Paleomagnetism and cunterclockwise tectonic rotation of the Eocene-Oligocene Lyre, Quimper, and Marrowstone formations, Northeast Olympic Peninsula, Washington State, USA

The Eocene-Oligocene Lyre, Quimper, and Marrowstone formations on the northeast Olympic Peninsula of Washington are an important sedimentary sequence whose age has long been controversial. They were thought to span the Eocene-Oligocene transition. In addition, previous paleomagnetic studies on the underlying volcanic rocks produced a counterclockwise tectonic rotation. We sampled three sections: West Indian Island, West Marrowstone Island, and near Woodman's Wharf on Discovery Bay. After both AF and thermal demagnetization, all three formations produced a single-component remanence held largely in magnetite which passes a reversal test and is rotated counterclockwise by about 35°. This is consistent with the results from the underlying lower Eocene Port Townsend volcanics. It is also consistent with counterclockwise rotations obtained from the lower Eocene Metchosin volcanics and upper Oligocene Sooke Formation on the southern tip of Vancouver Island, showing that the counterclockwise rotation is widespread in the region. Based on magnetobiostratigraphic correlations with other sections in the region, we correlate the Lyre Formation with late middle Eocene Chrons C15r-C16r (35.0-36.5 Ma). The Quimper Sandstone spans the Chron C15n and probably the lowermost part of Chron C13r (34.6-35.0 Ma), so the Eocene-Oligocene boundary probably lies in the middle of the Quimper Sandstone, as previously suspected. The Marrowstone Shale is correlated with late Chron C13r.