USING SURFACE MAGNETIC PROFILES TO MAP COLUMBIA RIVER BASALT STRATIGRAPHY IN OREGON

Using Surface Magnetic profiles to map Columbia River Basalt stratigraphy in Oregon. Madin, I. P., Blakely, R.J., and McConnell, V.M.

Magnetic polarity of hand specimens of Columbia River Basalt flows has been used as a field mapping criteria for decades. Collecting magnetic data using a fluxgate magnetomer in the field, however, is time consuming, requires good exposures, is susceptible to scrambling by lightning, and provides only gross polarity information. We are testing the use of detailed, ground-magnetic profiles collected with total field magnetometers to quickly map CRB at or near the topographic surface. Variations in the total field measured close to the surface are strongly influenced by the polarity of the underlying basalt flows, especially when in contact with other lithologies. Total-field measurements may even be sensitive to differences in inclination, declination or strength of magnetization between flows of the same magnetostratigraphic unit. If true, it might be possible to map CRB stratigraphy in areas of poor exposure by collecting detailed surface magnetic profiles. We used a proton precession magnetometer and a cesium-vapor magnetometer, both with GPS navigation, to collect profiles up or down slopes where magnetostratigraphic boundaries had been mapped by previous workers. This allows data to be collected as fast as one can walk. In profiles from a half dozen sites in Eastern and Western Oregon, we generally found strong signals associated with normal-reversed contacts, and also found substantial signal between flows in the same magnetostratigraphic unit. Simple modeling of the magnetic signals expected in crossing polarity contacts on slopes supports the validity of the field method. Further profiling of well-characterized sections is needed to fully develop this tool.