Giancarlo Paganelli asked about the Claibornian Age, which corresponds to
most of the middle Eocene in North America. Most people will probably not
want to read this technical answer, so hit that delete button if you like.

The type section is the bluff on the Alabama River at Claiborne, Monroe
County, Alabama, about 65 miles (~100 km) northeast of Mobile. Claibornian
strata in Alabama include, from base to top, the Tallahatta Formation,
Lisbon Formation, and Gosport Sand. All are fossiliferous. The upper Lisbon
and Gosport are exposed at Claiborne, along with younger strata.

According to Harland et al. (1982), the Claibornian correlates to foram
zones P11 to P14, that is, the later 2/3 of the Lutetian Age and the entire
Bartonian Age in Europe.

Lutetian strata include the Calcaire Grossier (Paris Basin), Bracklesham
Beds (England), and Brussels sands (Belgium).

Bartonian strata include, unsurprisingly, the Barton beds (England).

In works written before about 1975-1980, the Bartonian is correlated with
the Priabonian (Mediterranean area) and the Lutetian equated with the middle
Eocene. However, in the 70's it was discovered that the Bartonian is much
older than previously thought, correlating to the upper part of the old
"Lutetian." This caused consternation among stratigraphers, who decided that
the best way to minimize confusion was to reassign the upper part of the old
"Lutetian" to the Bartonian and say that Lutetian + Bartonian = middle
Eocene, which is 50.5 to 42.0 million years ago. The Priabonian = upper
Eocene is unchanged.

Andrew K. Rindsberg
Geological Survey of Alabama

Reference

Harland, W. B., 1982, A geologic time scale: Cambridge, Cambridge University
Press, 131 p.