MAGNETIC STRATIGRAPHY OF THE LATE EOCENE LA PORTE FLORA, NORTHERN SIERRAS, CALIFORNIA

The La Porte flora, from a tuff bed disconformably overlying old hydraulic mining excavations in the Ione Formation near the town of La Porte, Plumas County, California, is a famous fossil assemblage originally described in 1935. It yields about 41 species of fossil plants that suggest a relatively warm late Eocene mean annual temperature of about 22-24°C, and 1.65 m of average annual precipitation. Even though it is one of the youngest Eocene floras before the early Oligocene cooling event, its age is constrained only by a potassium-argon date of 34.275 Ma on the La Porte tuff at the top of the section. We sampled the entire 40 m of Ione Formation below the tuff, as well as the La Porte tuff itself. Most samples produced a single stable component of remanence held largely in magnetite, based on the low-coercivity response of the samples in alternating field demagnetization, and the lack of remanence above the Curie temperature of magnetite. The entire 40 m of section was reversed in polarity, with only a minor normal overprint on a few samples. The Ione formational mean direction was D = 186.6, I = -57.8, k = 5.4, alpha95 = 16.5, which is antipodal to the Eocene pole, and not significantly rotated or translated. Based on radiometric dates of 45 Ma on ashes in the Ione Formation elsewhere in the region, we correlate the Ione section at La Porte with Chron C20r (43-45.1 Ma). The radiometric date correlates the La Porte Tuff with Chron C13n (33.7-34.9 Ma), or very latest Eocene in age. It is very close to the age of the Florissant flora in Colorado and the Goshen flora near Eugene, Oregon, both of which also represent relatively warmer conditions just before the early Oligocene cooling event.