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Home , Cataclastic rock, Chickaloon Formation, Chugach Mountains, Geology of Alaska, Kings Mountain (Alaska), KMXS

GEOLOGY OF THE NORTHERN CHUGACH , SOUTHCENTRAL

BY L.E.Burns, G.H.Pessel, T.A. Little, T.L. Pavlis, R.J. Newberry, G.R. Winkler, and John Decker

Professional Report 94

Published by STATE OF ALASKA DEPARTMENT OF NATURAL RESOURCES DMSION OF GEOLOGICAL & GEOPHYSICAL SURVEYS Alaska Department of NATURAL RESOURCES OF THE NORTHERN ,

BY

L.E. Burns, G.H. Pessel, TA. Little, T.L. Pavlis, R.J. Newberry, G.R. Winkler, and John Decker

Cover: View looking east into of a tributary of Coal Creek in the Anchorage C-4 Quadrangle. Photo shows (Mzsh) and a very mixed unit of diorite-quam diorite (Jdqd) inmdcd by Creraceow rrondhjemite (Kt). A smaU -bounded sliver of Jurassic gabbroic rocks (Jgu) is imbedded in the rrondhjemite. A mkture of Jurassic plutonic rocks, including compositionsfrom gabbroic to quom diontic, crops our in the background. Foreground is composed of a rock (Qrg) and glacier (Qg). Photo by G.H. Pessel, 1983.

Professional Report 94 Division of Geological & Geophysical Surveys

Fairbanks, Alaska 1991 STATE OF ALASKA Walter J. Hickel, Governor DEPARTMENT OF NATURAL RESOURCES Harold C. Heinze, Commissioner DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS Thomas E. Smith, Acting Director and State Geologist

DGGS publications may be inspected at the following locations. Address mail orders to the Fairbanks office.

Alaska Division of Geological & Geophysical Surveys 794 University Avenue, Suite 200 400 Willoughby Avenue, 3rd floor Fairbanks, Alaska 99709-3645 Juneau, Alaska 99801-1796

U. S. Geological Survey Science Information Center 605 West 4th Avenue, Room G684 4230 University Drive, Room 101 Anchorage, Alaska 99501-2299 Anchorage, Alaska 99508-4664

This publication, released by the Division of Geological & Geophysical Surveys, was produced at a cost of $10 per copy which includes the text printed in Delta Junction, Alaska and the oversize maps which were printed in , D.C.Publica- tion is required by Alaska Statute 41, "to determine the potential of Alaskan land for production of metals, , ...[and] advance knowledge of the ." CONTENTS

Page

Abstract ...... Introduction ...... Description of the study area ...... Location, access, and topography ...... Previous work ...... Current study ...... Geologic setting ...... Peninsular ...... Chugach terrane ...... Rocks common to both ...... Descriptive geology of the Peninsular terranc ...... (?) and Mesozoic metamorphic rocks ...... and ...... Amphibolite and marble ...... Pelitic schist and ...... Mixed assemblages of metamorphic and plutonic rock ...... Alteration and ...... Age, correlation, and interpretation ...... Jurassic system igneous rocks ...... Volcanic rocks of the Talkeetna Formation ...... Plutonic rocks ...... Cumulate ultramafic rocks ...... Petrology and lithology ...... Mineralogy ...... Alteration and metamorphism ...... Deformation ...... Gabbroic rocks ...... Petrology and lithology ...... Mineralogy ...... Gabbronorite clan ...... -hornblende gabbronorite ...... Alteration and metamorphism ...... Age and correlation ...... Intermediate composition plutonic rocks ...... diorite and tonalite ...... Granodiorite within quartz diorite-tonalilc plutons ...... Small plutons of quartz monzodiorite and granodiorite ...... Age and correlation ...... Interpretation ...... sedimentary rocks ...... ...... Petrology and lithology ...... Age and correlation ...... Interpretation ...... Descriptive geology of the Chugach terrane ...... Cretaceous system ...... McHugh Complex ...... Petrology and lithology ...... Alteration and metamorphism ...... Age and correlation ...... Interpretation and deformation ...... Valdez Group ...... Petrology and lithology ...... Alteration and metamorphism ...... Age and correlation ...... Interpretation and deformation ...... Descriptive geology of rocks common to both terranes ...... Cretaceous system plutonic rocks ...... Tonalite-diorite-trondhjemite ...... Dikes in eastern part of map area ...... Plutons in western part of map area ...... Compositon of both eastcrn and western trondhjemites ...... Age, correlation, and interpretation...... Tertiary system ...... Sedimentary rocks of the Chickaloon Formation ...... Petrology and lithology ...... Conglomerate...... ...... Mudstonc ...... Clast types and sandstone petrography ...... Vertical and lateral variations ...... Age and correlation ...... Interpretation ...... Hypabyssal rocks ...... Peninsular terrane felsites ...... Chugach terrane felsites ...... Age, correlation, and interpretation ...... ...... Peninsular terrane ...... Deformation of dominantly Jurassic age ...... Metamorphic rocks of dominantly Jurassic age ...... Plutonic rocks ...... Volcanic rocks ...... Brittle deformation of probable Early Cretaceous age ...... Uplift, , and northward tilting of postJurassic age ...... Uplift, erosion, and faulting of latest Cretaceous-early age ...... Faulting and folding of post-early Eocene age ...... Faulting ...... Folding ...... Faulting that postdates intrusion of Late Eocene felsites ...... Chugach terrane ...... Cretaceous deformation affecting the McHugh Complex ...... Deformation affecting both the McHugh Complex and Valdez Group...... Valdez Group ...... S1-forming event ...... S2-forming event ...... Interpretation ...... Deformation of Chugach terrane rocks similar to that affecting early Eocene and older rocks in the Peninsular terrane ...... Faulting ...... Folding ...... Summary and conclusions ...... Acknowledgments ...... References ...... FIGUKES

Figure 1. Location map of southcentral Alaska, showing ranges, major valleys, and major faults ...... Generalized geologic map of Nelchina- ...... Map of Nelchina-Kings Mountain region showing quadrangle boundaries, Ranges fault and major surficial features ...... Harker diagrams for Talkeetna Formation volcanic rocks ...... AFM diagram for Talkeetna Formation volcanic rocks ...... Variation diagram (SO2 versus FcO*/MgO) for Talkeetna Formation volcanic rocks ...... Harker diagrams for gabbroic, quartz dioritic, and trondhjemitic rocks from the northcentral Chugach Mountains ...... Ternary diagram showing modal percentages of quartz-plagioclase-potassium- of a quartz diorite-tonalite pluton ...... Synoptic stereogram of combined poles to schistocity and bedding in prc- Jurassic metamorphic rocks of the Peninsular terrane ...... Contoured lower-hemisphere equal-area stereograms of poles to schistosity and in the Cretaceous complex ...... Stylized map sections showing present distribution of McHugh Complex, Peninsular terrane, and Valdez Group and inferred model in central part of study area ......

TABLES

Table 1. Major-oxide analyses and CIPW norms on volcanic rocks of the Talkeetna Formation ...... Major-oxide analyses and CIPW norms on gabbroic rocks from the north- central Chugach Mountains ...... Major-oxide analyses and CIPW norms on quartz diorite, tonalite, and granodiorite from the northcentral Chugach Mountains ...... K-Ar and U-Pb ages from intermediate composition plutonic rocks of Jurassic age from the northcentral Chugach Mountains ...... IS-Ar and U-Pb ages from Cretaceous tonalite and trondhjemite plutonic rocks of the northcentral Chugach Mountains ...... Major-oxide analyses and CIPW norms of trondhjemite from the northcentral Chugach Mountains ...... Major-oxide analyses and CIPW norms on hypabyssal rocks of the northcentral Chugach Mountains ...... K-Ar ages from hypabyssal rocks of the northcentral Chugach Mountains ......

SHEETS [In pocket]

Sheet 1. Geologic map of the northern Chugach Mountains, southcentral Alaska (west half) 2. Geologic map of the northern Chugach Mountains, southcentral Alaska (east half) GEOLOGY OF THE NORTHERN CHUGACH MOUNTAINS, SOUTHCENTRAL ALASKA

BY L.E. ~urns?G.H. ~essel?T.A. ~ittle?T.L. ~avlis?RJ, ~ewberr~tG.R. ~inkler: and John ~ecker~

ABSTRACT batholith. The intermediate rocks also occur as complexes in the Chugach Mountains. The Border Ranges fault system juxtaposes the Peninsular The arc bcca~~~eextinct by about and Chugach terranes, and is one of the major structural features time. Uplift and erosion produced an unconformity between the in southcentral Alaska. This fault system can be traced in an 'Talkeetna Formation and the overlying Cretaccous Matanuska arcuate pattern from Kodiak Island to southeastern Alaska along Formation, a sequence of marine sedimentary rocks. the northern part of the Chugach and Fairweather Mountains. In Between Middle Jurassic and late Early Cretaccous time, a the northcentral Chugach Mountains, the fault system shows convergent margin was formed along the southern edge of the evidence for direct compressional movement between the two extinct Peninsular terrane . Deposition of trench slope terranes, and younger dextral-wrench movement, similar to that deposits, and deformation of these sediments in a subduction present on the and Castle Mountain fault systems. wedge, formed the McIIugh Complex, a subduction melange. The Peninsular terrane crops out to the north of the Border Continued convergent plate motion pushed the McIIugh Complex Ranges fault system and is composed of the plutonic core and beneath the Peninsular tcrranc rocks, which probably caused extrusive portions of an intraoceanic island arc of Jurassic age and uplift, erosion, and northward tilting of rocks of the I'cninsular younger sedimentary rocks. The Chugach tcrrane to the south is terranc. The underthrusting formed large brittle zones with composed of trench-fill and trench slope deposits of largely cataclastic texture. l'hcsc shear zones and inferred thrusts are Cretaceous age. part of the original Bordcr Ranges fault system. The geologic mapping described in this report suggests that In latest Cretaccous - Paleocene time, continued mafic and ultramafic rocks of the island arc intruded upper convergence and sediment deposition caused thrust emplacement Paleozoic(?) metavolcanic and metasedimcntary rocks at of the Valdez Group, a thick sequence of trench fill deposits, moderately high pressures (15 to 30 km depth). Much multiple beneath the MclIugh Complex and dcposition of the Matanuska diking occurred, although large chambers of may also have Formation ceased because of uplift of the Peninsular and Chugach been present. The mafic and ultramafic rocks formed a terranes. South-side-up block faulting within the Bordcr Ranges coilsiderable thickness, possibly 10 km, by the end of arc activity. fault system was followed by deposition of humid-climate, alluvial Fractional crystallization and vcnting of the mafic fan sediments of the Chikaloon Formation. probably produced much of the associated with the In post-Early- to pre-late Eocene time, the Border Ranges overlying Talkeetna Formation volcanic rocks. Large plutons of fault system was activated as a dextral strike-slip fault zone, prior intermediate composition began intruding the mafic plutonic rocks to intrusion of widely distributed Eocene felsite dikes. No more before all gabbroic rock had completely cooled. The intermediate than a few tens of kilometers of dextral-slip accumulated during rocks crop out discontinuously from near Tonsina to near Kodiak this faulting event. I'hc youngest rocks in the area are felsite and Island. The intermediate rocks are similar to those in the basaltic dikes of mid- to late Eocene age which are common Talkeetna- batholith, and may be an edge of that throughout both terranes in the map area and locally intrude the faulted boundaly between these terranes. Movement on the Border Ranges fault system has probably been minor since '~laska Division of Geological and Geophysical Surveys, 3700 intrusion of these dikes in late Flcene time. Airport Way, Fairbanks, Alaska 99709-4699. 21)cpartment of Geology, University of , St. Lucia, Queensland, 4072. INTRODUCTION 3~epartmentof Geology-Geophysics, University of New Orleans, New Orleans, 70148. DESCRIPTION OF THE 4~cpartmentof Geology, University of Alaska, Fairbanks, Alaska STUDY AREA 99775. 'US. Geological Survey, MS 905, Box 25046, Federal Center, Denver, Colorado 80225-0046. The area mapped in this study, the Nelchina-Kings 6~rcoAlaska Inc., P.O. Box 100360, Anchorage, Alaska 99510- Mountain region, lies within the northern Chugach 0360. Mountains in southcentral Alaska (fig. 1). The

Geology of the Northern Chugach Mountains 3

Border Ranges fault system crosscs the region and . F.F. Barnes and Arthur juxtaposes two geologic terranes: the Peninsular Grantz of the U.S. Geological Survey conducted tcrrane, composed of the plutonic core and extrusive extensive detailed mapping northeast of the map portions of an intraoceanic island arc of Jurassic age area in the 1950s and 1960s, respectively (Barnes and and younger sedimentary rocks; and to the south, the Payne, 1956; Grantz, 1960a,b,c, 1961a,b, 1964, 1966; Chugach terrane, composed of highly deformed Barnes, 1962; Grantz and others, 1963). Part of their trench-fill, trench-slope, and -basin deposits mapping along the northernmost edge of the largely of Cretaceous age (fig. 1). Chugach Mountains has been modified and The almost continuous exposure in the incorporated into this report. S.H.B. Clark (1972a,b) Nelchina-Kings Mountain region allows detailed mapped in the area of the Wolverine ultramafic examination of the processes that formed the complex in 1970; Clark's map has been modified and magmatic part of an island arc and the faulting along is also incorporated. George Plafker of the U.S. major convergent plate margins. This report Geological Survey had obtained some scattered data concentrates on the plutonic rocks of the Jurassic between Glacier Creek and the Nelchina Glacier, island arc, Cretaceous and Tertiary sedimentary while working on a study of the regional extent of the rocks, and post-Jurassic structural events. Border Ranges fault system during 1973-75, and very kindly furnished his unpublished field maps to us LOCATION, ACCESS, AND when we started work in the Nelchina-Kings TOPOGRAPHY Mountain region in 1979.

The Nelchina-Kings Mountain region lies CURRENT STUDY between the Nelchina Glacier on the east and the Knik on the west; the region is bounded on the The geologic mapping in this report is a north by the Matanuska Valley, a structural compilation of field work done over the 5-yr period that separates the Chugach and Talkeetna from 1979 to 1983 by a team of geologists. The U.S. Mountains (fig. 1). The region name was chosen Geological Survey (USGS) and the Alaska Division from geographic landmarks; no geologic boundaries of Geological and Geophysical Surveys (DGGS) are associated with its edges. The region lies entirely have cooperated on an informal basis in the within the Anchorage Quadrangle. Nelchina-Kings Mountain region since DGGS The Nelchina-Kings Mountain region is roadless started work in the Chugach Mountains in 1979. and generally inaccessible except by aircraft. A few Although geologic mapping was continued on a f~ed-winglanding sites are located on sandbars limited basis in 1980 and 1981, most of the work in along the major streams in the area, but best access the study area was done during 1982 and 1983, with is by helicopter. There are some trails in the eastern helicopter support. and western parts of the area that are used by guided In 1981, the USGS started its Alaska Re- hunting parties with pack horses. source Appraisal Project (AMRAP), which involved Topography in the Nelchina-Kings Mountain 1:250,000-scale mapping in the Anchorage region is extremely steep. Mountains with thousands Quadrangle (Winkler, 1990), while DGGS was doing of feet of relief are incised by deep glacial valleys and detailed mapping at a scale of 1:25,000 in the steep-sided stream-cut gorges. Most of the major northern Chugach Mountains. Geologists from both streams have their source in descending organizations worked together on an informal basis from ice fields in the core of the Chugach Mountains. in parts of the area, sharing information, ideas, and Glaciation occurred over much of the area as logistical support; the geologic map of the current recently as late (Reger and Updike, report is simplified and reproduced in the Anchorage 1983); moraines, , horns, and other glacial AMRAP map (Winkler, 1990). Detailed geologic features are common. maps published as a result of this study include Pessel and others (1981), Burns and others (1983), PREVIOUS WORK Pavlis (1986), Little and others (1986b), and Burns (in press). Geologically, the Nelchina-Kings Mountain Several graduate theses and mineralization region was relatively unexplored prior to 1979. G.C. studies on the northern Chugach Mountains have Martin (1926) and S.R. Capps (1927) performed been done as a result of this study. These include early reconnaissance exploration in the Matanuska work by Burns (1981, 1983) on Jurassic plutonic Valley and along the edges of the Chugach and rocks, Pavlis (1982a) on Mesozoic structural geology, 4 Professional Report 94

McMillan (1984) on Jurassic volcanic rocks, These intermediate plutonic rocks may correlate with Monteverde (1984) on Cretaceous plutonism, Serfes rocks of the main Talkeetna-Aleutian Range (1984) on amphibolites of the western Chugach batholith (Hudson, 1979) to the north, as they consist Mountains, and Little (1988) on Tertiary structural of the same rock types and have similar geology. R.J. Newberry, under contract to DGGS, compositions, ages, and intrusive relationships. conducted an in-depth study of mineralization in the Tectonic deformation in the northern Chugach northern Chugach Mountains (Newberry, 1986). Mountains has tilted and faulted the arc sequence of Peninsular terrane, exposing structurally lower arc rocks to the south (Pessel and others, 1981; Winkler GEOLOGIC SETTING and others, 1981b; Burns and others, 1983). Although faulting is extensive, the general of the The northern Chugach Mountains comprise the arc sequence remains intact (Burns, 1985). plutonic core and extrusive portions of an Cumulate ultramafic and mafic rocks of the intraoceanic island arc of Jurassic age juxtaposed Peninsular terrane are located on the southern edge with faulted subduction complex rocks, principally of of the arc sequence and are structurally overlain and Cretaceous age, to the south. The island arc rocks intruded by the intermediate plutonic rocks; volcanic compose part of the Peninsular terrane, and the rocks of the Talkeetna Formation form the northern accreted rocks compose the Chugach terrane (fig. 1). edge of the arc sequence and are structurally the The two contrasting terranes are juxtaposed along highest unit (fig. 2). the Border Ranges fault system, which can be traced The ultramafic and mafic plutonic rocks probably in an arcuate pattern along the northern part of the crystallized near the crust-mantle boundary (Burns, Chugach and Fairweather Mountains, from Kodiak 1985). Tectonized with a mantle Island to southeastern Alaska. The term Peninsular deformation texture are present south of the terrane as used herein also includes Cretaceous cumulate rocks, at the eastern edge of the arc sedimentary rocks north of the Border Ranges fault sequence (Burns 1985; Coleman and Burns, 1985). that were derived from the Jurassic arc. Minor amounts of harzburgites also occur south of the cumulate ultramafic rocks in the Wolverine PENINSULAR TERRANE complex (fig. 2) (Pavlis, 1983; Newberry, 1986). These harzburgites were intensely deformed during Rocks formed in a Late (?)-Early Jurassic movement of the Border Ranges fault system and island arc constitute most of the Peninsular terrane may represent mantle rocks. of Jones and others (1981). This terrane extends in Mafic plutonic rocks of the island arc intrude discontinuous outcrops from the Talkeetna metavolcanic and metasedimentary rocks of Mountains and northern Chugach Mountains to the unknown affinities (fig. 2) (Pessel and others, 1981; present-day Aleutian Arc on the . Burns, 1983; Pavlis, 1983). The protoliths of these A fault-bounded slice of island arc rocks ranging metamorphic rocks were dominantly mafic volcanic from 2 to 15 km in width crops out between the rocks and siliceous sedimentary rocks. With the Border Ranges fault system and the Matanuska possible exception of the tectonized harzburgites, Valley. The arc sequence consists of the Talkeetna these metamorphic rocks are the oldest rocks in the Formation, ultramafic and gabbroic rocks, and Nelchina-Kings Mountain region. Their ages have intruding quartz diorite and tonalite plutons. The not been constrained more precisely than pre-Early Talkeetna Formation, largely andesitic and dacitic Jurassic; for this report, they have been assigned to a volcanic rocks and volcaniclastic sedimentary rocks, Paleozoic-Mesozoic age range. The protolith is Early Jurassic in age. The ultramafic and gabbroic volcanic and sedimentary rocks were penctratively rocks are penecontemporaneous with the volcanic deformed and metamorphosed in the and rocks and probably represent the fractional amphibolite facies during a regional metamorphic crystallization products of magma(s) which produced event, probably as the island arc formed. The the volcanic rocks (Burns, 1983, 1985). A suite of metamorphic rocks are of upper amphibolite facies intermediate composition plutonic rocks, including where intruded by the gabbroic rocks of the arc, and diorite, quartz diorite, tonalite, and minor amounts of amphibolite facies where intruded by of granodiorite, intruded mafic and volcanic rocks of plutons of quartz diorite and tonalite. Extensive the arc sequence during Middle to Late Jurassic time retrograde alteration in the epidote amphibolite and (Pessel and others, 1981; Winkler and others, greenschist facies of the metavolcanie and 1981a,b; Burns and others, 1983; Pavlis, 1983). metasedimentary rocks is common. The retrograde

6 Professional Report 94 event occurs locally along faults of post-Jurassic age ROCKS COMMON TO BOTH (Pavlis, 1982b) and may be in part related to TERRANES dispersal of fluids after or during intrusion of the Cretaceous tonalite-trondhjemite plutons. Plutons of leucocratic tonalite and trondhjemite The Matanuska Formation, a thick sequence of yielding Early Cretaceous K-Ar ages intrude both marine sedimentary rocks of age, the Chugach and Peninsular terranes in the unconformably overlies the arc rocks of the Anchorage C-4 and C-5 Quadrangles (fig. 2) (Pavlis, Peninsular terrane (Grantz, 1964). The Matanuska 1982a,b, 1983). Because these plutonic rocks cut Formation crops out at the northern edge of the rocks of both terranes, they constrain interpretation Chugach Mountains and consists mostly of of the structural development of the region. mudstone, siltstonc, sandstone, and pebbly to cobbly Moreover, their emplacement into the subduction conglomerate deposited as turbidites. The formation zone assemblage of the McHugh Complex apparently formed in a fore-arc basin (arc-trench apparently requires "near-trench" plutonism ) with the clastic detritus derived from erosion of (Marshak and Karig, 1977; and others, 1981; the Jurassic (Grantz, 1964). Pavlis and others, 1988). .Hypabyssal dikes and plugs of andesitic to dacitic composition that intruded Chugach and Peninsular terrane rocks during Eocene time (Winkler and CHUGACH TERRANE others, 1981b; Silberman and Grantz, 1984) represent one of the last intrusive episodes in the The Chugach terrarie crops out in an elongate arc Nelchina-Kings Mountain region. A few small, from the Shumagin , about 400 km southwest undated dikes of basaltic composition cut across the of Kodiak Island, to southeastern Alaska. The structure and stratigraphy of the map area and are Chugach terrane has been divided into two major possibly related to the extensive middle Tertiary lithotectonic units: an older, melange subterrane volcanism in the Talkeetna Mountains to the north exposed discontinuously along its northern inboard (Csejtey and others, 1978). Some mafic dikes in the edge, and a younger, deformed flysch subterrane Matanuska Valley yield late Eocene ages (Silberman constituting the bulk of the terrane (Plafker and and Grantz, 1984). others, 1976). The melange subterrane is known as The Chickaloon Formation of late Paleocene to the McHugh Complex in the Chugach Mountains early Eocene age (Wolfe, 1966; Little, 1988), which (Clark, 1973) and the Uyak Formation on Kodiak consists largely of fluvial sedimentary rocks, Island (Connelly, 1978). In the map area, the unconformably overlies the crystalline and volcanic McHugh Complex is a melange composed of blocks rocks of the Peninsular terrane and the sedimentary of various ages in a highly deformed matrix of rocks of the Matanuska Formation. The Chickaloon argillite, graywacke, and tuff. Formation includes mudstone, siltstone, sandstone, The flysch subterrane is referred to as the Valdez and conglomerate, as well as coal and carbonaceous Group and is composed largely of structurally sedimentary rocks. The bulk of the Chickaloon imbricated trench-fill turbidites (Moore, 1973; Formation overlies the Peninsular terrane, but a Plafker and others, 1977). Unlike the McHugh small outcrop of nonmarine conglomerate and Complex, the Valdez Group is not obviously sandstone, which probably correlates with the dismembered but is isoclinally folded and deformed Chickaloon Formation, unconformably overlies by a penetrative slaty or phyllitic that Valdez Group rocks of the Chugach terrane near the generally parallels original layering. Nelchina Glacier (sheet 2). The melange has yielded ranging in age from to middle Cretaceous () and is locally cut by plutons which yield Early Cretaceous DESCRIPTIVE GEOLOGY OF THE K-Ar age determinations (Pavlis, 1983). The age of PENINSULAR TERRANE the flysch is believed to be latest Cretaceous (Campanian-Maestrichtian) on the basis of K-Ar age PALEOZOIC(?) AND MESOZOIC determinations (Tysdal and Plafker, 1978; Winkler METAMORPHIC ROCKS and others, 1981b). Rocks of the Chugach terrane in the map area are metamorphosed to mineral The oldest known rocks in the Nelchina-Kings assemblages typical of the prehnite-pumpellyite or Mountain region are metasedimentary and greenschist facies. metavolcanic rocks of unknown correlation. These Geology of the Nortltent Cltrigaclt Morlntairls 7 rocks form a discontinuous east-west-trending belt and consist of thinly layered (0.5 to 10 cm) plagio- near the southern margin of the Peninsular terrane, clase-hornblende-quartz, plagioclase-clinopyroxene, but large screens or roof pendants are also present in garnet-clinopyroxene-calcite * quartz, hornblende- the plutonic rocks farther to the north. The plagioclase-biotite + garnet rocks, and garnet-pyrox- metamorphic rocks are most abundant in the ene skarn. The presence of metasomatic skarn is Anchorage C-4 and C-5 Quadrangles, where they indicated by garnets with anisotropic rims, by the crop out in a belt 3 km wide (figs. 2 and 3). prograde reaction of garnet-clinopyroxene formed Three main assemblages of metamorphic rocks directly from clinopyroxene-plagioclase layers, and occur in the Nelchina-Kings Mountain region. The by veins of garnet. first, an assemblage denoted as amphibolite (Mza), contains amphibolite, garnet-bearing amphibolite, Pelitic Schist and Quartzite and thin (<0.5 m) marble beds, a few of which are garnetiferous. The protolith of this assemblage is Fault-bounded sequences of pelitic and siliceous thought to be a sequence of mafic volcanic rocks schist (presumably metamorphosed siliceous argillite (flows?) with small, discontinuous beds of impure and ribbon ) with lesser amounts of quartzo- limestone and marl. The second assemblage (Mzh) feldspathic schist (metagraywacke and/or metatuff) is a heterogeneous assemblage composed of siliceous and minor amounts of metamorphosed marl, biotite- schist, pelitic schist, semipelitic schist, mafic schist, albite amphibolite, and garnet amphibolite (Mzsh) and minor amounts of amphibolite and marble. occur in the Anchorage C-3, C-4, and C-5 These rocks are interpreted to be metachert, Quadrangles (figs. 2 and 3). Most of these metagraywacke, metatuff, meta-pillow , and metamorphic rocks weather dark red-brown. In the argillaceous limestone. The third assemblage western third of the Anchorage C-5 Quadrangle (Mzmp) is an undifferentiated unit of metamorphic (near Jim Creek), siliceous schist grades into a rocks of the first two assemblages, complexly lower-grade sequence, where the protolith is clearly intruded by Jurassic gabbroic and(or) quartz dioritic chert with interbedded siliceous argillite. Within this rocks. unit, coarse-grained amphibolite is transitional into upper greenschist facies greenstones, which arc PETROLOGY AND LITHOLOGY weakly foliated. These rocks probably include both metatuff and pillow basalt (Pavlis, 1983) and have Arnphibolite and Marble been described in detail by Pavlis (1986) and Little and others (1986b). Sequences of amphibolite (Mza) commonly Siliceous and pelitic schist is generally thinly exceed 0.5 km in structural thickness and contain layered. Isolated, thick (to 10 m) layers of minor marble and calc-silicate rocks. In most of the amphibolite are interleaved with the quartz-rich elc -K&~S Mountain region, these amphibolites schist. Siliceous schist and quartzite are fine-grained are characterized by gneissic textures, grossularitic and contain 75 to 98 percent granoblastic-polygonal garnet layers, and a few thin beds of siliceous marble quartz. Locally the schist is strongly foliated. Most (probably representing recrystallized argillaceous schist contains 4 to 15 percent lepidoblastic red- limestones). In the western half of the Anchorage brown biotite, about 2percent magnetite and C-5 Quadrangle, the amphibolites are fine-grained hematite, and about 1 percent euhcdral garnet; many greenstones with a weak . Many of the samples contained as much as 7 percent white mica. amphibolitic rocks show a thin (1 to 3 cm) Garnet is usually concentrated along pelitic layers, compositional banding of lighter (plagioclase-rich) where it makes up as much as 20 percent of the and darker (hornblende-rich) layers. Calc-silicate layers. Minor amounts of plagioclase and(or) marble crops out in distinct layers and in lenses tens subhedral clinozoisite are present in some rocks. of meters thick. Garnet-rich marble layers are Retrograde metamorphism (post-S1) of the schist is usually < 1 m thick and can be traced discontinuously represented by a late generation of chlorite * epidote for 200 m or more (generally the length of an and by late stage veins of chlorite, carbonate, outcrop). prehnite, datolite, and quartz. Pelitic schist, Contacts between the amphibolitic rocks and the commonly graphitic and locally calcareous, is darker intrusive gabbroic rocks are well exposed near the than the siliceous schist and contains muscovite, Nelchina Glacier (Burns, 1983, in press) and in the biotite, quartz, garnet, + plagioclase. Typically, very Anchorage C-4 Quadrangle (figs. 2 and 3). Host thin, siliceous laminae (chert beds) are transposed metamorphic rocks in contact areas are granofelsic subparallel to the strong schistosity (Sl). Geology of the Northern Chugach Mountains 9

Fine-grained quartzo-feldspathic schist is less Thin, metamorphosed marl layers occur locally common than pelitic schist. The protolith of the and consist of laminated granofels containing quartzo-feldspathic rocks was probably graywacke, plagioclase, carbonate, tremolite, quartz, chlorite, tuff, or a combination of both. Compositional clinozoisite-epidote, magnetite, and sphene. Some layering in these rocks probably represents original plagioclase crystals are helicitic porphyroblasts. bedding. The are interbedded and (or) interlayered with thin to thick (1 to 10s of cms) units Mixed Assemblages of Metamorphic of pelitic schist and lesser amounts of epidote and Plutonic Rock amphibolite. The semi-pelitic schist typically contains 15 to 30 percent very pale green amphibole Mixed assemblages of the metamorphic rocks (actinolite?) needles which are commonly described above are complexly intruded by gabbroic nematoblastic. Very fine grained granoblastic and tonalitic magmas (Mzmp) in the Anchorage C-4 plagioclase composes 30 to 60 percent of the rock. and C-5 Quadrangles. Most commonly, the Most rocks also contain a small amount of relatively metamorphic rocks consist of amphibolites, some coarse, twinned plagioclase grains (oligoclase- granulites, and minor calc-silicate layers. The andesinc). Lcpidoblastic biotite and chlorite minerals produced during intrusion of the Jurassic compose 10 to 20 percent of the semi-pelitic schist; magmas are correlative with those in the pyroxene quartz, magnetite, titanite (sphene), clinozoisite- and hornblende hornfels facies. Burns (1983, in epidote, carbonate, and white mica are minor press) has shown that textural features near the constituents. The texture of the semi-pelitic schist is contact areas between gabbroic and metamorphic generally strongly foliated and nematoblastic with rocks exhibit criteria for catazonal intrusions as fine compositional lamination subparallel to the defined by Buddington (1959), such as (1) medium- foliation (Si). Locally, the texture is isotropic, to high-grade country rocks, (2) absence of sharp suggesting that the metamorphic recrystallization contacts, (3) extensive concordant magmatic border postdated or outpaced the deformation. zones, and (4) apparent syntectonic flow. In these In the Anchorage C-5 Quadrangle (fig. 3), discon- areas of complex intrusion, typical plutons consist of tinuous bodies of dark-brown- to reddish-brown- gneissic pods and lenses separated by screens of weathering mafic schist are almost as abundant as in which fabric is concordant with the siliceous schist (Pavlis, 1986). In the Anchorage that of the gabbroic (Burns, 1983, in press). C-4 Quadrangle, mafic schist is less abundant (Little Similar field evidence was observed near mixtures of and others, 1986b). Its composition is basaltic; it the quartz diorite and metamorphic rocks (Burns thus presumably represents part of a volcanic and others, 1983; Pavlis, 1986; Little and others, complex. Compositional variations include (1) calc- 1986b). These textural features indicate that the silicate-bearing "banded amphibolite," in which environment of intrusion, particularly that of the normal hornblende-epidote-plagioclase amphibolite gabbroic magma, was deep (15 to 25 km). is interlayered with diopside-zoisite-garnet rock, and (2) biotite-hornblende (garnet) amphibolite. The ALTERATION AND METAMORPHISM former may be unusual metamorphosed marl or mafic tuff interlayered with carbonate; the latter, The metamorphic rocks have been subject to at andesitic and basaltic volcanic rocks or volcanogenic least two periods of metamorphism. The first graywacke. The mafic schist varies markedly in tex- recognizable event occurred in the Jurassic, ture, depending upon composition and metamorphic synchronous with arc plutonism. Schist from a grade. Lower-grade (upper greenschist facies) rocks probably correlative sequence of rocks in the near Jim Creek consist of massive greenstone with Anchorage B-6 Quadrangle yielded a K-Ar age of widely spaced (to 10 m separations) domains of 173 Ma (Carden and Decker, 1977), and quartz weakly developed foliation. Typically, however, the diorite that cuts metamorphic rocks in the amphibolite exhibits moderately developed Anchorage C-5 Quadrangle yielded an Early Jurassic cataclastic foliation with a weakly to moderately (195 Ma) K-Ar age on hornblende (Pavlis, 1983). developed down-dip lineation. In thin section, the The peak produced were (1) amphibolite generally shows strong alignment of amphibolite and (2) pyroxene and hornblende hornblende with foliation and(or) lineation, but granulites (or pyroxene and hornblende hornfels). notable exceptions occur in "banded amphibolite" Whether some metamorphism occurred as a regional where calc-silicate minerals are randomly aligned event before intrusion of the Jurassic magmas, or within the calc-silicate layers. whether all metamorphism occurred during arc 10 Geology of tlze Northern Cliugach Mountairis formation, is not clear. Pavlis (1983, unpubl. data, accretionary prism. Pavlis (1983) thought that these 1985) reports that some of the metamorphic rocks rocks were then overprinted by metamorphism and were deformed prior to emplacement of the Jurassic penetrative deformation at high temperatures and plutons and peak metamorphic conditions. low or intermediate pressures prior to intrusion of Retrograde metamorphism occurs adjacent to Early to Middle Jurassic plutonic rocks. post-Early Jurassic faults and is more widespread in Although Pavlis (Pavlis and others, 1988) has the western part of the Nelchina-Kings Mountain proposed a differing interpretation; the other area than in the eastern part. The metamorphic authors of this paper consider all of the crystalline grade attained is greenschist and, less commonly, and metamorphic rocks discussed in this section to lower amphibolite facies. During the retrograde be part of the Peninsular terrane. metamorphism, clinopyroxene was replaced by quartz-calcite-chlorite * pyrite, and partial JURASSIC SYSTEM replacement of garnet and plagioclase by epidote, IGNEOUS ROCKS clinozoisite, chlorite, calcite, and minor pyrite is common. Hornblende is replaced by actinolite. Rocks of Jurassic age consist of three major Rocks in the center of fault zones commonly contain groups: (1) an upper sequence of volcanic and abundant prehnite. volcaniclastic sedimentary rocks of dominantly Much retrograde alteration may be Cretaceous in andesitic to dacitic composition (the Talkeetna age. Two K-Ar age determinations on hornblende Formation); (2) a basal part composed of ultramafic from retrograde metamorphic rocks in the and mafic plutonic rocks (the Border Ranges Anchorage C-5 Quadrangle were 121 and 107 Ma ultramafic and mafic complex [BRUMC] of Burns, (Pavlis, 1983). These rocks are considered by Pavlis 1985); and (3) a slightly younger, middle portion of (1986) to be a distinct assemblage, the " plutonic rocks of intermediate composition (largely terrane" (see footnote: Age, Correlation, and quartz diorite and tonalite). The gabbroic and Interpretation section). intermediate composition plutonic rocks intrude older Paleozoic(?) and Mesozoic metamorphic rocks AGE, CORRELATION, AND INTERPRETATION discussed above. The quartz diorites and tonalites intrude the Talkeetna Formation also. These three The age of the protolith of the metamorphic groups are interpreted to represent the intrusive and sequence is unknown. The metamorphic rocks are extrusive phases of a Jurassic magmatic arc. intruded by Early to Middle Jurassic plutonic rocks; The arc sequence, though interrupted by faults, is hence, the age of their protoliths is probably prc- structurally tilted in the northern flank of the Early Jurassic. Permian age fusulinids occur in Chugach Mountains, its upper parts lying to the marble from rocks in an equivalent structural north. From south to north, the sequence consists of position about 40 km west of the map area near the minor amounts of tectonized ultramafic rocks of Eklutna ultramafic complex (Clark, 1972a), mantle origin, cumulate ultramafic rocks, gabbroic suggesting a Paleozoic age. rocks, plutons of quartz diorite and tonalite, and The Nelchina-Kings Mountain metamorphic volcanic rocks. Because of extensive faulting, no rocks may be correlative with -Permian single area exhibits a complete transition from aee rocks such as the Strelna Formation in the &angall Mountains (Plafker and others, 1985), the Skolai Group (Smith and MacKevett, 1970; 7~avlisinterprets the southern half of the Jurassic plutonic rocks and part of the metamorphic rocks to be a distinct terrane, the MacKevett and others, 1978), and the Mankommen "Knik River terrane," (Pavlis and others, 1988) after Group in the eastern (Richter and terminology introduced by Carden and Decker (1977). In Dutro, 1975). They may also be correlative with Pavlis's nomenclature, the Knik River terrane would include all metamorphic rocks of probable Permian age that fault-bounded bodies of cqstalline rock that are not clearly crop out on the west side of (Magoon and associated with the Talkeetna Formation. This terminology others, 1976). arises from an attempt to distinguish between a system where The metamorphic rocks were interpreted to be a disparate rocks are juxtaposed along a major fault (as in subduction complex of Paleozoic or Triassic age by separate tectonostratigraphic terranes) and a system where minor faulting has obscured the primary contact relationships Pavlis (1982c), who argued that metamorphic rocks between basement and cover (see Pavlis, 1986, for a discussion). in the Anchorage C-5 Quadrangle were an oceanic Pavlis also believes there are sufficient ambiguities in correlation suite of rocks that had undergone an early brittle, or within the belt to justify separation into two distinct melange-like deformation, presumably within an structural/lithologic belts. Geology of the Northern Chugach Mountains 11 ultramafic to volcanic rocks, but the sequence from subdivision of the Talkeetna Formation has not been mantle rocks to extrusive rocks is shown on the map attempted, although lithologic variations have been and can be traced laterally for more than 1,000 km in identified on the detailed maps wherever exposure southcentral Alaska, from 100 km east of the map and accessibility allowed (Pessel and others, 1981; area to Kodiak Island (fig. 1; Burns, 1985). Burns and others, 1983; Little and others, 1986b). McMillan's (1984) study of the area VOLCANIC ROCKS OF THE provides the most complete stratigraphic information TALKEETNA FORMATION of any for a single volcanic center in the Talkeetna Formation. The Talkeetna Formation (Martin, 1926), a thick The structurally lowermost Talkeetna Formation sequence of dominantly andesitic flows and tuffs with exposed in the study region are basaltic to minor amounts of volcanic-derived sediments, crops andesitic flows. Unfaulted contacts with older units out on the Alaska Peninsula (Detterman and are not exposed; therefore, flow thicknesses can only Hartsock, 1966; Detterman and Reed, 1980), in the be estimated. The basal sequence is probably at Talkeetna Mountains (Grantz, 1960a,b; Csetjey and least 1 km thick. Pillowed units have not been others, 1978), and along the far northern edge of the observed, and sedimentary rocks are apparently not Chugach Mountains (Grantz, 1960c, 1961a,b; present in the lower part of the formation. The rocks Winkler and others, 1981b). These rocks are widely are characteristically thick-bedded, massive to considered to represent the volcanic rocks of an vaguely layered, and dark red-brown to dark gray- intraoceanic island arc (for example, Barker and green. Some hyaloclastic beds may be present, Grantz, 1982), and compositional diagrams from this although none have been reliably identified from study support that conclusion (figs. 43, and 6). petrographic observation. The flows are lighter Because of Mesozoic-Tertiary structural colored and more siliceous upsection, at least in a disruption and major lateral variations characteristic broad sense, although reversals are common. of all volcanic arcs, a complete stratigraphic section of the Talkeetna Formation is not exposed in the The thickest subunit of the Talkeetna Formation Chugach Mountains. However, a composite is in the middle of the formation and consists of stratigraphic section for the Talkeetna Formation in andesitic to rhyolitic tuffs, flows, and possible domes. the northcentral Chugach Mountains appears to The proportion of tuff to flow varies considerably consist of (from bottom to top) about 2 km of along the lateral extent of the Chugach Mountain basaltic to andesitic flows and volcanic (Jtb, front, but the presence of both is characteristic of the Jta), more than 2 km of andesitic to rhyolitic subunit. Thickly to massively bedded gray-green ignimbritic crystal-lithic lapilli tuffs, flows, and porphyritic andesite is generally the predominant possible domes (Jta, Jtat, Jtt, Jtd), and 3 km of constituent. Most conspicuous, however, are the tuff interlayered aquagene tuffs and marine sedimentary beds, 5 to 30 m thick, that consist of 0.2- to rocks (Jtt, Jts; sheets 1and 2) (Newberry and others, 2-m-diam blocks suspended in a volcaniclastic 1986). This sequence is best exposed in the area matrix, and the siliceous crystal-lithic (ignimbritic) between the South Fork of the and tuffs. Tuff breccias are common near the base of the the Matanuska Glacier, where an east-dipping middle subunit, whereas ignimbritic tuffs are present sequence of basaltic volcanic rocks and volcanic- near its top. The ignimbritic tuffs are pale green and derived sedimentary rocks crops out (fig. 3). Swarms contain variable. proportions of broken phenocrysts of basaltic and andesitic dikes also make up an (principally plagioclase and pyroxene) and lapilli-size important part of the Talkeetna Formation. lithic fragments. These tuffs range from dacitic to Several volcanic centers, spaced approximately 10 rhyodacitic in composition. The thickness of to 20 km apart, exist in the region. The stratigraphy ignimbritic tuffs reaches a maximum of 2 to 3 km of these centers is dominated by flows and coarse near Mount Wickersham (fig. 3), suggesting that breccias. Notable examples appear at Sheep some explosive volcanism centered in that area. The Mountain (Anchorage D-2 Quadrangle), in the area ignirnbritic tuffs are commonly interbedded with 1- west of the Nelchina River (Anchorage D-1 to 10-m-thick andesitic sills or flows, which suggests Quadrangle), in northeast Anchorage C-2 and D-2 that several explosive events took place; small Quadrangles, and east of Ninemile Creek (~0.1km2), lenticular masses of rhyolite also occur (Anchorage C-4 Quadrangle) (fig. 3). Each center structurally above the ignimbrite. Basaltic to appears to have a somewhat different volcanic andesitic flows commonly cap the ignimbrites; this sequence; consequently, no single detailed relationship is best exposed on Sheep Mountain stratigraphic description applies regionwide. Formal (fig. 3).

Figure 4. Harker diagrams showing SiOz plotted against (a)K20, (b) MgO, (c)FeO*, (d) CaO, (e) Ti03 u) Na20, and (g)A1203 (all in weight percent) for rocks of the Talkeetna Formation. High-4 medium-4 and low-K andesite fields for K20 plot from Gill (1981). Curve for MgO plot calculated using the Rayleigh fractional cystallization equation where 6 DRb = 0.03, CiRb = 7ppm, DM~O= 2.2, and CiMgO = 10.0 weight percent. Tick-marks indicate 10-percent intervals. 14 Professional Report 94

Volcaniclastic sedimentary rocks compose the Most of the volcanic rocks have been upper member of the Talkeetna Formation. These metamorphosed to the greenschist facies and are rocks are thin- to medium-bedded, sparsely to characterized by a variably altered phenocryst abundantly fossiliferous, and include light-gray-green assemblage of plagioclase, clinopyroxene, to black graywacke, sandstone, and mudstone. They titanomagnetite, and either orthopyroxene or altered are distinguished from overlying formations by their pigeonite. Primary amphiboles, alkali feldspar, and less friable character, their greater variability, and quartz are present in the most felsic rocks. Mafic the presence of zeolite minerals. An especially minerals are generally altered to chlorite, fibrous prominent lithology is a pale, very thinly bedded, amphibole, and(or) clay. Plagioclase is altered to moderately well sorted arkosic sandstone (water-laid clinozoisite(?), epidote, and albite. Titanomagnctite tuff?) with 10 to 20 percent lithic fragments. is replaced by a fine-grained mixture of pyrite, Contacts between the upper member and underlying sphene, and apatite. Zeolites are also common, lithologies are gradational. The lower part of the especially in sedimentary members of the Talkeetna upper member commonly consists of a sequence Formation (Hawkins, 1973). about 1 km thick of interbedded andesites and Whole-rock major-oxide analyses of 23 volcanic sedimentary rocks, capped by several kilometers of rocks from the Talkeetna Formation in the northern volcaniclastic sedimentary rocks with no volcanic Chugach Mountains and from Sheep rocks. In a few locations in the Nelchina-Kings from basalt to rhyolite ( 1). Rocks within the Mountain region, depositional contacts with the andesite range are low-K as defined by Gill (1981) overlying Cretaceous sedimentary section have been (fig. 4a). Large scatter in Harker diagrams may observed; additionally, an unconformable upper reflect the greenschist-facies metamorphism of the contact with the Tertiary Chickaloon Formation has Talkeetna Formation (figs. 4a-g). Some of the been mapped near Ninemile Creek (Anchorage C-4 scatter on the Harker diagrams could be produced by Quadrangle). In most areas, however, much of the magma mixing or by crystal accumulation (inset, upper Talkeetna Formation may have been removed fig. 4b). No systematic compositional difference or by erosion. trend is prescnt in the rocks analyzed from any one area. The rocks appear to be largely calc-alkaline on an AFM diagram (fig. 5). However, on a plot of FeO*/MgO versus SiO2 (fig. 6), the Talkeetna Formation rocks showed two compositional trcnds,

Figure 5. AFM diagrant for Talkeetrta Fomlatio~i volcanic rocks. Calc-alkaline trend (denoted by CA) from Irvine and Baragar (1971). A = lOOx Figure 6. Variation diagram sllowing SiOzplotted against (Nu20 t K20)/TOTAL,AFM; F = lOOx (0.9xFez03 FeO*/MgO for rocks from tlte Talkeetna Fomtatiott. t FeO)/TOTALA~~;M = 100~M~O/TOTALAFM, Pigeonitic and hypersflien ic rock series (PRS and wlzere TOTALAFM = Nu20 t K20 + 0.9~Fe203 t HRS) from Gill (1981) wlto took tltali front Krrno FeO t MgO. Tick-niarks indicate 10-percent (1968). Line for calc-alkalit~e(CA) arid tl~oleiitic intervals. (TH) trends taken from Miyashiro (1974). Geology of the Nortllern Chugach Mountains 15 similar to Kuno's (1968) average compositions of sizes (Burns, 1985, in press). The quartz diorite and pigeonitic and hypersthenic rock series in , and tonalite intruded the gabbroic rocks and appear to fell within both calc-alkaline and tholciitic fields of have crystallized structurally above the gabbroic Miyashiro (1974). Thus, at least some of the rocks as relatively homogeneous masses. Dikes of Talkeetna Formation rocks are probably tholeiitic in quartz diorite and tonalite in the gabbroic rocks are character. Barker and Grantz (1982) found that particularly abundant where the gabbroic rocks eight and andesites of the Talkeetna crystallized as dikes. Ductile deformation textures of Formation from the Talkeetna Mountains have contacts between gabbroic and quartz dioritic rocks tholeiitic characteristics and are medium-K by Gill's suggest that the gabbroic rocks were not completely (1981) classification. In summary, based on the crystallized when the quartz dioritic magma intruded existing analyses, the Talkeetna Formation south of (Burns, 1983, in press). the Matanuska Valley is similar to the Talkeetna Major exposures of gabbroic and ultramafic rocks Formation in the Talkeetna Mountains. along the Border Ranges fault system in the northern The age of the Talkeetna Formation on the Chugach Mountains include the Tonsina mafic- Alaska Peninsula is constrained by fossiliferous ultramafic assemblage; Nelchina River Gabbronorite Upper Triassic limestones stratigraphically beneath (formally named in Burns, in press); the Wolverine, and by Bajocian (early Middle Jurassic) Eklutna, and Kodiak complexes; and the directly above the volcanic rocks. In the Talkeetna Klanelneechena and Red Mountain klippen. These Mountains, sparsely fossiliferous lower members and rocks are collectively referred to as the Border abundantly fossiliferous upper members indicate Ranges ultramafic and mafic complex (BRUMC) Sinemurian (Early Jurassic) to late Bajocian ages (Burns, 1985). The Wolverine complex and the (Detterman and Hartsock, 1966), or approximately Nelchina River Gabbronorite, plus numerous 200 to 180 Ma (Palmer, 1983). No K-Ar agc unnamed fault slices of gabbroic rocks, crop out in determinations have been attempted, due to the the Nelchina-Kings Mountain area. These mafic and extensive alteration of the volcanic rock. ultramafic rocks are exposed as elongate structural blocks tens to hundreds of kilometers long. PLUTONIC ROCKS Cumulate Ultramafic Rocks Plutonic rocks of Jurassic age in the northern Chugach Mountains form both homogeneous and The Wolverine complex consists dominantly of heterogeneous plutons. The homogeneous plutons ultramafic rocks and crops out in the Anchorage C-4 occur in three major compositional types: and C-5 Quadrangles (figs. 2, 3) (Clark, 1972b, 1983; (1) cumulate , (2) gabbroic rocks, or Pavlis, 1982a, 1983; Little and others, 1986b). Minor (3) quartz diorite and tonalite. "Cumulate," used amounts of ultramafic rocks, including cumulate here as a textural term (as defined by Irvine, 1982), is , clinopyroxenite, troctolite, and lesser not meant to imply magmatic processes resembling , are also present in the southern part of sedimentation. the Nelchina River Gabbronorite in the Anchorage The heterogeneous plutonic units contain C-2 Quadrangle (fig. 3). The ultramafic rocks in the mixtures of these three major types of plutonic rock, Nelchina River Gabbronorite are not differentiated and are particularly abundant in the area mapped on on the map and are not discussed in this paper. sheet 1. The heterogeneous units were formed by repeated intrusion. A general sequence of multiple Petrology and Lithology.-The Wolverine intrusions changing from mafic to more silicic types complex is composed of fault-bounded blocks of is seen all along the northern flank of the Chugach ultramafic rocks (dunite - Jd; clinopyroxenite and Mountains. The ultramafic rocks are intruded by, websterite - Jcw; harzburgite and - Jp; and are in part transitional to, gabbronorites8 sheet 1) and mafic rock (gabbronorite and (Newberry and others, 1986), which, in turn, are hornblende - Jgu; sheet 1). Much of the intruded by dikes and large plutons of quartz diorite, layering and diking is too complex to be shown at tonalite, and minor granodiorite. The gabbroic rocks map scale; thus, a unit of ultramafic undifferentiated appear to have crystallized both in relatively large (Juu) and a unit of mafic-ultramafic undiffcrentiatetl magma chambers and as multiple dikes of varying (Jmu) are also shown on sheet 1. Fault-bounded blocks of amphibolite, metachert, and schist 8~lutonicrock names used in this report are consistent with Mzsh, and Mzm) are spatially associated with Streckeisen (1976). the ultramafic rocks (Pavlis, 1983; Newberry, 1986). 16 Professional Report 94

The fault blocks which make up the Wolverine orthopyroxene. Textures are generally adcumulate, complex are 60 to 600 m wide and trend northeast- with both present as cumulus phases. southwest. Ultramafic rocks predominate in the southeastern portion of the complex; gabbroic rocks Alteration and Metamorphism.-The Wolverine are more abundant to the northwest (Clark, 1972b; rocks are the most highly altered ultramafic rocks in Pavlis, 1983; Little and others, 1986b). Layering the BRUMC; they are the only ultramafic rocks in generally dips to the south. The ultramafic rocks the complex known to commonly contain abundant exhibit gradational and faulted contacts with secondary cummingtonite. Serpentinization, which gabbronorites to the north (Newberry, 1986). occurs at a lower temperature than that required for The bulk of the Wolverine complex is composed formation of cummingtonite, is also extensive. of ultramafic cumulates interlayered with and Alteration of rocks in the Wolverine area is intense, intruded by mafic rocks. Dunite, -bearing probably because of numerous faults which would dunite, wehrlite, olivine clinopyroxenite, clino- have provided access for fluids. Tonalite-trond- , gabbronorite, and hornblende gabbro are hjemite plutons of Cretaceous age may have the main rock types present. Small amounts of provided fluids and heat for this alteration. cumulate websterite and tectonized harzburgite occur in the Wolverine body, and the harzburgite may represent residual mantle; however, no studics Deformation.-The deformational history of the on rock fabrics have been done to separate cumulate rocks in the Wolverine body is complex deformation by Cretaceous faulting from that and, as yet, poorly understood. Olivine commonly occurring in the mantle. shows banding, and in many rocks, both types of pyroxenes show undulatory extinction. A small Dunite is commonly massive, but thin layers or amount of recrystallized olivine with marked grain- bands (1 cm to 1m thick) of clinopyroxenite and (or) size reduction is present in many rocks, and is wehrlite in dunite are also common. The layering indicative of dynamic recrystallization under commonly dips to the southeast but is disrupted by relatively high (Giradeau and Nicolas, 1981). faulting. Where abundant, chromite generally occurs Clinopyroxene and hornblende crystals are in the dunite as deformed, wispy layers meters to commonly recrystallized. These deformational tens of meters long (possibly to 100 m) (Newberry, features probably developed when the rocks were 1986). Dikes of gabbronorite and hornblende gabbro still hot; that is, the rocks are metacumulates as in 5 to 40m thick are present within the other ophiolitic-type complexes: for example, of clinopyroxenites and . Islands (Christensen and Salisbury, 1979); Troodos (George, 1978); and Mountain (AVC Lallemant, 1976). Mineralogy.-Dunites are composed predomi- nantly of fine- to coarse-grained (commonly 3 to Some deformation features of the Wolverine 5 mm diam) olivine. Disseminated chromite (about complex, however, are related to younger, more 2 percent) is common. Chromite forms rounded brittle deformation of probable Late Cretaceous- crystals, suggesting interaction with liquid and(or) early Tertiary age. Broken and brecciated olivine crystals. Clinopyroxene * orthopyroxene clinopyroxene and hornblende, and possibly strain crystals are present in some dunites as cumulus lamellae in clinopyroxene, are examples of the phases; however, most clinopyroxene crystals form younger deformation. Brittle deformation probably an intercumulate phase. Dunites are typically occurred in two stages: the first associated with serpentinized, obscuring textural relations. Many thrusting of the Peninsular terrane over the McHugh chromite grains are surrounded by a halo of mag- Complex, and the second associated with the Tertiary netite dust formed during serpentinization. high-angle fault system (see Structural Geology - - section). Some ductile deformation and Clinopyroxenites and olivine clinopyroxenites of recrystallization described above may have occurred the Wolverine Complex are composed dominantly of during early thrusting. coarse (3 to 7 mm) cumulate crystals of clinopyroxene. Olivine, orthopyroxene, and a brown- to-greenish spinel (pleonaste) are present in varying Gabbroic Rocks amounts. The clinopyroxene grains commonly contain growth twins, and, less commonly, narrow Gabbroic rocks are the most abundant plutonic exsolution lamellae of orthopyroxene. Websterite rocks in the northern Chugach Mountains. The generally contains more clinopyroxene than western part of the Nelchina River Gabbronorite, a Geology of the Northern Chugach Mountains 17 body composed dominantly of gabbroic rocks, crops in the Anchorage C-4 and C-5 Quadrangles. The out in the map area in the Anchorage C-1 and C-2 areas of multiple intrusion, referred to herein as dike Quadrangles. Gabbroic rocks crop out also in fault complexes, commonly include quartz diorite and contact at the base of the quartz diorite pluton in the tonalite dies, as well as many generations of Anchorage C-3 Quadrangle, as xenoliths in other gabbroic dikes. Stretched and boudinaged dikes and quartz diorite and tonalite plutons, and in association migmatitic-textured contacts between quartz diorite with large masses of ultramafic rock in the and gabbronorite indicate that the gabbroic rocks Anchorage C-5 Quadrangle (the Wolverine complex; were still hot and ductile during intrusion of the figs, 2,3). quartz diorites (Burns, 1983, in press).

Petrology and Lithology.-The undifferentiated Mineralogy.-On the basis of mineralogy, Burns gabbroic rock unit (Jgu) consists dominantly of (1983) classified the gabbroic rocks into two main gabbronorite (or two-pyroxene gabbro), magnetite categories: (1) the gabbronorite clan, and gabbronorite, and anorthositic gabbronorite; norites, (2) pyroxene-hornblende gabbronorite and horn- anorthosites, pyroxene-hornblende gabbronorites, blende gabbro. Rocks of the gabbronorite clan form and lesser hornblende and ultramafic rocks the majority of the undifferentiated gabbroic rock are also present. A garnet clinopyroxenite (most unit (Jgu) and contain very minor amounts of likely a reequilibrated gabbronorite [Burns, 19851) hornblende, whereas rocks in the second category was found within this gabbroic unit in a fault- contain > 10 modal percent coarse-grained magmatic bounded slice near the Nelchina Glacier (fig. 3). or early deuteric hornblende. The pyroxene- Other map units that contain gabbroic rocks form hornblende gabbronorite and hornblende gabbro complex intrusive mixtures composed of gabbroic form a large component of the diorite-gabbro unit rocks and either ultramafic rocks (Jmu), dioritic (Jdgu) as well as occurring in the undifferentiated rocks (Jdgu), quartz diorites (Jgqd), or migmatitic gabbroic rock unit (Jgu) and as a minor component textured metasedimentary and metavolcanic rocks in the diorite-quartz diorite unit (Jdqd). Major-oxide (Mzmp). These rock types also occur in very minor analyses and CIPW norms of sample from both these amounts within the main gabbroic rock unit. Dikes groups (table 2) showed the pyroxene hornblende and small veins of trondhjemite of Late Jurassic- gabbronorites to be slightly more differentiated than Early Cretaceous age and felsite dikes of Tertiary rocks of the gabbronorite clan. No other age are common in the zones of cataclastically compositional differences were apparent. More deformed gabbroic rocks. detailed petrographic descriptions of the gabbroic Some of the gabbroic rocks are massive with little rocks and mineral compositions than are given below or no directional fabric. Other gabbroic rocks are presented by Burns (1983,1985, in press). consist of layers and dikes. Layers range from 1 cm to 5 m thick. Thin layers pinch out and commonly Gabbronorite clan.-Most of the gabbroic rocks in have strike lengths of c20 m. Most of the layering is the Nelchina-Kings Mountain region are rocks of the defined by modal variation in pyroxene and gabbronorite clan and include gabbronorite, plagioclase content. Narrow (1 cm wide) layers of leucogabbronorite, anorthosite, magnetite anorthosite are commonly interlayered with the gabbronorite, and leuconorite. Fine- to medium- gabbronorites. Minor pegmatitic pods (to 15 cm grained gabbronorites form 45 to 55 percent of the long) of hornblende and plagioclase are present in exposed gabbroic rocks. Gabbronorites weather some of the layered gabbronorites and some locally reddish brown but are whitish gray where extensively hornblende-plagioclase veins cut across the layering fractured and altered to zeolites. Gabbronorites of the gabbroic rocks. The layering may be caused contain 15 to 30 percent orthopyroxene, 15 to by a combination of flow segregation, periodic 30percent clinopyroxene, 40 to 65 percent crystallization, and new pulses of magmatic injection. plagioclase (Anss-7s), 0 to 10 percent (commonly <5 Most layered rocks display igneous lamination, an percent) pargasitic hornblende, and to 10 percent alignment of "c" axis of plagioclase and pyroxene magnetite and ilmenite (commonly 2 to 6 percent). grains, which probably resulted from moderate Trace amounts of apatite are present in a few rocks. pressure during cooling. -rich green spinel (pleonaste) is present in the Layering, clearly produced by multiple intrusion, more magnesium- and calcium-rich gabbronorites. is present in the Nelchina transect (Burns, 1983, in Leucogabbronorite and lesser anorthosite form press), in the southern part of the Anchorage C-2 20 to 30 percent of the exposed gabbroic rocks. A Quadrangle (fig. 3), and in rnost of the gabbroic units small amount of leuconorite is also present. 18 Professional Report 94

Leucogabbronorite and leuconorite contain 65 to alteration adjacent to fault zones, are also recorded 90 percent plagioclase (An95.70) and 10 to 35 percent in the gabbroic rocks. pyroxenes. Hornblende, magnetite and ilmenite Deuteric assemblages are discussed here in terms generally form < 3 percent. The leucocratic rocks of decreasing temperatures and progressive are commonly medium grained, but layers with alteration. The least altered gabbroic rocks contain extremely coarse-grained (1.5 cm) plagioclase orthopyroxene (dominantly low-calcium crystals are also typical. Many of the leucocratic hypersthene), diopside-salite, and calcic plagioclase i rocks weather white, but where plagioclase grains are magnetite and lesser ilmenite i minor pargasitic calcic (Ango-95) and unaltered, the rock retains a hornblende. Temperature estimates based on the clear, greenish sheen in outcrop. two-pyroxene geothermometer of Ross and Huebner (1975) for host pyroxene compositions of the Pyroxene-hornblende gabbrononte.-Pyroxene- gabbroic rocks suggest that exsolution lamellae in hornblende gabbronorite is most common in dike pyroxene crystals probably formed as the rocks complexes, and composes about 10 to 15 percent of cooled to about 800 to 950 OC (Burns, 1985). The the exposed gabbroic rocks in the Nelchina-Kings pargasitic hornblende probably formed only during Mountain region. They crop out directly west of the reequilibration and does not appear to be a primary Nelchina Glacier in the Anchorage C-1 Quadrangle, magmatic phase. These mineral assemblages and in the southern part of the Anchorage C-2 Quad- compositions are characteristic of rocks which rangle, and in scattered places in the Anchorage C-4 reequilibrated in granulite facies conditions. and C-5 Quadrangles. These rocks commonly are As temperatures decreased, cumming- fine grained and weather black. Minor amounts of tonitelanthophyllite i magnetite i minor quartz hornblende gabbro and diorite(?) are commonly formed pseudomorphs after orthopyroxene. The associated with the pyroxene-hornblende gab- main minerals present in this second assemblage bronorite, particularly in the Anchorage C-4 and C-5 (diopside, cummingtonite/anthophyllite, and calcic Quadrangles. plagioclase i minor pargasitic hornblende) are The unit typically is composed of fine-grained similar to minerals formed in temperatures of upper gabbronorite and magnetite gabbronorite that amphibolite metamorphism (labradorite/bytownite contain coarse-grained hornblende, magnetite, amphibolite facies of Winkler, 1974). ilmenite, and plagioclase in veins 1to 2 cm wide and Lower amphibolite facies and greenschist facies in discontinuous, elongated pods. The resulting rock are represented by growth of clinozoisite in is composed of 15 to 30 percent hornblende, 40 to plagioclase and partial to complete replacement of 60 percent plagioclase, 10 to 30 percent pyroxenes, orthopyroxene crystals (or its alteration products) and commonly as much as 8 to 15 percent magnetite with chlorite i sphene. Clinopyroxene and and ilmenite. Hornblende is tan in thin section. hornblende remain unaltered in the gabbroic rocks Apatite is much more common in the pyroxene- except in areas of Tertiary faulting, near other hornblende gabbronorites than in the gabbronorites, intrusions, and in areas affected by hydrothermal but still composes c2 modal percent. circulation. On the basis of mineral growth, field Intense hydrothermal alteration has occurred in relationships, and petrographic textures, these rocks large areas of the gabbroic rocks at greenschist facies are interpreted to form at high temperatures by conditions. These gabbroic rocks are commonly circulation of vapor phases through gabbronorites present in dike complexes, and many had been and magnetite gabbronorites (Burns, 1983, in press). previously altered by vapor phases to the magnetite The vapor phase caused coarse-grained hornblende, pyroxene-hornblende gabbronorite discussed above. magnetite, ilmenite, and plagioclase to crystallize. In Hydrothermal alteration produced pervasive places, the younger mineralogy obscures the older. chloritization, chlorite veining, and abundant quartz- epidote veins. Ortho- and clinopyroxene (or their Alteration and Metamorphism.-Many alteration alteration products) altered to chlorite (+ epidote), assemblages caused both by deuteric and magnetite altered to chlorite, and clinozoisite altered metamorphic processes are present in the gabbroic to epidote. Minor veining of hornblende by chlorite rocks. Three major deuteric assemblages can be is also common, and calcite, albite, quartz, sphene, identified on the basis of mineralogy and mineral pyrite, and chalcopyrite are present in minor compositions. Several localized metamorphic events, amounts. Actinolite, generally considered to bc a including intrusion of quartz diorite-tonalite, characteristic mineral of mafic rocks in the intrusion of felsite plugs, and low temperature greenschist facies, is rarely present. Table 1. Major-oxide analyses and CZPWnoms on volcanic rocks of the Talkeetna Formation [Analyses performed at Alaska Division of Geological and Geophysical Suweys by N.C Veach and Kate Bull. FeO* and Fe203*, total molecular Fe; A, (Na2O +K20)/Na20 + K20 +FeO* + MgO); F, FeO*/(Na20 + K20 +FeO * + MgO); M, MgO/(Na20 + K20 +Fe0' +MgO); -, not applicable; norm An, an/(an + ab + or); Ab, ab/(an + ab +or); Or, or/(an + ab + or); SM, Sheep Mountain]

Quadtangle D4 D-2,SM D-2,SM D-2, SM C4 C-4 C-4 D4 C-2 C-2 DLSM C4 C4 D-1 C-2 D-2,SM D-2, SM C-4 D-4 D4 C-2 D-2 D-4 Field no. 82Ny249a 82Mn2.44 82Mn246 my141 83Ny21 83Ny17 my265 a3Ny14 82Ny191 a3Ny47 82Mn211 82NyZBZB 82NyZ53B 79DLM52 82Mn36 82Mn2.48~ 82Mn239 a3Ny15 83Ny19 83Ny3.3 83Ny48 82Ny54A 83Ny37a

Chemical analyses (weight percent)

Si02 &93 Fe2% Fa3 MgO CaO NaZo K20 Ti02 p205 MnO LO1 - Fea* -10.93 - 10.00 -10.86 -1137 -- -- -956 ------10.84 - 11.16 9.77 10.11 8.12 7.67 6.23 7.78 ------4.48 TOTAL 100.11 100.74 10032 10057 10028 98.91 10039 99.00 lW.20 100.23 100.75 100.41 100.02 10058 100.42 100.62 101.10 100.03 100.09 10022 98.81 10053 99.16

FCO* 9.82 898 9.75 1021 9.46 1023 858 938 9.73 7.67 10.02 877 9.08 729 6.89 559 6.99 5.16 452 4.20 2.80 4.02 137 FeO8/Mg0 1.4 13 1.9 25 2.6 2.0 1.8 2.6 2.1 3.0 2.1 4.0 3.9 32 2.6 2.8 22 3.0 2.7 3.0 4.7 33 105

ClPW nom(weight percent) qm 1.6 3.0 22 15.1 112 172 32 7.6 13.8 7.8 11.1 243 14.6 20.1 21.9 13.8 31.6 27.6 24.6 26.6 33.9 34.0 42.4 or 43 02 05 0.1 0.1 - 20 3.2 0.2 27 1.6 0.1 13 29 1.7 1.4 8.7 05 53 63 6.7 3.0 121 ab 20.4 16.6 27-4 05 20.1 8.1 30.8 26.6 11.7 38.4 35.4 135 44.7 243 29.1 49.0 28.4 37.2 42.1 42S 39.7 35.6 39.8 an 37.2 44.4 41.0 46.1 35.6 40.7 315 36.0 39.9 335 193 35.1 17.7 30.0 27.4 213 52 202 15.4 13.0 128 16.8 3.0 di - 5.0 8.0 192 18.1 11.6 10.0 4.9 11.0 13 - 12.6 0.0 9.9 3.4 0.7 - 55 3.0 3.0 20 3.1 -- hY 29.8 25.6 20.6 133 55 123 17.7 13.6 17.4 7.8 24.0 75 15.8 8.1 112 8.6 14.7 32 45 3.9 15 3.7 03 01 ------. - - -- mt 3.4 3.4 35 3.6 7.0 7.8 33 5.9 3.7 6.8 3.8 3.9 3.7 32 3.4 33 33 4A 35 3.0 22 2.8 0.9 d 15 15 1.7 1.7 1.8 2.0 13 1.7 1.9 13 19 22 1.8 13 1.6 15 I A 1.1 1.1 12 1.O 0.8 03 aP 02 02 02 03 0.8 02 02 0.4 03 0.4 03 0.8 0.4 03 03 0.4 0.2 0.4 0.4 05 02 02 0.0 mr 1.6 - - .------2.6 ------6.6 ------05 hm ------05 Table 2. Major-oxide analyses and CZPWnomzs on gabbroic rocks from tlte nortlzcentral Cltugaclz Mountains [Analyses performed at Alaska Division of Geological and Geophysical Sumeys by N.C Veach and Kate Bull. FeO* and Fe203*, total molecular Fe; A, (Na 0 + K20)/Na20 + K20 + FeO* + MgO); F, FeO*/(Na20 + K20 + FeO* + MgO); M, MgO/(Na20 + K20 + FeO' + MgO); -, not applicable; norm An, an/(an + ab + or); Ab, ab/(an + ab +or); Or, or/(an + ab + or$. Symbols for rock types are as follows: gn - gabbronorite (gabbronotite clan); phgn - pyroxene hornblende gabbronorite (pyroxene hornblende gabbronorite clan); hgb - hornblende gabbro; and dio - diorite]

Rebv gn P~P gn phgn phgn gn P gn hgb PhkY hgb bgb hgb P Phgn gn gn dio dio dio gn gn gn gn mafic mafic mafic dike dikc dikc Quadrangle C-1 C-1 C-1 GI C-l C4 C1 C4 C-1 C-2 C4 C4 C4 C-1 C-l C4 C4 C.4 C4 C4 C-4 C4 C-1 C-l C-1 C-1 C-1 Fieldno 79Pc66 81Bu57 81Bu6Sa 81Bu117 81BG 83Fn2.4 79DLM17 83FnlSb 81Bu7S 81Bu219 83Pc9 me6 83PeZ 81864 81Bu27b 83FD 83Fnlk 83Fn22 83Fn42 83Fn13 83Fnl9 83Fn17b 81Bul24 8lBu128 81Bu123 8181148 81BuR5

Chemical analyses (weight percent)

Si02 40.04 4090 41.12 41.74 4188 4208 43.43 43.70 44.02 4420 44.66 44.78 4493 45.04 45.60 45.93 4658 47.10 47.44 48.06 48.23 4830 4885 49.80 47.70 48.49 48.95 A1203 1683 17.77 19.14 19.67 20.26 18.11 17.72 15.60 2037 17.16 19.66 2020 8.72 2039 18.49 23.99 2737 21.06 2132 18.11 17.64 12.27 18.49 17.75 13.73 1396 15.92 FcO 9.43 8.48 9.12 7.62 8.96 - 822 - 738 734 - - - 5.75 4.73 ------639 6s 7.02 6-73 734 Fern 7.75 659 5.01 639 4d3 - 6.27 - 427 620 - - - 246 3A9 ------453 us 241 3.03 1.a MgO 7.49 731 651 4.89 5.n 7.11 6.94 856 5.72 7.25 7.79 885 16.66 735 7.83 5.13 333 7.85 8.90 840 933 12.98 5.66 6.09 1054 12.07 9.05 CaO 1231 14.19 1335 1332 1267 1185 1132 13.78 11.77 13.66 1196 11.46 10.47 1290 14Al 15.08 14.43 1261 13.15 1137 1216 837 7.98 7.81 11.72 lOAl 9.94 Nafl 1.23 0.65 1.14 1.82 1A5 1.00 205 OAl 1.92 135 1.84 1.13 036 1.77 126 1-73 2.60 2.00 1.15 2.09 1.19 087 279 3.66 1.95 2.08 230 Kfl 0.04 0.00 0.00 0.00 0.09 0.19 0.14 0.08 0.15 0.05 0.11 0.10 0.03 0.13 0.06 0.02 0.13 0.24 0.07 0.08 0.05 0.08 039 0.49 0.43 0.13 0A2 Ti02 1.19 1.04 1.06 151 1.04 1.15 1.14 0.89 0.SU 0.97 0.60 0.43 038 0.74 0.48 0 0.17 026 0.15 037 0.18 0.49 0.84 0.81 0.63 OM 0.65 P205 0.06 0.07 0.10 032 0.23 0.01 0.12 0.01 020 0.08 0.04 0.01 0.01 0.08 0.06 0.01 0.01 0.07 0.01 0.01 0.01 0.03 0.16 0.18 0.11 0.11 0.12 MnO 022 0.16 024 0.18 024 0.22 0.23 020 021 023 021 0.17 0.25 023 0.17 0.11 0.07 0.17 0.15 0.19 0.17 024 0.13 021 OM 0.18 0.18 LO1 236 206 238 Id5 2.18 206 W 1.77 241 1.10 1.99 239 3.47 1.78 1.94 279 80 222 1.12 133 282 3.43 220 199 223 1.40 160

NastKfl 127 0.65 1.14 1.82 154 1.19 219 0.49 2.07 1.4 1.95 1.23 039 19 132 1.61 273 224 I22 217 I24 0.95 3.18 4.E 238 221 272 FeO* 16.40 14A1 13.63 1337 12% 14.71 13.86 1420 1122 1292 1033 10.17 13.79 796 7.87 5.47 3.92 633 6.61 983 787 11.66 10.47 9.44 9.19 932 8.99 F~O*/M~O 219 197 2.07 273 m 2.07 200 1.66 196 1.78 133 1.15 om 1.08 1.01 1.07 1.18 0.81 0.75 1.17 0.84 0.90 1s 1s om 0.77 0.9

CIPW norms (weight percent) Geology of the Notiliem Chugach Mountains 21

Detailed geologic mapping and petrographic metamorphism, the primary pyroxene crystals and study of the gabbroic rocks intruded by quartz diorite dark-green hornblende are replaced by two or three indicates the gabbroic host rocks were compositions and(or) slightly different generations of metasomatically altered. Near intrusions of quartz fibrous, low-temperature amphiboles which are diorite, the gabbroic rocks contain much more randomly oriented. Plagioclase grains are hornblende than is typical elsewhere, pyroxenes that completely altered to clinozoisite and(or) epidote are ~artlyreplaced by magnesian hornblende, and and contain minor amounts of actinolitic amphiboles. locally contain minor amounts of interstitial quartz. Near most contacts, a complete gradation is present Age and Correlation.-Few age determinations in these gabbroic rocks from pyroxene with no have been made on the ultramafic and gabbroic hornblende, through pyroxene rimmed and partly rocks. Five K-Ar ages on hornblende and one on replaced by hornblende, to hornblende forming some clinopyroxene from gabbroic rocks near the eastern pyroxene pscudomorphs. Plagioclase crystals, which part of the BRUMC range from 154 to 188 Ma; only are not noticeably zoned in typical gabbroic rocks, one is younger than 165 Ma (Winkler and others, are variably zoned in the contact areas and 1981b). Field relationships support a Jurassic age, as commonly consist of broad calcic cores and narrower they suggest intrusion of Jurassic-age quartz dioritcs into the gabbroic rocks while they were still hot sodic rims. The sodic rims are much more prevalent enough to be ductile (Burns, 1983, in press). A few near quartz diorite contacts. The mineralogic older K-Ar ages (267 8 Ma and 419 +_ 21 Ma) on changes of gabbroic rocks in the contact zone are * gabbroic rocks from the eastern part of the BRUMC reflected in major-oxide analyses (tables 2 and 3; (Winkler and others, 1981b) have becn attributed to fig. 7). Gabbroic rocks of the Chugach Mountains excess argon (G. Sisson, written commun., 1987). generally contain <51 percent Si02; quartz diorites contain >62 weight percent SO2. The only Jurassic plutonic rocks in the study area to have SiOz values Intermediate Composition Plutonic Rocks between <51 and 62 weight percent are the gabbroic A voluminous suite of plutonic rocks of rocks intruded by quartz diorite (fig. 7). These intermediate composition is present in the northern analytical results support the hypothesis of Chugach Mountains and extends discontinuously from metasomatic contamination of the mafic host rock by Kodiak Island to near Tonsina (Hudson, 1979; Flynn the intruding quartz diorite. and Pessel, 1984). Large plutons are composed of Greenschist facies alteration of the gabbroic quartz diorite and tonalite, lesser amounts of rocks is localized near small fault zones and is granodiorite, and very minor trondhjemite (fig. 8; abundant near the many faults in the Anchorage C-4 table 3). A few small plutons of quartz monzodiorite and C-5 Quadrangles and in the southern part of the and granodiorite intrude the quartz diorite and tonalite Nelchina River Gabbronorite. This alteration is in the Anchorage C-5 Quadrangle (Pavlis, 1986). characterized by abundant, randomly oriented Mafic dike swarms are abundant in the quartz diorite actinolitic amphiboles which are superimposed on plutons. the alteration assemblages discussed above. The association of the actinolitic amphiboles and the Quartz Diorite and Tonalite.-Quartz diorite and high-angle faults suggests that the grecnschist facies tonalite are the most abundant intermediate alteration is a relatively young event, probably composition plutonic rocks in the northern Chugach associated with fluid circulation along Cretaceous or Mountains. These rocks occur together and crop out early Tertiary faults (see Structural Geology section). both as large structural blocks bounded by high-angle The greenschist facies alteration in the Anchorage faults, and as a continuous, relatively intact intrusive C-4 and C-5 Quadrangles may be caused by fluids complex beneath the Talkeetna Formation in the associated with intrusion of the Cretaceous tonalite Anchorage C-4 and C-5 Quadrangles (fig. 3). Dikes, plutons. sills, and small plutons of quartz diorite-tonalite In areas of slight greenschist facics intrude the Jurassic-age dioritic and gabbroic rocks, metamorphism, clinopyroxene is replaced by a the pre-Jurassic melasedimentary and metavolcanic fibrous, light-green amphibole (actinolitic rocks, and the Talkeetna Formation. Most of the hornblende?). The dark-green hornblende (a relic of original contacts between Talkeetna Formation and higher temperature alteration of probable Jurassic the quartz diorite-tonalite plutons are now obscured time) present as rims on some clinopyroxene crystals by high-angle faults, but intrusive contacts showing is not altered. Plagioclase grains are partly altered to this relationship are exposed in the Anchorage C-4 clinozoisite. In areas of intense greenschist facies and C-5 Quadrangles.

f SiO,

Figure 7. Harker diagrams showing Si02 plotted against (a) A1203 (b) CaO, (c) Na20, (d) K20, (e) FeO +0.9* Fe203, (f) MgO, and (g) Ti02 (all in weight percent) for gabbroic, quartz dioritic, and trondhjemific rocks from the northcentral Chugach Mountains. m, gabbroic rocks; q, quartz diorific rocks; q', altered quartz dioritic rocks; t, frondhjemifes; c, rocks known from field evidence to represent mechanical mixtures of gabbroic rock and quartz dioritic magma. Figure illustrates lack of intennediafe-composition ' rocks, besides those formed from mechanical mixtures, between gabbroic rocks and quartz diorifes. Table 3. Major-oxide analyses and CIPWnonns ott quartz dionte, tonalite, and grattodionte frotn tlte nortlzcentral Clliigaclz Mountains [Analyses performed at Alaska Division of Geological and Geophysical Surveys by N.C. Veach and Kate Bull. 17c0*and Fc203*, total molecular Fc; A, (Na20 t K20)/Na20 t K20 t FeO* t MgO); F, FeOa/(Na20 t K20 t FeO* t MgO); M, MgO/(Na20 t K20 t FcO* t Mg0); ---, not applicable; norm An, an/(an + ab tor); Ab, ab/(an t ab t or); Or, or/(an t ab t or). Symbols for rock type are as follows: hgb - hornblende gabbro; ahgb - altered hornblende gabbro; agn - altered gabbronorite; qd - quartz diorite; ton - tonalite; aqd - altered quartz diorite]

Rock type ahgb ahgb agn hgb hgb qd qd ton ton ton ton ton aqd Quadrangle C-4 C-4 C-4 C-1 C-2 C-4 C-4 C-1 C-1 C-4 C-1 C-4 C-1 Ficld no. 83Fn18 83Fn47 83Fn39b 81Bu50a 81Bu195 83Fn44 83Pe35 81Bu89 81Bu29 83Pn31 81Ru8 83Pe4 81Bu87 Chemical analyses (weight percent)

Si02 A1203 Fc 0 Fez03 MgO cao Na20 K20 Ti02 P205 MnO LO1 Fc203* ------TOTAL 100.56 100.54 100.64 98.38 99.07 100.11 100.55 98.89 99.31 99.85 98.72 100.21 99.14

ClPW norms (weight percent) qtz or ab an di "Y 01 mt il aP cor h nl SP" wo

Norm An 56.5 54.6 46.4 52.3 48.4 23.7 30.3 36.9 5.9 19.9 36.2 28.5 17.2 Ab 40.9 41.2 49.6 46.1 44.6 61.1 63.2 60.7 93.5 72.2 61.7 62.5 79.9 Or 2.6 4.2 4.0 1.6 7.0 15.1 6.6 2.4 0.5 7.8 2.0 9.0 3.0 Geology of the Northern Chugach Mountains 25

QUARTZ brown pleochroism. Biotite grains form adjacent to opaque minerals and are partially to completely replaced by chlorite and sphene in most rocks. Zones of highly altered, cataclastic quartz diorite and tonalite are common along many of the fault zones. Rocks in these zones contain abundant fine- grained, recrystallized quartz; mafic minerals are completely replaced by chlorite, and plagioclase is partially to completely replaced by epidote and sericite. Other alteration assemblages in these areas include prehnite * epidote * carbonate * laumontite and secondary quartz.

Granodiorite within Quartz Diorite-Tonalite POTASSIUM FELDSPAR PLAGIOCLASE Plutons.-Minor amounts of pink-weathering granodiorite (with 10 to 15 percent alkali feldspar) Figure 8. Ternary diagram showing modal percentages of are present in the quartz diorite and tonalite plutons, quartz-plagioclase-potassi11n1-feIdsparof the quartz and are too small to show on the map (sheets 1 and diorite-tonalite plutott in the Anchorage C-3 2). Contacts between the granodiorite and the quadrangle. The pAlton is typical of those nlapped as quartz diorite and tonalite are generally gradational, quartz dionte-tonalite in the northcentral Chugach but small dikes of granodiorite do intrude the quartz Mountains. Tick-marks indicate 10-percent intervals. diorite and tonalite. Pegmatitic dikes composed of (From Flynn, unpub. data.) potassium feldspar and quartz associated with granodiorite also intrude the quartz dioritic plutons. The quartz diorite and tonalite are typically The granodiorite is located in the upper parts of the medium grained, relatively homogeneous in quartz diorite-tonalite plutons and may be a late- composition and texture, and contain locally stage differentiate or a younger, unrelated intrusion. abundant patches and trains of xenoliths of more Granodiorite is generally massive, medium mafic Jurassic plutonic rocks. The quartz diorite and grained, and hypidiomorphic-granular to porphyritic. tonalite are typically composed of 45 to 65 percent Quartz grains are commonly dendritic with plagioclase, 10 to 35 percent strained, anhedral myrmekitic intergrowths of plagioclase. The quartz, and 15 to 25 percent hornblende. Anhcdral dominant mafic phase is biotite, which makes up biotite commonly forms 1 to 2 percent of the rock; about 5 percent of the rock. Biotite is partially to apatite, iron-oxides, zircon, and sphene are present completely replaced by chlorite. Trace amounts of in trace amounts. Locally, interstitial fine-grained green hornblende, apatite, and zircon are also crystals of potassium feldspar constitute <2 percent present. Plagioclase is heavily sericitized. Secondary of the rock. epidote forms granular masses and large, radiating Plagioclase grains are subhedral (0.5 to 2.5 mm) bladed grains along fractures. and display normal zoning, with andesine cores and oligoclase rims; the cores are commonly altered to Small Plutons of Quartz Monzodiorite and assemblages of clinozoisite, epidote, and sericite. Granodiorite.-Small plutons (<5 km2) ranging in Quartz grains are anhedral to subhedral (0.5 to composition from quartz monzodiorite to lesser 2.5 mm) with undulatory extinction, and some granodiorite intrude quartz diorite and tonalite in the crystals have interlocking boundaries. Hornblende Anchorage C-5 Quadrangle (Pavlis, 1982a, 1983). grains are mostly anhedral to subhedral (0.5 to Most of the quartz monzodiorite and granodiorite 3.0 mm) and display green and yellow pleochroism. bodies are roughly circular or elliptical with some Less commonly, hornblende is blue green to light irregular lobes. The plutons clearly intrude green, and seems to be associated with low- surrounding rocks, but no chilled margin or contact temperature alteration. In more mafic quartz aureole was recognized adjacent to the bodies diorite, hornblende is poikilitic, enclosing subhedral (Pavlis, 1986). quartz and plagioclase crystals. Relict clinopyroxene The rocks are fine to medium grained and forms the cores of some large hornblende grains in hypidiomorphic-granular; they generally contain both more mafic quartz diorite. Biotite grains are hornblende and biotite. Mafic phases and quartz anhedral (0.5 to 3.0 mm) with dark-brown to light- contents are 10 to 20 percent each. Plagioclase is Professional Report 94 zoned, which suggests either cooling at relatively Talkeetna-Aleutian Range batholith, the Kodiak- shallow crustal levels (Pavlis, 1982a, 1983) or Kenai plutonic belt, and the plutonic rocks of changing magmatic conditions, including intermediate composition in the northern Chugach fractionation and mechanical processes (for example, Mountains include rock type and relative Morse, 1980). abundances, regional setting, composition, and petrography. These similarities suggest that the Age and Correlation.-K-Ar age determinations quartz diorite and related rocks exposed in the on hornblende and biotite from the quartz diorites, northern Chugach Mountains may be the edge of the tonalites quartz monzodiorites and mafic dikes Talkeetna-Aleutian Range batholith, which has been indicate Jurassic ages (160 to 194 Ma) (table 4). A disrupted by Late Cretaceous(?) and Tertiary K-Ar whole-rock analysis on a mafic dike intruding faulting. quartz diorite yields an age of 130 Ma (table 4; Winkler and others, 1990). These ages determined from the intermediate INTERPRETATION rocks coincide with ages reported from the Talkeetna-Aleutian Range batholith and the Kodiak- The cumulate ultramafic and gabbroic rocks of Kenai plutonic belt (Csetjey and others, 1978; the BRUMC are interpreted by Burns (1985) and Hudson, 1979). Other similarities between the Newberry and others (1986) to represent the

Table 4. K-Ar (mineral and whole-rock) and U-Pb (zircon) ages from intermediate compositon plutonic rocks of Jurassic age from the northcentral Chugach Mountains [Analytical work for this study done under the direction of D.L. Turner, University of Alaska Geophysical Institute, Fairbanks, Alaska. Rock types given as in previous publications. Abbreviations include: hb - hornblende; bio -biotite; qtz - quartz; dio - diorite; cglm - conglomerate; ton - tonalite; --- no data]

Mineral Age % K20 Sample no. Unit Rock dated (m.~.) average Quadrangle Reference

Pavlis F Jdg Mb qtz dio Hornblende 194 t 7 --- Pavlis (1983) Pavlis E Jdg IIb qtz dio Hornblende 191 h 7 --- Pavlis (1983) Pavlis G Jdg Hb dio Hornblende 189 t 8 --- Pavlis (1983) 82Fn255b Jdg Qtz dio Biotite 187.3 * 5.6 0.600 This study 79AH38 (AK319) Jdg Qtz dio Zircon 183 concordant - Hudson & Arth (1989) 82Fn113b -- Qtz dio Biotite 182.8 t 5.5 2.423 This study 82Fn255a --- Qtz dio Hornblende 181.4 i 5.4 4.352 This study 81AWk14 Jqd Hb bio ton Hornblende 181 * 8 0.440 Winkler (1989) Biotite 172 * 5 0.432 mineral pair Qtz dio Biotite 177.5 i 5.3 2.700 This study Qtz dio Biotite 178.4 * 5.4 1.368 This study Qtz dio IIornblende 177.8 t 5.3 0.450 This study Hb dio Hornblende 177 t 11 --- Hudson & Arth (1989) cglm clast Hornblende 177.1 t 5.3 0.600 This study Hb bio ton Hornblende 175 * 8 0.421 Winkler (1989) Biotite 174 i 5 5.88 mineral pair 81JD874a Qtz dio Hornblende 174.3 * 5.2 0.298 This study 79AN52 (AK332) Hb dio IIornblende 172 i 11 --- Winkler (unpub. data) 79AII36 (AK317) Qtz dio Zircon 171 --- I Iudson & Arth (1989) 76AI-I58 (AK338) Hb qtz dio Hornblende 167 * 10 --- Hudson & Arth (1989) 82Fn120a Qtz dio Ilornblende 166.5 * 5.0 0.287 This study SI-IB Clark Hb bio qtz dio Biotite 167 k 9 --- Clark (1972) 79AH34 (AK316) Hb qtz dio Hornblende 167 t 9 --- Hudson & Arth (1989) 79AE-140 (AK320) Hb qtz dio Hornblende 165 * 10 --- Winkler (unpub. data) 82Fn113a Qtz dio Hornblende 163.6 i 4.9 0.363 This study 82Pnlllb Qtz dio Biotite 161.0 * 4.8 2.947 This study 82Fn202a-1 Qtz dio Hornblende 160.0 i 4.8 0.425 This study Mafie dikes 81AWk49a Basaltic dike Whole rock D -4 Winkler (unpub. data) Geology of the Northern Chugach Mountains

fractional crystallization product of magma(s) that CRETACEOUS SEDIMENTARY produced the Talkeetna Formation volcanic rocks. ROCKS This interpretation is based on mineralogy and petrology of the ultramafic and gabbroic rocks and of MATANUS KA FORMATION the volcanic rocks. Magma from the quartz dioritic intrusions may The Matanuska Formation, a marine sequence of have erupted and formed intermediate silicic to clastic sedimentary rocks (Km, sheets 1 and 2), is the siliceous volcanic rocks, but the bulk of these only Cretaceous age sedimentary sequence north of volcanic rocks, if they existed, has apparently been the Border Ranges fault in the Nelchina-Kings removed by erosion. The majority of the Talkeetna Mountain region. The Matanuska Formation was Formation appears to be basaltic to andesitic in deposited unconformably on rocks of the Peninsular character, and is thus more mafic than the quartz terrane and crops out in the northernmost edge of diorite and tonalite. Additionally, the intrusive the central Chugach Mountains, in the Matanuska relationship of the quartz diorite-tonalite into Valley, and in the southern Talkeetna Mountains Talkeetna Formation volcanic rocks indicates that (Grantz, 1961a,b; Barnes, 1962; Csetjey and others, the large quartz diorite-tonalite plutons are younger 1978) (figs. 1 and 2). The outcrops in the Chugach than the Talkeetna Formation in the northern Mountains occur along the faulted southern limb of Chugach Mountains. Therefore, we propose that the the Matanuska Valley synclinorium and generally dip quartz diorite-tonalite is not the intrusive equivalent moderately or steeply to the north or northwest of volcanic rocks in the Talkeetna Formation. (Grantz, 1961a,b). Clastic detritus in the Matanuska The quartz dioritic rocks also always intrude the Formation is apparently derived from erosion of gabbroic rocks. Abundant ductile deformation of Jurassic arc rocks of the Peninsular terrane (Grantz, adjacent quartz dioritic and gabbroic rocks (Burns, 1964). 1983, in press) and similar K-Ar ages for quartz The Matanuska Formation consists chiefly of diorite and gabbroic rocks suggest that the intrusions fine-laminated dark-gray claystone and siltstone, with of quartz diorite and gabbroic rocks are broadly subordinate greenish-gray lithic and arkosic contemporaneous. sandstone, and minor conglomerate and An attempt was made during this study to select a conglomeratic mudstone and sandstone (Grantz, continuous compositional suite of samples ranging 1964). Sole markings, flame structure, graded from gabbronorite to quartz diorite for major-oxide bedding and other features indicative of turbidite analysis. Petrographic study and field relationships, deposition are abundant in the formation, combined with analytical results, indicate that the particularly in the Matanuska Valley. diorite plutons which have been mapped by other workers in the Anchorage C-4 and C-5 Quadrangles Petrology and Lithology (Clark, 1972b; Pavlis, 1983, 1986) appear to have formed (in most instances) by mixing partially Along the north flank of the central Chugach solidified gabbroic and molten quartz dioritic Mountains, the Matanuska Formation consists magmas (see Little and others, 1986b). Only minor chiefly of a homogeneous, thick sequence of well- amounts of the diorite appear to have formed as a indurated, fissile, medium- to dark-gray claystone straight differentiate of the gabbroic magma or as a and mudstone. These mudstones are fine-laminated mafic fractionate of the quartz dioritic magma. It locally and contain some thin, laterally continuous would therefore appear that the more differentiated interbeds of siltstone and fine sandstone (Little and quartz diorite magma is also not a differentiate of the others, 1986b). Reddish-weathering concretions and magma that formed the gabbroic rocks. lenses of sandy limestone as thick as 30 cm within the This study then supports the interpretation of dark mudstone contain cone-in-cone structures. Burns (1985) that the quartz diorite-tonalite probably These rocks are probably equivalent to the crystallized from a different magma than the one that Campanian-Maestrichtian unit recognized to the east produced the gabbroic rocks, on the basis of and north (unit Krns of Grantz, 1961a,b). Unlike petrology, mineralogy, and compositional data. We Grantz's (1961b) Kms unit in the Matanuska Valley, suggest a less conventional petrogenetic mechanism which contains abundant channelized lenses of than fractional crystallization for formation of the conglomerate and sandstone, the exposures in the quartz dioritic rocks, such as anatexis of preexisting northern Chugach Mountains consist predominantly , although we have not quantitatively of mudstone with only minor interbedded sandstone. tested such a model. These shaly rocks may have been deposited in a 28 Professional Report 94 more distal basin- environment than those in rocks exposed. This section locally contains the Matanuska Valley described by Grantz (1964), glauconite and was probably deposited in littoral and which would suggest southward transport. nearshore environments (Grantz, 1964). Along the Chugach range front in the Anchorage To the east of Wolverine Creek, the Matanuska D-2 Quadrangle, the base of the Matanuska Formation unconformably overlies the Lower Formation consists of Cenomanian or age Jurassic Talkeetna Formation. Near Riley Creek pebbly sandstone that is overlain by a unit of (Anchorage D-4 Quadrangle), the basal Matanuska Turonian age siltstone, interbedded siltstone and Formation consists of about 25 m of volcaniclastic graded sandstone beds, sandstone, and conglomerate sandstone and pebble conglomerate overlain by (Grantz, 1961b). These Cenomanian-Turonian rocks about 80 m of dark-olive-gray and dark-gray siltstone are overlain by the Campanian-Maestrichtian unit and thin beds of greenish-gray fine-grained (Kms) of Grantz (1961a,b) described below and were sandstone (Grantz, 1964). These two clastic units of not recognized west of the Matanuska Glacier. Albian age are derived chiefly from underlying In the Matanuska Valley (Waring, 1936; Barnes volcanic rocks of the Talkeetna Formation (Capps, and Payne, 1956; Grantz, 1961a,b) and along the 1927; Grantz, 1964). Rocks similar to the gray north flank of the Chugach Mountains, the siltstone and sandstone unit described above overlie Matanuska Formation is unconformably overlain, the Talkeetna Formation along the range front east with only minor angular discordance, by fluvial of Ninemile Creek (Anchorage D-4 Quadrangle; sedimentary rocks of the Paleocene Chickaloon Grantz, 1964). In the Anchorage D-1 Quadrangle, Formation. This disconformity in the northern basal beds of the Matanuska Formation (Albian) Chugach Mountains is well exposed in the consist of conglomerate and crossbedded sandstone Anchorage D-3 Quadrangle in the between that grade upward into sandstone which is overlain O'Brien and Gravel Creeks, and on Coal Creek in by interbedded siltstone and sandstone (Grantz, the Anchorage D-4 Quadrangle (Little and others, 1961a). These beds are in turn overlain by hard 1986b). siltstone and claystone (Grantz, 1961a). Although the lower contact of the Matanuska The basal unconformity of the Matanuska Formation is commonly covered or faulted, there are Formation is well-exposed on the Chugach range several localities in the Nelchina-Kings Mountain front between Coal and Carbon Creeks (Anchorage region where a depositional contact is exposed. The D-4 Quadrangle), where it has about 1 m of local Matanuska Formation overlies volcanic rocks of the erosional relief (Little and others, 1986b). Lithic Lower Jurassic Talkeetna Formation in several lapilli ash tuff of the Talkeetna Formation is overlain places east of Wolverine Creek (Capps, 1927; by about 3 m of dark greenish-gray, very poorly Grantz, 1961a,b; Little and others, 1986b), and to the sorted pebble cbnglomerate with a coarse-grained west of the map area, overlies the Upper Jurassic sandy matrix containing subangular clasts (to 10 cm Naknek Formation at one location along Wolverine diam) from the underlying Talkeetna Formation Creek (Anchorage C-6 Quadrangle, fig. 3) (Grantz, (Little, 1988; Little and others, 1986b). These rocks 1964). are in fault contact with about 7 m of brown In lower Wolverine Creek (Anchorage C-6 mudstone. The mudstone is, in turn, overlain by a Quadrangle; fig. 3), late-early or late Albian age 20- to 30-m-thick sequence of medium- to thick- rocks of the Matanuska Formation unconformably bedded olive-green siltstone and fine-grained overlie siltstone of the Upper Jurassic Naknek sandstone resembling Grantz's (1964) description Formation (Grantz, 1964). At this location, the basal (above) of the claystone and siltstone unit. Matanuska Formation consists of 10 m of pebbly (locally containing boulders), coarse-grained Age and Correlation volcaniclastic sandstone that grades upward into pebbly, fine-grained sandstones containing sandy The Matanuska Formation contains fossils of limestone concretions (Grantz, 1964). These poorly Albian and Maeslrichtian ages (middle to Late sorted rocks are overlain by about 70 m of dark Cretaceous) and is part of an elongate belt of clastic greenish-gray to medium-dark-gray silty claystone sedimentary rocks of dominantly marine origin that containing thin and medium interbeds of sandstone extends from the Canadian border to the Alaska near the top of the sequence. Above the claystone Peninsula (Grantz, 1964). This belt overlaps the and siltstone, 13 m of thick-bedded, dark greenish- Wrangellia and Peninsular terranes (Jones and gray, fine- and medium-grained volcaniclastic Silberling, 1979). On the Alaska Peninsula, sandstone of Albian age represent the uppermost correlative Late Cretaceous rocks of the Peninsular Geology of the Northern Chugach Mountains 29 terrane are included in the Chignik and Hoodoo arc margin and which continued as the subduction Formations (Mancini and others, 1978). complex accreted southward. In the Nelchina-Kings Mountain area, the Interpretation Chugach terrane is composed of an older melange known as the McHugh Complex and a younger The Matanuska Formation consists of both flysch-like sequence known as the Valdez Group. shallow and deep marine sedimentary rocks. Grantz The McHugh Complex contains mafic volcanic rocks, (1964) concluded that the shoreline during and basin and trench-slope(?) deposits, all deformed Campanian-Maestrichlian time was only several into a tectonic melange. The Valdez Group consists miles north of the present day Matanuska Valley and chiefly of trench-fill turbidites and mudstone. In that, at least at certain times, a southern shoreline some areas, highly deformed, dark-gray meta- may also have existed in the present Chugach sedimentary rocks (argillite) in the McHugh Mountain area. If this is true, then the transition Complex are difficult to distinguish from similar from a nearshore to relatively deep marine rocks in the Valdez Group. Generally, however, environment was abrupt. This observation, together McHugh Complex rocks were distinguished from with the occurrence of several unconformities in the Valdez Group rocks on the basis of these criteria: Matanuska Formation, some of them angular (1) argillaceous rocks are typically slaty or phyllitic in (Grantz, 1964), suggests tectonic instability during the Valdez Group, and have a less penetratively deposition of the Matanuska Formation. deformed, more chaotic texture in the McHugh Late Cretaceous-Early Tertiary age plutonic and Complex; (2) green metatuff lenses and thin "wispy" volcanic igneous rocks representing a calc-alkaline layers of green tuffaceous rna~crialare typical of the magmatic arc coeval with the Matanuska Formation McHugh Complex, but generally absent in the are located north of the Matanuska Formation in the Valdez Group; (3) where tuff is present in the Alaska-Aleutian Range and in the Talkeetna Valdez Group, it tends to be interlayered with Mountains (Fisher and Magoon, 1977; Hudson, argillite (possibly original sedimentary layering) and 1979). The Chugach terrane, which is generally is traceable for hundreds of meters, whereas tuff in interpreted as an Early to Late Cretaceous the McHugh Complex is typically finely and subduction complex (Nilsen and Zuffa, 1982) lies chaotically intermixed with argillite; (4) exotic blocks south of the belt of Upper Cretaceous sedimentary of marble, chert, pillow , and serpentine occur rocks. This spatial relationship between the three locally in the McHugh Complex, but are absent in coeval convergent margin tectonic elements led the Valdez Group; and (5) geometry is more Moore (1973) and Fisher and Magoon (1977) to systematic and folds more easily 'recognized in the interpret the Upper Cretaceous part of the Valdez Group than in the McHugh Complex. Matanuska Formation as a basin deposit associated with a north-dipping subduction zone MCHUGH COMPLEX along what is now the southern margin of Alaska. The McHugh Complex, a melange composed of blocks of various ages in a highly deformed matrix of DESCRIPTIVE GEOLOGY OF argillite, graywacke, and tuff, was named by Clark THE CHUGACH TERRANE (1973) for a chaotic assemblage of weakly metamorphosed sedimentary and volcanic rocks CRETACEOUS SYSTEM exposed in the Chugach Mountains between Turnagain Arm and the Knik River. Winkler and The Chugach terrane (Berg and others, 1972) of others (1981b) assigned a similar assemblage in the Cretaceous age is exposed continuously for 2,000 km Valdez Quadrangle to the McHugh Complex. Our along the southern rim of Alaska and represents one mapping in the Nelchina-Kings Mountain area shows of the best-preserved ancient subduction complexes the McHugh Complex occurs in irregular fault- in the world (fig. 1) (Nilson and Zuffa, 1982). The bounded blocks between the Border Ranges fault Chugach terrane is the most landward, uplifted system and the thrust (sheets 1 and 2), portion of a wide accretionary prism which becomes and thus forms a Sink between the type area near generally younger toward the sea. Low-grade Anchorage defined by Clark (1973) and the rocks metamorphic rocks of the Chugach terrane have mapped by Winkler and others (1981b) in the Valdez been subjected to a complex sequence of ductile and Quadrangle. brittle deformational events, which began during The McHugh Complex is in fault contact with the initial accretion to the edge of the continental island Valdez Group (Kvs) to the south and with rocks of 30 Professional Report 94 the Peninsular terrane to the north. The northern The metasedimentary rock unit (Krns) of the boundary of the McHugh Complex has been referred McHugh Complex consists of dark-gray argillite and to as the Border Ranges fault (MacKevett and scaly argillite. The unit contains bodies of graywacke Plafker, 1974). In the Nelchina-Kings Mountain and(or) chert and local lenses of metatuff. area, the Border Ranges fault system extends in an The predominantly metavolcanic rock unit arcuate (concave to the southeast) pattern across the (Kmxv) is gradational in composition between the region and consists mainly of a series of steeply metavolcanic unit (Kmv) and the metasedimentary dipping high-angle faults. Motion on the faults unit (Kms) and consists of 50 to 80 percent appears to have been dominated in early Tertiary metavolcanic rock. The unit consists chiefly of light- time by extensional and dextral strike-slip green, weakly to moderately foliated metatuff displacement. Vestiges of an earlier, probably Early interlayered and chaotically intermixed with dark- Cretaceous age, north-dipping thrust boundary gray argillite. The unit is commonly massive, with a between the Peninsular and Chugach terranes are fabric characterized by swirls of green and gray. locally preserved. These thrust zones are now Layered metatuff commonly has a detrital clastic crosscut by Tertiary high-angle faults that dominate texture and contains abundant mud-chip rip-ups, the present structural grain of the Chugach range. suggesting transportation by submarine mass-flow The southern boundary of the McHugh Complex is mechanisms. the regionally north-dipping Eagle River thrust, The re dominantly metasedimentary rock unit which places the McHugh Complex on top of the (Kmxs) is gradational in composition between Valdez Group to the south. Locally, this fault has metavolcanic unit (Kmv) and metasedimentary unit been cut by younger, high-angle faults, or has been (Krns) and contains 50 to 95 percent rotated to a nearly vertical position. metasedimentary rock. Typically, the predominantly metasedimentary unit consists of dark-gray, scaly Petrology and Lithology argillite, with lenses and layers of green metatuff. Less commonly, it consists of thinly interlaminated In the Nelchina-Kings Mountain area, the green and gray . The unit locally contains McHugh Complex is composed of argillite, gray- blocks of marble, chert, and graywacke. green metatuff, thinly interlaminated gray and green The unit (Kmxu) consists of phyllite, graywacke, chert, pillow basalt, and lesser microbreccia and cataclasite with an inferred marble. Composition varies from dominantly protolith of McHugh Complex rocks. metatuff to dominantly argillite. Chaotic mixtures of The undifferentiated McHugh Complex (Kmu) light-green metatuff and dark-gray argillite are contains any of the previous five rock types in common in the McHugh Complex and were proportions that individual units could not be presumably formed, at least in part, by soft-sediment distinguished at map scale. The undifferentiated unit deformation. The complex commonly contains a typically consists of intermixed dark-gray, very fine block and matrix (melange) fabric, with a scaly grained argillite, gray-green metatuff, and gray and foliation defied by pervasive shear fractures. green phyllite, and it locally contains exotic blocks of The McHugh Complex is mapped as six units on graywacke, marble, phyllite, chert, and metadiorite. sheets 1 and 2: (1) metavolcanic rocks, (2) predomi- Additional rock associations in the undifferentiated nantly metavolcanic rocks, (3) metasedimentary McHugh Complex unit, too small to show at map rocks, (4) predominantly metasedimentary rocks, scale, include sequences of chert and argillite, (5) cataclasite, and (6) undifferentiated McHugh assumed to be part of the McHugh Complex because Complex. of their lithology and structural position. These The metavolcanic rock unit of the McHugh sequences consist of dark-gray argillite, silty argillite, Complex (Kmv; sheets 1 and 2) is massive, medium contorted slate, and thin- to very thin bedded shale green or slightly bluish green. These rocks consist of which typically contains interbeds of laminated light- light-green, weakly foliated metatuff that is locally gray chert. These rocks locally contain zones of interlayered or chaotically intermixed with minor cataclastic rock. amounts of dark-gray argillite. Locally, the unit contains blocks of recrystallized marble and bedded Alteration and Metamorphism chert as large as several meters. The rocks arc commonly highly shattered and locally have The McHugh Complex in the Nelchia-Kings abundant tectonically polished and slickensided Mountain area is typically metamorphosed to lower surfaces. or subgreenschist facies assemblages. Metamorphic Geology of tlie Nortllern Chugach Moutltains 3 1 minerals commonly include quartz, albite, chlorite, setting. Argillaceous material epidote, prehnite, pumpellyite, white mica, actinolite, most likely represents hemipelagic slope deposits; calcite, magnetite, and sphene. Late-stage veins are graywacke and conglomerate probably represent composed of quartz and calcite * prehnite. submarine canyon and base of slope deposits. The Regionally in the Chugach Mountains, the matrix common occurrence of diorite as clasts and exotic of the McHugh Complex melange is most commonly blocks in the McHugh Complex suggests derivation metamorphosed in the prehnite-pumpellyite facies, from nearby or island arc. The although blocks and belts of blueschist and presence of ocean-floor lithologies, including chert, greenschist occur locally in the Valdez Quadrangle metabasalt, limestone, and ultramafic rock, and the (Clark, 1973; Winkler and others, 1981b). In the presence of blueschist facies clasts and blocks within Nelchina-Kings Mountain region, the occurrence of the melange suggest deposition of the McHugh apparent relict pumpellyite and the structural fabric Complex in a subduction zone setting. observations discussed in the section on structure The first deformation to affect the McHugh suggest that the lower greenschist facies Complex produced a ductilc, chaotic fabric, and most metamorphism was superimposed upon the prehnite- likely was associated with submarine slumping and pumpellyite metamorphism (Decker, in Burns and sliding, but may in part be due to tectonic others, 1983; Little, 1988). deformation of unlithified strata during subduction. Subsequent deformation parallels closely the Age and Correlation deformation of the Valdez Group and is discussed in The age of the McHugh Complex was originally the Structural Geology section. designated as Late Jurassic and(or) Cretaceous (Clark, 1973). In actuality, formation of the McHugh VALDEZ GROUP Complex may have spanned most of Jurassic and Cretaceous time. The McHugh Complex contains The Valdez Group was originally named the blocks of fossiliferous rocks that range in age from "Valdes series" by Schrader (1899, 1900) for Mississippian to Early Cretaceous (Albian) (for rhythmically intcrbedded graywacke and slate in the example, Winkler and others, 1981b; Nelson and Port Valdez area. The name Valdez Group was others, 1986), and fossils found in the matrix material applied to similar rocks in the Anchorage Quad- range in age from Triassic to Early Cretaceous rangle by Clark (1972a). As now defined, the Valdez (Blome and others, written commun., 1989). The Group includes rocks from the southwestern end of youngest exotic blocks incorporated in the melange the to the Canadian border (Tysdal arc Albian, and therefore the age of at lcast part of and Plafker, 1978). In the Nelchina-Kings Mountain the McHugh Complex can be no older than Albian. area, the Valdez Group contains predominantly However, Winkler and others (1981b) found that the lower-greenschist-facies metagraywacke, argillite, McHugh Complex in the Valdez Quadrangle is phyllite, and minor grcenstone and conglomerate. composed of northern, middle, and southern bclts These rocks are contiguous with less metamorphosed which yield fossils of different ages. The oldest belt counterparts along the Kenai Peninsula and in the crops out discontinuously on the north and contains type area at Port Valdez. chert with Late Triassic to Early Jurassic radiolarian The inboard (northern) contact of the Valdez faunas. The central and southern belts yielded chert Group is the Eagle River thrust. This fault is a with Late Jurassic to Early Cretaceous, and mid- regionally shallow, north-dipping structure along Cretaceous radiolarian faunas respectively (Winkler which rock of the Valdez Group were thrust beneath and others, 1981b). Tonalite-trondhjemite plutons, the McHugh Complex. The Eagle River thrust was which intrude the McHugh Complex in the probably active during latest Cretaceous time and Anchorage C-5 Quadrangle, have been dated at 110 was later strongly modified by Tertiary age to 135 Ma (Pavlis, 1982b, 1983; Pavlis and others, deformation. The fault has been locally rotated to 1988). Therefore, deposition of at least part of the near vertical, and has been cut by numerous high- McHugh Complex would have occurred by angle faults. Where metamorphic gradients extend Cretaceous time, prior to the time of the tonalite- across the fault and similar rock types occur on trondhjemite intrusion. opposite sides, the distinction between the Valdez Interpretation and Deformation Group and the McHugh Complex is not clear. The southern boundary of the Valdez Group (the Contact The metasedimentary rocks of the McHugh Fault) does not occur in the Nelchina-Kings Complex probably were deposited in a marine Mountain area. 32 Professional Report 94

Petrology and Lithology Undifferentiated metasedimentary rocks (Kvs) of the Valdez Group occur east of the Anchorage C-5 The Valdez Group is highly deformed and Quadrangle and consist predominantly of gray characterized by a penetrative, phyllitic cleavage. In phyllite, argillite, and thin- to thick-bedded the Nelchina-Kings Mountain area, the Valdez metagraywacke. In this region, the Valdez Group is Group is a very thick sequence of phyllite, argillite, highly deformed and characterized by a continuous and thin- to thick-bedded metagraywacke, with phyllitic cleavage, S1, and a younger minor amounts of gray-green metatuff and stretched- cleavage, S2 (discussed further in Structural pebble conglomerate. In this area, volcanic rocks are Geology). most abundant in the northern portions of the The metatuff and tuffacccrus metaconglomerate Valdez Group, and argillite and graywacke are unit (Kvt) occurs in the Anchorage C-4 Quadrangle, largely metamorphosed to phyllite. where it forms two relatively persistent marker The Valdez Group was subdivided into three map horizons. The metatuff commonly is gray-green, units in the Anchorage C-5 Quadrangle. These units moderately well foliated and contains abundant lithic include: (1) black phyllite, (2) graywacke and fragments and locally common mud chips. The argillite, and (3) a conglomeratic graywacke. Farther tuffaceous metaconglomerate consists of stretched to the east, these units cannot be distinguished as pebbles and cobbles in a sandy tuffaceous matrix. mappable units, due in part to an increased Granitic clasts, limestone clasts, and mudstone metamorphic grade in that region. Locally, however, ripups are recognizable. protoliths similar to those in the Anchorage C-5 huadrangle can be recognized. and etat tuff Alteration and Metamorphism tuffaceous metaconglomerate occur within the Valdez Group in the Anchorage C-4 Quadrangle Metasedimentary rocks of the Valdez Group in near the head of Coal Creek and were mapped the Nelchina-Kings Mountain area are moderately to separately (unit Kvt). completely recrystallized. Detrital quartz, feldspar, The black phyllite unit (Kvp) consists of highly mica, and rock fragments are best recognized dcformed black phyllite interlayered with <30 farthest to the west, in the Anchorage C-5 percent very fine grained, silty metagraywacke. The Quadrangle. In the Anchorage C-2, C-3, and C-4 black phyllite unit is delineated only in the Quadrangles, a typical chlorite-zone greenschist- Anchorage C-5 Quadrangle, where the character of facies assemblage of albite, very fine grained white the deformational fabric varies along strike from a mica, chlorite, calcite, quartz, and sphene occur in slaty cleavage in lower Friday Creek to a phyllitic the metasedimentary rocks. Actinolite, chlorite, cleavage at the eastern end of the quadrangle. albite, calcite, quartz, and epidote group minerals are The graywacke and argillite unit (Kvg) consists of typical minerals in the metavolcanic rocks. interbedded graywacke and argillite in which the sandstone to shale ratio is at least 1:l. Lithologic Age and Correlation layering varies from thin (<1 cm) laminations to large scale interbedding (> 100 m) of predominantly Very sparse marine megafossil collections from sandstone or predominantly shale. The the Valdez Group on the Kenai Peninsula are all indicative of a Late Cretaceous (Maestrichtian) metagraywackes are locally coarse grained and depositional age (Jones and Clark, 1973). K-Ar ages conglomeratic. Argillite is indistinguishable from on the Valdez Group in the Valdez Quadrangle similar rocks of the black phyllite unit. The include one whole-rock age of 53.5 i 1.6 Ma on a graywacke and argillite unit is delineated only in the Anchorage C-5 Quadrangle, where it forms the metatuff, and a white mica age of 51.6 * 1.5 Ma on a structurally lowest levels of the area. post-metamorphic granitic stock (Winkler and The conglomeratic graywackc unit (Kvc) consists others, 1981b). In the Anchorage C-3 Quadrangle, of massive units of foliated, coarse-grained biotite which formed in a hornfels zone around a felsitic intrusion (Ti) yielded a K-Ar age of 52.1 Ma. metagraywacke in which large mud chips are These ages suggest that metamorphism of the Valdez interspersed. In these highly deformed rocks, mud Group and intrusion of felsites occurred soon after chips now form wafers along foliation surfaces. The deposition. conglomeratic graywacke unit is delineated only in the Anchorage C-5 Quadrangle, where two bands of Interpretation and Deformation massive conglomeratic graywacke mark the transition from structurally higher, shaly rocks (Kvp) to The Valdez Group was deposited as part of a structurally lower, sandier lithologies (Kvs). large trench-fill system extending from Chatham Geology of the Northern Chugach Mountains 33

Strait in southeastern Alaska to Sanak Island near in a narrow belt along the Border Ranges fault the end of the Alaska Peninsula (Moore, 1973; system in the western part of the mapped area from Nilsen and Zuffa, 1982). The graywackes are typical the Knik River to Ninemile Creek (sheet 1). These turbidites. Evidence in support of this conclusion plutons have been the subject of considerable was documented from the less metamorphosed rocks investigation (for example, Pavlis, 1982a,b, 1983; of the Valdez Group elsewhere in the Chugach Monteverde, 1984; Pavlis and others, 1988) and yield Mountains and on the Kenai Peninsula (Budnik, Early Cretaceous K-Ar ages ranging from 135 to 1974; Nilsen and Zuffa, 1982). In the Nelchina-Kings llOMa (Pavlis, 1982b, 1983; Winkler, personal Mountain area, relict interbedded sandstone and commun.). In the eastern part of the mapped area shale, flattened but still recognizable mud-chip rip- (Anchorage C-1 and C-2 Quadrangles), dikes of ups, and rare graded beds and flute marks are trondhjemite up to 1.5 km wide are abundant in the consistent with the marine turbidite interpretation. mixed metamorphic and gabbroic rocks (MzPzmp Subduction-related imbricate thrust faulting that unit; sheet 2) of the Peninsular terrane. A K-Ar date was in part contemporaneous with deposition was from a dike in the eastern part of the mapped area most likely responsible for the first deformational yields an age of 145 Ma, or Early Jurassic (table 5). event to affect the Valdez Group. The second, These dikes have not been studied in detail; more is regionally more extensive, deformation occurred in known about the dikes in the western part of the early Eocene time, about 55 Ma, and probably is mapped area. Because the mineralogy, K-Ar ages, related to emplacement of the structurally underlying and level of knowledge differ between trondhjemites Orca Group in the area. The in the western and eastern portions of the belt, they latest Cretaceous deposition of the Valdez Group are discussed separately. and its subduction, low-grade metamorphism, and penetrative deformation in latest Cretaceous to Dikes in Eastern Part of Map Area earliest Tertiary time was quickly followed by subaerial exposure and erosional beveling of those Trondhjemite dikes in the eastern part of the metamorphic rocks; they could thus be a source mapped area weather white and typically have sharp terrain for, and locally, a depositional basement for contacts, crosscutting the country rock. The the Chickaloon Formation. This orogenic event trondhjemite is generally composed of medium- to affecting the Valdez Group must have occurred coarse-grained plagioclase (60 to 75 percent) and within a short interval of time (probably less than quartz (25 to 40 percent), with <10 percent 5 m.y.) near the Cretaceous-Tertiary boundary (see hornblende, chlorite and opaque minerals. Structural Geology section). Trondhjemite is commonly extensively altered, and contains minor quartz-sericite veins, chlorite, muscovite, and contains abundant, fine-grained white DESCRIPTIVE GEOLOGY OF ROCKS mica which replaces plagioclase. Minor amounts of COMMON TO BOTH TERRANES coarse-grained muscovite are present in chlorite veins, and in places grossularitic garnet is present. The oldest rocks common to both the Peninsular The trondhjemite dikes are variably deformed but and the Chugach terranes belong to a plutonic suite typically not as deformed as the surrounding of diorite, tonalite, and trondhjemite of probable metamorphic and gabbroic rocks. Early Cretaceous age (Pavlis, 1982b; 1983; Pavlis and others, 1988). Tertiary rocks common to both Plutons in Western Part of Map Area terranes include sedimentary rocks of the Chickaloon Formation, hypabyssal dikes of dominantly andesitic The bulk of the Early Cretaceous-Late and dacitic composition, and coarse-grained basaltic Jurassic(?) intrusive rocks in the western part of the and minor lamprophyric dikes. map area are confined to five stocks with areal exposures of 2 to 12 km2. The rocks clearly intrude CRETACEOUS SYSTEM the foliated metamorphic rocks along the Border PLUTONIC ROCKS Ranges Fault System, the mafic/ultramafic complexes, and locally, the McHugh Complex. The TONALITE-DIORITE-TRONDHJEMITE rocks are weakly foliated locally, indicating some post-emplacement deformation. Lithologically, the A series of small stocks, sills, and dikes plutons range from tonalite to trondhjemite, composed dominantly of a distinctive leuco- although leuco-tonalite with bronze (altered) biotite tonalite/trondhjemite with minor diorite is exposed is characteristic. Professional Report 94

Table 5. K-Ar and U-Pb ages-. from Cretaceous tonalite and trondhjemiteplutonic rocks of the northcentral Chugach Mountains [Analytical work for this study done under the direction of D.L. Turner, University of Alaska Geophysical Institute, Fairbanks, Alaska. Rock types given as in previous publications. Abbreviations include: trond - trondhjemite; bio -biotite; mus - muscovite; hb - hornblende; dio - diorite; --- no data]

Sample Mineral Age % '020 no. Unit Rock dated (m.~.) average Quadrangle Reference

81JD867a Kt Trond Biotite C-2 This study 82ARM21 Kt 13io mus trond Biotite D -6 Winkler (1989) Muscovite mineral pair Pavlis C Kt Hb dio IIornblende C-5 Pavlis (1983) Pavlis 11 Kt IIb bio trond EIornblende C5 Pavlis (1983) Biotite mineral pair 81AWk44b Kt Trond Muscovite C-4 Winkler (1989) 79AEI62 (AK341) Kt Trond Zircon C-5 Winklcr (unpub. data) 81Pe125 Kt Trond White mica This study + chlorite

The intrusions are multi-staged, and range from 5 to 10 percent quartz. In the main plutonic phase, mafic to silicic in composition. These variations plagioclase (An5 to An3o) and quartz are the probably reflect magmatic differentiation rather than dominant igneous minerals, with typical modal distinct magmatic phases. The oldest (most mafic) percentages of 50 to 65 percent and 25 to 35 percent, igneous rocks are hornblende diorites and tonalites respectively, and biotite is the principal mafic phase, that occur largely as cognate xenoliths and as massive with modal percentages of about 10 to 15 percent. In igneous rocks at scattered localities along the the late trondhjemite dikes, dark minerals are plutonic margins. These mafic phases probably generally absent, although a few trondhjemites represent early crystallization products of a leuco- contain traces of biotite. In addition, the tonalitic magma, as they are characteristically coarse- trondhjemites locally contain up to 5 percent grained along intrusive contacts. These mafic to muscovite and(or) garnet. Apatite is a ubiquitous intermediate plutonic rocks are, in turn, intruded by accessory in all of the plutonic phases (Monteverde, the leuco-tonalite, the main plutonic phase of the 1984). igneous complex. The leuco-tonalite typically Hydrothermal alteration has variably affected the contains bronze biotite, but the rocks range from primary igneous mineralogies of all the Cretaceous relatively dark hornblende-biotite tonalite to biotite plutons, and includes both an alteration that trondhjemite. The compositional variations within permeated the rock and an alteration that formed the main plutonic mass are gradational. These younger zeolite veins. Plagioclase is variably variations may represent differentiation in a magma saussuritized or sericitized, although altered cores chamber; however, injection of distinct magmatic are commonly surrounded by a clear rim of pulses commonly forms gradational contacts. metamorphic(?) albite. Biotite is generally Finally, the youngest plutonic phase within the converted to brown chlorite intergrown with complex is a series of trondhjemite dikes, which radiating interstitial prehnite; this type of alteration range from thin, 1- to Zcm-thick stringers to gives rise to the characteristic bronze appearance of massive, 10- to 20-m-thick dikes. Because the biotite in hand sample. Amphiboles are not leucocratic dikes are restricted to the plutonic conspicuously altered, although many are actually complexes and are most abundant near the cores of actinolite in composition, suggesting these large plutons, they are presumed to be late hornblende grains were completely converted to differentiates of the main plutonic suite. actinolite under greenschist facies conditions The plutonic rocks show considerable variation in (Monteverde, 1984). mineralogy because of primary composition and The rocks show considerable textural variation hydrothermal alteration. In the early mafic phase of related to both the amount of strain and degree of the plutons, hornblende is the only Fe-Mg mineral alteration. The least altered rocks typically show phase and forms up to 50 percent of the rock. The little, if any, evidence of deformation and are remainder of the rock is composed of plagioclase and characterized by hypidiomorphic granular textures. Geology of tlze Nortkenz Cltclgach Mountai~~s 35

Near plutonic margins and laults, the rocks are with the death of the Jurassic arc or be an early part conspicuously foliated. The foliation is defined of the near-trench plutons produced during thrusting largely by aligned biotite and elongated quartz. On a (as discussed in Pavlis and others, 1988). microscopic scale, this foliation is generally K-Ar ages on trondhjemite from the southern associated with alteration of biotite to chlorite t Talkectna Mountains to the north of the map area prehnite, and its character as a deformational fabric (fig. 1) are similar to ages on the Chugach is clearly indicated by: (1) kinking of biotite, trondhjemites and range from 125 to 148.5 Ma, with (2) dynamic recrystallization of quartz to produce a concordant K-Ar age on biotite (143 Ma) and seriate textures, and (3) kinking and grain breakage muscovite (146 Ma) (Csetjey and others, 1978). The in plagioclase. relationship between the trondhjemites and related rocks of the Talkeetna and Chugach Mountains is not understood, but they may have formed from Con~positionof Both Eastern and similar processes. Western Trondhjemites

Seven trondhjemite samples were analyzed for TERTIARY SYSTEM major oxide composition (table 6). Harker diagrams based on additional analyses of trondhjemite and re- Rocks of 'Tertiary age in the Nelchina-Kings lated tonalite from plutons in the Anchorage C-4 and Mountain region include clastic sediments of the C-5 Quadrangles are presented in Pavlis and others Chickaloon Formation, dikes and hypabyssal ,plugs, (1988) and are not reproduced here. All of the of dacitic to andesitic composition, and mafic dikes. analyzed trondhjemites contained < 15 weight In general, the Chickaloon Formation percent A1203 and therefore correspond with the unconformably overlies rocks of the Peninsular low-aluminum trondhjemite field of Barker and terrane, but the unit also unconformably overlies others (1976). Two of the analyzed trondhjemites rocks of the Chugach terrane at one location. The (samples 81Bu135, 81Bu129, table 6) intrude pre- dikes and plugs of dacitic and andesitic rocks are Jurassic metamorphic rocks and Jurassic gabbroic generally referred to as fclsites; they intrude rocks of rocks; the other trondhjemites shown in table 6 are both the Chugach terrane and the Peninsular terrane from plutons in the Anchorage C-4 and C-5 (Winkler and others, 1981b; Silberman and Grantz, Quadrangles. None were compositionally distinct 1984; this study). from the trondhjemites presented in Pavlis and others (1988). SEDIMENTARY ROCKS OF THE CHICKALOON FORMATION Age, Correlation, and Interpretation A partial section, > 1 km thick, of late Paleocene- The compositional and mineralogical data do not early Eocene clastic sedimentary rocks is exposed constrain the environment in which the trondhjemite along the north flank of thc Chugach Mountains. rocks formed. The trondhjemite may have resulted These rocks consist of conglomerate, sandstone, from fractionation of mafic magma or quartz dioritic siltstone, and mudstone of chiefly fluvial origin, and magma, subsolidus alteration of quartz diorite, or are here correlated with the Chickaloon Formation from partial melting of metamorphic rocks, such as in the Matanuska Valley. Along the northern edge amphibolite or metagraywacke, or of gabbroic rocks. of the Chugach-St. Elias Mountains, exposures of Pavlis and others (1988) present convincing sedimentary rocks are uncommon. The arguments, based on age dates, for generation of the only known outcrops of such rocks are located in and plutons in the west during underthrusting of the near this study area. McHugh Complex beneath the Peninsular terrane. The present distribution of Chickaloon Radiometric ages (table 4 and 5) on these rocks Formation in the Nelchina-Kings Mountain region is suggest that trondhjemite intrusion may have structurally controlled. Chickaloon Formation is occurred in the Chugach Mountains during a time preserved (I) west of the Matanuska Glacier in a span of at least 20 m.y., from about 110 to 130 Ma. downdropped , bounded on the north by The oldest age, from a K-Ar determination on the Lower Jurassic Talkeetna Formation of the hornblende from a trondhjemite dike in the Peninsular terrane, and on the south by the Upper Anchorage C-2 Quadrangle, was 145 Ma (table 4; Cretaceous Valdez Group of the Chugach terranes; fig. 3). Assuming all the ages are roughly accurate, (2) farther north in the Peninsular terrane, where the the Jurassic age may either imply an event associated southern, north-dipping limb of the Matanuska Professional Report 94

Table 6. Major-oxide artalyses and CIPWnoms of trondlijeniite (trond) from the northcentral Chugacli Mountains [Analyses performed at the Alaska Division of Geological and Geophysical Surveys by N.C. Veach and Kate Bull. FeO* and Fe2O3*, total molecular Fe; A, (Na20 + K2O)/NazO + K20 + FeO* + Mg0); F, FeO*/(NafO + K20 + FeO* + MgO); M, MgO/(Na20 + K2O + FeO* + MgO); ---,not applicable; norm An, an/(an + ab tor); Ab, ab/(an + ab tor); Or, or/(an + ab tor)]

Rock type trond trond trond trond trond trond trond Quadrangle C-4 C-4 C-4 C-1 C1 C-4 C-4 Field no 83Pe7 83Fn39a 83Fn12 81~~135~ 81~~129~ 83Fn40b 83Fn41 Chemical analyses (weight percent)

Si02 A1203 FeO ~e203 MgO cao Na2O K20 Ti02 p205 MnO LO1 Fe2O3* TOTAL

ClPW norms (weight percent)

qtz or ab an di hy mt il aP cor hm wo

myses81Bu135 and 81Bu129 intrude the pre-Jurassic metamorphic rocks and the gabbroic rocks and are similar to the trondhjemite yielding of a K-Ar age 145 Ma (table 4).

Valley synclinorium emerges along the Chugach involvement in an early Tertiary deformational event range front; (3) in the Peninsular terrane in a narrow (see Structural Geology section). fault-slice between Glacier and Ninemile Creeks; and The type-section Chickaloon Formation in the (4) in two locations in the Chugach terrane (fig. 3, Matanuska Vdey unconformably overlies the Upper sheets 1 and 2). The Chickaloon Formation, in all of Cretaceous Matanuska Formation, with no more these exposures, is folded and faulted due to its than slight angular discordance along a poorly Geology of tlie Northern Chugach Mountains 37 exposed contact (Waring, 1936; Barnes and Payne, general upward fining of conglomerate beds suggests 1956; Grantz, 1961b; Barnes, 1962). Along the deposition during waning flow associated with northernmost edge of the Chugach Mountains, the progressive emergence of the bars in the wake of a Chickaloon Formation also overlies the Matanuska high-water event. Overlying sandy beds were Formation disconformably (Little and others, 1986b). probably deposited during lateral migration and Farther south in the Chugach Mountains, the abandonment of shallow river channels that were Chickaloon Formation generally overlies the Lower incised into the emergent gravel bars. Mudstone Jurassic Talkeetna Formation with slight to beds at the top of the sequence are interpreted as pronounced angular discordance, and locally overlies overbank sediments deposited in areas relatively Jurassic quartz diorite along a low-angle fault remote from axes. contact, probably a faulted nonconformity. In one location in the Anchorage C-1 Quadrangle, just west CongIomerate.-Conglomerates are poorly sorted, of the Nelchina Glacier, sedimentary rocks clast-supported mixtures of subrounded to lithologically and stratigraphically similar to the subangular pebbles with a matrix of subangular Chickaloon Formation unconformably overlie both coarse sand and granules. Typical maximum clast the McHugh Complex and Upper Cretaceous Valdez diameter ranges from 2 to 7 cm. Clasts with Group of the Chugach terrane (figs. 2 and 3; Pessel maximum diameters in the range of 10 to 15 cm are and others, 1981). These sedimentary rocks overlie common in some parts of the formation, and the latest Cretaceous age rocks of the Valdez Group boulders to 1.5 m diam occur locally. and are intruded by probable Eocene age felsite Conglomerate beds are generally thick or very dikes. Thus, a Paleocene-Eocene age seems likely. thick, ranging between about 0.75 and 2 m in The upper contact of the Chickaloon Formation thickness, but locally units to 5 m and over 10 m thick is not exposed in the Chugach Mountains. Eocene are present. Beds of conglomerate have erosive and younger sedimentary rocks that overlie the basal contacts and upper contacts. Beds are Chickaloon Formation in the Matanuska Valley most commonly massive, but bedding is defined (Barnes and Payne, 1956; Clardy, 1974) may also locally by a crude alignment of elongate clasts or by have been present in the region of the present interbeds of finer grained conglomerate. Bedding Chugach Mountains, but have been stripped away by includes both horizontal and cross-stratified types. erosion. Scour surfaces locally bound lenses, 10 to 30 cm thick, of coarse-grained sandstone that are Petrology and Lithology surrounded by pebble conglomerate. Molds and casts of fallen tree trunks are common along coarse- The Chickaloon Formation in the northern grained, sandy bases of conglomerate beds. Chugach Mountains generally consists of well- The average grain size in a conglomerate lens indurated pebble conglomerate and lesser sandstone varies vertically and laterally. Conglomerate beds interbedded with subequal amounts of mudstone and are usually normally graded and may grade upward minor siltstone. Rhythmic alternation of coarse- and from coarse pebble or cobble conglomerate at the fine-grained beds is characteristic of the formation. base to fme pebble or granular conglomerate. A thin Subunits in the formation are characterized by layer or erosionally truncated wedge of reverse- different average proportions and thicknesses of graded sandstone and conglomerate is common conglomerate beds relative to mudstone beds (see along the base of beds. In general, the average grain section on variations below). size in a given bed decreases systematically from a Conglomerate, sandstone, and mudstone beds are maximum in the thickest part of a lens to a minimum typically found in several-meter-thick, fining-upward, (commonly sandstone) at the feather edges of a rhythmic sequences. Very thick beds of cobble or lenticular channel. pebble conglomerate at the base of a sequence commonly grade vertically into granular Sandstone.-Sandstone in the Chickaloon conglomerate or sandstone. These beds are in turn Formation is fine to coarse grained and consists of abruptly to gradationally overlain by a bed of coarse- moderately sorted, subangular, chiefly lithic grains. to medium-grained sandstone or by a sequence of Sandstone is brown to greenish-gray and well beds of fine to coarse sandstone and(or) granular indurated. Greenish coloration is caused by pre- and conglomerate. Mudstone or siltstone occurs at the post-depositional alteration of volcanic lithic top of a sequence. Conglomerate was probably fragments to chlorite and by post-depositional deposited as longitudinal bars during floods. The growth of chlorite cement. and 38 Professional Report 94 deformation of lithic grains and diagenetic growth of Peninsular terrane. Clasts of metamorphic rocks chlorite t laumontite (Ca-Al zeolite) i clay i calcite similar to those in the Peninsular terrane, such as have resulted in low porosity. This diagenetic arnphibolite, are rare. Red radiolarian chert, alteration (or burial metamorphism), mean vitrinite "graywacke" sandstone and siltstone, low-grade reflectance values in the range of 0.5 to 0.9, and phyllite, and quartz are minor but ubiquitous apatite fission track data (Little, 1988; Little and constituents, and are probably derived from the Naeser, 1989) suggest that the Chickaloon Formation Chugach terrane. Some chert clasts contain poorly was once buried under a significant thickness of preserved radiolarian assemblages indicative of a younger sediments. Karnian (Late Triassic) age and others of Mesozoic Sandstone beds are generally <1 m thick and (probably Jurassic and Cretaceous) age (C. Blome, beds 10 to 70 cm thick are typical. Beds are as quoted in Little, 1988). These ages are generally lenticular in shape and are massive or comparable to those of chert blocks from the parallel-laminated, or cross-laminated into tabular or McHugh Complex in the Valdez Quadrangle trough-shaped sets. Low angle cross-strata of (Winkler and others, 1981b; Nelson and others, sandstone commonly lap onto or drape the margins 1986) and south of Anchorage (Karl and others, of channels incised into conglomerate. These 1979), which also include radiolarian assemblages of internally cross- or parallel-laminated bedforms are Late Triassic and Early Cretaceous age. bounded above and below by sharp bedding surfaces Conglomerates deposited unconformably on the and thicken and coarsen laterally to a maximum Chugach terrane west of the Nelchina Glacier along the floor of the channel, suggesting that they (fig. 3), contain the same clast types as those formed by lateral accretion of the channel margin. described above but are richer in low-grade Many sandstone beds are normally graded, from metamorphic clasts such as recrystallized chert and coarse-grained or granular sandstone near the base siliceous argillite, aphyric greenstone and metatuff, to fine- or medium-grained sandstone at the top, but and green and black low-grade phyllite. Distinctive this pattern often is complicated by thin to thick gray and mauve-colored hypabyssal or volcanic felsite interbeds of coarse sandstone or fine pebble clasts are locally quite abundant. The source of these conglomerate. felsite clasts is uncertain, but they may derive from supracrustal parts of the Paleogene Sanak-Baranof Mudstone.-Mudstone in the Chickaloon magmatic belt (Hudson and Plafker, 1982) in the Formation is a medium-brown to black, generally Chugach terrane to the south, an early phase of the non-fissile mixture of clay, silt, and fine-grained Tertiary felsite plutonism in the area, or Jurassic carbonaceous material. Mudstone beds are felsic rocks of the Peninsular terrane. approximately tabular in shape and range in Sandstones in the Chickaloon Formation are thickness from 5 cm to several tens of meters; beds lithic arenites. Impure chert (recrystallized), 0.5 to 2 m thick are most common. Very thick argilLite, "graywacke," phyllite, and other low-grade (> 1 m) mudstone beds commonly contain thin (< 10 metasedimentary fragments combined are about cm) interbeds of siltstone or fine sandstone, leaf three times as common as andesitic (and less impressions, and fragments of petrified wood. commonly basaltic) volcanic and metavolcanic lithics. Monocrystalline plagioclase and quartz are generally Clast Types and Sandstone Petrography minor constituents, but cataclastically deformed, altered grains of plagioclase (polycrystalline) are Conglomerates in the Chickaloon Formation are locally a major component of the rock and were polymictic and contain clasts of underlying probably derived from dioritic or gabbroic rocks of Peninsular terrane rocks as well as clasts of rocks the Peninsular terrane. Lithic fragments of diorite, from the Chugach terrane. Between the Matanuska microdiorite, and tonalite are minor but widespread Glacier and Ninemile Creek (fig. 3), conglomerate constituents, as are coarsely polycrystalline quartz clasts consist predominantly of recrystallized impure grains of probable metamorphic origin; potassium- chert and siliceous argillite with subordinate amounts feldspar is absent. Trace detrital constituents include of pyroclastic and porphyritic andesitic volcanic rocks chlorite, epidote, muscovite, carbonate, opaque identical to rocks in the Talkeetna Formation. Clasts oxide, hornblende, biotite, clinopyroxene, prehnite, of massive and foliated plutonic rock are somewhat pumpellyite, and, near the Nelchina Glacier, blue less common and consist of gabbronorite, sodic amphibole. Uncommon white-weathering hornblende gabbro, diorite, quartz diorite, and (laumontitic) sandstone beds contain a component of tonalite identical to the plutonic rocks of the first-cycle tuffaceous material, such as broken Geology of the Notfheni Chlcgach Moutltaitzs 39 crystals of volcanic quartz, sanidine, plagioclase, and dominated facies, in which conglomerate beds are devitrified fragments of volcanic glass. thin (about 1 m) and relatively fine grained (<2 cm) and mudstone beds are very thick (to 10 m or more). Vertical and Lateral Variations Between the Matanuska Glacier and Gravel Creek (fig. 3), over a distance of about 2 km A typical vertical section of the Chickaloon northward from the southern fault contact of the Formation between Glacier and Gravel Creeks (fig. formation with the Chugach terrane, the above- 3, sheet 2) is fine grained and mud-rich at the base, described stratigraphic sequence has marked lateral coarse grained and conglomeratic in the middle part, variation. This lateral variation may be summarized and fine grained near the exposed top. Near the as follows: (1) the basal mud-dominated unit and(or) base of the formation, the Chickaloon consists chiefly the well-beddcd unit above it abruptly thickens, of black, carbonaceous mudstone with minor thin (2) the massive coarse conglomerate unit near the beds of orange-weathering marl (probably fresh- middle of the formation pinches out, and (3) the water limestone). Coal is present locally and tabular-bedded unit above it becomes distinctly petrified wood is common. With the exception of a thinner bedded and more mudstone-rich. In the several-meter-thick basal conglomerate bed that same direction, the depositional basement for the unconformably overlies the Talkeetna Formation, formation changes from (1) Jurassic quartz diorite coarse clastic beds are uncommon in the basal part along the southern edge of exposure to (2) Talkeetna of the formation. Moreover, the conglomerate beds Formation along the north flank of the range to are seldom more coarse grained than a granular (3) Matanuska Formation at the edge of the conglomerate or thicker than 1 m. The ratio of Matanuska Valley. mudstone to conglomerate t sandstone is about 10:l. This basal mud-dominated unit (to 100 m Age and Correlation thick) grades upward into a unit about 150 m thick which is characterized by a mudstone to Chickaloon Formation sedimentary rocks contain conglomerate t sandstone ratio of about 1:l. abundant plant fossils of late Paleocene early Mudstone in this unit is generally brownish rather Eoccnc age that arc identical to those found in the than dark gray or black. Above this well-bedded unit type area of the Chickaloon Formation in the is a massive coarse-grained conglomerate unit that Wishbone Hill coal district of the Matanuska Valley locally exceeds 400 m in thickness. Mudstone and (Wolfe and others, 1966; Triplchorn and othcrs, sandstone beds are uncommon in this unit. A 1984; Wolfe, as quoted in Little, 1988). A middle to discontinuous marker bed of coarse, hematite- late Eocene minimum age for the Chickaloon cemented conglomerate to 20 m thick commonly Formation is indicated by zircon fission-track occurs near the top of the unit. Overlying this minimum ages of 43 to 48 Ma for felsite dikes that conglomerate-dominated unit is another intrude the Chickaloon Formation (Little and conglomeratic unit, >300 m thick, distinguished by Naeser, 1989; see Hypabyssal Rocks section). Such its tabular-bedded appearance. Fining-upward dikes commonly intrude the Chickaloon Formation, sequences of sandstone and mudstone, generally both in the Matanuska Vallcy and in the Chugach €60 cm thick, are present between 1- to 4-m-thick Mountains. conglomerate beds. Upsection, conglomerate beds in this unit are systematically thinner and mudstone Interpretation beds are thicker, so that the ratio of mudstone to conglomerate t sandstone ranges from about 1:8 Clastic detritus in Chickaloon Formation in the near the base to 1:3 near the top. Chugach Mountains was derived from both Peninsu- The Chickaloon Formation is characterized by lar and Chugach terranes (Little, 1988). Conglomer- rapid lateral facies changes and general lenticularity ate and sandstone compositions indicate that clastic of units, and by abrupt lateral variations in its detritus was partially derived from erosion of the thickness and in the lithology of its depositional mafic to intermediate plutonic and andesitic volcanic basement. Two facies end members are: (1) a rocks of the Peninsular terrane which commonly conglomerate-dominated facies, in which underlie the formation. The predominance of conglomerate units are very thick (to 10 m or more) chemically and mechanically unstable clasts of and coarse grained (maximum clast diameter weakly metamorphosed chert/argillite (which yield commonly 10 to 15 cm or more) and mudstone beds Mesozoic fossils), graywacke, and (locally) are thin (<0.5 m) or absent; and (2) a mud- greenstone that are lithologically indistinguishable Professional Report 94 from low-grade rocks of the McHugh Complex, as conglomerate with almost no interbedded sandstone well as clasts of graywacke and phyllite similar to and mudstone, probably reflects the northward Valdez Group metasedimentary rocks, indicate that progradation of an system. This fan both units of the Chugach terrane were important system flanked a scarp (or scarps) of the ancestral sediment sources also. A southern source for Border Ranges fault system, which underwent a Chickaloon Formation sediments from faulted and component of south-side-up displacement during uplifted rocks of the Chugach terrane is also deposition of the Chickaloon Formation. The suggested by lithofacies adjacent to the Border alluvial system, judging from the coarseness of its Ranges fault. The lithofacies contains greenstone, bedload and the morphologic complexity of its thin metatuff, and phyllite clasts, in addition to typical channel sequences, probably consisted of shallow, clasts found in the Chickaloon Formation. North of braided streams. Because the climate was humid the Border Ranges fault, the massive conglomerate and soils were thick and clayey, the streams were unit of the Chickaloon Formation contains chert supplied with a copious suspended load that was boulders 1.5 m diarn that could have been derived periodically deposited on extensive only from adjacent rocks of the Chugach terrane to peripheral to channel axes. Northward and westward the south. A trend of northward thinning and fining paleocurrent directions in Chickaloon Formation of of conglomerate beds, together with a corresponding the Chugach Mountains (Little, 1988), and regional increase in the thickness and proportion of mudstone isopach trends for Cretaceous and Tertiary rocks in beds and a predominance of northward and Cook Inlet (Kirschner and Lyon, 1973; Boss and westward paleocurrent directions, provides added others, 1976; Hartman and others, 1972) suggest that evidence for northward sediment transport (Little, alluvial fans flanking the proto-Chugach Mountains 1988). This trend indicates a more distal were tributary to a longitudinal drainage system environment to the north. In contrast to its along the present-day Cook Inlet-Matanuska Valley conglomeratic exposures in the Chugach Mountains, trend. Chickaloon Formation in the Matanuska Valley Like the Upper Cretaceous Matanuska consists chiefly of mudstone and coal, with subordi- Formation beneath it, the Chickaloon Formation was nate sandstone, and contains only minor fine-grained probably deposited in a complex . conglomerate near its base (Barnes and Payne, The late Paleocene to early Eocene age Chickaloon 1956). Chickaloon Formation sandstones in the Formation is presently situated between coeval Matanuska Valley (Winkler, 1978) are lithic arenites, magmatic arc rocks to the north and northwest (Moll but these sandstones are more quartzose than age- and Patton, 1983; Wallace and Engebretson, 1984) equivalent sandstones in the Chugach Mountains to and a subduction complex (the Orca Group) to the the south, contain K-feldspar, and contain a higher south (Helwig and Emmet, 1981). Airfall and ratio of volcanic to total lithic fragments. This lateral waterlaid siliceous tuff occurs locally in the difference in Chickaloon sandstone petrography Chickaloon Formation, both in the Matanuska Valley indicates that fluvial sands became compositionally (Conwell and others, 1981), and in the Chugach more mature and(or) increasingly diluted by other Mountains, suggesting that a distant magmatic arc volcanic and plutonic source to the north. was active during deposition of the formation. The In the Chugach Mountains, the Chickaloon disconformity between the Matanuska and Formation is chiefly of fluvial (locally lacustrine) Chickaloon Formations represents a transition origin (Little, 1988). Large deciduous leaf fossils and between marine (probably deep marine) and non- petrified wood throughout the formation indicate a marine depositional systems. The Cook terrigenous origin in a humid climate. The Inlet-Matanuska Valley forearc basin thus appears abundance of laterally continuous beds of to have been considerably uplifted, without carbonaceous mudstone and siltstone indicate that appreciable folding, during latest Cretaceous to poorly drained, swampy floodplains were an integral earliest Tertiary time, although the effect could be, in part of the fluvial system. The mudstone-dominated part, eustatic. Along the north flank of the Chugach basal unit of the formation was deposited in a Mountains, this uplift was accompanied by high- forested, swampy environment of low relief, angle block-faulting and erosion, as indicated by dissected by a meandering(?) system of shallow mapping of pre- to syndepositional faults beneath the stream channels and local . Above the Chickaloon Formation. South-side-up motion on mudstone-dominated unit, the upward thickening these faults has resulted in exposures of progressively and coarsening sequence of conglomerate beds, deeper structural levels of Jurassic and Cretaceous capped by the thick wedge of coarse-grained, massive basement overlapped in a southward direction by the Geology of the Northern Chugach Mountains 41

Chickaloon Formation (Little, 1988). This block- felsite intrusions have a fine-grained holocrystallinc faulting created a topographically positive area along texture. Some felsites are aphyric, but most contain the southern margin of the forearc basin. to 5 percent small (1 to 2 mm), subhedral to euhedral phenocrysts of plagioclase + lesser quartz HYPABYSSAL ROCKS and small amounts (to about 3 percent) of biotite and(or) hornblende phenocrysts. Although the Hypabyssal dikes and small plugs of tan-, buff-, plagioclase phcnocrysts arc now commonly albitized, and orange-weathering leucocratic rock of andcsitic, rclict oligoclase or andesine is prcscrvcd locally. dacitic, and possibly trondhjemitic composition Obvious zoning is uncommon. The fincr grained (table 7) intrude rocks of both tlic Chugach and quartz phenocrysts are generally clear, lack undulosc Pcninsular terranes in the Valdcz (Winklcr ancl extinction, and show resorbed or rclict beta-quark others, 1981b) and Anchorage Quadrangles. Felsitc outlines. The groundmass of the fclsitcs conln~only plutons and dikes are also common to the nclrih of contains 70 to 80 percent microlitic, less con~n~only the Chugach Mountains in the Matanuska Valley and equant, plagioclase subhedra and 20 to 30 percent northern Talkeetna Mountains, where they intrude interstitial quartz anhedra. Accessory minerals Mesozoic and Paleocene sedimentary rocks of the include opaque oxide (commonly hematite platelets), Peninsular terrane (Grantz, 1960a,b, 196la,b; Csetjey apatite, zircon, and sphene. Trachytoid (flow- and others, 1978; Fuchs, 1980; Silberman and Grantz, aligned) plagioclase microlites and minor patches of 1984). myrmekitic tcxture are present in some felsites. Locally, as in the region between Glacier and A relatively large (1 kni) lens-shaped composite Gravel Creeks (Anchorage C-3 Quadrangle; fig. 3), pluton at Kings Mountain (Anchorage D-4 and D-5 tabular-shaped felsite plutons clearly intrude the Quadrangles; fig. 3), probably represents a coarscr older fault boundary between the Peninsular and grained phase of the felsitic intrusives. Thc Kings Chugach terranes, indicating that at least some of the Mountain pluton consists largely of medium-grained, felsites post-date the latest major event of faulting hypidiomorphic-granular, hornblende granodiorite. between the two terranes. These plutons arc The rock contains about 40 percent anhedral quartz, petrographically similar to felsite plutons intruding and about 15 percent perthite. Poikilitic hornblende other parts of the Peninsular terrane. However, is an accessory mineral. Minor amounts of biotite, many of the felsites occurring entirely within the apatite, zircon, and sphene are also present (Littlc Chugach terrane have slightly different petrographic and others, 1986b). characteristics and older K-Ar ages (see below) than The felsite intrusions in the Peninsular terrane those in the Pcninsular tcrrane. Hence, more than are pervasively altered. Plagioclase phenocrysts arc one generation of silicic intrusives may be prcscnt in typically albitiied or partly replaced by a mottled the Nelchina-Kings Mountain region. intergrowth of laumontite (determination by X-ray diffraction). In addition, plagioclase grains are Peninsular Terrane Felsites generally clouded by fine-grained calcite, white mica, Felsites in the Peninsular terrane commonly and opaque dust. Fine-grained granules of cpidote intrude along or across high-angle faults and are a less common alteration product of plagioclase. cataclastic zones and are also present as discordant Hornblende and biotite grains are partly to dikes cutting normal, reverse, and strike-slip faults. completely replaced by clots of chlorite and calcite + Four large, irregular-shaped felsite plutons also white mica. The groundmass is altered by patches of intrude Peninsular terrane rocks in and north ol' thc white mica, calcite, and amorphous-looking seams of Matanuska Valley. Major-oxide compositions of amber-colored opaque dust which includes tiny three felsite intrusives which intrude the Peninsular disseminated grains of hematite. This material, terrane support the field observation that the rocks much of it an alteration product of pyrite, commonly are of andesitic to dacitic composition (table 7). In coats fractures and is responsible for the typical contrast to the Jurassic plutonic rocks, the felsites orange-staining of felsites. show very little brittle or cataclastic deformation, and are generally not cut by large displacemcnt faults. Chugach Terrane Felsites The high-level intrusion of the felsites is interpreted to postdate most of the faulting along the north flank Felsites intruding the Chugach terrane are most of the Chugach Mountains. abundant in the Valdez Group where they occur as Felsites in the Peninsular terrane typically have a sills that intrude the second foliation, S2 (see section microcrystalline groundmass. A few of the larger on structural geology). Locally, the felsites crosscut Table 7. Major-oxide analyses and CIPW norms on hypabyssal rocks of the northcentral Chugach Mountains. These felsites intrude the Peninsular terratle [Analyses performed at Alaska Division of Geologicai and Geophysical Surveys by N.C.Veach and Kate Bull. FeO* and Fe203*, total molecular Fe; A, (Na20 t KzO)/Na20 t K20 + FeO* t MgO); F, FeO*/(NazO t K2O + FeO* t MgO); M, MgO/(NazO t K20 t FeO* t Mg0); ---,not applicable; norm An, an/(an t ab t or); Ab, ab/(an t ab + or); Or, or/(an t ab t or)]

Rock type Andesite Dacite Dacite Quadrangle C-4 G1 C-4 Field no. 83Fn49 81Bu82 83Fn26 Chemical analyses (weight percent)

Si02 4203 FeO Fez03 MgO cao Na20 K20 Ti02 p205 MnO LO1 Fe2O3' TOTAL

Na20 t K20 FeO* FeO*/MgO

CIPW norms (weight percent)

'Itz or ab an di hy 01 mt il aP cor hm

Norm An 57.7 Ab 40.9 Or 1.4 Geology oj the Norlllent Cl~ugachMountains 43

this foliation as dikes. These intrusions are most Eocene K-Ar age of 37.7 +_ 1.1 Ma (table 8). Whole- commonly <2 m thick, but, as in the region between rock K-Ar ages of live other felsite intrusions in the Glacier and Gravel Creeks (Anchorage C-3 Matanuska Valley are also middle to late Eocene Quadrangle; fig. 3), may be as thick as 30 m. Locally, and range between 37.5 Ma and 45.5 Ma (Silbcrman these larger bodies have thermally metamorphosed and Grantz, 1984). Dacitic intrusions in the the surrounding phyllite to a biotite hornfels. Talkeetna Mountains have K-Ar ages on hornblende Many felsites intruding rocks of the Chugach ter- of 43.6 Ma and 39.7 Ma. rane, particularly those near the fault contact The source of the magma for the felsite intrusivcs between the two terranes, are indistinguishable from near the Matanuska Valley is uncertain. Th:: those in the Peninsular terrane, which suggests that primitive strontium isotopic composition for these they may be cogenetic. However, in contrast to felsites does not seem to reflect derivation by partial felsites in the Peninsular terrane, other Chugach melting of continental crust (Silberman and grant^, terrane felsites (I) tend to be fine grained rather 1984). than microcrystalline, (2) contain anhedral, equant Two K-Ar age determinations lor fclsitc plagioclase grains in the groundmass rather than intrusions in the Chugach terrane, the first on subhedral, elongate grains, (3) are crowded hornblende from a dike intruding the McHugh porphyries containing up to 25 percent strongly Complex, and the second on biotite from a pclitic zoned andesine phenocrysts (composition hornfels adjacent to a sill in the Valdez Group, yield determined by extinction angles), and (4) have early minimum ages of 57.2 +_ 1.7 Ma and 52.1 + 1.6 Ma, rather than late Eocene K-Ar agcs (see below). respectively (table 8). A felsite in the Valdcz Quadrangle yielded a K-Ar age on hornblentlc of Age, Correlation, and Interpretation 51.6 Ma (Winkler and others, 1981b). These agcs The age of felsites intruding the Peninsular are distinctly older than those obtained from fclsitcs terrane is relatively well constrained. Felsites in the in the Peninsular terrane to the north. These da~a Anchorage Quadrangle intrude the Chickaloon suggest that there may be at least two generations of' Formation of late Paleocene age, but have not been silicic intrusions in the northcentral Chugach found intruding the Eocene Wishbone Hill Mountains. In the Valdez Group, a medium-grained Formation in the Matanuska Valley (Barnes, 1962; equigranular suite of foliated tonalitic sills, possibly Clardy, 1974). Numerous K-Ar age determinations syntectonic with S2, may represent an even oldcr support the field evidence. Hornblende from the generation of silicic plutons (see below). Kings Mountain pluton, which intrudes the The 52-Ma to 57-Ma nonfoliatcd Chugach Cretaceous Matanuska Formation, has a latest terrane felsites may represent part of the Sanak-

Table 8. K-Ar ages front hypabyssal rocks of the nortlicentral Chugacla Mounlai~ts [Analytical work for this study done under the direction of D.L. Turner, University of Alaska Geophysical Institute, Fairbanks, Alaska. Abbreviations include: ti - titanium; hb - hornblende; --- not available]

Mineral Age % K20 Sample no. Unit Rock dated 0n.y.) average Quadrangle Reference

Tertiary felsite dikes 82JD386a Tf Andesite dike XIornblende 57.2 + 1.7 0.538 C-3? This study

82JD378c Tf I-Iornfels adja- Biotile 52.1 + 1.6 5.812 C-3? This study cent to ande- site dike suite

821,e 124 King Mtn Ti granodiorite Amphibole 37.7 * 1.1 0.450 D-4 This study

SIIB Clark Tf Ilb dacite IIornblende 33.9 + 2" --- A-8 Clark (1981)

Mafic dikes 81AWk51b TJa Basalt dike Whole rock 38 + 2 0.158 D-4 Winkler (1989)

"35 with IUGS constants 44 Professional Report 94

Baranof belt of "near-trench plutons that intrudes many faults within the Border Ranges fault system the Chugach and Prince William terranes (Hudson are part of a set of east- or northeast-striking, near- and Plalker, 1982). Strontium isotope data from vertical faults. Such faults locally cut Lower plutons of this belt of silicic plutons, and their Cretaceous tonalite-trondhjemite plutons, Lower association with high-grade metamorphic rocks of Cretaceous McHugh Complex, Upper Cretaceous the Valdez Group in the core of the St. Elias Matanuska Formation, Upper Cretaceous Valdez Mountains east of the , suggest that the Group, and late Paleocene-early Eocene Chickaloon plutons were derived from partial melting of deep- Formation (Pessel and others, 1981; Burns and seated rocks in the Chugach terrane (Hudson and others, 1983; Pavlis, 1986; Little and Naeser, 1989). Plafker, 1982). The Sanak-Baranof plutons of These post-Paleocene age faults are an important southern Alaska have K-Ar ages on biotite and younger element of the Border Ranges fault system hornblende between 47 Ma and 52 Ma in the St. and include components of both normal and dextral Elias Mountains of eastern Alaska (Sisson and strike-slip displacement. Hollister, 1985, 1986) and between 58 Ma and 60 Ma In the structural descriptions below, the in western Alaska. The 52-Ma and 57-Ma ages for Peninsular and Chugach terranes are treated felsite intrusions located between these two areas, separately, and structural events affecting each including those in the study area, fall between the terrane are described in inferred chronological order. older and younger age groups, which supports their correlation with plutons in the Sanak-Baranof belt. PENINSULAR TERRANE An older generation of tonalitic sills intrudes the higher grade parts of the Valdez Group east of the The Peninsular terrane has been involved in a Copper River (Sisson and Hollister, 1985, 1986). complex sequence of brittle and ductile structural These tonalitic sills are interpreted to have intruded at events ranging in age from Jurassic or older to a depth of about 7 km and occurred between 64 Ma Quaternary. The earliest recognized deformation of and 59 Ma (K-Ar on hornblende; Sisson and pre- to syn-Jurassic age occurred in an island arc Hollister, 1985, 1986) syntectonically with the second environment. Later structural events are probably foliation. These sills may be early forerunners of all related to Cretaceous inception of a convergent felsites in the northcentral Chugach. margin (or subduction zone) along the southern edge of the extinct Peninsular terrane arc (Pavlis, 1982b) STRUCTURAL GEOLOGY and to Paleogene transpression and wrench-faulting along this margin. In the Peninsular terrane, Rocks of the Peninsular and Chugach terranes structural events can be organized chronologically as have been subjected to a complex sequence of brittle follows: (1) Jurassic or older ductile deformation; and ductile deformation ranging in age from Jurassic (2) probable Early Cretaceous brittle deformation; or older to Quaternary. The principal fault system in (3) post-Jurassic uplift, erosion, and northward the Nelchina-Kings Mountain area, the Border tilting; (4) Late Cretaceous and early Tertiary uplift Ranges fault system, forms the contact between the and block faulting; (5) folding and faulting that Peninsular and the Chugach terrane (MacKevett and postdates the late Paleocene-early Eocene Plafker, 1974). The Border Ranges fault system is Chickaloon Formation and pre-dates intrusion of late actually a composite fault system and has a Eocene felsites; and (6) faulting that postdates protracted history of both dip- and strike-slip intrusion of late Eocene felsites. movement in late Mesozoic and Tertiary time. The term "Border Ranges fault system" is here used DEFORMATION OF DOMINANTLY descriptively to denote a system of faults which JURASSIC AGE (1) locally separate rocks of the Peninsular terrane to the north from those of the Chugach terrane to the Metamorphic Rocks of Dominantly south and (2) cut rocks of both terranes to the north Jurassic Age and south of that terrane boundary within a 5- to 10- km-wide zone of densely spaced faults. Some north- The metamorphic rocks interpreted to be and northwest-dipping faults within the Border dominantly Jurassic in age (map units Mzh, Mza, Ranges fault system are probably remnants of an Mzm, and Mzmp; sheets 1 and 2) are ductilely original Cretaceous age thrust contact between the deformed. Some of the deformation may have Chugach and the Peninsular terrane (MacKevett and occurred when the rocks were intruded by the Plafker, 1974; Pavlis, 1982b). As discussed below, Jurassic arc plutons. The rocks are characterized by Geology ofthe Northern Chugach Mountains 45 a pronounced foliation that is axial-planar to close- but some mineral growth continued during late- or or isoclinal-folds of compositional layering. Original post-kinematic stages (Pavlis, 1983; Little and others, bedding in thinly bedded chertlargillite sequences is 1986b). In the Anchorage C-1, C-3, C-4, and now generally transposed parallel to a strong phyllitic C-5 Quadrangles, these metamorphic rocks are foliation in the argillite (Pavlis, 1983; Little and intruded by dikes and stocks of gabbronorite, others, 1986b). The fabric in greenschist and epidote hornblende gabbro, and quartz diorite-tonalit&. amphibolite facies metabasaltic rocks and impure Inasmuch as all available geochronologic data (biotite or hornblende-bearing) quartzofeldspathic indicates an Early to Middle Jurassic age for plutonic rocks is weakly to strongly schistose. Amphibolite rocks of the Peninsular terrane, this regional facies metabasaltic rocks are schistose or contain a metamorphism, penetrative deformation, and strong gneissic layering defined by thin layers of isoclinal folding must have occurred in Jurassic time amphiboles ( pyroxenes) with either sodic or earlier. plagioclase (t quartz) or calc-silicates (Burns, 1983, Plutonic Rocks in press; Pavlis, 1983, 1986; Little and others, 1986b). The metamorphic foliation, S1, generally strikes in a Many of the Jurassic plutohic rocks are ductilely northeasterly direction and dips steeply to the deformed (Burns, 1983, in press; also see Peninsular northwest or southeast (fig. 9). F1 fold axes plunge terrane-Jurassic plutonic section). A strong foliation gently (generally to the northeast) within the plane of is present in plutonic rocks of the Peninsular terrane, the S1 foliation. F2 folds that locally warp the S1 especially where multiple diking of Jurassic magmas foliation commonly have a box or geometry has occurred (Burns, 1983, in press; Burns and with apical angles that range from open to isoclinal others, 1983; Pavlis, 1986; Little and others, 1986b). (Pavlis, 1983). These folds, of unknown age, are These die complexes range in composition from approximately co-axial with F1 folds but have more dominantly gabbroic to dominantly hornblende gently dipping axial planes (Pavlis, 1982b). Textural gabbro and tonalite. The foliation in these Jurassic relationships indicate that metamorphic minerals age plutonic rocks is defined by (1) segregation of grew synkinematically with the strong S1 foliation, mafic and leucocratic minerals into thin laminae and (2) preferred dimensional alignment of elongate hornblende and plagioclase grains, the latter of which are commonly internally strained and partly recrystallized along their rims. Quartz in foliated (gneissic) tonalitic rocks occurs in anastomosing laminae subparallel to the foliation and is commonly completely recrystallized into a granoblastic- polygonal mosaic. Near the Nelchina Glacier, the attitude of foliation in the pre-Jurassic metamorphic rocks is subparallel to the foliation in the plutonic rocks (Burns, 1983, in press). Elsewhere, the relationship of local foliation in plutonic rocks to the widespread S1 fabric in their associated metamorphic wall rocks is still poorly known. Many of the textures in the foliated plutonic rocks may represent flow foliation or in situ growth habits. Some of the foliation near the Nelchina Glacier (Anchorage C-1 Quadrangle) has been shown by Burns (1983, in press) to represent intrusions which were deformed 100 TRANSPOSED BEDDING and SCHISTOSITY: So and S1 while still partly molten. This concept has been further documented to the west in rocks of the N=343 Anchorage C-3 and C-4 Quadrangles by Burns and others (1983), Pavlis (1986), and Little and others Figure 9. Synoptic stereogram of combined poles to (1986b). schistocity (SI)and tightly folded bedding (So) in pre- Volcanic Rocks Jurassic metamorphic (Mzsh) rocks of the Peninsl~lar terrane. Contour intervals correspond to 2.0, 3.4, 4.7, A poorly understood deformation has affected and 7.3percentper 1 percent area. the Lower Jurassic rocks of the Talkeetna 46 Professional Report 94

Formation. Detailed mapping of the Talkeetna This fault is intruded locally by a swarm of Formation has shown that the map pattern of strikes trondhjemite dikes. A trondhjemite dike in the area and dips is compatible with broad, open folds about yields a K-Ar age on hornblende of 145 Ma (table 5), axes that trend in an approximately north-south which suggests that the shallow-dipping fault may be direction. These large-scale folds have wavelengths older than middle Cretaceous. Secondly, clasts of of 1 to 10 krn with axes that dip shallowly to the cataclastically deformed plutonic rocks containing north. The origin and timing of this deformation is prehnite or pumpellyite (typical of the uncertain, but it appears to p;re-date the Cretaceous metamorphism discussed below, associated with this brittle deformation. No evidence is seen for isoclinal brittle deformation) occur in the late Paleocene to folding within the Talkeetna Formation and, in Early Eocene age sedimentary rocks of the general, the unit has undergone little ductile Chickaloon Formation. deformation. The maximum age limit of the brittle deformation is provided by the ages of the youngest rocks affected BRITI'LE DEFORMATION OF PROBABLE by the deformation, which range from 177 Ma (K-Ar EARLY CRETACEOUS AGE age on hornblende in mafic dike intruding plutons of quartz diorite-tonalite) to 155 to 160 Ma (K-Ar ages Jurassic plutonic, volcanic, and metamorphic on hornblende in quartz diorite pluton). These rocks are cut by a complex network of low- and relationships suggest that pervasive brittle moderate-angle faults along the southern margin of deformation occurred in post-Middle Jurassic to pre- the Peninsular terrane. The rocks are intensely Late Cretaceous time. fractured, cataclastically deformed, and altered by Faults of probable Cretaceous age within the subgreenschist and greenschist facies mineral Peninsular terrane rocks include discrete faults with assemblages (Pavlis, 1983; Little, 1988). Pervasive narrow zones of cataclasis (~5m wide) and brittle brittle deformation and alteration is most intense shear zones to 1.5 krn wide consisting chiefly of within a several-kilometer-wide zone along the cataclastic rocks. The faults most commonly dip southern edge of the Peninsular terrane in the gently to moderately north. Displacement on these Anchorage C-1 and C-2 Quadrangles (Pessel and faults places Talkeetna Formation above quartz others, 1981; Burns and others, 1983), but also occurs diorite or gabbroic rocks, and quartz diorite above (at various scales) within the brittle shear zones gabbroic rocks. Such faults also isternally throughout the Anchorage C-3, C-4, and C-5 dismember the Jurassic plutonic rocks and locally Quadrangles (Burns and others, 1983; Pavlis, 1986; place Jurassic plutonic and metamorphic rocks of the Little and others, 1986b). The brittle deformation Peninsular terrane above Cretaceous melange of the can be classified into three groups: (I) low- to McHugh Complex, as in the area between moderate-angle faults and cataclastic zones dipping Monument and Niemile Creeks (fig. 3). Most of 30" to 70" to the north that separate different map these low- to moderate-angle north-dipping faults units; (2) low- to moderate-angle faults and along the southern edge of the Peninsular terrane cataclastic zones dipping to the north that are within are interpreted as thrust faults, even though they do map units; and (3) mesoscopic-scale faults and not always place older rocks over younger in the cataclastic zones (~0.5m thick). Jurassic arc rocks. Such faults presumably The age of many of these low- to moderate-angle constituted an original Cretaceous brittle megathrust faults and associated low-grade metamorphism is zone between the Peninsular terrane and the probably post-Middle Jurassic and pre-Late underthrust Chugach terranes (MacKevett and Cretaceous. However, at least part of the Plafker, 1974; Pavlis, 1982b). deformation is of Tertiary age (see section below). Many cataclastic zones are themselves mappable A Cretaceous minimum age limit is constrained by units at 1:25,000 scale. Mapped, cataclastic shear two observations: First, low- to moderate-angle zones within the Peninsular terrane include faulting and pervasive cataclasis generally does not cataclastic gabbroic rock (TKgc), quartz diorite affect Lower Cretaceous tonalite-trondhjemite (TKcu), Talkeetna Formation volcanic rocks (TKvc), plutons that locally intrude complexes of plutonic and sheared metamorphic rocks of pre-Jurassic rocks dismembered by brittle deformation (Pavlis, protolith age (TKsm). These zones can be traced 1982b; Burns and others, 1983). East of the discontinuously from the Anchorage C-5 Quadrangle Matanuska Glacier, a fault dipping gently to the (Pavlis, 1982b, 1986) eastward to the Valdez north places rocks of the Talkeetna Formation on Quadrangle (Winkler and others, 1981b) and top of a tectonic complex of gabbroic rocks (sheet 2). increase in width and degree of deformation in the Geology of the Northern Chugach Mountaitis 47

southern Peninsular terrane, where they gencrally lie against a melange composed of gabbroic rocks from within disrupted plutonic rock. the Peninsular terrane. Intermixing may occur. Widespread cataclastic deformation is typical in Pavlis (1986) reports a similar melange-like these tectonic complexes. The melange-like units admixture of Chugach and Peninsular terrane rocks consist of variable proportions of relatively intact along their contact in the Anchorage C-5 blocks or phacoids from 10 to 100 m in dimension, Quadrangle. The style of brittle deformation and surrounded by anastomosing zones of proto- very low-grade alteration in the TKmc unit is similar cataclasite and cataclasite as wide as 10 m. Original to that affecting disrupted Peninsular terrane rocks igneous textures and mineralogy are commonly to the north. The similarity in metamorphic grade preserved in the blocks and in small porphyroclasts between the cataclastic rocks of the McHugh in the matrix. The very fine grained greenish matrix Complex and the cataclastic rocks of the gabbroic is locally foliated, contains an abundance of complex is consistent with the idea that brittle anastomosing, slickensided surfaces, and ranges in deformation in the Peninsular terrane occurred texture from protocataclasite to ultracataclasite during underthrusting of the McHugh Complex (Sibson, 1977). Blocks and matrix are both altered along the Border Ranges fault system (Pavlis, by pre- to post-cataclastic growth of new minerals in 1982b). the prehnite-pumpellyite facies (Pavlis, 1982b; Little, 1988). In the Anchorage C-5 Quadrangle, Pavlis UPLIFT, EROSION, AND NORTHWARD (1982b, 1986) reported two brittle shear zones TILTING OF POST-JURASSIC AGE ("disrupted zones") as wide as 1.5 km, consisting of a braided network of moderately northwest-dipping A faulted cross section of the Jurassic arc is faults and cataclastic zones which are subparallel to present in the Nelchina-Kings Mountains area. the fault contact between the Peninsular and the Stratigraphically younger rocks of the arc crop out to Chugach terranes. Faults within these zones are the north. The rocks of the Peninsular terrane arc characterized by a central core of intense cataclastic are tilted between 60" and 90" to the north. The deformation, generally <50 m wide (Pavlis, 1982b). general sequence of rocks north of the Chugach Typically, the cataclastic core of a fault is surrounded terrane, from south to north, is pre-Jurassic protolith by a halo of subgreenschist facies altered rock. age, metamorphic rocks and(or) ultramafic cumulate Prehnite-pumpellyite facies with minor lower rocks, gabbroic rocks, quartz diorite and tonalite, and greenschist-facies assemblages are typical in the island-arc volcanic rocks. The volcanic rocks of the mafic to intermediate composition cataclastic rocks Jurassic arc, the Talkeetna Formation, are overlain occurring along faults throughout the southern by the Cretaceous Matanuska Formation, which Peninsular terrane. The assemblage of chlorite, crops out only in the northern part of the Nelchina- epidote, prehnite, albite, quartz, sphene, 5 Kings Mountain area, and by the late Paleocene- pumpellyite, * actinolite (optical and X-ray early Eocene Chickaloon Formation. diffraction data) alters and replaces original igneous Variations in this idealized sequence of the grains in the very fine grained cataclastic matrix. Jurassic rocks are probably related to lateral Laumontite and calcite occur as late-stage variations in the level of structural exposure and to fillings and may be a much younger alteration widespread dismemberment of the sequence by post- product. Blocks of ultramafic rocks are variably Jurassic faulting. Locally, as in the area just west of serpentinized. This and similar assemblages exist in the Matanuska Glacier, younger uplift has been less the altered cataclastic rocks across the map area, and significant than in adjacent areas, and sedimentary thus the physical conditions during early cataclasis of rocks of the Chickaloon Formation cover the the plutonic rocks were probably similar throughout southern edge of the Peninsular terrane. The a large portion of the belt. sequence of Jurassic and pre-Jurassic arc lithologies As first suggested by Pavlis (1982b), these low- to indicates decreasing levels of structural exposure to moderate-angle faults and cataclastic zones are the north and suggests that the Peninsular terrane interpreted as part of an original Cretaceous age was tilted to the north in post-Jurassic and pre- megathrust between the Peninsular and the Chugach Paleocene time. terranes. In the melange-like units east and west of Stratigraphic evidence suggests that much of the the Nelchina Glacier (TKmc and TKgk on sheet 2; northward tilting may have been associated with Pessel and others, 1981; Winkler and others, 1981b), uplift of the southern Peninsular terrane during post- blocks of McHugh Complex rocks, including Middle Jurassic and pre-Albian time. As suggested radiolarian chert and pillow basalt, are juxtaposed by Pavlis (1982b), the regional pre-Albian 48 Professional Report 94 unconformity at the base of the Matanuska uplift during the time of the unconformity and during Formation may represent an uplift of the Peninsular deposition of lower parts of the Chickaloon terrane during inception of the Border Ranges fault Formation was associated with south-side-up block- system. In the northern Chugach Mountains, the faulting. Evidence for such high-angle faulting conspicuous omission of about 7 km of Middle includes (1) abrupt changes in depositional basement Jurassic to Lower Cretaceous units which occur beneath the Chickaloon Formation to older or elsewhere in the Peninsular terrane (Grantz, 1960a,b; structurally deeper lithologies in a southward Detterman and Hartsock, 1966; Kirshner and Lyon, direction; (2) local occurrence of steeply dipping 1973; Detterman and Reed, 1980) suggests that the faults with south-side-up displacement, which bound southern edge underwent a greater amount of blocks of different basement type and which locally erosion in pre-Albian time than did other parts of the do not offset the lower parts of the Chickaloon Peninsular terrane (Little and Naeser, 1989). One Formation (Little, 1988); and (3) the observation that might expect that thrust emplacement of the the Chickaloon Formation was an alluvial fan system Chugach terrane beneath the Peninsular terrane derived largely from exposed metamorphic rocks of would have resulted in both uplift and northward the Chugach terrane to the south. Thus the Chugach tilting of the southern edge of the Peninsular terrane terrane was uplifted and subaerially exposed prior to on the hanging wall of the subduction zone (Karig and during deposition of the Chickaloon Formation. and Sharman, 1975). The northward tilting of the Moreover, the discontinuity in regional metamorphic Jurassic arc sequence of the Peninsular terrane may grade across the Border Ranges fault system with thus have occurred during Cretaceous inception of respect to Late Cretaceous rocks of the Valdez the Border Ranges fault system. Group (greenschist facies) and Matanuska The observed uplift and northward tilting of the Formation (zeolite facies) suggests that the Chugach Peninsular terrane in the northern Chugach terrane has undergone more post-Late Cretaceous Mountains is of composite and, in part, younger uplift and erosion than adjacent rocks of the origin (discussed below). Detailed mapping of Peninsular terrane. Uplift of the Valdez Group and Paleocene Chickaloon Formation and older rocks south-side-up block-faulting along the northern edge exposed along the present range front indicates that of the Chugach terrane may have been caused by some uplifting and northward tilting is the result of isostatic response to accretion of the large, south-side-up block-faulting and folding of latest tectonically thickened prism of low density Cretaceous and early Tertiary age. Moreover, an metasedimentary rocks now represented by the even younger Neogene uplift of the Chugach Range Valdez Group (Little and Naeser, 1989). has been postulated by Kirshner and Lyon (1973); Alternatively, regional uplift of the Chugach terrane Pavlis and Bruhn (1983); and Byrne (1986). may have been related to possible -trench interactions (such as subduction of the Kula-Pacific UPLIFT, EROSION, AND FAULTING OF ridge), as has been proposed by Marshak and Karig LATEST CRETACEOUS-EARLY EOCENE (1977) and Moore and others (1983). AGE FAULTING AND FOLDING OF A latest Cretaceous-late Paleocene age event of POST-EARLY EOCENE AGE uplift, erosion, and high-angle faulting in the Peninsular terrane can be inferred on the basis of Faulting sediientologic and structural data. The late Paleocene-early Eocene Chickaloon Formation Folded Paleocene rocks of the Chickaloon consists chiefly of fluvial rocks and disconformably Formation are uplifted along the north flank of the (locally unconformably) overlies marine sedimentary Chugach Mountains and cut by high-angle faults that rocks of the Late Cretaceous Matanuska Formation. strike subparallel to the trend of the range. The This erosional hiatus is exposed in the Matanuska Chickaloon Formation is preserved in several fault Valley and in the northernmost Chugach Mountains blocks in the central part of the mapped area and in (Waring, 1936; Grantz, 1961a,b; Barnes, 1962; Little a strip along the northern edge of the range. The and others, 1986b) and records an abrupt transition Anchorage C-2 and C-3 Quadrangles are the only from marine to non-marine deposition that area where such Tertiary age rocks are preserved in apparently occurred without significant folding or the northern Chugach Mountains. Most of our deformation other than block-faulting. Along the information regarding the nature of Tertiary faulting north flank of the Chugach Mountains, regional is therefore derived from detailed structural mapping Geology oj the Northern Chugach Mountairts 49 studies in this region (Little, 1988). Post-early contacts of the plutons (as deduced from map Eocene high-angle faults can be traced from this pattern), the mean attitude of foliation in the region into areas to the east and west, where only metamorphic rocks, ar~dthe fault itself are all nearly Jurassic and older crystalline rocks are exposed. vertical, whereas observed slickenlines are These Tertiary age faults are an important subhorizontal; the observed separation is therefore element of the Border Ranges fault system, as the probable result of strike-slip rather than dip-slip defined earlier. The average strike of these faults displacement. changes from approximately east-west in the eastern A second high-angle fault, with dextral strike part of the mapped area, to northeast-southwest in separation, occurs between Glacier and Gravel the western part. The change in strike occurs in the Creeks in the Anchorage C-3 Quadrangle (sheet 2). axial region of an apparent oroclinal bend (Carey, This east-striking, subvertical fault offsets an older 1955) that also affects the trend of the entire normal fault that dips moderately south-southwest. Chugach terrane along the southern rim of Alaska. The normal fault places Paleocene Chickaloon Most of these post-Paleocene faults are Formation, to the south, on Jurassic quartz diorite, to subvertical and are probably dominated by dextral the north, with Talkeetna Formation not present (wrench) displacements (Littlc, 1988). In most cases, (cross section H-H', sheet 2). The younger the amount of strike separation (or offset) of map subvertical fault apparently offsets the normal fault units across high-angle faults in the Border Ranges about 1.5 km in a right-lateral sense. This separation fault system cannot be determined because has resulted in a repetition (in plan view) of the trace (1) sedimentary units in the Peninsular terrane of the normal fault and emplacement of a narrow generally have an east-west stratigraphic strike, sliver of Chickaloon Formation within a mass of subparallel to the faults; (2) most of the rocks cut by quartz diorite (sheet 2). The younger subvertical the faults are plutonic units, which are discontinuous, fault surface is exhumed, where its trace crosses the nontabular, and without unique features; and west-flowing tributary to Gravel Creek. Here, that (3) many of the faults also have a significant east-striking fault surface contains slickenlines that component of dip-slip displacement (discussed p111nge within a few degrees of horizontal, suggesting below) which results in mismatching of map units that the observed separation is due to strike-slip across the fault. rather than dip-slip displacement. In the Anchorage Several high-angle faults in the Peninsular terrane C-3 and C-4 Quadrangles west of Gravel Creek, on the Anchorage C-3 and C-4 Quadrangles can be Little (1988) reports a small trondhjemite stock is shown, from map pattern alone, to have a probable displaced about 6 km across a fault-bounded sliver of right-lateral displacement. Near the head of Carbon Chickaloon Formation. Creek, in the western part of the Anchorage C-4 In the central Chugach Mountains, the fault Quadrangle and eastern part of the Anchorage C-5 contact between the Peninsular and the Chugach Quadrangle (fig. 3; sheet 1, cross section D-D'), terranes is probably a dextral oblique-slip fault with a Chugach terrane rocks of the McHugh Complex are south-side-up component of normal displacement. fault-bounded on the north by the greatest width of Between Gravel Creek and the Matanuska Glacier Peninsular terrane metamorphic rocks in the (sheet 2), rocks of the Chickaloon Formation occur mapped area (about 4 km instead of the more usual along the southern edge of the Peninsular terrane. 1 to 2 km). An east-northeast striking high-angle Here, the fault contact between the Peninsular and fault, cutting through the middle of the exposed the Chugach terranes dips 60" to 75 to the north- metamorphic rocks, separates two lithologically northwest (sheet 2; cross section J-J') and is similar sub-belts of metamorphic rocks. The demonstrably of post-early Eocene age, because it observed double width of metamorphic rocks downdrops sedimentary rocks of the Chickaloon appears to be the result of fault-repetition (in plan Formation against greenschist facies metamorphic view) of a Zkm-wide metamorphic belt by the high- rocks of the Valdez Group. The metamorphic rocks angle fault. Two other map units (Jgu and Kt; sheets have early Eocene K-Ar ages on metamorphic white 1 and 2) form a similar sequence (in plan view) on mica (Little and Naeser, 1989). The Chickaloon the north side of each of the two sub-belts of Formation has been completely eroded away from metamorphic rocks. Thus, a sequence of map units the southern block (Chugach terrane). Although the seems to be repeated in plan view across the fault terrane-bounding fault, on the basis of mapped (Little and others, 1986b, cross section A-A'). The relationships, vitrinite reflectance, and the mapped geometry (sheet 1) suggests a minimum of metamorphic mineral assemblage data, probably has about 12 km of right-lateral strike separation. The a large dextral strike-slip displacement, it also has at 50 Professional Report 94 least 3 km of normal throw of post-early Eocene age nearly upright axial planes. The folds are oblique to (Little and Naeser, 1989). In detail this fault is a east-northeast to east-west-striking high-angle faults braided zone of steeply dipping, complexly and are arranged in an en echelon pattern; these intershuffled fault slices (Little, 1988). folds die out rapidly to the north away from the Figure 10 shows one possible kinematic model to Border Ranges fault system. This geometry probably explain the distribution of rock units in a central part reflects development of these folds within a zone of of the map area that invokes fault-bend folding of distributed right-lateral wrench deformation along strike-slip fault blocks during an infcrred 13 km of the Border Ranges fault system. The folds have dextral-slip along a major nonplanar fault in the wavelengths on the order of 0.5 to 1 km and a Border Ranges fault system. Descriptions of fault concentric or chevron morphology. Except in areas geometries and kinematics in greater detail may be where local depressions and culminations affect the found in Little (1988). The change in mean strike of hinge lines of these folds, areas where the folds die post-early Eocene faults in the Border Ranges fault out into lateral terminations, or areas where they system across the map area corresponds to the have been deformed by demonstrably younger apparent hinge of southern Alaska's orocline; thus, folding, the folds are very nearly cylindrical in their the age of that structure appears to be post-early geometry. Axial traces form an acute angle of less Eocene (Little and Naeser, 1989). than 45" against adjacent east or east-northeast striking high-angle faults. The axial planes of some Folding nearly isoclinal folds form angles of 10" or less with adjacent faults and have probably been rotated Throughout the area of its exposure in the within zones of greater strain. Locally, small-scale northern Chugach Mountains, the Chickaloon transverse-normal faults and conjugate sets of strike- Formation is folded about gently plunging, east- slip faults s~mmetricallytransect the wrench folds, northeast to northeast-trending axes which lie within causing their axial elongation.

I00 INTERSECTION LINEATION: L2 CRENULATION CLEAVAGE: S2 ALSO AXES OF F2 CILENULATIONS OF S1 AND AXES OF F2 FOLDS OF EARLY QUARTZ VEINS N=2464

Figure 10. Contoured lower-ketnisphere equal-area stereoprlts of poles to scltistosity (S2) and lineation (L2) in the Cretaceous subduction conzplex (Clz~lgachterrane). Mean valiles rleten?iined by conzpccter using the eigenvahie of Woodcock (1977). A, L2 lirteatiorts in the Valdez Group defined by tlze intersection of Sl and S2 at contour ittlervals of 3.5, 5.8, 8.1, and 10.4percentper I percent area; asterisk locates rnean litteation. B, poles to schistosity (S2) in tlte Valdez Group at contour intervals of 1.0, 1.7, 2.4, 3.1, 3.8, and 4.4 percent per 1 percent area. u indicates best-fit B axis lo girdled dislribulion resultitzg front map-scale F4 cltevron. Geology of tlze Northerrt Clzz~gucl~Moitr~tairts 51

The enechelon folds in the carly Eocenc prescntly exposed roclcs in thc Chugach Mountains Chickaloon Formation are locally crosscut by faults wcrc not cooled below about 100" until early intruded by the late Eocenc felsites. Thus, the faults Mioccne time. This observation, coupled with the are probably complctely Eocene in age (Little, 1988). widespread occurrcncc of laumontitc in the Further north in the Matanuska Valley, a Chickaloon Formation, a mineral characteristic of synclinorium of broad, longer-wavelength, upright burial metamorphism, led Little and Naeser (1989) and with east-northeast trending to intcrprct ovcr 4 km of to recent uplift axial traces deforms the Matanuska and Chickaloon and erosion in the northern Chugach Mountains. Formations, and, locally, the post-early Eocene-age High-anglc faults of demonstrable Quatcrnary Wishbone Hill Formation (Barnes and Payne, 1956; age occur locally in the northern Chugach Mountains Grantz, 1961a,b; Barnes, 1962). Thcse folds arc (scc, for cxample, Updike and Ulery, 1983) and do probably younger than the en cchclon folds within not appcar to havc more than 1 to 2 m of the Border Ranges fault system and formed during a displacement. Thcse faults are gcncrally exprcsscd late Paleogene-Neogene period of north-south as trough-likc linear depressions cutting across shortening across the Matanuska Valley (Bruhn and -clad colluvial deposits. Such faults are most Pavlis, 1981). For a more detailed description of common along the crest of the Chugach Range front polyphase folding of thc Chickaloon Formation in the arca bctween Ninemile and Gravel Creeks. At within the Border Ranges fault system, see Little and this location, the Quatcrnary faults are high-angle Naeser (1989) and Little (1988). features, cutting down into the Chickaloon and Wrench and oblique-slip faulting and en echelon Talkectna Formations. On a ridgetop about 1 km folding of the Chickaloon Formation is probably no east of the Monument Crcck drainage, one such fault younger than Eocene. The principal evidence for offsets the surfacc of a steeply south-dipping slopc this conclusion is that felsite dikes with K-Ar ages on approximately 1.5 m vertically, with a south-sidc-up hornblende and biotite of 30 to 37 Ma (Silberman movemcnt, This sense of vertical separation is and Grantz, 1984; Little, unpubl. data) intrude or cut opposite to that which would be expcctcd for a across high-angle faults and only rarely are cut by simple land scarp. faults. The relationship of Eocene intrusions to folds is generally equivocal. West of the Matanuska CHUGACH TERRANE Glacier, a pluton of fclsite intrudcs the axial region of an in the Chickaloon Formation, which The Chugach terrane (Berg and others, 1972) of suggests that the fold controlled (and thcrcforc pre- Cretaceous age is exposed continuously for 2,000 km dates) intrusion of the pluton. Moreover, the along the southern rim of Alaska and represents one faulting and folding appear to be relatcd to the same of the best-preserved ancicnt subduction complexcs tectonic event; thus, the folding also is probably prc- in thc world (Nilson and Zuffa, 1982). The Chugach late Eocene in age. tcrrane is the most landward, uplifted portion of a wide accrctionary prism that, in a gcneral way, is FAULTING THAT POSTDATES INTRUSION younger in a seaward direction towards the present OF LATE EOCENE FELSITES trench. Low-grade metamorphic rocks of the Chugach tcrrane adjacent to the Border Ranges fault Faults that cut the late Eocene felsite intrusions system have bcen subjected to a complex sequcnce of but do not cut Quatcrnary deposits arc gcncrally ductilc and brittle deformational events occurring east-west striking, stceply dipping faults. Bccause initially during accretion to the lcading edge of the such faults are uncommonly obscrvcd, littlc is known continental margin arid subsequently within more about Neogcne faulting in the northern Chugach interior parts of the growing accrctionary prism. Mountains. The clastic stratigraphy of the Neogene Late-stage cvents include dextral wrench Kenai Group in Cook Inlet Basin (Calderwood and deformation of probable Tertiary age that is similar Fackler, 1972; Kirshner and Lyon, 1973; Boss and to that affecting early Eocene and older rocks in the others, 1976; Haycs and others, 1976) reflects Peninsular terrane to the north. Mioccne and Pliocene uplift of the Chugach Range. Structures in the Chugach terrane can be Pavlis and Bruhn (1983) summarize data from the organized by decreasing age into the following Kenai Peninsula for south-side-up high-angle faulting categories: (1) Cretaceous deformation affecting the along the Kenai-Chugach range front during this McHugh Complex; (2) latest Cretaceous-Eocene time. Apatite fission-track data in the Anchorage polyphase events affecting both units of the Chugach C-3, C-4, and D-4 Quadrangles indicate that tcrrane, but not the Peninsular terrane; and 52 Professional Report 94

(3) Paleogene faulting and folding of Chugach TLis "soft-sediment" deformation and occurrence terrane rocks similar to that affecting early Eocene of subgreenschist facies metamorphism in melange and older rocks in the Peninsular terrane. of the Chugach terrane has been interpreted as having formed in deeper parts of a subduction zone CRETACEOUS DEFORMATION AFFECTING (Moore and Connelly, 1977; Connelly, 1978; Decker, THE MCHUGH COMPLEX 1980). The age of early deformation in the accretionary McHugh Complex probably spans a The McHugh Complex in the Nelchina-Kings long period of time in the Early Cretaceous and Mountain region is highly deformed and possibly extends back into older Mesozoic time. characterized by stratigraphic discontinuity of units The K-Ar ages of 110 to 135 Ma on the intrusion of with a variety of brittle-to-ductile structural styles post-tectonic tonalite-trondhjemite (discussed (Clark, 1972a; Connelly, 1978; Decker, 1980; Burns earlier) suggest that deformation of the McHugh and others, 1983; Pavlis, 1982a, 1986; Little and Complex in a subduction zone and its underthrusting others, 1986b). The highly disrupted nature of the beneath the Peninsular terrane occurred prior to late McHugh Complex contrasts markedly with the Early Cretaceous time. This pre-Early Cretaceous Valdez Group, which is generally coherent at the accretionary event was probably associated with outcrop scale of observation. Except locally, as near formation of the densely spaced faults and cataclastic its thrust contact with underlying rocks of the Valdez zones that disrupt crystalline rocks along the Group, the McHugh Complex is generally not southern edge of the Peninsular terrane (Pavlis, characterized by a cleavage fabric. Common brittle 1982b). Inasmuch as the Valdez Group was not structural styles in the McHugh Complex include deposited until Late Cretaceous time, its broken formation (or densely faulted rock) and deformation is clearly younger than much of that block-in-matrix melange. Broken formation in the affecting the McHugh Complex. McHugh Complex is densely faulted on an outcrop scale, but .lacks a block-in-matrix structure. DEFORMATION AFFECTING BOTH THE Pervasive faulting has resulted in a complex mixing MCHUGH COMPLEX AND VALDEZ GROUP of fault blocks of greenstone, mafic metatuff or graywacke, lesser black argillite, and minor impure Later stages of deformation of the McHugh chert. Anastomosing networks of densely spaced Complex are similar to the deformation of the mesoscopic-scale faults, many of them tectonically Valdez Group. The first two of three remaining polished, are associated with cataclastic textures and deformational events are recognized only in the locally divide the McHugh Complex into lensoidal rocks of the Chugach terrane. These two events blocks or phacoids (Pavlis, 1985). Where the produced dominantly ductile deformation. For McHugh Complex is a true block-in-matrix melange, simplicity, the terminology for the Valdez Group the matrix consists of black argillite, fine-grained deformation (S1, S2, and so on) will be used when green metatuff, and, locally, thin-bedded siliceous discussing the similar deformation in the McHugh argillite or impure chert. Melange blocks consist of Complex. greenstone, metatuff, graywacke, chert, impure The age of the two-phase deformation sequence limestone, and locally dioritic to ultramafic plutonic to form S1 and S2 is post-Maestrichtian (age of rocks. Brittle deformation of uncertain age, possibly Valdez Group protolith) to early Eocene. A pre- Tertiary, is also represented by post-fold, late-stage early Eocene age for the F1 deformation is provided systems of quartz and calcite-filled veins, which are by the probable late Paleocene-early Eocene age of locally a significant feature in the overall strain in the Chickaloon Formation unconformably overlying S1- rocks (Pavlis, 1986). deformed rocks of the Valdez Group west of the The above-described brittle structures probably Nelchia Glacier. Elsewhere, phyllitic rock developed penecontemporaneously with other more fragments (probably S1-deformed rocks of the ductile structures in the McHugh Complex. Black Valdez Group) are a common constituent of the late argillaceous rocks typically contain rootless laminae Paleocene-early Eocene Chickaloon Formation or wisps of green metatuff that have a complexly (Little, 1988). An early Eocene upper age limit for swirled or disharmonically folded morphology which, the F2 deformational event in the central Chugach in the absence of any tectonic foliation, is suggestive Mountains is established from early Eocene K-Ar of "soft-sediment" deformation. Chert beds are ages of 52 Ma and about 57 Ma, respectively, for typically boudinaged or folded, or both. Such folds biotite and hornblende, ages of felsite dike intrusions are generally tightly appressed to isoclinal. that postdate S2 in the Valdez Group (table 8), and a Geology ofthe Northern Cllzqpch Mountains 53

K-Ar age of about 57 Ma on white mica growing south of the report area, near Mt. Marcus Baker, the along S2 (Little, 1988). presence of isoclinal Colds within imbricate thrust slabs is well established. Inasmuch as the outcrop Valdez Group width of Valdez Group rocks across this part of In contrast to widespread brittle deformation southern Alaska represents an immense structural affecting most other units in the Chugach Mountains, thickness of Maestrichtian age clastic sediments, deformation in the Valdez Group is ductile in duplication by imbricate thrusting of an originally nature, at least at a mesoscopic scale of observation. several-kilometer-thick sedimentary section is an As first noted by Decker (Burns and others, 1983), appealing hypothesis, but is here difficult to Valdez Group rocks in the central part of the demonstrate. northern chugach Mountains have been penetratively deformed twice and contain two Sz-forming event.-The cleavage of the first cleavages. Pavlis (1982b, 1986) also noted two (and deformational event is folded into disharmonic open locally three) cleavages in the western Chugach folds, with low amplitude to wavelength ratios, and Mountains. The intensity of the second and younger into compressed and isoclinal folds, with a locally cleavage, S2 (a crenulation cleavage), increases well developed axial-planar cleavage (S2). This markedly, from only partial or local development in second cleavage is predominantly a spaced- the southwest part of the Anchorage C-5 Quadrangle crenulation cleavage, but locally is penetrative and to widespread development in the eastern part breaks along micaceous surfaces. The second (Pavlis, 1985, 1986). Farther east, in the Anchorage cleavage (S2) is best observed where formed at a C-3 Quadrangle, S2 occurs pervasively and is the high angle to the first cleavage (Sl) and is predominant cleavage observed in the Valdez Group particularly well displayed in the rocks with a shaly (Little, 1988). S2 appears to decline in intensity east protolith. This crenulation cleavage, S2, is the most of the Matanuska Glacier (in the eastern Anchorage conspicuous mesoscopic fabric element of the Valdez C-2 Quadrangle) and is only a minor fabric in the Group throughout most of the central part of the Anchorage C-1 Quadrangle. Nelchina-Kings Mountain region. Development of S2 in the Valdez Group was contemporaneous with Si-forming event.-In the northern Chugach the lower greenschist facies regional metamorphism Mountains, the first deformational event in the (Decker, in Burns and others, 1983; Little, 1988). Valdez Group resulted in widespread development The spaced S2 cleavage is zonal to discrete, and of a penetrative cleavage (SI). S1 is characterized by planar to anastomosing in morphology (Borradaile thin (1 to 3 mm) laminae of phyllosilicate-rich and and others, 1982). Long limbs of asymmetric quartz-rich material that alternate with one another microfolds of S1 lie within cleavage domains and are to form a finely banded rock. Veins of white quartz rotated to small angles with respect to cleavage i albite(?) * chlorite up to 10 cm thick commonly domain boundaries. The short limbs and hinges of constitute 3 to 5 percent of the total rock volume and F2 microfolds commonly lie within microlithons and are generally subparallel to, but locally crosscut S1. intersect cleavage domain boundaries at relatively Several generations of white quartz veins probably high angles. Microlithons are generally about 1 cm formed penecontemporaneously with the S1 cleavage, wide or less, but locally they are as wide as 4 cm. which is, in part, a cleavage. Like S1, S2 is a differentiated cleavage for which Where bedding can be seen (as defined by silty or pressure solution was an important formation psammitic layers), it is generally subparallel to the process, and cleavage domains are markedly first foliation, suggesting that F1 folds, which depleted in quartz and feldspar with respect to presumably formed in association with the S1 adjacent microlithons. A few quartz veins appear to cleavage, are approximately isoclinal. Because much have formed penecontcmporaneously with S2. The of the Valdez Group in the mapped area consists of intersection of S1 and S2 commonly defines a faint phyllite, and original bedding is rarely traceable in lineation (L2), which is parallel to the axes of the field, little is known about the nature or geometry associated small-scale plications (Fz folds) of S1. In of F1 folds in this region. the central Chugach Mountains, the attitude of Sz is Pavlis (1986) implied that the S1- forming not uniform, because that cleavage has been folded deformational event in the western Chugach into younger map-scale folds (F3) with axes that Mountains may not have been accompanied by plunge gently west-southwest (sheet 2). Thus, on the significant (F1) folding and was, instead, limbs of these later folds, Sz generally dips either to characterized by imbricate thrusting. However, the northwest or southeast (fig. lob). 54 Professional Report 94

S2 is axial planar to F2 folds. F2 folds are defined (Moore, 1973, 1978; Plafker and others, 1977) was by (1) small-scale plications or microfolds of S1, as probably associated with development of SI and described above; and (2) mesoscopic folds of white resulted in accretion of the Valdez Group to the quartz veins subparallel to S1. In general, F2 fold continental margin along the inner wall of an oceanic axes (and L2 intersection lineations) plunge 10" to trench (Moore, 1973, 1978) or deeper in a 30" within S2, most commonly to the west-southwest subduction zone along a zone of duplex development (fig. 10a). These harmonic folds generally have and underplating (Sample, 1985). Where the unit is hinges that are angular to rounded in shape and largely unaffected by later deformations, the cleavage tectonically thickened with respect to adjacent, nearly generally dips to the north or northwest, as is straight fold limbs. Multiple orders of Fz folds are consistent with its development during northward present, and their wavelength and amplitude are a under-thrusting (Moore, 1973, 1978; Budnik, 1974; function of the thickness of the folded layers. Where Plafker and others, 1977; Nokleberg and others, several-centimeter-thick quartz veins are the 1985; Sample, 1985). The early slaty cleavage dominant member, Fz folds typically have affecting Chugach terrane flysch has been best wavelengths between 4 and 15 cm, and amplitudes studied on Kodiak Island, where there is evidence between 2 and 10 cm. These folds are close to that it developed when the rocks were in semi- isoclinal. More competent, silty or psammitic lithified state (Moore, 1973, 1978; Sample, 1985). lithologies are less tightly folded, more sinusoidal in S2 in the northern Chugach Mountains may be profile, and often have no Sz cleavage. analagous to the dominant schistosity or gneissic foliation affecting higher grade metamorphic rocks of Interpretation the Valdez Group in the eastern Chugach and St. Elias Mountains (Hudson and others, 1977; The McHugh Complex locally is affected by the Plafker and others, 1977; Hudson and Plafker, 1982). same two-phase deformation sequence described In this area, as in the northern Chugach Mountains, above (Decker, in Burns and others, 1983). The fact the dominant foliation overprints an earlier cleavage, that both Valdez Group S1 and S2 cleavages are which presumably developed during accretion of the present in the McHugh Complex indicates that the Valdez Group and is postdated by 43- to 52-Ma-old Valdez Group and the McHugh Complex were granodioritic intrusions of the Sanak-Baranof belt juxtaposed by S1 time, and that the origin of the two (Hudson and Plafker, 1982). In the eastern Chugach cleavages is the same in both units. and St. Elias Mountains, the age of this dominant Superposition of the S1 and S2 fabrics onto foliation is approximated by K-Ar ages of 48 to 53 McHugh Complex melange seems to have been Ma for synkinematic minerals in the Valdez Group accompanied by an overprint of lower greenschist (Winkler and others, 1981b; Hudson and Plafker, facies metamorphism. The restriction of the 1982). Sisson and Hollister (1985) reported foliated strongest such overprints affecting the McHugh tonalitic sills that intruded the Valdez Group prior to Complex melange to a narrow belt adjacent to its the Sanak-Baranof plutons and during development fault contact with the Valdez Group suggests that of S2. These older sills may be analagous to similar, this latest Cretaceous-Paleocene age polyphase foliated tonalitic sills intruding the Valdez Group in overprint was associated with episodes of fault the northern Chugach Mountains (see section on displacement between the Valdez Group and the Tertiary felsite intrusions). McHugh Complex. Moore and Wheeler (1978) have A second (crenulation) cleavage of early Tertiary described a similar fault-related foliation age seems to characterize the Valdez Group (and its overprinting Chugach terrane melange on Kodiak correlatives) throughout much of the area of its Island. There, a foliation overprints the Uyak exposure along the rim of southern Alaska. In Complex (a correlative of the McHugh Complex) in general, where such crenulation cleavage has not a narrow belt adjacent to its thrust contact with the been much affected by later deformations, it has Kodiak Formation (a correlative of the Valdez been described as dipping moderately or steeply to Group). the southeast of south (Budnik, 1974; Nokleberg and 111 the Chugach Mountains, the McHugh others, 1985; Pavlis, 1985; Sample, 1985). In the Complex commonly rests in low-angle thrust contact central part of the northern Chugach Mountains, above rocks of the Valdez Group (Clark, 1972a; post-F2 folding (discussed below) has obscured the Winkler and others, 1981b). Northward original orientation of the S2 cleavage. The S2 underthrusting of the Valdez Group along this fault cleavage in the Valdez Group is about the (or faults) in latest Cretaceous-Paleocene time appropriate age (early Eocene) to be related to Geology of the Northern Cltrlgach Mountains 55 accretion and deformation of Paleogene age over a bend in the fault (Little and others, 1986a). In turbidites of the Orca Group (Helwig and Emmet, the central part of the mapped area in the Anchorage 1981) against the continental margin in the area of C-3 and C-4 Quadrangles (fig. 3, sheets 1 and 2; near Prince William Sound. cross section E-E' and south of cross section G-GI), a right-stepping bend in the trace in the Border DEFORMATION OF CHUGACH TERRANE Ranges fault system is convex northward and ROCKS SIMILAR TO THAT AFFECTING coincides with the northern fault boundary of a EARLY EOCENE AND OLDER ROCKS lenticular body of the McHugh Complex (fig. 11). At IN THE PENINSULAR TERRANE the eastern end of this convexity, this lensoid mass of the McHugh Complex pinches out and is separated Post-Sz faulting and folding of the Chugach by a nearly vertical fault from the Valdez Group to terrane is similar to that seen in the Peninsular the south. To the north, the lenticular body is in terrane. Faults are generally steeply dipping and fault contact with the Chickaloon Formation and form part of the Tertiary Border Ranges fault other rocks of the Peninsular terrane. Detailed system. Folds of S2 in the Chugach terrane are structural mapping on the east side of the convexity generally large, upright, chevron-style folds (F3) with shows that the Z-shaped deflection of the Chickaloon southwest-trending axes and kilometer-scale Formation and a corresponding abrupt bending of wavelengths, similar to folds in the Chickaloon cleavage in the McHugh Complex is the result of Formation. Locally, small-scale parasitic (F3) folds large-scale kinking about several subvertical kink- also exist. band boundaries that strike to the northeast and northwest. The relationship (in map view) of these Faulting kinks to the high-angle fault north of the Valdez Group is similar to that observed (in vertical section) In the northern Chugach Mountains, rocks of the for fault-bend folds in the hanging wall of a thrust Chugach terrane are involved in high-angle faulting sheet to an underlying horizontal decollement along the Border Ranges fault system. Many of (Suppe, 1983). The shape and dimensions of this these faults are probably dextral wrench faults of fault-bend fold system suggest about 13 km of dextral post-early Eocene age, similar to those affecting displacement along the fault contact between the nearby rocks of the Peninsular terrane. Inasmuch as Valdez Group and the McHugh Complex. most high-angle faults cut the latest Cretaceous age Moreover, if the model is correct, it requires that this Valdez Group, they are probably of early Tertiary or subvertical fault continue westward with an abrupt, younger age. In the upper parts of Coal and Carbon left-stepping bend somewhere in the region of upper Creeks, an east-striking high-angle fault has about 4 Coal Creek. North-northeast-striking thrust contacts km of dextral strike separation with respect to the placing McHugh Complex above Valdez Group in trace of a low-angle fault (the Eagle River fault) that area may be transpressional features, as should placing McHugh Complex rocks above the Valdez be expected along such a restraining bend. Group (Decker, in Little and others, 1986b). Similarly, Pavlis (1986) noted that in the Folding Anchorage C-5 Quadrangle another northeast- striking high-angle fault crossculs the Eagle River An en echelon system of map-scale antiforms and fault and results in about 5 km of dextral strike synforms deforms S2 in the Valdez Group. This separation of the trace of the Eagle River fault. system of late upright folds within the Valdez Group Clearly, some of these strike separations could be was first described by Decker (in Burns and others, due to dip-slip movements on the fault, but, as noted 1983) and has been shown to be a regionally by Pavlis (1986), this fault is only part of a system of extensive class of structures within the Valdez Group high-angle faults that form a fault network consistent (Winkler and others, 1981b). The system is with east-northeast-striking master faults and east- described in detail by Little (1988). In the striking riedel shears developed along a dextral shear Anchorage C-2 and C-3 Quadrangles, these folds system. have gently plunging axes that trend west-southwest About 13 km of dextral displacement along a or southwest and lie within subvertical axial planes non-planar wrench fault near the boundary between (fig. 10B). Little (1988) considers the folds to be Fq, the Peninsular and Chugach terranes may have ranging from chevron to box in their profile shape. resulted in fault-bend folding of a block of McHugh Apparent step-like deflections in the axial trace of Complex and Peninsular terrane rocks as they rode these folds (sheet 2) are related in part to regional 56 Professional Report 94 strain variations affecting the original orientation of of metasedimentary and metavolcanic rocks. These folds and in part to local development of multi-hinge, rocks comprise a metamorphic suite of upper green- box-fold geometries. schist to amphibolite grade, and appear to have The geometry, orientation, and age of these folds formed in an oceanic environment. The age of these is indistinguishable from post-early Eocene age rocks is uncertain, but is probably pre-Jurassic, possi- wrench folds (F1) deforming the Chickaloon bly Triassic or late Paleozoic. The metamorphism is Formation. The age of F3 in the Valdez Group is probably attributable to Early Jurassic magmatism in constrained to post-early Eocene (probable age of S2 the Peninsular terrane. foliation) on the basis of K-Ar ages on white mica Magmatism and volcanism in an intraoceanic (Little, 1988). island arc setting occurred in the Peninsular terrane by Early Jurassic time. Early intrusion of basaltic magma probably started near the crust-mantle SUMMARY AND CONCLUSIONS boundary, and fractional crystallization produced ultramafic and mafic plutonic rocks of the Wolverine The Border Ranges fault system, one of the complex and the Nelchina River Gabbronorite. major structural features in south-central Alaska, Residual magma from this event, mixed with new cuts through the center of the Nelchina-King magma, probably moved to shallower levels in the Mountain region. This fault system separates the crust and differentiated to produce the volcanic rocks trench-slope and trench-fill deposits of the Chugach of the Talkeetna Formation. Successive intrusions of terrane from the plutonic and volcanic rocks of the basaltic magma contributed additional material to Peninsular terrane. the plutonic and volcanic rocks. The mineralogy and The Peninsular terrane includes the oldest rocks mineral composition of the plutonic rocks suggest in the region, a structurally dismembered sequence that they crystallized at 5 to 10 kilobars of pressure

Figure 11. Stylized map sections showing present distribution of McHzcglt Comnplq Pert inszilar terrait e, and Valdez Group in area of convex- northward bend of the Border Ranges fault system and inferred fault-bend folding model in central part of study area. Note left-stepping bend in strike- slip fault and its right-lateral offset of Eagle River tllrust fault at base of the McHugh Con~plex. Heavy dashed line in the McHzlgh Contplex is present-day trace of basal . Solid toothed line to the south is inferred original trace of thrust fault prior to uplift and erosion ...... Peninsular Terrane along left-stepping bend of the strike-slip fault; &hisdip-slip ntotion is inferred to ltave reduced tlte antount of rigltt-lateral Valdez Group separation of tlre tlrnlst trace across the strike-slip fault. Geology of the Northern Chugach Mountains 57

(15 to 30 km depth). Textures of the gabbroic rocks cleavage along with lower greenschist metamorphism and the associated metamorphic rocks also support in early Eocene time. the hypothesis of deep-seated intrusion. Rapid uplift and subaerial exposure of the Quartz diorites, tonalites, and minor Chugach terrane occurred in post-Maestrichtian and granodiorites intruded the mafic plutonic rocks and pre-late Paleocene time, during or soon after the volcanic rocks of the Talkeetna formation in later emplacement of the Valdez Group. Block-faulting, Jurassic time. Ductile deformation of the gabbroic with uplift to the south, also occurred along the rocks suggests that intrusion of the quartz diorites Border Ranges fault system. Alluvial fan sediments took place early in the cooling phase of the mafic of the Chickaloon Formation of late Paleocene to rocks. The source of the magma for the plutonic early Eocene age were deposited across this block- rocks of intermediate composition is not known. faulted region. The alluvial fans had local sources, in Possibilities include extreme differentiation of the part from the uplifted Chugach terrane to the south. original basaltic magma, partial melting of either new In post-early Eocene to pre-late Eocene, dextral mantle material or old oceanic crust, and mixing of wrench-faulting occurred in the zone of the Border these various materials. In any case, the plutonic Ranges fault system. Available data suggest a dis- suite of intermediate rocks may be associated with placement of no more than a few tens of kilometers. the Talkeetna-Aleutian batholith. The occurrence of the wrench-faulting indicates a The island arc became inactive by Early to change in the direction of convergent plate motions. Middle Jurassic time. Between Middle Jurassic and Felsic dies and sills of late Eocene age intrude late Early Cretaceous time, a convergent margin was both the Peninsular and Chugach terranes, including formed along the southern edge of the extinct island the shear zones along the Border Ranges fault arc of the Peninsular terrane. Deformation of system. Lack of deformation of these intrusives oceanic rocks, trench slope, and trench fill sediments indicates that little motion has occurred along the took place in a subduction wedge at the convergent fault system since that time. margin, and resulted in the subduction melange of Major faulting, associated with plate convergence, the McHugh Complex. Deformation in the has occurred to the south of the northern Chugach subduction wedge formed a large zone of rocks Mountains in post-Eocene time, along more exhibiting cataclastic textures and other features outboard parts of the accretionary prism. Some typical of brittle shear. Tectonism associated with minor faulting has occurred in the Nelchina-Kings this subduction zone probably caused the uplift, Mountain region since that time, but does not appear northward tilting, and erosion of the rocks of the to have been of very large displacement. The later extinct island arc in the southern edge of the phases of faulting have resulted in the uplift of the Peninsular terrane. Thrusting of crystalline rocks of soft sediments of the McHugh Complex and the Peninsular terrane over the subduction melange Valdez Group to an elevation higher than the represented by the McHugh Complex apparently crystalline rocks of the adjoining Peninsular terrane. occurred at this time. This thrust faulting represents This uplift defines the southern side of the the earliest phase of the Border Ranges fault system. system of the modern Matanuska Valley, at the Cessation of this tectonism at the southern edge northeastern edge of the Cook Inlet basin. of the Peninsular terrane is evidenced by intrusion of tonalite-trondhjemite plutons (125 to 100 Ma) into ACKNOWLEDGMENTS the shear zones associated with the subduction zone. During Late Cretaceous time, shelf deposits of the This research was made possible in part by Matanuska Formation were deposited on the eroded National Science Foundation Grant Numbers EAR surface of the Jurassic arc complex. Continental 80-01076 and EAR 81-15522, issued to N.H. Sleep. margin sedimentation and continued convergence to George Plafker of the USGS kindly provided the south of the Border Ranges fault system, resulted unpublished field data from stations along the in underthrusting of trench-fill sediments of the Border Ranges fault to help start the project. Valdez Group under the McHugh Complex in latest Discussions with R.G. Coleman, R.R. Compton, Cretaceous to Paleocene time. This underthrusting Arthur Grantz, E.L. Miller, Steve Nelson, N.H. was probably associated with the fust of two phases Sleep, and Fred Barker, proved invaluable. Diana N. of nearly isoclinal folding of the Chugach terrane Solie and Mark Robinson kindly reviewed the sediments. This event was associated with an axial- manuscript. We would like to thank Mike Arline of planar cleavage. A second folding event resulted in a TransAlaska Helicopters, our trusty, faithful second axial-planar cleavage and a crenulation helicopter pilot who was always there with coffee, 58 Professional Report 94

J.C. Pessel for many meals and hospitality, Mike and Society of America Bulletin, v. 70, no. 6, Mark Meekin of Alaska Wilderness Safaris and Don p. 671-747. Deering for camp logistics, and futed- and Budnik, R.T., 1974, The geologic history of the horseback transportation. Valdez Group, Kenai Peninsula, Alaska: Deposition and deformation at a Late Cretaceous consumptive plate margin: Los Angeles, -REFERENCES University of at Los Angeles, Ph.D. thesis, 166 p. Avi: Lallemant, H.G., 1976, Structure of the Canyon Burns, L.E., 1981, Gravity and aeromagnetic model- Mountain (Oregon) oghiolite complex and its ing of a large gabbro body in the northeast implications for sea-floor spreading: Geological Anchorage and north Valdez Quadrangles, Society of America Special Paper 173, 49 p. Alaska: Stanford, Stanford University, M.S. Barker, Fred, Arth, J.G., Peterman, Z.E., and thesis, 97 p. Friedman, Irving, 1976, The 1.7 to 1.8 b.y. old 1983, The Border Ranges ultramafie and trondhjemite-basalt suite of southwestern mafic complex: plutonic core of an intraoceanic Colorado and northern New Mexico: Geo- island arc: Stanford, Stanford University, Ph.D. chemistry and depths of genesis: Geological thesis, 150 p. Society of America Bulletin, v. 87, no. 2, 1985, The Border Ranges ultramafic and p. 189-198. mafic complex, south-central Alaska: Cumulate Barker, Fred, and Grantz, Arthur, 1982, Talkeetna fractionates of island-arc volcanics: Canadian Formation in the southeastern Talkeetna Moun- Journal of Earth Science, v. 22, p. 1020-1038. tains, southern Alaska: An Early Jurassic Geology of gabbroic rocks near the intraoceanic island arc [abs.]: Geological Society Nelchina Glacier, south-central Alaska: U.S. of America Abstracts with Programs, v. 14, no. 4, Geological Survey Bulletin [in press]. p. 147. Burns, L.E., Little, TA., Newberry, R.J., Decker, Barnes, F.F., 1962, Geologic map of lower John, and Pessel, G.H., 1983, Preliminary Matanuska Valley: U.S. Geological Survey, geologic map of parts of the Anchorage C-2, C-3, Miscellaneous Geologic Investigations Map 1-359, D-2, and D-3 Quadrangles, Alaska: Alaska scale 1:63,360, 1 sheet. Division of Geological and Geophysical Surveys Barnes, F.F., and Payne, T.G., 1956, The Wishbone Report of Investigations 83-10, scale 1:25,000, Hill district, Matanuska coal field, Alaska: U.S. 3 sheets. Geological Survey Bulletin 1016, 88 p., 20 plates. Byrne, Tim, 1986, Eocene underplating along the Berg, H.C., Jones, D.L., and Richter, D.H., 1972, Kodiak shelf, Alaska: , v. 5, p. 403-421. Gravina-Nutzolin belt: Tectonic significance of Calderwood, K.W., and Fackler, W.C., 1972, and upper Mesozoic sedimentary and volcanic Proposed stratigraphic nomenclature for Kenai sequence in southern and southeastern Alaska: Group, Cook Inlet basin, Alaska: American U.S. Geological Survey Professional Paper 800-D, Association of Petroleum Geologists Bulletin, p. Dl-D24. v. 56, no. 4, p. 739-754. Borradaile, G.H., Bayly, M.B., and Powell, C. McA., Capps, S.R., 1927, Geology of the upper Matanuska eds., 1982, Atlas of deformational and Valley, Alaska: U.S. Geological Survey Bulletin metamorphic rock fabrics: New York, Springer- 791,92 p. Verlag, 551 p. Carden, J.R., and Decker, John, 1977, Tectonic Boss, R.F., Lenoon, R.B., and Wilson, B.W., 1976, significance of the Knik River schist terrane, Middle Ground Shoal oil field, Alaska: in North south-central Alaska: Alaska Division of American oil and gas fields: American Geological and Geophysical Surveys Gcologic Association of Petroleum Geologists Memoir 24, Report 55, p. 7-9. p. 1-22. Carey, S.W., 1955, The orocline hypothesis in Bruhn, R.L., and Pavlis, T.L., 1981, Late geotectonics: Proceedings of the Royal Society of deformation in the Matanuska Valley, Alaska: , v. 89, p. 255-288. Three dimensional strain in a forearc region: Christensen, N.1, and Salisbury, M.H., 1979, Seismic Geological Society of America Bulletin, v. 92, anisotropy of the oceanic : Evidence no. 5, p. 282-293. from the Bay of Islands complex: Buddington, A.F., 1959, Granite emplacement with Journal of Geophysical Research, v. 84, no. B9, special reference to : Geological p. 4601-4610. Geology of the Northern Cl~zigachMout~tuit~s 59

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1966, Strike-slip faults in Alaska: U.S. Jones, D.L., Silberling, N.J., Berg, H.C., and Plafker, Geological Survey Open-file Report 267, George, 1981, Map showing tectonostratigraphic scale 1:96,000,4 sheets. terranes of Alaska, columnar sections, and Grantz, Arthur, Stern, T.H., ant1 ShcCfey, N.B., 1963, summary description of terranes: U.S. Geological Potassium-argon and Icacl-alpha agcs for Survey Open-file Report 81-792, 21 p., scale stratigraphically bracketed plutonic rocks in the 1:2,500,000,2 sheets. Talkeetna Mountains, Alaska: U.S. Geological Jones, D.L., and Clark, S.H.B., 1973, Upper Survey Professional Papcr 475-B, p. B56-B59. Cretaceous (Maestrichtian) fossils from the Hartman, D.C., Pessel, G.H., and McGee, D.L., Kenai-Chugach Mountains, Kodiak, and 1972, Preliminary report on stratigraphy of Kenai Shumagin Islands, southern Alaska: U.S. Group, Upper Cook Inlet, Alaska: Alaska Geological Survey Journal of Research, v. 1, Division of Geological and Geophysical Surveys no. 2, p. 125-136. Special Report 5,5 p., scale 1:500,000,7 sheets. Jones, D.L., and Silberling, N.J., 1979, Mesozoic Hawkins, D.B., 1973, Sedimentary zeolite deposits of stratigraphy - the key to tectonic analysis of the Upper Matanuska Valley, Alaska: Alaska southern and central Alaska: U.S. Geological Division of Geological and Geophysical Surveys Survey Open-file Report 79-1200,40 p. Special Report 6, 17 p., scale 1:63,360, 1sheet. Karig, D.E., and Sharman, G.F., 1975, Subduction Hayes, J.B., Harms, J.C., and Wilson, Thomas Jr., and accretion in trenches: Geological Society of 1976, Contrasts between braided and meandering America Bulletin, v. 86, no. 3, p. 377-389. stream deposits, Beluga and Sterling Formations Karl, S.M., Decker, John, and Jones, D.L., 1979, (Tertiary), Cook Inlet, Alaska, in Miller. T.P., ed., Early Cretaceous radiolarians from the McHugh Recent and ancient sedimentary environments in Complex, southcentral Alaska, in Johnson, K.M., Alaska: Alaska Geological Society, Symposium and Williams, J.R., eds., The U.S. Geological Proceedings, p. 51-517. Survey in Alaska: Accomplishments during 1978: Helwig, James, and Emmet, Peter, 1981, Structure of U.S. Geological Survey Circular 804-B, Early Tertiary Orca Group in Prince William p. B88-B90. Sound and some implications for the plate Kirschner, C.E., and Lyon, CA., 1973, Stratigraphic tectonic history of southern Alaska: Journal of and tectonic development of Cook Inlet the Alaska Geological Society, v. 1, p. 12-35. petroleum province, in Arctic Geology: American Hill, M.D., Morris, J.D., and Whelan, Joseph, 1981, Association of Petroleum Geologists Memoir 19, Hybrid granodiorites intruding the accretionary p. 396-407. prism, Kodiak, Shumagin, and Sanak Islands, Kuno, Hisashi, 1968, Origin of andesite and its : Journal of Geophysical bearing on the island arc structure: Bulletin Research, v. 86, no. 11, p. 10569-10590. Volcanologique, v. 32, p. 141-175. Hudson, T.G., 1979, Mesozoic plutonic belts in Little, TA., Blome, C.D., and Wolfe, JA., 1986a, southern Alaska: Geology, v. 7, p. 230-234. Paleocene-Eocene sedimentation and wrench Hudson, T.G., and Plafker, George, 1982, Paleogene tectonics along the Border Ranges fault system, metamorphism of an accretionary flysch terrane, north-central Chugach Mountains, Alaska [abs]: eastern : Geological Society of Geological Society of America Abstracts with America Bulletin, v. 93, no. 12, p. 1280-1290. Programs, v. 18, no. 2, p. 127. Hudson, T.G., Plafker, George, and Turner, D.L., Little, TA., Pessel, G.H., Newberry, R.J., Decker, 1977, Metamorphic rocks of the Yakutat-St. Elias John, and Burns, L.E., 1986b, Preliminary area, southcentral Alaska: U.S. Geological Survey geologic map of parts of the Anchorage C-4 and Journal of Research, v. 5, no. 2, p. 173-184. C-5 Quadrangles, Alaska: Alaska Division of Hudson, T.G., and Arth, J.G., 1989, Unpublished Geological and Geophysical Surveys Public-data U-Pb and K-Ar ages for samples of plutonic File 86-28, scale 1:25,000, 1sheet. rocks for the Anchorage Quadrangle, U.S. Little, TA., 1988, Geologic and structural history of Geological Survey. the north-central Chugach Mountains during Irvine, T.N., 1982, Terminology for layered Cretaceous and Early Tertiary time [Ph.D. intrusions: Journal of Petrology, v. 23, p. 127-162. dissertation]: Stanford University, Stanford, Irvine, T.N., and Baragar, W.RA., 1971, A guide to California, 343 p. the chemical classification of the common Little, TA., and Naeser, C.W., 1989, Tertiary volcanic rocks: Canadian Journal of Earth tectonics of the Border Ranges fault system, Science, v. 8, p. 523-548. southern Alaska: Deformation and uplift in a Geology of the Northern Chugach Mountains 61

forearc setting: Journal of Geophysical Research, Moore, J.C., Byrne, Tim, Plumley, P.W., Reid, Mary, v. 94, p. 4333-4360. Gibbons, Helen, and Coe, R.S., 1983, Paleogene MacKevett, E.M., Jr., and PlaBer, George, 1974, The evolution of the Kodiak Islands, Alaska: Border Ranges fault in south-central Alaska: U.S. Consequences of ridge-trench interaction in a Geological Survey Journal of Research, v. 2, more southerly latitude: Tectonics, v. 2, no. 3, no. 3, p. 323-329. p. 265-293. MacKevett, E.M., Jr., Smith, J.G., Jones, D.L., and Moore, J.C., and Connelly, William, 1977, Winkler, G.R., 1978, Geologic map of the Subduction, arc volcanism, and forearc McCarthy C-8 Quadrangle, Alaska: U.S. sedimentation during the early Mesozoic, in Geological Survey Geologic quadrangle Map Talwani, Manik, and Pitman, W.C., 111, eds., GQ-1418, scale 1:63,360. Island arcs, deep sea trenches and back-arc basins: Washington, D.C., American Geophysical Magoon, L.B., Adkison, W.L., and Egbert, R.M., Union, p. 71-82. 1976, Map showing geology, wildcat wells, Ter- Moore, J.C., and Wheeler, R.S., 1978, Structural tiary plant localities, K-Ar age dates, and fabric of a melange, Kodiak Islands, Alaska: petroleum operations, Cook Inlet area, Alaska: American Journal of Science, v. 278, p. 739-765. U.S. Geological Survey Miscellaneous Geologic Morse, SA., 1980, Basalts and phase diagrams: An Investigations Map 1-1019, scale 1:250,000, introduction to the quantitative use of phase 4 sheets. diagrams in igneous petrology: New York, Mancini, EA., Deeter, T.M., and Wingate, F.H., Springer-Verlag, 493 p. 1978, Upper Cretaceous arc-trench gap Nelson, S.W., Blome, C.D., Harris, A.G., Reed, sedimentation on the Alaska Peninsula: Geology, K.M., Wilson, F.H., 1986, Late Paleozoic and v. 6, no. 7, p. 437-439. early Cretaceous fossil ages from the McHugh Marshak, R.S., and Karig, D.E., 1977, Triple Complex: in Bartsch-Winkler, S. and Reed, K.M., junctions as a cause for anomalously near-trench eds., Geologic studies in Alaska by the U.S. igneous activity between the trench and volcanic Geological Survey during 1985: U.S. Geological arc: Geology, v. 5, p. 233-236. Survey Circular 978, p. 60-64. Martin, G.C., 1926, The Mesozoic stratigraphy of Newberry, R.J., 1986, Mineral resources of the Alaska: U.S. Geological Survey Bulletin 776, north-central Chugach Mountains, Alaska: 493 p. Alaska Division of Geological and Geophysical McMillan, S.L., 1984, The geology, alteration, and Surveys Report of Investigations 86-23,44 p. sulfide mineralization of Sheep Mountain, Newberry, R.J., Burns, L.E., and Pessel, G.H., 1986, Matanuska Valley, southcentral Alaska, [M.S. Volcanogenic massive sulfide deposits and the thesis]: University of Alaska, Fairbanks, Alaska, "missing complement" to the calc-alkaline trend: 112 p. Evidence from the Jurassic Talkeetna island arc Miyashiro, Akiho, 1974, Volcanic rock series in of southern Alaska: Economic Geology, v. 81, island arcs and active continental margins: no. 4, p. 951-960. American Journal of Science, v. 274, p. 321-355. Nilsen, T.H., and Zuffa, G.G., 1982, The Chugach Moll, E.J., and Patton, W.W., 1983, Late Cretaceous- terrane, a Cretaceous trench-fill deposit, southern Early Tertiary calc-alkalic volcanic rocks of Alaska, in Legget, J.K., ed., Trench and Forearc western Alaska: Geological Society of America Basin Sedimentation: Geological Society of Abstracts with Programs, v. 15, no. 5, p. 406. London Special Paper, [in press]. Monteverde, D.H., 1984, Tonalite and trondhjemite Nokleberg, W.J., Plafker, George, Winkler, G.R., plutonism in the western Chugach Mountains, Pessel, G.H., and Wallace, W.K., 1985, southern Alaska: an example of near trench Accretionary tectonics along the northern magmatism: Bethlehem, LeHeigh University, Chugach Mountains and southern Copper River M.S. thesis, 82 p. Basin, Alaska, [abs.]: Geological Society of Moore, J.C., 1973, Cretaceous continental margin America Abstracts with Programs, v. 17, no. 6, sedimentation, southwestern Alaska: Geological p. 397. Society of America Bulletin, v. 84, no. 2, p. 595- Palmer, A.R., 1983, The Decade of North American 613. Geology 1983, Geologic time scale: Geology, 1978, Orientation of underthrusting during V. 11, p. 503-504. latest Cretaceous and earliest Tertiary time, Pavlis, T.L., 1982a, Deformation along a Late Kodiak Islands, Alaska: Geology, v. 6, p. 209-213. Mesozoic convergent margin: The Border Ranges Professional Report 94

fault system, southern Alaska: Salt Lake City, Plafker, George, Ambos, E.L., Fuis, G.S., Mooncy, University of Utah, Ph.D. thesis, 250 p. W.D., Nokleberg, W.J., and Campbell, D.L., 1982b, Origin and age of the Border Ranges 1985, Accretion subduction and underplating fault of southern Alaska and its bearing on the along the southern Alaska Continental margin: late Mesozoic tectonic evolution of Alaska: Geological Society of America Abstracts with Tectonics, v. 1, no. 4, p. 343-368. Programs, v. 17, no. 7, p. 690. 1982c, A metamorphosed predurassic Reger, R.D., and Updike, R.G., 1983, Upper Cook subduction complex(?) in the western Chugach Inlet region and the Matanuska Valley: in Pewe, Mountains, Alaska, Cabs.]: Geological Society of T.L., and Reger, R.D., eds., Guidebook to America Abstracts with Programs, v. 14, no. 4, permafrost and Quaternary geology along the p. 224. Richardson and Glenn Highways between 1983, Pre-Cretaceous crystalline rocks of Fairbanks and Anchorage, Alaska: Alaska the western Chugach Mountains, Alaska: Nature Division of Geological and Geophysical Surveys of the basement of the Jurassic Peninsular Guidebook 1, p. 185-259. terrane: Geological Society of America Bulletin, Richter, D.H., and Dutro, .T.T., Jr., 1975, Revision of v. 94, no. 11, p. 1329-1344. the type Mankommen Formation (Pennsylvanian 1985, Structural styles in the Mesozoic and Permian), Eagle Creek area, eastern Alaska accretionary complex of the northern Chugach Range, Alaska: U.S. Gcological Survey Bulletin Mountains, Alaska, [abs.]: Geological Society of 1395-B. America Abstracts with Programs, v. 17, no. 6, p. 400. Ross, M., and Huebner, J.S., 1975, A pyroxene 1986, Preliminary geologic map of the geothermometer based on composition - Anchorage C-5 Quadrangle: Alaska Division of temperature relationships of naturally occurring Geological and Geophysical Surveys Public-data orthopyroxene, pigeonite, and augite, [abs.]: in File 86-7, scale 1:63,360, 1 sheet. Extended abstracts of the International Con- Pavlis, T.L., and Bruhn, R.L., 1983, Deep-seated flow ference of Geothermometry and Geobarometry: as a mechanism for the uplift of broad forearc Pennsylvania State University, University Park, and its role in the exposure of high P/T Pennsylvania, p. 4. metamorphic terranes: Tectonics, v. 2, no. 5, Sample, Jim, 1985, An ancient underplated sequence p. 473-497. preserved in the Kodiak Islands, Alaska: Pavlis, T.L., Monteverde, D.H., Bowman, J.R., Geological Society of America Abstracts with Rubenstone, J.L., and Reason, M.D., 1988, Early Programs, v. 17, no. 6, p. 406. Cretaceous near-trench plutonism in southern Schrader, S.C., 1899, Report on Prince William Alaska: A tonalite-trondhjemite intrusive Sound and the Copper River region: in Maps and complex injected during ductile thrusting along descriptions of routes of exploration in Alaska in the Border Ranges fault system: Tectonics, v. 7, 1898, with general information concerning the no. 6, p. 1179-1199. territory: U.S. Geological Survey Special Pessel, G.H., Henning, M.W., and Burns, L.E., 1981, Publication, p. 51-63. Preliminary geologic map of parts of the 1900, A reconnaissance of a part of Prince Anchorage C-1, C-2, D-1, and D-2 Quadrangles, William Sound and the Copper River district, Alaska: Alaska Division of Geological and Alaska in 1889: U.S. Geological Survey 20th Geophysical Surveys Open-file Report 121. Annual Report, part 7, p. 341-423. Plafker, George, Jones, D.L., Hudson, T.G., and Serfes, M.E., 1984, The Wolverine Creek Fault Berg, H.C., 1976, The Border Ranges fault System - A newly ecognized terrane boundary in system in the and the western Chugach Mountains of southcrn Alexander Archipelago: in Cobb, E.H., ed., The Alaska: Bethlehem, LcHigh University, M.S. U.S. Geological Survey in Alaska: thcsis, 67 p. Accomplishments during 1975: U.S. Geological Sibson, R.H., 1977, Fault rocks and fault Survey Circular 733, p. 14-16. mechanisms: Journal Geological Society of Plafker, George, Jones, D.L., and Pessagno, EA., Jr., London, v. 133, p. 191-213. 1977, A Cretaceous accretionary flysch and Silberman, M.L., and Grantz, Arthur, 1984, melange terrane along the Gulf of Alaska margin: Paleogene volcanic rocks of the Matanuska in Blean, K.M., ed., The U.S. Geological Survey Valley area and the displacement history of the in Alaska: Accomplishments during 1976: U.S. Castle Mountain fault: in Coonrad, W.L. and Geological Survey Circular 751-B, p. 41-43. Elliott, R.L., eds., The U.S. Geological Survey in Geology olllle Norilzenl Clzugucli Moutttairts 63

Alaska: Accomplishments during 1981: U.S. Waring, GA., 1936, Geology of the Anthracite Ridge Geological Survey Circular 868, p. 82-86. coal district: U.S. Geological Survey Bulletin 861, Sisson, V.B., and Hollister, L.S., 1985, Relation of 57 p., scale 1:62,500, 13 sheets. magma intrusion to deformation and uplift of Winkler, G.R., 1978, Framework grain mineralogy eastern Chugach Mountains, Alaska [abs]: and provenance of sandstone from the Arkose Geological Society of America Abstracts with Ridge and Chickaloon Formations, Matanuska Programs, v. 17, no. 6, p. 408. Valley: in Johnson, K.M., ed., The U.S. 1986, Rapid two-stage metamorphism of Geological Survey in Alaska: Accomplishments the eastern Chugach Mountains, southern Alaska during 1977: U.S. Geological Survey Circular [abs]: Geological Society of America Abstracts 772-B, p. B70-B73. with Programs, v. 18, no. 2, p. 186. Winkler, G.R, Miller, R.J., and Case, J.E., 1981a, Smith, J.G., and MacKevett, E.M., Jr., 1970, The Blocks and belts of blueschist and greenschist in Skolai Group in the McCarthy B-4, C-4, and C-5 the northwestern Valdez Quadrangle, Alaska: in Quadrangles, Alaska: in Contributions to Albert, N.R.D., and Hudson, Travis, eds., The stratigraphy: U.S. Geological Survey Bulletin U.S. Geological Survey in Alaska: 1274-Q, p. Q1-Q26. Accomplishments during 1979: U.S. Geological Streekeisen, Albert, 1976, To each plutonic rock its Survey Circular 823-B, p. B72-B74. proper name: Earth Science Review, v. 12, no. 1, Winkler, G.R., Silberman, M.L., Grantz, Arthur, p. 1-35. Miller, R.J., and MacKevett, E.M., Jr., 1981b, Suppe, John, 1983, Geometry and kinematics of Geologic map and summary geochronology of the fault-bend folding: American Journal of Science, Valdez Quadrangle, southern Alaska: U.S. V. 283, p. 684-721. Geological Survey Open-file Report 80-892-A, Triplehorn, D.M., Turner, D.L., and Naeser, C.W., scale 1:250,000,2 sheets. 1984, Radiometric age of the Chickaloon Winkler, G.R., comp., 1990, Preliminary geologic Formation of south-central Alaska: Location of map, cross sections, and summary geochronology Paleocene-Eocene boundary: Geological Society of the Anchorage Quadrangle, southern Alaska: of America Bulletin, v. 95, no. 6, p. 740-742. U.S. Geological Survey Open-file Report 90-83, Tysdal, R.G, and Plafker, George, 1978, Age and scale 1:250,000,2 sheets. continuity of the Valdez Group, southern Alaska: Winkler, Helmut G.F., 1974, Petrogenesis of in Sohl, N.F., and Wright, W.B., eds., Changes in metamorphic rocks, [4th ed.]: New York, stratigraphic nomenclature of the U.S. Geological Springer-Verlag, 334 p. Survey: U.S. Geological Survey Bulletin 1457-A, Wolfe, JA., 1966, Tertiary plants from the Cook p. A120-A124. Inlet region, Alaska: U.S. Geological Survey Updike, R.G., and Ulery, CA., 1983, Preliminary Professional Paper 398-B, p. B1-B32. geologic map of the Anchorage B-6 NW (Eklutna Wolfe, JA., Hopkins, D.M., and Leopold, E.B., 1966, Lake) Quadrangle: Alaska Division of Geological Tertiary stratigraphy and paleobotany of the and Geophysical Surveys Report of Investigation Cook Inlet region, Alaska: U.S. Geological 83-8, scale 1:10,000,2 sheets. Survey Professional Paper p. 398-A, p. A1-A29. Wallace, W.K., and Engebretson, D.C., 1984, Woodcock, N.H., 1977, Specification of fabric shapes Relationships between plate motions and Late using an eigenvalue method: Geological Society Cretaceous to Paleogene magmatism in of America Bulletin, v. 88, no. 9, p. 1231-1236. southwestern Alaska: in Correlation between plate motions and Cordilleran tectonics, Tectonics, v. 3, no. 2, p. 293-315. STAFF1 Thomas E. Smith, Acting State Geologist

Minerals and Energy Resources Engineering Geology

G.H. Pessel, Geologist2 R.A. Combellick, Geologist2 N.D. Bowman, Geology Assistant J.T. Kline, Geologist T.K. Bundtzen, Geologist G.D. March, Geologist L.E. Burns, Geologist R.J. Motyka, Geologist K.H. Clautice, Geologist C.J. Nye, Geologist J.G. Clough, Geologist R.D. Reger, Geologist W.G. Gilbert, Geologist M.A. Wiltse, Chemist E.E. Hams, Geologist J.J. Vohden, Chemist S.A. Liss, Geologist C.G. Mull, Geologist Administrative Services J.W. Reeder, Geologist R.R. Reifenstuhl, Geologist P.M. Yerosta, Administrative Assistant2 M.S. Robinson, Geologist K.E. Brown, Clerk Typist D.N. Solie, Geologist D.L. Chavez, Secretary R.A. Czajka, Clerk Typist Resource Information R.R. Groner, Accounting Technician M.G. Murphree, Clerk Typist G.M. Laird, Publications Specialist2 J.M. Robinson, Administrative Assistant R.A. Mm,Clerk J.A. Outten, Publications Technician A-L. Schell, Cartographer A.G. Sturrnann, Cartographer F.C. Tannian, Publications Specialist

'In addition to the permanent staff listing above, DGGS presently employs 14 students in the Department of Natural Resources Student Intern Program. 2Section chief.