Magnetic Polarity Stratigraphy and Biostratigraphy of Paleocene and Lower Eocene Continental Deposits, Clark's Fork Basin, Wyoming

Abstract

Paleomagnetic samples were collected at 273 sites within a 2012 m continental sedimentary sequence in the Clark's Fork Basin near Powell, Wyoming. The lower 1158 m is in the Polecat Bench Fm while the upper 854 m is within the Willwood Fm. Although significant secondary components of magnetization were present, alternating-field demagnetization to 300 oe peak field revealed the polarity of the primary component of the natural remanent magnetism. A well-defined polarity zonation is observed. The Polecat Bench Fm is dominantly of reversed polarity but does contain two normal polarity zones. The Willwood Fm in this section is entirely of reversed polarity. Abundant Tiffanian, Clarkforkian, and Wasatchian faunas indicate that the age range of the sediments is from late Paleocene through early Eocene. These age constraints, along with the characteristic polarity sequence, allow the magnetic polarity sequence to be correlated with the magnetic polarity time scale. The section correlates with the magnetic polarity time scale from the reversed polarity interval preceding anomaly 26 chron into the reversed polarity interval preceding anomaly 24 chron. Tiffanian fossils are found in sediments deposited during the reversed polarity interval preceding anomaly 26 chron up into sediments deposited during anomaly 25 chron. Clarkforkian faunas occur in sediments deposited during anomaly 25 chron into the base of the overlying reversed polarity zone. Wasatchian faunas are found within sediments deposited during the reversed polarity interval preceding anomaly 24 chron. The Paleocene/Eocene boundary occurs in the Clark's Fork Basin in the reversed polarity interval preceding anomaly 24. These data indicate that the age of anomaly 24 chron is early Eocene rather than late Paleocene.