SILURIAN TO EARLY DEVONIAN STRATIGRAPHIC HISTORY AND PROVENANCE OF THE FAREWELL TERRANE, SOUTHWESTERN ALASKA

The Farewell terrane of southwestern Alaska is located at the northernmost endpoint of the North American Cordillera and is defined by a four part succession consisting of 1) Neoproterozoic basement, 2) Neoproterozoic (?)-Devonian carbonate & subordinate siliciclastic carbonate platform strata of the Nixon Fork subterrane, 3) Cambrian-Devonian deepwater siliciclastics and carbonates of the Dillinger subterrane, and 4) Devonian-Permian(?) siliciclastic submarine fan to foreland basin strata of the Mystic subterrane. Previous studies suggest a displaced passive continental margin of mixed Siberian and Laurentian affinity for oldest parts of the Farewell but also suggest proximity to components of the Wrangellia and Alexander terrains for youngest parts of the Farewell. The provenance of the Dillinger subterrane has yet to be determined, but the Dillinger and Nixon Fork subterrane are believed to be lateral equivalents, in part, sharing the same Cambrian-Devonian depositional system.

Presented here are sedimentologic and stratigraphic field observations, modal composition analyses, U-Pb ages, and εHf values from Silurian-Early Devonian strata of the Dillinger subterrane (Terra Cotta Mountains Sandstone and Barren Ridge Limestone). Facies analyses of the Terra Cotta Mountains Sandstone indicate deposition by low- and high-density turbidity flows, within medial- to distal- deepwater submarine fan depositional environments. Point counts record abundant mono- and polycrystalline quartz, chert, plagioclase, and minor lithic fragments (metamorphics). New bulk U-Pb ages show a primary peak age at 436 Ma and secondary peaks at 950 and 1650 Ma, with isolated neoarchean to mesoarchean ages. εHf values range between -13.9 and +13.0 reflecting both juvenile and enriched crustal sources.

Facies analyses of the Barren Ridge Limestone indicate deposition by primarily low-density turbidity flows, in distal submarine fan to basin plain depositional environments. Point counts record abundant mono- and polycrystalline quartz, chert, lithic fragments (volcanic), and plagioclase. New bulk U-Pb ages show a primary peak age at 434 Ma and secondary peaks at 962, 1737, 1912, and 2052 Ma, with isolated neoarchean to mesoarchean ages.