CENOZOIC EVOLUTION OF THE CASCADIA FOREARC: CONSTRAINTS FROM PALEOMAGNETISM AND ANISOTROPY OF MAGNETIC SUSCEPTIBILITY OF SEDIMENTARY ROCKS EXPOSED NEAR CAPE ARAGO, CENTRAL OREGON COAST

Oriented samples for paleomagnetic analysis were collected from Middle Eocene to Late Miocene Fivemile Point Sandstone, Tyee, Beds of Sacchi Beach, Coaledo, Bastendorf Shale, Tunnel Point Sandstone, Tarheel, and Empire formations as part of a broader project to understand the Cenozoic evolution of the Cascadia forearc and test rotation models in coastal Oregon. To date, 161 drilled field sites have yielded 469 cores, 65 cores have been drilled from blocks at 53 block sites and 708 1-inch samples from 2-7 inch long cores, from both field and block sites, have been analyzed, focused on the Coaledo Formation.

Preserved remanence is highly variable but typically comprises a large, low-coercivity overprint that masks a smaller component more resistant to high AF and stepwise thermal demagnetization greater than ~350°C, with directions similar to the few samples with simple linear high-coercivity remanence. We are investigating grain size (claystone to medium sandstone), clastic grain composition, degree and type of cementation, presence of authigenic sulfides, presence of primary sedimentary structures, depth of core analyzed (up to 7 inches below outcrop surface), location of samples (supratidal cliff face vs. sub-tidal platforms), field cores vs. block cores, and orientation of cores relative to bedding to determine sources of sample quality variability to guide additional sampling and analysis. Fold, rip-up and rotated block tests are being applied to the data.

Restoring the bedding to paleohorizontal, the high-coercivity component in the Coaledo points in two, nearly antipodal, directions that are rotated clockwise 70+/-25° with respect to the expected Eocene field direction. We have also determined the mean axis of maximum magnetic susceptibility (a paleocurrent proxy) at 62 sites in the Coaledo Formation. The overall mean axis trends nearly north-south (170/350 +/- 9°), consistent with field indicators of paleocurrent orientation from five previous studies. If the ancient continental margin was oriented roughly north-south with transport trending offshore, the magnetic susceptibility data corroborate paleomagnetic evidence suggesting moderate (45° to 90° clockwise) tectonic rotation of the study area since the Eocene.