230 Recent Advances in Stratigraphy Geology I

Wednesday, 3 November 2010
8:00 AM - 12:00 PM
Room 303

THE EOCENE-OLIGOCENE BOUNDARY INTERVAL IN THE SE USA (1): A HIGH-RESOLUTION BIOSTRATIGRAPHIC STUDY TO DEFINE AND CORRELATE THE BOUNDARY INTERVAL AMONG OTHER CO-EVAL SEQUENCES

Paper number: 230-1
Program: Technical Sessions
Time: 8:00 AM – 8:15 AM

Authors

Abstract

To better 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, geochemical, and magnetic susceptibility (MS) measurements. The resulting composite section 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 and MS data, again at a ~5 cm interval, from 1 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 were able to identify the E-O boundary in the Pelham North Quarry, based on the last occurrence of the planktonic foraminiferal family Hantkeninidae. We identify the position of the E-O boundary to lie in the Shubuta Marl, ~0.3 m above a gradational transition with the underlying Pachuta Marl. In the Brooks Quarry, we establish that the Astrocyclina Zone extends above the basal Bumpnose unconformity and, furthermore, based on the LAD of rare specimens of Turborotalia cerroazulensis, we place the E-O boundary ~0.5 m above the base of the Bumpnose Limestone in the quarry. This placement means that the basal Shubuta Marl in the Pelham North Quarry, Alabama, is essentially co-eval with the basal Bumpnose Limestone in the Brooks Quarry, Florida.