THE PALEOCENE-EOCENE TRANSITION IN THE EASTERN GULF COASTAL PLAIN: PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY
In the eastern Gulf Coastal Plain of the United States, the Paleocene-Eocene boundary occurs within the Tuscahoma and Hatchetigbee Formations. This section is 160 m thick and includes several glauconitic shell beds (marls) containing planktonic and benthic foraminifera. For the past 30 years, the Paleocene-Eocene boundary in the Gulf Coast region has been placed above the highest occurrence of the planktonic foraminiferan Morozovella velascoensis in the Bells Landing Marl Member of the Tuscahoma Formation at a level corresponding to the the Tuscahoma-Hatchetigbee lithostratigraphic boundary. Identifying the boundary at this stratigraphic level is problematic as the highest occurrence of M. velascoensis in the Dabibaya section occurs above the Carbon Isotope Excursion in the lower Eocene.
The presence of Pseudohastergerina wilcoxensis in the Bear Creek Marl bed about 3 m above the base of the Tuscahoma suggests the entire Tuscahoma Formation is Eocene in age. The Bear Creek marl bed, the Greggs Landing Marl Member and the Bells Landing Marl Member of the Tuscahoma are assigned to Biozone E2 while the Bashi Marl Member and the upper marls of the Hatchetigbee are assigned to biozone E3. Evidence of re-working in the foramininferal fauna of the early Eocene Bear Creek marl and the absence of Paleocene planktonic foraminiferal biozone P5 in the eastern Gulf Coastal Plain indicates an unconformable Paleocene-Eocene boundary in the region.