PRELIMINARY EARLY EOCENE AGE MODEL AND TERRESTRIAL PALEOENVIRONMENT RECORD FROM THE SAN JUAN BASIN, NEW MEXICO, USA

The early Eocene was marked by a series of rapid hyperthermal events and abrupt global carbon cycle perturbations which dramatically altered terrestrial environments. However, previous work in western North America has focused on mid to high latitude localities with little work in southern North America hindering our understanding of latitudinal differences in early Eocene terrestrial ecosystems. The San Juan Basin, located in northwestern New Mexico, preserves previously understudied early Eocene terrestrial deposits making it an ideal location to address this gap. Here we present a preliminary early Eocene age model and paleoenvironment record from the San Juan Basin near Cuba, New Mexico using paleomagnetic and paleosol bulk geochemical samples from the Cuba Mesa and lower Regina members of the San Jose Formation.

Both members were correlated with magnetic polarity subchron C24r and the presence of “Hyracotherium” in the Cuba Mesa Member indicates earliest Eocene (Wasatchian) deposition. Forty paleosols were identified and divided into 8 pedotypes based on degree of pedogenesis determined by Munsell soil color, ped structure, horizonation, and the presence/absence of FeMn or pedogenic carbonate nodules. Paleosols from the Cuba Mesa Member were generally more gleyed, better developed, and had a higher proportion of paleosols with FeMn nodules while paleosols from the lower Regina Member were red in color, poorly developed, and had a higher proportion of soils with pedogenic carbonate nodules indicating a shift to better drained but more fluvially active floodplain through time. Similarly, the degree of channelization and grain size of channel deposits increases through time supporting a more fluvially active lower Regina Member, which may be coeval with similar deposits from western North America. The paleosol bulk geochemical ratios of Ba/Sr decreases through time indicating decreasing soil development and estimated soil pH using increases from an average of ~5.9 in the Cuba Mesa Member to ~6.3 in the lower Regina Member. These changes in paleosol characteristics and fluvial sedimentation, along with the first occurrence of “Hyracotherium”, are similar to patterns observed at the onset of the Eocene in other Laramide basins, suggesting the San Juan basin may preserve a coeval paleoenvironmental record.