An international geochronologic scale is needed in order to provide a single universal standard of reference for dating rock strata, or events recorded in rock strata, anywhere in the world.
There are many possible means of relating rock strata and the geologic events they record to the passage of timephysical relations of strata (law of superposition), degree of isotopic decay, stage of organic evolution indicated by fossils, and other methods. Each is useful, but each is fallible under certain circumstances. The ideal standard, therefore, is one which does not depend on any one method but allows and encourages the utilization of all methods of age determination and time correlation.
All methods of geologic age determination and time correlation must be based fundamentally on features of the rock strata; and thus the rock strata constitute the best register in which to inscribe the standard reference pointsstratotypesfor whatever units or other horizons we wish to recognize in a global geochronologic scale. Only thus, by the designation of standard unit stratotypes, boundary stratotypes, and other horizon stratotypes, can we establish unequivocal definitions of points on this scale in a manner which lends itself to the utilization of all methods of age determination and time correlation, reinforcing the evidence from each by that from all others, and limited for each only by its capacity to usefully contribute.
The practical value and utility of points on a global geochronologic scale is dependent on the extent and accuracy with which chronohorizons coincident with the stratotypes of these points can be traced or identified elsewhere in the world. Therefore, effort should be made to designate these stratotypes at places in the stratigraphic sequence which, through their coincidence with, or relation to time-significant features (isotopic dates, fossils, magnetic reversals, etc), particularly lend themselves to reliable widespread time correlation. Individual chronohorizons are no less important than chronostratigraphic units and their boundaries in the reference base for a geochronologic scale.
Some geologists have questioned a basic tenet of chronostratigraphytheoretical boundaries of a chronostratigraphic unit should be everywhere isochronouson the grounds that erosion or nondeposition has locally altered the position of these boundaries from those seen at the stratotypes. However in such cases, the boundary is still theoretically present at the same time level, although included within the time value of the hiatus or unconformity.
The stratotype concept should be applied to the global geochronologic scale, including both Quaternary and Precambrian.