stratigraphy

Stratigraphy is the branch of geology that studies rock layers and their distribution, deposition, and age relationships. It seeks to reconstruct the sequence of events that formed the Earth's crust by analyzing the arrangement and content of sedimentary, volcanic, and other layered rocks.

Key principles include the law of superposition, original horizontality, lateral continuity, the principle of cross-cutting relationships, and faunal succession. Together, these concepts allow geologists to establish a relative chronology for strata and to correlate layers between distant areas.

Stratigraphy uses different types of stratigraphic units such as formations, members, beds, and groups. Lithostratigraphy groups rocks by lithology; biostratigraphy uses fossils to identify and correlate units; chronostratigraphy links rock successions to actual intervals of geological time; sequence stratigraphy analyzes how depositional sequences respond to changes in relative sea level. Unconformities mark gaps in the record and can reflect erosion or non-deposition.

Dating and correlation combine relative methods with absolute dating tools. Relative dating places strata within a relative order; absolute dating provides age estimates with radiometric techniques. Correlation uses fossils (biostratigraphy), magnetism (magnetostratigraphy), chemical signatures (chemostratigraphy), and distinctive lithologies to align strata across regions.

Stratigraphy underpins much of geology and applied earth sciences. It informs petroleum geology, groundwater exploration, palaeoenvironmental reconstruction, archaeology, and natural hazard assessment by interpreting depositional settings, tectonic context, and long-term climate change.