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Katmai reported activity





Event Name : Katmai Lethe Assemblage/Lethe Tephra

Start: 16000 Years BP C-14 (raw)
Stop: 12000 Years BP C-14 (raw)

Tephrafall: BibCard BibCard BibCard BibCard BibCard BibCard BibCard BibCard
Lahar, debris-flow, or mudflow: BibCard BibCard
Pyroclastic flow, surge, or nuee ardente: BibCard BibCard
Eruption Type:Explosive
Eruption Product: andesite BibCard

Description: From Stevens (2012): "The informally named Lethe volcaniclastic deposits (Pinney and Begét 1990, 1991a, 1991b; Pinney 1993) comprise an extensive suite of dacitic deposits, including pyroclastic flows, lahars, lahar-runout flows, and primary and reworked fallout tephra. These deposits are exposed in river gorges in the Valley of Ten Thousand Smokes as far as 1.0 mi (1.5 km) up-valley from the mouth of Windy Creek and extend about 3.1 mi (5 km) past Windy Creek in Overlook valley (Figure 5 [in original text]). Lethe deposits overlie Iliuk drift in the Windy Creek area and are overlain by and incorporated into Ukak and Katolinat drifts (Pinney and Begét 1991b, Pinney 1993). Organic silt immediately underlying the younger glacial deposits yields a minimum age of 12,640+/-100 RC yr B.P. for Lethe deposits (Pinney and Begét 1991a, Pinney 1993). Hildreth and others (2003) correlate the Lethe volcaniclastics to a remnant of a proximal pumice-and-scoria fall in the Katmai caldera rim, suggesting that the deposits were erupted from Mount Katmai. Lethe ash has been used as an important stratigraphic marker horizon at Iliamna and Naknek lakes (Kaufman and Stilwell 1997), and 185 mi (300 km) away on the Kenai Peninsula (Reger and others 1996). Organic material beneath the Lethe tephra on the Kenai Peninsula gives a maximum age for the tephra of 16,480+/-170 RC yr B.P. (Reger and others 1996), and lake-core studies in lower Cook Inlet suggest a minimum age of 13,730 RC yr B.P. (Rymer and Sims 1982, Riehle and others 2008). These dates correlate well with the minimum date from the Windy Creek area and indicate that the Lethe volcaniclastics were erupted between about 16,600 and 13,730 RC yr B.P."

From Fierstein (2007): "...Plinian to sub-Plinian dacitic eruption 12 to 16 ka that sent remobilized pumiceous debris down glaciercovered slopes to pond in lowlands beyond (the Lethe Assemblage)..."

"Analyses of pumice clasts from the hcs and debris flows of the Lethe deposits overlap those of two pyroclastic units exposed on the rim of Katmai caldera (Fig. 20 [in original text]); (1) a 50-m-thick remnant of an orange-oxidized coarse proximal scoria fall zoned upward from dacite to andesite on the south rim, and (2) a sheet of grey agglutinated dacite fallout (described by Hildreth and Fierstein 2000) that widely caps the west rim, drapes the summit of peak 6128, and thickens into a pre-1912 crater (largely destroyed in 1912) (Figs. 3, 22 [in original text]). Pumice from these rim units are so similar that none of the whole-pumice or microprobe glass data discriminate between the two deposits; only the higher P2O5 in the Lethe pumice suggests an affinity with the south rim dacite. Fe-Ti oxide data, however, better discriminate between them. Oxide data from Lethe pumice deposits cluster tightly and yield higher MgO and lower TiO2 than oxides from pumice in the west-rim deposits (Fig. 21 [in original text]), suggesting the west rim agglutinate is not correlative with the Lethe tephra. Considering this, and that the whole-pumice and glass data show that the Lethe deposits did come from one of these two rim sources, and because the clasts on the south rim are oxidized orange like those in the remobilized deposits, we favor the south rim of Mount Katmai as the source for the Lethe volcaniclastics. Direct comparison with oxides from the Katmai south-rim deposits cannot be made because all are badly oxyexsolved and unsuitable for correlation. Glass shard data from what Reger and others (1996) call the "Lethe Tephra" -an ashfall layer widespread in late Pleistocene deposits on the Kenai Peninsula (260-380 km away)-are plotted with our data (Fig. 21a [in original text]) and corroborate their correlation with the "Lethe volcaniclastics". We thus prefer the term "Lethe Assemblage" (Hildreth and others 2000; Hildreth and Fierstein 2000, 2003) to refer to this collection of distal fall, medial remobilized deposits, and proximal agglutinate."

"The assemblage is widely overlain by several meters of Early Holocene till, and, wherever its base is exposed, it also rests directly on late Pleistocene till. From stratigraphic relationships in the Kenai Lowland, Reger and others (1996) inferred an age for the "Lethe Tephra" slightly younger than 16,000 14C years, B. P. For the Lethe Assemblage itself, we have obtained an older limiting age of 18,980+/-90 14C years B.P. for a rip-up clast of silty soil enclosed in the massive sand deposit near Windy Creek (Table 2 [in original text]). Pinney and Beget (1991) published an upper limiting age of 12,640+/-100 14C years B.P. for "organic silt" nearby, which they inferred from their interpretation of the local glacial deposits to be younger than the Lethe Assemblage. Thus, our best emplacement-age estimate for the Lethe Assemblage is between ?12,000 and 16,000 14C years B.P., when a valley glacier still occupied the VTTS."

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