Search

New data on Upper Triassic rocks in the eastern Coast Mountains show that it is Stikinia, not Wrangellia, that lies along the eastern margin of the Coast Plutonic Complex, at least as far south as latitude 51°N. These rocks constitute the upper Carnian–lower Norian Mt. Moore formation and the upper Norian Mosley formation. Clinopyroxene-phyric basaltic to andesitic breccia with lesser volcanic sandstone and rare carbonate compose the Mt. Moore formation. The Mosley formation comprises mafic volcaniclastic rocks and limestone. Correlation of these formations with Stikinia is based on similarities in age, stratigraphy, lithology, basalt geochemistry, and inferred tectonic setting.Recognition of Upper Triassic arc-related rocks of the Cadwallader terrane east of its previously known extent indicates that the Cadwallader terrane, rather than Stikinia, underlies much of the southern Intermontane Belt. The revised terrane distribution shows that Stikinia lay west of both the Cadwallader and Bridge River terranes prior to Cretaceous and Tertiary faulting. This configuration supports the idea that the Cadwallader and Stikine terranes represent fragments of a single early Mesozoic arc that was accreted during final closure of the Cache Creek – Bridge River ocean in Middle Jurassic time.

Several lower Mesozoic, fault-bounded units separate the Intermontane and Insular superterranes in southwestern British Columbia. Detailed study of one of these Mesozoic units, the Cadwallader Group, helps clarify the boundary between the superterranes and establish the tectonic evolution of southwestern British Columbia. The Cadwallader Group is the oldest unit in an Upper Triassic through Middle Jurassic volcanic and sedimentary tectono-stratigraphic terrane. Two formations, the Pioneer and the Hurley, compose the Cadwallader Group; the previously recognized Noel Formation is no longer considered valid. The Pioneer Formation contains pillow basalt, flows, and basalt breccia. Siltstone, sandstone, conglomerate, and minor amounts of limestone megabreccia and basalt belonging to the Hurley Formation conformably overlie the Pioneer. The Hurley spans latest Carnian or earliest Norian to middle Norian time. Two episodes of deformation affected the Cadwallader, and a thrust fault separates the group from slightly younger clastic rocks of the Tyaughton Group. Similarities in clastic rocks indicate the Tyaughton was deposited on the Cadwallader; together the units form the Cadwallader terrane. Basalts and clastic rocks in the terrane record deposition in or near a Carnian to earliest Norian volcanic arc. Volcanism waned later in the Norian, but presence of the arc is preserved in the clastic rocks.Oceanic rocks of the Middle Triassic to Middle Jurassic Bridge River terrane became juxtaposed with the Cadwallader terrane in Middle Jurassic time, after which the terranes functioned as a single tectonic block. Contrasting volcanic histories suggest that the Cadwallader terrane was not accreted to the Intermontane superterrane until Middle Jurassic or Early Cretaceous time, although the similar tectonic settings of Stikinia and the Cadwallader terrane allow a common earlier history. The Cadwallader terrane is not part of either the Alexander terrane or Wrangellia, and so the inboard margin of the Insular superterrane must lie west of the Cadwallader terrane.

Rocks formerly mapped as Leech River Formation can be subdivided into two partly coeval rock units with completely different histories. The Upper Jurassic – Lower(?) Cretaceous Pandora Peak unit, which comprises black mudstone, terrigenous greywacke, radiolarian ribbon chert, green tuff, metabasaltic greenstone, minor pebbly mudstone, and a few blocks of limestone, was probably deposited in small basins on a continental margin. Local stratal disruption occurred before sediments were lithified. A static, high-pressure, low-temperature metamorphism produced lawsonite-bearing assemblages in metaclastic rocks. The Pandora Peak unit was originally coextensive with the Pacific Rim complex on western Vancouver Island and the Constitution formation in the San Juan Islands of Washington. The Leech River complex consists of foliated metasandstone, phyllite, and minor metabasalt of probable Jurassic–Cretaceous age. Multiple folding, transposition, and synkinematic greenschist- to amphibolite-facies metamorphism culminated about 40 Ma.The Pandora Peak unit is separated from crystalline rocks of the Wrangellia terrane by major faults. In southeast Victoria, partly retrograded amphibolites of the Wark–Colquitz complex overlie locally cataclastic lawsonite-bearing Pandora Peak rocks along the newly discovered Trial Islands thrust. A similar thrust separates the two units in Finlayson Arm, but near Port Renfrew the Pandora Peak terrane and crystalline West Coast complex are juxtaposed along the high-angle San Juan Fault. In each of these areas, emplacement of the Pandora Peak unit postdated the high-pressure (lawsonite-grade) metamorphism, which occurred between late Albian – early Cenomanian and Santonian–Campanian time (approximately 99–83 Ma). The Pandora Peak terrane was emplaced during a major Late Cretaceous or early Tertiary tectonic event that modified and probably truncated the southern margin of the Wrangellia terrane. Following this event the Leech River complex was faulted against the southern margin of the Pandora Peak terrane near Port Renfrew and in Goldstream Park in the Late Eocene or Early Oligocene.