This report presents new paleomagnetic and palynologic data that confirm the Paleocene age of the Ojo Alamo Sandstone and its contained dinosaurs in the San Juan Basin as reported in
Fassett and Lucas (2000) and
Fassett et al. (2002). In addition, chemical analyses of new dinosaur bone samples from the Cretaceous Kirtland Formation and Paleocene Ojo Alamo Sandstone expand the existing geochemical data base for such samples in the basin and confirm the findings in
Fassett et al. (2002) that the dinosaur bone present in the Ojo Alamo has not been reworked. Challenges to the Paleocene age of the Ojo Alamo by
Sullivan, Lucas, and Braman (2005), when carefully analyzed, have been found to be unsupported by data.
Fassett et al. (2002) estimated that dinosaurs persisted into early Paleocene time for about 1 m.y. That estimate was based on the assumption that an 11 m thick paleomagnetic-normal interval in the Ojo Alamo Sandstone represented the entire C29n magnetochron. This report (as discussed in the "Paleomagnetism" section) extends the top of magnetochron C29n to well above the top of the Ojo Alamo Sandstone and into the lower part of the Nacimiento Formation (Figure 42). The 11 m thick magnetic-normal interval within the Ojo Alamo Sandstone is now interpreted to be only the lowermost part of chron C29n and is designated C29n.2n herein.
Figure 42 shows that C29n is now known to range from about 68 to 98 m thick.
This increase in the length of magnetochron C29n required a revised estimate of how long dinosaurs lived in the Paleocene in the San Juan Basin. Dinosaur bone has been found to be 8.2 m above the base of the Ojo Alamo in the southern San Juan Basin (Table 2 and
Figure 5) near the Barnum Brown Amphitheater locality (locality J of
Figure 4). At that locality, the base of chron C29n is close to the base of the Ojo Alamo Sandstone. The base of C29n has an age of 65.118 Ma (Gradstein et al. 2004), thus the age of the youngest Paleocene dinosaur fossil that can be linked directly to paleomagnetic data is now estimated to be about 65 Ma. The stratigraphically highest, in-place dinosaur fossil in the entire basin was found at the San Juan River locality (Figure
1.1 and Figure 34), 15.2 m above the base of the formation, however, with no geochronologic data available at that place to quantify the time represented by this stratigraphic interval, it is not possible to say if this bone is younger than the youngest dinosaur bones in the southern part of the basin. Additional paleomagnetic studies of the Ojo Alamo Sandstone at this locality could help resolve this problem.