Stratigraphic unit

A body of rock or sediment classified by its stratigraphic relationships, physical characteristics, fossil content, or age. Stratigraphic units provide a formal framework for describing and correlating Earth materials.

Lithostratigraphy: The classification of rock bodies by their observable lithologic characteristics and stratigraphic relationships. It defines units such as formations by rock properties rather than age.
Stratigraphic boundary: A surface separating two adjacent stratigraphic units or intervals. A unit's boundaries specify where its recognized body begins and ends.
Geologic map: A map showing the distribution, age, and relationships of geological materials at Earth's surface. Mapped units make the regional arrangement of rocks legible.
Lithodeme: A fundamental body of intrusive, highly deformed, or intensely metamorphosed rock classified by lithic characteristics. Unlike a formation, a lithodeme need not have stratified or mappable bedding.
Biostratigraphy: The subdivision and correlation of strata using their fossil content. Fossil assemblages distinguish biostratigraphic units within rock sequences.
Type section: A designated reference exposure that defines the characteristics and boundaries of a stratigraphic unit. A type section anchors a unit's definition to an observable reference.
Sequence stratigraphy: The analysis of genetically related sedimentary strata bounded by surfaces of erosion or nondeposition. It groups strata into sequences to interpret changing accommodation and sediment supply.
Geochronologic unit: A division of geologic time represented by rocks formed during a specified interval. It names an interval of time, whereas a chronostratigraphic unit is the rock body formed then.
Chronostratigraphy: The classification of rock bodies according to the time intervals in which they formed. It organizes units by geologic time rather than composition alone.
Stratigraphic nomenclature: The rules and conventions for naming and classifying stratigraphic units. Standardized names let geologists communicate about units consistently.

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