UNIQUE STRATIGRAPHIC RECORDS OF EXPLOSIVE VOLCANISM AND QUATERNARY GLACIATION, EMMONS LAKE CALDERA AND VICINITY, SOUTHWESTERN ALASKA

Uniquely preserved volcanic and glacial deposits near Emmons Lake caldera (ELC) on the Alaska Peninsula provide an opportunity to evaluate the characteristics and timing of explosive volcanism and glaciation. Caldera-forming eruptions at ELC produced at least 2 ignimbrite sheets and tephra that are locally interbedded with Quaternary glacial deposits. Deposits along the Leontovich River include a 1-m-thick bed of fine tephra overlain by an 8-m-thick bed of moderately welded dacitic tuff. The tephra-tuff sequence was erupted during the first of two caldera-forming eruptions (C-1) at ELC, about 238 ka. Glacial deposits that record significant ice advances onto the Alaska Peninsula coastal plain enclose C-1 deposits. Glacial deposits above C-1 tuff are weathered and featureless, and a diamicton beneath C-1 tephra is probably a lodgment till. Oxidation of the upper 20 cm of the lower till indicates subaerial exposure of the drift prior to eruption of the C-1 tephra-tuff sequence. Younger knob-and-kettle moraine and outwash in the Cathedral River valley is less extensive than glacial deposits on the C-1 tuff and represents the most recent glaciation of the valley. The terminal moraine in the Cathedral River valley overlies a lithic and pumice bearing tuff that is geochemically similar to C-2 pyroclastic deposits. Pumiceous pyroclastic-flow deposits on the south side of ELC, in the Caribou River drainage northeast of Pavlof Volcano, and in sea cliffs along the Bering Sea coast record a second major pyroclastic eruption of ELC (C-2). A buried soil immediately beneath the C-2 ignimbrite sheet exposed along the Bering Sea coast yielded a radiocarbon age of 27,110 +/- 790 yr. B.P., a maximum-limiting age for C-2. Subtle differences in trace-element glass chemistry and in Fe-Ti oxide content indicate that the C-1 event did not produce the well-known ca. 140-ka Old Crow tephra as we reported previously. Furthermore, 40Ar/39Ar ages on C-1 welded tuff indicate they were emplaced ca. 238 ka and are older than Old Crow tephra. The 14C age on the C-2 event indicates a major eruption of ELC less than 27 ka and compositional data on C-2 pyroclastic-flow deposits indicate that they correlate with the Dawson tephra, a distinctive bed of silicic ash found in west-central Yukon, Canada erupted ca. 22-12 ka.