NORTH of DENALI FAULT Pediment Surfaces

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DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP 1-1 174 UNITED STATES GEOLOGICAL SURVEY

GEOLOGIC MAP OF THE TALKEETNA QUADRANGLE, ALASKA By Bruce L. Reed and Steven W. Nelson

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS moraine have subdued geomorphic expres- sion and include alluvial, swamp, marsh, and ALLUVIAL AND MASS-WASTING DEPOSITS bog deposits. North of the Alaska Range, Q1 Young and old landslide deposits. - Chiefly end and lateral moraines are relatively fresh debris avalanches and large block glides in and include well-preserved pot-and-kettle areas of steep slopes. Particularly well topography exposed along headwaters of Dillinger and West Fork of Yentna Rivers Qpe Drift of pre-Eklutna age.-Recognized only as subdued lateral(?) and ground moraines on Qau Alluvium, undivided.-Includes alluvium on north side of Alaska Range, although glacial active flood plains, older alluvium on erratics and well-modified drift on top of terraces of major streams, alluvial fans, Yenlo Hills (T. 24 N., R. 11 W.) are prob- outwash from both Alaskan and Wisconsin ably deposits of this extensive glaciation in Glaciations, swamp, marsh, and bog deposits the Cook Inlet region (Karlstrom, 1964) along the larger streams in the southeast part of quadrangle, and alluvial deposits on broad NORTH OF DENALI FAULT pediment surfaces. It locally'includes collu- Sedimentary and volcanic rocks vium and other deposits on valley walls and hill slopes. The unit is chiefly boulders,gravel, Tc COAL-BEARING ROCKS.-A cumulative and sand with local areas of silt and clay thickness of about 30 m of subbituminous coal, with individual beds up to 9 m thick, Qr Active and recently active rock glaciers and crops out on the west bank of the Little associated talus aprons.-Chiefly rubble and Tonzona River (T.31 N., R. 20 W.). The diamicton; only larger rock glaciers shown coal-bearing rocks, which contain interbeds of ferruginous siltstone and silty shale, dip GLACIAL DEPOSITS about 55" to the north. The contact with Qa Drift of the Alaskan Glaciation.-Includes end unit Pzsl is covered but in this area is prob- and lateral moraines left after recession of ably a fault. These rocks are tentatively existing glaciers (Tunnel and Tustumena correlated to the east with the coal-bearing Stades); chiefly rubble and diamicton and group (Oligocene and Miocene) in the locally covered by vegetation; only larger Nenana coal field (Wahrhaftig and others, deposits shown 1969) and to the west with a sequence of middle or late Oligocene conglomerate, Qn Drift of the Naptowne Glaciation.-Includes sandstone, and shale (Reed and Elliott, end, lateral, and ground Moraine, postglacial 1968) alluvial, pond, and swamp deposits, fluvial- glacial deposits of all stages of Naptowne Ts CONTINENTAL SEDIMENTARY ROCKS.- Glaciation, and glaciolacustrine deposits Chiefly medium- to dark-gray phyllitic shale, formed during Knik and Naptowne Glacia- shale, sandstone, grit, and conglomerate with tions. At higher elevations, deposits merge minor carbonaceous shale and tuffaceous with and are covered by colluvium and sandstone. Shales are lithologically similar to locally contain drift of the Alaskan dark-gray shale in unit KJs, but close exam- Glaciation ination generally reveals a few Metasequoia fragments and carbonaceous debris. Light- to Qk Drift of the Knik Glaciation.-Well-developed medium-brown and maroon, fine- to coarse- end and lateral moraines shown only in grained sandstone beds 1-3 m thick typically northwest part of quadrangle. Possible drift are gradational into and interbedded with of this glaciation on south side of Alaska conglomerate and, in many places, contain Range is included with Naptowne and fragments of large broadleaf plant fossils. Eklutna deposits Resistant conglomerate beds generally form lenslike units from 2 to more than 200 m Qe Drift of the Eklutna Glaciation (Illinoian).- thick; clasts range in size from 0.5 to 20 cm, South of Alaska Range, lateral and ground are moderately well rounded, and consist chiefly of white quartz. Clasts of'qartzite, contact with unit KJs, nowhere within the quartz semischist, and varicolored phyllite quadrangle is the nature of the contact and chert suggest derivation from Pzm and between these two units precisely known. KJs terrane. Crossbedding and channel scour Owing to the extreme difference in compe- features are locally well developed. Lack of tence between the two units and to their granitic clasts in Ts indicates that Tonzona proximity to the Denali fault zone, the pluton (55 m.y.) was either not emplaced or contact is sheared. The basalt is, however, not unroofed during deposition. Poorly inferred to be part of a sequence which preserved plant fossils are best compared to includes unit KJs and to represent local species indicative of a Paleocene agd (J. A. extrusion within a marine basin. It is discon- Wolfe, written commun., 1977). The unit is tinuously exposed northeast to Straightaway believed to be an extensive continental Glacier and southwest to Farewell. Pillow sequence deposited during the Late Creta- basalts south of the Denali fault (Rbs) have a ceous and early Tertiary that now is similar major oxide content but occur within preserved in fault slivers chiefly along Denali a Paleozoic terrane fault zone. It is correlated with the Cantwell Formation (Paleocene) as restricted by KJs TJNDIVIDED MARINE SEDIMENTARY Gilbert and others (1976, p. 1). Tuffaceous ROCKS.-Moderate- to deep-water, inter- sandstone may reflect distal portions of the tonguing marine sedimentary rocks consist- Teklanika Formation of Gilbert and others ing chiefly of gray to black phyllite, argillite, (1976) and the onset of early Tertiary graywacke, and siltstone which are, in part, plutonism in this part of the Alaska Range. turbidite deposits; lesser amounts of gray, (Reed and Lanphere, 1 973a) green, and red chert, conglomerate, grit, and thin beds ~f limy mudstone and impure KJb PILLOW BASALT.-Dark-greenish-gray limestone. The conglomerate contains clasts elongate bodies of massive-weathering; of medium- to dark-gray chert, phyllite, and resistant pillow basalt. The unit includes sandstone in a fiae-grained sheared gray- interbeds and lenses of mudstone, shale, and wacke matrix. Clasts range in size from 6 to siltstone. Irregular patches and lenses of 25 mm. Fossils are extremely rare, but a few light-gray-weathering sparry calcite and fragments of belemnites, Buchiu, and reddish jasperoid breccia fillings are spor- occasional Inoceramus shell fragments adically developed. The basalt locally (T. 30 N., R. 19 W.) are Valanginian to includes gabbro which may represent its Barremian (Early Cretaceous) in age (D.L. feeder source. Pillows range in size from Jones, written commun., 1976). Fossils of 15 cm to 2 m in maximum dimension, are Jurassic age have not been found; thus, the generally porphyritic, and contain pheno- Jurassic age of its lower part is uncertain. crysts of altered plagioclase, subordinate The unit is in fault contact with units Rsv, clinopyroxene, and rare olivine in an inter- Pzsl, and Ts. Where observed, the contact granular to intersertal groundmass of plagio- with KJb is sheared, although bedding clase and pyroxene. Groundmass plagioclase attitudes and pillow flows appear conform- occasionally displays swallowtail and belt- able. The thickness of the unit is unknown buckle crystal shapes. Rare amygdules but is probably more than 1500 m. Rare consist of quartz, calcite, chlorite, and groove casts suggest a source either to the epidote. Chemically, the rocks range from north or south. These sedimentary rocks are hypersthene-olivine normative to hyper- intruded by units Tmt and TKi. Thermal sthene-quartz normative tholeiitic basalt. metamorphism has produced andalusite Trace element analyses indicate that the hornfels adjacent to larger intrusive bodies. basalts can be classified as ocean-floor basalt Sparse fossils indicate that this unit is, at (Pearce and Cann, 1973). The copper con- least in part, a time equivalent of unit KJs tent of nine samples of pillow basalt ranges south of the Denali fault. Lithologies of the from 200 to 500 ppm and averages about two units are broadly similar, but unit KJs 280 ppm. West of Pingston Creek, the flows south of the fault contains significantly generally face north, have a steep to sub- more sandstone, occasional plant fragments, vertical dip, are at least 1300 m thick, and and sedimentary structures that suggest local are thought to have considerable vertical shallow-water deposition. Unit KJs north of extent. In T. 32 N., R. 15 W. the pillow the Denali fault contains more chert and basalt appears to overlie unit KJs and has an shale and represents deeper marine deposi- apparent thickness of about 750 m. Tops tion. Northeast of T. 32 N., R. 14 W. the and bottoms of flows are locally brecciated; unit is chiefly dark-gray phyllitic shale and columnar joints are also present. Although argillaceous siltstone and is 1ithologicaUy the basalt is shown as having a normal similar to dark-gray phyllite (Pzp) Rsl SHALE AND LIMESTONE.-Marine sedi- bsu QUARTZITE, SEMISCHIST, AND META- mentary rocks that typically form a thin- to VOLCANIC ROCKS.-Tan to light-greenish- medium-interbedded sequence of black, gray, low-grade (greenschist facies), region- sooty, carbonaceous, and calcareous shale, ally metamorphosed quartzite, quartz semi- dark-gray calcareous siltstone, medium- to schist, quartz grit, metavolcanic rocks, and dark-gray limestone, and minor gray quartz- minor conglomerate, limestone, phyllite, and ite. Shale locally amounts to as much as 70 quartz-muscovite schist.The unit is isoclinally percent of the unit, is generally calcareous, folded and there is evidence of multiple and contains thin laminae of darkgray folding. Moderate to well-developed schistos- siltstone. Semiquantitative spectrographic ity is common. Schistosity and metamorphic analyses of eight samples of black shale from grade increase to northeast. The base is not the northwestern part of the quadrangle exposed; the unit is in fault contact with show the following average metal contents younger sedimentary rocks. Quartz-rich beds, (ppm): Ag=lO, Cu=l 1 1, Cr=194, Mo=24, which are more prevalent to the west, consist Ni=115, Pb=29, eU (equivalent uranium) chiefly of thin- to medium-bedded, poorly =29, V=344,Zn=688. Limestone is chiefly sorted quartz wacke, quartz semischist, micrite and contains varying amounts of quartzite, and minor conglomerate. Quartz subangular grains of quartz. Some beds emit grains are angular to rounded and consist of a fetid odor when freshly broken. Siltstone plutonic(?), vein, and metamorphic quartz and quartzite are notably different from in a matrix of quartz, muscovite, and minor quartzite in Pzsv in that they have calcite chlorite. Feldspar grains are noticeably matrix, lack a semischistose fabric, contain absent. Tourmaline and zircon are the rare plagioclase grains, and quartz grains lack predominant accessory minerals. Malachite fluid inclusions. The unit contains numerous has developed on schistosity planes east of dikes and sills of altered gabbro (Mzkg). Chedotlothna Glacier. East of Pingston Contact metamorphism is generally restric- Creek secondary copper staining and rare ted to within a few meters of the dikes and lenses (2-7 cm thick) of chalcocite are noted. sills, although where intrusive rocks are Quartz-muscovite schist and chlorite schist abundant, as for example, east of Pingston are locally present between Boulder Creek Creek, tan phyllite and dark argillite are and South Fork Kuskokwim River. Talc common. The unit is well exposed along the schist is developed near MzPzu serpentinite. northernmost foothills of the Alaska Range, Conglomerate consists of gray subrounded especially on Pingston Creek, where rocks quartz and sporadic dark siltstone clasts that range in size from 0.5 to 15 cm. Meta- form tight isoclinal folds (although cleavage volcanic rocks are chiefly light-green is not developed); abundant milky-white quartz veinlets are restricted to siltstone and metatuff. Small (<0.5 to 5 mm) lenticular masses of chlorite, calcite, epidote, and calcareous quartzite; limestone (in beds as quartz suggest collapsed pumice fragments much as 2 m thick) becomes more abundant or filled vesicles. Euhedral phenocrysts of to the south. It is believed to be alloch- plagioclase, quartz, and altered mafic thonous on unit hsv. In most places the trace minerals are locally present. Metavolcanic of the thrust has subsequently been rocks are noticeably more abundant east of refaulted to high-angle faults; however, a the Tonzona River where they are inter- lowdipping thrust fault is moderately well bedded with quartzite and phyllite. Lime- documented east of Pingston Creek stone occurs as gray, medium- to massive- (T. 31 N., R. 19 W.). This unit has been bedded lenticular units as much as 30 m traced northeast as far as the Foraker Glacier thick in quartzite and phyllite. Skarn is and discontinuously to the west as far as the developed adjacent to Tonzona pluton. Tan, Windy Fork area (McGrath quadrangle). It is lightgreenish-gray, and maroon phyllite is tentatively correlated to the northwest with interbedded with quartzite West of Tonzona the lower part of the Totatlanika Schist in River. East of Tonzona River phyllites are the Kantishna Hills (Bundtzen and others, chiefly gray to dark gray and interbedded 19751, which consists, in part, of graphitic with rnetavolcanic rocks. Andalusite is slate and schist and impure marble which, at present in phyllites adjacent to unit Tmt. Kantishna, are in fault contact with Birch The unit is inferred to correlate with so- Creek terrane. called Birch Creek Schist in the Kantishna Hills (Bundtzen and others, 1975), but rocks of equivalent age in the Alaska Range west of the quadrangle are not presently known. The unit is considered to be lower Paleozoic or possibly upper Precambrian Intrusive and ultramafic rocks pyroxene(?), occur in a polygonal mosaic of coarser antigorite. Antigorite polygons are Tmt TONZONA PLUT0N.-A small composite rimmed by opaque minerals. Irregular biotite-muscovite granite pluton of the patches and veins of calcite are present; McKinley sequence (see below), which crops chrysotile is rare. An irregular surface tex- out north of the Denali fault (T.32 N., ture produced by differential weathering R. 15 W.). Includes petrographically and suggests that the original rock was a perido- chemically similar rocks near Sheep Moun- tite containing both olivine and pyroxene. tain (T. 3 1 N., R. 17 W.) that may represent The serpentinite is chemically equivalent a cupola of underlying granitic rocks. Three (calculated water free) to pyroxene perido- (1) textural facies are recognized: coarse- tite. Chrome and nickel contents average grained, locally porphyritic biotite granite, 2000 ppm each; float samples of serpentinite (2) medium-grained biotite granite, and (3) a contain pyrite, chalcopyrite, and pent- late fine-grained, leucocratic, locally aplitic, landite(?) muscovite-tourmaline granite in which ovoid clusters of small black tourmaline crystals SOUTH OF DENALI FAULT give the rock a "dalmatian" appearance. The Sedimentary and volcanic rocks granite is classified as a tin granite; muscovite may exceed biotite in abundance; Tk KENAI GROUP, UNDIVIDED.-Fluvial sedi- accessory minerals include tourmaline with mentary rocks correlative with estuarine and lesser amounts of topaz, fluorite, garnet, nonmarine clastic sedimentary Tertiary zircon, and apatite. Late-stage greissen formations assigned to the Kenai Group in veinlets contain muscovite, topaz, tourma- the Cook Inlet basin by Calderwood and line, locally abundant beryl, and, Fackler (1972). Exposures of the Kenai occasionally, cassiterite. Fractures are healed Group in the Talkeetna quadrangle are with black tourmaline. Small, apparently limited to gently to moderately southeast- late, clots of beryl, topaz, muscovite, dipping beds along the foothills of the and quartz are present in granite boulders. Alaska Range, shallow-dipping beds along Modal analysis given in figure 1. Lead, silver, stream cutbanks in the lowlands to the and tin mineralization occurs in unit bsv southeast, and a small outcrop of subbitumi- along north and northeast contact nous coal in T. 3 1 N., R. 6 W. The group is locally divided into the Sterling(?) Forma- TKi UNDIVIDED INTRUSIVE ROCKS.-Inter- tion and the Tyonek(?) Formation. The mediate altered porphyry dikes in unit KJs; sterl'ing(?) unconformably overlies the most are too small to show at scale of map Tyonek(?), and the Tyanek(?) Formation Mzhg GABBRO AND QUARTZ DIOR1TE.-Dark- rests with angular unconformity on Meso- green rust-weathering coarse-grained altered zoic rocks gabbro and minor quartz diorite; restricted Tps STERLING(?) FORMATION (Pliocene).- to unit F'zsl. Age unknown. Gabbro is Orange, light-tan, or light-gray, massive- hypidiomorphic granular to porphyritic, in bedded conglomerate, distinguished from places subophitic, and consists chiefly of conglomerate in the Tyonek(?) by its color, clinopyroxene and altered plagioclase. Minor relative coarseness, and clast lithology. Clasts skeletal ilmenite, myrmekitic quartz, and are well rounded and equant and average secondary chlorite, biotite, and amphibole 5-10 cm in diameter, although clasts to are also present. Plagioclase is altered to 30 cm in diameter are not uncommon. The clouded masses of sericite and kaolinite(?), conglomerate is poorly to moderately well ilmenite partially altered to leucoxene, indurated with a clayey and, locally, ferrugi- clinopyroxene partially altered to chlorite, nous matrix. Clasts consist of the same green amphibole, and biotite. Average lithologies but occur in proportions different copper conrent of six samples is 350 ppm. from those of the conglomerate member of These intrusive rocks are chemically equiva- the Tyonek(?) Formation, which occurs at lent to gabbro, notable differences being the same locality. Coal fragments southwest that they contain more HzO' and FeO and of the Yentna River (T. 25 N., R. 14 W.) less MgO and Alz03 suggest that the Sterling(?) is at least partly &Pzu SERPENTIN1TE.-Orange- to light-brown- derived from the Tyonek(?) Formation. weathering, light- to dark-green, fault- Crossbed sets in orange and tan sandstone 'bounded body of serpentinite and minor talc lenses in conglomerate (T. 27 N., R. 13 W.) schist (T. 32 N., R. 15 W.); cut by Tonzona suggest that current directions were to the pluton. Patches of very fine grained bladed east or southeast. Bedding is approximately antigorite, commonly with a core zone of conformable on the Tyonek(?) Formation serpentine minerals pseudomorphic after where the contact can be observed, but the apparent thinning of the Tyonek(?) to the are smooth and well rounded and average 5 northwest, culminating in overlap of the to 10 cm in diameter, with maximum size Sterling(?) directly onto basement in the about 15 cm. The greatest number of chert foothills, and inclusion of probably and quartz clasts is found at Nakochna Tyonek(?) coal fragments in the Sterling(?) River; fewer are found to the northeast. suggest that its basal contact is an angular Igneous clasts reach a maximum abundance unconformity along the mountain front. at Fairview Mountain. The proportion of Maximum thickness at Fairview Mountain is shale and graywacke clasts steadily increases 770 m, but the thickness may be greater from Nakochna River to more than 90 beneath the lowlands percent at Ruth Glacier. Sandstone is coarse, poorly sorted and pebbly, and very poorly TYONEK(?) FORMATION (Miocene).-Divided indurated. Siltstone, claystone, and coal into two members: occur as interbedded units that grade up- Tts Sandstone member.-Approximately 80 ward from sandy siltstone to silty claystone percent sandstone, 20 percent siltstone and to coal. Coal beds range in thickness from claystone, and less than 1 percent conglom- 0.5 to 17 m but average 0.6-3 m thick. Coal erate, coal, and volcanic ash. The member is subbituminous, dull, black or brownish occurs in repetitive cycles 7-23 m thick that black, and contains clay or bone partings. grade upward from conglomerate or coarse One bed near the Nakochna River ( sec. 7, sandstone to finer grained sandstone, to T. 24 N., R. 15 W.) is partially burned interbedded silt and clay with coal or bony coal. Conglomerate occurs in beds as much Tvs UNDIVIDED VOLCANIC AND SEDIMEN- as 5 m thick; clasts have an average size of TARY ROCKS.-Interbedded tuffs, mafic 2-6 cm and a maximum size of 10 cm. volcanic flows, sandstone, shale, and minor Sandstone is tan or light gray, coarse to calcareous mudstone. Volcanic rocks are medium grained, poorly indurated, and chiefly medium- to coarse-grained greenish- consists of about 75-85 percent chert and gray crystal lithic lapilli tuffs and volcanic quartz grains, 10-20 percent feldspar, and rubble flows in units as much as 1SO m about 5 percent mafic grains which include thick. Tuffs are composed of broken biotite, hornblende, clinozoisite, and chlo- euhedral crystals of plagioclase with oscilla- rite. Sandstone beds are as much as 60 m tory zonation, serpentinized olivine, quartz, thick, although the thicker ones were prob- and lapilli-size fragments of crystal tuff in a ably not deposited as a single unit. Inter- brownish, slightly devitrified matrix. Lapilli bedded claystone and siltstone occur in units fragments contain altered plagioclase, as much as 15 m thick. Siltstone and clay- amphibole, and minor biotite. Tuffs are stone are light to medium gray and contain locally pyritized; eU content of four samples rootlets and coaly partings. Coal or bone, if averages 25 ppm; minor uranium prospecting present, is found in these units above clay was done in 1954 and in 1976. Interbedded beds. Coal, in beds as much as 3 m thick with tuff is well-sorted lithic sandstone with an average thickness of less than 1 m, is composed of angular clasts of dark chert, platy or papery, soft, and in many places metamorphic rock, and rare volcanic rocks, contains shaly or bony partings. Volcanic and angular grains of quartz and plagioclase, ash layers (1-30 cm thick) in coal beds are with minor amounts of muscovite; calcite composed of partially devitrified glass shards and chlorite form the interstitial cement. altered to illite(?) and less than 0.5 percent Shale and siltstone contain fragmentary biotite. Rootlets and other plant materials plant remains. The source area for volcanic also are present in the ash. The sandstone rocks is believed to be Round Mountain, member interfingers with or conformably 10 km to the northwest (T. 23 N., overlies the conglomerate member. About R. 19 W.). The volcanism is probably related 170 m of the unit crops out in the foothills, to early or middle Tertiary plutonism in this but the thickness is probably greater in the part of the Alaska Range (Reed and Lan- lowlands to the south phere, 1973a)

Ttc Conglomerate member.-At Fairview Moun- KJs UNDIVIDED MARINE SEDIMENTARY tain (T. 26 N., R. 12 W.), the member ROCKS.-Medium- to dark-gray, generally comprises at least 40 percent conglomerate, isoclinally folded, thick sequence of lithic 20 percent sandstone, and less than 40 graywacke, phyllite, and shale with local percent siltstone, claystone, and coal. Con- lenses of quartz-chert conglomerate. The glomerate is light brown, light gray, or bluish unit includes minor fossiliferous limestone, gray and poorly indurated. It occurs as radiolarian chert, and red ferruginous sand- massive beds as much as 25 m thick. Clasts stone and siltstone. Lithic graywacke is thin to massive bedded and locally shows graded the Dillinger River (Pzd), although in bedding, ripple marks, crossbedding, and T. 27 N., R. 20 W. the contact is an angular pull-apart structures. Graywacke is poorly to unconformity. The contact relation of the moderately well sorted and consists of Early Jurassic to Cretaceous strata is uncer- angular to subangular, fine to coarse sand- tain. The thickness of the unit is unknown size detritus, of which quartz and lithic but is probably more than 3000 m fragments comprise about SO percent. Lithic fragments are fine-grained recrystallized bbs PILLOW BASALT.-Chiefly dark-greenish-gray quartzite or metachert with lesser amounts massive pillow basalt and petrographically of dark siltstone or argillite, muscovite schist, similar coarse-grained pyroxene gabbro and and fine-grained volcanic rocks (including basaltic dikes and sills. The unit as mapped porphyry). Plagioclase, potassium feldspar, includes varying amounts of interstratified and detrital muscovite are subordinate. agglomerate, flow breccia, aquagene tuff, Common, although not abundant, heavy and thick, chaotic sequences of eugeo- minerals include zircon and tourmaline. synclinal sedimentary rocks. The last of Graywacke from the Yenlo Hills (T. 23 and these are especially abundant in the roughly 24 N., R. 1 1 W.) is significantly different in circular area northwest of Shellabarger Pass that the grains are extremely angular, and (T. 29 N., R. 19 W.), where about 600 m of oscillatory zoned plagioclase, volcanic rock submarine basalt flows forms the core of a fragments, hornblende, epidote, and calcite north-trending structural trough (Reed and grains are more abundant, suggesting that the Eberlein, 1972). Pillows as much as 0.6 m in sandstone may have been derived from the diameter show varying degrees of serpentini- Jurassic magmatic arc (Reed and Lanphere, zation and are typically porphyritic with 1973b) now covered by younger rocks in phenocrysts of plagioclase and lesser clino- Cook Inlet basin. Contemporaneous volcan- pyroxene. Olivine is extremely rare, but its ism is suggested by rare interbedded light- former presence is suggested by occasional colored tuffaceous sediments in the Yenlo rounded patches of chlorite and serpentine Hills. Argillaceous rocks are locally thermally minerals of unidentified composition. metamorphosed to spotted cordierite and an- Groundmass consists of very fine laths of dalusite hornfels. Graywacke is thermally plagioclase, some with swallewtail and metamorphosed to biotite-quartz hornfels. belt-buckle crystal shapes, birefringent clino- Elsewhere, low-grade metamorphism is indi- pyroxene variolites, and dark-brown devitri- cated by the presence of metamorphic chlo- fied and deuterically altered glass. Clino- rite and biotite. Hydrothermal activity, prob- pyroxene is both pigeonite and subcalcic ably related to emplacement of plutonic augite. Chemically the basalts are quartz- rocks at depth,locally has altered the gray- hypersthene normative tholeiites. Limited wackes to a soft orange-weathering quartz, trace-element data suggest that the basalts chlorite,sericite, iron-oxide rock that locally are of the ocean-floor type (Pearce and Cann, contains vuggy and brecciated quartz. Gold can 1973); associated rocks indicate local sub- usually be panned from streams draining these aerial volcanism. The basalts are petrograph- altered zones. Extremely rare Inoceramus ically and chemically distinguishable from have been found, and bioclastic limestones the Mesozoic pillow basalt by the presence of of Early Cretaceous age (late Hauterivian to pigeonite, lower K20, Ba, and Rb contents, early Barremian; D. L. Jones, oral commun., and higher FeO content. Copper content 1977) are locally present. Along the Happy ranges from 200 to 300 ppm. Locally the River (T. 22 N., R. 18 W.), dark-gray platy basalts are associated with massive cuprif- shale and sandstone contain indeterminate erous sulfide deposits (Reed and Eberlein, broadleaf plant fossils, and the rocks are lith- 1972). Because of the chaotic structural ologically similar to the Matanuska Formation features that characterize the Paleozoic of Late Cretaceous age. Although Upper Cre- sedimentary rocks (kus), the age of the taceous fossils have not yet been found, a pillow basalt is uncertain but inferred to be Lake Cretaceous age cannot be ruled out. On middle or late Paleozoic. A post-Late the north side of the Tatina River, reddish- Devonian age is locally indicated by distinc- brown-weathering sandstone and dark-gray tive spherical concretions of fluorapatite shale contain Early Jurassic ammonites, brach- present in sedimentary breccia and conglom- iopods, and pelecypods (R.W. Irnlay, written erate that apparently underlie pillow basalt commun., 1977). To the west, in the McGrath at Shellabarger Pass. The source of the quadrangle (T. 26 N., R. 21 W.), these rocks concretions is a phosphatic chert unit that include minor pillow basalt and volcanic flows. overlies the Middle and Upper Devonian In most places the Early Jurassic rocks are in limestone. A late Paleozoic (possibly Missis- fault contact with the sedimentary rocks of sippian) age is suggested by thin units of pillow basalt interbedded with flyschoid plant fossils of Middle Pennsylvanian age sedimentary rocks and limestone-chert (S. H. Mamay, written commun., 1977). conglomerate that are gradational upward Conglomerate is locally bleached grayish into the conglomerate of Mount Dall of white due to hydrothermal alteration assoc- Middle Pennsylvanian age. In T. 28 N., iated with intrusive rocks (Tcp). Limestone R. 18 W., a small unit of pillow basalt (not clasts are altered to cream-colored and shown on the geologic map) is present light-green diopside, idocrase, wollastonite, within a chaotic sequence of chert, shale, quartz, and calcite. This unit appears to be limy sandstone, and limestone that contains gradational downward to lithologically fossils of late Middle DevoniBn(?) age (J. T. similar conglomerate in unit Pzus that does Dutro, Jr., written commun., 1976) not contain varicolored chert clasts, is interbedded with shales that contain abun- bv VOLCANIC AND SEDIMENTARY ROCKS.- dant worm trails, and in which limestone Dark-greenish-gray massive-weathering mafic clasts are subordinate. The base of the unit volcanic rocks that include pillow basalt, consists chiefly of massive-bedded conglom- breccia, agglomerate, tuff, and massive basalt erate and sandstone. The top is not exposed, flows. Pillow basalts are porphyritic with but a minimum thickness of at least 1500 m phenocrysts of plagioclase and clinopyrox- is present near Mount Dall ene (some with both augite and pigeonite) set in a matrix of finer grained clinopyrox- bus UNDIVIDED SEDIMENTARY ROCKS.-A ene and lath-shaped plagioclase. Secondary depositionally and structurally complex minerals include sericite, calcite, serpen- terrane of chiefly marine flyschoid sedimen- tine(?), chlorite, and clinozoisite. Chemical tary rocks which include (I) trench analyses of two separate pillow basalts assemblages (and possibly intra-oceanic arc indicate that the rocks are quartz-hyper- deposits) characterized by terrigenous sthene and olivine-hypersthene normative turbidites, cherty pelagites, and basaltic tholeiitic basalt. Volcanic rocks commonly pillow lavas (all of which underwent com- contain interbeds of black phyllite, chert- plex undersea sliding and later multiple pebble conglomerate, light-green tuff, and thrusting and folding), (2) slope and shelf graywacke. The presence of massive basalt assemblages that include chert, shale, reefoid flows, breccia, and agglomerate suggests in limestone, and, locally, terrestrial conglom- part subaerial and (or) shallow-water ponded erate and redbeds, and (3) a thick, locally deposition. The unit is probably largely terrestrial conglomerate and sandstone equivalent to but is tentatively distinguished assemblage. Thtse dissimilar middle and late from unit Pzbs by presence of massive basalt Paleozoic depositional environments are now flows juxtaposed by large dislocated nappes and thrust faults on all scales. Structural features Pd CONGLOMERATE OF MOUNT DALL.- generally show a westerly vergency. The Informal name given to dark-brown to structural and stratigraphic relations of the yellowish-brown-weathering sequence of contrasting rock types are known in only a conglomerate, sandstone, siltstone, and shale few places. This terrane is allochthonous on mainly of continental origin that forms a unit Pzd, truncated on the north by Denali broad, open, east-plunging syncline. This fault, fault bounded on the south by unit KJs, unit is chiefly massive lenticular beds of intruded by dunite and associated ultramafic conglomerate and sandstone with numerous rocks (Mz PLd) along the southern contact cut-and-fill channels. Conglomerate forms with unit KJs, and elsewhere by units TKi, beds as much as 40 m thick; clasts range in Tcp,and the middle Tertiary Foraker pluton size from 2 to 30 cm and are chiefly gray (Tf). Intrusive rocks of McKinley sequence limestone and black chert. Limestone clasts (Reed and Lanphere, 1973a) and calc-alkaline contain Middle Devonian(?) fossils (W. A. volcanic and intrusive rocks of Paleozoic age Oliver, Jr., written commun., 1975) similar are notably absent. For convenience, these to those in Devonian limestone (Dl). Un- rocks are described in units that were locally common red and green chert, chert-pebble recognized in the field; however, none of conglomerate, and graded sandstone clasts these units is differentiated on the geologic also suggest unit Pzus as its source. No map. The unit locally includes thin pillow granitic or volcanic clasts were recognized. basalt (Pzbs), discontinuous and tectonically Sandstone beds are commonsly 2-3 cm thick emplaced limestone beds (Dl), and conglom- and have well-developed planar and trough erate of Mount Dall (lPd). The base of the crossbedding and slump structures suggestive terrane was not recognized; the thickness is of .fluvial deposition. Shale and sandstone unknown but believed to be in excess of beds 3-5 m thick locally contain abundant 2500 m Flysch deposits, believed to occur both corals and massive strornatoporoids. From above and below units hbs and Psv, are 40 to 60 m above the base of the limesfone, characteristic of the terrane and consist of the beds are composed of a reefoid mass of graywacke, shale, phyllite, grit, chert-pebble colonial rugose corals, ramose bryozoans, conglomerate, mudstone, chert, various and mamelons of large stromatoporoids. At types of volcanogenic sediments, wildflysch, 60 m, the limestone becomes more argilla- and minor bioclastic limestone. Wildflysch ceous, thinner bedded, and less fossiliferous locally contains house-sized blocks of and is composed of micrites. Thin (5 to bedded limestone (Dl, or Early to Middle 15 cm) dark-gray shale beds between lime- Devonian limestone units locally present in stone beds are common. A massive micritic, unit Pzus) chaotically distributed in volcano- poorly fossiliferous gray limestone is present genic mudstone and is spatially associated from 75-87 m. Massive stromatoporoid lime- with pillow basalts. Graded beds and slump stones are present from 90 to 95 m. The folds are common. Worm trails are character- stromatoporoid beds are overlain by dark- istic of the finer grained sediments. Clastic brown to black shales with thin limestone limestone beds (T. 29 N., R. 19 W.) contain interbeds that contain conodonts of Late echinoderm remains that suggest a late Early Devonian (upper half of Frasnian) age (A. G. or early Middle Devonian age (J. T. Dutro, Harris, written commun., 1977). The thick- Jr., written commun., 1975) for the lime- ness of the limestone varies through its area stone of exposure due to original depositional facies. It may represent a series of small Slope, shelf, and fluvial deposition is indi- patch reefs and interreef beds. Variation in cated by a 0- to I 0-m-thick redbed sequence thickness may also be due in part to tectonic of sandstone and conglomerate that contains factors. Limestone contains abundant coral plant and coalified wood debris. The redbeds and shelly faunas of late Middle and early are overlain by 125-250 m of sandstone, Late Devonian age (J. T. Dutro, Jr., and siltstone, and shale followed in turn by W. A. Oliver, Jr., written commun., 1974, 60-90 m of locally reefoid limestone (Dl). 1975, 1976). This unit locally includes The limestone is overlain by an unknown discontinuous gray limestone beds of late thickness of black shale and phosphatic Early or Middle Devonian age (No. 17, chert that locally contains distinctive spher- table 1) oidal concretions ("blackballs") of fluorapatite. Radiolarians from the concretions Pzp PHYLL1TE.-Prominent unit of dark-gray are late Devonian (Famennian) in age (D.L. phyllite, phyllitic shale, and argillaceous Jones, oral commun., 1978). The locally siltstone that lithologically is very similar to abundant fauna from the clastic rocks and the phyllitic shale in unit KJs both north limestone indicates a late Middle and early (T. 33 N., R. 13 and 14 W.) and south Late Devonian age (J. T. Dutro, Jr., and W. A. (T. 26 N., R. 20 W.) of the Denali fault. The Oliver, Jr., written commun., 1975, 1976) contact relation with unit PLUSis unknown but appears to be gradational southwest of Massive-bedded chert-limestone conglom- Pingston Creek (T. 29 N., R. 19 W.); the unit erate, sandstone, and shale, in part terres- may be allochthonous KJs. It contains worm trial, is thought to be gradational downward trails, which are abundant in unit Pzus but are into graywacke, shale, grit, minor pillow also locally present in unit KJs south of the basalt, and local beds of echinoderm-pellet W.) limestone and lime mudstone of Late Missis- Denali fault (T. 26 N., R. 20 sippian and Middle Pennsylvanian age (A. K. Armstrong, written cornmun., 1976). The SI LIMESTONE.-Light-gray to light-brown chert-limestone conglomerate and sandstone massive-weathering marbleized limestone and local areas of thin-bedded to laminated are gradational upward to the nonmarine conglomerate of Mount Dall of Middle limestone. Freshly broken surfaces of bedded limestone are dark gray and emit a Pennsylvanian age fetid odor. Marbleized limestone contains Dl LIMESTONE.-Middle and Upper Devonian probably Silurian dasycladacean algae and limestone near Shellabarger Pass is more corals (J. T. Dutro, Jr., and W. A. Oliver, Jr., than 95 m thick. On the West Fork of the written cornmun., 1977). Bedded limestone Yentna River (T. 29 N., R. 18 W.) the base contains rare graptolites of Silurian age of the limestone is transitional with yellow (Claire Carter, written commun., 1977). This to dark-gray siltstones and shales. The lower unit is mapped only as small allochthonous 10 m consists of thin-bedded gray micrites blocks of marbleized limestone in unit Pzus that grade upward into fossiliferous, massive along north side of Shellabarger Pass to reefoid, biostromal beds of colonial rugose (T. 28 N., R. 18 W.) and as a larger area of limestone on south side of pass. It may, in light-gray chlorite. Shale beds are locally part, be equivalent to Silurian and (or) phyllitic and average 0.15 m in thickness. A Devonian limestone in the Terra Cotta well-exposed sequence of interbedded Mountains (Churkin and others, 1977) sandstone, shale, and limestone on the east side of Jones River in the McGrath quad- Oc CHERT AND SHALE.-Chaotically folded rangle (T. 27 N., R. 22 W.) consists of an allochthonous sequence of interbedded upper 360-m-thick massive turbidite sub- graptolitic shale and chert and minor sand- graywacke sequence underlain by 250 m of stone and siltstone. Its thickness is well-bedded argillaceous and dolomitic lime unknown; some folds suggest penecontem- mudstone (micrite) containing Middle poraneous deformation. Chert beds average Silurian graptolites (Claire Carter, written about 7 cm thick and are rhythmically commun., 1977), and a lower 300-m-thick interbedded with black shale. Shale locally massive-bedded subgraywacke to proto- contains abundant Early to Middle Ordo- quartzite that contains interbeds of dark- vician graptolites (Claire Carter, written gray silty shale. The lower clastic unit, which commun., 1977). The distribution of Ordo- shows rhythmic and graded bedding and vician rocks on the north flank of Cathedral contains numerous shale fragments, is Spires (Tmc, T. 27 and 28 N., R. 19 W.) and interpreted as a turbidite. East-trending the nature of their contact with adjacent isoclinal folds in the McGrath quadrangle, sedimentary rocks (KJs and bus) is poorly disharmonic folds on all scales north of the understood because thermal metamorphism Tatina River, and multiple folds are also of Ordovician shale and siltstone (chert was present. Intruded by dark-green, partially not noted in this area) and KJs shale and altered mafic dikes. These rocks can be siltstone has produced essentially identical- traced westward into.the McGrath quad- appearing metamorphic rocks consisting of rangle for at least 50 km and may, in part, dark-gray phyllitic shale and argillite. The be equivalent to the Terra Cotta sequence presence of Ordovician rocks is indicated near the Post River (Churkin and others, only by rare, poorly preserved graptolites in 1977) T. 28 N., R. 19 W. In addition, it locally contains discontinuous units of recrystal- Intrusive and ultramafic rocks lized limestone 5-30m thick that may be tectonically emplaced slivers of unit S1. The Tf FORAKER PLUT0N.-Medium-grained, unit is tentatively correlated with Ordovician equigranular biotite and hornblende granodi- hemipelagic graptolitic shale and ribbon orite with a color index that averages about chert in the Terra Cotta Mountains (Churkin 13 but ranges from 9 to 20. Petrographic and others, 1977) descriptions and modal and chemical analyses are given in Reed and Lanphere (1974). Pzd SEDIMENTARY ROCKS OF DILLINGER This pluton has nearly identical mineralogy RIVER.-A thick sequence of sedimentary and chemistry to the McGonagall pluton in rocks presently recognized to consist of an the Mount McKinley quadrangle, and both undetermined thickness of interbedded lime plutons are considered to be parts of a single mudstone and shale, more than 900 m of igneous mass that has undergone right-lateral apparently unfossiliferous deep-water, displacement of about 38 km along the well-bedded lime mudstone (micrites), and McKinley segment of the Denali fault system at least 900 m of interbedded sandstone, since the igneous mass crystallized about shale, and limestone (Armstrong and others, 38 m.y. ago. Molybdenite-bearing quartz 1977). Due to complex structural features veins are associated with the Foraker pluton, and sparse megafossils, the relative age of and sparse cobbles of granodiorite on the these units is poorly known. Lime-mudstone Yentna and Lacuna Glaciers contain frac- and shale in the McGrath quadrangle (NW% tures filled with molybdenite, pyrite, and see. 14, T. 27 N., R. 21 W.) contain poorly chalcopyrite preserved Cardiolacea of late Wenlockian (Silurian) age (Jiri Kriz, written commun., McKINLEY SEQUENCE 1977). Limestone is medium to dark gray, A group of widely scattered, relatively large weathers light gray, is uniformly bedded, plutonic bodies generally restricted to unit and locally contains conodonts of Silurian KJs terrane south of the Denali fault. The age (A. G. Harris, written commun., 1977). Tonzona pluton, north of the fault, is also Beds range in thickness from 0.1 to 1.2 m, part of this sequence. The McKinley average about 0.2 m, locally contain well- sequence is assigned to the Late Cretaceous formed pyrite cubes that are partially and early Tertiary plutonic event in south- mantled by pressure fringes of quartz and central Alaska. Nine K-Ar ages on biotite and muscovite from the McKinley sequence grained biotite-hornblende granodiorite range from 52.3 to 56.2 m.y. Although (T. 31 N., R. 9 W.) is included with the previously classified as quartz monzonite Kahiltna pluton and granite (Reed and Lanphere, 1973a), the classification system used herein is that Tmr Ruth p1uton.-A large granitic mass that is recommended by the IUGS (Streckeisen, probably continuous at depth but is de- 1973), and the rocks fall within the granite scribed below as a north body, a south body, and granodiorite fields (fig. 1). In general, and the Chulitna body. The Chulitna body the rocks are coarse grained, fresh, withequi- (T. 30 N., R. 5 W.)is medium- to coarse- granular to porphyritic hypidiomorphic grained, weakly foliated, hypidiomorphic- textures.Biotite,generally inamounts between granular biotite granite. Rare hornblende is 3 and 10 volume percent, is the chief mafic present in one sample. Apatite, allanite, and mineral; hornblende is extremely rare or zircon are characteristic accessory minerals, absent from most plutons. Muscovite is and tourmaline, fluorite, and an unidentified present in some plutons but generally makes zeolite are present in some samples. The up less than 2 volume percent of the rock. south body (T. 3 1 N., R. 6 W.) is character- The plutons are epizonal and discordant and ized by coarse-grained hypidiomorphic- generally lack xenoliths, although they granular biotite granite and granodiorite that locally contain foundered blocks of country locally are intensely weathered to grus. Rare rock along their margins and apical parts muscovite or hornblende is present in some samples. The average color index of 10 Tm McKinley p1uton.-This pluton forms the samples is 10. Accessory minerals are the rugged and spectacular massif of Mt. Mc- same as in the Chulitna body. The north Kinley. Only one sample was collected from body (T. 33 N., R. 6 W.) is medium- to the massif, but glacier-transported boulders coarse-grained hypidiomorphic-granular from the pluton are uniformly similar in biotite and biotite-muscovite granite. Aver- composition and consist of medium- to age color index of seven samples is 6. The coarse-grained, hypidiomorphic-granular coarser grained granite typically weathers to biotite granite ghs. Tin anomalies from pan concentrate samples are spatially associated with iron- Tmc Cathedral p1uton.-Medium- to coarse-grained, stained, altered, an2 intensely weathered hypidiomorphic-granular biotite and biotite- zones in the granite. Tourmaline, fluorite, muscovite granite form the rugged and ,and topaz have been observed in some spectacular Cathedral Spires (T. 27 N., samples in addition to the characteristic R. 19 W.). The rocks also contain minor apatite and zircon phases of biotite-hornblende granodiorite. A small, petrographically similar body of Te GRANODIORITE OF MOUNT ESTELLE.- fine-grained biotite granite occurs in The northern part of the granodiorite of T. 26 N., R. 17 W. Average color index of Mount Estelle crops out in the southwestern six samples is 8; apatite and zircon are part of the quadrangle. Modal and chemical characteristic accessory minerals, but fluo- plots for these rocks are given in Reed and rite, tourmaline, and allanite are also present Lanphere (1973a) and in figure I. The plutonic rocks are classified as silicic gran- Tmk Kahiltna p1uton.-A relatively large mass of odiorites and are medium grained, hypidio- granite and granodiorite geographically morphic-granular. Plagioclase averages separated into two bodies by the Kahiltna between 35 and 45 percent anorthite, Glacier. The body to the southwest is fine- although individual zoned crystals have a to coarse-grained, hypidiomorphic-granular greater range in composition. Biotite equals biotite and biotite-muscovite granite and or exceeds hornblende by as much as four granodiorite. The average color index of nine times, and tourmaline is a characteristic samples is 12. Apatite and zircon are charac- accessory mineral. Gold-bearing quartz- teristic accessory minerals, but tourmaline, sulfide veins are locally present within the garnet, and allanite are locally present. pluton (Reed and Elliott, 1970). Hornblende Samples from the northeast body are ages of about 65 and 66 m.y. are similar in medium- to coarse-grained, hypidiomorphic- age to the composite plutons and pbbably granular biotite-muscovite granite. Minor tin reflect the same plutonic event in this part anomalies are associated with a quartz- of Alaska tourmaline-muscovite greissen zone at the head of Hidden Creek (T. 30 N., R. 10 W.). Tcp COMPOSITE PLUT0NS.-A series of nine The average color index of 10 samples is 8. relatively small plutonic bodies, previously A small satellitic body of fine- to coarse- assigned to the Yentna series (Reed and Lanphere, 1973a), that form a 65-km-long quartz monzonite and -porphyritic biotite curvilinear belt extending northeast from the granite with potassium feldspar phenocrysts granodiorite of Mt. Estelle to the headwaters as much as 2 cm in length of Cascade Creek (T. 28 N., R. 15 W.). The average composition of the plutons is quartz Upper Yentna pluton (T. 28 N., R. 17 W.)- monzonite (figure I), but the larger and Elongate composite body consisting of better exposed plutons are composite in medium-grained equigranular biotite perido- nature and consist of distinctly different tite, medium- to coarse-grained pyroxene compositions that range from peridotite to quartz monzodiorite, porphyritic biotite- granite. Where intrusive sequences have been hornblende quartz monzonite, and biotite- established, the more mafic phases were the pyroxene quartz syenite that contains blue first to be emplaced. Some ultramafic bodies chatoyant potassium feldspar phenocrysts as are distinctive in that they contain a, signifi- much as 2.5 cm in length cant amount of K-feldspar along with olivine, clinopyroxene, and biotite. Olivine is Cascade pluton (T. 28 N., R. 16 W.)- commonly rimmed by pyroxene and biotite Composite pluton consisting of potassium and, in the peridotites, is altered to fibrous feldspar-bearing peridotite, medium-grained amphibole and iron oxide minerals. Serpen- hornblende quartz monzodiorite and biotite tine minerals were not noted. Mafic minerals granodiorite, and a late phase of medium- to in the more felsic rocks are biotite, rare to coarse-grained porphyritic pyroxene-biotite abundant clinopyroxene or clino- and quartz monzonite. Copper and gold mineral- orthopyroxene, and locally olivine and ization indicated by stream sediment and hornblende. Apatite is a characteristic pan concentrate samples from Cascade accessory mineral Creek. K-Ar age date of 64.6 + 1.8 m.y. (Reed and Lanphere, 1973a) Ptarmigan stock (T. 24 N., R. 20 W.)- Medium-grained, hypidiomorphic-granular East Fork Yentna stock (T. 28 N., biotite granite R. 15 W.)-Medium-grained, hypidiomorphic-granular, pyroxene-bearing biotite Kohlsaat pluton (T. 25 N., R. 20 W.)- granodiorite Composite pluton consisting of medium- to coarse-grained, hypidiomorphic-granular Shellabarger stock (T. 28 N., R. 18 W.)- biotite granite, hornblende-biotite granite, Small, poorly exposed stock of altered and pyroxene-biotite monzonite. K-Ar age biotite-pyroxene gabbro northwest of the date of 65.6 * 1.9 may.(Reed and Lan- main composite plutonic belt. Relation to phere, 1973a) the composite plutons unknown. Locally contains abundant disseminations of spec- Threemile stock (T. 25 N., R. 19 W.)-Fine- ular hematite. Semiquantitative spectro- to medium-grained, locally porphyritic, graphic analyses of three grab samples pyroxene-bearing biotite quartz monzonite average 1000 ppm copper that contains altered phenocrysts of pyroxene TKk KICHATNA PLUT0NS.-Nine small plutons, some of which were previously assigned to Kichatna stock (T. 26 N., R. 18 W.)- the Yentna series (Reed and Lanphere, Composite stock consisting chiefly of 1973a), located southeast of the belt of medium- to coarse-grained, hypidiomorphic- composite plutons (TKc). The average granular olivine-bearing pyroxene quartz composition of these rocks is granodiorite, monzonite; locally contains disseminated but quartz monzodiorite and quartz diorite pyrite, pyrrhotite, and chalcopyrite are also present (fig. 1). The rocks are predominantly medium grained and hypidio- Fourth of July stock (T. 26 N., R. 17 W.)- morphic-granular; dark-reddish-brown Small pipelike body of porphyritic perido- biotite and light-green hornblende comprise tite containing euhedral phenocrysts of about 22 volume percent of the rocks. green clinopyroxene as much as 5 cm long in Biotite generally exceeds hornblende. Allan- a groundmass of clinopyroxene, bladed ite, apatite, and zircon are characteristic biotite, rounded olivine, and interstitial accessory minerals. A K-Ar age date of potassium feldspar 67.4 + 2 m.y. (Reed and Lanphere, 1973a) from a hornblende-biotite quartz diorite Lower Yentna pluton (T. 28 N., R. 17 W.)- (T. 23 N., R. 17 W.) suggests that these Elongate composite body of medium- to rocks may be slightly older than the com- coarse-grained pyroxene and (or) biotite posite plutons TONZONA PLUfON CATHEDRAL PLUTON

Southwest body North body Northeast body South body AChulitna body

KAHILTNA PLUTON RUTH PLUTON

0 Kohlsaat pluton Lower Yentna

Upper Yentna

Cascade pluton

GRANODIORITE OF MT ESTELLE COMPOSITE PLUTONS

KICHATNA PLUTONS FORAKER PLUTON

Quartz

Granite

diorite

PLUTONIC ROCK CLASSIFICATION

Figure 1.-Modal analyses of plutonic rocks in the Talkeetna quadrangle. Classification after Streckeisen (1973). TKi UNDIVIDED INTRUSIVE ROCKS.-Small exposures that have a rough-textured sur- intrusive bodies of Late Cretaceous or face. It is intruded by the main mass and Tertiary age with compositions that preclude dikes of Foraker pluton (Tf). Olivine definite assignment to the above intrusive is recrystallized and (or) altered to sequences. Owing to difficult accessibility, serpentine minerals, talc, and amphibole; many of these bodies could not be sampled; shearinp and deformation banding are their contacts were mapped from the air. common. Chromite occurs chiefly in dunite The petrography and composition of these sills as (1) disseminated rounded grains rocks are incompletely known. The rocks 1-3 mrn in diameter, (2) streaks and lenses, sampled range in composition from granite (3) disrupted and irregular pods of various to tonalite. The small body located along the shapes as much as 2 m in maximum dimen- Susitna River (T. 29 N., R. 4 W.) is a weakly sion that are cut by dunite, and (4) lenslike foliated, fine- to medium-grained, hypidio* bodies as much as 2 m thick and 20 m in morphic-granular biotite granodiorite. The length. Chrome content of typical dunite two small intrusive bodies of biotite granite ranges from 0.7 to 1 percent. The average of and biotite-hornblende granodiorite 3 microprobe analyses from one sample of (T.32N.,R.5W.,andT.33N.,R.4W., chromite (T. P. Thayer, written commun., respectively) may be satellitic bodies of the 1976) is 58.4 percent Cr203, 2 1.l percent Ruth pluton. At Little Peters Hills (T. 26 N., FeO, 8.9 percent MgO, and 9.7 percent R. 8 W.) a small body of equigranular medi- Alz 03.Serpentinite, composed of dark- um-grained biotite granite, thought to green serpentine minerals and magnetite, belong to the McKinley sequence, cuts locally contains relict olivine crystals altered argillite and graywacke hornfels. The small to antigorite and chrysotile. It is cut by veins body on the east side of the Kahiltna Glacier of finegrained carbonate and unidentified (T. 3 1 N., R. 10 W.) is a foliated, recrystal- clay minerals. Emerald-green chrome-nickel lized, fine- to medium-grained biotite-horn- garnets, locally present in calc-silicate rocks blende granodiorite. In thin section, horn- (Dl?) adjacent to dunite, suggest that the blende is dark bluish green in color. The dunite was emplaced at moderately high small intrusive body located in T. 30 N., temperatures R. 14 W. is a medium-grained porphyritic biotite tonalite and may be a satellitic body of the Foraker pluton. The intrusive mapped Chulitna sequence as TKi in T. 25 and 26 N., R. 4 W. is reported to be an altered biotite-hornblende The informal term "Chulitna sequence" refers quartz diorite (Bela Csejtey, oral commun., to an allochthonous sequence of upper 1976) Paleozoic limestone, tuff, and argillite, Lower and Upper Triassic limestone, basalt, &Pzi UNDIVIDED INTRUSIVE ROCKS.-Dark-gray and volcanic redbeds, and Upper Triassic and to greenish-gray dikes and sills that range in Lower Jurassic marine sandstone and argil- composition from basalt to granodiodte. lite that crop out on the south flank of the Most bodies are porphyritic and contain Alaska Range along the Chulitna Valley altered phenocrysts of plagioclase and northeast of the Talkeetna quadrangle amphibole in a matrix of fine-grained feld- (Jones and others, 1976, and written spar, quartz, and alteration minerals. Clino- commun., 1977; Hawley and Clark, 1974; pyroxene is rare. The age is unknown. Most Clark and others, 1972). In the Chulitna area bodies are too small to show at scale of map these rocks include a dismembered ophiolite (oceanic crust) sequence of Late Devonian Mbd UNDIVIDED DUNITE AND SERPENTIN- age (D. L. Jones, oral commun., 1977) 1TE.-Alpinetype ultramafic rocks and which has not been observed in the Talkeet- minor talc schist exposed discontinuously na quadrangle. Within the Talkeetna quad- for approximately 25 km along the Lacuna rangle the Chulitna sequence consists of and Yentna Glaciers and as small isolated Upper Triassic limestone and basalt, Upper occurrences in T. 3 1 N., R. 14 W. and Triassic volcaniclastic and clastic sedlmen- T. 30 N., R. 15 W. Chromite-bearing dunite tary rocks, and Upper Triassic(?) and Lower occurs as sills as much as 90 m thick and Jurassic massive- to well-bedded siliceous 7 km long (T. 3 1 N., R. 12 W.) and as a argillite, chert, and minor pillow basalt. larger irregular body of dunite and serpentin- These rocks structurally overlie Jurassic(?) ite in T. 30 N., R. 13 and 14 W. A magnetic and Cretaceous graywacke, shale, and phyl- high suggests that similar rocks may intrude lite (KJs) and are locally cut by small intru- Pzus at depth in T. 30 N., R. 14 W. Dunite sive bodies of biotite granite (Tmr) and forms massive light-orangebrown-weathering biotite-hornblende granodiorite (TKi) JTsv SEDIMENTARY AND VOLCANIC ROCKS.- , black chert and phyllite. Basaltic aqua- Massive cliff-forrhing outcrops of silicified gene(?) tuff is dark green, massive, and argillite, radiolarian chert, sil.tstone, minor locally semischistose and consists of lapilli- pillow basalt, and impure micritic limestone. size fragments of various types of volcanic Yellowish-gray to black silicified arglllite and rocks. Also present are coarse lenses of chert is well bedded, and individual beds chlorite and calcite with unidentified bire- average about 8 cm in thickness. Siltstone is fripgent microlites, and sparse plagioclase. dark gray, rust weathering, and well sorted Schistase basaltic tuffs contain lenses of and contains abundant angular quartz, clinopyroxene phenocrysts and amygdules feldspar, and metachert fragments that of calcite, quartz and epidote, and chlorite average 0.2 mm in size. Minor pillow basalt and quartz. L~callyabundant fossils from in units as much as 3 m thick is composed of this unit include Monatis subcircularis Gabb, medium-grained plagloclase and clinopyrox- Heterastridi~~n,and Cassianella of Late ene. Clinopyroxene is altered to blue-green Triassic (Norian) age (N. J. Silberling, amphibole and chlorite written commun., 1977)

VOLCANIC AND SEDIMENTARY ROCKS.- Distinctive light-green to maroon volcaniclastic rocks that range from tuffaceous shale to a quartz-pebble conglomerate with a REFERENCES CITED tuffaceous matrix. These rocks commonly contain interbeds of massive micritic and Armstrong, A. K., Reed, B. L., and Carter, Claire, 1977, bioclastic limestone, minor chert, argillite, Paleozoic sediments in the northwest part of the rust-weathering siltstone and sandstone, and Talkeetna quadrangle, Alaska Range, Alaska: U.S. basaltic tuff. The coarser grained tuffaceous Geological Survey Circular 75 1-8, p. B6 1. volcaniclastic beds average about 0.6 m in Bundtzen, T. K., Smith, T. E., and Tosdal, R. M., 1975, thickness and contain angular to subrounded Progress report: Geology and mineral deposits of the clasts of quartz, chert, volcanic rocks, and Kantishna Hills, Alaska: Alaska Division of Geological siltstone as much as 7 cm in maximum and Geophysical Surveys open-file report 98,80 p. dimension. In places the texture appears Calderwood, K. W., and Fackler, W. C., 1972, Proposed cataclastic, and in others the rock is a sedi- stratigraphic nomenclature for Kenai Group, Cook mentary breccia. Unit contains locally Inlet basin, Alaska: American Association of Petrole- abundant fossils including Thecosmilia, um Geologists Bulletin, v. 56, no. 4, p. 739-754. Heterastridium, and Septocardia(?) of Upper Churkin, Michael, Jr., Reed, B. L., Carter, Claire, and Triassic (Norian) age (N. J. Silberling, Winkler, G. R., 1977, Lower Paleozoic graptolitic wntten commun., 1977). Basaltic tuff section from the Terra Cotta Mountains, southern consists of plagioclase crystals and volcanic Alaska Range: U.S. Geological Survey Circular 75 1-B, rock fragments in a semischistose matrix of p. B37. chlorite and altered glass Clark, A. L., Clark, S. H. B., and Hawley, C. C., 1972, Significance of upper Palezoic oceanic crust in the LIMESTONE AND BASALT.-Upper Triassic Upper Chulitna district, west-central Alaska Range, in limestone and basalt form a distinctive unit Geological Survey research 1972: U.S. Geological northeast of the Eldridge Glacier (Clark and Survey Professional Paper 800-C, p. C95-C 10 1. others, 1972) and apparently grade south- Gilbert, W. G., Ferrell, V. M., and Turner, D. L., 1976, westward into interbedded gray limestone, The Teklanika Formatian-a new Paleocene volcanic dark-green pillow basalt, massive and locally formation in the central Alaska Range: Alaska semischistose basaltic tuff, calcareous sand- Division of Geological and Geophysical Surveys stone, and minor black chert and silty Geologic Report 47, 16 p. limestone. The limestone is discontinuously Hawley, C. C., and Clark, A. L., 1974, Geology and exposed and usually interbedded with or mineral deposits of the Upper Chulitna district, included within basalt. It is commonly Alaska: U.S. Geological Survey Professional Paper massive and marbleized but locally is well 758-B, p. BI-B47. bedded and contains volcanic rock frag- Jones, D. L., Pessagno, E. A., Jr., and Csejtey, Bela, Jr., ments. Dark-green basaltic rocks form 1976, Significance of Upper Chulitna ophiolite for massive-weathering outcrops. Recognizable the late Mesozoic evolution of southern Alaska: pillow basalt is subordinate to basaltic tuff, Geological Society of America Abstracts with Pro-

but pillows as much as 45 cm in diameter are & grams, v. 8, no. 3, p. 385-386. locally present. Irregular masses of chlorite, Karlstrom, T. N. V., 1964, Quaternary geology of the epidote, fibrous amphibole, and iron oxides Kanai Lowland and glacial history of the Cook Inlet suggest the former presence of mafic min- aegion, Alaska: U.S. Geological Survey Professional erals. The pillow basalt contains interbeds of Paper 443,69 p. Pearce, J. A., and Cann, T. R., 1973, Tectonic setting of -1973a, Alaska-Aleutian Range batholith: Geochron- basic volcanic rocks determined using trace element ology, chemistry, and relation to circumPacific analyses: Earth and Planetary Science Letters, v. 19, plutonism: Geological Society of America Bulletin, p. 290-300. v. 84, p. 2583-2610. Reed, B. L., and Elliott, R. L., 1968, Results of stream - 1973b, Plutonic rocks of the Alaska-Aleutian Range sediment sampling in parts of the southern Alaska batholith, in International Arctic Symposium, 2d, Range: U.S. Geological Survey Open-File Report, 9 p. Proceedings: American Association of Petroleum _ 1970, Reconnaissance geologic map, analyses of Geologists Memoir 15, p. 421-428. bedrock and stream sediment samples, and an aero- - 1974, Offset plutons and history of movement along magnetic map of parts of the southern Alaska Range: the McKinley segment of the Denali fault system, U.S. Geological Survey Open-File Report, 158 p. Alaska: Geological Society of America Bulletin, v. 85, Reed, B. L., and Eberlein, G. D., 1972, Massive sulfide p. 1883-1892. deposits near Shellabarger Pass, southern Alaska Streckeisen, A. L., chm., 1973, Classification and Range, Alaska: U.S. Geological Survey Bulletin nomenclature recommended by the IUGS Sub- 1342,45 p. commission on the systematics of igneous rocks: Reed, B. L., and Lanphere, M. A., 1972, Generalized Geotimes, v. 18, no. 10, p. 26-30. geologic map of the Alaska-Aleutian Range batholith Wahrhaftig, Clyde, Wolfe, J. A., Leopold, E, B., and showing potassium-argon ages of the plutonic rocks: Lanphere, M. A., 1969, The coal-bearing group in the U.S. Geological Survey Miscellaneous Field Studies Nenana coal field, Alaska: U.S. Geological Survey Map MF-372. Bulletin 1274-D, p. Dl-D28.

NOTE ADDED IN PRESS ...... Fieldwork in 1979 north bf the Denali fault indicates that Late Triassic conodonts are present in the sooty carbonaceous and calcareous shale, interbedded siltstone, limestone and quartzite (Pzsl) exposed along Pingston Creek in the S1/2 T, 45 N., R. 19 W.(D.L. Jones, N.J. Silberling, Bruce Wardlaw, and D.H. Richter, written commun., 1979). In the foothills northeast of Pingston Creek (NE1/4 T. 45 N., R. 19 W.), a lens of crinoidal limestone contains Early Pennsylvanian conodonts. These workers have divided unit Pzsl into two stratigraphic units. The units appear to be in fault contact, but the Triassic rocks are clearly infolded with the Pennsylvanian strata. In addition, the gabbro and quartz diorite dikes and sills (MzEg) that cut these sedimentary rocks must be Mesozoic or younger in age. Another important result of this recent investigation is the discovery of Permian conodonts and radiolarians of Permian and possibly Mississippian ages in chert exposed in the SE1/4 T. 45 N., R. 18 W. In this report the chert was mapped as part of the undivided marine sedimentary rocks (KJs). The chert is probably in fault contact with the adjacent clastic rocks. Although undivided KJs north of the fault locally contains rocks of Early Cretaceous age (locality 4, this report), its age is now known to extend into the upper Paleozoic.