A WINDOW INTO THE LATE CRETACEOUS PALEOGEOGRAPHY OF THE U. S. CORDILLERA: PROVENANCE ANALYSIS OF THE OCHOCO BASIN, CENTRAL OREGON (Invited Presentation)

Cretaceous forearc strata of the Ochoco basin in central Oregon preserve a record of magmatism, mountain-building, and possibly regional transpression within the Late Cretaceous North American Cordillera. Paleogeographic models for mid-Cretaceous time indicate that the Blue Mountains and the Ochoco sedimentary overlap succession experienced post-depositional, coast-parallel, dextral translation of either less than 500 km or as much as 1700 km. We combine sandstone petrography, detrital-zircon age and hafnium analysis, and whole-rock trace-element geochemistry to generate a detailed provenance signature for the Ochoco basin. Our results indicate that deposition of Ochoco strata was largely Late Cretaceous, from late Albian through at least Santonian time (ca. 105-86 Ma and younger), rather than limited to Albian-Cenomanian time (ca. 113-94 Ma). Comparison of the Ochoco basin provenance signature with potential source regions and with the provenance signatures of Albian and younger strata of the Hornbrook Formation, Great Valley Group, and Methow basin tests paleogeographic models and the potential extent of mid- to Late Cretaceous forearc deposition in the U.S. Cordillera. Provenance characteristics of the Ochoco basin are consistent with northern U.S. Cordilleran sources, and Ochoco strata may represent the destination of much of the mid- to Late Cretaceous Idaho arc that was intruded and eroded during and following rapid transpression along the western Idaho shear zone. Our provenance comparisons suggest that the Hornbrook Formation and Ochoco basin formed two sides of the same depositional system during Late Cretaceous time, which may have been linked to the Great Valley Group to the south by Coniacian time, but was not connected to the Methow basin. These results limit northward displacement of the Ochoco basin to less than 500 km relative to the North American craton and suggest that the anomalously shallow paleomagnetic inclinations documented in the Ochoco basin may result from inclination error, rather than deposition at low latitudes. Our results demonstrate that detailed provenance analysis of forearc strata permits improved understanding of Late Cretaceous Cordilleran paleogeography and complements the incomplete record of arc magmatism and tectonics preserved in bedrock exposures.