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
Dinosaur fossils are present in the Paleocene Ojo Alamo Sandstone and Animas Formation in the San Juan Basin, New Mexico, and Colorado. Evidence for the Paleocene age of the Ojo Alamo Sandstone includes palynologic and paleomagnetic data. Palynologic data indicate that the entire Ojo Alamo Sandstone, including the lower dinosaur-bearing part, is Paleocene in age. All of the palynomorph-productive rock samples collected from the Ojo Alamo Sandstone at multiple localities lacked Cretaceous index palynomorphs (except for rare, reworked specimens) and produced Paleocene index palynomorphs. Paleocene palynomorphs have been identified stratigraphically below dinosaur fossils at two separate localities in the Ojo Alamo Sandstone in the central and southern parts of the basin. The Animas Formation in the Colorado part of the basin also contains dinosaur fossils, and its Paleocene age has been established based on fossil leaves and palynology.
Magnetostratigraphy provides independent evidence for the Paleocene age of the Ojo Alamo Sandstone and its dinosaur-bearing beds. Normal-polarity magnetochron C29n (early Paleocene) has been identified in the Ojo Alamo Sandstone at six localities in the southern part of the San Juan Basin.
An assemblage of 34 skeletal elements from a single hadrosaur, found in the Ojo Alamo Sandstone in the southern San Juan Basin, provided conclusive evidence that this assemblage could not have been reworked from underlying Cretaceous strata. In addition, geochemical studies of 15 vertebrate bones from the Paleocene Ojo Alamo Sandstone and 15 bone samples from the underlying Kirtland Formation of Late Cretaceous (Campanian) age show that each sample suite contained distinctly different abundances of uranium and rare-earth elements, indicating that the bones were mineralized in place soon after burial, and that none of the Paleocene dinosaur bones analyzed had been reworked.
James E. Fassett. U. S. Geological Survey, Emeritus, 552 Los Nidos Drive, Santa Fe, New Mexico 87501
KEY WORDS: Paleocene dinosaurs; K-T interface, geochronology, palynology, paleomagnetism, vertebrate paleontology
PE Article Number: 12.1.3A
Copyright: U.S. Geological Survey, Public Domain April 2009
Submission: 13 December 2007. Acceptance: 28 January 2009