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*Present address: Malcolm Pirnie, 3101 Wilson Blvd, Suite 550, Arlington, Virginia, 22201, USA; jmanuszak@pirnie.com
†Corresponding author: ridge@purdue.edu

Upper Jurassic-Lower Cretaceous sedimentary strata of the Nutzotin basin, the Nutzotin Mountains sequence, crop out in the Nutzotin and Mentasta Mountains of the eastern Alaska Range. These strata represent one of the best-exposed and least-metamorphosed examples of a basin that is interpreted to have formed during collision of an allochthonous volcanic arc (i.e., the Wrangellia terrane) with a continental margin. New stratigraphic, geologic mapping, and provenance data indicate that the Nutzotin basin formed as a retroarc foreland basin along the northern margin (present coordinates) of the Wrangellia terrane. Coeval with basin development along the northern margin, sedimentary basins and plutons located along the southern margin of the Wrangellia terrane were being incorporated into a regional fold-and-thrust belt. This fold-and-thrust belt, located south of the Nutzotin basin, exposed multiple structural levels of the Wrangellia terrane that were eroded and provided sediment that was transported northward and deposited in the Nutzotin basin.

New sedimentologic and stratigraphic data from the ∼3 km thick (minimum thickness) Nutzotin Mountains sequence define a three-part stratigraphy. The lower part consists of Upper Jurassic (Oxfordian to Tithonian) conglomerate with outsized limestone clasts (>10 m in diameter) and interbedded sandstone and shale that grade basinward into mainly black shale with minor micritic limestone and isolated lenses of conglomerate. The middle part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) normal-graded sandstone and shale interbedded with massive tabular sandstone and lenticular conglomerate. The upper part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) mudstone with distinctive fossil-rich horizons and minor interbedded sandstone. The overall stratigraphy of the Nutzotin Mountains sequence represents a general upward-shallowing and upward-coarsening package that represents a general transition from distal mud-rich submarine-fan strata to more proximal sand-rich submarine-fan strata that are in turn overlain by marine shelf strata. Feldspathic sandstone compositions (Q6F67L27), eastward and northeastward directed paleocurrent indicators, diagnostic clasts in conglomerate, and detrital zircon U-Pb ages of 151–147 Ma (n = 8) and 159–156 Ma (n = 2) indicate that sediment in the Nutzotin basin was derived primarily from the Wrangellia terrane and the Chitina and Chisana arcs that intrude the Wrangellia terrane.

The stages of deformation documented in the Nutzotin Mountains sequence provide insight into the growth of collisional continental margins by the tectonic incorporation of basinal strata. Our data show that strata of the Nutzotin basin have been deformed into an accretionary wedge by north-dipping thrust faults and related overturned folds above a north-dipping décollement. Displacement on this décollement was the product of northward underthrusting of basinal strata beneath the former continental margin and resulted in southward tectonic transport of distal basinal strata of the Nutzotin Mountains sequence strata over both more proximal basinal strata and the Wrangellia terrane. Previously published K-Ar ages from plutons that cross-cut both the décollement and folded Nutzotin Mountains sequence strata indicate that contractional deformation ended between 117 and 105 Ma. Regionally, the Nutzotin Mountains sequence represents part of a series of Mesozoic sedimentary basins located along the inboard margin of the Wrangellia composite terrane that have similar depositional styles and were all subsequently incorporated into accretionary wedges that dip toward the former continental margin. These deformed strata define a continental-scale suture zone that extends along the northwestern Cordillera for over 2000 km.

INTRODUCTION

Collisional basins form along plate boundaries during arc-arc, arc-continent, and continent-continent collisions (Dewey, 1977; Miall, 1995). Several active collisional basins are forming near Taiwan, for example, due to the collision of the Luzon volcanic arc with the southeastern continental margin of China (Lundberg and Dorsey, 1988; Teng, 1990; Chen et al., 2001). In a similar tectonic setting, a series of sedimentary basins are forming near Papua New Guinea as a result of the collision of the Bismarck volcanic arc with the northeastern continental margin of Australia (Silver et al., 1991; Cullen, 1996; Galewsky and Silver, 1997). Ancient examples of collisional basins are often difficult to identify and study because their strata may be deformed, metamorphosed, and/or eroded during the final stages of collision. If preserved, however, the stratigraphy and deformation of collisional basins provide a record of processes associated with the growth of continents through suturing of island arcs, oceanic plateaus, and microcontinents onto continental margins.

Upper Jurassic-Lower Cretaceous sedimentary strata of the Nutzotin basin, the Nutzotin Mountains sequence, are part of a series of Upper Jurassic-Lower Cretaceous basinal strata located on or near the suture zone between arc-related rocks of the allochthonous Wrangellia composite terrane and the Mesozoic continental margin of western North America (Fig. 1; e.g., Pavlis, 1982; McClelland et al., 1992a; Manuszak, 2000; Ridgway et al., 2002; Hampton et al., this volume; Kalbas et al., this volume). The majority of the strata representing these Mesozoic basins have undergone a high degree of structural and metamorphic deformation (Berg et al., 1972). Metamorphism has destroyed most of the fossils in these strata, so lack of age control is a major obstacle in studying the development of these basins. Strata of the Nutzotin basin, fortunately, have undergone limited metamorphism, and preservation of marine fossils provides adequate age control (e.g., Richter, 1971, 1976; Richter and Jones, 1973; Richter and Schmoll, 1973; Manuszak and Ridgway, 2000).

Figure 1. (A) Map showing the composite terranes, Mesozoic sedimentary basins, and major faults of the northwestern North American Cordillera (adapted from Nokleberg et al., 1994b). Inset in the upper right shows the location of Figure 1A. The Yukon composite terrane represents the late Paleozoic-early Mesozoic continental margin of southern Alaska and the Yukon Territory. The boundary between the Yukon composite terrane and the allochthonous Wrangellia composite terrane is marked by the Denali fault, and a broad zone of deformation consisting of highly deformed sedimentary, igneous, and metamorphic rocks that is referred to as the Alaska Range suture zone in the text. The Nutzotin basin is located in the Alaska Range suture zone near the Alaska/Canada border. (B) Map showing composite terranes, Mesozoic sedimentary basins, and the axes of Jurassic-Cretaceous magmatic arcs in east-central Alaska and western Yukon Territory. Note the location of the Chitina thrust belt (labeled CTB) discussed in the text. Also note that the Nutzotin basin is located on the inboard (northern) margin of the Wrangellia composite terrane, whereas the Wrangell Mountains basin is located on the outboard (southern) margin. See key for explanation of abbreviations. BRF = Border Ranges fault.

Figure 1. (A) Map showing the composite terranes, Mesozoic sedimentary basins, and major faults of the northwestern North American Cordillera (adapted from Nokleberg et al., 1994b). Inset in the upper right shows the location of Figure 1A. The Yukon composite terrane represents the late Paleozoic-early Mesozoic continental margin of southern Alaska and the Yukon Territory. The boundary between the Yukon composite terrane and the allochthonous Wrangellia composite terrane is marked by the Denali fault, and a broad zone of deformation consisting of highly deformed sedimentary, igneous, and metamorphic rocks that is referred to as the Alaska Range suture zone in the text. The Nutzotin basin is located in the Alaska Range suture zone near the Alaska/Canada border. (B) Map showing composite terranes, Mesozoic sedimentary basins, and the axes of Jurassic-Cretaceous magmatic arcs in east-central Alaska and western Yukon Territory. Note the location of the Chitina thrust belt (labeled CTB) discussed in the text. Also note that the Nutzotin basin is located on the inboard (northern) margin of the Wrangellia composite terrane, whereas the Wrangell Mountains basin is located on the outboard (southern) margin. See key for explanation of abbreviations. BRF = Border Ranges fault.

Our investigation of the Nutzotin Mountains sequence is based on 26 measured stratigraphic sections (7500 m total thickness), clast counts from conglomerate (n = 1784), paleocurrent measurements (n = 221), U-Pb age determinations of detrital zircons (n = 10), petrographic and microprobe analyses of sandstone thin sections (n = 17), and geologic mapping. With these data, we reconstruct the evolution of depositional environments and stages of basin development, identify the provenance of detritus for the basin, define the structural geometry of the basin within the suture zone, and relate the development and deformation of the Nutzotin basin with collision of the Wrangellia composite terrane against the continental margin of western North America.

GEOLOGIC SETTING AND PREVIOUS STUDIES

Tectonostratigraphic Terranes

South-central Alaska consists of three composite terranes that are separated by major fault systems (e.g., Coney et al., 1980; Plafker and Berg, 1994). From north to south, these terranes are the Yukon composite terrane, the Wrangellia composite terrane, and the Southern Margin composite terrane (Fig. 1; Foster et al., 1994; Nokleberg et al., 1994a; Plafker et al., 1994; Plafker and Berg, 1994). The Yukon composite terrane consists of structurally dismembered Paleozoic metamorphic rocks that are para-autochthonous to North America and formed the late Paleozoic-early Mesozoic continental margin of southern Alaska and the Yukon Territory (Tempelman-Kluit, 1976; Hansen, 1990; Mortensen, 1992; Nokleberg et al., 1994a; Hansen and Dusel-Bacon, 1998). The southern boundary of the Yukon composite terrane is marked by a broad zone of deformation that consists of highly deformed sedimentary, igneous, and metamorphic rocks and is commonly referred to as the Alaska Range suture zone (Csejtey et al., 1982; Ridgway et al., 1997; Cole et al., 1999; Ridgway et al., 2002). The trace of the Denali fault follows the trend of the Alaska Range suture zone (Fig. 1; Ridgway et al., 2002). The Wrangellia composite terrane is juxtaposed against the southern margin of the Yukon composite terrane within the Alaska Range suture zone. The Wrangellia composite terrane is an amalgamation of three separate terranes (Wrangellia, Alexander, and Peninsular terranes; Fig. 1B) that were probably sutured together during late Paleozoic time (Jones et al., 1977; Jones and Silberling, 1979; Gehrels and Saleeby, 1987; Gardner et al., 1988; Plafker et al., 1989) and are presently exposed from western Alaska to southern British Columbia (Fig. 1). The terranes are interpreted to represent remnant volcanic arc assemblages and overlying flood basalts and platform carbonates that are allochthonous to North America (Packer and Stone, 1974; Hillhouse and Grommé, 1984; Hillhouse and Coe, 1994). The Wrangellia composite terrane was located near the equator during Late Triassic time, was translated northward relative to North America, and accreted to the western margin of North America sometime between Late Triassic and early Tertiary time (McClelland et al., 1992a; Plafker and Berg, 1994; Hillhouse and Coe, 1994; Cowan et al., 1997; Butler et al., 2001; Trop et al., 2002, 2005). The Southern Margin composite terrane is juxtaposed against the Wrangellia composite terrane along the Border Ranges fault (Fig. 1). This terrane consists of an Upper Triassic-Paleogene subduction-complex that was produced by northeast-ward to northwestward subduction (e.g., Plafker et al., 1994).

Mesozoic Sedimentary Basins

Mesozoic strata representing two sedimentary basins, the Nutzotin and Wrangell Mountains basins, were deposited on the Wrangellia composite terrane and are well preserved in south-central Alaska (Fig. 1B; Trop et al., 2002). The Nutzotin basin, the focus of this study, consists of at least 6 km of Upper Jurassic-Lower Cretaceous sedimentary and volcanic strata that are preserved along the inboard margin (cratonward side) of the Wrangellia terrane in a ∼35-km-wide and 250-km-long outcrop belt (Figs. 2, 3). The oldest strata of the basin, the Nutzotin Mountains sequence, disconformably overlie Triassic sedimentary and volcanic strata of the Wrangellia terrane along the southern margin of the basin and are in fault contact with this terrane at other locations (Figs. 3, 4; Richter, 1976; Manuszak, 2000). The disconformable contact is best exposed at Misty Mountain (Fig. 2B). The Nutzotin Mountains sequence consists of an ∼3-km-thick package (minimum thickness) of Upper Jurassic-Lower Cretaceous (Oxfordian–Valanginian) marine sedimentary strata (Fig. 3; Berg et al., 1972; Richter and Jones, 1973). These marine sedimentary strata are conformably overlain by the Chisana Formation, a ∼3-km-thick succession of Lower Cretaceous (Hauterivian-Aptian) lava flows, tuff, mud-stone, and volcaniclastic breccia (Fig. 3; Richter, 1976; Sandy and Blodgett, 1996). The Chisana Formation has a gradational contact with the Valanginian Buchia-bearing strata of the Nutzotin Mountains sequence (Manuszak, 2000). Stratigraphically higher in the Chisana Formation, Berg et al. (1972) reported the presence of the ammonite Shasticrioceras, which is indicative of an Early Cretaceous (Barremian) age. In addition, 40Ar-39Ar ages from samples collected 888 and 1036 m above the base of the Chisana Formation give ages of 116.7 ± 1.4 Ma and 113.4 ± 1.5 Ma (Short et al., 2005). The volcanic and related plutonic rocks of the Chisana Formation have been interpreted as the product of northward- to northeastward-directed subduction of an oceanic slab beneath the Wrangellia composite terrane (Barker, 1988; Plafker et al., 1989; Plafker and Berg, 1994). A relatively thin (<90 m) package of possibly Upper Cretaceous nonmarine, unnamed strata overlies the Chisana Formation along an angular unconformity (Fig. 3; Richter, 1976).

Figure 2. (A) Generalized geologic map of the Nutzotin basin. Map location indicated in Figure 1B. Black circles with white numbers mark the location of our 22 measured sections that are discussed in the text. Rose diagrams show paleocurrent data discussed in the text. Geology modified from Richter et al. (1975) and Richter (1976). (B) Cross section showing structural relationships between the Nutzotin Mountains sequence, Wrangellia terrane, and the Yukon composite terrane. The cross section has no vertical exaggeration. Line of cross section shown on Figure 2(A). Notice the small outlier of the Nutzotin Mountains sequence that is in depositional contact with the Wrangellia terrane in the center of the cross section at Misty Mountain. Also note that the Lost Creek décollement (black dashed line) is interpreted to have tectonically transported more distal strata of the Nutzotin Mountains sequence southward over the Wrangellia terrane. Point B on the cross section is roughly along the axis of a large anticline in the Wrangellia terrane; these regional folds are discussed in the text. See text for additional discussion.

Figure 2. (A) Generalized geologic map of the Nutzotin basin. Map location indicated in Figure 1B. Black circles with white numbers mark the location of our 22 measured sections that are discussed in the text. Rose diagrams show paleocurrent data discussed in the text. Geology modified from Richter et al. (1975) and Richter (1976). (B) Cross section showing structural relationships between the Nutzotin Mountains sequence, Wrangellia terrane, and the Yukon composite terrane. The cross section has no vertical exaggeration. Line of cross section shown on Figure 2(A). Notice the small outlier of the Nutzotin Mountains sequence that is in depositional contact with the Wrangellia terrane in the center of the cross section at Misty Mountain. Also note that the Lost Creek décollement (black dashed line) is interpreted to have tectonically transported more distal strata of the Nutzotin Mountains sequence southward over the Wrangellia terrane. Point B on the cross section is roughly along the axis of a large anticline in the Wrangellia terrane; these regional folds are discussed in the text. See text for additional discussion.

Figure 3. Stratigraphic chart showing the stratigraphy of the Nutzotin and Wrangell Mountains basins, stratigraphy of the upper part of the Wrangellia terrane, and the age ranges of associated volcanic arcs. Also shown is the composite lithostratigraphic section of the Nutzotin and Wrangell Mountains basins. Facies associations (FA1–FA5) discussed in the text for the Nutzotin basin are labeled next to the composite lithostratigraphic section. Numbered vertical black bars in the Nutzotin Mountains sequence on the stratigraphy column correspond to age ranges of Buchia species listed in key at bottom of figure. Rectangle marked DZ on the lithostratigraphic column marks the approximate stratigraphic position of sandstone sample from which detrital zircons were collected and dated by U-Pb geochronology. Chart represents a synthesis of data presented in Richter (1976), MacKevett (1978), Plafker et al. (1989), Nokleberg et al. (1994a), and Trop et al. (2002). Time scale is from Palmer and Geissman (1999). Key at bottom of figure shows lithologic patterns and age-diagnostic fossils of the Nutzotin Mountain sequence.

Figure 3. Stratigraphic chart showing the stratigraphy of the Nutzotin and Wrangell Mountains basins, stratigraphy of the upper part of the Wrangellia terrane, and the age ranges of associated volcanic arcs. Also shown is the composite lithostratigraphic section of the Nutzotin and Wrangell Mountains basins. Facies associations (FA1–FA5) discussed in the text for the Nutzotin basin are labeled next to the composite lithostratigraphic section. Numbered vertical black bars in the Nutzotin Mountains sequence on the stratigraphy column correspond to age ranges of Buchia species listed in key at bottom of figure. Rectangle marked DZ on the lithostratigraphic column marks the approximate stratigraphic position of sandstone sample from which detrital zircons were collected and dated by U-Pb geochronology. Chart represents a synthesis of data presented in Richter (1976), MacKevett (1978), Plafker et al. (1989), Nokleberg et al. (1994a), and Trop et al. (2002). Time scale is from Palmer and Geissman (1999). Key at bottom of figure shows lithologic patterns and age-diagnostic fossils of the Nutzotin Mountain sequence.

Figure 4. Simplified northwest-southeast stratigraphic cross section through the Nutzotin basin showing lithostratigraphic relationships in the Nutzotin Mountains sequence. Numbers above each measured stratigraphic section correspond to location on the map shown in Figure 2. The Nutzotin Mountains sequence has a depositional contact with Wrangellia along the south-central basin margin (measured sections 12, 13). Note that the Nutzotin Mountains sequence also has a fault contact with the Wrangellia terrane that is best documented in the northwestern part of the study area (measured sections 4, 5, 6). Also note that Facies Association 5 is only exposed in the southeastern part of the basin and that it has a gradational contact with the overlying Chisana Formation. Notice the prevalence of east-directed paleoflow indicators in Facies Associations 3 and 4 (north is to the top of the figure), whereas paleoflow indicators in Facies Association 1 are mainly to the northwest. Data for measured sections were collected as bed-by-bed measurements using a Jacob staff. For detailed measured stratigraphic sections, see Manuszak (2000).

Figure 4. Simplified northwest-southeast stratigraphic cross section through the Nutzotin basin showing lithostratigraphic relationships in the Nutzotin Mountains sequence. Numbers above each measured stratigraphic section correspond to location on the map shown in Figure 2. The Nutzotin Mountains sequence has a depositional contact with Wrangellia along the south-central basin margin (measured sections 12, 13). Note that the Nutzotin Mountains sequence also has a fault contact with the Wrangellia terrane that is best documented in the northwestern part of the study area (measured sections 4, 5, 6). Also note that Facies Association 5 is only exposed in the southeastern part of the basin and that it has a gradational contact with the overlying Chisana Formation. Notice the prevalence of east-directed paleoflow indicators in Facies Associations 3 and 4 (north is to the top of the figure), whereas paleoflow indicators in Facies Association 1 are mainly to the northwest. Data for measured sections were collected as bed-by-bed measurements using a Jacob staff. For detailed measured stratigraphic sections, see Manuszak (2000).

The Wrangell Mountains basin, located ∼80 km south of the Nutzotin basin, formed on the outboard (southern) margin of the Wrangellia composite terrane (WB in Fig. 1). Strata of this basin consist of a 7-km-thick sequence of Upper Triassic-Upper Cretaceous sedimentary strata that are exposed in a ∼55-km-wide and ∼120-km-long outcrop belt in the Wrangell Mountains (Grantz et al., 1966; MacKevett, 1969, 1978; Jones and MacKevett, 1969; Trop et al., 1999; Trop, 2000; Trop et al., 2002). The Chitina fold-and-thrust belt is well exposed along the southern margin of the Wrangell Mountains basin and was active during Late Jurassic-Early Cretaceous sedimentation in the basin (Fig. 1B; Gardner et al., 1986; Trop et al., 2002; Trop and Ridgway, this volume). The northwest-trending fold-and-thrust belt consists of southwest-dipping thrust faults that juxtapose different stratigraphic levels of the Wrangellia composite terrane and strata of the Wrangell Mountains basin (MacKevett, 1978; Gardner et al., 1986; Trop et al., 2002). Crustal shortening related to the Chitina fold-and-thrust belt also produced regional northeast-verging folds with wavelengths up to 15 km that have been mapped between exposures of the Nutzotin basin and exposures of the Wrangell Mountains basin (e.g., Richter, 1976; MacKevett, 1978; Trop et al., 2002). In this article, when we refer to the Chitina fold-and-thrust belt, we are including both the well-exposed thrust faults in the Wrangell Mountains and the regional folds that extend northward to the Nutzotin basin.

Volcano-Plutonic Arcs

The composite terranes of east-central Alaska are intruded and overlain by linear belts of Upper Jurassic-Upper Cretaceous igneous rocks that have been interpreted as representing magmatic arcs (LJ, EK, and LK in Figure 1B; Plafker et al., 1989; Nokleberg et al., 1994a). Upper Jurassic-lowermost Cretaceous calc-alkaline plu-tonic rocks of the Chitina arc intrude the southern margin of the Wrangellia composite terrane in south-centralAlaska (LJ in Fig. 1B; Plafker etal., 1989; Roeske etal., 2003), Yukon Territory (Doddsand Campbell, 1988), and southeastern Alaska (Karl et al., 1988). During late Early to early Late Cretaceous time, the Chisana arc (EK in Fig. 1B) formed inboard (northward) of the remnant Chitina arc. Plutonic and andesitic volcanic rocks of the Chisana arc are discontinuously exposed from south-central to southeastern Alaska (Berg etal., 1972; Plafker etal., 1989; Stowell etal., 2000; Snyder and Hart, 2002, this volume). During latest Cretaceous time, magmatism migrated farther inboard (northward), forming the Kluane arc (LK in Figure 1B; Plafker et al., 1989). Upper Cretaceous-Lower Tertiary plutonic and volcanic rocks of the Kluane arc are exposed from south-central Alaska to British Columbia (Monger et al., 1982; Brew and Ford, 1984; Plafker and Berg, 1994; Trop etal., 1999) and intrude both the Wrangellia and Yukon composite terranes.

Major Faults

The Denali and Totschunda faults are the two major strike-slip faults present in the study area (Fig. 2). The Denali fault is a dextral fault system that spans more than 2200 km in a broad arc from Alaska through British Columbia (Fig. 1A; Lanphere, 1978). Up to 400 km of Late Cretaceous-Tertiary displacement is interpreted along the Denali fault based partly on correlation of the Nutzotin basin as the offset equivalent of the Dezadeash basin (Fig. 1A; Eisbacher, 1976; Jones et al., 1982; Nokleberg et al., 1985; Plafker et al., 1989; Lowey, 1998). Much of this displacement is interpreted to have occurred during Eocene-Oligocene time based on the ages of strike-slip basins exposed along the fault system (Ridgway and DeCelles, 1993; Ridgway et al., 1995; Trop et al., 2004). The Totschunda fault trends northwestward for 200 km from Canada to its junction with the Denali fault (Fig. 2A; Richter and Matson, 1971). Displacement on the Totschunda fault is dominantly dextral with maximum offsets of up to 4 km; faulting began at ca. 1 Ma (Plafker et al., 1977; Lisowski et al., 1987), but possibly extends as far back as the Pleistocene time (Richter and Matson, 1971). Recent earthquakes and neotectonic studies indicate that the Denali and Totschunda faults are active structures (Eberhart-Phillips et al., 2003; Matmon et al., 2006; Plafker et al., 2006). GPS data indicate 8–9 mm/year dextral slip on the Denali fault, with some slip likely on parallel strands north of the main fault trace (Fletcher, 2002). Geodetic measurements across the Totschunda fault show shear strain consistent with ∼5 mm/year of dextral slip (Plafker et al., 1977).

SEDIMENTOLOIC AND STRATIGRAPHIC DATA

Our analysis of the sedimentologic and stratigraphic architecture of the Nutzotin Mountains sequence is based on 22 measured stratigraphic sections whose locations are shown on Figure 2. Five facies associations within the Nutzotin Mountains sequence have been identified based on grain size, geometry and thickness of bedding, presence or lack of macrofauna and ichnofauna, and types of sedimentary structures. General descriptions and depositional interpretations of the five facies associations are presented in this section. Representative measured sections showing the general stratigraphy of the Nutzotin Mountains sequence are shown in Figure 4. For a more detailed discussion of the sedimentologic and stratigraphic data and more detailed measured stratigraphic sections, see Manuszak (2000).

Facies Descriptions

Facies Association 1

Description: Lenticular, matrix- and clast-supported conglomerate with interbedded shale. Facies Association 1 consists of conglomerate that has an average bed thickness of ∼1–1.5 m, lenticular geometries, and matrix- and clast-supported framework (Fig. 5A). The conglomerate is poorly organized, has average maximum clast sizes ranging from 6 to 29 cm, and contains outsized limestone clasts exceeding 10 m in diameter (Fig. 5B). Upsection, the conglomerate is more commonly clast-supported, has poorly developed normal grading, has average maximum clast sizes of 6–8 cm, and contains interbedded shale and sandstone. Shale lithofacies interbedded with the conglomerate include minor thin limestone beds, disarticulated bivalve fossils, and carbonaceous plant debris. Sandstone lithofacies interbedded with the conglomerate range in thickness from 2 to 100 cm and commonly display normal grading, tabular geometries, and isolated pebble- to cobble-sized clasts. The lithofacies and bed thicknesses common in Facies Association 1 are illustrated in our measured stratigraphic section from Misty Mountain shown in Figure 6A.

Figure 5. (continued on the next page) Photographs of facies associations of the Nutzotin Mountains sequence. (A) Massive, unorganized conglomerate typical of Facies Association 1. Arrows point to boulder-size clasts. Bedded strata in lower part of photo are part of an individual outsized clast within the conglomerate. Person circled for scale. (B) Outsized limestone clast (white area in lower right) common in Facies Association 1. Black arrows point to edge of clast. Person (circled) standing on clast for scale; bar scale next to person is 1.7 m. Dashed white lines (upper left) represent bedding in conglomerate and shale that are laterally equivalent with outsized clast. (C) Interbedded black shale and silty limestone characteristic of Facies Association 2. Black arrows point to prominent resistant limestone beds. Hammer circled for scale. (D) Trace fossil Paleodictyon in sandstone of Facies Association 2. Black arrows point to honeycomb pattern characteristic of this trace fossil. Pencil (upper left) for scale. (E) Normal-graded sandstone and shale of Facies Association 3. This is the most common lithofacies in the Nutzotin Mountains sequence. Lighter colored beds are sandstone and siltstone; the darker beds are mudstone. Most sandstone beds in the photograph are <20 cm thick. Bedding in the photo dips to the left. Hammer circled for scale. (F) Closeup photograph of normal-graded sandstone/shale characteristic of Facies Association 3. Black arrows point to erosional base of sandstone that has scoured into underlying mudstone. Massive (Sm) and ripple-stratified (Sr) sandstone grade upward into ripple-laminated siltstone (Fsr) that is overlain by massive shale. Coin (right center) for scale. (G) Amalgamated tabular sandstone of Facies Association 4. These sandstones are massive to weakly normal-graded. Bedding dips to the right and dashed white lines partly outline the base and top of an individual 35-cm-thick bed. Black arrow points to hammer for scale. (H) Polymictic matrix-supported conglomerate characteristic of Facies Association 4 that is interbedded with massive sandstone. Clast types: L = limestone, D = diorite, C = chert, and Q = quartz. Pen for scale. (I) Bioturbated shale with interbedded thin tabular sandstone (black arrows) characteristic of Facies Association 5. Bedding is dipping to left. Person in lower center for scale. (J) Distinctive in situ fossil-rich horizon consisting of Buchia fossils that are common in Facies Association 5. Coin (white arrow) for scale. (K) Mudstone rip-up clasts in matrix of Buchia shell hash and sandstone. Scale is 10 cm. (L) U-shaped horizontal trace fossil common on bedding surfaces of shale of Facies Association 5. Coin (lower center) for scale.

Figure 5. (continued on the next page) Photographs of facies associations of the Nutzotin Mountains sequence. (A) Massive, unorganized conglomerate typical of Facies Association 1. Arrows point to boulder-size clasts. Bedded strata in lower part of photo are part of an individual outsized clast within the conglomerate. Person circled for scale. (B) Outsized limestone clast (white area in lower right) common in Facies Association 1. Black arrows point to edge of clast. Person (circled) standing on clast for scale; bar scale next to person is 1.7 m. Dashed white lines (upper left) represent bedding in conglomerate and shale that are laterally equivalent with outsized clast. (C) Interbedded black shale and silty limestone characteristic of Facies Association 2. Black arrows point to prominent resistant limestone beds. Hammer circled for scale. (D) Trace fossil Paleodictyon in sandstone of Facies Association 2. Black arrows point to honeycomb pattern characteristic of this trace fossil. Pencil (upper left) for scale. (E) Normal-graded sandstone and shale of Facies Association 3. This is the most common lithofacies in the Nutzotin Mountains sequence. Lighter colored beds are sandstone and siltstone; the darker beds are mudstone. Most sandstone beds in the photograph are <20 cm thick. Bedding in the photo dips to the left. Hammer circled for scale. (F) Closeup photograph of normal-graded sandstone/shale characteristic of Facies Association 3. Black arrows point to erosional base of sandstone that has scoured into underlying mudstone. Massive (Sm) and ripple-stratified (Sr) sandstone grade upward into ripple-laminated siltstone (Fsr) that is overlain by massive shale. Coin (right center) for scale. (G) Amalgamated tabular sandstone of Facies Association 4. These sandstones are massive to weakly normal-graded. Bedding dips to the right and dashed white lines partly outline the base and top of an individual 35-cm-thick bed. Black arrow points to hammer for scale. (H) Polymictic matrix-supported conglomerate characteristic of Facies Association 4 that is interbedded with massive sandstone. Clast types: L = limestone, D = diorite, C = chert, and Q = quartz. Pen for scale. (I) Bioturbated shale with interbedded thin tabular sandstone (black arrows) characteristic of Facies Association 5. Bedding is dipping to left. Person in lower center for scale. (J) Distinctive in situ fossil-rich horizon consisting of Buchia fossils that are common in Facies Association 5. Coin (white arrow) for scale. (K) Mudstone rip-up clasts in matrix of Buchia shell hash and sandstone. Scale is 10 cm. (L) U-shaped horizontal trace fossil common on bedding surfaces of shale of Facies Association 5. Coin (lower center) for scale.

Interpretation. We interpret Facies Association 1 to represent proximal, submarine-fan deposits. The matrix-supported conglomerate and large outsized clasts are interpreted to be the product of debris flows and rock-fall avalanches along proximal portions of submarine canyons based on the coarse grain size, outsized clasts, poorly organized internal fabrics, and disarticulated bivalve fossils (e.g., Lowe, 1982; Stow et al., 1996). Matrix strength and momentum from the gravity flows permitted transportation of coarse-grained detritus and outsized clasts. Deposition occurred by rapid mass emplacement (“freezing”) when the gravitational driving stress decreased below the matrix strength (e.g., Johnson, 1965, 1970).

Lenticular, clast-supported conglomerate is interpreted to have been deposited by gravelly debris flows transitional to density-modified grain flows (e.g., Lowe, 1976a, 1982). The minor grading and lack of stratification suggests that fluid turbulence was relatively unimportant as a clast-supporting mechanism. The sandstone of Facies Association 1 is interpreted to be the deposits of sandy turbidity currents based on the normal grading, grain size, and tabular geometries (e.g., Bouma, 1962; Middleton, 1967; Lowe, 1976b). Suspension fallout is the interpreted depositional mechanism for the interbedded shale. The occurrence of disarticulated bivalve fossils and plant debris within the shale is suggestive of general proximity to marine shelfal environments.

Facies Association 2

Description: Nonfossiliferous, black shale with minor amounts of red micritic limestone and lenticular conglomerate. Massive black shale, that lacks macrofauna, is the most common lithofacies in Facies Association 2 (Figs. 5C, 6B). Faint subhorizontal laminations and bioturbation are locally present throughout this facies association. Horizontal burrows less than a few millimeters in diameter are most common and resemble Chondrites type 1–3 (e.g., D'Alessandro et al., 1986). Thin vertical burrows are also observed but are uncommon. The Nereites trace fossil Palaeodictyon occurs locally (Fig. 5D; e.g., D'Alessandro et al., 1986). Red, silty limestone interbedded with the black shale is up to 30 cm thick, lacks sedimentary structures, and becomes less common upsection (Fig. 5C). Lenticular conglomerate interbedded with the black shale occurs in beds that are ∼10 m in width and a meter in thickness. These conglomerates are matrix-supported, poorly organized, and have clast sizes ranging from 3 to 50 cm. The lithologies and bed thicknesses that characterize Facies Association 2 are shown in our measured stratigraphic section from Lost Creek (Fig. 6B).

Figure 5. (continued)

Figure 5. (continued)

Figure 6. Representative measured stratigraphic sections for Facies Association 1 and Facies Association 2 that define the lower part of the stratigraphy for the Nutzotin basin. Proximal lenticular conglomerate of Facies Association 1 is interpreted to be laterally equivalent to the more distal black shale with isolated lenticular conglomerate of Facies Association 2. The inset for Facies Association 2 shows the details of the lithologies characteristic of this facies association. Facies Association 2 is dominated by shale with thin interbedded siltstone and silty limestone beds. Lithology descriptions for the stratigraphic columns are shown at the top of the figure. The stratigraphic sections do not represent the complete thickness of the facies associations. See text for additional discussion.

Figure 6. Representative measured stratigraphic sections for Facies Association 1 and Facies Association 2 that define the lower part of the stratigraphy for the Nutzotin basin. Proximal lenticular conglomerate of Facies Association 1 is interpreted to be laterally equivalent to the more distal black shale with isolated lenticular conglomerate of Facies Association 2. The inset for Facies Association 2 shows the details of the lithologies characteristic of this facies association. Facies Association 2 is dominated by shale with thin interbedded siltstone and silty limestone beds. Lithology descriptions for the stratigraphic columns are shown at the top of the figure. The stratigraphic sections do not represent the complete thickness of the facies associations. See text for additional discussion.

Interpretation. We interpret Facies Association 2 to be deposited mainly by deep-water hemipelagic sedimentation and/or suspension fallout related to muddy turbidity currents on distal parts of submarine-fan complexes (e.g., Stow et al., 1996). This interpretation is based on the lack of sedimentary structures indicative of tractive transport, and the bioturbation indicative of the Nereites ichnofacies (e.g., Pemberton and MacEachern, 1992). Nereites ichnospecies, such as Paleodictyon, in mudstone-siltstone intervals are indicative of interdepositional colonization and are common in deep-marine depositional environments (e.g., Seilacher, 1977; Walker, 1984; Buatois and Mangano, 2003). The conglomerate lithofacies is interpreted to represent gravel-rich channels that locally prograded into more distal parts of the submarine-fan system (e.g., Shanmugam and Moiola, 1991).

Facies Association 3

Description: Normal-graded sandstone and shale. This facies association consists of normal-graded sandstone and shale beds that range in thickness from 1 to 50 cm (Fig. 5E). The lower part of the graded beds is characterized by medium- to fine-grained sandstone that commonly contains horizontal stratification, ripple cross-stratification, rip-up clasts, and flute casts (Fig. 5F). The upper part of the graded beds is characterized by shale that often forms an abrupt contact with and less commonly is scoured into by the overlying coarser sandstone beds (Fig. 5F). The shale often contains horizontal and vertical burrows. The lithologies and bed thicknesses common in Facies Association 3 are illustrated in our measured stratigraphic section from Suslota Creek (Fig. 7A).

Figure 7. Representative measured stratigraphic sections for Facies Associations 3 to 5. The inset diagram for Facies Associations 3 and 4 shows closeup of normal-graded beds (black triangles) characteristic of this facies association. Volcanic strata at 290 m on Bonanza Creek section marks the contact with the overlying Chisana Formation. Stratigraphic sections do not represent the complete thickness of the facies associations.

Figure 7. Representative measured stratigraphic sections for Facies Associations 3 to 5. The inset diagram for Facies Associations 3 and 4 shows closeup of normal-graded beds (black triangles) characteristic of this facies association. Volcanic strata at 290 m on Bonanza Creek section marks the contact with the overlying Chisana Formation. Stratigraphic sections do not represent the complete thickness of the facies associations.

Interpretation. We interpret Facies Association 3 to represent deposition in medial to distal environments within submarine-fan systems (e.g., Shanmugam and Moiola, 1991). The normal-graded sandstone/shale beds that characterize this facies association are best classified by the Bouma (1962) model for turbidites and consist predominantly of Ta, Tb–d, and Te units. The medium- to fine-grained sandstone at the base (Ta) grades upward to ripple-laminated sandstone (Tc) and less commonly into laminated sandstone/ shale (Td) (Fig. 5F). These units are capped by bioturbated shale (Te). The massive to graded Ta units were most likely deposited rapidly by suspension fallout, whereas the rippled to laminated Tb–d units were deposited more slowly under tractive transport processes. The Te units are interpreted as being deposited predominantly by suspension settling from midwater or surface-water plumes, muddy turbidity currents, tails of sandy turbidity currents, and/or hemipelagic processes.

Facies Association 4

Description: Tabular sandstone and clast-supported lenticular conglomerate. Facies Association 4 consists of tabular, medium- to coarse-grained massive sandstone (Fig. 5G) interbedded with less common lenticular, pebble-cobble conglomerate (Fig. 5H). The sandstone consists of amalgamated beds with an average bed thickness of 0.5–1 m that are tabular on a scale of several kilometers. Interbedded conglomerate is moderately sorted and has average maximum clast sizes ranging from 5 to 20 cm. This facies association grades vertically and laterally into the graded sandstone and shale beds of Facies Association 3. Our measured stratigraphic section from Suslota Creek, shown on Figure 7A, is representative of Facies Association 4 and illustrates common lithologies, bed thicknesses, and the interbedded relationship with Facies Association 3.

Interpretation. The massive sandstone of Facies Association 4 is interpreted as the deposits of high-density turbidity currents and liquefied flows in broad channels within distinct lobes of the medial regions of submarine-fan systems (e.g., Clark and Pickering, 1996). This interpretation is based on their tabular geometries, amalgamated beds, lack of sedimentary structures, and their interbedded relationship to the graded beds of Facies Association 3 (e.g., Lowe, 1976b; Middleton and Hampton, 1976; Stow et al., 1996). The lack of well-developed grading suggests that sediment was deposited rapidly from turbulent suspensions with little time for lateral segregation of the grains within the current prior to deposition (e.g., Hein, 1982). Similar sandstone-rich facies are described from ancient submarine-fan deposits interpreted to have formed by sandy, high-density turbidity currents (e.g., Hiscott and Middleton, 1979; Lowe, 1982; Trop et al., 1999). The conglomerate interbedded with the massive sandstone is interpreted to represent deposits of high-density turbidity flows and/or pebbly debris flows (e.g., Lowe, 1982; Stow et al., 1996).

Facies Association 5

Description: Fossil-rich mudstone. This facies association consists predominantly of fossiliferous, bioturbated mudstone with minor amounts of sandstone, conglomerate, fossiliferous limestone, and lava flows (Fig. 5I). Distinct fossil-rich bivalve horizons (Fig. 5J), isolated articulated bivalve fossils, rip-up clasts (Fig. 5K), abundant horizontal feeding traces (probably Cruziana or Zoophycos; Fig. 5L), and vertical burrows, dewatering structures, and carbonaceous plant debris characterize this facies association. The fossil-rich horizons occur as both coquina (concentrations of disarticulated reworked shell fragments; Fig. 5J) and articulated, in situ horizons of the bivalve Buchia. The articulated bivalve horizons contain densely clustered Buchia shells that are uniform in length (∼4 cm down the long axis of the shell), and form 10–20 cm thick tabular beds. The coquina beds are up to 1 m thick, are sometimes interbedded with the articulated Buchia horizons, are locally lenticular over a distance of 5 m, commonly contain mudstone rip-up clasts that are up to 30 cm in length (Fig. 5K), and have horizontally stratified tops that grade upward into bioturbated, fossiliferous shale.

Sandstone, interbedded with the fossiliferous shale, generally occurs in tabular beds but locally is lenticular. Bed thickness ranges from 1 to 30 cm. Minor channelized pebbly conglomerate beds are also interbedded with the fossiliferous shale. Intercalated basaltic to andesitic lava flows occur throughout this facies association and increase upsection toward the gradational contact with the Chisana Formation (Fig. 7B). The common lithologies, bed thicknesses, and the gradational contact with the Chisana Formation that characterize Facies Association 5 are shown on our measured stratigraphic section from Bonanza Creek (Fig. 7B).

Interpretation. We interpret Facies Association 5 to have been deposited on a marine shelf based on the abundance of nontransported, open marine bivalve macrofauna (e.g., Zakharov, 1987; Kidwell, 1991), and the predominance of bioturbated shale containing plant debris (e.g., Leithold, 1989). The in situ deposits of Buchia and the coquina beds are interpreted to represent fair-weather and storm deposits, respectively, on a marine shelf. Similar deposits of Buchia have been described from other Jurassic-Cretaceous strata of Alaska and have been interpreted as forming in less than 150 m water depth (Miller and Detterman, 1985; Clautice et al., 2000; Trop et al., 2005). The presence of Cruziana or Zoophycos ichnofacies is also consistent with a shelf interpretation (e.g., Bottjer et al., 1987; Vossler and Pemberton, 1988; MacEachern et al., 2005). The intercalated volcanic flows that increase upsection in Facies Association 5 probably mark the beginning of volcanism associated with the overlying 3000 m of volcanic strata represented by the Chisana Formation. The Chisana Formation has been interpreted as representing a volcanic arc complex (Plafker and Berg, 1994). In this context, Facies Association 5 may represent the muddy marine shelf that flanked the embryonic Chisana arc.

Paleodrainage

Conglomerate imbrication, ripple stratification, and flute casts allow us to reconstruct sediment transport directions in the Nutzotin basin (Figs. 2A, 4). The paleocurrent data presented in Figure 2A were all collected from coarse-grained strata (Facies Associations 1, 3, and 4) and have been restored to horizontal based on bedding orientation. Conglomerate imbrication (n = 24) in Facies Association 1 along the southern part of the outcrop belt indicates predominantly northwestward sediment dispersal (Fig. 2A). These data indicate that in proximal depositional environments sediment transport was away from the Wrangellia terrane. Conglomerate imbrication (n = 66), ripple stratification (n = 121), and flute casts (n = 10) from Facies Associations 3 and 4 in the northwestern part of the outcrop belt record predominantly eastward sediment dispersal with a smaller component of south-eastward paleoflow (Figs. 2A, 4). The eastward paleoflow in Facies Associations 3 and 4 suggests transverse flow away from the Wrangellia terrane. The more southeasterly directed paleoflow may represent axial drainage within the basin. Paleocurrent data from ripple stratification (n = 212) from the southeastern part of the outcrop belt, reported from Kozinski (1985) within our Facies Associations 3 and 4, display northward sediment transport (Fig. 2A). These paleoflow data also support transverse flow away from the margin of the Wrangellia terrane. No paleocurrent indicators were observed from Facies Association 5; however, the lithology of clasts in conglomerate (discussed on next page) within this facies association suggests that detritus was also being derived from the Wrangellia composite terrane located to the south of the Nutzotin basin. Due to a lack of reliable paleomagnetic data from strata of the Nutzotin Mountains sequence or from any overlying strata, the possible role of block rotation about a vertical axis on the distribution of our paleocurrent data cannot be evaluated. Raw paleocurrent data are available in Manuszak (2000).

Summary: Stratigraphy and Depositional Systems

Our measured stratigraphic sections and geologic mapping data, combined with previous paleontologic and geologic mapping studies (Richter, 1971, 1976; Richter and Schmoll, 1973; Richter and Jones, 1973), define a three-part stratigraphy for the Nutzotin Mountains sequence as shown on Figure 4. The lowest part of the stratigraphy consists of proximal lenticular conglomerate of Facies Association 1 that is laterally equivalent to more distal black shale and isolated lenses of conglomerate of Facies Association 2. Facies Associations 1 and 2 are Upper Jurassic (Tithonian-Oxfordian) based on marine megafossils (Richter, 1971; Richter and Schmoll, 1973). Facies Association 1 is at least 500 m thick and is exposed only along the southern part of the outcrop belt (Figs. 2A, 4). Geologic mapping shows that Facies Association 1 has a depositional contact with the underlying Wrangellia terrane (Richter, 1971; Manuszak, 2000). This depositional contact is well exposed at Misty Mountain (Fig. 2B). Facies Association 2 is at least 300 m thick and also is only exposed along the southern part of the outcrop belt (Figs. 2, 4). At all localities where Facies Association 2 was mapped, it rests with a fault contact on top of Triassic strata of the Wrangellia terrane (Fig. 4; Manuszak, 2000). A lateral facies transition between Facies Associations 1 and 2 is inferred based on similarities in fossil ages, lithofacies, clast composition of conglomerate, and relative stratigraphic position with overlying facies associations. Stratigraphic correlation between Facies Associations 1 and 2 cannot be verified by physical correlation because of the lack of continuous exposure and the presence of several faults between outcrops of the two facies associations.

The middle part of the stratigraphy consists mainly of normal-graded sandstone and shale of Facies Association 3 (Fig. 4). Interbedded with the graded sandstone and shale are tabular coarse-grained sandstone and conglomerate of Facies Association 4 (Fig. 4). Facies Associations 3 and 4 are Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) based on marine megafossils (Richter, 1976). Facies Association 3 has a minimum thickness of 1000 m and is best exposed in the central and northern parts of the outcrop belt (Fig. 2A). This facies association gradationally overlies the strata of Facies Associations 1 and 2, is laterally equivalent with Facies Association 5 in the upper part of the basin fill, and locally grades directly upward into the overlying Chisana Formation based on field observations. Facies Association 4 is found throughout the outcrop belt (Figs. 2A, 4), but the thickest exposures (up to 300 m) are found in the central and southeastern part of the outcrop belt.

The upper part of the stratigraphy consists of fossiliferous, bioturbated shale of Facies Association 5. These strata are Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) based on marine megafossils (Richter, 1971; Richter and Jones, 1973). Facies Association 5 is best exposed in the southeastern part of the outcrop belt where it has a minimum thickness of 300 m (Figs. 2A, 4). An attempt to better define the upper age limit of Facies Association 5 by radiometrically dating lava flows at the base of the Chisana Formation was unsuccessful because of significant alteration of feldspar (P.W. Layer, 2000, personal commun.). The available age data, along with the gradational contact between Facies Association 5 and the Chisana Formation, indicate that the Tithonian-Valanginian Facies Association 5 grades into the Chisana Formation, which has an Aptian upper age limit (see discussion in Geologic Setting and Previous Studies).

We interpret the Nutzotin Mountains sequence as being deposited mainly by sand- and mud-rich submarine-fan systems (Facies Associations 1–4) that bordered a muddy marine shelf (Facies Association 5). The submarine-fan interpretation is based on (1) the dominance of sediment gravity flow deposits, especially turbidites and debris-flow lithofacies similar to those described from submarine-fan models (e.g., Mutti and Ricci Lucchi, 1972, 1975; Pickering et al., 1986; Mutti, 1992); (2) the lack of wave-formed cross-stratification or evidence of extensive reworking by wave or tidal currents in Facies Associations 1–4; and (3) the presence of transported open-marine megafossils in Facies Associations 1–4. Our data suggest that the submarine-fan systems probably developed along a base-of-slope setting where a confined inner fan channel (submarine canyon) broadened into a less confined basinal setting. Evidence for base-of-slope deposition includes the relative abundance of fan lobe deposits, the lack of syndepositional slump folds, abundant sediment gravity flow deposits, and the presence of rock-fall avalanche deposits (e.g., Galloway, 1998).

Proximal regions of the submarine-fan systems during Late Jurassic (Tithonian-Oxfordian) time were dominated by broad gravel-rich channels that were filled by debris flows (matrix-supported and clast-supported conglomerate) and rock-fall avalanches (large outsized clasts) of Facies Association 1. These strata grade basinward into the shale-rich deposits that contain isolated lenticular conglomerate of Facies Association 2. During Late Jurassic to Early Cretaceous (Tithonian-Valanginian) time, the medial parts of the submarine-fan systems were dominated by broad lobes that were on the scale of several kilometers wide and are represented by the tabular massive sandstone and clast-supported conglomerate of Facies Association 4. The coarser-grained lobes are interbedded with overbank medial and distal submarine-fan deposits that are represented by the thin-bedded, normal-graded turbidites of Facies Association 3. We interpret the medial overbank deposits as forming in response to overbank flows adjacent to the submarine lobes of Facies Association 4 (e.g., Mutti, 1977; Pickering, 1985; Nelson and Maldonado, 1988). Facies Association 5 represents the shallowest marine depositional environments in the Nutzotin Mountains sequence. The bivalve-rich shale that characterizes this lithofacies is interpreted as representing a muddy marine shelf that probably flanked the embryonic Chisana arc. The in situ Buchia faunas documented in Facies Association 5 have been interpreted in other strata as forming in less than 150 m water depth (Clautice et al., 2000; Trop et al., 2005).

COMPOSITIONAL AND PROVENANCE DATA

Provenance data from the Nutzotin Mountains sequence include clast composition of conglomerate (n = 1784), petrographic analysis of sandstone thin sections (n = 17), and detrital zircon geochronology (n = 10). Clast compositional data from conglomerate were collected by identifying ∼100 individual clasts within a delineated rectangular area within individual beds. Thin sections used for petrographic analysis were cut from medium- to coarse-grained sandstone samples and stained for both potassium and plagioclase feldspar. The stained thin sections were point counted with a polarizing microscope; ∼400 grains were counted from each thin section. We followed the Gazzi-Dickinson convention (e.g., Gazzi, 1966; Dickinson, 1970) whereby sand-sized monomineralic components of lithic fragments are tallied as individual mineral grains, and only aphanitic grains are classified as lithic fragments (e.g., Ingersoll et al., 1984). Detrital zircon grains were collected by pulverizing a medium-grained sandstone sample and separating out zircons from other minerals using standard magnetic, density, and chemical separation techniques. Detrital zircon age determinations were performed using standard isotope dilution-thermal ionization mass spectrometry as described by Gehrels et al. (1991). All zircon populations were analyzed to delineate different age groups.

Conglomerate Data

Clast types in conglomerate of the Nutzotin Mountains sequence consist mainly of metabasalt/greenstone, limestone, chert, quartz, granite/diorite, and fine-grained volcanic clasts. Conglomerate compositional data are plotted in Figure 8A according to their facies associations/stratigraphic position described in the previous section. The data represent compiled clast counts from multiple stratigraphic horizons within individual facies associations.

Figure 8. (A) Histograms showing the clast composition of conglomerate from the facies associations of the Nutzotin Mountains sequence. Note the relative upsection decrease in limestone, fine-grained volcanic, and chert clasts, as well as the upsection increase in metabasalt clasts. Conglomerate data collected from multiple stratigraphic horizons for each facies association in our measured sections labeled 1, 2, 4, 5, 6, 11, 12, and 19 on Figure 2. n = total number of clast counted. See text for additional discussion and for description of common clast types. (B) Histograms showing sandstone point-count compositional data from each facies association. Note upsection relative increase in feldspar and decrease in quartz and volcanic lithic grains. See text for additional discussion.

Figure 8. (A) Histograms showing the clast composition of conglomerate from the facies associations of the Nutzotin Mountains sequence. Note the relative upsection decrease in limestone, fine-grained volcanic, and chert clasts, as well as the upsection increase in metabasalt clasts. Conglomerate data collected from multiple stratigraphic horizons for each facies association in our measured sections labeled 1, 2, 4, 5, 6, 11, 12, and 19 on Figure 2. n = total number of clast counted. See text for additional discussion and for description of common clast types. (B) Histograms showing sandstone point-count compositional data from each facies association. Note upsection relative increase in feldspar and decrease in quartz and volcanic lithic grains. See text for additional discussion.

Metabasalt/greenstone clasts are dark gray green and exhibit aphanitic to porphyritic textures in hand sample. Petrographic analysis shows that these clasts have sericite and chlorite alteration and contain elongate phenocrysts of plagioclase and pyroxene. The percentage of metabasalt/greenstone clasts increases upsection in the Nutzotin Mountains sequence (Fig. 8A). Limestone clasts range from light to dark gray, have micritic textures, usually lack any observed macrofauna, and may contain calcite veins. The relative abundance of limestone clasts decreases upsection (Fig. 8A). Chert clasts are commonly black but some are dark gray. Chert clasts comprise small percentages of the overall clast types in each of the facies associations; however, they do show a relative upsection decrease (Fig. 8A). Quartz clasts are milky white and show a relative upsection decrease (Fig. 8A). Plutonic clasts are dominantly felsic, ranging in composition from diorite to granite. Identifiable phenocrysts include quartz, feldspar, amphibole, pyroxene, and biotite in a finegrained, white to light gray groundmass. Granite/diorite clasts increase upsection but are not observed in Facies Association 5. Volcanic clasts consist mostly of light gray, brown, and white fine-grained tuffs that often contain small white, black, green, or gray phenocrysts. The aphanitic volcanic clasts decrease upsection in relative abundance (Fig. 8A). Clasts in the “other” category occur in minor quantities (<5%) and include argillite, sandstone, mafic-intrusive, and metasedimentary clasts (Fig. 8A). Raw clast count data are available in Manuszak (2000).

Sandstone Data

Sandstones of the Nutzotin Mountains sequence can be classified as feldspathic wackes (e.g., Dott, 1964) or lithofeldspathic subquartzose sandstones (e.g., Dickinson, 1970). The sandstones point counted in this study are poorly to moderately sorted, medium- to coarse-grained, and often exhibit some clay and/or calcite alteration of framework grains. The framework modes of 17 sandstones from the Nutzotin Mountains sequence are shown graphically in Figures 8B and 9. Recalculated and raw point-count data, as well as photomicrographs of common framework grains in sandstones, are available in Manuszak (2000).

Figure 9. Ternary diagrams showing modal composition of sandstone from facies associations of the Nutzotin Mountains sequence. Dashed areas represent one standard deviation from the mean modal composition. Recycled orogen, magmatic arc, continental block-basement uplift, and mixed provenance fields from Dickinson et al. (1983). Note that the average sandstone composition for Nutzotin Mountains sequence overlaps the magmatic arc and continental block-basement uplift fields.

Figure 9. Ternary diagrams showing modal composition of sandstone from facies associations of the Nutzotin Mountains sequence. Dashed areas represent one standard deviation from the mean modal composition. Recycled orogen, magmatic arc, continental block-basement uplift, and mixed provenance fields from Dickinson et al. (1983). Note that the average sandstone composition for Nutzotin Mountains sequence overlaps the magmatic arc and continental block-basement uplift fields.

The modal compositions for sandstones of the Nutzotin Mountains sequence are Q6F67L27, Qm4F67Lt29, Qm5P95K0, Lv71Lm15Ls14, and Lvm72Lsm17Qp11 (Fig. 9). Note the low total quartz content (Q), that plagioclase (P) represents 95% of the total monocrystalline count, and that volcanic fragments (Lv) compose 71% of the unstable lithic grains. Plagioclase is most commonly observed as individual grains or as laths within volcanic lithic fragments. Common unstable lithic grains include volcanic grains with trachytic, pilotaxitic, hyalopilitic, or felsitic textures; sedimentary grains of siliceous mudstone, and brown to black mudstone; and metamorphic grains of quartz mica schist, mica schist, and foliated mudstone grains. Six of the seventeen thin sections analyzed in this study contain substantial amounts of pyroxene and hornblende grains (as high as 29% in one thin section). Microprobe analysis indicates that some of these grains are calcium- and magnesium-rich clinopyroxenes.

The average sandstone composition for the Nutzotin Mountains sequence shown on Figure 9 overlaps the magmatic arc and continental block-basement uplift provenance fields of Dickinson et al. (1983). Note that sandstone from low in the stratigraphic section (Facies Association 1) plots mostly in the magmatic arc provenance field (Fig. 9), whereas sandstone from the middle of the section (Facies Associations 3 and 4) overlaps both the magmatic arc and continental block-basement uplift provenance fields. Sandstone from the upper part of the section (Facies Association 5) plots entirely in the continental block-basement uplift provenance field (Fig. 9). Figure 8B shows the relative proportions of six common framework grain types based on facies association/stratigraphic position. Note the upsection relative increase in feldspar and decrease in volcanic lithic and quartz grains.

Detrital Zircon Geochronology

U-Pb ages were determined on ten detrital zircons extracted from a medium-grained sandstone of Facies Association 3 within the South Noyes measured section (section 7 on Figures 2, 4). The relative stratigraphic position of this sample within the Nutzotin Mountain sequence is shown on Figure 3. We extracted individual zircons from the sandstone, sieved them according to size, and divided them into populations based on color and degree of textural maturity. Ten grains were selected as representatives from the zircon populations and analyzed. Analytical details and raw data are shown on Table 1. The analysis of the ten grains shows two age groups of 151–147 (n = 8) and 159–156 Ma (n = 2) (Fig. 10). These interpreted ages are based on the 206Pb/238U ratios shown on Table 1.

TABLE 1. U-Pb ISOTOPIC DATA AND APPARENT AGES

Figure 10. U-Pb concordia plot for ten detrital zircons from Lower Cretaceous sandstone sample of Facies Association 3. Sample was collected from measured section labeled 7 on Figure 2. Note the two age groups of 147–151 (n = 8) and 156–159 Ma (n = 2). Detrital zircon ages overlap U-Pb zircon ages from Jurassic plutons in the Chitina arc (153–150 Ma; Plafker et al., 1989; Roeske et al., 2003). See text for discussion. See Table 1 and Manuszak (2000) for analytical details and raw data.

Figure 10. U-Pb concordia plot for ten detrital zircons from Lower Cretaceous sandstone sample of Facies Association 3. Sample was collected from measured section labeled 7 on Figure 2. Note the two age groups of 147–151 (n = 8) and 156–159 Ma (n = 2). Detrital zircon ages overlap U-Pb zircon ages from Jurassic plutons in the Chitina arc (153–150 Ma; Plafker et al., 1989; Roeske et al., 2003). See text for discussion. See Table 1 and Manuszak (2000) for analytical details and raw data.

Provenance Discussion

Our compositional data, combined with northward- and eastward-directed paleocurrent measurements (Fig. 2A), indicate that most of the Nutzotin Mountains sequence was derived from mainly Paleozoic-Triassic metavolcanic strata of the Wrangellia terrane, Upper Jurassic-Lower Cretaceous igneous rocks representing the Chitina and Chisana arcs, and possibly sedimentary strata of the Wrangell Mountains basin. The compositional data indicate exhumation and progressive erosion of deeper stratigraphic/structural levels of the Wrangellia terrane during deposition of the Nutzotin Mountains sequence. Conglomerates in the lower part of the section (Facies Associations 1 and 2), for example, are dominated by limestone and chert clasts (Fig. 8A) interpreted as being derived from the Upper Triassic-Lower Jurassic Chitistone and Nizina Limestones and the McCarthy Formation (Fig. 3). Conglomerates in the middle of the section (Facies Associations 3 and 4) contain a mixture of metabasalt/greenstone, limestone, and chert clasts (Fig. 8A). The metabasalt/greenstone clasts are petrographically similar to the metabasalt of the Lower to Middle Triassic Nikolai Greenstone and Pennsylvanian-Permian Skolai Group and reflect unroofing of deeper stratigraphic levels of the Wrangellia terrane. Conglomerate in the upper part of the basin fill (Facies Association 5) consists almost entirely of meta-basalt/greenstone clasts that record exhumation of the basaltic roots of the Wrangellia terrane during deposition of this part of the Nutzotin Mountains sequence. Detritus from the Chitina arc is represented by granite/diorite clasts, fine-grained volcanic clasts (Fig. 8A), and detrital zircons with concordant U-Pb ages of ca. 159–147 Ma (Fig. 10). These ages are consistent with previously published thermochronologic data from plutonic rocks of the Chitina arc, which yield Late Jurassic U-Pb zircon crystallization ages (153–150 Ma; Plafker et al., 1989; Roeske et al., 2003) and Late Jurassic-Early Cretaceous K-Ar and Ar-Ar cooling ages (146–138 Ma; MacKevett, 1978; Plafker et al., 1989; Roeske et al., 1992, 2003). Another possible source for the dated detrital zircons is Triassic-Jurassic diorites mapped by Richter (1976) that yielded a K-Ar age of 163 ± 4 Ma. These rocks are very limited in their distribution and only one radiometric age is available, but they crop out close to the Nutzotin Mountains sequence (labeled Jp on Fig. 2).

Our sandstone compositional data also record the unroofing of the Wrangellia terrane and associated arcs. Sandstones from the lower part of the Nutzotin Mountains sequence (Facies Association 1) show a relative increase in volcanic lithic fragments (Figs. 8B, 9) and relatively unaltered feldspars. We interpret these sandstones as being derived from the volcanic edifice of the Chitina arc. These sandstones plot mainly in the magmatic arc provenance fields (Fig. 9). Sandstones from higher in the section (Facies Associations 3 and 4) overlap both the magmatic arc and continental block-basement provenance fields suggesting a feldspar-rich source terrane. The most likely source terrane for these sandstones is the 3000-m-thick Nikolai Greenstone and the plutonic roots of the Chitina arc. Sandstones from the upper part of the Nutzotin Mountains sequence (Facies Association 5) have the highest percentage of feldspar and lowest percentage of volcanic lithic fragments. Feldspar in these sandstones is highly altered by chlorite and epidote suggesting derivation mainly from the mafic basement of the Wrangellia terrane (the Nikolai Greenstone and Skolai Group). Some of the feldspar in the upper part of the Nutzotin Mountains sequence may have also been derived from the deeper plutonic levels of the Chitina and Chisana arcs.

GEOLOGIC MAPPING AND CROSS SECTIONS

Figure 2B is a simplified regional cross section from the southern margin of the Yukon composite terrane, through the Nutzotin basin, and into the Wrangellia terrane based on geologic mapping during this study and by Richter (1976). This cross section illustrates the geometry of the suture zone between the allochthonous Wrangellia terrane and the former Mesozoic continental margin of North America (i.e., the Yukon composite terrane). Note that the suture zone contains a range of structural styles including reverse, normal, and strike-slip faults. Also note that the cross section shows regional folds that have wavelengths on the order of 15 km. Figure 11 is an enlargement of the regional cross section presented in Figure 2B that focuses on the structural configuration of strata of the Nutzotin basin and the major structures that deformed the Nutzotin Mountains sequence. This cross section shows a north-dipping regional décollement that transported the majority of the Nutzotin Mountains sequence southward over the Wrangellia terrane (labeled Lost Creek décollement in Fig. 11). It also shows north-dipping thrust faults and related folds within the Nutzotin Mountains sequence (e.g., area near Buck Creek pluton in Fig. 11); major strike-slip faults that include the Denali and Totschunda fault systems; and normal faults (e.g., area near southern end of cross section in Fig. 11). In the following sections, we discuss each of these structural styles and their possible timing based on geologic mapping data.

Figure 11. North-south cross section showing structural relationships between the Nutzotin Mountains sequence, the Wrangellia terrane, and the Yukon composite terrane. Line of cross section shown on Figure 2. Bedding attitudes from this study, Richter et al. (1975) and Richter (1976). Radio-metric dates are not available from the Buck Creek, Lost Creek, and Devil's Mountain plutons, but they are interpreted as equivalent to a suite of nearby plutons with 118–105 Ma K-Ar ages. See text for discussion.

Figure 11. North-south cross section showing structural relationships between the Nutzotin Mountains sequence, the Wrangellia terrane, and the Yukon composite terrane. Line of cross section shown on Figure 2. Bedding attitudes from this study, Richter et al. (1975) and Richter (1976). Radio-metric dates are not available from the Buck Creek, Lost Creek, and Devil's Mountain plutons, but they are interpreted as equivalent to a suite of nearby plutons with 118–105 Ma K-Ar ages. See text for discussion.

Contractional Structures

We have identified a décollement horizon that displaced distal basinal strata of the Nutzotin Mountains sequence southwestward (present coordinates) over both more proximal basinal strata of the Nutzotin Mountains sequence as well as the Wrangellia terrane. Figure 12 is a reconnaissance field map of the Lost Creek area, where the décollement is particularly well exposed. We informally refer to the décollement as the Lost Creek décollement because of the excellent exposures in this area. Attitudes collected from the Lost Creek décollement surface show that it has an average orientation of N70W/31NE. The footwall of the décollement includes the McCarthy Formation, which consists of ∼100 m of dark gray, micritic limestone interbedded with black chert, siliceous argillite, and carbonaceous shale (Fig. 12). The hanging wall consists of Facies Association 1 of the Nutzotin Mountains sequence. Overturned drag folds in the hanging wall of the décollement have axial surfaces that dip to the northeast (Figs. 12, 13A) indicating southwest-ward displacement of the hanging wall (e.g., Ramberg, 1963). This tectonic transport direction is corroborated by monoclinal kink bands in the footwall that show preferential development of central limbs that dip steeply to the northeast; this orientation is indicative of shear produced from the hanging wall moving to the southwest (Figs. 12, 13B; e.g., Pfaff and Johnson, 1989). For a more detailed discussion of the analysis of folds in the Nutzotin Mountain sequence see Manuszak (2000). Tertiary-Cretaceous dikes (Richter and Schmoll, 1973) and a radiometrically dated, undeformed pluton (Suslota Pass Pluton of Richter et al., 1975) crosscut the Lost Creek décollement indicating that the age of displacement along the décollement in this area must be older than ca. 117–105 Ma. The décollement is interpreted to have been truncated by the Totschunda and Denali strike-slip faults (Fig. 11).

Figure 12. Simplified geologic map adjacent to the Lost Creek décollement in the northwestern part of the Nutzotin basin. Location of map shown on Figure 2A. Photo location 1 is shown on Figure 13A; photo location 2 is shown on Figure 13B. See text for discussion.

Figure 12. Simplified geologic map adjacent to the Lost Creek décollement in the northwestern part of the Nutzotin basin. Location of map shown on Figure 2A. Photo location 1 is shown on Figure 13A; photo location 2 is shown on Figure 13B. See text for discussion.

Figure 13. (A) Closeup photograph of overturned fold in hanging wall of the Lost Creek décollement. Fold is verging to the southwest. Hammer (circled) for scale. Photo is from Location 1 shown on Figure 12. (B) Kink fold in the footwall of the Lost Creek décollement. Person (circled) for scale. View is to the southeast. Photo is from Location 2 shown on Figure 12. (C) Intrabasinal thrust fault that places Facies Associations 3 and 4 (labeled FA3, 4) in the hanging wall over Facies Association 5 (FA5) and the Chisana Formation (Kc) in the footwall (low-lying hills in midground). White barbed line shows fault trace; barbs on upthrown side of fault. Photo location is from area near measured section 18 on Figure 2A. View is to the northeast. (D) Normal fault that juxtaposes Triassic Nikolai Greenstone (Trn) against younger Triassic Nizina Limestone (Trl) and the Nutzotin Mountains sequence (KJs). Black line shows fault trace; black arrow shows relative displacement. Photo is from the Lost Creek area in the northwestern part of the basin along the southern basin margin. Person circled for scale. (E) Trace of the Totschunda strike-slip fault (white dashed line) in the headwaters of the Nabesna River. View is to the south. WCT = Wrangellia composite terrane. Note the juxtaposition of Facies Associations 1 and 2 (FA1, 2) with Facies Associations 3 and 4 (FA3, 4) along the fault system.

Figure 13. (A) Closeup photograph of overturned fold in hanging wall of the Lost Creek décollement. Fold is verging to the southwest. Hammer (circled) for scale. Photo is from Location 1 shown on Figure 12. (B) Kink fold in the footwall of the Lost Creek décollement. Person (circled) for scale. View is to the southeast. Photo is from Location 2 shown on Figure 12. (C) Intrabasinal thrust fault that places Facies Associations 3 and 4 (labeled FA3, 4) in the hanging wall over Facies Association 5 (FA5) and the Chisana Formation (Kc) in the footwall (low-lying hills in midground). White barbed line shows fault trace; barbs on upthrown side of fault. Photo location is from area near measured section 18 on Figure 2A. View is to the northeast. (D) Normal fault that juxtaposes Triassic Nikolai Greenstone (Trn) against younger Triassic Nizina Limestone (Trl) and the Nutzotin Mountains sequence (KJs). Black line shows fault trace; black arrow shows relative displacement. Photo is from the Lost Creek area in the northwestern part of the basin along the southern basin margin. Person circled for scale. (E) Trace of the Totschunda strike-slip fault (white dashed line) in the headwaters of the Nabesna River. View is to the south. WCT = Wrangellia composite terrane. Note the juxtaposition of Facies Associations 1 and 2 (FA1, 2) with Facies Associations 3 and 4 (FA3, 4) along the fault system.

In the hanging wall above the décollement, north-dipping thrust faults and overturned folds are common in the Nutzotin Mountains sequence (Fig. 11). Many of these structures were originally identified by the mapping of Richter and Jones (1973). Thrust faults with displacements on the order of a few kilometers are common throughout the area (e.g., Buck Creek fault in Fig. 11). Figure 13C, for example, shows a thrust fault in the southwestern part of the basin that juxtaposes Facies Association 3 in the hanging wall against Facies Association 5 and the Chisana Formation in the footwall. Upright and overturned folds with wavelengths of up to ∼2 km are common in the Nutzotin Mountains sequence and are usually associated with nearby faults. Relatively undeformed Cretaceous plutons with K-Ar ages of ca. 117–105 Ma (Richter et al., 1975) that crosscut both thrust faults and folds in the upper plate provide a minimum age for the end of contractional deformation.

Extensional Structures

Normal faults are common in the study area and offset both the Nutzotin Mountains sequence and the Chisana Formation (Figs. 11, 13D; Richter and Jones, 1973). Note the normal faults shown in the central and southern parts of the cross sections (e.g., near Misty Mountain on Figures 2B, 13D). The extensional deformation documented in the Nutzotin Mountains sequence needs much more additional study, but we tentatively interpret this deformation as correlative to better-documented Early to Late Cretaceous (Albian-Campanian) normal faulting in the Wrangell Mountains basin (Trop et al., 2002). In the Wrangell Mountains basin, Upper Lower to Upper Cretaceous siliciclastic strata show both abrupt changes in thickness and lithofacies across syndepositional normal faults (Trop, 2000; Trop et al., 2002).

Strike-Slip Structures

The Denali and Totschunda faults are the two dominant strike-slip faults in the study area (Fig. 2A), and are shown in the northern and central parts of the cross section in Figure 11. Both faults are interpreted as accommodating mainly Cenozoic displacement (Richter and Jones, 1973; Plafker and Berg, 1994; Eberhart-Phillips et al., 2003). The Denali fault juxtaposes the Nutzotin Mountains sequence against Devonian metamorphic rocks of the Yukon composite terrane (Fig. 11). The Totschunda fault has been interpreted to be a relatively young (ca. 1 Ma) dextral strike-slip fault that has accommodated primarily horizontal offsets (Fig. 13E; Plafker et al., 1977; Lisowski et al., 1987). Lithofacies mapping suggests that the Totschunda fault may have formed along a preexisting thrust fault that had juxtaposed Upper Jurassic strata of Facies Associations 1 and 2 against younger Cretaceous strata of Facies Associations 3 and 4 (Fig. 11).

DISCUSSION

Development of the Nutzotin Basin

Figure 14 illustrates our interpretation of the stages of basin development for the Nutzotin basin based on our new data as well as data from previous investigations (e.g., Berg et al., 1972; Richter and Jones, 1973; Richter, 1976; Kozinski, 1985). The earliest record of subsidence in the Nutzotin basin is represented by Upper Jurassic (Oxfordian-Tithonian) submarine-fan strata of Facies Associations 1 and 2. Proximal submarine-fan strata, consisting of conglomerate with outsized limestone clasts (Facies Association 1), were deposited unconformably over the northern margin (present coordinates) of the Wrangellia terrane (Fig. 14A). This unconformity represents a ca. 50 m.y. depositional hiatus between the underlying Upper Triassic-Lower Jurassic McCarthy Formation and the overlying Upper Jurassic part of the Nutzotin Mountains sequence (Fig. 3). The unconformity at the base of the Nutzotin Mountains sequence marks a change from deep-marine, fine-grained calcareous and siliceous strata (i.e., the McCarthy Formation) that contain no detritus demonstrably linked to erosion of the Wrangellia terrane to coarse-grained, submarine-fan strata of Facies Association 1 that clearly record exhumation and erosion of the Wrangellia terrane. We interpret this unconformity to partly represent a regional tectonic event that deformed and exhumed the Wrangellia terrane based on the boulder conglomerate that overlies the unconformity (Facies Association 1). The conglomerate of Facies Association 1 represents deposition by debris flows and rock-fall avalanches in submarine canyons and other proximal regions of submarine-fan systems (Fig. 14A). Northward-directed paleocurrent data from Facies Association 1 indicate that sediment dispersal was away from the northern margin of the Wrangellia terrane. The abundant limestone and chert clasts in conglomerate and the volcanic lithic-rich sandstone of Facies Association 1 (Figs. 8A, 9) indicate that the uppermost units of the Wrangellia terrane (i.e., the Nizina and McCarthy Limestones; Figure 3; Tc and Tm on Figure 14A) and the active Chitina arc were the primary sources of sediment for this first stage of basin development. Distal submarine-fan strata of Facies Association 2, represented by mainly black shale with thin micritic limestone and isolated conglomerate lenses, were deposited predominantly by fine-grained turbidity flows and hemipelagic processes in the distal regions of submarine-fan systems (Fig. 14A).

Figure 14. Simplified block diagrams showing stages of basin development for the Nutzotin basin. View is roughly to the northwest (present coordinates). See text for discussion. AP = Antler pluton; LP = Lost Creek pluton.

Figure 14. Simplified block diagrams showing stages of basin development for the Nutzotin basin. View is roughly to the northwest (present coordinates). See text for discussion. AP = Antler pluton; LP = Lost Creek pluton.

The Upper Jurassic-Lower Cretaceous normal-graded sandstone and shale of Facies Association 3 and the tabular sandstone and clast-supported conglomerate of Facies Association 4 represent the main phase of sedimentation and subsidence in the Nutzotin basin. The greater than 1000 m of strata associated with Facies Associations 3 and 4 are the product of well-developed transverse and axial submarine-fan systems (Fig. 14B). The bulk of the sediment deposited in the submarine-fan systems was derived from the Wrangellia terrane and the remnant Chitina arc (RA in Fig. 14B) based on our compositional and paleocurrent data. The exhumation of deeper stratigraphic levels of the Wrangellia terrane (i.e., the Nikolai Greenstone and Skolai Group) is recorded by the upsection relative increase of metabasalt/greenstone clasts in conglomerate of the Nutzotin Mountains sequence (Fig. 8A). The relative upsection increase in igneous clasts also reflects exhumation of deeper levels of the Chitina arc; this arc was built on the southern margin of the Wrangellia terrane (Fig. 14B). Similarly, detrital zircons from sandstone of Facies Association 3 yield concordant U-Pb ages that are consistent with previously published isotopic ages from plutons of the Chitina arc (e.g., Plafker et al., 1989; Roeske et al., 1992; Nokleberg et al., 1994a). Sandstone compositional data from Facies Associations 3 and 4 of the Nutzotin Mountains sequence indicate derivation from magmatic arc and uplifted basement sources (Fig. 9), which we interpret as being the volcanic-rich strata of the Wrangellia terrane and the Chitina arc, and the plutonic rocks of the Chitina arc, respectively. Eastward and southeastward paleoflow indicators from Facies Associations 3 and 4 are consistent with our compositional data and the interpretation of mainly Wrangellia terrane sources for sediment of the Nutzotin basin.

The next stage of basin development recorded in the Nut-zotin Mountain sequence is defined by Lower Cretaceous fossiliferous shale of Facies Association 5. The presence of sedimentary structures indicative of tractive transport and the abundance of nontransported open marine bivalve macrofauna indicate that Facies Association 5 represents deposition on a marine shelf (Fig. 14C). Exposures of these strata are limited to the southern margin of the Nutzotin basin. More proximal, shallow-water strata of Facies Association 5 are interpreted to grade basinward into the more distal, upper part of Facies Association 3. Both Facies Associations 5 and 3 have gradational contacts with the base of the overlying Chisana Formation. Clast compositional data from Facies Association 5 are dominated by metabasalt/greenstone clasts (Fig. 8A) suggestive of derivation from the basaltic roots of the Wrangellia terrane (i.e., the Nikolai Greenstone and Skolai Group). Overlying the sedimentary strata of Facies Association 5 with a gradational contact are ∼3000 m of volcanic strata of the Chisana Formation (Fig. 3). The Chisana Formation represents a late Early Cretaceous volcanic arc that was constructed ∼50 km northeast of the exhumed and erosionally dissected Late Jurassic Chitina arc (Fig. 1B; Berg et al., 1972; Plafker and Berg, 1994; Short et al., 2005).

Regional shortening of the Nutzotin Mountain sequence occurred on a series of north-dipping thrust faults and south-verging folds during Late early Cretaceous time (Fig. 11). These thrust faults are interpreted to sole into a basal décollement (e.g., the Lost Creek décollement; labeled LF in Fig. 14D). Displacement on this décollement resulted in the juxtaposition of more distal strata of the Nutzotin Mountains sequence over both more proximal strata as well as the Wrangellia terrane (Figs. 11, 14D). This crustal shortening must have occurred before emplacement of the 117–105 Ma undeformed granitic plutons that crosscut both the décollement and folded strata of the Nutzotin Mountains sequence (labeled LP and AP in Figure 14D; Richter, 1976; Manuszak, 2000).

Subaerial erosion, nonmarine deposition, and possibly normal faulting characterize the Nutzotin basin during latest Cretaceous time (Fig. 14E). Localized nonmarine strata containing conglomerate, shale, and coal were deposited over the already folded Nutzotin Mountain sequence and Chisana Formation. These unnamed sedimentary strata are poorly dated but are thought to be Upper Cretaceous in age (Richter, 1976). Normal faults that cut the entire Nutzotin Mountains sequence (Fig. 2B; Richter, 1976, this study) require a more detailed analysis but potentially may be related to Late Cretaceous extension that has been documented in the Wrangell Mountains basin (e.g., Trop et al., 2002).

In summary, our stratigraphic, compositional, and structural data from strata of the Nutzotin basin provide a record of a sedimentary basin that was at least in part deposited on the northern margin of the Wrangellia terrane and was filled with sediment derived from this terrane. Following deposition, these strata were deformed by north-dipping thrust faults and related folds above a north-dipping décollement. The stratigraphy that we have defined for the Nutzotin Mountain sequence represents a general upward-shallowing and upward-coarsening sedimentary package. The lower and middle parts of the Nutzotin Mountain sequence (Facies Associations 1–4 on Fig. 4) represent a general transition from distal mud-rich submarine-fan deposition to more proximal sandstone-rich submarine-fan deposition. These submarine-fan strata are in turn overlain by marine shelf strata (Facies Association 5 on Fig. 4). We interpret the Nutzotin basin, based on its stratigraphy and on its regional tectonic setting (discussed in the following section), as a retroarc foreland basin (Figs. 14A, 14B). In general, contractional basins, such as foreland basins, are characterized by upward-coarsening and upward-shallowing stratigraphies because in these settings as the source terrane is being exhumed it is also being tectonically transported by reverse faults toward the basin center (see Jordan, 1995, and Miall, 1995, for review). We rule out the Nutzotin basin as an extensional backarc basin that formed adjacent to the Wrangellia terrane because the stratigraphy of these types of basins is often characterized by upward-fining and upward-deepening sedimentary packages (see Marsaglia, 1995, for review). The reason for this is that in extensional backarc basins, the exhumed source terrane is generally being tectonically transported away from the basin center by normal faults. Another important distinction between the stratigraphy of the Nutzotin Mountain sequence and the stratigraphy of most extensional backarc basins is the ubiquitous presence of interbedded volcanic and volcaniclastic strata throughout the basin fill of extensional basins (e.g., Carey and Sigurdsson, 1984; Busby-Spera, 1988).

Regional Basin Configuration and Tectonic Setting

The Nutzotin Mountains sequence represents part of a series of sedimentary basins located along the inboard margin of the Wrangellia composite terrane (the Gravina, Dezadeash, and Kahiltna basins on Fig. 1A) that define a major suture zone in the northwestern Cordillera (Rubin et al., 1990; McClelland et al., 1992a; Rubin and Saleeby, 1992). To provide a perspective of the development of this suture boundary in south-central Alaska, our data from the Nutzotin basin, located on the inboard (northern) margin of the Wrangellia composite terrane, are integrated with geologic data from the Wrangell Mountains basin, a Late Triassic to Late Cretaceous depocenter that formed along the outboard (southern) margin of the terrane (WB in Figure 1B; MacKevett, 1978; Winkler et al., 1981; Trop et al., 1999; Trop, 2000; Trop et al., 2002; Trop and Ridgway, this volume). The strata of the Nutzotin and Wrangell Mountains basins both clearly rest depositionally on the Wrangellia composite terrane. Geologic and paleomagnetic data indicate that the Wrangellia composite terrane was located near the equator during late Paleozoic-Late Triassic time and was subsequently translated ∼30° northward relative to North America sometime between Late Triassic to early Tertiary time (Hillhouse and Grommé, 1984; Hillhouse and Coe, 1994; Plafker and Berg, 1994; Cowan et al., 1997; Stamatakos et al., 2001). The time of collision and the location of the collision with respect to North America during northward displacement of the Wrangellia composite terrane is unclear and highly debated (e.g., Umhoefer, 1987; McClelland and Gehrels, 1990; van der Heyden, 1992; Plafker and Berg, 1994; Cowan et al., 1997; Mahoney et al., 1999; Housen and Beck, 1999; Butler et al., 2001). Our paleogeographic model presented in this section assumes Mesozoic collision of the Wrangellia composite terrane at a northern paleolatitude followed by Cenozoic strike-slip fault shuffling; our model builds on reconstructions by Nokleberg et al. (1998), Ridgway et al. (2002), and Trop and Ridgway (this volume). In our reconstruction, collision of the Wrangellia composite terrane is time-transgressive, starting in southeastern Alaska and progressing northward to south-central Alaska (e.g., Pavlis, 1982; McClelland et al., 1992a; Ridgway et al., 2002; Trop et al., 2005; Kalbas et al., this volume). Whereas initial collision of the south-central Alaska segment of the composite terrane took place sometime during Late Jurassic-Cretaceous time, geologic data from southeastern Alaska and western Canada indicate close proximity of the composite terrane with the outboard margin of the Yukon-Tanana and Stikine terranes by Middle Jurassic time (e.g., McClelland and Gehrels, 1990; McClelland et al., 1992a; van der Heyden, 1992; Kapp and Gehrels, 1998; Saleeby, 2000; Gehrels, 2001).

Middle Jurassic (Bajocian-Callovian)

Middle Jurassic strata were not deposited or are not preserved in the Nutzotin basin. Strata of this age, however, are well preserved in the Wrangell Mountains basin. The Middle Jurassic Nizina Mountain Formation (Fig. 3) exposed in the southern Wrangell Mountains was deposited in a retroarc position, inboard (north) of Upper Jurassic arc plutons (WB on Figure 15A; Chitina arc of Plafker et al., 1989; Roeske et al., 1989, 2003; Trop et al., 2002). Previous investigations have interpreted the dominance of fine-grained arc-derived detritus, the localization of sediment accumulation adjacent to the Jurassic arc platform, and the presence of primary volcanic strata as indicators that Middle Jurassic sedimentation was a product of erosion of an oceanic arc prior to collision with inboard terranes (Fig. 15A; Trop et al., 2002, 2005; Trop and Ridgway, this volume). North-dipping oceanic subduction during this time interval is based on detailed geochemical and stratigraphic investigations of Jurassic arc-related igneous rocks (e.g., Burns, 1985; Plafker et al., 1989; DeBari and Coleman, 1989; DeBari and Sleep, 1991; Keleman et al., 2003; Rioux et al., 2005; Clift et al., 2005a, 2005b; Trop et al., 2005; Draut and Clift, 2006). The key point of this time interval for our discussion of the Nutzotin basin is that Middle Jurassic sedimentary strata accumulated in shallow-marine depocenters that fringed a south-facing oceanic island-arc along the southern margin of the Wrangellia composite terrane, whereas age-correlative strata were not deposited along the northern margin of the terrane in the area that would become the Nutzotin basin. See Trop and Ridgway (this volume) for a more detailed discussion of this paleogeographic time slice for south-central Alaska.

Figure 15. (continued on the next page) Sketch maps showing the inferred paleogeographic evolution of Mesozoic sedimentary basins in southern Alaska. Paleolatitudes are not shown due to uncertainty in the paleoposition of the Wrangellia composite terrane with respect to North America (see Cowan et al., 1997, for review). Current distance between towns of Anchorage (#A) and McCarthy (#M) is 390 km. See text for discussion on the evolution of the Nutzotin basin (NB) and the Wrangell Mountains basin (WB). See Trop and Ridgway (this volume) for a more detailed discussion of the geology of the Matanuska basin (MB) that is shown in this figure. Modified from Trop and Ridgway (this volume).

Figure 15. (continued on the next page) Sketch maps showing the inferred paleogeographic evolution of Mesozoic sedimentary basins in southern Alaska. Paleolatitudes are not shown due to uncertainty in the paleoposition of the Wrangellia composite terrane with respect to North America (see Cowan et al., 1997, for review). Current distance between towns of Anchorage (#A) and McCarthy (#M) is 390 km. See text for discussion on the evolution of the Nutzotin basin (NB) and the Wrangell Mountains basin (WB). See Trop and Ridgway (this volume) for a more detailed discussion of the geology of the Matanuska basin (MB) that is shown in this figure. Modified from Trop and Ridgway (this volume).

Late Jurassic (Oxfordian-Tithonian)

The marked change in grain size and composition of sediment that defines the first stage of basin development in the Nutzotin basin is also clear in the stratigraphy of the Wrangell Mountains basin (WB in Fig. 1B). In this basin, older Jurassic sedimentary strata (Nizina Mountain and Lower Root Glacier Formations in Fig. 3) contain abundant recycled primary volcanic detritus derived from the active Chitina arc, whereas Upper Jurassic sedimentary strata (Kotsina Conglomerate and Upper Root Glacier Formation in Fig. 3) contain metavolcanic, metaplutonic, metasedimentary, and sedimentary detritus derived from the Wrangellia terrane and the plutonic roots of the Chitina arc (Trop et al., 2002). The abrupt introduction of coarse-grained, mixed detritus into both the Nutzotin and Wrangell Mountains basins is interpreted as the erosional product of Late Jurassic shortening in the Chitina fold-and-thrust belt (CTB on Figures 1B, 15B). This fold-and-thrust belt, located along the southern margin of the Wrangell Mountains basin (MacKevett, 1978; Gardner et al., 1986), exposed multiple structural levels of the Wrangellia terrane that were eroded and transported northward into the Nutzotin and Wrangell Mountains basins (Trop et al., 2002).

We interpret the Late Jurassic introduction of coarse-grained detritus, the exhumation and erosion of the Wrangellia terrane and Chitina arc, and the northward transport of sediment documented in both the Nutzotin and Wrangell Mountains basins as the first sedimentary evidence of the early stages of collision of the Wrangellia composite terrane with inboard terranes that formed the Mesozoic continental margin of North America. Both of these basins were located inboard of the Chitina fold-and-thrust belt, which had formed adjacent to the active Chitina arc (Fig. 15B). Plutons representing this arc were foliated by pervasive syntectonic deformation during Late Jurassic time (MacKevett, 1978). In interpreting the Nutzotin basin as forming in a retroarc setting, we infer a dual plate subduction scenario for the northern Cordillera (e.g., Monger et al., 1982; Rubin et al., 1990; Stanley et al., 1990; McClelland and Mattinson, 2000; Trop and Ridgway, this volume) with subduction polarity of both plates dipping northward, one beneath the Wrangellia composite terrane and the other beneath the continental margin (see Figure 4B in Trop and Ridgway, this volume). Structurally emplaced slivers of ophiolitic rocks presently located in the northwestern corner of the study area (Fig. 2A), as well as other ophiolitic slivers found along the suture zone between the Wrangellia and Yukon composite terranes in south-central Alaska may represent fragments of the consumed oceanic plate (e.g., Richter, 1976; Nokleberg et al., 1994b, 1998; Patton et al., 1994). The size of this inferred oceanic plate is presently unknown; it may have been a substantial oceanic plate as suggested by Ridgway et al. (2002, see their Fig. 11A) or a small back-arc basin floored by transitional or oceanic crust as suggested by McClelland and Mattinson (2000, see their Fig. 10A).

Latest Jurassic-Early Cretaceous

In the Wrangell Mountains basin, the latest Jurassic-Early Cretaceous interval is defined by a regional angular unconformity (Fig. 3; MacKevett, 1978). This unconformity is a product of the incorporation of basinal strata into the Chitina fold-and-thrust belt that resulted in folding and structural imbrication of sedimentary strata of the Wrangell Mountains basin, igneous rocks of the Chitina arc, and the Wrangellia terrane (Fig. 15C; Trop et al., 2002). Regional shortening, exhumation, and erosion within the Chitina fold-and-thrust belt provided a large volume of sediment for the Nutzotin basin that is recorded in the more than 1000 m of strata in Facies Associations 3 and 4 of the Nutzotin Mountains sequence. Basinal subsidence needed to preserve such a thick package of strata may have been a flexural response to the load supplied by the Chitina fold-and-thrust belt and Chitina arc. Regional exhumation of the Wrangellia terrane in the Chitina fold-and-thrust belt also allowed unroofing and erosion of the deeper stratigraphic/structural levels of the terrane, which is well documented in our compositional and detrital zircon age data.

We interpret the Upper Jurassic-Lower Cretaceous Nutzotin Mountains sequence as being deposited in a retroarc foreland basin located inboard (cratonward) of the active Chitina arc and Chitina fold-and-thrust belt. Collectively, we interpret the main phase of coarse-grained sedimentation in the Nutzotin basin; the extensive development of the Chitina fold-and-thrust belt; and the regional shortening and exhumation of the Wrangell Mountains basin, Chitina arc, and the Wrangellia terrane to represent a main phase of oblique collision between the Wrangellia composite terrane and the inboard terranes that formed the outer continental margin. Inboard terranes, such as the Yukon composite terrane, are interpreted to have been linked to North America by Middle Jurassic time (e.g., Monger et al., 1982; Mihalynuk et al., 1994; Monger and Nokleberg, 1996). A weak point in our interpretation is that direct unequivocal sedimentological and compositional evidence linking strata of the Nutzotin basin with inboard terranes has not been found; this may be partly due to the lack of a detailed detrital zircon provenance study for the strata of this basin. In along-strike Upper Jurassic-Lower Cretaceous basins, however, there is direct evidence of sediment derivation from continental margin sources. In the Kahiltna basin of the Alaska Range (KBa in Fig. 1A), for example, detrital zircons and diagnostic clasts in conglomerate indicate derivation from inboard Paleozoic terranes (Eastham, 2002; Ridgway et al., 2002; Hampton et al., 2005, this volume; Kalbas et al., this volume). Kalbas et al. (this volume), for example, report a significant population of detrital zircons with Middle and Early Proterozoic ages from the Kahiltna assemblage in the western Alaska Range. East of the Nutzotin basin, Kapp and Gehrels (1998) have reported U-Pb age determinations from detrital zircons of the Upper Jurassic-Upper Cretaceous Gravina basin (GB in Fig. 1A) that best fit with derivation from the inboard Yukon composite terrane. In addition, geochemical and neodymium isotopic data from Upper Jurassic-Lower Cretaceous strata of the Dezadeash basin (DB in Fig. 1A) also suggest that a possible small component of continental margin crust provided sediment to the basin (Mezger et al., 2001).

Regionally, along-strike Upper Jurassic-Lower Cretaceous sedimentary strata have similar submarine-fan facies, depositional packages, and provenance as described for the Nutzotin Mountains sequence. For example, in the Kahiltna basin of the Talkeetna Mountains, located west of the Nutzotin basin (KBt in Fig. 1A), a similar unroofing of the Wrangellia composite terrane is clear in clast composition of conglomerate (Eastham and Ridgway, 2002; Ridgway et al., 2002). Strata of the Dezadeash basin located in the Yukon Territory (Fig. 1B) were also derived mainly from the Wrangellia composite terrane and transported eastward by submarine-fan systems (e.g., Eisbacher, 1976, 1985; Lowey, 2006). Strata of the Gravina basin (Fig. 1A) exposed in the vicinity of Juneau, Alaska, show a similar upward-coarsening package as documented in the Nutzotin basin with conglomerate derived from the Wrangellia composite terrane (e.g., Cohen and Lundberg, 1993, see their Fig. 3). Strata of the Gravina basin near Ketchikan, Alaska do not show an overall upward-coarsening depositional package but do contain metavolcanic and metaplutonic clasts derived from the Wrangellia composite terrane (e.g., Rubin and Saleeby, 1991). U-Pb zircon ages from plutonic clasts in this part of the Gravina basin have age ranges from 158 to 154 Ma (Rubin and Saleeby, 1991), roughly similar to the 159–147 Ma age ranges from detrital zircons of the Nutzotin Mountains basin (Fig. 10). In both cases, the zircons are interpreted to have been derived from exposed plutons of the Late Jurassic volcanic arc that was constructed on the Wrangellia composite terrane. The clast composition of conglomerates, the detrital zircon ages, and eastward/northward paleoflow away from the Wrangellia composite terrane in the Nutzotin, Gravina, Dezadeash, and Kahiltna basins all indicate that deeper stratigraphic/structural levels of the Wrangellia composite terrane, as well as plutons of the associated Late Jurassic arc were being regionally exhumed, eroded, and transported into adjacent basins along the inboard margin of the Wrangellia composite terrane by at least Early Cretaceous time. These inboard Upper Jurassic-Lower Cretaceous basins extended for ∼2000 km along the suture zone between the Wrangellia composite terrane and inboard terranes of continental margin affinities (Fig. 1A). As pointed out by Pavlis (1982), McClelland et al. (1992a), and Ridgway et al. (2002), these Late Jurassic-Early Cretaceous sedimentary basins probably represent a main phase of collision of the Wrangellia composite terrane with the Cordilleran continental margin. Our findings from the Nutzotin basin are consistent with such an interpretation.

There are, however, some differences in the Late Jurassic-Early Cretaceous sedimentary basins that formed along the inboard margin of the Wrangellia composite terrane that may be important for paleogeographic reconstructions of western North America. In southeastern Alaska, for example, strata of the Gravina basin are interpreted as having been deposited in extensional and/or transtensional backarc and intra-arc basins, possibly associated with a major dextral fault system, following Middle Jurassic collision (e.g., McClelland et al., 1992a, 1992b; among others). In the case of the Nutzotin basin, most of our data, in combination with data from the Wrangell Mountains basin, best fit with deposition in a retroarc foreland basin that formed due to flexural subsidence associated with the Chitina fold-and-thrust belt. Stratigraphic data from the Kahiltna basin, located west of the Nutzotin basin (Fig. 1A), also appear to be consistent with a contractional basin origin (Ridgway et al., 2002; Kalbas et al., this volume). The possible change from a Late Jurassic-Early Cretaceous strike-slip-dominated margin in southeastern Alaska to a convergent-dominated margin in southcentral and southwestern Alaska may be a function of along-strike changes in the tectonic character of the collisional zone, possibly related to diachronous closure of the suture zone (e.g., Pavlis, 1982; Nokleberg et al., 1998; Ridgway et al., 2002).

Early to Late-Early Cretaceous

Growth of the Chisana arc was coeval with renewed subsidence and siliciclastic sedimentation in the Wrangell Mountains basin (Berg Creek and Kuskulana Formations in Figure 3; Trop et al., 2002) but marked the end of widespread clastic deposition in the Nutzotin basin (Fig. 15D). Along strike in southeastern Alaska near Juneau, strata of the Gravina basin (Fig. 1A) show a similar transition from sedimentary to volcanic strata as documented in the Nutzotin basin (e.g., Cohen and Lundberg, 1993). Along-strike basins to the west, the Kahiltna basins (Fig. 1A), do not have Lower Cretaceous volcanic strata overlying Upper Jurassic-Lower Cretaceous sedimentary strata (Eastham, 2002; Kalbas et al., this volume). The significance of these along-strike differences is poorly understood but likely important for thorough paleogeographic reconstructions of the northwestern Cordillera.

Figure 15. (continued)

Figure 15. (continued)

Late-Early Cretaceous

We interpret the structural imbrication and the translation of strata of the Nutzotin basin over the Wrangellia terrane to mark the later stages of oblique Mesozoic terrane accretion in south-central Alaska. In our interpretation, strata of the Nutzotin retroarc foreland basin were incorporated into an accretionary wedge (Fig. 14D; e.g., Sengör and Okurogullari, 1991; Ingersoll et al., 1995) with the overriding North American continental margin providing the backstop against which the Nutzotin Mountains sequence was imbricated and subsequently thrust over the Wrangellia terrane (Fig. 15D). The structural style of north-dipping thrust faults and south-verging folds documented for the Nutzotin basin is consistent with an inferred northward-dipping subduction zone between the northern margin of the Wrangellia composite terrane and North America. The Nutzotin Mountain sequence, which had been deposited on the downgoing plate that was transporting the Wrangellia composite terrane toward the continental margin, was now tectonically incorporated into the overriding plate as an accretionary wedge.

Our interpretation for the Nutzotin Mountain sequence is consistent with previous studies of deformation of other Upper Jurassic-Lower Cretaceous basinal strata in the suture zone. Mapping of the Gravina basin (labeled GB in Figure 1A, for example) shows it to have been imbricated and underthrust to relatively deep crustal levels (∼25–30 km) beneath the Yukon composite terrane during middle Cretaceous time (McClelland et al., 1992a, 1992b, 1992c; McClelland and Mattinson, 2000). Crosscutting relationships and U-Pb age determinations from syn- and posttectonic plutons suggest that underthrusting of the Gravina basin began between 113 and 97.5 Ma and ended by 90 Ma in southeastern Alaska (Haeussler, 1992; McClelland et al., 1992c). West of the Nutzotin basin, magnetotelluric surveys and seismic lines across the suture zone in the central Alaska Range have been interpreted as suggesting that Upper Jurassic-Lower Cretaceous strata of the Kahiltna basin (labeled KB in Fig. 1A) were under-thrust beneath the Yukon composite terrane during middle Cretaceous time (Stanley et al., 1990; Beaudoin et al., 1992).

The thrust faults documented in the Nutzotin basin are probably part of a regionally extensive thrust system that has been documented along the northwestern Cordillera. This thrust system places Jurassic-Cretaceous sedimentary strata on top of rocks equivalent to the Wrangellia composite terrane (Rubin et al., 1990; Rubin and Saleeby, 1992). If the deformation recorded in the Nutzotin basin is laterally equivalent to this thrust system that has been described from Washington to southeastern Alaska, this thrust system extends for over 2000 km and represents a series of sedimentary basins that have been incorporated into accretionary wedges defining a continental-scale suture zone.

Latest Cretaceous-Tertiary

Strata of the Nutzotin and Wrangell Mountains basins were both gently folded and topographically inverted during latest Cretaceous-Tertiary time. This deformation is recorded by angular unconformities that separate Cretaceous and older strata from overlying Miocene-Quaternary strata in both basins (Fig. 3; Richter, 1976; MacKevett, 1978). The timing of basin inversion broadly overlapped with 1650 ± 890 km of northward translation of both basins along orogen-parallel dextral fault systems (e.g., Stamatakos et al., 2001). During northward translation, strata of the Nutzotin basin were dismembered by the Denali fault system (Fig. 14E; Richter and Jones, 1973; Eisbacher, 1976; Nokleberg et al., 1985). Much of this displacement is interpreted to have occurred during Eocene-Oligocene time based on ages of strike-slip basins along the fault system (e.g., Ridgway et al., 1999; Trop et al., 2004). Deformation of strata of the Nutzotin basin continues to the present as recorded by surface ruptures along the Totschunda fault associated with the 2003 Denali earthquake (e.g., Eberhart-Phillips et al., 2003) and offset late Pleistocene-Holocene geomorphic features (Matmon et al., 2006).

CONCLUSIONS

  1. Stratigraphic, compositional, and provenance data from the Upper Jurassic-Lower Cretaceous Nutzotin Mountains sequence are interpreted as representing deposition in a retroarc foreland basin that was located on the northern margin of the Wrangellia terrane. Coeval with basin development along the northern margin, sedimentary basins and plutons located along the southern margin of the Wrangellia terrane were being incorporated into a regional fold-and-thrust belt. Sediment eroded from this fold-and-thrust belt was transported northward and deposited in the Nutzotin basin.

  2. Compositional data and U-Pb detrital zircon ages from the Nutzotin Mountains sequence record exhumation and progressive erosion of the Wrangellia terrane, the strata of the Wrangell Mountains basin, and the Chitina arc. All these source areas were exposed in the fold-and-thrust belt located south of the Nutzotin basin. Comparison of stratigraphic and provenance data from the Nutzotin basin with along-strike Upper Jurassic-Lower Cretaceous basins, such as the Gravina, Dezadeash, and Kahiltna basins, shows that deeper stratigraphic/structural levels of the Wrangellia composite terrane, as well as plutons of associated Late Jurassic arcs, were being regionally exhumed and eroded and were providing sediment for basins along the inboard margin of the terrane by at least Early Cretaceous time.

  3. Collectively, we interpret the regional unconformity at the base of the Nutzotin Mountains sequence (ca. 50 m.y. hiatus), the abrupt introduction of coarse-grained detritus above the unconformity, the overall upward-coarsening and upward-shallowing stratigraphy of the Nutzotin Mountains sequence, the compositional record of exhumation and erosion of deeper levels of the Wrangellia terrane, and the development of the fold-and-thrust belt south of the Nutzotin basin as products of Late Jurassic-Early Cretaceous collision of the Wrangellia composite terrane with inboard terranes representing the continental margin of North America.

  4. The stages of deformation documented in strata of the Nutzotin basin also provide insight into the growth of collisional continental margins by the tectonic incorporation of basinal strata. In the case of the Nutzotin basin, strata were incorporated into an accretionary wedge related to underthrusting beneath the continental margin. Regionally, the Nutzotin Mountains sequence represents part of a series of sedimentary basins located along the inboard margin of the Wrangellia composite terrane that generally have similar depositional styles and were subsequently incorporated into accretionary wedges. These deformed strata define a continental-scale suture along the entire northwestern Cordillera.

This research was supported by Donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation. Partial support for Manuszak's M.S. research was provided by the GSA John T. Dillon grant, the AAPG Fred A. Dix grant, the Linda Horn Memorial scholarship from Purdue University, the Sigma Xi Alexander Bache Fund, and the Society of Petroleum Engineers' research scholarship. We thank Wrangell–St. Elias National Park and Preserve staff, especially Danny Rosenkrans, for their help and support of our field activities; Brian Lareau, Mike DePersia, and Shane Smith for assistance in the field; and Arvid Johnson, Scott King, Rick Hoy, and Kaj Johnson for useful discussions. Reviews by Jonathan Glen, Marc Hendrix, and Brad Ritts were extremely helpful for improving the manuscript. This paper is dedicated to the late Don Richter, who provided encouragement and helpful discussions on the geology of the eastern Alaska Range and Wrangell Mountains; Don's geologic map of the Nabesna quadrangle provided the foundation for our study.

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Figures & Tables

Contents

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