Magnetic Stratigraphy of the Upper Miocene Montesano Formation, Olympic Peninsula of Washington: Implications for Miocene Molluscan Biostratigraphy

The Montesano Formation in the Wishkah River near Aberdeen, WA, was the original basis for the late Miocene "Wishkahan" and "Graysian" molluscan stages of Addicott (1976). Yet the precise age of these formations has long been controversial. Magnetostratigraphic sampling was conducted on the 700 m thick section of the Montesano Formation on the middle fork of the Wishkah River. A stable remanence held mainly in magnetite which passed a reversal test was isolated. The 400 m thick Wishkahan Stage was mixed in polarity; the 350 m of the type Graysian was entirely reversed in polarity. Based on the diatom biostratigraphy of Barron (1981), this polarity pattern correlates with Chron C4Ar-C5n (9.2-10.5 Ma), which is entirely late Miocene. As suspected in 1981, the Wishkan and Graysian stages are not actually successive but coeval; Wishkahan molluscs are also known from the Empire Formation, Coos Bay, OR, which dates at 6.5-8.5 Ma. This makes the Wishkahan both younger and older than the supposedly successive Graysian. This discrepancy is probably due to the fact that Graysian molluscs are a local, shallow-water biofacies, and not a true time indicator.