Magnetic stratigraphy and tectonic rotation of the Oocene-Oligocene Keasey Formation, northwest Oregon

摘要

The upper Eocene-lower Oligocene Keasey Formation in northwestern Oregon consists of similar to 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 degrees +/- 4.7 degrees, which is consistent with rotation of the nearest volcanic basement rock reported by Wells and Coe [1985]. [References: 54]