New insights on the late Pleistocene Rio Grande-Rio Chama fluvial system from detrital zircon dating
— Marissa Repasch, Karl Karlstrom, Matt Heizler, and Daniel Koning

Detrital zircon analysis has excellent potential to elucidate the evolution of the Rio Grande-Rio Chama fluvial system by characterizing the provenance of sediment and assessing fluvial connectivity of Rio Grande rift basins through time. This study uses U-Pb geochronology of detrital zircon separated from modern river sediment, terrace deposits with the Lava Creek B ash (640 ka), and three intermediate terraces to reconstruct the provenance of sediment carried by the Rio Grande-Rio Chama fluvial system over the last 640 ka. U-Pb detrital zircon ages for Red River sediment are further supported by high precision ⁴⁰Ar/³⁹Ar detrital sanidine dating, which has the ability to identify age populations that are not resolved by lower precision detrital zircon dates.

The detrital zircon age spectra of a 640 ka Rio Grande terrace in the Albuquerque Basin and a 640 ka Rio Chama terrace show a number of diagnostic age peaks: 1) 23-25 Ma characteristic of the Latir Volcanic Field, 2) 27-28 Ma that could be either from the Latir or San Juan Volcanic Fields, 3) 34-35 Ma characteristic of the San Juan Volcanic Field, 4) Cretaceous-aged peaks, and 5) a “triple peak” consisting of 1.7, 1.4, and 1.1 Ga zircon derived from Precambrian basement, which is common in Phanerozoic sediments throughout the western U.S. Based on two samples, we interpret relatively abundant 28-36 Ma zircon in river sediment to be characteristic of the Rio Chama. These similarities are compatible with models showing that the Rio Chama – with San Juan Mountain headwaters – dominated the river system at 640 ka. Similar 28-35 Ma zircon in a 250-350 ka Rio Grande terrace in the Española Basin is compatible with models positing a San Juan Mountain source of detritus for the Rio Grande after spillover of Lake Alamosa ~430 ka, and with reworking of Santa Fe Group sediment. The 27-28 Ma temporal overlap between the San Juan and Latir Volcanic Fields is problematic for provenance, but modern river sand samples suggest it is possible to distinguish the 28 Ma San Juan Volcanic Field (Fish Canyon Tuff) zircon peak from the 22-27 Ma Latir Volcanic Field zircon peak. Although the Latir Volcanic Field has a 28.2 Ma tuff (Tetilla Peak Tuff), its associated zircon is much less abundant than younger zircon (e.g., the 25.49 Ma Amalia Tuff). The similar pattern of age peaks between 640 ka fluvial sediment in the Albuquerque Basin and the Rio Chama indicates that the Rio Chama dominated the river system at 640 ka. Preliminary detrital sanidine data for Red River sediment that was derived from the Latir Volcanic Field show that 23-25 Ma sanidine is twice as abundant as 27-29 Ma sanidine, and therefore abundant 27-29 Ma zircon in the Rio Grande system is interpreted to indicate fluvial connectivity to the San Juan Mountains.