PROVENANCE OF LATE JURASSIC TO LATE CRETACEOUS SANDSTONES IN SOUTHWEST OREGON

We present >450 ²⁰⁶Pb/²³⁸U detrital zircon LA-ICPMS ages from six samples of Late Jurassic to Late Cretaceous sedimentary rocks to document sediment sources and evaluate the depositional history of fault-bounded terranes in southwest Oregon. Samples were collected from the Dothan Formation in the Yolla Bolly terrane (YBt), the Houstanaden Creek and Hunter’s Cove Formation in the Gold Beach terrane (GBt), the Fivemile Point Formation in the Sixes terrane (St), and the Rocky Point Formation and Humbug Mountain Conglomerate in the Western Klamath terrane (WKt).

Our results indicate that Late Jurassic to Late Cretaceous sandstones in the GBt, YBt and WKt are dominated by Jurassic zircons with rarer Paleozoic and Proterozoic zircons. A sample from the Houstanaden Creek Formation (GBt) yielded a unimodal zircon population at 146 Ma. Jurassic zircons from the Dothan Formation (YBt) gave more varied Jurassic age populations of 144, 159, 185 Ma and Precambrian zircons with age distribution peaks at 1045, 1244, 1441, 1728 and 1816 Ma. Two samples from the Rocky Point Formation and the Humbug Mountain Conglomerate (WKt) yielded Jurassic peak populations at 156, 171, 181, 198 Ma, minor Triassic peaks at 226 and 234 Ma, and Precambrian age peaks at 1094 and 1208 Ma. These Jurassic and Triassic age populations are common in exposed plutonic rocks in the western US Cordillera (e.g., Klamath Mountains, Northern Sierras, Blue Mountains province) and likely indicate derivation from proximal sources with contribution from transcontinental sands shed from the greater Ouachita-Appalachian orogeny and southwest Laurentian cratonic sources.

Cretaceous sandstones from Fivemile Point (St) and the Hunter’s Cove Formation (GBt) are dominated by Jurassic and Early Cretaceous zircons. The Hunter’s Cove Formation contained peak age distributions of 92, 108, 127, 158, 193 and 1325 Ma. The Fivemile Point yielded peak age distributions of 103, 111, 118, 127, 159, 166, 198 and 209 Ma. Exposed plutonic rocks with Early and Middle Jurassic ages are preserved in the Klamath Mountains and northern Sierra Nevada, and Early Cretaceous plutonic rocks are ubiquitous in the central and southern Sierra Nevada. We suggest that Early to Late Cretaceous sands in southwest Oregon were likely sourced from southerly sources in the Klamath Mountains and Sierra Nevada.