MAGNETIC STRATIGRAPHY OF THE UPPER EOCENE FLORISSANT FORMATION, COLORADO

Florissant Fossil Beds National Monument in the Rocky Mountains just west of Colorado Springs is one of the world's classic Lagerstätten, famous for its extraordinary preservation of fossil plants, arthropods (especially insects), mollusks, and a few fossil vertebrates (fish, birds, and mammals). The mammals indicate a Chadronian (late Eocene) age, based on the co-occurrence of brontotheres and the horse Mesohippus. The fossil flora includes many types of leaves and huge in situ redwood stumps, and is thought to represent a warm-temperate forest, the final phase of the late Eocene warming before the early Oligocene climatic crash (represented by the nearby early Oligocene Antero flora). The Florissant Formation consists of about 70 m of lake shales and conglomerates, interbedded with and surrounded by volcanic ashes. In the summer of 2003, we sampled the exposed parts of the section with 17 paleomagnetic sites (3 samples per site). The single-component remanence was held mainly in magnetite with only minor overprinting. All but one of the sites was reversed in polarity and antipodal to the single normal site, so they pass a reversal test, and the remanence is primary. Based on a mean ⁴⁰Ar/³⁹Ar date of 34.07 ± 0.10 Ma on four tuffs in the upper part of the section, we correlate this long reversed magnetozone with Chron C13r (33.7-34.7 Ma), consistent with the Chadronian age (based on the overlapping range of brontotheres and Mesohippus) of the mammalian fauna.