Late Paleozoic Orogeny in Alaska’s Farewell Terrane

By

D.C. Bradley, J. Dumoulin (U.S. Geological Survey), P. Layer (University of Alaska, Fairbanks), D. Sunderlin (University of Chicago), J. Brinton (University of Alaska, Fairbanks), S. Roeske (University of California, Davis), B. McClelland (University of Idaho), T. Kusky (St. Louis University), and A.G. Harris (U.S. Geological Survey)

Alaska’s Farewell terrane was long regarded as a displaced piece of western Canada. A growing body of fossil evidence instead indicates an origin near Siberia, resurrecting the question of how and when the terrane came to rest in Alaska’s collage.We suggest this happened in the late Paleozoic, based on a previously unknown orogeny in two parts of the Farewell terrane.

The Farewell terrane consists of a metamorphic “basement” complex, platform carbonates (Cambrian-Devonian), deepwater facies (Cambrian-Devonian), and the heterogeneous Mystic sequence (Devonian-Jurassic). In the Kuskokwim Mountains, Proterozoic rocks in the basement complex include 980 Ma rhyolite and 851 Ma orthogneiss. 40Ar/39Ar white mica ages of 284–296 Ma from very low-grade to upper greenschist facies metasediments show that parts of this region were metamorphosed during the late Paleozoic. Lower Paleozoic carbonates were involved in this event. Meanwhile, siliciclastics of the Mystic Sequence were being deposited in a 150 by 200 km area to the south, in the Kuskokwim Mountains and Alaska Range. At Mt. Dall, a 1.5-km thick fluvial conglomerate succession has yielded an Early Permian north-temperate flora. Clasts are almost exclusively sedimentary types, derived, at least in part, from the Farewell terrane. Limestone clasts contain Pennsylvanian conodonts and Middle Devonian megafossils. Paleoflow was west to east, from a region underlain by older parts of the Farewell terrane. We hypothesize that the late Paleozoic areas of deformation and metamorphism represent the vestiges of a collisional orogen, and that the Mystic siliciclastic strata represent the fill of its now-dismembered foreland basin. 

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.