Late Hemingfordian-Barstovian Mammalian Biostratigraphy and Lithostratigraphy of the Tesuque Formation (Middle Miocene), Española Basin, North-central New Mexico

The middle Miocene was a time of significant climatic, ecological, and depositional transitions that shaped mammalian communities and basin evolution across North America. The Tesuque Formation of the Española Basin, north-central New Mexico, preserves a well-documented record of faunal and environmental changes spanning the late Hemingfordian to the Barstovian North American Land Mammal Ages (NALMA). However, the complex tectonic history of the Española Basin, a west-tilted half-graben within the Rio Grande Rift, has complicated stratigraphic correlation due to intrabasinal faulting and facies variability. To address this, a composite stratigraphic section (836 m thick) was developed through the Bureau of Land Management’s Sombrillo Area of Critical Environmental Concern (ACEC), targeting fossil-rich intervals and utilizing widespread volcanic ash beds to stratigraphic correlation across structurally complex areas. This lithostratigraphic framework provides critical context for understanding faunal turnover and paleoenvironmental evolution during the Miocene Climatic Optimum (MCO; ~17-14.5 Ma) and the subsequent Miocene Climatic Transition (MCT; ~14.5-13.9 Ma). During the MCO, the Española Basin experienced warm, humid conditions, coeval with alluvial fan deposition, and the presence of a diverse browsing-adapted fauna, including camelids, equids, rhinos, and oreodonts. As global climate cooling began near the end of the MCO, alluvial fan facies began to retrograde. Floral evidence suggests that while the basin remained relatively wet, cooling temperatures led to the local extinction of palms. This period also saw shifts in mammalian communities, including increased antilocaprid abundance and the appearance of gomphotheres. Four major faunal turnover events (17.1 Ma, 16.3 Ma, 15.3 Ma, and 14.3 Ma) align with climatic and depositional shifts, highlighting the interplay between environmental change and mammalian evolution. The final turnover at ~14.3 Ma coincides with the MCT and a transition from an ecosystem dominated by C3 feeders to an ecosystem with an abundance of mixed feeders incorporating some C4 plants in their diets. By integrating biostratigraphy with lithostratigraphy and paleoecological analyses, this study refines the temporal and environmental framework of the middle Miocene mammalian evolution in the southwestern United States. The results enhance regional biochronologic correlations within the late Hemingfordian to the Barstovian NALMAs and provide new insights into the role of climate in shaping terrestrial ecosystems during the middle Miocene.