DETRITAL ZIRCON ASSEMBLAGES IN SANDSTONE INDICATE REVISED LATE EARLY TO LATE CRETACEOUS AGE FOR THE DOTHAN FORMATION IN SOUTHWESTERN OREGON

The Dothan Formation is a flysch sequence exposed as broken formation in SW Oregon. Max-depositional ages from U-Pb LA-ICPMS zircon dating of 537 detrital zircon grains from Dothan sandstone range from 102 to 74 Ma (Early-Late Cretaceous), significantly younger than sparse Late Jurassic-Early Cretaceous fossils previously reported. Zircons were analyzed from Dothan sandstone collected from five widely dispersed inland and coastal sites. Inland exposures within the Dothan type area, along West Fork Cow Creek, return max-depositional ages of 74 Ma and 78 Ma respectively (2 SCJ 13, UTM NAD83 443879E, 4739240N; 1 SCJ 13, 449210E, 4739076N). West of the type area, in the Oregon Coast Range, Dothan sandstone returns a slightly older max-depositional age of 95 Ma (11 SCJ 13, 436831E, 4716884N). Coastal exposures of Dothan sandstone has max-depositional ages comparable to those of inland exposures, returning ages of 89 Ma at the Port Orford Marina (53 SCJ 13, 377221E, 4732986N) and 102 Ma near Brookings (215-11-1, 395965E, 4658639N). The contrast between previously reported Late Jurassic-Early Cretaceous fossil and Early-Late Cretaceous zircon ages likely relates to 1) inclusion of adjacent sections of older Buchia-bearing flysch in the Dothan, such as those in the Sixes River terrane, and those collectively referred to as "Galice" Formation in nearby Western Jurassic terranes; 2) inclusion of older blocks of chert and flysch in olistostromes; and 3) inclusion of older blocks in tectonic shear zones cutting the Dothan.

On the basis of new zircon ages, intense deformation characterizing the Dothan occurred during Latest Cretaceous-Late Paleocene time, largely after 74 Ma. The upper age of Dothan deformation is constrained by much less disturbed early Eocene sedimentary rocks onlapping the formation in the Coast Range of SW Oregon and late Paleocene Siletz River Volcanics exposed north of the Canyonville Fault. This deformation history, combined with an absence of Paleocene rocks elsewhere in SW Oregon, implies a Late Cretaceous min-depositional age for the Dothan. Other late Early-Late Cretaceous submarine fan sequences in SW Oregon include those in the upper part of the coastal Gold Beach terrane, the Hornbrook Formation exposed near Medford, and the Days Creek Formation exposed north of the Canyonville Fault.