Cordilleran Sedimentary Basins of Western Canada ...

Provided for non-commercial research and educational use only. Not for reproduction, distribution or commercial use. This chapter was originally published in the book Sedimentary Basins of the World, published by Elsevier, and the attached copy is provided by Elsevier for the author’s benefit and for the benefit of the author’s institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who know you, and providing a copy to your institution’s administrator.

All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier’s permissions site at: http://www.elsevier.com/locate/permissionusematerial From Brian D. Ricketts, Cordilleran Sedimentary Basins of Western Canada Record 180 Million Years of Terrane Accretion. In: K.J. Hsü, editor: Sedimentary Basins of the World, Vol 5, The Sedimentary Basins of the United States and Canada, Andrew D. Miall. The Netherlands: Elsevier, 2008, pp. 363–394. ISBN: 978-0-444-50425-8 © Copyright 2008 Elsevier BV. Elsevier

Author's personal copy CHAPTER 10

Cordilleran Sedimentary Basins of Western Canada Record 180 Million Years of Terrane Accretion Brian D. Ricketts

Contents 1. Introduction 2. The Cordilleran Morphogeological Belts 3. Terranes, Terrane Accretion and Associated Basins of the Canadian Cordillera 3.1. Intermontane superterrane 3.2. Insular superterrane 3.3. Magmatism, deformation, and relative plate motions 3.4. The modern plate boundary 3.5. The ‘‘Baja BC’’ debate 4. Sedimentary Basins Associated with Intermontane Superterrane 4.1. Whitehorse trough 4.2. Bowser Basin 4.3. Sustut ‘‘piggyback’’ Basin 4.4. Tyaughton–Methow basin 5. Basins Located along the Inboard Margin of Insular Superterrane 5.1. Nutzotin–Dezadeash–Gravina–Gambier basins 6. Basins Located along the Outboard Margin of Insular Superterrane 6.1. Queen Charlotte–Wrangell Mountains basins 7. Cenozoic Basins-Harbingers of the Modern Plate Boundary 7.1. Queen Charlotte-Georgia-Tofino basins 7.2. Tertiary Queen Charlotte Basin 7.3. Georgia (Nanaimo) Basin 7.4. Tofino Basin 7.5. Provenance linkages 8. Discussion Acknowledgments References

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Abstract The North American Cordillera is a collage of disparate and commonly far-traveled lithospheric blocks and slivers (terranes). Beginning in the early part of the Jurassic, terranes accreted to the western margin of the North American craton such that the margin has grown several hundred kilometres westward to its present position. Terrane accretion continues today. Jurassic to Recent foreland, forearc, backarc, wrench, and remnant-ocean basins in the Canadian Cordillera, west of the Foreland Belt, record complex relationships between terrane accretion to ancestral North America, and the crustlithosphere responses (subsidence, uplift, denudation) associated with collision, subduction, rifting, and wrench tectonics. Basin subsidence, driven for example by tectonic loading during terrane collision, may be terminated by accretion of successive terranes and terrane-amalgams (superterranes). The significance of successive accretion events for basin evolution is well illustrated in Bowser Basin where subsidence and sedimentation were associated with the interaction among Stikinia, Quesnellia, and Cache Creek terrane (components of the Intermontane superterrane) and ancestral North America, beginning in the early Middle Jurassic. Crustal shortening across Bowser Basin beginning in the Early Cretaceous, was likely driven by the docking farther west of Insular superterrane. Indeed, it is likely that tectonic Sedimentary Basins of the World, Volume 5 ISSN 1874-5997, DOI 10.1016/S1874-5997(08)00010-5

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loading and accumulation of major clastic wedges in the Western Canada foreland basin in Alberta and British Columbia also were mechanically linked to Intermontane and Insular superterrane accretion. It is generally accepted that most Cordilleran terranes traveled some distance prior to accretion. Some, like Stikinia, essentially traveled alone; others, like the Insular superterrane contained lithospheric blocks that were amalgamated before docking with North America. However, vigorous debate continues concerning the distances and paleolatitudes traversed by each terrane, and to some extent the timing of terrane accretion to the North American plate margin.

1. Introduction Sedimentary rocks making up the mountainous swath that is the Canadian Cordillera constitute basins or remnants of basins dating back to the Precambrian. However, it is only since the beginning of the Middle, and possibly Early Jurassic that any of these Cordilleran basins has formed in response to plate-tectonic processes acting directly on the ancient western margin of the North America. All Cordilleran basins older than this formed on plates remote from the North American margin and subsequently were accreted to it. The North American Cordillera is a collage of disparate and commonly far-traveled lithospheric blocks and slivers (terranes) that have accreted to the western craton margin such that the margin has grown several hundred kilometres westward to its present position (Figure 1). A terrane is defined as a fault-bounded block containing rocks that have a distinct geologic history compared with contiguous blocks. Howell (1995) defined different kinds of terranes as: stratigraphic representing fragments of continents, continental margins, volcanic arcs, and oceanic crust; disrupted characterized by pervasive shearing and penetrative deformation; and metamorphic. Terms like exotic and

Figure 1 (a) Map of the principal terranes discussed in this chapter, the accretion of which over the past 180--200 Myr has given rise to the Canadian Cordillera (adapted from Jones et al., 1986; Monger, 1989). Terrane abbreviations are listed in Table 2. (b) The ¢ve morphotectonic belts making up the Canadian Cordillera. The western limit is the modern plate boundary to North America; the eastern limit is the stable North American craton. The main characteristics of each belt are summarized in Table 1.

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suspect generally are used for terranes of unknown origin and, with the term allochthonous, imply tectonic transport. In contrast, pericratonic terranes contain rocks that are probably related to an autochthonous craton but may overlie attenuated crust and have uncertain paleogeographic affinities. Additional terms that are important to the Cordilleran story are composite or superterranes, which apply to two or more terranes that have amalgamated into a single lithospheric entity before final accretion to the North American margin. Two superterranes, Intermontane and Insular superterranes, contain most of the basins in the Cordilleran collage of western Canada. Plate-tectonic processes involving Cordilleran terranes were predominantly contractional, including subduction and obduction, collision, crustal-scale delamination, and transcurrent or strike-slip faulting. A sense of the magnitude of lithospheric convergence along the western margin of North American is illustrated by Engebretson et al. (1992) who calculated that about 13,000 line-km of mostly oceanic crust have been consumed in subduction zones over the last 200 Myr. The Jurassic and younger Cordilleran basins provide a record of these terrane-accretion processes in the following interrelated ways: the timing of basin-subsidence records crustal loading by contraction or extension; basins or stratigraphic assemblages that extend across adjacent terrane boundaries (also called overlap or successor basins) help to bracket the timing of the accretion events; sedimentation style and stratigraphic architecture provide critical information on paleogeography and the capacity for the basin to accommodate sediment; sediment composition provides a record of source rock and changes in source resulting from tectonic relocation or unroofing of terranes; paleontological and isotopic-radiometric records provide temporal constraints for accretion events; fossils also provide a measure of control on terrane paleolatitude prior to and during accretion. Paleomagnetic data provide independent constraints on terrane paleolatitudes. The remainder of this chapter is organized as follows: elucidation of the five Cordilleran morphogeological belts is followed by a conspectus of the major Cordilleran terranes, their accretion to North America, and the resulting sedimentary basins. The major basins are then examined in relation to associated terrane-accretion events in terms of their subsidence history, stratigraphic and sedimentologic architecture, and provenance (Figure 2).

2. The Cordilleran Morphogeological Belts The Canadian segment of the Cordillera has been divided into five belts. Each belt is identified by the congruence between its distinctive bedrock geology and its physiography; each belt is oriented parallel to the overall tectonic trend (Figure 1) (Wheeler and Gabrielse, 1972; Monger et al., 1972; Tipper et al., 1981). As plate-tectonic theory and subsequent models of terranes developed, it was recognized that the extent of the morphogeological belts correspond closely to the boundaries of two superterranes (the Insular and Intermontane superterranes), the two intervening swaths of plutonic and metamorphic rocks (the Omineca belt and Coast belt; Monger et al., 1982), and the easternmost Foreland belt that tapers eastward into undeformed rocks that overlie the stable craton (Figure 1). The main characteristics of each morphogeological belt are summarized in Table 1. Over much of their length the Coast and Omineca belts are bounded by fold-and-thrust belts. Folding and thrusting in the Foreland belt, including subsidence of the Alberta Foreland basin, is dynamically and mechanically linked to the Omineca belt (Price, 1973; Stockmal et al., 1992; Miall et al., Chapter 9, this volume). Likewise, the Coast Belt Thrust System is thought to be dynamically linked to accretion of the Insular Superterrane (Crawford et al., 1987; Journeay and Friedman, 1993). The Intermontane and Insular belts are underlain by large areas of volcanic (commonly arc and forearc assemblages) and sedimentary strata, most of which have been metamorphosed to sub-greenschist grades, in contrast to rocks in the adjacent ‘‘crystalline’’ belts. The sedimentary basins examined herein are located in or at the margins of the Intermontane and Insular belts (Figure 2). Development of each basin was linked to terraneaccretion events and associated crustal-scale contraction or extension, lithospheric flexure, and arc magmatism.

3. Terranes, Terrane Accretion and Associated Basins of the Canadian Cordillera The North American Cordillera is made up of more than 200 terranes (Jones et al., 1986; Monger 1989), ranging in size from a few kilometres to the length of British Columbia (Stikinia). The terranes themselves consist of one or more tectonic assemblages, bounded by faults or unconformities, in which strata have a common depositional setting (e.g., platform, shelf, volcanic arc or forearc) and/or a common structural fabric or tectonic history (Tipper et al., 1981; DNAG map 1712A). The defining characteristics of terranes relevant to this synopsis are shown in Table 2. An encyclopaedic compilation of Canadian Cordilleran geology is contained in the Decade of North American Geology Cordillera


Brian D. Ricketts

Figure 2 An outline of the sedimentary basins discussed in this chapter (the map underlay is from Figure 1a. Adapted from Eisbacher (1985), Monger (1989), McClelland et al. (1992). Terrane abbreviations are listed in Table 2.

Orogen volume and accompanying maps (Gabrielse and Yorath, 1991), in particular Terrane Map 1713A (DNAG). The timing of terrane–superterrane accretion and associated basin formation are summarized in Figure 3. Provenance linkages that correspond with these events are shown in Figure 4.

3.1. Intermontane superterrane Intermontane and Insular composite (super)terranes, their amalgamation, and eventual accretion to North America are central to understanding the history of Canadian Cordilleran basins. However, uncertainties and


Cordilleran Sedimentary Basins of Western Canada

Table 1

Defining characteristics of the five morphogeological belts of the Canadian Cordillera (see also Figure 1b).

Insular Belt West

Coast Belt

Inter-Montane Belt

Omineca Belt

Foreland Belt East

Western boundary at Mountainous terrain Mostly low relief, High relief uplifts of Middle Proterozoic to the base of the underlain by except in the Skeena plutonic and highPaleogene modern continental extensive plutonic & area of north British grade metamorphic sedimentary rocks, slope & the Queen metamorphic rocks. Columbia. E and W rocks, and slivers of overlying North Charlotte transform Plutons tend to boundaries coincide sedimentary strata America crystalline fault; the eastern become younger to with major changes in having affinities with basement deformed boundary extends the east. Mostly topographic relief the Foreland Belt. into an E-NE verging from east Vancouver I quartz diorite, diorite Sedimentary strata are Pervasive penetrative thrust stack. Up to to the Denali Fault & & tonalite; Sr initial mostly sub-greendeformation. Core 200 km shortening. Saint Elias Mts. in the ratios o/ ¼ 0.704. schist grade. zones of Western boundary is north. Generally, low Metamorphic grade Deformation style metamorphic and the N and S Rocky grades of ranges from greendepends on basement rock. Mountain Trenches metamorphism schist to amphibolite competency of rock and eastern Selwyn units; e.g., tight- to Mts. in the north. open-folds in the Sedimentation and Skeena Fold Belt deformation are linked to terrane accretion events Source: Information is from Monger et al. (1972, 1982) and Gabrielse et al. (1991)

debate remain over the timing of these events and the horizontal distances terranes may have traveled from some paleolatitude before accretion took place. Slide Mountain terrane, the easternmost component of Intermontane superterrane (Figure 1a), contains Upper Paleozoic and some Triassic volcanic, ultramafic, and sedimentary rocks that have ocean-basin affinities. Initial thrusting over Omineca belt and the pericratonic Kootenay and Cassiar terranes was probably Middle Jurassic in southern British Columbia, but may have been earlier farther north. Rare clasts of metamorphic rock having pericratonic affinities suggest close links between the Slide Mountain ocean and the adjacent craton margin. However, contradictory evidence from Permian faunas suggests that original paleolatitudes may have been as far south as northern Mexico (Monger et al., 1991). Slide Mountain terrane was separated from Cache Creek oceanic crust by Quesnellia (Figure 1a). Quesnellia volcanic and volcaniclastic assemblages have island-arc affinities spanning the Late Triassic through Middle Jurassic. Low strontium isotope initial ratios (o0.704) indicate a generally primitive magmatic character (Armstrong, 1988). Cache Creek terrane, located west of Quesnellia, is similar to Slide Mountain terrane in that it comprises igneous and sedimentary lithologies having ocean-basin affinities. In central British Columbia, Cache Creek ocean-basin strata have been deformed into a crustal-scale thrust stack that bears the hallmarks of an accretionary prism above a subduction complex (Struik et al., 2001). The complex forms part of the Pinchi suture which Struik et al. (2001) interpreted as the lithospheric-scale collisional boundary. Stikinia, the largest terrane in the Canadian Cordillera, is located west of Cache Creek terrane. Like Quesnellia, it consists predominantly of rocks having volcanic arc affinity. In northern British Columbia, Stikinia is overthrust by the Cache Creek terrane along the King Salmon thrust. The timing of Slide Mountain-Quesnellia-Cache Creek-Stikine terrane amalgamation (Figure 3) and the timing of accretion of the resulting Intermontane superterrane to the western margin of North America is still debated. Most agree that there was some interaction among the terranes as early as Late Triassic (e.g., Monger, 1989). Whitehorse Trough in the northern Intermontane belt (Figure 2), is an Early Jurassic arc-marginal or forearc basin that overlies Stikinia and Cache Creek terranes, is located northeast of an active Stikinian arc, and provides evidence of arc-subduction links as early as Sinemurian among Cache Creek, Stikinia, Quesnellia, and the ancient North American margin (Figure 3) (Tempelman-Kluit, 1979; Monger et al., 1991). These events may correlate in part with post-Sonoma (Orogeny) magmatic arc and successor-back-arc basin development in the southern USA Cordillera (Ingersoll, Chapter 11, this volume). However, the sedimentary record indicates that it was not until the late Early Jurassic to early Middle Jurassic that fundamental changes in the Cordilleran landscape occurred, with collision between Quesnellia and Stikinia, and subsequent delamination of the intervening Cache Creek oceanic crust during southwest-directed obduction over Stikinia (Monger et al., 1972, 1982; Eisbacher, 1981), possibly as early as Aalenian (Ricketts et al., 1992). Blueschist metamorphism of Cache Creek rocks as young as Early Jurassic is dated at 173.7 Ma and was followed


Brian D. Ricketts

Table 2 Principal characteristics of allochthonous and pericratonic terranes in the Canadian Cordillera. The modern ocean plates are included for completeness. Abbreviations are those used in Figure 1b. The descriptions are mainly from Monger et al. (1982), Monger (1989), Jones et al. (1986), Gabrielse et al. (1991), and other references cited herein. Allochthonous & Pericratonic Terranes

De¢ning characteristics

Alexander, AX

Possible composite of Precambrian (?) Paleozoic and Mesozoic volcanic, clastic sedimentary and limestone. Associated with WR in the Paleozoic. Translated from lower latitudes Mississippian to Lower Jurassic oceanic rocks, structurally complex, metamorphosed to blueschist facies. Overlain by Jurassic to L. Cretaceous sediments (Cayoosh Assemblage) Carboniferous to Jurassic rocks having oceanic affinities, including radiolarian chert, ophiolite; blueschist metamorphism Pericratonic; Precambrian to Devonian platform carbonates having affinities with the Western Canada miogeocline Triassic-Jurassic arc rocks; Triassic to Mid-Cretaceous carbonates & clastics Composite terrane; Upper Mesozoic flysch and melange, Paleogene flysch and volcanic rocks Eocene ophiolitic layered gabbro, submarine and subaerial lava flows. Located above the modern JDF Modern ocean plate. Subduction beneath North America Pericratonic; Metamorphosed Paleozoic carbonate and clastic sedimentary and volcanic rocks having affinities with the western North American miogeocline Triassic oceanic rocks, including ophiolites, overlain by Jurassic to Cretaceous clastic assemblages Pericratonic. Lower Proterozoic, high-grade metamorphic rocks Modern ocean plate. Separated from North America by Queen Charlotte transform Jurassic-Cretaceous continental margin chert, sandstone, conglomerate, volcanics, and metasediments; melange. Located above the modern JDF Devonian to Upper Paleozoic volcanics and carbonates; Triassic–Jurassic volcanic, plutonic and associated sedimentary rocks. Faunas indicate terrane originally at lower latitudes Upper Paleozoic mudrock and coarse-grained clastics, overlain by Alpine-type ultramafic rocks and carbonates. Permian fusulinids indicate paleolatitudes equivalent to northern Mexico or southern California The largest terrane in the Canadian Cordillera. Mainly Devonian to Jurassic volcanic and plutonic rocks and associated sediments. Faunas indicate derivation from lower latitudes Structurally complex, Upper Paleozoic to Lower Mesozoic volcaniclastics and basalt, limestone, and flysch Composite terrane, Upper Paleozoic to Jurassic magmatic complexes, limestone, and clastic sediments. Associated with AX in the Paleozoic. Translated from lower latitudes Transitional continental crust of metamorphosed Cretaceous to Paleogene sediments. Translated northwards along the Queen Charlotte-Fairweather transform Pericratonic, includes part of Omineca Belt; variably metamorphosed Paleozoic and possibly Precambrian to Jurassic sedimentary, volcanic and granitic rocks; similarities to K

Bridge River, BR Cache Creek, CC Cassiar, CA Cadwallader, CD Chugach, CH Crescent, CR Juan De Fuca, JDF Kootenay, K Methow, M Monashee, MO Pacific Ocean, PO Pacific Rim, PR Quesnellia, Q Slide Mountain, SM

Stikinia, ST Taku, T Wrangellia, WR Yakutat, Y Yukon-Tanana, YT

by rapid exhumation of the metamorphosed subduction assemblage (Mihalynuk et al., 2004). In their interpretation of deep reflection-seismic profiles across the northern Cordillera, Evenchick et al. (2005) show Stikinia overlying a westward-tapering wedge of North American crust. The upper contact of the crustal wedge is inferred to be the accretion surface resulting from the interaction between Stikinia, Cache Creek, and ancestral North America that includes pericratonic terranes (Figure 5). In north-central British Columbia, flexure of Stikinian crust beneath the Cache Creek terrane load, in concert with thermoisostatic cooling of Stikinia, resulted in a foredeep — Bowser Basin. Age constraints for this event are cemented where King Salmon fault and associated crustal-scale structures are cut by 160–172 Ma postkinematic plutons (Woodsworth et al., 1991). Collision of the contiguous Intermontane terrane with the North American margin took place about the same time (Monger et al., 1982; Murphy et al., 1995). On the basis of U/Pb dates from Kootenay Arc and Caribboo Mountains, Murphy et al. (1995) suggested overthrusting of the autochthon by Quesnellia in Mid-Toarcian time, and southwest-vergent deformation until the end of the Aalenian, the latter age bracket corresponding with Cache Creek obduction over Stikinia (Ricketts et al., 1992). Unterschutz et al. (2001) on the other hand contended that Quesnellia in southern British Columbia had depositional links with North America during the Late Triassic, based on Nd isotopic and geochemical data


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Figure 3 Chart showing the space--time relationships among major sedimentary basins of the Canadian Cordillera, the mor photectonic belts, associated terranes--superterranes and their accretion to North America (adapted from Monger, 1989; Yorath, 1991; and other references cited herein). Large arrows indicate the generalized polarity of sediment £ux into the basins.

(REEs) that indicate both a primitive sediment source (volcanic arc) and a more evolved source from the craton. This also implies that Quesnellia may be pericratonic and/or that accretion to North America was strongly diachronous. Additional sedimentological and Nd/U-Pb isotope data from the Nicola horst in southern British Columbia, indicate that mid-crustal Precambrian rocks there have continental affinities, supporting a pericratonic origin for part of Quesnellia (Erdmer et al., 2002). The Quesnel–Cache Creek–Stikinia interaction is linked thermally and mechanically to metamorphism and tectonic thickening of the Omineca belt (the ‘‘core zone’’ of the eastern Foreland belt; Brown et al., 1986). In the foreland basin these events correlate with deposition of the Kootenay-Fernie clast wedge (Figure 3) (Stockmal et al., 1992; Miall et al., Chapter 9, this volume). Thickening, in part is also the result of tectonic onlap of Slide Mountain and Quesnellia over the western Omineca belt during contraction (Gabrielse et al., 1991).


Brian D. Ricketts

Figure 4 A summary of the main provenance characteristics for each sedimentary basin (see text of relevant basins for sources of information). Black arrows indicate the generalized polarity of sediment supply into the basins.Terrane abbreviations are from Table 2. Additional abbreviations are: CB, Coast belt; Casc. Arc, Cascade arc;Volc., volcanic source; N. Calif., northern California; OM, Omineca belt.

In north-central British Columbia the stratigraphic record of these events lies within the Bowser Basin overlap assemblage. Farther south, the Tyaughton–Methow trough is preserved at the intersection of the Quesnel–Cache Creek–Stikine terrane amalgam, with two smaller terranes, the Bridge River and Cadwallader terranes which have oceanic and magmatic arc affinities, respectively. Middle Jurassic strata in Tyaughton–Methow basin directly overlie Bridge River and Cadwallader rocks and constitute an overlap assemblage (Kleinspehn, 1985; Umhoefer et al., 2002). McClelland et al. (1992) suggested that Bowser and Tyaughton–Methow basins were linked both depositionally and kinematically by an oblique dextral-slip fault system during the Late Jurassic to Early Cretaceous.

3.2. Insular superterrane In Canada, the Alexander terrane and Wrangellia are the main crustal elements comprising the amalgam of the Insular composite or superterrane (Table 2; Figures 2 and 3): Alexander terrane consists of Paleozoic (and ?Precambrian) arc, and possibly rift-related Mesozoic assemblages; Wrangellia itself is a composite terrane containing a variety of arc-related assemblages that extend from southern Alaska to beneath Vancouver Island (Jones et al., 1986). The Peninsula terrane in southern Alaska is also amalgamated with Wrangellia (Jones et al., 1986). Dated granite intrusions that stitch basement rocks from Alexander and Wrangellia terranes indicate that the two have probably been together since at least the Late Pennsylvanian (Monger, 1989). The earliest signals of accretion of the Alexander–Wrangellia–Peninsula terrane amalgam to the North American margin include 175 Ma metavolcanics in southeast Alaska, that overlap the Alexander and YukonTanana terranes (Gehrels, 2001). The older age limit for amalgamation of Insular and Intermontane superterranes is recorded by the Upper Jurassic–Lower Cretaceous Gravina–Nutzotin overlap assemblage in the northern Cordillera, and the Gambier overlap assemblage, that includes the Dezadeash and Gambier basins in central and southern British Columbia (Monger et al., 1982, 1994). These basins (Figure 2) formed on the inboard margin of the Insular Superterrane (McClelland et al., 1992); whether they were a continuous or discontinuous retroarc or forearc basin is still debated (McClelland et al., 1992). Nevertheless, there seems little doubt that they were


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Figure 5 Sketch from part of the SNORCLE re£ection seismic pro¢le (line 2b) showing the primary terrane components and main faults from the northwestern edge of Bowser basin north to the Cache Creek--Quesnellia terrane boundary (from Evenchick et al., 2005). Details of the seismic pro¢le are provided by Snyder et al. (2002) and Cook et al. (2004). The accentuated part of line 2b in the accompanying simpli¢ed geology-terrane map corresponds to the sketched portion of the seismic pro¢le shown here; vibration point 5,000 is located at the south end of the sketch.

associated with accretion of the Insular Superterrane to North America. In contrast, Wrangell Mountains and Queen Charlotte forearc basins formed on the outboard margin of the Insular Superterrane (Figure 2). Monger et al. (1982) and others maintain an Early Cretaceous time for the final docking of Intermontane and Insular superterranes, which were subsequently stitched together by Cretaceous–Paleogene granite plutons in northwestern and southwestern British Columbia. The accretion boundary along the eastern margin of Insular Superterrane (Alexander–Wrangellia terranes) is marked by a Mid-Cretaceous, west-verging thrust belt that extends from southern Alaska to the northwestern USA (Monger et al., 1982; McClelland et al., 1992; McClelland and Mattinson, 2000). Emplacement of the thrust belt also marked the collapse of the inboard Gravina–Nutzotin–Dezadeash–Gambier collisional, arc-related basins. Both the basinal strata and Alexander– Wrangellia basement rocks form the footwall to the east-dipping thrust system. In southeastern Alaska and northwestern British Columbia the thrust system hanging wall contains elements of Taku terrane, and a metamorphic assemblage that is correlated with Yukon-Tanana terrane rocks based on Nd-Sr isotope and detrital zircon characteristics (McClelland et al., 2000; Saleeby, 2000; Gehrels, 2001). The intervening welt of plutonic and metamorphic rocks, the Coast belt, contains mostly Cretaceous-Tertiary granitic rocks that evolved initially as a magmatic arc above an east-dipping subduction zone. Exposed granulite-grade metamorphic rocks in the Coast belt indicate significant crustal thickening and subsequent uplift of as much as 25 km. Mid-Cretaceous crustal shortening along the eastern margin of the Insular Superterrane was approximately coeval with accumulation of the Blairmore clastic wedge in the Alberta foreland Basin (Figure 4) (Stockmal et al., 1992; Miall et al., Chapter 9, this volume). In California at about this time, the Great Valley forearc basin was coupled to the Franciscan subduction complex above an east-dipping subduction zone (Ingersoll, Chapter 11, this volume). Separating the west-verging thrust belt and Late Cretaceous to Eocene plutons along the western Coast belt, is the Coast shear zone that extends about 700 km into southern Alaska and British Columbia. The steeply dipping


Brian D. Ricketts

shear zone developed after accretion of Insular Superterrane to North America and may have accommodated some of the transcurrent terrane displacements relative to the stable craton (McClelland et al., 2000). Debate continues about the timing of accretion events and about the crustal position of the basins involved prior to their demise following superterrane accretion. For example, the inboard Gravina–Nutzotin basins have been regarded as: (i) marginal to a Late Jurassic ocean basin (Monger et al., 1982); (ii) a Late Jurassic rift basin (e.g., Van der Heyden, 1992); (iii) a Late Jurassic dextral transtensional basin that was transported northwards (e.g., McClelland et al., 1992); (iv) a back-arc basin associated with oblique sinistral convergence (Monger et al., 1994); and (v) part of the Alexander–Wrangellia system that originated near Mexico (e.g., Irving et al., 1985). Kapp and Gehrels (1998) evaluated each hypothesis in light of zircon U-Pb isotope data and concluded that the Gravina–Nutzotin basin likely formed in a rift or transtensional setting between the composite Wrangellia– Alexander terrane and the Late Jurassic margin of North America that extended from Central America to Alaska. Trop et al. (2002) observed that basin subsidence and filling of the outboard Wrangell Mountains and Queen Charlotte basins is the reciprocal of the inboard Gravina–Nutzotin–Dezadeash basins, where the period of deposition in the latter (Late Jurassic–Early Cretaceous) corresponds to a significant hiatus in the Wrangell Mountains-Queen Charlotte basins. Likewise, Early to Late Cretaceous deposition in the basins outboard of Insular superterrane corresponds to non-deposition in the inboard Gravina–Nutzotin–Dezadeash basins. If correct, the interpretation implies that basin development on both the eastern and western margins of the Insular superterrane were linked by the dynamics of superterrane accretion. Beginning in the Early Cretaceous, regional shortening across Bowser Basin in north-central British Columbia produced the Skeena fold belt, that was broadly coeval and possibly dynamically linked to similarstyled deformation in the Western Canada foreland belt; deformation may also be linked to Coast belt development (Evenchick, 1991, 2001). Northwestward tectonic shortening in the fold belt is as much as 160 km (Evenchick, 1991). The resulting (piggyback) thrust-top basin, Sustut basin, evolved in concert with, and was subsequently deformed by the Skeena deformation. Mid-way through the Early Cretaceous, regional uplift and erosion of the Intermontane belt and OminecaForeland belts, coincided with major sub-Hauterivian/Barremian unconformities in Tyaughton–Methow and Bowser basins, and beneath the Blairmore clastic wedge in Alberta Basin (Figures 3 and 4) (Eisbacher, 1981; Yorath, 1991; McClelland et al., 1992). This event is also signaled by a significant decrease in intrusion and volcanism farther west (Armstrong, 1988) and by a decrease in plate velocities, in particular the westward motion of the North America craton relative to Farallon plate and Kula plate (Engebretson et al., 1985). Sediment derived from uplift and plutonism in Omineca belt was for the first time transported westward into Sustut and Skeena basins (first appearance of white micas), and Tyaughton–Methow trough (Figure 4). MidCretaceous plutonism in the southern Coast belt also shed sediment into Tyaughton–Methow trough. Arc-related sedimentation continued in Gambier Basin. Continued crustal shortening across the orogen during the Campanian through Paleocene revived uplift and erosion of the Foreland belt providing sediment for the Belly River and Paskapoo clastic wedges (Figure 3). Significant uplift in the Coast belt and continued tectonic shortening across the inverted Bowser Basin also resulted in the reversal of sediment transport to Sustut Basin, and westerly oriented sediment dispersal into the newly subsiding Georgia Basin that overlaps Wrangellia and the southern Coast belt. Tectonic shortening also continued across Tyaughton–Methow trough. This phase apparently was driven by accretion of three relatively small terranes: Chugach terrane in northern British Columbia and Alaska (Jones et al., 1986), and Pacific Rim and Crescent terranes on southwest Vancouver Island. Both Pacific Rim and Crescent terranes are presently wedged beneath Wrangellia, above the subducting Juan de Fuca plate (Figure 2) (Hyndman et al., 1990; Cook et al., 1991). Thus, in Late Cretaceous–Paleogene times, strain was distributed across the entire width of the Cordilleran orogen. During the Early Tertiary deposition continued along the west margin of Insular belt in Queen Charlotte and Georgia basins. Tofino Basin that encompasses most of the continental shelf west of Vancouver Island, contains Eocene to Recent marine clastic sediments that overlie, from east to west, Pacific Rim terrane, Crescent terrane, and the modern accretionary prism. Inland, non-marine sedimentation infilled fault-bounded basins and troughs that commonly were associated with regional, north-northwest-striking strike-slip faults (e.g., Northern and Southern Rocky Mountain trenches, Tintina fault and Denali fault). These fault systems also dismembered older basins, such as Dezadeash Basin which is truncated and offset 300–400 km by the Denali fault system (Eisbacher, 1976; Lowey, 1998).

3.3. Magmatism, deformation, and relative plate motions Ground-breaking studies of Coast belt magmatism by Armstrong (1988) revealed periods of intense intrusive and volcanic activity punctuated by intervals of relative magmatic quiescence. Furthermore, it has been demonstrated


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that these magmatic ‘‘cycles’’ exhibit a degree of correlation with periods of high or increased velocities of the Farallon, Kula, and Pacific plates relative to the North American craton (Engebretson et al., 1985; Woodsworth et al., 1991; Ward, 1995). In the Canadian Cordillera, inception and/or changes in the rate of basin subsidence, accompanied by high sediment flux, also appear to be broadly sympathetic with the acme of magmatic activity, and with major periods of supracrustal deformation in the Western Canada foreland fold and thrust belt, Skeena fold belt, and the Coast Mountains thrust belt (Figures 3 and 4).

3.4. The modern plate boundary The present relative motion of North America is toward the Pacific Ocean, relative to mantle hotspots (Riddihough and Hyndman, 1991); in the last 180 Ma the craton has moved through about 701 of longitude (Engebretson et al., 1985). Between northern California and Alaska the modern North American plate margin is bounded by two major transform faults, two subduction zones and two oceanic plates (Figure 6). From the south, the dextral San Andreas Fault separates the North American from the Pacific plate. The Mendocino triple junction marks the transition to the Juan de Fuca plate that extends north to about the southern tip of Queen Charlotte Islands; the Juan de Fuca Plate is moving eastward and is being subducted beneath North America at up to 46 mm/year relative to North America (Cascadia subduction zone). Deep reflection seismic shows that the top of the downgoing Juan de Fuca plate is about 30 km beneath western Vancouver Island (Yorath et al., 1985; Hyndman et al., 1990; Cook et al., 1991). The Juan de Fuca plate is separated from the Pacific plate by enechelon, northeast-trending spreading ridges (Juan de Fuca and Explorer ridges) and transform faults. A triple junction offshore Vancouver Island is hypothesized to be evolving at the junction of Juan de Fuca Ridge and Nootka fault (Rohr and Furlong, 1995). In this model Explorer microplate, born about 5 Ma, accounts for strain partitioning between Juan de Fuca plate and the Queen Charlotte transform. North of Vancouver Island, the Pacific plate, and the much smaller Yukatat terrane have a strong northward component of motion, relative to North America, that is taken up by the Queen Charlotte-Fairweather dextral transform fault. North of the British Columbia–Alaska border, this motion is almost orthogonal such that Pacific crust is being subducted beneath Alaska at the Aleutian Trench.

3.5. The ‘‘Baja BC’’ debate Whereas it is generally accepted that the Cordilleran terranes were laterally mobile, there is still considerable debate over how far they traveled before their accretion to North America. There are essentially two schools of thought: one that posits travel from afar, up to 5,000 km and usually from warmer southern climes; and the other that claims the terranes have traveled less than 1,000 km. The debate hinges on the interpretation of paleontological and paleomagnetic data and terrane tracks that help determine paleolatitudes, and piercing points that aim to correlate stratigraphy, sediment provenance, or magmatism across faults. The Baja BC controversy in particular has been reviewed by Cowan et al. (1997). Inferred paleolatitudes of some Late Paleozoic and Mesozoic faunal provinces in Cordilleran terranes are anomalous when compared with provinces in autochthonous strata. For example, Permian fusulinids (Monger and Ross, 1971) and low latitude Triassic corals and molluscs (Tozer, 1982) are found much farther north in strata of accreted terranes (e.g., Slide Mountain terrane) than in strata on the craton, and indicate significant northwards displacement. In contrast, latest Jurassic and Cretaceous molluscan and radiolarian faunas from the Insular superterrane indicate middle to high paleolatitudes and therefore minimal northward displacement (Haggart and Carter, 1994). Paleomagnetism data collected over the last half century (see the review by Irving and Wynne, 1991) indicate significant latitudinal displacements for (summarized by Keppie and Dostal, 2001): Baja Alaska, comprising Wrangellia, Peninsular, and Chugach terranes (B5,000 km); and Baja BC (including Insular superterrane B3,000 km, Stikinia B2,000 km, and Quesnellia B1,000 km). Insular and Intermontane superterranes appear to have amalgamated by Mid-Cretaceous time and according to the paleomagnetism data this amalgam was located about 2,000 km south of its present position (Irving and Wynne, 1991). However, even here there are significant discrepancies, as is the case for the southern block of the Methow terrane (displaced northwards 1,800 +/�500 km) compared with the northern block of the Methow terrane that was displaced B3,000 +/�450 km; syntectonic remagnetization of the southern block has been posited as an explanation for the contradictory data sets (Enkin et al., 2002). Structural tilting of crustal blocks has also been shown to generate discrepancies in paleomagnetically determined terrane paleolatitudes. For example, Neogene tilting of parts of southern Alaska and British Columbia mean that coastwise displacements of less than 1,000 km are required to explain the discrepancies in paleolatitude (Butzer et al., 2004). Very little lateral displacement of Baja BC has taken place since the Mid-Tertiary (Irving and Wynne, 1991).


Brian D. Ricketts

Figure 6 The modern plate boundary of western Canada and northwestern USA. Adapted from Monger (1989) and Riddihough and Hyndman (1991).


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Sediment provenance methods that are used to unravel the source rock links between terranes and western North America include whole-rock petrography, zircon U-Pb ages, REE isotopes and geochemistry (e.g., Garver, 1992; Garver and Brandon, 1994; Gehrels et al., 1995; Kapp and Gehrels, 1998). Unfortunately, arguments relating to terrane displacement based on these data frequently conflict not only with paleomagnetism data, but between the provenance data sets themselves. For example zircon ages from Mesozoic strata in Methow, Gravina, Queen Charlotte, and Georgia basins generally indicate derivation from source rocks that require little separation from their present positions (Kapp and Gehrels, 1998; Mahoney et al., 1999; DeGraaf-Surpless et al., 2003). This interpretation depends in part on a comparison of Precambrian zircons with particular segments of North American crystalline basement (e.g., Mahoney et al., 1999). However, these interpretations are challenged by Housen and Beck (1999) and Keppie and Dostal (2001), who suggest that components of the Precambrian zircon population could also be derived from northern Mexico and therefore support the paleomagnetic data. Keppie and Dostal (2001) also propose a test that involves correlation of plume magmatism as piercing points, for example where latest Cretaceous magmatism in northern Stikinia is correlated with the Yellowstone hotspot. Clearly, adjudication of this debate in a way that is acceptable to the broader geological community is not yet possible, but its resolution is important because there are significant implications for paleogeographic reconstructions.

4. Sedimentary Basins Associated with Intermontane Superterrane Sedimentary basins directly related to Cordilleran terrane accretion are treated in three sections that follow approximately the accretionary events from east to west: (1) Intermontane Superterrane accretion, (2) Insular Superterrane accretion, and (3) basins originating from Cenozoic plate-tectonic processes leading to the modern plate boundary.

4.1. Whitehorse trough 4.1.1. Tectonostratigraphic foundations The sedimentary fill of Whitehorse trough provides evidence of the oldest (Early Jurassic and possibly latest Triassic) linkages between Stikinia, Cache Creek terrane, Quesnellia, and North America (Monger, 1989). The basin is a structurally shortened and largely fault-bound remnant of an Early Jurassic, northeast-facing forearc basin that overlies both Stikinia and Cache Creek terranes (Eisbacher, 1985; Mihalynuk et al., 2004). It is about 500 km long and is located between the King Salmon fault in the south and west, and Nahlin fault (Figure 7). Outliers that unconformably overlie Stikinian arc rocks also are present south of King Salmon fault. Whitehorse trough formed above a southwest-dipping subduction zone in which Cache Creek oceanic crust was thrusted beneath Stikinia. Blueschist metamorphism between 191 Ma and about 177 Ma (Mihalynuk et al., 2004) during subduction was coincident with the earliest period of sedimentation in the forearc basin. Subsidence was sufficient to accommodate more than 4,000 m of Sinemurian to Bajocian clastic, mostly deep-water sediments (Laberge Group) (Eisbacher, 1974c, 1985; Tempelman-Kluit, 1979). The Laberge Group overlies at least 4,000 m of Stikinian arc-related rocks (Lewes River Group). Uplift in Middle Jurassic time resulted in the demise of the forearc, and concomitant basin shallowing with deposition of paralic, deltaic, and ultimately alluvial sediments in the upper part of the Laberge Group (Tanglefoot Formation) and the disconformably overlying Tantalus Formation. Progradation of these Upper Jurassic–Lower Cretaceous successor-basin assemblages is correlated with the final stages of Stikinia–Cache Creek terrane accretion (Dickie and Hein, 1995). Forearc basin subsidence in the Early Jurassic is delineated by the abrupt transition from lowest Triassic to Hettangian shallow-marine facies, to the relatively deep-water Laberge Group (Figure 7). The shallow water facies include bouldery coastal fan deltas that interfinger with sandy delta platform and shelf facies and local bioherms. The Sinemurian through Bajocian portion of the Laberge Group consists primarily of facies developed in slope, slope-apron, and submarine-fan environments (Dickie and Hein, 1995; Hart et al., 1995; Johannson et al., 1997). 4.1.2. Provenance linkages Significant changes in clast composition through the Laberge succession reflect fundamental changes in tectonic setting with respect to terrane accretion, and evolving source rocks (Figure 8). Petrofacies analysis of Sinemurian to Pliensbachian strata indicate that most sediment was derived from unroofing of older Stikinian arc rocks, and Early Pliensbachian volcanic rocks (Johannson et al., 1997). Uplift and incision of Pliensbachian granitic plutons (U-Pb B186 Ma) was rapid enough to supply large volumes of sediment during Pliensbachian deposition. U-Pb


Brian D. Ricketts

Figure 7 Map of Whitehorse trough separated from Cache Creek terrane (CC) by Nahlin fault, from Stikinia (ST) by the King Salmon fault, and in the north from Quesnellia (Q) by the Teslin fault. Note the stratigraphic contacts at the northern, and western limits of the trough. Modi¢ed from Johannson et al. (1997) and Hart et al. (1995).

Figure 8 Schematic summary of provenance trends for the Whitehorse trough. Source terranes are shown in the left column, and unroo¢ng pathways on the right column (Laberge Group). The fundamental shift in clast composition in Middle Jurassic strata re£ects the uplift and exhumation of Cache Creek terrane rocks following their earlier Jurassic blueschist-grade metamorphism. Adapted from Johannson et al. (1997) and Mihalynuk et al. (2004).


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zircon dating of granite boulders confirms the Stikinian arc as the likely source (Hart et al., 1995). Significantly, there is no evidence for Cache Creek-derived sediment in Whitehorse trough during the Early Jurassic. Early Jurassic unroofing of Stikinian rocks was coincident with blueschist metamorphism of the subducted Cache Creek terrane. Metamorphism of Pliensbachian to Toarcian Cache Creek chert has been dated at about 173 Ma (Mihalynuk et al., 2004). Rapid exhumation and unroofing of the metamorphic complex supplied chert and granulite clasts to Whitehorse trough before about 171 Ma (Early to Mid-Bajocian), a date that is constrained by the ages of cross-cutting, post-kinematic plutons (Figure 7) (Mihalynuk et al., 2004). These events were likely related to the closure of the Cache Creek Ocean between Stikinia and Quesnellia.

4.2. Bowser Basin 4.2.1. Tectonostratigraphic foundations Bowser Basin, the largest contiguous basin in the Canadian Cordillera, is located between Stikine and Skeena arches and extends about 400 km parallel to the orogen (Figure 9). The signal importance of Bowser Basin lies in its provenance links to obducted, uplifted and eroded Cache Creek terrane during the final stages of collision between Quesnellia and Stikinia. The bedrock foundations of the basin consist of the Triassic–Mid-Jurassic Hazelton volcanic assemblage, in particular the Early-Middle Jurassic Hazelton Group. Hazelton Group arc volcanism extended over much of Stikinia and began to decline in the Toarcian with subsequent thermal subsidence of Stikinian basement. The combined effects of thermal subsidence and crustal-scale loading by obducted Cache Creek resulted in widespread flexural subsidence (Monger, 1977; Eisbacher, 1985; Evenchick and Thorkelson, 2005). Initial subsidence is recorded in the Upper Spatsizi Formation (uppermost Hazelton Group; Figure 10) by extensive deep-water mudrocks (Quock Mbr.) and more locally condensed, organic-rich

Figure 9 Map of Bowser and Sustut basins, and the present relationships with Cache Creek terrane,Whitehorse trough, and Stikinian basement. Generalized structural trends are based on mapped fold axes and thrust traces (modi¢ed from Evenchick, 1991, 2001; Mihalynuk et al., 2004).Whitehorse trough is separated from Stikinia by the King Salmon fault (dashed line). ST, Stikinia; CC, Cache Creek terrane; Q, Quesnellia; NA, autochthonous North America; Su, Sustut basin; HB, Hotailuh batholith.


Brian D. Ricketts

Figure 10 General stratigraphy and schematic of Bowser Lake Group depositional model along an approximately southwestoriented pro¢le (modi¢ed from Eisbacher, 1981; Yorath, 1991; Ricketts and Evenchick, 1991, 1999; assemblages from Evenchick and Thorkelson, 2005). The Abou and Quock members at the base of the succession belong to the upper part of the Spatsizi Formation that underlies the Bowser Lake Group. CC, Overthrust Cache Creek terrane.

shale (Abou Mbr.) that overlie a sub-Aalenian erosional unconformity (Monger et al., 1991). This (Aalenian) starved-basin stage, represented by Quock-Abou sedimentation, was interpreted by Ricketts et al. (1992) to reflect initial flexural subsidence of Stikinia. However, Gabrielse (1991) has noted that collision may have begun as early as Toarcian based on stratigraphic relationships on Stikine arch. The Toarcian–Aalenian interval also corresponded with the period of significant convergence and tectonic shortening between Quesnellia and North American in southern British Columbia (Murphy et al., 1995). Regionally, Bowser Basin was a west-facing foredeep that continued to subside until the Early Cretaceous. The basin accommodated a broadly regressive, southwest-prograding marine and non-marine succession, up to 3,500–4,000 m thick, that can be divided into three main components (Figure 10): (1) the starved-basin stage (Upper Spatsizi Formation) dominated by mudrock; (2) the Bowser Lake Group that makes up most of the basin fill, and: (3) the Devils Claw Formation (conglomerate) that heralds deformation associated with shortening across the Skeena fold belt. The northern and eastern margins of the basin are overlain unconformably by the Cretaceous-Maastrichtian Sustut Group (Sustut Basin), a foredeep that piggybacked on the deformed Bowser Basin during Skeena fold belt contraction. The stratigraphic nomenclature of the basin has evolved such that formations are defined in some areas (Tipper and Richards, 1976; Bustin and Moffat, 1983), and facies or facies assemblages in other areas (Eisbacher, 1974a; Evenchick and Thorkelson, 2005). The problem with the more formal stratigraphic schemes is that formations and members are difficult to map beyond their type sections because of the structural complexity. In this synopsis, the more recent nomenclature of Evenchick and Thorkelson (2005) is used. Across the basin, the overall trend toward southwest-directed regression and regional offlap is depicted by Eisbacher (1974a, 1981) as a delta system with associated fluvial, slope, and basin-floor facies (Figure 10). However, the northern basin margin for much of the Bathonian to Oxfordian-Kimmeridgian interval was also characterized by coarse-grained facies (Eaglenest and Muskaboo Creek assemblages) that include Gilbert and


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braid deltas that prograded onto narrow, tectonically active sand-dominated shelves (Ricketts and Evenchick, 1999, 2007). Basinward, slope facies (Todagin assemblage) and basin-floor fans (Ritchie-Alger assemblage) were depositionally linked to the shelf — fan-delta systems by shelf-break gullies and gravel-filled submarine channels (Figure 11). Gravel mostly bypassed the shelves during sea-level lowstands. Submarine channels incised into the slope deposits locally occur as stacked and overlapping complexes, that are filled with gravel deposited as cohesive and cohesionless debris flows (Ricketts and Evenchick, 1991). Continued regression during the latest Jurassic to Early Cretaceous resulted in extensive delta plains that contained swamp, lacustrine, floodplain, and distributary and high-sinuosity channel facies (Groundhog– Gunanoot and related assemblages) (Eisbacher, 1981; Bustin and Moffat, 1983; MacLeod and Hills, 1990). The transition from the coarser-grained, predominantly marine assemblages farther north to the delta-plain assemblages may correspond to a decrease in the rate of foreland-driven subsidence and either: (i) a decrease in surface topography associated with the over-thrust Cache Creek, or (ii) the northwards retreat of exposed Cache Creek source rocks toward their present position in the hanging wall of King Salmon fault (Figure 9). The non-marine Devils Claw Formation, up to 1,000 m thick and containing 30–80% conglomerate (Evenchick and Thorkelson, 2005), is the youngest unit in the Bowser Lake Group. It abruptly but conformably overlies Groundhog–Gunanoot strata. Devils Claw strata herald a return to high-energy alluvial fan — bedload stream conditions (Eisbacher, 1981) that are equated with initial Skeena fold belt deformation in the late Early Cretaceous, and the cannibalizing of older Bowser Basin strata. Devils Claw strata in turn are unconformably

Figure 11 Inferred depositional setting for Callovian to Oxfordian conglomerate-dominated assemblages on the northern Bow ser basin margin. During this interval shelves were probably narrow and tectonically active, commonly overlain by large Gilbert deltas. Sediment £ux was high because of proximity to the obducted Cache Creek block. Coarse-grained sediment frequently bypassed the shelf during both highstands and lowstands of sea level, eventually being deposited as slope apron and basin £oor submarine fans. Adapted from Ricketts and Evenchick (1991, 1994).


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overlain by units of Sustut Basin. South toward Skeena Arch, approximately correlative Hauterivian to Aptian strata in the Skeena Group, of paralic and neritic origin (Tipper and Richards, 1976), overlie a transgressive surface on the Hazelton and Bowser Lake groups. These in turn are overlain by upper Lower Cretaceous subaerial volcanics and marine to non-marine sediments. Early studies of coalification in the northwestern part of Bowser Basin indicated anthracite to meta-anthracite levels that Bustin and Moffat (1989) equated to about 5,000 m of burial and an average heat flow of 80 mW/m2. However, recent evaluation of coal rank and oil preservation potential in the northern two-thirds of the basin indicate that the regional coal ranks lie within the oil–gas generation–preservation window (Osadetz et al., 2003). These recent discoveries have sparked renewed interest in Bowser Basin as a possible petroleum province. 4.2.2. Provenance linkages By far the most abundant clast-type making up Bowser Basin rocks is radiolarian chert in sandstone and conglomerate; W90% in many conglomerate outcrops. Conglomerate chert clasts are spherical to oblate and very well rounded, indicating a substantial degree of mechanical abrasion during transport; recycling of Bowser Basin sediment was also likely (Ricketts and Evenchick, 2007). Limestone clasts (from the Cache Creek terrane) are less common (o5%); metamorphic detritus is rare. The only conceivable source for such large volumes of chert is the oceanic Cache Creek terrane (Souther and Armstrong, 1966; Eisbacher, 1981; Gabrielse, 1991; Evenchick and Thorkelson, 2005). Recognition of Alpine-type spinals in the heavy mineral sediment fraction supports this interpretation (Cookenboo et al., 1997). Eisbacher (1981) also noted that the underlying Hazelton rocks provided relatively little sediment to the basin. The chert petrofacies provides a critical link among the lithosphere-scale processes of Cache Creek–Stikinia collision, overthrusting, and subsidence of the Bowser Basin. Notably, there is no evidence that the Omineca belt supplied sediment to Bowser Basin; this situation changed during formation of the overlying Sustut Basin (Figure 4).

4.3. Sustut ‘‘piggyback’’ Basin 4.3.1. Tectonostratigraphic foundations Whereas the Devils Claw Formation (uppermost Bowser Basin) heralds initial deformation in the Skeena fold belt, the overlying Sustut Group records uplift, erosion and cannibalizing of the older Bowser Basin deposits, and direct involvement of Sustut strata in Skeena fold belt contraction during the Mid- to Late Cretaceous (Figures 9 and 10). Sustut Basin is interpreted to be the foreland basin to Skeena fold belt (Evenchick, 1991). The southwest part of the basin hosts the triangle zone (Evenchick and Thorkelson, 2005), such that the basin has ‘‘piggybacked’’ with the frontal thrust. Note that Skeena fold belt contraction was driven by Insular superterrane accretion outboard (west) of the Stikinia–Cache Creek — North American amalgam, in contrast to Bowser Basin which was associated with Stikinia–Cache Creek terrane accretion. Upwards of 2,000 m of strata are divided into the Tango Creek (Barremian or Lower Albian to Upper Campanian) and Brothers Peak formations (Upper Campanian to Lower Maastrichtian) (Evenchick, 1991). Each formation represents a different stage of basin subsidence attending Skeena deformation (Eisbacher, 1974b, 1981). Basal Tango Creek conglomerate constitutes a pediment that unconformably overlies deformed Bowser Basin strata and Stikinian Hazelton volcanics (Eisbacher, 1974c). Fluvial and lacustrine deposits low in the formation accumulated in southwest-flowing drainage basins, changing to northeast flowing in the topmost strata (Figure 10). The coarse-grained Brothers Peak Formation unconformably overlies the Tango Creek, and heralds further changes in basin configuration. Alluvial fan and fluvial deposits accumulated where flow mostly was directed southeast along the basin axis. The basin axis at this time appears to have been strongly controlled by the predominantly southeast-trending Skeena folds and thrust faults. Tango Creek and Brothers Peak deposition overlap in time with the Blairmore and Paskapoo-Belly River clastic wedges, respectively; Evenchick (1991, 2001) has suggested that the deformation in Skeena fold belt and the Western Canada foreland belt were dynamically linked. 4.3.2. Provenance linkages There is a fundamental provenance shift in deposits of Sustut Basin compared with Bowser Basin that corresponds to different basin dynamics and regional tectonics. Well-rounded chert clasts derived from uplifted Bowser Basin predominate in the Tango Creek and Brothers Peak formations. Eisbacher (1974b) also suggests that some Brothers Peak sediment was cannibalized from Tango Creek rocks during Skeena fold belt deformation.


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Unroofing of the Omineca belt rocks provided sediment types not seen before in the northern Intermontane Superterrane, although they are encountered in Albian and younger Jackass and Pasayten groups in Tyaughton–Methow basin (Garver, 1992). Quartz clasts, locally up to 50%, abundant muscovite and foliated granite clasts were derived from high-grade metamorphic complexes and plutons, respectively. Andesite clasts in the Brothers Peak Formation indicate that uplift and erosion at this time also involved Stikinian bedrock (Eisbacher, 1981).

4.4. Tyaughton–Methow basin 4.4.1. Tectonostratigraphic foundations Remnants of the Tyaughton–Methow basin composite (Tyaughton Basin, Methow Basin) in southwest British Columbia are sandwiched between Intermontane superterrane and the eastern margin of the Coast belt (Figures 2 and 12). The basin is significant because: it helps bracket the timing of Intermontane and Insular superterrane interaction; it defines the timing of accretion of Bridge River–Cadwallader–Methow terranes; and it provides one of the earliest source rock links to unroofed plutonic rocks in the eastern Coast belt (Kleinspehn, 1985; Woodsworth and Monger, 1991; Monger et al., 1994; Riesterer et al., 2001; Umhoefer et al., 2002; DeGraaf-Surpless et al., 2003). It is still uncertain whether the faulted segments of the basin were once part of a contiguous basin or were separate depositional entities; they are presently separated by about 100 km of dextral displacement along the Fraser– Straight Creek fault. In the North Cascades (USA), Late Jurassic volcanic arc rocks (Newby Group) are linked to the western margin of Intermontane superterrane, and underlie Methow Basin strata (Mahoney et al., 2002). The arc rocks are intruded by 152.8 Ma granodiorite, indicating that Methow terrane was linked to Intermontane superterrane at least by the Late Jurassic. A minimum age for linkage between the Insular superterrane and Bridge River– Cadwallader–Methow terranes is provided where these terranes are thrust over Coast belt rocks and where the thrust imbricates are intruded by B93 Ma plutons (Journeay and Friedman, 1993). Correlation of the Harrison Lake Formation and Cayoosh Assemblage in southeastern British Columbia further suggest a Middle Jurassic overlap assemblage linking Wrangellia (part of the Insular superterrane amalgam) and Methow terranes (Journeay and Mahoney, 1994). Tyaughton–Methow strata form an overlap assemblage on Mesozoic oceanic and arc-related rocks composing Bridge River, Cadwallader, and Methow terranes. The three terranes were amalgamated by the Callovian, based on correlation of the Relay Mountain Group (Callovian to Upper Hauterivian) and related stratigraphic units (Rusmore et al., 1988; Umhoefer et al., 2002) (Figure 12). Rusmore et al. (1988) suggested Middle Jurassic amalgamation between Bridge River and Cadwallader terranes and Intermontane superterrane based on the timing of deformation; timing that is similar to Cache Creek obduction over Stikinia. Petrofacies analysis

Figure 12 Map and generalized stratigraphic composition of segments of Tyaughton and Methow basins, associated with Bridge River, Cadwallader, and Methow terranes. Adapted from Kleinspehn (1985), Garver (1992), Umhoefer et al. (2002), DeGraaf-Surpless et al. (2003). M, Methow terrane; CAD, Cadwallader terrane; BR, Bridge River terrane; WR,Wrangellia; J-K, plutonic rocks; TCG,Taylor Creek Group; RMG, Relay Mountain Group; JMG, Jackass Mountain Group; Intermontane terranes include Stikinia, Quesnellia, and Cache Creek terrane.


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indicates that early Relay Mountain Group deposition took place in a relatively deep, forearc-like setting, west of the Tyaughton–Methow basin, from Callovian to Oxfordian time (Garver, 1992), possibly even part of the Klamath Mountains forearc (Umhoefer et al., 2002; Ingersoll, Chapter 11, this volume). The basin became shallower, and even non-marine toward the end of the Valanginian. West-derived sediment is first recorded in the Valanginian–Hauterivian (B135–130 Ma) indicating a significant change in paleogeography at this time with some sediment possibly sourced from the Gambier arc assemblage (Woodsworth and Monger, 1991; Umhoefer et al., 2002). Umhoefer et al. (2002) further suggested that Methow Basin at this time may have evolved to a back-arc basin located east of the Ottarasko volcanic arc. Depositional links (but not necessarily continuous depocentres) along the western margin of Intermontane Superterrane have been postulated by Eisbacher (1985) to include Tyaughton–Methow, Bowser, and Gravina–Nutzotin basins. Local unconformities separate the older overlap assemblage from the Jackass Mountain, Taylor Creek groups and related units (Hauterivian to Albian). Polymict conglomerate and volcanic-lithic wackes were deposited in a variety of submarine-fan environments (Kleinspehn, 1985) within a forearc basinal setting (Woodsworth and Monger, 1991). Uplift during Late Albian to Santonian time resulted in deposition of coarse clastic strata, including red beds (Pasayten Group and related units), that interfingered with volcanic flows and were deposited in marginal marine, alluvial, and fluvial environments across much of Tyaughton–Methow basin (Garver, 1992).

4.4.2. Provenance linkages The Relay Mountain Group sediment was derived primarily from the underlying volcanic terranes (volcanic lithic). Umhoefer et al. (2002) recognized an unroofing sequence with a progressive increase in plutonic quartzfeldspar into the Oxfordian. During the Hauterivian to Albian interval, coarse volcanic and plutonic detritus was shed westwards from Intermontane Superterrane (Kleinspehn, 1985). Of particular significance is the early provenance linkage between Insular and Intermontane superterranes during the Albian, in concert with uplift and unroofing of the southern Omineca belt (Garver, 1992). Three petrofacies identified in Albian to Cenomanian strata (Upper Jackass to Lower Pasayten groups) include (Garver, 1992): a west-derived volcanic petrofacies found only in Tyaughton Basin (Taylor Creek Group); a chert petrofacies that contains minor blueschist and serpentinite detritus derived from Bridge River–Cadwallader source rocks; and an arkose petrofacies with quartz, feldspar, and muscovite derived from the Omineca belt. The arkose facies predominates in Methow Basin (strata 3–8 km thick) with only a thin unit extending to Tyaughton Basin. Detrital zircons from Mid-Albian to Santonian rocks overlying the Methow terrane also correspond best with southern Cordilleran source rocks, indicating little translation of Methow terrane relative to the North American margin (DeGraafSurpless et al., 2003). Contradistinct evidence based on paleomagnetism for Methow terrane and other components of Insular Superterrane posits upwards of 3,000 km of northwards displacement relative to the North American craton (Irving and Wynne, 1991; Keppie and Dostal, 2001; Enkin et al., 2002).

5. Basins Located along the Inboard Margin of Insular Superterrane 5.1. Nutzotin–Dezadeash–Gravina–Gambier basins 5.1.1. Tectonostratigraphic foundations Unifying elements in the debate concerning the timing of accretion and horizontal translation of Insular superterrane relative to North America, are the sediment depocentres sandwiched between Insular and Intermontane superterranes. Overlap assemblages that compose the Nutzotin, Dezadeash, Gravina, and Gambier successions (Figure 13), their provenance, and structural associations provide evidence for basin formation and the early stages of terrane collision, and eventual basin destruction following the final stages of Insular superterrane accretion to, and tectonic overlap of the Cordilleran margin. There is no definitive evidence that there was a contiguous locus of deposition along the 2,000+ km length of the Cordilleran margin. However, there is reasonable consensus that the basins were linked in terms of Insular superterrane accretion, based on their terrane overlap geometries, timing and depositional character, and their relationships with an active, western magmatic arc (Figure 14) (Eisbacher, 1985; Monger et al., 1994; McClelland et al., 2000; Trop et al., 2002). Berg et al. (1972) incorporated the Nutzotin, Dezadeash, and Gravina successions into what is commonly referred to as the Gravina–Nutzotin belt. Connection of these inboard basins with the western margin of Bowser Basin has also been hypothesized (Eisbacher, 1985; Yorath, 1991). Links with Tyaughton–Methow basin are more tenuous although sediment derived from Gambier arc may have been transported into that basin (Garver, 1992).


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Figure 13 Generalized stratigraphic columns for Nutzotin, Dezadeash, Gravina, and Gambier basins, located on the inboard margin of Insular superterrane. Adapted from Eisbacher (1976), Rubin and Saleeby (1991), Souther (1991), Lynch (1992, 1995), Kapp and Gehrels (1998), Gehrels (2001), McClelland et al. (2000),Trop et al. (2002). Contrasting, and to some extent reciprocal events in the outboard basins are shown in Figure 16.

Figure 14 Two versions of a reconstructed Jurassic North American margin showing the relationship of the inboard Nutzotin- Gambier forearc basins to Insular and Intermontane superterranes: (A) The inboard basins together with Bowser and Tyaughton-Methow (T-M) basins are linked tectonically by a regional, oblique dextral strike--slip shear couple (from McClelland et al.,1992). Present latitudes are shown asYukon--BC and Canada--USA borders. Paleolatitudes (501N, 401N and 301N in red) also are shown on the right. AX-WR-P, Alexander--Wrangellia--Peninsula terranes (Insular superterrane). Solid black cones represent the magmatic arc. Kahiltna basin is located in north Alaska. Other abbreviations as inTable 2. (B) Eisbacher’s (1985) interpretation shows a similar disposi tion of inboard basins from Middle Jurassic to Early Cretaceous, with a more explicit link to Bowser and Tyaughton--Methow basins. Solid black arrows indicate directions of regional sediment transport. Paleolatitudes are shown by successive displacement of the Canada--USA border that depict a total of 1,500--2,000 km of displacement relative to North America over this period.


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Approximately time-equivalent terrane and island-arc suturing events that herald the Nevadan orogeny in northern California are illustrated in Figure 12 (Ingersoll, Chapter 11, this volume). Characteristics that the four basins have in common include a predominance of argillaceous- and sanddominated siliciclastic and volcaniclastic lithologies, a high proportion of sediment gravity-flow deposits, and a close association with active calcalkaline and tholeiitic arc volcanism bordering the western basin margins (Figure 13). Volcanic flows, agglomerates, tuffs, and welded tuffs, which were the products of strato-volcanoes and explosive volcanism (Lynch, 1992), commonly interfinger with the sedimentary strata. Structural imbrication and dismembering of the basin successions accompanied Mid-Cretaceous crustal shortening during the final stages of Insular-Intermontane superterrane accretion (Journeay and Friedman, 1993). For example, the eastern boundary of Gravina Basin is overthrust by Stikine and metamorphosed Yukon-Tanana and Taku terrane rocks (Crawford et al., 1987; Rubin and Saleeby, 1991; McClelland et al., 1992).

5.1.2. Nutzotin Basin The bulk of Nutzotin Basin resides in Alaska with a small portion sneaking into the northern Canadian Cordillera. Westwards, the basin onlaps Wrangellia, whereas its eastern boundary is truncated by the dextral-slip Denali fault. More than 3,000 m of Oxfordian to Barremian, west-derived shale, volcaniclastic turbidites, and gravelly debris flows compose a large-scale coarsening-upwards submarine-fan succession (Nutzotin Mountains sequence) (Berg et al., 1972; Trop et al., 2002). Minor conglomerate near the base of the succession contains Triassic limestone clasts derived from Wrangellian bedrock. Volcanic detritus was derived from the active Chisana arc bordering the west margin (Figure 15). Unroofing of older and structurally deeper segments of the Chitina arc

Figure 15 Inferred paleogeographic evolution of Nutzotin (inboard; NB) and Wrangell Mountains (outboard; WMB) basins (southeast Alaska), the evolving magmatic arcs, and regional deformation associated with Insular Superterrane accretion (modi ¢ed from Trop et al., 2002). (A) The active Chitina arc lies west of Wrangell Mountains basin during the Late Jurassic--Early Cretaceous. Inception of Nutzotin basin during the Oxfordian marks the close association between the incoming Insular and Intermontane superterrane margins. (B) Arc activity jumps northeastwards during the Hauterivian with the Chisana arc forming the west margin of Nutzotin basin. (C) Thrusting, beginning in the Barremian, continues into the Mid-Cretaceous, terminating deposition in Nutzotin basin. NA, North America.


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farther west also provided igneous clasts to Lower Cretaceous conglomerates. Eastward migration of Chisana arc (relative to North America) is recorded by the gradual transition from the volcaniclastic succession to more than 3,000 m of effusive volcanics, and concomitant migration of the Nutzotin foredeep axis.

5.1.3. Dezadeash Basin The Dezadeash Formation is located northeast of Denali fault, about 300 km south of, and along strike from the Nutzotin Basin. Unlike the other three inboard basins, the preserved Dezadeash succession contains only sedimentary strata; about 3,000 m of predominantly sandy and argillaceous turbidites and other sediment gravityflow deposits (Eisbacher, 1976). The base and top of the succession are structurally truncated. Sediment transport was mostly toward the east and northeast from a volcanogenic source, with deposition in relatively deep-water submarine-fan lobes and channels (Lowey, 1992). The Oxfordian to Valanginian Dezadeash succession has been correlated with Nutzotin Basin rocks, wherein the Dezadeash strata are possibly the distal equivalents of a volcanogenic submarine-fan depositional system (Eisbacher, 1976). Limestone mega-boulders, derived from Triassic Wrangellian rocks, have been traced to both Nutzotin and Dezadeash successions straddling Denali fault and indicate about 370 km of strike-parallel offset (Lowey, 1998). The Kluane Schist (Kluane metamorphic assemblage of quartz-mica schist, gneiss, and serpentinite) structurally overlies, and has been posited as the metamorphosed equivalent of, the Dezadeash succession (Eisbacher, 1976). However, rare-earth element and isotopic analyses of rocks from both assemblages indicate that the Kluane Schist–Dezadeash linkage is less clear than hitherto thought (Mezger et al., 2001). Structural imbrication of the two assemblages is linked to the Mid-Cretaceous crustal shortening seen elsewhere in the Coast belt orogen (McClelland et al., 1992).

5.1.4. Gravina Basin Oxfordian to Albian strata making up Gravina Basin (southeastern Alaska) contain a complex, interfingering assemblage of volcaniclastic wackes, argillites, conglomerates, tuffs, and volcanic flows and breccias (Berg et al., 1972; Rubin and Saleeby, 1991; Kapp and Gehrels, 1998; Gehrels, 2001). The preserved western segments of the basin unconformably overlie Wrangellia and Alexander terrane rocks, whereas eastern exposures unconformably overlie the Taku terrane (Figure 1a). Strata are separated from Stikinia and Yukon-Tanana terranes by MidCretaceous, west-verging thrusts. Metamorphism to greenschist and lower amphibolite facies pervades most of the Gravina Basin rocks (Rubin and Saleeby, 1991). The lower part of the succession contains mostly volcanic and pillowed flows, pyroclastic flows and tuffs that possess island-arc geochemical signatures (Rubin and Saleeby, 1991). Like Nutzotin and Dezadeash basins, basal strata also contain conglomerate with metavolcanic and limestone clasts derived from Alexander terrane, and deposited in submarine-fan channels and fan lobes. Siliciclastic turbidites are present higher in the succession. Submarine-fan development was linked to the active volcanic arc along the western basin margin. Middle Jurassic through Albian diorite and granitoid intrusions (Figure 13) apparently were shallow enough to have contributed detritus to the basin.

5.1.5. Gambier Basin Gambier arc and its associated basin are generally considered to be the southern extension of the Gravina– Nutzotin arc. Gambier Basin was located along the eastern margin of Wrangellia during the Early Cretaceous (Lynch, 1992), and subsequently was dismembered by west-vergent thrusts (Journeay and Friedman, 1993; Lynch, 1995). Geochemical trends normal to the axis of the arc suggest that Gambier magmatism developed above a west- to southwest-dipping subduction zone (Lynch, 1995). Metamorphic grade ranges from prehnite pumpellyite to amphibolite facies, and penetrative deformation is relatively intense. The Gambier Group (Roddick, 1965) is similar to the other inboard-basin successions, and contains a mix of Lower Cretaceous arc-related volcanic rocks, and volcaniclastic and siliciclastic sediments (Figure 13). The succession is in thrust and nonconformable contact with Jurassic granodiorites. Basal conglomerates (Peninsula Formation) contain boulders of granodiorite, andesite, and basalt; trough crossbedding and thin coal layers indicate local non-marine to estuarine-type environments, that pass higher in the sequence to marine sandstoneshale dominated lithologies (Lynch, 1992, 1995).


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5.1.6. Provenance linkages Sedimentary strata of the inboard basins are composed predominantly of epiclastic volcanic detritus derived from adjacent active magmatic arcs. Additional components include older sedimentary, metamorphic, and igneous rocks derived primarily from Alexander and Wrangellia terranes (Eisbacher, 1976; Rubin and Saleeby, 1991; Lynch, 1992; Monger et al., 1994; Gehrels, 2001). More precise estimates of source rock composition and age are provided by analysis of detrital zircons. For example, zircon age populations in the Gravina succession include those derived from the adjacent magmatic arc, Alexander terrane, and Paleozoic to Precambrian zircons that were likely sourced from inboard Yukon-Tanana, Stikine, or northern Californian terranes (Kapp and Gehrels, 1998). These results imply that the inboard terranes must have been close to Gravina Basin, and places some restrictions on the Late Jurassic to Early Cretaceous paleolatitude of the inboard basins (Figure 14).

6. Basins Located along the Outboard Margin of Insular Superterrane 6.1. Queen Charlotte–Wrangell Mountains basins 6.1.1. Tectonostratigraphic foundations As terranes migrate across the globe, basins associated with the leading and trailing plate edges evolve in concert with their plate-tectonic environments. Triassic and Jurassic strata in Wrangell Mountains (south Alaska) and Queen Charlotte Islands (coastal British Columbia) record the passage of the Insular Superterrane from lower latitudes northwards toward its present position. Accretion of the Superterrane to North America during the Late Jurassic–Early Cretaceous is recorded in the inboard basins (see above) and their eventual tectonic demise. These events are also recorded along the outboard margin in the Wrangell Mountains and Queen Charlotte basins. However, Trop et al. (2002) have noted that the timing of major depositional and uplift-erosional events in the outboard basins (at the basin scale) are out of phase with similar events in the inboard basins (Figures 13 and 16).

6.1.2. Wrangell Mountains Basin Uplift of Wrangellia and associated magmatic arcs in the latest Jurassic and Early Cretaceous resulted in coarse clastic sediment being shed eastward into Nutzotin–Gravina basins (Figures 2 and 15). In Wrangell Mountains Basin, this event coincides with a regional, subaerial unconformity that records the initial collision of Insular Superterrane to North America (Trop et al., 2002). Subsequent growth of Chisana arc (Hauterivian to Barremian) was accompanied by subsidence of Wrangell Mountains forearc basin and the deposition of shallowmarine clastics overlain by deeper water submarine fans. Uplift of the outboard margin during the Aptian coincided with crustal shortening in Nutzotin Basin. Renewed subsidence of the southwest-facing forearc basin and northeast migration of the magmatic arc coincided with deposition of a 3 km thick, largely upward-coarsening, submarine-fan-slope-apron succession during the Albian and Campanian.

6.1.3. Queen Charlotte Basin The Cretaceous portion of Queen Charlotte Basin (see varying definitions of the basin by Shouldice, 1971; Thompson et al., 1991; Lyatsky and Haggart, 1993) is broadly similar to Wrangell Mountains Basin (Figures 16 and 17). Lower Cretaceous strata unconformably overlie Wrangellia, where a basal shallow-marine conglomerate is the oldest indication of forearc-basin subsidence and transgression, following regional uplift that spanned most of the Late Jurassic (Lewis et al., 1991). Clast compositions reflect derivation primarily from local Wrangellian bedrock. Subsidence that continued until the Early Maastrichtian accommodated at least 3 km of shallow-marine and deeper water deposits. Coarse-grained, shoreface, and shallow-shelf facies in the Longarm Formation are overlain by finer grained, outer-shelf, and slope facies in lower units of the Queen Charlotte Group. Continued basin subsidence and increasing water depths resulted in coarse-grained sediment bypassing the shelf and slope, and progradation of slope-apron and submarine-fan turbidites and gravelly debris flows that accumulated in two distinct depocenters on Graham and Moresby islands (Honna Formation, Upper Queen Charlotte Group; Haggart, 1991; Lewis et al., 1991). Sediment transport was mostly westward. This time interval corresponds with major thrusting along the inboard margin of Insular Superterrane that resulted in the demise of the Nutzotin to Gambier belt of basins (Figure 13).


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Figure 16 Generalized stratigraphic columns for Wrangell Mountains and Queen Charlotte basins along the outboard margin of Insular superterrane. Adapted from Lewis et al. (1991) and Trop et al. (2002). Compare the overall depositional--tectonic events with those of the inboard basins in Figure 13.

6.1.4. Provenance linkages Clast compositions in both outboard basins indicate that the primary sources of sediment were: (1) the underlying Wrangellia–Alexander terranes consisting of sedimentary, metasedimentary and old magmatic arc rocks, (2) detritus derived from active arcs and local plutons, and (3) in Queen Charlotte Basin, metamorphic and peraluminous plutonic clasts that point to links with the Coast belt (Lewis et al., 1991).

7. Cenozoic Basins-Harbingers of the Modern Plate Boundary 7.1. Queen Charlotte-Georgia-Tofino basins Accretion of Insular Superterrane to North America was essentially over by the Mid- to Late Cretaceous. Basins that formed subsequent to these events contain Upper Cretaceous to Holocene strata that overlie Wrangellia and Alexander terranes and structurally dismembered elements of Queen Charlotte Basin (Figure 17). The basins considered here include the Eocene–Pliocene component of Queen Charlotte Basin (Figure 16), Georgia– Nanaimo basins (Figure 17), and Tofino Basin, all located along coastal British Columbia (Figure 2). These younger basins also onlap uplifted and dissected Coast belt plutonic and metamorphic rocks. Subsidence and deformation of the coastal basins were driven by plate-tectonic processes that evolved throughout the Tertiary. Convergent motion between Farallon plate and North America persisted for much of the Late Cretaceous (Engebretson et al., 1985). Convergence continued until about Mid-Eocene time, when relative motion between Kula plate (the northern part of the older Farallon plate) and North America became


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Figure 17 Location of Queen Charlotte, Georgia, and To¢no basins (adapted from Hyndman et al., 1990; Rohr and Dietricht, 1992; Katnick and Mustard, 2003). The Cretaceous portion of Queen Charlotte Basin is exposed on eastern Queen Charlotte Islands; the fault-bounded Neogene portion is mostly o¡shore. For Queen Charlotte Basin stratigraphy see Figure 16. Nanaimo Group generalized stratigraphy (Cretaceous part of Georgia Basin) is from Mustard (1994). The generalized Paleogene--Neogene subsurface stratigraphy (Point Roberts, also part of Georgia Basin) is fromYorath (1991).

increasingly strike-slip, eventually giving rise to the modern Queen Charlotte transform that separates the Pacific plate from North America (from Queen Charlotte Islands north). These Early to Mid-Tertiary events coincided approximately with accretion of the Pacific Rim and Crescent terranes to Wrangellia (Figure 18), and widespread volcanism on Queen Charlotte and Vancouver islands (Hyndman et al., 1990; Lewis et al., 1991). Subduction of Juan de Fuca Plate (modified Kula Plate) continued to be largely orthogonal throughout the Neogene, with igneous activity migrating eastward to the Coast belt and culminating in the modern Cascadia arc. Modern Georgia Strait and adjacent Puget Sound occupy the forearc position above the Cascadia (Juan de Fuca) subduction zone (Brandon, 2004).

7.2. Tertiary Queen Charlotte Basin The Tertiary Queen Charlotte Basin extends offshore to Hecate Strait and Queen Charlotte Sound, west to the Queen Charlotte transform plate boundary (Figures 2 and 17), and represents a significant departure in tectonic environment compared to its Cretaceous forearc precursor. Basin subsidence was strongly influenced by rift-like stretching within an overall transform regime, where relative motion evolved progressively from transtensional to


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Figure 18 Pro¢le of the Juan de Fuca subduction zone across southernVancouver Island and Strait of Georgia, based on interpreted re£ection seismic from Hyndman et al. (1990), Cook et al. (1991), and England and Bustin (1998), showing the crustal position of Georgia and To¢no basins. CR, Crescent terrane; PR, Paci¢c Rim terrane.

pure strike-slip and ultimately transpressive (Higgs, 1991; Dehler et al., 1997; Irving et al., 2000). Thinning of the crust beneath Queen Charlotte Basin, which has accommodated up to 6 km of subsidence, has also been modeled as a lithosphere-scale, east-dipping shear zone, that was dynamically coupled to uplift of the adjacent Coast Mountains that have risen more than 3.5 km in the last 14 Myr (Rohr and Currie, 1997). Deformed Upper Cretaceous Queen Charlotte Basin strata are overlain unconformably by Paleocene to Miocene volcanic and volcaniclastic rocks, locally intercalated with non-marine sedimentary deposits. Higgs (1991) has compared these units to the syn-rift stage of the simple McKenzie (1978) stretching model. The thickest deposits are Miocene to Pliocene marine and non-marine clastics that accumulated in a complex array of extensional, or transtensional fault-bound sub-basins (Hole et al., 1993; Lyatsky and Haggart, 1993) (Figure 17). The stratigraphic architecture of the sub-basins is controlled by profound stratigraphic thickness variations that range from 200 m up to 6 km, as revealed in offshore reflection-seismic profiles (Hole et al., 1993).

7.3. Georgia (Nanaimo) Basin Georgia Basin extends across southeastern Vancouver Island, Strait of Georgia, and parts of the eastern and southern mainland (Figures 2 and 17). As such, the basin overlaps Wrangellia, the Coast belt farther east, and Cascade Mountains to the south; it postdates the Mid- to Late Cretaceous Coast Mountains thrust belt that pinned the Insular Superterrane to North America. The term Georgia Basin is used here to include Nanaimo Basin (see disputations in England and Bustin, 1998; Katnick and Mustard, 2003). Dispute also continues about the crustal position and origin of Georgia Basin. Early interpretations favor a peripheral foreland basin origin where crustal flexure was driven by west-verging, Mid- to Late Cretaceous thrusting (e.g., Brandon et al., 1988; Mustard, 1994). England and Bustin (1998) on the other hand, argued that thrusting was largely over by the time Georgia Basin sedimentation had begun, and that the main locus of sedimentation did not lie immediately outboard of the thrust belt, as is usually the case in foreland-type basins. Instead, they favored a forearc setting, where the basin was located between the subduction-related trench to the west, and the Coast belt magmatic arc. The forearc interpretation may be consistent with comparable depocenters in northwest Washington (Seattle and Everett basins) that also form part of the Cascadia subduction system (Lowe et al., 2003). Basin subsidence along a northwest-trending axis began in the Turonian, and by the Maastrichtian, was sufficient to accommodate more than 4 km of east-derived sediment in the Nanaimo Group (Figure 17) (Katnick and Mustard, 2003). The lower few 100 m of the Nanaimo Group contain alluvial and (economically important) coal-bearing strata that interfinger with shallow-marine deposits; complex facies changes were controlled primarily by paleotopography. The remainder of the succession contains submarine-fan deposits organized into laterally and vertically overlapping fan-lobe complexes, wherein inner-, mid-, and outer-fan facies are recognized (Mustard, 1994; Katnick and Mustard, 2003). Regional uplift at the end of the Cretaceous terminated deposition of the Nanaimo Group. Deformation of Nanaimo Group rocks and the underlying Wrangellian bedrock by southwest-verging imbricate thrusts resulted in 20–30% crustal shortening (Cowichan fold and thrust belt; England and Calon, 1991). Renewed subsidence in the Paleogene coincided with an eastward and southward shift in the Georgia Basin depocenter, and accumulation of 2–3 km of mainly non-marine clastics (Figure 17). Deposition continued into the Neogene with a major sedimentation focal point in the modern Fraser River delta.

7.4. Tofino Basin Tofino Basin is also linked to Cascadian subduction, but unlike modern Georgia Basin, it is located above the actively growing accretionary prism, seaward of Vancouver Island (Figures 17 and 18). The Pacific Rim


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(Mesozoic) and Crescent (Eocene volcanic) terranes, that overlie the downgoing Juan de Fuca plate, were thrust beneath, and accreted to Wrangellia by Mid-Eocene (B42 Ma; Hyndman et al., 1990). These terranes formed a backstop, against which oceanic crust and sediment scraped from the subducting slab have been stacked into an accretionary prism (Figure 18). Tofino Basin overlies the accretionary prism and the up-dip edges of the Pacific Rim and Crescent terranes, and covers most of the continental shelf west of Vancouver Island. Strata up to 4 km thick range in age from Middle Eocene to Holocene. Deformation and structural repetition of Tofino Basin strata are likely related to thrusting in the underlying accretionary prism, and possibly to breached compartments of overpressured fluid that, in a few exploration wells, reach lithostatic pressures (Shouldice, 1971). Unconformities and complex sedimentary facies changes are a consequence of the deformation. A basin, that is broadly similar to Tofino Basin in age and style of deformation, is located above Yakutat terrane in the Gulf of Alaska (Figures 2 and 6). Yakutat terrane lies outboard of the northern extension of Fairweather transform, and is presently being accreted to, and partly subducted beneath southern Alaska (Bruhn et al., 2004). Neogene to Holocene sediments are accumulating above the active Yakutat accretionary prism.

7.5. Provenance linkages Nanaimo Basin has become an important target of sediment provenance investigation because of arguments concerning the hypothesis that Insular Superterrane was displaced about 3,000 km during the period 90–50 Ma (Irving and Wynne, 1991; Enkin et al., 2001; Keppie and Dostal, 2001). Deposition of the Nanaimo Group spans the first half of this time interval and overlies Wrangellia and Coast belt rocks. Therefore, its sediment composition should reflect any changes in source resulting from horizontal displacement relative to the North American margin. Mahoney et al. (1999) argued that significant populations of polycyclic Archean and Proterozoic zircons in the Nanaimo Group can only have been derived from the Canadian Shield. However, Keppie and Dostal (2001) have contested this, noting that similar zircons are found in strata of varying ages at several locations in western North America and Mexico. The issue remains contentious.

8. Discussion Sedimentary basins in the Canadian Cordillera, west of the Foreland Belt, record complex relationships between terrane accretion to ancestral North America, and the crustal–lithosphere responses (subsidence, uplift, denudation) associated with collision, subduction, rifting, and wrench tectonics. That most Cordilleran terranes traveled from afar prior to accretion is generally accepted. Some, like Stikinia, essentially traveled alone; others, like the Insular superterrane, contained lithospheric blocks that were amalgamated before docking with North America. However, vigorous debate continues concerning the distances and paleolatitudes traversed by each terrane, and to some extent the timing of terrane accretion to the North American plate margin. The attributes of each Cordilleran sedimentary basin, at scales ranging from crustal-scale basin structure, stratigraphic architecture, sedimentary facies, sediment provenance, fossils, and remnant paleomagnetic records, to rare-earth signatures in single zircon crystals, all contain the key ingredients for solving these vexing questions. All these attributes will continue to play critical roles in future investigations. The Canadian Cordillera is a vast, rugged landscape; many of its geological complexities are still to be unraveled. An optimist’s position is that there remains a great deal of exciting geoscience to be discovered.

ACKNOWLEDGMENTS Thanks to Andrew Miall for the invitation to contribute this chapter, and for persisting with the North American Phanerozoic basins project. Ray Ingersoll did an excellent job of reviewing the chapter. I also want to acknowledge the staff in the Vancouver office of the Geological Survey of Canada, for discussions over the years, and the opportunity to work in one or two of the Cordilleran terranes. Carol Evenchick in particular provided a significant amount of information on Bowser Basin and Stikinia.

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