DEFINING THE LUTETIAN-BARTONIAN STANDARD EOCENE STAGE BOUNDARY AND IDENTIFYING A GLOBAL STRATOTYPE SECTION AND POINT (GSSP)

The Bartonian Stage is one of the standard stages of the Eocene. It has a complex history of usage dating from the mid-19th century. Although the stage was first applied to rocks in the Paris Basin, its name was derived from the Barton Clay in the Hampshire Basin of England. The Barton Clay is best known today from exposures at Alum Bay and Whitecliff Bay on the Isle of Wight and these exposures serve as a “unit stratotype” for the Bartonian. The Lutetian-Bartonian Boundary Working Group of the International Subcommission on Paleogene Stratigraphy (ISPS) has been focused on defining the base of the Bartonian and on identifying a Global Stratotype Section and Point (GSSP). The GSSP of the Lutetian-Bartonian boundary will be defined by a specific point in the rock within a designated section. While the boundary will always be recognizable in the designated stratotype section, its recognition away from the GSSP will depend on long distance correlation. This will mean that the definition will be tied to some globally recognized stratigraphic event or group of events. Efforts to define the Lutetian-Bartonian boundary to date have been hampered by the absence of globally recognized biostratigraphic datums and the lack of clear global paleoclimatic and paleoceanographic events. 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 is a widely recognized biolevel which correlates closely with the base of C19n. Among planktonic foraminifera, the lowest occurrences of Hantkenina alabamensis, Turborotalia pomeroli, and Guembelitriodes higginsi have generally been associated with the beginning of the Bartonian but detailed correlation with the magnetochronology remains to be completed. 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 region and the base of shallow benthic zone SBZ 17 in the Tethyan region. Current field efforts are focused on sections in the Umbrian Apennines of Italy.