HR: 1340h
AN: V33B-0681    [Abstracts]
TI: Prehistoric Lahar and Tephra Sequences on Mt. Cleveland, Islands of Four Mountains, Eastern Aleutian Islands
AU: * Pekar, K
EM: kpekar@ksu.edu
AF: Dept. of Geology, Kansas State University, 108 Thompson Hall, Manhattan, KS 66506
AU: Nicolaysen, K P
EM: knic@ksu.edu
AF: Dept. of Geology, Kansas State University, 108 Thompson Hall, Manhattan, KS 66506
AU: Bridges, D
EM: dbridges@ksu.edu
AF: Dept. of Geology, Kansas State University, 108 Thompson Hall, Manhattan, KS 66506
AU: Dehn, J
EM: jdehn@gi.alaska.edu
AF: University of Alaska, Fairbanks Geophysical Institute, and Alaska Volcano Observatory, Fairbanks, AK 99775
AB: We provide the first geochronology for prehistoric activity of Mt. Cleveland, which has erupted 23 times historically and as recently as June, 2005. The lowest stratigraphically-exposed dacite indicates Cleveland already had a significant volume by 1.4+/-0.3 Ma (1 s.d.) based on a total gas age of a groundmass separate analyzed by ⁴⁰Ar/³⁹Ar. We describe two locations on the northern vegetated flank of Mt. Cleveland that expose interbedded lahar and tephra sequences that provide geochronologically-constrained comparisons with modern lahar deposits. Four calibrated radiocarbon dates were obtained by accelerator mass spectrometry (1 s.d. quoted). Beach cliffs (9 m high) on the northeast shore expose at least four debris flows, a thick ash and a probable paleosol. Three debris flows form the base of the exposure and organic material from the top finest-grained lahar gives a calibrated age of 3652+/-43 yr BP. This layer grades upward to a 2-5 cm thick, extremely hard and oxidized layer with abundant root material, some of which is burned. We interpret this to be a soil baked by an overlying fine-grained ash (50 cm thick). Charred organics from the baked soil yielded an especially important age of 2758+/-20 yr BP giving a maximum age for the eruption of the ash. Root material in a debris flow from just above the ash gives a calibrated age of 1753+/-59 yr BP, thus bracketing the eruption of the ash. The second location, located in a ravine on the north flank, is a sequence of distal lahars with at least four interbedded tephras. Coarse sand layers, some of which are oxidized, separate these ashes. Also intercalated between two ashes is a fine silt layer 20 cm thick with preserved stems and grass leaves, which yielded a calibrated age of 9082+/-58 yr BP. Previous work showed that the 2001 eruptions on the western and southern flanks emplaced debris flows that are overlain by lava and highlights the role of snowfields in creating this sequence. Unlike the recent phase, prehistoric lahars on the northern flank of Mt. Cleveland are not interbedded with lava, but instead occur with tephra sequences that will help unravel Holocene activity of the volcanoes in the Islands of Four Mountains.
DE: 1105 Quaternary geochronology
DE: 1115 Radioisotope geochronology
DE: 8400 VOLCANOLOGY
DE: 8455 Tephrochronology (1145)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005