TESTING MODELS OF POST-CRETACEOUS TERRANE TRANSLATION USING MUDROCK GEOCHEMISTRY OF THE OCHOCO BASIN, CENTRAL OREGON

The North American Cordillera has a well-studied tectonic history of terrane accretion and translation, but the magnitude of coast-parallel translation since Cretaceous time remains uncertain. Various tectonic models predict moderate (<1000 km) or large-scale (>1000 km) terrane displacement since Cretaceous time. Because the Ochoco basin of present-day central Oregon was deposited unconformably on the Blue Mountains terranes during the Cretaceous Period, characterizing the provenance of the Ochoco basin may better constrain translation magnitude proposed by these models. For example, moderate displacement models suggest a combined Cretaceous Hornbrook-Ochoco Basin, while large-scale displacement models juxtapose the Ochoco basin with the San Joaquin basin of the southern Great Valley Group in California.

We analyzed 28 mudrock samples from the Mitchell Inlier, a submarine fan deposit within the Ochoco basin, using X-ray fluorescence and inductively-coupled plasma mass spectrometry at Washington State University. Geochemical results indicate an active arc source throughout deposition. Although samples show some variability, the absence of systematic changes with stratigraphic position suggests that sediment sources did not change significantly through time. Weathering trends derived from major-element compositions of Mitchell Inlier mudrocks suggest granodiorite and andesite source rocks. Rare-earth element (REE) plots consistently show a slight negative europium anomaly as well as light REE enrichment and near-flat heavy REE patterns, all typical of sediment derived from active margin settings. Trace-element data indicate that magmatic processes controlled source composition, rather than sedimentary recycling. Initial epsilon Nd values range from -6 to +2, indicating that sediment originated from both juvenile and evolved sources.

Our Ochoco basin geochemical results are consistent with mudrock geochemistry of the Hornbrook Basin, suggesting that these two basins shared similar sources. In contrast, Ochoco results do not overlap significantly with geochemistry of the southern Great Valley Group. Therefore, our Ochoco mudrock geochemistry results support moderate-translation tectonic reconstructions that posit a combined Cretaceous Hornbrook-Ochoco basin.