Early Paleocene fossil floras, paleoclimate, and magnetostratigraphy from the San Juan Basin, New Mexico, USA.

Abstract

The San Juan Basin, located in northwestern New Mexico, preserves an excellent early Paleocene floral and faunal records in North America making it an ideal location to study the tempo and processes of terrestrial ecosystem recovery following the Cretaceous-Paleogene mass extinction. However, the lack of precise age constraints for fossil localities and poor sampling of the early Paleocene floral record has hampered previous work. To address this issue, I constructed a high-resolution age model for the early Paleocene Ojo Alamo Sandstone and lower Nacimiento Formation in the San Juan Basin using magnetostratigraphy, biostratigraphy, and detrital sanidine ages. This age model demonstrated that the earliest Paleocene Ojo Alamo Sandstone deposition was time transgressive, the Nacimiento Formation records a shift from low to high accommodation and an increase in basin subsidence, the Puercan-Torrejonian North America Land Mammal Age faunal transition occurred between 65.03 – 64.66 Ma, and this faunal transition may have been diachronous across North America. Fossil leaves were collected from the Ojo Alamo Sandstone and lower Nacimiento Formation to evaluate changes in early Paleocene floral composition and paleoclimate. The fossil floras from San Juan Basin were diverse, laterally heterogeneous, and dominated by angiosperms. Leaf physiognomic paleoclimate estimates indicate warm temperatures and high precipitation through the entire record consistent with a modern tropical seasonal forest biome. The San Juan Basin flora had higher species richness and warmer paleoclimate estimates compared to contemporaneous northern North American localities indicating an early Paleocene latitudinal diversity and paleoclimate gradient. Early Paleocene floral diversity dynamics were modeled from western North American collections during the first ~1.5 Myr of the Paleocene. Model results indicate simultaneous and volatile changes in floral diversity across western North America, especially during the first ~800 Kyr of the Paleocene, suggesting a global driver in early Paleocene floral diversity. Intervals of decreasing diversity during the early Paleocene correspond with a negative bulk organic δ13C excursion and correlate with the onset of major Deccan Traps eruptive phases suggesting Deccan Traps volcanism impacted early Paleocene floral communities. These results highlight prolonged recovery and increased sensitivity to environmental perturbations in terrestrial ecosystems following the Cretaceous-Paleogene mass extinction.