THE PALEOCENE DINOSAURS OF THE OJO ALAMO SANDSTONE, SAN JUAN BASIN, NEW MEXICO AND COLORADO

The Ojo Alamo Sandstone of the San Juan Basin, NM and CO, is a striking, massive, conglomeratic, cliff-forming sandstone unit containing thin, interbedded mudstones that crops out mostly around the New Mexico part of the San Juan Basin and has a distinctive geophysical-log expression throughout the basin’s subsurface. The abundant dinosaur fossils of the Ojo Alamo have been known for nearly a century and thus, the Ojo Alamo was long thought to be Cretaceous in age. The conformable vs. unconformable nature of the basal Ojo Alamo contact has also been in dispute for many decades.

For the past thirty years, evidence for a Paleocene age for the Ojo Alamo has grown based on palynologic and paleomagnetic data. In addition, precise ⁴⁰Ar/³⁹Ar ages for altered volcanic ash beds in the underlying Kirtland Formation and the overlying Nacimiento Formation have helped clarify the relation of the Ojo Alamo to underlying Cretaceous strata. Fassett (2009) summarized these data finding that: 1) a hiatus of about 8 m.y. separates the Kirtland from the Ojo Alamo, 2) all evidence indicates that the Ojo Alamo is Paleocene in its entirety throughout the San Juan Basin, and 3) the dinosaur fossils in the Ojo Alamo represent the remains of animals that lived and died in Paleocene time. Fassett et al. (2011) found that a large sauropod-dinosaur femur from the Ojo Alamo has an age of 64.8±0.9 Ma using laser ablation U-Pb dating methods. This age places this dinosaur nearly 1.2 m.y. above the K-Pg boundary of 65.96 Ma. This direct dating of a Paleocene dinosaur bone thus provides convincing, or even conclusive evidence that some dinosaurs survived the end-Cretaceous mass extinction. In addition, this first-ever successful direct U-Pb dating of a vertebrate fossil has the potential to revolutionize the geochronology of vertebrate species.

References

Fassett, J.E. (2009), New geochronologic and stratigraphic evidence confirms the Paleocene age of the dinosaur-bearing Ojo Alamo Sandstone and Animas Formation in the San Juan Basin, New Mexico and Colorado: Palaeontologia Electronica, no. 1, 146 p. (on-line pub. at http://palaeo-electronica.org/splash/index12_1.html.

Fassett, J.E., Heaman, L.M., and Simonetti, A., 2011, Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico: Geology, v. 39, no. 2, p. 159-162.