SEDIMENTARY PROVENANCE ANALYSIS OF THE OCHOCO BASIN NEAR MITCHELL, OR: IMPLICATIONS FOR CRETACEOUS CORDILLERAN PALEOGEOGRAPHY

The Ochoco basin provenance in central Oregon may be a critical link in reconstructing the complex mid-Cretaceous paleogeography of the North American Cordillera. Ochoco basin sandstone petrography indicates a recycled orogen source and conglomerate clast analysis reveals >50% volcanic clasts, suggesting a combined recycled orogen-arc source. Chondrite-normalized trace element geochemical analysis also indicates a component of continental arc input. Detrital zircon analysis reveals that the source rocks for the Ochoco Basin include abundant Late Jurassic to Early Cretaceous plutons, with minor (<20%) input from pre-Mesozoic sources. Mesozoic peaks in the detrital zircon signature coincide with periods of magmatic activity in the Blue Mountains, Klamath Mountains, and the northern Sierra Nevada terranes, but no single source could have provided the full spectrum of ages. The Ochoco basin has certainly received sediment from the Blue Mountains province, but the basin also contains ages younger than those present in the Blue Mountains. Late Cretaceous detrital zircon could have been derived from the Idaho batholith or the main Cretaceous batholith of the Sierra Nevada Mountains. In addition, the youngest zircon peak age indicates that the Ochoco basin has a Cenomanian maximum depositional age, slightly younger than previously suggested.

Ochoco basin provenance data are consistent with previous suggestions that the Hornbrook Formation and the Ochoco basin were connected or closely related during their formation. Although conglomerate clast count data reveal a higher percentage of lithic volcanic clasts in the Ochoco basin than the Hornbrook Formation, sandstone petrography from both basins are broadly similar. Geochemical data from the Ochoco and the Hornbrook basins are also similar, indicating that the basins may have received sediment from related arc sources. Statistical comparison of detrital zircon age signatures from the Ochoco basin and Hornbrook Formation reveals high overlap and similarity between the two data sets, indicating that the Hornbrook Formation and the Ochoco basin received zircon from similar-age source regions. These data indicate that a combined Hornbrook-Ochoco fore-arc basin likely existed during the Cretaceous along the Pacific margin of the United States.