PROVENANCE ANALYSIS OF CRETACEOUS INLIERS OF EASTERN OREGON: ASSESSING THE EXTENT OF THE OCHOCO BASIN AND IMPLICATIONS FOR CORDILLERAN PALEOGEOGRAPHY

The Cordillera was constructed through terrane accretion and translation during Paleozoic and Mesozoic time. Debate continues regarding the latitude of accretion, and the amount and timing of subsequent terrane translation. Improved constraints on depositional age and provenance of middle Cretaceous sedimentary rocks deposited on terranes of the Blue Mountains in Eastern Oregon should help link sediment to northern or southern source regions within the Cordillera, and permit potential correlation between isolated exposures and the adjacent Mitchell Inlier to determine if they formed in a single Ochoco basin or separate basins. We analyzed eight sandstone samples from isolated Cretaceous exposures at Goose Rock, Antone Ranch, Dixie Butte, and Flat Creek, and one quartzite cobble from Goose Rock. Sandstone petrography reveals immature sediment with low polycrystalline/total quartz, high volcanic/total lithic, and low potassium feldspar/plagioclase ratios, indicating recycled orogenic to dissected arc sources. All samples contain Jurassic U-Pb age peaks (160 Ma and 180-200 Ma) and variable Cretaceous peaks (95-125 Ma), with maximum depositional ages of 95-103 Ma. Samples contain few Paleozoic and Precambrian grains (11.2%), with the exception of one Antone Ranch sample (27.4% Precambrian). The quartzite cobble contains two Precambrian age peaks (1.8 Ga and 2.1 Ga). Epsilon Hf from selected Jurassic grains (160-200 Ma) are similar and positive (mean of +9.6 ± 0.5), while Cretaceous grains (~90-120 Ma) show significant variation (-13.7 to +11.2). The petrographic, detrital zircon age and epsilon Hf similarities between samples suggest, but do not require, that these sandstone exposures were deposited within the same sedimentary system. Mesozoic ages match juvenile magmatism documented throughout the Mesozoic Cordilleran arc, and the presence of <105 Ma detrital zircon with evolved epsilon Hf values suggest sources beyond the proximal Blue Mountains terranes; potential sources include the northeastern Sierra Nevada and early Idaho Batholith suture zone. Paleomagnetic analysis of samples from these inliers, and comparison of these isolated exposures to the Mitchell Inlier and the Hornbrook Formation will help resolve the extent and original paleolatitude of the Cretaceous Ochoco basin.