SEDIMENTARY RECORDS PROVIDE NEW INSIGHT INTO THE EVOLUTION OF THE ARKANSAS RIVER IN THE RIO GRANDE RIFT

The northern Rio Grande Rift comprises the upper Arkansas River (UAR) Valley and the San Luis Basin, separated by the Poncha Block transfer zone (TZ). The Arkansas River flows north-south in the UAR Valley, then makes a sharp turn to the east at the Poncha Block TZ. We suggest that the topographic development associated with the growth of the Poncha Block TZ could have significantly influenced the formation of the modern Arkansas River. By studying sediment deposition rates, timing, and sources, we aim to gain a deeper understanding of rift basin evolution, including the factors that control fluvial processes and basin connectivity. New paleocurrent data, measured stratigraphic sections, detrital zircon U-Pb analyses, and Ar-Ar geochronology on ashes deposited in fluvial gravels on the uplifted Poncha Block, yield insights into paleo-depositional environments and potential pathways traveled by the paleo-Arkansas River. The Poncha Block appears to have notable differences in depositional environments and flow directions on the east side, compared to the west side of the uplifted block. The strata on the east side of the block contain evidence for fluvial environments, including flood plains with carbonate nodules, as well as paleo-flow directions ranging from northeast to northwest. In comparison, the west side of the block contains immature alluvial fan-type environments with large volcanic clasts and flow directions to the southwest. These results imply that although the fluvial gravels on the Poncha Block are all considered part of the same Dry Union Formation, there were different sedimentary processes occurring, which may have been influenced by active tectonics, like continental rifting, at the time of deposition. Furthermore, expected results from ashes deposited between fluvial gravels and thermochronology ages from the Poncha Fault will help to determine fault initiation timing, rates of exhumation, and the possible driving forces behind the potential redirection of the Arkansas River.