Magnetic stratigraphy of the lower portion of the middle Miocene Mascall Formation, central Oregon

Prothero, Donald; Draus, Elizabeth; Foss, Scott

The Mascall Formation in central Oregon consists of about 350 meters of volcaniclastic floodplain siltstones and sandstones exposed in numerous fault blocks in the John Day region of central Oregon. It yields a famous early Barstovian mammalian fauna (part of the Wood Committee's 1941 original concept of the Barstovian) that includes at least 33 species of mammals, as well as birds, turtles, fish, and freshwater gastropods. The most complete section in the type area was sampled using oriented block sampling. The samples were then subjected to both alternating field demagnetization at 25, 50, and 100 Gauss, followed by thermal demagnetization at 50°C steps from 200 to 630°C. Most samples yielded a stable single component of remanence that passed a reversal test, and was held largely in magnetite with minor goethite overprints. The lower half of the section is of reversed polarity, followed by shorter magnetozones of normal, reversed, and normal polarity to the top of the section. Based on dates of 16.2 ± 1.4 Ma at the base of the section, and 15.77 ± 0.07 Ma in the lower part of the section, we correlate the Mascall Formation with Chron C5Br to Chron C5Bn1n (14.8–16.0 Ma). This correlation is consistent with the early Barstovian age of the fauna, and it matches the pattern seen in other Barstovian magnetostratigraphic sections, such as those at Barstow, California; Virgin Valley and Massacre Lake, Nevada; and Pawnee Creek, Colorado.

Magnetic stratigraphy of the lower portion of the middle Miocene Mascall Formation, central Oregon

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