Hornbrook Formation, Oregon and California: A Sedimentary Record of the Late Cretaceous Sierran Magmatic Flare-Up Event

Early Late Cretaceous time was characterized by a major magmatic flare-up event in the Sierra Nevada batholith and early phases of magmatism in the Idaho batholith, but the sedimentary record of this voluminous magmatism in the U.S. Cordillera is considerably less conspicuous. New detrital zircon U-Pb ages from the Hornbrook Formation in southern Oregon and northern California reveal a significant and sustained influx of 100–85 Ma detrital zircons into the broader Hornbrook region beginning ca. 90 Ma. Detrital zircon ages and hafnium isotopic compositions, combined with whole-rock geochemistry, suggest that sediment was largely derived from the Sierra Nevada, requiring uplift and erosion of the Sierra Nevada batholith during and immediately following the Late Cretaceous magmatic flare-up event. Sediment derived from the eroded arc may have been transported northward along the axis of the arc, between a western drainage divide along the arc crest and the rising Nevadaplano to the east. Although the Klamath Mountains and Blue Mountains Province present more proximal potential sources of Jurassic and Early Cretaceous detrital zircons in the Hornbrook Formation than the Sierra Nevada, Late Cretaceous deposition on the Klamath Mountains 80 km west of Hornbrook Formation outcrops, and Late Cretaceous deep-water deposition on the Blue Mountains in the Ochoco Basin suggest that these regions were the locus of subsidence and sedimentation, rather than erosion, during Late Cretaceous time. The limited outcrop extent of the Hornbrook Formation may represent only a sliver of a much larger Late Cretaceous Hornbrook basin system. Complete characterization of the episodic magmatic history of continental arcs requires integration of age distributions from the arc itself and from detrital zircons eroded from the arc; it is critical to recognize the potential of drainage systems to transport sediment to depocenters not directly linked to present-day arc exposures.