Ignimbrites up to 80 m thick underlie the lower Bandelier Tuff in the southwestern part of the Jemez Mountains volcanic field, New Mexico. They were formed during silicic explosive eruptions that preceded the climactic events of the Bandelier Tuffs and collapse of the Valles (Toledo) and Valles II calderas. Source vents for these units, herein called the San Diego Canyon ignimbrites, probably underlie the southwestern portion of the Valles caldera. Small-scale caldera collapse may have been associated with the eruption of these ignimbrites, but evidence of an early caldera, or any related vent structure, has since been obliterated. This late Pliocene (about 2.5 Ma), high-silica rhyolite activity was contemporaneous with eruptions from the predominantly dacitic Tschicoma volcanic center in the northeastern part of the Jemez Mountains and suggests a significantly different history for the Plio-Pleistocene development of the Jemez Mountains volcanic field than has previously been considered. ¿ American Geophysical Union 1988