New Mexico Geological Society Annual Spring Meeting — Abstracts

Late Miocene tuffs and sediments of the Peralta Tuff Member of the Bearhead Rhyolite provide a partial record of high-silica rhyolite volcanism and contempoaneous sedimentation in the southeastern Jemez Volcanic Field. Study of 275 m of section in the Colle Canyon-peralta Canyon-Tent Rocks area documents the occurrence of fans constructed by emplacement of pyroclastic flows and surges and ephemeral braided stream deposited alluvium. Dipersal of pyroclast flows and was controlled by topography generated by synchronous faulting and emplacement of thick rhyolite domes and coulees. At least 30 rhyolitic eruptive episodes are recorded by discrete primary pyroclastic units separated by sediments and paleosols. Fourteen of these episodes emplaced pyroclastic-flow and/or surge deposits in the study area; at least three episodes involved activity in the Bearhead Peak area and one preceded emplacemnet of a dome near Tent Rocks. Other episodes are recorded only by falI deposits. An andesitic fall bed documents overlap of Paliza Canyon Formation andesitic volcanism with that of the Bearhead Rhyolite. Thick lava flows and domes strongly affected dispersal of pyroclastic flows and sediments until they were buried, producing noncorrelative stratigraphic intervals in closely spaced sections. Preferential occurrence of thin surge beds downslope of the couless and domes suggest that surges were produced from pyroclastic flows that impinged upon, or overtopped, the high-standing rhyolite landforms. Deposition was synchronous with NNW-trending down to the east normal faulting, requiring continued extension in the Canada de Cochiti fault zone into the latest Miocene and probably the PIiocene. Upward decrease in primary pyroclastic units documents the waning of Bearhead Rhyolite volcanism, which is also implied by an upward increase in the epiclast: pyroclast ratio of the sediments and a decrease in spacing of paleosols.