HR: 1340h
AN: V43E-1453    [Abstracts]
TI: Stratigraphy and Paleomagnetism of the Pine Creek and Castle Creek Eruptive Episodes, Mount St. Helens, Washington
AU: * Clynne, M A
EM: mclynne@usgs.gov
AF: U.S. Geological Survey, Volcano Hazards Team 345 Middlefield Rd. MS910, Menlo Park, CA 94025 United States
AU: Champion, D E
EM: dchamp@usgs.gov
AF: U.S. Geological Survey, Volcano Hazards Team 345 Middlefield Rd. MS910, Menlo Park, CA 94025 United States
AU: Wolfe, E W
EM: ewwolfe@usgs.gov
AF: U.S. Geological Survey, Volcano Hazards Team 2725 Boone Court, Prescott, AZ 86305 United States
AU: Gardner, C A
EM: cgardner@usgs.gov
AF: U.S. Geological Survey, Volcano Hazards Team 1300 SE Cardinal Court, Vancouver, WA 98683 United States
AU: Pallister, J S
EM: jpallist@usgs.gov
AF: U.S. Geological Survey, Volcano Hazards Team 1300 SE Cardinal Court, Vancouver, WA 98683 United States
AB: We present new paleomagnetic data, and we compile and re-evaluate published data for the Pine Creek and immediately following Castle Creek eruptive episodes at Mount St. Helens. For the first time, paleomagnetic sites are correlated with detailed volcanic stratigraphy for these late Holocene eruptive episodes. New and existing radiocarbon data and correlation with western North America geomagnetic secular variation data provide age control. Our work leads to a revised understanding of the eruptive sequence and suggests modifications to the traditional interpretation of Pine Creek and Castle Creek eruptive episodes. Previously, the beginning of the Castle Creek episode was defined as the first appearance of andesite in the Pine Creek- Castle Creek sequence, and a hiatus of a few hundred years was inferred to have been present between the two episodes (for example, see Mullineaux 1996, USGS PP 1563). However, a number of early Castle Creek andesitic pyroclastic deposits and lava flows have magnetic directions unlike later Castle Creek units. Instead, they are similar to Pine Creek dacitic pyroclastic flows. Correlation between Castle Creek age lava flows and pyroclastic deposits and Castle Creek tephras (B set of Mullineaux) shows that the hiatus defined by a gap in the paleomagnetic record occurs between tephras Bo and Bi instead of at the presently accepted boundary between the Pine Creek and Castle Creek eruptive episodes. Thus, Castle Creek tephras Bh and Bo and associated units have magnetic directions similar to Pine Creek, and tephras Bi and Bu and associated units have Castle Creek magnetic directions. Magnetic directions of both eruptive episodes are entirely consistent with the late Holocene western North America geomagnetic secular variation as presently understood. Pine Creek dacites erupted about 2,700-2,600 years B.P. Castle Creek andesites (and tephras Bh and Bo) probably erupted about 2,600-2,300 years B.P., and Castle Creek dacites and basalts (and tephras Bi and Bu) erupted between about 2,100-1,900 years B.P.
DE: 8400 VOLCANOLOGY
DE: 1527 Paleomagnetism applied to geologic processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting