FLUVIAL FAN PROGRADATION IN RESPONSE TO EXTREME SEASONALITY IN PRECIPITATION: AN EXAMPLE FROM THE EARLY EOCENE OF THE SAN JUAN BASIN, NEW MEXICO

The San Juan Basin (SJB) in New Mexico preserves the southwestern-most sedimentary record of the Paleocene-Eocene (P-E) boundary in North America, and represents a point for comparison with P-E sections preserved in Colorado and Wyoming. Here, we present evidence that the early Eocene Cuba Mesa Member of the San Jose Formation was deposited by a river system with highly variable discharge resulting from extreme seasonality of precipitation. We also discuss how the Cuba Mesa deposits differ from those of the underlying Nacimiento Formation and the overlying Regina Member, and hypothesize about what these differences imply about climate change in the SJB across the P-E boundary.

The Cuba Mesa Member consists of laterally and vertically amalgamated fluvial channels that are dominated by upper flow regime (UFR) and high deposition rate (HDR) sedimentary structures. These structures are characteristic of monsoonal and subtropical rivers that receive most of their annual (or in some cases, interannual) precipitation from one to few large flooding events. There is also evidence that channels underwent periods of prolonged dryness between flooding episodes. Furthermore, we interpret that the large spatial extent, downstream trends (decreasing degree of channel amalgamation, channel size and grain size), and evidence of channel avulsions in the Cuba Mesa Member are consistent with a large fluvial fan system (megafan). Studies of modern fluvial megafans have suggested that they can only form under both sufficient aggradation rates and moderate to extreme seasonal fluctuations in discharge that result from highly seasonal precipitation patterns.

Vertical trends indicate that prior to the progradation of the Cuba Mesa fan, the Paleocene channels were deposited under a more perennial precipitation regime. The return to more isolated channels in the overlying Regina Member indicate backstepping of the fan system and an overall trend toward drier conditions further into the early-Eocene. These changes in fluvial deposits are like those associated with the Paleocene-Eocene Thermal Maximum (PETM) and subsequent early Eocene hyperthermal events in Laramide basins to the north. These trends, combined with carbon isotope data, suggest that the SJB may provide an additional source of proxy data for early Eocene hyperthermal research.