A PALEOMAGNETIC RECONNAISSANCE STUDY OF THE POWDER RIVER VOLCANIC FIELD, UNION COUNTY, OREGON

The Powder River Volcanic Field is a series of middle Miocene extension-related lava flows exposed along the margins of the Grande Ronde Valley in northeastern Oregon. Oriented samples for a paleomagnetic reconnaissance study were collected from nine units at three sites in the PRVF. Samples were subjected to progressive alternating field demagnetization to determine primary paleomagnetic directions and to study the general magnetic behavior of the PRVF. Five of the units were lava flows; one was a volcaniclastic deposit: and three represented different horizons below a baked contact. Three of the lava flows and one of the units beneath the baked contact demagnetized smoothly and showed uni-vectorial decay toward the origin. Two of these units have normal inclinations (~65°), but southern declinations ranging from -105° to -140°. The other two units have reversed inclinations (~-70°) and southern declinations ranging from 140° to 200°. The median destructive fields (MDFs) ranged from 20mT to 30mT. The other lava units did not demagnetize fully, but also have normal inclinations and southern declinations. The volcaniclastic unit and other units beneath the baked contact did not demagnetize fully, but have reversed inclinations and southern declinations. Magnetic directions from the unit immediately below the baked contact indicate the unit had been heated above the Curie temperature by the overlying flow. The paleomagnetic results appear to be reliable but the normal inclinations/southern declinations are anomalous. One possibility is that these units were deposited during a geomagnetic reversal or excursion. Another possibility is that we sampled a small tectonic block that has been significantly rotated. These results indicate the PRVF is suitable for further paleomagnetic study and that additional work is needed to resolve the origin of the anomalous directions.