Abstract
The upper Eocene‐lower Oligocene Keasey Formation in northwestern Oregon consists of ∼700 m of deep marine forearc basin sediments exposed in stream beds and roadcuts. The abundant molluscs of this unit were the basis for the Keasey Stage of the Weaver et al. [1944] timescale, but the precise age of this fauna and formation have long been controversial. Analysis of over 130 magnetic samples from the Keasey in five sections yielded a stable remanence held mainly in magnetite which passed 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 early Oligocene. The magnetic directions suggest a tectonic rotation of 73.7°±4.7°, which is consistent with rotation of the nearest volcanic basement rock reported by Wells and Coe [1985].
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