2.5 Amendment and rejection of formal units
2.5.1 Amendment of formal units
Formally erected geological units can be redefined or revised.
Redefinition involves giving a new description of the content of a unit without changing its boundaries, rank or category. A formation, for example, may originally have been characterized as shale, whereas more recent investigation has shown that the unit chiefly consists of limestone. In the formal compound formation name, "shale" can be replaced by "limestone" without the proper name (geographical term) needing to be changed.
Revision involves (a) minor changes in the definition of one or more boundaries of the unit, (b) change in rank, (c) change in category, or (d) changes in two or more of a-c.
Minor changes in definition of boundaries may be desirable following new investigations. If such revision only alters a small portion of the original unit, its name can be retained. See Chapter 4 for biostratigraphical and time units.
A unit can be changed in rank without needing to have its boundaries redefined or the geographical part of its name changed. A unit can be revised in this way both within and outside its type area, or only outside it.
A unit can be changed in category, in which case its proper (geographical) name usually has to be changed. If a unit is redefined to a closely related category, for example from a metamorphic lithodemic unit to a metamorphic lithostratigraphical unit, the original proper name can be retained.
If a unit is divided into two or more units having the same rank as the original one, the original proper name must not be used for any of the new ones. The proper name of the original unit may be retained if the rank of that unit is raised following the new division.