New Mexico Bureau of Mines & Mineral Resources Bulletin 149 Field-trip guide to the geochronolgy of El Malpais National Monument and the Zuni-Bandera volcanic field, New Mexico

This field guide grew out of the 1993 Quaternary Dating Field Conference held in Grants, New Mexico, in April 1993. The Conference, sponsored by the New Mexico Geochronology Research Laboratory and the U.S. National Park Service, was convened to consider using the Zuni-Bandera volcanic field, including El Malpais National Monument and El Malpais National Conservation Area, as a test area for evaluating, calibrating, and improving the application of Quaternary dating techniques. Improvement of the accuracy and precision of these dating techniques is one of the major goals of the New Mexico Geochronology Research Laboratory, which was established in 1992 as a collaborative effort of the New Mexico Bureau of Mines & Mineral Resources, New Mexico Institute of Mining & Technology, and Los Alamos National Laboratory.

The Zuni-Bandera volcanic field is located in west-central New Mexico south of the town of Grants (Fig. 1). It lies near the center of the Jemez zone or lineament that extends at least from central Arizona to northeastern New Mexico. The Jemez lineament is a major flaw in the Earth's crust along which volcanoes have erupted for the past 16 Ma. Approximately 100 volcanoes have been identified in the Zuni-Bandera volcanic field. The youngest volcanic activity along the lineament occurred in the area now encompassed by El Malpais National Monument, where volcanoes have erupted from about 700 to 3 ka. Because of the youthfulness of these lava flows, they are well preserved, providing the Monument visitor with spectacular views of volcanic landforms and the geologist with an ideal area for investigating volcanic processes.

FIGURE 1.—Map of Zuni-Bandera volcanic field showing field-guide stops. (click on image for an enlargement in a new window)

One of the more important aspects of research in the Zuni-Bandera volcanic field is the geochronology of the many volcanic eruptions. Determination of the absolute ages of these basaltic volcanoes is fundamental to other geological, geochemical, and isotopic studies. Although there are several dating techniques that should be applicable to volcanic rocks of this age, their results have rarely been compared in a single area, and, where this has been attempted, they are often not in agreement. The Quaternary Dating Field Conference was convened to address this problem by assembling a group of researchers interested in applying their specific techniques to a common sequence of lava flows with the goal of improving their accuracy and precision, with the hope of improving our understanding of the significance of the results obtained by the different techniques. A secondary objective was the refinement of the geochronology of the area.