New Mexico Geological Society Annual Spring Meeting — Abstracts

The late Pliocene Puye Formation is a 2 to 3 my oId volcaniclastic alluvial fan sequence that formed contemporaneously with the later stages of activity of the Tschicoma volcanic center in the NE part of the Jemes Mountains, north-central New Mexico. Interstratified with fan sediments are significant volumes of primary and reworked pyroclastic fall, flow and surge deposits of both mafic and felsic composition. These provide datable times planes and allow fan evolution to be monitored In detail. The petrography, granulometric characteristics, and facies relationships of silcic pyroclastic deposits can be used to constrain a model of dome growth, during periods of explosive eruption and erosion of the volcanic center. The Puye fan is dominated by epiclastic material derived from intermediate to felsic volcanoes of the Tschicoma center, and to a lesser extent, by Rio Grande rift basalts. Pumice and ash are major components in many of the fluvial, lacustrine, and sediment-gravity flow deposits. Dacitic to rhyodacitic pyroclastic fall and surge deposits probably represent plinian-style eruptions intimately associated with effusive phases that produced voluminous lava flows and domes in the Tschicoma center. Also interstratified within the fan sequence are
several nonwelded pyroclastic flow deposits that can be related both to climactic explosive episodes and large-scale gravitational collapse of growing lava domes. Pyroclastics from the Tschlcoma center range in composition from dacitic with two-pyroxene-pl-mt-qt assemblages to hydrous rhyodacltic pl-qt-hbl-bt assemblages. Rhyolitic tephra containing qt-pl-ksp-mt forms deposits with abundant obsidian clasts believed to have been derived from the El Rechuelos center further north. The youngest fall deposits differ from the aforementioned compositions. They closely approximate Lower Bandelier tuff compositions and may have been derived from the same source.