Abundant evidence from many widely distributed stratigraphic sections in Colorado, New Mexico, Arizona, Wyoming, Montana, Idaho, Utah, Nevada, and elsewhere indicates the presence of a widespread unconformity at about a sub-Lower Mississippian position through most, if not all, of the United States Cordilleran region. This unconformity appears to be contiguous on the east with the regional unconformity occurring at about the same stratigraphic position over most of the continental interior and eastern part of the United States.

In most of the Cordilleran region, Lower Mississippian strata rest unconformably on rocks ranging in age from Precambrian to Late Devonian. Recognition of this unconformity in the western United States, especially where units above the break rest on Devonian rocks, substantiates the Piankasha-Tamaroa Sequence boundary as a major discontinuity distinct from the sub-Piankasha unconformity.

Another important regional unconformity, beneath Upper Devonian strata containing the Cyrtospirifer zone fauna, is widespread in the Cordilleran region and perhaps elsewhere. In the original definition of the Tamaroa Sequence, Wheeler (1963) alternately employed both the intra-Upper Devonian unconformity and the higher unconformity near the base of the Mississippian System as the lower boundary of the Tamaroa. The unconformity near the base of the Mississippian System is regarded as the more reasonable and practical lower boundary for the Tamaroa Sequence in the western United States because the regional relations of the intra-Upper Devonian unconformity are subject to different interpretations, and because the stratigraphically higher unconformity near the base of the Mississippian i more widespread in the Cordilleran region.