New Mexico Geological Society Annual Spring Meeting — Abstracts

In the San Juan Basin of northwestern New Mexico, Upper Cretaceous-Paleocene vertebrate fossil assemblages (assigned to the Kirtlandian, Edmontonian, Puercan and Torrejonian landvertebrate “ages” [LVAs]) allow precise correlations that are consistent with available radioisotopic dating and magnetostratigraphy. The Fruitland Formation and overlying Kirtland Formation yield the vertebrate assemblage used to define the Kirtlandian LVA, which is associated with radioisotopic ages of 73- 75.5 Ma and a relatively long normal polarity chron most reasonably identified as chron 33n. The base of the Naashoibito Member of the Ojo Alamo Formation is a substantial unconformity overlain by a vertebrate assemblage of Late Cretaceous age, although opinions diverge as to whether it is Edmontonian or Lancian. We prefer to assign it to the former, in part based on the association with a radioisotopic age of 69 Ma in Texas of the characteristic Naashoibito sauropod dinosaur Alamosaurus, and in part based on our re-evaluation of the mammals reported from the Naashoibito Member, some of which have been misidentified, and all of which do not provide conclusive evidence of a Lancian age.

The Naashoibito and overlying Kimbeto Member of the Ojo Alamo Formation are in an interval of reversed polarity, and palynostratigraphy has long been used to place the base of the Paleocene at the base of the Kimbeto Member. We regard all dinosaur bones in the Kimbeto Member as reworked, and we also recognize a substantial unconformity at its base. The oldest Paleocene vertebrates in the San Juan Basin are the fossil-mammal-dominated assemblages used to define the Puercan and Torrejonian LVAs. The Puercan and Torrejonian mammals are part of an extensive correlation web from Montana to New Mexico, correlated to magnetostratigraphy that assigns them to the youngest portion of chron 29r through the oldest portion of chron 26r. The integration of vertebrate biochronology and magnetostratigraphy in the San Juan Basin is consistent with other correlations in the Western Interior that place the youngest dinosaur fossils in uppermost Cretaceous, not Paleocene, strata.