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

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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. For permission to copy, contact [email protected] 47 © 2009 Geological Society of America Greene et al. 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 erup- tion 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
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