Description: From Davies and others (2016): "Ager (1983, 2003) reports a 5 cm thick grey tephra near the base of cores taken from Puyak and Zagoskin Lakes on St Michael Island (Fig. 5 [in original text]) in western Alaska. Although no geochemical data were published with the original reports, they were analysed but did not match any known tephra at the time (Ager, 2003; pers. comm.). The original samples from both locations have been analysed here and new data are shown in Fig. 7 and Table 3 [of original text], along with analyses for a younger tephra from Zagoskin Lake that is correlated to the mid-late Holocene caldera-forming eruption (CFE II) of Aniakchak volcano (Riehle and others, 1987; UA 1602-see Table 4 in Section 5 [in original text] for details). The basal tephra present in both Zagoskin and Puyak lakes are correlative, have a glass morphology predominantly comprised of highly vesicular pumice shards, and are andesitic in composition. They also correlate with Tephra D from the Ahklun Mountains (Kaufman and others, 2012), and hence shall be referred to as such (Fig. 7 [in original text]). This indicates Tephra D is distributed well beyond south-western Alaska. The zones outlined on Fig. 7 [in original text] show the geochemical range of major element data produced from whole rock samples of Aniakchak CFE I and II (Bacon and others, 2014). Although these data are not directly comparable to EPMA glass data due to the incorporation of minerals, it does show that the relations between Tephra D and Aniakchak CFE II glass data (UA 1602) mirror that between the whole rock CFE I and II data for multiple elements. The offset seen for wt% FeOt and SiO2 are as expected given from the presence of common phenocryst phases (e.g. plagioclase). This suggests that Tephra D may also be sourced from Aniakchak."
"Tephra D is given a maximum age of 15,610+/-220 14C yr BP at Puyuk Lake, and a minimum age of 14,970+/-170 14C yr BP at Zagoskin Lake, although these radiocarbon dates are likely to be older than expected as they are bulk sediment dates (Ager, 2003; pers. comm.) Tephra D has a mean age of 15,505+/-312 cal yr BP from two sites (Kaufman and others, 2012) and this is believed to be a more reliable estimate for the tephra as the constraining dates are produced from plant macrofossils. These ages do not support a correlation between Tephra D and Aniakchak CFE I, which has a suggested maximum age from VanderHoek and Myron (2004) of 9470+/-40 14C yr BP. The apparent geochemical associations do, however, suggest that Tephra D may originate from an earlier eruption of Aniakchak that has not been reported from the proximal records. This is potentially due to the glacial erosion of pre-Holocene age deposits surrounding the Aniakchak volcano."
From Bacon and others (2014): "Pleistocene Aniakchak was a composite volcano that grew over a period of several hundred thousand years atop Jurassic-Tertiary sedimentary rocks by effusion of basaltic andesite to dacite lava. The Aniakchak edifice was smaller than those of other volcanoes with ~10 km diameter calderas in the eastern Aleutian arc, and it was repeatedly sculpted by ice during glacial intervals so that deep valleys were present at the end of the last glaciation. Basaltic magmas, with a range of incompatible element concentrations characteristic of arcs, were drawn from the mantle wedge into the deep crust beneath Aniakchak, where they were modified through crystallization differentiation and interaction with their surroundings in a deep crustal hot zone. Fractionated magmas that escaped upward further differentiated and partially to completely solidified to gradually build an intrusive complex, or mush column, in the mid to upper crust that was well developed by the end of the last glaciation."
"The concept of late Pleistocene Aniakchak that emerges is of a composite volcano of moderate size whose glacially sculpted summit would have been south of the center of the present caldera. Bedrock morphology of glacial valleys heading in Aniakchak lavas would have been essentially as now, with the exception of that of The Gates and Aniakchak River that was extensively modified by draining of the intracaldera lake. Birthday Creek glacial valley cut deeply into the southwest flank of the edifice and extended as a significant declivity well into the area above the present southwest caldera floor. The width and east-west trend of Birthday Creek valley near the modern caldera rim suggest that ice of sufficient volume to carve this large valley may have been sourced in a small late Pleistocene caldera."
"The Pleistocene eruptive history preserved by the edifice lavas has yet to be studied in detail. Reconnaissance sampling and geochemical analysis of about 30 specimens provides a general compositional range for Aniakchak erupted magmas over the probable several hundred thousand years during which the edifice was constructed (Nye and others, 1993). There doubtless exist materials emplaced during glacial conditions, so that a record of volcanism during different climatic conditions could be obtained through additional field study and argon geochronology. For the present report, we are limited to employing the geochemistry of edifice lavas as context for compositional diversity among products of postglacial Aniakchak."