INITIATION OF THE CRETACEOUS, ANDEAN-TYPE MARGIN OF THE WESTERN U.S. CORDILLERA: INSIGHTS FROM DETRITAL-ZIRCON AGES OF THE COON HOLLOW FORMATION, IDAHO, U.S.A

New detrital-zircon U-Pb ages from Mesozoic sedimentary rocks in western Idaho clarify tectonic models for the Blue Mountains Province, and provide a crucial link to other Mesozoic basins of the western U.S. Historically, Middle Jurassic coral ages from the Coon Hollow Formation have led workers to interpret a stratigraphic overlap assemblage linking the Blue Mountains terranes by Early to Middle Jurassic time. However, three sandstone samples from the lower fluvial and upper marine member of the Coon Hollow Formation contain significant quantities of Late Jurassic to Early Cretaceous detrital zircons (ca. 154-142 Ma), suggesting that Middle Jurassic corals are reworked. In two samples, detrital-zircon ages define bimodal probability distributions that span 245-215 Ma and 160-140 Ma, yielding a Late Triassic peak at ca. 230 Ma and a Late Jurassic peak at ca. 150 Ma. A third sample contains dominantly Permo-Triassic grains (ca. 280-215 Ma) and has a prominent Late Jurassic peak at ca. 160 Ma. Precambrian detrital-zircon grains are largely absent from these samples. In comparison, Middle Jurassic rocks in the Izee terrane contain abundant Early to Middle Jurassic grains, few Triassic grains, and abundant ca. 2.0-1.0 Ga grains. The disparity in detrital-zircon ages, and the newly recognized difference in depositional ages between these successions, suggests that the Coon Hollow Formation does not belong to the same regional megasequence as Middle Jurassic deposits in the Izee area. Detrital-zircon ages of the Coon Hollow Formation are in striking agreement with ages from the Great Valley Group and Galice Formation in California, the Ingalls Graywacke and rocks of the Northwest Cascades in Washington, and the Gravina Belt in Alaska. We suggest that rocks of the Coon Hollow Formation are correlative with these successions and represent a basin-filling stage that is distinct from that of older deposits in the BMP. These successions record deposition following establishment of an Andean-type convergent margin along the western U.S. The transition from Precambrian-rich, detrital-zircon ages in Middle to Late Jurassic strata to dominantly Triassic-Jurassic ages in Early Cretaceous basins of the western U.S. records major reorganization of the tectonic setting and sediment-dispersal pattern of the Cordilleran margin.