Stratotype

Stratotype, in geology, is a formally designated representative stratigraphic section, or type section, that defines the characteristics of a stratigraphic unit such as a formation, member, or series. It provides the standard reference against which other sections are compared and is typically paired with a type locality—the geographic location where the section is best exposed and described.

Stratotypes aim to capture the essential features of the unit, including typical lithology, fossil content, thickness, and facies variation. They support correlation by providing a concrete, measurable sequence that can be traced elsewhere and related to other data such as biostratigraphy, chemostratigraphy, and, where available, radiometric ages.

Different kinds of stratotypes reflect different basis for unit definition. A lithostratigraphic stratotype characterizes a unit defined by rock type and stratigraphic position; a biostratigraphic stratotype emphasizes distinctive fossil assemblages. For higher-level chronostratigraphic boundaries, formal global reference points known as Global Boundary Stratotype Section and Point (GSSP) are used, tying local stratotypes to international stage boundaries.

Documentation for a stratotype includes a measured, continuous section, a detailed lithological description, the vertical sequence, fossil content, age implications, and correlations with other sections. The selection and any subsequent revisions involve peer review and, where applicable, approval by the relevant geological authority or commission.

Stratotypes are essential for consistent reference in geology, enabling robust global correlation, standardized unit naming, and clear communication of stratigraphic boundaries across regions.