biozone

A biozone is a stratigraphic unit defined by its distinctive fossil content and used in biostratigraphy to subdivide and correlate rock sequences. It represents a span of time during which particular fossils are present in the sediment, and its recognition relies on identifying and mapping the distribution of fossils within the rock record.

Biozones are typically bounded by fossil appearance events, such as the first appearance datum (FAD) and the last appearance datum (LAD) of diagnostic taxa. The lower boundary often corresponds to the FAD of a species or group, while the upper boundary may be defined by the LAD or by the FAD of a different taxon used as a regional marker. Boundaries are chosen to maximize consistency and applicability across comparable strata.

There are several main types of biozones. Range zones are defined by the continuous occurrence of a single taxon from its FAD to its LAD. Interval zones are defined by the interval between FADs (or LADs) of diagnostic taxa, effectively marking a specific span of time. Abundance or acme zones are defined by a peak in the relative abundance of a taxon within a stratigraphic interval. Lineage zones are defined by the presence or evolutionary stage of a particular lineage.

Applications of biozones include the regional and global correlation of sedimentary sequences, refinement of relative ages, and the construction of biostratigraphic frameworks that complement radiometric dating. Limitations include diachronous boundaries that vary by region, taphonomic biases, and uneven fossil preservation or sampling.

See also: Biostratigraphy, First appearance datum, Last appearance datum, Fossil, FAD/LAD concepts.