REASSEMBALING THE NUTZOTIN-DEZADEASH BASIN: NEW EVIDENCE FROM THE DEZADEASH FORMATION, YUKON, AND IMPLICATIONS CONCERNING THE ACCRETIONARY HISTORY OF THE WRANGELLIA COMPOSITE TERRANE

The Nutzotin Mountains sequence (Oxfordian-Valanginian) in southcentral Alaska and the Dezadeash Formation (Oxfordian-Valanginian) in southwestern Yukon are generally considered to be part of a Jura-Cretaceous sedimentary basin that was dismembered and displaced by ~370 km of dextral slip along the Denali fault system in post-Early Cretaceous time. Previous work on the ~2000 m thick Nutzotin Mountains sequence suggests that it originated mainly by axial submarine fan sedimentation in a retroarc basin. Recent work on the ~3000 m thick Dezadeash Formation does not support this interpretation. Based on over 100 sections totaling 16,500 m of measured strata, the Dezadeash Formation is dominated by thin- to thick-bedded sandstone-mudstone couplets containing thinly bedded hemipelagic and hemiturbidite limestones that are interpreted as lower submarine fan lobes. The lobes are vertically stacked and represent periodic avulsion events. Trace fossils, paleocurrent directions and the composition of sandstone and conglomerate beds indicate that the submarine fan prograded transversely into a bathyal to abyssal marine environment from a volcanoplutonic terrane to the west. In addition, zircons from interbedded vitric-rich epiclastic sediment gravity-flow deposits that are up to 8 m thick reveal a U-Pb isotopic age of 149.4 +/- 0.3 Ma, and hornblende from a diorite clast in a conglomerate bed reveals a K-Ar isotopic age of 144 +/- 4 Ma. Also, conodonts from a limestone megaboulder in a 20 m thick debrite, reveals a Late Triassic (i.e., Late Norian) age. The interpretation of the Dezadeash Formation as a deepwater turbidite system contemporaneous with volcanism is compatible with a backarc setting located between the allochthonous Wrangellia composite terrane and the former western margin of North America.