MAGNETOSTRATIGRAPHY OF THE WILLWOOD FORMATION, BIGHORN BASIN, WYOMING- NEW CONSTRAINTS ON THE LOCATION OF PALEOCENE EOCENE BOUNDARY

The lower Eocene Willwood Formation in the Bighorn Basin of Wyoming pr eserves a rich and diverse mammalian and floral record. The paleomagne tic behavior of the sequence of floodplain paleosols of varying degree s of maturation ranges from excellent to poor. We present a magnetostr atigraphic section for a composite section near Worland, Wyoming, by u sing a set of strict criteria for interpreting the step-wise alternati ng field and thermal demagnetization data of 266 samples from 90 sites throughout the composite section. Correlation to the geomagnetic reve rsal time scale was achieved by combining magnetostratigraphic and bio stratigraphic data from this section, from a section in the Clark's Fo rk Basin in northern Wyoming, and from DSDP Site 550, with the isotopi c date determined on a tuff near the top of our section. Our correlati on suggests that the Bighorn Basin composite section in the Worland ar ea spans from within Chron C24r to near the top of Chron C24n, or from approximately 55 to 52 Ma. This correlation places the Paleocene/Eoce ne boundary within the vicinity of the base of the section. Cryptochro n C24r.6 of Cande and Kent is tentatively identified some 100 m above the base of the section. The temporal framework provided here enables correlation of the mammalian biostratigraphy of the Bighorn Basin to o ther continental sequences as well as to marine records. It also provi des independent chronological information for the calculation of sedim ent accumulation rates to constrain soil maturation rates. We exclude an age as young as 53 Ma for the Paleocene/Eocene boundary and support older ages, as recommended in recent time scales. The location of a t uff dated at 52.8 +/- 0.3 Ma at the older boundary C24n.1 is consisten t with the, age of 52.5 Ma estimated by Cande and Kent and inconsisten t with that of 53.7 Ma, from Harland et al.