HR: 16:15h
AN: V44B-02    [Abstracts]
TI: Dynamics of a Phreatomagmatic Eruption at Narbona Pass Volcano, Navajo Volcanic Field, Navajo Nation, New Mexico
AU: * Brand, B D
EM: brittany.brand@asu.edu
AF: 1Department of Geological Sciences, Arizona State University, Box 1404, Tempe, AZ 85287-1404 United States
AU: Clarke, A B
EM: amanda.clarke@asu.edu
AF: 1Department of Geological Sciences, Arizona State University, Box 1404, Tempe, AZ 85287-1404 United States
AU: Semken, S C
EM: semken@asu.edu
AF: 1Department of Geological Sciences, Arizona State University, Box 1404, Tempe, AZ 85287-1404 United States
AU: Sandoval, C
V44B-02 AF: 2Division of Math, Science, and Technology, Din‚ College, PO Box 0580, Shiprock, NM 87420-0580 United States
AB: We incorporate field investigations and analytical techniques to reconstruct vent geometry and evolution, magma/water ratio, zone of fragmentation and efficiency of fragmentation of a mid-Tertiary phreatomagmatic eruption. Narbona Pass maar, located in the Chuska Mountains of northwest New Mexico, is a center in the (28-19 Ma) Navajo Volcanic Field. Minette magma erupted through a sandstone aquifer approximately 25 Ma (Cather et al., 2003) creating a phreatomagmatic sequence of bedded tuffs capped by trachybasalt lava flows. Nine stratigraphic columns were measured documenting the types, proportions and distribution of deposits around the crater. Inner crater units include massive tuff breccias, interpreted as debris flows back into the crater, and large, symmetrical cross-strata, interpreted as base surge deposits. Outer crater deposits consist of alternating bedded tuff sequences of large, symmetrical to anti-dune cross-strata, both normal and reverse graded, planar-bedded lapilli tuff, and massive lapilli tuffs and tuff breccias. Soft sediment deformation occurs only intermittently throughout the deposits and no accretionary lapilli were found, both indicative of a relatively dry phreatomagmatic eruption. The contact between pyroclastic deposits and the substrate indicates that the thickest sections were deposited in a paleo-valley (Ehrenberg, 1978). Paleo-valley fill deposits are coarser-grained than the rest of the sequence, but otherwise exhibit the same cross strata as previously described. The percentage and composition of accidental sandstone fragments remain vertically consistent through the tuff sequence, possibly suggesting a fixed zone of fragmentation. However, the accidental lithics vary in percentage depending on location around the edifice, suggesting eruption asymmetry. Zones rich in dike- and sill-sourced accidental lithics are also common in the sequence. Several angular unconformities were observed indicating partial cone collapse and enlargement of the crater during the eruption. The end of the pyroclastic sequence is characterized by fine-grained ash deposits with abundant cross-stratification and soft sediment deformation, suggesting there was no gradual drying out period before the final magmatic stage. Instead the eruption may have suddenly depleted the external water source, the zone of magma flux may have dropped below the aquifer, or the eruption may have paused before the final effusive phase.
DE: 8404 Volcanoclastic deposits
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8427 Subaqueous volcanism
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005