chronostratigraphy

Chronostratigraphy is the branch of stratigraphy that studies the relationship between rock layers and the time during which they were deposited. It defines units of rock by their time of origin and by boundary criteria that separate consecutive time intervals. The principal chronostratigraphic units, from largest to smallest, are eonothem, erathem, system, series, and stage. The corresponding geochronologic units are eon, era, period, epoch, and age.

Chronostratigraphy relies on correlation of rocks through time using fossils (biostratigraphy), magnetism (magnetostratigraphy), variations in chemical or isotopic composition (chemostratigraphy), and sedimentary architecture (sequence stratigraphy). Boundaries often coincide with recognizable marker horizons, such as mass extinctions, major faunal turnover, or isotope excursions. When possible, the base of a chronostratigraphic unit is formally defined at a Global Boundary Stratotype Section and Point (GSSP) to anchor the boundary to a specific section and time.

Calibrating chronostratigraphic boundaries with radiometric dating provides numerical ages, linking the chronostratigraphic time scale with the geochronologic scale. However, not all boundaries have precise numeric ages; many are diachronous in different regions. Chronostratigraphy complements lithostratigraphy and is essential for constructing geological time scales, correlating strata worldwide, and interpreting paleogeography, tectonics, and sedimentary processes.