The proposed Eocene-Oligocene stratotype, SW Alabama; not ideal due to magnetostratigraphic inconsistencies

A well-known stratigraphic section and a proposed IGCP stratotype for the Eocene-Oligocene boundary is exposed at St. Stephens Quarry, southwestern Alabama. On the evidence of planktonic foraminifera and nannofossils, the boundary is generally placed at or near the top of the Yazoo Clay (Shubuta Member). In the magnetic polarity time scale, the Eocene-Oligocene boundary is within a reversed polarity interval (Chron 13R) with normal polarity intervals 500,000-750,000 years below and above the boundary. This sequence, however, is not observed at St. Stephens Quarry, where paleomagnetic samples for the entire uppermost Eocene-lowermost Oligocene record normal polarity or are magnetically unstable. The samples contain glauconite and abundant authigenic pyrite, and in places both minerals show considerable late-stage alteration to hematite. The authigenic hematite is present throughout the section and dominates the magnetization, obscuring primary magnetization polarity boundaries and preventing correlation with the standard polarity time scale. Since magnetic polarity data may provide globally synchronous correlations, we recommend that the magnetic characteristics be evaluated in the recognition of stratotype sections. Therefore, because of remagnetization, the St. Stephens Quarry section is not an ideal stratotype for the Eocene-Oligocene boundary.