Magnetic Stratigraphy of the Lower-Middle Miocene Astoria Formation, Newport Embayment, Oregon

G. W. Moore¹, D. R. Prothero², C. Z. Bitboul², and A. R. Niem^1
1Geosciences, Oregon State Univ, Corvallis, OR
²Geology, Occidental College, Los Angeles, CA

The Astoria Formation exposed along the northwest coast of Oregon has long been known for its middle Miocene fossils of marine mammals, mollusks, crabs, and many other taxa. It was the basis ofWarren Addicott’s (1976) middle Miocene “Newportian Molluscan Stage.” In the Newport Embayment, the Astoria Formation consists of over 700 m of shallow marine sandstones and siltstones. Three sections were sampled through the Astoria Formation and the conformably underlying Nye Mudstone. Most samples yielded a stable remanence held in magnetite overprinted with goethite, which passed a reversal test and showed no statistically significant tectonic rotation. This is consistent with other middle Miocene results reported from the region. Based on biostratigraphic constraints and the age of the overlying basalts, we correlate the Astoria Formation with magnetic Chrons C5Bn2-C5Dr (15.1–17.7 Ma), making it latest early Miocene to early middle Miocene in age. This is slightly older than the age of the ?Astoria Formation in theWishkah River drainage of the southern Olympic Peninsula, Washington, which yields diatoms of an age of 14–15 Ma. It is slightly younger than the lower part of the type Astoria Formation near Astoria, which yields early Miocene (Pillarian) mollusks. However, the upper part of the type Astoria is probably coeval, because it yields Newportian mollusks.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California