A biostratigraphic study of the “Austin group” in Williamson County, Texas, shows that this group can be subdivided into six faunal zones. These are, in ascending order, the Inoceramus subquadratus, zone Inoceramus undulatoplicatus zone, Texanites internodosus zone, Gryphaea aucella zone, Exogyra laeviuscula zone, and Ostrea centerensis zone. The lower five zones are in the Austin chalk, the upper one is in the Burditt marl.
Correlation with standard Cretaceous Zones is very tentative, but the Burditt marl corresponds closely with the upper Santonian, and the Austin chalk is lower Santonian and Coniacian.
Named after characteristic species, the zones are based on faunal suites. Some fossils in each suite are abundant enough for detailed mapping.