HIGH-RESOLUTION LATEST CRETACEOUS TO EARLY PALEOCENE MAGNETOSTRATIGRAPHY OF THE SAN JUAN BASIN, NEW MEXICO, USA CONSTRAINS THE AGE OF DINOSAUR AND MAMMALIAN FAUNAS ACROSS THE K/PG BOUNDARY

The San Juan Basin (SJB), located in northwestern New Mexico and southwestern Colorado, preserves a nearly continuous sequence of Upper Cretaceous to lower Paleocene terrestrial deposits that documents a unique latest Cretaceous dinosaur fauna and the type sections for the early Paleocene Puercan (Pu) and Torrejonian (To) North American Land Mammal Ages. However, the lack of a high-resolution age model obscures the timing and pacing of events surrounding the Cretaceous-Paleogene (K-Pg) boundary within the SJB and inhibits comparison of the SJB to contemporaneous deposits across North America. Here we present high-resolution magnetostratigraphy spanning the latest Cretaceous to early Paleocene from multiple locations across the SJB.

Paleomagnetic samples were collected from the Upper Cretaceous Naashoibito Member of the Kirkland Formation and the lower Paleocene Ojo Alamo Sandstone and lower Nacimiento Formation. The Naashoibito Member correlates with the top of magnetic chron C30n and the base C29r of the geomagnetic polarity timescale (GTPS) indicating latest Maastrichtian deposition. In the northwest portion of the basin, the lower Paleocene Ojo Alamo Sandstone, which unconformably overlies the Naashoibito Member, was deposited entirely within magnetic C29r indicating the unconformity that removed the K-Pg boundary is relatively short (<400 kyr). In the southeast, the upper Ojo Alamo Sandstone was deposited in C29n indicating that the formation is time transgressive from north to south. The basal Nacimiento Formation correlates to magnetic chron C29r in the northwest and to C29n in the southeast. Above the diachroneity at the base of the formation, the Arroyo Chijuillita and Ojo Encino Members of the Nacimiento correlate to chrons C29n - C26r of the GPTS. Using our age model, the latest Cretaceous SJB dinosaur and mammalian faunas are temporally equivalent with the Hell Creek fauna of northern North America and early Paleocene Pu2 and Pu3 mammalian localities were correlated with C29n, To1 localities with C28n, To2 localities with C27r, and To3 with C27n. This revised age model allows comparison of SJB faunas to contemporaneous faunas from across North America to better understand patterns of regional faunal occurrence, extinction, and origination in the latest Cretaceous and early Paleocene.