Geologic implications of an 40AR/³⁹Ar single-crystal sanidine age for an altered volcanic ash bed in the Paleocene Nacimiento Formation in the southern San Juan Basin
— James E. Fassett, Matthew T. Heizler, and William C. McIntosh

The first ⁴⁰Ar/³⁹Ar single-crystal age for Paleogene strata in the southern San Juan Basin was obtained from sanidine crystals recovered from an altered volcanic ash bed in the Paleocene Nacimiento Formation at Mesa de Cuba near Cuba, NM. The age, 64.0 ± 0.4 Ma (2 sigma), is based on single crystal sanidine results relative to Fish Canyon sanidine at 28.02 Ma and a total 40K decay constant of 5.543E-10/a. Due to significant contamination by older K-feldspar crystals, this date is considered a maximum depositional age. Stratigraphically, the dated ash bed is 119 m above the Cretaceous-Tertiary contact and 76 m above the Nacimiento/Ojo Alamo Sandstone contact. This age places the ash bed near the bottom of magnetochron C28n. The lower part of chron C29n was identified within the underlying Ojo Alamo Sandstone at Mesa Portales, about 12 km south of the Mesa de Cuba ash-bed locality; the base of this chron is 98 m below the dated ash bed. This age determination allowed a calculation of the rate of sedimentation for underlying Paleocene strata of 73 m/m.y. (not corrected for compaction). Extrapolating this rate of deposition to overlying Nacimiento Formation strata to the base of the Eocene San Jose Formation places that formation boundary at 61.42 Ma. Because the Paleocene-Eocene contact is 55.8 Ma, a long-lasting hiatus of at least 5.6 m.y. must be present at the Paleocene-Eocene contact at Mesa de Cuba (assuming there are no significant, intervening unconformities present in Nacimiento strata overlying the dated ash bed). Puercan and (or) Torrejonian mammal fossils have been identified at numerous localities in the southern San Juan Basin, including at Mesa de Cuba. The boundary between these two land-mammal zones is estimated to be at about the top of magnetochron C29n with an age of about 64.4 Ma thus making the duration of the Puercan land-mammal age 1.1 m.y. in the San Juan Basin. This newly documented presence of a 5.6 m.y. hiatus between Paleocene and Eocene rocks in the San Juan Basin will require a revised evaluation of the burial history of the San Juan Basin during Tertiary time.