New Mexico Geological Society Annual Spring Meeting — Abstracts

The geochronology of alluvial deposits in the Albuquerque Basin are refined using 20 previously published and 58 new (single-crystal laser-fusion and incremental heating) ⁴⁰Ar/³⁹Ar age determinations and tephrochronologic correlations of (primary and reworked) tephra and lavas younger than 10 Ma. These data provide a robust geochronologic framework that was used to develop a reversal magnetic-polarity stratigraphy for Plio-Pleistocene fluviatile sandstone, conglomerate, and mudstone of the Ceja and Sierra Ladrones Formations (upper Santa Fe Group). Tilt-corrected means of all class I (highest quality) sites (normal polarity: D = 2.8º, I = 45.0º, n = 58; reverse polarity: D = 180.2º, I = -41.7º, n = 50) document nearly all polarity changes (longer than 20 ka) since 4.0 Ma and are suitable for correlation to the global polarity timescale. The Ceja Fm onlapped the late Miocene (ca. 6.3 Ma) Rincones paleosurface, and buried it by about 3.0 Ma. After 3.0-2.6 Ma, the Ceja Fm became considerably coarser-grained and carried scattered small boulders. Ceja deposition ended shortly after the Olduvai subchron (1.778 Ma) with the formation of a relict depositional surface (Llano de Albuquerque surface). Axial-fluvial deposits of the Sierra Ladrones Fm (ancestral Rio Grande) had entered the northern part of the basin by late Miocene time (ca. 7 Ma), and through-going axial-river drainage was established into southern New Mexico by early Pliocene time (ca. 4.8 Ma). The axial river once flowed 1-5 km west of the eastern basin-bounding fault system before migrating to the present position of the Rio Grande Valley after 1.8-1.6 Ma. Deposition of the Sierra Ladrones Fm ceased shortly after the beginning of the Brunhes polarity chron (0.781 Ma), and before deposition of the oldest known inset fluvial terrace deposit in the Rio Grande Valley near Albuquerque, which contains the 0.64 Ma Lava Creek B ash. Valley incision abandoned an early Pleistocene basinfloor and flanking piedmont-slopes of the Sunport, Las Huertas, and Llano de Manzano surfaces. Incision of the Rio Grande in central and southern New Mexico began before the 0.45 Ma age of breaching of Lake Alamosa (San Luis Basin) in the headwaters region of Colorado and may not be a direct result of drainage capture in the upper Rio Grande.