DETRITAL ZIRCON PROVENANCE OF A QUARTZITE COBBLE FROM THE CRETACEOUS GOOSE ROCK CONGLOMERATE, CENTRAL OREGON

The Cretaceous Goose Rock Conglomerate (GRC) in eastern Oregon lies unconformably on Permian-Triassic strata of the Blue Mountains Province and likely represents the terrestrial equivalent of deep-marine turbidites of the Mitchell Inlier (MI). Paleomagnetic analysis suggests >1000 km of post-depositional, coast-parallel, dextral translation of the MI and related rocks, but regional studies indicate translation of < 500 km. This study presents U-Pb ages of detrital zircon from a quartzite cobble within the GRC to differentiate between northern and southern Cordilleran sources.

The GRC quartzite cobble detrital zircon age distribution is characterized by a dominant peak at 1836 Ma, two smaller peaks at 1915 and 2078 Ma, and several Archean grains. The youngest grain in the quartzite cobble is ~1000 Ma. Comparison of the GRC cobble age distribution with published detrital zircon data from Cordilleran quartzites reveals that the GRC quartzite very closely resembles Ordovician quartzites found throughout the Cordillera. Ordovician sedimentary systems draining the western side of the transcontinental arch deposited the source of the GRC quartzite cobble along the Cordilleran passive margin. The dominant Paleoproterozoic detrital zircon population (1836 and 1915 Ma peaks) likely originated from the Trans-Hudson Orogen (1.8 – 2.0 Ga), while the 2078 Ma zircon may have been derived from Paleoproterozoic provinces accreted as a part of the Trans-Hudson Orogen, including the Hearne and Rae (~2.0 – 2.1 Ga) provinces. The Archean grains are likely sourced from the Wyoming Craton (2.5 – 3.0 Ga). In addition, a long-shore current may have transported sediment along the margin from as far north as the Peace River Arch (1.9 – 2.3 Ga).

The similarity of Ordovician detrital zircon age distributions throughout much of the Cordillera renders provenance distinctions problematic, but southern quartzites from the Sonoran region, including the Sierra Lopez and Sierra el Aliso, contain a prominent ~2700 Ma peak that is nearly absent in their equivalents in Nevada, Idaho, and British Columbia. The absence of 2700 Ma grains in the GRC cobble suggests that the MI region may have been more proximal to northern Cordilleran quartzites, indicating translation of < 500 km since Cretaceous time.