Magnetostratigraphy of the Lower Miocene Sandstone of Floras Lake and the Cape Blanco Flora, Oregon

The sandstone of Floras Lake contains a tuff unit yielding the Cape Blanco flora, at Cape Blanco on the southwestern coast of Oregon. Previous dating of these rocks was based on a Newportian molluscan fauna (roughly middle Miocene), but there is a single-crystal plagioclase ⁴⁰Ar/³⁹Ar date of 18.24 ± 0.86 Ma on the plant-bearing tuff layer near the top of the section. Thirteen magnetic sites were taken as oriented block samples spanning the 150 m of section, subsampled into at least 8 cores per site, and analyzed on a 2G cryogenic magnetometer with a Caltech-style automatic sample changer. After measurement of NRM, samples were AF demagnetized, then thermally demagnetized at 50°C steps from 100 to 650°C. Most samples showed a stable single-component magnetization, or only slight overprinting. Based on the coercivity behavior, the remanence is held mostly in magnetite. Both normal and reversed polarities were obtained, and passed a reversal test for stability. Inverting all the reversed directions gives a formational mean of D = 38.0, I = 43.6, k = 6.7, α₉₅ = 7.1, which suggests a clockwise tectonic rotation of 37.6° ± 7°, consistent with other results from southwestern Oregon. The basal 35 m of the section is reversed in polarity, followed by a 45-m-thick interval of normal polarity. From 80 to 115 m on the section is reversed, and the uppermost 40 m of the section (including the dated tuff) is normal in polarity. Based on the 18.24 Ma ash date near the top, we correlate these four polarity zones with Chrons C6r to Chron C5en, or 20.0-18.0 Ma (based on the new time scale of Lourens et al., 2004). Thus, the entire 150 m of section spans about 2 million years, and the flora is late early, not middle, Miocene.