New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Grande-Rio Chama (RG-RC) fluvial system preserves a record of southern Rocky Mountain erosion and sediment transport over the last 8 Ma. During this time the two rivers have evolved dramatically, undergoing channel migrations, drainage capture and integration events, carving and refilling of paleocanyons, and lake spillovers. Volcanism concurrent with the development of the river system provides a unique opportunity to apply multiple geochronometers to the study of its incision and drainage evolution. New 40Ar/39Ar basalt ages coupled with new detrital zircon and sanidine age population data for fluvial sediments document the birth of the ancestral RG by 4.8 Ma, with headwaters in the San Juan Mountains and extending to the Palomas Basin.
This paper reports on twelve detrital samples (zircon + sanidine) collected from RG-RC river alluvium ranging from ~8 Ma RC to ~3.5 Ma RG deposits. RC detrital zircon age spectra are distinct from ancestral RG, and are rich in San Juan Basin and San Juan volcanic detritus; this river is interpreted to be the main drainage before 5 Ma. Detrital zircon age spectra for ancestral RG alluvium underlying 4.7-4.5 Ma basalt flows in the Espanola Basin reveal peaks at 21 Ma (Servilleta Plaza volcanics), 28 Ma (southern San Juan Mountains), 30-35 Ma (northern San Juan Mountains), and 70-100 Ma (recycled San Juan Basin sediment). Detrital sanidine grains from these samples reveal similar age peaks at 19.8, 25.4 (Latir Volcanic Field), 28.72, 29.3, and 34.8 Ma. Although the 28 Ma peak could include input from small volume pre-Caldera Latir Field volcanics, and recycling of the Ojo Caliente Sandstone of the Santa Fe Group (5% 37-27 Ma grains) may have resulted in 28 and 35 Ma grains, the larger percentage (10-12%) of 37-27 Ma grains in river sediment at Black Mesa are interpreted to reflect a RG with headwaters in the San Juan Mountains at 4.8 Ma. Building of the Taos Plateau Volcanic Field precluded drainage from the upper San Luis Basin from ~5 Ma until the ~0.430 Ma spillover of Lake Alamosa re-integrated this drainage to the lower RG-RC system. 2.6 Ma ancestral RG sediment downstream of today’s RG-RC confluence (Totavi Lentil) has a similar age spectrum to the ancestral RC, while 1.6 Ma Totavi Lentil is more similar to the combined RG-RC, suggesting northward shift of the RG-RC confluence between 2.6 and 1.6 Ma during the building of the Jemez Mountain eruptive center.
This study highlights the influence of magmatic and tectonic processes on major river systems. Building of the Taos Plateau Volcanic Field from 5-3 Ma is interpreted to have driven downward integration of the RG to the Palomas Basin by 4 Ma. River damming events in the RG-RC system were driven by Taos Plateau and Jemez volcanism, while spillover events were driven by continued headwater uplift in the southern Rocky Mountains. Increased amplitudes of glacial and interglacial periods likely contributed to increased discharge and incision rates during latest stages of river evolution, but tectonic drivers dominate over the ~5 Ma time period.