Geologic stages and other international subdivisions of the Phanerozoic portion of the geologic scale are defined by their lower boundaries at Global Stratotype Sections and Points (GSSPs). The main criteria for a GSSP are that primary and secondary markers provide the means for global correlation. GSSP theory and criteria are outlined, the status of ratified GSSPs provided, and three examples discussed of prominent GSSPs. Subdivisions of the International Stratigraphic Chart are summarized and illustrated.
TIME AND ROCK
Geologic time and the observed rock record are separate but related concepts. A geologic time unit (geochronologic unit) is an abstract concept measured from the rock record by radioactive decay, Milankovitch cycles, or other means. A “rock-time” or chronostratigraphic unit consists of the total rocks formed globally during a specified interval of geologic time. The chronostratigraphic units are grouped into a hierarchy to subdivide the geologic record on Earth progressively. This chronostratigraphic scale was originally established from a combination of regional lithologic units (e.g. the Chalk of England defined the “Cretaceous,” and the “Triassic” was assigned to a trio of distinctive formations in Germany) and of unique, non-recurring events provided by biological evolution.
These fragmentary chapters in the history of life and regional sediment facies gave rise to the succession of the standard geologic periods and the subdivision of periods into stages that form the chronostratigraphic time scale.