EARLY MESOZOIC WALLOWA TERRANE, NORTHEASTERN OREGON AND ADJACENT IDAHO

The Wallowa terrane (WA), one of several tectonic units in the Blue Mountains Province, is arguably one of the best studied terranes of the American Cordillera. It records oceanic Triassic volcanism above the eroded roots of an older Paleozoic volcanic complex. During early Mesozoic time, the WA is linked to other terranes in the Blue Mountains region (Baker, Izee, Olds Ferry). Mesozoic fossils provide keys to age and paleogeography while stratigraphy chronicles a complex history of island arc volcanism long before Middle Cretaceous accretion to North America. Middle to Late Triassic volcaniclastic rocks contain microfossils and flat clams, revealing deep-water deposition during active volcanism. The Latest Triassic witnessed abrupt cessation of volcanism and carbonate deposition of the Martin Bridge Fm. At subtropical latitudes coral and sponge reefs developed and shelly invertebrates of the WA reveal Tethyan affinities. Similar fossils in the Olds Ferry succession link these two terranes. Tectonic activity created instability, triggering carbonate debris periodically shed into the basin. Late Triassic-Early Jurassic rocks, represented by the Hurwal Fm., record deeper water settings, increased shelf instability and drowning of the carbonate platform, followed by uplift and erosion. After an extensive gap, the Middle Jurassic (Bajocian-Callovian) succession of the Coon Hollow Fm. links WA with other terranes of the Blue Mountains, revealing renewed volcanism, marine to terrestrial ash-flow tuffs, conglomerate, sandstone and marine mudstone and sandstone. Non-Tethyan corals and bivalves show cratonal affinities with North America. The Coon Hollow succession ends the record of the WA but the flora and fauna indicate movement into cooler more boreal settings. Triassic location of the WA and relationship to other terranes are uncertain but closest faunal links exist with Wrangellia. Rock provenance and fossils link WA to the Olds Ferry terrane, so both these island arc terranes were close by Latest Triassic time. By Jurassic time, WA had shifted to higher latitudes and fossils indicate proximity to the craton.