GEOCHRONOLOGY OF LOWERMOST PALEOCENE NACIMIENTO AND OJO ALAMO FORMATIONS, SAN JUAN BASIN, NEW MEXICO

The first ⁴⁰Ar/³⁹Ar single-crystal sanidine date for Tertiary strata in the San Juan Basin has been obtained from an altered volcanic ash bed in the Paleocene Nacimiento Formation at Mesa de Cuba in the southeast part of the basin. This age is 64.67 ± 0.38 Ma (2 sigma) and is based on single crystal sanidine results relative to Fish Canyon Sanidine at 28.28 Ma and a total 40K decay constant of 5.53E-10/a. Due to significant contamination by older K-feldspar this date is considered a maximum depositional age. The dated ash is 163 m above the Cretaceous-Tertiary interface and 118 m above the base of the Nacimiento/top of the Ojo Alamo Sandstone. This age places the dated ash bed in the upper part of magnetochron C29n. The lower part of chron C29n (top not determined) has been identified within the underlying Ojo Alamo Sandstone, 123 m below the dated ash bed. This age determination allows the calculation of the rate of sedimentation for underlying Paleocene strata of 440 m/m.y (not corrected for compaction). Extrapolating this rate for overlying Paleocene strata to the base of the overlying Eocene San Jose Formation puts this contact at 64.33 Ma. Because the Paleocene-Eocene contact has been found to be 55.8 Ma, an enormous unconformity of 8.5 m.y. must be present at the Paleocene-Eocene contact at this locality (assuming there are no significant, intervening unconformities present in Nacimiento strata overlying the dated ash bed). The extremely high deposition rate of 440 m/m.y. calculated on the basis of this dated ash bed makes us suspect that the age of 64.67 Ma may be older than the actual age of the eruption that produced this ash fall. We are currently processing additional sanidine samples from this ash to try and verify its age. Palynological analyses of rock samples from the Nacimiento Formation are also currently under way as an independent geochronologic tool to date these strata.