IN PRESS

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1

ABSTRACT

Upper Cretaceous sedimentary strata exposed in south-central Alaska provide insight on tectonic processes that shaped the northern Pacifi c margin following accretion of the Wrangellia composite ter-rane, the largest addition of crust to North America over the past 100 m.y. Sedimento-logic, geochronologic, biostratigraphic, and petrographic data from the Matanuska For-mation permit reconstruction of the tectono-sedimentary history of strata in a forearc basin constructed upon accreted oceanic-arc crust. The Matanuska Formation consists of >3 km of sedimentary strata exposed in the northern Chugach Mountains, Matanuska Valley, and southern Talkeetna Mountains of interior south-central Alaska. Measured stratigraphic sections and lithofacies analy-ses demonstrate that mass slumps and slides, debris fl ows, and turbidity currents depos-ited Campanian–Maastrichtian sandstone, conglomerate, and mudstone on a gullied, trenchward-dipping submarine ramp. Ben-thic foraminifera, inoceramid bivalves, and Nereites ichnogenera indicate deposition mainly at bathyal water depths. Sandstone and conglomerate petrofacies are charac-terized by monocrystalline quartz, plagio-clase feldspar, and volcanic lithic fragments (Q39F40L21, Qm29F40Lt32, Lm25Lv42Ls32, and Qm42P54K4). Jurassic–Cretaceous arc plutons exposed north of the basin were an impor-tant sediment source, based on U-Pb zircon ages of granitoid clasts from conglomerate and detrital zircons from sandstone. Coeval arc plutons were unroofed relatively quickly, judging by the presence of 77–71 Ma detri-tal zircons in sandstone and 79–77 Ma gra-nitic clasts in conglomerate, together with Maastrichtian (71–65 Ma) ammonite and foraminifera fossils. Sparse Paleozoic–Trias-sic detrital zircons indicate minor sediment

contribution from inboard sources, including the Yukon-Tanana composite terrane and recycled Jurassic–Cretaceous sedimentary strata (Kahiltna assemblage).

New data from the upper Matanuska For-mation, together with recent studies from age-equivalent strata exposed in the Alaska Range and Wrangell Mountains, provide an exceptional example of basin development along a subduction margin shortly following accretion of an oceanic arc. Forearc basin development was dominated by subsidence and sediment gravity fl ow deposits enriched in plutonic and volcanic clasts eroded from both remnant- and coeval-arc plutons. Within the arc, newly recognized conglomerate in the northern Talkeetna Mountains records ero-sion of coeval- and remnant-arc source ter-ranes to the south and Precambrian–Paleo-zoic sources to the north. Farther inboard, syndepositional shortening prompted thrust-top basin development and accumulation of alluvial-lacustrine strata derived from both the former continental margin to the north and accreted oceanic rocks to the south. Regional subsidence and basin development terminated during late Maastrichtian–early Paleocene time, coincident with subduction of progressively younger oceanic lithosphere inboard of an oceanic spreading center.

Keywords: Alaska, detrital geochronology, fore-arc basin, Late Cretaceous, Matanuska Valley.

INTRODUCTION

Southern Alaska is considered an archetypal area for studying tectonic processes responsible for the growth of continental margins (e.g., Coney et al., 1980; Plafker and Berg, 1994; Trop and Ridgway, 2007). Subduction of oceanic lithosphere, accretion of allochthonous terranes, arc magmatism, and exhumation along regional strike-slip faults have produced the largest coastal range on Earth (Saint Elias Mountains), the largest topographic relief in North America

(Alaska Range), and the most explosive volca-nic fi eld in the Pacifi c region (Wrangell Moun-tains). Despite its tectonic signifi cance, however, much of the geologic history of southern Alaska remains poorly understood.

Growth of the southern Alaska continental margin is attributable to tectonic processes asso-ciated with two major collisional events: Meso-zoic collision of the Wrangellia composite terrane and Cenozoic collision of the Yakutat terrane. Collision of the Wrangellia composite terrane represents the largest addition of juvenile crust to western North America during the past 100 m.y. (e.g., Coney et al., 1980; Plafker and Berg, 1994). Recent studies provide improved constraints on crustal conditions before and during collision (e.g., Ridgway et al., 2002; Clift et al., 2005; Trop et al., 2005a; Amato et al., 2007; Hampton et al., 2007; Kalbas et al., 2007; Rioux et al., 2007). Models explaining the postcollisional, latest Cre-taceous–Paleocene tectonic framework postulate a continental-margin arc and paired accretionary prism related to northward subduction of oceanic lithosphere beneath the Wrangellia composite terrane (e.g., Plafker and Berg, 1994; Nokleberg et al., 1998). Geologic evidence for this model is based primarily on a >2000-km-long belt of 85–60 Ma calc-alkaline igneous rocks (black pat-tern in Fig. 1A), which crop out inboard (north) of age-equivalent accretionary prism deposits (Chugach terrane in Fig. 1A; Moll-Stalcup, 1994; Plafker et al., 1994).

Uppermost Cretaceous sedimentary deposits exposed throughout southern Alaska can be used to evaluate the aforementioned tectonic model (CB, MB, PB, PC, and WB in Fig. 1A). Campa-nian–Maastrichtian sedimentary strata exposed in the Matanuska Valley–Talkeetna Mountains (Matanuska Formation; MB in Fig. 1A) are tentatively interpreted as forearc basin deposits based on their position outboard (south) of 80–65 Ma calc-alkaline plutons and inboard (north) of a coeval accretionary prism (e.g., Trop and Ridgway, 2007). Despite the potential of the Matanuska Formation for providing an improved understanding of the tectonic development of the

Latest Cretaceous forearc basin development along an accretionary convergent margin: South-central Alaska

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

*E-mail: jtrop@bucknell.edu.

GSA Bulletin; Month/Month 2007; v. 119; no. xx/xx; p. XXX–XXX; doi: 10.1130/B26215.1; 10 fi gures; 2 tables; Data Repository Item 2007195.

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2 Geological Society of America Bulletin, Month/Month 2007

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CF

BF

Denali

Fault

PacificOcean

EK

Cook Inlet

Denali

PZ

Fig. 2

Pacific plate

55 mm/yr

TF

PC

KodiakIsland

BRF

Alaska

Fig. 1A

Fault

AR-TM belt

WB

0 200 km

C. Alaska

N. Talkeetna Mtns.

A

M

Wrangell Mtns.

Alaska

Peninsula

153oW

PZ

WF

AVX’

Ale

utia

n T

renc

h

X

BRF

A

N

Wrangellia composite terrane

PZ

Chugach Terrane

Terrane

Yakutat terrane

MOHO?

Approx. top of Benioff seismic zone25

50

75

25BRF Maastrichtian

strata

50

Paleogene strata

AVS.L.

Arc Forearc basin Accretionary prismS.L.Trench

XX’

Uppermost Cretaceous plutons Neogene accretionary prism and Yakutat terrane Paleogene accretionary prism (Prince William terrane) Mesozoic accretionary prism (Chugach terrane)Uppermost Cret. sedimentary strata (CB, MB, PB, PC, WB) Mesozoic accreted terrane (Wrangellia composite terrane)Paleozoic-PreCambrian terranes (includes Yukon-Tanana terrane)

Neogene strata

EXPLANATION

80–65 Ma K-Ar, Ar-Ar age80–65 Ma U-Pb age

Cretaceous plutons (145–75 Ma)Jurassic plutons (201–145 Ma)Fault (dotted where concealed)

75

WCT

B

Depth, in km

PZ

MC

WCT

Oceanic Crust

PB

Prince William

141oWCB

MB

61o

Figure 1. (A) Map showing the present distribution of accreted terranes, uppermost Cretaceous sedimentary deposits, accretionary-prism deposits, and Jurassic–Cretaceous magmatic belts in southern Alaska. This study emphasizes uppermost Cretaceous forearc (MB, PB, WB), intra-arc (PC), and retroarc (CB) deposits exposed outboard (south), within, and inboard (north) of coeval continental-arc plutons (black). Coeval accretionary prism deposits are preserved in the Chugach terrane. This paper integrates new data from the Matanuska Valley–Talk-eetna Mountains basin (MB) with recent studies from the Cantwell basin (CB), Pass Creek area (PC), and Wrangell Mountains basin (WB). Uppermost Cretaceous and younger strata and magmatic belts formed within a juvenile continental margin composed of the allochthonous Wrangellia composite terrane (WCT) and para-autochthonous Paleozoic–Mesozoic metamorphic rocks (PZ). Abbreviations not explained on map: A—Anchorage; AR-TM—Alaska Range–Talkeetna Mountains magmatic belt; AV—Augustine volcano; BRF—Border Ranges fault; CB—Cantwell basin; CF—Castle Mountain fault; M—McCarthy; PB—Peninsula deposits; PC—Pass Creek strata; TF—Taral Fault; WB—Wrangell Mountains basin; WF—West Fork fault. Modifi ed from Plafker et al. (1994) and Moll-Stalcup et al. (1994). Age data for uppermost Cretaceous plutons are from Magoon et al. (1976), Csejtey et al. (1978, 1992), Winkler (1992), Drake and Layer (2001), Harlan et al. (2003), and Davidson and McPhillips (2007). (B) Cross section showing generalized crustal structure and tectonic elements of the southern Alaska convergent margin. See A for line of section. Note active northward subduction, arc magmatism, and forearc basin subsidence inboard of the Aleutian trench, analogous to the inferred tectonic framework of the Matanuska Valley–Talkeetna Mountains region during latest Cretaceous time. S.L.—sea level. MC—McHugh Complex, pre-Maastrichtian part of accretionary prism. Adapted from Hudson and Magoon (2002).

Forearc basin development in southern Alaska

Geological Society of America Bulletin, Month/Month 2007 3

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northern Pacifi c margin, depositional and prov-enance characteristics of the strata are basically undocumented. The original U.S. Geological Survey mapping and paleontologic studies pro-vided a useful stratigraphic framework (Grantz, 1960a, 1960b, 1964; Jones, 1963, 1967), but subsequent sedimentologic and provenance studies have not been reported. The goal of this paper is to present (1) lithologic, petrographic, paleontologic, and geochronologic data from the upper Matanuska Formation, (2) reconstruc-tions of sediment sources and depositional con-ditions, and (3) interpretations of the latest Cre-taceous tectonic framework and stratigraphic linkages across the southern Alaska convergent margin. These new data represent the fi rst mea-sured stratigraphic sections, paleocurrent mea-surements, and geochronologic data reported from the upper Matanuska Formation. Results include >4000 m of bed-by-bed stratigraphic sections, >500 paleocurrent measurements, new biostratigraphic data, modal analyses of 36 sandstone thin sections, and U-Pb zircon ages from six igneous clasts and one sandstone sample. Collectively, these data indicate that the upper Matanuska Formation was deposited in an actively subsiding, deep-water forearc basin infl uenced by erosion of a juvenile continental-margin arc and accreted oceanic arc above a north-dipping subduction zone.

REGIONAL GEOLOGY

Matanuska Valley–Talkeetna Mountains Geology and Tectonic History

Middle Jurassic–Oligocene sedimentary strata as much as 6-km-thick crop out in a 90-km-long and 20–70-km-wide outcrop belt in the Mata-nuska Valley, southern Talkeetna Mountains, and northern Chugach Mountains (MB in Fig. 1A). These strata depositionally overlie Jurassic igne-ous rocks of the 7-km-thick Talkeetna oceanic arc, which forms part of the Wrangellia com-posite terrane (Fig. 1A). The Talkeetna arc col-lided with inboard terranes (e.g., Yukon-Tanana in Fig. 1A) sometime during the Triassic–early Late Cretaceous (e.g., Ridgway et al., 2002; Trop et al., 2005a; Clift et al., 2005; Hampton et al., 2007; Kalbas et al., 2007). Late Cretaceous–Paleocene calc-alkaline plutons and volcanic rocks overlap the Wrangellia composite terrane and Yukon-Tanana terrane, recording postcol-lisional arc magmatism (Fig. 1A; Moll-Stalcup, 1994; Plafker and Berg, 1994). Late Creta-ceous–Paleocene magmatism is attributable to north-dipping subduction of oceanic lithosphere based on the presence to the south of a coeval accretionary prism that decreases southward in both age and metamorphic grade (Figs. 1A, 1B;

Plafker and Berg, 1994; Sample and Reid, 2003; Haeussler et al., 2006). The Border Ranges fault system juxtaposes the accreted Talkeetna arc and younger rocks against the accretionary prism (BRF in Fig. 1A). This fault system represents a paleo-subduction-zone thrust that accommodated northward underthrusting of oceanic lithosphere during Early Jurassic–early Late Cretaceous time and postcollisional dextral-oblique slip during latest Cretaceous–Paleogene time (Roeske et al., 2003; Clift et al., 2005; Pavlis and Roeske, 2007). To the north, the Castle Mountain fault system disrupts Jurassic–Oligocene rocks in the southern Talkeetna Mountains (CF in Fig. 1A). Offset geologic features along the fault indicate a few tens to >100 km of dextral-oblique slip dur-ing Cretaceous–Holocene time (Haeussler et al., 2002; Pavlis and Roeske, 2007).

Matanuska Formation: Outcrop Distribution and Age

This paper concentrates on the stratigraphy of the upper Matanuska Formation, a >3-km-thick succession of Campanian–Maastrichtian silici-clastic sedimentary strata exposed in the Mata-nuska Valley and southern Talkeetna Mountains (MB in Fig. 1A). New measured sections from the eastern outcrop belt document a maximum preserved thicknesses of ~2100 m (e.g., section 20 in Fig. 2). In the eastern outcrop belt, the upper Matanuska Formation disconformably overlies Albian–Santonian nonmarine to shal-low-marine strata of the lower Matanuska For-mation (Bergquist, 1961). In the western outcrop belt (Matanuska Valley), the base of the upper Matanuska Formation is not exposed. Paleocene–Eocene sedimentary and volcanic strata overlie the upper Matanuska Formation along an angular unconformity (e.g., section 19 in Fig. 2).

Previous studies inferred a Campanian– Maastrichtian depositional age for the upper Matanuska Formation based on marine fossils collected within the context of geologic map-ping. Jones (1963) established two faunal zones on the basis of ammonite and bivalve fossils: a Campanian zone (Inoceramus schmidti) and a latest Campanian–Maastrichtian zone (Inoc-eramus kusiroensis, Diplomoceras notabile, Pachydiscus sp.). Bergquist (1961) developed three Campanian–Maastrichtian faunal zones based on detailed micropaleontological studies. During the present study, fossils were collected from stratigraphic sections that lacked previous age control. Ammonite fossils from Black Shale Creek (section 13 in Fig. 2) are similar to late Campanian–Maastrichtian taxa reported from the upper Matanuska Formation (Guadryceras tenuiliratum; Neophylloceras hetonaiens; Hag-gart, 2004). Foraminifera from sections 19 and

24 are diagnostic of Campanian–Maastrichtian faunal zones (e.g., Bathysiphon vitta, Cribrosto-moides cretaceous, Haplophragmoides impen-sus; M. Mickey, 2004, personal commun.). See the GSA Data Repository for foraminifera data1.

New geochronologic data support late Cam-panian–Maastrichtian deposition of the upper Matanuska Formation. The youngest and oldest concordant U-Pb ages from a sandstone sample collected within the Pachydiscus ammonite zone are 71.7 ± 2 Ma and 441 ± 8 Ma, respec-tively. The occurrence of 34 grains (41% of ana-lyzed 82 grains) with 84–71 Ma isotopic ages, together with 79–77 Ma granitoid clasts from conglomerate, indicates that the sampled inter-val was deposited no earlier than Campanian time (using Gradstein et al. [2005] time scale). Geochronologic data are discussed in more detail in the following text. (See footnote 1 for U-Pb analytical methods and data.)

SEDIMENTOLOGIC DATA

The sedimentology and inferred depositional environments of the upper Matanuska Formation are based on >4000 m of detailed measured sec-tions from 27 locations (Fig. 2). For space con-siderations, only generalized sections are pre-sented here (Fig. 3). (See footnote 1 for detailed sections.) Common lithofacies are described and interpreted in Table 1. One lithofacies consists of chemical deposits (facies 1), one lithofacies volcanic deposits (facies 2), and nine lithofacies are defi ned by grain size, bed thickness, and sed-imentary structures (facies 3–11). Genetically associated lithofacies are grouped into seven lithofacies associations that can be interpreted in terms of depositional processes and environ-ments. Six lithofacies associations dominate the stratigraphy and represent deposition on subma-rine slope and/or ramp subenvironments via sus-pension fallout (LA1), turbidity currents (LA2, 3), debris fl ows and/or slurries (LA4), mass slides and/or slumps (LA5), and channelized sediment gravity fl ows (LA6). A fl uvial lithofa-cies association (LA7) is spatially limited to the northern part of the outcrop belt.

Lithofacies Association 1: Marine Suspension-Fallout Deposits

DescriptionConcretionary horizons (lithofacies 1), tuff

(lithofacies 2), and gray to black mudstone

1GSA Data Repository Item 2007195, Analytical techniques, age data, detailed stratigraphic sections, and foraminifera data, is available at www.geoso-ciety.org/pubs/ft2007.htm. Requests may also be sent to editing@geosociety.org.

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4 Geological Society of America Bulletin, Month/Month 2007

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(lithofacies 3, 4) characterize this fi ne-grained association (Fig. 4A). Strata are typically structureless or parallel laminated with sparse starved-ripple laminae. Diverse open-marine foraminifera fossils and horizontal trace fos-sils, mainly Nereites and Chondrites, are pre-served within the mudstone. Many concre-tionary horizons contain intact ammonite and inoceramid bivalve fossils. Most tuffs are thin bedded, dominated by clay alteration products, and contain sparse relict glass shards.

InterpretationMudstones record suspension settling from

hemipelagic plumes and muddy sediment gravity fl ows. Concretionary horizons rep-resent diagenetic growth near the sediment-water interface, probably during reduced rates of sediment accumulation (e.g., Raiswell, 1987). Episodic volcanic eruptions produced ash layers that were preserved within this low-energy association and subsequently altered by diagenetic processes. Thick sections of this facies association refl ect episodes of rela-tively slow deposition by suspension settling interrupted by rapid deposition by dilute tur-bidity currents.

Lithofacies Association 2: Thin-Bedded Turbidites

DescriptionThis association consists of thin-bedded sedi-

mentation units (beds that each represent one gravity-fl ow event) characterized by rippled siltstone (Fig. 4B; lithofacies 4) and fi ne- to medium-grained sandstone fi ning upward to siltstone (lithofacies 5). Units are tabular, with sharp, low-relief bases (Figs. 4C, 4D). Normal grading and incomplete Bouma sequences are common (Fig. 4C). Mudstone and siltstone (lithofacies 3) separate individual sedimentation units (Fig. 4C). Finer-grained sedimentation units are nongraded to normal graded and plane laminated or current ripple laminated (lithofa-cies 4). Coarser-grained units exhibit erosive sole marks and basal lags of coarse-grained sandstone, wood fragments, and Inoceramus fossil fragments (Fig. 4D; lithofacies 5).

InterpretationSharp-based, normal-graded sedimentation

units are a characteristic product of turbid-ity currents (e.g., Lowe, 1982). Siltstone with starved ripple laminae and rare normal grading

(lithofacies 4) records weak tractive transport and suspension settling from lower concen-tration turbidity currents (e.g., Piper, 1978). Thicker, sandy units represent the deposits of higher concentration turbidity currents (litho-facies 5).

Lithofacies Association 3: Massive to Plane-Laminated, Thick-Bedded Turbidites

DescriptionThis association consists of medium- to

thick-bedded sandstone units that range from normal graded and parallel laminated (lithofa-cies 6) to structureless (lithofacies 7). Tabular, sheet-shaped layers dominate (Figs. 4E–4G), although thicker, amalgamated units locally fi ll low-relief incisions that are tens of meters across (Fig. 4H). Individual sandstone units are amalgamated or separated by structureless to laminated mudstone (lithofacies 3). Lower bounding surfaces are typically sharp, fl at, and nonerosional, although low-relief scours are present locally. Internally, these units are typi-cally structureless except for diffuse, discontin-uous parallel lamination (Fig. 5A) and dewater-ing features (Figs. 5B). Flute casts, groove casts,

Kg

1

5

4

N

Km

Km

Kg

uuu

78 9

20

MatanuskaValley

Talkeetna

Mountains

Fault

Castle

Mountains

1211

SheepMtn.

16

22

23

24

61o45'

25

Copper RiverBasin

148o149o

Border Ranges

Fault

10

1817

21

26 27

3

6

1413

Matanuska River

0 km 20

Tvi

Chugach

MzM

2

Mountain

Ju

Ju

Ju

JuJu

Ju

Ju

Ju

Ts

Kg Km

1519

Kl

Kl

Kv

Kv

Quaternary surficial deposits and iceCretaceous-Oligocene Sedimentary Strata (MB in Fig. 1A) Paleogene Wishbone, Chickaloon, Arkose Ridge, Tsadaka Fms.—conglomerate, sandstone, mudstone, tuff. Upper Cretaceous Matanuska Fm.—mudstone, sandstone, conglomerate, coal, limestone, tuff. Lower Cretaceous Nelchina Limestone, unnamed strata—calcareous sandstone, siltstone, mudstone.Cretaceous-Oligocene Igneous Rocks Eocene volcanic and intrusive rocks attributed to slab-window magmatism. Latest Cretaceous–Paleocene pluton attributed to arc magmatism; subordinate Paleogene intrusives.Accreted Jurassic Oceanic Arc Lower to Upper Jurassic plutonic and volcanic rocks, subordinate marine sedimentary strataMesozoic Accretionary Prism (Chugach terrane in Fig. 1A) Campanian–Maastrichtian Valdez Group Permian–Cretaceous McHugh Complex High-angle fault

(u-upthrown block)Measured stratigraphic section location RiverDextral-oblique fault 33u

TsKm

Ju

Kg

Kv

Kn

EXPLANATION

MzM

Tvi

Figure 2. Geologic map of the Matanuska Valley, southern Talkeetna Mountains, and northern Chugach Mountains. See Figure 1A for map location. Dark gray areas (Km) repre-sent Cretaceous sedimentary strata that are the focus of this study and are interpreted as forearc basin deposits. Num-bered circles (1–27) show loca-tions of measured stratigraphic sections discussed in text. Geol-ogy from Wilson et al. (1998).

Forearc basin development in southern Alaska

Geological Society of America Bulletin, Month/Month 2007 5

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2—GraniteCreek (m)

6-SheepValley (e)

7—Glenn Hwy. (m)

4—GlennHighway (m)

5—Red Mountain (m)

8—Mat. River (m)

9—GlennHighway (m)

3—MatanuskaRiver (e)

10-Glenn Highway (m)

11—Packsaddle Mountain (e)

19—BillyMountain

21—Squaw Creek-Syncline Mountain

24EurekaCreek

Measured sections north of Castle Mountain Fault

16—Mazuma Creek

18—LittleOshetnaRiver

14—Boulder Creek

*M562*24860

*8587*24214*M566

*24201*24208-12*24219

*24191-24195 *25949

*26030

EastWest

*24871*24227

26NelchinaRiver

*M593*M594

27NelchinaRiver

*M593*M594

20—Alfred Creek

*8596

*M577*M578*M580

*M6696

*M584

13—Black Shale Creek

Campanian Inoceramus schmidti zone

1—AlaskaRailroad (m)

Late Campanian–Early MaastrichtianPachydiscus kamishakensis zone

Measured sections south of Castle Mountain Fault EastWest

FA1

FA2 FA2 FA2

FA2

Tuff

676

m

150m

FA6 25m

Eocene conglomerate

erodederoded

eroded

141

m

17—Upper CardiocerasCreek

21m

Eocene cong.

7m

eroded

eroded eroded

erodederoded eroded

eroded eroded

109m

eroded

410m

440m

FA613

2mFA1 FA1

530m

170m

2000

+m

8m6m

286m

35m

eroded eroded eroded eroded eroded eroded

eroded

eroded

*8559

FA7FA7

FA1 FA1 FA1FA1

FA1

FA1 FA1FA1

Rootlet structures

Plant fragment fossil

Bivalve fossils

Ammonite fossils

Calcareous concretions

Mass slide/slump folds

Channel-fill complexes

Geochronology samples

*M56

Tuff

Claystone/siltstone

Siltstone/carb. shale

Sandstone

Conglomerate

USGS. fossil locality

Thrust fault

FA3 FA3 FA3

FA4

FA3

FA2/3

FA4

FA4

FA3

FA1/2

FA4

FA3

FA3FA3

FA3 FA3

FA2/3

FA3

FA2/3

FA1/2

184m

193m

266m

FA7

FA4FA3

FA1/2

22InoceramusCreek

Late Campanian–Early MaastrichtianPachydiscus kamishakensis zone

Campanian Inoceramus schmidti zone

Tuff18m

26m

FA1FA1 FA1FA1

Explanation

Sections north of Caribou Fault

200+

m

104-CC

MR3-CC

MR3-SS

104-CC

Figure 3. Simplifi ed measured stratigraphic sections from upper Matanuska Formation. Note concentration of thick-bedded sandstone turbi-dites (LA3) south of Castle Mountain fault and thick successions of mudstone, thin-bedded turbidites (LA2), and channel-fi ll deposits (LA6) north of Castle Mountain fault. See text for discussion and Figure 2 for section locations. Ck—Creek; USGS—U.S. Geological Survey.

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6 Geological Society of America Bulletin, Month/Month 2007

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prod marks, and bounce marks are common on bed bases (Figs. 5C, 5D). Sparse Inoceramus fossil fragments, disseminated carbonaceous debris, intraformational mudstone clasts, and truncated burrows occur sporadically.

InterpretationHigh-concentration turbidity currents depos-

ited this facies association. Normal-graded, plane-laminated sedimentation units (lithofacies 6) record waning fl ow in which stable upper fl ow regime conditions persisted long enough to organize the bedload. Massive sedimenta-tion units with water-escape structures (lithofa-cies 7) refl ect abrupt, en masse collapse of sand from dense sediment clouds (Lowe, 1982) or aggradation of the depositional interface dur-ing quasi-steady fl ow (Kneller and Branney, 1995). Intraformational mudstone clasts record upslope erosion by sediment gravity fl ows. Fluid-escape structures, load marks, and con-volute laminae indicate syndepositional to early

postdepositional deformation and escape of pore water that was trapped during rapid deposition.

Lithofacies Association 4: Debris-Flow and Slurry Deposits

DescriptionMassive, clast-rich pebble-cobble conglomer-

ate (lithofacies 8) and sandstone and mudstone with fl oating gravel clasts (Fig. 5E; lithofacies 9) constitute this coarse-grained association. Mudstone rip-up clasts with maximum diam-eters >60 cm are common within matrix-rich units. Inoceramus fossil fragments are locally preserved within the matrix.

InterpretationLithofacies 8 and 9 represent the deposits of

debris fl ows. Moderate to poor sorting, sand-stone matrix, and limited clay content suggest deposition of clast-rich units by debris fl ows that maintained clast support through frictional

dispersion from high basal shear stresses and clast buoyancy related to high sediment concen-trations (e.g., Nemec and Steel, 1984). Muddier units with fl oating sand grains and mudstone clasts are similar to the deposits of watery debris fl ows (slurry deposits).

Lithofacies Association 5: Mass Slump and/or Slide Deposits

DescriptionThis association consists of deformed thin- to

medium-bedded sandstone and mudstone units (lithofacies 5, 6, 7). Deformation is expressed as rounded, overturned folds and normal and reverse faults with <50 cm displacement (Figs. 5F–5H). Individual sandstone beds are locally thicker in fold hinges relative to fold limbs. At some loca-tions, syndepositional folds directly overlie cha-otic zones with sandstone blocks and distorted mudstone (Fig. 5G). Deformed horizons are ~8–25 m thick and bound by nondistorted strata.

TABLE 1. SUMMARY OF LITHOFACIES DESCRIPTIONS AND INTERPRETATIONS FOR THE UPPER MATANUSKA FORMATION (CAMPANIAN–MAASTRICHTIAN)

Lithology Textures Sedimentary structures

Bounding surfaces and bed thicknesses

Fossils and outsized clasts

Inferred process of deposition

1 Concretionary horizons

Calcareous ellipsoids in mudstone and/or siltstone

Cone-in-cone structures

Sharp bases, tops Beds up to 80 cm

InoceramusAmmonites Wood fragments

Diagenetic concretion formation during sediment starvation

2 Tuff Yellow-orange- weathering altered volcanic ash

Structureless Sharp bases Tops sharp to irregular Most beds <5 cm thick

None Suspension fallout of volcanic ash

3 Mudstone Gray to black mudstone and subordinate claystone, siltstone

Generally massive Rare parallel laminae

Gradational bases, tops Beds 0.2 to >10 m thick Individual beds often

indiscernible

InoceramusAmmonites

Hemipelagic suspensionfallout and low- concentration turbidity currents

4 Mudstone and/or siltstone heteroliths

Mudstone, siltstone, and fine sandstone interbeds

Generally massive, Sparse normal grading Parallel to undulatory laminationStarved ripple lamination

Sharp to gradational Laminae <1cm thick Amalgamated packages

up to 10 m

Sparse horizontal trace fossils

Hemipelagic suspensionand low-concentration turbidity currents

5 Thin, graded sandstone and/or siltstone

Gray to black siltstone; coarse to very fine sandstone

Normal grading Bouma Tcde, Tab, Tabc Ripple, parallel laminae Rare climbing ripples Groove, flute, load casts

Bases sharp; tops sharp to gradational

Most beds 3–30 cm thick

Sparse horizontal trace fossils

Wood, plant, and shelly debris

Hemipelagic suspensionand low-concentration turbidity currents

6 Structured sandstone

Medium to very fine sandstone, siltstone

Normal grading Bouma A-B divisions Plane lamination Groove, flute, load casts Dish/pillar structures Convolute lamination

Bases sharp and often erosional;

Tops sharp to gradational

Beds 15–75 cm thick

Truncated burrows Wood fragments Shell fragments Mudstone clasts

High-concentration turbidity currents

7 Massive sandstone

Coarse to very fine sandstone, siltstone

Nongraded to crude normal grading

Groove, flute, load casts Dish and/or pillar structures Convolute lamination

Sharp to erosive bases Sharp tops Beds 0.2–4.2 m thick Amalgamated packages

up to 30 m thick

Wood fragments Mudstone clasts

High-concentration turbidity currents

8 Clast-rich pebble-cobbleconglomerate

Moderately to poorly sorted, subrounded to rounded clasts

Rare, poorly developed normal grading

Sharp, irregular bases irregular tops

Beds 1–3 m thick

Rare wood and shell fragments

High-concentration debris flows

9 Matrix-rich pebble-cobbleconglomerate, muddy sandstone

Moderately to poorly sorted, subrounded to rounded clasts

sesab ralugerri ,prahS tnesbAGradational tops Beds 20–200 cm thick

Rare wood and shell fragments

Debris flows, slurry flows

10 Imbricated pebble-cobbleconglomerate

Moderately sorted, subrounded to rounded clasts

Clast imbrication Sharp, erosional bases Beds 2–4 m thick

None Unidirectional subaqueous flow

11 Greenish-gray siltstone

Siltstone Faint parallel lamination Rootlet (?) structures

Bases irregular, Tops eroded

Disseminated plant matter

Suspension fallout

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Geological Society of America Bulletin, Month/Month 2007 7

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Sc

Sm

M

M

SfSm

MSh

Sh

A B

C

E

G H

F

D

Figure 4. Representative photos of lithofacies 1–7. (A) Concretionary horizons (white arrows, lithofacies 1), gray mudstone (lithofacies 3), and thin-bedded sandstone (black arrows, lithofacies 5) characterize lithofacies association 1. Hammer (lower right, 30 cm) for scale. (B) Ripple-laminated sandstone (black arrows) and mudstone heteroliths (lithofacies 4). (C) Thin-bedded, normal-graded sedimentation units (light col-ored, lithofacies 5) separated by structureless mudstone (dark colored, lithofacies 3). Person (lower right) for scale. (D) Close-up of thin-bed-ded, normal-graded sandstone (lithofacies 5) encased in mudstone (M). Note sharp basal contact, normal grading from coarse (Sc) to medium (Sm) grained sandstone, and Inoceramus shell fragments (arrows). Pen (lower right, 14 cm) for scale. (E) Medium- to thick-bedded sedimenta-tion units (lithofacies 6, 7) that are amalgamated or separated by structureless mudstone (lithofacies 3). Outcrop is ~7 m high. (F) Close-up of medium-bedded, normal-graded sedimentation unit (lithofacies 6). Note sharp base (arrow) and upward fi ning from medium (Sm) to fi ne-grained (Sf) sandstone, mudstone (M), and shale (Sh). (G) Massive, sheet-shaped thick-bedded sedimentation units (lithofacies 7) separated by dark mudstone (lithofacies 3). Outcrop is ~18 m high. (H) Amalgamated thick-bedded sedimentation units (lithofacies 6, 7). Beds dip steeply to the right. Most individual units are amalgamated along discrete contacts (black arrows) or separated by structureless mudstone (lithofacies 3). Most units are tabular, but several units exhibit low-amplitude lenticular geometries (white arrows).

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8 Geological Society of America Bulletin, Month/Month 2007

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LA1,2

LA3

LA2

Sm

Sh

Sl

A B

C

F

D

G

E

H

Forearc basin development in southern Alaska

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InterpretationLarge-scale syndepositional deformation

associated with folding and faulting repre-sents mass failure of semiconsolidated bedded deposits, mainly through submarine sliding and slumping. Localized sandstone thickening in fold hinges is consistent with sand fl ow con-temporaneous with deformation. Basal chaotic zones represent the deposits of debris fl ows (olistostromes) generated during mass sliding (e.g., MacDonald et al., 1993). This association is comparable to type 1A mass-transport com-plexes of Pickering and Corregidor (2005).

Lithofacies Association 6: Channel-Fill Deposits

DescriptionChannel-fi ll deposits are typically 5–25 m

wide with erosional bases. These deposits occupy single, isolated channels that incised <10 m into underlying massive mudstone (lithofacies 3–4). Sediment gravity-fl ow depos-its (lithofacies 5–7) and subordinate mudstone (lithofacies 3–4) onlap gently to moderately dipping channel margins. Channel-fi ll deposits typically consist of crude upward-fi ning and upward-thinning successions.

InterpretationDocumented channel-fi ll deposits repre-

sent isolated conduits for downslope sediment transport prior to infi lling by subsequent mass fl ows. Channels were generated through mass slumping and/or sliding (lithofacies association 5) and erosive sediment-gravity fl ows (lithofa-cies association 4). Upward-fi ning and thinning successions are interpreted as the products of progressive channel abandonment. The scale, geometry, and deposits of the preserved chan-nels are similar to modern submarine-slope

gullies (e.g., Galloway, 1998). Larger subma-rine canyon-fi ll deposits may not be readily distinguished in the laterally discontinuous and structurally disrupted outcrops of the Mata-nuska Formation.

Lithofacies Association 7: Fluvial Deposits

DescriptionThis spatially limited facies association con-

sists of imbricated pebble-cobble conglomer-ate (lithofacies 10) and greenish-gray siltstone (lithofacies 11). Medium- to thick-bedded con-glomerate units are lenticular at the outcrop scale (several meters) and exhibit reactivation surfaces. Mudstones are massive with faint parallel lamination and bifurcating structures that may represent poorly preserved in situ plant rootlets.

InterpretationClast imbrication, reactivation surfaces, and

lenticular bed geometries indicate subaqueous unidirectional fl ow in shallow fl uvial channels and bar tops. Interbedded siltstone records suspension fallout, and possible poorly devel-oped pedogenesis, on bar tops and/or adjacent fl oodplains.

Lithofacies Distributions

Measured sections document spatial and temporal variations in lithofacies distributions (Fig. 3). The lower ~2100 m of the upper Mata-nuska Formation is dominated by mudstone, mudstone-siltstone heteroliths, and concre-tionary horizons that represent dilute sedi-ment-gravity fl ows and background suspen-sion fallout (lithofacies associations 1 and 2). The upper part of the stratigraphy (~600 m) contains thicker-bedded and coarser-grained

sediment gravity-fl ow deposits (LA2–LA5). The observed upward-coarsening trend is well documented in measured sections from the Fortress Mountain and Syncline Mountain area (e.g., section 20 in Figs. 2 and 3). Spatial trends in the distribution of lithofacies are also recognized. Fluvial strata (association 6) are exposed exclusively in the northernmost part of the outcrop belt (sections 16–18 in Figs. 2 and 3). Thick-bedded sandstone turbidites (LA3) are preserved exclusively south of the Castle Mountain fault in the southern part of the out-crop belt (sections 1–12). Channel-fi ll deposits (LA6) are concentrated along the central part of the outcrop belt near sections 6, 13, and 14 (Figs. 2 and 3).

Paleocurrent Data

Paleocurrent data from the upper Matanuska Formation consist of measurements (n = 508) made from groove casts, fl ute casts, and imbri-cated clasts (Fig. 6). Western outcrops exhibit sparse southeast-directed unidirectional indi-cators and abundant southeast-northwest– to east-west–trending bidirectional indicators. Northeastern outcrops yield sparse south- to southeast-directed unidirectional indicators and northwest-southeast– to northeast-southwest–oriented bidirectional indicators. Central out-crops contain abundant bidirectional indicators that indicate east-northeast to west-southwest and east-west paleofl ow. Detailed paleocurrent analysis is hampered by the paucity of unidirec-tional indicators and the possibility of postdepo-sitional vertical-axis rotations along faults with known Cenozoic strike-slip displacements (i.e., Castle Mountain fault). Paleomagnetic data from Eocene intrusions in center of the outcrop belt indicate Neogene counterclockwise rota-tion (Stamatakos et al., 1988).

Figure 5. Representative photos of sedimentary structures characteristic of lithofacies 5–7 (turbidites), conglomerate of lithofacies 8–9 (debris-fl ow and/or slurry deposits), and lithofacies association 5 (slump and/or slide deposits). (A) Normal-graded sedimentation unit (lithofacies 6) with massive sandstone (Sm) overlain by horizontally stratifi ed sandstone (Sl). Note sharp base above black shale (Sh, litho-facies 5). Hammer (lower right, 30 cm) for scale. (B) Water-escape structures formed by dewatering of thick-bedded sedimentation unit (lithofacies 7). Vertical to near-vertical pillar columns (white arrows) of structureless to swirled sandstone cross-cut massive sandstone. Teepee structures are present at top of units (black arrows). (C, D) Erosional structures common on bases of medium to thick-bedded sedimentation units (lithofacies 6, 7) include fl uted (C) to elongate (D) groove casts. Photograph in C is ~40 cm high. Hammer (lower left) in D for scale. (E) Poorly sorted, rounded clasts within a poorly sorted, sandstone-siltstone matrix (lithofacies 9). Pen (13 cm; lower right) for scale. (F) Folded thin-bedded sandstone turbidites (LA3—lithofacies association 3) directly overlie a chaotic zone that consists of sand-stone turbidite rafts (olistoliths) and contorted mudstone. Note gently dipping, undistorted strata above and below deformed interval (LA1, LA2—lithofacies associations 1, 2). Olistostromal unit and folded strata are interpreted as the product of sliding and debris fl ow of previ-ously deposited submarine-slope deposits. Increased abundance and thickness of sandstone turbidites (LA2) directly above deformed hori-zon is consistent with ponding within accommodation space created via earlier sliding. Outcrop is ~35 m high and located 1.2 km southeast of section 20. (G) Close-up of slide-generated folds and olistostrome shown in F. Note outsized sandstone blocks (arrows) in chaotic zone below folds. (H) Contorted sandstone turbidites (white arrows) and mudstone of lithofacies 6 and 3, respectively. Rootless nature of these folds is consistent with deformation via sliding. Outcrop is ~16 m high. See section 4 in Figure 2 for location.

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10 Geological Society of America Bulletin, Month/Month 2007

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accretionary prism

JTrt

TcTc

Tc

Tc

Tg

Tw

Border

Rangesfault

CastleMountain fault

Caribou

fault

fault

Mountain

Castle

n = 24

n = 40

n = 36

n = 10

n = 115

n = 72

n = 24

n = 65n = 10

n = 17

n = 9

n = 35

n = 35n = 62

n = 43

n = 12

Paleocurrent Data (n = 1052)

Paleogene Chickaloon, Arkose Ridge, Wishbone, Tsadaka Fms.

n = 130

9

n = 211

n = 6 n = 38

n = 60

n = 6

n = 5

n = 9

n = 84

n = 83

n = 6N

Groove castsPaleocurrent Data (this study, n = 508)

Campanian–Maastrichtian Upper Matanuska Formation

Flute Casts Imbrication

fault

Mountain

Castle

Border

Rangesfault

A

B

clast imbrication (Cole et al., 2006)clast imbrication (Trop et al., 2003) clast imbrication and planar cross-stratification (Trop et al., 2003)planar and trough cross-stratification (Trop et al., 2003)planar cross-stratification (Trop et al., 2003)ripple, planar, and trough cross-stratification (Little, 1988)trough cross-stratification (Clardy, 1974)

Cariboufault

CastleMountain fault

n = 138

n = 40

0 20 km

0 20 km

marine forearc basin

subaerially exposed accretionary prism

Eocene slab-window

magmatism

alluvial-flu

vial remnant forearc basin

Cretaceous arc magmatism

n = 51n = 15

Figure 6. Maps showing structurally restored paleocurrent data from Campanian–Maastrichtian (A) and Paleocene–Oligocene (B) strata in the Matanuska Valley–Talkeetna Mountains. Detailed reconstruction of Campanian–Maastrichtian sediment dispersal is hampered by the general paucity of unidirectional paleo-current indicators. (A) Sparse south-directed unidirectional data, together with detrital geochronologic ages that match the age of plutons spatially restricted to the north side of the Castle Mountain fault, indicate a component of southward- to southeastward-directed paleofl ow. Abundant east-west bidirectional indica-tors along the center of outcrop belt are interpreted as recording radial spreading of sediment gravity fl ows on sandy lobes. Postdepositional vertical-axis rotations are also possible, but robust paleomagnetic data are not available from these Campanian–Maastrichtian strata. (B) Paleocurrent indicators from Paleogene alluvial-fl uvial strata document south-directed paleofl ow along northern basin margin, westward-directed paleofl ow along basin axis, and north-directed paleofl ow along the southeastern basin margin. Regional late Maastrichtian–Paleocene deformation prompted subaerial uplift of the inboard margin of the accretionary prism. Also note semiradial paleodrainage away from volcanic center related to slab-window magmatism (Cole et al., 2006).

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Geological Society of America Bulletin, Month/Month 2007 11

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PROVENANCE DATA

Modal analysis of sandstone thin sections and age determinations of detrital zircons help resolve sediment provenance and the erosional history of igneous source terranes. Samples for petrography and detrital geochronology were collected from outcrops that yield late Campan-ian–Maastrichtian ammonite and bivalve fossils.

Sandstone Modal Analyses

Standard petrographic thin sections were made from 36 medium-grained sandstone samples that were obtained while measur-ing stratigraphic sections. Thin sections were stained for plagioclase and potassium feldspar, and point counted (400 grains per section) using the Gazzi-Dickinson method (Dickinson, 1970; Ingersoll et al., 1984). See Table 2 and footnote 1 for point-counting parameters and point-count data, respectively. Point-counted sandstones are moderately to poorly sorted, with angular to subrounded framework grains. Most samples contain fi ne-grained matrix, cal-cite or laumontite cement, and volcanic pseu-domatrix. Volcanic pseudomatrix was counted as volcanic lithic grains. Modal percentages are dominated by plagioclase feldspar, mono-crystalline quartz, volcanic lithic grains, and mudstone lithic grains (Fig. 7). Mean modal compositions are Q:F:L = 39:40:21 and Qm:F:Lt = 29:40:31. Subangular plagioclase feld-spars dominate the feldspar population (Qm:P:K = 42:54:4) and occur as individual grains or phenocrysts within volcanic and granitic lithic grains. Monocrystalline quartz and subordi-nate chert and polycrystalline quartz make up the quartz population. Lithic fragments include abundant volcanic grains and subordinate sedi-mentary and low-grade metamorphic grains (Lm:Lv:Ls = 25:43:32). Volcanic grains vary from felsic to mafi c varieties and commonly contain plagioclase feldspar laths and micro-lites. Mudstone, argillite, and minor quartz-ofeldspathic siltstone are common sedimentary lithic grains. Subordinate metamorphic lithic grains include mica schist, quartz-mica schist, and quartz tectonite. Pyroxene, olivine, amphi-bole, muscovite, biotite, chlorite, and zircon are the most common accessory minerals.

Zircon Geochronologic Data

Detrital U-Pb Zircon AgesA medium-grained sandstone sample was

collected for detrital zircon geochronologic analyses (see Fig. 3 for sample location). (For age data and analytical details, see footnote 1.) U-Pb zircon analyses of 82 individual grains

TABLE 2. CATEGORIES USED FOR POINT-COUNT DATA FOR UPPER MATANUSKA FORMATION SANDSTONE SAMPLES

Raw

Qm Monocrystalline quartz (includes single crystals in volcanic, plutonic, and gneissic rock fragments) Qpq Polycrystalline quartz C Chert P Plagioclase feldspar (includes single crystals in volcanic, plutonic, and gneissic rock fragments) K Potassium feldspar (includes single crystals in volcanic, plutonic, and gneissic rock fragments) Lvl Volcanic lithic fragments (lathwork textures) Lvf Volcanic lithic fragments (felsitic to microlitic textures) Lss Argillaceous sedimentary lithic fragments (shale, argillite, siltstone) Lsc Calcareous sedimentary lithic fragments Lmm Mica schist and subordinate chlorite schist lithic fragments, foliated Lmt Quartz-mica tectonite and subordinate quartz-mica-feldspar tectonite, foliated Accessory: Gl—glauconite; Cp—clinopyroxene; Ol—olivine; Am—amphibole; H—hornblende;

Mu—muscovite; Bi—biotite; Cl—chlorite Recalculated

Q Total quartzose grains (=Qm + Qpq + C) F Total feldspar grains (=P + K) L Total unstable lithic grains (=Lss + Lsc + Lvl + Lvf + Lmt + Lmm) Lv Total volcanic lithic fragments (=Lvl + Lvf) Ls Total sedimentary lithic fragments (=Lss + Lsc) Lm Total metamorphic lithic fragments (=Lmt + Lmm) Lt Total lithic grains (=Ls + Lv + Lm + Qpq + C)

Note: Sample locations and point-count data are in GSA Data Repository (see text footnote 1).

Qm

LtF

ContinentalBlock-

BasementUplift Mixed

DA

TransitionalArc Undissected

Arcn = 36

Qm

KP n = 36

Lm

n = 36Lv Ls

Q

LFUndissected

ArcTransitional

Arc

ContinentalBlock-

BasementUplift

NCSC

Dissected arcMR

CP

Recycled Orogen

Recycled Orogen

A

B

C

D

Figure 7. Sandstone point-count data from the upper Matanuska Formation plotted on (A) Qm-F-Lt, (B) Q-F-L, (C) Qm-P-K, and (D) Lm-Lv-Ls ternary diagrams. Q—monocrystalline and polycrystalline quartz; F—plagioclase and potassium feldspar; L—lithics, Qm—monocrystalline quartz; Lt—total lithics, including polycrystalline quartz; P—plagioclase feldspar; K—potas-sium feldspar; Lm—metamorphic lithics; Lv—volcanic lithics; and Ls—sedimentary lithics. Note that Matanuska Formation sandstone (black squares, this study, n = 36) overlaps the dissected arc provenance fi eld of Dickinson et al. (1983) on Qm-F-Lt and Q-F-L plots. Enrichment of plagioclase feld-spar relative to potassium feldspar on Qm-P-K plot is also consistent with an arc prov-enance. Also note variation in Q-F-L plot between forearc and retroarc sandstone. Forearc sandstone includes Matanuska For-mation (black squares) and MacColl Ridge Formation (MR, n = 40, Trop et al., 1999). Retroarc sandstone includes northern Cantwell Formation (NC, n = 44; Trop and Ridgway, 1997), southern Cantwell Forma-tion (SC, n = 42, Trop and Ridgway, 1997), and Caribou Pass Formation (CP, n = 22, Hampton et al., 2007). Gray polygons rep-resent one standard deviation from mean modal composition (solid circles). Gray tri-angles represent mean modal composition of age-equivalent accretionary prism deposits (Dumoulin, 1987). See text for discussion.

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12 Geological Society of America Bulletin, Month/Month 2007

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document mainly Mesozoic ages (94% of analyzed grains) and sparse Paleozoic ages (6%; Fig. 8A). Two Mesozoic populations dominate: 95–72 Ma (Late Cretaceous; 45% of analyzed grains) and 185–146 Ma (Early to Late Jurassic; 30%). Subordinate Mesozoic populations are 143–124 Ma (Early Creta-ceous; 7%) and 241–213 Ma (Triassic; 5%). Paleozoic grain populations include 330–326 Ma (Mississippian, 2%), 377–375 Ma (Devonian, 2%), and 441 Ma (Silurian; 1%).

Most analyzed zircons yield U/Th values <5 (Fig. 8B).

Igneous U-Pb Zircon AgesRepresentative granitoid clasts were sam-

pled from conglomerate for U-Pb zircon geo-chronologic analyses. See Figure 3 for sample locations. (For age data and analytical details, see footnote 1.) Reported ages are the weighted mean of ablation pit dates from 9 to 19 indi-vidual zircons per sample. Granitic clasts yield

latest Cretaceous (79.2 ± 1 Ma, 77.1 ± 1 Ma) and Middle to Late Jurassic (173.9 ± 1 Ma; ca. 166–150 Ma; 151.8 ± 1 Ma; 146.7 ± 1 Ma) U-Pb zircon ages (Fig. 9). Sample MR3-CCD exhibits a broad range of ages spanning 166–150 Ma (Fig. 9C). The distribution of ages and discordance on concordia plots indicate 165 Ma zircon crystallization and variable lead loss, or ca. 150 Ma zircon crystallization with a component of inheritance from older arc basement.

80

166176

185

0

5

10

15

20

25

30

0 100 200 300 400

Num

ber

206Pb/238U age

Rel

ativ

e pr

obab

ilityJurassic

Cenozoic

Triassic

Mesozoic Paleozoic

Cretaceous

YTCWrangelliaPeninsularAlexander

AR-TMTA

TBCA CH

Sandstone sample MR3-SS SHRIMP-RG zircon ages (n = 82)

100

200

300

400

500

U/Th

0

2

4

6

8

10

66 70 74 78 82 86 90

Num

ber

Age (Ma)

76

80

Sandstone sample MR3-SS

0

100

200 Granitic clastsMR3-CC1-B, C, D104-CC1-A, B, D

0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

500 Ma

206 P

b/23

8 U

age

(Ma)

206 P

b/23

8 U

age

(Ma)

SAA

B

Figure 8. Geochronologic data for detrital zircon grains from sandstone of the upper Matanuska Formation. Sample represents lower half of sharp-based, normal-graded sandstone turbidite. See section 1 in Figure 3 for sample location. Analytical techniques, age data, and detailed stratigraphic section are in GSA Data Repository (see footnote 1). (A) Age probability histogram showing distribution of U-Pb age determinations for 82 detrital zircon grains. Age determinations represent individual spot analyses from separate detrital zircon grains. U-Pb ages are plotted as histograms with a normalized relative-probability distribution (Lud-wig, 2003). Histograms represent 10 m.y. intervals. Relative heights of peaks correspond to statistical signifi -cance. Inset shows details of dominant age population. Black bars above plot represent age range of granitic plutons from the Wrangellia and Yukon composite terranes (YTC) in south-central Alaska. Abbreviations: AR-TM—Alaska Range–Talkeetna Mountains belt; CA—Chisana arc; CH—Chitina arc; TA—Talkeetna arc; TB—Taylor Mountains batholith; SA—Skolai arc. Unlabeled black bars represent unnamed plutonic belts. SHRIMP—sensitive high-resolution ion microprobe. (B) U/Th of spot analyses of detrital zircons from sandstone (squares) and granitic clasts (circles).

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Geological Society of America Bulletin, Month/Month 2007 13

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DISCUSSION

Inferred Depositional Environments

Documentation and interpretation of the sedimentology of the upper Matanuska Forma-tion provide the basis for a depositional model dominated by suspension fallout and sediment gravity fl ows on a submarine slope and/or ramp environment. Most strata record deposition via muddy suspension settling, mass failure (slump-ing and/or sliding), and sandy turbidity currents, consistent with deposition on an unstable slope (Figs. 10A, 10B). Abundant mass-failure fea-tures, dewatering structures, and sandstone dikes record syndepositional to early postdepositional reorganization of loosely consolidated strata in response to seismicity, oversteepening of the depositional slope, or mass-fl ow events. Syn-depositional slope failure and slumping created small gullies and depressions that were infi lled by subsequent gravity-fl ow deposits. Deposi-tion took place mainly under open-marine con-ditions in outer neritic to bathyal water depths (<1000 m), based on paleontologic data. Benthic macrofauna are dominated by intact to disarticu-lated inoceramid bivalves (Jones, 1963; Grantz, 1964), which are characteristic of neritic–upper bathyal paleoenvironments (e.g., Thiede and Dinkelman, 1977; Kauffman and Sageman, 1990; Henderson, 2004). Benthic foraminifera assemblages are comparable to those reported from outer shelf–slope strata. In particular, Ammodiscus spp., Bathysiphon spp., Cribros-tomoides cretaceous, and Haplophragmoides spp. indicate slope environments, whereas other recovered taxa (e.g., Dentalina spp., Gaudry-ina spp., Gydroidinoides spp., Nodosaria spp., Praebulimina spp., Pullenia spp., Silicosigmo-lina spp., Spiroplectammina spp.) commonly range from outer shelf to slope environments (Sliter and Baker, 1972; Sliter, 1975; Miller and Lohmann, 1982; Olsson and Nyong, 1984; Li et al., 1999; Maestas et al., 2003). Biogenic structures from the upper Matanuska Formation are also consistent with moderately deep water conditions. Observed ichnogenera include Cos-morhaphe(?), Nereites, Paleodictyon, Planolites, Zoophycos, and Urohelminthoida (Ekdale, 1979; this study), all of which are commonly reported from submarine-slope deposits (e.g., McCann and Pickerill, 1988; Pemberton et al., 1992).

Paleocurrent data, lithologic distributions, and detrital geochronologic data (discussed in following section) indicate deposition on a south-dipping basin fl oor. The concentration of fl uvial deposits, submarine channel fi lls, and thick-bedded, sheet-shaped turbidites in the northern, central, and southern outcrop belts, respectively, indicates south-directed sediment

transport. The lateral continuity of gravity-fl ow deposits, prevalence of thick-bedded sandstone turbidites, and paucity of lower bathyal to abys-sal fossil assemblages suggests deposition on a submarine slope and/or ramp system that was likely sourced from a broad line source (e.g., fl uvial-deltaic system) as opposed to a discrete point source (i.e., submarine canyon; Figs. 10A, 10B). Upper slope and/or ramp deposits are characterized by laterally persistent mudstone, contorted beds, and relatively narrow channel fi lls that record suspension fallout, mass slump-ing, and downslope bypass, respectively, of coarse-grained gravity fl ows (e.g., Heller and Dickinson, 1985; Reading and Richards, 1994; Galloway, 1998). Lower-slope and/or ramp deposits consist of sheet-like sandstone and mudstone deposited by relatively unconfi ned turbidity currents and debris fl ows. Some thick-bedded sandstones may refl ect deposition in base-of-slope lobes or braidplains characterized by large (kilometer wide) distributary channels (e.g., Kenyon and Millington, 1995; Harrison and Graham, 1999) that are not easily distin-guished in laterally discontinuous outcrops of the Matanuska Valley.

Provenance Interpretation

Compositional and geochronologic data from the upper Matanuska Formation indicate deriva-tion mainly from Jurassic–Cretaceous igneous source terranes that are currently exposed land-ward (north) of the forearc strata. The composi-tion of sandstone within the Matanuska Forma-tion is most similar to sandstone derived from dissected volcanic arcs. Specifi cally, the abun-dance of volcanic lithic grains and granitic rock fragments, plagioclase feldspar, and unstable accessory minerals indicate derivation mainly from volcanic and plutonic source terranes. Moreover, the low U/Th values documented from analyzed detrital zircon grains suggest derivation from igneous rather than metamor-phic source areas (Williams, 2001; Rubatto et al., 2001). Detrital zircon age populations from the Matanuska Formation match the known age range of plutons within the Wrangellia and Yukon-Tanana composite terranes. Latest Creta-ceous detrital zircons in sandstone (81–71 Ma) and granitic clasts (79–77 Ma) match the age of 80–60 Ma calc-alkaline plutons that crop out for >2000 km from southwestern to southeastern (Figs. 1A; Moll-Stalcup et al., 1994; Breitspre-cher and Mortensen, 2004a, 2004b). Plutons composing this belt in south-central Alaska are commonly referred to as the Alaska Range–Tal-keetna Mountains belt (AR-TM in Figs. 1A and 8A), which yields 80–59 Ma K-Ar and Ar-Ar ages (Moll-Stalcup, 1994; Moll-Stalcup et al.,

167

169

171

173

175

177

179

181

139

141

143

145

147

149

151

153

140

144

148

152

156

160

164

70

72

74

76

78

80

82

84

72

74

76

78

80

82

84

86

F. Granodiorite Clast 104-CC1-A

Mean age 77.1 ± 1 Ma (n = 11)

Granite Clast MR3-CC1-C

Granite Clast MR3-CC1-B

Mean age 79.2 ± 1 Ma (n = 9)

145

150

155

160

165

170Granite Clast MR3-CC1-D

Age 165-150 Ma (n = 19)

Mean age 173.9 ± 1 Ma (n = 13)

Mean age 146.7 ± 1 Ma (n = 11)

Mean age 151.8 ± 1 Ma (n = 9)

E. Diorite Clast 104-CC1-B

Tonalite Clast 104-CC1-D

23

8U

/20

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b A

ge

(M

a)

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155

137

23

8U

/20

6P

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a)

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a)

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ge

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a)

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a)

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Figure 9. Age histograms of zircon ages from granitoid clasts from conglomerate of the upper Matanuska Formation. Plots show individual spot analyses from separate zircon grains from sections 1 (A–C) and 4 (D–F). Error ellipses are 2σ. Analytical techniques, age data, and detailed stratigraphic sections are in GSA Data Repository (see footnote 1).

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14 Geological Society of America Bulletin, Month/Month 2007

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1994; Drake and Layer, 2001) and 73–63 Ma U-Pb zircon ages (Harlan et al., 2003; S. Har-lan, 2004, personal commun.; Davidson and McPhillips, 2007). A modern river that drains the Alaska Range–Talkeetna Mountains belt yields detrital zircons with 79–59 Ma U-Pb ages (Van Wyck and Norman, 2004). Upper Creta-ceous plutons have not been reported south of the Matanuska Formation outcrop belt along the Border Ranges fault or within the Chugach accretionary prism.

Subordinate Lower Cretaceous detrital zircon grains in sandstone (143–124 Ma) overlap the age range of 140–115 Ma plutons and volcanic rocks that make up the Chisana arc in eastern Alaska (Figs. 1A and 8A; Richter et al., 1975; Short et al., 2005; Snyder and Hart, 2007). Cre-taceous sedimentary strata exposed in the north-ern Talkeetna Mountains (Kahiltna assemblage) also contain abundant 130–115 Ma detrital

zircons that are interpreted as recording erosion of the Chisana arc (Hampton et al., 2007; Kal-bas et al., 2007). Lower Cretaceous detrital zir-cons may have also been sourced from spatially limited and generally younger (130–105 Ma) sills that crop out along the Border Ranges fault zone (Barnett et al., 1994).

Abundant Jurassic zircons from both sand-stone (185–146 Ma) and granitoid clasts (174–147 Ma) match the age range of plutons within the Wrangellia terrane and Talkeetna arc (Figs. 1A and 8A). Talkeetna arc plutons yield 202–153 Ma U-Pb crystallization ages, 189–146 Ma K-Ar cooling ages, and 131–75 Ma U-Th/He uplift ages (Rioux et al., 2007). Jurassic plutons within Wrangellia yield 153–150 Ma U-Pb zircon ages and 146–138 Ma Ar-Ar and K-Ar ages (Chitina Valley batholith; Roeske et al., 2003). Sparse Paleozoic–Triassic detrital zircon grains are interpreted as refl ecting erosion of

plutons exposed in the Yukon-Tanana terrane or poorly dated igneous rocks within the Wrangel-lia composite terrane. Minor 219–213 Ma U-Pb zircon ages overlap the age of 215–175 Ma plutons that constitute the Taylor Mountains batholith (Dusel-Bacon et al., 2002) and Upper Triassic volcanic rocks within the Wrangellia composite terrane (Amato et al., 2007; Hamp-ton et al., 2007). Sparse 377–326 Ma detrital grains match 380–330 Ma metaigneous rocks within the Yukon-Tanana terrane (Dusel-Bacon et al., 2004).

Compositional and geochronologic data from the Matanuska Formation indicate that subor-dinate detritus may have been recycled from Jurassic–Cretaceous sedimentary strata. The presence of minor rounded monocrystalline quartz grains in compositionally immature sand-stone indicates that part of the quartz population was recycled from older sedimentary strata.

Alluvial Fluvial Lacustrine

Depositional Features Magmatism Deformational FeaturesActive arc magmatism Active subduction zone

Active crustal shorteningTopographic upliftsActive

depozone

Former continental margin

accretionary prism oceanic crustVCSDF Valdez GroupUKHCF CB MB/WB

N/NE S/SE

DF—Denali faultHCF—Hines Creek faultCB—Cantwell Fm. (AK Range)VCS—Valdez Creek shear zone

UK—Upper Cretaceous plutonsPC—Pass Creek strata

Sea LevelPC

Submarine Ramp/Fan

Delta-Submarine Apron

Uplifted depozone

M

Latest Cretaceous subduction complex strata (Valdez Group)

Pre-latest Cretaceous subduction complex strata (McHugh Complex)

A

CB

DF

MBCRB

erodederoded

Kluane arc

UDB

Alluvial-fluvial-lacustrine deposition (lower Cantwell Fm.) in thrust-top basin (CB) inboard of active arc magmatism.

Marine sediment gravity flows in forearc basins (MB, CRB, WB); exhumation of coeval and remnant magmatic arcs.

Right-lateral displacement along Denali fault (DF), shuffling marine collisionalbasin strata (UKB, UNB, UDB).

Continental-arc magmatism across the Yukon-Tanana terrane (Kluane arc) and Wrangellia composite terrane (Alaska Range-Talkeetna Mtns. belt).

WB

Rapid deposition/accretion of submarine slope/fan strata (Valdez Group) into Chugach accretionary prism.

W

rang

ellia

com

posi

te te

rran

e

Chu

gach

te

rran

eYu

kon-

Tana

na

terr

ane

VCS

eroded

NUNB

UKB UKB

AK Range-Talkeetna Mtns. belt

Wrangellia composite terrane

Spreading ridge

<10 m.y. oceanic lithosphere

BRF

Oceanic Plate(Resurrection?)

Towns/Cities A, Anchorage; M, McCarthy; N, Nabesna

HCF

Remnant Middle to Upper Jurassic Talkeetna-Chitina arc

BRF—Border Ranges faultWB—MacColl Ridge Fm. MB—Matanuska Fm.

BRF

PC

forearc basinsmagmatic arc

Yukon-Tanana terrane

Fluvial-deltaic system

Southeast

SubmarineRamp

Gullied Slope

45 6

13 14

2120

22

1112

23

25

1519

1617

79810

1 2

18

24

26

3

??

BCampanian–Maastrichtian (83–66 Ma) Convergent-Margin Paleogeography

Fig. 10B

?

A

CD

Remnant Lower Jurassic Talkeetna arc

Remnant Cretaceous Chisana arc

Figure 10. Sketch maps (A, B) and cross section (C) showing inferred depositional and tec-tonic framework of the south-ern Alaska convergent margin during Campanian–Maastrich-tian time. (B) Depositional sys-tems of the upper Matanuska Formation based on this study. See A for location. Locations of measured sections shown in Figure 3 are marked 1–26. Reconstruction includes resto-ration of ~35 km of latest Cre-taceous–Cenozoic dextral slip along the southern segment of the Castle Mountain fault. Compare section positions in B and Figure 2. A and C are adapted from Trop and Ridg-way (2007). UKB—uplifted Kahiltna basin; UNB—uplifted Nutzotin basin; UDB—uplifted Dezadeash basin. (D) Explana-tion of symbols and abbrevia-tions shown in Figures 10A–C.

Forearc basin development in southern Alaska

Geological Society of America Bulletin, Month/Month 2007 15

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Lithic grains of deformed siltstone and argillite also suggest erosion of older sedimentary strata. Sediment recycling is consistent with the pres-ence of an angular unconformity between the Matanuska Formation and underlying Jurassic–Cretaceous sedimentary strata in the Matanuska Valley–Talkeetna Mountains (Winkler, 1992). Jurassic strata exposed along the northern part of the outcrop belt contain Jurassic detrital zir-cons (Trop et al., 2005a; Amato et al., 2007). Farther north, Jurassic–Cretaceous strata of the Kahiltna assemblage (UKB in Fig. 1A) yield Mesozoic detrital zircon populations (Hampton et al., 2007; Kalbas et al., 2007) that correspond with populations in the Matanuska Formation.

Integrated biostratigraphic and detrital zir-con age data from the upper Matanuska For-mation indicate brief time intervals between sediment depositional ages and the youngest zircons observed in the upper Matanuska For-mation. Robust Campanian–Maastrichtian bio-stratigraphic ages indicate that a maximum of 6–7 m.y. passed between zircon crystallization, pluton exhumation, and deposition of detrital zircons in moderately deep water environments (using 79–77 Ma granitoid clasts from conglom-erate, 77–71 Ma zircons from sandstone, and 71–65 Ma for the Maastrichtian age). Assuming pluton emplacement depths of ~6–12 km, typi-cal of silicic continental-arc plutons (Martel et al., 1998; Scaillet and Evans, 1999; Scaillet et al., 2001), estimated latest Cretaceous unroof-ing rates range from 0.75 to ~1.0 mm/yr. If only granitoid clasts (79–77 Ma) are considered reliable indicators of pluton exhumation, then potentially longer time gaps (6–12 m.y.) may exist between crystallization and deposition, suggesting slower unroofi ng rates (0.5–1.0 mm/yr). These rates are broadly consistent with inferred uplift rates reported from the adjacent Chugach accretionary prism (0.55 mm/yr; Lit-tle and Naeser, 1989) and southeastern Alaska (1 mm/yr; Crawford et al., 1987) for latest Cre-taceous–Paleogene time.

Convergent-Margin Tectonics and Basin Development

Results of this study provide new insight on the paleogeography and crustal conditions of a juvenile continental margin following terrane accretion. Tectonic models for the northern Cor-dillera margin invoke a latest Cretaceous conti-nental-margin arc and paired accretionary prism related to northward subduction (present coordi-nates) of oceanic lithosphere beneath the former continental margin (e.g., Moll-Stalcup, 1994; Plafker and Berg, 1994; Nokleberg et al., 1998). Individual arc segments include the Alaska Range–Talkeetna Mountains belt (south-central

Alaska), Kluane arc (eastern Alaska–Yukon Ter-ritory), and Coast Mountains belt (western Can-ada). New stratigraphic and provenance data from the Matanuska Formation, combined with basin analyses from other parts of Alaska, reveal a distinct episode of basin development out-board (south), within, and inboard (north) of the continental-margin arc. These latest Cretaceous strata are interpreted as the deposits of forearc, intra-arc, and retroarc depocenters, respectively. Figures 10A and 10C illustrate the inferred lat-est Cretaceous paleotectonic framework.

Forearc Development and Arc ExhumationCampanian–Maastrichtian sedimentary strata

were deposited in a forearc position between a continental-margin arc to the north and a trench-accretionary prism to the south (Fig. 10). Cam-panian–Maastrichtian forearc deposits crop out discontinuously for >800 km from the Alaska Peninsula to the Wrangell Mountains. Strata exposed in Cook Inlet and the Alaska Peninsula (PB in Fig. 1A) were deposited by fl uvial, shal-low-marine, shelf, and slope deposystems on a trenchward (southward) dipping basin fl oor (Fisher and Magoon, 1978; Mancini et al., 1978; Fisher et al., 1987; Detterman et al., 1996). In the Matanuska Valley, submarine-slope and/or ramp strata were deposited on a trenchward (southward) dipping basin fl oor (this study). In the Wrangell Mountains, Campanian sedi-ment-gravity fl ow deposits, mass-fl ow deposits, and open-marine microfl ora document subma-rine-fan deposition in the Wrangell Mountains (MacColl Ridge Formation; Trop et al., 1999). Provenance data throughout the outcrop belt indicate that forearc detritus was derived mainly from local igneous sources terranes, including plutons of the accreted Talkeetna-Chitina arc and the coeval continental-margin arc (Mancini et al., 1978; Trop et al., 1999; this study).

Outboard of the forearc depocenters, an estimated sediment volume of ~3–6 × 106 km3 was offscraped and accreted to the accretion-ary prism during Maastrichtian–Paleocene time (Plafker et al., 1994; Hudson and Magoon, 2002; Sample and Reid, 2003). Sedimentologic data from accretionary-prism strata exposed in south-central Alaska (Valdez Group) indicate deposition by southwest-directed sediment grav-ity fl ows on submarine-slope and base-of-slope environments (e.g., Bradley et al., 2003; Sample and Reid, 2003). Thus, submarine-slope envi-ronments that deposited the upper Matanuska Formation may have merged trenchward (south-ward) into deeper-water environments where strata composing the Valdez Group accumulated. However, topographic barriers locally separated the accretionary prism from forearc depocenters (Fig. 10; Trop et al., 1999). Detailed stratigraphic

correlations between forearc depocenters and the accretionary prism are hampered by postdeposi-tional right-lateral shuffl ing along the Border Range fault (Pavlis and Roeske, 2007).

Intra-arc or Proximal-Retroarc Basin Development

The proximal northern margin of the Alaska Range–Talkeetna Mountains magmatic belt ca. 80–70 Ma was characterized by shortening, metamorphism, and exhumation of Jurassic–Cretaceous marine sedimentary strata that were deposited earlier within the suture zone between the Wrangellia and Yukon composite terranes (UKB, UNB in Fig. 10). Metamorphic and geo-chronologic data record metamorphism of these deposits ca. 74 Ma within the Maclaren Gla-cier metamorphic belt and Valdez Creek shear zone (VCS in Fig. 10; Ridgway et al., 2002). A newly recognized succession of conglomerate and sandstone >1 km thick crops out within the shear zone near Pass Creek (PC in Fig. 10; East-ham, 2002). A latest Cretaceous depositional age is tentatively inferred based on the age of the youngest detrital zircon cluster (ca. 80–75 Ma; Trop et al., 2005b; Hampton et al., 2007). These unnamed strata are interpreted here as intra-arc or proximal-retroarc deposits based on their proximity to Late Cretaceous–Paleocene calc-alkaline plutons. Compositional data and detrital zircon age distributions from sandstone at Pass Creek indicate that detritus was eroded from coeval to remnant Mesozoic arc sources within the Talkeetna Mountains, as well as Paleozoic plutons within the former continental margin (Yukon-Tanana terrane; Hampton et al., 2005, 2007; Trop et al., 2005b).

Retroarc Basin DevelopmentRegional shortening, thrust-related subsid-

ence, and nonmarine deposition characterized crust inboard of the latest Cretaceous magmatic arc (Fig. 10). Sedimentary strata of the Campa-nian–Maastrichtian Cantwell Formation crop out in an east-west–trending, 45-km-wide and 135-km-long outcrop belt in the central Alaska Range (CB in Fig. 1A). Alluvial, fl uvial, lacus-trine, and minor marginal-marine strata were deposited in a thrust-top basin related to north-directed contractile deformation (CB in Fig. 10; Ridgway et al., 1997). Petrofacies and paleocur-rent data from the northern part of the basin doc-ument erosion of metamorphic rocks exposed within the Yukon-Tanana terrane (Trop and Ridgway, 1997). Along the southern part of the basin, growth footwall synclines record synde-positional displacement on south-dipping thrust faults (Ridgway et al., 1997). Thrust sheets along the side of the basin contributed recycled Meso-zoic marine sedimentary and volcanic detritus.

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16 Geological Society of America Bulletin, Month/Month 2007

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Erosion of quartzose metamorphic rocks and recycled sedimentary rocks prompted quartz enrichment in retroarc and intrarc sandstone relative to forearc sandstone (Fig. 7B).

Late Maastrichtian–Paleocene Uplift and Unconformity Development

Campanian–Maastrichtian basinal strata were deformed and partly eroded during late Maas-trichtian–early Paleocene time. In the Alaska Range, retroarc sedimentary strata (lower Cantwell Formation) were folded and partly eroded prior to deposition of 59 Ma volcanic rocks (upper Cantwell Formation; Cole et al., 1999). In Cook Inlet, Matanuska Valley, and the Wrangell Mountains, forearc submarine-slope strata were deformed and subaerially exposed prior to deposition of 59 Ma or younger nonma-rine strata (Fisher and Magoon, 1978; Fisher et al., 1987; Trop et al., 1999, 2003; Cole et al., 2006). Inferred base-level changes across the unconformity locally exceeded eustatic sea-level fl uctuations (<100 m; Haq et al., 1988), requiring tectonically driven surface uplift. Regional uplift and unconformity development are consistent with reconstructions that invoke northward subduction of progressively younger oceanic lithosphere beneath the northern Pacifi c margin from ca. 85 to 60 Ma, followed by dia-chronous subduction of a spreading ridge from ca. 61 to 50 Ma (e.g., Haeussler et al., 2003; Cole et al., 2006). Subduction of buoyant juve-nile crust inboard of a spreading ridge would be expected to prompt shortening and surface uplift via increased coupling between the sub-ducting crust and the overriding plate (e.g., Cloos, 1993; Taylor et al., 2005). Subduction of the topographically high spreading ridge would also be expected to generate surface uplift due to isostatic adjustments. Along modern conver-gent margins, Holocene underthrusting of large spreading ridges has contributed to net uplift rates that exceed the post–Last Glacial Maxi-mum sea-level rise (e.g., Cloos, 1993; Taylor et al., 2005; Fisher et al., 2004; Sak et al., 2004). Underplating of thick sediment wedges, which characterized the southern Alaska subduction zone during Campanian–Maastrichtian time (e.g., Clendenen et al., 2003), would have also contributed to surface uplift.

CONCLUSIONS

New sedimentological, paleontological, and geochronologic data from the upper Matanuska Formation document the latest Cretaceous depositional and tectonic framework of the Matanuska Valley–Talkeetna Mountains forearc basin. Sedimentologic data indicate that mass slides, slumps, and gravity fl ows deposited a

much as 3 km of clastic sediment on a gullied submarine slope. Sandstone compositional data document detritus enriched in plagioclase feld-spar, volcanic lithic grains, and monocrystalline quartz, consistent with a dissected arc prove-nance. The U-Pb detrital zircon ages from sand-stone and granitoid clasts from conglomerate record mainly Jurassic–Cretaceous populations, sparse Paleozoic grains, and no Precambrian grains. Spatial trends in lithofacies distributions, sparse unidirectional paleocurrent indicators, and detrital geochronologic data suggest that sediment was transported southward (trench-ward) from igneous source terranes within the Talkeetna Mountains and Alaska Range, includ-ing Jurassic oceanic-arc plutons of the accreted Talkeetna arc and Cretaceous plutons of the coeval continental-margin arc. Integration of these new data from the Matanuska Forma-tion with recent basin analyses in the Alaska Range and Wrangell Mountains demonstrates contemporaneous basin development outboard (south), within, and inboard (north) of a juvenile continental-margin arc following oceanic-arc collision. Within the framework of a south-fac-ing arc, these strata represent forearc, intra-arc, and retroarc basin development, respectively. Low-angle subduction of juvenile oceanic lith-osphere and a large spreading ridge prompted exhumation of basinal strata and development of a regional late Maastrichtian–early Paleocene unconformity. Results of this study indicate that uppermost Cretaceous deposits exposed in southern Alaska offer an exceptional record of crustal conditions within a juvenile continental margin following oceanic-arc collision and prior to ridge subduction.

ACKNOWLEDGMENTS

This research was funded by the Donors of the Petroleum Research Fund administered by the Ameri-can Chemical Society, the Forest Oil Corporation, Ans-hutz Corporation, and the Bucknell Program for Under-graduate Research. Dwight Bradley aided access to the Stanford-U.S. Geological Survey Sensitive High-Reso-lution Ion Microprobe-Reverse Geometry (SHRIMP-RG) facility and Joe Wooden facilitated geochronologic analyses. Jim Haggart and Mike Mickey identifi ed ammonite and foraminifera fossils, respectively. Aubri Jenson, Ryan Delaney, Clay Slaughter, and John Wit-mer contributed to fi eld work. I thank Dwight Bradley, Brian Hampton, Terry Pavlis, Matt Rioux, Ken Ridg-way, and Sarah Roeske for helpful discussions. Jeff Amato, Dwight Bradley, Ray Ingersoll, Cari Johnson, and Paul Kapp provided insightful reviews.

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