Significance of a near-source tephra-stratigraphic sequence to the eruptive history of Hayes Volcano, south-central Alaska

Bluffs along the Hayes River valley, 31 km northeast and 40 km downstream from Hayes Volcano, reveal volcanic deposits that shed new light on its eruptive history. Three thick (>10 cm) and five thin (<10 cm) tephra-fall deposits are dacitic in whole rock composition and contain high proportions of amphibole to pyroxene and minor biotite and broadly correlate to Hayes tephra set H defined by earlier investigators. Two basal ages for the tephra-fall sequence of 3,690±30 and 3,750±30 14C yr B.P. are also consistent with the Hayes tephra set H timeframe. Distinguishing among Hayes tephra set H units is critical because the set is an important timestratigraphic marker in south-central Alaska and this section provides a new reference section for Hayes tephra set H. Analysis of Fe-Ti oxide grains in the tephras shows promise for identifying individual Hayes deposits. Beneath the dacitic tephra sequence lies an older, poorly sorted tephra (tephra A) that contains dacite and rhyolite lapilli and whose basal age is 4,450±30 14C yr B.P. Immediately below the tephra-fall sequence (Unit III) lies a series of mass-flow deposits that are rich in rhyodacitic clasts (Unit II). Below Unit II and possibly coeval with it, is a 20–30 m thick pumiceous pyroclastic-flow deposit (Unit I) that extends to the valley floor. Here informally named the Hayes River ignimbrite, this deposit contains pumice clasts of rhyolite with quartz, sanidine, plagioclase, and biotite phenocrysts, an assemblage that is unique among known Quaternary volcanic products of Hayes and other Alaskan volcanoes. Units I, II, and tephra A of Unit III represent at least two previously unrecognized eruptions of Hayes Volcano that occurred prior to ~3,700 yr B.P. No compositionally equivalent distal tephra deposits correlative with Hayes Volcano rhyodacites or rhyolites have yet been identified, perhaps indicating that some of these deposits are pre-Holocene, and were largely removed by glacial ice during the last ice age. More field and analytical work is needed to further refine the eruptive history of Hayes Volcano. Introduction Hayes Volcano is a snowand ice-covered volcano in the northern Tordrillo Mountains, 140 km northwest of Anchorage, Alaska (fig. 1). The northernmost active volcano in the Aleutian-Alaskan volcanic arc, it was discovered in 1975 (Miller and Smith, 1976), and little is known about its eruptive history owing to the limited exposure of volcanic deposits. It has produced pyroclastic flows that descended into the Hayes River drainage, lahars that travelled down the Hayes River and into the Skwentna River (Waythomas and Miller, 2002), and most notably, a series of seven or eight closely spaced tephra-fall deposits between ~3,800–3,500 14C yr B.P. (Riehle, 1985, 1994; Riehle and others, 1990; Begét and others, 1991). These tephra-fall deposits, informally known as “Hayes tephra set H” (herein refered to as tephra set H), are the most widespread tephras of Holocene age in south-central Alaska (Riehle, 1994). The tephra set has a composite volume of about 10 km3 and is distributed as far as 650 km north, south, and east of Hayes Volcano. Tephras within tephra set H, as described by Riehle (1985) and Riehle and others (1990), are dacitic in composition, and contain rhyolitic glass and phenocrysts of plagioclase, amphibole, pyroxene, Fe–Ti oxides, and rare biotite. The high proportion of amphibole and trace amounts of biotite distinguish tephra set H tephras from other Holocene tephras erupted from Cook Inlet volcanoes south of Hayes (Spurr, Redoubt, Iliamna, and Augustine) (Riehle, 1985). Tephras from locations as widespread as the Kenai Peninsula (Combellick and Pinney, 1995; de Fontaine and others, 2007), Denali National Park (Child and others, 1998), the Susitna River valley (Dixon and Smith, 1990), and the Matanuska River valley (Fontana, 1988), have been correlated to tephra set H on the basis of mineralogy, major-element glass composition, and age. Begét and others (1991) recognized that the Jarvis Ash Bed and informal Cantwell and Tangle Lakes tephra deposits of interior Alaska, all likely formed during a single eruption during the tephra set H sequence. Tephras of tephra set H are present in the Anchorage area, and are the only tephras of Holocene age of any substantial thickness (as much as 2 cm) in and around Alaska’s most populated city (Riehle, 1985). Some ambiguity remains, however, regarding exactly how many eruptions occurred during the interval of time represented by the tephra set, and delineation of discrete deposits within the set awaits further work. Aside from tephra set H, evidence for other Holocene eruptions of Hayes Volcano is sparse (Waythomas and Miller, 2002). Riehle (1985) describes a possible Hayes tephra that 2 Significance of a Near-Source Tephra-Stratigraphic Sequence to the Eruptive History of Hayes Volcano is ~500–1,000 14C yr B.P. Additionally, archeological studies in the upper Susitna River valley revealed an older regional tephra deposit, informally named the Oshetna tephra (Child and others, 1998), that was erupted 5,790–5,960 14C yr B.P. and has been attributed to Hayes Volcano (J.E. Dixon and others, written commun(s). , 1985; Dilley, 1988; Dixon and Smith, 1990). A younger, fine ash bed informally known as the Devil tephra, also found throughout the upper Susitna River valley, was erupted 1,420–1,516 14C yr B.P. and is also a likely product of Hayes Volcano (Dixon and Smith, 1990). Only a few samples have been collected from the volcano itself, and none have been radiometrically dated. In this report, we describe a stratigraphic section 31 km northeast of Hayes Volcano, along the Hayes River immediately downstream from the terminus of the Hayes Glacier (fig. 1B). This section, referred to as the Hayes River outcrop (fig. 2), is significant because it contains a well-preserved sequence of dacite tephra deposits that generally correlates to tephra set H of Riehle and others (1990), but that also includes at least one additional tephra-fall deposit dated at 4,450±30 14C yr B.P. that is significantly different in composition. This tephra records an eruption of Hayes Volcano that occurred prior to the eruption of tephra set H. Lower in the section, the Hayes River outcrop contains an older rhyodacite-bearing flowage