Collision-deformed Paleozoic continental margin, western Brooks Range, Alaska

A structural sequence of Carboniferous, Permian, and Triassic argillaceous and cherty rocks in the northwestern Brooks Range, herein named the Kagvik sequence, is a key to the tectonic history of Arctic Alaska. The Kagvik sequence lies structurally below thrust-faulted sheets of coeval carbonate rocks, including the Lisburne Group, and structurally above quartz mica schist and greenstone of the southern Brooks Range. Sedimentary features of the shelly fossil-rich carbonate strata of the Lisburne in the upper thrust sheets indicate deposition in a shallow-water shelf environment. The underlying shale and chert of the Kagvik is an attenuated section about 500 m thick repeated by imbricate thrusts. Ubiquitous pelagic fossils (radiolaria and Nereites trace-fossil assemblages) and volcanic material locally forming andesitic tuff and flows point to an oceanic environment of deposition. The presence of some limestone turbidites interbedded with the ocean-floor sediments of the Kagvik suggests that a carbonate shelf was nearby and that a south-facing continental margin existed during late Paleozoic and early Mesozoic time near the present-day southern Brooks Range. Collapse of this long-standing continental margin appears to be related to collision and accretion of Paleozoic island arcs and microcontinental blocks now represented by the metamorphic terrane along the south flank of the Brooks Range. The structurally higher thrust slices of mafic and ultramafic rocks emplaced on top of the collapsed continental margin represent ophiolitic basement of the Yukon-Koyukuk basin.