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

Abstract

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