Revised geochronology and paleomagnetic interpretations of uppermost Cretaceous and lowermost Paleocene rocks in the southern San Juan Basin
James E. Fassett
The first radiometric age for the Nacimiento Formation in the San Juan Basin was from an altered volcanic ash bed at Mesa de Cuba, near Cuba, NM. This 40Ar/39Ar single-crystal sanidine age of 64.0 ± 0.4 Ma was determined relative to the Fish Canyon Sanidine (FCS) with an age of 28.02 Ma. The age of the FCS has recently been revised to 28.294 Ma which makes the adjusted age for the Nacimiento ash bed 64.63 Ma. Based on this age and a newly reported age for an ash bed in the lower part of the Nacimiento Formation of 65.70 ± 0.03 Ma the base of magnetochron C29n has been moved from within and near the base of the Ojo Alamo Sandstone to about the top of the Ojo Alamo. The base of chron C29n and the Nacimiento Formation-Ojo Alamo Sandstone contact appear to be coeval, with an age of 65.76 Ma. The short normal-polarity interval in the lower part of the Ojo Alamo Sandstone present at multiple localities in the southern San Juan Basin is no longer considered to be part of magnetochron C29n, but rather must be a hitherto unknown short-normal interval within chron C29r. The rate of deposition for the stratigraphic interval between the dated Nacimiento ash bed and the top of the Ojo Alamo Sandstone (base of chron C29n) is 57 m/m.y. Extrapolation over the 201 m interval, between the Nacimiento ash bed and the base of the overlying Eocene San Jose Formation results in an age of 61.1 Ma for the base of the San Jose. The Paleocene-Eocene contact is 56.34 Ma, thus a hiatus of at least 4.8 m.y. must be present at the Paleocene-Eocene contact at Mesa de Cuba (assuming there are no significant, intervening unconformities). Puercan and Torrejonian mammal fossils have been identified in the Nacimiento Formation at numerous localities in the southern San Juan Basin. The boundary between these land-mammal stages is estimated to be at about the top of magnetochron C29n with an age of 65.06 Ma. This makes the duration of the Puercan Stage in the southern San Juan Basin about 1 m.y. In addition, this study shows that a recently reported U-Pb age of 64.8 Ma for a dinosaur bone from the Ojo Alamo Sandstone is too young. However, robust palynologic data still strongly support the Paleocene age of the Ojo Alamo Sandstone and its contained dinosaur fossils throughout the San Juan Basin.
- Fassett, James E., 2013, Revised geochronology and paleomagnetic interpretations of uppermost Cretaceous and lowermost Paleocene rocks in the southern San Juan Basin, in: Geology of Route 66 region: Flagstaff to Grants, Zeigler, Kate; Timmons, J. Michael; Timmons, Stacy; Semken, Steve, New Mexico Geological Society, Guidebook, 64th Field Conference, pp. 215-222.