New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Chama is a major headwater of the Rio Grande, so deciphering its fluvial record is critical for understanding the evolution of the Rio Grande fluvial system. In this paper, we present evidence that early Rio Chama deposits are represented by the Hernandez Member of the Chamita Formation in the Española Basin, and the maximum age of this member lies between 12.3-11.5 Ma. The Hernandez Member is characterized by its light gray color and relatively abundant gravelly channel fills containing very coarse pebbles and cobbles. Gravels are subrounded to rounded and include high proportions of dark-colored dacites and andesites, together with minor amounts of rhyolite, welded tuff, and ≤26% quartzite.

The geographic and stratigraphic position of the Hernandez Member is consistent with a paleo-Rio Chama. This member is mapped between the modern Rio Grande and the Jemez Mountains in the south-central Española Basin as well as along the northern flank of the Jemez Mountains. Paleocurrent data west of Española are to the southeast, consistent with a southeast-flowing paleo-Rio Chama wrapping southward around the northeastern Jemez Mountains. As would be expected for a paleo-Rio Chama, the Hernandez Member interfingers eastward with sandy fluvial deposits derived from the southern San Luis Basin (Vallito Member of Chamita Formation) and westward with relatively monolithic, angular volcaniclastic detritus derived from the Jemez Mountains volcanic field.

Two Hernandez Member clast types are useful for identifying provenance. The dark-colored intermediate volcanic gravels are interpreted to be derived from the southwestern San Juan volcanic field. They are very similar to those seen in Quaternary Rio Chama deposits and are not observed in Miocene-Quaternary fluvial deposits derived from the San Luis basin. The dark-colored intermediate volcanic clast have returned ⁴⁰Ar/³⁹Ar ages of 29.98±0.09 Ma and 29.08±0.13 Ma (latter from Kempter et al., 2007). These ages are consistent with derivation from the San Juan volcanic field but are too old to be sourced in the Latir or Jemez volcanic fields. The Jemez Mountains lack quartzite, so the quartzite clasts likely are derived from the Tusas Mountains.

Radiometric age controls (primarily ⁴⁰Ar/³⁹Ar) in exposures and/or wells at 5 sites indicate the paleo-Rio Chama began 12.3-11.5 Ma. Near Española, the river subsequently shifted northeastward due to development of ca. 10 Ma volcanic edifices associated with the Lobato Formation of the Jemez volcanic field.

We attribute the initiation of the paleo-Rio Chama to development of the early Jemez volcanic field and shifting of tectonic strain from major normal faults near Abiquiu towards the Pajarito fault. The build-up of topographic highs in the volcanic field effectively blocked previously southeast-flowing streams heading in the eastern Colorado Plateau. These blocked streams converged into a single, high-competency river that flowed around the north end of the Jemez volcanic field. A shifting of strain to the Pajarito fault facilitated development of paleovalleys (up to 60 m deep) observed north of the Jemez Mountains. These paleovalleys may have migrated headward towards the Colorado Plateau, and ensuing stream-capture there may have resulted in further consolidation of discharge to the paleo-Rio Chama.