MAGNETIC STRATIGRAPHY OF THE UPPER MIOCENE (TYPE WISHKAHAN-GRAYSIAN) MONTESANO FORMATION, WASHINGTON, AND EMPIRE FORMATION, OREGON: IMPLICATIONS FOR MIOCENE MOLLUSCAN BIOSTRATIGRAPHY

The faunas of the Montesano Formation in the southern Olympic Peninsula were the basis for the late Miocene Wishkahan and Graysian molluscan stages of Addicott (1976). The faunas of the Empire Formation, Coos Bay, southwestern Oregon, were also referred to the Wishkahan. Paleomagnetic sampling of both of these formations yielded a stable remanence held mainly in magnetite which passed a reversal test. Neither formation shows significant tectonic rotation, consistent with other results from this region. Based on the diatom biostratigraphy, the type Wishkahan and Graysian Montesano section on the Wishkah River correlates with Chrons C4Ar to mid-Chron C5n (9.1-10.5 Ma). This is considerably older than the 6.5-8.5 Ma age estimate (Chrons C3Br-C4r, based on diatoms) for the referred Wishkahan rocks of the Empire Formation. Such results suggest that the Graysian Stage is probably a local, shallow-water facies partially coeval with the Wishkahan, rather than a discrete increment of time following the Wishkahan, as originally proposed. In addition, these data show that the stratotypes of these late Miocene molluscan stages span only a small portion (1.5 m.y. out of 6 m.y.) of the time they were originally thought to represent.