mesozoic and cenozoic tectonic growth of southern … and ridgway, 2007.pdf · 55 the geological...

55

The Geological Society of AmericaSpecial Paper 431

2007

Mesozoic and Cenozoic tectonic growth of southern Alaska: A sedimentary basin perspective

Jeffrey M. Trop*Department of Geology, Bucknell University, Moore Avenue, Lewisburg, Pennsylvania, 17837, USA

Kenneth D. Ridgway*Department of Earth and Atmospheric Sciences, 550 Stadium Mall Drive, Purdue University, West Lafayette, Indiana 47907-2051, USA

ABSTRACT

Mesozoic and Cenozoic sedimentary strata exposed throughout southern Alaska con-tain a rich archive of information on the growth of collisional continental margins throughthe processes of terrane accretion, magmatism, accretionary prism development, and sub-duction of oceanic spreading ridges. Two major collisional events define the tectonicgrowth of southern Alaska: Mesozoic collision of the Wrangellia composite terrane andCenozoic collision of the Yakutat terrane. The sedimentary record of these two collisionalevents can be summarized as follows. (1) Middle Jurassic volcaniclastic and sedimentarystrata represent shallow-marine deposition in narrow subbasins adjacent to the volcanicedifice of the south-facing, intraoceanic Talkeetna arc. (2) Late Jurassic syndepositionalregional shortening resulted in thick sections of conglomerate in proximal parts of bothretroarc and forearc basins. In distal retroarc depocenters, fine-grained turbidite sedimen-tation commenced in a series of basins that presently extend for .2000 km along strike.This time interval also marked cessation of magmatism and rapid exhumation of the Tal-keetna oceanic arc. We interpret these observations to reflect initial oblique collision,younging to the northwest, of the Wrangellia composite terrane with the continental mar-gin of western North America. (3) During Early Cretaceous time, Jurassic retroarc basinstrata were incorporated into an expanding north-verging thrust belt, and sediment wasbypassed into more distal collisional retroarc basins located within the suture zone. Com-positional data from these collisional basins show that the Wrangellia composite terranewas exhumed to deep stratigraphic levels. Detrital zircon ages from strata in these basinsrecord some sediment derivation from source areas with North American continentalmargin affinity. Our data clearly show that the Wrangellia composite terrane and the for-mer continental margin were in close proximity by this time. Accretion of this oceanic ter-rane and associated basinal deposits marked one of the largest additions of juvenile crustto western North America. The collision of the Wrangellia composite terrane also resultedin a change in subduction parameters that eventually prompted development of a newsouth-facing arc system, the Chisana arc. Construction of this arc was contemporaneouswith renewed forearc basin subsidence and sedimentation. (4) Late Early Cretaceous toearly Late Cretaceous time was characterized by regional deformation of retroarc colli-sional basin strata by south-verging thrust faults that are part of a regional thrust belt that

*[email protected]; [email protected]

Trop, J.M., and Ridgway, K.D., 2007, Mesozoic and Cenozoic tectonic growth of southern Alaska: A sedimentary basin perspective, in Ridgway, K.D., Trop, J.M.,Glen, J.M.G., and O’Neill, J.M., eds., Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska: Geological Society of America Spe-cial Paper 431, p. 55–94, doi: 10.1130/2007.2431(04). For permission to copy, contact [email protected]. ©2007 The Geological Society of America. All rightsreserved.

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56 Trop and Ridgway

INTRODUCTION

Mesozoic and Cenozoic strata exposed throughout southernAlaska provide a detailed record of collisional continental growththrough terrane accretion, sedimentation, magmatism, and accre-tionary prism development. The tectonic growth of southernAlaska is defined by two main collisional events. The first event,Mesozoic collision of the Wrangellia composite terrane, resulted inone of the largest additions of juvenile crust to the North AmericanCordillera margin. The second event, Cenozoic collision of theYakutat terrane, an excised continental fragment of western NorthAmerica, has generated the largest coastal mountain range onEarth. This paper synthesizes recent advances in our understandingof Mesozoic and Cenozoic depositional and structural processes inrelation to these two major collisional events. Previous synthesesof the geologic framework of southern Alaska emphasized the na-ture and distribution of accreted terranes and structural elements(e.g., Plafker et al., 1989; Plafker and Berg, 1994; Nokleberg et al.,2001) but did not discuss the collisional record contained withinsedimentary basins that formed between and upon colliding ter-ranes and adjacent to major structural elements. The goal of thispaper is to explore linkages between depositional and deforma-tional processes related to the collisional convergent margin tecton-

ics that formed much of the northwestern Cordillera. The first partof the paper provides a brief descriptive overview of Mesozoic andCenozoic basinal stratigraphy and basin-bounding structures basedon recent and ongoing basin studies and previously published datasets (mainly 1:250,000–scale U.S. Geological Survey geologicmaps). Our basin analysis studies provide sedimentologic, compo-sitional, geochemical, and geochronologic data within the contextof bed-by-bed measured stratigraphic sections, as well as targetedgeologic mapping of synorogenic structures. The second part of thepaper presents an interpretive model of Mesozoic and Cenozoicbasin development that reconstructs the evolution of depositionalenvironments, identifies the provenance of basinal strata, definesthe structural framework of depocenters through time, and relatessedimentary basin development to the collisional processes thatformed the southern margin of Alaska.

TECTONIC FRAMEWORK OF SOUTHERN ALASKA

Major Composite Terranes

Alaska consists of a complex collage of exhumed sedimentarybasins, accreted terranes, magmatic belts, and subduction complexstrata that were added to the former continental margin during

extends throughout the northwestern Cordillera. (5) Latest Cretaceous time was charac-terized by synorogenic sedimentation in forearc and retroarc basins related to regionalshortening and exhumation of a coeval continental-margin arc and older collisional basi-nal deposits. Forearc depocenters subsided into deep-water settings between the arc andexpanding accretionary prism. Nonmarine to marginal-marine strata accumulated inretroarc depocenters influenced by syndepositional thrust-fault deformation. (6) Growthof the southern Alaska continental margin during Paleocene to Early Eocene time isdefined by regional nonmarine deposition, magmatism within the suture zone, and ex-pansion of the accretionary prism. Oblique subduction of an oceanic spreading ridgeprompted diachronous deformation, synorogenic sedimentation, and magmatism. Sub-duction of progressively more buoyant, topographically higher lithosphere (the spreadingridge) followed by less buoyant, topographically lower lithosphere prompted coarse-grained alluvial-fluvial sedimentation and slab-window magmatism in remnant forearcbasins. (7) Regional transpressive deformation characterized southern Alaska duringMiddle Eocene to Oligocene time. This deformation generated coarse-grained alluvial-fluvial sedimentation in narrow fault-bound basins along major strike-slip faults, includ-ing the Denali and Castle Mountain faults. (8) A second major phase of terrane collisionand basin development shaped the southern margin of Alaska during latest Oligocene to Holocene time. Northward translation and collision of the Yakutat terrane, an excisedcontinental fragment of western North America, prompted growth of the largest coastalmountain range on Earth, construction of a new magmatic arc, exhumation of older fore-arc basinal strata, and renewed uplift of the Alaska Range. The sedimentary record of thiscollisional event is contained in a collisional foreland basin on the north side of the AlaskaRange, intra-arc basins in the Wrangell Mountains, and a collisional foreland basin withinthe Yakutat terrane. This phase of collision continues to the present as evidenced by activemountain building, large-magnitude earthquakes, and some of the highest sediment accu-mulation rates on Earth.

Keywords: Alaska, tectonics, sedimentary basins, Mesozoic, Cenozoic.


Mesozoic and Cenozoic tectonic growth of southern Alaska 57

Mesozoic and Cenozoic time (e.g., Jones et al., 1977; Coney et al.,1980; Nokleberg et al., 2001). Three composite terranes make upmost of south-central Alaska. From north to south, these includethe Yukon, Wrangellia, and Southern Margin composite terranes(Fig. 1D; Plafker et al., 1994; Nokleberg et al., 1994, 2001). TheYukon composite terrane consists of ductilely deformed and struc-turally dismembered Proterozoic-Paleozoic metamorphic rocks(Yukon-Tanana terrane) and arc-related rocks (Stikine terrane).Metamorphic rocks of the Yukon-Tanana terrane crop out in a sus-pect position, faulted between autochthonous or slightly displacedNorth American crust and outboard allochthonous terranes (e.g.,Mortensen, 1992; Foster et al., 1994; Nokleberg et al., 1994, 2001;Hansen and Dusel-Bacon, 1998; Dusel-Bacon et al., 2004). TheYukon-Tanana and Stikine terranes were probably attached to in-board terranes by Middle Jurassic time (Mihalynuk et al., 1994;Monger and Nokleberg, 1996; Gehrels, 2001).

The allochthonous Wrangellia composite terrane, one of thelargest accreted terranes in the North American Cordillera, is jux-taposed against both the southern margin of the Yukon compositeterrane and smaller continental terranes (Dillinger, Nixon-Fork)along a regional suture zone (Fig. 1D). This suture zone is com-monly referred to as the Alaska Range suture zone (Ridgway

et al., 2002) or as the megasuture zone (Jones et al., 1982) and ischaracterized by complexly deformed sedimentary, igneous, andmetamorphic rocks attributable to Jurassic-Cretaceous collisionalprocesses. The Denali fault bisects the suture zone (Fig. 1D) andhas accommodated up to 400 km of Late Cretaceous-Cenozoicright-lateral displacement (Eisbacher, 1976; Nokleberg et al.,1985; Lowey, 1998). The Wrangellia composite terrane consistsof three allochthonous terranes (Peninsular, Wrangellia, andAlexander terranes) that crop out discontinuously from westernAlaska to southern British Columbia (Plafker and Berg, 1994;Nokleberg et al., 2001). The Alexander and Wrangellia terraneswere joined together by Middle Pennsylvanian time (Gardneret al., 1988). These terranes were positioned ,20–30° south oftheir present latitude during Late Triassic time before being trans-lated northward to Alaska (Plafker et al., 1989; Hillhouse andCoe, 1994; Stamatakos et al., 2001). The Early to Late JurassicTalkeetna arc, which is part of the Peninsular terrane, was eitherconstructed upon the previously combined Paleozoic-Triassic crustof the Wrangellia-Alexander terrane, or it collided with the com-bined Wrangellia-Alexander terrane sometime during Permian toLate Jurassic time (Nokleberg et al., 2001; Trop et al., 2005a;Rioux et al., 2005; Clift et al., 2005b). Thus, two different tectonic

Figure 1. (continued on following page) (A, B) Index maps showing location of study area (black rectangle) in south-central Alaska within the con-text of Jurassic-Cretaceous (A) and latest Cretaceous-Cenozoic magmatic belts (B). Note inboard (northward) migration of magmatism from EarlyJurassic through Late Cretaceous time prior to trenchward (southward) retreat. Adapted from Plafker et al. (1994). (C) Explanation of map unitsshown in Figure 1D. (D) Generalized geologic map showing Mesozoic-Cenozoic sedimentary basins, terranes, faults, magmatic belts, and majorstructural elements of interior south-central Alaska. Abbreviation: LOF, Little Oshetna fault; MC, McCarthy; T, Totschunda. Thin black lines define1:250,000 quadrangles. Adapted from Wilson et al. (1998) and Bradley et al. (2003).

Quaternary deposits

Pliocene Nenana Gravel

Miocene Usibelli Group

Paleogene Tsadaka, Wishbone, Chickaloon,Middle Colorado Creek Fms.

Cretaceous Matanuska, Kennicott, ChitituSchultze, Moonshine Creek, MacColl Ridge,and lower Cantwell Fms.

Jurassic-Cretaceous Kahiltna assemblage, Nutzotin Mtns. sequence, and melange

Jurassic Tuxedni, Chinitna, Naknek, NizinaMountain, Root Glacier, and Kotsina Fms.

Mesozoic-Cenozoic Accretionary Prism

Paleozoic-Mesozoic accreted terranes

Mesozoic-Cenozoic Sedimentary Strata

Wrangellia composite terrane

Triassic-Paleozoic undifferentiated

Yukon composite terrane

Miocene-Quat. igneous rocks (Wrangell-Aleutian arc)

Paleocene-Oligocene igneous rocks

Upper Cretaceous igneous rocks

Lower Cretaceous igneous rocks (Chisana arc)

Upper Jurassic igneous rocks (Talkeetna-Chitina arc)

Middle Jurassic igneous rocks (Talkeetna arc)

Middle Jurassic-Quaternary igneous rocks

Paleocene-Eocene Orca Group

Upper Cretaceous Valdez Group

Permian-mid-Cret. McHugh Complex

Ice

Paleocene-Holocene Yakutat terrane

Cenozoic accreted terrane

Lake/Ocean

River

Fault (exposed)

Town/City

Fault (covered/concealed)

T

200 km

Present limit of Southern Margin Composit

e

Terra

ne

200 km

Aleutian

Megath

rust

Alaska Peninsula

ALA

SK

AC

AN

AD

A

ALA

SK

AC

AN

AD

A

Early Cretaceous Chisanaarc

Late Jurassic Chitinaarc

Middleto

Late

Jura

ssic

Talkeetna

Early

Jura

ssic

Talkeetna arc

Latest

Cret.

Kluane

arc

Fig.1D

Border

Ran

ges

FaultLatest Cretaceous

KuskokwimMtns.-A

laska

Range-Talkeetna

Mtns. belt

Cenoz

oic Ale

utia

nar

c

Cz.Wrangell arc

B

C

A

Reverse fault(barbs on hanging wall)

Strike-slip fault

Fig. 1D


58 Trop and Ridgway

models have been proposed to account for Mesozoic accretion ofthe Wrangellia composite terrane against inboard (northern) ter-ranes based on studies from south-central Alaska: (1) Late Juras-sic collision of the Talkeetna arc (Peninsular terrane) against thesouthern margin of the combined Wrangellia-Alexander terranefollowed by Early Cretaceous juxtaposition against the formercontinental margin (Clift et al., 2005b) and (2) Late Jurassic-EarlyCretaceous juxtaposition of the combined Wrangellia, Alexander,and Peninsular terranes against the former continental margin(Ridgway et al., 2002; Trop et al., 2002, 2005a).

The Border Ranges fault juxtaposes the outboard (southern)margin of the Wrangellia composite terrane against the northernedge of the Southern Margin composite terrane, which includesthe Mesozoic Chugach terrane and the Cenozoic Prince Williamand Yakutat terranes (Fig. 1D). Interpreted as a paleo-subduction-zone thrust, the Border Ranges fault accommodated northwardunderthrusting of oceanic crust beneath the Wrangellia compos-ite terrane during Early Jurassic to Late Cretaceous time (e.g.,Pavlis, 1982; Pavlis and Roeske, this volume). Latest Cretaceous-

Tertiary reactivation of the fault accommodated up to hundreds ofkilometers of right-lateral displacements along the fault (e.g., Lit-tle and Naeser, 1989; Little, 1990; Pavlis and Crouse, 1989; Smartet al., 1996; Roeske et al., 2003; Pavlis and Roeske, this volume).The Chugach and Prince William terranes consist of metamorphicrocks, mélange, and offscraped oceanic sedimentary and volcanicrocks interpreted as the products of a long-lived subduction com-plex (Plafker et al., 1994). The age of strata, degree of structuraldeformation, and grade of metamorphism decrease systemati-cally southward across the subduction complex, indicating pro-tracted northward subduction (Plafker et al., 1994). From north tosouth, the Chugach terrane includes (1) spatially limited blueschistbelts with Late Triassic to Early Jurassic metamorphic ages (Sis-son and Onstott, 1986; Roeske et al., 1989, 1992); (2) mélange(McHugh-Uyak Complex) with Triassic to Upper Cretaceous fos-sils (Plafker et al., 1994)—structural and geochronologic rela-tions indicate that most of the mélange is younger than ca. 125 Ma(Pavlis et al., 1988; Barnett et al., 1994); and (3) the ValdezGroup, a marine metasedimentary unit that bears latest Cretaceous

FIG1A

K Jk

T K g

M m

F A U L T

DENALI

K g

R iv e r

S T ?S T ?

Y T

Y T r

CopperRiverbasin

CookInlet

PrinceWilliamSound

BRF

TF

VS

Key to 1:250,000 Quads

Susitnabasin

Nutzotin basin

Kahiltna

basin

Broad

Pass

Northway basin

61o

62o

63o

60o

ValdezAnchorage

Tanana basin

Fairbanks

YakutatTerrane

Prince WilliamTerrane

Bor

der R

ange

sFa

ult

147o150

o

Contact

Wrangell Mtns.

Chitina Valley

N. Talkeetna Mtns.

S. Talkeetna Mtns.

Nab

esna

Riv

er

Riv

er

Chi

sana

WrangellVolcanoes

Susitna

River

Kahiltna

River

Yentna

River

Tanana River

Denali

Fault

Yukon Tanana Uplands

RobinsonMountains

C. Alaska Range

E. Alaska Range

Mt.McKinley(Denali)

Wes

tern

Chu

gach

Mtn

s.

Eastern Chugach Mtns.

Glenallen

T

LOF

Kantishna

Riv

er

Nen

ana

Riv

er

Union Nenana #1Arco Totek Hill #1

Kahiltnabasin

D 144o

Yukon Tanana UplandsTanana

River

St. Elias Mtns.

FaultChugach

FaultSt. Elias

Chitina

Thrust Belt

TaralFaultCastle

MountainFault

Cantwellbasin

9A

9B

9C

9D

7C

7A

7B

7D

Matanuska Valley

Bruin

Bay

Faul

t

Wrangellia composite terrane

Sou

ther

n

Margin composite terrane

Bor

der

RangesFault

Denali Fault

Wra

ngellia composite terrane

Yukon composite terrane0

80 km

Del

taR

iver

Yukon composite terrane

Nutzotin Mtns.

Northern Chugach Mtns.

Chugach Terrane

64o

MC

Talkeetna Fault

N

Figure 1 (continued)



fossils and latest Cretaceous-Paleocene uplift ages (Plafker et al.,1994; Sample and Reid, 2003; Clendenen et al., 2003). Juxta-posed against the southernmost strata of the Valdez Group is thePrince William terrane, which consists of offscraped Paleocene-Eocene marine sedimentary and volcanic rocks (e.g., GhostRocks Formation, Orca Group, Sitkalidak Formation, and Resur-rection Peninsula ophiolite; Plafker et al., 1994).

The southernmost part of the Southern Margin compositeterrane is characterized by the ongoing collision of the Yakutatterrane (Fig. 1D). The allochthonous Yakutat terrane is faultedagainst the southern margin of the Chugach and Prince Williamterranes along the Chugach-Saint Elias and Fairweather faults(Fig. 1D). The Yakutat terrane, a sliver of the western NorthAmerican continental margin, has been transported along the out-board margin of the Cordillera during the past 30 m.y. (e.g.,Plafker et al., 1978; Bruns, 1983; Plafker, 1987; Plafker and Berg,1994). The Yakutat terrane was located along the coast of south-eastern Alaska and British Columbia (Plafker et al., 1994) or thePacific Northwest (Bruns, 1983) when the transform fault bound-ary between the Pacific and North American plates movedinboard (ca. 30 Ma). Relocation of the transform boundary initi-ated northward tectonic transport of the Yakutat terrane and sub-sequent collision along the southeastern margin of Alaska (Bruhnet al., 2004; Pavlis et al., 2004). Several hundred kilometers of thenorthern margin of the terrane are inferred to have been subductedbeneath North America and the northeastern end of the Aleutiansubduction zone. Plafker et al. (1994) suggested that most of thesubducted crust was “typical” oceanic lithosphere, but a morerecent seismic study indicates that the subducted crust may havebeen an oceanic plateau (Ferris et al., 2003). In the tectonic modelproposed by Plafker (1987), the Yakutat terrane has been tecton-ically transported ,600 km to its present position by dextral dis-placement along the Queen Charlotte-Fairweather transform faultsystem. Tectonic transport started at ca. 30 Ma, and continuedfrom 30 to 25 Ma, prompting subductions of ,225 km of theYakutat terrane beneath southern Alaska. The oldest lavas of theWrangell volcanic field in eastern Alaska (Fig. 1D) record initia-tion of arc magmatism above the subduction zone at ca. 26 Ma(Richter et al., 1990). At ca. 10 Ma, the buoyant continental partof the Yakutat terrane entered the subduction zone and was par-tially subducted and underthrust beneath the North American con-tinental margin (Plafker, 1987; Ridgway et al., 1996). At ca. 5 Ma,an increase in the angle and rate of plate convergence promptedmore orthogonal convergence (Engebretson et al., 1985) and acorresponding increase in the rate of underthrusting of buoyantcontinental crust of the Yakutat terrane beneath the margin ofsouthern Alaska. This underthrusting is partly responsible forregional uplift of the Saint Elias Mountains, Chugach Mountains,and adjacent parts of Canada and Alaska (e.g., Plafker et al., 1994; Jaeger et al., 2001; Bruhn et al., 2004; Pavlis et al., 2004).Presently, the Yakutat terrane is moving 45–50 mm/yr northwestwith respect to interior Alaska (Sauber et al., 1997; Fletcher andFreymueller, 1999) while being internally deformed, partly sub-ducted beneath, and partly accreted to the North American plate(Bruhn et al., 2004; Pavlis et al., 2004).

Magmatic Belts

The composite terranes of southern Alaska are intruded andoverlain by plutonic and volcanic rocks attributable to collisionalorogenesis, subduction of oceanic lithosphere, and slab-windowmagmatism (e.g., Plafker et al., 1989; Richter et al., 1990; Nokle-berg et al., 1994; Hudson, 1994; Cole et al., 1999, 2006, this vol-ume; Bradley et al., 2000, 2003; Kelemen et al., 2003; Clift et al.,2005a, 2005b). Linear belts of Jurassic, Cretaceous, and Neogeneigneous rocks exhibit geochemical characteristics that suggestemplacement mainly within subduction-related arcs (Figs. 1A,1B). Arc rocks are dominated by plutons interpreted as the deeperroots of subvolcanic intrusive masses, although thick extrusivesuccessions are also preserved locally (e.g., Cretaceous ChisanaFormation, Jurassic Talkeetna Formation, Miocene-QuaternaryWrangell Lava). Geochronologic data document progressiveinboard (northward) migration of magmatism from Early Juras-sic to Late Cretaceous time (Talkeetna, Chisana, and Kluane arcson Figures 1A, 1B; Plafker et al., 1989), prior to trenchward (south-ward) retreat during Cenozoic plate reorganization (NeogeneAlaska-Aleutian and Wrangell arcs; Plafker et al., 1989; Moll-Stalcup, 1994; Preece and Hart, 2004). Most workers attributeJurassic-Neogene arc magmatism to north-dipping subduction(using present-day coordinates) based on age-equivalent subduc-tion complex strata exposed south of the arc rocks (e.g., Plafkeret al., 1989; Plafker and Berg, 1994; Ridgway et al., 2002; Cliftet al., 2005b). In contrast, Reed et al. (1983) infer a south-dippingsubduction zone during Early to Late Jurassic time based onwhole-rock chemical differentiation trends in plutons exposed onthe Alaska Peninsula. However, definitive age-equivalent sub-duction complex strata are not recognized north of the Jurassic arcrocks (Nokleberg et al., 1994). Moreover, Jurassic volcanic rocksyield spatial isotopic trends that support north-dipping subduction(Clift et al., 2005a).

Comprising a 1000-km-long outcrop belt from the easternCopper River basin to the Alaska Peninsula, Lower to UpperJurassic volcanic and plutonic rocks of the accreted oceanic Tal-keetna arc young northward across the Peninsular terrane(Fig. 1A; Plafker et al., 1989; Nokleberg et al., 1994; Rioux et al.,2005). Upper Jurassic plutons also intrude the southern margin ofWrangellia from the eastern Copper River basin to southeasternAlaska (Chitina arc of Plafker et al., 1989), possibly representingan eastern extension of the Talkeetna arc (Trop et al., 2005a). Thenext major arc is represented by Lower Cretaceous volcanic andplutonic rocks that crop out along the inboard margin of theWrangellia terrane for ,450 km from south-central to southeast-ern Alaska (Berg et al., 1972; Richter et al., 1975; Plafker et al.,1989; McClelland et al., 1992a). In south-central Alaska, igneousrocks associated with this arc are referred to as the Chisana arc(Fig. 1A), whereas correlative strata in southeastern Alaska arecalled the Gravina belt. No direct evidence suggests that Gravina-Chisana rocks depositionally overlap inboard terranes; the Denalifault juxtaposes these rocks against the Yukon-Tanana terrane.Upper Cretaceous-Paleocene plutonic and volcanic rocks (mainly80–55 Ma) intrude both the Wrangellia and Yukon composite

GSA_SPE431_04_Trop.qxd 7/13/07 8:05 AM 9

60 Trop and Ridgway

terranes from western and central Alaska (Kuskokwim Moun-tains-Talkeetna Mountains belt of Moll-Stalcup, 1994) to easternAlaska and British Columbia (Fig. 1B; Kluane arc of Plafkeret al., 1989). Regional geologic data suggest that this arc formedafter accretion of the Wrangellia composite terrane to inboardterranes (Plafker et al., 1989). Geochemical and lithologic char-acteristics of these rocks support emplacement within a conti-nental-margin arc (Plafker et al., 1989; Moll-Stalcup, 1994).

Paleocene-Eocene volcanic rocks up to 3 km thick overlap theWrangellia and Peninsular terranes from the southern TalkeetnaMountains to the central Alaska Range (Fig. 1D). Unlike Mesozoicigneous rocks of south-central Alaska, these strata crop out in anorthwest-southeast-trending belt and yield geochemical signaturesconsistent with slab-window magmatism, probably in response to

subduction of an oceanic spreading ridge (Cole and Stewart, 2005;Cole et al., 2006). Bisecting the outboard margin of the Wrangel-lia composite terrane, Miocene-Recent volcanic and plutonic rocksof the Aleutian-Wrangell arc (Fig. 1B) record subduction of thenorthern edge of the Pacific plate and the Yakutat terrane beneaththe continental margin of southern Alaska (Richter et al., 1990;Moll-Stalcup, 1994; Preece and Hart, 2004).

EXHUMED AND ACTIVE SEDIMENTARY BASINS

In this section, we synthesize the characteristics of Mesozoicand Cenozoic sedimentary basinal strata of south-central Alaska.Figure 2 categorizes the basins in terms of their relationship to col-

Age

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Maastrich.

Campan.

Albian

Aptian

Barremian

Hauteriv.

Valangin.

Berriasian

TithonianKimm.Oxfordian

Callovian

Bathonian

Bajocian

Aalenian

Toarcian

Norian

Carnian

Ladinian

NIZINA LIMESTONEmarine limestone

Ma

Per

iod

Q

TERTIARY

CRETACEOUS

JURASSIC

TRIASSIC

ConiacianSantonian

Turonian

Holo./Pleist.

NUTZOTINMOUNTAINSSEQUENCE24

McCARTHY FM.25

NIZINA LIMESTONE8

LUBBE CK FM.31

MacCOLL RIDGE FM.

27

KUSK. PASS AND

BERG CK. FMS.29

KENNICOTT FM.28

CHITITU FM.28

SchultzeFm.28

MoonshineCk. Fm.28

LOWER ROOT GL. FM.31

KOTSINACONG.30

NIZINA MOUNTAIN FM.31

FREDERIKA FM.26

WRANGELL LAVA21

CHISANA FM.23

UNNAMED22

Pliensbach.

Sinemurian

Hettangian

UPPER MATANUSKA FM.

14

LOWER MATANUSKA FM.

15

NELCHINA LIMESTONE AND

UNNAMED STRATA16

CHINITNA FM.18

TUXEDNI FM.19

TALKEETNA FM.20

TSADAKA FM.10

WISHBONE FM.11

ARKOSE RIDGE12

AND CHICKALOONFMS.

13

WrangellMountains

Matanuska Valley-Talkeetna Mtns.

KAHILTNAASSEMBLAGE

7

(Talkeetna Mtns.)

KAHILTNAASSEMBLAGE

7

(Alaska Range)

Middle/UpperCOLORADO CK. STRATA

3

LOWER CANTWELL FM.

5

HONOLULU PASS FM.6

USIBELLI GROUP2

CentralAlaska Range

Eastern Alaska Range

NENANA GRAVEL1

NIZINALIMESTONE8

McCARTHY FM.25

CHITISTONE LIMESTONE8

NAKNEK FM.17

?

Pre

--W

CT-

colli

sion

Syn

-WC

T-co

llisi

onY

akut

at-c

ollis

ion

?



NIZINALIMESTONE8

UPPERCANTWELL FM.

4

Cenomanian

?

NIKOLAI GREENSTONE9

NIKOLAI GREENSTONE9

WRANGELL LAVA21

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

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KULTHIETHFM.32

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RobinsonMountains

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POUL CREEK FM.32

YAKATAGAFM.32

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Figure 2. Stratigraphic chart showing theage ranges of Mesozoic-Cenozoic sedi-mentary and subordinate volcanic strataalong the inboard and outboard margins ofthe Wrangellia composite terrane. GSAtimescale from Palmer and Geissman(1999). Age ranges based mainly on previ-ously published megafossils and recentisotopic and palynologic data. Sources:1—Ager et al., 1994; Ridgway et al., thisvolume; 2—Wolfe and Tanai, 1980;Leopold and Liu, 1994; 3—Trop et al.,2004; 4—Cole et al., 1999; 5—Ridgwayet al., 1997; 6—Hampton et al., 2003, thisvolume; 7—Jones et al., 1986; Csejteyet al., 1992; 8—Armstrong et al., 1969;MacKevett, 1970c; 9—MacKevett, 1978;10—Clardy, 1974; 11—Clardy, 1974; Tropet al., 2003; 12—Silberman and Grantz,1984; 13—Wolfe et al., 1966; Triplehorn et al., 1984; 14—Jones, 1964, Trop et al.,2005b; 15—Jones, 1967; Trop and Ravn,2003; 16—Jones, 1973; 17—Imlay, 1981;Trop et al., 2005a; 18—Imlay, 1975, 1980;Winkler, 1992; 19—Imlay, 1982, 1984;Winkler, 1992; 20—Imlay and Detterman,1973; Winkler, 1992; 21—Richter et al.,1990; 22—Richter, 1976; 23—Berg et al.,1972; Short et al., 2005; 24—Berg et al.,1972; Richter and Jones, 1973; 25—MacKevett, 1970a, 1970b, 1971; 26—MacKevett, 1971; Tidmore et al., 2005;27—Trop et al., 1999; 28—Jones andMacKevett, 1969; 29—Grantz et al., 1966;30—Grantz et al., 1966; Trop et al., 2002;31—MacKevett, 1969; 32—Plafker, 1987.

GSA_SPE431_04_Trop.qxd 7/13/07 8:05 AM 0


lision of the Wrangellia composite terrane and the Yakutat terraneand by their location in terms of being inboard (cratonward) or out-board (oceanward) with respect to the Wrangellia composite ter-rane. Refer to Figure 1D for the map location of basinal strata and Table 1 for details on formation names, ages, thicknesses, andlithologies. Space limitations prevent acknowledging all sourcesof data and interpretations. Cited references emphasize recent stud-ies that contain more extensive bibliographies. Geologic time units

are those of Palmer and Geissman (1999). Orientations of geologicand geographic features refer to present geographic coordinates.

Inboard Margin Basins and Structures

Mesozoic and Cenozoic sedimentary strata are discontinu-ously exposed along the inboard (northern) margin of the Wrangel-lia composite terrane in the Alaska Range, northern Talkeetna

TABLE 1. SUMMARY OF MIDDLE JURASSIC-PLIOCENE SEDIMENTARY STRATA OF INTERIOR SOUTH-CENTRAL ALASKA

Common Depositional Inferred Basin/Formations Age Thickness Lithologies Environments Basin Setting

Cantwell Basin

Upper Cantwell Fm. Paleocene- 3000 m Basalt, andesite, rhyolite, Subaerial volcanic Successor/Early Eocene tuff, breccia eruptions strike slip

Lower Cantwell Fm. Campanian- 4000 m Conglomerate, sandstone, Alluvial, fluvial, Thrust-top Maastrichtian mudstone lacustrine basin

Colorado Creek BasinLower member Late Cretaceous 30 m Sandstone, Shallow marine Collisional

mudstone shelf foreland*Middle member Early Oligocene 330 m Conglomerate, sandstone, Alluvial, fluvial, Continental

mudstone lacustrine strike slipUpper member Early Oligocene- 55 m Lava flows, tuff Subaerial volcanic Continental

Eocene(?) eruptions strike slip

Kahiltna Basin

Kahiltna Assemblage Valanginian- .3000 m Mudstone, sandstone, Submarine Remnant (AK Range) Cenomanian limestone fans ocean basin

Caribou Pass Fm. Albian- .250 m Sandstone, mudstone, Fluvial channels/ Collisional (Talkeetna Mtns.) Cenomanian conglomerate floodplain foreland*

Kahiltna Assemblage Kimmeridgian- .3000 m Mudstone, sandstone, Submarine Collisional (Talkeetna Mtns.) Albian limestone fans foreland*

Nutzotin Basin

Unnamed strata Cretaceous(?)- 90 m Sandstone, conglomerate, Sandy fluvial Collisional Tertiary tuff, coal systems foreland*

Chisana Fm. Hauterivian- 3000 m Lava flows, Subaerial volcanic Collisional Aptian breccia, tuff eruptions foreland*

Nutzotin Mtns. Seq. Oxfordian- 3000 m Mudstone, sandstone, Submarine Collisional Valanginian conglomerate fans foreland*

Tanana Basin

Conglomerate, Alluvial fan, Retroarc Nenana Fm. Pliocene 1040 m sandstone braided fluvial foreland

Laminated mudstone, Retroarc Grubstake Fm. Miocene 23–450 m sandstone Lacustrine, fluvial foreland

Conglomerate, Fluvial channels/ Retroarc Lignite Creek Fm. Miocene 160 m sandstone, coal floodplain foreland

Fluvial channels/ Retroarc Suntrana Fm. Miocene 205 m Sandstone, coal floodplain foreland

Retroarc Sanctuary Fm. Miocene 30 m Laminated mudstone Lacustrine forelandHealy Creek Fm. Oligocene-Miocene 125 m Sandstone, mudstone, coal Fluvial channels/ Retroarc

floodplain foreland

White Mountain BasinUnnamed strata Late Oligocene 440 m Conglomerate, sandstone, Fluvial channels/ Continental

mudstone floodplain strike slip

(continued)

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TABLE 1. SUMMARY OF MIDDLE JURASSIC-PLIOCENE SEDIMENTARY STRATAOF INTERIOR SOUTH-CENTRAL ALASKA (continued )

Matanuska Valley-Talkeetna Mountains Basin

Tsadaka Fm. Oligocene 200 m Conglomerate, sandstone Stream-dominated Remnant alluvial forearc

Wishbone Fm. Eocene 1100 m Conglomerate, sandstone, Stream-dominated Remnant mudstone alluvial forearc

Arkose Ridge Fm. Paleocene-Eocene 1600 m Sandstone, mudstone, Alluvial, fluvial, Remnant lava, tuff estuarine forearc

Chickaloon Fm. Paleocene-Eocene 1500 m Mudstone, sandstone, Fluvial, lacustrine, Remnant coal, tuff estuarine forearc

Upper Matanuska Fm. Campanian- 2000 m Sandstone, mudstone, Sandy submarine Forearc Maastrichtian conglomerate fans

Lower Matanuska Fm. Albian-Santonian 500 m Sandstone, mudstone, coal, tuff Fluvial, shoreface, Forearc marine shelf

Nelchina Limestone Valanginian- 200 m Calcareous sandstone, Calcareous marine Forearc Barremian mudstone shelf

Naknek Fm. Oxfordian- 700 m Conglomerate, sandstone, Marine fan-delta, ForearcTithonian mudstone slope, shelf

Chinitna Fm. Bathonian-Callovian 300 m Mudstone, sandstone, limestone Marine shelf, slope Forearc Tuxedni Fm. Bajocian-Bathonian 150 m Sandstone, mudstone, conglomerate Shoreface, foreshore, Forearc

shelf

Wrangell Mountains Basin

Frederika Fm. Middle to Late 600 m Sandstone, mudstone, Alluvial, fluvial, Intra-arcMiocene tuff, cong. lacustrine

MacColl Ridge Fm. Campanian 1150 m Sandstone, cong., Sandy submarine Forearc mudstone, tuff fans

Chititu Fm. Albian-Campanian 1100 m Mudstone, limestone, Submarine slope Forearc sandstone

Schultze Fm. Albian-Cenomanian 75 m Siliceous mudstone, Marine shelf Forearcporcellanite

Moonshine Creek Fm. Albian-Cenomanian 1000 m Sandstone, mudstone Shoreface, foreshore, shelf Forearc

Kennicott Fm. Albian 250 m Sandstone, conglomerate, Shoreface, mudstone foreshore, shelf Forearc

Berg Ck./ Hauterivian- 300 m Calcareous sandstone, Calcareous marine Forearc Kusk. Pass Fm. Barremian mudstone shelf

Root Glacier Fm. Oxfordian-Tithonian 1100 m Mudstone, sandstone, Submarine slope/ Retroarc conglomerate fans foreland

Kotsina Conglomerate Kimmeridgian- 600 m Conglomerate, sandstone Fan-delta RetroarcTithonian foreland

Nizina Mountain Fm. Bathonian-Callovian 250 m Mudstone, sandstone, tuff Marine shelf Backarc

Yakutat Basin

Yakataga Fm. Miocene-Holocene 5000 m Sandstone, siltstone, diamictite Glacial marine Collisionalforeland*

Poul Creek Fm. Oligocene-lower 2000 m Mudstone, sandstone, tuff Marine shelf Collisional Miocene foreland*

Kulthieth, Stillwater, Upper Paleocene 3000 m Sandstone, mudstone, coal Fluvial-deltaic, Continental Token Fms. Eocene shallow marine margin

*Regional arc magmatism partially overlapped with collisional orogenesis and deposition in these foreland basins

** Watana and MacCallum Creek basins are not listed due to paucity of stratigraphic/sedimentologic data

Key references for environmental and basin setting interpretations

Tanana - Wahrhaftig et al., 1969; Stevens, 1971; Buffler and Triplehorn, 1976; Wolfe and Tanai, 1980; Selleck and Panuska, 1983; Stanley et al., 1991; Ager et al., 1994;

Leopold and Liu, 1994; Ridgway et al., 1999a, 2002; Cantwell - Wolfe and Wahrhaftig, 1970; Gilbert et al., 1976; Ridgway et al., 1997; Cole et al., 1999;

Kahiltna - Wallace et al., 1989; Csejtey et al., 1992; Eastham and Ridgway, 2002; O’Neill et al., 2003; Hampton et al., this volume; Kalbas et al., this volume

Nutzotin - Berg et al., 1972; Richter, 1976; Manuszak and Ridgway, 1999; Manuszak et al., this volume; Colorado Creek - Csejtey et al., 1984, 1992; Trop et al., 2004;

Matanuska Valley-Talkeetna Mountains - Clardy, 1974; Grantz, 1964; Little, 1988; Trop et al., 2003, 2005a, b; Trop, 2006

Wrangell Mountains - Grantz et al., 1966; MacKevett, 1978; Winkler et al., 1981; Trop et al., 1999, 2002; Tidmore, 2004; Tidmore et al., 2005; Yakutat - Plafker (1987)

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Mountains, and Mentasta-Nutzotin Mountains (Fig. 1D; Table 1).Strata crop out within a regional suture zone between accretedMesozoic and older oceanic island-arc rocks of the Wrangelliacomposite terrane and Paleozoic and older continental marginrocks of the Yukon-Tanana terrane (e.g., Pavlis, 1982; Coney andJones, 1985; Jones et al., 1982; Nokleberg et al., 1994; Wilsonet al., 1998; Ridgway et al., 2002).

Kahiltna BasinIn south-central Alaska, the Kahiltna assemblage is exposed

in a 100-km-wide and .300-km-long outcrop belt in the AlaskaRange, and a 60-km-wide and 150-km-long outcrop belt in thenorthern Talkeetna Mountains (Fig. 1D; Eastham and Ridgway,2002; Ridgway et al., 2002). Broad Pass, a major topographic lin-eament, separates the two Kahiltna outcrop belts. In the northernTalkeetna Mountains, the Kahiltna assemblage disconformablyoverlies Upper Triassic marine volcanic and sedimentary strata(Honolulu Pass Formation of Hampton et al., this volume). TheKahiltna assemblage in this area contains Kimmeridgian-Valanginian marine megafossils, but detrital zircon ages indicatethat deposition extended at least into Albian time (Table 1;Hampton et al., 2005, this volume). In the northeasternmost Tal-keetna Mountains, the Kahiltna assemblage is locally metamor-phosed to kyanite-garnet schist and gneiss (Maclaren Glaciermetamorphic belt; Nokleberg et al., 1985; Davidson et al., 1992).Albian-Cenomanian nonmarine sedimentary strata informallyreferred to as the Caribou Pass Formation overlie the Kahiltnaassemblage in the northwestern Talkeetna Mountains (Hamptonet al., 2003, this volume).

The Kahiltna assemblage in the Alaska Range is juxtaposedagainst the Yukon-Tanana terrane along the Denali and HinesCreek faults (Fig. 1D). The lower contact of the Kahiltna assem-blage in the central Alaska Range has not been observed by theauthors or by previous mapping studies. To the southwest in thewestern Alaska Range, however, the Kahiltna assemblage depo-sitionally overlies Upper Triassic to Lower Jurassic marine vol-canic and volcaniclastic strata (Kalbas et al., this volume).Submarine-fan strata .5.5 km thick characterize the Kahiltnaassemblage in the Alaska Range (Fig. 2; Eastham and Ridgway,2002; Kalbas et al., 2003, this volume). The Kahiltna assemblagein the Alaska Range contains Valanginian-Cenomanian fossils.South of the Denali fault in the central Alaska Range, unnamedSantonian-Campanian marginal-marine strata and Oligocene non-marine strata overlie the Kahiltna assemblage above an angularunconformity (Trop et al., 2004). North of the Denali fault, theKahiltna assemblage is overlain by Campanian-Maastrichtiannonmarine and marginal-marine strata (lower Cantwell Forma-tion; Figures 1D, 2; Ridgway et al., 1997).

Nutzotin BasinExhumed strata of the Nutzotin basin crop out in a 15- to 35-

km-wide and 250-km-long belt in the Nutzotin and MentastaMountains of east-central Alaska (Fig. 1D; Richter, 1976; Mac-

Kevett, 1978). The basin fill consists of three stratigraphic units:a 3-km-thick Upper Jurassic-Lower Cretaceous sedimentary suc-cession dominated by submarine-fan strata (Nutzotin Mountainssequence), a 3-km-thick Lower Cretaceous volcanic succession(Chisana Formation), and ,90 m of unnamed Paleogene(?) non-marine sedimentary and volcaniclastic strata (Fig. 2; Table 1;Berg et al., 1972; Richter, 1976; Manuszak and Ridgway, 1999;Manuszak et al., this volume). South of the Totschunda fault,lowermost strata of the Nutzotin Mountains sequence discon-formably overlie Upper Triassic strata of the Wrangellia terrane.Throughout most of the outcrop belt, however, a north-dippingdécollement or the Totschunda fault juxtaposes the NutzotinMountain sequence against the Wrangellia terrane (Manuszak,2000; Manuszak et al., this volume). The Denali fault juxtaposesthe Nutzotin Mountains sequence against metamorphic rocks ofthe Yukon-Tanana terrane along the northern boundary of the out-crop belt (Fig. 1D). Estimates of up to 400 km of Late Cretaceous-Cenozoic right-lateral displacement along the Denali fault arebased partly on the interpretation that the Nutzotin Mountainssequence is laterally offset from the Dezadeash Formation, whichis exposed in the Yukon Territory on the opposite (north) side ofthe fault (Eisbacher, 1976; Nokleberg et al., 1985; Lowey, 1998).

Cantwell BasinThe Cantwell basin is defined by the 45-km-wide and 135-

km-long outcrop belt of the Cantwell Formation in the centralAlaska Range (Fig. 1D). Two distinct lithologic units make up the Cantwell Formation, a .3-km-thick Upper Cretaceous(Campanian-Maastrichtian) sedimentary unit (lower CantwellFormation; Figure 2; Ridgway et al., 1997) and a .3-km-thickPaleocene-Eocene volcanic unit (upper Cantwell Formation; Fig-ure 2; Cole et al., 1999). Locally, an angular unconformity sepa-rates the sedimentary and volcanic units records a 10–20 m.y.hiatus (Cole et al., 1999). Nonmarine to marginal-marine strata of the lower Cantwell Formation depositionally overlie marinestrata of the Kahiltna assemblages along a regional angularunconformity. The southern structural limit of the basin is definedby a series of south-dipping thrust faults that are truncated by theeast-west-trending Denali fault (Ridgway et al., 1997). Northernbasin margin strata of the Cantwell basin are juxtaposed againstPaleozoic metamorphic rocks of the Yukon-Tanana terrane alongthe Hines Creek fault (Fig. 1D; Trop and Ridgway, 1997).

Tanana BasinThe Holocene Tanana basin, a 22,000 km2 alluvial and

swampy lowland, is located north of the Alaska Range and southof the Yukon-Tanana uplands (Fig. 1D). Large braided-stream sys-tems flow transverse to the Alaska Range (Kantishna, Nenana, andDelta rivers) and merge into an axial drainage (Tanana River) ori-ented subparallel to the Alaska Range (Fig. 1D; Lesh, 2002; Leshand Ridgway, this volume). Deposits of the Tanana basin includealluvial, fluvial, and lacustrine strata with a cumulative thickness.1.6 km (Usibelli Group, Nenana Gravel, and Quaternary surfi-cial deposits; Figure 2; Table 1). The Usibelli Group and Nenana


64 Trop and Ridgway

Gravel crop out in a ,225-km-long and 30- to 50-km-wide outcropbelt in the northern foothills of the Alaska Range; correlative strataoccupy the subsurface of the basin based on exploratory drillholes(Nenana #1 and Totek Hills #1 on Fig. 1D) and geophysical sur-veys (Stanley et al., 1990a; Ridgway et al., 2002, this volume). TheUsibelli Group is .800 m thick and consists of five sedimentaryformations with Late Eocene to Late Miocene age ranges, althoughthe bulk of the strata are Miocene age (Fig. 2; Wolfe and Tanai,1980; Leopold and Liu, 1994). North of the Hines Creek fault, anangular unconformity separates the Usibelli Group from under-lying metamorphic rocks of the Yukon-Tanana terrane. South of thefault, spatially limited outcrops of the Usibelli Group overlie theCantwell Formation along an angular unconformity (Csejtey et al.,1992; Thoms, 2000). Pliocene conglomerate up to 1200 m thick(Nenana Gravel) and Quaternary proglacial deposits overlie theUsibelli Group (Ridgway et al., 2002, this volume).

Northway BasinThe Holocene Northway basin is a fluvial and swampy low-

land area of ,3000 km2 located between the Wrangell-St. EliasRange to the south and the Yukon-Tanana uplands to the north(Fig. 1D). Proglacial braided rivers flow transverse to theWrangell-St. Elias and Nutzotin-Mentasta ranges (Chisana andNabesna rivers on Fig. 1D) and merge into an axial fluvial system(Tanana River on Fig. 1D) that drains the basin. Pleistocene-Holocene strata were deposited in alluvial, fluvial, eolian, andlacustrine environments (Richter, 1976). Basinal strata are lessthan 1000 m thick, overlie metamorphic rocks of the Yukon-Tanana terrane, and are juxtaposed against strata of the Nutzotinbasinby the Denali fault (Fig. 1D; Richter, 1976; Kirschner, 1994).

Outboard Margin Basins

Jurassic-Miocene sedimentary basin strata are exposed alongthe outboard (southern) margin of the Wrangellia composite ter-rane in the Wrangell Mountains and in the Matanuska Valley-southernTalkeetnaMountainsarea(Fig.1D).Correlativestrataarepreserved in the subsurface of the Holocene Susitna and CopperRiver basins (Fig. 1D) based on exploratory drillholes and gravitysurveys.Cenozoic sedimentarybasinstrataarealsoexposedon theYakutat terrane in the Robinson Mountains (Fig. 1D).

Matanuska Valley–Talkeetna Mountains BasinThe Matanuska Valley–Talkeetna Mountains basin consists

of an ,90-km-long and 20- to 70-km-wide belt of Mesozoic-Cenozoic sedimentary strata that crop out in the Matanuska Val-ley, southern Talkeetna Mountains, and northern ChugachMountains (Fig. 1D). Middle Jurassic-Upper Cretaceous marinestrata .3800 m thick and Paleocene-Oligocene nonmarine strata.2900 m thick unconformably overlie Lower to Middle Jurassicigneous rocks of the accreted Talkeetna oceanic arc (Table 1;Grantz, 1964; Clardy, 1974; Fuchs, 1980; Little, 1988; Flores andStricker, 1993; Trop and Ridgway, 1999; Trop et al., 2003, 2005a,

2005b; Trop and Plawman, 2006). The Border Ranges fault placessouthern basin margin strata in the hanging wall against theChugach subduction complex in the footwall (Fig. 1D). Northern-most strata of this basin are juxtaposed against Jurassic arcplutons along the Little Oshetna fault (LOF on Figure 1D; Tropet al., 2005a). The Castle Mountain fault system bisects the basin(Fig. 1D).

Wrangell Mountains BasinStrata of the Wrangell Mountains basin crop out in a ,55-km-

wide and ,120-km-long belt in the Wrangell-Saint Elias Moun-tains and Chitina Valley (Fig. 1D). Basinal strata include .4,500 mof Middle Jurassic-Upper Cretaceous marine sedimentary strataand .600 m of Miocene nonmarine sedimentary and volcanicstrata (Fig. 2; MacKevett, 1978; Trop et al., 1999, 2002; Tidmoreet al., 2005). These strata unconformably overlie the Wrangelliaand Alexander terranes; they crop out north of Upper Jurassic arcrocks (Talkeetna-Chitina arc) and south of Lower to Upper Creta-ceous arc rocks (Figs. 1A, 1D; Chisana and Kluane arcs). Southernbasin margin strata are faulted against the Chugach subductioncomplex along the Border Ranges fault (Fig. 1D). North of the Bor-der Ranges fault, south-dipping thrust faults (Chitina thrust belt)juxtapose Jurassic arc rocks and the Wrangellia terrane againstJurassic sedimentary strata. Regional folds and the Totschundafault separate northern basin margin strata of the Wrangell Moun-tains basin from the Nutzotin basin to the north. Strata of the Nut-zotin and Wrangell Mountains basins record different depositionalhistories in separate depocenters (Trop et al., 2002; Manuszak et al., this volume).

Susitna BasinThe Holocene Susitna basin is a fluvial and swampy lowland

of ,13,000 km2 located between the Alaska Range on the northand west, the Talkeetna Mountains on the east, and Cook Inletbasin on the south (Fig. 1D). Modern glacially influenced fluvialsystems (e.g., Susitna, Kahiltna, and Yentna rivers) flow south-ward from the Alaska Range and merge into a major trunk system(Susitna River) that drains into the upper Cook Inlet estuary. Con-sidered a northern extension of Cook Inlet basin, the Susitna basinis bisected by the Castle Mountain fault (Haeussler, 1998; Haeus-sler et al., 2002). Exploratory drillholes and gravity surveys pene-trate .4 km of Paleocene-Miocene conglomerate, carbonaceoussandstone, mudstone, and coal deposited in fluvial-lacustrineenvironments (e.g., Hackett, 1977; Merritt, 1986; Meyer et al.,1996; Meyer and Boggess, 2003b). Quaternary deposits up to180 m thick record deposition in fluvial and glacial environ-ments influenced by at least five Pleistocene glacial episodes(Karlstrom, 1964). Tertiary strata unconformably overlie a 4- to6-km-thick succession of Mesozoic-Paleozoic sedimentary strata(Hackett, 1977).

Copper River BasinThe Holocene Copper River basin is a 4500 km2 fluvial-

lacustrine lowland that separates the Matanuska Valley-Talkeetna



Mountains and Wrangell Mountains basins (Fig. 1D). Eleven ex-ploratory drillholes encountered up to 580 m of Tertiary strata andup to 1715 m of Upper Jurassic-Upper Cretaceous strata (AlaskaGeological Society, 1970a, 1970b; Wilson et al., 1998). Maxi-mum thicknesses of 1200 m and 7000 m are inferred for the Ter-tiary and Jurassic-Cretaceous sequences, respectively, based onseismic profiles (Fuis and Plafker, 1989), aeromagnetic data(Case et al., 1986; Meyer and Saltus, 1995), and gravity profiles(Andreasen et al., 1964; Barnes et al., 1994; Meyer and Boggess,2003a).

Yakutat BasinSedimentary strata of the Yakutat basin depositionally over-

lie the Yakutat terrane and crop out in an ,50-km-wide and,190-km-long belt in the Robinson Mountains along the south-eastern coastline of Alaska (Figs. 1D, 2; Miller, 1961, 1971;Plafker, 1967, 1987; Winkler and Plafker, 1993). Cenozoic strataof theYakutat basin include: (1) the Upper Paleocene-Eocene non-marine to shallow-marine coal-bearing Kulthieth Formation andlateral submarine-slope facies equivalents (Stillwater and TokunFormations) that locally exceed 3 km in thickness, (2) Oligocene-Lower Miocene marine and tuffaceous strata of the Poul CreekFormation (,2 km thick), and (3) Middle Miocene-Holoceneglacial marine strata of the Yakataga Formation (up to ,5-km-thick; Fig. 2; Table 1). These strata have been incorporatedinto a south-verging, thin-skinned fold-and-thrust belt attributableto underthrusting of the Yakutat terrane beneath the southernAlaska margin along the Chugach-Saint Elias fault (Fig. 1D;Bruhn et al., 2004).

TECTONIC AND PALEOGEOGRAPHIC MODEL FORBASIN DEVELOPMENT AND CONTINENTALGROWTH

This section summarizes our interpretation of the tectonic andpaleogeographic development of southern Alaska, emphasizingthe evolution of sedimentary basins and related syndepositionalstructures. Our interpretations build upon previous tectonic mod-els (e.g., Plafker and Berg, 1994; Nokleberg et al., 2001) by incor-porating the results of recent detailed investigations of sedimentarybasins, syndepositional structures, and magmatic belts. Schematicpaleogeographic reconstructions and cross sections summarizingkey aspects of Mesozoic and Cenozoic tectonics and basin devel-opment are shown in Figures 3 and 4, respectively. These new fig-ures emphasize our preferred tectonic interpretations based on thepresently available geologic information; alternative interpreta-tions are discussed within the text. Higher-resolution stratigraphic,geochronologic, and structural investigations are needed to de-velop more detailed palinspastic reconstructions. Some geologicevents span the time range shown on specific maps and/or crosssections; that is, there is a continuum in some cases between dis-tinct stages shown on Figures 3 and 4 and those discussed in thetext. The time scale and absolute ages are from Palmer and Geiss-man (1999).

Middle Jurassic (Bajocian-Callovian; 176–159 Ma):Intraoceanic Arc Construction and Sedimentation

Middle Jurassic sedimentary strata accumulated in shallow-marine depocenters that fringed a south-facing oceanic island-arc(Talkeetna arc; Figs. 3A, 4A). An oceanic subduction setting isbased on detailed geochemical and stratigraphic investigations ofJurassic arc-related igneous rocks (e.g., Burns, 1985; Plafkeret al., 1989; DeBari and Coleman, 1989; DeBari and Sleep, 1991;Kelemen et al., 2003; Clift et al., 2005a, 2005b; Draut and Clift,2006). North-dipping subduction is also indicated by the locationof Jurassic arc rocks inboard (north) of age-equivalent (but poorlydated) north-dipping subduction complex strata (McHugh Com-plex on Figs. 3A, 4A; Plafker et al., 1994); moreover, geo-chemical data from arc rocks (Talkeetna Formation) also indicatenorthward reduction in subduction influences (Clift et al., 2005a).In the Matanuska Valley-Talkeetna Mountains basin, MiddleJurassic sedimentary strata (Tuxedni and Chinitna Formations)were deposited in a forearc setting (MB on Figs. 3A, 4A), out-board (south) of Middle to Upper Jurassic arc plutons and inboard(north) of the subduction complex and remnant Lower Jurassicarc plutons (Nokleberg et al., 1994; Trop et al., 2005a). In thesouthern Wrangell Mountains, the Middle Jurassic Nizina Moun-tain Formation records deposition in a backarc position, inboard(north) of Upper Jurassic arc plutons (WB on Figs. 3A, 4A;Chitina arc of Plafker et al., 1989; Roeske et al., 1989, 2003). Age-correlative strata were not deposited or are not preserved alongthe inboard (northern) margin of the Wrangellia composite ter-rane (Figs. 2, 3A, 4A).

In general, forearc and retroarc Middle Jurassic sedimentarystrata exposed in the southern Talkeetna Mountains and WrangellMountains, respectively, were deposited in relatively low-gradientmarine shelf environments based on sedimentological and paleon-tological data (Imlay and Detterman, 1973; Detterman et al., 1996;Nokleberg et al., 1994; Trop et al., 2002). Measured stratigraphicsections document mainly mudstone and sandstone along withsparse tuff, and conglomerate. Sandstone and conglomerate com-positional data demonstrate that these strata contain mainly vol-canic and minor plutonic lithic detritus, consistent with erosion of a partially dissected magmatic arc (Fig. 5A). Volcaniclasticsandstone from the southern Talkeetna Mountains yields zirconswith exclusively Early to Middle Jurassic isotopic ages (Amato etal., this volume, chapter 11), overlapping isotopic ages of nearbyTalkteene arc rocks (Rioux et al., 2005). Sparse tuff interbeddedthroughout the section records intermittent volcaniclastic eruptions(Fig. 6A). The dominance of juvenile igneous detritus, localizationof sediment accumulation within the Jurassic arc platform, andpresence of primary volcanic strata are all consistent with Mid-dle Jurassic sedimentation being related to erosion of adjacentoceanic arc rocks prior to collision with inboard terranes (Tropet al., 2002, 2005a; Clift et al., 2005b).

Middle Jurassic sedimentation in the Matanuska Valley-Talkeetna Mountains basin is also interesting because it is linkedwith a shift in the locus of arc magmatism. Arc plutons exposed


Oceanic Plate

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DC

subduction erosion?

subduction erosion?

conjectural Lower Jurassic forearc deposits

DBBLF

CTB

?

?

Onset of subsidence and sedimentationalong suture zone. Sediment transport away from Wrangellia composite terrane on submarine-fan environments (NB, DB).

Onset of subsidence and sedimentation along suture zone. Sediment transport away from Wrangellia composite terrane on submarine fans (KB; Kahiltna basin in the northern Talkeetna Mtns).

Rapid exhumation of Jurassicarc plutons along Chitina thrust belt (CTB). Fan-deltadeposition in forearc (MB) and retroarc (WB) depocenters. Possible displacement on Bruin Bay-Little Oshetna fault (BLF).

Oceanic Plate

M

A

N

A 176-159 MaMiddle JurassicBajocian-Callovian

FKB

Wra

ngel

liaco

mpo

site

terr

ane

FKB

Future position of U.Jurassic plutons

separation unknown

separation unknown

FNB

FNB

Inboard Terranes

NB

Initial collision and underthrusting of inboard margin of Wrangellia composite terrane along regional suture zone.

?

?

?

WB

MB

Coast Plutonic Complex

Inboard Terranes (Yukon-Tanana/Stikine?)

MB

Initial deposition of McHugh Complex?

conjectural Lower Jurassic forearc depositsFine-grained deposition in localized marine depocenters within intra-arc (MB) and retroarc (WB) basins within an oceanic arc (Talkeetna-Chitina arc).

Figure 3. (continued on following pages)Sketch maps showing the inferred paleo-geographic evolution of the Wrangelliacomposite terrane and associated tectonicelements. See Figure 4 for companionpaleotectonic cross sections. See Table 2for explanation of abbreviations and pat-terns. Paleolatitudes are not shown due to uncertainty in the paleoposition of the Wrangellia composite terrane withrespect to North America (see Cowan et al., 1997, for review). Current distancebetween the towns of Anchorage (#A) and McCarthy is ,390 km. See text fordiscussion.

Oceanic Plate

A

N

C 144-112 MaEarly Cretaceous Berriasian-Aptian

KB

Wra

ngel

liaco

mpo

site

terr

ane

Yuk

on-T

anan

ate

rran

e?

UNB

DC

DB

BLF

CTB

?

?

Continued subsidence and submarinefan deposition in Nutzotin basin along suture zone. Sediment transport away from Wrangellia composite terraneon submarine-fan environments (NB, DB).

Continued subsidence and submarinefan deposition in Kahiltna basin (KB) along suture zone. Sediment transport away from Wrangellia composite terrane.

Underthrusting of Wrangellia composite terrane along suture zone. Retrograde metamorphism and development of regional anticlinorium.

Future CastleMountain fault

Inboard Terranes

WBM

NBMB

Localized arc magmatism (Chisana arc) along inboard margin of Wrangellia terrane.

Continued shortening along outboard margin prompts upliftof remnant retroarc (WB) andforearc basins (MB). Minorcalcareous shallow-marinedeposition locally.

Conjectural Jurassic forearc basin deposits


Oceanic Plate

M

A

Jurassic-Cretaceous subduction complex deposits (McHugh Complex)

N

D 112-83 MaEarly Late CretaceousAlbian-Santonian

WB

KB UKB

?

?

Wra

ngel

liaco

mpo

site

terr

ane

Chu

gach

terr

ane

Renewed deposition in outboard basins (WB,MB) influenced by normal faults. Southward sediment transport from remnant Jurassic-Cretaceous arcs uplifted along suture zone.

Yuk

on-T

anan

ate

rran

e

UNB

Southward thrusting and folding of marinecollisional basin deposits (UNB, UDB, UKB)and Chisana arc rocks along north-dipping decollement (LD). Folded basinal strata locally intruded by 110-90 Ma plutons.

UDB

Diachronous westward closure andsubaerial uplift of Kahiltna collisional/remnant ocean basin deposits (UKB).

LDFuture CastleMountain fault

Westward sediment transport in the Kahiltna basin (KB) on fluvial, shallow-marine, and submarine-fan deposystems. Detritus derived from Yukon-Tanana terrane and Wrangellia composite terrane.

M

TKF

T

MB


F 61-33 MaLate Paleocene-Eocene

Resurrection

PlateRidgeKula

Plate

Ridge migration

UWBM

A

MB

UNB

VCS

CB

Cenozoic subduction complex

Uplifted Mesozoic subduction complex

50 Ma52 Ma

57 Ma58 Ma

60 Ma

62 Ma

Orocline

Hinge

Area

Spreading

?

CV

CIB

TK

DF

30o-50

orotation of

southwestern Alaska

Arc magmatism (TK) until ca. 56 Ma.

Alluvial-fluvial-estuarine deposition in southwestward-tilted, two-sided forearc basin (CIB, MB). Diachronous uplift and conglomerate deposition in response to ridge subduction.

Diachronous near-trench plutonism within subduction complex during northeastward subduction of spreading ridge oriented obliquely to trench.

M

Latest Cretaceous subduction complex strata (Valdez Group)

Pre-latest Cretaceous subduction complex strata (McHugh Complex)

A

E 83-68 MaLatest CretaceousCampanian-Maastrichtian

CB

DF

MB

Oceanic Plate

CRB

erodederoded

Kluane arc

UDB

Alluvial-fluvial-lacustrine deposition (lower Cantwell Fm.) in thrust-top basin (CB). Detritus derived fromformer continental margin and uplifted marinecollisional basin deposits (UKB).

Coarse-grained submarine fandeposition in forearc basins (MB, WB) record erosion ofcoeval and remnant arcs.

Probable onset of major right-lateral displacement along Denali fault (DF), shuffling marine collisional basin strata (UKB, UNB, UDB).

Continental arc magmatism across both the Mesozoic continental margin (Yukon-Tanana terrane) and accreted Wrangelliacomposite terrane.

WB

Rapid deposition/accretionof submarine-fan strata (Valdez Group) into Chugach subduction complex.

Wra

ngel

liaco

mpo

site

terr

ane

Chu

gach

terr

ane

Yuk

on-

Tana

nate

rran

e

Wra

ngel

liaco

mpo

site

terr

ane

Yuk

on-T

anan

ate

rran

e

Chugach Terrane


VCS

eroded

N

UNB

UKBUKB

AK Range-Talkeetna Mtns. arc

CTV

JV

UKB

UKBMagmatism (60-55 Ma, CB; 60-50 Ma; JV) within remnant collisional basins along suture zoneattributable to remnant mantle wedge from earlier arc magmatism. Slab-window magmatism

(CTV, CV, MI, PP)attributable to spreadingridge subduction.

PI

T

MI


TB

F

M

A

NOB

DF

WV

SB

DF

CMF

CRB

CIB

BHF

Yakutat terrane

TAF

BHF

Alluvial-fluvial-lacustrine deposition inretroarc foreland basins (TB, NOB) inboard of Denali fault (DF) and Alaska Range suture zone.

Dextral transpressive shortening along Denali fault(DF) prompts uplift of Mesozoic collisional basins (UCB, UKB, UNB). Area of high seismicityand large earthquakes.

Continental arc magmatism(WV, AA) and alluvial-lacustrinedeposition in intra-arc basins.

Collision and underthrusting of Yakutat terrane against subduction complex. High seismicity/large earthquakes. Development of collisional foreland basins with high sedimentation rates.

H 26-0 MaLatest Oligocene-

Neogene

Localized alluvial-fluvialdeposition in partitioned forearc basins (CVB, MB, CIB).AA

MB

Pacific

Plate

CVB

Uplifted Mesozoic subduction complex (CT)

Cenozo

ic subduction complex (PWT)

Cenozoic subduction complex (PWT)

UWBM

UNB

UCB

DF

A

Proto-YakutatTerrane

BHF

MB

G 33-26 MaOligocene

Alluvial-fluvial-lacustrine deposition (Healy Ck. Fm.) in Tanana foreland basin (TB), inboard of Denali fault (DF) and uplifted collisional basins (UCB, UKB).

CMF

MCB

Coarse-grained alluvial deposition (Tsadaka Fm.) in remnant forearc basin (MB) coeval with oblique right-lateral displacement on Castle Mtn. fault (CMF).

Continental arc magmatism in Aleutian-Alaska arc (43-30 Ma) and alluvial-fluvial deposition (Hemlock Conglomerate,Tyonek Fm.) in Cook Inlet forearc basin.

DF

Pacific

Plate

Wra

ngel

liaco

mpo

site

terr

ane

Yuk

on-T

anan

ate

rran

e

CIBChugach Terrane


TF

YTT

YTT

Uplifted Mesozoic subduction complex (CT)

TKF

Possible subsidenceand nonmarine depositionin Copper River basin

UKB

CCBUKB

UKB

UNB

UCB

TKF

TB

WCB

TB

Regionally extensive dextral displacement along Denali fault (DF) prompts alluvial-fluvial-lacustrine deposition in narrow fault-bound basins (CCB, MCB).

WMB




immediately north of the Border Ranges fault yield Early Juras-sic crystallization ages (ca. 201–181 Ma), whereas plutonsexposed to the north in the Talkeetna Mountains yield Middle toLate Jurassic crystallization ages (ca. 177–156 Ma; Figures 1A,3A, 4A; Clift et al., 2005b; Rioux et al., 2005). The northwardshift in magmatism was coeval with the onset of Middle Jurassicsedimentation based on the oldest age-diagnostic fossils from theTuxedni Formation (Fig. 2; Bajocian; ca. 177 Ma; Imlay, 1982,1984). Middle Jurassic forearc basin strata unconformablyonlapped marine volcanic strata of the remnant Lower Jurassic arcplatform. Middle Jurassic northward migration of arc magmatismand deposition is attributable to shallowing of the subducting slab(e.g., Plafker et al., 1989) and/or tectonic removal of Lower Juras-sic forearc strata by subduction erosion (Clift et al., 2005b).

Subduction shallowing and tectonic erosion may alsoaccount for the apparent absence of Jurassic forearc basin stratain the southern Wrangell Mountains region, where Upper Juras-sic plutons are juxtaposed directly against the subduction com-plex (Fig. 1D). Inferred Lower to Middle Jurassic arc plutons andassociated forearc basin strata in the Wrangell Mountains regionare interpreted to have been tectonically removed via exhuma-tion, strike-slip truncation, and/or subduction erosion (Figs. 3A,

3B; Pavlis et al., 1988; Plafker et al., 1989; Roeske et al., 1989,1992; Trop et al., 2005a; Clift et al., 2005b).

Late Jurassic (Oxfordian-Tithonian; 159–144 Ma):Collisional Orogenesis and Oceanic Arc Exhumation

Regional basin development and coarse-grained sedimenta-tion throughout southern Alaska commenced during Late Juras-sic (Oxfordian-Kimmeridgian) time in response to shortening,uplift, and erosion of multiple structural levels of the Wrangelliacomposite terrane (Manuszak and Ridgway, 1999; Trop et al.,2002, 2005a; Eastham and Ridgway, 2002; Hampton et al., 2003,this volume; Manuszak et al., this volume). Early to Late Jurassicoceanic-arc magmatism ceased during latest Jurassic time, con-current with crustal-scale shortening, exhumation, and coarse-grained sedimentation (Clift et al., 2005b; Amato et al., thisvolume, chapter 11; Figs. 3B; 4B). In the Wrangell Mountains, the Upper Jurassic Kotsina Conglomerate and Upper Root Glac-ier Formation record development of a thrust belt and narrowretroarc foreland basin inboard (north) of Jurassic arc plutons(Chitina thrust belt/CTB on Figures 3B, 4B). Conglomerate locallycovers south-dipping faults within the thrust belt, recording Late

Geographic references

A - Anchorage

F - Fairbanks

M - McCarthy

N - Nabesna

Outboard Margin BasinsCIB - Cook InletCRB - Copper River CVB - Chitina ValleyMB - Matanuska - S. Talkeetna Mtns.SB - SusitnaWB - Wrangell Mountains(Prefixed with U=Uplifted)(Prefixed with F=Future position)

TerranesAT - Alexander CT - ChugachPT - Peninsular PWT - Prince WilliamWT - WrangelliaYT - Yakutat YTT - Yukon-Tanana

Alluvial

Fluvial

Lacustrine

Fan-Delta/

Inboard Margin BasinsCB - CantwellCCB - Colorado CreekDB - Dezadeash KB - Kahiltna MCB - MacCallum CreekNOB - Northway NB - NutzotinTB - TananaWCB - Watana Creek(Prefixed with U=Uplifted)(Prefixed with F=Future position)

Faults, Shear Zones BHF - Border Ranges-Hanagita faultBLF - Bruin Bay-Little Oshetna faultCMF - Castle Mountain faultCTB - Chitina thrust beltDF - Denali faultHCF - Hines Creek faultLD - Lost Creek decollementTF - Totschunda faultTAF - Taral faultTKF - Talkeetna faultVCS - Valdez Creek shear zone

Volcanic Fields, Plutonic BeltsAA - Aleutian-Alaska arcCV - Caribou Creek volcanicsCTV - Central Talkeetna Mtns. volcanicsJV - Jack River volcanics/plutonsMI - Matanuska Valley intrusivesPI - Prince William Sound intrusivesTK - Talkeetna Mountains-Kluane arcWV - Wrangell volcanic field

Depositional Features

Magmatism

Deformational Features

Locus of active volcanism

Paleocene-Eocene intrusives

Mid-Cretaceous Chisana arc rocks

Upper Jurassic Talkeetna-Chitina arc rocks

Middle Jurassic Talkeetna arc rocks

Lower Jurassic Talkeetna arc rocks

Active subduction zone (barb on downgoing plate)

Active crustal shortening (barb on thrust sheets)

Regional topographic uplifts via folding/faulting

Active crustal extension via normal faults

Regional dextral strike-slip fault

Strike-slip movement away ( ) or toward ( )

Uplifted basinal strata

Active depositional basin

Marine environments

Submarine Fans

Prodelta Slope

Marine Shelf

TABLE 2. EXPLANATION FOR PALEOGEOGRAPHIC MAPS AND CROSS SECTIONS SHOWN IN FIGURES 3 AND 4


70 Trop and Ridgway

NAM LJ

NF

UJ

?

Former continental margin Wrangellia oceanic crustPeninsular terrane

RGF KCTA/CA BLF

UJ - Upper Jurassic plutonsMJ - Middle Jurassic plutonsLJ - Lower Jurassic plutonsTA/CA - Talkeetna/Chitina arc

RGF - Root Glacier Fm.KC - Kotsina ConglomerateCTB - Chitina thrust beltBLF - Bruin Bay-Little Oshetna faultNF - Naknek Fm.

MJUJUJ LK

Former continental margin Wrangellia acc. prism oceanic crustPeninsular terrane

uplift, unconformity development

LJ

LK - Lower Cretaceous plutonsUJ - Upper Jurassic plutonsMJ - Middle Jurassic plutonsLJ - Lower Jurassic plutonsCTB - Chitina thrust belt

YTT?MJLK UJ

Former continental margin Wrangellia oceanic crustPeninsular terrane

KF MCF/SF CF

N/NE S/SW

LK - Lower Cretaceous plutonsUJ - Upper Jurassic plutonsMJ - Middle Jurassic plutonsLJ - Lower Jurassic plutonsLD - Lost Creek decollement

KF - Kennicott Fm.MCF - Moonshine Creek Fm.SF - Schultze Fm.CF - Chititu Fm.

D 112-83 MaEarly Late Cretaceous(Albian-Santonian)

B 159-144 MaLate Jurassic(Oxfordian-Tithonian)

C 144-112 MaEarly Cretaceous (Berriasian-Aptian)

MJNAM LJCPC

McHugh Complex?

Former continental margin Wrangellia oceanic/transitional

crust?

acc.prism oceanic crustPeninsular terrane

Talkeetna Oceanic ArcMJ - Middle Jurassic plutonsLJ - Lower Jurassic plutons

Arc-Related Marine Basins Intra-arc Deposition - MB (Chinitna, Tuxedni Fms.)Retroarc Deposition - WB (Nizina Mountain Fm.)

A 176-159 MaMiddle Jurassic(Bajocian-Callovian)

WB MB

CPC - Middle Jurassic plutons;Coast Plutonic Complex?

acc.prism

CTB

LD

CTB

McHugh Complex?

McHugh Complex?

McHugh Complex?

Nutzotin and Kahiltna basins

T

Anticlinorium

CPC - Late Jurassic plutons;Coast Plutonic Complex?

CPC



N/NE

N/NE

N/NE

MJ

S/SW

S/SW

S/SW

YTT?

YTT-Yukon-Tanana terrane?

YTT-Yukon-Tanana terrane?

Figure 4. (continued on the following page) Schematic cross sections showing generalized tectonic development of south-central Alaska duringMesozoic-Cenozoic time. See Figure 1D for explanation of patterns. See text for discussion and references. See Figure 3 for companion paleo-geographic maps.

Jurassic syndepositional thrusting (Figs. 6B, 7A; Trop et al.,2002). Deposition occurred on fan-deltas (Kotsina Conglomer-ate) that merged northward into submarine fans (Upper RootGlacier Formation). Compositional and detrital geochronologicdata document exhumation of Upper Jurassic plutons and multi-ple structural levels of the Wrangellia terrane (Fig. 6C). LateJurassic retroarc thrusting and synorogenic sedimentation alongthe Chitina thrust belt was coeval with regional downflexure andforeland basin development ,60–100 km inboard of Jurassicarc plutons in the Kahiltna and Nutzotin basins (KB, NB on Fig-ure 3B; Manuszak, 2000; Manuszak et al., this volume; Eastham,2002; Eastham and Ridgway, 2002; Hampton et al., 2003, thisvolume). Sedimentologic data indicate sediment flux away from

the Wrangellia composite terrane. Paleocurrent and lithologicdata document northward- to northwestward-directed sedimentgravity flows on submarine fans in the Kahiltna basin of thenorthern Talkeetna Mountains (KB on Fig. 3B) and northward- toeastward-directed sediment gravity flows on submarine fans inthe Nutzotin basin (NB on Fig. 3B). Paleocurrent, detritalgeochronologic, and paleoslope data from correlative strata in theYukon Territory (Dezadeash Formation) also indicate sedimentflux away from the Wrangellia composite terrane (DB on Fig. 3B;Eisbacher, 1976; Lowey, 2006).

Rapid exhumation and coarse-grained sedimentation alsocharacterized the outboard margin of the Wrangellia composite ter-rane during Late Jurassic time (Figs. 3B, 4B). In the Matanuska



Valley-Talkeetna Mountains basin, coarse-grained successionsfrom 800 to 3000 m thick were deposited in a forearc basin setting(Naknek Formation; MB on Figure 3B), judging from their loca-tion between Middle to Upper Jurassic arc plutons to the north andChugach subduction complex deposits to the south. Sedimento-logic, compositional, and geochronologic data from the UpperJurassic forearc strata indicate rapid exhumation of Middle toUpper Jurassic felsic plutons of the remnant Talkeetna arc (Fig. 5A;Detterman et al., 1996; Trop et al., 2005a; Clift et al., 2005a). Ma-rine sediment gravity flows transported sediment southward on un-stable, trenchward-dipping fan deltas (Figs. 6D, 7B). Exhumationand sedimentation may have been influenced by syndepositionaldisplacement on arcward-dipping reverse faults, including theBruin Bay and Little Oshetna faults (BLF on Figures 3B, 4B).

Two general tectonic models have been proposed to accountfor regionally extensive Late Jurassic shortening, exhumation, and

sedimentation in south-central Alaska: (1) collision of the combinedWrangellia, Alexander, and Peninsular terranes with inboard ter-ranes (i.e., Yukon-Tanana and Stikine; Nokleberg et al., 2001; Ridg-way et al., 2002; Trop et al., 2002, 2005a) and (2) collision of theTalkeetna arc (Peninsular terrane) with the previously combinedWrangellia and Alexander terranes, prior to Cretaceous collisionagainst inboard terranes (Clift et al., 2005a, 2005b). A potentialweak point in the latter model is that the Peninsular and Wrangelliaterranes appear to have a shared history prior to the proposed colli-sion event. For example, cross-cutting intrusions demonstrate amal-gamation of the Wrangellia and Alexander terranes by MiddlePennsylvanian time (Gardner et al., 1988), and overlap assemblageslink the Peninsular and Wrangellia terranes by Late Jurassic time(Plafker et al., 1989; Trop et al., 2005a). In addition, the Wrangelliaand Peninsular terranes apparently contain lithologically similarPermian-Triassic strata, indicating a potentially longer shared

CCBTC

Former continental margin Wrangellia accretionary prism oceanic crustPeninsular terrane

DF Orca GroupCMF

DF - Denali faultCCB - Colorado Creek strataCMF - Castle Mountain faultTC - Tsadaka Conglomerate

?

NAM

Former continental margin Wrangellia accretionary prism oceanic crustPeninsular terrane

VCSDF Valdez GroupUKHCF LCF CPF MR/MF

HCF - Hines Creek faultDF - Denali faultLCF - Lower Cantwell Fm.CPF - Caribou Pass Fm.VCS - Valdez Creek shear zone

MR - MacColl Ridge Fm.MF - Upper Matanuska Fm.UK - Upper Cretaceous plutons(Talkeetna Mtns.-AK Range mag. belt)

accretionary prism

DF CMFTB WV/FF

UG - Usibelli groupNG - Nenana GravelDF - Denali faultWV/FF - Frederika Fm./Wrangell LavaCMF - Castle Mountain fault

Oceanic crust

ARF/WF CKF

Former continental margin Wrangellia accretionary prism oceanic crustPeninsular

UCF DF Orca Group

UCF - Upper Cantwell Fm.DF - Denali faultVCS - Valdez Creek shear zone

BHF

ARF - Arkose Ridge Fm.WF - Wishbone Fm.CKF - Chickaloon Fm.BHF - Border Ranges fault

Yakutat terrane

Yakutat terrane

H 26-0 MaLatest Oligocene-Neogene

G 33-26 MaOligocene

F 61-33 MaLate Paleocene-Eocene

E 83-68 MaLatest CretaceousCampanian-Maastrichtian

VCS

Growing Wrangell arcN/NE

N/NE

N/NE

N/NE

S/SW

S/SW

S/SW

S/SW

Collisional Shortening/Sedimentation

near-trench intrusives

UG/NG



0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360

Mesozoic Paleozoic Precambrian

1000 1400 1800

206Pb/238U Age (Ma)

Num

ber

ofG

rain

s

LA-ICPS-MS zircon ages (n=71)Northern Talkeetna Mountains Youngest detrital zircons = 74-75 Ma

Accreted arc

Coeval arc

Formercont.

margin

60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440

SHRIMP-RG zircon ages (n=81)Matanuska ValleyMatanuska Formation (sandstone)Maastrichtian fossils and zirconsYoungest detrital zircon = 72-71 Ma

206Pb/238U Age (Ma)

Num

ber

ofG

rain

s

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Accreted arc

Coeval arc

0

o

oo

o

o

oo

oo

o

ooooooo

ooo

oo

oo

oo

oo

o

o

ooo

o

100

200

300

400

500

600

1300

1200

1100

1000

900

800

700

Percent (%)10 20 30 40 50 60 70 80 90 1000

Str

atig

raph

icP

ositi

on(m

)

Healy Creek Fm.

Lignite Creek

Fm.

Suntrana Fm.

Nenana Gravel

Meta-sedimentary

Quartz

Schist

Volcanic

Plutonic/Greenstone

Chert

OtherSed.

n=301

G L C Q P O V

80

60

40

20

0

80%

0%

20%

%

Lower Jurassic Talkeetna Fm.

n=40780

60

40

20

0

63%

9%28%

%

Middle Jurassic Chinitna Fm.

80

60

40

20

0

29%

0% 0% 1%

24%

44%

2%

n=1,339

%

Upper Jurassic Naknek Fm.

0%0%0%0%

1%0%0% 0%

Usibelli Group

Cantwell Fm.Northern basin

margin

Cantwell Fm.Southern basin

margin

NenanaGravel

n=144

Q

F L

C

D F

E

100

80

60

40

20

0

97%

0% 1% 0% 0% 0%2%

G L C Q P O V

80

60

40

20

05%

53%

15% 8% 4%10%5%

80

60

40

20

0

44%27%

9% 3%13%

3%1%

%

%

%G=metabasalt, L=limestone, C=chertQ=quartz, P=granite/dioriteO=other, V=fine-grained volcanic

Upper Nutzotin Mtns. Sequence(Lower Cretaceous; n=102)

Middle Nutzotin Mtns. Sequence(Upper Jurassic-Lower Cretaceous)

n=913

Lower Nutzotin Mtns. Sequence(Upper Jurassic)

n=769

100

80

60

40

20

08%

60%

8%17%

7%

L V A C S

80

60

40

20

0

22%

3%

53%

0%11%

80

60

40

20

0

86%

2% 9% 0% 2%

%

%

%

Kahiltna Assemblage 410 m above base

n=142

Kahiltna Assemblage 320 m above base

n=155

Kahiltna Assemblage 59 m above base (n=116)

L=LimestoneV=VolcanicA=Argillite C=ChertS=Siltstone

A B

G=metabasalt, L=limestone, C=chertQ=quartz, P=granite/dioriteO=other, V=fine-grained volcanic

Sandstone/Conglomerate

Sanctuary Fm.

Grubstake Fm.



history between the two terranes (Plafker et al., 1989; Nokleberget al., 1994). A potential weak point of the first model is that detri-tus with unequivocal affinity with inboard terranes has not beendocumented in the Upper Jurassic stratigraphy of sedimentarybasins exposed along the inboard margin of the Wrangellia com-posite terrane (i.e., Kahiltna, Nutzotin, and Dezadeash basins). Ifthe Wrangellia composite terrane did not interact with inboard ter-ranes until Cretaceous time, Upper Jurassic basinal deposits withinthe suture zone should lack abundant metamorphic detritus as wellas .900 Ma detrital zircon grains, features diagnostic of sedimentcontribution from inboard terranes (i.e., Yukon-Tanana terrane).These types of provenance indicators are common in Lower Cre-taceous strata of the Gravina and Kahiltna basins (Kapp andGehrels, 1998; Kalbas et al., this volume; Hampton et al., 2005, thisvolume). Unfortunately, detrital zircon analyses have yet not beenreported from Upper Jurassic strata of the inboard basins. In eithertectonic model, collision of the Wrangellia composite terrane waslikely time-transgressive, starting in southeastern Alaska andprogressing northward to south-central Alaska (Pavlis, 1982;McClelland et al., 1992a; Ridgway et al., 2002; Trop et al., 2005a).Whereas initial collision of the south-central Alaska segment of the composite terrane took place sometime during Late Jurassic-Cretaceous time, geologic relations in southeastern Alaska andwestern Canada indicate close proximity of the composite terranewith the outboard margin of the Yukon-Tanana and Stikine ter-ranes by Middle Jurassic time (e.g., McClelland and Gehrels, 1990;McClelland et al., 1991; van der Heyden, 1992, Kapp and Gehrels,1998; Saleeby, 2000; Gehrels, 2001).

Early Cretaceous (Berriasian-Aptian; 144–112 Ma):Diachronous Suturing and Collisional Basin Development

Lower Cretaceous strata on both the inboard and outboard mar-gins of the Wrangellia composite terrane record continued regionalshortening, cessation of arc magmatism, and erosion/recycling ofolder marine basinal strata (Fig. 3C; Ridgway et al., 2002; Tropet al., 2002, 2005a; Clift et al., 2005b). Outboard of Middle to UpperJurassic plutons in the Talkeetna Mountains, for example, a regionaldisconformity separates the Naknek Formation from the overlying

Nelchina Limestone and unnamed strata (Fig. 2). This forearc basinunconformity marks subaerial uplift and partial erosion of Juras-sic marine forearc basin strata. Inboard of Jurassic plutons in theWrangell Mountains, northward propagation of shortening in theChitina thrust belt prompted subaerial uplift and erosion of proxi-mal retroarc Jurassic strata (Figs. 3C, 4C, 8D; Trop et al., 2002).Shortening and uplift within the Chitina thrust belt was coeval withcontinued subsidence and submarine-fan deposition along the in-board margin of the terrane in the Kahiltna and Nutzotin basins(Figs. 6E and 6F). Upsection variations in clast composition in thesebasins record progressive unroofing of sedimentary and volcanicrocks of the Wrangellia composite terrane (Figs. 5B and 5C; Manus-zak and Ridgway, 1999; Eastham and Ridgway, 2002; O’Neill et al.,2003; Ridgway et al., 2002). Detrital zircons from Lower Creta-ceous sandstone of the Nutzotin basin yield Late Jurassic isotopicages (Manuszak, 2000; Manuszak et al., this volume) that are con-sistent with ages reported from plutons to the south that had been in-corporated into the Chitina thrust belt (Chitina segment ofTalkeetna-Chitina arc; Plafker et al., 1989; Roeske et al., 1992,2003). Similarly, Kahiltna assemblage sandstone exposed in thenorthern Talkeetna Mountains yields Middle to Late Jurassic detri-tal zircons that match zircon ages from plutons exposed south of thebasin (Talkeetna segment of the Talkeetna-Chitina arc; Hampton et al., 2005, this volume; Rioux et al., 2005). We attribute regionalshortening, subaerial uplift of Jurassic marine depocenters, ex-humation of Jurassic plutons, and voluminous clastic sedimentationin the Nutzotin and Kahiltna basins as the product of collision of theWrangellia composite terrane with inboard terranes. Alternatively,collisional orogenesis and sedimentation have been interpreted toreflect accretion of the Peninsular terrane with the previously com-bined Wrangellia and Alexander terranes, as discussed in the pre-vious section. We, in contrast, interpret the .2000-km-long belt of siliciclastic detritus represented by the Kahiltna, Nutzotin, andGravina basins, as well as the detrital zircons with “continentalmargin” ages in these strata, as more indicative of a regional colli-sional event along the former continental margin rather than a lo-calized collision between terranes.

During late Early Cretaceous time, magmatism was reestab-lished ,30–50 km northward of remnant Jurassic arc plutons

Figure 5. (A) Compositional data for Jurassic strata in the Matanuska Valley-southern Talkeetna Mountains basin. Note upsection increase in plutonicclasts and upsection decrease in volcanic clasts in response to uplift and dissection of the Talkeetna oceanic arc. n 5 number of clasts counted (fromTrop et al., 2005a). (B) Compositional data from Upper Jurassic-Lower Cretaceous conglomerate of the Nutzotin Mountain sequence. Note upsectionincrease in volcanic/metavolcanic clasts in response to unroofing of deeper levels of Wrangellia. n 5 total number of clast counted (data fromManuszak, 2000, and Manuszak et al., this volume). (C) Histograms showing clast composition of conglomerate in the lower, middle, and upper partsof the Kahiltna assemblage in the northern Talkeetna Mountains. Note upsection increase in volcanic/metavolcanic clasts in response to unroofing ofdeeper levels of Wrangellia. n 5 number of clasts counted (data from Eastham, 2002, and Eastham and Ridgway, 2002). (D) Histograms showingdetrital zircon ages from Cretaceous sandstone (data from Hampton et al., 2005, and Trop and Plawman, 2006). (E) QFL ternary diagram showingmodal composition of sandstone from strata of the Cantwell and Tanana basins. Q 5 quartz, F 5 feldspar, L 5 lithic grains. Gray areas represent onestandard deviation from the mean modal composition (solid circles; data from Trop and Ridgway, 1997, and Ridgway et al., 1999a). (F) Clast compo-sition data for conglomerate of the Usibelli Group and Nenana Gravel of the Tanana basin. Circles on the left mark stratigraphic position of 36 clastcounts. Total number of clasts counted 5 3420. Quartz, schist, and metasedimentary clasts are most common low in the section, whereas sandstone/conglomerate and plutonic/greenstone clasts are more abundant higher in the section (data from Ridgway et al., 1999a).


JkJk

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Figure 6. (A) Tuffs in the Middle Jurassic Nizina Mountain Formation of the Wrangell Mountains basin. White arrows point to thin-bedded tuffs thatare interbedded with fine-grained sandstone and bioturbated mudstone. These strata were deposited in a retroarc basin inboard of Jurassic arc plu-tons. Person lower left for scale. (B) Upper Jurassic syndepositional thrust of the Chitina thrust belt (black barbed line) with Triassic Nizina Lime-stone (Trn) in the hanging wall and Upper Jurassic Kotsina Conglomerate (Jk) in the footwall. Locally, the Kotsina Conglomerate covers similarthrust faults in the China thrust belt (See Figure 7A). Black circle and arrow point to person for scale. Tadpole symbols indicate bedding orientation.(C) Kotsina Conglomerate in the Wrangell Mountains retroarc basin indicates exhumation of multiple stratigraphic levels of the Wrangellia terranecoeval with displacement along the Chitina thrust belt. Clast types: L 5 limestone, G 5 granite, M 5 metabasalt (greenstone). Hammer for scale.(D) Upper Jurassic coarse-grained strata (Naknek Formation) of the Matanuska Valley-Talkeetna Mountains basin. These strata record exhumationof the Talkeetna arc and deposition of marine sediment gravity flows in proximal forearc depositional environments. (E) Lower Cretaceous sand-stone and mudstone of the Kahiltna assemblage in the western Alaska Range near the Farewell Lake area. These strata contain detrital zircons thathave ages indicating derivation from continental margin strata of the Yukon-Tanana terrane as well as from remnant arc rocks of the Wrangellia com-posite terrane. Tadpole symbols show dip of bedding. (F) Close-up of the Kahiltna assemblage of the Kahiltna basin exposed in the northwesternTalkeetna Mountains. Alternating beds of sandstone (lighter color) and mudstone (darker color) are interpreted as submarine-fan strata. Hammer forscale (white box).



(Figs. 3C, 4C). Northward migration of magmatism is consistentwith shallowing of the subducting slab (Plafker et al., 1989)and/or tectonic erosion/removal of forearc crust. In south-centralAlaska, construction of the Chisana arc is recorded in the Nut-zotin basin by an upsection transition from marine sedimentarystrata (Nutzotin Mountains sequence) to ca. 130–113 Ma marinevolcanic strata of the Chisana Formation (Figs. 1A, 2, 8A; Berget al., 1972; Short et al., 2005); coeval felsic plutons crop out inthe eastern Alaska Range and Nutzotin Mountains (e.g., Richter,1976). Chisana Formation lavas and spatially associated graniticintrusions exhibit geochemical traits, particularly initial Nd andSr isotopic compositions, consistent with a subduction petrogen-esis in which recycling of continental crust was minimal (Barker,1988; Snyder and Hart, 2002, 2005, this volume; Short et al.,2005). Instead, source materials responsible for the production ofmagmas are limited to the depleted mantle wedge above thedowngoing slab 6 juvenile lithosphere (Snyder and Hart, 2005,this volume). Correlative volcanic rocks in southeastern Alaskayield similar lithologic and geochemical traits (Berg et al., 1972;Rubin et al., 1990; McClelland et al., 1992a; Stowell et al., 2000).The Chisana arc was an important sediment source based on com-positional and detrital geochronologic data from the Kahiltna,Nutzotin, and Matanuska Valley-Talkeetna Mountains basins.Cretaceous sandstone from those basins contains a 130–113 Madetrital zircon age cluster that overlaps isotopic ages from theChisana arc (e.g., Kalbas et al., this volume; Hampton et al., 2005,this volume; Trop et al., 2005b). Sandstone detrital modes areenriched in plagioclase feldspar and volcanic lithic grains, consis-tent with erosion of remnant and coeval igneous source terranes.

Re-establishment of arc magmatism was coeval with renewedsubsidence and sedimentation in outboard depocenters. The BergCreek and Kuskalana Pass Formations in the Wrangell Mountains(Fig. 2) and the Nelchina Limestone in the southern TalkeetnaMountains (Fig. 2) were deposited in a forearc basin position be-tween the Chisana arc to the north and Chugach accretionaryprism to the south. Shallow-marine depositional systems trans-ported recycled sedimentary, metavolcanic, and metaplutonic de-tritus southward from the coeval Chisana arc and remnant upliftsattributable to Late Jurassic-Early Cretaceous deformation (Tropet al., 2002, 2005a). Conglomerate along the southern margin ofthe forearc basin in the Chitina Valley contains diagnostic red chertand black argillite clasts that are consistent with local subaerialuplift and erosion of the Chugach accretionary prism by this timerange locally. Renewed volcanism and marine deposition docu-mented in south-central Alaska may have been equivalent to thepost-collisional, transtensional setting inferred for age-equivalentbasinal strata in southeastern Alaska (e.g., Gehrels and Saleeby,1985; McClelland et al., 1992a; Monger et al., 1994).

Late Early Cretaceous sedimentation and arc magmatismoverlapped with accretion of the McHugh Complex mélange in theChugach subduction complex and emplacement of 125–115 Manear-trench intrusive rocks along the Border Ranges fault (e.g.,Pavlis et al., 1988; Barnett et al., 1994; Bradley et al., 2000). Theserocks are currently located .100 km trenchward (southward) of

the contemporaneous subduction-related arc rocks (Chisana arc).Emplacement of this near-trench intrusive suite was synchro-nous with southward thrusting of the Wrangellia composite ter-rane against the subduction complex along the Border Rangesfault (Pavlis et al., 1988). Previous workers attribute near-trenchmagmatism to shallow melting of metamorphic rocks along thejuvenile subduction zone or slab-window magmatism associatedwith subduction of an oceanic spreading ridge (Barnett et al.,1994). We suggest that the regional unconformity spanning ca.130–115 Ma in forearc basinal strata from the Matanuska Valleyto the Wrangell Mountains may record subduction of this oceanicspreading ridge. Stratigraphically, this unconformity is definedby late Early Albian and younger marine sedimentary strata (Kenni-cott and Matanuska Formations on Fig. 2) that unconformablyoverlie Berriasian marine sedimentary strata (Kuskalana Pass andNelchina Formations). We interpret this unconformity as poten-tially recording uplift of the forearc region concurrent with thethrusting and near-trench magmatism along the Border Rangesfault. Subduction of progressively more buoyant, topographi-cally higher lithosphere (the spreading ridge) followed by lessbuoyant, topographically lower lithosphere could have promptedinitial uplift of the forearc basin floor (Aptian to Early Albianunconformity) followed by extension, subsidence, and renewedmarine sedimentation (Kennicott and lower Matanuska Forma-tions; Fig. 2).

Early Cretaceous to Late Cretaceous (Albian-Santonian;112–83 Ma): Diachronous Subaerial Uplift of Suture Zone

Continued Late Cretaceous oblique suturing of the Wrangel-lia composite terrane to the continental margin prompted di-achronous shortening and subaerial uplift of marine forelandbasins positioned along the inboard margin of the compositeterrane. In the eastern Alaska Range, marine strata of the Nut-zotin basin were deformed by southwest-verging folds andthrust faults that sole into a basin-wide décollement (Figs. 3D,4D, 9A; Manuszak, 2000; Manuszak et al., this volume). Thesedeformed strata are intruded by undeformed plutons with117–105 Ma K-Ar ages (e.g., Richter, 1976; Manuszak et al.,this volume). Concurrently, retrograde metamorphism and de-velopment of a regional anticlinorium occurred along the south-ern margin of the Yukon-Tanana from ca. 115–106 Ma (Fig. 4D;Nokleberg et al., 1992; Ridgway et al., 2002). South- to south-west-verging asymmetric folds within the metamorphic zoneindicate southward thrusting (Nokleberg et al., 1992), consis-tent with the structural vergence in strata of the nearby Nutzotinbasin (Figs. 4D, 9A).

Coeval with shortening and uplift of the Nutzotin basin, ma-rine sedimentation continued to the northwest in the Kahiltnabasin through Late Albian-Cenomanian time (Fig. 3D; Csejteyet al., 1982; Hampton et al., 2003, this volume; Kalbas et al., 2003,this volume). Albian-Cenomanian sedimentation in the Kahiltnabasin was characterized by sediment input from both metamor-phic rocks of the former continental margin and oceanic rocks of


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Figure 7. Cross sections showing key structural relations from sedimentary basins along the outboard margin of the Wrangellia composite terrane. SeeFigure 1D for locations of cross sections. (A) Structural cross section through part of the Chitina thrust belt in the Wrangell Mountains. Upper JurassicKotsina Conglomerate (Jk) is exposed in the footwall of thrust faults and locally covers thrust faults (inset), indicating syndepositional displacement(from Trop et al., 2002). WT—Wrangellia terrane. (B) Geologic cross section showing faulted northern margin of the Matanuska Valley-southern Tal-keetna Mountains basin. Northwest-dipping Little Oshetna fault juxtaposes Jurassic arc-related igneous rocks (PT) against conglomerate of the UpperJurassic Naknek Formation (Jn) and underlying Middle Jurassic Chinitna Formation (Jc). Proximal conglomerate consists of volcanic and plutonic clastswith isotopic ages that match the age of igneous rocks exposed north of the fault. Proximal fan-delta conglomerate merges southward into distal prodeltasandstone and mudstone (from Trop et al., 2005a). (C) Cross section showing angular unconformity between the Lower Cretaceous Kennicott For-mation (black bed) and folded Upper Triassic-Lower Jurassic strata of the Wrangell Mountains basin and Wrangellia terrane (WT). Note relative south-ward thickening of the Kennicott (black bed) and Chititu (Ks) Formations across southward-dipping normal faults (from Trop et al., 2002). (D) Crosssection through the Castle Mountain fault along the northern part of the Matanuska Valley-Talkeetna Mountains basin. Note that immediately south ofthe Castle Mountain fault, attitudes of beds decrease progressively upsection (from ,80° to 10°), and a distinct angular unconformity separates theEocene Wishbone Formation (Tw) from overlying Tertiary volcanic strata (Tv; from Trop et al., 2003).


the Wrangellia composite terrane (Eastham, 2002; Hampton et al.,2005, this volume; Kalbas et al., 2003, this volume). Evidence ofsediment contribution from the Wrangellia composite terranecomes from detrital zircon peaks in sandstone of the Kahiltna as-semblage in the Alaska Range that are consistent with age datafrom magmatic belts exposed south of the Kahiltna basin, includ-ing the Cretaceous Chisana arc (Snyder and Hart, 2002, this vol-ume; Short et al., 2005), Jurassic Talkeetna arc (Rioux et al., 2005),and Upper Triassic igneous rocks (Nikolai Greenstone; Nokleberget al., 1994). All of these igneous successions are part of theWrangellia composite terrane. Evidence for sediment contributionfrom the former continental margin includes detrital zircon grains.900 Ma in sandstone of the Kahiltna assemblage that are inter-preted as reflecting erosion of Precambrian-Paleozoic source ter-ranes (e.g., Yukon-Tanana, Dillinger, and Pingston terranes), whichare spatially limited to regions north of the Kahiltna basin. In addi-tion, limestone clasts in conglomerate of the Kahiltna assemblagecontain Paleozoic conodonts that are correlative with conodontassemblages from inboard terranes with continental affinities(Ridgway et al., 2002). Limited paleocurrent indicators and fa-cies transitions from strata of the Kahiltna basin exposed in theAlaska Range are consistent with axial (southwestward-directed)sediment transport coeval with oblique closure of the basin and de-velopment of topographic relief along the suture zone between theYukon-Tanana and Wrangellia composite terranes (Fig. 3D; East-ham, 2002; Eastham and Ridgway, 2002; Ridgway et al., 2002;Kalbas et al., 2003, this volume).

Strata of the Kahiltna basin were shortened, subaerially ex-posed, and possibly partially underthrust beneath the Yukon-Tananaterrane during Cenomanian-Campanian time. In the northern Tal-keetna Mountains, Albian-Cenomanian nonmarine strata overliemarine strata of the Kahiltna assemblage (Caribou Pass forma-tion on Figures 2, 8C; Hampton et al., this volume). Farther north,Coniacian-Campanian marginal-marine strata (lower member ofColorado Creek basin on Fig. 2) overlie deformed marine strata of the Kahiltna assemblage along a prominent angular unconformity(Fig. 8B, 9D; Trop et al., 2004). North of the Denali fault, non-marine strata of the Campanian-Maastrichtian lower Cantwell For-mation overlie folded marine strata of the Kahiltna assemblagealong an angular unconformity (Ridgway et al., 1997).

Regional late Early to early Late Cretaceous shortening andsubaerial uplift of the Kahiltna and Nutzotin basins of south-central Alaska may be part of a regional thrust belt that extendssoutheastward to the state of Washington (e.g., Rubin et al., 1990;Manuszak et al., this volume). This thrust belt is defined by aseries of inboard- (northward- and eastward-) dipping thrustfaults that juxtapose Upper Jurassic-Upper Cretaceous marinebasinal strata above the Wrangellia composite terrane. In south-eastern Alaska for example, marine strata of the Gravina belt wereimbricated and underthrust to relatively deep crustal levels(,25–30 km) beneath the Yukon-Tanana terrane, starting at ca. 113–98 Ma and ending ca. 90 Ma (McClelland et al., 1992b;Haeussler, 1992). Similarly, interpretations of seismic refractionand magnetotelluric data across the central Alaska Range suggestthat the Kahiltna assemblage and Wrangellia terrane have been

underthrust beneath the Yukon-Tanana terrane (Stanley et al.,1990b; Beaudoin et al., 1992).

Regionaldeformationofsedimentarybasinsalongtheinboardmargin of the Wrangellia composite terrane was coeval with re-newed sediment accumulation in outboard basins (WB, MB onFigure3D;KF,MCF,SF,CFonFigure4D).Apparently,southwardthrusting and uplift along the suture zone of the Wrangellia com-posite terraneenhancedsubsidenceandsouthwardsedimentdeliv-ery to forearcdepocenters. In theWrangellMountainsbasin, sandyshoreface and inner shelf environments (northernmost exposuresof theKennicott andMoonshineCreekFormations)mergedsouth-ward with muddy outer shelf environments (Fig. 7C; SchultzeFormation and southern exposures of the Moonshine Creek For-mation) and submarine slope environments that were influencedby normal faults (Chititu Formation; Trop et al., 2002). In theMatanuska Valley Talkeetna Mountains basin, strata of the lowerMatanuska Formation record deposition in swampy fluvial, shore-face, shelf, and slope depositional environments (Fig. 3D, 8E;Grantz and Jones, 1960, 1967; Trop and Plawman, 2006).

Late Cretaceous (Campanian-Maastrichtian; 83–68 Ma):Post-collisional Suturing and Continental-Margin ArcConstruction

Latest Cretaceous basinal strata provide a regional sedimen-tary record of Cretaceous continental-margin arc construction re-lated to northward subduction of oceanic crust beneath southernAlaska (Figs. 3E, 4E). Coalescing 80–60 Ma plutons and subordi-nate volcanic rocks occupy an outcrop belt up to 150 km wide and.3000 km long, comprising the Alaska Range-Talkeetna Moun-tain magmatic belt in south-central Alaska, Kluane arc in easternAlaska-Yukon Territory, and Coast arc in western Canada (Fig. 1B;Wallace and Engebretson, 1984; Plafker et al., 1989; Moll-Stalcup,1994; Nokleberg et al., 2001). Latest Cretaceous Continental-Margin arc rocks stitch accreted oceanic terranes (Wrangellia com-posite terrane) and the former continental margin (Yukon compos-ite terrane). Geochemical compositions from the Upper Cretaceousigneous rocks are typical of continental-margin arc rocks (Moll-Stalcup, 1994). Construction of the continental-margin arc wascontemporaneous with rapid expansion of the Chugach accre-tionary prism in response to accretion of extensive submarineslope/fan deposits via north-directed subduction beneath the con-tinental margin (Valdez Group on Figures 1D, 4E; Plafker et al.,1994). Stratigraphic data indicate that the main phase of accre-tionary prism growth took place during Early Maastrichtian time,probably over a time interval of less than four million years (Sam-ple and Reid, 2003). Petrographic and geochemical data from LateCretaceous accretionary prism strata, including Nd isotopic com-positions, indicate that they were derived from a recycled orogencomposed of Proterozoic rocks, as well as continental-margin vol-canic arc sources (Sample and Reid, 2003).

Forearc depocenters, positioned between the accretionaryprism to the south and continental-margin arc to the north, werecharacterized by submarine sediment-gravity flows and mass-slidesin the Matanuska Valley (upper Matanuska Formation on Figures 2,



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8F) and Wrangell Mountains (MacColl Ridge Formation on Fig-ures 2, 8G). Isotopic ages from interbedded tuff and detrital zirconages from sandstone demonstrate that forearc basin detritus was de-rived from the coeval continental-margin arc and remnant accretedarcs. Synvolcanic detritus is evidenced by numerous thin- to thick-bedded tuffs, sandstone with embayed quartz, feldspar laths, andvolcanic lithic grains, and detrital zircon grains that overlap in agewith source terranes exposed inboard of the basin, including coevaland remnant arc rocks (Fig. 5D; Trop et al., 1999, 2005b; Trop andPlawman, 2006). Conglomerate compositional data and Jurassic-Early Cretaceous detrital zircons indicate contribution from rem-nant magmatic belts as well (Talkeetna-Chitina, Chisana arcs;Fig.5D). Subsidence of the forearc region into deeper-water settingsmay reflect flexural subsidence of the forearc under the growingload of the Chugach accretionary prism, which expanded markedlyduring latest Cretaceous time (Plafker et al., 1994); isostatic subsi-dence under the growing sediment load, which added .4 km ofstrata during Late Cretaceous time; and thermotectonic subsidenceas a result of magmatism in the adjacent Alaska Range-TalkeetnaMountains belt. However, a detailed understanding of the subsi-dence history is presently hampered by a lack of reliable constraintson key parameters known to influence forearc basin subsidence, in-cluding changes in thickness of the subduction complex, changes inthe thermal regime of the arc region, and variations in the buoyancyof the subducting oceanic slab (Dickinson, 1995).

Within and inboard of theAlaska Range-Talkeetna Mountainsegment of the latest Cretaceous continental-margin arc, the suturezone between the Wrangellia composite terrane and inboard ter-ranes was characterized by deformation and exhumation of marinebasinal strata and deposition of coarse-grained nonmarine strata(Pass Creek strata). Deformation of marine strata culminated withmetamorphism within the Maclaren Glacier metamorphic belt andValdez Creek shear zone (VCS on Figures 3E, 4E, 9B; Csejteyet al., 1982; Nokleberg et al., 1985; Davidson et al., 1992; Ridg-way et al., 2002). A .1000-m-thick succession of conglomerateand sandstone exposed within the Valdez Creek shear zone recordserosion of coeval, continental arc rocks, as well as remnant oceanicarc source terranes based on detrital zircon geochronology (Fig. 5D)

and conglomerate compositional data; subordinate detritus wasderived from the former continental margin (e.g., Yukon-Tananaterrane; Hampton et al., 2005; Trop et al., 2005b). However, directage data are presently lacking from these sedimentary strata; aCampanian or younger depositional age is inferred based on theage of the youngest detrital zircon cluster (ca. 80–75 Ma).

Inboard of the locus of arc magmatism, northward thrusting of the Kahiltna assemblage and Wrangellia composite terraneagainst the Yukon-Tanana terrane prompted development of theCantwell thrust-top basin. Campanian-Maastrichtian strata up to4000 m thick (lower Cantwell Formation on Fig. 2) were depositedunconformably above deformed marine strata of the Kahiltna as-semblage. Sedimentological, paleocurrent, and compositional datadocument a two-sided basin characterized by alluvial, fluvial, lacus-trine, and minor marginal-marine deposystems (Fig. 8H; Wolfe andWahrhaftig, 1970; Ridgway et al., 1997; Trop and Ridgway, 1997).Along the southern margin of the Cantwell basin, footwall growthsynclines indicate syndepositional displacement on southward-dipping thrust faults (Fig. 9C). Compositional data from southernbasin margin strata document erosion of oceanic source terranesexposed within south-dipping thrust sheets in the suture zone, in-cluding recycled marine strata of the Kahiltna basin and Triassicvolcanic rocks (Fig. 5E; Trop and Ridgway, 1997). Along the north-ern basin margin, syndepositional displacement along the HinesCreek fault prompted erosion of quartz-rich metamorphic sourceterranes within the Yukon-Tanana terrane (Fig. 5E). The dominanceof nonmarine strata and evidence for detritus eroded from bothoceanic and continental margin sources indicate that deposition ofthe lower Cantwell Formation was coeval with regional subaerialuplift of the suture zone between the Yukon and Wrangellia com-posite terranes (Ridgway et al., 2002).

Late Maastrichtian-Early Eocene (68–61 Ma): ContinentalMargin Uplift and Regional Unconformity Development

Sedimentary basins on both the inboard and outboard mar-gins of the Wrangellia composite terrane were subaerially up-lifted, shortened, and partially eroded during Late Maastrichtian

Figure 8. (A) Pillow basalts of the 3-km-thick Lower Cretaceous Chisana Formation in the Nutzotin basin. Lithologic, paleontologic, and geochemi-cal data from these bedded volcanic rocks indicate proximal submarine deposition in an arc-related setting. Hammer (upper right) for scale. (B) Photo-graph showing key stratigraphic relationships in the Colorado Creek basin. Note that steeply dipping sedimentary strata of the Kahiltna assemblage(JKk) are overlain by gently dipping Coniacian-Lower Campanian (Kcb) and Lower Oligocene (Tcb) strata. Black line shows position of angularunconformity; tadpole symbols show dip of beds. (C) Plant fossil leaves (white arrows) from the Albian-Cenomanian section of the Caribou Pass For-mation in the northwestern Talkeetna Mountains. Coin (penny, circled) for scale. (D) Photograph of angular unconformity (white line) in the WrangellMountains forearc basin. Shallow dipping strata of the Lower Cretaceous Kennicott and Schultze Formations (Ks) depositionally overlie folded strataof the Chitistone and Nizina Limestones (Trl), and Nikolai Greenstone (Trn). View is toward the west. Exposed face is ,500 m high. (E) Albian coal(c) and sandstone (Sm) of the lower Matanuska Formation deposited in coastal depositional environments. Person for scale (arrow, center). (F) Deformedlatest Cretaceous strata of the upper Matanuska Formation in the Matanuska Valley-Talkeetna Mountains basin. Syndepositional folds formed in responseto mass sliding of interbedded mudstone and turbidite sandstone on an unstable submarine slope. Exposed face is ,20 m thick. Undisturbed strata dipto the left (white tadpole symbol, lower right). (G) Thick-bedded sandstone, mudstone, and tuff (white arrows) of the latest Cretaceous MacColl RidgeFormation of the Wrangell Mountains basin. (H) Boulder and cobble conglomerate of the Upper Cretaceous lower Cantwell Formation along the north-ern margin of the Cantwell basin. These quartz-rich (white clasts) conglomerates were deposited in stream-dominated alluvial fans derived from nearby“continental margin” metamorphic rocks of the Yukon-Tanana terrane. Person (lower center) for scale.


overprintedkyanite schists

Grt

Stl-

Kykyanite

schists

tonalite

Valdez Creek shear zone (VCS)Kahiltna Assemblage

2000m

1000m

KJkKJk KJk

0 5 km

NW SE

0 20 km

NW SE

B

SE

Triassiclava

Intraformationalangular unconformity

Intraformationalangular unconformity

0 2 km

?

Denali fault

Totschunda fault

Lost Creek

decollement2400m

-2400m

SL

BuckCreek pluton

A TA T

Qs

BuckCreek fault

Wrangellia terrane

YT

Devil's Mountain

pluton

Lost Creek pluton

C

NW

A

Wrangellia terrane

Wrangelliaterrane

KJnKJn

Kp

Wrangelliaterrane

Kcs Kcs Kcs

Figure 9. (continued on the following page) Cross sections showing key structural relations from sedimentary basins along the inboard margin ofthe Wrangellia composite terrane. See Figure 1D for locations. (A) Cross section showing structural relationships between the Nutzotin Moun-tains sequence (KJn), the Wrangellia terrane (WT), and the Yukon composite terrane (YT). Note that the Nutzotin Mountains sequence has beenthrust over the Wrangellia terrane along the north-dipping Lost Creek décollement. Geochronologic ages are not available from plutons shown oncross section (Kp), including the Buck Creek, Lost Creek, and Devil’s Mountain plutons. These plutons are interpreted to be equivalent to a suiteof nearby plutons that yield 117–105 Ma K-Ar ages. Qs 5 Quaternary surficial deposits. Bedding attitudes (tadpole symbols) are from Manuszak(2000), Richter et al. (1975) and Richter (1976). The Totschunda and Denali faults are interpreted to have accommodated oblique displacement:A 5 block moving away from viewer, T 5 block moving toward the viewer (from Manuszak et al., this volume. (B) Cross section showing ValdezCreek shear zone in the northeastern corner of the Kahiltna basin. In this area, metamorphosed strata of the Kahiltna basin are part of the MaclarenGlacier metamorphic belt. Solid lines are bedding, and dashed lines are the orientation of axial-planar cleavage. Note that kyanite schist in thehanging wall of the Valdez Creek shear zone is juxtaposed against relatively undeformed Kahiltna assemblage in the footwall. Abbreviations: Grt 5 garnet-in isograd, Stl 6 Ky 5 staurolite-kyanite-in isograd, KJk 5 Kahiltna assemblage, WCT 5 Wrangellia composite terrane (from Ridg-way et al., 2002). (C) Cross section along southern margin of the Cantwell basin showing bedding orientation and location of intraformationalunconformities in the lower Cantwell Formation (Kcs). Cross section represents line interpretation of a photomosaic that was used during fieldmapping. Wavy black lines indicate intraformational unconformities, and tadpole symbols represent dip of bedding. Note northward decrease indip away from thrust-fault that forms the southern boundary of the Cantwell basin. Also, note change in dip across intraformational unconformi-ties (wavy lines) that are interpreted as a product of progressive tilting of strata by syndepositional thrust-fault displacement. Adapted from Ridg-way et al. (1997).



to Early Paleocene time. Along the outboard margin, an un-conformity records deformation and subaerial uplift of marinesedimentary strata deposited on the forearc side of the Late Cretaceous-Paleocene magmatic arc (Fig. 2). In the MatanuskaValley-Talkeetna Mountains basin, Campanian-Maastrichtianmarine sediment-gravity-flow deposits (upper Matanuska For-mation) were subaerially exposed prior to onlap by Paleocene-Eocene fluvial-estuarine strata (Chickaloon and Arkose RidgeFormations; Winkler, 1992; Little and Naeser, 1989; Trop et al.,2003). The timing of unconformity development is constrainedby ammonite fossils and detrital zircon ages from the upperMatanuska Formation (ca. 72–71 Ma; Trop et al., 2005b) in com-bination with plant fossils and isotopic ages from felsic tuffs inthe Chickaloon and Arkose Ridge Formations (ca. 59 Ma;Triplehorn et al., 1984; D. Bradley, unpublished data) from theoverlying Chickaloon and Arkose Ridge formations. Alongstrike, in the Wrangell Mountains, Campanian submarine-fandeposits were subaerially uplifted prior to intrusion of Mio-cene igneous stocks and nonmarine deposition of the MioceneFrederika Formation (Fig. 2). The timing of unconformity de-velopment in this area is constrained by 79–78 Ma isotopic ages from tuff of the MacColl Ridge Formation (Trop et al.,1999) and 11–10 Ma isotopic ages from volcanic strata in theFrederika Formation (Denton and Armstrong, 1969; Tidmore et al., 2005).

Along the inboard margin of the Wrangellia composite ter-rane, strata of the Kahiltna and Cantwell basins were deformedand uplifted by north-dipping thrusts and associated folds withinthe Alaska Range suture zone (Ridgway et al., 2002; Hampton et al., this volume). Campanian-Maastrichtian sedimentary strataof the Cantwell basin were folded and partly eroded prior to theonset of volcanism at ca. 59 Ma (upper Cantwell Formation; Coleet al., 1999). Approximately, 100 km south of the Cantwell basin,metamorphosed rocks of the Valdez Creek shear zone and Mac-Claren Glacier metamorphic belt cooled through the biotite clo-sure temperature by ca. 62 Ma (Ridgway et al., 2002). Regionalshortening and exhumation were contemporaneous with rapidaccretion and cooling of metamorphic and plutonic rocks withinthe Chugach accretionary prism (Clendenen et al., 2003).

From a regional perspective, Late Maastrichtian-Early Pa-leocene deformation of the continental margin of south-centralAlaska was contemporaneous with crustal shortening and rapiduplift throughout southeastern Alaska and western Canada(Wood et al., 1991; McClelland et al., 1992a; McClelland andMattinson, 2000; Haeussler et al., 2003). Widespread deformationis attributable to rapid low-angle subduction of oceanic crust andassociated dextral transpression of the previously amalgamatedWrangellia and Yukon composite terranes along orogen-parallel,strike-slip faults (Plafker and Berg, 1994; Smart et al., 1996;Roeske et al., 2003). This model is consistent with publishedplate motion reconstructions that infer oblique subduction alongthe northeast Pacific margin from ca. 85 to 50 Ma (Engebretsonet al., 1985; Stock and Molnar, 1988; Haeussler et al., 2003). Up-lift in south-central Alaska may have been a consequence of sub-duction of increasingly younger, more buoyant oceanic crustassociated with ridge subduction. Geologic evidence documentswest-to-east subduction of an oceanic spreading ridge orientedsubparallel to the margin from 61 to 50 Ma (Fig. 3F; Bradley etal., 2000, 2003; Haeussler et al., 2000, 2003; Sisson et al., 2003;Cole et al., 2006). Buoyancy considerations (Cloos, 1993) sug-gest that subduction of ,10 Ma oceanic crust inboard of the ap-proaching spreading ridge (Fig. 4E) would have promptedhigher shear stresses between the subducting crust and overrid-ing plate (i.e., stronger coupling) and may have been manifestedby uplift and erosion throughout the continental margin.

Late Paleocene-Early Eocene (61–49 Ma): OceanicSpreading Ridge Subduction

Regional continental-margin arc magmatism ceased insouth-central Alaska during Late Paleocene time (Fig. 1B). Sedi-mentation was limited to remnant forearc depocenters exposed inthe Matanuska Valley and southern Talkeetna Mountains (MB onFig. 3F). Upper Paleocene-Eocene nonmarine sedimentary strata(Chickaloon, Arkose Ridge, and Wishbone Formations on Fig-ures 2, 3F, 4F, 10A, 10B) were deposited across Upper Creta-ceous and older deep-marine strata. Gravelly alluvial-fluvialdeposystems prograded from both the northern and southern

1500m

2000mNW SE

D

Denalifault

A T

TcvKcs

Cretaceous-TertiaryColorado Creek strata (CCB)

McKinley and Dillinger terranes

0 5 km

KJk

KJk

Angularunconformity

Angularunconformity

Figure 9 (continued). (D) Simplified structural cross section through the Colorado Creek and Cantwell basins adjacent to the Denali fault in the cen-tral Alaska Range. Note unconformity between the Kahiltna assemblage (KJk) and overlying strata in both the Colorado Creek and Cantwell basins.Abbreviations: Kcs 5 Cretaceous lower Cantwell Formation; Tcv 5 Paleocene upper Cantwell Formation; A 5 block moving away from viewer;T 5 block moving toward the viewer (modified from Trop et al., 2004).


TfTf

TwTw

TymTym

TydTyd

TymTym

A B

G

C D

TaTa

Tv

Tw

FE

CC

CC

CC

RR

RR

QQ AA

AA

AA

H


Figure 10. (A) Photograph of coarse-grained alluvial-fan strata (Wishbone Formation, Tw) and overlying volcanic strata (Tv) that were depositedalong the northern margin of the Matanuska Valley–Talkeetna Mountains basin during Eocene time. Compositional and detrital geochronologic dataindicate that detritus was derived from remnant to coeval magmatic belts along the northern basin margin. Exposure is ,200 m thick. (B) Carbona-ceous fluvial and overbank strata (Chickaloon Formation) that were deposited along the axis of the Matanuska Valley-Talkeetna Mountains basinduring Paleocene-Eocene time. Person circled in lower right for scale. (C) Paleocene-Eocene conglomerate deposited along the southern margin ofthe Matanuska Valley-Talkeetna Mountains basin contains abundant red chert (R), vein quartz (Q), and black argillite (A) derived from the Chugachaccretionary prism. (D) Interbedded conglomerate and coal (dark units, white arrows) of the Amphitheater Formation (TA) in the Eocene-OligoceneBurwash basin. This strike-slip basin formed along the Duke River fault, an eastern segment of the Denali fault system. The fault is located at thebase of the snow-covered mountains in the background. Tent (black arrow) for scale in lower right of photo. (E) Yakataga Formation of the Robin-son Mountains, which was deposited during Miocene-Recent collision of the Yakutat terrane along the southern continental margin of Alaska. Resis-tant bed in midground (white arrows mark the base) is a lenticular bed of glacial diamictite (Tyd) that is interbedded with marine mudstone (Tym).Two people circled (lower left) for scale. (F) Photograph of Miocene sedimentary strata of the Frederika Formation (Tf) and lava flows of the WrangellLava (Tw), which were deposited in an intra-arc basin within the Wrangell continental arc. Note boulders .2 m long (white arrows). Exposure is,180 m thick. (G) Photograph of interbedded coal and coarse-grained sandstone in the Miocene Usibelli Group in the Tanana foreland basin. Coalbeds up to 20 m thick are present in this basin. Spruce forest at bottom of photo for scale. (H) Conglomerate of the Pliocene Nenana Gravel in theTanana foreland basin. Diagnostic conglomerate clasts (C) and paleocurrent data indicate exhumation and recycling of latest Cretaceous nonmarinestrata (lower Cantwell Formation) exposed in thrust sheets to the south in the Alaska Range. Hammer for scale.

basin margins across swampy basin-axis fluvial environments(Fig. 3F; Clardy, 1974; Little, 1988; Trop et al., 2003). Along thesouthern basin margin, alluvial-fluvial strata record erosion ofmetavolcanic and metasedimentary source terranes of the adja-cent Chugach accretionary prism (Fig. 10C), coeval with displace-ment along the Border Ranges fault (Little and Naeser, 1989; Little,1990). Along the northern basin margin, alluvial-fluvial stratarecord erosion of dissected Jurassic-Cretaceous magmatic arcs and coeval Paleocene-Eocene igneous rocks synchronous withdextral-oblique displacement along the Castle Mountain faultsystem (Fig. 7D; Flores and Stricker, 1993; Trop et al., 2003).

Paleocene-Eocene sedimentary strata were deposited duringcomplex convergent margin tectonism, including oblique sub-duction of oceanic crust and a spreading ridge (Haeussler et al.,2003), right-lateral displacements on orogen-parallel, strike-slipfaults (Smart et al., 1996; Cole et al., 1999; Roeske et al., 2003),and oroclinal bending of western and interior southern Alaska(Coe et al., 1985). During Paleogene-Neogene time, arc magma-tism shifted southward to form the Aleutian and Wrangell arc sys-tems, possibly in response to break-off or roll-back of thesubducting slab following terminal suturing of the Wrangelliacomposite terrane (Plafker and Berg, 1994; Cole and Stewart,2005; Cole et al., 2006). During this reorganization, west-to-eastsubduction of an oceanic spreading center influenced a .2000-km-long segment of the margin from south-central Alaska towestern Canada (Haeussler et al., 2003). Effects of ridge subduc-tion within the accretionary prism include near-trench magmatism(Bradley et al., 2000); high-temperature, low-pressure metamor-phism (Sisson et al., 1989, 2003); generation of lode-gold de-posits (Haeussler et al., 1995), and ophiolite accretion (Kusky andYoung, 1999).

Less well understood are potential geologic effects of ridgesubduction inboard of the near-trench intrusive belt. Distinctepisodes of uplift, coarse-grained nonmarine sedimentation, andmagmatism took place inboard of the subduction complex dur-ing near-trench magmatism, indicating potential links with ridge

subduction processes (e.g., Bradley et al., 2003). In the Matan-uska Valley, subduction of progressively more buoyant, topo-graphically higher lithosphere (juvenile oceanic crust and thespreading ridge) followed by less buoyant, topographically lowerlithosphere (progressively older crust) may have prompted sub-aerial exposure of the formerly marine forearc basin, followedby Eocene coarse-grained nonmarine sedimentation (e.g., Wish-bone Formation on Fig. 2; Trop et al, 2003). Eocene volcanic andintrusive rocks interfinger with and intrude Eocene sedimentaryrocks in the Talkeetna Mountains (CTV, CV on Fig. 3F) and theMatanuska Valley (MB on Fig. 3F). Geochemical compositionsfrom the igneous rocks indicate derivation of magmas from a de-pleted magma source, consistent with an upper-mantle slab win-dow associated with subduction of a spreading ridge (Silbermanand Grantz, 1984; Amos and Cole, 2003; Cole et al., 2006). Fur-ther inboard, coeval volcanic rocks exposed in the northern Tal-keetna Mountains (JV on Figure 3F; Cole and Stewart, 2005;Cole et al., this volume) and the central Alaska Range (CB onFigure 3F; upper Cantwell Formation; Cole et al., 1999) yieldgeochemical compositions that reflect derivation from a more en-riched mantle source, likely the remnant mantle wedge from LateCretaceous-Paleocene continental-margin arc magmatism (Coleet al., 1999; Cole et al., this volume).

Late Eocene-Late Oligocene (49–26 Ma): DextralTranspression and Strike-Slip Basin Development

Transpressional tectonics, dextral displacement along orogen-parallel faults, and localized nonmarine sedimentation character-ized south-central Alaska during Late Eocene-Late Oligocene time(Figs. 3G, 4G). An angular unconformity separating the Paleocene-Eocene Wishbone Formation and Oligocene Tsadaka Formation(Fig. 2) records deformation along the outboard margin of theWrangellia composite terrane during this interval. Limited sedi-mentologic data from the Tsadaka Formation document deposition



84 Trop and Ridgway

of boulder-rich conglomerate on south-sloping humid alluvial fansin the Matanuska Valley (Fig. 3G; Clardy, 1974; Trop et al., 2003).Compositional and paleocurrent data from these strata indicateerosion of dissected Jurassic-Paleocene felsic plutons exposed inthe southern Talkeetna Mountains, including the remnant Tal-keetna oceanic arc and the Alaska Range-Talkeetna Mountainsmagmatic belt. Syndepositional dextral-oblique displacementalong the Castle Mountain fault probably contributed to erosion of these igneous source terranes (Figs. 3G, 4G; Fuchs, 1980; Tropet al., 2003).

Oligocene synorogenic sedimentation in the MatanuskaValley-Talkeetna Mountains was coeval with formation of smallfault-bound basins along the Denali fault system within the pre-viously formed suture zone between the Wrangellia compositeterrane and inboard terranes (Figs. 1D; 3G). In western Canada,Upper Eocene-Oligocene alluvial-fluvial strata up to 750 mthick are interpreted as the product of right-lateral displacementon the eastern Denali fault along the Duke River and Dalton seg-ments of the Denali fault system (Fig. 10D; Bates Lake, Bur-wash, Sheep Creek, and Three Guardsmen basins; Ridgwayet al., 1992; Cole and Ridgway, 1993); most strike-slip displace-ment took place between 40 and 30 Ma (Ridgway et al., 1995,1996). In the central Alaska Range, Lower Oligocene alluvial-fluvial strata .700 m thick (Tcb on Fig. 8B) are linked withright-lateral displacement along the McKinley segment of theDenali fault (middle member of Colorado Creek basin on Figure 2,Table 1; CCB on Figures 3G, 9D; Trop et al., 2004) and alongthe Talkeetna fault (WCB on Figure 3G; Hardy, 1986). In west-ernAlaska, Upper Oligocene strata .440 m thick record alluvial-fluvial deposition along the Farewell segment of the Denali faultin the White Mountain basin (WMB on Figure 3G; Ridgwayet al., 1999b); other Eocene-Oligocene nonmarine sedimentarysuccessions exposed along the Farewell segment of the Denalifault system (Talkeetna and McGrath basins; Reed and Nelson,1980; Dickey, 1984) may also reflect right-lateral displacement,but structural controls on sedimentation are unclear in thesebasins given the available published information. Collectively,these nonmarine basinal strata record Late Eocene to LateOligocene right-lateral displacement and synorogenic sedimen-tation along much of the 2,100 km length of the Denali fault sys-tem from western Canada to southwestern Alaska.

This distinct episode of right-lateral, strike-slip tectonismprompted lateral shuffling along the suture zone, including tec-tonic escape (extrusion) of accreted terranes, sedimentary basins,and magmatic belts along the Denali, Nixon Fork, and Kaltagfaults in western Alaska (Scholl et al., 1992a, 1992b). Matchinginferred source lithologies north of the Denali fault with conglom-erate clast types south of the fault (Trop et al., 2004) and offsetEocene igneous belts (Reed and Lanphere, 1974; Cole, 1999)indicates ,30–40 km of post-Eocene, right-lateral displacementalong the central Denali fault, part of up to 400 km of LateCretaceous–Cenozoic right-lateral displacement along the east-ern and central segments of the Denali fault system (e.g., Eis-bacher 1976; Nokleberg et al., 1985; Lowey, 1998).

Latest Oligocene-Recent (26–0 Ma): Yakutat Collision and Regional Transpression

A second major phase of terrane collision and basin devel-opment characterized the southern margin of Alaska during lat-est Oligocene to Holocene time (Figs. 3H, 4H). Sedimentarybasins record deformation, magmatism, and exhumation associ-ated with oblique collision of the allochthonous Yakutat terrane.The stratigraphic record of this Neogene collisional event is wellexposed in outcrops in the Robinson Mountains of the Yakutatbasin, in the Wrangell Mountains, and the Tanana basin north ofthe central Alaska Range (Fig. 1D). Subsurface deposits in theCopper River, Northway, Susitna, and Cook Inlet basins alsorecord latest Oligocene-Recent sedimentation. The outboardsedimentary record of Neogene collision of the Yakutat terraneis recorded in the Middle Miocene-Holocene glacial-marinestrata of theYakataga Formation exposed in the Robinson Moun-tains (Figs. 3H, 10E). This formation is ,5 km thick and con-tains abundant evidence of syndepositional deformation, such asgrowth structures and progressive unconformities (Plafker 1967,1987; Miller, 1971). Ongoing collision of the Yakutat terranehas prompted incorporation of the Yakataga Formation into anoceanward-verging fold-and-thrust belt (Figs. 3H, 4H; Bruhnet al., 2004; Pavlis et al., 2004). Collision of the Yakutat terranein this part of the Gulf of Alaska has produced the largest con-centration of peaks higher than 4300 m on the North Americancontinent, including the second highest peak in North America(Mt. Logan). Sediment flux from this mountainous coastal set-ting via glaciers and proglacial streams has produced thick pack-ages of sediment in offshore basins with some of the highestsediment accumulation rates on Earth (e.g., Hallet et al., 1996;Merrand and Hallet, 1996; Meigs and Sauber, 2000; Jaeger et al.,2001; Sheaf et al., 2003).

In the Wrangell Mountains of east-central Alaska, thick sec-tions of interbedded volcanic and sedimentary strata record con-struction of a continental-margin arc that formed in response to oblique collision and subduction of the Yakutat terrane be-neath the continental margin (Richter et al., 1990; Preece andHart, 2004; Tidmore et al., 2005). Magma genesis commencedin western Canada and eastern Alaska ca. 26 Ma, coeval withnorthward translation of the Yakutat terrane along the QueenCharlotte fault. Magmatism migrated northwestward duringMiocene-Holocene time, progressively shutting down along the southeastern part of the arc in Canada as continental crust of the Yakutat terrane started to collide with the continentalmargin from 16 to 10 Ma (Richter et al., 1990; Ridgway et al.,1996). Oligocene-Miocene volcanic strata located in the Cana-dian part of the volcanic field have geochemical signaturesconsistent with magma genesis along leaky transform faults(Skulski et al., 1991). Miocene-Holocene volcanic strata in theAlaska part of the field, in contrast, have geochemical traits at-tributable to northward subduction of Pacific plate oceanic crustand the Yakutat terrane along the leading edge of the Yakutat ter-rane (Richter et al., 1990; Skulski et al., 1991; Preece and Hart,



2004). Successions of interbedded sedimentary and volcanic strata.500 m thick accumulated in localized intra-arc extensionalbasins (Frederika Formation and White River tillite; Eyles andEyles, 1989; Tidmore, 2004; Tidmore et al., 2005; Delaney,2006). Interbedded cobble-boulder conglomerate (Fig. 10F),volcanic-lithic sandstone, and lava flows were deposited onvent-proximal alluvial fans and braided rivers characterized byeffusive volcanic eruptions, lahars, debris flow, and stream flow(Fig. 3H). Pebble-cobble conglomerate, volcanic-lithic sand-stone, carbonaceous mudstone, coal, and minor limestone weredeposited in basin-axis meandering fluvial channels, swampyfloodplains, and lacustrine environments. Sandstone and con-glomerate compositional data document erosion of coeval erup-tive centers, Jurassic-Cretaceous marine basinal strata, andaccreted Triassic oceanic rocks. Lavas of the Frederika Forma-tion exhibit geochemical characteristics that are typical of sub-duction-related arc volcanic suites and are indistinguishablefrom those observed for a subset of ,5 Ma western Wrangell arclavas interpreted to have been emplaced in an intra-arc exten-sional setting. Arc magmatism has been relatively dormant overthe past 200,000 years, and the underlying Wadati-Benioff zonepresently exhibits weak seismicity, attributes that may reflect ac-commodation of Yakutat terrane plate motion along activestrike-slip faults (e.g., Fairweather and Totschunda faults;Richter et al., 1990).

To the west, Pliocene-Recent transpression along the CastleMountain-Bruin Bay fault system partitioned the outboard mar-gin of the Wrangellia composite terrane into localized uplifts andfault-bound nonmarine depocenters (Fig. 3H; Detterman et al.,1974; Fuchs, 1980; Bruhn and Pavlis, 1981; Lahr, et al., 1986;Haeussler et al., 2000, 2002). Deformation of remnant forearc-basin deposits continues to the present. The Castle Mountainfault, which truncates strata of the forearc basin, has a historicalrecord of two right-lateral earthquakes, and paleoseismologicstudies have interpreted four major earthquakes during the past2700 years (Detterman et al., 1974; Lahr et al., 1986; Haeussleret al., 2002).

The Neogene Tanana basin contains the inboard sedimentaryrecord of the collision of the Yakutat terrane. Strata in this forelandbasin are 2–3 km thick and have been deformed and exhumed inthrust faults that form the foothills along the north side of theAlaska Range (Figs. 3H, 4H; Ridgway et al., 2002, this volume).The lower part of the sedimentary package, the Usibelli Group,consists of 800 m of mainly Miocene strata that were deposited influvial, lacustrine, and peat-bog environments of the foredeep de-pozone of the foreland-basin system (Table 1; Fig. 10G). Compo-sitional data, as well as recycled Upper Cretaceous palynomorphs,indicate that the Miocene foreland-basin system was suppliedwith increasing amounts of sediment from lithologies currentlyexposed in thrust sheets located south of the basin (Fig. 5F;Ridgway et al., 1999a, this volume). The upper part of the sedi-mentary package, the Nenana Gravel, consists of 1200 m of mainlyPliocene strata that were deposited in alluvial-fan and braidplainenvironments in the wedge-top depozone of the foreland-basin

system (Thoms, 2000; Ridgway et al., this volume). Composi-tional data from conglomerate and sandstone (Figs. 5F, 10H), aswell as isotopic dating of detrital feldspar grains in sandstone andgranitic clasts in conglomerate, indicate that lithologies exposedin the central Alaska Range provided most of the detritus to thePliocene foreland-basin system. The age distribution of detritalfeldspar grains of the Nenana Gravel records progressive north-ward exhumation of plutons located south of the PlioceneTanana basin.

The modern deposystems of the Tanana basin have been re-organized by extensive Pleistocene glaciation and ongoing defor-mation. Rivers in the active wedge-top depozone are presentlyincised through deformed strata of the Nenana Gravel and aretransporting detritus eroded from the core of the central AlaskaRange northward into the active foredeep depozone of the Tananaforeland-basin system.We attribute Neogene contractile deforma-tion, uplift, and related flexural subsidence of the Tanana basin toregional transpressive shortening along the central Denali faultin response to oblique subduction of the Pacific plate, as well asunderthrusting of the Yakutat terrane (e.g., Plafker et al., 1994;Pavlis et al., 2004). Active deformation within the transpressivezone includes folding and thrust-fault deformation of proximalforeland basin strata (Lesh et al., 2001; Lesh, 2002; Ridgwayet al., 2002, this volume; Bemis, 2004; Bemis and Wallace,this volume; Lesh and Ridgway, this volume) and recent large-magnitude earthquakes along the Denali fault (Mw 6.7 andMw 7.9 in 2002; Eberhart-Phillips et al., 2003). GPS data indi-cate 8–9 mm/year of dextral-oblique slip on the active Denalifault, with some slip likely on parallel strands north of the mainfault trace (Fletcher, 2002).

SUMMARY: SEDIMENTARY RECORD OF COLLISIONAL TECTONICS

The sedimentary basin record of south-central Alaska con-tributes to our growing understanding of the complex processesassociated with crustal growth and recycling within the NorthAmerican Cordillera. The tectonic growth of southern Alaska isdefined by Mesozoic collision of the Wrangellia composite ter-rane and Cenozoic collision of the Yakutat terrane. The sedimen-tary basinal record of these two collisional events is summarizedas follows.

(1) Middle Jurassic marine volcaniclastic sandstone, mud-stone, chert, and tuff were deposited in narrow forearc andbackarc basins positioned south and north, respectively, of thevolcanic edifice of the Talkeetna-Chitina arc (Figs. 3A, 4A). Thissouth-facing oceanic arc system was probably located at substan-tially lower paleolatitudes during deposition of these strata.

(2) Late Jurassic syndepositional regional shortening andcoarse-grained sedimentation mark a change in the tectonicconfiguration of the northwestern Cordillera. Over 700 m ofcoarse-grained, clastic detritus was deposited in a forearc basin(Matanuska Valley-Southern Talkeetna Mountains) coeval with


86 Trop and Ridgway

final phases of oceanic-arc magmatism. In proximal retroarcbasins (Wrangell Mountains), ,600 m of syndeformational con-glomerate was deposited in the footwall of the south-dippingChitina thrust belt (Figs. 3B, 4B). In more distal retroarc depocen-ters, fine-grained turbidite sedimentation was initiated in a seriesof collisional foreland basins that presently extend for .2000 kmfrom British Columbia to southwestern Alaska (Kahiltna, Nut-zotin, Dezadeash, and Gravina basins). This time interval alsomarked the demise of Talkeetna-Chitina arc magmatism. The ces-sation of magmatism, exhumation of the Talkeetna-Chitina arc,and introduction of coarse-grained clastic detritus to bothretroarc and forearc basins may reflect either initial accretion ofthe previously combined Wrangellia and Peninsular terranes tothe continental margin of western North America or amalgama-tion of the Peninsular and Wrangellia terranes prior to collisionwith the former continental margin. We prefer the former inter-pretation. Oblique collision of the Wrangellia composite terrane,younging to the northwest, is inferred based on the diachronousages of Jurassic-Cretaceous deformation, uplift, and sedimenta-tion along the continental margin from British Columbia to south-western Alaska.

(3) During earliest Cretaceous time, Jurassic retroarc basi-nal deposits in the Wrangell Mountains basin were incorporatedinto an expanding, north-verging thrust belt associated with con-tinued collision of the Wrangellia composite terrane (Figs. 3C,4C). This regional deformation is marked by an angular uncon-formity across which sediment was bypassed into collisionalforeland basins (Kahiltna and Nutzotin basins) along the inboardmargin of the composite terrane. This tectonic configuration ledto exhumation and partial erosion of the Wrangell Mountainsforearc basin and the Wrangellia composite terrane, and pro-duced distinct upward-coarsening megasequences in collisionalforeland basins (Kahiltna and Nutzotin basins). The first sedi-ment with unequivocal North American continental marginaffinity was deposited in these basins during Early Cretaceoustime. The Early Cretaceous (Valanginian-Aptian) southern mar-gin of Alaska was reconfigured through development of theChisana arc. Construction of this arc marked the resumption ofsubsidence and deposition of clastic sediments in forearc basins.Forearc deposition was influenced by a series of south-dipping,syndepositional normal faults.

(4) Late Early Cretaceous to early Late Cretaceous time wascharacterized by regional deformation of retroarc basins by south-verging thrusts that are part of a regional thrust belt that extendsfor .2000 km from south-central Alaska to southern BritishColumbia (Figs. 3D, 4D). This thrust belt merged along strike intoa collisional basin that is recorded by strata of the Kahiltna assem-blage in south-central and southwestern Alaska.

(5) The Late Cretaceous tectonic setting of southern Alaskawas marked by folding, metamorphism, and exhumation ofretroarc basinal strata in a broad continental suture zone betweenoceanic rocks of the Wrangellia composite terrane and the quartz-rich metamorphic rocks of the former continental margin (Figs. 3E,4E). Subsequently, a latest Cretaceous-Paleocene continental arc

(Kluane arc) intruded and stitched the remnant oceanic arcs, de-formed retroarc basin strata, and the former continental margin.Locally, Latest Cretaceous post-collisional retroarc basinal strata(Pass Creek strata, lower Cantwell Formation) unconformablyoverlie deformed older syncollisional retroarc strata (Kahiltna as-semblage) and record sediment input from both coeval and rem-nant arc assemblages to the south of the suture zone, as well as fromthe former continental margin to the north. Northward convergenceand arc emplacement prompted development of a retroarc thrustbelt that produced thrust-top basins, such as the Cantwell basin ofthe central Alaska Range. These nonmarine to marginal-marinedeposits reflect subaerial exposure of the suture zone contempora-neous with persistent marine sedimentation in outboard forearcdepocenters. Forearc depocenters subsided into deep-water set-tings and were characterized by detritus derived from remnant andcoeval magmatic arcs.

(6) Paleocene-Neogene nonmarine forearc, intra-arc, andretroarc foreland basin strata contain the record of post-collisionalprocesses related to the Wrangellia composite terrane and docu-ment erosion of the now amalgamated continental arc rocks, rem-nant accreted oceanic arcs, and subaerially exposed segments ofthe subduction complex (Figs. 3F–3H, 4F–4H). Growth of thesouthern Alaska continental margin during Paleocene-EarlyEocene time is defined by emplacement of the McKinley plutons,mainly nonmarine deposition in forearc and retroarc regions, andcontinued expansion of the accretionary prism. Accretionaryprism and forearc basin deposits record coarse-grained deposi-tion, near-trench magmatism, and high-temperature metamor-phism linked to subduction of an oceanic spreading ridge from 60to 50 Ma.

(7) Middle Eocene-Oligocene time was characterized byregional transpressive deformation along regional strike-slipfaults (Figs. 3G, 4G). The sedimentary record of this deforma-tion is contained in localized strike-slip basin deposits exposedalong several segments of the Denali and Talkeetna faults dur-ing this interval. Transpressive deformation throughout interiorsouth-central Alaska resulted in a regional uplift that providedabundant sediment for the actively subsiding Cook Inlet forearcbasin.

(8) Asecond major phase of terrane collision and basin devel-opment characterized the southern margin of Alaska during latestOligocene-Holocene time (Figs. 3H, 4H). Northward translationand collision of the Yakutat terrane resulted in the growth of thelargest coastal mountain range on Earth (St. Elias Mountains),construction of a new continental arc (Wrangell-St. Elias Moun-tains), deformation of remnant forearc basinal deposits in theWrangell and Talkeetna Mountains, and renewed uplift of theAlaska Range. The sedimentary record of this collision is con-tained in the Tanana retroarc foreland basin north of the AlaskaRange, intra-arc basins in the Wrangell Mountains, and in colli-sional foreland basins on the Yakutat terrane and offshore Gulf ofAlaska. This phase of collision continues to the present as evi-denced by seismicity, geodetic data, and some of the highest sed-iment accumulation rates on Earth.



ACKNOWLEDGMENTS

Our studies in southern Alaska would not have been possiblewithout the original geologic mapping by Bela Csejtey, ArthurGrantz, Davey Jones, Ed MacKevett, Warren Nokleberg, GeorgePlafker, Don Richter, Clyde Wahrhaftig, and Gary Winkler, amongmany others. We thank the U.S. Geological Survey, Wrangell-St. Elias National Park, Denali National Park, and the Bureau ofLand Management for supporting our research. Assistance fromNational Park geologists Phil Brease and Danny Rosenkrans hasbeen especially important. Primary funding was provided by theNational Science Foundation, Donors of the Petroleum ResearchFund administered by the American Chemical Society, AnschutzCorporation, Forest Oil Corporation, and the U.S. GeologicalSurvey. Additional support was provided by the Geological Soci-ety of America, American Association of Petroleum Geologists,Sigma Xi, Purdue Research Foundation, and Bucknell Programfor Undergraduate Research. This synthesis includes importantcontributions from many collaborators, including geochronology(George Gehrels, Paul Layer, Matt Rioux, Terry Spell), palynol-ogy (Rob Ravn, Art Sweet, James White), paleontology (RobertBlodgett, Jim Haggart, Anita Harris, Mike Mickey, Scott Wing),igneous petrology and geochemistry (Ron Cole, Bill Hart, DarinSnyder, Tom Skulski), metamorphic petrology (Cam Davidson,Sarah Roeske), structural geology (Mike O’Neill, Terry Pavlis),and paleomagnetic analysis (John Stamatakos). We thank cur-rent and former students at Purdue University (Kevin Eastham,Brian Hampton, Jay Kalbas, Paul Landis, Mark Lesh, and JeffManuszak) and Bucknell University (Ryan Delaney, Aubri Jensen,Emily Short, Darren Szuch, Clay Slaughter, Rob Tidmore, andJohn Witmer) for their many contributions. KDR also thanksDwight, Lauren, Alice, and Dan Bradley for their generous hos-pitality in letting the Purdue basin analysis group use their homeas a base camp.

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