substages

Substages are subdivisions within a geologic stage in stratigraphy, used to describe shorter intervals of geological time contained within a single stage. In chronostratigraphy, a stage represents a broad interval of rock and time; substages allow finer subdivision when the available stratigraphic record or research questions require greater resolution. Substages are more common in regional or local chronostratigraphic schemes than in the globally formal framework, and their use varies by region and discipline.

Boundaries between substages are typically defined by recognizable changes in the rock record, such as shifts in fossil assemblages, magnetostratigraphic signals, isotopic events, or other stratigraphic markers. When a globally recognised reference section exists, a Global Boundary Stratotype Section and Point (GSSP) may be used to anchor a substage boundary; otherwise, substages may be defined by regional criteria and correlations.

Nomenclature for substages can be ordinal (Substage 1, Substage 2) or regionally named, often reflecting the geographic area or a characteristic fossil assemblage. Importantly, substages are nested within a stage; the compilation of substages should sum to the full duration of the stage.

Substages facilitate more precise correlation of rock records and environmental reconstructions, but they can also introduce complexity and potential inconsistencies across regions. They are one tool among chronology, biozones, and other stratigraphic units used to interpret Earth’s history.