THE OCHOCO BASIN, CENTRAL OREGON: ANOTHER PIECE OF THE LATE CRETACEOUS CORDILLERAN FOREARC SYSTEM

The Ochoco Basin contains marine turbidites deposited unconformably on the Blue Mountains Province. Detrital zircon from exposures near Mitchell, OR consistently yield robust maximum depositional ages of 98.2±1.9 Ma at the base, 85.9±3.0 Ma near the middle, and 85.1±3.0 Ma near the top of the section. These ages are younger than the accepted Albian-Cenomanian biostratigraphic age. However, latest Albian to earliest Cenomanian foraminifera occur in the lower third of the strata, and correlative Albian-Campanian strata are documented in a well 70 km south of Mitchell. Several conglomerate units in the Mitchell strata contain rounded cobbles of fossiliferous sandstone similar to Albian-Cenomanian shallow-marine deposits located 50 km east. Older macrofossils may have been transported and resedimented in younger deposits, with the bulk of the Ochoco strata deposited in Cenomanian through Campanian(?) time.

Sandstone petrography, conglomerate clast compositions, whole-rock geochemistry, and detrital zircon ages point to a source dominated by a Jura-Cretaceous arc, and primarily SW-directed paleocurrents suggest provenance in the Blue Mountains Province. However, the appearance of 100-85 Ma detrital zircon in Santonian and younger strata indicates a source in the Sierra Nevada batholith to the SE. Drainage systems reaching the Ochoco basin may have shifted from NE to SE sources in early Santonian time. Alternatively, the Ochoco provenance signature is consistent with an entirely northern Sierran source if paleocurrents record within-basin transport on an overall SW-paleoslope, but do not reflect source-to-basin transport.

Albian-Campanian deposition of Ochoco strata permits correlation with coeval strata of the Hornbrook Formation in southwest OR, which also record an influx of Sierran-derived zircon in Santonian and younger strata. These results suggest that the Ochoco-Hornbrook strata formed an extensive Late Cretaceous basin deposited on subsiding basement rocks of the Blue Mountains and Klamath Mountains. This Ochoco-Hornbrook system likely formed the northern extension of the Great Valley forearc basin, and records similar shifts in provenance that reflect Late Cretaceous tectonic uplift and denudation of the Late Cretaceous batholith in the northern Sierra Nevada.