EARLY PALEOCENE PLANT COMMUNITIES FROM THE LOWER NACIMIENTO FORMATION (SAN JUAN BASIN, NEW MEXICO, USA) DOCUMENT RELATIVELY LONG-TERM ECOSYSTEM INSTABILITY FOLLOWING THE CRETACEOUS-PALEOGENE EXTINCTION EVENT

Following the Cretaceous-Paleogene (K-Pg) boundary, early Paleocene fossil floras from southern Canada to central Colorado have been described as relatively low diversity and dominated by long lived, cosmopolitan, mire-adapted taxa for millions of years after the extinction event. However, these floral records are derived from the Northern Great Plains in North America and little is known about the floras of the southern North American terrestrial basins limiting our understanding of the regional floral response following the K-Pg boundary. The Nacimiento Formation in the San Juan Basin (SJB), located in northwestern New Mexico, contains a nearly continuous early Paleocene record with abundant fossil leaves making it an ideal location to examine the early Paleocene floral record of a southern North American basin. Here we present a detailed assessment of the early Paleocene plant communities in the San Juan Basin.

Fossil leaves were collected from the lower Nacimiento Formation from strata that was deposited from ~65.7-64.3 Ma and corresponds with the Puercan 2 (Pu2), Pu3, and Torrejonian 1 (To1) North American Land Mammal “age” (NALMA) biozones. The lower Nacimiento Formation flora is dominated by dicot angiosperms, many of which are endemic to the San Juan Basin, and is considerably more diverse that contemporaneous floras from the Northern Great Plains. Additionally, there is a notable facies affect between localities indicating that the flora was laterally heterogeneous. These results indicate a variable floral response to the K-Pg boundary across North America and suggest early Paleocene floras may have been considerably more diverse than previously suggested. Additionally, there is a major floral turnover coincident with the Pu3-To1 faunal transition. This turnover in both the floral and faunal records at ~64.9 Ma suggests that there may have been external processes, such as climate change, that led to a large-scale ecosystem change ~1 myr into the Paleocene and that there may have been relatively long-term ecosystem instability in plant and animal communities following the K-Pg boundary.