THE EARLY EOCENE SAN JUAN BASIN FLORA: AN INVESTIGATION INTO THE EARLY CENOZOIC HISTORY OF THE MOUNTAIN WEST

Human influence is driving the Earth towards a hothouse climate similar to what has been observed in the Early Paleogene. Understanding regional variations in Early Paleogene climate, biodiversity, and ecology is critical to understand the climate and ecology of our likely future. The goal of this study is to assess the paleoclimate and ecology of the San Juan Basin in New Mexico by collecting and analyzing fossil flora found in the Cuba Mesa Formation. Samples were collected from two separate sites, the latest Paleocene “Clear as Mud” locality and the early Eocene “Twin Pines” locality. In between the two sites is the Paleocene-Eocene Thermal Maximum (PETM), a geologically abrupt global warming event caused by a major perturbation to the carbon cycle.

The Paleocene site consisted of five separate mud clasts found in the face of a Cuba Mesa sandstone cliff. Fossil leaves were collected from cm scale layers of alternating light and dark gray siltstones. The leaf-bearing layer of our Eocene site is a fine-grained carbonaceous shale lens bounded by two massive sandstones of the Cuba Mesa Formation. A one meter squared quarry was excavated, revealing an Averrhoites affinis- dominated swamp assemblage. Preliminary examination of this Eocene flora suggests the presence of thermophilic species— that is, species that are present in the San Juan Basin in the earliest Eocene, but do not occur in more northerly latitudes until later, when global temperatures warmed to a Cenozoic maximum. Our study suggests a distinct early Eocene temperature gradient between northwestern New Mexico and other well collected Eocene basins of Colorado and Wyoming.