Paper No. 6
Presentation Time: 9:20 AM


DUMOULIN, Julie A., N/a, U.S. Geol Survey, 4210 University Dr, Anchorage, AK 99508, BRADLEY, Dwight C., U.S. Geological Survey, 11 Cold Brook Rd, Randolph, NH 03593, TILL, Alison B., USGS, 4210 University Dr, Anchorage, AK 99508, BOX, Stephen E., U.S. Geological Survey, 904 W. Riverside Ave, Room 202, Spokane, WA 99201 and JONES III, James V., U.S. Geological Survey, Alaska Science Center, 4210 University Drive, Anchorage, AK 99508,

The Farewell terrane of interior Alaska is a continental fragment made up of a Proterozoic basement complex overlain by Neoproterozoic-Mesozoic strata. U-Pb detrital zircon age spectra from 14 Farewell samples show an up-section age progression like that seen in the Arctic Alaska-Chukotka terrane.

Five samples come from a Neoproterozoic succession in central Farewell at Lone Mt. The basal “ferruginous unit” has a spectrum like that of correlative(?) strata in SW and NE Farewell, with a prominent detrital zircon age peak at ~2000 Ma. The overlying Windy Fork Fm. has a similar age cluster with an additional major peak at ~600 Ma. Three samples from the youngest unit, the Lone Fm., yield grains mainly between ~750-550 Ma, with minor older Proterozoic grains.

Nine samples were collected from deep water early Paleozoic rocks in SE Farewell. Siltstone in the Post River Fm., interbedded with Middle-Upper Ordovician graptolitic shale, has a major age peak at ~490 Ma and subordinate peaks between ~785-575 Ma that overlap the age peaks in the Lone Fm. Five turbidite samples from the overlying Terra Cotta Mts. Sandstone (Silurian) show distinctly different spectra—they have major peaks between ~450-420 Ma and numerous grains between 2000-900 Ma. Three samples from the Devonian Barren Ridge Limestone have spectra similar to those of the Terra Cotta, plus some Early Devonian grains. No igneous sources within the Farewell terrane for 785-550 Ma zircons are known; known local igneous sources of 450-420 Ma zircons are limited to a 431-Ma tuff in NE Farewell. The Silurian shift in detrital zircon spectra coincides with a major influx of siliciclastic sediment suggestive of an as yet unknown tectonic event involving the Farewell terrane.

Detrital zircon data from the Nome Complex on Seward Peninsula in northern Alaska (Arctic Alaska-Chukotka terrane) show a similar up-section age progression. Spectra from older (Ordovician?) strata are dominated by 740-550 Ma grains, likely in part derived from local sources. Devonian (and younger?) rocks yield abundant 450-420 Ma and ~1250-900 Ma grains—local sources for these grains are unknown. Parallel shifts in detrital zircon spectra in Farewell and Arctic Alaska support paleontologic evidence for a shared early Paleozoic history of the two terranes.