A prominent unconformity, present across shallow shelf areas of the Euramerican paleoequatorial basins, is used to demark the boundary between the Mississippian and Pennsylvanian subsystems. This unconformity, the mid-Carboniferous eustatic event, is generally attributed to a major glacio-eustatic sea-level fall. Although a Mississippian-Pennsylvanian unconformity is recognized throughout most of the Appalachian region, the record of the mid-Carboniferous eustatic event in the structurally deepest part of the basin has been controversial. Based on early reports that suggested the most complete Pennsylvanian section was present in southern West Virginia, various conceptual depositional models postulated continuous sedimentation between the youngest Mississippian Bluestone Formation and the oldest Pennsylvanian Pocahontas Formation. In contrast, tabular-erosion models envisioned axial drainage systems that evolved in response to changing basin dynamics. These models predicted a Mississippian-Pennsylvanian unconformity. All these models suffered from a lack of biostratigraphic control. The presence of a sub-Pocahontas paleovalley, herein named the Lashmeet paleovalley, has been confirmed in southern West Virginia. The Lashmeet paleovalley was incised over 35 m into Bluestone strata and filled by lithic sands derived from the Appalachian orogen to the northeast and east. The polygenetic Green Valley paleosol complex marks the Bluestone-Pocahontas contact on associated interfluves. Together, these features indicate a substantial period of subaerial exposure and argue strongly in favor of a Mississippian-Pennsylvanian unconformity. Paleontologic data from the Bluestone Formation, including marine invertebrates and conodonts from the marine Bramwell Member and paleofloral data, support a late, but not latest, Arnsbergian age assignment. Marine fossils are not known from the Pocahontas Formation, but macrofloral and palynomorph taxa support a Langsettian age for most of the Pocahontas. The biostratigraphic, sedimentologic, and paleogeographic data support the presence of an early Pennsylvanian (middle to late Namurian) disconformity in the Appalachian Basin that corresponds to the mid-Carboniferous eustatic event.

Abstract Paleodrainage mapping of the Mississippian-Pennsylvanian unconformity in northwestern West Virginia verifies the existence of an incised, sub-Pennsylvanian paleovalley system there that extends for over 130 km. The paleovalleys are filled mostly with quartzose sandstones of the New River Formation. This paleovalley system was carved by a southwest-draining network of rivers that was rejuvenated during the mid-Carboniferous. Bedload-dominated streams that occupied the paleovalleys deposited most of the valley-fill sediment. Regional paleodrainage data indicate that the sub-Pennsylvanian paleovalleys in northwestern West Virginia form the middle reach of a major paleoriver system which includes the Middlesboro paleovalley in eastern Kentucky, the Sharon paleovalley in eastern Ohio, and the Perry paleovalley in southeastern Ohio. This regional paleodrainage network (herein named the Middlesboro-Sharon-Perry paleovalley system) transported sediment from the craton north of the central Appalachian basin to the mid-Carboniferous depocenter in southwestern Virginia. Although it has been previously suggested that maximum erosional development of the Mississippian-Pennsylvanian unconformity occurred during the Early Pennsylvanian, paleoslope considerations rule against an Early Pennsylvanian age for carving of the Middlesboro-Sharon-Perry paleovalley system. Existing biostratigraphic data from the mid-Carboniferous depocenter in southern West Virginia support the existence of an Upper Mississippian (Chokierian-Alportian stages of the Namurian Series) hiatus there, suggesting that incision of the Middlesboro-Sharon-Perry paleovalley system was dominantly a Late Mississippian (Chokierian-Alportian) event. Uplift of the craton north of the central Appalachian basin combined with subsidence within the basin that increased in rate toward the mid-Carboniferous depocenter in southern West Virginia created the generally south-dipping paleoslope which the Middlesboro-Sharon-Perry paleoriver system drained. The regional paleodrainage picture rules against tectonic uplift of the Cincinnati arch as a key factor in driving the incision of the Middlesboro-Sharon-Perry paleovalley system. Carving of this paleovalley system apparently was driven by a previously documented, Late Mississippian (Chokierian-Alportian) eustatic sea-level drop. Regional tectonic uplift during the Early Pennsylvanian may have influenced erosional development of the Mississippian-Pennsylvanian after the Late Mississippian incision of the Middlesboro-Sharon-Perry paleodrainage system.