THE EOCENE-OLIGOCENE BOUNDARY INTERVAL IN THE SE USA (2): A HIGH-RESOLUTION MAGNETIC SUSCEPTIBILITY (MS), GEOCHEMICAL AND GAMMA RAY (GR) SPECTROSCOPY STUDY TO DEFINE AND CORRELATE THE BOUNDARY INTERVAL AMONG OTHER CO-EVAL SEQUENCES

To further define the Eocene-Oligocene boundary exposed in the classic Pelham North portion of the St. Stephens Quarry in southwestern Alabama, we collected unweathered samples at ~5 cm intervals through each of three cleaned exposures. These samples were used for biostratigraphic, elemental and isotope geochemistry, and magnetic susceptibility (MS) measurements. Carbon and oxygen isotopic analyses were performed on 10 Cibicidoides spp. specimens picked from each sediment sample analyzed. In addition, gamma Ray (GR) measurements were made at ~5 cm intervals directly on the outcrop. The resulting composite section in the Pelham North Quarry ranges from ~2 m below an unconformity within the Pachuta Marl, and extends to ~1 m above the unconformable top of the Bumpnose Limestone, in a unit we refer to as the Red Bluff Clay. We correlate these results with strata exposed at the Brooks Quarry near Marianna, Florida, where the Bumpnose Limestone was formally defined. In the Brooks Quarry we collected samples for biostratigraphic, geochemical and MS data as well as GR measurements, again at a ~5 cm interval, from 2 meter below an unconformity in the Eocene Ocala Limestone, to a second unconformity at the top of the Bumpnose Limestone, overlain by the Marianna Limestone at this locality.

We are able to correlate between the two locations using MS and lithostratigraphic methods, and independently from the biostratigraphic information, pinpoint the E-O boundary. In addition, using MS data, we tie the southeastern USA data sets to the Global Boundary Stratotype Section and Point (GSSP) in Italy. Finally, using Fourier analysis of the MS data sets, we argue for climatically induced cyclicity in continental erosion and runoff that drives MS cyclicity in the USA sections.