New Mexico Geological Society Annual Spring Meeting — Abstracts

Upper Miocene pyroclastic-fall deposits in the southeastern Jemez Mountains consist of a distinct tripartite stratigraphy. The lower white layer is a 0.5-3.0 m thick, poorly-to-moderatelysorted, coarse-grained, pumice-Iapilli layer. The middle pink layer is an 8.0-12.0 cm thick, moderately-sorted, laminated fine ash with minor dispersed coarser fragments. The top gray layer is 15.0 cm- 1.1 m thick, stratified, and composed of well-sorted, poorly-vesicular perlite fragments that were originally obsidian. At least four pyroclastic fall deposits contain this stratigraphy, suggesting a repetitive process of changing eruption dynamics that was common to several different eruptions. Two of these deposits, both located in a tributary to Peralta Canyon which lies northwest of Tent Rocks, are being studied. Grain-size data, clast-petrography data, and SEM evaluations and observations of grain shape and vesicularity support the following hypothesis of a three-stage eruption history that is responsible for the white-pink-gray layering: The first eruptive stage that produced the lower white layers resulted from the fallout of coarser pumice-Iapilli and lithic fragments from a plinian-style eruption column. The second eruptive stage that produced the proceeding pink layers resulted from ash-cloud deposition from pyroclastic flows that are not deposited at these particular outcrops. Lastly, the third eruptive stage that produced the top gray layers was due to several explosions ejecting partially-to-Iargely degassed magma and/or dome fragments. For one tephra, the middle pink ash has been laterally traced into a pyroclastic-flow deposit, supporting the proposed origin for the pink layers.