Event Name : Hayes Oshetna TephraThis is a questionable event.
|Start: 7090 (± 30 Years) || Years BP C-14 (raw) || |
|Stop: 5850 (± 25 Years) || Years BP C-14 (raw) || |Description:
|Eruption Product: || other ||
From Dixon and Smith (1990): "The Oshetna tephra consists of a uniform layer of light brownish gray (2.5Y 6/2) sandy silt typically 3 to 5 cm thick although a maximum thickness of 8 cm was observed. Microscopically the Oshetna tephra is characterized by the presence of quartz, feldspar, glass shards, blue-green hornblende, and orthopyroxene."
From Davies and others (2016): "The Oshetna tephra has been reported at several sites within central Alaska, particularly in association with archaeological sites in the Susitna valley (e.g. Dixon, 1985). While its source was not identified, it has recently been suggested that it may originate from the Hayes volcano (Wallace and others, 2014)."
From Wallace and others (2015): "The Oshetna tephra, informally named by Child and other (1998) for the stream valley in which it was first identified during cultural resource investigations conducted from 1979-85, is widespread in the Susitna River valley in south-central Alaska (fig. 1A [in original text]; J.E. Dixon and others, written commun(s)., 1985). The Oshetna tephra is 3-5 cm thick in this region, and is attributed to Hayes Volcano based on similarities in mineralogy and major-element glass composition to Hayes Volcano tephras. The Oshetna tephra was erupted 5,960-5,790 14C yr B.P. (Child and others, 1998), which makes it older than tephra set H at the Hayes River outcrop. Numerous additional radiocarbon analyses from J.E. Dixon and others (written commun(s)., 1985) corroborate this age. Glass analyses for pumice from the Hayes River ignimbrite do not match glass data for the Oshetna tephra, which have lower SiO2 and K2O, and higher CaO, MgO, and TiO2 than Hayes River ignimbrite glass (fig. 10 [in original text]). Similar amounts of amphibole and biotite in the Oshetna tephra and the rhyodacite clasts from Unit II at the Hayes River outcrop are suggestive of a possible correlation, though we do not have glass analyses of the rhyodacite to test this hypothesis."
From Mulliken (2016): "Glass geochemical analyses indicate that the Oshetna tephra is heterogeneous with at least four populations of volcanic glass. Previous analyses of the Oshetna tephra, however, have only identified two population of volcanic glass in the deposit (Child and others 1998; Dilley 1988) (Tables 2.1 and 2.2 [in original text]). Because the source, or sources, of the volcanic glass shards comprising the Oshetna tephra are unknown, it is difficult to evaluate its deposit in terms of the number of volcanic events represented. The Oshetna could be a product of Hayes Volcano, based on the proximity and thickness of the deposit; however, the deposit lacks biotite (Romick and Thorson 1983), which is a mineral characteristic to Hayes Volcano. Therefore, another possibility is that the deposit is reworked and representative of multiple tephra depositional events from different volcanoes. Ashfalls from nearby Cook Inlet volcanoes as recorded in lacustrine cores (de Fontaine and others 2007; Schiff and others 2008) suggest that numerous tephra deposits could be present in the mSRV. Redoubt, Augustine, Iliamna, and Spurr volcanoes could have deposited Holocene aged tephra in the mSRV and future correlation efforts will aid in understanding the Oshetna deposit."