Abstract 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. Areas 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 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.

At several localities in southwestern Nevada and adjacent California the Mississippian-Pennsylvanian boundary coincides with the contact between the Eleana Formation and the overlying Tippipah Limestone, or their equivalent strata. Near Red Canyon southeast of the Eleana Range, southern Nye County, Nevada, a thin limestone in the Eleana containing the ammonoids Cravenoceras hesperium Miller and Furnish and C. merriami Youngquist of late Chester age is separated by approximately 100 ft of shale and minor quartzite from platy limestone of the Tippipah, containing Diaboloceras aff. D. neumeieri Quinn and Carr of Morrow age. Three inches of conglomeratic limestone marks the base of the Tippipah. In the hills northwest of Frenchman Flat, southern Nye County, 14 ft of shale separates a limestone in the Eleana containing late Chester brachiopods from the basal Tippipah containing the ammonoids Bisatoceras, Diaboloceras, and Stenopronorites. In the hills southwest of Indian Springs, northwestern Clark County, Nevada, we refer a similar sequence (the Indian Springs Member of Longwell and Dunbar, 1936, of the Bird Spring Formation) to rocks equivalent to part of the Chainman Shale and Tippipah Limestone. The Chainman equivalent is about 85 ft thick and contains Mississippian (Chester) brachiopods 25 ft below the top. The basal Tippipah equivalent contains Bisatoceras, Diaboloceras, Stenopronorites, and Syngastrioceras. One foot of conglomeratic limestone marks the base of the Pennsylvanian. The same lithologic sequence is recognized on the northeast flank of the Nopah Range, southeastern Inyo County, California. The Tippipah ammonoids appear closest to late Morrow forms, indicating a hiatus at the base of the Pennsylvanian in this region. A Mississippian-Pennsylvanian disconformity is further suggested by the abrupt lithologic change and the conglomeratic limestones.