Biostratigraphy

Biostratigraphy is the subdivision and correlation of rock units based on their fossil content. It is a core component of stratigraphy that uses the orderly appearance, persistence, and disappearance of particular fossil groups to establish relative ages and to link rocks from different locations.

The approach hinges on the principle of faunal succession, the observation that fossil assemblages change through time in a reproducible sequence. Index fossils, typically widespread yet short-lived species, are especially valuable because their first appearance and last appearance datums (FADs and LADs) define boundaries for biostratigraphic zones. These zones, or biozones, can be based on ranges of a single taxon, assemblages of taxa, or intervals defined by particular fossil occurrences, and they provide a practical framework for correlating rock sequences.

Biostratigraphic analysis involves identifying and dating fossils within rock samples, constructing zonal schemes, and integrating these with other stratigraphic information. While biostratigraphy excels in constraining relative ages and correlating distant outcrops, it does not directly provide absolute numerical ages; radiometric dating and magnetostratigraphy often calibrate biostratigraphic results.

Applications range from petroleum exploration and petroleum geology to paleogeography and environmental reconstruction. Limitations include diachronous appearances of taxa in different regions, reworking of fossils, uneven preservation, and gaps in the fossil record. Despite these challenges, biostratigraphy remains a fundamental tool for interpreting Earth's history, enabling detailed timelines of sedimentary successions and the synchronization of geological events across space.