HR: 0830h
AN: GP31D-0774    [PDF]
TI: Jaramillo, Cobb Mtn., or Punaruu? - Re-examination of "Cobb Mtn. Event" Lavas at the Coso Volcanic Field, CA.
AU: * Pluhar, C J
EM: cpluhar@es.ucsc.edu
AF: University of Califronia, Earth Science Dept. 1156 High St., Santa Cruz, CA 95064-1077 United States
AU: Nomade, S
EM: snomade@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Glen, J M
EM: jglen@usgs.gov
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Glen, J M
EM: jglen@usgs.gov
AF: United States Geological Survey, 345 Middlefield Road, Palo Alto, CA 94025 United States
AU: Coe, R S
EM: rcoe@es.ucsc.edu
AF: University of Califronia, Earth Science Dept. 1156 High St., Santa Cruz, CA 95064-1077 United States
AB: Mankinen and Gromme (1982) hypothesized that two flows from the region were erupted during the Cobb Mtn event since they yielded dates of 1.07$\pm$0.14 Ma and 1.07$\pm$0.12 Ma (Duffield et al., 1980). Given advances in the magnetic polarity timescale, it is now unclear whether to assign these flows to the Jaramillo subchron (0.99-1.07 Ma), the Punaruu Event (1.122$\pm$0.010 Ma), or the Cobb Mtn. Event (1.194$\pm$0.014 Ma). Therefore, we have undertaken improved radiometric and paleomagnetic studies to resolve this uncertainty and provide additional age constraints to the magnetic polarity timescale. We have dated by Ar/Ar geochronology the four widely separated units (3 basalts, 1 rhyolite) described by Mankinen and Gromme. All ages are reported here at 2 sigma error and relative to the monitor FCs (28.02 Ma) and/or ACs (1.192 Ma). Paleomagnetically, we confirm that two of these are of normal polarity and two reversed, but our age constraints remain as equivocal as before. Fresh glass was extracted from one normal-polarity basalt sample in order to avoid the xenocrysts common in rocks of this suite. The degassing spectrum is discordant, with rising ages at higher temperatures. The isochron age (1.11$\pm$0.11 Ma, trapped $^{40}$Ar/$^{36}$Ar equal to 298$\pm$2, not distinct from atmospheric) yield error bars too large to distinguish between Cobb Mtn., Jaramillo, and Punaruu. Data from the other normal-polarity basalt indicates an age (c.a 0.97 Ma) younger than previously reported. This may correlate with the Jaramillo subchron. The reversed polarity units were also dated in order to bracket the normal polarity flows. One of these ages, 1.140$\pm$0.008 Ma, from a rhyolite dome falls between the Punaruu and Cobb Mtn. Events. In short, paleomagnetic data confirm previous results on the polarity of these flows. However, radiometric dating suggests that these units should no longer be referred to as constraints for the Cobb Mtn. Event unless future geochronology shows such a correlation more convincingly.
DE: 1035 Geochronology
DE: 1513 Geomagnetic excursions
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
DE: 9350 North America
DE: 9604 Cenozoic
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting