Palynomorph Zonation for the Eocene-Oligocene Boundary in the Southeastern U.S.: An Alternate Stratigraphic Framework Applicable to Offshore Gulf of Mexico Studies

Eocene/Oligocene sequences in Alabama are stratigraphically complex. Deposits in this region are composed of clastic sediments to the northwest intergrading with carbonates to the southeast. The St. Stephens Quarry (SSQ) in Alabama is one of the few, easily accessible quarries along the Gulf Coast where the Eocene-Oligocene boundary is visible in outcrop. Despite the abundance of research projects conducted in the SSQ, many controversies arise when attempting to place the Eocene-Oligocene boundary, and more specifically, tying Gulf coast sequences with the Global stratotype in Italy. Indeed, as established by the International Geological Congress in 1992, the extinction of the Hantkeninidae family defines the Eocene-Oligocene boundary in the Massignano section in Italy. The lack of Hantkenina sp.
(Miller et. al., 2008) in core from near the SSQ and the complex stratigraphy there contribute to the controversy in boundary placement.

To circumvent the rarity of foraminifera biomarkers in the section, we are working to develop a high-resolution organic-walled microfossil zonation for the boundary interval. Here we present the results of this new palynological evaluation and integrate it with new magnetostratigraphic susceptibility (MSS) data. The goals of this project are three fold: 1) to build an alternate biostratigraphic zonation based on organic-walled microfossils that ties with new but limited foraminifera data to increase biostratigraphic control in the Gulf Coast region, 2) locate the Eocene-Oligocene boundary to better control correlation to the GSSP in Italy with the new MSS data, and 3) supplement and assist biosteering when foraminifera are not present.

Of the 38 boundary samples analyzed for palynomorphs, a strong palynological zonation has been developed for the area using acmes and first (FAD) and last (LAD) appearance datums. Critical LADs (Operculodinium divergens and Corrudinium incompostium) and one FAD (Nematosphaeropsis lemniscata) have been identified. Other Eocene-Oligocene related events include an acme of Homotryblium floripes and the presence of Deflandrea phosphoritica, all providing strong organic-walled biozones for biosteering.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California