mississippian system-.continued ...... I Locat ion. size. and acccss ill i li t y vf rc.$ion Lisburne group continued ...... Previuus invest igac ions 2 Utukok formation (new) ...... Natureandscopeof tlleirlvrstigatir)n 3 Distribution and outcrop ...... Field operations. 1447-1953 5 Character and thickness ...... Ar.knoi~ledgments. 8 Stratigraphic relationsl~ips...... I.eoyr.rptiy 9 Kogruk formation (new) ...... ~rcl~rolo~y.vegetation. wildlife. climate ...... 9 Distribution and outcrop ...... Topography and drainage I1 Character and thickness ......

Stratigraphic relationsiiips ......

Tupik formation (new) ...... Dcvorliaa system...... 19 Distribution and otltcrop ...... Kugururok formation (new) ...... 19 Character and thickness ...... Uistribution and outcrop ...... 19 Carboniferous-Permian systems ...... Character and tl>ickness...... 20 Cherty and shaly rocks ...... Stratigraphic relationships 22 liistribution and outcrop ...... Fossils and age ...... 23 Character and thickness ...... Elode of deposit inn ...... 24 St rilLjgrnl,lbic relationstrips ...... Undifferentiated Dvvonian rocks ...... 25 Fossils r~nriaye ...... blississippian system ...... 29 Nuki~iornia t ion ...... I.~S~IITI~L*group ...... 30 llist rihut iur~;~rlri rwi.urc'llcc......

Con tents--continued

Page The information contained in this report was collected during field

Igneous rocks--continued investigations between 1947 and 1953. The text was written In 1957, and

Mode of occurrence and age ...... 214 the generalized geologic map that accompanies the report (fig. 1) was

Structural geology ...... 216 compiled at that time. Although some of the statements or conclusions

Folded belt...... 218 contained herein may seem outdated when compared to modern theories, the

Folded and faulted belt...... 221 report contains information that has not been published elsewhere, to-

Overthrust belt...... 225 gether with early interpretations that support the current hypotheses

Interpretation and age of faulting ...... 230 regarding the development of the Brooks Range orogen.

Economicgeology ...... 232 Names and inferred age designations of stratigraphic units have I Petroleum possibilities...... 232 been changed from the original report to correspond with current usage. Referencescited ...... 237 Locations of sections that are given by geodetic coordinates are from 1 planimetric maps used in mapping and compilation during the original

work during the 1950's. Discussion of the Nuka Formation does not

consider the much-studied section at Nuka Ridge (Tailleur, Mamet, and

1 ILLUSTRATIONS Dutro, 197?), and the Nuka Formation as described herein probably in- cludes Lower Mississippian, Permian, and Triassic strata of the Nuka Figure 1. Generalized geologic and structural map of the Kukpowruk-Nuka Rivers region, Northwestern Alaska Ridge exposures. The pelecypod Buchia reported herein was previously

assigned to the genus Aucella. Both are considered to be of Early

Cretaceous (Ncocomian) age, but in earlier reports, fine-ribbed Aucella

was considered to be of Valanglnian age and coarse-ribbed Aucella was

thought to be Berriasian. In the sec Lion entitled "Structural Geology1',

the "(hrert\~rustbelt" is generally equated with the "Disturbed belt"

oi later writers (Tailleur and Llrosgb, 1970).

vii viii ill Y .4 w V] 0 a0 Y P P c S .rl m w 4 T a u; I ill 0 P a *A ill 4 rl0 .c m ill 'Y E 2 v] -rl Q rl E: P u 0 2 al m M 3 ri 2 v al m ?I h &J m w m u m e U Li m b ?. rl U u r: w ill u P U ri * rl Vc-. '+ E Li M ul m Y 30 U m > ill .A al s 0 X U a U Lrr rl m e h 3 d Y z CO w 4 Oy m m U u m u Li z i, d

.o"n Li Yl .OU uc .='A 0 'rlU .cum C ?1 e c cam 0 .?+ .A Y b. WJw me- € '3, 0-L.., rrmm s25 u 0 E* I;IBLL>CY 01,. 'l'lil? KUKl1(IWHUt;-NULI 1:I VISHS I

b Y in the soutllern fuorliills and along the mounti~in front. A gravel air-

Edward C. Sable, .J. Tl~ui~tdsDutro, Jr., M.jrvio I). Mangus, strip, along the east side of the Utukok River 4 miles north of Driftwood

and Rubcrt H. Morrls Creek was constructed in 1950, but because of the low position near the

river it may now be unsuitable for wheel landings. 1NTKOl)iIC'~L Foot travel in the foothills and mountains is not difficult except Location, Size, and Accessibility of Region in the swampy lowlands of the foothills. Although the east- and north-

Tlie Kukpouruk-Nuka Rivers region includes more than 3,200 square miles in east-trending ridges afford the best foot travel, they are mostly

the western part of the DeLong Mountains, a part of the flruoks Range province, and discontinuous. Tracked vellicle travel is easily accomplished through-

in the Arctic Foothills province north of the mountains. Most of the out the loothills and along most stream valleys to the mountain divide.

region lies in the Southern Foottlills section and a lesser part in Routes must be selected with care through low swampy terrain, for

the Northern Foothills section. The north- example, the area east of Lake Noluk and along the Colville River. ern part of the area overlaps with the Utukok-Corwin region (Chapman Streams flowing south from the divide are deeply incised, streun with

and Sable, 1960) and the eastern part lies within NavaL Petroleum large houLders and, in some places flow over small waterfalls. In

Reserve No. 4. Explorations during the years 1947 through 1953 were these valleys, vehicle travel is difficult or impossible. The divide

undertaken as a part of the U.S. Geological Survey's program of irivesti- was crossed in one place with tracked vehicles, at Weasel Pass between

gatiocs for the U.S. Deportment of the Navy in Naval Petroleum Keserve the Utukok and Kliguroruk Rivers. No suitable route over the divide is

No. 4 and adjacent lands. known in the region east of this pass.

The Kukpowruk-Nuka region can be reached by air Irum Kotzebue, Major rivers ;ire navigable to within 10 miles of the divide by

Point Hope, and Point Lay, the nearest permanent settlements. Uistances boats having a dralt of less than one foot. During low water, however,

to the center of the region from these towns are about 130, 130, and shallow channels and rifllrs make boat travel almost impossible. In

80 miles, respectively. In the winter season ski-airplane landings the mountains even the major strean~scan be crossed easily on foot at

are possible at many places. During the sumlcr only a few lakes are selected places.

known to be suitable for float-airplane landings; these include Lake Previous Investigations

Noluk, Kvkolik Lake, and the large lake 5 miles suuth of Igluo Elr>untain. F(l1lowing the estiihlisl~n~r~ltof Naval Petroleum Reserve No. 4

hy Enctrutive Order in February. 1923, n part of the Kukpowruk-Nuka regioo w.2~iirst pmpltrr~d in :\ug~rsl .rnJ St-pt~.mlrpri~i 1924. A U.S.

(:c~llc~~i~.~~l 5lll.v~.): j'drty 1l11~1c.rWi 1 l irllu 1'. I:LII-:JI~.IWL.~~~CCI stutly ctte stratigra[>hy and structrJre, and to evaluate rhe petroleum the Utuk~~k Kivcr and Ilisr%l>pvi~rl~nc~~tCreek in scaitrclj of a rtlurtl through possibilities of tlre area. The region had been only partly covered the DcLol~g >lourltijins. 'The p.lrty crossed the Colvillt. Kiver, traveled by rapid geologic reconnaissance during the earlier explorations sr~ut11w:ird to Ldke Nuluk. port;ljied across the divide to the Ninliuktuk of 1924-1926. Kiver, and co~~tinucddown the Noatilk Hiver to its moutlt. Very little The project was srlpt~rvisedby (:<,(,rgt.0. I:;ites jn 1547, by Ralph geologic work was accomplished a11 this trip because of logistic dl f f i- L. Miller in 1949-1950, and by George Gryc from 1951 to 1953. cultics. During field studies which covered about 70 percent of the area. In April and flay, 1925, a U.S. Geological Survey party under the geology was mapped on available aerial photographs, taken I:erald Fitzgerald, topographic engineer, with Walter R. Smith, Geologist, by the U.S. Air Force. These cover the entire map area and consist of asce~ided the Kugururok River by dog team from che Moatak River, portaged trimetrogon photographs flown in 1943, 1947, and 1949, and vercical across tlie divide to OtukoL Kiver drainage, and reached the upper Col- photographs of approximately 1:20,000 scale flown in 1948 and 1950. ville Xivar. The geology and topography along the route were mapped, Low-angle oblique photographs along portions of the upper Kukpowruk, and the parry continued down the Colville River by canoe. Kokolik, Utukok, Colville, and Nuka Rivers were taken by the U.S. In Hay and June, 1926, a U.S. Ceological Survey party under Phillip . Navy in 1949. In addition, the photographs were used for photogeologic S. Smith made an overland journey by dog team from Kivalina, crossing interpretation beyond the limits of field control and, to some extent, the headwaters of the Kukpuwcuk Hiver to the upper Kokolik River. The to calculate structural and stratigraphic measurements. Data were party then canoed down theKokolikRiver. Geology and topography along transferred from photographs to 1:48,000- and 1:96,000-scale planimetric the route were mapped. maps compiled by the U.S. Geological Survey mostly from the trimetrogon More detailed accounts of these expeditions and resulcs of the photographs and from older sources. Because modern topographic maps investigations are presented by Smith and Mertte (1930). of the area are now available, geology was transicrrt~dto ttlasc Nature and Sropc. of the lnvcstigatil~n 1:63,360-scale maps. As a result of these transfer procedures, the Field studies in the Kukpowruk-Nukn region were under taken dnsring 1947. resulting positions of geologic features are not of the highest degree 1949, 1950. 1951, and 1953, as parts oF the U.S. Ceological Survey's program of accuracy. of exploration for the U.S. Navy Ilepartment in and near Naval Petroleum Reserve During field work, vertical elevation control was achieved mostly No. 4. The purpose of the work was tu produce a geologic map, to by use of airplane altimeters. Elevations were tied in a few places to iriangulat ion neLs ot iaurtll-order ;lccur:~cy c.s t;iblisl~udin 1947 or~d geologist, Ihle A. Hnuck and Gordon W. tlerreid, field assistants, Paul 1949 along the Utukok and Ktlkul ik Rivers and, in 1950, in the vicinity H. Shannon, cook, and Kalph S. Soleckl, archeologist, Smithsonian of the Driftwood anticlinal area. A topograpliic map of tl~cDriftwood Institution, was lartdrd by single-rngined ski-equipped airplane near anticlinal area was prepared from the 1950 trianguldtion net and altim- the Kukpowruk River about 16 miles from its headwaters. A triangulation eter traverse data. Approximate altitudes, with respect to mean sed net was begun at 1949 camp 1 and extended downriver; and a geologic level, were obtained by a combinatiun of tlte U.S. Geological Survey reconnaissance was made in the vicinity of camp 1 and southward to triangulation nets and level-lines establisiled by the United Geophysical the head of the Kukpowruk. The downriver traverse, in three 18-foot Company Inc. during seismic surveys. folding canvas boats, was begun on June 13, and mapping was carried Field Operations, 1947-l953 On May 12 and 17, 1947, a party consisting of Raymond M. along the river northward into the Utukok-Comin region. The mouth of 'Thumpson, geologist and party chief, William L. Barksdale, the river was reached on July 30. On July 31 and August 1 the party geologist, Edward G. Sable, field assiscant, Charles T. was flown to ~~k~likLake,about 16 miles north the headwaters Marrow, camphand, and Ernest H. Wadsworth, cook, was landed by sinyle- the Kokolik River. A triangulation net was established at 1949 camp sngined, ski-equipped airplane near rhe Utukok Kiver about 8 miles 17 and carried northward. Geologic reconnaissance in this part of north of its headwaters. A geologic reconnaissance was made in tile the area was mainly limited to the vicinity of Kokolik Lake and the vicinity of, and about 7 miles upriver from, I947 camp 1. A downrivc~ outcrops along the river north of the lake, The downriver traverse was begun on May 26 in three 18-foot folding canvas boats to boat traverse, begun on August 11, also continued into the Utukok- the northern limit of the Kukpowruk-Nuka region, and the party continued (:orwin region. its work northward into the Utukok-Corwin area (Chapman and Sable, The bulk of geologic work in the Kukpowruk-Nuka region was done 1960). A triangulation net was established over snme of thr. route. in 1950 and 1951. 011 May 29, 1950, a party consisting of Edward G. The geologic work in the Kukpuwruk-Nuka region was of general recon- S~hle,geologist and parry chief, Marvin D. Mangus, geologist, Charles I rlaissance nature dnd was mostly resrricted to the vicinity of the river L. tlummcl and Par~lIi. Shimnun, field assista~~ts,Robert D. Gerard. v'11lry. cook, and Lloyd E. Hall, Arctic Contractors mechanic, landed on Lake 'I'tie Kukpowruk and Kokolik Kivcrs were t raverscd ill a similar I Noluk by twin-engined ski-cqrripped airplane. On June 2 and 4 the party 1 fashion in 1949. 011 May 17 and 27 tl~cfield party crlnsisting of I was rransportrd by a single-cngii~e airplane on wheels to rlie Utukok Robert M. Chapman, genlogist ;~ndparty chiei, Edward C. Sable, Kiver north of its jurtct i~~nwith Urf Etuood Creek. After careful mapping of ttlc Driftwuud ant icl in's, n deteilcd yculi~gicrct:nunaissal~cc ill the In .Jur~eand July, 1953, Sable and Irvin L. Tailleur, geologists.

southern foutlli l ls and UeLdng Elountains was undertaken. About 600 square and 14. Glcnn Richards, field assistant, extended earlier mapping by

miles was mapped hrtwren the Kokolik River and Uriftwook Creek. Tailleur westward into the Kukpowruk-Nuka regiun, mainly along the middle

Amplljbious full-tracked vehicles, "weasels", were used for transportation. and lower parts of the Nuka River. "Weasels" were used for transporta-

'The pdrty cuncluded the field season on September 3, Allan N. Kover tion, and a tie-in with geologic mapping in the Kiligwa River area to

and George Cryc, Survey geologists, spent several days examining rock the east was effected. exposures south of Lake Ploluk during the summer of 1950. All the above parties were flown from Umiat, the main base for

Between May 29 and September 2, 1951, a party consisting of field operations 200 miles east of the mapped area. Food and some

Edward C. Sable, geologist and pary chief, J. Thomas Dutro. Jr., supplies were cached by ski-equipped airplane at prearranged locations

geologist, Robert H. Morris, geologist, Harald R. Drewes and Robert during April and early Hay. This hazardous operation, handled in

H. Meade, field assistants, William L. Nystrom, cook, and Leo C. 1947 by R. F. Thurrell and Sable, in 1949 by Mangus and E. Lebert, in

Sutliff, Arctic Contractors mechanic, extended the 1950 work eastward 1950 by Tailleur and A. S. Keller, and in 1951 by Mangus and R. L.

f ram Driftwood Creek to the Nuka River, and mapped about 700 square Detcerman, was carried out with complete success. The food was cached miles of country. The party was landed on a gravel airstrip in 55-gallon steel drums co protect it from bears. Mail and a few prepared by Arctic Contractors in 1950 along the Utukok River north additional supplies were dropped to the parties from airplanes during of Driftwood Creek. "bieasels" were again used for transportation during the field season, except where suitable float Landings were possible.

field work, and one boat traverse was made down the upper part of the The parties maintained daily communication with Umiat by two-way radio.

Colville River. Seismic work by United Geophysical Company Inc., in tlie vicinity

Ln August, 1951, exposures along the upper Colvifle River were of tttc Driftwood anticlines consisted of refraction studies in 1950

examined by George Gryc, geologist, and Lloyd A. Spetzman, field assis- and refraction and reflection studies in 1952 (Uoolson and others,

tant. A canvas boat was used for transportation, and the work included 1962. p. 22). Acknowledgments reexamination of cutbank exposures near the mouth of Reynard Creek In a program of the size and scope of the Naval Petroleum which had been studied on August 2, 1948,by Edward J. Webber, U.S. Reserve No. 4 expluratidns, many people contributed to

Geological Survey geologist. the completion of the geologic work. Navy, Arctic Contractors, and

U.S. Geological Survey personnel in Fairbanks and on the Reserve all provided helpful support without which tl~cparties could not II.IVL' workccl ~~kpowrukRivers (Solecki, 1950a, 1950b). effectively. The vegetation In the area, dwarfed by climatic conditions and Fossil identifications include those of post-Triassic Mesozoic typicill of tundra regions, has few plants exceeding 1 foot in height. invrrtebrates by Ralph W. Ialay, Triassic invertebrates by Bernhard Willows arc usually less than 5 feet high, and, itlthough they are rela- Ktrmmef and 3. 8. Reeside. Jr., Paleozoic bracl~iopodsand miscellaneous tively scarce, enough wood can be gathered along the Larger streams fossils by J. Thomas Dutro, Jr., with preliminary identifications by to provide several days firewood for small camps. In the foothills, Arthur L. Bowsher, corals by Helen M. Duncan, and cephalopods by tufted cortongrass is the dominant evident form of vegetation in inter- MacKenzic Cordon, Jr. Most of the microfossils were identified by stream areas where it is associated with sedges, grasses, mosses, and Harlan K. Bergquist, some by Helen L. Tappan, and the conodonts were heath communities. In the mountains, mosses. lichens, and flowering restudied by W. H. Hass. Finally, the work of the earlier geologists, plants provide a partial covering. P. S. Smith, W. R. Smith, W. T. Foran, and others was very helpful The most abundant animal is the caribou, several thousand of in the planning and compilation of the fieldwork. which were seen each summer by the field parties. Ground squirrels The skilled bush pilots of Wien Alaska Airlines, Alaska Airlines, are ubiquitous. Wolves, grizzly bears, red and cross foxes, wolverines, and Trans-Ocean Airlines rendered invaluable support in caching and small rodents and hoary marmots were occasionally seen. Dall mountain other transportation activities throughout the field seasons. sheep are present in the DcLong Mountains in the headwaters of the CEOGKAPIIY Utukok and Nuka Rivers. The Arctic grayling and small trout were the Archeology, Vegetation, Uildlife, Climate only fish noted in the streams, and larger trout are present in some No settlements, roads, or established trails are present in thc of the small mountain lakes. Ptarmigan, hawks, falcons, jaegers, Kukpowruk-Nuka region. Some of the coastal Eskimos hunt in the area plovers, and many song birds are abundant during the summer months; a during the winter, as is evidenced by the recent campsites seen along all few eagles, owls, and water fowl were observed. Mosquitoes are very of the major rivers. Flint-chipping sires are fairly common on hiLltops in abundant and annoying during late Jrine, July, and early August; flies the foothills, and most of the chippings and projectile points are probably and gnats are common during the same period. of recent Eskimo origin. Ralph S. Solecki, Smithsonian Institution archeolo- Tile climate is arctic and semiarid. Only general weather obser- gist, accompanied the 1949 field party and gathered data along the Kokolik vations were made by the field parties from June through August in 1950

and 1951. In 1950, temperatures averaged about 50°F., with highs of 82O~.;uld lows of 26O~. I'recipit;~tia~~ill the iorm ol rain squalls. defined by Payne and nthcrs (1951). Within the mapped drizzle, snow and, rare Ly, hail was usually of short duration; prolonged area, the Hrooks Range province consists oE the northwestern part of rains lasting more than a day occurred only twice. Strong winds, esti- the DeLong Mountains. Physiographic boundary divisions are shown on mated to be at least 40 miles per hour with gusts probably as much as the index map.

80 miles per hour, were predominantly from the soutll and east and were The Northern Foothills section is characterized by belts of most common in June. In 1951, temperatures averaged about 45O~., with prominent cuesta-ridges and mesas, with as much as l,5OO feet of relief, friglls of about 7z°F. and IDUS of 25O~.; precipitation was much more separated by wide lowland belts. The lowlands are almost entirely frequent and in periods of longer duration than in 1950; strong winds, covered by tundra and are underlain mostly by shale. The ridges and most common in June and August, were predominantly from the south. The mesas contain good rock outcrops and traces which reflect siltstone nortliern foothills and the soutl~side of the LkLong Mountains could and sandstone. often be seen to have clear weather whlle storms, overcast, conditions The northern part of the Southern Foothills section, separated and precipitation were occurring In the southern foothills and along from the northern foothills by a belt of lowlands, is largely a lowland tile north side of the mountain divide. Thunderstorms were infrequent region interrupted by rounded hills and whaleback ridges with relief and usually resulted in little rainfall. of about 1,000 feet. Cutbank rock exposures are most abundant along

The area lies in the zone of continuuus permafrost (PCwB, 1954, the Colville, Nuka, and Utukok Rivers, Adventure Creek, and parts of p. 317, fig. 69). The permafrost table lies at I foot or more in the Tingmerkpuk and Kukpowruk Rivers. Altitude increases southward, depth in late summer, depending on factors such as the amount of and the southern part of tlrt: southern foothills is characterized by vegetation cover and drainage. Soil creep and flow features are discontinuous, mostly north-facing hogback ridges, rounded rubble- common on hill slopes. Abundant talus along the valley sides and covered Ilills, and wiideback ridges. The ridges, composed largely oE rock rubble on hills and mcluntainv nrtcsts to tlre importance LI~mcchan- sandstone, conglomerate, and chert are separ~tedby nearly exposureless ical weathering in the area. lowlands. Kclicl is irs n~uchas 4800 fret, and altitude ranges up

'Topr~~raphy .ind 1)rs i iq!,c to about LSOO feet. Cutbitnk exposures are most numerous along the

The Kukpowruk-Ntiko rrgia~nI irs in thr, Arctic Foothills ant1 Nuka Hivcr, l'lruudcr, I)ri ftwt>od, ;~nd Iligluruk Creeks, and parts of the

Ur(>c>ksKange physiogr;ipf~ic prt,vii~rcs, the iormcr bcing divjdcd Kukpowruk 1< iver. into the Northurn Foothills and Soutlrtirn I.'oothills sections, as

I I The uortl~wrstcrnpart of tllc I)el.n!lg Mount.tins Is cll;lracterfzed by In the Kukpowruk-Nukd regiurl, altitudes in

and erosion of widely different and well exposed rock types in the

DeLongs produces a variety of topographic features and color patterns

of considerable scenic interest.

Major north-flowing river systems with headwaters in the Kukpowruk-

Nuka region are the Colville, Utukok, Kokolik, and Kukpowruk. Major

tributaries of the Colville River ere the Nuka River, Storm and Thunder

Creeks; of the Utukok, Driftwood Creek; of the Kokolik, Iligluruk Creek;

and of the Kukpowruk, Eagle Creek,and an unnamed creek. The major rivers

rise in the DeLong Mountains at altitudes of about 2,000 to 2,500 feet.

From ttre drainage divide to the northern limits of the southern foot-

hills, the gradient oE toe Utukok River averages 30 feet per mile, that

of the Kukpowruk River about 36 feet per mile, and the gradients of

the Kokolik, upper part of the Colville, and the Nuka Rivers are roughly

25 tc~30 feet per mile. The Utukok and Kokolik are sttallow with numer-

ous I~rnidtxtl areas; main char~ncl depths at low wdrer are estimated to

range from h inches ro 5 feer. Tllr Kukpowruk, Colville, and Nuka Rivers

arc more rloticrab ly incised rid contain fewer braided areas. Stream

widths range Irum 50 to 100 feet and f land plains are as much as 2,000

Cect wide. Sprin!: I>re.~kup oi rl~~,rivers tlci-ura ;~ll~lutmid-El;~y, ;rftcr wlbicl~ Corwin region bCha~racmdSjrb/C, 1960) are also enlarging their drainage

tl~e.itrc.~~iis tlrirl) s~.ver;~lfret and are clear a1111 v~-ry10w during rn~)St areas castward by headward erosion. Thus, the northwest directions of of rh~.ssumncr. Prolungcd or, Inure rarely, sudden rainfall brings ttie the streams west of the Colville appear to have been the result of

st rt~~unlevels up several teet, but Tl~slrfloods are unknown. relatively recent uplift along a north- or northeast-trending line in Tlbe major rivers appear, for LII~most part, tu be anteccdcnt the eastern part of the Kukpowruk-Nuka region and east of the Utukok- str~~drnsin tt~csuuther~~ footl~ills and mountains; they commonly flow Corwin region, and the tributaries are in part consequent streams t ransuerst! to the east- and northeast- trending topographic features flowing away from the line of uplift. North of the Kukpouruk-Nuka wt~ictlparallel the structural grain. The major tributaries also flow region high-level gravel deposits of Pleistocene age are known to across clie grain over most of their courses but, in part, they cut slope westward (Chapman and Sable, 1960); the slope may reflect regional against dnd parallel certain high bedrock ridges. Drainage patterns structural tilting like that indicated in the western part of the Kok- of Storm Creek, Driftwood Creek, Iligluruk Creek, Eagle Creek, and the powruk-Huka area. upbwrmr)st part of the Kukpowruk River show a striking similarity, arc South of the mountain divide, only headwater portions of tl~cmajor nearly parallel, and flow generally northwest. The Nuka River and its st)utlr-flowing streams, the Kelly, Kugururok, and Nirniuktuk Rivers, lie major triburliries flow northeasf. within the Kukpowruk-Nuka region. Their lieadwater tributaries are The present major tributaries between the Kukpowruk and Nuka deeply incised and lie several hundred feet below the level of north- Rivers were probably parts of the ancient east-flowing Colville; they flo\~ingdrainages whic11 they head against. They are actively capturing were subsequently captured by the present north-flowing major rivers. tlie north-Clowiug streams by headward erosion. The lower portions of This is indicated by che fact that west-flowing streams west of tlic the main tributaries, and the valleys of the major rivers which they Colvi llr are enlarging their drainage areas eastward by headward enter, 11avc uodergone extensive valley glaciation. During Pleistocene erasion. I)riFtwood Creek, whicti lies at u mucl~ lower i~ltitudcLhnn glaciatiuns, the Dei.nng Huuntai~~sdivide was south of its prese~lt the Colvillc, is capturing Crllvillc drail~i~yc.Plcistocene(?i gravel posit ion. The active hcadwiird curt ing of south-flowing streams prob- drposi ts, whicll l ic we1 i ;ibr~vi?prescnL stream levels, extend f runt ;it ably rrsultcd from the deepening of the major river valleys by Icnst Uri Ftwt)rrd Crcck trr the i:alvi llc Hivcr and i~rdicntci that (:r>lville gli~ciitl;i~.tion ilnrk tlrc consequent stccpenrd headwater gradients. drainage once extended farthtbr wcbsl 111;ln i L dues Lthrli~y. Trit>r~l;~ry C I at-iat ion

ICxtrnsivr v.rllry fil;at.i.~tir,r~al.)ng the struth sid~of the Del,+>ng

- mount;^ ins is rcfltlcted by wide U-shaped valleys, cirques,

16 arete ridges, and extensive dcpos its of gliic iill urigln in tile mnjor river valleys. 011 tile north side of tile muuntaios, however, there is little fresh evidence of glacial actLvity except in the highest portiirr~snear the mountain divide. At these Ilighrr altitudes, sm;~ll

'~bandoned cirques are present in the Ireadwaters of the Nuka, CoLvillt. and Urukok Rivers at altitudes of about 1500 to 4D00 feet. One small cirque glacier, less than 4000 feet in diameter, lies at about 4,000 feet near the dividi! between Storm Creek and Nirniuktuk River drainages.

Terminal moraines, identified on aerial photographs, lie along the small tributaries which head in the cirques. These moraines represent were observed as far west as the headwaters of the Kukpowruk River. In relatively Late stages of glaciation in the area and may correspond to addition, modified arcuate drainage patterns of certain tributary streams the Alapat~Mountain and Fan Mounrain glaciations recognized by netterman, in the rnour~tninsand the southern part of the foothills, may reflect Bowsher, and Dutro (1958) in the central Brooks Range. initiaL control hy morainal deposits. On the north side of the mountains, ulder glacial features havr 'These features point to one or more ice advances considerably been greatly modified by erosion and solifluctiun. No erratics, till, ulder than the later stages. Valley glaciers probably extended into or murainal deposits were recognized with certainty anywhere in the the southernmost part of the foothills in the area between the Nuka area. Unconsolidated deposits are either of Eluvial origin or consist and Utukok Rivers. Farther west, glacial extent may have been more of rubble derived from nearby bedrock, Nevertheless, the valleys of restricted. The apparent absence of ice-contact deposits may mean some major north-flowing streams within tho Ue1.ong Mountains show that the ice transported a relatively small amount of material or that a modified U-shaped cross-section and rock benches into which the the deposits may have been removed subsequently by erosive agents. streams havr been incised. The upper parts of Driftwood Creek, Storm The widespread high-level gravel deposits along the Colville River Creek, and Kidney Creek show these CeaLures. Small north-facing may contain reworked glacial and glaciofluvial material derived cirques, also considerably modified, occur at many localities between from of der glacial advances. altitudes of $500 and 3Ll00 feet. One such is occupied by the lake along

a tributary of Driftwood Creek. Cirques at this general altitude . --Character and thickness (;Kn~.ocY The Kugururok Formation in the type section is largely dolomite Devonian Sys tcm and limestone, with shaly to conglomeratic clastic rocks in the lowel

' part. At the type locality, three informal lithologic members are * Rocks OF know Devonian age crop out in four widely separated reco~nized. The lower member is dominantly clastic (shale with localities in the Kukpowruk-Nuka region. They occur on a tributary Interbedded sandstone, granule con~lomerate,siltstnne, and Iimentnn~l of Iligluruk Creek, at approximate lat. 68°35'40'1 N., long. 161°37' W. ; and is about 389 feet thick: the ItnneT mmnhpr is 1ioht:r~lored. - . in the headwaters of Iligluruk Creek at lat. 68°36'15" H*, long. 161'17'00" W.; laminated to cross-bedded dolomite, The yellowish-weathering ("creamy") along the upper Nuka River, at lat . 68O40' N., long. 160°09' W, , and color of the dolomite is a distinctive gross feature of the formation. in Mount aastille and vicinity south of che upper Kugururok River at Section 1. Kugururok Formation, type section, exposed on north lat. 68O28' N., long. 160°53' W. The Iligluruk Creek exposures consist side of Mount Bastille, approximately lat. 6B028*N., long. of a narrow, east-trending band not more than a few hundred feet 160°50' W. Measured by E. G. Sable and J. T. Dutro, Jr.. wide and 1% miles long; the exposures on the upper Nuka River occur in

July 14, 1951( see j1.j r) a southwest-trending band about 1 mile long and a quarter of a mile wide; in contrast, those at Mount Bastille make up the main mass of -Unit Thickness in feet the mountain and can be traced on aerial photographs for at least 6 Top of section. Shales of probable Jurassic or miles east and 2 miles west of the mountain. Early Cretaceous age. Kuaururok Formation 1. Dolomite, light gray, very fine grained, saccharofdal Distribution and outcrop , rnAV, ,3.i 1 texture, massive; partly rubble covered...... 235+ The Kugururok Formation, typically exposed in Mount Bastille,and adjacent 2. Dolomite, yellowish gray to light brownish gray, mountainous areas. was named for the Kugururok River (Sable and Dutro, 1961). medium grained, weathers dark yellowish orange to The formation can he traced for several miles east and west of the type dark yellowish brown; beds 2-6 inches thick, mostly locality in outcrops south of the Kugurkirok River. Rocks in part similar cross bedded with conspicuous black bands produced to those in the type section are exposed on the west side of the Kugururok by weathering along bedding planes; laminated in Hiver valley about 25 miles to the southwest hryund the limits of the map area. upper part; cliff-former ...... 25 3. Dolomite, light gray to light brownisli gray, beds mostly 2-3 inches chick, hut as much as

weatl~erspale orange to grayish orange, saccljaroidal 8 inches thick...... 30

texture. Blocky irregular fracture with partings 7. Intrrbedded shale, sands tone, conglomerate, and

4-6 inches apart; talus 113s conspicuous creamy ct~lor; limcstone, Shale, black, silty, noncalcareous.

lower part massive with beds as much as 6 feet thick. ... 12Oi fissile to hackly; in sets of beds as much as 10 feet percent 4. Linlcstonr and sandstone (dolomitic?), interbedded in thick; 601 of unit. Sandstone and granule conglomerate, percent platy beds 112 inrh to 4 inches thick. Limestone, 50 percent 20-30/ of section, are medium dark gray; medium- to

of unit, is medium dark gray. very fine grained, coarse-grained calcareous matrix, with granules of dark

laminated; about 50 feet of bioclastic limestone in gray siltstone(?), pelletal grains, and crystalline

upper part of unit contains a few lenses of chert and calcite; weather pale reddish brown to grayish red;

poorly preserved horn corals and brachiopods. Sand- some elongate concretions of ferruginous claystone as

stone is dark gray, very fine grained; weathers moderate much as 2 inches long and half an inch thirk. Limestone,

yellowish brown, finely cross bedded; contains poorly medium dark gray, very fine grained; contains shell

preserved brachiopods (At rypa sp. and spiriferoid types). . 350t fragments aligned parallel with bedding ...... 380+

Fault?

5. Limestone, light brownish gray to medium dark gray, Thickness measured...... 1.370+

weathers light bluish gray; medium grained; calcarenite. Fault

with some conglomeratic phases containing small lime- Stratigrapl~i~relatic~nships

stone pebbles which are generally darker than the The type section is in a thrust plate that overlies probable Mississippian

matrix; a massive unit...... 23M- c~rbu~urcrocks .and is unconformably overlain by dark shales of possible Fault? Mesozoic age. On the Kugururok River 25 miles southwest of the type locality, the 6. Limestone, dark gray in lower part, weathers medium Devonian sequence is separated by faults from underlying ~riassicrocks

gray, very fine grained to sublithographic; medium and overlying Mississippian rocks (3. T. Dutro, Jr., oral comun.,

gray in upper part, weathers pale yellowish orange; 1962). The lithologic character and fossils in that section of the (Urchinella) ranges from Early Devonian through early Late Devonian. Kugurur~rk Furmdti~ln, suggest that tlie Lotnl rllickness of the lijrmat ion The two collections from thc Mount Bastille section are of either latest exceeds 2,0011 tcrt. A 400-ioot-thick basal unit ol dolomite, perhaps Middle Devonian or earliest Late Devonian age. equivalent to the upper 380 fret of the type section is overlain by Fossils from high it1 the sequence in the Kugururok Valley are 30 fret uf beds transitional to 100 feet of dark-gray to brownish-gray, considered to indicate a Famennian age and include brachiopod assem- medium- to very fine grained limestone. This Is succeeded by more than glages rich in Cyrtiopsis? and Cyrtospirifer. The Kugururok Formation 400 feet of thick-bedded. light-gray, aphanitic limestone which is a is a time equivalent of a part of the Upper Devonian rocks in the cliff-forming unit overlain by 15 feet of thin-bedded, brownish-gray, Shahin Lake area (Bowsher and Dutro, 1957). Lithologically, it appears medium-grained calcarenite. The uppermost unit is an 80-foot thickness more closely related to the unnamed sandstone and shale formation ui brownish-gray coarse calcarenite which, in places, is a nearly solid than to the probably nonmarine Kanayut Conglomerate of the Shainin Lake area brijchiopod coquina, Fossils and a* Mode of deposition Fossils from unit of the type section, identi- 4 Rocks of the Kugururok Formation suggest fied by Dutro, include: cystiphyllid corals, undet., Thamnopora sp., marine inshore deposition in their basal part; upper beds may represent Cypidula? sp.. Atrypa sp., Euryzone sp., Strobeus? sp., and orthoceratid shallow offshore platform conditions. Within a probably east-trending and cy rtochoani tic cephalopods. Late Devonian seaway, the depositional basin of Kugururok sediments 51ASa210f (USGS locality 3512-Sf)) (Collected from limestone may have bordered a large-scale deltaic outpouring of clastic rocks and conglomerate in unit 7, section 1) (Kanayut) which crop out to the east. The locations of source areas Ambocoelia? sp. for Upper Devonian clastic rocks are not known, but distributional Dechinella (Dechinella) sp. p- pat terns suggest a northerly or northeasterly source. A positive area, ostracode fragment, indet. perhaps with an extension as a south-trending peninsula in the vicinity The gastropods were identified by .T. Brooks Knight (oral commun., of the present Alaska-Yukon boundary, may have shed clastic debris 1952) who indicated that the genera are present in the Eurapean Middle both southwestward (Kanayut Conglomerate) and southeastward (Imperia3 Devonian. The faunal assemblage suggests an approximate correlation Formation equivalent) during the Famennian (see also Martin, 1959. with limestones in the western Brooks Range which have yielded coral especially p. 2442-2445). faunas of Givetian or early Frasnian age. The trilobite Dechjnclla in part of Mississippian age.

1. Clay shale, moderate yellowish brown, fissilc ...... 10 'Tli~. I>~wo~~i,lnrocks on a tributary of Tligluruk Creek consist of 2. Rubble of clay shale, as above...... 7 two 6-~nclibeds of ~nediunr-1igllt-gray to medium-gray, finely to coarsely 3. Clay shale as above, contains a few siltstone nodules . . 20 crystallint. limcstonc which protrude tliruugh vegetation and mud-cover 4. Covered...... 20 kin ihc WC-Z~ side of tlic creek. The total possible thickness of Ilevunian 5. Clay shale, dark gray to grayish brown, soft, fissile; sc~.tionat this locality, includirlg tire covered inccrvdls, is 45 Iret. contains nodules and lenses of reddish-brown weathering, Ucvu~~iaurocks are faulted over rucks of probably Plesozoic age and are cnlcareous claystone and siltstone as much as 5 feet ~wcrlair] 1)y fossi liferous black dlcrt , shale, and limestone of pruh;at)lc long and 1 foot thick; one 2-inch hed of coaly shale; ?lississipliiarl age. These exposures lie in a complexly folded and faulted contains conodonts ...... 130% belt at the mountdin front. Although ~Iwydre, in general, along strike 6. Shale and limestone, interbedded. Shale grayish wirh the Devonian rocks exposed in the headwaters of Iligluruk Creek. brown, Eissile; limestone, olive-gray to light tt~cvconnot be traced into thesc exposursu. greenish gray, Einc grained, soft, in lenses and beds I:t~ssilsfrom the iimtlstonc hcds were idcn~ifirdby J. T. Llutru, Jr.: as much as 4 feet thick, locally coquinoid; contains llSt;S loc. 4740-S1l (5nrlS;167) : spiriferoid and rhynchonelloid brachiopods and 'iunnane lla sp. convdoz~ts ...... 15 Parapugnax sp.

Sinotectirostrum_ cE. 5. rlordeggi (Kindle) Thickness measured...... 202% Cyrtospjrifrr aff. 5. aninrascnsis (Cirty) Fault Section 2. Devonin~~rocks, undifferentiated. Section of These Devonian rocks are interpreted to lie in the south-dipping Devonian rocks rixposed in the lieddwnters oC Iligluruk Creck; north limb of an overturned anticline. A high-angle faulr cuts the measured by E. C. Snhlc rln August 9, 1950. anticline so rhat sout1,-dipping Triassic and l'ermlan(?) rocks overlie Unit Thickness in Ccet the Devonian sequence. Top(?) of section - Overlying rocks consjst of black Fossils from unit 6 include brachiopods and conodonts. Collection to dark-gray siltstone, shale, chert, and limcstrine, x t'w m m a,. + m m E n C m m P 0 I- -0 (P3 Lt R r.'I A

E ;F.

n %- m P C

m0 0 R r. 0 1 All the I-<)] lcctiilus from the undi lrf~.rci~tinte~l 1)evuninn rocks divide, alorig the Nontak River and its tributaries, in the Kivalina probably represent parts of o single Late Dcvclnia11 (Famennian) fossil valley, and in rhe Cape Lisburne region (Smith and Mertie, 1930, p, 168- assemblage. The bracf~iopodsare similar to those found in the upper 169). Subsequently, the Lisburne was raised to group status by Bowsher part of the Palliser Fornlatiun of the Rot-ky b!ounrnins of AIherta and Durro (1957, p. 3). 1.isburne Croup (McLaren, Norris and Mci;regor, 1962) and also contairi elements of high The 1,jsburne Group in the Shainin Lake area, abuut Devonian assemblages from the western United States. According to 250 miles east of the Kukpowruk-Muka region, was defined by

Eticharl House, Cyrtoclymrnia plicata is characteristic of the middle Bowsher and Dutro (1957) to include rocks of Mississippian age which

Famcnniin Clymenia Zone of the Upper Devonian in western Europe. overlie the Kayak Shale of Early Mississippian age and underlie the

Devonian rocks on the Nuka River, together with the associated Siksikpuk Formation of Early Permian(?) age. In the Kukpowruk-Nuka igneous rocks, are interpreted as a klippen from a thrust plate which region, a sequence of Mississippian rocks believed possibl y to be at least has overridden Jurassic(?) and Mississippian(?) rocks at this locality. 4,500 feet thick and consisting largely of limestone, sandstone, chert,

Tlie Devonian limestones are lithologically similar to those expused shale, and quartzite, underlies rocks ranging from probable Permian along Iligluruk Creek but do not resemble the Devonian rocks exposed to Early Cretaceous age. The Kayak Shale is not known to be present oc Mount Ilastille. Identifications of megafossils indicate that most or all of this

Mississippian System sequence is probably equivalent to Imrt of the Lishurne Group uf Bowsher

and Hutr~r,.~nd the scqucrlr.c is Ilcrr. included in the 1.isburne Group. Mississippian rocks exposed in the Kukpowruk-Nuka region wcre first Mississippian rocks of the Llsburne Group in the Kukpowruk-Nuka reported by Smith and Mertie (1930, p. 168, 171-132). In the treadwaters region are divided into three named formations and dark-colored cherty of the Utukok River, light-colored limestone containing "a considerat)le units whose precise correlation to the Mississippian rocks of the amount of silica" was cal led the Lisburne formatir~n(Srl~rndcr , 1904, Shsinin Ldke area is iiot known. 'The three formations, from oldest to p. 63) by Smith and Merti,,. 'I'lic 1,isburnr W;IS rcpr~rLot1to ~vcrlicil youngest, have been named tl~eUtukuk, Kogruk, and Tupik Formations sandstone believed to rcprcscnt thc Noa~akiorn~iit irrn (Smith, 1917, (Sable and Dutro, 1961). Other rocks of Mississippian age consist p. 69-75). Crearrrcolorrd l irnt:stclnc, c;~llcrlI.i sl)urnu l irnrstone or largely of black and dark-gray chert, shale, limestone and siltstone Lisburne ir>rrlation, was also rt-por~crlto he cxpr,scd st>uth and st~uthwcst are mapped as the dark-cr~lored cllerty units, and rocks of probable of the K~tkpuwruk-Nuka region, in tt~chills near the Kukpuk-Kukpowruk Permiall nl;" .IS undivided ch~,rtv tinils, .~ndwi th tlj~lNulid Form;~tirxl. certain1y Ut ukok I:ormmt itjn were seen to compr isc wide belts of mountain

posit it,lls oi ourcrop brlts and rclationshlps of Elississippian rucks reflect exposures alol~gthe Kelly River and its tributaries. These rocks are

ov<.rtlbrt~st fiiult ir~gwi tll tilt. resultant jwtaposi tiun of strata or exposed in tl~raxial zones of broad anticlines and are overlain

the same general age but of different rock facies. by Kogruk Formatir~n.

Utukok Format ion East of the Utukok River, three belts of Utukok Formation strike

The Utukok Formation is named for the Utukok River near which wcll- generally east and southeast; the southernmost belt, north of Trail

exposed sections of this formation occur. The type se.ctiun of the formation Creek, narrows and strikes souttjeastward heyond the limits of the

is exposed in the headwaters of the Utukok River, on the north face and mapped area.

along the top of Tupik PIountain about 2 1/2 miles northwest nf the junction The Utukok Formation along Chertchip Creek is topographically

of Tupik and Kogruk Creeks. expressed as a high rubble-covered ridge about 1.2 miles long which

Distribution and outcrop.-- Most of the exposures of the Utukok strikes east. The formation does not extend continuously east or

Formation are in the DeLong Mountains, west and south of the Utukok west of these exposures; there are, however, several small outcrops

River headwaters. The formation also extends southeastward from the and rubble mounds of rock types like those of the Utukok that are

Utukok River and one belt of exposures is present on Chertchip Creek, discontinuously exposed along the same structural belt as far west as a tributary of the Nuka River. West of tl~cUtukok River, the formation Storm Creek. These exposures may constitute isolated thrust plate is exposed in two distinct outcrop belts which trend west to southuesc remnants far north of the main outcrop of the Formation. and whose northern limits are nearly everywhere defined by thrust faults. The Utukok Formation is a resistant unit west of the Utukok River

In the northern belt, which is relatively narrow and discontinuously which is typically exposed in high, mostly rubble- and talus-covered, exposed from the Utukok River westward for about 12 miles, only the mountains. Bedrock exposures are mostly limited to ridge tops between uppermost part of the formation is exposed on the north-facing frtrnts mountain pulleys, altl~oughthe resistant units uphold benches which oi the mountain ridges. The suutilern belt, which is as muclr as 2 miles can be traced along the mountain sides. Cutbank exposures of the wide, strikes west and southwest from the Utukok Iiiver to beyond the Connation are best exposed along Kogruk Creek. The weathering colors

southern limits of tlre nrca. Ihlring an aerial reconi~aissancesouth OC outcrops and rubble or the formation are distinctive moderate to of the Kukpowruk and kokol ik 1:ivers in 1949, rocks which are almost d.~i-k yellowis11 brown as contrasted to the grayish weathering colors of

tt~rKogruk. On tl~ebasis ai SI-IISS urilthering colors, the Utukok Forlnation is unlike other rock units in the area with the exception The Utukok differs from other units in the Lisburne Croup in the of parts of the Nuka Formation. On aerial photographs, ft~eUtukok silty and argillaceous character of the limestone, the presence of

Formation usually appears medium to light gray with a two-toned quartzite and other clastic rocks, the absence of chert, and in its splotchy appearance due tu differential weathering. characteristic. yellowish-brown-weathering colors.

Character and thickness .-- The Utukok Furniarion consists largely Section 3. Utukok Formation, type section. Exposed on north face of n~cdiurn-dark-gray silty, argi llsceoi~s,f crruginuus and finely and along top of Tupik Mountain, about 2.5 miles northwest of crystalline limestone which wealhers distinctive dark yellowish brown, junction of Tupik and Kugruk Creeks; measured by E. G. Sable, dark vclluwisli orange, and olive-gray. Limestor~e, rougl~ly80 percent August, 1950. of the formation, is fissile, platy, blocky, and massive, and occurs Unit Thickness in feet in beds as much as 1 foot thick. A few thin beds of dark-gray, Kogruk Formation. Contact believed to be grayish-red-weather*, silty limestone occur in the lower part of the gradational. formation. Limestone beds are locally fussiliferous and contain 1. Rubble. Limestone, silty, medium to dark gray, crinoidal debris, brachiopods, gastropods, prlecypods, cephalopods, hard to soft, weathers moderate yellowish brown and trilobites. to medium gray. platy to blocky ...... 75

Limy siltstone, quartzitic siltsrone, very fine grained limy 2. Limestone and quartzitic sandstone interbedded. sandstone and quartzite are interbedded with the limestone and comprise Limestone similar to but more sandy than that in unit roughly 20 percent of the formation. These clastfc rocks are dark grily 1, about 50 percent of unit. Sandstone, medium dark to medium gray, in part argillaceous, and occur in platy, blocky, and gray, silty to very fine grained, platy to blocky, massive beds which average about 1 1/2 teet thick. Quartzite finely laminated and cross bedded, ferruginous, in part beds as much as 10 feet thick are most abundant in the lower part of calcareous, weathers yellowish brown and light olive-

Weathering colors are simil,lr to those of the limestone the formation. gray. Spiri feroid and productid brachiopods in but are commonly lighter. Although limy shale appears to comprise a limestone. Unit exposed in wide saddle ...... 390 minor part of the formation, it may be more abundant than indicated 3. Sandstone with interbedded limestone similar to rocks here, particularly in units which are talus covered. in units 1 and 2, but sandstone weathers light brown

to pale yellowish brown, and comprises about 70 percent of bruwn, platy, beds as mucli as 2 inches tllirk. Iri talus ...... 540 par1 fossiliferous. (Talus-covered units probably 4, Sn~ldstilu~,and limestone as iu unit 2, but sit~idstonc percrn t partly underlain by calcareous shale.) ...... 2 5 about 40/ of section. Poorly preserved and meeger 10. Limestone and coquinoid limestone, medium to dark fauna consiscil~gof crirtoidal debris. brachiopods, gray, ferruginous, weathers yellowish brown; sandy to and gastrq~ods ...... 150 finely crystatline, platy, beds as much as 4 inches 5. Limestone with few intrrhrds oE quartzitic sandstone. thick. Laminated and finely cross bedded. Fossils Similar to rocks in unit 2, but contains a few thin include spiriferoid and productid bracttiopods ..... 8 5 beds of medium-gray, finely crystalline limestone, 11. Sandstone and sandy limestone, similar to unit 2. wit11 thin beds of coquinoid limestone throughout unit. Few fossils like those in unit 10...... 50 Crinoidal debris abundant; cephalopods, spiriferoid 12. Talus. Sandstone and limestone as in unit 11...... 65 brachiopods. and gastropods common...... 60 13. Sandstone and limestone interbedded, similar to unit h. Sandstone and limestone, similar to unit 2, inter- percent 2, but beds are lenticular. Sandstone beds as much as bedded. Sandstone about 40/ of sectior~,blocky in , 3 feet thick, limestone beds as much as 3 inches thick. Abundant spiriferoid brachiopods in b~dsas rn~lctr as 2 feet thick. Few spiriferoid and 1imes tone...... 20 pruductid brachiopods ...... 30 14. Largely talus of Limestone and sandstone similar to unit 7. Covered. Rubble of platy lin~estone,similar to 2, in approximately equal amounts. Limestone, in part that in unit 1 ...... 10 cuquinoid, contains crinoidal debris, hrachiopods, and 8. Limestone, medium dark gray, weathers dark yellowish gastropods. Limestone percentage increases in upper brown and reddish brown; dense, platy, in irrrguLar part of unit. Prominent rubble-covered bench occurs about beds. I.vwer 10 feet highly fossj lifcro~~s,coiltains 415 feet below top of unit ...... 1,215 spirifcruid and productid bractriopuds ...... 40 Fault? 9. Talus-covered. Sandy 1 inws tone, niedium dark gray, 15. Quartzitic sandstone and limestone interbedded. ferruginous, weathers liglrt bruwn Lrr pale yellow Lines tone weathers dark yelldwish orange ; laminated ; Total thickness mcasurcd ...... 2,99qt_ beds as n~ucli 3s 4 inclws thick ...... Fault 16. Covrrcd by quartzitic sandstone and limestone rubble Onc thin section from the Utukok Formation, was examined under and talus. l'ranirlent rubble-covered bench .... the petrographic microscope. In hand specimen, the rock is a medium- 17. Calcareous shale with thin interbeds of silty limc- dark-gray tu brownish-gray, silty to f ine-grained sandy 1imrsrune, stone, calcarcous siltstone, and qiiartzitr. Sllnlc in sets platy, laminated,md in part quartzitic. In thin section, it is dark gray, platy, !lard, with a quarter nf an inch partings,/ calcareous orthoquartzitt: (Pettijohn). The rock has a sandy texture of beds as much as 4 i~~cl~esthick. Worm trails and and granular structure. The cement, 30 tu 40 percent of the thin iucoidal markings on surfaces of shale ...... 30 section, is predominantly carbonate, with less than 10 percent crypto- 18. Covered. Quartzitic sandstone and limestone talus ... 20fi crystalli~~rquartz. Limonite and hematite are also present in small 19. Quartzitic ferruginous snndstonc and silty limestone percent amounts, mostly along grain boundaries and fractures. A black opaque irbtrrbeddcd. Sandstone 80 / of unj t in beds as much substance, possibly carbonaceous matter, occurs along grain boundaries as 10 feet thick, silty to fine grained, in part slightly of as small particles. and is less than 5 percent of the section. to modrralely calcareous. IIassive, cliff-forming unit . . 100 Subround to subangular, mostly equidimensional, grains from .02 to 20. guartzitic sandstone and limestone interbedded. .16 mm in diameter with a modal size of about .08 mm are closely Ferruginous sandstone, medium light to dark gray, packed or separated from each other by as much as 0.1 mm. The grains weathers light brown, yellowish brown, and light consist predominantly of clear quartz (about 90 percent), crypto- olive-gray; in part calcareous ; finely cross bedded crystalline quartz (Less than 10 percent), and scattered grains of and laminated to blocky and massive; contains lew percent mica, zircon, and tourmaline (less than 2 percent). Edges of some fossils on bedding surfaces; forms about 701 of unit. quartz grains appear to have been etched by the carbonate cement, and Limestone, medium gray weathers moderate yellowish some grains have been partially replaced by calcite. Pore space Is brown, in beds averaging about 4 inches thick but as less than 5 percent. much as 8 incllrs in upper part of unit, has irregular No structural breaks were observed in the type section of the bedding surfaces; contains abundant bract~iopods, Utukok Formation; all units appear to be conformable with little gastropods, and pelccypods ...... change in strike or dip. The belt of outcrop, however, narrows depositional breaks were noled in exposures of the lltukok Formntion. Kngruk carbonate rocks weather chalky white to light gray in contrast to the darker weathering colors adjacent rocks, Ridges of similar Tile base of the formation was nowliere seen; and the bases of measured of can seciions of the Utukok are all in fault relationsllip with younger rock appearanre be traced along the same outcrop belt as far as 7 miles units. The uppermost part uf the Lltukok coriforrnably underlies thta Kogruk east of tlrc Utukok River.

Formntion. In [tie sectior~sexamined, ;~(>[~roximatelytl~c uppermost 50 The southern helt of Kogruk for ma ti or^ exposures, which is arcuate feet of Utukok consists of interberldcd li~ncstoneof the yellowis11- and convex northward in plan view, extends to beyond the east and west hrown-weathering silty and sandy types common in thc Utukok and the Lirnits of the map area. The formation crops out in much the same griiy-weathering crystalliue types cllaracteristic uf the Kogruk. 'The fashion as in the northern belt, except that outcrops in the southern contact is considered to be gradatio~~al. belt are less conspicuous than those of the northern belt. This is partly because tlrr grayish-weathering colors of the southern belt rocks -Kogruk Form-te2 are generally darker than those to the north, and partly because the The Kogruk Formation is named after Kogruk Creek. the wesL-flowing Utukok Formation makes up most of the prominent mountains along the major headwater tributary of the Utukok River. The type section of the southern helt west of the Utukok River. East of the Utukok River. Eonnation is exposed in mountain-top outcrops adjacent to and souti, of the Utukok Formation is not as conspicuous and the Kogruk Formation I tlre Urukok Formation type section. composes many of the more prominent ridges. The Kogruk is well exposed Distribution and outcrop.-- Tlre L;r)gr~~kFormation in the Kukprjwruk- in cutbanks along the Utukok River from about 1.5 to 2.6 miles above Nuka region lies along the same belts of the mouth of Kogruk Creek. outcrops as the Utukok Formation. In the northern belt of outcrops In addition to these well-defined belts of exposure, rocks mapped west of the Utukok River, exposures uf the Kogruk Formation extcrid at as Kogruk Formation are exposed north of and adjacent to the Utukok least as far as the headwaters of the Kokolik River. In this belt, exposures atClrerrr:llip Creek. and occur at many localities along two the Kogri~k crops out in high, distlontiriuous en echelon hogback ridges complex structural belts. Ttw first strikes west from the Chertchip which strike southwest and whose steepest slopes face north. Strata Creck exposures at least as far as Elbow Creek; farther north, a are exposed mostly as ledges and cliffs nrr the north sides of the second belt stretches from the eastern Limits of the area, at about ridges; the south slopes are mostly rubble covered. The ridges are Lat. hS047' N., southwest at least as far as Driftwood Creek. Most prominent landmarks and are conspicuous on aerial photographs because of these localities consist of rubble patches and bands containing sparsc outL.rol,s of tossi l i fcrous and 111tios.~i1 i idrous rarhunate rucks black; some is light bluish gray and white in color. Some of the black

resembliug those of tllc Kogruk. They are discontinuous, irregularly cherts emit a fetid odor when freshly broken. In most exposures, the spaced, and range from a iew fret to several Iiundrcd Eect in width. chert is not concentrated at persistent stratigraphic horizons but is

Nut all of them are shown on the geologic map. randomly distributed throughout the f omation ; however, in some sections

Charact rr and thickness. -- The Kogruk Forma L ion consists almost the percentage of chert is higher in the uppermost part of the formation entirely of interbedded limestone and chert. Limestone, about 85 per- Some of the chert contains silicified fossils similar to those found cent of the formation, ranges in color from dark gray and brownish in the limestone. gr.ly fo nearly white, weathers n~ediun~to light gray and grayish orange, Thin beds of limy shale are interbedded with iimestone in some and is finely to coarsely crystalline and silty to granular. Most sections, but constitute a minor part of t:e formation. Mediumgray of tlle limestone is well indurated and is in part siliceous althougll dolomitic limestone in beds less than 1 foot thick also occurs in some some bioclastic varieties are friable and soft. Much of the limestone sections but does not seem abundant or widely distributed in the emits a bituminous odor from freshly Eractured surfaces. Beds range Kukpowruk-Nuka Rivers region. from abouc 1 to 5 feet in thickness, altllough sume arc as mucl~as 20 Section 4. Kogruk Formation, type section. Measured by E. C. fret thick; and fracture in blocky, massive, and platy fragments. 'l'he Sable, August, 1950. strata are locally fossiliferous, containing abundant crinoidnl debris Unit Thickness in feet which constitutes as much as 75 percent of some beds. Horn corals, Tupik Format ion colonial corals, brachiopods, and bryozoans, in part siliciiied, are Fault? also common. The thickest beds, and those of lighter color, are 1. Limestone and chert interbedded. Limestone, dark generally composed of coarsely crystalline Limestone. In many exposures, gray to medium light gray, platy to massive, dense, lilncstonc beds arc highly Eractured and siliceous rcccmentdtion of silty; crinoidal debris common. Sf liceous 1 imestone

fr;isturcs occurs in several loc;~l i 1 ics. in upper 25 feet of unit contains silicified spiriferoid Cllcrt occurs with the linlestor~cas Icnstbs, irregular nodules bracl~iopodsand horn corals. Chert, dark gray to and interbeds averaging abut~t 'I irlrhch in thit.knu.ss; ut.cnsi~~r~iilly medium gray, in irregular beds and lenses, comprises ~llcyare as much as I foot thick. 'l'l~t*ctlcrt is ~rhostly dark gray to about 30 percent of lower half of unit, and about 20 percent in upper half, Krsistar~tunit ...... 1305 SectLon 5. Kogruk Formation, mountain exposures at lat. 68°34'45"N., 2. Limestone with feu interbedded irregulz~rchert long. 160'50' N., measured and described by J. T. DuEro, Jr., on nodules. Limestone includes medium-gray bioclastic July 10, 1951. types, in massive beds as much as 8 feet thick, inter- Unit Thickness in Eeet bedded with dark-gray, thin-bedded, platy to lissilr, Top of section, overlain by Okpikruak Formation. very finely crystalline and silty limestone. Contains 1. Chert, dark gray, granular, iron stdined on upper crinoidal debris and scat tercd spiriferoid and surface...... 0.3 product id brachiopods ...... 220% 2. Silicified linestone, light brownish gray ...... 2.7 3. Rubble of limestone and chert similar to that in unit 1. 3. I.imestor~e, light brownish gray, coarse I:rained, but Less resistant. Some thin-bedded, platy, contains silicified crinoidal debris and shell frag- argi 1 laceous limestone ...... ments; about 5 percent black chert nodules in lower 4. Limestone and chert interbedded. Limestone, medium 10 feet ...... 37% dark to dark gray, weathers medium light gray and 4. Limestone as in unit 3, with lensatic concentrations grayish orange; fine to medium crystalline, soft, in and silicified crinoidal debris ...... 5 beds averaging about 4 inches thick, with fetid odor 5. Limestone, brownish black, fine to medium grained, on fresh fracture surfaces. Contains abundant weathers medium gray, contains 40 percent black crinoidal debris and horn corals, and less common chert nodules and 10 to 20 percent fine bioclastic spiriferoid brachiopods, produc tid brachiopods, material ...... and colonial corals. Chert, dark gray to black, 6. Partly covered. Limestone, dark brownish gray to notabundant,poorlyexposedunit...... 20+ brownish black, weatllers medium gray, with 10

percent granule-size bioclastic material ...... 8 'Potal ~i~easuredthj cknuss ...... 520+ 7. Limestone as in unit 6, beds 0.1 foot thick ..... 3 Utukrlk For~nntioo. Ctrn~actbe1 icvcd tr, he gradational. 8. Limestone as in unit 6. hut beds 1 to 3 feet thick with

thinner interbeds 0.1 to 11.2 fmt thick ...... 9 9. Lilnestun~?;is in unit 6; co~ltainsSyringoporii ci)lony grained, sandy with 40 to 50 percent crinojdal granule-

1 foot in diameter and 1 Eoot thick at top of unit .... 2 size fragments and limestone, brownish gray, fine

10. I.imestone, light brownish gray, fine grainad, wit11 grained sdndy , weathers medium gray, with lamellar

60 to 75 percent very coarse crinoidal debris and fluted surfaces. Crinoidal beds, about 70 percent in

ot1rr.r fossil fragments in lenses as much as 0.3 lower 20 feet, disappear upward ...... 60

foot thick ...... 4 16. Talus covered ...... 75+

1 Limestone, brounish gray, s~ibapt~anitic,mostly in

massive beds 5 to 8 feet thick, contains about 25 Measured thickness ...... 780+

percent small pebble-size crinoidal debris. Thinner Fau 1 t

limestone beds 0.1 to 0.3 foot thick 6 to 8 feet Two thin sections of limestones from the Kogruk Formation were below top of unit, contain black chert nodules ...... 25 examined under the petrographic microscope. In hand specimen, sample 12. Limcstune and chert interbedded. Limes tone medium 50ASaL95 is very light gray to yellowish gray, coarse grained, crinoidal, dark gray, coarse grained, weatlwrs light bluish gray, and massive to blocky. In thin section the rock can be termed a very in beds 1 to 2 feet thick. Black chert, about 10 coarse grained fossiiiferouu calcarenite {Pettijohn, 1957). The rock perccnt of unit, in beds 0.3 to 0.5 foot thick and nodules has a granular structure, with no preferred orientation of grains. about 0.3 foot Long and 0.1 foot thick, elongated parallel Carbonate cement, probably entirely calcite, makes up about 50 percent to bedding...... 27 of the section. Poorly sorted detrital grains consist mostly of crinoid 13. Limestone as in unit 12, but medium grained, in beds debris and shell fragments and subround, angular, or irregularly 0.2 to 0.6 1 mt thick ...... 29.5 shaped carbonate, as much as 1.5 mm in diameter with a modal size ot

about 9.5 mm. A few crinoid columnals have been replaced by crypto- T1iicknc.s~measured ...... 168.52 crystalline quartz. Smaller carbonate f ragmrnts may also be comminuted Utukok Formation? shell and crinoid debris. A few tiny circular black opaque grains, 14. Covered ...... 6455 possibly carbonaceous material, are also present. 15. Limestune, interbedded, medium dark gray, Cine Sample 5OASa238 Crcln~ tlte Kugruk Form.iLii~n in hand specimen is a is at Least 5100 feet thick. In the most southeasterly exposures, east medium-dark to olive-gray medium-grained limestone. In thin section of Kogruk Creek, a much thinner Kogruk overlies the Utukok Formation it is similar to 50ASa195 except that it is finer grained wit11 constl- and is overlain by Cretaceous rocks. There the thickness of Kogruk tuents less than 0.75 mm in diameter and grains are about 30 percent Formation is variable and ranges from 50 to 100 feet. of tltr sectiorl. tlematite, limonite, and a moderale yellow opaque In the northern belt exposures, no reliable total thicknesses mineral are present as coatings on some carbonate grains, alu~lgirregu- of the Kogruk were obtained. In the western part of the belt along lar fractures, and firlrly disseminated in the matrix. A few small the upper part of Spike Creek, Kogruk sections underlying the TupFk euiieclrol carbonate crystals, possibly dolomite, are present. A small Formation are as much as k5OO feet thick; but the sections may be amount of black opaque material is present in some fractures. ['ore repeated by faulting and maximum reliable stratigraphic thicknesses space is less than 5 percent. The rock is a medium- to coarse-grained are not more than 800 feet. Farther east, in the headwaters of fussiliferous calcarenite Iligluruk Creek, the Kogruk overlies the Utukok, and is overlain by bleasurernents believed to represent the total thickness of the Cretaceous rocks. There, thicknesses of the Kogruk Formation are not Ko;;rr~k Formation were obtained at a feu localities. These sequences, more than 600 feet. From upper Iligluruk Creek to the Utukok River, however, may be cut by faults which were not recognized during field thicknesses of Kogruk overlying the Utukok Formation thin progressively studies; they lie in structurally ct~mplexbelts and show considerable eastward to about 50 feet at the Utukok Hiver. One mile east of the variation in thickness within relatively short distances. The thickest Utukok River, however, a partial section thickness of A200 to 4400 feet section of Kogruk, measured by H. U. Hangus 5 miles west of the type is estimated. Several sections of Kogruk Formation, overlying the section, is about go0 feet thick. Seven miles to the southwust, Utukok Formation east of this section, show progressive eastward ;A section was estimated by E. (;. Sable to be 1,400 feet thick. The ttiirtning and are overlain by rocks of Cretaceous age. type secrion, however, and a section ~neasurcdby Sable 3 ~llilcs Sfrnti~raphicrelationships.-- The lower contact of the Kogruk cast-northeast of the typc section art! about 520 ~nd590 fe~atthick, Formntion witfi the Utukok Formation seems gradational. The upper respectively . Thesc Lwt, sect tons arc over lain by tlte Tupik Formation contact with the Tupik Formation appears to be conformable, although dud undcrlain by the Iltukok Format im. East oi thcsc srctjr)ns, it1 ;I the lithologic citange is abrupt. The base of the Tupik Formation may serlucrlre measured Ily M;lngus olr)ng Kogruk Creek, the Kogruk Formation represent an erosional or non-depositional hiatus or fault which would

nodules cormnonly less than 2 inches in thickness hut as much as 1 foot

thick. Locally, both the limestone and chert contain scattered fossil

Rocks whirti resemble those of the Tupik Formation, mapped as remains which consist mostly of rhynchonelloid, orbiculoid, and lingu- i the dark-colored farirs of the Lisburne Grnup, and are known from loid brachiopods, and crinoid fragments. fossils to br in part equivalent to ttre 'Tupik, are exposed both west Section 6. Tupik Formation, type section. Measured by E, G. I and east ot the Utukok River in belts north and so~lttrof the outcrop Sable, August, 1950. I hc 11s discussed above. The ~lortherrin~ostbelt of these rocks, exposed Unit Thickness in feet in discontinuous en echefon ridges, exrends eastward from beyond the Ir~complete. Top is ilolocene erosion surf ace.

I~cddwatersoE the Kukpowruk River, subparaliel LO the northern belt 1. Limestone and chert, interbedded. Limestone, mrdi-

I of Kogruk and Utukok Formations, nearly to the Utukok River. It urn dark gray to grayish black, silty to aphanitic,

strikes toward the complex structural belts east of the Utukok River dense, weatliers light olive-gray, mostly in platy

which contain similar rocks in several belts of outcrop and it extends beds averaging less than 6 inches in thickness but

beyoad the Nuka River east of the map area. The southernmost belt with some massive beds as much as 1.5 feet thick.

of rocks resembling tlrr Tupik Formotitn~ is composed of mountain expo- Chart, grayish black, in irregular beds and lenses

sures in the Utukok and Kugururok Rjucrs I~cadwnters,hut their areiil as much as b jncl,es thick, occurs mostly with thicker

extent is not known. The belt probahly uxtc~~dssoutheastward in the limestone beds. Chert and limestone in upper 65 feet

Del.or~g Huuntains beyond tile mapped area. of unit form resistant sets of beds as much as 10 feet

Character and thickness.-- The 'Tupik Formation consists mostly of hick sep~irntedby beds of platy limestone. Silicified

SO to 70 percent ~hin-beddedplaty limestone with thin interbeds of br;iclr iopods and small amount of crinoidal debris in u-dY rlrert. Limestone is mostly dnrk/to grayis11 black, weatliers light limestone and cllert in upper 65 fret of unit. Platy

ulive gray to light grayish hrown, ;ind is rrplianitic, fi~ielycrys~allinc, limestone about 50 percent of unit, massive limestone,

or silty. Tlrr ljmastone is well indurated, dense, in part siliceous. 30 percent, chert, 20 percent ...... 135%

I)iscorlfunnity or Fault?

average ahout 2 inches in thickness, but some ;ire as rnuc11 as 2 feet Kogruk Vomhat ion

thick. Gray ish-hlack cl~crtuccurs as i rrcgular beds, lenses, and One thin sactio~>of limestone €rum thc Tupik Fornntion, sample graphs, lies in the headwaters of Kugururok River at about lat. 68O

50AS~242was examined under the yctrogral>I~icmicroscope. Fn hand 32'30" M., long. 161°23' W. and is about 700 feet thick. There the a specimro the rock Isldark gray to grayish black, dense, and silty. In Tupik is overlain with apparent conformity by rocks of probable late

thin section, the rock is silty calcilutite (Pettijohn). In thin Paleozoic and Triassic age. Elsewhere, measured sections of the Tupik

section, a yellowish gray in color is imparted by undetermined sub- Formation are less than 300 feet thick.

microscopic material in the carbonate cement which makes up about 70 Carboniferous-Permian Systems

percent of the section. Well-sorted grai~tsof carbonate and quartz Cherty and Shaly Rocks

with average size of 0.02 mm are imbedded in the carbonate matrix. There is considerable uncertainty regarding the classification Bounddries of quartz grains are irregular, and some of the quartz has of cherty and shaly rocks which are older than the Triassic Shuhlik been extensively replaced by carbonate. A few crinoidal and shelly Formation and, in part. equivalent to and younger than the Mississippian ( fragments as much as 0.7 nun long and 0.2 m vide, some of which are 1.isburne Croup. These sparsely fossiliferous rocks are complexly folded partially replaced by quartz, are imbedded in the cement. Small grains with and faulted against various other ruck units and are themselves of hematite, limonite, and a black opaque substance, probably organic highly deformed. They contain rock types similar to those of the 1I matter, make up less than 2 percent of the section. Several straight Shublik, Siksikpuk and Tupik Formations. In some parts of the area carbonate-filled fractures intersect the bedding nearly at right I they are dominantly bedded bldck chert. Fossils collected from these ' angles. A black opaque substance extends along bedding for short beds suggest that they range in age from Early Mississippian to distances away from sowe of the fractures, and probably was introduced Permian. £rum the fractures before they were filled. Isolated pore spaces make In some localitfes,three generally different units are recognized. up about 5 percrnt of the section. I Elsewhere, only one unlt is exposed along an outcrop belt. Some out- The total thickness a£ the Tupik Formation is not known with crop belts are structurally iuo complex to allow differentiation of I certainty. In most sequences the upper part of the formation is not i these individual units un the geologic map. Consequently, while they well exposed; in other sections the formation is missing because of have been mapped separately in some parts of the area, in other places pre-Cretaceous erosion or raulting. The thickest sequence of the they are mapped as one undivided unit. These Carboniferous and Permian 't'upik, computed from field data and measurements on aerial photo- rocks, lying in overtl~rustplates, are of great importance in the under- standing of the regional structure because the recogill tion or their The stratigraphic position of some of these rocks has been determined proper position in the stratigraphic column would aid greatly in with a fair degree of certainty; that of others is not well known. interpreting the magnitude of thrust faulting. rutore Three general lithologic units, both consisting dominantly of wurk should place emphasis on their differentiation and possible corre- chert with lesser amounts of claystone or shale, limestone, dolomitic lation with Paleozoic rocks of different lithologic character, princi- sil tstune, and siliceous siltstone occur above Mississippian Lisburne pally he formationsof the Lisburne Group and the Nuka Formation. Croup or below the Triassic Shublik Formation rocks at several locali- Distribution and outr ro~ - .------ties. No complete sequence is known anywhere in the area, however, The Carbonifernus-Permian rtlert y and shal y rorks and the relationships of the partial sections allow only a general are conlined almost entirely to the belts ut thrust taulting in the southern part of the Southern Foothills and the DeLong Mountains. stratigraphic Lnterpretation. The three units are distinguished on

They are widely distributed and crop out either in narrow linear belts the basis of color, thickness of individual chert beds, relative which can be traced for as much as 20 miles, or in irregularly shaped abundance of dark-colored limestone,. character and abundance of sparse areas as much as 8 miles vide. 'fie resistant cherty rocks form either fossil assemblages, and differences in the proportion of shale. They high hogback ridges, commonly with several hundred feet of rclicf, or are referred to as the dark-colored facies of the Lisburne Group, the mountainous masses more than 1,500 feet high. These cherty rocks are dark-colored cherty unit, and the light-colored cherty unit. The highly fractured and their rubble covers most of the slopes. Outcrups dark-colored units are probably in part at least time equivalent; the occur along crests of ridges and in cutbanks where resistant beds light-colored unit is younger than these. protrude through rubble cover. Shaly rocks are poorly exposcd except The dark-colored facies uf the Lisburne Croup closely resembles

in widely scattered stream cuts. Gray and black lichens commtlrlly the Tupik I.'ormation in many exposures but Ln some outcrops it cuntains accentuate the dark colors of the rock rubble to pruducc black ridges more abundant shale, thicker beds of chert, and more rarely, sandstone. and tiills which contrasr with the lighter weathering colors of the Rocks in sections 9 and 10 are representative of the unit. other rock units. These rocks are also associated with mafic igneous The dark-colored cherty unit is 50 to 90 percent black and dark

rqcks in many areas. gray vitreous to dull chert with interbeds of black shale and minor and Character thickness dark limestone, dolomite, and siltstone. The chert is mostly even

Several distinctive rork types t-t)nsE it ute the bedded; beds average 4 to 6 inches thick, although some are as much as Carboniferous-Permian rocks in the Kukpowruk-Nuka region. shale is Eissi le to I~locky. 111 su111e uulcrops, thr c~,loratioacorresponds -UII it Thickness in feet to bedding; in others it crosses bedding iri an irregular fashicm. In l'op ui section. Over lain by Okpikruak Formation. most measured sections wl~frethis facies is exposed, it is less than A. t.ipllt-colored cherty unit.

25 fret tilick. At one isolated locality in the headwaters of Chertchip 1. Mostly covered. Scattered rubble of chert and

Creek, 1lowrvt.r. mvre than 125 fecr is exposed. The reddish and greenish siliceous shale, grayish green, olive-gray . ~olorsmay not be primary scdi1nent;iry features. Reddish chert and bluish gray, medium to dark gray ...... 200 limestone have been observed in exposures which are indisputably 2. Silceous 8hale and chert; grayish red and

Tri-,~ssic . . . Shublik Formation. At these pldces the colors are clearly grayish green, nodular to fissile, irregular post-depusitional. The irregular color distributioll in some of the beds; a few thin grayish-red siltstone lenses . . . . 10

Carboniferous-Permian upper unit rocks suggests that at least some of 3. Chert and siliceous shale; medium gray, dull; the colors are post-depositional. noduLar ciiert in beds less than 4 inches thick . . . . 15

'The total thickness of the light-colored cherty unit is not known 4. Chert, with minor thin shaly interbeds; chert because nf faulting and pre-Cretaceous erosion. From a comparison of mostly medium gray and bluish gray with some parLial sections, the average thickzrsa of the unit is estimated to be interheddcd black and greenish-gray beds; uniform about 500 feet. to undulating bedding surfaces; unit contains

The following partial sections of Carboniferous-Permian cherty thin bed of sedimentary breccia composed of and shaly rocks are representative of these units in the Kukprwruk- angular chert fragments, less thanhalf an inch in

Nuka r~.gion: didmeter, in a gray chert groundmass ...... 40

Sect ion 7. Carboniferous-Permian cherty rocks, exposed in hills U. 1)ark-colored cherty unit.

I mile north oE Tupik Creek, lat. 6~~33'30"N.. long. 161°07' W. 5. Mostly covered. Rubble of black chert and

Measured by E. C. Sabic, August 12, 1950. shale ...... 10e

6. Chert, hlack; massive beds from I/? inch to

2 feer thick; thin interbeds of black shale;

bright iron-stained weathering on fracture surfaces. . 35 7. Cl~frtdlrd sifict.uus dulomltc, i~rtcrbcddcd;r.tx!rt Section 8. Comlmsitc section of Carboniferous-I'ermii~n-Triassic?

,ib~~ut50 percent of unit in upper part, increases rocks exposed along Spike Creek and tributary, lac. 68"35'301'N.,

to 70 percent in lower part; black to dark gray, lorrg. 161°45' W. Measured by E. G. Sable, July, 1950.

vitreous to dull, in part silty, ill irregular Unit -Thickness in feet beds as muctr as feet thick and lenses as Top of section. Overlain by Shublik Formation.

much as 2 inches thick and 2 fret long; dolomite A. Light-colored cherry unit.

medium dark gray, weathers grayish brow], oiive- 1. Largely rubble covered. Chert and shale;

gray. and moderate yellowish brown; silty, in chert grayish green, black, gray; shale

part laminated, irregular beds as much as 5 inches mostly dark gray; minor thin limestone

thick; contains a few orbiculoid brachiopods beds in upper part ...... 20% (50ASa219f); resistant unit ...... 2. Chert, grayish green, uniformly bedded;

8. Shale, dark gray; with thin lenses and beds beds less than 6 inches thick, average

of olive-gray to dark-gray silrstone and lime- 2 to 3 inches; thin incerbeds of shale;

stone weathering bright yellowish orange ..... 1-foot bed of light gray siliceous

Base of section. Fault limestone at top of unit ......

3. Argillite, dark greenish gray, blocky; beds

Total thickness ...... 555+ less than 1 foot thick; gradational to thin

Triassic rocks are exposed near the above section, hut their beds of chert and shale ...... contact with the CarboniFerous-Permian units is believed to lie 4. Chert, greenish gray and medium gray. along a fault. Farther west, in the headwaters of Spike Creek, a interbedded; thin shale interbeds ...... 25 so~newhat different sequence of C;lrhoniferous-Permian rocks is ovrr- 5. Covered. Fdulc?...... 20 lain by Triassic rocks. The upper p.irt oC this sequence may ini lude 8. Dark-colored cherty unit or dark-colored Cacjes of Lisburne Group. rocks of the Shublik Formation. h, lntrrbedded chert, limt~stone, and shale.

Chert and limestone 70 percent of unit, blark

trl grayish black, irregular lensing beds as

18. 'Talus, lirnestunt?;~ndshalc...... 60 irregular to nodular bcdd ings; organic odor from

29. Interbedded black chert, shale, and limestone ; beds freshly fractured surfaces ...... 5

less than 6 inches thick ...... 6 Chert, limestone, and shale ~IIterbedded. dark gray

30. TLI~I~S,sl~itle. limestone, andchert asabove ..... 5W to black; limeslone, in part, in thin platy beds; in part

Base of section. Fault. 1i)ssiliierous (Leiorhynchus sp. and other brachiopods. .. 5

Limestone, as inunit 3 ...... 8

Total thickness ...... 15155 Chert, black, beds as much as 4 inches thick; inter-

Alor~gthe mountain Front between the Utukok River and Spike bedded with minor amounts of black sl~ale...... 3

Creek, the dark-colored facies of the Lisburne Group is in part of Limestnne, as in unit 3, but with thin intrrbeds of

Early Mississippian agc and overlies Devonian rocks. Basal sandstone black sl~alr ...... 9

and shale may be Kayak Shale (Bowsher and l)c~tro, 1957) equivalents. Chert and shale interbeddrd, black, weathers yellowish

Sect ion 10. Dark-colored facies of he Lisburne Croup exposed gray, jn beds as much as 4 inches thick; upper part of

in cutbanks on west side of Iligluruk Creek tributary, lat. 68' unit dominantly chert with nodules of medium-gray lime-

36' N., long. 161°1h' W. Measured by E. C. Sable, August 9, 1950. stone; lower part dominantly shale with nodules of chert . . 16

Unit Tliickriclss in fs Silty and clay shale, black, weathers to "sulfur yellow"

Top of sect ion. Fault, dipping 55O south. color; contains nodules of limy quartzitic siltstone,

1. Chert and shale interbedded, black; with a few dark- weatltrring yellowish orange; nodules of olive-gray arilla-

gray limestone nodules ...... 8 crous limestone, as much as 1 foot thick, near base of I 2. Chert, Limestone, and shale interbedded; black ti) unit...... 62

dark gray; chert 70 percer>t of unit; lirnestol~efinely Rubble covered, black sl~illcand siltstnne; black limy

crystalline to silty, in beds as much as 2 Errt thick spherical concretions at top of unit associated with shale;

and as irregular nodules in ct~ert ...... goniiltite, Muensteroceras sp. found in concretion...... 50%

3. Limestone, medium dark gray, weathers medium Sandstone, dark gray to black, weathers yellowish orange

gray, silty to finely crystalline, dense, massive, and light brown, dense; beds as much as 1 foot thick;

some sand grains are iron stained...... 15 S~~~i~i~r-i>~~~--Iat iunships 4 12. Covered ...... , . . . . In ~lncht~,~dwnr crs of Spike ;III

17. Shale, black ;~ndrnrdium dark gray. with thin iuterbrds (:;~rhonif ~.rc~us-I'tarnbianr:herty and sl~illy UII 1 ts over lit-

of siltstorle similar to sandstone as in unit 11 . , . . . 9 the Tupik Formation at several localities, and along the belts of

14. Salldstone,asinunit 1) ...... 20 Lisburne carbonate rock exposures the cherty units are interpreted to

Base of section. Underlain by Devonian rocks oi section 2. lie between the Lisburne Group and Triassic rocks. North and east of

the main Lisburne Group exposures, however. the base of the dark-

Total tl~ickness...... 234+ colored facies of the Lisburne Group is interpreted to normally overlie

Four samples of siliceous rocks form the Carboniferous-Permian Devonian rocks at two localities at the DeLong Mountain front. These

sequence were examined under the petrographic microscope. The first exposures are in cutbanks on Iligluruk Creek (Section 10). and a

two samples are from the lower unit. tributary of Spike Creek. At the former locality, a Muensteroceras

Tn hand specimen, sample 51ASak29 is black chert containing of Early Mississippian age was found in a sequence of black shale and

lenses of dark-gray, silty-appear- siliceous material. In thin sil tstone over lying brownish shale and limestone of Devonian age. Less

section, the siliceous material is composer! of about 30 percent dolomite than 100 feet of well-exposed section separates the Nuensteroceras beds

rhombs in a finely crystalline quartz groundmass. No sharp contact from fossiliferous Devonian limestone. Black chert and limestone I00

between the dolomitic portion and that cwnposed entirely of quartz feet above the Muensteroceras occurrence contains leiorhynchid

can be seen. Tiny black opaque grains, probably cnrbonaceous matter, brachiopods of probable Late Mississippian age.

are scattered through the section. At the Spike Creek locality relationships are not as clear.

Sample 51ASa52 in hand specimen is a dark-gray, silty-tn waxy- Ilere, 60 feet of black chert, shale, and 1Lmestone contains a few

appearing chert in which gradations in grain size can be seen with a orbiculoid brachiopods and lies within 20 feet of Devonian limestone.

hand lens. In thin section, angular graLns of quartz, plagioclase These relationships suggest that the dark-colored rocks of

feldspar, opaque minerals, and spheroidal grains 01 an isotropic the Carboniferous-Permian cherty rocks include time equivalents of all

substance surrounded by brownish aurenles occur in a groundmass of of the Mississippian Lisburne Group rocks exposed farther south and

almost isotropic, very finely crystalline quarl z. that the entire Mississippian sequence here may be largely chert and

shale. If this interpretation is correct, then the Utukok, Kogruk, ;~udTupik Fornlat idn~have probably becii supcrpused unto rile cherty tlrr lower unit and from rocks which are lithalogically similar to it

units by large-scale overthr~lsting. point to a late Paleozoic age, and most collections are Early to Late

Within the Carboniferous-Permian cherty sequence no major Mississippian in age. Rljynchonelloid, orbiculoid, chonetid, and lingu-

str11i.tura1 or rruslonal breaks were recognized. Tl~csignificance loid brachiopods are tile more common types often associated with

of the chin sedimentary breccia beds or reddish zones is not kl~owii. scattered crinoidnl debris. Some of the collect ions resemble those

Triassic rocks overlie the cllrrty unit in several widely from the Tupik Formation.

scattered Localities but, where examined, the contact between ttrc No fossils of definite Pennsylvanian age have been recognized

two sequences is obscured by unfossili£erous shale and chert rubble in rocks or the Kukpowruk-Nuka rcgir~rl.

wl~icll is similar to rock types in both sequences. The upper unit of the Carboniferous-Permian sequence is unfossilif-

The upper cherty unit contains some rock types like those of erous except for a few gastropod and brachiopod shells, and undiagnostic

the Siksikpuk Formation exposed in areas farther east (Patton, 1958) shell fragments which may represent leiorhynchid brachiopods or pectenoid

but the scarcity of diagnostic fossils In these units in the Kukpowruk- pelecypods. From its general stratigraphic position, and, in part,

Nuka region makes specific correlation difficult. General strailgrapl~ic lithologic similarity to the Siksikpuk Formation of eastern area, the

posit ion and rare fossils, houevrr, suggest that they arc equivdle~,ts unit may he Early Permian in age. it is also possibly equivalent, in

of tllc Siksikpuk Formation and/ur the Nukd Fornlation. Because the part, to the Nuka Formation. Finally, the uppermost part of the sequence

Siksikpuk Formation is also exposcd to the west at Cape LLsburnc may include rocks of Triassic age at a few localities.

(Campbell, 1967), it is likely that its equlvafents would be prcseitt Nuka Formation

in the Kukpowruk-Nuka region. Rocks of Permian and probable Permian age include cherty and Fossils and age -.-- ;~rgill;lcroussequences described in the foregoing section of this Megafossils €r>llnd at scattrrcd 1oralitit.s in Lhr report. Some of thcsc are lithol,~gjcnLlysimilar to the Siksikpuk (:;~rhorli fcrrj~~s-l'ermionr.hcrty rods drtb almust entire1y ~.rrufi nt-d to the dark-colored uni ts and occur mostly in secr ions conrnir~i~jg Porm;~t ion (Pat ton, 1958), ;and :II*. mapped with Carboniferous-Permian

carbonate rocks. Except For a few locnljties, it has not tleru pcjssit>lt. cherty and shaly rocks in the Kukpowruk-Nuka region. Other Permian 1 LO sa~irfact~rilycorrelar~ tl~ese sections hccil~sv1.1 tile conpier rttcks, i11 large part litl~nlngic;,lly different from the Siksikpuk struclur~~drtd lack of strntigraplric dt.tai1. All ni the fossils irtlrn i 1:ormnt ion, are assigned to the Nuka Formation (Tailleur and Sable, 1963). rescmbli~tllose of the Kogruk Format ion.

This formtion has been directly traced into the Kukpowruk-Nuka region rlllr;u.tc j111d ~IIi11k11es~~ from adjoining arras farther east where the type section is exposed. The Nukn Fi~rmation, as exposed in ~IIL.Kukpuwruk- us described here Rocks of tt~rmapped unit/correspond to the original definition of the N~bka reg ion, consisLs largely of arkosic sands tone and con-

Nuka Formation (Tailleur and Sable, 1963) and not to the restricted glomerati~.sandstone, platy limestone, shale, and siltstone. Chert, usage of TaiLlcur, Mamet, and Dutro (1972). sandy dolomite, and conglomerate make up minor parts of the formation.

The coarser clastic rocks are unlike other clastic rocks in the area;

Tl~rmain ~lutcropsof the Nt~k~lF~rm~~tion lie in the a~~uthc.:~sternpart of the they are relatively clean and well sorted, consist almost entirely

Kukpowruk-Nuko region, mostly betwee11 lats. 68'40' N. and h8"43' N. Kcsisri~nt of well-rounded to subangular quartz and feldspar grains, and include units ~>fclw format ion are exposed in Iiigh north-facing hills and in linear. some glauconicic sandstones. Sandstones and conglomerates, which mrlsr ly rr~bhle-covered ridges. 1,owl;inris between the hills arid ridges are comprise about 50 percent of the sections examined, are mostly fine tundra covered. West of these exposures, the formation is present at to very coarse grained, very light to medium gray and yellowish gray many localities in several complex structural belts as far west as the in color, and weather gray to moderate yellow-brown. They are quartz- headwaters of Elbow Creek. These exposures consist of linear ridges Ltic to friable and contain scattered granules and small pebbles of and rubble-covered patches not connected with the larger main exposures. clear to frosted quartz, feldspar, and black chert. Fossils include

The westernmost known exposures Lie near southern boundary of the area, scattered crinoid columnals and shell fragments and, more rarely, near Cairn Creek, a tributary of the Kugururok River. The northernmost whole shells, mostly of productid brachiopods. Some of the coarse known exposures consist of a few isolated and mostly rubble-covered clastic rocks are medium dark gray, probably due to organic matter, and patches in the vicinity of Storm Creek at about lat. 68'44' N- others are a distinctive gray-green color due to the presence of

Weathered exposures of ct~eNuka Formation are mostly pale to glauconite and chlorite. The sandstones and conglomerates occur in moderate yellowish brown and light to medium gray. From a distance, blocky to massive beds averaging ahout 2 feet thick, but as rnucl~ as some of the outcrops resemble the Utukok Formation in weathering rolor. 10 fret thick, and form resistant units as much as 100 feet in thickness

Resistant units of the Nuka form ledges and cliffs and talus of these They are massively crossbedded (foreset beds inclined as much as 20~ units is composed of dominantly boulder-size blocks. On aerial photo- from topset beds), laminated, and uniform in appearance, and graphs, the outcrops and rubble are light gray to white in tone and beds are commonly lenticular. I.imt~stu~~eand limy silLstone are ~nutuallvinterbedded and are to finely crusbedded and are, in part, calcareous. intrlrheddrd with sandstone units, The limestones are mostly dark gray Four thin sections of Nuka Formation clastic rocks were examined and si1ty, a1 thuugl~some light-gray and olive--gray limestones and under the petrographic microscope. Two sections are of granule conglm- finely crystalline to sublithographic cherty limestones are present *rate. Sample 51ASa62, in hand specimen is medium light gray and

Crayish-green, glauconitic Limestoile is jnterbeddrd wIth sandstone light olive-gray, massive, quartzitic, wlth iron-stairrd weathering and other cypes of limesto~iein some exposures of the formation. Some surfaces. Subround, frosted grains of quartz and feldspar are as much limesttme beds contain scattered granules of quartz and black chert. as 5 m In diameter. In chin section, the rock is intermediate between

Fosailiferous limestone contains brachiopuds (mainly productid types), a quartzitic feldspathic sandstone and a quartzitic arkose. Detrital bryozoans, horn corals and crinoidal debris, in part silicified. grains, estimated to comprise 90 percent of ttte section, are poorly

The limestone beds are commonly less than 1 inch thick, but may reach sorted and tightly packed. They range to as much as 4.4 m in diameter as much as 1 foot in thickness, They are commo111y laminated, with with modal size of 0.25 mm. They consist largely of quartz, about thln platy laminations an ci~hthof an inch to ;h quartcr of ;in inch thick. Much of the/ 80 percent, mostly in single grains but with some composite grains, limestone resembles that of tllr Ti~pikFormation, but associated chert w11ich may be vein quartz or chert. Microcline (10 percent) and plagio- is more common in tile Tupik. 111 the Nuko Formation, chert interbedded clase (about 5 percent) (Albite, AbgO-Ab10 and Oligoclase, An75-An25) , witti the limestunes is medium dark gray, uniformly bedded, and in beds and a few grains, possibly orthoclase, comprise the remainder. Cemtnt , as much as 6 inches thick. Rubble exposures of bluisll-gray, greenish- about 20 percent of section, is cryptocrystalline quartz with minor gray,and grayish-red chert, commonly associated wlth the clastjc locies carbonate. A yellowish-gray earthy mineral (limonite?) is conspicuous of the formation, are mapped with the Nuka. in the cement and along intergrain contacts. Sample 51ASa231 in

Unfossiliferous shale and siltstone, assnciated with and appearing hand specimen is similar to 5lASa62, but is medium dark gray in color. to both overlie and underlie snme or the resistant sandstone and lime- In thin section, the rock is also similar to 5iASa62, except that mineral stone sequences described above, are also mapped as Nuka Formation. gralns range from subround to angular, quartz grains (about 70 percent)

Dark-gray, fissile to hackly, silty shale comprises 70 to 80 percent exhibit irregular etched edges, feldspars (30 percent) are in part of these rocks. It is interbedded witti thin lenses and beds uf dark- sericitized or replaced by carbonate and include larger proportaion of

gray and medium-dark-gray siltstone whjch weathers bright yelluwish plagioclase than SiASa62. Cement (about 20 percent of section) includes

brown and yellowish orange. Both the shale and siltstone are laminated minor amounts of carbonate. A dark-brownish-black substance, probably urg;lr~ico~.itt~.r, is c~~rumt)ualong intt~r~r.3111<-LHI~LLI'~S .i11(1 ill the ccinrllt rxl>cIsurcs, seq~lc.n(-c.srumposed dtminant ly of the resist ant coarse

,~~tdgives tlw rock its dark color. clastics and limestones range frvm a few tens of feet to about 450 Tvo very lint. grained sandstones the Nuka Formation were from feet thick. 3kso ex.imirlerl under the microscope. SampLc. 51ASa124 (Secticln 11. unit Secticlo 11. Huka Furmatlc~n, headwaters of Kidney Creek. L?) in hand specimen is grayish green and well indurated. In tllill Hrasured by E. C. Sable on July 4, 1951. section the rock is a calcareous ieldspathic sandstone. Moderately --Unit Thickness In Ceet we1 1-sorted, subangular to angular griiins of quartz, plagioclase, Tup of section. Overlain by rubble and microcline in relative order ui ahundar~ce are the major graila 1. Silty shale and siltstone incerbedded. Shale dark const iruents. Some feldspars are in part serici tized and replaced gray, fissile, hard, about 70 percent of unit. Silt- by carbonate, others are clean in appearance. Clauconitel?), tour- stone dark gray and medium gray, weathers bright maline(?), opaque minerals, artd bryuzoan(?) fragments replaced by yelluwish brown, argillaceous, finely laminated and carbonate are minor elements. Modal size of grains is .06 mm. Ccmenr crossbedded, in lenticular beds '1 lncl~t~r 1 inch ttttck. . . 20% comprises about 25 perctrnt of the section and is dominantly calcite 2. Covered...... , ...... =of_ with some green chlorite and Limonite. 3. Rubble. Limestone, quartzit lc siltstone, and quartz- Sample 51ASa315 in lbilnd specimen is yellwisl* gray and uelL it ic sandstone. Limestone medium gray, weathers indurated. In thin sect inn ir resembles 51ASa124, but grains are pale yellow-brown, platy, contains scattered subround of sn inch in more ~111seIypacked, tiny scattered grains of t~cma~itcare I>rcsrl>i, trl subangular black chert pebbles as much ds a (lu.~rter/ and the chlorite is dominantly brovn in colnr. diameter and few brachiopods. Siltstone medium No single complete sequence uf the Nuka Formaciiln is k11cw11 to be dark gray, calcareous. Sandstone very light gray, exposed in the area, The formation has been overthrust onto rucks f Ine to coarse grained, well-rounded grains appear as young as Early Cretaceous and is c~mm~nlyoveriain uncunfi)rn~ably tu be clear tu frosted white quartz ...... 25+ by Early Cretacruus rucks. In the st~uthrastcrnpart of the arv,t, thc 4. Ru1,blr. Limestone, medium dark gray, sandy, silty, maximum exposed tl~icknessof the formation is probably more than and argillaceous, platy to blocky ...... lo+_ 5200 feet. In the complex structural belts west and 11orLt1 ul Li~*.sc 5. Limestone as in unit 4, beds 3 inches to 1 foot thick, contains bractiiopods, bryozoans, and crinoid coarser lenses containing granules and

debris. Interbedded with small amount of medium small subround pebbles of quartz and black

crystalline fossiliferous Itmestone. One 6-inch chert; about 35 percent of unit. Limestone, dark

bed of medium-dark-gray chert 25 feet above base ...... 30 gray, platy, finely crystalline, about 5 percent of

6. Siltstone, olive-gray, platy, calcareous, contains unit, contains poorly preserved brachiopods and

poorly preserved brachiopods and ct~rinoiddebris ...... 5 crinoid debris. Small amount of dark-gray bedded

7. Mostly covered. Limestone, dark gray, sandy, chert in talus ...... 20

platy, with interbedded siltstone as in unit 6 ...... 17 13. Sandy limestone and limy siltstone, light to dark

8. Sandstone and quartzite, very light gray, weathers gray, weathers moderate yellowish brown, contains

yellowish brown to white, clean, calcareous, in scattered quartz grains, blocky to platy ...... 5 lenticular massive beds 1 to 4 feet thick; in part 14. Talus covered ...... 255

massively crossbedded. Unit thins eastward to Fault

25 feet thick within GOO0 feet distance ...... 40

9. Talus covered, sandstone, quartzite, and limestone Measured thickness...... 477+

as in units 8 and 9 ...... 15 The descriptions of Nuka Formation rocks apply to the northern

10. Sandstone, grayish green, very fine to coarse grained, belts of outcrop of the formation. Farther south, exposures of

contains white quartz grains, massive to platy, calcar- shale, siltsrone, sparsely fossififrruus platy limestone, limestone

eous, slightly micaceous, friable to well indurated .... 5 conglomerate, and chert, are present in the headwaters of Kidney

11. Talus covered. Sandstone, quartzite, and limestone Creek and in a belt extending at least from the headwaters of Trail

as in units 8 and 10 ...... 30 Creek as far west as the mountains in the vicinity of Cairn Creek.

12. Interbedded sandstone, conglomeratic sandstone, The shale, siltstone, and limestone strongly resemble the less-

and quartzite. Grayish-green sandstone as in unit 8, in resistdnt rock types described above. Quartzose sandstone rubble

beds 1 to 5 feet chick, 60 percent of unit. Sandstone, similar to that described above is exposed along part of the latter

medium dark gray, very fine grained, calcareous, with belt and, if not faulted, overlies or is interbedded with a dominantly

shale-siltstone sequence. The sequence appears to be underlain by black chert and limrstone of probable Mississippian age; it is few rhynchonelloid brachiopods. Chert, black, as intruded by mafic igneous rock and is in fault contact or uncoilformably irregular nodules and beds. Unit has banded appearance ... 20 overlain by rocks of Jurassic or Cretaceous age. A description of 10. Lirny,silty shale and limy,argillaceous siltstone, one section of this sequence, which is mapped as the questionable Nuka interbedded; medium to medium dark gray, Formation, is given below, weathers yellowish gray and yellowish brown, Section 12. Nuka Formation(?) exposed along mountain side on fissile to platy, laminated, contains scattered west side of Trail Creek. Measured by E. G. Sable, July 13, 1951. pebbles to boulders of limestone, and lenses and Unit Thickness in feet irregular beds of light-to medium-gray crinoidal Overlain by Okpikruak Formation. limestone; scattered crinold debris in shale...... 45% 1. Rubble, gray and black chert ...... ? 11. Limestone conglomerate. Matrix dark-gray, very 2. Chert, grayish green, bedded ...... if? finely crystalline soft limestone; as much as 70 Fault? percent. Subround pebbles to 5-foot boulders of 3. Chert and argillite, grayish green, uniformly bedded, several limestone types including medium-to dark- beds 1 to 4 inches thick. Minor amount of grayish- gray,medium-grained limestone and white friable red weathering chert ...... 45 limestone, and pebbles of black chert. Massive 4. Covered ...... 20 unit...... 40 5. Microgabbro sill, grayish green, medium crystalline. ... 50 12. Limestone, dark gray, platy, sublithographic, 6. Marhle, light gray, very finely crystalline...... 4 weathers light gray; massive unit ...... 15 7. MicrogabbrosiLl,similiartounit5...... 5

8. Chert, black, silty, weathers yellowish orange, beds Measured thickness...... 277% 2 to 6 inches thick. Recrystallized zone of granular Unfossilif erous shaly sequences, similar to shales in the sections chert 4 inches thick at top...... 13 discussed above, are exposed in the headwaters of the Nuka River. 9. Limestone and chert interbedded; limestone dark gray, Kidney Creek, Trail Creek, and Cairn Creek. There is as much as sublithographic platy to blocky, weathers light olive- several hundred feet of dun~inantlysilty shale with interbedded yellow- gray and brownish gray; 70 percent of unit; contains ish-orange and yellowish-brown weathering siltstone and limestone. Exact relatio~lstlipswith the other shaly sequences are not known but, Rubble covered. Siliceous shale, dark gray, weachers

on a litllolugic basis, and because in part they lie along the same pale yellowish gray ...... 10

belts of outcrup, the shaly units are believed tu be generally correla- Chert and argillite interbedded. Chert, bluish gray

tive. Most outcrops of the unfussiliferous shaly units underlie grayish- and greenlsh gray, interbanded and uniformly colored

green and grayish-red chert, siliceous slrnl e, and argillite of possible beds 1 to 3 inches thick, 70 percent of unit. Argillite, inch Permian age. Along a tributary of Cairn Creek, they overlie black chert greenish gray, beds 1/8/to 2 inches thick ...... 20

and limestone of probable Flississipian age. Mostly rubble covered. Interbedded silty shale, limy

A section representative of this sequence, including overlying shale, and si ltstone. Shale dark gray, micaceous,

Permian(?) chert, siliceous shale, and argillite, is described below: laminated to finely crossbedded, weathers moderate

Section 13. Nuka Formation(?) exposed along south side of Trail yellowish brown and yellowish orange, organic odor

Creek. Measured by E. G. Sable, July 12, 1951. from freshly fractured surfaces; about 90 percent of

-Unit Thickness in feet unit. Siltstone, medium dark gray, weathers moderate

Okpikruak Formation; fault or angular uncunformity. yellowisl%brown, argillaceous, in part calcareous, in

1. Poorly exposed. Chert, medium gray, glassy tu lensing beds ss much as 3 feet thick...... 65 inch to 3 granular, beds 1/2/ inches thick, weathers Siltstone and silty shale interbedded as in unit 7, but

moderate yellowish green ...... 35 siltstone dominant in large lenses as much as 20 feet

2. Chert, bluish gray, glassy. beds 1 to 3 inches thick ... 8 thick...... 38

3. Chert, grayish red, grayish green, and bluish gray; Rubble covered. Silty shale and siltstone as in unit 7 . . 10

grayish-red chert about 60 percent of unit in beds 112 inch Slltstone similar to that in unit 7, laminated...... 15

to 3 inches thick; reddish coloration also crosses Silty shale, limy shale and siltstone interbedded as

bedding of greenish and bluish cherts...... 35% in unit 7; shale dominant ...... ?

I 4. Chert, medium dark gray, granular to suhvitreuus, Ruhble covered. Silty shale, limy shale, and siltstone

weathers light gray, 80 percent of unit. Creenish- as in unit 7; shale dominant...... 12

1 gray and bluish-gray chert, 20 perce.nt of unit ...... 2ie Limy shale, limy siltstone, and silty shale interbedded similar to unit 7; st~oledominant ...... 35 8. Limestone, medium gray, weathers yellowish broun to olive-gray. very finely crystalline to

Measured thickness ...... 3032 silty, thin bedded, finely laminated and crossbedded. Section 14. Composite section of Huka Formation(?), exposed on Contains orbiculoid and linguloid hrachiopods ...... 65

mountain slopes and tributaries of Cairn Creek, Kugururok River. 9. Siltstoneandshale, poorly exposed...... 10

Measured by E. G. Sable, AugusL 1950. 10. Limestone, medium gray, finely crystalline, massive .... 2

Unit Thickness in Eel

Top section. Overlain by mafic igneous rock. Base of section

1. Clay shale and silty shale, dark gray, weathers Total thickness ...... 357+ yellowish orange and reddish orange, fissiLr. Stratigraphic relationships and mode nf deposition

Contains scattered concretions and lenses of Mo significant depositional or erosional breaks were observed in exposures

reddish-weathering clay ironstone and bright Nuka Formation in Kukpouruk-Nuka region, but relationships of units with-

yellouish-orange-weathering siltstone ...... 752 in the formation are not well understood. This is a marine deposit,

2. Siltstone, medium gray, weathers yellowish in which the clastic constituents have been relatively well sorted and brown, laminated, probably a lense...... lot reworked. Frosted grains, high-angle and tangential crossbedding, and 3. Share with lenses of siltstone as in unit I ...... 65 the clean character and lenticular nature of the sandstone beds suggest

4. Shale, black, silty. contains scattered nodules an aeolian origin, in part, with reworking in a relatively shallow,

of clay ironstone ...... 60 clear sea. lnterbedded lfmestones represent intervals of relative

5. Sandstone rubble; light gray, clean, frtable, quiescence of shallow marine vaters during which abundant brachiopods

quartzose (position of this unit uncertain) ...... 20? and crinoid assemblages existed. The nearly unfossiUfrrous shale and

6. Break in section. Covered...... 20? argillaceous siltstone sequences represent deposit Lon from turbid water

7. Chert, black, massive beds as much as 1 Foot with sluggish current activity.

thick. Scattered crinoidal debris...... 30 Interpretation of lateral facies differences is difficult because

of the present structural positions of units referred to the Nuka Format ion. The m,lin rxpasurtks oi the form;~ti~urare interpreted to 1 ir by Smith and Mert ie arc now interpreted to be Carbonj ferous, Permian, in .II> ~~v~~t-ll~r~~~tl>lt>~.k. 'I't~t,ir origi~r.~ I p,~.si 1 ~LVI ~I-~oI-1'1 f~ult in>; is 13ot Triassic in age. The strata here referred to as the kr~owli c.s~.c.p~tll~t it is il~tcrllrclti~j.LO II;IVL* I.I~I~south OF tlrc present Shublik Formation are rocks which in part are similar to those in the outcrop ltr<.ilitic.s. Tl~emrlrr. soutlrcrn, do~ni~rantlyslr.ile, setluc.ut.cs ;lrtL exposures of the type Shublik, but they also include some considerably intc.rl)rc.t~bd ti] belong ttl tile furnr;ic iul~,,tnd suggest rclat ivcly qule t, different rock types. deeper water deposition. They may represent either an overthrust block Distrlhr~tionand outrrol ------. which originated south of the more northern exposures, or a relatively Altho~agh the Shublik Formation is widely dist rlbuted in the Kukpowruk-Nuka region, most of its exposures autoc~~tt~onousblock over which mare northerly rocks moved. are of small areal extent. The main belts of Triassic outcrop extend If this assumption is correct, along the front of the DeLong Mountains from the eastern limit of tllc source area lay to the north. Clastic constituents indicate that the area at about lat. 6E047' N- southwestward beyond the Kukpowruk the source area was rich in felsic rocks, and that only the stable Hiver. These belts are composed of several discontinuous bands lighter and more resistant minerals survived transportation and depo- expressed as linear or curving ridges of moderate relief. Farther north sition. and west, similar Triassic outcrops are scattered. These also trend Triassic System generally southwest and are associated with rocks of several ages. Shublik Format ion Soutlr of the main belts of Triassic exposures, and west and north of

the Nuka River, fussiliferous Triassic rocks were seen only at a few Thc name Shublik Formation was first applied by Leffingwell localities. In the headwaters of the Utukok River and west, however. (1919, p. 115-118) to a sequence of dominantly dark shale, limestone, Triassic outcrops, although discontinuuus, are more abundant. They and sandstone of Late Triassic age in northeastern Alaska. Trlassic rocks in the Kukpowruk-Nuka region, first reported by Smith and Mertie lle for the most part between the main northern and southern belts of Lisburnr Group and south of the Lisburne Group southern belt (1930, p. 188-189) at the heads of the Utukok and Colville Rivers and The sourlrernmost locality of Triassic rocks in the region lies on the in the Kukpowruk River valley, include chert, sandstone, shale. and north side of Mounr Bastille. Triassic rocks have heen reported limestone. Smith and Mertie recognized that these rocks differed south of the Kukpowruk-Nuka region along the Ntmiuktuk and Kugururok lithologically from those in the type Shohlik Format ion. More recent Rivers (.I. T. nutro , Jr., 1951, written commun.), as far west as the investigations have shown that many of Llle outcrops mapped as Triassic vicinity of Cape Lisburne (Smith and Mertie, 1930, p. 187), and at many otticr Locnlit ies tl~rouglir~ur northern Alaska (Paynr ~tdotl~ers, 145 1) , the weathered surfaces. Large masses of red jasperoid quartz-veined ThcSttuI~itkFormationis generally not well exposed. Hills and chert, as much as 50 feet thick. in whichbedding is not discernible, ridges underlain by ttte formation are largely rubble covered, and beds are associated and are mapped with the Shublik Formation rocks at a in most cutbank exposures are extremely contorted. broken by faulting, few localities. and Largely covered by talus. Many outcrops can be easily recognized, Shale comprises 10 to 40 percent of the sections examined and from a distance, because of the distinctive grayish-orange and pale- is mostly black, grayish green, and olive-gray. Black sooty shale, yellwish-orange- ("buff'q weathering colors of fossiliferous, cherty interbedded with black limestone and chert, occurs in units as much limestone that is associated with dark chert and shale. On aerial as 15 feer thick. In some localities it contains spherical black photographs, the light-weathering limestane traces contrast sharply with cherty concretions as much as 6 inches in diameter. Some shales have the dark cherts and distinguish many exposures of the Shublik from a high organic content and can be ignited. One exposure of black adjacent rocks. Traces are commonly sinuous or, where broken by faulting, limy shale on the Kukpowruk River yielded abundant pelacypods of the are discontinuous and lie at angles to the regional strike of the genus Halobia. The greenish and grayish shales are mostly siliceous Character and thickness and col~lonlyoccur in thin units interbedded with chert. Some shale Chert, shale, siliceous shale. and limestone, in units in the Shublik Formation contain conodonts, approximate order of abundance, comprise the dominant rocks Limestones of two distinctly different types occurs in the Shublik of the Shublik Formation in the Kukpowruk-Nuka region. Chert is variable Formation. A distinctive easily recognizable zone, consisting of in abundance, comprises 50 to 90 percent of the sections examined, olive-gray- to mediuurgray-, "buf f'Lweathering, dense, lithographic, and is mostly evenly bedded. Beds average less than 3 inches thick, cherry limestone interbedded with variegated chert, occurs in the upper but may be as much as 6 inches thick; locally they contain fossils on part of tlic formation. Ft varies from a few feet to 35 feet in thick- bedding surfaces. In addition, chert occurs as nodules and lenses in ness. Limestone beds, commonly less than 2 inches thick, range to 4 shale and as irregular lenses interbedded with limestone. The cherts inches in thickness. Some of the limestone is fossiliferous and are commonly bluish-gray, olive-gray, medium light gray, and black. coqulnoid, bearing the pelecypod Monotis. The shells are silicified Grayish-red, grayish-green, and grayish-olive chert is less common. in many of the limestone beds and the zone contains a correspondingly Weathering colors, commonly various shades of grayish yellow, yellowish below greater amount of chert than where si1icification is not present. In orange, grayish green, and light gray, extend as much as a quarter t)f ,In inch/ some sections, Mor~otis-bearing chert occupies the stratigraphic position the rocks are fossiliferous or exposed in sect ions where associations of the limestone. These cherts are of secondary origin, the result of with other rock types are recognized, great uncertainty exists as replacement of carbonate by silica. Ln whether they represent the Shublik Formation or rocks of late The second type of limestone is dark gray to black and very fine Paleozoic age. Even the criterion of association breaks down at some grained to finely crystalline. It occurs in beds and lenses averaging localities where these rock types are unfossillferous and are in fault less than 2 inches, but as much as 6 inches thick. The limestone is contact with other units, Some of the cherty sequences which are mapped interbedded with black chert and black shale in one or more zones as Shublik Formation may locally include units of late Paleozoic age. below the "buff"-weathering limestone and chert. Dark limestone beds An absolute range of thickness for the Shublik Formation in the are locally fossiliferous and contain shells of the pelecypods mapped area is not known. Lack of well-exposed sections in which Daonella and Halobia. These dark zones resemble rocks in some units structural repetition of beds can be ruled out, post-Triassic erosion, of late Paleozoic age and, where unfossiliferous or with poorly preserved and uncertainty about the lower contact make thickness measurements or fragmental fossils, the Paleozoic and Triassic rocks ace difficult difficult. The thickest,uell-exposed section bounded by fossiliferous to distinguish. Shublik chert beds are not as thick as many of the beds is 86 feet. Zn other more poorly exposed sections Triassic fossils

Psleozoic cherts, however, and Shublik Formation dark limestone and Thicknesses seem resista~lt are present through as much as 100 section-feet. chert units are thinner and less/than those in the Paleozoic sequences. highly variable, and in many parts of the area the Shublik has been Minor rock varieties in some sequences of the Shublik Formation completely eroded prior to deposition of younger Mesozoic beds. Fre- include light-gray, dense, black-veined cherty marble associated with served sequences of clle formation are believed to be at least 260 feet, the "buff"-weathering limestone and chert zone, and olive-gray, yellowish- and perhaps as great as 400 feet thick. orange-weathering,thin-bedded siltstone that is interbedded with chert The following section is representative of the formation in the and limestone. mapped area: The rock types of the Shublik, with the exception of the "bufftL Section 15. Shublik Formation, exposed in cutbank on west side weathering Monotis-bearing limestone beds, are in themselves not of a tributary of Thunder Creek, lat. 680.45' N., long. 160°19' W. distinctive of this formation. Carboniferous and Permian units Measured by J. T. Dutro, Jr., July 25, 1951. contain similar appearing cherts, sliales, and dark limestones. Unless -Unit Thickness in feet 7. Interbedded chert, Limestone, and shale; chert,

Top of section. black. in 0.1-0.2-foot-thick beds, 70 perccnr

1. Fossiliferous limestone, medium gray and of unit; sandy limestone, very fine grained,

light olive-gray , weathers "buff" color, dark gray to brownish gray, 20 percent of

dense, very finely crystalline to litho- unit; black shale 10 percent of unit; scattered

graphic, thin-bedded 0.1-0.2 foot; contains pyritic nodules ...... 7.5

irregular nodules of bluish-gray chert ; in 8. Chert and limestone, black; one massive bed; large

part f ossiliferous and coquinoid; with shells of Daonella cf. E, dubIa Gabb and

Monotis subcircularis (Gabb) OSGS loc. 24104) Daonella cf. D. moussoni Merian (USGS loc. 24103) ..... 0.6

2. Chert, medium bluish gray, olive-gray , and 9. Chert, limestone and shale, as in unit 7...... 8

dark gray. uniform to banded, glassy to Fault

dull, conchoidal fracture, thin bedded; IO. Chert and shale; chert, medium gray, olive-gray, and

contains few pyrite grains; interbedded blue-gray, glossy to dull. beds 0.05-0.1 foot thick;

black shale less than 5 percent of unit ...... 22 shale olive-gray, silty about 5 to 10 percent of unit ... 15

3. Interbedded chert and limestone, dark gray Total thickness measured...... 103+

to grayish black, beds 0.1-0.2 foot thick ...... 10 Base of section. Fault.

4. Rubble covered. Black shale ...... 15 A sequence somewhat unlike the above described exposures was

5. Chert, olive-gray with black streaks, beds examined in the southern part of the area a quarter of a mile north of

as much as 0.2 foot thick; interbedded the Mount Bastille Devonian Kugururok Formation-type section. Chert

with dark-gray clay shale and olive-gray silty shale. ... 10 and limestone are the dominant rock types but the section also contains

6. Interbedded limy siltstone and chert; siltstone, granule conglomerate and sandstone. Fragmental shells identified as

light olive-gray, weathers grayish orange; chert, Monotis subcircularis (Gabb) by Bernhard Kummel occur within the

medium gray to medium dark gray...... 1 conglomerates. w c E .+ 0 5 4 u p*"? cm.rlm .d~da m 3 3 '0 yl u fi* ~umo Wb CwuU 3.cb- U u '+ 45es rl P 22$? mn:o YWSW cmequ E 3 UPS VIw+ U4@J= 22e5 :";m .3 em w 5 4 w cl4ua, m c a! 0. umw WWWz "-I MO " 0, !4 V1 h "2Sm "re:C I.I U91Y UJ a! 5 :gtb uoaC E : O4uIhC rmPb a! U 111 b) mCO. m - togettler in rubble of dark and Light limestone. In that place they and conodonts. Nodosaria ~hublikensis Tappan and o ther long-ranging are either %ittiin 10 section-feet of each other or are contemporaneous. foraminiferawere reported from 51ADu145 and 148, accordtng to H. R.

On the Kukpowruk River, Hiilobia cf. g. ornatissima Smith was found in Bergquist (written commun., 1952). The same samples contained the black limy shale below the "buf ftLweatherLng limestone. conodont Condolella. in additLon to unidentified bladelike and bar-

Precise locations of Triassic fossil collections made by P. S. like conodonts, according to W, ti. Hass (written commun., 1957).

Smith in 1926 in the upper Kukpowruk and Kokolik River drainages (Smith Another sample, from about 10 feet below the Daonella beds, contained and Mertie, 1930, p. 193) are not known. All of these were reported fragments of Gondolella and bladelike and barlikr conodonts, according to contain Pseudomonotis subcircularis (Gabb) (SmltIi and Mertie, 1930, to Hass. Of five samples whose stratigraphic position is uncertain, p. 193). The name of this pelecypod has since been changed to Monotis four were barren and the filth contained the conodonts Gondolella, subcircularis (Gabb) (Muller, 1938). Hindeoella and bladelike, barlikc*. and platelike conodonts.

Pelecypods identified by Bernhard Kummel as Daonella cf. 2, Nodosaria shublikensis -ar?an was described from the type

Cabh and Daonella cf. D. moussoni Merian were cullected from a black Shublik Formation (Tappan, 195 , p. 11) where it is known to be of limestone and chert bed of Section 15, about 65 feet below limestone Late Triassic age, but it may range into older Triassic beds. beds bearing Monotis subcircularis (Cabb). According to Kumm~l, the Hass reports that Gondalella ranges from Middle Pennsynvanian genus Daonella is generally a good marker for the Middle Triassic. tl~roughMiddle Triassic and perhaps into the Upper Cretaceous.

All Triassic megafossils in the Kukpowruk-Nuka rvgiun therefore Mode of drposi t ion Shallow, quicscr~~tseas probahly covered the entire point to a Late and Middle Triassic age for the Shublik Formation. Kukpr>wruk-Nuka region during Middle and Late Triassic time. The However, as far as can be determined, all of these fossils are from dark limestone, chert, and shale in the lower and middle parts of the upper or, possibly, middle part of the formation. It may be that the Shublik may represent deposition in relatively deep water or where unfossfliferous rocks directly below thesr beds are of Early Triassic current action was weak. The light limestones and cl~ertsin the upper pfrbably \&4-~n~LIrbay age. part of the formation represent shallow ~helf~depositsin clear seas, Hicrofossils uere recovered from four of ci ght samples. 'Two during which time the pelrcyll~dMonotis flourished. The origin of samples, collected from beds between the Monotis-bearing "buff" the Iwddcd chert is a problem yet uusolved, although much of the chert limestones and the Daonella beds (set? Section 15). yielded foraminifera appears to be prrstdrpusitiunal. The locations of source areas for tlrc fi~icclastic rucks in tt~cShuhlili arr! iilsa slut known, but tl~cynl:ly unit and two wackc, sandstone, mudstune, and conglomerate units. have loill to the soutt~,as inferred from the presence of granule-size Sedimentary rocks that are now known or regarded to be of Early sedimentary breccia and cunglirmerate in the southernmost part of the Cretaceous age in the Kukpowruk-Nuka region were considered by Smith area near Mount Bastille. and Mertie (1930, pl. 1, p. 196-232) to belong to both the Lower

Jurassic(?) and/or Cretaceous Systems Cretaceous and Upper Cretaceous Series. Their division was based

upon differences In lithology and fossil content between the clastic Several thick units uf terrigeltously derived clasric rocks in the 4 rocks exposed in the northern part of the area as compared to those Kukpovruk-Nuka region are assigned to the Cretaceous system. These in the southern part, and upon the interpretation that the less-deformed rocks are of drastically different aspect from those of Paleozoic and northern sequence overlies che structurally complex southern facies earlier Mesozoic ages, and indicate a radical change in sedimentation with pronounced angular unconformity. As a result of more recent processes, tectonic framework, and depositional environments from studies, five Cretaceous rock units In the Kukpowruk-Nuka region can those represented by the older rocks. be correlated with named units defined elsewhere in northern Alaska. No Jurassic rocks were recognized in tile Kukpowruk-Nuka region These units are the Okpikruak Formatton, argillaceous unit, and Fortress during early investigations (Smith and Mertie, 1930, p. 195). Studies Mountain, Torok, Kukpowruk, and Corwin Formations. All of these units, since then have shown that, in parts of the area, sparsely Cossiliferous with the possible exception of part of the Coruin Formation are of beds. largely wacke and mudstone, underlie rocks of known Early Cretaceous Early Cretaceous (Neocomian and Albian) age. (tleucomian) age and over lie 'friassic, Permian, and probably Mississippian Thq agesand correlations of other unnamed units described below rocks. These strata may include rocks of Jurassic age, although no are are uncertain; they occur in different belts of exposure most or all diagnostic fossils were found In them. The rocks/in structurally com- of which probably occur in different allochthon~@thrust sheets. plex interrelationships and relationships with adjoining rocks of Informally named units considered to be Late Jurassic or earliest various ages, and are associated with cherty and shaly rock types which Cretaceous in age are herein called the wacke with cannonball are similar to Triassic and some upper PaLrozoic rocks. Some of these concretions unit, the argillaceous unit, two wacke, sandstone, mudstone, units resemble the fossiliferous Okpikruak Formation of Cretaceous age and conglomerate units, and the micaceous sandstone unit. described below, and it is possible that all of them are units of

Early Cretaceous age. The units are the wacke with cannonball concretions Uacke with Cannonball Concretions siltstonc are also present in some locaIit~rs,but thelr stratigraphic Dislribution and outcrop relationships to the above rocks are not known. Extrusive and intrusive The wncke with cannonball ~.~,r~i-rctir>nsunit is expost,d ill mafic igneous rocks are associated with exposures of the formation, sev~r~ilstructurd 1 b~kltsin thr S~>uthc.rr~Fut,rl~il 1s particularly along the Nuka River and Driftwood Creek. At some locali- and DeLong Mountains from the eastern limit of the Kukpowruk-Nuka ties they intrude or are interbedded with sk~ale,graywacke, and chert region, ai about lat. 6a043' N., westward and southwestward to the head- units. Although many of the clastic rocks in the unit are similar in waters of Elbow Creek and Kogruk Creek. The northernmost exposures of composition to those in the Okpikruak Formation of Early Cretaceous the formation lie east of the Nuka River at about lac. 6a047' N., and age, the wacke with cannonball concretions unit generally lacks the the southernmost exposures lie about 1.5 miles south of Kogruk Creek more uniform and rhythmic bedding that characterizes many exposures at ldng. ~60~48'W. of the Okpikruak. The unit also contains bedded chert and mafic Rocks of the unit are not well exposed and, except for conglomeratic rocks, neither of which has been recognized in the Okpikruak Formation. and cherty strata, and associated mafic rocks, they are poorly resistant, Mudstone, which comprises 40 to 60 percent of the exposures. is and are topographically expressed as low, mostly rubble-covered hi 11s mostly dark gray but also may be medium gray, greenish gray, and black. and discontinuous ridges or tundra-covered lowlands. Cutbank exposures Strata are platy to hackly. and occur in units as much as 400 feet thick. are common along the upper part of the Nuka River and its north-flowing Some mudstone units weather to a "gun-metalu-blue color. tributaries and along Driftwood Creek west of Thunder Mountain. These Locally they contain a wide variety of nodules, concretions, and lenses, which include are also largely rubble covered. however, and beds are highly folded dark-gray, grayish-olive, and grayish-green,dull chert occurriagrnostly and cut by numerous f~tults. As seen on aerial photographs, the surface as nodules and lenses; greenish-gray to grayish-black silty and sandy expression of the formation is not distinctive, and exposures resemble spheroidal "cannonball" concretions, some of which have concentric other poorly resistant rocks of Cretaceous age. Character and thickness structure, which average 4 inches but may be as much as 2 feet in Interbedded mudstone, siltstone, grapacke, and diameter; dark-gray silty limestone lenses, some with cone-and-cqne conglomerate comprise about 70 percent of the w;icke with structure ; and argillaceous lenses and nodules which commonly weather cannonball concretions unit exposures in the Kukpowruk-Nuka region. reddish brown. Bedded chert, conglomeratic mudstone, and siliceous shale are inter-

bedded or associated with ttiese clastic rocks. Variegated stiale and Poorly stratified conglomeratic mudstone, in units as much as 40 fect thick, contair~sscatr~.r~d silbruund to subangu1;rr granules, pebbles, Graywacke conglomerate, conglomeratic sandstone, and scattered and cobbles made up largely of 1,lack and gray-green chert, with lesser boulders in fine-grained, clastic rocks are interbedded with or occur amounts uE argillite, sandstone, limestone, mudstone, and mafic igneous in the graywacke, siltstone,and shale sequence. Conglomerates are 'Ingular to subangular boulders as much as 8 feet in width also rock. variable in thickness and composition within single outcrops. They occur in these shale units, and consist mostly of chert, chert breccia, are poorly sorted and contain as much as 70 percent granule- to boulder- and limestone breccia. Some of these boulders may be fault blocks sizeconstituentsenclosedinclay tocoarse sand-sizematrix, which do not represent deposition contemporaneous with that of the Some of thc conglomerate beds, believed to lie at or near the base of

shale. Others, however, are completely enclosed in mudstone,and there the formation, are composed almost entirely of black and greenish little doubt that they and the shale were deposited contemporaneously. is chert and argillice. Other beds, believed to lie higher in the sequence,

These exotic conglomeratic mudstones resemble the "wildflysch" deposits also contain sandstone, limestone, shale, conglomerate, silty "cannon-

and almost certainly represent sedimentation attendant to orogeny. ball" concretions, and a few mafic and felsic rocks. Granule- to Grayuacke, ranging from silt ro coarse sand size and locally pebble-size fragments are commonly subround to subangular although conglomeratic, constitutes about 20 to 30 percent of the exposures. some of the conglomerates are composed dominantly of angular chert It is greenish gray and medium dark gray, weathers dark reddish brown, fragments. Cobble- and boulder-size fragments are commonly angular to moderate brown, and olive-gray, and is poorly stratified. The graywacke subangular, mostly less than 3 feet in diameter, but as much as 20 feet commonly occurs in lenticular beds, mostly less than 5 feet hut in places Long and 8 feet wide. In general, the coarsest conglomerates lie in as much as 25 feee thick, inrerbedded with shale and siltstone in units the southern part of the area in the upper parts of the Nuka River and as thick as 200 feet. Beds are massive to blocky and highly fractured. Drif tvood Creek drainage. Those conglomerates exposed farther north The rock is well indurated, generally dense, locally quartzitic, and are composed of cobble-size or finer material. in part calcareous. Some beds contain common to abundant spheroidal Chert. interbedded with shale, graywacke, and siltstone, probably "cannonball" concretions which average about 6 inches in diameter but makes up less than 10 percent of the formation, and occurs in lenticular may be as much as 5 feet across. The concretions are sandy or silty, masses averaging less than 15 feet, but in places as much as 40 feet more calcareous than the enclosing rock, dense, generally resernbIe thick. The chert consists mostly of grayish-green, grayish-olive, and the graywacke matrix, and are like those found Ln the shale. dark-gray interbanded and uniform varieties, but bluish-gray and gray chert is also present. Irregu1,lr and lenticular beds are comnwnly less poorly sorted, as much as 0.8 mm in mean diameter with a modal size

than 3 inches thick, ueattrer light gray, grayish green, or yellowish of iibout 0.4 m. They consist of quartz (about 60 percent) in single

gray, and are rowonly iron stLljnd. Thin units of siliceous shale and crystal grains and composite grains grading downward in fineness to

argillite are interbedded uith the chert. Unless associated with the chert, plagioclase (20 percLnt), rock fragments (15 percent) and black,

clastic rocks described above, cherty units of the wacke with '~annonbal~' brown, and yellowish-gray opaque minerals (less than 5 percenr). Crains

concretions unit are difficult. or sometimes impossible, to distinguish of mica. carbonate, and tourmaline(?) are relatively rare. Some grains

from other cherty units of Trlassic and Late Paleozoic ages. of plagioclase are clear; others are extensively sericitized and replaced

Sectiors111 which there is no repetition of beds by faulting or by calcite. Most quart z-grain boundaries are sharp, but some boundaries

folding are rare. These range from a few feet to about 400 feet thick. with the chlorite and calcite are irregular and etched. of Because tll~lack of known key horizons, discontinuous exposures, One thin section of a "cannonball" concretion (51ADu29) was examined complex structural relationships, and rapid lateral facies variation, under the petrographic microscope. In hand specimen the spheruidal it has not been possible to correlate many of these sections. A reliable crlncrrtion is 2 inches in diameter, uith concentric structure, medium

total thickness for the unit in the mapped area has not been obtained. dark gray to dark greenish gray, and weathers grayish orange. In thin As inferred from the diverse rock types which make up scattered sect ion, the ruck is similar to the sectloll of graywacke described above, sections along Nuka River tributaries, the unit Is believed to be mure but the cement is about 50 percent of the rock and the percentage of than 1.00 feet thick; it is possibly as much as 5500 feet thick in some carbonate In the cement increases toward the center of the concretion. places, and is absent in other places due to erosion prior to depusition Grains are as much as 0.6 mm in width with modal size of about 0.1 to of Early Cretaceous rocks. 0.2 mm. A few subround grains of greenish mineral, perhaps glauconite, A representative sample of graywacke from the wacke with 'C.mnonbalP1 are also present. concretions unit (51ASa250) in hand specimen is greenish gray, weathers St r.~ti&rapt~icrelations1iips and nlod~-r)LIr_epnsiri[In mtrderate brown, is fine to medium grained, dense, blucky. and slightly [nterrelationshfps of rtu-ks tlrnt compose the wnckt. witlr cannonball concre- calcareous. In thin section, the rock contains about 40 percrnt tions unit and relationships of this formtion with rlt11c.r mop units are poorly chlorite and carbonate cement, in part sericitic, will) some micro- known. The unit overlies rocks ranging from Triassic to probably crystalline quartz. Crains are predominantly angular to subangular, Mississippian age, and it is associated with Devonian rocks in the mid others upper part of tlie Nuka River. Ilowever, the nature of the lower ctlnract sition into the Colville geosyncline (Paynr/ 1951). Pronounced lateral

was nowhere defined with cert.llnty. It is likely that many of the Eacies changes, poorly stratified, lenticular clastic beds, graywacke

relationsliips with older rocks may be due to faulting. Nevertheless, conglomerate containing a large proportion of angular fragments and,

it seems that tlre unit lies unconfornlably on older rocks, and that locally, boulder-size constituents. and sandy "cannonball" concretions,

several different facies represent its basal heds. Some beds inter- suggest rapid, chaotic deposition.

preted to be basal consist of chert ronglomerate and sandstone; others Although the presence of coarse wacke-type clastic rocks suggests

are shale, and still others are graywacke and shale containing the a nearby source area, it is not certain that these rocks are actually

distinctive sandy "cannonball" concretions. These may be contemporaneous close to their original positions of deposition. Some or all of them

local facies or they may represent different times of deposition are in overthrust blocks which have moved north from their original

interrupted by periods of erosion. It is possible that these strata source. In part, rocks of the unit are interpreted to have been

were deposited on an erosional surface of considerable relief or a overridden by other overthrust plates. Although evidence that the

terrain composed of complexly folded and faulted older rocks. Because source areas for these sediments lay south of the Kukpovruk-Pluka region

the unit has been involved in the thrust faulting which characterizes is fairly certain, the actual positions of these areas are not known. much of the regional structural pattern, the different basal facies Volcanfc activity during deposition of these beds is indicated

may represent different overthrust plates, each far removed from its by the presence of mafic extrusive rocks within the sequence. Other

original depositional position. mafic igneous rocks, which almost certainly are sill-like intrusive with The uacke 'CannonbalF'concretions unit underlies the Okpikruak bodies, have been intruded subsequent to at least part of deposition

Formation in almost certain unconformable angular relationship along but preceding deposition of Okpikruak Formation rocks.

the Nuka River and its tributaries. The Okpikruak rests on several The mode of origin of the bedded charts is not known. They may

different facies of the unit along this outcrop belt, but most commonly be all or in part silicified tuft beds associated with the volcanic

overlies the "cannonball" concretion-bearing graywacke and shale. activity. Examination of chert samples under the petrographic micro-

The diverse rock types which comprise the unit suggest rapidly scope has neither substantiated nor disproved this hypothesis. The

changing sedimentary conditions, and tlie dominant clastic rocks are origin of these and other cherty rocks poses an interesting problem orogenic types which represent the initial stages of northward depo- which remains to be solved Wacke, Sandstone, Muds tune, and Cunglomeriltu Units skclctal structure, are associated with these rocks in rubble. Conglo-

merate consists of granule- to pebble-sized angular chert fragments Unfossiliferous clastic rocks, exposed in several structural belts embedded in a very fine to medium-grained sandy micaceous matrix. in the soutliern foothills and DeLong Mountains south of the belt of the One thin section of the conglomerate was examined under the petro- micaceous sandstone unit and north of fossiLiferous Okpikruak Formation graphic microscope. The rock is a graywacke. Poorly sorted angular rocks, are mapped as two units of wackr, sandstone, mudstone, and con- and subangular chert fragments 1 to 6 mm in width comprise about 50 glomerate. Lithic types are nut particularly distinctive. The strata percent of the section. These are surrounded by closely packed, uvurlie rocks ranging from Devonian(?) to Trlassic age, and underlie angular to subround grains, with modal size of about 0.3 mm, consisting the Okpikruak Formation in some places. Bedding relationships with mostly of clear quartz in single crystal grains, chert, and mica, with Triassic and Paleozoic rocks range from conformable to as much as 45" lesser amounts of plagioclase, carbonate, and rock fragments. Cement discordance. The clastic rocks may represent equivalents of cunsists of carbonate, chlorite, and sericitic material. The carbonate the Okpikruak or, in part, Fortress Mountain Formations; they contain appears to have wholly or partially replaced some feldspar grains, and rock types which are found in these formations. some quartz grains have irregular outlines against the cement. The clastic rocks are poorly exposed and mostly rubble covered Alon" the front oE the UeLong Mountains, between the Kokolik River in ttre mapped area. They consist of interbedded sandstone, wacke, and Storm Creek, these strata in part appear to represent a coarser siltstone, and shale, with minor amounts of conglomeratic sandstone facies of the micaceous sandstone unit. Included in these highly and conglomerate. Mudstone and claystone comprise about 60 percent deformed rocks are sedimentary breccias in which angular fragments of these rocks and is dark gray to grayish black, hard, and micaceous, consist almost entirely of chert and argillite. In addition, there in units averaging about 5 feet in thickness. The sandstone and silt- are medium- to dark-gray , brownish-weathering graywacke and subgraywacke stone are medium gray, medium dark gray and greenish gray, weather light sandstnne, siltstone, mudstone, and conglomerate containing sedimentary to moderate brown and olive-gray, thin to medium bedded, generally and igneous r0c.k fragments. weli indurated,highly micaceous, slightly to lrighly calcareous, and in Pebble to boulder conglomerate of two types are mapped with these part crossbedded. Shale and sandstone-siltstone units are repeated unils; one type is characterized hy contained clasts of chert, sandstone, in roughly rhythmic pattern. Clear, ueli-formed quartz crystals, mostly and mafic igneous rocks; the other by a lrigh proportion of mafic and less than 1 inch but as much as 3 inches in didmeter and exhibiting €elsic ignc,c>us rocks. A section includil~gthe latter type of conglomerate Measured tt~ickness...... , ...... 77e is described below. Fault? Section overlies highly folded rocks of wacke

Succion 17. Exposed along north front and top of Thunder Mountain. with %cannonball1 concretions unit.

Measured by J. T. Durro, Jr., June 23, 1951. Total tllickness of these units are unknovn but are estimated to

Unit Thickness in feet exceed 4000 feet. Two units are differentiated mainly on the basis

Top of section. Overlain by Okpikruak Formation, of association with different suites of older rock types and apparently

rhyttlmically interbedded wacke and mudstone facies more abundant and coarser grained conglomerates in the eastern unit

containing coarsely ribbed Buchia shells. which extends from the eastern borders of the region to the Utukok

1. Craywacke-type sandstone, medium gray to greenish gray, River.

weathers moderate brown, fine to coarse grained, Ln Of the 43 shale samples collected from these units for microfossil four Q.2 beds to 0.4 fmt thick, flaggy, sligfitly undulating content, all but / were barren. The four samples contained furaminifera 525% bedding surfaces, but generally evenly bedded ...... considered by H. R. Bergquist to be nondiagnostic for precise age

2. Conglomerate, matrix of dark gray to dark grayish determinations. green, fine to medium grained, dense sandstone, Micaceous Sandstone Unit about 20 percent of rock; mostly subrounded to The micsceous sandstone unit sequence consists largely of dark- rounded pebbles and cobbles as much as 3 inches gray to grayish-black siltstone and shale, and lesser amounts of sand- in diameter, about 90 percent of which are of msfic stone. It overlies Triassic rocks and underlies the wacke and conglomerate and felsic igneous rocks, which from hand speci- member of the Fortress Mountian Formation in the Kukpowruk-Nuka region. men are greenish-gray diorite, dark-green diabase, The age of the unit and its exact relationships to other rocks in the medium-dark-green coarsely crystallir~egabbro, light- area are not known. gray and pink granite and granuphyre, and gray Distribution~ndoutcrop quarcz diorite. Smaller pebbles of the above types KoCks of the micaceous sandstone unit arc exposed are msrly subangular. Conglomerate contains less in tltfi,iouthern foott~ills,in a southwest-trending belt mile about / ro 8 miles wide, from at least the Nuka River at lac. 68O than 10 perccnt subangular pebbles of fine-grained 46' N. to west of Spike Creek. The belt adjoins the main belt of the sandstone and angular fragments of black shale...... conglomerate member of the Fr~rtressMountd in Formac ion on the north> and Siltstone and sandstone are mostly medium dark gray to medium gray,

on the s,>utti, adjoins a belt of high hills composed of rocks of various weather light olive-gray to brownish hues, and are jn part ironstained

ages wliich are mostly faulted against the mlcaceous sandstone unit. or with Ivgunmetal" blue stain. Beds are mostly less than 1 foot, but

Rocks of the unit weather to lincar ridges end hills which are may bc as much as 5 feet chick, platy to massive, and commonly exhibit Rubble traces of generally lower than those of the adjacent belts. fine crossbedding and graded bedding. A granule-size chert conglomer-

siltstone and sandstone give a finely striped or banded appearance to ate layer, lens than i inch thick, occurs at the base of some sandstone

many of the hills. Tile less resistant shale weathers to mud. and beds. In addition, common features are small irregular ripple marks,

fluwage oE tl~rmud and rock rubble produces a finely streaked appear- casts of worm trails, borings, indeterminate organic markings and

ance to hillsides. Zn general, the appearance of these hills is similar small-scale scour markings.

to thost? underlain by the sandstone mcmber of the Fortress Mountain In some localities, fractures in sandstone and siltstone are

Formation. Cutbank exposures of the unlt are best exposed along filled with calcite or drusy quartz. Some greenish-gray, reddish-brown

Driftwood Creek and its tributar i rs and along the Utukok River. weathering sandstone is interbedded with the above rocks, probably -Character- -. -- dnd thickness-- - in the upper part of the formation. Sandstone beds also appear to be Silty shale and clay shale, siltstonc, and very fine grained sands tone, in decreasing drtlrr of alllkndance, more abundant in the upper part of the unit. constitute the major rock types of the micaceous sandstone unit. Thcse Section 18. Section of micaceous sandstone unit. East bank are typically medium to dark gray and grayish black, well stratified of Driftwood Creek, lat. 68O43'N., long. 160°30' W, Measured and rhy~llmically intrrbedded. Beds are mostly hard to quartzitic by P. W. Shannon, September 2, 1950. and moderately to hjghly micaceous. They contaLn pyritic and dense -Ilni t Thickness in feet Eerruginous claystone nodules and, rarely, carbonaceous fragments. Top of section, fault contact with Jurassic(?) rocks. The shale, mostly grayish black bur containing some greenish-gray 1. Siltstone and interhedded shale. Siltstone, and grayish -red beds in the lower part. is fissile, in part paper dark gray, hard; shale, grayish black ...... 4 thin, hackly, and nodular, and generally resembles other dark shales 2. Siltstone, dark gray, very hard ...... - ...... 1 in tile area. Some beds are iron stair1t.d and show "gunmetal" blue- 3. Siltstone, with thin interbeds of soft grayish- stained surfaces. black silty shale; siltstong medium dark gray 0 'I;.

0 lu P mog - U YiiZ OUOmuci m9.xgum u lo +C3e~u 3

Siltstone, mediurc dark gray, micaceous, hard, 109. Clay shale and silty shale, grayish red, fissile ..... 16 in 2-inch beds; about 10 percent of thinly beddedshale...... 7 Measured thickness...... SO@

Siltstone and silty shale, intergradational...... 2 Bottom of section, anticllnal axis Note: Silty shale...... 2 (In units not described, siltstones are medium dark gray to dark

Siltstone, medium gray, weathers light olive- gray, weather light olive-gray, are hard and slightly to moderately gray, hard; calcite-filled fractures ...... 1 rnicaceous; the lower contacts of many siltstone units are abrupt

Siltstone and silty shale, intergradational...... 6 and the beds contain mud markings and indeterminate organic(?)

Siltstone,asinunit97 ...... I markings on their lower surfaces; shale is dark gray to grayish

Siltstone and silty shale, intergradational...... 2 black. )

Siltstone, as in unit 97 ...... 1 Two thin sections of sandstone and siltstone from the micaceous

Siltstone and shale, intergradat ional, sandstone member were examined under the petrographic microscope. In fissile, grades downward into unit 103 ...... 8 hand specimen, sample 50ASa166 is a fine-grained platy-micaceous sand-

Shaly siltstone, medium gray ...... 1 stone that weathers olive-gray. The grains appear well sorted. In

Clay shale, medium dark gray; lower 1 foot thin section, the rock is intermediate between a quartzitic subgraywacke sheared...... 10 and a quartzitic calcarrnite. Cement and matrix comprises about 30

Siltstone, medium dark gray, weathers olive- percent of the section. The cement consists of: microcrystalline gray, poorly indurated. dusky-blue stain ...... 1 quartz containing small euhedral carbonate crystals; black opaque

Clay shale, greenish gray...... 0.3 metallfc minerals; crystal aggregates of pyrite, black nonmetallic

Silty shale, dark gray, grayish black, and material and limonite; and sericitic paste containing carbonate and dusky blue; in part,sheared, ...... 46% chlorite. Grains are mostly subangular, but range to angular and

Clay shale, greenish gray, fissile ...... 5 subround and are fairly well sorted; modal size is about 0.15 mm but

may reach 0.3 mm in diameter. Grains consist of: clear quartz

(estimated at 40 percent) partly sericitized and replaced by carbonate; along Kidney Creek, Driftwood Creek, and trtbutaries of Storm Creek. carbonate (30 percent) as subround grains and euhedral crystals; chert and distribution (20 percent); muscovite; plagioclase (Andesine, Ab6*Anh0); rock fragments; Ho identifiable megafossils were found in the mica- and, mostly, limonitic opaque minerals. ceous sandstone member. Worm trails, borings , some carbona- The siltstone sample (50ASa318) in hand specimen resembles the ceous fragments and indeterminate organic(?) markings are present in sandstone but is more micaceous and weathers pale olive to light .brown. some beds. One fossil collection containing the fine-ribbed pelecypod In thin section too, it is slmilar to the sandstone. The modal grain Buchia (USGS loc. no. 23566) was found in limy nodules size is 0.05 mm, cement contains minor amounts of metallic minerals interbedded with wacke and mudstone at the south edge of the micaceous but more carbonate than the sandstone and the rock contains about 20 sandstone unit belt of exposures, along a tributary of Storm Creek. percent muscovite. This exposure, mapped as Okpikruak Formation, overlies Triassic rocks In the two partial stratigraphic sections measured thicknesses of and appears to underlie beds of the Driftwood Formation exposed scarcely the unit were 900 feet along the Utukok River and 800 in the type section 50 feet to the north. Contacts are obscured by rubble. If the on Driftwood Creek. An estimated total thickness, based in part upon Okpikruak rocks normally underlie the micaceous sandstone unit at this structural reconstruction across the belt of exposures, is between locality, the unit is younger than, or equivalent to, rocks of the IQOO and WOO feet. Okpikruak Formation exposed farther south. The micaceous sandstone No well-exposed contacts between the micaceous sandstone unit unit is tentatively assigned an Early Cretaceous age. and the wacke and conglomerate member of the Fortress Mountain Formation Of 21 shale samples collected from the micaceous sandstone unit were seen during field studies. Along the west bank of the Utukok five for microfossil content, all but / were barren. The five samples con- River, the unit overlies the wacke and conglomerate tained foraminifera which are considered by H. R. Bergquist to be member, but the contact is interpreted to lie along a high-angle either too poorly preserved or nondiagnostlc for age determination. reverse fault with the micaceous sandsrone unit in the upthrown block. Crrtaceuus System Elsewhere, rocks of both units are highly folded near their contact. Argillaceous Unir Along Elbow Creek. Driftwood Creek and near Storm Creek, the wacke and conglomerate member dips dominantly north, away from the belt of Fossiliferous rocks of Lower Cretaceous (Neocomian) age which lie

Driftwood Formation exposures. Within a few hundred feet of the con- along structural belts north of Okpikruak Formation exposures are tact, beds in the two units appear conformable at several localities mapped as the argillaceous unit. These include exposures in the Kukpowruk River valley 8 miles southwest of Igloo Mountain; discontin- Section 19. Argilldceous unit, Driftwood Creek, measured by uous exposures between the Utukok River and Thunder Creek, best exposed E. G. Sable and J. T. Dutro, Jr., June 3, 1951. along Driftwood Creek at lat. 68O46'40" N., long. 160~42'W.; and strata Unit Thickness in feet

in the vicinity of Storm Creek, 10 miles southwest of Lake Noluk. Top? of section. Possible synclinal axis.

Rocks mapped as the argillaceous unit in the Kukpowruk River 1. Mostly covered, probably underlain by shale; valley are poorly exposed in a cutbank on the south side of the river rubble of coquinoid 1imestone , greenish gray and across the river in several cutbanks along an abandoned channel. to light gray, weathers grayish red to pale brown,

The exposures are interpreted to lie in a structurally complex anti- locally contains well-rounded chert granules, and cllnal fold and are overlain by rocks of the Fortress Mountain Formation closely packed shells of Buchia sp. .... 25+ sandstone member. Rocks of the argillaceous unit at this locality are 2. Clay shale, greenish gray, hard, poorly exposed ...... 17+ not more than a few hundred feet thick and consist of mostly inter- 3. Coquinoid limestone, as In unit 1, a single bed, bedded soft black and dark-gray claystone, medium-gray, fine-grained with finely ribbed Buchia ...... 8 sandstone, brownish to black coquinoid claystone with finely ribbed 4. Clay shale, dark gray, fissile, with shiny surfaces,

Buchia (Valanginian), and unusual rock types such as gray clay, sep- contains two 6-inch lenses of coquinoid limestone . . . . . 6 tarian and marcasite concretions, limestone breccia, pebble conglomerate, 5. Coquinoid limestone, as in unit 1; one lenticular

limy spherical claystone "cannonball" concretions, limestone with cone- bed, with finely ribbed Buchia...... 5

in-cone structure, ferruginous siltstone, and claystone concretions 6. Clay shale, as in unit 4, contains several with coarsely ribbed Buchia and coquinoid limestone lenses of medium-dark-gray silty limestone, with finely ribbed Buchia. as much as 2% feet long ...... 11

The exposures of argillaceous unit along Driftwood Creek are of 7. Rubble of dark-gray and greenish-gray shale with

small areal extent and consist predominantly of shale with interbedded medium- to dark-gray chert nodules...... 7

coquinoid limestone beds and lenses, and cherty nodules near the assumed 8. Clay shale, black and greenish gray, iron stained,

base of the section. hard, contains carbonaceous wood fragments and medium to dark-gray chert lenses and beds less than 1 inch thick. . . 7 member of the Olcpikruak.

Uacke Member

Fault? Distribution and outcrop.-- Rocks of this member are widely

Measured thickness...... , ...... 862 distributed in the southern foothills and DeLong Mountains. They

This sequence is interpreted to lie on the north limb of a are exposed in fairly continuous belts, as much as 251 miles wide, synclinal fold along a fault, and is flanked by conglomeratic sandstone in the southern part of the area ueet of the Utukok River. The and shale of the Fortress Mountain Formation. The south limb of the outcrop belts strike southvest beyond the southern limits of the syncline contains a sequence nearly identical to that described above, area, probably at least to the Kivalina River (Smith and Mertie, and fossils collected from coquinoid limestone beds on these include 1930, p. 200). They are also present east of the Utokok River in several discontinuous outcrop belts,as much as 3 miles wide which finely ribbed Buchia and a belemnite, Cylindroteuthis sp. strike generally east and northeast beyond the limits of th& map area.

Okpikruak Formation The uacke member is commonly expressed in high, rounded

to moderately rugged hills and mountains. Most of them, when seen The Okpikruak Formation of Neocomian age, named by Patton (b Gryc from a distance, are a dark-reddish-brovn color. This weathering and 1951, p. 159-160) at its type section about 160 miles color is particularly distinctive in outcrop belts in the DeLong east of the Kukpouruk-Nuka region, consists of interbedded graywacke Mountains south of the northernmost main exposures of Kogruk Formation, sandstone and shake. It is characterized by a rhythmic alternation of and in outcrops along and south of the Nuka River. Farther north. beds, conglomerate beds locally near the base, and early Lower Cretaceous similar rocks questionably referred to the Okpikruak Formation weather Buchia pelecypods. In the Kukpowruk-Nuka region, a rock sequence of to lighter hues of reddish brown and light brown. Well-elrposed but Neocomian age which rests on Mississippian to ~urassih?'rocks, is mapped structurally complex sections of the member are present in cutbanks as Okpikruak Formation. Two distinctly different facies of Neocomian in the headwaters of Spike Creek and along the Nuka River; structurally rocks, occupying different outcrop belts, have been recognized. The simple sections of similar, but unfossiliferous strata, are exposed southern facies is similar to that described by Patton. It is named in mountain-side exposures along Trail and Seagull Creeks. the wacke member of the Okpikruak Formation in the Kukpowruk-Nuka On aerial photographs, the general appearance of Okpikruak region. The northern facies consists of relatively clean quartzite, exposures south of the northernmost Kogruk Formation exposures contrasts sandstone, coguinoid siltstone, and shale and is here called the quartzite sharply with that of adjoining rocks. The hills end mountains composed facies overlain by finer grained unfosailiPeroua rocks resembling the of Okpikruak Formation are medium gray in tone, even textured, and are rhythmically bedded facies. In other exposures weat of the Utukok cut by well-incised streams in subdendritic drainage patterns. North River, rhythmic bedding is not conspicuous in the lower part of tbe and east of these belts, the appearance of . wacke. member is similar formation; instead, a relatively greater proportion of fine clastice where the belts of exposure are uide, but in narrow outcrop belts the with Lenticular beda of graywacke is preaent. member is difficult ro distinguish from otlber clestic rocks. Conglomeratic fades.-- Conglomeratic mudstone, siltstone, and

Character and thickness.--In general, three rock facies represent graywacke ranging from less than a foot to at least 30 feet in thiclorena the uacke member in the Kukpowruk-Nuka regb% These include: 1) a local locally represent the basal bed8 of the wacke member. These are best conglomerate facies consisting of grayuacke conglomerate and conglomeratic exposed in the headwaters of the Utukok River, but are also preeent mudstone (06 shale, skltutor~e,and sandstone); 2) a more or less rhyth- in many localities from Spike Creek east to the Nuka River. The

mically bedded facies consisting of st~ale,siltstone, and graywacke; conglomeratic mudstone is dark gray, alley, commonly limy, with poorly and 3) a vackr facies consisting of graywacke, conglomeratic grayuacke, developed bedding. It locally contains minor amounts of grayish- siltstone and shale. The wacke member in the DeLong Mountains has a green sandstone in lenses as much a3 1 foot by 5 feet in dimensions. higher percentage of coarser grained rocks than exposures in the northern Ir contains randomly scattered, subround to angular, granule- to cobble- part of the area where more shale is preeent. The conglomerate facies sized fragments of bluish, grayish, greenish, and black chert, gray is lucally present in the basal or near-basal beds of the formation; limestone, cherty limestone. and mtrfic igneous rocks. Reworked Missis- the rhyclimically bedded facies and the grayuacke facies probably inter- sippian crinoidal debris and horn corals are present in sane exposures. tongue- Neither appears to maintatn a cunscant position in the member The shale locally contains Early Cretaceous Buchfa pelecypods. Small throughout the area. tiowever, along the Nuka River and in the belt that pyrite crystals and aggregates are present in some exposures of the extends from Trail Creek across Seagull Creek, the rhythmically bedded mudstone. Medium-dark-grey to dark-grayish-green conglomeratic silt-

Eacics consistently underlies the coarser wacke facies. In those stone and conglomeratic grayvacke represent lateral coarsening of the

1ocalLties it represents the lower part of the formation. On the other mudstone beds in some exposures. At some localities where conglomeratic hand, west of the Utukok River in the headwaters of Spike Creek, the beds of the Okpikruak Formation. rest on, or are within several thousand

lower part of the Eormatiou is represented by rocks of the wacke lateral feet of, Mississippian limestotie exposures (for example, in the hills north of Kogruk Creek at approximate lat. 6B034'N., long. and boulders aa much as 2 feet in diameter (5 percent); grayiah- 160°50'~.), tlie conglomeratic beds contain angular pebbles and green eandstone cobbles, subround to round, as much aa 6 inches in cobbles of limestone and black chert identical to the Mississippian diameter (less than 5 percent); white quartz cobbles, subround to round, beds. In some places these rocks contain as much as 20 percent as much as 4 inches in diameter (less than 5 percent); and subround crinoidal debris. These conglomerate beds were doubtless locally schistose quartzite and black slaty argillite cobble8 (less than 5 derived and were deposited within shorr distances of the source rocks. percent). No fossils were found in this erposure. but along the vest Some may represent but slightly reworked residual deposits. side of the Utukok River, 11( miles north, a conglomerate exposure Conglomerate units composed of granule- to boulder-size material like that described above, except that the maximum clast sizes are well exposed in the HrooFs Range province at the headwaters of are slightly smaller, contains abundant shell fragment8 and shells of ILigluruk Creek and the Utukk River. A conglomerate unit, estimted Buchia ap. preserved in the matrix. to be as as feet thick along Tupik Creek, believed to muc11 500 is Other similar expoaurea in the headwaters of Iligluruk Creek

represent a local facie6 of the basal part of the Okpikruak Formation. end Kugururok River drainages with boulders as much as I5 feet in The massive conglomerate overlies cherts of late Paleozoic age and diameter are mapped as Okpikruak Formation. The matrix of some of

crops out as huge pinnacles and spires along the creek. The matrix, these conglomerates contains shell fragments of Buchirr sp.; others composed of dark-greenish-gray and dark-gray sandy, silty, and clay are apparently unfosailiferous. The conglnmeratea are not persistent .

material, is 20 to 30 percent of the rock. Granule- to boulder-sized units but appear to grade laterally into either interbedded graywacke in fragments as muc11 as 4 feet diameter, wich average size of about sandstone and shale or unirs composed dominantly oE shale. East and 6 inches, are angular to well rounded, poorly sorted, and lie in northeast of the headwaters of the Ucukok River, many isolated exposures chaotic to roughly bedded positions. Coarse constituents are: black of conglomerate, as much as several hundred but commonly less than and grayish chert and fossiliferous Triassic limestone (estimated at 30 feet thick, are present in the southern foocltills and mour~tains. 45 percent of the contituents) fn subangular to well-rounded granules, No fossils were found in these conglomerates, however. These conglm-

, pebbles, and cobbles; sobround to round pebbles and cobbles of mafic erates of uncertain correlation are mapped in the nacke. eandatone, and felslc igneous rock (20 percent); Mississippian limestone and chert, mudstone, and conglomerate units discussed elsevhere in this report. in part fossiliferous, iti angular to subdnqular cobbles and boulders up to fiytl~micallybedded faclea.-- The rhythmically bedded facie6 of 4 feet in diameter (20 percent); quartzite and sandy limestone (possibly

Utukok Formation) in angular and subdrwjular pebbles, cobbles, the Okpikruak Formatioil is dominantly of silty and clay ahale, with shale in units ae much as 35 feet ttiick. The unit ie 250 to 400 feet intrrbedded graywacke and subgrayvacke sandstor~eand ailtstone, ferru- thick along the Nuka River. ginous limestone, and a feu thin conglomerate lenses. Rocks typical In some expoaures, such as tho6e along Trail and Seagull Creeks, of this unit are exposed in several cutbanks along the Nuka River, unfossiiiferoue sequences about 500 feet thick contafn rocks nearly and 2 to b miles southwest of 1951 camp 9. Here, it consists of 60 to 80 identical to those described above/are mapped as questionable Okpikruak percent medium-dark-gray to grayish-black shale in unit8 commonly less Formation, These contain small amounts of interbedded than 3 feet, but as much as ,15 feet, ttiick. The shale is platy, hackly, greenish-gray shale. In these localities, and in some exposures of I and noduIar, and some beds contain well-rounded siltstone pebbles and fossilfferous Okpikruak Formation like those immediately north of shells of the prlecypod Buchia. Beds of fine-grained sandstone and the Nuka River, the rocks have been metamorphosed to qwrtzites. siltstone average less than 1 foot, but are as much as 3 feet thick argillites with "pencil" fracture, and siliceous limestone. and are mostly even bedded. The salldstone and siltstone are commonly Wacke fades.-- The wacke facies is well exposed in the headvaters dark gray, greenish gray, and olive-gray, contain claystone clasts and of Spike Creek, in the headwaters of the Utukok River, and north OE carbonaceous fragments, and are laminated to finely crossbedded. the Nuka River. It consists of variable amounts of interbedded gray-

Graded bedding is a common feature and the lower surface of many wacke sandatone, ailtstone, sl~ale,conglomeracicgrayuacke sandstone, sandstone beds contains sole markings, which, tiowever, arenot as common and grayuacke conglomerate. The coarser grained rock typea comprise or prominent as those in rocks of the Fortress Plountain Formation. from 50 to 80 percent of the sections examined. They are mostly dark

Sulnr thin sandstone and siltstone beds arc coquinoid types, locally grayish green, greenish gray, and medium dark to dark gray. commonly colnposed of 10 to 30 percent Buct~lasllells. Ferruginous limestone weather dark reddish brown, light brown, and dark yellwish orange, lenses and elongate nodules comprise 5 to 10 percent of the exposuces, and are very fine to coarse grained. The rocks are moderately indurated are silty, dense, medium to dark gray, and comnlonly weather dark to hard, calcareous to quartzitic, micacrous, and massive to platy. reddish brown and light brown. Many of these .nodules are fossilif erous. Although they commonly arc present as lenticular and massively crous-

Hocks of the rhytIimically bedded facies exposed on the south side bedded units 2 to 5 feet of Tliunder Mountain are at least 410 feet thick. They represent the ill thinner, well-s tratif led, laminated or finely crossbedded units. lower part of the formdtlon and contain as much as 70 to 80 percellt Graded bedding is a common feature and claystone clasts and carbonacruus fragments are present in some' heds. Shell fragments and ahells of (AbboAn4o), albite or oligoclase (Abg@nlO), and possible bytovnite

1 several species of the peierypod luchla are scattered throughout the oranorthite CAblOAngo); and carbonaceous matter, ore mlnerala, chlorite.

, coarse clastic rocks of some sections. Lenticular conglomerate and and cellophane(?). Buchia shells (lesa than 10 percent) are deformed

conglomeratic sandstone beds, interbedded with the sandstone, contain and are composed mostly of carbonate which has been partially replaced

granules and pebbles moatly of chert and cherty argillite with a feu by quartz.

igneous and Limestone fragments. Silty shale and, lesa commoniy , All sections of the vacke faciea, with the poasible exception

clay shale ere interbedded with the coarser rocks as units generally of that examined in the headwaters of Spike Creek, are incomplete as

less than 2 feet thick. The shale is dark gray to grayish black. a result of faulting or recent erosion. The Spike Creek section,

fissile, nodular, and blocky and in some sections weathers a gun-metal more than 4600 feet and possibly YO0 feet thick, overlies rocks of

blue color. Locally fossilife!kous, ferruginous limestone lenses and Jurassic or Triassic age. It is gradational upwards into at least 500

nodules are interbedded with the shale and coarser elastics. They are feet of unfossiliferous beds consisting mostly of thinly bedded shale

similar to those in the rhythmically bedded member, but are less and siltstone with interbeds of gray- and greeniah-gray grayvacke

abundant in the wacke member. sandstone as much aa 25 feet thick. Similar unfossiliferous sections

One thin section of coquinoid graywacke (SlASa258), from the upper vhich, in part, resemble the rhythmically bedded and vacke members

pare of the member along the Nuka River, was exmined under the are mapped as questionable Okpikruak Formation. Other unfossiliferous

petrographic microscope. In hand specimen the rock is dark greenish sections resembling the wacke member, all of vhich,gradationally over- member, gray, ueathera light brown, and contains very fine to granule-size lie strata resembling the rhythmically bedded /include a measured

fragments and shells of liucl~ia ap. In thin section, cement (about thickness of 4640 feet along Trail Creek (section 20). a measured

50 percent) consists of cryptocrystalline quartz, calcite, chlorite, and and computed section of 750 feet along Seagull Creek. and an estimated

clay material. Angular to subround, mostly elongate grains are thickness of 500 feet north of the Nuka River.

scattered and poorly sorted together vith granules as much as 1 by 4 mm Stratigraphtc ~elationnhipsand mode of deposition.--There is

in size. Grains consist of: rock fragments (estimated at 40 percent); no doubt that an erosional hiatus of considerable magnitude preceded

quartz and chert (30 percent); carbonate (10 percent); fresh-appearing deposition of Okpikruak sediments and, from fossil evidence, the break

biotite (5 percent); plagtoclaae (5 percent) including andesine probably represents . Jurassic time. The vacke member of the Okpikruak Fomatioii overlies a wrde variety of rock units ranging from Triassic absent and the base of Okpikruak Formation ia even more irregular; in to Misslssippian in age. In general, in the southernmost exposures several places gulleylfke indentations as much as 10 feet deep in the ttic formation overlies older rocks such as the Lisburne Group of Kogruk are filled with Okpikruak rocks, and the maximum thickness of Mississippian age. Farther north, the Okpikruak more commonly overlies the Kogruk, about 250 feet, thins to 10 feet uithtn 5000 feet along rocks of late PaleozoLc, Triassic, and Jurassic ages. Because single strike. In these exposures, and at many other localities where the beds of conglomerate in the Okpikruak contain fossiliferous angular Okpikruak overlie8 the Kogruk Formation, the Miasissippian limestones rock fragments of older formations these older rocks apparently were are almost completely silicified within 1 to 3 feet of the contact. exposed to erosional agents at about the same time. The vacke member represents continued filling of he Colville The unconformity at the base of the Okpikruak represent6 a tectonic geosyncline (Payne, 1951), following an orcgenic interval ingat4 break as vell as an.erosiona1 interval. In most localities vhere rela- Jurassic time, Source areas lay to the south end the orogaoic belt tionships of the Okpikruak vith older rocks can be observed, the Okpik- of the Brooks Range geonticline may have shifted northward from its ruak is conformable or nearly conformable with the older rocks; but position during it. Jurassic time. Depositional conditi~ns'probabl~ in some places, for example,along the upper part of Driftwood Creek varled considerably in different parts of the Kukpowruk-Nuka region but, the formation rest8 on older rocks with angular discordance of as much in general, deposition of the most coarsely clastic'rocks is believed

a$ 35'. In addition, the map pattern in several parts of the area * to have occurred in the southern part. suggests angular relationships with the alder rocks. In the upper Local basal and near-basal conglomerate acies represenc rapid parts of the Nuka River, Elbov Creek, and the Utukok River the formation \ and chaotic dumping from nearby source areas. ' Interbedded graywacke overlies several older rock units along the same belts of ourcrop. and shale beds represent typical flysch sedimentation, but massive In a few localities, tl~elowercontactof the Okpikruak Formation crossbedding in many of the graywackes suggests that they vere deposited is uell exposed. North of Kogruk Creek, at lat. 68'35' N., long. 160' in relatively shallow water wit11 subsequent rapid sinking of the sea bottom. 49' tile Okpikruak disconformably overlies the Kogruk Formation.. The rhythmically bedded facies suggests turbidite deposition; coquinoid bed' and fine crossbedding indicate relatively shallow-water conditions Ilere the erosional surface ia undulating, with local relief of about during which currents were relatively weak and sediment supply was I 5 feet within a few hundred feet along strike. Along this contact, limited. the maximum thickness of the Kogruk Formation is about 170 feet

(section 5), but 1% miles east of thia section, the Kogruk is locally

I Character and rhicknea8.-- The quartzite member of the Okpikruak Quart zl te Hembe r is typified by highly siliceous claatic rocka. Host prominent is DLstrLbution and outcrop. -- The quartzite member of the Okpikruak very fine grained to ailty quartzite, uith interbedded siliceous silt- Formation is exposed in the Southern Foothills province along two out- atone, hard shale and, locally, coquinoid or limy siltstone. Quartzite crop belts which strike southwest from Adventure Creek to beyond the comprises about 70 percent.of the exposures examined. It ia light gray, Kukpouruk River. The northern belt, typified by prominent north-facing, light olive-gray to dark gray, and grayish-green, and occura in uell- discontinuous ridgea, extends southwest from Adventure Creek at Lat. atrat,ified maasive to platy beda mostly less than 2 feet but as much 68O44' H. across Eligluruk Creek. The southern belt, en echelon to the as 20 feet thick. It commonly weathers light brown, moderate yellowish northern belt, ia exposed in discontinuous high hills and ridges from broun, and yellowish orange, or appear8 bleached with light-gray tones west of Spike Creek at lat. 68°37' N. to beyond the Kukpowruk River. on weathering surfaces. Some beds exhibit a "gun-metal'lblue surface Several small mountainous masses cmposed chiefly of the quartzite an eighth of an stain which extends as much aa / inch into the rock. Most of the member and surrounded by lwer hills, lie along this belt. Among these quartzites appear uniformly bedded, but some beds are "gnarled" is Tingmerkpuk Mountain. and irregular and contain worm trails. mud lumps, hexagonal impressions. Although rocks of the quartzite member are resiatant, they ere and carbonaceous fragments on bedding surfaces. Thin-bedded,dark-gray, poorly exposed in cutbanks along Iligluruk Creek. Most of the ridges grayi~h-green and grayish-red shale and dark-gray to grayish-green and hills are largely rubble covered, except at their crests and at a siliceous siltstone, together uith a feu thin beds of friable gray few localities along their north sides. The topographic expression sandstone, are interbedded uith the quartzite. The sandstones are and general appearance of these ridges and hills contrasts sharply with locally ripple marked, laminated to finely crossbedded, and some beds lover hills composed of claatic rocks of the Fortress Mountain Formation. 4 contain small silty concretions. At least three beds of mediumto From a distance, hills of the quartzite meniber are black, orange, and Foot dark-gray limy coquinoid siltsfone, from about $/to 2 feet thick. are yellowish in color, and to some extent resemble the exposures of interbedded with the shale and siltatone. They contain finely ribbed Paleozoic cl~ertyrocks exposed farther south. Rock rubble fs commonly Buchia pelecy pods. covered by black lichens. On aerial photographs, exposures of the These descriptions apply to the rocks exposed in the hills east quartzite member are much darker in tone than those in the surrounding of Jligluruk Creek and west of Spike Creek. The exposures near the hills, and some have a rougl>ly banded appearance. Kukpowruk River consist almost entirely of massive blocks of quartzite two localities east of Illgluruk Creek. Contacts betveen the Shublik

rubble. Little quartzite bedrock is exposed and no Iosaila were found and Okpikruak are rubble covered. The fosniliferous Triassic beds at

at these localities. the eastern localiry are interprered as a complex anticlinal structure

One thin section of quartzite from the quartzite member (eample fltlnked by rubble of gray and reddish-gray shale and limy coquinoid I 50AEig63) was examined urider the petrograpl~icmicroscope. In hand siltstone af the quartzite member. If this interpretation is correct. specimen tile rock is Hght olive-gray and dark gray, very fine grained, the Okpikruak liee directly on the Shublik at this locality. East of an eighth of an with gun-metal-blue weathering uhtch extends as much as / inch below Iligluruk Creek, the quartzite member appears to be overlain by rocks !i the surface. In thin section, the cement (about 10 percent of the of the sandstone member of the Fortress Mountain Formation. lko inter-

section) ia composed of microcrystalline quartz, some chlorite, brownish pretations of tha relationships between the quartzite member and the

ferruginous(?) films along grain boundaries, and bluish-black metallic Fortress Mountain Formation are considered equally plausible. The

material. Closely packed, roughly equidimensional subangular to subround simplest interpretation is that the rocks of the quortztte member are

grains, from about 0.02 to 0.2 mm in diameter with modal size of 0.1 m, in the axial zone of a tightly folded anticline in which the south limb

consist of quartz and lesser amounts of chert (estimated 80 percent locally has overridden the north limb. Some structural observations .

of gralns), black opaque minerals (15 percent), chlorite, muscovite, along this outcrop belt confirm the presence of anticlinal axes exposing

zircon, and limonitic grains. Tile rock is classified as a protoquartzite Triassic rocks. According to thi~interpretation, the quartzite

(Pettijohn). This may represent a reworked grayvacke and, as such, member normally underlies the Fortress Mountain Formation. Some

probably was a winnowed shallou-water deposit. evidence, however, points to a second interpretation that the quartzite

At least 225 feet of the quartzite member is exposed in ridges menbcr and associated Triassic rocks represent remnants of overthrust east of Iligluruk Creek but t11is measurement probably does not represent I fault blocks which now are resting on rocks of the Fortress Plountain I I the total thickness. West of Spike Creek no field meaeureruents were Formation. The evidence includes tile discontinuous and scattered obtained but tlirre the member appears to be thicker and rough measure- character of the quartzite member belts of exposure; small isolated

ments from aerial phorograpfis give a minimum thickness of 500 feet. exposures of Triassic rocks along these belts, all surrounded by rubble

Stratigraphic relationships and mode of deposition.-- Rocks of the of Fortress Mountain Formation; the fact that most of the exposures of

quartzite member are associated with the Trinseic Shublik Formation at quartzite member represent the tiigltest topographic features along this

147 outcrop belt; a topographic. benchlikc break, along both the north and do not reflect much change in environmental conditiona except, perhape. south sides of some ridges composed of the quartzite member, which, if that current action was weaker. The interbedded reddish and greenish projected under the ridges does not exceed an apparent slope of 10' shales may represent intervals of slov deposition and contemporaneous soutI1; and the local divergence of strike between the ridges composed oxidation. of quartzite member and traces of the Fortress Mountain Formarion, The linear but dfscontfnuous outcrop pattern, clean and uell-sorted which in some localities appear to strike under both the north and south character of the sands, geograptilc position of these rocks, and absence sides of the quartzite ridges. Available information on the regional of Jurassic rocb suggest that they my have been local offshore bar seructural features along these belts does not conclusively favor either deposits along a topographic high within the basin of Okpikruak deposi- interpretation. Along Adventure Creek the belt appears to lie in a tion. general anticlinal zone, bud) along Iligluruk Creek the zone may be Posaila and age.--R. W. Imlay (1961), after examiafng fosail synclinal. In many isolated exposures, older rocks associated with collections from many areas in northern Alaska including those from the outcrop belts of the Fortress Mountain Formation may be in high-angle- Kukpouruk-Nuka region, concluded that the coarsely ribbed Buchia pelecypods fault relationship with the younger rocks. On the other hand, chey may probably represent earlieat Cretaceous (Iierriasian) age, and that the represent large-scale overthrusting. Solution of this problem is of finely ribbed Buchia are probably Valanginian. Field relationships in great import in deciphering the regional structural ana stratigraphic most localities tend to bear out Imlay's conclusion in that beds relationships but it has not been satisfactorily resolved. containing finely ribbed Buchia generally occupy a higher stratigraphic

Beds of the quartzite member, composed of sand and silt-sized position than beds with coarsely ribbed Buchfa. In some sections, however, material. are relatively clean and well sorted. This is in sharp finely ribbed Buchia occurs near the base of the uacke membir of Okpikruak. contrast to the grayuackes exposed farther eouth. However. BOW of The absence of foeblils along some belts of Okpikruak(?), such as the features commonly found in the rhythmically bedded facies, including the one south of Trail Creek, may reflect factors such as increased fine crossbedding and lamination, are found also In the quartzite depth, severe current agitation, or unfavorable bottom conditions. The member. The sandy sediments were probably deposited in relatively probable telescoping of lithologically similar fossiliferous and un- shallow water with current nctlon sufficient to allw winnowing of the fossiliferous rock facies by thrust faulting probably explaimthia sand. The interbedded shale and Buchia-bearing siltstone probably ,*..y absence of fossils. Three collecrLons of pelecypods were made from coquinoid siltstone and other Mesozoic clastic units. The Fortress Mountain Formation is beds in the quartzite member of the Okpikruak Formation (fig. 12 and here divided into three informal meubers, the wacke and conglomerate, fable 4). All three contain finely ribbed Buchia of Early Cretaceous sandstone, and shale m+nbers, which outcrop respectively in separate

age, nccording to R. W. Imlay. Therefore, south to north linear belts. The etratigraphic relntionships of the they would appear to be equivalent to some part of the wacke member of members is uncertain, but the conglomerate and sandstone member8 are the Okpikruak. Two of these collections (USGS Hes. Loc. 22481 and believed to be in part equivalent and appear to be overlain by the

22497) are from beds estimated to lie within 200 stratigraphic feet shale member. The sandstone member is also overlain by the Torok Q I of Triassic rocks. Formation farther north. All of these rocks were previousLy mapped in

the Lower Cretaceous series by Smith and Hertie 11930).

Wacke and conglomerate member

Distribution and outcrop.-- The wacke and conglomerate member of A total-of 35 shale samples were collected for microfossil content the Fortress Mountain Formation is expressed in the southern Eoothllls from the Okpikruak Formation in the Kukpouruk-Nuka region. Only nine as a linear belt, 1/2 mile to 5 miles wide, which extends aouthuest- of these contain microfossils vl~ich,according to t1, R. Bergquist, are ward from the Nuka River at about 1st. 68'45' El., to vest of Spike Creek foraniinifera representing nondfagnoscic forms. The foraminifera1 at lac. 68'38' N- The conglomerate member was not seen in the vicinity assembfages have a Cretaceous aspect, but are not diagnostic for precise of the Kukpowruk River. The belt narrows both eastward and westward age determination. and attains Lta maxirnum wldrh becveen Storm Creek and Driftwood Creek. Fortress Mountain Barma tl or1 The narrowing of the belt near the Nuka River is interpreted to be due The name Fortress Mountain Formation (Petton, 2 Gryc and others, In part to scructure, and it1 part to stratigrap11ic thinning of the member

1956, p. 219-221) is applied to a thick sequence of nearly unfossiliferous The narrowing of the belt at Spike Creek is believed to be due mostly clastic rocks consisting of several Cacies which outcrop in the central to stratigraphic thinning. northern parts of the Kukpouruk-Nuka region. These rocks in part under- Crnwacke conglomerate and sandstone, the coarse clastic rocks of may lie and be in part equivalent to the lower part of the Torok the member, are topographically expressed as narrov hogbacks and linear

Formation; locally they overlie the Shublik and Okpikruak Formations ridges which have relief of as muell as J000 feet near Driftwood Creek. Finer grained mudstone nr~dsiltstone, intercalated with the coarser thick; they are interbedded and intergraded in reaistant units aa much elastics, are less resistant and are expressed in linear belts of low as 75 feet thick. Strata are commonly micaceoua and calcareous and, relief between the ridges and hills. The northern boundary of the uacke locally, contain clay pellets, carbonized wood and reed fragments, and conglomerate metuber is expressed as an abrupt topographic break, small pyrite nodules and, more rarely, sandy spherical concrerions a in sharp contrast to the nearly exposureless lowlands to the north. few inches in diameter, Subround to subangular granule- to large-

The southern boundary merges into lower hills underlain by Mesozoic pebble-size rock fragments are locally abundant and form conglomeratic clastic rocks and chert. Cutbank outcrops of the member are best beds as much as 10 feet thick. The fragments consist mostly of chert exposed along the Nuka River, Thunder Creek, Driftwood Creek, and the and argillite, shale, and lesser amounts of uhite quartz, gray lime-

Utukok River. stone, and mafic igneous rock. In a few localities, subround cobbles

4 Character and thickness'!-- The vacke and conglomerate member con- of combustibie bituminous shale are found in conglomerate. In general. sists of about 40 to 60 percent graywacke and subgraywacke sandstone, the coarsest phases and highest percentage of coarse clastic rocks in conglomeratic sandstone, and corrglomerate, with intercalated units of the conglor~eratemember occur between the Nuka River and Driftwood interbedded siltstone and sudstone almost identical to those in the wacke Creek, the widest part of the belt of exposures. facies of the Okpikruak Formation. The recognizable differences are Very fine grained sandstone, niltstone and silty mudstone, with that the Fortress Mountain Formation does not contain the dark-grayish- lesser amounts of claystone. art? both mutually interbedded and inter- green graywackrs common in many exposures of the Okpikruak. has less bedded uith the coarser clastics. These finer rocks are nearly vlvid weathering colors, atld contains very feu fossils. identical to those of the sandstone member as mapped farther north.

Massive, interlensing beds, lateral and upward vertical gradation Medium-dark-gray silty limescone lenses as much as 4 inches thick to finer clastics, well-developed graded bedding, and crossbedding are interbedded with the fine clastics of tha conglomerate member; in the finer grained sandstones are typical of the coarse clastics of in some exposures they represent the uppermost unit of graded bedding

the wacke and conglolneratc nwmber. These rocks are dominantly greenish sequences. The lin~estonelenses weather llgllt brown or "cream" pray, but also range from light olive-gray to medium dark gray, wcatller colored, are predominantly lluninated or finely crossbedded, and in

light brown to ligllt olive-gray, are moderately to well indurated, and some places contain carbonized wood fragments and scattered well-

occur in le~~tlcularbeds conunonly less than 5, but as much as 30, feet rounded chert and quartz gral~uleu. Graded bedding is a conunon feature in rocks of both the wacke -Unit Thickness in feet and conglomerate and sandstone members of the Fortreas Mountain Top of section. Probable fault contact with argillaceous

Formation. The louer contact of a typical graded sequence ia abrupt unit of section 19- and irregular; conglomerate or conglomeratic sandstone overlies clay- 1. Sandstone, siltstone, and mudatone incerbedded. stone, and the bottom surface of the conglomeratic bed contains reverse Sandstone greenish gray, ffne to medium grained, impressions of the initial mud surface, abundant sole markings such as in lenticular beds...... 13 flute, groove, and striation casta, casts of lobate or elongate scour- 2. Silty mudseone, sandstone, and ailtstone interbedded. $ filli~lgs,mud flows, worm trails and boringa. and elongate or circular Nudstone 70 percent of section. Some medium-dark-gray fucoldal markings. Ultl~inthe conglomeratic beds numerous clay pellets silty limestone nodules in rubble ...... 15 and carbonaceous fragments are randomly distributed. The conglomerate 3. Sandstone and siltstone interbedded, grades upwards grades upward into successively finer sandstones of a more uniform into medium-dark-gray silty mudstone. Sandstone texture which exhibit "curly" bedding in cross section. Succeeding very fine grained, greenish gray...... 4 sandstones and siltstones contain small-scale crossbedding and are 4. Interbedded grayuacke-type granule conglomerate. overlain by lmninated siltstone or silty limestone, eilty mudseone and, eandstone, siltstone, and mudstone...... 5 finally, claystone. The claystone is again abruptly overlain by a 5. Mudstone, siltstone, and sandstone tnterbedded; conglomeratic bed, Sequences such as this are roughly rhythmic and similar rock types to those in unit 1 . .Shale range from a few inches to many feet in thickness. Although this ideal 50 percent of unit...... 8 sequence is not present in all exposureu, some graded bedding feafures 6. Grayuacke-type conglomerate, sandstone and ailtstone

are present in most outcrops and are reliable criteria for the deter- interbedded; greenish gray, in beds as much as 3 feet mltiiltion of superposition in a succession of strata. tliick, contains subround granules and pebbles of

Section 20: Wacke conglomerate member.of Fortress Mountain chert. Massive unit...... 14

Formation. Cutbank, east side of Driftwood Creek; measured by 7. Silty mudstone, clay mudstone, conglomerate sandstone,

E. C. Sable and J. T. Dutro, Jr., June 3, 1951. and siltstone interbedded. Pebble conglomerate at

base...... 14 8. Similar to untt 6, Iaut wit11 6-inch interbeds of 16. Hostly mudstone and siltstone interbedded, medium mudstone. Massive unit 15 ...... dark gray, platy, laminated, thin bedded Pew 9. Similar to miit 6, but with lenses of mudstone as much as beds of greenish sandstone as much as 1 foot thick. .... 15 6 inches thick and 15 feet long interbedded with coarser 17. Interbedded granule conglomerate, sandstone, siltstone. clastics 10 ...... and mudstone. Conglomerate and sandstone greenish gray 10. Mudstone, grading downwards into sandstone and granule to medium dark gray, 70 percent of unit, in massive conglomerate. Abrupt contact with unit 11. 3 ...... beda as much as 1% feet thick. Abrupt lower contact 11. Similar to unit 10. Abrupt contact with unit 12. 2.5 ..... of conglomerate with unit 18, underside of conglomerate 12. Siltstone, medium dark gray, rnicaceous, in 1/2-inch to bed contains longitudinal rill-like markings striking 3-inch beda. contains karbonaceous plant fragments north and northeast ...... 19 and angular claystone pebbles 7 ...... 18. Claystone and silty mudvtone interbedded. Fev 13. Mudstone in upper 2 feet, grading downward into thin sandstone beds in upper 2 feet of unit ...... 10 interbedded granule conglomerate, sandstone. and 19. Sandstone and mudstone interbedded in rough rhythmic siltstone. Irregular bedding surfaces at base of fashion. Sands cone, medium gray, very fine grained. conglomerate beds !O ...... beds less than 6 Inches chick, contain claystone pebbles; 14. Granule conglomerate, sandstone, silts tone and mudstone shale, medium dark gray, silty...... 5 in rltythmically interbedded graded sequences 1 to 3 20. Mostly sandstone and granule conglomerate, with feet thick. 13 ...... thin interbeds of siltstone and mudstone. Abrupt 15. Hus tly sandstone and granule conglomerate, contacc with unit 21...... 4 greenish gray, in massive beds as much as 3 feet 21. Interbedded sandstone, granule conglomerate, thick, contains well-rounded chert granules, siltstone, and mudstone. Sandstone and carlonaceous fragments, and angular to subround conglomerate of graywacke type, greenish gray mudstone pebbles, grades into mudstonr in upper and olive-gray, weather light olive-gray, sliglttly 2 feet of unit. Pew 3- to 6-lrtclt mudstone lenses calcareous, generally fine grained. contain iuterbedded ...... 10 granules of greenisl), black, and brownist~ Sample 51ASa3, in hand specinhen. ia a granule conglomerate, greenish chert, and angular claystone pebbles as much gray to light olive-gray, with silty to coarse-grained matrix, ia as 2 inches in diameter. Mudstoile, medium to calcareous, and exhibits graded bedding. In thin aection, carbonate dark gray. platy to blucky, about 40 percent cement comprises about 20 percent of the rock. Subround to angular, of unit. Siltstone finely crossbedded. 10 ...... poorly sorted grains as much as 1.5 mm in diameter consist of 22. Similar to unit 21, but mudstone about 10 percent of rock fragments, mostly chert and argillaceous rock (estimated to be unit. Chert pebble conglomerate rubble at base Q 85 percent of the grains). clear quartz (about 5 percent), black of unit 28 ...... opaque minerals and unidentified minerals with whitish coating End of exposure. (5 percent) and lesser amounts of carbonate, plagioclase, and microcline. Total thickness : 234+ ...... The total thicknesa of the uacke and conglomerate member of the Descriptions of two graywacke sandstones, considered to be Fortress Mountain Formation in the Kukpowruk-Huka region is not known. representavive of the conglomerate member, are based on examination The major structural features In the belt of exposures consist of a under the petrographic microscope. In hand specimen, sample 50AMgl75, narrow northward overturned synclinorium on the south edge of the belt the rock is dark greenish gray,. wrattters light brown and olive-gray, and a larger normal anticlinorium to the north. Thesg structures and is very fine to coarse grained. Visible fragments are angular contain numerous tight folds with thrust faults of unknovn displacement and-roughly oriented alonl: bedding. In thin section, the matrix, and are cut by many of transverse faults. No distinctive marker about 20 percent of tlte rock, consists of a very fine cloy paste horizons are known in the meolber, and even thick resistant units and cttlorite. Angular to subroulld grains from about .05 to 1 mm cannot be traced for more than a few miles before they are cut by faults Fn diameter are poorly sorted and consist rock fragments, mostly of or lose their surface expression. Partial sections measured on limbs ctiart and argillaceous rock (estimated at 70 percent), clear quartz of minor structures include 1,300 feet along the Utukok River ar 1950 (10 percent), and about (4 percent each) of plngioclase (albite or camp 9, 1,700 feet on Thunder Creek, and sections several hundred ollgoclasr and andesenc), carbonate, biotite, muscovite, and opaque feet thick along Driftuood Creek and the Nuka Kfvrr. Some thickness minerals. A small amount of ctrlorite occurs as grains. estillliltrs made frolo structural reconstructions across the belt of

exposures, include possible thicknesses of 200 feet along Spike Creek, feet along Utukok River, feet along Thunder Creek, and WOO the 4,000 2. Conglomerate, dark greenish gray, weathers to00 feet along the Nuka River. From these estimates it seems that light brown. grayvackp type, contains subround the wacke and conglomcrare member thins both eastward and westward pebbles and small cobblea of matic and felsic frou~the Thunder Creek vicinity. igneous rocks, shale, chert, and sandstone...... 25 South of the main outcrop belt of Fortress Mountain wacke and 3. Sandstone, yellowish gray, pale olive, and conglomerate member, a relatively narrov belt of small areal extent light olive-gray. weathers graylsh orange, consisting of irregular hills and ridges is mapped as the conglomerate light brovn, and chalky greenish gray, mostly member from about 4 miles south to 7 miles southwest of Lake Noluk. fine to medium grained, moderately indurated,

The rocks in this belt coqsist of light-colored sandatone interbedded fairly clean; in part exhibits laminated gradations 4 with grayuacke conglomerate which locally overlies rocks of the Okpikruak in grain size, flaggy to blocky ...... 150 The sandstone and conglomerate beds are gently dipping while Formation. 4. Conglomerate similar to unit 2. Massive ...... 75 the Okpikruak beds directly underlying these exposures are steeply 5. Sandstone, similar to unit 3...... 200 dipping and isoclinally folded. A generalized section of these rocks,

N.. exposed at lac. 6B043' long. 160~12'W, Is described below: Total thickness...... 525+- Sectior~21. Fortress Mountain Formation?. Examined by One thin section of a sandstone from unit 5 of the above sectiop J. T. Ducro, Jr., duly 19, 1951. (sample 51ADu128) was examined under the petrographic microscope. Unit Thickness in feet - (estimated) The rock in hand specimen is light olive-gray to yellowish gray, weathers pale to dark yellwish brown, medium to coarse grained, Top of section, eroded. with numerous pore spaces. The cement, comprising about 50 percent 1. Sandstone, yellmish gray and light of the rock, is mainly crypcocrystalllne quartz with finely divided olive-gray , veachers moderate brown to clay material, probably chlorite. fore space is estimated to be about light brown. Fine to medium grained, 15 percent, but pores are not interconnected. Subangular to subround, thin bedded, flaggy. . , . . , ...... 75 much fractured gralns as much as .6 mm in diameter uirh modal

size of .3 mm ore largely quartz and chert (estimated 50 percent of grains), mostly siliceous and argillaceous rock fragments exposures of Triassic and associated rocks in belts of the Fortresa

(20 percent), plagioclase (15 percent) consisting of andesene Mountain Formation, it Is uncertian whether these rocks everyvhere

(Ab60An40) and albite or oligoclase (Abg@lo), and minor amounts underlie the Fortress Mountain, or vhether they represent remnanta of mica, probably muscovite. Plagioclase is Fresh to sericitized. of thrust blockg resting on the Fortress Mountain rocks. In the belt The rock a quartzitic subgraywocke. is of wacke and conglomerate member exposures the Triassic rocks generally . Stratigraphic relationstlips. -- Although the wacke and conglomerate appear to lie along the crest of complex anticlinal structures and member locally overlies !lie micaceous sandstone unit with apparent con- they are thus interpreted to normally underlie the rocks of the con- 4 may formity, as along Driftwood Creek, the base of the member represent glomerate member. an unconfomity of considerable magnitude. Between the Utukok and There are one or more stratigraphic breaks within the wacke and

Nuka Rivers, the member appears to cross structural trends in the conglomerate member, but their significance and positions in the belt of exposures of the mtcaceous sandstone unit which narrows east- sequence are not known. Evidence for these breaks consists of dis- ward. Along the Utukok River, the contact with the micaceous sand- cordant bedding relatiorishipa observed in several isolated outcrops stone unit is interpreted to represent a reverse Fault. Elsewhere, where rock unite are truncated by overlying beds vich angular dip along ttie southern edge of the conglomerate member, contacts are not discordance of as much aa 15'. At one localLty, on the southwest well exposed, and outcrops within a few hundred feet of the contact limb of the small syncline exposed 3% miles southeast of Lake Noluk, arc hLghly folded. several hundred feet of section in the lower beds of the syncline

Farther north, uLthin and generally parallel to the belt of are truncated by overlying beds with angular etrike discordance of vackr and conglomerate member exposures, discontinuous exposures of as much as 45O. These angular relationships probably do nor represent Triassic rocks extend Erom south of Lake Noluk across the Utukok large time intervals of uplift accompanied by subaerial erosion, but

River. In some outcrops, the Triassic rocks appear to normally may be the result of slumping or shifting of local centers of depo- underlie the Fortress Mountain rocks in aqticlinal relationstlip. sition and subsequent scouring. In otliar exposures, however, the Triassic rocks are folded in Precise relationstlips between the wacke and conglomerate nember aynclinrs and, where exposed, concilcts between the member and the and the shale member of thu Fortress Mountain Formation are not known. Triassic rocks are interpreted to lie along 'faults. As with other Beds near their contact belt of exposures are complexly folded, but the dominant dip is north aid the wacke and conglomerate member appears basins of depoaftfon with deposition from point sources along the to underlie the shale member. The boundary betueen the two members i.9 north front of the Brooks Range geanticline. mapped mostly on the basia of hopographic expression between ridges Fossils and age.-- The rocks of the vacke and conglomerate member

composed of coarse clastic rocks to the south and the lowlands under- are very sparaely foasiliferous. One ammonite fragment found in gray- lain by finer clastica to the north. The northern front of the wacke wacke sandstone along the upper part of Adventure Creek (USGS Mes. loc. and conglomerate member is a relatively straight line and locally no. 22496) vas identified by R. W. Imlay as Lytoceras? sp. One shell appears to truncate small-scale structures,in rocks of this member; fragment of Buchia sp. was found in a limy nodule associated vith it may, in part, be the expression of one or more faults. rubble of sandstone and shale east of Driftwood Creek (USGS Nes. loc.

Rocks of the wacke and conglomerate member represent relatively no. 224941, but fossiliferous rocka of the Okpikruak Formation are Q rapid deposition of marine gl'aywackes along the south edge of the exposed near this locality and the shell fragment may be from Okpikruak

Colville geosyncline, in a belt farther north than similar sediments beds. Vom trails and fucoidal markings are common in some fine

in the Okpikruak Formation and other pre-Fortress Mountain clastic clastic beds and indeterminate carbonaceous vood and reed fragments units. It is inferred that the rnobile belt represented by the Brooks are common in the coaraer clastics.

Range geanticline had moved farther north from its position ic Although its age is not known from direct fossil evidence, the

earliest Cretaceous time . Clastic rocks of earlier Cretaceous uacke and conglomerate member is believed to be at least in part

age, as well as Triassic and Paleozoic rocks. contributed material to equivalent to the sandstone member of the Fortress Mountain Formation

the sediments of the conglomerate member. The lateral thinning of the exposed farther north which contains some beds of Albian age. The

member both to the east and west, and the absence of significant amounts wacke and conglomerate member may also be in part equivalent to the

of these rocks along the Kukpowruk River, suggcsts that deposition Okpikruak Formation and may, therefore, represent the entire fnterval

was restricted to a relatively local depocenter in which the tllfckest between and including Valanginian to Albian time. No fossils repre-

accumulation of the sediments and maximum subsidence of the basin floor senting Hauterivian, Barremian, or Aptian times have been found in northern Alaska, but fossiliferous Albian rocks have been recognized occurred between Storm and Driftwood Creeks. East of the Kukpowruk- in Inany areas. On these bases, the uacke and conglomerate member of

Nuka region, other belts of similar Fortress Mountain Formation con- the Fortress Mountain Forutation in the Kukpowruk-Nuka region is tenta-

glomerate rocks in the svuthern foothills may also represent local tively assigned to the late Lower Cretaceous {Albian), and is probably

not younger than middle Albian, Outcrops of the member are mainly limited to acactered stream cuts, Sandstone member although bedding traces and rock ledges are locally common on ridge Distribution and outcrop.-- The sandstone member of the Fortress tops. The ueathering colors of rubble and traces, generally dull gray Mountain Formation is exposed in the southern foothills no linear and olive hues, are not particularly distinctive. Cutbank exposures belts which extend beyond tkie eastern and western limits of the area. are more common along Iligluruk Creek drainages and the Colville and Several poorly exposed belts strike roughly west between the Nuka and Nuka Rivers. Colville Rivers, and a nearly continuous belt 2 to 8 miles wide extends Character and thicknese.-- Interbedded mudstone, clayscone, silt- from DriEtwood Creek at lat!, 68'47' N. southwestward to the Koklik stone. sandstone. and conglomerate In approximate decreasing order of River. West of the Koklik River, the sandstone member ia exposed in abundance consticute the sandstone member of the Fortress Hountain a wide belt which strikes southwest, crosses the Kukpowruk River, and Formation in the Kukpowruk-Nuka region. The relative amounts of these extends beyond the limits of the mapped area. North of tlleee relatively rock types are variable; shale and tt~inlybedded siftseone are estimated persistent belts, three isolated areaa of the sandstone member expo- to comprise about 40 to 60 percent of the rocks, and are most abundant sures lie in the northern part of the Southern Foothills province. in the northernmost exposures. The sandstone member differs from the Rocks of the sandstone member form rounded, mostly mud-,rubble-, shale member of the Fortress Mountain Formation and the* Torok Forma- and vegetation-covered tlills and long, whaleback ridges which are tion, both of which adjoin belts of the sandstone member, in its greater about 700 to l,OOO feet Iilgh in the northern areas of exposures and abundance of sandstone and the presence of thin conglomeratic beds, range to about 4500 feet of relief in the more southerly belts. As and differs from the wacke and conglomerate member of the Fortress seer) on aerial pllotographs, light-appcariltg ridge tops are commonly Mountain in that it contains only minor amounts of conglomerate, the bare, with subdued rock traces, wud heavings, and fine linear features sandstones are generally cleaner and better sorted. and are gray and caused by flowage and creep of rock detrirus. The appearance of these olive-gray rather than the greenish-gray colors common in the conglom- hIlls and ridges dlffers from tllose composed of Nanuslluk Croup rocks; erate member. tl~rrris an absence oE cucuta- and mesa-like topography and a lack of The mare resistant elastic rocks of tlie sandstone member, sand- sharply defined and persistent rock traces. As seen on aerial photo- stone, siltstone, and conglomerate, are scattered as individual graphe, the rock traces exhibit rather chaotic,inteneely folded and units throughout the ntrmber. Where they are tlie dominant rock types, faulted pat terns wliic11 resemble s tructur&s due to plastic flow. they Eorm resistant units a9 much as several hundred feet thick. ence of fine intergranular material. Irregular ripple marks were

There are two such units in the DrlEtwood anticlines, one in the observed on some beds. Most of the sandstone beds are generally well uppernost part and the other roughly 3,500 feet belw the top. atratified although some lenticular beds were observed; the beds are

The sandstone beds increase in abundance southward, and many resistant commonly less than 3 feet but range to more than 10 feet in thicknese, unlts are presenr in the southern belts of exposure. The units are lenticular, however, and can not be traced for more than a and resistant unite are as much as 110 feet thick. The sandstone feu miles. weathers to platy, blocky, and masaive fragments.

The sandstone Is mostly 'medium gray and light olive-gray, but Lenses of grayvacke conglomerate and conglomeratic sandstone, ranges from light to dark gray, olive-gray, and more rarely greenish commonly only a few inches but as much as 3 feet thick, occur at the gray; it weathers to dull shades of olive-gray, light brown, hale to base of or within some sandstone beds. The conglomeratic rocks resemble moderate yellowish brown, a& commonly contains chalk-white travertine- those in the uacke and conglomerate member. The matrix of clay to like carbonate deposits on weathered surfaces. Most of the sandstone coarse sand-size particles 1s greenish gray to olive-gray, and encloses is very fine to medium grained, moderately well aorted, uniform to scattered subround granules and small pebbles of chert, carbonaceous gradationally laminated or with small-scale crossbedding and "curly" fragments, white quartz, greenish sandstone, igneous rocks, and gray bedding restricted to layers less than asfeu inches chick. Coarse- limestone. grained sandstone commonly exhibits aalr-and-pepper texture. The Rocks of the sandatone member seem to represent a similar but, sandstone is commonly calcareous, argillaceous, and micaceous, and in part, finer grained and cleaner Iacies of the grayvackea exposed many beds contain rounded pellets and angular fragments of mudstone, farther south. The rocka of the sandstone member are also more a feu chert granules and pebbles. sandy iron-stainednodules, and calcareous. pyrite nodules and concretions. Abundant small carbonaceous fragments Tnterbedded silty mudstone and claystone are most commonly medium are locally concentrated along bedding surfaces, and the fragments are dart to dark gray and rarely yellowish gray, nodular blocky and fissile, also scattered througllout many of the sandstone beds. Craded bedding soft to hard, and slightly to noncalcareous. .They are well stratl-

it; commun, and the bottom surfaces OF the beds contain abundant sole fied, occur in beds less than 2 iucher in thickness, and are commonly markings. Must of the s~ndstoneis well indurated, but some beds are interbedded with thin siltstone beds in units as much as several

poorly indurated to friable, altIioug11 of low porosity due to the pres- hundred feet thick. Small mud cracks occur at a few localities, and small pyritic nodules art present in some beds. The mudstone and dominantly medium gray, weather moderate yellowieh brown, are commonly claystone weather to olive-gray and yellwish-gray mud associated vith micaceous, and commonly contain fucoidal impressions and sole markings rubble of coarser grained rocks along ridges and hilltops. Some beds on bottoms of beds. cont ail] small carbonaceous fragments and, more rarely, larger fragments 1. Sandstone, medium gray, weathers moderate of coaly wood. In a small cutbank on Adventure Creek, about 94 miles yellowitih brown, fine to coarse grained, poorly lnduratrd to friable, 112- to 2-inch platy above the mouth, a 1/4-inch bed of clean vitreous coal, asso- fracture, contains ecatteced carbonaceous wood ciated with highlyiron-stained shale, occurs in a 50-foot section of fragments and chert-granule conglomerate as r, claystone interbedded with thin siltstone beds; there is also one thin interbeds...... 6

4-incl~ layer of conglo~neraticmudstone containing round to aubround 2. Claystone, dark gray, poorly exposed...... 10 pebbles and irregularly shaped cobbles of dark-gray silty limestone. 3. Sandstone, medium gray, weathers moderate

The siltstone, interbedded and incergraded vith finer and coaraer yellovish brown, very fine grained. massive clastic rocks, is wostly medlum gray to dark gray, and light olive- toplaty,wellindurateb...... 6 gray, commonly calcareous, and in pert iron etained. Beds range to 4. Mostly rubble covered. Pew thin beds of clayetone about 2 feet in thickness and are commonly laminated or finely and medium-dark-gray siltstone...... 35 crossbedded. Thin beds and lenses of silty limestone are similar 5. Sandstone, eimilar to unit 3, but fine grained. in appearance to the siltstone and constitute a minor part of the sequence. 6. Rubble of cloystone ...... 7

Section 22. Sandstone member of the Fortress Mountain Formation, 7. Sandstone, similar to u~iit3 but highly micaceous ..... 4

west side of the Spike Creek, measured by E. G. Sable, duly 12, 8. Rubble of claystone ...... l8+

9, Siltstone, medium dark gray, platy...... 4

Unit Thickness in feet 10. Rubble of claystone ...... 4

Top of section, covered. Note: Although not described in all individual 11. Siltstone as in unit 9, finely crossbedded...... 3 arc unl ts, mudsconc and claystone/commonly medium dark to dark gray, 12. Mostly covered, some rubble of claystone...... 35

I isslle to blocky, and micaceuus. S;andstonc and siltstor~earc 13. Sandstone, niediilm gray, weathers moderate yollowisll brown, thin bedded wit11 hrds 1/8 to 26. Claystone, poorly exposed ...... I lnch thick, poorly indurated...... 27. Siltstone, finely crovsbedded ...... Covered. Fault?...... 28. Claystone, silty shale, and siltstone interbedded. Mostly covered, some rubble of claystone...... Claystone 80 percent of unit......

Sandstone, similar to unit 1, micaceous, platy 29. Sandstone, very fine %rained, poorly indurated. in lower part of unit, massive .in upper part...... Abrupt contact with unit 30 ...... ; .. Sandstone, similar to unlt 3, but medium 30. Mostly covered. Claystone and silty mudstone rubble. ... grained and laminated ...... 31. Siltstone ......

Rubble of claystone ...... 32. Rubble of clay shale...... r, Sandstone, similar to unit I, but fine grained...... 33. Sandstone, fine grained, platy to fissile, Covered ...... poorly indurated......

Siltsrone and sandstone inrrrbeddrd. Lower 15 feet 34. Covered...... of unit very fine grained, friable, ulth ffssile 35. Sandstone, similar to unit 33. Contains scattered parting, upper 10 feet fine to medium grained, carbonaceous fragments, very micaceous...... blocky, iron stained, contains abundant carbonaceous 36. Rubble of claystone ...... fragments on bedding surfaces ...... 37. Sandy siltstone, poorly indurated ......

Claystone with interbedded thin siltstone 38. Covered...... beds, poorly exposed. Claystone 95 percent 39. Sandy ailtstone as in unit 37 ...... of unit ..,...... 40. Mostly covered. Rubble of claystone......

Sandstone, very fine grained. and siltstone, 41. Sandstone, medium light gray. very fine to

"curly" bedding in lower 1 foot of unit ...... fine grained, hard to poorly indurated, Claystone, poorly exposed ...... contains carbonaceous fragments ......

Sandstone, very fine grained, lower 1 foot 42. Hostly coveted. Some rubble, dominantly clay- fissfle, upper 2 feet platy. Pourly indurated...... stone, but also siltstone and silty shale ...... wood anclclinrs vicinfcy,'are suggestive of volcanic activity. may be as much as YO0 feet thick between the Kokolik and Kukpouruk

However, no tuffaceous or positively identified bentonitic material Rivers. was recognized in the sandstone seuber. Stratigraphic relationships and rode of deposition.-- Within its

Sample 50ASa49. collected In the southern part of the belt of southern belts of exposure, the aandstone member of the Fortress exposures east of Iligluruk Creek, in hand specimen ia medium light Mountain Formation is interpreted to locally overlie rocka of the gray, weathers moderate yellowish brown, fine grained with salt-and- Shublik Formation and the Creteceoue (Neocomian) rocks. Contacta pepper texture, exhibits graded bedding, and is highly calcareous. between beds of the sandstone member and these rocks are obscured by

Cenrent caioprlses about 30 percent of the rock and consists of fine- rubble and some of the Shublik and Okpikruak rocks may actually overlie grained carbonate with a small aioount of chlorite. Mostly subangular the sandstone member in thrust fault relationship. In the Kukpwruk to angular fairly well sol?.ted grains form less than .02 to .2 Valley, 6 to 8 miles southwest of Igloo Mountain. the sandstone member

in diameter, with modal size of .08 consist of car- overlies rocks of the argillaceous unit. Contacts are not exposed, bonate (estimated to be 35 percent of the grains), clear quartz but bedding traces of the sandstone member discordantly truncate

(30 percent), chert (25 percent), muscovite (less than 10 percent), traces of the older strata. The contact may either represent an and rock fragments, opaque minerals, and plagioclase. The rock is angular unconformity or lie along a fault in this vicinity. West of intermediate between a subgrayuacke and calcarenite. the Nuka River, 3 miles west of the mouth of Pilly Fork. the sandstone

No ~omplete~unfaultedsection of the sandstone member is known member appears locally to overlie a complexly folded anticlinal to be exposed in the Kukpouruk-Nuka region, and cl~etotal thickness of structure containing grayuacke sandstone, shale, and associated Shublik the member is not knovn. Partlal thicknesses include about 4,400 feet Formation rocka. liere, the general strike of the sandstone member in the axial zone of the East Driftwood anticline, based upon struc- beds is at considerable variance to those of the underlying rocks, tural computations from scattered dip readings along the Utukok River, and an angular unconformity is inferred. Along the south side of and about 3100 feet along Spike and Iligluruk Creeks, where the rocks the southern belt of exposure, the sandstone member apparently under- are better exposed. However, the thickness of the member is believed lies tibe shale member of the Fortress Mountain Formation. No clear to vary considerably within the area; it Is inferred to reach a relationsbips between the two were observed, however, and their

nilx xi mum in the southern and western parts of the exposure belts and relative positions are based oil the structural interpretation that the shale member lies in a synclinal structure. Happed contacts between that unit. The sandstone and uacke and congloolerate members are at the two members are drauir along obscure topographic breaks marked by least in part equivalent and the upper part and northernmost exposures hills containing the coarser and more resistant clastic rocks of the of the sandstone member may be younger than any conglomerate member sandstone mrmher. Along the north side of the southern belts of rocks. exposure of the sandstone member, the contact with rocks mapped as Fortress Mountain Formation rocks probably represent a progr~ding~ Torok Formation also lies for the most part along topographic breaks deltaic geosynclinal sequence. Tl~estrata indicate conditions of . whictt are not everywhere well ,defined. Brtueen Driftwood Creek and rapid deposition in wliich turbidity currents were important agents 4 Tingmerkpuk Kivrr, this contact is a relatively straight line and may, of transportation. Source areas lay to the south In the Brooks Range in part, reflect one or more faults. geanticline. Although the sandstone member is interpreted to be a In the northern areas of exposures, the sandstone member occurs northerly distal facies of the uacke and conglomerate member, it may in wltat appear to be ilr~ticlinal.structures flanked by rocks of tlte in part be a younger progrlrdational unit in which reworked sediments Torok Formatiori. Along the Driftwood anticlines the member directly of the uacke and conglomerate member were incorporated as the geanticline underlies the Torot Formation and, although the actual contact is not axis migrated northward such as it did later fn the Cretaceous (Payne wnd'others exposed, the rocks appear to be conformable. The mapped contacts are 1951; Chapman and Sable, 1960). The more widespread character and ; drawn at tlie top of the uppermost resistant ridge-forming clastic i apparent greater thickness of the eandstone member are features which unit of the sandstone member, along a break in slope. Similar seem to support this assumption. Following deposition of the sand- I relations1iips between the sandstone member and the Torok Formation are etone member, the supply of coarse sediments diminished and the finer interpreted far those rocks in the other northern areas of exposures sediments making up the shale member were deposited. although the coritacts in part lie along faults. Fossils and age.-- Megafossils in the sandstone member of the Lvidc~~cefor relationships between the sandstone and wacke and Fortress Mountain Pormatlon are very rare. Surfaces of sandstone and co~iglomeratc~neinbers of the Fortress Mountain Formation is indirect siltstone beds locally contafn worm trails. indeterminate circular and consists uf co~nparativclit!~ulogic character and features of and elongate markings, hexagonal impressions identified by R. W. rocks in the two ur~its,their relative stratigraphic positions with flrown as Re tipt~ycushex;rgonale Ulricl~,possibly a plant form, regard to the sl~alemember of the Coro~ationboth 'a1)pearfng to underlle ! These structures, now called Paleodictyon, are interpreted as feeding trails by Srilaclier (1954). An aamonite the sandstone member but are generally very well indurated. Along identified as Colvillin crasvicostato by R. W. Imlay, was collected Thunder Creek, and in some exposurea further ueat, the surfaces of from a sandstone bed along the Colville River (USCS loc. no. 23558). these beds are brightly ironstained. Fine-graded bedding and cross- According to Imlay (1962), Colvillia has been found also In the lower bedding, mud markings, small pyrite concretions. and small ripple part oE the Torok Formation from other areas in northern Alaska and marks are characteristic of many of the sandstone and siltstone beds, probably represents late Early Cretaceous (Albian) time. which are cowonly less than 4 inches thick, and rarely as much as I

Shale member 10 feet thick. Along the Nuka and Coiville Rivers, the lover part

Diutributinn and outcrop.-- The shale member of the Fortress of the formation contains a higher percentage of sandstone and ailt-

Mountain Formation is exposed in the southern foathills in a persistent stone and grades upward into predominantly argillaceous rocks (sectionl-3). i mile vest-and southwest-trending belt about $/to 4 mileg wide which extends Along Spike Creek, exposures of the member are composed almost entirely

from east of the Nuka River to beyond Spike Creek. of claystone and mudstone.

Rocks of the nonresistant shale member are topographically The total thickness of the shale member is not known, as no rocks known to be younger than the member were recognized along belt of expressed as relatively featureless swampy lowlands. Only a feu its

rock traces are exposed and, except along the Nuka River and near the exposures. The tl~ickestknown section, about $50 feet, is about 80

junction of Storm and Thunder Creeks, cutbank exposures are rare and percent exposed in cutbanks along the Nuka River and Pilly Fork. and .

scattered. liea in the north limb of a major syncline, A feu small drag folds

Character and thicknet;s.--Mudstone and claystone comprise about were observed in the upper and lover parts of the section. but the

50 to 80 percent of the rocka in the shale member. These, Similar to section is not believed to be cut by mafor displacements.

that present in other units of the Fortress Mountain Formation, commonly Section 23. Shale member of Fortress Hountain Formatipn, vest

are dark gray, fissile, blocky and nodular, and locally iron stained. side of Nuka River and F'illy Fork; measured by %. H. Horrie and E. G. Sable, August 22-23, 1951. A few beds exhiblt small mud cracks. Claystone and mudstone occur Thickness in feet in units comm?nly less than 1 foot but as much as 30 feet thick, Unit

intcrbeddcd with mostly tl~fnalltstonc and very fine to fine-grained Top of section, eroded. Syncline axis.

s;u~Jstunt:beds. The siltstunr and sandstone arc similar to those in 1. Silty shale, medium to dark gray, fissile to blocky, thlnbdded. about 90 percent of unit. 7. Similar to unit 5. Sandstone. very fine to fine Thin interbeds of siltstone. medium gray, grained, in beds as much as 2 inchea chick. Shale weathers light gray, micaceous, calcareous, inch beds +/to 4 inches thick with l/&to $-inch partings. ... 26% in beds less than 1 inch thick, finely cross bedded.

Beds well stratified; unit cuntairrs few small Total flticknesra ...... 165% drag folds. Unit about 80 percent exposed...... \80% Srratigraphic relationships.-- The shale member is interpreted 2. Silty shale and siltstone interbedded as in unit 1, to lie in a aynelinorial structure between complex anticlinoria of the !i but shale about 70 percent of unit, platy, in part sandstone and wacke and conglomerate members of the formation. The laminated. Feu beds of very fine grained sand- south limb of the synclinorim is highly folded and the member may be stone less than l .inch thick. About 70 percent in fault contact with the wacke and conglomerate member. On the north exposed...... 63% side of the outcrop belt along the Nuka River, however, the north limb 3. Similar to unit 2. but includes soft sandy shale, is relatively simple and the sequence appears-to grade downward into platy, interbedded with I-inch beds of very fine the sandstone member. No ocf~erbasal contacts of the ahale member grained sandstone ...... 70% with older rocks were seen anywhere in the area. 4. Sandstone, medium dark gray, weathers moderate Ttie shale member may be a allghtly coarser grained equivalent yellowish brown to light olive-gray, micaceous, hard. ... 2 of the lower part of the Torok Formation exposed farther north vhere 5. Interbedded silty sitale, clay shale, siltstone, and the Torok overlies the sandstone member of the Fortress Mountain sandstone. Clay shale dark gray, fissilr to nodular, Formation. In part laminaced, snall mud cracks seen on one bed. inches Fossils and age.-- No megafossils were found in che shale member. Sandstone mostly less than l$/but as much as 4 incites Thirty-six shale samples were collected for microfossil content and thick. Shale about 60 to 70 percent of unit...... 25% of these 30 samples were barren. The meager microfauna in the 11 6. Sa~idstone,medium gray, weatliers light ollve-gray , fossiliferaus samples of 105 total samples from all three members of fine grained, micaceous, cor~tainsscattered carbon- the Fortress Hountain Formation.identi£ied by H. R. Brrgquist include aceous fragments, massive to blucky , bedding the three formis Ihthysipht~nbrosaci 'Tappan, Huplophragmoides partings 1 to 6 iriciles...... 132 Nauss, and Conorbina?. These are also present in the louer part of recent studiea show that the rock or the Torok Formation are not the Torok PornlaT;on, but are not known from other rock units in the younger than middle Albian age, but the sequence may contain one or liukpowruk-~uka region. more depositional breaks occupying the same general position as the

Torok Format ion one suggested by Smith and Mertie, Distribution and outcrop The Torok Formation, first described by Gryc, Patton, and Payne The Torok' Formation underlies vide lowlands in the

( 19511, was reief ined by Patton ,to include "the predominantly shale Southern and Northern Foothills provinces in the northern part of the Kukpowruk-Nuka region. Lowlands extend across the entire sequence that underlies the Nanushuk group in northern Alaska1' (in area and appear monoton~uslyfeatureless in contrast to the hills and Gryc and others, 1956, p. 222). At least the upper part of the forma- ridges underlain by rocks of the Portresa Mountain Formation and tion can be traced westward from the type area for a distance of more 9 Nanushuk Group; however, they contain many small hills and ridges as than 200 miles and, in the northern part of the Kukpwruk-Nuka region high as 100 fret. There are a few mud- and rock-rubble traces on the the sequence underlies rocks of the Kukpowruk Formation and overlies hills and ridges, but for the most part the lovlands are covered by the sandstone member of the Fortress Mountain Formation. tundra and post-Cretaceous gravel deposits. Outcrops are mainly Torok Formation rocks exposed in the northernmost part of the limited to amall disconnected cutbank exposures that are most numerow mapped area and those farther north in the Utukok-Corwin area have along the Colville River, Driftwood and Adventure Creeks, and Ting~erkpukRiver, been previously discussed by Chapriian and Sable (1960). Other exposures Character and thickness of the Torok Formation in the Kukpovruk-Nuka region contain rocks which Claystone, mudstone, and silry shale are the preS- are similar to those described by Chapman and Sable, but sock sequences dominant rock types of the Torok Formation; interbedded are not well exposed and the beds are tightly folded and faulted; little siltstone, sandstone, limestone and varled nodules and concretions new stratigrapi~icor lithologic information is included in this report. comprise less than 15 percent of the sequence. The shale and thin-bedded Kocks uittlln the sequence mapped as Torok Formation had previously coarser rocks are uniformly bedded but, thick beda of sandstone and been considered to mark the dividing line between Lower and Upper siltscone, where well exposed, are mostly lenticular. Cretaceous Series by earlier in~estigutorv(Smith and Mertie, 1930, Claystone and mudstone are similar to those in the Fortress PI. 1) wllo interpreted the Late Cretaceous rocks to overlie the Hounrain Formation. They are mostly medium to dark gray; some are Early Cretaceous rocks with pronounced angular uncoriformity. More olive-gray and yellowish gray, and rarely greenish gray; the shales and otherwise resembles the more thinly bedded sandstone and ailtstone. weather grayish yellow, and some beds exhibit "gun-metall'-blue The thick-bedded sandstones more closely resemble similar rock types ueachering surfaces. 'Illese fine-grained rocks occur in well stratified in the Nanushuk Group than those in the Fortress Mountain Formation; and, in part, laminated blocky, fiasile, and nodular beds cornonly they are relatively clean, well aorted. contain relatively little less than 1 inch thick; units rarlge from a few inches to more than 100 visible mica, and commonly weather rusty and yellowish brown in contrast feet thic$. The shale is in part calcareous and locally contains small to the dull olive-gray and brounlsh colors characteristic of the Fortresa ripple marks and mud cracks, mud markings, whitish or yellowish Mountain sandstone beds. These Torok sandy unit8 are scattered through- \ efflorescence along fractures. layered carbonaceous wood fragments out the upper part of the formation. particularly in the upper iOOO associated with irorrstafoed shale, and ferruginous, limy, clayey, feet, but sandy units also occur In the middle part of the formation and pyritic nodules and concretions. about 3000 to WOO feet below the top. Sandy units, locally 75 feet

Siltstone and very fine to fine-grained sandstone, mutually thick, were recognized along the east-flowing part of the Colville interbedded and interbedded with shale in platy- to blocky-fracturing River, on Plunge Creek, Adventure Creek, and Iligluruk Creek. These beds commol~ly less than 4 incl~esthick and in unlt~mostly less than may represent a relatively per'sistent aandy horizon which extends

2 feet thick, are light to dark gray and yellowish gray, weather throughout the belts of exposures; but because of the limited outcrops yellowish brown, olive-gray and in part irortstained, They are hard. and uncertain structural relationships, the persistence of the sand-

fairly clean, well sorted, locally cnlcareouu, and some beds contain stones as a marker horizon has not been demonstrated. Some sandstone

ripple marks, scour markings, ferruginous and pyritic nodules, scattered beds also occur in the lower part of the Torok Formation; but where

and layered carbonaceous plant fragments and mica flakes, clay present, as along Adventure Creek, they are relatively thin and pellets, burrows, trails, indeterminate organic markings and a few resemble strata of the sandstone member of the Fortress Mountain

pelecypods . Formation.

Fine- to mediuargrained sandscone nls? occurs in sontewliat thicker, A 550-foot sequence exposed along the north lfmb of the East

platy to massive Lenticular bed* and units cunmonly less than 5, Driftwood anticline, described by Chapman and Sable, represents the

but as much as 75, feet tliick. Sandstone is cornahonly medium gray, basal part of the formation at this locality and consists mostly of to u~odcrnteyfllowlsl~ brown, wlth salt-and-pepper texture, weatl~rrspdle dark-gray clay shale wltich con~ainsa small amount of dark-greenish-gray sholc, lenses of ye1lowisk1-brown cloystone, dark-gray silty limestone beds of limy claysrone, light gray. weathers pale witti cone-in-cone structure, and septaritrn concretions less than 2 yellowish orange...... 252 inches in diameter. Part of this sectiunub mapped as the argillaceouu 4. Largely rubble. Shale 75 percent of unit. sandstone unit (p, 130-134). Ten miles east of this section, on the west side of ttie Colville River. similar rocks exposed in three cutbanks may 15 percent, siltstone 10 percent. Feu nodules of moderate represent the same horizon. In the southernmost cutbank at approximate red-weathering dense ferruginoua claystone (ironstone), lac. 68°51:30", rubble of ligt~t-yellowish-gray clay associated with and Fragments of clayscone with cone-in-cone structure. .. 5005 greenish-gray and grayish-red shale overlies siltstone, sandstone, Bottom of section. End of exposures and shale of the sandstone member of the Fortress Hountain Formatian on the north limb of a small anticline. One mile downstream, the Total thickness ...... 7765 foliowing section is exposeh on the south limb of an anticline. On the north limb of the same anticline, an isolated exposure

Section 24. Torok Formation? West side of Colville River. Includes 30 feet of dark-gray clay shale, the upper IS feet of which

Examined by E. G. Sable, August 12, 1951. contains irregular nodules of medium-gray. calcite-veined. Silty lime- -Unit Thickness in feet atone as much aa 1 inch thick and 4 inches long. This shale overlie8 Tap of section, end of exposure several hundred feet of interbedded siltstone, sandstone, and shale,

1. Largely claystone, 80 percent of unit, dark gray to in which the coarser grained rocks, typical of the sandstone member of

medium dark gray, nodular to platy, in sets of beds the Portresa Mountain Formation. comprise 60 percent of che exposure. .

as much as 1 foot thick. Interbcdded siltstone and North of this exposure, scattered cucbanks consist predominantly of

small amount of fine-grnined sandstone, medium shale and the beds dip northward towards the axis of the Meat Hountain

light gray to light olive-gray, calcareous, finely cross- syncline.

bedded, in beds less than 1 inch thick...... 250 The rocks interpreted ta represent basal beds of the Torok

2. Sandstone, medium light gray, weaLbers dark yellouish Formation to some extent resemble strata of ttie argillaceous unit

orange, Line grained, calcareous...... 1 exposed in the Kukpouruk River Valley. However, no megafossils were

3. Sltale, slltstone, and sandstone interbedded as in unit 1, found In the rocks along the East Driftwood anticline or on the Colville

but sandstone beds as much as 6 inclies thick; few 2-inch River. Microfossil samples from the Colville River exposures were

barren, but one aggregate lutcrofossil sample (Sample 51~Sa306)from

188 the Driftwood exposures contai~leda microfauna of posaible Cretaceous River with apparent conformity, although the actual conracts are not age, according to 11. R. Bcrgquist. some exposed. The Torok underlies the Kukpowruk Formation of the Nanushuk of these rocks may represent argillaceous unit equivalents. Croup, and the contact is gradational and intertonguing. Because of Tliickness measurements of the Torok Formation are approxiloate; this intertonguing. the upper part of the Torok Formation in the eastern the formation is not con~pletelyexposed anywhere in the area, and, part of the urea is interpreted to be at least in part equivalent to altllough the upper limits of the formation are fairly well defined in several thousand feet of the Kukpouruk Formation in the western part a few places where Nanushuk Group rocks are present, the lower limits (Chapman and Sable, 1960). h are urlcertain. Section mealiurcments were calculated at localitiee where On the baais of rock types and stratigraphic position, at least no major structural break is apparent. Heasuremrnte were made on vertical the lower part of the Torok Farmtion may be equivalent to the shale aerial photographs using field information from well-exposed but member of the Fortress Mountain Formation, and Borne of the Torok beds scattered outcrops. In each of these sections a feu outcrops uhich lie may also represent a northern fine-grained facies of the coarser near maJor aqticlinal axes showed evidence of minor folding and faulting clastic rocks of the sandstone and conglomerate members of the Fortress of unknovn magnitude. The thicknesses were measured at three localitiea: 1 Mountain. Microfossils from the lover part of the Torok end the sand- along Adventure Creek south of the East Driftwood anticline; between stone member of the Fortress Mountain are generally aimi!ar but other Heat Mountaln syncline and East Driftwood anticline; and north of cite fossil evidence is lacking. If these units are equivalent. the north- Colville River along long. 160~10'W., on the south limb of Foggy ward facies shift represents the normally expected decreased current syncline. The Meat Mountain 6,400-foot-thick section, is believed to competency in more distal areas of the geosyncline, and the vertical represent the total thickness of the Torob Formation at that locality. facies shift may represent both slower rate of uplift in the Brooks The Adventure Creek section, about 5,200 feet thick n~eaaured from the Range geanticline and a general southuard marine transgression during bott~mof the formation, and the Foggy syncline section, about 6,000 Torok' time. feet thick measured from the top of the formation, do not represent Discordant bedding: relationships within the Tarok Formation were total tl~icknesses. observed in a feu iaolated localities but they could not be traced Stratigrztphic relationslrlps and made nC deposition beyond the exposures in wliich they occur. One example, along a Tllr Torok Formation overlies tlic sandstone mrotber of the Fortress Mountain tributary of the upper part of the Colville River, is discussed by formatlull along the East and W~stIlrlftwocd anticlines and along ttte Colville u: 0 4 w ace 52" 22%" 9 d camUPIS mru w P zam- a42 a 3 0 mt; m-22 b C bdm oms UhC Yam bCZ PI PL gwu PI Ey

Gm* ua ClOJ b 9 3- m l4 w m w C PIum Y .,dud fn OI L. oz2 au- E 111 a 0 m-tc mum Fossils and age

Tlw fcw a~c.gaCossiltlfound ill the Torok Form*tion the Utukok-Comin region, farther north. In themselves, the foseils in the Kukpowruk-Nuka region corasist of pelccypods, the worm Ditrupn, are not dfagnoatic of a specific age, but the stratigraphic poaition and hexugorral markings. Three collections from sandstone beds in the of the Torok Formation between the Fortress Mountain and Kukpovruk

Torok Formation or probable Torok Formation include one sample collected Formations, both considered to be of Albian age, indicates an Albian by P. S. Smich in 1926, previously identified by J. B. Reeside, Jr., age for the Torok. and reexamlried by R. W. Imlay in 1953, and two samples collected in A more extensive microfaunal assemblage is recorded from the

1950 and identified by Imlay. Torok Formation in the Kukpowruk-Nuka region than the combined assemblages

13717 (26AS37) Igloo Mountain, near Kukpowruk River. of the other rock units. Although this may be partly due to the greater Torok Approximate Lat. 68'46' N.. long. 162O50' W. number of shale samples collected from the Torok, it is more probable

Formation? Upper part? that the Torok containa a greater abundnace of forms and variety of

Ditrupa n. sp. species.

pelecypods indc L. A total of 146 shale samples were collected from the lower part,

22480 (5OASa32) Plunge Creek, north of West DriEtwood anti- representing approximately the lower 400 feet. Of these, 40 samples 68°53'24"N. cline, west of Utukok River. lat. were barren. Microfossils in the remaining samples include Dorothfa

long. 161°21' W. Torok Pormacion, upper part? chandlerensis Tappan, Baplophragrnoides Nauss, Bathysiphon brosgei

Arctica? ap. Tappan, and Trochamina eilece Tappan. Dorothia chandlerensis and I. 3. Fanope? kissou~ni (McLearn) Aaplophragmoides linki are forms which are present in the lower part

Inoccrarn~~ssp. Juv. of the Torok Formation elsewhere in northern Alaska and are not

Pleuromya sp. certainly known co be present in the upper part of the formation

50ASa18. West side of Adventure Creek, lat. 6a051' N., (Bergquist, H. R., 1957, written commun.).

Torok Formation, middle part? About 10 shale samples were obcained from the upper $000 feet

Paleodlctyon hexagonale (Ulrich) {hexagonal markings) of the Torok Formatlon in the Kukpowruk-Nuka region. These were

W~LIIthe exception of Paler~llctyonand Inoceramus, the other fossils either barren or contained long-ranging speclev of Forminifera and

are also present in the overlying rocks of the Kukpowruk Formation in cannot be adequately compared to those from the lower part of the Torok. Hicrofossils from the upper part of the formation farther north in the is of a general nature. Part of the geologic map of Chapman and Sable

Utukok-Corwln region (Chapman and Sable, 1960) lack Dorothia chandler- is here incorporated in the northernmoat part of the Kukpovruk-Nuka ensls and Hnploplrragmoldes and contain a somewhat different region. Rocks of the Nanushuk Croup had previously been included in the assemblage than those from the Kukpouruk-Nuka region, and more closely Upper Cretaceous Series by Smith end Mertie (1930). resemble assernblagea in the overlying Nanushuk Group. Although a Diatriburion and outcrop. -- Several large simple synclinee microfaunal break vithin the Torok Formation may be present, faunal composed of Nanuahuk Group rocks, topographically expressed as mesa8 data are too fragmentary to be more ~hansuggestive. Other evidence and cuestaa, lie in the Northern Foothills province along the northern for a possible nondepositional break within the formation has been boundary of the Kukpowruk-Nuka map area. These include Foggy, Heat discussed above. but the specific stratigraphic interval uhich may Houncain, Poko Hountain, and Igloo Hountain synclines, and synclines represent the break is not well exposed and has not been sampled. along the Kokolik and Kukpowruk Rivers. The mesas and cueatas'rise

Lover and Upper(?) Cretaceous Series to about 900 feet above lowlands underlain by the Torok Formation.

Nanushuk Croue Their slopes contain relatively persistent rock ledges and rubble

~uk~owrukand Cowin Formations traces of resistant sandstone units uhich alternate with rundra-

and rubble-covered bands representing less resistant rock types. The Nanushuk Group is a sequence of marine and nomarine rocks Cutbank exposures are restricted to scattered outcrops along the major in northern Alaska that directly overlies the Torok Formation and streams, the Kokolik and Kukpovruk Rivers. underlies the Colville Group (Cryc, Patton, and Payne, 1951, p. 162- Character end thickness.--The Kukpowruk and Corvin Formations 164). and is of Early and early Late Cretaceous age (Detterman, ill consist largely of sandstone, siltstone, and shale, parts of a north- Gryc and others, 1956. p. 233). In the Utukok-Cowin region, north of I ward prograding deltaic complex. The ~uk~ourukFormation in the and overlapping with the Kukpowruk-Nuka region, the group has been Kukpowruk-Nuka region contains about 35 percent subgraywacke to relatively divided into two major rock units, a lower dominantly marine unit, the clean sandstone. sandy siltstone, and thin lenses of conglomeratic Yukpowruk Formation, and an upper dominantly nonmarine unit, the Coruin sandstone and subround pebble conglomerate which are mostly gray, Forlnation (Sable, 1956). Rocks of tlaese formations exposed in the yellowish gray, and lig11t olive-gray, and commonly weather yellowish Gukpowruk-Nuka region have already been described in detail by Chapman brown. These occur in resistant units as much as 100 fret thick. which and Sable (1960), and discussion of these units in the present report weacber flaggy to masstve and exlliblt large-scale crossbedding, current The Conrin becomes progreaaively more nonmarine upward in the aequence.

and oscillation ripple marks, and graded bedding. Intercalated units No complete sections of the Corwin Formation are exposed in the of interbedded dark-gray shale, claystone, and thinly bedded siltstone Kukpowruk-Nuka region; tlle upper part of the formation hae everyuhere resemble those in the Torok Formation, Carbonaceous shale, thin coaly been eroded. Partial thicknesses oE the formation include about 3500

beds, and nodules of clay and silty ironstone are present in minor feet on the south limb of the syncline along the Kukpowruk River (Coke

amounts. Fossils, locally present in sandstone beds, consist mostly baein in Chapman and Sable, 1960). and warm casts or trails. Clay pellets and carbonaceous of pelecypods Stratigraphic relationships and mode of deposition.-- Because the

Fragments are common throughout the fomation. Kukpowruk and Torok Formations are gradational and intertonguing, Along the nortl~ernboundary of the Kukpowruk-Nuka region, ttte the two are in part equivalent. The basal contact of the Kukpowruk is Kukpouruk Formation thins eastward from about 4,500 feet on the Kukpou- placed at the base of the lowest persistent sandstone beds vhich form Partial sequences ruk River to about 2,750 feet an the Kokolik River. a break in slope above lowlands underlain by the Torok Formation.

from wliicll the top of the formation has been eroded include measured The Kukpowruk and the Coruin Formations are also gradational and inter-

thicknesses of &out 2,300 feet at Meat Mountain, 3,700 feet in Poko tonguing, and the position of their contact is not clearly defined.

Mountain syncline and estimated thicknesses of &300 and $000 feet, The base of the Corwin Formation Is mapped in a transitional zone as

respectively, at Igloo Mountain and Foggy synclines. much as several hundred feet thick, above which nonmarine rocka are

The dominantly nonmariue rocks of tile Corvin Formation exposed dominant. Although local erosional breakspave been recognized within

in the syncllnes along the Kukpowruk River consist of rock types similar both the Kukpouruk and Corvin Formarions (Chapmn and Sable, 1960),

to those in the Kukpowruk Formation but include more abundant and these rocka are believed to represent an essentially continuous

thicker beds of coal, carbonaceous shale, conglomerate, and bentonite depositional sequence.

beds. Tlie formation contains abundant carbonaceous fragments, carbon- The Kukpowruk Formation is interpreted to represent an inshore

ized wood remains, plant impressions, and ironstme nodules and lenses. delta plain and delta front facies deposited in s shallow sea with a

The orange, reddish, and yellouish weathering colors of the Coruin, widely fluctuating shoreline wlllch, in general, regressed nortliward.

when seen from a distance, are distinguishing gross features. in The ciraracrer and bedding features of the rocks indicate shelf-type

conCrast KO the darker yellowlsls-brown-weathering hues of the Kukpowruk. deposition rather than true geosynclinnl dumping, and the Kukpowruk I and have been deposited during recent periods of stream activity. however, suggesta a considerably older period of stream activity. The High-level terrace deposits higher deposits may be Pleistocene in age, possibly correlative with

High-level gravel terraces include the deposit8 generally the probable Pleistocene depoaita expoeed north of the area (Chapman more then 20 feet above present stream levelsitheselie roughly along and Sable, 1960, p. 126). Gravel depoaita at considerably higher level than thoae listed the screams but their spatial arrangement is more irregular; in parts a above include the extensive gravel cover which extends northeastward of the area they form a widespread gravel cover. Two general levela of high terraces are recongized. - from Driftwood Creek along the Colville River, and scattered depo8ita -J in small areas near the Nuka River, north of Iligluruk Creek, south Gtelower general level about 20 to 50 feet above present streams is generally confined to lovland areas near of Poko Mountain, and at several localities 3 to 4 miles north of the cunfluences af major tributaries. Where examined, the terraces at mountain front between the Kokollk River and Driftwood Creek. Where ttris level consist of 5 to 10 feet of gravel and include: examined. these gravels are less than 25 feet thick. '

1. Two terraces, 20and 25 feet above the Nuka River east of Lake The Driftwood Creek-ColvLlle River deposits constitute a single

Nolik at approximately \950 feet altitude; terrace which lies at about 2,000-foot altitude 450 feet above Driftwood

2. One terrace at roughly ;3I00 feet altitude and less than 50 Creef, and at about the same altitude 65 feet above the north-flowing feet above Storm Creek, west of Lake Noluk; portion of the Colville River. They probably represent deposits of

3. One terrace between Driftwood Creek and the Utukok River at the ancient Colville River, whose headwaters once extended farther \ 4550 feet altitude, 40 feet above the Utukok River; west and have been captured by Utukok River'drainages; their age Is

4. One or more terraces between Iligluruk Creek and Tingmerkpuk probably Pleistocene. Although the deposits consist dominantly of

River at about 4150 feet altitude north of Kokolik Lake; gravel-size constituents, poorly developed drainage on many of the

5. One or more terraces along Kukpowruk River drainages south of terrace surfaces suggests' that the gravels may in part be covered by

Igloo Mountain on uhic11 no data are avaliable. a thin mantle of silt or loess.

The distribution of these deposits suggests that they were deposited The highest level terraces along the Nuka River have been mapped by screams essentially following the same drainage patterns as those mostly fron aerial photographs. 'I')ley appear to lie more than I00 feet of today, The position relatlve to the low-level terrace deposits, above the Nuka River and are outlined by a distinct break i.n topographic slope. North of Iligluruk Creek and south of Poko Mountain, scattered Rivers eastward and northeaatuard to beyond the limits of rhe map area. terrace rcmnsnts lie at about 4200-and 1,400-foot altitudes 200 to 450 The igneous rocks lie entirely south of belts in which the Fortress feet above the present streams. Two smdll terraces between the Utukok Hountain Formation is exposed. Trends of individual igneous bodies

River and Elbow Creek lie at about 2300-feet altitude, 800 feet above are mostly parallel or subparallel to the atrucrural belts in vhich che Utukok. they lie, bur when the igneous rocks are considered together, their

The relationship between the Driftwood Creek-Colville River' pattern of distribution crosses these structural belte from aouthuest terrace and the high terrace remnants exposed elsewhere in the area ia to northeast; east of Storm Creek, the igneoua rocks occur in belts not known. Detailed studies of these deposits may show that they all adjacent to Portresa Hountain Pornation exposures. Exposurea of represent deposition by the ancient Colville River which formerly similar igneous rocks have been recognized in areas adjacent to the drained a much larger area; this explanation is favored by the authors. Kukpwuk-Huka region, both to the eaat (Tailleur, Kent and Reieer,

The deposits could also have been made by northwestward-flowing streams 1966) and south (3. T. Dutro, Jr., 1951, vritten commun.). aimilar to rhos$ now in existence. The 1argest.igneous bodiea in the-mapped area lie in the head-

IGNEOUS ROCKS waters of the Kugururok River. south of the IItukok River, at about

Bodies of incrualve and extrusive mafic rocks, although individually lac. 68030' N. Tvo dark jagged mountain masses with 2,qO to 3000 of small size and areal extent, ere locally abundant in the southeastern feet relief ere capped by sill-like bodies of mafic rock which probably part of the Kukpowruk-Nuka region, and increase in abundance from represent one original igneous body more than 10 pquare milea in area. southvest to northeast. Sills and sheetlike bodies, dikes, and small Outcrops occur as sharp pinnacles and sawtooth ridges and exhibit irregularly shaped intrusives, all of gabbroic to dioritic composition, rough, highly fractured surfaces. Elsewhere in the mountsina,sills k were probably intruded at relatively sl~alloudepths. Volcanic flows outcrop as resistant ledges like those in the Nuka River headwaters, of basalt and microgabbro, some of wt~ichcontain pillow structures, or as less conspicuous rubble traces and knobs. Within che foothills, constitute the remainder OF the igneoua bedrock exposures. ieolated ridges and knobs of igneous rock are rmatly less than 100 feet

Distribution and Outcrop high, but where associated. with resistant chert and limestone they

Tl~eexposures of igneous rocks are scattered along- locally form hilla of impressive proportions. Some of the high ridges

several structural belts in the DeLong Hountains and southern near the Nuka River south of Lake Noluk are of this type.

foothills from the headwaters of the Utukok and Kugururok

20 4 The igneous rocks weather hrwn or reddish brown. Where asaocinted are estimated to be 500 feet or more thlck; dikes, where recognized, with Paleozoic sedimentary rocks, they can be distinguished by their are relatively thin, and may represent offshoots from sills; domes or weathering colors or by their darker hue8 on aerial photographs. In plugs are as much as aewral hundred feet in diameter; and flws,

localities where they are associated with Hesozoic clastic rocks, mostly lesa than 20 feet thick, range to more than 100 feet in thick-

however, igneous rocks cannot in moat cases be distinguished from tire ness. Intrusive Rocks coarser clastics except by field examination. In these localities,

aerial photographs proved valueless in recognizing igneous rock exposurea Gnbbro and rocks intermediate between gabbro and diorite are during preliminary interpretations of the area. Thunder Mountain was the most abundant intrusive rock types in the area. Diorite is less thought to be composed of mafic sllla both from photograph study and comon and occurs in relatively thin bodies. No chronological vhen seen from a distance; not until foot traverses across these expo- sequence wes recognized during atudiee of these rocks. Likevise, age sures were made was it determined that.the mountain coneiated of relationships between the intrusive and volcanic rocks are not known, clastic sedimentary rocks. although at least some of them are probably contemporaneous. Character and Size The igneous rocks of the Kukpouruk-Nuka area are Gabbro of the intrusive bodlea ia lpoderate to dark grayish green, inostly tabular bodies of mafic composition. The rocks medium to finely crystalline, and granitoid, ophitic, or porphyritic probably intruded at shallow depths include gabbroic and dioritic in texture. Hicrogabbro is sugary-textured, fin+to medium-grained varieties in the form of sills, sheetlike bodies, and dikes. Igneous gabbro comonly containing phenocrysts of'ipyroxene or feldspar less stacks, present in some 1ocalities in the footl~illseast of Storm than half an inch in diameter. The less common coarse-grained and Creek, may represent small domes or plugs; but evidence other than micropegmatitic varieties are composed predominantly of pyroxene topographic expresssion is lacking. Basalt and microgabbro represent and feldspar crystals as much as three-quarters of an inch in diameter. flows in some outcrops, as evidenced by pillow structures, planar Verj coarsely crystalline varieties, in which the crystals exceed flow structure, and incorporated rounded fragments of the same rock type 1 inch, occur within thick sill-like bodies of gabbro. Calcite- interpreted to be partially assimilated flow breccia. Sills range filled amygdules are present in some of the fine- to medium-grained from a few feet to 200 feet in ttilckness; the larger sheetlike gatbro. and are distributed haphazardly or concentrated along the bodies exposed in the headwaters of the Utukok and Kugururok Rivers upper parts of sills. Chlorite, epidote, and zeolites(?) also occur

207

exhlbit zonal growth, are bent and st~rared.and contain inclusions are reddish brown to light olive-gray. Individual bodies are fairly

vhfch are altered t~ chlorite. Calcite hns partially or almost wholly uniform in texture. massive, and some shov planar flaw structure from

replaced plngioclasr, with possible 1ibe;ntion of silica which occurs a few inches to 2 feet thick. Amygdules and veeicules are common

as interstitial secondary quartz In some sections. Plagloclase is in basalt and occur scattered or roughly concentrated in horizons within

also altered to sericite and kaolinite. Ore minerals comprise 4 to 8 or at the tops of the bodiea. Amygdule fillings are composed dominantly

percent of the sections. and include magnetite, titaniferous magnetite, of calcite, and also include epfdote, zoelites(?) and chlorite. Por-

ilmenite. and pyrite. Accessories include a feu grains of zircon and phyritic basalts contain scattered phenocry~csof pyroxene as much as

apatite. Serpentine. In aome sections, is possibly an alteration half an inch acrosa, and in a feu bodies alignment parallel to the sides

product of olivine and pyroxenea. Calcite and quartz are present is believed to represent planar flow structure. Plllows, commonly

as veinlet fillings, and leucoxene, titanite. and limonite as altera- 2 feet uide end 1 foot thick, but as much as 3 feet by 2 feec, ate

tion products of ore mineralsmN fairly common but ere often difficult to see in the highly fractured

Extrusive Rocks and sheared bodies. Where well developed in microgabbro and basalt,

the pillows have chilled borders and radial fractures extending outward Basalt and microgabbro in tabular bodies varying from a few feet from their centers. Pillows and pillov layers are commonly separated to more than 100 feet thick outcrop sporadically along belta extending by thin layers of shaly material. Sharply angular fragments of limestone, east-northeast from the headwaters of the Utukok River, and along I chert, siltstone, basalt, and microgabbro as much a's 2 feet in diameter I the belts of the uacke with cannonball concretions unit exposures are present in some bodies, including those with pillow structure. south of the Huka River. The largest body of basalt at long. The fragments appear to be haphazardly distributed and are rarely 160°, lat . 6~~41'is about 2 miles lung and half mile wide at '$ abundant enough to constitute a breccia. Perhaps they were of explosive I its widest point; it is at least 100 and possibly as much as several origin and uere dropped into submarine flwa, or in part talus fragments hund~edfeet thick. This is interpreted to be a fragment of an and Fractured bedrock incorporated in subaerial flows. allocl~thonousthrust plate. Other smaller bodies, however, appear to I Thin sections from t~o'~i1larbasalts uere examined by R. H. Morris. be interbedded with Mesozoic(?) clastic rocks. In one sample, the fine-grained rock consists of a mass of labradorite Most of the basalt and rnicroyahbro is dark grayish green to dark (Ab55An45) laths and irregular chloritc aald calcite patches which hove , grly, altlrough some bluish gray basalt is present. Weathering colors but elsewhere, am south of the Nub River, Nuka Formation claetic rocks

appear to be free of igneoua intrusion. The few contacts between intrusive completely replaced the ferromagnesian minerals. Some skeletons of rocks and the Nuka clastics may lie along faults. Triassic rocks, particularly flmenitr crystals have been partially or completely altered to leucoxene. as well- as those in the northeastern part of t11c area east of the Nuka River, are intruded,/ The second sample is a porpllyritic amygdaloidal basalt with phenocryats shale, graywacke, and conglom&rate of ~urassiporEarly ~retaceoud~ki~e. of pyroxene (pigeonice to dlopside) and occasional phenocrysta of Basalt and microgabbro extrusivea are interbedded or associated plagloclave (andesinr?) in a ground~assof fine, needlelike laths of uith shale, grapacke, and chert of the wacke with cannonball concretions plagioclase and interstitial ghes. Larger plagioclase laths enclose unit, and are believed to be entirely restricted to this sequence. abundant secondary chlorite. Some feldspar laths are oriented in a con- Basalt and microgabbro are also associated uith Triassic and older centric pattern around ernygdules which are filled by calcite and chlorite. rocks in a feu localities, but jt ia not clear whether they actually Pyrite occurs as primary(?) grains and specks, and as secondary vein represent extrusive activity or are shallow intrusive bodies. Most of fillings associated with calcite. the contacts with these older rocks are obscure and some probably lie Some of the basaltic and dlabasic bodies may represent shallow along faults. intrusives. Zt fs difficult to classify many of these rocks as No igneous activity is recognized in rocks of known Cretaceous age. volcanics or intrusives because they have been extensively fractured Some uniosslliferous clastic rocks associated uith mafic rocks may be end sheared by later orogenic activity, and because their textural Cretaceous, and it has not been proved that such activity did not and strtictural features do not clearly reflect the mode of origin. extend into at least earliest Cretaceous time. Nevertheless, wherever Mode of Occurrence and Age fossiliferous rocks of Cretaceous age are e~counteredin the area. 1 they are free of igneous bodies. Furthermore, the Cretaceous rocks Hqfic rocks intrude sedimentary rocks of Mississippian to Early almost certainly overlie the wacke with cannonball concretions unit Cretaceous(?) age. The cherty and shaly rocks of Mississippian and and its associated igneous rocks uith angular unconfomity in several Carbor.if erous-Permain(?) age, particularly the black chert and lio~estone parts of the area, and the Cretaceous clastic rocks contain a high facies, seem to be most commonly al;sociated with sills and dikes. These

sedi~~entaryrocks are relatively even bedded and form structurally percentage of relatively fresh igneous detritus rnineralogically

competent units vhich probably yielded by fracturing so that the magma similar to the mafic intrusives.

gaitled easy entry along bedding planes and joints. The shaly rocks

questionably referred to the Nuka ~orma&bn between the Utukok and

Kugrlrurok Rivers may Ilavc been intruded by large n~aficsheetlike bodies. If igneous rocke of Cretaceous age are present in the Kukpouruk- from south to north,the regional structural dip is south. High-angle Nuka region, it is believed that they represent e waning atage of igneous thrust faults and folded overthrust plates are cotmnon in the DeLong activity, not repeated again until late Cretaceous time when volcanoes Mountains and parts of the southern foothills. Nearly all fault plane8 of unknown location contributed bentonite and tuff beds to sediments dip south, as do the axial places oE most Folds in these provinces. of the Nanushuk Group. Regional strike is aboiat N. 70 E. and parallels the trend of the The evidence points to marlmum igneous activity preceding the mountain front. The entire map area lies on the west side of a major, deposition of earliest Cretaceous age sediments. However, it is not northwardly convex structural salient which culminates east of the known whether more than one stage of such activity occurred and the Nuka River. The convexity may reflect a major north-trending negative specific times of the activity are not clearly understood. Igneous element which offered less resistance to overriding of thrusr blocks clasta of mafic and acidic igneous rock in pre-Okpikruak Formation from the south. conglomerates point to igneoua intrusion prior to Cretaceous time. Excepr for the northern part of the area, the rocks are eo highly However, the source areas of these pebbles are not known. If some of folded, Faulted, and contorted, that in places even the major struc- the mafic bodies intruding Paleozoic and Triassic rocks ere pre-Jurassic, rural relationehipa are not well understood. Stratigraphic sections one would expect detrital feldspars in the Nuka Formation feldspathic in which repetition by faulting and isoclinal folding can be discounted sediments to indicate thia; in reslity they show no similarity to those are rarely more than a few hundred feet thick, and consistent rela- in the mafic rocks. It seems unlikely that the Triassic eediments. tionships of many major rock units are limited to relatively local of strikingly similar shelf facie8 throughout the area, accumulated parts of the area. Chaotic melange-style structure in which rock unit during a period of extensive intrusion. The time of the mafic igneous relationships are unknown characterizes part$ of the area easc of the i activity within the area therefore appears restricted to late Jurassic, Utukok River; elsevhere, relatively simple structural fcaturea break or earliest Cretaceoutl time. down into structures with inexplicable relationships when followed STRUCTURAL GEOLOGY along strike. Except for parts of the Lisburne Group and Shublik

and Okpikruak Formations, rock units are sparsely Iossilfferous. Northward tangential stress has been the dominant factor in major In addition. recognition of overturned sections in cherty, ahaly, and producing the structural features now exposed in the Kukpouruk-Nuka

region. hlttiough in general, progressively younger rocks are encountered carbonate rock units is difficult because of lack of diagnostic bedding features.

217 The Kukpouruk-Nuka region is divided into three major structural Group thickens from 2300 feet at Meat Mountain ayncline to more than

belts. From north to south, the belts are the folded belt, the folded $000 feet along the Kukpowruk River. As stated by Chapman and Sable

and faulted belt, and the overtlarust belt. (1960), this is good evidence of the existence of a major north-trending

Folded Belt arch east of the Utukok River which he6 persisted from Hanushuk time

Restricted to the northern foothills and nortl~ernpart of the south- to the present.

ern foothills, the folded belt includes stru,ctures exposing rocka of the South of these structures, several complex anticlinal structures

Hanuslluk Croup, Torok Formation, and oandstonr member of the Fortress which expose rocks of the Fortress Mountain Formation in their axial

Mountain Formation. Okpikruak Formation equivalents are exposed in an areas are also di~tributeden echelon. These include Kukpwruk and

aaticlinal structure on tl~eKukpovruk River, and one knovn exposure Tingnerkpuk bulges, lligluruk high, and the Driftwood anticlines,

of Sttublik Formation occurs in tlie vicinity of the Kukpovruk River all of whfch strike generally east to northeast. In detail they

(I. L. Tailleur, oral commun, 1974). contain steeply dipping rocka, minor folds, and fauits of unknown dis-

Broad, gently dipping synclinea containing Nanusl~ukGroup rocka placement, most of whfch appear to be high-angle thrust faults with

alternate with more complex anticlinal structures. These synclines south side upthroun. The Kukpowruk and Tingrnerkpuk bulges are struc-

.at Igloo, Poko and Meat Mountains, along the Kukpowruk River, and north turally continuous vith the wider folded and faulted belt to the south

of the Colville River, strike east, and are characterized by relatively which contains similar structural fractures. The surface expressions

low dips. Some are slightly asyt~metricak tlie south limb being steeper. of the Iligluruk and Driftwood structures, hpwever, cannot be traced

They are arranged in en echelon parallel to the general strike of the into the more southerly belts. Although much structural complexity

structural trends farther south. The anticlinal structures in which lies in thk intervening structures exposing Torok Formation, the folded

Torok Formation is exposed contain steeply dipping and closely folded belt is considered to be essentially structurally continuous with the

beds near their axial areas and some may be surface expressions of folded and faulted belt.

titrust faults (Chapman and Sable, 1960). The thick shales of the 'Che folded belt is interpreted essentially as a relatively auto-

Torok Fnrnlation are relatively incompetent and much of the close folding chthonous structural unit. Folding has resulted in some foreshortening

is probably the reault of shale flowage. and one or more rlirust faults may be present along anticlinal struc-

A westward regional plunge is shown by the deeper folding of ttte tures in tllc Torok Formation as suggested by geophysical data here in Nanushuk Croup in that direction; preserved sections of the Nanushuk areas farther nortl~(Chapman and Sable, 1960). It is epeculated that

The simplc structure and east-striking pattern of individual structural conditions during deposition of the Fortress Mountain

folds in the Narlushuk Croup and upper pnit of the Torok Formation Formation were not the same as those during Nanushuk Group depoeition.

conr rast sl~arplywith the structural complexity and dominant east- The presence of an erosional or nondepositional break within the Torok This is in part due to the decreased northeast trends farther south. Formation has been suggested by Chapman and Sable (1960) from several

intensity of orogrnlc effects northward. The structurally incompetent lines of evidence. If such a break exists, it may reflect the time

Torok Formation probably absorbed and dissipated much tectonic energy intekal during which a major change in the broad structural features

through shale flowage. In addition, a subsurface zone of flexure or took place. ,

faulting may lie along the northernmost exposures of the Fortress Folded and Faulted Belt

Mountain Formation, beyond which less stress was transmitted. Fortress This structural element lies within the southern foothills, between Efountain rocks are in general more highly disturbed than the equally the folded belt and the thrust beluof the DeLong Mountains, varies competent Nanushuk Group rocks; and interpretation of geophysical from 8 to 15 milea in width, and strikes abouc k4.70~ E. It is believed data shows that the subsurface structure of the Driftwood anticlines to comprise an eesentially relatively autochthonous unit which is is highly complex as coupared to the relatively simple structures 2 to generally continuous with the folded belt but whose southern boundaries 3 miles farther north. are marked by major thrust and reverse faults. Structures in the southwrscern part of the folded belt do not In comparison to the folded belt, the folded and faulted belt is indicate a westward deepening of a major basin during deposition of characterized by its linear pattern, dominant northeast trend, Fortress Mountain sediments. generally parallel linear belts of Lower Cre'raceous clastic rocks,

structures of greater complexity. and many narrow belts of older rocks

such as the Triasaic Shublik Formation.

Dominant structural features within the folded and faulted belt

include highly folded and steeply dipping beds, high-angle reverse

faults, and some thruat faults. The intensity of deformation increases

southward, and in the southern part of the belt south dips are predomi-

nant. Somewhat broader structures are present in the southvestera part of the belt between the Kokolik &lid Kukpowruk Rivers, but east- one which could fonn the sole of an overridhg thrust plate. as mapped ward, the outcrop belts are more tightly compressed. Structural relationships of the quartzite member/are interpreted

Described From north to south, the major structural features in tile simplest manner, as anticlinal atructures in part faulted vltllin the folded and faulted belt consist of: against younger rock sequences.

1. Fortress Mountain sandstone member in a complex anticlinorium. Although the folded and faulted belt is considered to be an

2. Fortress EIoulatain shale member in a narrow synclinorlum. essentially autocllthonous unit, high-angle thrust faulta are common.

3. Fortress Mountain conglomerate member in an anticlinorium and patterns of some faults suggest overthruating within the belt. along the northern part of Its belt of exposures and a syncIinorium The fault patterna within the conglomerate unit west of the Ucukok along the southern part. River are of this type. The essentially etraight-line outcrop pattern

4. Shublik Formation and micoceous sandstone unit in a complex along the north front oE the belt, and the linear patterns of the major anticlinorium. clastic rock units also suggest that faults may be present along the

In addftion, most of the isolated intermittent exposures of north side and along contacts of the major units within the belt. .

Sllublik Formation along linear subparallel trends are interpreted Faults are mapped along parts of these contacts and, wirere exposed. to represent ar~ticlinalstructures, in part ruptured along their axes, appear to be either high-angle reverse faulta vith south side upthrovn vith the south limbs thrust over the north limbs. Other Shublik or soutt~uard-dipping thrust faults in which the fault plane is at a

Formation exposures appear to be associated with synclinal trends and high angle. Although the author recogniz< fhe possibility that some tt~eirstructural style is unknown. They may represent small remnants major thrust faulting may be expressed by these relationships, field of overthrust faults or topograpt~icallyhigh areas on which the over- evidence points to generally normal relationships among the major lying clastic rocks were deposited. clastic units; crustal forestlortening by faulting is interpreted to be

The Okpikruak Formation quartzite facies occurs in two en ecllelou of a limited nature. Despite the large amounts of sandstone and con- belts of exposure within the folded and faulted belt. Relationships glomerate in the clastic units, considerable interbedded shale is also between this and adjoining rock units have been previously described, present. Tile units therefore do not appear to be highly competent, and the evidence for and against their occurrence as an overthrust and in tl~emselvesprobably would not form major thrust plates. If remnant reviewed. Tile quartzite member is a highly competent unit, their boundaries represented offshoots of a major sole fault at depth, then older rocks might be expected to occur along them, but none were highly competent rocks. Although highly fractured and tightly folded secn. Most of the Triassic rocks within the clastic units can be in places, it has yielded-mainly by faulting; between tlie Kokolik and most simply explained as occurring in normal or ruptured anticlines. Kukpwruk Rivera the faulting appears to be of the imbricate thrust

Original fault planes. if present, have probably been modiffed by type- folding as in the structural belt farther south. Another factor affecting deformation in this belt, other than

The proxfdty of rock units in the folded and faulted belt to the relative competency and thickness of rock units and nearness to the belts of intense deformation farther south is the major factor the maximum atreas pointti, is the northward extent of possible over- in producing the deformation of these rocks. The relative structural thrusting plates onto this belt. vllich have since been eroded. Over- competency of the major rock units exposed in this belt is also a riding thrust plates co~nposedof competent rocks vould impart a con- factor, particularly in detetminlng the degree of deformation. The siderable degree of complexity to the structures in the underlying higher proportion of sandstone in the Fortress Mountain Formation block. vest of the Utukok River, however, may be important in determining the Overthrust Belt some~hatbroader folding of thia unit west of the Kokolik River. The The overthrust belt lies moatly within the DeLong Mountain8 and wacke and conglomerate member is perhaps somewhat more ccanpetent than extends south of the mapped area. It ie here divided into two the sandstone member. but it lies closer to the areas of maximuin stress structural elements. the Kugururok-liukpourob block, and the Utukok- and is, therefore, mare intensely deformed and cut by a larger number Nuka block, which have many similarities but which also contain unlike of thrust and reverse faults. The micaceous sandstone unit, south structural features. of the uacke and conglomerate member, is highly contorted, because of h Extreme structural complexity characterites much of thia belt. its sbaly character, and nearness to the thrust belts farther south. Intense deformation has resulted in numerous thrust faults including In addition, it may have undergone more stages of deformation than the folded overthrusta, close folding and shearing of incompetent rock overlying conglomerate member. The Triassic rocks, particularly those units, transverse faulting and, in parts of the area, chaotic melange-style in the southern part of the belt, are very severely deformed, and many structure and igneous intrusion . exposures present a picture of bewildering structural complexity. The belt as a whole is largely an allochthonous cover of Paleo- The quartzite member of the Okpikruak Formation is composed of zoic and Mesozoic rocks overlying highly contorted relatively aucmh- thonous Paleozoic and Mesozoic beds. Direct evidence for this are unmistakably present. Exposures in the vicinity of major

interpretation consists of: structural breab are characterized by intense contortion and shearing

1. Older rock units overlying younger rocks in fault relationship of underlying incompetent sequences, often combined with a series of

on a large areal scale. Some of these relationships extend as much thrusts in which fault planes cornonly dip at angles of 30' to 60' S.

1 as 30 miles along strike. These faults are characterized by ahear zones in which gouge ranges

2. Presence of klippen consisting of Paleozoic rocks overlying from a few inches to more than a foot in vidth and contains minor amounts

Hesozoic rocks, and wLndous in which highly deformed rocka are exposed. of breccia. Faults between competent units are expressed as silicified I 3. Isolated fault alivers of rocks-considerably older than those and brecciated zones. All thrust faulta observed were parallel or they adjoin along the northern front of parts of the belt. at slight angles to bedding in the upper plate. Tl~emoat consistent

4. Imbricate thr"sting of structurally competent rock units topographic expression of major faults is a break in slope on the north 1 interpreted as attendant to major overthrusting. side of the south-dipping thrust sequence. s {pheak firdsfi I Indirect evidence consists of: The exact locatiot of mafoj,. .. . at the north frodt of the

1. Linear arcuate patterns along the front of and within the belt &+interpretive; in the Frontal portion it is represented by a I belt, and the truncation of srructural trends by thee patternsi zone of faulting in which many high-angle thrusts' rind highly folded I 2. Superposition of considerably different lithologic types rocks occur. Rocks belonging to the folded and faulted belt are believed to be time equivalents; probably infolded in the frontal portion. f nqluding Triassic rocka and I 3. Lack of orderly trends and stratigraphic relationships; younger rocks. Structural relationships are most complex in the

4. Chaotic structures; eastern part of the belt near the northward bulge, and decrease

5. Major structural features which geem difficult t1.r impossible in intensity towards the Rukpouruk River.

to explain by normal folding and small-scale faulting; The maximum northward extent of the overthrust belt lies east 6, Presence of intrusive magmatic rocks in some overthrust plates i of the Nuka River. Farther east, the front of the belt trends south- I and their absence in the overridden block in parta of the area. east, so that a northward bulge lies at the east limit of the Kukpowruk- I Actual fault planes of low-angle overthrusts were not seen Huka region. If the belt is interpreted as a major overthrust block, I during fieldwork, but in areas of high relief, such relationships the bulge represents either a concentration of northward tangential farces or o north-trending structural Iw. Dominant fold plunges in common in the overthrust sheets in the eastern part of the area. Tlie the Fortress Mountain Fonation conglomerate member west of the bulge relatively thin but competent Shublik Fowatioo has locally formed the are eastward as are the plunges of synclinal folds uithin the thrust sole of thrust plate8 but in general is not important in this respect. Faulted belt. This points to the presence of a structural low east The Okpikruak Formation, where folded to excessive thickness, has locally of the Nuka Rlver over which the overriding fault block moved with acted as a thrust unit, but the vacke with cannonball concretions unit relative ease. As discussed previously, however, the dominant struc- contains a high proportion of shale, and except for some thick conglo- tural plunge in the folded belt is west, suggesting the presence of a merate unite asaigned to thia formation has rarely acted thus. major north-trending Mesozoic-Cenozoic high. the Heade Brch, east of An unusual structural feature that i8 repeated in several places the Utukok River (Payne, 1955). Either the Meade hrch is not represented in the thrust faulted belt is folded lou-angle thrust faults resulting In the thrust faulted belt in the eastern part of the Kukpouruk-Nuka in eaat-plunging en echelon synclinal structures resembling structures region, or the bulge east of the Nuk8 River is a minor structural exposed north of the region but of greater complexity. The rocks com- feature on the west side of the Meade arch. 5 prising the upper plate,of the faults vary in age and lithology. Some The relative degree of structural competency of the rock units of the thrust platea appear to be relatively thin sheeta'less than 500 involved in thrust faulting is an important factor in determining the hrch y feet thick; others arefithick&. A striking small-scale example in the expression of faulting in various parts of the belt and the relative Kukpwruk-Kugururok block is the synclinal structure consisting of amount of deformation uithin the different -stratigraphic units. Highly Triassic chert, limestone and shale which overlies and is over@& by competent rocks, relatively undeformed thick reaistant units which rocks of the Okpikruak Formation, located 11 miles west of the Utukok form overthrust plates. include the Lisburne Croup, Carboniferous- River at lat. 6S034' N. The synclinal axis continues southwestward Permian cherty units. and the Kugururok and Nuka Formations. The 'i in the Okpikruak Formation, which contains lines of faulting similar igneous rocks are the most competent units in the area, but they occur to that described above, and is reflected in the synclinal structure mostly in thin bodies. Thicker sill-like units have been overthruat of the Lisburne Group farther southwest. The folding in this instance in the headwaters of the Nuka River and form parts of overthrust is later than the overthrus t faulting. 0the'r structures which approxi- sheets in Kugururok River drainages. The combination of igneous sills mate this pattern are tile imbricate unit fault structures in which and Carboniferous-Permian chercs results in extremely competent units, cherty units and Okpikruak Formation are exposed 4 miles southwest of Tupik Creek and. to a lesser degree, the overall structural pattern dark-colored facies of thh Lisburne Group. between Kogruk Creek and the Utukok River. 2. Uacke, sandstone, and mudstone unit, Shublik Formation Light-

The east-plunging syncline-type structure is also well expressed colored and dark-colored cherty rocks. Devonian rocks.

farther east: just east of the Utukok River, in the amaller structures 3. Okpikruak Formation wacke member, wacke with cannonball

10 miles east of the Utukok, and alon~the east-flowing part of the concretions, Nuka Formation, mafic extrusive and intrusive rocks,

Nuka River. En these structures the relationships are not as clearly cherty units. expressed as those in the western areas. Rather, the rock units are 4. Okpikruak Formation vacke member, Shublik Formation, cherty

highly contorted and structure is difficult to decipher. Hwever, units, Tupik, Kogruk, and Utukok Formations of Lisburne Group.

they are inrerpreted in the same way as those described above; namely, 5. Nuka Formation(?) shaly unit, mafic sheet-like bodies. as low-angle thrust plates which have been subsequently folded into 6. Kugururok Formation, rnafic sheet-like bodies. complex synclines. Number 1 above, exposed in the eastern and southeastern parts of

Interpretation and Age of Faulting the region seems to correspond to the Ipnavik sequence of Tailleur,

Kent, and Reiser; numbers 4 and 6 to their Nuka Ridge sequence; and A useful approach to scructoral. interpretation in the overthrust numbers 3 and S to their foothills sequence. belt is one by Tailleur, Kent, and Reiser (1966) in the adjoining Interrelationships of these thrust plates are not everyuhere Nuka-Etivluk region. They recognize several lithologically different each other by sheetlike over thrusting clear because areas of melange-type clastic structure such as that but time equivalent stratigraphic sequences which have been superimposed on/ between Elbow Creek and Driftwood Creek probably represent a mixture and subsequently strongly deformed. In the Kukpowruk-Nuka region a of highly disturbed relatively autoct~thonous lower plate rocks and similar interpretation provides a key to understanding the bewildering embedded fragments of the upper plate, and because one or more episodes complexity of the atructure and stratigraphy. In the overtllrust of post-thrusting tectonism have blurred evidence of specific limits of and folded and faulted belts there seems to be sfx different lithologic tl~rustplates. associations,eacl~believed to belong to a different overthrust plate.

They are, from north to south:

I. Okpikruak Formation(?) wecke, sandstone, and conglomerate

unit, mafic igneous sills, light- and dark-colored cherty rocks, and Distances of overthrusting are unknown. Consf dering the variation one dolomite eample tested for porosity and permeability showed a in fncies within the different plates, however, the hypothesis of total porosity of only 0.7 percent and was impermeable. The small exposures northward movement of 100 miles proposed by Tailleur, Kent, and Reiser of Devonian rocks farther north contain larger amounta of shale, and

(1966) does not seem unreasonable. if they are representative of Devonian rocks in this part of the area,

ECONOMIC GEOLOGY appear to have lees favorable reservoir potencial than those of the

Petroleum Possibilities Kugururok Formation.

Hiseisaippian rocks of the Lfsburne Croup in the DeLong Mountains, Fn general the Kukpouruk-Nuka region does not appear to offer particularly the Kogruk and tltukok Formations, may also contain suitable attractive possibilities for petroleum in the rock units and structures reaervolr units. Some sandy limestone and limy sandstone beds in the that have been observed or can be reasonably inEerred. Extensive Lltukok Formation appear to have visible porosity. but examination of faulting, tight folding, generally poor reservoir rocks, presence thin aections showed that intergranular spaces between the well-sorted of igneous rocks, and the extent of early Mesozoic eroaion in the area grains are mostly filled with carbonate. The thick Kogruk Formation are all unfavorable factors. No evidence of surface accumulation of or gas limestones are somewhat more favorable as potenrial reservoir rocks. oil/was found during the explorations. 'On the basis of available In the northernmost exposures of the formation, beds as much as 20 data, however, the area cannot be dismissed as having no petroleum feet thick are composed almost wholly of crinoidal debris and these potential. Except for the seismic work covering a very small part of rocks are extrememly friable. These beds were not found in all sections the area, no subsurface exploration has been undertaken. The char- of the Kogruk Formation, however, and they may be only local in acter of subsurface structures which underlie the overthrust rocks occurrence. The larger part of the Kogruk Formation is composed of in the southern part of the area, and the lithologic character of 'i well-indurated limestone. Three representative samples showed an Paleozoic rocks which may underlie most of the foothills farther effective porosity of less than 1.7 percent, and were impermeable. north are unknown; they may possessqualities which are favorable The dense limestones and chert8 of the Tupik Formation and other dark- to petroleum accumulation. colored rocks in the Lisburne Croup are not considered to have good The subsurface distribution and character of Devonian rocks in reservoir possibilitic?~but could, under the proper structural condi- the area ueunknown. The Kugururok Formation contains dolomite and tions, afford a suitable cap rock overlying the Kogruk Formation. No limestone beds wl~ichmay be favorable for oil accumulation. Ilowever, millidarcy permeability. The sandstone member of the Fortress Mountain The Carboniferous-Permian units consisting mostly of chert, shale, Formation contains somewhat cleaner sandstones but theaecontain consid- and dense limestone are also unfavorable resewoir rocks. The Nuka erable interstitial calcite. Pour samples collected along and weet Formation contains clean,relatively well sorted sandstones of suffi- of the Utukok River in the main belt of sandstone member exposurea cLent thickness to afford good reservoir beds. Some beds are friable; yielded maximum porosity of 7.93 percent and 1.2 millidarcys permeability ochers are quartzitic. In thin sections of these rocka, a large A sample in the Driftvood anticlinal area sllowed porosity of 10.9 proportion of the intergranular spaces is filled with quartz or calcite. percent but the degree of permeability was not tested. The Torok Three samples of this formation tested ahowed porosities of 6.9, Formation consiata almost entirely of shale, and rocks of the Nanushuk 6.3, and 2.5 percent, and permeabilities of 31, 5.7, and 0 millidarcye. Group do not occur in the subsurface oE the Kukpovruk-Nuka region. respectively. The distribution and character of the formation can An additional indication of unfavorable reservoir cohditiona in only be surmised. It is known to be present only in overthrust plates the Mesozoic clastic rocks is the observation that these units become possibly Far removed from its source and may not be present in the .rocks finer grBined northward. ~*rrrsti~~,. .. ,and O'kpikruak Formation, are subsurf ace. probably represented by dominantly shale equivalents in the subsurface The cherty and shaly rocks of tile Shublik Formation and the shale north of their surface expoaures. and dense siltstones of the Driftwood Formation show no promise of Although the degrees of porosity and permeability in the rocks oil accumulatlon. are uniformly low, the structurally competent qaleozoic rock units Tlie clastic sequences such as the Okpikruak and Fortress are highly fractured. Such conditions, if present in the subsurface. Mountain Formations and units of similar lithologic composition are might prove favorable for the migration and accumulation of petroleum. thick units, but the well-indurated graywacke and sandstones and In several thin sectione of the Tupik Formation and Carboniferous- conglomerates which characterize these formations appear to be poor Permian cherty rocks. a brown opaque residue, possibly a hydrocarbon, reservoir rocks, 'Rie quartzite member of the Okpikruak Formation is present along the fractures. Hany of the fractures have been contains clastics which are well sorted, but these are dense and quart- subsequently filled with carbonate and quartz, however, and vein zitic. Four samples from the Fortress Mountain Formation conglomerate fillings by these minerals are relatively common in many exposures in member showed a maximum porosity of 7.32 percent and less than 1 the area. REFERENCES CITED

Baker, Marcus. 1906. Geographic dictionary of Alaska: U.S. Geological Potential source rocks for petroleum in the Kukpowruk-Nuka region Survey Bulletin 299, 690 p. occur in many of the Paleozoic and Heso~oicsequences. Organic Bowaher, Arthur L.. and Dutro. J. Thomas, Jr., 1957, The Paleozoic section marine stiales and limestones are abundant in the Tupik Formation and in the Shainin Lake area, central Brooks Range, Alaska: U.S. parts of the Carboniferous-Permian cherty unite and the Shublik Geological Survey Professional Paper 303-A, p. 1-39. Formation. The Mesozoic clastic rocks also contain abundant dark shales. Campbell, R. H., 1967, Areal geology in the vicinity of the Chariot site,

Liaburne Peninsula, northwestern Alaska: U.S. Geological Survey

Profeseional Paper 395, 71 p., 2 pls., 28 figs., 2 tables.

Chapman. R. M., and Sable, E. G., 1960, Geology of the Utukok-Cordin region,

northweetern Alaska: U.S. Geological Survey Professional Paper 3034,

167 p., 14 pla., 19 figs., 12 tablea.

Detterman, Robert L., Bowsher, Arthur L..,and Dutro, J. Thomas. Jr., 1958,

Glaciation on the Arctic Slope of the Brooks Range, Northern Alaska:

Arctic. Journal of Arctic Institute of North America. v. 11, p. 43-61.

Gordon. Mackenzie, Jr.. 1958. Mississippian cephalopods of northern and

eastern Alaska: U.S. Geological Survey Professional Paper 283, 61 p.

Gryc, George, Patton, W. W., Jr., and Payne, T. G., 1951, Present Cretaceous

nomenclature of northern Alaska: Washington Academy of Science Journal,

v. 41, no. 5, p. 159-167. 'i

Gryc, George, and others, 1956. Mesozoic sequence in Colville River region,

northern Alaska: American Association of Petroleum Geologists

Bulletin, v. 40, no. 2, p. 209-254.

Smlay, Ralph W., 1955, Characteristic Jurassic mollusks From northern Alaska:

U.S. Geological Survey Professional Paper 274-D, 96 p. Zmlay, Ralph W., 1961, Characteristic Lower Cretaceous megafossils from PlwA, T. L., 1954, Effect of permafrost on cultivated fields, Fairbanks northern Alaska: . U.S. Ceological Survey Professional Paper 335, p. area, Alaska: U.S. Geological Survey Bulletin 989-F, 351 p. 1-74. pls. 1-24. fig. 1, tables 1-14 (1962). Sable, Edward G., 1956, New and redefined Cretaceous formation8 in ueatern Lef f Lnguell. E: de K., 1919. The Canning River region, northern Alaska: part of northern hlaska:' American Association of Petroleum Geologists U.S. Geological Survey Professional Paper 109, 251 p., 33 figa. 35 pls. Bulletin, v. 40, p. 2635-2643. Martin, L. J., 1959, Stratigraphy and depositional tectonics of North Yukon- Sable, E. C., and Dutro, J. T., Jr., 1961, Nev Devonian and Missistrippian Lower Mackenzie area [Northwest Territories], Canada: American Asso- Formations in the DeLong Mountains, northern Alaska: American Associ- ciation of Petroleum Geologists Bulletin, v. 43, no. 10, p. 2399-2455. ation of Petroleum Ceologiets Bulletin, v. 45. no. 5, p. 585-593. HcLaren. D. J., Norris, A. W., and McGregor. D. C., 1962, Iliustrations of Schrader. F. C., 1904, A reconnaissance of northern Alaeka: U.S. Geolo- Canadian foasils, Devonian of western Canada: Ceological Survey of gical Survey Professional Paper 20, 139 p. Canada Paper 62-4. 34 p., 16 pls. Smith, P. S., 1913, The Noatak-Kobuk region, Alaaka: U.S. Ceological Huller, SLmeon W., 1938, Upper Triassic pelecypod genus Monotis: Geological . Survey Bulletin 536. 160 p. Society oE America Bulletin, v. 49, pi 1893. Smith, P. S., and Mertie. J. B.. Jr., 1930, Geology and mineral resources Patton, W. U., Jr., 1954, A new upper Paleozoic formation, central Brooks of northuestern Alaska: U.S. Geological Survey Bulletin 815, 351 p. Range. Alaska. Chapter 13 ofArea1 Geology, Pt. 3 Exploration of Solecki, R. S., 1950, A preliminary report of an archaeological recon- Naval Petroleum Reserve No. 4 and adjacent areas, Northern Alaska naissance of the Kukpowruk and Kokolik River in northwestern Alaska: 1944-53: U.S. Geological Survey Professional Paper 303-B, p. IV, 41-45. American Antiquity, v. 16, p. 66-69. ' 'i 1958, A new Upper Paleozoic formatioa, central Brooks Range, Alaska: 1950, Archaeology and ecology on the Arctic Slope of Alaska: U.S. Geological Survey Professional Paper 303-8, p. 41-43 Smithsonian Institute Annual Report for 1950, p. 469-496. Pnyne, T. G., 1955, Mesozoic and Cenozoic tectonic elements of Alaska: Tailleur, I. L., Kent, 8. H., and Reiser, H. N., 1965, Outcrop-geologic U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-84. mapa of the Nuka Etivluk region, northern Alaska: U.S. Geological fayne, T. G., and otl~ers,1951, Geology of the Arctic Slope of Alaska: Survey Open-File Report, 7 sheets. U.S. Geological Survey Oil and Gas Investigations Map OM-126, 3 sheets. Tailleur, I. L., Mamet, B. L., and Dutro, J. T... Jr., 1972, Revised age and Pettijohn, F. J., 1957, Sedimentary rocks (2d ed.) : New York, Harper and structural interpretions of the Nuka Formation at Nuka Ridge, north- Brothers, 718 p. western Alaska: U.S. Ceological Suwey Open-File Report 547, 8 p. Tailleur, I. L., and Sable, E. C., 1963, Nuka Formations of Late Hississip-

pian to Late Permian age, new formation in northern Alaska: American

Association of Petroleum Geologists Bulletin, v. 47, no. 4, p. 632-642.

Tappan, H. N., 1951, Foraminifera from the Arctic Slope of Alaska: U.S.

Geological Survey Professional Paper 236-A, 20 p.

Woolson, .I. R., and others, 1962, Seismic and gravity surveys of Naval

Petroleum Reserve No. 4 and adjoining areas, Alaaka: U.S. Geological

Survey Professional Paper 304-A, 25 p.