AGE AND FLUVIAL EVOLUTION OF THE PALEO-RIO CHAMA, THE MAIN HEADWATER OF THE LATE MIOCENE AXIAL RIVER IN THE RIO GRANDE RIFT, USA

The Rio Chama is the main tributary to the upper Rio Grande, joining the latter river in the Española Basin of the Rio Grande rift. Heading in the southwestern San Juan Mountains, the Rio Chama drains the boundary region of the upper Rio Grande rift and northeastern Colorado Plateau. Deposits of the paleo-Rio Chama are represented by the light gray, conglomeratic Hernandez Member of the Chamita Formation. Gravels are cobble-rich and include high proportions of dark-colored dacites and andesites, together with minor amounts of rhyolite, welded tuff, and ≤26% quartzite. We interpret that the Hernandez Member reflects paleo-Rio Chama deposition because of its gravel composition (which is similar to that observed in Quaternary-age, Rio Chama terrace deposits), southeastward paleoflow, and geographic extent. Furthermore, previously published ⁴⁰Ar/³⁹Ar dating of two intermediate volcanic clasts yield 29-30 Ma. These ages are consistent with derivation from the San Juan volcanic field but are too old to be sourced in the Latir volcanic field, the latter which would have been drained by a south-flowing paleo-Rio Grande upstream of the paleo-Rio Chama confluence. Radiometric age control (primarily ⁴⁰Ar/³⁹Ar) in exposures and wells places the earliest deposits of the paleo-Rio Chama at 12.3-11.5 Ma.

We attribute the initiation of the Rio Chama to development of the Jemez volcanic field and inward shifting of rift-related strain. Prior to the paleo-Rio Chama and Jemez volcanic field, the eastern Colorado Plateau was drained by a series of southeastward-flowing, ephemeral(?) rivers that carried high amounts of sandy bedload into the southwestern Española Basin and the adjoining Albuquerque Basin. Build-up of topography associated with the Jemez volcanic field effectively caused some of these drainages to converge into a single, high-competency river that flowed around the north end of the field. Shifting of strain to the Pajarito fault, located near the center of the Española Basin, facilitated development of paleovalleys up to 60 m deep. Upstream migration of paleovalley incision and related stream capture may have further consolidated discharge to the paleo-Rio Chama. Based on clast size comparisons, competence and probably discharge on the paleo-Rio Chama exceeded that of the upstream paleo-Rio Grande.