LATE PALEOZOIC STRATIGRAPHIC HISTORY AND PROVENANCE OF THE FAREWELL TERRANE, SOUTHWESTERN ALASKA

Alaska’s Farewell terrane represents the northernmost endpoint of the North American Cordillera in southwestern Alaska. The Farewell terrane is exposed both north and south of the Denali fault in the western Alaska Range and is primarily composed of three subterranes that include (1) dominantly Upper Paleozoic siliciclastic components of the Mystic subterrane, (2) Upper Paleozoic mixed siliciclastic and carbonate units of the Dillinger subterrane, and (3) Lower Paleozoic carbonate strata of the Nixon Fork subterrane.

New geologic mapping, measured stratigraphic section, and provenance data of Devonian–Permian strata that make up the lower parts of the Sheep Creek Formation (and equivalent strata) of the Mystic subterrane reveal a continuous record of siliciclastic sedimentation. Strata consist largely of interbedded fine- to coarse-grained sandstone and mudstone that are laterally extensive (>100 m) and exhibit tabular geometries. Individual units are 0.02–1.0 m thick and sandstone facies are characterized by plane beds, ripple cross- stratification and massive beds. Mudstone rip-up clasts are common at the base of beds. Matrix-supported conglomerate occurs sporadically throughout this unit. Individual clasts are rounded to subrounded and range from 1–10 cm in diameter. Preliminary provenance reveals clasts and grains consisting of lithic sedimentary fragments (sandstone, limestone, and mudstone), mono- and polycrystalline quartz, and abundant green, gray, and black chert. Overall, this unit appears to coarsen upward and may grade stratigraphically into overlying marginal-marine to non-marine, fossil leaf-bearing strata of the Pennsylvanian–Permian Mt. Dall conglomerate.

Age-equivalent Devonian–Permian strata of the Sheep Creek Formation occur sporadically across a >250 km-long transect of the Alaska Range in southwest Alaska both north and south of the Denali fault. Future study in this unit that is specifically aimed at the stratigraphic history and provenance of these strata will provide a basis for understanding the early stages of orogenesis in the northernmost parts of the North American Cordillera.