Revised Chronostratigraphy and Biostratigraphy of the John Day Formation (Turtle Cove and Kimberly Members), Oregon, with Implications for Updated Calibration of the Arikareean North American Land Mammal Age

Abstract

Although the Arikareean North American land mammal age was first typified in the Great Plains, the succession there contains significant unconformities, a generally poor magnetic record, relatively sparse radioisotopic calibration, and a major faunal hiatus. In the John Day Valley of central Oregon, however, is a thick, remarkably complete sequence of Oligocene through early Miocene strata (the John Day Formation) potentially amenable to addressing these shortcomings and long known to harbor one of the richest records of mid‐Tertiary mammals in North America. Since Prothero and Rensberger’s first magnetostratigraphic study of the John Day Formation in 1985, new advances in geochronology, together with a more comprehensive suite of paleomagnetic sections keyed to new radioisotopic and biostratigraphic data, have greatly enhanced chronostratigraphic precision. In our attempt to refine John Day chronostratigraphy, we sampled nearly 300 sites for magnetostratigraphy over a 500‐m‐thick interval and used several radioisotopically dated volcanic tuffs for our correlation with the geomagnetic polarity timescale. Many of the rocks analyzed showed unusual magnetic behavior, possibly due to the known zeolitization in this region, thereby precluding an abundance of class 1 polarity determinations. Nevertheless, preliminary results indicate that the Turtle Cove Member stratigraphically upward through the lower Kimberly Member extends from late chron C12n through C7n.1r, or from about 30.6 to 24.1 Ma. Intensive radioisotopic and magnetostratigraphic characterization of these strata provides a framework by which the associated biostratigraphy is assessed for biochronological significance relative to fossiliferous successions of the Great Plains, in turn resulting in reassessment of Arikareean subbiochron (Ar1–Ar4) boundaries. We present a revision of those boundaries that differs from their traditional timing as a hypothesis for testing in other locations.