EXAMINING THE TORREJONIAN (To2-To3) BOUNDARY IN THE NACIMIENTO FORMATION, SAN JUAN BASIN, NEW MEXICO

The early Paleocene Nacimiento Formation of the San Juan Basin, New Mexico contains one of the longest and most continuous records of mammalian succession for the early Paleocene. It is also home to the type faunas that were originally used to define the Puercan and Torrejonian North American Land Mammal Ages (NALMAs), This record captures most of the early and most dynamic portion of the adaptive radiation of mammals following the K-Pg mass extinction event. Although models of adaptive radiation predict high, but relatively even rates of turnover, the Nacimiento Formation record deviates from this by containing several brief episodes of unusually high turnover in mammal species. One of these episodes occurs within the Torrejonian NALMA interval and marks boundary between To2 and To3 subdivisions.

In order to better understand the nature of this event, we worked intensively in the area of Torreon Wash, near the southeastern part of the San Juan Basin where the To2-To3 interval of the Nacimiento Formation is well exposed. There we collected vertebrate fossils that bracket this boundary. We also employed screenwashing and other methods to obtain microvertebrates to compensate for previous collecting biases. We tied fossil localities to measured sections obtained from both the east and west flanks of Torreon Wash. This allows direct correlation between the mammal fossil record and potential independent climate proxies preserved in strata such as megafossil plants and paleosols. Moreover, the measured sections were correlated to existing paleomagnetic data that allow correlation to the Paleocene global time scale. With these new data, we hope to test for possible correlation between this early Paleocene episode of mammal turnover and a possible shift in regional or global climate.