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The most widespread of all Holocene tephra deposits in the Cook Inlet region of south-central Alaska is a set of deposits from Hayes volcano. Because of their unique phenocryst content—biotite in rare amounts and a high proportion of amphibole to pyroxene—the deposits are readily identifiable at all but the most distant sites where they are very fine grained. Eighteen radiocarbon dates from eight upland sites limit the age of the tephra set to between about 3500 and 3800 yr. The set originated at Hayes volcano in the Tordrillo Mountains 150 km northwest of Anchorage; seven or possibly eight closely succeeding deposits, low-silica dacite in composition, compose two main lobes that extend northeast for 400 km and south for at least 250 km from the vent. We estimate the total tephra volume to be 10 km3; multiple layers imply four to six larger and two or three smaller eruptions. The deposits are a nearly isochronous marker horizon that should be useful in future archeologic, geologic, and palynologic studies in the region.

Several Holocene tephra deposits of Hayes volcano constitute a marker horizon in southern and east-central Alaska. Their identification is aided by high amphibole/pyroxene ratio and biotite in trace amounts, unique among Holocene tephra deposits of the region. However, correlations are obscured by chemical heterogeneity of the glass which occurs at a scale less than the size of a lapilli. Single-shard analyses confirm that the heterogeneity is due neither to fractionation nor to plagioclase microlites. The heterogeneity may be due to mixing of magmas prior to eruption. It is proposed that the deposits be informally called Hayes tephra set H.

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