NEW U-PB (CA-TIMS) AGE CONSTRAINTS ON THE DEPOSITION OF THE WEATHERBY FORMATION, BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON

The Connor Creek thrust fault is the apparent terrane boundary between the Baker accretionary complex and the eastern continental-fringing Olds Ferry volcanic arc terrane of the Blue Mountains Province. In its southern-most exposures, the fault juxtaposes mélange sediments, greenstones, and mafic plutonic rocks of the Baker terrane with marine flysch of the Weatherby Formation, part of the Izee basin sedimentary onlap sequence. The Weatherby Formation of the Izee basin thus records the interaction of the Baker and Olds Ferry terranes during accretion to the North American margin. This study systematically applies U-Pb zircon geochronology to volcanic units of the Weatherby Formation to constrain the depositional history of the Izee basin and subsequent deformation associated with movement on the Connor Creek fault. Previous work on a crystal tuff horizon from a structurally simple section near the base of the Weatherby Formation at Dennet Creek, Idaho has established an initiation of basin subsidence around 180.6 Ma. A welded tuff sampled about 60 m stratigraphically above the base of the Weatherby Formation gave an age of about 173.9 Ma. A new U-Pb (CA-TIMS) single grain zircon age for a structurally higher tuffaceous horizon in the Weatherby Formation from Oregon has yielded a crystallization age of 175.3 Ma, indicating the need for more structural complexity within the basin than has previously been recognized. Further geochronology will be presented on the structurally highest volcanic sediments of the Weatherby Formation in the vicinity of the Connor Creek fault, and our results will be compared to the chronostratigraphy of the type Izee basin west of our field area.