Graham T. Nixon

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

The accreted Wrangellia flood basalts and associated sedimentary rocks that compose the prevolcanic and postvolcanic stratigraphy provide an unparalleled view of the architecture, eruptive environment, and accumulation and subsidence history of an oceanic plateau. This Triassic large igneous province extends for ∼2300 km in the Pacific Northwest of North America, from central Alaska and western Yukon (Nikolai Formation) to Vancouver Island (Karmutsen Formation), and contains exposures of submarine and subaerial volcanic rocks representing composite stratigraphic thicknesses of 3.5–6 km. Here we provide a model for the construction of the Wrangellia oceanic plateau using the following information and visualization tools: (1) stratigraphic summaries for different areas of Wrangellia; (2) new 40Ar/39Ar geochronology results; (3) compilation and assessment of geochronology and biostratigraphy for Wrangellia; (4) compiled digital geologic maps; (5) an online photographic archive of field relationships; and (6) a Google Earth file showing the mapped extent of Wrangellia flood basalts and linked field photographs. Based on combined radiometric (U-Pb, 40Ar/39Ar, K-Ar), paleontological, and magnetostratigraphic age constraints, the Wrangellia flood basalts were emplaced during a single phase of tholeiitic volcanism ca. 230–225 Ma, and possibly within as few as 2 Myr, onto preexisting submerged arc crust. There are distinct differences in volcanic stratigraphy and basement composition between Northern and Southern Wrangellia. On Vancouver Island, ∼6 km of high-Ti basalts, with minor amounts of picrites, record an emergent sequence of pillow basalt, pillow breccia and hyaloclastite, and subaerial flows that overlie Devonian–Mississippian (ca. 380–355 Ma) island arc rocks and Mississippian–Permian marine sedimentary strata. In contrast, Alaska and Yukon contain 1–3.5-km-thick sequences of mostly subaerial high-Ti basalt flows, with low-Ti basalt and submarine pillow basalts in the lowest parts of the stratigraphy, that overlie Pennsylvanian–Permian (312–280 Ma) volcanic and sedimentary rocks. Subsidence of the entire plateau occurred during and after volcanism, based on late-stage interflow sedimentary lenses in the upper stratigraphic levels and the presence of hundreds of meters to >1000 m of overlying marine sedimentary rocks, predominantly limestone. The main factors that controlled the resulting volcanic architecture of the Wrangellia oceanic plateau include high effusion rates and the formation of extensive compound flow fields from low-viscosity, high-temperature tholeiitic basalts, sill-dominated feeder systems, limited repose time between flows (absence of weathering, erosion, sedimentation), submarine versus subaerial emplacement, and relative water depth (e.g., pillow basalt–volcaniclastic transition).