The role of stratotypes in stratigraphy: Part 1. Stratotype functions

Because our concepts of various “natural” geological events and entities are always subject to modification with new data, we must provide a principled answer to the question: “How much can the scope of a stratigraphic or temporal unit change and yet still be called by the same name?” This issue is clarified by noting that stratotypes have three distinct functions in stratigraphy—the boundary-defining, example-providing, and name-bearing functions. Suggested names to denote these functions are “boundary-defining stratotype” (including unit- and boundary stratotypes), “exemplary stratotype,” and “nominal stratotype.” The analogy between type specimens in biology and type sections in stratigraphy is valid for nominal and exemplary stratotypes, but invalid for boundary-defining stratotypes. The terms boundary-defining, exemplary, and nominal refer to the functions performed by a given stratotype, whereas the terms holo-, para-, lecto-, neo-, and hypostratotype refer mainly to the historical circumstances under which a given stratotype is designated. and boundary stratotypes delimit an authorʹs concept of the boundaries of a given stratigraphic entity in a particular section at a given time, but generally cannot be expected to permanently fix those boundaries. Important exceptions are the strict boundary stratotypes known as Global Stratotype Sections and Points (GSSPs). Exemplary stratotypes serve as examples of an authorʹs concept of a given stratigraphic entity, but neither define nor constrain the boundaries of that entity. Nominal stratotypes constrain, but do not define the boundaries of a stratigraphic entity. They are divided into “loose” and “strict” subcategories, the former being appropriate for lithostratigraphic units, the latter for biochronologic units with geographic names. Both kinds of nominal stratotypes are also relevant to standard global chronostratigraphy. signation of boundary-defining stratotypes for biostratigraphic units with binomial names is possible but essentially pointless, and such units are better characterized by exemplary stratotypes. For several reasons, however, biostratigraphic units with binomial names cannot have nominal stratotypes. ernative method of attaching a name to a span of time involves what are here called nominal points, which are best used for the nominal definition of provincial biochronologic units with geographic names. While permitting much instability in our concept of a given biochron, the nominal point method does not suffer from the problem of potential temporal overlap inherent in the strict nominal stratotype approach. These distinctions should be made in future revisions of stratigraphic codes and guides.