MAGNETIC STRATIGRAPHY OF THE MIDDLE MIOCENE (EARLY BARSTOVIAN) MASCALL FORMATION, CENTRAL OREGON

The Mascall Formation in the John Day region of central Oregon consists of about 200 m of fluvial-lacustrine siltstones and sandstones, lying below an angular unconformity with the Rattlesnake Formation. It has been famous for over a century for its early Barstovian fossil mammals. We collected paleomagnetic samples covering the entire fossiliferous portion of the type Mascall Formation in the Rattlesnake Creek area. Samples were demagnetized with both alternating field and thermal demagnetization. Most samples showed a single component of remanence held in a mixture of magnetite and hematite. The entire lower half of the Mascall Formation is reversed in polarity, but the Mascall tuff bed is normal in polarity, and the the upper Mascall has three short zones of reversed-normal-reversed polarity. Based on the Ar/Ar date of 15.8 Ma on a tuff just below the Mascall tuff bed, we correlate the lower reversed magnetozone with Chron C5B4 (15.1-16.0 Ma). The normally magnetized Mascall tuff bed correlates with Chron C5Bn2n, and the upper R-N-R sequence with Chrons C5ADr to C5Bn1r. Thus, the total sampled section spans 14.7-16.0 Ma. This is also consistent with a K-Ar date of 16.6 +/- 1.4 Ma on the basal Mascall tuff, and the date of 16.0 Ma on the interbedded Columbia River basalts.