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The boundary relationship between the Mississippian and Pennsylvanian Systems has been one of the most controversial aspects of Carboniferous stratigraphy in "the central Appalachian basin. It has provided the impetus and subject matter for numerous papers, field trips, and symposia featuring widely divergent points of view concerning the position and character of the boundary. Regionally, this boundary relationship is exacerbated by variations in the lithology, depositional environment, and age of associated strata and by the nature of the boundary contact. For example, involved strata vary lithically from marine limestone and shale to terrestrial coal-bearing elastics and, in age, from Early to latest Mississippian subjacent to the boundary and from oldest to Middle Pennsylvanian superjacent to the boundary. At the craton edge, the boundary is an unconformity with a significant hiatus but in the trough of the geosyncline, it is conformable in a continuously deposited sequence. Where unconformable, strata below the boundary exhibit incised drainage patterns, deep sink holes, and upland paleosol development. In the conformable trough area, the boundary is transcended by a coal-bearing facies of Late Mississippian to Early Pennsylvanian age. Biostratigraphically, the boundary is at the base of zone 4 of Read and Mamay (1964), specifically at the first appearance of Neuropteris pocahontas D. White. Below the boundary, terrestrial beds contain a newly recognized floral zone intermediate between floral zones 3 and 4 (Pfefferkorn and Gillespie, 1981a) and marine beds contain the youngest known Mississippian invertebrate fauna (Gordon and Henry, 1981). Stratigraphically, the systemic boundary in the geosynclinal trough is placed at the gradational contact between the Bluestone and Pocahontas Formations, where the terrestrial coal-bearing sequence assigned to the Pocahontas (Lower Pennsylvanian) attains its maximum thickness of about 700 ft. However, the lower sandstone member or tongue of the Pocahontas Formation wedges out northwestward and, at its terminus, the boundary passes into the upper part of the Bluestone Formation at the contact between the Bramwell Member (Upper Mississippian), a well-defined marine unit, and the upper member (Lower Pennsylvanian) of the Bluestone Formation. This gradational nature of strata across the systemic boundary prevails in the geosyclinal trough but only where the Pocahontas and Bluestone Formations are present. These formations together with underlying Upper Mississippian units are truncated progressively northwestward by an unconformity that originates in the lower part of the overlying New River Formation (Lower Pennsylvanian). As a result of this truncation, the systemic boundary at the western edge of the basin is delineated by an unconformity that places Middle Pennsylvanian rocks assigned to the Lee and Breathitt Formations in contact with strata as old as the Borden Formation (Lower Mississippian). 1U.S. Geological Survey, Reston, Virginia 22092

Characteristics of the Mississippian-Pennsylvanian boundary and associated coal-bearing rocks in the southern Appalachians