Biostratigraphy

Biostratigraphy dates and correlates rock layers using their fossil content and the succession of fossil assemblages.

Biozone: A body of rock defined by its fossil content or by the range of one or more taxa. Biozones are the basic units used to organize fossil successions.
Fossil: Preserved remains, traces, or evidence of past organisms. Fossils provide the biological evidence on which biostratigraphic divisions are based.
Chronostratigraphy: The branch of stratigraphy that organizes rock bodies by the time intervals in which they formed. Biostratigraphic correlations help establish the relative timing of rock units.
Radiometric dating: A method for determining ages by measuring radioactive isotopes and their decay products. It can provide numerical ages, whereas biostratigraphy primarily establishes relative age and correlation.
Index fossil: A fossil of a widespread, abundant, and relatively short-lived species useful for dating and correlating rocks. Its limited time range and broad distribution make correlation more precise.
Stratigraphy: The study of rock layers, their relationships, and their history. Biostratigraphy is one method within the broader study of strata.
Sequence stratigraphy: The study of sedimentary strata organized into packages bounded by surfaces of erosion or nondeposition. Fossil ages help constrain the timing of sedimentary sequences and their boundaries.
Magnetostratigraphy: A stratigraphic method that uses recorded changes in Earth's magnetic field to correlate rocks. It correlates magnetic polarity patterns rather than fossil succession.
First appearance datum: The lowest stratigraphic occurrence of a taxon in a succession of rocks. Its position can mark a biozone boundary or a point of correlation.
Law of superposition: In an undisturbed sequence of sedimentary rocks, lower layers are older than layers above them. Superposition orders layers before their fossil contents are compared.

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