An unconformity-bounded unit is a body of rock bounded above and below by specifically designated discontinuities in the stratigraphic succession (angular unconformities, disconformities, and so on), preferably of regional or interregional extent. Being bounded by stratigraphic discontinuities is the single diagnostic criterion used to establish and recognize unconformity-bounded units; they should be extended only as far as both of its bounding unconformities are identifiable. Unconformity-bounded units can be composed of diverse kinds of any class or classes of rocks—sedimentary, igneous, metamorphic, or any combination of two or more of these classes. Neither the lithologic character of the rocks that compose the unit, nor their fossil content, nor the time span it represents, however, enters into its definition and recognition. Unconformity-bounded units, therefore, are not lithostratigraphic, biostratigraphic, or chronostratigraphic units. They are what their name indicates—unconformity-bounded units, a distinct and separate kind of stratigraphic unit that requires separate recognition. The basic unconformity-bounded unit is the synthem. Synthems are given geographic names derived from features at or near the location where the unit is well developed.

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