The Architecture of Oceanic Plateaus Revealed by the Volcanic Stratigraphy of the Accreted Wrangellia Oceanic Plateau

The architecture of oceanic plateaus revealed by the volcanic stratigraphy of the accreted Wrangellia oceanic plateau

Andrew R. Greene* James S. Scoates Dominique Weis Paciﬁ c Centre for Isotopic and Geochemical Research, Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T1Z4, Canada Erik C. Katvala Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N1N4, Canada Steve Israel Yukon Geological Survey, 2099-2nd Ave, Whitehorse, Yukon Y1A2C6, Canada Graham T. Nixon British Columbia Geological Survey, P.O. Box 9320, Station Provincial Government, Victoria, British Columbia V8W9N3, Canada

ABSTRACT gellia fl ood basalts were emplaced during a from low-viscosity, high-temperature tholei- single phase of tholeiitic volcanism ca. 230– itic basalts, sill-dominated feeder systems, The accreted Wrangellia fl ood basalts and 225 Ma, and possibly within as few as 2 Myr, limited repose time between fl ows (absence associated sedimentary rocks that compose onto preexisting submerged arc crust. There of weathering, erosion, sedimentation), sub- the prevolcanic and postvolcanic stratigraphy are distinct differences in volcanic stratig- marine versus subaerial emplacement, and provide an unparalleled view of the architec- raphy and basement composition between relative water depth (e.g., pillow basalt– ture, eruptive environment, and accumula- Northern and Southern Wrangellia. On volcaniclastic transition). tion and subsidence history of an oceanic Vancouver Island, ~6 km of high-Ti basalts, plateau. This Triassic large igneous province with minor amounts of picrites, record an INTRODUCTION extends for ~2300 km in the Pacifi c North- emergent sequence of pillow basalt, pil- west of North America, from central Alaska low breccia and hyaloclastite, and subaerial Approximately 3% of the ocean fl oor is and western Yukon (Nikolai Formation) to fl ows that overlie Devonian–Mississippian covered with oceanic plateaus or fl ood basalts, Vancouver Island (Karmutsen Formation), (ca. 380–355 Ma) island arc rocks and mostly in the western Pacifi c and Indian Oceans and contains exposures of submarine and sub- Mississippian–Permian marine sedimen- (Fig. 1; Coffi n and Eldholm, 1994). These aerial volcanic rocks representing composite tary strata. In contrast, Alaska and Yukon regions can rise thousand of meters above the stratigraphic thicknesses of 3.5–6 km. Here contain 1–3.5-km-thick sequences of mostly ocean fl oor and rarely exhibit magnetic lin- we provide a model for the construction of subaerial high-Ti basalt fl ows, with low-Ti eations like the surrounding seafl oor (Ben- the Wrangellia oceanic plateau using the fol- basalt and submarine pillow basalts in the Avraham et al., 1981). Oceanic plateaus are lowing information and visualization tools: lowest parts of the stratigraphy, that over- produced from high volumetric output rates (1) stratigraphic summaries for different areas lie Pennsylvanian–Permian (312–280 Ma) of magma generated by high-degree melting of Wrangellia; (2) new 40Ar/39Ar geochronol- volcanic and sedimentary rocks. Subsidence events that are distinct from melting beneath ogy results; (3) compilation and assessment of of the entire plateau occurred during and mid-ocean ridges. Most of what we know about geochronology and biostratigraphy for Wran- after volcanism, based on late-stage interfl ow the architecture of oceanic plateaus is based on gellia; (4) compiled digital geologic maps; sedimentary lenses in the upper stratigraphic obducted portions of oceanic plateaus, drilling (5) an online photographic archive of fi eld levels and the presence of hundreds of meters of the volcanic sequences of extant oceanic pla- relationships; and (6) a Google Earth fi le to >1000 m of overlying marine sedimentary teaus, and geophysical studies of oceanic pla- showing the mapped extent of Wrangellia rocks, predominantly limestone. The main teaus. Study of oceanic plateaus improves our fl ood basalts and linked fi eld photographs. factors that controlled the resulting volca- understanding of the relationship between large Based on combined radiometric (U-Pb, nic architecture of the Wrangellia oceanic igneous provinces (LIPs) and large melting 40Ar/39Ar, K-Ar), paleontological, and mag- plateau include high effusion rates and the events, mass extinctions, and continental growth netostratigraphic age constraints, the Wran- formation of extensive compound fl ow fi elds (e.g., Kerr, 2003; Wignall, 2001).

*Present address: Department of Geology and Geophysics, University of Hawai’i at Mānoa, 1680 East-West Road, Honolulu, Hawai’i 96822, USA; [email protected].

Geosphere; February 2010; v. 6; no. 1; p. 47–73; doi: 10.1130/GES00212.1; 9 ﬁ gures; 6 tables; 11 supplemental ﬁ les.

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Figure 1. Map showing the distribution of Phanerozoic large igneous provinces (Mahoney and Coffi n, 1997). Transient LIPs (large igneous provinces) are indicated in yellow (continental) and orange (oceanic) with peak eruption ages. Red areas are fl ood basalts and ocean islands. Major oceanic plateaus and fl ood basalts are mostly concentrated in the western Pacifi c and Indian Oceans. Map modifi ed from base map by A. Goodlife and F. Martinez (in Mahoney and Coffi n, 1997). The peak eruption ages of each LIP are from the compiled references in Courtillot and Renne (2003) and Taylor (2006). Shatsky Rise age from Mahoney et al. (2005). CAMP—Central Atlantic Magmatic Province.

Oceanic plateaus form near spreading ridges, British Columbia, to south-central Alaska. In this defi ning unit of Wrangellia, in conjunction with extinct arcs, fragments of continental crust, contribution we integrate new observations on underlying and overlying Triassic sediments with and in intraplate settings. They form crustal the volcanic stratigraphy and prevolcanic and age-diagnostic fossils (Jones et al., 1977). The emplacements 20–40 km thick with fl ood basalt postvolcanic stratigraphy of Wrangellia with pre- fl ood basalts are defi ned as the Karmutsen Forma- sequences as much as 6 km thick, with individ- viously published data, including geochronology tion on Vancouver Island and the Queen Charlotte ual plateaus extending as much as 2 million km2 and biostratigraphy, to evaluate the construction Islands (Haida Gwaii), and the Nikolai Forma- in area (e.g., Ontong Java Plateau; Coffi n and and age of the Wrangellia oceanic plateau. This tion in southwest Yukon and south-central Alaska Eldholm, 1994). Current understanding of the material is presented as descriptions of Wrangel- (Fig. 2). Smaller elements of Middle to Late Tri- construction of oceanic plateaus is based on the lia stratigraphy, compiled geologic maps, photo- assic basalt stratigraphy in southeast Alaska are age, composition, and stratigraphy of the vol- graphic databases, an interactive Google Earth also believed to correlate with the Wrangellia canic rock in conjunction with observations of fi le, and a review and compilation of previous fl ood basalts (Plafker and Hudson, 1980). interrelationships between sedimentation, ero- research on Wrangellia. The maps, photographs, sion, and magmatism (Saunders et al., 2007). and archiving of information offer tools to visu- Geographic Distribution and Aerial Extent Stratigraphic and geochronological studies of an alize and fully explore the large body of infor- of the Wrangellia Flood Basalts obducted oceanic plateau, where the base and mation about Wrangellia, bringing together past top of the volcanic stratigraphy are exposed, as and present research to provide an overview of The Wrangellia fl ood basalts in British well as the underlying and overlying sediments, the architecture of the Wrangellia oceanic plateau Columbia, Yukon, and Alaska have been iden- provide a means for evaluating the construc- (Greene et al., 2008, 2009a, 2009b). This contri- tifi ed by geologic mapping and regional geo- tion of an oceanic plateau (e.g., emplacement bution highlights the prominent geologic features physical surveys. Exposures of Wrangellia fl ood of fl ows, eruption environment, tectonic set- of the Wrangellia oceanic plateau, and compares basalts extend ~2300 km from Vancouver Island ting during formation, time scale of volcanism, them to other oceanic plateaus, to provide infor- to south-central Alaska (Fig. 2; Supplemental paleoenvironment preceding and following mation that can be used to test and refi ne models Google Earth File1). Exposures of the Karmut- eruption, uplift and subsidence history). on the genesis of oceanic plateaus. sen Formation cover ~58% of Vancouver Island, Wrangellia fl ood basalts represent one of the British Columbia. In southern Alaska, elements best-exposed accreted oceanic plateaus on Earth. WRANGELLIA FLOOD BASALTS: THE of Wrangellia have a well-defi ned northern Wrangellia is a rare example of an accreted oce- VOLCANIC STRATIGRAPHY OF AN boundary along the Denali fault and extend anic plateau where parts of the entire volcanic OCEANIC PLATEAU southward to the outboard Peninsular terrane stratigraphy are exposed, as well as the prevol- (Fig. 2). In southeast Alaska and southwest canic and postvolcanic stratigraphy. This oce- A large part of Wrangellia formed as an oceanic Yukon, Wrangellia is mostly limited to slivers anic plateau is exposed in numerous fault-bound plateau, or transient LIP, that accreted to western of intensely dissected crustal fragments with blocks in a belt extending from Vancouver Island, North America. Wrangellia fl ood basalts are the minimal aerial extent (Fig. 2).

48 Geosphere, February 2010

Alaska 4 Valdez 5

2 Yukon 1

Haines 3 6 British Columbia 1 Peninsular Juneau 2 Northern Wrangellia

7 3 Alexander 4 Southern Wrangellia 8

1 Talkeetna Mountains 4 2 Clearwater Mountains Ketchikan 3 Amphitheater Mountains

4 Wrangell Mountains Prince Rupert 5 Kluane Ranges British Columbia 6 Chilkat Peninsula

7 Chichagof & Baranof Islands

8 Southeast Alaska (within Alexander terrane) 9

9 Queen Charlotte Islands

10 Vancouver Island 126°00' W 51°00' N Port McNeill Scale

0 500 km 10 Nanaimo

Figure 2. Simplifi ed map showing the distribution of Wrangellia fl ood basalts in Alaska, Yukon, and British Columbia (green). Basalts indicated in southeast Alaska are mostly part of the Alexander terrane. Map derived from Wilson et al. (1998), Israel (2004), Massey et al. (2005a, 2005b), Wilson et al. (2005), and D. Brew (2007, written commun.). Inset shows northwest North America with Wrangellia fl ood basalts, and outlines for the Peninsu- lar (orange) and Alexander (blue) terranes. Purple lines are faults in Alaska and parts of Yukon. Circled numbers indicated in the legend refer to areas mentioned in the text.

1Supplemental Google Earth File. ALL Wrangellia fi les. This .kmz fi le consists of georeferenced information that is designed to be viewed with the satellite imagery in Google Earth. The Google Earth application is available for free download at http://earth.google.com/. Information about Google Earth can be found at this web address. Please use the most current version of Google Earth for viewing these fi les (version 5.0 as of June 2009). The Supplemental Google Earth File contains ten separate folders: (1) Mapped Wrangellia fl ood basalts. This is a red transparent layer that shows the distribution of the Wrangellia fl ood basalts in Alaska, Yukon, and British Columbia. The map was derived from data in Wilson et al. (1998, 2005), Israel (2004), Massey et al. (2005a, b), and D. Brew (2007, written commun.). (2) Major faults in Alaska and Yukon. (3) Major faults in southwest BC. These folders show the location of faults in parts of Alaska, Yukon, and BC. These fi les were fi ltered from original fi les found at the following locations: Alaska from http://www.asgdc.state.ak.us/; Yukon from in Israel (2004); BC faults from Massey et al. (2005a, b). (4) Alaska sample locations. (5) Yukon sample locations. (6) Vancouver Island sample locations. These fi les show the locations, sample numbers, and fl ow type of samples of Wrangellia fl ood basalts collected during fi eld work. (7) Alaska Range photograph locations. (8) Wrangell Mountains photograph locations. (9) Yukon photograph locations. (10) Vancouver Island photograph locations. These four folders contain small versions of georeferenced photographs. Multiple photographs are referenced to a single coordinate. Therefore, in order to view all of the photographs from a single coordinate it is necessary to open the subfolders for each region in the My Places menu so individual photographs can be selected and viewed. Zooming in also helps to distinguish photograph locations. These photographs and others can be viewed in higher resolution online at the following link: http://www.eos.ubc.ca/research/wrangellia/. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00212.S1 or the full-text article on www.gsapubs.org to view the Supplemental Google Earth File.

Geosphere, February 2010 49

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From recently compiled digital geologic and areas in British Columbia are referred to and Yole, 1972; Hillhouse, 1977; Hillhouse maps, the aerial exposure of the Wrangellia as Southern Wrangellia. The Wrangellia com- and Gromme, 1984), and Late Triassic bivalves fl ood basalts is ~20 × 103 km2 on Vancouver posite terrane refers to three distinct terranes indicate an eastern Panthalassan position in the Island, ~800 km2 in southwest Yukon and south- (Wrangellia, Alexander, Peninsular; Fig. 2) that Late Triassic (Newton, 1983). A mantle plume east Alaska, and ~2000 km2 across southern share similar elements or have a linked geologic origin for the Wrangellia fl ood basalts was Alaska (Table 1). The original areal distribution history (as defi ned by Plafker et al., 1989, 1994; proposed by Richards et al. (1991) and is sup- was considerably greater, and these estimates Nokleberg et al., 1994; Plafker and Berg, 1994). ported by ongoing geochemical and petrologi- of outcrop extent do not consider areas of fl ood The connections between the Wrangellia, Alex- cal studies (Greene et al., 2008, 2009a, 2009b). basalt covered by younger strata and surfi cial ander, and Peninsular terranes are not well estab- Late Triassic–Early Jurassic arc magmatism deposits. The boundaries and crustal structure lished, although a single Pennsylvanian pluton is preserved as intrusions within and volcanic of Wrangellia in southern Alaska, as defi ned in Alaska is proposed to link the Proterozoic– sequences overlying Wrangellia fl ood basalts by recent magnetic and gravity surveys, indi- Triassic Alexander terrane to Wrangellia by the throughout areas on Vancouver Island. Paleobio- cate a distinct magnetic high interpreted as an late Pennsylvanian (Gardner et al., 1988). This geographic studies indicate that Wrangellia was expression of the presence of thick dense crust paper specifi cally focuses on Wrangellia and located in the northeast Pacifi c Ocean during the from Triassic mafi c magmatism (Glen et al., does not examine the relationship of Wrangellia Early Jurassic (Smith, 2006). Wrangellia prob- 2007a, 2007b; Saltus et al., 2007). Wrangellia to the Alexander and Peninsular terranes. ably accreted to western North America in the crust beneath Vancouver Island (~25–30 km Wrangellia has a geologic history span- Late Jurassic–Early Cretaceous (e.g., Csejtey et thick) has seismic properties corresponding to ning a large part of the Phanerozoic prior to al., 1982; McClelland et al., 1992; Nokleberg mafi c plutonic rocks extending to depth that are its accretion with western North America. The et al., 1994; Umhoefer and Blakey, 2006; Trop underlain by a strongly refl ective zone of high geologic record beneath the fl ood basalts com- and Ridgway, 2007), and was caught up in the velocity and density (Clowes et al., 1995). This prises Paleozoic oceanic arc and sedimentary Cretaceous–Eocene thermal events of the Coast refl ective zone was interpreted by Clowes et al. sequences with a rich marine fossil assemblage. Mountains, as well as partly covered by younger (1995) as a major shear zone where lower Wran- Paleontological studies indicate that South- sediments (Hollister and Andronicos, 2006). gellia lithosphere was detached. ern Wrangellia was located in cool-temperate waters at northern paleolatitudes (~25°N) and SUMMARY OF THE STRATIGRAPHY Geologic History of Wrangellia not far from the North American continent dur- OF WRANGELLIA ing the Pennsylvanian–Early Permian (Katvala In this paper Wrangellia (or the Wrangellia and Henderson, 2002). The geologic record is A full description of prominent features of terrane) refers to fault-bound sections of the sparse to absent during the Middle Permian– Wrangellia stratigraphy and a summary of upper crust that contain diagnostic successions Early Triassic throughout Wrangellia. A major, previous research are presented in the supple- of Middle to Late Triassic fl ood basalts and short-lived phase of tholeiitic fl ood volcanism mental fi gure, map, methods, and data fi les. Triassic sedimentary strata that overlie Paleo- occurred in the Middle to Late Triassic in sub- Table 2 provides a synthesis of Wrangellia stra- zoic units (as originally defi ned by Jones et al., marine and subaerial environments. Paleo- tigraphy. Supplementary Data Files 12 and 23 1977). The areas of Wrangellia in Alaska and magnetic studies of Wrangellia fl ood basalts also contain a compilation of ~500 references Yukon are referred to as Northern Wrangellia indicate eruption in equatorial latitudes (Irving related to Wrangellia.

TABLE 1. AREAL EXTENT AND VOLUMETRIC ESTIMATES FOR THE WRANGELLIA FLOOD BASALTS Estimated thickness Estimated volume Region and/or Area Area 3 2 (km) (km ) Rel. volume (%) quadrangle (km ) (%) minimum maximum minimum maximum British Columbia Southern VI (NM10) 10,581 41.9 4 6 42,323 63,485 45.2 Northern VI (NM9) 8,561 33.9 4 6 34,245 51,368 36.6 Queen Charlotte Is 1,773 7.0 3 4.5 5,319 7,979 5.7 (NN8) Queen Charlotte Is 1,827 7.2 3 4.5 5,481 8,221 5.9 (NN9) Northern BC (NO8) 86 0.3 1 3 86 257 0.2 Yukon Kluane South 705 2.8 1 3 705 2,115 1.5 Alaska McCarthy 720 2.9 3 4 2,161 2,882 2.1 Nabesna 275 1.1 3 4 824 1,098 0.8 Valdez 55 0.2 1 2 55 110 0.1 Gulkana 7 0.0 1 2 7 14 0.0 Mt Hayes 356 1.4 3 4.5 1,067 1,601 1.1 Healy 205 0.8 3 4.5 616 924 0.7 Talkeetna Mtns 105 0.4 3 4.5 314 471 0.3 Total 25,256 93,204 140,525 Note: Area estimates for British Columbia are calculated from Massey et al. (2005). Area estimates for Yukon are calculated from Israel et al. (2004). Area estimates for Alaska are calculated from Wilson et al. (1998, 2005). Rel.—relative; VI—Vancouver Island; BC—British Columbia; Is—Island; Mtns—mountains.

50 Geosphere, February 2010

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Geosphere, February 2010 51

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Northern Wrangellia age from Late Triassic to Late Jurassic, overlies posed as the type section for the Karmutsen the Nikolai basalts in the Wrangell Mountains. Formation, as this is where the most complete The accretion and northward tectonic migra- From the Nutzotin Mountains along the Alaska- stratigraphic section is preserved (~6 km thick; tion of parts of Wrangellia, followed by the Yukon border to southeast Alaska, Wrangellia Figs. 5 and 6; Yorath et al., 1999). Recent map- oroclinal bending of Alaska, has left Wran- forms a thin northwest- to southeast-trending ping on northern Vancouver Island has delin- gellia fl ood basalts exposed in an arcuate belt belt in the southwest corner of Yukon (Fig. 2). eated the three-part volcanic stratigraphy of extending ~450 km across south-central Alaska The best exposures of Wrangellia in Yukon are pillowed lava sequences, hyaloclastite, and sub- (Fig. 3). Wrangellia stratigraphy forms most in the Kluane Ranges, where the stratigraphy aerial fl ows of the Karmutsen Formation (Fig. 6; of the Wrangell Mountains in the eastern part is similar in most aspects to stratigraphy in the Greene et al., 2009b; Nixon et al., 2008). Picritic of southern Alaska and extends westward in a Wrangell Mountains, and it has been described pillow basalts occur near the top of the subma- wide belt immediately south of the Denali fault, using the same nomenclature (Muller, 1967; rine basalt stratigraphy on northern Vancouver along the southern fl ank of the eastern Alaska Read and Monger, 1976). Triassic basaltic and Island (Greene et al., 2006, 2009b). A photo- Range and in the northern Talkeetna Mountains sedimentary rocks with similarities to Wrangel- graphic archive of fi eld relationships in Wran- (Fig. 3). The northwestern boundary of Wran- lia sequences in southern Alaska are exposed as gellia is available as an abbreviated version in gellia corresponds with a prominent steeply dip- elongate fault-bound slivers within a large and the Supplemental Figure File4 and in full online ping structure (Talkeetna suture zone), which complex fault system in the Alexander terrane at http://www.eos.ubc.ca/research/wrangellia/. may represent the original suture between in several areas of southeast Alaska. Additional geologic maps for areas of Alaska, Wrangellia and the former continental margin Yukon, and Vancouver Island can be found in (Glen et al., 2007a). Mesozoic and Cenozoic Southern Wrangellia the Supplemental Map File5. sedimentary basins along the inboard margin of Wrangellia, adjacent to and overlying the Northern and central Vancouver Island AGE OF WRANGELLIA FLOOD Talkeetna suture zone and Denali fault, record is underlain by Wrangellia, which forms BASALTS the uplift and collisional history of Wrangellia the uppermost sheet of a thick sequence of and the former continental margin, north of the northeast-dipping thrust sheets that constitute Previous Geochronology for Wrangellia Denali fault (Trop et al., 2002; Ridgway et al., the upper crust of Vancouver Island (Fig. 5; Flood Basalts and Related Plutonic Rocks 2002; Trop and Ridgway, 2007). Monger and Journeay, 1994; Yorath et al., Some of the best exposures of Wrangellia 1999). Wrangellia lies in fault contact with the Samples of Wrangellia fl ood basalts and fl ood basalt stratigraphy are preserved as part Pacifi c Rim terrane and Westcoast Crystalline related plutonic rocks from British Columbia, of an east-west–trending synform underlain Complex to the west, and is intruded by the pre- Yukon, and Alaska were previously dated in by mafi c and ultramafi c plutonic rocks in the dominantly Cretaceous Coast Plutonic Complex eight separate studies; these results are sum- Amphitheater Mountains (Figs. 3 and 4). There to the east (Wheeler and McFeely, 1991). The marized in Table 3 (all ages below are quoted are ~3 km of subaerial basalt fl ows overlying cumulative thickness of Wrangellia stratigraphy with 2σ analytical uncertainty). A total of <800 m of submarine stratigraphy in the Amphi- exposed on Vancouver Island is >10 km (Yor- 15 ages (4 U-Pb, 9 40Ar/39Ar, and 2 K-Ar) from theater Mountains. Wrangellia fl ood basalts also ath et al., 1999). Two prominent northwest- to the literature include 4 basalts and 11 plutonic form two prominent northwest- to southeast- southeast-trending anticlinoria (Buttle Lake rocks. In Southern Wrangellia, 3 U-Pb ages trending belts along the northeast and southwest and Cowichan) are cored by Paleozoic rocks, from gabbroic rocks on southern Vancouver margins of the Wrangell Mountains, between which are not exposed on northern Vancouver Island are available, including (1) a single con- the Totschunda fault and Chitina thrust belt Island (Fig. 5; Brandon et al., 1986; Yorath et cordant analysis of a multigrain baddeleyite (Fig. 3). The Nikolai Formation in the Wrangell al., 1999). Wrangellia also forms a large part of fraction that yielded a 206Pb/238U age of 227.3 ± Mountains is estimated to be ~3.5 km in total the southern Queen Charlotte Islands, where it 2.6 Ma (Parrish and McNicoll, 1992), and thickness and is composed almost entirely of is mostly unsampled. (2) two unpublished 206Pb/238U baddeleyite ages subaerial fl ows (MacKevett, 1978). The Nikolai The deepest stratigraphic levels of Wrangel- of 226.8 ± 0.5 Ma (5 fractions) and 228.4 ± Formation consists of low-titanium basalts that lia, which are exposed in the Buttle Lake and 2.5 (2 fractions) (Table 2; Sluggett, 2003). A form the lowest ~400 m of volcanic stratigraphy Cowichan anticlinoria, comprise the lower to single whole rock of Karmutsen basalt from in the Alaska Range, and the remainder of the middle Paleozoic Sicker Group and the upper Buttle Lake on Vancouver Island yielded a volcanic stratigraphy in the Alaska Range and Paleozoic Buttle Lake Group (Fig. 5). The 40Ar/39Ar plateau age of 224.9 ± 13.2 Ma (Las- all of the sampled stratigraphy in the Wrangell combined total thickness of the Sicker and siter, 1995). Mountains are high-titanium basalt (Greene et Buttle Lake Groups is estimated to be ~5000 m In Northern Wrangellia, the U-Pb results al., 2008). A cumulative thickness of >3.5 km (Massey, 1995; Yorath et al., 1999; Fig. 5).The for zircon separated from a gabbro sill possi- of marine sedimentary rocks, which range in basalt stratigraphy around Buttle Lake is pro- bly related to the Nikolai basalts in southwest

4Supplemental Figure File. This PDF fi le contains nine fi gures with captions. These fi gures accompany Supplemental Data File 7 and illustrate some of the excep- tional exposures of the volcanic stratigraphy of the Wrangellia plateau in Alaska, Yukon, and British Columbia. Additional georeferenced photographs are part of the Supplemental Google Earth File and also can be found at the following website dedicated to research on Wrangellia: http://www.eos.ubc.ca/research/wrangellia/. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00212.S4 or the full-text article on www.gsapubs.org to view the Supplemental Figure File.

5Supplemental Map File. This fi le contains four maps of different parts of Wrangellia in a single pdf fi le with references. Two maps are simplifi ed regional geologic maps of southwest Yukon and southeast Alaska showing the distribution of Triassic fl ood basalts. Two maps are detailed geologic maps of areas in the Amphithe- ater Mountains in the Alaska Range and on northern Vancouver Island. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/ 10.1130/GES00212.S5 or the full-text article on www.gsapubs.org to view the Supplemental Map File.

52 Geosphere, February 2010

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/1/47/3337977/47.pdf by guest on 29 September 2021 Volcanic Stratigraphy of the Wrangellia Oceanic Plateau sandstone. — Formation; ss — al., 1992). Fm units on the south side of the Wrangell Mountains Wrangell units on the south side of ood basalts (black) and stratigraphic columns. Map is derived from Wilson et al. Wilson ood basalts (black) and stratigraphic columns. Map is derived from ed map of eastern south-central Alaska showing the distribution of Wrangellia ﬂ Wrangellia Alaska showing the distribution of ed map of eastern south-central Figure 3. Simpliﬁ Figure Schmidt et al. (2003b). Stratigraphic columns depict late Paleozoic–Jurassic et al. (2005), with mapping from Wilson (1998) and Nokleberg et Alaska Range (derived partly from Smith and MacKevett, 1970; 1978), the east-central (derived from

Geosphere, February 2010 53

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/1/47/3337977/47.pdf by guest on 29 September 2021 Greene et al. Hasen Creek Hasen Creek — ows overlain by Chitistone Limestone Limestone (taken by Ed MacKevett, Jr.). Limestone (taken by Ed MacKevett, Jr.). in Fig. 3). (A) Sediment-sill complex and base aph by Ed MacKevett, Jr. (Phc aph by Ed MacKevett, Jr. beds). (D) Photograph of ~1000 m subaerial basalt ﬂ Daonella Middle Triassic Middle Triassic — Golden Horn Limestone Lentil; TRd Golden Horn Limestone Lentil; — ood basalts on the west side of Lower Tangle Lake. (B) Photograph of the top Nikolai Formation and overlying Chitistone Tangle ood basalts on the west side of Lower (dashed orange line indicates the contact). Several faults offset the contact. (C) Paleozoic arc rocks and marine sedimentary sequences underlying Nikolai basalts. Photogr rocks Several faults offset the contact. (C) Paleozoic arc Formation; Pgh of ﬂ Figure 4. Photographs of the base of the Nikolai Formation in the Alaska Range and Wrangell Mountains, Alaska (locations shown Mountains, Wrangell Alaska Range and 4. Photographs of the base Nikolai Formation in Figure

54 Geosphere, February 2010

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Yukon yielded an age of 232.2 ± 1.0 Ma (aver- teau and isochron ages of variable precision tic rocks, mafi c and ultramafi c intrusive rocks age 207Pb/206Pb age of 3 discordant [1.6%– have been determined for hornblende and and dikes, and limestone that is distinct from 2.4%] analyses from multigrain zircon frac- biotite separates from mafi c and ultramafi c the stratigraphy of the Nikolai Formation; these tions) (Table 3; Mortensen and Hulbert, 1991). plutonic rocks in the Amphitheater Mountains units may be older than the Wrangellia fl ood K-Ar dating of biotite from peridotite in the in the Alaska Range (Table 3). Three of these basalts (Bittenbender et al., 2003) or may be Kluane mafi c-ultramafi c complex provided samples, with ages of 225.2 ± 6.5 Ma, 225.7 ± younger (Nokleberg et al., 1992). Two gabbros ages of 224 ± 8 Ma and 225 ± 7 Ma (Campbell, 2 Ma, and 228.3 ± 1.1 Ma, are from the Rainy related to Wrangellia fl ood basalts in the Tan- 1981). In Alaska, three samples of Wrangel- Creek area, which is to the north across a major gle Lakes area of the Amphitheater Mountains lia fl ood basalts from the Wrangell Mountains fault from typical volcanic stratigraphy of have reported 40Ar/39Ar ages of 230.4 ± 2.3 and yielded whole rock 40Ar/39Ar plateau ages of Wrangellia fl ood basalts (Bittenbender et al., 231.1 ± 11 Ma (Bittenbender et al., 2003; 228.3 ± 5.2 Ma, 232.8 ± 11.5 Ma, and 232.4 ± 2007). The Rainy Creek area is a steeply dip- Schmidt and Rogers, 2007); however, analytical 11.9 Ma (Lassiter, 1995). Five 40Ar/39Ar pla- ping sequence of picritic tuff and volcaniclas- information is not available for these samples.

Figure 5. Simplifi ed map of Vancouver Island, British Columbia, showing the distribution of the Karmutsen Formation (green) and underlying Paleozoic formations (black) (after Massey et al., 2005a, 2005b). The main areas of fi eld study are indicated with boxes or circles with uppercase letters (see legend). Stratigraphic columns are shown for northern and central Vancouver Island. Column for northern Vancouver Island is adapted from Nixon and Orr (2007). Column for central Vancouver Island is derived from Carlisle (1972), Juras (1987), Massey (1995), and from fi eld work. Fm.—Formation; ss—sandstone. Red lines indicate the axes of anticlinoria.

Geosphere, February 2010 55

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/1/47/3337977/47.pdf by guest on 29 September 2021 Greene et al. ow marine fl ct between limestone of the upper Paleozoic ct between limestone of the upper by pillow basalt in ow with hackly jointing surrounded to Karmutsen basalts, and overlain by pillow basalt of the Formation. (B) Photograph massive sub c sills, related Figure 6. Field photographs of the Karmutsen Formation on Vancouver Island (locations shown in Fig. 5). (A) Photograph of conta Vancouver 6. Field photographs of the Karmutsen Formation on Figure intruded by mafi Buttle Lake Group draped over thick sequence of pillow basalt in the Alice-Nimpkish Lake area. (C) Photograph of a massive submarine fl Alice-Nimpkish Lake area. thick sequence of pillow basalt in the draped over the Schoen Lake area. (D) Photograph of contact between top the Karmutsen Formation and overlying Quatsino limestone. the Schoen Lake area.

56 Geosphere, February 2010

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t n ilable. Analysis at University of at University ilable. Analysis e an alteration age, maximum age for m m Ar released, integrated age: 226.0 ± Ar released, integrated age: 226.4 ± o 39 39 C Pb age of 3 discordant (1.6 to 2.4%) analyses from 206 Pb/ Ar released; n = 5/7 Ar released; n = 5/7 Ar released; n = 4/7 U ages of other fractions are 219.8 ± 0.7, 223.3 ± 0.5, 221.9 ± 39 39 39 207 U age for 1 of 5 fractions, interpreted age. as the crystallization U ages for 2 concordant fractions are 229.4 ± 2.5 and 228.4 ± 1.6 U age from a single concordant analysis of a multigrain 238 Ar step-heating technique; inverse isochron age: 228.1 ± 11.8 Ma; Ar step-heating technique; inverse isochron age: 224.8 ± 5.2 Ma; Ar step-heating technique; inverse isochron age: 230.5 ± 27.8 Ma; Ar step-heating technique; inverse isochron age: 213.7 ± 20.5; 69% 238 238 238 Ar; n = 5/7 39 39 39 39 39 Pb/ Pb/ Pb/ Pb/ Ar/ Ar/ Ar/ Ar/ 1.1 Ma; interpreted as age of magmatism multigrain zircon fractions J.E. Harakal British Columbia by 0.6, and 233.9 ± 1.4 Ma, which were interpreted to indicate (USGS) baddeleyite fraction baddeleyite of post-crystallization Pb-loss post-crystallization 58% of 57% of 2.0 Ma; Interpreted as cooling age following crystallization Ma. Crystallization age interpretedMa. Crystallization to be 228.4 ± 2.5 Ma 69% of 206 (USGS). 62.7°N, 148.5°W a reset event. Impure separate, excess Ar 0 40 40 206 206 Average K-Ar age. informationNo analytical ava 40 206 K-Ar age. informationNo analytical M. by Lanphere available. Analysis 4 Plateau age, 8 fractions, 97% of Total fusion age. informationNo analytical L. Snee available. Analysis by Plateau age, 5 fractions, 81% of Invserse isochron age; interpreted as

Ar ages are reported relative to biotite standard FCT-3 (28.03 ± 0.16 Ma, Renne n o 39 i t a Ar/ c 40 o l

d n a

n o i t p i r c s e d

k c Ranges Ranges Kluane Ranges Crofton, southern Vancouver Island Island Island upper third of the volcanic stratigraphy; Glacier Creek lower part of the basalt part of lower stratigraphy; Skolai Creek stratigraphy; Skolai Creek Lake, central Vancouver Island Complex above the base of the basalt including olivine; Rainy Creek Rainy including olivine; Complex southwestern Grizzly Ridge southwestern Grizzly Complex o R Mount Tuam gabbro; Saltspring Nikolai basalt flow from the Nikolai basalt flow Peridotite; Tatamagouche Creek, Peridotite; White River, Kluane Porphyritic gabbroic rock; Maple Creek gabbro; Kluane Mount Tuam gabbro; Saltspring Nikolai basalt flow, ~100 m Nikolai basalt flow, Gabbro with poikilitic biotite, poikilitic Gabbro with Gabbro; Tangle Lakes area No analytical information available Olivine basalt; Rainy Creek Olivine basalt; Rainy Nikolai basalt flow, from the Nikolai basalt flow, Olivine peridotite; Rainy Creek Olivine peridotite; Rainy Karmutsen basalt flow, Buttle Karmutsen basalt flow, Gabbro; Tangle Lakes area,

§ § § §

e c

n † † † † e r e f e 1992 2007 Hulbert, 1992 2007 2003 2007 2007 R Parrish and McNicoll, Bittenbender et al., Bittenbender et al., Mortensen and Bittenbender et al., Bittenbender et al., Schmidt and Rogers,

l a i r e t a : Analyses preformed at the Geochronology Laboratory at the University of Alaska, Fairbanks. Monitor mineral MMhb-1 (Samson and 2002); n = number of heating steps used/total. M

d o h t e M

. USGS—U.S. Geological Survey. σ TABLE 3. COMPILATION OF PREVIOUSLY PUBLISHED GEOCHRONOLOGY OF WRANGELLIA FLOOD BASALTS AND ASSOCIATED PLUTONIC ROCKS 225.7 4 Hornblende Ar-Ar 232.2 1 Zircon U-Pb Errors are 2

: Analyses provided of courtesy R. Duncan (Oregon State University), interpreted originally in Lassiter (1995): Analyses provided courtesy of P. Bittenbender Note † § 84822-1C 227.3 2.6 Baddeleyite U-Pb HDB88- TAT22 Vancouver Island 02-CS-11 226.8 0.5 Zircon U-Pb 2003 Sluggett, HBD-2003- Hornblende* 29 13 Ar-Ar 10830 225.2 SC1 224 8 K-Ar Biotite Campbell, 1981 1981 Campbell, Biotite 8 Campbell, Biotite K-Ar SC1 224 7 K-Ar SC2 225 92LNG-42 232.8 11.5 Ar-Ar Whole rock Lassiter, 1995 Yukon 02-CS-14 228.4 2.5 Zircon U-Pb 2003 Sluggett, 92LNG-47 232.4 11.9 Ar-Ar Whole rock Lassiter, 1995 AK25515 228.3 2.2 Biotite Ar-Ar

00AG022 231.1 11 Hornblende Ar-Ar 91LKB-46 224.9 13.2 Ar-Ar Whole rock Lassiter, 1995 92LNG-19 228.3 5.2 Ar-Ar Whole rock Lassiter, 1995 Wrangell Mountains et al., 1998), calculated ArArCalc (Koppers, with number Age Error ± 2

PB1 230.4 2.3 Ar-Ar Biotite Ar-Ar 2.3 PB1 230.4 Alaska Range 1987) anwith age of 513.9 Ma (Lanphere and Dalrymple, 2000) was used to monitor neutron flux. Sample *Replaces clinopyroxene.

Geosphere, February 2010 57

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40Ar/39Ar Geochronological Results for a gabbro from the Alice–Nimpkish Lake ater Mountains show disturbed 40Ar/39Ar sys- area display a disturbed saddle-shaped age tematics (Fig. 7D). Analytical methods for 40Ar/39Ar dating spectrum and may have been affected by excess are described in Supplemental Methods File6. 40Ar (e.g., Lanphere and Dalrymple, 1976; Har- Interpretation of New 40Ar/39Ar Petrographic textures and major and trace ele- rison and McDougall, 1981). The crosscutting Geochronology ment chemistry for all of the geochronological mafi c dike (sample 4718A3) yields a plateau samples are listed in Supplemental Data File age of 193 ± 26 Ma (54% of the 39Ar released) Among the 13 samples of Wrangellia fl ood 37. Age spectra are shown in Figure 7 and the and a broadly concordant isochron age of 186 ± basalt that were analyzed in this study, the analytical results are summarized in Table 4; the 13 Ma (Fig. 7A; Table 4). results for eight samples satisfy the spectra and analytical data are available in Supplemental Plagioclase separates from fi ve basalt fl ows isochron criteria to be geologically interpretable Data File 48. All ages indicated below are inter- and one mafi c sill in Alaska, including two 40Ar/39Ar ages (see Supplemental Data File 4 preted to represent cooling ages that correspond samples from the Wrangell Mountains and four [see footnote 8]). The only sample inferred to

to the bulk closure temperature (Tcb) of the dif- samples from the Alaska Range, yield a total of have retained a magmatic age is the peridotite ferent minerals to Ar diffusion (~200 °C plagio- three interpretable ages based on plateau ages from Yukon for which the biotite separate yields clase; ~550 °C hornblende; ~350 °C biotite; as (80%–99% of the 39Ar released): 191 ± 11 Ma, a plateau age of 227.5 ± 1.2 Ma. This age, which summarized in Hodges, 2003). We processed 160.7 ± 1.3 Ma, and 169.0 ± 2.4 Ma (Fig. 7B; corresponds to cooling of the ultramaﬁ c intru-

20 mineral separates, including 14 plagioclase Table 4). For sample 5810A4, the plateau age sion through the Tcb of biotite (~350 °C) and is separates prepared from the basalt and intrusive (137.3 ± 8.5 Ma) is considerably younger than thus a minimum age of crystallization, is con- samples, 5 hornblende separates, and 1 biotite the integrated age (160.57 ± 8.64 Ma), refl ect- sistent with the range of previously published separate, from 19 samples from throughout ing the younger ages of the lower temperature and reported ages from 227 to 233 Ma for Wrangellia for 40Ar/39Ar dating; 13 samples steps included in the plateau age. The individ- emplacement of the Wrangellia fl ood basalts were Wrangellia fl ood basalts or intrusive ual step-heating results for sample 5719A5 are described above. The remaining seven ages equivalents and 6 were from younger, crosscut- associated with relatively large errors, and two from the Wrangellia basalts range from 191 ting intrusive rocks. Of the 13 Wrangellia fl ood samples of basalt fl ows (5715A1, 5810A10) to 73 Ma, indicating open-system behavior of basalts or intrusive samples, 9 were basalt fl ows show strongly disturbed 40Ar/39Ar systematics the 40Ar/39Ar systematics (Table 4). The Kar- and 4 were mafi c sills or gabbroic rocks. One (Fig. 7B). mutsen basalts on Vancouver Island underwent biotite separate was from an ultramafi c plutonic The biotite separate from a peridotite in prehnite-pumpellyite facies metamorphic con- rock from the Kluane Ranges, Yukon (sample Yukon yields a plateau based on results from ditions (1.7 kbar, ~300 °C) and higher meta- 05-SIS-751). The fi ve hornblende separates the higher temperature steps (60% of the 39Ar morphic gradients where proximal to granitoid were all from younger dikes and intrusions that released), with an age of 227.5 ± 1.2 Ma and an intrusions (Cho and Liou, 1987), and the min- crosscut Wrangellia basalts and were selected inverse isochron age of 226.1 ± 3 Ma (Fig. 7C; eralogy of Nikolai basalts in Alaska indicates a as they provide minimum ages for eruption. The Table 4). Plagioclase separates from two basalt similar degree of metamorphism. Plagioclase,

samples that are younger crosscutting intrusive fl ows from different levels of the volcanic stra- which has a low Tcb for Ar diffusion (<200 °C; units are clearly distinguishable from the Wran- tigraphy of the Nikolai Formation in Yukon dis- Cohen, 2004), was analyzed from each of these gellia fl ood basalts by their different textures play strongly disturbed age spectra (Fig. 7C). samples, thus these samples likely degassed 40Ar and whole-rock chemistry. Hornblende separates from four younger during thermal resetting at some period after Plagioclase separates from three basalt fl ows intrusive samples from Alaska, including two their emplacement. (two submarine and one subaerial fl ow) and samples from the Wrangell Mountains and two Despite resetting, the 40Ar/39Ar ages of these one gabbro from Vancouver Island were ana- samples from the Alaska Range, yield plateau basalts are geologically meaningful and pro- lyzed. The incremental heating data of the three ages (54%–95% of the 39Ar released) of 29.7 ± vide temporal constraints on the geologic his- basalt fl ows form plateaus over 67%–96% of 1.1 Ma, 123.13 ± 0.77 Ma, 148.80 ± 0.83 Ma, tory of the Wrangellia terrane. Three of the reset the 39Ar released with ages of 72.5 ± 1.2 Ma, and 149.9 ± 0.93 Ma (Fig. 7D; Table 4); the 40Ar/39Ar ages (191, 181, and 161 Ma) of Kar- 180.9 ± 3.1 Ma, and 161.1 ± 7.3 Ma (Fig. 7A; plateau ages for these samples correspond with mutsen basalts from Vancouver Island are within Table 4); inverse correlation diagrams (39Ar/40Ar their respective isochron ages. The results for the age range of Bonanza arc intrusions and versus 36Ar/40Ar) yield isochron ages that are two hornblende analyses from an amphibolite volcanic sequences (197–167 Ma) that intrude concordant with the plateau ages. The results dike in the Rainy Creek area of the Amphithe- and overlie the Karmtusen basalts on Vancouver

6Supplemental Methods File. This Microsoft Word ﬁ le summarizes the analytical methods used for 40Ar/39Ar analyses. If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://dx.doi.org/10.1130/GES00212.S6 or the full-text article on www.gsapubs.org to view the Supplemental Methods File.

7Supplemental Data File 3. Geochemistry for 40Ar/39Ar samples. This Microsoft Excel ﬁ le contains the analytical data for whole-rock analyses for the 19 samples dated by the 40Ar/39Ar dating method. Analyses were performed by ActLabs and the analytical methods are summarized in Greene et al. (2009b). Additional information in this table includes: UTM coordinates, geographic location, lithology, mineral proportion, and texture. If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://dx.doi.org/10.1130/GES00212.S7 or the full-text article on www.gsapubs.org to view Supplemental Data File 3.

8Supplemental Data File 4. 40Ar/39Ar analytical data. This Microsoft Excel fi le contains the analytical data for the 40Ar/39Ar dating method for the 20 mineral separates analyzed at the Noble Gas Laboratory in the Pacifi c Centre for Isotopic and Geochemical Research at University of British Columbia. Age spectra and isochron diagrams for each of the samples are shown along with all the analytical data from incremental step-heating. The analytical methods are summarized in the Supple- mental Methods File. Where multiple analyses of a single sample were made, all the results for each sample are included in a single worksheet in this workbook. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00212.S8 or the full-text article on www.gsapubs.org to view Supplemental Data File 4.

58 Geosphere, February 2010

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/1/47/3337977/47.pdf by guest on 29 September 2021 Volcanic Stratigraphy of the Wrangellia Oceanic Plateau ve plagioclase and plateau steps ve hornblende sep- σ errors) are indicated are errors) ned in Supplemental σ Ar age spectra for pla- age spectra for Ar 39 inverse. (B) Age spectra inverse. (B) ood basalts, and samples — Ar/ 40 Ar Released (hornblende) 39 ood basalts); asterisks after theood basalts); asterisks after

% ood basalt or intrusive related to the intrusive related ood basalt or gioclase, hornblende, and biotite min- basaltsWrangellia eral separates of from intrusive rocks and crosscutting Alaska. and Yukon, Island, Vancouver fi Age spectra for (A) Vancouver mineral separates from Island. Inv. six plagioclase mineral separatesfor oneAge spectra for Alaska. (C) from biotite (05SI-75-1) and two plagioclase Sample Yukon. mineral separates from 05SI-75-1 is a peridotite and samples Nikolai basalts. are 4808A8 and 4811A1 fi Age spectra for (D) that intrude plutonic rocks arates from onAlaska. Errors basalts in Wrangellia 2 plateau ages are or red included in age calculation are plateau steps are blue (samples with red fl Wrangellia Wran- not with blue plateau steps are gellia fl Wrangellia indicate sample number fl younger samples are basalts. Other intrusive units. Samples with all white asteps do not meet the criteria for plateau age, defi Methods File (see footnote 6). Inverse ages (2 isochron Ana- comparison with plateau ages. for in Supple- presented lytical data are mental Data File 4 (see footnote 8) and 4. Geographic Table summarized in type, and type of minerallocation, rock indicated in each panel. separate are Figure 7. Figure Rainy Creek, Amphitheater Mountains Rainy amphibolite 5808A6 0 20406080100 0 40 80 200 160 120 Ar Released 39

Ar Released (hornblende) (hornblende) % 137.3 ± 8.5 Ma 39 29.7 ± 1.1 Ma % 191 ± 11 Ma 148.80 ± 0.83 Ma Tangle Lake, Amphitheater Mtns. Lake, Tangle basalt (plagioclase) pillow 5810A10* 5719A5* 5719A5* Skolai Creek, Wrangell Mountains Wrangell Creek, Skolai basalt (plagioclase) massive 189 ± 29 Ma isochron age: Inv. 5810A4* Inv. isochron age: 121 ± 89 Ma isochron age: Inv. Tangle Lake, Amphitheater Mtns. Lake, Tangle mafic sill (plagioclase) Inv. isochron age: 147.4 ± 2.1 Ma isochron age: Inv. Nugget Creek, Wrangell Mountains Wrangell Nugget Creek, mafic dike mafic dike 5712A4 0 20 40 60 80 100 020406080100 0 20406080100 0 0 Inv. isochron age: 26.3 ± 2.2 Ma isochron age: Inv. 5729A1 5729A1 Glacier Gap Lake, Amphitheater Mtns. Glacier Gap Lake, intrusion felsic 80 300 200 100 280 240 200 160 120 600 400 200 0 20 40 60 80 100 020406080100 0 0 80 40 60 40 20 280 240 200 160 120

Ma (hornblende) Ar Released 39 Ar Released 160.7 ± 1.3 Ma

169.0 ± 2.4 Ma 39 % % 149.86 ± 0.93 Ma 123.13 ± 0.77 Ma sochron age: 149 ± 1.4 sochron age: 5715A1* Hidden Creek, Wrangell Mountains Wrangell Hidden Creek, massive basalt (plagioclase) massive 5810A6* Amphitheater Mountains Lake, Tangle basalt (plagioclase) pillow 166.6 ± 3.5 Ma isochron age: Inv. 5802A6* Tangle Lake, Amphitheater Mountains Lake, Tangle basalt (plagioclase) pillow 160.3 ± 2.1 Ma isochron age: Inv. 5725A5 5725A5 Inv. isochron age: 122.0 ± 1.8 Ma isochron age: Inv. Tangle Lake, Amphitheater Mtns. Lake, Tangle intrusion felsic 5712A1 Mountains Wrangell Nugget Creek, (hornblende) mafic dike i Inv. 020406080100 0 20406080100 D 0 20406080100 0 020406080100 020406080100

80 80 40 0 0

80 40

280 240 200 160 120 200 160 120 80 40

360 320 280 240 200 160 120

Apparent Age (Ma) Age Apparent Apparent Age (Ma) Age Apparent

Apparent Age (Ma) Age Apparent 200 160 120 300 200 100 Apparent Age (Ma) Age Apparent Apparent Age (Ma) Age Apparent B 193 ± 26 Ma Ar Released Ar Released 39 39

161.1 ± 7.3 Ma

% %

Inv. isochron age: 160.5 ± 8.7 Ma isochron age: Inv. 4723A13* Alice-Nimpkish Lake picritic basalt (plagioclase) pillow 20 40 60 80 100 20 40 60 80 100 Donjek River, Kluane Ranges Donjek River, massive basalt (plagioclase) massive 4811A1* Inv. isochron age: 186 ± 13 Ma isochron age: Inv. 4718A3 mafic dike (plagioclase) mafic dike Mount Arrowsmith 0 20406080100 0 0 0 0 0 800 600 400 200 500 400 300 200 100 300 200 100 (biotite) Ar Released Ar Released 72.5 ± 1.2 Ma 39 39

% 227.5 ± 1.2 Ma (plagioclase) 180.9 ± 3.1 Ma % Inv. isochron age: 226.1 ± 3 Ma isochron age: Inv. Kluane Ranges ultramafic intrusionultramafic 05SI-75-1* 4718A2* 4718A2* 4720A6* Inv. isochron age: 69 ± 18 Ma isochron age: Inv. Inv. isochron age: 180.3 ± 5.4 Ma isochron age: Inv. Mount Arrowsmith Schoen Lake pillow basalt (plagioclase) pillow massive flow (plagioclase) flow massive Tatamagouche Creek, Kluane Ranges Tatamagouche basalt (plagioclase) massive 4808A8* 5615A6* Alice-Nimpkish Lake gabbro gabbro 020406080100 020406080100 020406080100 020406080100 020406080100 0 0 0

50 80 40

75 50 25 90 70

250 200 150 100 400 360 320 280 240 200 160 120

280 240 200 160 120 100 Apparent Age (Ma) Age Apparent Apparent Age (Ma) Age Apparent 150 130 110 Apparent Age (Ma) Age Apparent Apparent Age (Ma) Age Apparent Apparent Age (Ma) Age Apparent A C

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TABLE 4. 40AR/39AR DATING RESULTS FOR WRANGELLIA FLOOD BASALTS AND INTRUSIONS FROM THE WRANGELLIA TERRANE

† § Integrated age Plateau age Inverse isochron Sample Mineral # Plateau information** †† Note (Ma) (Ma) (ages in Ma) Vancouver Island (2σ) (2σ) 5 steps 69 ±18 39 40 36 4718A2* Plag 76.31 ± 1.32 72.5 ± 1.2 67.6% of Ar released ( Ar/ Ar)i = 307 ± 65 tholeiitic, pillowed flow MSWD = 1.3 MSWD = 1.14, n = 5 3 steps 186 ± 13 39 40 36 4718A3 Plag (1) 255.99 ± 10.86 193 ± 26 54.3% of Ar released ( Ar/ Ar)i = 322.4 ± 9.6 mafic dike MSWD = 5.1 MSWD = 2.6, n = 7 177.1 ± 3.1 4718A3 Plag (2) 224.51 ± 10.33 NP (40Ar/36Ar) = 327.5 ± 6.2 last step: 231 ± 13 Ma i MSWD = 1.02, n = 6 177.6 ± 3.0 combined high-T run #1, 4718A3 Plag (C) 255.99 ± 10.86 NP (40Ar/36Ar) = 325.3 ± 4.5 all run #2 inverse i MSWD = 1.4, n = 9 isochron 6 steps 180.3±5.4 39 40 36 4720A6* Plag 182.31 ± 3.20 180.9 ± 3.1 95.6% of Ar released ( Ar/ Ar)i = 307 ± 16 tholeiitic, massive flow MSWD = 0.93 MSWD = 0.39, n = 7 4 steps 160.5 ± 8.7 slightly upstepping 39 40 36 4723A13* Plag 150.24 ± 8.50 161.1 ± 7.3 79.1% of Ar released ( Ar/ Ar)i = 280 ± 15 plateau, MSWD = 1.3 MSWD = 1.5, n = 7 picritc pillow lava coarse-grained gabbroic 5615A6* Plag 220.98 ± 2.15 NP NI rock Yukon 9 steps 226.1 ± 3 39 40 36 9 steps in plateau, 05SI-75-1* Biotite 226.08 ± 0.53 227.5 ± 1.2 60.1% of Ar released ( Ar/ Ar) = 462 ± 380 i peridotite MSWD = 1.6 MSWD = 1.7, n = 9 4808A8* Plag 114.88 ± 2.60 NP NI high-Ti massive flow 4811A1* Plag 361.37 ± 11.01 NP NI low-Ti massive flow Wrangell Mountains 6 steps 146.5 ± 2.1 39 40 36 hbl-phyric hypabyssal 5712A1 Plag 144.23 ± 1.08 145.6 ± 1.2 84.6% of Ar released ( Ar/ Ar) = 267 ± 48 i rock MSWD = 0.91 MSWD = 0.72, n = 6 6 steps 149.0 ± 1.4 39 40 36 hbl-phyric mafic intrusive 5712A1 Hbl 150.71 ± 0.53 149.86 ± 0.93 53.5% Ar released ( Ar/ Ar) = 359 ± 23 i rock MSWD = 0.56 MSWD = 1.3, n = 9 6 steps 147.4 ± 2.1 39 40 36 hbl-phyric mafic intrusive 5712A4 Hbl 152.51 ± 0.63 148.80 ± 0.83 61.4% of Ar released ( Ar/ Ar) = 378 ± 96 i rock MSWD = 0.67 MSWD = 2.9, n = 12 5715A1* Plag 131.21 ± 1.92 NP NI high-Ti massive flow 6 steps 189 ± 29 39 40 36 5719A5* Plag 187.20 ± 10.44 191 ± 11 82.4% of Ar released ( Ar/ Ar)i = 296 ± 21 high-Ti massive flow MSWD = 0.48 MSWD = 0.16, n = 6 Alaska Range 6 steps 122.0 ± 1.8 39 40 36 hbl-phyric felsic intrusive 5725A5 Hbl 124.38 ± 0.69 123.13 ± 0.77 95.2% of Ar released ( Ar/ Ar) = 334 ± 18 i rock MSWD = 1.5 MSWD = 0.51, n = 9 U-Pb zircon age§§ 5 steps 26.3 ± 2.2 39 40 36 i 31.2 ± 0.2 Ma, hbl- 5729A1 Hbl 37.59 ± 2.41 29.7 ± 1.1 82.8% Ar released ( Ar/ Ar) = 362 ± 20 phyric felsic intrusive MSWD = 1.9 MSWD = 0.46, n = 8 rock 7 steps 160.3 ± 2.1 5802A6* Plag 161.86 ± 1.31 160.7 ± 1.3 98.6% of 39Ar released (40Ar/36Ar)i = 299 ± 16 low-Ti sill MSWD = 1.13 MSWD = 0.88, n = 7 5808A6 Hbl (1) 92.77 ± 0.88 NP NI low-Ti dike, Rainy Creek 5808A6 Hbl (2) 87.14 ± 0.6 NP NI low-Ti dike, Rainy Creek 3 steps 121 ± 89 integrated age is 39 40 36 5810A4* Plag 160.57 ± 8.64 137.3 ± 8.5 68.3% of Ar released ( Ar/ Ar)i = 379 ± 470 preferred, MSWD = 1.7 MSWD = 0.17, n = 3 low-Ti sill 5 steps 166.6 ± 3.5 39 40 36 5810A6* Plag 180.30 ± 3.65 169.0 ± 2.4 80.3% of Ar released ( Ar/ Ar)i = 321 ± 21 low-Ti pillow basalt MSWD = 1.3 MSWD = 0.80, n = 8 5810A10* Plag 325.39 ± 26.68 NP NI high-Ti pillow basalt *Wrangellia flood basalt or intrusive equivalent, based on field relations, petrography, and geochemistry. †Sample number: last digit year, month, day, initial, sample, except sample 05SI-75-1. §Mineral separate and run number in parentheses, or (C) for combined result of multiple runs. Mineral abbreviations: hbl, hornblende; plag, plagioclase. #Plateau age is preferred age, error reported for ± 2σ. NP—no plateau. **Number of steps used for calculating age. Criteria for plateau age are described in the analytical methods in the Supplemental Methods File. ††Inverse isochron (36Ar/40Ar vs. 39Ar/40Ar) age, initial 40Ar/36Ar ratio, and MSWD. n—number of points included in isochron. MSWD—mean sum of the weighted deviates, which is a measure of the scatter compared to that which is expected from analytical uncertainties. §§Unpublished data (A. Greene, 2009). NI—no isochron. See Supplemental Data File 3 (see footnote 7) for petrographic and geochemical data for all samples, and coordinates for sample locations. Supplemental Data File 4 (see footnote 8) presents the complete analytical results for 40Ar/39Ar analyses as an Excel workbook.

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Island (Table 4; Nixon and Orr, 2007). The three couver Island samples, thus this gabbroic sill Magnetostratigraphy in the Newark rift basin reset ages of Nikolai basalts from the Amphi- may represent an earlier phase of magmatism. of eastern North America has provided a high- theater Mountains (169, 161, and 161 Ma) are The relatively narrow range of ages for Wran- resolution geomagnetic polarity time scale similar to ages of felsic plutonic rocks of the gellia fl ood basalts and associated intrusive that refl ects nearly idealized geomagnetic fi eld Early to Middle Jurassic Talkeetna arc, in close rocks indicates that the duration of the major- reversal behavior in the Late Triassic (Kent and proximity (<30 km) to the south (168–150 Ma; ity of the magmatic activity was likely <5 Myr, Olsen, 1999). A total of 59 polarity reversals Rioux et al., 2007). In the Amphitheater and occurring between ca. 230 and 225 Ma. occurred between 233 and 202 Ma; the mean Talkeetna Mountains, Schmidt et al. (2003a) duration of polarity intervals is estimated to reported that reset plagioclase 40Ar/39Ar ages Paleontological Age Constraints be 0.53 Myr; the longest polarity interval is (174–152 Ma) for eight Nikolai basalts and ~2 Myr and the shortest polarity interval is gabbroic rocks are similar to the crystallization Fossils in sedimentary strata directly underly- ~0.02 Myr (Kent and Olsen, 1999). All of the ages for these felsic plutonic rocks, which are ing and overlying the Wrangellia fl ood basalts recorded polarity intervals between 225 and assigned to the Peninsular terrane. A K-Ar iso- also provide constraints on the age and duration 230 Ma were at least ~0.5 Myr in duration. chron age of 112 ± 11 Ma was determined from of volcanism. MacKevett et al. (1964) reported Based on this, the Wrangellia basalts likely seven Nikolai basalts in the Wrangell Moun- that shale directly beneath Nikolai basalts on erupted within a period of only 2 Myr and pos- tains, indicating resetting of K-Ar systematics Golden Horn Peak in the Wrangell Mountains sibly in a considerably shorter time span. during tectonism related to northward transport contained abundant Middle Triassic index of Wrangellia (MacKevett, 1978; Plafker et al., fossils identifi ed as Daonella frami Kittl. In DISCUSSION 1989). The hornblende 40Ar/39Ar ages are coin- 1971, a mineral exploration party discovered a cident with regional magmatic events reported sedimentary layer (<2 m thick) of fi ssile black Comparison of Northern and Southern from other studies in the areas in which these shale between sills of Karmutsen basalts on Wrangellia samples were collected. Two Late Jurassic ages Mount Schoen on Vancouver Island that con- of 148.8 ± 0.83 and 149.9 ± 0.93 Ma from mafi c tained imprints of Daonella tyrolensis (Carlisle, The stratigraphy and age of different areas dikes in the Wrangell Mountains correspond 1972). As part of the present study, samples of of the Wrangellia oceanic plateau provide with ages of Late Jurassic plutons of the Chi- Daonella were collected from Golden Horn constraints on the construction of the volcanic tina arc (140–160 Ma; most ages between 145 Peak in Alaska and on Mount Schoen on Van- stratigraphy, the paleoenvironments existing at and 150 Ma), ages that are synchronous with a couver Island. The Daonella specimens from the time of eruption, and the duration of volca- major regional orogeny related to subduction both localities appear to have similar forms nism. To aid in the following discussion, a sum- (Grantz et al., 1966; MacKevett, 1978; Hudson, and are closely related to Daonella frami (C.A. mary of observed and previously reported fi eld 1983; Dodds and Campbell, 1988; Plafker et al., McRoberts, 2006, personal commun.). The relationships of Wrangellia fl ood basalts, and 1989; Roeske et al., 2003). Daonella specimens appear older than Upper the prevolcanic and postvolcanic rock record, In summary, the ages of Wrangellia fl ood Ladinian forms, and are likely of middle Ladin- is presented in Table 5, and a compilation of basalts and intrusive rocks with associated ana- ian age (Poseidon Zone, ca. 235–232 Ma; C.A. ages and biostratigraphy for Paleozoic– Triassic lytical errors that are <±10 Myr (n = 9) are in McRoberts, 2006, personal commun.). Recov- rocks of Wrangellia is presented in Figure 8 and the range from 222 to 233 Ma (Fig. 8 inset; ery of the conodont Neospathodus from these Supplemental Data File 59. We establish the Tables 3 and 4). The three U-Pb ages from mafi c Daonella beds on Vancouver Island agrees with similarities and differences in the stratigraphy of sills from Vancouver Island are within error of this age. Northern and Southern Wrangellia and use them the four 40Ar/39Ar ages of basalts from Alaska in subsequent sections to interpret the eruption that meet this precision criterion, as well as the Magnetostratigraphic Age Constraints environments and accumulation subsidence 40Ar/39Ar plateau age of 227.5 ± 1.2 Ma given histories of different parts of this vast accreted by biotite from a peridotite in this study. The Magnetic reversals were common through- oceanic plateau. slightly older age of 232.2 ± 1 Ma for a gabbro out the Late Triassic; however, only a single The basement of Wrangellia has different age from Yukon (Mortensen and Hulbert, 1991) is magnetic polarity reversal is preserved in the strata in Alaska and Yukon than on Vancouver the only age that is outside the range for the Van- Wrangellia fl ood basalts (Hillhouse, 1977). Island (Fig. 8, Supplemental Data Files 610 and

9Supplementary Data File 5. Wrangellia ages and biostratigraphy. This Microsoft Excel ﬁ le contains three worksheets. (1) The Age worksheet contains ~750 isotopic ages for units assigned to Wrangellia. Most of this data was extracted from the CordAge 2004 database and was supplemented by ~50 ages not in the CordAge database, mostly from Alaska. Samples of the Coast and Insular Belts are included, based on their location, and caution should be used in deciding whether samples are actually located within Wrangellia. CordAge 2004 (a database of isotopic age determinations for rock units from the Canadian Cordillera) is an MS-Access based database, consisting of the merged datasets of the publically available products BCAge 2004A-1 (released October 2004; Breitsprecher and Mortensen (2004a)) and YukonAge 2004 (released July, 2004; Breitsprecher and Mortensen (2004b)). The compilation contains all reported non-proprietary isotopic age determinations for bedrock units from British Columbia and Yukon Territory, respectively: 9321 age determinations from 5997 rock samples, summarizing 778 published articles, theses, reports or unpublished sources. Katrin Breitsprecher offered assistance with extracting this information. The user is referred to Breitsprecher and Mortensen (2004a, 2004b) for information about the rating system of ages (Rel_rating column; column H) and other details. The database should not be cited as the source of the age. Age determinations should be cited to the original source, which is provided in each record of the database. (2) The Age_refs worksheet contains the information of the references for the age compilation listed in the Age worksheet (described above). The ref no column (column A) in the Age_refs worksheet refers to the Ref No column (column L) in the Age worksheet. (3) The Biostratigraphy worksheet is a compilation of 75 fossil age determinations from published literature related to Wrangellia. The full reference where each determination is published is included in column B and the region is shown in column E. All of the age ranges are plotted in Figure 8 according to the region indicated in column E. The low and high ages for each age range use the age boundaries for epochs and stages from Gradstein et al. (2004). Age boundaries for the Triassic are from Ogg (2004) and revised based on Furin et al. (2006). If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://dx.doi.org/10.1130/GES00212.S9 or the full-text article on www.gsapubs.org to view Supplemental Data File 5.

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ably older (ca. 380–355 Ma) than dated volcanic lenses are common in the upper parts of the Kar- Figure 8. Summary of ages and biostratigra- sequences in Alaska (ca. 312–280 Ma, mostly mutsen Formation and the overlying limestone phy in fi ve areas of Wrangellia. Radiometric from the Wrangell Mountains; see Fig. 8 and contains late Carnian–Norian fossils. Within ages are white circles. Fossil ages are colored references therein). Paleozoic limestone beneath upper Nikolai Formation stratigraphy in North- according to the formation they are found Wrangellia basalts in Alaska contains Early ern Wrangellia, interfl ow sedimentary lenses in the stratigraphic column. Circles that are Permian bryozoans, brachiopods, foraminifera, occur in southwest Yukon and the Clearwater multicolored green and blue indicate fos- and corals (Smith and MacKevett, 1970). On Mountains, and Nikolai basalts are overlain by sils from interfl ow sedimentary lenses, and Vancouver Island, conodonts in the Buttle Lake limestone with age-diagnostic late Carnian– other multicolored circles indicate fossils Group indicate Mississippian–Permian ages early Norian fossils. Sedimentary strata extend found near the contact between two for- (Orchard fi de Brandon et al., 1986; Orchard, up through the Triassic-Jurassic boundary in mations. Information for age data and bio- 1986; Fig. 8; Supplemental Data File 5 [see Northern and Southern Wrangellia. The inter- stratigraphy are presented in Supplemental footnote 9]). fl ow limestone lenses and overlying sedimen- Data File 5 (see footnote 9). Age probabil- The volcanic stratigraphy on Vancouver tary rocks are important time and depth mark- ity density distribution plots for each area Island consists of a tripartite succession of sub- ers for estimating the subsidence and mantle are calculated from the plotted ages using marine fl ows (50%–60% of total thickness), thermal history of Wrangellia (see following AgeDisplay (Sircombe, 2004). The ages for volcaniclastics, and subaerial fl ows, whereas discussion). Based on the similar age, high effu- the period boundaries are from Gradstein volcanic stratigraphy in Alaska and Yukon is sion rate, tectonic history, and trace element and et al. (2004). Ages for epoch boundaries of predominantly subaerial fl ows (>90%; Table 5; isotopic geochemistry of Wrangellia basalts the Triassic are adjusted using Furin et al. Greene et al., 2008, 2009a, 2009b). In areas of (Greene et al., 2008, 2009a, 2009b), the fl ood (2006). Inset is a summary diagram showing central Vancouver Island, the stratigraphically basalts of Northern and Southern Wrangellia 40Ar/39Ar and U-Pb ages of Wrangellia fl ood lowest pillow basalts were emplaced on uncon- are interpreted to have formed from the same basalts and plutonic rocks with analytical solidated fi ne-grained sediments, and mafi c sills plume-related magmatic event. uncertainty <10 Myr total. The results are intrude marine strata with Daonella beds. In shown from north (top) to south (bottom) other areas of central and southern Vancouver Eruption Environment for Wrangellia and are distinguished by region. Analyti- Island, basal basalt fl ows overlie Permian lime- Flood Basalts cal details are presented in Tables 3 and 4. stone and sills intrude this limestone (Fig. 6). Two schemes are shown for the boundar- The submarine section on Vancouver Island Northern Wrangellia ies of part of the Triassic, from Furin et al. (~3000 m) is substantially thicker than in the The Nikolai basalts in Alaska were emplaced (2006) and Ogg (2004). Errors are 2σ. Gray Alaska Range (<800 m). In the Alaska Range, as effusive eruptions in a shallow marine and fi elds in the inset and main diagram indicate the stratigraphically lowest submarine fl ows subaerial environment (Fig. 9). Sedimentary 230–225 Ma. were emplaced on nonfossiliferous black shale. rocks directly beneath the fl ood basalts in the In the Wrangell Mountains, a basal fl ow con- Alaska Range are mostly siliceous argillites, glomerate directly overlies shale with Daonella carbonaceous black shales, and mudstones; the 711). The basement of Wrangellia was originally beds and contains erosional remnants from the upper part of the 200–250 m sequence has a defi ned as a Pennsylvanian–Permian volcanic underlying Paleozoic units. Laterally discon- higher proportion of black carbonaceous shale arc sequence that may have been deposited on tinuous zones of conglomerate along the base and carbonate (Blodgett, 2002). In this area, oceanic crust (Jones et al., 1977), and this has of the Nikolai Formation in Yukon have been Blodgett (2002) interpreted the total absence been maintained by numerous authors (Smith interpreted as syntectonic deposits and fl ows of fossils and biogenic sedimentary structures and MacKevett, 1970; MacKevett, 1978; Coney related to the formation of grabens (Israel et al., (trace fossils), and the even, parallel laminations et al., 1980; Monger et al., 1982; Saleeby, 1983; 2006). The fl ood basalts in Northern Wrangellia (indicating lack of bioturbation), as indicative Beard and Barker, 1989). However, whereas are low-Ti basalts (<400 m) overlain by high-Ti of deposition in a starved, anoxic shallow sub- Pennsylvanian–Permian volcanic arcs are pre- basalts, whereas almost all of the basalt stratig- marine environment. The higher proportion of served in Alaska (Smith and MacKevett, 1970; raphy in Southern Wrangellia is high-Ti basalt black carbonaceous shale and calcareous com- MacKevett, 1978; Beard and Barker, 1989), and includes an area 30 km in diameter with ponent for sediments higher in the sequence older Paleozoic rocks make up much of Vancou- abundant picritic pillow basalts (Greene et al., may indicate a shallower depositional environ- ver Island (Muller, 1977; Brandon et al., 1986), 2008, 2009a, 2009b). ment for the younger sediments than for the and there is no signifi cant arc volcanism in the The overlying limestone and interfl ow older sediments, possibly due to uplift prior to Pennsylvanian–Permian of Vancouver Island sedimentary lenses in Northern and Southern eruption (Blodgett, 2002). The pillow basalts in (Fig. 8; Yole, 1969; Massey and Friday, 1988; Wrangellia are lithologically similar and have the Alaska Range (~500 m) are highly vesicu- Yorath et al., 1999). The Paleozoic volcanic a similar range of fossil ages (Table 5; Fig. 8). lar, consistent with eruption in shallow water sequences on Vancouver Island are consider- In Southern Wrangellia, interfl ow sedimentary (Jones, 1969; Kokelaar, 1986). Volcaniclastic

10Supplemental Data File 6. Previous research on Wrangellia. This Microsoft Word ﬁ le is a summary of previous research on parts of the Wrangellia terrane in Alas- ka, Yukon, and British Columbia that includes many references. These references and others are listed in Supplemental Data Files 1 and 2. If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://dx.doi.org/10.1130/GES00212.S10 or the full-text article on www.gsapubs.org to view Supplemental Data File 6.

11Supplemental Data File 7. Stratigraphy of Wrangellia. This Microsoft Word fi le presents a detailed description of the stratigraphy of Wrangellia. See the Supple- mentary Figure File for nine fi gures that accompany this summary of the stratigraphy. If you are viewing the PDF of this paper or reading it offl ine, please visit http:// dx.doi.org/10.1130/GES00212.S11 or the full-text article on www.gsapubs.org to view Supplemental Data File 7.

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TABLE 5. COMPARISON OF GEOLOGY AND AGES OF NORTHERN AND SOUTHERN WRANGELLIA Northern Wrangellia Southern Wrangellia Rocks overlying the Nikolai Formation Rocks overlying the Karmutsen Formation Clastic sedimentary sequences above Nikolai span Triassic- Clastic sedimentary sequences above Karmutsen span Jurassic boundary Triassic-Jurassic boundary Late Carnian to early Norian fossils in overlying limestone Late Carnian to Early Norian fossils in overlying limestone Overlying limestone grades upward from shallow water to Overlying limestone grades upward from shallow water to deeper water facies deeper water facies Nikolai basalts interbedded with limestone and argillite Limestone and sedimentary rocks of the Kunga and (Yukon) Maude Groups on Haida Gwaii Kunga Formation contains identical fossils to Quatsino Limestone is commonly brecciated near the base limestone Karmutsen basalt, or younger basalt, locally intrudes the Quatsino limestone Thin layer of siltstone in several locations along Nikolai- Mostly micritic limestone directly overlying the basalts Chitistone contact Occurrences of regolith between the top of the Nikolai and Thin (<25 cm) layer of siltstone immediately overlying the base of the Chitistone basalt in several locations Little evidence of erosion between Karmutsen and Quatsino limestone Nikolai Formation Karmutsen Formation 4 U-Pb and Ar-Ar ages (uncertainty <±10 Myr) of 225– 3 U-Pb ages (uncertainty <±10 Myr) of 226–228 Ma 230 Ma (with one Yukon outlier) Late Carnian to early Norian fossils in limestone lenses in Regolith above uppermost flow Yukon Trachytic-textured plagioclase-rich flows near top of Nikolai Intraflow limestone and/or clastic lenses near top formed in Wrangell Mtns in shallow submarine areas Late Carnian to early Norian fossils in sedimentary lenses Predominantly massive subaerial flows within upper Karmutsen Plagioclase-megacrystic flows near the top of the basalt succession Alaska Range (Amphitheater and Clearwater Mountains) ~7% of lowest part of stratigraphy is pillow basalt (<500 m), Vancouver Island and ~3000 m subaerial flows Pillows tend to have mostly small diameter (<1 m) Flood basalts form emergent basalt sequence Pillows have abundant vesicles (20–30 vol%) indicating Subaerial stage—subaerial flows (>2500 m on CVI, eruption in shallow water (<800 m) <1500 m on NVI) Vesicles in pillows are spherical (<1 mm) and evenly Shallower water stage—pillow breccia and/or hyaloclastite distributed throughout pillows (400–1500 m on NVI) Deeper water stage—pillowed and massive basalt flows Megapillows rare in Alaska (>2500 m) Minor amounts of breccia and tuff within submarine section Interbedded limestone and/or clastic near top of Nikolai in Pillow basalts erupted in deeper marine setting than in Clearwater Mtns and in Yukon Northern Wrangellia Thick sediment-sill complex Most pillows are commonly large diameter (>1 m) Large area of complementary plutonic rocks represent Pillows are more commonly nonvesicular or have radially feeder system for flood basalts oriented pipe vesicles No sheeted dike complexes, sill-dominated feeder system Megapillows (>4 m) within pillowed flows on VI Volcaniclastic rocks formed primarily via cooling- Rare dikes contraction granulation Black shale within sediment-sill complex, nonfossiliferous Volcaniclastics mostly between pillow basalt and (starved, anoxic environment) subaerial flow units Picritic pillow lava near base of submarine section in Peperites in submarine section Clearwater Mtns Pillow basalt engulfs fine-grained sediment in lowermost Subaerial flow unit is conformable with the underlying pillowed flows hyaloclastite unit Picritic tuff found outside the main area of flood basalts (possibly related to Nikolai basalts) (continued)

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TABLE 5. COMPARISON OF GEOLOGY AND AGES OF NORTHERN AND SOUTHERN WRANGELLIA (continued) Northern Wrangellia Southern Wrangellia Rare occurrences of interflow sediments, aside from Wrangell Mountains and Kluane Ranges lenses between upper flows Occurrences of regolith between top of Nikolai and overlying Columnar jointing is rare, irregular jointing in massive Chitistone limestone submarine flows Rare thin argillite (<10 cm) on top of uppermost flow Rare dikes, no sheeted dike complexes Intraflow limestone lenses (1–30 m thick) near top formed in Thick sediment-sill complexes where base is exposed shallow submarine areas on VI Plagioclase-megacrystic flows near the top of the basalt Increase in vesicularity and proportion of volcaniclastics succession upwards in submarine unit Almost no pillow basalt in basalt stratigraphy (only in Pahoehoe structures (e.g., ropy festoons) within flows lowermost ~70 m) and just below Quatsino limestone Pillow breccia and polymictic flow-conglomerate in Piciritc pillow lavas in upper part of the submarine section lowermost ~70 m on NVI Basal flow-congolmerate directly overlies argillite with fissile Marine fossils in pillow breccia, Buttle Lake area Daonella beds Abundant rounded clasts derived from underlying Paleozoic Pillow basalt engulfs fine-grained sediment in lowermost formations pillowed flows Larger pendants (>100 m long) of Paleozoic rocks in Submarine basalts form volumetrically minor part of the lowermost flows subaerial stratigraphy No clear indication of uplift prior to eruption on VI ~3500 m of subaerial sheet flows; columnar jointing is rare Queen Charlotte Islands (Haida Gwaii) Very rare occurrences of interflow sediments, other than One measured section (~4300 m): 95% submarine, pillow lenses in upper part to fragmental ratio (8:2) Quartz-pebble conglomerate in sediments beneath Nikolai in Local tuffaceous crinoidal limestone lenses in lowermost Talkeetna Mtns flows Rocks underlying the Nikolai Formation Rocks underlying the Karmutsen Formation Paleozoic arc sequences not exposed in Talkeetna Mtns Permian chert, carbonate, and volcaniclastic rocks form beneath Nikolai deepest level of exposure Wrangell Mountains and Kluane Ranges Evidence of erosional surface beneath Nikolai, rounded Daonella imprints in shale within sediment-sill complex on clasts from underlying sequences Mount Schoen, VI Rounded pebble- to cobble-size clasts indicate subaerial or Daonella imply dysoxic bottom waters typical of mud- shallow submarine reworking dwellers Indication of uplift prior to eruption in Wrangell Mountains Coarse-grained mafic intrusions intrude and deform Coarse-grained mafic rocks intrude and deform underlying sedimentary sequences underlying sedimentary sequences Daonella imprints in fissile shale underlying lowermost pillow Mafic sills intrude siliceous argillite, shale, chert, and breccia limestone Erosional unconformity between Daonella beds and underlying Paleozoic limestone Formation of laterally discontinuous grabens along base in Kluane Ranges Paleozoic sequences mostly exposed in two anticlinoria Alaska Range (Amphitheater and Clearwater Mountains) on central and southern VI Pre-Nikolai sediments grade upward from siliceous argillite to carbonaceous black shale Pre-Nikolai black shale deposited in starved, anoxic marine Conodonts indicate Mississippian to Permian ages in the setting Buttle Lake Group No bioturbation (parallel laminations) and no biogenic structures (trace fossils) Coarse-grained mafic rocks intrude and deform underlying Paleozoic arc sequences underlying Karmutsen basalts sedimentary sequences with ages of 380–355 Ma Limestone (with Early Permian fossils), chert, argillite Oldest known rocks on Vancouver Island are the underlying Nikolai basalts Devonian Duck Lake Formation Paleozoic arc sequences underlying Nikolai basalts with ages of 312–280 Ma No rocks dated older than Pennsylvanian in Wrangellia in Alaska Note: References not included in this table, see supplementary data files for references.VI—Vancouver Island; CVI— central Vancouver Island; NVI—northern Vancouver Island.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/1/47/3337977/47.pdf by guest on 29 September 2021 Greene et al. ows and is not shown in a separate diagram fl depict the eruption environment and growth of and growth depict the eruption environment te downward progression of time sequence in dia- te downward progression ood basalts are overlain by Late Triassic limestone. Triassic overlain by Late ood basalts are c age and biostratigraphic information on units and Table 5 for detailed descriptions of 5 for Table c age and biostratigraphic information on units ows emplaced onto pelagic sediments and all of the fl ood basalts on Vancouver Island and in Alaska and Yukon. Prevolcanic and postvolcanic sedimentary sequences underlying Prevolcanic Yukon. Alaska and Island and in Vancouver ood basalts on eld relationships. eld for space consideration. Most of the units in the sketches are labeled, so there is no accompanying legend. White arrows indica White arrows is no accompanying legend. labeled, so there space consideration. Most of the units in sketches are for initiated with submarine fl activity in each of the areas Volcanic grams. Figure 9. Interpretative diagrams showing the construction of the Wrangellia oceanic plateau. From top to bottom, the diagrams oceanic plateau. From Wrangellia diagrams showing the construction of 9. Interpretative Figure the volcanic stratigraphy for areas of Wrangellia fl Wrangellia of areas the volcanic stratigraphy for Island is indicated by subaerial Vancouver The emergent stage of the Karmutsen Formation on also shown. units are Paleozoic arc See interpretation of eruption environments for further discussion. See Figure 8 for specifi 8 for discussion. See Figure further for of eruption environments See interpretation fi

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fl ows intercalated with pillow basalt also indi- lowed fl ows are interconnected tubes and lobes vesicularity with decreasing depth in the subma- cate eruption in shallow water; this transition that contain large-diameter pillows (>2 m) and rine lava fl ows (Garcia et al., 2007). Intrusions typically occurs in <200 m water depth for tho- have low abundances of amygdules. The mas- are more common in the lower parts of the stra- leiitic magmas (Kokelaar, 1986). The subaerial sive fl ows may represent some of the master tigraphy within both submarine sections and, fl ows (>3000 m) were emplaced as infl ated tubes for delivery to distal parts of fl ow fi elds or although more diffi cult to identify, they appear compound pahoehoe fl ow fi elds during pro- locally increased effusive rates, due to topogra- to be less common within the subaerial sections. longed, episodic eruptions similar to those in phy, as evinced by concave basal contacts. The In contrast, fl ow morphologies and thicknesses most continental fl ood basalts (e.g., Self et al., basalts increase in vesicularity, as does the pro- are distinct in Karmutsen stratigraphy and 1997). The fl ows erupted from a limited number portion of volcaniclastics, upward in the subma- Hawaiian volcanoes. of eruption sites that are rarely observed, except rine stratigraphy (Nixon et al., 2008). for a large sill-dominated eruptive center in the The shallow-water stage of the Karmutsen Accumulation and Subsidence of the Amphitheater Mountains. preserves an increasing proportion of volcani- Wrangellia Basalts In the Wrangell Mountains and Yukon, the clastic units (pillow breccia and hyaloclastite) basal fl ow conglomerate and pillow brec- conformably overlying mostly close-packed pil- The geology, age, and biostratigraphy of cia (<100 m thick) erupted in shallow water lows. The pillow breccias are commonly associ- Wrangellia can be used to estimate the rate of (<200 m; Fig. 9). Rounded clasts in the basal ated with pillowed fl ows and contain aquagene accumulation of Wrangellia basalts and the fl ow conglomerate indicate an area of relief near tuff (Carlisle, 1963) or redeposited hyaloclas- subsidence of the Wrangellia oceanic plateau. sea level in the Wrangell Mountains. Above tite. The transition from close-packed pillowed Carlisle and Suzuki (1974) originally estimated the basal fl ow unit in the Wrangell Mountains fl ows to pillow breccia and hyaloclastite proba- an accumulation rate for the Karmutsen basalts are ~3500 m of subaerial sheet fl ows that lack bly occurred in <500 m water depth; however, in of 0.17–0.27 cm/yr over 2.5–3.5 Myr. This features of submarine emplacement. The pro- certain areas on northern Vancouver Island the yielded a total erupted volume of basalt of 3.7– portion of amygdules in the massive fl ows is volcaniclastic unit is >1500 m thick (Nixon et 4.0 × 105 km3 (they assumed an area 400 km × variable, but generally high, similar to many al., 2008). Sedimentary structures (e.g., graded 150 km, roughly the size of Vancouver Island, continental fl ood basalts. In contrast, submarine bedding, fl uidization structures) are present or 60 × 103 km2, and a stratigraphic thick- sheet fl ows exposed in accreted portions of the locally and indicate resedimentation processes. ness of 6 km) and a volumetric output rate of Ontong Java Plateau in the Solomon Islands of Pyroclastic deposits containing lapilli tuff 0.10–0.16 km3/yr (Carlisle and Suzuki, 1974). the western Pacifi c Ocean rarely have vesicles and volcanic bombs do not appear to be com- The area of exposure of Karmutsen basalts in (Petterson, 2004); pillowed and massive fl ows mon (Carlisle, 1963). The volcaniclastic rocks this study was calculated using digital geology are preserved throughout the stratigraphy of the likely formed primarily via cooling-contraction maps for exposures of the Karmutsen Formation Ontong Java Plateau. granulation, magma-water-steam interaction, on Vancouver Island and the Queen Charlotte autobrecciation, and mass wasting, rather than Islands, and represents a minimum estimate of Southern Wrangellia pyroclastic eruption. surface area. The estimated total erupted vol- The tripartite Karmutsen stratigraphy on The emergent subaerial stage is marked by ume and volumetric output rate were calculated Vancouver Island formed as an emergent the relative absence of volcaniclastic and pil- using a stratigraphic thickness of 6 km and basalt sequence that passed through a deeper lowed fl ow units and dominance of massive duration of volcanism of 5 Myr. Our estimate of water, shallow water, and subaerial stage, simi- amygdaloidal sheet fl ows. The sheet fl ows were minimum volcanic output rate is ~0.03 km3/yr lar in ways to those described in the forma- emplaced as infl ated compound pahoehoe fl ow and that of minimum total erupted volume of tion of emergent seamounts (Fig. 9; Schmidt fi elds atop an enormous oceanic plateau. There Karmutsen basalts is 1.4–1.5 × 105 km3. Even and Schmicke, 2000) and Hawaiian volcanoes are isolated sections of submarine fl ood basalts using this very conservative estimate for the (Garcia et al., 2007). Sills obscure relation- (<200 m thick) within the uppermost subaerial area of exposure and age, the volumetric output ships at the sediment-basalt interface at the Karmutsen stratigraphy (Surdam, 1967; Carlisle rate is comparable to recent estimates of long- base of the Karmutsen Formation, and there and Suzuki, 1974); these units form a volumetri- term volumetric eruption rates for ocean islands may have been sediments on top that are no cally minor component of the subaerial stratig- such as Iceland (0.02–0.04 km3/yr) and Hawaii longer preserved, but most of the fi ne-grained raphy. The intra-Karmtusen sedimentary lenses (0.02–0.08 km3/yr) (White et al., 2006). strata underlying the Karmutsen were deposited formed in isolated, low-lying areas in a predom- Subsidence of the Karmutsen basalts dur- below storm wave base. Carlisle (1972) reported inantly subaerially exposed plateau (Carlisle ing volcanism was recorded by the deposition that pre-Karmutsen sediments show a progres- and Suzuki, 1974). of interfl ow sedimentary lenses between the sive change from coarse bioclastic limestone There are similarities between the stratigra- upper fl ows during the waning stages of vol- to laminated and silicifi ed shale, indicating a phy of Karmutsen basalts and the stratigraphy canism as low-lying areas of the plateau were transition from an organic-rich, shallow-water of the Hawaii Scientifi c Drilling Project core submerged. The occurrence of interfl ow lenses environment to a starved, pelagic deeper water (HSDP2; Garcia et al., 2007). In both the Kar- indicates that by the end of Karmutsen fl ood depositional environment prior to initiation of mutsen basalts and HSDP2, predominantly pil- volcanism most of the top of the basalt plateau volcanism. Daonella fossils in fi ne black shale lowed fl ows in the lower parts of the submarine had subsided and submerged below sea level. from near the top of this unit imply dysoxic bot- stratigraphy give way to increasing proportions This implies that over the duration of subaerial tom waters typical of mud dwellers that fl oat on of volcaniclastic units upsection, below the volcanism, the rate of accumulation of basalt soupy sediments (Schatz, 2005). submarine-subaerial transition. Vesiculated pil- fl ows was comparable to the rate of subsidence. The deeper water stage (>200 m water depth) lows of the Karmutsen Formation on Northern Carbonate deposition was preserved during the of the Karmutsen Formation was dominated Vancouver Island are more common near the waning stage of volcanism, and there was no by effusive activity that formed pillowed and top of the pillow unit and in the hyaloclastite signifi cant break after volcanism ceased. There massive fl ows (Greene et al., 2009). The pil- unit. HSDP2 drill core shows an increase in are few signs of erosion between the Quatsino

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limestone and Karmutsen Formation (in places basaltic volcanism and the end of the Triassic IMPLICATIONS FOR THE only a <25 cm thick siltstone and/or sandstone (~25 Myr), ~2000 m of shallow- to deep-water ARCHITECTURE OF OCEANIC layer is preserved), and interfl ow lenses have marine sediments accumulated on top of the PLATEAUS fossils identical to those of the lower part of the Nikolai basalts. Neglecting sediment compac- Quatsino limestone. tion, this indicates a minimum subsidence rate of The architecture of the erupted products Postvolcanic subsidence of the Wrangel- ~80 m/Myr. This subsidence decreased sub- of oceanic plateaus provides one of the most lia oceanic plateau is recorded by hundreds of stantially, to <20 m/Myr, in the Early Jurassic direct ways of evaluating the largest magmatic meters to >1000 m of Late Triassic marine sedi- (Saltus et al., 2007). Initial subsidence rates of events on Earth. Construction of oceanic pla- mentary rocks overlying the basalt stratigraphy. 50 m/Myr were also estimated by Richards et teaus has implications for the age and duration The Quatsino limestone was deposited on top al. (1991) based on the thickness and age of car- of magmatism, the association of plume- or of the plateau as it began to submerge beneath bonate rocks overlying the Nikolai Formation. plate-controlled processes, uplift and subsid- sea level, and while volcanism waned. Depo- Several oceanic plateaus and ocean islands ence history related to large magmatic events, sition continued as the plateau became fully worldwide preserve evidence of rapid subsid- mantle melting processes and thermal evolu- submerged and the sea transgressed over the ence after their formation (Detrick et al., 1977), tion, and the effect of eruptions on the environ- entire plateau. Initially, intertidal to supratidal and mantle plume models predict subsidence ment (e.g., Ito and Clift, 1998; Wignall, 2001; limestones were deposited in shallow-water, following the formation of oceanic plateaus Sager, 2005; Saunders et al., 2007). The Wran- high-energy areas, some of the limestones (e.g., Campbell and Griffi ths, 1990). Subsid- gellia plateau provides an unmatched view of refl ecting quieter, subtidal conditions (Carlisle ence may result from dispersion of the mantle the preeruption, syneruption, and posteruption and Suzuki, 1974). Upsection, the Quatsino For- buoyancy anomaly above a plume; decay of the phases of the growth of plume-related oceanic mation refl ects a slightly deeper water deposi- thermal anomaly and cooling and contraction plateaus on Earth. Here we compare Wrangel- tional environment and abruptly grades into the of the lithosphere; removal of magma from the lia to other oceanic plateaus, and to continental overlying Parson Bay Formation (Carlisle and plume source, causing defl ation of the plume fl ood basalts and modern ocean island hotspots, Suzuki, 1974; Nixon and Orr, 2007). head; and/or depression of the surface from to highlight the conditions and processes that In Alaska and Yukon, the carbonate rocks loading of volcanic and plutonic material on the control the formation of oceanic plateaus. overlying the Nikolai basalts (>1000 m; Chiti- lithosphere (e.g., Detrick et al., 1977; Camp- Based on the stratigraphic and temporal con- stone and Nizina limestones) preserve a record bell and Griffi ths, 1990). Hawaiian volcanoes straints outlined in this study, and the petrology of the gradual submergence of the extensive undergo rapid subsidence during their growth and geochemistry of the Wrangellia basalts Nikolai basalt platform. There are only rare due to the response from loading of intrusive from Greene et al. (2008, 2009a, 2009b), the occurrences of thin (<0.5 m) weathered zones and extrusive magmas on the lithosphere (e.g., main factors that controlled the construction or discontinuous, intervening clastic deposits at Moore and Clague, 1992). Pacifi c Cretaceous of the Wrangellia oceanic plateau include high the top of ~3500 m of subaerial basalt fl ows in plateaus, such as Ontong Java, Manihiki, and effusion rates and the formation of extensive the Wrangell Mountains (Fig. 3; Armstrong et Shatsky Rise, underwent signifi cantly less compound fl ow fi elds from low-viscosity, high- al., 1969). The absence of signifi cant erosion subsidence than predicted by current models, temperature tholeiitic basalts, sill-dominated or deposition of clastic sediment and the age of with post–fl ood basalt subsidence compara- feeder systems, low volatile content, protracted the Chitistone limestone (Fig. 8) indicate only ble to that of normal ocean seafl oor (Ito and eruption (perhaps >10 yr for individual fl ow a brief interval of nondeposition between the Clift, 1998; Roberge et al., 2005). Drilling of fi elds), limited repose time between fl ows end of volcanism and carbonate deposition. Fol- the Kerguelen Plateau indicates that subaerial (absence of weathering, erosion, sedimenta- lowing volcanism, several cycles of shaley to fl ows, perhaps originally 1–2 km above sea tion), submarine versus subaerial emplacement, argillaceous limestone were deposited in a high- level, subsided below sea level and paleoenvi- and relative water depth (e.g., pillow basalt– energy, intertidal to supratidal (sahbka) environ- ronments of the overlying sediments changed volcaniclastic transition). Combined, these fac- ment, similar to the modern Persian Gulf, and from intertidal to pelagic (Coffi n, 1992; Frey et tors contributed to the production of 4–6 km form the lowest 100 m of the Chitistone lime- al., 2000; Wallace, 2002). Subsidence estimates of basalt stratigraphy with limited magnetic stone (Armstrong et al., 1969). Limey mud- for the Northern Kerguelen Plateau (Ocean reversals, a lack of fossil remains in associated stone and wackestone with abundant disinte- Drilling Program Leg 183, Site 1140) indicate sediments, and minimal evidence of eruptive grated shelly material (~300 m thick) indicate a that ~1700 m of subsidence occurred since explosivity. In Table 6, the major features of the gradual transition to low-energy shallow-water eruption of the basalts at 34 Ma (~50 m/Myr; Wrangellia, Ontong Java, and Caribbean oce- deposition with intermittent high-energy shoal- Wallace, 2002). The subsidence history of anic plateaus are compared. From this compari- ing deposition. The upper part of the Chitistone Wrangellia may be more fully reconstructed by son, several commonalities are evident, includ- and overlying Nizina limestones refl ect deeper future studies that estimate the depth of sedi- ing high effusive rates, sill-dominated feeder water deposition on a drowned carbonate plat- mentation and age from microfossils collected systems, largely submarine eruptions (but vary- form (Armstrong et al., 1969). Gray to black from sediments overlying the Karmutsen and ing amounts of subaerial eruptions), presence shale and chert of the overlying McCarthy For- Nikolai Formations. The well-preserved and of high-MgO lavas, absence of alkalic lavas mation represent submergence of the carbonate accessible sedimentary strata overlying the during the main eruptive phase, and relatively platform below the carbonate compensation accreted Wrangellia plateau has the potential short periods of repose. Short repose intervals depth (Armstrong et al., 1969). A thin tuff bed to affect our understanding of the thermal his- are indicated by the absence of weathering, ero- in the lower part of the McCarthy Formation has tory of oceanic plateaus from plume-controlled sion, sedimentation, and magnetic reversals, in been dated at 209.9 ± 0.07 Ma (J. Trop, 2006, magmatic events that are not associated with addition to age constraints. personal commun.), and the Triassic-Jurassic mid-ocean ridge processes, such as several Distinct phases of volcanism, characterized boundary is preserved in the upper McCar- oceanic plateaus in the Pacifi c (e.g., Shatsky by changes in the chemistry of the basalts, thy Formation (Fig. 8). Between the end of Rise; Sager, 2005). occur during the construction of some oceanic

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TABLE 6. SUMMARY OF THE ARCHITECTURE OF THREE MAJOR OCEANIC PLATEAUS Wrangellia Ontong Java Caribbean (overlying flood basalts) (overlying flood basalts) (overlying flood basalts) Vancouver Island and Alaska Primarily limestone, chert, shale (Late Triassic to Lower Aptian (Lower Cretaceous) to Late Cretaceous and Tertiary pelagic Early Jurassic) Miocene pelagic sediment (>1000 m) carbonate in Caribbean Sea Foraminifera-nannofossil ooze; rare shallow <2 km thick on VI, ~2 km thick in areas of Alaska Pelagic sediment deposits in low-energy, water fossils submarine environment above the CCD Overlying limestone grades upward from shallow Silicified limestone and chert less common to deeper water facies Late Carnian to early Nornian (Triassic) fossils in overlying limestone Thin (<25 cm) layer of siltstone immediately Nannofossil studies reveal six Black shales indicative of oceanic anoxia overlying basalt unconformities Little evidence of erosion between basalt and Cenomanian-Turonian boundary extinction overlying limestone Oceanic Anox ic Event 1a suggested to be event linked to plateau formation Triassic-Jurassic boundary preserved in shale in result of eruption of plateau basalts Alaska

(flood basalts) (flood basalts) (flood basalts) Multiple eruption phases, main initial Multiple eruption phases, main initial Single eruption phase (231–225 Ma) episode (122 ± 3 Ma) episode (93–89 Ma) ca. 90 Ma volcanism is minor Minor volcanism at 76–72 Ma High proportion of subaerial flows (~40% on VI, Low proportion of subaerial flows Low proportion of subaerial flows ~90% in Alaska) Very little sediment between flows (high eruption Very little sediment between flows (high Very little sediment between flows (high rate) eruption rate) eruption rate) Formed mostly in deep water (mostly below Initial eruptions in deep water (VI) Deep- and shallow-water eruptions CCD) Rare dikes, sills present on Malaita (1–50 m Rare dikes, sills abundant near base Rare dikes, sills? thick; Solomon Islands) Continental detritus absent Continental mate rial absent Continental material absent Predominantly tholeiitic basalt (no alkalic basalt Predominantly tholeiitic basalt (no alkalic Predominantly tholeiitic basalt (no alkalic or felsic lavas) basalt or felsic lavas) basalt or felsic lavas) High-MgO pillow lavas in submarine section High-MgO picritic lavas present High-MgO picritic lavas present Submarine flows mostly pillowed with lesser Submarine flows mostly pillowed with rare massive, jointed flows massive flows Rare plagioclase-megacrystic basalt in upper stratigraphy Evidence of erosion in shallow-marine Rare spheruloidal basalt Rare spheruloidal basalt environment Increasing vesicularity and volcaniclastics Volcaniclastics in Caribbean indicate Amygdules are rare upward in stratigraphy subaerial and/or shallow-marine eruptions Vancouver Island-Karmutsen Formation Rare 1–5 mm noncalcareous pelagic Coral fossils in volcanic tuffs within (~6000 m) sediment between flows Columbia basalts (shallow water) Local interflow limestone in upper stratigraphy Volcaniclastic sequences, oxidized Curaçao basalt stratigraphy >5 km thick horizons, wood fragments indicate (picrite lower in section, evolved basalt Upper—subaerial basalt flows (>2500 m on CVI, subaerial and/or shallow marine eruptions higher in succession, >2 km hyaloclastite) <1500 m on NVI) Middle—pillow breccia and hyaloclastite (400– Thin (<1 m) and thick (>50 m) flows Picritic and tholeiitic flows commonly 1500 m on NVI) separated by hyaloclastite on Curacao Keogh Lake picrite (pillow lavas, NVI) Columnar jointing rare in massive flows Stratigraphy (based on Kerr et al., 1998): Lower—pillowed and massive submarine flows Smooth upper surface capped by several Thin pelagic sediments (>2500 m) large seamounts Extrusive sections-homogeneous pillow Trap-like topography on Malaita lavas, intrusive sheets (sills) Heterogeneous picrite, komatiite, and basalt 95% flows <25 m thick, 50% flows 5–10 m Alaska-Nikolai Formation (~3500 m) flows thick Predominantly subaerial basalt flows Intrusive sections—isotropic gabbro and gabbronorite Pillow basalt (<800 m) in Alaska Range Layered gabbroic rocks Flow conglomerate, pillow breccia along base in Ultramafic rocks—dunite with bands of Wrangell Mtns (<100 m) (deepest level of exposure) pyroxenite and lherzolite (continued)

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TABLE 6. SUMMARY OF THE ARCHITECTURE OF THREE MAJOR OCEANIC PLATEAUS (continued) Wrangellia Ontong Java Caribbean (stratigraphy beneath basalts is not (stratigraphically beneath flood basalts) (stratigraphically beneath flood basalts) exposed on Malaita) Middle Triassic Daonella beds and mafic sills Seismic and magnetic surveys indicate: Mafic intrusions beneath the flood basalts Layered and isotropic gabbros and Silicified shale, chert, minor limestone Basalts erupted on oceanic crust, possibly ultramafic plutonic rocks exposed in near spreading ridge Columbia (cumulates from flood basalt Evidence of erosion (rounded clasts) of Estimated total thickness of extrusive and volcanism) sequences beneath Nikolai intrusives of ~33 km Mafic intrusions related to flood basalts in underlying sedimentary sequences Formation of laterally discontinuous grabens Basalts erupted on oceanic crust, possibly along base in Yukon near spreading ridge Estimated total thickness of extrusive and Vancouver Island intrusives of ~20 km Mississippian–Early Permian limestone and argillite (Buttle Lake Group) Devonian–Early Mississippian arc-derived volcanic rocks (Sicker Group) Mafic sills and intrusive suites Alaska (Skolai Group) Early Permian bioclastic limestone, chert, shale (Hasen Creek Fm.) Pennsylvannian–Early Permian arc-derived volcanics (Station Creek Fm.) (deepest level of exposure) (deepest level of exposure) Note: CCD is carbonate compensation depth. NVI, northern Vancouver Island; CVI, central Vancouver Island; Fm, Formation. See supplementary data files for references.

plateaus and many continental fl ood basalts. from basaltic sections sampled on Vancouver shield-like features that form from the lower In the latter, three main phases of volcanism Island and in Yukon and Alaska (Greene et al., volume eruptions on Hawaiian, and other hot- include initial low-volume transitional to alka- 2008, 2009a, 2009b), even for samples taken spot, volcanoes. The morphology of fl ows in lic eruptions, a main phase of large volume of from the uppermost stratigraphic sections just oceanic plateaus is also distinct from fl ows on tholeiitic fl ows that produced large fl ow fi elds below covering sedimentary rocks. Perhaps Hawaiian volcanoes. Oceanic plateaus consist from fi ssure eruptions, and a waning phase of parts of the Wrangellia plateau are missing or of thick (10–50 m), tabular and laterally exten- decreased-volume eruptions from widely dis- are not exposed, or Wrangellia may represent a sive fl ows extending tens of kilometers, whereas tributed, localized volcanic centers with higher purely tholeiitic large igneous province formed Hawaiian volcanoes form thin (<5 m) lobes with silica and/or alkali lavas and explosivity (Jer- during a single-stage (<5 Myr) interval of time. large local variation and features like volcanic ram and Widdowson, 2005). In contrast, oce- The high eruption rates, relatively short peri- cones, craters, and channels. Despite compara- anic plateaus appear to largely form during a ods of repose, and basaltic lava compositions bly high estimates of eruption rates for modern single, main tholeiitic phase, such as observed of the Wrangellia and other oceanic plateaus eruptions at ocean island hotpots over brief peri- for Wrangellia, although both the Ontong Java are comparable to those of continental fl ood ods (e.g., Laki, 1983–1984; Self et al., 1997), and Caribbean Plateaus reveal distinct phases of basalts, which have estimated eruption rates there is no modern analogue for the formation of late-stage volcanism (Table 6). The evolution of orders of magnitude greater than present-day submarine sections of oceanic plateaus. the Ontong Java, Manihiki, and Hikurangi pla- hotspots such as Hawaii (e.g., Thordarson and teaus (Fig. 1), which are perhaps dispersed parts Self, 1996; White et al., 2006). The volumetric CONCLUSIONS of a single giant oceanic plateau (Taylor, 2006), output of fl ow fi elds in continental fl ood basalts, show similarities with a main tholeiitic stage which is likely comparable for oceanic plateaus, The volcanic stratigraphy of the obducted (ca. 126–116 Ma) followed by late-stage vol- is on the order of hundreds to thousands of cubic Wrangellia fl ood basalts records the construc- canism lasting more than 30 Myr (Davy et al., meters per second, and individual fl ow fi elds tion of a major oceanic plateau and provides a 2008). The late-stage volcanism on Hikurangi persist for years to perhaps more than a decade rare view of the preeruption, syneruption, and and Manihiki includes seamounts and volcanic (although volumes of individual eruption events posteruption phases and paleoenvironments ridges that are mostly alkalic in composition, can be highly variable; Jerram, 2002). For of an oceanic plateau that originated from a whereas late-stage volcanism on Ontong Java example, the fl ow rate estimated for the Roza plume-related magmatic event not associated is mostly tholeiitic (although alkalic rocks are fl ow fi eld in the Columbia River Basalt Group with mid-ocean ridge–controlled processes. The present; Hoernle et al., 2008). To date, there is (~4000 m3/s) is orders of magnitude higher Wrangellia basalts are now exposed over ~27 × no evidence for a clear gap in volcanism in the than the fl ow of lava during the past 26 yr on 103 km2 in Alaska, Yukon, and British Columbia formation of the Wrangellia oceanic plateau, Kilauea Volcano on the island of Hawaii (Vancouver Island and Queen Charlotte Islands) although additional high-precision geochronol- (~2 m3/s; <6 m3/s; Heliker and Mattox, 2003). along the western edge of North America, ogy is required to better evaluate the onset and The higher sustained fl ow rates for the tholeiitic although the original extent was signifi cantly termination of magmatism. There is also no evi- basaltic lavas that dominated oceanic plateaus greater. Geochronologic, paleontological, and dence for, or trend toward, alkalic compositions produce compound fl ow fi elds as opposed to magnetostratigraphic constraints indicate that

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