MAGNETIC STRATIGRAPHY AND TECTONIC ROTATION OF THE OLIGOCENE-MIOCENE ALSEA, YAQUINA, AND NYE FORMATIONS, CENTRAL OREGON COAST

The Alsea Siltstone, Yaquina Sandstone, and Nye Mudstone make up a sequence of almost 3300 m of Oligocene-Miocene marine strata. It is one of the thickest and most continuous records of the mid-Cenozoic in Oregon, yet dating of these rocks has been imprecise in the past. Magnetic samples were taken spanning all three formations on the north shore of Yaquina Bay, and at the type section of the Alsea Siltstone on Alsea Bay. The samples yielded a stable remanence held mainly in magnetite which passes a reversal test, and shows a tectonic rotation of 45-80 degrees clockwise, consistent with previous studies in the area. We correlate the Alsea Siltstone with earliest Oligocene Chrons C12r-C13r (30.6-33.7 Ma), the Yaquina Sandstone with early and late Oligocene Chrons C12n-C9r (30.6-28.0 Ma), and the lower part of the Nye Mudstone with late Oligocene Chrons C9n-C8n (28.0-26.0 Ma). The upper part of the Nye Mudstone is too poorly exposed for dense sampling or confident correlation, but must be as young as early Miocene, based on Zone M1a planktonic microfossils.