MAGNETIC STRATIGRAPHY AND TECTONIC ROTATION OF THE MIDDLE EOCENE COALEDO FORMATION, SOUTHWESTERN OREGON

The Coaledo Formation in Coos Bay on the southern coast of Oregon consists of over 2000 m of deltaic sediments that filled a forearc basin during the middle Eocene. This unit yields molluscs of the "Cowlitz-Coaledo fauna," one of the oldest and best-studied faunas of the Pacific Northwest, but its precise age has been controversial. Over 100 magnetic samples spanning the formation were analyzed using both AF and thermal demagnetization. They yielded a stable remanence held mainly in magnetite which passed a reversal test and showed a tectonic rotation of 105 +/- 5 degrees. This is greater than any rotation previously reported from the region. It is consistent with the previously observed trends that the tectonic rotations of the Coast Ranges of Oregon and Washington increase with the increasing age of rock unit, and increase toward the southwest. The magnetic polarity pattern of the Coaledo Formation best correlates with Chrons C18r-C20r (40.0-44.0 Ma), which is significantly older than the magnetic pattern in the Cowlitz Formation (36.5-38.3 Ma). This suggests that the "Cowlitz-Coaledo fauna" is very long-lived (at least 36.5-44.0 Ma), spanning most of the middle Eocene.