New Mexico Geological Society Annual Spring Meeting — Abstracts

New 1:24,000 scale geologic mapping combined with 40Ar/39Ar studies are revealing significant new findings on intracaldera stratigraphy and structure, initial development of intracaldera lakes, and the duration of resurgence within the ca. 1.25 Myr Valles caldera. The caldera is about 22 km in diameter, containing a resurgent dome that was uplifted more than 1000 m. Uplift and faulting have exposed large, rootless megabreccia blocks composed of precaldera rocks submerged in densely welded, intracaldera upper Bandelier Tuff. The largest blocks are 0.2 to 2.0 km long, consisting of Pennsylvania through Quaternary rocks. Evidence that an initial lake developed within the caldera depression is preserved in finely laminated lacustrine beds and interbedded rhyolitic, hydromagmatic tuffs that contain accretionary pepperite and pillow textures. The combined evidence indicates that this first Valles lake was widespread and relatively shallow.

40Ar/39Ar dating of Deer Canyon and Redondo Creek rhyolites yields ages that are statistically indistinguishable from the age of underlying upper Bandelier Tuff (1.256±0.010 Ma, 2σ). These results indicate that the intracaldera lake developed immediately after the caldera formed and that the resurgent dome rose out of the lake. In contrast, initial rhyolite lavas of the first post-caldera moat complex, Cerro del Medio (about 1.229±0.017 Ma) show no apparent deformation due to resurgence, providing an upper time constraint on uplift. Thus, 40Ar/39Ar dates indicate that resurgence lasted 27±27 ka, taking place within 54 ka. Resurgence averages 3.7 cm/yr and probably not less than 1.9 cm/yr.