STABLE AND CHAOTIC ERAS: AN EARLY EOCENE TERRESTRIAL RECORD FROM THE SAN JUAN BASIN, NEW MEXICO, USA

The rapid hyperthermal events and global carbon cycle perturbations during the early Eocene are excellent deep-time analogs to modern anthropogenic warming and help us understand how terrestrial environments responded to abrupt climatic changes in the past. In North America, these hyperthermal events intensified the hydrologic cycle causing the deposition of laterally extensive, amalgamated channel bodies which intermingle with more stable floodplain deposits. However, previous work in North America has focused on the mid-high latitudes, with relatively little work further south, inhibiting our understanding of how hyperthermal events effected lower latitudes. The San Juan Basin (SJB), located in northwestern New Mexico, preserves an extensive early Eocene terrestrial record and represents an ideal location to study the effects of early Eocene climate change in mid-low latitude North America. Samples were collected from the southeastern flank of the SJB from the Cuba Mesa and Regina Members of the San Jose Formation. New mammalian biostratigraphy and magnetostratigraphy constrain deposition to the Wasatchian North American Land Mammal Age during magnetic chron 24r (~56.0 to 53.9 Ma). Eight basin-wide, coarse grained sediment packages formed time equivalent paleolandscapes that are likely analogous to amalgamated channel bodies found elsewhere in North America and may represent hydrologic cycle intensification in the SJB. Preliminary sandstone petrography shows shifts in sediment source from feldspar-rich to lithic-rich sandstones up section. Paleosols are poorly drained and developed in the Cuba Mesa Member, but better drained and developed in the overlying Regina Member potentially showing lowering of the relative water table through time. Mean annual temperature estimates using clumped isotope paleothermometry (Δ₄₇) from pedogenic carbonates and mean annual precipitation estimates using paleosol bulk geochemistry indicate warm (25.9 ± 2.1 to 48.2 ± 4.6 °C) and relatively wet (1047 ± 458 to 1765 ± 379 mm/yr) climate, with an overall trend to warmer and wetter estimates in the Regina Member. This ongoing work demonstrates that the SJB represents a potential record of mid-low latitude response to early Eocene climate change.