Stratigraphic Principles and Correlation

A handful of simple principles — superposition, original horizontality, lateral continuity, and faunal succession — let geologists order layered rocks in time and match them across distant regions.

Definition

Stratigraphic principles are the rules — superposition, original horizontality, lateral continuity, cross-cutting relationships, and faunal succession — used to establish the relative ages of rock layers, and correlation is the matching of strata of equivalent age or rock type between separate locations.

Scope

This topic covers the foundational principles of relative dating in layered rock and the methods used to correlate strata between locations, including lithostratigraphy, biostratigraphy using fossil successions, and the recognition of unconformities. It supplies the time framework that sedimentology and stratigraphy depend on.

Core questions

• What principles establish the relative ages of rock layers?
• How do fossils allow rocks of the same age to be correlated worldwide?
• What do unconformities reveal about gaps in the rock record?

Key theories

Principles of relative dating

Steno's principles of superposition, original horizontality, and lateral continuity, together with cross-cutting relationships, allow the order of geological events to be reconstructed from the geometry of rock bodies.

Faunal succession and biostratigraphy

Smith demonstrated that distinctive fossil assemblages occur in a consistent vertical order, so strata in different places can be correlated by their contained fossils, the basis of biostratigraphy.

Mechanisms

Because sediment is deposited in successive horizontal layers, undisturbed strata preserve a chronological order that can be read by superposition. Cross-cutting features such as faults and intrusions postdate the rocks they cut. Fossils that evolved, spread, and went extinct provide time markers that allow distant successions to be correlated, while unconformities mark intervals of erosion or non-deposition that interrupt the record.

Clinical relevance

Stratigraphic correlation is the working framework of petroleum and mineral exploration, groundwater mapping, and geological hazard assessment, and it provides the relative time scale into which absolute ages from geochronology are placed.

History

Steno set out the basic principles of stratigraphy in 1669. In the early nineteenth century William Smith, working as a canal surveyor, recognized that fossils could correlate strata and produced the first geological map of a country, founding biostratigraphy and applied stratigraphy.

Key figures

• Nicolas Steno
• William Smith
• Georges Cuvier

Seminal works

• steno1669
• smith1815

Frequently asked questions

What is the law of superposition?

The law of superposition states that in an undisturbed sequence of sedimentary rocks, each layer is younger than the one beneath it and older than the one above, so the order of layers records the order in which they were deposited.