Assembling an ignimbrite: Compositionally defined eruptive packages in the 1912 Valley of Ten Thousand Smokes ignimbrite, Alaska

Geological Society of America Bulletin
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The 1912 Valley of Ten Thousand Smokes (VTTS) ignimbrite was constructed from 9 compositionally distinct, sequentially emplaced packages, each with distinct proportions of rhyolite (R), dacite (D), and andesite (A) pumices that permit us to map package boundaries and flow paths from vent to distal extents. Changing pumice proportions and interbedding relationships link ignimbrite formation to coeval fall deposition during the first ∼16 h (Episode I) of the eruption. Pumice compositional proportions in the ignimbrite were estimated by counts on ≥100 lapilli at multiple levels in vertical sections wherever accessible and more widely over most of the ignimbrite surface in the VTTS. The initial, 100% rhyolite ignimbrite package (equivalent to regional fall Layer A and occupying ∼3.5 h) was followed by packages with increasing proportions of andesite, then dacite, emplaced over ∼12.5 h and equivalent to regional fall Layers B1-B3. Coeval fall deposits are locally intercalated with the ignimbrite and show parallel changes in R:D (rhyolite:dacite) proportions, but lack significant amounts of andesite. Andesite was thus dominantly a low-fountaining component in the eruption column and is preferentially represented in packages filling the VTTS north of the vent. The most extensive packages (3 and 4) occur in B1 and early B2 times where flow mobility and volume were optimized; earlier all-rhyolite flows (Package 1) were highly energetic but less voluminous, while later packages (5-9) were both less voluminous and emplaced at lower velocities. Package boundaries are expressed as one or more of the following: sharp color changes corresponding to compositional variations; persistent finer-grained basal parts of flow units; compaction swales filled by later packages; erosional channels cut by the flows that fill them; lobate accumulations of one package; and (mostly south of the vent) intercalated fall deposit layers. Clear flow-unit boundaries are best developed between ignimbrite of non-successive packages, indicating time breaks of tens of minutes to hours. Less well-defined stratification may represent rapidly emplaced successive flow units but often changes over short distances and indicates variations in localized depositional conditions.

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Publication type Article
Publication Subtype Journal Article
Title Assembling an ignimbrite: Compositionally defined eruptive packages in the 1912 Valley of Ten Thousand Smokes ignimbrite, Alaska
Series title Geological Society of America Bulletin
DOI 10.1130/B25621.1
Volume 117
Issue 7-8
Year Published 2005
Language English
Contributing office(s) Volcano Hazards Program
Description 14 p.
Larger Work Title Geological Society of America Bulletin
First page 1094
Last page 1107
Country United States
State Alaska
Other Geospatial Valley of Ten Thousand Smokes
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