Investigating links between climate perturbations, river discharge variability and fluvial fans in the Paleogene San Juan Basin, New Mexico, USA

Abstract

The San Juan Basin in New Mexico preserves the southernmost sedimentary record of the Paleocene and early-Eocene in North America, and thus represents a critical point for comparison for climatic and biotic change during the Paleogene. This dissertation combines basin-scale 3D outcrop analyses of fluvial architecture and preliminary stable carbon isotope records from the Paleocene upper Nacimiento Formation and the early-Eocene San Jose Formation to investigate the link between Paleogene climate perturbations and depositional trends. Through these investigations, we 1) identify the succession as deposits of variable discharge river systems, 2) observe a long-term stratigraphic trend toward increasingly well-drained floodplain deposits, and thus successively more arid conditions from Paleocene into the early-Eocene that we suggest may indicate a long-term shift from a monsoonal climate in the Paleocene to fluctuating humid and arid subtropical and then semi-arid/arid conditions in the early Eocene, 3) identify the Paleocene uppermost Nacimiento Formation and early-Eocene Cuba Mesa and Regina Members of the San Jose Formation as deposits of a large fluvial fan and identify at least two vertical packages of fan progradation, and 4) identify at least two negative carbon isotope excursions that may record the Paleocene-Eocene Thermal Maximum (PETM) and one or more post-PETM hyperthermal events. By combining the stratigraphic analyses with the carbon isotope records, we observe that the packages of fan progradation coincide with the negative carbon isotope excursions, suggesting that changes in water discharge patterns linked to climate were a key control in sedimentary delivery to the basin. These findings have implications for paleoclimate interpretations, variable discharge river and fluvial fan facies models, and improved understanding of San Juan Basin landscape evolution. Furthermore, the recognition of negative carbon isotope excursions suggests that the San Juan Basin may provide a new dataset for studying climatic and biotic changes associated with the Paleocene-Eocene Thermal Maximum and post-PETM hyperthermal climate perturbations.