40AR/39AR SANIDINE AGES CONFIRM THE ~250 KY DURATION OF DEPOSITION OF THE PALEOCENE, DINOSAUR-BEARING, OJO ALAMO SANDSTONE IN THE SOUTHERN SAN JUAN BASIN, NEW MEXICO

The Ojo Alamo Sandstone of the San Juan Basin, New Mexico and Colorado, is early Paleocene in age in its entirety throughout the basin based on contained palynologic data from numerous localities in the basin. The Ojo Alamo also contains abundant dinosaur fossils found in multiple localities in the basin. Conclusive evidence shows that these dinosaur bones have not been reworked and were fossilized in place in the Ojo Alamo Sandstone as evidenced by trace-element geochemistry of large suites of these fossil bones. An earlier report, based on existing ⁴⁰Ar/³⁹Ar sanidine ages bracketing the Ojo Alamo, suggested that it was deposited from between about 65.95 Ma and 65.70 Ma or over about 250 ky in the southern San Juan Basin. (The Ojo Alamo Sandstone is on the order of 25-30 m thick in that area; the Ojo Alamo ranges in thickness up to 120 m in places in the northern part of the basin and in those areas its depositional interval could represent as much as one million years.) The previously reported age for the top of the Ojo Alamo was based on a ⁴⁰Ar/³⁹Ar sanidine age of 65.7 Ma age for an altered volcanic ash bed from the lower part of the overlying Nacimiento Formation, however the distance above the top of the Ojo Alamo Sandstone was not known at that time. It is now known that this ash bed has an age of 65.5 Ma and that the dated ash bed is 13 m above the top of the Ojo Alamo Sandstone. Because the rate of deposition of the lowermost Nacimiento Formation in the southern San Juan Basin is about 60 m/my, the top of the Ojo Alamo must be about 65.7 Ma based on the more precise data discussed above. Thus the previous determination of the time represented by Ojo Alamo Sandstone deposition in the southern San Juan Basin of about a quarter-million years has now been confirmed by more precise data. However, in those parts of the basin where the Ojo Alamo is thicker, the time interval for its deposition would have probably been proportionally greater and its upper boundary would be commensurately younger. A published U/Pb age for a dinosaur bone from the lower part of the Ojo Alamo Sandstone of 64.8+/- 0.9 Ma would have a maximum uncertainty age of 65.7 Ma and is thus too young – dinosaur bones in the lower part of the Ojo Alamo must be on the order of 65.9 Ma, based on the available data discussed above.