Unconformity-bounded units are bodies of rocks bounded above and below by significant unconformities.

Unconformity-bounded units are generally composed of diverse types of any kind or kinds of rocks (sedimentary, igneous, metamorphic, or any combination of two or more of these kinds). Unconformity-bounded units are mappable stratigraphic units differentiated and set apart from underlying and overlying units only by being separated from them by their bounding stratigraphic discontinuities. Lithologic properties, fossil content, and chronostratigraphic span of the rocks on either side of a bounding unconformity are significant for the recognition of an unconformity-bounded unit only to the extent that they serve to recognize the boundary unconformity.

Lithostratigraphic, biostratigraphic, and chronostratigraphic units will continue to be those most frequently used in stratigraphic work, but in certain areas and for certain purposes, unconformity-bounded units are invaluable stratigraphic units, almost “natural” units, which the stratigrapher may be able to use for a clear and pragmatic approach to stratigraphic analysis and for a descriptive, lucid interpretation of geologic history. Unconformity-bounded units lend themselves, for instance, to the expression of those aspects of the geologic development of the Earth dealing with its orogenic episodes, its epeirogenic cycles, and its phases of eustatic sea-level changes. These geological events are commonly recorded by unconformities in the stratigraphic succession. Unconformity-bounded units, for this reason, have sometimes been considered to be equivalent to “sedimentary cycles” or tectonically controlled stratigraphic units: stratotec-tonic, tectostratigraphic, tectonostratigraphic, or tectogenic units; tectonic cycles; tectosomes; structural or tectonic stages; and so on. All of these types