Magnetic Polarity Stratigraphy and Biostratigraphy of Middle-Late Paleocene Continental Deposits of South-Central Montana

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Abstract
Exposures of the Fort Union Formation on the divide between Hunt Creek and Cub Creek in the northern Clark''s Fork Basin, Carbon County, Montana, were selected for magnetostratigraphic study of the transition between the Torrejonian and Tiffanian Land-Mammal Ages. Paleomagnetic samples were collected from 25 sits within a 160 m-thick section of the Fort Union Formation at that location. Rock-magnetic analyses indicate that alternating-field demagnetization to peak fields in the 10 to 40 mT interval successfully removed secondary components of natural remanent magnetism (NRM). Characteristic NRM directions define three polarity zones, a 50 m-thick normal polarity zone bracketed by two reversed polarity zones. The Cub Creek local faunule CC-2 (To3 or Ti1) occurs within the upper portion of the normal polarity zone. Cub Creek local faunules, CC-1, CC-3, and Eagle Quarry (all Ti1) occur in the upper reversed polarity zone. These data, along with faunal and magnetostratigraphic data from the San Juan Basin, New Mexico, and the southern Clark''s Fork Basin, Wyoming, allow the transition between the Torrejonian and Tiffanian Land-Mammal Ages to be correlated with the later portion of chron 27. Paleomagnetic and paleontologic data from isolated quarries in the southern Clark''s Fork Basin allow Mantua Quarry (Pu1) to be correlated with chron 29r, while Rock Bench Quarry correlates with the later portion of chron 27r. Data from the Crazy Mountain Basin in Montana indicate that Silberling Quarry (To3) correlates with chron 27r, while Douglass Quarry (Ti1), Scarritt Quarry (Ti2) and Locality 13 (Ti3) correlate with chron 26r.