THE MYSTIC SUBTERRANE (PARTLY) DEMYSTIFIED: NEW DATA FROM THE FAREWELL TERRANE, INTERIOR ALASKA

The Farewell terrane of interior Alaska consists of a Proterozoic basement complex, a Neoproterozoic-Devonian passive margin sequence, and enigmatic Devonian-Jurassic rocks of the Mystic subterrane. New U-Pb detrital zircon (DZ), fossil, chemical, and petrographic data from Mystic strata illuminate their nature and origin, and provide the first documentation of phosphate in the Farewell terrane.

The Mystic is mainly siliciclastic strata with lesser volcanics and rare carbonates. Six samples of the Devonian-Permian Sheep Creek unit (central part of Mystic), including 2 from beds with late Paleozoic fossils, yield youngest DZ of Devonian age, major peaks between ~460-420 Ma, and overall age spectra very similar to those from the underlying Dillinger sequence. Samples are sandstones rich in sedimentary lithics (Ls), and differ in composition and DZ spectra from coeval(?), previously studied Mystic strata to the east that are rich in volcanic lithics (Lv) and have abundant late Paleozoic DZ. A sample taken between these study areas has both Ls and Lv and youngest DZ of Mississippian age.

Mesozoic strata from 4 central Mystic localities have DZ spectra much like those of the Sheep Creek, with major age peaks between ~440-420 Ma: 2 outcrops of carbonate±chert-clast conglomerate (Late Triassic youngest DZ) and 2 sites with quartz-rich sandstone (Early Jurassic youngest DZ). Quartz-rich strata also occur at 2 locales with newly identified Early Jurassic fossils (pelecypods and early Sinemurian ammonites) and at coeval fossil locales to the east.

Phosphate has been reported from Mystic strata but not chemically or petrographically characterized. We document cm-size phosphatic nodules in Late Devonian chert in the eastern Mystic, roughly coeval with barite deposits (e.g. Gagaryah) to the west, and phosphatic peloids in quartzose sediments of Early Jurassic(?) age in the central Mystic, associated with radiolarians and siliceous sponge spicules. Phosphatic Jurassic sediments in the Mystic may also occur to the west and east.

Our findings suggest that highly productive oceanographic conditions (upwelling?) bracketed Mystic siliciclastic deposition (Devonian-Permian) and rift-related volcanism (Triassic-Jurassic) and that DZ spectra like those of the Dillinger persist into the Mesozoic in the central Mystic.