Silicic phreatomagmatism in the Snake River Plain: the Deadeye Member |
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| Authors: | B Ellis M J Branney |
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| Institution: | (1) Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH, England, UK;(2) Present address: School of Earth and Environmental Sciences, Washington State University, Pullman, WA 99163, USA |
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| Abstract: | Non-welded rhyolitic pyroclastic units in the central Snake River Plain are interbedded with the much better exposed, large-volume
‘Snake-River type’ rheomorphic welded rhyolitic ignimbrites and rhyolite lavas. We document one such unit to investigate why
it is so different from the interbedded welded ignimbrites. The newly recognised Deadeye Member of southern Idaho is a soil-bounded
eruption-unit that comprises ashfall layers and a 4-m-thick ignimbrite that extends for >35 km. The ignimbrite is non-welded,
lithic-clast poor and varies from massive to diffuse low-angle cross-bedded. It contains abundant angular clasts of non-vesicular
black glass, and upper parts contain accretionary lapilli. The ashfall layers above it contain coated ash pellets and ash
clumps, which record moist aggregation of fine ash. The magmas of the Deadeye eruption were closely similar in composition
and temperature to those that generated the intensely welded rheomorphic ignimbrites of the central Snake River Plain. We
infer that the marked contrast in physical appearance of the Deadeye ignimbrite compared to the other, more typical Snake-River-type
welded ignimbrites was the result of emplacement at relatively low temperatures during an eruption in a lacustrine environment.
Magmatic volatile-driven fragmentation of the rhyolitic magma was influenced by interaction with lake water that also led
to cooling. The Deadeye Member is the first-recorded example of explosive silicic phreatomagmatism in the central Snake River
Plain. |
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