MAGNETOSTRATIGRAPHY AND MAGNETIC FABRICS OF THE MIOCENE COLUMBIA RIVER BASALTS IN SOUTHEASTERN WASHINGTON

The Columbia River Basalt Group (CRBG) covers eastern Oregon, western Idaho, Washington, and northern Nevada. The main phase of eruptions was fed by a series of north-northwest trending linear fissures, known as the Chief Joseph Dike System, which migrated from near the Idaho-Oregon border to the north. The geochronology, stratigraphy, and geochemistry of these flows has been extensively researched. However, the correlation between flow members and source fissures is poorly constrained. In addition, there are very few studies that have utilized magnetic fabrics or anisotropy of magnetic susceptibility (AMS) to study the CRBG. AMS has been shown to reflect the dimensional orientation of magnetic minerals with magnetic foliation paralleling the flow plane and magnetic lineation paralleling lava flow directions (Lerner et al., 2022), so AMS studies may be potential tools to aid in flow correlation and improve understanding of flow dynamics. For this study fieldwork was concentrated in the United States Umatilla National Forest (southeast Washington) during the summer and fall of 2023. Flow measurements, oriented blocks, and oriented cores were collected along Patrick Grade Road where a stratigraphic stack of CRBG flows is present. This study conducts a detailed study of AMS on all discrete samples to estimate the direction of each lava flow. Paleomagnetic methods will be utilized to constrain the magnetostratigraphy of the sampled section and general rock-magnetic methods (magnetic susceptibility, magnetic hysteresis, and Curie temperature experiments) will be utilized to access the magnetic mineralogy and magnetic mineral concentration for each site. To summarize, the AMS results, when combined with the paleomagnetic directions, should yield some detailed information about lava flow directions which will aid in determining the emplacement direction of the lava flows.