RECOGNIZING THE BARTONIAN STAGE IN THE GULF COASTAL PLAIN

Bartonian age rocks have been identified in the Gulf Coastal Plain for nearly half a century. As generally understood, the Bartonian includes a diverse package of lithostratigraphic units including all or parts of the Cook Mountain, Lisbon, Gosport, Moodys Branch, Yazoo, and Ocala Formations from Texas to Florida. Correlation of these sections to the global geochronologic scale is a challenge given the shallow water nature of the Gulf Coast sections and the likely location of the Lutetian-Bartonian Global Stratotype Section and Point (GSSP) in the deep water sections of the Umbrian Apennines of Italy. To date, attention has been focused on the base of magnetochron C19n as a potential criterion for the definition of the base of the Bartonian. Significant biostratigraphic events occur near the base of magnetochron C19n. The lowest occurrence of the calcareous nannofossil Reticulofenestra reticulata, the lowest occurrences of planktonic foraminifera Hantkenina alabamensis, Turborotalia pomeroli, and the highest occurrence Guembelitriodes higginsi are widely recognized biolevels which correlate closely with the base of C19n. Other biostratigraphic events of a more regional nature associated with the base of the Bartonian include the first appearance of the macroforaminiferal genus Lepidocyclina in the Gulf and Caribbean. The Bartonian-Priabonian boundary will likely be correlated with the first occurrence of the calcareous nannofossil Chiasmolithus oamaruensis. In the Gulf Coastal Plain, the presence of regional unconformities makes the planktonic microfossil highest and lowest occurrences unreliable. The most reliable correlation criterion is the lowest occurrence of the macroforaminiferal genus Lepidocyclina. Using this biohorizon, the upper Lisbon and upper Cook Mountain Formation, and all of the Ocala Formation are correlated with the Bartonian. Although less certain, the upper limit of the Bartonian in the Gulf Coastal Plain is in the middle Yazoo Clay.