Volcanic ash beds: Recorders of Upper Cenozoic silicic pyroclastic volcanism in the western United States

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Abstract
Stratigraphic relationships and tephrochronologic information for volcanic ash beds (4.0–0.1 m.y.) of the Western Interior allow the recognition of at least 68 different ash falls. The average reoccurrence rate, based on the succession of the isotopically dated ash beds, is near one eruption every 57,000 years. The ash beds were grouped into two chemical types, rhyolitic and dacitic; the main property that distinguishes dacitic from rhyolitic ashes is the larger Ca and Mg and smaller K content in dacitic ashes. Dacitic volcanic ashes were defined to have more than 0.55 wt % Ca. Rhyolitic ashes were subdivided further into two types, W‐ and G‐type, based on the presence of biotite in the W‐type and its absence in G‐type. W‐type rhyolitic ashes are chalky white, have less than 0.55 wt % Fe, have colorless glass shards of pumiceous habit. G‐type rhyolitic ashes are light to medium gray, have from 0.55 to 2.0 wt % Fe in the glass shards, lack biotite, have colorless, platy, bubble wall, and bubble junction glass shards. Dacitic ashes are light gray to grayish brown, have more than 0.55 wt % Fe and Ca, and have a complex mixture of colorless to light brown shards, including pumiceous, fibrous, chunky, and bubble wall and bubble junction types. W‐type rhyolitic ashes possibly formed from magmas that were cooler (695° to 830°C) and thus more viscous than the hotter (870° to 1000°C) magma progenitors of G‐type ashes based on Fe‐Ti oxide mineral compositions. Primary microphenocrysts in the ash beds comprise assemblages of the following: quartz, sanidine, plagioclase, biotite, amphibole, clinopyroxene, orthopyroxene, fayalite, zircon, apatite, allanite, chevkinite, sphene, magnetite, and ilmentite. Fayalite and chevkinite only have been found in G‐type; biotite and sphene only have been found in W‐type. Rhyolitic ashes are highly evolved and contain 74–79 wt % SiO2; dacitic ashes vary from 67–77 wt % SiO2. Al ranges from 10.5 to 14.5 wt % Al2O3, Mg ranges from 0.06 to 1.0 wt % MgO, and Fe (as Fe3O4) ranges from 0.50 to 4.1 wt %. Dacitic ash beds have the most (0.80–3.8 wt % as Fe3O4) and rhyolitic ashes have the least (0.50–1.9 wt % as Fe3O4). Ca varies from an unusually low amount (0.25 wt % CaO) in a few rhyolitic ashes to as much as 2.2% in some dacitic ashes. Rhyolitic glasses show larger Eu deficiencies compared to dacitic glasses on rare‐earth element diagrams. W‐type rhyolitic glasses have moderately large La to Sm ratios and have La contents from about 30 to 100 times chondrite values; G‐type rhyolitic glasses also have moderately large La to Sm ratios and contain somewhat more La than do the W‐type rhyolitic ashes, generally from 100 to 300 times chondrites amounts.