Once the core has been extruded from the earth, it is analyzed and evaluated by a team of geologists. These professionals pay close attention to all the features within the core including; taxa, mineralogy, and lithology.

Once all the taxa have been correctly identified (and compared against other studies), biozones are created. It is from these zones where marker beds (or biohorizons) are determined. These entities will serve as the basis for correlating strata between wells. Correlations can be done within basins, or between separate basins.

Source: http://www.odp.tamu.edu/public/