Unconformities have been used commonly as boundaries of stratigraphic units; such unconformity-bounded units have played an important role in the development of stratigraphy and will continue to do so. However, their value has been impaired by failure to understand that they constitute an independent category of stratigraphic units — that of unconformity-bounded units — with a distinctive significance of its own, and that they should not be confused with other commonly accepted kinds of stratigraphic units.
Many unconformity-bounded units have been regarded as lithostratigraphic units, even though they may be characterized by being bounded by unconformities. In order to maintain the original concept of lithostratigraphic units, this usage should be avoided. Similarly, many unconformity-bounded units are considered to be chronostratigraphic units in spite of the fact that unconformity surfaces inevitably cut across isochronous horizons and hence cannot be true chronostratigraphic boundaries. If unconformity-bounded units are to be the basis for chronostratigraphic units, their boundaries should be redefined elsewhere in continuously deposited sections. Otherwise, they should be regarded not as chronostratigraphic units but as belonging to the here-proposed independent category of unconformity-bounded units.
Probably the most widely used unconformity-bounded units are those bounded by unconformities of regional or interregional magnitude and comparable in thickness to supergroups and in time span to one or more stratigraphic systems. The formal term “synthem” is recommended for these units when they are of major rank. Synthems are useful in cratonic areas where stratigraphic schemes consisting of units of this kind provide a basis for tectonic correlation and best reveal geologic (particularly tectonic) history. The use of synthems also promotes the recognition of natural geologic provinces and aids in the establishment of natural stratigraphic classifications.
The term “interthem” is proposed for minor disconformity-bounded units comparable in thickness with a formation or in time span with a stage. The prefixes “sub” and “super” may be used if further ranks of classification are needed.