Magnetic Polarity Stratigraphy of the Early Tertiary Black Peaks Formation, Big Bend National Park, Texas

The fossiliferous Black Peaks Formation consists of fluviatile mudstones, silts, and sands. The faunal succession of this deposit indicates an age ranging from middle Paleocene (Torrejonian) to early Eocene (Clarkforkian). A minimum of three separately oriented paleomagnetic samples were collected from each of 53 sites in the type and one reference section. Depending upon pilot studies and lithology, in the laboratory the paleomagnetic specimens were treated with either AF or thermal demagnetization (at, respectively, a minimum regime of 25 mT or 200° and 400°C), which appeared to isolate the characteristic component of magnetization. The magnetic polarity zonation is predominantly reversed punctuated by several normal intervals. Based on the faunal constraints and magnetostratigraphic pattern, the Black Peaks is correlated to the reversed zone below chron 26, to the base of chron 24, or an absolute interval of time ranging from about 62 to 53 Ma. Previous hypotheses that the Javelina-Black Peaks and Black Peaks-Hannold Hill lithological boundaries approximate, respectively, the Cretaceous-Paleocene and Paleocene-Eocene boundaries, need to be revised in light of the present study. Rates of sediment accumulation represented in the Black Peaks Formation appear to have been relatively constant with a mean value of about 1.8 cm/1000 yrs. The transitions between Torrejonian-Tiffanian and Tiffanian-Clarkforkian faunas occurs in respectively, the reversed zones below chron 26 and chron 24, consistent with the situation in the Clark's Fork Basin of Wyoming.