GEOLOGY OF THE M C G R A T H A - 2 QUADRANGLE, ALASKA B y T.K. Bundtzen, J-T-Kline, T.E. Smith, and M.D. Albanese

PROFESSIONAL REPORT 91

A summary o f the stratgraphy, lithology, and mineral resources of a structurally deformed 450 kmP ( 1 75 mi2) region o n the north flank of the western Alaska Range, Alaska

I i ALASKA DIVISION O F GEOLOGICAL ((r GEOPIIYSICAL S U R V E Y S , F A I K U A N K S : 1988

STATE OF ALASKA Steve Cowper , Governor

I)EPAIl'I'MICN'I' OF NAT'UltAI, RI3SOURCES Jud i th M. Hrady, Conlrnissio~zcv-

DIVISION OF GEOLOGICAL & GEOPHYSICAL SUI tVEYS Rober t B. Forbes , Ilirector a n d S ta t e Geologist

Division of Geological & Geophysicai Surveys publications a r e available a t t h e following 1ocat.ions:

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INTRODUCTION

The Mc(;rath A-2 Quadrangle is composed of ruggcd mountains with numerous glacier-scoured, U-shaped valleys on the northwest Ilank 01 the southern Alaska Range. Elevations range from 1,750 fi (534 m) at Post River to 7,880 ft (2,402 m) in the west-central part of the quadrangle. No roads exist in the arca; however, a 1,700-m-long gravel airstrip is rniiii.rt:rincd ai ihc Farewell Fcdcr;rl Aviation Atlministration station 35 km to ~ h c north. Fiscd- wing airplanes can also land on an unrnaintained airstrip at Dowser Creek, ,on gravcl flood plains, and on Post Lakc.

Sedimentary rocks of the McGrath area form irrcgular, rolling hillslopcs; igneous plutons, dike swarms, and thermally altered rocks form prominant, ruggcd, circular n1;rssifs. Valley floors and hillslopcs hclow 4,000-St (1,220 m) clcvation arc mantled with surlicial deposits and vcgctation. Colluvir~nl covcrs many hillslopcs, and bedrock exposures arc conccntratcd on ritigc lops and cirque headwalls and in steep canyons. Bcdrock and surlicial control is generally cxccllcnt. Although the metric system is uscd in this report, elevations are also expressed in fcct to bc consistent with the published topographic maps.

We thank D.N. Solie, K.M. Bull, M.S. l,ockwood, K.M. MacDonald, irnd J.E. Ebel for thcir assistance during the 1982 field season. Helpful discussions with B.L. Rccd (U.S. (;eological Survey) and W.(;. Gilbert (D(;(;S) improved

1 the study. Wc arc grateful to Clairc Carter and J.W. Millcr (U.S. (icologic;il Survey) and R.B. Blodgetl (Oregon State University) for providing faunal 1 control in the iirea. We appreciate thc accommodations providcd hy Dcnoi\ Harmcs (Alaskan S'ifaris, Ltd.) at Post Eakc during (lie 1983 and 1084 licld investigations. We also thank John Decker, W.G. (;ilbcrt, and K.D. Kcgcr (DGGS) for rcvlcwin~ rhc manuscript.

Sixtccn mappable rock units that range in agc from Cambrian(?) l o Cretaceous arc exposed in the McGrath A-2 Quadrangle. Figure 1 (map inset) shows a simplified version of the bedrock gcolog. Most layered units were originally dcscril)cd by Brooks (1911) as the Talinil Group after cspoaurcs on the Tatina River in the Mc(3rath B-1 Quadrangle 12 km northeast of I'ost Lakc. Armstrong and othcrs (1977) later referred to thcsc rocks as the 'scclimcntary rocks of the Dillinger River.' Jones and others (1982) assigncd the term 'Dillinger icrranc' to those deepwater rocks of lower lo mid-l'alco~oic irgc in the study arca; Bundtzcn and Gilbert (1983) used (he term 'Dillingcr group' lor

rhcsc same rock units.' (iilberl and Bundtzen (1984) referred to most pre- .lurassic laycrcd rocks south of the Farewell fault in the southern Alaska Range as the 'Dillingcr-Mystic succession.' The Dillinger tcrrane consists of Cambrian and Ordovician ;imesrone turbidite deposits (OEIs) with minor greenstone sills (OCg); Ordovician and Lower Silurian interbeddcd shale, chert, and minor 1irneslone (SOsh, ISI); Middle io Upper Silurian laminated limestone (mSI, uS1, uSsl), sandstone turbidite (Ss, n~Ss), and shale (mSa); and Upper Silurian to Lower Devonian limestone (DSI) and limestone breccia and calcareous sandstone (DSls). Bundtzen and Gilbert (1983) suggesred that the Dillinger tcrrane rctlccts a sI~nlPo\ving-upwards marinc regime that includes basinal, turt>iditc-fan, and foreslope depositional cnvironmcnts on a Paleozoic cot~~ilicnt:rl margin. I.in~itcd p;ileocurrcnt data (tablc 1) and g)rol,ablc facies ri.l;~~ionsl~il)s wi~h the Nison Fork tcrranc (Uundtzcn and (ii1bcr.1, 1983) irldicatc that during the Pi~lcozoic Era, a shorclinc existed northwest o f the study area. Bearings of the palcocurrcn~s are dominantly southeasterly in lower Paleozoic units, but are dominantly northeasterly in the Silurian turbiditc fan. The latter t)c:iring was probably dcrivcd from a paleocurrent that was directed down the axes of a sedimentary trough parallel to the continental margin. The Dillinger group rangcs in thickness from 2,750 to 3,000 m in the study arca; the base is not exposed.

'rhc Ilillingcr terranc is overlain by a 1,000-m-thick section of sublithic quartz sandstone, limestone, and fluvial(?) conglomerate (IPDs,IPDl, TPDc) of probable Middle Devonian through Late Pennsylvanian age. Gilbert and Bundtzen (1984) suggested that these units form a southwestern extension of the Mystic tcrrane as described by Jones and others (1982) in the Talkeetna Qu:tdran?lc. Gilbcrt and Bundtzen (1984) also suggested that the Mystic tcrr;tr~c is :i con~intl:riior~ of the scdimcnt,~ry t)nsin that I;,rmcd in carly I',ilco/oic lime (Dillingcr tcrri~nc).

A deformed package of lithic sandstone, siltstone, and conglomerate (KJs) of Latc Jurassic to Early Cretaceous age is in tectpnic contact with Paleozoic rocks. 'The deformed unit is part of a 50,000-km' turbidite-dominated basin that (rends from southcentral Alaska near Cantwell (Smirh, 1981, p. 15-17) 300 km west-southwest to Lake Clrirk (Eakins and others, 1978). Jones and others (1982) referred to this llysch sequence as the Kahiltna terrane after exposures in thc Talkcctna Quadrangle to the east.

Seven units of hypabyssal rocks crop out in the study arca (Tim, Tif, Tids, Tqm, Tqp, Tia, Td). They include the Bowser and Post River intrusive complexes and an extensive east-west-trending dike swarm in the westcentral

'1:or dcscriptivc convenience only, the following tcxt uses the tcrm 'Dillingcr tcrranc.'

'!.able !. I'alcocurrcni datan from i'alcozoic scdimcnt:~ry rocks in thc ~Mc(ir;tIh 12-2 Quadrangle, Alaska.

-- . --- -. -, .- - -- - . . -- .-

fizimuth i n ' hlap Itclck 1:Iow (corrccl cd (;r:trxI no. - I:icld no. rcgirnet' for l i l t ) II~C:III

a Mcasurcmcnts takcn and intcrprctcd by 'I3.K. l iundtzcn. Cllu~.hin 2nd olhcrs (1'177) :11st)

prcscnl p:~lcoctirrcnt data fro111 strldy arca. " ~ p p c r floiv-rcgimc s~ruc t i~rcs includc flutes, st~.iation casts. ;111d longirudinal ripples. I.ouzcr

flow-rcgimc structt~rcs includc cross-l'eds.

5

p,rrt of rhc map area. The Bowser complex consists of igneous breccia with a quariL inonzoniie core and quartz porphyry intrusive rocks that cul the igneous lithologics. The Post Kivcr pluton is a porphyritic quart/ mon/onilc. 130th plutons havc s h i p high-angle contacts with host rock units.

I'rominant dike swarms and scaitcrctl dikes oi augitc basalt, andesire, and rhyolite cut layered rocks in the area; basaltic and andesitic dikes arc most abundant. Rhyolite (likes consistently cut the more mafic varieties.

Two K-Ar analyses from intrusivc rocks in the s:udy arca (table 2) yield early Tertiary ages consistent with those reported by Reed and Lanphere (1972). However, some dikes in adjrrcent map areas yield ages as young as 20 m.y. (Bl~ndtzcn ;rntl others, in prcp:tration). Solie anti others (1082) believed that ; i l l iyncous rocks ;ire part of :rt Icast two calc-alk:ilinc suldi~ction-rclHled s t i i ~ ~ ~ s a~.tivc J u r i n ~ ri'crli:~~-y t i n ~ c (tirl~lc 3) .

'I.al,lc 2. Anulytical d;rt;i lor " K ~ O A ~ agc dctcrmina~ions.~' I

kiinplc wi (g)

4 0 ~ r l r : ~ d )

Samplc

Iloch rypc

Agc (111 y.)

I Quartz porphyry (2ua1.t~ n j o n z o ~ ~ i t c

a~onstants used: f \n ;~ly~cd by 1):11?icl Krummcnachcr, S;ln I>icgo Statc University, S;ln I>ic,c ." I , (:;tlifornia.

Ana lycd hy J.1). I3 lunl . l)(i(iS-University of Alaska Coopcrarivc Gcocl1ro11~11o~y

E!c~cn C][~;ilcr;~;iry u i~i (s wcsc n~:tp~)cd i n i l ~ c Mc~; r ;~ t I i A-2 (]II;I(~I.:III::~L.; these units inclurlc Wisconsin and k loloccnc clrit'~, ice-contirct, o~it\v;rsll, ; I I I ~ t i l l deposits anti tloloccnc colluviu!n ;:nd ;:lll:viui~i. Mtiltisioricct c r o - \ , ; ~ l i c y profiles, pcrcl~cd sitlc-gl;~ci:rl notchcs, f;rcctctl spurs, l~igll lcvcl crr;~tic t)o~iltlcis, and multiple I;~tcrirl itntl end moraines dcnro!~s!ratc r!l;:i t l ~ c stiltly ;ii<.:t iindcrwent scvcriil m:rjor cpisodcs of glaciation. A more c-olul,lcte ~lar i i i l chronology of the gcncral Farewell arca is summarized in Klinc and Uundtxcrl (1986). Bet'orc late Wisconsin time, mosr of the McGrath A-2 Quaclranglc was overlain by an cxtcnsive icc licld with only a few nunataks protruding ahovc ice. Two major gliici;rl cpisodcs of Wisconsin ;tgc arc prinllirily rcspo~lsil)lc lor the current morphology and unconsolidatcti surllci;rl tlcposits.

Evidcr~<.c I'or [)rc-LVi~co~~sin gI:ici:rtion i n tllc hlc(it-;ill\ i\-2 C)u ; \d r i i~~s l~ consists ol' sc.;t~tcrctl c r r a~ ics tha t arc t'oi~ntl at clcv;rtions iil,ovc 4,500 f t (1,372 rn) and rclict viillcy morphology; however, most olticr deposits havc twcn eroded from t l ~ c rugged terrain. The oldest cxtcnsivc glacial tlcposits in the McGrath A-2 Q u a d r a n ~ l c (Odt in high-level terraces) arc inl'crrcd lo bc roughly corrc1:rtivc with early Wisconsin deposits of Knik rise in the up[)u" (.:oak Inlet bitsin (Kiirlstrom, 1004) and 'I'alkccrna Ouirtiranglc ( N c l s o ~ ~ and I<cctl, 1978) basetl o n n~orpliology, clcgrcc 01' wca~hcr ing and cstcnt. 'l'[lesc r:l;rcial deposits may at50 corrc1;rtc with deposits of Sclatna ; y e alony thc l3ig 1iivc.r t o the norrhwcsr ;inti of I:;~rcwcll I ;igc along ihc Sonth I:ork Kuskohwin~ Itivcr (originally dcscritlcd Ijy t:crnirltl, 1900). 'I'hc irpparcnt t1iscrcp;incy in t l ~ c 1;rltcr corrclation with previously n1;rppcd drift units is dtrc to :r rcccnt rccvirlu;ttio~l o f Fcrnald's c;rriicr valley-to-v;dlcy correlations ol' drilt limits (Klinc ; i ~ ~ d Bundtzen, 1!)SO).

I n the I\.lc(;r;rtll A-2 Quadrangle, late Wisconsin clriil is roughly cor.rcliitivc with Naptownc-;tgc dcptlsits of the uppcr Cook Inlci \,asin (Karlsfr.orn, 1904) and Talkcctna ()ua~lran$c (Nelson ;ind Reed, 1978) ;tnd with Fiircwcll 11 ;rgc drifl (Fcrn;rld, IOtiO) ihat occurs along the ~nount;rin front irl :tc!jaccn: quadrangles.

Multiple rcccssio11;il mor;lincs that occur in major southerly tribu[ar'ics to the Post Kivcr rescmble the Iritest Wisconsin and early i loloccnc dcposi(s of (he uppcr Cook Inict hasin ;rncl Kcn:ii I'cninsula that Knrlstroni (I!)(i4) rcfcsrctl to ;is Skilak a11d 'r;ir~y:i rc:rdv:rnccs of 1;rtc N;iptownc tirnc. 1,alc I lolocc:~~c moraines that cxlcnd a few kilonictcrs clow11-vallcy from cirtluc hc;iilu';~lls throughout the study arca wcrc deposited well after tllc main trunk-g1;tcicr systems coll;ti)scti. Ncsrctl ncoglacial moraincs indic;~tc at Icirsi two, and possil~ly nlorc, Iirtc l loloccnc pulses of gl;~cicr activity.

Modern ;rll~~ilial units (Uu , ()at, ();it) include tcrritcc, I'an, and motlcrn- stream deposits. Colluvial processes and vigorous stream erosion c;auscd hy

l'ablc 3. Major-oxide an;ilyscs a~ld CII'W norms of igncous rocks.

Map no. 1 2 3 Ik ld no. 8115l'lW 821Tr165 u21ri-183

12ock typc Olivine gabbro (iabbro dike I<hyolire

SiO 45.53 52.02 77.31

A1203 11.06 17.0-2 11.84 1% 0 3.45 4.09 0.13

2 3 I:cO 5.95 3.61 1.33

hln0 0.16 0.16 0.05

hl$) 6.15 4.77 0.12

(::I<) 6.77 6.69 1 . 3

N:3,0 3.31 3.78 3.43 K,c) 4.25 0.89 2.06 'l'xl, 1 .0j 0.95 0.10

0.70 0.20 0.10

ll,O, 0 . 3 0.16 0.05 l ,o l 4.77 4.81 1.32

4 5 6 7 831Tl'300 82151'323 82133275 8213'1'181

Andcsire dike 1);rcite Quartz rnonzonitc I<hyolitc

56.01 67.00 68.98 77.58 15.18 16.81 14.39 14.16 3.19 2.01 0!!6 0.11 7.32 0.71 1.80 1.26 0.19 0.03 0.05 0.02 1.53 1.71 0.69 0.15 4.80 5.05 2.55 0.36 4.28 3.61 3.07 0.15 3.38 0.93 3.18 2.60 1.09 0.54 0.46 0.03 0.85 0.33 0.17 0.03 0.30 0.40 9.32 0.00 0.94 1.01 0.88 3.16

I~v i~ i c and Alkalic Suba1k;rlic Sul)alkalic 'I'holciitic Sul)al!ialic Sul>alkalic Iaw-calcium Ihr:\g;ir (1971) t rachyl?asal t andcsitc rhyolite andcsite andcsire adarncllitc rhyolite cI;issific;~tion (quartz monzonite)

iipli!'t and lowcring of local base lcvcl by glacial scour continue to modify surlicial and bedrock units of the landscape.

STNUCTUKE

Layered rocks of the study area have undergone isoclinal to subisoclinal folding and synkinematic thrusting (see cross sections A, B, C, D). The Terra Cotta and Sheep Creek anticlines can be traced south and north of the study area for at least 100 krn and 70 km alon strike, respectively (fig. 1, map inset). 6 Most subisoclinal folds trcnd N. 10'-40 E., plunge to the northeast, and are locally overturned t o the norrhwest. However, west of the Sheep Crcek fault (fig. I ) , Toltl ;isymmcrry is reversed, and axes plunge to the southwest and are ovcrtllrncd to ~ h c southci\\[. Tlic trend of' fold axes sharply turns 20' E. ncnr llowscr CIrcck (fig. 1). Three major overturned folds in rhc McGrati~ A-2 (juadranglc arc rcsponsil)lc for an estimated 20 to 25 km of crustal shortening. Locally, minor slip has occurred along thrust faults. Isoclinal folding and thrust faulting occurred after Lower Cretaceous units ( U s ) were deposited and before rocks wcrc intruded by plurons and dikes during Paleocene time. These plutonic rocks apparently were intruded during extensional processes in early Tertiary rime. Later, east-west to northeast broad warping locally steepened thrust faults and isoclinal folds. The Tatina fault (after Recd and Lanpherc, 1972) separates the Mesozoic flyschoid rocks from the Dillinger tcrrane; the fault trace is obscured by Quaternary deposits throughout the map arca.

Northwest-trending, high-angle fauits, such as the Bowser Crcek fault, cut lxdrock units and older structures; however, lateral and vertical displacement along ihcse youthfi~l faults only amounts ro a few hundred meters.

ECONOMIC GEOLOGY

Mineraiization in the McGrath A-2 Quadrangle consists of iead-zinc- silver-copper skarn replacement bodies in limestone; veins and stockwork that contain zinc, tungsten, tin, and silver in igneous rocks; cobalt- and nickcl- bearing massive pyrrhotite replacement deposits; and anomalous lead, zinc, and silver in shale (SOsh). The assay data in table 4 are not derived from channel samples and hencc do not necessarily show average metal values of mineral deposits in the area.

Base-metal skarns at Bowser Creek (table 4) are typical of low- temperature, fracture-controlled lead-zinc skarns described by Einaudi and Burt (1982). Recd and Elliott (1968a,b) described the Bowser Creek occurrences and presented assay and prospect map information in addition to that reported here. The main Bowser Creek deposit (table 4, map no. 2) consists of replacement deposits of pyrrhotite, sphalerite, galena, and

chalcopyrirc in a hedenbergite-johannsenite skarn next to a fcEsite dike. The main pyrrhotite-sphalerite occurrences are in a johannsenitc-rich exoskarn in the marble froni; howevcr, important fissure-controlled, silver-rich, galena- tetrahedrite-pyrrhotite-calcite mineralization occurs in the marble 5 to 25 nl

from the marble front. The fissure deposits have been mined for silvcr, and modest amounts of ore were marketed before 1973. Son~e nxtasomatic fluids have replaced felsic dikes with anhedrrrl garnet and pyroxcne and produced a small area of cndoskarn. Skarn deposits northeast of Bowser Creek (table 4, map no. 3) are apparently associated with quartz monzonitc porphyry. A sphalerite-pyrrhotit+hannsenite skarn zone 3 to 10 m thick continues at distances of 14 to 40 m within the marble away from the intrusive contact. Assay data (table 3) for both deposits show positive correlation of silvcr with lead content.

Fracture-controlled, coball-and-nickel-bearing massive pyrrhotite deposits cut hornfeised shale (SOsh) in a bedrock canyon near Post Lake (table 4, map no. 5). Several n~assive pyrrhotite zones 0.5 to 2 rn thick and 20 to 80 m long trend N. 15'-25' W. and parallel the joint-cleavage set in the host rock. A small quartz monzonite plug is exposed 1 km north of the occurrcnccs. Skarn mineralogy is notably absent. Pyrrhotite samples contain up io 570 ppm copper, 156 ppm cobalt, and 637 ppm nickel, well below commercial assay Icvels. However, the occurrences are similar to thc Chip-Loy nickel-cobalt deposit (McGrath A-3 Quadrangle) that contains up to 2.25 percent nickcl and 0.18 percent cobalt (Hcrrcid, 1968; Gilbert and Solie, 1083).

Veins and srockwork in igneous rocks occur at the head of Bowser Creek and in the Post River pluton. The Bowser mineralization (table 4, map no. 4, fig. 2, and map inset) occurs as 3- to 20-cm-thick veins of pyrrhotite, sphaleritc, and minor chalcopyrite in a felsic intrusion (Tqp) that is similar in composition to the felsic dike exposed in the skarn described above. Sulfide-bearing veins in the intrusion are locali~ed along steeply dipping joints that trend N. 65-70" W. Skarn mineralogy is notably absent. Occurrences in the Post River plulon (table 4, map no. 6a7b) consist of pyrite 2 scheelite-chalcopyrite-quartz-calcik veinlets in hornfelsed roof pendants, in fracture zones in the intrusion, or in the contact zones between intrusions and country rock. The mineralized areas are poorly exposed and of unknown extent. A sample of mineralized vein found in the rubble yielded 1.1 perccnt copper, 0.089 percent tin, 98 ppnl silvcr, and 0.028 perccnt tungsten. Scheelite occurs in pan concentrates from ravines that drain the south side of the intrusion. The Post River intrusion is similar to ,r tungsten-bearing quartz mon~onite plug in the McGrath B-2 Quitdranglc (Bundtzen and oihers, in preparation) and is also similar to tungsten-bearing intrusions that cut identical Paleozoic rock units (Rabbit Kettle Formation) in the Selwyn Basin, Canada (Anderson, 1982).

'I'aOle 4. 1)escriptions and analytical results of sclccted mines and minerals prospects and occurrences in the McGralh A-2 Ouadrangle, Alaska (sanlple data in ppni).

Main Ilowscr Crcck

Northeast I3owscr Creek

I'ost I ,a kc PI-ospcct

I'ost Ilivcr pluton

llandom chip channel; geochemical samples of sulfurous (SOsh) shale.

Skarn with disseminated sulfides. Average of six high-grade samples of massive pyrrhotite-galena orcs. Sphalerite-rich orcs; contain 0.10 and 0.1 170 cadmiuni, respcctivcly. (See map inset, fig. 2).

Selected sample of ctalcna- yrrliotitc ore. Sphalerite-rich vcinoin rnargle.

Selected samples trom pyrrhotitc-

Grab sample of pyrrhotitc 'ore.' Selected 0.5-rn-long chip channel. 1-rn-long chi channel massive pyrrhotitc contains 333% ~ c . ' 0.5-m-long chip channel; contains 13.1% It. Grab sample of massive pyrrhotitc. (Sec map inset, fig. 3).

-1-rn-wide pyrite zone in skarn. Chalcopyrltequartz-scheelite vein.

Sulfide vein in intrusive rock (T m Contact mne between Tqm ?nd%dkh. Quartz vein in Tqm; 0.1-m-wde chip sample. Conccntratc sample below Tqni-SOsh contact creck ravine.

Random chi sdmplcs of sulfur-rich sh:rlc J. '. (contains S sh).

Ihrec 10-cm-thick ~(ratiform pyrite l2ycl.s with disscn1illalcd sphalcrite.

Chip samples of sulfur-rich shale (SOsh). Samc as above.

Analyses by M.R. Ashwell, I'.A. Iknjanlin, and M.K. I'olly using X-ray fluorescence and atomic-absorption spectrophotometry, I)G(;S Minerals laboratory. N1) = Not detcctcd. - = Not ;~n:~ly~cd.

Composite grab sarnpfes of sulfur- and iyon-rich shale (SBsh) locally contain anomalous lead, zinc, silver, and molybdenum (table 4, map no. 1, 7a,b, 9). The occurrence at map number 9 (table 4) Is coincident with a significant barium itnomiily reported in stream-sediment sample data by A;tmondt and others (1979)).

Selected shale samples were analyzed for Rock-Eval pyrolysis geochemistry. Four samples of shale (SOsh) contain 1,20 to 1.93 percent total organic carbon and 0.05 miIl/g free hydrocarbons. Three samples of Silurian shale (mSs) average only 0.37 percent total organic carbon and 0.03 mill/g free hydrocarbon. Hence, the shale (SOsh) has fair to good potential as a hycirocarbon source. However, all Paleozoic units are overmature and contain conodonts with maturation indexes that range from 3 to 5.

REFERENCES CITED

Aamondt, P.L., Jacobsen, S.I., and Hill, D.E., 1979, Uranium hydrogeochemical and stream sediment reconnaissance of the McGrath and Talkeetna NTMS Quadrangles, Alaska, including concentrations of 43 additional elements: Los Alamos, New Mexico, Los Alamos Scientific Laboratory Report LA-7350, 214 p., scale 1:250,000, 12 sheets.

Anderson, R.G., 1982, The relationship of the southeastern Selwyn plutonic suite to tungsten skarn mineralization in southeastern Selwyn Basin: <;cological Survey of Canada Paper 83-13,22 p.

Annstrong, A.K., Harris, A.G., Kced, B.L., and Carter, Claire, 1977, Paleozoic sedimentary rocks in the northwest part of Talkeetna Quadrangle, Alaska Range, Alaska, it2 Blean, K.M., ed., The U.S. Geolocical Survev - accomplisl~ments during 1976: U.S. Geological Survcy Information Circular 751-B. D. 61-62.

Blusson, S.L., 1976,'ielvqn Basin, Yukon and district of MacKenzic: Report of activities: Geological Survey of Canada Paper 764, p. 131-133.

Brooks, A.H., 1911, The MI. McKinley region, Alaska: U.S. C;eological Survey Professional Paper 70, p. 69-77. '

Uundtzen, T.K., and Gilbert, W.G., 1983, Outline of geolosy and mineral resources of upper Kuskokwim region, Alaska, bl Reed, K.M., and Mull, C.G., eds., Western Alaska geology and resource potential: Journal of the Alaska Geological Society, v. 3, p. 101-117.

Bundtzcn, T.K., Kline, J.T., and Clough, J.G., 1982, Preliminary geology of McGrath B-2 Quatirangle, Alaska: Alaska Division of Gcological and (icophysical Survcys Open-file Rcport 149, 22 p., scale 1:63,360, 1 shcct.

Bi~nclt;~cn, T.K., Kline, J.T., and Clough, J.G., Geologic map of McCirath B-2 Quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys unpublished report [in preparation].

Churkin, Michael, Jr., Reed, B.L., Carter, Claire, and Winklcr, (;.it., 1977, Lower Paleozoic gfaptolite section in Terra Cotta Mountains: U.S. Geological Survcy C~rcular 751-B, p. 37-38.

Dickinson, W.K., and Suc7ek, C.A., 1979, Piatc tectonics and sandstone compositions: American Association of Petroleum C;cologisls Bulletin, V. 63, no. 12, p. 2164-2182.

Eakins, G.R., Gilbert, W.G., and Bundtzen, T.K., 1978, Preliminary t)cdrock geology of west-central Lake Clark Quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Open-file Report 118, 15 p., scale 1:63,360,2 sheets.

Einaudi, M.T., and Burt, D.M., 1982, Pntroduction to terminology, classific:rtion, and composition of skaan deposits: Economic Geology, v. 77, no. 4, p. 745- 755

Fernald, A.T., 1960, (ieomorphology of upper Kuskokwin~ region, Alaska: U.S. Geological Survey Bulletin 1071-B, p. 191-275.

Folk, R.L., 1965, Petrology of sedimentary rocks: Austin, Tcxas, llcrn[)hills Book Store, 170 p.

Gabrielse, H., Blusson, S.L., and Roddick, J.A., 1973, Cicology of F la~ Kivcr, Glacier Lake, and Wriglcy Lake map areas: District of Mackcnzic and Yukon Territory: C;cological Survey of Canada Memoir 366, in portlolio (text, figs., and shcct at 1:250,000).

Gilbert, W.G., and Bundtzen, T.K., 1984, Stratigraphic relationship 1)ctwccn Dillingcr and Mystic tcrranes, western Alaska Range, Alaska tabs.]: Geological Society of America, Cordilleran Section, Abstracts with Progran~s, v. 16, no. 5, p. 286.

Gilbert, W.G., and Solic, D.N., 1983, Preliminary bedrock geology of McGrath A-3 Quadrangle, Alaska: Alaska Division of Geological and Geophysical Survcys Rcport of Investigations 83-7, scale 1:40,000, 1 shcct.

Gilbert, W.G., Solie, D.N., and Dickey, D.B., 1982, Preliminary geologic map of the Mc(;ralh B-3 Quadrangle, Alaska: Alaska Division of Geological anti Geophysical Surveys Open-file Report 148, scale 1:40,000, 1 shecl.

Hantzschel, Walter, 1975, Trace fossils and problcmaiics (2nd cd.), itt 'Trcatisc on invcrtcbrate paleontology, part W, supplement 1: (;eological Society of America, 269 p.

Herreid, Gordon, 1968, (;eulogical and geochemical investigaliorls southwcsi or Farewell, Alaska: Alaska Division of Geological and Geophysical Surveys C;eologic Kcport 26, 24 p., scale 1:75,000, 2 sheets.

Irvine, T.N., and Barnger, W.R.A., 1971, A guide to the chemical classification of common volcanic roc,ks: Canadian Journal of Earrh Sciences, V. 8, no. 6, p. 523-535.

Karlstrorn, T.N.V., 1904, Quaternary geology of tllc Kcnai lowland and 2liici:tl history of the Cook Inlet rcgion, Alaska: U.S. (;eological Survcy Prolessional Papcr 443,O p.

Klinc, J.T., and Bundtzen, T.K., 1986, Two glacial records from west-central Alaska, in Hamilton, T.D., Reed, K.M., and Thorson, eds., Glaciation in Alaska - The geologic record: Alaska C;eological Society, p. 123-150.

Jones, DL., Silberling, N.J., C;ilber~, W.G., and Coney, P.N., 1982, Character, distributiqn, and tectonic significance of accretionary terranes in central Alaska Range, Alaska: Journal of Geophysical Research, v. 87, no. B5, p. 3709-3717.

MacDonald, K.M., 1983, Stratigraphy of the Ordovician and Silurian 'Black Shale' unit of McGrath Quadrangle [abs.], in New developments in the Paleozoic geology of Alaska and the Yukon: Alaska Geological Society Symposium, Anchorage, 1983, Program and Abstracts, p. 19.

Nelson, S.W., and Reed, B.L., 1978, Slmrficial deposits map of the Talkeetna Quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF 870-J, scale 1:250,000, 1 sheet.

Kccd, B.L., and Elliott, R.L., 1968a, Lead, zinc, and silver deposits at Bowser Creek, Mc(;rath A-2 Quadrangle, Alaska: U.S. C;eological Survey Circular 559, 17 p.

Rccd, B.L., and Elliott, R.L., l%G8b, Geochemical anomalies and metallifcrous dcposits between. Windy Fork and Post River, southern Alaska: U.S. Geological Survey Circular 596, 22 p.

Kectf, EL., and Lanphere, MA., 1972, Generalized geologic map of the Alaska - Aleutian Range batholith showing potassium-argon ages of plulonic rocks: U.S. Geological Survey Miscellaneous Field Studies Map M F-372, scale 1 ,OOO,000, 2 sheets.

Reed, B.L., and Nelson, S.W., 1980, Geologic map of the Talkeetna Quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Map 1-1174, scale 1:250,000,1 sheet.

Smith, T.E., 1981, C;cology of the Clearwater Mountains, south-central Alaska: Alaska Division of (;cological and Geophysical Surveys Geologic Report 60, 72 p., scale 1:63,360,3 sheets.

Solie, D.N., Bundtzen, T.K., and Gilbert, W.G., 1982, Upper Cretaceous - Lower Tertiary volcanic rocks near Farewell, Alaska [abs.], itt Science in the North: Alaska Science Conference, 331-4 Fairbanks, 1982, Proceedings, American Association for the Advancement of Science, Arctic Division, p. 138.

'1'al)lc 5 . I'ossil idcnlificatio~is from Ihc McGrath A-2 Quailrangle, Alaska.

Unive~-si[y of

Map 1:icld Al;~ska no. I no. I Musci~m n o . 1 1)escription

1 82137'152 A-IS41 Grapiolites in calcarcous sandstone of mS1. 13.is1iograprrcs cf. 1'. drrbirrs, Wc~ll( )c.Li;~ n

(Middle Silurian) agc.

2 82M'390 - - Poorly preserved, ribbed arrypacearl braclliopod in

mSI; suggests Silurian lo early lare 1)cvoni;ln range.

3 6213'1'175 A-1864 Graptolites midway througll SOsh; mixed col lcct~o~l of D. clecorntrrs and C. bicorr~is zoncs, Middle Ordovocian age; 11. iiccornrrts zonc conlains Pfcroppnrs cf. 1'. c lqor~s ( f ) , C~~~)~o~yrcrj>cris

sc11nfcr.i (r), I)ic!\?r~ob?'npnrs sp. (r), ~;lo.r.ro~qr-~r~~rrrs

cf. G. ncnrirkrrs ( f ) , C;. cilinnrs (c), (;. cilii~rtrr cf. subsp. nr~rcr~rtnrir.~ (r), I'I~~~llogr.nj)rr~.c ci. I?

nrtgrrsrifolirrs elor~gntrrs (r). C. bicor.rri.c zonc contains C l i r r ~ n c o ~ n ~ ~ n r s bicorrric (c), I ) i c c I I ~ i ~ ~ ~ ~ r n cf. 1.). di\,nr.ic-crrrrs tli\~trr.i<.tr~rir (r). 11. cf. D. r1i~~nrico1rr.r snIopiol.ris (c).

4 8213'1'176A A-IS71 (iraptolitc collection made at lop o f SOsI1 0 111

stratigraphically I~clow ISI; contains Morro,~rrr~)/rrr

cf, M. j ) Inr~rr,~ (r), I : I ~ c I , l ~ ~ ~ l d o v e r i ~ ~ ~ l (I'IIC I ,:;I rly Silurian) agc.

5 82131'17-i A-I851 (:ollcction made midway t l~ ro i~gh SOsll sccrioll contains graptolitcs o f P. rerlrnclrlnnrs zonc, lalc Early Ordoviciall age. Incli~dcs I .so~~v~rj~/~i.r

/or.cipi/onrlis lnrrrs (r), BI-ociiiopq~rrrs c.rni,r?rri.~

( f), CI?~)IOS".~~~)IIIS sch(rfcri ( r), ~ ' / ~ ~ I l o ~ r ~ ( r j ) r r ~ . s r~obilic (c), (;4y)rogrc1/)111s cf. (;. ~rrr.rrr.o~k~~r~(rrr,r

(r), ~;(\y)/o~qr(r,)rir.~ sp. ( f ) , l ) ic I~/r~o~qr(r j~~r~.s sp. (1').

6 82131'173 A-1846 C:ollcclion 11l;lde ncar core of Sliecp (:~.cck ;1111icl11lc on west side of SOsh; corltains gr;iptolilcs of 011cogn1)nrs zonc, Early Ordovician age. Include Isopnprrts forci~)iforrrri.r (r), I?iilyrrrognprrrs cf. I ) . r~-dcj7ccvrrs (r), I)itiJPtropr~~rrrz sp. (r).

Table 5. (con.) Table 5. (con.)

University of AI:tska

P\luscuni no. Map

8

9

10

I I

12

13

I4

15

I6

18

University of Alaska

h4useurn no.

Map no. -

Field no.

Collection hi SOsh co~i~:iins ( . ' I ~ I ~ I ~ I c ~ ~ ~ I ~ ~ I ~ ~ I I ~ s 11ic.o111i~ (c), Olthopfll,r~lrrs aft'. 0. colcornrrrs s.1. (I-).

Gfy~)ro6myi11s sp. (f), I~iccIlopci~)ir~.r cf. I). d i ~ ~ ~ r i c n n r s snlopierasis (c), I)icrn/iogrnl)r~i.s'! sp.(r) of C. bicorrtb zone, Middlc 01-dovici:rli a;c.

Collection made in aiis of Shccp CI-cck anliclinc in SOsh; graptolitcs arc of 7: nppro.virl?clr~rs and '1'. fruricosrrs zones, earlicst Ordovician age, includcs 7'errnpnprrrs np~~r.o.vir?rnrrrs (r).

Plryllogmpnrs? sp. (r) in SOsh cast of axis of Sheep Creek anticline. Ilasc of SOsh contilins ~~.!I~I~I,w/)/II \ cf. 'I:

q~rndribrncltinms (r), lkrrly Ordivicia~i age.

<iraptolitc collcction from SOsh 11c;ir IS1 contact; contains poorly prcscrvcd diplograplids inclr~ding Clirttoco~oj)rris? and C;!\g>roh~tlprrrs?, RZiJdlc or I.;llc Ordovician agc.

Upscction fro111 map no. 18 in S O ~ I I ; cont:iI11\ Glossosnt,~rrs sp. ( I ) l ' / l o r ~ i . sp. (r).

CIi l7 l~ lco~o~~f1is sp. (c), ~ ; ~ \ ~ ) r o , ~ r f r ~ ) l l ~ ~ sp. (0, Uidyr?~ogfl[)rris:' sp. (r) of I). iic~cornnr.~ zone. C ; I I . I Y Middlc Ordovician agc. Graptolitc collection from niSI contains

I'risriopoprrrs cf. 1'. dlrbirrr (a), rc~iolirid (r), Wcnlockian (Middlc Silurian) agc. I:tagmcnts of l ) icI~r~ro~r.o~~r~rs? sp. indictitcs I:;rrly to

Middlc Ordovicii~~l age.

<;raptolilc collection from IS1 contains Qyrogmyrrrs cc~irrifr~g~is, late Ihr ly Si1uri;in agc. Upper portion o f SOsh scctiorl yiclcls I'/it/io,yr(~f)fi/.t

sp. (r), M ~ I I O ~ ~ I I I . I At. ( > . V ~ ~ I I I I S (I-), M O ~ l O b ~ ~ ~ ~ l l ~ . s sp. (f), l.l;~~ldovcri:tn (Ilaily Sili11-1:11,) Scree slopc in SOsh contains Cyr.rog~.crprrrs sp. (r),

Dicellopl~trrs? sp., Middlc 0rdovici;jn age. age.

(:ollcction in c;arl)on;~tc layer above IS1 yiclilb Aforio,m.opnrs aff. hl. priodo~r, I'ilrly to hlidtilc Silurian age.

1'r.isriogi.npnrs dubius, Wcnlockian (hqitldlc Silurian) age.

Scrcc slope in SOsh contains unding~loslic diplogr;rptid graptolitcs, Middlc o r 1,;rlc 0rdovici;in age.

mSI contains comprcsscd orllioconc n;rr~r~l(>id hhcllb. probable Silurian age.

0 C l s contains abilndant branching tracc losh~ls or1 shale partings si11iil;lr to 7hnlnssi1ioidcc. and ?hn.o~.nr~gcn from hasinal dcposirs in Aus~t. ;~l i :~. I <uropc, and Nol-1 I) Amc~.ic;l.

SOsh contains graptolilcs (.'Ii~irflco~r~r~)r~is c.rrrr(lirrrrs (c), Glypro~~ol)rr~s? sp. ( r), 01?hog'c7l)nrs ;I f f . 0. c(~lc(>rf~t~rs s.1. (r), I~i~iy~?~ogrn~~rrrs ' ! sp. (r) or C. I I I ~ I I ~ ~ ~ C ~ I I S zone, late Middlc Ortlovician age.

I,owcsl c;~~-l)o~l;cccous al~;llc Ilol.izo11 in SObIi )iclil> 1i.rr.opnptlrs cf. 7: nly)rn.rb?rirt~rs? and I)ic!~~iro;t.rr~)/r~~ ltrnrs ncq11q1i.c o f 7: /r.rririco.sic j-o~lc, I ~ : ; I I ~ ~ J Ordo\~ici;ili age.

Small limcstonc unit 20 nl al>ovc IS1 contains Cqropnprrrs sp. (r), Mo~rogrnpcrls sp. aff. At. fil7?rrrs (c), M. sp. (pr.ioiIo~7 type) (c), Mo~ro~Ii~~rc~ci ,s sp., carlicst Wenlochin (Middle Siluvirtn) ;~gc.

Table 5. (con.)

Univcl-sity of I I:icld

Muscum no. 1)escription

26 li2MK18211 A-lS76a-1) SOsh yields Gl~sro~?.f lp~l~s cj: G. cilinrrrs (f), Anlple.~ogrnprrrs fnllav (r), O,?Aogrnyirrs cnlcnrnr~rs ncirrtrs (r), Clir~~ncopnj>rrrs sp. (r), Q~.rlto~?.nprra cnlcnrnrrrs oclrrrrs (r), Cli111ncopr7j)rrrs sp. (r), and Glyj~fogrnj~rus sp. Dic!\7?1ob~nl,nrs? sp. (r) of C bicorrtis zonc, Middlc Ordovician age.

1 7 S1~1KI03 A-1910 (:arbonalc unit in mSs yiclcls Aior~o~yrvrj)nrs of priotlar~ type; Middlc Silurian(?) agc.

I 28 1213'1172 . - Cli~~~nropnj~ri , . ( bicorrtir, Middlc Ordovician agc.

20 811<1'.192 A-1901 SOsh yiclds 7i.rrogroj)rlrs i/r~criirih~~trchitr~r~~ (c), Early Ordovician age.

30 82hIA320 A- 11.13 SOsh yiclds (.'li~~rncog~oprrrs bicorrii.~, Middlc Ordovician age.

31 8113'1'321 . . Uppcrmost part of SOsli bclow IS1 yiclds Mor~o,m.n~>rrlssj>irnlis, Early Silurian agc.

32 82hlS1.61 . - Kls yiclds Ir?occr.n~~l:rs sp. wilh ribbing similar to I~~occrnr?rlrs j)clri/or.r,~is pocAinlny~~ct~ of 141rIy Crctaccous (Ilauterivian) agc.

33 SlhlAi00 . - . ~

Ihnlnssir~oitlc~s(?) Ir;~cc fossil; very similar to map no. 23.

1:ossils collcclcd I)y 'I'.K. 13und1zc1l (13'1'), K.M. Mac1)onald (MI;), M.S. 1,ockwood (MSL), and M I ) . Allxlncsc (MA). (i~.aplolitc identifications hy Claire Carter, U.S. Geological Sutvcy. I'clccypod. (m:~p 110. 32) idcnlilicd by .l.W. Millcr, U.S. <;cological Survcy. Othcr nicgarossil cr;llninalions I>y 11.13. l3lodgctt, Orcgon Slalc University. Trace-fossil idcntific;ltioiis I>y I3undtzcn aflcr I la~itzsclicl (1975). Most fossil collcciions arc archived ai tlic University of Alaska M~~scitrii i l l 1:. ' . ' 1 1 1 t~111ks.

(:I) :: :11>1111<1;1111 (I') = I ' ~ c c ~ " c " I (c) = C0I11111011