ERUPTIVE PROCESSES RESPONSIBLE FOR FALL TEPHRA IN THE UPPER MIOCENE PERALTA TUFF, JEMEZ MOUNTAINS, NEW MEXICO

Upper Miocene pyroclastic-fall deposits in the southeastern Jemez Mountains consist of a distinct tripartite stratigraphy. The lower white layer is a thick (~0.5-3.0 m), massive, moderately-sorted, coarse-grained, pumice-lapilli layer. The proceeding pink layer is a much thinner (~8-12 cm), moderately-sorted, laminated fine ash with minor dispersed coarser fragments. The top gray layer (~15 cm-1.1m) is stratified and composed of well-sorted, poorly-vesicular perlite fragments. 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, are being studied in detail to test the hypothesis that a three-stage eruptive history is responsible for the white-pink-gray layering. These layers are believed to be produced by the following three stages: (1) the fallout of denser pumice lapilli and lithic fragments from a plinian-style eruption column, followed by (2) ash-cloud deposition from pyroclastic flows that are not deposited at these particular outcrops, and lastly by (3) several explosions ejecting partially-to-largely 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. Currently, grain-size, clast-petrography, grain-shape and texture, vesicularity, and microlite crystallinity data are being obtained to further test the hypothesis. In addition, XRF analyses are being performed to see if the various layers relate to compositional changes in the magma. The results from these analyses will be presented.