MAGNETIC STRATIGRAPHY OF THE LOWER-MIDDLE MIOCENE ASTORIA FORMATION, NEWPORT EMBAYMENT, OREGON

The Astoria Formation, one of the world’s richest sections of Miocene fossiliferous marine rocks, crops out in seacliffs north and south of Yaquina Head at Newport, Oregon. It contains mollusks, crabs, marine mammals, and many other taxa, and in 1976 Warren Addicott made it the basis of his Newportian Molluscan Stage. The upper part of the formation north of the head is 190 meters thick, and the lower part south of the head is 130 meters. A total of 57 paleomagnetic sample sites were spaced through the two parts of the section, which we think do not overlap, and a gap of several tens of meters lies between them. The base of the upper part and the top of the lower part are both magnetically reversed. Six normal and five reversed intervals characterize the total Astoria Formation, and the top of the Astoria is normal. Most samples yield a stable remanence held in magnetite overprinted with goethite. The reversal pattern does not immediately suggest a direct correlation with the worldwide Miocene magnetic reversal stratigraphy. The Astoria is closely overlain by the Cape Foulweather (Ginkgo) Basalt, which has been dated at 15.4 ± 0.3 Ma. We suggest two possible correlations of the Astoria Formation at Newport with the worldwide paleomagnetic scale. One most compatible with the early to middle Miocene age of the molluscan fossils is from chron C5Bn2 to C5Cr (15.1 to 19.2 Ma). One most compatible with our present understanding of the radiometric age of the overlying basalt is from C5Cn1 to C6An (16.0 to 21.1 Ma).