Event Name : Hayes River Ignimbrite
|Start: 40000 || Years BP K-Ar || |
|Stop: 4450 (± 30 Years) || Years BP C-14 (raw) || |
|Lahar, debris-flow, or mudflow: ||
|Pyroclastic flow, surge, or nuee ardente: ||
|Eruption Product: || rhyolite ||
From Wallace and others (2015): "The basal unit of the Hayes River outcrop is a light gray (N7) to white (N9) pumiceous pyroclastic-flow deposit, the Hayes River ignimbrite (fig. 3 [in original text]). This deposit is 20-30 m thick, and forms the most visually striking part of the Hayes River outcrop (fig. 2 [in original text]). The stratigraphically lowest and largest portion of the Hayes River ignimbrite is a 20-m-thick, lithic-poor (<1 percent) pumiceous, nonwelded pyroclastic-flow deposit (fig. 4A [in original text]). It consists of subangular to subround pumice clasts typically as large as 10 cm (rarely 50 cm) in a fine- to coarse-ash, crystal-rich matrix. The pumice clasts are of two types, (1) dominantly white (N9), friable, biotite-sanidine-plagioclase-quartz rhyolite, with notable foliation caused by alignment of biotite grains, and (2) light gray (N7), finely vesicular rhyolite, less friable than type (1) but with the same mineralogy. Some pumice clasts are texturally banded and are gradational between the two main types. Phenocrysts of feldspar and quartz are as large as 6 mm across. In addition, dense white holocrystalline blocks, also rhyolitic in composition and with the same mineralogy, make up <1 percent of the deposit. The upper 10 m of the Hayes River ignimbrite is lithic-rich, and in some places appears gradational with Unit II."
"The total volume of the Hayes River ignimbrite is not known, as it has been recognized only at the Hayes River outcrop. A minimum estimate of 2 km3 is derived from the total area of the Hayes Glacier drainage upstream (90 km2) and a minimum thickness at the outcrop of 20 m."
"Immediately overlying the Hayes River ignimbrite of Unit I is a thick sequence of complex mass-flowage deposits that are variable in thickness along the 600 m wide outcrop (fig. 2 [in original text]). The relation between Units I and II is unclear, as there is some evidence they may be coeval, but for the sake of clarity we describe these flowage deposits as a separate unit."
"Immediately above the Hayes River ignimbrite in some places is a 2-4 m-thick sequence of laterally discontinuous, coarse diamicton that includes rounded to angular boulders as much as 2 m in diameter, which are composed of mostly intrusive rocks (granite, granodiorite) and some indurated clasts of volcanic breccia (figs. 4B, C [in original text]). No obviously juvenile material was observed in this part of Unit II. The contact between Unit II and the Hayes River ignimbrite below is erosional in some places but graditional in others (figs. 4B, C [in original text]). Where the Hayes River ignimbrite is in direct contact with the overlying diamicton, oxidized gas-escape pipes at the contact and upward into the basal boulder diamicton suggest that the ignimbrite may have been hot when the diamicton was emplaced (fig. 4C [in original text])."
"The upper several meters of Unit II are a poorly sorted, vaguely bedded, matrix-supported, angular-pebble gravel with an oxidized silt-rich upper 25 cm (fig. 4D [in original text]). The dominant clast type in this upper portion of Unit II is rounded dense rhyodacite with prominent quartz and feldspar phenocrysts in a light gray (N7) matrix."
"We have no direct evidence for the eruption age of the Hayes River ignimbrite, however, it must be older than the 4,450+/-30 14C yr B.P. soil beneath Unit III. The limited extent of Hayes River ignimbrite deposits may be the result of erosion by glacial ice. Given the thickness, particle size, and composition of the Hayes River ignimbrite, it likely records a substantial eruption of Hayes Volcano, and we would expect to find correlative tephra deposits in many areas of south-central Alaska if the ignimbrite were emplaced during a Holocene eruption. Radiometric dating of sanidine, zircon, and monazite from the Hayes River ignimbrite’s rhyolite pumice indicate crystallization ages of ~40-30 ka (Calvert, Coombs, and Vazquez, USGS, unpub. data, 2014), thus representing a maximum possible age. Preservation of pre-Holocene pyroclastic deposits in south-central Alaska is extremely limited, presumably due to emplacement on ice or erosion by glacial ice."