Notes on geochronologic and chronostratigraphic units

There is no need to regard the term "geochronology" as a edundant synonym o f either "geologic time scale" or "radiometric dating." This term may reaso nably be used in reference to all principles and methods employed in geolog y to determine the relative and numerical ages of all kinds of rocks. The t erm "chronostratigraphy" would best be restricted to the process of definin g 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 a bsolute numerical age before present), and biochronologic units (spans of t ime defined theoretically by paleobiological events). A chronostratigraphic unit is a set of material, existing, stratified rock that was formed durin g a given span of time (geochronologic unit), Therefore, a geochronologic u nit must be defined before the corresponding chronostratigraphic unit can e ven be conceptualized. Chronostratigraphic units are generally not isochronous bodies of rock, bec ause owing to the unconformities that exist over any appreciable geographic area, their upper and lower material boundaries are not everywhere synchro nous. Thus, particular systems, series, and stages are material stratigraph ic 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 conventi onal function, and will rarely if ever need to be redefined. We can continu e to discover and debate important events in Earth history without insistin g that such events must be used to constantly redefine the boundaries of ou r global temporal pigeonholes.