PALEOMAGNETIC STUDY OF THE GRAYS RIVER VOLCANICS, SOUTHWESTERN WASHINGTON AND NORTHWESTERN OREGON

Southwestern Washington and Northwestern Oregon are home to the Middle Eocene Gray River Volcanics (GRV), a widespread rock unit containing mafic volcanic flows and minor pyroclastic rocks interlayered with clastic sediments. This study uses paleomagnetism to examine the structural/tectonic history of these lava flows. Nine sites, each consisting of at least 10 samples from a single lava flow from the GRV, were sampled in three areas. Six sites along Germany Creek north of Stella, WA, and two sites east of Clatskanie, OR were sampled in July 2009. One site from a quarry on Fossil Creek Road, about 2 miles east of Grays River,WA, was sampled in October, 2008. Alternating field and thermal demagnetization studies were performed on samples from all sites. Germany Creek samples yield inconsistent magnetic directions within flows and from flow to flow. Germany Creek flows are structurally deformed, exhibiting moderately steep dips. Samples from gently-dipping flows near Clatskanie and Fossil Creek exhibit well-behaved demagnetizations and demonstrate consistent directions. Preliminary directions from these three sites indicate significant clockwise rotation of the GRV of about 20 degrees since the Middle Eocene. Analyses of samples from other GRV flows near Clatskanie and Bebe Mountain are underway to confirm or refute this result. In addition, polished sections of samples from all locations are being prepared for optical examination to determine if the magnetic minerals present are primary or secondary. This may help explain the scatter in directions from Germany Creek samples.