Biozones

Biozones are stratigraphic units defined on the basis of fossil content rather than rock type. In biostratigraphy, a biozone is the interval of strata characterized by a particular fossil assemblage, a single taxon, or the co-occurrence of several taxa. The concept relies on the idea that fossils record the temporal evolution of life and can be used to correlate rock layers across space and time. Biozones are independent of lithology and can cut across lithostratigraphic units, enabling cross-regional correlations.

Common types include taxon-range zones, defined by the first and last appearances (FAD and LAD) of a species; assemblage zones, defined by a distinctive co-occurrence of multiple taxa; and concurrent-range or interval zones, which use overlapping ranges of two or more taxa to delimit a period. Some practitioners utilize abundance- or rarity-based zones as well. Determining zones requires careful sampling, accurate taxonomy, and robust zonal markers such as index fossils.

Applications include dating and correlating sedimentary sequences, constructing regional and global stratigraphic frameworks, and guiding hydrocarbon exploration by identifying intervals with favorable faunal assemblages. Biozones also support paleoenvironmental reconstructions by tracing faunal turnover through time.

Limitations stem from diachronous first appearances across regions, reworking and taphonomic biases, and gaps in the fossil record. Taxonomic revisions can redefine zones, and biostratigraphic boundaries often require corroboration from other methods such as lithostratigraphy, magnetostratigraphy, or chemostratigraphy.

Examples: ammonite, conodont, and foraminifer zones are among the most developed, especially in Mesozoic and Paleozoic marine rocks. Brachiopod and nannofossil zones are used in older and Cenozoic sequences. The term index fossil refers to a species that serves as a diagnostic marker for a biozone.