40AR/39AR GEOCHRONOLOGY, MAGNETIC-POLARITY STRATIGRAPHY, AND
TEPHROCHRONOLOGY OF PLIO-PLEISTOCENE DEPOSITS IN THE
ALBUQUERQUE BASIN, RIO GRANDE RIFT, NEW MEXICO,

CONNELL, S.D., New Mexico Bureau of Geology and Mineral Resources-Albuquerque
Office, New Mexico Institute of Mining and Technology, 2808 Central Ave., SE, Albuquerque,
New Mexico 87106; SMITH, G.A., and GEISSMAN, J.W., Department of Earth and Planetary
Sciences, MSC03 2040, University of New Mexico, Albuquerque, New Mexico 87131;
McINTOSH, W.C., DUNBAR, N.W., LOVE, D.W., and CATHER, S.M., New Mexico Bureau of
Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy
Place, Socorro, New Mexico 87801


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.