L. Tauxe et al., MAGNETOSTRATIGRAPHY OF THE WILLWOOD FORMATION, BIGHORN BASIN, WYOMING- NEW CONSTRAINTS ON THE LOCATION OF PALEOCENE EOCENE BOUNDARY, Earth and planetary science letters, 125(1-4), 1994, pp. 159-172
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.