PROVENANCE OF THE EOCENE PAYNE CLIFFS FORMATION, SOUTHERN OREGON

The Payne Cliffs Formation crops out in southwestern Oregon, but it is unclear how it fits into the regional geology. Deposited in a fluvial system, the Payne Cliffs Formation overlies the Cretaceous Hornbrook Formation, is overlain by the Late Eocene Colestin Formation and the Early Oligocene Roxy Formation, and is cross-cut by a 36.9 Ma basaltic dike that provides a minimum depositional age. Detrital zircon dating gives a maximum depositional age of Middle Eocene, greatly improving depositional age resolution. Conglomerates of the Payne Cliffs Formation consist mainly of quartzite clasts, felsic, intermediate, and mafic vocanic clasts, and metavolcanic clasts, and sandstone petrography shows dissected and transitional magmatic arc provenance. Detrital zircon ages in the Payne Cliffs Formation range from Proterozoic to Eocene, indicating that the fluvial system tapped multiple source regions. The lower Payne Cliffs Formation contains a mid- to Late Cretaceous age peak, a large Late Jurassic peak, and smaller peaks as old as Middle to Early Jurassic. The middle Payne Cliffs Formation is dominated by one large Middle Eocene peak, and a scattering of ages as old as Early Jurassic, with no pre-Mesozoic ages. The upper Payne Cliffs Formation has two major peak ages, Middle Eocene and mid- to Late Cretaceous, with a broad spread of ages back to Early Jurassic, and small Paleozoic and Proterozoic peaks. A granodiorite clast was 157 Ma, and quartzite clasts were dominated by 1700-1900 Ma ages, with smaller peaks at 660-700 Ma, 1060-1080 Ma, 1390-1450 Ma, 2480-2560 Ma, and 2620-2660 Ma. The Middle Eocene ages could be from the Clarno Formation, reflecting the onset of Eocene volcanism in Oregon, and the mid- to Late Cretaceous peak from either the Idaho Batholith, as previously suggested, or the main Cretaceous Batholith of the Sierra Nevadas. The most likely source of the Middle Jurassic peak is within the proximal Klamath Mountains, but the northern Sierran terranes and Blue Mountains could also provide these ages. The quartzite age peak of 1700-1900 Ma is most consistent with the proximal Antelope Mountain Quartzite (AMQ) in the Eastern Klamaths, but also includes ages not associated with the AMQ. Thus, provenance analysis indicates Middle Eocene deposition for the Payne Cliffs Formation, with both proximal and distal sources.