Magnetostratigraphy and tectonic rotation of the Eocene-Oligocene Makah and Hoko River Formations, northwest Washington, USA

Abstract

The Eocene-Oligocene Makah Formation and subjacent middle Eocene Hoko River Formation of the northwestern Olympic Peninsula, Washington, yield mollusks, crustaceans, foraminifera, and early neocete whales; their age has never been precisely established. We sampled several sections; most samples showed a stable single-component remanence held largely in magnetite and passed a Class I reversal test. The upper Refugian (late Eocene) and lower Zemorrian (early Oligocene) rocks at Baada Point correlate with Chron C13r (33.7-34.7 Ma) and Chron C12r (30-33 Ma). The Ozette Highway section of the Makah Formation spanned the early Refugian to late Refugian, with a sequence that correlates with Chrons C15r-C13r (33.7-35.3 Ma), and a long reversed early Zemorrian section that correlates with Chron C12r (30-33 Ma). The type section of the Hoko River Formation correlates with Chron C18r (40.0-41.2 Ma). The area sampled shows about 45 ° of post-Oligocene counterclockwise tectonic rotation, consistent with results obtained from the Eocene-Oligocene rocks in the region. © 2009 Donald R. Prothero et al.