RECOGNIZING THE BARTONIAN STAGE IN THE GULF COASTAL PLAIN

The Bartonian Stage does not yet have an approved GSSP. There are two proposed primary correlation events for the base of the Bartonian Stage: the base or the top of magnetic polarity chronozone C19n. Recent stratigraphic work in the type area of the Bartonian in the Hampshire Basin of southern England has demonstrated the validity of both proposed guide events. The challenge now becomes to demonstrate correlation into other regions.

The Bartonian Stage in the Gulf Coastal Plain consists of a series of unconformity bound sequences. These sequences are recognized in outcrop and in the subsurface from Florida to Texas within the upper Claiborne and lower Jackson Groups. The units are richly fossiliferous with abundant and diverse invertebrates and microfossils. Lutetian and Bartonian units can be separated by paleontology but the actual base of the Bartonian in the region is a sequence boundary. The transition from the Lutetian to the Bartonian occurs locally within the Avon Park, Lisbon, Cook Mountain, and Crockett Formations. Only limited magnetostratigraphic data has been collected from this interval making recognition of either proposed primary guide event in the Gulf Coastal Plain problematic.

Correlation of Gulf Coast Bartonian sections to the eventual GSSP will depend on secondary guide events. Planktonic foraminifera across this interval in the Gulf Coastal Plain have only limited use for global correlation. The most reliable correlation event for the base of the Bartonian in the region is the lowest stratigraphic occurrence of the calcareous nannofossil Reticulofenestra reticulata. This horizon is recognizable from Alabama to Texas. Closely associated with this event in the Lisbon, Cook Mountain, and Crockett Formations is the highest stratigraphic occurrence of the smaller benthic foraminiferan Ceratobulimina exima. This horizon has long been used for subsurface correlation in the Gulf Coastal Plain and can be used effectively with the lowest occurrence of R. reticulata to identify the Lutetian-Bartonian boundary in the region.

The Gulf Coastal Plain represents a setting where correlation with deep marine magnetostratigraphic and biostratigraphic events is challenging. The use of secondary and local biostratigraphic horizons can be used to establish correlation with the eventual Bartonian GSSP.