Chronostratigraphic

Chronostratigraphy is a branch of stratigraphy that studies the relationship between rock layers and time, providing a framework to organize the geological record into time-bound units. It defines chronostratigraphic units that represent the rocks deposited during a specific interval of geologic time and links them to the corresponding geochronologic units, thereby tying the rock record to absolute ages.

The main chronostratigraphic units, from largest to smallest, are eonothem, erathem, system, series, and stage. Each unit is bounded by boundaries defined at a Global Boundary Stratotype Section and Point (GSSP), which marks the base of the unit in a globally recognized section. The base ages and correlations to time are given by the associated geochronologic unit (for example, the Cambrian System corresponds to the Cambrian Period). The Global Chronostratigraphic Chart, maintained by the International Commission on Stratigraphy (ICS), standardizes unit names, boundary ages, and cross-regional correlations.

Chronostratigraphy often integrates with other methods such as biostratigraphy, magnetostratigraphy, and radiometric dating. Fossil assemblages help define biozones that support correlations between sections, while radiometric dating and astrochronology constrain absolute ages.

Differences from related disciplines are noted: lithostratigraphy groups rocks by lithology, biostratigraphy by fossil content, while chronostratigraphy groups rocks by their time interval. The chronostratigraphic approach is essential for precise time-scale calibration, global correlation, and interpretation of Earth's history, including paleoenvironments, climate changes, and tectonic events. The adjective chronostratigraphic describes aspects related to this time-based classification of rocks.