Distribution and Origin of the 1883 Augustine Tephra-Fall Deposits, South-Central Alaska

HR: 08:35h
AN: V21C-03 [Abstracts]
TI: Distribution and Origin of the 1883 Augustine Tephra-Fall Deposits, South-Central Alaska
AU: * Gardner, C A
EM: cgardner@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Court, Vancouver, WA 98683 United States
AU: BegＵ, J E
AF: University of Alaska, 900 Yukon Drive, Fairbanks, AK 99755 United States
AB: Augustine Volcano, an island in south-central Alaska, is the most frequently active volcano in Cook Inlet basin where almost half the population of Alaska lives. It has had at least five eruptions in historical time (past 200 years; 1812(?), 1883, 1935, 1964, 1976, and 1986), the most notable of which occurred in 1883. The 1883 eruption consisted of a debris avalanche and resultant tsunami, lava dome, block-and-ash pyroclastic flows, pyroclastic surges, and tephra fall. It is the only historical eruption that does not contain pumice; nonetheless, tephra fall from 1883 may be the most widespread of the historical tephras. 1883 tephra-fall deposits are tentatively identified as far south as the Kamishak River on the west side of the inlet and as far north as the Kenai River on the east side of the inlet; deposit thickness off island is generally 2 to 4 cm. Tephra-fall deposits are everywhere fine grained. Particles rarely exceed 1 mm on island (most are finer) and are generally $<=0.25$ mm distally. Tephras on island contain plagioclase, hypersthene, augite, and magnetite in order of decreasing abundance. We speculate that the origin of the tephra fall is chiefly from ash clouds elutriated from the pyroclastic flows. Field identification of 1883 tephra is based on it being the first non-pumiceous tephra within a few centimeters below the 1912 Katmai ash, a widespread prominent white marker bed in Cook Inlet. The only other Cook Inlet eruption that could occupy that position is tephra fall from the 1902 Redoubt eruption of which little is known. Other potential sources are volcanoes on the Alaska Peninsula, but eruptions from those volcanoes would need to be of significant volume to be several centimeters thick in the upper Cook Inlet basin. Studies are underway to look at mineral assemblage, glass chemistry, and oxide compositions to strengthen correlations. Historical eruptions from Augustine contain mixed-magma clasts of andesite and dacite and most glasses contain numerous microlites; thus it may be difficult to obtain satisfactory geochemical correlations for these tephras.
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting