Stratigraphie

Stratigraphy is the branch of geology that studies layered rocks (strata) and their relationships in time and space. It aims to reconstruct the geological history of a region by describing, classifying, and correlating rock sequences.

Key principles include the law of superposition, original horizontality, lateral continuity, cross-cutting relationships, and faunal succession. Together these principles enable geologists to determine relative ages and connect strata across the landscape.

Stratigraphic practice uses units such as beds and formations (and their subdivisions into members and groups) and, at higher levels, formations, groups, and supergroups. Methods fall into lithostratigraphy (rock type), biostratigraphy (fossil content), chronostratigraphy (age dating), magnetostratigraphy (magnetic polarity), chemostratigraphy (chemical signatures), cyclostratigraphy (orbital cycles), and sequence stratigraphy (depositional sequences tied to sea level).

Chronostratigraphy links rocks to ages and forms the basis of the geologic time scale. Biostratigraphy uses fossils, especially index fossils, to date and correlate strata. Other methods provide independent age constraints and permit correlation across regions.

The field originated in the 18th–19th centuries with pioneers such as Steno and William Smith and has grown to play a central role in geology, paleontology, and natural resource exploration. Stratigraphy underpins petroleum geology, groundwater studies, engineering geology, and archaeological stratigraphy, where layered deposits provide relative chronology and context for cultural remains.