PHASED DEVELOPMENT OF A SUBAERIAL PALEOKARST PLANE IN UPPER PENNINGTON FORMATION LIMESTONES (MISSISSIPPIAN–LATEST CHESTERIAN) AND ASSOCIATED PALEOKARST FEATURES

The uppermost Pennington Formation (Mississippian – latest Chesterian) consists of 3-4 m of limestone deposits, which, in some areas, contain evidence of subaerial paleokarst. A 100 m long continuous outcrop of the upper Pennington Formation was studied at the Big South Fork National River and Recreation Area, in Scott County, Tennessee. Field and petrographic methods were used to demonstrate subaerial exposure as the mechanism for localized paleokarst features and the phased development of a karst plain. Paleokarst features associated with subaerial exposure were mapped and catalogued according to three scales of features: macroscale, mesoscale, and microscale. Macroscale refers to major features expressed as paleo-topographic variation along exposure surface, such as paleo-dolines, the largest of which has a width of approximately 30m, and a depth of 4m. Other features include a shallow and flat-floored kamenitza with a width of 7m and a depth of only 0.5m. Mesoscale features include mantling limestone breccia-conglomerate, red and green claystone paleosols, limestone breccia clasts, and iron oxide and oxy-hydroxide crusts. Microscale features include meteoric cements, grain micritization, reddening of the limestone, and extensive pore development. Five phases of paleokarst are recognized: Phase 1 is the initial development of paleokarst features, particularly at the microscale. Phase 2 is the development of macroscale paleo-dolines and paleo-kamenitzas. Phase 3 occurred as soil and vegetation developed on the karst plain. Phase 4 marked pulses of early Pennsylvanian sedimentation, during which time, paleo-channels developed, mantling deposits, including a residual breccia-conglomerate, began to drape exposed limestone surfaces, and a collapse at the paleo-exposure surface occurred. Finally, phase 5 involved the movement of reducing fluids along the unconformity and ultimately precipitation of burial cements. These phases of paleokarst offer evidence of a prolonged period of subaerial exposure and landscape stability in a weathering regime in which the limited stratigraphic thickness of soluble limestone limited the depth and extent of paleokarst features.