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 1650 mm of average annual precipitation. Even though it considered one of the youngest Eocene floras before the early Oligocene cooling event, its age is constrained only by a questionable 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 compo- nent 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, 95 = 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). If taken at face value, the radiometric date would correlate the La Porte Tuff with Chron C13r (33.7-34.9 Ma), or very latest Eocene in age. However, if the La Porte ash date is suspect then the La Porte flora might be between 37 Ma (tropical Kummerian stage) and 35 Ma old (beginning of Goshen-type stage) and possibly then aligns with Chron C15r (35.1-35.3 Ma) or Chron C16r (36.3-36.5). This could better explain the discrepancies between the La Porte flora and other latest Eocene Goshen- type floras, and make the La Porte flora transitional between typical Chalk Bluffs floras and late Eocene Goshen- type floras.