New Mexico Geological Society Annual Spring Meeting — Abstracts

A 40Ar/39Ar age for an altered volcanic ash bed from the Nacimiento Formation at Mesa de Cuba in the southern San Juan Basin was reported in Fassett et al. (2010). This 40Ar/39Ar single-sanidine-crystal age of 64.0 ± 0.4 Ma was determined relative to the Fish Canyon Sanidine (FCS) age of 28.02 Ma. A recent publication (Renee et al., 2013) has revised the FCS age to 28.294 Ma making the revised age of the Nacimiento ash bed 64.63 Ma. This new age for the Nacimiento ash bed plus adjustments to the levels of the top and base of magnetochron C29n moves the base of C29n from near the base (as shown in Fassett et al., 2010) to the top of the Ojo Alamo Sandstone. The base of C29n is 65.76 Ma (cf. Gradstein et al., 2012) and the top of the Ojo Alamo Sandstone is approximately this same age. This means that the Ojo Alamo Sandstone was deposited between 66.04 Ma (Renee et al., 2013) and 65.76 Ma. At a rate of deposition of 57 m/m.y. (obtained for the lower part of the Nacimiento Formation) for the overlying Nacimiento to to the base of the Eocene San Jose Formation, results in an age for the top of the Nacimiento of 61.1 Ma. Because the Paleocene-Eocene contact is 56.34 Ma, a hiatus of at least 4.8 m.y. must exist 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 in the Nacimiento Formation at numerous localities in the southern San Juan Basin, including at Mesa de Cuba. 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 making the duration of the Puercan Stage in the southern San Juan Basin about 1 m.y. These revisions indicate that paleomagnetic data no longer confirm the Paleocene age of the Ojo Alamo Sandstone. However, robust palynologic data still strongly support the Paleocene age of the Ojo Alamo Sandstone and its contained dinosaur fossils.

References:

1. Fassett, J.E., Heizler, M.T., and McIntosh, W.C., 2010, Geologic implications of a 40Ar/39Ar single-crystal sanidine age for an altered volcanic ash bed from the Paleocene Nacimiento Formation in the southern San Juan Basin, in Fassett, J.E., and Zeigler, K.E., eds., Geology of the Four Corners Country: New Mexico Geological Society 61st Field Conference Guidebook, p. 147-156.
2. Renne, P.R., Deino, A.L., Hilgen, F.J., Kuiper, K.F., Mark, D.F., Mitchell, W.S, III, Morgan, L.E., Mundil, R., and Smit, J., 2013, Time scales of critical events around the Cretaceous-Paleogene boundary: Science, v. 339, p. 684-687.