Unconformities are conspicuous stratigraphic features in the Phanerozoic strata of the Western Interior, U.S.A. Important unconformity traps for petroleum are found in strata of Ordovician, Mississippian, Permo-Pennsylvanian, and Cretaceous ages. The role of the unconformity in trapping petroleum is principally by truncation of porous zones and by providing a seal for the trap. Lenticular zones of porosity and permeability in sandstones immediately above the erosional surface are also important stratigraphic traps, both in marine and nonmarine strata.

An unconformity is defined as a sedimentary structure in which two sets of strata (or groups of rocks) are separated by an erosional surface where the erosion may be by subaerial or submarine processes. Factors to be considered in evaluating unconformities are structural discordance, nature of contact, hiatus, duration of erosion, areal distribution, and cause.

Principal causes for unconformities are sea-level changes, tectonics, or a combination of both. Two examples of unconformities controlling petroleum occurrences are as follows: (1) porosity beneath a paleokarst surface at the top of the Mississippian carbonates (Kevin-Sunburst field, Montana), and (2) the fluvial sandstones of Early Cretaceous age (Muddy Formation) that fill an incised drainage system resulting from a sea-level drop and subsequent rise approximately 97 m.y.B.P. (Recluse field, Powder River basin, and Third Creek field, Denver basin).

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