MAGNETIC STRATIGRAPHY OF THE MIDDLE EOCENE COWLITZ FORMATION, WASHINGTON AND OREGON

The Cowlitz Formation consists of about 800 m of deltaic sediments which filled a forearc basin in southwestern Washington and northwestern Oregon. Its abundant molluscan fossils were the basis for the "Cowlitz-Coaledo fauna," one of the oldest and best studied molluscan assemblages in the Pacific Northwest. Paleomagnetic sections were taken at the type section at Olequa Creek, at the Big Bend of the Cowlitz River, at Coal Creek and Germany Creek, and several places in the Cowlitz/Hamlet sequences in northwestern Oregon. Using AF and thermal demagnetization, the paleomagnetic samples yielded a stable remanence held mainly in magnetite which passed a reversal test. The lower Cowlitz Formation (members 2 and 3) was mostly reversed in polarity, while the upper Cowlitz (members 4 and 5) was of normal polarity. Based on an argon/argon date of 38.9 +/- 0.01 Ma on the base of the Olequa Creek section, the formation correlates with Chron C17n-C18n (36.7-38.7 Ma). This is considerably younger than the Coaledo Formation of southwestern Oregon (40-44 Ma), even though they both produce the classic "Cowlitz-Coaledo fauna." The Cowlitz Formation shows a tectonic rotation of 97 +/- 11 degrees, consistent with similar-aged rocks in the region.