Age, Provenance, and Structure of the Weatherby Formation, Eastern Izee Sub-Basin, Blue Mountains Province, Oregon and Idaho

The terranes of the Blue Mountains province of eastern Oregon and west-central Idaho provide an important link between the geology of the North American Cordillera exposed in Canada and Alaska in the north and Nevada and California in the south. The Izee basin is a volcanic-rich sedimentary sequence onlapping the volcanic arc and accretionary mélange terranes of the Blue Mountains, and comprises the marine flysch Weatherby Formation at its eastern extent. This study sought to establish the stratigraphic and structural relationships between the Weatherby Formation and the proximal Baker and Olds Ferry terranes. Small-scale mapping, structural measurements, and geochronological data have been used to interpret the Weatherby Formation as the synformal remnant of a larger forearc basin deposited onto both the Olds Ferry and Baker terranes, and deformed by movement along the Connor Creek fault during amalgamation of the Wallowa and Olds Ferry terranes. New high precision U-Pb zircon ages of volcanic-rich turbidite horizons found within the marine flysch of the Weatherby Formation provide new constraints on activity in the Olds Ferry volcanic arc. The youngest primary volcaniclastic horizons within the Weatherby Formation are early Middle Jurassic (Aalenian), ca.173 Ma. Detrital zircon analysis of sediments stratigraphically above the youngest primary volcaniclastic horizon indicate reworking of Middle Jurassic zircons and no input of new volcanic material into the basin after about 170 Ma, implying Olds Ferry volcanism continued into, but no later than, the Aalenian. Field observations coupled with new volcanic ages provide new constraints on the deformation of the Weatherby Formation and confirm the synformal structural model developed from mapping of bedding and cleavage orientations. The southern flanks of the basin at the unconformity between the upper Huntington and Weatherby Formations conform to an upright, open folded stratigraphic section of sediments, whereas the northern flanks of the basin in the vicinity of the Connor Creek fault display tight folding and overturning. High precision U-Pb geochronology of primary volcaniclastic rocks corroborates the structural measurements, and demonstrates that the Weatherby Formation has been folded into a synformal structure via movement along the Connor Creek fault. A 192 Ma volcanic exposed near the Connor Creek fault is interpreted as a fault-bounded slice of the underlying Huntington Formation riding along a splay of the Connor Creek fault system. The Connor Creek fault is interpreted as a significant contractional structure related to the internal amalgamation of the terranes. U-Pb zircon dating of granitoids in the southern Blue Mountains province demonstrates that a belt of southwest-northeast trending plutons consistently youngs from 129.4 Ma in the southwest to 123.8 Ma in the northeast. Of these the Lookout Mountain pluton stitches the Connor Creek fault, indicating the movement along the fault occurred after the deposition of ca.170 Ma volcanics, and before ca.129 Ma intrusions of these Early Cretaceous plutons. Compared with the western Izee sub-basin, however, the Weatherby Formation lacks Oxfordiansediments, suggesting the propagation of the Connor Creek fault occurred as early as the Callovian, leading to termination of basin development. Detrital zircon geochronology of sandstones from the Weatherby Formation strengthens provenance correlations between the eastern and western Izee sub-basin sediments. Similar to Early to Middle Jurassic sediments in the western Izee sub-basin (LaMaskin et al., 2011b), the Weatherby Formation contains Neoproterozoic and Mesoproterozoic zircons in all samples. These Precambrian zircons indicate a Laurentian influence, suggesting the basin was proximal to the western North American margin throughout its Jurassic depositional history. At the same time, over 65% of the zircons from each sample indicate a Mesozoic influence primarily from the late Early Jurassic to Middle Jurassic, suggesting a local influx of Jurassic volcanism from the Olds Ferry volcanic arc. This provenance confirms the model of the Weatherby as a forearc basin to the Olds Ferry fringing arc along the North American plate margin. Tighter constraints on the depositional histories of the Huntington Formation and the Weatherby Formation provide better temporal correlations with other Late Triassic to Middle Jurassic forearc systems in the Canadian Cordillera, including the Ashcroft basin in southern British Columbia, the Bowser basin in central British Columbia, and the Whitehorse Trough in northern British Columbia and southern Yukon