In the type regions for the Mississippian and Pennsylvanian Systems, the boundary between the two systems occurs at a hiatus that varies laterally in age. The section across the boundary seems to be complete in the central Appalachian region, where a reference section has been established for the Pennsylvanian in West Virginia. However, the scarcity of marine beds in the Lower Pennsylvanian in that area makes it extremely difficult to establish detailed correlations with areas of predominantly marine deposition. Search has been underway by a working group studying this boundary for a section of continuous (and principally marine) deposition from Late Mississippian into Early Pennsylvanian, which could serve as a boundary stratotype for the transition between the two. Area where deposition seems to have been continuous across the boundary are known in Tennessee, Alabama, Arkansas, Oklahoma, Utah, Nevada, Idaho, Wyoming, and Alaska.

The Lower-Middle Carboniferous boundary of the official classification (FOOTNOTE 4) in the Soviet Union approximates the Mississippian-Pennsylvanian boundary. Field trips of the Eighth Carboniferous Congress, in 1975, provided the opportunity for geologists to study sections in the Moscow basin, southern Ural Mountains, Donets basin, northern Caucasus, Kuznetsk basin, and parts of Middle Asia. Large hiatuses are present at this boundary in the Moscow and Kuznetsk basins. A hiatus is also present in the southern Urals at the boundary between the Lower and Middle Carboniferous (base of the Bashkirian Stage as revised in November, 1974). In other sections, particularly one near Chimkent in south-central Asia, deposition appears to have been continuous across this boundary.

At present, a radiometric date of roughly 320 m.y. for the Mississippian-Pennsylvanian boundary is reached by extrapolation of rather meager data. Before the age of the boundary itself can be determined accurately, a boundary stratotype must be designated.