Cratonal contributions to a “classic” molasse: The Carboniferous Pottsville Formation of eastern Pennsylvania revisited

Abstract

The Carboniferous Pottsville Formation of eastern Pennsylvania in the central Appalachian foreland basin is considered a classic orogenic molasse, typical of other Acadian-Alleghanian clastic wedges along the eastern margin of Laurentia. Although the Pottsville conglomerates are thought to be derived entirely from early Alleghanian highlands, we have documented an unrecognized dramatic change in the sediment-dispersal pattern within the formation; northwest-directed paleocurrent indicators in older units (implying an orogenic provenance) are superseded abruptly by southwestward-directed paleocurrent indicators (implying a cratonal provenance). This alteration in the sediment-dispersal pattern is attributed to basin reorganization during Early Pennsylvanian time as tectonic loads migrated southward during orogenesis. The paleocurrent data warrant a revision of Early Pennsylvanian paleogeography and demonstrate that molasse facies can record a complex interplay between sediment dispersal systems along both active and cratonal margins of a foreland basin.