HR: 1340h
AN: GP43B-0861    [Abstracts]
TI: Magnetic Excursion Recorded in Basalt at Newberry Volcano, Central Oregon
AU: * Champion, D E
EM: dchamp@usgs.gov
AF: U.S. Geological Survey, MS-910 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Donnelly-Nolan, J M
EM: jdnolan@usgs.gov
AF: U.S. Geological Survey, MS-910 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Lanphere, M A
EM: alder@usgs.gov
AF: U.S. Geological Survey, MS-910 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Ramsey, D W
EM: dramsey@usgs.gov
AF: U.S. Geological Survey, MS-910 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: Paleomagnetic study of basalt flows on the north flank of Newberry Volcano has identified a major eruptive episode that occurred during a magnetic excursion. The measured direction of the basalt flows erupted during the excursion shallows from $81\deg$ to $76\deg$ inclination along a declination of $\sim$ $155\deg$. The Virtual Geomagnetic Pole also shallows from $29\deg$ to $19\deg$ paleolatitude, along a paleolongitude of $\sim$ $250\deg$, and is located off the west coast of Mexico. Geologic evidence combined with limited argon dating indicate that the basalt erupted from multiple sites about 80,000 years ago, probably during the time of anomalous magnetic directions recorded by ($\sim$80 ka) ocean sediments in the Norwegian Sea and the North Atlantic. The westernmost flows erupted from spatter vents located a few km south of the city of Bend, and flowed north through lava tube(s) which form Stevens Cave, Horse Cave, and Redmond Cave among others. This western lobe flowed more than 50 km to the north, over NW-trending faults of the Tumalo Fault Zone that cut the adjacent and underlying basalt of Bend (40Ar/39Ar plateau age of 78$\pm$9 ka; isochron age of 77$\pm$19 ka); it is overlain by the basaltic andesite of Klawhop Butte (40Ar/39Ar plateau age of 39$\pm$6 ka). One sample of the transitional magnetic direction basalt has a K-Ar age of 77$\pm$40 ka; another sample has a 40Ar/39Ar plateau age of 92$\pm$25 ka and an isochron age of 73$\pm$24 ka. The eastern lobe erupted from vents at and near Lava Top Butte, located approximately 15 km SE of the western vents. These eastern lavas flowed through Arnold Cave and formed a broad ~10-12 km rootless shield known as the Badlands, the NE extent of which is about 30 km from Lava Top Butte. The west and east lobes each cover about 150 km$^{2}$, and comprise an estimated volume of 3-5 km$^{3}$. Newly acquired 10-meter DEM's and compilation of the mapping in ArcGIS will allow more precise calculation of the total area covered and the volume erupted. Chemical analyses of multiple samples from both lobes, together with the paleomagnetic directional variations, suggest that most of the western and eastern lobes erupted at approximately the same time. Basalt erupted early has 49-50 wt.% SiO2, but the stratigraphically youngest eastern lavas are more silicic (50.5-51.5 wt.% SiO2). We propose to name these combined basalt flows as the basalt of the Badlands, and the transitional paleomagnetic behavior as the Badlands Excursion.
DE: 1513 Geomagnetic excursions
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting