LATERAL FLORAL VARIABILITY IN THE EARLY PALEOCENE NACIMIENTO FORMATION, SAN JUAN BASIN, NM

The San Juan Basin (SJB), located in northwest New Mexico, preserves one of the best early Paleocene mammalian evolution, spanning the Puercan and Torrejonian North American Land Mammal “ages” (NALMA). In addition to the remarkable mammal record through the Nacimiento Formation, fossil leaves are also very common. However, the fossil plant record is relatively understudied. We present a description of two laterally equivalent early Paleocene floras from different facies from the basal Nacimiento Formation in SJB and a comparison of these leaf localities to understand lateral variability in plant communities.

Fossil leaf samples were collected from the Nacimiento Formation during the summers of 2011 and 2014. This research focuses on two sites, DP1108 and DP1109, which have an absolute age of 65.47 Ma and are located ~150 m apart from each other. DP1108 was collected from a carbonaceous shale, interpreted as a swamp facies, and DP1109 was collected from a siltstone interpreted as a pond facies. Despite their close proximity, the two sites have only one morphotype in common. DP1109 has a higher species richness and is dominated by two dicot angiosperm morphotypes, SJ46 and SJ82, which make up almost 70% of the sample collection. A fern morphotype, SJ57, and a sycamore-like morphotype, SJ36, are also relatively common. These morphotypes suggest the presence of trees and/or shrubs around a ponded environment. In contrast, site DP108 has lower species richness and is not dominated by any single morphotype. The common occurrence of an equisetum-like morphotype, SJ62, suggests the presence of standing water. The morphotype composition of the two floras indicate considerable lateral floral heterogeneity across a small geographic area and suggest that facies significantly influenced local plant community composition of the Nacimiento Formation.