New Mexico Geological Society Annual Spring Meeting — Abstracts

Several episodic pulses of volcanic and tectonic activities that climaxed in the Late Miocene make up the Jemez volcanic field in north-central New Mexico. The volcanic field is mostly confined to the shoulder and the adjacent western margin and to the western part of the floor of the Espanola Basin of the Rio Grande rift. The volcanic field, which represents various types of mafic to silicic flows, formed at the intersection of three tectonic features, consisting of the N-S-trending Rio Grande rift, the NE-SW-trending Jemez Lineament, and the Colorado Plateau.

At least five major pulses of volcanic activities that ranged in age from the late Oligocene (25.5 Ma) to the late Pleistocene (68.3 ka) are known within the Jemez volcanic field. Even though andesitic and basaltic rocks of late Miocene ages dominated the volcanic assemblage, it was a series of caldera-forming silicic eruptions that altered the current landscape during the late Pleistocene (≤1.8 Ma). With an estimated volume of 800-950 km³ silicic pyroclastic products from the Valles-Toledo caldera complex, the Bandelier Tuff (1.2-1.6 Ma) is the dominant rock type within the volcanic field. More localized silicic lava and pyroclastic eruptions continued from domes confined to the Valles caldera ring fracture.

Most of the Valles caldera-bound pyroclastic and lava eruptions do not appear to pose volcanic hazard concerns in north-central New Mexico because they have been inactive for a long time. However, the latest volcanic activity from the southern moat of the Valles caldera is too young to be ignored. The youngest pyroclastic and lava eruptions belong to the East Fork Member, comprised of El Cajete Pyroclastic Beds, the Battleship Rock Ignimbrite, and the Banco Bonito lava that range in age from 74.7 to 68.3 ka. With an estimated eruptive volume of about 10 km³, outcrops of the El Cajete Pyroclastic Beds were identified ≥30 miles to the southeast of the vent. More than a meter of bedded fallout deposits were identified in topographically protected areas within the northwestern part of the Cerros del Rio volcanic field east of the Rio Grande. Similar deposits, ranging in thicknesses from 0.7 m along a road cut on State Road 502 east of the Rio Grande and up to 2-m thick tephra beds were noted within the southern half of the Pajarito Plateau, including within the Los Alamos National Laboratory boundaries. The current distribution of the El Cajete Pyroclastic Beds provides a valuable conservative estimate for gauging potential impacts of the likelihood of similar-magnitude future tephra eruptions from the Valles caldera.