NEW AGE CONSTRAINTS ON THE LATE CRETACEOUS THROUGH EARLY PALEOCENE ROCKS IN THE SAN JUAN BASIN, NEW MEXICO

Upper Cretaceous and lower Paleocene rocks in the San Juan Basin, New Mexico, contain a robust record of dinosaur and mammal evolution, diversification, and extinction. Despite this rich fossil record, the ages and durations of the Upper Cretaceous Naashoibito Member of the Kirtland Formation and the lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation have been relatively poorly constrained. These poor age constraints have limited the ability to accurately correlate these vertebrate records to others across North America and to assess rates of speciation and extinction of dinosaurs in the Cretaceous and mammals before and after the Cretaceous-Paleogene (K-Pg) mass extinction. Further, the ages of the Naashoibito and the Ojo Alamo are contentious and have been interpreted to range from Campanian to early Paleocene.

Here we present new geochronologic results that combine magnetostratigraphy and ⁴⁰Ar/³⁹Ar dating of detrital sanidine from sedimentary units and sanidine phenocrysts from a volcanic ash to constrain the ages of the Naashoibito Member, the Ojo Alamo Sandstone, and the lower Nacimiento Formation. Coupled detrital sanidine dates and magnetostratigraphy indicate that the Naashoibito correlates to chrons C31n – C29r, suggesting a protracted depositional history with multiple disconformities. Further, our results indicate that the youngest Cretaceous sedimentary rocks in the San Juan Basin were likely deposited within the last 300 kyr of the Cretaceous. A ⁴⁰Ar/³⁹Ar sanidine date of 65.59±0.01 Ma (1S, analytical error only) from an ash within the Nacimiento demonstrates that biozone Pu2 (2^nd biozone of the Puercan Land Mammal “age”) began within ~440 kyr of the K-Pg boundary. A probable volcanic ash coincident with the first occurrence of Pu3 mammals yielded an age of 65.43±0.04 Ma, tentatively suggesting that Pu2 was only ~150 kyr long. These dates and our magnetostratigraphy indicate that the Ojo Alamo was deposited in chron C29r and the lower Nacimiento in chrons C29r – C28r. This new geochronology helps to constrain the ages of the first occurrence of the Pu2 and Pu3 faunas in the San Juan Basin and indicates that deposition of basal Paleocene strata in the basin began <300 kyr after the K-Pg boundary.

All argon dates are relative to FC-2 sanidine at 28.201 Ma and 40K decay constant of 5.463e-10 /a.