MAGNETOSTRATIGRAPHY OF THE LOWER PALEOCENE UPPER NACIMIENTO FORMATION, KUTZ CANYON, SAN JUAN BASIN, NEW MEXICO

Lower Paleocene alluvial foreland basin deposits of the Upper Nacimiento Formation in the San Juan Basin, New Mexico contain an excellent record of early Cenozoic mammalian evolution. The Nacimiento Formation contains a record of early Paleocene faunal mammalian faunal dynamics, and a significant faunal turnover of therian mammals is found between Tj5-Tj6 biozones (corresponding to Torrejonian [To]2 and To3 North American Land Mammal Age [NALMA]). The Tj5-Tj6 faunal turnover is best documented at three locales across the basin: Torreon West, Torreon East, and Kutz Canyon. However, to understand the possible causes of the turnover and to determine if it occurs synchronously across the basin, high resolution geochronology is needed from all of the locales. Previous magnetostratigraphic analyses at Torreon East and Torreon West indicate that the Tj5-Tj6 transition occurs near the top of C27r. However, a detailed magnetostratigraphic study of the Tj5-Tj6 interval has not been conducted at Kutz Canyon limiting our ability to correlate the section, and thus the record of faunal turnover, to the Torreon East and West locales. Kutz Canyon is located near the center of the San Juan Basin and contains one of the thickest outcrops (~65 m) of the Tj5 – Tj6 interval, and thus a key section for reconstructing the paleoenvironment and paleoclimate through the Tj5-Tj6 transition. In this study, we construct a local polarity history of the Kutz Canyon strata, which will be used to develop a precise age model for the section to constrain the ages of the biozones and to correlate it to the Torreon East and West sites. High temperature susceptibility measurements indicate titanohematite and titanomagnetite are the dominant magnetic minerals present in the deposits and demonstrate the suitability of the sediments for magnetistratigraphic analyses. Preliminary polarity stratigraphy for the section indicates an ~35 m thick interval of normal polarity bracketed by intervals of reversed polarity, which we correlate to subchrons C27r, C27n, and C26r of the geomagnetic polarity time scale. The development of this local polarity stratigraphy for Kutz Canyon indicates that it is correlative to the Torreon East and West sections and suggests that the Tj5-Tj6 faunal turnover is synchronous across the San Juan Basin.