REVISITING THE PALEOCENE-EOCENE BOUNDARY IN THE SAN JUAN BASIN OF NEW MEXICO

The San Juan Basin (SJB) of New Mexico and Colorado is the southwestern-most Laramide basin known to contain a succession of sedimentary deposits spanning the Paleocene-Eocene (P-E) boundary. Nevertheless, the basin has been overlooked as interest in climate change across the P-E boundary has increased in recent years. Here, we present preliminary results from an ongoing investigation of the Paleocene Nacimiento Formation and the early-Eocene San Jose Formation (Cuba Mesa - Regina Members) in the SJB. The long-term goals of this study are to (1) refine the chronostratigraphy of the P-E boundary and determine whether records of early-Eocene hyperthermals are preserved, (2) describe climatic conditions and their impact on fluvial sedimentation, and (3) compare data collected from the SJB with published datasets from other Laramide basins.

Preliminary biostratigraphic data suggest a late-Paleocene age for the upper Nacimiento Formation and an early-Eocene age for the upper-most Cuba Mesa Member. Variations in carbon isotope ratios of lignitic wood fragments from the Cuba Mesa Member are indicative of carbon isotope excursions, however, additional data are needed to determine if records of the Paleocene-Eocene Thermal Maximum (PETM) or other hyperthermal events are preserved. There is a transition from isolated fluvial channel deposits bounded by drab mudstones in the Nacimiento Formation to laterally-continuous, amalgamated fluvial channel complexes in the Cuba Mesa Member. Changes in channel deposits are consistent with an increase in deposition and discharge rates into the Eocene, while associated changes in mudstone floodplain deposits from drab to increasingly variegated (grey-purple-red) mudstones are consistent with improved soil drainage. These changes in fluvial deposits are similar to those associated with the P-E boundary in Laramide basins further to the north. Overall, these initial results from the SJB suggest that continued study of the sedimentological, biological, and geochemical records in the area should provide useful data to further research of the P-E boundary in the Southern Cordillera of North America.