New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

The Rio Grande, in conjunction with its associated landscapes, such as the Taos Gorge, provide an ideal environment to study important geomorphic processes that have shaped the region over the past 5 million years. ⁴⁰Ar/³⁹Ar geochronology of paleoriver deposits and lava flows can help test competing hypotheses for the evolution of the Rio Grande. Wells et al. (1987) suggest that the Ancestral Rio Grande had its headwaters in the modern Red River drainage until 600-300 ka. More recently, Machette et al. (2013) proposed that the upper Rio Grande was only integrated into its modern configuration following spillover of the Ancestral Lake Alamosa at ~400 ka. Repasch et al. (2017) hypothesize that the Ancestral Rio Grande was established as a through-going river at least as far south as the Socorro area by ~5 Ma, with headwaters both in the San Juan Volcanic Field (SJVF) and Latir Volcanic Field (LVF). Furthermore, Lake Alamosa spillover, and geomorphic resolution of other river-damming episodes related to Servilleta lava eruptions between about 4.5 and 2.5 Ma, represent reintegration of the Rio Grande.

⁴⁰Ar/³⁹Ar detrital sanidine (DS) geochronology yield maximum deposition ages (MDA) of river deposits, and lava geochronology bracket the age of river deposits between lavas. In addition, DS dates and associated K/Ca values yield sediment provenance that delineate source material derived from the SJVF versus the LVF. Published and new DS analysis on a river deposit collected between underlying 4.52 ± 0.20 Ma and overlying 3.57 ± 0.02 Ma Servilleta lavas near the confluence of the Rio Grande and Red River at La Junta Point show a dominance of dates between 26 and 20 Ma. Based on age and K/Ca values, this distribution correlates to a LVF source. The DS distribution did not reproduce published detrital zircon (DZ) data that had a significant 40–27 Ma mode; thought to be most consistent with detritus from the SJVF. A gravel deposit sample collected 1.3 km north of the confluence along the Rio Grande Gorge that overlies a 3.57 ± 0.02 Ma lava, yields a youngest single grain MDA of 1.21 ± 0.20 Ma and also contains a strong mode between 26 and 20 Ma. This current DS dataset supports a Red River headwaters for the Ancestral Rio Grande in this area between ~4 Ma and 1.2 Ma, but does not explain the published older DZ ages. Importantly, the DS-derived 1.21 Ma MDA for the gravel defining the present surface at La Junta Point indicates that the modern Taos Gorge formed after 1.21 Ma and constrains a minimum average river incision rate of ~230 m/Ma. The DS-derived timing for Taos Gorge carving is consistent with the Machette et al. (2013) estimate. Ongoing geochronology will further constrain the complex interplay of volcanism and Rio Grande drainage evolution in the iconic landscape of northern New Mexico and other comparable areas of global interest.