Notes on geochronologic and chronostratigraphic units

Abstract

There is no need to regard the term “geochronology” as a redundant synonym of either “geologic time scale” or “radiometric dating.” This term may reasonably be used in reference to all principles and methods employed in geology to determine the relative and numerical ages of all kinds of rocks. The term “chronostratigraphy” would best be restricted to the process of defining the formal subdivisions of the geologic time scale. “Geochronologic unit” is best used as a general category term. It includes aurichronologic units (spans of time defined operationally by golden spikes), geochronometric units (spans of time defined theoretically in terms of absolute numerical age before present), and biochronologic units (spans of time defined theoretically by paleobiological events). A chronostratigraphic unit is a set of material, existing, stratified rock that was formed during a given span of time (geochronologic unit). Therefore, a geochronologic unit must be defined before the corresponding chronostratigraphic unit can even be conceptualized. Chronostratigraphic units are generally not isochronous bodies of rock, because owing to the unconformities that exist over any appreciable geographic area, their upper and lower material boundaries are not everywhere synchronous. Thus, particular systems, series, and stages are material stratigraphic units of geographically variable time value within the maximum temporal limits of the corresponding periods, epochs, and ages. Geochronologic boundaries that have been formally ratified serve a conventional function, and will rarely if ever need to be redefined. We can continue to discover and debate important events in Earth history without insisting that such events must be used to constantly redefine the boundaries of our global temporal pigeonholes.