UNITED STATES DEPARTmNT OF THE INTERIOR GEOLOGICAL SURVEY

RECONNAISSANCE 6EOLOGIC MAP AND GEOCHRQNDLOGY , TALKEETNA MOUNTAINS QUADRANGLE, NORTHERN PART OF ANCHORAGE QUAORANGLE, AND SOUTHYEST CORNER OF HEALY QUADRANGLE, AWW

OPEN-FILE REPORT 78-558k.A

This report is prel lminary and has not been edited or viewed for canf'emi ty wi th -01 ogi ca t Survey standards and nomncl atvre Men1 e Park, Cal Iforni a 1978 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY

RECONNAISSANCE GEOLOGIC MAP AND GEOCHRONOLOGY , TALKEmJA MOUNTAINS QUADRANGLE, NORTHERN PART Of ANCHORAGE 'QUADRANGLE, AND SOUTHWEST

CORNER OF HEALY QUADRANGLE, ' ALASKA BY ~blaCsejtey, Jr, , W. H. Nelson, Q, L. Jones, N, J. Sll bert ing, R. M. Dean, M, S. Morris, M, A, L~nphere, J. G, SrnJth, and M. L. Sllbemn

Description of map unf ts, Structure, Tectonics, Reference 1 1st, and tables to accompany Ilpen-file Report

This report Is prelimf nary and has not been edited or reviewed for conformity wf th Geological Survey standards and namenc7 ature DESCRIPTION OF MAP UNITS

SEDIMENTARY AND VOLCANIC ROCKS

SURFICIAL DEPOSITS, UNDI FFEREHTIATED (Quaternary) --GI acia1 and alluvfal deposits, chiefly unconsot idated gravel, sand, and clay. VOLCANIC ROCKS, UNOIFFEREfCTIATED (Paleocene to Mlocene , uppemst part may be as young as Pleistocene)--Over 1,500-m-thick sequence

of fel sic to mfic subaerial volcanic rocks and related shallow intrusives. Lower part of sequence consists of small stocks, irregular dikes, lenticular flows, and thf ck layers of pym- clastfc rocks; made up domfnantly of medlum- to fine-gralned, general 1y medi urn-gray quartz latite, rhyal 4 te, and l atite. A few dikes and intercalated flows of brown andesfte are also pres- ent. Rocks of the 1ower part of the sequence, occurring mstly in the upper Tal keetna River area, are interpreted to be vent facies deposf ts and near vent deposits of stratovolcanos. The upper part of the sequence consists of gently dipping brown andem stte and basalt flows interlayered wSth mSnor amounts of tuffs. A few lenses of fluviatjle conglomerate are also present. Locally, at Yellonjacket Creek for instance, the feeder dikes of the mafjc flows make up more than half the vol me of the underlying country rocks, According to E. M. Nckvett, Jr. (oral cornnun., 1975), the andesite and basalt flows are 1 ithalogically identi- cal to the basal andesites aP the Wrangell Lava in eastern Alaska. Contact between the dominantly felsic lower part and mafic upper part of the sequence is gradational through intertongui ing of the two rock types. The three samples for potassium- argon age determinations (map numbers 7, 8, 13 in table I), indicating Paleocene and Eocene ages, were obtained from ande- sfte flows near the middle of thfs sequence. Tim HY PABYSSAL MAFIC INTRUSIVES (Pal eocene to Miocene, youngest rocks may be Pleistocene)--$ma1 1 stocks and f rregular di kes of dto- rf te porphyry, diabase, and basalt, They probably are the sub- volcanfc equivalents of the andesSte and basalt flows of unlt Tv. HYPABYSSAL FELSIC INTRUSIVES (Pal eocene to Miocene, some rocks may be as young as Pleistocene)--Small stocks and irregular dikes of rhyolfte, quartz latite, and latite. Lithologically, they are identfcal to, and thus probably carrelative with the felsic subrolcanic rocks of unit TY.

TSADAKA (Miocene) AND WISHBONE (Pal eocene and Eocene) FORMATIONS, UNDIVIDED--Tsadaka Formation, occurring only at Wishbone Hi1 1 , consists of cobble-boulder conglomerate wlth thin interbeds of sandstone, $51 tstone; and shale; abaut 200 m thick. The Wish- bone Fomtfm, which unconfombly underlies the Tsadaka, corn- prf ses we1 1-indurated fl uviatile conglomerate with thf ck inter- beds of sandstone, siltstone, and claystone; about 600 to 900 m thick (Dettemn and others, 1976; Barnes, 1962). The present

map unit also includes over 150 m of fluvf atile conglamerate and coaly sandstone (unit Tf of Grantr, 19603, b) in the east- ern Tal keetna Mountains. Tc CHICKALOON FORMATION (Paleocene)--We1 1-Indurated, continental, domfnantly fluviatile sequence of massive feldspathfc sandstone, siltstone, claystone, and conglomrate,contnining numerous beds

of bitminous coa 1 ; over 1 ,500 m thick (Barnes, 1 962). Tsu SEDIMENTARY ROCKS, UNDIFFERENTIATED (Tertiary)--F1 uvlatfle con- glomerate, sandstone, and claystone wlth a few thin fnterbeds of 1 ignitf c coal. Lithological ly, these rocks look simf 1ar to

the TertIary sedimentary rocks of the southern Tal keetna Moun- tafns, but lack of fossil evidence does not pemSt move defini- tfve correlatjon. The largest exposure of these rocks is along Watana Creek, and, according to SmIth (1974a), the sequence is over 160m thick. L.lthologically, it resembles the Paleocene Chf ekaloon Formation of the Matanus ka Val 1 ey.

PLUTONIC AND METAMORPHIC ROCKS

Tgd TERTIARY GRANODIORITE (Eocene)--Conta4ns hornblende and biotfte, This granodiorite is part of a small pluton along the northern edge of the map area. Turner and Smith (1974) report an Eocene age for this pluton, determined by the potassium-argon method on bietite (48.8y. 5 m.y. 1 and on hornblende (44.83.3 m.y. ) fm a sample just north of the present map area. Thgd BIOTITE-HORNBLENDE GRANODIORITE {Pal eocene , in part may be Eocene)-- Rocks of this unit occur in one large and several smaller, , poorly exposed pl utons in the western and northern Tal keetna Mountains. A1 1 of the plutons were forcibly intruded in the eplzone of Buddington (1 959). Granodiori te is the dominant rock, but locally it grades into adamel 1i te (= granite with plagieclase and a1 kal i feldspar in approxfmately equal propor- tions), tonalite, and quartz diorite. A11 these rocks are medim to dark gray, medium grained, generally strvctureless, and have granitic to seriate textures. In all of them, horn- blende is the chief maffc mineral. Biotite- and hornblende- rfch xenoliths of reconstituted country rock are corn; in every pluton. The I jthologf c compositions and available age determinations (see table 1) indicate that these granitic rocks are the plutonic equivalents of some of the Pel sic rocks in the lower portion of unit Tv. mgd BIOf IT€ GRANODIORITE (Paleocene, in part may be Eocene) --Bioti te granodiori te and adamel 1 Ste in approximately equal proportions. Blotfte fs the chief mafic mfneral, hornblende is occas.lonally pwsent. Color is 1 ight to medfum gray, grain sire is from dimto coarse, texture Is granitic to serfate. Very faint flow structures have developed only locally. These rocks occur in shal lw, forclbly emplaced epf zonal plutons in the north- western Tal keetna Mountains. Apl jtic and pegmatt tic dlkes are comn in all the plutbns. Just north of the map area, these plulonic rocks grade into fel sf c volcanic rocks. Potassium- argon age determinations (see table 1) f ndicate that the bio-

tite granodiorite and adamellite of the present unit are essen-

tially of the same age as the biotlte-hornblende granodiorf te (unit Thgd), Thus, the rocks of these two units, in view of thef r spatial proximity, probably are the products of dlfferen- tiatfon of the same parent magma, either in situ or at some deeper levels in the Earth's crust, The biotite granodforf te

intrusive5 are a1 so considered to be the plutonic equivalents of some of the felsic volcanic rocks in the lower portion of unlt Tv. Tsmg SCHIST, MIGMATITE, AND GRANITE (Paleocene fntrusive and neetamarm phic ages )--Undi fferentiated terrane of andal usite and (or) silljmanite-bearing pelitic schist, llt-par-lit type migmatite, and small granitic bodies with moderately to well-developed flow foliation. These rocks occur in approximately equal pro- portions, and the contacts between them are general 1y gradational, as .Is the contact between the schPst and its unmetamrphased pel Stic rock equivalents {unit kg) outside the present map unit. The pelitSc schist is medium to dark gray, medium grained,

has we1 I-developed but wavy fa1 iation, and contains 1 i t-par-1 it

type granitic injections In greatly varying amounts. Rock-

forming mineral s of the schjst include biotite (pl eochroism

Nz = dark reddish brown, Nx = pale brown), quartz, plagiaclase, minor K-feldspar, muscovite, garnet, and sill imnite which locally coexists with andal usite.

The 1 it-par-1 it type grani tfc Injections within the sehi st are medf urn gray, medium grained, and consist of feldspar, quartz,

and biatt te.

The rocks of the small, granitic bodies range in composi-

tion from bfotite adamellite to bfotfte-hornblende granadiorite. They are medlum gray and medium grafned, generally have grani- tic textures, and, in addition to the flow foliation, locally display flow banding of felsic and mafjc minerals. These grani- tic bodies appear to be the source of the 1 it-par-1 it intrusions. The proximity of the schfst to the small granitfc bodies, the occurrence of the l i t-par-7 f t f nSections, and the presence of andal us4 te In the schist indicate that the schist is the resul t of contact metamorphi sm. Perhaps thi s metamorphism took place in the roof zone of a large pluton, the cupolas of which may be the small granitfc bodies. TKt TONALITE (Upper Cretaceous and Lower Paleucene) --Domi nantl y bdo t ite- hornblende tonal f te, 1ocal ly grades Snto quartz diorlte, The tonatite is medium gray, coarse to medium grained, has a grani- tic texture and a fairly we1 1-developed prfmary Pol iation, It occurs in a large, possibly composite, bath01 ith, approximately 75 to 61 my. old (see table I), which was emplaced in the epi-

zone and mesozone of Buddington (1959). The tanal ite is described fn more detail in Csejtey (1974).

6 TKa ADAMELLITE (Upper Cretaceous and Lower Paleocene)--Occurs in a

large epizonal pluton in the southwestern part of the map area. The dominant rock type is adamellite but locally includes gran-

odfori ts. Blot1 te is the chief MIIc mineral , museovi te occurs in subordfnate amounts. The typical adamelltte Is medium to 1 ight gray, medium to coarse grained, its texture ranges fm granitic to seriate. The adantellfte appears to be fntrusive into the tonal ite (unit TKt) , but concordant potassium-argon ages an one sample (map no. 24, table 1) Sndlcate the adamell ite to be essentjally the same age as the tonalfte. These rocks apparently are comaptic. TKgr GRANITIC ROCKS3 UNDIVIDED (Cretaceous and (or) Tertiary )--These rocks of uncertafn age occur in four smaller epjtonal pl utons of granodforite and tonal ite. Their color is medium to dark gray, gnSn size is medium, texture is granf tic, Mafic minerals are hbmbf ende and (or) biotite, The largest of these plutons, in the northeast corner of the map area, is reported by Smith and others (I 975) to be of Cretaceous age. TK1 g LEUCOMBBRO (Cretaceous and (or) Tertiary ) --%a1 I , poor1 y exposed intrusive of uncertain age in west-central part of map area, essentially consisting of pl agfoclase (around An70 and about 80 percent of volume), and pale-green hornblende. The leuco- gabbn, is medium to light gray, coarse to medium grained, with a granitic to serfate texture. 7 SOUTHEASTERN TALKEETNA 140UNTAINS Sedimentary and volcanlc rocks Kar ARKOSE RIDGE FORMATION (Cretaceous)--Arkosic sandstone* conglom- erate, graywac ke, s ll tstone , and s ha 1 e (Detterman and others, 1976; Grantz and Molfe, 1961). Clastic components. consist chlefly of granitf c and metamorphic rock fragments, quartz, Pel dspar, and biotlte, indfcating a dominantly plutonic and, to a lesser extent, metamorphic provenance (G. R, Winkler, oral comnun., 1977). Mumeraus plant fragments suggest a dominantly terrestrial origin. Recent fie1 d and petrographic studies Fsejtey and others, 1977) f ndicate that this fomatfon is of

Cretaceous age. A pre-Tertiary age is also indicated by a potassium-argon age determination on biotfte (map no. 37, table I). The biotlte was separated from a sample of g~aywackewith secondary bf otite, obtaf ned from near the tonal 1 te pluton (unf t TKt). The formation rests unc6nfomably an Jurassf c granf tic and metamorphic rocks and Ss as much as 700 m thick. In this report the Arkose Ridge is considered to be a dominantly non- marine facf es of the Cretaceous Matanus ka Formation.

MATANUSKA FORMATION (tower and Upper Cretaceous )--Wet 1-Z ndura ted shale, sf1tstonc, sandstone, graywacke, with subordinate eon- glomerate interbeds; occurs along the southern edge of the map area, mostly in the Matanuska Valley. These rocks, having a total thickness in excess of 1,200 m, are general ly dark gray and thinly bedded, and for the most part were deposited in a marine environment of moderate to shallow depths, Some of the sandstone beds contain fragmentary plant remains. Age of the formation ranges from Maestrichtfan at the top to A1 bian at the base (Grantz, 19641, The fornation rests wfth a pronounced angular unconf'omfty on Lower Cretaceous and 01 der strata. In part, the Matanuska formation correlates with the Kennicott, the Shulze, the Chitltna, and the MacCall Ridge Fomtfons of the southern Wrangell Mountains (Jones, 1967). SEDIMENTARY ROCKS, UHDIV I OED (Lower Cretaceous)--A shall ow water marine sequence of thf nly bedded cal careous sandstone, si1 tstone, claystone, minor conglomerate, and thfck-bedded to massfve elastic 1 imestone; interpreted as a continental shelf-type deposit; over 100 m thick, These strata occur in the south- eastern Tal keetna Mountains, and they have been previously mapped

and dated by Grantz (1960a, b). The present undivided unit includes Grants' units Ks, Kc, and the Nelchina Limestone. The contact between these strata and the under1ying Jurassic Naknek Fomtfon (unlt Jn) f s a slightly angular uneonformity. The Nelchina Limestone correlates with the Berg Creek Formation of the southern Wrangell Mountafns (E. M. MacKevett, Jr., oral comnun. , 1977). WKNEK FORMATION (Upper Jurassic)--Shal low water marine, thfn to

thick bedded, intercalated strata of foss5liferous gray siltstone, shale, sandstone, and conglomerate; over 1,400 m

thick, Previously mapped and dated by Grantz (1960a, b). The

Naknsk Fomtlon is restricted to the southeastern Tal keetna

Mountains, lacks any contemporaneous vol canf c materf a?, and appears to have been deposited in a continental shelf environ- ment, Its contact with the underlying Chinjtna Formation is a very sl ightly angular unconfomity. The Na knek correlates with the Root Glacfer Formatjon of the southern Wrangell Mountains (E. M. MacKevett, Jr,, oral comnun., 1977). CHINITNA FORMATION (Upper Jurassic) AND TUXEDNI GROUP (Middl e

Jurassic) , UNDIVIDED--The Chlnitna Formati on cons1 sts of a shal- low marine, Intercalated sequence of dark-gray shale, sjltstone, and subordfnate graywacke; contains numerous large limestone

concretions; it is as much as 600 m thick. The Tuxednf Group

unconfombly under1 ies the Chin1tna, and consists of shall ow marine, we1 1-indurated, thinly to thickly bedded graywacke, sandstone, and massive conglomerate in Its lower part, and thinly to thickly bedded dark siltstone and shale in its upper part. The Tuxedni is about 300 to 400 m thick. Both the Chinitna and Tuxedni have been prevlously mapped and dated by Grantz (1960a, b; 3961a, b), by Grantz and othe~s(19631, and by Petterm man and others (1976). Both formations occur in the southeast- ern part of the map area, are devoid of coeval volcanic material , and are interpreted to have been deposited in a continental shelf environment. The contact between the Tuxedni

Group and the underlying Tal keetna Formation (unit Jtk) is a major angular unconformity. The Chinitna and Tuxedn i are partly correlative with the Mizina Mountain Formation of the southern Wrangell Mountains (E, M. MacKevett, Jr. , oral cormun. , 1977). Jtk TALKEETNA FORMATI ON (Lower Jurassic) --Andes itic fl ows , fl ow brec- cia, tuff, and agglomerate; subordf nate interbeds of sandstone, siltstone, and 1lmestone (mapped separately as unit J7 s] , espe- cially in upper part of the formatfan. A dominantly shallow marine sequence, about 1,000 to 2,000 m thick (Grantz 1960a, b; 1961a, b; Grantz and others, 1963; Dettemn and others, 1976).

This fomtfon occurs only in the southeastern half of the mapped

area and fts base is nowhere exposed. The occurrence of marble I (units Jmb and Mr) within the pl utenic and metamorphic rocks just northwest of the Talkeetna Formation outcrop area suggests that the formation is underlain by valcanogenfc rocks of Triassic (unit Wv) and of Paleozoic age (unit Pzv).

LIMESTONE (Lower Jurassic)--Light- to dark-gray, f f ne- to medi MI- gralned unfossilfferous 1Smestane; near granitic rocks recry- stallized to medium- to coarse-grained marble. Forms discontln- uous lenticular bodies, as much as 30 m thick, within Tal keetna Formatjon. Plutonic and metamorphic rocks SERPENTTNIZED ULTRAMAFLC ROCKS (Lower and (or) Upper Cretaceous)-- These rocks occur in small, tectonlcally emplaced, discordant bodies (protrusions1 within the probably Lower to Mf ddl e Juras- sic pel itic schist (unit Jps) near Willow Creek. They are

medium greenish gray to black in color, and are composed of aphani tic masses of serpenttne, talc, minor amounts of actinoli tea tremol ite, chlorite, and opaque minerals. Re1 ict textures were nowhere observed, and a1 1 these bodies are strongly sheared.

SemiquantitatSve spectrographf c analyses indicate chromi urn con-

tents to be between 1,000 and 5,000 ppm and nickel between 1,000

and 2,000 ppm (analyses by D. F. Sjems and J. M. Motooka, 1973).

Fire assay analyses of ten samples show both platlnum and pat- ladium contents to range from 0.0 ppm to 0.030 ppm (analyses

by R. R. Carlson, 1973). However, the average platinum to pal-

ladium ratfo is only about three to one. PotassSm-argon age determinations on actinol ite-tremol S te from two samples yielded early Late Cretaceous minimum ages (map nos. 32, 36, tab1e 1 ) . These minimum ages coincide in time wlth a middle to Late Cre- taceous period of Intense, a1pine-type orogen jc deformation (see Structure and Tectonics sections) of the Taf keetna Moun-

talns region. Thus, the serpentjnite bodles, whose original age is unknown, are assumed to have been emplaced during this orogeny . 12 Jtr TRONDHJDIITE (Upper Jurass ic)--Forms a di scordant , northeast- trending, el ongate, epizonal pl uton of fairly uniform 'I 1tho1 ogy in the central fa7 keetna Mountains. Large portlons of the pl uton have been sheared and saussuritized, Typically, the trondhjemite is 1 ight gray, medium to coarse grained with a granitic texture. A faint flow foliation is locally developed. Major rock form-

ing mineral s are plagiocl ase (01 igocl ase to sodlc andesf ne) , quartz, K-feldspar (between 0 to 10 percent of volume), and

biotite, wt th subordinate amounts of muscovite, and opaque min-

erals. Color fndex ranges from 3 to 9. Average oxide pertent- ages, by welght, of seven trondhjemlte analyses are: Sf02 - 70.30, A1203 - 16.74, 50 - 1.27, NapO - 5.07, Ca0 - 3.33. Potasslum-argon age determinations (map nos. 21, 22, 26, 31,

table I) from the southern part of the pluton show considerable variation fn age, which is attributed to resetting. However, three age detem-inations fram the northern half of the pluton

(map nos. 10, 11, 14, table I), including concordant ages on a mfneral pa5 r of muscovite and biatite, yielded very simf lar num- bers indicating the empl acoment of the troridbjemi te pl uton between 145 to 150 m.y. ago. The trandhjemlte is the youngest member of a group of Jurassfc plutonic and metamrphfc rocks in the Tal keetna Mountains.

GRANODIORITE (Middle to Upper JurassCc)--Dominantly granodiorite but Includes minor amounts of tonal ite and quartz diarite. these epfzonal plutonic rocks, underlying considerable areas Sn the central and eastern Tal keetna Mountains, were probably empl aced as mu1 tiple intrusion of consanguineous magmas. They

are medf urn ta dark gray, medi um graf ned, and f n undefomed rocks the texture is granj tic. Hafic minerals are hornblende and blotf te f n various proportions. A1 dng the northwestern border of its exposure area, the granodforite and related rocks have been catacl astical 1y defomd, resul ting in a pronounced north- east-trending secondary foliation and, to a lesser degree, Ifneation. The width of the deformed zone varies from about

2 km to 25 km. Isotopic age determinations (map numbers 15-17,

27, tables 1, 2) from four separate localfties indicate that emplacement, pmbabf y mu1 tip1 e intrusions, took place approxi - nately 150 and 175 my. ago. While the Upper Jurassic trond- hjemi te intrudes the granodiorf te, the granodiori te 1tsel f f ntrudcs the Tal keetna Format ion of Lower Jurassic age (Grantz and others, 1963). MIGMATITIC BORDER ZONE OF GRANODIORITE (Middle to Upper Jurassic)-- Forms a temane of poor1y exposed, intrf catel y 1ntemi xed con- tact schist, amphi bol f te, and smal I di ker and velnlets of gran- odiorjte; all of these twck types occur in appmxfmately equal proportions.

The contact schist is dark to medfum gray, medium grained;

rock-forming mineral s are quartz, bf oti te, and subordinate pl agioclase.

14 The amphlbolite is dark gray, medium grained, and consists

of hombl ende and plagiocl ase; megascopic schistosi ty is sel dom conspicuous. The granodlorite is the same as that of unit Jgd; most of the vein1 ets have been f ntruded along fol iatlon planes. The metamorphic rocks of this unit were probably derived from either the Tat keetna Formation (unit Jtk) or from the upper Paleozoic volcanogenic sequence (untt Pzv), or possibly in part from the Upper TriassSc basal tic sequence (unit TRv). Jmrb MARBLE (Middle to Upper Jurassf c metamorphic age)--Contact meta- morphosed marble bed more than 40 m thlck wfthin mdgmatitfc border zone (unit Jgdm). The marble is poorly exposed and occurs only along John Creek, a tributary of upper Kosina Creek. The rock 3s white, coarse to medium grained, and contains num- erous porphyrebl astic crystal s of andradite garnet and diopside. The marble was derived from a 1 (mestone bed, probably within the upper Paleozoic volcanogenic sequence (unlt Pzv) or ps- sSbly within the Upper Triassic basaltic sequence (unit 7R v}. QUARTZ DIORITE {Lower to Middle Jurassf c)--Epironal intrusf ve in

the southern Tal keetna Mobntaf ns. DomSnantly quartz diori te but a1 so includes diorite and tonal ite, Large portions of this rack have been sheared and intensively altered. The fresh quartz diorf te is medium to dark greenish gray, medium to coarse

gralned, and has a granitic texture. Rock-forming minerals are

pl agiocl ase (andesine ) , quartz, hornblende, subordinate biotite and K-fel dspar. Where -a1tered, the quartz diorite consists of

mineral aggregates of epidote, chlorite, and serici te, as we1 1 as some -ants of the primary mineral s. The age of the

quartz diorite is probably 1ate Early Jurassic or early Middle Jurassic because it intrudes the Talkeetna Formation and is intruded by the Middle to Upper Jurassic granodiorite of unit Jgd . Jam AMPHIBOLITE (Lower to Middle Jurassic metamorphic age)--Forms a

metamorphic terrane consisting dominantly of amphi bol ite but

includes subordinate amounts of greenschi st and Sol iated diori te. This metamorphic terrane also includes several interbeds of coarsely crystal 3 f ne marble which are mapped and described separ-

ately (unit Jmb).

The amphibofite is generally dark greenZsh gray, medium to coarse grained, but fine-grained varietfes also occur. Foli- ation and lineation are generally poorly developed, and segre- gatfon layering is rare. Major rock-fomlng minerals ape, in approximately equal proportions, anhedral to euhedral hombl ende

(Z = dart green to brownish green, occasionally bf uf sh green) and an hedral , general 1y twlnned pl agiocl ase ranging from 1 abra- dorite ta cal cfc andesine, Accessory mfneral s are quartz,

garnet, sphcne, apatite, opaques, occasional epidote, and, f n

some of the rocks, shreds of biotite. The greenschist Is dark greenlsh gray, ffne to medium grained, with a moderately we1 1-developed schi stosi ty. Major minerals are actinol ite, untwinned pl agioclase (probably al bite) , epidote, chlorite, quartz, and opaques. Some of the actinol ite-1 ike amphibole may actually be aluminous hornblende, thus some of these rocks may be transi tionat to amphibolite. ' The foliated diorite is very similar to the amphibolite in appearance. It is dark greenish gray, medium to coarse grained, with a generally we1 1-developed shear fol iation. A remnant granitic texture is always discernible in thin section. Rock- forming mineral s are hornbl ende, twinned and occasional 1y zoned plagiocl ase (andesine to sodic 1 abradorf te), with subordf nate amounts of chlorite and epidote, minor quartz and biotite, and opaques. A71 of the above rocks, as well as the quartz diorite of unit Jqd, apparently are the earliest products of a Jurassic plutonic and metamorphic event which appears to have started in the Tal keetna Mountains in late Early Jurassic time after the deposition of the Tal keetna Formation (unit Jtk). A pstas- sium-argon age determination on hornblende of a diorite or amphibolfte sample (map no, 5, table 1) from the northeast part of the map area yZel ded an age of 776.6 my. (Turner and Smith, 1974), suggesting an Early to MIddle Jurassdc age fir the amphf - bol ite and associated rocks. The quartz dforlte of unit Jqd in the southern Tal keetna Mountains is probably correlative with the sheared df orite of the amphf bol ite terrane. The metamorphic rocks of the amphlbolf te terrane probably were derived from any or a1 l of the following dominantly basic

volcanic formations: Talkeetna Formation (unit Jtk), upper Pal eotoic vol canogen ic sequence (unl t Pzv) , or the Upper Trjas- sic basaltic sequence (unit R v) , The pods of greenschist, fntemalated with the amphibolite, suggest that the metamr- phi sm in the amphibol i te terrane was not of unf form intensity. Jmb MARBLE (Lower to Middl e Jurassic metamorphic age)--Whi te, medi m-

to coarse-grained marble. It occurs f n massive interbeds, as much as 30 m thick, within the amphiboljte terrane of unit Jam. The marble contains subordinate amounts of garnet and diopsi de. Its parent rock was a limestone bed, probably within the Tal- keetna Fornation (unit Jtk) or wfthin the upper Paleozoic vol- eanbgenjc sequence (unit Pzv), or, least likely, within the Upper Triassf c basal tic sequence (unit TI v) . Jmf AMPHIBOLIYE AND QUARn DIORITE (Lower to Middle Jurassic meta- mrphic and plutonfc ages)--Forms a terrane of f ntri cate7y intermixed amphi bol i te and quartz diorite in about equal amounts in the southern Talkeetna Mountains,

The amphlbol jte is very $$milar to the amphi bol ite of unit Jam, thus the two amphSbollter are considered ta be correlative, and no description is given here. One difference is that segre- tion layering of mafjc and felsic components is more prevalent

in the amphi bol ite of unit hi. A thin wedge of blotfte-quartz- feldspar gneiss, probably derSved from a nonvol canic elastic

18 Interbed, is intercalated with the amghibol ite along lower

Granite Creek (Dettemn and others, 1976; Travis Hudson, oral comnun. , 1 978). The quartz diorite is petrographically identical to the quartz dlorfte in adjacent unit Jqd (see rock description there), and the two rocks are considered to be corrcl ative. The quartz

diorite of the present unft is generally more a1 tered than that of unit Jqd.

GREENSTONE (Probably Lower to Middle Jurassic metamorphic age)-- The basSc metavolcanic rocks of this unit form small, isolated

hflls along the eastern edge of the map area near the Susitna

Ri~er. The typical greenstone is a dark greenish gray, fine

grained, generally structure7 ess rock. Original rock-forming mineral s were pyroxene, amphf bol e, and pl agioclase (andesine to Inbradorlte) which more or less altered to chlorite, epidote, serpentine, calcite, and minor ssrici te and quartz. The prox-

{mi ty of the amphibol ite terrane (unit Jam) strongly suggests that the metavo7canic greenstones of the present unit represent a low-grade faci es of the same metamorphism which produced the

amphi bol ite. The re1 ative position of the greenstone within the nortl~easterlystwctural trend of the Tal keetna Mountains suggests that the greenstone was probably derived f~omthe Talkeetna Formation (unit Jtk) or, possibly, from either the upper Pal eozoic vol canogenic sequence (unit Pzv) or the Upper Trf assic basal tic sequence (unit 1R v). 19 PELITIC MICA SCHIST (Probably Lower to Middl e Jurassic metamor-

phic age)--This rock occurs only in the southwestern comer of the map area near the headwaters of Willow Creek. The schist

is medium to dark gray, medium grained, wfth uniform lSthology throughout its exposure area. Its ubiquitous mineral consti - tuents are quartz, muscovite, a1 bite, chlorite, chlori tlzed crystals of garnet and subordinate biotite, Very thfn laminae of carbonaceous material occur sparsely. Small open folds and

crenul ations form an incipient slip cleavage at a 1 arge angle to the primary schistosi ty. Numerous thin veins and stringers of hydrothermal quartz occur throughout the schist, Detailed petrographic descriptions of the mica schlst are given in Ray

(1 954). The present mineralogy of the schist is indicative of the

greenschist metamorphic facies of Turner (1 968). However, St is probably retrograde from higher metamorphism, possj bly the amphi - bolfte facies. Evidence for this is the chleritfzed garnet and biotite crystals and the sparse mineral out1 ines consf sting of chlorite whf ch probably are pseudomorphs after hornbl ende. The age of the schist is imperfectly known, but, based on reglonat geol ogle f nterpretations, the primary metamorphism is

considered to be Early to Middle Jurasslc f n age. Thus, the schist and the amphibol ite of unit Jam are interpreted to be

the products of the same metamorphism. The retrograde metamor- phism is assumed to be of middle to Late Cretaceous in age and related to an alpfne-type orogeny in the Talkeetna Mountains at that time, However, the Late Cretaceous Arkose Rjdge Formation,

which lies unconformably on the schist, has not been affected by this retrograde metamorphism. The three potassium-argon age

detennfnatlons, measured on muscovite from the schist (map nos. 33-35, tabl e 1 ) , yiel ded obvf ovsl y wset Pal eocene ages. The parent rock of the schlst is unknown because no pelitic rocks of comparable thtckness (the schist is at least severaJ hundred meters thick) are known to occur in the pre-Middle Jurassic rocks 'of the Tal keetna Mountains.

Jpm~ PLUTONIC AND METAMORPHIC ROCKS, UNDIFFERENTIATED (Lower to Upper Jurassic plutonf c and metamorphic ages)--This unit consists of

an Intricately intemjxed mosaic of most of the previously dis- cussed Jurassic metamorphic and plutonic rocks (units Jtr, Jgd, Jgdm, Jqd, Jam, Jgs, and Jps) . Wf thf n the terrane of the pres- ent unit, the exposure area sf an individual rock type ?s not more than a few square kilometers. Two rack types, amphybolfte and sheared quartz diori te, comprise approxf matel y 60 percent of the terrane. Next in jmportance are sheared granodiorlte and associated migmtS tes. Subordinate amounts of pel itic mica

schist and greenstone also occur. Numerous apaphyses of trond- hjmite, as much as several meters thick, occur along the east-

ern edge of the terrane adjacent to the large trondhjemfte pluton (unit Jtr). All of these rocks are litholog~callyvery simflar to their correlative map units, and they will not be

described here. At two local ities, the sheared granodiori te

(unft Jgd) was mapped separately to show the proxfmity of

sheared Jurassic granitic rocks to the Late Cretaceous and early Pal eocene unsheared tonal ite (un It TKt ) .

NORTHWESTERN TALKEETNA MOUNTAINS AND UPPER CHVL ITNA RIVER AREA Sedimentary and volcanlc rocks; rocks of each column occur in separ- ate fault blocks. Central and northern Tal keetna Mountains Rv BASALTIC METAVOLCAHIC ROCKS (Upper Triassic)--This shallow water mrSne unit consists of amygdaloidal mtabasal t flows with very subordfnate amounts of thin interbeds of mtachert, argil 1 ite, metavol canicl astic rocks, and marbl e (Smf th and others, 1 975). Rocks of this unlt have been mapped only in the northeast por- tion of the map area. However, small blocks of the basal tic

rocks may occur within the complexly deformed late Paleoaofc

vol canogen f c sequence (un it Pzv) toward the southwest. The basal tlc rocks rest with angular unconfonnity on the late Paleo-

zoic volcanics (unlt Pzv); the top of the basalts is unexposed, The minimal thickness of the basaltic metavolcanic rocks is

800 m. The individual metabasal t flows are as much as 10 m thick, and, according to Smith and others (I9751, display col mar jointing and locally pf 1 low structures. The typjcal metabasal t is dark greenfsh gray, ffne grained, and generally contains numerous amygdules. Thin sections show the metabasal ts to consist of labrodorlte, augi te, and opaques in an intergranular or sub- ophl tic texture. Secondary mineral s are chlorite, epi dote, el tnoroisite, very subordinate a1 lanite, sericite, and possibly some kaolln. The amygdules consist of chlorfte, silfca, and xeolftes. The present mineralogy is probably the result af deuteric a1 teration and low-grade regional metamorphf sm which apparently did not reach the intensity of the greenschfst facies of Turner ( 1968). From a marble interbed In upper Walana Creek (1 ocal 1ty 1 , table 31, T. E. Smith (unpub. data, 1974) collected fossil speci- mens whfch were identifjed and interpreted by K. M. Nlehol s and N, J, Silberling to be Hafobia cf. H. synmetrfca Smith, indicat- ing a latest Karnian or early Norian age. Previously, Smith (1 974a) and SmI th and others (1 975) have correlated the basal tic metavoleanic rocks of the present unit wlth the Amphi theater Group of the central Alaska Range. Accordingly, the fossils collected by 1. E. Smith suggest that the Amphftheater Group

is younger than, and thus not correlative wt th the 7 ithological 1y very similar Nikolai Greenstone of pre-late Karn-lan age in eastern Alaska (Jones and others, 1977). Pzv BASALT1C TO ANDES IfIC METAVOLCANOGEN IC ROCKS ( Pmnsyl van inn(? ) and Early Permian)--Rocks of this unit occur in a northeast- trendf ng be1 t across the center of the Tal keetna Mountains, and they form an inter1 ayered heterogeneous, dominantly marl ne sequence over 5,000 m thick. The base of the sequence 1s

nowhere exposed, and the contact with the overlyt ng Triassfc metabasal ts is an angvl ar unconfomity. The metavolcanogenic sequence consists domi'nantly of metamorphosed flows and tuffs of basal tic to andesi tic composition, and of coarse- to Fine- grained metavol caniclastic rocks with el asts composed chf efly of mf lc vol canfc rocks. Mudstone, bioelastic marble (mapped and described separately as unit PIS), and dark-gray to black phylli te are subordinate. The various rock types of the sequence form ccrnfombl e but 1entf cular uni is of 1 imi ted areal extent. The crudely layered and poor1 y sorted metavol can icl ast ic un its

have thicknesses in excess of 1,000 m, and the thfckness of the phyll f tes ranges from a few meters to several hundred meters. The whole sequence has been tightly folded and complexly faulted, and the rocks have been regionally metamorphosed into mineral assemblages mostly of the greenschi st and the prehnite-pumpel- lyite facies, but locally along Tsisj Creek of the amphybolite facies of Turner (1 968). Detaf 1ed petrographic descriptions of these rocks were given by Csejtey (19741,

The age of the metamorphl sin f s uncertain. The most inten- sive metamorphism In the mapped area probably took place in Early to Mf ddl e Jurassic time, contemporaneously with the development of the amphi bo7 ite terrane (unit Jam). Subsequent but 1 ess severe metamorphism, primarily shearing, occurred probably in middle to Late Cretaceous tlme during the alpine-

type orogenfc deformation of the Talkeetna Mountains (see

discussf ons In Structure and Tectonf cs sect1 ons) ,

The composition and lithologtc character of the rnetavol- canogenfe sequence strongly suggest that thfs sequence is a

rmant of b compl ex vol can ic arc system ( CseJ tey , 1 974, 1976). Fossil evidence (see description of unit PIS) from a marble interbed near the top of the sequence indicates an Early Per- 'mian age. However, because of the considerable thickness of the sequence, its lowemst port-ion may be as old as Late Pennsyl van f an.

MARBLE (Pennsylvanian(?) and Early PermJan)--Forms lenticular Interbeds, as much as a few tens of meters thick, within the basal tic to andesi tIc late Paleozoic mtavalcanogenic sequence

(unit Pzv). Most af the rock is light gray to white, medium to coarse grained, thick-bedded to massive marble, but some less metamorphosed varieties a1so occur, Sti13 di scerni bl e organic -ins and bedding features indicate that the marble inter- beds were derlved from bfoclastic I inmestone which probably was deposited by hlgh energy currents on sha1 1 ow banks of 1 f mited areal extent. A number of the marble interbeds contain poorly preserved and generi cal ly unfdentifiabl e crinoid col umnal s , brachiopods, bryozoans, and rarely corals (see tab1 e 3) of late Paleozoic or probable late Paleozoic ages. However, one of the marble interbeds near the top of the sequence (locality 8, tab1e 3) yielded we7 l -preserved brachjopods and crS not d columnaf s which were identiffed and fnterpreted by J, T. Dutro, Jr. (Csejtey, 1976) to be late Early Penjanr that is, late

, Leonardian to early Guadalupian in age. The regfonal correla- tjon of these rocks and that of the late Paleozoic metavolcano- genic sequence (unit Pzv) has been previously discussed by

Csej tey (1 976). Northern Watana Creek area SEDIMENTARY AND VOLCANIC ROCKS, UNDlVlOED (Upper Jurass f c )--These

rocks only occur in a small, apparently tectonic sliver along

the northern edge of the tnap area. They compri se a sectfan of intercalated argillite and graywacke, pebble conglomerate, and flows and dlkes of andesitic to latf tic feldspar porphyry. Some of these rocks are sheared but some, mostly the pebble conglom- erates, are not sheared. The argfll ite and f ine-grained graywacke are thinly to mod- erately thfckly bedded and generally are dark gray. Mowever, dark-green ish-gray varletj es a1 so occur, suggesting the presence of volcanic ash or fine-grained tuffaceaus mterfal, The con- glomerates are massire, and the well-rounded to subrounded peb- bles consist chfefly of unmetamorphosed andesite, latite, and

26 subordinate amounts of dacite. A mjnorf ty of the pebbles are

composed of dark-gray argill ite and white quartz. The feldspar porphyry is dark gray, with flow al fned phenocrysts of zoned andesfne and 01 igoclase as much as I cm long, and some horn- blende and biotite, in an aphanitic matrix. An argjtfite bed at the top of the 5,053-ft hill in the Healy A-2 quadrang7 e, just north of the present map area

yielded well-preserved fossils of Buchia Lvgosa (Fischer)*

indfcatjng a Late Jurassic age for these rocks ED. L. Jones, oral cormnun., 7977). On the basis of lithology and age, the rocks of the present unit are considered to be the westernmost occurrence of the Gravina-Nutzotin terrane of Berg and others (1 9721, Northwest Tal keetna Mountains

Kag ARGILLITE AND LITHIC GWYWACKE (Lower Cretaceous)--These rocks occur in a monotonous, f ntensely deformed flyschl ike turbjdi te sequence, probably several thousand meters thick, in the north- west part of the mapped area, north of the Talkeetna thrust fault. fhewhole sequence has been compressed into tfght and t socl tnal fol ds and probably has been cempl exly faul ted as we1 1. The rocks are highly Indurated, and many are sheared and per- vasively cleaved as a result of low-grade dynmmetamrphism,

the intensity of which is only locally as high as the lowermost

portion of the greenschf st metamorphic facies of Turner (1 968). Most of the cleavage is probably axial plane cleavage. Neither

the base nor the top of the sequence is exposed and, because of

the intense deformation, even its minimal thickness is only an estimate. The argillite is dark gray or black. Commonly it contains

small grains of detrital mica as much as 1 nun in diameter.

Because of the dynamometamorphism, in large areas the argillite is actually a slate or fine-grained phyllite. Thin sections show that some of the argillites are derived from very fine grained sil tstone and that they contain considerable carbona- ceous material.

The typical lithic graywacke is dark to medium gray, fine to medium grained, and occurs intercalated with the argillite in graded beds ranging in thickness from laminae to about 1.5 m.

The individual graywacke beds are not uniformly distributed throughout the whole sequence, of which they comprise about 30

to 40 percent by volume, but tend to be clustered in zones 1

to 5 m thick. Thin sections of graywacke samples show them to be composed of angular or subrounded detrital grains of lithic fragments, quartz, moderately fresh plagioclase, and some, generally a1 tered, mica in a very fine grained matrix; euhedral opaque grains, probably authigenic pyrite, are present in most

thin sections. The lithic fragments consist in various propor- tions of little altered, fine-grained to aphanitic volcanic

rocks of mafic to intermediate composition; fine-grained, weakly foliated low-grade metamorphic rocks; chert; and some fine-

2 8 grained unmetamorphosed sedikentary roc kr possf bly of intrafor- matf onal orf gin. Ho carbonate grains were seen. The matrix

constitutes about 20 to 30 percent of the rock by volume, gen- erally contains some secondary serfcite and chlorite, and, in the more metamorphosed rocks, biotite and possibly some

amphl bol e. Analyses of pal eocuwent features, such as small -scale cross-strati ficatlon , found In several exposures near the west- em edge of the mapped area, suggest that deposl tional currents came from the east or northeast (A. T. Ovenshlne, oral comun., 1974). Because fossils are extremely sparse, the exact age of the argfllfte and ffthfc graywacke sequence is imperfectly known, A poor specimen of rnoceramua sp. of Cretaceous age was found just west of the map area between the Chulitna and Susitna Rivers,

and a block of ~uchfa-bearlng 1 imestone of: Val anginian age was found in float near Caribou Pass in the Healy quadrangle north of the mpped area (D, L. Jones, oral comun., 1978). Northwestern ?a1 keetna Mountains

R vs MTABASALT AMD SLATE (Upper TrIass4c)--$ha1 low water marhe, Interbedded sequence of amygdaloidal metabasaf t flows and slate,

found only in twa allochthonous klippen near the northwest cor- ner of the mapped area. The sequence is tdlghtly folded, along

with the underlying Cretaceous racks (unit ~ag),and is sl ightly metamorphosed and unevenly sheared, The basalt and slate are fntemal ated In approximately equal proportions in individual units as much as 15 m thick.

The metabasatt is dark greenish gray, aphanitic, with num- erous amygdules. In thin sect1 ens the primary minerals are twlnned I abradorite, augite, and opaques whf ch probably are, for the most part, ilmnfte. Secondary minerals are chlorite (much of it after glass), epl dote, el inozoi site, minor zoIsi te, calcite, 1 eucoxene, very minor serici te, very fine grained fel ty amphj bal e (probably ural ite after augfte) , and possibly some very subordinate a1 bite. The original texture was inter- sertaf and subophi tic. The amygdul es consist of chlorite, zeo- 11tes (prfmarf ly prehnite) , quartz, and some feldspar. The slate is dark gray to black, Thin sections show that some of the rack is fine-grained mtasfltstone. A17 of the rocks conta4n consfderabl e carbonactlous material and some amounts of ffne-grained, secondary serict te. Secondary blot1te is pre- sent fn some of the slates. The secondary mineral assemblages suggest that, in addi- tion to deuterf c a1 teratf on, the metabasal t and slate sequence underwent very 1ow grade regional metamorphism. The metabasalt and slate sequence has been dated in the Healy quadrangle, north of the present map area, near the East

Fork of the Chul f tna Rlver where a. L. Jones and N. J. Silberl ing

(oral cmn., 19773 found upper Morian fossi 1 s of ~~rncrtissub- circaaris and Heterostrfdium sp. in slightly metamorphosed 30 argillaceous beds. Thus, the age of the present sequence is simflar to, and the lithology of fts basalt is identical to, that of the Upper Triassic metabasaltfc sequence (unit 7Rv) in the northeast Talkeetna Mountafns, These two rock sequences

may represent different facies, brought closer by thrusting, of the same geologic terrane. Upper Chuf itna River area DSga GRAYWACKE, ARGILLITE , AND SHALE ( Sf 1 urf an (?) to Mi ddl e ~evonian) --These rocks occur in an apparently a1 1 ocht honous tectonic block along the western side of the Chuf 1 tna Valley and corn- pri se a poorly and inaccessibly exposed, camp1 ex1y def omed and sheared sequence. As a result, the sequence is poorly known; it was brlefly examined in outcrop only along Long Creek. There the component rocks are medium to dark gray, sheared and tight1y folded with vertical dips, and occur intercalated in beds as

much as 1 m thlck. The graywackes are ffne grained and appear to con ta4 n same vol canogen f c detritus . Reconna issance ff el d checktng by D. L. Jones (oral comn., 1977) further to the north indicates that the sequence a1 so includes some chert, cherty tuff, and phyllite. In Long Creek, two fossil ifemus limestone beds (mapped and described separately as unit DS15) were found; they prob- ably are in depositional contact with, and thus date, the envel- oping unfossi 1 i Perous cl astjc rocks. It is passf bl e, however,

that some of the lfmestone contacts are tectontc and that some of the enveloping rocks are of a different age.

DSI s LIMESTONE (Si 1 urian (7) to Middle Devonian)--Massi ve to thfck- bedded, medium-gray, ffne-grained, moderately sheared bioclastic 1 imestone, probably formed in patch reefs. It occurs at three separate 1 ocal i ties, in apparent depositional Interbeds as much

as 20 m thick, within fine-grained clastic rocks (unit DSga). Of the two 1 imestone beds f n Long Creek, one yielded fossils of Devonian, probably Middle Devonian,age, the other of Silurian or Devonian age (map nos. 12, 13, respectively, table 3). The fossils a1 so indicate ~hallohrmarine deposf tion. The types of fossll s and the characteristics of the hast 1imestones and the enveloping ct astic rocks suggest deposf tion a1 ong an ancient

continental margin. These continental margin-type deposits crop out only about 6 km to the southeast of Upper Devonian ophiolitic rocks (unit Dbs) that arc indicative of ocean floor deposition. The praxjmity of these rocks that are close in age but different in deposi t5onal environment is add?tionat evidence for large-scale Alpine-type arogenic deformatfan in south- central Alaska (Csejtey and others, 1977; Jones and others, 1978).

Upper Chul j tna River area CRYSTAL TUFF, ARGILL ITE , CHERT, GRAVWACKE , AND LIMESTONE (Lower to Upper Jurassic)--Sha? low to moderately deep marine sequence, tightly folded and internally faulted, at least several thou-

sand meters thick. These rocks are interpreted to occur f n a thrust block along the western slope of the upper Chulitna Valley. Four-fifths of the sequence is comprised of the mas- sIve, cl iff-forming crystal tuff, while the remaining rocks form only a narrow outcrop be1 t along the western margin of

the map unft, The contact between these two groups of rocks may be tectonic.

The crystal tuff 1s 1 1 ght to dark gray, 1ocal ly w4 th a greenish tlnt, and weathers to various shades of brown. It is massive with obscure rhythm1 c laminations and thin bedding. The tuff is composed of abundant small feldspar crystals (at bite?) set in a very flne grain4 matrlx of devltrified vol- canic glass fn which some shards can be recognized, Sparse but midentiff abl e fragments of radio1 arfa were a1 so found. A thin interbed of volcanielastic sandstone yielded the fol low-

ing fossils: htoasteroceras jeletskyi Frebold, Saltechioceras

(Qrthechiocbras?) sp. , and weyla sp. (Jones and others, 1978; fossil locality in Silberling and others, 1978). Accordfng to R. H. Iml ay (written comun . to D. L, Jones, 19761, these fos- sils indicate a late Sinemurfan age. The argll 1 ite, chert, graywacke, and 1 imestone occur inter- bedded in varfous proportions Sn individual units as much as several tens of meters thick, The argilf ite and chert are dark gray to black; the graywacke is medium to dark gray, very fine to medium grained, locally with graded bedding. The limestone Is medium gray, generally phosphatic, in part sandy, locally

f s associated with 7 imy si 1 tstone and conglomerate; forms blocks and lenticular beds as much as several kilometers in extent. Some of the chert beds yielded radiolaria of late Kimneridgian or early Tithoninn age (Late Jurassic), and at five dffferent localities, the lfmy rocks yfelded Early JurassIc amnite faunas of early Sinemurian age (Jones and others, 1978; fossil localities in Silberling and others, 1978). Probably these Lower and Upper Jurassic rocks originally formed

a coherent stratigraphic sequence which subsequently was di3- rupted by folding and faulting. Ohio Creek area Dsb SERPENTINITE, BASALT, CHERT, AND MBBRO (Upper Devon S an ) --Tecton- ical ly f ntemtxed assemblage that forms a northeast-trending belt of apparent thrust slivers in the northwest comer of the mapped area. Sheared serpentinite fs the most abundant rock type; the remaining component rocks occur in various proportions fn lenticuf ar and podifom tectonic blocks as much as several hundred meters in extent. Many chert lenses occur intercalated with basalt flows which locally show poorly preserved pi1 low structures. Rocks of this map unit have been previously de- seri bed and interpreted as a dismmbered ophlol ite assembl age by Clark and others (1972) and by Jones and others (1978). The serpentinite is dark gray to dark greenish gray, a1 ways sheared, and consists a1most entirely of cl inochrysotil e and lizardfte with subordinate brucite, talc, and chrornite. Sparse re1 1 et ol ivlne crystals and a bastite texture suggest that the serpent inf te original ly was a pyroxene-07 ivine ul tra- mafic rock. Basalt is dark gray, aphanitic to fine grained with a few phenocrysts, as muck as 4 m in maxfmum dimension, of a1tered plagioclase, pyroxene, and el (vine. The rock is local Jy vesi c- ular or amygdaloidal and generally is fragmental; many of the fragments are palagonite. Some of the vesicles and amygdules are concentrated along spherf cal surfaces which may be parts of pi 1 low structures. DeposS tional ly intercalated marine chert beds further fndScate that the basal ts were formed as submarfne flows. The chert is generally red, but reddish-brown and greenish- gray varieties also occur. It is conrmonly in beds a few milli- meters to a few centimeters in thjckness,and contains abundant radi ol arta The gabbm is medium to dark greenish gray, fine ta coarse grajned, and is composed of a1 tered plagioclase, pyroxene, ol ivf ne, and opaques. Compositional layering, interpreted to be cumulate textures, is comn, and the layers range in thick- ness from a few millimeters to a few centimeters. The best exposed gabbro occurs in a lens about TOO m thick and about 1 km long on the ridge north of the unnamed northern branch of Shotgun Creek. Age determinations of radf 01 arDa and conodon ts in chert samples from eight separate locatitles ref iably indicate a Late Devon'Ean (Famennian) age for the ophiol f tjc rocks (Jones and others, 1978; Si7 berl ing and others, 1978). Long Creek area Rr RED BEDS (probably Upper TriassSc)--Red sandstone, sf 1 tstone, argSlljt@, and conglomerate similar to the red beds of unit JlR rs. Clasts of gabbro, serpentinf te, and foss f l Sferous Per- mJan(?) 1 imestone are present fn these racks but have not been identified In rocks of unit Jlffs. Also, a thin conglomerate. bed containing angular clasts af rhyolite is locally present at the base. These rocks I le with deposit5onal uncanfomity on

late Paleozojc, possibly Triassic, and older strata In the map area. Just north of the map area, the red beds rest on Lower Triassf c 1 Jmes tone (Jones and ethers, 1978). The red beds 1 ack fossils and, therefore, have not been dated, but they are assumed to be equivalent in age to the Upper Triassic red beds of unit J7Rrs (Jones and others, 1978).

Pzsv VOLCANOGENIC ' AND SEDIMENTARY ROCKS, UNDIVIDED (Upper Devon f an to

tower Pemfan)--Heterogeneous intercalated sequence of greenish- gray ta black tuffaceous chert, lesser amounts of maroon volcanic mudstone, breccia composed 1argely of basal tic detritus, 1 ami-

nated flyschl ike graywacke and shale, and large 1enses of 1 ight- gray, thick-bedded 7 imestone. Fosslls from the thick-bedded 1 imestone are Early Permian in age; brachiopods from the con- glomerate are also of Early Permian age; and fossils from the chert are Devonian and Carboniferous, but some poorly preserved fossils may possibly, though not likely, be as young as Trfassic (Jones and ethers, 1978). The stratigraphic and structural relations between these dtverse rocks are obscured by abundant folds and poor exposures. A detailed discussion of these rocks is glvm by Jones and others (19781, and fossil local lties are shown 5n Sf 1 berl ing and others (1 978). Ohio Creek area

RED AND BROWN SEDIMEt4TARY ROCKS AND BASALT, UNDIVIDED (Upper TriassPc and Lower Jurassic)--The basal part of this unjt con-

sists of a red-colored sequence of sandstone, sll tstane, argll- lite, and conglomerate, with a few thSn interbeds of brown fos- si 1 lferous sandstone, pink to I tght-gray dense 1 imestane, and intercalated massfve basalt flows. This red bed sequence grades upward into highly fossil f Perous bmwn sandstone, which in turn grades upward fnto brownish-gray siltstone with yellowish-brown I fmy concretions. Clasts in the red beds are dominantly basalt grains and pebbles which probably were derived from basalt flows of unit

Rlb that lies unconfombfy below the red beds and from massive basalt flows within the red bed sequence, Subordfnate amounts of the clasts consist of white, in part So1 iated, metaquartzite pebbles; flakes of white mica which, along with the metaquartz- ite, must have been derived from an unidentified siltceous mtamorphfc terrane; and red radio1arfan chert pebbl es and grains, which probably were derived from the ophf 01 itic rocks of unft # Dsb. tie other clasts that can be identified as coming from the ophf 01 itic rocks have been recogni zed. Fossils from the I fmestane and the overlying brown sand- stone are of Upper TrtassSc age, and those from the ye1 I awfsh- brown 1imy concretions are of Upper Triassic and Lower Juras- sic age. Detailed discusrions of both the red and brown beds are given by Jones and athers (19781, and fossil localities are shown in Sllbe~l ing and others (1978), lR 1b LIMESTONE AND BASALT (Upper Triassf c)--Inter1 ayered sequence of 1 imstone, partly recrystal 1 ired to marble, and flows of a1 tered amygdaloidal basalt, Individual units are as much as several tens of meters thick. These rocks occur in a complexly faulted . xnne in the northwest comer of the mapped area. The limestone is medium gray, massive to thick bedded,

but locally it has a1 tered to fine- to medium-grained marble. It contains sparse fragments of poorly preserved corals and thick-shelled ~e~aloc?ontid(?)bivalves up to 20 em in length.

A single specimen of spondylospir4 sp., in conjunction with the Megalodentid bivalves, suggests a Norian age far the sequence (Jones and others, 1978; fossil localjties shown in Silberling and others, 1 978). The amygdaloidal basalt is dark gray to greenish gray, aphanltic* with numerous amygdules. Locally, it dtsplays well- developed pi11 ow structures. Primary rock-forming mineral s are fine-grained 1abradori te, titanf um-rich augi te, and opaques in an origf nal 7y intersortal or subophf ti c texture. The original mineral assemblage has been more or less altered to an aggre- gate of chlorite (much of it after glass), epidote, calcite, serici te, and sme zeol ite, probably prehni te. The amygdul es consist of chlorite, calct te, prehnl te, and mlnor quartz. Host of the secondary mS neral s are probably the result of deuteric alteration, but some might be the product of very low-grade regional metamorphism. Fifteen chemical analyses of least a1 tered basalt samples indicate that the basal ts are somewhat low in silica (normalized Si02 contents average 46.7 percent by weight, ranging from 43.7 to 48.7 percent), high in alkalis (normalized NapO contents average 3.06 percent by weight, rang- ing from 1.3 to 5.2 percent; and normalized 50 contents aver- age 0.47 weight percent, rangf ng from 0.07 to 1.5 percent), and are high in titanium (normalized TiOe contents average 3.8 weight percent, ranging from 2.5 to 5.0 percent). The chemistry and mineralogy suggest that these basalls had af kal i afffnlties prfor to a1teration. The fossfls and the lithologies of the limestones and the basal ts indicate shal low water marine deposition. The pmbabl e a1 kal i affinfty of the basal ts further suggests that they either were part of an ocean island shield volcano, perhaps associated with a barrier reef, or that they were formed on a continental margf n. Upper Copeland Creek area KJs ARGTLLITE, CHERT, SANDSTONE, AND LIMESTONE (Upper Jurassic and Lower Cretaceous)--This unSt consists of dark-gray argill ite, dark-gray to green ish-gray bedded chert , t hic k-bedded sandstone, thfn-bedded gray sandstone, and rare thin beds of shelly lfme- stone. Both Upper Jurassic and Lower Cretaceous radio1 arias were obtained from the chert. The thick-bedded sandstone cnn-

bins abundant fragments of ~noceramussp. of Hauterivian to

Barranfan age, and some of the 1 imestone beds contain sucwa

mtblaevis of Valanginfan age, Some of the thin-bedded sandstone contains abundant detri tal white mica and may be as young as A1 bian (mf d-Cretaceous 1. Thicknesses and the stratS g~aphic relatjans within these rocks and with adjacent rocks are unknown because of complex folding and faultfng and poor exposures, A more detafled dtscussfon of these rocks is given by Jones and others (1978), and fossil localities are shown in Silberling and others 11978). Structure

The rocks of the Talkeetna Mountains region have undergone complex and Intense thrusting, folding, faul ting, shearf ng, and dffferential up1 lfting with associated reglonal metamorphism and plutonf n. At least three major periods of deformation are recognfzed: a period of lntense metamrphlsm, plutonirm, and uplifting fn the late Early to Middle Jurassf c, the plutontc phase of which persisted into Late Jurassic; a rnlddle to Late Cretaceous a1 pine-type orogeny, the most Intense and impor- tant of the three; and a period of normal and hjgh-angl e reverse Paul ting and mjno~fof dlng Jn the middle Tertiary, possibly extending into the Qua t ema ry . Most of the structural features in the Tal keetna Mountains region are the result of the Cretaceous orogeny which produced a pronounced northeast-southwest-I~ending structural grain of the regton. The verg- ence Ol thls structural grain 1s steeply to moderately toward the north- west, but across the Chulftna Valley in the northwest part of the map area, f t abruptly reverses toward the southeast with steep attitudes, This Cretaceous defonnatjon is most intense in the central and northwest- ern part af the map area, and it rapidly decreases toward the southeast. The complex fault pattern along and near the southern edge of the Tal- keetna Mountains Ss part of the late Cenozoic Castle Mountain-Caribau faul t systems, consisting chiefly of high-angl e reverse and normal fnvl ts of probably local sf gni ffcanec.

Evidence fir the Jurassic deformation is provided by the post- Talkeetna Formation major uneonfomity and the apparently coeval regional

42 metamorphism, up to the amphi bollte grade, and associated pl utoni c rocks (all the Lower to Mlddle Jurassic metamorphic and plutonic units). The hf gher crustal 1 eve1 manifestation of this Jurassf c tectonic event was regfonal uplift and consequent rapid denudation of the intruded epizonal pl u tons. Compl ex foI ding produced by the Cretaceous orogeny is espec ia1 1y pronounced in the areas northwest of the be1 t of Jurassic metamrphfc: and plutonic rocks. The folds are chiefly tight or Isoclinal, with ampli- tudes of several hundred to several thousand meters. The lfmbs are gener- ally sheared out or faulted out. As a result, no fndividual beds can be traced in the ffeld for more than a few kilometers. Many of the large folds, especial 1y in the Cretaceous argil 1 i tes and graywackes (unit Kag ) . have a we? l -devel oped axf a1 plane slaty cleavage. Fine-grained sericite and biotite are comnonly developed along these cleavages, The fsldSng must have taken place in several episodes during the nrogeny because thrust faults not only truncate folds wfthin both the upper and lower plates but are themselves folded. The Qol ded thrusts are especially evident 4n the CRul ltna area where, in contrast to the reg1 onal northwest vergence, the axial planes of the folds steeply dip toward the northwest. Most promtnent of the Cretaceous faults is the Talkeetna thrust which has placed Pat eozoic, Trf assic, and, locally, Jurassic rocks over Creta- ceous sedjmentary rocks across the whole map area. The thrust is gener- al ly poorly exposed except near the Lower fa1 keetna River, There f t 43 dips steeply toward the southeast, Another thrust, the one delineating the klippe of rocks of unit TRys, has been sharply folded. The thrusts in the northwest corner of the map area are very complex, also have been intensely folded, and are more numerous than could be shown on the present map* A number of them are not fully understood, and thus their subsurface conffguration is speculative. It 1s certain, however, that these thrusts stack and bring together on top of the Kag unit a wide variety of rock sequences of different ages and depositional environment. The root zone of all the thrusts in the northwest half of the map area is herein Inter- preted to be the Talkeetna thrust (see cross sectton). Another Cretaceous feature is an intense shear zone, locally as much as 25 km wide, trendlng across the Tal keetna Mountains, parallel to, but southeast of the Tal keetna thrust, A1 though not supported by any evj - dence, it is possible that the shear zone mrks a thrust zone of slgni- Picant displacement. (The center of thls shear zone is show as a postu- lated thrust on the map.) The dips in the zone are generally southeasterly. The shearing 3s penetrative, and its most rpectacular result 1 s that por- tions of all the Jurassic plutonic rocks, including the Upper Jurassic tmndhjemite, have been transformed to calaclastic gneiss, The 75 to 61 m. y. 01 d Upper Cretaceous and I ower Pal eocene tonal ite pl uton (unl t TKt) truncates this shear zone and is not affected by it.

The age of the Cretaceous orogeny, or at least its majar phase, is rather we1 1 bracketed by stratigraphic evfdence. The youngest rocks invol vad are the Cretaceous argill ites and graywackes (unl t Kag) whlch are as young as Valanginian or possibly even younger in age. A maximum upper age bracket Is pravlded by the late Paleocene granitic plutons, which arc structural 1y unaffected, and intrude the a1 ready foldad and faulted country rocks in the northwest half of the map area. Two of the Creta- ceous thrusts, including the Tal keetna thrust, are actual ly intruded by these plutons. A sllghtly older upper age bracket is provided by the pre- viously discussed 61 to 75 m.y. old tonal l te p'luton (unit TKt) that cuts and is unaffected by the prominent shear zone in the central Talkeetnas. Thus, the most important orogenlc deformation in the Tal keetna Mountains region must have taken place during middle to tats Cretaceous time. Such an age asalgnment for the orogeny is further supported by potassium-argon age determinations of 88 and 91 m,y. for the serpentinite protrusions 'in the southwest comer of the map area (unf t Km). The dominant features of the middle Tertiary to Quaternary defoma- t-ion are the already mentioned Castle Mountain-Caribou fault systems, along which the southern Talkeetna Mountains have been uplifted locally as much as 2,800 m (Detterman and others, 1976). The only other features of this Cenozoic defomtion recognized within the map area are the two poorly exposed noml faults In the Chul f tna River val I ey (see map and cross section). In addltion to field observations, the existence of these favl ts is a1 so supported by grari ty data (R. L. Win, oral comn. , 1977; N. 6. Harris, oral carnun,, 1977). No other Cenozoic faults, or any other faults with obvious Recent movement, were observed within the map area. Tectonics

The Talkeetna Mountains and adjacent areas are part of the dominantly alfochthonous terrane of southern Alaska. Previously, this terrane has been 1nterpreted to have developed by accretion of a11 ochthonous eon ti - nental blocks to the ancSent North American plate (Richter and Jones, 1973; Csejtey, 1974) fn late Mesozoic time (Csejtey, 1976; Jones and others, 1978). Although the exact number or even the extent of these allochthonous blocks is still fmperfectly known, they appear to Rave moved northward consjderabte distances prior to their coll isf on with the North American pf ate. For one of the blocks in eastern Alaska (hangell ia of Jones and others, 19771, a probable northward movement of several thousand kilometers has been shown by Hillhouse (1977). The results of the present investi- gatdons and those of Jones and others (1978) not only lend credence to the accretionary concept of southern Alaska but also provide addjtional evS dence for the time, method, and direction of emplacement. One of the keys to the tectonic history of the Talkeetna Mountains reglon , and to southern A1 aska as we1 1 , is the occurrence of the tecton- ically emplaced diverse rock packages in the Chulitna area in the north- west part of the map area, Most sf the Triassic and Jurassf e racks there, especially the Triassic red beds, do not occur anywhere 127512 in Alaska, and the f'ossll faunas and 1 1thologic characteristics of these Mesozof c rocks strongly suggest deposition In warm water at low paleolatitudes (Jones and others, 1978). Furthermore, the pre-middl e Cretaceous roe ks above the Tal keetna thrust, above the root zone of the Chwl itna faults, are either structural ly part of the a1 1 ochthonous Wrangel 1 ia terrane of Jones and others (1977) or belong to a different terrane lying south (that is outboard) of Wrangellia. Thus, all available evidence strongly indi- cates that, with the exception of vni t Kag, all pre-middle Cretaceous rocks of the Talkeetna Mountains region are a7lochlhonous, and, after the collislan of their papent continental blocks with the middle Creta- ceous North American contlnent, they were thrust upon, that is obducted onto the mrgfn of the continent. In turn, the middle Cretaceous Alaskan margin of the continental North American plate itsel f probably developed by still earl jer accretions (D. L. Jones, oral comun., f 977). The dls- tance the allochthonous rocks of the ~alkeetnaMountains region were thrustdbeyond the edge sf the continent Is not known with certainty, but it must be at least several hundred kttometers, In accordance with the present obductlon concept, all the tectonic and depositional rack assemblages nemlly associated with the continental upper plate of a subductf ng system, especial 1 y trench depbsS ts and volcanic arc rocks, are now hidden by the overthrust rock masses, Possibly the small tectonic sliver of Upper Jurassic sedimentary and volcanic rocks (unit 3s) along the Talkeetna thrust 3s the only exposed remnant of these hidden assem- blages. As shown an the cross sectdon, the mafn thrust along which most movement presumably occurred f s the Tal keetna thrust, and a1 1 other thrusts northwest of it are interpreted to be slivers below j t.

The northeast-southwest-trend1ng compressional structural Features, that is the folding and thrusting, indicate a general northwestward 47 tectonic transport. This is further supported by the sharp character of the suture zone in eastern Alaska, along which the alloehthonous racks of southern Alaska, especially the Urangeltia terrane, are in contact with the pre-rnfddle Cretaceous North Amerfcan continent. This suture zone in eastern Alaska trends northwesterly and is dwotd of the struc- tural cornplexfties of the Chulitna area, This part of the suture, the part southeast of Paxson, which also cofncides with the mlddle Tertiary to Holocene hnal i fault, is thus interpreted to have been a transform or

a wrench fault. In contrast, the great variety of tectonically juxta- posed rock packages in the Chulitna area may be the result of "bulldoztng"

by a large continental block drffting toward the northwest. The age of this orogenlc period of continental cof 1 ist on and sub- sequent abduction fs indqcated by the age of its structural features, which are djscussed in the Structure section, to be middle to Late Creta- ceous, In sumnary, southern Alaska is interpreted to have developed geologi- cal ly by the accretion of an fndetemlnate number of northwestward drift- lng continental blocks to the North American continent. After coll ision , at least parts of these blocks were thrust several hundred k.I'Iometers onto the North Amerf can contf nent in middle to Late Cretaceous time. The resulting structural features are truly alpine in character and compare favorably with the classic structures of the Alps in their grandeur and compl ex5 ty.

48 A cowl lary of the present tectonic interpretation of southern Alaska is that the present Denall fault, a middle Tertiary and younger feature

(Rf chter and Jones, 3973). has not played a sf gnificant role in the tec- tonic development of southern Alaska. The eastern, that i.s strfke-slip portion of the Denali fault (Csejtey, 19761, may not have more than a fw tens of kilometers of total movement. An interesting, but st11 1 unresolved, tectonic problem in the Tale keetna Mountains region is the shallow depth of the present Benioff zone (Lahr, 1975). The 50-ian contour (below sea level) for the upper surface of the Bentoff zone strlkes northeasterly and is approximately below the Jurassf c trondhjemite bat hol f th (unit Jtr) . The 100-h contour, a1so striking northeasterly, Is 1ocated approximately under the northwest corner of the map area, Accordfng to plate tectonic concepts, in conjunction with a subducting system, the top of the undergoing slab should descend at least 100 km below sea Icvel for magma generation. It appears that in the Talkeetna Mountains region there fs not enough thickness of upper plate for magma to fam. For the Jurassic and older igneous rocks the problem can be explained that th~erocks are allochthonous and have been tecton- ically cut off and transported away from their mats, However, for the Upper Cretaceous and younger igneous rocks, this mechanism cannot be invoked. Tm, explanations are possible, First, that the present shallow posjtlon of the B~nfoPfzone 1s a relatively recent phenomenon achieved by shearing and cutting away of the base of the upper plate by the down- going slab. Perhaps the development of the present Denall fault and

49 other mfddle Tertiary and younger faults of southern Alaska could be related to this process. The other possibility is that all the Upper

Cretaceous and younger igneous rocks of the Tal keetna Mountains region were formed in a thin upper plate by exceptionally high heat flow of unknown origin and mechanism (atectonic anatexis -by Red and Lanphere, 1974). References cited

Anderson, R. E,, 1969, Geology and geochemistry of the Diana Lakes area, western Tal keetna Mountains, A1 aska: Alaska Div. Mines and Geology

Geol. Rept. 34, 27 p. Banes, F. F., 1962, GealogSc map of lower Matanuska Valley, Alaska: U.S. Geol. Survey Misc. Geol. Inv. Map 1-359. Berg, H. C., Jones, D. t., and Rlchter, D, H., 1972, Gravfna-Nutzotfn be1 t--Tectonic signf ficance of an upper Mesorot c sedimentary and volcanf c sequence in southern and southeastern A1 aska, _in Geol ogf cal Survey research 1 972 : U. S. Geol . Survey Prof. Paper 800-D, p. Dl-024. Buddf ngton, A. F., 1959, Granlte emplacement with special reference to North America: Geol. Soc. America Bull., v. 70, p. 671-747. Clark, A. L., Clark, S. H. B., and Hawley, C. C., 1972, Significance of upper Paleozoic oceanic crust in the Upper Chul itna district, west-central Alaska Range, Geological Surrey research 1972: U. S. Gwl . Survey Prof. Paper 800-C, p. C95-C101. Csejtey, BQI a, Jr., 1974, Reconnaissance geologf c Investiga t tons in the Talkeetna Mountains, Alaska: U.S. Geol. Survey Open-ftle Rept. 74-147, 48 p. -. 1976, Tectonic Imp1 hatfans of a late Pal eozaic volcanic arc tn the Talksetna Mountains, south-central Alaska: Gealogy, v. 4, p. 49-52. Cse jtey, 0kl a, Jr., Nel son, W. H., Eberl ein , G. D. , Lanphere, M. A. and Smith, J. G., 1977, New data concernjng age of the Arkose Rldge Fomtlon, south-central Alaska, Blwn, K. M., ed., The Unf ted States Geological Survey in A1 as ka : Accompl ishments durf ng 1976: U.S. Geol. Survey Clrc. 751 -B, p, 862-064. Petterman, R. L., Plafker, George, Jysdal , R. G., Hudson, Travis, 1976, Geology and surface features along part of the fa1 keetna segment of the Cast1e Mountains-Cart bou fault system, A1 as ka: U. S. Geol . Surrey Map MF- 738. Detterman, R. L., Reed, B. L., and Lanphere, M. A,, 1965, Jurassjc plutonirm in the Cook Inlet region, Alaska, jn Geological Survey research, 1965: U.S. Geol. Survey Prof. Paper 525-D, p. 016-D21. Evernden, J. F., Curtf s, G. H. , Obradovich, J. D. , and Kist1er, R. W. , 1961, &I the evaluation of glauconite and ill ite for datfng sedi- mentary rocks by the potassiumargon method: Geochim. et Cosmochim. Acta, v. 23, p. 78-99. Grantz, Arthur, 1960a, GeoJogf e map of the Tal keetna Mountaf ns A-1 quadrangle and the south third of the Talkeetna Mountafns B-1 quadrangle, Alaska: U,S. Gwl. Survey Mfsc. Geol. Inv, Map 1-314, scale 1 :48,000. -1960b, Gmlogic map of the Tal keetna Mountains A-2 quadrangle, Alaska and the contiguous area to the north and northwest: U,S. Geol. Survey Misc. Geol. Inv. Map 1-313, scale 1:48,000. Grantz, Arthur, 1961a, Geologic map of the north two-thirds of Anchorage 0-2 quadrangle, Alaska: U.S. Geol . Survey Misc. GcoJ . Inv. Map 1-343, scal e 1 :48,000. -1961 b, Geologic map and cross sectfons of the Anchorage 0-2 quad- rangle and northernmost part of the Anchorage D-3 quadrangle, Alaska: U.S. Geol . Survey Mlsc. Geal . Inv. Mp 1-342, scale 1 :48,000. -1964, StratigraphSc reconnaissance of the Matanuska Formation in the Matanuska Valley, Alaska: U.S. Geol. Survey Bull. 1181-I, 33 p. Grantz, Arthur, Thomas, Herman, Stem, T, W., and Sheffey, N. B., 1963, Potasslm-argon and lead-alpha ages for strati graphically bracketed plutonic rocks in the Talkeetna Mountains, Alaska, -in U.S, a- logical Survey Research 1963: U,S. Geol. Survey Prof. Paper 475-8, p, 856-B59. Grantz, Arthur, and Wolfe, J. A., 7961, Age of Arkose RSdge formation, south-central Alaska: dm, Assoc. Petrol em Geologists Bull., v. 45, p. 1762-1765. Hi11 house, 3, W., 1977, Paleomagneti vn of the Trfassic Mi kol ai Green- stone, McCarthy quadrangl e, A1 aska: Canadf an Jaur, Earth Sci ., v, 14, p. 257892592. Jones, D. L., 1967, Cretaceous amnjtes Prom the lower part of the Matanuska Fomtion, southern Alaska: U.S. -1. Survey Prof. Paper 547, 49 p, 53 Jones, D, L., Silberling, N. J., Csejtey, ~hla,Jr., Nelson, W. H.,

and Blome, C. D., 1978, Age and structural sf gnf ficance of the

Chul itna ophiol ite and adjoining rocks, south-central A1 as ka t U.S. Geol. Survey Prof. Paper. (in press). Jones, D. L., Sflberling, M. J.; and Hillhouse, John, 1977, Wrangellia --A df splaced terrane in northwestern North America: Canadian JOUF. Earth Sci., v. 14, p. 2565-2577. tahr, J. C, , 1975, Detailed seisrnlc investigation of Paciffc-North Amerlcan plate interaction in southern Alaska: Hew York CS ty,

Columbia Univ., Ph.D. thesis, 141 p.

Ray, R. G., 1954, Geology and ore deposits of the Willow Creek minfng

dlstrfet, Alaska: U.S. Geol. Survey Bull. 1004, 86 p. Reed, 0, L. , and Lanphere, M. A., 1974, Chemical variatdons across the Alas ka-A1 eutian Range bath01 ith: U. S. Geol . Survey Jour. Research, v. 2, p. 343-352. Richter, D. H., and Jones, D. L., 1973, Structure and stratigraphy of eastern Alaska Range, A1 aska: Am. Assoc. Petrol eum Geologists Mem, 19, p, 408-420. Rase, A. W, , 1967, Geology of an area on the upper Tat keetna River, Tat keetna Ffountajns quadrangle, Alaska: Alaska Div. Mines and Minerals Geol . Rept, 32, 7 p, Sllberling, N. J., Jones, D. L., Csejtey, ~hla,Jr., and Nelson, W. H.,

1978, Interpret f ve bedrock geologic map of part of the Upper C hul f tna district (~ealyA-6 quadrangle), Alaska Range, Alaska: U.S. Geol. Survey Open-f 11e Rept . 54 3113 th, T. E., 1974a, Newly discovered Tertiary sedimentary bast n near Denall: Alaska Div. Geol, and Geophys. Surveys Ann. Rept,, 1973,

p. 19. -1974b, Regf onal geology of the Susi tna-MacLaren River area : Alaska Piv. -1. and Geophys. Surveys Ann. Rept., 1973, p. 3-6. Smith, T. E., Bundtzen, T. K., and Trlble, T. C., 1975, Stratabound copper-go1 d occurrence, ~orthernTal keetna Mountains, A1 aska : A1 aska Dir. Geol , and Geophys . Surveys MI sc, Paper 3, 7 p, Turner, D. L., and Smith, T. E., 1974, Geochronelogy and generalfzed geology of the central Alaska Range, Clearwater Mountafns, and northern Tatkeetna Mountains: Alaska DQv, Geol. and Geophys. Surveys Open-ffle Rept. 72, 11 p. Turner, F. J., 1968, Metamorphic petrology, mPneralogica1 and ff eld aspects: New Vark, McGraw-Hfll, 403 p, ad-n z 2 2 ggg *--

latest Karnlan K. M. f4ichols md 7. E. S~nlth, wl~iLt~11 ur Early Horlan N. J. Silb~rlii~g cnusrm.. 1974.

2 76AC.y- Crlnoid coluim~rls2 cm In dla~ter;echlno- bte Paleoaolt 0. L. JOI~P Tlbts report. PIS 47 dew and brachiopt debris. Inilet.

3 73ASt- Echlnohm debrts, Indat. Late Paleozoic, T. J, Outro, dr. T. E. Lltlb, WI-1ttctl Plr t4W P1atycrlrltld. ldt. (prtlal cup and pr&hly Permian em., 19H. columwlr). llorn coral, Indet. Femse~*Semd -H bmns. tndet. (dh~snt). ' ~~rlcarat~~raffp. Rhync hmeliold bnchlopod. hlet. Produc tat6 brMklapal (Irrge. In&temlmte). Sl~lrifcruld bracll lopad [prhdp~Splrir,rwllaJ. Harllnlold brrcbiupd {(yeplwps krarduurtlnad. Pllrlcndothyrtd brdclrlopod frqiwnts, f&t. Punctate brachlollod (perhdp5 IIUY tlwfid ) . Pelecypds, Itldct. (srver~lkl11d5). Pactenoid petecylads. inrlet.

4 72ACy- Crlnotd colrunsls as lurch as 1.6 cm In PmbLly A. K. Amstrong EseJtay, 1971. PIs 35. dlar*eter. late Palmmlc IZACy- B~-YUZO~II, Indrt, 21 Brachtupod and coral framts. 5 131St- Echlooder~ldebrls, tndet. Llte Paleozoic, J. '1. htro, Jr, 1. E. Srnith,d

V Wb mrridadaf sp, Pl r 10 Spirtfernid brachloppd. Crlnald coluw~dl~a5 wuch as 1.5 cm in djanrter. h a 74w- hn'titrdtd SP. Early Pe~nlrn d. T. Dutrn, Jr. Csejtay, 1916. PIr 16 wrfdmla SP. SplrlturuJka sp. Cchlnodpt. debris. Indet, CrtnuLOld tol~mdksIS UCll as 2.5 E. 1m Hamter, p-bly palm- A. I. Mrnstrolbg This report. m lc

12 1- #hd~k~~d?sp. Oer~nlm.prob- U. A. Oltver. Jr. -----h.-*-- wls 75 lbrslva st-toprold. Indet. &IF Ml&la OwQnl~