Geology of southern Prince of Wales Island, southeastern Alaska

Abstract

Geologic mapping on southern Prince of Wales Island combined with U-Pb (zircon) geochronologic, geochemical, and conodont studies yield new information about the early Paleozoic geologic framework and evolution of the southern Alexander terrane. The oldest rocks recognized consist of arc-type(?) meta-volcanic and metasedimentary rocks (Wales metamorphic suite) which were derived from late Proterozoic(?)-Cambrian protoliths. These rocks were deformed, metamorphosed, and uplifted during the Middle Cambrian-Early Ordovician Wales orogeny. Beginning soon after this orogenic event and continuing into Early Silurian time, basaltic to rhyolitic rocks and marine clastic strata (Descon Formation) were deposited, and dioritic to granitic plutons were emplaced. A variety of geochemical data suggest that these Ordovician-Early Silurian rocks formed in a volcanic-arc environment. The middle Silurian-earliest Devonian Klakas orogeny marked the end of arc-type magmatism; it is manifest on southern Prince of Wales Island by regional imbrication on southwest-vergent thrust faults, penetrative brecciation and pervasive hydrothermal alteration of pre-Devonian rocks in some areas, and at least several kilometres of structural uplift. Lower Devonian strata (Karheen Formation) record the waning stages of this event: conglomeratic redbeds low in the section were deposited in topographically rugged, subaerial to shallow-marine environments, whereas strata high in the section were deposited in more distal marine environments. Post-Devonian rocks on southern Prince of Wales Island include the Bokan Mountain Granite (Jurassic), basaltic to dacitic(?) dike swarms, and mid-Cretaceous granodiorite and diorite bodies. Prior to emplacement of the mid-Cretaceous plutons, the Devonian and older rocks were offset along several sets of strike-slip faults and then downdropped along the Keete Inlet fault.

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