September 3, 2020
Bureau of Geology Associate Director for Labs and Senior Geochronologist Dr. Matthew Heizler co-authored a recent paper “Early Paleocene Magnetostratigraphy and Revised Biostratigraphy of the Ojo Alamo Sandstone and Lower Nacimiento Formation, San Juan Basin, New Mexico, USA” in the September/October 2020 issue of GSA Bulletin.
The study used a combination of magnetostratigraphy and 40Ar/39AR geochronology to generate a high-resolution age model for the Ojo Alamo Sandstone and lower Nacimiento Formation in the San Juan Basin in northwestern New Mexico, spanning approximately 65.7 to 62.6 million years ago during the Paleocene epoch.
The Ojo Alamo Sandstone and the Nacimiento Formation contain arguably the best preserved early Paleocene mammalian fossil record in North America, including type localities for the Puercan and Torrejonian North American land mammal ages, fossil assemblages used to produce age estimates for sediments across North America. The revised age model suggests that the appearance of Torrejonian mammals following the end-Cretaceous mass extinction occurred at different times, first appearing in Montana and North Dakota, then in the middle latitudes in Utah, and lastly in New Mexico.