MAGNETIC STRATIGRAPHY AND TECTONIC ROTATION OF THE EOCENE-OLIGOCENE KEASEY AND PITTSBURG BLUFF FORMATIONS, NORTHWEST OREGON

The upper Eocene-lower Oligocene Keasey and Pittsburg Bluff Formations in northwestern Oregon consists of over 1000 m of deep- to shallow-marine forearc basin sediments exposed in streambeds and roadcuts. The abundant molluscs of the Keasey Formation were the basis for the "Keasey Stage" of Weaver et al. (1944), and the Pittsburg Bluff molluscs were considered the classic Oligocene of the region, but the precise age of these formations has been controversial. Analysis of samples from five sections in the Keasey Formation and four in the Pittsburg Bluff yielded a stable remanence held mainly in magnetite which passes a reversal test. On the basis of calibration by benthic foraminifera and molluscs, the Keasey Formation is correlative with Chrons C15r-C12r (35.0-33.0 Ma), so the unit spans the latest Eocene and earliest Oligocene. The overlying Pittsburg Bluff Formation correlates with Chrons C12r-C11n (29.5-33.0 Ma); this is corroborated by an argon-argon date of 29.83 +/- 0.03 Ma on the upper Scaponia Member. The magnetic directions suggest a tectonic rotation of 73.7 +/- 4.7 degrees for the Keasey, and 65 +/- 12 degrees for the Pittsburg Bluff, consistent with the rotations for the nearest middle Eocene volcanic basement rock reported by Wells and Coe (1985).