EVIDENCE FOR A COMBINED HORNBROOK-OCHOCO BASIN: A COMPLEX LATE CRETACEOUS FOREARC SYSTEM

The Hornbrook Formation of northern California and southern Oregon and the Ochoco basin of central Oregon were deposited unconformably on the Klamath Mountains and the Blue Mountains, respectively, during the Late Cretaceous. Several workers have postulated that the Ochoco basin represents a continuation of the Hornbrook Formation that has since been separated due to clockwise rotation of the Blue Mountains terranes on which it rests. The Hornbrook basin deepened from nonmarine and shallow marine to deep marine turbidite deposition as subsidence progressed from northwest to southeast. Detrital zircon age spectra and whole-rock sedimentary geochemistry indicate that the basal Hornbrook Formation likely was derived from the underlying Eastern Klamath terranes, but younger arc sources contributed sediment to the younger members of the Hornbrook Formation. A 98 Ma detrital zircon age peak and increased volcanic input indicated by sedimentary petrography and geochemistry suggest a Sierra Nevada Mountains provenance for the Rocky Gulch Sandstone and Blue Gulch Mudstone members of the Hornbrook Formation. Whole rock epsilon Nd values for the Hornbrook Formation match well with those in the northern Great Valley Group, also sourced by the northern Sierra Nevada Mountains.

The Ochoco basin contains a deep marine turbidite sequence deposited in a submarine fan setting. Ochoco basin petrography and whole-rock geochemistry indicate a mixed recycled orogen and arc provenance, and the basin also contains a 98 Ma detrital zircon age peak. These Ochoco basin provenance results match well with provenance of the younger members of the Hornbrook Formation and are distinct from the basal Hornbrook Formation. Thus, sandstone petrography, whole-rock sandstone and mudstone geochemistry, and detrital zircon age spectra are consistent with, but do not require, a combined Hornbrook-Ochoco basin during the Late Cretaceous, with sediment derived from both eastern (Blue Mountains and Sierra Nevada Mountains) and western (Klamath Mountains) sources. The Hornbrook Formation and Ochoco basin may have been separate systems with distinct provenance signals initially, but the similar provenance characteristics of the younger strata in both basins suggest the development of a single basin system.