New Mexico Geological Society Annual Spring Meeting — Abstracts

Claims of Paleocene dinosaur fossils in the San Juan Basin have been largely based on palynostratigraphy---namely, that in situ Paleocene-age palynomorphs are found stratigraphically below in situ dinosaur fossils. One key locality is Barrel Springs in the west-central San Juan Basin (sec. 17, T24N, R11W), where a carbonaceous shale bed at the top of the De-na-zin Member of the Kirtland Formation supposedly yielded Paleocene-age palynomorphs below the in situ dinosaur fossils in the overlying Naashoibito Member of the Ojo Alamo Formation. We have repeatedly sampled this carbonaceous shale bed, most recently in 2009, and it yields only Cretaceous palynomorphs. Thus, the presence of Tschudypollis thalmanni and Equisetosporites multipartitus at this locality indicates a Late Cretaceous age, probably Maastrichtian. In 2009, we also sampled this carbonaceous shale bed at the top of the Kirtland Formation at Willow Wash, ~ 6 km to the northwest, and here it also yields a Cretaceous palynomorph assemblage. The presence of abundant Tschudypollis thalmanni and T. retusus at this second locality also suggests a Late Cretaceous age for the sample, probably Maastrichtian. We therefore consider the report of Paleocene palynomorphs from this bed at Barrel Spring a non-repeatable result, so it is not reliable for age assignment. Palynostratigraphy provides no support for assigning a Paleocene age to dinosaur fossils in the west-central San Juan Basin, New Mexico.