LATE PENNSYLVANIAN CARBONATES OF THE NORTHERN APPALACHIAN BASIN: CRITERIA TO DISTINGUISH BRACKISH AND FRESHWATER CONDITIONS

The depositional parameters of the Pennsylvanian cyclothems of the northern to central Appalachian Basin, including Ohio, Pennsylvania, West Virginia, and Kentucky, have consistently been subject to controversy. Nonmarine limestones within these cyclothems have traditionally been referred to as freshwater to brackish units; detailed sedimentologic analyses of these limestones are rare. An overview study contrasting the depositional paleoenvironments of freshwater and brackish limestones of the Pennsylvanian cyclothems throughout the northern to central Appalachian Basin has never been attempted. Marine limestones within the northern to central Appalachian Basin range from lower to upper Pennsylvanian, while nonmarine carbonates occur stratigraphically from the Allegheny Group (middle Pennsylvanian) through the Conemaugh and Monongahela Groups (upper Pennsylvanian) into the Dunkard Group (lower Permian). This study focuses upon limestones within the Conemaugh and Monongahela Groups sampled from outcrops in Ohio, Pennsylvania, West Virginia, and Kentucky. The units were selected due to their lateral continuity across the basin, offering the potential to display a transition from marine into freshwater depositional regimes. The key question is whether the carbonates represent truly freshwater, brackish-marine conditions, or both. This will aid in a more precise determination of paleoenvironment and distance to shoreline within cyclothemic sequences. Textures of limestones across marine to nonmarine transects in the proximal to distal Appalachian foreland basin were studied to identify trends in sedimentary structures, diagenetic features, and fossil occurrences. Textures of brackish-water carbonate sediments including matrix-supported siliciclastic micro-breccias, suggest marine incursions, perhaps during storms. These disturbed textures can contain Spirorbis, a member of the family Serpulidae. These polychaete worms have been in existence for roughly 500 million years and secrete a carbonate tube about their body, attaching to a fixed object, such as a shell, rock, or plant. Spirorbids found in Carboniferous cyclothems have been interpreted as freshwater animals; however, their physiology cannot be supported in a freshwater milieu.